Internalization of transthyretin - Evidence of a novel yet unidentified receptor-associated protein (RAP)-sensitive receptor
Internalization of transthyretin - Evidence of a novel yet unidentified receptor-associated protein (RAP)-sensitive receptor
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DOI：
10.1074/jbc.m010869200
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发表时间：
2001-04-27
影响因子：
4.8
通讯作者：
Saraiva, MJ
中科院分区：
文献类型：
--
作者：
Sousa, MM;Saraiva, MJ
甲状腺素运载蛋白 (TTR) 是甲状腺素和视黄醇结合蛋白 (REP) 的血浆载体，虽然肝脏是 TTR 降解的主要部位，但其细胞摄取却知之甚少。我们利用肝癌和原代肝细胞探索了 TTR 的摄取，并显示了特定受体的内化作用。 REP 与 TTR 复合导致 TTR 内化减少 70%，而 TTR 与甲状腺素结合导致 TTR 内化增加 20%。不同的 TTR 突变体在摄取方面表现出差异，表明受体识别依赖于 TTR 的结构。使用肝癌和 I-125-TTR 的交联研究揭示了与与其受体结合的 I-125-TTR 相对应的类似 90-kDa 复合物。鉴于先前的证据表明； TTR 的一部分与高密度脂蛋白 (HDL) 相关，并且在肾脏中，低密度脂蛋白受体家族 (LDLr) 的成员巨蛋白内化 TTR，我们假设 TTR 和脂蛋白可能共享相关的降解途径。在摄取测定中使用缺脂血清，没有观察到显着变化，表明 TTR 摄取不依赖于脂蛋白或由 TTR-脂蛋白复合物引起。然而，竞争研究表明脂蛋白抑制 TTR 内化。根据不同细胞系统的评估，清道夫受体 SR-BI（一种 HDL 受体）和已知的 LDLr 家族肝受体并不介导 TTR 摄取。有趣的是，受体相关蛋白（RAP）是 LDLr 所有成员的配体，能够抑制 TTR 内化。此外，通过交联获得的类似于90-kDa TTR-受体复合物对RAP的存在敏感，为了证实在肝癌中观察到的RAP敏感性并不代表正常细胞中不存在的机制，对原代肝细胞进行了测试，并获得了类似的结果。 RAP 敏感的 TTR 内化以及脂蛋白对 TTR 摄取的置换，进一步表明 TTR 和脂蛋白代谢之间可能存在共同途径，并且尚未鉴定的 RAP 敏感受体介导 TTR 摄取。
Transthyretin (TTR) is a plasma carrier of thyroxine and retinol-binding protein (REP), Though the liver is the major site of TTR degradation, its cellular uptake is poorly understood. We explored TTR uptake using hepatomas and primary hepatocytes and showed internalization by a specific receptor. REP complexed with TTR led to a 70% decrease of TTR internalization, whereas TTR bound to thyroxine led to a 20% increase. Different TTR mutants showed differences in uptake, suggesting receptor recognition dependent on the structure of TTR. Cross-linking studies using hepatomas and I-125-TTR revealed a similar to 90-kDa complex corresponding to I-125-TTR bound to its receptor. Given previous evidence that; a fraction of TTR is associated with high-density lipoproteins (HDL) and that in the kidney, megalin, a member of the low-density lipoprotein receptor family (LDLr) internalizes TTR, we hypothesized that TTR and lipoproteins could share related degradation pathways. Using lipid-deficient serum in uptake assays, no significant changes were observed showing that TTR uptake is not lipoprotein-dependent or due to TTR-lipoprotein complexes. However, competition studies showed that lipoproteins inhibit TTR internalization. The scavenger receptor SR-BI, a HDL receptor, and known LDLr family hepatic receptors did not mediate TTR uptake as assessed using different cellular systems. Interestingly, the receptor-associated protein (RAP), a ligand for all members of the LDLr, was able to inhibit TTR internalization. Moreover, the similar to 90-kDa TTR-receptor complex obtained by cross-linking was sensitive to the presence of RAP, To confirm that RAP sensitivity observed in hepatomas did not represent a mechanism absent in normal cells, primary hepatocytes were tested, and similar results were obtained. The RAP-sensitive TTR internalization together with displacement of TTR uptake by lipoproteins, further suggests that a common pathway might exist between TTR and lipoprotein metabolism and that an as yet unidentified RAP-sensitive receptor mediates TTR uptake.