SUBCELLULAR-LOCALIZATION AND ENDOCYTIC FUNCTION OF LOW-DENSITY-LIPOPROTEIN RECEPTOR-RELATED PROTEIN IN HUMAN GLIOBLASTOMA CELLS

The low density lipoprotein receptor-related protein (LRP) is a multif unctional cell surface receptor that binds and endocytoses several str ucturally and functionally distinct ligands. Several of the ligands fo r LRP participate in both normal physiology and pathophysiology of the central nervous system. To begin to gain insights into the role of LR P in the central nervous system, we have analyzed the expression, subc ellular distribution, and endocytic function of LRP in human glioblast oma U87 cells. These cells express an abundance of LRP at both the mRN A and protein levels. A 39-kDa protein, which copurifies with LRP and regulates its ligand binding activity, is also highly expressed in U87 cells. The subcellular localization of LRP and the 39-kDa protein was analyzed using scanning laser confocal and electron microscopy combin ed with immunolabeled U87 cells. At the plasma membrane, LRP was large ly confined to clathrin-coated pits. Within cells, LRP and the 39-kDa protein partially colocalized within rough endoplasmic reticulum and t he Golgi complex, suggesting a potential intracellular interaction bet ween the two proteins. Little 39-kDa protein was found in endosomes in which LRP occurred abundantly. In examining the functional role of LR P in U87 cells, we found that LRP at the cell surface and along the ce llular processes was functional in the binding and endocytosis of its ligands, and its activity therein was regulated by the 39-kDa protein. Using truncated recombinant 39-kDa protein constructs, we also demons trated that distinct regions of the 39-kDa protein were responsible fo r inhibiting the binding of different LRP ligands on U87 cells. Our re sults thus strongly suggest several potential roles for LRP in brain p rotein and lipoprotein metabolism, as web as control of extracellular protease activity.