The LRP1/CD91 ligands, tissue-type plasminogen activator, α-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-macroglobulin (αM); and S-PrP, a soluble derivative of nonpathogenic cellular prion protein (PrP). 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 αM 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 also functioned as a co-receptor for EI-tPA and αM, decreasing the ligand concentration required for cell-signaling and LPS antagonism; however, when the concentration of EI-tPA or αM was sufficiently increased, cell-signaling and LPS antagonism occurred independently of PrP. S-PrP is the only LRP1 ligand in this group that activated cell-signaling independently of membrane-anchored PrP. EI-tPA, αM, 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.