Idiopathic pulmonary fibrosis (IPF) is a progressive and fatal lung disease of unknown origin, characterised by alveolar epithelial cell damage, increased deposition of extracellular matrix (ECM) in the lung interstitium, enhanced fibroblast/myofibroblast proliferation and activation, which ultimately leads to the distortion of normal lung architecture and loss of respiratory function. The interstitial fibroblast/myofibroblast represents the key effector cell responsible for the increased ECM deposition characteristic of IPF. Fibroblasts secrete large amounts of fibrillar collagens, which are the key ECM proteins, which exhibit elevated expression in this disease. The TGF-beta is the primary and most potent profibrotic mediator involved in fibroblast activation and differentiation, and subsequent collagen production and deposition. Thus, it was hypothesised that the expression of TGF-beta system components is altered in IPF, ultimately affecting the fibroblast activation and collagen synthesis.In this study, the expression levels of ALK1, ALK5, TGF-betaRII and endoglin, as well as Smads and TGF-beta target genes, were analysed in the context of human pulmonary fibrosis. The expression of ALK1 was significantly downregulated in human lung homogenates from fibrotic lungs when compared to those from healthy subjects. Expression of other TGF-beta system components was not altered in the disease. Furthermore, ALK1 and ALK5 mRNA and protein expression was localised to epithelial cells, endothelial cells, smooth muscle cells and fibroblasts, and the expression of ALK1 and ALK5 was decreased in primary fibroblasts isolated from human fibrotic lung tissue, compared to healthy controls, as assessed by quantitative RT-PCR and immunohistochemistry. The human fibroblast cell lines HFL1 and IMR-90 were selected for functional assays because these cell lines express TGF-beta system components, and demonstrate active TGF-beta and BMP signalling characterised by the phosphorylation of Smad2/3 and Smad1/5/8, respectively. Finally, treatment of human lung fibroblast cell lines with the siRNA specific for ALK1 attenuated collagen deposition, which was rescued by TGF-beta1 stimulation. However, the impact of ALK1 on fibroblast activation and collagen deposition may not be primary, as the other signalling pathways might be involved.These results demonstrated that ALK1 was expressed and functional in lung fibroblasts. The lack of ALK1 might be involved in the activation of fibroblasts thus leading to the collagen production, therefore being involved in the pathogenesis of pulmonary fibrosis.
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