These data are the first to provide direct experimental evidence that ECM is dynamically regulated in the repair response to selective epithelial cell injury and that epithelial repair defects are sufficient to drive excessive matrix deposition typically observed in chronic fibroproliferative lung diseases. Increased deposition ofTnchas recently been shown to be a marker of chronic fibroproliferative lung disease, such as obliterative bronchiolitis (OB) (30). after injury and was remodeled to basement membrane subtending the bronchiolar epithelium during epithelial repair. Epithelial restitution was accompanied by a decrease inTncmRNA and protein expression to steady-state levels. In contrast, abortive repair using a transgenic model allowing ablation of all reparative cells led to a progressive increase inTncmRNA within lung tissue and accumulation of its gene product within the subepithelial mesenchyme of both conducting airways and alveoli. These data demonstrate that this ECM is usually dynamically remodeled in response to selective epithelial cell injury and that this process is usually activated without resolution in the setting of defective airway epithelial repair. Keywords:airway epithelium, repair, Clara cell, extracellular matrix, fibrosis == CLINICAL RELEVANCE == This study demonstratesin vivothat airway epithelial reparative capacity regulates extracellular matrix (ECM) deposition and turnover. As such, this study suggests that excessive ECM deposition in chronic fibroproliferative lung disease may be a result of defective epithelial repair. The epithelialmesenchymal trophic unit (EMTU) is usually a complex arrangement of epithelial cells, fibroblasts, easy muscle, and extracellular matrix (1). Dynamic Rabbit Polyclonal to PHLDA3 interactions between these components are critical for the process of lung development and continue into adulthood for regulation of normal tissue homeostasis and repair (2,3). Chronic lung injury leads to permanent alterations to both cellular and extracellular components of the EMTU that lead to permanent declines in lung function. This is clearly seen in chronic lung diseases such as asthma and chronic obstructive pulmonary disease. Pathological changes to lung tissue that are associated with these diseases include fibroproliferation, accumulation and remodeling of extracellular matrix components, and altered epithelial cell function, which together lead to airway obstruction (46). However, the complexity of these interactions in the intact lung, and the inability to effectively model these interactionsin vitro, have led to difficulties in identifying critical factors that initiate the process of pathological tissue remodeling. Roles for epithelial cell dysfunction in the initiation and/or progression of this process have been proposed, although not testedin vivo(1,7). The airway epithelium provides both a physical and biological barrier that functions to protect the host against invading microorganisms and inhaled pollutants (812). The epithelium of bronchiolar airways turns over with very slow kinetics and is maintained in the steady-state by the infrequent proliferation of an abundant facultative progenitor cell that is commonly referred to as the Clara cell (1316; A. Giangreco and coworkers, unpublished data). Injury resulting in the depletion of Clara cells, such as that resulting from exposure to the aromatic hydrocarbon naphthalene, is usually repaired through the activation of local tissue stem cells residing at airway branch point associated neuroepithelial bodies (NEBs) and the bronchioalveolar duct junction (BADJ) (14,1720). Further studies have revealed that stem cells localized at the BADJ, termed bronchioalveolar stem cells (BASCs), can be identifiedin vivoby co-expression of Clara cell secretory protein (CCSP) and sufactant protein C (SPC), isolated according to cell surface phenotype, and may have thein vitrocapacity to generate cells of both bronchiolar and alveolar lineages (20). Further research is necessary to understand the microenvironmental cues that regulate stem NMS-P515 cellmediated epithelial repair. Roles for matrix remodeling in regulation of airway repair are suggested from studies investigating mouse models carrying null alleles for selected matrix metalloproteinases (MMPs). MMPs are a family of membrane-associated or secreted proteases that, upon activation, have the ability to degrade components of the ECM, epithelial cell junctions, and liberate tethered growth and chemotactic factors in response to wounding (21). In recent work, MMPs have been shown to play key roles in regulating epithelial repair in the NMS-P515 lung (22,23). Chen and coworkers exhibited that tissue inhibitor of metalloproteinase (TIMP)1 functions to inhibit airway epithelial repair, raising the possibility that increased levels of TIMP1 that are observed in diseases such as obliterative bronchiolitis may both attenuate epithelial reparative capacity and promote fibrosis (23). These studies reinforce the concept that epithelial defects within the EMTU may initiate uncontrolled fibroproliferation and ECM deposition (1).In vitrostudies have shown that inhibition of epithelial repair can augment pro-fibrotic signaling pathways, thereby supporting this model (24). However, this paradigm has yet to be tested directly. In this study we test the hypothesis that epithelial reparative capacity is usually a key regulator of extracellular matrix remodeling. We used previously validatedin vivomodels of Clara cell ablation to demonstrate that ECM deposition, as measured by Tenascin C (Tnc) mRNA NMS-P515 and protein expression, is usually dynamically and reversibly regulated during repair of epithelial lesions in airways. In contrast, we show that loss of epithelial reparative capacity results in continued deposition of ECM without resolution. This study provides the firstin vivoevidence that ECM deposition is usually a dynamic component of the EMTU during productive airway epithelial repair and suggests that defects to epithelial.