Abstract
BackgroundHepatitis B virus (HBV) transcription and replication are essentially restricted to hepatocytes. Based on the HBV enhancer and promoter complex that links hepatic glucose metabolism to its transcription and replication, HBV adopts a regulatory system that is unique to the hepatic gluconeogenic genes. CRTC2, the CREB-regulated transcription coactivator 2, is a critical switch modulating the gluconeogenic program in response to both hormonal and intracellular signals. However, the relationship between CRTC2 and HBV transcription and replication remains unclear.MethodsTo analyze the influence of CRTC2 on HBV transcription and replication, CRTC2 expression construct or siRNA was cotransfected with plasmids containing enhancer II/core promoter complex-controlled luciferase or 1.3× wtHBV genome in Huh-7 cells. Luciferase activity, HBV core protein expression, HBV transcripts, and DNA replication intermediates were measured by luciferase assays, western blots, real-time polymerase chain reaction (PCR), and Southern blots, respectively. Forskolin (FSK) or phosphorylation-defective CRTC2 mutants were further utilized to elucidate the potential mechanism. siRNA against peroxisome proliferator-activated receptor-γ coactivator 1α (PGC1α) was also used to examine the mediator involved in CRTC2-regulated HBV biosynthesis in Huh-7 cells.ResultsCRTC2 overexpression increased HBV transcription and replication in Huh-7 cells, including levels of core protein expression, mRNA, and DNA replication intermediates. Correspondingly, CRTC2 knock down by siRNA reduced HBV biosynthesis. FSK treatment strongly enhanced the effect of CRTC2 through triggering the dephosphorylation and nuclear entry of CRTC2. The phosphorylation-defective mutant (S171A/S275A) of CRTC2 localized in the nucleus and was constitutively active, which dramatically promoted HBV transcription and replication similar to FSK-treated wild-type CRTC2. Knock down of PGC1α, whose expression was induced by CRTC2, greatly compromised the enhancing effect of CRTC2 on HBV transcription and replication.ConclusionsOur results clearly indicate that non-phosphorylated CRTC2 strongly enhances HBV biosynthesis through inducing PGC1α expression. Further study of the mechanisms will elucidate the importance of metabolic signals on HBV transcription and replication, and offer insight into potential targets for developing anti-HBV agents.
Highlights
Hepatitis B virus (HBV) transcription and replication are essentially restricted to hepatocytes
Camp response element-binding protein-regulated transcription coactivator 2 (CRTC2) enhances HBV transcription Due to its enhancer and promoter composition, HBV transcription and replication can be regulated by nutritional signals that control hepatic glucose metabolism [2]
As a critical switch promoting the hepatic gluconeogenic program, CRTC2 attracted our interest for its role in HBV transcription and replication
Summary
Hepatitis B virus (HBV) transcription and replication are essentially restricted to hepatocytes. CRTC2, the CREB-regulated transcription coactivator 2, is a critical switch modulating the gluconeogenic program in response to both hormonal and intracellular signals. The cAMP-responsive element-binding protein (CREB)-regulated transcription coactivator 2 (CRTC2) is a critical switch promoting the gluconeogenic program in the liver [5]. The calcineurin inhibitor cyclosporine A (CsA) disrupts the cAMP-induced dephosphorylation and nuclear translocation of CRTC2, but it has no effect on a phosphorylation-defective CRTC2 mutant (S171A/S275A) [8]. Because it cannot be phosphorylated at these two sites, the S171A/S275A mutant remains active in the nucleus [11,12]
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