Regulation of cholesterol 7 alpha-hydroxylase gene expression in Hep-G2 cells. Effect of serum, bile salts, and coordinate and noncoordinate regulation with other sterol-responsive genes

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.

Intestinal Flora Derived Metabolites Affect the Occurrence and Development of Cardiovascular Disease

In recent years, increasing evidence has shown that the gut microbiota and their metabolites play a pivotal role in human health and diseases, especially the cardiovascular diseases (CVDs). Intestinal flora imbalance (changes in the composition and function of intestinal flora) accelerates the progression of CVDs. The intestinal flora breaks down the food ingested by the host into a series of metabolically active products, including trimethylamine N-Oxide (TMAO), short-chain fatty acids (SCFAs), primary and secondary bile acids, tryptophan and indole derivatives, phenylacetylglutamine (PAGln) and branched chain amino acids (BCAA). These metabolites participate in the occurrence and development of CVDs via abnormally activating these signaling pathways more swiftly when the gut barrier integrity is broken down. This review focuses on the production and metabolism of TMAO and SCFAs. At the same time, we summarize the roles of intestinal flora metabolites in the occurrence and development of coronary heart disease and hypertension, pulmonary hypertension and other CVDs. The theories of "gut-lung axis" and "gut-heart axis" are provided, aiming to explore the potential targets for the treatment of CVDs based on the roles of the intestinal flora in the CVDs.

Di(2-ethylhexyl) phthalate disturbs cholesterol metabolism through oxidative stress in rat liver

Di(2-ethylhexyl) phthalate (DEHP) is widely used and has been implicated in hepatotoxicity, although the mechanism is unclear. Here, we investigated the effect of DEHP on hepatic cholesterol metabolism in SD rats exposed to 0 and 300 mg/kg/day DEHP for 12 weeks. An RNA-Seq analysis was performed to describe the hepatic responses to long-term DEHP exposure in combination with serological and oxidative stress parameter measurements. DEHP increased the serum levels of total cholesterol (TC), high-density lipoprotein (HDL), and alanine transaminase (ALT). Moreover, DEHP increased the content of malondialdehyde (MDA) and decreased antioxidant enzyme activities in the liver. Transcriptomic results revealed that DEHP dramatically changed the cholesterol metabolism pathway and oxidation-reduction process and depressed gene expression involved in cholesterol efflux and monooxygenase activity. Total antioxidant capacity (T-AOC) positively correlated with Abcg5 and Abcg8. Overall, this study showed the mechanisms underlying hepatotoxicity caused by DEHP, providing new insights into understanding DEHP poisoning.

Polyphenol Effects on Cholesterol Metabolism via Bile Acid Biosynthesis, CYP7A1: A Review

Atherosclerosis, the main contributor to coronary heart disease, is characterised by an accumulation of lipids such as cholesterol in the arterial wall. Reverse cholesterol transport (RCT) reduces cholesterol via its conversion into bile acids (BAs). During RCT in non-hepatic peripheral tissues, cholesterol is transferred to high-density lipoprotein (HDL) particles and returned to the liver for conversion into BAs predominantly via the rate-limiting enzyme, cholesterol 7 α-hydroxylase (CYP7A1). Numerous reports have described that polyphenol induced increases in BA excretion and corresponding reductions in total and LDL cholesterol in animal and in-vitro studies, but the process whereby this occurs has not been extensively reviewed. There are three main mechanisms by which BA excretion can be augmented: (1) increased expression of CYP7A1; (2) reduced expression of intestinal BA transporters; and (3) changes in the gut microbiota. Here we summarise the BA metabolic pathways focusing on CYP7A1, how its gene is regulated via transcription factors, diurnal rhythms, and microRNAs. Importantly, we will address the following questions: (1) Can polyphenols enhance BA secretion by modulating the CYP7A1 biosynthetic pathway? (2) Can polyphenols alter the BA pool via changes in the gut microbiota? (3) Which polyphenols are the most promising candidates for future research? We conclude that while in rodents some polyphenols induce CYP7A1 expression predominantly by the LXRα pathway, in human cells, this may occur through FXR, NF-KB, and ERK signalling. Additionally, gut microbiota is important for the de-conjugation and excretion of BAs. Puerarin, resveratrol, and quercetin are promising candidates for further research in this area.

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.

Human cytochrome P450 enzymes 5-51 as targets of drugs and natural and environmental compounds: mechanisms, induction, and inhibition - toxic effects and benefits

Cytochrome P450 (P450, CYP) enzymes have long been of interest due to their roles in the metabolism of drugs, pesticides, pro-carcinogens, and other xenobiotic chemicals. They have also been of interest due to their very critical roles in the biosynthesis and metabolism of steroids, vitamins, and certain eicosanoids. This review covers the 22 (of the total of 57) human P450s in Families 5-51 and their substrate selectivity. Furthermore, included is information and references regarding inducibility, inhibition, and (in some cases) stimulation by chemicals. We update and discuss important aspects of each of these 22 P450s and questions that remain open.

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.

Thyroid-stimulating hormone regulates hepatic bile acid homeostasis via SREBP-2/HNF-4α/CYP7A1 axis

BACKGROUND & AIMS

Bile acids (BAs) play a crucial role in dietary fat digestion and in the regulation of lipid, glucose, and energy metabolism. Thyroid-stimulating hormone (TSH) is a hormone produced by the anterior pituitary gland that directly regulates several metabolic pathways. However, the impact of TSH on BA homeostasis remains largely unknown.

METHODS

We analyzed serum BA and TSH levels in healthy volunteers under strict control of caloric intake. Thyroidectomized rats were administered thyroxine and injected with different doses of TSH. Tshr(-/-) mice were supplemented with thyroxine, and C57BL/6 mice were injected with Tshr-siRNA via the tail vein. The serum BA levels, BA pool size, and fecal BA excretion rate were measured. The regulation of SREBP-2, HNF-4α, and CYP7A1 by TSH were analyzed using luciferase reporter, RNAi, EMSA, and CHIP assays.

RESULTS

A negative correlation was observed between the serum levels of TSH and the serum BA levels in healthy volunteers. TSH administration led to a decrease in BA content and CYP7A1 activity in thyroidectomized rats supplemented with thyroxine. When Tshr was silenced in mice, the BA pool size, fecal BA excretion rate, and serum BA levels all increased. Additionally, we found that HNF-4α acts as a critical molecule through which TSH represses CYP7A1 activity. We further confirmed that the accumulation of mature SREBP-2 protein could impair the capacity of nuclear HNF-4α to bind to the CYP7A1 promoter, a mechanism that appears to mediate the effects of TSH.

CONCLUSIONS

TSH represses hepatic BA synthesis via a SREBP-2/HNF-4α/CYP7A1 signaling pathway. This finding strongly supports the notion that TSH is an important pathophysiological regulator of liver BA homeostasis independently of thyroid hormones.

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.

CYP7A1 (-204 A>C; rs3808607 and -469 T>C; rs3824260) promoter polymorphisms and risk of gallbladder cancer in North Indian population

Cholesterol 7-alpha hydroxylase (CYP7A1), which is a rate-limiting enzyme for cholesterol catabolism and bile acid synthesis, may affect cholesterol homeostasis and result in gallstone formation that is a major risk factor for gallbladder cancer (GBC) pathogenesis. Genetic variations in CYP7A1 may influence its expression and thus may affect the risk of gallstone disease and GBC. We aimed to study the association of 2 promoter polymorphisms of CYP7A1 (-204 A>C [rs3808607] and -469 T>C [rs3824260]) in gallstone and GBC susceptibility in North Indian population. The study included 185 GBC patients, 195 symptomatic gallstone patients, and 200 healthy controls. Genotyping for both polymorphisms was done by polymerase chain reaction-restriction fragment length polymorphism method. Although the CC genotype of CYP7A1 -204 A>C was not significantly associated with gallstone disease (P = .083, odds ratio [OR] = 1.69, 95% confidence interval [CI] = 0.9-3.0), it was conferring higher risk for GBC (P = .018, OR = 2.05, 95% CI = 1.1-3.7). However, CYP7A1 -469 T>C was not associated with gallstone disease and GBC risk in our population. After subgroup stratifications on the basis of sex and gallstone status, CC genotype and variant allele of CYP7A1 -204 A>C imparted higher risk for GBC in women (P = .003, OR = 3.30, 95% CI = 1.5-7.2) and patients without gallstones (P = .045, OR = 1.91, 95% CI = 1.2-3.6). Haplotype analysis of the 2 polymorphisms showed that C,T (P = .045, OR = 1.84, 95% CI = 1.0-3.3) and C,C (P = .0001, OR = 3.10, 95% CI = 1.6-6.0) haplotypes had elevated risk of GBC predisposition. CYP7A1 -469 T>C is not associated with gallstone disease or GBC risk. Although CYP7A1 -204 A>C might play a modest role in gallstone susceptibility, it is an independent risk factor for GBC in North Indian population. Underlying mechanism for GBC susceptibility by CYP7A1 (-204 A>C and -469 T>C) haplotype appears to be independent of gallstone pathway and is believed to involve genotoxicity resulting from subnormal bile acid production.

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.

Hypolipidaemic effect of maize starch with different amylose content in ovariectomized rats depends on intake amount of resistant starch

The effect of amylose content on digestibility of starch in the small intestine and on the concentration of plasma lipid were studied in ileorectostomized rats and in ovariectomized rats, respectively. Seven kinds of starch with different amylose content (0, 27, 54, 62, 76, 79, 86 %) were used as test starch, which contained 0·4, 5·6, 37·1, 40·2, 45·6, 36·9 and 36·1 % resistant starch (RS), respectively. Rats were fed one of test diets containing 30 % test starch with different amylose content for 14 d in ileorectostomized and for 21 d in ovariectomized rats. Food intake was not significantly different among the groups. In ileorectostomized rats, the small intestinal starch digestibility decreased with increasing intakes of amylose and RS. In ovariectomized rats, body weight gain was lower on the higher amylose maize starch diets. The concentrations of plasma TAG and cholesterol decreased with increasing intake of RS. The concentrations of liver total lipids and TAG decreased with increasing intake of RS, but that of liver cholesterol did not. There was significant positive correlation between the level of sterol regulatory element-binding protein-1c mRNA and concentration of liver TAG. Total SCFA amount in the caecum increased logarithmically with increasing dry weight of caecal contents. The amount of bile acids in the small intestinal content and the excretions of bile acids and neutral steroids in faeces increased with increasing RS intake. These results show that starch rich in RS is more effective in preventing ovarian hormone deficiency-induced hyperlipidaemia.

Activation of the farnesoid X receptor represses PCSK9 expression in human hepatocytes

The purpose of this study was to determine whether bile acids (BAs) modulate hepatic pro-protein convertase subtilisin/kexin 9 (PCSK9) gene expression. Immortalized human hepatocytes were treated with various BAs. Chenodeoxycholic acid (CDCA) treatment specifically decreased both PCSK9 mRNA and protein contents. Moreover, activation of the BA-activated farnesoid X receptor (FXR) by its synthetic specific agonist GW4064 also decreased PCSK9 expression. Of functional relevance, coadministration of CDCA counteracted the statin-induced PCSK9 expression, leading to a potentiation of LDL receptor activity. This study suggests that a transcriptional repression of PCSK9 by CDCA or FXR agonists may potentiate the hypolipidemic effect of statins.

The pharmacological exploitation of cholesterol 7alpha-hydroxylase, the key enzyme in bile acid synthesis: from binding resins to chromatin remodelling to reduce plasma cholesterol

Mammals dispose of cholesterol mainly through 7alpha-hydroxylated bile acids, and the enzyme catalyzing the 7alpha-hydroxylation, cholesterol 7alpha-hydroxylase (CYP7A1), has a deep impact on cholesterol homeostasis. In this review, we present the study of regulation of CYP7A1 as a good exemplification of the extraordinary contribution of molecular biology to the advancement of our understanding of metabolic pathways that has taken place in the last 2 decades. Since the cloning of the gene from different species, experimental evidence has accumulated, indicating that the enzyme is mainly regulated at the transcriptional level and that bile acids are the most important physiological inhibitors of CYP7A1 transcription. Multiple mechanisms are involved in the control of CYP7A1 transcription and a variety of transcription factors and nuclear receptors participate in sophisticated regulatory networks. A higher order of transcriptional regulation, stemming from the so-called histone code, also applies to CYP7A1, and recent findings clearly indicate that chromatin remodelling events have profound effects on its expression. CYP7A1 also acts as a sensor of signals coming from the gut, thus representing another line of defence against the toxic effects of bile acids and a downstream target of agents acting at the intestinal level. From the pharmacological point of view, bile acid binding resins were the first primitive approach targeting the negative feed-back regulation of CYP7A1 to reduce plasma cholesterol. In recent years, new drugs have been designed based on recent discoveries of the regulatory network, thus confirming the position of CYP7A1 as a focus for innovative pharmacological intervention.

Enhancing effect of taurine on CYP7A1 mRNA expression in Hep G2 cells

Taurine has been reported to enhance cholesterol 7alpha-hydroxylase (CYP7A1) mRNA expression in animal models. However, no in vitro studies of this effect have been reported. The Hep G2 human hepatoma cell line has been recognized as a good model for studying the regulation of human CYP7A1. This work characterizes the effects of taurine on CYP7A1 mRNA levels of Hep G2 cells in a dose- and time-dependent manner. In the dose-dependent experiment, Hep G2 cells were treated with 0, 2, 10 or 20 mM taurine in the presence or absence of cholesterol 0.2 mM for 48 h. In the time-dependent experiment, Hep G2 cells were treated with 0 or 20 mM taurine for 4, 24 and 48 h with and without cholesterol 0.2 mM. Our data revealed that taurine showed time- and dose-response effects on CYP7A1 mRNA levels in Hep G2 cells. However, glycine - a structural analogue of taurine - did not have an effect on CYP7A1 gene expression. These results show that, in agreement to previous studies on animal models, taurine induces the mRNA levels of CYP7A1 in Hep G2 cells, which could enhance cholesterol conversion into bile acids. Also, Hep G2 cell line may be an appropriate model to study the effects of taurine on human cholesterol metabolism.

Induction of human cholesterol 7α-hydroxylase in HepG2 cells by 2,4,6-trihydroxyacetophenone

In animal the plasma cholesterol-lowering activity of 2,4,6-trihydroxyacetophenone (THA) is due to enhanced cholesterol 7alpha-hydroxylase (CYP7A1) activity. We have examined the effect of THA on CYP7A1 activity and mRNA level in HepG2 cells. THA stimulated CYP7A1 activity in a concentration- and time-dependent manner. After exposure for 24 h, 1 muM THA induced CYP7A1 activity 160+/-8% and mRNA level 166+/-21% (mean+/-S.E.M.) of control. Moreover THA antagonized the inhibitory regulation of chenodeoxycholic acid on CYP7A1 mRNA expression. These results indicated that THA increases CYP7A1 activity in human HepG2 cells by stimulating mRNA transcription.

Adult sterol metabolism is not affected by a positive sterol balance in the neonatal Golden Syrian hamster

Dietary components impact metabolism early in life. Some of the diet-induced effects are long lasting and can lead to various adult-based diseases. In the current studies, we examined the short-term effects of dietary cholesterol on neonatal hepatic sterol metabolism and the long-term effects that those early-life diets had on sterol metabolism in adulthood. Neonatal hamsters began consuming solid food as a supplement to milk by 5 days of age; diets contained 0 or 2% added cholesterol (wt/wt). By 10 days of age, plasma and liver cholesterol concentrations were 3.2- and 2.5-fold greater, respectively, in the neonates fed cholesterol. Hepatic sterol synthesis rates were suppressed 65% in cholesterol-fed neonates compared with control neonates. By 20 days of age, plasma and liver cholesterol concentrations were still greater and sterol synthesis rates were now suppressed maximally in neonates fed cholesterol compared with control neonates. The expression level of an apolipoprotein B-containing lipoprotein receptor (low-density lipoprotein receptor-related protein) was greater and the mature form of the sterol regulatory element-binding protein-2 was similar in livers of 20-day-old control neonates compared with control neonates at 10 days of age. To test whether the change in sterol balance in the neonatal period had a lasting effect on hepatic sterol metabolism, all animals were weaned on a low-cholesterol diet. At 70 days of age, hepatic sterol synthesis rates, plasma lipoprotein and liver cholesterol concentrations, and bile acid pool sizes and compositions were measured. Sterol balance in the adults was similar between animals fed either diet early in life, as demonstrated by a lack of difference in any parameter measured. Thus, even though dietary cholesterol suppressed hepatic sterol synthesis rates dramatically in the neonatal hamster, the change has little impact on sterol balance later in life.

Pitavastatin, a Potent Hydroxymethylglutaryl Coenzyme a Reductase Inhibitor, Increases Cholesterol 7 .ALPHA.-Hydroxylase Gene Expression in HepG2 Cells

BACKGROUND

The effect of pitavastatin on the mRNA levels of apolipoprotein (apo) A-I, peroxisome proliferator-activated receptor alpha (PPARalpha), cholesterol 7alpha-hydroxylase (CYP7A1), and farnesoid X receptor (FXR) in HepG2 cells was examined to establish whether pitavastatin affects bile acid synthesis and if so, to determine a possible molecular mechanism.

METHODS AND RESULTS

HepG2 cells were cultured in serum-free Dulbecco's modified Eagle medium for 18 h before drug treatment. Total RNA was extracted at set times and mRNA levels were quantified by reverse transcription-real time polymerase chain reaction. Pitavastatin at 0.1, 1, 5, and 10 micromol/L increased the mRNA levels of apo A-I, PPARalpha, CYP7A1, and FXR in a dose-dependent manner. The mRNA levels of apo A-I, PPAR alpha, CYP7A1, and FXR similarly increased with increasing doses of pitavastatin. Coincubation of mevalonate (4 mmol/L) with pitavastatin (5 micromol/L) reversed the inductive effects of pitavastatin on the mRNA levels of these genes, indicating that the inductive effects of pitavastatin were related to its inhibition of HMG-CoA reductase.

CONCLUSIONS

Pitavastatin increased the mRNA levels of CYP7A1 in HepG2 cells, suggesting that increased conversion of cholesterol to bile acids may be the mechanism for its potent low-density lipoprotein cholesterol-lowering effects.

Mice expressing the human CYP7A1 gene in the mouse CYP7A1 knock-out background lack induction of CYP7A1 expression by cholesterol feeding and have increased hypercholesterolemia when fed a high fat diet

Cholesterol 7alpha-hydroxylase (CYP7A1) catalyzes the rate-limiting step in the pathway responsible for the formation of the majority of bile acids. Transcription of the gene is regulated by the size of the bile acid pool and dietary and hormonal factors. The farnesoid X receptor and the liver X receptor (LXR) are responsible for regulation by bile acids and cholesterol, respectively. To study the effects of dietary cholesterol and fat upon expression of the human CYP7A1 gene, mice were generated by crossing transgenic mice carrying the human CYP7A1 gene with mice that were homozygous knock-outs (CYP7A1(-/-)). The mice (mCYP7A1(-/-)/hCYP7A1) expressed the human gene at much higher levels than did the transgenics bred in the wild-type background. A diet containing 1% cholic acid reduced the expression of the human gene in mCYP7A1(-/-)/hCYP7A1 mice to undetectable levels. Cholestyramine (5%) increased the level of expression of the human gene and the mouse gene. Thus, farnesoid X receptor-mediated regulation was preserved. A diet containing 2% cholesterol increased expression of the mouse gene in wild-type mice, but it did not affect expression of the human gene in mCYP7A1(-/-)/hCYP7A1 mice. None of the diets altered the serum cholesterol or triglyceride levels in these mice; 1% cholic acid caused a redistribution of cholesterol from the high density lipoprotein to the low density lipoprotein density in the humanized mice but not in wild-type mice. A diet containing 30% saturated fat and 2% cholesterol caused a decrease in CYP7A1 levels in mCYP7A1(-/-)/hCYP7A1 mice. The serum cholesterol levels rose in all mice fed this diet. The increase was greater in the mCYP7A1(-/-)/hCYP7A1 mice. Together, these data suggest that the lack of an LXR element in the region from -56 to -49 of the human CYP7A1 promoter may account for some of the differences in response to diets between humans and rodents.

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

Recent studies have indicated that bile acids regulate the expression of several genes involved in bile acid and lipid metabolism as ligands for the farnesoid X receptor (FXR). We report here that bile acids are directly able to govern cholesterol metabolism by a novel mechanism. We show that chenodeoxycholic acid (CDCA) enhances low density lipoprotein (LDL) receptor gene expression in human cultured cell lines (HeLa, Hep G2, and Caco-2). The proteolytic activation of sterol regulatory element-binding protein-2 (SREBP-2), a major regulator for LDL receptor gene expression, is not affected by CDCA. Both deoxycholic acid and lithocholic acid as well as CDCA, but not ursodeoxycholic acid, increase the mRNA level for the LDL receptor, even when Hep G2 cells are cultured with 25-hydroxycholesterol, a potent suppressor of gene expression for the LDL receptor. Although it seems possible that FXR might be involved in genetic regulation, both reporter assays with a reporter gene containing the LDL receptor promoter as well as Northern blot analysis reveal that FXR is not involved in the process. On the other hand, inhibition of mitogen-activated protein (MAP) kinase activities, which are found to be induced by CDCA, abolishes the CDCA-mediated up-regulation of LDL receptor gene expression. We further demonstrate that CDCA stabilizes LDL receptor mRNA and that the MAP kinase inhibitors accelerate its turnover. Taken together, these results indicate that bile acids increase LDL uptake and the intracellular cholesterol levels through the activation of MAP kinase cascades in conjunction with a down-regulation of bile acid biosynthesis by FXR. This work opens up a new avenue for developing pharmaceutical interventions that lower plasma LDL by stabilizing LDL receptor mRNA.

The role of orphan nuclear receptors in the regulation of cholesterol homeostasis

Cholesterol balance is maintained by a series of regulatory pathways that control the acquisition of cholesterol from endogenous and exogenous sources and the elimination of cholesterol, facilitated by its conversion to bile acids. Over the past decade, investigators have discovered that a family of membrane-bound transcription factors, sterol regulatory element-binding proteins (SREBPs), mediate the end-product repression of key enzymes of cholesterol biosynthesis. Recently orphan members of another family of transcription factors, the nuclear hormone receptors, have been found to regulate key pathways in bile acid metabolism, thereby controlling cholesterol elimination. The study of these orphan nuclear receptors suggests their potential as targets for new drug therapies.

Correlation of farnesoid X receptor coactivator recruitment and cholesterol 7alpha-hydroxylase gene repression by bile acids

Cholesterol conversion to bile acids in the liver is regulated by the rate-limiting enzyme cholesterol 7alpha-hydroxylase (CYP7A1). CYP7A1 activity is regulated by feedback repression by bile acids at the transcriptional level. The farnesoid X receptor (FXR), a member of the nuclear hormone receptor superfamily, was recently demonstrated to function as the bile acid receptor and its high level of expression in the liver implicates it in the transcriptional regulation of CYP7A1. This study compares the potencies of various bile acids in their ability to mediate recruitment of the transcriptional coactivator protein, steroid receptor coactivator-1 (SRC-1), to the FXR ligand binding domain with their ability to repress CYP7A1 expression in HepG2 cells. A mammalian two-hybrid assay was utilized to assess the ability of FXR to recruit SRC-1 in a ligand-dependent manner. Chenodeoxycholic acid (CDCA) was the most potent and efficacious compound in the SRC-1 recruitment assay (EC(50) = 11.7 microM) followed by deoxycholic acid (DCA; EC(50) = 19.0 microM). Ursodeoxycholic acid (UDCA) displayed minimal activity while cholic acid (CA) was inactive. In order to directly compare the potencies of the bile acids in the coactivator recruitment assay to their ability to repress CYP7A1 expression, a branched DNA assay was developed to rapidly measure CYP7A1 mRNA levels from HepG2 cells cultured in 96-well plates. The rank order and absolute potency was conserved (CDCA IC(50) = 8.7 microM, DCA IC(50) = 27.2 microM, UDCA and CA inactive) consistent with bile acid repression of CYP7A1 being mediated by FXR.

Bile acid induction of cytokine expression by macrophages correlates with repression of hepatic cholesterol 7alpha-hydroxylase

In the studies reported herein, we show that two complementary experimental models: inbred strains of mice (i.e. C57BL/6 and C3H/HeJ), and a differentiated line of rat hepatoma cells (i.e. L35 cells), require the activation of cytokines by monocyte/macrophages to display bile acid negative feedback repression of cholesterol 7alpha-hydroxylase (CYP7A1). Feeding a bile acid-containing atherogenic diet for 3 weeks to C57BL/6 mice led to a 70% reduction in the expression of hepatic CYP7A1 mRNA, whereas no reduction was observed in C3H/HeJ mice. The strain-specific response to repression of CYP7A1 paralleled the activation of hepatic cytokine expression. Studies using cultured THP-1 monocyte/macrophages showed that the hydrophobic bile acid chenodeoxycholate, a well established potent repressor of CYP7A1, induced the expression of mRNAs encoding interleukin 1 (IL-1) and tumor necrosis factor alpha (TNFalpha). In contrast, the hydrophilic bile acid ursodeoxycholate, which does not repress CYP7A1, did not induce cytokine mRNA expression by THP-1 cells. Chenodeoxycholate activation of cytokines by THP-1 cells was blocked by the peroxisome proliferator-activated receptor gamma agonist rosiglitazone. The expression of cytokines (e.g. IL-1 and TNFalpha) by THP-1 cells paralleled with the ability of these cells to produce conditioned medium that when added to rat L35 hepatoma cells, repressed CYP7A1. Moreover, rosiglitazone, which blocks cytokine activation by macrophages, also blocked the repression of CYP7A1 normally exhibited by C57BL/6 mice fed the bile acid-containing atherogenic diet. The combined data indicate that the activation of cytokines may mediate CYP7A1 repression caused by feeding mice an atherogenic diet containing bile acids.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Expression of the human cholesterol 7alpha-hydroxylase gene in transgenic mice

We have generated transgenic mice expressing human CYP7A1 transgenes. Only 1.5 kilobases (kb) of 5' upstream sequence and 6.5 kb of 3' sequence were sufficient for hepatic transcription of the transgenes. However, the 5' end segment alone was not sufficient to direct liver expression, suggesting that additional hepatic regulatory elements reside in the 3' extension or within introns. The level of expression of these transgenes was low in comparison to the levels of the endogenous mouse CYP7A1 mRNA. To generate mice expressing higher levels of CYP7A1 mRNA, we injected a large human genomic PAC clone, extending up to -105 kb 5' of the structural gene and about 50 kb 3' of the gene. These transgenic mice expressed CYP7A1 mRNA at higher levels, suggesting that additional hepatic regulatory elements are found either 5' of -1520 or beyond 6.5 kb 3' of the gene.

Deoxycholic acid treatment in patients with cholesterol gallstones: failure to detect a suppression of cholesterol 7alpha-hydroxylase activity

Hillebrant C-G, Nyberg B, Angelin B, Axelson M, Björkhem I, Rudling M, Einarsson C (Huddinge University Hospital and Karolinska Hospital, Karolinska Institute, Stockholm, Sweden). Deoxycholic acid treatment in patients with cholesterol gallstones: failure to detect a suppression of cholesterol 7alpha-hydroxylase activity. J Intern Med 1999; 246: 399-407.

OBJECTIVES

Based on animal studies, hydrophobic bile acids have been postulated to be particularly strong inhibitors of bile acid synthesis. The present study was undertaken to characterize in humans the effects of one of the most hydrophobic of the common bile acids, deoxycholic acid (DCA), on the transcriptional regulation and activity of the cholesterol 7alpha-hydroxylase, on hepatic cholesterol metabolism and on biliary lipid metabolism and plasma lipids. DESIGN, SUBJECTS AND SETTINGS: Thirteen patients with cholesterol gallstone disease were treated with DCA (750 mg day-1) for 3 weeks prior to cholecystectomy. Blood samples were collected before and during treatment. At operation, a liver biopsy and gallbladder bile were obtained. Twenty-eight untreated gallstone patients undergoing cholecystectomy served as controls. The study was carried out at a university hospital.

RESULTS

Deoxycholic acid comprised 72 +/- 6% (mean +/- SEM) of total biliary bile acids in DCA-treated patients (n = 8), and 21 +/- 2% in the controls (n = 16; P < 0.001). Cholesterol saturation of gallbladder bile averaged 102% in both treated (n = 7) and untreated (n = 16) patients. Cholesterol 7alpha-hydroxylase and HMG CoA reductase activities and mRNA levels were not different between DCA-treated and untreated gallstone patients. The LDL receptor mRNA levels were similar in both groups of patients. Plasma levels of total cholesterol were lowered by 10% upon DCA treatment (P < 0.05).

CONCLUSIONS

Treatment with DCA did not significantly affect mRNA levels and activity of hepatic cholesterol 7alpha-hydroxylase or HMG CoA reductase in patients with cholesterol gallstones. There was no effect on the saturation of gallbladder bile, Thus, the present study could not verify that the hydrophobicity of the bile acid pool is a major factor regulating human hepatic cholesterol 7alpha-hydroxylase activity.

Hepatocyte nuclear factor 1 binds to and transactivates the human but not the rat CYP7A1 promoter

Cholesterol 7alpha-hydroxylase (CYP7A1), a liver-specific enzyme, catalyzes the rate-limiting step in the degradation pathway of cholesterol to bile acids, and thus plays a key role in cholesterol homeostasis. To elucidate the mechanisms that control hepatic expression of the human CYP7A1 gene, we are studying the promoter region. Initially, we observed that up to 40% of the overall transcriptional activity of the promoter in HepG2 cells was associated with DNA sequences from -65 to -1 of the human gene. Within this region, a binding site for the liver-enriched transcription factor HNF-1 (-56 to -49) has been identified. Binding of HNF-1 to this site leads to transcriptional activation of the human promoter. The corresponding segment from the rat CYP7A1 gene does not bind HNF-1; instead, it is bound by the orphan receptors ARP-1 (COUP-TFII) and LXRalpha, that are implicated in dietary regulation.

Complex feedback regulation of bile acid synthesis in the hamster: the role of newly synthesized cholesterol

Hepatic bile acid synthesis is regulated by recirculating bile acids, possibly by modulating the availability of newly synthesized and preformed cholesterol. Because data in the hamster on this mechanism are lacking, we fitted these animals with an extracorporeal bile duct and administered tritiated water intraperitoneally to label newly formed cholesterol. After interruption of the enterohepatic circulation, physiological and double-physiological doses of conjugated cholate (25 or 50 micromol/100 g. h) or of unconjugated deoxycholate (6 or 12 micromol) were infused intraduodenally for 54 hours and compared with controls. De novo and preformed cholesterol directly secreted into bile or used for cholate and chenodeoxycholate synthesis were quantitated by high-pressure liquid chromatography (HPLC)-liquid scintillation. Directly after depletion of the bile acid pool (6-9 hours) at nearly physiological conditions, chenodeoxycholate synthesis was significantly reduced by cholate and deoxycholate by up to 45% to 51%, whereas cholate formation decreased by approximately 22% during deoxycholate. This short-term effect was mainly mediated by reduced synthesis from preformed cholesterol. After long-term bile depletion (30-54 hours), bile acid synthesis returned to control levels during 25 micromol of cholate and of both deoxycholate doses. In contrast, only 50 micromol of cholate prevented derepression of bile acid synthesis. This long-term effect was mainly attributed to a diminished formation from de novo cholesterol exceeding the reduced synthesis from preformed cholesterol. In summary, short- and long-term regulation of bile acid synthesis in hamsters differs with respect to availabilities of preformed and de novo cholesterol.

Endogenous bile acids are ligands for the nuclear receptor FXR/BAR

The major metabolic pathway for elimination of cholesterol is via conversion to bile acids. In addition to this metabolic function, bile acids also act as signaling molecules that negatively regulate their own biosynthesis. However, the precise nature of this signaling pathway has been elusive. We have isolated an endogenous biliary component (chenodeoxycholic acid) that selectively activates the orphan nuclear receptor, FXR. Structure-activity analysis defined a subset of related bile acid ligands that activate FXR and promote coactivator recruitment. Finally, we show that ligand-occupied FXR inhibits transactivation from the oxysterol receptor LXR alpha, a positive regulator of cholesterol degradation. We suggest that FXR (BAR) is the endogenous bile acid sensor and thus an important regulator of cholesterol homeostasis.

Regulation of bile acid biosynthesis

This article provides a review of the pathways through which cholesterol is degraded to bile acids. Regulation of key enzymes in the bile acid biosynthestic pathways is discussed. The important role of these pathways in the maintenance of cholesterol homeostasis and the possible therapeutic implications for the treatment of hypercholesterolemia are emphasized.

Analysis of human CYP7A1 mRNA decay in HepG2 cells by reverse transcription-polymerase chain reaction

The conversion of cholesterol to bile acids is the major pathway through which cholesterol is removed from the body. The initial and rate-limiting step in this catabolic pathway is catalyzed by the liver-specific enzyme cholesterol 7alpha-hydroxylase (CYP7A1). The HepG2 cell line has been used as a model to study human CYP7A1. The levels of CYP7A1 mRNA, however, are quite low in this cell line and require the use of poly(A)+ mRNA for detection using standard Northern analysis. As an alternative, we established a reverse transcription-polymerase chain reaction (RT-PCR) assay that can be used to study CYP7A1 mRNA in HepG2 cells. Using RT-PCR, we analyzed the influence of cell culture conditions on CYP7A1 mRNA levels. We observed an increase in CYP7A1 mRNA levels as the density of the cell culture increased. This rise in CYP7A1 was accompanied by a reciprocal drop in the levels of the proto-oncogene c-myc. Since c-myc expression is strongly associated with cell growth status, this inverse relationship suggests that conditions that favor reduced cell proliferation result in higher levels of CYP7A1 expression. We also established the validity of using RT-PCR for the measurement of mRNA decay rates using c-myc and glyceraldehyde-3-phosphate dehydrogenase mRNAs as a model: The same half-life value was obtained for the c-myc mRNA using either Northern analysis or RT-PCR. Using our RT-PCR method we determined that human CYP7A1 mRNA decays with a half-life of 4.6 +/- 0.9 h (n = 8) in HepG2 cells. We show that the protein synthesis inhibitor cycloheximide prolonged the CYP7A1 mRNA half-life, suggesting that translation is required for mRNA decay. Dexamethasone treatment, however, did not alter CYP7A1 mRNA decay rate but it increased CYP7A1 steady-state mRNA levels, suggesting that the effect of this glucocorticoid in HepG2 cells may be transcriptional.

Hepatic microsomal triglyceride transfer protein messenger RNA concentrations are increased by dietary cholesterol in hamsters

In hamsters fed high fat diets enriched in trimyristin, tripalmitin or tristearin, increased dietary cholesterol content was associated with increased plasma concentrations of very low density lipoprotein (VLDL) cholesterol and triacylglycerol (p < 0.0001 and p = 0.0017, respectively). Hepatic microsomal triglyceride transfer protein (MTP) mRNA concentration also increased (p < 0.0001), independent of the nature of dietary fat, and was significantly correlated with the plasma VLDL lipid concentrations (p = 0.0002 and p = 0.0106 for cholesterol and triacylglycerol, respectively) and hepatic cholesterol concentrations. Increased expression of the MTP gene may be part of a coordinated response to hepatic cholesterol accumulation leading to increased VLDL lipid secretion.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.

Bile acids in the assessment of hepatocellular function

Bile acids, which are synthesized in the liver from cholesterol, are important in the production of bile flow, excretion of cholesterol, and intestinal digestion and absorption of fats and fat-soluble vitamins. Increases and/or alterations in concentrations of bile acids in serum are specific and sensitive indicators of hepatobiliary disorders. Synthesis of bile acids in hepatocytes involves steps in endoplasmic reticulum, cytosol, mitochondria, and peroxisomes. Other important hepatocellular processes involving bile acids include active uptake by the basolateral membrane, intracellular transport, P-450-mediated conjugations and hydroxylations, and canalicular secretion. Hydrophobic bile acids produce hepatotoxicity in vivo and in vitro. In experimental and epidemiologic studies, some of these forms have been identified as causative agents in the development of colon and liver (experimental only) cancer. Conversely, several hydrophilic forms, primarily ursodeoxycholic acid, have demonstrated cytoprotective properties in a variety of clinical and experimental hepatobiliary diseases and disorders. Because bile acids can have dramatically different properties and effects, determination of mechanisms of action of these compounds has become an active area of research. Primary isolated hepatocytes provide an opportunity to investigate bile acid-related functions and effects in well-designed, carefully controlled studies. Short-term cultures have been used to study a variety of issues related to bile acids, including cytotoxicity, synthesis, and hepatocellular processing. With these systems, however, many functions of mature hepatocytes, including those pertaining to bile acids, can be lost when cultures are maintained for more than several days. Recent developments in culture techniques permit long-term maintenance of functionally stable, differentiated cells. Pertaining to bile acid research, these systems remain to be fully characterized but, in appropriate situations, they should provide important alternatives to in vivo studies and short-term in vitro assays.

Human cell lines in pharmacotoxicology. An introduction to a panel discussion

Various types of cells lines are used in pharmacotoxicology. Established cell lines are easily available, with few ethical restrictions. Some specific properties are preserved, although they have kept the phenotype of the original tissue, which is frequently a tumor phenotype. They are usually more resistant to toxic compounds than freshly isolated cells. Some drug-metabolizing enzymes are expressed and regulated in these cells. Immortalized cell lines are also of interest in toxicology. They are mainly examined for their potential in mutagenicity testing. These cells and numerous others of animal or human origin can be transfected with cDNA coding for human enzymes. They are used for determination of the individual enzyme involved in a particular metabolic pathway, or, when multiple transfections are successfully achieved, for mutagenicity testing. Regulation studies are also possible in such cells after transfection of DNA elements regulating gene transcription.

Failure of intravenous infusion of taurocholate to down-regulate cholesterol 7 alpha-hydroxylase in rats with biliary fistulas

BACKGROUND/AIMS

The decrease in cholesterol 7 alpha-hydroxylase induced by intraduodenal infusion of taurocholate in bile fistula rats may be indirect, i.e., mediated through release or absorption of an intestinal factor in response to the presence of bile salts in the intestine. The aim of this study was to determine if negative feedback regulation of cholesterol 7 alpha-hydroxylase can be shown when equimolar concentrations of taurocholate are administered intravenously, thus bypassing the intestine.

METHODS

After 96 hours of biliary diversion, taurocholate (36 mumol.h-1.100 g, rat-1) was infused into the rats either intravenously or intraduodenally for the final 24 hours. Livers were then harvested for analysis of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase specific activity, cholesterol 7 alpha-hydroxylase specific activity, messenger RNA levels, and transcriptional activity.

RESULTS

Intraduodenally administered taurocholate significantly decreased HMG-CoA reductase and cholesterol 7 alpha-hydroxylase specific activity by more than 50% and cholesterol 7 alpha-hydroxylase steady-state messenger RNA levels and transcriptional activity by 50%-75%. In contrast, intravenous administration of taurocholate failed to down-regulate either cholesterol 7 alpha-hydroxylase or HMG-CoA reductase.

CONCLUSIONS

Passage of taurocholate through the intestine strongly potentiates negative feedback regulation of cholesterol 7 alpha-hydroxylase. A putative intestinal factor, released or absorbed in the presence of bile acids in the intestinal lumen, may play a role in the regulation of bile acid synthesis.

Characteristics and regulation of bile salt synthesis and secretion by human hepatoma HepG2 cells

Bile salt uptake, synthesis and secretion by the human hepatoma-derived cell line HepG2 were studied. The cells transported and secreted bile salts largely by means of passive mechanisms. The cells synthesized and secreted the normal human primary bile salts. The ratio of cholate to chenodeoxycholate was 1.5:1. The degree of conjugation, about 35%, was lower than normal, and the glycine-to-taurine ratio was abnormal (4.5:1). This was not due to amino acid deficiency in the medium. Contrary to the report of others, little 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestan-26-oic acid was secreted. This was confirmed by gas chromatography-mass spectrometry. The total rate of synthesis was about 33% that of normal liver. The specific activity of bile salts synthesized from [3H]mevalonate was about 20 times higher than that of the cellular cholesterol derived from the same precursor. The regulation of bile salt synthesis by two compounds that could alter the precursor pool of cholesterol was studied. After a 24-hr incubation in serum-free medium, the compound 25(OH)cholesterol inhibited the rate of bile salt synthesis compared with control values, possibly by depleting the intracellular free cholesterol pool. Surprisingly, however, progesterone, which inhibits cholesterol esterification and should have expanded this pool, also inhibited bile salt synthesis under those conditions. The effect of these compounds on the level of mRNA for cholesterol 7 alpha-hydroxylase was also determined by Northern-blot analysis. The cholesterol 7 alpha-hydroxylase mRNA was 3.7 kb, similar to that in the rat. The incubation of cells in 25(OH)cholesterol or progesterone, as above, resulted in a decreased level of mRNA. The reduction was proportional to the reduction in bile salt synthesis, suggesting that these compounds act at a pretranslational level. Taken together, these results suggest that our particular subclone of HepG2 cells will be useful for studies of the regulation of bile salt synthesis, but not of transport, by human liver-derived tissue.