Cell Communication 1

Chapter 16 Cell Communication

The major signalingpathways relevantto cancer

You will not be responsible for: Specific downstream

signaling pathways

Questions in this chapter you should be able to answer:

Chapter 16:1 - 10 11all but e, 12,13,16,17,18, 19, 20, 22, 24, 25

Cell Communication 2

How do cells communicate with each other?

Signaling mechanisms

Signaling responses

Cell Communication 3

What types of molecules carry signals to cells?

1) Gases (really small)NO, H2S, CO

2) ‘Smallish’ organic moleculessteroidsneurotransmitters[drugs/poisons (nicotine, phytohormones, etc)]

3) Peptide hormones (much bigger)

EGF

Cell Communication 4

Where are the receptors?

Intracellular receptorsvs

Cell-surface receptors

Cell Communication 5

How do cell surface receptors function?

Signal transductionPathways

Signaling proteins

Secondary Signals -- cAMP, Ca++, DAG, IP3

FSH & Receptor

Cell Communication 6

Signaling pathways can interact

Multiple signals

Processed simultaneously

Activating or inhibiting

Signal integration

Cell Communication 7

What are the three types of cell surface receptors?

= Ligand-gated channel

Cell Communication 8

How are G-proteins activated?

“7-pass” receptors-- Hundreds of different types-- triggering enumerable different

cytoplasmic processes

Examples

Glucagon – activates glucose release by liver

Lutenizing Hormone (LH) – triggers progesteronerelease from ovary

Adrenalin (epinephrine) – increases heart rate

Allergen – mast cell degranulation

G-protein-linked receptors

Cell Communication 9

Acetylcholine acts at a G-protein-linked receptor on heart muscle to make the heart beat more slowly by the effect of the G protein on a K+ channel, as shown in this Figure. Which one or more of the following would enhance this effect of acetylcholine? Explain.

(a) A high concentration of a non-hydrolyzable analog of GTP.

(b) Mutations in the acetylcholine receptor that weaken the interaction between the receptor and acetylcholine.

(c) Mutations in the G protein α-subunit that speed-up the hydrolysis of GTP.

(d) Mutations in the K+ Channel that make the βγ-subunit bind tighter

Cell Communication 10

How do activated G-proteins trigger release of ‘secondary messenger’ molecules?

-- open channels-- activate enzymes

Secondary messengers include:cAMP, Ca++, DAG, IP3

Some toxins interfere with G-proteins

Cholera toxin Inhibits GTPase activity of α-subunit

-- causes Na+ efflux into intestine -- water flow into intestine

Pertussis toxinPrevents GDP/GTP exchange

-- GTP locked in off state-- mucous secretion into lungs

cAMP Signaling

Cell Communication 11

Downstream effects can be

Downstream enzyme activation (can be very rapid)

-- effect of adrenaline

Changes in gene expression(slower)

There can be many other types of responses

Block gene expressionActivate exocytosis

-- allergic responses-- insulin release

or endocytosis-- phagocytic cells

Cell Communication 12

Retina contains G-protein coupled light receptorsRod cellsActivation ↓ Na+ flow Na channel gated by cGMP

Rhodopsin

Transducin (G-protein)-- activates cGTP phosphodiesterase

Δ Membrane potential

Question 16-8P 556

Cell Communication 13

How do enzyme-linked receptors function?

Receptor Tyrosine Kinases (RTK)

Dimerization

Autophosphorylation

Activated signaling proteins

Cell Communication 14

RTK Signaling often occurs through Ras

A “monomeric” GTP-binding protein

RAS activates a kinase “cascade” (MAP Kinase module)

Cell Communication 15

Signaling Pathways and Cancer

Oncogenes -- Deregulated cell proliferation-- mitogens and growth factors

Constitutive Activation/Signaling

RAS mutations are common in cancers

Cell Communication 16

How are complex signally pathways ‘dissected’?

Genetically engineer cells to contain…

-- Knockout mutations

-- Constitutive expression mutations

How do these 5 experiment establish signaling sequence of RAS, X and Y?

Cell Communication 17

When activated by the signal, the platelet-derived growth factor (PDGF) receptor phosphorylates itself on multiple tyrosines (as indicated below by the circled Ps; the numbers next to

these Ps indicate the amino acid number of the tyrosine). These phosphorylated tyrosines serve as docking sites for proteins (A, B, C, and D) that interact with the activated PDGF-receptor. Binding of PDGF activates the PDGF-receptor leading to an increase in DNA synthesis.

To determine whether protein A, B, C, and/or D are responsible for activation of DNA synthesis, you construct mutant versions of the PDGF-receptor that retain one or more tyrosine phosphorylation sites. In the cells, the various versions of the PDGF-receptor become phosphorylated on whichever tyrosines remain. You measure the level of DNA synthesis in cells that express the various mutant receptors and obtain the data shown below.

A. From these data, which, if any, of these proteins A, B, C, and D are involved in the stimulation of DNA synthesis by PDGF? Why?

B. Which, if any, of these proteins inhibit DNA synthesis? Why?

C. Which, if any, of these proteins appear to play no detectable role in DNA synthesis? Why?

D. What is the effect of the binding of A on the effect of B?