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DHA down-regulates phenobarbital-induced cytochrome P450 2B1 gene expression in rat primary hepatocytes by

Chien-Chun Li1, Chong-Kuei Lii, Kai-Li Liu

  • 1Department of Nutrition, Chung Shan Medical University, Taichung, Taiwan.

Toxicology and Applied Pharmacology
|October 2, 2007
PubMed

Insights

Docosahexaenoic acid (DHA) down-regulates phenobarbital-induced CYP 2B1 expression by inhibiting the translocation of the constitutive androstane receptor (CAR) into the nucleus, impacting its binding to gene regulatory sites.

Area of Science:

  • Molecular Pharmacology
  • Hepatocyte Biology
  • Drug Metabolism

Background:

  • Constitutive androstane receptor (CAR) regulates detoxifying enzymes like cytochrome P450 2B (CYP 2B).
  • Phenobarbital (PB) induces CYP 2B expression via CAR.
  • Polyunsaturated fatty acids (PUFAs), particularly DHA, were previously found to down-regulate PB-induced CYP 2B1 expression, but the mechanism was unclear.

Purpose of the Study:

  • To investigate if altered CAR translocation mediates the down-regulation of PB-induced CYP 2B1 expression by n-6 and n-3 PUFAs in rat primary hepatocytes.
  • To elucidate the molecular mechanism by which DHA affects CAR activity and CYP 2B1 expression.

Main Methods:

  • Primary rat hepatocytes were treated with PB and various PUFAs (arachidonic acid, linoleic acid, eicosapentaenoic acid, DHA).
  • CAR translocation was assessed by analyzing its distribution in cytosolic and nuclear fractions.
  • CYP 2B1 expression levels were quantified.
  • Immunoprecipitation and Electrophoretic Mobility Shift Assay (EMSA) were used to study CAR/RXR heterodimer binding to the CYP 2B1 gene promoter.

Main Results:

  • PB induced CAR translocation from the cytosol to the nucleus in a dose- and time-dependent manner.
  • DHA significantly decreased nuclear CAR accumulation and increased cytosolic CAR levels, in a dose-dependent manner.
  • DHA treatment dose-dependently reduced PB-induced CYP 2B1 expression and attenuated the binding of CAR to the NR-1 site of the CYP 2B1 gene promoter.

Conclusions:

  • Attenuation of CAR translocation from the cytosol to the nucleus is a key mechanism by which DHA down-regulates PB-induced CYP 2B1 expression.
  • DHA reduces the binding of the CAR/RXR heterodimer to the PB-responsive enhancer module (PBREM) of the CYP 2B1 gene.
  • These findings provide novel insights into the regulation of drug-metabolizing enzymes by dietary fatty acids at the level of nuclear receptor signaling.

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