The vitronectin RGD motif regulates TGF-β-induced alveolar epithelial cell apoptosis

Abstract

Transforming growth factor-β (TGF-β) is a critical driver of acute lung injury and fibrosis. Injury leads to activation of TGF-β, which regulates changes in the cellular and matrix makeup of the lung during the repair and fibrosis phase. TGF-β can also initiate alveolar epithelial cell (AEC) apoptosis. Injury leads to destruction of the laminin-rich basement membrane, which is replaced by a provisional matrix composed of arginine-glycine-aspartate (RGD) motif-containing plasma matrix proteins, including vitronectin and fibronectin. To determine the role of specific matrix proteins on TGF-β-induced apoptosis, we studied primary AECs cultured on different matrix conditions and utilized mice with deletion of vitronectin (Vtn(-/-)) or mice in which the vitronectin RGD motif is mutated to nonintegrin-binding arginine-glycine-glutamate (RGE) (Vtn(RGE/RGE)). We found that AECs cultured on fibronectin and vitronectin or in wild-type mouse serum are resistant to TGF-β-induced apoptosis. In contrast, AECs cultured on laminin or in serum from Vtn(-/-) or Vtn(RGE/RGE) mice undergo robust TGF-β-induced apoptosis. Plasminogen activator inhibitor-1 (PAI-1) sensitizes AECs to greater apoptosis by disrupting AEC engagement to vitronectin. Inhibition of integrin-associated signaling proteins augments AEC apoptosis. Mice with transgenic deletion of PAI-1 have less apoptosis after bleomycin, but deletion of vitronectin or disruption of the vitronectin RGD motif reverses this protection, suggesting that the proapoptotic function of PAI-1 is mediated through vitronectin inhibition. Collectively, these data suggest that integrin-matrix signaling is an important regulator of TGF-β-mediated AEC apoptosis and that PAI-1 functions as a natural regulator of this interaction.

Efferocytosis of apoptotic alveolar epithelial cells is sufficient to initiate lung fibrosis

“…Uhal and colleagues identified evidence of increased alveolar epithelial cell apoptosis in tissue sections from patients with fibrotic lung disease, implicating cell death as a mechanism to explain the denuded epithelium overlying the fibrotic lesion 24 . AEC apoptosis has also been identified as a prominent feature in the bleomycin animal model of lung fibrosis 9 , 20 , 25 . In our present study, we employed transgenic mice with restricted expression of the diphtheria toxin receptor on their type II AECs.…”

“…This consistent observation of increased AEC apoptosis in human fibrotic lung disease and in distinct animal models certainly implicates this event as a critical element of dsiease pathogenesis. Further evidence substantiating the link between AEC apoptosis and fibrogenesis comes from studies in which fibrosis is ameliorated through interventions that reduce apoptosis of the epithelium 9 , 26 . Despite the strong correlation between AEC apoptosis and fibrosis, the precise mechanism linking these processes has been unclear.…”

“…Surface expression of phosphatydylserine was determined using TACS Annexin V-FITC Apoptosis detection kit (R&D Systems) per manufacturer’s protocol. Terminal deoxynucleotidyl transferase dUTP-mediated nick-end labeling (TUNEL) was performed on lung sections using the in situ death detection kit and TMR red as previously described 9 , 20 .…”

“…Mounting evidence implicates defects in the type II alveolar epithelial cell (AEC) in disease pathogenesis 6 . For example, histopathologic abnormalities of the epithelium including apoptosis are observed in tissue sections from IPF patients and in animal models of pulmonary fibrosis 7 – 9 . Furthermore, mutations in type II AEC genes including surfactant proteins A and C are linked to familial disease 10 .…”

Bacterial Outer Membrane Vesicles Induce Vitronectin Release Into the Bronchoalveolar Space Conferring Protection From Complement-Mediated Killing

“…It protects human cells and tissues from self-damage by inhibiting formation of the membrane attack complex ( Sheehan et al, 1995 ). Other functions of vitronectin are associated with cellular attachment and migration, tissue healing, and regulation of apoptosis ( Preissner, 1991 ; Wheaton et al, 2016 ). Vitronectin is also an effector associated with inflammatory processes, as evidenced by increased levels of the glycoprotein in the bronchial lumen of patients with chronic lung disease ( Eklund et al, 1992 ; Pohl et al, 1993a ; Teschler et al, 1993 ; Carpagnano et al, 2003 ).…”