UBC Theses and Dissertations
Activation of the NRF2-mediated phase 2 enzyme response as a prophylactic strategy for treatment of stroke and neurodegeneration Shih, Andy Yi-An
The transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) coordinates expression of genes required for free radical scavenging, detoxification of xenobiotics, and maintenance of redox potential. We hypothesized that the multifaceted Nrf2 response was a viable neuroprotective strategy for neurodegeneration and stroke. We initially examined the role of Nrf2 in primary cortical neuron/astrocyte cultures. Nrf2 activity was robust in the astrocytes, but limited in neurons. Augmentation of astrocytic antioxidant capacity using Nrf2 over-expression conferred potent neuroprotection during in vitro toxicity paradigms involving oxidative stress. Nrf2 overexpression coordinated upregulation of many Phase 2 enzymes. In particular, Nrf2-dependent production of astrocyte GSH was both necessary and sufficient for neuroprotection. Activation of endogenous astrocyte Nrf2 by the small molecule antioxidant response element (ARE) inducer, tert-butylhydroquinone (tBHQ), also protected against oxidative stress. Nrf2 function was critical for neuroprotection during in vivo neurodegeneration. Nrf2⁻ʹ⁻ mice exhibited hypersensitivity to the mitochondrial toxin 3-nitropropionic acid (3-NP) displaying severe motor deficits and larger striatal lesions than identically treated Nrf2⁺ʹ⁺ and Nrf2⁺ʹ⁻ controls. Dietary supplementation with tBHQ increased brain GSH content and attenuated 3-NP toxicity in Nrf2⁺ʹ⁻ mice, but not Nrf2⁻ʹ⁻. Increased Nrf2 activity alone was sufficient to protect animals from 3-NP toxicity because intrastriatal adenovirus-mediated Nrf2 overexpression reduced lesion size compared to GFP over-expressing controls. Nrf2 activation was also essential for neuroprotection during in vivo cerebral ischemia. Acute pretreatment with tBHQ increased cortical GSH levels and reduced cortical damage and sensorimotor deficit even 1 month after ischemia-reperfusion in rats. Conversely, Nrf2⁻ʹ⁻ mice exhibited larger ischemic infarcts than Nrf2⁺ʹ⁺ controls due to decreased survival of penumbral tissue. Neuronal death caused by an endothelin-1 based "penumbral" model of stroke could be attenuated by tBHQ administration to Nrf2⁺ʹ⁺, but not Nrf2⁻ʹ⁻ mice. We conclude that increased Nrf2 activity is highly neuroprotective in both in vitro and in vivo toxicity paradigms that mimic aspects of neurodegeneration and ischemic injury. Nrf2 pathways may be accessed in vivo by treatment with small molecule inducers through multiple routes of administration, providing an effective prophylactic strategy for combating neuronal death caused by neurodegenerative disease and stroke.
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