A novel arachidonic acid-related thioesterase involved in acute steroidogenesis

Role of Protein Phosphorylation and Tyrosine Phosphatases in the Adrenal Regulation of Steroid Synthesis and Mitochondrial Function

In adrenocortical cells, adrenocorticotropin (ACTH) promotes the activation of several protein kinases. The action of these kinases is linked to steroid production, mainly through steroidogenic acute regulatory protein (StAR), whose expression and activity are dependent on protein phosphorylation events at genomic and non-genomic levels. Hormone-dependent mitochondrial dynamics and cell proliferation are functions also associated with protein kinases. On the other hand, protein tyrosine dephosphorylation is an additional component of the ACTH signaling pathway, which involves the "classical" protein tyrosine phosphatases (PTPs), such as Src homology domain (SH) 2-containing PTP (SHP2c), and members of the MAP kinase phosphatase (MKP) family, such as MKP-1. PTPs are rapidly activated by posttranslational mechanisms and participate in hormone-stimulated steroid production. In this process, the SHP2 tyrosine phosphatase plays a crucial role in a mechanism that includes an acyl-CoA synthetase-4 (Acsl4), arachidonic acid (AA) release and StAR induction. In contrast, MKPs in steroidogenic cells have a role in the turn-off of the hormonal signal in ERK-dependent processes such as steroid synthesis and, perhaps, cell proliferation. This review analyzes the participation of these tyrosine phosphates in the ACTH signaling pathway and the action of kinases and phosphatases in the regulation of mitochondrial dynamics and steroid production. In addition, the participation of kinases and phosphatases in the signal cascade triggered by different stimuli in other steroidogenic tissues is also compared to adrenocortical cell/ACTH and discussed.

Rodent StAR mRNA is substantially regulated by control of mRNA stability through sites in the 3'-untranslated region and through coupling to ongoing transcription

The steroidogenic acute regulator (StAR) gene is transcribed to 1.6 kb and 3.5 kb mRNAs that differ only through the length of the 3'-untranslated region (3'-UTR). These forms result from alternative polyadenylation sites in exon 7. These sites are utilized similarly in unstimulated adrenal cells whereas Br-cAMP selectively stimulates 3.5 kb mRNA. After removal of Br-cAMP, 3.5 kb mRNA declines rapidly (t(1/2) = 2 h) while 1.6 kb mRNA responds more slowly. This selective degradation is more evident in testis MA10 cells and is seen even in the presence of Br-cAMP. Transfection of Y-1 cells with CMV promoted StAR vectors confirmed that the 3.5 kb form is less stable and that Br-cAMP slowly increases this instability. Basal instability resides solely in the extended 3'-UTR which contains AU-rich elements. Br-cAMP enhances this degradation of 3.5 kb mRNA but additionally requires translated and 5'-UTR sequences. Degradation of both forms is arrested by inhibitors of transcription or translation, indicating that mRNA stability is coupled to these processes independent of the extended 3'-UTR. Br-cAMP stimulates substantial selective synthesis of 3.5 kb StAR mRNA despite this instability. The preferential generation of the unstable form may facilitate rapid increases and decreases of StAR activity in response to external changes.

The role Acyl-CoA thioesterases play in mediating intracellular lipid metabolism

Acyl-CoA thioesterases are a group of enzymes that catalyze the hydrolysis of acyl-CoAs to the free fatty acid and coenzyme A (CoASH), providing the potential to regulate intracellular levels of acyl-CoAs, free fatty acids and CoASH. These enzymes are localized in almost all cellular compartments such as endoplasmic reticulum, cytosol, mitochondria and peroxisomes. Acyl-CoA thioesterases are highly regulated by peroxisome proliferator-activated receptors (PPARs), and other nutritional factors, which has led to the conclusion that they are involved in lipid metabolism. Although the physiological functions for these enzymes are not yet fully understood, recent cloning and more in-depth characterization of acyl-CoA thioesterases has assisted in discussion of putative functions for specific enzymes. Here we review the acyl-CoA thioesterases characterized to date and also address the diverse putative functions for these enzymes, such as in ligand supply for nuclear receptors, and regulation and termination of fatty acid oxidation in mitochondria and peroxisomes.

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.

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.

Concentration-dependent, biphasic effect of prostaglandins on avian corticosteroidogenesis in vitro

Previous work with mammalian and frog adrenocortical tissue and cells indicates that prostaglandins (PGs) can directly stimulate corticosteroidogenesis. However, work with avian adrenal preparations is absent. Therefore, the present studies with isolated chicken (Gallus gallus domesticus) and turkey (Meleagris gallopavo) adrenal steroidogenic cells were conducted to determine whether PGs can directly influence avian corticosteroidogenesis as well. Cells (1 x 10(5) cells/ml) were incubated with a wide range of concentrations of PGs in the presence of indomethacin (1 microg/ml) (to attenuate endogenous PG production) and 1-methyl-3-isobutylxanthine (0.5 mM) [to preserve cyclic AMP (cAMP)] for 2 h. Corticosterone and cAMP production were measured by highly specific radioimmunoassay. PGI2 was without effect. With the exception of PGF2alpha, which had a slight stimulation in chicken but not in turkey cells, the influence of the other PGs on corticosterone production was biphasic. For the stimulatory phase (up to a concentration of 5 x 10(-5) M), there were prostanoid structural and avian species differences in both potency and efficacy of PGs. Overall, PGs were 11 times more potent in turkey cells than in chicken cells. However, the order of potency for stimulation was similar for both chicken and turkey cells: for chicken cells the order was PGE2 > PGE1 > PGA1 > PGB2 > PGB1 > PGF2alpha and for turkey cells it was PGE2 > PGE1 > PGA1 > PGB2 = PGB1. In contrast, PG efficacy for stimulation was greater for chicken cells. In addition, the orders of efficacy were different from the orders of potency. In chicken cells, the order of efficacy was PGE2 = PGA1 > PGE1 > PGB2 > PGB1 > PGF2alpha and for turkey cells it was PGB2 = PGE2 > PGA1 > PGE1 > PGB1. Because of the greater maximal corticosterone response over basal production of chicken cells to PGs, they were used to assess the interaction of PGs with ACTH and to examine more fully the inhibitory phase of PGs. Cells were incubated with PGs in the presence of threshold (2.5 x 10(-11) M), half-maximal (1 x 10(-10) M), and maximal (1 x 10(-7) M) steroidogenic concentrations of ACTH. With the exception of PGF2alpha, the average efficacy of PGs to elevate corticosterone was increased 55% by a threshold steroidogenic concentration of ACTH. However, with higher concentrations of ACTH, this enhancement of efficacy disappeared as did the stimulatory effect of some PGs. The results suggest that the steroidogenic actions of PGs and ACTH converge on the same pool of steroidogenic enzymes leading to corticosterone. At concentrations greater than 5 x 10(-5) M, several PGs (notably PGA1, PGA2, PGB1, and PGB2) inhibited both ACTH-induced and basal corticosterone production. PGA1 and PGA2 were the most potent inhibitors. Corticosterone and cAMP production were closely associated in the biphasic action of PGs, suggesting that the effect of PGs was mediated by the changing levels of intracellular cAMP. Collectively, these data suggest that PGs may be important modulators of corticosteroidogenesis in the avian adrenal gland.