p53-mediated up-regulation of CD95 is not involved in genotoxic drug-induced apoptosis of human breast tumor cells

Induction of CD95 (Fas/APO-1) and CD95 ligand during chemotherapeutic treat ment may contribute to the death by apoptosis of some tumor cells, In this study, we have analyzed the role of the CD95 system in genotoxic drug-induc ed death of human breast tumor cells, Incubation of the breast tumor cell l ines MCF-7 and EVSA-T with doxorubicin or methotrexate caused apoptosis aft er 48 h of treatment, These drugs induced a marked increase in the level of CD95 mRNA and protein in wild-type p53-expressing MCF-7 cells. On the cont rary, the breast cancer cell line EVSA-T that expresses high levels of an i nactive form of p53, did not up-regulate CD95 upon drug treatment. Elevatio n of CD95 expression by DNA-damaging drugs was notably blocked in MCF-7 cel ls expressing the human papillomavirus type 16 E6 protein (E6 cells) which prevented p53 accumulation upon DNA damage, However, E6 cells were still ki lled by the drugs. Furthermore, the genotoxic drugs did not induce the expr ession of CD95 ligand in MCF-7 cells at doses that caused apoptosis in thes e breast tumor cells. Moreover, drug-induced apoptosis of breast tumor cell s was not prevented in the presence of either a CD95 antagonistic antibody or a CD95 ligand blocking antibody. We also observed a strong synergism bet ween lower doses of DNA-damaging drugs and CD95 agonistic antibody in the i nduction of apoptosis in MCF-7 cells. In summary, our data indicate that dr ug-induced apoptosis of breast tumor cells occurs by a CD95/CD95L-independe nt mechanism although by elevating the tumor suppressor proteins p53 and CD 95, genotoxic drugs may sensitize breast tumor cells to CD95-mediated apopt osis.