Hepatitis B and related viruses that infect mammalian hosts encode the "X" protein that has been shown to contribute importantly to the pathogenesis of chronic liver disease (CLD) and to the evelopment of hepatocellular carcinoma (HCC). In a variety of tissue culture systems, hepatitis B virus (HBV) X antigen, or HBxAg, has been shown to trigger apoptosis, while other evidence suggests that HBxAg inhibits apoptosis and stimulates the cell cycle by constitutively activating a number of signaling pathways that are important for hepatocellular growth and survival. These apparently contrasting properties of HBxAg may be associated with differences in the X protein itself, since carboxy-terminaltruncated forms of HBxAg appear to be associated with HCC lesions. Alternatively, or in addition, these differences may be due to the cell type, state of cell differentiation, and whether expression occurs in resting or dividing cells. Further, the association between HBxAg expression and chromosomal instability, may also contribute to the apparently contrasting fates of HBxAg positive cells. It is proposed that in many of these systems, the different outcomes of HBxAg expression may be due to the nature of the cellular response to HBxAg, and not due to differences in the fundamental properties of HBxAg, the latter of which promote cell survival, cell cycle rogression, and the development of HCC.

Polymerase chain reaction (PCR) select complementary DNA (cDNA) subtraction of hepatitis B x antigen (HBxAg)-positive compared with -negative HepG2 cells resulted in the up-regulated expression of a cellular gene that encodes a transcript of 745 bases and a polypeptide 99 amino acids long. GenBank analysis revealed extensive homology with the amino terminal domain of cellular multidrug resistant proteins (MRP), although overexpression of this gene did not confer an MRP phenotype. In situ ybridization and immunostaining showed colocalized expression with HBxAg in the liver of hepatitis B carriers. Overexpression of this protein stimulated the growth of HepG2 cells in serum-free medium, and partially protected cells from anti-Fas-mediated killing, but did not promote growth in soft agar or tumor formation in nude mice. Introduction of the dominant negative inhibitor of nuclear factor B (I B) into HBxAgpositive HepG2 cells decreased the levels of messenger RNA (mRNA) and protein, uggesting that its up-regulation is nuclear factor B (NF-B) dependent. Hence, HBxAg activation of NF-B may result in the up-regulation of a cellular protein that promotes growth factor-ndependent survival and protects against Fas-mediated killing. This factor may contribute to the persistence of infected hepatocytes during chronic infection, which is important for the later development of epatocellular carcinoma (HCC). (HEPATOLOGY 2001; 34: 146-157.)

HepG2 cells stably transfected with a full-length, infectious hepatitis C virus (HCV) cDNA demonstrated consistent replication of HCV for more than 3 years. Intracellular minus strand HCV RNA was present. Minus strand synthesis was NS5B dependent, and was sensitive to interferon alpha (IFNa) treatment. NS5B and HCV core protein were detectable. HCV stimulated HepG2 cell growth and survival in culture, in soft agar, and accelerated tumor growth in SCID mice. These mice becameHCVRNApositive in blood, where the virus was also sensitive to IFNa. The RNA banded at the density of HCV, and was resistant to RNase prior to extraction. Hence,HCVstably replicates inHepG2cells, stimulates epatocellular growth and tumorigenesis, and is susceptible to IFNa both in vitro and in vivo. J. Cell. Physiol. 201: 447-458, 2004.

The development of hepatocellular carcinoma (HCC) is a multistep process associated with changes in host gene expression, some of which correlate with the appearance and progression of tumor. reneoplastic changes in gene expression result from altered DNA methylation, the actions of hepatitis B and C viruses, and point mutations or loss of heterozygosity (LOH) in selected cellular genes. Tumor progression is characterized by LOH involving tumor suppressor genes on many chromosomes and by gene ampli