Subcellular distribution of urokinase and urokinase receptor in human neutrophils determined by immunoelectron microscopy

A high-affinity receptor for urokinase-type plasminogen activator (uPAR) ha s been identified on the plasma membrane of a number of different cell type s, and has been shown to be important for plasminogen activation, cell adhe sion, and possibly signal transduction. uPAR and uPA cosediment with secret ory vesicles and specific granules by subcellular fractionation and translo cate to the plasma membrane upon activation of neutrophils. Here the subcel lular distribution of uPAR and uPA is studied by electron microscopy of neu trophils using immunogold double labeling for uPAR and uPA and a set of mar kers for well-defined subtypes of granules: matrix metalloproteinase type-9 (MMP-9) for gelatinase granules, lactoferrin (LF) for specific granules, a nd myeloperoxidase (MPO) and neutrophil elastase (NE) for primary granules. With this technique uPAR colocalizes with uPA in 71% of labeled granules. In granules containing uPAR the degree of coexpression with MMP-9, MPO and NE was 19. 66, and 74%, respectively. In granules labeled for uPA the corre sponding overlap with MMP-9, MPO and NE was 24, 64. and 51%, respectively. Low levels of co-localization were found for uPAR and LF (7%) and for uPA a nd lactoferrin (5%). The results indicate that uPAR and uPA are present in gelatinase granules and primary granules, but rarely in specific granules. The demonstration of uPAR and uPA in primary granules is of particular inte rest, and may indicate that uPAR and uPA participate in the activation of l atent hepatocyte growth factor of neutrophils.