Levels of Superoxide Dismutases, Glutathione, and Poly(ADP‐ribose) Polymerase in Radioresistant Human KB Carcinoma Cell Line

In order to investigate the radioresistance mechanism of human carcinoma cells, we measured intracellular manganese‐ (Mn‐) and copper/zinc‐ (Cu/Zn‐) superoxide dismutases (SODs), glutathi‐one (GSH) and poly(ADF‐ribose) polymerase (PARP) in radioresistant N10 and its parental KB cell lines. The Mn‐SOD level was 1.3‐fold less in N10 than in KB, but Mn‐SOD was induced at 1.3 to 1.5‐fold higher level in N10 than in KB by X‐irradiation (4‐ Gy). Cu/Zn‐SOD in N10 showed a higher level than that in KB both without and with irradiation. In addition, N10 had a 1.65‐fold higher GSH level than did KB and became radiosensitive on treatment with buthionine sulfoximine, an inhibitor of GSH. Furthermore, PARP mRNA was highly expressed in N10 as compared to KB under unirradiated conditions. X‐Irradiation reduced the PARP mRNA level in KB in a time‐dependent manner, whereas the PARP mRNA level in N10 was still high at 6 h postirradiation. Assay for PARP activity demonstrated an approximately 3‐fold higher activity in N10 than in KB under unirradiated conditions. X‐Irradiation caused a rapid induction of PARP activity within 1 h in both cell lines, but treatment of cells with nicotinamide, a PARP inhibitor, markedly reduced the enzyme induction in N10, but not in KB, and potentiated the radiosensitivity in N10. These factors may all contribute to the radioresistance of the N10 cell line.