CELLULAR-LOCALIZATION OF THE INHIBITORY-ACTION OF ABRUQUINONE-A AGAINST RESPIRATORY BURST IN RAT NEUTROPHILS

1 The possible mechanisms of action of the inhibitory effect of abruqu inone A on the respiratory burst in rat neutrophils in vitro was inves tigated. 2 Abruquinone A caused an irreversible and a concentration-de pendent inhibition of formylmethionyl-leucyl-phenylalanine (fMLP) plus dihydrocytochalasin B (CB)- and phorbol 12-myristate 13-acetate (PMA) -induced superoxide anion (O-2(.-)) generation with IC50 values of 0.3 3+/-0.05 mu g ml(-1) and 0.49+/-0.04 mu g ml(-1), respectively. 3 Abru quinone A also inhibited O-2 consumption in neutrophils in response to fMLP/CB and PMA. However, abruquinone A did not scavenge the generate d O-2(.-) in xanthine-xanthine oxidase system and during dihydroxyfuma ric acid (DHF) autoxidation. 4 Abruquinone A inhibited both the transi ent elevation of [Ca2+](i) in the absence of [Ca2+](o) (IC50 7.8+/-0.2 mu g ml(-1)) and the generation of inositol trisphosphate (IP3) (IC50 10.6+/-2.0 mu g ml(-1)) in response to fMLP. 5 Abruquinone A did not affect the enzyme activities of neutrophil cytosolic protein kinase C (PKC) and porcine heart protein kinase A (PKA). 6 Abruquinone A had no effect on intracellular guanosine 3':5'-cyclic monophosphate (cyclic GMP) levels but decreased the adenosine 3':5'-cyclic monophosphate (cy clic AMP) levels. 7 The cellular formation of phosphatidic acid (PA) a nd phosphatidylethanol (PEt) induced by fMLP/CB was inhibited by abruq uinone A with IC50 values of 2.2+/-0.6 mu g ml(-1) and 2.5+/-0.3 mu g ml(-1), respectively. Abruquinone A did not inhibit the fMLP/CB-induce d protein tyrosine phosphorylation but induced additional phosphotyros ine accumulation on proteins of 73-78 kDa in activated neutrophils. 8 Abruquinone A inhibited both the O-2(.-) generation in PMA-activated n eutrophil particulate NADPH oxidase (IC50 0.6+/-0.1 mu g ml(-1)) and t he iodonitrotetrazolium violet (INT) reduction in arachidonic acid (AA )-activated cell-free system (IC50 1.5+/-0.2 mu g ml(-1)). 9 Collectiv ely, these results indicate that the inhibition of respiratory burst i n rat neutrophils by abruquinone A is mediated partly by the blockade of phospholipase C (PLC) and phospholipase D (PLD) pathways, and by su ppressing the function of NADPH oxidase through the interruption of el ectron transport.