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Plasmin cleaves betaglycan and releases a 60 kDa transforming growth factor-beta complex from the cell surface
J Lamarre1, J Vasudevan, S L Gonias
1Department of Biomedical Sciences, University of Guelph, Ontario, Canada.
Abstract:
Plasmin regulates the activity and distribution of transforming growth factor beta (TGF-beta) and other growth factors. The purpose of the present investigation was to determine the effects of plasmin on cellular receptors for TGF-beta. AKR-2B fibroblasts were affinity-labelled with 125I-TGF-beta 1 and 125I-TGF-beta 2, demonstrating betaglycan, the type-I TGF-beta receptor and the type-II TGF-beta receptor. Treatment of TGF-beta-affinity-labelled cells with plasmin (10-100 nM) for 1 h profoundly and selectively decreased recovery of TGF-beta-betaglycan complex. The type-I and type-II receptors were not plasmin substrates. A radiolabelled complex with an apparent mass of 60 kDa was detected by SDS/PAGE in both the medium and cell extracts of plasmin-treated affinity-labelled cells. In order to demonstrate that plasmin cleavage of betaglycan did not require prior exposure of the betaglycan to cross-linking agent, AKR-2B cells were treated with plasmin first and then affinity-labelled. Markedly decreased TGF-beta binding to cellular betaglycan was observed. Although plasmin treatment of AKR-2B cells decreased overall binding of 125I-TGF-beta 1 and 125I-TGF-beta 2, the rate at which the cells degraded bound 125I-TGF-beta at 37 degrees C was not changed. AKR-2B cells treated with plasmin demonstrated slightly increased [3H]thymidine incorporation; the plasmin-treated cells retained their ability to respond to TGF-beta. Conditioned medium from plasmin-treated AKR-2B cells contained increased amounts of active TGF-beta as determined in Mv 1 Lu epithelial-cell-proliferation assays. Specific cleavage of betaglycan represents a novel mechanism whereby plasmin may regulate the assortment of receptors available for TGF-beta. In addition, plasmin may facilitate transfer of active TGF-beta between neighbouring cells by releasing the active growth factor from the cell surface.
Insights
Plasmin cleaves betaglycan, a key receptor for transforming growth factor beta (TGF-beta), reducing TGF-beta binding to cells. This mechanism regulates TGF-beta availability and may facilitate its release between cells.
Area of Science:
- Cell biology
- Biochemistry
- Molecular biology
Background:
- Plasmin regulates growth factor activity and distribution.
- Transforming growth factor beta (TGF-beta) signaling is crucial for cellular processes.
- Understanding TGF-beta receptor regulation is vital for cell communication research.
Purpose of the Study:
- To investigate the effects of plasmin on cellular receptors for TGF-beta.
- To determine if plasmin directly interacts with TGF-beta receptors.
- To elucidate the mechanism by which plasmin influences TGF-beta binding and signaling.
Main Methods:
- Affinity labeling of AKR-2B fibroblasts with 125I-TGF-beta 1 and 125I-TGF-beta 2.
- Treatment of cells with varying concentrations of plasmin.
- Analysis of receptor-ligand complexes using SDS/PAGE.
- Assessment of TGF-beta binding and degradation rates.
- Cell proliferation assays to measure TGF-beta response.
Main Results:
- Plasmin selectively decreased the recovery of the TGF-beta-betaglycan complex.
- Type-I and Type-II TGF-beta receptors were not degraded by plasmin.
- Plasmin treatment reduced TGF-beta binding to betaglycan.
- Cellular degradation rates of TGF-beta remained unchanged.
- Plasmin-treated cells showed increased [3H]thymidine incorporation and retained TGF-beta responsiveness.
- Conditioned medium from plasmin-treated cells contained higher amounts of active TGF-beta.
Conclusions:
- Plasmin specifically cleaves betaglycan, a novel mechanism for regulating TGF-beta receptor availability.
- Plasmin may facilitate the transfer of active TGF-beta between cells by releasing it from the cell surface.
- This study reveals a new pathway for plasmin-mediated regulation of growth factor signaling.