Phosphoinositide 3-Kinase Catalytic Subunit Deletion and Regulatory Subunit Deletion Have Opposite Effects on Insulin Sensitivity in Mice

Studies ex vivo have shown that phosphoinositide 3-kinase (PI3K) activity is necessary but not sufficient for insulin-stimulated glucose uptake. Unexpectedly, mice lacking either of the PI3K regulatory subunits p85 or p85 exhibit increased insulin sensitivity. The insulin hypersensitivity is particularly unexpected in p85 / p55 / p50 / mice, where a decrease in p110 and p110 catalytic subunits was observed in insulinsensitive tissues. These results raised the possibility that decreasing total PI3K available for stimulation by insulin might circumvent negative feedback loops that ultimately shut off insulin-dependent glucose uptake in vivo. Here we present results arguing against this explanation. We show that p110 / p110 / mice exhibit mild glucose intolerance and hyperinsulinemia in the fasted state. Unexpectedly, p110 / p110 / mice showed a 50% decrease in p85 expression in liver and muscle. Consistent with this in vivo observation, knockdown of p110 by RNA interference in mammalian cells resulted in loss of p85 proteins due to decreased protein stability. We propose that insulin sensitivity is regulated by a delicate balance between p85 and p110 subunits and that p85 subunits mediate a negative role in insulin signaling independent of their role as mediators of PI3K activation. Type 2 diabetes is a serious public health problem that is growing rapidly in the developed world. The disease is characterized by insulin resistance of the peripheral organs and hyperglycemia. Initially, patients compensate by secreting elevated insulin, but over the course of the disease, the pancreatic beta cells fail to produce compensating levels of insulin. Based on ex vivo experiments, phosphoinositide 3-kinase (PI3K) is required for insulin-induced glucose uptake into muscle and fat and inhibition of glucose production in the liver (10, 30, 35, 38). Polymorphism in the human PI3K regulatory isoform p85 gene has been associated with increased risk for developing type 2 diabetes (5). However, the consequences of the polymorphism on the expression level or function of p85 have not yet been analyzed. Many of the metabolic effects of insulin require activation of the PI3K downstream target Akt. While constitutively active Akt induces translocation of the glucose transporter GLUT4 to the plasma membrane in adipocytes, dominant negative Akt inhibits it (13). Consistent with this, targeted disruption of Akt2 in the mouse resulted in insulin resistance and severe diabetes, and RNA interference (RNAi) for Akt2 inhibited insulin-dependent glucose uptake in 3T3L1 cells (12, 19, 28). An inactivating mutation in human AKT2 has been associated with severe insulin resistance and diabetes mellitus, demonstrating a pivotal role of AKT signaling in glucose homeostasis in humans (20). Surprisingly, deletion of PI3K regulatory subunits in mice