Functional interaction of hepatic nuclear factor-4 and peroxisome proliferator-activated receptor-gamma coactivator 1alpha in CYP7A1 regulation is inhibited by a key lipogenic activator, sterol regulatory element-binding protein-1c

The distinct mechanism regulating taurine homeostasis in mice: Nutrient availability affects taurine levels in the liver and energy restriction influences it in the intestine

AIMS

Our previous findings indicate that caloric restriction (CR) stimulates the production and secretion of taurine-conjugated bile acids in mice. Subsequent processing by gut microbiota leads to increased levels of deconjugated bile acids, taurine, and various taurine conjugates in the intestine. Furthermore, we demonstrated that carbohydrate restriction and protein restriction, to a smaller extent, mirror the impact of CR in terms of hepatic production of bile acids but not their secretion. We hypothesized that modulating dietary macronutrient levels would influence taurine homeostasis in the liver and intestine of ad libitum-fed and CR animals.

MATERIALS AND METHODS

Ad libitum-fed male mice were allocated to receive either a control, low-protein (LP), low-fat (LF), or low-carbohydrate (LC) diet. Meanwhile, CR groups were given 80 % of their regular voluntary food intake as a control, high-protein (HP), high-fat (HF), or high-carbohydrate (HC) diet.

KEY FINDINGS

While CR did not affect the taurine levels and its conjugates in the liver, alteration in carbohydrates and protein intake impacted it. Conversely, in the intestine, CR increased the amount of free and conjugated taurine, whereas the various diets did not affect it or disrupt the CR-specific phenotype. Notably, variations in diet composition impacted the expression of the taurine transporter (Slc6a6) and glutathione-S transferases (GST) in the intestine as well as cysteine dioxygenase (Cdo) in the liver.

SIGNIFICANCE

The liver and the intestine show distinct responses to dietary interventions, with hepatic taurine being affected by the diet composition, while intestinal taurine is governed by energy availability.

Peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) family in physiological and pathophysiological process and diseases

Peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) family (PGC-1s), consisting of three members encompassing PGC-1α, PGC-1β, and PGC-1-related coactivator (PRC), was discovered more than a quarter-century ago. PGC-1s are essential coordinators of many vital cellular events, including mitochondrial functions, oxidative stress, endoplasmic reticulum homeostasis, and inflammation. Accumulating evidence has shown that PGC-1s are implicated in many diseases, such as cancers, cardiac diseases and cardiovascular diseases, neurological disorders, kidney diseases, motor system diseases, and metabolic disorders. Examining the upstream modulators and co-activated partners of PGC-1s and identifying critical biological events modulated by downstream effectors of PGC-1s contribute to the presentation of the elaborate network of PGC-1s. Furthermore, discussing the correlation between PGC-1s and diseases as well as summarizing the therapy targeting PGC-1s helps make individualized and precise intervention methods. In this review, we summarize basic knowledge regarding the PGC-1s family as well as the molecular regulatory network, discuss the physio-pathological roles of PGC-1s in human diseases, review the application of PGC-1s, including the diagnostic and prognostic value of PGC-1s and several therapies in pre-clinical studies, and suggest several directions for future investigations. This review presents the immense potential of targeting PGC-1s in the treatment of diseases and hopefully facilitates the promotion of PGC-1s as new therapeutic targets.

Energy and macronutrient restriction regulate bile acid homeostasis

As we reported previously, caloric restriction (CR) results in an increased concentration of bile acids (BA) in the intestinal mucosa. We now investigated the background of this phenotype, trying to identify nutrition-related factors modulating BA levels. Male mice were submitted to various types of restrictive diets and BA levels and expression of associated factors were measured. We found that BA concentration is increased in the liver of CR mice, which corresponds to reduced expression of the Shp gene and elevated mRNA levels of Cyp27a1, Bal, and Ntcp, as well as CYP7A1 protein and gene expression. Correlation between decreased concentration of BAs in the feces, increased BAs levels in plasma, and elevated gene expression of BAs transporters in the ileum mucosa suggests enhanced BA uptake in the intestine of CR mice. Corresponding to CR upregulation of liver and ileum mucosa, BA concentration was found in animals submitted to other types of prolonged energy-restricting dietary protocols, including intermittent fasting and fasting-mimicking diet. While over-night fasting had negligible impact on BAs levels. Manipulation of macronutrient levels partly affected BA balance. Low-carbohydrate and ketogenic diet increased BAs in the liver but not in the intestine. Carbohydrate restriction stimulates BA synthesis in the liver, but energy restriction is required for the increase in BA levels in the intestine and its uptake.

SREBP Regulation of Lipid Metabolism in Liver Disease, and Therapeutic Strategies

Sterol regulatory element-binding proteins (SREBPs) are master transcription factors that play a crucial role in regulating genes involved in the biogenesis of cholesterol, fatty acids, and triglycerides. As such, they are implicated in several serious liver diseases, including non-alcoholic fatty liver disease (NAFLD), non-alcoholic steatohepatitis (NASH), fibrosis, and hepatocellular carcinoma (HCC). SREBPs are subject to regulation by multiple cofactors and critical signaling pathways, making them an important target for therapeutic interventions. In this review, we first introduce the structure and activation of SREBPs, before focusing on their function in liver disease. We examine the mechanisms by which SREBPs regulate lipogenesis, explore how alterations in these processes are associated with liver disease, and evaluate potential therapeutic strategies using small molecules, natural products, or herb extracts that target these pathways. Through this analysis, we provide new insights into the versatility and multitargets of SREBPs as factors in the modulation of different physiological stages of liver disease, highlighting their potential targets for therapeutic treatment.

A crosstalk between epigenetic modulations and non-alcoholic fatty liver disease progression

Non-alcoholic fatty liver disease (NAFLD) has recently emerged as a major public health concern worldwide due to its rapidly rising prevalence and its potential to progress into end-stage liver disease. While the precise pathophysiology underlying NAFLD remains incompletely understood, it is strongly associated with various environmental triggers and other metabolic disorders. Epigenetics examines changes in gene expression that are not caused by alterations in the DNA sequence itself. There is accumulating evidence that epigenetics plays a key role in linking environmental cues to the onset and progression of NAFLD. Our understanding of how epigenetic mechanisms contribute to NAFLD pathophysiology has expanded considerably in recent years as research on the epigenetics of NAFLD has developed. This review summarizes recent insights into major epigenetic processes that have been implicated in NAFLD pathogenesis including DNA methylation, histone acetylation, and microRNAs that have emerged as promising targets for further investigation. Elucidating epigenetic mechanisms in NAFLD may uncover novel diagnostic biomarkers and therapeutic targets for this disease. However, many questions have remained unanswered regarding how epigenetics promotes NAFLD onset and progression. Additional studies are needed to further characterize the epigenetic landscape of NAFLD and validate the potential of epigenetic markers as clinical tools. Nevertheless, an enhanced understanding of the epigenetic underpinnings of NAFLD promises to provide key insights into disease mechanisms and pave the way for novel prognostic and therapeutic approaches.

Transcriptome shifts triggered by vitamin A and SCD genotype interaction in Duroc pigs

BACKGROUND

The composition of intramuscular fat depends on genetic and environmental factors, including the diet. In pigs, we identified a haplotype of three SNP mutations in the stearoyl-coA desaturase (SCD) gene promoter associated with higher content of monounsaturated fatty acids in intramuscular fat. The second of these three SNPs (rs80912566, C > T) affected a putative retinol response element in the SCD promoter. The effect of dietary vitamin A restriction over intramuscular fat content is controversial as it depends on the pig genetic line and the duration of the restriction. This study aims to investigate changes in the muscle transcriptome in SCD rs80912566 TT and CC pigs fed with and without a vitamin A supplement during the fattening period.

RESULTS

Vitamin A did not affect carcass traits or intramuscular fat content and fatty acid composition, but we observed an interaction between vitamin A and SCD genotype on the desaturation of fatty acids in muscle. As reported before, the SCD-TT pigs had more monounsaturated fat than the SCD-CC animals. The diet lacking the vitamin A supplement enlarged fatty acid compositional differences between SCD genotypes, partly because vitamin A had a bigger effect on fatty acid desaturation in SCD-CC pigs (positive) than in SCD-TT and SCD-TC animals (negative). The interaction between diet and genotype was also evident at the transcriptome level; the highest number of differentially expressed genes were detected between SCD-TT pigs fed with the two diets. The genes modulated by the diet with the vitamin A supplement belonged to metabolic and signalling pathways related to immunity and inflammation, transport through membrane-bounded vesicles, fat metabolism and transport, reflecting the impact of retinol on a wide range of metabolic processes.

CONCLUSIONS

Restricting dietary vitamin A during the fattening period did not improve intramuscular fat content despite relevant changes in muscle gene expression, both in coding and non-coding genes. Vitamin A activated general pathways of retinol response in a SCD genotype-dependant manner, which affected the monounsaturated fatty acid content, particularly in SCD-CC pigs.

Negative Regulation of Human Hepatic Constitutive Androstane Receptor by Cholesterol Synthesis Inhibition: Role of Sterol Regulatory Element Binding Proteins

The squalene synthase inhibitor squalestatin 1 (Squal1) is a potent and efficacious inducer of CYP2B expression in primary cultured rat hepatocytes and rat liver. To determine whether Squal1 is also an inducer of human CYP2B, the effects of Squal1 treatment were evaluated in primary cultured human hepatocytes, differentiated HepaRG cells, and humanized mouse livers. Squal1 treatment did not increase CYP2B6 mRNA levels in human hepatocytes or HepaRG cells and only slightly and inconsistently increased CYP2B6 mRNA content in humanized mouse liver. However, treatment with farnesol, which mediates Squal1's effect on rat CYP2B expression, increased CYP2B6 mRNA levels in HepaRG cells expressing the constitutive androstane receptor (CAR), but not in cells with knocked-down CAR. To determine the impact of cholesterol biosynthesis inhibition on CAR activation, the effects of pravastatin (Prava) were determined on CITCO-mediated gene expression in primary cultured human hepatocytes. Prava treatment abolished CITCO-inducible CYP2B6 expression, but had less effect on rifampicin-mediated CYP3A4 induction, and CITCO treatment did not affect Prava-inducible HMG-CoA reductase (HMGCR) expression. Treatment with inhibitors of different steps of cholesterol biosynthesis attenuated CITCO-mediated CYP2B6 induction in HepaRG cells, and Prava treatment increased HMGCR expression and inhibited CYP2B6 induction with comparable potency. Transfection of HepG2 cells with transcriptionally active sterol regulatory element binding proteins (SREBPs) reduced CAR-mediated transactivation, and inducible expression of transcriptionally active SREBP2 attenuated CITCO-inducible CYP2B6 expression in HepaRG cells. These findings suggest that Squal1 does not induce CYP2B6 in human hepatocytes because Squal1's inhibitory effect on cholesterol biosynthesis interferes with CAR activation. SIGNIFICANCE STATEMENT: The cholesterol biosynthesis inhibitor squalestatin 1 induces rat hepatic CYP2B expression indirectly by causing accumulation of an endogenous isoprenoid that activates the constitutive androstane receptor (CAR). This study demonstrates that squalestatin 1 does not similarly induce CYP2B6 expression in human hepatocytes. Rather, inhibition of cholesterol biosynthesis interferes with CAR activity, likely by activating sterol regulatory element binding proteins. These findings increase our understanding of the endogenous processes that modulate human drug-metabolizing gene expression.

Hepatic cholesterol transport and its role in non-alcoholic fatty liver disease and atherosclerosis

Non-alcoholic fatty liver disease (NAFLD) is a quickly emerging global health problem representing the most common chronic liver disease in the world. Atherosclerotic cardiovascular disease represents the leading cause of mortality in NAFLD patients. Cholesterol metabolism has a crucial role in the pathogenesis of both NAFLD and atherosclerosis. The liver is the major organ for cholesterol metabolism. Abnormal hepatic cholesterol metabolism not only leads to NAFLD but also drives the development of atherosclerotic dyslipidemia. The cholesterol level in hepatocytes reflects the dynamic balance between endogenous synthesis, uptake, esterification, and export, a process in which cholesterol is converted to neutral cholesteryl esters either for storage in cytosolic lipid droplets or for secretion as a major constituent of plasma lipoproteins, including very-low-density lipoproteins, chylomicrons, high-density lipoproteins, and low-density lipoproteins. In this review, we describe decades of research aimed at identifying key molecules and cellular players involved in each main aspect of hepatic cholesterol metabolism. Furthermore, we summarize the recent advances regarding the biological processes of hepatic cholesterol transport and its role in NAFLD and atherosclerosis.

Dietary anthocyanins as potential natural modulators for the prevention and treatment of non-alcoholic fatty liver disease: A comprehensive review

Nonalcoholic fatty liver disease (NAFLD) refers to a metabolic syndrome linked with type 2 diabetes mellitus, obesity, and cardiovascular diseases. It is characterized by the accumulation of triglycerides in the hepatocytes in the absence of alcohol consumption. The prevalence of NAFLD has abruptly increased worldwide, with no effective treatment yet available. Anthocyanins (ACNs) belong to the flavonoid subclass of polyphenols, are commonly present in various edible plants, and possess a broad array of health-promoting properties. ACNs have been shown to have strong potential to combat NAFLD. We critically assessed the literature regarding the pharmacological mechanisms and biopharmaceutical features of the action of ACNs on NAFLD in humans and animal models. We found that ACNs ameliorate NAFLD by improving lipid and glucose metabolism, increasing antioxidant and anti-inflammatory activities, and regulating gut microbiota dysbiosis. In conclusion, ACNs have potential to attenuate NAFLD. However, further mechanistic studies are required to confirm these beneficial impacts of ACNs on NAFLD.

Transcriptional Regulation in Non-Alcoholic Fatty Liver Disease

Obesity is the primary risk factor for the pathogenesis of non-alcoholic fatty liver disease (NAFLD), the worldwide prevalence of which continues to increase dramatically. The liver plays a pivotal role in the maintenance of whole-body lipid and glucose homeostasis. This is mainly mediated by the transcriptional activation of hepatic pathways that promote glucose and lipid production or utilization in response to the nutritional state of the body. However, in the setting of chronic excessive nutrition, the dysregulation of hepatic transcriptional machinery promotes lipid accumulation, inflammation, metabolic stress, and fibrosis, which culminate in NAFLD. In this review, we provide our current understanding of the transcription factors that have been linked to the pathogenesis and progression of NAFLD. Using publicly available transcriptomic data, we outline the altered activity of transcription factors among humans with NAFLD. By expanding this analysis to common experimental mouse models of NAFLD, we outline the relevance of mouse models to the human pathophysiology at the transcriptional level.

LXRα Regulates ChREBPα Transactivity in a Target Gene-Specific Manner through an Agonist-Modulated LBD-LID Interaction

The cholesterol-sensing nuclear receptor liver X receptor (LXR) and the glucose-sensing transcription factor carbohydrate responsive element-binding protein (ChREBP) are central players in regulating glucose and lipid metabolism in the liver. More knowledge of their mechanistic interplay is needed to understand their role in pathological conditions like fatty liver disease and insulin resistance. In the current study, LXR and ChREBP co-occupancy was examined by analyzing ChIP-seq datasets from mice livers. LXR and ChREBP interaction was determined by Co-immunoprecipitation (CoIP) and their transactivity was assessed by real-time quantitative polymerase chain reaction (qPCR) of target genes and gene reporter assays. Chromatin binding capacity was determined by ChIP-qPCR assays. Our data show that LXRα and ChREBPα interact physically and show a high co-occupancy at regulatory regions in the mouse genome. LXRα co-activates ChREBPα and regulates ChREBP-specific target genes in vitro and in vivo. This co-activation is dependent on functional recognition elements for ChREBP but not for LXR, indicating that ChREBPα recruits LXRα to chromatin in trans. The two factors interact via their key activation domains; the low glucose inhibitory domain (LID) of ChREBPα and the ligand-binding domain (LBD) of LXRα. While unliganded LXRα co-activates ChREBPα, ligand-bound LXRα surprisingly represses ChREBPα activity on ChREBP-specific target genes. Mechanistically, this is due to a destabilized LXRα:ChREBPα interaction, leading to reduced ChREBP-binding to chromatin and restricted activation of glycolytic and lipogenic target genes. This ligand-driven molecular switch highlights an unappreciated role of LXRα in responding to nutritional cues that was overlooked due to LXR lipogenesis-promoting function.

Delayed Feeding Alters Transcriptional and Post-Transcriptional Regulation of Hepatic Metabolic Pathways in Peri-Hatch Broiler Chicks

Hepatic fatty acid oxidation of yolk lipoproteins provides the main energy source for chick embryos. Post-hatching these yolk lipids are rapidly exhausted and metabolism switches to a carbohydrate-based energy source. We recently demonstrated that many microRNAs (miRNAs) are key regulators of hepatic metabolic pathways during this metabolic switching. MiRNAs are small non-coding RNAs that post-transcriptionally regulate gene expression in most eukaryotes. To further elucidate the roles of miRNAs in the metabolic switch, we used delayed feeding for 48 h to impede the hepatic metabolic switch. We found that hepatic expression of several miRNAs including miR-33, miR-20b, miR-34a, and miR-454 was affected by delaying feed consumption for 48 h. For example, we found that delayed feeding resulted in increased miR-20b expression and conversely reduced expression of its target FADS1, an enzyme involved in fatty acid synthesis. Interestingly, the expression of a previously identified miR-20b regulator FOXO3 was also higher in delayed fed chicks. FOXO3 also functions in protection of cells from oxidative stress. Delayed fed chicks also had much higher levels of plasma ketone bodies than their normal fed counterparts. This suggests that delayed fed chicks rely almost exclusively on lipid oxidation for energy production and are likely under higher oxidative stress. Thus, it is possible that FOXO3 may function to both limit lipogenesis as well as to help protect against oxidative stress in peri-hatch chicks until the initiation of feed consumption. This is further supported by evidence that the FOXO3-regulated histone deacetylase (HDAC2) was found to recognize the FASN (involved in fatty acid synthesis) chicken promoter in a yeast one-hybrid assay. Expression of FASN mRNA was lower in delayed fed chicks until feed consumption. The present study demonstrated that many transcriptional and post-transcriptional mechanisms, including miRNA, form a complex interconnected regulatory network that is involved in controlling lipid and glucose molecular pathways during the metabolic transition in peri-hatch chicks.

Role of SREBPs in Liver Diseases: A Mini-review

Sterol regulator element binding proteins (SREBPs) are a family of transcription factors involved in the biogenesis of cholesterol, fatty acids and triglycerides. They also regulate physiological functions of many organs, such as thyroid, brain, heart, pancreas and hormone synthesis. Beside the physiological effects, SREBPs participate in some pathological processes, diabetes, endoplasmic reticulum stress, atherosclerosis and chronic kidney disease associated with SREBP expression changes. In the liver, SREBPs are involved in the pathogenesis of nonalcoholic fatty liver disease, nonalcoholic steatohepatitis, hepatitis and hepatic cancer. There are several SREBP inhibitors that have potential for treating obesity, diabetes and cancer. This review assesses the recent findings about the roles of SREBPs in the physiology of organs' function and pathogenesis of liver diseases.

Spexin Acts as Novel Regulator for Bile Acid Synthesis

Spexin is a novel hormone involved in obesity and diabetes while its biofunctional significance in lipid metabolism is still to be comprehended. Global metabolomic analysis in the present study revealed multiple metabolic pathways altered by spexin intraperitoneal (i.p.) injection in rat serum, which are highlighted by the changes in several bile acid metabolites. In rats, spexin (300 μg/kg) could dramatically reduce hepatic and circulating total bile acids (TBA) level compared with the controls. Correspondingly, treatment with spexin by i.p. injection for 28 days led to significant decrease in serum TBA and gallbladder weight in C57BL/6J mice. In enterohepatic circulation system, spexin effectively reduced TBA levels in mouse liver and gallbladder but not the intestine. Hepatic cholesterol 7α-hydroxylase 1 (CYP7A1) expression, unsurprisingly, was suppressed by spexin injection. Both GALR2 and GALR3 antagonists reversed the inhibitory effects of spexin on concentrations of serum TBA and 7 α-hydroxy-4-cholesten-3-one (C4), and hepatic CYP7A1 expression. Finally, negative correlations were observed between serum spexin and total cholesterol (TC), total bile acid (TBA), tauro-chenodeoxycholate (TCDCA), as well as glycochenodeoxycholate (GCDCA) in 91 healthy volunteers. These findings illuminate the intrinsic importance of spexin in the regulation of bile acid synthesis and metabolism.

SREBP-regulated lipid metabolism: convergent physiology - divergent pathophysiology

Cellular lipid metabolism and homeostasis are controlled by sterol regulatory-element binding proteins (SREBPs). In addition to performing canonical functions in the transcriptional regulation of genes involved in the biosynthesis and uptake of lipids, genome-wide system analyses have revealed that these versatile transcription factors act as important nodes of convergence and divergence within biological signalling networks. Thus, they are involved in myriad physiological and pathophysiological processes, highlighting the importance of lipid metabolism in biology. Changes in cell metabolism and growth are reciprocally linked through SREBPs. Anabolic and growth signalling pathways branch off and connect to multiple steps of SREBP activation and form complex regulatory networks. In addition, SREBPs are implicated in numerous pathogenic processes such as endoplasmic reticulum stress, inflammation, autophagy and apoptosis, and in this way, they contribute to obesity, dyslipidaemia, diabetes mellitus, nonalcoholic fatty liver disease, nonalcoholic steatohepatitis, chronic kidney disease, neurodegenerative diseases and cancers. This Review aims to provide a comprehensive understanding of the role of SREBPs in physiology and pathophysiology at the cell, organ and organism levels.

SREBP1c-CRY1 signalling represses hepatic glucose production by promoting FOXO1 degradation during refeeding

SREBP1c is a key lipogenic transcription factor activated by insulin in the postprandial state. Although SREBP1c appears to be involved in suppression of hepatic gluconeogenesis, the molecular mechanism is not thoroughly understood. Here we show that CRY1 is activated by insulin-induced SREBP1c and decreases hepatic gluconeogenesis through FOXO1 degradation, at least, at specific circadian time points. SREBP1c^−/− and CRY1^−/− mice show higher blood glucose than wild-type (WT) mice in pyruvate tolerance tests, accompanied with enhanced expression of PEPCK and G6Pase genes. CRY1 promotes degradation of nuclear FOXO1 by promoting its binding to the ubiquitin E3 ligase MDM2. Although SREBP1c fails to upregulate CRY1 expression in db/db mice, overexpression of CRY1 attenuates hyperglycaemia through reduction of hepatic FOXO1 protein and gluconeogenic gene expression. These data suggest that insulin-activated SREBP1c downregulates gluconeogenesis through CRY1-mediated FOXO1 degradation and that dysregulation of hepatic SREBP1c-CRY1 signalling may contribute to hyperglycaemia in diabetic animals.

The clock protein Cry regulates hepatic glucose metabolism. Here the authors show that SREBP1c, activated by insulin signalling after feeding, directly regulates Cry transcription at specific circadian time points, and that Cry represses hepatic glucose production by promoting proteasomal degradation of Foxo1.

FXR Primes the Liver for Intestinal FGF15 Signaling by Transient Induction of β-Klotho

The bile acid (BA)-sensing nuclear receptor, farnesoid X receptor (FXR), regulates postprandial metabolic responses, including inhibition of BA synthesis, by inducing the intestinal hormone, fibroblast growth factor (FGF)15 (FGF19 in human). In this study, we tested a novel hypothesis that FXR not only induces intestinal FGF15 but also primes the liver for effectively responding to the signal by transcriptional induction of the obligate coreceptor for FGF15, β-Klotho (βKL). Activation of FXR by a synthetic agonist, GW4064, in mice increased occupancy of FXR and its DNA-binding partner, retinoid X receptor-α, at FGF15-signaling component genes, particularly βKL, and induced expression of these genes. Interestingly, mRNA levels of Fgfr4, the FGF15 receptor, were not increased by GW4064, but protein levels increased as a result of βKL-dependent increased protein stability. Both FGF receptor 4 and βKL protein levels were substantially decreased in FXR-knockout (KO) mice, and FGF19 signaling, monitored by phosphorylated ERK, was blunted in FXR-KO mice, FXR-KO mouse hepatocytes, and FXR-down-regulated human hepatocytes. Overexpression of βKL in FXR-lacking hepatocytes partially restored FGF19 signaling and inhibition by FGF19 of Cyp7a1, which encodes the rate-limiting BA biosynthetic enzyme. In mice, transient inductions of intestinal Fgf15 and hepatic βKL were temporally correlated after GW4064 treatment, and pretreatment of hepatocytes with GW4064 before FGF19 treatment enhanced FGF19 signaling, which was abolished by transcriptional inhibition or βKL down-regulation. This study identifies FXR as a gut-liver metabolic coordinator for FGF15/19 action that orchestrates transient induction of hepatic βKL and intestinal Fgf15/19 in a temporally correlated manner.

Chlamydia pneumoniae acute liver infection affects hepatic cholesterol and triglyceride metabolism in mice

OBJECTIVE

Chlamydia pneumoniae has been linked to atherosclerosis, strictly associated with hyperlipidemia. The liver plays a central role in the regulation of lipid metabolism. Since in animal models C. pneumoniae can be found at hepatic level, this study aims to elucidate whether C. pneumoniae infection accelerates atherosclerosis by affecting lipid metabolism.

METHODS

Thirty Balb/c mice were challenged intra-peritoneally with C. pneumoniae elementary bodies and thirty with Chlamydia trachomatis, serovar D. Thirty mice were injected with sucrose-phosphate-glutamate buffer, as negative controls. Seven days after infection, liver samples were examined both for presence of chlamydia and expression of genes involved in inflammation and lipid metabolism.

RESULTS

C. pneumoniae was isolated from 26 liver homogenates, whereas C. trachomatis was never re-cultivated (P < 0.001). C. pneumoniae infected mice showed significantly increased serum cholesterol and triglycerides levels compared both with negative controls (P < 0.001 and P = 0.0197, respectively) and C. trachomatis infected mice (P < 0.001). Liver bile acids were significantly reduced in C. pneumoniae compared to controls and C. trachomatis infected mice. In C. pneumoniae infected livers, cholesterol 7α-hydroxylase (Cyp7a1) and low-density lipoprotein receptor (Ldlr) mRNA levels were reduced, while inducible degrader of the low-density lipoprotein receptor (Idol) expression was increased. Hypertriglyceridemia was associated to reduced expression of hepatic carnitine palmitoyltransferase-1a (Cpt1a) and medium chain acyl-Coenzyme A dehydrogenase (Acadm). Pro-inflammatory cytokines gene expression was increased compared to negative controls. Conversely, in C. trachomatis infected animals, normal serum lipid levels were associated with elevated pro-inflammatory cytokines gene expression, linked to only a mild disturbance of lipid regulatory genes.

CONCLUSION

Our results indicate that C. pneumoniae mouse liver infection induces dyslipidemic effects with significant modifications of genes involved in lipid metabolism.

Influence of dietary sugar on cholesterol and bile acid metabolism in the rat: Marked reduction of hepatic Abcg5/8 expression following sucrose ingestion

Previous studies have indicated that dietary intake of sugar may lower bile acid production, and may promote cholesterol gallstone formation in humans. We studied the influence of dietary sucrose on cholesterol and bile acid metabolism in the rat. In two different experiments, rats received high-sucrose diets. In the first, 60% of the weight of standard rat chow was replaced with sucrose (high-sucrose diet). In the second, rats received a diet either containing 65% sucrose (controlled high-sucrose diet) or 65% complex carbohydrates, in order to keep other dietary components constant. Bile acid synthesis, evaluated by measurements of the serum marker 7-alpha-hydroxy-4-cholesten-3-one (C4) and of the hepatic mRNA expression of Cyp7a1, was markedly reduced by the high-sucrose diet, but not by the controlled high-sucrose diet. Both diets strongly reduced the hepatic - but not the intestinal - mRNA levels of Abcg5 and Abcg8. The differential patterns of regulation of bile acid synthesis induced by the two sucrose-enriched diets indicate that it is not sugar per se in the high-sucrose diet that reduces bile acid synthesis, but rather the reduced content of fiber or fat. In contrast, the marked reduction of hepatic Abcg5/8 observed is an effect of the high sugar content of the diets.

A GAPDH-mediated trans-nitrosylation pathway is required for feedback inhibition of bile salt synthesis in rat liver

BACKGROUND & AIMS

Bile salts inhibit their own production by inducing the nuclear receptor small heterodimer partner (SHP) (encoded by NR0B2), which contributes to repression of the gene encoding cholesterol 7α-hydroxylase (CYP7A1), a key enzyme for the control of bile salt synthesis. On the other hand, bile salts stimulate hepatic synthesis of nitric oxide. We investigated the role of nitric oxide signaling in the control of CYP7A1 expression and the involvement in this process of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), which participates in intracellular propagation of nitric oxide signals.

METHODS

We studied the effects of inhibitors of nitric oxide synthesis (L-NG-nitroarginine methyl ester [L-NAME]) or protein nitrosylation (via dithiothreitol) on bile salt homeostasis in male Wistar rats placed on a cholate-rich diet for 5 days and in cultured primary hepatocytes. S-nitrosylation of GAPDH was assessed using a biotin-switch assay. Interacions of SHP with other proteins and with the Cyp7a1 promoter sequence were studied using immunoprecipitation and chromatin immunoprecipitation (ChIP) assays. We reduced the GAPDH levels in H35 cells with small interfering RNAs. GAPDH nitrosylation was assessed in normal and cholestatic rat and human livers.

RESULTS

Rats placed on cholate-rich diets and given L-NAME had increased intrahepatic and biliary levels of bile salts, and deficiency in repression of CYP7A1 (at the messenger RNA and protein levels) in liver tissue, despite preserved induction of SHP. In cultured hepatocytes, L-NAME or dithiothreitol blocked cholate-induced down-regulation of CYP7A1 without impairing SHP up-regulation. In hepatocytes, cholate promoted S-nitrosylation of GAPDH and its translocation to the nucleus, accompanied by S-nitrosylation of histone deacetylase 2 (HDAC2) and Sirtuin 1 (SIRT1), deacetylases that participate, respectively, in the formation of Cyp7a1 and Shp repressor complexes. Knockdown of GAPDH prevented repression of CYP7A1 by cholate, and blocking nuclear transport of nitrosylated GAPDH reduced cholate-induced nitrosylation of HDAC2 and SIRT1; this effect was accompanied by abrogation of Cyp7a1 repression. Cholate induced binding of SHP to HDAC2 and its recruitment to the Cyp7a1 promoter; these processes were inhibited by blocking nitric oxide synthesis. Levels of nitrosylated GAPDH and nitrosylated HDAC2 were increased in cholestatic human and rat livers reflecting increased concentrations of bile salts in these conditions.

CONCLUSIONS

In rat liver, excess levels of bile salts activate a GAPDH-mediated transnitrosylation cascade that provides feedback inhibition of bile salt synthesis.

Epithelial SCAP/INSIG/SREBP signaling regulates multiple biological processes during perinatal lung maturation

Pulmonary surfactant is required for lung function at birth and throughout postnatal life. Defects in the surfactant system are associated with common pulmonary disorders including neonatal respiratory distress syndrome and acute respiratory distress syndrome in children and adults. Lipogenesis is essential for the synthesis of pulmonary surfactant by type II epithelial cells lining the alveoli. This study sought to identify the role of pulmonary epithelial SREBP, a transcriptional regulator of cellular lipid homeostasis, during a critical time period of perinatal lung maturation in the mouse. Genome wide mRNA expression profiling of lung tissue from transgenic mice with epithelial-specific deletions of Scap (Scap^Δ/Δ, resulting in inactivation of SREBP signaling) or Insig1 and Insig2 (Insig1/2^Δ/Δ, resulting in activation of SREBP signaling) was assessed. Differentially expressed genes responding to SREBP perturbations were identified and subjected to functional enrichment analysis, pathway mapping and literature mining to predict upstream regulators and transcriptional networks regulating surfactant lipid homeostasis. Through comprehensive data analysis and integration, time dependent effects of epithelial SCAP/INSIG/SREBP deletion and defined SCAP/INSIG/SREBP-associated genes, bioprocesses and downstream pathways were identified. SREBP signaling influences epithelial development, cell death and cell proliferation at E17.5, while primarily influencing surfactant physiology, lipid/sterol synthesis, and phospholipid transport after birth. SREBP signaling integrated with the Wnt/β-catenin and glucocorticoid receptor signaling pathways during perinatal lung maturation. SREBP regulates perinatal lung lipogenesis and maturation through multiple mechanisms by interactions with distinct sets of regulatory partners.

SREBP-2 negatively regulates FXR-dependent transcription of FGF19 in human intestinal cells

Sterol regulatory element-binding protein-2 (SREBP-2) is a basic helix-loop-helix-leucine zipper transcription factor that positively regulates transcription of target genes involved in cholesterol metabolism. In the present study, we have investigated a possible involvement of SREBP-2 in human intestinal expression of fibroblast growth factor (FGF)19, which is an endocrine hormone involved in the regulation of lipid and glucose metabolism. Overexpression of constitutively active SREBP-2 decreased FGF19 mRNA levels in human colon-derived LS174T cells. In reporter assays, active SREBP-2 overexpression suppressed GW4064/FXR-mediated increase in reporter activities in regions containing the IR-1 motif (+848 to +5200) in the FGF19 gene. The suppressive effect disappeared in reporter activities in the region containing the IR-1 motif when the mutation was introduced into the IR-1 motif. In electrophoretic mobility shift assays, binding of the FXR/retinoid X receptor α heterodimer to the IR-1 motif was attenuated by adding active SREBP-2, but SREBP-2 binding to the IR-1 motif was not observed. In chromatin immunoprecipitation assays, specific binding of FXR to the IR-1-containing region of the FGF19 gene (+3214 to +3404) was increased in LS174T cells by treatment with cholesterol and 25-hydroxycholesterol. Specific binding of SREBP-2 to FXR was observed in glutathione-S-transferase (GST) pull-down assays. These results suggest that SREBP-2 negatively regulates the FXR-mediated transcriptional activation of the FGF19 gene in human intestinal cells.

Increased bile acids in enterohepatic circulation by short-term calorie restriction in male mice

Previous studies showed glucose and insulin signaling can regulate bile acid (BA) metabolism during fasting or feeding. However, limited knowledge is available on the effect of calorie restriction (CR), a well-known anti-aging intervention, on BA homeostasis. To address this, the present study utilized a "dose-response" model of CR, where male C57BL/6 mice were fed 0, 15, 30, or 40% CR diets for one month, followed by BA profiling in various compartments of the enterohepatic circulation by UPLC-MS/MS technique. This study showed that 40% CR increased the BA pool size (162%) as well as total BAs in serum, gallbladder, and small intestinal contents. In addition, CR "dose-dependently" increased the concentrations of tauro-cholic acid (TCA) and many secondary BAs (produced by intestinal bacteria) in serum, such as tauro-deoxycholic acid (TDCA), DCA, lithocholic acid, ω-muricholic acid (ωMCA), and hyodeoxycholic acid. Notably, 40% CR increased TDCA by over 1000% (serum, liver, and gallbladder). Interestingly, 40% CR increased the proportion of 12α-hydroxylated BAs (CA and DCA), which correlated with improved glucose tolerance and lipid parameters. The CR-induced increase in BAs correlated with increased expression of BA-synthetic (Cyp7a1) and conjugating enzymes (BAL), and the ileal BA-binding protein (Ibabp). These results suggest that CR increases BAs in male mice possibly through orchestrated increases in BA synthesis and conjugation in liver as well as intracellular transport in ileum.

Prox1 directly interacts with LSD1 and recruits the LSD1/NuRD complex to epigenetically co-repress CYP7A1 transcription

Cholesterol 7α-hydroxylase (CYP7A1) catalyzes the first and rate-limiting step in the classical pathway of bile acids synthesis in liver and is crucial for maintaining lipid homeostasis. Hepatocyte nuclear factor 4α (HNF4α) and α₁-fetoprotein transcription factor (FTF) are two major transcription factors driving CYP7A1 promoter activity in hepatocytes. Previous researches have shown that Prospero-related homeobox (Prox1) directly interacts with both HNF4α and FTF and potently co-represses CYP7A1 transcription and bile acid synthesis through unidentified mechanisms. In this work, mechanisms involved in Prox1-mediated co-repression were explored by identifying Prox1-associated proteins using immunoprecipitation followed by mass spectrometry (IP-MS) methodology. Multiple components of the epigenetically repressive lysine-specific demethylase 1 (LSD1)/nucleosome remodeling and histone deacetylase (NuRD) complex, most notably LSD1 and histone deacetylase 2 (HDAC2), were found to be associated with Prox1 and GST pulldown assay demonstrated that Prox1 directly interacts with LSD1. Sequential chromatin immunoprecipitation (ChIP) assays showed that Prox1 co-localizes with HNF4α, LSD1 and HDAC2 on CYP7A1 promoter in HepG2 cells. Furthermore, by using ChIP assay on HepG2 cells with endogenous Prox1 knocked down by RNA interference, Prox1 was shown to recruit LSD1 and HDAC2 onto CYP7A1 promoter and cause increased H3K4 demethylation. Finally, bile acids treatment of HepG2 cells, which significantly repressed CYP7A1 transcription, resulted in increased Prox1 and LSD1/NuRD complex occupancy on CYP7A1 promoter with a concurrent increase in H3K4 demethylation and H3/H4 deacetylation. These results showed that Prox1 interacts with LSD1 to recruit the repressive LSD1/NuRD complex to CYP7A1 promoter and co-represses transcription through epigenetic mechanisms. In addition, such Prox1-mediated epigenetic repression is involved in the physiologically essential negative feedback inhibition of CYP7A1 transcription by bile acids.

Hypocholesterolemic effect of daily fisetin supplementation in high fat fed Sprague-Dawley rats

We aimed to test whether fisetin could modulate cholesterol homeostasis in rats with diet-induced hypercholesterolemia, and further investigated the underlying mechanisms by which fisetin exerts its cholesterol lowering effect. Blood lipid profile, hepatic cholesterol content, as well as gene expressions in cholesterol metabolism were examined. Elevated levels of total cholesterol and LDL-cholesterol, along with hepatic cholesterol content in a high fat group were found to be significantly reduced by fisetin. The high fat diet significantly decreased hepatic mRNA levels of LDLR, SREBP2, HMGCR and PCSK9 in comparison to the control diet, however, fisetin did not further elicit any changes in mRNA levels of the same genes. The high fat diet dramatically increased the transcript levels of CYP7A1, which was subsequently reversed by the fisetin. In HepG2 cells, fisetin was found to increase the levels of a nuclear form of SREBP2 and LDLR. In conclusion, fisetin supplementation displayed hypocholesterolemic effects by modulating the expression of genes associated with cholesterol and bile acid metabolism.

Complex interplay between the lipin 1 and the hepatocyte nuclear factor 4 α (HNF4α) pathways to regulate liver lipid metabolism

Lipin 1 is a bifunctional protein that serves as a metabolic enzyme in the triglyceride synthesis pathway and regulates gene expression through direct protein-protein interactions with DNA-bound transcription factors in liver. Herein, we demonstrate that lipin 1 is a target gene of the hepatocyte nuclear factor 4α (HNF4α), which induces lipin 1 gene expression in cooperation with peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) through a nuclear receptor response element in the first intron of the lipin 1 gene. The results of a series of gain-of-function and loss-of-function studies demonstrate that lipin 1 coactivates HNF4α to activate the expression of a variety of genes encoding enzymes involved in fatty acid catabolism. In contrast, lipin 1 reduces the ability of HNF4α to induce the expression of genes encoding apoproteins A4 and C3. Although the ability of lipin to diminish HNF4α activity on these promoters required a direct physical interaction between the two proteins, lipin 1 did not occupy the promoters of the repressed genes and enhances the intrinsic activity of HNF4α in a promoter-independent context. Thus, the induction of lipin 1 by HNF4α may serve as a mechanism to affect promoter selection to direct HNF4α to promoters of genes encoding fatty acid oxidation enzymes.

The extended TILAR approach: a novel tool for dynamic modeling of the transcription factor network regulating the adaption to in vitro cultivation of murine hepatocytes

BACKGROUND

Network inference is an important tool to reveal the underlying interactions of biological systems. In the liver, a complex system of transcription factors is active to distribute signals and induce the cellular response following extracellular stimuli. Plenty of information is available about single transcription factors important for the different functions of the liver, but little is known about their causal relations to each other.

RESULTS

Given a DNA microarray time series dataset of collagen monolayers cultured murine hepatocytes, we identified 22 differentially expressed genes for which the corresponding protein is known to exhibit transcription factor activity. We developed the Extended TILAR (ExTILAR) network inference algorithm based on the modeling concept of the previously published TILAR algorithm. Using ExTILAR, we inferred a transcription factor network based on gene expression data which puts these important genes into a functional context. This way, we identified a previously unknown relationship between Tgif1 and Atf3 which we validated experimentally. Beside its known role in metabolic processes, this extends the knowledge about Tgif1 in hepatocytes towards a possible influence of processes such as proliferation and cell cycle. Moreover, two positive (i.e. double negative) regulatory loops were predicted that could give rise to bistable behavior. We further evaluated the performance of ExTILAR by systematic inference of an in silico network.

CONCLUSIONS

We present the ExTILAR algorithm, which combines the advantages of the regression based inference algorithm TILAR, like large network sizes processable and low computational costs, with the advantages of dynamic network models based on ordinary differential equation (i.e. in silico knock-down simulations). Like TILAR, ExTILAR makes use of various prior-knowledge types such as transcription factor binding site information and gene interaction knowledge to infer biologically meaningful gene regulatory networks. Therefore, ExTILAR is especially useful when a large number of genes is modeled using a small number of experimental data points.

Chronic exposure to rifaximin causes hepatic steatosis in pregnane X receptor-humanized mice

Rifaximin, a nonsystemic antibiotic that exhibits low gastrointestinal absorption, is a potent agonist of human pregnane X receptor (PXR), which contributes to its therapeutic efficacy in inflammatory bowel disease. To investigate the effects of long-term administration of rifaximin on the liver, PXR-humanized mice were administered rifaximin for 6 months; wild-type and Pxr-null mice were treated in parallel as controls. Histological analysis revealed time-dependent intense hepatocellular fatty degeneration and increased hepatic triglycerides in PXR-humanized mice and not in wild-type and Pxr-null mice. After long-term treatment, PXR target genes were induced in small intestine and liver, with significant up-regulation in the expression of hepatic genes related to triglyceride synthesis and lipid accumulation. However, no significant hepatic accumulation of rifaximin was found, even after 6 months of treatment, in PXR-humanized mice. Genes in the small intestine that are involved in the uptake of fatty acids and triglycerides were induced along with increased triglyceride accumulation in intestinal epithelial cells of PXR-humanized mice; this was not observed in wild-type and Pxr-null mice. These findings suggest that long-term administration of rifaximin could lead to PXR-dependent hepatocellular fatty degeneration as a result of activation of genes involved in lipid uptake, thus indicating a potential adverse effect of rifaximin on liver function after long-term exposure.

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.

Insulin-dependent suppression of cholesterol 7α-hydroxylase is a possible link between glucose and cholesterol metabolisms

Cholesterol 7α-hydroxylase (CYP7A1) regulates the balance between cholesterol supply and metabolism by catalyzing the rate-limiting step of bile acid biosynthesis. The transcriptional activity of CYP7A1 is tightly controlled by various nuclear receptors. A forkhead transcription factor O1 (FOXO1) plays a critical role in metabolism, and insulin inactivates FOXO1 through Akt-dependent phosphorylation and nuclear exclusion. We investigated the role of insulin- Akt-FOXO1 signaling pathway in CYP7A1 transcriptional regulation since we found putative insulin-response elements, FOXO1 binding sequences, in both rat and human CYP7A1 promoters. However, ectopic expression of FOXO1 increased the rat CYP7A1-, but mildly reduced human CYP7A1-promoter activities in a dose-dependent manner. Similarly to bile acids, insulin treatment increased small heterodimer partner (SHP) mRNA rapidly and transiently, leading to the suppression of CYP7A1 transcription in both human and rodents. Chromatin immunoprecipitation showed that FOXO1 directly bound to rat CYP1A1 promoter in the absence of insulin. FOXO1 binding to the rat promoter was diminished by insulin treatment as well as by expression of SHP. Our results suggest that the stimulation of insulin- signaling pathway of Akt-FOXO1 and SHP expression may regulate cholesterol/bile acid metabolisms in liver, linking carbohydrate and cholesterol metabolic pathways. A prolonged exposure of insulin in hyperinsulinemic insulin resistance or diabetic status represses CYP7A1 transcription and bile acid biosynthesis through SHP induction and FOXO1 inactivation, leading to impairment of the hepatic cholesterol/bile acid metabolisms.

Mechanisms underlying the development of hepatic steatosis-related insulin resistance in Chinese hamsters with type 2 diabetes

AIM: To investigate the mechanisms underlying the development of hepatic steatosis-related insulin resistance in Chinese hamsters with type 2 diabetes.

METHODS: Insulin-resistant obese Chinese hamsters and Chinese hamsters with type 2 diabetes were generated by feeding a high-fat diet with or without low-dose streptozotocin. Then we investigated the alterations in hepatic gene expression profiles by microarray analysis followed by real-time RT-PCR confirmation.

RESULTS: Microarray analysis indicated that, in insulin-resistant obese Chinese hamsters and those with type 2 diabetes, differentially expressed metabolism-related genes were mainly associated with hepatic glycolipid metabolism and related signaling pathways. Real-time RT-PCR analysis verified that the expression of hepatic sterol regulatory element-binding proteins (SREBPs) and peroxisome proliferator-activated receptors (PPAR) γ was increased (all P < 0.05), the expression of liver X receptor α (LXRα), PPARα, and PPARβ/δ was decreased (all P < 0.05), and the expression of LXR β was unchanged in the liver of hamster models. The expression of hepatic LXR α, SREBPs, PPARs and their target genes in insulin-resistant hamsters significantly differed from that in type 2 diabetic hamsters (all P < 0.05).

CONCLUSION: Altered expression of LXR α, SREBPs and PPARs may be involved in the development of hepatic steatosis-related insulin resistance in type 2 diabetic Chinese hamsters.

Activated sterol regulatory element-binding protein-2 suppresses hepatocyte nuclear factor-4-mediated Cyp3a11 expression in mouse liver

Sterol regulatory element-binding protein-2 (SREBP-2) is a key transcription factor for the cholesterol homeostasis. Recent studies have suggested the association of CYP3A enzymes, major drug-metabolizing enzymes, with cholesterol metabolism. In the present study, we have investigated a possible involvement of SREBP-2 in hepatic Cyp3a11 expression. Feeding a low-cholesterol diet (LCD) to mice activated hepatic SREBP-2 whereas it attenuated hepatic Cyp3a11 expression. These phenomena were reversed by cholesterol supplementation to LCD. In reporter assays, the overexpression of constitutively active SREBP-2 reduced Cyp3a11 reporter activity through the region from -1581 to -1570 of Cyp3a11. This region contained a putative hepatocyte nuclear factor-4α (HNF-4α) binding motif, and HNF-4α, but not SREBP-2, bound to the motif in in vitro binding assays. With the mutation or deletion of this motif, the SREBP-2-dependent suppression of Cyp3a11 expression disappeared in reporter assays. In pull-down assays and coimmunoprecipitation assays, SREBP-2 bound to peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α), a major coactivator for HNF-4α, via its transactivation domain and inhibited the interaction between HNF-4α and PGC-1α in vitro. A mutant SREBP-2 lacking the transactivation domain consistently failed to reduce Cyp3a11 reporter activity. Furthermore, PGC-1α overexpression relieved the SREBP-2-mediated reduction of Cyp3a11 reporter activity. Finally, chromatin immunoprecipitation assays demonstrated that the extent of PGC-1α binding to the Cyp3a11 promoter was reduced by LCD-feeding in mouse livers. In conclusion, activated SREBP-2 interacts with PGC-1α in mouse livers at reduced cholesterol intake. This results in the reduced PGC-1α recruitment to HNF-4α on the Cyp3a11 promoter and the subsequent down-regulation of Cyp3a11 expression.

SIRT1 deacetylates and inhibits SREBP-1C activity in regulation of hepatic lipid metabolism

The SIRT1 deacetylase inhibits fat synthesis and stimulates fat oxidation in response to fasting, but the underlying mechanisms remain unclear. Here we report that SREBP-1c, a key lipogenic activator, is an in vivo target of SIRT1. SIRT1 interaction with SREBP-1c was increased during fasting and decreased upon feeding, and consistently, SREBP-1c acetylation levels were decreased during fasting in mouse liver. Acetylated SREBP-1c levels were also increased in HepG2 cells treated with insulin and glucose to mimic feeding conditions, and down-regulation of p300 by siRNA decreased the acetylation. Depletion of hepatic SIRT1 by adenoviral siRNA increased acetylation of SREBP-1c with increased lipogenic gene expression. Tandem mass spectrometry and mutagenesis studies revealed that SREBP-1c is acetylated by p300 at Lys-289 and Lys-309. Mechanistic studies using acetylation-defective mutants showed that SIRT1 deacetylates and inhibits SREBP-1c transactivation by decreasing its stability and its occupancy at the lipogenic genes. Remarkably, SREBP-1c acetylation levels were elevated in diet-induced obese mice, and hepatic overexpression of SIRT1 or treatment with resveratrol, a SIRT1 activator, daily for 1 week decreased acetylated SREBP-1c levels with beneficial functional outcomes. These results demonstrate an intriguing connection between elevated SREBP-1c acetylation and increased lipogenic gene expression, suggesting that abnormally elevated SREBP-1c acetylation increases SREBP-1c lipogenic activity in obese mice. Reducing acetylation of SREBP-1c by targeting SIRT1 may be useful for treating metabolic disorders, including fatty liver, obesity, and type II diabetes.

Modification in oxidative stress, inflammation, and lipoprotein assembly in response to hepatocyte nuclear factor 4alpha knockdown in intestinal epithelial cells

Hepatocyte nuclear factor 4α (HNF4α) is a nuclear transcription factor mainly expressed in the liver, intestine, kidney, and pancreas. Many of its hepatic and pancreatic functions have been described, but limited information is available on its role in the gastrointestinal tract. The objectives of this study were to evaluate the anti-inflammatory and antioxidant functions of HNF4α as well as its implication in intestinal lipid transport and metabolism. To this end, the HNF4A gene was knocked down by transfecting Caco-2 cells with a pGFP-V-RS lentiviral vector containing an shRNA against HNF4α. Inactivation of HNF4α in Caco-2 cells resulted in the following: (a) an increase in oxidative stress as demonstrated by the levels of malondialdehyde and conjugated dienes; (b) a reduction in secondary endogenous antioxidants (catalase, glutathione peroxidase, and heme oxygenase-1); (c) a lower protein expression of nuclear factor erythroid 2-related factor that controls the antioxidant response elements-regulated antioxidant enzymes; (d) an accentuation of cellular inflammatory activation as shown by levels of nuclear factor-κB, interleukin-6, interleukin-8, and leukotriene B4; (e) a decrease in the output of high density lipoproteins and of their anti-inflammatory and anti-oxidative components apolipoproteins (apo) A-I and A-IV; (f) a diminution in cellular lipid transport revealed by a lower cellular secretion of chylomicrons and their apoB-48 moiety; and (g) alterations in the transcription factors sterol regulatory element-binding protein 2, peroxisome proliferator-activated receptor α, and liver X receptor α and β. In conclusion, HNF4α appears to play a key role in intestinal lipid metabolism as well as intestinal anti-oxidative and anti-inflammatory defense mechanisms.

cAMP-mediated regulation of HNF-4alpha depends on the level of coactivator PGC-1alpha

Hepatocyte nuclear factor-4 alpha (HNF-4alpha) is a member of the nuclear receptor superfamily with important roles in hepatic metabolism. Fasting induces the cAMP/protein kinase A (PKA)-signaling pathway. The mechanisms whereby cAMP regulates HNF-4alpha transcriptional activity are incompletely understood. We have therefore investigated the role of cAMP/PKA in regulation of HNF-4alpha in COS-1 cells and the hepatoma HepG2 cell line. cAMP/PKA inhibited the transcriptional activity of HNF-4alpha in COS-1 cells, whereas a stimulatory effect was observed in HepG2 cells. The cAMP-induced inhibition of HNF-4alpha in COS-1 cells was counteracted by overexpression of the nuclear receptor coactivator PGC-1alpha, and cAMP/PKA-dependent induction of the PGC1A gene in HepG2 cells seems to explain the cell specific differences. This was further supported by knock-down of PGC-1alpha in HepG2 cells, which abolished the stimulatory effect of PKA on HNF-4alpha transcriptional activity. Similar to the cAMP/PKA-mediated regulation of HNF-4alpha, overexpression of the cAMP-response element binding protein (CREB) inhibited the transcriptional activity of HNF-4alpha in COS-1 cells, regardless of cAMP/PKA activation and CREB phosphorylation. Moreover, activation of CREB by cAMP/PKA further stimulated HNF-4alpha transactivation in HepG2 cells. cAMP induced the expression of the HNF-4alpha target genes PCK1 and G6Pase in these cells. In conclusion, our results suggest that the level of PGC-1alpha determines whether the cAMP/PKA-pathway overall stimulates or inhibits HNF-4alpha transcriptional activation.

ADD1/SREBP1c activates the PGC1-alpha promoter in brown adipocytes

Cold adaptation elicits a paradoxical simultaneous induction of fatty acid synthesis and beta-oxidation in brown adipose tissue. We show here that cold exposure coordinately induced liver X receptor alpha (LXRalpha), adipocyte determination and differentiation-dependent factor 1 (ADD1)/sterol regulatory element-binding protein-1c (SREBP1c) and peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC1alpha) in brown and inguinal white adipose tissues, but not in epididymal white adipose tissue. Using in vitro models of white and brown adipocytes we demonstrate that beta-adrenergic stimulation induced expression of LXRalpha, ADD1/SREBP1c and PGC1alpha in cells with a brown-like adipose phenotype. We demonstrate that ADD1/SREBP1c is a powerful inducer of PGC1alpha expression via a conserved E box in the proximal promoter and that beta-adrenergic stimulation led to recruitment of ADD1/SREBP1c to this E box. The ability of ADD1/SREBP1c to activate the PGC1alpha promoter exhibited a striking cell type dependency, suggesting that additional cell type-restricted factors contribute to ADD1/SREBP1c-mediated activation. In conclusion, our data demonstrate a novel role of ADD1/SREBP1c as a regulator of PGC1alpha expression in brown adipose tissue.

Activated polyamine catabolism leads to low cholesterol levels by enhancing bile acid synthesis

Transgenic mice with activated polyamine catabolism due to overexpression of spermidine/spermine N(1)-acetyltransferase (SSAT) have significantly reduced plasma total cholesterol levels. In our study, we show that low cholesterol levels were attributable to enhanced bile acid synthesis in combination with reduced cholesterol absorption. Hepatic cholesterol 7alpha-hydroxylase (CYP7A1), the rate-limiting enzyme catalyzing the conversion of cholesterol to bile acids, plays an important role in the removal of excess cholesterol from the body. We suggest that by reducing activity of Akt activated polyamine catabolism increased the stability and activity of peroxisome proliferator-activated receptor gamma co-activator 1alpha, the critical activator of CYP7A1. This is supported by our finding that the treatment with SSAT activator, N (1) ,N(11)-diethylnorspermine, reduced significantly the amount of phosphorylated (active) Akt in HepG2 cells. In summary, activated-polyamine catabolism is a novel mechanism to regulate bile acid synthesis. Therefore, polyamine catabolism could be a potential therapeutic target to control hepatic CYP7A1 expression.

Functional specificities of Brm and Brg-1 Swi/Snf ATPases in the feedback regulation of hepatic bile acid biosynthesis

Bile acid homeostasis is critical in maintaining health and is primarily regulated by the nuclear receptors farnesoid X receptor (FXR) and small heterodimer partner (SHP). Bile acid-activated FXR indirectly inhibits expression of cholesterol 7alpha hydroxylase (CYP7A1), a key enzyme in conversion of cholesterol to bile acids, by induction of SHP. We recently demonstrated that SHP inhibits CYP7A1 transcription by recruiting chromatin-modifying cofactors, including Brm-Swi/Snf. Swi/Snf complexes contain either Brm or Brg-1 ATPases, and whether these subunits have distinct functions remains unclear. We have examined the role of these subunits in regulation of bile acid metabolism under physiological conditions by FXR and SHP. Brg-1 interacted with FXR and enhanced FXR-mediated transactivation of SHP, whereas Brm interacted with SHP and enhanced SHP-mediated repression of CYP7A1 and, interestingly, auto-repression of SHP. Chromatin immunoprecipitation and remodeling studies revealed that after treatment with FXR agonists, Brg-1 was recruited to the SHP promoter, resulting in transcriptionally active accessible chromatin, whereas Brm was recruited to both CYP7A1 and SHP promoters, resulting in inactive inaccessible chromatin. Our studies demonstrate that Brm and Brg-1 have distinct functions in the regulation of two key genes, CYP7A1 and SHP, within a single physiological pathway, feedback inhibition of bile acid biosynthesis, by differentially targeting SHP and FXR.

Bile acid signaling pathways increase stability of Small Heterodimer Partner (SHP) by inhibiting ubiquitin-proteasomal degradation

Small Heterodimer Partner (SHP) inhibits activities of numerous transcription factors involved in diverse biological pathways. As an important metabolic regulator, SHP plays a key role in maintaining cholesterol and bile acid homeostasis by inhibiting cholesterol conversion to bile acids. While SHP gene induction by increased bile acids is well established, whether SHP activity is also modulated remains unknown. Here, we report surprising findings that SHP is a rapidly degraded protein via the ubiquitin-proteasomal pathway and that bile acids or bile acid-induced intestinal fibroblast growth factor 19 (FGF19) increases stability of hepatic SHP by inhibiting proteasomal degradation in an extracellular signal-regulated kinase (ERK)-dependent manner. SHP was ubiquitinated at Lys122 and Lys123, and mutation of these sites altered its stability and repression activity. Tandem mass spectrometry revealed that upon bile acid treatment, SHP was phosphorylated at Ser26, within an ERK motif in SHP, and mutation of this site dramatically abolished SHP stability. Surprisingly, SHP stability was abnormally elevated in ob/ob mice and diet-induced obese mice. These results demonstrate an important role for regulation of SHP stability in bile acid signaling in normal conditions, and that abnormal stabilization of SHP may be associated with metabolic disorders, including obesity and diabetes.

Down-regulation of hepatic HNF4alpha gene expression during hyperinsulinemia via SREBPs

Mutations in the coding region of hepatocyte nuclear factor 4alpha (HNF4alpha), and its upstream promoter (P2) that drives expression in the pancreas, are known to lead to maturity-onset diabetes of the young 1 (MODY1). HNF4alpha also controls gluconeogenesis and lipid metabolism in the liver, where the proximal promoter (P1) predominates. However, very little is known about the role of hepatic HNF4alpha in diabetes. Here, we examine the expression of hepatic HNF4alpha in two diabetic mouse models, db/db mice (type 2, insulin resistant) and streptozotocin-treated mice (type 1, insulin deficient). We found that the level of HNF4alpha protein and mRNA was decreased in the liver of db/db mice but increased in streptozotocin-treated mice. Because insulin increases the activity of sterol regulatory element-binding proteins (SREBP)-1c and -2, we also examined the effect of SREBPs on hepatic HNF4alpha gene expression and found that, like insulin, ectopic expression of SREBPs decreases the level of hepatic HNF4alpha protein and mRNA both in vitro in primary hepatocytes and in vivo in the liver of C57BL/6 mice. Finally, we use gel shift, chromatin immunoprecipitation, small interfering RNA, and reporter gene analysis to show that SREBP2 binds the human HNF4alpha P1 promoter and negatively regulates its expression. These data indicate that hyperinsulinemia down-regulates HNF4alpha in the liver through the up-regulation of SREBPs, thereby establishing a link between these two critical transcription factor pathways that regulate lipid and glucose metabolism in the liver. These findings also provide new insights into diabetes-associated complications such as fatty liver disease.

The p300 acetylase is critical for ligand-activated farnesoid X receptor (FXR) induction of SHP

The primary bile acid receptor farnesoid X receptor (FXR) maintains lipid and glucose homeostasis by regulating expression of numerous bile acid-responsive genes, including an orphan nuclear receptor and metabolic regulator SHP. Using SHP as a model gene, we studied how FXR activity is regulated by p300 acetylase. FXR interaction with p300 and their recruitment to the SHP promoter and acetylated histone levels at the promoter were increased by FXR agonists in mouse liver and HepG2 cells. In contrast, p300 recruitment and acetylated histones at the promoter were not detected in FXR-null mice. p300 directly interacted with and acetylated FXR in vitro. Overexpression of p300 wild type increased, whereas a catalytically inactive p300 mutant decreased, acetylated FXR levels and FXR transactivation in cells. While similar results were observed with a related acetylase, CBP, GCN5 did not enhance FXR transactivation, and its recruitment to the promoter was not increased by FXR agonists, suggesting functional specificity of acetylases in FXR signaling. Down-regulation of p300 by siRNA decreased acetylated FXR and acetylated histone levels, and occupancy of FXR at the promoter, resulting in substantial inhibition of SHP expression. These results indicate that p300 acts as a critical coactivator of FXR induction of SHP by acetylating histones at the promoter and FXR itself. Surprisingly, p300 down-regulation altered expression of other metabolic FXR target genes involved in lipoprotein and glucose metabolism, such that beneficial lipid and glucose profiles would be expected. These unexpected findings suggest that inhibition of hepatic p300 activity may be beneficial for treating metabolic diseases.

Interplay of pregnane X receptor with other nuclear receptors on gene regulation

Human body needs to protect itself from a diverse array of harmful chemicals. These chemicals are also involved in drug metabolism, enzyme induction, and can cause adverse drug-drug interactions. Being a member of nuclear receptors (NRs), pregnane X receptor (PXR) has recently emerged as transcriptional regulators of cytochrome P450 (CYP) and transporters expression so as to against xenobiotics exposure. This review describes some common nuclear receptors, i.e. farnesoid X receptor (FXR), small heterodimer partner (SHP), hepatocyte nuclear factor-4alpha (HNF-4alpha), liver X receptor (LXR), glucocorticoid receptor (GR), constitutive androstane receptor (CAR) that crosstalk with PXR and involvement of coregulators thus control target genes expression.