CYCLIC ADENOSINE-3',5'-MONOPHOSPHATE REGULATES GLUT4 AND GLUT1 GLUCOSE-TRANSPORTER EXPRESSION AND STIMULATES TRANSCRIPTIONAL ACTIVITY OF THE GLUT1 PROMOTER IN MUSCLE-CELLS

We have previously reported that innervation-dependent basal contracti le activity regulates in an inverse manner the expression of GLUT1 and GLUT4 glucose transporters in skeletal muscle. Based on the facts tha t muscle innervation decreases and muscle denervation increases cAMP l evels, we investigated whether cAMP might mediate the effects of inner vation/denervation on glucose transporter expression. Treatment of L6E 9 myotubes with 8-bromo-cAMP, forskolin, or monobutyryl-8-bromo-cAMP l ed to a marked decrease in GLUT4 protein levels; 8-bromo-cAMP also dim inished GLUT4 messenger RNA (mRNA), suggesting pretranslational repres sion. In contrast, L6E9 myoblasts and myotubes responded to 8-bromo-cA MP or forskolin by increasing the cell content of GLUT1 protein. Induc tion of GLUT1 protein was a consequence of the activation of different mechanisms in myoblast and myotube cells; whereas 8-bromo-cAMP treatm ent caused a substantial increase in GLUT1 mRNA in myoblasts, no chang e in GLUT1 mRNA was detected in myotubes. The increase in GLUT1 mRNA i n L6E9 myoblasts induced by 8-bromo-cAMP was the result of transcripti onal activation, as concluded from transfection analysis of 2.1 kiloba ses of the rat GLUT1 gene promoter fused to the bacterial chlorampheni col acetyltransferase gene. Furthermore, the stimulatory effect of 8-b romo-cAMP on the transcriptional activity of the GLUT1 promoter requir ed a 33-bp sequence lying 5' upstream of the transcription start site. In all, cAMP inversely regulates GLUT4 and GLUT1 glucose transporter expression in muscle cells. Furthermore, our results suggest that down -regulation of GLUT4 expression and up-regulation of GLUT1 expression in muscle associated with denervation are partly attributable to cAMP.