ACCUMULATION OF P53 INDUCED BY THE ADENOVIRUS E1A PROTEIN REQUIRES REGIONS INVOLVED IN THE STIMULATION OF DNA-SYNTHESIS

It has been known for some time that expression of the 243-residue (24 3R) human adenovirus type 5 (Ad5) early region 1A (E1A) protein causes an increase in the level of the cellular tumor suppressor p53 and ind uction of p53-dependent apoptosis, Deletion of a portion of conserved region 1 (CR1) had been shown to prevent apoptosis, suggesting that bi nding of p300 and/or the pRB retinoblastoma tumor suppressor and relat ed proteins might be implicated, To examine the mechanism of the E1A-i nduced accumulation of p53, cells were infected with viruses expressin g E1A-243R containing various deletions which have well-characterized effects on p300 and pRB binding, It was found that in human HeLa cells and rodent cells, complex formation with p300 but not pRB was require d for the rise in p53 levels, However, in other human cell lines, incl uding MRC-5 cells, E1A proteins which were able to form complexes with either p300 or pRB induced a significant increase in p53 levels. Only E1A mutants defective in binding both classes of proteins were unable to stimulate p53 accumulation, This same pattern was also apparent in p53-null mouse cells coinfected by Ad5 mutants and an adenovirus vect or expressing either wild-type or mutant human p53 under a cytomegalov irus promoter, indicating that the difference in importance of pRB bin ding may relate to differences between rodent and human p53 expression , The increase in p53 levels correlated well with the induction of apo ptosis and, as shown previously, with the stimulation of cellular DNA synthesis. Thus, it is possible that the accumulation of p53 is induce d by the induction of unscheduled DNA synthesis by EIA proteins and th at increased levels of p53 then activate cell death pathways.