Endocrine Abstracts

Type I phosphoinositide 3-kinases (PI3Ks)are regulated by tyrosine kinases, Ras and G-protein coupled receptors and generate the lipid second messenger, phosphatidylinositol (3,4,5)-trisphosphate (PIP3). PIP3 regulates a bewildering variety of cellular responses primarily through binding and hence activating or targeting a range of proteins each of which posseses a specific PIP3 binding module, most commonly a pleckstrin homology (PH) domain. In terms of cell survival the key players are phosphoinositide dependent protein kinase 1, which phosphorylates and activates protein kinase B (PKB) in a PIP3-dependent manner. PKB has many substrates including BAD, the binding partner of Bcl-2.
PIP3 is a minor membrane phospholipid which has in the past proved very difficult to measure directly. My group has exploited lipid binding protein domains to generate probes suitable for the analysis of PIP3 in enzyme assays, in cell and tissue extracts, in single,live cells and at the ultrastructural level using quantitative immuno-electron microscopy. These probes are beginning to reveal the true temporal and spatial complexity of signalling via PI3Ks.
Metabolism of PIP3 is carried out by at least two enzyme classes. SH2 domain containing 5-phosphatases remove the 5-phosphate converting PIP3 to a product (phosphatidylinositol 3,4-bisphosphate) which may have its own role as a signal. By contrast the tumour suppressor protein, PTEN, directly coutermands the PI3K signalling pathway by selectively dephosphorylating the 3-position of PIP3. We have shown that PTEN regulation is complex, involving targeting and allosteric activation as well as transcriptional control.