Transcriptional regulation of human oxysterol 7alpha-hydroxylase by sterol response element binding protein

Regulation and targeting of SREBP-1 in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is an increasing burden on global public health and is associated with enhanced lipogenesis, fatty acid uptake, and lipid metabolic reprogramming. De novo lipogenesis is under the control of the transcription factor sterol regulatory element-binding protein 1 (SREBP-1) and essentially contributes to HCC progression. Here, we summarize the current knowledge on the regulation of SREBP-1 isoforms in HCC based on cellular, animal, and clinical data. Specifically, we (i) address the overarching mechanisms for regulating SREBP-1 transcription, proteolytic processing, nuclear stability, and transactivation and (ii) critically discuss their impact on HCC, taking into account (iii) insights from pharmacological approaches. Emphasis is placed on cross-talk with the phosphatidylinositol-3-kinase (PI3K)-protein kinase B (Akt)-mechanistic target of rapamycin (mTOR) axis, AMP-activated protein kinase (AMPK), protein kinase A (PKA), and other kinases that directly phosphorylate SREBP-1; transcription factors, such as liver X receptor (LXR), peroxisome proliferator-activated receptors (PPARs), proliferator-activated receptor γ co-activator 1 (PGC-1), signal transducers and activators of transcription (STATs), and Myc; epigenetic mechanisms; post-translational modifications of SREBP-1; and SREBP-1-regulatory metabolites such as oxysterols and polyunsaturated fatty acids. By carefully scrutinizing the role of SREBP-1 in HCC development, progression, metastasis, and therapy resistance, we shed light on the potential of SREBP-1-targeting strategies in HCC prevention and treatment.

Up to date on cholesterol 7 alpha-hydroxylase (CYP7A1) in bile acid synthesis

Cholesterol 7 alpha-hydroxylase (CYP7A1, EC1.14) is the first and rate-limiting enzyme in the classic bile acid synthesis pathway. Much progress has been made in understanding the transcriptional regulation of CYP7A1 gene expression and the underlying molecular mechanisms of bile acid feedback regulation of CYP7A1 and bile acid synthesis in the last three decades. Discovery of bile acid-activated receptors and their roles in the regulation of lipid, glucose and energy metabolism have been translated to the development of bile acid-based drug therapies for the treatment of liver-related metabolic diseases such as alcoholic and non-alcoholic fatty liver diseases, liver cirrhosis, diabetes, obesity and hepatocellular carcinoma. This review will provide an update on the advances in our understanding of the molecular biology and mechanistic insights of the regulation of CYP7A1 in bile acid synthesis in the last 40 years.

Re-adopting classical nuclear receptors by cholesterol metabolites

Since the first cloning of the human estrogen receptor (ER) α in 1986 and the subsequent cloning of human ERβ, there has been extensive investigation of the role of estrogen/ER. Estrogens/ER play important roles not only in sexual development and reproduction but also in a variety of other functions in multiple tissues. Selective Estrogen Receptor Modulators (SERMs) are ER lignds that act as agonists or antagonists depending on the target genes and tissues, and until recently, only synthetic SERMs have been recognized. However, the discovery of the first endogenous SERM, 27-hydroxycholesterol (27HC), opened a new dimension of ER action in health and disease. In addition to the identification of 27HC as a SERM, oxysterols have been recently demonstrated as indirect modulators of ER through interaction with the nuclear receptor Liver X Receptor (LXR) β. In this review, the recent progress on these novel roles of oxysterols in ER modulation is summarized.

Box C/D small nucleolar RNA (snoRNA) U60 regulates intracellular cholesterol trafficking

Mobilization of plasma membrane (PM) cholesterol to the endoplasmic reticulum is essential for cellular cholesterol homeostasis. The mechanisms regulating this retrograde, intermembrane cholesterol transfer are not well understood. Because mutant cells with defects in PM to endoplasmic reticulum cholesterol trafficking can be isolated on the basis of resistance to amphotericin B, we conducted an amphotericin B loss-of-function screen in Chinese hamster ovary (CHO) cells using insertional mutagenesis to identify genes that regulate this trafficking mechanism. Mutant line A1 displayed reduced cholesteryl ester formation from PM-derived cholesterol and increased de novo cholesterol synthesis, indicating a deficiency in retrograde cholesterol transport. Genotypic analysis revealed that the A1 cell line contained one disrupted allele of the U60 small nucleolar RNA (snoRNA) host gene, resulting in haploinsufficiency of the box C/D snoRNA U60. Complementation and mutational studies revealed the U60 snoRNA to be the essential feature from this locus that affects cholesterol trafficking. Lack of alteration in predicted U60-mediated site-directed methylation of 28 S rRNA in the A1 mutant suggests that the U60 snoRNA modulates cholesterol trafficking by a mechanism that is independent of this canonical function. Our study adds to a growing body of evidence for participation of small noncoding RNAs in cholesterol homeostasis and is the first to implicate a snoRNA in this cellular function.

Interplay between cholesterol and drug metabolism

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

Involvement of the PI3K/Akt pathway in estrogen-mediated regulation of human CYP7B1: identification of CYP7B1 as a novel target for PI3K/Akt and MAPK signalling

The steroid hydroxylase CYP7B1 metabolizes neurosteroids, cholesterol derivatives, and estrogen receptor (ER) ligands. Previous studies identified CYP7B1 as a target for regulation by estrogen. The present study examines the mechanism for estrogen-mediated regulation of the human CYP7B1 gene promoter. Treatment with LY294002, a specific inhibitor of phosphatidylinositol 3-kinase (PI3K), abolished ER-mediated up-regulation of a CYP7B1 promoter-luciferase reporter in HepG2 cells, whereas overexpression of PI3K or Akt significantly increased estrogenic up-regulation of CYP7B1. Overexpression of dominant-negative mutant Akt abolished ER-mediated stimulation of CYP7B1 in HepG2 cells. Data indicated no binding of ER to CYP7B1 promoter sequences, suggesting that ER interacts with the PI3K/Akt pathway without binding to the gene. At low ER levels, overexpression of Akt suppressed CYP7B1 promoter activity, suggesting that its effect on CYP7B1 is different when estrogens are absent. In HEK293 cells, CYP7B1 transcription was much less affected by Akt, indicating that the mechanism for up-regulation of CYP7B1 is different in different cell types. Other experiments indicated that MAPK signalling may affect basal CYP7B1 levels. The current results, indicating that regulation of CYP7B1 by ER can be mediated via the PI3K/Akt signal pathway, a regulatory pathway important for cellular survival and growth, suggest an important role for CYP7B1 in cellular growth, particularly in connection with estrogenic signalling.

Glucocorticoid receptor-mediated upregulation of human CYP27A1, a potential anti-atherogenic enzyme

Sterol 27-hydroxylase (CYP27A1) is required for the hepatic conversion of cholesterol into bile acids and for production of 27-hydroxycholesterol which affects cholesterol homeostasis in several ways. Dexamethasone increases hepatic bile acid biosynthesis and CYP27A1-mediated enzyme activity in HepG2 cells. This study examines the mechanism of the dexamethasone-induced effect on the human CYP27A1 promoter. Dexamethasone treatment of HepG2 cells overexpressed with glucocorticoid receptor alpha (GRalpha) increased the CYP27A1 promoter activity more than four-fold as compared with untreated cells. The GR-antagonist mifepristone almost completely abolished the dexamethasone-induced effect on the promoter activity. Progressive deletion analysis of the CYP27A1 promoter indicated that sequences involved in GR-mediated induction by dexamethasone are present in a region between -1094 and -792. Several putative GRE sites could be found in this region and EMSA experiments revealed that two of these could bind GR. Site-directed mutagenesis of GR-binding sequences in the CYP27A1 promoter identified a GRE at -824/-819 important for GR-mediated regulation of the transcriptional activity. Endogenous and pharmacological glucocorticoids may have a strong impact on several aspects of cholesterol homeostasis and other processes related to CYP27A1-mediated metabolism. The glucocorticoid-mediated induction of human CYP27A1 transcription is of particular interest due to the anti-atherogenic properties ascribed to this enzyme.

Cholesterol-lowering effect of bezafibrate is independent of peroxisome proliferator-activated receptor activation in mice

The hypocholesterolemic potential of peroxisome proliferator-activated receptor (PPAR) pan-activator bezafibrate has been documented. However, in addition to uncertainty about the contribution of PPAR alpha to its effect, there is a marked discrepancy in bezafibrate dosages used in previous rodent experiments (> or = 50 mg/kg/day) and those in clinical use (< or = 10 mg/kg/day). To investigate the association between bezafibrate-induced cholesterol reduction and PPAR alpha activation, wild-type and Ppar a-null mice were treated with bezafibrate at high (100 mg/kg/day) or low (10 mg/kg/day) doses and analyzed. High-dose treatment decreased hepatic cholesterol content in wild-type mice, but increased serum cholesterol concentration. In liver samples, simultaneous increases in the expression of numerous proteins involved in cholesterol biosynthesis and catabolism, as well as cholesterol influx and efflux, were observed, which made interpretation of phenotype changes subtle. These complicated responses were believed to be associated with intensive PPAR activation and accompanying up-regulation of liver X receptor alpha, farnesoid X receptor, and sterol regulatory element-binding protein 2 (SREBP2). In contrast, low-dose bezafibrate treatment decreased serum and hepatic cholesterol concentrations in a PPAR alpha-independent manner, probably from suppression of SREBP2-regulated cholesterogenesis and enhancement of cholesterol catabolism due to elevated 7alpha-hydroxylase levels. Interestingly, the low-dose treatment did not affect the expression of PPAR target genes or number of peroxisomes, suggesting the absence of PPAR activation. These results demonstrate that the action of bezafibrate on cholesterol metabolism may vary with dosage, and that the cholesterol-reducing effect found in mice at dosages similar to those administered to humans is independent of significant PPAR activation.

Regulation of human CYP27A1 by estrogens and androgens in HepG2 and prostate cells

The regulation of the human CYP27A1 gene by estrogens and androgens was studied in human liver-derived HepG2 and prostate cells. Our results show that the promoter activity, enzymatic activity and mRNA levels of CYP27A1 in HepG2 cells are downregulated by estrogen in presence of ERalpha or ERbeta. Similar effects by estrogen were found in RWPE-1 prostate cells. In contrast, estrogen markedly upregulated the transcriptional activity of CYP27A1 in LNCaP prostate cancer cells. 5alpha-Dihydrotestosterone and androgen receptor upregulated the transcriptional activity of CYP27A1 in HepG2 cells. Progressive deletion experiments indicate that the ERbeta-mediated effects in HepG2 and LNCaP cells are conferred to the same region (-451/+42) whereas ERalpha-mediated effects on this promoter are more complex. The results indicate that the stimulating effect of androgen in HepG2 cells is conferred to a region upstream from -792 in the CYP27A1 promoter. In summary, we have identified the human CYP27A1 gene as a target for estrogens and androgens. The results imply that expression of CYP27A1 may be affected by endogenous sex hormones and pharmacological compounds with estrogenic or androgenic effects.

Activation of LXRs prevents bile acid toxicity and cholestasis in female mice

Liver X receptors (LXRs) have been identified as sterol sensors that regulate cholesterol and lipid homeostasis and macrophage functions. In this study, we found that LXRs also affect sensitivity to bile acid toxicity and cholestasis. Activation of LXRalpha in transgenic mice confers a female-specific resistance to lithocholic acid (LCA)-induced hepatotoxicity and bile duct ligation (BDL)-induced cholestasis. This resistance was also seen in wild-type female mice treated with the synthetic LXR ligand TO1317. In contrast, LXR double knockout (DKO) mice deficient in both the alpha and beta isoforms exhibited heightened cholestatic sensitivity. LCA and BDL resistance in transgenic mice was associated with increased expression of bile acid-detoxifying sulfotransferase 2A (Sult2a) and selected bile acid transporters, whereas basal expression of these gene products was reduced in the LXR DKO mice. Promoter analysis showed that the mouse Sult2a9 gene is a transcriptional target of LXRs. Activation of LXRs a l so suppresses expression of oxysterol 7alpha-hydroxylase (Cyp7b1), which may lead to increased levels of LXR-activating oxysterols.

CONCLUSION

We propose that LXRs have evolved to have the dual functions of maintaining cholesterol and bile acid homeostasis by increasing cholesterol catabolism and, at the same time, preventing toxicity from bile acid accumulation.

Cytochrome P450s and cholesterol homeostasis

By participating in pathways of cholesterol biosynthesis and elimination, different cytochrome P450 (P450 or CYP) enzymes play an important role in maintenance of cholesterol homeostasis. CYP51 is involved in cholesterol biosynthesis, whereas CYP 7A1, 27A1, 46A1, 7B1, 39A1, and 8B1 are the key enzymes in cholesterol catabolism to bile acids, the major route of cholesterol elimination in mammals. Cholesterol transformations to steroid hormones are also initiated by the P450 enzyme CYP11A1. Finally, one of the major drug-metabolizing P450s CYP3A4 seems to contribute to bile acid biosynthesis as well. The 9 P450s will be the focus of this review and assessed as drug targets for cholesterol lowering.

Hypercholesterolemia in rats with hepatomas: increased oxysterols accelerate efflux but do not inhibit biosynthesis of cholesterol

Hypercholesterolemia is an important paraneoplastic syndrome in patients with hepatoma, but the nature of this defect has not yet been identified. We investigated the molecular mechanisms of hypercholesterolemia in a hepatoma-bearing rat model. Buffalo rats were implanted in both flanks with Morris hepatoma 7777 (McA-RH7777) cells. After 4 weeks, tumor weight was 5.5+/-1.7 g, and serum cholesterol level increased from 60+/-2 to 90+/-2 mg/dL. Protein and mRNA expression of the ATP-binding cassette transporters A1 and G1 (ABCA1 and ABCG1) was markedly higher in tumors than in livers. These increases were associated with activation of liver X receptor alpha (LXRalpha) as a result of the increased tissue oxysterol concentrations. The accumulation of oxysterols in the hepatomas appeared to be caused mainly by the upregulation of cholesterol biosynthesis, despite the increased tissue sterol concentrations. Overexpression of the sterol regulatory element-binding protein (SREBP) processing system relative to sterol concentration contributed to the resistance to sterols in this tumor. In addition, bile acid biosynthesis was inhibited despite the reduced expression of the small heterodimer partner (SHP) and activated LXRalpha, which also appeared to contribute to the accumulation of oxysterols followed by the acceleration of cholesterol efflux. In conclusion, hypercholesterolemia in McA-RH7777 hepatoma-bearing rats was caused by increased cholesterol efflux from tumors as a result of activation of LXRalpha. Overexpression of the SREBP processing system contributed to the activation of LXRalpha by maintaining high oxysterol levels in tissue.

Estrogen-mediated regulation of CYP7B1: a possible role for controlling DHEA levels in human tissues

The current study examines regulation of CYP7B1, a DHEA 7alpha-hydroxylase, by sex hormones. Transfection with estrogen receptor alpha and treatment with 17beta-estradiol in human embryonic kidney 293 cells significantly increased CYP7B1 catalytic activity and mRNA, and stimulated a human CYP7B1 reporter gene. Transfection with estrogen receptor beta showed similar but less significant effects. In the absence of receptors, 17beta-estradiol suppressed CYP7B1 activity, suggesting that estrogenic effects may be different in cells not expressing receptors. Quantitation of CYP7B1 mRNA in adult and fetal human tissues showed markedly higher CYP7B1 mRNA levels in fetal tissues compared with the corresponding adult ones, except in the liver. This indicates a tissue-specific, developmental regulation of CYP7B1 and suggests an important function for this enzyme in fetal life. DHEA secreted by fetal adrenals is an essential precursor for placental estrogen formation. Since CYP7B1 diverts DHEA from the sex hormone biosynthetic pathway, estrogen receptor-mediated up-regulation of CYP7B1 should lead to less DHEA available for sex hormone synthesis and may help to maintain normal levels of estrogens and androgens in human tissues, especially during fetal development. Regulation by estrogens may also be of importance in other processes where CYP7B1 is involved, including cholesterol homeostasis, cellular proliferation, and CNS function.

Regulation of steroid hydroxylase CYP7B1 by androgens and estrogens in prostate cancer LNCaP cells

The present study reports effects of androgens and estrogens on human CYP7B1 transcription in prostate cancer LNCaP cells. Studies with rodents have suggested a role for the CYP7B1 enzyme in balancing cellular hormone levels important for prostate growth. Little is, however, known about the regulation of human CYP7B1. The current study showed strong suppression of a human CYP7B1 luciferase reporter gene by dihydrotestosterone (DHT) in prostate cancer LNCaP cells. Also, DHT and overexpression of androgen receptor (AR) suppressed CYP7B1 promoter activity and CYP7B1-mediated catalysis in kidney-derived HEK293 cells. Effects on CYP7B1 transcription were observed also by estrogen receptors (ER). The effects appeared different for different estrogens. CYP7B1 was stimulated by synthetic ER agonists but suppressed by 17beta-estradiol and 5alpha-androstane-3beta,17beta-diol in LNCaP cells. Our data indicate an important role for CYP7B1 in balancing prostate hormone levels in human cells. In particular, the data suggest that androgens may control intraprostatic levels of estrogen via regulation of CYP7B1-mediated metabolism.

Hepatic Gene Expression Changes in Mouse Models with Liver-specific Deletion or Global Suppression of the NADPH-Cytochrome P450 Reductase Gene

NADPH-cytochrome P450 reductase (CPR) is an essential component for the function of many enzymes, including microsomal cytochrome P450 (P450) monooxygenases and heme oxygenases. In liver-Cpr-null (with liver-specific Cpr deletion) and Cpr-low (with reduced CPR expression in all organs examined) mouse models, a reduced serum cholesterol level and an induction of hepatic P450s were observed, whereas hepatomegaly and fatty liver were only observed in the liver-Cpr-null model. Our goal was to identify hepatic gene expression changes related to these phenotypes. Cpr-lox mice (with a floxed Cpr gene and normal CPR expression) were used as the control. Through microarray analysis, we identified many genes that were differentially expressed among the three groups of mice. We also recognized the 12 gene ontology terms that contained the most significantly changed gene expression in at least one of the two mouse models. We further uncovered potential mechanisms, such as an increased activation of constitutive androstane receptor and a decreased activation of peroxisomal proliferator-activated receptor-alpha by precursors of cholesterol biosynthesis, that underlie common changes (e.g. induction of multiple P450s and suppression of genes for fatty acid metabolism) in response to CPR loss in the two mouse models. Additionally, we observed model-specific gene expression changes, such as the induction of a fatty-acid translocase (Cd36 antigen) and the suppression of carnitine O-palmitoyltransferase 1 (Cpt1a) and acyl-CoA synthetase long chain family member 1 (Acsl1), that are potentially responsible for the severe hepatic lipidosis and an altered fatty acid profile observed in liver-Cpr-null mice.

Nuclear receptors constitutive androstane receptor and pregnane X receptor ameliorate cholestatic liver injury

Cholestasis is associated with accumulation of bile acids and lipids, and liver injury. The constitutive androstane receptor (CAR) and pregnane X receptor (PXR) are xenobiotic nuclear receptors that coordinate protective hepatic responses to potentially toxic stimuli, including bile acids. We investigated the role of these receptors in the regulation of bile acid and lipid metabolism in a bile duct ligation (BDL) model of cholestasis applied to receptor knockout mice. Hepatic damage from bile acid accumulation was increased in both CAR knockout (CARKO) and PXR knockout mice, but bile acid concentrations were lower in CARKO mice. High-density lipoprotein (HDL) cholesterol was elevated in CARKO mice, and serum total cholesterol increased less in CARKO or PXR knockout mice than WT mice after BDL. Gene expression analysis of the BDL knockout animals demonstrated that, in response to cholestasis, PXR and CAR both repressed and induced the specific hepatic membrane transporters Oatp-c (organic anion transporting polypeptide C) and Oatp2 (Na+-dependent organic anion transporter 2), respectively. Induction of the xenobiotic transporter multidrug resistance protein 1 in cholestasis was independent of either PXR or CAR, in contrast to the known pattern of induction of multidrug resistance protein 1 by xenobiotics. These results demonstrate that CAR and PXR influence cholesterol metabolism and bile acid synthesis, as well as multiple detoxification pathways, and suggest their potential role as therapeutic targets for the treatment of cholestasis and lipid disorders.

The Liver X Receptor Ligand T0901317 Down-regulates APOA5 Gene Expression through Activation of SREBP-1c

Alterations in the expression of the recently discovered apolipoprotein A5 gene strongly affect plasma triglyceride levels. In this study, we investigated the contribution of APOA5 to the liver X receptor (LXR) ligand-mediated effect on plasma triglyceride levels. Following treatment with the LXR ligand T0901317, we found that APOA5 mRNA levels were decreased in hepatoma cell lines. The observation that no down-regulation of APOA5 promoter activity was obtained by LXR-retinoid X receptor (RXR) co-transfection prompted us to explore the possible involvement of the known LXR target gene SREBP-1c (sterol regulatory element-binding protein 1c). In fact, we found that co-transfection with the active form of SREBP-1c down-regulated APOA5 promoter activity in a dose-dependent manner. We then scanned the human APOA5 promoter sequence and identified two putative E-box elements that were able to bind specifically SREBP-1c in gel-shift assays and were shown to be functional by mutation analysis. Subsequent suppression of SREBP-1 mRNA through small interfering RNA interference abolished the decrease of APOA5 mRNA in response to T0901317. Finally, administration of T0901317 to hAPOA5 transgenic mice revealed a significant decrease of APOA5 mRNA in liver tissue and circulating apolipoprotein AV protein in plasma, confirming that the described down-regulation also occurs in vivo. Taken together, our results demonstrate that APOA5 gene expression is regulated by the LXR ligand T0901317 in a negative manner through SREBP-1c. These findings may provide a new mechanism responsible for the elevation of plasma triglyceride levels by LXR ligands and support the development of selective LXR agonists, not affecting SREBP-1c, as beneficial modulators of lipid metabolism.