The hepatic circadian clock regulates the choline kinase α gene through the BMAL1-REV-ERBα axis

The circadian timing system adapts most of the mammalian physiology and behaviour to the 24 h light/dark cycle. This temporal coordination relies on endogenous circadian clocks present in virtually all tissues and organs and implicated in the regulation of key cellular processes including metabolism, transport and secretion. Environmental or genetic disruption of the circadian coordination causes metabolic imbalance leading for instance to fatty liver, dyslipidaemia and obesity, thereby contributing to the development of a metabolic syndrome state. In the liver, a key metabolic organ, the rhythmic regulation of lipid biosynthesis is known, yet the molecular mechanisms through which the circadian clock controls lipogenesis, in particular, that of phospholipids, is poorly characterised. In this study, we show that the wild-type mice display a rhythmic accumulation of hepatic phosphatidylcholine with a peak at ZT 22-0 while clock-deficient Bmal1(-/-) mice show elevated phosphatidylcholine levels in the liver associated with an atherogenic lipoprotein profile. Profiling of the mRNA expression of enzymes from the Kennedy and phosphatidylethanolamine N-methyltransferase pathways which control the production of hepatic phosphatidylcholine revealed a robust circadian pattern for Chk alpha while other mRNA showed low amplitude (Chk beta and Pemt) or no rhythm (Cct alpha and Chpt1). Chk alpha mRNA expression was increased and no longer rhythmic in the liver from clock-deficient Bmal1(-/-) mice. This change resulted in the upregulation of the CHK alpha protein in these animals. We further show that the robust circadian expression of Chk alpha is restricted to the liver and adrenal glands. Analysis of the Chk alpha gene promoter revealed the presence of a conserved response element for the core clock transcription factors REV-ERB and ROR. Consistent with the antiphasic phase relationship between Chk alpha and Rev-erb alpha expression, in cotransfection experiments using HepG2 cells we show that ROR alpha 4-dependent transactivation of this element is repressed by REV-ERB alpha Correspondingly, Rev-erb alpha(-/-) mice displayed higher Chk alpha mRNA levels in liver at ZT 12. Collectively, these data establish that hepatic phosphatidylcholine is regulated by the circadian clock through a Bmal1-Rev-erb alpha-Chk alpha axis and suggest that an intact circadian timing system is important for the temporal coordination of phospholipid metabolism.