The Role of PPARs in Lung Fibrosis.

Pulmonary fibrosis is a group of disorders characterized by accumulation of scar tissue in the lung interstitium, resulting in loss of alveolar function, destruction of normal lung architecture, and respiratory distress. Some types of fibrosis respond to corticosteroids, but for many there are no effective treatments. Prognosis varies but can be poor. For example, patients with idiopathic pulmonary fibrosis (IPF) have a median survival of only 2.9 years. Prognosis may be better in patients with some other types of pulmonary fibrosis, and there is variability in survival even among individuals with biopsy-proven IPF. Evidence is accumulating that the peroxisome proliferator-activated receptors (PPARs) play important roles in regulating processes related to fibrogenesis, including cellular differentiation, inflammation, and wound healing. PPARalpha agonists, including the hypolidipemic fibrate drugs, inhibit the production of collagen by hepatic stellate cells and inhibit liver, kidney, and cardiac fibrosis in animal models. In the mouse model of lung fibrosis induced by bleomycin, a PPARalpha agonist significantly inhibited the fibrotic response, while PPARalpha knockout mice developed more serious fibrosis. PPARbeta/delta appears to play a critical role in regulating the transition from inflammation to wound healing. PPARbeta/delta agonists inhibit lung fibroblast proliferation and enhance the antifibrotic properties of PPARgamma agonists. PPARgamma ligands oppose the profibrotic effect of TGF-beta, which induces differentiation of fibroblasts to myofibroblasts, a critical effector cell in fibrosis. PPARgamma ligands, including the thiazolidinedione class of antidiabetic drugs, effectively inhibit lung fibrosis in vitro and in animal models. The clinical availability of potent and selective PPARalpha and PPARgamma agonists should facilitate rapid development of successful treatment strategies based on current and ongoing research.

Aryl hydrocarbon receptor-deficient mice develop heightened inflammatory responses to cigarette smoke and endotoxin associated with rapid loss of the nuclear factor-kappaB component RelB.

The transcription factor aryl hydrocarbon receptor (AhR) plays an important role in the response to environmental pollutants. However, its role in normal physiology is unclear. To investigate the role of AhR in acute lung inflammation, control and AhR knockout (KO) mice were exposed to inhaled cigarette smoke or bacterial endotoxin. Smoke-induced lung inflammation was twofold to threefold more severe in AhR KO mice than controls. Intriguingly, levels of tumor necrosis factor-alpha and interleukin-6 in the bronchoalveolar lavage of air-exposed KO mice were equal to the levels seen in smoke-exposed controls, suggesting that AhR-deficient mice are inflammation prone. AhR KO mice challenged with inhaled endotoxin, which does not contain AhR ligands, also developed greater lung neutrophilia than controls, and bronchoalveolar lavage cells from AhR KO mice produced elevated levels of tumor necrosis factor-alpha and interleukin-6 when treated with endotoxin in vitro. Nuclear factor-kappaB DNA-binding activity was elevated in smoke-exposed AhR KO mice compared with controls and was associated with a rapid loss of RelB only in the KO mice. We propose that AhR is a previously unrecognized regulator of inflammation that interacts with nuclear factor-kappaB so that in the absence of AhR RelB is prematurely degraded, resulting in heightened inflammatory responses to multiple proinflam-matory stimuli.

Oropharyngeal aspiration of a silica suspension produces a superior model of silicosis in the mouse when compared to intratracheal instillation.

Instillation of crystalline silica into the lungs of mice is a common experimental model of pulmonary fibrosis. Typically, a suspension of silica in saline is injected into the trachea via intubation or surgical tracheostomy. These techniques require a high degree of technical skill, have a lengthy training period, and can suffer from a high failure rate. In oropharyngeal aspiration, a droplet of liquid is placed in the animal's mouth while simultaneously holding its tongue (to block the swallow reflex) and pinching its nose shut, forcing it to breathe through its mouth, aspirating the liquid. To determine whether oropharyngeal aspiration (OA) could replace intratracheal instillation (IT) in a model of silica-induced fibrosis, a comparison was performed. Crystalline silica was introduced into the lungs of male C57BL/6 mice by the IT or OA procedure, and the resulting inflammation and fibrosis was assessed after 3 weeks. IT and OA instillation of silica both resulted in neutrophilic inflammation and fibrotic changes, including interstitial fibrosis and dense fibrotic foci. Mice treated via IT demonstrated a few large lesions proximal to conducting airways with little involvement of the distal parenchyma and large interanimal variability. In contrast, OA resulted in a diffuse pathology with numerous fibrotic foci distributed throughout the lung parenchyma, which is more representative of human fibrotic lung disease. OA- but not IT-treated mice exhibited significantly increased lung collagen content. Furthermore, the interanimal variability within the OA group was significantly less than in the IT group. Oropharyngeal aspiration should be considered as an alternative to intratracheal instillation of silica and other particulates in studies of respiratory toxicity and lung disease.

PPARgamma agonists inhibit TGF-beta induced pulmonary myofibroblast differentiation and collagen production: implications for therapy of lung fibrosis.

Pulmonary fibrosis is a progressive life-threatening disease for which no effective therapy exists. Myofibroblasts are one of the key effector cells in pulmonary fibrosis and are the primary source of extracellular matrix production. Drugs that inhibit the differentiation of fibroblasts to myofibroblasts have potential as antifibrotic therapies. Peroxisome proliferator-activated receptor (PPAR)-gamma is a transcription factor that upon ligation with PPARgamma agonists activates target genes containing PPAR response elements. PPARgamma agonists have anti-inflammatory activities and may have potential as antifibrotic agents. In this study, we examined the abilities of PPARgamma agonists to block two of the most important profibrotic activities of TGF-beta on pulmonary fibroblasts: myofibroblast differentiation and production of excess collagen. Both natural (15d-PGJ2) and synthetic (ciglitazone and rosiglitazone) PPARgamma agonists inhibited TGF-beta-driven myofibroblast differentiation, as determined by alpha-smooth muscle actin-specific immunocytochemistry and Western blot analysis. PPARgamma agonists also potently attenuated TGF-beta-driven type I collagen protein production. A dominant-negative PPARgamma partially reversed the inhibition of myofibroblast differentiation by 15d-PGJ2 and rosiglitazone, but the irreversible PPARgamma antagonist GW-9662 did not, suggesting that the antifibrotic effects of the PPARgamma agonists are mediated through both PPARgamma-dependent and independent mechanisms. Thus PPARgamma agonists have novel and potent antifibrotic effects in human lung fibroblasts and may have potential for therapy of fibrotic diseases in the lung and other tissues.