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A.M. Bernstein, S.S. Twining, K.A. Vaughan, S.K. Masur; Urokinase Pathway Regulation in Corneal Fibroblasts and Myofibroblasts . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2304.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Upon corneal wounding, the normally quiescent cells in the stroma differentiate into fibroblasts, which secrete proteases that degrade and remodel the extracellular matrix as they migrate. The urokinase–type plasminogen activator (uPA) is an extracellular serine protease expressed in the stroma of wounded corneas. When uPA's precursor, pro–uPA, binds to its cell–surface receptor, uPAR, it is cleaved into active uPA which in turn cleaves plasminogen into the broad–spectrum protease, plasmin. Plasmin remodels extracellular matrix, activating latent growth factors and stimulating cell migration. Using an in vitro wounding model that reproduces the transition from keratocyte to fibroblast to myofibroblast, we have investigated differences in the uPA/uPAR pathway that correspond with these stromal phenotypes. Methods: Human corneal fibroblasts were seeded onto collagen and cultured for three days in DMEM/F12 with supplements plus FGF–2/heparin for fibroblasts or TGFß1 for myofibroblasts. uPAR cleavage and PAI–1 expression were detected by western blot. uPA activity was evaluated by zymography and a colorimetric activity assay. Results: We previously demonstrated that uPA/uPAR expression is upregulated in fibroblasts and that the binding of uPA to uPAR promotes uPAR's association with the actin cytoskeleton through the integrin, αvß3 (IOVS, 45:2967, 2004). We now report that FGF–2/heparin stimulated an increase in cell–associated uPA activity and a decrease in secreted pro–uPA. In contrast, myofibroblasts significantly down–regulated the uPA/uPAR pathway by 1) promoting the cleavage of uPAR's domain 1 into a form that can no longer bind to pro–uPA, 2) increased pro–uPA secretion and decreased cell–associated uPA activity (possibly because cleaved uPAR can not bind uPA) and 3) increased secretion of PAI–1 (uPA/uPAR inhibitor). The TGFß1 induced uPAR cleavage was blocked by 1% FBS or FGF–2/heparin treatment in serum–free media but not by a serine protease inhibitor (aprotinin), or metalloprotease inhibitor (GM6001).Conclusions:Together our data support a model in which fibroblasts are able to migrate into a wound using the uPA/uPAR/αvß3/vitronectin pathway. Myofibroblast differentiation correlates with inhibition of the uPA/uPAR pathway through cleavage of uPAR into a non–uPA binding form. PAI–1, by promoting the endocytosis of uPA/uPAR, further down–regulates uPA/uPAR activity. By this mechanism, myofibroblasts may become non–motile and fulfill their role in promoting wound closure.
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