Carcinogenesis Advance Access published online on October 28, 2009
Carcinogenesis, doi:10.1093/carcin/bgp256
Differential effects of reactive nitrogen species on DNA base excision repair initiated by the alkyladenine DNA glycosylase
a Department of Pharmaceutical and Biomedical Sciences, South Carolina College of Pharmacy, University of South Carolina, Columbia SC 29208
b Department of Pharmacology & Chemical Biology, University of Pittsburgh School of Medicine & University of Pittsburgh Cancer Institute, Hillman Cancer Center; Pittsburgh, PA 15213
c Department of Human Genetics, University of Pittsburgh Graduate School of Public Health, Pittsburgh, PA 15213
d Laboratory of Human Carcinogenesis, Center for Cancer Research, National Cancer Institute
e Molecular and Clinical Nutrition Section, NIDDK, NIH, Bethesda, MD 20892, USA
M.G.E.: Molecular and Clinical Nutrition Section, NIDDK, NIH, Bethesda, MD 20892, USA; Tel: 1 301 402 6513; fax: 1 301 402 6436. E-mail address: sp{at}nih.gov L.J.H. & M.D.W.: Department of Basic Pharmaceutical and Biomedical Sciences, University of South Carolina, 715 Sumter Street, Columbia SC 29208, Tel: 1 803 777 6627; fax: 1 803 777 8356. E-mail address: hofseth{at}sccp.sc.edu, Tel: 1 803 777 0856; fax: 1 803 777 8356. E-mail address: wyatt{at}sccp.sc.edu
Chronic generation of reactive nitrogen species (RNS) can cause DNA damage and may also directly modify DNA repair proteins. RNS-modified DNA is repaired predominantly by the base excision repair (BER) pathway, which includes the alkyladenine DNA glycosylase (AAG). The AAG active site contains several tyrosines and cysteines that are potential sites for modification by RNS. In vitro, we demonstrate that RNS differentially alter AAG activity depending on the site and type of modification. Nitration of tyrosine 162 impaired 1,N6-ethenoadenine (
A)-excision activity, whereas nitrosation of cysteine 167 increased A-excision. To understand the effects of RNS on BER in vivo, we examined intestinal adenomas for levels of inducible nitric oxide synthase (iNOS) and AAG. A striking correlation between AAG and iNOS expression was observed (r = 0.76, p = 0.00002). Interestingly, there was no correlation between changes in AAG levels and enzymatic activity. We found AAG to be nitrated in human adenomas, suggesting that this RNS modification is relevant in the human disease. Expression of key downstream components of BER, apurinic/apyrimidinic endonuclease 1 (APE1) and DNA polymerase β (POLβ), was also examined. POLβ protein was increased in nearly all adenomas compared to adjacent non-tumor tissues, whereas APE1 expression was only increased in 
half of the adenomas and also was re-localized to the cytoplasm in adenomas. Collectively, the results suggest that BER is dysregulated in colon adenomas. RNS-induced post-translational modification of AAG is one mechanism of BER dysregulation, and the type of modification may define the role of AAG during carcinogenesis.
Key Words: Nitric Oxide • Base Excision Repair • Inflammation • Colorectal Cancer
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Received August 4, 2009; revised October 8, 2009; accepted October 15, 2009.