Functional and structural correlates of PCSK9 association with lipoproteins

摘要 枯草杆菌前蛋白转换酶9(PCSK9)是一种循环蛋白，与低密度的 脂蛋白受体(LDLR)，诱导其降解，并增加低密度脂蛋白-胆固醇(LDL-C)水平。因此， PCSK9是增加低密度脂蛋白受体表达和降低血浆低密度脂蛋白-C水平的治疗靶点，PCSK9 抑制剂最近已被批准在美国和欧洲使用。血浆PCSK9水平很高 与低密度脂蛋白水平相关，两者都是因为PCSK9水平升高通过降低血浆低密度脂蛋白水平 由于高达40%的血浆PCSK9在物理上与低密度脂蛋白有关，这一发现最初是在 我们的实验室，后来得到了其他人的证实。血浆PCSK9主要有两种形式，一种是完整的形式(62 Kda)和一个呋喃裂解形式(55 KDa)。我们发现与低密度脂蛋白相关的PCSK9主要存在于完整的62- 血浆PCSK9的其余部分主要以55 kDa的呋喃裂解片段形式存在。完好无损的低密度脂蛋白- 相关的PCSK9似乎具有更强的结合和降解LDLR的能力。我们的初步结果 同时也表明，低密度脂蛋白可以保护PCSK9免受呋喃的切割。然而，目前仍不清楚是什么推动了 循环PCSK9分子形式的区划，以及是否存在功能相关 对这种隔断。因此，我们建议进行研究，以澄清血浆PCSK9与 它们的脂蛋白结合的分子驱动因素，以及低密度脂蛋白的降解活性。此外，我们还建议 建立高通量检测血浆中低密度脂蛋白结合PCSK9的方法及其相关性研究 与其他含载脂蛋白B的脂蛋白，如Lp(A)，以及与细胞内的载脂蛋白B。最后，我们将研究 导致初始PCSK9-LDLR接触和 LDLR的最终降解，因为这似乎是由于PCSK9的复杂细胞内路线， 包括重新分泌/回收步骤。这一提议的中心假设是，低密度脂蛋白具有显著的 完整血浆PCSK9形式的一部分(62 KDa)，在LDLR结合和 与另一种主要形式的血浆PCSK9(55 KDa)相比，PCSK9的降解能力更强。这些研究的结果 将加深我们对PCSK9与脂蛋白相关的生理作用和 抑制PCSK9产生治疗收益的机制，并可能为替代战略提供参考 通过阻断PCSK9与脂蛋白的结合，抑制PCSK9对低密度脂蛋白受体的影响。 。

Summary Proprotein convertase subtilisin kexin type 9 (PCSK9) is a circulatory protein that binds to the low-density lipoprotein receptor (LDLR), induces its degradation, and increases LDL-cholesterol (LDL-C) levels. Thus, PCSK9 is a therapeutic target to increase LDLR expression and reduce plasma LDL-C levels, and PCSK9 inhibitors have recently been approved for use in the US and Europe. Plasma PCSK9 levels are highly correlated to LDL-C levels, both because elevated PCSK9 levels increase plasma LDL levels by decreasing LDLR and because up to 40% of plasma PCSK9 is physically associated with LDL, a discovery initially made in our laboratory and later confirmed by others. Plasma PCSK9 is found in two main forms, an intact form (62 kDa) and a furin-cleaved form (55 kDa). We show that PCSK9 associated with LDL is mainly in the intact 62- kDa form, whereas the rest of plasma PCSK9 is mainly as the 55 kDa furin-cleaved fragment. The intact, LDL- associated PCSK9 seems to have an increased capacity to bind and degrade LDLR. Our preliminary results also suggest that LDL protects PCSK9 from cleavage by furin. However, it is still unclear what drives the compartmentalization of the molecular forms of circulating PCSK9, and whether there are functional correlates to this compartmentalization. Thus, we propose studies to clarify the connection between plasma PCSK9 forms, molecular drivers of their lipoprotein association, and LDLR degradation activity. Moreover, we propose to develop a high-throughput method to detect LDL-bound PCSK9 in plasma and to study PCSK9 association with other apoB-containing lipoproteins, such as Lp(a), and with intracellular apoB. Finally, we will study the mechanisms leading to the unexpectedly large time delay between the initial PCSK9-LDLR contact and the eventual degradation of the LDLR, as this seems to be due to a complex intracellular routing of PCSK9 which includes a re-secretion/recycling step. The central hypothesis of this proposal is that LDL carries a significant portion of the intact plasma PCSK9 form (62 kDa), which may behave differently in LDLR binding and degradation compared with the other major form of plasma PCSK9 (55 kDa). The results from these studies will enhance our understanding of both the physiologic role of PCSK9 association with lipoproteins and the mechanism by which PCSK9 inhibition produces therapeutic gains, and may inform alternative strategies to inhibit the effect of PCSK9 on LDLR through interruption of PCSK9 association with lipoproteins. .