Abstract 2122: Chronic inflammatory levels of nitric oxide/reactive nitrogen species down-regulate BRCA1 protein expression and decrease homologous recombination
Authors: Yakovlev Vasily A. 
Published: August 2012

The link between inflammation and cancer was proposed more than 150 years ago when Virchow suggested that malignancies tend to arise at sites of chronic inflammation. Recent studies have established that elevated levels of nitric oxide/reactive nitrogen species (NO/RNS) generated in the inflammatory microenvironment are associated with several conditions critical for initiation and promotion of cancer. The BRCA1 protein contributes to cell viability in multiple ways, including DNA repair, cell cycle checkpoint control, transcription, and regulation of chromosome segregation. The BRCA1 gene mutations have been identified and associated with an increased risk of cancer. Sporadic breast carcinomas, on the other hand, rarely show mutations of this gene. Instead, BRCA1 protein expression is frequently reduced in sporadic cases. BRCA1 expression is negatively regulated at the transcriptional level by the repressive complex of retinoblastoma-like protein 2 (RBL2) and E2F4. Formation of the repression RBL2/E2F4 complex is accelerated by RBL2 dephosphorylation. Recently, protein phosphatase 2A (PP2A), an enzyme responsible for RBL2 dephosphorylation, was shown to be nitrated and activated by NO/RNS (Ohama T., et al., JBC 285, 2010). In the present investigation MCF-10A (human mammary epithelial cells) and A549 (human lung carcinoma cells) were incubated with 0.05-0.1 mM of the NO-donor SNAP, concentrations that mimic the chronic inflammation level of intracellular NO/RNS. These levels of NO/RNS induce substantial dephosphorylation of RBL2 in a PP2A dependent way. RBL2 dephosphorylation promoted a repressive RBL2/E2F4 complex formation, with accumulation of this complex into nuclei. RBL2/E2F4 inhibitory complex binds to the single E2F site in the proximal promoter of the BRCA1 gene, replaces the activator E2F1, and block BRCA1 proliferation. To measure the impact of RNS-dependent BRCA1 downregulation on the level of DNA Homologous Recombination Repair (HRR) we used DR-GFP system, which has been described by many investigators (R. S. Bindra et al., Cancer Res 65, 2005; S. E. Golding et al., JBC 279, 2004; A. J. Pierce, R. D. Johnson, L. H. Thompson, M. Jasin, Genes Dev 13, 1999). The preliminary results showed that inflammatory levels of NO/RNS significantly decreased the HRR frequency for the both types of cells. Interesting, the same concentration of SNAP also promotes MCF-10A and A549 cells proliferation. These data revealed that NO/RNS-dependent decrease in BRCA1 expression can lead to genetic instability by shifting the balance between the high-fidelity HRR and the error-prone non-homologous end-joining pathways of the DNA repair. This mechanism can play a significant role in the maintenance of genetic instability and stimulation of carcinogenesis under chronic inflammatory conditions.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2122. doi:1538-7445.AM2012-2122