Hepatitis viruses and signal transduction: True to the core?

Hepatitis viruses, in general, are not cytolytic; thus, the mechanisms by which they induce liver injury and hepatocyte death and promote hepatocellular carcinoma are of considerable research interest. One prevailing thought is that viral proteins possess various accessory functions (i.e., functions that are not directly related to viral replication, but, nonetheless, affect host functions) that target host proteins and thus alter normal cellular growth properties. By use of the yeast 2-hybrid screening and/or other research tools, an amazing array of cellular proteins has now been shown to interact with various proteins of hepatitis B virus (HBV) and hepatitis C virus (HCV), the hepatitis viruses that cause chronic infection. Presumably, the functions of these cellular proteins are altered as a result. For HBV, the X protein has been a particular focus. The list of cellular proteins found to interact with the X protein under various circumstances includes p53 and a number of general and specific transcription factors (for review, see Murakami1); however, other HBV proteins, such as the polymerase, may also bind cellular proteins (e.g., heat shock protein 90). 2 For HCV, the core, ns5a, and ns3 proteins have received particular scrutiny.3 The cellular proteins that interact with the various HCV proteins include tumor necrosis factor (TNF) receptor; lymphotoxin-b receptor; RNA helicase; heterogeneous nuclear ribonucleoprotein K; apolipoprotein A2; 14-3-3 protein; leucine zipper protein; protein kinases k (PKk), PKA, and PKC; growth factor receptorbound 2 protein; snf2-related CBP activator protein transcription factor; karyopherin-b3; and the vesicle transport protein VAP33. The list goes on and continues to grow. Even the HCV RNA itself can bind cellular proteins, including polypyrimidine-tract-binding protein, La antigen, heterogeneous nuclear ribonucleoprotein L, poly(C)-binding protein, and glyceraldehyde-3-phosphate dehydrogenase. These cellular proteins are involved in a wide range of normal functions, including transcriptional regulation and growth control, which presumably are affected by the viral proteins. Unfortunately, because of the lack of a tissue culture system for HCV, it is not clear whether most of the cellular proteins on the list indeed interact with the viral proteins in virus-infected cells. Furthermore, the consequences of the interactions between the viral and cellular proteins remain mostly ambiguous even under the best of culture conditions, let alone in the infected patients.4-8 How do these protein interactions lead to the reported effects on cells? Intuition suggests that various signal transduction pathways must mediate these effects. Previous studies have shown many signal pathways to be affected by HBV or HCV proteins. Nuclear factor kB (NF-kB), PKC, activator protein 1 (AP-1), Ras-GTP (mitogen-activated protein kinases), and nuclear factor of activated T cells are but a few examples of signaling molecules that were reported to be activated by either HBV (primarily the X protein) or HCV proteins.2 Many promoters or enhancers containing the binding sites of various transcription factors (AP-1, AP-2, serum response factor, CAAT/enhancer-binding protein, activated transcription factor 1, specificity protein 1, cyclic AMPresponsive element binding protein, p53, etc.) also have been shown to be activated by some of these viral proteins. These factors will lead to a series of downstream responses. The complexity of this tangled web of potential targets for HBV and HCV either portends or clouds understanding (take your pick) of how these viruses cause hepatitis. It is not clear what the relative importance of each effect is or how all of these events could possibly take place at the same time. In this issue of HEPATOLOGY, Kato et al.9 have probed these issues, studying the effects of HBV and HCV proteins on 5 disparate signal transduction pathways associated with cellular proliferation, differentiation, and apoptosis. As a reporter system, they used a luciferase gene under the control of promoters that contain the cyclic AMP response element, the serum response element, or the binding sites of NF-kB, AP-1, or serum response factor. These promoters were regulated at the strategic convergent points of several interconnecting signaling pathways. The authors studied 7 HCV proteins and 4 HBV proteins by expressing each of the viral proteins individually. They found that some of the HCV proteins and the HBV X protein were able to activate several promoters responsive to the signal transducers tested in this study, the HCV core protein being the most potent of all. Other HCV proteins (nonstructural protein 4b [ns4b] and ns5a) had less pronounced effects. These findings confirm and extend the findings of many previous studies. By exploring almost all of the HBV and HCV proteins and several signaling pathways simultaneously, this study provides a broader perspective of the viral infections as a whole. However, it examined only the constitutive, basal level activity of these signal transducers, and the nature of the upstream events that lead to the activation of the signal transducers was not addressed. Presumably, the activation of reporter gene expression is a result of interactions between the viral proteins and the upstream signal transducers. Because the list of the signaling pathways covered in this study is by no means exhaustive and each reporter promoter may be activated by several pathways, additional studies will be needed to fill in the missing links between the Abbreviations: HBV, hepatitis B virus; HCV, hepatitis C virus; TNF, tumor necrosis factor; NF-kB, nuclear factor kB; PK, protein kinase; AP-1, activator protein 1; ns, nonstructural protein. From the Howard Hughes Medical Institute and Department of Molecular Microbiology and Immunology, University of Southern California School of Medicine, Los Angeles, CA. Received June 14, 2000; accepted June 15, 2000. Address reprint requests to: Michael M. C. Lai, M.D., Ph.D., Howard Hughes Medical Institute, Department of Molecular Microbiology and Immunology, University of Southern California School of Medicine, 2011 Zonal Avenue, Los Angeles, CA 90033-1054. E-mail: michlai@hsc.usc. edu; fax: 323-342-9555. Copyright © 2000 by the American Association for the Study of Liver Diseases. 0270-9139/00/3202-0035$3.00/0 doi:10.1053/jhep.2000. 9856