Ketoconazole and 25-hydroxycholesterol produce reciprocal changes in the rate of transcription of the human LDL receptor gene

Tissue-specific regulation of pregnane X receptor in cancer development and therapy

As a ligand-dependent transcription factor of the nuclear hormone receptor superfamily, the pregnane X receptor (PXR) has a multitude of functions including regulating xenobiotic and cholesterol metabolism, energy homeostasis, gut mucosal defense, and cancer development. Whereas the detoxification functions of PXR have been widely studied and well established, the role of PXR in cancer has become controversial. With more than 60% of non-prescription and prescription drugs being metabolized by cytochrome P450 enzyme 3A4 (CYP3A4), a transcriptional target of PXR, insights into the regulation of PXR during systemic administration of novel treatment modalities will lead to a better understanding of PXR function in the context of human disease. Previous studies have suggested that PXR activation decreases drug sensitivity and augments chemoresistance in certain colon cancers mainly through the upregulation of CYP3A4 and multidrug resistance protein-1 (MDR1). Later studies suggest that downregulation of PXR expression may be oncogenic in hormone-dependent breast and endometrial cancers by reducing estrogen metabolism via CYP3A4; thus, higher estradiol concentrations contribute to carcinogenesis. These results suggest a differential role of PXR in tumor growth regulation dependent on tissue type and tumor microenvironment. Here, we will summarize the various mechanisms utilized by PXR to induce its diverse effects on cancerous tissues. Moreover, current approaches will be explored to evaluate the exploitation of PXR-mediated pathways as a novel mechanistic approach to cancer therapy.

Circadian regulation of low density lipoprotein receptor promoter activity by CLOCK/BMAL1, Hes1 and Hes6

Low density lipoprotein receptor (LDLR) plays an important role in the cholesterol homeostasis. We examined the possible circadian regulation of LDLR and mechanism(s) underlying it. In mice, blood glucose and plasma triglyceride, total and high density lipoprotein cholesterol varied distinctively throughout a day. In addition, LDLR mRNA oscillated in the liver in a functional clock-dependent manner. Accordingly, analysis of human LDLR promoter sequence revealed three putative E-boxes, raising the possible regulation of LDLR expression by E-box-binding transcription factors. To test this possibility, human LDLR promoter reporter constructs were transfected into HepG2 cells and the effects of CLOCK/BMAL1, Hes1, and Hes6 expression were analyzed. It was found that positive circadian transcription factor complex CLOCK/BMAL1 upregulated human LDLR promoter activity in a serum-independent manner, while Hes family members Hes1 and Hes6 downregulated it only under serum-depleted conditions. Both effects were mapped to proximal promoter region of human LDLR, where mutation or deletion of well-known sterol regulatory element (SRE) abolished only the repressive effect of Hes1. Interestingly, hes6 and hes1 mRNA oscillated in an anti-phasic manner in the wild-type but not in the per1-/-per2 -/- mouse. Comparative analysis of mouse, rat and human hes6 genes revealed that three E-boxes are conserved among three species. Transfection and site-directed mutagenesis studies with hes6 reporter constructs confirmed that the third E-box in the exon IV is functionally induced by CLOCK/BMAL1. Taken together, these results suggest that LDLR expression is under circadian control involving CLOCK/BMAL1 and Hes family members Hes1 and Hes6.

Activated pregnenolone X-receptor is a target for ketoconazole and its analogs

PURPOSE

Variations in biotransformation and elimination of microtubule-binding drugs are a major cause of unpredictable side effects during cancer therapy. Because the orphan receptor, pregnenolone X-receptor (PXR), coordinately regulates the expression of paclitaxel metabolizing and transport enzymes, controlling this process could improve therapeutic outcome.

EXPERIMENTAL DESIGN

In vitro RNA-, protein-, and transcription-based assays in multiple cell lines derived from hepatocytes and PXR wild-type and null mouse studies were employed to show the effects of ketoconazole and its analogues on ligand-activated PXR-mediated gene transcription and translation.

RESULTS

The transcriptional activation of genes regulating biotransformation and transport by the liganded human nuclear xenobiotic receptor, PXR, was inhibited by the commonly used antifungal ketoconazole and related azole analogs. Mutations at the AF-2 surface of the human PXR ligand-binding domain indicate that ketoconazole may interact with specific residues outside the ligand-binding pocket. Furthermore, in contrast to that observed in PXR (+/+) mice, genetic loss of PXR results in increased (preserved) blood levels of paclitaxel.

CONCLUSIONS

These studies show that some azole compounds repress the coordinated activation of genes involved in drug metabolism by blocking PXR activation. Because loss of PXR maintains blood levels of paclitaxel upon chronic dosing, ketoconazole analogues may also serve to preserve paclitaxel blood levels on chronic dosing of drugs. Our observations may facilitate new strategies to improve the clinical efficacy of drugs and to reduce therapeutic side effects.

Inhibition of drug metabolism by blocking the activation of nuclear receptors by ketoconazole

Individual variation in drug metabolism is a major cause of unpredictable side effects during therapy. Drug metabolism is controlled by a class of orphan nuclear receptors (NRs), which regulate expression of genes such as CYP (cytochrome)3A4 and MDR-1 (multi-drug resistance-1), that are involved in this process. We have found that xenobiotic-mediated induction of CYP3A4 and MDR-1 gene transcription was inhibited by ketoconazole, a commonly used antifungal drug. Ketoconazole mediated its effect by inhibiting the activation of NRs, human pregnenolone X receptor and constitutive androstene receptor, involved in regulation of CYP3A4 and MDR-1. The effect of ketoconazole was specific to the group of NRs that control xenobiotic metabolism. Ketoconazole disrupted the interaction of the xenobiotic receptor PXR with the co-activator steroid receptor co-activator-1. Ketoconazole treatment resulted in delayed metabolism of tribromoethanol anesthetic in mice, which was correlated to the inhibition of PXR activation and downmodulation of cyp3a11 and mdr-1 genes and proteins. These studies demonstrate for the first time that ketoconazole represses the coordinated activation of genes involved in drug metabolism, by blocking activation of a specific subset of NRs. Our results suggest that ketoconazole can be used as a pan-antagonist of NRs involved in xenobiotic metabolism in vivo, which may lead to novel strategies that improve drug effect and tolerance.

Mitochondrial cholesterol transport: A possible target in the management of hyperlipidemia

Sterol 27-hydroxylase (CYP27A1) may defend cells against accumulation of excess cholesterol, making this enzyme a possible target in the management of hyperlipidemia. The study objective was to analyze cholesterol homeostatic responses to increases in CYP27A1 activity in HepG2 cells and primary human hepatocytes. Increasing CYP27A1 activity by increasing enzyme expression led to significant increases in bile acid synthesis with compensatory increases in HMG-CoA reductase (HMGR) activity/protein, LDL receptor (LDLR) mRNA, and LDLR-mediated cholesterol uptake. Under these conditions, only a small increase in cellular 27-hydroxycholesterol (27OH-Chol) concentration was observed. No changes were detected in mature sterol regulatory element-binding proteins (SREBP) 1 or 2. Increasing CYP27A1 activity by increasing mitochondrial cholesterol transport (i.e., substrate availability) led to greater increases in bile acid synthesis with significant increases in cellular 27OH-Chol concentration. Mature SREBP 2 protein decreased significantly with compensatory decreases in HMGR protein. No change was detected in mature SREBP 1 protein. Despite increasing 27OH-Chol and lowering SREBP 2 protein concentrations, LDLR mRNA increased significantly, suggesting alternative mechanisms of LDLR transcriptional regulation. These findings suggest that regulation of liver mitochondrial cholesterol transport represents a potential therapeutic strategy in the treatment of hyperlipidemia and atherosclerosis.

Coadministration of ketoconazole and cyclosporine for kidney transplant recipients: long-term follow-up and study of metabolic consequences

In a prospective randomized study including 100 kidney transplant recipients, we previously reported on the safety and financial benefits of the coadministration of ketoconazole (keto) to cyclosporine (CsA)-treated kidney transplant recipients. In this study, we report on the long-term follow-up of these patients and their control group, as well as possible metabolic consequences of this drug combination. Evaluation of 51 keto-treated patients and their control group (49 patients) included graft function, lipogram, fasting blood glucose, liver function tests, serum calcium, phosphorus, and radiological and histopathologic assessments. Follow-up of these patients for 54 months showed that the CsA dose reduction was 72.9% at 12 months, decreased to 69.3% at the last follow-up. We also found that the mean keto dose required for CsA dose reduction decreased to 82.8 +/- 24.1 mg/d compared with the starting dose (100 mg/d). Diagnosis of acute rejection episodes was similar in both groups. However, in the control group, rejection episodes were more recurrent, with poorer response to treatment. Acute CsA nephrotoxicity was more common in the keto group, but this was encountered more at keto induction and was rapidly reversed on further reduction of CsA doses. Chronic graft dysfunction was statistically significantly less in the keto group during the first year. However, by the end of the study, the difference was not statistically significant. In this study, hepatotoxicity was similar in the two groups. On studying the metabolic consequences, we found that serum cholesterol, low-density lipoprotein, and triglyceride levels were lower in the keto group. Bone mineral contents in both groups were less than the mean values for age- and sex-matched healthy controls. From this study, we conclude that long-term use of low-dose keto in CsA-treated kidney transplant recipients is safe and cost-saving and may induce better graft function. Bone mineral contents, vitamin D blood levels, and lipid profiles are not affected by long-term keto coadministration in CsA-treated kidney transplant recipients.

Oxysterols: modulators of cholesterol metabolism and other processes

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

Up-regulation of low density lipoprotein receptor by a novel isobenzofranone derivative, MD-700

Stimulatory effects of a novel isobenzofranone, MD-700, on low density lipoprotein (LDL) receptor activity were investigated in vitro and in vivo. MD-700 at 0.03 microg/ml elevated the expression of LDL receptor in HepG2 cells within 4 h. Corresponding to this, uptake of fluorescent labeled-LDL (3,3'-dioctadecylindocarbocyanine-LDL) by the cells increased linearly in time- and dose-dependent manner by MD-700 for up to 12 h. In the experiment using HepG2 cells transiently transfected with promoter-luciferase gene constructs, MD-700 increased luciferase activity in a dose-dependent manner from 0.03 to 0.1 microg/ml. In contrast, luciferase activity was not stimulated by MD-700 in construct with a deleted sterol regulatory element (SRE)-1, suggesting importance of SRE-1 in stimulation of the LDL receptor gene promoter by MD-700. Binding experiments on liver membranes from MD-700-treated hamsters showed about a 60% increase in 125I-labeled LDL binding. A Scatchard plot revealed that MD-700 increased the maximal binding without affecting binding affinity. In contrast to findings with an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, pravastatin, MD-700 had no effect on the sterol synthesis in hamster liver homogenates. These results suggest that MD-700 stimulates the expression of LDL receptor, presumably in a manner independent of change in sterol metabolism, and thereby promotes LDL clearance. Hypocholesterolemic actions of MD-700 in hamsters were then examined. MD-700 lowered serum cholesterol levels in hamsters fed normal chow or a high-fat diet. Fractionation of serum lipoproteins demonstrated that MD-700 selectively decreased LDL and very low density lipoprotein cholesterol. Dose-dependent decrease in serum cholesterol was also seen in hypercholesterolemic rats. Thus, the hypocholesterolemic action of MD-700 may be attributed to up-regulation of the LDL receptor, based on stimulation of the transcription of the LDL receptor gene. Although pravastatin stimulates LDL uptake and lowers serum cholesterol in a manner similar to that seen with MD-700, the mechanism responsible for hypocholesterolemic action appears to differ.

Effect of coffee lipids (cafestol and kahweol) on regulation of cholesterol metabolism in HepG2 cells

We studied the effect of the coffee diterpene alcohols, cafestol and kahweol, on cholesterol metabolism in HepG2 cells. Uptake of 125I-tyramine cellobiose-labeled LDL was decreased by 15% to 20% (P < .05) after 18 hours of preincubation with cafestol (20 micrograms/mL), whereas 25-hydroxycholesterol reduced uptake by 55% to 65% (P < .05). Degradation of LDL in the presence of cafestol was decreased by 20% to 30% (P < .05) under the same conditions. The effect of cafestol (20 micrograms/mL) on uptake and degradation of LDL was greatest (35% to 40%, P < .05) after 6 and 10 hours of preincubation, respectively. Furthermore, the effect of cafestol was also dependent on its concentration, and a significant decrease in the LDL uptake (19%) was observed at 10 micrograms/mL (P < .05). Specific binding of LDL was reduced by 17% (P < .05) and 60% (P < .05) after preincubation with cafestol (20 micrograms/mL) and 25-hydroxycholesterol (5 micrograms/mL) for 6 hours, respectively, compared with control cells. Analysis of LDL binding showed that cafestol reduced the number of binding sites for LDL on the cell surface (capacity) by 35% (P < .05). In contrast, no significant effect on the level of mRNA for the LDL receptor was observed after incubation with cafestol, whereas 25-hydroxycholesterol reduced the mRNA level for the LDL receptor by 40% to 50% (P < .05). A fusion gene construct consisting of a synthetic sterol regulatory element-1 (SRE-1) promoter for the human LDL receptor coupled to the reporter gene for chloramphenicol acetyltransferase (CAT) was transfected into HepG2 cells. No change was observed in CAT activity in SRE-1-transfected cells after incubation with cafestol, whereas 25-hydroxycholesterol reduced CAT activity by 30% to 40% (P < .05). Incorporation of [14C]acetate into unesterified cholesterol and 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity were unaffected in cells incubated with cafestol as well as the cafestol-kahweol mixture compared with control cells. Moreover, cafestol and the cafestol-kahweol mixture did not promote increased incorporation of radiolabeled [14C]oleic acid into cholesteryl esters after short-term incubation compared with control cells. On the other hand, 25-hydroxycholesterol caused a 70% to 90% reduction of cholesterol synthesis (P < .05) and HMG-CoA reductase activity (P < .05), decreased HMG-CoA reductase mRNA level by 70% to 80% (P < .05), and promoted a twofold increase in cholesterol esterification (P < .05). Finally, no effect of the coffee diterpenes on bile acid formation was observed. These results suggest that cafestol (and kahweol) may reduce the activity of hepatic LDL receptors and thereby cause extracellular accumulation of LDL.

New low-density lipoprotein receptor upregulators acting via a novel mechanism

The synthesis and biological activity of a new series of benzamides and related compounds that upregulate the expression of the low-density lipoprotein (LDL) receptor in human hepatocytes (HepG2 cells) by a novel mechanism are described. The lead compound, N-[5-[(3-cyclohexylpropionyl)amino]-2-methylphenyl]-4-hydroxybe nzamide (1, RPR102359), increased the expression of the LDL receptors in HepG2 cells by 80% when tested at a concentration of 3 microM. Mevinolin (lovastatin) was found to increase the LDL receptor expression by 70% at the same concentration. In contrast to mevinolin, 1 was found to have no effect on cholesterol biosynthesis in liver homogenates or in HepG2 cells at doses where substantial upregulation of the LDL receptor was observed and thus stimulated LDL receptor expression by a novel mechanism.

Review of progress in sterol oxidations: 1987-1995

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

Clinical observations of metabolic changes occurring in renal transplant recipients receiving ketoconazole

Metabolism of cyclosporine is reduced by ketoconazole binding to the monooxygenase responsible for cyclosporine degradation. This isozyme of cytochrome P450, along with other similar monooxygenases, is involved in the regulation of the synthesis and degradation of important metabolic pathways of cholesterol. Monooxygenases throughout these pathways are inhibited by ketoconazole binding causing a decreased metabolism of calcitriol, bile acids, and steroid hormones, and can thereby potentiate altered lipid metabolism, bone metabolism, and weight status of transplant recipients. A group of renal transplant recipients taking ketoconazole (n=25) was compared with a matched cohort not receiving ketoconazole for metabolic changes during the first six months posttransplantation. Lower LDL cholesterol levels were seen in the ketoconazole group (109 +/- 8 mg/dl) than the no ketoconazole group (140 +/- 8 mg/dl) at one month but this difference was not sustained at six months. More bone loss occurred in the ketoconazole group as demonstrated by significant changes in bone density as well as a greater urinary appearance of bone collagen crosslink, deoxy-pyridinoline (29 +/- 4 nmol dpd/mmol creatinine and 18 +/- 4 at six months for the ketoconazole group versus the no ketoconazole group, respectively, P<0.05). Weight gain changes were different between the ketoconazole group and no ketoconazole group (6.4 +/- 1.4 kg versus 5.0 +/- 1.3 kg) at six months and an increased rate of weight gain over time in the ketoconazole group (0.02 kg/day at one month versus 0.05 kg/day at six months, P<0.007). Effectiveness of ketoconazole inhibition of cyclosporine is valuable, but inhibition of other metabolic pathways should be evaluated as well.

Tyrosine kinase inhibitors potentiate the induction of low density lipoprotein receptor gene expression by hepatocyte growth factor

The role of tyrosine kinase, protein kinase C, cyclic nucleotide- and Ca(2+)-calmodulin-dependent protein kinase second messenger pathways in the induction of LDL receptor gene expression by hepatocyte growth factor (HGF) was studied in the human hepatoma cell line Hep-G2. Incubation with media containing HGF increased the level of LDL receptor mRNA by 6.5-fold. Co-incubation with HGF and either of two tyrosine kinase inhibitors genistein (2.0-20.0 micrograms/ml) and herbimycin A (0.5-500.0 ng/ml) increased the level of LDL receptor mRNA above that observed with HGF alone by 40-60%. Incubation with HGF in the presence of the calmodulin antagonist W7 (10-30 microM) also super-induced the level of LDL receptor mRNA by nearly 230%. The protein kinase C and A inhibitors chelerythrine (0.1-10.0 microM) and H8 (0.5-5.0 microM), respectively, had no significant effects on the induction of LDL receptor mRNA by HGF. Taken together, these data suggest that tyrosine kinase, protein kinases C and A, and Ca(2+)-calmodulin dependent protein kinase activities are not essential for activation of LDL receptor gene expression in Hep-G2 cells by HGF.