Hepatitis B Virus X Protein Differentially Activates RAS-RAF-MAPK and JNK Pathways in X-transforming Versus Non-transforming AML12 Hepatocytes

• Published in: The Journal of biological chemistry 2001-09, Vol.276 (37), p.34671-34680
• Main Authors: Tarn, Chi, Lee, Sook, Hu, Yong, Ashendel, Curtis, Andrisani, Ourania M.
• Format: Article
• Language: English
• Subjects: 3pX-1 cells; 4pX-1 cells; Cell Division; Cell Line; Cell Transformation, Neoplastic; Cyclic AMP Response Element-Binding Protein - metabolism; Cyclin A - genetics; hepatitis B virus; Hepatocytes - drug effects; Hepatocytes - pathology; JNK Mitogen-Activated Protein Kinases; JNK protein; mitogen-activated protein kinase; Mitogen-Activated Protein Kinases - metabolism; Phosphorylation; Proto-Oncogene Proteins c-jun - metabolism; Proto-Oncogene Proteins c-raf - metabolism; Raf protein; ras Proteins - metabolism; Trans-Activators - toxicity; Viral Regulatory and Accessory Proteins; X protein
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M104105200

The hepatitis B virus (HBV) X protein (pX) is implicated in hepatocarcinogenesis of chronic HBV patients by an unknown mechanism. Activities of pX likely relevant to hepatocyte transformation include activation of the mitogenic RAS-RAF-MAPK and JNK pathways. To assess the importance of mitogenic pathway activation by pX in transformation, we employed a cellular model system composed of two tetracycline-regulated, pX-expressing cell lines, constructed in AML12-immortalized hepatocytes. This system includes the differentiated 3pX-1 and the de-differentiated 4pX-1 hepatocytes. Our studies have demonstrated that conditional pX expression transforms only 3pX-1 cells. Here, comparative in vitro kinase assays and variousin vivo analyses demonstrate that pX affects an inverse activation of RAS-RAF-MAPK and JNK pathways in 3pX-1 versus4pX-1 cells. Sustained pX-dependent RAS-RAF-MAPK pathway activation is observed in pX-transforming 3pX-1 cells, whereas sustained pX-dependent JNK pathway activation is observed in pX non-transforming 4pX-1 cells. This differential, pX-dependent mitogenic pathway activation affects differential activation of cAMP-response element-binding protein and c-Jun and determines the proliferative response of 3pX-1 and 4pX-1 cells. Furthermore, tetracycline-regulated, pX-NLS-expressing cell lines demonstrate that expression of the nuclear pX-NLS variant minimally activates the RAS-RAF-MAPK pathway and results in markedly reduced transformation. These results link sustained, pX-mediated activation of RAS-RAF-MAPK pathway to hepatocyte transformation.