8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
1/57
Journal of Environmental Science and Health, 23:105161, 2005
Copyright C Taylor & Francis Inc.
ISSN: 1059-0501 (Print); 1532-4095 (Online)
DOI: 10.1080/10590500500234970
Human Melanocyte Biology,Toxicology, and Pathology
William H. Tolleson
National Center for Toxicological Research, U.S. Food and Drug Administration,Jefferson, AR, USA
The human melanocytes of the skin, hair, eyes, inner ears, and covering of the
brain provide physiologic functions important in organ development and maintenance.
Melanocytes develop from embryonic neural crest progenitors and share certain traits
with other neural crest derivatives found in the adrenal medulla and peripheral nervous
system. The distinctive metabolic feature of melanocytes is the synthesis of melanin pig-
ments from tyrosine and cysteine precursors involving over 100 gene products. These
complex biochemical mechanisms create inherent liabilities for melanocytic cells if in-
tracellular systems necessary for compartmentalization, detoxification, or repair are
compromised. Melanocyte disorders may involve pigmentation, sensory functions, au-
toimmunity, or malignancy. Environmental factors such as ultraviolet radiation and
chemical exposures, combined with heritable traits, represent the principal hazards
associated with melanocyte disorders.
Key Words:Melanocytes; Melanin; Melanoma; Ocular toxicity; Ototoxicity; Oxidative
stress
MELANOCYTE PHYSIOLOGY
Melanocytes are melanin-producing somatic cells that provide important phys-
iological functions in the skin, eyes, inner ear, and meninges (Table 1). Rec-ognized functions associated with melanin production in humans include pho-
toprotection, trapping reactive oxygen species, sequestering metal ions, and
binding certain drugs and organic chemicals (1, 2). Moreover, exfolliation pro-
vides a minor route for the elimination of toxic materials bound to melanin
embedded in the hair and outer skin layers. Human melanogenesis also
Address correspondence to William Tolleson, National Center for ToxicologicalResearch, U.S. Food and Drug Administration, Jefferson, AR 72079, USA; E-mail:[email protected]
This article is not an official guidance or policy statement of U.S. Food and DrugAdministration (FDA). No official support or endorsement by the U.S. FDA is intendedor should be inferred.
105
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
2/57
106 W. H. Tolleson
Table 1: Typical melanocyte functions.
Function Example Relevant melanocyte properties
Photoprotection Intrafollicular cutaneousmelanocytes
Melanin biosynthesis anddistribution to neighboringkeratinocytes
Scavenge organic andinorganic cations
Leptomeningealmelanocytes
Affinity of melanin fordrugs/chemicals
Elimination of excessmetal ions
Cutaneousmelanocytes
Shedding melanized hair shaftsbearing sequestered metals
Defense againstphotooxidative stress
Cutaneous and uvealmelanocytes
Consumption by melanin ofsinglet oxygen, hydroxylradical, and superoxideanion
Vision Uveal melanocytes;also retinal pigmentepithelium
Pigmented cells absorb straylight preventing internalreflections
Organogenesis Choroidal melanocytes Development of the iris andciliary
Organ maintenance Otic melanocytes Maintenance of the scalamedia of the cochlea
Antigen presentation Cutaneousmelanocytes
Phagocytosis; expression ofMHC II and ICAM-1;stimulate proliferation ofcytotoxic T-lymphocytes
influences interactions between individuals and cultures; the moral, social, andpsychological implications of this function have been addressed by others (35).
Additional functions of melanin are apparent in other species, such as creating
an opaque barrier that helps marine cephalopods evade potential predators,
imparting structural strength to seed pods in plants, and providing antibiotic
properties for insect immune systems (1). The relevance of these properties to
human melanocytes is unclear.
Unlike epithelial and mesenchymal constituents of the skin and eyes,
melanocytes are not responsible for the principal functions of these tissues.
Instead, melanocytes take up residence in between other cell types to provide
accessory functions, typically involving pigmentation. Melanoblasts, the embry-
onic precursors of melanocytes, migrate to different sites during development
to establish the melanocyte populations from which various melanocytic lesions
may arise (for review, see Piris and Rosai [6]). This review will emphasize four
populations of melanocytes located in the skin, eyes, inner ears, and covering
of the brain (Figure 1).
Cutaneous Melanocytes
The largest numbers of melanocytes are found in the skin and hair follicles.
The majority of proliferating cutaneous melanocytes in normal, out-bred, adult,
furred mammals are present in the papillary bulbs of hair follicles in an ap-
proximate one-to-five ratio with keratinocytes (Figure 2). Human interfollicular
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
3/57
Human Melanocyte Biology, Toxicology, and Pathology 107
Figure 1: Distribution of human melanocytes. Human melanocytes populate theintrafollicular epidermis, hair follicles, uveal tract of the eye, stria vascularis of the cochlea,and meninges covering the brain and spinal cord.
Figure 2: Hair follicle structure. Melanocytes are concentrated around the papillary bulb ofthe hair shaft. Additional melanocytes are present in the inner and outer root sheaths.
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
4/57
108 W. H. Tolleson
melanocytes are particularly abundant in the face, scalp, and the genital
regions, but are also found throughout the epidermis, including mucosal epithe-
lia such as the oral cavity and the anus, but are less abundant in the thicker
afollicular epidermis of palms and foot soles. Interfollicular melanocytes are
less common in the keratinized epidermis of nonhuman species. In human skin,
interfollicular melanocytes are found within the basal layer of the epidermis,
attached to up to 36 neighboring keratinocytes that comprise the epidermal
pigmentation unit (Figure 3) (7). Melanin is packaged within specialized vesi-
cles known as melanosomes for distribution to the surrounding keratinocytes
of the epidermal melanin unit. This occurs via a network of dendritic processes
originating from a melanocyte located within the basal layer of the epidermalstratified epithelium.
Figure 3: Human epidermal pigmentation unit. Melanosomes are distributed toneighboring keratinocytes via dendritic processes originating from a melanocyte residing in
the basal layer of the stratified epithelium. Viable pigment-carrying keratinocytes of thebasal layer undergoing differentiation are pushed upwards through the skin to form thestratum spinosum. Nuclear degeneration, crosslinking of the cell envelope, andkeratinization occur in the stratum granulosum. Fully keratinized, enucleated squams areexfollilated from the stratum corneum.
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
5/57
Human Melanocyte Biology, Toxicology, and Pathology 109
In contrast to the continually renewing epidermal pigmentation unit of
the intrafollicular epidermis, a different pigmentation mechanism is apparent
within the hair follicle (reviewed in ref. [8]). The follicular melanin unit un-
dergoes a cyclic pattern of melanogenesis synchronized with the hair cycle.
The human scalp hair cycle is comprised of a 35 year growth period (anagen
stages IIII), a three week regression phase (catagen), and a three month rest-
ing phase (telogen). Undifferentiated amelanotic melanocytes or melanoblasts
are recruited to the dermal papilla at beginning of anagen from a supply of
immature melanocytes located in the upper, permanent portion of the outer
root sheath. The immature melanocytes in the anagen hair bulb differentiate
and form dendritic connections with keratinocytes of the hair shaft cortex dur-ing anagen I and II. Melanogenesis and transfer of melanosomes to precortical
keratinocytes occurs primarily during anagen III and continues through cata-
gen, when melanogenesis decreases and the dendrites of bulbar melanocytes
retract. Extensive apoptosis occurs within the hair bulb during late catagen,
but the preexisting fully pigmented club hair shaft is usually retained in the
follicle through the end of telogen. After telogen it is often displaced by a newly
formed hair shaft produced in the subsequent anagen cycle.
The proliferation of cutaneous melanocytes is normally suppressed by close
physical association with epithelial cells. The co-expression of the E-cadherin
cell adhesion molecule by melanocytes and keratinocytes facilitates gap junc-tion formation and contributes to suppressing melanocyte proliferation (9).
Some cutaneous melanocytes switch from E-cadherin to N-cadherin expression;
melanocytes expressing N-cadherin are released from growth suppression via
attachment to epithelial cells, allowing them to proliferate and self-aggregate
to form moles or nevi, latin for nests. Switching from E- to N-cadherin expres-
sion confers increased resistance to apoptosis and also permits the migration of
melanocytes among N-cadherin-expressing dermal fibroblasts (10). The more
rapidly proliferating keratinocytes of the epidermal pigmentation unit retain
melanin granules donated to them by melanocytes as they continue to progress
through their characteristic program of terminal differentiation to establish
the keratinized epithelium of the skin. This process creates a sheltered envi-
ronment for epidermal melanocytes, protecting them under a stratified canopy
of melanized keratinocytes that provides a tough, waterproof, and pigmented
physical barrier. Also, because intrafollicular cutaneous melanocytes are lo-
cated primarily within the basal layer of the epidermis, they experience dimin-
ished oxygen tension and have restricted access to other factors transported via
the capillary blood supply of the dermis.
Ocular Melanocytes
Another large population of melanocytes resides within the uveal tract of
the eye, comprised of the choroid, ciliary body, and iris (Figure 4). The choroid,
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
6/57
110 W. H. Tolleson
Figure 4: Ocular anatomy.
located in the posterior portion of the eye, is separated from the retinal pigment
epithelium and neural retina by Bruchs membrane and is surrounded on its
outer boundary by the densely fibrous sclera. The cillary body extends from
the choroid in the anterior direction and forms the site of attachment for the
lens. The pigmentation of the iris provides photoprotection for the system of
capillaries, muscles, and motor nerves that regulate light entering the pupil.
Cells of the iris, ciliary, and retinal pigment epithelium are also pigmented, but
they are not true melanocytes, and the adenomas and adenocarcinomas that
arise from pigment epithelium cells are rare in humans (1114).
The microenvironment of uveal melanocytes differs significantly from that
of cutaneous melanocytes. The uveal tract is very highly vascularized, which
creates a higher oxygen tension than that of the central nervous system.
Melanocytes of the choroid have ready access to blood-borne factors and are
in physical contact with fibroblasts that form the sclera and with vascular en-
dothelial cells that form the choriocapillaris. Uveal melanocytes are typically
attached to other melanocytes, unlike cutaneous intrafollicular melanocytes
that are distributed singly among clusters of keratinocytes. The anterior
structures of the eye that overlay choroidal melanocytes (e.g., Bruchs mem-
brane, retinal pigment epithelium, photoreceptor layer, neural retina, vitre-
ous humor, lens, aqueous humor, and cornea) absorb or scatter the major-
ity of UV, visible, and infrared radiation found in sunlight (1517). Thus,
the potential for direct photodamage is greatest for interfollicular cutaneous
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
7/57
Human Melanocyte Biology, Toxicology, and Pathology 111
melanocytes, followed in decreasing order by follicular cutaneous melanocytes,
conjunctival melanocytes, and iris melanocytes. Melanocytes of the choroid and
cililary body receive virtually no exposure to UV or visible light. Unlike cuta-
neous melanocytes, which distribute pigment to adjacent keratinocytes, uveal
melanocytes retain the melanosomes they produce (18). Until recently it was
believed that melanin synthesis occurred continuously in interfollicular cuta-
neous melanocytes, cyclically during the anagen phase of the hair cycle in fol-
licular cutaneous melanocytes, but only during prenatal development by uveal
melanocytes (19). However, because optic treatments with the prostaglandin
analog latanoprost result in heterochromia and increased pigmentation of the
iris, it is apparent that melanin synthesis can be induced in adult eyes (20, 21).Further evidence that uveal melanocytes retain the potential for melanin syn-
thesis in adults is apparent in the observations that: (1) induction of tyrosinase
activity by latanoprost in cultured adult human uveal melanocytes is blocked
by the tyrosinase inhibitor 1-methyltyrosine, (2) the artificial melanin precur-
sor [3H]-methimazole labels the uveal tract of pigmented adult DBA mice, and
(3) tyrosinase activity is evident in the uveal tract of bovine eyes (2224). Thus,
the potential for melanin synthesis is present in adult ocular melanocytes.
Otic MelanocytesMelanocytes are also found in the stria vascularis of the cochlea. The stria
vascularis forms the lateral wall of the scala media, the endolymph-filled cham-
ber that houses the organ of Corti, which is the principal sound-detecting com-
ponent of the inner ear (Figure 5). The melanocytes of the stria vascularis are
termed intermediate cells and provide functions that are necessary for nor-
mal hearing (Figure 6). Deafness results from loss of these cells via trauma,
infection, or perhaps noise (25).
Ototoxic agents, such as ticarcillin, may damage intermediate cells, result-
ing in hearing loss (26). The range of functions provided by intermediate cells
has not been characterized extensively but it is known that the stria vascularis
is responsible for endolymph maintenance (27). Unlike most other bodily fluids,
the major cation of endolymph is not sodium, but potassium supplied by the
stria vascularis. Potassium transport is regulated to maintain the endocochear
potential at +80 mV via ion-selective channels expressed by cells of the scala
media (28). Expression of the Kir4.1 potassium channel is restricted to interme-
diate cells, suggesting that this melanocyte function is essential for endolymph
maintenance and hearing (29, 30).
Cephalic Melanocytes
Melanocytes are distributed through the meninges covering the central ner-
vous system, particularly within the ventrolateral leptomenigenes overlying
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
8/57
112 W. H. Tolleson
Figure 5: Structure of the inner ear. Otic melanocytes are found within the stria vascularis(SV) which covers the spiral ligament and bony labyrinth of the cochlea. The scala mediais the endolymph-filled chamber that houses the organ of Corti (OC). The scala media isseparated from the scala vestibuli by Reissers membrane (RM) and from the scalatympanium by the basilar membrane. (Guinea pig, second turn of cochlea, H&E stain, bypermission A.N. Salt, Ph.D., Washington Univ., St. Louis, MO, http://oto.wustl.edu/cochlea/accessed Feb. 2005).
the pons and medulla oblongata (31) (Figure 7). In addition to their well known
role as light-absorbing sunscreens for tissues exposed to the environment,melanosomes can scavenge potentially toxic organic and inorganic cations and
free radical species (32, 33). The physiological functions of leptomeningeal
melanocytes have not been established, but the tendency of melanotic cells
to cluster around blood vessels within the pia mater of other species (cats (34)
and Zucker rats (35)) suggests that they have a protective role, probably by
sequestering toxic materials from the circulation.
MELANOCYTIC CELLS DURING EMBRYOGENESIS
Melanocytes are derived from neural crest cells that migrate after neural tube
closure. The initial set of migratory neural crest cells follow a ventral pathway
and ultimately give rise to the peripheral nervous system, craniofacial bones
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
9/57
Human Melanocyte Biology, Toxicology, and Pathology 113
Figure 6: Melanocytes of the stria vascularis. Melanocytes of the stria vascularis are termedintermediate cells (IC). Dendritic intermediate cells are found between the pigmentedmarginal cells (MC) and the basal cells (BC), which are attached to the spiral ligament
(SL). A capillary (C) is also shown.
Figure 7: Diagram of meninges covering the medulla. Melanocytes are more prevalent inthe leptomeninges covering the pons and medulla in humans.
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
10/57
114 W. H. Tolleson
Figure 8: Embryonic origins of human melanocyte populations. Neural ectodermdifferentiates to form the neural plate and neural fold. Upon closure of the neural fold,neural crest cells migrate through dorsolateral pathways to form the peripheral nervoussystem, adrenal medulla, craniofacial structures, cardiac structures, and variousmelanoblast subpopulations. Melanocyte progenitor cell populations are shown in boldtype.
and cartilage, cardiac structures, and adrenal medulla. Later migrating neural
crest cells include melanoblasts, which follow a dorsolateral migratory path-
way between the mesodermal and ectodermal layers (Figure 8). Melanoblast
migration is directed vectorially by chemotactic signal gradients and patterns
of cell surface molecules and receptors expressed by the mobilized melanoblasts
or present within the extracellular matrix through which they move. Migrating
melanoblasts are responsive to various signals including: endothelin, stem cell
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
11/57
Human Melanocyte Biology, Toxicology, and Pathology 115
factor (c-kit ligand), ephrin, fibronectin, Wnt3a, bone morphogenic protein-4
(BMP4), transforming growth factor- (TGF), retinoic acid, and hepatocyte
growth factor/scatter factor (HGF/SF) (3642). Melanoblast migration is termi-
nated and mesenchymal-epithelial transition occurs as melanoblasts encounter
the proper signals, such as those expressed by epithelial cells of the embry-
onic periderm or mesenchymal cells of the periocular mesoderm and the otic
vesicle.
Neural crest cells arise from the primitive neuroectoderm. Later, in prepa-
ration for migration, neural crest cells undergo a characteristic epithelial-to-
mesenchymal transition. Migratory cells arriving at their destinations undergo
the reverse process, mesenchymal-to-epithelial transformation, and initiatetheir specific differentiation programs. Neural crest derivatives may acquire
enhanced motility via a second epithelial-to-mesenchymal transition cycle
instituted in the process of carcinogenic transformation. The tendency of
some melanomas to develop an aggressive metastatic phenotype recapitulates
aspects of embryonic melanoblast programming, including responsiveness to
chemotactic signals, particularly those mediated by the c-met: HGF/SF and
Eph : ephrin receptor systems (42, 43)
Melanocytes share similarities with some neural crest derivatives, partic-
ularly the adrenal medulla and dopaminergic neurons, but are clearly dis-
tinct from others, such as those that give rise to craniofacial bones and car-tilage. For example, migrating dendritic neural crest cells communicate with
neighboring cells via gap junctions involving connexin-43 (44). Many neu-
ral crest derivatives function as secretory cells to produce and release neu-
rotransmitters or pigment. A significant biochemical feature that these cells
share is the utilization of tyrosine to generate the catechol intermediate L-
3,4-dihydrophenylalanine (DOPA), a precursor in the synthesis of dopamine,
epinephrine, norepinephrine, melanin, pheomelanin, and neuromelanin. Cate-
chol biosynthesis is compartmentalized within specialized organelles in these
cell types, limiting exposure to dangerous catechol, quinone, and reactive oxy-
gen species formed as intermediates or by-products. It has been suggested
that inherited mutations or chemical treatments may compromise the contain-
ment or scavenging of toxic derivatives generated during catechol biosynthe-
sis, contributing to disorders such as contact/occupational vitiligo or melanoma
progression (45, 46).
MELANOSOME ASSEMBLY AND MATURATION
Melanin biosynthesis in mammals has been the subject of numerous chemical,
biochemical, and molecular biological studies. At least 127 genetic loci involved
in pigmentation have been identified in mice and other species (47). Genes in-
volved in human pigmentation were recently reviewed by Sturm et al. (48), and
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
12/57
116 W. H. Tolleson
Spritzet al. (49) reviewed disorders of human and mouse pigmentation. Slo-
miniskyet al. (8) provided a comprehensive review of hormonal regulation of
melanogenesis. The roles of many genes in the expression of a fully pigmented
phenotype have been elucidated or implicated using biochemical and molecu-
lar biological techniques. Inherited pigmentation patterns often follow simple
Mendelian genetics. The genes associated with human oculocutaneous albinism
types 14 (tyrosinase, pink-eyed dilution, TRP-1, and MATP) are representative
of autosomal recessive traits. Piebaldism (c-kit), Waardenburg syndrome types-
1 (PAX3), -2 (MITF), -3 (PAX3), and -4 (SOX10, endothelin 3, and endothelin-B
receptor) exhibit autosomal dominance.
Melanogenesis is tightly coordinated with genesis of melanosomes in hu-man melanocytes. The process of melanosome assembly in relation to melanin
biosynthesis was recently reviewed by Hearing (50). Melanosome develop-
ment begins when spherical stage I melanosomes, or premelanosomes, bud
from the endoplasmic reticulum (ER). The major scaffold protein Pmel 17, also
known as gp 100 (encoded by the silver locus in mice), is transferred to Stage
I melanosomes. Proteolytic cleavage of Pmel 17 activates its condensation to
form the characteristic internal fibers of melanosomes and marks the transi-
tion to ellipsoidal Stage II melanosomes. Tyrosinase, essential for melanogen-
esis, is synthesized within the ER, and abnormal transport of tyrosinase from
the ER has been established as a mechanism for some forms of oculocutaneousalbinism. Inactivation of the tyrosinase gene is responsible for oculocutaneous
albinism type-1 (OCA-1). The genetic basis for oculocutaneous albinism type
2 (OCA-2), the most common form of albinism globally, is associated with mu-
tations of the pink-eyed dilution gene, the p locus in mice (51); yet, until re-
cently, the intracellular localization and functions of the OCA-2 protein product
(P protein) were uncertain. Current evidence establishes that the P protein is
required for proper transit of tyrosinase from the ER and that the majority
of P protein is located within the ER (52). Other studies show that increased
expression of the wild type P protein is associated with decreased intracellu-
lar glutathione levels and decreased resistance to arsenic (53). Earlier studies
detected P protein associated with melanosomes (54). Peptide sequence homolo-
gies between the P protein and bacterial Na+/H+ transporters suggested that it
might function in tyrosine transport into melanosomes (55), but tyrosine trans-
port is evident within Pnull melanocytes, arguing against involvement of the P
protein (56).
While within the lumen of the ER, tyrosinase is folded properly via
chaperone-type interactions with TRP-1 (tyrosinase-related protein 1, also
called catalase B) and calnexin (57, 58). Oculocutaneous albinism type 3 (OCA-
3) is associated with TRP-1 mutations that prevent tyrosinase export from the
ER (59, 60). Watabe and coworkers (61) showed that abnormal acidification of
the ER also prevents tyrosinase export from the ER, resulting in albinism via
proteasome-dependent digestion of tyrosinase but that raising the pH of the ER
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
13/57
Human Melanocyte Biology, Toxicology, and Pathology 117
reversed this effect. Thus, the pH of the ER exerts a regulatory effect on tyrosi-
nase expression. Following its initial glycosylation step in the ER, tyrosinase is
processed through the cis-Golgi and trans-Golgi where it is glycosylated further.
Tyrosinase acquires its two essential copper ions by the point it reaches the
trans-Golgi, based on DOPA-positive staining of this organelle (62). Mature ty-
rosinase is first transported via AP3 (adapter protein 3) sorting vesicles to early
or late endosomes, and finally delivered to Stage I and Stage II melanosomes.
Oculocutaneous albinism type 4 is associated with inactivating mutations of
MATP (membrane associated transporter protein), resulting in abnormal extra-
cellular delivery of melanosome-specific proteins after normal transit through
the ER, Golgi, and endosomal compartments (63, 64). Other melanosome-specific components, such as TRP-1, TRP-2 (tyrosinase-related protein-2, also
called dopachrome tautomerase), MART-1 (also called melan A), and OA-1 (ocu-
lar albinism-1) proteins, are delivered to maturing melanosomes via alternative
sorting pathways that have not been characterized completely (57, 6567).
Pigment synthesis begins within Stage III melanosomes. Stage IV melanosomes
are mature, fully pigmented, and ready for transfer to neighboring cells.
BIOSYNTHESIS OF MELANINS
Tyrosinase, a mono-oxygenase Type-3 copper protein with o-phenol oxidase and
catechol oxidase activities, catalyzes the synthesis of dopaquinone via hydrox-
ylation and oxidation of L-tyrosine or L-DOPA (Figure 9). Inactive tyrosinase,
which cannot bind molecular oxygen, is termed deoxy-met-tyrosinase. The two
active site copper centers of deoxy-met-tyrosinase are both in the Cu(II) oxi-
dation state and are liganded by two clusters of three histidine residues (68)
(Figure 10). Reduction of the two active site Cu(II) centers to the Cu(I) state is
typically accomplished via the sacrificial oxidation of L-DOPA to dopaquinone
yielding deoxy-tyrosinase. Deoxy-tyrosinase readily binds molecular oxygen
as a bridged 2 2 peroxy complex, forming active oxy-tyrosinase prior
to binding either tyrosine or DOPA substrates (69). It was previously ac-
cepted that dopaquinone was formed from tyrosine sequentially in two steps:
(1) hydroxylation at C-3 followed by (2) two-electron oxidation to convert the
3,4-catechol to the o-quinone final product (70). Recent studies showed that
oxy-tyrosinase converts tyrosine to dopaquinone directly (Figure 9, step 1), per-
haps via a bidentate copper/catechol complex that decomposes with oxidation
of the catechol to the o-quinone (71, 72). Fenoll et al. (73) have challenged
this view and, among other considerations, proposed that formation of a dead-
end complex between tyrosine and met-tyrosinase alters the flux of interme-
diates formed in tyrosinase-mediated reactions. Computer simulations using
rate constants for intermediate steps in dopaquinone synthesis indicated that
separate hydroxylation and oxidation reactions of tyrosine provided the best fit
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
14/57
118 W. H. Tolleson
Figure 9: Biosynthesis of melanin pigments from L-tyrosine and L-cysteine.
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
15/57
Human Melanocyte Biology, Toxicology, and Pathology 119
Figure 10: Catalytic cycle of the two copper centers in the active site of humantyrosinase. Each copper is liganded by histidinyl residues (his). The enzyme is representedby the plane passing through the two copper centers.
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
16/57
120 W. H. Tolleson
to their experimental data. Regardless of whether tyrosine is converted in one
or two steps, formation of dopaquinone is the common end result. Alternatively
acting as a catechol oxidase, oxy-tyrosinase binds L-DOPA and catalyzes its
two-electron oxidation to form the o-quinone product (Figure 9, step 2). The
catechol oxidase activity of tyrosinase is also required to recycle L-DOPA gen-
erated non-enzymatically as a by-product in later steps of melanogenesis.
The rate constants for spontaneous chemical reactions of dopaquinone in-
volved in melanin synthesis have been determined using spectrophotometry
and pulse radiolysis techniques (7476), providing important mechanistic ra-
tionales for inter-individual differences in the type and quantities of melanin
produced (for review, see [7]). Dopaquinone has three potential chemical fatesthat alternatively generate black, brown, or reddish-yellow pigment forms. The
synthetic pathway favored and the type of pigment produced depends primarily
on the relative influx of L-tyrosine and L-cysteine into the melanosome, the ac-
tivities of the three melanogenic enzymes (tyrosinase, TRP-1, and TRP-2), and
the pH of the melanosome. Formation of 5-S-cysteinyldopa from dopaquinone
via a Michael addition is favored in the presence of excess L-cysteine (Figure 9,
step 3). Pheomelanin, a reddish-yellow polymeric pigment ultimately formed
from 5-S-cysteinyldopa, is predicted to be the predominant product when the
intramelanosomal cysteine concentration is >107 M (77). In the absence of
thiols, dopaquinone cyclizes (Figure 9, step 4) to form leukodopachrome (cy-clodopa), which is readily oxidized nonenzymatically by excess dopaquinone to
form dopachrome and L-DOPA (Figure 9, step 5). Tyrosinase catalyzes the ox-
idative recycling of L-DOPA to dopaquinone (Figure 9, step 6). Dopachrome
tautomerizes spontaneously at a moderate rate to form dihydroxyindole-2-
carboxylic acid (DHICA) (Figure 9, step 7) or partially decarboxylates to form
dihydroxyindole (DHI) (Figure 9, step 8). Alternatively, in the presence of
dopachrome tautomerase (TRP-2/DCT), dopachrome derived from dopaquinone
is converted to DHICA more rapidly, and without decarboxylation. Polymeriza-
tion of DHI generates a dark colored, high molecular weight, insoluble pig-
ment, termed DHI-melanin (Figure 9, step 9). Polymerization of DHICA yields
a lighter colored, lower molecular weight pigment, called DHICA-melanin
(Figure 9, step 10), which is slightly more soluble than DHI-melanin. DHI-
melanin and DHICA-melanin are collectively termed eumelanins. In mice, an-
other enzyme, TRP-1, catalyzes the further oxidation of DHI and DHICA cate-
chols to their respective o-quinone forms (Figure 9, steps 1112). The absence
of this activity confers a brown color to eumelanin pigments. This function of
TRP-1 is absent in humans, where DHI and DHICA oxidation can be catalyzed
by human tyrosinase instead (78).
In summary, essentially no melanin pigment is formed within human
melanosomes in the absence of active tyrosinase (oculocutaneous albinism
types 14). Pheomelanin synthesis, conferring red or yellow color, is favored
when cysteine is readily available. Active TRP-2 accelerates dopachrome
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
17/57
Human Melanocyte Biology, Toxicology, and Pathology 121
tautomerization, thereby increasing the efficiency of eumelanin production and
the generation of darker pigments. Tyrosinase (or TRP-1 in mice) catalyzes fur-
ther oxidation of eumelanin precursors to indolequinones, yielding the darkest
black pigments. Although a mixture of pigments ranging from yellow (blonde)
to jet black can be produced by human cutaneous melanocytes, the presence of
pheomelanin may be visually masked by the darker eumelanins. Pheomelanin
pigments are visually dominant when TRP-2 levels are lower, due to diminished
flow of dopaquinone through dopachrome to eumelanins.
Other factors are required for proper melanogenesis and/or melanosome
development in addition to expression of the three melanogenic enzymes
tyrosinase, TRP-1, and TRP-2 and the intracellular vesicular traffickingsystems described above. Basrur et al. (79) used a combination of SDS gel
electrophoresis, trypsin digestion, and tandem mass spectrometric analysis of
peptides separated by liquid chromatography to identify 68 proteins associated
with Stage III melanosomes obtained from cultured melanocytes. These pro-
teins included 6 novel melanosomal proteins, 56 proteins shared with other
organelles, and the 6 melanosomal proteins known previously. The majority
of proteins identified possess functions consistent with the overall nature of
melanosomes and melanin synthesis. On the other hand, some factors known
to play critical roles in melanogenesis, such as the OCA-2 protein, were not
detected in melanosomes, supporting the view that the majority of P-proteinis located within the ER where it functions in tyrosinase export. Both tyro-
sine and cysteine are necessary substrates for melanogenesis and saturable
melanosomal transporter systems for both amino acids have been demonstrated
(56, 8083). The identities of the protein components of the melanosomal tyro-
sine and cysteine import systems have not yet been determined genetically or
biochemically. The nature of these amino acid transport systems, particularly
with regards to their selectivity for substrates and inhibitors, could be relevant
to mechanisms of chemically-induced depigmenting disorders.
Contact between cutaneous keratinocytes and melanocytes affects the ratio
of pheomelanin to eumelanin. Using reconstructed human epidermis systems
and melanocyte/keratinocyte co-culture experiments in vitro, Duvalet al.(84)
showed that synthesis of eumelanin increases concomitantly with decreased
pheomelanogenesis under conditions favoring keratinocyte/melanocyte contact.
The majority of Caucasians with red hair are hetero- or homozygous for variant
alleles of the melanocortin-1 receptor (MC1-R), the receptor for -melanocyte
stimulating hormone (-MSH) (48). Normally,-MSH binding by MC1-R stim-
ulates adenylate cyclase which increases cAMP levels, and among its other
physiological effects, stimulates the expression of TRP-2 expression and eume-
lanin synthesis. Common MC1-R variations occurring in humans account for
either decreased affinity for -MSH or diminished coupling to adenylate cy-
clase (85). Interestingly, the anesthetic requirement is increased in redheads
homozygous for variant MC1-R alleles and compound heterozygotes, perhaps
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
18/57
122 W. H. Tolleson
due to depressed cAMP levels (85, 86). The human agouti signaling protein
acts as an endogenous MC1-R antagonist that blocks MC1-R dependent signal-
ing and suppresses eumelanin synthesis (87). A genetic polymorphism for the
agouti signaling protein was found to be more common among West Africans,
African-Americans, and East Asians than among American Caucasians, and
was associated with dark hair and brown eyes, but not melanoma (88, 89).
PROPERTIES OF MELANINS AND MELANOSOMES
Human melanocytes from very darkly pigmented tissues, such as the choroid,
are enriched with ellipsoidal melanosomes, approximately 0.9 0.3m, con-taining eumelanins as the predominant pigment. Melanosomes of this type,
sometimes called eumelanosomes, exhibit the classic stages IIV of melanosome
development and the characteristic well-ordered structural features described
in the previous section. Pheomelanosomes are spherical melanosomes (0.7m
diameter) and exhibit pheomelanin as their primary pigment; these organelles
are more abundant within the melanocytes of red or blonde hair (48). The fin-
gerprint pattern of regularly spaced internal fibers found in Stage II eume-
lanosomes is replaced with an irregular vesiculoglobular matrix in developing
pheomelanosomes (18).
A variety of model systems have been utilized to study the fine structureof melanosomes: (a) native melanosomes examined in situ within fixed human
tissue specimens, (b) intact melanosomes isolated from marine invertebrates
or human melanocytic tissues, and (c) synthetic melanin polymers formed in
vitro. Particular advantages and limitations are associated with these model
systems so a complete view of melanosome ultrastructure is gained by consid-
ering studies of each of them.
Human melanosomes embedded in melanocytic tissues studied using elec-
tron microscopy provide standard reference information regarding their na-
tive structure. Electron microscopes adapted to perform electron energy loss
spectroscopy (EELS) allow elemental analyses of ultrastructural features.
Muller and Bereiter-Hahn (90) demonstrated calcium accumulation within
melanosomes of dermal melanocytes in Xenopus laevis using EELS. Nagai
and coworkers (91) applied EELS to oral melanosis and melanoma to com-
pare relative sulfur levels within eumelanosomes and pheomelanosomes in
situ. Secondary ion mass spectrometry (SIMS) has been used to image sulfur
distribution in hair shaft cross sections (0.20.5 m probe beams) (92) and to
detect iodobenzamide, a melanoma tracer compound, within the melanosomes
(0.05m resolution) of metastatic B16 melanomas colonizing lung tissue (93).
Hallegot and coworkers (94) applied in-situ imaging mass spectrometry, a
multi-ion technique related to SIMS, to study cryosections of human hair. By
imaging the distribution of 16O, 12C 14N, 32S, and 34S ions simultaneously across
the hair shaft, the chemical composition of individual melanin granules could
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
19/57
Human Melanocyte Biology, Toxicology, and Pathology 123
be determined. Melanin composition contrasted with that of other architec-
tural features of the hair shaft, including the medulla, cortex, endocuticle, and
exocuticle. Analytical techniques such as these should continue to provide in-
formation regarding the chemical environment of melanocyte substructures
within melanocytic tissues.
Eumelanosomes can be isolated from cultured melanocytic cells or the ink
sacs of cuttlefish (Sepia officinalis) using relatively mild techniques, minimizing
the potential for damage to delicate ultrastructural features. Although eume-
lanin isolated from Sepia ink sacs is chemically similar to that produced by
human melanocytes, Sepia eumelanosomes are spherical and smaller (0.2 m
diameter) than ellipsoidal human eumelanosomes (0.9 0.3 m). The basisfor the morphological differences in human and Sepia eumelanosomes is not
known, although differences in metal ion and protein composition are appar-
ent. However, both human and Sepia mature eumelanosomes contain similar
internal ultrastructural features to be described later.
Pigmented hair contains abundant melanin granules, and human eume-
lanosomes have been extracted from hair using acid, base, and/or peroxide
treatments. The chemical composition of human melanin has been studied
using these preparations, but recent studies showed that bound metal ions
were altered by the aggressive chemical treatments necessary to disrupt the
hair matrix. Novellino and coworkers (95) discovered that appropriate deter-gents and exhaustive proteolysis allowed the recovery of undamaged eume-
lanosomes from bovine irides and human hair. Adapting this approach allowed
intact eumelanosomes and pheomelanosomes to be isolated from black or red
human hair samples, respectively, facilitating chemical analyses and detailed
ultrastructural analyses using electron microscopy and atomic force microscopy
(Figure 11, Table 2) (96).
Figure 11: Atomic force microscopic images of melanosomes isolated by proteolyticmethod from human hair. (A) Eumelanosomes isolated from dark hair, (B)pheomelanosomes isolated from red hair. Atomic force microscopy, tapping mode, bars200 nm. (by permission, Liu et al. Photochem Photobiol. 2005;17(3):262269 [96])
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
20/57
124 W. H. Tolleson
Table 2: Composition of purified melanosomesa.
Chemical analyses
Eumelanosomes Pheomelanosomes Eumelanosomes(dark human hair)b,c (red human hair)b (Sepia ink sac)
Amino acid (w/w%)d 14.115.3 43.8 5.1Total melanin/mge 58006240 1430 10810PTCA (ng/mg) f 27002900 750 166004-AHP (ng/mg)g
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
21/57
Human Melanocyte Biology, Toxicology, and Pathology 125
Atomic force microscopy of eumelanosomes isolated from Sepia and darkly
pigmented human hair reveal three categories of fine structural features: (a) fil-
aments approximately 4.5 nm diameter and variable lengths, (b) smaller par-
ticles 415 nm high with diameters 20 nm, and (c) larger roughly spherical
particles approximately 150 nm in diameter (Figure 12) (96, 105). The melanin
filament category was detected in native melanosome preparations but could
also be recovered by ultrafiltration from the 10003000 Da fraction of son-
icated Sepia eumelanosomes. The 10003000 Da fraction correlates with the
estimated size of melanin protomolecule aggregates (2250 Da). Thus, the 4.5 nm
diameter filaments detected by atomic force microscopy, which are somewhat
larger than the pores of the ultrafiltration membranes used (13 nm), likelyrepresent reassembled structures, reminiscent of 6.6 nm diameter filaments
formed from synthetic melanin in the presence of copper ions (104). The physi-
cal integrity of the larger 150 nm melanosome particles was probed using atomic
force microscopy. These particles appeared to be composed of smaller substruc-
tures that correlated roughly with the dimensions of the 20 nm particles and fil-
aments. Thus, the current model of eumelanin structure proposes that melanin
protomolecule aggregates are arranged to form the ultrastructural features de-
tected in intact eumelanosomes, and that the smaller ultrastructural features
may represent components of larger 150 nm melanin particles.
The biophysical mechanisms controlling the deposition and organization ofnascent melanin polymers to form the distinctive features of human pheo- and
eumelanosomes are not completely understood. It is clear that melanosomal
proteins and metal ions influence melanin polymerization and organization.
The spectrum of functional groups provided by melanin monomers includes
potential hydrogen bond donors and acceptors, hard and soft ligands for
metal binding, aromatic groups capable of -stacking, and reactive centers
for Michael-type addition reactions. Sharma and coworkers (106) showed that
melanosomal proteins affect melanogenesis by facilitating or inhibiting poly-
merization of melanin precursors and the binding of melanin polymers to
melanosomal proteins. Chakraborty et al. (107) and Lee et al. (108) showed
that polymerization of 5,6-dihydroxyindole-2-carboxylic acid was facilitated by
the Pmel 17 melanosomal scaffolding protein in a superoxide-dependent man-
ner. Amino acid analysis of human eu- and pheomelanosomes isolated from
hair revealed that arginine residues were the most abundant, suggesting that
this residue could play a special role in melanogenesis. It has already been
mentioned that copper ion concentration influences the assembly of synthetic
melanin into sheet-like or rod-like structures (104), and it is well known that
bound metals and protein contribute considerably to the mass of human eu-
and pheomelanosomes (Table 2).
A recent comparison was performed of metals bound to eumelanosomes
obtained from Sepia ink sacs or human eu- and pheomelanosomes isolated
from hair samples using proteolytic enzymes (96). Ca(II) and Mg(II) were the
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
22/57
126 W. H. Tolleson
Figure 12: Hypothetical model of human melanosome fine structure. Melanin precursors(bottom right) are polymerized to form melanin protomolecules, which associate via-stacking to form 1.5 nm 1.2 nm aggregates. Melanin aggregrates organize to formrods (4.5 nm diameter), 20 nm particles, and 150 nm particles which comprise substructuresdetected in human eumelanosomes using atomic force microscopy. (by permission,Clancy and Simon, Biochemistry, 2001;40(44):1335313360).
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
23/57
Human Melanocyte Biology, Toxicology, and Pathology 127
most abundant bound ions in each case. Melanosomes from human hair con-
tained higher levels of the transition metals Fe(III), Cu(II), and Zn(II), with
particularly higher levels of bound Fe(III) present in pheomelanosomes. The
ion exchange properties and metal binding capacity of eumelanin was studied
using Sepia melanosomes washed extensively with EDTA (109). Cu(II) and
Fe(III) ions were able to bind EDTA-stripped melanosomes under acidic condi-
tions, presumably to hydroxyl or amine functional groups. Mg(II), Ca(II), and
Zn(II) binding occurred above the pKa for carboxyl groups, implicating DHICA
as the primary ligand for these metals. Fe(III) and Cu(II) binding to EDTA-
washed melanosomes reached saturation at 1.2 and 1.1 mmol/g, representing
maximum metal binding capacities of 6.7 and 7.0% by mass, respectively. As-suming a DHICA:DHI ratio of 1:1.12 for Sepia melanosomes and combining the
parameters from Table 2, yields the rough estimation that up to 20% of avail-
able melanin monomers can be recruited for metal binding, as described by
Potts and coworkers (110). This deduction reveals that the structure of Sepia
melanosomes must efficiently allow for metal ion access to potential binding
sites.
Melanins readily bind inorganic metal cations, neutral organic compounds,
and organic cations (111117). Borel et al. (118) showed that line width mea-
surements from high resolution magic angle spinning proton NMR could be
used to determine weak binding constants (Kd 1.818.0 mM) of compoundsto synthetic melanin. The ability of melanin to sequester toxic metal cations
and hazardous organic compounds is believed to provide a protective effect for
melanocytic tissues and impart other biochemical properties. It has been argued
by various investigators that drug binding to melanin is, or is not, relevant to
ocular phototoxicity and to ototoxicity (113, 116, 119121). UV-induced lipid
peroxidation is inhibited by eumelanin (122). However, superoxide-mediated
lipid peroxidation is enhanced when Fe(III)-adenosine diphosphate complexes
are combined with synthetic melanin and incubated with microsomal phospho-
lipids (123). The presence of melanin-bound Cu(II) and Fe(III) ions increases
single stranded breaks in supercoiled plasmid DNA incubated aerobically
either in the dark or with UV-irradiation (109).
MELANOCYTE PHOTOBIOLOGY
Photoprotection via absorption of biologically dangerous solar UV radiation is
the primary function for melanin in the skin and eyes. Solar radiation pen-
etrates exposed tissues in a wavelength-dependent fashion (16, 17, 124, 125)
(Figure 13). The very small amount of UVC (100280 nm) in sunlight that is not
filtered by atmospheric ozone is typically absorbed by the outermost cornefied
layers of the epidermis or cornea. UVB (280320 nm) is far more abundant in
terrestrial sunlight than UVC. UVB is able to reach viable keratinocytes of the
stratum spinosum and penetrates the anterior chamber of the eye where it is
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
24/57
128 W. H. Tolleson
Figure 13: Tissue penetrating properties of sunlight. (A) skin, (B) eye. (by permission, Tollesonet al. Int. J. Env. Science Pub. Health, 2005;2(1):147155).
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
25/57
Human Melanocyte Biology, Toxicology, and Pathology 129
absorbed by the outer layer of the lens. UVA (320400 nm) is also more abundant
in sunlight, penetrates more deeply into the lens, and penetrates the full depth
of the epidermis to the level of the papillary dermis. Visible light (400760 nm)
and IR (76010,000 nm) constitute the majority of solar energy at sea level.
Visible light reaches the ascending loops of the capillary bed in the dermis and
penetrates the eye to be absorbed completely by the retina and retinal pigment
epithelium. IRA (7601400 nm) penetrates the skin completely and reaches the
subcutis. IRA can also penetrate the eye deeply but IRB (1,4003,000 nm) and
IRC (3,00010,000 nm) are absorbed by water and do not penetrate beyond the
superficial layers of the cornea. Acute overexposure of unprotected skin or eyes
to visible or IR radiation via lasers is typically associated with thermal damage,although photodynamic or photo-thermal effects can result in the presence of
photosensitizing agents (126, 127).
The absorption spectrum of eumelanin isolated from human hair reveals
that it absorbs the most hazardous UV components of solar radiation very
efficiently. Furthermore, melanin is recognized as an efficient scavenger of su-
peroxide radical anions, hydroxyl radicals, and singlet oxygen generated un-
der conditions of photooxidative stress (32). However, reactive oxygen species
are released when synthetic melanin or melanosomes isolated from Sepia
or human hair containing normal levels of bound transition metals are ex-
posed to UV in the presence of oxygen (128, 129). Dontsov and coworkers(130) detected increased lipid peroxidation when melanosomes isolated from
the retinal pigment epithelium of human eyes were irradiated with an argon
laser. Addition of synthetic melanin increased the UVA-induced formation of
8-hydroxy-2-deoxyguanosine in calf thymus DNA (131). Interestingly, the ab-
sorption spectrum of the lower molecular weight fraction (1,000 Da), matches the action spectrum
for this effect (129, 132). Lower molecular weight melanin fractions isolated
from melanosomes have a tendency to reassemble spontaneously over time.
Incubation of the low molecular weight melanin fraction (
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
26/57
130 W. H. Tolleson
for blonde hair lightening that implicates UV-induced melanosome structural
damage, perhaps via attack of reactive oxygen species on melanosomal mem-
brane components. Landet al. (135) and Pilaset al. (136) reported that UV irra-
diation of DOPA generated dopa semiquinone, dopaquinone, and dopachrome,
but UV irradiation of 5-cysteinyldopa led to S-CH2 photohomolysis in which
carbon centered alanyl radicals and aryl thiyl radicals were formed. Koch and
Chedekel (137) showed that the pheomelanogenic precursor 5-S-cysteinyldopa
was approximately 10-fold more susceptible to UV photolysis than DOPA and
that it promoted more photoinitiated DNA single strand breaks and photomod-
ification of calf thymus DNA. These results show that the pheomelanogenic
precursor 5-S-cysteinyldopa is particularly labile to UV and is more effectivein promoting UV-induced DNA damage than the corresponding eumelanogenic
precursor, DOPA.
UV radiation participates in a mixture of beneficial and deleterious
photoreactions in the skin. UV is required for synthesis of cholecalciferol, a
precursor for the active form of vitamin D3,1,25-dihydrocholecalciferol. Pho-
tooxidation by UVA or visible light converts the catechol melanin constituents
DHI and DHICA into darker colored 5,6-indole quinone and 5,6-indole quinone-
2-carboxylic acid components. This constitutes the mechanism for the imme-
diate tanning response, a transient increase in pigmentation that occurs im-
mediately in people who tan normally when reexposed to sunlight. Unlike thedelayed tanning response, the effects of the immediate tanning response begin
to fade when photoexposure stops. UVB is more effective than UVA in stimu-
lating the delayed tanning response, in which synthesis of additional melanin
and melanosomes provides a longer lasting pigmented phenotype.
The UV component of sunlight exerts a suppressive effect on the im-
mune system. Suppression of immune survellience is believed to be relevant to
development of melanoma and nonmelanoma skin cancers in humans (138).
UV-treated humans (139) and laboratory animals (140, 141) exhibit local and
systemic immune suppression, which can be quantified using contact hy-
persensitivity and delayed hypersensitivity assays. Chemical mediators for
photoimmune suppression include: photoinduced formation of pyrimidine
dimers in DNA, photoisomerization oftrans- to cis-urocanic acid (UCA), and
photogeneration of free radicals and lipid peroxidation products (138). Strate-
gies that scavenge these mediators also block UV-induced suppression of the
immune system. Damian et al. (142) showed that broad-spectrum sunscreens
blocking both UVA and UVB were required to prevent photoimmune suppres-
sion in human volunteers.
Immune survellience of the epidermis begins with dendritic Langerhans
cells that patrol the epidermis, phagocytizing antigenic materials, and migrat-
ing to local draining lymph nodes to present antigens to T-cells (143). UV-
treatment blocks the ability of Langerhans cells to present antigens to the
immune system, thus thwarting its ability to mount a contact hypersensitivity
reaction (144, 145). Murine Langerhans cells previously exposed to antigens
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
27/57
Human Melanocyte Biology, Toxicology, and Pathology 131
can be isolated from lymph nodes and injected into recipient mice to provide
passive immunity (dendritic cell vaccine). UV-treatment of isolated Langerhans
cells prior to injection destroys the transferred passive immunity they would
have provided otherwise (146). However, if UV-induced pyrimidine dimers in
the isolated Langerhans cells are first repaired using the T4N5 bacteriophage
excision repair enzyme delivered via artificial liposomes, the effectiveness of
the transplanted cells to provide passive immunity is restored (147).
Histidase (histidine ammonia lyase) activity in the stratum corneum of the
skin converts L-histidine to trans-urocanic acid that can accumulate in large
amounts due to the lack of urocanase (148). UV stimulates isomerization of
trans-UCA to cis-UCA, which inhibits contact hypersensitivity and delayed typehypersensitivity (149, 150). Sleijffers et al. (151) correlated decreased specific
cellular immunity against hepatitis B surface antigen with increased cutaneous
cis-UCA levels in human volunteers exposed to five daily UVB treatments fol-
lowed by a standard two-dose hepatitis B vaccination protocol.
The hypothesis that photogeneration of radicals and lipid peroxides
contributes to immune suppression stemmed from the observation that antiox-
idants blocked UV-induced suppression of contact hypersensitivity and immune
tolerance (152, 153). Ullrich (138) proposes that cutaneous UV treatments
initiate a signaling cascade that begins with generation of platelet activating
factor signaling (PAF) and PAF-like molecules derived from lipid peroxidationof membrane lipids (154). PAF and PAF-like molecules stimulate PAF receptors
on adjacent cells, which then amplify the signal by activating phospholipase
A2 to generate additional PAF. Activation of the PAF receptor also stimulates
transcription of the COX-2 gene (155). Induction of COX-2 leads to increased
prostaglandin E2levels and increased transcription of IL-10, which suppresses
the immune system (156). In fact, injecting mice with PAF-like molecules
generated by UV-treating phosphatidylcholine induced immune suppression
that could be blocked by prior injections with PAF receptor antagonists (157).
The melaninization of skin blocks UV-dependent accumulation of pyrimi-
dine dimers,cis-UCA, and PAF-like molecules that mediate photoimmunosup-
pression. Melanin granules transferred from epidermal melanocytes to neigh-
boring keratinocytes should inhibit generation of PAF-like molecules primarily
via absorption of UV and secondarily via scavenging reactive oxygen species.
Conversely, UV exposure of unpigmented skin impairs the ability of the immune
system to mount cell-mediated defenses against potential tumor antigens such
as those related to melanoma.
MELANOCYTE PATHOLOGIESGenetic factors play a major role in many melanocyte pathologies, but pho-
toexposures, hazardous chemicals, and drugs also contribute. Melanocyte-
specific traits, such as the development of melanosomes and melanogenesis,
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
28/57
132 W. H. Tolleson
Table 3: Representative melanocyte disorders.
MelanocyteRelevant functions
Disease Characteristics melanocytes affected
Vitiligo andalbinism
Depigmentingdisease; variouscauses
All melanocytes All functions
Vogt-Koyanagi-Haradadisease
Inflammatory diseaseinvolvingmelanocytes of themeninges, eyes,inner ear, and skin
All melanocytes Vision, hearing,photoprotection
Drug-induced
ototoxicity
Tinnitus, hearing loss Intermediate
cells of thestria vascularis
Hearing
Microphthalmia Inactivation ofmicrophthalmiagene; absence ofuveal tract; failure ofthe eye to develop
Uvealmelanocytes
Vision,organogenesis
Tietz albinism-deafnesssyndrome
Mutation ofmicrophthalmiagene; congenitaldeafness, mildalbinism
Intermediatecells of thestria vascularis
Hearing,photoprotection
Uvealmelanoma
Intraocular cancer ofthe eye
Uvealmelanocytes
Vision
Cutaneousmelanoma
Melanoma of the skin Cutaneousmelanocytes
Congenitalnevussyndrome
Abnormally largecongenital moles
Cutaneousmelanocytes
provide unique vulnerabilities for pigment-producing cells. In the cases of con-
tact/occupational vitiligo and drug-induced uveitis, exposure to chemicals or
drugs can induce syndromes similar to those typically associated with genetic
or autoimmune dysfunctions (45, 158). Representative pathologies involving
human melanocytic tissues include heritable or induced defects of pigmenta-tion, developmental abnormalities, melanocyte-specific immune disorders, sen-
sitivities to chemicals or drugs, impaired vision or hearing, and melanomas
(Table 3). Although the biochemical mechanisms have not been well character-
ized in some circumstances, melanocytes can provide organ-specific functions
unrelated to their obvious role in melanin biosynthesis; impaired organ func-
tion is associated with abnormal or absent melanocytes in these cases.
Melanocytosis
Disorders that perturb melanocyte functions not only lead to primary syn-
dromes or diseases but may induce additional sequelae in some cases. For ex-
ample, oculodermal melanocytosis, also called nevus of Ota, is a proliferation
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
29/57
Human Melanocyte Biology, Toxicology, and Pathology 133
of melanocytes within the facial dermis and mucosa innervated by the first and
second branches of the trigeminal nerve. Nevus of Ota occurs more frequently in
women, who represent 80% of cases, and is more common among Asians, where
it accounts for 0.20.8% of dermatology visits (159161). Nevus of Ota and ne-
vus of Ito, a related syndrome that affects parts of the body other than the face,
are characterized by a blue-grey or blue-black pigmentation and melanocyte
proliferation deep within the dermis. The unusual persistence of melanocytes
within the dermis and exclusion from their normal destinations within the
intrafollicular epidermis of the face is suggestive that failure of melanoblast
precursors to complete the final phase their migratory journey from the neu-
ral crest during embryogenesis may be part of the etiology of oculodermalmelanocytosis. Approximately 10% of patients with nevus of Ota develop glau-
coma due to proliferation of melanocytes within the anterior chamber of the
eye (162). Moreover, nevus of Ota is also associated with an increased risk
for melanoma of the skin, eye, and brain, particularly when it occurs in Cau-
casians (163, 164), or it may lead to tinnitus or deafness in a few individuals
(164, 165).
Immunity, Autoimmunity, and Melanocytes
A recent review by Spritz and coauthors (49) noted that human disor-ders of depigmentation can be grouped into diseases of melanocyte develop-
ment (piebaldism and Waardenburg syndrome), function (oculocutaneous al-
binism types 14, Hermansky-Pudlak syndrome, Chediak-Higashi syndrome,
and Griscelli syndrome), or survival (vitiligo). The absence of melanocytes may
impact adversely the neighboring cells within their microenvironment through
diminished availability of factors required for tissue growth, maturation, or
maintenance. For example, melanocytes are required during embryonic organo-
genesis of the eyes and inner ears and are important postnatally for organ
function and maintenance. The relationship between melanocyte survival and
organ function is illustrated in the case of autoimmune disorders targeting
melanocytes of the inner ear in which tinnitus and/or loss of hearing can
result. In particular, Vogt-Koyanagi-Harada syndrome and sympathetic oph-
thalmia, a closely related syndrome, are inflammatory disorders that target
the four melanocytic organs described in this review (skin, eyes, inner ears,
and meninges). Specific autoimmunity against melanocytes and melanocyte
antigens is associated with Vogt-Koyanagi-Harada syndrome (166169), sym-
pathetic ophthalmia, (170) and vitiligo (171). Vogt-Koyanagi-Harada syndrome
and sympathetic ophthalmia are grouped among other uveo-meningeal syn-
dromes including Behcet disease, Wegeners granulomatosis, sarcoidosis, and
acute posterior multifocal placoid pigment epithliopathy (167, 172); the latter of
these are inflammatory autoimmune disorders for which melanocyte specificity
has not been associated, although clinical effects are evident within melanocytic
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
30/57
134 W. H. Tolleson
organs. Likewise, Cogan syndrome and autoimmune inner ear disease,
disorders that cause sensorineural hearing loss, have not been associated with
melanocyte immunospecificity; however, no serologic or immunologic tests pro-
vide sufficient diagnostic accuracy for these disorders and antigen specificity
is unclear (173). Although the etiologies of these syndromes have not been es-
tablished definitively, the effects produced in at least some cases of vitiligo,
Vogt-Koyanagi-Harada syndrome, and sympathetic ophthalmia are consistent
with systemic, depigmenting immune reactions attacking melanocytes in the
skin (vitilgo, poliosis, alopecia), eyes (photophobia, uveitis, retinal detachment),
inner ear (tinnitus, dysacusis, vertigo), and meninges (headache, stiff neck,
nausea) (167).Interestingly, an accessory role for melanocytes in immune defense has been
established. This raises the question: do normal or transformed melanocytes
participate in presentation of melanoma tumor-specific antigens to the immune
system? Cultured mouse cutaneous melanocytes exhibit traits normally asso-
ciated with components of the immune system, including: phagocytic capac-
ity (174), expression of MHC I and II and intracellular adhesion molecule-1
(175, 176), antigen-presentation to immune cells (177), and stimulation of T-
lymphocyte proliferation (178). The ability of melanocytes to act as professional
antigen-presenting cells provides another paradigm relevant to both autoim-
munity and cancer. Auto-antibodies reactive against the melanocyte-specificantigens tyrosinase, Pmel 17, TRP-1, and TRP-2 have been detected in the
serum of some vitiligo patients, and cytotoxic T-lymphocytes reactive against
the melanocyte antigen melan A are present in peripheral blood samples from
some vitiligo patients (171). Peptide-based and dendritic cell-based immuniza-
tion strategies have been developed for melanoma therapy exploiting these
melanocyte specific traits (179, 180). Krone and coworkers (181) present an
intriguing hypothesis that immunization with Bacille Calmette-Guerin or vac-
cinia vaccines, or prior infection by certain pathogens, provides cross-reacting
immune protection against melanoma due to peptide sequence homologies with
the melanoma antigen HERV-K-MEL, derived from theenvgene of the HERV-
K human endogeneous retrovirus. The HERV-K-MEL antigen is not detectable
in normal skin or eyes but is expressed by cutaneous and ocular melanomas,
presented by HLA-2 molecules, and recognized by cytotoxic T-lymphocytes
from melanoma patients (182). A similar phenomenon is evident in murine
melanomas which, unlike normal mouse melanocytes, express the p15e env
antigen encoded by an endogenous retrovirus (183) recognized by mouse cyto-
toxic lymphocytes (184). Immunization against this antigen provides protection
against melanoma (185). Thus, the expression of antigens such as HERV-K-
MEL by transformed or damaged human melanocytes marks these cells for
elimination by the immune system, potentially providing a survival advantage
for the organism.
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
31/57
Human Melanocyte Biology, Toxicology, and Pathology 135
Vitiligo
Disorders of pigmented cells are prevalent in humans. Cutaneous and oc-
ular melanocytes are more numerous in the body and, due to their anatomic
placement, experience greater exposure to chemical and physical hazards than
the more highly-shielded melanocytes located within the inner ear or the
meninges. Thus, disorders of cutaneous and ocular melanocytes are more com-
mon and are more easily detected. Nevertheless, rare disorders derived from
extracutaneous and extraocular melanocytes also occur in humans, including
neurocutaneous melanosis (186) and melanomas of the cerebellopontine angle
(187).
Approximately 32 million individuals globally are affected by vitiligo, com-prising 0.5% of the world population (188). Vitiligo, a depigmentation disorder
that is particularly noticeable in the skin and eyes, can lead to increased photo-
sensitivity and night blindness due to depletion of ocular melanocytes. Active
cases of vitiligo may progress beyond the skin and eyes and affect melanocytes
in other sites, such as the inner ear (189191) and leptomeninges (31). High
frequency hearing defects are more common among cases of active vitiligo than
among healthy subjects, particularly among males (192). In a review of vitiligo,
Njoo and Westerfof (188) compared six hypotheses of vitiligo pathogenesis:
(1) genetic, (2) autoimmunity, (3) neural, (4) self-destruction, (5) growth fac-
tor defect, and (6) convergence; the latter presumes that combinations of theformer five mechanisms are required for acquisition of vitiligo. Compelling ev-
idence is available in support of each of these theories. Perhaps, as in the case
of carcinogenesis for which a diversity of targets are related to the disease,
no single risk factor accounts for all cases of vitiligo (Table 4). Boissy and
Manga (45) proposed that unspecified genetic factors render melanocytes fragile
and thus susceptible to contact/occupational vitiligo. This model speculates that
vitiligo is induced by chemical agents activated by melanocyte-specific enzymes;
particularly tyrosinase or tyrosinase-related protein-1, to form toxic products
that attack melanocytic cells. In fact, the majority of proven chemical induc-
ers of vitiligo are aromatic or aliphatic phenols and catechols with structural
similarities to melanin precursors or intermediates. Thiol compounds analo-
gous to cysteine, thiol-reactive substances, certain drugs, and other miscella-
neous compounds have been associated with occupational vitiligo (Table 5). In
most cases of chemically-induced depigmentation, a direct association of the
depigmenting agent with melanin or melanin synthesis is apparent or can be
inferred.
Albinism
Oculocutaneous albinism types 14, ocular albinism, Hermansky-Pudlak
syndrome, and Chediak-Higashi syndrome are heritable pigmentation
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
32/57
136 W. H. Tolleson
Table 4: Vitiligo pathogenesisa.
Proposedmechanism Evidence
Genetic family history for vitiligo in 6.2538% cases vitiligo in monozygotic twins correlations with VIT1, catalase, tenascin, and FOXD mutations various alleles for HLA antigens associated with vitiligo among
different study populationsAutoimmune coincidence with other autoimmune diseases
presence of anti-organ or melanocyte specific antibodies(tyrosinase, TRP-1, or TRP-2)
infiltrating mononuclear cells and T-cells in vitiligo lesions response to corticosteroid treatments
Neural segmental vitiligo confined to body zones definedneurologically
onset of vitiligo associated with periods of emotional stress association with neurologic disorders (viral encephalitis,
multiple sclerosis with Horners syndrome, and peripheralnerve injury)
Self-destruct metabolic precursors of melanin are cytotoxic organic chemicals activated by melanogenic enzymes
induce contact/occupational vitiligo (e.g.,4-(phenylmethoxy)phenol, 4-tert-butylphenol,4-tert-butylcatechol)
association with hyperpigmented skin and sun-exposure association with defective pterin homeostasis, MAO activity,
redox cycling, increased H2O2, and cytotoxic pterinby-products response to UVB+pseudocatalase therapy (pilot study)
Growth factordefect
cultured melanocytes from vitiligo lesions respond to growthfactors derived from fetal lung fibroblasts
melanocytes from repigmenting lesions proliferate in culture response to melagenine, an extract from human placenta
Convergence different clinical forms of vitiligo and different histories vitiligo cases respond to various, unrelated therapeutic modes
aCompiled from reviews by Njoo and Westerhof (Am. J. Clin. Dermatol. 2001;2:167181) andBoissy and Manga (Pigm. Cell Res. 2004;17:208214).
disorders with no apparent association with environmental factors. However,
albinism caused by these genetic traits is associated with additional squelae
relevant to depigmentation disorders, such as vitiligo, for which environmen-
tal causes are sometimes involved. Ophthalmic disorders common among al-
binos include photophobia, reduced visual acuity, astigmatism, nystagmus,
strabismus, misrouting of axons at the optic chiasm, and, occasionally, ambly-
opia (193). Furthermore, the incidence of albinism affects significant number of
individuals globally (Table 6). The incidence of oculocutaneous albinism among
South African Bantu is particularly high, 1:3,800, and is associated with a very
high rate of squamous cell cancer, 504 per 100,000, a rate 15-fold higher than
that estimated for the US, 34 per 100,000 (194, 195).
Ocular albinism type 1 (OA1) is an X-linked recessive trait (Xp22.3) that af-
fects human males in which there is relatively normal pigmentation of the skin
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
33/57
Human Melanocyte Biology, Toxicology, and Pathology 137
Table 5: Chemicals associated with contact/occupational vitiligoa.
Phenols:
p-tert-butylphenolp-tert-amylphenolp-octylphenolp-nonylphenolp-phenylphenolhydroquinone (1,4-dihydroxybenzene; 1,4-benzenediol; quinol; p-hydroxyphenol)p-cresol (1,3-methylphenol)monomethyl ether of hydroquinone (p-methoxyphenol, p-hydroxyanisole)monoethyl ether of hydroquinone (p-ethoxyphenol)monobenzyl ether of hydroquinone (4-(phenylmethoxy)phenol,
p-(benzyloxy)phenol)butylated hydroxytoluene (BHT)butylated hydroxyanisole (BHA)Catechols:p-tert-butylcatecholp-isopropylcatecholp-methylcatecholpyrocatechol (1,2-benzenediol)Sulfhydryls:-mercaptoethylamine hydrochloride (cysteamine)N-(2-mercaptoethyl)-dimethylamine hydrochloridesulfanolic acidcystamine dihydrochloride3-mercaptopropylamine hydrochloride
Miscellaneous:MercurialsArsenicCinnamic aldehydeP-PhenylenediamineBenzyl alcoholAzaleic acidCorticosteroidsOptic preparations:Eserine (physostigmine)Diisopropyl fluorophosphate(N,N,N-triethylene-thiophosphoramide)GuanonitrofuracinSystemic medications:ChloroquineFluphenazine (prolixin)aCompiled from Boissy and Manga (Pigm. Cell Res. 2004, 17: 208214).
and hair but reduced or absent fundus pigmentation in the eyes, photophobia,
nystagmus, strabismus, reduced visual acuity, iridal translucency, decussing of
axons at the optic chiasm, and lack of foveal reflex (193, 196, 197). The periph-
eral regions of the retinas often exhibit a characteristic mottled or mud spat-
tered pigmentation pattern in female carriers of the trait (198). The normal
OA1 gene product is a seven helix transmembrane G-protein coupled receptor
melanosomal protein (199). The absence of function OA1 protein inhibits proper
G-protein dependent budding of stage I melanosomes from the ER, resulting in
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
34/57
138 W. H. Tolleson
Table 6: Albinism.
Human GlobalDisease chromosome Gene product incidence
Oculocutaneous albinism type 1 11q14q21 tyrosinase 1:40,000Oculocutaneous albinism type 2 15q11.2q12 P locus,
pink-eyeddilution
1:15,000
Oculocutaneous albinism type 3 9q23 TRP-1 unknownOculocutaneous albinism type 4 5p MATP,
underwhiteunknown
Ocular albinism 1 Xp22.3 OA1 1:50,000Chediak-Higaski Syndrome 1q42.1q42.2 Lysosomal
protein, LystExtremely
rareHermansky-Pudlak Syndrome type 1 10q23.1 HSP1,
lysosomalprotein
Hermansky-Pudlak Syndrome type 2 5q14.1 HSP2, AP3B1,pearllocus
Hermansky-Pudlak Syndrome type 3 3q24 HSP3,cocoalocus
Rare
Hermansky-Pudlak Syndrome type 4 22q11.2q12.2 HSP4, lightear locus
Opthalmic complications associated with human albinismreduced visual acuity
photophobianystagmusstrabismusdecussing axons of the optic chiasmincreased risk for skin cancers
the formation of the macromelanosomes that are diagnostic for this condition
(197, 200, 201). Cutaneous pigmentation is clinically normal, but reduced com-
pared to unaffected first degree relatives and macromelanosomes are apparent
within cutaneous melanocytes of OA1 patients (198).
Melanoma
The age-adjusted incidence rate for melanoma of the skin is 2.7 per 100,000
globally, accounting for 160,000 cases, but the age-adjusted incidence is much
higher in the Australia and the US (68.0 and 29.3 per 100,000, respectively)
(202). Melanoma of the skin accounts for approximately 4% of new cancers
detected in the US, where the incidence rate increased by 6% per year from
19751981 and continued to increase at 3% per year until the present (195).
Increased risk for cutaneous melanoma does not exhibit a direct relationship
with chronic photoexposure in the same manner as nonmelanoma skin cancers
(Table 7). Epidemiological studies of cutaneous melanomas in humans reveal
that increased risk is related to intermittent over-exposure of unprotected and
unconditioned skin to solar radiation, particularly during adolescence (OR 1.5,
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
35/57
Human Melanocyte Biology, Toxicology, and Pathology 139
Table 7: Risk factors for cutaneous melanoma.
Risk factors OR Cl Ref.
Intermittent sun exposure, meta-analysis 1.5 1.21.8 (205)Chronic sun exposure, meta-analysis 1.1 0.91.4 (205)Exposure to arsenic 2.1 1.43.3 (206)Regular swimmers in chlorinated pools 2.2 1.14.6 (211)Exposure to 50 Hz magnetic fields, 0.2 Tesla 2.7 1.45.0 (209)Occupation in metal industry 2.6 1.07.1 (212)Occupation in electronics 2.0 0.66.6 (212)Common nevi (101120), meta-analysis 6.9 4.610.3 (215)Atypical nevi, meta-analysis 6.4 3.810.3 (215)Red hair, data pooled from 8 studies 2.4 1.93.0 (250)
Blonde hair, data pooled from 8 studies 1.8 1.52.2 (250)Light brown hair, data pooled from 8 studies 1.5 1.31.7 (250)Freckling, data pooled from 6 studies 2.3 2.02.5 (250)Family history of melanoma, data pooled from 8 studies 2.2 1.82.9 (251)Blue eyes, adolescents 4.5 1.513.6 (252)Inability to tan, adolescents 4.7 0.924.6 (252)
95% Cl 1.21.8; OR 1.1, 95% Cl 0.91.4 for intermittent or chronic sun exposure,
respectively, from meta-analysis of 19 case control studies) (203205).
Exposure to industrial and environmental carcinogens have not been identi-
fied as important risk factors for cutaneous melanoma, with the exception of tworeports associating exposure to arsenic with cutaneous melanoma (206, 207) A
study by Guoet al.(208) evaluated skin cancer cases recorded in the National
Cancer Registry (Taiwan, ROC) reported an association between exposure to
arsenic and nonmelanoma skin cancers, but failed to detect an association with
cutaneous melanoma. A recent study by Beane-Freeman et al.(206) found ele-
vated arsenic levels detected in toenail clippings (0.084g/g) from cutaneous
melanoma cases reported to the lowa Cancer Registry compared with those from
matched control colorectal cancer patients (OR 2.1, 95% Cl 1.43.3). Kennedy
et al. (207) also detected an association between arsenic exposure (ever vs.
never) among a Dutch population and increased risk for cutaneous melanoma
(OR 7.1), although the relatively wide confidence interval renders that conclu-
sion more equivocal (95% Cl 1.145.5). An association between exposure to elec-
tromagnetic fields and cutaneous melanoma is more controversial. Although
residential exposure to 0.2 Tesla was found to increase risk for cutaneous
melanoma in a Norwegian population study (OR 2.7, 95% Cl 1.45.0), a lack of
association was found in an earlier Finnish cohort study (OR 1.1, 95% Cl 0.9
1.2) (209, 210). Inconclusive or equivocal associations with risk for cutaneous
melanoma have also been proposed for exposure to chlorinated swimming pools
and occupations in electronics and the metal industry (211, 212)
Phenotypic risk factors contributing to cutaneous melanoma include pres-
ence of dysplastic or abundant nevi and freckling (213215). It has been es-
timated that an affected first or second degree relative can be identified for
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
36/57
140 W. H. Tolleson
approximately 10% of melanoma cases. This observation suggests a role for her-
itable risk factors in melanoma. Hayward (216) identified two high penetrance
genes associated with increased risk for cutaneous melanoma: CDKN2A tumor
suppressor locus (9p21) and CDK4 (12q14) and described four other low pene-
trance melanoma susceptibility genes: MC1R (16q24.3), EGF (4q25), GST M1
(1p13.3), and CYP2D6 (22q13.1). The CDKN2A tumor suppressor gene encodes
the overlapping p16ink4a and p14Arf genes, which share exons 2 and 3 but uti-
lize separate promoters and first exons. The p16ink4a tumor suppressor protein
functions in the retinoblastoma signaling pathway through inhibition of cyclin
dependent kinases 4 and 6 (CDK4 and CDK6). CDK4 is activated during G1
phase of the cell cycle through binding cyclin D where it phosphorylates the Rbtumor suppressor protein to allow transcription of S-phase genes. The p14Arf
tumor suppressor influences the p53 signaling pathway by inhibiting MDM2-
dependent ubiquitinylation and degradation of p53. The MC1R protein is the
receptor for-MSH and regulates the expression of melanogenic traits. Numer-
ous MC1R polymorphisms have been identified and the majority of Caucasians
with red hair are homo- or heterozygous for variant alleles (48). Melanocyte
proliferation is stimulated by epidermal growth factor (EGF) and a variant
EGF allele is more common in melanoma cases (P < 0.0001) (216). Many Cau-
casians do not possess a functional glutathione S-transferase GSTM1 gene, but
Kanetskyet al. (217) found that individuals with red or blonde hair were 2.2-fold(95% Cl 1.24.2) more likely than controls to be GSTM1 null and 9.5 fold (95%
Cl 1.273.0) more likely than controls to be null for both GSTM1 and GSTT1.
These authors speculate that these GST polymorphisms could contribute to
increased risk for melanoma among blondes and red heads. The association
between cytochrome P450 2D6 (CYP2D6) polymorphisms and melanoma is not
well supported, with one study of Northern European Caucasians detecting an
association (P = 0.039) and two others failing to detect a clear association with
melanoma (218220).
Ocular melanomas represent two types of tumors, those that arise within
the uveal tract (iris, ciliary body, and choroid) or others that arise within the
conjunctiva, the thin layer of mucous epithelium overlying the surface of the eye
and the inner (posterior) surface of the eyelid (11). The incidence of conjuncti-
val melanomas in the US increased by 5.5% from 1973 through 1999, especially
among men, and currently accounts for 0.046 cases per 100,000 (221). Thus, the
increase in conjunctival melanoma incidence parallels the increased incidence
of cutaneous melanoma; this correlation is presumed by some to reflect com-
monality in etiology related to changing sun exposure behavior over time (11,
221). On the other hand, the incidence rate for uveal melanomas has remained
stable in the US from 1973 to 1997 at 0.430.46 cases per 100,000 (222). The
role of UV and sun exposure in uveal melanoma has been debated, with some
epidemiologic studies demonstrating increased risk with increased photoexpo-
sure, but others disputing those results (Table 8) (11, 222228). However, it is
8/12/2019 Tolleson JESHC 2005 melanocyte review.pdf
37/57
Human Melanocyte Biology, Toxicology, and Pathology 141
Table 8: Risk factors for uveal melanoma.
Risk factors OR Cl Ref.
Atypical cutaneous nevi 7.3 2.719.4 (253)Occupational exposure to uv (artificial sources - welders) 7.3 2.620.1 (223)Snow blindness, welders burn or sunburn of the eye 7.2 2.520.6 (225)Family history of ocular melanoma 6.9 6.767.4 (224)Chemists, chemical technicians, or chemical engineers 5.9 1.622.7 (254)Nevus of the iris 3.2 1.85.4 (253)Exposure to formaldehyde 2.9 1.27.0 (254)Exposure to asbestos 2.4 1.53.9 (254)Exposure to antifreeze, 2.5 1.06.5 (254)Iris color: green/gray/hazel iris 2.5 1.83.5 (255)
Exposure to carbon tetrachloride 2.3 1.14.7 (254)Exposure to pesticides 2.3 1.34.1 (254)Personal history of cutaneous melanoma 2.4 0.96.6 (224)Iris color: blue/gray iris 1.9 1.32.9 (256)Tendency to sunburn easily 1.8 1.32.5 (225)Personal history of nonmelanoma skin cancer 1.5 1.02.3 (224)
clear that virtually all of the UV and visible light present in sunlight is scat-
tered or absorbed by the anterior structures of the eye (cornea, aqueous humor,
lens, vitreous humor, neural retina, retinal pigment epithelium, and Bruchs
membrane) overlying the posterior choroid, where the majority of uveal
melanomas arise. Thus, if an association between photoexposures and uveal
melanoma is valid, it is highly unlikely to be due to direct absorption of UV
by melanocytes in the posterior choroid. Nevertheless, it can be presumed that
UV photoexposures could facilitate uveal melanoma by an indirect mecha-
nism, such as photoimmunosuppression, which does not require the absorp-
tion of light by the target cells. Hypothetically, longer wavelengths of sunlight,
which have the potential for deeper tissue penetration, could elicit a photo-
dynamic effect within the choroid through interaction with photosensitizing
chromophores. The very high optical density of the choroid would make this
hypothesis untenable except in cases of very lightly pigmented retinas. Inter-estingly, only 26 cases of cutaneous melanomas and no cases of uvea