The role of arachidonic acid on LH-stimulated steroidogenesis and steroidogenic acute regulatory protein accumulation in MA-10 mouse Leydig tumor cells

Role of Omega-3 Fatty Acids in Improving Metabolic Dysfunctions in Polycystic Ovary Syndrome

Polycystic ovary syndrome (PCOS) is a common endocrine disorder that impacts women of reproductive age. In addition to reproductive and psychological complications, women with PCOS are also at a higher risk of developing metabolic diseases such as obesity, type 2 diabetes and cardiovascular disease. While weight reduction can help manage these complications in overweight or obese women, many weight loss interventions have been ineffective due to weight stigma and its psychological impact on women with PCOS. Therefore, exploring alternative dietary strategies which do not focus on weight loss per se is of importance. In this regard, omega-3 polyunsaturated fatty acids of marine origin (n-3 PUFAs), which are known for their hypotriglyceridemic, cardioprotective and anti-inflammatory effects, have emerged as a potential therapy for prevention and reversal of metabolic complications in PCOS. Several clinical trials showed that n-3 PUFAs can improve components of metabolic syndrome in women with PCOS. In this review, we first summarize the available clinical evidence for different dietary patterns in improving PCOS complications. Next, we summarize the clinical evidence for n-3 PUFAs for alleviating metabolic complications in PCOS. Finally, we explore the mechanisms by which n-3 PUFAs improve the metabolic disorders in PCOS in depth.

Effect of dietary n-3 polyunsaturated fatty acid supplementation on the expression of genes involved in progesterone biosynthesis in the corpus luteum of goat (Capra hircus)

Emerging evidence indicates that dietary n-3 polyunsaturated fatty acids (PUFA) alter the fatty acid composition of corpus luteum (CL) and directly affect the luteal function in the cow, which is independent of the inhibitory effect on the endometrial PGF_2α production. The present study, thus, investigated the effects of n-3 PUFA rich fish oil (FO) supplementation on the transcriptional modulation of genes involved in the biosynthesis of progesterone (P₄ ) in the CL collected during the luteolytic phase of oestrous cycle in the goat. On the day of synchronized oestrus, goats (n = 6/group) were fed an isocaloric diet supplemented with either FO or palm oil (PO). The dose of oil supplementation was 0.6 mlkg^-1 body weight, and the duration was 55-57 days. The FO provided 156 mgkg^-1 body weight of n-3 eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The CL was collected by laparotomy on day 16 post-oestrus, and the relative abundance of P450 side-chain cleaving enzyme, steroid acute regulatory protein (StAR) and 3β-hydroxy steroid dehydrogenase (3β-HSD) genes was quantitated by real-time PCR. The results indicated that the dietary FO significantly upregulated the expression of 3β-HSD by 1.13-fold and downregulated StAR by ~2-fold as compared to PO group (p < .05). It is concluded that dietary FO differently affected the expression of genes involved in P₄ synthesis in the CL during the luteolytic window of the oestrous cycle in the goat.

Lambda-cyhalothrin delays pubertal Leydig cell development in rats

Lambda-cyhalothrin (LCT) is a widely used broad-spectrum pyrethroid insecticide and is expected to cause deleterious effects on the male reproductive system. However, the effects of LCT on Leydig cell development during puberty are unclear. The current study addressed these effects. Twenty-eight-day-old male Sprague Dawley rats orally received LCT (0, 0.25, 0.5 or 1 mg/kg body weight/day) for 30 days. The levels of serum testosterone, luteinizing hormone, and follicle-stimulating hormone, Leydig cell number, and its specific gene and protein expression were determined. LCT exposure lowered serum testosterone levels at doses of 0.5 and 1 mg/kg and luteinizing hormone levels at a dose of 1 mg/kg, but increased follicle-stimulating hormone levels at doses of 0.5 and 1 mg/kg. LCT lowered Star and Hsd3b1 mRNA or their protein levels at a dose of 1 mg/kg. Immature Leydig cells were purified from pubertal rats and treated with different concentrations of LCT for 24 h and medium androgen levels, Leydig cell mRNA and protein levels, the mitochondrial membrane potential (△Ψm), and the apoptotic rate of immature Leydig cells were investigated. LCT inhibited androgen production at 5 μM and downregulated Scarb1 at 0.05 μM, Hsd3b1 and Hsd11b1 at 0.5 μM, and Cyp11a1 at 5 μM. LCT also decreased △Ψm at 0.5 and 50 μM. In conclusion, LCT can influence the function of Leydig cells.

Involvement of the thromboxane A2 receptor in the regulation of steroidogenic acute regulatory gene expression in murine Leydig cells

Recent studies suggested an involvement of thromboxane A2 in cyclooxygenase-2-dependent inhibition of steroidogenic acute regulatory (StAR) gene expression. The present study further investigated the role of thromboxane A2 receptor in StAR gene expression and steroidogenesis in testicular Leydig cells. The thromboxane A2 receptor was detected in several Leydig cell lines. Blocking thromboxane A2 binding to the receptor using specific antagonist SQ29548 or BM567 resulted in dose-dependent increases in StAR protein and steroid production in MA-10 mouse Leydig cells. The results were confirmed with Leydig cells isolated from rats. StAR promoter activity and StAR mRNA level in the cells were also increased after the treatments, suggesting an involvement of the thromboxane A2 receptor in StAR gene transcription. Furthermore study indicated that blocking the thromboxane A2 receptor reduced dosage sensitive sex reversal-adrenal hypoplasia congenita critical region on the X chromosome, gene 1 protein, a transcriptional repressor of StAR gene expression. Specific binding of the antagonists to the receptors on cellular membrane was demonstrated by binding assays using (3)H-SQ29548 and binding competition between (3)H-SQ29548 and BM567. Whereas SQ29548 enhanced cAMP-induced StAR gene expression, in the absence of cAMP, it was unable to increase StAR protein and steroidogenesis. However, when the receptor was blocked by the antagonist, subthreshold levels of cAMP were able to induce maximal levels of StAR protein expression, suggesting that blocking the thromboxane A2 receptor increase sensitivity of MA-10 cells to cAMP stimulation. Taken together, the results from the present and previous studies suggest an autocrine loop, involving cyclooxygenase-2, thromboxane A synthase, and thromboxane A2 and its receptor, in cyclooxygenase-2-dependent inhibition of StAR gene expression.

The orphan nuclear receptor NUR77 regulates hormone-induced StAR transcription in Leydig cells through cooperation with Ca2+/calmodulin-dependent protein kinase I

Cholesterol transport in the mitochondrial membrane, an essential step of steroid biosynthesis, is mediated by a protein complex containing the steroidogenic acute regulatory (StAR) protein. The importance of this transporter is underscored by mutations in the human StAR gene that cause lipoid congenital adrenal hyperplasia, male pseudohermaphroditism, and adrenal insufficiency. StAR transcription in steroidogenic cells is hormonally regulated and involves several transcription factors. The nuclear receptor NUR77 is present in steroidogenic cells, and its expression is induced by hormones known to activate StAR expression. We have now established that StAR transcription in cAMP-stimulated Leydig cells requires de novo protein synthesis and involves NUR77. We found that cAMP-induced NUR77 expression precedes that of StAR both at the mRNA and protein levels in Leydig cells. In these cells, small interfering RNA-mediated NUR77 knockdown reduces cAMP-induced StAR expression. Chromatin immunoprecipitation assays revealed a cAMP-dependent increase in NUR77 recruitment to the proximal StAR promoter, whereas transient transfections in MA-10 Leydig cells confirmed that NUR77 can activate the StAR promoter and that this requires an element located at -95 bp. cAMP-induced StAR and NUR77 expression in Leydig cells was found to require a Ca2+/calmodulin-dependent protein kinase (CaMK)-dependent signaling pathway. Consistent with this, we show that within the testis, CaMKI is specifically expressed in Leydig cells. Finally, we report that CaMKI transcriptionally cooperates with NUR77, but not steroidogenic factor 1, to further enhance StAR promoter activity in Leydig cells. All together, our results implicate NUR77 as a mediator of cAMP action on StAR transcription in steroidogenic Leydig cells and identify a role for CaMKI in this process.

Expression and regulation of the SCD2 desaturase in the rat ovary

Despite the significant role of the lipid reserve in cell structure and function, very few studies have provided detailed descriptions of unsaturated fatty acid synthesis in the ovary. In the present study, we have shown by RT-PCR, Northern blot, and Western blot analyses the mRNA and protein expression of SCD2 (stearoyl-coenzyme A desaturase 2; also named delta 9 desaturase) in rat ovary. We also have localized Scd2 mRNA by in situ hybridization, mainly in granulosa cells of antral follicles, cumulus oophorus, and corpus luteum. Interestingly, either no or very weak SCD2 expression was observed in primordial follicles and oocytes. After eCG injection for 24 h in immature rats (age, 22 days), the level of SCD2 expression and SCD activity in ovary was increased by approximately fourfold (P < 0.05), and the response was further increased 48 h after hCG treatment. As expected, eCG/hCG treatment increased expression of the steroidogenesis enzymes (CYP11A1 and HSD3B) and STAR. We also found a decrease in the SCD2 expression and SCD activity in the corpus luteum at Days 10 and 15 compared to Day 3 of gestation, paralleled by a decrease in the expression of the steroidogenesis enzymes and STAR. To investigate the molecular mechanisms involved in the regulation of SCD2 expression in ovary, we performed primary culture of rat granulosa cells. We observed that both insulin-like growth factor 1 (IGF1) (7.5 x 10(-8)g/ml) and FSH (350 x 10(-8)g/ml) increased SCD2 expression and SCD activity by approximately threefold. Using specific inhibitors, we demonstrated that the MAPK3/MAP1 and PIK3R1/AKT pathways are involved in the IGF1- and FSH-induced SCD2 expression, respectively. The SCD2 is expressed and active in rat ovary, and it may be involved in the regulation of follicular growth and/or the oocyte maturation.

Effect of ethanol on follicle stimulating hormone-induced steroidogenic acute regulatory protein (StAR) in cultured rat granulosa cells

Steroidogenic acute regulatory protein (StAR) plays a critical role in trophic hormone-stimulated steroid biosynthesis by facilitating the transfer of cholesterol across the mitochondrial membrane, where the cytochrome P450scc enzyme resides to initiate steroid hormone biosynthesis. Because follicle stimulating hormone (FSH) is a critically important regulator of estradiol (E2) synthesis in granulosa cells and because ethanol is known to suppress gonadotropin-stimulated ovarian steroidogenesis, we evaluated the effects of ethanol on FSH-stimulated StAR in ovarian granulosa cells. Granulosa cells from immature rats pretreated with pregnant mare serum gonadotropin were cultured for 24 h in serum-free medium, either alone (medium only) or with FSH (25 ng/ml) in the presence or absence of ethanol (50 mM). Real-time polymerase chain reaction (PCR) analysis showed increased (p < 0.01) expression of the StAR transcript in FSH-treated cells, when compared with cells that received medium only. The FSH stimulation of StAR transcript was blocked (p < 0.01) by the presence of ethanol. This effect coincided with a decrease in E2 secretion into the culture medium. We also examined whether ethanol could affect the production of cyclic AMP (cAMP), the main second messenger that mediates gonadotropin action within the ovary. FSH treatment of granulosa cells markedly increased (p < 0.001) cAMP levels, an effect that was not altered by ethanol. Importantly, FSH induced an increase (p < 0.01) in the release of prostaglandin E2 (PGE2), an effect that was blocked by ethanol. Real-time PCR analysis showed that ethanol had no effect on the expression of cyclooxygenase-1 (COX-1), but blocked (p < 0.01) FSH-stimulated expression of COX-2. These results demonstrate that ethanol is capable of inhibiting FSH-induced ovarian StAR and thus, contributing to suppressed E2 secretion, at least in part, through an inhibitory action on the COX-2-PGE2 pathway.

Dose-response modeling in reproductive toxicology in the systems biology era

Systems biology approaches for modeling cellular signaling networks affected by chemical exposures should soon produce integrated methodologies capable of predicting dose-response relationships for developmental toxicants and for other toxic responses. This paper outlines an emerging strategy for systems biology approaches in dose-response modeling. Genome-wide functional screens, bioinformatic tools, and network mapping technologies together can provide directed graph representations of the cellular signaling networks. The graphical representations can be converted into mathematical models that permit predicting the shapes of dose-response curves for altered cell signaling by test compounds during development. Systems biology approaches require interdisciplinary teams with expertise in reproduction, cell biology, signal transduction, mathematical/biomedical modeling, and risk assessment. In addition to outlining a systems approach for dose-response research, this paper discusses initial stages of application of this strategy to examine inhibition of steroidogenesis in testes by phthalate esters.

Involvement of protein kinase C and cyclic adenosine 3',5'-monophosphate-dependent kinase in steroidogenic acute regulatory protein expression and steroid biosynthesis in Leydig cells

This study investigated the roles of the protein kinase C (PKC) and protein kinase A (PKA) pathways in regulating constitutive steroidogenesis and steroidogenic acute regulatory (STAR; herein designated by its common name, StAR) protein in R2C Leydig tumor cells. Inhibition of PKC and phospholipase C resulted in significant decreases in steroid production, phosphorylation of cAMP-responsive element binding (CREB) protein, and Star gene transcription under basal conditions in R2C cells. These observations were corroborated in MA-10 and mLTC-1 Leydig tumor cell lines, in which activation of PKC by phorbol-12-myristate-13-acetate (PMA, 10 nM) increased CREB phosphorylation and total StAR (tot-StAR) protein expression. However, induction of StAR protein by PMA did not result in the expected concomitant increase in steroids because PKC failed to phosphorylate StAR, the biologically active form of the protein. However, in conjunction with PMA, minor increases in PKA activity using submaximal doses of (Bu)2cAMP (0.05-0.1 mM; a concentration range insufficient for induction of StAR), were able to stimulate dramatic increases in both phospho-StAR (P-StAR) and steroid production. Human chorionic gonadotropin stimulation also resulted in a further enhancement in P-StAR and progesterone production when added to PMA-treated MA-10 cells. Similar results for tot-StAR and P-StAR expression were observed in primary cultures of immature rat Leydig cells treated with PMA and submaximal doses of (Bu)2cAMP. In summary, the present study demonstrates that basal activities of both PKC and PKA play important roles in the constitutive steroidogenic characteristics of R2C cells. This study also demonstrates for the first time a role for PMA-induced PKC in StAR protein regulation and the requirement for submaximal doses of cAMP to produce steroids in Leydig cells.

Inhibition of cyclooxygenase-2 activity enhances steroidogenesis and steroidogenic acute regulatory gene expression in MA-10 mouse Leydig cells

To study the mechanism for the regulatory effect of arachidonic acid (AA) on steroidogenesis, the role of cyclooxygenase (COX) in steroid production and steroidogenic acute regulatory (StAR) gene expression was investigated. Although stimulation with 0.05 mM dibutyryl cAMP (Bt(2)cAMP) did not increase StAR protein or progesterone in MA-10 mouse Leydig cells, the addition of 1 microM of the COX inhibitor indomethacin increased StAR protein expression and progesterone production by 5.7-fold and 34.3-fold, respectively. In the presence of indomethacin, the level of Bt(2)cAMP required for maximal steroidogenesis was reduced from 1.0 mM to 0.25 mM. Similar results were obtained in studies on StAR promoter activity and in Northern blot analyses of StAR mRNA expression, suggesting that inhibition of COX activity enhanced StAR gene transcription. COX2 (an inducible isoform of COX) was constitutively detected in MA-10 cells. Although SC560, a selective COX1 inhibitor, did not affect steroidogenesis, the COX2 inhibitor NS398 significantly enhanced Bt(2)cAMP-stimulated StAR protein expression and steroid production. Overexpression of the COX2 gene in COS-1 cells significantly inhibited StAR promoter activity. The results of the present study suggest that inhibition of COX2 activity increases the sensitivity of steroidogenesis to cAMP stimulation in MA-10 Leydig cells.

Involvement of 5-lipoxygenase metabolites of arachidonic acid in cyclic AMP-stimulated steroidogenesis and steroidogenic acute regulatory protein gene expression

To understand the mechanism for the role of arachidonic acid (AA) in steroidogenic acute regulatory (StAR) gene transcription, sections of the -1/-966 StAR promoter were deleted to produce constructs of -1/-426, -1/-211, -1/-151, and -1/-110 and inserted into the PGL3 vector to drive luciferase expression. Results indicated that -1/-151 StAR promoter contains the elements that are most responsive to AA. Electrophoretic mobility shift assays using nuclear extracts from AA-treated MA-10 Leydig tumor cells showed that AA enhanced specific binding of the nuclear extract to a 30bp (-67/-96) sequence of the StAR promoter. Also, HPLC was used to identify AA metabolites involved in StAR gene transcription. It was found that 1mM N6,2-O-dibutyryladenosine 3:5-cyclic monophosphate (dbcAMP) significantly increased the 5-lipoxygenase metabolites, 5-hydroperoxyeicosatetraenoic acid (5-HPETE) and 5-hydroxyeicosatetraenoic acid (5-HETE). Moreover, in the presence of 0.2mM dbcAMP addition of 20 microM 5-HPETE or 5-HETE significantly enhanced StAR protein expression and progesterone production (P<0.05). Similar results were obtained for StAR gene transcription with StAR mRNA levels and StAR promoter activities being significantly increased (P<0.05) when 5-HPETE was added to MA-10 cell cultures. In summary, the present studies demonstrated that cyclic AMP (cAMP) stimulated the production of the AA metabolites, 5-HPETE and 5-HETE, and showed that these metabolites enhanced StAR gene expression and steroid hormone production. The results further suggested that the AA-responsive element resides in the -67/-96 region of the StAR promoter.

Interaction between arachidonic acid and cAMP signaling pathways enhances steroidogenesis and StAR gene expression in MA-10 Leydig tumor cells

Previous studies have demonstrated that trophic hormone stimulation induced cyclic AMP (cAMP) formation and arachidonic acid (AA) release from phospholipids and that both these compounds were required for steroid biosynthesis and steroidogenic acute regulatory (StAR) gene expression in MA-10 mouse Leydig tumor cells. The present study further investigates the synergistic effects of the AA and cAMP interaction on steroidogenesis. To demonstrate cAMP-induced AA release, MA-10 cells were pre-loaded with 3H-AA and subsequently treated with dibutyryl cyclic AMP (dbcAMP). Stimulation with dbcAMP significantly induced AA release in MA-10 cells to a level 145.7% higher than that of controls. Lowering intracellular cAMP concentration by expressing a cAMP-phosphodiesterase significantly reduced human chorionic gonadotrophin (hCG)-induced AA release. The dbcAMP-induced AA release was inhibited significantly by the phospholipase A(2) (PLA(2)) inhibitor dexamethasone (Dex) and also by the protein kinase A (PKA) inhibitor H89, suggesting the involvement of PKA phosphorylation and/or PLA(2) activation in cAMP-induced AA release. The effect of the interaction between AA and cAMP on StAR gene expression and steroid production was also investigated. While 0.2 mM dbcAMP induced only very low levels of StAR protein, StAR mRNA, StAR promoter activity and steroid production, all of these parameters increased dramatically as AA concentration in the culture medium was increased from 0 to 200 microM. Importantly, AA was not able to induce a significant increase in steroidogenesis at any concentration when used in the absence of dbcAMP. However, when used in concert with submaximal concentrations of dbcAMP (0.05 mm to 0.5 mm), AA was capable of stimulating StAR gene expression and increasing steroid production significantly. The results from this study demonstrate that AA and cAMP act in a highly synergistic manner to increase the sensitivity of steroid production to trophic hormone stimulation and probably do so by increasing StAR gene expression.

StAR protein and the regulation of steroid hormone biosynthesis

Steroid hormone biosynthesis is acutely regulated by pituitary trophic hormones and other steroidogenic stimuli. This regulation requires the synthesis of a protein whose function is to translocate cholesterol from the outer to the inner mitochondrial membrane in steroidogenic cells, the rate-limiting step in steroid hormone formation. The steroidogenic acute regulatory (StAR) protein is an indispensable component in this process and is the best candidate to fill the role of the putative regulator. StAR is expressed in steroidogenic tissues in response to agents that stimulate steroid production, and mutations in the StAR gene result in the disease congenital lipoid adrenal hyperplasia, in which steroid hormone biosynthesis is severely compromised. The StAR null mouse has a phenotype that is essentially identical to the human disease. The positive and negative expression of StAR is sensitive to agents that increase and inhibit steroid biosynthesis respectively. The mechanism by which StAR mediates cholesterol transfer in the mitochondria has not been fully characterized. However, the tertiary structure of the START domain of a StAR homolog has been solved, and identification of a cholesterol-binding hydrophobic tunnel within this domain raises the possibility that StAR acts as a cholesterol-shuttling protein.

Elements involved in the regulation of the StAR gene

The steroidogenic acute regulatory protein (StAR) mediates the transfer of cholesterol from the outer to the inner mitochondrial membrane, the regulated step in steroidogenesis. A most interesting facet of this protein is the manner in which its expression is acutely regulated. In this regard, a number of studies have concentrated on the search for consensus cis regulatory elements within its promoter, and, more importantly, on whether these elements are involved in its expression. This short review will summarize some of the findings that have been reported concerning the nature of how the expression of this gene is regulated.

Econazole and miconazole inhibit steroidogenesis and disrupt steroidogenic acute regulatory (StAR) protein expression post-transcriptionally

The imidazole antifungal drugs econazole and miconazole have previously been shown to disrupt steroidogenesis in Leydig and adrenal cells by inhibiting 17alpha-hydroxylase/17,20-lyase (P450c17) enzyme activity, thus reducing the conversion of progesterone to androstenedione. However, a recent study in Y-1 adrenal cells indicated that these compounds may also reduce the availability of cholesterol to the cytochrome P450 side chain cleavage (P450(scc)) enzyme, the first enzyme in the steroidogenic pathway. Since the steroidogenic acute regulatory protein (StAR) mediates the transfer of cholesterol from the outer to the inner mitochondrial membrane where the P450(scc) enzyme resides, an action which constitutes the rate-limiting and acutely-regulated step in steroidogenesis, we hypothesized that these drugs may also reduce StAR expression and/or activity. Our studies demonstrate that these drugs reversibly inhibited (Bu)(2)cAMP-stimulated progesterone production in a dose- and time-dependent manner in MA-10 cells without affecting total protein synthesis or P450(scc) and 3beta-hydroxysteroid dehydrogenase (3beta-HSD) enzyme expression or activity. In contrast, they dramatically decreased (Bu)(2)cAMP-stimulated StAR protein expression post-transcriptionally. This study indicates that StAR protein is susceptible to inhibition by at least some imidazole compounds that inhibit steroidogenesis.

Role of arachidonic acid metabolism in ACTH-stimulated cortisol secretion by bovine adrenocortical cells

We have studied the effects of inhibitors of arachidonic acid (AA) metabolism, nordihydroguaiaretic acid (NDGA), a lipoxygenase (LPX) inhibitor, and indomethacin (INDO), a cyclooxygenase (COX) inhibitor, on cortisol secretion and StAR protein in primary cultures of bovine adrenal zona fasciculata (ZF) cells. NDGA inhibited cortisol secretion in response to both 10(-12) M and 10(-8) M ACTH. AA (10(-4) M) partially reversed the inhibition of cortisol secretion by NDGA at 10(-12) M ACTH but not at 10(-8) M ACTH. On the other hand, INDO potentiated the cortisol response to 10(-12) M ACTH. Neither NDGA nor INDO significantly affected StAR protein levels. These results suggest a StAR protein-independent role for the LPX and COX pathways in acute cortisol secretion, and support the hypothesis that LPX products of AA metabolism are key cellular signals when bovine ZF cells are acutely stimulated by physiological concentrations of ACTH (10(-12) M).

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.

Cyclic AMP and arachidonic acid: a tale of two pathways

Increasing evidence in recent years has demonstrated the regulatory effects of arachidonic acid and its metabolites on steroid hormone production in various steroidogenic tissues. In trophic hormone-stimulated steroidogenesis, arachidonic acid is rapidly released from phospholipids. This release is dependent upon hormone-receptor interaction and inhibition of arachidonic acid release results in an inhibition of steroidogenesis. Several of the earlier studies indicated that arachidonic acid acts at the rate-limiting step of steroid biosynthesis, the transfer of substrate cholesterol to the inner mitochondrial membrane, but the manner in which this occurred was not clear. Recently it has been demonstrated that arachidonic acid release can participate in the regulation of gene expression of the steroidogenic acute regulatory (StAR) protein which mediates cholesterol transfer to the inner mitochondrial membrane. These studies suggest that this fatty acid may be instrumental in transducing a signal from trophic hormone/receptor interaction to the nucleus utilizing a pathway different from the reported cyclic AMP pathway. It is possible that these two pathways cooperate and serve to co-regulate transcription factors, resulting in StAR gene expression and subsequent steroid production. This hypothesis may serve to explain and co-ordinate previous observations on the roles of cyclic AMP (cAMP) and arachidonic acid in steroid hormone biosynthesis.