Urokinase activates calcium‐dependent potassium channels in U937 cells via calcium release from intracellular stores

The urokinase receptor (uPAR) is highly expressed in the human promyelocytic cell line U937 and contributes to transmembrane signalling. However, the signalling mechanisms are poorly understood. We used the patch‐clamp technique to demonstrate that urokinase (uPA) binds to uPAR and thereby stimulates Ca 2+ ‐activated K + channels in U937 cells. uPA transiently increased K + currents within 30 s. The K + currents were pertussis toxin‐sensitive and were also observed in Ca 2+ ‐free solution. However, when cells were dialysed with EGTA, uPA did not affect K + currents. The intracellular Ca 2+ response to uPA was independent of extracellular Ca 2+ , was pertussis toxin‐sensitive, and was blocked by both thapsigargin and the phospholipase C inhibitor U‐73122. The uPA‐induced increase in intracellular Ca 2+ was independent of uPA proteolytic activity. Furthermore, uPA initiated a rapid formation of inositol 1,4,5‐trisphosphate [Ins(1,4,5) P 3 ]. The amino‐terminal uPA fragment and uPA inactivated with diisopropyl fluorophosphate or with inhibitory antibody, elicited the same Ca 2+ signal. On the other hand, Ca 2+ signalling required the intact uPAR because the effects were abrogated by PtdIns‐specific phospholipase C, which removes the uPAR from the cell surface. The prevention of glycosyl phosphatidylinositol moiety synthesis and interference with uPAR anchoring to the cell surface using mannosamine also abolished Ca 2+ signals. Taken together, our findings indicate that uPA binds to uPAR and stimulates the production of Ins(1,4,5) P 3 via a G‐protein‐ and phospholipase C‐dependent mechanism. Ins(1,4,5) P 3 in turn liberates Ca 2+ from intracellular stores, which leads to the stimulation of Ca 2+ ‐activated K + channels.