The LRP1/CD91 ligands, tissue-type plasminogen activator, α(2)-macroglobulin, and soluble cellular prion protein have distinct co-receptor requirements for activation of cell-signaling

LDL Receptor-related Protein-1 (LRP1/CD91) binds diverse ligands, many of which activate cell-signaling. Herein, we compared three LRP1 ligands that inhibit inflammatory responses triggered by lipopolysaccharide (LPS), including: enzymatically-inactive tissue-type plasminogen activator (EI-tPA); activated α(2)-macroglobulin (α(2)M); and S-PrP, a soluble derivative of nonpathogenic cellular prion protein (PrP(C)). In bone marrow-derived macrophages, the N-methyl-D-aspartate receptor was essential for all three LRP1 ligands to activate cell-signaling and inhibit LPS-induced cytokine expression. Intact lipid rafts also were essential. Only α(2)M absolutely required LRP1. LRP1 decreased the EI-tPA concentration required to activate cell-signaling and antagonize LPS but was not essential, mimicking its role as a S-PrP co-receptor. Membrane-anchored PrP(C) also functioned as a co-receptor for EI-tPA and α(2)M, decreasing the ligand concentration required for cell-signaling and LPS antagonism; however, when the concentration of EI-tPA or α(2)M was sufficiently increased, cell-signaling and LPS antagonism occurred independently of PrP(C). S-PrP is the only LRP1 ligand in this group that activated cell-signaling independently of membrane-anchored PrP(C). EI-tPA, α(2)M, and S-PrP inhibited LPS-induced LRP1 shedding from macrophages, a process that converts LRP1 into a pro-inflammatory product. Differences in the co-receptors required for anti-inflammatory activity may explain why LRP1 ligands vary in ability to target macrophages in different differentiation states.