TYROSINE PHOSPHATASE INHIBITORS, VANADATE AND PERVANADATE, STIMULATE GLUCOSE-TRANSPORT AND GLUT TRANSLOCATION IN MUSCLE-CELLS BY A MECHANISM INDEPENDENT OF PHOSPHATIDYLINOSITOL 3-KINASE AND PROTEIN-KINASE-C

Vanadate and pervanadate (pV) are protein tyrosine phosphatase (PTP) i nhibitors that mimic insulin to stimulate glucose transport. To determ ine whether phosphatidylinositol (PI) 3-kinase is required for vanadat e and pV, as it is for insulin, cultured L6 myotubes were treated with vanadate and pV. The two compounds stimulated glucose transport to le vels similar to those stimulated by insulin; however, while PI 3-kinas e activity and the increase in the lipid products PI 3,4-bisphosphate and PI 3,4,5-trisphosphate were inhibited by wortmannin after stimulat ion by all three agents-insulin, vanadate, and pV-wortmannin blocked g lucose transport stimulated by insulin but not vanadate or pV. Vanadat e and pV stimulated the translocation of GLUTs from an intracellular c ompartment to the plasma membrane; this stimulation was not blocked by wortmannin, but insulin-induced GLUT translocation was inhibited. Sim ilar results were obtained in cultured H9c2 cardiac muscle cells in wh ich wortmannin did not inhibit glucose transport or the vanadate-induc ed translocation of GLUT4 in c-myc-GLUT4 transfected cells. The ser/th r kinase PKB (Akt/PKB/RAC-PK) is activated by insulin, lies downstream of PI 3-kinase, and has been implicated in signaling of glucose trans port. Insulin and pV stimulated PKB activity, and both were inhibited by wortmannin. In contrast, vanadate, at concentrations that maximally stimulated glucose transport, did not significantly increase PKB acti vity. To determine the potential role of protein kinase C (PKC), L6 ce lls were incubated chronically with phorbol myristate acetate (PMA) or acutely with the PKC inhibitors calphostin C and bisindolylmaleimide. There was no inhibition of glucose transport stimulation by insulin, vanadate, or pV, and a combination of wortmannin and PKC inhibitors al so failed to block the effect of vanadate and pV. In contrast, disasse mbly of the actin network with cytochalasin D blocked the stimulation of glucose transport by all three agents. In conclusion, vanadate and pV are able to stimulate glucose transport and GLUT translocation by a mechanism independent of PI 3-kinase and PKC. Similar to that by insu lin, glucose transport stimulation by vanadate and pV requires the pre sence of an intact act;in network.