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 art erial 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. Thes e disorders may be related to CSA-induced abnormalities in platelet fu nction. We report here that CSA causes increased ADP-stimulated aggreg ation in isolated platelet suspensions indicating that CSA has a direc t effect on platelet function, independent of CSA interactions with pl asma factors. Maximal hyperaggregability of ADP-stimulated platelets o ccurred following a 1 h preincubation period with CSA. Hyperaggregabil ity of platelets due to the presence of CSA was dose-dependent and app roached plateau between 200-500 ng/ml CSA. We determined that CSA exer ted its effects through a signal transduction pathway involving the ph osphorylation of two intracellular proteins, a 40 kD substrate of PKC (p47) and the 20 kD light chain of myosin (p20), a substrate of calciu m/calmodulin dependent kinase. Preincubation with CSA resulted in a 20 0% increase in the phosphorylation of these proteins in platelets stim ulated with ADP. We conclude that CSA enhances ADP-induced platelet ag gregation 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 ath erogenesis observed in CSA-treated patients.