Chapter 6 Cytoplasmic Signaling Circuitry Programs Many of the Traits of Cancer - 5.9 - - 6.2 ~ 6.9 - Apr 3 & 12, 2007

Chapter 6

Cytoplasmic Signaling Circuitry Programs

Many of the Traits of Cancer

- 5.9 -- 6.2 ~ 6.9 -

Apr 3 & 12, 2007

5.9 The Ras protein functions as a G protein

- The ras oncogene triggers many of the same changes in cells which are transformed by erbB (truncated EGF-R) or sis (PDGF-B).

- Could Ras be found somewhere downstream of erbB and sis ?

- Do the signals emitted by EGF-R and PDGF-R converge on some common molecule ?

(guanosine diphosphate)(guanosine triphosphate)

GRB2: growth factor receptor- bound protein 2

SH2, SH3: Src homolog 2 or 3

Sos: son of sevenless

Ras

growth factor receptor

Regulation of Ras (a GTPase) activity

GTPase-Activating(or Accelerating) Protein

Guanine nucleotide Exchange Factor

e.g., Sos, which catalyzes conversion of inactive GDP-bound Ras to the active GTP-bound form.

(mitogenic signals)

Regulation of Ras (a GTPase) activity

GTPase-Activating(or Accelerating) Protein

Guanine nucleotide Exchange Factor

e.g., Sos, which catalyzes conversion of inactive GDP-bound Ras to the active GTP-bound form.

(mitogenic signals)

(e.g., H-Ras)

Figure 5.31 The Biology of Cancer (© Garland Science 2007)

The structure of the Ras protein

- Two most frequently altered a.a. residues found in human oncoproteins are glycine 12 and glutamine 61, which are closed associated with the γ-phosphate of GTP.

EGF/EGFR-mediated Ras activation

Two classes of GTPase switch proteins (guanine nucleotide-binding proteins) :

1. trimeric (large) G proteins, which directly bind to and activated by receptors, e.g., Smoothened, Frizzled.

2. monomeric (small) G proteins, such as Ras and various Ras- like proteins. Ras doesn’t bind to receptors. It indirectly links to receptors via adapter proteins (such as GRB2) and GEF proteins (such as Sos).

6.2 The Ras protein stands in the middle of a complex signaling cascade

The story of son of sevenless (sos)- the genetics of eye development in Drosophila


In the absence of sevenless, the 7th cell in each ommatidium fail

to form.

6.3 Tyrosine phosphorylation controls the location and thereby the actions of many cytoplasmic signaling proteins

Why is phosphorylation critical?

- protein relocalization.

Figure 6.7a The Biology of Cancer (© Garland Science 2007)

Domain structure of Src protein

catalytic SH1 domain

substrate recognition- proline rich

(SH3 domain)

substrate recognition- p-tyrosine (SH2 domain)


Structure and function of SH2 domain

act as a modular plug


Signal-transducing proteins attach to

phosphorylated receptors

a phosphotyrosine phosphatase (PTP)


Signaling molecules with SH2 and SH3 domains

Figure 6.10b The Biology of Cancer (© Garland Science 2007)

Molecular “ligand domains” and “receptor domains” in order to facilitate intermolecular interactions

Table 6.2 The Biology of Cancer (© Garland Science 2007)


6.4 SH2 groups explain how growth factors activate Ras

and acquire signaling

Mammalian Ras proteins have been studied in great detail because mutants Ras proteins are associated with many types of human cancer. These mutant proteins, which bind but cannot hydrolyze GTP, are permanently in the “on” state and contribute to neoplastic transformation.

Most oncogenic, constitutively active Ras protein contain a mutation at position 12. Replacement of the normal glycine-12 with other amino acid blocks the functional binding of GAP, and in essence “lock” Ras in the active GTP-bound state.


Sidebar 6.3 The SH2 domain of Src has two alternative functions

inactive dephosphorylation of Y527 phosphorylation of Y416 SH2/SH3 bind to p-receptor exposure of the catalytic cleft

6.5 A cascade of kinases forms one of three important signaling pathways downstream of Ras



The Ras effector loop can bind Raf, PI3K and

Ral-GEF


MEK: MAP/Erk kinase

Erk: extracellular signal- regulated kinases 1 and 2 (Erk1/2)

Ras → Raf → MAP (Mitogen-activated protein) kinase pathway

Raf, MEK and Erk1/2 are serine/threonine kinases

phosphotyrosines attract signaling partners → relocalization of partners

phosphorylation of serines/threonines → shift in structure → functional activation

Ras/Raf/MAP Kinase Pathway

dimer


Activated MAPK induces gene transcription

transcription factor TCF: ternary complex factor

SRE:serum response element

transcription factor SRF:serum response factor

(early response gene) → enter the cell cycle


The Ras-PI3 kinase (PI3K) pathway

(cytoplasm)

Generation of the 2nd messengers – DAG and IP3


PI3K converts PIP2 into PIP3

PI3K can bind to phosphotyrosine and Ras.


PH domain: pleckstrin homology

PIP2

Docking of PH domains of Akt/PKB to PIP3

PTEN:phosphatase and tensin homolog



Islet cells expressing constitutively active Akt/PKB. The cells are fourfold larger than normal.

Normal islet cells

AKT/PKB enhances the growth of cells


Sidebar 6.4 Ras is the prototype of a large family of similar proteins

Small G proteins – 35 similarly structured proteins e.g., Ras, Ral, Rac, Ran, Rho…

- All small G proteins operate like a binary switch, using a GTP-GDP-GTP cycle to flip back and forth between an on and an off state.

- Each small G protein has its own specialized guanine nucleotide exchange factor (GEF) to activate it and its own GTPase-activating proteins (GAP)


(Ral guanine nucleotide exchange factors)

(Ral-A and Ral-B are Ras-like proteins)

(Rho family proteins)

6.7 A third Ras-regulated pathway acts through Ral, a distinct cousin of Ras

(control of cytoskeleton)

play key roles in the motility that enables cancer cells to invade and metastasize


6.8 The Jak-STAT pathway allows signals to be transmitted from the plasma membrane directly to the nucleus cytoki

nes


6.9 Cell adhesion receptors emit signals that converge with those released by growth factor receptors


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Chapter 6 Cytoplasmic Signaling Circuitry Programs Many of the Traits of Cancer - 5.9 - - 6.2 ~ 6.9 - Apr 3 & 12, 2007