PI 3-kinase, PKB, protein synthesis and cellular survival

PI 3-kinase, PKB, protein synthesis and

cellular survival

IJsbrand Kramer i.kramer@iecb.u-bordeaux.fr

Modifications of phosphoinositol lipids by: a kinase (PI 3-kinase), two phosphatases (PTEN, SHIP) and one lipase (phospholipase C). All products

are signaling molecules (second messengers).

PDGF-R

PDGF A/B

ATPATP

y751

Y741

PI 3-kinase acts downstream of a number of tyrosine-kinase containing receptors. Here we show the example of the PDGF-R.

Importanlty, insulin signals through PI 3-kinase and this is essential for the regulation of the glucose metabolism (together with the regulation by PKA).

intracellular

extracellular

The adoptor/regulatory protein p85 recruits the catalytic subunit p110 (or to the membrane. This way it places the enzyme in the vincinity of its substrate (inositol phospholipids). Compare this with the role of Grb for the recruitment of Sos).

Binding of p85 to the receptor may also have an allosteric effect that promotes kinase activity of the catalytic subunit.

pTyr751

Met755

P85SH2 domain(C-terminal)

PDGF-R(aa751-755)

PI 3-kinase adaptor protein p85

PDGF-R

PDGF A/B

ATPATP

y751

Y741 p85

P110 PI 3-kinase

PI 3-kinase p110 can also be recruited to the membrane by an interaction with RasGTP.

Both Ras and p85 have a membrane recruitment and an allosteric role in the regulation of activity of the catalytic subunit (p110)

Activation of PI 3-kinase downstream of the insulin receptor

Numerous PI 3-kinases, classified as type I, II or II, and their regulators.

Numerous interactions between PI 3-kinase and other signalling components. PI 3-kinase may control Cdc42 and Rac activity

through its BCR/GAP domain.

a) The core of type 1 PI 3-kinase has a typical kinase fold; with its small (N-terminal) and big (C-terminal) lobes separated by the catalytic cleft that binds ATP (in red) and substrate (inositol-1,4,5-trisphosphate).

b) The helix domain forms a scaffold to which RBD, the catalytic and the C2-domain are firmly attached.

Wortmannin and Lilly294002 are PI 3-kinase inhibitors that occupy the catalytic cleft.

Domain architecture of the family of protein kinase B and its upstream kinase PDK1

Molecular structure of inactive and active PKB (panel a and b). The position of the HM domain in panel (a) is hypothetical. Note the subtle cascade of events

that render PKB catalytically competent (panel c)

a) Activation of PKB by membrane attachment followed by two phosphorylationsb) Members of the AGC family of protein kinases that are activated in a similar

manner

PKB controls protein synthesis through activation of mTOR. This leads to activation of S6K1 as well as assocation of initiation factors with

mRNA

mTOR regulates protein synthesis by phosphorylating and removing BP. This signals the assembly of the ribosomal initiation complex. S6K1, primed by

mTOR, is activated by PDK1 and this too contributes to the process of initiation of protein synthesis

Dual role of Ras and PI 3-K/PKB in the activation of protein synthesis

Adhesion of cells to matrix contributes to cellular survival

PKB protects against apoptosis in a number of ways

Initiation of apoptosis by leakage of cytochrome c into the cytoplasm: a process facilitated by the pro-apoptotic mediator Bad

PKB inhibits activity of GSK3,thereby preventing the breakdown of cyclin D and thus stimulating the entry into the G1 phase of the cell cycle

Downystream of the Insulin receptor; PKB plays a role in the metabolism of glucose; activation of glycogen synthase b

An essential task of PKB is to inactivate GSK-3.

PTEN classifies as a tyrosine phosphatase but it prefers inositol substrates. It opposes the action of PI 3-kinase and its loss is associated with cell

transformation, hence its label « tumour suppressor ».

PTEN is inhibited by the action of growth factors. This occurs through the activation of Src which phosphorylates PTEN on two tyrosine residues. This reduces its half-live and inhibits membrane localisation (no access to substrate)

The tumour suppresive effect of PTEN is not limited to counteracting PI 3-kinase. It also plays a role in the nucleus, independent of its phosphatase

action, where it associates with proteins that warrant « chromosome integrity ».