Intermediate role of adenosine 3':5'-cyclic monophosphate and protein kinase during gonadotropin-induced steroidogenesis in testicular interstitial cells

ERK5 Cooperates With MEF2C to Regulate Nr4a1 Transcription in MA-10 and MLTC-1 Leydig Cells

Leydig cells produce hormones required for the development and maintenance of sex characteristics and fertility in males. MEF2 transcription factors are important regulators of Leydig cell gene expression and steroidogenesis. ERK5 is an atypical member of the MAP kinase family that modulates transcription factor activity, either by direct phosphorylation or by acting as a transcriptional coactivator. While MEF2 and ERK5 are known to cooperate transcriptionally, the presence and role of ERK5 in Leydig cells remained unknown. Our goal was to determine whether ERK5 is present in Leydig cells and whether it cooperates with MEF2 to regulate gene expression. We found that ERK5 is present in Leydig cells in testicular tissue and immortalized cell lines. ERK5 knockdown in human chorionic gonadotrophin-treated MA-10 Leydig cells reduced steroidogenesis and decreased Star and Nr4a1 expression. Luciferase assays using a synthetic reporter plasmid containing 3 MEF2 elements revealed that ERK5 enhances MEF2-dependent promoter activation. Although ERK5 did not cooperate with MEF2 on the Star promoter in Leydig cell lines, we found that ERK5 and MEF2C do cooperate on the Nr4a1 promoter, which contains 2 adjacent MEF2 elements. Mutation of each MEF2 element in a short version of the Nr4a1 promoter significantly decreased the ERK5/MEF2C cooperation, indicating that both MEF2 elements need to be intact. The ERK5/MEF2C cooperation did not require phosphorylation of MEF2C on Ser387. Taken together, our data identify ERK5 as a new regulator of MEF2 activity in Leydig cells and provide potential new insights into mechanisms that regulate Leydig cell gene expression and function.

The Urokinase-Type Plasminogen Activator Contributes to cAMP-Induced Steroidogenesis in MA-10 Leydig Cells

Leydig cells produce androgens which are essential for male sex differentiation and reproductive functions. Steroidogenesis, as well as expression of several genes in Leydig cells, are stimulated by LH/cAMP and repressed by AMP/AMPK. One of those genes is Plau, which codes for the urokinase-type plasminogen activator (uPA), a secreted serine protease. The role of uPA and the regulation of Plau expression in Leydig cells remain unknown. Using siRNA-mediated knockdown, uPA was required for maximal cAMP-induced STAR and steroid hormone production in MA-10 Leydig cells. Analysis of Plau mRNA levels and promoter activity revealed that its expression is strongly induced by cAMP; this induction is blunted by AMPK. The cAMP-responsive region was located, in part, in the proximal Plau promoter that contains a species-conserved GC box at −56 bp. The transcription factor Krüppel-like factor 6 (KLF6) activated the Plau promoter. Mutation of the GC box at −56 bp abolished KLF6-mediated activation and significantly reduced cAMP-induced Plau promoter activity. These data define a role for uPA in Leydig cell steroidogenesis and provide insights into the regulation of Plau gene expression in these cells.

The association between caffeine intake and testosterone: NHANES 2013-2014

Background

Caffeine is one of the most commonly used psychoactive drugs in the world, and provides many health benefits including alertness, improved memory, and reducing inflammation. Despite these benefits, caffeine has been implicated in a number of adverse health outcomes possibly due to effects within the endocrine system, effects that may contribute to impaired reproductive function and low testosterone in men. Previous studies have investigated associations between caffeine consumption and testosterone levels in men, although the quantity and generalizability of these studies is lacking, and the results between studies are conflicting and inconclusive.

Methods

Using data from a cross-sectional study of 372 adult men in the 2013–2014 NHANES survey cycle, the researchers set out to characterize the association between serum testosterone levels, caffeine, and 14 caffeine metabolites.

Results

Multivariable, weighted linear regression revealed a significant inverse association between caffeine and testosterone. Multivariable, linear regression revealed significant, inverse associations between 6 xanthine metabolic products of caffeine and testosterone. Inverse associations were observed between 5-methyluric acid products and testosterone, as well as between 5-acetlyamino-6-amino-3-methyluracil and testosterone. A significant, positive association was observed for 7-methyl xanthine, 3,7-dimethyluric acid, and 7-methyluric acid. Logistic regression models to characterize the association between 2 biologically active metabolites of caffeine (theobromine and theophylline) and odds of low testosterone (< 300 ng/dL) were non-significant.

Conclusions

These findings suggest a potential role for caffeine’s contribution to the etiology of low testosterone and biochemical androgen deficiency. Future studies are warranted to corroborate these findings and elucidate biological mechanisms underlying this association.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12937-022-00783-z.

Physiological and Pharmacological Roles of PTH and PTHrP in Bone Using Their Shared Receptor, PTH1R

Parathyroid hormone (PTH) and the paracrine factor, PTH-related protein (PTHrP), have preserved in evolution sufficient identities in their amino-terminal domains to share equivalent actions upon a common G protein-coupled receptor, PTH1R, that predominantly uses the cyclic adenosine monophosphate-protein kinase A signaling pathway. Such a relationship between a hormone and local factor poses questions about how their common receptor mediates pharmacological and physiological actions of the two. Mouse genetic studies show that PTHrP is essential for endochondral bone lengthening in the fetus and is essential for bone remodeling. In contrast, the main postnatal function of PTH is hormonal control of calcium homeostasis, with no evidence that PTHrP contributes. Pharmacologically, amino-terminal PTH and PTHrP peptides (teriparatide and abaloparatide) promote bone formation when administered by intermittent (daily) injection. This anabolic effect is remodeling-based with a lesser contribution from modeling. The apparent lesser potency of PTHrP than PTH peptides as skeletal anabolic agents could be explained by lesser bioavailability to PTH1R. By contrast, prolongation of PTH1R stimulation by excessive dosing or infusion, converts the response to a predominantly resorptive one by stimulating osteoclast formation. Physiologically, locally generated PTHrP is better equipped than the circulating hormone to regulate bone remodeling, which occurs asynchronously at widely distributed sites throughout the skeleton where it is needed to replace old or damaged bone. While it remains possible that PTH, circulating within a narrow concentration range, could contribute in some way to remodeling and modeling, its main physiological role is in regulating calcium homeostasis.

PTH1R Actions on Bone Using the cAMP/Protein Kinase A Pathway

After the initial signaling action of parathyroid hormone (PTH) on bone was shown to be activation of adenylyl cyclase, its target was found to be cells of the osteoblast lineage, to the exclusion of osteoclasts and their precursors. This led to the view that the osteoblast lineage regulated osteoclast formation, a proposal that was established when the molecular mechanisms of osteoclast formation were discovered. This is in addition to the effect of PTH1Rv signaling throughout the osteoblast differentiation process to favour the formation of bone-forming osteoblasts. Initial signaling in the PTH target cells through cAMP and protein kinase A (PKA) activation is extremely rapid, and marked by an amplification process in which the later event, PKA activation, precedes cAMP accumulation in time and is achieved at lower concentrations. All of this is consistent with the existence of "spare receptors", as is the case with several other peptide hormones. PTH-related protein (PTHrP), that was discovered as a cancer product, shares structural similarity with PTH in the amino-terminal domain that allows the hormone, PTH, and the autocrine/paracrine agent, PTHrP, to share actions upon a common G protein coupled receptor, PTH1R, through which they activate adenylyl cyclase with equivalent potencies. Studies of ligand-receptor kinetics have revealed that the PTH/PTH1R ligand-receptor complex, after initial binding and adenylyl cyclase activation at the plasma membrane, is translocated to the endosome, where adenylyl cyclase activation persists for a further short period. This behavior of the PTH1R resembles that of a number of hormones and other agonists that undergo such endosomal translocation. It remains to be determined whether and to what extent the cellular effects through the PTH1R might be influenced when endosomal is added to plasma membrane activation.

Angiotensin II stimulation promotes mitochondrial fusion as a novel mechanism involved in protein kinase compartmentalization and cholesterol transport in human adrenocortical cells

In steroid-producing cells, cholesterol transport from the outer to the inner mitochondrial membrane is the first and rate-limiting step for the synthesis of all steroid hormones. Cholesterol can be transported into mitochondria by specific mitochondrial protein carriers like the steroidogenic acute regulatory protein (StAR). StAR is phosphorylated by mitochondrial ERK in a cAMP-dependent transduction pathway to achieve maximal steroid production. Mitochondria are highly dynamic organelles that undergo replication, mitophagy and morphology changes, all processes allowed by mitochondrial fusion and fission, known as mitochondrial dynamics. Mitofusin (Mfn) 1 and 2 are GTPases involved in the regulation of fusion, while dynamin-related protein 1 (Drp1) is the major regulator of mitochondrial fission. Despite the role of mitochondrial dynamics in neurological and endocrine disorders, little is known about fusion/fission in steroidogenic tissues. In this context, the present work aimed to study the role of angiotensin II (Ang II) in protein subcellular compartmentalization, mitochondrial dynamics and the involvement of this process in the regulation of aldosterone synthesis. We demonstrate here that Ang II stimulation promoted the recruitment and activation of PKCε, ERK and its upstream kinase MEK to the mitochondria, all of them essential for steroid synthesis. Moreover, Ang II prompted a shift from punctate to tubular/elongated (fusion) mitochondrial shape, in line with the observation of hormone-dependent upregulation of Mfn2 levels. Concomitantly, mitochondrial Drp1 was diminished, driving mitochondria toward fusion. Moreover, Mfn2 expression is required for StAR, ERK and MEK mitochondrial localization and ultimately for aldosterone synthesis. Collectively, this study provides fresh insights into the importance of hormonal regulation in mitochondrial dynamics as a novel mechanism involved in aldosterone production.

Acute decrease in plasma testosterone and appetite after either glucose or protein beverages in adolescent males

OBJECTIVE

Chronic testosterone blood concentrations associate with food intake (FI), but acute effects of testosterone on appetite and effect of protein and glucose consumption on testosterone response have had little examination.

METHODS

In a randomized, crossover study, twenty-three adolescent (12-18 years old) males were given beverages containing either: (a) whey protein (1 g/kg body weight), (b) glucose (1 g/kg body weight) or (c) a calorie-free control (C). Plasma testosterone, luteinizing hormone (LH), GLP-1 (active), ghrelin (acylated), glucose, insulin and subjective appetite were measured prior (0) and at 20, 35 and 65 minutes after the consumption of the beverage. FI at an ad libitum pizza meal was assessed at 85 minutes.

RESULTS

Testosterone decreased acutely to 20 minutes after both protein and glucose with the decrease continuing after protein but not glucose to 65 minutes (P = 0.0382). LH was also decreased by both protein and glucose, but glucose had no effect at 20 minutes in contrast to protein (P < 0.001). Plasma testosterone concentration correlated positively with LH (r = 0.58762, P < 0.0001) and negatively with GLP-1 (r = -0.50656, P = 0.0003). No associations with appetite, ghrelin or glycaemic markers were found. Food intake was not affected by treatments.

CONCLUSION

Protein or glucose ingestion results in acute decreases in both plasma testosterone and LH in adolescent males. The physiological significance of this response remains to be determined as no support for testosterone's role in acute regulation of food intake was found.

Expression patterns of C1ql4 and its cell-adhesion GPCR Bai3 in the murine testis and functional roles in steroidogenesis

C1q-like 4 (C1QL4), a novel member of the C1q- and TNF-related protein family, was found to be highly expressed in rodent and human testis. However, the localization, developmental, and hormonally regulated expression and biologic function of C1ql4 in the testis have not been investigated. Here, we demonstrated that C1ql4 mRNA and protein levels in murine testes gradually increased from the postnatal period to the adult stage and were up-regulated by LH in vivo. In situ hybridization demonstrated that the distribution and expression levels of C1ql4 mRNA varied at different developmental stages, although C1ql4 mRNA was detected in the seminiferous tubule and interstitial Leydig cells. Recombinant C1QL4 did not affect cell proliferation but did increase testosterone production in TM3 Leydig cells, as well as in cultured seminiferous tubules. C1QL4-induced testosterone secretion in Leydig cells was accompanied by increased expression of steroidogenic acute regulatory (StAR) protein and steroidogenic enzymes. During this process, the c-Raf/extracellular signal-regulated protein kinase kinases 1 and 2/ERK1/2/mitogen- and stress-activated protein kinase-1 and cAMP/PKA/cAMP-responsive element binding protein signaling cascades were activated by C1QL4. The cell-adhesion GPCR brain-specific angiogenesis inhibitor 3 (BAI3), a putative receptor of C1QL4, was detected in the seminiferous tubule and interstitial Leydig cells during testicular development. Knockdown of Bai3 expression in Leydig cells led to a reduction in Star expression, accompanied by increases in phosphorylation of ERK1/2 and intercellular cAMP levels. However, C1QL4-induced StAR expression was not completely suppressed in the Bai3-deficient Leydig cells, and phosphorylation of ERK1/2 and intercellular cAMP levels were not significantly changed before and after C1QL4 stimulation. Our results suggested that although BAI3 played a role in C1QL4-induced steroidogenesis, there was an unidentified receptor that mediated C1QL4-activated testosterone secretion in Leydig cells through phosphorylation of ERK1/2 and up-regulation of intracellular cAMP levels. Taken together, our results showed, for the first time to our knowledge, that C1QL4 served as a novel acute regulator of testosterone secretion, and BAI3 functioned as a new receptor that is involved in steroidogenesis in Leydig cells. BAI3-independent ERK1/2 activation and cAMP activation mediated C1QL4-induced testosterone secretion. This study expanded the reproductive roles and mechanisms of C1QL4 and BAI3 signaling pathways.-Tan, A., Ke, S., Chen, Y., Chen, L., Lu, X., Ding, F., Yang, L., Tang, Y., Yu, Y. Expression patterns of C1ql4 and its cell-adhesion GPCR Bai3 in the murine testis and functional roles in steroidogenesis.

Human LH and hCG stimulate differently the early signalling pathways but result in equal testosterone synthesis in mouse Leydig cells in vitro

BACKGROUND

Human luteinizing hormone (LH) and chorionic gonadotropin (hCG) are glycoprotein hormones regulating development and reproductive functions by acting on the same receptor (LHCGR). We compared the LH and hCG activity in gonadal cells from male mouse in vitro, i.e. primary Leydig cells, which is a common tool used for gonadotropin bioassay. Murine Leydig cells are naturally expressing the murine LH receptor (mLhr), which binds human LH/hCG.

METHODS

Cultured Leydig cells were treated by increasing doses of recombinant LH and hCG, and cell signaling, gene expression and steroid synthesis were evaluated.

RESULTS

We found that hCG is about 10-fold more potent than LH in cAMP recruitment, and slightly but significantly more potent on cAMP-dependent Erk1/2 phosphorylation. However, no significant differences occur between LH and hCG treatments, measured as activation of downstream signals, such as Creb phosphorylation, Stard1 gene expression and testosterone synthesis.

CONCLUSIONS

These data demonstrate that the responses to human LH/hCG are only quantitatively and not qualitatively different in murine cells, at least in terms of cAMP and Erk1/2 activation, and equal in activating downstream steroidogenic events. This is at odds with what we previously described in human primary granulosa cells, where LHCGR mediates a different pattern of signaling cascades, depending on the natural ligand. This finding is relevant for gonadotropin quantification used in the official pharmacopoeia, which are based on murine, in vivo bioassay and rely on the evaluation of long-term, testosterone-dependent effects mediated by rodent receptor.

Identification of an orally active small-molecule PTHR1 agonist for the treatment of hypoparathyroidism

Parathyroid hormone (PTH) is essential for calcium homeostasis and its action is mediated by the PTH type 1 receptor (PTHR1), a class B G-protein-coupled receptor. Hypoparathyroidism and osteoporosis can be treated with PTH injections; however, no orally effective PTH analogue is available. Here we show that PCO371 is a novel, orally active small molecule that acts as a full agonist of PTHR1. PCO371 does not affect the PTH type 2 receptor (PTHR2), and analysis using PTHR1–PTHR2 chimeric receptors indicated that Proline 415 of PTHR1 is critical for PCO371-mediated PTHR1 activation. Oral administration of PCO371 to osteopenic rats provokes a significant increase in bone turnover with limited increase in bone mass. In hypocalcemic rats, PCO371 restores serum calcium levels without increasing urinary calcium, and with stronger and longer-lasting effects than PTH injections. These results strongly suggest that PCO371 can provide a new treatment option for PTH-related disorders, including hypoparathyroidism.

Hypoparathyroidism and osteoporosis can be treated with parathyroid hormone, but frequent injections are required. Here the authors develop a small-molecule agonist for the parathyroid hormone type I receptor that can be administered orally, and demonstrate its efficacy in rats.

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

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

Molecular regulation of steroidogenesis in endocrine Leydig cells

Steroid hormones regulate essential physiological processes and inadequate levels are associated with various pathological conditions. Consequently, the process of steroid hormone biosynthesis is finely regulated. In the testis, the main steroidogenic cells are the Leydig cells. There are two distinct populations of Leydig cells that arise during development: fetal and adult Leydig cells. Fetal Leydig cells are responsible for masculinizing the male urogenital tract and inducing testis descent. These cells atrophy shortly after birth and do not contribute to the adult Leydig cell population. Adult Leydig cells derive from undifferentiated precursors present after birth and become fully steroidogenic at puberty. The differentiation of both Leydig cell populations is controlled by locally produced paracrine factors and by endocrine hormones. In fully differentially and steroidogenically active Leydig cells, androgen production and hormone-responsiveness involve various signaling pathways and downstream transcription factors. This review article focuses on recent developments regarding the origin and function of Leydig cells, the regulation of their differentiation by signaling molecules, hormones, and structural changes, the signaling pathways, kinases, and transcription factors involved in their differentiation and in mediating LH-responsiveness, as well as the fine-tuning mechanisms that ensure adequate production steroid hormones.

The spatial and temporal regulation of the hormonal signal. Role of mitochondria in the formation of a protein complex required for the activation of cholesterol transport and steroids synthesis

The mitochondria are critical for steroidogenesis since the ability of cholesterol to move into mitochondria to be available for cytochrome P450, CYP11A1, determines the efficacy of steroid production. Several proteins kinases, such as PKA, MEK and ERK which are essential to complete steroidogenesis, form a mitochondria-associated complex. The protein-protein interactions between kinases and key factors during the transport of cholesterol takes place in the contact sites between the two mitochondrial membranes; however, no mitochondrial targeting sequence has been described for these kinases. Here we discuss the possibility that mitochondrial reorganization may be mediating a compartmentalized cellular response. This reorganization could allow the physical interaction between the hormone-receptor complex and the enzymatic and lipidic machinery necessary for the complete steroid synthesis and release. The movement of organelles in specialized cells could impact on biological processes that include, but are not limited to, steroid synthesis.

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.

Dihydrolipoamide dehydrogenase and cAMP are associated with cadmium-mediated Leydig cell damage

Cadmium (Cd) directly inhibits testosterone production in Leydig cells, but its mechanism is still unclear. To further explore the signaling pathway of Cd-mediated toxicity to Leydig cells, various concentrations of Cd were cultured with R2C cells for 24h, and two-dimensional gel electrophoresis (2DE)-based proteomics profiling was used to analyze the change of protein expressions. Cd caused a concentration-dependent inhibition of cell viability with IC(25), IC(50) and IC(75) of 2.42×10(-5)M, 4.83×10(-5)M and 7.39×10(-5)M, respectively. Cd significantly reduced progesterone production and mitochondrial membrane potential (ΔΨ(m)) in a concentration-dependent manner. 2DE-based proteomics showed 34 protein spots with altered expression by 2-folds or more, and dihydrolipoamide dehydrogenase (DLD) was the hub in the network of these altered proteins. Real-time polymerase chain reaction (PCR) and Western blotting showed that Cd downregulated the expression of DLD. Cd also decreased intracellular levels of cyclic adenosine monophosphate (cAMP). The results suggest that DLD and cAMP may be key elements related to Cd toxicity to Leydig cells.

Tyrosine phosphatases as key regulators of StAR induction and cholesterol transport: SHP2 as a potential tyrosine phosphatase involved in steroid synthesis

The phospho-dephosphorylation of intermediate proteins is a key event in the regulation of steroid biosynthesis. In this regard, it is well accepted that steroidogenic hormones act through the activation of serine/threonine (Ser/Thr) protein kinases. Although many cellular processes can be regulated by a crosstalk between different kinases and phosphatases, the relationship of Ser/Thr phosphorylation and tyrosine (Tyr)-dephosphorylation is a recently explored field in the regulation of steroid synthesis. Indeed in steroidogenic cells, one of the targets of hormone-induced Ser/Thr phosphorylation is a protein tyrosine phosphatase. Whereas protein tyrosine phosphatases were initially regarded as household enzymes with constitutive activity, dephosphorylating all the substrates they encountered, evidence is now accumulating that protein tyrosine phosphatases are tightly regulated by various mechanisms. Here, we will describe the role of protein tyrosine phosphatases in the regulation of steroid biosynthesis, relating them to steroidogenic acute regulatory protein, arachidonic acid metabolism and mitochondrial rearrangement.

Luteinizing hormone (LH) acts through PKA and PKC to modulate T-type calcium currents and intracellular calcium transients in mice Leydig cells

LH increases the intracellular Ca(2+) concentration ([Ca(2+)](i)) in mice Leydig cells, in a process triggered by calcium influx through T-type Ca(2+) channels. Here we show that LH modulates both T-type Ca(2+) currents and [Ca(2+)](i) transients through the effects of PKA and PKC. LH increases the peak calcium current (at -20mV) by 40%. A similar effect is seen with PMA. The effect of LH is completely blocked by the PKA inhibitors H89 and a synthetic inhibitory peptide (IP-20), but only partially by chelerythrine (PKC inhibitor). LH and the blockers induced only minor changes in the voltage dependence of activation, inactivation or deactivation of the currents. Staurosporine (blocker of PKA and PKC) impaired the [Ca(2+)](i) changes induced by LH. A similar effect was seen with H89. Although PMA slowly increased the [Ca(2+)](i) the subsequent addition of LH still triggered the typical transients in [Ca(2+)](i). Chelerythrine also does not avoid the Ca(2+) transients, showing that blockage of PKC is not sufficient to inhibit the LH induced [Ca(2+)](i) rise. In summary, these two kinases are not only directly involved in promoting testosterone synthesis but also act on the overall calcium dynamics in Leydig cells, mostly through the activation of PKA by LH.

A calcium-induced calcium release mechanism supports luteinizing hormone-induced testosterone secretion in mouse Leydig cells

Leydig cells are responsible for the synthesis and secretion of testosterone, processes controlled by luteinizing hormone (LH). Binding of LH to a G protein-coupled receptor in the plasma membrane results in an increase in cAMP and in intracellular Ca2+ concentration ([Ca2+]i). Here we show, using immunofluorescence, that Leydig cells express ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP3Rs). Measurements of intracellular calcium changes using the fluorescent calcium-sensitive dye fluo-3 and confocal microscopy show that both types of receptors are involved in a calcium-induced calcium release (CICR) mechanism, which amplifies the initial Ca2+ influx through plasma membrane T-type calcium channels (CaV3). The RyRs and IP3Rs are functional, as judged from both their activation by caffeine and IP3 and block by ryanodine and 2-aminoethoxydiphenyl borate (2-APB), respectively. RyRs are the principal players involved in the release of Ca2+ from the endoplasmic reticulum, as evidenced by the fact that global Ca2+ changes evoked by LH are readily blocked by 100 μM ryanodine but not by 2-APB or xestospongin C. Finally, steroid production by Leydig cells is inhibited by ryanodine but not by 2-APB. These results not only broaden our understanding of the role played by calcium in Leydig cells but also show, for the first time, that RyRs have an important role in determining testosterone secretion by the testis.

Participation of signaling pathways in the derepression of luteinizing hormone receptor transcription

The luteinizing hormone receptor (LHR) transcription is subject to an epigenetic regulatory mode whereby the proximal Sp1 site acts as an anchor to recruit histone deacetylases (HDAC)1/2 and the Sin3A co-repressor complex. This results in promoter-localized histone hypo-acetylation that contributes to the silencing of LHR transcriptional expression. Chromatin changes resulting from site-specific acetylation and methylation of histones regulate LHR gene expression. The HDAC inhibitor TSA-induced cell-specific phosphatase release from the promoter, which serves as an 'on' mechanism for Sp1 phosphorylation by phosphatidylinositol 3-kinase/protein kinase Czeta (PI3K/PKCzeta) at Ser641, leading to p107 repressor derecruitment and LHR transcriptional activation. The methylation status of the promoter provides another layer of modulation in a cell-specific manner. Maximal derepression of the LHR gene is dependent on complete DNA demethylation of the promoter in conjunction with histone hyperacetylation and release of repressors (p107 and HDAC/Sin3A). Independently, the PKC-alpha/Erk pathway, participates in LHR gene expression through induction of Sp1 phosphorylation at Ser site(s) other than Ser641. This causes dissociation of the HDAC1/mSin3A from the promoter, recruitment of TFIIB and Pol II, and transcriptional activation. Collectively, these findings demonstrate that LHR gene expression at the transcriptional level is regulated by complex and diverse networks, in which coordination and interactions between these regulatory effectors are crucial for silencing/activation of LHR expression.

Gonadotropin-regulated testicular RNA helicase (GRTH/DDX25): a multifunctional protein essential for spermatogenesis

Male germ cell maturation is governed by the expression of specific protein(s) in a precise temporal sequence during development. Gonadotropin-regulated testicular RNA helicase (GRTH/DDX25), a member of the Glu-Asp-Ala-Glu (DEAD)-box protein family, is a testis-specific gonadotropin/androgen-regulated RNA helicase that is present in germ cells (meiotic spermatocytes and round spermatids) and Leydig cells. GRTH is essential for completion of spermatogenesis as a posttranscriptional regulator of relevant genes during germ cell development. Male mice lacking GRTH are sterile with spermatogenic arrest due to failure of round spermatids to elongate, where striking structural changes and reduction in size of chromatoid bodies are observed. GRTH also plays a central role in preventing germ cell apoptosis. In addition to its inherent helicase unwinding/adenosine triphosphatase activities, GRTH binds to specific mRNAs as an integral component of ribonuclear protein particles. As a shuttle protein, GRTH transports target mRNAs from nucleus to the cytoplasm for storage in chromatoid bodies of spermatids, where they await translation during spermatogenesis. GRTH is also associated with polyribosomes to regulate target gene translation. The finding of a missense mutation associated with male infertility, where its expression associates with loss of GRTH phosphorylation, supports the relevance of GRTH to human germ cell development. We conclude that the mammalian GRTH/DDX25 is a multifunctional RNA helicase that is an essential regulator of spermatogenesis and is highly relevant for studies of male infertility and contraception.

Hormonal activation of a kinase cascade localized at the mitochondria is required for StAR protein activity

It is known that ERK1/2 and MEK1/2 participate in the regulation of Star gene transcription. However, their role in StAR protein post-transcriptional regulation is not described yet. In this study we analyzed the relationship between the MAPK cascade and StAR protein phosphorylation and function. We have demonstrated that (a) steroidogenesis in MA-10 Leydig cells depends on the specific of ERK1/2 activation at the mitochondria; (b) ERK1/2 phosphorylation is driven by mitochondrial PKA and constitutive MEK1/2 in this organelle; (c) active ERK1/2 interacts with StAR protein, leads to StAR protein phosphorylation at Ser(232) only in the presence of cholesterol; (d) directed mutagenesis of Ser(232) (S232A) inhibited in vitro StAR protein phosphorylation by ERK1; (e) transient transfection of MA-10 cells with StAR S232A cDNA markedly reduced the yield of progesterone production. We show that StAR protein is a substrate of ERK1/2, and that mitochondrial ERK1/2 is part of a multimeric complex that regulates cholesterol transport.

Cross-talk between G protein-coupled and epidermal growth factor receptors regulates gonadotropin-mediated steroidogenesis in Leydig cells

Gonadal steroid production is stimulated by gonadotropin binding to G protein-coupled receptors (GPCRs). Although GPCR-mediated increases in intracellular cAMP are known regulators of steroidogenesis, the roles of other signaling pathways in mediating steroid production are not well characterized. Recent studies suggest that luteinizing hormone (LH) receptor activation leads to trans-activation of epidermal growth factor (EGF) receptors in the testes and ovary. This pathway is critical for LH-induced steroid production in ovarian follicles, probably through matrix metalloproteinase (MMP)-mediated release of EGF receptor (EGFR) binding ectodomains. Here we examined LH and EGF receptor cross-talk in testicular steroidogenesis using mouse MLTC-1 Leydig cells. We demonstrated that, similar to the ovary, trans-activation of the EGF receptor was critical for gonadotropin-induced steroid production in Leydig cells. LH-induced increases in cAMP and cAMP-dependent protein kinase (PKA) activity mediated trans-activation of the EGF receptor and subsequent mitogen-activated protein kinase (MAPK) activation, ultimately leading to StAR phosphorylation and mitochondrial translocation. Steroidogenesis in Leydig cells was unaffected by MMP inhibitors, suggesting that cAMP and PKA trans-activated EGF receptors in an intracellular fashion. Interestingly, although cAMP was always needed for steroidogenesis, the EGFR/MAPK pathway was activated and necessary only for early (30-60 min), but not late (120 min or more), LH-induced steroidogenesis in vitro. In contrast, 36-h EGF receptor inhibition in vivo significantly reduced serum testosterone levels in male mice, demonstrating the physiologic importance of this cross-talk. These results suggest that GPCR-EGF receptor cross-talk is a conserved regulator of gonadotropin-induced steroidogenesis in the gonads, although the mechanisms of EGF receptor trans-activation may vary.

Mitochondrial A-kinase anchoring protein 121 binds type II protein kinase A and enhances steroidogenic acute regulatory protein-mediated steroidogenesis in MA-10 mouse leydig tumor cells

The expression of the steroidogenic acute regulatory protein (STAR) is regulated by PKA in response to trophic hormone stimulation through the second messenger cAMP. However, in steroidogenic cells, the concentrations of hormone necessary to maximally induce cAMP synthesis and PKA activity are often significantly higher than is necessary to achieve maximum steroidogenesis. One general mechanism believed to make PKA signaling more effective is the use of A-kinase anchoring proteins (AKAPs) to recruit PKA to discrete subcellular compartments, which coordinates and focuses PKA action with respect to its substrates. The characterization of AKAP121 has suggested that it enhances the posttranscriptional regulation of STAR by recruiting both Star mRNA and PKA to the mitochondria, thereby permitting more effective translation and phosphorylation of STAR. Testing this hypothesis revealed that cAMP-induced STAR expression and steroidogenesis closely followed AKAP121 abundance when this AKAP was silenced or overexpressed in MA-10 cells but that these changes were effected posttranscriptionally. Moreover, silencing AKAP121 expression in these cells specifically altered the localization of type II PKA regulatory subunit alpha (PKAR2A) at the mitochondria but did not affect its relative expression within the cell. Affinity purification experiments showed that PKAR2A preferentially associated with AKAP121, and cAMP analogs that activate type II PKA induced STAR phosphorylation more efficiently than analogs stimulating type I PKA. This suggests that AKAP121 and PKAR2A serve to enhance steroidogenesis by directing the synthesis and activation of STAR at the mitochondria in response to cAMP.

Tyrosine phosphatases in steroidogenic cells: regulation and function

In adrenocortical and Leydig cells PKA activation by trophic hormones increases the activity of protein tyrosine phosphatases and also induces the expression of MAP kinase phosphatase 1 (MKP-1), a dual activity protein phosphatase (serine/threonine and tyrosine). This work summarizes the knowledge on the regulation and the role played by cAMP-activated tyrosine phosphatases as well as MKP-1 in the hormonal activation of the acute and chronic phases of steroidogenesis.

Profiling of functional phosphodiesterase in mesangial cells using a CRE-SEAP-based reporting system

1. Phosphodiesterases (PDEs) are critically implicated in the regulation of mesangial cell function, but profile of functional PDEs in mesangial cells is still unclear. In this study, we investigated roles of individual PDEs in the regulation of mesangial cell behavior by the cAMP pathway. 2. Reporter mesangial cells that express secreted alkaline phosphatase (SEAP) under the control of the cAMP response element (CRE) were exposed to selective PDE inhibitors in the presence or absence of cAMP, and activity of CRE, expression of CRE-regulated protein, mitogenesis and cell survival were examined. 3. Exposure of reporter cells to cAMP-elevating agents resulted in time- and concentration-dependent activation of CRE. Treatment of the cells with any PDE inhibitors alone did not induce CRE activation. Under stimulation with 8-bromo-cAMP or 8-bromo-cGMP, however, inhibitors of PDE2, PDE3, PDE4 and PDE5 enhanced activation of CRE. Inhibition of PDE1 or PDE6 did not affect the CRE activation. 4. Among different combinations tested, only inhibitors of PDE3 and PDE4 cooperatively increased the level of intracellular cAMP, activity of protein kinase A, activation of CRE, and CRE-regulated protein, connexin43. 5. Concomitant inhibition of PDE3 and PDE4 attenuated mitogen-induced activation of extracellular signal-regulated kinases and cell proliferation. Under serum deprivation, combinational inhibition of PDE3 and PDE4 exclusively caused activation of caspase-3 and apoptosis. 6. The present data elucidated that PDE3 and PDE4 play critical roles in the regulation of mesangial cell function. PDE3 and PDE4 were identified as the novel, antiapoptotic machinery that supports survival of mesangial cells.

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

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

Functional plasticity of cyclic AMP hydrolysis in rat adenohypophysial corticotroph cells

Characterisation of cyclic nucleotide-hydrolysing phosphodiesterases (PDEs) in recombinant systems has highlighted regulatory properties indicative of distinct physiological roles for these enzymes. The present study investigated the role of PDEs in the adenosine 3'5'-monophosphate (cAMP) response to the hypothalamic neuropeptides corticotrophin-releasing factor (CRF) and arginine vasopressin (AVP) in acutely dispersed rat adenohypophysial cells. Ca(2+)-activated PDE (PDE1) and Ca(2+)-independent, rolipram-sensitive PDE (PDE4) accounted for close to 90% of cAMP-hydrolysing activity in the adenohypophysis. Messenger RNA transcripts of PDE1 (isotypes 1A and 1C) and PDE4 (isotypes B and D3) were detected by RT-PCR. The PDE blockers rolipram and IBMX enhanced cAMP accumulation induced by CRF or CRF and AVP. Vinpocetine, an inhibitor of low K(m) PDE1 isotypes, did not alter the response to CRF but enhanced the effect of the combined CRF/AVP stimulus. Thus, PDE4s terminate the cAMP response to moderate stimulation, while low-affinity PDE1 becomes important when the concentrations of CRF and AVP are characteristic of exposure to intensive stress.

Mutations of gonadotropins and gonadotropin receptors: elucidating the physiology and pathophysiology of pituitary-gonadal function

The recent unraveling of structures of genes for the gonadotropin subunits and gonadotropin receptors has provided reproductive endocrinologists with new tools to study normal and pathological functions of the hypothalamic-pituitary-gonadal axis. Rare inactivating mutations that produce distinctive phenotypes of isolated LH or FSH deficiency have been discovered in gonadotropin subunit genes. In addition, there is a common polymorphism in the LHbeta subunit gene with possible clinical significance as a contributing factor to pathologies of LH-dependent gonadal functions. Both activating and inactivating mutations have been detected in the gonadotropin receptor genes, a larger number in the LH receptor gene, but so far only a few in the gene for the FSH receptor. These mutations corroborate and extend our knowledge of clinical consequences of gonadotropin resistance and inappropriate gonadotropin action. The information obtained from human mutations has been complemented by animal models with disrupted or inappropriately activated gonadotropin ligand or receptor genes. These clinical and experimental genetic disease models form a powerful tool for exploring the physiology and pathophysiology of gonadotropin function and provide an excellent example of the power of molecular biological approaches in the study of pathogenesis of diseases.

Corticotropin increases protein tyrosine phosphatase activity by a cAMP-dependent mechanism in rat adrenal gland

Corticotropin signal transduction pathway involves serine/threonine protein phosphorylation. Recent reports suggest that protein tyrosine dephosphorylation may also be an integral component of that pathway. The present study was performed to investigate the role played by protein tyrosine phosphatases (PTPs) on acute response to corticotropin and the hypothetical regulation of PTPs by this hormone. We have used two powerful cell permeant PTP inhibitors, phenylarsine oxide (PAO) and pervanadate (PV), in order to examine the relevance of PTP activity on hormone-stimulated and 8-bromo-adenosine 3',5'-phosphate (8Br-cAMP is a permeant analogue of adenosine 3',5'-phosphate)-stimulated steroidogenesis in adrenal zona fasciculata (ZF) cells. In both cases, PAO and PV inhibited the steroid production in a dose-dependent fashion, and had no effect on steroidogenesis supported by a permeant analogue of cholesterol. The effect of hormonal stimulation on PTP activity was analyzed in rat adrenal ZF. In vivo corticotropin treatment reduced phosphotyrosine content in endogenous proteins and produced a transient increase of PTP activity in the cytosolic fraction, reaching a maximum (twofold) after 15 min. Incubation of adrenal ZF with 8Br-cAMP also produced PTP activation, suggesting that it can be mediated by cAMP-dependent protein kinase (PKA)-dependent phosphorylation. Detection of PTP activity in an in-gel assay showed three corticotropin-stimulated soluble PTPs with molecular masses of 115, 80 and 50 kDa. In summary, we report for the first time a hormone-dependent PTP activation in a steroidogenic tissue and provide evidence that PTP activity plays an important role in corticotropin signal pathway, acting downstream of PKA activation and upstream of cholesterol transport across the mitochondrial membrane.

Signaling role of PDE isozymes in pathobiology of glomerular mesangial cells. Studies in vitro and in vivo

Mesangial cells (MC) of renal glomeruli respond to immune-inflammatory injury by accelerated proliferation and generation of reactive oxygen metabolites (ROM). We studied in vivo and in vitro roles of cAMP-protein kinase A (PKA) signaling in modulation of these pathobiologic processes with focus on PDE isozymes. Mitogenic synthesis of DNA in mesangial cells grown in primary culture was blocked by forskolin and dibutyryl cyAMP. Incubation of MC with PDE-3 inhibitors, cilostamide and lixazinone, inhibited (> 50%) mitogenesis, whereas inhibitors of PDE-4, rolipram and denbufylline, caused little or no inhibition. Conversely, inhibitors of PDE-4 suppressed generation of ROM in MC, whereas inhibitors of PDE-3 had no effect. Incubation of mesangial cells with cilostamide or with rolipram increased in situ activity of PKA, and effects of the two inhibitors were additive. PDE inhibitors also decreased activity of mitogen-activated protein kinase. The efficacy of PDE isozyme inhibitors (IC50) to suppress mitogenesis or ROM generation paralleled IC50 for inhibition of cAMP hydrolysis by extracts from mesangial cells. Administration of lixazinone or lixazinone in combination with rolipram to rats with mesangial proliferative glomerulonephritis induced by antithymic serum suppressed proliferation of mesangial cells and also reduced other histopathologic manifestations of the disease. Based on these observations, we propose that in MC, a cAMP pool that is hydrolyzed by PDE-3 inhibits by negative crosstalk via activation of PKA, mitogen-activated protein kinase (MAPK) pathway, and mitogenesis; whereas cAMP pool linked to PDE-4 inhibits, also via activation of PKA, ROM generation in mesangial cells. Results also suggest that PDE isozyme inhibitors, in particular inhibitors of PDE-3, should be investigated for potential use for "signal transduction pharmacotherapy" of glomerulonephritis.

The luteinizing hormone receptor

The luteinizing hormone receptor (LHR) is a member of the subfamily of glycoprotein hormone receptors within the superfamily of G protein-coupled receptor (GPCR)/seven-transmembrane domain receptors. Over the past eight years, major advances have been made in determining the structure and function of the LHR and its gene. The hormone-binding domain has been localized to exons 1-7 in the extracellular (EC) domain/region of the receptor, which contains several leucine-rich repeats. High-affinity binding of LH and human chorionic gonadotrophin (hCG) causes secondary hormone or receptor contacts to be established with regions of the EC loop/transmembrane module that initiate signal transduction. Models of hormone-receptor interaction have been derived from the crystal structures of hCG and of the ribonuclease inhibitor, which also contains leucine-rich repeats. Such models provide a framework for the interpretation of mutational studies and for further experiments. The extracellular domain of the receptor has been overexpressed in vitro, which will facilitate crystallographic resolution of the structure of the receptor-binding site. The transmembrane domain/loop/cytoplasmic module transduces the signal for coupling to G proteins. Several constitutive, activating mutations that cause human disease have been found in helix VI and adjacent structures. These mutations have provided valuable information about mechanisms of signal transfer and G protein coupling. The structure of the LHR gene has been elucidated, and the regulation of its transcription is beginning to be understood. Valuable insights into receptor evolution have been derived from analysis of sequence homologies, the gene structure of glycoprotein hormone receptors and other members of the GPCR family, and the glycoprotein hormone receptor-like precursors identified in several invertebrate species.

Compartmentalization of cAMP signaling in mesangial cells by phosphodiesterase isozymes PDE3 and PDE4. Regulation of superoxidation and mitogenesis

Some major pathobiologic processes in renal mesangial cells, elicited in response to immunoinflammatory stimuli, are modulated via cAMP-protein kinase A (PKA) signaling pathways; namely, generation of reactive oxygen metabolites (ROM) and accelerated proliferation of mesangial cells. We investigated the role of cAMP phosphodiesterase (PDE) isozymes in these regulatory mechanisms. Generation of ROM in cultured rat mesangial cells was inhibited by selective inhibitors of PDE4, rolipram and denbufylline, whereas PDE3 inhibitors, cilostamide and lixazinone, had no effect. Conversely, cilostamide or lixazinone suppressed mitogenic synthesis of DNA in mesangial cells, but 1 microM rolipram or 1 microM denbufylline showed no inhibitory effect. The efficacy of PDE isozyme inhibitors (IC50) to suppress [3H]thymidine incorporation or ROM generation paralleled IC50 values for inhibition of cAMP PDE. Incubation of mesangial cells with either rolipram alone or with cilostamide alone increased significantly in situ activity of PKA in mesangial cells, assessed by (-cAMP/+cAMP) PKA activity ratio, and the stimulatory effects were additive. Results indicate that in mesangial cells a cAMP pool that is metabolized by PDE4 activates PKA and thereby inhibits ROM generation; another cAMP pool that is metabolized by PDE3 activates another PKA (isozyme or pool) which suppresses proliferation of mesangial cells. We propose that in mesangial cells, a cAMP-PKA pathway that regulates mitogenesis is determined by activity of PDE3, whereas another cAMP-PKA pathway is directed by activity of PDE4 and controls ROM generation. Therefore, two PDE isozymes within one cell type compartmentalize distinct cAMP signaling pathways.

Effect of modification of the beta-hairpin and long loops simultaneously in both alpha- and beta-subunits on the function of human choriogonadotropin: part II

According to the X-ray diffraction, human choriogonadotropin has four beta-hairpin and two long loops, equally distributed in each of the alpha and beta subunits. Radical mutations such as the replacement of alpha 18Phe and alpha 74Phe with Thr in the alpha1 and alpha3 loops respectively and the replacement of alpha 45Lys with Asp in the alpha2 loop in the alpha-subunit were introduced while the loop sequences in the beta-subunit were replaced with the corresponding sequences in hFSH beta. Nine different double mutants with simultaneous mutations in both the alpha and beta loops including hCG alpha1 beta1, hCG alpha1 beta2, hCG alpha1 beta3, hCG alpha2 beta1, hCG alpha2 beta2, hCG alpha2 beta3, hCG alpha3 beta1, hCG alpha3 beta2 and hCG alpha3 beta3 were partially purified from insect High-Five cells. As previously reported (Shao et al., 1996, Mol. Cell. Endocrinol. 122, 173-182), the mutation in the alpha1 loop in the mutant, hCG alpha1 beta, the mutants hCG alpha1 beta1 and hCG alpha1 beta3 caused 200%, increase in the receptor binding, cAMP and progesterone stimulation. The mutant, hCG alpha1 beta2 and all other mutants behaved like the recombinant hCG (rehCG) in the receptor binding and post-receptor signaling activities. The molecular cause for this increase is probably due to a conformational change in the heterodimers caused by the mutation in the alpha1 loop. This conclusion is based on the results of the dissociation studies of the mutants heterodimers which indicated a decreased affinity between the subunits. The first order rate constants for the dissociation of the mutants hCG alpha1 beta1, hCG alpha1 beta2 and hCG alpha beta3 were 3.7 x 10(-2) min(-1), 1.4 x 10(-2) min(-1) and 4.6 x 10(-2) min(-1) respectively, as compared with 4.6 x 10(-3) min(-1) for the rehCG. It seems from the data that alpha 18Phe is located in, or in proximity to the receptor binding site and probably plays a critical role in maintaining either directly or indirectly its conformational integrity.

Requirement of cysteine residues in exons 1-6 of the extracellular domain of the luteinizing hormone receptor for gonadotropin binding

The functional importance of cysteine residues in the extracellular domain and the extracellular loops (EL1 and EL2) to hormone binding of the rat luteinizing hormone receptor (LHR) was investigated. For this purpose, cysteines in the seven-transmembrane holoreceptor (Form A) and its hormone-binding splice variant (Form B) were replaced by serine residues, and mutant receptors were expressed in COS1 and/or insect cells. Within the extracellular domain, individual replacement of all four cysteines from Exon 1 abolished hormone binding activity, and replacement of Cys-109 and Cys-134 from exons 5 and 6 caused a 75% decrease in both cell surface and total cellular solubilized LHR hormone binding activity. Mutations of Cys-257 and -258 (Exon 9), Cys-321 and -331, and Cys-417 and -492 of EL1 and EL2, respectively (Exon 11), showed no surface hormone binding activity on intact cells, but exhibited wild type levels of total hormone binding activity when recovered from detergent-solubilized cellular extracts. This finding indicated that expression of high affinity LHR binding activity at the cell surface is independent of the acquisition of the high affinity binding conformation. Other cysteine residues, including Cys-282 (exon 10), and Cys-314 (exon 11) were not essential for hormone binding activity or plasma membrane insertion. This study demonstrates that the functional hormone binding domain utilizes all cysteines N-terminal to exon 7 and localizes the binding site to this N-terminal region of the extracellular domain.

Inhibitors of cyclic nucleotide phosphodiesterase isozymes type-III and type-IV suppress mitogenesis of rat mesangial cells

We studied interactions between the mitogen-activated protein kinase (MAPK) signalling pathway and cAMP-protein kinase (PKA) signaling pathway in regulation of mitogenesis of mesangial cells (MC) determined by [3H]thymidine incorporation, with or without added EGF. Forskolin or dibutyryl cAMP strongly (by 60-70%) inhibited [3H]thymidine incorporation into MC. Cilostamide, lixazinone or cilostazol selective inhibitors of cAMP-phosphodiesterase (PDE) isozyme PDE-III, inhibited mitogenesis to similar extent as forskolin and DBcAMP and activated in situ PKA, but without detectable increase in cAMP levels. Cilostamide and cilostazol were more than three times more effective at inhibiting mesangial mitogenesis than rolipram and denbufylline, inhibitors of isozyme PDE-IV, even though PDE-IV was two times more abundant in MC than was PDE-III. On the other hand, when incubated with forskolin, rolipram-enhanced cAMP accumulation was far greater (10-100x) than with cilostamide. EGF increased MAPK activity (+300%); PDE isozyme inhibitors which suppressed mitogenesis also inhibited MAPK. PDE isozyme inhibitors also suppressed PDGF-stimulated MC proliferation. We conclude that cAMP inhibits the mitogen-dependent MAPK-signaling pathway probably by decreasing the activity of Raf-1 due to PKA-catalyzed phosphorylation. Further, we surmise that minor increase in the cAMP pool metabolized by PDE-III is intimately related to regulation of mesangial proliferation. Thus, PDE isozyme inhibitors have the potential to suppress MC proliferation by a focused effect upon signaling pathways.

Purification of a novel 43-kDa protein (p43) intermediary in the activation of steroidogenesis from rat adrenal gland

In previous reports we have demonstrated the presence of a soluble factor that responds to cAMP signals to induce steroid synthesis in adrenocortical tissue. Here, we describe the purification of this factor from adrenal zona fasciculata cells by using a five-step procedure that includes DEAE-cellulose, gel filtration, Mono Q HPLC and Superose HPLC, and elution of the protein from SDS/PAGE. This procedure results in the purification to homogeneity of a protein of 43-kDa that retains the capacity to stimulate steroid synthesis in an in vitro recombination assay. This activity is inhibited by the use of phospholipase A2 inhibitors. Antipeptide antibodies against the N-terminal region recognize p43 as a double band on SDS/PAGE that resolves in different spots on two-dimensional gel electrophoresis. Adrenocorticotropin treatment of adrenal glands results in the appearance of multiple spots that migrated towards a lower pH compared to controls, suggesting the presence of phosphorylated and dephosphorylated forms of p43. Sequencing of the N-terminal region and internal peptides reveals no significant similarities with other proteins, suggesting that p43 is a novel protein. We conclude from our data that the isolated protein (p43) is a novel, soluble protein that acts as intermediary in adrenocorticotropin-induced stimulation of arachidonic acid release and steroid synthesis.

Formation of reactive oxygen metabolites in glomeruli is suppressed by inhibition of cAMP phosphodiesterase isozyme type IV

Several independent studies indicate that synthetic inhibitors of cyclic-3',5'-nucleotide phosphodiesterase (PDE) isozymes, especially inhibitors of PDE-IV, are potent agents which suppress generation of reactive oxygen metabolites (ROM) by NADPH oxidase in leukocytes. Recent studies also show that NADPH oxidase is present in all cell types populating glomeruli. In view of this, we investigated PDE isozymes and their relation to ROM in isolated rat glomeruli. Glomeruli have the capacity to hydrolyze cAMP by isozymes PDE-II, PDE-III and PDE-IV, whereas cGMP is hydrolyzed by PDE-I and PDE-V. Inhibitor of PDE-IV rolipram inhibited significantly (cca 40 to 50%) ROM generation in response to stimulation by phorbol myristate acetate (PMA). Inhibitor of PDE-III cilostamide had only minor suppressive effects and inhibitors of other PDE isozymes did not influence ROM generation. Rolipram (3 microM) suppressed ROM generation without detectable increase in cAMP content. Incubation of glomeruli with forskolin, which increased cAMP content in glomeruli tenfold, inhibited ROM generation to a similar degree as rolipram. The suppression of ROM generation by rolipram was prevented by Rp-cAMPS, a specific inhibitor of protein kinase A (PKA) activity. Further, incubation of glomeruli with rolipram elicited marked in situ activation of PKA (+ 100%), as documented by increase in the (-cAMP/+cAMP) PKA activity ratio. We suggest that selective inhibitor of PDE-IV rolipram acted via the cAMP-signaling pathway and suppressed ROM generation possibly via phosphorylating ras-type GTP-binding protein component of NADPH oxidase and thereby blocking assembly of functional NADPH oxidase complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of the S. cerevisiae Cdc25 in response to glucose results in its dissociation from Ras

In the yeast Sacchromyces cerevisiae, addition of glucose to starved cells triggers a transient rise in the intracellular level of cyclic AMP that induces a protein phosphorylation cascade. The glucose signal is processed by the Cdc25/Ras/adenylyl cyclase pathway, where the role of Cdc25 is to catalyse the GDP-GTP exchange on Ras. The molecular mechanisms involved in the regulation of the activity of Cdc25 are unknown. We report here the use of highly selective anti-Cdc25 antibodies to demonstrate that Cdc25 is a phospho protein and that in response to glucose it is hyperphosphorylated, within seconds, by the cyclic AMP-dependent protein kinase. It is also demonstrated that, concomitantly with hyperphosphorylation, Cdc25 partially relocalizes to the cytoplasm, reducing its accessibility to membrane-bound Ras. These results are of general significance because of the highly conserved sequence of Ras-guanyl nucleotide exchange factors from yeasts to mammals.

Diabetes mellitus/male infertility

Diabetes mellitus (DM), associated with very subtle disorders, affects, either directly or indirectly, various functions of the reproductive system. Adequate, regular, and timely therapy may prevent or delay these disorders. The T synthesis disorder is caused by molecular changes at the level of Leydig cells and may lead to other disorders in all target organs and tissues. The close correlation between Leydig and Sertoli cells function, i.e., between spermatogenesis and second sex glands function, results in certain anomalies in diabetic patients' spermiograms. Parallel lesions associated with DM, through CNS (hypothalamus-hypophysis), and endocrine profile are indirectly intensified or induced by these disorders, which reflect dysfunction of homeostatic balance in carbohydrate metabolism. Sexual dysfunction in all its forms (reduced erection, impotence, and other libido dissociations) is an accompanying phenomenon of the diabetic disease. However, manifestations of these disorders are related to the regulation of carbohydrate metabolism and to the duration of disease. The duration of disease is not necessarily correlated with sexual dysfunction. Even carbohydrate metabolism remains within normal range in addition to other lesions, diabetes leads gradually but progressively to premature aging of body cells.

Physiology of the male reproductive system: endocrine, paracrine and autocrine regulation

This presentation reviews the male reproductive system, concentrating on newer advances in our knowledge of its physiology, biochemistry, and regulation, and introduces the topic of male reproductive toxicology. GnRH is the hypothalamic peptide responsible for the stimulation of LH and FSH release from the pituitary. It is synthesized as a pro-hormone, processed in the hypothalamus and released into the portal system in a pulsatile fashion. The timing of these pulses is critical to the release of LH and FSH into the general circulation. While LH and FSH are the main trophic hormones for the testis, we now realize the importance of not only endocrine control, but also of paracrine and autocrine regulation. Specifically, the local control of Leydig cells, Sertoli cells, and germ cells appears to be modulated by numerous growth factors and local regulators arising from within the testis. This point is emphasized both during a discussion of the interaction of the various cell types in the testis and during a discussion of spermatogenesis, where techniques which show stage-specific secretions are highlighted. Newest advances in the mechanism of action of steroidal and peptide hormones are also emphasized with special reference to the possible interaction between toxicants and endocrine control of the reproductive system. This update of the reproductive system "sets the stage" for an in-depth examination of the site and mechanism of action of reproductive toxicants.

Biological and immunological activity of human chorionic gonadotrophin in maternal serum during early pregnancy

Serum chorionic gonadotrophin (hCG) levels were measured by three different assay methods in 20 women admitted to legal abortion and 21 patients having insulin-dependent diabetes, both during 8 to 14 weeks of pregnancy. Biological activity of hCG was determined with a mouse Leydig cell in-vitro bioassay, binding activity of the hormone to LH receptors by rat testis radioreceptorassay, and immunoreactivity by fluorimmunoassay. Bioassay and radioreceptorassay gave 1.4 and 1.7-fold higher hCG concentrations than fluoroimmunoassay using the same standard. Correlations between hCG levels measured by bioassay and fluoroimmunoassay (r = 0.81; P less than 0.01) and radioreceptorassay and fluoroimmunoassay (r = 0.95; P less than 0.01) were good. The results emphasize the heterogeneity of hCG in the pregnancy serum. Different domains of the molecules are recognized by assay methods based on antigenicity, receptor binding and biopotency of the hormone.

Early effects of hCG on human testicular cyclic AMP content, protein kinase activity, in-vitro progesterone conversion and the serum concentrations of testosterone and oestradiol

Ten infertile men underwent testicular biopsy. Cyclic AMP concentration and cAMP-dependent protein kinase activity were determined in biopsies obtained before, and at 3, 10, 20 and 30 min after an intravenous injection of hCG (1500-5000 IU). The in-vitro conversion of progesterone by testicular tissue, and the serum concentrations of testosterone and oestradiol were then studied before and at 30 min after hCG injection. Intravenous injection of hCG induced a rapid increase in cAMP concentration and in the activity of cAMP-dependent protein kinase. The kinetics of this response indicated that cAMP and cAMP-dependent protein kinase mediate hCG effects on the human testis, presumably via effects on the Leydig cells. No stimulatory effect on steroid conversion in vitro or on the serum concentrations of testosterone and oestradiol were seen after 30 min.