Plasminogen Activator Inhibitor-1 Deficiency Augments Visceral Mesothelial Organization, Intrapleural Coagulation, and Lung Restriction in Mice with Carbon Black/Bleomycin–Induced Pleural Injury

Abstract

Local derangements of fibrin turnover and plasminogen activator inhibitor (PAI)-1 have been implicated in the pathogenesis of pleural injury. However, their role in the control of pleural organization has been unclear. We found that a C57Bl/6j mouse model of carbon black/bleomycin (CBB) injury demonstrates pleural organization resulting in pleural rind formation (14 d). In transgenic mice overexpressing human PAI-1, intrapleural fibrin deposition was increased, but visceral pleural thickness, lung volumes, and compliance were comparable to wild type. CBB injury in PAI-1 2/2 mice significantly increased visceral pleural thickness (P , 0.001), elastance (P , 0.05), and total lung resistance (P , 0.05), while decreasing lung compliance (P , 0.01) and lung volumes (P , 0.05). Collagen, a-smooth muscle actin, and tissue factor were increased in the thickened visceral pleura of PAI-1 2/2 mice. Colocalization of a-smooth muscle actin and calretinin within pleural mesothelial cells was increased in CBB-injured PAI-1 2/2 mice. Thrombin, factor Xa, plasmin, and urokinase induced mesothelial-mesenchymal transition, tissue factor expression, and activity in primary human pleural mesothelial cells. In PAI-1 2/2 mice, D-dimer and thrombin-antithrombin complex concentrations were increased in pleural lavage fluids. The results demonstrate that PAI-1 regulates CBB-induced pleural injury severity via unrestricted fibrinolysis and cross-talk with coagulation proteases. Whereas overexpression of PAI-1 augments intrapleural fibrin deposition, PAI-1 deficiency promotes profibrogenic alterations of the mesothelium that exacerbate pleural organization and lung restriction.

Mesomesenchymal transition of pleural mesothelial cells is PI3K and NF-κB dependent

“…Recently, we reported that expression of tissue factor pathway inhibitor is phosphatidylinositol-3-kinase (PI3K)/Akt and NF-B dependent and that these signaling pathways are linked in HPMCs (19). We therefore inferred that PI3K/Akt/NF-B signaling could contribute to the progression of MesoMT, a possibility further supported by prior reports linking these pathways to mesenchymal transition in alternative cell types (10,15,18).…”

“…Our work and that of others has shown that pleural mesothelial cells (PMCs) contribute to the progression of PF by undergoing a process called mesothelial (Meso) mesenchymal transition (MT) (1,2,13,14,19), through which PMCs assume characteristics of myofibroblasts. Mesothelial MT (MesoMT) is characterized by mesenchymal changes in cell morphology.…”

“…These myofibroblasts promote the accumulation of extracellular matrix proteins such as collagen and fibronectin. In a recent publication (19), we reported that pleural fibrosis induced by carbon black and bleomycin (CBB) in mice is characterized by the presence of a visceral pleural rind composed of thickened mesothelium and submesothelial tissue. This thickened rind contributes to reduced lung volume, decreased lung compliance, and lung restriction that closely approximates fibrosing pleural injury and fibrothorax in humans (19).…”

“…Mesothelial MT (MesoMT) is characterized by mesenchymal changes in cell morphology. Furthermore, increased expression of ␣-SMA and collagen 1 (Col-1) are hallmarks of myofibroblasts (1,2,14,19). Both ␣-SMA and Col-1 were upregulated in mesothelial cells undergoing MesoMT in vivo in a CBB-mediated model of pleural injury in which thrombin and plasmin were upregulated within the injured pleural compartment (19).…”

“…Furthermore, these same factors promote MT in human (H) PMCs. Although transforming growth factor ␤-1 (TGF-␤) has been shown to potently promote MT in diverse cell types including PMCs (11,14,17,19,22,23), the mechanism by which thrombin and plasmin contribute to this process in PMCs has not, to our knowledge, been previously investigated.…”