Differential early viral gene expression in two stages of human papillomavirus type 16 DNA-induced malignant transformation.

Human papillomavirus (HPV) type 16 DNA induces progressive transformation in NIH 3T3 cells. Two types of cell lines, PM3T3G0 and PM3T3Fo, were isolated by G418 or focus selection, respectively, after transfection of cells by a recombinant HPV 16 DNA carrying the neo gene. These cell lines exhibited distinct phenotypes compared with controls. Saturation densities of PM3T3G0 and PM3T3Fo lines were two- to three- and five- to sevenfold greater than that of control NIH 3T3 cells, respectively. Neither cell type required high serum for growth, in contrast to NIH 3T3 cells. PM3T3G0 lines were premalignant, whereas PM3T3Fo lines manifested tumorigenicity within 2 weeks. Subpopulations of three PM3T3G0 lines underwent progressive transformation as reflected by focus formation. Analysis of HPV 16-specific mRNA species demonstrated that high levels of early and late gene expression were detected in premalignant PM3T3G0 lines, whereas relatively low quantities of selected gene messages were expressed in malignant transformants. Thus, high levels of viral gene expression are not crucial for malignant transformation.