1. Int J Radiat Oncol Biol Phys. 2010 Apr;76(5):1520-7.

Activated macrophages as a novel determinant of tumor cell radioresponse: the
role of nitric oxide-mediated inhibition of cellular respiration and oxygen
sparing.

Jiang H, De Ridder M, Verovski VN, Sonveaux P, Jordan BF, Law K, Monsaert C, Van 
den Berge DL, Verellen D, Feron O, Gallez B, Storme GA.

Vrije Universiteit Brussel, Cancer Research Unit, Brussels, Belgium.

PURPOSE: Nitric oxide (NO), synthesized by the inducible nitric oxide synthase
(iNOS), is known to inhibit metabolic oxygen consumption because of interference 
with mitochondrial respiratory activity. This study examined whether activation
of iNOS (a) directly in tumor cells or (b) in bystander macrophages may improve
radioresponse through sparing of oxygen. METHODS AND MATERIALS: EMT-6 tumor cells
and RAW 264.7 macrophages were exposed to bacterial lipopolysaccharide plus
interferon-gamma, and examined for iNOS expression by reverse transcription
polymerase chain reaction, Western blotting and enzymatic activity. Tumor cells
alone, or combined with macrophages were subjected to metabolic hypoxia and
analyzed for radiosensitivity by clonogenic assay, and for oxygen consumption by 
electron paramagnetic resonance and a Clark-type electrode. RESULTS: Both tumor
cells and macrophages displayed a coherent picture of iNOS induction at
transcriptional/translational levels and NO/nitrite production, whereas
macrophages showed also co-induction of the inducible heme oxygenase-1, which is 
associated with carbon monoxide (CO) and bilirubin production. Activation of iNOS
in tumor cells resulted in a profound oxygen sparing and a 2.3-fold
radiosensitization. Bystander NO-producing, but not CO-producing, macrophages
were able to block oxygen consumption by 1.9-fold and to radiosensitize tumor
cells by 2.2-fold. Both effects could be neutralized by aminoguanidine, a
metabolic iNOS inhibitor. An improved radioresponse was clearly observed at
macrophages to tumor cells ratios ranging between 1:16 to 1:1. CONCLUSIONS: Our
study is the first, as far as we are aware, to provide evidence that iNOS may
induce radiosensitization through oxygen sparing, and illuminates NO-producing
macrophages as a novel determinant of tumor cell radioresponse within the hypoxic
tumor microenvironment.

PMID: 20338478 [PubMed - indexed for MEDLINE]