The role of clock genes and rhythmicity in the liver

Regulation of Adenine Nucleotide Metabolism by Adenylate Kinase Isozymes: Physiological Roles and Diseases

Adenylate kinase (AK) regulates adenine nucleotide metabolism and catalyzes the ATP + AMP ⇌ 2ADP reaction in a wide range of organisms and bacteria. AKs regulate adenine nucleotide ratios in different intracellular compartments and maintain the homeostasis of the intracellular nucleotide metabolism necessary for growth, differentiation, and motility. To date, nine isozymes have been identified and their functions have been analyzed. Moreover, the dynamics of the intracellular energy metabolism, diseases caused by AK mutations, the relationship with carcinogenesis, and circadian rhythms have recently been reported. This article summarizes the current knowledge regarding the physiological roles of AK isozymes in different diseases. In particular, this review focused on the symptoms caused by mutated AK isozymes in humans and phenotypic changes arising from altered gene expression in animal models. The future analysis of intracellular, extracellular, and intercellular energy metabolism with a focus on AK will aid in a wide range of new therapeutic approaches for various diseases, including cancer, lifestyle-related diseases, and aging.

The circadian rhythm regulates branched-chain amino acids metabolism in fast muscle of Chinese perch (Siniperca chuatsi) during short-term fasting by Clock-KLF15-Bcat2 pathway

As an internal time-keeping mechanism, circadian rhythm plays crucial role in maintaining homoeostasis when in response to nutrition change; meanwhile, branched-chain amino acids (BCAA) in skeletal muscle play an important role in preserving energy homoeostasis during fasting. Previous results from our laboratory suggested that fasting can influence peripheral circadian rhythm and BCAA metabolism in fish, but the relationship between circadian rhythm and BCAA metabolism, and whether circadian rhythm regulates BCAA metabolism to maintain physiological homoeostasis during fasting remains unclear. This study shows that the expression of fifteen core clock genes as well as KLF15 and Bcat2 is highly responsive to short-term fasting in fast muscle of Siniperca chuatsi, and the correlation coefficient between Clock and KLF15 expression is enhanced after fasting treatment. Furthermore, we demonstrate that the transcriptional expression of KLF15 is regulated by Clock, and the transcriptional expression of Bcat2 is regulated by KLF15 by using dual-luciferase reporter gene assay and Vivo-morpholinos-mediated gene knockdown technique. Therefore, fasting imposes a dynamic coordination of transcription between the circadian rhythm and BCAA metabolic pathways. The findings highlight the interaction between circadian rhythm and BCAA metabolism and suggest that fasting induces a switch in KLF15 expression through affecting the rhythmic expression of Clock, and then KLF15 promotes the transcription of Bcat2 to enhance the metabolism of BCAA, thus maintaining energy homoeostasis and providing energy for skeletal muscle as well as other tissues.

Obese Neotomodon alstoni mice exhibit sexual dimorphism in the daily profile of circulating melatonin and clock proteins PER1 and BMAL1 in the hypothalamus and peripheral oscillators

Obesity is a global health threat and a risk factor for several metabolic conditions. Though circadian dysfunction has been considered among the multiple causes of obesity, little work has been done to explore the relationship between obesity, circadian dysfunction, and sexual dimorphism. The Neotomodon alstoni mouse is a suitable model for such research. This study employed N. alstoni mice in a chronobiological analysis to determine whether there is circadian desynchronization of relative PER1 and BMAL1 protein levels in the hypothalamus, liver, visceral white adipose tissue, kidney, and heart. It also compared differences between sexes and lean and obese N. alstoni adult mice, by recording behavior and daily circulating serum melatonin as markers of circadian output. We found that obese mice display reduced locomotor activity. Additionally, Cosinor analyses of the relative expression of PER1 and BMAL1 show differences between lean and obese mice in a sex-linked manner. The PER1 24 h rhythm was absent in all tissues of obese males and significant in the tissues of obese females. The BMAL1 24 h rhythm also was significant in most of the tissues tested in lean males, whereas it was significant and shifted the acrophase (peak time of rhythm) in most of the tissues in obese females. Both lean male and female mice showed a rhythmic 24 h pattern of circulating serum melatonin. This daily profile was not only absent in obese mice of both sexes but showed sexual dimorphism. Obese male mice showed lower circulating levels of melatonin compared to lean male mice, but they were higher in obese females compared to lean females. Our results suggest that obesity in N. alstoni is associated with an internal circadian desynchronization in a sex-dependent manner. Overall, this study reinforces the need for further research on the neuroendocrinology of obesity and circadian rhythms using this biological model.

Chronobiology and Chronotherapy in Depression: Current Knowledge and Chronotherapeutic Promises

Background:

Depression is a heavily prevalent mental disorder. Symptoms of depression extend beyond mood, cognition, and behavior to include a spectrum of somatic manifestations in all organic systems. Changes in sleep and neuroendocrine rhythms are especially prominent, and disruptions of circadian rhythms have been closely related to the neurobiology of depression. With the advent of increased research in chronobiology, various pathophysiologic mechanisms have been proposed, including anomalies of sleep architecture, the effects of clock gene polymorphisms in monoamine metabolism, and the deleterious impact of social zeitgebers. The identification of these chronodisruptions has propelled the emergence of several chronotherapeutic strategies, both pharmacological and non-pharmacological, with varying degrees of clinical evidence.

Methods:

The fundamental objective of this review is to integrate current knowledge about the role of chronobiology and depression and to summarize the interventions developed to resynchronize biorhythms both within an individual and with geophysical time.

Results:

We have found that among the non-pharmacological alternatives, triple chronotherapywhich encompasses bright light therapy, sleep deprivation therapy, and consecutive sleep phase advance therapy-has garnered the most considerable scientific interest. On the other hand, agomelatine appears to be the most promising pharmacological option, given its unique melatonergic pharmacodynamics.

Conclusions:

Research in chronotherapy as a treatment for depression is currently booming. Novel interventions could play a significant role in adopting new options for the treatment of depression, with Tripe Cronotherapy standing out as the most promising treatment.

Prebiotic supplementation (beta-glucan and inulin) attenuates circadian misalignment induced by shifted light-dark cycle in mice by modulating circadian gene expression

Circadian rhythm governs multiple behavioural and physiological processes and its disruption is closely associated with various pathological conditions. In this study, the effects of dietary intervention by prebiotics including beta-glucan and inulin on attenuating circadian desynchrony in C57BL/6J mice subjected to weekly shifted light-dark cycle under a high fat diet was investigated. Using RT-qPCR and rhythmicity analysis, our study revealed that beta-glucan (0.2 g/day) and inulin (0.2 g/day) modulated the expression and phase of circadian-clock genes, explicitly reversed the phase delay of Period 1 and Period 3 in the hypothalamus, and reversed the phase delay of Period 2 in the liver of the mice. In the shifted mouse group, inulin also exhibited its reversal effects on the phase advance of Brain and muscle-Arnt-like 1 in the hypothalamus. These findings indicated that prebiotic supplementation can be a novel dietary approach for attenuating circadian misalignment.

Effect of melatonin on monochromatic light-induced changes in clock gene circadian expression in the chick liver

Light is the most prominent zeitgeber of the circadian system, which contains central and peripheral oscillators. Our previous studies found that light wavelength could influence the rhythms of melatonin synthesis and clock gene expression in the central oscillator of chicks. However, the effect of monochromatic light on the peripheral oscillator and the role of melatonin have yet to be clarified. In this study, 216 newly hatched chicks were divided into three groups (intact, sham operation and pinealectomy) and were raised under white (WL), red (RL), green (GL) or blue (BL) light for 14 days. Their plasma and livers were sampled at 6 time points with 4-h intervals. Plasma melatonin concentration and liver clock gene expression (cClock, cBmal1, cBmal2, cCry1, cCry2, cPer2, cPer3) were measured for circadian rhythm analysis. In intact and sham operation chicks under WL, all liver clock genes showed circadian expression along with oscillations in plasma melatonin. However, positive clock genes peaked at subjective night along with melatonin, while negative clock genes peaked at subjective day or the shifting time of day-night. Chick exposure to monochromatic light led to an unaltered circadian rhythmicity in plasma melatonin and liver clock genes; however, their rhythmic parameters were notably influenced. Compared to WL, GL enhanced the mesor and amplitude of melatonin and all kinds of clock genes, whereas RL had the opposite effect. Pinealectomy significantly decreased expression of liver clock genes, which was consistent with the reduction in plasma melatonin concentration, especially for the GL group, and resulted in the expression of liver clock genes showing low-mesor and low-amplitude oscillations as well as no statistically significant differences among the monochromatic light groups. Thus, we speculated that melatonin plays a key role in the effects of light wavelength on clock gene rhythm in the chick liver.

Tick-tock hedgehog-mutual crosstalk with liver circadian clock promotes liver steatosis

BACKGROUND & AIMS

The mammalian circadian clock controls various aspects of liver metabolism and integrates nutritional signals. Recently, we described Hedgehog (Hh) signaling as a novel regulator of liver lipid metabolism. Herein, we investigated crosstalk between hepatic Hh signaling and circadian rhythm.

METHODS

Diurnal rhythms of Hh signaling were investigated in liver and hepatocytes from mice with ablation of Smoothened (SAC-KO) and crossbreeds with PER2::LUC reporter mice. By using genome-wide screening, qPCR, immunostaining, ELISA and RNAi experiments in vitro we identified relevant transcriptional regulatory steps. Shotgun lipidomics and metabolic cages were used for analysis of metabolic alterations and behavior.

RESULTS

Hh signaling showed diurnal oscillations in liver and hepatocytes in vitro. Correspondingly, the level of Indian Hh, oscillated in serum. Depletion of the clock gene Bmal1 in hepatocytes resulted in significant alterations in the expression of Hh genes. Conversely, SAC-KO mice showed altered expression of clock genes, confirmed by RNAi against Gli1 and Gli3. Genome-wide screening revealed that SAC-KO hepatocytes showed time-dependent alterations in various genes, particularly those associated with lipid metabolism. The clock/hedgehog module further plays a role in rhythmicity of steatosis, and in the response of the liver to a high-fat diet or to differently timed starvation.

CONCLUSIONS

For the first time, Hh signaling in hepatocytes was found to be time-of-day dependent and to feed back on the circadian clock. Our findings suggest an integrative role of Hh signaling, mediated mainly by GLI factors, in maintaining homeostasis of hepatic lipid metabolism by balancing the circadian clock.

LAY SUMMARY

The results of our investigation show for the first time that the Hh signaling in hepatocytes is time-of-day dependent, leading to differences not only in transcript levels but also in the amount of Hh ligands in peripheral blood. Conversely, Hh signaling is able to feed back to the circadian clock.

The circadian protein CLOCK regulates cell metabolism via the mitochondrial carrier SLC25A10

Physiological function and metabolic regulation are the most important outputs of circadian clock controls in mammals. Mitochondrial respiration and ROS production show rhythmic activity. Mitochondrial carriers, which are responsible for mitochondrial substance transfer, are vital for mitochondrial metabolism. Clock (Circadian Locomotor Output Cycles Kaput) is the first core circadian gene identified in mammalian animals. However, whether CLOCK protein can regulate mitochondrial functions via mitochondrial carriers is unclear. Here, we showed that CLOCK can bind to the mitochondrial carrier SLC25A10. For further analysis, we established a Slc25a10^-/--Hepa1-6 cell line using CRISPR/Cas9 gene-editing technology. Slc25a10^-/--Hepa1-6 cells showed disordered glucose homeostasis, increased oxidative stress levels, and damaged electron transport chains. Next, using an immunoprecipitation assay, we found that amino acids 43-84 and 169-210 in SLC25A10 are key sites that respond to CLOCK binding. Finally, forced expression of wild-type SLC25A10 in Slc25a10^-/--Hepa1-6 cells could compensate for the loss of SLC25A10; the decreased glucose metabolism, severe oxidative stress and damaged electron transport chain were recovered. In addition, a mutant Slc25a10 with changes in two key sites did not show a rescue effect. In conclusion, we identified a new protein-protein interaction mechanism in which CLOCK can directly regulate cell metabolism via the mitochondrial membrane transporter SLC25A10. Our study might provide some new insights into the relationship between circadian clock and mitochondrial metabolism.

Effects of cross-fostering and developmental exposure to mixtures of environmental contaminants on hepatic gene expression in prepubertal 21 days old and adult male Sprague-Dawley rats

The notion that adverse health effects produced by exposure to environmental contaminants (EC) may be modulated by the presence of non-chemical stressors is gaining attention. Previously, our lab demonstrated that cross-fostering (adoption of a litter at birth) acted as a non-chemical stressor that amplified the influence of developmental exposure to EC on the glucocorticoid stress-response in adult rats. Using liver from the same rats, the aim of the current study was to investigate whether cross-fostering might also modulate EC-induced alterations in hepatic gene expression profiles. During pregnancy and nursing, Sprague-Dawley dams were fed cookies laced with corn oil (control, C) or a chemical mixture (M) composed of polychlorinated biphenyls (PCB), organochlorine pesticides (OCP), and methylmercury (MeHg), at 1 mg/kg/day. This mixture simulated the contaminant profile reported in maternal human blood. At birth, some control and M treated litters were cross-fostered to form two additional groups with different biological/nursing mothers (CC and MM). The hepatic transcriptome was analyzed by DNA microarray in male offspring at postnatal days 21 and 78-86. Mixture exposure altered the expression of detoxification and energy metabolism genes in both age groups, but with different sets of genes affected at day 21 and 78-86. Cross-fostering modulated the effects of M on gene expression pattern (MM vs M), as well as expression of energy metabolism genes between control groups (CC vs C). In conclusion, while describing short and long-term effects of developmental exposure to EC on hepatic transcriptomes, these cross-fostering results further support the consideration of non-chemical stressors in EC risk assessments.

Time-Lag in Feeding Schedule Acts as a Stressor That Alters Circadian Oscillators in Goldfish

The circadian system controls temporal homeostasis in all vertebrates. The light-dark (LD) cycle is the most important zeitgeber (“time giver”) of circadian system, but feeding time also acts as a potent synchronizer in the functional organization of the teleost circadian system. In mammals is well known that food intake during the rest phase promotes circadian desynchrony which has been associated with metabolic diseases. However, the impact of a misalignment of LD and feeding cycles in the entrainment of fish circadian oscillators is largely unknown. The objective of this work was to investigate how a time-lag feeding alters temporal homeostasis and if this could be considered a stressor. To this aim, goldfish maintained under a 12 h light-12 h darkness were fed at mid-photophase (SF6) or mid-scotophase (SF18). Daily rhythms of locomotor activity, clock genes expression in hypothalamus, liver, and head kidney, and circulating cortisol were studied. Results showed that SF6 fish showed daily rhythms of bmal1a and clock1a in all studied tissues, being in antiphase with rhythms of per1 genes, as expected for proper functioning clocks. The 12 h shift in scheduled feeding induced a short phase advance (4–5-h) of the clock genes daily rhythms in the hypothalamus, while in the liver the shift for clock genes expression rhythms was the same that the feeding time shift (∼12 h). In head kidney, acrophases of per genes underwent a 12-h shift in SF18 animals, but only 6 h shift for clock1a. Plasma cortisol levels showed a significant daily rhythm in animals fed at SF6, but not in SF18 fish fed, which displayed higher cortisol values throughout the 24-h. Altogether, results indicate that hypothalamus, liver, and head kidney oscillate in phase in SF6 fish, but these clocks are desynchronized in SF18 fish, which could explain cortisol alterations. These data reinforce the hypothesis that the misalignment of external cues (daily photocycle and feeding time) alters fish temporal homeostasis and it might be considered a stressor for the animals.

Impact of Short-Term Fasting on The Rhythmic Expression of the Core Circadian Clock and Clock-Controlled Genes in Skeletal Muscle of Crucian Carp (Carassius auratus)

The peripheral tissue pacemaker is responsive to light and other zeitgebers, especially food availability. Generally, the pacemaker can be reset and entrained independently of the central circadian structures. Studies involving clock-gene expressional patterns in fish peripheral tissues have attracted considerable attention. However, the rhythmic expression of clock genes in skeletal muscle has only scarcely been investigated. The present study was designed to investigate the core clock and functional gene expression rhythms in crucian carp. Meanwhile, the synchronized effect of food restrictions (short-term fasting) on these rhythms in skeletal muscle was carefully examined. In fed crucian carp, three core clock genes (Clock, Bmal1a, and Per1) and five functional genes (Epo, Fas, IGF1R2, Jnk1, and MyoG) showed circadian rhythms. By comparison, four core clock genes (Clock, Bmal1a, Cry3, and Per2) and six functional genes (Epo, GH, IGF2, Mstn, Pnp5a, and Ucp1) showed circadian rhythms in crucian carp muscle after 7-day fasting. In addition, three core clock genes (Clock, Per1, and Per3) and six functional genes (Ampk1a, Lpl, MyoG, Pnp5a, PPARα, and Ucp1) showed circadian rhythms in crucian carp muscle after 15-day fasting. However, all gene rhythmic expression patterns differed from each other. Furthermore, it was found that the circadian genes could be altered by feed deprivation in crucian carp muscle through the rhythms correlation analysis of the circadian genes and functional genes. Hence, food-anticipatory activity of fish could be adjusted through the food delivery restriction under a light⁻dark cycle. These results provide a potential application in promoting fish growth by adjusting feeding conditions and nutritional state.

Are Type 2 Diabetes Mellitus and Depression Part of a Common Clock Genes Network?

In recent years, there has been an increased prevalence of type 2 diabetes mellitus (T2DM) and depression across the world. This growing public health problem has produced an increasing socioeconomic burden to the populations of all affected countries. Despite an awareness by public health officials and medical researchers of the costs associated with these diseases, there still remain many aspects of how they develop that are not understood. In this article, we propose that the circadian clock could be a factor that coordinates both the neurobehavioral and metabolic processes that underlie depression and T2DM. We propose further that this perspective, one which emphasizes the regulatory effects of clock gene activity, may provide insights into how T2DM and depression interact with one another, and may thus open a new pathway for managing and treating these disorders.

Clock gene Per2 as a controller of liver carcinogenesis

Environmental disruption of molecular clocks promoted liver carcinogenesis and accelerated cancer progression in rodents. We investigated the specific role of clock gene Period 2 (Per2) for liver carcinogenesis and clock-controlled cellular proliferation, genomic instability and inflammation. We assessed liver histopathology, and determined molecular and physiology circadian patterns in mice on chronic diethylnitrosamine (DEN) exposure according to constitutive Per2 mutation. First, we found that Per2^m/m liver displayed profound alterations in proliferation gene expression, including c-Myc derepression, phase-advanced Wee1, and arrhythmic Ccnb1 and K-ras mRNA expressions, as well as deregulated inflammation, through arrhythmic liver IL-6 protein concentration, in the absence of any DEN exposure. These changes could then make Per2^m/m mice more prone to subsequently develop liver cancers on DEN. Indeed, primary liver cancers were nearly fourfold as frequent in Per2^m/m mice as compared to wild-type (WT), 4 months after DEN exposure. The liver molecular clock was severely disrupted throughout the whole carcinogenesis process, including the initiation stage, i.e. within the initial 17 days on DEN. Per2^m/m further exhibited increased c-Myc and Ccnb1 mean 24h expressions, lack of P53 response, and arrhythmic ATM, Wee1 and Ccnb1 expressions. DEN-induced tumor related inflammation was further promoted through increased protein concentrations of liver IL-6 and TNF-α as compared to WT during carcinogenesis initiation. Per2 mutation severely deregulated liver gene or protein expressions related to three cancer hallmarks, including uncontrolled proliferation, genomic instability, and tumor promoting inflammation, and accelerated liver carcinogenesis several-fold. Clock gene Per2 acted here as a liver tumor suppressor from initiation to progression.

NcRNAs and Cholangiocarcinoma

Cholangiocarcinoma (CCA) is the most common primary biliary malignancy with poor prognosis. Less understanding of its etiology and pathogenesis makes the diagnosis and therapy difficult. Recently, accumulating evidences have demonstrated that deregulated expression of non-coding RNAs (ncRNAs) is closely associated with the etiopathogenesis of CCA. NcRNAs which lack open reading frame are a heterogeneous class of transcribed RNA molecules, including microRNAs, long non-coding RNAs and circular RNAs. Several studies have shown ncRNAs dysregulation is a common central event occurring in CCA and has the potential of being therapy targets. Moreover, ncRNAs can be easily detected in cancer tissues and biofluids, representing valuable tools for diagnosis. In this review, we illustrate the role of ncRNA in the CCA and discuss their potential clinical value.

Evaluating the associations between human circadian rhythms and dysregulated genes in liver cancer cells

Network analysis is a useful approach in cancer biology as it provides information regarding the genes and proteins. In our previous study, a network analysis was performed on dysregulated genes in HepG2 cells, a hepatoblastoma cell line that lacks the viral infection, compared with normal hepatocytes, identifying the presence of 26 HUB genes. The present study aimed to identify whether these previously identified HUB genes participate in the network that controls the human circadian rhythms. The results of the present study demonstrated that 20/26 HUB genes were associated with the metabolic processes that control human circadian rhythms, which supports the hypothesis that a number of cancer types are dependent from circadian cycles. In addition, it was revealed that the CLOCK circadian regulator gene was associated, via cytoskeleton associated protein 5 (CKAP5), with the HUB genes of the HepG2 network, and that CKAP5 was associated with three other circadian genes (casein kinase 1ε, casein kinase 1δ and histone deacetylase 4) and 10 HepG2 genes (SH2 domain containing, ZW10 interacting kinetochore protein, aurora kinase B, cell division cycle 20, centromere protein A, inner centromere protein, mitotic arrest deficient 2 like 1, baculoviral IAP repeat containing 5, SPC24 NDC80 kinetochore complex component and kinesin family member 2C). Furthermore, the genes that associate the circadian system with liver cancer were demonstrated to encode intrinsically disordered proteins. Finally, the results of the present study identified the microRNAs involved in the network formed by the overlapping of HepG2 and circadian genes.

Critical cholangiocarcinogenesis control by cryptochrome clock genes

A coordinated network of molecular circadian clocks in individual cells generates 24-hr rhythms in liver metabolism and proliferation. Circadian disruption through chronic jet lag or Per2 clock gene mutation was shown to accelerate hepatocarcinoma development in mice. As divergent effects were reported for clock genes Per and Cry regarding xenobiotic toxicity, we questioned the role of Cry1 and Cry2 in liver carcinogenesis. Male WT and Cry1^-/- Cry2^-/- mice (C57Bl/6 background) were chronically exposed to diethylnitrosamine (DEN) at ZT11. Rest-activity and body temperature rhythms were monitored using an implanted radiotransmitter. Serum aspartate and alanine aminotransferases (AST and ALT) were determined on four occasions during the progression stage. After 7 months, serum alkaline phosphatases (ALP) were determined, and livers were sampled for microscopic tumor nodule counting and histopathology. Five months after initiation of DEN treatment, we found that Cry1^-/- Cry2^-/- mice developed severe liver dysplasia, as evident from the increased AST, ALT and ALP levels, as compared to WT mice. DEN exposure induced primary liver cancers in nearly fivefold as many Cry1^-/- Cry2^-/- mice as compared to WT mice (p = 0.01). Microscopic study revealed no difference in the average number of hepatocarcinomas and a nearly eightfold increase in the average number of cholangiocarcinomas in Cry1^-/- Cry2^-/- mice, as compared to WT mice. This study validated the hypothesis that molecular circadian clock disruption dramatically increased chemically induced liver carcinogenesis. In addition, the pronounced shift toward cholangiocarcinoma in DEN exposed Cry1^-/- Cry2^-/- mice revealed a critical role of the Cry clock genes in bile duct carcinogenesis.

Ghrelin induces clock gene expression in the liver of goldfish in vitro via protein kinase C and protein kinase A pathways

The liver is the most important link between the circadian system and metabolism. As a food-entrainable oscillator, the hepatic clock needs to be entrained by food-related signals. The objective of the present study was to investigate the possible role of ghrelin (an orexigenic peptide mainly synthesized in the gastrointestinal tract) as an endogenous synchronizer of the liver oscillator in teleosts. To achieve this aim, we first examined the presence of ghrelin receptors in the liver of goldfish. Then, the ghrelin regulation of clock gene expression in the goldfish liver was studied. Finally, the possible involvement of the phospholipase C/protein kinase C (PLC/PKC) and adenylate cyclase/protein kinase A (AC/PKA) intracellular signalling pathways was investigated. Ghrelin receptor transcripts, ghs-r1a, are present in the majority of goldfish hepatic cells. Ghrelin induced the mRNA expression of the positive (gbmal1a, gclock1a) and negative (gper genes) elements of the main loop of the molecular clock machinery, as well as grev-erbα (auxiliary loop) in cultured liver. These effects were blocked, at least in part, by a ghrelin antagonist. Incubation of liver with a PLC inhibitor (U73122), a PKC activator (phorbol 12-myristate 13-acetate) and a PKC inhibitor (chelerythrine chloride) demonstrated that the PLC/PKC pathway mediates such ghrelin actions. Experiments with an AC activator (forskolin) and a PKA inhibitor (H89) showed that grev-erbα regulation could be due to activation of PKA. Taken together, the present results show for the first time in vertebrates a direct action of ghrelin on hepatic clock genes and support a role for this hormone as a temporal messenger in the entrainment of liver circadian functions.

Rhodiola crenulata extract regulates hepatic glycogen and lipid metabolism via activation of the AMPK pathway

Background

Metabolic syndrome may lead to many complications, such as nonalcoholic fatty liver disease (NAFLD). A natural and effective therapeutic agent for patients with NAFLD is urgently needed. In a previous study, we showed that Rhodiola crenulata root extract (RCE) regulated hepatic gluconeogenesis through activation of AMPK signaling. However, the manner in which RCE regulates hepatic lipid and glycogen metabolism remains unclear. The current study was conducted to investigate the effects of RCE on hepatic glycogen and lipid metabolism, as well as the mechanisms underlying such effects.

Methods

Human hepatoma HepG2 cells were treated with RCE for 6 h under high glucose conditions, after which glycogen synthesis, lipogenesis, and relative gene expression were examined. In addition, lipogenesis-related genes were investigated in vivo.

Results

RCE significantly increased glycogen synthesis and inhibited lipogenesis, while regulating genes related to these processes, including glycogen synthase kinase 3β (GSK3β), glycogen synthase (GS), fatty acid synthase (FAS), CCAAT/enhancer-binding protein (C/EBP), and sterol regulatory element-binding protein 1c (SREBP-1c). However, the effects caused by RCE were neutralized by compound C, an AMPK antagonist. Further studies showed that expression levels of lipogenic genes decreased at the protein and mRNA levels in the rat liver.

Conclusions

Our results demonstrate that RCE regulates hepatic glycogen and lipid metabolism through the AMPK signaling pathway. These results suggest that RCE is a potential intervention for patients with NAFLD.

Muscle-specific loss of Bmal1 leads to disrupted tissue glucose metabolism and systemic glucose homeostasis

BACKGROUND

Diabetes is the seventh leading cause of death in the USA, and disruption of circadian rhythms is gaining recognition as a contributing factor to disease prevalence. This disease is characterized by hyperglycemia and glucose intolerance and symptoms caused by failure to produce and/or respond to insulin. The skeletal muscle is a key insulin-sensitive metabolic tissue, taking up ~80 % of postprandial glucose. To address the role of the skeletal muscle molecular clock to insulin sensitivity and glucose tolerance, we generated an inducible skeletal muscle-specific Bmal1 (-/-) mouse (iMSBmal1 (-/-)).

RESULTS

Progressive changes in body composition (decreases in percent fat) were seen in the iMSBmal1 (-/-) mice from 3 to 12 weeks post-treatment as well as glucose intolerance and non-fasting hyperglycemia. Ex vivo analysis of glucose uptake revealed that the extensor digitorum longus (EDL) muscles did not respond to either insulin or 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) stimulation. RT-PCR and Western blot analyses demonstrated a significant decrease in mRNA expression and protein content of the muscle glucose transporter (Glut4). We also found that both mRNA expression and activity of two key rate-limiting enzymes of glycolysis, hexokinase 2 (Hk2) and phosphofructokinase 1 (Pfk1), were significantly reduced in the iMSBmal1 (-/-) muscle. Lastly, results from metabolomics analyses provided evidence of decreased glycolytic flux and uncovered decreases in some tricarboxylic acid (TCA) intermediates with increases in amino acid levels in the iMSBmal1 (-/-) muscle. These findings suggest that the muscle is relying predominantly on fat as a fuel with increased protein breakdown to support the TCA cycle.

CONCLUSIONS

These data support a fundamental role for Bmal1, the endogenous circadian clock, in glucose metabolism in the skeletal muscle. Our findings have implicated altered molecular clock dictating significant changes in altered substrate metabolism in the absence of feeding or activity changes. The changes in body composition in our model also highlight the important role that changes in skeletal muscle carbohydrate, and fat metabolism can play in systemic metabolism.

The Liver in the Eyes of a Chronobiologist

Our understanding of the molecular underpinnings of the mammalian circadian oscillator and its tight connection to physiology has progressed tremendously during the past decades. The liver is considered the prototypic experimental model tissue for circadian research in peripheral organs. Studies on liver clocks have been highly productive and yielded information about widely different aspects of circadian biology. The liver, as one of the largest organs in the body, has often been used for the identification of core clock and auxiliary clock components, for example, by biochemical purifications. Because the liver is also a major metabolic hub, studies addressing the interplay between circadian clocks and metabolism have been insightful. In addition, the use of liver-specific loss-of-function models for clock components highlighted not only specific physiological roles of the hepatic clock but also its interplay with systemic cues and oscillators in other organs. Recently, technological advances in omics approaches have been successfully applied on the liver, providing a comprehensive depiction of pervasive circadian control of gene expression and protein and metabolite accumulation. In this review, we chose to illuminate specific examples that demonstrate how different experimental approaches--namely, biochemical, metabolic, genetic, and omics methodologies--have advanced our knowledge regarding circadian liver biology and chronobiology in general.

The liver of goldfish as a component of the circadian system: Integrating a network of signals

The circadian system drives daily physiological and behavioral rhythms that allow animals to anticipate cyclic environmental changes. The discovery of the known as "clock genes", which are very well conserved through vertebrate phylogeny, highlighted the molecular mechanism of circadian oscillators functioning, based on transcription and translation cycles (∼ 24 h) of such clock genes. Studies in goldfish have shown that the circadian system in this species is formed by a net of oscillators distributed at central and peripheral locations, as the retina, brain, gut and liver, among others. In this work we review the existing information about the hepatic oscillator in goldfish due to its relevance in metabolism, and its key role as target of a variety of humoral signals. Different input signals modify the molecular clockwork in the liver of goldfish. Among them, there are environmental cues (photocycle and feeding regime) and different encephalic and peripheral endogenous signals (orexin, ghrelin and glucocorticoids). Per clock genes seem to be a common target for different signals. Thus, this genes family might be important for shifting the hepatic oscillator. The physiological relevance of the crosstalking between metabolic and feeding-related hormones and the hepatic clock sets the stage for the hypothesis that these hormones could act as "internal zeitgebers" communicating oscillators in the goldfish circadian system.

Daily rhythms in expression of genes of hepatic lipid metabolism in Atlantic salmon (Salmo salar L.)

In mammals, several genes involved in liver lipid and cholesterol homeostasis are rhythmically expressed with expression shown to be regulated by clock genes via Rev-erb 1α. In order to elucidate clock gene regulation of genes involved in lipid metabolism in Atlantic salmon (Salmo salar L.), the orphan nuclear receptor Rev-erb 1α was cloned and 24 h expression of clock genes, transcription factors and genes involved in cholesterol and lipid metabolism determined in liver of parr acclimated to a long-day photoperiod, which was previously shown to elicit rhythmic clock gene expression in the brain. Of the 31 genes analysed, significant daily expression was demonstrated in the clock gene Bmal1, transcription factor genes Srebp1, Lxr, Pparα and Pparγ, and several lipid metabolism genes Hmgcr, Ipi, ApoCII and El. The possible regulatory mechanisms and pathways, and the functional significance of these patterns of expression were discussed. Importantly and in contrast to mammals, Per1, Per2, Fas, Srebp2, Cyp71α and Rev-erb 1α did not display significant daily rhythmicity in salmon. The present study is the first report characterising 24 h profiles of gene expression in liver of Atlantic salmon. However, more importantly, the predominant role of lipids in the nutrition and metabolism of fish, and of feed efficiency in determining farming economics, means that daily rhythmicity in the regulation of lipid metabolism will be an area of considerable interest for future research in commercially important species.

Liver zonation: Novel aspects of its regulation and its impact on homeostasis

Liver zonation, the spatial separation of the immense spectrum of different metabolic pathways along the liver sinusoids, is fundamental for proper functioning of this organ. Recent progress in elucidating localization and interactions of different metabolic pathways by using “omics” techniques and novel approaches to couple them with refined spatial resolution and in characterizing novel master regulators of zonation by using transgenic mice has created the basis for a deeper understanding of core mechanisms of zonation and their impact on liver physiology, pathology and metabolic diseases. This review summarizes the fascinating technical achievements for investigating liver zonation and the elucidation of an emerging network of master regulators of zonation that keep the plethora of interrelated and sometimes opposing functions of the liver in balance with nutritional supply and specific requirements of the entire body. In addition, a brief overview is given on newly described zonated functions and novel details on how diverse the segmentation of metabolic zonation may be. From these facts and developments a few fundamental principles are inferred which seem to rule zonation of liver parenchyma. In addition, we identify important questions that still need to be answered as well as interesting fields of research such as the connection of zonation with circadian rhythm and gender dimorphism which need to be pushed further, in order to improve our understanding of metabolic zonation. Finally, an outlook is given on how disturbance of liver zonation and its regulation may impact on liver pathology and the development of metabolic diseases.

Daily patterns and adaptation of the ghrelin, growth hormone and insulin-like growth factor-1 system under daytime food synchronisation in rats

Daytime restricted feeding promotes the re-alignment of the food entrained oscillator (FEO). Endocrine cues which secretion is regulated by the transition of fasting and feeding cycles converge in the FEO. The present study aimed to investigate the ghrelin, growth hormone (GH) and insulin-like growth factor (IGF)-1 system because their release depends on rhythmic and nutritional factors, and the output from the system influences feeding and biochemical status. In a daily sampling approach, rats that were fed ad lib. were compared with rats on a reversed (daytime) and restricted feeding schedule by 3 weeks (dRF; food access for 2 h), also assessing the effect of acute fasting and refeeding. We undertook measurements of clock protein BMAL1 and performed somatometry of peripheral organs and determined the concentration of total, acylated and unacylated ghrelin, GH and IGF-1 in both serum and in its main synthesising organs. During dRF, BMAL1 expression was synchronised to mealtime in hypophysis and liver; rats exhibited acute hyperphagia, stomach distension with a slow emptying, a phase shift in liver mass towards the dark period and decrease in mass perigonadal white adipose tissue. Total ghrelin secretion during the 24-h period increased in the dRF group as a result of elevation of the unacylated form. By contrast, GH and IGF-1 serum concentration fell, with a modification of GH daily pattern after mealtime. In the dRF group, ghrelin content in the stomach and pituitary GH content decreased, whereas hepatic IGF-1 remained equal. The daily patterns and synthesis of these hormones had a rheostatic adaptation. The endocrine adaptive response elicited suggests that it may be associated with the regulation of metabolic, behavioural and physiological processes during the paradigm of daytime restricted feeding and associated FEO activity.

Synchronization by food access modifies the daily variations in expression and activity of liver GABA transaminase

Daytime restricted feeding (DRF) is an experimental protocol that influences the circadian timing system and underlies the expression of a biological clock known as the food entrained oscillator (FEO). Liver is the organ that reacts most rapidly to food restriction by adjusting the functional relationship between the molecular circadian clock and the metabolic networks. γ-Aminobutyric acid (GABA) is a signaling molecule in the liver, and able to modulate the cell cycle and apoptosis. This study was aimed at characterizing the expression and activity of the mostly mitochondrial enzyme GABA transaminase (GABA-T) during DRF/FEO expression. We found that DRF promotes a sustained increase of GABA-T in the liver homogenate and mitochondrial fraction throughout the entire day-night cycle. The higher amount of GABA-T promoted by DRF was not associated to changes in GABA-T mRNA or GABA-T activity. The GABA-T activity in the mitochondrial fraction even tended to decrease during the light period. We concluded that DRF influences the daily variations of GABA-T mRNA levels, stability, and catalytic activity of GABA-T. These data suggest that the liver GABAergic system responds to a metabolic challenge such as DRF and the concomitant appearance of the FEO.

Neonatal capsaicin treatment in rats affects TRPV1-related noxious heat sensation and circadian body temperature rhythm

The transient receptor potential vanilloid 1 (TRPV1) is a cation channel that serves as a polymodal detector of noxious stimuli such as capsaicin. Therefore, capsaicin treatment has been used to investigate the physiological function of TRPV1. Here, we report physiological changes induced by treating neonatal rats with capsaicin. Capsaicin (50mg/kg) (cap-treated) or vehicle (vehicle-treated) was systemically administered to newborn SD rat pups within 48 h after birth. TRPV1 expression, intake volume of capsaicin water, and noxious heat sensation were measured 6 weeks after capsaicin treatment. Circadian body temperature and locomotion were recorded by biotelemetry. Expression of Per1, Per2, Bmal1 and Hsf1 (clock genes) was also investigated. Neonatal capsaicin treatment not only decreased TRPV1 expression but also induced desensitization to noxious heat stimuli. Circadian body temperature of cap-treated rats increased significantly compared with that of vehicle-treated rats. Additionally, the amplitude of the circadian body temperature was reversed in cap-treated rats. Expression of the hypothalamic Hsf1 and liver Per2 clock genes followed a similar trend. Therefore, we suggest that these findings will be useful in studying various physiological mechanisms related to TRPV1.

Differential modulation of clock gene expression in the suprachiasmatic nucleus, liver and heart of aged mice

Studies on the molecular clockwork during aging have been hitherto addressed to core clock genes. These previous investigations indicate that circadian profiles of core clock gene expression at an advanced age are relatively preserved in the master circadian pacemaker and the hypothalamic suprachiasmatic nucleus (SCN), and relatively impaired in peripheral tissues. It remains to be clarified whether the effects of aging are confined to the primary loop of core clock genes, or also involve secondary clock loop components, including Rev-erbα and the clock-controlled genes Dbp and Dec1. Using quantitative real-time RT-PCR, we here report a comparative analysis of the circadian expression of canonical core clock genes (Per1, Per2, Cry1, Cry2, Clock and Bmal1) and non-core clock genes (Rev-erbα, Dbp and Dec1) in the SCN, liver, and heart of 3month-old vs 22month-old mice. The results indicate that circadian clock gene expression is significantly modified in the SCN and peripheral oscillators of aged mice. These changes are not only highly tissue-specific, but also involve different clock gene loops. In particular, we here report changes of secondary clock loop components in the SCN, changes of the primary clock loop in the liver, and minor changes of clock gene expression in the heart of aged mice. The present findings outline a track to further understanding of the role of primary and secondary clock loop components and their crosstalk in the impairment of circadian output which characterizes aging.

Exploitation of host clock gene machinery by hepatitis viruses B and C

Many aspects of cellular physiology display circadian (approximately 24-h) rhythms. Dysfunction of the circadian clock molecular circuitry is associated with human health derangements, including neurodegeneration, increased risk of cancer, cardiovascular diseases and the metabolic syndrome. Viruses triggering hepatitis depend tightly on the host cell synthesis machinery for their own replication, survival and spreading. Recent evidences support a link between the circadian clock circuitry and viruses’ biological cycle within host cells. Currently, in vitro models for chronobiological studies of cells infected with viruses need to be implemented. The establishment of such in vitro models would be helpful to better understand the link between the clock gene machinery and viral replication/viral persistence in order to develop specifically targeted therapeutic regimens. Here we review the recent literature dealing with the interplay between hepatitis B and C viruses and clock genes.

Tumor necrosis factor and transforming growth factor β regulate clock genes by controlling the expression of the cold inducible RNA-binding protein (CIRBP)

The circadian clock drives the rhythmic expression of a broad array of genes that orchestrate metabolism, sleep wake behavior, and the immune response. Clock genes are transcriptional regulators engaged in the generation of circadian rhythms. The cold inducible RNA-binding protein (CIRBP) guarantees high amplitude expression of clock. The cytokines TNF and TGFβ impair the expression of clock genes, namely the period genes and the proline- and acidic amino acid-rich basic leucine zipper (PAR-bZip) clock-controlled genes. Here, we show that TNF and TGFβ impair the expression of Cirbp in fibroblasts and neuronal cells. IL-1β, IL-6, IFNα, and IFNγ do not exert such effects. Depletion of Cirbp is found to increase the susceptibility of cells to the TNF-mediated inhibition of high amplitude expression of clock genes and modulates the TNF-induced cytokine response. Our findings reveal a new mechanism of cytokine-regulated expression of clock genes.

Dark nights reverse metabolic disruption caused by dim light at night

OBJECTIVE

The increasing prevalence of obesity and related metabolic disorders coincides with increasing exposure to light at night. Previous studies report that mice exposed to dim light at night (dLAN) develop symptoms of metabolic syndrome. This study investigated whether mice returned to dark nights after dLAN exposure recover metabolic function.

DESIGN AND METHODS

Male Swiss-Webster mice were assigned to either: standard light-dark (LD) conditions for 8 weeks (LD/LD), dLAN for 8 weeks (dLAN/dLAN), LD for 4 weeks followed by 4 weeks of dLAN (LD/dLAN), and dLAN for 4 weeks followed by 4 weeks of LD (dLAN/LD).

RESULTS

After 4 weeks in their respective lighting conditions both groups initially placed in dLAN increased body mass gain compared to LD mice. Half of the dLAN mice (dLAN/LD) were then transferred to LD and vice versa (LD/dLAN). Following the transfer dLAN/dLAN and LD/dLAN mice gained more weight than LD/LD and dLAN/LD mice. At the conclusion of the study dLAN/LD mice did not differ from LD/LD mice with respect to weight gain and had lower fat pad mass compared to dLAN/dLAN mice. Compared to all other groups dLAN/dLAN mice decreased glucose tolerance as indicated by an intraperitoneal glucose tolerance test at week 7, indicating that dLAN/LD mice recovered glucose metabolism. dLAN/dLAN mice also increased MAC1 mRNA expression in peripheral fat as compared to both LD/LD and dLAN/LD mice, suggesting peripheral inflammation is induced by dLAN, but not sustained after return to LD.

CONCLUSION

These results suggest that re-exposure to dark nights ameliorates metabolic disruption caused by dLAN exposure.

Circadian variation of Valproic acid pharmacokinetics in mice

Valproic acid (VPA) is currently one of the most commonly used antiepileptic drugs. This study aims to investigate whether VPA pharmacokinetics varied according to circadian dosing-time. A single dose of VPA (350 mgkg(-1)) was administered by intraperitonally (i.p.) route to a total of 132 mice synchronized for 3 weeks to 12h light (rest span) and 12 h dark (activity span). Four different circadian times (1, 7, 13 and 19 HALO) of drug injection were used (33 mice/circadian time). At each circadian time, blood samples were withdrawn at (0 h) and at 0.083, 0.166, 0.25, 0.5, 0.75, 1, 1.25, 1.5, 2 and 3h after VPA injection. Plasma VPA concentrations were determined by an EMIT method. There were no significant differences in T(max) of VPA whatever the circadian-time of injections (T(max)=0.166 h). However, there were relevant differences in C(max) between the four circadian groups (p<0.005), it varied between 386 ± 30.86 mg L(-1) in mice treated at 7 HALO and 824 ± 39.85 mg L(-1) in mice treated at 19 HALO. The AUC(0-∞) was significantly two times higher when VPA was administered at 19 HALO as compared to the injection at 7 HALO. Drug dosing at 7 HALO resulted in highest Cl(T) value: 0.405 ± 0.006 L h(-1)kg(-1), whereas Cl(T) was significantly slower when VPA was administered at 19 HALO (0.157 ± 0.009 L h(-1)kg(-1)) (p<0.0001). The AUC(0-∞) was significantly 2-fold higher when VPA was administered at 19 HALO (2216.65 ± 138.91 mg h(-1)L(-1)) as compared to the injection at 7 HALO (864.09 ± 16.82 mg h(-1)L(-1)) (p<0.0001). Cosinor analysis showed circadian rhythm in different pharmacokinetic parameters. C(max) and AUC(0-∞) have a significant circadian rhythm with an acrophase located at 20.16 HALO ± 0.16 h (the middle of the active span) (p<0.001), whereas Cl(T) and Vd showed a significant circadian rhythm with an acrophase located respectively at 7.86 HALO ± 0.57 h and 6.13 HALO ± 0.07 h (the middle of the rest span) (p<0.001). The large circadian variation of VPA pharmacokinetic processes might be involved in the mechanisms of circadian rhythm in murine toxicity since the optimal tolerance corresponded to the time which induces lowest C(max) and AUC values.

Impact of fasting on the rhythmic expression of myogenic and metabolic factors in skeletal muscle of adult mice

Circadian rhythms and metabolism are tightly integrated, and rhythmic expression of metabolic factors is common in homeostatic processes. We measured the temporal changes in the expression of myogenic regulatory factors and expression and activity level of molecules involved in protein metabolism in skeletal muscles and livers in mice and examined the impact of fasting. Tissues were collected over 24 h (at zeitgeber times ZT1, ZT5, ZT9, ZT13, ZT17, ZT21, and ZT1 the following day) from adult male C57Bl/6J mice that had been either freely fed or fasted for 24 h. In skeletal muscle, there was a robust rise in the mRNA expression of the myogenic regulatory factors MyoD and myogenin during dark hours which was strongly suppressed by fasting. Circadian pattern was observed for mRNA of MuRF1, Akt1, and ribosomal protein S6 in muscles in fed and fasted mice and for Fbxo32 in fed mice. Activity (phosphorylation) levels of Akt(Ser473) displayed temporal regulation in fasted (but not fed) mice and were high at ZT9. Fasting caused significant reductions in phosphorylation for both Akt and S6 in muscles, indicative of inactivation. Hepatic phosphorylated Akt(Ser473) and S6(Ser235/236) proteins did not exhibit daily rhythms. Fasting significantly reduced hepatic Akt(473) phosphorylation compared with fed levels, although (unlike in muscle) it did not affect S6(Ser235/236) phosphorylation. This in vivo circadian study addresses for the first time the signaling activities of key molecules related to protein turnover and their possible cross-regulation of expression of genes related to protein degradation.

Shift work or food intake during the rest phase promotes metabolic disruption and desynchrony of liver genes in male rats

In the liver, clock genes are proposed to drive metabolic rhythms. These gene rhythms are driven by the suprachiasmatic nucleus (SCN) mainly by food intake and via autonomic and hormonal pathways. Forced activity during the normal rest phase, induces also food intake, thus neglecting the signals of the SCN, leading to conflicting time signals to target tissues of the SCN. The present study explored in a rodent model of night-work the influence of food during the normal sleep period on the synchrony of gene expression between clock genes and metabolic genes in the liver. Male Wistar rats were exposed to forced activity for 8 h either during the rest phase (day) or during the active phase (night) by using a slow rotating wheel. In this shift work model food intake shifts spontaneously to the forced activity period, therefore the influence of food alone without induced activity was tested in other groups of animals that were fed ad libitum, or fed during their rest or active phase. Rats forced to be active and/or eating during their rest phase, inverted their daily peak of Per1, Bmal1 and Clock and lost the rhythm of Per2 in the liver, moreover NAMPT and metabolic genes such as Pparα lost their rhythm and thus their synchrony with clock genes. We conclude that shift work or food intake in the rest phase leads to desynchronization within the liver, characterized by misaligned temporal patterns of clock genes and metabolic genes. This may be the cause of the development of the metabolic syndrome and obesity in individuals engaged in shift work.

Timed high-fat diet in the evening affects the hepatic circadian clock and PPARα-mediated lipogenic gene expressions in mice

A long-term high-fat diet may result in a fatty liver. However, whether or not high-fat diets affect the hepatic circadian clock is controversial. The objective of this study is to investigate the effects of timed high-fat diet on the hepatic circadian clock and clock-controlled peroxisome proliferator-activated receptor (PPAR) α-mediated lipogenic gene expressions. Mice were orally administered high-fat milk in the evening for 4 weeks. The results showed that some hepatic clock genes, such as Clock, brain-muscle-Arnt-like 1 (Bmal1), Period 2 (Per2), and Cryptochrome 2 (Cry2) exhibited obvious changes in rhythms and/or amplitudes. Alterations in the expression of clock genes, in turn, further altered the circadian rhythm of PPARα expression. Among the PPARα target genes, cholesterol 7α-hydroxylase (CYP7A1), 3-hydroxy-3-methylglutaryl-coenzyme A reductase, low-density lipoprotein receptor, lipoprotein lipase, and diacylglycerol acyltransferase (DGAT) showed marked changes in rhythms and/or amplitudes. In particular, significant changes in the expressions of DGAT and CYP7A1 were observed. The effects of a high-fat diet on the expression of lipogenic genes in the liver were accompanied by increased hepatic cholesterol and triglyceride levels. These results suggest that timed high-fat diets at night could change the hepatic circadian expressions of clock genes Clock, Bmal1, Per2, and Cry2 and subsequently alter the circadian expression of PPARα-mediated lipogenic genes, resulting in hepatic lipid accumulation.

An association between clock genes and clock-controlled cell cycle genes in murine colorectal tumors

Disruption of circadian machinery appears to be associated with the acceleration of tumor development. To evaluate the function of the circadian clock during neoplastic transformation, the daily profiles of the core clock genes Per1, Per2, Rev-Erbα and Bmal1, the clock-controlled gene Dbp and the clock-controlled cell cycle genes Wee1, c-Myc and p21 were detected by real-time RT-PCR in chemically induced primary colorectal tumors, the surrounding normal tissue and in the liver. The circadian rhythmicity of Per1, Per2, Rev-Erbα and Dbp was significantly reduced in tumor compared with healthy colon and the rhythmicity of Bmal1 was completely abolished. Interestingly, the circadian expression of Per1, Per2, Rev-Erbα and Dbp persisted in the colonic tissue surrounding the tumor but the rhythmic expression of Bmal1 was also abolished. Daily profiles of Wee1, c-Myc and p21 did not exhibit any rhythmicity either in tumors or in the colon of healthy animals. The absence of diurnal rhythmicity of cell cycle genes was partially associated with ageing, because young healthy mice showed rhythmicity in the core clock genes as well as in the Wee1 and p21. In the liver of tumor-bearing mice the clock gene rhythms were temporally shifted. The data suggest that the circadian regulation is distorted in colonic neoplastic tissue and that the gene-specific disruption may be also observed in the non-neoplastic tissues. These findings reinforce the role of peripheral circadian clockwork disruption for carcinogenesis and tumor progression.

Altered feeding differentially regulates circadian rhythms and energy metabolism in liver and muscle of rats

Energy metabolism follows a diurnal pattern responding to the light/dark cycle and food availability. This study investigated the impact of restricting feeding to the daylight hours and feeding a high fat diet on circadian clock (bmal1, dbp, tef and e4bp4) and metabolic (pepck, fas, ucp3, pdk4) gene expression and markers of energy metabolism in muscle and liver of rats. The results show that in chow-fed rats switched to daylight feeding, the peak diurnal expression of genes in liver was shifted by 6-12h while expression of these genes in muscle remained in a similar phase to rats feeding ad libitum. High fat feeding during the daylight hours had limited effect on clock gene expression in liver or muscle but shifted the peak expression of metabolic genes (pepck, fas) in liver by 6-12h. The differential effects of daylight feeding on gene and protein expression in muscle and liver were accompanied by an 8% reduction in whole body energy expenditure, a 20-30% increased glycogen content during the light phase in muscle of day-fed rats and increased adipose tissue deposition per gram food consumed. These data demonstrate that a mismatch of feeding and light/dark cycle disrupts tissue metabolism in muscle with significant consequences for whole body energy homeostasis.

Rhythmic nucleotide synthesis in the liver: temporal segregation of metabolites

The synthesis of nucleotides in the body is centrally controlled by the liver, via salvage or de novo synthesis. We reveal a pervasive circadian influence on hepatic nucleotide metabolism, from rhythmic gene expression of rate-limiting enzymes to oscillating nucleotide metabolome in wild-type (WT) mice. Genetic disruption of the hepatic clock leads to aberrant expression of these enzymes, together with anomalous nucleotide rhythms, such as constant low levels of ATP with an excess in uric acid, the degradation product of purines. These results clearly demonstrate that the hepatic circadian clock orchestrates nucleotide synthesis and degradation. This circadian metabolome timetable, obtained using state-of-the-art capillary electrophoresis time-of-flight mass spectrometry, will guide further investigations in nucleotide metabolism-related disorders.

The circadian clock component PERIOD2: from molecular to cerebral functions

The circadian clock is based on a molecular oscillator, which simulates the external day within nearly all of a body's cells. This "internalized" day then defines activity and rest phases for the cells and the organism by generating precise rhythms in the metabolism, physiology, and behavior. In its perfect state, this timing system allows for the synchronization of an organism to its environment and this may optimize energy handling and responses to daily recurring challenges. However, nowadays, we believe that desynchronization of an organism due to its lifestyle or problems with its circadian clock not only causes discomfort but also may aggravate conditions such as depression, metabolic syndrome, addiction, or cancer. In this review, we focus on one simple cogwheel of the mammalian circadian clock, the PERIOD2 (PER2) protein. Originally identified as an integral part of the molecular mechanism that yields overt rhythms of about 24h, more recently multiple other functions have been identified. In essence, the PER proteins, in addition to their important function within the molecular oscillator, can be seen not only as integrators on the input side of the circadian clock but also as mediators of clock output. This diversity in their function is possible, because the PER proteins can interact with a multitude of other proteins transferring oscillator timing information to the latter. In this fashion, the circadian clock synchronizes many rhythmic processes.