INOSITOL PHOSPHOLIPID 3-KINASE IS ACTIVATED BY CELLULAR STRESS BUT ISNOT REQUIRED FOR THE STRESS-INDUCED ACTIVATION OF GLUCOSE-TRANSPORT IN L6 RAT SKELETAL-MUSCLE CELLS

A characteristic response of cells subjected to a stress stimulus is a rapid activation of cellular glucose transport, The mechanisms govern ing this increase in glucose transport are poorly understood, but it h as been suggested that the response may involve the intracellular-sign aling components that also participate in the hormonal activation of g lucose transport. In skeletal muscle and fat tissue, inositol phosphol ipid 3-kinase plays an integral role in the regulation of both basal a nd insulin-stimulated glucose transport. In this study, we have invest igated whether inositol phospholipid 3-kinase is activated by chemical stress and, if so, whether it has a role to play in the stress-induce d increase in glucose transport in L6 muscle cells, Furthermore, we ha ve attempted to assess the basis by which inositol phospholipid 3-kina se may participate in the regulation of basal glucose transport. Acute exposure (30 min) of L6 muscle cells to 0.5 mM arsenite induced an 80 % stimulation in glucose transport. This activation was due to a rise in the number of cell-surface glucose transporters, based on an increa se in the V-max of glucose transport and the observation that arsenite increases the plasma membrane content of GLUT1 and GLUT4 glucose tran sporters by 95% and 60%, respectively, from an intracellular compartme nt. Arsenite induced rapid activation (<2 min) of inositol phospholipi d 3-kinase with an approximately fourfold increase in phosphatidylinos itol 3,4,5-trisphosphate (PtdIns3,4,5P(3)). In contrast, phosphatidyli nositol 3-phosphate (PtdIns3P) levels were unaffected. Prior treatment of L6 cells with 100 nhl wortmannin suppressed the arsenite-induced i ncrease in PtdIns3,4,5P(3) and reduced the cellular content of PtdIns3 P by 50%. Under these conditions however, wortmannin failed to prevent the stress-induced activation of glucose transport, but suppressed ba sal glucose transport by 60% with an IC50 of about 10 nM. In the absen ce of arsenite, wortmannin caused a dose-dependent inhibition in the c ellular levels of PtdIns3P and PtdIns3,4,5P(3), with IC50 values of ab out 10 nM and 100 nM, respectively. In summary, the present results de monstrate that chemical stress activates inositol phospholipid 3-kinas e and glucose transport in L6 muscle cells, but unlike the hormonal re sponses of these cells the activation of inositol phospholipid 3-kinas e is not responsible for the stress-induced increase in glucose transp ort. This implies that stress-induced and hormonal stimulated increase s in PtdIns3,4,5P(3) levels are functionally distinct. By contrast, th e maintenance of PtdIn3P levels, presumably involving a PtdIns-specifi c, wortmannin-sensitive inositol phospholipid 3-kinase may be required to support basal glucose transport.