Transforming Growth Factor β1 Induces αvβ3 Integrin Expression in Human Lung Fibroblasts via a β3 Integrin-, c-Src-, and p38 MAPK-dependent Pathway

Abstract

In response to transforming growth factor ␤1 (TGF␤) stimulation, fibroblasts modify their integrin repertoire and adhesive capabilities to certain extracellular matrix proteins. Although TGF␤ has been shown to increase the expression of specific ␣v integrins, the mechanisms underlying this are unknown. In this study we demonstrate that TGF␤1 increased both ␤3 integrin subunit mRNA and protein levels as well as surface expression of ␣v␤3 in human lung fibroblasts. TGF␤1-induced ␣v␤3 expression was strongly adhesion-dependent and associated with increased focal adhesion kinase and c-Src kinase phosphorylation. Inhibition of ␤3 integrin activation by the Arg-Gly-Asp tripeptide motif-specific disintegrin echistatin or ␣v␤3 blocking antibody prevented the increase in ␤3 but not ␤5 integrin expression. In addition, echistatin inhibited TGF␤1-induced p38 MAPK but not Smad3 activation. Furthermore, inhibition of the Src family kinases, but not focal adhesion kinase, completely abrogated TGF␤1-induced expression of ␣v␤3 and p38 MAPK phosphorylation but not ␤5 integrin expression and Smad3 activation. The TGF␤1-induced ␣v␤3 expression was blocked by pharmacologic and genetic inhibition of p38 MAPKbut not Smad2/3-, Sp1-, ERK-, phosphatidylinositol 3-kinase, and NF-B-dependent pathways. Our results demonstrate that TGF␤1 induces ␣v␤3 integrin expression via a ␤3 integrin-, c-Src-, and p38 MAPK-dependent pathway. These data identify a novel mechanism for TGF␤1 signaling in human lung fibroblasts by which they may contribute to normal and pathological wound healing.One of the key events in wound repair is the infiltration of fibroblasts from surrounding tissue to the extracellular matrix (ECM) 2 in which they proliferate and differentiate into myofibroblasts. Under normal conditions myofibroblasts play a crucial role in ECM deposition and subsequent wound contraction and then disappear as the fibrotic response diminishes and normal structure and function are achieved (1). However, their retention, uncontrolled proliferation, and excessive synthesis of ECM proteins represents a pathologic process that ultimately results in fibrosis (2). Both fibroblast proliferation and differentiation, as well as ECM protein synthesis, are profoundly influenced by growth factors such as TGF␤ as well as cell adhesion (3-6). Adhesion of cells to ECM is mediated by a family of transmembrane proteins known as integrins that are expressed on the cell surface as ␣/␤ heterodimers (7, 8). Importantly, integrins not only support cell attachment but also act in concert with receptors for several growth factors, including TGF␤, to regulate survival, migration, proliferation, and differentiation of fibroblastic, epithelial, and endothelial cells (reviewed in Refs. 7,8). Over the past few years a close relationship between ␣v integrins (recognizing RGD motif) and TGF␤ signaling pathways has been identified (9). These include activation of latent TGF␤ complexes by ␣v␤6 and ␣v␤8 integrins in airway epithelium (8, 10), augmented TGF␤ signaling by ␣v␤3 and ␣v...

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