Phosphoinositide 3-kinase

Phosphoinositide 3-kinases (PI 3-kinases or PI3Ks) are a family of related enzymes that are capable of phosphorylating the 3 position hydroxyl group of the inositol ring of phosphatidylinositol (PtdIns).

Classes
PI3Ks interact with the IRS (Insulin receptor substrate) in order to regulate glucose uptake through a series of phosphorylation events.

The phosphoinositol-3-kinase family is composed of Class I, II and Class III, with Class I the only ones able to convert PI(4,5)P2 to PI(3,4,5)P3 on the inner leaflet of the plasma membrane.

Class I
Class I PI3K are heterodimeric molecules composed of a regulatory and a catalytic subunit; they are further divided between IA and IB subsets on sequence similarity. Class IA PI3K are composed of one of five regulatory p85α, p55α, p50α, p85β or p55γ subunit attached to a p110α, β or δ catalytic subunit. The first three regulatory subunits are all splice variants of the same gene (Pik3r1), the other two being expressed by other genes (Pik3r2 and Pik3r3, p85β and p55γ, respectively). The most highly expressed regulatory subunit is p85α, all three catalytic subunits are expressed by separate genes (Pik3ca, Pik3cb and Pik3cd for p110α, p110β and p110δ, respectively). The first two p110 isoforms (α and β) are expressed in all cells, but p110δ is primarily expressed in leukocytes and it has been suggested it evolved in parallel with the adaptive immune system. The regulatory p101 and catalytic p110γ subunits comprise the type IB PI3K and are encoded by a single gene each.

The majority of the research on PI 3-kinases has focused on the Class I PI 3-kinases. Class I PI 3-kinases are composed of a catalytic subunit known as p110 and a regulatory subunit either related to p85 or p101. The p85 subunits contain SH2 and SH3 domains.

Class II and Class III
Class II and III PI3K are differentiated from the Class I by their structure and function.

Class II comprises three catalytic isoforms (C2α, C2β, and C2γ), but unlike Classes I and III, no regulatory proteins. These enzymes catalyse the production of PI(3)P from PI (may also produce PI(3,4)P2 from PI(4)P), however little is known about their role in immune cells. C2α and C2β are expressed throught the body, however expression of C2γ is limited to hepatocytes. The distinct feature of Class II PI3Ks is the C-terminal C2 domain. This domain lacks critical Asp residues to coordinate binding of Ca2+, which suggests class II PI3Ks bind lipids in a Ca2+ independent manner.

Class III are similar to II in that they bias the production of PI(3)P from PI, but are more similar to Class I in structure, as they exist as a heterodimers of a catalytic (Vps34) and a regulatory (p150) subunits. Class III seems to be primarily involved in the trafficking of proteins and vesicles. There is, however, evidence that they are able to contribute to the effectiveness of several process important to immune cells, not least phagocytosis.

Mechanism
The various 3-phosphorylated phosphoinositides that are produced by PI 3-kinases (PtdIns3P, PtdIns(3,4)P2, PtdIns(3,5)P2 and PtdIns(3,4,5)P3) function in a mechanism by which an assorted group of signalling proteins, containing PX domain, pleckstrin homology domains (PH domains), FYVE domains and other phosphoinositide-binding domains, are recruited to various cellular membranes.

Inhibition
All PI 3-kinases are inhibited by the drugs wortmannin and LY294002, although certain member of the class II PI 3-kinase family show decreased sensitivity.

Functions
PI 3-kinases have been linked to an extraordinarily diverse group of cellular functions, including cell growth, proliferation, differentiation, motility, survival and intracellular trafficking. Many of these functions relate to the ablilty of class I PI 3-kinases to activate protein kinase B (PKB, aka Akt). The class IA PI 3-kinase p110α is mutated in many cancers. Many of these mutations cause the kinase to be more active. The PtdIns(3,4,5)P3 phosphatase PTEN which antagonises PI 3-kinase signalling is absent from many tumours. Hence, PI 3-kinase activity contributes significantly to cellular transformation and the development of cancer. The p110δ and p110γ isoforms regulate different aspects of immune responses. PI 3-kinases are also a key component of the insulin signaling pathway. Hence there is great interest in the role of PI 3-kinase signaling in Diabetes mellitus.

AKT is activated as a result of PI3-kinase activity, because AKT requires the formation of the PtdIns(3,4,5)P3 (or "PIP3") molecule in order to be translocated to the cell membrane. At PIP3, AKT is then phosphorylated by another kinase called PDK1, and is thereby activated. The "PI3-k/AKT" signaling pathway has been shown to be required for an extremely diverse array of cellular activities - most notably cellular proliferation and survival.

In addition to AKT and PDK1, one other related serine threonine kinase is bound at the PIP3 molecule created as a resulte of PI3-kinase activity, SGK.

PI 3-kinases as protein kinases
Many of the PI 3-kinases appear to have a serine/threonine kinase activity in vitro; however, it is unclear whether this has any role in vivo.

In addition to the class I – class III PI 3-kinases there is a group of more distantly related enzymes that are sometimes referred to as class IV PI 3-kinases. The class IV PI 3-kinases family is composed of ataxia telangiectasia mutated (ATM), ataxia telangiectasia and Rad3 related (ATR), DNA-dependent protein kinase (DNA-PK) and mammalian Target Of Rapamycin (mTOR). These members of the PI 3-kinase superfamily are protein serine/threonine kinases.

PI 3-kinases inhibitors as therapeutics
As wortmannin and LY294002 are broad inhibitors against PI 3-kinases and a number of unrelated proteins at higher concentrations they are too toxic to be used as therapeutics. A number of pharmaceutical companies have recently been working on PI 3-kinase isoform specific inhibitors including the class I PI 3-kinase, p110δ isoform specific inhibitors, IC486068 and IC87114, ICOS Corporation.