Reconstitution of Insulin-sensitive Glucose Transport in Fibroblasts Requires Expression of Both PPARg and C/EBPa*

Adipocyte differentiation is regulated by at least two major transcription factors, CCAAT/enhancer-binding protein a (C/EBPa) and peroxisome proliferator-activated receptor g (PPARg). Expression of PPARg in fibroblasts converts them to fat-laden cells with an adipocyte-like morphology. Here, we investigate the ability of PPARg to confer insulin-sensitive glucose transport to a variety of murine fibroblast cell lines. When cultured in the presence of a PPARg ligand, Swiss-3T3 and BALB/c3T3 cells ectopically expressing PPARg accumulate lipid droplets, express C/EBPa, aP2, insulin-responsive aminopeptidase, and glucose transporter isoform 4 (GLUT4), and exhibit highly insulin-responsive 2-deoxyglucose uptake. In contrast, PPARg-expressing NIH-3T3 cells, despite similar lipid accumulation, adipocyte morphology, and aP2 expression, do not express C/EBPa or GLUT4 and fail to acquire insulin sensitivity. In cells ectopically expressing PPARg, the development of insulin-responsive glucose uptake correlates with C/EBPa expression. Furthermore, ectopic expression of C/EBPa in NIH-3T3 cells converts them to the adipocyte phenotype and restores insulin-sensitive glucose uptake. We propose that the pathway(s) leading to fat accumulation and morphological changes are distinct from that leading to insulin-dependent glucose transport. Our results suggest that although PPARg is sufficient to trigger the adipogenic program, C/EBPa is required for establishment of insulin-sensitive glucose transport.