INOSITOL PHOSPHO-OLIGOSACCHARIDES FROM RAT FIBROBLASTS AND ADIPOCYTES STIMULATE 3-O-METHYLGLUCOSE TRANSPORT

Inositol phospho-oligosaccharides (IPOs), which are released from liver membranes upon stimulation by insulin, mimic a wide spectrum of insulin effects in different cells, but not the stimulation of glucose transport. We investigated whether other insulin-sensitive tissues release glucose transport-stimulating IPOs and whether this is related to the human insulin receptor isoform-A or -B (HIR-A or HIR-B). Rat1 fibroblasts overexpressing HIR-A or -B (rat1-HIR cells) were labelled with [H-3]-glucosamine, [H-3]mannose or myo-[H-3]inositol. IPOs from the cell supernatant were partially purified by an AG1X2 anion-exchange column, and fractions were eluted at different pH values (pH 3, pH 2 and pH 1.3). The label from glucosamine, mannose and myo-inositol appeared predominantly in the pH 2 fraction. The biological activity of the fractions was determined by measuring 3-O-methylglucose transport and lipogenesis in fat cells. Using the pH 2 fraction from the supernatant of rat1-HIR fibroblasts, insulin increased the release of 3-O-methylglucose-transport-stimulating activity (HIR-A: without insulin, 22.4 +/- 5.4 %; with insulin 54.0 +/- 8.4 %; HIR-B: without insulin 21.6 +/- 7.5 %, with insulin, 44.7 +/- 10.6 %, given as a percentage of equilibrium glucose transport reached after 4 s) and lipogenesis-stimulating activity (HIR-A: without insulin, 1.24 +/- 0.17; with insulin, 4.69 +/- 0.2; HIR-B: without insulin, 1.34 +/- 0.18; with insulin, 4.98 +/- 0.31, given as nmol of [H-3]glucose converted into lipids/min per 10(6) cells). Analogous experiments were performed with isolated rat fat cells expressing the physiological level of insulin receptors. Upon insulin stimulation of fat cells in the presence of 2.5 mM mannose, the release of 3-O-methylglucose-transport-stimulating activity was detected (for purified supernatant of adipocytes without insulin, 6.9 +/- 1.12 %; with insulin, 41.0 +/- 3.6%) and lipogenesis-stimulating activity (without insulin, 0.93 +/- 0.17, with insulin 2.96 +/- 0.31 nmol/min per mg). These data suggest (1) that adipocytes and rat1-HIR fibroblasts release IPOs that are able to stimulate glucose transport, (2) that both insulin receptor isoforms (HIR-A and HIR-B) mediate the effect of insulin on IPO release, and (3) that overexpression of insulin receptors increases the basal release of IPOs.