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

Abstract

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.

• Authors: Heng Jiang, Mark De Ridder, Valeri N Verovski, Pierre Sonveaux, Bénédicte F Jordan, Kalun Law, Christinne Monsaert, Dirk L Van den Berge, Dirk Verellen, Olivier Feron, Bernard Gallez, Guy A Storme
• Journal: International journal of radiation oncology, biology, physics (Int J Radiat Oncol Biol Phys) Vol. 76 Issue 5 Pg. 1520-7 (Apr 2010) ISSN: 1879-355X [Electronic] United States
• PMID: 20338478 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)

Chemical References

• Guanidines
• Lipopolysaccharides
• Nitric Oxide
• Interferon-gamma
• Nitric Oxide Synthase
• Nitric Oxide Synthase Type II
• Heme Oxygenase-1
• pimagedine

Topics

• Animals
• Cell Hypoxia (physiology)
• Cell Line, Tumor
• Cell Respiration
• Electron Spin Resonance Spectroscopy
• Enzyme Activation
• Guanidines (pharmacology)
• Heme Oxygenase-1 (biosynthesis)
• Interferon-gamma (pharmacology)
• Lipopolysaccharides (pharmacology)
• Macrophage Activation
• Macrophages (enzymology)
• Mice
• Nitric Oxide (biosynthesis)
• Nitric Oxide Synthase (antagonists & inhibitors)
• Nitric Oxide Synthase Type II (antagonists & inhibitors, metabolism)
• Oxygen Consumption (physiology)
• Radiation Tolerance (physiology)