STIMULATION OF A GLYCOSYL-PHOSPHATIDYLINOSITOL-SPECIFIC PHOSPHOLIPASEBY INSULIN AND THE SULFONYLUREA, GLIMEPIRIDE, IN RAT ADIPOCYTES DEPENDS ON INCREASED GLUCOSE-TRANSPORT

Lipoprotein lipase (LPL) and glycolipid-anchored cAMP-binding ectoprot ein (Gce1) are modified by glycosyl-phosphatidylinositol (GPI) in rat adipocytes, however, the linkage is potentially unstable. Incubation o f the cells with either insulin (0.1-30 nM) or the sulfonylurea, glime piride (0.5-20 mu M), in the presence of glucose led to conversion of up to 35 and 20%, respectively, of the total amphiphilic LPL and Gce1 to their hydrophilic versions. Inositol-phosphate was retained in the residual protein-linked anchor structure. This suggests cleavage of th e GPI anchors by an endogenous GPI-specific insulin- and glimepiride-i nducible phospholipase (GPI-PL). Despite cleavage, hydrophilic LPL and Gce1 remained membrane associated and were released only if a competi tor, e.g., inositol-(cyclic)monophosphate, had been added. Other const ituents of the GPI anchor (glucosamine and mannose) were less efficien t. This suggests peripheral interaction of lipolytically cleaved LPL a nd Gce1 with the adipocyte cell surface involving the terminal inosito l-(cyclic)monophosphate epitope and presumably a receptor of the adipo cyte plasma membrane. In rat adipocytes which were resistant toward gl ucose transport stimulation by insulin, the sensitivity and responsive ness of GPI-PL to stimulation by insulin was drastically reduced. In c ontrast, activation of both GPI-PL and glucose transport by the sulfon ylurea, glimepiride, was not affected significantly Inhibition of gluc ose transport or incubation of rat adipocytes in glucose-free medium c ompletely abolished stimulation of GPI-PL by either insulin or glimepi ride. The activation was partially restored by the addition of glucose or nonmetabolizable 2-deoxy-glucose. These data suggest that increase d glucose transport stimulates a GPI-PL in rat adipocytes.