INSULIN DOES NOT INDUCE THE HYDROLYSIS OF A GLYCOSYL PHOSPHATIDYLINOSITOL IN RAT FETAL HEPATOCYTES

An inositol phosphoglycan that is the polar head group of a glycosyl p hosphatidylinositol has been considered as a putative mediator of insu lin action. To gain insight into the functions of this hormone during development, the relationships between insulin, insulin receptors, gly cosyl phosphatidylinositol, and inositol phosphoglycan were studied. G lycosyl phosphatidylinositol was isolated and characterized in fetal l iver as early as day 15 of intrauterine life. In isolated hepatocytes from fetal and adult rats labeled with [H-3]glucosamine, [H-3]galactos e, or [H-3]myo-inositol, these molecules were incorporated into glycos yl phosphatidylinositol. In hepatocytes labeled with [H-3]glucosamine and then allowed to react with [1-C-14]IAI, the [H-3]glycosyl phosphat idylinositol was purified as the C-14-labeled amidinated lipid. Glycos yl phosphatidylinositol molecules from fetal and adult cells were sens itive to hydrolysis by a phosphatidylinositol-specific phospholipase C from B. cereus. The product of this hydrolysis inhibits the activity of a cAMP-dependent protein kinase, whereas this effect was abolished by nitrous acid deamination. In isolated hepatocytes from adult animal s, an inverse correlation between extracellular insulin and the number of insulin receptors and the cellular content of glycosyl phosphatidy linositol was observed. However, in fetal hepatocytes insulin failed t o reduce the glycosyl-phosphatidylinositol content when labeled either with [1-C-14]isethionyl acetimidate or [H-3]glucosamine, whereas insu lin-like growth factor I produced a significant hydrolysis of glycosyl phosphatidylinositol. Fetal and adult hepatocytes were incubated with insulin or inositol phosphoglycan after which glycogen phosphorylase activities were determined. Inositol phosphoglycan mimicked the action of insulin on both forms of the enzyme from adult hepatocytes, wherea s in fetal cells insulin did not change, and purified inositol phospho glycan reduced the activities of glycogen phosphorylase. These finding s suggest a dissociation between Insulin receptor occupancy and the ex pected hormonal effects in fetal hepatocytes. This could be related to alterations at a postreceptor level.