1. ¹Department of Neurology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA
2. ²Department of Neurosurgery, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA
3. ³RS Dow Neurobiology Laboratories, Legacy Research and Department of Neurology, Physiology and Pharmacology, Oregon Health Science University, Portland, Oregon, USA
4. ⁴Department of Pharmacology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA
5. ⁵Geriatric Research, Educational and Clinical Center, Veterans Affairs Pittsburgh Health Care System, Pittsburgh, Pennsylvania, USA

Correspondence: Dr J Chen, Department of Neurology, S-507, Biomedical Science Tower, University of Pittsburgh School of Medicine, Pittsburgh, PA 15213, USA. E-mail: chenj2@upmc.edu

Received 10 February 2005; Revised 10 May 2005; Accepted 20 May 2005; Published online 6 July 2005.

Abstract

The development of ischemic tolerance in the brain, whereby a brief period of sublethal 'preconditioning' ischemia attenuates injury from subsequent severe ischemia, may involve the activation of multiple intracellular signaling events that promote neuronal survival. In this study, the potential role of inducible DNA base-excision repair (BER), an endogenous adaptive response that prevents the detrimental effect of oxidative DNA damage, has been studied in the rat model of ischemic tolerance produced by three episodes of ischemic preconditioning (IP). This paradigm of IP, when applied 2 and 5 days before 2-h middle cerebral artery occlusion (MCAO), significantly decreased infarct volume in the frontal–parietal cortex 72 h later. Correlated with this protective effect, IP markedly attenuated the nuclear accumulations of several oxidative DNA lesions, including 8-oxodG, AP sites, and DNA strand breaks, after 2-h MCAO. Consequently, harmful DNA damage-responsive events, including NAD depletion and p53 activation, were reduced during postischemic reperfusion in preconditioned brains. The mechanism underlying the decreased DNA damage in preconditioned brain was then investigated by measuring BER activities in nuclear extracts. Beta-polymerase-mediated BER activity was markedly increased after IP, and this activation occurred before (24 h) and during the course of ischemic tolerance (48 to 72 h). In similar patterns, the activities for AP site and 8-oxodG incisions were also upregulated after IP. The upregulation of BER activities after IP was likely because of increased expression of repair enzymes beta-polymerase, AP endonuclease, and OGG1. These results suggest that the activation of the BER pathway may contribute to IP-induced neuroprotection by enhancing the repair of endogenous oxidative DNA damage after ischemic injury.