Plasminogen activation stimulates an increase in intracellular calcium in human synovial fibroblasts

Both plasminogen (Pg) and urinary-type Pg activator (u-PA), but not tissue-type Pg activator (t-PA), bind to normal and rheumatoid arthritis (RA) human synovial fibroblasts in culture with high affinity and in a dose-dependent manner. Single cell intracellular Ca2+ responses to Pg and u-PA were studied using Fura-2 and digital imaging fluorescence microscopy. Pg activation by u-PA on the surface of RA synovial fibroblasts induces a significant rise in cytosolic free Ca2+ concentration ([Ca2+]i) within 90 s. Pg kringle 4 and the alpha 2,3-linked sialic acid in the carbohydrate chain bound to Thr245 are involved in mediating the increases in [Ca2+]i. This response is not observed in normal synovial fibroblasts, suggesting that RA synovial fibroblasts have altered responses to the binding and activation of Pg on their surfaces.