Th. Holmstrom et al., Inhibition of mitogen-activated kinase signaling sensitizes HeLa cells to Fas receptor-mediated apoptosis, MOL CELL B, 19(9), 1999, pp. 5991-6002
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.