Farnesylcysteine analogues inhibit store-regulated Ca2+ entry in human platelets: evidence for involvement of small GTP-binding proteins and actin cytoskeleton

We have investigated the mechanism of Ca2+ entry into fura-2-loaded human p latelets by preventing the prenylation of proteins such as small GTP-bindin g proteins. The farnesylcysteine analogues farnesylthioacetic acid (FTA) an d N-acetyl-S-geranyl-geranyl-L-cysteine (AGGC), which are inhibitors of the methylation of prenylated and geranylgeranylated proteins respectively, si gnificantly decreased thrombin-evoked increases in intracellular free Ca2concentration ([Ca2+](i)) in the presence, but not in the absence, of exter nal Ca2+, suggesting a relatively selective inhibition of Ca2+ entry over i nternal release. Both these compounds and N-acetyl-S-farnesyl-L-cysteine, w hich had similar effects to those of FTA, also decreased Ca2+ entry evoked by the depletion of intracellular Ca2+ stores with thapsigargin. The inacti ve control N-acetyl-S-geranyl-L-cysteine was without effect. Patulin, an in hibitor of prenylation that is inert with respect to methyltransferases, al so decreased store-regulated Ca2+ entry. Cytochalasin D, an inhibitor of ac tin polymerization, significantly decreased store-regulated Ca2+ entry in a time-dependent manner. Both cytochalasin D and the farnesylcysteine analog ues FTA and AGGC inhibited actin polymerization; however, when evoking the same extent of decrease in actin filament formation, FTA and AGGC showed gr eater inhibitory effects on Ca2+ entry, indicating a cytoskeleton-independe nt component in the regulation of Ca2+ entry by small GTP-binding-protein. These findings suggest that prenylated proteins such as small GTP-binding p roteins are involved in store-regulated Ca2+ entry through actin cytoskelet on-dependent and cytoskeleton-independent mechanisms in human platelets.