Escape from premature senescence is not sufficient for oncogenic transformation by Ras

Resistance of primary cells to transformation by oncogenic Ras has been att ributed to the induction of replicative growth arrest(1-3). This irreversib le 'fail-safe mechanism' resembles senescence and requires induction by Ras of p19(ARF) and p53 (refs 3-5). Mutation of either p19ARF or p53 alleviate s Ras-induced senescence and facilitates oncogenic transformation by Ras(3, 6,7). Here we report that, whereas Rb and p107 are each dispensable for Ras -induced replicative arrest, simultaneous ablation of both genes disrupts R as-induced senescence and results in unrestrained proliferation. This occur s despite activation by Ras of the p19(ARF)/p53 pathway, identifying pRb an d p107 as essential mediators of Ras-induced antiproliferative p19ARF/p53 s ignalling. Unexpectedly, in contrast to p19ARF or p53 deficiency, loss of R b/p107 function does not result in oncogenic transformation by Ras, as Ras- expressing Rb-/-/p107(-/-) fibroblasts fail to grow anchorage-independently in vitro and are not tumorigenic in vivo. These results demonstrate that i n the absence of both Rb and p107 cells are resistant to p19(ARF)/p53-depen dent protection against Ras-induced proliferation, and uncouple escape from Ras-induced premature senescence from oncogenic transformation.