Abstract 1324: Modulation of IR induced NFKB activity by manipulation of NOS coupling

Abstract The transcriptional activity of NFκβ is in part regulated by nitric oxide (NO) and reactive nitrogen species (RNS). Previously we demonstrated that low doses of ionizing radiation (IR) activate constitutive, Ca2+ dependent NO Synthases (NOS) and the generation of RNS, resulting in protein Tyr nitration and/or Cys s-nitrosylation. Previous studies in our lab demonstrated that nitration of IκBα is critical for the IR-induced activation of NFκβ whereas others have shown that s-nitrosylation of the p65 subunit leads to the inhibition of NFκβ. Here we show that exposing MCF-7 cells to 5Gy IR results not only in Tyr nitration of IκBα but also Cys s-nitrosylation of NFκβ both with a maximum at 30min. post IR. One explanation for the discordant results is that NOS activity is uncoupled, resulting in the production of increased levels of superoxide anion as opposed to NO. Tetrahydrobiopterin (BH4) is a necessary cofactor that stabilizes the enzyme and allows the generation of NO through oxidation of L-arginine. However, if BH4 levels become limiting as found in chronic inflammatory or oxidative conditions, electron transfer is instead shuttled to molecular oxygen resulting in the production of superoxide anion, a scenario in which NOS is described as “uncoupled.” When NOS is uncoupled the resulting increase in superoxide can lead to enhanced levels of peroxynitrite formation, resulting in enhanced IκBα Tyr nitration and NFκβ activity. This evidence has led to the hypothesis that by altering cellular levels of BH4, the levels of Tyr nitration and Cys s-nitrosylation can be manipulated, leading to changes in NFκβ activity. To determine the effects of exogenous BH4 on IκBα nitration in MCF-7 cells, cells were treated with varying concentrations of cell permeable sepiapterin (SP), a precursor of BH4, and exposed to 5Gy IR. The results show that between 100nM-500nM IκBα nitration is all but completely eliminated, resulting in decreased NFκβ activity as assessed by luciferase assay. Initial experiments looking at the effects of SP on p65 s-nitrosylation have been variable. At a minimum, levels of s-nitrosylation remain the same as untreated cells; however, enhancement of s-nitrosylation is seen in some experiments. Studies are ongoing to clarify SP's effects on p65 s-nitrosylation as well as studies to determine SP's effects on tumor cell survival post IR. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1324.