Regulation of human lung fibroblast phenotype and function by vitronectin and vitronectin integrins

Myofibroblasts, characterised by high expression of alpha -smooth muscle ac tin (alpha -SMA), are important and transient cells in normal wound healing but are found in increased number in various pathological conditions of th e lung including asthma and pulmonary fibrosis. The mechanisms that regulat e the myofibroblast phenotype are unknown but are likely to involve signals from the extracellular matrix transmitted via specific integrins. Vitronec tin is a glycoprotein released during inflammation and has been shown to re gulate the phenotype of vascular smooth muscle cells via alphav and betaI i ntegrins. In the current study we have examined whether vitronectin influen ces the phenotype and function of normal human lung fibroblasts (HFL-1). In cubation of HFL-1 cells with vitronectin induced a concentration-dependent reduction in alpha -SMA expression. By contrast, function-blocking monoclon al antibodies to the vitronectin integrins alphav, alpha1, alphav beta3 and alphav beta5 induced the expression of alpha -SMA and its organization int o stress fibers. Expression of alpha -SMA induced by all function-blocking monoclonal antibodies was abrogated by inhibition of protein kinase C and p hosphatidylinositol-3 kinase, but the effects of inhibition of other signal ling pathways was integrin dependent. Exposure to other extracellular matri x proteins such as fibronectin, collagen or their integrins did not influen ce expression of alpha -SMA. The expression and organization of alpha -SMA induced by exposure to function-blocking antibodies was translated into an augmented capacity of HFL-1 cells to contract fibroblast populated collagen gels. By contrast, contraction of collagen gels following incubation with vitronectin was not significantly different to control. This study has show n that vitronectin influences the phenotype and behaviour of HFL-1 cells by downregulating the expression of alpha -SMA and reducing their contractile ability. By contrast, occupancy of specific integrins by function-blocking antibodies upregulated the expression of alpha -SMA and induced the format ion of functional stress fibers capable of contracting collagen gels. These results suggest that vitronectin modulates the fibroblast-myofibroblast ph enotype, implying an important role in the remodelling process during lung development or response to injury.