Early gene expression and cellular DNA synthesis after stimulation of quiescent NIH3T3 cells with serum or purified simian virus 40.

Like growth factors or hormones, the DNA tumor virus simian virus 40 can stimulate quiescent cells to re-enter the S-phase. Time course experiments revealed similar kinetics for the stimulation of cellular DNA synthesis by growth factors or SV40 infection, although there was a delay in DNA synthesis of about 2 h in the case of the SV40 infection. The analysis of the transcriptional activation of proto-oncogenes in cells stimulated by serum or SV40 infection revealed an enhanced transcription of a common set of proto-oncogenes, including c-myc and c-fos. Early on after infection of quiescent cells SV40 induced in addition the transcription of the c-sis gene which did not occur with UV-irradiated SV40 virus. Furthermore, we could demonstrate the presence of growth stimulating activities in medium from SV40 infected quiescent cells, which triggered quiescent cells to re-enter the cell cycle. These data suggest that SV40 might stimulate quiescent cells by inducing the transcription and ultimately the release of growth-factors.