Immunohistochemical detection of hepatocyte nuclear factor-4α in vertebrates

Hepatocyte nuclear factor-4α (HNF4α) presents in multiple isoforms generated using alternative promoter (P1 and P2) and splicing. Neither conservation of tissue distribution of HNF4α isoforms, nor presence of alternative promoter usage is known. In this study, to detect the expression of HNF4α in some species of animals, we have applied monoclonal antibodies against P1 (K9218) and P2 (H6939) promoter-driven and P1/P2 promoter-driven H1415 HNF4α for immunohistochemistry and western blot analysis. Antibody K9218 was observed in the hepatocytes, proximal tubules of the kidney, and epithelial cells in the mucosa of the small intestine and colon of rats, chicken, and tortoise, whereas antibody H6939 signal were detected in the stomach, pancreas, bile duct, and pancreatic duct of human and rats. The signal for antibody K9218 was recognized in tissues of a wide range of mammals, bird, reptile, amphibian, and fish as well. Antibody H1415 displayed a positive reaction in hepatocytes and intestinal epithelial cells in chicken and tortoise, whereas the bile duct, mucosal epithelial cells in the stomach, or pancreas in these animals were negative. Western blotting showed the binding of the antibody with HNF4α protein from each animal. The sequence of human HNF4α was 100% identical to murine and rat HNF4α, 88.9% to chicken, 77.8% to Xenopus HNF4α, and 81.5% to medaka. However, the specific part of human and invertebrate Drosophila HNF4 shares only 14.8% sequence identity. This antibody is useful for detecting HNF4α isoforms in a wide range of vertebrates, and suggests many insights into animal evolution.

Effect of Cytochrome P450 7A1 (CYP7A1) Polymorphism on Lipid Responses to Simvastatin Treatment

BACKGROUND

Identifying patients with high risk of low response to statin therapy is important for optimization of lipid-lowering therapy. Cholesterol 7α-hydroxylase, a rate-limiting enzyme encoded by cytochrome P450 7A1 (CYP7A1) gene, is considered to be associated with statin efficacy. This study aimed to investigate the association between a novel CYP7A1 single nucleotide polymorphism rs3824260 and statin treatment response for hypercholesteremic patients in Chinese Han population.

METHODS

A total of 336 subjects were prescribed with simvastatin for 12 weeks after enrollment. Plasma lipid parameters were measured at enrollment and after 12-week simvastatin treatment separately. Subjects were classified into high- and low-response groups depending on their total cholesterol, low-density lipoprotein cholesterol (LDL-C) and TG changes and increase or reduction groups according to their high-density lipoprotein cholesterol (HDL-C) levels changing after simvastatin treatment. The CYP7A1 rs3824260 was genotyped from blood samples with a SNaPshot assay.

RESULTS

At baseline, the LDL-C level and TG level were significantly higher in the AA genotype, while the HDL-C level was significantly higher in the GG genotype of CYP7A1 rs3824260. Patients carrying AA genotype are at an increased risk of low response for LDL-C reduction (odds ratio = 2.295, 95% confidence interval = 1.164-4.524, P = 0.016). Furthermore, the GG genotype of rs3824260 was significantly associated with a high risk of HDL-C reduction response after simvastatin therapy (odds ratio = 2.240, 95% confidence interval = 1.137-4.413, P = 0.025).

CONCLUSIONS

The CYP7A1 gene polymorphism rs3824260 is related to inappropriate response of simvastatin treatment for hypercholesterolemia patients in Chinese Han population.

Cholesterol-lowing effect of taurine in HepG2 cell

BACKGROUND

A number of studies indicate that taurine promotes cholesterol conversion to bile acids by upregulating CYP7A1 gene expression. Few in vitro studies are concerned the concentration change of cholesterol and its product of bile acids, and the molecular mechanism of CYP7A1 induction by taurine.

METHODS

The levels of intracellular total cholesterol (TC), free cholesterol (FC), cholesterol ester (EC), total bile acids (TBA) and medium TBA were determined after HepG2 cells were cultured for 24/48 h in DMEM supplemented with taurine at the final concentrations of 1/10/20 mM respectively. The protein expressions of CYP7A1, MEK1/2, c-Jun, p-c-Jun and HNF-4α were detected.

RESULTS

Taurine significantly reduced cellular TC and FC in dose -and time-dependent ways, and obviously increased intracellular/medium TBA and CYP7A1 expressions. There was no change in c-Jun expression, but the protein expressions of MEK1/2 and p-c-Jun were increased at 24 h and inhibited at 48 h by 20 mM taurine while HNF4α was induced after both of the 24 h and 48 h treatment.

CONCLUSION

Taurine could enhance CYP7A1 expression by inducing HNF4α and inhibiting MEK1/2 and p-c-Jun expressions to promote intracellular cholesterol metabolism.

Variants in the promoter region of CYP7A1 are associated with neuromyelitis optica but not with multiple sclerosis in the Han Chinese population

Multiple sclerosis (MS) and neuromyelitis optica (NMO) are common autoimmune demyelinating disorders of the central nervous system. The exact etiology of each remains unclear. CYP7A1 was reported to be associated with NMO in Korean patients, but this is yet to be confirmed in other populations. In this study, we used Sanger sequencing to detect SNPs in the promoter region of CYP7A1 in a population consisting of unrelated patients and controls from the Han Chinese population (129 MS; 89 NMO; 325 controls). Two known SNPs, -204A>C (rs3808607) and -469T>C (rs3824260), and a novel SNP (-208G>C) were identified in the 5'-UTR of CYP7A1. The -204A>C was in complete linkage with -469T>C and both were associated with NMO but not with MS. Results suggest that the CYP7A1 allele was associated with NMO. NMO and MS have different genetic risk factors. This further supports the emerging evidence that MS and NMO are distinct disorders.

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

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

Physiology of FGF15/19

This chapter will review the various biological actions of the mouse fibroblast growth factor 15 (Fgf15) and human fibroblast growth factor 19 (FGF19). Unlike other members of the fibroblast growth factor (FGF) family, the Fgf15 and FGF19 orthologs do not share a high degree of sequence identity. Fgf15 and FGF19 are members of an atypical subfamily of FGFs that function as hormones. Due to subtle changes in tertiary structure, these FGFs have low heparin binding affinity enabling them to diffuse away from their site of secretion and signal to distant cells. FGF signaling through the FGF receptors is also different for this sub-family, requiring klotho protein cofactors rather than heparin sulfate proteoglycan. Mouse Fgf15 and human FGF19 play key roles in enterohepatic signaling, regulation of liver bile acid biosynthesis, gallbladder motility and metabolic homeostasis.

CYP7A1 genotypes and haplotypes associated with hypertension in an obese Han Chinese population

This study investigated the association between single-nucleotide polymorphisms (SNPs; rs3808607 and rs1125226) within the CYP7A1 promoter and hypertension susceptibility in a Han Chinese population. From 2003 through 2006, a population-based case-control study was performed in a cohort of 1187 randomly selected Han Chinese subjects. A sib-pair study for a transmission disequilibrium test analysis was carried out in 76 hypertensive (HT) families (n=312) from northeastern Liaoning province. SNPs were detected using real-time PCR. No significant differences were found in the genotype or allele frequencies of either SNP (P>0.05), with no excessive allele sharing. For rs3808607, the frequency of the AA genotype in obese hypertensive patients was 31.91%, significantly higher than in normotensive (NT) subjects (12.73%; odds ratio (OR)=3.21, 95% confidence interval (CI)=1.35-7.66). For rs3808607, the AA genotype frequency was significantly higher in obese male HT subjects (27.87%) than in matched NTs (7.41%; OR=4.83, 95% CI=1.03-22.65). After adjustment for environmental risk factors in obese participants, the AA genotype was associated with hypertension (OR=3.395, 95% CI=1.412-8.162). Among subjects with body mass index 28 kg m(-2), the HT and NT groups had significantly different frequencies of Hap I (C/C) and Hap IV (A/A). The frequencies of rs3808607 alleles in the CYP7A1 gene differed significantly between obese HT and NT men. Haplotypes I and IV were associated with hypertension in obese participants.

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.

FGF15/FGFR4 integrates growth factor signaling with hepatic bile acid metabolism and insulin action

The current studies show FGF15 signaling decreases hepatic forkhead transcription factor 1 (FoxO1) activity through phosphatidylinositol (PI) 3-kinase-dependent phosphorylation. The bile acid receptor FXR (farnesoid X receptor) activates expression of fibroblast growth factor (FGF) 15 in the intestine, which acts through hepatic FGFR4 to suppress cholesterol-7alpha hydroxylase (CYP7A1) and limit bile acid production. Because FoxO1 activity and CYP7A1 gene expression are both increased by fasting, we hypothesized CYP7A1 might be a FoxO1 target gene. Consistent with recently reported results, we show CYP7A1 is a direct target of FoxO1. Additionally, we show that the PI 3-kinase pathway is key for both the induction of CYP7A1 by fasting and the suppression by FGF15. FGFR4 is the major hepatic FGF receptor isoform and is responsible for the hepatic effects of FGF15. We also show that expression of FGFR4 in liver was decreased by fasting, increased by insulin, and reduced by streptozotocin-induced diabetes, implicating FGFR4 as a primary target of insulin regulation. Because insulin and FGF both target the PI 3-kinase pathway, these observations suggest FoxO1 is a key node in the convergence of FGF and insulin signaling pathways and functions as a key integrator for the regulation of glucose and bile acid metabolism.

Transient gene delivery for functional enrichment of differentiating embryonic stem cells

There is a critical need for new sources of hepatocytes, both clinically to provide support for patients with liver failure and in drug discovery for toxicity, metabolic and pharmacokinetic screening of new drug entities. We have reported previously a variety of methods for differentiating murine embryonic stem (ES) cells into hepatocyte-like cells. One major challenge of our work and others in the field has been the ability to selectively purify and enrich these cells from a heterogeneous population. Traditional approaches for inserting new genes (e.g., stable transfection, knock-in, retroviral transduction) involve permanent alterations in the genome. These approaches can lead to mutations and involve the extra costs and time of developing, validating and maintaining new cell lines. We have developed a transient gene delivery system that uses fluorescent gene reporters for purification of the cells. Following a transient transfection, the cells are purified through a fluorescence-activated cell sorter (FACS), re-plated in secondary culture and subsequent phenotypic analysis is performed. In an effort to test the ability of the reporters to work in a transient environment for our differentiation system, we engineered two non-viral plasmid reporters, the first driven by the mouse albumin enhancer/promoter and the second by the mouse cytochrome P450 7A1 (Cyp7A1) promoter. We optimized the transfection efficiency of delivering these genes into spontaneously differentiated ES cells and sorted independent fractions positive for each reporter 17 days after inducing differentiation. We found that cells sorted based on the Cyp7A1 promoter showed significant enrichment in terms of albumin secretion, urea secretion and cytochrome P450 1A2 detoxification activity as compared to enrichment garnered by the albumin promoter-based cell sort. Development of gene reporter systems that allow us to identify, purify and assess homogeneous populations of cells is important in better understanding stem cell differentiation pathways. And engineering cellular systems without making permanent gene changes will be critical for the generation of clinically acceptable cellular material in the future.

Role of genetic variant A-204C of cholesterol 7alpha-hydroxylase (CYP7A1) in susceptibility to gallbladder cancer

Gallbladder carcinoma (GBC) usually arises in the background of gallstone disease. Cholesterol 7alpha-hydroxylase (CYP7A1) is a rate-limiting enzyme for cholesterol catabolism and bile acid synthesis. A-204C genetic polymorphism in CYP7A1 may influence gene expression and thus affect the risk of gallstone disease and GBC. We aimed to study the association of A-204C variation of CYP7A1 gene promoter polymorphism in GBC patients, gallstone patients and healthy subjects. The study included 141 histopathologically proven GBC patients, ultrasonographically proven 185 symptomatic gallstone patients and 200 gallstone-free healthy subjects. Genotyping was done by PCR-RFLP method. CYP7A1 A-204C genotypes in control population were in Hardy-Weinberg equilibrium. The CC genotype conferred marginally significant risk for gallstone disease (p=0.051; OR=1.54; 95% CI=0.9-3.4). In GBC patients, the CYP7A1 A-204C polymorphism conferred high risk for GBC at genotype (p=0.005; OR=2.78; 95% CI: 1.3-5.6) as well as allele levels (p=0.008; OR=1.58 and 95% CI: 1.1-2.2). After stratification of GBC patients on the basis of presence or absence of gallstones, CC genotype imparted higher risk for GBC without stones (p=0.002; OR=4.44: 95% CI=1.7-11.3). The association of the polymorphism with GBC was more pronounced in female GBC patients, and also in cancer patients who developed GBC at advanced age. The CC genotype of CYP7A1 is an independent genetic risk factor for GBC but plays a modest role in susceptibility to gallstone disease. The GBC pathogenesis by CYP7A1 polymorphism appears to be independent of gallstone pathway and probably involves genotoxicity due to lipid peroxidation mechanisms.

Peroxisome proliferator-activated receptor-gamma coactivator-1alpha activation of CYP7A1 during food restriction and diabetes is still inhibited by small heterodimer partner

Cholesterol 7alpha-hydroxylase (CYP7A1) catalyzes the rate-limiting step in the classic pathway of hepatic bile acid biosynthesis from cholesterol. During fasting and in type I diabetes, elevated levels of peroxisome proliferator-activated receptor gamma-coactivator-1alpha (PGC-1alpha) induce expression of the Cyp7A1 gene and overexpression of PGC-1alpha in hepatoma cells stimulates bile acid synthesis. Using Ad-PGC-1alpha-RNA interference to induce acute disruption of PGC-1alpha in mice, here we show that PGC-1alpha is necessary for fasting-mediated induction of CYP7A1. Co-immunoprecipitation and promoter activation studies reveal that the induction of CYP7A1 is mediated by direct interaction between PGC-1alpha and the AF2 domain of liver receptor homolog-1 (LRH-1). In contrast, the very similar PGC-1beta could not substitute for PGC-1alpha. We also show that transactivation of PGC-1alpha and LRH-1 is repressed by the small heterodimer partner (SHP). Treatment of mice with GW4064, a synthetic agonist for farnesoid X receptor, induced SHP expression and decreased both the recruitment of PGC-1alpha to the Cyp7A1 promoter and the fasting-induced expression of CYP7A1 mRNA. These data suggest that PGC-1alpha is an important co-activator for LRH-1 and that SHP targets the interaction between LRH-1 and PGC-1alpha to inhibit CYP7A1 expression. Overall, these studies provide further evidence for the important role of PGC-1alpha in bile acid homeostasis and suggest that pharmacological targeting of farnesoid X receptor in vivo can be used to reverse the increase in CYP7A1 associated with adverse metabolic conditions.

Biosynthesis of bile acids in mammalian liver

The biosynthesis of bile acids in mammalian liver and its regulation, together with the physiological role of bile acids, are reviewed in this article. Bile acids are biosynthesized from cholesterol in hepatocytes. Several steps are involved including epimerisation of the 3beta-hydroxyl group, reduction of the delta4 double bond to the 5beta-H structural arrangement, introduction of alpha-hydroxyl groups at C7 or C7 and C12 and, finally, oxidative degradation of the side chain by three carbon atoms. This gives the primary bile acids, cholic and chenodeoxycholic acids. Cholesterol-7alpha-hydroxylation is the rate determining step in the biosynthesis of cholic and chenodeoxycholic acids. Feedback regulation of cholesterol biosynthesis occurs by various mechanisms including termination of the synthesis of specific cytochromes P-450, modulation of specific cytosol proteins, short-term changes in the process of phosphorylation-dephosphorylation and changes in the capacity of the cholesterol pool as a substrate. Prior to being exported from the liver, bile acids are conjugated with glycine and taurine to produce the bile salts. After excretion into the intestinal tract, primary bile acids are partly converted to secondary bile acids, deoxycholic and lithocholic acids, by intestinal microorganisms. The majority of bile acids is absorbed from the intestinal tract and returned to the liver via the portal blood, so that only a small fraction is excreted in the feces. Bile acids returned to the liver can be reconjugated and reexcreted into the bile in the process of enterohepatic recycling. In addition to the physiological function of emulsifying lipids in the intestinal tract, bile acids are particularly important in respect of their ability to dissolve and transport cholesterol in the bile.

Pharmacogenetic determinants of variability in lipid-lowering response to pravastatin therapy

Pravastatin is mainly taken up from the circulation into the liver via organic anion-transporting polypeptide 1B1 (SLCO1B1 gene product). We examined the contribution of genetic variants in the SLCO1B1 gene and other candidate genes to the variability of pravastatin efficacy in 33 hypercholesterolemic patients. In the initial phase of pravastatin treatment (8 weeks), heterozygous carriers of the SLCO1B1*15 allele had poor low-density lipoprotein cholesterol (LDL-C) reduction relative to non-carriers (percent reduction: -14.1 vs -28.9%); however, the genotype-dependent difference in the cholesterol-lowering effect disappeared after 1 year of treatment. Cholesterol 7alpha-hydroxylase (CYP7A1) and apolipoprotein E (APOE) are known to contribute to lipid metabolism. Homozygous carriers of the CYP7A1 -204C allele or heterozygotes for both CYP7A1 -204C and APOE epsilon4 alleles showed significantly poorer LDL-C reduction compared to that in other genotypic groups after 1 year of treatment (-24.3 vs -33.1%). These results suggest that the SLCO1B1*15 allele is associated with a slow response to pravastatin therapy, and the combined genotyping of CYP7A1 and APOE genes is a useful index of the lipid-lowering effect of pravastatin.

Regulation of bile acid biosynthesis by hepatocyte nuclear factor 4alpha

Hepatocyte nuclear factor 4alpha (HNF4alpha) regulates many genes that are preferentially expressed in liver. Mice lacking hepatic expression of HNF4alpha (HNF4alphaDeltaL) exhibited markedly increased levels of serum bile acids (BAs) compared with HNF4alpha-floxed (HNF4alphaF/F) mice. The expression of genes involved in the hydroxylation and side chain beta-oxidation of cholesterol, including oxysterol 7alpha-hydroxylase, sterol 12alpha-hydroxylase (CYP8B1), and sterol carrier protein x, was markedly decreased in HNF4alphaDeltaL mice. Cholesterol 7alpha-hydroxylase mRNA and protein were diminished only during the dark cycle in HNF4alphaDeltaL mice, whereas expression in the light cycle was not different between HNF4alphaDeltaL and HNF4alphaF/F mice. Because CYP8B1 expression was reduced in HNF4alphaDeltaL mice, it was studied in more detail. In agreement with the mRNA levels, CYP8B1 enzyme activity was absent in HNF4alphaDeltaL mice. An HNF4alpha binding site was found in the mouse Cyp8b1 promoter that was able to direct HNF4alpha-dependent transcription. Surprisingly, cholic acid-derived BAs, produced as a result of CYP8B1 activity, were still observed in the serum and gallbladder of these mice. These studies reveal that HNF4alpha plays a central role in BA homeostasis by regulation of genes involved in BA biosynthesis, including hydroxylation and side chain beta-oxidation of cholesterol in vivo.

Binding sites for metabolic disease related transcription factors inferred at base pair resolution by chromatin immunoprecipitation and genomic microarrays

We present a detailed in vivo characterization of hepatocyte transcriptional regulation in HepG2 cells, using chromatin immunoprecipitation and detection on PCR fragment-based genomic tiling path arrays covering the encyclopedia of DNA element (ENCODE) regions. Our data suggest that HNF-4alpha and HNF-3beta, which were commonly bound to distal regulatory elements, may cooperate in the regulation of a large fraction of the liver transcriptome and that both HNF-4alpha and USF1 may promote H3 acetylation to many of their targets. Importantly, bioinformatic analysis of the sequences bound by each transcription factor (TF) shows an over-representation of motifs highly similar to the in vitro established consensus sequences. On the basis of these data, we have inferred tentative binding sites at base pair resolution. Some of these sites have been previously found by in vitro analysis and some were verified in vitro in this study. Our data suggests that a similar approach could be used for the in vivo characterization of all predicted/uncharacterized TF and that the analysis could be scaled to the whole genome.

A promoter polymorphism in cholesterol 7alpha-hydroxylase interacts with apolipoprotein E genotype in the LDL-lowering response to atorvastatin

Bile-acid biosynthesis is a key determinant of intracellular cholesterol and, in turn, cholesterol synthesis rate in hepatocytes. This suggests that variation in the cholesterol 7alpha-hydroxylase gene (CYP7A1), a key enzyme in bile-acid biosynthesis, may influence the statin response. To test this hypothesis, a promoter polymorphism (A-204C) in CYP7A1 was examined in 324 hypercholesterolemic patients treated with atorvastatin 10mg. The variant C allele was significantly and independently associated with poor LDL cholesterol reductions; -39% in wild type allele homozygotes, -37% in variant allele heterozygotes, and -34% in variant allele homozygotes (p<0.0001 for trend). Differences were more striking in men, and were enhanced by the coexistence of common variants of apolipoprotein E gene (APOE), epsilon2 or epsilon4. In subjects having wild type alleles at both loci, the mean reduction in LDL cholesterol was -40%, while the value in subjects having two CYP7A1 variant alleles and at least one variant APOE allele was -31% (p<0.0001). Combination analysis of these two loci more accurately predicted the achievement of goal LDL cholesterol, than did both single locus analysis. We concluded that the CYP7A1 A-204C promoter variant was associated with poor response to atorvastatin, which were additively enhanced by common variants in another locus, APOE.

Common polymorphisms in the CYP7A1 gene do not contribute to variation in rates of bile acid synthesis and plasma LDL cholesterol concentration

Transcriptional regulation of the cholesterol 7alpha-hydroxylase (CYP7AI) gene is of critical importance for bile acid and cholesterol metabolism. We evaluated the physiological significance of two common polymorphisms (-203C/A and -469T/C) in the promoter region of the CYP7AI gene. No evidence was found for physiological differences between either the -203C and -203A alleles or the -469T and -469C alleles in transient transfection studies using native 834bp promoter constructs. Moreover, no association was observed between the CYP7AI promoter polymorphisms and the hepatic cholesterol 7alpha-hydroxylase activity and parameters of bile acid synthesis rates, as analyzed in subjects with gallstone disease. In addition, no relationships were found between the promoter polymorphisms and plasma LDL cholesterol concentration in association studies conducted in three different groups of middle-aged Swedish men. Finally, near complete allelic association was found between the two promoter polymorphisms and the IVS6+363G/A polymorphism at the 3' end of the CYP7AI gene (|D'|=0.98), indicating strong linkage disequilibrium across the whole CYP7AI gene. It is concluded that common polymorphisms of the CYP7A1 gene do not contribute to variation in cholesterol 7alpha-hydroxylase activity, rates of bile acid synthesis and plasma LDL cholesterol concentration in humans.

Species-specific mechanisms for cholesterol 7alpha-hydroxylase (CYP7A1) regulation by drugs and bile acids

The gene encoding cholesterol 7alpha-hydroxylase (CYP7A1) is tightly regulated in order to control intrahepatic cholesterol and bile acid levels. Ligands of the xenobiotic-sensing pregnane X receptor inhibit CYP7A1 expression. To retrace the evolution of the molecular mechanisms underlying CYP7A1 inhibition, we used a chicken hepatoma cell system that retains the ability to be induced by phenobarbital and other drugs. Whereas bile acids regulate CYP7A1 via small heterodimer partner and liver receptor homolog-1, mRNA expression of these nuclear receptors is unchanged by xenobiotics. Instead, drugs repress chicken hepatic nuclear factor 4alpha (HNF4alpha) transcript levels concomitant with a reduction in CYP7A1 expression. Importantly, no reduction of HNF4alpha levels is found in mouse liver in vivo and in human primary hepatocyte cultures, respectively. Thus, besides the importance of HNF4alpha in CYP7A1 regulation in all species, birds and mammals use different signaling pathways to adjust CYP7A1 levels after exposure to xenobiotics.

Orphan nuclear receptor small heterodimer partner represses hepatocyte nuclear factor 3/Foxa transactivation via inhibition of its DNA binding

Small heterodimer partner (SHP; NR0B2) is an atypical orphan nuclear receptor and acts as a coregulator of various nuclear receptors. Herein, we examined a novel cross talk between SHP and a forkhead transcription factor HNF3 (hepatocyte nuclear factor 3/Foxa. Transient transfection assay demonstrated that SHP inhibited the transcriptional activity of all three isoforms of HNF3, HNF3alpha, beta, and gamma. In vivo and in vitro protein interaction studies showed that SHP physically interacted with HNF3. Adenovirus-mediated overexpression of SHP significantly decreased the mRNA levels of glucose-6-phosphase (G6Pase), cholesterol 7-alpha-hydroxylase (CYP7A1), and phosphoenolpyruvate carboxykinase (PEPCK) in HepG2 cells and rat primary hepatocytes. Moreover, the mRNA level of G6Pase was notably increased by down-regulation of SHP with small interfering RNA. Interestingly, HNF3 transactivity was still repressed by SHPDelta128-139 that fails to repress nuclear receptors. Mapping of interaction domain revealed that SHP interacted with forkhead DNA binding domain of HNF3alpha. Gel mobility shift and chromatin immunoprecipitation assays demonstrated that SHP inhibits DNA binding of HNF3. These results suggest that SHP is involved in the regulation of G6Pase, CYP7A1, and PEPCK gene expression via novel mechanism of inhibition of HNF3 activity and expand the role of SHP as a coregulator of other family of transcription factors in addition to nuclear receptors.

In vivo regulation of murine CYP7A1 by HNF-6: a novel mechanism for diminished CYP7A1 expression in biliary obstruction

Disruption of the enterohepatic bile acid circulation during biliary tract obstruction leads to profound perturbation of the cholesterol and bile acid metabolic pathways. Several families of nuclear receptor proteins have been shown to modulate this critical process by regulating hepatic cholesterol catabolism and bile acid synthesis through the transcriptional control of cholesterol 7-alpha hydroxylase (CYP7A1). Hepatocyte nuclear factor (HNF) 6 (also known as OC-1) is a member of the ONECUT family of transcription factors that activate numerous hepatic target genes essential to liver function. We have previously shown that hepatic expression of mouse HNF-6 messenger RNA (mRNA) and protein significantly decrease following bile duct ligation. Because CYP7A1 contains potential HNF-6 binding sites in its promoter region, we tested the hypothesis that HNF-6 transcriptionally regulates CYP7A1. Following bile duct ligation, we demonstrated that diminished HNF-6 mRNA levels correlate with a reduction in CYP7A1 mRNA expression. Increasing hepatic levels of HNF-6 either by infection with recombinant adenovirus vector expressing HNF-6 cDNA by growth hormone treatment leads to an induction of CYP7A1 mRNA. To directly evaluate if HNF-6 is a transcriptional activator for CYP7A1, we used deletional and mutational analyses of CYP7A1 promoter sequences and defined sequences -206/-194 to be critical for CYP7A1 transcriptional stimulation by HNF-6 in cotransfection assays. In conclusion, the HNF-6 protein is a component of the complex network of hepatic transcription factors that regulates the expression of hepatic genes essential for bile acid homeostasis and cholesterol/lipid metabolism in normal and pathological conditions.

The human organic anion transporting polypeptide 8 (SLCO1B3) gene is transcriptionally repressed by hepatocyte nuclear factor 3beta in hepatocellular carcinoma

BACKGROUND/AIMS

The organic anion transporting polypeptides (OATPs) mediate the uptake of numerous amphipathic compounds into hepatocytes. Our aim was to study the expression and regulation of OATP8 (OATP1B3, SLC21A8/SLCO1B3) and OATP-C (OATP1B1, SLC21A6/SLCO1B1) in hepatocellular carcinomas (HCC).

METHODS

RNA and protein levels in 13 paired HCC and adjacent non-tumor liver samples were quantified by real-time polymerase chain reaction or Western blot, respectively. The OATP8 and OATP-C gene promoters were characterized by luciferase reporter assays and electrophoretic mobility shift assays (EMSA).

RESULTS

The expression of OATP8 was decreased in 60% of HCC compared to surrounding non-tumor liver tissue, on both the mRNA and protein levels. Expression of the liver-enriched transcription factor hepatocyte nuclear factor 3beta (HNF3beta) was increased in 70% of HCC and correlated inversely with OATP8 mRNA (r=-0.75, P<0.05) and protein. In contrast to OATP8, expression of OATP-C was not significantly decreased in HCC. In transfected Huh7 cells, OATP8 promoter activity was inhibited by 70% when HNF3beta was cotransfected. An HNF3beta binding site was located at nt -39/-23 by EMSA. The OATP-C promoter was not inhibited by HNF3beta.

CONCLUSIONS

HNF3beta represses transcription of the OATP8 but not the OATP-C gene, providing a mechanism for reduced expression of OATP8 in HCC.

Nuclear receptors and the control of metabolism

The metabolic nuclear receptors act as metabolic and toxicological sensors, enabling the organism to quickly adapt to environmental changes by inducing the appropriate metabolic genes and pathways. Ligands for these metabolic receptors are compounds from dietary origin, intermediates in metabolic pathways, drugs, or other environmental factors that, unlike classical nuclear receptor ligands, are present in high concentrations. Metabolic receptors are master regulators integrating the homeostatic control of (a) energy and glucose metabolism through peroxisome proliferator-activated receptor gamma (PPARgamma); (b) fatty acid, triglyceride, and lipoprotein metabolism via PPARalpha, beta/delta, and gamma; (c) reverse cholesterol transport and cholesterol absorption through the liver X receptors (LXRs) and liver receptor homolog-1 (LRH-1); (d) bile acid metabolism through the farnesol X receptor (FXR), LXRs, LRH-1; and (e) the defense against xeno- and endobiotics by the pregnane X receptor/steroid and xenobiotic receptor (PXR/SXR). The transcriptional control of these metabolic circuits requires coordination between these metabolic receptors and other transcription factors and coregulators. Altered signaling by this subset of receptors, either through chronic ligand excess or genetic factors, may cause an imbalance in these homeostatic circuits and contribute to the pathogenesis of common metabolic diseases such as obesity, insulin resistance and type 2 diabetes, hyperlipidemia and atherosclerosis, and gallbladder disease. Further studies should exploit the fact that many of these nuclear receptors are designed to respond to small molecules and turn them into therapeutic targets for the treatment of these disorders.

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.

A distinct thyroid hormone response element mediates repression of the human cholesterol 7alpha-hydroxylase (CYP7A1) gene promoter

We examined the molecular basis by which T3 regulates the human cholesterol 7alpha-hydroxylase gene (CYP7A1) promoter. L-T3 decreased chloramphenicol acetyltransferase activity in hepatoma cells cotransfected with a plasmid encoding the T3 receptor (TR) alpha [NR1a1] and a chimeric gene containing nucleotides -372 to +61 of the human CYP7A1 gene fused to the chloramphenicol acetyltransferase structural gene. Deoxyribonuclease I footprinting revealed that recombinant TRalpha protected two regions in this segment of the human CYP7A1 gene promoter. In EMSAs, TRalpha bound to both regions. The binding was competed by oligonucleotides bearing an idealized TRalpha binding motif and abolished by mutation of these elements. In assays of promoter function, mutation of only one of the TRalpha binding sites blocked repression by T3. The results indicate that T3-dependent repression of human CYP7A1 gene expression is mediated via a novel site in the human CYP7A1 gene promoter.

A predictive model for regulatory sequences directing liver-specific transcription

The identification and interpretation of the regulatory signals within the human genome remain among the greatest goals and most difficult challenges in genome analysis. The ability to predict the temporal and spatial control of transcription is likely to require a combination of methods to address the contribution of sequence-specific signals, protein-protein interactions and chromatin structure. We present here a new procedure to identify clusters of transcription factor binding sites characteristic of sequence modules experimentally verified to direct transcription selectively to liver cells. This algorithm is sufficiently specific to identify known regulatory sequences in genes selectively expressed in liver, promising acceleration of experimental promoter analysis. In combination with phylogenetic footprinting, this improvement in the specificity of predictions is sufficient to motivate a scan of the human genome. Potential regulatory modules were identified in orthologous human and rodent genomic sequences containing both known and uncharacterized genes.

Elevated levels of hepatocyte nuclear factor 3beta in mouse hepatocytes influence expression of genes involved in bile acid and glucose homeostasis

The winged helix transcription factor, hepatocyte nuclear factor-3beta (HNF-3beta), mediates the hepatocyte-specific transcription of numerous genes important for liver function. However, the in vivo role of HNF-3beta in regulating these genes remains unknown because homozygous null HNF3beta mouse embryos die in utero prior to liver formation. In order to examine the regulatory function of HNF-3beta, we created transgenic mice in which the -3-kb transthyretin promoter functions to increase hepatocyte expression of the rat HNF-3beta protein. Postnatal transgenic mice exhibit growth retardation, depletion of hepatocyte glycogen storage, and elevated levels of bile acids in serum. The retarded growth phenotype is likely due to a 20-fold increase in hepatic expression of insulin-like growth factor binding protein 1 (IGFBP-1), which results in elevated levels in serum of IGFBP-1 and limits the biological availability of IGFs required for postnatal growth. The defects in glycogen storage and serum bile acids coincide with diminished postnatal expression of hepatocyte genes involved in gluconeogenesis (phosphoenolpyruvate carboxykinase and glycogen synthase) and sinusoidal bile acid uptake (Ntcp), respectively. These changes in gene transcription may result from the disruptive effect of HNF-3beta on the hepatic expression of the endogenous mouse HNF-3alpha,-3beta, -3gamma, and -6 transcription factors. Furthermore, adult transgenic livers lack expression of the canalicular phospholipid transporter, mdr2, which is consistent with ultrastructure evidence of damage to transgenic hepatocytes and bile canaliculi. These transgenic studies represent the first in vivo demonstration that the HNF-3beta transcriptional network regulates expression of hepatocyte-specific genes required for bile acid and glucose homeostasis, as well as postnatal growth.

Identification and characterization of a 315-base pair enhancer, located more than 55 kilobases 5' of the apolipoprotein B gene, that confers expression in the intestine

We recently reported that an 8-kilobase (kb) region, spanning from -54 to -62 kb 5' of the human apolipoprotein B (apoB) gene, contains intestine-specific regulatory elements that control apoB expression in the intestines of transgenic mice. In this study, we further localized the apoB intestinal control region to a 3-kb segment (-54 to -57 kb). DNaseI hypersensitivity studies uncovered a prominent DNaseI hypersensitivity site, located within a 315-base pair (bp) fragment at the 5'-end of the 3-kb segment, in transcriptionally active CaCo-2 cells but not in transcriptionally inactive HeLa cells. Transient transfection experiments with CaCo-2 and HepG2 cells indicated that the 315-bp fragment contained an intestine-specific enhancer, and analysis of the DNA sequence revealed putative binding sites for the tissue-specific transcription factors hepatocyte nuclear factor 3beta, hepatocyte nuclear factor 4, and CAAT enhancer-binding protein beta. Binding of these factors to the 315-bp enhancer was demonstrated in gel retardation experiments. Transfection of deletion mutants of the 315-bp enhancer revealed the relative contributions of these transcription factors in the activity of the apoB intestinal enhancer. The corresponding segment of the mouse apoB gene (located -40 to -83 kb 5' of the structural gene) exhibited a high degree of sequence conservation in the binding sites for the key transcriptional activators and also exhibited enhancer activity in transient transfection assays with CaCo-2 cells. In transgenic mouse expression studies, the 315-bp enhancer conferred intestinal expression to human apoB transgenes.

The nuclear matrix protein CDP represses hepatic transcription of the human cholesterol-7alpha hydroxylase gene

To date, the molecular mechanisms that govern hepatic-specific transcription of the human cholesterol 7alpha-hydroxylase (CYP7A1) gene are poorly understood. We recently reported that the region extending from -1888 to +46, which includes the promoter, is not capable of conferring expression to human CYP7A1 promoter lacZ transgenes in the livers of mice, but that expression is observed with transgenes containing the entire structural gene. To locate liver-specific elements in other segments of the human gene, DNase I hypersensitivity studies were performed with transcriptionally active, liver-derived HepG2 cells and with transcriptionally inactive HeLa cells. Three DNase I hypersensitivity sites were detected within the first intron of the human CYP7A1 gene, but only in HepG2 cells. Transient transfection experiments with HepG2 cells revealed a transcriptional repressor within intron 1. Five binding sites for the CAAT displacement protein (CDP) were detected within intron 1. Since CDP is a nuclear matrix protein, two methods were employed to localize nuclear matrix attachment sites within intron 1 of the human CYP7A1 gene. A matrix attachment site was found throughout the entirety of intron 1. Gel retardation experiments and cell transfection studies provided evidence for the repression mechanism. Repression is achieved by displacement by CDP of two hepatic activators, namely HNF-1alpha and C/EBPalpha, that bind to three different sites within intron 1. Additionally, CDP represses transactivation mediated by these two activators.

The murine and human cholesterol 7alpha-hydroxylase gene promoters are differentially responsive to regulation by fatty acids mediated via peroxisome proliferator-activated receptor alpha

We determined if fatty acids can regulate the murine Cyp7a1 and human CYP7A1 gene promoters via peroxisome proliferator-activated receptor alpha (PPARalpha)/9-cis-retinoic acid receptor alpha (RXRalpha). In transfected cells, the murine Cyp7a1 gene promoter displayed markedly lower basal activity, but greater sensitivity to fatty acid- or WY 14,643-activated PPARalpha/RXRalpha when compared with the human CYP7A1 gene promoter. PPARalpha/RXRalpha can bind to a site (Site II) located within the region at nucleotides -158 to -132 of both promoters. Mutagenesis of the human CYP7A1 Site II element abolished the response to activated PPARalpha/RXRalpha. The murine Cyp7a1 gene promoter contains an additional PPARalpha/RXRalpha-binding site (Site I) located within nucleotides -72 to -57. Replacement of a single residue in human CYP7A1 Site I with that found in the murine Cyp7a1 Site I sequence enabled PPARalpha/RXRalpha binding, and this mutation resulted in reduced basal activity, but substantially improved the response to activated PPARalpha/RXRalpha in transfected cells. We conclude that fatty acids can regulate the cyp7a gene promoter via PPARalpha/RXRalpha. The differential response of the murine Cyp7a1 and human CYP7A1 gene promoters to PPARalpha activators is attributable to the additional PPARalpha/RXRalpha-binding site in the murine Cyp7a1 gene promoter.

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.

P450 gene induction by structurally diverse xenochemicals: central role of nuclear receptors CAR, PXR, and PPAR

The biochemistry of foreign compound metabolism and the roles played by individual cytochrome P450 (CYP) enzymes in drug metabolism and in the toxification and detoxification of xenochemicals prevalent in the environment are important areas of molecular pharmacology and toxicology that have been widely studied over the past decade. Important advances in our understanding of the mechanisms through which foreign chemicals impact on these P450-dependent metabolic processes have been made during the past 2 years with several key discoveries relating to the mechanisms through which xenochemicals induce the expression of hepatic P450 enzymes. Roles for three "orphan" nuclear receptor superfamily members, designated CAR, PXR, and PPAR, in respectively mediating the induction of hepatic P450s belonging to families CYP2, CYP3, and CYP4 in response to the prototypical inducers phenobarbital (CAR), pregnenolone 16alpha-carbonitrile and rifampicin (PXR), and clofibric acid (PPAR) have now been established. Two other nuclear receptors, designated LXR and FXR, which are respectively activated by oxysterols and bile acids, also play a role in liver P450 expression, in this case regulation of P450 cholesterol 7alpha-hydroxylase, a key enzyme of bile acid biosynthesis. All five P450-regulatory nuclear receptors belong to the same nuclear receptor gene family (family NR1), share a common heterodimerization partner, retinoid X-receptor (RXR), and are subject to cross-talk interactions with other nuclear receptors and with a broad range of other intracellular signaling pathways, including those activated by certain cytokines and growth factors. Endogenous ligands of each of those nuclear receptors have been identified and physiological receptor functions are emerging, leading to the proposal that these receptors may primarily serve to modulate hepatic P450 activity in response to endogenous dietary or hormonal stimuli. Accordingly, P450 induction by xenobiotics may in some cases lead to a perturbation of endogenous regulatory circuits with associated pathophysiological consequences.

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.

CPF: an orphan nuclear receptor that regulates liver-specific expression of the human cholesterol 7alpha-hydroxylase gene

Cholesterol 7alpha-hydroxylase is the first and rate-limiting enzyme in a pathway through which cholesterol is metabolized to bile acids. The gene encoding cholesterol 7alpha-hydroxylase, CYP7A, is expressed exclusively in the liver. Overexpression of CYP7A in hamsters results in a reduction of serum cholesterol levels, suggesting that the enzyme plays a central role in cholesterol homeostasis. Here, we report the identification of a hepatic-specific transcription factor that binds to the promoter of the human CYP7A gene. We designate this factor CPF, for CYP7A promoter binding factor. Mutation of the CPF binding site within the CYP7A promoter abolished hepatic-specific expression of the gene in transient transfection assays. A cDNA encoding CPF was cloned and identified as a human homolog of the Drosophila orphan nuclear receptor fushi tarazu F1 (Ftz-F1). Cotransfection of a CPF expression plasmid and a CYP7A reporter gene resulted in specific induction of CYP7A-directed transcription. These observations suggest that CPF is a key regulator of human CYP7A gene expression in the liver.

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.

Two 7 alpha-hydroxylase enzymes in bile acid biosynthesis

The addition of a 7-hydroxyl group is an early and often rate-limiting step in the synthesis of bile acids. This reaction is catalysed by two cytochrome P450 enzymes known as cholesterol 7 alpha-hydroxylase and oxysterol 7 alpha-hydroxylase. cDNAs encoding these proteins have been isolated and used to define two evolutionarily conserved pathways that produce 7 alpha-hydroxylated bile acids.