Macrophages enhance the radiosensitizing activity of lipid A: A novel role for immune cells in tumor cell radioresponse

Abstract

This study examines whether activated macrophages may radiosensitize tumor cells through the release of proinflammatory mediators. RAW 264.7 macrophages were activated by lipid A, and the conditioned medium (CM) was analyzed for the secretion of cytokines and the production of nitric oxide (NO) through inducible nitric oxide synthase (iNOS). EMT-6 tumor cells were exposed to CM and analyzed for hypoxic cell radiosensitivity. The role of nuclear factor (NF)-kappaB in the transcriptional activation of iNOS was examined by luciferase reporter gene assay. Clinical immunomodulator lipid A, at a plasma-relevant concentration of 3 microg/mL, stimulated RAW 264.7 macrophages to release NO, tumor necrosis factor (TNF)-alpha, and other cytokines. This in turn activated iNOS-mediated NO production in EMT-6 tumor cells and drastically enhanced their radiosensitivity. Radiosensitization was abrogated by the iNOS inhibitor aminoguanidine but not by a neutralizing anti-TNF-alpha antibody. The mechanism of iNOS induction was linked to NF-kappaB but not to JAK/STAT signaling. Interferon-gamma further increased the NO production by macrophages to a level that caused radiosensitization of EMT-6 cells through the bystanding effect of diffused NO. We demonstrate for the first time that activated macrophages may radiosensitize tumor cells through the induction of NO synthesis, which occurs in both tumor and immune cells.