Effects of antioxidants on X-ray- or hyperthermia-induced apoptosis in human lymphoma U937 cells

Hydroxyl radicals (·OH) and superoxide anion radicals (O2·−) are known to play cardinal roles in cell killing and various types of cell damage. In order to elucidate the mechanism of the involvement of both free radicals on apoptosis, the correlation between anti-apoptotic effects and free radical scavenging abilities of anti-oxidants was studied. As an indicator of anti-apoptotic effects, C1/2 (antioxidant concentration to inhibit DNA fragmentation by 50%) was evaluated in human lymphoma cell line U937 cells 6 hr after X-ray (10 Gy) or hyperthermia (44°C, 30 min) treatment. Rate constants of the reactions between antioxidants and ·OH or O2·− were calculated as the scavenging ability of the antioxidants with graded concentration estimated by EPR spectroscopy. No apparent correlation between C1/2 obtained in apoptosis induced by X-rays or hyperthermia and the rate constants of antioxidants for ·OH or O2·− was observed. On the other hand, the partition coefficients in 1-octanol/water of the antioxidants, an indicator of hydrophobicity, revealed a correlation with the C1/2 of the agents with hyperthermia, but not with X-ray irradiation. These results indicate that the prevention of apoptosis by an antioxidant is not simply associated with its scavenging ability for ·OH or O2·−. The hydrophobicity of the antioxidant, among other possible factors, is involved in the inhibition of hyperthermia- induced apoptosis.