Fatty genotype-induced increase in GLUT4 promoter activity in transfected adipocytes: delineation of two fa-responsive regions and glucose effect.

We have previously shown that adipocytes from genetically obese rats exhibited increases in GLUT4 protein and mRNA expressions. In order to elucidate the responsible defect we examined here the activity of GLUT4 promoter using adipocytes transiently transfected with -2212 + 164 rat GLUT4 gene fragment fused to luciferase reporter gene. GLUT4 promoter activity was several-fold higher in obese than in lean rat adipocytes (x4 in suckling 16 days old and x6 in weaned 30 days old rats), demonstrating the implication of transcription factors in the fatty genotype effect on GLUT4 expression. 5' deletion analysis of GLUT4 promoter allowed us to delineate two regions, -907 to -502 and -151 to -68, critical to the genotype effect, suggesting that they harbor fa-response element(s). In lean rat adipocytes GLUT4 promoter activity was not affected by the presence of glucose in the culture medium. In contrast, in obese rat adipocytes GLUT4 transcriptional activity was increased 3-fold from 0 to 5mM glucose, the concentration required for the full genotype effect, suggesting interactions between glucose and fa-dependent GLUT4 transactivator(s).