Rat 3Y1 cell lines that express either adenovirus type 12 ElA 13S mRNA or 12S mRNA in response to dexamethasone treatment were established by introduction of recombinant vector DNA containing the EIA 13S-or 12S-mRNA cDNA placed downstream of the hormone-inducible promoter of mouse mammary tumor virus. These cell lines were growth arrested, and the induction of cell cycle progression was analyzed by flow cytometry after switch on of the cDNA by the addition of dexamethasone. The results indicate that the 13S-or 12S-mRNA product alone has the ability to cause progression of the cell cycle at a similar rate. The simultaneous addition of epidermal growth factor accelerated the rate of cell cycle progression in the transition from the GOGI phase to the S phase.

Several lines of rat 3Y1 cells in which expression of the adenovirus type 12 ElA gene can be regulated by dexamethasone were established by introduction of recombinant vector DNA containing the adenovirus type 12 ElA gene placed downstream of the hormone-inducible promoter of mouse mammary tumor virus. These cell lines (gMA cells), produced low basal levels of the ElA transcripts and proteins in normal medium and much higher levels upon addition of dexamethasone to the medium. When dexamethasone was added to densityarrested cells, DNA synthesis was induced in 10 to 40% of the cells, the percentage depending on the cell line. DNA synthesis was increased to up to 60% of the cell population by further addition of epidermal growth factor. Indirect immunofluorescence detection of ElA proteins in gMA cells treated with dexamethasone indicated that the intensity of fluorescence in cells varied and that the proportion of cells synthesizing DNA was correlated with the proportion that exhibited strong fluorescence. These results indicate that the ElA gene has a function to trigger the synthesis of cellular DNA. The adenovirus ElA gene can immortalize primary cells in culture and render the cells susceptible to transformation by second transforming genes for adenovirus E1B or polyoma middle-T antigen, or' by the human mutated c-ras gene.

Rat 3Y1 cell lines that express either adenovirus type 12 E1A 13S mRNA or 12S mRNA in response to dexamethasone treatment were established by introduction of recombinant vector DNA containing the E1A 13S- or 12S-mRNA cDNA placed downstream of the hormone-inducible promoter of mouse mammary tumor virus. These cell lines were growth arrested, and the induction of cell cycle progression was analyzed by flow cytometry after switch on of the cDNA by the addition of dexamethasone. The results indicate that the 13S- or 12S-mRNA product alone has the ability to cause progression of the cell cycle at a similar rate. The simultaneous addition of epidermal growth factor accelerated the rate of cell cycle progression in the transition from the G0/G1 phase to the S phase.