High-fat diet-induced hyperinsulinemia and tissue-specific insulin resistance in Cry-deficient mice

Circadian influences on feeding behavior

Feeding, like many other biological functions, displays a daily rhythm. This daily rhythmicity is controlled by the circadian timing system of which the central master clock is located in the hypothalamic suprachiasmatic nucleus (SCN). Other brain areas and tissues throughout the body also display rhythmic functions and contain the molecular clock mechanism known as peripheral oscillators. To generate the daily feeding rhythm, the SCN signals to different hypothalamic areas with the lateral hypothalamus, paraventricular nucleus and arcuate nucleus being the most prominent. With respect to the rewarding aspects of feeding behavior, the dopaminergic system is also under circadian influence. However the SCN projects only indirectly to the different reward regions, such as the ventral tegmental area where dopamine neurons are located. In addition, high palatable, high caloric diets have the potential to disturb the normal daily rhythms of physiology and have been shown to alter for example meal patterns. Around a meal several hormones and peptides are released that are also under circadian influence. For example, the release of postprandial insulin and glucagon-like peptide following a meal depend on the time of the day. Finally, we review the effect of deletion of different clock genes on feeding behavior. The most prominent effect on feeding behavior has been observed in Clock mutants, whereas deletion of Bmal1 and Per1/2 only disrupts the day-night rhythm, but not overall intake. Data presented here focus on the rodent literature as only limited data are available on the mechanisms underlying daily rhythms in human eating behavior.

Effect of circadian clock disruption on type 2 diabetes

Introduction

Type 2 diabetes (T2D) is the predominant form of diabetes mellitus and is among the leading causes of death with an increasing prevalence worldwide. However, the pathological mechanism underlying T2D remains complex and unclear. An increasing number of studies have suggested an association between circadian clock disruption and high T2D prevalence.

Method

This review explores the physiological and genetic evidence underlying T2D symptoms associated with circadian clock disturbances, including insulin secretion and glucose metabolism.

Results and Discussion

Notably, circadian clock disruption reduces insulin secretion and insulin sensitivity and negatively affects glucose homeostasis. The circadian clock regulates the hypothalamic-pituitary-adrenal axis, an important factor that regulates glucose metabolism and influences T2D progression. Therefore, circadian clock regulation is an attractive, novel therapeutic approach for T2D, and various circadian clock stabilizers play therapeutic roles in T2D. Lastly, this review suggests novel therapeutic and preventive approaches using circadian clock regulators for T2D.

Eating Around the Clock: Circadian Rhythms of Eating and Metabolism

The time of day that we eat is increasingly recognized as contributing as importantly to overall health as the amount or quality of the food we eat. The endogenous circadian clock has evolved to promote intake at optimal times when an organism is intended to be awake and active, but electric lights and abundant food allow eating around the clock with deleterious health outcomes. In this review, we highlight literature pertaining to the effects of food timing on health, beginning with animal models and then translation into human experiments. We emphasize the pitfalls and opportunities that technological advances bring in bettering understanding of eating behaviors and their association with health and disease. There is great promise for restricting the timing of food intake both in clinical interventions and in public health campaigns for improving health via nonpharmacological therapies.

Histone methylation: at the crossroad between circadian rhythms in transcription and metabolism

Circadian rhythms, essential 24-hour cycles guiding biological functions, synchronize organisms with daily environmental changes. These rhythms, which are evolutionarily conserved, govern key processes like feeding, sleep, metabolism, body temperature, and endocrine secretion. The central clock, located in the suprachiasmatic nucleus (SCN), orchestrates a hierarchical network, synchronizing subsidiary peripheral clocks. At the cellular level, circadian expression involves transcription factors and epigenetic remodelers, with environmental signals contributing flexibility. Circadian disruption links to diverse diseases, emphasizing the urgency to comprehend the underlying mechanisms. This review explores the communication between the environment and chromatin, focusing on histone post-translational modifications. Special attention is given to the significance of histone methylation in circadian rhythms and metabolic control, highlighting its potential role as a crucial link between metabolism and circadian rhythms. Understanding these molecular intricacies holds promise for preventing and treating complex diseases associated with circadian disruption.

Meal timing and its role in obesity and associated diseases

Meal timing emerges as a crucial factor influencing metabolic health that can be explained by the tight interaction between the endogenous circadian clock and metabolic homeostasis. Mistimed food intake, such as delayed or nighttime consumption, leads to desynchronization of the internal circadian clock and is associated with an increased risk for obesity and associated metabolic disturbances such as type 2 diabetes and cardiovascular diseases. Conversely, meal timing aligned with cellular rhythms can optimize the performance of tissues and organs. In this review, we provide an overview of the metabolic effects of meal timing and discuss the underlying mechanisms. Additionally, we explore factors influencing meal timing, including internal determinants such as chronotype and genetics, as well as external influences like social factors, cultural aspects, and work schedules. This review could contribute to defining meal-timing-based recommendations for public health initiatives and developing guidelines for effective lifestyle modifications targeting the prevention and treatment of obesity and associated metabolic diseases. Furthermore, it sheds light on crucial factors that must be considered in the design of future food timing intervention trials.

Long non-coding RNA LncCplx2 regulates glucose homeostasis and pancreatic β cell function

OBJECTIVE

Numerous studies have highlighted the role of clock genes in diabetes disease and pancreatic β cell functions. However, whether rhythmic long non-coding RNAs involve in this process is unknown.

METHODS

RNA-seq and 3' rapid amplification of cDNA ends (RACE)-PCR were used to identify the rat LncCplx2 in pancreatic β cells. The subcellular analysis with qRT-PCR and RNA-Scope were used to assess the localization of LncCplx2. The effects of LncCplx2 overexpression or knockout (KO) on the regulation of pancreatic β cell functions were assessed in vitro and in vivo. RNA-seq, immunoblotting (IB), Immunoprecipitation (IP), RNA pull-down, and chromatin immunoprecipitation (ChIP)-PCR assays were employed to explore the regulatory mechanisms through LncRNA-protein interaction. Metabolism cage was used to measure the circadian behaviors.

RESULTS

We first demonstrate that LncCplx2 is a conserved nuclear long non-coding RNA and enriched in pancreatic islets, which is driven by core clock transcription factor BMAL1. LncCplx2 is downregulated in the diabetic islets and repressed by high glucose, which regulates the insulin secretion in vitro and ex vivo. Furthermore, LncCplx2 KO mice exhibit diabetic phenotypes, such as high blood glucose and impaired glucose tolerance. Notably, LncCplx2 deficiency has significant effects on circadian behavior, including prolonged period duration, decreased locomotor activity, and reduced metabolic rates. Mechanistically, LncCplx2 recruits EZH2, a core subunit of polycomb repression complex 2 (PRC2), to the promoter of target genes, thereby silencing circadian gene expression, which leads to phase shifts and amplitude changes in insulin secretion and cell cycle genes.

CONCLUSIONS

Our results propose LncCplx2 as an unanticipated transcriptional regulator in a circadian system and suggest a more integral mechanism for the coordination of circadian rhythms and glucose homeostasis.

White adipose tissue: Distribution, molecular insights of impaired expandability, and its implication in fatty liver disease

We are far behind the 2025 World Health Organization (WHO) goal of a zero increase in obesity. Close to 360 million people in Latin America and the Caribbean are overweight, with the highest rates observed in the Bahamas, Mexico, and Chile. To achieve relevant progress against the obesity epidemic, scientific research is essential to establish uniform practices in the study of obesity pathophysiology (using pre-clinical and clinical models) that ensure accuracy, reproducibility, and transcendent outcomes. The present review focuses on relevant aspects of white adipose tissue (WAT) expansion, underlying mechanisms of inefficient expandability, and its repercussion in ectopic lipid accumulation in the liver during nutritional abundance. In addition, we highlight the potential role of disrupted circadian rhythm in WAT metabolism. Since genetic factors also play a key role in determining an individual's predisposition to weight gain, we describe the most relevant genes associated with obesity in the Mexican population, underlining that most of them are related to appetite control.

Transcription Repression of CRY2 via PER2 Interaction Promotes Adipogenesis

The circadian clock is driven by a transcriptional-translational feedback loop, and cryptochrome 2 (CRY2) represses CLOCK/BMAL1-induced transcription activation. Despite the established role of clock in adipogenic regulation, whether the CRY2 repressor activity functions in adipocyte biology remains unclear. Here we identify a critical cysteine residue of CRY2 that mediates interaction with Period 2 (PER2). We further demonstrate that this mechanism is required for repressing circadian clock-controlled Wnt signaling to promote adipogenesis. CRY2 protein is enriched in white adipose depots and robustly induced by adipogenic differentiation. Via site-directed mutagenesis, we identified that a conserved CRY2 cysteine at 432 within the loop interfacing with PER2 mediates heterodimer complex formation that confers transcription repression. C432 mutation disrupted PER2 association without affecting BMAL1 binding, leading to loss of repression of clock transcription activation. In preadipocytes, whereas CRY2 enhanced adipocyte differentiation, the repression-defective C432 mutant suppressed this process. Furthermore, silencing of CRY2 attenuated, while stabilization of CRY2 by KL001 markedly augmented adipocyte maturation. Mechanistically, we show that transcriptional repression of Wnt pathway components underlies CRY2 modulation of adipogenesis. Collectively, our findings elucidate a CRY2-mediated repression mechanism that promotes adipocyte development, and implicate its potential as a clock intervention target for obesity.

Rescue of Comorbid Behavioral and Metabolic Phenotypes of Arrhythmic Mice by Restoring Circadian Cry1/2 Expression in the Suprachiasmatic Nucleus

Background

Psychiatric and metabolic disorders occur disproportionately often comorbidly, which poses particular hurdles for patients and therapists. However, the mechanisms that promote such comorbidities are largely unknown and therefore cannot yet be therapeutically targeted for the simultaneous treatment of both conditions. Because circadian clocks regulate most physiological processes and their disruption is a risk factor for both psychiatric and metabolic disorders, they may be considered as a potential mechanism for the development of comorbidities and a therapeutic target. In the current study, we investigated the latter assumption in Cry1/2-/- mice, which exhibit substantially disrupted endogenous circadian rhythms and marked metabolic and behavioral deficits.

Methods

By targeted virus-induced restoration of circadian rhythms in their suprachiasmatic nucleus, we can restore behavioral as well as several metabolic processes of these animals to near-normal circadian rhythmicity.

Results

Importantly, by rescuing suprachiasmatic nucleus rhythms, several of their anxiety-like behavioral as well as diabetes- and energy homeostasis-related deficits were significantly improved. Interestingly, however, this did not affect all deficits typical of Cry1/2-/- mice; for example, restlessness and body weight remained unaffected.

Conclusions

Taken together, the results of this study demonstrate, on the one hand, that restoration of disturbed circadian rhythms can be used to simultaneously treat psychiatric and metabolic deficits. On the other hand, the results also allow us to distinguish processes that depend more on local canonical clocks from those that depend more on suprachiasmatic nucleus rhythms.

Age-Related Changes in Circadian Rhythm and Association with Nutrition

PURPOSE OF REVIEW

Considering the increase in life expectancy, there is a time-related decline in biological functions. Age-related changes are also observed in the circadian clock which directly leads to appropriate rhythms in the endocrine and metabolic pathways required for organism homeostasis. Circadian rhythms are affected by the sleep/wake cycle, environmental changes, and nutrition. The aim of this review is to show the relationship between age-related changes in circadian rhythms of physiological and molecular processes and nutritional differences in the elderly.

RECENT FINDINGS

Nutrition is an environmental factor that is particularly effective on peripheral clocks. Age-related physiological changes have an impact on nutrient intake and circadian processes. Considering the known effects of amino acid and energy intakes on peripheral and circadian clocks, it is thought that the change in circadian clocks in aging may occur due to anorexia due to physiological changes.

Research progresses on the effects of heavy metals on the circadian clock system

Environmental pollution with heavy metals is widespread, thus increasing attention has been paid to their toxic effects. Recent studies have suggested that heavy metals may influence the expression of circadian clock genes. Almost all organs and tissues exhibit circadian rhythms. The normal circadian rhythm of an organism is maintained by the central and peripheral circadian clock. Thus, circadian rhythm disorders perturb normal physiological processes. Here, we review the effects of heavy metals, including manganese, copper, cadmium, and lead, on four core circadian clock genes, i.e., ARNTL, CLOCK, PER, and CRY genes.

The interaction between CRY1 Polymorphism and Alternative Healthy Eating Index (AHEI) on cardiovascular risk factors in overweight women and women with obesity: a cross-sectional study

BACKGROUND

According to some studies, diet can be interaction with CRY1 polymorphism and may be related to obesity and the risk of cardiovascular diseases (CVD). So, this study examined the interaction between CRY1 polymorphism and AHEI on cardiovascular risk factors in overweight women and women with obesity.

METHODS

This cross-sectional study was performed on 377 Iranian women with overweight and obesity aged 18-48(BMI ≥ 25 kg/m2). Dietary intake was evaluated by the use of a food frequency questionnaire (FFQ) with 147 items. The AHEI was calculated based on previous studies. Anthropometric and biochemical measurements were assessed and the bioelectrical impedance analysis method was used for body analysis. The rs2287161 was genotyped by the restriction fragment length polymorphism (PCR-RFLP) method. Objects were divided into three groups based on rs2287161 genotypes.

RESULTS

Our findings determined that the prevalence of the C allele was 51.9% and the G allele was 48.0%. The mean age and BMI were 36.6 ± 9.1years and 31 ± 4 kg/m2 respectively. After controlling for confounders (BMI, age, total energy intake, and physical activity), this study demonstrated that there was a significant interaction between CC genotype and adherence to AHEI on odds of hyper LDL (OR = 1.94, 95% CI = 1.24-3.05, P for interaction = 0.004), hypertension (OR = 1.80, 95% CI = 1.11-2.93, P for interaction = 0.01) and hyperglycemia (OR = 1.56, 95% CI = 0.98-2.47, P for interaction = 0.05).

CONCLUSIONS

This study indicated that adherence to AHEI can reduce the odds of hyper LDL, hypertension, and hyperglycemia in the CC genotype of rs2287161.

Circadian Dysfunction in Adipose Tissue: Chronotherapy in Metabolic Diseases

Essential for survival and reproduction, the circadian timing system (CTS) regulates adaptation to cyclical changes such as the light/dark cycle, temperature change, and food availability. The regulation of energy homeostasis possesses rhythmic properties that correspond to constantly fluctuating needs for energy production and consumption. Adipose tissue is mainly responsible for energy storage and, thus, operates as one of the principal components of energy homeostasis regulation. In accordance with its roles in energy homeostasis, alterations in adipose tissue's physiological processes are associated with numerous pathologies, such as obesity and type 2 diabetes. These alterations also include changes in circadian rhythm. In the current review, we aim to summarize the current knowledge regarding the circadian rhythmicity of adipogenesis, lipolysis, adipokine secretion, browning, and non-shivering thermogenesis in adipose tissue and to evaluate possible links between those alterations and metabolic diseases. Based on this evaluation, potential therapeutic approaches, as well as clock genes as potential therapeutic targets, are also discussed in the context of chronotherapy.

Circadian mechanisms in adipose tissue bioenergetics and plasticity

The circadian clock plays an essential role in coordinating feeding and metabolic rhythms with the light/dark cycle. Disruption of clocks is associated with increased adiposity and metabolic disorders, whereas aligning feeding time with cell-autonomous rhythms in metabolism improves health. Here, we provide a comprehensive overview of recent literature in adipose tissue biology as well as our understanding of molecular mechanisms underlying the circadian regulation of transcription, metabolism, and inflammation in adipose tissue. We highlight recent efforts to uncover the mechanistic links between clocks and adipocyte metabolism, as well as its application to dietary and behavioral interventions to improve health and mitigate obesity.

Transcription repression of Cry2 via Per2 interaction promotes adipogenesis

The circadian clock is driven by a transcriptional-translational feedback loop, and Cryptochrome 2 (Cry2) represses CLOCK/Bmal1-induced transcription activation. Despite the established role of clock in adipogenic regulation, whether the Cry2 repressor activity functions in adipocyte biology remains unclear. Here we identify a critical cysteine residue of Cry2 that mediates interaction with Per2, and demonstrate that this mechanism is required for clock transcriptional repression that inhibits Wnt signaling to promote adipogenesis. Cry2 protein is enriched in white adipose depots and was robustly induced by adipocyte differentiation. Via site-directed mutagenesis, we identified that a conserved Cry2 Cysteine at 432 within the loop interfacing with Per2 mediates heterodimer complex formation that confers transcription repression. C432 mutation disrupted Per2 association without affecting Bmal1 binding, leading to loss of repression of clock transcription activation. In preadipocytes, whereas Cry2 enhanced adipogenic differentiation, the repression-defective C432 mutant suppressed this process. Furthermore, silencing of Cry2 attenuated, while stabilization of Cry2 by KL001 markedly augmented adipocyte maturation. Mechanistically, we show that transcriptional repression of Wnt pathway components underlies Cry2 modulation of adipogenesis. Collectively, our findings elucidate a Cry2-mediated repression mechanism that promotes adipocyte development, and implicate its potential as a clock intervention target for obesity.

The Circadian Clock, Nutritional Signals and Reproduction: A Close Relationship

The circadian rhythm, which is necessary for reproduction, is controlled by clock genes. In the mouse uterus, the oscillation of the circadian clock gene has been observed. The transcription of the core clock gene period (Per) and cryptochrome (Cry) is activated by the heterodimer of the transcription factor circadian locomotor output cycles kaput (Clock) and brain and muscle Arnt-like protein-1 (Bmal1). By binding to E-box sequences in the promoters of Per1/2 and Cry1/2 genes, the CLOCK-BMAL1 heterodimer promotes the transcription of these genes. Per1/2 and Cry1/2 form a complex with the Clock/Bmal1 heterodimer and inactivate its transcriptional activities. Endometrial BMAL1 expression levels are lower in human recurrent-miscarriage sufferers. Additionally, it was shown that the presence of BMAL1-depleted decidual cells prevents trophoblast invasion, highlighting the importance of the endometrial clock throughout pregnancy. It is widely known that hormone synthesis is disturbed and sterility develops in Bmal1-deficient mice. Recently, we discovered that animals with uterus-specific Bmal1 loss also had poor placental development, and these mice also had intrauterine fetal death. Furthermore, it was shown that time-restricted feeding controlled the uterine clock's circadian rhythm. The uterine clock system may be a possibility for pregnancy complications, according to these results. We summarize the most recent research on the close connection between the circadian clock and reproduction in this review.

Aryl hydrocarbon receptor affects circadian-regulated lipolysis through an E-Box-dependent mechanism

An internal circadian clock regulates timing of systemic energy homeostasis. The central clock in the hypothalamic suprachiasmatic nucleus (SCN) directs local clocks in peripheral tissues such as liver, muscle, and adipose tissue to synchronize metabolism with food intake and rest/activity cycles. Aryl hydrocarbon receptor (AhR) interacts with the molecular circadian clockworks. Activation of AhR dampens rhythmic expression of core clock genes, which may lead to metabolic dysfunction. Given the importance of appropriately-timed adipose tissue function to regulation of energy homeostasis, this study focused on mechanisms by which AhR may influence clock-controlled adipose tissue activity. We hypothesized that AhR activation in adipose tissue would impair lipolysis by dampening adipose rhythms, leading to a decreased lipolysis rate during fasting, and subsequently, altered serum glucose concentrations. Levels of clock gene and lipolysis gene transcripts in mouse mesenchymal stem cells (BMSCs) differentiated into mature adipocytes were suppressed by the AhR agonist β-napthoflavone (BNF), in an AhR dependent manner. BNF altered rhythms of core clock gene and lipolysis gene transcripts in C57bl6/J mice. BNF reduced serum free fatty acids, glycerol and liver glycogen. Chromatin immunoprecipitation indicated that BNF increased binding of AhR to E-Box elements in clock gene and lipolysis gene promoters. These data establish a link between AhR activation and impaired lipolysis, specifically by altering adipose tissue rhythmicity. In response to the decreased available energy from impaired lipolysis, the body increases glycogenolysis, thereby degrading more glycogen to provide necessary energy.

Obstructive Sleep Apnea, Circadian Clock Disruption, and Metabolic Consequences

Obstructive sleep apnea (OSA) is a chronic disorder characterized by recurrent episodes of apnea and hypopnea during sleep. It is associated with various cardiovascular and metabolic complications, including type 2 diabetes mellitus (T2DM) and obesity. Many pathways can be responsible for T2DM development in OSA patients, e.g., those related to HIF-1 and SIRT1 expression. Moreover, epigenetic mechanisms, such as miRNA181a or miRNA199, are postulated to play a pivotal role in this link. It has been proven that OSA increases the occurrence of circadian clock disruption, which is also a risk factor for metabolic disease development. Circadian clock disruption impairs the metabolism of glucose, lipids, and the secretion of bile acids. Therefore, OSA-induced circadian clock disruption may be a potential, complex, underlying pathway involved in developing and exacerbating metabolic diseases among OSA patients. The current paper summarizes the available information pertaining to the relationship between OSA and circadian clock disruption in the context of potential mechanisms leading to metabolic disorders.

Complex physiology and clinical implications of time-restricted eating

Time-restricted eating (TRE) is a dietary intervention that limits food consumption to a specific time window each day. The effect of TRE on body weight and physiological functions has been extensively studied in rodent models, which have shown considerable therapeutic effects of TRE and important interactions among time of eating, circadian biology, and metabolic homeostasis. In contrast, it is difficult to make firm conclusions regarding the effect of TRE in people because of the heterogeneity in results, TRE regimens, and study populations. In this review, we 1) provide a background of the history of meal consumption in people and the normal physiology of eating and fasting; 2) discuss the interaction between circadian molecular metabolism and TRE; 3) integrate the results of preclinical and clinical studies that evaluated the effects of TRE on body weight and physiological functions; 4) summarize other time-related dietary interventions that have been studied in people; and 4) identify current gaps in knowledge and provide a framework for future research directions.

Circadian Regulation of Hormonal Timing and the Pathophysiology of Circadian Dysregulation

Circadian rhythms are endogenously generated, daily patterns of behavior and physiology that are essential for optimal health and disease prevention. Disruptions to circadian timing are associated with a host of maladies, including metabolic disease and obesity, diabetes, heart disease, cancer, and mental health disturbances. The circadian timing system is hierarchically organized, with a master circadian clock located in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus and subordinate clocks throughout the CNS and periphery. The SCN receives light information via a direct retinal pathway, synchronizing the master clock to environmental time. At the cellular level, circadian rhythms are ubiquitous, with rhythms generated by interlocking, autoregulatory transcription-translation feedback loops. At the level of the SCN, tight cellular coupling maintains rhythms even in the absence of environmental input. The SCN, in turn, communicates timing information via the autonomic nervous system and hormonal signaling. This signaling couples individual cellular oscillators at the tissue level in extra-SCN brain loci and the periphery and synchronizes subordinate clocks to external time. In the modern world, circadian disruption is widespread due to limited exposure to sunlight during the day, exposure to artificial light at night, and widespread use of light-emitting electronic devices, likely contributing to an increase in the prevalence, and the progression, of a host of disease states. The present overview focuses on the circadian control of endocrine secretions, the significance of rhythms within key endocrine axes for typical, homeostatic functioning, and implications for health and disease when dysregulated. © 2022 American Physiological Society. Compr Physiol 12: 1-30, 2022.

Circadian rhythms and pancreas physiology: A review

Type 2 diabetes mellitus, obesity and metabolic syndrome are becoming more prevalent worldwide and will present an increasingly challenging burden on healthcare systems. These interlinked metabolic abnormalities predispose affected individuals to a plethora of complications and comorbidities. Furthermore, diabetes is estimated by the World Health Organization to have caused 1.5 million deaths in 2019, with this figure projected to rise in coming years. This highlights the need for further research into the management of metabolic diseases and their complications. Studies on circadian rhythms, referring to physiological and behavioral changes which repeat approximately every 24 hours, may provide important insight into managing metabolic disease. Epidemiological studies show that populations who are at risk of circadian disruption such as night shift workers and regular long-haul flyers are also at an elevated risk of metabolic abnormalities such as insulin resistance and obesity. Aberrant expression of circadian genes appears to contribute to the dysregulation of metabolic functions such as insulin secretion, glucose homeostasis and energy expenditure. The potential clinical implications of these findings have been highlighted in animal studies and pilot studies in humans giving rise to the development of circadian interventions strategies including chronotherapy (time-specific therapy), time-restricted feeding, and circadian molecule stabilizers/analogues. Research into these areas will provide insights into the future of circadian medicine in metabolic diseases. In this review, we discuss the physiology of metabolism and the role of circadian timing in regulating these metabolic functions. Also, we review the clinical aspects of circadian physiology and the impact that ongoing and future research may have on the management of metabolic disease.

Circadian regulation of digestive and metabolic tissues

The circadian clock is a self-sustained molecular timekeeper that drives 24-h (circadian) rhythms in animals. The clock governs important aspects of behavior and physiology including wake/sleep activity cycles that regulate the activity of metabolic and digestive systems. Light/dark cycles (photoperiod) and cycles in the time of feeding synchronize the circadian clock to the surrounding environment, providing an anticipatory benefit that promotes digestive health. The availability of animal models targeting the genetic components of the circadian clock has made it possible to investigate the circadian clock's role in cellular functions. Circadian clock genes have been shown to regulate the physiological function of hepatocytes, gastrointestinal cells, and adipocytes; disruption of the circadian clock leads to the exacerbation of liver diseases and liver cancer, inflammatory bowel disease and colorectal cancer, and obesity. Previous findings provide strong evidence that the circadian clock plays an integral role in digestive/metabolic disease pathogenesis, hence, the circadian clock is a necessary component in metabolic and digestive health and homeostasis. Circadian rhythms and circadian clock function provide an opportunity to improve the prevention and treatment of digestive and metabolic diseases by aligning digestive system tissue with the 24-h day.

Effects of gestational exposure to bisphenol A on the hepatic transcriptome and lipidome of rat dams: Intergenerational comparison of effects in the offspring

Our previous studies demonstrated that prenatal bisphenol A (BPA) exposure affected the hepatic transcriptome and lipidome in rat offspring in a sex- and age-dependent manner. In this study, we investigated the effects of gestational exposure to BPA on the rat dams, after weaning period, and compared them with those of their offspring. Our results showed alterations in hepatic transcriptome related to insulin signaling, circadian rhythm, and infectious disease pathways in BPA-treated dams even 4 weeks after the exposure, whereas slight modifications on the lipid profile were found. Alterations in lipid and transcriptome profiles were more prominent in the prenatally BPA-exposed offspring at postnatal day (PND) 1 and 21 than those in the dams, suggesting that in utero exposure to BPA is more serious than exposure in the adulthood. Cryptochrome-1 (Cry1) and peroxisome proliferator-activated receptor delta (Ppard) were commonly altered in both dams and offspring. Nevertheless, the results of DIABLO (Data Integration Analysis for Biomarker discovery using Latent cOmponents), showed that multi-omics data successfully distinguished the exposed dams from the corresponding controls and their offspring with a high level of accuracy. The accuracy rates in BPA50 models (including control and 50 μg BPA/kg bw/day exposed groups) were smaller than those in BPA5000 models (control and 5000 μg BPA/kg bw/day exposed groups), suggesting dose-dependent severity in BPA effects. Palmitic acid and genes related to circadian rhythm, insulin responses, and lipid metabolism (e.g., 1-acylglycerol-3-phosphate O-acyltransferase 2 (Agpat2), B-cell CLL/lymphoma 10 (Bcl10), Cry1, Harvey rat sarcoma virus oncogene (Hras), and NLR family member X1 (Nlrx1)) were identified through DIABLO models as novel biomarkers of effects of BPA across two generations.

Sleep Loss Causes Dysfunction in Murine Extraorbital Lacrimal Glands

Purpose

Sleep loss markedly affects the structure and function of the lacrimal gland and may cause ocular surface disease as a common public health problem. This study aims to investigate the circadian disturbance caused by sleep loss leading to dysfunction of extraorbital lacrimal glands (ELGs).

Methods

A mouse sleep deprivation (SD) model for sleep loss studies was built in C57BL/6J male mice. After four weeks, the ELGs were collected at three-hour intervals during a 24-hour period. The Jonckheere-Terpstra-Kendall algorithm was used to determine the composition, phase, and rhythmicity of transcriptomic profiles in ELGs. Furthermore, we compared the non-sleep-deprived and SD-treated mouse ELG (i) reactive oxygen species (ROS) by fluorescein staining, (ii) DNA damage by immunostaining for γ-H2Ax, and (iii) circadian migration of immune cells by immunostaining for CD4, CD8, γδ-TCR, CD64, and CX3CR1. Finally, we also evaluated (i) the locomotor activity and core body temperature rhythm of mice and (ii) the mass, cell size, and tear secretion of the ELGs.

Results

SD dramatically altered the composition and phase-associated functional enrichment of the circadian transcriptome, immune cell trafficking, metabolism, cell differentiation, and neural secretory activities of mouse ELGs. Additionally, SD caused the ROS accumulation and consequent DNA damage in the ELGs, and the ELG dysfunction caused by SD was irreversible.

Conclusions

SD damages the structure, function, and diurnal oscillations of ELGs. These results highlight comprehensive characterization of insufficient sleep–affected ELG circadian transcriptome that may provide a new therapeutic approach to counteract the effects of SD on ELG function.

Time-of-day dependent effect of proanthocyanidins on adipose tissue metabolism in rats with diet-induced obesity

BACKGROUND

Grape-seed proanthocyanidin extract (GSPE) improve white adipose tissue (WAT) expansion during diet-induced obesity. However, because adipose metabolism is synchronized by circadian rhythms, it is plausible to speculate that the bioactivity of dietary proanthocyanidins could be influenced by the time-of-day in which they are consumed. Therefore, the aim of the present study was to determine the interaction between zeitgeber time (ZT) and GSPE consumption on the functionality of WAT in rats with diet-induced obesity.

METHODS

Male Wistar rats were fed a cafeteria diet for 9 weeks. After 5 weeks, the animals were supplemented with 25 mg GSPE/kg for 4 weeks at the beginning of the light/rest phase (ZT0) or of the dark/active phase (ZT12). Body fat content was determined by nuclear magnetic resonance and histological analyses were performed in the epididymal (EWAT) and inguinal (IWAT) fat depots to determine adipocyte size and number. In addition, the expression of genes related to adipose metabolism and circadian clock function were analyzed by qPCR.

RESULTS

GSPE consumption at ZT0 was associated with a potential antidiabetic effect without affecting adiposity and energy intake and downregulating the gene expression of inflammatory markers in EWAT. In contrast, GSPE consumption at ZT12 improved adipose tissue expansion decreasing adipocyte size in IWAT. In accordance with this adipogenic activity, the expression of genes involved in fatty acid metabolism were downregulated at ZT12 in IWAT. In turn, GSPE consumption at ZT12, but not at ZT0, repressed the expression of the clock gene Cry1 in IWAT.

CONCLUSIONS

The interaction between ZT and GSPE consumption influenced the metabolic response of WAT in a tissue-specific manner. Understanding the impact of circadian clock on adipose metabolism and how this is regulated by polyphenols will provide new insights for the management of obesity.

Bredemolic acid restores glucose utilization and attenuates oxidative stress in palmitic acid-induced insulin-resistant C2C12 cells

Objective. Due to insulin resistance and oxidative stress that are associated with type 2 diabetes mellitus (T2DM), T2DM has become a prevalent metabolic disorder that presents various side effects. However, alternative antidiabetic treatment has commonly been used in treating diabetes mellitus in diabetic patients. In our previous studies, bredemolic acid has been reported as an antidiabetic agent that improves glucose uptake, ameliorates insulin resistance, and oxidative stress in the liver, heart, kidney, and skeletal muscle of prediabetic rats. However, these effects have not been validated in vitro. Therefore, this study was aimed to investigate the effects of bredemolic acid on insulin-mediated glucose utilization, lipid peroxidation, and the total antioxidant capacity (TOAC) in palmitic acid-induced insulin-resistant C2C12 skeletal muscle cells in vitro. Methods. Insulin resistance was induced in the skeletal muscle cells after 4 h of exposure to palmitic acid (0.5 mmol/l). Different cell groups were incubated in culture media DMEM supplemented with fetal calf serum (10%), penicillin/streptomycin (1%), and L-glutamine (1%) and then treated with either insulin (4 µg/ml) or bredemolic acid (12.5 mmol/l) or with both. Thereafter, the cells were seeded in 24- or 96-well plates for determination of the cell viability, glucose utilization, glycogen formation, and antioxidant capacity. Results. The results showed that bredemolic acid significantly improved TOAC and promoted glucose utilization via attenuation of lipid peroxidation and increased glycogen formation in the insulin-resistant cells, respectively. Conclusion. This study showed that bredemolic acid restored the insulin resistance through improved glucose utilization, glycogen formation, and TOAC in the skeletal muscle cells.

Evaluation of the effects of bredemolic acid on selected markers of glucose homeostasis in diet-induced prediabetic rats

CONTEXT

Pentacyclic triterpenes (such as maslinic acid) are natural anti-diabetic agents that ameliorate glucose metabolism in diet-induced prediabetes. However, the effects of bredemolic acid (BA), maslinic acid isomer, is yet unknown in prediabetic (PD) conditions.

OBJECTIVES

To investigate the effects of BA on some glucose homeostasis parameters in high-fat high-carbohydrate (HFHC) diet-induced PD rats.

METHODS

Thirty-six (36) male rats (150-180 g) were divided into two groups, the normal diet (ND) non-prediabetic, NPD (n = 6) and the HFHC diet PD groups (n = 30). The PD animals were further sub-divided into five groups (n = 6) where they were treated with BA for 12 weeks while monitoring changes in blood glucose, caloric intake, and body weight.

RESULTS

Diet-induced prediabetes resulted in increased body weight, caloric intake, glycated haemoglobin, and glucose tolerance. BA treatment ameliorated glucose tolerance, lowered plasma insulin and increased expression of glucose transporter 4 (GLUT 4) in rats.

CONCLUSIONS

BA administration restored glucose homeostasis in diet-induced prediabetes regardless of diet intervention.

A little goes a long way: Neurobiological effects of low intensity rTMS and implications for mechanisms of rTMS

Repetitive transcranial magnetic stimulation (rTMS) is a widespread technique in neuroscience and medicine, however its mechanisms are not well known. In this review, we consider intensity as a key therapeutic parameter of rTMS, and review the studies that have examined the biological effects of rTMS using magnetic fields that are orders of magnitude lower that those currently used in the clinic. We discuss how extensive characterisation of "low intensity" rTMS has set the stage for translation of new rTMS parameters from a mechanistic evidence base, with potential for innovative and effective therapeutic applications. Low-intensity rTMS demonstrates neurobiological effects across healthy and disease models, which include depression, injury and regeneration, abnormal circuit organisation, tinnitus etc. Various short and long-term changes to metabolism, neurotransmitter release, functional connectivity, genetic changes, cell survival and behaviour have been investigated and we summarise these key changes and the possible mechanisms behind them. Mechanisms at genetic, molecular, cellular and system levels have been identified with evidence that low-intensity rTMS and potentially rTMS in general acts through several key pathways to induce changes in the brain with modulation of internal calcium signalling identified as a major mechanism. We discuss the role that preclinical models can play to inform current clinical research as well as uncover new pathways for investigation.

CRYPTOCHROMES promote daily protein homeostasis

The daily organisation of most mammalian cellular functions is attributed to circadian regulation of clock-controlled protein expression, driven by daily cycles of CRYPTOCHROME-dependent transcriptional feedback repression. To test this, we used quantitative mass spectrometry to compare wild-type and CRY-deficient fibroblasts under constant conditions. In CRY-deficient cells, we found that temporal variation in protein, phosphopeptide, and K+ abundance was at least as great as wild-type controls. Most strikingly, the extent of temporal variation within either genotype was much smaller than overall differences in proteome composition between WT and CRY-deficient cells. This proteome imbalance in CRY-deficient cells and tissues was associated with increased susceptibility to proteotoxic stress, which impairs circadian robustness, and may contribute to the wide-ranging phenotypes of CRY-deficient mice. Rather than generating large-scale daily variation in proteome composition, we suggest it is plausible that the various transcriptional and post-translational functions of CRY proteins ultimately act to maintain protein and osmotic homeostasis against daily perturbation.

Variants of the cry 1 gene may influence the effect of fat intake on resting metabolic rate in women with overweight of obesity: a cross-sectional study

BACKGROUND

Previous studies have shown that the minor allele (C allele) for Cry 1 rs2287161, may be associated with increased risk of cardiovascular diseases (CVDs). Low resting metabolic rate (RMR) caused by the diet has been shown to have, potentially, unfavorable effects on obesity. This study sought to investigate the interactions between the Cry 1 Gene and fat intake on RMR in women with overweight of obesity.

METHODS

This comparative cross-sectional study was conducted on 377 Iranian women with overweight of obesity. A food frequency questionnaire (FFQ), with 147 items, was used to assess dietary intake. Individuals were categorized into two groups based on the rs2287161 genotype. Body composition, dietary intake, and RMR were assessed for all participants.

RESULTS

There was a significant difference between genotypes for fasting blood sugar (FBS) (P = 0.04), fat free mass (FFM) (P = 0.0009), RMR per FFM (P = 0.05), RMR per body mass index (BMI) (P = 0.02), and RMR deviation (P = 0.01). Our findings also showed significant interactions between total fat and C allele carrier group on RMR per kg body weight, RMR per body surface area (BSA), RMR per FFM, and RMR deviation (P for interaction < 0.1), in addition to a significant interaction between CC + CG group genotype and polyunsaturated fatty acids (PUFA) intake on RMR per BMI (P for interaction =0.00) and RMR per kg (P for interaction = 0.02) and RMR per BSA (P = 0.07), compared to the GG group, after control for confounder factors.

CONCLUSION

These results highlight that dietary compositions, gene variants, and their interaction, should be acutely considered in lower RMR.

Interconnections between circadian clocks and metabolism

Circadian rhythms evolved through adaptation to daily light/dark changes in the environment; they are believed to be regulated by the core circadian clock interlocking feedback loop. Recent studies indicate that each core component executes general and specific functions in metabolism. Here, we review the current understanding of the role of these core circadian clock genes in the regulation of metabolism using various genetically modified animal models. Additionally, emerging evidence shows that exposure to environmental stimuli, such as artificial light, unbalanced diet, mistimed eating, and exercise, remodels the circadian physiological processes and causes metabolic disorders. This Review summarizes the reciprocal regulation between the circadian clock and metabolism, highlights remaining gaps in knowledge about the regulation of circadian rhythms and metabolism, and examines potential applications to human health and disease.

Circadian Clock and Liver Cancer

Simple Summary

The circadian coordination of metabolism is tightly regulated, and its alteration can trigger several diseases, including liver steatohepatitis and cancer. Many factors (such as diet and jet lag) shape both the liver molecular clock and the circadian transcription/translation of genes related to different metabolic pathways. Here, we summarize our current knowledge about the molecular mechanisms that control this circadian regulation of liver metabolism.

Abstract

Circadian clocks control several homeostatic processes in mammals through internal molecular mechanisms. Chronic perturbation of circadian rhythms is associated with metabolic diseases and increased cancer risk, including liver cancer. The hepatic physiology follows a daily rhythm, driven by clock genes that control the expression of several proteins involved in distinct metabolic pathways. Alteration of the liver clock results in metabolic disorders, such as non-alcoholic fatty liver diseases (NAFLD) and impaired glucose metabolism, that can trigger the activation of oncogenic pathways, inducing spontaneous hepatocarcinoma (HCC). In this review, we provide an overview of the role of the liver clock in the metabolic and oncogenic changes that lead to HCC and discuss new potentially useful targets for prevention and management of HCC.

Eat, sleep, repeat - endocrine regulation of behavioural circadian rhythms

The adaptation of organisms to a rhythmic environment is mediated by an internal timing system termed the circadian clock. In mammals, molecular clocks are found in all tissues and organs. This circadian clock network regulates the release of many hormones, which in turn influence some of the most vital behavioural functions. Sleep-wake cycles are under strict circadian control with strong influence of rhythmic hormones such as melatonin, cortisol and others. Food intake, in contrast, receives circadian modulation through hormones such as leptin, ghrelin, insulin and orexin. A third behavioural output covered in this review is mating and bonding behaviours, regulated through circadian rhythms in steroid hormones and oxytocin. Together, these data emphasize the pervasive influence of the circadian clock system on behavioural outputs and its mediation through endocrine networks.

Blockade of alcohol excessive and "relapse" drinking in male mice by pharmacological cryptochrome (CRY) activation

RATIONALE

Metabolic dysfunction, mood disorders, anxiety disorders, and substance abuse disorders are associated with disruptions in circadian rhythm and circadian clock gene machinery. While the effects of alcohol on several core components of the clock genes have been described in rodent models, pharmacological activation or inhibition of clock gene functions has not been studied on alcohol drinking behaviors.

OBJECTIVES

We investigated whether cryptochrome (CRY1/2) activator KL001 altered alcohol intake in mice in excessive and relapse-like alcohol drinking models.

METHODS

Mice, subjected to 3 weeks of chronic intermittent alcohol drinking (IAD) (two-bottle choice, 24-h access every other day) developed excessive alcohol intake and high preference. We evaluated the pharmacological effects of KL001 after either 1-day acute withdrawal from IAD or 1-week chronic withdrawal using the alcohol deprivation effect (ADE) model.

RESULTS

Single pretreatment with KL001 at 1-4 mg kg-1 reduced alcohol intake and preference after acute withdrawal in a dose-related manner. The effect of KL001 on reducing excessive alcohol consumption seems alcohol specific, as the compound does not alter sucrose (caloric reinforcer) or saccharin (noncaloric reinforcer) consumption in mice. Both single- and multiple-dosing regimens with an effective dose of KL001 (4 mg kg-1) prevented the ADE after chronic withdrawal, with no tolerance development after the multi-dosing regimen.

CONCLUSIONS

Pretreatment with KL001 (a CRY1/2 activator) reduces excessive and "relapse" alcohol drinking in mice. Our in vivo results with a CRY activator suggest a possible novel target for alcohol treatment intervention.

Time-restricted feeding regulates molecular mechanisms with involvement of circadian rhythm to prevent metabolic diseases

Lifestyle and genetic perturbation of circadian rhythm can trigger the incidence and severity of metabolic diseases. Time-restricted feeding (TRF) regulates the circadian rhythm of food intake that protects against metabolic disorders induced by adverse nutrient intake. TRF also executes host metabolism from nutrient availability to optimize nutrient utilization. Circadian clock and nutrient-sensing pathways coordinate to regulate metabolic health through the feeding/fasting cycle. Concurrently, TRF imposes diurnal rhythm in nutrient utilization, thereby preserving cellular homeostasis. However, modulation of daily feeding and fasting periods calibrates the circadian clock, which protects against the lethal effects of nutrient imbalance on metabolism. Therefore, TRF also improves and restores metabolic rhythms that ultimately lead to better fitness by reversing the alteration in genotype-specific gene expression. The aim of this review was to summarize that TRF is an emerging dietary approach that maintains robust circadian rhythms in support of a steady daily feeding and fasting cycle. TRF also encourages the coordination between circadian clock components and nutrient-sensing pathways via molecular effectors that exert a protective role in the prevention of metabolic diseases.

Discovery of : A Novel Potent and Peripherally Preferred Melatonin Receptor Agonist that Reduces Liver Triglycerides and Steatosis in Diet-Induced Obese Rats

The modulation of melatonin signaling in peripheral tissues holds promise for treating metabolic diseases like obesity, diabetes, and nonalcoholic steatohepatitis. Here, several benzimidazole derivatives have been identified as novel agonists of the melatonin receptors MT1 and MT2. The lead compounds 10b, 15a, and 19a demonstrated subnanomolar potency at MT1/MT2 receptors, high oral bioavailability in rodents, peripherally preferred exposure, and excellent selectivity in a broad panel of targets. Two-month oral administration of 10b in high-fat diet rats led to a reduction in body weight gain similar to dapagliflozin with superior results on hepatic steatosis and triglyceride levels. An early toxicological assessment indicated that 10b (also codified as ACH-000143) was devoid of hERG binding, genotoxicity, and behavioral alterations at doses up to 100 mg/kg p.o., supporting further investigation of this compound as a drug candidate.

Clocks, cancer, and chronochemotherapy

The circadian clock coordinates daily rhythmicity of biochemical, physiologic, and behavioral functions in humans. Gene expression, cell division, and DNA repair are modulated by the clock, which gives rise to the hypothesis that clock dysfunction may predispose individuals to cancer. Although the results of many epidemiologic and animal studies are consistent with there being a role for the clock in the genesis and progression of tumors, available data are insufficient to conclude that clock disruption is generally carcinogenic. Similarly, studies have suggested a circadian time-dependent efficacy of chemotherapy, but clinical trials of chronochemotherapy have not demonstrated improved outcomes compared with conventional regimens. Future hypothesis-driven and discovery-oriented research should focus on specific interactions between clock components and carcinogenic mechanisms to realize the full clinical potential of the relationship between clocks and cancer.

Cross-species physiological interactions of endocrine disrupting chemicals with the circadian clock

Endocrine disrupting chemicals (EDCs) are endocrine-active chemical pollutants that disrupt reproductive, neuroendocrine, cardiovascular and metabolic health across species. The circadian clock is a transcriptional oscillator responsible for entraining 24-hour rhythms of physiology, behavior and metabolism. Extensive bidirectional cross talk exists between circadian and endocrine systems and circadian rhythmicity is present at all levels of endocrine control, from synthesis and release of hormones, to sensitivity of target tissues to hormone action. In mammals, a range of hormones directly alter clock gene expression and circadian physiology via nuclear receptor (NR) binding and subsequent genomic action, modulating physiological processes such as nutrient and energy metabolism, stress response, reproductive physiology and circadian behavioral rhythms. The potential for EDCs to perturb circadian clocks or circadian-driven physiology is not well characterized. For this reason, we explore evidence for parallel endocrine and circadian disruption following EDC exposure across species. In the reviewed studies, EDCs dysregulated core clock and circadian rhythm network gene expression in brain and peripheral organs, and altered circadian reproductive, behavioral and metabolic rhythms. Circadian impacts occurred in parallel to endocrine and metabolic alterations such as impaired fertility and dysregulated metabolic and energetic homeostasis. Further research is warranted to understand the nature of interaction between circadian and endocrine systems in mediating physiological effects of EDC exposure at environmental levels.

Sleep Disturbance Induces Increased Cholesterol Level by NR1D1 Mediated CYP7A1 Inhibition

Disturbed sleep is closely associated with an increased risk of metabolic diseases. However, the underlying mechanisms of circadian clock genes linking sleep and lipid profile abnormalities have not been fully elucidated. This study aimed to explore the important role of the circadian clock in regulating impaired cholesterol metabolism at an early stage of sleep deprivation (SD). Sleep disturbance was conducted using an SD instrument. Our results showed that SD increased the serum cholesterol levels. Concentrations of serum leptin and resistin were much lower after SD, but other metabolic hormone concentrations (adiponectin, glucagon, insulin, thyroxine, norepinephrine, and epinephrine) were unchanged before and after SD. Warning signs of cardiovascular diseases [decreased high density lipoprotein (HDL)-cholesterol and increased corticosterone and 8-hydroxyguanosine levels] and hepatic cholestasis (elevated total bile acids and bilirubin levels) were observed after SD. Cholesterol accumulation was also observed in the liver after SD. The expression levels of HMGCR, the critical enzyme for cholesterol synthesis, remained unchanged in the liver. However, the expression levels of liver CYP7A1, the enzyme responsible for the conversion of cholesterol into bile acids, significantly reduced after SD. Furthermore, expression of NR1D1, a circadian oscillator and transcriptional regulator of CYP7A1, strikingly decreased after SD. Moreover, NR1D1 deficiency decreased liver CYP7A1 levels, and SD could exacerbate the reduction of CYP7A1 expression in NR1D1^-/- mouse livers. Additionally, NR1D1 deficiency could further increase serum cholesterol levels under SD. These results suggest that sleep disturbance can induce increased serum cholesterol levels and liver cholesterol accumulation by NR1D1 mediated CYP7A1 inhibition.

Mediators of Host–Microbe Circadian Rhythms in Immunity and Metabolism

Simple Summary

Circadian rhythms serve as the body’s internal metronome, driving responses to environmental cues over a 24-h period. Essential to nearly all life forms, the core circadian clock gene network drives physiological outputs associated with metabolic and immune responses. Modern-day disruptions to host circadian rhythms, such as shift work and jet lag, result in aberrant metabolic responses and development of complex diseases, including obesity and Type 2 Diabetes. These complex diseases are also impacted by interactions between gut microbes and the host immune system, driving a chronic low-grade inflammatory response. Gut microbes exhibit circadian dynamics that are closely tied to host circadian networks and disrupting microbial rhythmicity contributes to metabolic diseases. The underlying mediators that drive communication between host metabolism, the immune system, gut microbes, and circadian networks remain unknown, particularly in humans. Here, we explore the current state of knowledge regarding the transkingdom control of circadian networks and discuss gaps and challenges to overcome to push the field forward from the preclinical to clinical setting.

Abstract

Circadian rhythms are essential for nearly all life forms, mediated by a core molecular gene network that drives downstream molecular processes involved in immune function and metabolic regulation. These biological rhythms serve as the body’s metronome in response to the 24-h light:dark cycle and other timed stimuli. Disrupted circadian rhythms due to drastic lifestyle and environmental shifts appear to contribute to the pathogenesis of metabolic diseases, although the mechanisms remain elusive. Gut microbiota membership and function are also key mediators of metabolism and are highly sensitive to environmental perturbations. Recent evidence suggests rhythmicity of gut microbes is essential for host metabolic health. The key molecular mediators that transmit rhythmic signals between microbes and host metabolic networks remain unclear, but studies suggest the host immune system may serve as a conduit between these two systems, providing homeostatic signals to maintain overall metabolic health. Despite this knowledge, the precise mechanism and communication modalities that drive these rhythms remain unclear, especially in humans. Here, we review the current literature examining circadian dynamics of gut microbes, the immune system, and metabolism in the context of metabolic dysregulation and provide insights into gaps and challenges that remain.

Peroxisome Proliferator-Activated Receptors as Molecular Links between Caloric Restriction and Circadian Rhythm

The circadian rhythm plays a chief role in the adaptation of all bodily processes to internal and environmental changes on the daily basis. Next to light/dark phases, feeding patterns constitute the most essential element entraining daily oscillations, and therefore, timely and appropriate restrictive diets have a great capacity to restore the circadian rhythm. One of the restrictive nutritional approaches, caloric restriction (CR) achieves stunning results in extending health span and life span via coordinated changes in multiple biological functions from the molecular, cellular, to the whole-body levels. The main molecular pathways affected by CR include mTOR, insulin signaling, AMPK, and sirtuins. Members of the family of nuclear receptors, the three peroxisome proliferator-activated receptors (PPARs), PPARα, PPARβ/δ, and PPARγ take part in the modulation of these pathways. In this non-systematic review, we describe the molecular interconnection between circadian rhythm, CR-associated pathways, and PPARs. Further, we identify a link between circadian rhythm and the outcomes of CR on the whole-body level including oxidative stress, inflammation, and aging. Since PPARs contribute to many changes triggered by CR, we discuss the potential involvement of PPARs in bridging CR and circadian rhythm.

Bile acid metabolism and circadian rhythms

Circadian rhythms are biological systems that synchronize cellular circadian oscillators with the organism's daily feeding-fasting or rest-activity cycles in mammals. Circadian rhythms regulate nutrient absorption and utilization at the cellular level and are closely related to obesity and metabolic disorders. Bile acids are important modulators that facilitate nutrient absorption and regulate energy metabolism. Here, we provide an overview of the current connections and future perspectives between the circadian clock and bile acid metabolism as well as related metabolic diseases. Feeding and fasting cycles influence bile acid pool size and composition, and bile acid signaling can respond to acute lipid and glucose utilization and mediate energy balance. Disruption of circadian rhythms such as shift work, irregular diet, and gene mutations can contribute to altered bile acid metabolism and heighten obesity risk. High-fat diets, alcohol, and gene mutations related to bile acid signaling result in desynchronized circadian rhythms. Gut microbiome also plays a role in connecting circadian rhythms with bile acid metabolism. The underlying mechanism of how circadian rhythms interact with bile acid metabolism has not been fully explored. Sustaining bile acid homeostasis based on circadian rhythms may be a potential therapy to alleviate metabolic disturbance.

An in-depth neurobehavioral characterization shows anxiety-like traits, impaired habituation behavior, and restlessness in male Cryptochrome-deficient mice

Many psychiatric disorders, for example, anxiety, are accompanied by disturbances of circadian rhythms, including disturbed sleep/wake cycles, changes in locomotor activity, and abnormal endocrine function. Conversely, alternations of circadian rhythms are a risk factor for the development of psychiatric disorders. This assumption is supported by animals with clock gene mutations which often display behaviors that resemble human psychiatric disorders. In this study, we performed an in-depth behavioral analysis with male mice lacking the central clock genes Cryptochrome 1 and 2 (Cry1/2^-/- ), which are thus unable to express endogenous circadian rhythms. With wild-type and Cry1/2^-/- mice, we performed an extensive behavioral analysis to study their cognitive abilities, social behavior, and their expression of depression-like and anxiety-like behavior. While Cry1/2^-/- mice showed only mild abnormalities at cognitive and social behavioral levels, they were consistently more anxious than wildtype mice. Anxiety-like behavior was particularly evident in reduced mobility in new environments, altered ability to habituate, compensatory behavior, and consistent restless behavior across many behavioral tests. In line with their anxiety-like behavioral phenotype, Cry1/2^-/- mice have higher c-Fos activity in the amygdala after exposure to an anxiogenic stressor than wild-type mice. In our study, we identified Cry1/2^-/- mice as animals that qualify as a translational mouse model for anxiety disorder in humans because of its consistent behavior of restlessness, increased immobility, and dysfunctional habituation in new environments.

Circadian clock and liver energy metabolism

Circadian rhythm, generated by the circadian clock, is an internal rhythm that the body evolved to adapt to the diurnal changes in the external environment. Under its influence, mammals have distinct feeding and fasting cycles, which cause rhythmic changes in nutrient supply and demand. In recent years, many studies have shown that biorhythms are closely related to body metabolism. The liver, as the metabolism center of the body, is affected by circadian rhythm. However, with the acceleration of the pace of modern life and the change of life styles, the body's original rhythm is disrupted, resulting in a significant increase in the incidence of liver related metabolic diseases. Meanwhile, the disorder of circadian rhythm can also promote the occurrence and development of these diseases, and affect their prognosis and outcome. This paper reviews the relationship between the function of liver clock genes and the metabolism of liver glucose, lipids, bile acids, protein, etc.

Circadian Rhythm in Adipose Tissue: Novel Antioxidant Target for Metabolic and Cardiovascular Diseases

Obesity is a major risk factor for most metabolic and cardiovascular disorders. Adipose tissue is an important endocrine organ that modulates metabolic and cardiovascular health by secreting signaling molecules. Oxidative stress is a common mechanism associated with metabolic and cardiovascular complications including obesity, type 2 diabetes, and hypertension. Oxidative stress can cause adipose tissue dysfunction. Accumulating data from both humans and experimental animal models suggest that adipose tissue function and oxidative stress have an innate connection with the intrinsic biological clock. Circadian clock orchestrates biological processes in adjusting to daily environmental changes according to internal or external cues. Recent studies have identified the genes and molecular pathways exhibiting circadian expression patterns in adipose tissue. Disruption of the circadian rhythmicity has been suggested to augment oxidative stress and aberrate adipose tissue function and metabolism. Therefore, circadian machinery in the adipose tissue may be a novel therapeutic target for the prevention and treatment of metabolic and cardiovascular diseases. In this review, we summarize recent findings on circadian rhythm and oxidative stress in adipose tissue, dissect the key components that play a role in regulating the clock rhythm, oxidative stress and adipose tissue function, and discuss the potential use of antioxidant treatment on metabolic and cardiovascular diseases by targeting the adipose clock.

MCD diet-induced steatohepatitis generates a diurnal rhythm of associated biomarkers and worsens liver injury in Klf10 deficient mice

A large number of hepatic functions are regulated by the circadian clock and recent evidence suggests that clock disruption could be a risk factor for liver complications. The circadian transcription factor Krüppel like factor 10 (KLF10) has been involved in liver metabolism as well as cellular inflammatory and death pathways. Here, we show that hepatic steatosis and inflammation display diurnal rhythmicity in mice developing steatohepatitis upon feeding with a methionine and choline deficient diet (MCDD). Core clock gene mRNA oscillations remained mostly unaffected but rhythmic Klf10 expression was abolished in this model. We further show that Klf10 deficient mice display enhanced liver injury and fibrosis priming upon MCDD challenge. Silencing Klf10 also sensitized primary hepatocytes to apoptosis along with increased caspase 3 activation in response to TNFα. This data suggests that MCDD induced steatohepatitis barely affects the core clock mechanism but leads to a reprogramming of circadian gene expression in the liver in analogy to what is observed in other experimental disease paradigms. We further identify KLF10 as a component of this transcriptional reprogramming and a novel hepato-protective factor.

Association of insulin resistance with polymorphic variants of Clock and Bmal1 genes: A case-control study

Background: Little information is available in the literature for the correlation of insulin resistance (IR) and CLOCK gene polymorphism in Chinese population. This study aimed to investigate the relationship of HOMA-IR (homeostasis model assessment of insulin resistance) to polymorphic variants of Clock and Bmal1 genes in Chinese patients with essential hypertension.Methods: A total of 334 outpatients with essential hypertension (103 patients of HOMA-IR positive and 231 patients of HOMA-IR negative) were recruited to analyze Clock T3111C and Bmal1 A1420G genotypes with DNA sequencing approach.Results: Waist circumference, body mass index, glycated hemoglobin, total cholesterol, triglyceride, and plasminogen activator inhibitor-1 were significantly increased, while high-density lipoprotein cholesterol was significantly decreased in patients with HOMA-IR positive (P < .05-0.001 vs. patients with HOMA-IR negative). Twenty-four-hour ambulatory blood pressure monitoring showed that 24-h mean systolic blood pressure (SBP), especially nightime SBP, was higher in patients with HOMA-IR positive (P < .05 vs. patients with HOMA-IR negative). Notably, compared with the negative group, the distribution frequency of C allele of Clock T3111C and GG genotype of Bmal1 A1420G were significantly higher in the HOMA-IR positive group (29.1 vs. 10.8% P < .000 and 43.7 vs. 27.7% P = .007, respectively). Logistic regression analysis showed that C allele of Clock T3111C (OR = 4.128, CI 95% 2.313-7.368, p = .000) and GG genotype of Bmal1 A1420G (OR = 1.983, CI 95% 1.117-3.521, p = .019) were independent risk factors for potential HOMA-IR in Chinese patients with essential hypertension.Conclusion: Our results indicated that Chinese hypertensive patients with C allele of Clock T3111C or GG genotype of Bmal1 A1420G might be susceptible to IR and are more likely to develop high nighttime SBP.

Metabolic implications of circadian disruption

Circadian rhythms are generated by the circadian clock, a self-sustained internal timing system that exhibits 24-h rhythms in the body. In mammals, circadian rhythms are driven by a central clock located in suprachiasmatic nucleus and various peripheral clocks located in different tissues and organs of the body. Many cellular, behavioral, and physiological processes are regulated by the circadian clock in coordination with environmental cues. The process of metabolism is also under circadian regulation. Loss of synchronization between the internal clock and environmental zeitgebers results in disruption of the circadian rhythms that seriously impacts metabolic homeostasis leading to changed eating behavior, altered glucose and lipid metabolism, and weight gain. This in turn augments the risk of having various cardio-metabolic disorders such as obesity, diabetes, metabolic syndrome, and cardiovascular disease. This review sheds light on circadian rhythms and their role in metabolism with the identification of gaps in the current knowledge that remain to be explored in these fields. In this review, the molecular mechanisms underlying circadian rhythms have been elaborated first. Then, the focus has been kept on explaining the physiological significance of circadian rhythms in regulating metabolism. Finally, the implications for metabolism when these rhythms are disrupted due to genetic mutations or social and occupational needs enforced by modern lifestyle have been discussed.

Long-term environmental exposure to microcystins increases the risk of nonalcoholic fatty liver disease in humans: A combined fisher-based investigation and murine model study

Microcystins (MCs) produced by cyanobacteria pose serious threats to human health. However, the contribution of long-term exposure to MCs to the development of nonalcoholic fatty liver disease (NAFLD) remains poorly documented. In this study, we estimated the environmental uptake of MCs by a small population of fishers who have lived for many years on Meiliang Bay of Lake Taihu, where cyanobacterial blooms occur frequently. Serum biochemical indices of liver function and their relationships with MC contamination in these people were also investigated. Moreover, to mimic the long-term effects of MC on the livers of fishers, an animal model was established in which mice were exposed to MC-LR at an environmentally relevant level, a reference level (the no-observed adverse effect level, NOAEL), and three times the NOAEL through drinking water for 12 months. We estimated the total daily intake of MCs by fishers through contaminated lake water and food to be 5.95 μg MC-LReq, far exceeding the tolerable daily intake (2.40 μg MC-LReq) proposed by the World Health Organization (WHO). More than 80% of participants had at least one abnormal serum marker. The indices of aspartate aminotransferase (AST)/alanine aminotransferase (ALT), triglyceride (TG), globulin (GLB), and lactate dehydrogenase (LDH) had close positive associations with MC contamination, indicating that both liver damage and lipid metabolism dysfunction were induced by chronic MC exposure. Furthermore, the animal experimental results showed that long-term exposure to MC-LR at the environmentally relevant level led to hepatic steatosis with molecular alterations in circadian rhythm regulation, lipid metabolic processes, and the cell cycle pathway. Exposure to MC-LR at or above the NOAEL worsened the pathological phenotype towards nonalcoholic steatohepatitis disease (NASH) or fibrosis. These results suggest that prolonged exposure to the reference level (NOAEL) of MC-LR could cause severe liver injury to mammals. People with long-term environmental exposure to MCs might be at high risk for developing NAFLD.