Introduction to the Theme "New Insights, Strategies, and Therapeutics for Common Diseases".

The reviews in Volume 62 of the Annual Review of Pharmacology and Toxicology (ARPT) cover a diverse range of topics. A theme that encompasses many of these reviews is their relevance to common diseases and disorders, including type 2 diabetes, heart failure, cancer, tuberculosis, Alzheimer's disease, neurodegenerative disorders, and Down syndrome. Other reviews highlight important aspects of therapeutics, including placebos and patient-centric approaches to drug formulation. The reviews with this thematic focus, as well as other reviews in this volume, emphasize new mechanistic insights, experimental and therapeutic strategies, and novel insights regarding topics in the disciplines of pharmacology and toxicology. As the editors of ARPT, we believe that these reviews help advance those disciplines and, even more importantly, have the potential to improve the health care of the world's population.

Introduction to the Theme "Old and New Toxicology: Interfaces with Pharmacology".

The theme of Volume 61 is "Old and New Toxicology: Interfaces with Pharmacology." Old toxicology is exemplified by the authors of the autobiographical articles: B.M. Olivera's work on toxins and venoms from cone snails and P. Taylor's studies of acetylcholinesterase and the nicotinic cholinergic receptor, which serve as sites of action for numerous pesticides and venoms. Other articles in this volume focus on new understanding and new types of toxicology, including (a) arsenic toxicity, which is an ancient poison that, through evolution, has caused most multicellular organisms to express an active arsenic methyltransferase to methylate arsenite, which accelerates the excretion of arsenic from the body; (b) small molecules that react with lipid dicarbonyls, which are now considered the most toxic oxidative stress end products; (c) immune checkpoint inhibitors (ICIs), which have revolutionized cancer therapy but have numerous immune-related adverse events, including cardiovascular complications; (d) autoimmunity caused by the environment; (e) idiosyncratic drug-induced liver disease, which together with the toxicity of ICIs represents new toxicology interfacing with pharmacology; and (f) sex differences in the development of cardiovascular disease, with men more susceptible than women to vascular inflammation that initiates and perpetuates disease. These articles and others in Volume 61 reflect the interface and close integration of pharmacology and toxicology that began long ago but continues today.

Introduction to the Theme "New Therapeutic Targets".

"New Therapeutic Targets" is the theme of articles in the Annual Review of Pharmacology and Toxicology, Volume 59. Reviews in this volume discuss targets for a variety of conditions in need of new therapies, including type 2 diabetes, heart failure with preserved ejection fraction, obesity, thyroid-associated ophthalmopathy, tinnitus, multiple sclerosis, Parkinson's disease and other neurodegenerative diseases, pain, depression, post-traumatic stress disorder, muscle wasting diseases, cancer, and anemia associated with chronic renal disease. Numerous articles in this volume focus on the identification, validation, and utility of novel therapeutic targets, in particular, ones that involve new or unexpected molecular entities. This theme complements several previous themes, including "New Approaches for Studying Drug and Toxicant Action: Applications to Drug Discovery and Development," "Precision Medicine and Prediction in Pharmacology," and "New Methods and Novel Therapeutic Approaches in Pharmacology and Toxicology."

The 9th Santorini Conference: Systems Medicine, Personalised Health and Therapy. "The Odyssey from Hope to Practice", Santorini, Greece, 30 September⁻3 October 2018.

The 9th traditional biannual conference on Systems Medicine, Personalised Health & Therapy-"The Odyssey from Hope to Practice", inspired by the Greek mythology, was a call to search for practical solutions in cardio-metabolic diseases and cancer, to resolve and overcome the obstacles in modern medicine by creating more interactions among disciplines, as well as between academic and industrial research, directed towards an effective 'roadmap' for personalised health and therapy. The 9th Santorini Conference, under the Presidency of Sofia Siest, the director of the INSERM U1122; IGE-PCV (www.u1122.inserm.fr), University of Lorraine, France, offered a rich and innovative scientific program. It gathered 34 worldwide distinguished speakers, who shared their passion for personalised medicine with 160 attendees in nine specific sessions on the following topics: First day: The Odyssey from hope to practice: Personalised medicine-landmarks and challenges Second day: Diseases to therapeutics-genotype to phenotype an "-OMICS" approach: focus on personalised therapy and precision medicine Third day: Gene-environment interactions and pharmacovigilance: a pharmacogenetics approach for deciphering disease "bench to clinic to reality" Fourth day: Pharmacogenomics to drug discovery: a big data approach and focus on clinical data and clinical practice. In this article we present the topics shared among the participants of the conference and we highlight the key messages.

The Sterolgene v0 cDNA microarray: a systemic approach to studies of cholesterol homeostasis and drug metabolism.

BACKGROUND: Cholesterol homeostasis and xenobiotic metabolism are complex biological processes, which are difficult to study with traditional methods. Deciphering complex regulation and response of these two processes to different factors is crucial also for understanding of disease development. Systems biology tools as are microarrays can importantly contribute to this knowledge and can also discover novel interactions between the two processes. RESULTS: We have developed a low density Sterolgene v0 cDNA microarray dedicated to studies of cholesterol homeostasis and drug metabolism in the mouse. To illustrate its performance, we have analyzed mouse liver samples from studies focused on regulation of cholesterol homeostasis and drug metabolism by diet, drugs and inflammation. We observed down-regulation of cholesterol biosynthesis during fasting and high-cholesterol diet and subsequent up-regulation by inflammation. Drug metabolism was down-regulated by fasting and inflammation, but up-regulated by phenobarbital treatment and high-cholesterol diet. Additionally, the performance of the Sterolgene v0 was compared to the two commercial high density microarray platforms: the Agilent cDNA (G4104A) and the Affymetrix MOE430A GeneChip. We hybridized identical RNA samples to the commercial microarrays and showed that the performance of Sterolgene is comparable to commercial arrays in terms of detection of changes in cholesterol homeostasis and drug metabolism. CONCLUSION: Using the Sterolgene v0 microarray we were able to detect important changes in cholesterol homeostasis and drug metabolism caused by diet, drugs and inflammation. Together with its next generations the Sterolgene microarrays represent original and dedicated tools enabling focused and cost effective studies of cholesterol homeostasis and drug metabolism. These microarrays have the potential of being further developed into screening or diagnostic tools.

Regulatory cross-talk between drug metabolism and lipid homeostasis: constitutive androstane receptor and pregnane X receptor increase Insig-1 expression.

Activation of pregnane X receptor (PXR) and constitutive androstane receptor (CAR) by xenobiotic inducers of cytochromes P450 is part of a pleiotropic response that includes liver hypertrophy, tumor promotion, effects on lipid homeostasis, and energy metabolism. Here, we describe an acute response to CAR and PXR activators that is associated with induction of Insig-1, a protein with antilipogenic properties. We first observed that activation of CAR and PXR in mouse liver results in activation of Insig-1 along with reduced protein levels of the active form of sterol regulatory element binding protein 1 (Srebp-1). Studies in mice deficient in CAR and PXR revealed that the effect on triglycerides involves these two nuclear receptors. Finally, we identified a functional binding site for CAR and PXR in the Insig-1 gene by in vivo, in vitro, and in silico genomic analysis. Our experiments suggest that activation Insig-1 by drugs leads to reduced levels of active Srebp-1 and consequently to reduced target gene expression including the genes responsible for triglyceride synthesis. The reduction nuclear Srebp-1 by drugs is not observed when Insig-1 expression is repressed by small interfering RNA. In addition, observed that Insig-1 is also a target of AMP-activated kinase, the hepatic activity of which is increased by activators of CAR and PXR and is known to cause a reduction of triglycerides. The fact that drugs that serve as CAR or PXR ligands induce Insig-1 might have clinical consequences and explains alterations lipid levels after drug therapy.

Effects of rifampicin on global gene expression in human small intestine.

OBJECTIVES: The small intestinal wall serves as an important barrier for the entry of foreign substances into the organism. Of particular importance are enzymes and transporters that can inactivate or prevent the uptake of many xenobiotics including drugs. Some of the genes encoding these proteins are transcriptionally activated by xenobiotics, a response well studied in liver but less so in the intestine. The effect of the inducer drug rifampicin on intestinal cells was therefore evaluated both in vivo and in vitro. METHODS: Seven healthy volunteers were treated with rifampicin for 9 days and the global gene expression profile was analysed in RNA from duodenal biopsies taken before and after drug treatment. The gene expression profile was also assessed in LS174T cells derived from a human colon adenocarcinoma after exposure to 10 micromol/l rifampicin for 24 h. RESULTS: We identified 32 genes that were upregulated and two genes that were downregulated by rifampicin treatment in vivo. The list of rifampicin regulated transcripts expectedly included drug metabolizing enzymes and drug transporters, but also genes involved in lipid and amino acid metabolism as well as genes not previously recognized to be part of the adaptation of intestinal cells to xenobiotic exposure. Only a limited number of these rifampicin-regulated transcripts were however also regulated by rifampicin in LS174T cells. CONCLUSION: The similarities and differences of changes in gene expression after rifampicin treatment between duodenal biopsies and cell culture provide a new assessment of the extent and diversity of systems affected by drug exposure.

Dehydroepiandrosterone induces human CYP2B6 through the constitutive androstane receptor.

Dehydroepiandrosterone (DHEA), the major precursor of androgens and estrogens, has several beneficial effects on the immune system, on memory function, and in modulating the effects of diabetes, obesity, and chemical carcinogenesis. Treatment of rats with DHEA influences expression of cytochrome P450 (P450) genes, including peroxisome proliferator-activated receptor alpha (PPAR alpha)- and pregnane X receptor (PXR)-mediated induction of CYP4As and CYP3A23, and suppression of CYP2C11. DHEA treatment elevated the expression and activities of CYP3A4, CYP2C9, CYP2C19, and CYP2B6 in primary cultures of human hepatocytes. Induction of CYP3A4 in human hepatocytes was consistent with studies in rats, but induction of CYP2Cs was unexpected. The role of PXR in this response was studied in transient transfection assays. DHEA activated hPXR in a concentration-dependent manner. Because CYP2B6 induction by DHEA in human hepatocytes might involve either PXR or constitutive androstane receptor (CAR) activation, we performed experiments in primary hepatocytes from CAR knockout mice and observed that CAR was required for maximal induction of Cyp2b10 by DHEA. Furthermore, CAR-mediated Cyp2b10 induction by DHEA was inhibited by the inverse agonist of CAR, androstanol (5 alpha-androstan-3 alpha-ol). Further evidence for CAR activation was provided by cytoplasmic/nuclear transfer of CAR upon DHEA treatment. Elucidation of CAR activation and subsequent induction of CYP2B6 by DHEA presented an additional mechanism by which the sterol can modify the expression of P450s. The effect of DHEA on the activation of the xenosensors PPAR alpha, PXR, and CAR, and the consequent potential for adverse drug/toxicant interactions should be considered in humans treated with this nutriceutical agent.

Stimulation of AMP-activated protein kinase is essential for the induction of drug metabolizing enzymes by phenobarbital in human and mouse liver.

Our previous studies have suggested a role for AMP-activated protein kinase (AMPK) in the induction of CYP2B6 by phenobarbital (PB) in hepatoma-derived cells (Rencurel et al., 2005). In this study, we showed in primary human hepatocytes that: 1) 5'-phosphoribosyl-5-aminoimidazol-4-carboxamide 1-beta-d-ribofuranoside and the biguanide metformin, known activators of AMPK, dose-dependently increase the expression of CYP2B6 and CYP3A4 to an extent similar to that of PB. 2) PB, but not the human nuclear receptor constitutive active/androstane receptor (CAR) ligand 6-(4-chlorophenyl)imidazol[2,1-6][1,3]thiazole-5-carbaldehyde, dose-dependently increase AMPK activity. 3) Pharmacological inhibition of AMPK activity with compound C or dominant-negative forms of AMPK blunt the inductive response to phenobarbital. Furthermore, in transgenic mice with a liver-specific deletion of both the alpha1 and alpha2 AMPK catalytic subunits, basal levels of Cyp2b10 and Cyp3a11 mRNA were increased but not in primary culture of mouse hepatocytes. However, phenobarbital or 1,4 bis[2-(3,5-dichloropyridyloxy)]benzene, a mouse CAR ligand, failed to induce the expression of these genes in the liver or cultured hepatocytes from mice lacking hepatic expression of the alpha1 and alpha2 subunits of AMPK. The distribution of CAR between the nucleus and cytosol was not altered in hepatocytes from mice lacking both AMPK catalytic subunits. These data highlight the essential role of AMPK in the CAR-mediated signal transduction pathway.

Nutritional regulation of hepatic heme biosynthesis and porphyria through PGC-1alpha.

Inducible hepatic porphyrias are inherited genetic disorders of enzymes of heme biosynthesis. The main clinical manifestations are acute attacks of neuropsychiatric symptoms frequently precipitated by drugs, hormones, or fasting, associated with increased urinary excretion of delta-aminolevulinic acid (ALA). Acute attacks are treated by heme infusion and glucose administration, but the mechanisms underlying the precipitating effects of fasting and the beneficial effects of glucose are unknown. We show that the rate-limiting enzyme in hepatic heme biosynthesis, 5-aminolevulinate synthase (ALAS-1), is regulated by the peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha). Elevation of PGC-1alpha in mice via adenoviral vectors increases the levels of heme precursors in vivo as observed in acute attacks. The induction of ALAS-1 by fasting is lost in liver-specific PGC-1alpha knockout animals, as is the ability of porphyrogenic drugs to dysregulate heme biosynthesis. These data show that PGC-1alpha links nutritional status to heme biosynthesis and acute hepatic porphyria.

Three haplotypes associated with CYP2A6 phenotypes in Caucasians.

The human cytochrome P450 2A6 (CYP2A6) enzyme metabolizes several xenobiotic compounds of clinical or toxicological importance. We aimed to identify genetic variants and major CYP2A6 haplotypes associated with CYP2A6 phenotypic variation. CYP2A6 mRNA level, protein level, activity and haplotypes were determined in Caucasian liver samples via real-time polymerase chain reaction, Western blot, coumarin 7-hydroxylation, DNA sequencing and genotyping, respectively. Phenotypes were then analyzed for associations with haplotypes. CYP2A6 transcript, protein and activity levels were correlated among each other. In 45 African-American, 156 Caucasian, 47 Chinese, 50 Japanese and 47 Korean DNA samples, we detected 95 different polymorphisms in the CYP2A6 gene, 49 of which had not been described previously. Caucasian variants formed 33 haplotypes which built four clades. Allele *9B and the CYP2A7/2A6 partial deletion allele CYP2A6*12B were both associated with decreased expression. The latter haplotype extends at least over 147 kb up into the CYP2B6 gene. A haplotype almost identical to allele *1A was associated with decreased expression and activity of CYP2A6 compared to all other haplotypes. In summary A CYP2A6*1A-like allele, *9B and *12B are major genetic determinants of CYP2A6 phenotype variation in Caucasians.

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.

The evolution of drug-activated nuclear receptors: one ancestral gene diverged into two xenosensor genes in mammals.

BACKGROUND: Drugs and other xenobiotics alter gene expression of cytochromes P450 (CYP) by activating the pregnane X receptor (PXR) and constitutive androstane receptor (CAR) in mammals. In non-mammalian species, only one xenosensor gene has been found. Using chicken as a model organism, the aim of our study was to elucidate whether non-mammalian species only have one or two xenosensors like mammals. RESULTS: To explore the evolutionary aspect of this divergence, we tried to identify additional xenobiotic sensing nuclear receptors in chicken using various experimental approaches. However, none of those revealed novel candidates. Ablation of chicken xenobiotic receptor (CXR) function by RNAi or dominant-negative alleles drastically reduced drug-induction in a chicken hepatoma cell line. Subsequently, we functionally and structurally characterized CXR and compared our results to PXR and CAR. Despite the high similarity in their amino acid sequence, PXR and CAR have very distinct modes of activation. Some aspects of CXR function, e.g. direct ligand activation and high promiscuity are very reminiscent of PXR. On the other hand, cellular localization studies revealed common characteristics of CXR and CAR in terms of cytoplasmic-nuclear distribution. Finally, CXR has unique properties regarding its regulation in comparison to PXR and CAR. CONCLUSION: Our finding thus strongly suggest that CXR constitutes an ancestral gene which has evolved into PXR and CAR in mammals. Future studies should elucidate the reason for this divergence in mammalian versus non-mammalian species.

Nuclear receptors constitutive androstane receptor and pregnane X receptor activate a drug-responsive enhancer of the murine 5-aminolevulinic acid synthase gene.

Nuclear receptors have been implicated in the transcriptional regulation of expression of a growing number of genes, including cytochromes P450 and 5-aminolevulinate synthase (ALAS1), the first and rate-limiting enzyme in the heme biosynthesis pathway. Although drugs that induce cytochromes P450 also induce ALAS1, the regulatory mechanisms governing these pathways have not been fully elucidated. We have identified a drug-responsive enhancer in the murine ALAS1 gene. This sequence mediates transcriptional activation by a wide range of compounds including typical cytochrome P450 pan-inducers phenobarbital and metyrapone, as well as specific activators of the pregnane X receptor and the constitutive androstane receptor. ALAS1 drug-responsive enhancer sequences were identified by transient transfection of reporter gene constructs in the drug-responsive leghorn male hepatoma cell line. Using the NUBIScan algorithm, DR4 nuclear receptor binding sites were identified within the elements and their roles in mediating transcriptional activation of ALAS1 were confirmed by site-directed mutagenesis. Electrophoretic mobility shift assays demonstrate clear interactions of mouse pregnane X receptor and constitutive androstane receptor on the ADRES. Transactivation assays in CV-1 cells implicate the nuclear receptors as major contributors to transcriptional activation of ALAS1. Moreover, in vivo studies in knock-out animals confirm the induction of ALAS1 is mediated at least in part by nuclear receptors. These studies are the first to explain drug induction via drug response elements for mammalian ALAS1.

Molecular cloning and characterization of chicken orphan nuclear receptor cTR2.

Orphan nuclear receptors belong to the nuclear receptor superfamily of liganded transcription factors, whose ligands either do not exist or remain to be identified. We report here the cloning and characterization of the chicken orphan nuclear receptor, cTR2 (chicken testicular receptor 2). The cTR2 gene encodes a protein of 569 amino acids which shows approximately 72% overall identity with TR2 (NR2C1) and 95% identity in the DNA-binding domain (DBD). The cTR2 gene is expressed in almost all adult tissues and embryonic stages examined unlike its mammalian relative TR2, which is specifically expressed in testis. Electrophoretic mobility shift assays demonstrate that cTR2 binds the canonical direct repeat DNA recognition sequences spaced by one, four, and five nucleotides (DR1, DR4, and DR5), and in consistence with the results with canonical DNA-binding sequences, cTR2 forms specific DNA-protein complex with chicken phenobarbital response elements containing DR4 motifs. Both in vitro and in vivo interaction studies demonstrate that cTR2 forms homodimer. Moreover, transient transfection studies reveal its capability to transactivate canonical DR1, DR4, and DR5 sequences and the constitutive activity of cTR2 is mapped to the N-terminal region of this orphan receptor. Finally, cTR2 represses transactivation of estrogen receptor in a dose-dependent manner.

Drugs mediate the transcriptional activation of the 5-aminolevulinic acid synthase (ALAS1) gene via the chicken xenobiotic-sensing nuclear receptor (CXR).

Heme is an essential component in oxygen transport and metabolism in living systems. In non-erythropoietic cells, 5-aminolevulinate synthase (ALAS1) is the first and rate-limiting enzyme in the heme biosynthesis pathway. ALAS1 expression and heme levels are increased in vivo by drugs and other chemical inducers of cytochrome P450 hemoproteins through mechanisms that are poorly understood. In the present studies, a chicken genomic cosmid library was employed to isolate a major portion of the ALAS1 gene. Two drug-responsive enhancer sequences, 176 and 167 base pairs in length, were identified in the 5'-flanking region of the gene in reporter gene assays in the hepatoma cell line LMH. The relative potency of inducers to activate these enhancers corresponds to induction of ALAS1 mRNA levels in LMH cells. Analysis of putative transcription factor binding sites within the enhancers revealed DR5 and DR4 type recognition sequences for nuclear receptors. Drug activation of the enhancer elements was reduced at least 60% after mutagenesis of individual nuclear receptor binding sites and was virtually eliminated following alteration of both recognition sites within the respective elements. Electrophoretic mobility shift assays and transactivation studies demonstrate direct interactions between the nuclear receptor binding sites and the recently described chicken xenobiotic-sensing receptor, (CXR) implicating drug activation mechanisms for ALAS1 similar to those found in inducible cytochrome(s) P450. This is the first report describing direct transcriptional activation of ALAS1 by drugs via drug-responsive enhancer sequences.

NUBIScan, an in silico approach for prediction of nuclear receptor response elements.

Nuclear receptors (NRs) are transcription factors activated by a multitude of hormones, other endogenous substances, and exogenous molecules. These proteins modulate the regulation of target genes by contacting their promoter or enhancer sequences at specific recognition sites. The identification of these response elements is the first step toward detailed insight into the regulatory mechanisms affecting a gene. We have developed NUBIScan, a computer algorithm to predict DNA recognition sites for NRs in the regulatory regions of genes. The algorithm is based on weighted nucleotide distribution matrices and combines scores from both half-sites necessary for NR dimer binding. It provides more specific identification of functional sites than previous in silico approaches, as evidenced by scanning published regulatory regions of drug-inducible genes and comparing the obtained predictions with experimental results. In prospective analyses, NUBIScan consistently identified new functional NR binding sites in sets of large sequences, which had eluded previous analyses. This is exemplified by the detailed functional analysis of the flanking region of two genes. This approach therefore facilitates the selection of likely sites of gene regulation for subsequent experimental analysis.

Human organic anion transporting polypeptide 8 promoter is transactivated by the farnesoid X receptor/bile acid receptor.

BACKGROUND & AIMS: OATP8 (gene symbol: SLC21A8) is a multispecific uptake system for organic anions, xenobiotics, and peptides expressed at the basolateral (sinusoidal) membrane of human hepatocytes. We investigated whether OATP8 gene expression is regulated by the nuclear receptors farnesoid X receptor/bile acid receptor (FXR/BAR; NR1H4), pregnane X receptor (PXR), or liver X receptor (LXR). METHODS: OATP8 promoter function was studied in reporter assays. OATP8 expression in cells was quantitated by real-time polymerase chain reaction. RESULTS: The bile acid chenodeoxycholic acid (CDCA), a ligand of FXR/BAR, but not clotrimazole or 25-hydroxycholesterol, ligands of PXR or LXR, respectively, induced OATP8 promoter activity. An inverted hexanucleotide repeat motif (IR-1 element) in the promoter sequence was shown by electrophoretic mobility shift assays to bind the FXR (9-cis-retinoic acid receptor [RXRalpha]) heterodimer. Targeted mutagenesis of the IR-1 element abolished inducibility of the OATP8 promoter by CDCA, confirming its role as a bile acid response element. CDCA treatment increased OATP8 messenger RNA levels in human hepatoma cells, suggesting a physiologic role for FXR-mediated OATP8 gene regulation. CONCLUSIONS: OATP8 gene expression is regulated by bile acids via FXR/BAR. Induction of OATP8 could serve to maintain hepatic extraction of xenobiotics and peptides in conditions of increased intracellular bile acids.

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.