Insulin-induced phospho-oligosaccharide stimulates amino acid transport in isolated rat hepatocytes

Incorporation of ethanolamine into insulin-sensitive glycosylated phosphatidylinositol of chick embryo fibroblasts

Insulin sensitive glycosylated phosphatidylinositol (GPI) from chick embryo fibroblasts was isolated and partially characterized. [(3)H]Ethanolamine was incorporated into lipids different from phosphatidylethanolamine, as shown by two sequential thin layer chromatographies (TLC) using an acidic solvent system followed by a basic solvent system. Other isotopes, myo-[(3)H]inositol, [(3)H]glucosamine, [(3)H]galactose, and [(3)H]palmitic acid were also incorporated into these lipids. These lipids were separated into two peaks on the second basic TLC, designated as peaks I and II from the origin. Insulin stimulation of cells caused a rapid breakdown of these two lipids. These two lipids were treated by nitrous acid and phosphatidylinositol-specific phospholipase C (PI-PLC). The radioactivity of peak I lipid was decreased by both treatments, and that of peak II lipid was also decreased by PI-PLC treatment but not significantly by nitrous acid treatment. Peak II lipid did not fulfill the criteria for GPI. Tritium released by the treatment of PI-PLC of peak I lipid was recovered in the aqueous phase. [(3)H]Ethanolamine-labeled peak I lipid was hydrolyzed by acid treatment and the hydrolysis products were analyzed by TLC and high performance liquid chromatography (HPLC). Tritium label was recovered as native label at the rate of 95%. [(3)H]Ethanolamine of peak I lipid was reductively methylated completely with formaldehyde and cyanoborohydride, as shown by HPLC analysis. The results indicate that peak I lipid contains primary ethanolamine as a glycan component and is insulin-sensitive free GPI.

Insulin second messengers

The molecular pathways for insulin's signal transduction from its cell surface receptor to the cell's interior metabolic machinery remain in many ways uncharted. Lately two molecules have been proposed as second messengers transducing the insulin signal into the target cell. One is a phospho-oligosaccharide/inositolphosphoglycan and the other is diacylglycerol, both deriving from the same plasma membrane glycolipid, which is hydrolysed in response to insulin treatment. The phospho-oligosaccharide appears to mediate many metabolic effects of insulin through control of the phosphorylation state of key regulatory metabolic enzymes. Diacylglycerol may mediate insulin's stimulation of glucose transport over the plasma membrane. The glycolipid precursor of these putative second messengers, as well as the receptor for insulin, appear to be localized in caveolae microdomains of the plasma membrane, and glucose transporters accumulate in caveolae in response to insulin treatment, suggesting a focal role for caveolae in insulin signalling.

Cell signalling by inositol phosphoglycans from different species

The discovery of glycosyl-phosphatidylinositol (GPI) molecules and their products has given new insight into the field of signal transduction. In the last decade a novel mechanism of protein attachment to membranes has emerged, which involves a covalent linkage of the protein to the glycan moiety of a GPI. The discovery that GPI-anchored proteins are ubiquitous throughout the eukaryotes was followed by the observation that uncomplexed GPI molecules are implicated in signal transduction for a diversity of hormones and growth factors. The hydrolysis of free-GPI generates a novel second messenger: the inositol phosphoglycan (IPG). The aim of this article is to review the role of IPG and IPG-like molecules in signal transduction and to discuss future research directions.

Multifunctional roles of glycosyl-phosphatidylinositol lipids

The results presented here indicate that GPI lipids are a structurally and functionally diverse molecular family. Despite new detailed information on the structures of GPI-anchored proteins, there is relatively scant information on the structure of free-GPI. Thus, little is known of the relationships between GPI structures and the mechanism of their biological effects. For example, there is no distinction at the structural level between hormone-sensitive free-GPI and those that serve as precursors for protein-GPI. Nor is there precise biochemical data on the mechanism and importance of free-GPI in hormone signaling, or the signaling roles that GPI anchors play in protein function. The T-cell activation cascade is an ideal system for studying both forms of GPI and their derivatives. The study of GPI molecules in T lymphocytes offers the exciting possibility of addressing questions on the structure, function, genesis, and regulation of both free- and protein-GPI molecules in a single cell type. The detection of multiple protein-GPI and free-GPI forms, and of hormone-sensitive GPI, provides the first approach to these issues. For the moment, the potential for biochemical signaling by intact GPI or its metabolites is enormous. If significant progress is to be made, the structures of hormone sensitive free-GPI must be elucidated. Only then can we precisely define the roles of these molecules in the regulation of cell metabolism and proliferation.

Does oligosaccharide-phosphatidylinositol (glycosyl-phosphatidylinositol) hydrolysis mediate prolactin signal transduction in granulosa cells?

Initial biosynthetic radiolabelling experiments with cultured granulosa cells revealed the presence of an oligosaccharide-phosphatidylinositol (glycosyl-phosphatidylinositol; (Ose)nPtdIns) structurally related to (Ose)nPtdIns-lipids isolated from other cell types. Prolactin (PRL) stimulated [3H]glucosamine-(Ose)nPtdIns turnover and the rapid generation of [3H]myristoyl-diacylglycerol in cultured follicle-stimulating hormone-(FSH)-primed granulosa cells endowed with PRL receptors. In parallel experiments performed with [3H]myo-inositol-labelled granulosa cells, treatment with PRL stimulated (Ose)nPtdIns hydrolysis in a similar manner, whereas no effect on phosphoinositide (PtdIns, PtdInsP and PtdInsP2) turnover could be observed. These results strongly suggest that the cleavage of (Ose)nPtdIns by phosphodiesterase followed by the subsequent generation of diacylglycerol and a soluble phosphoinositol-oligosaccharide (inositol-phosphoglycan; (Ose)nInsP) moiety could be part of the signal-transduction mechanism linking PRL receptors to their biological effects in granulosa cells. To test this hypothesis, we examined the effect of PRL and purified (Ose)nInsP moiety (from rat liver membranes) on granulosa cell 3 beta-hydroxysteroid dehydrogenase/delta 5-4 isomerase (3 beta-HSD) enzyme activity. Results presented show that, in FSH-primed granulosa cells, PRL (40 nM) and (Ose)nInsP (5 microM) prevented gonadotropin-stimulated 3 beta-HSD activity. Furthermore, in undifferentiated granulosa cells where PRL receptors are absent, no effect of the hormone on 3 beta-HSD activity could be observed, whereas (Ose)nInsP (1-10 microM) inhibited enzyme activity in a dose-dependent manner.

Changes in the insulin-sensitive glycosyl-phosphatidyl-inositol signalling system with aging in rat hepatocytes

An inositol-phosphate glycan (InsP glycan), which is the polar head group of an insulin-sensitive glycosyl-phosphatidylinositol (glycosyl-PtdIns), has been reported to mimic some insulin actions when added to different types of cells. In connection with this, a specific, time-dependent and energy-dependent transport system for this InsP glycan has been identified in isolated rat hepatocytes [Alvarez, J. F., Sánchez-Arias, J. A., Guadaño, A., Estevez, F., Varela, I., Felíu, J. E. & Mato, J.M. (1991) Biochem. J. 274, 369-374]. Here we have investigated the glycosyl-PtdIns-dependent insulin-signalling system in hepatocytes isolated from either 3-month-old or 24-month-old rats. Aging reduced the stimulatory effect of insulin on [U-14C]glucose incorporation into glycogen, caused a significant decrease in basal glycosyl-PtdIns levels and blocked the insulin-mediated hydrolysis of this lipid. In 24-month-old rats, we also observed a diminution in the rate of hepatocyte InsP-glycan uptake and a marked reduction of the stimulatory effect of this compound on glycogen synthesis. These results support the hypothesis that insulin resistance associated with aging is accompanied by an impairment of the glycosyl-PtdIns-dependent cellular signalling system.

Relationship between insulin-mediated turnover of glucosamine-labeled lipids and activity of the insulin receptor tyrosine kinase

We studied the effects of insulin on the turnover of glucosamine-labeled lipids in embryonic RAT fibroblasts that overexpressed either normal human insulin receptors or insulin receptors with defective tyrosine kinase domains. Fractionation of organic extracts by thin layer chromatography in chloroform/acetone/methanol/acetic acid/water (50/20/10/10/5, v/v) revealed two insulin-sensitive glucosaminyl lipid fractions, the TLC origin (the Rf 0.0 fraction) and a fraction that migrated with Rf 0.18-0.2 (the Rf 0.2 fraction). The insulin-sensitive molecules in both fractions could also be labeled with D-[6-3H]galactose, but not with myo-[2-3H]inositol. Methanolysis and exposure to methylamine, phospholipase A2, or phosphatidylinositol-specific PLC destroyed the insulin-sensitive lipids in the Rf 0.0 fraction, but had no effect on the Rf 0.2 fraction lipid. The Rf 0.2 fraction lipid was destroyed by endoglycoceraminidase. Insulin caused a rapid loss of label from the Rf 0.0 fraction and an equally rapid increased labeling of the Rf 0.2 fraction, with similar time courses and dependencies on insulin concentration. The turnover of both lipids exhibited the same the insulin dose-response characteristics in cultures which overexpressed insulin receptors with defective tyrosine kinase domains as in cultures that overexpressed normal human insulin receptors. This result supports the conclusions that a number of signaling pathways diverge from the insulin receptor and that not all of those pathways are regulated by the insulin receptor tyrosine kinase.

Role of the glycosylphosphatidylinositol/inositol phosphoglycan system in human fibroblast proliferation

The involvement of the glycosylphosphatidylinositol/inositol phosphoglycan (gly-PtdIns/IPG) system in the stimulation of macromolecular syntheses in human fibroblasts has been investigated. The study demonstrates that an insulin sensitive gly-PtdIns/IPG system is present in human fibroblasts, that IPG can significantly stimulate DNA, RNA, and protein synthesis, and that the action of insulin on DNA synthesis as well as that of IPG can be significantly reduced by a specific anti-IPG antibody. These results strongly support the hypothesis that the gly-PtdIns/IPG system is involved in the signal transduction pathway leading to the stimulation of cell proliferation.

A phosphatidylinositol-linkage-deficient T-cell mutant contains insulin-sensitive glycosyl-phosphatidylinositol

Glycosyl-phosphatidylinositol molecules, acting as both signal transduction elements and membrane protein anchors, have been proposed to play a role during T-cell activation. The MVB2 cell line is a mutant, derived from the wild-type T-T hybrid YH.16.33, which has a defect in the biosynthesis of PtdIns-protein linkages. As a consequence, MVB2 mutants are defective in activation through the T-cell receptor. Despite the lack of glycosyl-PtdIns anchors in the mutant MVB2 cells, a comparison of the levels and structural features of the insulin-sensitive glycosyl-PtdIns between the MVB2 and YH.16.33 lineages indicates that both cell lines are identical in this respect. The time course for insulin-responsiveness coincides in both cell lines, with maximal hydrolysis 30 s after insulin addition. The ultimate localization of insulin-regulated glycosyl-PtdIns at the outer surface of the cell membrane is also similar. These data indicate that the glycosyl-PtdIns whose hydrolysis is regulated by insulin is not anchoring proteins at the cell surface of T-lymphocytes.

Transport in isolated rat hepatocytes of the phospho-oligosaccharide that mimics insulin action. Effects of adrenalectomy and glucocorticoid treatment

The addition to intact cells of an inositol phospho-oligosaccharide (POS), which is the polar head-group of an insulin-sensitive glycosylphosphatidylinositol, mimics and may mediate some of the biological effects of this hormone. Here we report the existence of a POS transport system in hepatocytes. This POS transport system is specific and time- and dose-dependent. Insulin-resistance caused by dexamethasone administration to rats was accompanied by a decrease in the hepatocyte POS transport system. In contrast, bilateral adrenalectomy provoked a significant increase in the transport of POS. Both the temporal uptake of POS and the regulation of this process by conditions known to modify the sensitivity to insulin suggest that this novel transport system might be involved in the insulin signalling mechanism.

Inositol phospho-oligosaccharide stimulates cell proliferation in the early developing inner ear

The ability of an inositol phospho-oligosaccharide (POS) to mimic the mitogenic effects of nerve growth factor (NGF) and insulin on the early development of the inner ear was investigated. POS (10 microM) stimulated the incorporation of [3H]thymidine into the cochleovestibular ganglion by 3.9-fold. NGF (50 ng/ml) stimulation was 4.7-fold. POS and NGF showed no additivity. Cells induced to proliferate by POS overlapped with those expressing NGF receptors. POS, like insulin, potentiated the mitogenic effect of bombesin on the otic vesicle epithelium. DNA synthesis in the presence of bombesin (100 nM) plus POS (10 microM) was increased by 6.4-fold. POS stimulation was not additive with insulin. The results suggest that POS may play a role in growth factor regulation of cell proliferation during embryonic development.