From the Department of Biochemistry, Weill Cornell Medical College, New York, NY (R.K.S., A.S.H., A.A., V.C.B.-L., I.G., H.F.C., F.R.M.).
Vascular Biology Program, Boston Children's Hospital and Department of Surgery, Harvard Medical School, Boston, MA (Y.X., T.H.).
Arterioscler Thromb Vasc Biol. 2020 Jan;40(1):86-102. doi: 10.1161/ATVBAHA.119.313200. Epub 2019 Oct 10.
Aggregation and modification of LDLs (low-density lipoproteins) promote their retention and accumulation in the arteries. This is a critical initiating factor during atherosclerosis. Macrophage catabolism of agLDL (aggregated LDL) occurs using a specialized extracellular, hydrolytic compartment, the lysosomal synapse. Compartment formation by local actin polymerization and delivery of lysosomal contents by exocytosis promotes acidification of the compartment and degradation of agLDL. Internalization of metabolites, such as cholesterol, promotes foam cell formation, a process that drives atherogenesis. Furthermore, there is accumulating evidence for the involvement of TLR4 (Toll-like receptor 4) and its adaptor protein MyD88 (myeloid differentiation primary response 88) in atherosclerosis. Here, we investigated the role of TLR4 in catabolism of agLDL using the lysosomal synapse and foam cell formation. Approach and Results: Using bone marrow-derived macrophages from knockout mice, we find that TLR4 and MyD88 regulate compartment formation, lysosome exocytosis, acidification of the compartment, and foam cell formation. Using siRNA (small interfering RNA), pharmacological inhibition and knockout bone marrow-derived macrophages, we implicate SYK (spleen tyrosine kinase), PI3K (phosphoinositide 3-kinase), and Akt in agLDL catabolism using the lysosomal synapse. Using bone marrow transplantation of LDL receptor knockout mice with TLR4 knockout bone marrow, we show that deficiency of TLR4 protects macrophages from lipid accumulation during atherosclerosis. Finally, we demonstrate that macrophages in vivo form an extracellular compartment and exocytose lysosome contents similar to that observed in vitro for degradation of agLDL.
We present a mechanism in which interaction of macrophages with agLDL initiates a TLR4 signaling pathway, resulting in formation of the lysosomal synapse, catabolism of agLDL, and lipid accumulation in vitro and in vivo.
LDL（低密度脂蛋白）的聚集和修饰促进其在动脉中的保留和积累。这是动脉粥样硬化形成过程中的一个关键起始因素。巨噬细胞通过一种特殊的细胞外水解隔室——溶酶体突触来代谢 agLDL（聚集的 LDL）。通过局部肌动蛋白聚合形成隔室，并通过胞吐作用输送溶酶体内容物，促进隔室酸化和 agLDL 的降解。代谢产物（如胆固醇）的内化促进泡沫细胞的形成，这一过程推动了动脉粥样硬化的发生。此外，越来越多的证据表明 TLR4（Toll 样受体 4）及其衔接蛋白 MyD88（髓样分化初级反应 88）参与了动脉粥样硬化的形成。在这里，我们研究了 TLR4 在溶酶体突触和泡沫细胞形成中对 agLDL 代谢的作用。
使用来自敲除小鼠的骨髓来源的巨噬细胞，我们发现 TLR4 和 MyD88 调节隔室形成、溶酶体胞吐作用、隔室酸化和泡沫细胞形成。使用 siRNA（小干扰 RNA）、药理学抑制和敲除骨髓来源的巨噬细胞，我们发现 SYK（脾酪氨酸激酶）、PI3K（磷酸肌醇 3-激酶）和 Akt 在使用溶酶体突触的 agLDL 代谢中起作用。通过 LDL 受体敲除小鼠的骨髓移植和 TLR4 敲除骨髓，我们表明 TLR4 的缺乏可保护巨噬细胞免受动脉粥样硬化过程中的脂质积累。最后，我们证明体内的巨噬细胞形成细胞外隔室并胞吐溶酶体内容物，类似于体外观察到的用于降解 agLDL 的情况。
我们提出了一种机制，即巨噬细胞与 agLDL 的相互作用引发了 TLR4 信号通路，导致溶酶体突触的形成、agLDL 的代谢以及体内外的脂质积累。
