Increased growth of permanent mouse fibroblasts in soft agar after transfection with hepatitis B virus DNA

Previously we have shown that a nontumorigenic mouse hepatocyte line harboring simian virus 40 large tumor antigen (SV 40 TAg) could be converted to a full-malignant phenotype by transfection with HBV DNA. Using a permanent SV 40 TAg-negative mouse fibroblast cell line (LTK-), we studied whether the in vitro-oncogenicity of HBV was dependent on simultaneous expression of SV 40 TAg or not. Three fibroblast lines stably transfected by full-length HBV DNA formed four times more colonies of large size in soft agar than nontransfected LTK- cells. All three clones expressed high levels of HBx protein, but variable levels of other HBV proteins. A second type of clone that was transfected by a partial HBV genome and that expressed HBV surface but no HBx proteins, did not acquire increased growth in soft agar. These data reveal that HBV DNA can enhance malignant growth independent of SV 40 TAg and suggest that HBx protein may act as an HBV oncogene at least in vitro.