Regulation of adipocyte gene expression by retinoic acid and hormones: effects on the gene encoding cellular retinol-binding protein.

Our laboratory has reported that adipose tissue and adipocytes are importantly involved in retinoid storage and metabolism. To gain further insight, we examined factors that may regulate CRBP mRNA levels in primary cultures of rat epididymal and murine BFC-1 beta adipocytes. Northern blot analysis revealed that retinoic acid is a potent inducer of CRBP mRNA, causing a 7.5-fold rise in mRNA levels in primary adipocytes and a 9.5-fold rise in BFC-1 beta adipocytes. This induction of CRBP mRNA was dose-dependent at retinoic acid concentrations ranging between 10(-8) and 10(-5) M. Retinoic acid induction of CRBP mRNA levels showed a short lag period (6 h) and reached a maximal level of induction by 12 h in BFC-1 beta adipocytes and by 48 h in primary epididymal adipocytes. Nuclear run-on transcription assays of retinoic acid-induced BFC-1 beta adipocytes indicated that the rate of CRBP gene transcription is enhanced 3.6- to 4.3-fold by retinoic acid. In contrast, dexamethasone markedly down-regulated CRBP expression in a dose-dependent manner at concentrations ranging between 10(-9) and 10(-6) M. CRBP mRNA levels in primary and BFC-1 beta adipocytes declined, respectively, by 90% and 80% when adipocytes were exposed to 10(-6) M dexamethasone for 24 h. Studies of mRNA half-life indicated that dexamethasone acts to lessen CRBP expression through the specific destabilization of CRBP mRNA. Treatment of both primary and BFC-1 beta adipocytes with triiodothyronine alone had no effect on CRBP mRNA levels; however, when adipocytes were treated with a mixture of triiodothyronine and retinoic acid, the induction of CRBP mRNA levels by retinoic acid was reduced. In summary, these studies indicate that CRBP gene expression is regulated by retinoic acid, dexamethasone, and triiodothyronine; thus suggesting that retinol uptake, intracellular transport, and metabolism are dynamically regulated in adipocytes.