The amyloid-β rise and γ-secretase inhibitor potency depend on the level of substrate expression
The amyloid-β rise and γ-secretase inhibitor potency depend on the level of substrate expression
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DOI：
10.1074/jbc.m804175200
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发表时间：
2008-08-22
影响因子：
4.8
通讯作者：
Toyn, Jeremy H.
中科院分区：
文献类型：
--
作者：
Burton, Catherine R.;Meredith, Jere E.;Toyn, Jeremy H.
淀粉样蛋白β (A β)肽可能在阿尔茨海默病中起关键作用，它是由淀粉样蛋白β前体蛋白（APP）通过β位点APP切割酶和γ -分泌酶的连续蛋白水解裂解而产生的。出乎意料的是，γ -分泌酶抑制剂在某些情况下可以增加A -肽的分泌。这种“A - β上升”现象已经被广泛观察到，即同一种抑制剂在低浓度下导致A - β增加，但在高浓度下抑制。在这里，我们发现A β的升高取决于β分泌酶衍生的APP c端片段（β CTF）或C99水平，低水平引起升高。相比之下，由α分泌酶切割形成的A β的n端截短形式，称为“p3”，并没有表现出上升。除了A β升高外，低CTF或C99表达降低了γ -分泌酶抑制剂的效力。这种“效力转移”可能是由于在低底物条件下酶与底物的比率相对较高，因为需要增加抑制剂的浓度来影响底物的周转。与这一假设相一致，γ -分泌酶抑制剂放射配体占用研究表明，在低底物表达条件下，高水平的占用与a β的抑制相关。γ分泌酶抑制剂BMS-299897给药后，大鼠脑内A β升高。因此，A - β升高和效力变化是γ -分泌酶抑制剂发展的相关因素，可以通过适当选择动物和细胞培养模型进行评估。讨论了A β上升的假设机制，包括“不完全加工”和内吞模型。
The amyloid-beta(A beta) peptide, which likely plays a key role in Alzheimer disease, is derived from the amyloid-beta precursor protein (APP) through consecutive proteolytic cleavages by beta-site APP-cleaving enzyme and gamma-secretase. Unexpectedly gamma-secretase inhibitors can increase the secretion of A beta peptides under some circumstances. This "A beta rise" phenomenon, the same inhibitor causing an increase in A beta at low concentrations but inhibition at higher concentrations, has been widely observed. Here we show that the A beta rise depends on the beta-secretase-derived C-terminal fragment of APP (beta CTF) or C99 levels with low levels causing rises. In contrast, the N-terminally truncated form of A beta, known as "p3," formed by alpha-secretase cleavage, did not exhibit a rise. In addition to the A beta rise, low beta CTF or C99 expression decreased gamma-secretase inhibitor potency. This " potency shift" may be explained by the relatively high enzyme to substrate ratio under conditions of low substrate because increased concentrations of inhibitor would be necessary to affect substrate turnover. Consistent with this hypothesis, gamma-secretase inhibitor radioligand occupancy studies showed that a high level of occupancy was correlated with inhibition of A beta under conditions of low substrate expression. The A beta rise was also observed in rat brain after dosing with the gamma-secretase inhibitor BMS-299897. The A beta rise and potency shift are therefore relevant factors in the development of gamma-secretase inhibitors and can be evaluated using appropriate choices of animal and cell culture models. Hypothetical mechanisms for the A beta rise, including the "incomplete processing" and endocytic models, are discussed.