Frequent transmeridian flights or predominant work at night can increase cancer risk. Altered circadian rhythms also predict for poor survival in cancer patients, whereas physical destruction of the suprachiasmatic nuclei (SCN), the hypothalamic circadian pacemaker, accelerates tumor growth in mice. Here we tested the effect of functional disruption of circadian system on tumor progression in a novel experimental model of chronic jet lag. B6D2F₁ mice were synchronized with 12 hours of light and 12 hours of darkness or underwent repeat 8-hour advances of the light/dark cycle every 2 days before inoculation of Glasgow osteosarcoma. The 24-hour changes were assessed for plasma corticosterone, clock protein mPER1 expression in the SCN, and mRNA expression of clock genes mPer2 and mRev-erbα in liver and tumor. Time series were analyzed by spectral analysis and/or Cosinor. Differences were compared with analysis of variance (ANOVA). The 24-hour rest/activity cycle was ablated, and the rhythms of body temperature, serum corticosterone, and mPER1 protein expression in the SCN were markedly altered in jet-lagged mice as compared with controls (ANOVA, P < 0.001 for corticosterone and P = 0.01 for mPER1). Tumor grew faster in the jet-lagged animals as compared with controls (ANOVA, P < 0.001), whereas exposure to constant light or darkness had no effect (ANOVA, P = 0.66 and P = 0.8, respectively). The expression of mPer2 and mRev-erbα mRNAs in controls showed significant circadian rhythms in the liver (P = 0.006 and P = 0.003, respectively, Cosinor) and in the tumor (P = 0.04 and P < 0.001). Both rhythms were suppressed in the liver (P = 0.2 and P = 0.1, respectively, Cosinor) and in the tumor (P = 0.5) of jet-lagged mice. Altered environmental conditions can disrupt circadian clock molecular coordination in peripheral organs including tumors and play a significant role in malignant progression.

Recent evidence has emerged that peroxisome proliferator-activated receptor α (PPARα), which is largely involved in lipid metabolism, can play an important role in connecting circadian biology and metabolism. In the present study, we investigated the mechanisms by which PPARα influences the pacemakers acting in the central clock located in the suprachiasmatic nucleus and in the peripheral oscillator of the liver. We demonstrate that PPARα plays a specific role in the peripheral circadian control because it is required to maintain the circadian rhythm of the master clock gene brain and muscle Arnt-like protein 1 (bmal1) in vivo. This regulation occurs via a direct binding of PPARα on a potential PPARα response element located in the bmal1 promoter. Reversely, BMAL1 is an upstream regulator of PPARα gene expression. We further demonstrate that fenofibrate induces circadian rhythm of clock gene expression in cell culture and up-regulates hepatic bmal1 in vivo. Together, these results provide evidence for an additional regulatory feedback loop involving BMAL1 and PPARα in peripheral clocks.

Most living organisms show circadian (approximately 24 h) rhythms in physiology and behavior. These oscillations are generated by endogenous circadian clocks, present in virtually all cells where they control key biological processes. Although circadian gating of mitosis has been reported for many years in some peripheral tissues, the underlying molecular mechanisms have remained poorly understood. Here we show that the cell cycle inhibitor p21WAF1/CIP1 is rhythmically expressed in mouse peripheral organs. This rhythmic pattern of mRNA and protein expression was recapitulated in vitro in serum-shocked differentiated skeletal muscle cells. p21WAF1/CIP1 circadian expression is dramatically increased and no longer rhythmic in clock-deficient Bmal1-/- knock-out mice. Biochemical and genetic data show that oscillation of p21WAF1/CIP1 gene transcription is regulated by the antagonistic activities of the orphan nuclear receptors REV-ERBalpha/beta and RORalpha4/gamma, which are core clock regulators. Importantly, p21WAF1/CIP1 overexpressing Bmal1-/- primary hepatocytes exhibit a decreased proliferation rate. This phenotype could be reversed using small interfering RNA-mediated knockdown of p21WAF1/CIP1. These data establish a novel molecular link between clock and cell cycle genes and suggest that the G1 progression phase is a target of the circadian clock during liver cell proliferation.

Abnormalities contributing to the pathogenesis of non-insulin-dependent diabetes mellitus include impaired beta cell function, peripheral insulin resistance, and increased hepatic glucose production. Glucocorticoids are diabetogenic hormones because they decrease glucose uptake and increase hepatic glucose production. In addition, they may directly inhibit insulin release. To evaluate that possible role of glucocorticoids in beta cell function independent of their other effects, transgenic mice with an increased glucocorticoid sensitivity restricted to their beta cells were generated by overexpressing the glucocorticoid receptor (GR) under the control of the insulin promoter. Intravenous glucose tolerance tests showed that the GR transgenic mice had normal fasting and postabsorptive blood glucose levels but exhibited a reduced glucose tolerance compared with their control littermates. Measurement of plasma insulin levels 5 min after intravenous glucose load demonstrated a dramatic decrease in acute insulin response in the GR transgenic mice. These results show that glucocorticoids directly inhibit insulin release in vivo and identify the pancreatic beta cell as an important target for the diabetogenic action of glucocorticoids.

Mutations of clock genes can lead to diabetes and obesity. REV‐ERBα, a nuclear receptor involved in the circadian clockwork, has been shown to control lipid metabolism. To gain insight into the role of REV‐ERBα in energy homeostasis in vivo , we explored daily metabolism of carbohydrates and lipids in chow‐fed, unfed, or high‐fat‐fed Rev‐erb α ^−/− mice and their wild‐type littermates. Chow‐fed Rev‐erb α ^−/− mice displayed increased adiposity (2.5‐fold) and mild hyperglycemia (∼10%) without insulin resistance. Indirect calorimetry indicates that chow‐fed Rev‐erb α ^−/− mice utilize more fatty acids during daytime. A 24‐h nonfeeding period in Rev‐erb α ^−/− animals favors further fatty acid mobilization at the expense of glycogen utilization and gluconeogenesis, without triggering hypoglycemia and hypothermia. High‐fat feeding in Rev‐erb α ^−/− mice amplified metabolic disturbances, including expression of lipogenic factors. Lipoprotein lipase (Lpl) gene, critical in lipid utilization/storage, is triggered in liver at night and constitutively up‐regulated (∼ 2‐fold) in muscle and adipose tissue of Rev‐erb α ^−/− mice. We show that CLOCK, up‐regulated (2‐fold) at night in Rev‐erb α ^−/− mice, can transactivate Lpl . Thus, overexpression of Lpl facilitates muscle fatty acid utilization and contributes to fat overload. This study demonstrates the importance of clock‐driven Lpl expression in energy balance and highlights circadian disruption as a potential cause for the metabolic syndrome.—Delezie, J., Dumont, S., Dardente, H., Oudart, H., Gréchez‐Cassiau, A., Klosen, P., Teboul, M., Delaunay, F., Pévet, P., Challet, E. The nuclear receptor REV‐ERBα is required for the daily balance of carbohydrate and lipid metabolism. FASEB J. 26, 3321–3335 (2012). www.fasebj.org