Insulin activates the α isoform of class II phosphoinositide 3-kinase

The novel class II phosphoinositide (PI) S-kinases are characterized by the presence of a C-terminal C2 domain, but little is known about their regulation, We find insulin causes a rapid 2-3-fold increase in the activity of PI S-kinase C2 alpha (PI3K-C2 alpha) in CHO-IR cells, 3T3-L1 adipocytes, and fully differentiated L5L6 myotubes. No insulin-induced activation of PI3K-C2 alpha was observed in cell types known to have low responsiveness to insulin including HEK 293 cells, 3T3-L1 preadipocytes, and undifferentiated L5L6 myoblasts, The mechanism of activation of PI3K-C2a by insulin differs from that of class Ia PI S-kinases in that insulin stimulation did not cause PI3K-C2a to associate with IRS-1 or insulin receptor, PI3K-C2a existed as a doublet, and insulin stimulation caused a redistribution from the lower molecular weight band to the higher molecular weight band, suggesting phosphorylation-induced bandshift. Consistent with this, in vitro phosphatase treatment reduced the intensity of the upper band back to that seen in unstimulated cells, This suggests that insulin-induced phosphorylation could play a role in regulation of the activity of PI3K-C2 alpha. The finding that insulin activates PI3K-C2 alpha in cell types known to possess a wide range of responses to insulin suggests that PI3K-C2 alpha is a novel component of insulin-stimulated signaling cascades.