Prospective influences of circadian clocks in adipose tissue and metabolism

Intermittent fasting shifts the diurnal transcriptome atlas of transcription factors

Intermittent fasting remains a safe and effective strategy to ameliorate various age-related diseases, but its specific mechanisms are not fully understood. Considering that transcription factors (TFs) determine the response to environmental signals, here, we profiled the diurnal expression of 600 samples across four metabolic tissues sampled every 4 over 24 h from mice placed on five different feeding regimens to provide an atlas of TFs in biological space, time, and feeding regimen. Results showed that 1218 TFs exhibited tissue-specific and temporal expression profiles in ad libitum mice, of which 974 displayed significant oscillations at least in one tissue. Intermittent fasting triggered more than 90% (1161 in 1234) of TFs to oscillate somewhere in the body and repartitioned their tissue-specific expression. A single round of fasting generally promoted TF expression, especially in skeletal muscle and adipose tissues, while intermittent fasting mainly suppressed TF expression. Intermittent fasting down-regulated aging pathway and upregulated the pathway responsible for the inhibition of mammalian target of rapamycin (mTOR). Intermittent fasting shifts the diurnal transcriptome atlas of TFs, and mTOR inhibition may orchestrate intermittent fasting-induced health improvements. This atlas offers a reference and resource to understand how TFs and intermittent fasting may contribute to diurnal rhythm oscillation and bring about specific health benefits.

Analysis of co-expression gene network associated with intracranial aneurysm and type 2 diabetes mellitus

To screen for common target genes in intracranial aneurysms (IA) and type 2 diabetes mellitus (T2DM), construct a common transcriptional regulatory network to predict clusters of candidate genes involved in the pathogenesis of T2DM and IA, and identify the common neurovascular markers and pathways in T2DM causing IA. Microarray datasets (GSE55650, GSE25462, GSE26969, GSE75436, and GSE13353) from the GEO database were analyzed in this research. Screening of the IA and the T2DM datasets yielded a total of 126 DEGs, among which 78 were upregulated and 138 were downregulated. Functional enrichment analysis revealed that these DEGs were enriched for a total of 68 GO pathways, including extracellular matrix composition, coagulation regulation, hemostasis regulation, and collagen fiber composition pathways. We also constructed transcriptional regulatory networks, and identified key transcription factors involved in both the conditions. Univariate logistic regression analysis showed that ARNTL2 and STAT1 were significantly associated with the development of T2DM and IA, acting as the common neurovascular markers for both the diseases. In cellular experiments, hyperglycemic microenvironments exhibited upregulated STAT1 expression. STAT1 may be involved in the pathogenesis of IA in T2DM patients. Being the common neurovascular markers, STAT1 may acts as novel therapeutic targets for the treatment of IA and T2DM.

Microbiome diurnal rhythmicity and its impact on host physiology and disease risk

Host-microbiome interactions constitute key determinants of host physiology, while their dysregulation is implicated in a wide range of human diseases. The microbiome undergoes diurnal variation in composition and function, and this in turn drives oscillations in host gene expression and functions. In this review, we discuss the newest developments in understanding circadian host-microbiome interplays, and how they may be relevant in health and disease contexts. We summarize the molecular mechanisms by which the microbiome influences host function in a diurnal manner, and inversely describe how the host orchestrates circadian rhythmicity of the microbiome. Furthermore, we highlight the future perspectives and challenges in studying this new and exciting facet of host-microbiome interactions. Finally, we illustrate how the elucidation of the microbiome chronobiology may pave the way for novel therapeutic approaches.

Circadian clock rhythms in different adipose tissue model systems

Circadian clock-controlled 24-h oscillations in adipose tissues play an important role in the regulation of energy homeostasis, thus representing a potential drug target for prevention and therapy of metabolic diseases. For pharmacological screens, scalable adipose model systems are needed that largely recapitulate clock properties observed in vivo. In this study, we compared molecular circadian clock regulation in different ex vivo and in vitro models derived from murine adipose tissues. Explant cultures from three different adipose depots of PER2::LUC circadian reporter mice revealed stable and comparable rhythms of luminescence ex vivo. Likewise, primary pre- and mature adipocytes from these mice displayed stable luminescence rhythms, but with strong damping in mature adipocytes. Stable circadian periods were also observed using Bmal1-luc and Per2-luc reporters after lentiviral transduction of wild-type pre-adipocytes. SV40 immortalized adipocytes of murine brown, subcutaneous and epididymal adipose tissue origin showed rhythmic mRNA expression of the core clock genes Bmal1, Per2, Dbp and REV-erbα in pre- and mature adipocytes, with a maturation-associated increase in overall mRNA levels and amplitudes. A comparison of clock gene mRNA rhythm phases revealed specific changes between in vivo and ex vivo conditions. In summary, our data indicate that adipose culture systems to a large extent mimic in vivo tissue clock regulation. Thus, both explant and cell systems may be useful tools for large-scale screens for adipose clock regulating factors.

Role of cardiotrophin-1 in the regulation of metabolic circadian rhythms and adipose core clock genes in mice and characterization of 24-h circulating CT-1 profiles in normal-weight and overweight/obese subjects

Cardiotrophin (CT)-1 is a regulator of glucose and lipid homeostasis. In the present study, we analyzed whether CT-1 also acts to peripherally regulate metabolic rhythms and adipose tissue core clock genes in mice. Moreover, the circadian pattern of plasma CT-1 levels was evaluated in normal-weight and overweight subjects. The circadian rhythmicity of oxygen consumption rate (Vo₂) was disrupted in aged obese CT-1-deficient (CT-1^-/-) mice (12 mo). Although circadian rhythms of Vo₂ were conserved in young lean CT-1^-/- mice (2 mo), CT-1 deficiency caused a phase shift of the acrophase. Most of the clock genes studied (Clock, Bmal1, and Per2) displayed a circadian rhythm in adipose tissue of both wild-type (WT) and CT-1^-/- mice. However, the pattern was altered in CT-1^-/- mice toward a lower percentage of the rhythm or lower amplitude, especially for Bmal1 and Clock. Moreover, CT-1 mRNA levels in adipose tissue showed significant circadian fluctuations in young WT mice. In humans, CT-1 plasma profile exhibited a 24-h circadian rhythm in normal-weight but not in overweight subjects. The 24-h pattern of CT-1 was characterized by a pronounced increase during the night (from 02:00 to 08:00). These observations suggest a potential role for CT-1 in the regulation of metabolic circadian rhythms.-López-Yoldi, M., Stanhope, K. L., Garaulet, M., Chen, X. G., Marcos-Gómez, B., Carrasco-Benso, M. P., Santa Maria, E. M., Escoté, X., Lee, V., Nunez, M. V., Medici, V., Martínez-Ansó, E., Sáinz, N., Huerta, A. E., Laiglesia, L. M., Prieto, J., Martínez, J. A., Bustos, M., Havel, P. J., Moreno-Aliaga, M. J. Role of cardiotrophin-1 in the regulation of metabolic circadian rhythms and adipose core clock genes in mice and characterization of 24-h circulating CT-1 profiles in normal-weight and overweight/obese subjects.

Subtype prediction in primary aldosteronism: measurement of circadian variation of adrenocortical hormones and 24-h urinary aldosterone

OBJECTIVE

Currently, adrenal venous sampling (AVS) is the only reliable method to distinguish unilateral from bilateral hyperaldosteronism in primary aldosteronism (PA). However, AVS is costly and time-consuming compared with simple blood tests. In this study, we conducted a retrospective study to determine whether circadian variation in plasma adrenocortical hormone levels (i.e. aldosterone, cortisol and ACTH) and a 24-h urinary aldosterone could contribute to the clinical differentiation between unilateral hyperaldosteronism (UHA) and bilateral hyperaldosteronism (BHA).

DESIGN

In 64 patients who were diagnosed with PA and underwent AVS, 32 and 22 patients were diagnosed with UHA and BHA, respectively. Plasma adrenocortical hormone levels at 0:00, 6:00, 12:00 and 18:00 and 24-h urinary aldosterone under a condition of 6 g daily dietary sodium chloride intake were measured.

RESULTS

Baseline plasma aldosterone concentration (PAC) and 24-h urinary aldosterone level in patients with UHA were significantly higher than in patients with BHA, particularly at 6:00. The area under the ROC curve for PAC at 0:00, 6:00, 12:00 and 18:00 and 24-h urinary aldosterone to discriminate UHA and BHA was 0·839 [95% confidence interval (CI); 0·73-0·95], 0·922 (95% CI; 0·85-1·00), 0·875 (95% CI; 0·78-0·97), 0·811 (95% CI; 0·69-0·93), 0·898 (95% CI; 0·81-0·99), respectively.

CONCLUSIONS

PAC at different blood sampling times and 24-h urinary aldosterone level may be diagnostically helpful in discriminating between UHA and BHA. We believe that these tests could reduce the number of unnecessary AVS procedures.

Characterization of ultradian and circadian rhythms of core body temperature based on wavelet analysis

This study was motivated by the needs of precise characterization for the ultradian and circadian rhythmicity of human core body temperature (CBT). The CBT data, two-whole-days' data of two female bed-ridden old aged suffering from cerebral infarction sequelae, was detrended to eliminate the long-term components with periods longer than two days and normalized at first. It was then analyzed by the stationary wavelets transform (SWT) to get the time-frequency information. In the step of SWT, symlet 6 was used, and the approximation waveforms in the 5th, 6th and 7th levels were used to reveal the targeted rhythmicity. The results of the SWT show that SWT can faithfully reveal the time-frequency information of feature elements (peaks and troughs) of waveforms and rhythmicity can be characterized by analyzing temporal information of feature elements.

Pinealectomy interferes with the circadian clock genes expression in white adipose tissue

Melatonin, the main hormone produced by the pineal gland, is secreted in a circadian manner (24-hr period), and its oscillation influences several circadian biological rhythms, such as the regulation of clock genes expression (chronobiotic effect) and the modulation of several endocrine functions in peripheral tissues. Assuming that the circadian synchronization of clock genes can play a role in the regulation of energy metabolism and it is influenced by melatonin, our study was designed to assess possible alterations as a consequence of melatonin absence on the circadian expression of clock genes in the epididymal adipose tissue of male Wistar rats and the possible metabolic repercussions to this tissue. Our data show that pinealectomy indeed has impacts on molecular events: it abolishes the daily pattern of the expression of Clock, Per2, and Cry1 clock genes and Pparγ expression, significantly increases the amplitude of daily expression of Rev-erbα, and affects the pattern of and impairs adipokine production, leading to a decrease in leptin levels. However, regarding some metabolic aspects of adipocyte functions, such as its ability to synthesize triacylglycerols from glucose along 24 hr, was not compromised by pinealectomy, although the daily profile of the lipogenic enzymes expression (ATP-citrate lyase, malic enzyme, fatty acid synthase, and glucose-6-phosphate dehydrogenase) was abolished in pinealectomized animals.

Adipose hypothermia in obesity and its association with period homolog 1, insulin sensitivity, and inflammation in fat

Visceral fat adiposity plays an important role in the development of metabolic syndrome. We reported previously the impact of human visceral fat adiposity on gene expression profile of peripheral blood cells. Genes related to circadian rhythm were highly associated with visceral fat area and period homolog 1 (PER1) showed the most significant negative correlation with visceral fat area. However, regulation of adipose Per1 remains poorly understood. The present study was designed to understand the regulation of Per1 in adipose tissues. Adipose Per1 mRNA levels of ob/ob mice were markedly low at 25 and 35 weeks of age. The levels of other core clock genes of white adipose tissues were also low in ob/ob mice at 25 and 35 weeks of age. Per1 mRNA was mainly expressed in the mature adipocyte fraction (MAF) and it was significantly low in MAF of ob/ob mice. To examine the possible mechanisms, 3T3-L1 adipocytes were treated with H2O2, tumor necrosis factor-α (TNF-α), S100A8, and lipopolysaccharide (LPS). However, no significant changes in Per1 mRNA level were observed by these agents. Exposure of cultured 3T3-L1 adipocytes to low temperature (33°C) decreased Per1 and catalase, and increased monocyte chemoattractant protein-1 (Mcp-1) mRNA levels. Hypothermia also worsened insulin-mediated Akt phosphorylation in 3T3-L1 adipocytes. Finally, telemetric analysis showed low temperature of adipose tissues in ob/ob mice. In obesity, adipose hypothermia seems to accelerate adipocyte dysfunction.

Glycemic control is impaired in the evening in prediabetes through multiple diurnal rhythms

AIMS

Recent studies suggest that circadian rhythms regulate glucose metabolism, weight loss, and even drug efficacy. Moreover, molecules targeted at the circadian clock show promise in treating metabolic disease. Therefore, this study set out to better characterize interactions among diurnal rhythms in prediabetes.

METHODS

Ten subjects with prediabetes completed oral glucose tolerance tests at 0700h and 1900h on the same day. Lipids and hormones were also measured.

RESULTS

Two-hour and three-hour glucose tolerances were worse in the evening by 40±52mg/dl (p=0.02) and 62±46mg/dl (p=0.001), respectively. These impairments were explained by lower insulin sensitivity (OGIS; 5.14±1.02 vs. 4.74±0.77mg/kg/min; p=0.03) and 2-hour AUC insulin levels (87.4±37.6 vs. 69.8±40.2mU∙hr/l; p=0.02) in the evening. Intriguingly, more insulin resistant subjects had weaker rhythms in insulin sensitivity (r=-0.66; p=0.04) but enhanced rhythms in insulin (r=-0.67; p=0.03) and cortisol (r=-0.78; p=0.008) levels. Importantly, the rhythms in cortisol primarily but also insulin sensitivity drove the declines in evening glucose tolerance (r=0.86; p=0.002).

CONCLUSIONS

Glycemic control is dramatically impaired in the evening in people with prediabetes, particularly when the cortisol rhythm is weak, but is unrelated to the rhythm in insulin levels. Therefore, food intake at dinnertime may need to be curbed in prediabetes.

Mice deficient in cryptochrome 1 (cry1 (-/-)) exhibit resistance to obesity induced by a high-fat diet

Disruption of circadian clock enhances the risk of metabolic syndrome, obesity, and type 2 diabetes. Circadian clocks rely on a highly regulated network of transcriptional and translational loops that drive clock-controlled gene expression. Among these transcribed clock genes are cryptochrome (CRY) family members, which comprise Cry1 and Cry2. While the metabolic effects of deletion of several core components of the clock gene machinery have been well characterized, those of selective inactivation of Cry1 or Cry2 genes have not been described. In this study, we demonstrate that ablation of Cry1, but not Cry2, prevents high-fat diet (HFD)-induced obesity in mice. Despite similar caloric intake, Cry1 (-/-) mice on HFD gained markedly less weight (-18%) at the end of the 16-week experiment and displayed reduced fat accumulation compared to wild-type (WT) littermates (-61%), suggesting increased energy expenditure. Analysis of serum lipid and glucose profiles showed no difference between Cry1 (-/-) and WT mice. Both Cry1 (-/-) and Cry2 (-/-) mice are indistinguishable from WT controls in body weight, fat and protein contents, and food consumption when they are allowed unlimited access to a standard rodent diet. We conclude that although CRY signaling may not be essential for the maintenance of energy homeostasis under steady-state nutritional conditions, Cry1 may play a role in readjusting energy balance under changing nutritional circumstances. These studies reinforce the important role of circadian clock genes in energy homeostasis and suggest that Cry1 is a plausible target for anti-obesity therapy.

Adipocytes and the regulation of bone remodeling: a balancing act

Throughout life, a balance exists within the marrow cavity between adipose tissue and bone. Each tissue derives from a common progenitor cell known both as a "bone marrow-derived multipotent stromal cell" and as a "mesenchymal stem cell" (BMSC). The majority of in vitro and in vivo data suggest that BMSCs differentiate into adipocytes or osteoblasts in a reciprocal manner. For example, while ligand induction of the transcription factors peroxisome proliferator-activated receptor γ initiates BMSC adipogenesis, it suppresses osteogenesis. Nevertheless, this hypothesis may oversimplify a complex regulatory paradigm. The picture may be further complicated by the systemic impact of extramedullary adipose depots on bone via the secretion of protein adipokines and lipid metabolites. This review focuses on past and current literature examining the mechanisms governing the adipose-bone interface.

Circadian clock-dependent and -independent rhythmic proteomes implement distinct diurnal functions in mouse liver

Diurnal oscillations of gene expression controlled by the circadian clock underlie rhythmic physiology across most living organisms. Although such rhythms have been extensively studied at the level of transcription and mRNA accumulation, little is known about the accumulation patterns of proteins. Here, we quantified temporal profiles in the murine hepatic proteome under physiological light-dark conditions using stable isotope labeling by amino acids quantitative MS. Our analysis identified over 5,000 proteins, of which several hundred showed robust diurnal oscillations with peak phases enriched in the morning and during the night and related to core hepatic physiological functions. Combined mathematical modeling of temporal protein and mRNA profiles indicated that proteins accumulate with reduced amplitudes and significant delays, consistent with protein half-life data. Moreover, a group comprising about one-half of the rhythmic proteins showed no corresponding rhythmic mRNAs, indicating significant translational or posttranslational diurnal control. Such rhythms were highly enriched in secreted proteins accumulating tightly during the night. Also, these rhythms persisted in clock-deficient animals subjected to rhythmic feeding, suggesting that food-related entrainment signals influence rhythms in circulating plasma factors.

Metabolite and transcriptome analysis during fasting suggest a role for the p53-Ddit4 axis in major metabolic tissues

BACKGROUND

Fasting induces specific molecular and metabolic adaptions in most organisms. In biomedical research fasting is used in metabolic studies to synchronize nutritional states of study subjects. Because there is a lack of standardization for this procedure, we need a deeper understanding of the dynamics and the molecular mechanisms in fasting.

RESULTS

We investigated the dynamic changes of liver gene expression and serum parameters of mice at several time points during a 48 hour fasting experiment and then focused on the global gene expression changes in epididymal white adipose tissue (WAT) as well as on pathways common to WAT, liver, and skeletal muscle. This approach produced several intriguing insights: (i) rather than a sequential activation of biochemical pathways in fasted liver, as current knowledge dictates, our data indicates a concerted parallel response; (ii) this first characterization of the transcriptome signature of WAT of fasted mice reveals a remarkable activation of components of the transcription apparatus; (iii) most importantly, our bioinformatic analyses indicate p53 as central node in the regulation of fasting in major metabolic tissues; and (iv) forced expression of Ddit4, a fasting-regulated p53 target gene, is sufficient to augment lipolysis in cultured adipocytes.

CONCLUSIONS

In summary, this combination of focused and global profiling approaches provides a comprehensive molecular characterization of the processes operating during fasting in mice and suggests a role for p53, and its downstream target Ddit4, as novel components in the transcriptional response to food deprivation.

Association of Rev-erbα in adipose tissues with Type 2 diabetes mellitus amelioration after gastric bypass surgery in Goto-Kakizaki rats

We estimated the key molecules related to Type 2 diabetes mellitus (T2DM) in adipose, liver, and muscle tissues, from nonobese diabetic Goto-Kakizaki (GK) rats and their Wistar controls, by computationally analyzing the expression profiles in open source data. With the aid of information from previous reports, Rev-erbα in adipose tissue emerged as one of the most plausible candidates. Here, in animal models, including GK rats surgically treated to ameliorate T2DM, we examined the association of Rev-erbα in adipose tissue with T2DM progression. After analyses of the Rev-erbα mRNA expression in the adipose tissue of our animal models, we compared the Rev-erbα protein expression levels in the adipose, liver, and muscle tissues of GK and Wistar controls at the ages of 1 mo (M), 3M, and 6M. The Rev-erbα protein levels in adipose tissue showed a distinctive pattern, with the negative correlation of an increasing trend in GK rats, and a decreasing trend in Wistar rats during aging, from those in liver and muscle tissues. Moreover, dysregulation of the circadian Rev-erbα expression in the adipose tissue of 6-mo-old GK rats was also observed. In particular, we ameliorated T2DM in GK rats by gastric bypass surgery, and revealed that T2DM amelioration in diabetic GK rats was associated with improved circadian Rev-erbα expression, in a comparison between the surgically treated and untreated GK rats. The roles of Rev-erbα in adipose tissue were further investigated by observations of Rev-erbα-related molecules, with reference to previous reports.

Circadian disruption leads to insulin resistance and obesity

BACKGROUND

Disruption of circadian (daily) timekeeping enhances the risk of metabolic syndrome, obesity, and type 2 diabetes. While clinical observations have suggested that insulin action is not constant throughout the 24 hr cycle, its magnitude and periodicity have not been assessed. Moreover, when circadian rhythmicity is absent or severely disrupted, it is not known whether insulin action will lock to the peak, nadir, or mean of the normal periodicity of insulin action.

RESULTS

We used hyperinsulinemic-euglycemic clamps to show a bona fide circadian rhythm of insulin action; mice are most resistant to insulin during their daily phase of relative inactivity. Moreover, clock-disrupted Bmal1-knockout mice are locked into the trough of insulin action and lack rhythmicity in insulin action and activity patterns. When rhythmicity is rescued in the Bmal1-knockout mice by expression of the paralogous gene Bmal2, insulin action and activity patterns are restored. When challenged with a high-fat diet, arhythmic mice (either Bmal1-knockout mice or wild-type mice made arhythmic by exposure to constant light) were obese prone. Adipose tissue explants obtained from high-fat-fed mice have their own periodicity that was longer than animals on a chow diet.

CONCLUSIONS

This study provides rigorous documentation for a circadian rhythm of insulin action and demonstrates that disturbing the natural rhythmicity of insulin action will disrupt the rhythmic internal environment of insulin sensitive tissue, thereby predisposing the animals to insulin resistance and obesity.

Molecular mechanism of circadian rhythmicity of seizures in temporal lobe epilepsy

The circadian pattern of seizures in people with epilepsy (PWE) was first described two millennia ago. However, these phenomena have not received enough scientific attention, possibly due to the lack of promising hypotheses to address the interaction between seizure generation and a physiological clock. To propose testable hypotheses at the molecular level, interactions between circadian rhythm, especially transcription factors governing clock genes expression, and the mTOR (mammalian target of rapamycin) signaling pathway, the major signaling pathway in epilepsy, will be reviewed. Then, two closely related hypotheses will be proposed: (1) Rhythmic activity of hyperactivated mTOR signaling molecules results in rhythmic increases in neuronal excitability. These rhythmic increases in excitability periodically exceed the seizure threshold, displaying the behavioral seizures. (2) Oscillation of neuronal excitability in SCN modulates the rhythmic excitability in the hippocampus through subiculum via long-range projections. Findings from published results, their implications, and proposals for new experiments will be discussed. These attempts may ignite further discussion on what we still need to learn about the rhythmicity of spontaneous seizures.

A pilot investigation of visceral fat adiposity and gene expression profile in peripheral blood cells

Evidence suggests that visceral fat accumulation plays a central role in the development of metabolic syndrome. Excess visceral fat causes local chronic low-grade inflammation and dysregulation of adipocytokines, which contribute in the pathogenesis of the metabolic syndrome. These changes may affect the gene expression in peripheral blood cells. This study for the first time examined the association between visceral fat adiposity and gene expression profile in peripheral blood cells. The gene expression profile was analyzed in peripheral blood cells from 28 obese subjects by microarray analysis. Reverse transcription-polymerase chain reaction (RT-PCR) was performed using peripheral blood cells from 57 obese subjects. Obesity was defined as body mass index (BMI) greater than 25 kg/m² according to the Japanese criteria, and the estimated visceral fat area (eVFA) was measured by abdominal bioelectrical impedance. Analysis of gene expression profile was carried out with Agilent whole human genome 4×44 K oligo-DNA microarray. The expression of several genes related to circadian rhythm, inflammation, and oxidative stress correlated significantly with visceral fat accumulation. Period homolog 1 (PER1) mRNA level in blood cells correlated negatively with visceral fat adiposity. Stepwise multiple regression analysis identified eVFA as a significant determinant of PER1 expression. In conclusion, visceral fat adiposity correlated with the expression of genes related to circadian rhythm and inflammation in peripheral blood cells.

Daily rhythms of plasma melatonin, but not plasma leptin or leptin mRNA, vary between lean, obese and type 2 diabetic men

Melatonin and leptin exhibit daily rhythms that may contribute towards changes in metabolic physiology. It remains unclear, however, whether this rhythmicity is altered in obesity or type 2 diabetes (T2DM). We tested the hypothesis that 24-hour profiles of melatonin, leptin and leptin mRNA are altered by metabolic status in laboratory conditions. Men between 45–65 years old were recruited into lean, obese-non-diabetic or obese-T2DM groups. Volunteers followed strict sleep-wake and dietary regimes for 1 week before the laboratory study. They were then maintained in controlled light-dark conditions, semi-recumbent posture and fed hourly iso-energetic drinks during wake periods. Hourly blood samples were collected for hormone analysis. Subcutaneous adipose biopsies were collected 6-hourly for gene expression analysis. Although there was no effect of subject group on the timing of dim light melatonin onset (DLMO), nocturnal plasma melatonin concentration was significantly higher in obese-non-diabetic subjects compared to weight-matched T2DM subjects (p<0.01) and lean controls (p<0.05). Two T2DM subjects failed to produce any detectable melatonin, although did exhibit plasma cortisol rhythms comparable to others in the group. Consistent with the literature, there was a significant (p<0.001) effect of subject group on absolute plasma leptin concentration and, when expressed relative to an individual’s 24-hour mean, plasma leptin showed significant (p<0.001) diurnal variation. However, there was no difference in amplitude or timing of leptin rhythms between experimental groups. There was also no significant effect of time on leptin mRNA expression. Despite an overall effect (p<0.05) of experimental group, post-hoc analysis revealed no significant pair-wise effects of group on leptin mRNA expression. Altered plasma melatonin rhythms in weight-matched T2DM and non-diabetic individuals supports a possible role of melatonin in T2DM aetiology. However, neither obesity nor T2DM changed 24-hour rhythms of plasma leptin relative to cycle mean, or expression of subcutaneous adipose leptin gene expression, compared with lean subjects.

Regulation of circadian behaviour and metabolism by synthetic REV-ERB agonists

Synchronizing rhythms of behavior and metabolic processes is important for cardiovascular health and preventing metabolic diseases. The nuclear receptors REV-ERBα and REV-ERBβ play an integral role in regulating the expression of core clock proteins driving rhythms in activity and metabolism. Here we describe the identification of potent synthetic REV-ERB agonists with in vivo activity. Administration of synthetic REV-ERB ligands alters circadian behavior and the circadian pattern of core clock gene expression in the hypothalami of mice. The circadian pattern of expression of an array of metabolic genes in the liver, skeletal muscle, and adipose tissue was also altered resulting in increased energy expenditure. Treatment of diet-induced obese mice with a REV-ERB agonist decreased obesity by reducing fat mass and markedly improving dyslipidemia and hyperglycemia. These results suggest that synthetic REV-ERB ligands that pharmacologically target the circadian rhythm may hold utility in the treatment of sleep disorders as well as metabolic diseases.

Adipose circadian rhythms: translating cellular and animal studies to human physiology

Emerging links between circadian rhythms and metabolism promise much for the understanding of metabolic physiology and pathophysiology, in which white adipose tissue (WAT) plays a prominent role. Many WAT endocrine molecules, termed adipokines, display rhythmic plasma concentration. Moreover, similar to most other tissues, WAT exhibits widespread 24-h variation in gene expression, with approximately 20% of the murine adipose transcriptome estimated to undergo daily variation. A major limitation to human chronobiology research is the availability of physiologically defined peripheral tissues. To date most analyses of in vivo human peripheral clocks has been limited to blood leucocytes. However, subcutaneous adipose tissue represents a novel opportunity to study peripheral molecular rhythms that are of clearly defined metabolic relevance. This review summarises basic concepts of circadian and metabolic physiology before then comparing alternative protocols used to analyse the rhythmic properties of human adipose tissue.

Metabolism: what causes the gut's circadian instincts?

What mechanisms control circadian rhythms in the gastrointestinal tract and how does this impact nutrient metabolism? The deadenylase and leucine zipper protein Nocturnin is now shown to play a central role.

Circadian rhythms in adipose tissue: an update

PURPOSE OF REVIEW

Over the past decade, evidence has accumulated from basic science, clinical and epidemiological studies linking circadian mechanisms to adipose tissue biology and its related comorbidities, diabetes, metabolic syndrome and obesity. This review highlights recent in-vitro and in-vivo findings from murine, human and model organism studies.

RECENT FINDINGS

High-fat diets attenuate circadian mechanisms in murine adipose depots and these effects appear to be due to obesity rather than hyperglycemia. Deletion of circadian regulatory genes such as AMPK1 and nocturnin alter the circadian biology of adipose tissue. Unlike the mouse, circadian gene oscillation in human adipose tissue appears to be independent of BMI and diabetes status, suggesting that circadian mechanistic variation occurs across species. Clues for future directions in this emerging field come from studies of the hibernation and torpor state in mammals and infection models involving the Drosophila metabolic organ or 'fat body'.

SUMMARY

There is a growing consensus that circadian rhythms and metabolism are tightly regulated in adipose tissue and peripheral metabolic organs. Although central mechanisms are critical, autonomous clocks exist within the adipocytes themselves. Future circadian advances are likely to result from the studies of adipose tissue-specific gene deletions.