Differences in the target specificity of the transactivating factors MHBst and HBx of hepatitis B virus

Abstract

Experiments with tissue culture cells revealed that carboxy-terminaUy truncated middle surface antigen (MHBs t) and HBx of hepatitis B virus (HBV) are able to act as transcriptional transactivators (Twu & Schloemer, 1987;Wollersheim et al., 1988;Zahm et al., 1988;Caselmann et al., 1990; Kekul6 et al., 1990). The transactivating function of MHBs t and HBx is directed to many known viral and cellular enhancer and promoter elements. Synopsis of epidemiological and molecular studies led to the assumption that, possibly based on their transactivating property, MHBs t and HBx contribute to the onset or development of hepatocellular carcinoma in individuals who, as a consequence of previous infection with HBV, carry chromosomally integrated viral DNA (for reviews see Tiollais et al., 1985;Ganem & Varmus, 1987;Beasley, 1988;Koshy & Meyer, 1992).The molecular mechanism that underlies the transactivating function of MHBs t and HBx is not known. Recent observations suggested that the generation of reactive oxygen intermediates is required for the activation of nuclear factor-(NF) xB by MHBs t and HBx, and that protein kinase C is involved in the stimulation of activator protein-1 (AP-1) by HBx (Meyer et al., 1992; Kekul~ et al., 1993). It has not been directly examined * Author for correspondence. Present address: German Heart Center Munich, Department of Experimental Cardiology, Lothstrasse 11, D-80335 Munich, Germany. Fax +49 891209549.whether MHBs t and HBx have differences in their target specificities. In theory, the existence of at least one enhancer element that can be stimulated only by one but not by the other of the HBV-derived transactivators would indicate the existence of, at least in part, alternate cellular signal transducing pathways for MHBs t and HBx.To address this issue, we directly compared the ability of MHBs t and HBx to perform their transactivating function, using selected, transcription factor-binding sequence elements present in the enhancer of simian virus 40 (SV40) and in the enhancer of the human metallothionein IIA (hMT IIA) gene. Chang liver cells (Chang, 1954) were cotransfected with transactivator plasmid or non-transactivating control plasmid and one of several reporter plasmids. Transactivator plasmids were pMHBs ta°l, encoding MHBs t and pHBV824, encoding HBx. Control plasmids were pMHBs, encoding fulllength MHBs, and pHBV824fs, encoding, in theory, a short amino-terminal fragment of HBx and aminoterminally truncated fragments of HBx initiating at methionine residues 79 and 103 (Fig.

The hepatitis B virus MHBst167 protein is a pleiotropic transactivator mediating its effect via ubiquitous cellular transcription factors.

“…Besides activation of viral elements like the simian virus 40 (SV40) early enhancer , the LTRs of Rous sarcoma virus (RSV), human immunodeficiency virus-1 (HIV-1) and human T cell lymphotropic virus type I (HTLV-I ; Meyer et al, 1992 ;Lauer et al, 1994), stimulation of the promoter sequences of the oncogenes c-myc Natoli et al, 1992 ;Lauer et al, 1994), c-fos (Meyer et al, 1992, Natoli et al, 1992Lauer et al, 1994 ;Schlu$ ter et al, 1994) and the interleukin-6 gene (Meyer et al, 1992) has been shown. Furthermore, MHBs t mediates its transactivating effect via factors binding to the serumresponsive element (SRE), to 12-O-tetradecanoylphorbol 13-acetate-responsive elements (TREs ; Natoli et al, 1992), NF-κB- (Meyer et al, 1992 ;Natoli et al, 1992 ;Lauer et al, 1994 ;Mu$ hlbauer & Koch, 1995), AP1- (Hildt et al, 1993 ;Lauer et al, 1994 ;Mu$ hlbauer & Koch, 1995) and AP2- (Lauer et al, 1994) binding sites.…”

Interaction of Hepatitis B Virus with Cellular Processes in Liver Carcinogenesis

“…While there may be some detailed differences between the transactivators, such as transactivation through specific sequences of the SV40 enhancer, 198 elucidation of the mechanisms of HBVencoded proteins in carcinogenesis (LHBs, truncated HBs proteins, and HBx protein) shows a multifunctional role of these proteins: interference with cellular mechanisms is both through transcriptional mechanisms (modulating TBP, TFIIH-activity, altering the binding specificity of transcription factors) and through the modulation of signaling cascades (NFΚB activation, AP1 activation). Furthermore, cellular anti-tumor pathways (p53, nucleotide excision repair, apoptosis) are directly influenced by these transactivators.…”