SELECTIVITY OF PROTEIN-KINASE INHIBITORS IN HUMAN INTACT PLATELETS

The specificity of commonly used protein kinase inhibitors has been evaluated in the intact human platelet. Protein kinase C (PKC) and cyclic AMP-dependent protine kinase (PKA) were activated selevtively by treating platelets with phorbol dibutyrate (PDBu) or prostacyclin (PG12). PKC activity was quantitated by measuring PBDu-specific phosphorylation of a 47, 000 molecular protein, and PKA activity monitored by measuring protacyclin-dependent phosphorylation of a 22,000 molecular weigth protein. Staurosporine and 1-(5-isoquinolinlsulphonyl)-2-methyl-piperazine (H-7) were found to be non-specific inhibitors in the intact platelet, consistent with their effects on the isolated enzymes. Tamoxifen inhibited PKC activity (IC50 = 80 μM) but increased PKA-dependent protein phosphorylation. These results support the use of human platelets for measuring the specifity of protein kinase inhibitors and indicate that tamoxifen might have value for experimental purposes as a relatviely selective PKC inhibitor. © 1990.