Cortese Katia, Sahores Macarena, Madsen Chris D, Tacchetti Carlo, Blasi Francesco
Centro di Ricerca MicroSCoBio/IFOM, FIRC Institute of Molecular Oncology, Dipartimento di Medicina Sperimentale, Sezione di Anatomia Umana, Università di Genova, Genova, Italy.
PLoS One. 2008;3(11):e3730. doi: 10.1371/journal.pone.0003730. Epub 2008 Nov 14.
The urokinase receptor (uPAR/CD87) is highly expressed in malignant tumours. uPAR, as a GPI anchored protein, is preferentially located at the cell surface, where it interacts with its ligands urokinase (uPA) and the extracellular matrix protein vitronectin, thus promoting plasmin generation, cell-matrix interactions and intracellular signalling events. Interaction with a complex formed by uPA and its inhibitor PAI-1 induces cell surface down regulation and recycling of the receptor via the clathrin-coated pathway, a process dependent on the association to LRP-1.
METHODOLOGY/PRINCIPAL FINDINGS: In this study, we have found that along with the ligand-induced down-regulation, uPAR also internalizes and recycles constitutively through a second pathway that is independent of LRP-1 and clathrin but shares some properties with macropinocytosis. The ligand-independent route is amiloride-sensitive, does not require uPAR partitioning into lipid rafts, is independent of the activity of small GTPases RhoA, Rac1 and Cdc42, and does not require PI3K activity. Constitutively endocytosed uPAR is found in EEA1 positive early/recycling endosomes but does not reach lysosomes in the absence of ligands. Electron microscopy analysis reveals the presence of uPAR in ruffling domains at the cell surface, in macropinosome-like vesicles and in endosomal compartments.
CONCLUSIONS/SIGNIFICANCE: These results indicate that, in addition to the ligand-induced endocytosis of uPAR, efficient surface expression and membrane trafficking might also be driven by an uncommon macropinocytic mechanism coupled with rapid recycling to the cell surface.
尿激酶受体（uPAR/CD87）在恶性肿瘤中高表达。uPAR作为一种糖基磷脂酰肌醇（GPI）锚定蛋白，优先定位于细胞表面，在那里它与其配体尿激酶（uPA）和细胞外基质蛋白玻连蛋白相互作用，从而促进纤溶酶生成、细胞 - 基质相互作用及细胞内信号转导事件。与由uPA及其抑制剂PAI - 1形成的复合物相互作用会诱导受体通过网格蛋白包被途径进行细胞表面下调和再循环，这一过程依赖于与低密度脂蛋白受体相关蛋白1（LRP - 1）的结合。
方法/主要发现：在本研究中，我们发现除了配体诱导的下调外，uPAR还通过一条独立于LRP - 1和网格蛋白但与巨胞饮作用具有一些共同特性的第二条途径进行组成型内化和再循环。这条不依赖配体的途径对阿米洛利敏感，不需要uPAR分配到脂筏中，独立于小GTP酶RhoA、Rac1和Cdc42的活性，并且不需要磷脂酰肌醇3 - 激酶（PI3K）活性。组成型内吞的uPAR存在于早期/再循环内体抗原1（EEA1）阳性的早期/再循环内体中，但在没有配体的情况下不会到达溶酶体。电子显微镜分析显示uPAR存在于细胞表面的褶皱区域、类巨胞饮小泡和内体区室中。
结论/意义：这些结果表明，除了uPAR的配体诱导内吞作用外，有效的表面表达和膜运输也可能由一种不常见的巨胞饮机制驱动，并伴有快速再循环至细胞表面。
