THE ROLE OF THE GLUT-4 TRANSPORTER IN REGULATING RAT MYOBLAST GLUCOSE-TRANSPORT PRECESSES

Previous studies revealed an inverse relationship between GLLTT 1 and GLUT 4 expression in rat myoblasts [L. Xia, Z. Lu, T.C.Y. Lo, J. Biol. Chem., 268 (1993) 23258-.23266]. It was not clear whether these were coincidental or causal occurrences. To examine the regulatory roles of the GLUT 4 isoform, rat L6 myoblasts were transfected with full lengt h GLUT 4 cDNAs (2.5 kb) in the sense or antisense orientation. L6 myob lasts transfected with the GLUT 4 sense cDNA (L6/G4S transfectants) po ssessed much elevated levels of both endogenous GLUT 4 transcripts (1. 4 kb and 2.8 kb). Transport and immunofluorescence studies showed that this GLUT 4 sense cDNA was responsible for a functional GLUT 4 transp orter. L6 cells transfected with the GLUT 4 antisense cDNA (L6/G4A tra nsfectants) possessed only 6% of the L6 level in day 6 cultures. These antisense transfectants were essentially devoid of any functional GLU T 4 transporter. The activation of transcription of the endogenous GLU T 4 gene in L6/G4S myoblasts suggested auto-regulation of GLUT 4 expre ssion. GLUT 3 expression and activity were not altered in both sense a nd antisense GLUT 4 transfectants. More interestingly, GLUT 1 expressi on was reduced in L6/G4S myoblasts, whereas it was elevated in L6/G4A myoblasts. This was the first direct evidence indicating GLUT 4 might play an important role in suppressing GLUT I expression. (C) 1998 Else vier Science B.V. All rights reserved.