Shepherd V L, Abdolrasulnia R, Stephenson J, Crenshaw C
VA Medical Center, Nashville, TN 37212.
Biochem J. 1990 Sep 15;270(3):771-6. doi: 10.1042/bj2700771.
Macrophages express a receptor on the cell surface that functions to clear glycoproteins from the extracellular milieu. The activity of this receptor is sensitive to treatment with trypsin. In inflammatory situations, macrophages are activated and exposed to increased levels of extracellular proteases. Under these conditions, mannose receptor activity on the macrophages is diminished. We therefore decided to study the effects of trypsin treatment on the structure and activity of cell-associated and purified receptor that might contribute to the activation-associated receptor down-regulation. Trypsin treatment (1 microgram/ml for 3 h) resulted in the production of a 140 kDa, trypsin-resistant fragment from both intact cells and isolated receptor. This fragment was no longer able to bind ligand. The remaining 35 kDa fragment apparently is further degraded into smaller fragments, since no evidence of this domain was found on Coomassie Blue-stained gels. The 140 kDa fragment retained immunoreactivity and contained at least a portion of the iodinated tyrosine residues following surface labelling with Na125I. Neither calcium nor ligand protected the receptor from proteolysis. In addition, prior treatment with oxidants did not increase the susceptibility of the receptor to trypsin digestion. We conclude from these results that the macrophage mannose receptor is clipped by the serine protease trypsin at the cell surface, resulting in the release and further degradation of the binding domain, and the production of a membrane-associated 140 kDa fragment. This trypsin-mediated down-regulation of receptor activity might be important in controlling glycoprotein clearance during inflammation.
巨噬细胞在细胞表面表达一种受体，其功能是清除细胞外环境中的糖蛋白。该受体的活性对胰蛋白酶处理敏感。在炎症情况下，巨噬细胞被激活并暴露于细胞外蛋白酶水平升高的环境中。在这些条件下，巨噬细胞上的甘露糖受体活性降低。因此，我们决定研究胰蛋白酶处理对细胞相关和纯化受体的结构和活性的影响，这可能有助于与激活相关的受体下调。胰蛋白酶处理（1微克/毫升，处理3小时）导致完整细胞和分离受体均产生一个140 kDa的抗胰蛋白酶片段。该片段不再能够结合配体。剩余的35 kDa片段显然进一步降解为更小的片段，因为在考马斯亮蓝染色的凝胶上未发现该结构域的证据。140 kDa片段保留了免疫反应性，在用Na125I进行表面标记后含有至少一部分碘化酪氨酸残基。钙和配体均不能保护受体免受蛋白水解。此外，用氧化剂预处理并未增加受体对胰蛋白酶消化的敏感性。我们从这些结果得出结论，巨噬细胞甘露糖受体在细胞表面被丝氨酸蛋白酶胰蛋白酶切割，导致结合结构域的释放和进一步降解，并产生一个与膜相关的140 kDa片段。这种胰蛋白酶介导的受体活性下调在控制炎症期间的糖蛋白清除中可能很重要。
