Differential regulation of steroid hormone biosynthesis in R2C and MA-10 Leydig tumor cells: role of SR-B1-mediated selective cholesteryl ester transport

High-Density Lipoprotein Metabolism and Function in Cardiovascular Diseases: What about Aging and Diet Effects?

Cardiovascular diseases (CVDs) have become the leading global cause of mortality, prompting a heightened focus on identifying precise indicators for their assessment and treatment. In this perspective, the plasma levels of HDL have emerged as a pivotal focus, given the demonstrable correlation between plasma levels and cardiovascular events, rendering them a noteworthy biomarker. However, it is crucial to acknowledge that HDLs, while intricate, are not presently a direct therapeutic target, necessitating a more nuanced understanding of their dynamic remodeling throughout their life cycle. HDLs exhibit several anti-atherosclerotic properties that define their functionality. This functionality of HDLs, which is independent of their concentration, may be impaired in certain risk factors for CVD. Moreover, because HDLs are dynamic parameters, in which HDL particles present different atheroprotective properties, it remains difficult to interpret the association between HDL level and CVD risk. Besides the antioxidant and anti-inflammatory activities of HDLs, their capacity to mediate cholesterol efflux, a key metric of HDL functionality, represents the main anti-atherosclerotic property of HDL. In this review, we will discuss the HDL components and HDL structure that may affect their functionality and we will review the mechanism by which HDL mediates cholesterol efflux. We will give a brief examination of the effects of aging and diet on HDL structure and function.

Rutin inhibits androgen synthesis and metabolism in rat immature Leydig cells in vitro

In the early stage of androgen-sensitive prostate cancer, cancer cells require androgens to grow. Hormone therapy that lowers androgen output or blocks androgen receptor can suppress the growth of this type of prostate cancer. Rutin, a flavonoid derivative of many plants, has numerous pharmacological effects. The objective of this study was to investigate the effect of rutin on androgen biosynthesis in Leydig cells isolated from the testes of pubertal rats. Immature Leydig cells isolated from 35 days-old male Sprague-Dawley rats were cultured in vitro with 0.5-50 μM rutin for 3 hr. Rutin significantly inhibited androgen secretion at 0.5, 5 and 50 μM under basal condition (medium only). At 50 μM, rutin also markedly compromised androgen secretion stimulated by 10 ng/ml luteinising hormone and 10 mM 8-bromoadenosine 3', 5'-cyclic monophosphate. Further analysis demonstrated that rutin compromised the transcript levels of Scarb1, Cyp11a1 and Hsd3b1 and their proteins expression. Rutin directly inhibited rat testicular CYP17A1, HSD17B3 and AKR1C14 activities at 50 μM. Rutin did not alter mitochondrial membrane potential at up to 50 μM. In conclusion, rutin suppresses androgen biosynthesis in Leydig cells through multiple mechanisms, thereby having benefits for the treatment of androgen-sensitive prostate cancer.

Identification of novel genes and pathways regulated by the orphan nuclear receptor COUP-TFII in mouse MA-10 Leydig cells†

In males, Leydig cells are the main producers of testosterone and insulin-like 3 (INSL3), two hormones essential for sex differentiation and reproductive functions. Chicken ovalbumin upstream promoter-transcription factors I (COUP-TFI/NR2F1) and COUP-TFII (NR2F2) belong to the steroid/thyroid hormone nuclear receptor superfamily of transcription factors. In the testis, COUP-TFII is expressed and plays a role in the differentiation of cells committed to give rise to fully functional steroidogenic adult Leydig cells. Steroid production has also been shown to be diminished in COUP-TFII-depleted Leydig cells, indicating an important functional role in steroidogenesis. Until now, only a handful of target genes have been identified for COUP-TFII in Leydig cells. To provide new information into the mechanism of action of COUP-TFII in Leydig cells, we performed microarray analyses of COUP-TFII-depleted MA-10 Leydig cells. We identified 262 differentially expressed genes in COUP-TFII-depleted MA-10 cells. Many of the differentially expressed genes are known to be involved in lipid biosynthesis, lipid metabolism, male gonad development, and steroidogenesis. We validated the microarray data for a subset of the modulated genes by RT-qPCR. Downregulated genes included Hsd3b1, Cyp11a1, Prlr, Shp/Nr0b2, Fdx1, Scarb1, Inha and Gsta3. Finally, analysis of the Gsta3 and Inha gene promoters showed that at least two of the downregulated genes are potentially new direct targets for COUP-TFII. These data provide new evidence that further strengthens the important nature of COUP-TFII in steroidogenesis, androgen homeostasis, cellular defense, and differentiation in mouse Leydig cells.

Reconstituted high density lipoprotein (rHDL), a versatile drug delivery nanoplatform for tumor targeted therapy

rHDL is a synthesized drug delivery nanoplatform exhibiting excellent biocompatibility, which possesses most of the advantages of HDL. rHDL shows almost no toxicity and can be degraded to non-toxic substances in vivo. The severe limitation of the application of various antitumor agents is mainly due to their low bioavailability, high toxicity, poor stability, etc. Favorably, antitumor drug-loaded rHDL nanoparticles (NPs), which are known as an important drug delivery system (DDS), help to change the situation a lot. This DDS shows an outstanding active-targeting ability towards tumor cells and improves the therapeutic effect during antitumor treatment while overcoming the shortcomings mentioned above. In the following text, we will mainly focus on the various applications of rHDL in tumor targeted therapy by describing the properties, preparation, receptor active-targeting ability and antitumor effects of antineoplastic drug-loaded rHDL NPs.

Suppression of testosterone production by nanoparticulate TiO2 is associated with ERK1/2-PKA-PKC signaling pathways in rat primary cultured Leydig cells

Background

Nanoparticulate titanium dioxide (nano-TiO₂) enters the body through various routes and causes organ damage. Exposure to nano-TiO₂ is reported to cause testicular injury in mice or rats and decrease testosterone synthesis, sperm number, and motility. Importantly, nano-TiO₂ suppresses testosterone production by Leydig cells (LCs) and impairs the reproductive capacity of animals.

Methods

In an attempt to establish the molecular mechanisms underlying the inhibitory effect of nano-TiO₂ on testosterone synthesis, primary cultured rat LCs were exposed to varying concentrations of nano-TiO₂ (0, 10, 20, and 40 µg/mL) for 24 hours, and alterations in cell viability, cell injury, testosterone production, testosterone-related factors (StAR, 3βHSD, P450scc, SR-BI, and DAX1), and signaling molecules (ERK1/2, PKA, and PKC) were investigated.

Results

The data show that nano-TiO₂ crosses the membrane into the cytoplasm or nucleus, triggering cellular vacuolization and nuclear condensation. LC viability decreased in a time-dependent manner at the same nano-TiO₂ concentration, nano-TiO₂ treatment (10, 20, and 40 µg/mL) decreased MMP (36.13%, 45.26%, and 79.63%), testosterone levels (11.40% and 44.93%), StAR (14.7%, 44.11%, and 72.05%), 3βHSD (26.56%, 50%, and 79.69%), pERK1/2 (27.83%, 63.61%, and 78.89%), PKA (47.26%, 70.54%, and 85.61%), PKC (30%, 50%, and 71%), SR-BI (16.41%, 41.79%, and 67.16%), and P450scc (39.41%, 55.26%, and 86.84%), and upregulated DAX1 (1.31-, 1.63-, and 3.18-fold) in primary cultured rat LCs.

Conclusion

Our collective findings indicated that nano-TiO₂-mediated suppression of testosterone in LCs was associated with regulation of ERK1/2–PKA–PKC signaling pathways.

Scavenger receptor B type 1: expression, molecular regulation, and cholesterol transport function

Cholesterol is required for maintenance of plasma membrane fluidity and integrity and for many cellular functions. Cellular cholesterol can be obtained from lipoproteins in a selective pathway of HDL-cholesteryl ester (CE) uptake without parallel apolipoprotein uptake. Scavenger receptor B type 1 (SR-B1) is a cell surface HDL receptor that mediates HDL-CE uptake. It is most abundantly expressed in liver, where it provides cholesterol for bile acid synthesis, and in steroidogenic tissues, where it delivers cholesterol needed for storage or steroidogenesis in rodents. SR-B1 transcription is regulated by trophic hormones in the adrenal gland, ovary, and testis; in the liver and elsewhere, SR-B1 is subject to posttranscriptional and posttranslational regulation. SR-B1 operates in several metabolic processes and contributes to pathogenesis of atherosclerosis, inflammation, hepatitis C virus infection, and other conditions. Here, we summarize characteristics of the selective uptake pathway and involvement of microvillar channels as facilitators of selective HDL-CE uptake. We also present the potential mechanisms of SR-B1-mediated selective cholesterol transport; the transcriptional, posttranscriptional, and posttranslational regulation of SR-B1; and the impact of gene variants on expression and function of human SR-B1. A better understanding of this unique pathway and SR-B1's role may yield improved therapies for a wide variety of conditions.

Cyanidin-3-O-glucoside promotes the biosynthesis of progesterone through the protection of mitochondrial function in Pb-exposed rat leydig cells

Cyanidin-3-O-glucoside (C3G) is an anthocyanin that has been reported to reduce the toxicity of heavy metals. In the present study, the protection effects of C3G on the biosynthesis of progesterone, the precursor of testosterone, against lead (Pb) in R2C rat Leydig cells were examined. Treatment of R2C cells with 100 μM Pb resulted in a significant decrease in progesterone production. After being cultured in a medium containing C3G and Pb, R2C cells exhibited an increase in progesterone concentration compared with the Pb treatment, as a result of up-regulation of the expression of the steroidogenic enzymes steroidogenic acute regulatory protein (StAR), 3β-hydroxysteroid dehydrogenase (3β-HSD) and cytochrome P450 enzyme cholesterol side chain cleavage enzyme (CYP11A1). Pb-induced inhibition of extracellular regulated Kinase 1/2 (ERK 1/2) phosphorylation and down-regulation of protein kinase A (PKA) expression were alleviated by C3G. In addition, Pb-induced reactive oxygen species (ROS) overproduction led to mitochondrial depolarization, resulting in a decrease in progesterone biosynthesis, while C3G intervention reduced the ROS level and increased progesterone production. In conclusion, C3G may alleviate the Pb-induced decrease of progesterone biosynthesis by modulating the dysfunction of mitochondria, including decreasing oxidative stress and regulating expression of steroidogenic enzyme proteins.

Cyanidin-3-O-Glucoside Protects against 1,3-Dichloro-2-Propanol-Induced Reduction of Progesterone by Up-regulation of Steroidogenic Enzymes and cAMP Level in Leydig Cells

1,3-Dichloro-2-propanol (1,3-DCP) is a food processing contaminant and has been shown to perturb male reproductive function. Cyanidin-3-O-glucoside (C3G), an anthocyanin antioxidant, is reported to have protective effects on many organs. However, it remains unclear whether C3G protects against chemical-induced reproductive toxicity. The present study was therefore to investigate the intervention of C3G on 1,3-DCP-induced reproductive toxicity in R2C Leydig cells. Results demonstrated that C3G inhibited the 1,3-DCP-induced cytotoxicity and cell shape damage with the effective doses being ranging from 10 to 40 μmol/L. In addition, 1,3-DCP (2 mmol/L) exposure significantly increased the ROS level and mitochondrial membrane potential damage ratio, leading to a decrease in progesterone production, while C3G intervention reduced the ROS level, and increased the progesterone production after 24 h treatment. Most importantly, C3G intervention could up-regulate the cyclic adenosine monophosphate (cAMP) level and protein expression of steroidogenic acute regulatory protein and 3β-hydroxysteroid dehydrogenase. It was concluded that C3G is effective in reducing 1,3-DCP-induced reproductive toxicity via activating steroidogenic enzymes and cAMP level.

Plasma Membrane Origin of the Steroidogenic Pool of Cholesterol Used in Hormone-induced Acute Steroid Formation in Leydig Cells

Hormone-sensitive acute steroid biosynthesis requires trafficking of cholesterol from intracellular sources to the inner mitochondrial membrane. The precise location of the intracellular cholesterol and its transport mechanism are uncertain. Perfringolysin O, produced by Clostridium perfringens, binds cholesterol. Its fourth domain (D4) retains cholesterol-binding properties but not cytotoxicity. We transfected steroidogenic MA-10 cells of mouse Leydig cell tumors with the mCherry-D4 plasmid. Tagged D4 with fluorescent proteins enabled us to track cholesterol. The staining was primarily localized to the inner leaflet of the plasma membrane and was partially released upon treatment with dibutyryl-cAMP (Bt₂cAMP), a cAMP analog. Inhibitors of cholesterol import into mitochondria blocked steroidogenesis and prevented release of D4 (and presumably cholesterol) from the plasma membrane. We conclude that the bulk of the steroidogenic pool of cholesterol, mobilized by Bt₂cAMP for acute steroidogenesis, originates from the plasma membrane. Treatment of the cells with steroid metabolites, 22(R)-hydroxycholesterol and pregnenolone, also reduced D4 release from the plasma membrane, perhaps evidence for a feedback effect of elevated steroid formation on cholesterol release. Interestingly, D4 staining was localized to endosomes during Bt₂cAMP stimulation suggesting that these organelles are on the route of cholesterol trafficking from the plasma membrane to mitochondria. Finally, D4 was expressed in primary rat Leydig cells with a lentivirus and was released from the plasma membrane following Bt₂cAMP treatment. We conclude that the plasma membrane is the source of cholesterol for steroidogenesis in these cells as well as in MA-10 cells.

Current status and future perspectives: TSPO in steroid neuroendocrinology

The mitochondrial translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), has received significant attention both as a diagnostic biomarker and as a therapeutic target for different neuronal disease pathologies. Recently, its functional basis believed to be mediating mitochondrial cholesterol import for steroid hormone production has been refuted by studies examining both in vivo and in vitro genetic Tspo-deficient models. As a result, there now exists a fundamental gap in the understanding of TSPO function in the nervous system, and its putative pharmacology in neurosteroid production. In this review, we discuss several recent findings in steroidogenic cells that are in direct contradiction to previous studies, and necessitate a re-examination of the purported role for TSPO in de novo neurosteroid biosynthesis. We critically examine the pharmacological effects of different TSPO-binding drugs with particular focus on studies that measure neurosteroid levels. We highlight the basis of key misconceptions regarding TSPO that continue to pervade the literature, and the need for interpretation with caution to avoid negative impacts. We also summarize the emerging perspectives that point to new directions that need to be investigated for understanding the molecular function of TSPO, only after which the true potential of this therapeutic target in medicine may be realized.

Targeting SR-BI for Cancer Diagnostics, Imaging and Therapy

Scavenger receptor class B type I (SR-BI) plays an important role in trafficking cholesteryl esters between the core of high density lipoprotein and the liver. Interestingly, this integral membrane protein receptor is also implicated in the metabolism of cholesterol by cancer cells, whereby overexpression of SR-BI has been observed in a number of tumors and cancer cell lines, including breast and prostate cancers. Consequently, SR-BI has recently gained attention as a cancer biomarker and exciting target for the direct cytosolic delivery of therapeutic agents. This brief review highlights these key developments in SR-BI-targeted cancer therapies and imaging probes. Special attention is given to the exploration of high density lipoprotein nanomimetic platforms that take advantage of upregulated SR-BI expression to facilitate targeted drug-delivery and cancer diagnostics, and promising future directions in the development of these agents.

Reconstituted high density lipoprotein mediated targeted co-delivery of HZ08 and paclitaxel enhances the efficacy of paclitaxel in multidrug-resistant MCF-7 breast cancer cells

In the past decades, reconstituted high density lipoprotein (rHDL) has been successfully developed as a drug carrier since the enhanced HDL-lipids uptake is demonstrated in several human cancers. In this paper, rHDL, for the first time, was utilized to co-encapsulate two hydrophobic drugs: an anticancer drug, paclitaxel (PTX), and a new reversal agent for P-gp (P-glycoprotein)-mediated multidrug resistance (MDR) of cancer, N-cyano-1-[(3,4-dimethoxyphenyl)methyl]-3,4-dihydro-6,7-dimethoxy-N'-octyl-2(1H)-isoquinoline-carboximidamide (HZ08). We proposed this drug co-delivery strategy to reverse PTX resistance. The study aimed to develop a biomimetic nanovector, reconstituted high density lipoprotein (rHDL), mediating targeted PTX-HZ08 delivery for cancer therapy. Using sodium cholate dialysis method, we successfully formulated dual-agent co-delivering rHDL nanoparticles (PTX-HZ08-rHDL NPs) with a typical spherical morphology, well-distributed size (~100nm), high drug encapsulation efficiency (approximately 90%), sustained drug release properties and exceptional stability even after storage for 1month or incubation in 10% fetal bovine serum (FBS) DMEM for up to 2days. Results demonstrated that PTX-HZ08-rHDL NPs significantly enhanced anticancer efficacy in vitro, including higher cytotoxicity and better ability to induce cell apoptosis against both PTX-sensitive and -resistant MCF-7 human breast cancer cell lines (MCF-7 and MCF-7/PTX cells). Mechanism studies demonstrated that these improvements could be correlated with increased cellular uptake of PTX mediated by scavenger receptor class B type I (SR-BI) as well as prolonged intracellular retention of PTX due to the HZ08 mediated drug-efflux inhibition. In addition, in vivo investigation showed that the PTX-HZ08-rHDL NPs were substantially safer, have higher tumor-targeted capacity and have stronger antitumor activity than the corresponding dosage of paclitaxel injection. These findings suggested that rHDL NPs could be an ideal tumor-targeted nanovector for simultaneous transfer of insoluble anticancer drug and drug resistance reversal agents. The PTX-HZ08-rHDL NPs co-delivery system might be a new promising strategy to overcome tumor drug resistance.

Role of the steroidogenic acute regulatory protein in health and disease

Steroid hormones are an important class of regulatory molecules that are synthesized in steroidogenic cells of the adrenal, ovary, testis, placenta, brain, and skin, and influence a spectrum of developmental and physiological processes. The steroidogenic acute regulatory protein (STAR) predominantly mediates the rate-limiting step in steroid biosynthesis, i.e., the transport of the substrate of all steroid hormones, cholesterol, from the outer to the inner mitochondrial membrane. At the inner membrane, cytochrome P450 cholesterol side chain cleavage enzyme cleaves the cholesterol side chain to form the first steroid, pregnenolone, which is converted by a series of enzymes to various steroid hormones in specific tissues. Both basic and clinical evidence have demonstrated the crucial involvement of the STAR protein in the regulation of steroid biosynthesis. Multiple levels of regulation impinge on STAR action. Recent findings demonstrate that hormone-sensitive lipase, through its action on the hydrolysis of cholesteryl esters, plays an important role in regulating STAR expression and steroidogenesis which involve the liver X receptor pathway. Activation of the latter influences macrophage cholesterol efflux that is a key process in the prevention of atherosclerotic cardiovascular disease. Appropriate regulation of steroid hormones is vital for proper functioning of many important biological activities, which are also paramount for geriatric populations to live longer and healthier. This review summarizes the current level of understanding on tissue-specific and hormone-induced regulation of STAR expression and steroidogenesis, and provides insights into a number of cholesterol and/or steroid coupled physiological and pathophysiological consequences.

Notch signaling represses GATA4-induced expression of genes involved in steroid biosynthesis

Notch2 and Notch3 and genes of the Notch signaling network are dynamically expressed in developing follicles, where they are essential for granulosa cell proliferation and meiotic maturation. Notch receptors, ligands, and downstream effector genes are also expressed in testicular Leydig cells, predicting a potential role in regulating steroidogenesis. In this study, we sought to determine if Notch signaling in small follicles regulates the proliferation response of granulosa cells to FSH and represses the up-regulation steroidogenic gene expression that occurs in response to FSH as the follicle grows. Inhibition of Notch signaling in small preantral follicles led to the up-regulation of the expression of genes in the steroid biosynthetic pathway. Similarly, progesterone secretion by MA-10 Leydig cells was significantly inhibited by constitutively active Notch. Together, these data indicated that Notch signaling inhibits steroidogenesis. GATA4 has been shown to be a positive regulator of steroidogenic genes, including STAR protein, P450 aromatase, and 3B-hydroxysteroid dehydrogenase. We observed that Notch downstream effectors HEY1, HEY2, and HEYL are able to differentially regulate these GATA4-dependent promoters. These data are supported by the presence of HEY/HES binding sites in these promoters. These studies indicate that Notch signaling has a role in the complex regulation of the steroidogenic pathway.

A cell-autonomous molecular cascade initiated by AMP-activated protein kinase represses steroidogenesis

Steroid hormones regulate essential physiological processes, and inadequate levels are associated with various pathological conditions. In testosterone-producing Leydig cells, steroidogenesis is strongly stimulated by luteinizing hormone (LH) via its receptor leading to increased cyclic AMP (cAMP) production and expression of the steroidogenic acute regulatory (STAR) protein, which is essential for the initiation of steroidogenesis. Steroidogenesis then passively decreases with the degradation of cAMP into AMP by phosphodiesterases. In this study, we show that AMP-activated protein kinase (AMPK) is activated following cAMP-to-AMP breakdown in MA-10 and MLTC-1 Leydig cells. Activated AMPK then actively inhibits cAMP-induced steroidogenesis by repressing the expression of key regulators of steroidogenesis, including Star and Nr4a1. Similar results were obtained in Y-1 adrenal cells and in the constitutively steroidogenic R2C cells. We have also determined that maximum AMPK activation following stimulation of steroidogenesis in MA-10 Leydig cells occurs when steroid hormone production has reached a plateau. Our data identify AMPK as a molecular rheostat that actively represses steroid hormone biosynthesis to preserve cellular energy homeostasis and prevent excess steroid production.

High-density lipoprotein: a novel target for antirestenosis therapy

Restenosis is an integral pathological process central to the recurrent vessel narrowing after interventional procedures. Although the mechanisms for restenosis are diverse in different pathological conditions, endothelial dysfunction, inflammation, vascular smooth muscle cell (SMC) proliferation, and myofibroblasts transition have been thought to play crucial role in the development of restenosis. Indeed, there is an inverse relationship between high-density lipoprotein (HDL) levels and risk for coronary heart disease (CHD). However, relatively studies on the direct assessment of HDL effect on restenosis are limited. In addition to involvement in the cholesterol reverse transport, many vascular protective effects of HDL, including protection of endothelium, antiinflammation, antithrombus actions, inhibition of SMC proliferation, and regulation by adventitial effects may contribute to the inhibition of restenosis, though the exact relationships between HDL and restenosis remain to be elucidated. This review summarizes the vascular protective effects of HDL, emphasizing the potential role of HDL in intimal hyperplasia and vascular remodeling, which may provide novel prophylactic and therapeutic strategies for antirestenosis.

Scavenger receptor class B type I (SR-BI): a versatile receptor with multiple functions and actions

Scavenger receptor class B type I (SR-BI), is a physiologically relevant HDL receptor that mediates selective uptake of lipoprotein (HDL)-derived cholesteryl ester (CE) in vitro and in vivo. Mammalian SR-BI is a 509-amino acid, ~82 kDa glycoprotein that contains N- and C-terminal cytoplasmic domains, two-transmembrane domains, as well as a large extracellular domain containing 5-6 cysteine residues and multiple sites for N-linked glycosylation. The size and structural characteristics of SR-BI, however, vary considerably among lower vertebrates and insects. Recently, significant progress has been made in understanding the molecular mechanisms involved in the posttranscriptional/posttranslational regulation of SR-BI in a tissue specific manner. The purpose of this review is to summarize the current body of knowledge about the events and molecules connected with the posttranscriptional/posttranslational regulation of SR-BI and to update the molecular and functional characteristics of the insect SR-BI orthologs.

Antioxidant mediated ameliorative steroidogenesis by Commelina benghalensis L. and Cissus quadrangularis L. against quinalphos induced male reproductive toxicity

Quinalphos (QP) is speculated to cause endocrine disruption through the generation of reactive oxygen species (ROS) by oxidative stress (OS). Exposure of QP decreased testosterone level considerably which resulted in reduced viable sperms in mice. The QP induced toxicity is initiated by the formation of free radicals as it is evidenced from the increased Lipid peroxidation (LPO) and diminution of antioxidant enzymes in testicular tissue. Increased serum cholesterol and reduced testicular cholesterol indicated the inhibition of cholesterol transport and biosynthesis in testicular tissues. Lack of cholesterol in testicular tissue impaired the steroidogenesis by down-regulating the expression of StAR protein, Cytochrome P450, 3β-HSD and 17β-HSD leading to reduced testosterone level. Treatment of Commelina benganlensis (CBE) and Cissus quadrangularis (CQE) significantly recovered the alterations in antioxidant profiles as well as increased LPO, thereby recovering the decreased mRNA expression levels of intermediate enzymes. However, CQE effectively protected the OS and prevented the inhibition of steroidogenesis thereby preventing male infertility.

Templated high density lipoprotein nanoparticles as potential therapies and for molecular delivery

High density lipoproteins (HDLs) are dynamic natural nanoparticles best known for their role in cholesterol transport and the inverse correlation that exists between blood HDL levels and the risk of developing coronary heart disease. In addition, enhanced HDL-cholesterol uptake has been demonstrated in several human cancers. As such, the use of HDL as a therapeutic and as a vehicle for systemic delivery of drugs and as imaging agents is increasingly important. HDLs exist on a continuum from the secreted HDL-scaffolding protein, apolipoprotein A-1 (Apo A1), to complex, spherical "mature" HDLs. Aspects of HDL particles including their size, shape, and surface chemical composition are being recognized as critical to their diverse biological functions. Here we review HDL biology; strategies for synthesizing HDLs; data supporting the clinical use and benefit of directly administered HDL; a rationale for developing synthetic methods for spherical, mature HDLs; and, the potential to employ HDLs as therapies, imaging agents, and drug delivery vehicles. Importantly, methods that utilize nanoparticle templates to control synthetic HDL size, shape, and surface chemistry are highlighted.

Regulation of expression and function of scavenger receptor class B, type I (SR-BI) by Na+/H+ exchanger regulatory factors (NHERFs)

Scavenger receptor class B, type I (SR-BI) binds HDL and mediates selective delivery of cholesteryl esters (CEs) to the liver, adrenals, and gonads for product formation (bile acids and steroids). Because relatively little is known about SR-BI posttranslational regulation in steroidogenic cells, we examined the roles of Na(+)/H(+) exchanger regulatory factors (NHERFs) in regulating SR-BI expression, SR-BI-mediated selective CE uptake, and steroidogenesis. NHERF1 and NHERF2 mRNA and protein are expressed at varying levels in model steroidogenic cell lines and the adrenal, with only low expression of PDZK1 (NHERF3) and NHERF4. Dibutyryl cyclic AMP decreased NHERF1 and NHERF2 and increased SR-BI mRNA expression in primary rat granulosa cells and MLTC-1 cells, whereas ACTH had no effect on NHERF1 and NHERF2 mRNA levels but decreased their protein levels in rat adrenals. Co-immunoprecipitation, colocalization, bimolecular fluorescence complementation, and mutational analysis indicated that SR-BI associates with NHERF1 and NHERF2. NHERF1 and NHERF2 down-regulated SR-BI protein expression through inhibition of its de novo synthesis. NHERF1 and NHERF2 also inhibited SR-BI-mediated selective CE transport and steroidogenesis, which were markedly attenuated by partial deletions of the PDZ1 or PDZ2 domain of NHERF1, the PDZ2 domain of NHERF2, or the MERM domains of NHERF1/2 or by gene silencing of NHERF1/2. Moreover, an intact COOH-terminal PDZ recognition motif (EAKL) in SR-BI is needed. Transient transfection of hepatic cell lines with NHERF1 or NHERF2 caused a significant reduction in endogenous protein levels of SR-BI. Collectively, these data establish NHERF1 and NHERF2 as SR-BI protein binding partners that play a negative role in the regulation of SR-BI expression, selective CE transport, and steroidogenesis.

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.

Involvement of Src family of kinases and cAMP phosphodiesterase in the luteinizing hormone/chorionic gonadotropin receptor-mediated signaling in the corpus luteum of monkey

BACKGROUND

In higher primates, during non-pregnant cycles, it is indisputable that circulating LH is essential for maintenance of corpus luteum (CL) function. On the other hand, during pregnancy, CL function gets rescued by the LH analogue, chorionic gonadotropin (CG). The molecular mechanisms involved in the control of luteal function during spontaneous luteolysis and rescue processes are not completely understood. Emerging evidence suggests that LH/CGR activation triggers proliferation and transformation of target cells by various signaling molecules as evident from studies demonstrating participation of Src family of tyrosine kinases (SFKs) and MAP kinases in hCG-mediated actions in Leydig cells. Since circulating LH concentration does not vary during luteal regression, it was hypothesized that decreased responsiveness of luteal cells to LH might occur due to changes in LH/CGR expression dynamics, modulation of SFKs or interference with steroid biosynthesis.

METHODS

Since, maintenance of structure and function of CL is dependent on the presence of functional LH/CGR its expression dynamics as well as mRNA and protein expressions of SFKs were determined throughout the luteal phase. Employing well characterized luteolysis and CL rescue animal models, activities of SFKs, cAMP phosphodiesterase (cAMP-PDE) and expression of SR-B1 (a membrane receptor associated with trafficking of cholesterol ester) were examined. Also, studies were carried out to investigate the mechanisms responsible for decline in progesterone biosynthesis in CL during the latter part of the non-pregnant cycle.

RESULTS AND DISCUSSION

The decreased responsiveness of CL to LH during late luteal phase could not be accounted for by changes in LH/CGR mRNA levels, its transcript variants or protein. Results obtained employing model systems depicting different functional states of CL revealed increased activity of SFKs [pSrc (Y-416)] and PDE as well as decreased expression of SR-B1 correlating with initiation of spontaneous luteolysis. However, CG, by virtue of its heroic efforts, perhaps by inhibition of SFKs and PDE activation, prevents CL from undergoing regression during pregnancy.

CONCLUSIONS

The results indicated participation of activated Src and increased activity of cAMP-PDE in the control of luteal function in vivo. That the exogenous hCG treatment caused decreased activation of Src and cAMP-PDE activity with increased circulating progesterone might explain the transient CL rescue that occurs during early pregnancy.

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.

Effects of apigenin on steroidogenesis and steroidogenic acute regulatory gene expression in mouse Leydig cells

Previous studies reported that the age-related decline in testosterone biosynthesis is associated with a decrease in the steroidogenic acute regulatory (StAR) protein which regulates the rate-limiting step of testosterone biosynthesis. To explore the possibility of delaying this decline using a dietary approach, we have examined the effect of a natural flavonoid, apigenin, on StAR gene expression in mouse Leydig cells. Incubation of these cells with the flavonoid enhanced cyclic adenosine monophosphate (cAMP)-induced steroidogenesis and StAR protein expression. The results from the analyses of StAR mRNA by reverse transcription-polymerase chain reaction and the luciferase assays of StAR promoter activity indicated that this flavonoid enhanced StAR gene expression at the level of transcription. Further studies showed that apigenin blocked the thromboxane A2 receptor and interrupted the signaling through the cyclooxygenase-2-thromboxane A synthase-thromboxane A2-receptor pathway, resulting in a reduction of DAX-1 (dosage sensitive sex reversal-adrenal hypoplasia congenita critical region on the X chromosome, gene-1) protein, a transcriptional repressor of StAR gene expression. When DAX-1 protein was reduced, the sensitivity of the Leydig cells was dramatically enhanced, with sub-threshold level of cAMP being able to induce maximal levels of StAR protein expression and steroid hormone production. The present study suggests a potential application of apigenin to improve StAR protein expression and steroidogenic sensitivity of aging Leydig cells.

Regulation of cytochrome P450 1B1 expression by luteinizing hormone in mouse MA-10 and rat R2C Leydig cells: role of protein kinase A

In the present study, we investigated the signaling pathway involved in luteinizing hormone (LH)-mediated regulation of testicular CYP1B1 in mouse MA-10 and rat R2C Leydig cells. CYP1B1 mRNA and protein levels were measured in MA-10 and R2C cells treated with LH and protein kinase activators or inhibitors. Treatment with LH or 8-bromo-cAMP, a protein kinase A (PRKA) activator, increased CYP1B1 expression and PRKA activity in a concentration-dependent manner in both cell lines, albeit to different extents. Treatment with 8-(4-chlorophenylthio)adenosine-3',5'-cyclic monophosphorothioate, Rp-isomer, a PRKA inhibitor, decreased basal CYP1B1 expression and attenuated LH-elicited increases in CYP1B1 mRNA and protein levels and PRKA activity. In contrast, treatment with a protein kinase G activator or an inhibitor of protein kinase C had no effect on basal or LH-induced CYP1B1 expression in MA-10 or R2C cells. Collectively, the results identify PRKA as the major signaling pathway involved in the LH-mediated regulation of testicular CYP1B1 expression in Leydig tumor cells.

Deficiency of scavenger receptor class B type I negatively affects progesterone secretion in human granulosa cells

Our goal was to examine the effect of deficiency of the lipoprotein receptor, scavenger receptor class B type I (SR-BI), on progesterone secretion in human granulosa cells (HGL5). Scrambled or SR-BI small interfering RNA [knockdown (KD)] cells were exposed to dimethylsulfoxide [DMSO, vehicle for forskolin (Fo)], Fo, serum, high-density lipoprotein, low-density lipoprotein (LDL), or Fo plus lipoproteins or serum for 24 h. Progesterone secretion was lower in all of the SR-BI KD cells regardless of treatment. We examined progesterone secretion in SR-BI KD, LDL receptor KD, and double KD cells incubated with DMSO, Fo, LDL, or Fo + LDL for 6-24 h. As compared with scrambled cells, progesterone secretion was lower in SR-BI and double KD cells regardless of treatment; whereas progesterone secretion was only lower in LDL receptor KD cells incubated with LDL and Fo + LDL. We measured phosphorylation of hormone-sensitive lipase (pHSL) expression, intracellular total cholesterol (TC) mass, and progesterone secretion in scrambled and SR-BI KD cells incubated with DMSO or Fo for 2-24 h. The expression of pHSL was similar between the cells and conditions. The mean change in TC mass and progesterone secretion was lower in SR-BI KD cells exposed to DMSO and Fo. Incubating SR-BI KD cells with 22-hydroxy cholesterol did not overcome the reduction in progesterone secretion. At different time points, RNA expression of steroidogenic acute regulatory protein, side-chain cleavage, and 3β-hydroxysteroid dehydrogenase was significantly lower in SR-BI KD cells incubated with Fo. In conclusion, SR-BI protein deficiency, in part, might explain progesterone deficiency in some infertile women.

Upregulation of peripubertal rat Leydig cell steroidogenesis following 24 h in vitro and in vivo exposure to atrazine

Atrazine is currently one of the most widely used herbicides in the United States and elsewhere. Here we examined 24 h in vitro and in vivo effects of atrazine on androgen production and on expression and activity of steroidogenic enzymes and regulatory proteins involved in cyclic adenosine monophosphate (cAMP)-signaling pathway in peripubertal rat Leydig cells. When in vitro added, 1-50 μM atrazine increased basal and human chorion gonadotropin-stimulated testosterone production and accumulation of cAMP in the medium of treated cells. The stimulatory action of atrazine on androgen production but not on cAMP accumulation was abolished in cells with inhibited protein kinase A. Atrazine also stimulated the expression of mRNA transcripts for steroidogenic factor-1, steroidogenic acute regulatory protein, cytochrome P450 (CYP)17A1, and 17β-hydroxysteroid dehydrogenase (HSD), as well as the activity of CYP17A1 and 17βHSD. The stimulatory effects of atrazine on cAMP accumulation and androgen production were also observed during the first 3 days of in vivo treatment (200 mg/kg body weight, by gavage) followed by a decline during further treatment. These results indicate that atrazine has a transient stimulatory action on cAMP signaling pathway in Leydig cells, leading to facilitated androgenesis.

Blocking L-type calcium channels reduced the threshold of cAMP-induced steroidogenic acute regulatory gene expression in MA-10 mouse Leydig cells

Previous studies have reported the roles of Ca(2+) in steroidogenesis. The present study has investigated an inhibitory effect of Ca(2+) influx through L-type Ca(2+) channels on gene expression of steroidogenic acute regulatory (STAR) protein that regulates the transfer of substrate cholesterol to the inner mitochondrial membrane for steroidogenesis. Blocking Ca(2+) influx through L-type Ca(2+) channels using the selective Ca(2+) channel blocker, nifedipine, markedly enhanced cAMP-induced STAR protein expression and progesterone production in MA-10 mouse Leydig cells. This was confirmed by utilization of different L-type Ca(2+) channel blockers. Reverse transcription-PCR analyses of Star mRNA and luciferase assays of Star promoter activity indicated that blocking Ca(2+) influx through L-type Ca(2+) channels acted at the level of Star gene transcription. Further studies showed that blocking the Ca(2+) channel enhanced Star gene transcription by depressing the expression of DAX-1 (NR0B1 as listed in the MGI Database) protein, a transcriptional repressor of Star gene expression. It was also observed that there is a synergistic interaction between nifedipine and cAMP. Normally, sub-threshold levels of cAMP are unable to induce steroidogenesis, but in the presence of the L-type Ca(2+) channel blocker, they increased STAR protein and steroid hormone to the maximal levels. However, in the absence of minimal levels of cAMP, none of the L-type Ca(2+) channel blockers are able to induce Star gene expression. These observations indicate that Ca(2+) influx through L-type Ca(2+) channels is involved in an inhibitory effect on Star gene expression. Blocking L-type Ca(2+) channel attenuated the inhibition and reduced the threshold of cAMP-induced Star gene expression in Leydig cells.

Changes in gene expression in somatic cells of rat testes resulting from hormonal modulation and radiation-induced germ cell depletion

Although gonadotropins and androgen are required for normal spermatogenesis and both testosterone and follicle-stimulating hormone (FSH) are responsible for the inhibition of spermatogonial differentiation that occurs in irradiated rats, it has been difficult to identify the specific genes involved. To study specific hormonally regulated changes in somatic cell gene expression in the testis that may be involved in these processes, without the complication of changing populations of germ cells, we used irradiated LBNF(1) rats, the testes of which contain almost exclusively somatic cells except for a few type A spermatogonia. Three different groups of these rats were treated with various combinations of gonadotropin-releasing hormone antagonist, an androgen receptor antagonist (flutamide), testosterone, and FSH, and we compared the gene expression levels 2 wk later to those of irradiated-only rats by microarray analysis. By dividing the gene expression patterns into three major patterns and 11 subpatterns, we successfully distinguished, in a single study, the genes that were specifically regulated by testosterone, by luteinizing hormone (LH), and by FSH from the large number of genes that were not hormonally regulated in the testis. We found that hormones produced more dramatic upregulation than downregulation of gene expression: Testosterone had the strongest upregulatory effect, LH had a modest but appreciable upregulatory effect, and FSH had a minor upregulatory effect. We also separately identified the somatic cell genes that were chronically upregulated by irradiation. Thus, the present study identified gene expression changes that may be responsible for hormonal action on somatic cells to support normal spermatogenesis and the hormone-mediated block in spermatogonial development after irradiation.

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

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

Atrazine oral exposure of peripubertal male rats downregulates steroidogenesis gene expression in Leydig cells

In the present study, we investigated the effects of oral dosing of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) to peripubertal male rats (50 and 200 mg/kg body weight daily from postnatal days 23-50) on ex vivo Leydig cell steroidogenesis. Leydig cells from treated rats were characterised by significant decline in mRNA transcripts of several genes responsible for steroidogenesis: luteinizing hormone receptor (LHR), scavenger receptor-B1, steroidogenic acute regulatory protein, translocator protein, steroidogenic factor-1, phosphodiesterase 4B, 3beta-hydroxysteroid dehydrogenase (HSD), CYP17A1, and 17betaHSD. In the presence of human chorion gonadotropin, the dose-dependent decrease in extracellular cAMP level and accordingly strong inhibition of androgenesis were obtained. The transcription of LHR gene in Leydig cells of atrazine-treated rats was downregulated in a dose-dependent manner, which could be the reason for reduction in cAMP level and expression of cAMP-dependent genes. To clarify the activity of the steroidogenic enzymes responsible for androgenesis, purified Leydig cells were challenged with different steroid substrates (22OH-cholesterol, pregnenolone, progesterone, and Delta(4)-androstenedione), and the obtained results indicated inhibition of androgen production in Leydig cells isolated from atrazine-treated animals in the presence of all those substrates. However, when Leydig cells were challenged with 22OH-cholesterol, the progesterone level in the incubation medium was unchanged, indicating that decrease in cholesterol transport and/or CYP17A1 and 17betaHSD activity are most probably responsible for inhibition of androgen production after the addition of different substrates. Our results demonstrated that in vivo exposure to atrazine affects Leydig cell steroidogenesis via the inhibition of steroidogenesis gene expression, which is accompanied by decreased androgenesis.

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.

Role of dosage-sensitive sex reversal, adrenal hypoplasia congenita, critical region on the X chromosome, gene 1 in protein kinase A- and protein kinase C-mediated regulation of the steroidogenic acute regulatory protein expression in mouse Leydig tumor cells: mechanism of action

Dosage-sensitive sex reversal, adrenal hypoplasia congenita, critical region on the X chromosome, gene 1 (DAX-1) is an orphan nuclear receptor that has been demonstrated to be instrumental to the expression of the steroidogenic acute regulatory (StAR) protein that regulates steroid biosynthesis in steroidogenic cells. However, its mechanism of action remains obscure. The present investigation was aimed at exploring the molecular involvement of DAX-1 in protein kinase A (PKA)- and protein kinase C (PKC)-mediated regulation of StAR expression and its concomitant impact on steroid synthesis using MA-10 mouse Leydig tumor cells. We demonstrate that activation of the PKA and PKC pathways, by a cAMP analog dibutyryl (Bu)2cAMP [(Bu)2cAMP] and phorbol 12-myristate 13-acetate (PMA), respectively, markedly decreased DAX-1 expression, an event that was inversely correlated with StAR protein, StAR mRNA, and progesterone levels. Notably, the suppression of DAX-1 requires de novo transcription and translation, suggesting that the effect of DAX-1 in regulating StAR expression is dynamic. Chromatin immunoprecipitation studies revealed the association of DAX-1 with the proximal but not the distal region of the StAR promoter, and both (Bu)2cAMP and PMA decreased in vivo DAX-1-DNA interactions. EMSA and reporter gene analyses demonstrated the functional integrity of this interaction by showing that DAX-1 binds to a DNA hairpin at position -44/-20 bp of the mouse StAR promoter and that the binding of DAX-1 to this region decreases progesterone synthesis by impairing transcription of the StAR gene. In support of this, targeted silencing of endogenous DAX-1 elevated basal, (Bu)2cAMP-, and PMA-stimulated StAR expression and progesterone synthesis. Transrepression of the StAR gene by DAX-1 was tightly associated with expression of the nuclear receptors Nur77 and steroidogenic factor-1, demonstrating these factors negatively modulate the steroidogenic response. These findings provide insight into the molecular events by which DAX-1 influences the PKA and PKC signaling pathways involved in the regulation of the StAR protein and steroidogenesis in mouse Leydig tumor cells.

Hormone-sensitive lipase expression and IHC localization in the rat ovary, oviduct, and uterus

Hormone-sensitive lipase (HSL) is a key regulator of cholesterol esters metabolism. The aim of this study was to determine HSL localization in rat female reproductive organs during the ovarian cycle by IHC methods. HSL was located in the ovarian epithelium. The granulosa cells and oocytes of primordial follicles were immunonegative. In mature follicles, HSL was found in oocytes and theca and granulosa cells. However, HSL expression in theca cells and oocytes decreased during follicular atresia. Luteal cells showed HSL staining in cytoplasm during proestrus and estrus, in the nucleus during metestrus, and in cytoplasm and the nucleus during diestrus. In the tubaric ampulla, HSL was located in the epithelial cells nuclei and in the cilia during proestrus and estrus but mainly in the nucleus during metestrus and diestrus. In the isthmus, cells showed HSL immunolabeling in the nucleus and cilia during proestrus, but only in the cilia during estrus, metestrus, and diestrus. In the uterus, HSL was found in the epithelial cells nuclei. HSL-immunoreactive bands at 84, 67, 54, and 43 kDa were found in rat female reproductive organs. HSL labeling in the nucleus of epithelial and germ cells suggests an as yet unknown function for this protein, probably related to oogenesis and cell proliferation.

Mono-(2-ethylhexyl) phthalate stimulates basal steroidogenesis by a cAMP-independent mechanism in mouse gonadal cells of both sexes

Phthalates are widely used as plasticizers in a number of daily-life products. In this study, we investigated the influence of mono-(2-ethylhexyl) phthalate (MEHP), the active metabolite of the frequently used plasticizer di-(2-ethylhexyl) phthalate (DEHP), on gonadal steroidogenesisin vitro. MEHP (25–100 μM) stimulated basal steroid synthesis in a concentration-dependent manner in immortalized mouse Leydig tumor cells (MLTC-1). The stimulatory effect was also detected in KK-1 granulosa tumor cells. MEHP exposure did not influence cAMP or StAR protein levels and induced a gene expression profile of key steroidogenic proteins different from the one induced by human chorionic gonadotropin (hCG). Simultaneous treatment with MEHP and a p450scc inhibitor (aminoglutethimide) indicated that MEHP exerts its main stimulatory effect prior to pregnenolone formation. MEHP (10–100 μM) up-regulated hormone-sensitive lipase and 3-hydroxy-3-methylglutaryl coenzyme A reductase, suggesting that MEHP increases the amount of cholesterol available for steroidogenesis. Our data suggest that MEHP, besides its known inhibitory effect on hCG action, can directly stimulate gonadal steroidogenesis in both sexes through a cAMP- and StAR-independent mechanism. The anti-steroidogenic effect of DEHP has been proposed to cause developmental disorders such as hypospadias and cryptorchidism, whereas a stimulation of steroid synthesis may prematurely initiate the onset of puberty and theoretically affect the hypothalamic–pituitary–gonadal axis.

Inhibition of thromboxane a synthase activity enhances steroidogenesis and steroidogenic acute regulatory gene expression in MA-10 mouse Leydig cells

The cyclooxygenase-2 (COX2)-dependent inhibition of Leydig cell steroidogenesis has been demonstrated. To understand the mechanism for this effect of COX2, the present study examined the role of an enzyme downstream of COX2, namely thromboxane A synthase (TBXAS), in steroidogenesis. Inhibition of TBXAS activity with the inhibitor furegrelate induced a concentration-dependent increase in cAMP-induced steroidogenic acute regulatory (StAR) protein in MA-10 mouse Leydig cells. The increase in StAR protein occurred concomitantly with a significant increase in steroid hormone production. Similar results were obtained in StAR promoter activity assays and RT-PCR analyses of StAR mRNA levels, suggesting that inhibition of TBXAS activity enhanced StAR gene transcription. These observations were corroborated when TBXAS expression was specifically inhibited by RNA interference. Although the RNA interference reduced mRNA levels of TBXAS, it increased StAR mRNA levels, StAR protein, and steroidogenesis. Additional studies indicated that inhibition of TBXAS activity reduced DAX-1 protein, a repressor in StAR gene transcription. In the absence of cAMP, inhibition of TBXAS activity did not induce a significant increase in steroid hormone and StAR protein. However, addition of a low level of cAMP analogs dramatically increased steroidogenesis. Lastly, inhibition of protein kinase A activity essentially abolished the steroidogenic effect of the TBXAS inhibitor. Thus, the results from the present study suggest that a minimal level of protein kinase A activity is required for the steroidogenic effect of the TBXAS inhibitor and that inhibition of TBXAS activity or its expression increase the steroidogenic sensitivity of MA-10 mouse Leydig cells to cAMP stimulation.

Insulin-like growth factor-I, regulating aromatase expression through steroidogenic factor 1, supports estrogen-dependent tumor Leydig cell proliferation

The aim of this study was to investigate the role of estrogens in Leydig cell tumor proliferation. We used R2C rat Leydig tumor cells and testicular samples from Fischer rats with a developed Leydig tumor. Both experimental models express high levels of aromatase and estrogen receptor alpha (ERalpha). Treatment with exogenous 17beta-estradiol (E(2)) induced proliferation of R2C cells and up-regulation of cell cycle regulators cyclin D1 and cyclin E, the expression of which was blocked by addition of antiestrogens. These observations led us to hypothesize an E(2)/ERalpha-dependent mechanism for Leydig cell tumor proliferation. In determining the molecular mechanism responsible for aromatase overexpression, we found that total and phosphorylated levels of transcription factors cyclic AMP-responsive element binding protein and steroidogenic factor 1 (SF-1) were higher in tumor samples. Moreover, we found that tumor Leydig cells produce high levels of insulin-like growth factor I (IGF-I), which increased aromatase mRNA, protein, and activity as a consequence of increased total and phosphorylated SF-1 levels. Specific inhibitors of IGF-I receptor, protein kinase C, and phosphatidylinositol 3-kinase determined a reduction in SF-1 expression and in IGF-I-dependent SF-1 recruitment to the aromatase PII promoter. The same inhibitors also inhibited aromatase expression and activity and, consequently, R2C cell proliferation. We can conclude that one of the molecular mechanisms determining Leydig cell tumorigenesis is an excessive estrogen production that stimulates a short autocrine loop determining cell proliferation. In addition, cell-produced IGF-I amplifies estrogen signaling through an SF-1-dependent up-regulation of aromatase expression. The identification of this molecular mechanism will be helpful in defining new therapeutic approaches for Leydig cell tumors.

Effect of hypoxia on the release of vascular endothelial growth factor and testosterone in mouse TM3 Leydig cells

Hypoxia has been shown to stimulate the expression of vascular endothelial growth factor (VEGF), which is a major mediator for angiogenesis and vasculogenesis. During hypoxia, VEGF promotes angiogenesis in the testis. However, the effect of VEGF on the steroidogenesis of testosterone and the cell proliferation in Leydig cells is unclear. To assess the effects and the action mechanisms of hypoxia, a mouse TM3 Leydig cell line was employed in the present study. The Leydig cells were incubated in an incubator chamber (95% N2-5% CO2) for 1-24 h. The cultured media were collected and assayed by testosterone RIA and VEGF enzyme immunoassay. 3-(4,50-Dimethylthiazol-2-yl)-2.5-diphenyltetrazolium bromide assay was used to detect the proliferation of Leydig cells. The present results showed that the proliferation of Leydig cells was enhanced significantly by hypoxia. The basal VEGF release was increased, and the response of VEGF production to human chorionic gonadotropin (hCG) was also enhanced in hypoxic condition. During hypoxia, administration of hCG or VEGF stimulated proliferation of Leydig cells, but the stimulatory effect was abolished by the administration of anti-VEGF antibody. Higher doses of VEGF stimulated testosterone release in a dose-dependent manner. Administration of anti-VEGF antibody abolished the stimulatory effect of VEGF on testosterone release. These data suggest that hypoxia stimulates cell proliferation and testosterone release in Leydig cells via an increase of VEGF production.

cAMP increases mitochondrial cholesterol transport through the induction of arachidonic acid release inside this organelle in Leydig cells

We have investigated the direct effect of arachidonic acid on cholesterol transport in intact cells or isolated mitochondria from steroidogenic cells and the effect of cyclic-AMP on the specific release of this fatty acid inside the mitochondria. We show for the first time that cyclic-AMP can regulate the release of arachidonic acid in a specialized compartment of MA-10 Leydig cells, e.g. the mitochondria, and that the fatty acid induces cholesterol transport through a mechanism different from the classical pathway. Arachidonic acid and arachidonoyl-CoA can stimulate cholesterol transport in isolated mitochondria from nonstimulated cells. The effect of arachidonoyl-CoA is inhibited by the reduction in the expression or in the activity of a mitochondrial thioesterase that uses arachidonoyl-CoA as a substrate to release arachidonic acid. cAMP-induced arachidonic acid accumulation into the mitochondria is also reduced when the mitochondrial thioesterase activity or expression is blocked. This new feature in the regulation of cholesterol transport by arachidonic acid and the release of arachidonic acid in specialized compartment of the cells could offer novel means for understanding the regulation of steroid synthesis but also would be important in other situations such as neuropathological disorders or oncology disorders, where cholesterol transport plays an important role.

Molecular Mechanisms of Insulin-like Growth Factor-I Mediated Regulation of the Steroidogenic Acute Regulatory Protein in Mouse Leydig Cells

Growth factors are known to play diverse roles in steroidogenesis, a process regulated by the mitochondrial steroidogenic acute regulatory (StAR) protein. The mechanism of action of one such growth factor, IGF-I, was investigated in mouse Leydig tumor (mLTC-1) cells to determine its potential role in the regulation of StAR expression. mLTC-1 cells treated with IGF-I demonstrated temporal and concentration-dependent increases in StAR expression and steroid synthesis. However, IGF-I had no effect on cytochrome P450 side-chain cleavage or 3beta-hydroxysteroid dehydrogenase protein levels. IGF-I was capable of augmenting N,O'-dibutyrl-cAMP-stimulated steroidogenic responsiveness in these cells. The steroidogenic potential of IGF-I was also confirmed in primary cultures of isolated mouse Leydig cells. IGF-I increased phosphorylation of ERK1/2, an event inhibited by the MAPK/ERK inhibitors, PD98059 and U0126. Interestingly, inhibition of ERK activity enhanced IGF-I-mediated StAR protein expression, but phosphorylation of StAR was undetectable, an observation in contrast to that seen with N,O'-dibutyrl-cAMP signaling. Further studies demonstrated that these events were tightly correlated with the expression of dosage-sensitive sex reversal, adrenal hypoplasia congenita, critical region on the X chromosome, gene 1 and scavenger receptor class B type 1. Whereas both protein kinase A and protein kinase C signaling were involved in the IGF-I-mediated steroidogenic response, the majority of the effects of IGF-I were found to be mediated by the protein kinase C pathway. Transcriptional activation of the StAR gene by IGF-I was influenced by several transcription factors, its up-regulation being dependent on phosphorylation of the cAMP response element-binding protein (CREB) and the activator protein 1 family member, c-Jun. Conversely, StAR gene transcription was markedly inhibited by expression of nonphosphorylatable CREB (Ser(133)Ala), dominant negative A-CREB, and dominant negative c-Jun (TAM-67) mutants. Collectively, the present studies identify molecular events in IGF-I signaling that may influence testicular growth, development, and the Leydig cell steroidogenic machinery through autocrine/paracrine regulation.

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

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

Novel signaling stimulated by arsenite increases cholesterol metabolism through increases in unphosphorylated steroidogenic acute regulatory (StAR) protein

Cholesterol metabolism to pregnenolone is dependent on the steroidogenic acute regulatory protein (StAR), which activates mitochondrial transfer of cholesterol to cytochrome CYP450scc. In mouse Y-1 adrenal cells and testis MA10 cells stimulation by 8-Bromo-cAMP (Br-cAMP) is augmented by a novel signaling initiated by low concentrations of arsenite (3-20 microM) and anisomycin (0.2 microM), a more selective stress agent. Each elevated StAR mRNA (three-fold after 6 h treatment) even with simultaneous stimulation by Br-cAMP. Arsenite produced parallel increases in StAR protein expression and cholesterol metabolism, but not for P450scc-mediated metabolism of 20alpha-hydroxycholesterol. Although arsenite and anisomycin each stimulated the phosphorylation of p38, the p38 inhibitor SB203580 (SB) produced additive increases in StAR expression. Cholesterol metabolism increased in parallel but without the increased StAR protein phosphorylation produced by Br-cAMP. Arsenite and anisomycin each elevated StAR mRNA but preferentially increased the 3.5 kb form relative to the 1.6 kb form. Arsenite and anisomycin each enhanced the stability of the more labile 3.5 kb mRNA which contains AU-rich elements that control mRNA stability. Although there were increases in both forms of StAR mRNA, arsenite did not stimulate a StAR promoter-reporter that exhibited a typical three-fold response to Br-cAMP. Arsenite and anisomycin may therefore activate a novel SB-independent MAP kinase which in part increases StAR expression through stabilizing the 3.5 kb mRNA but which may also activate a mechanism that by-passes transcription factors detected by the reporter. SB stimulation, which was completely blocked by a MEK inhibitor, was also selective towards the 3.5 kb StAR mRNA suggesting a second pathway for mRNA stabilization. These activations contrast with inhibition of StAR expression by arsenite at higher concentrations or longer incubation times.

Testicular cell lines

The range of in vivo or in vitro immortalized cell lines currently available provides a variety of model systems for studies of normal and pathological cell functions. The cell lines have been derived from spontaneous or experimentally induced tumors, or through in vitro immortalization. The transgenic (TG) techniques provide a powerful approach, allowing the production of in vivo animal models for a variety of diseases, including malignant tumors, through tissue-specific expression of oncogenes or other tumor-promoting genes. The TG techniques also enable the production of cell lines with specific characteristics, through insertion of desired genes into specific cell types, which can then be immortalized upon cell culture. The use of temperature-sensitive immortalizing genes offers an additional advantage of controlling gene expression, including the proliferation and differentiation of the cells to be immortalized. As regards the male reproductive system, a number of cell lines of testicular somatic cells are currently available. This review covers mainly the immortalized cell lines of testicular Leydig and Sertoli cells, with special reference to murine cell lines for the study of testicular endocrine function and tumorigenesis. These cell lines also provide useful tools to investigate the molecular basis of hormone actions and testicular cell interactions.