Distinct Regulation of Hepatitis B Virus Biosynthesis by Peroxisome Proliferator-Activated Receptor γ Coactivator 1α and Small Heterodimer Partner in Human Hepatoma Cell Lines

Abstract

Hepatitis B virus (HBV) replicates efficiently in hepatocytes in vivo and in hepatoma cells in culture because these cells have the appropriate composition of transcription factors to support the expression of the 3.5-kb viral pregenomic transcript, which is reverse transcribed to generate the relaxed circular genomic DNA present in infectious viral particles (4,11,26,27,29). In contrast, HBV transcription and replication does not occur in the majority of nonhepatic tissues in vivo or nonhepatoma cell lines in culture due to the absence of transcription of the 3.5-kb viral pregenomic RNA (11,27). However, complementation of nonhepatoma cells with several nuclear receptors but not other liver-enriched transcription factors leads to robust HBV transcription and replication (see Fig. 1 to 3) (18a, 27). These observations have led to the suggestion that transcriptional regulation of HBV biosynthesis is an important determinant of viral tropism, which may be just as significant as the presumptive cell type-specific expression of the viral receptor (22, 27).The molecular characterization of the various steps in the viral life cycle has been largely elucidated, utilizing human hepatoma cell lines (10, 24). In particular, the human hepatoma cell lines HepG2 and Huh7 have been utilized because they can support viral replication when transfected with genomic HBV DNA (4, 26). In addition, these two cell lines lack integrated HBV DNA which is commonly associated with other human hepatoma cell lines (4, 26, 28). As products from integrated HBV sequences might have unidentified effects on viral transcription and replication, the majority of studies aimed at understanding the various aspects of HBV transcription and replication have utilized HepG2 and Huh7 cells. The majority of observations using these two cell lines have been similar with regard to HBV biosynthesis, leading to the general assumption that they are essentially equivalent with respect to HBV biosynthesis (4,24,26). However, it is clear that these hepatoma cell lines are morphologically distinct and, consequently, probably display distinct patterns of gene regulation.Recent studies have indicated that 3.5-kb pregenomic HBV RNA expression and viral replication are regulated by a variety of nuclear receptors (see Fig. 1 to 3) (18a, 27). The observation that several nuclear receptors can potentially contribute to the level of viral biosynthesis raised the critical question of their relative importance both in vivo and in cell culture. In an attempt to address this issue, the effect of expressing the coactivator peroxisome proliferator-activated receptor ␥ coactivator 1␣ (PGC1␣) and the corepressor small heterodimer partner (SHP) on HBV transcription and replication in both human hepatoma and nonhepatoma cells was quantitatively evaluated with a view to establishing the potential role of distinct nuclear receptors in regulating viral biosynthesis in the HepG2 and Huh7 cells. This analysis demonstrated that each nuclear receptor displayed a unique pattern of responsi...

Limited Effects of Bile Acids and Small Heterodimer Partner on Hepatitis B Virus Biosynthesis In Vivo

“…In contrast, HNF4␣ was shown to be essential for the developmental expression of HBV transcripts and, hence, HBV replication in vivo (24). RXR␣ plus FXR␣ were shown to enhance HBV transcription and replication in the Huh7 human hepatoma cell line (40) and to support viral biosynthesis in nonhepatoma cells (35,44). This finding suggests that bile acids, the natural FXR␣ ligands (30,39), may modulate HBV biosynthesis in vivo (Fig.…”

“…1) (28, 56). As PGC1␣ is a coactivator that can counteract the effect of SHP on HBV biosynthesis in cell culture (35,36), and its expression was previously shown to be downregulated in vivo by bile acids (29), the effect of the feeding of cholic acid on this transcript in HBV transgenic mice was examined (Fig. 2E).…”

“…As SHP expression is activated by FXR, and it is a negative regulator of nuclear receptor-mediated transcription (13,28), it could potentially be an in vivo inhibitor of bile acid-and nuclear receptor-mediated HBV transcription and replication ( Fig. 1) (35)(36)(37).…”

“…Cytochrome P450 7A1 (CYP7A1) gene expression is regulated by multiple nuclear receptors and encodes the enzyme mediating the rate-limiting step in bile acid biosynthesis (7,13,28). Nuclear receptors activate the synthesis of HBV RNA and DNA replication intermediates (RI) (35,36,44).…”

“…However, the relative importance of the various nuclear receptors in governing HBV biosynthesis in vivo has not been extensively investigated (14,24). Additionally the coactivator peroxisome proliferator-activated receptor ␥ coactivator 1␣ (PGC1␣) and the corepressor small heterodimer partner (SHP) differentially modulate nuclear receptor activities and appear to represent important regulators of HBV biosynthesis (34)(35)(36).…”