Role of nitric oxide in radiation-induced initiation of mammary tumorigenesis in rats

Nitric oxide (NO) and its reaction products have been shown to cause DNA damage and to be mutagenic. To elucidate whether NO produced by irradiation participates in the initiation of mammary tumorigenesis, we performed experiments using the nitric oxide-specific scavenger Fe2+-diethyidithiocarbamate complex (Fe(DETC)(2)) or a selective inhibitor for inducible nitric oxide synthase (iNOS), S, S'-(4-phenylene-bis(1,2-ethanedinyl))bis-isothiourea (1,4-PB-ITU). Mother rats at day 21 of lactation were injected simultaneously with diethyldithiocarbamate intraperitoneally and Fe2+-citrate subcutaneously to form Fe(DETC)(2), in vivo, and then irradiated with 1.5 Gy gamma-rays immediately after the injection. An additional injection of chemicals followed twice at 8 and 24 h after the irradiation in the same manner. Both control and treated rats were then implanted with diethylstilbestrol pellets as a tumor promoter. The mammary tumor incidence in the experimental group was significantly reduced to one-fourth of that in the irradiated-alone group as the control. On the other hand, when mother rats took drinking water containing 0.005% 1,4-PB-ITU for 6 days from 3 days prior to irradiation at day 21 of lactation, a low tumor incidence in the iNOS inhibitor-treated groups was observed in the 1-year period. This report is the first to show that the NO derived from iNOS is an important radical for radiation-induced initiation of tumorigenesis of mammary glands in rats. (C) 2002 Elsevier Science (USA). All rights reserved.