p53-dependent effects of RAS oncogene on chromosome stability and cell cycle checkpoints

Mutations activating the function of ras proto-oncogenes are often observed in human tumors. Their oncogenic potential is mainly due to permanent stim ulation of cellular proliferation and dramatic changes in morphogenic react ions of the cell. To learn more on the role of ras activation in cancerogen esis we studied its effects on chromosome stability and cell cycle checkpoi nts. Since the ability of ras oncogenes to cause cell transformation may be dependent on activity of the p53 tumor-suppressor the cells with different p53 state were analysed. Ectopic expression of N-ras(asp12) caused in p53- deficient MDAH041 cell line an augmentation in the number of chromosome bre aks in mitogenic cells, significant increase in the frequency of metaphases showing chromosome endoreduplication and accumulation of polyploid cells. Similar effects were induced by different exogenous ras genes (N-ras(asp12) , H-ras(leu12), N-ras proto-oncogene) in Rat1 and Rat2 cells which have a d efect in p53-upstream pathways. In contrast, in REF52 and human LIM1215 cel ls showing ras-induced p53 upregulation, ras expression caused only slight increase in the number of chromosome breaks and did not enhance the frequen cy of endoreduplication and polyploidy. Inactivation in these cells of p53 function by transduction of dominant-negative C-terminal p53 fragment (gene tic suppressor element #22, GSE22) or mutant p53s significantly increased t he frequency of both spontaneous and uas-induced karyotypic changes. In con cordance with these observations we have found that expression of uns oncog ene caused in p53-defective cells further mitigation of ethyl-metansulphona te-induced G1 and G2 cell cycle arrest, but did not abrogate G1 and G2 cell cycle checkpoints in cells with normal p53 function. These data indicate t hat along with stimulation of cell proliferation and morphological transfor mation uas activation can contribute to cancerogenesis by increasing geneti c instability.