Idiopathic pulmonary fibrosis (IPF) is a severe epithelial-fibroblastic disease with poor prognosis, characterised by excessive deposition of collagen in the pulmonary interstitium. Several matrix metalloproteinases (MMPs) and the tissue inhibitors of metalloproteinases (TIMPs) have been demonstrated to be strongly up-regulated in human and experimental lung fibrosis, thus underscoring their dynamic regulation of extracellular matrix and of remodeling processes in the lung. To investigate the spatial regulation, nature of MMP activity and the contribution of TIMPs in the local inhibition of MMPs in IPF, we examined IPF in comparison to donor lung tissue for regulation of MMPs and TIMPs, and the relative contribution of single MMPs to the fibrotic lung repair process.
We identified MMP-7 and the collagenases, MMP-1 and -13, as key proteases upregulated in human IPF. A prominent and novel finding of our work is the upregulation of pro- and active MMP-13 protein in IPF patients. MMP-13 protein was localized in alveolar and bronchiolar epithelial cells, the alveolar septae and extracellular matrix of remodelled lung tissue. TIMP proteins were not differentially regulated; an observation that somehow contrasted with the increased hydroxyproline content, especially in the subpleural areas of IPF. We have shown by a combination of immunohistochemistry and in situ zymography that collagenolytic and gelatinolytic activities did not exclusively co-localize with a single MMP antigen signal in the lung tissues, suggesting predominant MMP activity within the airways and weaker activity in the scar regions.
Since mice lack the orthologue of human MMP-1, we investigated further the relevance of MMP-13 in the context of fibrotic lung diseases by examining the fibrogenic response to bleomycin in MMP-13-/- mice and wt littermates. In response to bleomycin challenge and likewise to their littermate wt controls, MMP-13-/- mice were characterized by elevated total cell counts in BALF caused largely by an influx of granulocytes and lymphocytes. Seven and fourteen days after bleomycin treatment, both wt and MMP-13-/- mice developed patchy areas of inflammation throughout the lung parenchyma, with MMP-13-/- mice again displaying a more severe inflammatory response to bleomycin that persisted for a longer time period. Likewise, the extent of fibrosis was more prominent in MMP-13-/- versus wt mice, with increased levels of hydroxyproline and more significant histologic signs of fibrosis. It is currently unclear if this augmented fibrotic response was caused by the more extensive inflammation or the absence of MMP-13, especially when considering that MMP-7 was found to be similarly upregulated in both MMP-13-/- and wt mice and the TIMPs were not dramatically altered in MMP-13-/- versus wt mice.
In conclusion, in an attempt to define the spatial regulation and the nature of MMP activity in IPF, the contribution of TIMPs in the local inhibition of MMPs and the relative contribution of single MMPs in the process of fibrotic repair in IPF, we could show that (a) TIMPs are not excessively upregulated as compared to MMPs in IPF lungs; (b) the collagenolytic activity of IPF tissue homogenates is higher as compared to donor lungs, thus the increased collagen deposition seems to be largely due to excessive matrix synthesis and deposition rather than blockade of MMPs, (c) collagenolytic and gelatinolytic activities do not colocalize completely with single MMP antigen signals in the lung tissues, suggesting the predominance of MMP activity within the airways and weaker activity in the scar regions (possibly providing one explanation for the process of honeycombing) and (d) MMP-13 seems to play a significant role in IPF, as this collagenase is dramatically upregulated in tissues from IPF patients and as excessive inflammation and fibrosis are encountered in bleomycin challenged MMP-13-/- mice versus control mice lungs.
Our data therefore suggest that MMP-13 plays an important, (if not pivotal) role in the remodeling process in IPF, both in view of matrix deposition in the scarring areas, as well as in view of the development of honeycomb cysts, similar to the induction of emphysema formation in MMP-1 overexpressing mice. However, the mechanism of action of MMP-13 together with related mediators remains a subject for further investigation.
Verknüpfung zu Publikationen oder weiteren Datensätzen
