Tumor cell apoptosis by irradiation-induced nitric oxide production in vascular endothelium

We examined the effects of X-ray irradiation on endothelial nitric oxide (NO) production in bovine aortic endothelial cells (BAECs). Single irradiation of up to 60 Gy did not affect the expression of endothelial NO synthase (eNOS) mRNA, as assessed by reverse transcription-PCR, in BAECs. The basal level of intracellular Ca²⁺ concentration and Ca²⁺ mobilization induced by thapsigargin and ATP were also not affected by single irradiation. However, eNOS-mediated, Ca²⁺-dependent constitutional NO production could not be examined directly because irradiated BAECs showed continuous NO production, as measured by diaminofluorescein-2 fluorescence, without agonist stimulation. Expression of inducible NO synthase (iNOS) mRNA and protein was markedly increased by 2 Gy of irradiation, thereby indicating that the basal and continuous NO production in irradiated BAECs was due to the expression of iNOS. Hepatoma HepG2 cells cocultured with irradiated BAECs showed apoptosis in the presence of L-arginine, and the apoptosis was prevented by L-NAME (N^ω-nitro-L-arginine methyl ester). These results indicate that single irradiation does not affect Ca²⁺ mobilization and eNOS expression but induces the expression of iNOS in BAECs, and the latter property would be beneficial to induce apoptosis in the adjacent tumor cells.