Induction of Interleukin-6 Release by Interleukin-1 in Rat Anterior Pituitary Cellsin Vitro: Evidence for an Eicosanoid-Dependent Mechanism*

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Abstract
We have reported previously that a subpopulation(s) of anterior pituitary cells released IL-6 and that this release was stimulated by interleukin-1 (IL-1) through a non-cAMP-dependent mechanism. We now report that IL-1 induces IL-6 release from anterior pituitary cells in an eicosanoid-dependent manner. Dispersed rat anterior pituitary cells were briefly prelabeled (2-3 h) with [3H]arachidonic acid (AA) to esterify the fatty acid within the lipid pool. Incubation of these prelabeled cells with 25 ng/ml IL-1 beta caused an increase only within 1-2 min in the amount of free [3H]AA detected in the extracts of the cells. During 15- to 30-min incubations, IL-1 beta (25 ng/ml) caused an increased accumulation of [3H]AA in the incubation medium which reached levels similar to those induced by 100 nM TRH. Perifused anterior pituitary cells responded to IL-1 beta (25 ng/ml) with a rapid (less than 2 min), biphasic, and reversible efflux of [3H]AA. The [3H]AA appears to have been derived from choline phospholipids, as formation of [3H]glycerophosphorylcholine was substantially increased by exposure of [3H]choline-prelabeled cells to either IL-1 alpha (171%) or IL-1 beta (236%); in addition, the complete deacylation of phosphatidylcholine suggests that other fatty acid species are liberated as a consequence of IL-1 receptor activation and, thus, may also contribute to the actions of IL-1 alpha and IL-1 beta. However, the levels of [3H]phosphorylcholine and [3H]choline were unchanged as well as those of catabolites of other lipid species. These data suggested an involvement of phospholipase-A2 (PLA2) in mediating the IL-1 induction of IL-6 release. Subsequently, we used inhibitors of the PLA2, cyclooxygenase, and lipoxygenase enzymes to investigate a possible role for the generation of AA and its subsequent enzymatic conversion in the signal transduction pathway activated by IL-1. The PLA2 inhibitor aristolochic acid (10 microM) blocked IL-1 beta-induced IL-6 release and the release of IL-6 caused by Pyrularia pubera thionin (5 micrograms/ml), a stimulator of PLA2 activity. The cyclooxygenase inhibitor indomethacin (10 microM) did not inhibit IL-1 beta-induced IL-6 release. In contrast, the general lipoxygenase inhibitor nordihydroguaiaretic acid (10 microM) and the more specific 5-lipoxygenase inhibitors AA861 and RHC5901 (both 10 microM) reduced basal and blocked IL-1 beta-induced IL-6-release.(ABSTRACT TRUNCATED AT 400 WORDS)

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