Hepatitis B virus X antigen promotes transforming growth factor-β1 (TGF-β1) activity by up-regulation of TGF-β1 and down-regulation of α 2-macroglobulin

Abstract

Hepatitis B virus (HBV) X antigen (HBxAg) may contribute to the development of hepatocellular carcinoma (HCC) by activation of signalling pathways such as NF-κB. To identify NF-κB target genes differentially expressed in HBxAg-positive compared to -negative cells, HepG2 cells consistently expressing HBxAg (HepG2X cells) were stably transfected with pZeoSV2 or pZeoSV2-IκBα. mRNA from each culture was isolated and compared by PCR select cDNA subtraction. The results showed lower levels ofα2-macroglobulin (α2-M) in HepG2X-pZeoSV2 compared to HepG2X-pZeoSV2-IκBαcells. This was confirmed by Northern and Western blotting, and by measurement of extracellularα2-M levels. Elevated transforming growth factor-β1 (TGF-β1) levels were also seen in HepG2X compared to control cells. Serum-free conditioned medium (SFCM) from HepG2X cells suppressed DNA synthesis in a TGF-β-sensitive cell line, Mv1Lu. The latter was reversed when the SFCM was pretreated with exogenous, activatedα2-M or with anti-TGF-β. Since elevated TGF-β1 promotes the development of many tumour types, these observations suggest that the HBxAg-mediated alteration in TGF-β1 andα2-M production may contribute importantly to the pathogenesis of HCC.