Cellular and molecular mechanisms involved in insulin's potentiation of glycogen synthase activity by metformin

By taking advantage of the Xenopus oocyte model, we recently confirmed the in vitro enhancing effect of metformin (MET) on glycogen synthase (GS) activity when induced by insulin (INS). We now investigated some mechanistic aspects of its modulatory role upon the hormonal regulation of this rate-limiting enzyme. The action of 20 microM MET (approximately 3.3 microg/mL) was measurable at early steps in the intracellular metabolic pathway: the amount of adenosine 3',5'-cyclic monophosphate (cAMP) was markedly decreased in the presence of the biguanide plus 50 nM INS (to about 60% of control vs 25% with INS alone). The injection of tyrphostin B46, a potent inhibitor of insulin receptor (IR)-associated tyrosine kinase activity, led to a drastic reduction in MET-stimulated GS activity in the presence of INS. MET failed to increase the activity of type 2 protein phosphatases whether INS was present or not. However, a specific inhibitor of type 1 phosphatases, when microinjected, blocked both the hormonal effect on GS and its potentiation by MET. The salient feature of this study was that there was almost no accumulation of radiolabeled MET in oocytes: less than 0.1% was found in the cytosol of cells which had been exposed to MET at a therapeutic dose (10 microM) for up to 16 hr. Moreover, a lack of detectable intracellular MET after a 60-min incubation nevertheless correlated with its sustained action on INS-regulated GS activity. From these results, it could be inferred that the major site of MET action may reside within some membrane components of a signaling complex most likely linked to the IR, but in any case located upstream of the branching of reactions which tightly control GS activity.