Plasmin Triggers a Switch-Like Decrease in Thrombospondin-Dependent Activation of TGF-β1

Transforming growth factor-beta 1 (TGF-beta 1) is a potent regulator of extracellular matrix production, wound healing, differentiation, and immune response, and is implicated in the progression of fibrotic diseases and cancer. Extracellular activation of TGF-beta 1 from its latent form provides spatiotemporal control over TGF-beta 1 signaling, but the current understanding of TGF-beta 1 activation does not emphasize cross talk between activators. Plasmin (PLS) and thrombospondin-1 (TSP1) have been studied individually as activators of TGF-beta 1, and in this work we used a systems-level approach with mathematical modeling and in vitro experiments to study the interplay between PLS and TSP1 in TGF-beta 1 activation. Simulations and steady-state analysis predicted a switch-like bistable transition between two levels of active TGF-beta 1, with an inverse correlation between PLS and TSP1. In particular, the model predicted that increasing PLS breaks a TSP1-TGF-beta 1 positive feedback loop and causes an unexpected net decrease in TGF-beta 1 activation. To test these predictions in vitro, we treated rat hepatocytes and hepatic stellate cells with PLS, which caused proteolytic cleavage of TSP1 and decreased activation of TGF-beta 1. The TGF-beta 1 activation levels showed a cooperative dose response, and a test of hysteresis in the cocultured cells validated that TGF-beta 1 activation is bistable. We conclude that switch-like behavior arises from natural competition between two distinct modes of TGF-beta 1 activation: a TSP1-mediated mode of high activation and a PLS-mediated mode of low activation. This switch suggests an explanation for the unexpected effects of the plasminogen activation system on TGF-beta 1 in fibrotic diseases in vivo, as well as novel prognostic and therapeutic approaches for diseases with TGF-beta dysregulation.