The isoquinoline sulfonamide H7 attenuates radiation-mediated protein kinase C activation and delays the onset of x-ray-induced G2 arrest

Protein kinase C activation by ionizing radiation in human tumor cell lines participates in the transcriptional activation of genes which may be associated with the phenotypic response of cells to x-rays. We γ-irradiated cell line RIT-3 (radiation-induced human sarcoma) and quantified the phosphorylating capacity of protein kinase C. Protein kinase C activity increased rapidly and transiently in these cells. The selective protein kinase C inhibitor H7 attenuated radiation-mediated protein kinase C activation when added to cells prior to irradiation. To determine whether protein kinase C activation is associated with radiation-induced G2 arrest, we analyzed the cell cycle distribution of cells following γ-irradiation. Following irradiation, RIT-3 cells rapidly progressed through GI and S and subsequently underwent a dose dependent G2 arrest. At concentrations which are selective for protein kinase C inhibition, H7 delayed the onset of radiation-induced G2 arrest. However, there was no difference in the duration of G2 arrest following the addition of inhibitor as compared to cells irradiated without inhibitor. We propose that protein kinase C activation by ionizing radiation is associated with radiation-mediated cell cycle regulation.