Hepatitis B virus X protein transactivates inducible nitric oxide synthase gene promoter through the proximal nuclear factor [kappa ]B[ndash ]binding site: Evidence that cytoplasmic location of X protein is essential for gene transactivation

Abstract

Nitric oxide appears to play a central role in the pathogenesis of many inflammatory disorders. We have previously shown that there is an enhanced intrahepatic expression of the inducible nitric oxide synthase (iNOS) gene during chronic hepatitis B virus infection, and that viral X protein (HBx) transcriptionally activates this cellular gene, but the molecular basis for this activation remains to be defined. We aimed to explore the involvement of different cis-acting elements of the human iNOS promoter in the HBx- Nitric oxide (NO) is a highly reactive gaseous molecule that plays an important role in the nonspecific immune response to infection, acting as antimicrobial. 1 However, a long-term production of large amounts of NO due to overexpression of the inducible nitric oxide synthase (iNOS) gene can have detrimental consequences, such as cytotoxicity and carcinogenesis. 2,3 Further supporting this assumption, we have recently shown that the expression of iNOS was markedly enhanced by hepatitis B virus (HBV) infection in hepatocytes from patients with chronic hepatitis B (CHB) 4 and, as expected, an intrahepatic accumulation of NO-derived reactive species closely associated with histologic severity of liver disease was also observed in patients with chronic HBV infection, 5 suggesting that NO could mediate important pathogenic events in the course of HBV infection.Cloning and functional analysis of the human iNOS gene promoter identified 2 copies of nuclear factor B (NF-B) response elements, one located Ϫ102 base pairs proximal to the transcription initiation site and the other as distal as Ϫ8,270 base pairs upstream. Furthermore, 2 copies of activator protein 1 (AP-1) binding sites have also been found. 6 Although it has been suggested that HBV X protein (HBx) transactivates the iNOS gene promoter through the induction of NF-B, 7 the mechanism of this activity remains only partially understood. It is conceivable, therefore, that understanding how HBx activates the iNOS gene transcription can provide insights into the pathogenic role of the enhanced intrahepatic iNOS expression that we observed in patients with CHB. 4,5 The HBV genome encodes a small protein known as HBx, 8 which is essential for viral infectivity 9 and is a potential cofactor in HBV-mediated carcinogenesis. 10,11 In addition, it has been shown that HBx behaves as a transcriptional transactivator of a number of viral and cellular gene promoters through direct interaction with transcriptional factors, such as the RPB5 subunit of RNA polymerase II, 12 TATA-binding protein 13 and ATF/CREB, 14 and activation of signal transduction pathways, such as the Ras/Raf/MAP kinase cascade. 15 In this regard, the role of the intracellular distribution of HBx still remains controversial, but data from Doria et al. 16 suggest that HBx could have a dual function on transcription machinery, acting on signal transduction pathways in the cytoplasm and transcription factors in the nucleus. The same investigators have shown that the cytoplasmic distribution of HBx wa...

Activating mechanism of transcriptor NF-kappaB regulated by hepatitis B virus X protein in hepatocellular carcinoma

“…The expression of HBx in samples with serum HBV-negative implicated quite a few patients were once infected with HBV. Moreover, the cytoplasmic location of HBx was consistent with the previous reports [15] . NF-κB plays a vital role in almost all aspects of cell regulation such as immune cell activation, proliferation, apoptosis, stress response, differentiation and oncogenic transformation.…”

Stimulation of Inducible Nitric Oxide by Hepatitis B Virus Transactivator Protein HBx Requires MTA1 Coregulator

“…Specifically, HBx activates NF-B signaling (12,13) and enhances the expression of its downstream inflammatory targets, including inducible nitric-oxide synthase (iNOS) (9, 14 -16). Inducible NOS is both an NF-B-responsive (17)(18)(19)(20) and an HBx-regulated gene (14). Inducible NOS generates nitric oxide (NO) from L-arginine.…”

Subcellular Mislocalization of Mutant Hepatitis B X Proteins Contributes to Modulation of STAT/SOCS Signaling in Hepatocellular Carcinoma

“…The functional activity and subcellular distribution of HBx seemed to be controlled by its phosphorylation status, e.g., mediated by ERK1/2 [18,40] . It has been reported that cytoplasmic HBx is involved in the activation of iNOS, a factor that plays an important role in inflammatory disorders and regulation of the Ras-Map kinase signaling [35,41,42] . Nuclear localization of the HBx protein has been shown to modulate the activity of cellular factors like AP-1, NF-B, tumor suppressor P53, DDB proteins (damaged DNA binding protein), and p21(WAF1/Cip1) [40,43] .…”