Distinct molecular mechanisms of Fas resistance in murine B lymphoma cells

A panel of murine B lymphoma cell lines, which express different levels of Fas, was extensively studied for sensitivity to Fas-mediated death signals via an anti-Fas mAb and Fas ligand-bearing cell lines. Expression of the Fas receptor on the B lymphoma cell lines did not correlate with their capacity to undergo Fas-mediated apoptosis. Moreover, Fas-associated death domain protein recruitment to the death-inducing signaling complex (DISC) complex occurred in all cell lines expressing Fas, regardless of whether they were sensitive to Fas-mediated death. Interestingly, the protein synthesis inhibitor, cycloheximide, and protein kinase C inhibitors, such as bisindolylmaleimide, rendered one of the resistant cell lines, CH33, sensitive to signals from the Fas receptor, although the levels of Fas were unchanged. This suggests that constitutive PKC activation plays a role in Fas resistance, perhaps by up-regulating NF-kappa B or Bcl-2 family members. Interestingly, CH33 demonstrated caspase 8 activity upon engagement of the Fas receptor in the absence of pharmacological manipulation, suggesting that the block in apoptosis is downstream of the DISC complex. In contrast, the fact that Fas-associated death domain protein was recruited to the DISC complex in other resistant lines, such as WEHI-231, with no caspase 8 activation indicates that these cells may be blocked within the DISC complex. Indeed, Western blot analysis showed that WEHI-231 expressed an isoform of FLICE-like inhibitory protein (cFLIP(L)), an antiapoptotic protein within the DISC. These studies provide evidence that murine B lymphoma cells utilize different molecular mechanisms along the Fas-signaling cascade to block apoptosis.