The role and functional domain of hepatitis B virus (HBV) X protein (HBx) in regulating HBV transcription and replication were investigated with a transient transfection system in the human hepatoma cell line HepG2 using wild-type or HBx-minus HBV genome constructs and a series of deletion or mutation HBx expression plasmids. We show here that HBx has augmentation effects on HBV transcription and replication as a HBV mutant genome with defective X gene led to decreased levels of 3.5-kb HBV RNA and HBV replication intermediates and that these decreases can be restored by either transient ectopic expression of HBx or a stable HBx expression cell line. The C-terminal two-thirds (amino acids [aa] 51 to 154), which contain the transactivation domain, is required for this function of HBx; the N-terminal one-third (aa 1 to 50) is not required. Using the alanine scanning mutagenesis strategy, we demonstrated that the regions between aa 52 to 65 and 88 to 154 are important for the augmentation function of HBx in HBV replication. By the luciferase reporter gene analysis, we found that the transactivation and coactivation activities of HBx coincide well with its augmentation function in HBV transcription and replication. These results suggest that HBx has an important role in stimulating HBV transcription and replication and that the transcriptional transactivation function of HBx may be critical for its augmentation effect on HBV replication.

Hepatitis B virus (HBV) possesses a 3.2-kb partially double-stranded DNA genome that is generated inside the nucleocapsid by the reverse transcription of the 3.5-kb pregenomic viral transcript. The initial steps in viral replication involve the recognition of an encapsidation signal termed epsilon (ε) at the 5'-end of the pregenomic RNA by the HBVpolymerase. The polymerase-bound pregenomic RNA is subsequently incorporated into an immature nucleocapsid particle and minus-strand HBVDNA synthesis is initiated utilizing the bulge region of ε as a template and a tyrosine residue in the amino-terminal region of the polymerase as a primer. Three nucleotides complementary to the 3'-end of the bulge region of ε are synthesized and subsequently translocated with the polymerase molecule to the acceptor site located in the DR1 sequence present at the 3'-end of the pregenomic RNA. Using mutagenesis analysis, a sequence element designated phi (ε) located upstream of the 3 ε DR1 sequence has been identified that is complementary to ε and is important for efficient viral replication. This element may bring the 3' DR1 sequence into proximity with the three nucleotide primer synthesized at the bulge of ε and facilitate primer translocation to the 3' DR1 acceptor sequence. Sequence elements with similar proximity to the 3' DR1 sequences and complementarity to ε are present in the woodchuck hepatitis virus (WHV) and duck hepatitis B virus (DHBV), suggesting the ε regulatory element may be phylogenetically conserved due to its functional importance in hepadnavirus minus-strand DNA synthesis.

The nuclear hormone receptors hepatocyte nuclear factor 4 (HNF4) and the retinoid X a (RXRa) plus the peroxisome proliferator-activated receptor a (PPARa) heterodimer support hepatitis B virus (HBV) replication in nonhepatoma cells. Hepatocyte nuclear factor 3 (HNF3) inhibits nuclear hormone receptor-mediated viral replication. Inhibition of HBV replication by HNF3a is associated with the preferential reduction in the level of the pregenomic RNA compared with that of precore RNA. Hepatitis B e antigen (HBeAg), encoded by the precore RNA, mediates part of the inhibition of viral replication by HNF3a. The amino-terminal transcriptional activation domain of HNF3a is essential for the inhibition of HBV replication. The activation of transcription by HNF3 from HBV promoters downstream from the nucleocapsid promoter appears to contribute indirectly to the reduction in the steady-state level of 3.5-kb HBV RNA, possibly by interfering with the elongation rate of these transcripts. Therefore, transcriptional interference mediated by HNF3 may also regulate HBV RNA synthesis and viral replication.

Hepadnavirus replication occurs in hepatocytes in vivo and in hepatoma cell lines in cell culture. Hepatitis B virus (HBV) replication can occur in nonhepatoma cells when pregenomic RNA synthesis from viral DNA is activated by the expression of the nuclear hormone receptors hepatocyte nuclear factor 4 (HNF4) and the retinoid X receptor a (RXRa) plus peroxisome proliferator-activated receptor a (PPARa) heterodimer. Nuclear hormone receptor-dependent HBV replication is inhibited by hepatocyte nuclear factor 3 (HNF3). In contrast, HNF3 and HNF4 support duck hepatitis B virus (DHBV) replication in nonhepatoma cells, whereas the RXRa-PPARa heterodimer inhibits HNF4-dependent DHBV replication. HNF3 and HNF4 synergistically activate DHBV pregenomic RNA synthesis and viral replication. The conditions that support HBV or DHBV replication in nonhepatoma cells are not able to support woodchuck hepatitis virus replication. These observations indicate that avian and mammalian hepadnaviruses have distinct transcription factor requirements for viral replication.

Hepatotropism is a prominent feature of hepatitis B virus (HBV) infection. Cell lines of nonhepatic origin do not independently support HBV replication. Here, we show that the nuclear hormone receptors, hepatocyte nuclear factor 4 and retinoid X receptor a plus peroxisome proliferator-activated receptor a, support HBV replication in nonhepatic cells by controlling pregenomic RNA synthesis, indicating these liver-enriched transcription factors control a unique molecular switch restricting viral tropism. In contrast, hepatocyte nuclear factor 3 antagonizes nuclear hormone receptormediated viral replication, demonstrating distinct regulatory roles for these liver-enriched transcription factors.