Conditional ectopic expression of C/EBP beta in NIH-3T3 cells induces PPAR gamma and stimulates adipogenesis.

ABSTRACT

Activation of adipogenesis in 3T3 preadipocytes by exposure to the adipogenic inducers dexamethasone, methylisobutylxanthine, insulin, and fetal bovine serum is accompanied by a transient burst of C/EBP beta protein expression that precedes the induction of the fat gene program. In this study we have investigated the role of C/EBP beta in initiating the adipogenic program by overexpressing C/EBP beta in multipotential NIH-3T3 fibroblasts. Conditional ectopic expression of C/EBP beta was accomplished by using an artificial transcriptional regulatory system based on the Escherichia coli tetracycline repressor to generate a stable cell line, beta 2, that expresses C/EBP beta mRNA and protein in a tightly controlled tetracycline dose-dependent manner. Induction of C/EBP beta DNA-binding activity in NIH-3T3 beta 2 cells exposed to dexamethasone in the presence of insulin and fetal bovine serum activates the expression of an adipocyte-specific nuclear hormone receptor, PPAR gamma, that stimulates the conversion of these fibroblasts into committed preadipocytes. Either ectopic expression of C/EBP beta or treatment with dexamethasone alone is incapable of inducing PPAR gamma expression, but when present together, they have a synergistic effect on the adipogenic program. Exposure of these stimulated cells to a PPAR activator 5,8,11,14-eicosatetraynoic acid (ETYA) results in the accumulation of fat droplets and expression of the adipocyte-enriched genes aP2 and glycerol phosphate dehydrogenase (GPD). The number of beta 2 cells that can differentiate into adipocytes is related to the concentration of tetracycline and, therefore, the amount of the exogenous C/EBP beta protein expressed. C/EBP beta can induce PPAR gamma mRNA in the absence of ETYA; however, expression of aP2 mRNA and maximum fat deposition is dependent on the PPAR activator. Our results suggest that enhanced expression of C/EBP beta converts multipotential mesenchymal precursor cells into preadipocytes that respond to adipogenic inducers, including dexamethasone and PPAR activators to differentiate into adipocytes.