DNA demethylation during the differentiation of 3T3-L1 cells affects the expression of the mouse GLUT4 gene.

GLUT4 is the major glucose transporter in adipose tissue and skeletal and cardiac muscles. We examined the mechanisms underlying GLUT4 gene expression in 3T3-L1 cells, which express the gene during their differentiation from preadipocytes to adipocytes. In transient transfections, the activity of a mouse GLUT4 promoter extending to -100 bp in the 5'-flanking region did not differ significantly between 3T3-L1 preadipocytes and adipocytes. Promoter activity up to -590 bp in preadipocytes and adipocytes showed a 70% lower and 228% higher activity, respectively, than promoter activity extending to -100 bp. We also examined methylation status of the GLUT4 promoter. Up to -100 bp, there were five CpG sites at -11, -30, -58, -63, and -75 bp. Two CpG sites at -11 and -30 bp were highly methylated in preadipocytes (60 and 92%, respectively) and highly demethylated in adipocytes (28.6 and 25%, respectively). Conversely, three CpG sites at -58, -63, and -75 bp were highly demethylated in both preadipocytes and adipocytes (<12%). In gel mobility-shift assays, a fragment extending from -40 to -1 bp generated a methylation-sensitive band with nuclear extracts from both preadipocytes and adipocytes when the CpG sites were methylated. Southwestern analysis identified a protein of approximately 55 kDa that bound strongly to the methylated probe. Furthermore, methylation of the CpG sites inhibited promoters extending to -50 or -70 bp. These results suggest that in addition to cell type-specific transcription factor, methylation of specific CpG sites and the methylation-sensitive transcription factor contribute to GLUT4 gene regulation during 3T3-L1 differentiation.