The role of arachidonic acid in steroidogenesis and steroidogenic acute regulatory (StAR) gene and protein expression.

This study was conducted to examine the mechanism for arachidonic acid (AA) regulation of steroidogenic acute regulatory (StAR) protein expression and the relationship between AA and cAMP in hormone-induced steroidogenesis. Dibutyryl cyclic AMP (Bt(2)cAMP)-stimulated MA-10 Leydig cells were treated with AA and/or the phospholipase A(2) inhibitor, dexamethasone. Dexamethasone significantly reduced Bt(2)cAMP-stimulated progesterone production, StAR promoter activity, StAR mRNA, and StAR protein. The inhibitory effects of dexamethasone were reversed by the addition of 150 microm AA to MA-10 cells. In addition, MA-10 cells were treated with the lipoxygenase inhibitor, nordihydroguaiaretic acid (NDGA), the 5-lipoxygenase inhibitor, AA861, the epoxygenase inhibitor, miconazole, and the cyclooxygenase inhibitor, indomethacin. Both NDGA and AA861 inhibited progesterone production and StAR protein expression. AA861-inhibited progesterone synthesis and StAR protein were partially reversed by addition of the 5- lipoxygenase metabolite, 5(S)-hydroperoxy-(6E,8Z,11Z, 14Z)-eicosatetraenoic acid. Inhibition of epoxygenase activity inhibited progesterone production significantly, but StAR protein was only slightly reduced. Indomethacin enhanced StAR protein expression and significantly increased progesterone production. Inhibition of AA release or lipoxygenase activities did not affect protein kinase A activity, whereas inhibition of protein kinase A activity using H89 reduced Bt(2)cAMP-induced StAR protein. AA alone did not induce StAR protein expression nor steroid production. These results demonstrate the essential role of AA in steroid biosynthesis and StAR gene transcription and suggest the possible involvement of the lipoxygenase pathway in steroidogenesis. This study further indicates that AA and cAMP transduce signals from trophic hormone receptors to the nucleus through two separate pathways and act to co-regulate steroid production and StAR gene expression and indicates that both pathways are required for trophic hormone-stimulated steroidogenesis.