Viral and cellular oncogene expression during progressive malignant transformation of SV40 transformed human fibroblasts

In vitro investigation of the multistep neoplastic progression which occurs during transformation of human cells has been hindered by resistance of human cells to both immortalization and tumorigenicity (Mut. Res. 199; 273, 1988). Previously our laboratory established a cell line, HSF4‐T12, by transfection of normal human foreskin fibroblasts with the plasmid pSV3‐neo which contains the early genes of simian virus 40 (SV40). A multi‐step progression in karyotypic alterations and transformed phenotype occurred resulting in a neoplastic cell line that was immortal, transformed, and tumorigenic. We have examined changes in the SV40 proteins, large T (T‐antigen) and small t (t‐antigen) antigens, and in the cellular protein, p53, during progressive transformation of these cells. Total viral protein expression relative to total cellular protein increased following immortalization of HSF4‐T12 as did the ratio of T‐antigen to t‐antigen. Interestingly, no significant change in DNA content accompanied immortalization. However, during the progressive in vitro transformation of HSF4‐T12 which occurred primarily post‐immortalization, DNA index increased to 1.6 but only small additional increases in T‐antigen expression were seen. No consistent or critical role for t‐antigen in development of the tumorigenic phenotype was found in this system.