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Production of second messengers following chemotactic and mitogenic urokinase-receptor interaction in human fibroblasts and mouse fibroblasts transfected with human urokinase receptor.

We studied urokinase-type plasminogen activator (u-PA)-dependent chemotaxis and DNA synthesis in both human fibroblasts and LB6 mouse fibroblasts transfected with human u-PA receptor (u-PAR) gene (LB6 clone 19). Both cell lines have receptors for the amino-terminal fragment of u-PA (u-PA-ATF). We observed that u-PA and u-PA-ATF stimulated chemotactic migration of both LB6 clone 19 cells and human fibroblasts, which could be impaired by down-regulation of protein kinase C (PKC) with phorbol myristate acetate (PMA). While LB6 clone 19 cells were unable to undergo mitosis following exposure to either u-PA or u-PA-ATF, human fibroblasts were stimulated to mitosis by exogenous addition of native u-PA, and u-PA-ATF was ineffective. The mitogenic activity of u-PA on human fibroblasts could also be impaired by down-regulation of PKC with PMA. We studied second messenger formation following u-PAR stimulation. Neither inositol lipid metabolism nor intracellular Ca2+ content were affected, while an increase of diacylglycerol (DAG) generation was observed. Such DAG formation was related to de novo synthesis from glucose and was dependent on ligand-receptor interaction. Both u-PA-ATF and the native u-PA molecule were able to stimulate DAG formation, u-PA being from three to fourfold more efficient than ATF. These data suggest that u-PAR stimulation per se is sufficient to trigger DAG formation. The native molecule confers on the cell an additional stimulus, possibly related with the activation of a u-PA-catalytic site-dependent substrate. Such stimulation allows the cell to reach the DAG threshold level required to trigger DNA synthesis.