Role of hepatitis B virus X repression of C/EBPβ activity in the down-regulation of glutathioneS-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/EBPbeta 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/EBPbeta activator), but not that by tert-butylhydroquinone (t-BHQ, Nrf2 activator). Moreover, HBx transfection completely inhibited GSTA2 reporter gene activity induced by C/EBPbeta, but failed to inhibit that by Nrf2. 4. Gel shift assays identified that HBx inhibited the increase in C/EBPbeta-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/EBPbeta. Moreover, chromatin immunoprecipitation assays confirmed HBx inhibition of C/EBPbeta 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 gamma-glutamylcysteine synthase. 5. These results demonstrate that HBx inhibits the induction of phase II detoxifying enzymes, which is mediated by its interaction with C/EBPbeta, but not Nrf2, substantiating the specific role of HBx in phase II detoxifying capacity.

Changes in Glutathione Content in Liver Diseases: An Update

“…Furthermore, HBV infection is able to change the expression levels of antioxidant enzymes in an Nrf2-/ARE independent fashion. For example, HBx inhibited GSTA2, GSTM1 and GSTM2 gene expression by C/EBP-mediated mechanism [275]. Another way for HBx to induce oxidative stress is the suppression of proteins indirectly involved in the antioxidant defence, as observed in HBx-transfected HepG2 cells for the inhibition of selenoprotein P (SeP) [276] and in HBV-transgenic mice for selenium-binding protein 2 (Selenbp2) [274].…”

Oxidative stress, a trigger of hepatitis C and B virus-induced liver carcinogenesis

“…HBx can inhibit expression of phase II enzymes not only by epigenetic mechanisms, but also by interfering with regulatory elements/factors other than Nrf2/ARE. For example, HBx was shown to prevent the expression of genes encoding phase II enzymes in response to agents that activate C/EBP elements in their promoters [296]. Interestingly, for GSTP1, such inhibition occurs in the case of genotype D of the virus, but not genotypes A-C [294].…”

Effect of Hepatitis Viruses on the Nrf2/Keap1-Signaling Pathway and Its Impact on Viral Replication and Pathogenesis

“…In light of the relevance of the Nrf2/ARE system for the process of liver regeneration and of the pathogenesis of an active chronic HBV infection that is associated with fibrosis and cirrhosis [278], it is obvious that the effect of HBV on the Nrf2/ARE system is of major interest. The literature describing this point is conflicting but this depends in part on the experimental systems that were used [119,279,280,281,282]. Therefore, several experimental settings have to be distinguished.…”