High-affinity binding of LDL receptor-related protein 1 to matrix metalloprotease 1 requires protease:inhibitor complex formation.

Matrix metalloprotease (MMP) activation contributes to degradation of the extracellular matrix (ECM), resulting in a multitude of pathologies. Low-density lipoprotein receptor-related protein 1 (LRP1) is a multifaceted endocytic and signaling receptor responsible for internalization and lysosomal degradation of diverse proteases, protease inhibitors and lipoproteins along with numerous other proteins. In the current study, we identified MMP-1 as a novel LRP1 ligand. Binding studies employing surface plasmon resonance revealed that both proMMP-1 and active MMP-1 bind to purified LRP1 with equilibrium dissociation constants (KD) of 19 nM and 25 nM, respectively. We observed that human aortic smooth muscle cells readily internalize and degrade 125I-labeled proMMP-1 in an LRP1-mediated process. Our binding data also revealed that all tissue inhibitors of metalloproteases (TIMPs) bind to LRP1 with KD values ranging from 23 nM to 33 nM. Interestingly, the MMP-1/TIMP-1 complex bound to LRP1 with 30-fold higher affinity (KD = 0.6 nM) than either component alone, revealing that LRP1 prefers the protease:inhibitor complex as a ligand. Of note, modification of lysine residues on either proMMP-1 or TIMP-1 ablated the ability of the MMP-1/TIMP-1 complex to bind to LRP1. LRP1's preferential binding to enzyme:inhibitor complexes was further supported by higher binding affinity for proMMP-9/TIMP-1 complexes compared with either of these two components alone. LRP1 has four clusters of ligand-binding repeats, and MMP-1, TIMP-1 and MMP-1/TIMP-1 complexes bound to cluster III most avidly. Our results reveal an important role for LRP1 in controlling ECM homeostasis by regulating MMP-1 and MMP-9 levels.