The Enigmatic X Gene of Hepatitis B Virus

Abstract

The exact function of HBx during HBV replication and its influence in HBV-associated hepatocellular carcinoma are still undefined, but common themes are emerging. Most studies agree that, if not absolutely essential for HBV infections, HBx at the very least plays a critical role in viral replication. HBx functions in the cytoplasm to activate various signaling pathways, many of which are controlled by modulation of cytosolic calcium. A number of groups have now demonstrated that HBx activities somehow involve modulation of cytosolic calcium, although the exact molecular mechanism is undefined. In the nucleus, HBx can regulate transcription through a direct interaction with different transcription factors and, in some cases, enhance their binding to specific transcription elements. Clearly, HBx can influence apoptotic and cell cycle regulatory pathways, but the exact consequences of these activities of HBx are likely affected by the cell types in which the studies have been conducted. In this regard, it is interesting to note that much of the seeming confusion or controversy regarding HBx activities is likely the result of the different assays performed, and the findings of different studies may ultimately be more compatible than expected. In most experiments, HBx activities have been analyzed based on the end product of what are long and complex pathways. For example, transcriptional reporter readout, apoptotic assays, cell cycle regulation, and mutagenic studies are judged based on an analysis of the final product of a given pathway without considering common effectors. HBx may initiate overlapping and very similar effectors in all the studies conducted, but the consequence of this will likely vary depending on the specific cellular phenotype and the assay involved. Moreover, the frequency or longevity of this stimulus may vary depending on the amount of HBx present, which in turn influences the results. Fluctuations in the frequency or longevity of a HBx-induced stimulus, such as calcium signaling, can have a dramatic influence on the final effect on a cell (reviewed in reference 19). Equally interesting is the possibility that HBx has different consequences for hepatocyte physiology as HBV-infected cells are targeted by the immune system or as hepatocytes in which HBx is expressed undergo transformation and progression to HCC. Both these processes are dynamic, and it is likely that cellular physiology is consequently altered. The demonstration that different signaling pathways can be activated based on the differentiation state of hepatocytes derived from the same parental cell line and suggestions that the cellular status of p53, p21, and p27 can affect HBx-dependent regulation of cell cycle control supports the notion that the consequence of HBx expression may change as hepatocytes become transformed (2, 62, 93). It remains to be determined whether similar results will be observed in the context of an authentic HBV infection. Such observations will add to the complexity of identifying and understanding the precise molecular mechanism of HBx activities during viral replication. However, there is the exciting possibility that the end product of HBx activities may change as the cell responds to the infection. Finally, studies of HBx activities during HBV replication should help clarify the many properties that have been ascribed to HBx expression, including those that are required for viral replication and those that may influence cellular transformation.