Induction of tissue transglutaminase in the trabecular meshwork by TGF-beta 1 and TGF-beta 2

PURPOSE. TO study whether human trabecular meshwork (HTM) cells are capable of expressing and secreting tissue transglutaminase (tTgase), an enzyme cr oss-linking extracellular matrix (ECM) proteins, and whether tTgase and syn thesis of cross-linked fibronectin are increased after treatment of HTM cel ls with transforming growth factor (TGF)-beta 1 or -beta 2. METHODS. Anterior segments of six normal human eyes were stained with antib odies to tTgase. Tissues from three eyes were analyzed for tTgase using Wes tern blot analysis. Monolayer cultures of HTM cells from eyes of five human donors were treated with 1.0 ng/ml TGF-beta 1, -beta 2, or 5 x 10(-7) M de xamethasone (DEX) for 12 to 96 hours. Induction of tTgase was investigated by Western and Northern blot analysis. External tTgase activity was measure d by the ability to form polymerized fibronectin and the incorporation of b iotinylated cadaverine into fibronectin. RESULTS. Labeling for tTgase was observed throughout the entire HTM. Cultur ed HTM cells expressed tTgase intra- and extracellularly. Treatment of cult ured HTM cells with TGF-beta 1 and -beta 2 increased the tTgase mRNA and pr otein levels, whereas DEX had no effect. TGF-beta-treated HTM cells showed a significant increase in polymerized and unpolymerized fibronectin. Incorp oration of biotinylated cadaverine was markedly increased when HTM cells we re treated with TGF-beta for 24 hours before seeding. CONCLUSIONS. The enzyme tTgase is expressed in the HTM and is inducible by TGF-beta 1 or -beta 2 in cultured HTM cells. Extracellular tTgase is able t o polymerize fibronectin. Increased levels of TGF-beta 2 in the aqueous hum or may lead to an increase of tTgase expression and activity in the HTM, ca using an increase of irreversibly cross-linked ECM proteins. This mechanism might play a role for the increased outflow resistance seen in glaucomatou s eyes.