Sirtuin 2 Isoform 1 Enhances Hepatitis B Virus RNA Transcription and DNA Synthesis through the AKT/GSK-3β/β-Catenin Signaling Pathway

Abstract

Sirtuin 2 (Sirt2), a NAD-dependent protein deacetylase, is overexpressed in many hepatocellular carcinomas (HCCs) and can deacetylate many proteins, including tubulins and AKT prior to AKT activation. Here, we found that endogenous Sirt2 was upregulated in hepatitis B virus (HBV) WT-replicating cells, leading to tubulin deacetylation; however, this was not the case in HBV replication-deficient mutant-transfected cells and 1.3mer HBV WT-transfected plus reverse transcriptase inhibitor (entecavir or lamivudine) treated cells but all HBV proteins are expressed. In HBV WT-replicating cells, upregulation of Sirt2 induced AKT activation, which consequently downregulated glycogen synthase kinase (GSK-3β) and increased β-catenin levels; however, downregulation of Sirt2 in HBV non-replicating cells impaired AKT/GSK-3β/β-catenin signaling. Overexpression of Sirt2 isoform 1 stimulated HBV transcription and consequently HBV DNA synthesis, which in turn activates AKT and consequently increases β-catenin levels, possibly through physical interactions with Sirt2 and AKT. Knockdown of Sirt2 by shRNAs or inhibition by AGK2 or dominant-negative mutant expression inhibited HBV replication, reduced AKT activation, and decreased β-catenin levels. Through HBV infection, we demonstrated that Sirt2 knockdown inhibited HBV replication from transcription. Although HBx itself activates AKT and upregulates β-catenin, Sirt2-mediated signaling and upregulated HBV replication were HBx-independent. Since constitutively active AKT inhibits HBV replication, the results suggest that upregulated Sirt2 and activated AKT may balance HBV replication to prolong viral replication, eventually leading to development of HCC. Also, the results indicate that Sirt2 inhibition may be a new therapeutic option for controlling HBV infection and preventing HCC.Even though Sirt2, a NAD-dependent protein deacetylase, is overexpressed in many HCCs, and overexpressed Sirt2 promotes hepatic fibrosis and associates positively with vascular invasion by primary HCCs via AKT/GSK-3β/β-catenin signaling, the relationships between Sirt2, HBV, HBx, and/or HBV-associated hepatocarcinogenesis are unclear. Here, we show that HBV DNA replication, not HBV expression, correlates positively with Sirt2 upregulation and AKT activation. We demonstrate that overexpression of Sirt2 further increases HBV replication and increases AKT activation, downregulates GSK-3β, and increases β-catenin levels. Conversely, inhibiting Sirt2 decreases HBV replication, reduces AKT activation, and decreases β-catenin. Although HBx activates AKT to upregulate β-catenin, Sirt2-mediated effects were not dependent on HBx. The results also indicate that a Sirt2 inhibitor may control HBV infection and prevent development of hepatic fibrosis and HCC.

SIRT2 Contributes to the Regulation of Intestinal Cell Proliferation and Differentiation

“…For example, SIRT2 activates Wnt/b-catenin through Akt/GSK-3 in hepatocellular carcinomas. 42,43 In contrast, SIRT2 inhibition of Wnt/b-catenin has been shown in certain types of cells. [44][45][46] SIRT2 is a deacetylase of E-cadherin, and acetylation of nuclear E-cadherin attenuates its interaction with b-catenin, which releases bcatenin from the complex, resulting in increased expression of its downstream genes and accelerated tumor growth and migration in colorectal cancer cells.…”

NAD+ metabolism: pathophysiologic mechanisms and therapeutic potential

“…304,313 Regarding the redox role of NADH/NADPH in eliminating ROS, the depletion of NAD + /NADP + pool exacerbates the oxidative damage during virus infection. 306,308,[314][315][316] NAD + contributes to the bactericidal activity. Bacterial infection induces rapid production of intracellular ROS either by NOXs or mitochondria that are, in turn, crucially required by macrophages to clear bacteria.…”

GSK3: A Kinase Balancing Promotion and Resolution of Inflammation

“…On the other hand, the Karposi's sarcoma-associated herpes virus-derived protein LANA is able to interact with GSK3β to favor nuclear GSK3β localization, and to increase both β-catenin [69,179] and Myc concentrations [180], which implies a LANA-dependent inactivation of GSK3β. Hepatitis B virus (HBV) replication is associated with increased GSK3β-Ser9 phosphorylation [181,182], indicating a beneficial effect of GSK3β inactivation for HBV replication. Mechanistically, anti-viral GSK3β effects may be associated in certain cases with its ability to enhance the anti-viral capacity of the zinc-finger anti-viral protein (an inhibitor of the replication of certain viruses) by sequential phosphorylation [183].…”