Dexamethasone stimulates the expression of GLUT1 and GLUT4 proteins via different signalling pathways in L6 skeletal muscle cells.

It was recently demonstrated that dexamethasone treatment of L6 skeletal muscle cells resulted in an elevation of GLUT1 protein. However, the level of GLUT4 protein under these conditions was not examined. In addition, the signalling mechanism(s) leading to dexamethasone-induced expression of GLUT1 protein was not investigated. In the present study we investigated the effect of dexamethasone on the expression of GLUT1 and GLUT4 proteins in differentiated L6 muscle cells and the signalling mechanism(s) via which dexamethasone may act. Dexamethasone (300 nM) treatment for 24 h elevated GLUT1 and GLUT4 proteins by 68% and 94%, respectively, above control levels. These increases were due to de novo synthesis as shown by metabolic labelling with [35S]methionine. Incubation of cells with 100 nM wortmannin or 30 ng/ml rapamycin prevented the dexamethasone-stimulated elevation of GLUT1 protein. In contrast, neither of these inhibitors affected the elevation of GLUT4 protein by dexamethasone. Furthermore, dexamethasone down-regulated insulin receptor substrate-1 protein content by 42% and insulin-induced tyrosine phosphorylation of insulin receptor substrate-1 by 28%. The p70 ribosomal S6 kinase was not activated by dexamethasone and instead, dexamethasone attenuated the stimulation of this enzyme activity by insulin. These results suggest that dexamethasone induces the expression of GLUT1 and GLUT4 protein by independent signalling mechanisms with a concomitant depression of intracellular signalling by insulin.