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Disruption of circadian clock timekeeping due to changes in the photoperiod enhances the risk of lipid metabolism disorders and metabolic syndrome. However, the effects of altered photoperiods on the circadian clock and lipid metabolism are not well understood. To explore the effects of altered photoperiods, we developed a rat model where rats were exposed to either short-day or long-day conditions. Our findings demonstrated that altered photoperiods mediated circadian clocks by partly disrupting rhythmicity and shifting phase values of clock genes. We also showed that compared to long-day conditions, rats under short-day conditions exhibited more photoperiodic changes in a variety of physiological outputs related to lipid metabolism, such as significant increases in serum triglyceride (TG), high-density lipoprotein, and leptin levels, as well as increased body weight, fat:weight ratio, and hepatic TG levels. These increments were gained possibly through upregulated expression of forkhead box O1 (FoxO1), which partly mediates the expression of peroxisome proliferator-activated receptorα (PPARα) to increase the expression of phosphoenolpyruvate carboxykinase (PEPCK), peroxisome proliferator-activated receptor-g coactivator-1β (PGC1β), and fatty acid synthase (Fasn). In addition, the oscillation rhythms of FoxO1, PEPCK, PGC1β, and Fasn expression levels in the livers of rats exposed to a short-day photoperiod were more robust than those exposed to a long-day photoperiod. These findings suggest that a change in photoperiod can partly disrupt the circadian rhythmcity of clock genes, impair lipid metabolism, and promote obesity.

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In mammals, the approximate 24-h rhythm in behavior and physiology is generally existed, such as wake/ sleep cycle, blood pressure, body temperature and concentration of melatonin. The circadian rhythms result from the cell-autonomous and self-sustained oscillators (circadian clocks), which rely on interlocking transcription/translational feedback loops involving a series of clock genes (Clock, Bmal1, Per, Cry, Dec etc.) and their proteins (1). The mammalian circadian clocks are composed of a master clock located in the hypothalamic suprachiasmatic nucleus (SCN), and of many peripheral clocks in tissues and extra-SCN brain regions (2). The master clock is mainly entrained by the light cue, while the peripheral clocks are not only affected by the SCN clock but also entrained by the food cue (3). Moreover, feeding could take over part of the SCN signaling, and affect internal synchrony between the master clock and the peripheral clocks. Nutrient Components and Circadian RhythmsNutrient components have been identified as important factors to entrain the peripheral circadian clocks (4).Glucose is a particularly potent entraining factor for peripheral clocks (5). Min-Dian Li et al. found that high concentration of glucose increased the transcription levels of Bmal1 and Cry1 genes without the alteration of the phase of Bmal1 cycling, and low concentration of glucose delayed the phase of BMAL1 protein accumulation via western blot (6). How could glucose influence the circadian clock and further affect the following metabolic process aroused extensive attention. Some researchers revealed that cellular nutrient sensors such as nuclear receptors were proposed as candidates for the circadian clock entrainment by nutrient components (7,8). A recent study showed that the O-GlcNAc signaling entrained the circadian clock by inhibiting BMAL1/ CLOCK ubiquitination, which may be the molecular mechanism underlying the glucose entrainment of the circadian clock (6).Intraperitoneal injection of amino acids combined with glucose delayed the phase of the liver clock as similar as the effect of delayed feeding (9). l-carnosine is a dipeptide of the amino acids b-alanine and l-histidine, which is identified to be related with various physiological alterations (including blood glucose, blood pressure etc.) through the autonomic nerves. Bilateral lesions of the SCN in rat results in destructive effects on the carnosine induced physiological alterations mentioned above (10). In our study, we found that l-carnosine administration could accelerate the resetting rate of peripheral clock genes in the hearts, which is regulated by the autonomic nervous system (11).Alcohol is another entraining factor for circadian clocks. Chronic alcohol administration induced the hepatic steatosis and disturbed the circadian clocks in the liver (12). Studies in human indicated that the expression level of clock genes in leukocytes of male alcoholic patients were lower than the healthy men (13). Timed Nutrients and Circadian RhythmsTimed nutrients are as impor...