Function and stimulus-specific effects of phorbol 12-myristate 13-acetate on human polymorphonuclear neutrophils: Autoregulatory role for protein kinase C in signal transduction

Exposure of polymorphonuclear neutrophils (PMNs) to phorbol 12-myristate 13-acetate (PMA) resulted in a concentration-dependent (1–10 ng/ml) inhibition of granule exocytosis induced with the receptor-specific ligands,N-formylmethionyl-leucyl-phenylalanine (FMLP), pepstatin A, 5(S),12(R)-dihydroxy-6, 14-cis-8, 10-trans-eicosatetraenoic acid (LTB₄), and acetyl-sn-glyceryl-3-phosphorylcholine (AGEPC), PMA exerted a marginal inhibitory effect on calcium ionophore A23187-induced PMN degranulation, and the PMA analog, 4-phorbol 12, 13-didecanoate (4-PDD), was inactive. However, PMA potentiated AGEPC, pepstatin A, FMLP, LTB₄, and A23187-stimulated Superoxide anion (O ₂ ^– ) production. The mobilization of intracellular sequestered calcium (Ca²⁺) by the receptor-specific ligands, as reflected by a rise in the cytosolic-free Ca²⁺ concentration (Ca²⁺]_i) in PMNs loaded with the CA²⁺-sensitive dye, Fura-2, was suppressed by PMA. A protein kinase C (PKC) inhibitor, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) reversed the PMA-mediated inhibition of PMN degranulation and intracellular CA²⁺ mobilization. However, another, but less potent PKC inhibitor,N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA 1004), had no effect on the inhibition of PMN activation by PMA.