Role of hepatitis B virus X repression of C/EBPβ activity in the down-regulation of glutathione S-transferase A2 gene: implications in other phase II detoxifying enzyme expression

Abstract

1. A genome-wide in silico screening rendered the genes of phase II enzymes in the rat genome whose promoters contain the putative DNA elements interacting with CCAAT/enhancer binding protein (C/EBP) and NF-E2-related factor (Nrf2). The hepatitis B virus X (HBx) protein strongly modulates the transactivation and/or the repression of genes regulated by some bZIP transcription factors.

2. This study investigated the effects of HBx on the induction of phase II enzymes with the aim of elucidating the role of HBx interaction with C/EBPβ or Nrf2 bZIP transcription factors in hepatocyte-derived cells.

3. Immunoblot and reporter gene analyses revealed that transfection of HBx interfered with the constitutive and inducible GSTA2 transactivation promoted by oltipraz (C/EBPβ activator), but not that by tert-butylhydroquinone (t-BHQ, Nrf2 activator). Moreover, HBx transfection completely inhibited GSTA2 reporter gene activity induced by C/EBPβ, but failed to inhibit that by Nrf2.

4. Gel shift assays identified that HBx inhibited the increase in C/EBPβ−DNA complex formation by oltipraz, but not the increase in Nrf2−DNA complex by t-BHQ. Immunoprecipitation and immunoblot assays verified the direct interaction between HBx and C/EBPβ. Moreover, chromatin immunoprecipitation assays confirmed HBx inhibition of C/EBPβ binding to its binding site in the GSTA2 gene promoter. HBx repressed the induction of other phase II enzymes including GSTP, UDP-glucuronyltransferase 1A, microsomal epoxide hydrolase, GSTM1, GSTM2, and γ-glutamylcysteine synthase.

5. These results demonstrate that HBx inhibits the induction of phase II detoxifying enzymes, which is mediated by its interaction with C/EBPβ, but not Nrf2, substantiating the specific role of HBx in phase II detoxifying capacity.

Keywords::

• In silico promoter analysis
• Phase II detoxifying enzymes
• Hepatitis B virus X (HBx)
• CCAAT/enhancer binding protein β (C/EBPβ)
• NF-E2 related factor 2 (Nrf2)

Acknowledgments

This work was supported by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (Grant No. R11-2007-107-01001-0), Korea.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.