Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, IN 46225.
Proc Natl Acad Sci U S A. 2023 Feb 14;120(7):e2214081120. doi: 10.1073/pnas.2214081120. Epub 2023 Feb 10.
Triglyceride (TG) metabolism is highly regulated by angiopoietin-like protein (ANGPTL) family members [Y. Q. Chen , , 1203-1220 (2020)]. During feeding, ANGPTL8 forms complexes with the fibrinogen-like domain-containing protein ANGPTL4 in adipose tissue to decrease ANGPTL3/8- and ANGPTL4-mediated lipoprotein lipase (LPL)-inhibitory activity and promote TG hydrolysis and fatty acid (FA) uptake. The ANGPTL4/8 complex, however, tightly binds LPL and partially inhibits it in vitro. To try to reconcile the in vivo and in vitro data on ANGPTL4/8, we aimed to find novel binding partners of ANGPTL4/8. To that end, we performed pulldown experiments and found that ANGPTL4/8 bound both tissue plasminogen activator (tPA) and plasminogen, the precursor of the fibrinolytic enzyme plasmin. Remarkably, ANGPTL4/8 enhanced tPA activation of plasminogen to generate plasmin in a manner like that observed with fibrin, while minimal plasmin generation was observed with ANGPTL4 alone. The addition of tPA and plasminogen to LPL-bound ANGPTL4/8 caused rapid, complete ANGPTL4/8 cleavage and increased LPL activity. Restoration of LPL activity in the presence of ANGPTL4/8 was also achieved with plasmin but was blocked when catalytically inactive plasminogen (S760A) was added to tPA or when plasminogen activator inhibitor-1 was added to tPA + plasminogen, indicating that conversion of plasminogen to plasmin was essential. Together, these results suggest that LPL-bound ANGPTL4/8 mimics fibrin to recruit tPA and plasminogen to generate plasmin, which then cleaves ANGPTL4/8, enabling LPL activity to be increased. Our observations thus reveal a unique link between the ANGPTL4/8 complex and plasmin generation.
甘油三酯（TG）代谢受到血管生成素样蛋白（ANGPTL）家族成员的高度调节[Y. Q. Chen,, 1203-1220 (2020)]。在进食期间，ANGPTL8 在脂肪组织中与纤维蛋白原样结构域包含蛋白 ANGPTL4 形成复合物，以降低 ANGPTL3/8 和 ANGPTL4 介导的脂蛋白脂肪酶（LPL）抑制活性，并促进 TG 水解和脂肪酸（FA）摄取。然而，ANGPTL4/8 复合物紧密结合 LPL 并在体外部分抑制其活性。为了尝试协调 ANGPTL4/8 的体内和体外数据，我们旨在寻找 ANGPTL4/8 的新结合伴侣。为此，我们进行了下拉实验，发现 ANGPTL4/8 与组织型纤溶酶原激活物（tPA）和纤溶酶原，即纤溶酶的前体结合。值得注意的是，ANGPTL4/8 以类似于与纤维蛋白观察到的方式增强 tPA 对纤溶酶原的激活，生成纤溶酶，而单独使用 ANGPTL4 时则很少观察到纤溶酶生成。将 tPA 和纤溶酶原添加到与 LPL 结合的 ANGPTL4/8 中会导致 ANGPTL4/8 的快速、完全切割，并增加 LPL 活性。在存在 ANGPTL4/8 的情况下，也可以通过纤溶酶恢复 LPL 活性，但当向 tPA 添加无催化活性的纤溶酶原（S760A）或当向 tPA+纤溶酶原添加纤溶酶原激活物抑制剂-1 时，这种恢复被阻断，表明纤溶酶原转化为纤溶酶是必不可少的。总之，这些结果表明，与 LPL 结合的 ANGPTL4/8 模拟纤维蛋白，募集 tPA 和纤溶酶原生成纤溶酶，然后切割 ANGPTL4/8，从而增加 LPL 活性。我们的观察结果因此揭示了 ANGPTL4/8 复合物与纤溶酶生成之间的独特联系。
