Inducible expression of tissue inhibitor of metalloproteinases-resistant matrix metalloproteinase-9 on the cell surface of neutrophils

Matrix metalloproteinase (MMP)-9 secreted by activated polymorphonuclear neutrophils (PMN) may play roles in mediating lung injury by degrading extracellular matrix proteins. However, the mechanisms by which MMP-9 retains activity in the presence of tissue inhibitors of metalloproteinases (TIMPs) are not known. We show that MMP-9 is also expressed on the cell surface of PMN, and proinflammatory mediators induce up to 10-fold increases in cell surface expression of MMP-9. Stimulated human PMN express active forms of cell surface MMP, which cleave the MMP substrate, McaPLGLDpaAR. Loss-of-function studies employing PMN from mice genetically deficient in MMP-9 (MMP-9(-/-)) demonstrate that membrane-bound MMP-9 contributes substantially to MMP-mediated surface-bound cleavage of McaPLGLDpaAR (similar to 50%) and gelatin (similar to 70%) by stimulated PMN. Like soluble MMP-9, membrane-bound MMP-9 cleaves McaPLGLDpaAR (K-cat/K-M 82,000 M(-1)s(-1)), gelatin, type IV collagen, elastin, and alpha(1)-proteinase inhibitor. However, in contrast to soluble MMP-9, membrane-bound MMP-9 is substantially resistant to inhibition by TIMPs. The IC50 for inhibition of membrane-bound MMP-9 by TIMP-1 and TIMP-2 are similar to 21-fold and similar to 68-fold higher, respectively, than those for inhibition of soluble MMP-9. The binding of MMP-9 to the plasma membrane of PMN enables it to evade inhibition by TIMPs, and thereby may after the pericellular proteolytic balance in favor of extracellular matrix degradation. Membrane-bound MMP-9 on PMN may play pathogenetic roles in inflammatory lung diseases.