CYCLOSPORINE-A ENHANCES AGONIST-INDUCED AGGREGATION OF HUMAN PLATELETS BY STIMULATING PROTEIN-PHOSPHORYLATION

Use of the immunosuppressant drug cyclosporine A (CSA) has resulted in improved renal graft survival. However, an increased incidence of arterial and venous thrombotic diseases, hemolytic-uremic type syndrome, and findings resembling vasculitis in the kidneys of patients with CSA nephrotoxicity and accelerated atherogenesis have been reported. These disorders may be related to CSA-induced abnormalities in platelet function. We report here that CSA causes increased ADP-stimulated aggregation in isolated platelet suspensions indicating that CSA has a direct effect on platelet function, independent of CSA interactions with plasma factors. Maximal hyperaggregability of ADP-stimulated platelets occurred following a 1 h preincubation period with CSA. Hyperaggregability of platelets due to the presence of CSA was dose-dependent and approached plateau between 200-500 ng/ml CSA. We determined that CSA exerted its effects through a signal transduction pathway involving the phosphorylation of two intracellular proteins, a 40 kD substrate of PKC (p47) and the 20 kD light chain of myosin (p20), a substrate of calcium/calmodulin dependent kinase. Preincubation with CSA resulted in a 200% increase in the phosphorylation of these proteins in platelets stimulated with ADP. We conclude that CSA enhances ADP-induced platelet aggregation and secretion, in part, by potentiating the phosphorylative response of specific intracellular proteins to stimulation by agonists. This process may be responsible for the increased thrombosis and atherogenesis observed in CSA-treated patients.