Plasminogen Activator System in Osteoclasts

To determine which genes of the plasminogen activator (PA) system were expressed in osteoclasts, RNA extracted from microisolated mouse osteoclasts was used as template for reverse transcribed polymerase chain reaction (RT‐PCR) with gene‐specific primer pairs. Using this approach, the expression of RNAs for tissue‐type plasminogen activator, urokinase‐type plasminogen activator, plasminogen activator inhibitor‐1, plasminogen activator inhibitor‐2, protease nexin, and urokinase receptor isoform 1 (uPAR1) were detected in mouse osteoclasts. The expression of uPAR RNA in osteoclasts was confirmed by in situ hybridization with a uPAR1 probe. RNA encoding the uPAR isoform 2 was not detected in mouse osteoclasts, but a novel unspliced uPAR RNA variant was detected in these cells. The novel uPAR variant and uPAR1 RNA were also detected in mouse calvarial osteoblasts, kidney, muscle, and the mouse macrophage cell line J774A.1 by RT‐PCR. The presence of RNAs for most of the components of the PA system in osteoclasts suggests that it may have a functional role in this cell type.