Insulin-mimetic signalling of synthetic phosphoinositolglycans in isolatedrat adipocytes

A set of synthetic phosphoinositolglycan (PIG) compounds has been demonstra ted to exert insulin-mimetic activity on glucose and lipid metabolism in ra t adipocytes differing considerably in potency [compound 41 > 37 > 45 much greater than 7 > 1; W. Prick, A. Bauer, J. Bauer, S. Wied and G. Muller, G. (1998) Biochemistry 37, 13421-13436]. In the present study we examine whet her these differences are based on the capability of the PIG compounds to s timulate signalling components which are thought to mediate metabolic insul in action. Studies using a tyrosine kinase inhibitor and introduction into adipocytes of anti-phosphotyrosine or inhibitory anti-insulin receptor beta -subunit antibodies demonstrated dependence on tyrosine phosphorylation but independence of insulin receptor kinase activation of the insulin-mimetic signalling and metabolic activity of the PIG compounds. The five compounds elicited in rat adipocytes a significant increase in tyrosine phosphorylati on of both insulin receptor substrate 1 (IRS-1) and IRS-3 and, to a minor d egree, IRS-2, in IRS-1/3-associated phosphatidylinositol 3-kinase (PI 3-K) protein as well as activity, and in protein kinase B (PKB) activity as well as phosphorylation. This was most pronounced for compound 41, approaching 65-95 % of the maximal insulin response (MIR) at 20 mu M, and declined in t he order of compounds 37, 45, 7 and 1. The same ranking was true for the ma ximal inhibition of glycogen synthase kinase 3 activity (GSK-3) (41, 75 % o f MIR; compound 37, 65 %;compound 7, 25 %; compound 1, 10 %) and GSK-3 auto phosphorylation. The half-maximal concentrations effective for signalling ( compound 41, 2-5 mu M; compound 37, 10-20 mu M) corresponded well to those stimulating glucose and lipid metabolism. Interestingly, compounds 37 and 4 1 stimulated mitogen-activated protein kinase (MAPK) and protein synthesis in rat adipocytes to only about 20-30% (at 50 mu M) of MIR. We conclude tha t in rat adipocytes : (i) the potency of PIG compounds to regulate glucose/ lipid metabolism depends on the activation of PI 3-K and PKB and inhibition of GSK-3; (ii) initiation of tyrosine phosphorylation of IRS-1/3 is suffic ient and activation of the PI 3-K cascade is required for insulin-mimetic m etabolic signalling; and (iii) PIG compounds are quite selective for the PI 3-K compared to the MAPK cascade, (iv) PIC compounds seem to use the same signalling components downstream of PI 3-K (including Rab4) for stimulation of glucose transport as does insulin. Thus the early signalling step(s) us ed by PIG, but not by insulin, may represent a target for the treatment of insulin-resistant states.