Constitutive activation of 5-lipoxygenase in the lungs of patients with idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive disorder characterized by inflammation, fibroblast proliferation, and accumulation of extracellular matrix proteins. Leukotrienes (LTs) are pro-inflammatory and pro-fibrogenic mediators derived from the 5-lipoxygenase (5-LO) pathway of arachidonic acid metabolism. They are thought to play a role in a number of disease processes, but have received relatively little attention in investigations into the pathogenesis of IPF. In this study, we measured the levels of immunoreactive LTs B(4) and C(4) in homogenates of lung tissue obtained from patients with newly diagnosed, untreated IPF, as compared to levels measured in homogenates of uninvolved nonfibrotic lung tissue from patients undergoing resectional surgery for bronchogenic carcinoma. Compared to homogenates on nonfibrotic control lung, homogenates from IPF patients contained 15-fold more LTB(4) and 5-fold more LTC(4). IPF homogenate levels of LTB(4) were significantly correlated with histologic indices of both inflammation (r=0.861) and fibrosis (r=0.926). Activation of 5-LO is known from in vitro studies to be associated with localization of the enzyme at the nuclear membrane. Immunohistochemical staining for 5-LO protein in alveolar macrophages (AMs) demonstrated that such an "activated" localization pattern was significantly more frequent in IPF lung (19.2+/-3.3% of cells) than in control lung (9.3+/-0.9%); this localization pattern was rarely seen (3.2%) in sections from a truly normal transplant donor lung. Consistent with these data, AMs obtained from IPF patients by bronchoalveolar lavage, purified by adherence, and cultured in the absence of a stimulus for 16 h elaborated significantly greater amounts of LTB(4) and LTC(4) than did control AMs obtained from normal volunteers. These data indicate that the 5-LO pathway is constitutively activated in the lungs of patients with IPF, and the AM represents at least one cellular source of LT overproduction in this disorder. We speculate that LTs participate in the pathogenesis of IPF, and their overproduction in this disorder may be amenable to specific pharmacotherapy.

Cultured lung fibroblasts isolated from patients with idiopathic pulmonary fibrosis have a diminished capacity to synthesize prostaglandin E2 and to express cyclooxygenase-2

Prostaglandin E2 (PGE2) inhibits fibroblast proliferation and collagen synthesis. In this study, we compared lung fibroblasts isolated from patients with idiopathic pulmonary fibrosis (F-IPF) and from patients undergoing resectional surgery for lung cancer (F-nl) with respect to their capacity for PGE2 synthesis and their expression and regulation of cyclooxygenase (COX) proteins. Basal COX activity, assessed by quantitating immunoreactive PGE2 synthesized from arachidonic acid, was twofold less (P < 0.05) in F-IPF than F-nl. In F-nl, incubation with the agonists PMA, LPS, or IL-1 increased COX activity and protein expression of the inducible form of COX, COX-2, and these responses were inhibited by coincubation with dexamethasone. By contrast, F-IPF failed to demonstrate increases in COX-2 protein expression or COX activity in response to these agonists. Under conditions of maximal induction, COX activity in F-IPF was sixfold less than that in F-nl (P < 0.05). Our data indicate that F-IPF have a striking defect in their capacity to synthesize the antiinflammatory and antifibrogenic molecule PGE2, apparently because of a diminished induction of COX-2 protein. This reduction in the endogenous capacity of F-IPF to down-regulate their function via PGE2 may contribute to the inflammatory and fibrogenic response in IPF. Moreover, we believe that this represents the first description of a defect in COX-2 expression in association with a human disease.

Expression and role of cyclooxygenase isoforms in alveolar and peritoneal macrophages

Prostaglandin synthesis represents one means by which macrophages modulate inflammation. The initial enzyme in the metabolism of arachidonic acid to prostaglandins is cyclooxygenase (COX). Both constitutive (COX-1) and inducible (COX-2) isoforms are recognized. We previously showed that COX activity of rat peritoneal macrophages (PM) exceeds that of alveolar macrophages (AM). In this study, we correlated the steady-state levels of COX-1 and COX-2 proteins with COX activity in resident AM and PM. Freshly obtained AM contained lower levels of COX-1 than did fresh PM. Neither contained substantial amounts of COX-2 in the basal state, but both cell types demonstrated induction when cultured with lipopolysaccharide; once again, COX-2 levels in PM exceeded those in AM. Despite COX-2 induction under these circumstances, its contribution to prostaglandin production appeared to be modest. We conclude that, although both isoforms of COX are expressed in rat AM and PM, COX-1 is responsible for the majority of enzyme activity in both the basal and stimulated states. The lesser prostaglandin synthetic capacity of AM than of PM appears to be the consequence of lower steady-state levels of both COX proteins.

Upper airway muscle activity and the thoracic volume dependence of upper airway resistance

Although a thoracic volume dependence of upper airway resistance and caliber is known to exist in seated subjects, the mechanisms mediating this phenomenon are unknown. To test the hypothesis that actively altered end-expiratory lung volume (EELV) affects upper airway resistance in the supine position and to explore the mechanisms of any EELV-induced resistance changes, we studied five normal males during wakefulness. Supraglottic upper airway resistance (Ruaw) was calculated at an inspiratory flow of 0.1 l/s. The genioglossal electromyogram was obtained with indwelling wire electrodes and processed as moving time average. End-tidal CO2 was monitored by infrared analyzer. Observations were made during four 20-breath voluntary maneuvers: two at high and two at low EELV in each subject. Each maneuver was preceded by a control period at functional residual capacity. At high lung volume the EELV was increased by 2.23 +/- 0.54 (SD) liters; Ruaw decreased to 67.8 +/- 35.1% of control, while tonic and phasic genioglossal activities declined to 79.0 +/- 23.1 and 72.4 +/- 29.8%, respectively. At low lung volume the EELV was decreased by 0.86 +/- 0.23 liters. Ruaw increased to 178.2 +/- 186.8%, while tonic and phasic genioglossal activities increased to 243.0 +/- 139.3 and 249.1 +/- 146.3%, respectively (P less than 0.0001 for all). The findings were not explained by CO2 perturbations or respiratory pattern. Multiple linear regression analysis indicated that the genioglossal responses blunted the EELV-induced changes in upper airway patency.(ABSTRACT TRUNCATED AT 250 WORDS)