Exploitation of the Ligand-Binding Properties of the Mannose 6-Phosphate/Insulin-Like Growth Factor II (IGF-II) Receptor to Inhibit IGF-II-Dependent Growth of Cancer Cells

The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) is a multifunctional, type I transmembrane receptor that is a member of the P-type lectin family. A large, extracytoplasmic (EC) region of the M6P/IGF2R binds various ligands, allowing the receptor to regulate multiple biological functions, including the role as a tumor suppressor. Two major classes of ligands, M6P-glycosylated (i.e. any proteins that bear M6P due to post-translational modification in the trans-Golgi network (TGN)) and non-glycosylated (i.e., the mitogen insulin-like growth factor II (IGF-II)), bind within distinct regions of the EC of the receptor and are trafficked to the lysosome. The M6P/IGF2R as well as the cation-dependent mannose 6-phosphate receptor (CD-MPR) are mostly involved in lysosomal biogenesis, trafficking newly synthesized lysosomal enzymes from the TGN to the early endosomes, where the vesicles mature into lysosomes. The receptors undergo recycling during the late endosomal phase where they are retrograde transported back to the TGN for another round of trafficking. However, a fraction of the receptors is found on the cell surface, where the M6P/IGF2R, but not the CD-MPR, is able to bind extracellular ligands. Through this action, IGF-II can bind to the M6P/IGF2R and will be degraded in the lysosome, reducing the bioavailability of the growth factor for the mitogenic insulin-like growth factor I receptor (IGF1R); thus, the M6P/IGF2R is considered a clearance receptor and tumor suppressor. Due to its growth suppressive function, the M6P/IGF2R is believed to play a role in cancer biology. High-affinity, bivalent M6P-based ligands, such as lysosomal enzymes, bind and stabilize the dimeric M6P/IGF2R at the cell surface, leading to its internalization at a faster rate than when there is no M6P-based ligand bound. Therefore, the major goal of our work is to produce a panel of M6P-based ligands capable of bi- or multivalent binding to the M6P/IGF2R that could suppress IGF-II-dependent growth of cancer cells. Additionally, the M6P receptors (MPR) are well conserved through evolution, with the earliest form of “true” MPR known to date in the invertebrates such as mollusk. However, the social amoeba, D. discoideum, produces lysosomal enzymes that bind to the M6P/IGF2R, a discovery that predated identification of a receptor capable of transporting these acid hydrolases within this organism. We provide evidence of a putative MPR protein that retains all the necessary components of a M6P receptor homology domain that also binds M6P. The studies presented herein further our understanding of the origin of the M6P/IGFR as well as exploiting this receptor as a novel therapeutic target against IGF-II-dependent cancers