Paclitaxel triggers cell death primarily via caspase-independent routes in the non-small cell lung cancer cell line NCI-H460.

Purpose: Here we report on the role of mitochondria, death receptors (DRs), and caspases in exerting the cytotoxic effect of clinically relevant concentrations of paclitaxel in the non-small cell lung cancer cell line NCI-H460.

Experimental Design: We have characterized paclitaxel-induced cell death with annexin V, propidium iodide staining, and poly(ADP-ribose) polymerase cleavage assays. The involvement of the mitochondria pathway was studied by monitoring cytochrome c release and using H460 cells stable in overexpressing Bcl-2 or Bcl-x(L). DR dependency was analyzed in FADD dominant-negative or cytokine response modifier A-overexpressing cells, and a possible role for DR4 and DR5 was investigated by antagonistic antibodies. Caspase activity and cleavage assays and treatment with the synthetic inhibitor zVAD-fmk were used to determine the involvement of caspases.

Results: Paclitaxel-treated cells displayed several features of apoptosis, including annexin V staining and poly(ADP-ribose) polymerase cleavage. The sequence of events suggested the involvement of a DR, as indicated by an early role for Fas-associated death domain and caspase-8, followed by cleavage of Bid and the disruption of mitochondria; nonetheless, we failed to demonstrate the involvement of DR4 and DR5. Interestingly, inhibition of either one of these routes only resulted in a 30% reduction of cell death that was in line with the observed small effect of caspase inhibition by zVAD-fmk on H460 cell survival.

Conclusion: Paclitaxel triggers cell death in H460 cells mainly via a currently unidentified caspase-independent mechanism in which the basic apoptotic machinery is merely coactivated. This finding is in sharp contrast with the largely caspase-dependent response elicited by DNA-damaging agents in these cells. We speculate on therapeutic implications.