W-7 primes or inhibits the fMLP-stimulated respiratory burst in human neutrophil by concentration-dependent dual expression of the formyl peptide receptors on cell surface

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It was investigated why the fMLP-stimulated respiratory burst in human neutrophils was enhanced by N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), a considered calmodulin antagonist, at lower concentration but inhibited at higher concentration. Flow cytometric analysis on binding of the receptor to the fluorescence-labeled formyl peptide and the polymerization of actin in cells showed that the drug inhibited actin polymerization and promoted expression of the fMLP receptors on cell membrane at lower concentration, while promoted the actin polymerization and depressed the receptor expression at higher concentration. As intracellular Ca 2+ ([Ca 2+] i) is elevated, polymerization of actin decreases and the receptor expression increases. At normal physiological and two moderately high intracellular calcium levels, the dual effect of W-7 became less significant as [Ca 2+] i was elevated indicating that the dual effect is calcium-dependent. Under two extreme conditions that the intracellular calcium was either depleted or highly elevated, the dual effect disappeared but only an inhibitory effect on actin polymerization was observed. Colchicine and taxol study showed that disruption or stabilization of microtubules had no effect on formyl peptide receptor expression. The results suggest that W-7 primes the fMLP stimulation by direct action on actin leading to breakdown of microfilaments and more expression of formyl peptide receptors, and inhibits the stimulation by indirect action on actin through inactivation of some Ca 2+-dependent proteins resulting in assembly of actin into microfilaments. Which action is favorable depends on the drug concentration.