Alterations of endothelial nucleotide levels by mycophenolic acid result in changes of membrane glycosylation and E-selectin expression

The effect of the inhibitor of inosine-5'-monophosphate dehydrogenase (IMPDH). mycophenolic acid, on intracellular nucleotides and the synthesis of cellular glycoproteins was evaluated in human umbilical vein endothelial cells. A clinically attainable concentration (10 mu mol/l) of mycophenolic acid decreased guanosine-5'-triphosphate (GTP) levels significantly and led to a strong elevation of uridine-5'-triphosphate (UTP), whereas intracellular adenosine-5'-triphosphate (ATP) pools remained unaffected. The staining of the endothelial cell membranes with lectins specific for fucose and mannose (Ulex europaeus- and Galanthus nivalis agglutinin, respectively) was reduced, reflecting an inhibition of fucose and mannose incorporation into endothelial glycoproteins. The surface expression of E-selectin, an important determinant for leuko-endothelial interactions decreased significantly. Guanine and guanosine prevented the actions of mycophenolic acid and reversed the drug-induced decrease in GTP and its associated effects. The findings that mycophenolic acid produces alterations in the formation off glycoproteins and in the membrane architecture are indicative of metabolic lesions induced by an agent that depresses guanine nucleotide synthesis through inhibtion of IMPDH. The pronounced reduction of E-selectin surface expression on endothelial cells accompanied by changes of endothelial cell fucosylation, a prerequisite for the contact with lymphocytic L-selectin, indicates an inhibitory effect of mycophenolic acid in the rolling phase of leukocyte recruitment and strongly implies a new and additional immunosuppressive mechanism of this agent.