Large Isoform of Hepatitis Delta Antigen Activates Serum Response Factor-associated Transcription

Tadashi Goto,Naoya Kato,Suzane Kioko Ono-Nita,Hideo Yoshida,Motoyuki Otsuka,Yasushi Shiratori,Masao Omata

doi:10.1074/jbc.m002947200

Tadashi Goto, Naoya Kato + Show 5 more

Open Access

https://doi.org/10.1074/jbc.m002947200

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Abstract

Hepatitis delta virus infection sometimes causes severe and fulminant hepatitis as a coinfection or superinfection along with the hepatitis B virus. To elucidate the underlying mechanism of injury caused by hepatitis delta virus, we examined whether two isoforms of the hepatitis delta antigen (HDAg) had any effect on five well defined intracellular signal transduction pathways: serum response factor (SRF)-, serum response element (SRE)-, nuclear factor kappaB-, activator protein 1-, and cyclic AMP response element-dependent pathways. Reporter assays revealed that large HDAg (LHDAg) activated the SRF- and SRE-dependent pathways. In contrast, small HDAg (SHDAg) did not activate any of five pathways. LHDAg enhanced the transcriptional ability of SRF without changing its DNA binding affinity in an electrophoretic mobility shift assay. In addition, LHDAg activated a rat SM22alpha promoter containing SRF binding site and a human c-fos promoter containing SRE. In conclusion, LHDAg, but not SHDAg, enhances SRF-associated transcriptions. Despite structural similarities between the two HDAgs, there are significant differences in their effects on intracellular signal transduction pathways. These results may provide clues that will aid in the clarification of functional differences between LHDAg and SHDAg and the pathogenesis of delta hepatitis.

Highlights

To elucidate the underlying mechanism of injury caused by hepatitis delta virus, we examined whether two isoforms of the hepatitis delta antigen (HDAg) had any effect on five well defined intracellular signal transduction pathways: serum response factor (SRF), serum response element (SRE), nuclear factor ␬B, activator protein 1, and cyclic AMP response element-dependent pathways
Hepatitis delta antigen (HDAg)1 was discovered as an antigen localized in the nuclei of hepatocytes in a patient who had already been infected with the hepatitis B virus (HBV) [1]
Cell Line-independent and Dose-dependent Activation of SRFdependent Pathway by large HDAg (LHDAg)—We investigated whether LHDAg activated the SRF-dependent pathway in other cell lines

Summary

Introduction

Hepatitis delta antigen (HDAg)1 was discovered as an antigen localized in the nuclei of hepatocytes in a patient who had already been infected with the hepatitis B virus (HBV) [1]. PFR-Luc (Stratagene) has a firefly-luciferase gene controlled by five yeast GAL4 upstream activation sequences that was utilized as a reporter to examine transcriptional activation of GAL4 DNA-BD-fused Elk1 or SRF by HDAg. Cell Culture—HeLa cells (human cervical carcinoma cell line), HuH-7 cells (human hepatocellular carcinoma cell line), and HepG2 cells (hepatoblastoma cell line) were obtained from the Riken cell bank (Tsukuba, Japan).

Results

Conclusion