He Guiqiong, Qing Hong, Cai Fang, Kwok Clement, Xu Huaxi, Yu Gang, Bernstein Alan, Song Weihong
Department of Psychiatry, Brain Research Center, The University of British Columbia, Vancouver, British Columbia, and Samuel Lunenfeld Research Institute, Program in Molecular Biology and Cancer, Mount Sinai Hospital, Toronto, Ontario, Canada.
J Neurochem. 2006 Dec;99(5):1403-12. doi: 10.1111/j.1471-4159.2006.04184.x. Epub 2006 Oct 24.
Gamma-secretase catalyzes intramembraneous proteolysis of several type I transmembrane proteins, including beta-amyloid precursor protein (APP), to generate amyloid beta protein (Abeta), a key player in the pathogenesis of Alzheimer's disease (AD). The critical components of the gamma-secretase complex include presenilin (PS), nicastrin (NCT), presenilin enhancer-2 (PEN-2) and anterior pharynx defective-1 (APH-1). Abnormalities of the ubiquitin-proteasome pathway have been implicated in the pathogenesis of AD; while PS and PEN-2 turnover is regulated by this pathway, it is unknown whether the ubiquitin-proteasome pathway is also involved in the degradation of APH-1 protein. In this study, we found that the expression of endogenous and exogenous APH-1 significantly increased in cells treated with proteasome-specific inhibitors. The effect of the proteasome inhibitors on APH-1 was dose- and time-dependent. APH-1 protein was ubiquitinated. Pulse-chase metabolic labeling experiments showed that the degradation of newly synthesized radiolabeled APH-1 proteins was inhibited by lactacystin. Disruption of the PS1 and PS2 genes did not affect the degradation of APH-1 by the ubiquitin-proteasome pathway. Furthermore, over-expression of APH-1 and inhibition of proteasomal APH-1 degradation facilitated gamma-secretase cleavage of APP to generate Abeta. These results demonstrate that the degradation of APH-1 protein is mediated by the ubiquitin-proteasome pathway.
γ-分泌酶催化包括β-淀粉样前体蛋白（APP）在内的多种I型跨膜蛋白的膜内蛋白水解，以生成淀粉样β蛋白（Aβ），它是阿尔茨海默病（AD）发病机制中的关键因素。γ-分泌酶复合物的关键成分包括早老素（PS）、尼卡斯特林（NCT）、早老素增强子2（PEN-2）和咽前缺陷蛋白1（APH-1）。泛素-蛋白酶体途径异常与AD的发病机制有关；虽然PS和PEN-2的周转受该途径调控，但泛素-蛋白酶体途径是否也参与APH-1蛋白的降解尚不清楚。在本研究中，我们发现用蛋白酶体特异性抑制剂处理的细胞中，内源性和外源性APH-1的表达显著增加。蛋白酶体抑制剂对APH-1的作用具有剂量和时间依赖性。APH-1蛋白被泛素化。脉冲追踪代谢标记实验表明，乳胞素抑制了新合成的放射性标记的APH-1蛋白的降解。PS1和PS2基因的破坏并不影响泛素-蛋白酶体途径对APH-1的降解。此外，APH-1的过表达和蛋白酶体对APH-1降解的抑制促进了APP的γ-分泌酶切割以生成Aβ。这些结果表明，APH-1蛋白的降解是由泛素-蛋白酶体途径介导的。
