Cyclooxygenase-2 (COX-2) inhibitors sensitize tumor cells specifically to death receptor-induced apoptosis independently of COX-2 inhibition

Cyclooxygenase-2 (COX-2) is involved in diverse processes such as inflammation, carcinogenesis and apoptosis. As COX-2 inhibitors interfere with these processes, inhibition of COX-2 has been suggested as a promising anticancer treatment. However, the role of COX-2 in modulation of apoptosis as well as the death pathways affected by COX-2 inhibitors are poorly characterized. Here we demonstrate that the selective COX-2 inhibitors NS-398 and nimesulide increased TNF sensitivity of TNF-resistant HeLa H21 and TNF-sensitive HeLa D98 cells, although this cytokine induced significant COX-2 activity, as judged by prostaglandin E2 (PGE2) production, only in H21 cells. TNF did also not induce PGE2 production in MCF-7/casp-3 cells stably expressing COX-2; however, nimesulide strongly enhanced TNF-induced apoptosis in these cells. Furthermore, COX-2 activity in HeLa H21 cells could be inhibited by NS-398 concentrations that were 10 000-fold lower compared to those required for the induction of cell death. Most intriguingly, sensibilization to apoptosis was specifically observed in response to activation of death receptors. Not only TNF-induced cell death but also apoptosis triggered by the CD95 and TRAIL receptors was enhanced by nimesulide. In contrast, apoptosis induced by the anticancer drugs doxorubicine and etoposide that target the mitochondrial death pathway remained unaffected. Together, our data suggest that COX-2 inhibitors overcome apoptosis resistance and selectively sensitize tumor cells to the extrinsic death receptor-induced apoptotic pathway independently of COX-2.