Expression of the phosphoenolpyruvate carboxykinase gene in 3T3-F442A adipose cells: opposite effects of dexamethasone and isoprenaline on transcription.

The enzyme phosphoenolpyruvate carboxykinase (PEPCK) plays a key role in gluconeogenesis in liver and in glyceroneogenesis in adipose tissue. These processes, and PEPCK, are regulated by a number of hormones, some of which have different effects on the enzyme in liver and adipose tissue. To explore this phenomenon, PEPCK gene expression was studied in 3T3-F442A adipocytes maintained in a serum-free medium. The beta-adrenergic agonist isoprenaline (isoproterenol) and a cyclic AMP analogue (8-CPT-cAMP) increased PEPCK mRNA. A maximal 3-fold induction occurred in 2 h. Dexamethasone decreased PEPCK mRNA by 80% in 4 h. Dexamethasone also counteracted the inductive effects of isoprenaline and 8-CPT-cAMP. Run-on transcription experiments showed that the isoprenaline and dexamethasone actions were, at least in part, exerted at the level of PEPCK gene transcription. These effects were further analysed by using transient and stable transfection of adipocytes with a plasmid containing bp -2100 to 69 of the PEPCK gene promoter fused to the chloramphenicol acetyltransferase (CAT) gene. In such cells isoprenaline stimulated CAT expression, an effect that was prevented if the cells were also exposed to dexamethasone.