Redistribution of glycolipid raft domain components induces insulin-mimetic signaling in rat adipocytes

Caveolae and caveolin-containing detergent-insoluble glycolipid-enriched ra fts (DIG) have been implicated to function as plasma membrane microcompartm ents or domains for the preassembly of signaling complexes, keeping them in the basal inactive state. So far, only limited in vivo evidence is availab le for the regulation of the interaction between caveolae-DIG and signaling components in response to extracellular stimuli, Here, we demonstrate that in isolated rat adipocytes, synthetic intracellular caveolin binding domai n (CBD) peptide derived from caveolin-associated pp59(Lyn) (10 to 100 muM) or exogenous phosphoinositolglycan derived from glycosyl-phosphatidylinosit ol (GPI) membrane protein anchor (PIG; 1 to 10 muM) triggers the concentrat ion-dependent release of caveolar components and the GPI-anchored protein G ce1, as well as the nonreceptor tyrosine kinases pp59(Lyn) and pp125(Fak), from interaction with caveolin (up to 45 to 85%), This dissociation, which parallels redistribution of the components from DIG to non-DIG areas of the adipocyte plasma membrane (up to 30 to 75%), is accompanied by tyrosine ph osphorylation and activation of pp59(Lyn) and pp125(Fak) (up to 8- and 11-f old) but not of the insulin receptor. This correlates well to increased tyr osine phosphorylation of caveolin and the insulin receptor substrate protei n 1 (up to 6- and 15-fold), as well as elevated phosphatidylinositol-3; kin ase activity and glucose transport (to up to 7- and 13-fold). Insulin-mimet ic signaling by both CBD peptide and PIG as well as redistribution induced by CBD peptide, but not by PIG, was blocked by synthetic intracellular cave olin scaffolding domain (CSD) peptide, These data suggest that in adipocyte s a subset of signaling components is concentrated at caveolae-DIG via the interaction between their CBD and the CSD of caveolin, These inhibitory int eractions are relieved by PIG. Thus, caveolae-DIG may operate as signalosom es for insulin-independent positive cross talk to metabolic insulin signali ng downstream of the insulin receptor based on redistribution and accompany ing activation of nonreceptor tyrosine kinases.