Bone matrix degradation by the plasminogen activation system; possible mechanism of bone destruction in arthritis

The observed increase in urokinase-type plasminogen activator (u-PA) and its receptor (u-PAR) in synovial tissue of patients with rheumatoid arthritis (RA) suggests pathophysiological involvement of the plasminogen activation PA system in inflammatory joint disease.
In the present study we investigated the capacity of the PA system to degrade non-mineralized and mineralized bone-like matrix in vitro as a model for bone destruction. Transfected mouse LB5 cell lines that expressed either human u-PA or u-PAR were cultured separately and simultaneously on radiolabelled bone matrix in the presence of plasminogen. Osteoblast-like murine calvarial MC2T2-E0 cells were used to produce a well-characterized highly organized bone-like matrix that could be mineralized in the presence of b-glycerol phosphate . Bone matrix degradation was followed by the release of radioactivity in the culture medium . u-PA-producing cells in contrast to u-PAR producing cells degraded both non-mineralized and mineralized bone matrix . This e}ect could be inhibited by anti-u-PA antibodies as well as by tranexamic acid and by aprotinin indicating that the degrading activity is u-PA mediated and plasmin dependent . Co-cultivation of a small portion of u-PA-producing cells with u-PAR-expressing cells resulted in a marked increase in degradation activity . Reduction of this potentiating e}ect by suramin or the amino-terminal fragment of u-PA both competitive inhibitors of u-PA receptor binding shows that this synergistic e}ect is due to binding of u-PA to u-PAR . u-PAR must be cell associated as binding of u-PA to a soluble u-PAR prevented this enhancement . The capability of the PA system to degrade bone matrix in vitro and the previously demonstrated increased expression of u-PA and u-PAR in synovial tissue of patients with RA further support a role for the PA system in the development of bone erosions.