Regulation of arachidonic acid release in steroidogenesis: role of a new acyl-CoA thioestrase (ARTISt)

Phospholipid and LC-PUFA metabolism in Atlantic salmon (Salmo salar) testes during sexual maturation

The importance of dietary lipids in male reproduction are not as well understood as in females, in which dietary lipids, such as phospholipids (PL) and associated fatty acids (FA), are important structural components of the eggs and provide energy for their offspring. In mammals, lipids are suggested to be important for spermatogenesis and to structural components of the spermatozoa that could improve fertilization rates. New knowledge of how lipids affect sexual maturation in male Atlantic salmon (Salmo salar), an important global aquaculture species, could provide tools to delay maturation and/or improve reproductive success. Therefore, changes in testicular composition of lipids and gene transcripts associated with spermatogenesis and lipid metabolism were studied in sexually maturing male salmon compared to immature males and females. An increase in total testis content of FA and PL, and a shift to higher PL composition was observed in maturing males, concomitant with increases in mRNA levels for genes involved in spermatogenesis, FA uptake and synthesis, and production of long chain-polyunsaturated fatty acids (LC-PUFA) and PL. A particularly interesting finding was elevated testis expression of acyl-CoA synthetase 4 (acsl4), and acyl-CoA thioesterase 2 (acot2), critical enzymes that regulate intra-mitochondrial levels of 20:4n-6 FA (arachidonic acid), which have been associated with improved cholesterol transport during steroidogenesis. This suggested that FA may have direct effects on sex steroid production in salmon. Furthermore, we observed increased testis expression of genes for endogenous synthesis of 16:0 and elongation/desaturation to 22:6n-3 (docosahexaenoic acid) in sexually maturing males relative to immature fish. Both of these FA are important structural components of the PL, phosphatidylcholine (PC), and were elevated concomitant with increases in the content of phosphatidic acid, an important precursor for PC, in maturing males compared to immature fish. Overall, this study suggests that, similar to mammals, lipids are important to spermatogenesis and serve as structural components during testicular growth and maturation in Atlantic salmon.

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.

Modulation of adrenocorticotrophin hormone (ACTH)-induced expression of stress-related genes by PUFA in inter-renal cells from European sea bass (Dicentrarchus labrax)

Dietary fatty acids have been shown to exert a clear effect on the stress response, modulating the release of cortisol. The role of fatty acids on the expression of steroidogenic genes has been described in mammals, but little is known in fish. The effect of different fatty acids on the release of cortisol and expression of stress-related genes of European sea bass (Dicentrarchus labrax) head kidney, induced by a pulse of adenocorticotrophin hormone (ACTH), was studied. Tissue was maintained in superfusion with 60 min of incubation with EPA, DHA, arachidonic acid (ARA), linoleic acid or α-linolenic acid (ALA) during 490 min. Cortisol was measured by RIA. The quantification of stress-related genes transcripts was conducted by One-Step TaqMan real-time RT-PCR. There was an effect of the type of fatty acid on the ACTH-induced release of cortisol, values from ALA treatment being elevated within all of the experimental period. The expression of some steroidogenic genes, such as the steroidogenic acute regulatory protein (StAR) and c-fos, were affected by fatty acids, ALA increasing the expression of StAR after 1 h of ACTH stimulation whereas DHA, ARA and ALA increased the expression of c-fos after 20 min. ARA increased expression of the 11β-hydroxylase gene. Expression of heat shock protein 70 (HSP70) was increased in all the experimental treatments except for ARA. Results corroborate previous studies of the effect of different fatty acids on the release of cortisol in marine fish and demonstrate that those effects are mediated by alteration of the expression of steroidogenic genes.

New enzymes involved in the mechanism of action of epidermal growth factor in a clonal strain of Leydig tumor cells

The studies presented herein were designed to investigate the effect of mouse epidermal growth factor (mEGF) on arachidonic acid (AA) release in a clonal strain of cultured murine Leydig cells (designed MA-10). In MA-10 cells, mEGF promotes AA release and metabolism to lipoxygenated products to induce the steroidogenic acute regulatory (StAR) protein. However, the mechanism by which mEGF releases AA in these cells is not totally elucidated. We show that mEGF produces an increment in the mitochondrial AA content in a short-term incubation (30 min). This AA is released by the action of a mitochondrial acyl-CoA thioesterase (Acot2), as demonstrated in experiments in which Acot2 was down or overexpressed. This AA in turn regulates the StAR protein expression, indirect evidence of its metabolism to lipoxygenated products. We also show that mEGF induces the expression (mRNA and protein) of Acot2 and an acyl-CoA synthetase that provides the substrate, arachidonyl-CoA, to Acot2. This effect is also observed in another steroidogenic cell line, the adrenocortical Y1 cells. Taken together, our results show that: 1) mEGF can induce the generation of AA in a specific compartment of the cells, i.e. the mitochondria; 2) mEGF can up-regulate acyl-CoA synthetase and Acot2 mRNA and protein levels; and 3) mEGF-stimulated intramitochondrial AA release leads to StAR protein induction.

Enzymes involved in arachidonic acid release in adrenal and Leydig cells

Stimulation of receptors and subsequent signal transduction results in the activation of arachidonic acid (AA) release. Once AA is released from phospholipids or others esters, it may be metabolized via the cycloxygenase or the lipoxygenase pathways. How the cells drive AA to these pathways is not elucidated yet. It is reasonable to speculate that each pathway will have different sources of free AA triggered by different signal transduction pathways. Several reports have shown that AA and its lipoxygenase-catalyzed metabolites play essential roles in the regulation of steroidogenesis by influencing cholesterol transport from the outer to the inner mitochondrial membrane, the rate-limiting step in steroid hormone biosynthesis. Signals that stimulate steroidogenesis also cause the release of AA from phospholipids or other esters by mechanisms that are not fully understood. This review focuses on the enzymes of AA release that impact on steroidogenesis.

Protein tyrosine phosphatases regulate arachidonic acid release, StAR induction and steroidogenesis acting on a hormone-dependent arachidonic acid-preferring acyl-CoA synthetase

The activation of the rate-limiting step in steroid biosynthesis, that is the transport of cholesterol into the mitochondria, is dependent on PKA-mediated events triggered by hormones like ACTH and LH. Two of such events are the protein tyrosine dephosphorylation mediated by protein tyrosine phosphatases (PTPs) and the release of arachidonic acid (AA) mediated by two enzymes, ACS4 (acyl-CoA synthetase 4) and Acot2 (mitochondrial thioesterase). ACTH and LH regulate the activity of PTPs and Acot2 and promote the induction of ACS4. Here we analyzed the involvement of PTPs on the expression of ACS4. We found that two PTP inhibitors, acting through different mechanisms, are both able to abrogate the hormonal effect on ACS4 induction. PTP inhibitors also reduce the effect of cAMP on steroidogenesis and on the level of StAR protein, which facilitates the access of cholesterol into the mitochondria. Moreover, our results indicate that exogenous AA is able to overcome the inhibition produced by PTP inhibitors on StAR protein level and steroidogenesis. Then, here we describe a link between PTP activity and AA release, since ACS4 induction is under the control of PTP activity, being a key event for AA release, StAR induction and steroidogenesis.

Sterol Regulatory Element-Binding Protein-2 modulates human brain acyl-CoA hydrolase gene transcription

The brain shows high catalyzing activity during hydrolysis of long-chain acyl-CoAs into fatty acids and CoA-SH. Brain acyl-CoA hydrolase (BACH) is responsible for most of the long-chain acyl-CoA hydrolyzing activity in the brain and is localized exclusively in neurons. We analyzed the human BACH gene promoter, focusing on transcriptional regulation by Sterol Regulatory Element-Binding Protein-2 (SREBP-2), which is a transcription factor that activates genes involved in cholesterol biosynthesis and uptake. When the nuclear form of SREBP-2 gene was transfected into human neuroblastoma cells, transcription of a BACH gene promoter-luciferase reporter gene was activated through a sterol regulatory element (SRE) motif. Moreover, a gel shift assay demonstrated that SREBP-2 specifically bound to the SRE motif. These results suggest that transcription of the BACH gene is activated by SREBP-2. This study also provides insights into BACH function in the interaction between the metabolism of acyl-CoAs and cholesterol in neurons.

beta-Adrenergic stimulation controls the expression of a thioesterase specific for very-long-chain fatty acids in perfused hearts

Arachidonic acid is not freely stored in the cells. A number of different pathways for the mobilization of this compound have been proposed, including a novel mechanism that involves the release of arachidonic acid from arachidonoyl-CoA by a thioesterase with substrate specificity for very-long-chain fatty acids. In rat heart, the acyl-CoA thioesterase activity can be regulated by a mechanism that involves beta-adrenoceptors. In this paper we demonstrate that beta-adrenergic agonists also regulate the acyl-CoA thioesterase mRNA levels. Isoproterenol (10(-7)M)-a concentration known to exert physiological responses-increases in a time-dependent manner the acyl-CoA thioesterase mRNA levels, an effect blocked by a specific beta-adrenoceptor antagonist. In addition, our results show that cAMP is involved in this process. The acyl-CoA thioesterase mRNA levels are also increased by fasting, but not by di(2-ethylhexyl)phthalate, a peroxisome proliferator. These results may suggest the existence of a beta-adrenoceptor-activated regulatory pathway for arachidonic acid release in cardiac tissue.

Concerted regulation of free arachidonic acid and hormone-induced steroid synthesis by acyl-CoA thioesterases and acyl-CoA synthetases in adrenal cells

Although the role of arachidonic acid (AA) in the regulation of steroidogenesis is well documented, the mechanism for AA release is not clear. Therefore, the aim of this study was to characterize the role of an acyl-CoA thioesterase (ARTISt) and an acyl-CoA synthetase as members of an alternative pathway in the regulation of the intracellular levels of AA in steroidogenesis. Purified recombinant ARTISt releases AA from arachidonoyl-CoA (AA-CoA) with a Km of 2 micro m. Antibodies raised against recombinant acyl-CoA thioesterase recognize the endogenous protein in both adrenal tissue and Y1 adrenal tumor cells by immunohistochemistry and immunocytochemistry and Western blot. Stimulation of Y1 cells with ACTH significantly stimulated endogenous mitochondrial thioesterases activity (1.8-fold). Nordihydroguaiaretic acid (NDGA), an inhibitor of AA release known to affect steroidogenesis, affects the in vitro activity of recombinant ARTISt and also the endogenous mitochondrial acyl-CoA thioesterases. ACTH-stimulated steroid synthesis in Y1 cells was significantly inhibited by a synergistic effect of NDGA and triacsin C an inhibitor of the AA-CoA synthetase. The apparent IC50 for NDGA was reduced from 50 micro m to 25, 7.5 and 4.5 micro m in the presence of 0.1, 0.5 and 2 micro m triacsin C, respectively. Our results strongly support the existence of a new pathway of AA release that operates in the regulation of steroid synthesis in adrenal cells.

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.

Mitochondrial processing of newly synthesized steroidogenic acute regulatory protein (StAR), but not total StAR, mediates cholesterol transfer to cytochrome P450 side chain cleavage enzyme in adrenal cells

The metabolism of cholesterol by cytochrome P450 side chain cleavage enzyme is hormonally regulated in steroidogenic tissues via intramitochondrial cholesterol transport. The mediating steroidogenic acute regulatory protein (StAR) is synthesized as a 37-kDa (p37) precursor that is phosphorylated by protein kinase A and cleaved within the mitochondria to generate 30-kDa forms (p30, pp30). The effectiveness of modified recombinant StAR forms in COS-1 cells without mitochondrial import has led to a prevailing view that cholesterol transport is mediated by p37 StAR via activity on the outer mitochondrial membrane. The present study of the activation of cholesterol metabolism by bromo-cAMP in adrenal cells in relation to (35)S-StAR turnover indicates that targeting of pp30 to the inner membrane provides the dominant cholesterol transport mechanism. We show that 1) only newly synthesized StAR is functional, 2) phosphorylation and processing of p37 to pp30 occurs rapidly and stoichiometrically, 3) both steps are necessary for optimum transport, and 4) newly synthesized pp30 exhibits very high activity (400 molecules of cholesterol/StAR/min). Segregation of cAMP activation and synthesis of StAR from cholesterol metabolism showed that very low levels of newly synthesized StAR (1 fmol/min/10(6) cells) sustained activated cholesterol metabolism (0.4 pmol/min/10(6) cells, t(1/2) = 70 min) long after complete removal of p37 (t(1/2) = 5 min). This activity was highly sensitive to inhibition of processing by CCCP only until sufficient pp30 was formed. Maximum activation preceded bromo-cAMP-induced StAR expression, indicating other limiting steps in cholesterol metabolism.