Review
Induction of drug metabolism by nuclear receptor CAR: molecular mechanisms and implications for drug research

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Abstract

Recent findings indicate that induction of drug metabolism is regulated by activation of specific members of the nuclear receptor gene family. This minireview deals with the mechanisms by which phenobarbital and phenobarbital-type chemicals induce cytochrome P450 and other genes, and summarises the knowledge on the role of the constitutively active receptor CAR in the induction process. The potential implications of CAR-mediated induction for drug research and possible uses of CAR are also discussed.

Introduction

The metabolism by cytochrome P450 (CYP) enzymes is a widely employed defense mechanism against exposure to xenochemicals including drugs (Conney, 1982). A central part of this defense is the adaptive increase of CYP gene expression (=induction) which leads to enhanced metabolism and termination of the pharmacological action of drugs (Okey, 1990). The induction mechanisms for major drug metabolising CYP genes have been studied intensively, and recent findings indicate that a common general pathway is utilised: Exposure to drugs activates specific members of the nuclear receptor (NR) superfamily which in turn bind to their cognate DNA elements and stimulate the CYP target gene transcription (Johnson et al., 1996, Kliewer et al., 1999, Honkakoski and Negishi, 2000). This leads to increased synthesis of CYP enzymes and enhanced metabolism and clearance of the drugs (Fig. 1). Similar mechanisms regulate CYP enzymes that metabolise endogenous compounds such as retinoids, vitamin D3, and cholesterol (Honkakoski and Negishi, 2000).

Because CYP enzymes can either eliminate drugs and xenochemicals or activate them to more harmful products, CYP induction may lead to drug–drug interactions, especially with the prominently expressed CYP3A enzymes (Thummel and Wilkinson, 1998), or to toxic and carcinogenic reactions. In broader context, drug exposure may also affect cell signalling, cell growth, and tumour promotion. These processes may be modulated by NR-mediated signalling or by altered CYP-mediated metabolism of endobiotic signalling molecules (Nebert, 1991, Whysner et al., 1996, Gonzalez et al., 1998).

Section snippets

Background

Phenobarbital (PB) has long been known to induce drug metabolism in laboratory animals and humans. Many other structurally diverse drugs and chemicals display similar profiles of enzyme induction (Okey, 1990, Waxman and Azaroff, 1992). PB induction mainly involves mammalian CYP forms in gene families 2B and 3A but expression of other drug metabolising enzymes and genes of unknown function are also modulated (Waxman and Azaroff, 1992, Frueh et al., 1997, Honkakoski and Negishi, 1998). It was

CAR is activated by multiple chemicals

PBREM is activated by virtually all PB-type inducers ranging from drugs to pesticides to solvents in mouse and rat primary hepatocytes (Honkakoski et al., 1998a, Ganem et al., 1999) with excellent correlation with induction of endogenous CYP2B mRNA. Consistently, PB-type inducers could reactivate the transfected PBREM-reporter and endogenous CYP2B6 genes in 3α-androstenol-suppressed CAR-expressing HepG2 cells (Sueyoshi et al., 1999) and translocate cytoplasmic CAR to the nucleus in mouse

Species and inter-individual differences

While the ligand binding specificity and induction mechanism of mouse CAR are becoming quite established, the properties of human CAR are less clear. As expected from the high degree of similarity within their DNA binding domains, both mouse and human CAR bind to similar DNA elements (Baes et al., 1994, Choi et al., 1997, Honkakoski et al., 1998b, Sueyoshi et al., 1999). Amino acid differences in the ligand binding domain are significant (72% amino acid identity) which implies that the ligand

Conclusions

The nuclear receptor CAR has emerged as one of the major regulators that activates CYP and other genes involved in drug metabolism. The mechanisms and roles of CAR in induction of drug metabolism and other biological processes are being actively pursued and this research is expected to bring insights into how organisms sense and react to exposure by drugs. Potential applications of CAR include novel methods to screen for induction of drug metabolism and perhaps a switch in protocols requiring

Acknowledgements

The authors thank their colleagues at NIEHS and University of Kuopio for collaboration and fruitful discussions. A part of this paper was presented at the International Symposium on the Induction of Drug Metabolism: From Molecular Mechanisms to Drug Approval, Stockholm, December 9–10, 1999. PH is supported by the Academy of Finland (award no. 66655).

References (42)

  • R. Ramsden et al.

    Phenobarbital responsiveness conferred by the 5′-flanking region of the rat CYP2B2 gene in transgenic mice

    Gene

    (1999)
  • T. Sueyoshi et al.

    The repressed nuclear receptor CAR responds to phenobarbital in activating the human CYP2B6 gene

    J. Biol. Chem.

    (1999)
  • E. Trottier et al.

    Localization of a phenobarbital-responsive element (PBRE) in the 5′-flanking region of the rat CYP2B2 gene

    Gene

    (1995)
  • J. Whysner et al.

    Phenobarbital mechanistic data and risk assessment: enzyme induction, enhanced cell proliferation, and tumor promotion

    Pharmacol. Ther.

    (1996)
  • R.V. Abruzzese et al.

    Ligand-dependent regulation of plasmid-based transgene expression in vivo

    Hum. Gene Ther.

    (1999)
  • M. Baes et al.

    A new orphan member of the nuclear hormone receptor superfamily that interacts with a subset of retinoic acid response elements

    Mol. Cell. Biol.

    (1994)
  • A.H. Conney

    Induction of microsomal enzymes by foreign chemicals and carcinogenesis by polyaromatic hydrocarbons

    Cancer Res.

    (1982)
  • B.M. Forman et al.

    Androstane metabolites bind to and deactivate the nuclear receptor CAR-β

    Nature

    (1998)
  • F.W. Frueh et al.

    Extent and character of phenobarbital-induced changes in gene expression in the liver

    Mol. Pharmacol.

    (1997)
  • F.J. Gonzalez et al.

    Mechanism of action of the nongenotoxic peroxisome proliferators: role of the peroxisome proliferator-activator receptor α

    J. Natl. Cancer Inst.

    (1998)
  • D.B. Gower et al.

    Olfaction in humans with special reference to odorous 16-androstenes: their occurrence, perception and possible social, psychological and sexual impact

    J. Endocrinol.

    (1993)
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