Growth factor induction by the adenovirus type 5 E1A 12S protein is required for immortalization of primary epithelial cells.

The 12S protein encoded by the adenovirus E1A region induces cellular DNA synthesis in and proliferation and immortalization of primary rat epithelial cells in the presence or absence of serum. It also induces the production of a growth factor(s) that stimulates epithelial cell proliferation. We have undertaken a mutational analysis of the 12S gene to determine the sequences required for these functions. We found that a region near the C-terminus of the 12S protein was required for growth factor induction. No activities have been defined previously for this region. Furthermore, we show that growth factor production was necessary for epithelial cells to survive past their normal life span in culture and to become immortalized. The ability to induce growth factor production required prior expression of E1A activities encoded by the N-terminus of the 12S protein, including activation of quiescent cells into the cell cycle, and an unknown activity that required expression of the first 13 amino acids of the gene. In addition, examination of the subcellular localization of mutant 12S polypeptides suggested new regions that affect the nuclear localization of E1A proteins.