Circadian and glucocorticoid regulation of Rev-erbα expression in liver

Rev-erb alpha [NR1D1], a member of the nuclear receptor superfamily, is an orphan receptor that constitutively represses gene transcription. Rev-erb alpha has been shown to play a role in myocyte differentiation and to be induced during adipogenesis. Furthermore, Rev-erb alpha is a regulator of lipoprotein metabolism. It was recently shown that Rev-erb alpha messenger RNA (mRNA) levels oscillate diurnally in rat liver. Here, we report that the circadian rhythm of Rev-erb alpha in liver is maintained in primary cultures of rat hepatocytes. Because glucocorticoids have been shown to regulate other transcription factors with circadian expression, it was furthermore examined whether hepatic Rev-erb alpha expression is also regulated by glucocorticoids. Treatment of rats with dexamethasone resulted in a decrease of Rev-erb alpha mRNA levels by 70% after 6 h. Furthermore, dexamethasone decreased Rev-erb alpha expression in rat primary hepatocytes in a dose-dependent fashion. This effect was mediated by the glucocorticoid receptor because simultaneous addition of the glucocorticoid antagonist RU486 prevented the decrease in Rev-erb alpha mRNA levels by dexamethasone. Protein synthesis inhibition with cyclohexamide markedly induced Rev-erb alpha mRNA levels; however, this induction was reduced by dexamethasone supplementation in both rat and human primary hepatocytes. Treatment with actinomycin D blocked the repression of Rev-erb alpha expression by dexamethasone in rat hepatocytes, suggesting that glucocorticoids regulate Rev-erb alpha expression at the transcriptional level. Transient transfection experiments further indicated that Rev-erb alpha promoter activity is repressed by dexamethasone in the presence of cotransfected glucocorticoid receptor. Taken together, these data demonstrate that Rev-erb alpha expression is under the control of both the circadian clock and glucocorticoids in the liver.