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 agonist antibody. The human myeloma cell line, RPMI 8226, has limited sensitivity to CD95-mediated apoptosis, with a maximum of 65% of the population responding. To determine the source of the limited sensitivity to CD95-mediated apoptosis, we isolated multiple clones from the RPMI-8226 cell line by limiting 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, whereas cells with low levels of expression had less than 10% cell death. In contrast, no correlative differences were identified for other members of the DISC complex, or for members of the anti-apoptotic Bcl-2 family. We further examined the sensitivity of the 8226 clones to various cytotoxic agents. Although modest clonal variability was demonstrated in response to the chemotherapeutic drugs, doxorubicin, etoposide (VP-16), and vincristine, there was no correlation between CD95 function and sensitivity to chemotherapeutic drugs. These results indicate that in this cell line, receptor expression is rate limiting in CD95-mediated apoptosis, whereas CD95 expression was not a determinant in drug-induced programmed cell death.