Cross talk between substance P and melittin-activated cellular signaling pathways in rat lactotroph-enriched cell cultures.

We have investigated the possible interaction (cross talk) between the phospholipase A2 (PLA2) and inositol 1,4,5-trisphosphate/protein kinase C (PKC) signaling pathways in rat lactotroph-enriched cell cultures. Melittin, a bee venom peptide, stimulated release of [3H]arachidonic acid ([3H]AA) from [3H]AA-labeled enriched lactotrophs in a dose-dependent manner. Moreover, melittin and exogenous AA induced a redistribution of PKC catalytic activity and PKC alpha and beta immunoreactivity from the soluble to the particulate fraction in resting and substance P (SP)-stimulated cells. Melittin had no effect on phospholipase C (PLC) activity. Pretreatment of cell cultures with the PLA2 inhibitors quinacrine and aristolochic acid resulted in a dose-dependent inhibition of melittin-stimulated PKC isozyme translocation as did the inhibitor of lipoxygenase, nordihydroguaiaretic acid, whereas the cyclooxygenase inhibitor indomethacin had no effect. SP and the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) dose-dependently increased levels of [3H]AA released from cells. Pretreatment of cell cultures with quinacrine reduced the effect of SP on [3H]AA formation. After long-term treatment (24 h) of cells with TPA, the effect of TPA on [3H]AA production was not different from control, whereas SP still displayed [3H]AA-releasing abilities although not at full scale. Pretreatment of cells with thapsigargin, U 73122, methoxyverapamil, and RHC 80267, an inhibitor of diacylglycerol lipase, all resulted in reduced SP-stimulated [3H]AA liberation. Treatment of cell cultures with pertussis toxin (PTX) reduced the release of [3H]AA induced by SP, whereas PTX had no effect on SP-stimulated generation of 3H-inositol phosphates. On the basis of these results, it is concluded that (1) the PLA2 pathways interfere with the phosphoinositide-PLC signaling system at the level of PKC isozymes alpha and beta, the product responsible for this interaction being either AA or a metabolite produced by the action of lipoxygenase; (2) SP and TPA are able to activate the PLA2 pathway at a level at or beyond PLA2, and this effect is mediated, in part, through PKC alpha and beta species and (for SP) intracellular Ca2+ recruited from internal stores as well as from external sources; and (3) SP also activates PLA2 through a PTX-sensitive pathway distinct from the one coupled to phosphoinositide-PLC, which is PTX insensitive.