Bile acids enhance low density lipoprotein receptor gene expression via a MAPK cascade-mediated stabilization of mRNA

Bile acids and incretins as modulators of obesity-associated atherosclerosis

Obesity is one of the major global health concerns of the 21st century, associated with many comorbidities such as type 2 diabetes mellitus (T2DM), metabolic dysfunction-associated steatotic liver disease, and early and aggressive atherosclerotic cardiovascular disease, which is the leading cause of death worldwide. Bile acids (BAs) and incretins are gut hormones involved in digestion and absorption of fatty acids, and insulin secretion, respectively. In recent years BAs and incretins are increasingly recognized as key signaling molecules, which target multiple tissues and organs, beyond the gastro-intestinal system. Moreover, incretin-based therapy has revolutionized the treatment of T2DM and obesity. This mini review highlights the current knowledge about dysregulations in BA homeostasis in obesity with a special focus on atherosclerosis as well as athero-modulating roles of incretins and currently available incretin-based therapies.

Role of Bile Acid Receptors in the Development and Function of Diabetic Nephropathy

Diabetic nephropathy (DN) is a prevalent microvascular complication that occurs often in individuals with diabetes. It significantly raises the mortality rate of affected patients. Therefore, there is an urgent need to identify therapeutic targets for controlling and preventing the occurrence and development of DN. Bile acids (BAs) are now recognized as intricate metabolic integrators and signaling molecules. The activation of BAs has great promise as a therapeutic approach for preventing DN, renal damage caused by obesity, and nephrosclerosis. The nuclear receptors (NRs), farnesoid X receptor (FXR), pregnane X receptor (PXR), vitamin D receptor (VDR); and the G protein-coupled BA receptor, Takeda G-protein-coupled receptor 5 (TGR5) have important functions in controlling lipid, glucose, and energy metabolism, inflammation, as well as drug metabolism and detoxification. Over the past 10 years, there has been advancement in comprehending the biology and processes of BA receptors in the kidney, as well as in the creation of targeted BA receptor agonists. In this review, we discuss the role of BA receptors, FXR, PXR, VDR, and TGR5 in DN and their role in renal physiology, as well as the development and application of agonists that activate BA receptors for the treatment of kidney diseases.

Chenodeoxycholic Acid-Modified Polyethyleneimine Nano-Composites Deliver Low-Density Lipoprotein Receptor Genes for Lipid-Lowering Therapy by Targeting the Liver

Lipid-lowering drugs, especially statins, are extensively utilized in clinical settings for the prevention of hyperlipidemia. Nevertheless, prolonged usage of current lipid-lowering medications is associated with significant adverse reactions. Therefore, it is imperative to develop novel therapeutic agents for lipid-lowering therapy. In this study, a chenodeoxycholic acid and lactobionic acid double-modified polyethyleneimine (PDL) nanocomposite as a gene delivery vehicle for lipid-lowering therapy by targeting the liver, are synthesized. Results from the in vitro experiments demonstrate that PDL exhibits superior transfection efficiency compared to polyethyleneimine in alpha mouse liver 12 (AML12) cells and effectively carries plasmids. Moreover, PDL can be internalized by AML12 cells and rapidly escape lysosomal entrapment. Intravenous administration of cyanine5.5 (Cy5.5)-conjugated PDL nanocomposites reveals their preferential accumulation in the liver compared to polyethyleneimine counterparts. Systemic delivery of low-density lipoprotein receptor plasmid-loaded PDL nanocomposites into mice leads to reduced levels of low-density lipoprotein cholesterol (LDL-C) and triglycerides (TC) in the bloodstream without any observed adverse effects on mouse health or well-being. Collectively, these findings suggest that low-density lipoprotein receptor plasmid-loaded PDL nanocomposites hold promise as potential therapeutics for lipid-lowering therapy.

Bile acid receptors and renal regulation of water homeostasis

The kidney is the key organ responsible for maintaining the body's water and electrolyte homeostasis. About 99% of the primary urine filtered from the Bowman's capsule is reabsorbed along various renal tubules every day, with only 1-2 L of urine excreted. Aquaporins (AQPs) play a vital role in water reabsorption in the kidney. Currently, a variety of molecules are found to be involved in the process of urine concentration by regulating the expression or activity of AQPs, such as antidiuretic hormone, renin-angiotensin-aldosterone system (RAAS), prostaglandin, and several nuclear receptors. As the main bile acid receptors, farnesoid X receptor (FXR) and membrane G protein-coupled bile acid receptor 1 (TGR5) play important roles in bile acid, glucose, lipid, and energy metabolism. In the kidney, FXR and TGR5 exhibit broad expression across all segments of renal tubules, and their activation holds significant therapeutic potential for numerous acute and chronic kidney diseases through alleviating renal lipid accumulation, inflammation, oxidative stress, and fibrosis. Emerging evidence has demonstrated that the genetic deletion of FXR or TGR5 exhibits increased basal urine output, suggesting that bile acid receptors play a critical role in urine concentration. Here, we briefly summarize the function of bile acid receptors in renal water reabsorption and urine concentration.

Role of the Gut Microbiome in the Development of Atherosclerotic Cardiovascular Disease

Atherosclerotic cardiovascular disease (ASCVD) is the primary cause of death globally, with nine million deaths directly attributable to ischemic heart diseases in 2020. Since the last few decades, great effort has been put toward primary and secondary prevention strategies through identification and treatment of major cardiovascular risk factors, including hypertension, diabetes, dyslipidemia, smoking, and a sedentary lifestyle. Once labelled “the forgotten organ”, the gut microbiota has recently been rediscovered and has been found to play key functions in the incidence of ASCVD both directly by contributing to the development of atherosclerosis and indirectly by playing a part in the occurrence of fundamental cardiovascular risk factors. Essential gut metabolites, such as trimethylamine N-oxide (TMAO), secondary bile acids, lipopolysaccharides (LPS), and short-chain fatty acids (SCFAs), have been associated with the extent of ischemic heart diseases. This paper reviews the latest data on the impact of the gut microbiome in the incidence of ASCVD.

Role of FXR in Renal Physiology and Kidney Diseases

Farnesoid X receptor, also known as the bile acid receptor, belongs to the nuclear receptor (NR) superfamily of ligand-regulated transcription factors, which performs its functions by regulating the transcription of target genes. FXR is highly expressed in the liver, small intestine, kidney and adrenal gland, maintaining homeostasis of bile acid, glucose and lipids by regulating a diverse array of target genes. It also participates in several pathophysiological processes, such as inflammation, immune responses and fibrosis. The kidney is a key organ that manages water and solute homeostasis for the whole body, and kidney injury or dysfunction is associated with high morbidity and mortality. In the kidney, FXR plays an important role in renal water reabsorption and is thought to perform protective functions in acute kidney disease and chronic kidney disease, especially diabetic kidney disease. In this review, we summarize the recent advances in the understanding of the physiological and pathophysiological function of FXR in the kidney.

Dietary supplementation of porcine bile acids improves laying performance, serum lipid metabolism and cecal microbiota in late-phase laying hens

Due to the exceptional laying performance of hens, the demand on lipid metabolism and oxidation in vivo is vigorous, resulting in excessive lipid accumulation in late-phase hens, which lowers the production performance. Bile acids regulate lipid metabolism and gut microbiota in humans and animals. However, the effect of porcine bile acids on lipid metabolism and cecal microbiota in laying hens in the late phase is still unclear. A total of 360 healthy 45-week-old laying hens were chosen for a 24-week feeding trial, where 0, 30, 60 and 90 mg/kg porcine bile acids were added to a basal diet, respectively. The results showed that dietary supplementation of 60 mg/kg bile acids increased egg production and feed conversion (P < 0.05). Also, 60 and 90 mg/kg porcine bile acids reduced abdominal fat percentage and body weight (P < 0.05). The levels of serum total cholesterol, triglyceride, and low-density lipoprotein cholesterol of hens decreased (P < 0.05) in bile acids supplement groups. As for cecal microbiota, bile acids supplementation did not affect the alpha diversity of cecal microbiota at the genus level. Moreover, dietary supplementation of 90 mg/kg bile acids resulted in an increase in the abundance of beneficial bacteria in the cecum, such as Lactobacillus, Bifidobacterium and Turicibacter. The changes in the cecal microbiota caused by bile acids supplementation correlated with serum lipid indexes. According to KEGG pathway analysis, dietary supplementation of 60 and 90 mg/kg bile acids promoted structural transformation of the cecal microbiota to down-regulate steroid biosynthesis, up-regulate fatty acid degradation and up-regulate unsaturated fatty acid biosynthesis. Meanwhile, bile acids bio-isomerization function of cecal microbiota was enhanced in 60 and 90 mg/kg bile acids treatment, and the short-chain fatty acid metabolism was also affected. In conclusion, the present study revealed dietary supplementation of porcine bile acids enriched probiotics in the gut and improved serum lipid metabolism of laying hens. These findings demonstrate that porcine bile acids can be a potential gut beneficial promoter for late-phase laying hens.

Mechanisms for Bile Acids CDCA- and DCA-Stimulated Hepatic Spexin Expression

Spexin (SPX) is a novel peptide involved in glucose and lipid metabolism and suppresses hepatic total bile acid levels by inhibiting hepatic cholesterol 7α-hydroxylase 1 expression. As important mediators for glycolysis/gluconeogenesis and lipid metabolism, the effects of bile acids on SPX expression is yet to be understood. By using SMMC7721 and BEL-7402 cell lines, we screened the effects of bile acids and found that chenodeoxycholic acid (CDCA) and deoxycholic acid (DCA) can stimulate SPX gene transcription. Both CDCA and DCA were able to stimulate SPX mRNA expression in the liver but not colon and ileum in mice. In SMMC7721 and BEL-7402 cells, CDCA- and DCA-induced SPX promoter activity was mimicked by bile acid receptor FXR and TGR5 activation and suppressed by FXR and TGR5 silencing. Adenylate cyclase (AC)/cyclic adenosine monophosphate (cAMP) activators significantly increased SPX promoter activity whereas the inhibitors for AC/CAMP/protein kinase A (PKA) and mitogen-activated protein kinases (MAPK) pathway attenuated CDCA- and DCA-induced SPX transcription. Thus, CDCA and DCA stimulate SPX expression at the hepatic level through FXR and TGR5 mediated AC/cAMP/PKA and MAPK cascades.

Triciribine Engages ZFP36L1 and HuR to Stabilize LDLR mRNA

An increased understanding of low-density lipoprotein receptor (LDLR) and its regulation may facilitate drug development for the treatment of hypercholesterolemia. Triciribine (TCN), which is a highly selective AKT inhibitor, increases the stability of LDLR mRNA downstream of extracellular signal-regulated kinase (ERK) in human hepatoma cells (HepG2). Here, a candidate approach was used in order to determine whether the RNA-binding proteins (RBPs) ZFP36 ring finger protein like 1 (ZFP36L1) and Hu antigen R (HuR) play a role in TCN-mediated stabilization of LDLR mRNA. The depletion of HuR led to a reduction of LDLR mRNA stability, an event that was more pronounced in TCN-treated cells. TCN was found to induce the translocation of nuclear HuR to cytoplasm in an ERK-dependent manner. ZFP36L1 depletion increased the stability of LDLR mRNA consistent with its destabilizing role. However, in contrast to HuR, TCN had no effect on LDLR mRNA turnover in ZFP36L1-depleted cells. TCN induced the phosphorylation of ZFP36L1 in an ERK/RSK-dependent manner and promoted its dissociation from the CCR4-NOT complex. In sum, these data suggest that TCN utilizes ERK signaling to increase the activity of HuR and inhibit ZFP36L1 to stabilize LDLR mRNA in HepG2 cells.

Role of the Sterol Regulatory Element Binding Protein Pathway in Tumorigenesis

Metabolic changes are a major feature of tumors, including various metabolic forms, such as energy, lipid, and amino acid metabolism. Sterol regulatory element binding proteins (SREBPs) are important modules in regulating lipid metabolism and play an essential role in metabolic diseases. In the previous decades, the regulatory range of SREBPs has been markedly expanded. It was found that SREBPs also played a critical role in tumor development. SREBPs are involved in energy supply, lipid supply, immune environment and inflammatory environment shaping in tumor cells, and as a protective umbrella to support the malignant proliferation of tumor cells. Natural medicine and traditional Chinese medicine, as an important part of drug therapy, demonstrates the multifaceted effects of SREBPs regulation. This review summarizes the core processes in the involvement of SREBPs in tumors and provides a comprehensive understanding of the pathways through which natural drugs target the SREBP pathway and regulate tumor progression.

Cholic Acid Stimulates MMP-9 in Human Colon Cancer Cells via Activation of MAPK, AP-1, and NF-κB Activity

Matrix metalloproteinase-9 (MMP-9) plays a crucial role in cell invasion and cancer metastasis. In this study, we showed that cholic acid (CA), a major primary bile acid, can induce MMP-9 expression in colon cancer HT29 and SW620 cells. CA increased reactive oxygen species (ROS) production and also activated phosphorylation of ERK1/2, JNK, and p38 MAPK. Specific inhibitors and mutagenesis studies showed that ERK1/2 and JNK functioned as upstream signals in the activation of AP-1, and p38 MAPK functioned as an upstream signal in the activation of NF-κB. N-acetyl-L-cysteine (NAC, an ROS scavenger) and diphenyleneiodonium chloride (DPI, an NADPH oxidase inhibitor) inhibited CA-induced activation of ERK1/2, JNK, and p38 MAPK, indicating that ROS production by NADPH oxidase could be the furthest upstream signal in MMP-9 expression. Colon cancer cells pretreated with CA showed remarkably enhanced invasiveness. Such enhancement was partially abrogated by MMP-9-neutralizing antibodies. These results demonstrate that CA could induce MMP-9 expression via ROS-dependent ERK1/2, JNK-activated AP-1, and p38-MAPK-activated NF-κB signaling pathways, which in turn stimulate cell invasion in human colon cancer cells.

Why Bile Acids Are So Important in Non-Alcoholic Fatty Liver Disease (NAFLD) Progression

Non-alcoholic fatty liver disease (NAFLD) is a complex disease, affecting not just the liver, but also all other organs in the body. Despite an increasing amount of people worldwide developing NAFLD and having it progress to non-alcoholic steatohepatitis (NASH) and potentially cirrhosis, there is still no approved therapy. Therefore, huge efforts are being made to find and develop a successful treatment. One of the special interests is understanding the liver-gut axis and especially the role of bile acids in the progression of NAFLD. Farnesoid X receptor (FXR)-agonists have been approved und used in other liver diseases, such as primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC), and have shown signs of being able to decrease inflammation and potentially steatosis. This review will mainly focus on targets/ligands that play an important role in bile acid metabolism and give an overview of ongoing clinical as well as pre-clinical trials. With the complexity of the issue, we did not aim at giving a complete review, rather highlighting important targets and potential treatments that could be approved for NAFLD/NASH treatment within the next few years.

MicroRNA-10a Impairs Liver Metabolism in Hepatitis C Virus-Related Cirrhosis Through Deregulation of the Circadian Clock Gene Brain and Muscle Aryl Hydrocarbon Receptor Nuclear Translocator-Like 1

The circadian rhythm of the liver plays an important role in maintaining its metabolic homeostasis. We performed comprehensive expression analysis of microRNAs (miRNAs) using TaqMan polymerase chain reaction of liver biopsy tissues to identify the miRNAs that are significantly up‐regulated in advanced chronic hepatitis C (CHC). We found miR‐10a regulated various liver metabolism genes and was markedly up‐regulated by hepatitis C virus infection and poor nutritional conditions. The expression of miR‐10a was rhythmic and down‐regulated the expression of the circadian rhythm gene brain and muscle aryl hydrocarbon receptor nuclear translocator‐like 1 (Bmal1) by directly suppressing the expression of RA receptor‐related orphan receptor alpha (RORA). Overexpression of miR‐10a in hepatocytes blunted circadian rhythm of Bmal1 and inhibited the expression of lipid synthesis genes (sterol regulatory element binding protein [SREBP]1, fatty acid synthase [FASN], and SREBP2), gluconeogenesis (peroxisome proliferator‐activated receptor gamma coactivator 1 alpha [PGC1α]), protein synthesis (mammalian target of rapamycin [mTOR] and ribosomal protein S6 kinase [S6K]) and bile acid synthesis (liver receptor homolog 1 [LRH1]). The expression of Bmal1 was significantly correlated with the expression of mitochondrial biogenesis‐related genes and reduced Bmal1 was associated with increased serum alanine aminotransferase levels and progression of liver fibrosis in CHC. Thus, impaired circadian rhythm expression of Bmal1 by miR‐10a disturbs metabolic adaptations, leading to liver damage, and is closely associated with the exacerbation of abnormal liver metabolism in patients with advanced CHC. In patients with hepatitis C‐related liver cirrhosis, liver tissue miR‐10a levels were significantly associated with hepatic reserve, fibrosis markers, esophageal varix complications, and hepatitis C‐related hepatocellular carcinoma recurrence. Conclusion: MiRNA‐10a is involved in abnormal liver metabolism in cirrhotic liver through down‐regulation of the expression of the circadian rhythm gene Bmal1. Therefore, miR‐10a is a possible useful biomarker for estimating the prognosis of liver cirrhosis.

Farnesoid X Receptor Activation by Obeticholic Acid Elevates Liver Low-Density Lipoprotein Receptor Expression by mRNA Stabilization and Reduces Plasma Low-Density Lipoprotein Cholesterol in Mice

Objective- The objective of this study was to determine whether and how activation of farnesoid X receptor (FXR) by obeticholic acid (OCA), a clinical FXR agonist, modulates liver low-density lipoprotein receptor (LDLR) expression under normolipidemic conditions. Approach and Results- Administration of OCA to chow-fed mice increased mRNA and protein levels of LDLR in the liver without affecting the sterol-regulatory element binding protein pathway. Profiling of known LDLR mRNA-binding proteins demonstrated that OCA treatment did not affect expressions of mRNA degradation factors hnRNPD (heterogeneous nuclear ribonucleoprotein D) or ZFP36L1 but increased the expression of Hu antigen R (HuR) an mRNA-stabilizing factor. Furthermore, inducing effects of OCA on LDLR and HuR expression were ablated in Fxr^-/- mice. To confirm the post-transcriptional mechanism, we used transgenic mice (albumin-luciferase-untranslated region) that express a human LDLR mRNA 3' untranslated region luciferase reporter gene in the liver. OCA treatment led to significant rises in hepatic bioluminescence signals, Luc-untranslated region chimeric mRNA levels, and endogenous LDLR protein abundance, which were accompanied by elevations of hepatic HuR mRNA and protein levels in OCA-treated transgenic mice. In vitro studies conducted in human primary hepatocytes and HepG2 cells demonstrated that FXR activation by OCA and other agonists elicited the same inducing effect on LDLR expression as in the liver of normolipidemic mice. Furthermore, depletion of HuR in HepG2 cells by short interfering RNA transfection abolished the inducing effect of OCA on LDLR expression. Conclusions- Our study is the first to demonstrate that FXR activation increases LDLR expression in liver tissue by a post-transcriptional regulatory mechanism involving LDLR mRNA-stabilizing factor HuR.

Lactobacillus mucosae DPC 6426 as a bile-modifying and immunomodulatory microbe

Background

Lactobacillus mucosae DPC 6426 has previously demonstrated potentially cardio-protective properties, in the form of dyslipidaemia and hypercholesterolemia correction in an apolipoprotein-E deficient mouse model. This study aims to characterise the manner in which this microbe may modulate host bile pool composition and immune response, in the context of cardiovascular disease. Lactobacillus mucosae DPC 6426 was assessed for bile salt hydrolase activity and specificity. The microbe was compared against several other enteric strains of the same species, as well as a confirmed bile salt hydrolase-active strain, Lactobacillus reuteri APC 2587.

Results

Quantitative bile salt hydrolase assays revealed that enzymatic extracts from Lactobacillus reuteri APC 2587 and Lactobacillus mucosae DPC 6426 demonstrate the greatest activity in vitro. Bile acid profiling of porcine and murine bile following incubation with Lactobacillus mucosae DPC 6426 confirmed a preference for hydrolysis of glyco-conjugated bile acids. In addition, the purified exopolysaccharide and secretome of Lactobacillus mucosae DPC 6426 were investigated for immunomodulatory capabilities using RAW264.7 macrophages. Gene expression data revealed that both fractions stimulated increases in interleukin-6 and interleukin-10 gene transcription in the murine macrophages, while the entire secretome was necessary to increase CD206 transcription. Moreover, the exopolysaccharide elicited a dose-dependent increase in nitric oxide and interleukin-10 production from RAW264.7 macrophages, concurrent with increased tumour necrosis factor-α secretion at all doses.

Conclusions

This study indicates that Lactobacillus mucosae DPC 6426 modulates both bile pool composition and immune system tone in a manner which may contribute significantly to the previously identified cardio-protective phenotype.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1403-0) contains supplementary material, which is available to authorized users.

Pharmacological Applications of Bile Acids and Their Derivatives in the Treatment of Metabolic Syndrome

Apart from well-known functions of bile acids in digestion and solubilization of lipophilic nutrients and drugs in the small intestine, the emerging evidence from the past two decades identified the role of bile acids as signaling, endocrine molecules that regulate the glucose, lipid, and energy metabolism through complex and intertwined pathways that are largely mediated by activation of nuclear receptor farnesoid X receptor (FXR) and cell surface G protein-coupled receptor 1, TGR5 (also known as GPBAR1). Interactions of bile acids with the gut microbiota that result in the altered composition of circulating and intestinal bile acids pool, gut microbiota composition and modified signaling pathways, are further extending the complexity of biological functions of these steroid derivatives. Thus, bile acids signaling pathways have become attractive targets for the treatment of various metabolic diseases and metabolic syndrome opening the new potential avenue in their treatment. In addition, there is a significant effort to unveil some specific properties of bile acids relevant to their intrinsic potency and selectivity for particular receptors and to design novel modulators of these receptors with improved pharmacokinetic and pharmacodynamic profiles. This resulted in synthesis of few semi-synthetic bile acids derivatives such as 6α-ethyl-chenodeoxycholic acid (obeticholic acid, OCA), norursodeoxycholic acid (norUDCA), and 12-monoketocholic acid (12-MKC) that are proven to have positive effect in metabolic and hepato-biliary disorders. This review presents an overview of the current knowledge related to bile acids implications in glucose, lipid and energy metabolism, as well as a potential application of bile acids in metabolic syndrome treatment with future perspectives.

FXR activation by obeticholic acid or nonsteroidal agonists induces a human-like lipoprotein cholesterol change in mice with humanized chimeric liver

Obeticholic acid (OCA) is a selective farnesoid X receptor (FXR) agonist that regulates bile acid and lipid metabolism. FXR activation induces distinct changes in circulating cholesterol among animal models and humans. The mechanistic basis of these effects has been elusive because of difficulties in studying lipoprotein homeostasis in mice, which predominantly package circulating cholesterol in HDLs. Here, we tested the effects of OCA in chimeric mice whose livers are mostly composed (≥80%) of human hepatocytes. Chimeric mice exhibited a human-like ratio of serum LDL cholesterol (LDL-C) to HDL cholesterol (HDL-C) at baseline. OCA treatment in chimeric mice increased circulating LDL-C and decreased circulating HDL-C levels, demonstrating that these mice closely model the cholesterol effects of FXR activation in humans. Mechanistically, OCA treatment increased hepatic cholesterol in chimeric mice but not in control mice. This increase correlated with decreased SREBP-2 activity and target gene expression, including a significant reduction in LDL receptor protein. Cotreatment with atorvastatin reduced total cholesterol, rescued LDL receptor protein levels, and normalized serum LDL-C. Treatment with two clinically relevant nonsteroidal FXR agonists elicited similar lipoprotein and hepatic changes in chimeric mice, suggesting that the increase in circulating LDL-C is a class effect of FXR activation.

5-Azacytidine engages an IRE1α-EGFR-ERK1/2 signaling pathway that stabilizes the LDL receptor mRNA

Hepatic low-density lipoprotein receptor (LDLR) is the primary conduit for the clearance of plasma LDL-cholesterol and increasing its expression represents a central goal for treating cardiovascular disease. However, LDLR mRNA is unstable and undergoes rapid turnover mainly due to the three AU-rich elements (ARE) in its proximal 3'-untranslated region (3'-UTR). Herein, our data revealed that 5-azacytidine (5-AzaC), an antimetabolite used in the treatment of myelodysplastic syndrome, stabilizes the LDLR mRNA through a previously unrecognized signaling pathway resulting in a strong increase of its protein level in human hepatocytes in culture. 5-AzaC caused a sustained activation of the inositol-requiring enzyme 1α (IRE1α) kinase domain and c-Jun N-terminal kinase (JNK) independently of endoplasmic reticulum stress. This resulted in activation of the epidermal growth factor receptor (EGFR) and extracellular signal-regulated kinase1/2 (ERK1/2) that, in turn, stabilized LDLR mRNA. Systematic mutation of the AREs (ARE1-3) in the LDLR 3'UTR and expression of each mutant coupled to a luciferase reporter in Huh7 cells demonstrated that ARE1 is required for rapid LDLR mRNA decay and 5-AzaC-induced mRNA stabilization via the IRE1α-EGFR-ERK1/2 signaling cascade. The characterization of this pathway will help to reveal potential targets to enhance plasma LDL clearance and novel cholesterol-lowering therapeutic strategies.

Peroxisome proliferator-activated receptor gamma co-activator 1 gene Gly482Ser polymorphism is associated with the response of low-density lipoprotein cholesterol concentrations to exercise training in elderly Japanese

Muscle peroxisome proliferator-activated receptor gamma co-activator 1 (PGC-1)α gene expression is influenced by the Gly482Ser gene polymorphism, which is a candidate genetic risk factor for diabetes mellitus and obesity. This study investigated the effects of PGC-1 gene Gly482Ser polymorphisms on alterations in glucose and lipid metabolism induced by exercise training. A 12-week intervention study was performed for 119 participants who were more than 65 years of age and completed exercise training at lactate threshold intensity. Total cholesterol and low-density lipoprotein cholesterol were significantly reduced in Gly/Gly but not in Gly/Ser and Ser/Ser participants after exercise. The Gly/Gly genotype of the PGC-1 gene Gly482Ser polymorphism influences the effects of moderate-intensity exercise training on low-density lipoprotein cholesterol and total cholesterol concentrations in older people.

Treatment with the natural FXR agonist chenodeoxycholic acid reduces clearance of plasma LDL whilst decreasing circulating PCSK9, lipoprotein(a) and apolipoprotein C-III

BACKGROUND

The natural farnesoid X receptor (FXR) agonist chenodeoxycholic acid (CDCA) suppresses hepatic cholesterol and bile acid synthesis and reduces biliary cholesterol secretion and triglyceride production. Animal studies have shown that bile acids downregulate hepatic LDL receptors (LDLRs); however, information on LDL metabolism in humans is limited.

METHODS

Kinetics of autologous ¹²⁵ I-LDL were determined in 12 male subjects at baseline and during treatment with CDCA (15 mg kg^-1 day^-1 ). In seven patients with gallstones treated with CDCA for 3 weeks before cholecystectomy, liver biopsies were collected and analysed for enzyme activities and for specific LDLR binding. Serum samples obtained before treatment and at surgery were analysed for markers of lipid metabolism, lipoproteins and the LDLR modulator proprotein convertase subtilisin/kexin type 9 (PCSK9).

RESULTS

Chenodeoxycholic acid treatment increased plasma LDL cholesterol by ~10% as a result of reduced clearance of plasma LDL-apolipoprotein (apo)B; LDL production was somewhat reduced. The reduction in LDL clearance occurred within 1 day after initiation of treatment. In CDCA-treated patients with gallstones, hepatic microsomal cholesterol 7α-hydroxylase and HMG-CoA reductase activities were reduced by 83% and 54%, respectively, and specific LDLR binding was reduced by 20%. During treatment, serum levels of fibroblast growth factor 19 and total and LDL cholesterol increased, whereas levels of 7α-hydroxy-4-cholesten-3-one, lathosterol, PCSK9, apoA-I, apoC-III, lipoprotein(a), triglycerides and insulin were reduced.

CONCLUSIONS

Chenodeoxycholic acid has a broad influence on lipid metabolism, including reducing plasma clearance of LDL. The reduction in circulating PCSK9 may dampen its effect on hepatic LDLRs and plasma LDL cholesterol. Further studies of the effects of other FXR agonists on cholesterol metabolism in humans seem warranted, considering the renewed interest for such therapy in liver disease and diabetes.

The role of bile acids in nonalcoholic fatty liver disease and nonalcoholic steatohepatitis

Nonalcoholic fatty liver disease is growing in prevalence worldwide. It is marked by the presence of macrosteatosis on liver histology but is often clinically asymptomatic. However, it can progress into nonalcoholic steatohepatitis which is a more severe form of liver disease characterized by inflammation and fibrosis. Further progression leads to cirrhosis, which predisposes patients to hepatocellular carcinoma or liver failure. The mechanism by which simple steatosis progresses to steatohepatitis is not entirely clear. However, multiple pathways have been proposed. A common link amongst many of these pathways is disruption of the homeostasis of bile acids. Other than aiding in the absorption of lipids and lipid-soluble vitamins, bile acids act as ligands. For example, they bind to farnesoid X receptor, which is critically involved in many of the pathways responsible for maintaining bile acid, glucose, and lipid homeostasis. Alterations to these pathways can lead to dysregulation of energy balance and increased inflammation and fibrosis. Repeated insults over time may be the key to development of steatohepatitis. For this reason, current drug therapies target aspects of these pathways to try to reduce and halt inflammation and fibrosis. This review will focus on the role of bile acids in these various pathways and how changes in these pathways may result in steatohepatitis. While there is no approved pharmaceutical treatment for either hepatic steatosis or steatohepatitis, this review will also touch upon the multitude of potential therapies.

Epigallocatechin gallate induces an up-regulation of LDL receptor accompanied by a reduction of PCSK9 via the annexin A2-independent pathway in HepG2 cells

SCOPE

In animal studies, epigallocatechin gallate (EGCG), the dominant catechin in green tea, has been shown to improve cholesterol metabolism. However, the molecular mechanisms of EGCG underlying these functions have not been fully understood. In this study, we aimed to clarify the molecular mechanisms of the effect of EGCG on cholesterol metabolism mainly in HepG2 cells.

METHODS AND RESULTS

We found that EGCG induced a reduction of the extracellular proprotein convertase subtilisin/kexin 9 (PCSK9) level accompanied by an up-regulation of the LDL receptor (LDLR) in HepG2 cells. The EGCG-induced up-regulation of LDLR occurred via the extracellular signal-regulated kinase (ERK) signaling pathway. Moreover, we showed that EGCG induced a significant early reduction of the extracellular PCSK9 protein level. However, there were no significant changes in the PCSK9 mRNA and the intracellular PCSK9 protein levels induced by EGCG. Annexin A2 knockdown affected the basal LDLR expression and did not affect the EGCG-induced reduction of the extracellular PCSK9 protein level or the up-regulation of LDLR.

CONCLUSION

Annexin A2 possesses an essential function for the basal LDLR expression in HepG2 cells. But, EGCG induces the suppression of PCSK9 accompanied by an up-regulation of LDLR in an annexin A2-independent manner. EGCG attenuates the statin-induced an increase in PCSK9 level.

Differential Feedback Regulation of Δ4-3-Oxosteroid 5β-Reductase Expression by Bile Acids

Δ4-3-oxosteroid 5β-reductase is member D1 of the aldo-keto reductase family 1 (AKR1D1), which catalyzes 5β-reduction of molecules with a 3-oxo-4-ene structure. Bile acid intermediates and most of the steroid hormones carry the 3-oxo-4-ene structure. Therefore, AKR1D1 plays critical roles in both bile acid synthesis and steroid hormone metabolism. Currently our understanding on transcriptional regulation of AKR1D1 under physiological and pathological conditions is very limited. In this study, we investigated the regulatory effects of primary bile acids, chenodeoxycholic acid (CDCA) and cholic acid (CA), on AKR1D1 expression. The expression levels of AKR1D1 mRNA and protein in vitro and in vivo following bile acid treatments were determined by real-time PCR and Western blotting. We found that CDCA markedly repressed AKR1D1 expression in vitro in human hepatoma HepG2 cells and in vivo in mice. On the contrary, CA significantly upregulated AKR1D1 expression in HepG2 cells and in mice. Further mechanistic investigations revealed that the farnesoid x receptor (FXR) signaling pathway was not involved in regulating AKR1D1 by bile acids. Instead, CDCA and CA regulated AKR1D1 through the mitogen-activated protein kinases/c-Jun N-terminal kinases (MAPK/JNK) signaling pathway. Inhibition of the MAPK/JNK pathway effectively abolished CDCA and CA-mediated regulation of AKR1D1. It was thus determined that AKR1D1 expression was regulated by CDCA and CA through modulating the MAPK/JNK signaling pathway. In conclusion, AKR1D1 expression was differentially regulated by primary bile acids through negative and positive feedback mechanisms. The findings indicated that both bile acid concentrations and compositions play important roles in regulating AKR1D1 expression, and consequently bile acid synthesis and steroid hormone metabolism.

Bile acids and nonalcoholic fatty liver disease: Molecular insights and therapeutic perspectives

Nonalcoholic fatty liver disease (NAFLD) is a burgeoning health problem worldwide and an important risk factor for both hepatic and cardiometabolic mortality. The rapidly increasing prevalence of this disease and of its aggressive form nonalcoholic steatohepatitis (NASH) will require novel therapeutic approaches to prevent disease progression to advanced fibrosis or cirrhosis and cancer. In recent years, bile acids have emerged as relevant signaling molecules that act at both hepatic and extrahepatic tissues to regulate lipid and carbohydrate metabolic pathways as well as energy homeostasis. Activation or modulation of bile acid receptors, such as the farnesoid X receptor and TGR5, and transporters, such as the ileal apical sodium-dependent bile acid transporter, appear to affect both insulin sensitivity and NAFLD/NASH pathogenesis at multiple levels, and these approaches hold promise as novel therapies. In the present review, we summarize current available data on the relationships of bile acids to NAFLD and the potential for therapeutically targeting bile-acid-related pathways to address this growing world-wide disease. (Hepatology 2017;65:350-362).

Kaempferol stimulates gene expression of low-density lipoprotein receptor through activation of Sp1 in cultured hepatocytes

A high level of plasma low-density lipoprotein (LDL) cholesterol is considered a risk factor for atherosclerosis. Because the hepatic LDL receptor (LDLR) is essential for clearing plasma LDL cholesterol, activation of LDLR is a promising therapeutic target for patients with atherosclerotic disease. Here we demonstrated how the flavonoid kaempferol stimulated the gene expression and activity of LDLR in HepG2 cells. The kaempferol-mediated stimulation of LDLR gene expression was completely inhibited by knockdown of Sp1 gene expression. Treatment of HepG2 cells with kaempferol stimulated the recruitment of Sp1 to the promoter region of the LDLR gene, as well as the phosphorylation of Sp1 on Thr-453 and Thr-739. Moreover, these kaempferol-mediated processes were inhibited in the presence of U0126, an ERK pathway inhibitor. These results suggest that kaempferol may increase the activity of Sp1 through stimulation of Sp1 phosphorylation by ERK1/2 and subsequent induction of LDLR expression and activity.

Chenodeoxycholic acid reduces feed intake and modulates the expression of hypothalamic neuropeptides and hepatic lipogenic genes in broiler chickens

Bile acids have recently become an emerging research hot spot in mammals due to their roles as metabolic regulators and molecular signatures controlling whole-body metabolic homeostasis. Such effects are still unknown in avian (non-mammalian) species. We, therefore, undertook this study to determine the effect of chenodeoxycholic acid (CDCA) on growth performance and on the expression of hypothalamic neuropeptides and hepatic lipogenic genes in broiler chickens. Chickens fed with diet-containing 0.1% or 0.5% CDCA for two weeks exhibited a significant and a dose dependent reduction of feed intake and body weight compared to the control (standard diet). These changes were accompanied with a significant decrease in plasma glucose levels at d10 and d15 post-treatment. At molecular levels, CDCA treatment significantly up-regulated the expression of feeding-related hypothalamic neuropeptides (NPY, AgRP, ORX, CRH, Ghrl, and MC1R) and down-regulated the hypothalamic expression of SOCS3. CDCA treatment also decreased the mRNA levels of key hepatic lipogenic genes (FAS, ACCα, ME, ATPcl, and SCD-1) and their related transcription factors SREBP-1/2 and PPARα. In addition, CDCA reduced the hepatic expression of FXR and the adipokine, visfatin, and adiponectin genes compared to the control. Together, our data provide evidence that CDCA alters growth performances in broilers and modulates the expression of hypothalamic neuropeptides and hepatic lipogenic and adipocytokine genes.

Piperine Induces Hepatic Low-Density Lipoprotein Receptor Expression through Proteolytic Activation of Sterol Regulatory Element-Binding Proteins

Elevated plasma low-density lipoprotein (LDL) cholesterol is considered as a risk factor for atherosclerosis. Because the hepatic LDL receptor (LDLR) uptakes plasma lipoproteins and lowers plasma LDL cholesterol, the activation of LDLR is a promising drug target for atherosclerosis. In the present study, we identified the naturally occurring alkaloid piperine, as an inducer of LDLR gene expression by screening the effectors of human LDLR promoter. The treatment of HepG2 cells with piperine increased LDLR expression at mRNA and protein levels and stimulated LDL uptake. Subsequent luciferase reporter gene assays revealed that the mutation of sterol regulatory element-binding protein (SREBP)-binding element abolished the piperine-mediated induction of LDLR promoter activity. Further, piperine treatments increased mRNA levels of several SREBP targets and mature forms of SREBPs. However, the piperine-mediated induction of the mature forms of SREBPs was not observed in SRD–15 cells, which lack insulin-induced gene–1 (Insig–1) and Insig–2. Finally, the knockdown of SREBPs completely abolished the piperine-meditated induction of LDLR gene expression in HepG2 cells, indicating that piperine stimulates the proteolytic activation of SREBP and subsequent induction of LDLR expression and activity.

L-Cysteine-induced up-regulation of the low-density lipoprotein receptor is mediated via a transforming growth factor-alpha signalling pathway

Sulphur-containing amino acids regulate plasma cholesterol levels in animals and humans. However, their mechanism of action remains unclear. Low-density lipoprotein receptor (LDLR) plays an important role in cholesterol metabolism. We therefore investigated the effects of sulphur-containing amino acids on the expression of LDLR in hepatocytes. HepG2 cells were cultured in Dulbecco's Modified Eagle's Medium with or without sulphur-containing amino acids and cysteine-containing compounds. We found that L-cysteine increased LDLR mRNA and enhanced LDLR gene promoter activity through the extracellular-signal-related kinase and p38 mitogen-activated protein kinase signalling pathways in HepG2 cells. Moreover, we observed that L-cysteine stimulated the release of transforming growth factor-alpha (TGF-α) and that TGF-α increased the LDLR mRNA levels. This study provides a report of the L-cysteine mediated up-regulation of the LDLR expression via TGF-α signalling pathway. Our findings provide insights into cholesterol homeostasis and amino acid signalling.

Effects of the genetic pattern defined by low-density lipoprotein receptor and IL28B genotypes on the outcome of hepatitis C virus infection

The aim of this study was to assess the impact of the genetic pattern (GP) defined by the single nucleotide polymorphisms (SNPs) rs14158 of low-density lipoprotein receptor (LDLR) and rs12979860 of interleukin-28B (IL28B) genes on the outcome and features of hepatitis C virus (HCV) infection in patients with and without human immunodeficiency virus (HIV) coinfection. 314 HIV/HCV-coinfected and 109 HCV-monoinfected patients treated with pegylated interferon (Peg-IFN) plus ribavirin (RBV), as well as 51 patients with HCV spontaneous clearance (SC), were included. Variations in both SNPs were determined by the TaqMan polymerase chain reaction (PCR) assay. In the 286 patients chronically infected by HCV genotypes 1 or 4, both rs14158 CC and rs12979860 CC were associated with a higher rate of sustained virological response (SVR), and these effects were complementary in both HCV-monoinfected and HIV/HCV-coinfected patients. Thus, 24 % of patients with rs14158/rs12979860 TT-TC/TT-TC, 33 % with TT-TC/CC, 44.2 % with CC/TT-TC, and 75.8 % harboring CC/CC attained SVR (p < 0.001). SC was associated with the IL28B genotype (66.7 % CC in SC vs. 42.6 % among those with chronic infection, p < 0.001) but not with the LDLR genotype. There was no association between GP and the plasma level of alanine aminotransferase (ALT) or the presence of advanced fibrosis. There is a complementary effect between the IL28B and LDLR genotypes on the probability of achieving SVR after Peg-IFN/RBV therapy in patients with HCV 1 or 4. Thus, the predictive value of IL28B genotype is modulated by the LDLR genotype in both HCV-monoinfected and HIV/HCV-coinfected patients. This complementary effect of both genotypes is also observed on the plasma levels of low-density lipoprotein cholesterol (LDL-C).

The critical role of mRNA destabilizing protein heterogeneous nuclear ribonucleoprotein d in 3' untranslated region-mediated decay of low-density lipoprotein receptor mRNA in liver tissue

OBJECTIVE

Previous studies showed that low-density lipoprotein receptor (LDLR) mRNA 3' untranslated region (UTR) contains regulatory elements responsible for rapid mRNA turnover in hepatic cells and mediates the mRNA stabilization induced by berberine (BBR). Here, we elucidate the underlying mechanism of BBR's action by characterizing mRNA-binding proteins that modulate LDLR mRNA decay via 3'UTR in liver tissue in vivo.

APPROACH AND RESULTS

We generated a transgenic mouse model (Alb-Luc-UTR) that expresses Luc-LDLR3'UTR reporter gene driven by the albumin promoter to study 3'UTR function in mediating LDLR mRNA decay in liver tissue. We show that treating Alb-Luc-UTR mice with BBR led to significant increases in hepatic bioluminescence signals, Luc-UTR mRNA, and LDLR mRNA levels as compared with control mice. These effects were accompanied by specific reductions of mRNA decay-promoting factor heterogeneous nuclear ribonucleoprotein D (hnRNP D) in liver of BBR-treated mice. Knockdown and overexpression studies further demonstrated that hnRNP D p37 isoform plays a major role in promoting hepatic LDLR mRNA degradation. In addition, we examined LDLR mRNA half-life, Luc-UTR reporter activity, and hnRNP D expression levels in cell lines derived from extrahepatic tissues. We demonstrated that strengths of 3'UTR in promoting mRNA degradation correlate with hnRNP D cellular abundances in nonhepatic cell lines, thereby suggesting its involvement in LDLR mRNA degradation beyond liver tissue.

CONCLUSIONS

hnRNP D is critically involved in LDLR mRNA degradation in liver tissue in vivo. The inverse relationship of hnRNP D abundance with LDLR mRNA levels after BBR treatment suggests the potential of hnRNP D of being a novel therapeutic target for LDL cholesterol lowering.

Bile acid receptors in non-alcoholic fatty liver disease

Accumulating data have shown that bile acids are important cell signaling molecules, which may activate several signaling pathways to regulate biological processes. Bile acids are endogenous ligands for the farnesoid X receptor (FXR) and TGR5, a G-protein coupled receptor. Gain- and loss-of-function studies have demonstrated that both FXR and TGR5 play important roles in regulating lipid and carbohydrate metabolism and inflammatory responses. Importantly, activation of FXR or TGR5 lowers hepatic triglyceride levels and inhibits inflammation. Such properties of FXR or TGR5 have indicated that these two bile acid receptors are ideal targets for treatment of non-alcoholic fatty liver disease, one of the major health concerns worldwide. In this article, we will focus on recent advances on the role of both FXR and TGR5 in regulating hepatic triglyceride metabolism and inflammatory responses under normal and disease conditions.

Activation of the farnesoid X receptor induces hepatic expression and secretion of fibroblast growth factor 21

Previous studies have shown that starvation or consumption of a high fat, low carbohydrate (HF-LC) ketogenic diet induces hepatic fibroblast growth factor 21 (FGF21) gene expression in part by activating the peroxisome proliferator-activated receptor-α (PPARα). Using primary hepatocyte cultures to screen for endogenous signals that mediate the nutritional regulation of FGF21 expression, we identified two sources of PPARα activators (i.e. nonesterified unsaturated fatty acids and chylomicron remnants) that induced FGF21 gene expression. In addition, we discovered that natural (i.e. bile acids) and synthetic (i.e. GW4064) activators of the farnesoid X receptor (FXR) increased FGF21 gene expression and secretion. The effects of bile acids were additive with the effects of nonesterified unsaturated fatty acids in regulating FGF21 expression. FXR activation of FGF21 gene transcription was mediated by an FXR/retinoid X receptor binding site in the 5'-flanking region of the FGF21 gene. FGF19, a gut hormone whose expression and secretion is induced by intestinal bile acids, also increased hepatic FGF21 secretion. Deletion of FXR in mice suppressed the ability of an HF-LC ketogenic diet to induce hepatic FGF21 gene expression. The results of this study identify FXR as a new signaling pathway activating FGF21 expression and provide evidence that FXR activators work in combination with PPARα activators to mediate the stimulatory effect of an HF-LC ketogenic diet on FGF21 expression. We propose that the enhanced enterohepatic flux of bile acids during HF-LC consumption leads to activation of hepatic FXR and FGF19 signaling activity and an increase in FGF21 gene expression and secretion.

Bile acid receptors as targets for the treatment of dyslipidemia and cardiovascular disease

Dyslipidemia is an important risk factor for cardiovascular disease (CVD) and atherosclerosis. When dyslipidemia coincides with other metabolic disorders such as obesity, hypertension, and glucose intolerance, defined as the metabolic syndrome (MS), individuals present an elevated risk to develop type 2 diabetes (T2D) as well as CVD. Because the MS epidemic represents a growing public health problem worldwide, the development of therapies remains a major challenge. Alterations of bile acid pool regulation in T2D have revealed a link between bile acid and metabolic homeostasis. The bile acid receptors farnesoid X receptor (FXR) and TGR5 both regulate lipid, glucose, and energy metabolism, rendering them potential pharmacological targets for MS therapy. This review discusses the mechanisms of metabolic regulation by FXR and TGR5 and the utility relevance of natural and synthetic modulators of FXR and TGR5 activity, including bile acid sequestrants, in the treatment of the MS.

Lipid-sensing nuclear receptors in the pathophysiology and treatment of the metabolic syndrome

Metabolic syndrome (MS) is a cluster of different diseases, namely central obesity, hypertension, hyperglycemia, and dyslipidemia, together with a pro-thrombotic and pro-inflammatory state. These metabolic abnormalities are often associated with an increased risk for cardiovascular disease (CVD) and cancer. Dietary and lifestyle modifications are currently believed more effective than pharmacological therapies in the management of MS patients. Nevertheless, the relatively low grade of compliance of patients to these recommendations, as well as the failure of current therapies, highlights the need for the discovery of new pharmacological and nutraceutic approaches. A deeper knowledge of the patho-physiological events that initiate and support the MS is mandatory. Lipid-sensing nuclear receptors (NRs) are the master transcriptional regulators of lipid and carbohydrate metabolism and inflammatory responses, thus standing as suitable targets. This review focuses on the physiological relevance of the NRs (peroxisome proliferator-activated receptors, liver X receptors, and farnesoid X receptor) in the control of whole-body homeostasis, with a special emphasis on lipid and glucose metabolism, and on the relationships between metabolic unbalances, systemic inflammation, and the onset of CVD. Future perspectives and possible clinical applications are also presented.

FXR an emerging therapeutic target for the treatment of atherosclerosis

Atherosclerosis is the leading cause of illness and death. Therapeutic strategies aimed at reducing cholesterol plasma levels have shown efficacy in either reducing progression of atherosclerotic plaques and atherosclerosis-related mortality. The farnesoid-X-receptor (FXR) is a member of metabolic nuclear receptors (NRs) superfamily activated by bile acids. In entero-hepatic tissues, FXR functions as a bile acid sensor regulating bile acid synthesis, detoxification and excretion. In the liver FXR induces the expression of an atypical NR, the small heterodimer partner, which subsequently inhibits the activity of hepatocyte nuclear factor 4α repressing the transcription of cholesterol 7a-hydroxylase, the critical regulatory gene in bile acid synthesis. In the intestine FXR induces the release of fibroblast growth factor 15 (FGF15) (or FGF19 in human), which activates hepatic FGF receptor 4 (FGFR4) signalling to inhibit bile acid synthesis. In rodents, FXR activation decreases bile acid synthesis and lipogenesis and increases lipoprotein clearance, and regulates glucose homeostasis by reducing liver gluconeogenesis. FXR exerts counter-regulatory effects on macrophages, vascular smooth muscle cells and endothelial cells. FXR deficiency in mice results in a pro-atherogenetic lipoproteins profile and insulin resistance but FXR^−/– mice fail to develop any detectable plaques on high-fat diet. Synthetic FXR agonists protect against development of aortic plaques formation in murine models characterized by pro-atherogenetic lipoprotein profile and accelerated atherosclerosis, but reduce HDL levels. Because human and mouse lipoprotein metabolism is modulated by different regulatory pathways the potential drawbacks of FXR ligands on HDL and bile acid synthesis need to addressed in relevant clinical settings.

Chenodeoxycholic acid stabilization of LDL receptor mRNA depends on 3'-untranslated region and AU-rich element-binding protein

Human low-density lipoprotein receptor (LDLR) mRNA is unstable and contains four AU-rich elements (AREs) in the 3'-untranslated region (3'-UTR). The aim of this study was to verify the involvement of the 3'-UTR in the rapid degradation of LDLR mRNA. This study revealed that the 3'-UTR is necessary and sufficient for the degradation, and that the 1st ARE (ARE1) close to the stop codon associates with cytoplasmic proteins, and is primarily responsible for the degradation. Chenodeoxycholic acid (CDCA) treatment stabilized chimeric GFP-LDLR 3'-UTR mRNA and accompanied mitogen-activated protein kinase (MAPK) activation. The UV cross-linking assays showed that a protein of 80kDa increasingly binds to the region including the ARE1 in response to CDCA-mediated MAPK activation.

Stabilization of small heterodimer partner mRNA by grape seed procyanidins extract in cultured hepatocytes

SCOPE

Consumption of dietary grape seed procyanidins extract (GSPE) has improved the plasma lipid profile in humans and experimental animals. The effect of GSPE on the reduction of the postprandial plasma triglyceride (TG) levels has been attributed to the activation of the small heterodimer partner (SHP). GSPE increases SHP gene expression in rat liver and the TG-lowering effect of GSPE is abolished in SHP-deficient mice. However, the mechanism by which GSPE increases SHP mRNA levels remains unclear. This study addressed the effect of GSPE on SHP mRNA stability.

METHODS AND RESULTS

The present study shows for the first time that SHP mRNA is rapidly degraded, as measured by actinomycin D-based mRNA chase experiments, and GSPE transiently stabilizes SHP mRNA in HepG2 cells. This degradative effect was completely abolished with 2 h of prolonged treatment with GSPE. However, treatment of fresh HepG2 cells with a pretreated GSPE-containing medium also stabilized SHP mRNA, indicating that GSPE inactivation is not responsible for the transient effects that GSPE has on SHP mRNA stability.

CONCLUSION

SHP expression is intricately controlled by mRNA stabilization, which is transiently increased by GSPE, along with at the transcriptional and posttranslational levels.

A role of the bile salt receptor FXR in atherosclerosis

This study reviews current insights into the role of bile salts and bile salt receptors on the progression and regression of atherosclerosis. Bile salts have emerged as important modifiers of lipid and energy metabolism. At the molecular level, bile salts regulate lipid and energy homeostasis mainly via the bile salt receptors FXR and TGR5. Activation of FXR has been shown to improve plasma lipid profiles, whereas Fxr(-/-) mice have increased plasma triglyceride and very-low-density lipoprotein levels. Nevertheless, high-density lipoprotein cholesterol levels are increased in these mice, suggesting that FXR has both anti- and proatherosclerotic properties. Interestingly, there is increasing evidence for a role of FXR in "nonclassical" bile salt target tissues, eg, vasculature and macrophages. In these tissues, FXR has been shown to influence vascular tension and regulate the unloading of cholesterol from foam cells, respectively. Recent publications have provided insight into the antiinflammatory properties of FXR in atherosclerosis. Bile salt signaling via TGR5 might regulate energy homeostasis, which could serve as an attractive target to increase energy expenditure and weight loss. Interventions aiming to increase cholesterol turnover (eg, by bile salt sequestration) significantly improve plasma lipid profiles and diminish atherosclerosis in animal models. Bile salt metabolism and bile salt signaling pathways represent attractive therapeutic targets for the treatment of atherosclerosis.

Interplay between cholesterol and drug metabolism

Cholesterol biosynthetic and metabolic pathways contain several branching points towards physiologically active molecules, such as coenzyme Q, vitamin D, glucocorticoid and steroid hormones, oxysterols, or bile acids. Sophisticated regulatory mechanisms are involved in maintenance of the homeostasis of not only cholesterol but also other cholesterogenic molecules. In addition to endogenous cues, cholesterol homeostasis needs to accommodate also to exogenous cues that are imported into the body, such as chemicals and medications. Steroid and nuclear receptors together with sterol regulatory element-binding protein (SREBP) mediate the fine tuning of biosynthetic and metabolic routes as well as transports of cholesterol and its derivatives. Similarly, drug/xenobiotic metabolism is the subject to the feedback regulation of cytochrome P450 enzymes and transporters. The regulatory mechanisms that maintain the homeostasis of cholesterogenic molecules and are involved in drug metabolism share similarities. Cholesterol and cholesterogenic compounds (bile acids, glucocorticoids, vitamin D, etc.) regulate the xenosensor signaling in drug-mediated induction of the major drug-metabolizing cytochrome P450 enzymes. The key cellular receptors, pregnane X receptor (PXR), constitutive androstane receptor (CAR), vitamin D receptor (VDR), and glucocorticoid receptor (GR) provide a functional cross-talk between the pathways maintaining cholesterol homeostasis and controlling the expression of drug-metabolizing enzymes. These receptors serve as metabolic sensors, resulting in a coordinate regulation of cholesterogenic compounds metabolism and of the defense against xenobiotic and endobiotic toxicity. Herein we present a comprehensive review of functional interactions between cholesterol homeostasis and drug metabolism involving the main nuclear and steroid receptors.

Molecular pathways and agents for lowering LDL-cholesterol in addition to statins

Recent guidelines in North America and Europe recommend lowering low density lipoprotein associated cholesterol (LDLC) to achieve optimal coronary heart disease risk reduction. Statins have been the therapy of choice and proven successful and relatively safe. However, we are now facing new challenges and it appears that additional or alternative drugs are urgently needed. This boosts research in the field, reopening old cases like other inhibitors of cholesterol synthesis or making attractive tools from the latest technologies like gene silencing by anti-sense oligonucleotides. LDLs are cholesterol-enriched lipoproteins stabilized by the hepatic apolipoprotein B100, and derived from TG rich very low density lipoprotein. This review focuses on the molecular pathways involved in plasma LDLC production and elimination, in particular cholesterol absorption and the hepatobiliary route, apoB100 and VLDL production, and LDL clearance via the LDL receptor. We will identify important or rate-limiting proteins (including Niemann-Pick C1-like 1 (NPC1L1), microsomal TG transfer protein (MTP), acyl-coenzyme A/cholesterol acyltransferase (ACAT), Acyl-CoA:diacylglycerol acyltransferases 2 (DGAT2), proprotein convertase subtilisin kexin type 9 (PCSK9)), and nuclear receptors (farnesoid X receptor (FXR), thyroid hormone receptor (TR)) that constitute interesting therapeutic targets. Numerous compounds already in use modulate these pathways, such as phytosterols, ezetimibe, bile acids sequestrants, niacin, and fibrates. Many pathways can be considered to lower LDLC, but the road has been paved with disappointments and difficulties. With new targets identified and diversification of the drugs, a new era for better LDLC management is plausible.

Chronic alcohol consumption disrupted cholesterol homeostasis in rats: down-regulation of low-density lipoprotein receptor and enhancement of cholesterol biosynthesis pathway in the liver

BACKGROUND

Chronic alcohol consumption causes alcoholic liver disease, which is associated, or initiated, with dysregulated lipid metabolism. Very recent evidence suggested that dysregulated cholesterol metabolism plays an important role in the pathogenesis of alcoholic fatty liver diseases, however, the effects of chronic alcohol exposure on cholesterol homeostasis have not been well studied and underlying mechanisms behind are still elusive.

METHODS

Male Sprague-Dawley rats weighing 250 +/- 5.5 g (mean +/- SEM) divided into 2 groups (8 rats per group) and pair-fed with liquid diets containing (in percent of energy intake) 18% protein, 35% fat, 12% carbohydrate, and 35% either ethanol (ethanol diet) or an isocaloric maltose-dextrin mixture (control diet), according to Lieber and De Carli, for 4 weeks.

RESULTS

Long-term excessive alcohol feeding to rats caused fatty liver and liver injury, which was associated with disrupted cholesterol homeostasis, characterized by increased hepatic cholesterol levels and hypercholesterolemia. Hepatic cholesterol increases were concomitant with constantly activated sterol regulatory element-binding protein-2 (SREBP-2) in the liver and increased expression of 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase, a rate-limiting enzyme for cholesterol de novo synthesis, indicating enhanced cholesterol biosynthesis. Alcohol-induced hypercholesterolemia was accompanied by decreased LDL receptor (LDLr) levels in the liver. Further investigations revealed that chronic alcohol exposure increased hepatic proprotein convertase subtilisin/kexin type 9 (PCSK9) contents to down-regulate LDLr via a post-translational mechanism. Moreover, alcohol feeding suppressed extracellular signal-regulated kinase (ERK) activation in the liver. In vitro studies showed that inhibition of ERK activation was associated with decreased LDLr expression in HepG2 cells.

CONCLUSIONS

Our study provides the first evidence that both increased PCSK9 expression and suppressed ERK activation in the liver contributes to alcohol-induced hypercholesterolemia in rats.

Regulation of the proprotein convertase subtilisin/kexin type 9 in intestinal epithelial cells

Proprotein convertase subtilisin/kexin type 9 (PCSK9) posttranslationally promotes the degradation of the low-density lipoprotein receptor (LDLr) in hepatocytes and increases plasma LDL cholesterol. It is not clear, however, whether PCSK9 plays a role in the small intestine. Here, we characterized the patterns of variations of PCSK9 and LDLr in fully differentiated Caco-2/15 cells as a function of various potential effectors. Cholesterol (100 microM) solubilized in albumin or micelles significantly downregulated PCSK9 gene (30%, P<0.05) and protein expression (50%, P<0.05), surprisingly in concert with a decrease in LDLr protein levels (45%, P<0.05). Cells treated with 25-hydroxycholesterol (50 microM) also displayed significant reduction in PCSK9 gene (37%, P<0.01) and protein (75% P<0.001) expression, whereas LDLr showed a decrease at the gene (30%, P<0.05) and protein (57%, P<0.01) levels, respectively. The amounts of PCSK9 mRNA and protein in Caco-2/15 cells were associated to the regulation of 3-hydroxy-3-methylglutaryl-CoA reductase and sterol regulatory element binding protein-2 (SREBP-2) that can transcriptionally activate PCSK9 via sterol-regulatory elements located in its proximal promoter region. On the other hand, depletion of cholesterol content by hydroxypropyl-beta-cyclodextrin upregulated PCSK9 transcripts (20%, P<0.05) and protein mass (540%, P<0.001), in parallel with SREBP-2 protein levels. The addition of bile acids (BA) taurocholate and deoxycholate to the apical culture medium lowered PCSK9 gene expression (25%, P<0.01) and raised PCSK9 protein expression (30%, P<0.01), respectively, probably via the modulation of farnesoid X receptor. Furthermore, unconjugated and conjugated BA exhibited different effects on PCSK9 and LDLr. Altogether, these data indicate that intestinal PCSK9 is highly modulated by sterols and emphasize the distinct effects of BA species.

A synthetic farnesoid X receptor (FXR) agonist promotes cholesterol lowering in models of dyslipidemia

The nuclear hormone receptor farnesoid X receptor (FXR) plays a critical role in the regulation of bile acid, triglyceride (TG), and cholesterol homeostasis. WAY-362450 (FXR-450/XL335) is a potent synthetic FXR agonist as characterized in luciferase reporter assays and in mediating FXR target gene regulation in primary human and immortalized mouse hepatocytes. In vivo, WAY-362450 dose dependently decreased serum TG levels after 7 days of oral dosing in western diet-fed low-density lipoprotein receptor-/- mice and in the diabetic mouse strains KK-Ay and db/db comparable to that achieved with the peroxisome proliferator activated receptor-alpha agonist, fenofibrate. WAY-362450 treatment also reduced serum cholesterol levels via reductions in LDLc, VLDLc, and HDLc lipoprotein fractions that were not accompanied by hepatic cholesterol accumulation. This cholesterol lowering was dependent on FXR as demonstrated in a hypothyroid-induced hypercholesterolemia setting in FXR-/- mice. In fructose-fed models, WAY-362450 also decreased TG and VLDLc levels in rats and hamsters but significantly increased HDLc levels in rats while reducing HDLc levels in hamsters. The differential effect of WAY-362450 on HDLc is likely due to a murine-specific induction of endothelial lipase and scavenger receptor-BI that does not occur in rats. These studies demonstrate a consistent ability of WAY-362450 to lower both serum TG and cholesterol levels and suggest that synthetic FXR agonists may have clinical utility in the treatment of mixed dyslipidemia.

Role of bile acids and bile acid receptors in metabolic regulation

The incidence of the metabolic syndrome has taken epidemic proportions in the past decades, contributing to an increased risk of cardiovascular disease and diabetes. The metabolic syndrome can be defined as a cluster of cardiovascular disease risk factors including visceral obesity, insulin resistance, dyslipidemia, increased blood pressure, and hypercoagulability. The farnesoid X receptor (FXR) belongs to the superfamily of ligand-activated nuclear receptor transcription factors. FXR is activated by bile acids, and FXR-deficient (FXR(-/-)) mice display elevated serum levels of triglycerides and high-density lipoprotein cholesterol, demonstrating a critical role of FXR in lipid metabolism. In an opposite manner, activation of FXR by bile acids (BAs) or nonsteroidal synthetic FXR agonists lowers plasma triglycerides by a mechanism that may involve the repression of hepatic SREBP-1c expression and/or the modulation of glucose-induced lipogenic genes. A cross-talk between BA and glucose metabolism was recently identified, implicating both FXR-dependent and FXR-independent pathways. The first indication for a potential role of FXR in diabetes came from the observation that hepatic FXR expression is reduced in animal models of diabetes. While FXR(-/-) mice display both impaired glucose tolerance and decreased insulin sensitivity, activation of FXR improves hyperglycemia and dyslipidemia in vivo in diabetic mice. Finally, a recent report also indicates that BA may regulate energy expenditure in a FXR-independent manner in mice, via activation of the G protein-coupled receptor TGR5. Taken together, these findings suggest that modulation of FXR activity and BA metabolism may open new attractive pharmacological approaches for the treatment of the metabolic syndrome and type 2 diabetes.

Identification of mRNA binding proteins that regulate the stability of LDL receptor mRNA through AU-rich elements

The 3'untranslated region (UTR) of human LDL receptor (LDLR) mRNA contains three AU-rich elements (AREs) responsible for rapid mRNA turnover and mediates the stabilization induced by berberine (BBR). However, the identities of the specific RNA binding proteins involved in the regulation of LDLR mRNA stability at the steady state level or upon BBR treatment are unknown. By conducting small interfering RNA library screenings, biotinylated RNA pull-down, mass spectrometry analysis, and functional assays, we now identify heterogeneous nuclear ribonucleoprotein D (hnRNP D), hnRNP I, and KH-type splicing regulatory protein (KSRP) as key modulators of LDLR mRNA stability in liver cells. We show that hnRNP D, I, and KSRP interact with AREs of the LDLR 3'UTR with sequence specificity. Silencing the expression of these proteins increased LDLR mRNA and protein levels. We further demonstrate that BBR-induced mRNA stabilization involves hnRNP I and KSRP, as their cellular depletions abolished the BBR effect and BBR treatment reduced the binding of hnRNP I and KSRP to the LDLR mRNA 3'UTR. These new findings demonstrate that LDLR mRNA stability is controlled by a group of ARE binding proteins, including hnRNP D, hnRNP I, and KSRP. Our results suggest that interference with the ability of destabilizing ARE binding proteins to interact with LDLR-ARE motifs is likely a mechanism for regulating LDLR expression by compounds such as BBR and perhaps others.