Idiopathic pulmonary fibrosis (IPF) is a progressive disease of unknown cause. The pathogenesis of the disease is characterized by fibroblast accumulation and excessive transforming growth factor-β (TGF-β) activation. Although TGF-β activation is a complex process involving various protein interactions, little is known of the specific routes of TGF-β storage and activation in human lung. Here, we have systematically analyzed the expression of specific proteins involved in extracellular matrix targeting and activation of TGF-β. Latent TGF-β-binding protein (LTBP)-1 was found to be significantly upregulated in IPF patient lungs. LTBP-1 expression was especially high in the fibroblastic foci, in which P-Smad2 immunoreactivity, indicative of TGF-β signaling activity, was less prominent. In cultured primary lung fibroblasts and epithelial cells, short-interfering-RNA-mediated downregulation of LTBP-1 resulted in either increased or decreased TGF-β signaling activity, respectively, suggesting that LTBP-1-mediated TGF-β activation is dependent on the cellular context in the lung. Furthermore, LTBP-1 was shown to colocalize with fibronectin, fibrillin-1 and fibrillin-2 proteins in the IPF lung. Fibrillin-2, a developmental gene expressed only in blood vessels in normal adult lung, was found specifically upregulated in IPF fibroblastic foci. The TGF-β-activating integrin β8 subunit was expressed at low levels in both control and IPF lungs. Alterations in extracellular matrix composition, such as high levels of the TGF-β storage protein LTBP-1 and the re-appearance of fibrillin-2, probably modulate TGF-β availability and activation in different pulmonary compartments in the fibrotic lung.
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We thank Eva Sutinen, Anne Remes, Sami Starast and Tiina Marjomaa for excellent technical assistance.
This work was supported by the Academy of Finland, the Finnish Cancer Foundations, the Sigrid Jusélius Foundation, the Yrjö Jahnsson Foundation, the Magnus Ehrnrooth Foundation, the Ida Montin Foundation, the Paulo Foundation, the Jalmari and Rauha Ahokas Foundation, the Finnish Anti-Tuberculosis Association Foundation, Biocentrum Helsinki, the Pulmonary Association Finland (Heli), a special governmental subsidy for health and sciences research, the Finnish Cultural Foundation and the University of Helsinki. The sponsors had no role in study design, data collection, analysis, interpretation, writing the report, or the decision to submit the paper for publication.
O.L. carried out the experiments, analysed the data and generated the figures. M.M. and K.K. conceived and carried out experiments and analysed and interpreted the data. C.S. carried out experiments and analysed data. K.S. collected the samples and took part in data analysis and interpretation. V.K. and J.K.-O. interpreted data. O.L., M.M. and K.K. drafted the paper. All authors reviewed the paper with critical comments and gave final approval to the submitted version.
The authors have no conflicts of interest to report.
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Leppäranta, O., Sens, C., Salmenkivi, K. et al. Regulation of TGF-β storage and activation in the human idiopathic pulmonary fibrosis lung. Cell Tissue Res 348, 491–503 (2012). https://doi.org/10.1007/s00441-012-1385-9
- Idiopathic pulmonary fibrosis
- Transforming growth factor-beta (TGF-β)
- Latent TGF-β-binding protein (LTBP)
- Lung fibroblast
- Extracellular matrix