Arachidonic acid inhibits luteinizing hormone-stimulated progesterone production in hen granulosa cells

Arachidonic acid has been proposed to function as a hormone-induced second messenger in a variety of mammalian endocrine tissues. The present studies were conducted to evaluate whether arachidonic acid, either added exogenously or released endogenously following treatment with physiologic (phospholipase A₂) or pharmacologic (melittin) agents, influences basal and/or luteinizing hormone (LH)-induced cyclic adenosine 3',5'-monophosphate (cAMP) and progesterone production in granulosa cells from domestic hens. Phospholipase A₂ (PLA₂) and melittin treatments failed to alter basal concentrations of progesterone, whereas arachidonic acid had a slight stimulatory effect (only at the 50-μM dose) on progesterone levels, and no effect on cAMP. By contrast, arachidonic acid, PLA₂, and melittin each inhibited LH-promoted progesterone production in a dose-dependent fashion. The inhibitory effects of arachidonic acid on the progesterone response were determined to occur both prior and subsequent to cAMP formation since cAMP levels in arachidonic acid-treated cells were attenuated after treatment with 10 ng LH or 100 μM forskolin (at 10- to 100-μM doses of arachidonic acid), and progesterone production was decreased in the presence of 1 mM 8-bromo-cAMP (with 50 and 100 μM arachidonic acid). The post-cAMP mechanism of action is characterized by the inability of cells to convert 25-hydroxycholesterol, but not pregnenolone, to progesterone. The effects of arachidonic acid are probably direct, since pharmacologic inhibitors of the lipoxygenase (nordihydroguaiaretic acid) and cyclooxygenase (indomethacin) pathways of arachidonic acid metabolism failed to alter the suppression of LH-stimulated progesterone production. Furthermore, several related fatty acids (pentaenoic, eicosatrienoic, oleic, and palmitoleic acids), which do not enter the cyclooxygenase or lipoxygenase metabolic pathway, replicated this response. We conclude that arachidonic acid may represent an additional second messenger that can modulate LH-stimulated steroidogenesis in the ovary of the hen.