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, D ibutyryl cyclic AMP (Bt(2)cAMP)-stimulated MA-10 Leydig cells were treated with AA and/or the phospholipase A, inhibitor, dexamethasone, Dexamethasone significantly reduced Bt(2)cAMP-stimulated progesterone production, StAR p romoter activity, StAR mRNA, and StAR protein. The inhibitory effects of de xamethasone were reversed by the addition of 150 mu M AA to MA-10 cells. In addition, MA-10 cells were treated with the lipoxygenase inhibitor, nordih ydroguaiaretic acid (NDGA), the 5-lipoxygenase inhibitor, AA861, the epoxyg enase inhibitor, miconazole, and the cyclooxygenase inhibitor, indomethacin , Both NDGA and AA861 inhibited progesterone production and StAR protein ex pression. AA861-inhibited progesterone synthesis and StAR protein were part ially reversed by addition of the 5-lipoxygenase metabolite, 5(S)-hydropero xy-(6E,8Z,11Z, 14Z)-eicosatetraenoic acid. Inhibition of epoxygenase activi ty inhibited progesterone production significantly, but StAR protein was on ly slightly reduced. Indomethacin enhanced StAR protein expression and sign ificantly increased progesterone production. Inhibition of AA release or li poxygenase activities did not affect protein kinase A activity, whereas inh ibition of protein kinase A activity using H89 reduced Bt(2)cAMP-induced St AR protein. AA alone did not induce StAR protein expression nor steroid pro duction. These results demonstrate the essential role of AA in steroid bios ynthesis and StAR gene transcription and suggest the possible involvement o f the lipoxygenase pathway in steroidogenesis. This study further indicates that AA and cAMP transduce signals from trophic hormone receptors to the n ucleus through two separate pathways and act to co-regulate steroid product ion and StAR gene expression and indicates that both pathways are required for trophic hormone-stimulated steroidogenesis.