Phosphoinositide 3-kinases 1 are a subfamily of lipid kinases that catalyze the addition of a phosphate molecule specifically to the 3-position of the inositol ring of phosphoinositides. Phosphatidylinositol (PtdIns), the precursor of all phosphoinositides (PI), constitutes less than 10% of the total lipid in eukaryotic cell membranes (Fig. 1). Approximately 5% of cellular PI is phosphorylated at the 4-position (PtdIns-4-P), and another 5% is phosphorylated at both the 4-and 5-positions (PtdIns-4,5-P 2 ). However, less than 0.25% of the total inositol-containing lipids are phosphorylated at the 3-position, consistent with the idea that these lipids exert specific regulatory functions inside the cell, as opposed to a structural function. To date, nine members of the PI 3-K family have been isolated from mammalian cells. They are grouped, as suggested by Domin and Waterfield (1), into three classes according to the molecules that they preferentially utilize as substrates. Four different lipid products can be generated by the different PI 3-K members: the singly phosphorylated form PtdIns-3-P; the doubly phosphorylated forms PtdIns-3,4-P 2 and PtdIns-3,5-P 2 ; and finally the triply phosphorylated form PtdIns-3,4,5-P 3 (Fig. 1).PI 3-K was first described as a PI kinase activity associated with the viral oncoproteins, v-Src, v-Ros, and polyomavirus middle T. Mutational studies of these oncoproteins more than 10 years ago indicated a critical role for the associated PI kinase in cell transformation (reviewed by Ref. 2).Recent advances in the field have been achieved by the development of new techniques to probe for the direct targets of PI 3-K lipid products. The chemical synthesis of short chain fatty acid versions of these lipids (3-5) has been a crucial step in determining the specificity of lipid-binding proteins. Additionally, new cloning strategies have been developed to isolate new lipid-binding proteins (6). Here we will review the most recent advances in our understanding of the role of PI 3-K in cell function by dissecting the contribution of each of its lipid products. PtdIns-3-PRegulation-PtdIns-3-P is constitutively present in both mammalian and yeast cells (7,8). It can be produced in vitro via phosphorylation of PtdIns by Class I, II, or III PI 3-Ks (Fig. 1). However, the majority of PtdIns-3-P in mammalian cells is probably produced by Class III PI 3-K (9). The mammalian Class III enzyme is highly related to the yeast Vps34 gene product (10) and, like the yeast enzyme, is specific for PtdIns and will not phosphorylate PtdIns-4-P or PtdIns-4,5-P 2 (11).Targets-PtdIns-3-P was recently shown to specifically interact with a 70-residue protein module called the FYVE finger domain. This domain is a special type of RING zinc finger that is characterized by two zinc-binding sites and a highly conserved stretch of basic residues surrounding the third zinc-coordinating cysteine. Liposomes containing PtdIns-3-P were shown to associate with several FYVE domains (15)(16)(17). Other phosphoinositides bound poorly ...

Phagosomes acquire their microbicidal properties by fusion with lysosomes. Products of phosphatidylinositol 3-kinase (PI 3-kinase) are required for phagosome formation, but their role in maturation is unknown. Using chimeric fluorescent proteins encoding tandem FYVE domains, we found that phosphatidylinositol 3-phosphate (PI[3]P) accumulates greatly but transiently on the phagosomal membrane. Unlike the 3′-phosphoinositides generated by class I PI 3-kinases which are evident in the nascent phagosomal cup, PI(3)P is only detectable after the phagosome has sealed. The class III PI 3-kinase VPS34 was found to be responsible for PI(3)P synthesis and essential for phagolysosome formation. In contrast, selective ablation of class I PI 3-kinase revealed that optimal phagocytosis, but not maturation, requires this type of enzyme. These results highlight the differential functional role of the two families of kinases, and raise the possibility that PI(3)P production by VPS34 may be targeted during the maturation arrest induced by some intracellular parasites.

We report that knocking down the expression of inositol polyphosphate 4-phosphatase type II (INPP4B) in human epithelial cells, like knockdown of PTEN, resulted in enhanced Akt activation, anchorage-independent growth, and enhanced over all motility. In xenograft experiments overexpression of INPP4B resulted in reduced tumor growth. INPP4B preferentially hydrolyzes phosphatidylinositol-3,4-bisphosphate (PI(3,4)P2) with no effect on phosphatidylinositol-3.4.5-triphosphate (PI(3,4,5)P3), suggesting that PI(3,4)P2 and PI(3,4,5)P3 may cooperate in Akt activation and cell transformation. Dual knockdown of INPP4B and PTEN resulted in cellular senescence. Finally, we find loss-of-heterozygosity (LOH) at the INPP4B locus in a majority of basal-like breast cancers as well as in a significant fraction of ovarian cancers, which correlated with lower over all patient survival, suggesting that INPP4B is a tumor suppressor.

Summary Dysregulation of the phosphatidylinositol 3-kinase (PI3K) signaling pathway occurs commonly in human cancer. PTEN tumor suppressor or PIK3CA oncogene mutations both direct PI3K-dependent tumorigenesis largely through activation of the AKT/PKB kinase. However, here we show through phospho-protein profiling and functional genomic studies that many PIK3CA-mutant cancer cell lines and human breast tumors exhibit only minimal AKT activation, and a diminished reliance on AKT for anchorage-independent growth. Instead, these cells retain robust PDK1 activation and membrane localization, and exhibit dependency on the PDK1 substrate SGK3. SGK3 undergoes PI3K- and PDK1-dependent activation in PIK3CA-mutant cancer cells. Thus, PI3K may promote cancer through both AKT-dependent and AKT-independent mechanisms. Knowledge of differential PI3K/PDK1 signaling could inform rational therapeutics in cancers harboring PIK3CA mutations.