OVEREXPRESSION OF P21(WAF1) LEADS TO INCREASED INHIBITION OF E2F-1 PHOSPHORYLATION AND SENSITIVITY TO ANTICANCER DRUGS IN RETINOBLASTOMA-NEGATIVE HUMAN SARCOMA-CELLS

The effect of overexpression of p21(waf1) On drug sensitivity was stud ied in an osteosarcoma cell line (SaOs-2) lacking both p53 and functio nal retinoblastoma protein using a tetracycline (TC)-inducible express ion system, p21(waf1) expression was barely detectable in SaOS-2 cells incubated in the presence of TC. After TC withdrawal, high levels of p21(waf1) were induced in these cells. These p21(waf1)-induced cells s howed increased sensitivity to doxorubicin, tomudex, and methotrexate as compared to uninduced cells; this condition is associated with incr eased apoptosis. Expression of p21(waf1) reduced cyclin A-associated k inase activity and, surprisingly, resulted in inhibition of phosphoryl ation of E2F-1 and increased E2F-1 binding activity, An S-G(2) cell cy cle arrest/delay and an increase in expression of E2F-responsive genes (dihydrofolate reductase and thymidylate synthase) was correspondingl y observed, Overexpression of p21(waf1) in cells lacking functional re tinoblastoma protein may mediate sensitivity to anticancer drugs by in hibiting E2F-1 phosphorylation, which may contribute to increased S-G( 2) cell cycle delay and increased cell susceptibility to apoptosis.