AGENTS THAT CAUSE DNA DOUBLE-STRAND BREAKS LEAD TO P16(INK4A) ENRICHMENT AND THE PREMATURE SENESCENCE OF NORMAL FIBROLASTS

The occurrence of DNA double strand breaks induces cell cycle arrest i n mortal and immortal human cells, In normal, mortal fibroblasts this block to proliferation is permanent, It depends on the growth regulato r p53 and a protein p53 induces, the cyclin dependent kinase inhibitor , p21. We show here that following DNA damage in mortal fibroblasts, t he induction of p21 and p53 is to a large degree shortlived. By 8 days after a brief exposure to DNA strand breaking agents, bleomycin or ac tinomycin D, p53 protein is at baseline levels, while the p53 transact ivation level is only slightly above its baseline, By this time the co ncentration of p21 protein, which goes up as high as 100-fold shortly after treatment, is down to just 2-4-fold over baseline levels, Follow ing the drop in p21 concentration a large increase in the expression l evel of the tumor suppressor gene p16(INK4a) is observed. This scenari o, where a transient increase in p21 is followed by a delayed inductio n of p16(INK4a), also happens with the permanent arrest that occurs wi th cellular senescence. In fact, these cells treated with agents that cause DNA double strand breaks share a number of additional markers wi th senescent cells, Our findings indicate that these cells are very si milar to senescent cells and that they have additional factor(s) besid e p21 and p53 that maintain cell cycle arrest.