Determinants of the nucleocytoplasmic shuttling of muscle glycogen synthase.

Muscle glycogen synthase (MGS) presents a nuclear speckled pattern in primary cultured human muscle and in 3T3-L1 cells deprived of glucose and with depleted glycogen reserves. Nuclear accumulation of the enzyme correlates inversely with cellular glycogen content. Although the glucose-induced export of MGS from the nucleus to the cytoplasm is blocked by leptomycin B, and therefore mediated by CRM1, no nuclear export signal was identified in the sequence of the protein. Deletion analysis shows that the region comprising amino acids 555-633 of human MGS, which encompasses an Arg-rich cluster involved in the allosteric activation of the enzyme by Glc6P, is crucial for its nuclear concentration and aggregation. Mutation of these Arg residues, which desensitizes the enzyme towards Glc6P, interferes with its nuclear accumulation. In contrast, the known phosphorylation sites of MGS that regulate its activity are not involved in the control of its subcellular distribution. Nuclear human MGS co-localizes with the promyelocytic leukaemia oncoprotein and p80-coilin, a marker of Cajal bodies. The subnuclear distribution of MGS is altered by incubation with transcription inhibitors. These observations suggest that, in addition to its metabolic function, MGS may participate in nuclear processes.

The antidiabetic agent sodium tungstate activates glycogen synthesis through an insulin receptor-independent pathway.

Sodium tungstate is a powerful antidiabetic agent when administered orally. In primary cultured hepatocytes, tungstate showed insulin-like actions, which led to an increase in glycogen synthesis and accumulation. However, this compound did not significantly alter the insulin receptor activation state or dephosphorylation rate in cultured cells (CHO-R) or in primary hepatocytes, in either short or long term treatments. In contrast, at low concentrations, tungstate induced a transient strong activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) after 5-10 min of treatment, in a similar way to insulin. Moreover, this compound did not significantly delay or inhibit the dephosphorylation of ERK1/2. ERK1/2 activation triggered a cascade of downstream events, which included the phosphorylation of p90rsk and glycogen synthase-kinase 3beta. Experiments with a specific inhibitor of ERK1/2 activation and kinase assays indicate that these proteins were directly involved in the stimulation of glycogen synthase and glycogen synthesis induced by tungstate without a direct involvement of protein kinase B (PKB/Akt). These results show a direct involvement of ERK1/2 in the mechanism of action of tungstate at the hepatic level.