Cellular localization of the inhibitory action of abruquinone A against respiratory burst in rat neutrophils

The possible mechanisms of action of the inhibitory effect of abruquinone A on the respiratory burst in rat neutrophils in vitro was investigated. Abruquinone A caused an irreversible and a concentration‐dependent inhibition of formylmethionyl‐leucyl‐phenylalanine (fMLP) plus dihydrocytochalasin B (CB)‐ and phorbol 12‐myristate 13‐acetate (PMA)‐induced superoxide anion (O 2 .− ) generation with IC 50 values of 0.33±0.05 μg ml −1 and 0.49±0.04 μg ml −1 , respectively. Abruquinone A also inhibited O 2 consumption in neutrophils in response to fMLP/CB and PMA. However, abruquinone A did not scavenge the generated O 2 .− in xanthine‐xanthine oxidase system and during dihydroxyfumaric acid (DHF) autoxidation. Abruquinone A inhibited both the transient elevation of [Ca 2+ ] i in the absence of [Ca 2+ ] o (IC 50 7.8±0.2 μg ml −1 ) and the generation of inositol trisphosphate (IP 3 ) (IC 50 10.6±2.0 μg ml −1 ) in response to fMLP. Abruquinone A did not affect the enzyme activities of neutrophil cytosolic protein kinase C (PKC) and porcine heart protein kinase A (PKA). Abruquinone A had no effect on intracellular guanosine 3′: 5′‐cyclic monophosphate (cyclic GMP) levels but decreased the adenosine 3′: 5′‐cyclic monophosphate (cyclic AMP) levels. The cellular formation of phosphatidic acid (PA) and phosphatidylethanol (PEt) induced by fMLP/CB was inhibited by abruquinone A with IC 50 values of 2.2±0.6 μg ml −1 and 2.5±0.3 μg ml −1 , respectively. Abruquinone A did not inhibit the fMLP/CB‐induced protein tyrosine phosphorylation but induced additional phosphotyrosine accumulation on proteins of 73–78 kDa in activated neutrophils. Abruquinone A inhibited both the O 2 .− generation in PMA‐activated neutrophil particulate NADPH oxidase (IC 50 0.6±0.1 μg ml −1 ) and the iodonitrotetrazolium violet (INT) reduction in arachidonic acid (AA)‐activated cell‐free system (IC 50 1.5±0.2 μg ml −1 ). Collectively, these results indicate that the inhibition of respiratory burst in rat neutrophils by abruquinone A is mediated partly by the blockade of phospholipase C (PLC) and phospholipase D (PLD) pathways, and by suppressing the function of NADPH oxidase through the interruption of electron transport.British Journal of Pharmacology (1997) 120 , 917–925; doi: 10.1038/sj.bjp.0700974