Proteoglycan expression during transforming growth factor beta-induced keratocyte-myofibroblast transdifferentiation

Keratocytes of the corneal stroma secrete a unique population of proteoglyc an molecules considered essential for corneal transparency. In healing corn eal wounds, keratocytes exhibit a myofibroblastic phenotype in response to transforming growth factor beta (TGF-beta), characterized by expression of a-smooth muscle actin. This study examined proteoglycan and collagen expres sion by keratocytes in vitro during the TGF-beta induced keratocyte-myofibr oblast transition. TGF-beta treated primary bovine keratocytes developed my ofibroblastic features, including actin stress fibers anchored to paxillin- containing focal adhesions, cell-associated fibronectin, a, integrin, and a -smooth muscle actin. Collagen I and III protein and mRNA increased in resp onse to TGF-beta. Secretion of [S-35]sulfate-labeled keratan sulfate proteo glycans decreased markedly in response to TGF-beta. Dermatan sulfate proteo glycans, however, increased in size and abundance. Protein and mRNA transcr ipts for normal stromal proteoglycans (lumican, keratocan, mimecan, and dec orin) all decreased in response to TGF-beta, but protein expression and mRN A for biglycan, a proteoglycan present in fibrotic tissue, was markedly up- regulated. These results show that TGF-beta in vitro induces a proteoglycan expression pattern similar to that of corneal scars in vivo. This altered proteoglycan expression occurred coordinately with transdifferentiation. of keratocytes to the myofibroblastic phenotype, implicating these cells as t he source of fibrotic tissue in nontransparent corneal scars.