Department of Pharmacology, University of Tennessee Health Science Center, College of Medicine, 874 Union Avenue, Memphis TN, 38163, USA.
Mol Neurodegener. 2011 Mar 3;6:17. doi: 10.1186/1750-1326-6-17.
It is well established that both cerebral hypoperfusion/stroke and type 2 diabetes are risk factors for Alzheimer's disease (AD). Recently, the molecular link between ischemia/hypoxia and amyloid precursor protein (APP) processing has begun to be established. However, the role of the key common denominator, namely nitric oxide (NO), in AD is largely unknown. In this study, we investigated redox regulation of BACE1, the rate-limiting enzyme responsible for the β-cleavage of APP to Aβ peptides.
Herein, we studied events such as S-nitrosylation, a covalent modification of cysteine residues by NO, and H2O2-mediated oxidation. We found that NO and H2O2 differentially modulate BACE1 expression and enzymatic activity: NO at low concentrations (<100 nM) suppresses BACE1 transcription as well as its enzymatic activity while at higher levels (0.1-100 μM) NO induces S-nitrosylation of BACE1 which inactivates the enzyme without altering its expression. Moreover, the suppressive effect on BACE1 transcription is mediated by the NO/cGMP-PKG signaling, likely through activated PGC-1α. H2O2 (1-10 μM) induces BACE1 expression via transcriptional activation, resulting in increased enzymatic activity. The differential effects of NO and H2O2 on BACE1 expression and activity are also reflected in their opposing effects on Aβ generation in cultured neurons in a dose-dependent manner. Furthermore, we found that BACE1 is highly S-nitrosylated in normal aging brains while S-nitrosylation is markedly reduced in AD brains.
This study demonstrates for the first time that BACE1 is highly modified by NO via multiple mechanisms: low and high levels of NO suppress BACE1 via transcriptional and post translational regulation, in contrast with the upregulation of BACE1 by H2O2-mediated oxidation. These novel NO-mediated regulatory mechanisms likely protect BACE1 from being further oxidized by excessive oxidative stress, as from H2O2 and peroxynitrite which are known to upregulate BACE1 and activate the enzyme, resulting in excessive cleavage of APP and Aβ generation; they likely represent the crucial house-keeping mechanism for BACE1 expression/activation under physiological conditions.
脑灌注不足/中风和 2 型糖尿病都是阿尔茨海默病（AD）的风险因素，这一点已得到充分证实。最近，缺血/缺氧与淀粉样前体蛋白（APP）加工之间的分子联系开始建立。然而，关键的共同因素，即一氧化氮（NO）在 AD 中的作用在很大程度上尚不清楚。在这项研究中，我们研究了 BACE1 的氧化还原调节，BACE1 是负责 APP 的β切割产生 Aβ肽的限速酶。
在此，我们研究了 S-亚硝基化等事件，S-亚硝基化是由 NO 引起的半胱氨酸残基的共价修饰，以及 H2O2 介导的氧化。我们发现，NO 和 H2O2 以不同的方式调节 BACE1 的表达和酶活性：低浓度（<100 nM）的 NO 抑制 BACE1 的转录及其酶活性，而高浓度（0.1-100 μM）的 NO 诱导 BACE1 的 S-亚硝基化，从而使酶失活而不改变其表达。此外，对 BACE1 转录的抑制作用是由 NO/cGMP-PKG 信号介导的，可能通过激活 PGC-1α。H2O2（1-10 μM）通过转录激活诱导 BACE1 表达，从而增加酶活性。NO 和 H2O2 对 BACE1 表达和活性的不同影响也反映在它们对培养神经元中 Aβ生成的剂量依赖性的相反影响上。此外，我们发现 BACE1 在正常衰老大脑中高度被 S-亚硝基化，而在 AD 大脑中 S-亚硝基化明显减少。
这项研究首次证明，BACE1 被 NO 通过多种机制高度修饰：低水平和高水平的 NO 通过转录和翻译后调节抑制 BACE1，而 H2O2 介导的氧化则上调 BACE1。这些新的 NO 介导的调节机制可能保护 BACE1 免受过度氧化应激的进一步氧化，因为众所周知，H2O2 和过氧亚硝酸盐会上调 BACE1 并激活酶，导致 APP 的过度切割和 Aβ的产生；它们可能代表生理条件下 BACE1 表达/激活的关键维持机制。
