Endo. 4 Detecting and signalling Cell surface receptors: G protein linked and tyrosine kinase receptors: second messengers, phosphorylating kinases, activation.

Endo. 4 Detecting and signalling

Cell surface receptors: G protein linked and tyrosine kinase receptors: second messengers, phosphorylating kinases, activation of transcriptionIntracellular steroid receptors; transcription factorsReceptor regulationHormone receptors and disease

Hormone receptorsHormone receptors

Peptide and protein hormones

Water soluble cell surface receptors

Activate second messengers and/or enzymes

Cytoplasmic and nuclear effects

Steroid hormones

Lipophilic intracellular receptors in

cytoplasm or nucleus. Cell surface receptors?

Receptors are transcription factors

General types of cell surface receptors

Protein and peptide hormone receptors

G-protein linked

Gs, Gi, Gq, G12

• Open ion channels

• Activate enzymessecond messengers• activation of

tyrosine kinases activation of serine/

threonine kinases

Enzyme linked or associated

• Tyrosine kinase on

receptor or on

molecule associated

with receptoractivation of serine/threonine kinases

G-protein linked receptors

Receptors with or associated with tyrosine kinase domains

I and E loops 1-3

G-protein linked receptors

Activity of trimeric G proteins after hormone binding to a G-protein linked receptor

Second messengers in G-protein linked receptors

• Adenyl cyclase cAMP Protein kinase A

DAG Protein kinase C

• Phospholipase C IP2

IP3 Ca2+

• Calcium Calmodulin Ca-CAM protein kinase

Signalling at the TSH receptor

cAMP signalling system

The phosphinositide

signalling system

Signalling at the vasopressin receptor

A CASCADE OF KINASESG-protein receptors

cAMP. Phospholipase C, Ca2+/calmodulin

Protein kinase A, C, or CaCM cytoplasm

Serine/threonine kinases

MEK MAP pathway nucleus

Tyrosine kinases in intracellular signalling

G-protein linked receptors

Receptors with or associated with tyrosine kinase domains

Signalling at a receptor with tyrosine kinase activity

IRS1,2,3

P

P

P

P

PP

Insulin

PI3K

PDK1

Increased glycogen synthase

PKB

GLUT-4vesicles

GRB/SHP

SOS

RAS

RAF, MEK, MAPK

FOS, ELK--> gene transcription

SHCP

Endosome

Insulin signaling

in muscle

P

JAK-STAT signalling pathway - receptor with

tyrosine kinase associated molecule

Signalling pathways at G-protein linked and tyrosine kinase receptors

STEROID RECEPTORS

Intracellular• Type I - in cytoplasm combined with

hsp e.g sex steroids, glucocorticoids. Form homodimers

• Type II - in nucleus, often bound to DNA e.g. thyroid hormones,vitamin D

Form homo- and hetero-dimers

Membrane receptors

STEROID HORMONE RECEPTORS

Steroid hormone receptors are a family of transcrtiption factors

Different functional regions of the receptor are defined as domains - A-F

The C domain is the DNA binding region and is highly conserved

Both the A/B domains and E/F domains have transcriptional activity

Different steroid receptors are continually being discovered

Hormone crosses cell membrane

Heat shock protein dissociates from receptor

Hormone binds to receptor and dimerization occurs

Dimerized receptors translocate to the nucleus

Binds to the hormone response element on the DNA

Along with other transcription factors transcription is initiated

hsp

Steroid hormone binding to its receptor

The C domain

The DNA binding region is made up of 2 zinc fingers

Each finger contains 3 helical regions, I, II and III

The first helix contains the P box which recognizes specific base sequences on the DNA - the HRE

The amino acids indicated in brown are those concerned with dimerization of two receptors

Cartoon showing two dimerized receptors linking into the DNA helix. The base sequences of the estrogen and glucocorticoid response elements are shown below

Activation of transcription by steroid

hormone receptors

Binding of steroid hormone receptors (transcription factors) to the hormone response element on the DNA induces chromatin remodelling

There is binding of additional transcription factors and acetylation of histones

RNA polymerase is activated and transcription is stimulated

When histones are deacetylated transcription is repressed

HistonesDNA

RECEPTOR REGULATION• Affinity of receptors - postitive and

negative co-operativity

• Number of receptors - up and down regulation

• Desensitization of receptors - receptor phosphorylation, uncoupling

Homologous and heterologous regulation

Receptor regulationPhosphorylation of receptor

Binding of arrestin

complete desensitization

Phosphatases remove phosphate and may reactivate receptor

Hormone receptors and disease• Genetic mutations of receptors - vitamin D

resistant rickets. • Auto-antibodies against membrane receptors

- Graves’ disease.

• Inability to couple receptors with signal transduction pathways - pseudohypoparathyroidism

• Receptor regulation in excess deficiency - obesity and insulin resistance

• Inappropriate receptor interaction

(specificity) - LH/TSH, prolactin/GH

Grave’s disease