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Evidence for Circadian Regulation of Activating Transcription Factor 5 But Not Tyrosine Hydroxylase by the Chromaffin Cell Clock

Division of Neuroscience (D.R.L., L.G., S.R.O., H.F.U.), Oregon National Primate Research Center, Beaverton, Oregon 97006; Department of Physiology (D.R.L.), Faculty of Medicine, University of Buenos Aires, 1121 Buenos Aires, Argentina; Vaccine and Gene Therapy Institute (L.T.), Oregon Health and Science University, Beaverton, Oregon 97006; Department of Cancer Biology (M.P.A.), Lerner Research Institute, The Cleveland Clinic Foundation, Cleveland, Ohio 44109; and Departments of Physiology and Pharmacology and Behavioral Neuroscience (H.F.U.), Oregon Health and Science University, Portland, Oregon 97239

Address all correspondence and requests for reprints to: Henryk F. Urbanski, Division of Neuroscience, Oregon National Primate Research Center, 505 Northwest 185th Avenue, Beaverton, Oregon 97006. E-mail: urbanski{at}ohsu.edu.

In mammals, adrenal medulla chromaffin cells constitute a fundamental component of the sympathetic nervous system outflow, producing most of the circulating adrenaline. We recently found that the rhesus monkey adrenal gland expresses several genes in a 24-h rhythmic pattern, including TH (the rate-limiting enzyme in catecholamine synthesis) and Atf5 (a transcription factor involved in apoptosis and neural cell differentiation) together with the core-clock genes. To examine whether these core-clock genes play a role in adrenal circadian function, we exposed rat pheochromocytoma PC12 cells to a serum shock and found that it triggered rhythmic oscillation of the clock genes rBmal1, rPer1, rRev-erb, and rCry1 and induced the circadian expression of Atf5 but not TH. Furthermore, we found that the CLOCK/brain and muscle Arnt-like protein-1 (BMAL1) heterodimer could regulate Atf5 expression by binding to an E-box motif and repressing activity of its promoter. The physiological relevance of this interaction was evident in Bmal1 –/– mice, in which blunted circadian rhythm of Atf5 mRNA was observed in the liver, together with significantly higher expression levels in both liver and adrenal glands. Although we found no compelling evidence for rhythmic expression of TH in chromaffin cells being regulated by an intrinsic molecular clock mechanism, the Atf5 results raise the possibility that other aspects of chromaffin cell physiology, such as cell survival and cell differentiation, may well be intrinsically regulated.

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