RAPAMYCIN AND P53 ACT ON DIFFERENT PATHWAYS TO INDUCE G1 ARREST IN MAMMALIAN-CELLS

Certain growth regulatory kinases contain domain related to the phosph o-inositol 3 (PI-3) kinase catalytic site, These include the ATM gene product, DNA-PKcs, and the target of rapamycin (TOR in yeast: and FRAP in mammalian cells), Rapamycin inhibits growth factor signalling and induces G1 arrest in many cell types, Some growth regulatory PI-3 kina ses appear functionally linked to p53 and we have explored potential l inks between cellular effects induced by rapamycin and p53. In p53 nul l cells rapamycin inhibited cell cycling but did not induce G1 arrest, In cells which showed selective G1 arrest in response to rapamycin, r apamycin had no effect on basal levels of p53 protein, Similarly p21(W AF1) protein,vas not induced by rapamycin, The kinetics of the cellula r p53/p21(WAF1) response to ionising radiation was unaffected by rapam ycin; and the ability of growth factor to protect against p53-mediated apoptosis in response to DNA damage was also unaffected by rapamycin. The ATM gene is mutated in the cancer susceptibility syndrome ataxia telangiectasia (AT) but such mutant cells showed a similar sensitivity to rapamycin compared to their normal counterparts, RKO cell lines of common genetic background, but with different levels of functional p5 3 protein, also responded similarly to rapamycin, Thus, although rapam ycin and p53 are each able to induce G1 arrest, they appear to act thr ough independent growth regulatory pathways.