Contraction-Mediated Release of TGF-β1

Abstract

Wound healing requires the contraction of the tissue, which in turn involves the conversion of fibroblasts into myofibroblasts, which exhibit high contractility generated by α smooth muscle actin stress fibers. In fibrotic diseases, myofibroblasts contribute to tissue fibrosis. Transforming growth factor-β1 (TGF-β1) and mechanical stress are both required for myofibroblast formation. Wipff et al . show that myofibroblasts also contribute to the activation of latent TGF-β1, which is secreted and sequestered in the extracellular matrix (ECM) bound to latency associated protein (LAP) and latent TGF-β binding protein 1 (LTBP-1), through a contraction-mediated mechanism. The authors developed a culture system consisting of fibroblasts or myofibroblasts, transformed mink lung reporter cells (TMLCs) that produce luciferase in the presence of active TGF-β1, and extracted ECM containing latent TGF-β1. They used this system to show that myofibroblasts stimulated the release of active TGF-β1 to a greater extent than did other types of fibroblasts. Furthermore, agonists that triggered myofibroblast contraction promoted TGF-β1 release, whereas drugs that inhibited contraction prevented contraction-stimulating agonists from stimulating TGF-β1 release without affecting basal TGF-β1 release. Proteolysis is one known mechanism for releasing active TGF-β1 from the latent complex bound to the ECM; however, protease inhibitors failed to block TGF-β1 release triggered by myofibroblast contraction. TGF-β1 release triggered by myofibroblast contraction was blocked by interfering with the interaction between integrins and LAP or by function-blocking antibodies to αvβ5 integrin. By culturing the myofibroblasts on silicone membranes and then subjecting the cells to stretch, the authors showed that stretch stimulated TGF-β1 release and that this required an intact cytoskeleton. Substrate stiffness was also important for contraction-mediated release of TGF-β1. Monitoring the movement of LTBP-1 in response to myofibroblast contraction showed that LTBP-1 was displaced toward the center of the myofibroblast when present in soft substrates. In stiffer substrates, LTBP-1 was not displaced toward the center and, instead, sometimes the LTBP-1-labeled fibers were straighter after myofibroblast contraction. The authors propose a model whereby integrins on the myofibroblast grab LAP and then when the myofibroblast contracts, this pulls against the ECM and releases TGF-β1. Thus, the stiffness of the ECM and ECM remodeling at the wound site may contribute to TGF-β1 release. P.-J. Wipff, D. B. Rifkin, J.-J. Meister, B. Hinz, Myofibroblast contraction activates latent TGF-β1 from the extracellular matrix. J. Cell Biol. 179 , 1311-1323 (2007). [Abstract] [Full Text]