Demethoxyviridin and wortmannin block phospholipase C and D activation in the human neutrophil

1 The fungal metabolite, wortmannin, has recently been shown to inhibit fMet‐Leu‐Phe‐stimulated superoxide production and phospholipase D (PLD) activation in the human neutrophil. 2 We have found that a close structural analogue of wortmannin, demethoxyviridin, has a similar inhibitory profile but in addition blocks phosphatidylinositol 4,5‐bisphosphate‐specific phospholipase C and hence inositol 1,4,5‐trisphosphate (IP3) formation. 3 Inhibition of fMet‐Leu‐Phe‐stimulated PLD by demethoxyviridin was characteristically non‐competitive (IC50 = 31 ± 10 nm). 4 Inhibition of fMet‐Leu‐Phe‐stimulated IP3 formation required concentrations almost 10 times higher (IC50 = 250 ± 130 nm). 5 Surprisingly, demethoxyviridin only inhibited fMet‐Leu‐Phe‐induced intracellular calcium mobilization at concentrations 100 times greater than those needed to block IP3 formation. 6 Demethoxyviridin also inhibited PLD activation induced by sodium fluoride or phorbol myristate acetate (PMA) but the concentrations required were 100 times those needed to block fMet‐Leu‐Phe‐stimulated PLD. 7 These observations support the contention that PLD plays an important role in signal transduction in the human neutrophil and indicate that wortmannin and demethoxyviridin inhibit PLD activation at a common step in the signalling pathway. 8 Furthermore, these results suggest that demethoxyviridin may block the interaction between the chemotactic peptide receptor and a GTP‐binding protein that is intimately involved in PLD activation.