Expression of antisense E1A in 293 cells results in altered cell morphologies and cessation of proliferation

To determine whether continued E1A expression is required to maintain immortalization, 293 cells, a cell line that has been immortalized and transformed by adenovirus E1A and E1B, respectively, were transfected by an antisense E1A expression vector. In the presence of low serum, 293 cells underwent a transient, viable, nonproliferative phase wherein cells with altered morphologies were detected. However, only those clones which did not integrate the antisense sequences and exhibited a morphology similar to the original 293 cells were able to survive. High serum concentrations resulted in fewer clones, which rapidly discarded the antisense DNA. Similar experiments with an antisense CAT vector did not give rise to cells with either altered morphologies or growth rates. Furthermore, CAT antisense DNA was integrated into the 293 cell DNA. Hela cells, which were not immortalized or transformed by E1, were unaffected by the expression of antisense E1A and integrated the antisense DNA into genomic DNA. Consistently, the addition of synthetic antisense, but not sense oligonucleotides resulted in a transient inhibition of 293 cell DNA synthesis. These data demonstrate that even after extended periods of time in culture, cells immortalized by E1A still require E1A expression to activate the cell cycle and prevent them from senescing.