Lipoprotein lipase (LPL) and glycolipid-anchored cAMP-binding ectoprotein (Gce1) are modified by glycosyl-phosphatidylinositol (GPI) in rat adipocytes, however, the linkage is potentially unstable. Incubation of the cells with either insulin (0.1-30 nM) or the sulfonylurea, glimepiride (0.5-20 microM), 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 the GPI anchors by an endogenous GPI-specific insulin- and glimepiride-inducible phospholipase (GPI-PL). Despite cleavage, hydrophilic LPL and Gce1 remained membrane associated and were released only if a competitor, e.g., inositol-(cyclic)monophosphate, had been added. Other constituents of the GPI anchor (glucosamine and mannose) were less efficient. This suggests peripheral interaction of lipolytically cleaved LPL and Gce1 with the adipocyte cell surface involving the terminal inositol-(cyclic)monophosphate epitope and presumably a receptor of the adipocyte plasma membrane. In rat adipocytes which were resistant toward glucose transport stimulation by insulin, the sensitivity and responsiveness of GPI-PL to stimulation by insulin was drastically reduced. In contrast, activation of both GPI-PL and glucose transport by the sulfonylurea, glimepiride, was not affected significantly. Inhibition of glucose transport or incubation of rat adipocytes in glucose-free medium completely abolished stimulation of GPI-PL by either insulin or glimepiride. The activation was partially restored by the addition of glucose or nonmetabolizable 2-deoxyglucose. These data suggest that increased glucose transport stimulates a GPI-PL in rat adipocytes.
An inositol phosphate glycan derived from a Trypanosoma brucei glycosyl-phosphatidylinositol mimics some of the metabolic actions of insulin.
