Nitric oxide(NO) is a major cytotoxic effector molecule of activated macrophages. Although the antitumor activity of NO produced by activated macrophages was demonstrated in in vitro experiments, the in vivo role of tumor infiltrating macropheges in tumor growth is currently uncertain. It is also unknown that tumor infiltrating macrophages produce NO, and thereby contribute either a host antitumor immune defense mechanism or a promoting mechanism of in vivo tumor growth by suppressing immune function. The purpose of the current study was to evaluate whether the NO synthesizing pathway is activated in in vivo growing tumor tissue, and thereby the produced NO inhibits tumor growth using various murine skin tumor models. Further experiments were designed to test whether tumor infiltrating macrophages are the major source of NO produced by the in vivo growing tumor tissue. Freshly diesociated murine skin tumors were found to have much higher levels of nitric axide secretion into culture medi- um than long-term cultured tumor. Immunomagnetic depletion of macrophages from freshly dissociated tumor cells using anti-Mac-1 antibody decreased NO production(17.5+-1.5 vs 5.0+-1.1 uM nitrite). Addition of the nitric oxide synthase inhibitor N-monomethyl-L-arginine(MLA) resulted in a dose-dependent decrease in nitric oxide synthesis in freshly dissociated tumor. There was a reciprocal relationship of NO synthesis with tritiated thymidine incorporation in these cultures. Strong iron-dithiol-dinitrosyl and heme-nitrosyl electron paramagnetic resonance signals were observed in freshly dissociated, but not long-term cultured tumor cells. Inhibition of NO synthesis using in vivo MLA administration resulted in a trend of increased growth rate of RD-995 murine skin cancer. This study demonstrates the role of macrophage NO synthesis as a host defense against tumor in vivo.
PDF Links
Full text via DOI 
Download Citation 
