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 protein kinase (PKA) were activated selectively by treating platelets with phorbol dibutyrate (PDBu) or prostacyclin (PGl2). PKC activity was quantitated by measuring PDBu-specific phosphorylation of a 47,000 molecular weight protein, and PKA activity monitored by measuring prostacyclin-dependent phosphorylation of a 22,000 molecular weight protein. Staurosporine and 1-(5-isoquinolinylsulphonyl)-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 microM) but increased PKA-dependent protein phosphorylation. These results support the use of human platelets for measuring the specificity of protein kinase inhibitors and indicate that tamoxifen might have value for experimental purposes as a relatively selective PKC inhibitor.