Centre for Discovery Brain Sciences, University of Edinburgh, Hugh Robson Building, George Square, Edinburgh, EH8 9XD, UK; Behavioral Genetics Unit, Department of Genetics, King Faisal Specialist Hospital and Research Center, P.O Box 3354, Riyadh, 11211, Saudi Arabia.
Centre for Discovery Brain Sciences, University of Edinburgh, Hugh Robson Building, George Square, Edinburgh, EH8 9XD, UK.
Redox Biol. 2021 Nov;47:102158. doi: 10.1016/j.redox.2021.102158. Epub 2021 Oct 2.
The transcription factor Nrf2 is a stress-responsive master regulator of antioxidant, detoxification and proteostasis genes. In astrocytes, Nrf2-dependent gene expression drives cell-autonomous cytoprotection and also non-cell-autonomous protection of nearby neurons, and can ameliorate pathology in several acute and chronic neurological disorders associated with oxidative stress. However, the value of astrocytic Nrf2 as a therapeutic target depends in part on whether Nrf2 activation by disease-associated oxidative stress occludes the effect of any Nrf2-activating drug. Nrf2 activation classically involves the inhibition of interactions between Nrf2's Neh2 domain and Keap1, which directs Nrf2 degradation. Keap1 inhibition is mediated by the modification of cysteine residues on Keap1, and can be triggered by electrophilic small molecules such as tBHQ. Here we show that astrocytic Nrf2 activation by oxidative stress involves Keap1-independent non-canonical signaling. Keap1 deficiency elevates basal Nrf2 target gene expression in astrocytes and occludes the effects of tBHQ, oxidative stress still induced strong Nrf2-dependent gene expression in Keap1-deficient astrocytes. Moreover, while tBHQ prevented protein degradation mediated via Nrf2's Neh2 domain, oxidative stress did not, consistent with a Keap1-independent mechanism. Moreover the effects of oxidative stress and tBHQ on Nrf2 target gene expression are additive, not occlusive. Mechanistically, oxidative stress enhances the transactivation potential of Nrf2's Neh5 domain in a manner dependent on its Cys-191 residue. Thus, astrocytic Nrf2 activation by oxidative stress involves Keap1-independent non-canonical signaling, meaning that further Nrf2 activation by Keap1-inhibiting drugs may be a viable therapeutic strategy.
转录因子 Nrf2 是一种应激反应性的抗氧化、解毒和蛋白质稳态基因的主要调节因子。在星形胶质细胞中，Nrf2 依赖性基因表达驱动细胞自主细胞保护，也驱动附近神经元的非细胞自主保护，并可以改善几种与氧化应激相关的急性和慢性神经疾病的病理学。然而，星形胶质细胞 Nrf2 作为治疗靶点的价值部分取决于与疾病相关的氧化应激引起的 Nrf2 激活是否会掩盖任何 Nrf2 激活药物的作用。Nrf2 的经典激活涉及 Nrf2 的 Neh2 结构域与 Keap1 之间相互作用的抑制，该相互作用指导 Nrf2 的降解。Keap1 的抑制是通过 Keap1 上半胱氨酸残基的修饰介导的，并且可以被诸如 tBHQ 等亲电小分子触发。在这里，我们表明氧化应激引起的星形胶质细胞 Nrf2 激活涉及 Keap1 非依赖性非经典信号传导。Keap1 缺陷会增加星形胶质细胞中基础 Nrf2 靶基因的表达，并阻断 tBHQ 的作用，但氧化应激仍然会诱导 Keap1 缺陷型星形胶质细胞中强烈的 Nrf2 依赖性基因表达。此外，虽然 tBHQ 可防止通过 Nrf2 的 Neh2 结构域介导的蛋白质降解，但氧化应激不会，这与 Keap1 非依赖性机制一致。此外，氧化应激和 tBHQ 对 Nrf2 靶基因表达的影响是相加的，而不是相互排斥的。从机制上讲，氧化应激以依赖于其 Cys-191 残基的方式增强 Nrf2 的 Neh5 结构域的转录激活潜力。因此，氧化应激引起的星形胶质细胞 Nrf2 激活涉及 Keap1 非依赖性非经典信号传导，这意味着通过 Keap1 抑制药物进一步激活 Nrf2 可能是一种可行的治疗策略。
