Mechanisms of Extracellular Matrix Proteoglycan Degradation by Human Neutrophils

Alterations in proteoglycans (PG) located in the pulmonary interstitium may influence extracellular matrix (ECM) structure and assembly during the development of diseases in which increased numbers of neutrophils enter the lung. To evaluate potential mechanisms of PG degradation, neutrophils or purified neutrophil products were incubated with ECM that had been produced by cultured neonatal rat vascular smooth muscle cells (SMC) or lung fibroblasts (LF) and metabolically labeled with 35SO4. Matrix PG solubilization was expressed as a percentage of the spontaneous [35SO4]PG solubilization that occurred in the presence of buffer alone. Solubilization by unstimulated neutrophils was 105.8 ± 3.1% (mean ± SEM, n = 6) and 101.7 ± 3.05 (n = 8) using ECM that had been produced by LF and SMC, respectively. Solubilization by neutrophils that had been stimulated with formyl-methionine-leucine-phenylalanine (FMLP) in the presence of cytochalasin B (CB) was 189.7 ± 5.8% and 298.2 ± 26.2% using ECM produced by LF and SMC, respectively. Matrix that had been produced by SMC was used to evaluate which neutrophil products were responsible for the degradation of PG. Addition of a specific inhibitor of neutrophil elastase (NE) to stimulated polymorphonuclear leukocytes (PMN) reduced PG solubilization by 88.3 ± 4.8% (n = 8). Addition of an inhibitor of cathepsin G (CG), as well, did not further reduce PG degradation. Purified CG and myeloperoxidase solubilized significantly more PG, 125.8 ± 6.2% (n = 9) and 143.2 ± 8.1% (n = 6), respectively (P < 0.01), than was solubilized spontaneously. However, degradation of PG by a similar quantity of NE was 227.5 ± 221.6%, suggesting that NE was the major source of PG-degrading activity in neutrophils. Degradation of PG by NE and myeloperoxidase appeared to result from cleavage of the core protein rather than the associated glycosaminoglycans. Similar mechanisms may lead to degradation of interstitial PG in the lung when neutrophils enter the interstitium acutely as in the adult respiratory distress syndrome or chronically during the development of emphysema.