Both increased cell proliferation and apoptosis play important roles in the malignant growth of
glioblastomas. We have demonstrated recently that the differential expression of
protein kinase C (
PKC)-eta increases the proliferative capacity of
glioblastoma cells in culture; however, specific functions for this novel PKC
isozyme in the regulation of apoptosis in these
tumors has not been defined. In the present study of several
glioblastoma cell lines, we investigated the role of
PKC-eta in preventing UV- and gamma-irradiation-induced apoptosis and in
caspase-dependent signaling pathways that mediate cell death. Exposure to UV or gamma irradiation killed 80% to 100% of
PKC-eta-deficient nonneoplastic human astrocytes and U-1242 MG cells, but had little effect on the
PKC-eta-expressing U-251 MG and U-373 MG cells.
PKC-eta appears to mediate resistance to irradiation specifically such that when
PKC-eta was stably expressed in U-1242 MG cells, more than 80% of these cells developed resistance to irradiation-induced apoptosis. Reducing
PKC-eta expression by transient and stable expression of antisense
PKC-eta in wild-type U-251 MG cells results in increased sensitivity to UV irradiation in a fashion similar to U-1242 MG cells and nonneoplastic astrocytes. Irradiation of
PKC-eta-deficient
glioblastoma cells resulted in the activation of
caspase-9 and
caspase-3, cleavage of
poly (ADP-ribose) polymerase (PARP), and a substantial increase in subdiploid
DNA content that did not occur in
PKC-eta-expressing
tumor cells. A specific inhibitor (
Ac-DEVD-CHO) of
caspase-3 blocked apoptosis in
PKC-eta-deficient U-1242 MG cells. The data demonstrate that resistance to UV and gamma irradiation in
glioblastoma cell lines is modified significantly by
PKC-eta expression and that
PKC-eta appears to block the apoptotic cascade at
caspase-9 activation.