The acidic tumor microenvironment promotes the reconversion of nitrite into nitric oxide: towards a new and safe radiosensitizing strategy.
OBJECTIVE
The biological status of nitrite recently evolved from an inactive end product of nitric oxide catabolism to the largest intravascular and tissue storage of nitric oxide (NO). Although low partial O(2) pressure favors enzymatic reconversion of nitrite into NO, low pH supports a nonenzymatic pathway. Because hypoxia and acidity are characteristics of the tumor microenvironment, we examined whether nitrite injection could preferentially lead to NO production in tumors and influence response to treatments.


METHODS
The effects of nitrite were evaluated on arteriole vasorelaxation, tumor cell respiration and tumor blood flow, oxygenation, and response to radiotherapy.


RESULTS
We first showed that a small drop in pH (-0.6 pH unit) favored the production of bioactive NO from nitrite by documenting a higher cyclic guanosine 3',5'-monophosphate-dependent arteriole vasorelaxation. We then documented that an i.v. bolus injection of nitrite to tumor-bearing mice led to a transient increase in partial O(2) pressure in tumor but not in healthy tissues. Blood flow measurements failed to reveal an effect of nitrite on tumor perfusion, but we found that O(2) consumption by nitrite-exposed tumor cells was decreased at acidic pH. Finally, we showed that low dose of nitrite could sensitize tumors to radiotherapy, leading to a significant growth delay and an increase in mouse survival (versus irradiation alone).


CONCLUSIONS
This study identified low pH condition (encountered in many tumors) as an exquisite environment that favors tumor-selective production of NO in response to nitrite systemic injection. This work opens new perspectives for the use of nitrite as a safe and clinically applicable radiosensitizing modality.
Frérart
Françoise
F
Unit of Pharmacology and Therapeutics, Université catholique de Louvain, UCL-FATH, Brussels, Belgium.
Sonveaux
Pierre
P
Rath
Géraldine
G
Smoos
Alexandra
A
Meqor
Ahlam
A
Charlier
Nicolas
N
Jordan
Bénédicte F
BF
Saliez
Julie
J
Noël
Agnès
A
Dessy
Chantal
C
Gallez
Bernard
B
Feron
Olivier
O
D000818:Animals; D001808:Blood Vessels / Q000187:drug effects; D006863:Hydrogen-Ion Concentration; D008113:Liver Neoplasms / Q000098:blood supply / Q000378:metabolism* / Q000532:radiotherapy; D008297:Male; D051379:Mice; D008815:Mice, Inbred Strains; D009569:Nitric Oxide / Q000378:metabolism*; D009573:Nitrites / Q000008:administration & dosage / Q000378:metabolism* / Q000494:pharmacology; D010101:Oxygen Consumption / Q000187:drug effects; D011838:Radiation-Sensitizing Agents; D014664:Vasodilation / Q000187:drug effects
D009573:Nitrites; D011838:Radiation-Sensitizing Agents; D009569:Nitric Oxide