Low Density Lipoprotein (LDL) Receptor-related Protein 1B Impairs Urokinase Receptor Regeneration on the Cell Surface and Inhibits Cell Migration

Abstract

The low density lipoprotein (LDL) receptor-related protein 1B (LRP1B) is a newly identified member of the LDL receptor family and is closely related to LRP. It was discovered as a putative tumor suppressor and is frequently inactivated in lung cancer cells. In the present study, we used an LRP1B minireceptor (mLRP1B4), which mimics the function and trafficking of LRP1B, to explore the roles of LRP1B on the plasminogen activation system. We found that mLRP1B4 and urokinase plasminogen activator receptor (uPAR) form immunoprecipitable complexes on the cell surface in the presence of complexes of uPA and its inhibitor, plasminogen activator inhibitor type-1 (PAI-1). However, compared with cells expressing the analogous LRP minireceptor (mLRP4), cells expressing mLRP1B4 display a substantially slower rate of uPA⅐PAI-1 complex internalization. Expression of mLRP1B4, or an mLRP4 mutant deficient in endocytosis, leads to an accumulation of uPAR at the cell surface and increased cell-associated uPA and PAI-1 when compared with cells expressing mLRP4. In addition, we found that expression of mLRP1B or the mLRP4 endocytosis mutant impairs the regeneration of unoccupied uPAR on the cell surface and that this correlates with a diminished rate of cell migration. Taken together, these results demonstrate that LRP1B can function as a negative regulator of uPAR regeneration and cell migration.The plasminogen activation system consists of a cascade of enzymes and plays a central role in many physiological processes requiring the degradation of basement membrane and components of the extracellular matrix. When the regulation of this system is disrupted, as occurs in the pathogenesis of cancer, malignant cells are able to invade surrounding tissue and metastasize to distant body regions (1-3). Urokinase plasminogen activator (uPA) 1 catalyzes the formation of plasmin from its inactive precursor, plasminogen. The activity of uPA is regulated by two proteins, the glycosylphosphatidylinositollinked uPA receptor (uPAR) and plasminogen activator inhibitor type 1 (PAI-1). The binding of uPA to uPAR at the cell surface greatly increases its catalytic rate. In contrast, uPA is inactivated by binding to PAI-1. When active uPA is bound to uPAR, it is not internalized but remains at the cell surface. However, when receptor-bound uPA is complexed to its inhibitor, PAI-1, the complex is rapidly internalized and degraded. The mechanism by which this occurs was unknown until it was determined that low density lipoprotein receptor-related protein (LRP) is responsible for this process (4). Following the internalization, uPAR and LRP recycle back to the cell surface, while uPA and PAI-1 are degraded in lysosomes (4 -7). The regeneration of unoccupied uPAR at the cell surface is thus critical for the maintenance of plasminogen activation and for regulation of cellular migration and invasion (8 -10).LRP1B is a recently discovered member of the LDLR family (11, 12). The LDLR family previously contained two large members, LRP (LRP1), a dimer of 515-and...

LRP1B Deletion in High-Grade Serous Ovarian Cancers Is Associated with Acquired Chemotherapy Resistance to Liposomal Doxorubicin

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Chromosomal, epigenetic and microRNA-mediated inactivation of LRP1B, a modulator of the extracellular environment of thyroid cancer cells

“…Aside from modulating matrix metalloproteinase 2, as shown here, LRP1B has also been reported to regulate the uPA system (Liu et al, 2001). Cells expressing LRP1B display a substantially slower rate of uPA/plasminogen activator inhibitor type-1 complex internalization (Knisely et al, 2007), which impairs the regeneration of unoccupied urokinase plasminogen activator receptor on the cell surface and correlates with a diminished rate of cell migration (Li et al, 2002;Tanaga et al, 2004). Our results highlight LRP1B as an unconventional and yet key tumor suppressor acting as a regulator of the 'extracellular proteome' and constraining the abundance of critical members of proteolytic systems in the tumor microenvironment.…”

Lipoprotein Receptor–Related Protein 1 Regulates Collagen 1 Expression, Proteolysis, and Migration in Human Pleural Mesothelial Cells

“…Previous studies have shown that LRP-1 function directly affects expression of uPA, PAI-1, and uPAR (39). In this study, LRP-1-expressing MeT5A PMCs and LRP-1-deficient REN were used in internalization assays.…”