A link between circadian rhythm and metabolism has long been discussed. Circadian rhythm is controlled by positive and negative transcriptional and translational feedback loops composed of several clock genes. Among clock genes, the brain and muscle Arnt-like protein-1 (BMAL1) and circadian locomotor output cycles kaput (CLOCK) play important roles in the regulation of the positive rhythmic transcription. In addition to control of circadian rhythm, we have previously shown that BMAL1 regulates adipogenesis. In metabolic syndrome patients, the function of BMAL1 is dysregulated in visceral adipose tissue. In addition, analysis of SNPs has revealed that BMAL1 is associated with susceptibility to hypertension and type II diabetes. Furthermore, the significant roles of BMAL1 in pancreatic β cells proliferation and maturation were recently reported. These results suggest that BMAL1 regulates energy homeostasis. Therefore, in this study, we examined whether loss of BMAL1 function is capable of inducing metabolic syndrome. Deficient of the Bmal1 gene in mice resulted in elevation of the respiratory quotient value, indicating that BMAL1 is involved in the utilization of fat as an energy source. Indeed, lack of Bmal1 reduced the capacity of fat storage in adipose tissue, resulting in an increase in the levels of circulating fatty acids, including triglycerides, free fatty acids, and cholesterol. Elevation of the circulating fatty acids level induced the formation of ectopic fat in the liver and skeletal muscle in Bmal1 -/- mice. Interestingly, ectopic fat formation was not observed in tissue-specific (liver or skeletal muscle) Bmal1 -/- mice even under high fat diet feeding condition. Therefore, we were led to conclude that BMAL1 is a crucial factor in the regulation of energy homeostasis, and disorders of the functions of BMAL1 lead to the development of metabolic syndrome.

Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor regulating the expression of genes involved in xenobiotic response. Recent studies have suggested that AhR plays essential roles not only in xenobiotic detoxification but also energy metabolism. Thus, in this study, we studied the roles of AhR in lipid metabolism. Under high fat diet (HFD) challenge, liver-specific AhR knock-out (AhR LKO) mice exhibited severe steatosis, inflammation, and injury in the liver. Gene expression analysis and biochemical study revealed that de novo lipogenesis activity was significantly increased in AhR LKO mice. In contrast, induction of suppressor of cytokine signal 3 (Socs3) expression by HFD was attenuated in the livers of AhR LKO mice. Rescue of the Socs3 gene in the liver of AhR LKO mice cancelled the HFD-induced hepatic lipotoxicities. Promoter analysis established Socs3 as novel transcriptional target of AhR. These results indicated that AhR plays a protective role against HFD-induced hepatic steatosis and the subsequent lipotoxicity effects, such as inflammation, and that the mechanism of protection involves the direct transcriptional regulation of Socs3 expression by AhR.

The circadian clock network is an evolutionarily conserved system involved in the regulation of metabolic homeostasis; however, its impacts on skeletal metabolism remain largely unknown. We herein demonstrated that the circadian clock network in the intestines plays pivotal roles in skeletal metabolism such that the lack of the Bmal1 gene in the intestines ( Bmal1 _Int ^–/– mice) caused bone loss, with bone resorption being activated and bone formation suppressed. Mechanistically, Clock protein interaction with the vitamin D receptor (VDR) accelerated its binding to the VDR response element by enhancing histone acetylation in a circadian-dependent manner, and this was lost in Bmal1 _Int ^–/– mice because nuclear translocation of Clock required the presence of Bmal1. Accordingly, the rhythmic expression of VDR target genes involved in transcellular calcium (Ca) absorption was created, and this was not observed in Bmal1 _Int ^–/– mice. As a result, transcellular Ca absorption was impaired and bone resorption was activated in Bmal1 _Int ^–/– mice. Additionally, sympathetic tone, the activation of which suppresses bone formation, was elevated through afferent vagal nerves in Bmal1 _Int ^–/– mice, the blockade of which partially recovered bone loss by increasing bone formation and suppressing bone resorption in Bmal1 _Int ^–/– mice. These results demonstrate that the intestinal circadian system regulates skeletal bone homeostasis.

Brain and muscle arnt-like protein 1 (BMAL1), is a transcription factor known to regulate circadian rhythm. BMAL1 was originally characterized by its high expression in the skeletal muscle. Since the skeletal muscle is the dominant organ system in energy metabolism, the possible functions of BMAL1 in the skeletal muscle include the control of metabolism. Here, we established that its involvement in the regulation of oxidative capacity in the skeletal muscle. Muscle-specific Bmal1 KO mice (MKO mice) displayed several physiological hallmarks for the increase of oxidative capacity. This included increased energy expenditure and oxygen consumption, high running endurance and resistance to obesity with improved metabolic profiles. Also, the phosphorylation status of AMP-activated protein kinase and its downstream signaling substrate acetyl-CoA carboxylase in the MKO mice were substantially higher than those in the Bmal1^flox/flox mice. In addition, biochemical and histological studies confirmed the substantial activation of oxidative fibers in the skeletal muscle of the MKO mice. The mechanism includes the regulation of Cacna1s expression, followed by the activation of calcium—nuclear factor of activated T cells (NFAT) axis. We thus conclude that BMAL1 is a critical regulator of the muscular fatty acid level under nutrition overloading and that the mechanism involves the control of oxidative capacity.

Adiponectin is a cytokine secreted from adipocytes and regulates metabolism. Although serum adiponectin levels show diurnal variations, it is not clear if the effects of adiponectin are time-dependent. Therefore, this study conducted locomotor activity analyses and various metabolic studies using the adiponectin knockout (APN (−/−)) and the APN (+/+) mice to understand whether adiponectin regulates the circadian rhythm of glucose and lipid metabolism. We observed that the adiponectin gene deficiency does not affect the rhythmicity of core circadian clock genes expression in several peripheral tissues. In contrast, the adiponectin gene deficiency alters the circadian rhythms of liver and serum lipid levels and results in the loss of the time dependency of very-low-density lipoprotein-triglyceride secretion from the liver. In addition, the whole-body glucose tolerance of the APN (−/−) mice was normal at CT10 but reduced at CT22, compared to the APN (+/+) mice. The decreased glucose tolerance at CT22 was associated with insulin hyposecretion in vivo. In contrast, the gluconeogenesis activity was higher in the APN (−/−) mice than in the APN (+/+) mice throughout the day. These results indicate that adiponectin regulates part of the circadian rhythm of metabolism in the liver.