Rotundo R F, Rebres R A, Mckeown-Longo P J, Blumenstock F A, Saba T M
Department of Physiology and Cell Biology, Albany Medical College, NY 12208, USA.
Hepatology. 1998 Aug;28(2):475-85. doi: 10.1002/hep.510280227.
It has been postulated that the in vivo removal of many plasma glycoproteins after desialylation is mediated by their interaction with a specific endocytic receptor on hepatocytes called the asialoglycoprotein receptor (ASGP-R), which is known to have a high affinity for specific carbohydrate residues, such as galactose. However, this mechanism has never been proven in vivo, nor has a naturally occurring ligand for the ASGP-R been identified. We investigated the influence of the terminal galactose residues on plasma fibronectin (pFn) on its liver deposition and turnover in adult rats, using neuraminidase to remove sialic acid residues to expose galactose residues. We also tested the hypothesis that the normal presence of a large amount of terminal galactose residues in cellular Fn (cFn) may allow cFn to serve as a natural ligand readily able to interact with the ASGP-R. In contrast to the slow clearance of normal pFn from the blood, cFn and desialylated pFn (aFn) displayed a rapid plasma clearance (P < .001) with greater than 50% of both the 125I-cFn or 125I-aFn depositing in the liver within 15 minutes. The enhanced plasma removal and liver deposition of both 125I-cFn and 125I-aFn was competitively inhibited (P < .01) by prior intravenous infusion of excess asialofetuin, which can selectively bind to the ASGP-R. The enzymatic addition of terminal sialic acid residues onto cFn to "mask" or "cap" the normally exposed galactose residues delayed the rapid plasma removal of cFn. Accelerated degradation of 125I-aFn and 125I-cFn as compared with 125I-pFn was demonstrated in vitro by both primary cultures of normal rat hepatocytes or incubated (37 degrees C) tissue slices of livers harvested from normal rats after in vivo preloading with tracer 125I-Fn forms. Thus, the ASGP-R appears to directly participate in the rapid in vivo removal of cFn from the blood, while native pFn may be removed by an alternative pathway unless it can become desialylated in vivo. These findings suggest that cFn may be a naturally occurring ligand that does not require desialylation before removal by the ASGP-R on hepatocytes.
据推测，许多血浆糖蛋白在去唾液酸化后在体内的清除是由它们与肝细胞上一种特定的内吞受体相互作用介导的，这种受体称为去唾液酸糖蛋白受体（ASGP-R），已知其对特定的碳水化合物残基如半乳糖具有高亲和力。然而，这一机制从未在体内得到证实，也未鉴定出ASGP-R的天然配体。我们利用神经氨酸酶去除唾液酸残基以暴露半乳糖残基，研究了血浆纤连蛋白（pFn）上的末端半乳糖残基对其在成年大鼠肝脏中的沉积和周转的影响。我们还检验了这样一个假设，即细胞纤连蛋白（cFn）中大量末端半乳糖残基的正常存在可能使cFn作为一种天然配体，能够轻易地与ASGP-R相互作用。与正常pFn从血液中缓慢清除不同，cFn和去唾液酸化的pFn（aFn）显示出快速的血浆清除率（P <.001），在15分钟内，125I-cFn或125I-aFn中有超过50%沉积在肝脏中。预先静脉输注过量的去唾液酸胎球蛋白可竞争性抑制125I-cFn和125I-aFn的血浆清除率增强和肝脏沉积（P <.01），去唾液酸胎球蛋白可选择性地与ASGP-R结合。通过酶促将末端唾液酸残基添加到cFn上以“掩盖”或“封闭”正常暴露的半乳糖残基，可延迟cFn快速的血浆清除。与125I-pFn相比，在体外，正常大鼠肝细胞的原代培养物或在体内预先用示踪剂125I-Fn形式预加载后从正常大鼠收获的肝脏的孵育（37摄氏度）组织切片均证明125I-aFn和125I-cFn加速降解。因此，ASGP-R似乎直接参与了cFn从血液中的快速体内清除，而天然pFn可能通过另一种途径清除，除非它能在体内去唾液酸化。这些发现表明，cFn可能是一种天然存在的配体，在被肝细胞上的ASGP-R清除之前不需要去唾液酸化。
