Inhibition of mitogen-activated kinase signaling sensitizes HeLa cells to Fas receptor-mediated apoptosis

The Fas receptor (FasR) is an important physiological mediator of apoptosis in various tissues and cells. However, there are also many FasR-expressing cell types that are normally resistant to apoptotic signaling through this receptor. The mitogen-activated protein kinase (MAPK) signaling cascade ha s, apart from being a growth-stimulating factor, lately received attention as an inhibitory factor in apoptosis. In this study, we examined whether MA PK signaling could be involved in protecting FasR-insensitive cells, To thi s end, we used different approaches to inhibit MAPK signaling in HeLa cells , including treatment with the MAPK kinase inhibitor PD 98059, serum withdr awal, and expression of dominant-interfering MAPK kinase mutant protein. Al l of these treatments were effective in sensitizing the cells to FasR-induc ed apoptosis, demonstrating that MAPK indeed is involved in the control of FasR responses. The MAPK-mediated control seemed to occur at or upstream of caspase 8, the initiator caspase in apoptotic FasR responses. Transfection with the constitutively active MAPK kinase abrogated FasR-induced apoptosi s also in the presence of cycloheximide, indicating that the MAPK-generated suppression of FasR-mediated apoptotic signaling is protein synthesis inde pendent, In cells insensitive to FasR-induced apoptosis, stimulation of the FasR with an agonistic antibody resulted in significant MAPK activation, w hich was inhibited by PD 98059. When different cell types were compared, th e FasR-mediated MAPK activation seemed proportional to the degree of FasR i nsensitivity. These results suggest that the FasR insensitivity is likely t o be a consequence of FasR-induced MAPK activation, which in turn interfere s with caspase activation.