The differential regulation of steroidogenic acute regulatory protein-mediated steroidogenesis by type I and type II PKA in MA-10 cells

Disruptions to protein kinase A localization in adrenal pathology

Cell signaling fidelity requires specificity in protein-protein interactions and precise subcellular localization of signaling molecules. In the case of protein phosphorylation, many kinases and phosphatases exhibit promiscuous substrate pairing and therefore require targeting interactions to modify the appropriate substrates and avoid cross-talk among different pathways. In the past 10 years, researchers have discovered and investigated how loss of specific interactions and subcellular targeting for the protein kinase A catalytic subunit (PKAc) lead to cortisol-producing adenomas and the debilitating stress disorder adrenal Cushing's syndrome. This article reviews classical studies regarding PKA localization in glucocorticoid-producing adrenal cells and synthesizes recent evidence of disrupted PKA localization and selective regulatory interactions in adrenal pathology.

Advancing 3Rs: The Mouse Estrus Detector (MED) as a Low-Stress, Painless, and Efficient Tool for Estrus Determination in Mice

Determining the estrous cycle stages in mice is essential for optimizing breeding strategies, synchronizing experimental timelines, and facilitating studies in behavior, drug testing, and genetics. It is critical for reducing the production of genetically unmodified offspring in the generation and investigation of genetically modified animal models. An accurate detection of the estrus cycle is particularly relevant in the context of the 3Rs-Replacement, Reduction, and Refinement. The estrous cycle, encompassing the reproductive phases of mice, is key to refining experimental designs and addressing ethical issues related to the use of animals in research. This study presents results from two independent laboratories on the efficacy of the Mouse Estrus Detector (MED) from ELMI Ltd. (Latvia) for the accurate determination of the estrus phase. The female mice of five strains/stocks (CD1, FVB/N, C57Bl6/J, B6D2F1, and Swiss) were used. The results showed that the MEDProTM is a low-traumatic, simple, rapid, and painless method of estrus detection that supports the principles of the 3Rs. The use of the MEDProTM for estrus detection in mice caused minimal stress, enhanced mating efficiency, facilitated an increase in the number of embryos for in vitro fertilization, and allowed the production of the desired number of foster animals.

Regulation of retinoid mediated StAR transcription and steroidogenesis in hippocampal neuronal cells: Implications for StAR in protecting Alzheimer's disease

Retinoids (vitamin A and its derivatives) play pivotal roles in diverse processes, ranging from homeostasis to neurodegeneration, which are also influenced by steroid hormones. The rate-limiting step in steroid biosynthesis is mediated by the steroidogenic acute regulatory (StAR) protein. In the present study, we demonstrate that retinoids enhanced StAR expression and pregnenolone biosynthesis, and these parameters were markedly augmented by activation of the PKA pathway in mouse hippocampal neuronal HT22 cells. Deletion and mutational analyses of the 5'-flanking regions of the StAR gene revealed the importance of a retinoic acid receptor (RAR)/retinoid X receptor (RXR)-liver X receptor (LXR) heterodimeric motif at -200/-185 bp region in retinoid responsiveness. The RAR/RXR-LXR sequence motif can bind RARα and RXRα, and retinoid regulated transcription of the StAR gene was found to be influenced by the LXR pathway, representing signaling cross-talk in hippocampal neurosteroid biosynthesis. Steroidogenesis decreases during senescence due to declines in the central nervous system and the endocrine system, and results in hormone deficiencies, inferring the need for hormonal balance for healthy aging. Loss of neuronal cells, involving accumulation of amyloid beta (Aβ) and/or phosphorylated Tau within the brain, is the pathological hallmark of Alzheimer's disease (AD). HT22 cells overexpressing either mutant APP (mAPP) or mutant Tau (mTau), conditions mimetic to AD, enhanced toxicities, and resulted in attenuation of both basal and retinoid-responsive StAR and pregnenolone levels. Co-expression of StAR with either mAPP or mTau diminished cytotoxicity, and concomitantly elevated neurosteroid biosynthesis, pointing to a protective role of StAR in AD. These findings provide insights into the molecular events by which retinoid signaling upregulates StAR and steroid levels in hippocampal neuronal cells, and StAR, by rescuing mAPP and/or mTau-induced toxicities, modulates neurosteroidogenesis and restores hormonal balance, which may have important implications in protecting AD and age-related complications and diseases.

Apigenin Isolated from Carduus crispus Protects against H2O2-Induced Oxidative Damage and Spermatogenic Expression Changes in GC-2spd Sperm Cells

Testicular oxidative stress is one of the most common factors underlying male infertility. Welted thistle, Carduus crispus Linn., and its bioactive principles are attracting scientific interest in treating male reproductive dysfunctions. Here, the protective effects of apigenin isolated from C. crispus against oxidative damage induced by hydrogen peroxide (H2O2) and dysregulation in spermatogenesis associated parameters in testicular sperm cells was investigated. Cell viabilities, ROS scavenging effects, and spermatogenic associated molecular expressions were measured by MTT, DCF-DA, Western blotting and real-time RT-PCR, respectively. A single peak with 100% purity of apigenin was obtained in HPLC conditions. Apigenin treated alone (2.5, 5, 10 and 20 µM) did not exhibit cytotoxicity, but inhibited the H2O2-induced cellular damage and elevated ROS levels significantly (p < 0.05 at 5, 10 and 20 µM) and dose-dependently. Further, H2O2-induced down-regulation of antioxidant (glutathione S-transferases m5, glutathione peroxidase 4, and peroxiredoxin 3) and spermatogenesis-associated (nectin-2 and phosphorylated-cAMP response element-binding protein) molecular expression in GC-2spd cells were attenuated by apigenin at both protein and mRNA levels (p < 0.05). In conclusion, our study showed that apigenin isolated from C. crispus might be an effective agent that can protect ROS-induced testicular dysfunctions.

Transcriptional regulation of HIF1α-mediated STAR expression in murine KK1 granulosa cell line involves cJUN, CREB and CBP-dependent pathways

Gonadal function is connected to hypoxia, with hypoxia-inducible factor (HIF) 1α, as a component of HIF1-complexes, regulating cellular adaptation to hypoxic conditions. In the ovary, it regulates follicular maturation, ovulation and luteal development. At the cellular level, HIF1-complexes coordinate the expression of steroidogenic acute regulatory protein (STAR), and thereby ovarian steroidogenesis. The functionality of STAR is associated with the cAMP/PKA-dependent pathways. In vitro, HIF1α is required for basal and cAMP-induced STAR expression, under ambient and reduced oxygen (O₂) tension. Lowering O₂ increases the responsiveness of the Star promoter towards cAMP and PKA mediates activation/phosphorylation (P) of several transcriptional factors, including cJUN and cAMP response element-binding protein (CREB), whose functionality is linked to HIF1 through utilization of CREB-binding protein (CBP). Since the mechanisms underlying HIF1α-dependent expression of STAR remain unknown, we investigated the involvement of HIF1α in CREB-, cJUN- and CBP-mediated expression of STAR using a well-characterized steroidogenic model, murine KK1 granulosa cells; ambient and lowered (10%) O₂ were applied. Our main findings were that while functional suppression of the α-subunit of HIF1 lowered STAR/P-STAR and steroidogenic output from granulosa cells, surprisingly the levels of P-CREB and its transcriptional activity were strongly induced. However, its association with the Star promoter was decreased, indicating dissociation of P-CREB from the promoter. Further, suppression of HIF1 activity ultimately diminished the expression of cJUN/P-cJUN and CBP. Finally, the study suggests that HIF1-complex: (1) regulates cJUN expression in granulosa cells, (2) is involved in regulating the recruitment of P-CREB to the Star promoter in (3) a mechanism which possibly involves the HIF1-dependent regulation of CBP expression.

The Effects of Separate and Combined Treatment of Male Rats with Type 2 Diabetes with Metformin and Orthosteric and Allosteric Agonists of Luteinizing Hormone Receptor on Steroidogenesis and Spermatogenesis

In men with type 2 diabetes mellitus (T2DM), steroidogenesis and spermatogenesis are impaired. Metformin and the agonists of luteinizing hormone/human chorionic gonadotropin(hCG)-receptor (LH/hCG-R) (hCG, low-molecular-weight allosteric LH/hCG-R-agonists) can be used to restore them. The aim was to study effectiveness of separate and combined administration of metformin, hCG and 5-amino-N-tert-butyl-2-(methylsulfanyl)-4-(3-(nicotinamido)phenyl)thieno[2,3-d]pyrimidine-6-carboxamide (TP3) on steroidogenesis and spermatogenesis in male rats with T2DM. hCG (15 IU/rat/day) and TP3 (15 mg/kg/day) were injected in the last five days of five-week metformin treatment (120 mg/kg/day). Metformin improved testicular steroidogenesis and spermatogenesis and restored LH/hCG-R-expression. Compared to control, in T2DM, hCG stimulated steroidogenesis and StAR-gene expression less effectively and, after five-day administration, reduced LH/hCG-R-expression, while TP3 effects changed weaker. In co-administration of metformin and LH/hCG-R-agonists, on the first day, stimulating effects of LH/hCG-R-agonists on testosterone levels and hCG-stimulated expression of StAR- and CYP17A1-genes were increased, but on the 3-5th day, they disappeared. This was due to reduced LH/hCG-R-gene expression and increased aromatase-catalyzed estradiol production. With co-administration, LH/hCG-R-agonists did not contribute to improving spermatogenesis, induced by metformin. Thus, in T2DM, metformin and LH/hCG-R-agonists restore steroidogenesis and spermatogenesis, with metformin being more effective in restoring spermatogenesis, and their co-administration improves LH/hCG-R-agonist-stimulating testicular steroidogenesis in acute but not chronic administration.

Gαs-Protein Kinase A (PKA) Pathway Signalopathies: The Emerging Genetic Landscape and Therapeutic Potential of Human Diseases Driven by Aberrant Gαs-PKA Signaling

Many of the fundamental concepts of signal transduction and kinase activity are attributed to the discovery and crystallization of cAMP-dependent protein kinase, or protein kinase A. PKA is one of the best-studied kinases in human biology, with emphasis in biochemistry and biophysics, all the way to metabolism, hormone action, and gene expression regulation. It is surprising, however, that our understanding of PKA's role in disease is largely underappreciated. Although genetic mutations in the PKA holoenzyme are known to cause diseases such as Carney complex, Cushing syndrome, and acrodysostosis, the story largely stops there. With the recent explosion of genomic medicine, we can finally appreciate the broader role of the Gαs-PKA pathway in disease, with contributions from aberrant functioning G proteins and G protein-coupled receptors, as well as multiple alterations in other pathway components and negative regulators. Together, these represent a broad family of diseases we term the Gαs-PKA pathway signalopathies. The Gαs-PKA pathway signalopathies encompass diseases caused by germline, postzygotic, and somatic mutations in the Gαs-PKA pathway, with largely endocrine and neoplastic phenotypes. Here, we present a signaling-centric review of Gαs-PKA-driven pathophysiology and integrate computational and structural analysis to identify mutational themes commonly exploited by the Gαs-PKA pathway signalopathies. Major mutational themes include hotspot activating mutations in Gαs, encoded by GNAS, and mutations that destabilize the PKA holoenzyme. With this review, we hope to incite further study and ultimately the development of new therapeutic strategies in the treatment of a wide range of human diseases. SIGNIFICANCE STATEMENT: Little recognition is given to the causative role of Gαs-PKA pathway dysregulation in disease, with effects ranging from infectious disease, endocrine syndromes, and many cancers, yet these disparate diseases can all be understood by common genetic themes and biochemical signaling connections. By highlighting these common pathogenic mechanisms and bridging multiple disciplines, important progress can be made toward therapeutic advances in treating Gαs-PKA pathway-driven disease.

S-allyl Cysteine Enhances Testosterone Production in Mice and Mouse Testis-Derived I-10 Cells

Hypogonadism, associated with low levels of testosterone synthesis, has been implicated in several diseases. Recently, the quest for natural alternatives to prevent and treat hypogonadism has gained increasing research interest. To this end, the present study explored the effect of S-allyl cysteine (SAC), a characteristic organosulfur compound in aged-garlic extract, on testosterone production. SAC was administered at 50 mg/kg body weight intraperitoneally into 7-week-old BALB/c male mice in a single-dose experiment. Plasma levels of testosterone and luteinizing hormone (LH) and testis levels of proteins involved in steroidogenesis were measured by enzymatic immunoassay and Western blot, respectively. In addition, mouse testis-derived I-10 cells were also used to investigate the effect of SAC on steroidogenesis. In the animal experiment, SAC significantly elevated testosterone levels in both the plasma and the testis without changing the LH level in plasma and increased phosphorylated protein kinase A (p-PKA) levels. Similar results were also observed in I-10 cells. The findings demonstrating the increasing effect of SAC on p-PKA and mRNA levels of Cyp11a suggest that SAC increases the testosterone level by activating the PKA pathway and could be a potential target for hypogonadism therapeutics.

Low dose of fire retardant, 2,2',4,4'-tetrabromodiphenyl ether (BDE47), stimulates the proliferation and differentiation of progenitor Leydig cells of male rats during prepuberty

2,2',4,4'-Tetrabromodiphenyl ether (BDE47), a flame retardant, is extensively distributed in the food chain. However, whether BDE47 affects Leydig cell development during prepuberty remains unclear. BDE47 was daily gavaged to 21-day-old Sprague-Dawley male rats with 0 (corn oil), 0.1, 0.2, and 0.4 mg/kg for 14 days. BDE47 did not affect the body weight or testis weight of rats. It significantly increased serum testosterone level at 0.4 mg/kg, but decreased luteinizing hormone (LH) level without affecting estradiol level. BDE47 induced Leydig cell hyperplasia (the number of CYP11A1-positive Leydig cells increased), and up-regulated the expression of Scarb1, Star, Hsd11b1, Pcna, and Ccnd1 in the testis. BDE47 significantly reduced p53 and p21 levels but increased CCND1 level. It also markedly increased the phosphorylation of AKT1, AKT2, ERK1/2, and CREB. BDE47 significantly up-regulated the expression of Scarb1, Star, and Hsd11b1 and stimulated androgen production by immature Leydig cells from rats under basal, LH, and 8Br-cAMP stimulated conditions at 100 nM in vitro. In conclusion, BDE47 increased Leydig cell number and up-regulated the expression of Scarb1 and Star, thereby leading to increased testosterone synthesis.

Comparative Study of the Steroidogenic Effects of Human Chorionic Gonadotropin and Thieno[2,3-D]pyrimidine-Based Allosteric Agonist of Luteinizing Hormone Receptor in Young Adult, Aging and Diabetic Male Rats

Low-molecular-weight agonists of luteinizing hormone (LH)/human chorionic gonadotropin (hCG) receptor (LHCGR), which interact with LHCGR transmembrane allosteric site and, in comparison with gonadotropins, more selectively activate intracellular effectors, are currently being developed. Meanwhile, their effects on testicular steroidogenesis have not been studied. The purpose of this work is to perform a comparative study of the effects of 5-amino-N-tert-butyl-4-(3-(1-methylpyrazole-4-carboxamido)phenyl)-2-(methylthio)thieno[2,3-d] pyrimidine-6-carboxamide (TP4/2), a LHCGR allosteric agonist developed by us, and hCG on adenylyl cyclase activity in rat testicular membranes, testosterone levels, testicular steroidogenesis and spermatogenesis in young (four-month-old), aging (18-month-old) and diabetic male Wistar rats. Type 1 diabetes was caused by a single streptozotocin (50 mg/kg) injection. TP4/2 (20 mg/kg/day) and hCG (20 IU/rat/day) were administered for 5 days. TP4/2 was less effective in adenylyl cyclase stimulation and ability to activate steroidogenesis when administered once into rats. On the 3rd–5th day, TP4/2 and hCG steroidogenic effects in young adult, aging and diabetic rats were comparable. Unlike hCG, TP4/2 did not inhibit LHCGR gene expression and did not hyperstimulate the testicular steroidogenesis system, moderately increasing steroidogenic proteins gene expression and testosterone production. In aging and diabetic testes, TP4/2 improved spermatogenesis. Thus, during five-day administration, TP4/2 steadily stimulates testicular steroidogenesis, and can be used to prevent androgen deficiency in aging and diabetes.

Genome-wide estrogen receptor-α binding and action in human endometrial stromal cells

OBJECTIVE

To investigate the gene targets of estradiol (E2)-estrogen receptor-α (ESR1) in human endometrial stromal cells.

DESIGN

Basic science.

SETTING

University research center.

PATIENT(S)

Premenopausal women with or without endometriosis.

INTERVENTION(S)

Primary cultures of human endometrial stromal cells from healthy endometrium, with or without small-interfering RNA (siRNA) knockdown of ESR1 expression, were treated with E2 or vehicle control.

MAIN OUTCOME MEASURE(S)

Genome-wide RNA expression by RNA sequencing was compared in endometrial stromal cells with or without siRNA knockdown of ESR1 in the presence or absence of E2. Genome-wide recruitment of ESR1 to chromatin was assessed by chromatin immunoprecipitation sequencing. Gene expression by real-time qualitative polymerase chain reaction of a potential E2-ESR1 target gene was determined in endometrial stromal cells and endometriotic stromal cells.

RESULT(S)

We identified several important pathways that are dependent on E2-ESR1 signaling in endometrial stromal cells, including progesterone signaling, cell-matrix adhesion, and cytoskeleton rearrangement, as well as paracrine signaling by members of the fibroblast growth factor family. We detected a total of 709 ESR1 target sites on chromatin. By integrating data on genome-wide transcriptomic changes and E2-ESR1 binding sites, we identified inositol polyphosphate phosphatase type II (INPP4B) as a candidate E2-mediated suppressor of proliferation in healthy endometrial cells. INPP4B was downregulated in endometriosis-derived stromal cells.

CONCLUSION(S)

E2-ESR1 activates genes involved in human endometrial stromal cell cycle regulation, progesterone response, and production of stromal growth factors. Understanding the direct role of estrogen on the endometrial stroma and identifying downstream targets of E2-ESR1 can inform the development of targeted therapies for endometriosis and diminished endometrial receptivity.

Current knowledge on the acute regulation of steroidogenesis

How rapid induction of steroid hormone biosynthesis occurs in response to trophic hormone stimulation of steroidogenic cells has been a subject of intensive investigation for approximately six decades. A key observation made very early was that acute regulation of steroid biosynthesis required swift and timely synthesis of a new protein whose role appeared to be involved in the delivery of the substrate for all steroid hormones, cholesterol, from the outer to the inner mitochondrial membrane where the process of steroidogenesis begins. It was quickly learned that this transfer of cholesterol to the inner mitochondrial membrane was the regulated and rate-limiting step in steroidogenesis. Following this observation, the quest for this putative regulator protein(s) began in earnest in the late 1950s. This review provides a history of this quest, the candidate proteins that arose over the years and facts surrounding their rise or decline. Only two have persisted-translocator protein (TSPO) and the steroidogenic acute regulatory protein (StAR). We present a detailed summary of the work that has been published for each of these two proteins, the specific data that has appeared in support of their role in cholesterol transport and steroidogenesis, and the ensuing observations that have arisen in recent years that have refuted the role of TSPO in this process. We believe that the only viable candidate that has been shown to be indispensable is the StAR protein. Lastly, we provide our view on what may be the most important questions concerning the acute regulation of steroidogenesis that need to be asked in future.

The Essential Role of GATA6 in the Activation of Estrogen Synthesis in Endometriosis

Endometriotic stromal cells synthesize estradiol via the steroidogenic pathway. Nuclear receptor subfamily 5, group A, member 1 (NR5A1) is critical, but alone not sufficient, in activating this cascade that involves at least 5 genes. To evaluate whether another transcription factor is required for the activation of this pathway, we examined whether GATA Binding Protein 6 (GATA6) can transform a normal endometrial stromal cell (NoEM) into an endometriotic-like cell by conferring an estrogen-producing phenotype. We ectopically expressed GATA6 alone or with NR5A1 in NoEM or silenced these transcription factors in endometriotic stromal cells (OSIS) and assessed the messenger RNAs or proteins encoded by the genes in the steroidogenic cascade. Functionally, we assessed the effects of GATA6 expression or silencing on estradiol formation. In OSIS, GATA6 was necessary for catalyzing the conversion of progesterone to androstenedione (CYP17A1; P < .05). In NoEM, ectopic expression of GATA6 was essential for converting pregnenolone to estrogen (HSD3B2, CYP17A1, and CYP19A1; P < .05). However, simultaneous ectopic expression of both GATA6 and NR5A1 was required and sufficient to confer induction of all 5 genes and their encoded proteins that convert cholesterol to estrogen. Functionally, only simultaneous knockdown of GATA6 and NR5A1 blocked estradiol formation in OSIS ( P < .05). The presence of both transcription factors was required and sufficient to transform endometrial stromal cells into endometriotic-like cells that produced estradiol in large quantities ( P < .05). In summary, GATA6 alone is essential but not sufficient for estrogen formation in endometriosis. However, simultaneous addition of GATA6 and NR5A1 to an endometrial stromal cell is sufficient to transform it into an endometriotic-like cell, manifested by the activation of the estradiol biosynthetic cascade.

A single cell level measurement of StAR expression and activity in adrenal cells

The Steroidogenic acute regulatory protein (StAR) directs mitochondrial cholesterol uptake through a C-terminal cholesterol binding domain (CBD) and a 62 amino acid N-terminal regulatory domain (NTD) that contains an import sequence and conserved sites for inner membrane metalloproteases. Deletion of the NTD prevents mitochondrial import while maintaining steroidogenesis but with compromised cholesterol homeostasis. The rapid StAR-mediated cholesterol transfer in adrenal cells depends on concerted mRNA translation, p37 StAR phosphorylation and controlled NTD cleavage. The NTD controls this process with two cAMP-inducible modulators of, respectively, transcription and translation SIK1 and TIS11b/Znf36l1. High-resolution fluorescence in situ hybridization (HR-FISH) of StAR RNA resolves slow RNA splicing at the gene loci in cAMP-induced Y-1 cells and transfer of individual 3.5 kB mRNA molecules to mitochondria. StAR transcription depends on the CREB coactivator CRTC2 and PKA inhibition of the highly inducible suppressor kinase SIK1 and a basal counterpart SIK2. PKA-inducible TIS11b/Znf36l1 binds specifically to highly conserved elements in exon 7 thereby suppressing formation of mRNA and subsequent translation. Co-expression of SIK1, Znf36l1 with 3.5 kB StAR mRNA may limit responses to pulsatile signaling by ACTH while regulating the transition to more prolonged stress.

ACTH Action on StAR Biology

Adrenocorticotropin hormone (ACTH) produced by the anterior pituitary stimulates glucocorticoid synthesis by the adrenal cortex. The first step in glucocorticoid synthesis is the delivery of cholesterol to the mitochondrial matrix where the first enzymatic reaction in the steroid hormone biosynthetic pathway occurs. A key response of adrenal cells to ACTH is activation of the cAMP-protein kinase A (PKA) signaling pathway. PKA activation results in an acute increase in expression and function of the Steroidogenic Acute Regulatory protein (StAR). StAR plays an essential role in steroidogenesis- it controls the hormone-dependent movement of cholesterol across the mitochondrial membranes. Currently StAR's mechanism of action remains a major unanswered question in the field. However, some insight may be gained from understanding the mechanism(s) controlling the PKA-dependent phosphorylation of StAR at S194/195 (mouse/human StAR), a modification that is required for function. This mini-review provides a background on StAR's biology with a focus on StAR phosphorylation. The model for StAR translation and phosphorylation at the outer mitochondrial membrane, the location for StAR function, is presented to highlight a unifying theme emerging from diverse studies.

SCAP/SREBP pathway is required for the full steroidogenic response to cyclic AMP

Luteinizing hormone (LH) stimulates steroidogenesis largely through a surge in cyclic AMP (cAMP). Steroidogenic rates are also critically dependent on the availability of cholesterol at mitochondrial sites of synthesis. This cholesterol is provided by cellular uptake of lipoproteins, mobilization of intracellular lipid, and de novo synthesis. Whether and how these pathways are coordinated by cAMP are poorly understood. Recent phosphoproteomic analyses of cAMP-dependent phosphorylation sites in MA10 Leydig cells suggested that cAMP regulates multiple steps in these processes, including activation of the SCAP/SREBP pathway. SCAP [sterol-regulatory element-binding protein (SREBP) cleavage-activating protein] acts as a cholesterol sensor responsible for regulating intracellular cholesterol balance. Its role in cAMP-mediated control of steroidogenesis has not been explored. We used two CRISPR (clustered regularly interspaced short palindromic repeat)-Cas9 (CRISPR associated protein 9) knockout approaches to test the role of SCAP in steroidogenesis. Our results demonstrate that SCAP is required for progesterone production induced by concurrent inhibition of the cAMP phosphodiesterases PDE4 and PDE8. These inhibitors increased SCAP phosphorylation, SREBP2 activation, and subsequent expression of cholesterol biosynthetic genes, whereas SCAP deficiency largely prevented these effects. Reexpression of SCAP in SCAP-deficient cells restored SREBP2 protein expression and partially restored steroidogenic responses, confirming the requirement of SCAP-SREBP2 in steroidogenesis. Inhibitors of 3-hydroxy-3-methylglutaryl-Coenzyme A reductase and isoprenylation attenuated, whereas exogenously provided cholesterol augmented, PDE inhibitor-induced steroidogenesis, suggesting that the cholesterol substrate needed for steroidogenesis is provided by both de novo synthesis and isoprenylation-dependent mechanisms. Overall, these results demonstrate a novel role for LH/cAMP in SCAP/SREBP activation and subsequent regulation of steroidogenesis.

The ATPase Inhibitory Factor 1 (IF1): A master regulator of energy metabolism and of cell survival

In this contribution we summarize most of the findings reported for the molecular and cellular biology of the physiological inhibitor of the mitochondrial H(+)-ATP synthase, the engine of oxidative phosphorylation (OXPHOS) and gate of cell death. We first describe the structure and major mechanisms and molecules that regulate the activity of the ATP synthase placing the ATPase Inhibitory Factor 1 (IF1) as a major determinant in the regulation of the activity of the ATP synthase and hence of OXPHOS. Next, we summarize the post-transcriptional mechanisms that regulate the expression of IF1 and emphasize, in addition to the regulation afforded by the protonation state of histidine residues, that the activity of IF1 as an inhibitor of the ATP synthase is also regulated by phosphorylation of a serine residue. Phosphorylation of S39 in IF1 by the action of a mitochondrial cAMP-dependent protein kinase A hampers its interaction with the ATP synthase, i.e., only dephosphorylated IF1 interacts with the enzyme. Upon IF1 interaction with the ATP synthase both the synthetic and hydrolytic activities of the engine of OXPHOS are inhibited. These findings are further placed into the physiological context to stress the emerging roles played by IF1 in metabolic reprogramming in cancer, in hypoxia and in cellular differentiation. We review also the implication of IF1 in other cellular situations that involve the malfunctioning of mitochondria. Special emphasis is given to the role of IF1 as driver of the generation of a reactive oxygen species signal that, emanating from mitochondria, is able to reprogram the nucleus of the cell to confer by various signaling pathways a cell-death resistant phenotype against oxidative stress. Overall, our intention is to highlight the urgent need of further investigations in the molecular and cellular biology of IF1 and of its target, the ATP synthase, to unveil new therapeutic strategies in human pathology. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

Disruption of steroidogenesis: Cell models for mechanistic investigations and as screening tools

In the modern world, humans are exposed during their whole life to a large number of synthetic chemicals. Some of these chemicals have the potential to disrupt endocrine functions and contribute to the development and/or progression of major diseases. Every year approximately 1000 novel chemicals, used in industrial production, agriculture, consumer products or as pharmaceuticals, are reaching the market, often with limited safety assessment regarding potential endocrine activities. Steroids are essential endocrine hormones, and the importance of the steroidogenesis pathway as a target for endocrine disrupting chemicals (EDCs) has been recognized by leading scientists and authorities. Cell lines have a prominent role in the initial stages of toxicity assessment, i.e. for mechanistic investigations and for the medium to high throughput analysis of chemicals for potential steroidogenesis disrupting activities. Nevertheless, the users have to be aware of the limitations of the existing cell models in order to apply them properly, and there is a great demand for improved cell-based testing systems and protocols. This review intends to provide an overview of the available cell lines for studying effects of chemicals on gonadal and adrenal steroidogenesis, their use and limitations, as well as the need for future improvements of cell-based testing systems and protocols.

Regulation of StAR by the N-terminal Domain and Coinduction of SIK1 and TIS11b/Znf36l1 in Single Cells

The cholesterol transfer function of steroidogenic acute regulatory protein (StAR) is uniquely integrated into adrenal cells, with mRNA translation and protein kinase A (PKA) phosphorylation occurring at the mitochondrial outer membrane (OMM). The StAR C-terminal cholesterol-binding domain (CBD) initiates mitochondrial intermembrane contacts to rapidly direct cholesterol to Cyp11a1 in the inner membrane (IMM). The conserved StAR N-terminal regulatory domain (NTD) includes a leader sequence targeting the CBD to OMM complexes that initiate cholesterol transfer. Here, we show how the NTD functions to enhance CBD activity delivers more efficiently from StAR mRNA in adrenal cells, and then how two factors hormonally restrain this process. NTD processing at two conserved sequence sites is selectively affected by StAR PKA phosphorylation. The CBD functions as a receptor to stimulate the OMM/IMM contacts that mediate transfer. The NTD controls the transit time that integrates extramitochondrial StAR effects on cholesterol homeostasis with other mitochondrial functions, including ATP generation, inter-organelle fusion, and the major permeability transition pore in partnership with other OMM proteins. PKA also rapidly induces two additional StAR modulators: salt-inducible kinase 1 (SIK1) and Znf36l1/Tis11b. Induced SIK1 attenuates the activity of CRTC2, a key mediator of StAR transcription and splicing, but only as cAMP levels decline. TIS11b inhibits translation and directs the endonuclease-mediated removal of the 3.5-kb StAR mRNA. Removal of either of these functions individually enhances cAMP-mediated induction of StAR. High-resolution fluorescence in situ hybridization (HR-FISH) of StAR RNA reveals asymmetric transcription at the gene locus and slow RNA splicing that delays mRNA formation, potentially to synchronize with cholesterol import. Adrenal cells may retain slow transcription to integrate with intermembrane NTD activation. HR-FISH resolves individual 3.5-kb StAR mRNA molecules via dual hybridization at the 3'- and 5'-ends and reveals an unexpectedly high frequency of 1:1 pairing with mitochondria marked by the matrix StAR protein. This pairing may be central to translation-coupled cholesterol transfer. Altogether, our results show that adrenal cells exhibit high-efficiency StAR activity that needs to integrate rapid cholesterol transfer with homeostasis and pulsatile hormonal stimulation. StAR NBD, the extended 3.5-kb mRNA, SIK1, and Tis11b play important roles.

Stimulation of StAR expression by cAMP is controlled by inhibition of highly inducible SIK1 via CRTC2, a co-activator of CREB

In mouse steroidogenic cells the activation of cholesterol metabolism is mediated by steroidogenic acute regulatory protein (StAR). Here, we visualized a coordinated regulation of StAR transcription, splicing and post-transcriptional processing, which are synchronized by salt inducible kinase (SIK1) and CREB-regulated transcription coactivator (CRTC2). To detect primary RNA (pRNA), spliced primary RNA (Sp-RNA) and mRNA in single cells, we generated probe sets by using fluorescence in situ hybridization (FISH). These methods allowed us to address the nature of StAR gene expression and to visualize protein-nucleic acid interactions through direct detection. We show that SIK1 represses StAR expression in Y1 adrenal and MA10 testis cells through inhibition of processing mediated by CRTC2. Digital image analysis matches qPCR analyses of the total cell culture. Evidence is presented for spatially separate accumulation of StAR pRNA and Sp-RNA at the gene loci in the nucleus. These findings establish that cAMP, SIK and CRTC mediate StAR expression through activation of individual StAR gene loci.

StAR Protein Stability in Y1 and Kin-8 Mouse Adrenocortical Cells

The steroidogenic acute regulatory protein (STAR) protein expression is required for cholesterol transport into mitochondria to initiate steroidogenesis in the adrenal and gonads. STAR is synthesized as a 37 kDa precursor protein which is targeted to the mitochondria and imported and processed to an intra-mitochondrial 30 kDa protein. Tropic hormone stimulation of the cAMP-dependent protein kinase A (PKA) signaling pathway is the major contributor to the transcriptional and post-transcriptional regulation of STAR synthesis. Many studies have focused on the mechanisms of cAMP-PKA mediated control of STAR synthesis while there are few reports on STAR degradation pathways. The objective of this study was to determine the effect of cAMP-PKA-dependent signaling on STAR protein stability. We have used the cAMP-PKA responsive Y1 mouse adrenocortical cells and the PKA-deficient Kin-8 cells to measure STAR phosphorylation and protein half-life. Western blot analysis and standard radiolabeled pulse-chase experiments were used to determine STAR phosphorylation status and protein half-life, respectively. Our data demonstrate that PKA-dependent STAR phosphorylation does not contribute to 30 kDa STAR protein stability in the mitochondria. We further show that inhibition of the 26S proteasome does not block precursor STAR phosphorylation or steroid production in Y1 cells. These data suggest STAR can maintain function and promote steroidogenesis under conditions of proteasome inhibition.

The role of hypoxia and HIF1α in the regulation of STAR-mediated steroidogenesis in granulosa cells

The adaptive responses to hypoxia are mediated by hypoxia-inducible factor 1 alpha (HIF1α). Its role, however, in regulating steroidogenesis remains poorly understood. We examined the role of hypoxia and HIF1α in regulating steroid acute regulatory protein (STAR) expression and steroidogenesis in immortalized (KK1) mouse granulosa cells under progressively lowering O2 concentrations (20%, 15%, 10%, 5%, 1%). Basal and dbcAMP-stimulated progesterone synthesis was decreased under severe hypoxia (1% and 5% O2). The partial hypoxia revealed opposing effects, with a significant increase in steroidogenic response at 10% O2 in dbcAMP-treated cells: Star-promoter activity, mRNA and protein expression were increased. The hypoxia-stimulated STAR expression was PKA-dependent. Binding of HIF1α to the Star-promoter was potentiated under partial hypoxia. Inhibition of the transcriptional activity or expression of HIF1α suppressed STAR-expression. HIF1α appears to be a positive regulator of basal and stimulated STAR-expression, which under partial hypoxia is capable of increasing the steroidogenic capacity of granulosa cells.

Estradiol modulates translocator protein (TSPO) and steroid acute regulatory protein (StAR) via protein kinase A (PKA) signaling in hypothalamic astrocytes

The ability of the central nervous system to synthesize steroid hormones has wide-ranging implications for physiology and pathology. Among the proposed roles of neurosteroids is the regulation of the LH surge. This involvement in the estrogen-positive feedback demonstrates the integration of peripheral steroids with neurosteroids. Within the female hypothalamus, estradiol from developing follicles stimulates progesterone synthesis in astrocytes, which activate neural circuits regulating gonadotropin (GnRH) neurons. Estradiol acts at membrane estrogen receptor-α to activate cellular signaling that results in the release of inositol trisphosphate-sensitive calcium stores that are sufficient to induce neuroprogesterone synthesis. The purpose of the present studies was to characterize the estradiol-induced signaling leading to activation of steroid acute regulatory protein (StAR) and transporter protein (TSPO), which mediate the rate-limiting step in steroidogenesis, ie, the transport of cholesterol into the mitochondrion. Treatment of primary cultures of adult female rat hypothalamic astrocytes with estradiol induced a cascade of phosphorylation that resulted in the activation of a calcium-dependent adenylyl cyclase, AC1, elevation of cAMP, and activation of both StAR and TSPO. Blocking protein kinase A activation with H-89 abrogated the estradiol-induced neuroprogesterone synthesis. Thus, together with previous results, these experiments completed the characterization of how estradiol action at the membrane leads to the augmentation of neuroprogesterone synthesis through increasing cAMP, activation of protein kinase A, and the phosphorylation of TSPO and StAR in hypothalamic astrocytes.

The contribution of serine 194 phosphorylation to steroidogenic acute regulatory protein function

The steroidogenic acute regulatory protein (StAR) facilitates the delivery of cholesterol to the inner mitochondrial membrane, where the cholesterol side-chain cleavage enzyme catalyzes the initial step of steroid hormone biosynthesis. StAR was initially identified in adrenocortical cells as a phosphoprotein, the expression and phosphorylation of which were stimulated by corticotropin. A number of in vitro studies have implicated cAMP-dependent phosphorylation at serine 194 (S194, S195 in human StAR) as an important residue for StAR activity. To explore the importance of S194 phosphorylation in StAR function in vivo, we developed a transgenic model using a bacterial artificial chromosome expressing either wild-type (WT) StAR or StAR mutation S194A to rescue StAR knockout (KO) mice. Despite StAR protein expression comparable to or higher than amounts seen with control animals or rescue with WT StAR, S194A StAR did not rescue the neonatal lethality and only partially rescued the sex reversal in male mice observed uniformly in StAR KO mice. Like the StAR KO mice, the adrenal cortex and testicular Leydig cells contained abundant lipid deposits when stained with oil red O. Adrenal StAR from S194A rescue animals lacks an acidic species, which appears upon corticotropin stimulation in animals rescued with WT StAR, consistent with defective StAR phosphorylation. These findings demonstrate that S194 is an essential residue for normal StAR function in the adrenal cortex and testes of mice.

Genome-wide DNA methylation analysis predicts an epigenetic switch for GATA factor expression in endometriosis

Endometriosis is a gynecological disease defined by the extrauterine growth of endometrial-like cells that cause chronic pain and infertility. The disease is limited to primates that exhibit spontaneous decidualization, and diseased cells are characterized by significant defects in the steroid-dependent genetic pathways that typify this process. Altered DNA methylation may underlie these defects, but few regions with differential methylation have been implicated in the disease. We mapped genome-wide differences in DNA methylation between healthy human endometrial and endometriotic stromal cells and correlated this with gene expression using an interaction analysis strategy. We identified 42,248 differentially methylated CpGs in endometriosis compared to healthy cells. These extensive differences were not unidirectional, but were focused intragenically and at sites distal to classic CpG islands where methylation status was typically negatively correlated with gene expression. Significant differences in methylation were mapped to 403 genes, which included a disproportionally large number of transcription factors. Furthermore, many of these genes are implicated in the pathology of endometriosis and decidualization. Our results tremendously improve the scope and resolution of differential methylation affecting the HOX gene clusters, nuclear receptor genes, and intriguingly the GATA family of transcription factors. Functional analysis of the GATA family revealed that GATA2 regulates key genes necessary for the hormone-driven differentiation of healthy stromal cells, but is hypermethylated and repressed in endometriotic cells. GATA6, which is hypomethylated and abundant in endometriotic cells, potently blocked hormone sensitivity, repressed GATA2, and induced markers of endometriosis when expressed in healthy endometrial cells. The unique epigenetic fingerprint in endometriosis suggests DNA methylation is an integral component of the disease, and identifies a novel role for the GATA family as key regulators of uterine physiology-aberrant DNA methylation in endometriotic cells correlates with a shift in GATA isoform expression that facilitates progesterone resistance and disease progression.

Synergistic activation of steroidogenic acute regulatory protein expression and steroid biosynthesis by retinoids: involvement of cAMP/PKA signaling

Both retinoic acid receptors (RARs) and retinoid X receptors (RXRs) mediate the action of retinoids that play important roles in reproductive development and function, as well as steroidogenesis. Regulation of steroid biosynthesis is principally mediated by the steroidogenic acute regulatory protein (StAR); however, the modes of action of retinoids in the regulation of steroidogenesis remain obscure. In this study we demonstrate that all-trans retinoic acid (atRA) enhances StAR expression, but not its phosphorylation (P-StAR), and progesterone production in MA-10 mouse Leydig cells. Activation of the protein kinase A (PKA) cascade, by dibutyrl-cAMP or type I/II PKA analogs, markedly increased retinoid-responsive StAR, P-StAR, and steroid levels. Targeted silencing of endogenous RARα and RXRα, with small interfering RNAs, resulted in decreases in 9-cis RA-stimulated StAR and progesterone levels. Truncation of and mutational alterations in the 5'-flanking region of the StAR gene demonstrated the importance of the -254/-1-bp region in retinoid responsiveness. An oligonucleotide probe encompassing an RXR/liver X receptor recognition motif, located within the -254/-1-bp region, specifically bound MA-10 nuclear proteins and in vitro transcribed/translated RXRα and RARα in EMSAs. Transcription of the StAR gene in response to atRA and dibutyrl-cAMP was influenced by several factors, its up-regulation being dependent on phosphorylation of cAMP response-element binding protein (CREB). Chromatin immunoprecipitation studies revealed the association of phosphorylation of CREB, CREB binding protein, RXRα, and RARα to the StAR promoter. Further studies elucidated that hormone-sensitive lipase plays an important role in atRA-mediated regulation of the steroidogenic response that involves liver X receptor signaling. These findings delineate the molecular events by which retinoids influence cAMP/PKA signaling and provide additional and novel insight into the regulation of StAR expression and steroidogenesis in mouse Leydig cells.

Steroid hormone synthesis in mitochondria

Mitochondria are essential sites for steroid hormone biosynthesis. Mitochondria in the steroidogenic cells of the adrenal, gonad, placenta and brain contain the cholesterol side-chain cleavage enzyme, P450scc, and its two electron-transfer partners, ferredoxin reductase and ferredoxin. This enzyme system converts cholesterol to pregnenolone and determines net steroidogenic capacity, so that it serves as the chronic regulator of steroidogenesis. Several other steroidogenic enzymes, including 3β-hydroxysteroid dehydrogenase, 11β-hydroxylase and aldosterone synthase also reside in mitochondria. Similarly, the mitochondria of renal tubular cells contain two key enzymes participating in the activation and degradation of vitamin D. The access of cholesterol to the mitochondria is regulated by the steroidogenic acute regulatory protein, StAR, serving as the acute regulator of steroidogenesis. StAR action requires a complex multi-component molecular machine on the outer mitochondrial membrane (OMM). Components of this machine include the 18 kDa translocator protein (TSPO), the voltage-dependent anion chanel (VDAC-1), TSPO-associated protein 7 (PAP7, ACBD3), and protein kinase A regulatory subunit 1α (PKAR1A). The precise fashion in which these proteins interact and move cholesterol from the OMM to P450scc, and the means by which cholesterol is loaded into the OMM, remain unclear. Human deficiency diseases have been described for StAR and for all the mitochondrial steroidogenic enzymes, but not for the electron transfer proteins or for the components of the cholesterol import machine.

Membrane receptor cross talk in steroidogenesis: recent insights and clinical implications

Steroid production by all three major steroidogenic tissues, the adrenals, testes, and ovaries, is critical for survival and reproduction of all animals. As such, the pathways that regulate steroidogenesis are conserved between these tissues, from the steroidogenic enzymes and cofactors that synthesize steroids, to the intracellular signaling molecules and Gαs-coupled receptors that mediate the activity of these enzymes. Recent work has revealed another important conserved pathway in steroidogenesis: crosstalk between membrane G protein-coupled receptors and membrane receptor tyrosine kinases. Luteinizing hormone (LH) or adrencorticotropic hormone (ACTH) binding to their cognate Gαs-coupled membrane receptors in the gonads and adrenals, respectively, leads to cAMP-induced trans-activation of the epidermal growth factor (EGF) receptor, followed by activation of Akt and Erk signaling. These kinase signals then activate Steroidogenic Acute Regulatory (StAR) protein, which promotes steroid production. Inhibition of this pathway abrogates both LH- and ACTH-induced steroidogenesis. Interestingly, LH-induced transactivation of the EGF receptor in the ovary uniquely requires matrix metalloproteinase-mediated release of EGF receptor ligands, and inhibition of these proteases blocks LH-induced steroidogenesis. Given this unique need for matrix metalloproteinases in ovarian steroidogenesis, MMP inhibition may prove to be useful when treating diseases of excess ovarian steroid production, such as polycystic ovary syndrome.

Prostaglandin E2 functions as a luteotrophic factor in the dog

The luteal phase in dogs is governed by many poorly understood regulatory mechanisms. Functioning of the corpus luteum (CL) is unaffected by hysterectomy. Recently, the role of prostaglandins in regulating canine CL function was addressed suggesting a luteotrophic effect of prostaglandin E2 (PGE2) during the early luteal phase. However, compelling functional evidence was lacking. The potential of PGE2 to stimulate steroidogenesis was tested in canine primary luteal cells isolated from developing CL of non-pregnant dogs. In addition, the luteal expression of prostaglandin transporter (PGT) and steroidogenic acute regulatory protein (STAR) was demonstrated and characterized in CL from non-pregnant bitches during the course of dioestrus as well as from pregnant animals during the pre-implantation, post-implantation and mid-gestation periods of pregnancy and during luteolysis; the luteal expression of PGE2 receptors (EP2 and EP4) has been investigated at the protein level throughout pregnancy. Our findings show that PGE2 is an activator of STAR expression in canine luteal cells from early luteal phase, significantly up-regulating STAR promoter activity and protein expression resulting in increased steroidogenesis. The 3βHSD (HSD3B2) and P450scc (CYP11A1) expression remained unaffected by PGE2 treatment. The expression of PGT was confirmed in CL during both pregnancy and dioestrus and generally localized to the luteal cells. After initial up-regulation during the earlier stages of the CL phase, its expression declined towards the luteal regression. Together with the demonstration of EP2 and EP4 throughout pregnancy, and the decline in EP2 at prepartum, our findings further support our hypothesis that intra-luteal PGE2 may play an important role in regulating progesterone secretion in the canine CL.

Mechanisms of action of hormone-sensitive lipase in mouse Leydig cells: its role in the regulation of the steroidogenic acute regulatory protein

Hormone-sensitive lipase (HSL) catalyzes the hydrolysis of cholesteryl esters in steroidogenic tissues and, thus, facilitates cholesterol availability for steroidogenesis. The steroidogenic acute regulatory protein (StAR) controls the rate-limiting step in steroid biosynthesis. However, the modes of action of HSL in the regulation of StAR expression remain obscure. We demonstrate in MA-10 mouse Leydig cells that activation of the protein kinase A (PKA) pathway, by a cAMP analog Bt2cAMP, enhanced expression of HSL and its phosphorylation (P) at Ser-660 and Ser-563, but not at Ser-565, concomitant with increased HSL activity. Phosphorylation and activation of HSL coincided with increases in StAR, P-StAR (Ser-194), and progesterone levels. Inhibition of HSL activity by CAY10499 effectively suppressed Bt2cAMP-induced StAR expression and progesterone synthesis. Targeted silencing of endogenous HSL, with siRNAs, resulted in increased cholesteryl ester levels and decreased cholesterol content in MA-10 cells. Depletion of HSL affected lipoprotein-derived cellular cholesterol influx, diminished the supply of cholesterol to the mitochondria, and resulted in the repression of StAR and P-StAR levels. Cells overexpressing HSL increased the efficacy of liver X receptor (LXR) ligands on StAR expression and steroid synthesis, suggesting HSL-mediated steroidogenesis entails enhanced oxysterol production. Conversely, cells deficient in LXRs exhibited decreased HSL responsiveness. Furthermore, an increase in HSL was correlated with the LXR target genes, steroid receptor element-binding protein 1c and ATP binding cassette transporter A1, demonstrating HSL-dependent regulation of steroidogenesis predominantly involves LXR signaling. LXRs interact/cooperate with RXRs and result in the activation of StAR gene transcription. These findings provide novel insight and demonstrate the molecular events by which HSL acts to drive cAMP/PKA-mediated regulation of StAR expression and steroidogenesis in mouse Leydig cells.

Short RNA molecules with high binding affinity to the KH motif of A-kinase anchoring protein 1 (AKAP1): implications for the regulation of steroidogenesis

One of the key regulators of acute steroid hormone biosynthesis in steroidogenic tissues is the steroidogenic acute regulatory (STAR) protein. Acute regulation of STAR production on the transcriptional level is mainly achieved through a cAMP-dependent mechanism, which is well understood. However, less is known about the posttranscriptional regulation of STAR synthesis, specifically the factors influencing the destiny of the Star mRNA after it leaves the nucleus. Here, we show that the 3'-untranslated region of Star mRNA interacts with the heterogeneous nuclear ribonucleoprotein K-homology (KH) motif of the mitochondrial scaffold A-kinase anchoring protein 1 (AKAP1) in vitro with a moderate affinity as measured by EMSAs. A mutation that mimics the phosphorylation state of the KH motif at a specific serine either did not alter, or had a negative impact on, protein-RNA binding under these conditions. The KH motif of AKAP1 binds short pyrimidine-rich RNA molecules with a stable hairpin structure as demonstrated by in vitro selection. AKAP1 also interacts with STAR mRNA in a dibutyryl-cAMP-stimulated human steroidogenic adrenocortical carcinoma cell line in vivo. Therefore, we propose a model in which AKAP1 anchors Star mRNA at the mitochondria, thus stabilizing the translational complex at this organelle, a situation that might affect STAR production and steroidogenesis. In addition, we suggest that the last 216 amino acid residues of AKAP1 might participate in the degradation of STAR and other nuclear-encoded mitochondrial mRNAs through interaction with a RNA-induced silencing complex, specifically with the argonaute 2 protein.

Cisplatin inhibits testosterone synthesis by a mechanism that includes the action of reactive oxygen species (ROS) at the level of P450scc

Cisplatin (Cs) is a chemotherapeutic agent able to generate reactive oxygen species (ROS) which are linked to several side effects of the drug. Even when it is known that Cs produces Leydig cell dysfunction, it is unknown whether this particular side effect is mediated by ROS. The aim of this study was to evaluate the in vitro effects of Cs on testosterone production and the participation of ROS in this effect. We demonstrate that Cs promotes the generation of ROS in a time-, and concentration-dependent fashion, not only in mouse testicular interstitial cells but also in MA-10 Leydig cells. Also, Cs inhibits testosterone synthesis in a concentration-dependent fashion (5-50 μM for 4 h) and to a similar extent, in cells exposed to human chorionic gondadotropin hormone (hCG), to an analog of the second messenger cAMP (8Br-cAMP) or to a freely diffusible cholesterol analog (22R-hydroxycholesterol). However, this treatment does not inhibit the conversion of pregnenolone to testosterone. These data suggest that Cs exerts its inhibitory action on testosterone synthesis by an action at the level of P450scc. We also demonstrated that an antioxidant impairs the inhibitory effect of Cs on the conversion of the cholesterol analog into pregnenolone and that Cs does not change the expression level of P450scc mRNA. Therefore, it is concluded that Cs inhibits testosterone synthesis by a mechanism that includes the inhibition of P450scc by ROS.

Cordycepin stimulated steroidogenesis in MA-10 mouse Leydig tumor cells through the protein kinase C Pathway

Cordycepin (3'-deoxyadenosine) is an adenosine analogue isolated from Cordyceps sinensis , which is a Chinese herbal medicine known to have many benefits, including adjustment of the physical condition, an anticancer effect, and enhancement of sexual performance. It was previously demonstrated that cordycepin could simultaneously activate steroidogenesis and apoptosis in MA-10 mouse Leydig tumor cells. However, the mechanism remains elusive. Thus, aim of the present study was to investigate the steroidogenic and apoptotic mechanism of cordycepin in MA-10 cells. MA-10 cells were treated with cordycepin at various dosages and time courses plus different protein kinase inhibitors. Steroid production, protein expression, and cell viability were then determined. Results illustrated that cordycepin stimulated MA-10 cell steroidogenesis in dose- and time-dependent relationships. However, cordycepin could not induce steroidogenic acute regulatory (StAR) protein expression. However, cordycepin did activate the phospholipase C/protein kinase C (PLC/PKC), but not PKA and PI3K, pathway to induce MA-10 cell steroidogenesis. Moreover, cordycepin could stimulate the phosphorylation of PKC, extracellular signal-regulated kinase 1/2 (ERK1/2), and c-Jun N-terminal kinase (c-JNK), but not p38, in MA-10 cells. In addition, cordycepin could activate the PKC pathway to induce MA-10 cell death, and this death effect was not caused by cordycepin-stimulated progesterone from MA-10 cells. In conclusion, cordycepin stimulated intracellular PLC/PKC and MAPK signal transduction pathways to induce steroidogenesis and cell death in MA-10 mouse Leydig tumor cells.

cAMP-specific phosphodiesterases 8A and 8B, essential regulators of Leydig cell steroidogenesis

Phosphodiesterase (PDE) 8A and PDE8B are high-affinity, cAMP-specific phosphodiesterases that are highly expressed in Leydig cells. PDE8A is largely associated with mitochondria, whereas PDE8B is broadly distributed in the cytosol. We used a new, PDE8-selective inhibitor, PF-04957325, and genetically ablated PDE8A(-/-), PDE8B(-/-) and PDE8A(-/-)/B(-/-) mice to determine roles for these PDEs in the regulation of testosterone production. PF-04957325 treatment of WT Leydig cells or MA10 cells increased steroid production but had no effect in PDE8A (-/-)/B(-/-) double-knockout cells, confirming the selectivity of the drug. Moreover, under basal conditions, cotreatment with PF-04957325 plus rolipram, a PDE4-selective inhibitor, synergistically potentiated steroid production. These results suggest that the pool(s) of cAMP regulating androgen production are controlled by PDE8s working in conjunction with PDE4. Likewise, PDE8A (-/-)/B(-/-) cells had higher testosterone production than cells from either PDE8A(-/-) or PDE8B(-/-) mice, suggesting that both PDE8s work in concert to regulate steroid production. We further demonstrate that combined inhibition of PDE8s and PDE4 greatly increased PKA activity including phosphorylation of cholesterol-ester hydrolase (CEH)/hormone-sensitive lipase (HSL). CEH/HSL phosphorylation also was increased in PDE8A(-/-)/B(-/-) cells compared with WT cells. Finally, combined inhibition of PDE8s and PDE4 increased the expression of steroidogenic acute regulatory (StAR) protein. Together these findings suggest that both PDE8A and PDE8B play essential roles to maintain low cAMP levels, thereby suppressing resting steroidogenesis by keeping CEH/HSL inactive and StAR protein expression low. They also suggest that in order for PDE inhibitor therapy to be an effective stimulator of steroidogenesis, both PDE8 isozymes and PDE4 need to be simultaneously targeted.

Early steps in steroidogenesis: intracellular cholesterol trafficking

Steroid hormones are made from cholesterol, primarily derived from lipoproteins that enter cells via receptor-mediated endocytosis. In endo-lysosomes, cholesterol is released from cholesterol esters by lysosomal acid lipase (LAL; disordered in Wolman disease) and exported via Niemann-Pick type C (NPC) proteins (disordered in NPC disease). These diseases are characterized by accumulated cholesterol and cholesterol esters in most cell types. Mechanisms for trans-cytoplasmic cholesterol transport, membrane insertion, and retrieval from membranes are less clear. Cholesterol esters and "free" cholesterol are enzymatically interconverted in lipid droplets. Cholesterol transport to the cholesterol-poor outer mitochondrial membrane (OMM) appears to involve cholesterol transport proteins. Cytochrome P450scc (CYP11A1) then initiates steroidogenesis by converting cholesterol to pregnenolone on the inner mitochondrial membrane (IMM). Acute steroidogenic responses are regulated by cholesterol delivery from OMM to IMM, triggered by the steroidogenic acute regulatory protein (StAR). Chronic steroidogenic capacity is determined by CYP11A1 gene transcription. StAR mutations cause congenital lipoid adrenal hyperplasia, with absent steroidogenesis, potentially lethal salt loss, and 46,XY sex reversal. StAR mutations initially destroy most, but not all steroidogenesis; low levels of StAR-independent steroidogenesis are lost later due to cellular damage, explaining the clinical findings. Rare P450scc mutations cause a similar syndrome. This review addresses these early steps in steroid biosynthesis.

The Effect of Cordycepin on Steroidogenesis and Apoptosis in MA-10 Mouse Leydig Tumor Cells

Cordycepin is a natural pure compound extracted from Cordyceps sinensis (CS). We have demonstrated that CS stimulates steroidogenesis in primary mouse Leydig cell and activates apoptosis in MA-10 mouse Leydig tumor cells. It is highly possible that cordycepin is the main component in CS modulating Leydig cell functions. Thus, our aim was to investigate the steroidogenic and apoptotic effects with potential mechanism of cordycepin on MA-10 mouse Leydig tumor cells. Results showed that cordycepin significantly stimulated progesterone production in dose- and time-dependent manners. Adenosine receptor (AR) subtype agonists were further used to treat MA-10 cells, showing that A₁, A _2A , A _2B , and A₃, AR agonists could stimulate progesterone production. However, StAR promoter activity and protein expression remained of no difference among all cordycepin treatments, suggesting that cordycepin might activate AR, but not stimulated StAR protein to regulate MA-10 cell steroidogenesis. Meanwhile, cordycepin could also induce apoptotic cell death in MA-10 cells. Moreover, four AR subtype agonists induced cell death in a dose-dependent manner, and four AR subtype antagonists could all rescue cell death under cordycepin treatment in MA-10 cells. In conclusion, cordycepin could activate adenosine subtype receptors and simultaneously induce steroidogenesis and apoptosis in MA-10 mouse Leydig tumor cells.

cAMP stimulation of StAR expression and cholesterol metabolism is modulated by co-expression of labile suppressors of transcription and mRNA turnover

The steroidogenic acute regulatory (StAR) protein is generated in rodents from 1.6 kb and 3.5 kb mRNA formed by alternative polyadenylation. The zinc finger protein, TIS11B (also Znf36L1), is elevated by cAMP in adrenal cells in parallel with StAR mRNA. TIS11b selectively destabilizes the 3.5 kb mRNA through AU-rich sequences at the end of the 3'UTR. siRNA suppression shows that TIS11b surprisingly increases StAR protein and cholesterol metabolism. StAR transcription is directly activated by PKA phosphorylation. cAMP responsive element binding (CREB) protein 1 phosphorylation is a key step leading to recruitment of the co-activator, CREB binding protein (CBP). A second protein, CREB regulated transcription coactivator (TORC/CRTC), enhances this recruitment, but is inhibited by salt inducible kinase (SIK). Basal StAR transcription is constrained through this phosphorylation of TORC. PKA provides an alternative stimulation by phosphorylating SIK, which prevents TORC inactivation. PKA stimulation of StAR nuclear transcripts substantially precedes TORC recruitment to the StAR promoter, which may, therefore, mediate a later step in mRNA production. Inhibition of SIK by staurosporine elevates StAR transcription and TORC recruitment to maximum levels, but without CREB phosphorylation. TORC suppression by SIK evidently limits basal StAR transcription. Staurosporine and cAMP stimulate synergistically. SIK targets the phosphatase, PP2a (activation), and Type 2 histone de-acetylases (inhibition), which may each contribute to suppression. Staurosporine stimulation through SIK inhibition is repeated in cAMP stimulation of many steroidogenic genes regulated by steroidogenic factor 1 (SF-1) and CREB. TIS11b and SIK may combine to attenuate StAR expression when hormonal stimuli decline.

Reactive oxygen species (ROS) play a critical role in the cAMP-induced activation of Ras and the phosphorylation of ERK1/2 in Leydig cells

Activation of the LH receptor (LHR) in Leydig cells results in the phosphorylation of ERK1/2 by cAMP-dependent and cAMP-independent pathways. Here we examine the mechanisms by which cAMP stimulates ERK1/2 phosphorylation. We show that the stimulation of steroidogenesis is not necessary or sufficient to stimulate the phosphorylation of ERK1/2 but that other cAMP-dependent mitochondrial functions are involved. Using MA-10 cells as a model, we showed that cAMP analogs increase reactive oxygen species (ROS) formation and that an uncoupler of oxidative phosphorylation and a ROS scavenger prevent this increase. These two compounds also inhibit the increase in ERK1/2 phosphorylation provoked by cAMP analogs, thus suggesting that the cAMP-induced phosphorylation of ERK1/2 is mediated by mitochondrial ROS. In agreement with this hypothesis we also show that a reduction in glutathione levels, which alters the redox state of MA-10 cells, potentiates the effect of cAMP on ERK1/2 phosphorylation. Measurements of the dephosphorylation of ERK and the activation of Ras showed that the ROS scavenger prevents the cAMP-provoked activation of Ras and that cAMP, with or without a ROS scavenger, has little or no effect on the dephosphorylation of ERK. Lastly, we show that the uncoupler of oxidative phosphorylation and the ROS scavenger also prevent the ability of cAMP analogs to increase ERK1/2 phosphorylation in primary cultures of mouse Leydig cells. We conclude that, in Leydig cells, cAMP enhances the phosphorylation of ERK1/2 via a mitochondria-derived, ROS-dependent activation of Ras.

The involvement of specific PKC isoenzymes in phorbol ester-mediated regulation of steroidogenic acute regulatory protein expression and steroid synthesis in mouse Leydig cells

Protein kinase C (PKC) is a multigene family of serine/threonine kinases. PKC is involved in regulating adrenal and gonadal steroidogenesis; however, the functional relevance of the different PKC isoenzymes remains obscure. In this study, we demonstrate that MA-10 mouse Leydig tumor cells express several PKC isoforms to varying levels and that the activation of PKC signaling, by phorbol 12-myristate 13-acetate (PMA) elevated the expression and phosphorylation of PKCα, -δ, -ε, and -μ/protein kinase D (PKD). These responses coincided with the expression of the steroidogenic acute regulatory (StAR) protein and progesterone synthesis. Targeted silencing of PKCα, δ, and ε and PKD, using small interfering RNAs, resulted in deceases in basal and PMA-mediated StAR and steroid levels and demonstrated the importance of PKD in steroidogenesis. PKD was capable of controlling PMA and cAMP/PKA-mediated synergism involved in the steroidogenic response. Further studies pointed out that the regulatory events effected by PKD are associated with cAMP response element-binding protein (CREB) and c-Jun/c-Fos-mediated transcription of the StAR gene. Chromatin immunoprecipitation studies revealed that the activation of phosphorylated CREB, c-Jun, and c-Fos by PMA was correlated with in vivo protein-DNA interactions and the recruitment of CREB-binding protein, whereas knockdown of PKD suppressed the association of these factors with the StAR promoter. Ectopic expression of CREB-binding protein enhanced the trans-activation potential of CREB and c-Jun/c-Fos in StAR gene expression. Using EMSA, a -83/-67-bp region of the StAR promoter was shown to bind PKD-transfected MA-10 nuclear extract in a PMA-responsive manner, targeting CREB and c-Jun/c-Fos proteins. These findings provide evidence for the presence of multiple PKC isoforms and demonstrate the molecular events by which selective isozymes, especially PKD, influence PMA/PKC signaling involved in the regulation of the steroidogenic machinery in mouse Leydig cells.

Vasoactive intestinal peptide (VIP)-mediated expression and function of steroidogenic acute regulatory protein (StAR) in granulosa cells

VIP is a peptide hormone capable of activating the cAMP/PKA pathway and modifying gonadal steroidogenic capacity. Less is known about the molecular mechanisms of VIP-mediated steroidogenesis and its role in regulating the steroidogenic acute regulatory protein (STAR). We examined the impact of VIP on STAR expression and function in immortalized (KK1) and primary mouse granulosa cells, where VIP strongly upregulated STAR expression and steroidogenesis. Inhibitors of the PKA and PKC pathways suggested that both are activated by VIP. VIP did not efficiently phosphorylate STAR (P-STAR); however, VIP together with cAMP-analogs that activate Type II PKA increased P-STAR and further increased steroidogenesis. Our results suggest that VIP-induced STAR expression and function in granulosa cells result from the preferential activation of Type I PKA. Furthermore, the PKA and PKC pathways appear to converge at regulating VIP-mediated Star transcription and translation.

Inhibitory actions of mibefradil on steroidogenesis in mouse Leydig cells: involvement of Ca(2+) entry via the T-type Ca(2+) channel

Intracellular cAMP and Ca(2+) are involved in the regulation of steroidogenic activity in Leydig cells, which coordinate responses to luteinizing hormone (LH) and human chorionic gonadotropin (hCG). However, the identification of Ca(2+) entry implicated in Leydig cell steroidogenesis is not well defined. The objective of this study was to identify the type of Ca(2+) channel that affects Leydig cell steroidogenesis. In vitro steroidogenesis in the freshly dissociated Leydig cells of mice was induced by hCG incubation. The effects of mibefradil (a putative T-type Ca(2+) channel blocker) on steroidogenesis were assessed using reverse transcription (RT)-polymerase chain reaction analysis for the steroidogenic acute regulatory protein (StAR) mRNA expression and testosterone production using radioimmunoassay. In the presence of 1.0 mmol L(-1) extracellular Ca(2+), hCG at 1 to 100 IU noticeably elevated both StAR mRNA level and testosterone secretion (P < 0.05), and the stimulatory effects of hCG were markedly diminished by mibefradil in a dose-dependent manner (P < 0.05). Moreover, the hCG-induced increase in testosterone production was completely removed when external Ca(2+) was omitted, implying that Ca(2+) entry is needed for hCG-induced steroidogenesis. Furthermore, a patch-clamp study revealed the presence of mibefradil-sensitive Ca(2+) currents seen at a concentration range that nearly paralleled those inhibiting steroidogenesis. Collectively, our data provide evidence that hCG-stimulated steroidogenesis is mediated at least in part by Ca(2+) entry carried out by the T-type Ca(2+) channel in the Leydig cells of mice.

Regulation of the steroidogenic acute regulatory protein gene expression: present and future perspectives

Steroid hormones are synthesized in the adrenal gland, gonads, placenta and brain and are critical for normal reproductive function and bodily homeostasis. The steroidogenic acute regulatory (StAR) protein regulates the rate-limiting step in steroid biosynthesis, i.e. the delivery of cholesterol from the outer to the inner mitochondrial membrane. The expression of the StAR protein is predominantly regulated by cAMP-dependent mechanisms in the adrenal and gonads. Whereas StAR plays an indispensable role in the regulation of steroid biosynthesis, a complete understanding of the regulation of its expression and function in steroidogenesis is not available. It has become clear that the regulation of StAR gene expression is a complex process that involves the interaction of a diversity of hormones and multiple signaling pathways that coordinate the cooperation and interaction of transcriptional machinery, as well as a number of post-transcriptional mechanisms that govern mRNA and protein expression. However, information is lacking on how the StAR gene is regulated in vivo such that it is expressed at appropriate times during development and is confined to the steroidogenic cells. Thus, it is not surprising that the precise mechanism involved in the regulation of StAR gene has not yet been established, which is the key to understanding the regulation of steroidogenesis in the context of both male and female development and function.