Van Hoof Dennis, Rodenburg Kees W, Van der Horst Dick J
Department of Biochemical Physiology and Institute of Biomembranes, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
J Cell Sci. 2002 Nov 1;115(Pt 21):4001-12. doi: 10.1242/jcs.00113.
The lipoprotein of insects, high-density lipophorin (HDLp), is homologous to that of mammalian low-density lipoprotein (LDL) with respect to its apolipoprotein structure. Moreover, an endocytic receptor for HDLp has been identified (insect lipophorin receptor, iLR) that is homologus to the LDL receptor. We transfected LDL-receptor-expressing CHO cells with iLR cDNA to study the endocytic uptake and intracellular pathways of LDL and HDLp simultaneously. Our studies provide evidence that these mammalian and insect lipoproteins follow distinct intracellular routes after receptor-mediated endocytosis. Multicolour imaging and immunofluorescence was used to visualize the intracellular trafficking of fluorescently labeled ligands in these cells. Upon internalization, which can be completely inhibited by human receptor-associated protein (RAP), mammalian and insect lipoproteins share endocytic vesicles. Subsequently, however, HDLp evacuates the LDL-containing endosomes. In contrast to LDL, which is completely degraded in lysosomes after dissociating from its receptor, both HDLp and iLR converge in a nonlysosomal juxtanuclear compartment. Colocalization studies with transferrin identified this organelle as the endocytic recycling compartment via which iron-depleted transferrin exits the cell. Fluorescently labeled RAP is also transported to this recycling organelle upon receptor-mediated endocytosis by iLR. Internalized HDLp eventually exits the cell via the recycling compartment, a process that can be blocked by monensin, and is re-secreted with a t(1/2) of approximately 13 minutes. From these observations, we conclude that HDLp is the first non-exchangeable apolipoprotein-containing lipoprotein that follows a transferrin-like recycling pathway despite the similarities between mammalian and insect lipoproteins and their receptors.
昆虫的脂蛋白，即高密度脂蛋白（HDLp），在载脂蛋白结构方面与哺乳动物的低密度脂蛋白（LDL）同源。此外，已鉴定出一种HDLp的内吞受体（昆虫脂蛋白受体，iLR），它与LDL受体同源。我们用iLR cDNA转染表达LDL受体的CHO细胞，以同时研究LDL和HDLp的内吞摄取及细胞内途径。我们的研究提供了证据，表明这些哺乳动物和昆虫的脂蛋白在受体介导的内吞作用后遵循不同的细胞内途径。多色成像和免疫荧光用于观察这些细胞中荧光标记配体的细胞内运输。内化后，这一过程可被人受体相关蛋白（RAP）完全抑制，哺乳动物和昆虫的脂蛋白共享内吞小泡。然而，随后HDLp从含LDL的内体中排出。与LDL不同，LDL与其受体解离后在溶酶体中完全降解，而HDLp和iLR都聚集在一个非溶酶体的近核区室。与转铁蛋白的共定位研究确定这个细胞器为内吞循环区室，通过它缺铁的转铁蛋白离开细胞。荧光标记的RAP在iLR介导的受体介导内吞作用后也被运输到这个循环细胞器。内化的HDLp最终通过循环区室离开细胞，这一过程可被莫能菌素阻断，并以约13分钟的半衰期重新分泌。从这些观察结果中，我们得出结论，HDLp是第一种含不可交换载脂蛋白的脂蛋白，尽管哺乳动物和昆虫的脂蛋白及其受体存在相似性，但它遵循类似转铁蛋白的循环途径。
