Effect of Antiepileptic drugs on plasma lipoprotein (a) and other lipid levels in childhood

AhR, PXR and CAR: From Xenobiotic Receptors to Metabolic Sensors

Traditionally, xenobiotic receptors are known for their role in chemical sensing and detoxification, as receptor activation regulates the expression of various key enzymes and receptors. However, recent studies have highlighted that xenobiotic receptors also play a key role in the regulation of lipid metabolism and therefore function also as metabolic sensors. Since dyslipidemia is a major risk factor for various cardiometabolic diseases, like atherosclerosis and non-alcoholic fatty liver disease, it is of major importance to understand the molecular mechanisms that are regulated by xenobiotic receptors. In this review, three major xenobiotic receptors will be discussed, being the aryl hydrocarbon receptor (AhR), pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). Specifically, this review will focus on recent insights into the metabolic functions of these receptors, especially in the field of lipid metabolism and the associated dyslipidemia.

Atypical functions of xenobiotic receptors in lipid and glucose metabolism

Xenobiotic receptors are traditionally defined as xenobiotic chemical-sensing receptors, the activation of which transcriptionally regulates the expression of enzymes and transporters involved in the metabolism and disposition of xenobiotics. Emerging evidence suggests that "xenobiotic receptors" also have diverse endobiotic functions, including their effects on lipid metabolism and energy metabolism. Dyslipidemia is a major risk factor for cardiovascular disease, diabetes, obesity, metabolic syndrome, stroke, nonalcoholic fatty liver disease (NAFLD), and nonalcoholic steatohepatitis (NASH). Understanding the molecular mechanism by which transcriptional factors, including the xenobiotic receptors, regulate lipid homeostasis will help to develop preventive and therapeutic approaches. This review describes recent advances in our understanding the atypical roles of three xenobiotic receptors: aryl hydrocarbon receptor (AhR), pregnane X receptor (PXR), and constitutive androstane receptor (CAR), in metabolic disorders, with a particular focus on their effects on lipid and glucose metabolism. Collectively, the literatures suggest the potential values of AhR, PXR and CAR as therapeutic targets for the treatment of NAFLD, NASH, obesity and diabetes, and cardiovascular diseases.

Assessment of asymmetric dimethylarginine and homocysteine in epileptic children receiving antiepileptic drugs

BACKGROUND

Epilepsy is a neurological disease that requires long-term antiepileptic drugs (AEDs). The old generation of AEDs may affect serum homocysteine and asymmetric dimethylarginine (ADMA) and disturb lipid levels. The aim of the study was to evaluate serum ADMA, homocysteine, lipid profile, and carotid intima-media thickness (CIMT) in epileptic children.

METHODS

This study was implemented on 159 epileptic children who were subdivided into 3 subgroups, with 53 receiving sodium valproate, 53 receiving levetiracetam, and 53 receiving polytherapy, for over 6 months and 53 healthy children.

RESULTS

Low-density lipoprotein, triglycerides, and cholesterol levels were increased in epileptic children (p < 0.001), which were higher in those receiving multidrug followed by a valproate receiver. While high-density lipoprotein was lower in those receiving multidrug more than those receiving valproate. ADMA and homocysteine levels increased in epileptic patients than in controls (p < 0.001). Higher ADMA was also observed in the multidrug receiver (5.78 ± 0.62), followed by the levetiracetam group (5.56 ± 0.61). Homocysteine levels were significantly higher in multidrug and valproate-treated children than those treated with levetiracetam. CIMT was significantly higher in multidrug and valproate-treated patients (p < 0.001).

CONCLUSIONS

Long-term use of AEDs, especially old-generation polytherapy, can elevate lipid profiles, homocysteine, ADMA levels, and carotid intima-media thickness compared to the minimal effect of new AEDs.

IMPACT

The long-term use of antiepileptic drugs, especially old-generation polytherapy, can increase lipid profiles, homocysteine levels, ADMA, and carotid intima thickness compared to the minimal effect of new antiepileptic generation. A routine follow-up of these markers and a lifestyle modification are recommended to avoid cerebrovascular events as much as possible.

Activation of pregnane X receptor induces atherogenic lipids and PCSK9 by a SREBP2-mediated mechanism

BACKGROUND AND PURPOSE

Many drugs and environmental contaminants induce hypercholesterolemia and promote the risk of atherosclerotic cardiovascular disease. We tested the hypothesis that pregnane X receptor (PXR), a xenobiotic-sensing nuclear receptor, regulates the level of circulating atherogenic lipids in humans and utilized mouse experiments to identify the mechanisms involved.

EXPERIMENTAL APPROACH

We performed serum NMR metabolomics in healthy volunteers administered rifampicin, a prototypical human PXR ligand or placebo in a crossover setting. We used high-fat diet fed wild-type and PXR knockout mice to investigate the mechanisms mediating the PXR-induced alterations in cholesterol homeostasis.

KEY RESULTS

Activation of PXR induced cholesterogenesis both in pre-clinical and clinical settings. In human volunteers, rifampicin increased intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL) and total cholesterol and lathosterol-cholesterol ratio, a marker of cholesterol synthesis, suggesting increased cholesterol synthesis. Experiments in mice indicated that PXR activation causes widespread induction of the cholesterol synthesis genes including the rate-limiting Hmgcr and upregulates the intermediates in the Kandutsch-Russell cholesterol synthesis pathway in the liver. Additionally, PXR activation induced plasma proprotein convertase subtilisin/kexin type 9 (PCSK9), a negative regulator of hepatic LDL uptake, in both mice and humans. We propose that these effects were mediated through increased proteolytic activation of sterol regulatory element-binding protein 2 (SREBP2) in response to PXR activation.

CONCLUSION AND IMPLICATIONS

PXR activation induces cholesterol synthesis, elevating LDL and total cholesterol in humans. The PXR-SREBP2 pathway is a novel regulator of the cholesterol and PCSK9 synthesis and a molecular mechanism for drug- and chemical-induced hypercholesterolemia.

The Short-Term Effect of Ketogenic Diet on Carotid Intima-Media Thickness and Elastic Properties of the Carotid Artery and the Aorta in Epileptic Children

The aim of this prospective study is to investigate the effect of a 6-month-long ketogenic diet on carotid intima-media thickness, carotid artery, and aortic vascular functions. Thirty-eight drug-resistant epileptic patients who were being treated with ketogenic diet were enrolled. Fasting total cholesterol, high-density lipoprotein, low-density lipoprotein, triglycerides, total cholesterol, and glucose concentrations were measured and echocardiography was performed in all patients before the beginning of ketogenic diet and at the sixth month of treatment. The body weight, height, body mass index, serum levels of triglyceride, total cholesterol, and low-density lipoprotein increased significantly at month 6 when compared to baseline values (P < .05). Carotid intima-media thickness, elastic properties of the aorta, and carotid artery did not change at the sixth month of therapy compared to baseline values. A 6-month-long ketogenic diet has no effect on carotid intima-media thickness and elastic properties of the carotid artery and the aorta.

Effect of Carbamazepine and Valproate Monotherapy on Cardiovascular Risks in Epileptic Children

OBJECTIVE

We determined the effects of carbamazepine and valproic acid on the serum lipids and apolipoprotein A and B in epileptic children on long-term monotherapy and 3 months after drug discontinuation.

METHOD

Thirty-three epileptic children (17 boys, 16 girls, mean age 9.79 ± 2.5 years) were evaluated for serum lipids and lipoprotein results at the initiation of antiepileptic drug tapering and 3 months after cessation of antiepileptic therapy.

RESULTS

In the carbamazepine group (n = 13), there was no significant difference in the lipid profile at the end of therapy or at 3 months after the discontinuation, whereas in the valproate group (n = 20), triglycerides and apoprotein B and high-density lipoprotein cholesterol increased significantly 3 months after discontinuation. The ratios of total cholesterol:high-density lipoprotein improved but low-density lipoprotein:high-density lipoprotein and apolipoprotein:apolipoprotein remained unchanged.

CONCLUSION

Because these ratios are better predictor of atherosclerosis risk than the absolute values, the overall risk is not increased by the long-term use of carbamazepine and valproate.

Role of Omega-3 fatty acids in preventing metabolic disturbances in patients on olanzapine plus either sodium valproate or lithium: a randomized double-blind placebo-controlled trial

Background

Metabolic and cardiovascular side effects have been noted with the use of second generation antipsychotics (SGAs) and mood stabilizers. Since Omega-3 fatty acids have been known to prevent some cardiovascular risks, this preliminary study was designed to evaluate the cardiovascular benefits of omega-3 when added to the combinations of olanzapine with mood stabilizers.

Methods

This study was a randomized, double-blind, placebo-controlled, within-subject trial in adult psychiatric patients who were receiving olanzapine combined with lithium (Li) or valproate sodium (VPA). Omega-3 as fish oil with less than 1 g/day of EPA/DHA or its placebo was added to patients’ olanzapine and mood stabilizer regimens for 6 weeks. Metabolic parameters including anthropometric variables, lipid profile, metabolic syndrome indices, C-reactive protein, fibrinogen and lipoprotein (a) [(Lp) (a)] were assessed for participants.

Results

Forty one participants completed this study; 20 patients received omega-3 and 21 patients received placebo, added to their regimen of SGA and mood stabilizer. Omega-3 addition did not modulate anthropometric, metabolic syndrome and lipid parameter changes in 6 weeks. However, fibrinogen levels significantly decreased, Lp (a) did not increase and non-high-density lipoprotein cholesterol (non-HDL-C) did not go beyond its target level after omega-3 supplementation. Additionally, a significant inter-group effect was noted for Lp(a).

Conclusions

This study suggests that use of short-term omega-3 supplementation added to a combined regimen of olanzapine and mood stabilizer may have a small modulating effect on some cardiovascular risk factors. Trials in longer periods of time and with larger number of patients are needed to further evaluate the effects of omega-3 supplements on preventing cardiovascular risk factors.

This trial is registered at irct.ir and its Identifier is as following: IRCT138712231764N1

Evaluation of serum lipids and carotid artery intima media thickness in epileptic children treated with valproic acid

The aim of this study is to evaluate the carotid artery intima media thickness and serum lipids in pediatric patients with epilepsy treated with valproic acid. The study included 44 pediatric epileptic and 40 healthy children. Intima media thickness of left common carotid artery and fasting lipid profile (total cholesterol, triglycerides, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol) were assessed. Although we did not observe any differences regarding serum lipid profiles, intima media thickness of common carotid artery was significantly higher in epileptic patients treated with valproic acid. We suggest that this increase in intima media thickness of common carotid artery may be due to epilepsy and/or valproic acid treatment.

Thrombophilic risk factors in epileptic children treated with valproic Acid

There are few reports on valproic acid related to thrombophilia. Thrombophilic risk factors were investigated in 21 children (age range, 1-13 years) diagnosed with epilepsy and newly treated with valproic acid monotherapy. None of the children had been previously treated with any anticonvulsant agent. Before starting valproic acid therapy, homocysteine, lipoprotein(a), factor VIII, factor IX, protein C, protein S, antithrombin III levels, and activated protein C resistance levels were evaluated in all patients, with repeat evaluation after 9 months or 1 year of the therapy. Thrombosis gene mutations (factor V Leiden and prothrombin G20210A) were also evaluated in all patients before therapy. There was statistically significant elevation in lipoprotein(a) levels and reduction in fibrinogen levels after treatment. Reduction in protein C levels and elevation in homocysteine levels were also observed, but without statistical significance. Before therapy, no thrombotic event had occurred, despite thrombotic risk factors in some patients. Valproic acid can increase lipoprotein(a) and decrease fibrinogen, which may increase the risk of stroke or other thrombotic events. No clinical adverse effects resulted from changes in the levels or activity of thrombophilic factors associated with valproic acid treatment. Thus, routine investigation of factors implicated in thrombosis prior to initiation of valproic acid is not warranted for all patients. Nonetheless, caution is advised when initiating valproic acid treatment in children who have had prior stroke or thrombotic events.

Lipid profile, apolipoproteins A and B in children with epilepsy

In this study, lipid profile was assessed, and the intimal wall thickness of both carotid arteries in children with idiopathic epilepsy was measured. The study comprised 22 children with idiopathic epilepsy on either carbamazepine or valproate. A profile including triglycerides, total cholesterol, low-density and high-density lipoproteins cholesterol, apolipoproteins AI and B, and Duplex estimation of intimal wall thickness of carotid arteries were performed. Patients on carbamazepine showed increase in total cholesterol, low-density and high-density lipoproteins cholesterol, and decrease in apolipoprotein AI levels compared with controls. Levels of total cholesterol, low-density lipoproteins cholesterol, and apolipoprotein AI were reduced in patients receiving valproate compared with controls, whereas total cholesterol, high-density lipoproteins cholesterol, and apolipoprotein AI were reduced compared with those receiving carbamazepine. Carotid arteries intimal wall thickness was not significantly changed in any of the studied groups. Although atherogenic ratios are not changed, the lowered apolipoprotein AI levels may suggest an increased risk for coronary heart disease.

Growing old with epilepsy: the neglected issue of cognitive and brain health in aging and elder persons with chronic epilepsy

The purpose of this review is to examine what is known about cognitive and brain aging in elders with chronic epilepsy. We contend that much remains to be learned about the ultimate course of cognition and brain structure in persons with chronic epilepsy and concern appears warranted. Individuals with chronic epilepsy are exposed to many risk factors demonstrated to be associated with abnormal cognitive and brain aging in the general population, with many of these risk factors present in persons with chronic epilepsy as early as midlife. We suggest that a research agenda be developed to systematically identify and treat known modifiable risk factors in order to protect and promote cognitive and brain health in aging and elder persons with chronic epilepsy.

Early and persistent increase in serum lipoprotein (a) concentrations in epileptic children treated with carbamazepine and sodium valproate monotherapy

PURPOSE

The aim of this study was to investigate by a prospective, self-controlled method, whether treatment with carbamazepine (CBZ) and sodium valproate (VPA) monotherapy may alter serum lipoprotein (a) [Lp(a)] concentrations in epileptic children.

METHODS

Serum Lp(a) concentrations have been determined in 18 epileptic children before and at 6, 12 and 24 months of treatment with CBZ monotherapy and in 30 epileptic children before and at 6, 12 and 24 months of treatment with VPA monotherapy. Serum total cholesterol, triglycerides, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, apolipoproteins A-I and B concentrations and serum concentrations of biochemical markers of liver and renal function were also measured in the study participants.

RESULTS

Serum Lp(a) concentrations were significantly increased at 6, 12 and 24 months of CBZ and VPA monotherapy. There were no significant correlations between serum Lp(a) and serum lipids, lipoproteins, apolipoproteins, concentrations of biochemical markers of liver and renal function or antiepileptic-drugs concentrations.

CONCLUSIONS

Children who receive CBZ or VPA monotherapy may have significant and persistent increase in serum lipoprotein (a) concentrations, occuring early in the course of therapy. It may be useful to measure serum Lp(a) concentrations routinely in epileptic children taking these antiepileptic drugs, especially in those that are already at higher atherosclerotic risk.

Lipids, Lipoproteins, Apolipoproteins, Selected Trace Elements and Minerals in the Serum of Children on Valproic Acid Monotherapy

We evaluated the serum levels of lipids, lipoproteins, apolipoproteins, along with a number of minerals and trace elements such as Ca, Mg, Cu and Zn in a group of children after 6 months of valproic acid monotherapy. Thirty patients with seizures, mean age, 9.8+/-2.6 years and 79 healthy children (controls), mean age, 10.9+/-3.2 years, formed the two styd groups. The patient group was treated with valproic acid (27.9+/-14.8 mg/kg/24 hr). Patients underwent clinical and laboratory evaluations including liver function tests, NH3, lipid, mineral and selected trace element levels before and after six months on valproic acid treatment, whereas controls only one evaluation. Liver function data and NH3 levels were found to be elevated in the group of patients, whereas albumin level was reduced. Triglycerides, total cholesterol, HDL-C, apolipoprotein (ApoA)-1, Apo B and Ca concentrations were found relative to control values, LDL-C, VLDL-C, Mg, Cu, Zn, were measured significantly altered (P<0.0001) compared to controls. The ratios ApoA-1/ApoB, HDL-C/ApoA-1, LDL-C/Apo B, which were closely related to the size of LDL particles, where correlated with Zn/Cu (P<0.001). Serum lipid profile, especially LDL size, indirectly evaluated for the first time and metal levels were found to be significantly changed, after six months on valproic acid monotherapy, suggesting a possible risk of developing coronary heart disease. Since valproic acid is a long-term treatment, it could be recommended that the incorporation of measurements of lipids, lipoproteins, apolipoproteins and trace elements in the "follow up" laboratory testing could be a preventive measure.

Evolution of serum lipids and lipoprotein (a) levels in epileptic children treated with carbamazepine, valproic acid, and phenobarbital

The concentration levels of serum lipids and lipoprotein (a) were measured in 20 children receiving carbamazepine, 25 children receiving valproic acid, and 5 children receiving phenobarbital at the following times: (1) during chronic treatment while eating a normal diet, (2) during chronic treatment while eating a low-fat diet (children treated with carbamazepine and phenobarbital with high levels of total cholesterol, high-density lipoprotein cholesterol, and low-density lipoprotein cholesterol), and (3) 3 months after the end of treatment with antiepileptic drugs. Patients during chronic treatment and eating a normal diet revealed significant changes in lipids, but when we reevaluated the groups of children treated with carbamazepine and phenobarbital when they were eating a low-fat diet and reevaluated the three groups of children 3 months after the end of treatment, a complete return to normal of all parameters was observed. These data demonstrate that the changes induced by these drugs are transient, reversible, and influenced by a low-fat diet. (J Child Neurol 2006;21:48-53).

Effect of antiepileptic drugs on plasma lipids, lipoprotein (a), and liver enzymes

We conducted a study to assess the effect of phenobarbital, carbamazepine, and valproate on serum lipid profiles and lipoprotein (a) in 64 children with epilepsy (aged between 1 and 15 years) admitted to the child neurology outpatient clinic between July 2000 and July 2002. The children were separated as group 1 (18 children), treated with phenobarbital, 5 mg/kg/day; group 2 (22 children), treated with carbamazepine, 10 to 15 mg/kg/day; and group 3 (24 children), treated with sodium valproate, 20 mg/kg/day. Plasma lipoprotein (a), total cholesterol, triglycerides, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, apolipoprotein A and apolipoprotein B levels, and liver enzymes alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, and gamma-glutamyltransferase were determined before the initiation of the treatment and at 3, 6, and 12 months of the treatment period. The mean age of children in group 1 was significantly low compared with those in groups 2 and 3 (P <.05). The mean pretreatment lipid levels among the groups were not significantly increased. The mean lipoprotein (a) levels were significantly increased in all groups at 3, 6, and 12 months of the treatment period (P <.05). The increase in alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, total cholesterol, low-density lipoprotein cholesterol, and high-density lipoprotein cholesterol at 3, 6, and 12 months was statistically significant in group 1 (P <.05). The higher levels in lipoprotein (a) (mean > 30 mg/dL) were observed only in carbamazepine-treated patients at 6 and 12 months. The percentage of children with lipoprotein (a) levels over 30 mg/dL was 44%, 63%, and 33% in the phenobarbital-, carbamazepine-, and valproate-treated children, respectively. Antiepileptic drugs significantly increase the level of lipoprotein (a), which is a major risk factor for atherosclerosis, and also have variable effects on other lipid parameters. Lipoprotein (a) levels should be closely followed in patients receiving antiepileptic drugs. (J Child Neurol 2006;21:70-74).

Expression of the pregnane X receptor in mice antagonizes the cholic acid-mediated changes in plasma lipoprotein profile

OBJECTIVE

Modification of lipoprotein metabolism by bile acids has been mainly explained by activation of the farnesyl X receptor (FXR). The aim of the present study was to determine the relative contribution of the pregnane X receptor (PXR), another bile acid-activated nuclear receptor to changes in plasma lipoprotein profile.

METHODS AND RESULTS

Wild-type mice, Pxr-deficient mice, and Pxr-null mice expressing human PXR (Pxr-null SXR-Tg mice) were fed a cholic acid-containing diet, and consequences on plasma lipoprotein profiles and target gene expression were assessed. Cholic acid produced significant decreases in high-density lipoprotein (HDL) cholesterol, plasma apolipoprotein (apo)A-I and hepatic apoA-I mRNA in wild-type mice. Interestingly, the effect of cholic acid was significantly more pronounced in Pxr-deficient mice, indicating that PXR contributes to the weakening of the effect of bile acids on lipoprotein metabolism. Reciprocally, changes in HDL/apoA-I profiles were abolished in Pxr-null SXR-Tg mice in which PXR-responsive genes, particularly those involved in bile acid detoxification were readily activated after cholic acid treatment.

CONCLUSIONS

PXR expression in mice antagonizes the cholic acid-mediated downregulation of plasma HDL cholesterol and apoA-I, and magnification of PXR/SXR-mediated changes may constitute a new mean to counteract the effects of bile acids on plasma lipoproteins.

LXR deficiency and cholesterol feeding affect the expression and phenobarbital-mediated induction of cytochromes P450 in mouse liver

Metabolic transformation by the superfamily of cytochromes P450 (CYPs) plays an important role in the detoxification of xenobiotics such as drugs, environmental pollutants, and food additives. Endogenous substrates of CYPs include fatty acids, sterols, steroids, and bile acids. Induction of CYPs via transcriptional activation by substrates and other xenobiotics is an important adaptive mechanism that increases the organism's defense capability against toxicity. Numerous in vivo and in vitro data have highlighted the concept that the molecular mechanism of hepatic drug induction is linked to endogenous regulatory pathways. In particular, in vitro data suggest that oxysterols via the liver X receptor (LXR) inhibit phenobarbital (PB)-mediated induction of CYPs. To study the link between LXR, cholesterol homeostasis, and drug induction in vivo, we designed experiments in wild-type, LXRalpha-, LXRbeta-, and LXRalpha/beta-deficient mice. Our data expose differential regulatory patterns for Cyp2b10 and Cyp3a11 dependent on the expression of LXR isoforms and on challenge of cholesterol homeostasis by excess dietary cholesterol. Our results suggest that, in the mouse, liver cholesterol status significantly alters the pattern of expression of Cyp3a11, whereas the absence of LXR leads to an increase in PB-mediated activation of Cyp2b10.

Effect of serum cholesterol on the mRNA content of amyloid precursor protein in rat livers

Genes that showed mRNA content profiles, which correlated with serum concentrations of total cholesterol (T.CHO), were screened from the microarray data of phenobarbital (PB)- or clofibrate (CLO)-treated rat livers, and the correlation was evaluated based on Spearman's correlation coefficient. Many genes involved in the cholesterol or bile acid metabolism were highly correlated such as UDP-glucuronosyltransferase-21, apolipoprotein A-I and cMOAT. The mRNA content of the amyloid precursor protein (APP) showed the 5th highest correlation among the 8799 probes in the Affymetrix Rat Genome U34 Array. In the livers of rats fed a high-cholesterol (1%) diet for 33 days, serum T.CHO levels increased by 4.6-fold, and the hepatic APP mRNA content also increased by 1.9-fold compared to the control group. These data suggest that the hepatic APP mRNA content was affected by serum T.CHO, and that hepatic APP was involved in cholesterol metabolism in rat livers.

The role of the nuclear receptor CAR as a coordinate regulator of hepatic gene expression in defense against chemical toxicity

The nuclear receptor CAR (constitutive active receptor) mediates the induction of transcription of cytochrome P450 (CYP) genes by phenobarbital (PB) and PB-type inducers. A recent study using CAR-null mice has shown that CAR regulates not only the CYP genes but also other genes encoding various drug/steroid-metabolizing enzymes. In addition to coordinating these enzymes, CAR plays other roles in hepatic gene expression: CAR represses various genes including carnitine palmitoyltransferase 1a and phosphoenolpyruvate carboxykinase 1 in response to PB, and the receptor regulates the constitutive expression of genes such as squalene epoxidase. On the other hand, induction of certain genes such as amino levulinate synthase 1 by PB is not regulated by CAR. Here we describe diverse roles of CAR in hepatic gene expression with a particular focus on endogenous substances such as cholesterol, bilirubin, and steroid hormones.

Cholesterol and bile acids regulate xenosensor signaling in drug-mediated induction of cytochromes P450

Cytochromes P450 (CYP) constitute the major enzymatic system for metabolism of xenobiotics. Here we demonstrate that transcriptional activation of CYPs by the drug-sensing nuclear receptors pregnane X receptor, constitutive androstane receptor, and the chicken xenobiotic receptor (CXR) can be modulated by endogenous cholesterol and bile acids. Bile acids induce the chicken drug-activated CYP2H1 via CXR, whereas the hydroxylated metabolites of bile acids and oxysterols inhibit drug induction. The cholesterol-sensing liver X receptor competes with CXR, pregnane X receptor, or constitutive androstane receptor for regulation of drug-responsive enhancers from chicken CYP2H1, human CYP3A4, or human CYP2B6, respectively. Thus, not only cholesterol 7 alpha-hydroxylase (CYP7A1), but also drug-inducible CYPs, are diametrically affected by these receptors. Our findings reveal new insights into the increasingly complex network of nuclear receptors regulating lipid homeostasis and drug metabolism.

A Link between cholesterol levels and phenobarbital induction of cytochromes P450

Squalestatin1 (SQ1), a potent inhibitor of squalene synthase produced a dose-dependent induction of cytochromes P450 CYP2H1 and CYP3A37 mRNAs in chicken hepatoma cells. The effect of SQ1 was completely reversed by 25-hydroxycholesterol. Bile acids elicited an induction of CYP3A37 and CYP2H1 mRNA. Bile acids also reduced the phenobarbital induction of CYP2H1 but not of CYP3A37 mRNA. The effects of SQ1 and its reversal by 25-hydroxycholesterol and the effects of bile acids were reproduced in reporter gene assays with a phenobarbital-responsive enhancer unit of CYP2H1. These data suggest that an endogenous molecule related to cholesterol homeostasis regulates induction of drug-inducible CYPs.

Diverse roles of the nuclear orphan receptor CAR in regulating hepatic genes in response to phenobarbital

Phenobarbital (PB) induces various gene encoding drug/steroid-metabolizing enzymes such as cytochromes P450 (P450s) and transferases. Although the nuclear orphan constitutive active receptor (CAR) has been identified as a key transcription factor that regulates the induction of CYP2B, the full scope of CAR-regulated genes still remains a major question. To this end, reverse transcriptase-polymerase chain reaction and cDNA microarray techniques were employed to examine gene expression in wild-type and CAR-null mice. The results show that a total of 138 genes were detected to be either induced or repressed in response to PB treatment, of which about half were under CAR regulation. Including CYP2B10, CYP3A11, and NADPH-CYP reductase, CAR regulated a group of the PB-induced drug/steroid-metabolizing enzymes. Enzymes such as amino levulinate synthase 1 and squalene epoxidase displayed CAR-independent induction by PB. Cyp4a10 and Cyp4a14 represented the group of genes induced by PB only in CAR-null mice, indicating that CAR may be a transcription blocker that prevents these genes from being induced by PB. Additionally, the group of genes encoding enzymes and proteins involved in basic biological processes such as energy metabolism underwent the CAR-dependent repression by PB. Thus, CAR seems to have diverse roles, both as a positive and negative regulator, in the regulation of hepatic genes in response to PB beyond drug/steroid metabolism.