Insulin Does Not Induce the Hydrolysis of a Glycosyl Phosphatidylinositol in Rat Fetal Hepatocytes

Abstract

An inositol phosphoglycan that is the polar head group of a glycosyl phosphatidylinositol has been considered as a putative mediator of insulin action. To gain insight into the functions of this hormone during development, the relationships between insulin, insulin receptors, glycosyl phosphatidylinositol, and inositol phosphoglycan were studied. Glycosyl phosphatidylinositol was isolated and characterized in fetal liver as early as day 15 of intrauterine life. In isolated hepatocytes from fetal and adult rats labeled with [3H]glucosamine, [3H]galactose, or [3H]myo-inositol, these molecules were incorporated into glycosyl phosphatidylinositol. In hepatocytes labeled with [3H]glucosamine and then allowed to react with [1-14C]IAI, the [3H]glycosyl phosphatidylinositol was purified as the 14C-labeled amidinated lipid. Glycosyl phosphatidylinositol molecules from fetal and adult cells were sensitive 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 animals, an inverse correlation between extracellular insulin and the number of insulin receptors and the cellular content of glycosyl phosphatidylinositol was observed. However, in fetal hepatocytes insulin failed to reduce the glycosyl-phosphatidylinositol content when labeled either with [1-14C]isethionyl acetimidate or [3H]glucosamine, whereas insulin-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, whereas in fetal cells insulin did not change, and purified inositol phosphoglycan reduced the activities of glycogen phosphorylase. These findings suggest a dissociation between insulin receptor occupancy and the expected hormonal effects in fetal hepatocytes. This could be related to alterations at a postreceptor level.

The Biomedical Uses of Inositols: A Nutraceutical Approach to Metabolic Dysfunction in Aging and Neurodegenerative Diseases

“…However, in adult rat hepatocytes, insulin mediates GPI cleavage and IPG formation, and it has been observed that fetal hepatocyte formation of IPGs is dependent on IGF-1 but not insulin activity. Furthermore, the addition of isolated IPG-P, but not insulin, has reduced the activity of glycogen phosphorylase (the rate limiting enzyme for glycogen hydrolysis) [ 119 ].…”

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

“…In many types of cells from peripheral tissues Suzuki et al, 1993) it has been reported that insulin initiates a rapid hydrolysis of a GPI, resulting in the release of IPG and DAG, that may act as second biochemical signals. Indeed, IPG mimics the direct effects of insulin in acting on the phosphorylation and dephosphorylation of several target proteins (Alemany et al, 1987), in modulating enzyme activities Ruiz-Albusac et al, 1993), and in regulating gene expression (Alvarez et al, 1991), whereas DAG significantly increases glucose uptake (Strdlfors, 1988). At present, it is unknown whether the action of insulin in neural cells involves GPI hydrolysis and hence the generation of chemical mediators, as has been described for nonneural tissues.…”

“…GPIs have been implicated in cell signaling by insulin, as well as by other hormones, growth factors, and interleukins Mérida et al, 1990;Ruiz-Albusac et al, 1993). In many types of cells from peripheral tissues Suzuki et al, 1993) it has been reported that insulin initiates a rapid hydrolysis of a GPI, resulting in the release of IPG and DAG, that may act as second biochemical signals.…”