Signaling Pathways

In the absence of specific internal stimuli, what will be automatically degraded?

p53

What is p53?

a master regulator of both apoptosis and proliferation.

In what two ways can apoptosis be triggered?

-intrinsically by mitochondrial cytochrome C

-extrinsically by a death receptor

What are cell surface receptors that can be stimulated by extracellular signals to engage signaling cascades?

-receptor tyrosine kinases (RTKs)

-G protein-couples receptors (GPCRs)

What does genomic damage signal?

it prevents p53 degradation.

What two things does p53 inhibit?

cyclin CDK

anti-apoptotic Bcl2

When anti-apoptotic Bcl2 is inhibited, what occurs?

effector Bcl2 can form a complex which leaks cytochrome C into the cytosol creating a caspase cascade which leads to apoptosis.

Besides cytochrome C, how can caspase cascade be activated?

via external death signals (Fas ligand, FasL) binding to the death receptors that engage the extrinsic pathway for apoptosis.

How do survival signals usually act on the cell?

via cell-surface RTKs.

Once the survival signal is in the cell, what does this engage?

internal PI3K/Akt cascade, which relies on Akt downstream to promote anti-apoptotic Bcl2 activity.

How do mitogens act on the cell?

via cell-surface RTKs or GPCRs

Once mitogens are in the cell, what does this engage?

the internal Ras/MAPK cascade which will in turn promote inhibitor of apoptosis proteins.

What will mitogen-activated MAPKs do?

promote proliferation by stimulating S phase cyclins. (G1/S and S-Cdks)

How do growth factors enter the cell? What do they do?

via the PI3K branch of the PI3K/Atk cascade.

-increases overall polypeptide synthesis (hypertrophy)

What is RTK?

receptor tyrosine kinase.

When a extracellular signal molecule binds to an RTK, what initially occurs?

dimerization of transmembrane RTK proteins.

After dimerization of transmembrane RTK proteins, what occurs?

dimerized signal-bound RTK kinase domains (cytosolic) catalyze ATP-dependent autophosphorylation.

-In easier terms, the signal causes the RTK to phosphorylate itself.

In the RTK, where are phosphoryl groups added?

to tyrosine amino acid side chains in the proteins structure, making the kinase domain now tyrosine kinase (TK)

In the RTK, what do the resulting phospho-tyrosine sites do?

they further interact with and activate additional intracellular effectors/effector cascades.

What is Ras? What does it do?

Ras is a small G protein.

-it has GTPase "molecular switch" domain that operates on cycles of GTP binding and hydorlysis

What is required to activate Ras? How does this occur?

Ras-GEF (guanine NT exchange factor)

Ras-GDP + GTP --> Ras-GTP + GDP

What is required to inactivate Ras? How does this occur?

Ras-GAP (GTPase activating protein)

Ras-GTP --> Ras-GDP + Pi

What can active Ras-GTP activate?

MAPK cascade

What is MAPK?

mitogen activated protein kinase

What are most MAPK cascades a sequence of?

3-step sequence.

-beginning with MAPK kinase kinase (MAP3K) and running through a MAPK kinase (MAP2K)

In a typical Ras/MAPK cascade what must occur first?

activated (signal bound) receptor must FIRST directly or indirectly activate a Ras-GEF

When a Ras-GEF is directly or indirectly activated, what occurs?

Guanine NT exchange.

Ras-GDP (inactive) into Ras-GTP (active and can interact with MAP3K)

When Ras-GTP interacts with MAP3K, what occurs?

MAP3K phosphorylates and activates MAP2K, which in turn does the same for the MAPK

What does the final product (MAPK) of the Ras/MAPK cascade do?

phosphorylates a variety of downstream target effectors, which in turn drives cell cycle progression and/or suppress apoptosis directly.

What are two ways Ras can be activated?

by RTKs

by GPCRs

How do GPCRs usually activate Ras?

via the dissociated G(beta)(gamma) component of the heterotrimeric G protein.

What can activated RTKs directly bind to and activate?

PI3Ks

When RTKs bind to PI3Ks, what do the PI3Ks do?

add additional phosphoryl groups to existing Pis in the cytosolic side of the plasma membrane

What do the extra phosphorylated Pis (created by PI3Ks) do?

serve as molecular scaffolds bringing other sets of effectors together which can then activate each other.

What is an example of the extra phosphorylated Pis working?

a kinase called PDK can be activated by binding to the extra phosphorylated Pis.

active PDK can then phosphorylated and activate another kinase called Akt.

What can the steps with ATK lead to?

activation of TOR

-stimulation of increased protein synthesis and hypertrophy

-ATK can hit targets like anti-apoptotic Bcl2

What is GPCR?

G-protein coupled receptor

What does the GPCR look like?

multiple ins and outs across the plasma membrane

What does the binding of an extracellular signal molecule to the GPCR do?

drives conformational change in the GPCR allowing it to interact with heterotrimeric G protein

What is heterotrimeric G protein?

a membrane anchored protein in the cytosolic side of the plasma membrane

What does the activated signal-bound receptor function as? For what?

as the GEF for the heterotrimeric G protein, causing guanine NT exchange in its alpha subunit followed by dissociation into G(alpha)-GTP and G(beta)(gamma) compenents.

In the GPCR, what do the G(alpha)-GTP and G(beta)(gamma) component do?

they can then each activate different sets of intracellular effectors; resetting the hetero G protein to inactive requires a separate GAP.

What are the most frequently-occurring types of heterotrimeric G protein signaling in animals?

cyclic AMP signaling (down stream of Gs and Gi) and calcium signaling (downstream of Gq)

After G(alpha)s-GTP is activated, what will it interact with?

it will interact with and activate an enzyme called adenylyl (or sometimes (adenylate) cyclase

What does adenylyl cyclase do?

converts molecules of ATP into cyclic AMP (cAMP), which is a powerful so-called second messenger in the cytosol

What is the primary impact of cAMP?

to trigger activation of protein kinase A (PKA)

What can activated PKA so?

phosphorylate various potential PKA target proteins, linking eventually to the downstream signaling effect on cell function

In Ca2+ signaling, G(alpha)q-GTP will do what?

interact with and activate an enzyme called phospholipase C (PLC)

What does PLC do?

cleaves existing phosphoinositides into diacylglycerol and IP3