TRANSGLUTAMINASE ACTIVITY IN AGING ARTICULAR CHONDROCYTES AND ARTICULAR-CARTILAGE VESICLES

Objective. Transglutaminases (TGases) (E.C. 2.3.2.13) catalyze a postt ranslational modification of proteins and are associated with biominer alization in growth plate cartilage. Type II TGase participates in the activation of latent transforming growth factor beta (TGF beta), a cr ucial factor for both normal cartilage mineralization and the patholog ic mineralization that results in calcium pyrophosphate dihydrate (CPP D) crystal formation in aging articular cartilage. To explore a possib le association between TGase levels and CPPD crystal formation in matu re articular cartilage, TGase activity in articular chondrocytes from old and young pigs and in the articular cartilage vesicle (ACV) fracti on of porcine articular cartilage was examined, In addition, the effec ts of TGase inhibitors on the production of inorganic pyrophosphate (P Pi), a process necessary for CPPD crystallogenesis, were determined. M ethods. TGase activity was measured with a radiometric assay in cultur ed articular chondrocytes from the knee joints of old (3-5 years old) and young (2-6 weeks old) pigs and in the ACVs. PPi levels were measur ed in chondrocyte-conditioned media in the presence of TGase inhibitor s or control compounds. Results. Levels of TGase activity in the cytos olic fraction of old chondrocytes were 7-fold higher than those in ide ntically cultured young chondrocytes. The mean+/-SD activity level in the membrane fraction of lysed chondrocytes was 6.0+/-0.6 units/mg pro tein in old articular chondrocytes and was undetectable in young chond rocytes. In ACVs, the mean+/-SD TGase activity level was 1.23+/-0.1 un its/mg protein. Type II TGase protein tvas present in chondrocyte cyto sol and in ACVs. TGase activity was increased by TGF beta to 120% of c ontrol values (P <0.01), and decreased by insulin-like growth factor 1 to 80% of control values (P <0.01). TGase inhibitors blocked media ac cumulation of PPi, an essential precursor of CPPD crystal formation, a nd a sensitive marker of TGF beta effect. Conclusion. These data sugge st a potential link between TGase activity and processes of pathologic biomineralization that result in CPPD crystal formation in aging arti cular cartilage.