Macrophages enhance the radiosensitizing activity of lipid A: a novel role for immune cells in tumor cell radioresponse.
OBJECTIVE
This study examines whether activated macrophages may radiosensitize tumor cells through the release of proinflammatory mediators.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
De Ridder
Mark
M
Academic Hospital, Free University Brussels, Oncology Center, Cancer Research Unit, Brussels, Belgium.
Verovski
Valeri N
VN
Darville
Martine I
MI
Van Den Berge
Dirk L
DL
Monsaert
Christinne
C
Eizirik
Decio L
DL
Storme
Guy A
GA
D000818:Animals; D015687:Cell Hypoxia; D045744:Cell Line, Tumor; D017077:Culture Media, Conditioned / Q000737:chemistry; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D006146:Guanidines / Q000494:pharmacology; D007371:Interferon-gamma / Q000494:pharmacology; D007378:Interleukins / Q000032:analysis; D008050:Lipid A / Q000494:pharmacology*; D008262:Macrophage Activation; D008264:Macrophages / Q000187:drug effects* / Q000276:immunology / Q000378:metabolism; D015674:Mammary Neoplasms, Animal / Q000378:metabolism / Q000532:radiotherapy*; D051379:Mice; D008807:Mice, Inbred BALB C; D016328:NF-kappa B / Q000378:metabolism; D009569:Nitric Oxide / Q000096:biosynthesis*; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors / Q000378:metabolism; D052247:Nitric Oxide Synthase Type II; D009573:Nitrites / Q000032:analysis; D011836:Radiation Tolerance; D014409:Tumor Necrosis Factor-alpha / Q000032:analysis / Q000096:biosynthesis
D017077:Culture Media, Conditioned; D004791:Enzyme Inhibitors; D006146:Guanidines; D007378:Interleukins; D008050:Lipid A; D016328:NF-kappa B; D009573:Nitrites; D014409:Tumor Necrosis Factor-alpha; D009569:Nitric Oxide; D007371:Interferon-gamma; D019001:Nitric Oxide Synthase; D052247:Nitric Oxide Synthase Type II; C496317:Nos2 protein, mouse; C004479:pimagedine