Modulation of NADPH oxidase by ionizing radiation and its role in radiation-induced oxidative stress and inflammation in brain endothelium.

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Progressive dementia occurs in some 20-50% of brain tumor patients who are long-term survivors after treatment with brain irradiation. The need to both understand and minimize the side effects of brain irradiation is exacerbated by the ever-increasing number of patients with brain metastases that require treatment with large field or whole brain irradiation (WBI); some 200,000 cancer patients/year receive large field or WBI. At the present time, there are no successful treatments for radiation-induced brain injury, nor are there any known effective preventive strategies. Data support a role for the renin-angiotensin system (RAS) in radiation-induced late effects in kidney, lung; both angiotensin-converting enzyme inhibitors (ACEI) and angiotensin II (Ang II) receptor antagonists (ATRA) have proved effective. However, the pathogenic mechanism(s) involved remains unknown. Ang II signaling is mediated by generation of reactive oxygen species (ROS) via activation of nicotinamide adenosine dinucleotide phosphate (NADPH) oxidase. NADPH oxidase is a multi-subunit enzyme localized to cell membranes whose activation requires the translocation of cytosolic subunits p47^phox,p67^phox, and Rac1. Activated NADPH oxidase converts molecular oxygen (O₂) to the superoxide anion (O₂^.-). We hypothesize that brain irradiation leads to activation of the intrinsic RAS and the subsequent induction of proinflammatory mediators regulated via the downstream NADPH oxidase signaling pathway. We report that ionizing radiation upregulated components of NADPH oxidase, namely Nox-4, p22^phox, and p47^phox. We further report that ionizing radiation increased i] expression of the proinflammatory transcription factor NFκB and ii] expression of proinflammatory mediators PAI-1, ICAM-1, and VCAM-1. This increase appeared to be NADPH oxidase-dependent; pharmacological (apocynin) inhibition of NADPH oxidase blocked the radiation-mediated upregulation of these proinflammatory mediators. These results suggest that the brain RAS may play a role in radiation-induced brain injury through activation of NADPH oxidase and subsequent upregulation of proinflammatory mediators.