Tissue transglutaminase as a modifying enzyme of the extracellular matrix in PVR membranes

PURPOSE. Proliferative vitreoretinopathy (PVR) is characterized by the development of epi- and subretinal fibrocellular membranes containing modified retinal pigment epithelial (RPE) cells among others. In the present study, the role of transglutaminases in accumulation of extracellular matrix (ECM) proteins in these membranes was investigated. Transglutaminases are enzymes capable of cross-linking ECM proteins to proteolysis-resistant complexes. METHODS. PVR membranes were incubated with dansyl-cadaverine to demonstrate active transglutaminase. Localization of tissue transglutaminase (tTgase), its reaction product epsilon-(gamma-glutamyl)-lysine, and fibronectin was investigated immunohistochemically. Colocalization was studied with a confocal laser scanning microscope. PVR membranes were also analyzed by RT-PCR for the presence of tTgase mRNA. In vitro, RPE cells were treated with transforming growth factor-beta2 (TGF-beta2), basic fibroblast growth factor, interleukin-6, and interleukin-1beta. Their,effect was studied using immunohistochemistry and Northern and Western blot analyses. RESULTS. Transglutaminase activity and expression of tTgase were present in all PVR membranes. Staining was most prominent at the rim of the membranes. The enzyme was colocalized with epsilon-(gamma-glutamyl)-lysine and fibronectin. No staining differences were found between epi- and subretinal membranes. Although native RPE cells contained only a basal level of tTgase mRNA, the expression and activity of tTgase was increased under culture conditions and further stimulated by TGF-beta2 treatment. CONCLUSIONS. The findings demonstrate that in PVR membranes tTgase is present and functionally active. The amount and activity of this enzyme appears to be related to the differentiation state of the RPE cells and their stimulation by TGF-beta2, a growth factor known to be increased in the vitreous of PVR. Intervention at this pathway may open a new approach for PVR prevention and therapy.