Multiple mechanisms regulate circadian expression of the gene for cholesterol 7alpha-hydroxylase (Cyp7a), a key enzyme in hepatic bile acid biosynthesis

Parenteral nutrition results in peripheral ileal to hepatic circadian discordance in mice

We have developed a mouse model of parenteral nutrition-associated liver disease (PNALD) in which parenteral nutrition (PN) infusion results in cholestatic liver injury. In the liver, the master circadian genes Arntl/Bmal drive rhythmic gene expression and regulate circadian expression of hepatic functions including bile acid synthesis. The aim of this study was to examine the effect of continuous PN on ileal and hepatic expression of circadian regulatory (CR) genes, farnesoid X receptor (FXR) signaling, and bile acid synthesis in mice. Wild-type mice were exposed to ad libitum Chow or continuous soy oil lipid emulsion-based PN infusion through a central venous catheter for 4 days (PN). Water was provided ad libitum, but no nutrients were provided enterally. On day 4, separate groups of Chow and PN-fed mice were euthanized every 6 h (7 AM, 1 PM, 7 PM, and 1 AM), and ileal, hepatic tissue and serum harvested. From tissue samples, the relative expression of circadian transcription factors and FXR signaling was assessed. Administration of 4-day PN increased hepatic injury, inflammatory cytokine expression, and gut permeability. In the ileum, PN activated FXR and induced expression of Fgf15 and Nr0b2. In the liver, expression of FXR-downstream targets was dysregulated. PN administrations impacted hepatic and ileal circadian transcription factor mRNA expression, which was discordant between the two organs. Dysregulation of circadian regulatory machinery is in part due to discordance of the gut-liver axis during PN. Pharmacological targeting of CR as a therapeutic strategy for PNALD thus deserves further investigation.NEW & NOTEWORTHY This study used a novel short-term model of parenteral nutrition (PN) that is translationally relevant. We find that short-term PN is sufficient to induce hepatic and ileal changes in circadian transcription factor expression and to prevent normal concordant coordination of circadian transcription factors between the ileum and liver. These data suggest that targeting circadian transcription may have some clinical benefit in patients receiving parenteral nutrition.

The Biological Clock of Liver Metabolism in Metabolic Dysfunction-Associated Steatohepatitis Progression to Hepatocellular Carcinoma

Circadian rhythms are endogenous behavioral or physiological cycles that are driven by a daily biological clock that persists in the absence of geophysical or environmental temporal cues. Circadian rhythm-related genes code for clock proteins that rise and fall in rhythmic patterns driving biochemical signals of biological processes from metabolism to physiology and behavior. Clock proteins have a pivotal role in liver metabolism and homeostasis, and their disturbances are implicated in various liver disease processes. Encoded genes play critical roles in the initiation and progression of metabolic dysfunction-associated steatohepatitis (MASH) to hepatocellular carcinoma (HCC) and their proteins may become diagnostic markers as well as therapeutic targets. Understanding molecular and metabolic mechanisms underlying circadian rhythms will aid in therapeutic interventions and may have broader clinical applications. The present review provides an overview of the role of the liver's circadian rhythm in metabolic processes in health and disease, emphasizing MASH progression and the oncogenic associations that lead to HCC.

The roles of nuclear receptors in cholesterol metabolism and reverse cholesterol transport in nonalcoholic fatty liver disease

As the most prevalent chronic liver disease globally, NAFLD encompasses a pathological process that ranges from simple steatosis to NASH, fibrosis, cirrhosis, and HCC, closely associated with numerous extrahepatic diseases. While the initial etiology was believed to be hepatocyte injury caused by lipid toxicity from accumulated triglycerides, recent studies suggest that an imbalance of cholesterol homeostasis is of greater significance. The role of nuclear receptors in regulating liver cholesterol homeostasis has been demonstrated to be crucial. This review summarizes the roles and regulatory mechanisms of nuclear receptors in the 3 main aspects of cholesterol production, excretion, and storage in the liver, as well as their cross talk in reverse cholesterol transport. It is hoped that this review will offer new insights and theoretical foundations for the study of the pathogenesis and progression of NAFLD and provide new research directions for extrahepatic diseases associated with NAFLD.

A ketogenic diet containing medium-chain triglycerides reduces REM sleep duration without significant influence on mouse circadian phenotypes

Ketogenic diets (KDs) affect the circadian rhythms of behavior and clock gene expression in experimental animals. However, these diets were designed to simulate a fasting state; thus, whether these effects are caused by diet-induced ketogenesis or persistent starvation is difficult to distinguish. The present study aimed to define the effects of a KD containing medium-chain triglycerides (MCT-KD) that increase blood ketone levels without inducing carbohydrate starvation, on circadian rhythms and sleep regulation. Mice were fed with a normal diet (CTRL) or MCT-KD for 2 weeks. Blood β-hydroxybutyrate levels were significantly increased up to 2 mM by the MCT-KD, whereas body weight gain and blood glucose levels were identical between the groups, suggesting that ketosis accumulated without carbohydrate starvation in the MCT-KD mice. Circadian rhythms of wheel-running activity and core body temperature were almost identical, although wheel-running was slightly reduced in the MCT-KD mice. The circadian expression of the core clock genes, Per1, Per2, Bmal1, and Dbp in the hypothalamus, heart, liver, epididymal adipose tissues, and skeletal muscle were almost identical between the CTRL and MCT-KD mice, whereas the amplitude of hepatic Per2 and adipose Per1 expression was increased in MCT-KD mice. The MCT-KD reduced the duration of rapid-eye-movement (REM) sleep without affecting the duration of non-REM sleep and the duration of wakefulness. These findings suggested that the impact of ketone bodies on circadian systems are limited, although they might reduce locomotor activity and REM sleep duration.

Role of circadian rhythm and autonomic nervous system in liver function: a hypothetical basis for improving the management of hepatic encephalopathy

Hepatic encephalopathy (HE) is a common, incapacitating complication of cirrhosis that affects many patients with cirrhosis. Although several therapies have proven effective in the treatment and prevention of this condition, several patients continue to suffer from covert disease or episodes of relapse. The circadian rhythm has been demonstrated to be pivotal for many body functions, including those of the liver. Here, we explore the impact of circadian rhythm-dependent signaling on the liver and discuss the evidence of its impact on liver pathology and metabolism. We describe the various pathways through which circadian influences are mediated. Finally, we introduce a novel method for improving patient response to drugs aimed at treating HE by utilizing the circadian rhythm. A digital system that introduces a customization-based technique for improving the response to therapies is presented as a hypothetical approach for improving the effectiveness of current medications used for the treatment of recurrent and persistent hepatic encephalopathy.

Dim light in the evening causes coordinated realignment of circadian rhythms, sleep, and short-term memory

In modern societies, people are regularly exposed to artificial light (e.g., light-emitting electronic devices). Dim light in the evening (DLE) imposes an artificial extension of the solar day, increasing our alertness before bedtime, delaying melatonin timing and sleep onset, and increasing sleepiness in the next morning. Using laboratory mice as a model organism, we show that 2 wk of 4-h, 20-lux DLE postpones rest–activity rhythms, delays molecular rhythms in the brain and body, and reverses the diurnal pattern of short-term memory performance. These results highlight the biological impact of DLE and emphasize the need to optimize our evening light exposure if we are to avoid shifting our biological clocks.

Light provides the primary signal for entraining circadian rhythms to the day/night cycle. In addition to rods and cones, the retina contains a small population of photosensitive retinal ganglion cells (pRGCs) expressing the photopigment melanopsin (OPN4). Concerns have been raised that exposure to dim artificial lighting in the evening (DLE) may perturb circadian rhythms and sleep patterns, and OPN4 is presumed to mediate these effects. Here, we examine the effects of 4-h, 20-lux DLE on circadian physiology and behavior in mice and the role of OPN4 in these responses. We show that 2 wk of DLE induces a phase delay of ∼2 to 3 h in mice, comparable to that reported in humans. DLE-induced phase shifts are unaffected in Opn4^−/− mice, indicating that rods and cones are capable of driving these responses in the absence of melanopsin. DLE delays molecular clock rhythms in the heart, liver, adrenal gland, and dorsal hippocampus. It also reverses short-term recognition memory performance, which is associated with changes in preceding sleep history. In addition, DLE modifies patterns of hypothalamic and cortical cFos signals, a molecular correlate of recent neuronal activity. Together, our data show that DLE causes coordinated realignment of circadian rhythms, sleep patterns, and short-term memory process in mice. These effects are particularly relevant as DLE conditions―due to artificial light exposure―are experienced by the majority of the populace on a daily basis.

Liver X receptors and skeleton: Current state-of-knowledge

The liver X receptors (LXR) is a nuclear receptor that acts as a prominent regulator of lipid homeostasis and inflammatory response. Its therapeutic effectiveness against various diseases like Alzheimer's disease and atherosclerosis has been investigated in detail. Emerging pieces of evidence now reveal that LXR is also a crucial modulator of bone remodeling. However, the molecular mechanisms underlying the pharmacological actions of LXR on the skeleton and its role in osteoporosis are poorly understood. Therefore, in the current review, we highlight LXR and its actions through different molecular pathways modulating skeletal homeostasis. The studies described in this review propound that LXR in association with estrogen, PTH, PPARγ, RXR hedgehog, and canonical Wnt signaling regulates osteoclastogenesis and bone resorption. It regulates RANKL-induced expression of c-Fos, NFATc1, and NF-κB involved in osteoclast differentiation. Additionally, several studies suggest suppression of RANKL-induced osteoclast differentiation by synthetic LXR ligands. Given the significance of modulation of LXR in various physiological and pathological settings, our findings indicate that therapeutic targeting of LXR might potentially prevent or treat osteoporosis and improve bone quality.

Hepatic metabolic regulation by nuclear factor E4BP4

Discovered as a b-ZIP transcription repressor 30 years ago, E4 promoter-binding protein 4 (E4BP4) has been shown to play critical roles in immunity, circadian rhythms, and cancer progression. Recent research has highlighted E4BP4 as a novel regulator of metabolisms in various tissues. In this review, we focus on the function and mechanisms of hepatic E4BP4 in regulating lipid and glucose homeostasis, bile metabolism, as well as xenobiotic metabolism. Finally, E4BP4-specific targets will be discussed for the prevention and treatment of metabolic disorders.

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.

Determination of genetic changes of Rev-erb beta and Rev-erb alpha genes in Type 2 diabetes mellitus by next-generation sequencing

BACKGROUND

The nuclear receptors Rev-erb alpha and Rev-erb beta are transcription factors that regulate the function of genes in glucose and lipid metabolism, and they also form a link between circadian rhythm and metabolism. We evaluated the variations in Rev-erb alpha and Rev-erb beta genes together with biochemical parameters as risk factors in type 2 diabetic (T2DM) patients.

METHODS

Molecular analyses of Rev-erb alpha and Rev-erb beta genes were performed on genomic DNA by using next-generation sequencing in 42 T2DM patients (21 obese and 21 non-obese) and 66 healthy controls.

RESULTS

We found 26 rare mutations in the study groups, including 13 missense mutations, 9 silent mutations, 3 5'UTR variations, and a 3'UTR variation, of which 9 were novel variations (5 missense and 3 silent and 1 5'UTR). Six common variations were also found in the Rev-erb genes; Rev-erb beta Chr3:24003765 A > G, Rev-erb beta rs924403442 (Chr3:24006717) G > T, Rev-erb alpha Chr17:38253751 T > C, Rev-erb alpha rs72836608 C > A, Rev-erb alpha rs2314339 C > T and Rev-erb alpha rs2102928 C > T. Of these, Rev-erb beta Chr3:24003765 A > G was a novel missense mutation (p.Q197R), while others were identified as intronic variants. T2DM patients with Rev-erb beta rs924403442 T allele had lower body surface area (BSA) than noncarriers (GG genotype) (p = 0.039). Rev-erb alpha rs72836608 A allele and Rev-erb alpha rs2314339 CC genotype were associated with decreased serum HDL-cholesterol levels in T2DM patients (p = 0.025 and p = 0.027, respectively). In our study, different effects of Rev-erbs polymorphisms were found according to gender and presence of obesity. Rev-erb alpha rs72836608 (C > A) and rs2314339 (C > T) and Rev-erb alpha rs2102928 (C > T) were associated with low HDL-C levels in male T2DM patients. In female patients, Rev-erb alpha rs2102928 (C > T) was associated with high microalbuminuria and Rev-erb beta rs9244403442 G > T was associated with low HDL and high BSA values. In addition, Rev-erb alpha Chr17: 38,253,751 (T > C), rs72836608 (C > A), and rs2314339 (C > T) and Rev-erb beta Chr3:24003765 (A > G) were associated with increased serum GGT levels in obese T2DM patients. In non-obese patients, Rev-erbs SNPs had no effect on serum GGT levels.

CONCLUSION

Our findings indicate that variations in the Rev-erb alpha and Rev-erb beta genes can affect metabolic changes in T2DM and these effects may vary depending on gender and obesity.

Circadian Clock Component Rev-erbα Regulates Diurnal Rhythm of UDP-Glucuronosyltransferase 1a9 and Drug Glucuronidation in Mice

UDP-glucuronosyltransferases (UGTs) are a family of phase II enzymes that play an important role in metabolism and elimination of numerous endo- and xenobiotics. Here, we aimed to characterize diurnal rhythm of Ugt1a9 in mouse liver and to determine the molecular mechanisms underlying the rhythmicity. Hepatic Ugt1a9 mRNA and protein displayed robust diurnal rhythms in wild-type mice with peak levels at zeitgeber time (ZT) 6. Rhythmicity in Ugt1a9 expression was confirmed using synchronized Hepa-1c1c7 cells. We observed time-varying glucuronidation (ZT6 > ZT18) of propofol, a specific Ugt1a9 substrate, consistent with the diurnal pattern of Ugt1a9 protein. Loss of Rev-erbα (a circadian clock component) downregulated the Ugt1a9 expression and blunted its rhythm in mouse liver. Accordingly, propofol glucuronidation was reduced and its dosing time dependency was lost in Rev-erbα ^-/- mice. Dec2 (a transcription factor) was screened to be the potential intermediate that mediated Rev-erbα regulation of Ugt1a9. We confirmed Rev-erbα as a negative regulator of Dec2 in mice and in Hepa-1c1c7 cells. Based on promoter analysis and luciferase reporter assays, it was found that Dec2 trans-repressed Ugt1a9 via direct binding to an E-box-like motif in the gene promoter. Additionally, regulation of Ugt1a9 by Rev-erbα was Dec2-dependent. In conclusion, Rev-erbα generates and regulates rhythmic Ugt1a9 through periodical inhibition of Dec2, a transcriptional repressor of Ugt1a9. Our study may have implications for understanding of circadian clock-controlled drug metabolism and of metabolism-based chronotherapeutics. SIGNIFICANCE STATEMENT: Hepatic Ugt1a9 displays diurnal rhythmicities in expression and glucuronidation activity in mice. It is uncovered that Rev-erbα generates and regulates rhythmic Ugt1a9 through periodical inhibition of Dec2, a transcriptional repressor of Ugt1a9. The findings may have implications for understanding of circadian clock-controlled drug metabolism and of metabolism-based chronotherapeutics.

The Disruption of Liver Metabolic Circadian Rhythms by a Cafeteria Diet Is Sex-Dependent in Fischer 344 Rats

Circadian rhythms are ~24 h fluctuations of different biological processes that are regulated by the circadian clock system. They exert a major influence on most of the metabolism, such as the hepatic metabolism. This rhythmicity can be disrupted by obesogenic diets, fact that is considered to be a risk factor for the development of metabolic diseases. Nevertheless, obesogenic diets do not affect both genders in the same manner. We hypothesized that the circadian rhythms disruption of the hepatic metabolism, caused by obesogenic diets, is gender-dependent. Male and female Fischer 344 rats were fed either a standard diet or a cafeteria diet and sacrificed at two different moments, at zeitgeber 3 and 15. Only female rats maintained the circadian variations of the hepatic metabolism under a cafeteria diet. Most of those metabolites were related with the very low density lipoprotein (VLDL) synthesis, such as choline, betaine or phosphatidylcholine. Most of these metabolites were found to be increased at the beginning of the dark period. On the other hand, male animals did not show these time differences. These findings suggest that females might be more protected against the circadian disruption of the hepatic metabolism caused by a cafeteria diet through the increase of the VLDL synthesis at the beginning of the feeding time.

Circadian Clock-Controlled Drug Metabolism: Implications for Chronotherapeutics

Dependence of drug metabolism on dosing time has long been recognized. However, only recently are the underlying mechanisms for circadian drug metabolism being clarified. Diurnal rhythmicity in expression of drug-metabolizing enzymes is believed to be a key factor determining circadian metabolism. Supporting the notion that biological rhythms are generated and maintained by the circadian clock, a number of diurnal enzymes are under the control of the circadian clock. In general, circadian clock genes generate and regulate diurnal rhythmicity in drug-metabolizing enzymes via transcriptional actions on one or two of three cis-elements (i.e., E-box, D-box, and Rev-erb response element or RAR-related orphan receptor response element). Additionally, cycling or clock-controlled nuclear receptors such as hepatocyte nuclear factor 4α and peroxisome proliferator-activated receptor γ are contributors to diurnal enzyme expression. These newly discovered mechanisms for each of the rhythmic enzymes are reviewed in this article. We also discuss how the rhythms of enzymes are translated to circadian pharmacokinetics and drug chronotoxicity, which has direct implications for chronotherapeutics. Our discussion is also extended to two diurnal transporters (P-glycoprotein and multidrug resistance-associated protein 2) that have an important role in drug absorption. Although the experimental evidence is lacking in metabolism-based chronoefficacy, circadian genes (e.g., Rev-erbα) as drug targets are shown to account for diurnal variability in drug efficacy. SIGNIFICANCE STATEMENT: Significant progress has been made in understanding the molecular mechanisms for generation of diurnal rhythmicity in drug-metabolizing enzymes. In this article, we review the newly discovered mechanisms for each of the rhythmic enzymes and discuss how the rhythms of enzymes are translated to circadian pharmacokinetics and drug chronotoxicity, which has direct implications for chronotherapeutics.

Small Heterodimer Partner Regulates Circadian Cytochromes p450 and Drug-Induced Hepatotoxicity

The role of small heterodimer partner (SHP) in regulation of xenobiotic detoxification remains elusive. Here, we uncover a critical role for SHP in circadian regulation of cytochromes P450 (CYPs) and drug-induced hepatotoxicity.

Methods: The mRNA and protein levels of CYPs in the livers of wild-type and SHP^-/- mice were measured by quantitative real-time polymerase chain reaction and Western blotting, respectively. Regulation of CYP by SHP was investigated using luciferase reporter, mobility shift, chromatin immunoprecipitation, and/or co-immunoprecipitation assays.

Results: The circadian rhythmicities of xenobiotic-detoxifying CYP mRNAs and proteins were disrupted in SHP-deficient mice. Of note, SHP ablation up-regulated Cyp2c38 and Cyp2c39, whereas it down-regulated all other CYP genes. Moreover, SHP regulated the expression of CYP genes through different mechanisms. SHP repressed Lrh-1/Hnf4α to down-regulate Cyp2c38, E4bp4 to up-regulate Cyp2a5, Dec2/HNF1α axis to up-regulate Cyp1a2, Cyp2e1 and Cyp3a11, and Rev-erbα to up-regulate Cyp2b10, Cyp4a10 and Cyp4a14. Furthermore, SHP ablation sensitized mice to theophylline (or mitoxantrone)-induced toxicity. Higher level of toxicity was correlated with down-regulated metabolism and clearance of theophylline (or mitoxantrone). In contrast, SHP ablation blunted the circadian rhythmicity of acetaminophen-induced hepatotoxicity and alleviated the toxicity by down-regulating Cyp2e1-mediated metabolism and reducing formation of the toxic metabolite. Toxicity alleviation by SHP ablation was also observed for aflatoxin B1 due to reduced formation of the toxic epoxide metabolite.

Conclusion: SHP participates in circadian regulation of CYP enzymes, thereby impacting xenobiotic metabolism and drug-induced hepatotoxicity.

E4bp4 regulates carboxylesterase 2 enzymes through repression of the nuclear receptor Rev-erbα in mice

Carboxylesterases (CES) are a family of phase I enzymes that play an important role in xenobiotic clearance and lipid metabolism. Here, we investigate a potential role of E4 promoter-binding protein 4 (E4bp4) in regulation of Ces and CPT-11 (irinotecan, a first-line drug for treating colorectal cancer) pharmacokinetics in mice. Mouse hepatoma Hepa-1c1c7 cells were transfected with Rev-erbα expression plasmid or siRNA targeting E4bp4. The relative mRNA and protein levels of Ces enzymes in the cells or the livers of wild-type and E4bp4-deficient (E4bp4^-/-) mice were determined by qPCR and Western blotting, respectively. Transcriptional regulation of Ces by E4bp4/Rev-erbα were investigated using luciferase reporter, mobility shift, and co-immunoprecipitation (Co-IP) assays. Pharmacokinetic studies were performed with wild-type and E4bp4^-/- mice after intraperitoneal injection of CPT-11. E4bp4 ablation down-regulated an array of hepatic Ces genes in mice. E4bp4^-/- mice also showed reduced Ces-mediated metabolism and elevated systemic exposure of CPT-11, a well-known Ces substrate. Consistently, E4bp4 knockdown reduced the expression of Ces genes (Ces2b, Ces2e and Ces2f) in Hepa-1c1c7 cells. Furthermore, Rev-erbα repressed the transcription of Ces2b, whereas E4bp4 antagonized this repressive action. Co-IP experiment confirmed a direct interaction between E4bp4 and Rev-erbα. Through a combination of promoter analysis and mobility shift assays, we demonstrated that Rev-erbα trans-repressed Ces (Ces2b) through its specific binding to the -767 to-754 bp promoter region. In conclusion, E4bp4 regulates Ces enzymes through inhibition of the transrepression activity of Rev-erbα, thereby impacting the metabolism and pharmacokinetics of Ces substrates.

Transcriptomic and proteomic analysis of potential therapeutic target genes in the liver of metformin‑treated Sprague‑Dawley rats with type 2 diabetes mellitus

The main actions of metformin are as follows: To reduce hyperglycemia via the suppression of gluconeogenesis, improve glucose uptake and insulin sensitivity, and stimulate activation of adenosine monophosphate-activated protein kinase during the treatment of diabetes mellitus. It is well known that metformin acts via complex mechanisms, including multitarget and multipathway mechanisms; however, the multi-targeted antidiabetic genes of metformin remain obscure. The present study aimed to perform transcriptomic and proteomic analysis of potential therapeutic target genes in the liver of metformin-treated Sprague-Dawley rats with type 2 diabetes mellitus. The type 2 diabetes rat model was established using streptozotocin. Fasting blood glucose, hemoglobin A1c, serum insulin and biological parameters were subsequently measured. Differentially expressed genes (DEGs) and proteins were identified in the rat livers by expression profile analysis and isobaric tags for relative and absolute quantitation (iTRAQ). A 1.5-fold alteration in gene expression, as determined using chip-based expression profile analysis, and a 1.2-fold alteration in protein expression, as determined using iTRAQ, were considered physiologically significant benchmarks, which were used to identify DEGS in metformin-treated rats with type 2 diabetes mellitus. The DEGs were verified using quantitative polymerase chain reaction (qPCR) and western blot analysis. Numerous hepatic genes involved in various metabolic pathways were affected by metformin; in particular, genes associated with lipid metabolism were markedly affected. Expression profile analysis and iTRAQ analysis suggested that carboxylesterase 1C subunit (Ces1C) and cholesterol 7α-hydroxylyase (Cyp7a1) may serve as important DEGs, which were validated by qPCR and western blot analysis. Ces1C and Cyp7a1 are the main enzymes in cholesterol metabolism, yet the result of western blotting was not consistent with qPCR. The present study demonstrated that metformin may affect the expression of numerous hepatic genes involved in metabolic pathways, particularly the lipid and cholesterol metabolic pathways. Ces1C and Cyp7a1 may be considered novel therapeutic target genes in the liver, which are involved in the antidiabetic effects of metformin.

REV-ERBα Regulates CYP7A1 Through Repression of Liver Receptor Homolog-1

Nuclear heme receptor reverse erythroblastosis virus (REV-ERB) α (a transcriptional repressor) is known to regulate cholesterol 7α-hydroxylase (CYP7A1) and bile acid synthesis. However, the mechanism for REV-ERBα regulation of CYP7A1 remains elusive. Here, we investigate the role of LRH-1 in REV-ERBα regulation of CYP7A1 and cholesterol metabolism. We first characterized the tertiary amine N-(4-chloro-2-methylbenzyl)-N-(4-chlorobenzyl)-1-(5-nitrothiophen-2-yl)methanamine (GSK2945) as a highly specific Rev-erbα/REV-ERBα antagonist using cell-based assays and confirmed expression of Rev-erbα in mouse liver. GSK2945 treatment increased hepatic mouse cholesterol 7α-hydroxylase (Cyp7a1) level and lowered plasma cholesterol in wild-type mice. Likewise, the compound increased the expression and microsomal activity of Cyp7a1 in hypercholesterolemic mice. This coincided with reduced plasma and liver cholesterol and enhanced production of bile acids. Increased levels of Cyp7a1/CYP7A1 were also found in mouse and human primary hepatocytes after GSK2945 treatment. In these experiments, we observed parallel increases in Lrh-1/LRH-1 (a known hepatic activator of Cyp7a1/CYP7A1) mRNA and protein. Luciferase reporter, mobility shift, and chromatin immunoprecipitation assays revealed that Lrh-1/LRH-1 was a direct Rev-erbα/REV-ERBα target gene. Furthermore, conditional deletion of Lrh-1 in the liver abrogated the regulatory effects of Rev-erbα on Cyp7a1 and cholesterol metabolism in mice. In conclusion, Rev-erbα regulates Cyp7a1 and cholesterol metabolism through its repression of the Lrh-1 receptor. Targeting the REV-ERBα/LRH-1 axis may represent a novel approach for management of cholesterol-related diseases.

Dietary fish oil differentially ameliorates high-fructose diet-induced hepatic steatosis and hyperlipidemia in mice depending on time of feeding

Chrononutrition is the science of nutrition based on chronobiology. Numerous epidemiological studies have shown that fish oil (FO) reduces the risk of cardiovascular events through various actions such as lowering triglycerides. The present study aimed to determine the time of day when the hypertriglyceridemia-decreasing ability of FO is optimal in mice. A high-fructose diet (HFrD) that induces hyperlipidemia in mice was replaced with the same diet containing 4% FO (HFrD-4% FO) at different times of the day for 2 weeks as described below. Mice were fed with HFrD alone (CTRL) or with HFrD containing 4% FO for 12 h around the time of activity onset [breakfast (BF)-FO] or offset [dinner (DN)-FO]. Plasma and liver concentrations of triglycerides and total cholesterol were reduced in BF-FO but not in DN-FO mice compared with CTRL mice. The temporal expression of genes associated with fatty acid synthesis such as Fasn, Acaca, Scd1 and Acly in the liver was significantly suppressed in both BF-FO and DN-FO mice. Expression levels of Scd1 in epididymal adipose tissue were significantly suppressed only in the BF-FO mice. Plasma concentrations of docosahexaenoic acid and eicosapentaenoic acid were far more increased in BF-FO than in DN-FO mice. Significantly more of these n-3 polyunsaturated fatty acids (PUFAs) were excreted in the feces of DN-FO than of BF-FO mice. These findings suggest that dietary FO exerts more hypolipidemic activity at the time of breakfast than dinner because the intestinal absorption of n-3 PUFAs is more effective at that time.

Profiling of the circadian metabolome in thioacetamide-induced liver cirrhosis in mice

Liver cirrhosis can disturb circadian rhythms, decreasing patient quality of life. Changes in metabolic products in cirrhosis are poorly understood. We evaluated changes in liver metabolism products using a thioacetamide‐induced mouse model of liver cirrhosis exhibiting circadian rhythm disturbance. Principal component analysis indicated that the circular progression found in the control group was disrupted in the thioacetamide group, and Jonckheere‐Terpstra‐Kendall analysis showed an imbalanced pattern of oscillating metabolic products. In addition to changes in serotonin and other vitamin A–related metabolites, differences in metabolic products associated with energetics, redox homeostasis, bile acid production, inflammation, and other processes were identified. Carbohydrate metabolism showed a reduction in metabolic products associated with the tricarboxylic acid cycle, suggesting up‐regulation of glycolysis and reduced mitochondrial activity. Lipid metabolism showed an increase in ω‐oxidation products, suggesting decreased β‐oxidation. Conclusion: These data will be useful for chronotherapy and modulation of circadian rhythms in patients with liver damage. (Hepatology Communications 2017;1:704–718)

The sweet tooth of the circadian clock

The endogenous circadian clock is a key regulator of daily metabolic processes. On the other hand, circadian clocks in a broad range of tissues can be tuned by extrinsic and intrinsic metabolic cues. The bidirectional interaction between circadian clocks and metabolism involves both transcriptional and post-translational mechanisms. Nuclear receptors exemplify the transcriptional programs that couple molecular clocks to metabolism. The post-translational modifications of the core clock machinery are known to play a key role in metabolic entrainment of circadian clocks. O-linked N-acetylglucosamine modification (O-GlcNAcylation) of intracellular proteins is a key mediator of metabolic response to nutrient availability. This review highlights our current understanding of the role of protein O-GlcNAcylation in mediating metabolic input and output of the circadian clock.

Atorvastatin alters the expression of genes related to bile acid metabolism and circadian clock in livers of mice

Aim

Atorvastatin is a HMG-CoA reductase inhibitor used for hyperlipidemia. Atorvastatin is generally safe but may induce cholestasis. The present study aimed to examine the effects of atorvastatin on hepatic gene expression related to bile acid metabolism and homeostasis, as well as the expression of circadian clock genes in livers of mice.

Methods

Adult male mice were given atorvastatin (10, 30, and 100 mg/kg, po) daily for 30 days, and blood biochemistry, histopathology, and gene expression were examined.

Results

Repeated administration of atorvastatin did not affect animal body weight gain or liver weights. Serum enzyme activities were in the normal range. Histologically, the high dose of atorvastatin produced scattered swollen hepatocytes, foci of feathery-like degeneration, together with increased expression of Egr-1 and metallothionein-1. Atorvastatin increased the expression of Cyp7a1 in the liver, along with FXR and SHP. In contract, atorvastatin decreased the expression of bile acid transporters Ntcp, Bsep, Ostα, and Ostβ. The most dramatic change was the 30-fold induction of Cyp7a1. Because Cyp7a1 is a circadian clock-controlled gene, we further examined the effect of atorvastatin on clock gene expression. Atorvastatin increased the expression of clock core master genes Bmal1 and Npas2, decreased the expression of clock feedback genes Per2, Per3, and the clock targeted genes Dbp and Tef, whereas it had no effect on Cry1 and Nr1d1 expression.

Conclusion

Repeated administration of atorvastatin affects bile acid metabolism and markedly increases the expression of the bile acid synthesis rate-limiting enzyme gene Cyp7a1, together with alterations in the expression of circadian clock genes.

Hypoxic repression of CYP7A1 through a HIF-1α- and SHP-independent mechanism

Liver cells experience hypoxic stress when drug-metabolizing enzymes excessively consume O2 for hydroxylation. Hypoxic stress changes the transcription of several genes by activating a heterodimeric transcription factor called hypoxia-inducible factor- 1α/β (HIF-1α/β). We found that hypoxic stress (0.1% O2) decreased the expression of cytochrome P450 7A1 (CYP7A1), a rate-limiting enzyme involved in bile acid biosynthesis. Chenodeoxycholic acid (CDCA), a major component of bile acids, represses CYP7A1 by activating a transcriptional repressor named small heterodimer partner (SHP). We observed that hypoxia decreased the levels of both CDCA and SHP, suggesting that hypoxia repressed CYP7A1 without inducing SHP. The finding that overexpression of HIF-1α increased the activity of the CYP7A1 promoter suggested that hypoxia decreased the expression of CYP7A1 in a HIF-1-independent manner. Thus, the results of this study suggested that hypoxia decreased the activity of CYP7A1 by limiting its substrate O2, and by decreasing the transcription of CYP7A1. [BMB Reports 2016; 49(3): 173-178].

Circadian regulation of metabolic homeostasis: causes and consequences

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

Nuclear receptors in acute and chronic cholestasis

BACKGROUND

Nuclear receptors (NRs) form a family of 48 members. NRs control hepatic processes such as bile acid homeostasis, lipid metabolism and mechanisms involved in fibrosis and inflammation. Due to their central role in the regulation of hepatoprotective mechanisms, NRs are promising therapeutic targets in cholestatic disorders.

KEY MESSAGES

NRs can be classified into five different physiological clusters. NRs from the 'bile acids and xenobiotic metabolism' and from the 'lipid metabolism and energy homeostasis' clusters are strongly expressed in the liver. Furthermore, NRs from these clusters, such as farnesoid X receptor α (FXRα), pregnane X receptor (PXR) and peroxisome proliferator-activated receptors (PPARs), have been associated with the pathogenesis and the progression of cholestasis. The latter observation is also true for vitamin D receptor (VDR), which is barely detectable in the whole liver, but has been linked to cholestatic diseases. Involvement of VDR in cholestasis is ascribed to a strong expression in nonparenchymal liver cells, such as biliary epithelial cells, Kupffer cells and hepatic stellate cells. Likewise, NRs from other physiological clusters with low hepatic expression, such as estrogen receptor α (ERα) or reverse-Erb α/β (REV-ERB α/β), may also control pathophysiological processes in cholestasis.

CONCLUSIONS

In this review, we will describe the impact of individual NRs on cholestasis. We will then discuss the potential role of these transcription factors as therapeutic targets.

Bile acid signaling in metabolic disease and drug therapy

Bile acids are the end products of cholesterol catabolism. Hepatic bile acid synthesis accounts for a major fraction of daily cholesterol turnover in humans. Biliary secretion of bile acids generates bile flow and facilitates hepatobiliary secretion of lipids, lipophilic metabolites, and xenobiotics. In the intestine, bile acids are essential for the absorption, transport, and metabolism of dietary fats and lipid-soluble vitamins. Extensive research in the last 2 decades has unveiled new functions of bile acids as signaling molecules and metabolic integrators. The bile acid-activated nuclear receptors farnesoid X receptor, pregnane X receptor, constitutive androstane receptor, vitamin D receptor, and G protein-coupled bile acid receptor play critical roles in the regulation of lipid, glucose, and energy metabolism, inflammation, and drug metabolism and detoxification. Bile acid synthesis exhibits a strong diurnal rhythm, which is entrained by fasting and refeeding as well as nutrient status and plays an important role for maintaining metabolic homeostasis. Recent research revealed an interaction of liver bile acids and gut microbiota in the regulation of liver metabolism. Circadian disturbance and altered gut microbiota contribute to the pathogenesis of liver diseases, inflammatory bowel diseases, nonalcoholic fatty liver disease, diabetes, and obesity. Bile acids and their derivatives are potential therapeutic agents for treating metabolic diseases of the liver.

Circadian rhythms in liver metabolism and disease

Mounting research evidence demonstrates a significant negative impact of circadian disruption on human health. Shift work, chronic jet lag and sleep disturbances are associated with increased incidence of metabolic syndrome, and consequently result in obesity, type 2 diabetes and dyslipidemia. Here, these associations are reviewed with respect to liver metabolism and disease.

Non-alcoholic fatty liver disease: the role of nuclear receptors and circadian rhythmicity

Non-alcoholic fatty liver disease (NAFLD) is the accumulation of triglycerides in the hepatocytes in the absence of excess alcohol intake, and is caused by an imbalance between hepatic synthesis and breakdown of fats, as well as fatty acid storage and disposal. Liver metabolic pathways are driven by circadian biological clocks, and hepatic health is maintained by proper timing of circadian patterns of metabolic gene expression with the alternation of anabolic processes corresponding to feeding/activity during wake times, and catabolic processes characterizing fasting/resting during sleep. A number of nuclear receptors in the liver are expressed rhythmically, bind hormones and metabolites, sense energy flux and expenditure, and connect the metabolic pathways to the molecular clockwork throughout the 24-h day. In this review, we describe the role played by the nuclear receptors in the genesis of NAFLD in relationship with the circadian clock circuitry.

Identification of transcription factors and single nucleotide polymorphisms of Lrh1 and its homologous genes in Lrh1-knockout pancreas of mice

BACKGROUND

To identify transcription factors (TFs) and single nucleotide polymorphisms (SNPs) of Lrh1 (also named Nr5a2) and its homologous genes in Lrh1-knockout pancreas of mice.

METHODS

The RNA-Seq data GSE34030 were downloaded from Gene Expression Omnibus (GEO) database, including 2 Lrh1 pancreas knockout samples and 2 wild type samples. All reads were processed through TopHat and Cufflinks package to calculate gene-expression level. Then, the differentially expressed genes (DEGs) were identified via non-parametric algorithm (NOISeq) methods in R package, of which the homology genes of Lrh1 were identified via BLASTN analysis. Furthermore, the TFs of Lrh1 and its homologous genes were selected based on TRANSFAC database. Additionally, the SNPs were analyzed via SAM tool to record the locations of mutant sites.

RESULTS

Total 15683 DEGs were identified, of which 23 was Lrh1 homology genes (3 up-regulated and 20 down-regulated). Fetoprotein TF (FTF) was the only TF of Lrh1 identified and the promoter-binding factor of FTF was CYP7A. The SNP annotations of Lrh1 homologous genes showed that 92% of the mutation sites were occurred in intron and upstream. Three SNPs of Lrh1 were located in intron, while 1819 SNPs of Phkb were located in intron and 1343 SNPs were located in the upstream region.

CONCLUSION

FTF combined with CYP7A might play an important role in Lrh1 regulated pancreas-specific transcriptional network. Furthermore, the SNPs analysis of Lrh1 and its homology genes provided the candidate mutant sites that might affect the Lrh1-related production and secretion of pancreatic fluid.

Understanding circadian gene function: animal models of tissue-specific circadian disruption

Circadian rhythms are the daily patterns that occur within an organism, from gene expression to behavior. These rhythms are governed not only externally by environmental cues but also internally, with cell-autonomous molecular clock mechanisms present nearly ubiquitously throughout the cells of organisms. In more complex organisms, it has been suggested that the clock mechanisms serve varied functions depending on the tissue in which they are found. By disrupting core circadian gene function in specific tissues of animal models, the various roles of the circadian clock in differing tissues can begin to be defined. This review provides an overview of the model organisms used to elucidate tissue-specific functions of the molecular circadian clock.

Disturbances in the murine hepatic circadian clock in alcohol-induced hepatic steatosis

To investigate the role of the circadian clock in the development of alcohol-induced fatty liver disease we examined livers of mice chronically alcohol-fed over 4-weeks that resulted in steatosis. Here we show time-of-day specific changes in expression of clock genes and clock-controlled genes, including those associated with lipid and bile acid regulation. Such changes were not observed following a 1-week alcohol treatment with no hepatic lipid accumulation. Real-time bioluminescence reporting of PERIOD2 protein expression suggests that these changes occur independently of the suprachiasmatic nucleus pacemaker. Further, we find profound time-of-day specific changes to the rhythmic synthesis/accumulation of triglycerides, cholesterol and bile acid, and the NAD/NADH ratio, processes that are under clock control. These results highlight not only that the circadian timekeeping system is disturbed in the alcohol-induced hepatic steatosis state, but also that the effects of alcohol upon the clock itself may actually contribute to the development of hepatic steatosis.

Loss of von Hippel-Lindau protein (VHL) increases systemic cholesterol levels through targeting hypoxia-inducible factor 2α and regulation of bile acid homeostasis

Cholesterol synthesis is a highly oxygen-dependent process. Paradoxically, hypoxia is correlated with an increase in cellular and systemic cholesterol levels and risk of cardiovascular diseases. The mechanism for the increase in cholesterol during hypoxia is unclear. Hypoxia signaling is mediated through hypoxia-inducible factor 1α (HIF-1α) and HIF-2α. The present study demonstrates that activation of HIF signaling in the liver increases hepatic and systemic cholesterol levels due to a decrease in the expression of cholesterol hydroxylase CYP7A1 and other enzymes involved in bile acid synthesis. Specifically, activation of hepatic HIF-2α (but not HIF-1α) led to hypercholesterolemia. HIF-2α repressed the circadian expression of Rev-erbα, resulting in increased expression of E4BP4, a negative regulator of Cyp7a1. To understand if HIF-mediated decrease in bile acid synthesis is a physiologically relevant pathway by which hypoxia maintains or increases systemic cholesterol levels, two hypoxic mouse models were assessed, an acute lung injury model and mice exposed to 10% O2 for 3 weeks. In both models, cholesterol levels increased with a concomitant decrease in expression of genes involved in bile acid synthesis. The present study demonstrates that hypoxic activation of hepatic HIF-2α leads to an adaptive increase in cholesterol levels through inhibition of bile acid synthesis.

Circadian rhythms in liver physiology and liver diseases

In mammals, circadian rhythms function to coordinate a diverse panel of physiological processes with environmental conditions such as food and light. As the driving force for circadian rhythmicity, the molecular clock is a self-sustained transcription-translational feedback loop system consisting of transcription factors, epigenetic modulators, kinases/phosphatases, and ubiquitin E3 ligases. The molecular clock exists not only in the suprachiasmatic nuclei of the hypothalamus but also in the peripheral tissues to regulate cellular and physiological function in a tissue-specific manner. The circadian clock system in the liver plays important roles in regulating metabolism and energy homeostasis. Clock gene mutant animals display impaired glucose and lipid metabolism and are susceptible to diet-induced obesity and metabolic dysfunction, providing strong evidence for the connection between the circadian clock and metabolic homeostasis. Circadian-controlled hepatic metabolism is partially achieved by controlling the expression and/or activity of key metabolic enzymes, transcription factors, signaling molecules, and transporters. Reciprocally, intracellular metabolites modulate the molecular clock activity in response to the energy status. Although still at the early stage, circadian clock dysfunction has been implicated in common chronic liver diseases. Circadian dysregulation of lipid metabolism, detoxification, reactive oxygen species (ROS) production, and cell-cycle control might contribute to the onset and progression of liver steatosis, fibrosis, and even carcinogenesis. In summary, these findings call for a comprehensive study of the function and mechanisms of hepatic circadian clock to gain better understanding of liver physiology and diseases.

Nuclear receptor control of enterohepatic circulation

Enterohepatic circulation is responsible for the capture of bile acids and other steroids produced or metabolized in the liver and secreted to the intestine, for reabsorption back into the circulation and transport back to the liver. Bile acids are secreted from the liver in the form of mixed micelles that also contain phosphatidylcholines and cholesterol that facilitate the uptake of fats and vitamins from the diet due to the surfactant properties of bile acids and lipids. Bile acids are synthesized in the liver from cholesterol by a cascade of enzymes that carry out oxidation and conjugation reactions, and transported to the bile duct and gall bladder where they are stored before being released into the intestine. Bile flow from the gall bladder to the small intestine is triggered by food intake in accordance with its role in lipid and vitamin absorption from the diet. Bile acids are further metabolized by gut bacteria and are transported back to the circulation. Metabolites produced in the liver are termed primary bile acids or primary conjugated bile salts, while the metabolites generated by bacterial are called secondary bile acids. About 95% of bile acids are reabsorbed in the proximal and distal ileum into the hepatic portal vein and then into the liver sinusoids, where they are efficiently transported into the liver with little remaining in circulation. Each bile acid is reabsorbed about 20 times on average before being eliminated. Enterohepatic circulation is under tight regulation by nuclear receptor signaling, notably by the farnesoid X receptor (FXR).

Retinoic acid-related orphan receptor α regulates diurnal rhythm and fasting induction of sterol 12α-hydroxylase in bile acid synthesis

Sterol 12α-hydroxylase (CYP8B1) is required for cholic acid synthesis and plays a critical role in intestinal cholesterol absorption and pathogenesis of cholesterol gallstone, dyslipidemia, and diabetes. In this study we investigated the underlying mechanism of fasting induction and circadian rhythm of CYP8B1 by a cholesterol-activated nuclear receptor and core clock gene retinoic acid-related orphan receptor α (RORα). Fasting stimulated, whereas restricted-feeding reduced expression of CYP8B1 mRNA and protein. However, fasting and feeding had little effect on the diurnal rhythm of RORα mRNA expression, but fasting increased RORα protein levels by cAMP-activated protein kinase A-mediated phosphorylation and stabilization of the protein. Adenovirus-mediated gene transduction of RORα to mice strongly induced CYP8B1 expression, and increased liver cholesterol and 12α-hydroxylated bile acids in the bile acid pool and serum. A reporter assay identified a functional RORα response element in the CYP8B1 promoter. RORα recruited cAMP response element-binding protein-binding protein (CBP) to stimulate histone acetylation on the CYP8B1 gene promoter. In conclusion, RORα is a key regulator of diurnal rhythm and fasting induction of CYP8B1, which regulates bile acid composition and serum and liver cholesterol levels. Antagonizing RORα activity may be a therapeutic strategy for treating inflammatory diseases such as non-alcoholic fatty liver disease and type 2 diabetes.

Sex differences in the circadian variation of cytochrome p450 genes and corresponding nuclear receptors in mouse liver

Sex differences and circadian variation are two major factors that affect the expression of drug-processing genes. This study aimed to examine sex differences in the circadian variation of hepatic cytochrome P450 (Cyp) genes and corresponding nuclear receptors. Adult mice were acclimated to environmentally controlled facilities for 2 wks, and livers were collected every 4 h during a 24-h period. Total RNA and protein were isolated and subjected to real-time reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis. The mRNA expression of the aryl hydrocarbon receptor (AhR) and AhR-regulated Cyp1a1 and Cyp1a2 were higher in females and higher during the light phase. The mRNA expression of constitutive and rostane receptor (CAR) and CYP2B10 protein was female-predominant and higher in the dark phase. Pregnane X receptor (PXR) peaked around 18:00 h, but PXR-regulated Cyp3a11 and Cyp3a25 were higher at 10:00 h, without apparent sex dimorphism at protein levels. Peroxisome proliferator-activated receptor-α (PPARα), Cyp4a10, and Cyp4a14 were higher in females and peaked between 14:00 and 18:00 h. The mRNA levels of farnesoid X receptor (FXR), Cyp7a1, and Cyp27a1 peaked around 18:00 h and CYP7A1 protein was higher during the dark phase and higher in females. Cyp7b1(male-predominant) and Cyp2a4 (female-predominant) both showed circadian variation. Circadian variation of hepatic clock genes such as nuclear receptor Rev-erbα, cryptochrome 1 (Cry1), and brain muscle ARNT-like protein 1 (Bmal1) showed distinct patterns. Sex differences and circadian rhythmicity of Cyp genes and corresponding nuclear receptors exist in mouse liver that could impact xenobiotic metabolism and toxicity at different times of the day.

Circadian clocks and metabolism

Circadian clocks maintain periodicity in internal cycles of behavior, physiology, and metabolism, enabling organisms to anticipate the 24-h rotation of the Earth. In mammals, circadian integration of metabolic systems optimizes energy harvesting and utilization across the light/dark cycle. Disruption of clock genes has recently been linked to sleep disorders and to the development of cardiometabolic disease. Conversely, aberrant nutrient signaling affects circadian rhythms of behavior. This chapter reviews the emerging relationship between the molecular clock and metabolic systems and examines evidence that circadian disruption exerts deleterious consequences on human health.

11β-Hydroxysteroid dehydrogenase type 1 contributes to the regulation of 7-oxysterol levels in the arterial wall through the inter-conversion of 7-ketocholesterol and 7β-hydroxycholesterol

The atherogenic 7-oxysterols, 7-ketocholesterol (7-KC) and 7β-hydroxycholesterol (7βOHC), can directly impair arterial function. Inter-conversion of 7-KC and 7βOHC has recently been shown as a novel role for the glucocorticoid-metabolizing enzyme 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1). Since this enzyme is expressed in vascular smooth muscle cells, we addressed the hypothesis that inter-conversion of 7-KC and 7βOHC by 11β-HSD1 may contribute to regulation of arterial function.

Incubation (4–24 h) of aortic rings with either 7-KC (25 μM) or 7βOHC (20 μM) had no effect on endothelium-dependent (acetylcholine) or -independent (sodium nitroprusside) relaxation. In contrast, exposure to 7-KC (but not to 7βOHC) attenuated noradrenaline-induced contraction (E_max) after 4 h (0.78 ± 0.28 vs 0.40 ± 0.08 mN/mm; p < 0.05) and 24 h (2.28 ± 0.34 vs 1.56 ± 0.48 mN/mm; p < 0.05). Both 7-oxysterols were detected by GCMS in the aortic wall of chow-fed C57Bl6/J mice, with concentrations of 7-KC (1.41 ± 0.81 ng/mg) higher (p = 0.05) than 7βOHC (0.16 ± 0.06 ng/mg). In isolated mouse aortic rings 11β-HSD1 was shown to act as an oxo-reductase, inter-converting 7-KC and 7βOHC. This activity was lost in aorta from 11β-HSD1^−/− mice, which had low oxysterol levels. Renal homogenates from 11β-HSD1^−/− mice were used to confirm that the type 2 isozyme of 11β-HSD does not inter-convert 7-KC and 7βOHC.

These results demonstrate that 7-KC has greater effects than 7βOHC on vascular function, and that 11β-HSD1 can inter-convert 7-KC and 7βOHC in the arterial wall, contributing to the regulation of 7-oxysterol levels and potentially influencing vascular function. This mechanism may be important in the cardioprotective effects of 11β-HSD1 inhibitors.

Does the circadian system regulate lactation?

Environmental variables such as photoperiod, heat, stress, nutrition and other external factors have profound effects on quality and quantity of a dairy cow's milk. The way in which the environment interacts with genotype to impact milk production is unknown; however, evidence from our laboratory suggests that circadian clocks play a role. Daily and seasonal endocrine rhythms are coordinated in mammals by the master circadian clock in the hypothalamus. Peripheral clocks are distributed in every organ and coordinated by signals from the master clock. We and others have shown that there is a circadian clock in the mammary gland. Approximately 7% of the genes expressed during lactation had circadian patterns including core clock and metabolic genes. Amplitude changes occurred in the core mammary clock genes during the transition from pregnancy to lactation and were coordinated with changes in molecular clocks among multiple tissues. In vitro studies using a bovine mammary cell line showed that external stimulation synchronized mammary clocks, and expression of the core clock gene, BMAL1, was induced by lactogens. Female clock/clock mutant mice, which have disrupted circadian rhythms, have impaired mammary development and their offspring failed to thrive suggesting that the dam's milk production was not adequate enough to nourish their young. We envision that, in mammals, during the transition from pregnancy to lactation the master clock is modified by environmental and physiological cues that it receives, including photoperiod length. In turn, the master clock coordinates changes in endocrine milieu that signals peripheral tissues. In dairy cows, it is clear that changes in photoperiod during the dry period and/or during lactation influences milk production. We believe that the photoperiod effect on milk production is mediated, in part by the 'setting' of the master clock with light, which modifies peripheral circadian clocks including the mammary core clock and subsequently impacts milk yield and may impact milk composition.

Genome-wide location analysis reveals distinct transcriptional circuitry by paralogous regulators Foxa1 and Foxa2

Gene duplication is a powerful driver of evolution. Newly duplicated genes acquire new roles that are relevant to fitness, or they will be lost over time. A potential path to functional relevance is mutation of the coding sequence leading to the acquisition of novel biochemical properties, as analyzed here for the highly homologous paralogs Foxa1 and Foxa2 transcriptional regulators. We determine by genome-wide location analysis (ChIP-Seq) that, although Foxa1 and Foxa2 share a large fraction of binding sites in the liver, each protein also occupies distinct regulatory elements in vivo. Foxa1-only sites are enriched for p53 binding sites and are frequently found near genes important to cell cycle regulation, while Foxa2-restricted sites show only a limited match to the forkhead consensus and are found in genes involved in steroid and lipid metabolism. Thus, Foxa1 and Foxa2, while redundant during development, have evolved divergent roles in the adult liver, ensuring the maintenance of both genes during evolution.

The duplication of a gene from a common ancestor, resulting in two copies known as paralogs, plays an important role in evolution. Newly duplicated genes must acquire new functions in order to remain relevant, otherwise they are lost via mutation over time. We have performed genome-wide location analysis (ChIP–Seq) in adult liver to examine the differences between two paralogous DNA binding proteins, Foxa1 and Foxa2. While Foxa1 and Foxa2 bind a number of common genomic locations, each protein also localizes to distinct regulatory regions. Sites specific for Foxa1 also contain a DNA motif bound by tumor suppressor p53 and are found near genes important to cell cycle regulation, while Foxa2-only sites are found near genes essential to steroid and lipid metabolism. Hence, Foxa1 and Foxa2 have developed unique functions in adult liver, contributing to the maintenance of both genes during evolution.

Expression and regulation of cholesterol 7 alpha-hydroxylase: An update

Cholesterol 7-alpha hydroxylase (CYP7A1) is the first and rate-limiting enzyme in the neutral pathway of bile acids synthesis. The expression of CYP7A1 can be regulated not only by diurnal rhythm, but also by gene polymorphism, diet, hormones, cytokines and drugs. CYP7A1 gene polymorphism is associated not only with some diseases but also with response to drug therapy. A cascade network consisting of multiple nuclear receptors is involved in the regulation of CYP7A1 expression to control bile acid synthesis and lipid metabolism.

Identification of a binding motif specific to HNF4 by comparative analysis of multiple nuclear receptors

Nuclear receptors (NRs) regulate gene expression by binding specific DNA sequences consisting of AG[G/T]TCA or AGAACA half site motifs in a variety of configurations. However, those motifs/configurations alone do not adequately explain the diversity of NR function in vivo. Here, a systematic examination of DNA binding specificity by protein-binding microarrays (PBMs) of three closely related human NRs—HNF4α, retinoid X receptor alpha (RXRα) and COUPTF2—reveals an HNF4-specific binding motif (H4-SBM), xxxxCAAAGTCCA, as well as a previously unrecognized polarity in the classical DR1 motif (AGGTCAxAGGTCA) for HNF4α, RXRα and COUPTF2 homodimers. ChIP-seq data indicate that the H4-SBM is uniquely bound by HNF4α but not 10 other NRs in vivo, while NRs PXR, FXRα, Rev-Erbα appear to bind adjacent to H4-SBMs. HNF4-specific DNA recognition and transactivation are mediated by residues Asp69 and Arg76 in the DNA-binding domain; this combination of amino acids is unique to HNF4 among all human NRs. Expression profiling and ChIP data predict ∼100 new human HNF4α target genes with an H4-SBM site, including several Co-enzyme A-related genes and genes with links to disease. These results provide important new insights into NR DNA binding.

Regulation of basic helix-loop-helix transcription factors Dec1 and Dec2 by RORα and their roles in adipogenesis

DEC1 and DEC2, members of the basic helix-loop-helix superfamily, are involved in various biological phenomena including clock systems, cell differentiation and metabolism. In clock systems, Dec1 and Dec2 expression are up-regulated by the CLOCK:BMAL1 heterodimer via E-box (CACGTG), exhibiting a circadian rhythm in the suprachiasmatic nucleus (SCN), the central circadian pacemaker and other peripheral tissues. In this study, using assays of luciferase reporters, electrophoretic mobility shift and chromatin immunoprecipitation, we identified novel nuclear receptor response elements, ROR response elements (RORE), in Dec1 and Dec2 promoters. These ROREs responded to the transcriptional activator RORα, but not to the repressor REVERBα, although the Bmal1 promoter responded to both RORα and REVERBα. Therefore, RORα, but not REVERBα, is involved in the regulation of Dec1 and Dec2 expression without significantly affecting their rhythmicity. Since RORα, DEC1 and DEC2 reportedly suppressed adipogenic differentiation, we examined expression of Rorα, Dec1, Dec2 and other clock-controlled genes in differentiating 3T3-L1 adipocytes. The results suggested that RORα suppresses adipogenic differentiation at a later stage of differentiation by RORE-mediated stimulation of Dec1 and Dec2 expression.

Role of liver X receptors in cholesterol efflux and inflammatory signaling (review)

Liver X receptors (LXRs) are nuclear receptors that play a central role in cholesterol metabolism. When activated, LXRs induce a series of genes that are involved in cholesterol efflux, absorption, transport and excretion. In recent studies, LXRs have also been shown to play an important role in inflammatory signaling. LXR agonists show promise as potential therapeutics, given their anti-atherogenic and anti-inflammatory properties. The function of LXRs in cholesterol efflux and inflammatory signaling make them attractive as therapies for cardiovascular and inflammatory diseases.

Glucose and insulin induction of bile acid synthesis: mechanisms and implication in diabetes and obesity

Bile acids facilitate postprandial absorption of nutrients. Bile acids also activate the farnesoid X receptor (FXR) and the G protein-coupled receptor TGR5 and play a major role in regulating lipid, glucose, and energy metabolism. Transgenic expression of cholesterol 7α-hydroxylase (CYP7A1) prevented high fat diet-induced diabetes and obesity in mice. In this study, we investigated the nutrient effects on bile acid synthesis. Refeeding of a chow diet to fasted mice increased CYP7A1 expression, bile acid pool size, and serum bile acids in wild type and humanized CYP7A1-transgenic mice. Chromatin immunoprecipitation assays showed that glucose increased histone acetylation and decreased histone methylation on the CYP7A1 gene promoter. Refeeding also induced CYP7A1 in fxr-deficient mice, indicating that FXR signaling did not play a role in postprandial regulation of bile acid synthesis. In streptozocin-induced type I diabetic mice and genetically obese type II diabetic ob/ob mice, hyperglycemia increased histone acetylation status on the CYP7A1 gene promoter, leading to elevated basal Cyp7a1 expression and an enlarged bile acid pool with altered bile acid composition. However, refeeding did not further increase CYP7A1 expression in diabetic mice. In summary, this study demonstrates that glucose and insulin are major postprandial factors that induce CYP7A1 gene expression and bile acid synthesis. Glucose induces CYP7A1 gene expression mainly by epigenetic mechanisms. In diabetic mice, CYP7A1 chromatin is hyperacetylated, and fasting to refeeding response is impaired and may exacerbate metabolic disorders in diabetes.

R-α-lipoic acid does not reverse hepatic inflammation of aging, but lowers lipid anabolism, while accentuating circadian rhythm transcript profiles

To determine the effects of age and lipoic acid supplementation on hepatic gene expression, we fed young (3 mo) and old (24 mo) male Fischer 344 rats a diet with or without 0.2% (wt/wt) R-α-lipoic acid (LA) for 2 wk. Total RNA isolated from liver tissue was analyzed by Affymetrix microarray to examine changes in transcriptional profiles. Results showed elevated proinflammatory gene expression in the aging liver and evidence for increased immune cell activation and tissue remodeling, together representing 45% of the age-related transcriptome changes. In addition, age-related increases in transcripts of genes related to fatty acid, triglyceride, and cholesterol synthesis, including acetyl-CoA carboxylase-β (Acacb) and fatty acid synthase (Fasn), were observed. Supplementation of old animals with LA did not reverse the necroinflammatory phenotype but, intriguingly, altered the expression of genes governing circadian rhythm. Most notably, Arntl, Npas2, and Per changed in a coordinated manner with respect to rhythmic transcription. LA further caused a decrease in transcripts of several bile acid and lipid synthesis genes, including Acacb and Fasn, which are regulated by first-order clock transcription factors. Similar effects of LA supplementation on bile acid and lipid synthesis genes were observed in young animals. Transcript changes of lipid metabolism genes were corroborated by a decrease in FASN and ACC protein levels. We conclude that advanced age is associated with a necroinflammatory phenotype and increased lipid synthesis, while chronic LA supplementation influences hepatic genes associated with lipid and energy metabolism and circadian rhythm, regardless of age.

The harmala alkaloid harmine is a modulator of circadian Bmal1 transcription

Biological rhythms are orchestrated by a cell-autonomous clock system that drives the rhythmic cascade of clock genes. We established an assay system using NIH 3T3 cells stably expressing the Bmal1 promoter-driven luciferase reporter gene and used it to analyse circadian oscillation of the gene. Modulators of PKC (protein kinase C) revealed that an activator and an inhibitor represented short- and long-period phenotypes respectively which were consistent with reported effects of PKC on the circadian clock and validated the assay system. We examined the effects of the alkaloid harmine, contained in Hoasca, which has a wide spectrum of pharmacological actions, on circadian rhythms using the validated assay system. Harmine dose dependently elongated the period. Furthermore, EMSA (electrophoretic mobility-shift assay) and Western-blot analysis showed that harmine enhanced the transactivating function of RORα (retinoid-related orphan receptor α), probably by increasing its nuclear translocation. Exogenous expression of RORα also caused a long period, confirming the phenotype indicated by harmine. These results suggest that harmine extends the circadian period by enhancing RORα function and that harmine is a new candidate that contributes to the control of period length in mammalian cells.

Intestinal FXR-mediated FGF15 production contributes to diurnal control of hepatic bile acid synthesis in mice

Hepatic bile acid synthesis is subject to complex modes of transcriptional control, in which the bile acid-activated nuclear receptor farnesoid X receptor (FXR) in liver and intestine-derived, FXR-controlled fibroblast growth factor 15 (Fgf15) are involved. The Fgf15 pathway is assumed to contribute significantly to control of hepatic bile acid synthesis. However, scientific evidence supporting this assumption is primarily based on gene expression data. Using intestine-selective FXR knockout mice (iFXR-KO), we show that contribution of intestinal FXR-Fgf15 signalling in regulation of hepatic cholesterol 7α-hydroxylase (Cyp7A1) expression depends on time of the day with increased hepatic Cyp7A1 expression in iFXR-KO mice compared with controls exclusively during the dark phase. To assess the physiological relevance hereof, we determined effects of intestine-selective deletion of FXR on physiological parameters such as bile formation and kinetics of the enterohepatic circulation of bile acids. It appeared that intestinal FXR deficiency leads to a modest but significant increase in cholic acid pool size, without changes in fractional turnover rate. As a consequence, bile flow and biliary bile acid secretion rates were increased in iFXR-KO mice compared with controls. Feeding a bile acid-containing diet or treatment with a bile acid sequestrant similarly affected bile formation in iFXR-KO and control mice and induced similar changes in Cyp7A1 and Cyp8B1 expression patterns. In conclusion, this study is the first to demonstrate the physiological relevance of the contribution of the intestinal FXR-Fgf15 signalling pathway in control of hepatic bile acid synthesis. Fgf15 contributes to the regulation of hepatic bile acid synthesis in mice mainly during the dark phase. Expansion of the circulating bile acid pool as well as bile acid sequestration diminishes the contribution of intestinal FXR-Fgf15 signalling in control of hepatic bile acid synthesis and bile formation.