Hepatitis B virus X protein activates proteasomal activator 28 gamma expression via upregulation of p53 levels to stimulate virus replication

Abstract

Proteasomal activator gamma (PA28γ), frequently overexpressed in hepatocellular carcinoma, is believed to play important roles in tumourigenesis. However, the underlying mechanism of PA28γ overexpression and its possible roles in hepatitis B virus (HBV) replication are largely unknown. In the present study, we found that hepatitis B virus X protein (HBx) activates PA28γ expression by upregulating p53 levels in human hepatoma cells. The elevated PA28γ levels in turn repressed seven in absentia homologue 1 expression via downregulation of p53 levels, thereby inhibiting ubiquitin-dependent proteasomal degradation of HBx, which ultimately led to upregulation of HBx levels. The correlation among HBx, p53 and PA28γ was exactly reproduced in a 1.2-mer HBV replicon system, mimicking the natural course of HBV infection. In particular, knockdown of either p53 or PA28γ in HepG2 cells downregulated HBx levels and thereby inhibited HBV replication, whereas overexpression of p53 or PA28γ in Hep3B cells upregulated HBx levels, which stimulated HBV replication, indicating that p53 and PA28γ act as activators of HBV replication. In conclusion, HBx levels are upregulated via a positive feedback loop involving p53 and PA28γ to stimulate HBV propagation.

Hepatitis B virus X protein stimulates cell growth by downregulating p16 levels via PA28γ-mediated proteasomal degradation

“…1b). These results support previous findings demonstrating that HBx upregulates PA28γ levels via activation of p53 in human hepatoma cells [21].…”

“…PA28γ knockdown also downregulated HBx and HBc levels in infected cells (Fig. 3e, g), results consistent with a previous report [21] demonstrating the role of PA28γ as a positive regulator of HBV replication. Together, the results demonstrate that HBx-mediated PA28γ upregulation is responsible for the stimulation of cell growth during HBV replication.…”

“…HBV replication by the replicons was confirmed through detection of viral proteins, such as HBx and HBV core antigen (HBc). Consistent with previous reports [21,27], the levels of HBV proteins derived from the 1.2-mer WT that encodes HBx were higher than those obtained with 1.2-mer HBx-null, from which HBx could not be formed (Fig. 3d).…”

“…In addition, p53 activates PA28γ transcription through p53 response elements located within its promoter [20]. Based on these observations, we have recently demonstrated that HBx stimulates PA28γ expression in human hepatoma cells via activation of its transcriptional activator p53 [21]. In the present study, we further explored the biological significance of the p53-dependent PA28γ activation in HBx-associated tumorigenesis, focusing on the PA28γ-mediated p16 proteasomal degradation and its effects on the Rb-E2F pathway and cell proliferation.…”

“…Several studies have shown that HBx upregulates p53 levels to stimulate the expression of target genes, such as p21, Siah-1, PA28γ and several pro-apoptotic genes, including Bax, Fas and Nox [18,19,21,23,28]. In addition to this effect, HBx alters the mitochondrial membrane potential to increase levels of cellular reactive oxygen species [29,30].…”