Clonal variability in CD95 expression is the major determinant in Fas-mediated, but not chemotherapy-mediated apoptosis in the RPMI 8226 multiple myeloma cell line

CD95 (Fas/APO-1) is a member of the TNFR superfamily that induces apoptosis following cross-linking with its cognate ligand, CD95L (FasL/APO-1L) or ag onist antibody. The human myeloma cell line, RPMI 8226, has limited sensiti vity to CD95-mediated apoptosis, with a maximum of 65% of the population re sponding. To determine the source of the limited sensitivity to CD95-mediat ed apoptosis, we isolated multiple clones from the RPMI-8226 cell line by l imiting dilution. Analysis of these clones demonstrated that sensitivity to CD95-mediated cell death directly correlated with CD95 expression. Clones with high levels of CD95 expression had greater than 90% cell death, wherea s cells with low levels of expression had less than 10% cell death. In cont rast, no correlative differences were identified for other members of the D ISC complex, or for members of the anti-apoptotic Bcl-2 family, We further examined the sensitivity of the 8226 clones to various cytotoxic agents. Al though modest clonal variability was demonstrated in response to the chemot herapeutic drugs, doxorubicin, etoposide (VP-16), and vincristine, there wa s no correlation between CD95 function and sensitivity to chemotherapeutic drugs. These results indicate that in this cell line, receptor expression i s rate limiting in CD95-mediated apoptosis, whereas CD95 expression was not a determinant in drug-induced programmed cell death.