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Brian Wasita,dr.,Ph.D
Bagian Patologi Anatomi FK UNS
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A fundamental property of cancer cells is their ability to evade the
apoptotic cellular death program.
Apoptosis can be initiated through the extrinsic or intrinsic pathways
Both pathways result in the activation of a proteolytic cascade of
caspases that destroys the cell.
Mitochondrial outer membrane permeabilization is regulated by the
balance between pro-apoptotic (e.g., BAX, BAK) and anti-apoptotic
molecules (BCL2, BCL-XL).
BH-3-only molecules activate apoptosis by tilting the balance in favor
of the pro-apoptotic molecules
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In cancer, oncogenes overcome these protective cellular responses by taking
advantage of cooperating, additional mutations in apoptosis signaling molecules,
resulting in the abnormal proliferation and survival/antiapoptosis of the tumor cell.
Mechanism of evasion from apoptosis:
1. Reduced levels of CD95 may render the tumor cells less susceptible to
apoptosis by Fas ligand (FasL).
2. High levels of FLIP, a protein that can bind death-inducing signaling complex
and prevent activation of caspase 8.
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3. Overexpression of the BCL2 protein. This in turn increases the BCL2/BCL-
XL buffer, protecting lymphocytes from apoptosis and allowing them to
survive for long periods. Approximately 85% of B-cell lymphomas of the
follicular type carry a characteristic t(14;18) (q32;q21) translocation.
4. p53 is an important pro-apoptotic gene that induces apoptosis in cells that
are unable to repair DNA damage.The actions ofp53are mediated in part
by transcriptional activation of BAX, but there are other connections as
well between p53 and the apoptotic machinery.
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In normal cells, which lack expression of telomerase, the shortenedtelomeres generated by cell division eventually activate cell cycle
checkpoints, leading to senescence and placing a limit on the
number of divisions a cell may undergo.
In cells that have disabled checkpoints, DNA repair pathways are
inappropriately activated by shortened telomeres, leading to massive
chromosomal instability and mitotic crisis. Tumor cells reactivatetelomerase, thus staving off mitotic catastrophe and achieving
immortality.
4. Limitless Replicative Potential
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Most normal human cells have a capacity of 60 to 70 doublings. After
this, the cells lose the capacity to divide and enter senescence. This
phenomenon has been ascribed to progressive shortening of
telomeresat the ends of chromosomes.
Short telomeres seem to be recognized by the DNA repair machinery
as double-stranded DNA breaks, and this leads to cell cycle arrest
mediated byp53and RB.
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Cells in which the checkpoints are disabled by p53 or RB
mutations, the nonhomologous end-joining pathway is activated as
a last-ditch effort to save the cell, joining the shortened ends of two
chromosomes.
This inappropriately activated repair system results in dicentric
chromosomes that are pulled apart at anaphase, resulting in new
double-stranded DNA breaks.
The resulting genomic instability from the repeated bridge-fusion-
breakage cycles eventually produces mitotic catastrophe,
characterized by massive cell death. If during crisis a cell manages to reactivate telomerase, the bridge-
fusion-breakage cycles cease and the cell is able to avoid death.
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Telomerase, active in normal stem cells, is normally absent from, or at very
low levels in, most somatic cells.
By contrast, in 85% to 95% of cancers telomere is preserved due to up-
regulation of the enzyme telomerase.
A few tumors use other mechanisms, termed alternative lengthening of
telomeres, which probably depend on DNA recombination.
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Vascularization of tumors is essential for their growth and is controlled
by the balance between angiogenic and anti-angiogenic factors that areproduced by tumor and stromal cells.
Hypoxia triggers angiogenesis through the actions of HIF1.
VHL acts as a tumor suppressor gene because of its ability to degrade
HIF1and thus prevent angiogenesis.
Inheritance of germ-line mutations of this gene causes VHL syndrome,
characterized by the development of a variety of tumors. Many other factors regulate angiogenesis; for example, p53 induces
synthesis of the angiogenesis inhibitor thrombospondin-1.
5. Development of Sustained Angiogenesis
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Tumor angiogenesis is controlled by the balance between angiogenic
factors and factors that inhibit angiogenesis
The molecular basis of the angiogenic switch involves increased
production of angiogenic factors and/or loss of angiogenesis inhibitors.
These factors may be produced directly by the tumor cells themselves or
by inflammatory cells (e.g., macrophages) or other stromal cells
associated with the tumors.
The angiogenic switch is controlled by several physiologic stimuli, such as
hypoxia. Relative lack of oxygen stimulates production of a variety of pro-
angiogenic cytokines, such as vascular endothelial growth factor (VEGF),
through activation of hypoxia-induced factor-1 (HIF1), an oxygen-
sensitive transcription factor
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Both pro- and anti-angiogenic factors are regulated by many other
genes frequently mutated in cancer.
For example, in normal cells, p53 can stimulate expression of anti-
angiogenic molecules, such as thrombospondin-1, and repress
expression of pro-angiogenic molecules, such as VEGF.
Thus, loss of p53 in tumor cells not only removes the cell cycle
checkpoints listed above, but also provides a more permissive
environment for angiogenesis.
The transcription of VEGF is also influenced by signals from the RAS-
MAP kinase pathway, and mutations of RAS or MYC up-regulate the
production of VEGF.
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6. Invasion and Metastasis
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Steps of Invasion :
1. Detachment of tumor cells from each otherE-cadherin function is lost in almost all epithelial cancers, either by
mutational inactivation of E-cadherin genes, by activation of -catenin
genes, or by inappropriate expression of the SNAIL and TWIST
transcription factors, which suppress E-cadherin expression.2. Degradation of ECM
Tumor cells may either secrete proteolytic enzymes themselves or
induce stromal cells (e.g., fibroblasts and inflammatory cells) to
elaborate proteases. Multiple different families of proteases, such as
matrix metalloproteinases (MMPs), cathepsin D, and urokinase
plasminogen activator. Overexpression of MMPs and other proteases
have been reported for many tumors
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(Wasita et al, 2009)
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3. Attachment to novel ECM components
Cleavage of the basement membrane proteins collagen IV and
laminin by MMP-2 or MMP-9 generates novel sites that bind to
receptors on tumor cells and stimulate migration.4. Migration of tumor cells
Migration is a complex, multistep process that involves many
families of receptors and signaling proteins that eventually impinge
on the actin cytoskeleton. Such movement seems to be potentiated
and directed by tumor cell-derived cytokines, such as autocrine
motility factors.In addition, cleavage products of matrix components (e.g.,
collagen, laminin) and some growth factors (e.g., insulin-like growth
factors I and II) have chemotactic activity for tumor cells. Stromal
cells also produce paracrine effectors of cell motility, such as
hepatocyte growth factor/scatter factor (HGF/SCF), which bind to
receptors on tumor cells.
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Metastatic cascade:
1. Invasion of ECM and vascular dissemination
In the bloodstream, some tumor cells form emboli by aggregating andadhering to circulating leukocytes, particularly platelets; aggregated tumor cells
are thus afforded some protection from the antitumor host effector cells.
Most tumor cells, circulate as single cells.
Extravasation of free tumor cells or tumor emboli involves adhesion to the
vascular endothelium, followed by egress through the basement membrane into
the organ parenchyma by mechanisms similar to those involved in invasion.2.Homing of tumor cells
The site of extravasation and the organ distribution of metastases
generally can be predicted by the location of the primary tumor and its vascular
or lymphatic drainage.
Many tumors arrest in the first capillary bed they encounter (lung and liver, most
commonly).
Some tumors show organ tropism, probably due to expression of adhesion or
chemokine receptors whose ligands are expressed by the metastatic site
preferentially on the endothelium of target organs
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.
Another mechanism of site-specific homing involves chemokines and
their receptors. Chemokines participate in directed movement (chemotaxis) of
leukocytes, and it seems that cancer cells use similar tricks to home in
on specific tissues.
Human breast cancer cells express high levels of the chemokine
receptors CXCR4 and CCR7. The ligands for these receptors (i.e.,
chemokines CXCL12 and CCL21) are highly expressed only in those
organs where breast cancer cells metastasize.
After extravasation, tumor cells are dependent on a receptive stroma for
growth. Thus, tumors may fail to metastasize to certain target tissues
because they present a nonpermissive growth environment. Despite the
foregoing considerations, the precise localization of metastases cannotbe predicted with any form of cancer
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Candidates metastasis oncogenes are SNAIL and TWIST, which
encode transcription factors whose primary function is to promote aprocess called epithelial-to-mesenchymal transition (EMT).
In EMT, carcinoma cells down-regulate certain epithelial markers (e.g.,
E-cadherin) and up-regulate certain mesenchymal markers (e.g.,
vimentin and smooth muscle actin).
These changes are believed to favor the development of a
promigratory phenotype that is essential for metastasis.
Loss of E-cadherin expression seems to be a key event in EMT, and
SNAIL and TWIST are transcriptional repressors that promote EMT by
down-regulating E-cadherin expression.
EMT has been documented mainly in breast cancers
Molecular Genetics of Metastasis
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Individuals with inherited mutations of genes involved in DNA repair systems
are at a greatly increased risk of developing cancer.
Example:
1. Patients with HNPCC syndrome have defects in the mismatch repair system
and develop carcinomas of the colon.
2. Patients with xeroderma pigmentosum have a defect in the nucleotide
excision repair pathway and are at increased risk for the development of
cancers of the skin exposed to UV light, because of an inability to repair
pyrimidine dimers.
3. Patient with familial breast cancers have mutation in BRCA1 and BRCA2,
which involved in DNA repair.
7. Genomic Instability-Enabler of Malignancy
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