Insulin Promotes the Hydrolysis of a Glycosyl Phosphatidylinositol in Cultured Rat Astroglial Cells

Glycosyl phosphatidylinositols have been implicated in insulin signaling through their action as precursors of second messenger molecules in peripheral tissues. In the present study, cultured rat astrocytes were used to investigate whether glycosyl phosphatidylinositol might be involved in the mechanism of insulin signal transduction in neural cells. A glycosyl phosphatidylinositol sensitive to hydrolysis by both phosphatidylinositol‐specific phospholipase C and glycosyl phosphatidylinositol‐specific phospholipase D and to nitrous acid deamination was purified. When astrocytes were exposed to 10 n M insulin, a rapid and significant reduction in the content of glycosyl phosphatidylinositol was observed within 1–2 min. In addition, an inverse concentration‐dependent relationship between glycosyl phosphatidylinositol and diacylglycerol levels was found, suggesting a phospholipase C‐mediated hydrolysis of glycosyl phosphatidylinositol in response to insulin. The effects of insulin were mediated through its own receptors and not through insulin‐like growth factor (IGF)‐I and/or IGF‐II receptors, as demonstrated by affinity cross‐linking studies. Also, the effects of 5 n M IGF‐I or 5 n M IGF‐II on glycosyl phosphatidylinositol and diacylglycerol levels were different from those caused by insulin and were not essentially modified by pretreatment of the cells with either platelet‐derived growth factor (PDGF) or epidermal growth factor (EGF). When cells were sequentially incubated with PDGF and EGF, a reduction in both glycosyl phosphatidylinositol and diacylglycerol contents was observed; the diacyl‐glycerol but not the glycosyl phosphatidyl content was reversed after incubation with IGF‐I, and especially with IGF‐II, for 10 min. Despite the remarkable homology among insulin, IGF‐I, and IGF‐II, our results indicate that in astrocytes these compounds probably use different signal transduction pathways.