Glucocorticoids repress induction by thiazolidinediones, fibrates, and fatty acids of phosphoenolpyruvate carboxykinase gene expression in adipocytes

Phosphoenolpyruvate carboxykinase (PEPCK) exerts a glyceroneogenic function in adipocytes in which transcription of its gene is increased by unsaturated fatty acids and fibrates. We used cultured rat adipose tissue fragments and 3T3‐F442A adipocytes to show that the antidiabetic thiazolidinedione BRL 49653, a ligand and an activator of the γ isoform of peroxisome proliferator activated receptors (PPARγ), is a potent inducer of PEPCK mRNA. In 3T3‐F442A adipocytes, the effect of BRL 49653 is rapid and concentration dependent, with a maximum reached at 1 μM and a half‐maximum at 10–100 nM. PEPCK mRNA is similarly induced by the natural ligand of PPARγ, the 15‐deoxy‐Δ 12–14 prostaglandin J2. These observations strongly suggest that PPARγ is a primary regulator of PEPCK gene expression in adipocytes. Dexamethasone at 10 nM repress induction of PEPCK mRNA by 1 μM BRL 49653, 0.32 mM oleate, or 1 mM clofibrate, in a cycloheximide‐independent manner. The antiglucocorticoid RU 38486 prevents dexamethasone action, demonstrating involvement of the glucocorticoid receptor. Stable transfectants of 3T3‐F442A adipocytes bearing −2100 to +69 base pairs of the PEPCK gene promoter fused to the chloramphenicol acetyltransferase (CAT) gene respond to 1 μM BRL 49653 or 1 mM clofibrate by a large increase in CAT activity, which is prevented by the simultaneous addition of 10 nM dexamethasone. Hence, in adipocytes, glucocorticoids act directly through the 5′‐flanking region of the PEPCK gene to repress, in a dominant fashion, the stimulation of PEPCK gene transcription by thiazolidinediones and fibrates. J. Cell. Biochem. 68:298–308, 1998. © 1998 Wiley‐Liss, Inc.