An inhibitor in human lung macrophages active against human neutrophil elastase

Discriminating between the activities of human neutrophil elastase and proteinase 3 using serpin-derived fluorogenic substrates

Human neutrophil elastase (HNE) has long been linked to the pathology of a variety of inflammatory diseases and therefore is a potential target for therapeutic intervention. At least two other serine proteases, proteinase 3 (Pr3) and cathepsin G, are stored within the same neutrophil primary granules as HNE and are released from the cell at the same time at inflammatory sites. HNE and Pr3 are structurally and functionally very similar, and no substrate is currently available that is preferentially cleaved by Pr3 rather than HNE. Discrimination between these two proteases is the first step in elucidating their relative contributions to the development and spread of inflammatory diseases. Therefore, we have prepared new fluorescent peptidyl substrates derived from natural target proteins of the serpin family. This was done because serpins are rapidly cleaved within their reactive site loop whether they act as protease substrates or inhibitors. The hydrolysis of peptide substrates reflects the specificity of the parent serpin including those from alpha-1-protease inhibitor and monocyte neutrophil elastase inhibitor, two potent inhibitors of elastase and Pr3. More specific substrates for these proteases were derived from the reactive site loop of plasminogen activator inhibitor 1, proteinase inhibitors 6 and 9, and from the related viral cytokine response modifier A (CrmA). This improved specificity was obtained by using a cysteinyl residue at P1 for Pr3 and an Ile residue for HNE and because of occupation of protease S' subsites. These substrates enabled us to quantify nanomolar concentrations of HNE and Pr3 that were free in solution or bound at the neutrophil surface. As membrane-bound proteases resist inhibition by endogenous inhibitors, measuring their activity at the surface of neutrophils may be a great help in understanding their role during inflammation.

Sequence and molecular characterization of human monocyte/neutrophil elastase inhibitor

cDNA encoding human monocyte/neutrophil elastase inhibitor (EI), a M(r) approximately 42,000 protein with serpin-like functional properties, has been sequenced. The 1316-base-pair sequence was obtained from overlapping clones and amplified DNA from libraries of monocyte-like and neutrophil-like cells. Hybridization with EI cDNA identified three EI mRNA species of 1.5, 1.9, and 2.6 kilobases in U937 monocyte-like cells and no hybridizing mRNA in lymphoblastoid cells lacking detectable EI. The cDNA open reading frame encodes a 379-amino acid protein, of which 167 residues were confirmed by tryptic peptides. Although EI may function extracellularly as well as intracellularly, its deduced sequence lacks a typical cleavable N-terminal signal sequence. Sequence analysis established that EI is a member of the serpin superfamily. EI has greatest homology (50.1% identity of amino acids) with plasminogen activator inhibitor 2, also a monocyte protein, and ovalbumin and gene Y, which were previously grouped as an ancient branch of the serpin superfamily. The extent of EI identity with the functionally related serpin alpha 1 antitrypsin is only 30.1%. Sequence alignment indicates that the reactive center P1 residue is Cys-344, consistent with abrogation of elastase inhibitory activity by iodoacetamide and making EI a naturally occurring Cys-serpin. The cleavable bond, Cys-Met, suggests an oxidation-sensitive molecule capable of inhibiting more than one serine protease. Oxidation sensitivity would limit the place of action of EI to the immediate vicinity of carrier cells. The molecular structure will help clarify the likely role of EI in regulating protease action and preventing tissue damage by phagocytic cells.

Elastase inhibitor. Characterization of the human elastase inhibitor molecule associated with monocytes, macrophages, and neutrophils

A fast-acting inhibitor of serine elastase has been detected at high levels in human neutrophils, fresh monocytes, matured monocytes, and macrophages. The elastase inhibitor was isolated from large scale cultures of the monocyte-like cell line U937 by DNase chromatography, disulfide exchange, Phenyl-Sepharose, Red A-agarose, and DEAE HPLC chromatography with an average yield of 480 micrograms from 1.8 x 10(10) cells. The isolated polypeptide was verified as elastase inhibitor by its ability to (a) form a covalent complex with elastase; and (b) inhibit the elastinolytic activity of elastase. The purified elastase inhibitor molecule is unique, i.e., physiochemical and/or functional properties distinguish it from all other serine proteinase inhibitors. Treatment with iodoacetamide abrogates the ability of the molecule to form a complex with elastase, thereby providing evidence for the presence of an essential cysteine residue. Based on functional criteria, this elastase inhibitor has been grouped with the proteinase inhibitors of the serpin superfamily. The purified elastase inhibitor is a single polypeptide of Mr approximately 42,000. The NH2 terminus appears to be blocked. Compositional analyses indicates five cysteine residues per molecule of approximately 360 amino acid residues. Negligible levels of carbohydrate were detected on gas-liquid chromatography. This finding and the insensitivity of the molecule to peptide N-glycosidase F treatment strongly indicate that the elastase inhibitor is a nonglycosylated protein.

Inhibition of human neutrophil elastase by bacterial polyanions

We previously demonstrated that pneumococcal extracts contain a highly specific inhibitor of human neutrophil elastase (HNE). We now show that the active inhibitor in these extracts is a high-molecular-weight, heat-stable substance that appears to be RNA, since inhibitory activity of pneumococcal extracts is decreased by incubation with ribonuclease but not by incubation with deoxyribonuclease or proteinase K. Moreover, metabolically labeled ([3H]uridine) pneumococcal RNA, isolated by phenol extraction followed by ethanol precipitation, strongly inhibits HNE. Pneumococcal capsular polysaccharide, although polyanionic, is only weakly inhibitory toward HNE and is not a major source of elastase-inhibitory activity in pneumococcal extracts. On the other hand, the capsule of Haemophilus influenzae type b contains polyribosylribitol phosphate. This highly charged polyanion possesses HNE-inhibitory activity, but only under special circumstances to be discussed below. Pneumococci (type I, type II smooth, type II rough) and H. influenzae (type b) all release HNE-inhibitory activity into their culture medium during growth. By contrast, Klebsiella pneumoniae and Staphylococcus aureus release little (if any) stable HNE-inhibitory activity during growth. We propose that some bacterial pneumonias may spare host tissue because polyanions released by the invading microorganisms (e.g. RNA from autolysing pneumococci) inhibit elastase released from inflammatory neutrophils and thereby modulate accompanying tissue proteolysis. Pneumonias caused by microorganisms that do not release stable polyanionic inhibitors of HNE (e.g., Staphylococcus and Klebsiella) may be correspondingly more injurious to the lung.

A fast-acting elastase inhibitor in human monocytes

A proteinase inhibitor active against neutrophil and pancreatic elastase was detected in extracts of cultured human monocytes and the human monocyte-like cell line U937. This component forms a covalent complex with the active site of elastase; the complex is stable in boiling sodium dodecyl sulfate solution, and is susceptible to nucleophilic cleavage. The activity of the elastase inhibitor is not detected in extracts of freshly isolated monocytes, but becomes detectable when the monocytes are allowed to mature in culture, with maximum levels occurring at 5-7 d. The monocyte inhibitor is fast-acting; its reaction with 125I-labeled elastase is complete in less than 1 min at 37 degrees C. Analysis by electrophoresis and studies using a heteroantiserum to alpha 1-proteinase inhibitor demonstrated that the elastase inhibitor of monocytes/U937 cells is not identical to alpha 1-proteinase inhibitor, the major elastase inhibitor of blood plasma. The extent of conversion of 125I-elastase to the 125I-elastase-inhibitor complex is proportional to the amount of U937 extract or cultured monocyte extract, indicating that this reaction can serve to quantify the elastase inhibitor. The elastase inhibitor is an abundant component in mature monocytes, with greater than or equal to 1.5 X 10(6) molecules/cell (greater than or equal to 12 micrograms per 10(8) cells, greater than 0.1% of total cell protein). Its mol wt is estimated at 50,000. Thus, the monocyte inhibitor should be classified as a putative regulator of neutrophil (and monocyte) elastase activity at inflammatory sites. This designation is based on the properties of the molecule, including its high concentration in maturing monocytes, its affinity for elastase, and its fast reaction with this enzyme.

Inhibition of human leucocyte elastase by polynucleotides

We have found that nanomolar range concentrations of transfer RNA inhibit human leucocyte elastase activity against synthetic or natural substrates. Titration curves give a stoichiometry of 9 +/- 1 elastase molecules inhibited per tRNA molecule. The interaction seems essentially electrostatic since similar inhibitions were measured with poly r(A) or calf thymus DNA, and because the increase in ionic strength leads to a decrease of inhibition. This observation appears to be specific of human leucocyte elastase since porcine pancreatic elastase, and both human and bovine chymotrypsins are not inhibited by tRNA.

Isolation of the major serine protease inhibitor from the 5-day serum-free conditioned medium of human embryonic lung cells and demonstration that it is fetuin

Human embryonic lung (HuEL) cells in culture produce large amounts of the enzyme, plasminogen activator, and thus generate substantial amounts of active plasmin. Despite the presence of plasmin, however, HuEL cells grow in ordered, flattened, adherent sheets. It seemed of interest to characterize protease inhibitors that might be present in HuEL cultures and which might account for this apparent contradiction. This paper reports the isolation and purification of the major serine protease inhibitor in 5-day serum-free conditioned medium (CM) from HuEL cells, and the purification of an identical molecule from fetal bovine serum (FBS). Both the CM-derived inhibitor and the FBS-derived inhibitor are identical with fetuin, the major glycoprotein of FBS. The CM-derived inhibitor is apparently derived from the FBS used to supplement the growth medium of HuEL cells between serum-free CM collection periods. It is not labeled metabolically with 3H-leucine. Its electrophoretic behavior is indistinguishable from that of standard fetuin in SDS-PAGE, non-SDS basic pH,PAGE, and isoelectric focusing. The CM-derived inhibitor and standard fetuin inhibit trypsin and plasmin with similar efficiencies, but neither inhibits chymotrypsin, pancreatic elastase, or plasminogen activator. They are immunologically indistinguishable. The suggestion is made that fetuin, and possibly other protease inhibitors present in HuEL cell cultures, may be concentrated locally by HuEL cells and gradually released back into the medium in the absence of serum. These molecules may serve to protect HuEL cells against proteases they generate.

Irreversible bronchial goblet cell metaplasia in hamsters with elastase-induced panacinar emphysema

A single intratracheal instillation of pancreatic elastase in hamsters induces a lesion resembling human panacinar emphysema. This paper reports the occurrence of irreversible goblet cell metaplasia in the bronchial epithelium of hamsters similarly exposed to elastase. The goblet cell change was dose related; a dose of 0.1 mg/100 g body wt or less at 16 days, produced slight or moderate goblet cell metaplasia in fewer than half the animals, whereas 84% of animals treated with a dose between 0.2 and 0.5 mg/100 g body wt developed goblet cell metaplastic lesions, more than half of which were considered to be severe. The percentage of goblet cells in the epithelium of elastase-treated hamsters (32.5) was significantly higher (P less than 0.005) than that of unexposed (12.2) and saline-exposed controls (18.7). All hamsters examined 6 and 12 mo after elastase treatment showed the lesion. The pathogenesis of the changes is unclear but the possibility is raised that the bronchial changes may be due to disturbance of an endogenous protease-antiprotease system. The findings suggest the hypothesis that, under appropriate circumstances, a single pulmonary insult in man could lead to a permanent lung injury demonstrating the anatomic lesions of both chronic bronchitis and panacinar emphysema.

Alpha-1-antitrypsin content in the serum, alveolar macrophages, and alveolar lavage fluid of smoking and nonsmoking normal subjects

The content of alpha-1-antitrypsin in the serum, alveolar lavage fluid, and alveolar macrophages of smokers and nonsmokers was studied. Bronchoalveolar lavage was used to obtain alveolar fluid and macrophages from normal volunteers, and alpha-1-antitrypsin and albumin were measured using the electroimmunodiffusion technique. The serum level of inhibitor was not different between the two groups, while the total lavage fliud content of alpha-1-antitrypsin was increased in the smokers. The level of alpha-1-antitrypsin was also significantly greater (P less than 0.001) in the alveolar macrophages of the smokers suggesting the possibility of chronically increased alveolar levels in the cigarette smoker as a possible protective mechanism against proteolysis.

Interrelationships between the human alveolar macrophage and alpha-1-antitrypsin

Alveolar macrophages lavaged from human lungs contain protease activity at an optimum pH of 3.0 and possibly a lesser peak of activity at pH 5.5. Protease activity measured at pH 4.1 is inhibited by purified alpha-1-antitrypsin. Fluorescent antibody studies of human alveolar macrophages showed that alpha-1-antitrypsin is present in normal alveolar macrophages. In addition, macrophages from a patient with a homozygous deficiency of alpha-1-antitrypsin exhibited less fluorescence when incubated in autologous serum than the same macrophages incubated in normal serum. Macrophages from normal subjects showed maximal fluorescence when removed from the lung and additional incubation with serum did not increase fluorescence. These results implicate the human alveolar macrophage as a possible source of an enzyme that may cause emphysema in patients deficient in alpha-1-antitrypsin. They also show that alpha-1-antitrypsin has access to the alveolus in normal subjects.