Liver clock protein BMAL1 promotes de novo lipogenesis through insulin-mTORC2-AKT signaling

MicroRNA-10a Impairs Liver Metabolism in Hepatitis C Virus-Related Cirrhosis Through Deregulation of the Circadian Clock Gene Brain and Muscle Aryl Hydrocarbon Receptor Nuclear Translocator-Like 1

The circadian rhythm of the liver plays an important role in maintaining its metabolic homeostasis. We performed comprehensive expression analysis of microRNAs (miRNAs) using TaqMan polymerase chain reaction of liver biopsy tissues to identify the miRNAs that are significantly up‐regulated in advanced chronic hepatitis C (CHC). We found miR‐10a regulated various liver metabolism genes and was markedly up‐regulated by hepatitis C virus infection and poor nutritional conditions. The expression of miR‐10a was rhythmic and down‐regulated the expression of the circadian rhythm gene brain and muscle aryl hydrocarbon receptor nuclear translocator‐like 1 (Bmal1) by directly suppressing the expression of RA receptor‐related orphan receptor alpha (RORA). Overexpression of miR‐10a in hepatocytes blunted circadian rhythm of Bmal1 and inhibited the expression of lipid synthesis genes (sterol regulatory element binding protein [SREBP]1, fatty acid synthase [FASN], and SREBP2), gluconeogenesis (peroxisome proliferator‐activated receptor gamma coactivator 1 alpha [PGC1α]), protein synthesis (mammalian target of rapamycin [mTOR] and ribosomal protein S6 kinase [S6K]) and bile acid synthesis (liver receptor homolog 1 [LRH1]). The expression of Bmal1 was significantly correlated with the expression of mitochondrial biogenesis‐related genes and reduced Bmal1 was associated with increased serum alanine aminotransferase levels and progression of liver fibrosis in CHC. Thus, impaired circadian rhythm expression of Bmal1 by miR‐10a disturbs metabolic adaptations, leading to liver damage, and is closely associated with the exacerbation of abnormal liver metabolism in patients with advanced CHC. In patients with hepatitis C‐related liver cirrhosis, liver tissue miR‐10a levels were significantly associated with hepatic reserve, fibrosis markers, esophageal varix complications, and hepatitis C‐related hepatocellular carcinoma recurrence. Conclusion: MiRNA‐10a is involved in abnormal liver metabolism in cirrhotic liver through down‐regulation of the expression of the circadian rhythm gene Bmal1. Therefore, miR‐10a is a possible useful biomarker for estimating the prognosis of liver cirrhosis.

Old and New Roles and Evolving Complexities of Cardiovascular Clocks

The cardiovascular (CV) system has been established to be significantly influenced by the molecular components of circadian rhythm. Oscillations of circadian rhythm occur within the circulation to affect thrombosis and blood pressure and within CV tissues including arteries, heart, and kidney to control function. Physiologic and molecular oscillations of circadian rhythm have been well connected via global, tissue-specific, and transgenic reporter mouse models of key core clock signals such as Bmal1, Period, and Clock, which can produce both pathology and protection with their mutation. With different nuances of CV clock action continuing to emerge in studies of the cardiovascular system, new questions are raised in both new and old mouse model system observations that underscore the importance, complexity, and continued study of the circadian clock mechanism in cardiovascular disease.

Diurnal Profiles of N-Acylethanolamines in Goldfish Brain and Gastrointestinal Tract: Possible Role of Feeding

N-acylethanolamines (NAEs) are a family of endogenous lipid signaling molecules that are involved in regulation of energy homeostasis in vertebrates with a putative role on circadian system. The aim of this work was to study the existence of daily fluctuations in components of NAEs system and their possible dependence on food intake. Specifically, we analyzed the content of oleoylethanolamide (OEA), palmitoylethanolamide (PEA), stearoylethanolamide (SEA), their precursors (NAPEs), as well as the expression of nape-pld (NAEs synthesis enzyme), faah (NAEs degradation enzyme), and pparα (NAEs receptor) in gastrointestinal and brain tissues of goldfish (Carassius auratus) throughout a 24-h cycle. Daily profiles of bmal1a and rev-erbα expression in gastrointestinal tissues were also quantified because these clock genes are also involved in lipid metabolism, are PPAR-targets in mammals, and could be a link between NAEs and circadian system in fish. Gastrointestinal levels of NAEs exhibited daily fluctuations, with a pronounced and rapid postprandial increase, the increment being likely caused by food intake as it is not present in fasted animals. Such periprandial differences were not found in brain, supporting that NAEs mobilization occurs in a tissue-specific manner and suggesting that these three NAEs could be acting as peripheral satiety signals. The abundance of pparα mRNA displayed a daily rhythm in the intestine and the liver, suggesting a possible rhythmicity in the NAEs functionality. The increment of pparα expression during the rest phase can be related with its role stimulating lipid catabolism to obtain energy during the fasting state of the animals. In addition, the clock genes bmal1a and rev-erbα also showed daily rhythms, with a bmal1a increment after feeding, supporting its role as a lipogenic factor. In summary, our data show the existence of all components of NAEs system in fish (OEA, PEA, SEA, precursors, synthesis and degradation enzymes, and the receptor PPARα), supporting the involvement of NAEs as peripheral satiety signals.

The Contribution of the Circadian Gene Bmal1 to Female Fertility and the Generation of the Preovulatory Luteinizing Hormone Surge

In rodents, the preovulatory LH surge is temporally gated, but the timing cue is unknown. Estrogen primes neurons in the anteroventral periventricular nucleus (AVPV) to secrete kisspeptin, which potently activates GnRH neurons to release GnRH, eliciting a surge of LH to induce ovulation. Deletion of the circadian clock gene Bmal1 results in infertility. Previous studies have found that Bmal1 knockout (KO) females do not display an LH surge at any time of day. We sought to determine whether neuroendocrine disruption contributes to the absence of the LH surge. Because Kiss1 expression in the AVPV is critical for regulating ovulation, we hypothesized that this population is disrupted in Bmal1 KO females. However, we found an appropriate rise in AVPV Kiss1 and Fos mRNA at the time of lights out in ovariectomized estrogen-treated animals, despite the absence of a measureable increase in LH. Furthermore, Bmal1 KO females have significantly increased LH response to kiss-10 administration, although the LH response to GnRH was unchanged. We then created Kiss1- and GnRH-specific Bmal1 KO mice to examine whether Bmal1 expression is necessary within either kisspeptin or GnRH neurons. We detected no significant differences in any measured reproductive parameter. Our results indicate that disruption of the hypothalamic regulation of fertility in the Bmal1 KO females is not dependent on endogenous clocks within either the GnRH or kisspeptin neurons.

Coupling the Circadian Clock to Homeostasis: The Role of Period in Timing Physiology

A plethora of physiological processes show stable and synchronized daily oscillations that are either driven or modulated by biological clocks. A circadian pacemaker located in the suprachiasmatic nucleus of the ventral hypothalamus coordinates 24-hour oscillations of central and peripheral physiology with the environment. The circadian clockwork involved in driving rhythmic physiology is composed of various clock genes that are interlocked via a complex feedback loop to generate precise yet plastic oscillations of ∼24 hours. This review focuses on the specific role of the core clockwork gene Period1 and its paralogs on intra-oscillator and extra-oscillator functions, including, but not limited to, hippocampus-dependent processes, cardiovascular function, appetite control, as well as glucose and lipid homeostasis. Alterations in Period gene function have been implicated in a wide range of physical and mental disorders. At the same time, a variety of conditions including metabolic disorders also impact clock gene expression, resulting in circadian disruptions, which in turn often exacerbates the disease state.

The hepatic BMAL1/AKT/lipogenesis axis protects against alcoholic liver disease in mice via promoting PPARα pathway

Alcohol liver disease (ALD) is one of the major chronic liver diseases worldwide, ranging from fatty liver, alcoholic hepatitis, cirrhosis, and potentially, hepatocellular carcinoma. Epidemiological studies suggest a potential link between ALD and impaired circadian rhythms, but the role of hepatic circadian proteins in the pathogenesis of ALD remains unknown. Here we show that the circadian clock protein BMAL1 in hepatocytes is both necessary and sufficient to protect mice from ALD. Ethanol diet-fed mice with liver-specific knockout (Bmal1-LKO) or depletion of Bmal1 develop more severe liver steatosis and injury as well as a simultaneous suppression of both de novo lipogenesis and fatty acid oxidation, which can be rescued by the supplementation of synthetic PPARα ligands. Restoring de novo lipogenesis in the liver of Bmal1-LKO mice by constitutively active AKT not only elevates hepatic fatty acid oxidation but also alleviates ethanol-induced fatty liver and liver injury. Furthermore, hepatic over-expression of lipogenic transcription factor ChREBP, but not SREBP-1c, in the liver of Bmal1-LKO mice also increases fatty acid oxidation and partially reduces ethanol-induced fatty liver and liver injury. Conclusion: we identified a protective role of BMAL1 in hepatocytes against ALD. The protective action of BMAL1 during alcohol consumption depends on its ability to couple ChREBP-induced de novo lipogenesis with PPARα-mediated fatty oxidation. (Hepatology 2018).

Nighttime light exposure enhances Rev-erbα-targeting microRNAs and contributes to hepatic steatosis

OBJECTIVE

The exposure to artificial light at night (ALAN) disrupts the biological rhythms and has been associated with the development of metabolic syndrome. MicroRNAs (miRNAs) display a critical role in fine-tuning the circadian system and energy metabolism. In this study, we aimed to assess whether altered miRNAs expression in the liver underlies metabolic disorders caused by disrupted biological rhythms.

RESULTS

We found that C3H/HePas mice exposed to ALAN developed obesity, and hepatic steatosis, which was paralleled by decreased expression of Rev-erbα and up-regulation of its lipogenic targets ACL and FAS in liver. Furthermore, the expression of Rev-erbα-targeting miRNAs, miR-140-5p, 185-5p, 326-5p and 328-5p were increased in this group. Consistently, overexpression of these miRNAs in primary hepatocytes reduced Rev-erbα expression at the mRNA and protein levels. Importantly, overexpression of Rev-erbα-targeting miRNAs increased mRNA levels of Acly and Fasn.

CONCLUSION

Thus, altered miRNAs profile is an important mechanism underlying the disruption of the peripheral clock caused by exposure to ALAN, which could lead to hepatic steatosis.

Caloric restriction effects on liver mTOR signaling are time-of-day dependent

The regulation of mechanistic target of rapamycin (mTOR) signaling contributes to the metabolic effects of a calorie restriction (CR) diet. We assayed the effect of CR on the activity of mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) in the liver of mice at six different times across the day. CR effects on mTORC1 and mTORC2 activities were time-of-day dependent. CR induced mTORC1 activity at one time, reduced at two times and has no effect during other times. CR induced mTORC2 activity at one time of the day and has no effects at other times. Circadian clocks are implemented in the regulation of mTOR signaling in mammals and mechanisms of CR. We assayed the effect of CR on mTOR signaling in the liver of mice deficient for circadian transcriptional regulators BMAL1 and CRYs. The CR induced suppression of mTORC1 activity was observed in both clock mutants, while up regulation of mTORC2 was observed in the liver of CRY deficient but not in the liver of BMAL1 deficient mice. Our finding revealed that CR has different time dependent effect on the activity of mTOR complexes 1 and 2 and suggest that circadian clock protein BMAL1 is involved in the up regulation of mTORC2 upon CR in mammals.

Circadian rhythms in mitochondrial respiration

Many physiological processes are regulated with a 24-h periodicity to anticipate the environmental changes of daytime to nighttime and vice versa. These 24-h regulations, commonly termed circadian rhythms, among others control the sleep-wake cycle, locomotor activity and preparation for food availability during the active phase (daytime for humans and nighttime for nocturnal animals). Disturbing circadian rhythms at the organ or whole-body level by social jetlag or shift work, increases the risk to develop chronic metabolic diseases such as type 2 diabetes mellitus. The molecular basis of this risk is a topic of increasing interest. Mitochondria are essential organelles that produce the majority of energy in eukaryotes by converting lipids and carbohydrates into ATP through oxidative phosphorylation. To adapt to the ever-changing environment, mitochondria are highly dynamic in form and function and a loss of this flexibility is linked to metabolic diseases. Interestingly, recent studies have indicated that changes in mitochondrial morphology (i.e., fusion and fission) as well as generation of new mitochondria are dependent on a viable circadian clock. In addition, fission and fusion processes display diurnal changes that are aligned to the light/darkness cycle. Besides morphological changes, mitochondrial respiration also displays diurnal changes. Disturbing the molecular clock in animal models leads to abrogated mitochondrial rhythmicity and altered respiration. Moreover, mitochondrial-dependent production of reactive oxygen species, which plays a role in cellular signaling, has also been linked to the circadian clock. In this review, we will summarize recent advances in the study of circadian rhythms of mitochondria and how this is linked to the molecular circadian clock.

Daily cycle in hepatic lipid metabolism in obese mice, Neotomodon alstoni: Sex differences

Disruption of circadian rhythms influences the pathogenesis of obesity, particularly with the basic regulation of food intake and metabolism. A link between metabolism and the circadian clock is the peroxisome proliferator-activated receptors (PPARs). The Neotomodon alstoni mouse, known as the "Mexican volcano mouse," may develop obesity if fed a normo-caloric diet. This manuscript documents the changes in part of the hepatic lipid homeostasis in both sexes of lean and obese N. alstoni mice, comparing the daily changes in the BMAL1 clock protein, in regulators of lipid metabolism (PGC-1α, PPARα-γ, SREBP-1c, and CPT-1α) and in free fatty acid (FFA) and hepatic triacylglyceride (TAG) metabolites in light-dark cycles. Hepatic tissue and blood were collected at 5, 10, 15, 19, and 24 h. Samples were analyzed by western blotting to determine the relative presence of protein. The results indicate that obesity affects daily changes in lipid metabolism and the BMAL1 profile in females considerably more than in males. These results suggest that the impact of obesity on lipid metabolism has important differences according to sex.

An individual 12-h shift of the light-dark cycle alters the pancreatic and duodenal circadian rhythm and digestive function

In mammals, behavioral and physiological rhythms are controlled by circadian clocks which are entrained by environmental light and food signals. However, how the environmental cues affect digestive tract's circadian clock remains poorly understood. Therefore, in order to elucidate the effect of light cue on the resetting of the peripheral clocks, we investigated the expressions of clock genes (Bmal1, Cry1, Rev-erbα, Per1, and Per2) and digestive function genes (Cck, Cck-1r, Sct, Sctr, and Ctrb1) in the pancreas and duodenum of rats after the light-dark (LD) cycle reversal for 7 days. We found that both the clock genes and digestive function genes exhibited a clear and similar daily rhythmicity in the pancreas and duodenum of rats. After reversal of the LD cycle for 7 days, the expressions of clock genes in pancreas, including Bmal1, Cry1, and Rev-erbα were affected; whereas the expression of Per1 gene failed to fit the cosine wave. However, in the duodenum the shifted genes were Bmal1, Rev-erbα, and Per2; in parallel, the Per1 gene expression also lost its circadian rhythm by reversal of the LD cycle. Therefore, the acrophases of the clock genes were shifted in a tissue- and gene-specific manner. Furthermore, the profiles of the digestive function genes, including Sctr and Ctrb1, were also affected by changes in LD cycle. These observations suggest that the mechanisms underlying the pancreatic and duodenal clocks are distinct, and there may be a potential linkage between the circadian clock system and the digestive system.

Genetic disruption of the cardiomyocyte circadian clock differentially influences insulin-mediated processes in the heart

Cardiovascular physiology exhibits time-of-day-dependent oscillations, which are mediated by both extrinsic (e.g., environment/behavior) and intrinsic (e.g., circadian clock) factors. Disruption of circadian rhythms negatively affects multiple cardiometabolic parameters. Recent studies suggest that the cardiomyocyte circadian clock directly modulates responsiveness of the heart to metabolic stimuli (e.g., fatty acids) and stresses (e.g., ischemia/reperfusion). The aim of this study was to determine whether genetic disruption of the cardiomyocyte circadian clock impacts insulin-regulated pathways in the heart. Genetic disruption of the circadian clock in cardiomyocyte-specific Bmal1 knockout (CBK) and cardiomyocyte-specific Clock mutant (CCM) mice altered expression (gene and protein) of multiple insulin signaling components in the heart, including p85α and Akt. Both baseline and insulin-mediated Akt activation was augmented in CBK and CCM hearts (relative to littermate controls). However, insulin-mediated glucose utilization (both oxidative and non-oxidative) and AS160 phosphorylation were attenuated in CBK hearts, potentially secondary to decreased Inhibitor-1. Consistent with increased Akt activation in CBK hearts, mTOR signaling was persistently increased, which was associated with attenuation of autophagy, augmented rates of protein synthesis, and hypertrophy. Importantly, pharmacological inhibition of mTOR (rapamycin; 10days) normalized cardiac size in CBK mice. These data suggest that disruption of cardiomyocyte circadian clock differentially influences insulin-regulated processes, and provide new insights into potential pathologic mediators following circadian disruption.

PTEN Mediates Activation of Core Clock Protein BMAL1 and Accumulation of Epidermal Stem Cells

Tissue integrity requires constant maintenance of a quiescent, yet responsive, population of stem cells. In the skin, hair follicle stem cells (HFSCs) that reside within the bulge maintain tissue homeostasis in response to activating cues that occur with each new hair cycle or upon injury. We found that PTEN, a major regulator of the PI3K-AKT pathway, controlled HFSC number and size in the bulge and maintained genomically stable pluripotent cells. This regulatory function is central for HFSC quiescence, where PTEN-deficiency phenotype is in part regulated by BMAL1. Furthermore, PTEN ablation led to downregulation of BMI-1, a critical regulator of adult stem cell self-renewal, and elevated senescence, suggesting the presence of a protective system that prevents transformation. We found that short- and long-term PTEN depletion followed by activated BMAL1, a core clock protein, contributed to accumulation of HFSC.

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PTEN downregulation leads to the enrichment of stem cells in the niche

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PTEN activates core clock protein BMAL1

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BMAL1 plays a role in PTEN-associated stem cell accumulation via AKT

Hepatic gene therapy rescues high-fat diet responses in circadian Clock mutant mice

OBJECTIVE

Circadian Clock gene mutant mice show dampened 24-h feeding rhythms and an increased sensitivity to high-fat diet (HFD) feeding. Restricting HFD access to the dark phase counteracts its obesogenic effect in wild-type mice. The extent to which altered feeding rhythms are causative for the obesogenic phenotype of Clock mutant mice, however, remains unknown.

METHODS

Metabolic parameters of wild-type (WT) and ClockΔ19 mutant mice (MT) were investigated under ad libitum and nighttime restricted HFD feeding. Liver circadian clock function was partially rescued by hydrodynamic tail vein delivery of WT-Clock DNA vectors in mutant mice and transcriptional, metabolic, endocrine and behavioral rhythms studied.

RESULTS

Nighttime-restricted feeding restored food intake, but not body weight regulation in MT mice under HFD, suggesting Clock-dependent metabolic dysregulation downstream of circadian appetite control. Liver-directed Clock gene therapy partially restored liver circadian oscillator function and transcriptome regulation without affecting centrally controlled circadian behaviors. Under HFD, MT mice with partially restored liver clock function (MT-LR) showed normalized body weight gain, rescued 24-h food intake rhythms, and WT-like energy expenditure. This was associated with decreased nighttime leptin and daytime ghrelin levels, reduced hepatic lipid accumulation, and improved glucose tolerance. Transcriptome analysis revealed that hepatic Clock rescue in MT mice affected a range of metabolic pathways.

CONCLUSION

Liver Clock gene therapy improves resistance against HFD-induced metabolic impairments in mice with circadian clock disruption. Restoring or stabilizing liver clock function might be a promising target for therapeutic interventions in obesity and metabolic disorders.

Bmal1 interference impairs hormone synthesis and promotes apoptosis in porcine granulosa cells

In mammals, granulosa cell proliferation, differentiation, luteinization, apoptosis, and hormone synthesis are tightly related to oocyte maturation, follicular development and ovarian function. In current study, we investigated the role of the key circadian clock gene, brain and muscle arnt-like protein-1 (Bmal1), on porcine granulosa cell hormone secretion and apoptosis. The transcription levels of circadian clock genes, including Bmal1 and period circadian clock 2 (Per2), were detected by RT-qPCR. We found that the circadian clock genes exhibited rhythmic change and were further enhanced by dexamethasone synchronization in granulosa cells. Bmal1 knockdown reduced transcriptional levels of hormone receptor genes, including follicle stimulating hormone receptor (Fshr), luteinizing hormone/choriogonadotropin receptor (Lhcgr) and estrogen receptor 2 (Esr2), and decreased the mRNA and protein levels of cytochrome P450 family 11 subfamily A member 1 (Cyp11a1), cytochrome P450 family 19 subfamily A member 1 (Cyp19a1) and steroidogenic acute regulatory protein (Star), which are the key enzymes involved in hormone synthesis. Synthesis of progesterone and estradiol were also inhibited by Bmal1 siRNA treatment in granulosa cells. Moreover, flow cytometry analysis demonstrated suppressing Bmal1 promoted granulosa cells apoptosis. Western blot analysis showed that Bmal1 interference inactivated the PI3K/Akt/mTOR signaling pathway. In conclusion, Bmal1 plays a critical role in secretion of hormone and apoptosis of porcine granulosa cells via the PI3K/Akt/mTOR signaling pathway.

Glucose Homeostasis: Regulation by Peripheral Circadian Clocks in Rodents and Humans

Most organisms, including humans, have developed an intrinsic system of circadian oscillators, allowing the anticipation of events related to the rotation of Earth around its own axis. The mammalian circadian timing system orchestrates nearly all aspects of physiology and behavior. Together with systemic signals, emanating from the central clock that resides in the hypothalamus, peripheral oscillators orchestrate tissue-specific fluctuations in gene expression, protein synthesis, and posttranslational modifications, driving overt rhythms in physiology and behavior. There is increasing evidence on the essential roles of the peripheral oscillators, operative in metabolically active organs in the regulation of body glucose homeostasis. Here, we review some recent findings on the molecular and cellular makeup of the circadian timing system and its implications in the temporal coordination of metabolism in health and disease.

Circadian Influence on Metabolism and Inflammation in Atherosclerosis

Many aspects of human health and disease display daily rhythmicity. The brain's suprachiasmic nucleus, which interprets recurring external stimuli, and autonomous molecular networks in peripheral cells together, set our biological circadian clock. Disrupted or misaligned circadian rhythms promote multiple pathologies including chronic inflammatory and metabolic diseases such as atherosclerosis. Here, we discuss studies suggesting that circadian fluctuations in the vessel wall and in the circulation contribute to atherogenesis. Data from humans and mice indicate that an impaired molecular clock, disturbed sleep, and shifting light-dark patterns influence leukocyte and lipid supply in the circulation and alter cellular behavior in atherosclerotic lesions. We propose that a better understanding of both local and systemic circadian rhythms in atherosclerosis will enhance clinical management, treatment, and public health policy.

Brain and muscle aryl hydrocarbon receptor nuclear translocator-like protein-1 cooperates with glycogen synthase kinase-3β to regulate osteogenesis of bone-marrow mesenchymal stem cells in type 2 diabetes

Type 2 diabetes mellitus (T2DM) is associated with inhibited osteogenesis of bone marrow mesenchymal stem cells (BMSCs). Brain and muscle ARNT-like protein 1 (BMAL1) has been linked to the T2DM-related bone remodeling, however, the specific mechanism is still unclear. Herein, we aimed to determine the role of BMAL1 in T2DM-induced suppression of BMSCs osteogenesis. Inhibited osteogenesis and BMAL1 expression were showed in diabetic BMSCs. And while β-catenin and T cell factor (TCF) expression were decreased, the glycogen synthase kinase-3β (GSK-3β) and nemo-like kinase (NLK) expression were increased in diabetic BMSCs. Moreover, over-expression of BMAL1 led to recovered osteogenesis ability and activation of Wnt/β-catenin pathway, which was partially due to inhibition of GSK-3β caused by over-expression of BMAL1. Taken together, our findings provide new insights into the role of BMAL1 in T2DM-induced suppression of BMSCs osteogenesis. Over-expressed BMAL1 could recover BMSCs osteogenesis in T2DM partially by decreasing GSK-3β expression to activate Wnt/β-catenin pathway. BMAL1 may have a potential use in repairing diabetic bone metabolic disorders.

Phosphorylation Is a Central Mechanism for Circadian Control of Metabolism and Physiology

Circadian clocks are self-sustainable endogenous oscillators, present in virtually every cell, driving daily cycles of metabolism and physiology. The molecular mechanism of the circadian clock is based on interconnected transcriptional and translational feedback loops. While many studies have addressed circadian rhythms of the transcriptome and, to a lesser extent, the proteome, none have investigated the phosphoproteome. We apply mass spectrometry-based phosphoproteomics to obtain the first global in vivo quantification of circadian phosphorylation in mammals. Of more than 20,000 phosphosites, 25% significantly oscillate in the mouse liver, including novel sites on core clock proteins. The extent and amplitude of phosphorylation cycles far exceeds those observed in RNA and protein abundance. Our data indicate a dominant regulatory role for phosphorylation-dependent circadian tuning of signaling pathways. This allows the organism to integrate different signals and rapidly and economically respond to daily changes in nutrient availability and physiological states.

CREBH Couples Circadian Clock With Hepatic Lipid Metabolism

The circadian clock orchestrates diverse physiological processes critical for health and disease. CREB, hepatocyte specific (CREBH) is a liver-enriched, endoplasmic reticulum (ER)-tethered transcription factor known to regulate the hepatic acute phase response and energy homeostasis under stress conditions. We demonstrate that CREBH is regulated by the circadian clock and functions as a circadian regulator of hepatic lipid metabolism. Proteolytic activation of CREBH in the liver exhibits typical circadian rhythmicity controlled by the core clock oscillator BMAL1 and AKT/glycogen synthase kinase 3β (GSK3β) signaling pathway. GSK3β-mediated phosphorylation of CREBH modulates the association between CREBH and the coat protein complex II transport vesicle and thus controls the ER-to-Golgi transport and subsequent proteolytic cleavage of CREBH in a circadian manner. Functionally, CREBH regulates circadian expression of the key genes involved in triglyceride (TG) and fatty acid (FA) metabolism and is required to maintain circadian amplitudes of blood TG and FA in mice. During the circadian cycle, CREBH rhythmically regulates and interacts with the hepatic nuclear receptors peroxisome proliferator-activated receptor α and liver X receptor α as well as with the circadian oscillation activator DBP and the repressor E4BP4 to modulate CREBH transcriptional activities. In conclusion, these studies reveal that CREBH functions as a circadian-regulated liver transcriptional regulator that integrates energy metabolism with circadian rhythm.

Bmal1 induces osteoblast differentiation via regulation of BMP2 expression in MC3T3-E1 cells

AIMS

Mammalian circadian rhythms regulate many metabolic processes. Recent studies suggest that brain and muscle Arnt-like 1 (BMAL1), an important component of mammalian circadian rhythm, is associated with insulin signaling. Several studies have shown that insulin is associated with bone metabolism; however, the relationship between BMAL1 and osteoblasts remains unclear.

MAIN METHODS

Expression of osteogenic markers and Bmal1 in MC3T3-E1 cells was measured by RT-PCR and Western blotting. Alizarin red S staining was performed to assess matrix mineralization in MC3T3-E1 cells.

KEY FINDINGS

mRNA levels of osteogenic genes and Bmal1 were up-regulated in MC3T3-E1 cells upon insulin treatment. In addition, Bmal1 overexpression increased the expression of osteogenic genes including inhibitor of DNA binding (Id1), Runt-related transcription factor 2 (Runx2), and osteocalcin (OC). Interestingly, expression of Bone morphogenetic protein-2 (BMP2), an important upstream factor of Id1, Runx2, and OC, was markedly increased by Bmal1. Finally, we confirmed that insulin-induced BMP2 expression was attenuated in Bmal1 knockout (KO) cells. PCR analysis and alizarin red S staining showed that insulin-mediated increases gene expression and calcium deposition were reduced in Bmal1 KO cells compared to wild-type cells.

SIGNIFICANCE

Taken together, these results demonstrate that Bmal1 promotes osteoblast differentiation by regulating BMP2 expression in MC3T3-E1 cells.

CLOCK-BMAL1 regulate the cardiac L-type calcium channel subunit CACNA1C through PI3K-Akt signaling pathway

The heterodimerized transcription factors CLOCK-BMAL1 regulate the cardiomyocyte circadian rhythms. The L-type calcium currents play important role in the cardiac electrogenesis and arrhythmogenesis. Whether and how the CLOCK-BMAL1 regulate the cardiac L-type calcium channels are yet to be determined. The functions of the L-type calcium channels were evaluated with patch clamping techniques. Recombinant adenoviruses of CLOCK and BMAL1 were used in the expression experiments. We reported that the expressions and functions of CACNA1C (the α-subunit of the L-type calcium channels) showed circadian rhythms, with the peak at zeitgeber time 3 (ZT3). The endocardial action potential durations 90 (APD90) were correspondingly longer at ZT3. The protein levels of the phosphorylated Akt at threonine 308 (pAkt T308) also showed circadian rhythms. Overexpressions of CLOCK-BMAL1 significantly reduced the levels of CACNA1C while increasing the levels of pAkt T308 and pik3r1. Furthermore, the inhibitory effects of CLOCK-BMAL1 on CACNA1C could be abolished by the Akt inhibitor MK2206 or the PDK1 inhibitor GSK2334470. Collectively, our findings suggested that the expressions of the cardiac CACNA1C were under the CLOCK-BMAL1 regulation, probably through the PI3K-Akt signal pathway.

Insulin post-transcriptionally modulates Bmal1 protein to affect the hepatic circadian clock

Although food availability is a potent synchronizer of the peripheral circadian clock in mammals, the underlying mechanisms are unclear. Here, we show that hepatic Bmal1, a core transcription activator of the molecular clock, is post-transcriptionally regulated by signals from insulin, an important hormone that is temporally controlled by feeding. Insulin promotes postprandial Akt-mediated Ser42-phosphorylation of Bmal1 to induce its dissociation from DNA, interaction with 14-3-3 protein and subsequently nuclear exclusion, which results in the suppression of Bmal1 transcriptional activity. Inverted feeding cycles not only shift the phase of daily insulin oscillation, but also elevate the amplitude due to food overconsumption. This enhanced and reversed insulin signalling initiates the reset of clock gene rhythms by altering Bmal1 nuclear accumulation in mouse liver. These results reveal the molecular mechanism of insulin signalling in regulating peripheral circadian rhythms.

Genetic and Environmental Models of Circadian Disruption Link SRC-2 Function to Hepatic Pathology

Circadian rhythmicity is a fundamental process that synchronizes behavioral cues with metabolic homeostasis. Disruption of daily cycles due to jet lag or shift work results in severe physiological consequences including advanced aging, metabolic syndrome, and even cancer. Our understanding of the molecular clock, which is regulated by intricate positive feedforward and negative feedback loops, has expanded to include an important metabolic transcriptional coregulator, Steroid Receptor Coactivator-2 (SRC-2), that regulates both the central clock of the suprachiasmatic nucleus (SCN) and peripheral clocks including the liver. We hypothesized that an environmental uncoupling of the light-dark phases, termed chronic circadian disruption (CCD), would lead to pathology similar to the genetic circadian disruption observed with loss of SRC-2 We found that CCD and ablation of SRC-2 in mice led to a common comorbidity of metabolic syndrome also found in humans with circadian disruption, non-alcoholic fatty liver disease (NAFLD). The combination of SRC-2(-/-) and CCD results in a more robust phenotype that correlates with human non-alcoholic steatohepatitis (NASH) and hepatocellular carcinoma (HCC) gene signatures. Either CCD or SRC-2 ablation produces an advanced aging phenotype leading to increased mortality consistent with other circadian mutant mouse models. Collectively, our studies demonstrate that SRC-2 provides an essential link between the behavioral activities influenced by light cues and the metabolic homeostasis maintained by the liver.

E4BP4 is an insulin-induced stabilizer of nuclear SREBP-1c and promotes SREBP-1c-mediated lipogenesis

Upon food intake, insulin stimulates de novo lipogenesis (DNL) in hepatocytes via the AKT-mTORC1-sterol regulatory element-binding protein (SREBP)-1c pathway. How insulin maintains the maximal SREBP-1c activities during the entire feeding state remains elusive. We previously reported that insulin induced b-ZIP transcription factor, E4-binding protein 4 (E4BP4), in hepatocytes. In the current study, we show that insulin injection increases hepatic E4bp4 expression by activating the AKT-mTORC1-SREBP-1c pathway in hepatocytes. E4bp4-deficient hepatocytes not only fail to maintain robust DNL but also become resistant to SREBP-1c-induced lipogenesis. In vivo, acute depletion of E4bp4 in the liver by adenoviral shRNA reduces the expression of lipogenic enzymes and results in reduced levels of serum triglycerides and cholesterol during the postprandial phase. In hepatocytes, E4BP4 interacts with nuclear SREBP-1c to preserve its acetylation, and subsequently protects it from ubiquitination-dependent degradation. In conclusion, the current studies uncover a novel positive feedback pathway mediated by E4BP4 to augment SREBP-1c-mediated DNL in the liver during the fed state.

Transcriptional regulation of hepatic lipogenesis

Fatty acid and fat synthesis in the liver is a highly regulated metabolic pathway that is important for very low-density lipoprotein (VLDL) production and thus energy distribution to other tissues. Having common features at their promoter regions, lipogenic genes are coordinately regulated at the transcriptional level. Transcription factors, such as upstream stimulatory factors (USFs), sterol regulatory element-binding protein 1C (SREBP1C), liver X receptors (LXRs) and carbohydrate-responsive element-binding protein (ChREBP) have crucial roles in this process. Recently, insights have been gained into the signalling pathways that regulate these transcription factors. After feeding, high blood glucose and insulin levels activate lipogenic genes through several pathways, including the DNA-dependent protein kinase (DNA-PK), atypical protein kinase C (aPKC) and AKT-mTOR pathways. These pathways control the post-translational modifications of transcription factors and co-regulators, such as phosphorylation, acetylation or ubiquitylation, that affect their function, stability and/or localization. Dysregulation of lipogenesis can contribute to hepatosteatosis, which is associated with obesity and insulin resistance.

Altered cellular redox status, sirtuin abundance and clock gene expression in a mouse model of developmentally primed NASH

Background

We have previously shown that high fat (HF) feeding during pregnancy primes the development of non-alcoholic steatohepatits (NASH) in the adult offspring. However, the underlying mechanisms are unclear.

Aims

Since the endogenous molecular clock can regulate hepatic lipid metabolism, we investigated whether exposure to a HF diet during development could alter hepatic clock gene expression and contribute to NASH onset in later life.

Methods

Female mice were fed either a control (C, 7% kcal fat) or HF (45% kcal fat) diet. Offspring were fed either a C or HF diet resulting in four offspring groups: C/C, C/HF, HF/C and HF/HF. NAFLD progression, cellular redox status, sirtuin expression (Sirt1, Sirt3), and the expression of core clock genes (Clock, Bmal1, Per2, Cry2) and clock-controlled genes involved in lipid metabolism (Rev-Erbα, Rev-Erbβ, RORα, and Srebp1c) were measured in offspring livers.

Results

Offspring fed a HF diet developed NAFLD. However HF fed offspring of mothers fed a HF diet developed NASH, coupled with significantly reduced NAD⁺/NADH (p < 0.05, HF/HF vs C/C), Sirt1 (p < 0.001, HF/HF vs C/C), Sirt3 (p < 0.01, HF/HF vs C/C), perturbed clock gene expression, and elevated expression of genes involved lipid metabolism, such as Srebp1c (p < 0.05, C/HF and HF/HF vs C/C).

Conclusion

Our results suggest that exposure to excess dietary fat during early and post-natal life increases the susceptibility to develop NASH in adulthood, involving altered cellular redox status, reduced sirtuin abundance, and desynchronized clock gene expression.

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Offspring of mothers fed a high fat diet show severe fatty liver in later life.

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HF feeding is associated with altered cellular redox status and reduced sirtuin gene expression.

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HF feeding desynchronises the expression of core clock genes and lipogenic transcription factors.

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Exposure to a HF diet during development causes changes in liver metabolism that precede severe fatty liver disease.

Evolving roles of circadian rhythms in liver homeostasis and pathology

Circadian clock in mammals is determined by a core oscillator in the suprachiasmatic nucleus (SCN) of the hypothalamus and synchronized peripheral clocks in other tissues. The coherent timing systems could sustain robust output of circadian rhythms in response to the entrainment controlled environmentally. Disparate approaches have discovered that clock genes and clock-controlled genes (CCGs) exist in nearly all mammalian cell types and are essential for establishing the mechanisms and complexity of internal time-keeping systems. Accumulating evidence demonstrates that the control of homeostasis and pathology in the liver involves intricate loops of transcriptional and post-translational regulation of clock genes expression. This review will focus on the recent advances with great importance concerning clock rhythms linking liver homeostasis and diseases. We particularly highlight what is currently known of the evolving insights into the mechanisms underlying circadian clock . Eventually , findings during recent years in the field might prompt new circadian-related chronotherapeutic strategies for the diagnosis and treatment of liver diseases by coupling these processes.

Interdependence of nutrient metabolism and the circadian clock system: Importance for metabolic health

BACKGROUND

While additional research is needed, a number of large epidemiological studies show an association between circadian disruption and metabolic disorders. Specifically, obesity, insulin resistance, cardiovascular disease, and other signs of metabolic syndrome all have been linked to circadian disruption in humans. Studies in other species support this association and generally reveal that feeding that is not in phase with the external light/dark cycle, as often occurs with night or rotating shift workers, is disadvantageous in terms of energy balance. As food is a strong driver of circadian rhythms in the periphery, understanding how nutrient metabolism drives clocks across the body is important for dissecting out why circadian misalignment may produce such metabolic effects. A number of circadian clock proteins as well as their accessory proteins (such as nuclear receptors) are highly sensitive to nutrient metabolism. Macronutrients and micronutrients can function as zeitgebers for the clock in a tissue-specific way and can thus impair synchrony between clocks across the body, or potentially restore synchrony in the case of circadian misalignment. Circadian nuclear receptors are particularly sensitive to nutrient metabolism and can alter tissue-specific rhythms in response to changes in the diet. Finally, SNPs in human clock genes appear to be correlated with diet-specific responses and along with chronotype eventually may provide valuable information from a clinical perspective on how to use diet and nutrition to treat metabolic disorders.

SCOPE OF REVIEW

This article presents a background of the circadian clock components and their interrelated metabolic and transcriptional feedback loops, followed by a review of some recent studies in humans and rodents that address the effects of nutrient metabolism on the circadian clock and vice versa. We focus on studies in which results suggest that nutrients provide an opportunity to restore or, alternatively, can destroy synchrony between peripheral clocks and the central pacemaker in the brain as well as between peripheral clocks themselves. In addition, we review several studies looking at clock gene SNPs in humans and the metabolic phenotypes or tendencies associated with particular clock gene mutations.

MAJOR CONCLUSIONS

Targeted use of specific nutrients based on chronotype has the potential for immense clinical utility in the future. Macronutrients and micronutrients have the ability to function as zeitgebers for the clock by activating or modulating specific clock proteins or accessory proteins (such as nuclear receptors). Circadian clock control by nutrients can be tissue-specific. With a better understanding of the mechanisms that support nutrient-induced circadian control in specific tissues, human chronotype and SNP information might eventually be used to tailor nutritional regimens for metabolic disease treatment and thus be an important part of personalized medicine's future.

Circadian regulation of metabolic homeostasis: causes and consequences

Robust circadian rhythms in metabolic processes have been described in both humans and animal models, at the whole body, individual organ, and even cellular level. Classically, these time-of-day-dependent rhythms have been considered secondary to fluctuations in energy/nutrient supply/demand associated with feeding/fasting and wake/sleep cycles. Renewed interest in this field has been fueled by studies revealing that these rhythms are driven, at least in part, by intrinsic mechanisms and that disruption of metabolic synchrony invariably increases the risk of cardiometabolic disease. The objectives of this paper are to provide a comprehensive review regarding rhythms in glucose, lipid, and protein/amino acid metabolism, the relative influence of extrinsic (eg, neurohumoral factors) versus intrinsic (eg, cell autonomous circadian clocks) mediators, the physiologic roles of these rhythms in terms of daily fluctuations in nutrient availability and activity status, as well as the pathologic consequences of dyssynchrony.

Circadian rhythms in glucose and lipid metabolism in nocturnal and diurnal mammals

Most aspects of energy metabolism display clear variations during day and night. This daily rhythmicity of metabolic functions, including hormone release, is governed by a circadian system that consists of the master clock in the suprachiasmatic nuclei of the hypothalamus (SCN) and many secondary clocks in the brain and peripheral organs. The SCN control peripheral timing via the autonomic and neuroendocrine system, as well as via behavioral outputs. The sleep-wake cycle, the feeding/fasting rhythm and most hormonal rhythms, including that of leptin, ghrelin and glucocorticoids, usually show an opposite phase (relative to the light-dark cycle) in diurnal and nocturnal species. By contrast, the SCN clock is most active at the same astronomical times in these two categories of mammals. Moreover, in both species, pineal melatonin is secreted only at night. In this review we describe the current knowledge on the regulation of glucose and lipid metabolism by central and peripheral clock mechanisms. Most experimental knowledge comes from studies in nocturnal laboratory rodents. Nevertheless, we will also mention some relevant findings in diurnal mammals, including humans. It will become clear that as a consequence of the tight connections between the circadian clock system and energy metabolism, circadian clock impairments (e.g., mutations or knock-out of clock genes) and circadian clock misalignments (such as during shift work and chronic jet-lag) have an adverse effect on energy metabolism, that may trigger or enhancing obese and diabetic symptoms.

Dual attenuation of proteasomal and autophagic BMAL1 degradation in Clock Δ19/+ mice contributes to improved glucose homeostasis

Circadian clocks orchestrate essential physiology in response to various cues, yet their mechanistic and functional plasticity remains unclear. Here, we investigated Clock(Δ19/+) heterozygous (Clk/+) mice, known to display lengthened periodicity and dampened amplitude, as a model of partially perturbed clocks. Interestingly, Clk/+ mice exhibited improved glycemic control and resistance to circadian period lengthening under high-fat diet (HFD). Furthermore, BMAL1 protein levels in Clk/+ mouse liver were upregulated compared with wild-type (WT) mice under HFD. Pharmacological and molecular studies showed that BMAL1 turnover entailed proteasomal and autophagic activities, and CLOCKΔ19 attenuated both processes. Consistent with an important role of BMAL1 in glycemic control, enhanced activation of insulin signaling was observed in Clk/+ mice relative to WT in HFD. Finally, transcriptome analysis revealed reprogramming of clock-controlled metabolic genes in Clk/+ mice. Our results demonstrate a novel role of autophagy in circadian regulation and reveal an unforeseen plasticity of circadian and metabolic networks.

Palmitate Inhibits SIRT1-Dependent BMAL1/CLOCK Interaction and Disrupts Circadian Gene Oscillations in Hepatocytes

Elevated levels of serum saturated fatty acid palmitate have been shown to promote insulin resistance, increase cellular ROS production, and trigger cell apoptosis in hepatocytes during the development of obesity. However, it remains unclear whether palmitate directly impacts the circadian clock in hepatocytes, which coordinates nutritional inputs and hormonal signaling with downstream metabolic outputs. Here we presented evidence that the molecular clock is a novel target of palmitate in hepatocytes. Palmitate exposure at low dose inhibits the molecular clock activity and suppresses the cyclic expression of circadian targets including Dbp, Nr1d1 and Per2 in hepatocytes. Palmitate treatment does not seem to alter localization or reduce protein expression of BMAL1 and CLOCK, the two core components of the molecular clock in hepatocytes. Instead, palmitate destabilizes the protein-protein interaction between BMAL1-CLOCK in a dose and time-dependent manner. Furthermore, we showed that SIRT1 activators could reverse the inhibitory action of palmitate on BMAL1-CLOCK interaction and the clock gene expression, whereas inhibitors of NAD synthesis mimic the palmitate effects on the clock function. In summary, our findings demonstrated that palmitate inhibits the clock function by suppressing SIRT1 function in hepatocytes.

Impact of the circadian clock on the aging process

The increase of life expectancy and the decline of biological functions with advancing age are impending obstacles for our society. In general, age-related changes can be separated into two processes. Primary aging is based on programs governing gradual changes which are generally not harmful. On the other hand, secondary aging or senescence is more aleatory in nature and it is at this stage that the progressive impairment of metabolic, physiological, and neurological functions increases the risk of death. Exploiting genetic animal models, we obtain more and more information on the underlying regulatory networks. The aim of this review is to identify potential links between the output of the circadian oscillator and secondary aging. The reasons to suspect such links rely on the fact that the mouse models without functional circadian clocks sometimes exhibit reduced life expectancy. This may be due to their inability to properly control and synchronize energy expenditure, affecting, for example, the integrity of neurons in the brain. Hence, it is tempting to speculate that re-synchronization of metabolic and physiological functions by the circadian clock may slow down the aging process.