Phosphatidylinositol-5-phosphate 4-kinase beta modulates insulin sensitivity

Phosphorylated derivatives of the lipid phosphatidylinositol are known to play critical roles in insulin response. Phosphatidylinositol-5-phosphate 4-kinases (PI-5-P 4-kinases) produce phosphatidylinositol-4,5-bis -phosphate (PI-4,5-P2) by phosphorylating the novel lipid phosphatidylinositol-5-phosphate (PI-5-P). To understand the physiological role of these enzymes, we generated mice lacking PI-5-P 4-kinase beta (PI5P4Kbeta) (Chapter 2). We found that PI5P4Kbeta-/- mice are hypersensitive to insulin and mildly growth retarded. Additionally, male mice lacking PI5P4Kbeta have less fat than wild type littermates. These results indicate that PI5P4Kbeta modulates insulin response and adiposity in vivo and suggest that inhibitors of PI5P4Kbeta may be useful in the treatment of obesity and type 2 diabetes.; The product of PI5P4Kbeta, PI-4,5-P2, is phosphorylated by phosphoinositide 3-kinase (PI3K) to produce phosphatidylinositol-3,4,5- tris-phosphate (PI-3,4,5-P3), which is required for insulin-stimulated glucose uptake. Thus, it was surprising that the loss of PI5P4Kbeta increased insulin responsiveness in mice. The majority of PI-4,5-P2 in mammalian cells is formed by phosphorylation of phosphatidylinositol-4-P (PI-4-P) by PI-4-P 5-kinases. PI-5-P 4-kinases make a small amount of PI-4,5-P2 in comparison. The purpose of a separate pathway for the production of PI-4,5-P 2 is unknown. PI-5-P 4-kinases may produce a spatially localized pool of PI-4,5-P2; alternatively, PI-5-P may act as a second messenger and PI-5-P 4-kinases may affect signaling by regulating PI-5-P.; To understand the role of PI-5-P 4-kinases in intracellular insulin signaling, we examined signaling downstream of insulin or insulin-like growth factor (IGF-1) in cultured cells with altered levels of expression of PI-5-P 4-kinases (Chapter 3). The protein kinase Akt is phosphorylated in response to insulin-stimulated production of PI-3,4,5-P3. Consistent with our results in PI5P4Kbeta -/- mice, we found that Akt phosphorylation in response to IGF-1 was sustained longer in fibroblasts derived from PI5P4Kbeta -/- mouse embryos than in fibroblasts derived from wild type embryos. Conversely, expression of PI-5-P 4-kinases in CHO-IR cells caused reduced insulin-induced Akt phosphorylation. Thus, PI5P4Kbeta alters insulin signaling in a cellautonomous manner and modulates insulin sensitivity in vivo and in cell culture. These studies are consistent with a model in which PI-5-P either directly or indirectly inhibits a phosphatase that breaks down PI-3,4,5-P3 and thus acts as a regulator of the PI3K/AKT pathway (Discussed in Chapter 4).