Divergent Regulation of Alveolar Type 2 Cell and Fibroblast Apoptosis by Plasminogen Activator Inhibitor 1 in Lung Fibrosis

Increased apoptosis sensitivity of alveolar type 2 (ATII) cells and increased apoptosis resistance of (myo)fibroblasts, the apoptosis paradox, contributes to the pathogenesis of idiopathic pulmonary fibrosis (IPF). The mechanism underlying the apoptosis paradox in IPF lungs, however, is unclear. Aging is the greatest risk factor for IPF. In this study, we show, for the first time, that ATII cells from old mice are more sensitive, whereas fibroblasts from old mice are more resistant, to apoptotic challenges, compared with the corresponding cells from young mice. The expression of plasminogen activator inhibitor 1 (PAI-1), an important profibrogenic mediator, was significantly increased in both ATII cells and lung fibroblasts from aged mice. In vitro studies using PAI-1 siRNA and active PAI-1 protein indicated that PAI-1 promoted ATII cell apoptosis but protected fibroblasts from apoptosis, likely through dichotomous regulation of p53 expression. Deletion of PAI-1 in adult mice led to a reduction in p53, p21, and Bax protein expression, as well as apoptosis sensitivity in ATII cells, and their increase in the lung fibroblasts, as indicated by in vivo studies. This increase was associated with an attenuation of lung fibrosis after bleomycin challenge. Since PAI-1 is up-regulated in both ATII cells and fibroblasts in IPF, the results suggest that increased PAI-1 may underlie the apoptosis paradox of ATII cells and fibroblasts in IPF lungs.

Apoptosis of alveolar type II (ATII) epithelial cells is evident in idiopathic pulmonary fibrosis (IPF).1, 2, 3 In contrast, (myo)fibroblasts, the major producers of extracellular matrix proteins, from IPF lungs, are resistant to apoptosis.3, 4, 5 Recurrent injury to alveolar epithelium (apoptosis), followed by sustained activation of (myo)fibroblasts (resistance to apoptosis), is a key factor in the initiation and progression of IPF. However, the mechanism underlying the increased apoptosis sensitivity of ATII cells and increased apoptosis resistance of lung fibroblasts in IPF, referred to as the apoptosis paradox,⁶ is unclear. As broadly targeting apoptosis may worsen fibrosis, identifying the pathways that control the apoptosis paradox in IPF lungs will be critical not only for elucidation of the etiology but also for the development of effective treatments for IPF.The incidence of and mortality due to IPF increases with advanced age,7, 8, 9 suggesting that aging is a major risk factor for IPF. The mechanism underlying the age-related susceptibility to IPF is unclear. Plasminogen activator inhibitor 1 (PAI-1) is a primary inhibitor of urokinase-type and tissue-type plasminogen activators, which convert plasminogen into plasmin, a serine protease involved in fibrinolysis. Besides inhibition of fibrinolysis, PAI-1 is also involved in the regulation of cell adhesion, migration, senescence, and apoptosis, dependent and independent of its anti-protease activity. More importantly, PAI-1 expression increases with age in humans and in wild-type and aging model mice as well as in aging-related diseases, including IPF.10, 11, 12, 13 An increased PAI-1 expression is also detected in experimental fibrosis models induced by different stimuli.13, 14, 15 Deletion of PAI-1 or inhibition of PAI-1 activity attenuates, whereas overexpression of PAI-1 enhances, lung fibrotic responses.^13,15, 16, 17, 18, 19, 20 Together, these data suggest that increased PAI-1 may contribute to the age-related susceptibility to IPF, although the underlying mechanism remains elusive.As PAI-1 modulates the sensitivity of different types of cells to apoptosis^13,15,21, 22, 23, 24, 25 and PAI-1 expression is increased in both ATII cells²⁰ and lung fibroblasts²² in IPF, we hypothesize that increased PAI-1 expression underlies the apoptosis paradox of ATII cells and fibroblasts in IPF lung. Whether ATII cells from old mice were more sensitive, and fibroblasts from old mice were more resistant, to apoptotic stimuli, compared with the corresponding cells from young mice, was investigated. Whether increased PAI-1 plays a role in the apoptosis paradox of ATII cells and fibroblasts was tested using genetic and pharmacologic approaches to modulate PAI-1 expression/activity in vitro and in vivo. Our results provide new insights into the potential mechanisms underlying the apoptosis paradox observed in IPF and aging lungs.