Agonist function of the neurokinin receptor antagonist, [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P, in monocytes

G-protein-coupled bombesin receptors are capable of signaling through the G(i) protein even when receptor-coupling to G(q) is blocked by [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P (SpD), a neurokinin-1 receptor antagonist and "biased" agonist to bombesin receptors. As bombesin is a monocyte and tumor cell attractant, we were interested in the effects of SpD on cell migration. Chemotaxis of monocytes was tested in micropore filter assays. SpD was a dose-dependent agonist in monocyte migration and was not inhibited by antagonists to neurokinin-1 or -2 receptors. SpD failed to inhibit chemotaxis toward bombesin, suggesting that inhibition of bombesin receptor coupling to G(q) with SpD does not impair migratory responses elicited by bombesin. As pertussis toxin inhibited migration, coupling of receptors to G(i) may signal migration. Chemotaxis toward SpD was inhibited by bombesin receptor antagonists as well as by blocking signaling enzymes downstream of G(q) (phospholipase-3 and protein kinase C with wortmannin and bisindolylmaleimide, respectively), suggesting transactivation of G(q)-mediated chemotaxis signaling by SpD via bombesin receptors. Protein kinase C that induces sphingosine kinase activation and production of sphingosine-1-phosphate, which may lead to G(q)-dependent chemoattraction, was involved in SpD-dependent migration. Inhibition of sphingosine-1-phosphate production with dimethylsphingosine inhibited monocyte migration toward SpD. Data suggest that SpD induces migration in monocytes and signaling events involving activation of sphingosine kinase in a G(i) protein- and protein kinase C-dependent fashion. "Biased" agonism of SpD at bombesin receptors may affect normal and tumor cell migration.