Granulocyte-Macrophage Colony-Stimulating Factor Rescues TF-1 Leukemia Cells From Ionizing Radiation-Induced Apoptosis Through a Pathway Mediated by Protein Kinase Cα

Abstract

Protein kinase C (PKC) activity has a recognized role in mediating apoptosis. However, the role of individual PKC isoforms in apoptosis is poorly defined. Therefore, we investigated the translocation of individual PKC isoforms during radiation-induced apoptosis with and without rescue from apoptosis by granulocyte-macrophage colony-stimulating factor (GM-CSF) in the human erythroleukemia cell line TF-1. PKCα was translocated from the particulate to cytosolic fraction of TF-1 cells within 5 minutes of treatment with apoptosis-inducing levels of ionizing radiation. However, this postirradiation translocation did not occur when cells were rescued from apoptosis by GM-CSF. Furthermore, treatment of cells with Gö6976, an inhibitor of classical PKC isoforms, abrogated the rescue effect of GM-CSF. The calcium-independent novel PKC isoform, PKCδ appeared to be degraded in both the particulate and cytosolic fractions of TF-1 cells after treatment with apoptosis-inducing levels of ionizing radiation in either the presence or absence of GM-CSF rescue. Levels of ceramide, a lipid mediator of apoptosis, were measured at 2, 4, 8, 10, and 60 minutes after treatment with ionizing radiation and were substantially reduced in TF-1 cells rescued from apoptosis by GM-CSF compared with apoptotic TF-1 cells. The largest decrease in ceramide production seen was at 4 minutes postirradiation, with a 46% reduction in ceramide levels in TF-1 cells rescued from apoptosis by GM-CSF compared with those in apoptotic TF-1 cells. Because ceramide has been shown to affect PKCα subcellular distribution, these data implicate a role for ceramide in mediating the rapid postirradiation translocation and inhibition of PKCα in TF-1 cells not rescued from apoptosis by GM-CSF. Expression of the antiapoptotic protein Bcl-2 doubled in TF-1 cells rescued from apoptosis by GM-CSF, but did not increase in unrescued cells. Our findings suggest that activated PKCα and increased expression of Bcl-2 after γ irradiation determine survival in TF-1 cells rescued from apoptosis with GM-CSF and that PKCδ plays a role in mediating signals involved in sensing cellular damage and/or regulation of cell damage repair.