Unique resistance of breast carcinoma cell line T47D to TRAIL but not anti-Fas is linked to p43cFLIP_L

The majority of breast cancer cell lines are resistant to tumor necrosis factor -related apoptosis inducing ligand (TRAIL) induced apoptosis. TRAIL and Fas receptor death-inducing signaling complex (DISCs) formation are similar and involve ligand-dependent recruitment of FADD and caspase-8. We have found that the breast carcinoma cell line T47D is an unusual example of selective sensitivity to anti-Fas mAb treatment but resistant to TRAIL. Therefore, a detailed comparison of these two signaling pathways in one cell line should provide insight into the mechanism of TRAIL resistance. We observed that only anti-Fas mAb induces caspase activation and cell death in T47D. Further, FADD and caspase-8 interact with both TRAIL-R1 and TRAIL-R2, and that the amount of caspase-8 recruited by Fas-, TRAIL-R1 and TRAIL-R2 are the same. cFLIP_S and cFLIP_R isoforms block death receptor-induced apoptosis by inhibiting caspase-8 activation at the DISC; the role of cFLIP_L at the DISC is still controversial. It has been suggested that the presence of the cleaved form of FLIP_L-p43 at the DISC prevents caspase-8 cleavage. We found that both TRAIL and anti-Fas mAb-induced DISCs contain the cleaved form of p43 cFLIP_L and its amount at the Fas DISC was higher compared to the TRAIL DISC. We also found that inhibition of cFLIP_L expression in T47D cells decreased Fas-mediated caspase-8 activation and activation of effector caspases. We propose that in T47D p43 cFLIP_L in the Fas-DISC may promote caspase-8 activation. The mechanism by which different amounts of p43cFLIP_L regulates caspase-8 activation remains to be investigated.