PLASMIN CLEAVES BETAGLYCAN AND RELEASES A 60-KDA TRANSFORMING GROWTH-FACTOR-BETA COMPLEX FROM THE CELL-SURFACE

Plasmin regulates the activity and distribution of transforming growth factor beta (TGF-beta) and other growth factors. The purpose of the p resent investigation was to determine the effects of plasmin on cellul ar receptors for TGF-beta. AKR-2B fibroblasts were affinity-labelled w ith I-125-TGF-beta 1 and I-125-TGF-beta 2, demonstrating betaglycan, t he type-I TGF-beta receptor and the type-II TGF-beta receptor. Treatme nt 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 a ffinity-labelled cells. In order to demonstrate that plasmin cleavage of betaglycan did not require prior exposure of the betaglycan to cros s-linking agent, AKR-2B cells were treated with plasmin first and then affinity-labelled. Markedly decreased TGF-beta binding to cellular be taglycan was observed. Although plasmin treatment of AKR-2B cells decr eased overall binding of I-125-TGF-beta 1 and I-125-TGF-beta 2, the ra te at which the cells degraded bound I-125-TGF-beta at 37 degrees C wa s not changed. AKR-2B cells treated with plasmin demonstrated slightly increased [H-3]thymidine incorporation; the plasmin-treated cells ret ained their ability to respond to TGF-beta. Conditioned medium from pl asmin-treated AKR-2B cells contained increased amounts of active TGF-b eta as determined in Mv 1 Lu epithelial-cell-proliferation assays. Spe cific cleavage of betaglycan represents a novel mechanism whereby plas min may regulate the assortment of receptors available for TGF-beta. I n addition, plasmin may facilitate transfer of active TGF-beta between neighbouring cells by releasing the active growth factor from the cel l surface.