25. Effect of X-lrradiation on NOS Activity in L1210 Cells

Toyoko Arimoto, Hidehiko Nakagawa, Keizo Takeshita, Hideyuki Majima, Junichi Ueda, Nobuo Ikota and Toshihiko Ozawa

Keywords: nitric oxide, nitric oxide synthase (NOS), irradiation, -tocopherol

Nitric oxide (NO) is an important mediator of cellular communication in several biological systems. NO is formed by nitric oxide synthase (NOS), which requires 02- dependent oxidation of L-arginine to citrulline and NO. An inducible type of NOS (iNOS) is expressed by cytokines, such as tumor necrosis factor- (TNF-) and interleukin 1 (IL-1), as well as lipopolysaccharide (LPS) in many types of cells including macrophages and smooth muscle cells. Overproduction of NO by iNOS in response to inflammatory cytokines has been observed to trigger apoptotic cell death in several types of cells.

This study was conducted to determine the effect of exposure to X-irradiation on NOS activity in L1210 cells. When cells were irradiated with 6 Gy, the level of nitrite in the culture medium increased 48 h after the irradiation. L-mono methyl-L-arginine (L-NMMA), a potent inhibitor of NOS, suppressed the nitrite production, suggesting that iN0S was expressed. The expression of iNOS requires activation of NF- B, a pleiotropic transcriptional regulatory factor responsive to reactive oxygen species (ROS). Activation of this factor is essential for iNOS gene induction by cytokines and LPS. To examine whether ROS are involved in the accumulation of nitrite after exposure to X-irradiation, the effect of the antioxidant, - tocopherol (-Toc), on X-ray-induced nitrite production was examined. Addition of -Toc to the medium of L1210 cells, no significant effect on nitrite production was observed. The mechanism underlying the effect of exposure to X-irradiation on NOS activity in L1210 cells requires examination.