Activation of endothelial cell phospholipase D by polycations

Naturally occurring polycations and cationic proteins are implicated in vascular disorders. It is known that activated leukocytes and platelets release polycations, such as polylysine (PLys), of varying molecular sizes into the vasculature, and some of these have been described to be bactericidal. Polycations interact with endothelial cells (ECs) and cause alterations in permeability and cellular functions. The precise mechanism(s) by which polycations bring about cellular changes is unknown. Here, we report that the polycations PLys and polyarginine (PArg) induce phospholipase D (PLD) activation in ECs. Polycation-mediated PLD activation was both time and concentration dependent, and activation of PLD was not due to cytotoxicity. PArg was more potent compared with PLys of the same molecular weight in stimulation of PLD. Treatment with bisindolylmaleimide, a specific protein kinase C (PKC) inhibitor, and heparin attenuated polycation-mediated PLD activation. Furthermore, downregulation of PKC by 12-O-tetradecanoylphorbol-13-acetate (100 nM, 18 h) also blocked polycation-mediated PLD stimulation. These data suggest that polycation-mediated PLD stimulation probably involves PKC and may represent an important cellular response to leukocyte/platelet activation in the vascular endothelium.