INSULIN STIMULATES THE BIOSYNTHESIS OF CHIRO-INOSITOL-CONTAINING PHOSPHOLIPIDS IN A RAT FIBROBLAST LINE EXPRESSING THE HUMAN INSULIN-RECEPTOR

HIRc-B cells (rat fibroblasts expressing the human insulin receptor) were incubated with myo-[H-3]inositol for 48 hr, and the biosynthesis of chiro-[H-3]inositol was investigated in the absence and presence of insulin following a time course up to 60 min. After phase separation, treatment with insulin for 15 min caused a 2.2-fold increase in the specific radioactivity of chiro-[H-3]inositol-containing phospholipids in contrast to a 1.2-fold increase in the specific radioactivity of myo-[H-3]inositol-containing phospholipids. No insulin-mediated change in the specific radioactivity was observed in the inositol phosphates or free inositols. Further detailed analysis of individual [H-3]inositol-containing phospholipids demonstrated marked increases in specific activity of the chiro-[H-3]inositol phospholipids after 15 min of incubation with insulin: phosphatidylinositol 4-phosphate and 4,5-bisphosphate, 4.2-fold; lysophosphatidylinositol, 1.5-fold; phosphatidylinositol, 3.2-fold. In contrast, myo-[H-3]inositol-containing phospholipids demonstrated relatively small increases (1.1- to 1.4-fold) after 5 min of incubation with insulin. These findings indicate that insulin stimulates de novo synthesis of chiro-inositol-containing phospholipids at the inositol phospholipid level.