LRP-1–CD44, a New Cell Surface Complex Regulating Tumor Cell Adhesion

Abstract

The low-density lipoprotein receptor-related protein 1 (LRP-1) is a large endocytic receptor mediating the clearance of various molecules from the extracellular matrix. In the field of cancer, LRP-1-mediated endocytosis was first associated with antitumor properties. However, recent results suggested that LRP-1 may coordinate the adhesion-deadhesion balance in malignant cells to support tumor progression. Here, we observed that LRP-1 silencing or RAP (receptor-associated protein) treatment led to accumulation of CD44 at the tumor cell surface. Moreover, we evidenced a tight interaction between CD44 and LRP-1, not exclusively localized in lipid rafts. Overexpression of LRP-1-derived minireceptors indicated that the fourth ligand-binding cluster of LRP-1 is required to bind CD44. Labeling of CD44 with EEA1 and LAMP-1 showed that internalized CD44 is routed through early endosomes toward lysosomes in a LRP-1-dependent pathway. LRP-1-mediated internalization of CD44 was highly reduced under hyperosmotic conditions but poorly affected by membrane cholesterol depletion, revealing that it proceeds mostly via clathrin-coated pits. Finally, we demonstrated that CD44 silencing abolishes RAP-induced tumor cell attachment, revealing that cell surface accumulation of CD44 under LRP-1 blockade is mainly responsible for the stimulation of tumor cell adhesion. Altogether, our data shed light on the LRP-1-mediated internalization of CD44 that appeared critical to define the adhesive properties of tumor cells.

ACKNOWLEDGMENTS

This work was supported by grants from CNRS, Ligue Nationale Contre le Cancer (CCIR-GE, Conférence de Coordination Interrégionale du Grand Est 2012), CPER 2007-2013, and Fonds National pour la Santé ACI 2008 (Cancéropôle Grand-Est Project). G.P. and E.S. were recipients of grants from Région Champagne-Ardenne (2008-2011 and 2010-2011, respectively). M.D. and M.K. acknowledge support from the Agence National pour la Recherche (ANR-08-MNPS-042-04 and ANR-09-BIOT-015-01, TIMPAD and VECtoBrain projects, respectively).

We acknowledge the expert technical support of C. Terryn (SFR CAP-Santé, Reims, France) and H. Bobichon (FRE CNRS/URCA no. 3481) for confocal microscopy. We thank F. Rabenoelina and L. Parent for technical assistance and S. Wright for editorial assistance. We are grateful to M. S. Nielsen (Department of Medical Biochemistry, University of Aarhus, Denmark) for kindly providing us with the pT7H6FX-RAP vector.