Induction of tissue transglutaminase in the trabecular meshwork by TGF-beta1 and TGF-beta2

To study whether human trabecular meshwork (HTM) cells are capable of expressing and secreting tissue transglutaminase (tTgase), an enzyme cross-linking extracellular matrix (ECM) proteins, and whether tTgase and synthesis of cross-linked fibronectin are increased after treatment of HTM cells with transforming growth factor (TGF)-beta1 or -beta2.

Anterior segments of six normal human eyes were stained with antibodies to tTgase. Tissues from three eyes were analyzed for tTgase using Western blot analysis. Monolayer cultures of HTM cells from eyes of five human donors were treated with 1.0 ng/ml TGF-beta1, -beta2, or 5 X 10(-7) M dexamethasone (DEX) for 12 to 96 hours. Induction of tTgase was investigated by Western and Northern blot analysis. External tTgase activity was measured by the ability to form polymerized fibronectin and the incorporation of biotinylated cadaverine into fibronectin.

Labeling for tTgase was observed throughout the entire HTM. Cultured HTM cells expressed tTgase intra- and extracellularly. Treatment of cultured HTM cells with TGF-beta1 and -beta2 increased the tTgase mRNA and protein levels, whereas DEX had no effect. TGF-beta-treated HTM cells showed a significant increase in polymerized and unpolymerized fibronectin. Incorporation of biotinylated cadaverine was markedly increased when HTM cells were treated with TGF-beta for 24 hours before seeding.

The enzyme tTgase is expressed in the HTM and is inducible by TGF-beta1 or -beta2 in cultured HTM cells. Extracellular tTgase is able to polymerize fibronectin. Increased levels of TGF-beta2 in the aqueous humor may lead to an increase of tTgase expression and activity in the HTM, causing an increase of irreversibly cross-linked ECM proteins. This mechanism might play a role for the increased outflow resistance seen in glaucomatous eyes.