Ionizing radiation potentiates the induction of nitric oxide synthase by IFN-γ and/or LPS in murine macrophage cell lines: role of TNF-α

Macrophages are activated to become cytotoxic by a highly coordinated set of cytokine signals. Ionizing radiation can mimic cytokine signals and lead to enhanced states of activation. We tested the ability of gamma-radiation, alone and with interferon-gamma (IFN-gamma) and/or lipopolysaccharide (LPS), to induce nitric oxide (NO) production in J774.1 and RAW264.7 murine macrophages. NO was induced weakly, moderately, or strongly by IFN-gamma alone, LPS alone, or IFN-gamma + LPS, respectively. Radiation alone (0.5-50 Gy) did not induce NO, but enhanced NO production in a dose-dependent manner (0.5-5 Gy) when cells were exposed to IFN-gamma or LPS 24 h post-irradiation. Immunoblots showed parallel induction of nitric oxide synthase (NOS2). Application of anti-tumor necrosis factor alpha (TNF-alpha) antibody before irradiation blocked induction of NO by IFN-gamma. We conclude (1) that irradiated cells produce more NO in response to either IFN-gamma or LPS and (2) that the increase is mediated by induction of TNF-alpha.