Role of Plasminogen Activator in Degradation of Extracellular Matrix Protein by Live Human Alveolar Macrophages

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Abstract
Recent evidence indicates that human alveolar macrophages can degrade purified elastin in vitro by a cell contact-dependent process involving acidic proteinases of the cysteine proteinase class. It is unclear to what extent these cells can degrade elastin within a natural extracellular matrix. To address this question, we cultured live human alveolar macrophages on elastin-rich, 3H-lysine-labeled, extracellular matrices deposited by rat smooth muscle cells in vitro. Under various culture conditions, we then measured release of total radioactivity from the matrices during co-culture with cells as well as net loss of desmosine/isodesmosine as a specific marker of elastin degradation. Live macrophages adhered to and progressively solubilized matrix protein at a slow rate (.apprx. 5 .mu.g/106 cells/24 h) but the rate of solubilization increased more than 15-fold in the presence of plasminogen. The elastin component of the complicated matrix was not measurably degraded in the absence of plasminogen, but in medium containing plasminogen, 3.5 .times. 106 macrophages degraded 25 .+-. 8 .mu.g of elastin in 72 h. After pretreatment of matrices with trypsin to remove glycoprotein elements, live cells degraded 16 .+-. 4 .mu.g of elastin under plasminogen-free conditions. The addition of serum to the medium (1 to 5%) inhibited degradation of elastin within whole matrices (.apprx. 50% compared to serum-free medium containing plasminogen) but had no effect on degradation of elastin in trypsin-pretreated matrices. An active site inhibitor of cysteine proteinases, Z-phenylalanine-phenylanaine-diazomethylketone, blocked .apprx. 50% of the elastin degradation. Because of the critical role of plasminogen in proteolysis by live alveolar macrophages, we questioned whether human lung actually contains measurable plasminogen in the alveolar or interstitial compartments. Immunoblots of concentrated lavage protein of normal subjects revealed that most of the antigen reacting with an antiplasminogen antibody was in the form of plasmin. Immunohistologic analysis of lung biopsy specimens also demonstrated the presence of immunoreactive antigen within the interstitial and connective tissue areas. These data provide further evidence that human alveolar macrophages can degrade glycoprotein and elastin elements of extracellular matrices by an enzymatic process involving cooperation between neutral and acidic proteinases. The data show the critical importance of plasminogen in this process and verify that the process can potentially occur in human lung. If degradation of elastin and associated proteins by macrophages is important to the pathogenesis of emphysema, then these data suggest that determinants of plasminogen activator and plasmin activities within the lung may also be important to the development of this disease.