Summary
Transglutaminases are a family of enzymes that catalyze the formation of ɛ-(γ-glutamyl)lysine
isopeptide bonds in proteins, an activity that has been implicated in the pathogenesis
of cartilage matrix mineralization in degenerative arthritis. Type II transglutaminase
and thrombin-activatable factor XIII have been identified in articular cartilage.
Thrombin, a coagulation protease, is found in pathological synovial fluids, and is
known to stimulate transglutaminase activity in non-articular tissues. We investigated
the effects of thrombin on transglutaminase activity in porcine articular chondrocytes.
Direct addition of thrombin to chondrocyte lysates resulted in increased transglutaminase
activity due to proteolytic conversion of factor XIII to XIIIa. Thrombin-treated chondrocyte
cultures (0.001 to 2.0U/ml) also showed increased transglutaminase activity. Thrombin
treatment of chondrocyte cultures increased transglutaminase activity as early as
15 minutes after addition, an effect that we attributed to factor XIII activation.
Additional stimulatory effects of thrombin were observed in cultured chondrocytes
at 4 and 24 hours. A thrombin receptor agonist peptide (TRAP) which activates the
PAR1 thrombin receptor mimicked these later effects. Thrombin treatment of chondrocyte
cultures increased factor XIII mRNA and protein levels, without affecting levels of
type II transglutaminase. Thus, thrombin stimulates transglutaminase activity in articular
cartilage by directly cleaving factor XIII and by receptor-mediated upregulation of
factor XIII synthesis. Such increases in potential transglutaminase activity may facilitate
pathological matrix calcification in degenerative arthritis.
Keywords
Degenerative arthritis - calcium pyrophosphate crystals - mineralization - proteases
- articular cartilage