Disease relevance of CYP7A1

• Additionally, infection of cultured HepG2 cells with a recombinant adenovirus expressing PGC-1alpha directly activates CYP7A1 gene expression and increases bile acid biosynthesis as well [1].
• We tested this hypothesis by inducing hypo- and hyperthyroidism in transgenic mice expressing the human CYP7A1 gene [2].
• MATERIALS AND METHODS: Surgical liver biopsies were obtained from 11 patients with untreated cholesterol cholelithiasis and nine gallstone-free subjects; mRNA levels of cholesterol 7alpha-hydroxylase (CYP7A1) and related nuclear receptors and coactivators were assayed by quantitative real-time RT-PCR [3].
• Studies of hepatoma cells overexpressing CYP7A1, the rate-limiting enzyme controlling bile acid synthesis, show that as a result of increased mature SREBP1, there is a coordinate induction of lipogenesis and the assembly and secretion of VLDL [4].
• Surprisingly, human cholesterol 7 alpha-hydroxylase, CYP7A, expressed as a recombinant in Escherichia coli and COS cells, was active toward 20(S)-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol [5].

High impact information on CYP7A1

• The expression of LDL receptors was increased by 180 percent (P less than 0.005), whereas the activities of cholesterol 7 alpha-hydroxylase (which governs bile acid synthesis) and of acyl-coenzyme A:cholesterol O-acyltransferase (which regulates cholesterol esterification) were unaffected by treatment [6].
• Although bile acids activate the farnesoid X receptor (FXR), the mechanism underlying bile acid-mediated repression of CYP7A1 remained unclear [7].
• The catabolism of cholesterol into bile acids is regulated by oxysterols and bile acids, which induce or repress transcription of the pathway's rate-limiting enzyme cholesterol 7alpha-hydroxylase (CYP7A1) [8].
• Human cholesterol 7alpha-hydroxylase (CYP7A1) deficiency has a hypercholesterolemic phenotype [9].
• DNA sequencing revealed two linked polymorphisms in the 5' flanking region of CYP7 [10].

Chemical compound and disease context of CYP7A1

• We examined if stabilization of mRNA decay could account for the >20-fold increase in the expression of CYP7A1 mRNA without a detectable change in transcription following dexamethasone treatment of rat hepatoma cells (L35 cells) [11].
• 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 [12].
• 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 [13].
• Competitive inhibition of bile acid synthesis by endogenous cholestanol and sitosterol in sitosterolemia with xanthomatosis. Effect on cholesterol 7 alpha-hydroxylase [14].
• Patients with sitosterolemia with xanthomatosis, who have elevated microsomal cholestanol and sitosterol, showed reduced cholesterol 7 alpha-hydroxylase activity relative to the activity in control subjects (13.9 and 14.7 vs. 20.3 +/- 0.9 pmol/nmol P-450 per min, P less than 0.01) [14].

Biological context of CYP7A1

• Human primary hepatocytes and HepG2 cells were used as models to study chenodeoxycholic acid (CDCA) and interleukin-1 beta (IL-1 beta) regulation of human CYP7A1 gene expression via real-time polymerase chain reaction, reporter assays, co-immunoprecipitation and chromatin immunocipitation (ChIP) assays [15].
• IL-1beta inhibited human CYP7A1 reporter activity via the HNF4 alpha binding site [15].
• Furthermore, we show that PGC-1alpha activates the CYP7A1 promoter directly in transient transfection assays in cultured cells [1].
• Expression of CAR inhibited HNF-4 transactivation of CYP7A1, a key gene in bile acid synthesis, in HepG2 cells, and mutation of the DNA binding domain of CAR impaired this inhibition [16].
• Our data reveal a fundamental difference in the regulation of CYP7A1 in rodent and human hepatocytes and provide evidence that different species employ distinct molecular strategies to regulate cholesterol homeostasis [17].

Anatomical context of CYP7A1

• IL-1 beta and CDCA reduced CYP7A1 but induced c-Jun messenger RNA expression in human primary hepatocytes [15].
• These results suggest that MLN64, in its full-length form, is not responsible for the transport of cholesterol to the mitochondria or the endoplasmic reticulum, where CYP27A1 or CYP7A1 is located, respectively [18].
• Conversely, accumulation of the oxysterol 7alpha-hydroxycholesterol in the microsomes after CYP7A1 overexpression was correlated with a decrease in HMGR activity [19].
• Prox1 strongly inhibited HNF4alpha and peroxisome proliferators-activated receptor gamma coactivator-1alpha co-activation of the CYP7A1 and PEPCK genes [20].
• Prox1, an early specific marker for developing liver and pancreas in foregut endoderm has recently been shown to interact with alpha-fetoprotein transcription factor and repress cholesterol 7alpha-hydroxylase (CYP7A1) gene transcription [20].

Associations of CYP7A1 with chemical compounds

• Addition of GCDCA, GDCA, and GCA (100 micromol/L) markedly decreased cholesterol 7alpha-hydroxylase (CYP7A1) mRNA levels to 2% +/- 1%, 2% +/- 1%, and 29% +/- 11% of controls, respectively, whereas GUDCA had no effect [21].
• Here we show that PGC-1alpha and CYP7A1 are also co-induced in livers of mice in response to streptozotocin induced diabetes [1].
• We also measured CYP27A1 mRNA and CYP7A1 mRNA in liver of humans subjected to treatment with chenodeoxycholic acid, ursodeoxycholic acid, hydroxymethylglutaryl (HMG)-CoA reductase inhibitor and a combination of HMG-CoA reductase inhibitor and cholestyramine [22].
• However, another nuclear receptor, hepatocyte nuclear factor 4alpha (HNF4alpha), was induced 2.9-fold in CTX, which was associated with enhanced mRNA levels of HNF4alpha target genes, CYP7A1, 7alpha-hydroxy-4-cholesten-3-one 12alpha-hydroxylase, CYP27A1, and NTCP [23].
• The data indicate that thyroid hormone can repress the human CYP7A1 gene in transgenic mice, but this effect is dependent on gender in this in vivo model [2].

Physical interactions of CYP7A1

• In particular we show that the hamster CYP7A1 insulin response sequence is part of a complex unit involved in specific interactions with multiple transcription factors such as members of the HNF-3 family; this region does not bind very strongly to HNF-3 and as a consequence partly contributes to the transactivation of the gene [24].

Regulatory relationships of CYP7A1

• The objective of this study is to elucidate the mechanism by which PXR inhibits CYP7A1 gene transcription [25].
• Recent studies show there is an "FXR/SHP-independent" mechanism that also represses CYP7A1 expression [26].
• The goal of this study is to elucidate the mechanism of CYP7A1 transcription by bile acid-activated farnesoid X receptor (FXR) in its native promoter and cellular context and to identify FXR response elements in the gene [27].
• In the liver, the liver X receptor alpha induces the cholesterol 7alpha-hydroxylase (CYP7A1) gene, which controls the rate-limiting step in bile acid synthesis, the major cholesterol excretion pathway [28].
• This DR1 sequence was mapped previously as a binding site for the hepatocyte nuclear factor 4 (HNF-4) which stimulates CYP7A1 transcription [29].

Other interactions of CYP7A1

• There was a 60-fold variation in the levels of CYP7A1 mRNA but only a 5-fold variation in the levels of CYP27A1 mRNA [22].
• Hepatic CYP7A1 and CYP7B1 mRNA levels increased several-fold with age [30].
• 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) [31].
• Interactions between common genetic polymorphisms in ABCG5/G8 and CYP7A1 on LDL cholesterol-lowering response to atorvastatin [32].
• Rifampicin inhibited CYP7A1 reporter activity and a PXR binding site was localized to the bile acid response element-I [25].

Analytical, diagnostic and therapeutic context of CYP7A1

• To characterize the coordinate regulation of cholesterol metabolism in human liver, we simultaneously quantified mRNA levels of cholesterol 7alpha-hydroxylase (CYP7A1), 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR), and low- density lipoprotein receptors (LDLRs) in liver biopsies from 76 patients undergoing cholecystectomy [33].
• In the present work we focused on identification of determinants of the CYP7A1 substrate specificity inside the active site using a homology model with a novel P450-fold, site-directed mutagenesis, and substrate-binding and kinetic studies [34].
• The mRNA expression levels of CYP7A1 and several nuclear receptors known to regulate the CYP7A1 gene were assayed in human primary hepatocytes by quantitative real-time PCR (Q-PCR) [25].
• We therefore performed transient transfection assays with CYP7A1 promoter/luciferase chimaeras mutated at putative response elements and studied protein-DNA interactions by means of gel electrophoresis mobility-shift assay [24].
• 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 [35].

References