Parallel inhibition of platelet-activating factor-induced protein phosphorylation and serotonin release by K-252a, a new inhibitor of protein kinases, in rabbit platelets.

K-252a, (8R*,9S*,11S*)-(-)-9-hydroxy-9-methoxycarbonyl-8-methyl-2,3,9,10-tetr ahy dro-8,11-epoxy-1H,8H,11H-2,7b,11a-triazadi benzo[a,g]cycloocta[c,d,e]triden-1-one, an indole carbazol compound isolated from microbial origin, potently inhibits protein kinase C in partially purified enzyme and intact platelets. We examined the effects of this compound on platelet-activating factor [1-O-alkyl-alpha-acetyl-sn-glycero-phosphocholine (AGEPC)] induced protein phosphorylation, serotonin release and a rise in intracellular free calcium using washed rabbit platelets. In Ca2+-containing medium (1 mM CaCl2), AGEPC at 10(-10) and 10(-9) M markedly phosphorylated two proteins having molecular weights of 40,000 daltons (40 K protein) and 20,000 daltons (20 K protein) and evoked a marked rise in cytosolic free calcium. K-252a at 3 and 10 microM caused a concentration-dependent inhibition in the 20 K protein phosphorylation but caused only slight inhibition in the 40 K protein phosphorylation. K-252a inhibited the basal phosphorylation of 20 K protein obtained in non-stimulated platelets, and caused no significant alteration in the rise of intracellular free calcium evoked by AGEPC. It can be considered, from this evidence, that K-252a may act directly on myosin light chain kinase, resulting in the inhibition of 20 K protein phosphorylation. In Ca2+-free medium [1 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA)], AGEPC at 10(-8) M predominantly phosphorylated 40K protein, although phosphorylation of 20K protein and cytosolic free calcium were increased slightly. K-252a at 1-10 microM caused a concentration-dependent inhibition in the 40K protein phosphorylation. These results indicate that K-252a functions as an inhibitor of both protein kinase C and myosin light chain kinase in rabbit platelets. In AGEPC-stimulated platelets, the inhibition of 20K protein phosphorylation in Ca2+-containing medium and of 40K protein phosphorylation in Ca2+-free medium was closely correlated with the inhibition of serotonin release by K-252a. These results strongly suggest that the phosphorylation of these two proteins may be a prerequisite for serotonin release in AGEPC-stimulated platelets.