Plasmin and the regulation of TGF-β1 bioavailability in asthma

Abstract

Rationale TGF-β exists in an inactive complex form bound to tissue proteoglycans, such as decorin, and requires release to exert its biological effects. Plasmin, a product of plasminogen activation, has been implicated in activating latent TGF-β and will be generated in association with tissue inflammation. We investigated the effects of plasmin on cytokine and TGF-β1 release and gene expression in primary fibroblast cultures from asthmatics and healthy controls. Methods Fibroblasts were grown from endobronchial biopsies of volunteers and used at passages 3-5. They were serum deprived for 24 hours, then challenged with plasmin for a further 24 hours. Cell supernatants were assayed for growth factors and cytokines by ELISA. Fibroblast mRNA was reverse transcribed and subjected to real-time PCR using Taqman probes. Results Plasmin induced a concentration-dependent (P<0.001 by ANOVA) increase in TGF-β1 levels in fibroblast supernatants, causing a 3-fold up-regulation at 3 μg/ml plasmin (P=0.001). However, the enzyme did not increase IL-6, IL-8 or RANTES measurements. Analysis of mRNA revealed no change in TGF-β1 gene expression, suggesting that the TGF-β1 measured by ELISA may have been liberated from the cell surface. However, a mean 8-fold increase in CTGF mRNA was noted, indicative of enhanced TGF-β bioactivity. Conclusions Plasmin promoted the liberation of TGF-β1, a cytokine associated with remodeling, but did not induce pro-inflammatory cytokines. The plasmin-induced release of immunomodulatory TGF-β1 may serve to regulate the inflammatory response and its effects on structural cells will promote tissue remodeling.