Rapid non-genomic actions of progesterone stimulate Ca2+ influx and the acrosome reaction in human sperm

The sirtuin 1 activator YK 3-237 stimulates capacitation-related events in human spermatozoa

RESEARCH QUESTION

Does sirtuin-1 (SIRT1) have a role in the human spermatozoa capacitation process?

DESIGN

Human spermatozoa were incubated for 6 h in a capacitating medium in presence or absence of the specific SIRT1 activator, YK 3-237. Several sperm parameters were determined by flow cytometry: viability, acrosome reaction and mitochondria membrane status. Sperm motility was determined objectively by computer-assisted semen analysis. Sperm capacitation status was evaluated by the extent of protein tyrosine phosphorylation and by the percentage of spermatozoa with the acrosome reacted by a calcium ionophore challenge.

RESULTS

SIRT1 was detected in the connecting piece of human spermatozoa where a lysine acetylation pattern was mainly found along the sperm tail. SIRT1 activation accelerates the occurrence of a phenotype associated with human sperm capacitation, with no differences seen in the lysine acetylation pattern. After 1 h of co-incubation of YK 3-237 with human spermatozoa, tyrosine phosphorylation levels were comparable to control levels after 6 h of incubation in capacitating conditions. In addition, the activator improved sperm responsiveness to a Ca²⁺ ionophore (A23187) challenge determined by an increase in acrosome-reacted spermatozoa (P = 0.025). Importantly, sperm viability and mitochondrial activity-related parameters assessed by flow cytometry were not affected by YK 3-237.

CONCLUSION

YK 3-237 induces capacitation-related events in human spermatozoa such an increase of tyrosine phosphorylation levels and acrosome-reacted spermatozoa after the ionophore challenge. Together, these results show that YK 3-237 affects human spermatozoa capacitation-related events by a mechanism independent of protein lysine acetylation but dependent on bicarbonate and calcium.

Proteins from male and female reproductive tracts involved in sperm function regulation

Spermatogenesis is a dynamic process that culminates in the production of mature spermatozoa in the seminiferous tubules of sexually mature animals. Although sperm leaving the testis are fully differentiated, they must further undergo two additional maturation steps before acquiring the capability to fertilize the egg. Such processes take place during the epididymal residency and transport in the seminal fluid during ejaculation and, after delivery into the female reproductive tract, during the journey aiming the encountering the egg in the oviduct. Throughout this trip, spermatozoa are exposed to different reproductive fluids whose molecular compositions regulate the progress towards obtaining a fertilized competent cell. This review summarizes the evidence obtained so far supporting the participation of male and female reproductive tract-derived proteins in the modulation of sperm fertilizing ability and discusses the mechanisms by which such regulation may be accomplished.

Factors and pathways involved in capacitation: how are they regulated?

In mammals, fertilization occurs via a comprehensive progression of events. Freshly ejaculated sperm have yet to acquire progressive motility or fertilization ability. They must first undergo a series of biochemical and physiological changes, collectively known as capacitation. Capacitation is a significant prerequisite to fertilization. During the process of capacitation, changes in membrane properties, intracellular ion concentration and the activities of enzymes, together with other protein modifications, induce multiple signaling events and pathways in defined media in vitro or in the female reproductive tract in vivo. These, in turn, stimulate the acrosome reaction and prepare spermatozoa for penetration of the egg zona pellucida prior to fertilization. In the present review, we conclude all mainstream factors and pathways regulate capacitation and highlight their crosstalk. We also summarize the relationship between capacitation and assisted reproductive technology or human disease. In the end, we sum up the open questions and future avenues in this field.

The steroid hormone 20-hydroxyecdysone upregulates calcium release-activated calcium channel modulator 1 expression to induce apoptosis in the midgut of Helicoverpa armigera

Animal steroid hormones stimulate extracellular Ca²⁺ influx into cells; however, the mechanism remains unclear. In this study, we determined that the Ca²⁺ influx induced by steroid hormone 20-hydroxyecdysone (20E) is mediated by the calcium release-activated calcium channel modulator 1 (CRACM1/Orai1). The Orai1 mRNA is highly expressed during midgut programmed cell death in the lepidopteran insect Helicoverpa armigera. 20E upregulated the expression of Orai1 in H. armigera larvae and in an epidermal cell line (HaEpi). Knockdown of Orai1 in HaEpi cells blocked 20E-induced Ca²⁺ influx, and the inhibitor of inositol 1, 4, 5-trisphosphate receptor (IP₃R) Xestospongin (XeC) blocked 20E-induced Ca²⁺ influx, suggesting that 20E, via Orai1, induces stored-operated Ca²⁺ influx. Orai1 interacts with stromal interaction molecule 1(Stim1) to exert its function in 20E-induced Ca²⁺ influx. 20E promotes Orai1 aggregation through G-protein-coupled receptors, phospholipase C gamma 1, and Stim1. Knockdown of Orai1 in the HaEpi cell line repressed apoptosis and maintained autophagy under 20E regulation. Knockdown of Orai1 in larvae delayed pupation, repressed midgut apoptosis, maintained the midgut in an autophagic state, and repressed 20E-pathway gene expression. These results revealed that steroid hormone 20E, via Orai1, induces Ca²⁺ influx to promote the transition of midgut from autophagy to apoptosis.

Modulation of Human Sperm Capacitation by Progesterone, Estradiol, and Luteinizing Hormone

Sperm residency in female reproductive tract is essential to undergo functional changes that allow the cell to encounter the oocyte and fertilize it. Those changes, known as capacitation, are modulated by molecules located in the uterotubal surface and fluids. During the fertile window, there is a notable increase in some reproductive hormones such as progesterone, estradiol, and luteinizing hormone in the female reproductive tract, so spermatozoa are exposed to these hormones in an environment that must favor gamete encountering and fusion. This spatiotemporal coincidence suggests that they are suitable candidates to modulate sperm function in order to synchronize the events that ultimately allow the success of fertilization. The presence of receptors for these hormones in the human sperm has been described, but their physiological relevance and mechanisms of action have been either subject of controversy or not properly investigated. This review intends to summarize the evidence that support the participation of these hormones in the regulation of sperm capacitation.

An in vitro study of the effect of mifepristone and ulipristal acetate on human sperm functions

Ulipristal acetate (UPA) and mifepristone are currently well-established agents for emergency contraception. Both drugs are selective progestogen receptor modulators which have been shown to have better efficacy than the widely used levonorgestrel in prevention of pregnancy. However, there is only limited information on the action of UPA on sperm function. The present study compared the in vitro biological effects of mifepristone and UPA on human sperm functions. Spermatozoa from semen samples with normal semen parameters were isolated. Capacitated spermatozoa were pre-incubated with 0.04, 0.4, 4 and 40 μM mifepristone or UPA for 1 h. Sperm motility, viability, DNA integrity, capacitation, spontaneous acrosome reaction, spontaneous hyperactivation, zona pellucida (ZP) binding capability and intracellular calcium concentration ([Ca(2+)]i) were determined. The effects of mifepristone and UPA on progesterone-induced acrosome reaction, hyperactivation and [Ca(2+)]i were also studied. Our results showed that mifepristone and UPA dose-dependently suppressed progesterone-induced acrosome reaction, hyperactivation and [Ca(2+)]i at concentrations ≥0.4 μM in human spermatozoa. Both compounds did not affect sperm motility, viability, DNA integrity, capacitation, spontaneous acrosome reaction, spontaneous hyperactivation, ZP binding capability and [Ca(2+)]i. This study demonstrated that UPA and mifepristone modulate human sperm functions by acting as progesterone antagonists. The results enable us to gain a better understanding of the mechanisms by which mifepristone and UPA work for emergency contraception, and provide a scientific basis for their clinical application.

Genomic analysis of the ecdysone steroid signal at metamorphosis onset using ecdysoneless and EcRnullDrosophila melanogaster mutants

Steroid hormone gene regulation is often depicted as a linear transduction of the signal, from molecule release to the gene level, by activation of a receptor protein after being bound by its steroid ligand. Such an action would require that the hormone be present and bound to the receptor in order to have target gene response. Here, we present data that presents a novel perspective of hormone gene regulation, where the hormone molecule and its receptor have exclusive target gene regulation function, in addition to the traditional direct target genes. Our study is the first genome-wide analysis of conditional mutants simultaneously modeling the steroid and steroid receptor gene expression regulation. We have integrated classical genetic mutant experiments with functional genomics techniques in the Drosophila melanogaster model organism, where we interrogate the 20-hydroxyecdysone signaling response at the onset of metamorphosis. Our novel catalog of ecdysone target genes illustrates the separable transcriptional responses among the hormone, the pre-hormone receptor and the post-hormone receptor. We successfully detected traditional ecdysone target genes as common targets and also identified novel sets of target genes which where exclusive to each mutant condition. Around 12 % of the genome responds to the ecdysone hormone signal at the onset of metamorphosis and over half of these are independent of the receptor. In addition, a significant portion of receptor regulated genes are differentially regulated by the receptor, depending on its ligand state. Gene ontology enrichment analyses confirm known ecdysone regulated biological functions and also validate implicated pathways that have been indirectly associated with ecdysone signaling.

In vivo treatment with progestogens causes immunosuppression of carp Cyprinus carpio leucocytes by affecting nitric oxide production and arginase activity

In this study, carp Cyprinus carpio were injected with various steroid compounds, including synthetic and natural progestogens and the glucocorticoid cortisol, to investigate effects on leucocytes isolated from their kidneys. Injection of cortisol led to an increased spleeno-somatic index (I(S)) on day 21 post-injection (pi) and immunosuppressive effects measured as decreased nitric oxide (NO) production and increased arginase activity in isolated leucocytes on days 14 and 21 pi, respectively. Moreover, reduced NO production was also observed after injection of the synthetic progestogens, levonorgestrel (LEV) and medroxyprogesterone acetate. In addition, LEV influenced arginase activity in head kidney cells on day 14 and day 21 pi. This study is the first demonstration in fishes that the application of these steroid compounds in vivo affects NO production and arginase activity of isolated leucocytes.

Progestogens cause immunosuppression of stimulated carp (Cyprinus carpio L.) leukocytes in vitro

The involvement of steroid hormones in direct and indirect regulation and modulation of immune responses is well recognized in mammals. Here, we demonstrate that progestogens are capable of influencing the innate immunity in fish as well. Therefore, we confirmed the known immunosuppressive effects of natural progesterone (P4), and compared them to influences of 17alpha,20beta-dihydroxy progesterone (DHP4) and the synthetic progestins, medroxyprogesterone acetate (MPA) and levonorgestrel (LEV), on NO release by in vitro-stimulated carp leukocytes derived from both, head and trunk kidney, respectively. DHP4 known as the main maturation-inducing steroid in many teleosts potently inhibited the NO release by carp leukocytes. The synthetic progestin MPA, which may also be environmentally relevant due to its world-wide use in hormonal contraception, significantly decreased NO formation by head and trunk kidney cells. In contrast, LEV showed no significant influence on NO release by head and trunk kidney leukocytes. The observed immunosuppressive actions of progestogens on NO production were compared to the known impairment by natural and synthetic glucocorticoids. Determining the potential impact of progestogens on mRNA expression of iNOS by means of semi-quantitative reverse transcription polymerase chain reactions (RT-PCR) revealed downregulation of proinflammatory type I immune response characteristics at high concentrations. These findings demonstrate for the first time that similar to the known effects of natural progesterone synthetic progestogens are also able to influence immune signaling cascades in fish, and provide evidence that these steroids are capable of influencing mRNA expression of iNOS. The induction of a regulatory type II immune response by progestogens is a striking example of interference of female steroid-mediated events with the piscine immune system. Furthermore, the identification of a partial sequence of a membrane-associated progestogen receptor (mPR) in carp leukocytes by RT-PCR indicates a specific mechanism underlying the observed effects of progestogens on these immune cells.

Effect of progesterone and oestradiol on sperm-zona binding and acrosome reaction in bovine spermatozoa after thawing

This study was conducted assess spermatozoa binding capacity to the oocyte in the presence of 0.1 or 0.5 microg/ml progesterone (P4) or a combination of 0.5 microg/ml P4 with 0.1 microg/ml oestradiol (OE). The number of oocyte-bound spermatozoa in the presence of progesterone was significantly higher (p < 0.05 to p < 0.001) when compared with the control samples, that were incubated in the absence of P4. Spermatozoa binding to the zona pellucida (ZP) of the oocyte were concentration-dependent - significantly higher numbers of spermatozoa were bound in the presence of 0.5 microg/ml P4, when compared with that of 0.1 microg/ml P4. Oestradiol at 0.1 microg/ml concentration used impaired the effect of progesterone-mediated sperm-oocyte binding. The incidences of acrosome-reacted (AR) spermatozoa bound to the ZP - following 0, 60, 120 and 180 min. incubation in the presence and absence of 1 microg/ml progesterone was also assessed. Only at 180 min of incubation a higher (p < 0.001) incidence of the AR-spermatozoa was found in sperm-ZP complexes incubated in the presence or absence of progesterone, being 56.5 +/- 11.1 and 43.2 +/- 8.8 % respectively. In conclusion, progesterone enhances the sperm-ZP binding capacity. Progesterone affects the incidences of AR on zona-bound spermatozoa only after prolonged co-culture.

Binding and activation of the seven-transmembrane estrogen receptor GPR30 by environmental estrogens: a potential novel mechanism of endocrine disruption

A wide variety of environmental contaminants have been shown to exert estrogenic actions in wildlife and laboratory animals through binding to nuclear estrogen receptors (ERs) and subsequent transcription of estrogen responsive genes. We show here that several of these environmental estrogens also bind to the novel seven-transmembrane estrogen receptor, GPR30, to activate alternative estrogen signaling pathways in an ER-negative cell line (HEK293) stably transfected with the receptor. Genestein was the most effective competitor for the receptor (IC(50) 133 nM), with a relative binding affinity (RBA) 13% that of estradiol-17beta (E2). Bisphenol A, zearalonone, and nonylphenol also had relatively high binding affinities for GPR30 with RBAs of 2-3%. Kepone, p,p'-DDT, 2,2',5',-PCB-4-OH and o,p'-DDE had lower affinities with RBAs of 0.25-1.3%, whereas o,p'-DDT, p,p'-DDE, methoxychlor and atrazine caused less than 50% displacement of [(3)H]-E2 at concentrations up to 10 microM. Overall, the binding affinities of these compounds for GPR30 are broadly similar to their affinities to the ERs. Environmental estrogens with relatively high binding affinities for GPR30 (genestein, bisphenol A, nonylphenol and Kepone) also displayed estrogen agonist activities in an in vitro assay of membrane-bound adenylyl cyclase activity, a GPR30-dependent signaling pathway activated by estrogens. The results indicate that nontraditional estrogen actions mediated through GPR30 are potentially susceptible to disruption by a variety of environmental estrogens.

Sex hormone receptors in the human eye

Dissimilarities in ocular physiopathology exist between human males and females. These differences can be observed in the lacrimal and other eye-associated glands, the ocular surface, the crystalline lens, and the retinochoroid complexes. Literature on the subject revealed that because of sex steroid hormone (estrogen, progesterone, and androgen) actions, various physiological conditions, such as age, menstrual cycles, pregnancy, and menopause or andropause, where the hormone milieu changes, affect vision. Well-designed scientific studies are lacking on the subject, although such studies hold much potential value. This review analyzes the relatively new area of hormones and vision.

Biological basis for human capacitation

More than 50 years ago Austin and Chang defined mammalian sperm capacitation as a period of time that sperm must reside in the female reproductive tract before they acquire the ability to fertilize oocytes. Since then numerous investigations have attempted to more clearly define the molecules and processes that are a part of capacitation. The data that have provided a more clear definition of capacitation were primarily derived from in vitro experiments. This is particularly true for studies on human sperm capacitation. While ethical constraints have limited an equal balance of in vivo studies there are those data that when coupled with some of the in vitro data allow for the formulation of a biological framework for human sperm capacitation in vivo. This review will put forth the biological basis for human capacitation.

Rates of suppression and recovery of human sperm output in testosterone-based hormonal contraceptive regimens*

BACKGROUND

Practical hormonal male contraceptive regimens are likely to have delayed onset and offset of reliable contraception dictated by the length of the spermatogenic cycle and clearance rate of pre-formed sperm from the ductular system. While delayed onset of contraceptive efficacy is an accepted feature of vasectomy, reliable time estimates for a hormonal male contraceptive of time to onset and offset of reliable contraception and of resumption of normal male fertility are required.

METHODS AND RESULTS

We utilized the sperm output data from three male contraceptive efficacy studies to define quantitative estimates of suppression and recovery rates from an androgen alone (testosterone enanthate) and an androgen/progestin (testosterone/depot medroxyprogesterone acetate) study. Using nearly 14,000 semen samples from World Health Organization (WHO) studies #85921 and #89903 with identical protocols, the rate of suppression of sperm output was best modelled as a two-parameter, single exponential decay function with effective half-time to suppression of 5.5 weeks and times of 6.8 weeks to 10 x 10(6)/ml, 8.7 weeks to 5 x 10(6)/ml, 10.0 weeks to 3 x 10(6)/ml and 13.0 weeks to 1 x 10(6)/ml. The rate of recovery using absolute sperm concentration was best modelled as a three-parameter, sigmoidal curve with effective time to reach half of the recovery plateau of 10.5 weeks and times of 9.0 weeks to 3 x 10(6)/ml, 9.9 weeks to 5 x 10(6)/ml, 11.5 weeks to 10 x 10(6)/ml, and 13.6 weeks to 20 x 10(6)/ml. Using relative sperm output, defined as a percentage of the participants' own baseline, recovery approached an asymptotic plateau of approximately 85% of geometric mean pre-treatment sperm concentration. In the combination androgen/progestin study, suppression rate was significantly faster (effective time to reach half maximal suppression of 3.0 weeks) and recovery significantly slower (effective time to reach half of recovery plateau of 14.7 weeks) and less complete (asymptotic recovery plateau of 43% of baseline) than in the androgen-alone WHO studies.

CONCLUSION

These findings therefore provide large sample estimates of the suppression and recovery rates from an androgen-alone hormonal male contraceptive regimen as a basis for comparison with other second-generation combination androgen/progestin regimens that are the most promising approach to developing practical male hormonal regimens.

Calcium Channels and Ca2+ Fluctuations in Sperm Physiology

Generating new life in animals by sexual reproduction depends on adequate communication between mature and competent male and female gametes. Ion channels are instrumental in the dialogue between sperm, its environment, and the egg. The ability of sperm to swim to the egg and fertilize it is modulated by ion permeability changes induced by environmental cues and components of the egg outer layer. Ca(2+) is probably the key messenger in this information exchange. It is therefore not surprising that different Ca(2+)-permeable channels are distinctly localized in these tiny specialized cells. New approaches to measure sperm currents, intracellular Ca(2+), membrane potential, and intracellular pH with fluorescent probes, patch-clamp recordings, sequence information, and heterologous expression are revealing how sperm channels participate in fertilization. Certain sperm ion channels are turning out to be unique, making them attractive targets for contraception and for the discovery of novel signaling complexes.

Arabidopsis membrane steroid binding protein 1 is involved in inhibition of cell elongation

A putative Membrane Steroid Binding Protein (designated MSBP1) was identified and functionally characterized as a negative regulator of cell elongation in Arabidopsis thaliana. The MSBP1 gene encodes a 220-amino acid protein that can bind to progesterone, 5-dihydrotestosterone, 24-epi-brassinolide (24-eBL), and stigmasterol with different affinities in vitro. Transgenic plants overexpressing MSBP1 showed short hypocotyl phenotype and increased steroid binding capacity in membrane fractions, whereas antisense MSBP1 transgenic plants showed long hypocotyl phenotypes and reduced steroid binding capacity, indicating that MSBP1 negatively regulates hypocotyl elongation. The reduced cell elongation of MSBP1-overexpressing plants was correlated with altered expression of genes involved in cell elongation, such as expansins and extensins, indicating that enhanced MSBP1 affected a regulatory pathway for cell elongation. Suppression or overexpression of MSBP1 resulted in enhanced or reduced sensitivities, respectively, to exogenous progesterone and 24-eBL, suggesting a negative role of MSBP1 in steroid signaling. Expression of MSBP1 in hypocotyls is suppressed by darkness and activated by light, suggesting that MSBP1, as a negative regulator of cell elongation, plays a role in plant photomorphogenesis. This study demonstrates the functional roles of a steroid binding protein in growth regulation in higher plants.

In vitro effect of gamma-aminobutyric acid on bovine spermatozoa capacitation

Sperm capacitation is defined as the maturational changes that render a sperm competent for fertilization and occurs in the female reproductive tract. Identification of the factor/s that regulate sperm capacitation would allow the understanding of these phenomena. Among these factors, gamma-aminobutyric acid (GABA) has recently become as a putative modulator of sperm function. The aim of this study was to explore the presence of a GABAergic regulation of bovine sperm capacitation as well as the possible intracellular mechanisms involved. GABA was detected in fresh semen by a sensitive radioreceptor assay (spermatozoa, 0.064 +/- 0.003 nmoles/10(6) cells; seminal plasma, 23.21 +/- 1.16 nmoles/ml). Scatchard analysis of [(3)H]-muscimol binding to sperm membranes yielded a linear plot consistent with a single population of binding sites (K(d) = 3.87 nM, B(max) = 417 fmol/mg prot.). [(3)H]-muscimol specific binding to sperm membranes was significantly inhibited by the GABA A receptor (GABA A-R) antagonist bicuculline and by the agonists muscimol and isoguvacine. Addition of GABA to the incubation medium resulted in a concentration-dependent increase in the percentage of capacitated spermatozoa (chlortetracycline assay). We observed a significant increment on intracellular calcium and cyclic 3',5' adenosine monophosphate (cAMP) concentrations induced by GABA, being the cation influx abolished when the cell suspensions were coincubated with the antagonists bicuculline or picrotoxin. It is concluded that GABA induces sperm capacitation through an intracellular mechanism dependent on calcium influx and cAMP accumulation mediated by a specific GABA A-R.

Regulation of sperm function by reactive oxygen species

Sperm capacitation can be increased by the addition of reactive oxygen species (ROS) and decreased by antioxidants. Broadly consistent results have been achieved with a wide variety of methods and across different species. Exposure to ROS increases protein tyrosine phosphorylation consequent on an increase in cAMP and activation of tyrosine kinase and inhibition of tyrosine phosphatase. The measurement of ROS production by sperm is complicated by contamination of suspensions by leukocytes, laying many studies open to doubt. In human sperm the observation that extracellular NADPH could support superoxide production detected with the chemiluminescent probe lucigenin and had physiological effects similar to hydrogen peroxide led to the suggestion that they contained NADPH oxidase activity to generate ROS to support capacitation. However, the realization that lucigenin can signal superoxide artefactually, combined with failure to detect superoxide production using spin trapping techniques or to detect NADPH oxidase components in mature sperm, and confirmation of old reports that NADPH solution contains substantial amounts of hydrogen peroxide due to autoxidation, have undermined this hypothesis. Although the presence of significant NADPH oxidase activity in mature human sperm now seems less likely, other observations continue to suggest that they can make ROS in some way. There is stronger evidence that animal sperm can make ROS although these may be mainly of mitochondrial origin.

Effect of Exogenous Progesterone on Post-thaw Capacitation and Acrosome Reaction of Bovine Spermatozoa

The aim of this work was to study the effect of progesterone (P4) on capacitation and acrosome reaction (AR) of post-thaw bovine spermatozoa in vitro. Spermatozoa were incubated (0-180 min) in capacitation medium supplemented with 0, 0.1, 1.0 and 10.0 microg/ml of P4. At different time intervals aliquots were taken to determine sperm plasma membrane lipid destabilization, or capacitation (AR induced by lysophosphatidylcholine) in spermatozoa. The second experiment aimed to study the effects of P4, as potential inducer of AR in heparin-capacitated spermatozoa. The acrosomal status and viability of spermatozoa were evaluated under an epifluorescence microscope using Ethidium homodimer/peanut agglutinin fluorescein isothiocyanate staining method. Plasma membrane scrambling in spermatozoa was assessed by a flow cytometer, using merocyanine staining. The results show that P4 at the concentrations used had no negative effects on sperm viability. Progesterone significantly enhanced sperm capacitation (p < 0.001), but had no effect on plasma membrane lipid stability (p > 0.05) and did not significantly increase the AR of heparin-capacitated spermatozoa (p > 0.05). Progesterone displayed its effects in a dose-dependent manner with a maximum effect of 10 microg/ml P4 at 180 min of incubation. The results demonstrate that in cryopreserved bovine semen, P4 acts as capacitating, but not as an AR-inducing agent.

Formation and dynamic alterations of horizontal microdomains in sperm membranes during progesterone-induced acrosome reaction

Capacitated mammalian spermatozoa undergo a fusion response of their head plasma membrane and the outer acrosomal membrane leading to vesiculation classically known as acrosome reaction. Acrosome reaction occurs in response to various acrosome reaction inducers including zona pellucida proteins, calcium ionophore, dibutyryl cAMP, progesterone, etc. All the acrosome reaction inducers cause a transient of calcium influx into the sperm through voltage-dependent cation channels. Efflux of chloride, stimulation of activity of phospholipases, and phosphorylation of proteins are other known changes introduced by acrosome reaction inducers. Macromolecular organization and dynamics of sperm membranes during the progression of this vesiculation are largely unexplored. In this study, we report that progesterone induced the formation of horizontal microdomains within the exofacial surfaces of sperm membranes, which showed progressive and independent alterations in molecular dynamics. In the light of this observation, we propose that sperm membrane rafts may contain both horizontal and vertical microdomains.

Rapid inhibition of the contraction of rat tail artery by progesterone is mediated by inhibition of calcium currents

Progesterone induced rapid relaxation of KCl-contracted tail artery helical strips from rats. The effect was dose dependent, with an IC50 (inhibitory concentration which produces 50% of the maximal response) of 8.9 microM progesterone. The actions of progesterone were not blocked by bicuculline, indicating that in this tissue the non-genomic actions of progesterone were not mediated via a gamma-aminobutyric acid (GABA)-A receptor. Fura-2 was used to measure intracellular calcium levels ([Ca(2+)](i)) in isolated vascular smooth muscle cells (VSMC). Incubation of cultured VSMC for 15 min with progesterone (10 microM) resulted in an inhibition of the KCl-induced [Ca(2+)](i )increase. The whole-cell patch-clamp technique was used to examine Ca(2+)-channel currents in the membrane of isolated VSMC. Progesterone suppressed the L-type Ca(2+)-channel currents in cells held at a potential of -40 mV. The effects of progesterone were quickly reversed by washout in all three experimental protocols suggesting that these effects on vascular tissues are non-genomic. The correlation of the effects on all these preparations, their time course and reversibility suggested that the rapid relaxation of the rat tail artery induced by progesterone is mediated at least in part by inhibition of L-type calcium channels, leading to inhibition of calcium responses in the VSMC of this tissue.

Characterization of membrane steroid binding protein mRNA and protein in lens epithelial cells

Epithelial cells of the ocular lens contain a 28 kDa membrane protein which is proposed to mediate high affinity binding of steroid hormones and rapid non-genomic actions of steroid hormones. It has been named membrane steroid binding protein (MSBP). Our purpose was to further characterize this protein from cultured bovine lens epithelial cells (BLEC) and compare it to similar forms of the protein present in other species and tissues. The size of the protein's mRNA was examined by Northern blot analysis using a digoxigenin-labelled antisense riboprobe. The sequence of the mRNA was obtained by RT-PCR amplification of poly A+ RNA recovered from cultured BLEC. PCR amplification was conducted using three sets of nested sense and antisense primers, one set at a time. The amino acid sequence of the lens protein was deduced from the revealed cDNA sequence. The hydropathy of the protein was examined by Kyte-Doolittle plots. The sequence of the lens protein's cDNA (about 1.7 kb total) described an open reading frame of 582 residues which coded for a protein of 194 amino acids. The presence of a C-terminal isoprenylation motif suggested by earlier work was not found in the coding region. The deduced amino acid sequence of the lens protein was extremely similar to those of other species and tissues, being 95-98% homologous with that of the other members. All of the MSBPs apparently contain a single membrane spanning domain in the amino terminal. The highly conserved nature of this protein implies a useful function to the cell. We speculate that the protein is a receptor which mediates rapid actions of steroids on lens epithelial cells, such as calcium mobilization, and that the protein plays a role in the mechanism of steroid induced cataracts.

Is there a glucocorticoid receptor in the bovine lens?

Prolonged glucocorticoid therapy is a risk factor for cataract development. The mechanism remains unknown. If cataract results from the direct effect of steroids on lens function, a glucocorticoid receptor is required. In order to determine whether such a receptor was present in the bovine lens, metabolic and steroid binding experiments were undertaken. Cultured bovine lens epithelial cells were exposed to 10(- 4)and 10(-8) M dexamethasone or prednisolone and the uptake and incorporation of(14)C leucine,(14)C glucose and(3)H thymidine, examined. Neither glucocorticoid affected cell protein synthesis or glucose uptake. Both dexamethasone concentrations and the lower concentration of prednisolone had no effect on thymidine uptake or incorporation, however, the 10(-4) M prednisolone exposure reduced these by 15 +/- 5%. This regulation is thought to be due to membrane fluidity changes and not the action of the glucocorticoid receptor. As the glucocorticoid receptor is very heat labile in vitro, the effects of increasing temperature on dexamethasone binding by proteins from lens epithelium, lens nucleus and liver were examined. At 0 degree C, lens epithelial extract bound nine-fold more dexamethasone than liver extract. After exposure to 37 degrees C, liver binding decreased by 66% whereas that for lens epithelium increased by 18%. For both lens extracts, steroid binding increased with temperature up to 50 degrees C. Scatchard analysis of the steroid binding kinetics showed there to be no high affinity sites in lens epithelial extract, with the binding best described as a non-specific partitioning event. Western blotting with a specific glucocorticoid receptor antibody revealed protein bands of approximately 94 and 79 kDa in liver, which is known to contain significant levels of receptor. No immunoreactivity was observed for lens epithelial extract. Therefore, within the limits of detection, these results suggest the bovine lens does not contain a glucocorticoid receptor. This raises questions about the validity of receptor-mediated mechanisms proposed for cataract development.

Localization and topology of ratp28, a member of a novel family of putative steroid-binding proteins

We have cloned ratp28, a membrane protein from rat liver homologous to the previously described hpr6.6, a putative steroid-binding protein in humans. Ratp28 has a type II topology as determined by protease digestion experiments on intact and detergent-solubilized membranes. Subcellular fractionation by sucrose density centrifugation revealed a distribution for ratp28 identical to Bip as a marker for membranes of the endoplasmic reticulum. In these experiments no association was found with markers for Golgi or plasma membranes, indicating that ratp28 is localized to the endoplasmic reticulum.

Activation of Src kinase in skeletal muscle cells by 1, 1,25-(OH(2))-vitamin D(3) correlates with tyrosine phosphorylation of the vitamin D receptor (VDR) and VDR-Src interaction

The rapid effect of 1 alpha,25(OH(2))-vitamin D(3) [1 alpha, 25(OH(2))D(3)] on tyrosine kinase Src and its relationship to the vitamin D receptor (VDR) was investigated to further characterize the hormone signaling mechanism in chick muscle cells. Exposure of cultured myotubes to 1 alpha,25(OH(2))D(3) caused a time-dependent increase in Src activity, which was evident at 1 min (one-fold) and reached a maximum at 5 min (15-fold). Immunoblotting with anti-phosphotyrosine antibody of immunoprecipitated Src showed that the hormone decreased Src tyrosine phosphorylation state with maximal effects at 5 min. Using a database for protein consensus motifs we found a putative tyrosine phosphorylation site (amino acids 164-170: KTFDTTY) within the primary sequence of the chick VDR. When the myotube VDR was immunoprecipitated it appeared onto SDS-PAGE gels as a single band of 58 kDa recognized by an anti-phosphotyrosine antibody. Prior treatment of cells with (1)alpha,25(OH(2))D(3) significantly increased tyrosine phosphorylation of the VDR (two- to three-fold above basal levels). In agreement with Src being a SH2-domain containing protein involved in recognition of tyrosine-phosphorylated targets, immunoprecipitation with anti-Src antibody under native conditions followed by blotting with anti-VDR antibody, or using the antibodies in inverse order, showed that the VDR co-precipitates with Src, thus indicating the existence of a VDR/Src complex. Stimulation with the cognate VDR ligand significantly increased formation of the complex with respect to basal conditions. These results altogether provide the first evidence to date for 1 alpha,25(OH(2))D(3) activation involving Src association to tyrosine phosphorylated VDR.

Testicular endocrine function, seasonality and semen quality of the stallion

To gain further information on gonadal function of the stallion, concentrations of testicular steroids in blood plasma (bpl) and seminal plasma (spl) and their distribution in the ejaculate were determined. Blood and semen samples from a total of 11 stallions were collected from November to July. Estrone (E1), estrone sulfate (E1S), estradiol-17beta (E2beta) and testosterone (T) were determined in bpl and spl, and in addition androstenedione (A), dehydroepiandrosterone (DHEA) and 5alpha-dihydrotestosterone (5alpha-DHT) were measured in spl. At certain points of time, aliquots of an ejaculate were centrifuged, washed and the distribution of E1, E1S, E2beta and T into seminal plasma and the sperm fraction was assessed. Hormone assay was by RIA, partly after prior separation by HPLC. Mean concentrations (X(g) x DF) were as follows: E2beta (bpl) 31.1 (1.16), (spl) 24.2 (1.42) pg ml(-1); E1 (bpl) 143.3 (1.21), (spl) 117.7 (1.53) pg ml(-1); E1S (bpl) 157.3 (1.44), (spl) 2.92 (1.42) ng ml(-1); T (bpl) 570.6 (1.43), (spl) 23.1 (1.68) pg ml(-1); A (spl) 17.9 (1.39) pg ml(-1); DHEH (spl) 12.4 (1.51) pg ml(-1); 5alpha-DHT (spl) 9.7 (1.29) pg ml(-1). Except for E2beta and A in seminal plasma, a seasonal pattern was established for all other steroids with lowest mean values occurring from November to April. From the semen parameters determined, only motility was correlated to season. There was a higher correlation among oestrogen in blp than in spl and the only correlation identified between oestrogenic and androgenic steroids was between T and E2beta in blp. In spl, T was correlated with A and 5alpha-DHT. T was the dominant free steroid in bpl while it was E1 in spl; T and E1S concentrations were about 23- and 54-fold lower in spl compared to bpl with E1S, however, showing the highest absolute values in both fluids. In the fractionated ejaculate an association of free oestrogens, particularly E2beta, with spermatozoa was observed.

Effect of progesterone on acrosome reaction, hypoosmotic swelling test, and DNA stability in human spermatozoa

The association between different sperm parameters, an in vitro effect of progesterone, has not been studied satisfactorily. In this article, the effect of progesterone on acrosome reaction (AR), plasma membrane integrity, and chromatin stability has been assessed in human spermatozoa with normal morphology and motility. Semen samples were obtained by masturbation from 25 patients. Two criteria of classification were utilized in this study: high motility group and normal morphology group incubated with progesterone. The effect of progesterone on AR, plasma membrane integrity, and chromatin stability in human spermatozoa with normal morphology and motility was realized. The results suggest that only the subpopulation of spermatozoa with normal morphology is able to undergo the progesterone-induced AR. It is possible that in the reproductive female tract it takes place a high selection of sperm with chromatin stability determined and optimal plasma membrane to undergo the AR prerequisite for the fecundation.

1alpha,25-dihydroxy-vitamin-D3-induced store-operated Ca2+ influx in skeletal muscle cells. Modulation by phospholipase c, protein kinase c, and tyrosine kinases

In skeletal muscle cells the steroid hormone 1alpha, 25-dihydroxy-vitamin-D3 (1,25(OH)2D3) nongenomically promotes Ca2+ release from intracellular stores and cation influx through both L-type and store-operated Ca2+ (SOC) channels. In the present work we evaluated the regulation and kinetics of the 1, 25(OH)2D3-stimulated SOC influx in chick muscle cells. Stimulation with 10(-9) M 1,25(OH)2D3 in Ca2+-free medium resulted in a rapid (40-60 s) but transient [Ca2+]i rise, which correlated with sterol-dependent inositol 1,4,5-trisphosphate production. The SOC influx stimulated by the hormone was insensitive to both L-type channel antagonists and polyphosphoinositide-specific phospholipase C (PPI-PLC) inhibitors but was fully inhibitable by La3+ and Ni2+. PPI-PLC blockade prior to 1,25(OH)2D3 stimulation suppressed both the [Ca2+]i transient and the SOC influx. 1,25(OH)2D3-induced SOC entry was markedly increased after 3 min of treatment (30% above basal) and then rapidly reached a steady-state level. The sterol-stimulated SOC influx was prevented by protein kinase C and tyrosine kinase inhibitors but unaffected by blockade of the protein kinase A pathway. None of these inhibitors altered the thapsigargin-induced SOC entry, suggesting the operation of a signaling mechanism different from that for sterol-dependent SOC influx. The present results indicate that 1,25(OH)2D3-induced activation of PPI-PLC is upstream to Ca2+ influx through SOC channels and point for a role of both protein kinase C and tyrosine kinases but not protein kinase A in the regulation of the sterol-dependent SOCE pathway.

A nongenomic mechanism for progesterone-mediated immunosuppression: inhibition of K+ channels, Ca2+ signaling, and gene expression in T lymphocytes

The mechanism by which progesterone causes localized suppression of the immune response during pregnancy has remained elusive. Using human T lymphocytes and T cell lines, we show that progesterone, at concentrations found in the placenta, rapidly and reversibly blocks voltage-gated and calcium-activated K+ channels (KV and KCa, respectively), resulting in depolarization of the membrane potential. As a result, Ca2+ signaling and nuclear factor of activated T cells (NF-AT)-driven gene expression are inhibited. Progesterone acts distally to the initial steps of T cell receptor (TCR)-mediated signal transduction, since it blocks sustained Ca2+ signals after thapsigargin stimulation, as well as oscillatory Ca2+ signals, but not the Ca2+ transient after TCR stimulation. K+ channel blockade by progesterone is specific; other steroid hormones had little or no effect, although the progesterone antagonist RU 486 also blocked KV and KCa channels. Progesterone effectively blocked a broad spectrum of K+ channels, reducing both Kv1.3 and charybdotoxin-resistant components of KV current and KCa current in T cells, as well as blocking several cloned KV channels expressed in cell lines. Progesterone had little or no effect on a cloned voltage-gated Na+ channel, an inward rectifier K+ channel, or on lymphocyte Ca2+ and Cl- channels. We propose that direct inhibition of K+ channels in T cells by progesterone contributes to progesterone-induced immunosuppression.

Ion-channels reconstituted into lipid bilayer from human sperm plasma membrane

Ion environment and ionic fluxes through membrane are thought to be important in the spermatozoa's maturation, capacitation, and the initiating process of gamete interaction. In this work, the membrane proteins isolated from human sperm plasma membrane were reconstituted into planar lipid bilayers via fusion, and the ion channels activities were observed under voltage clamp mode. In cis 200//trans 100 mM KCl solution, a TEA-sensitive cation-selective channel with a unit conductance of 40 pS was recorded. In a gradient of 200//100 mM NaCl solutions, a Na(+)-selective channel with a unit conductance of 26 pS was recorded. In both cases, reversal potential was about-18 mV, which is close to the predicated value of a perfect Nernst K+ or Na+ electrode. In 50//10 mM CaCl2 solution, a cation channel activity with a unit conductance of 40 pS and reversal potential of about -20 mV was usually observed. In 200//100 mM NMDG(N-methyl-D-glucamine)-Cl solution, where the cation ions were substituted with NMDG, a 30-pS anion-selective channel activity was also detected. The variety in the types of ion channels observed in human spermatozoa plasma membrane suggests that ion channels may play a range of different roles in sperm physiology and gamete interaction.

Isolation and characterization of an estrogen binding protein which may integrate the plethora of estrogenic actions in non-reproductive organs

A putative estrogen receptor (pER) from mouse liver has been characterized. The heterodimer protein (81-84 kDa) consists of two covalently bound subunits (61-67 and 17-27 kDa) with following characteristics: sedimentation constant--4.9 S; IP--4.8; dissociation constant (Kd) for estradiol-17beta binding--0.7 nmol; binding sites--0.746 pmol/mg protein; relative binding affinity--estradiol-17beta--100, estrone--80 and estriol--30; specificity--does not bind, other natural steroids, synthetic estrogens, antiestrogens and bioflavonoids. Importantly, immunosuppressants, neuroleptic and carcinogens influence 3H-estradiol-17beta binding to pER. Interestingly, pER is a serine phosphatase and this may have relevancy to estrogen action in Alzheimer's disease. The polyclonal anti-pER antibody does not react with estrogen receptors (ER). ER antibody does not react with pER. Remarkably, anti-pER antibody reacts with calcineurin, a brain phosphatase and anti-calcineurin antibody reacts with pER. Immunohistochemical analyses showed that pER is undetectable in reproductive organs (except ovary). It is localized on the plasma or the nuclear membranes in some, in cytoplasm and/or nucleus in other cells of non-reproductive organs (skeletal, neural, vascular, hair and retina), and in tumors (mammary, endometrial and prostate cancers, and prostatic hyperplasia). The information presented justifies the proposition that pER may mediate the estrogenic actions in non-reproductive organs.

Increase of intracellular calcium is not a cause of pentoxifylline-induced hyperactivated motility or acrosome reaction in human sperm

OBJECTIVE

To investigate the effects of the phosphodiesterase inhibitor pentoxifylline on hyperactivated motility and acrosome reaction in human sperm and to determine whether its stimulatory effects occur via increased intracellular calcium levels.

DESIGN

Prospective study.

SETTING

Academic tertiary care facility.

PARTICIPANT(S)

Healthy male donors.

INTERVENTION(S)

The effects of pentoxifylline on hyperactivated motility, acrosome reaction, and intracellular calcium were studied and compared with the effects of progesterone. Thapsigargin, a known mobilizer of intracellular calcium, also was used as positive control.

MAIN OUTCOME MEASURE(S)

Hyperactivated motility was assessed by computer-assisted sperm motion analysis using the HTM-IVOS, acrosome reaction was evaluated with the fluorescent probe fluorescein isothiocyanate-labeled Pisum sativum agglutinin, and intracellular calcium was determined by fura-2 using spectrofluorometry.

RESULT(S)

Pentoxifylline significantly increased both hyperactivated motility and acrosome reaction. Enhancement of hyperactivated motility by pentoxifylline in the capacitation medium persisted for up to 5 hours after pentoxifylline was washed from the medium. It also enhanced the percentage of acrosome-reacted spermatozoa after 4 hours of incubation. These effects occurred in the presence of a marginally significant decrease in intracellular calcium.

CONCLUSION(S)

Pentoxifylline stimulates hyperactivated motility and acrosome reaction in spermatozoa from fertile men. Its stimulatory effects occur through mechanism(s) other than increase in intracellular calcium.

Characterization of high affinity progesterone-binding membrane proteins by anti-peptide antiserum

A chemically synthesized 15-mer oligopeptide derived from the N terminus of high affinity progesterone-binding membrane site(s) from porcine liver was used to generate site-specific antibodies. Western blotting experiments confirmed the specificity of the anti-peptide serum obtained. In further investigations this antiserum was used for the identification of the native progesterone-binding membrane protein complex that represents an oligomer with an apparent molecular mass of about 200 kDa. In temperature-induced Triton X-114 phase separation experiments combined with Western-blotting, the progesterone-binding site was identified as an hydrophobic (integral) membrane protein. In addition, in Western blotting analyses the antiserum reacted with the progesterone-binding or related proteins in membrane fractions from a wide array of different tissues in various species.

Effects of gamma-aminobutyric acid, progesterone and ionophore A23187 on acrosome reaction of tree shrew sperm in vitro: examination of acrosome reaction with an improved fluorescence microscopy

A number of acrosome reaction (AR) initiators have been found to be effective in inducing AR of human, laboratory and domestic animal sperm. Using an improved simple fluorescence microscopy, effects of gamma-aminobutyric acid (GABA), progesterone and ionophore A23187 on sperm AR of tree shrew, a useful animal model in biomedical research, have been investigated. Spontaneous AR in 4.92-7.53% of viable sperm was observed. Complete AR in 10.31-18.25% of viable tree shrew sperm was obviously induced by 5 microM and 10 microM calcium ionophore A23187, 1 mM GABA, and 5 microM progesterone, and there were no significant differences between their abilities to initiate complete AR. No significant differences of AR percentages between 1- and 2-h treatments with A23187, progesterone and/or GABA were observed. These results suggested that the responses of tree shrew sperm to these AR initiators are similar to that of human and other mammalian sperm.

1 alpha,25-(OH)2-vitamin D3 stimulates the adenylyl cyclase pathway in muscle cells by a GTP-dependent mechanism which presumably involves phosphorylation of G alpha i

To further understand the mechanism underlying 1,25(OH)2D3 activation of the cAMP pathway, the effect of the hormone on adenylyl cyclase (AC), GTPase and protein kinase A (PKA) activities as well as on the phosphorylation of G alpha i was studied in membranes from chick skeletal muscle cells. The sterol stimulated AC activity in a dose (0.1-10 nM) and time (1-5 min.) dependent fashion, provided GTP (10 microM) was present in the assay. High affinity GTPase activity was unaffected by the hormone. In the absence of GTP or in the presence of Mn2+ (20 mM), 1,25(OH)2D3 effects on AC were abolished. PKA activity was increased (+120%) in cells pretreated (1 nM, 5 min.) with the sterol. Moreover, immunoprecipitation of G alpha i from [32P]-labeled myoblast membranes showed that 5 min. exposure to 1 nM 1,25(OH)2D3 increased (1.5-2 fold) the phosphorylation of its alpha subunit. The present data suggest that in muscle cells, 1,25(OH)2D3 activates AC by a non direct, GTP-dependent action which could imply amelioration of Gi function by sterol-induced alpha i phosphorylation.

RU 3117 a steroidal compound with high affinity for sigma sites in rat testis membranes

RU 3117 belongs to a new series of steroids which exhibited a high relative binding affinity (RBA) for (+)[3H]PPP sites in rat testis membranes; its RBA was about 40 times higher than that of progesterone. Furthermore, it is devoid of any binding to classical steroid receptors; therefore in order to study its binding parameters on rat testis membranes it was tritiated. [3H]RU 3117 bound at least two distinct sites with Ka values of 0.4 +/- 0.06 x 10(9) M(-1) and 1.3 +/- 0.2 x 10(7) M(-1). Using this marker, competition studies with cold haloperidol showed that a part of this binding was haloperidol-sensitive, whereas another part was haloperidol-resistant. Interestingly, progesterone described as a sigma ligand competes with [3H]RU 3117 binding, with an RBA of 1.6%. When haloperidol was preincubated (250 nM) with rat testis membranes, in order to mask the sigma sites, we observed that DTG (1,3-di-O-tolylguanidine) and haloperidol displayed a very low RBA (< 0.1%) and were not able totally to displace the [3H]RU 3117 binding up to 50 microM. Furthermore, benztropine exhibited a significant RBA of 19% but its displacement curve showed a plateau (500-50,000 nM). These results showed that part of the haloperidol-resistant sites was benztropine sensitive but another part was displaced neither by haloperidol nor by benztropine. The presence of these remaining binding sites was confirmed by preincubating a mixture of haloperidol and benztropine with testis membranes. Under these conditions, [3H]RU 3117 displayed a Ka of 1.0 +/- 0.01 x 10(7) M(-1), and we observed that these sites were recognized, up to now, only by the steroids RU 1968 and RU 54173 which are also devoid of any binding to classical nuclear steroid receptors.

Purification and partial sequencing of high-affinity progesterone-binding site(s) from porcine liver membranes

High-affinity progesterone-binding sites have been identified, characterized in and purified from porcine liver membranes. They were functionally solubilized by the non-denaturing zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid (Chaps, 20 mM. detergent/protein mass ratio 4:1) at a yield of 75-80%. Using [3H]progesterone as radioligand, binding studies showed high-affinity and low-affinity binding sites in microsomal preparations with an apparent Kd2 of 11 nM and an apparent Kd2 of 286 nM. In solubilized fractions the high-affinity binding sites were present at an apparent Kd2 of 69 nM. In both preparations, progesterone binding was time-dependent, saturable, reversible, and showed a similar hierachy of affinities for related steroids. A purification scheme was developed based on anion-exchanger procedures. The purified fraction as identified by maximum specific progester-one-binding activity contained two major polypeptides of apparent molecular masses (SDS/PAGE) of 28 kDa and 56 kDa, respectively. Sequencing of both polypeptides showed an identical amino terminus without significant identity in the amino acid sequence to any known protein primary structure.

Progesterone triggers rapid transmembrane calcium influx and/or calcium mobilization from endoplasmic reticulum, via a pertussis-insensitive G-protein in granulosa cells in relation to luteinization process

We investigated the early effects (5-60 s) of progesterone (1 pM-0.1 microM) on cytosolic free calcium concentration ([Ca2+]i) and inositol 1,4,5-trisphosphate (InsP3) formation in nonluteinized and in vitro luteinized porcine granulosa cells (pGCs). Progesterone increased [Ca2+]i and InsP3 formation within 5 s in both cell types. Progesterone induced calcium mobilization from the endoplasmic reticulum via the activation of a phospholipase C linked to a pertussis-insensitive G-protein. This process was controlled by protein kinases C and A. In contrast, only nonluteinized pGCs showed a Ca2+ influx via dihydropyridine-insensitive calcium channel. In both cell types, the nuclear progesterone receptor antagonist RU-38486 did not inhibit the progesterone-induced increase in [Ca2+]i, progesterone immobilized on bovine serum albumin, which did not enter the cell, increased [Ca2+]i within 5 s and was a full agonist, but less potent than the free progesterone; pertussis toxin did not inhibit progesterone effect on InsP3. In conclusion, progesterone may interact with membrane unconventional receptors that belong to the class of membrane receptors coupled to a phospholipase C via a pertussis toxin-insensitive G-protein. The source of the Ca2+ for the progesterone-induced increase in [Ca2+]i also depends on the stage of cell luteinization.

Steroid specificity of the human sperm membrane progesterone receptor

The aim of this study was to evaluate the effect of several abeopregnane, steroidal heterocycles (A/B-transandrostano [2,3-d]isoxazole, and 17-spiroandrostano[2,3-c]furazan), and 6 alpha, 11 beta, 16 alpha-trisubstituted 19-norpregnadienedione on the influx of extracellular Ca2+ in human sperm. These steroidal compounds had minimal genomic progestational, androgenic, or estrogenic activity with the exception of 16 alpha-ethyl-6 alpha-methyl-11 beta-(4-N,N-dimethylaminophenyl)-19- norpregna-4,9-diene-3,20-dione which was four times more progestational than progesterone. Some of the steroidal compounds, e.g., 16 alpha-ethyl-6 alpha-methyl-11 beta-(4-N,N-dimethylaminophenyl)-19-nor- pregna-4,9-diene-3,20-dione and 2',3',4',5'-tetrahydrospiro[furan-2' beta, 17-androstano] [2,3-c]furazan produced an influx of Ca2+ into human spermatozoa. These studies indicate that high (10 microM) concentrations of certain steroidal compounds are selective for the sperm membrane progesterone receptor, since most of them have minimal genomic activity. The steroidal compounds that elicited an influx of Ca2+ caused an initial high influx but were not as potent as progesterone, since no effects were observed below 1 microM, whereas progesterone at 1 microM produced a maximum effect. Progesterone as well as the steroidal compounds caused a modest increase in the number of acrosome-reacted spermatozoa. Molecular modeling revealed that 5 alpha-dihydro-2,3-fused and 4,4-dimethyl-5-ene-2,3-fused steroidal heterocycles possessing different conformations compared to that of progesterone are responsible for elevation of Ca2+. In conclusion, a unique non-genomic progesterone receptor is present on human spermatozoa and several steroidal compounds that do not have progestational effects may activate this sperm membrane receptor, resulting in Ca2+ influx.

Effects of progesterone on human spermatozoa prepared for in-vitro fertilization

Progesterone has been tested in vitro with human spermatozoa to verify its physiological effects and its possible therapeutic use in cases of male infertility. Progesterone induced a rapid, dose-dependent influx of calcium in capacitated and non-capacitated spermatozoa with a half-maximally effective dose of 30 nM. The agonist, 19-nortestosterone, was much less potent that progesterone itself. Progesterone-induced calcium influx was not inhibited by a similar concentration (0.1 microgram/ml) of RU 486, a classical progesterone antagonist. The increase in intracytoplasmic calcium levels was unable to induce the acrosome reaction (AR) even after incubation for 5 h, when this was evaluated by double staining, using a monoclonal antibody GB24 raised against the inner acrosome membrane and ethidium homodimer as a vital probe. However, after incubation for 5 h, progesterone was able to enhance the tyrosine phosphorylation of a 95 kD sperm protein, which is phosphorylated progressively during capacitation in well-defined culture media. Incubation of spermatozoa with 1 or 10 micrograms/ml progesterone for 3 or 30 min did not induce major modifications of hyperactivated movement when analysed by computer-assisted semen analysis. Progesterone secreted by cumulus cells may physiologically increase sperm intracytoplasmic free calcium during capacitation. This priming effect may facilitate the acrosome reaction, induced by binding to the zona pellucida, without enhancing spontaneous acrosome reaction prematurely. It therefore seems useful to propose progesterone as a means of accelerating capacitation during in vitro fertilization in cases of male infertility.

Actions of progesterone on human sperm: a model of non-genomic effects of steroids

Non-genomic actions of steroids have been extensively studied in the last few years. Among these actions, the non-genomic effect of progesterone (P) on human spermatozoa appears to be very promising, in view of the dramatic effect of this steroid on intracellular calcium, activation of tyrosine kinase, and induction of acrosome reaction. We have shown that the ability of spermatozoa to respond to P increases during the process of capacitation and is not counteracted by the P-receptor antagonist RU486 nor by the GABAA antagonists bicuculline and picrotoxin. We have also shown that P increases tyrosine phosphorylation of a sperm protein of about 97 kDa, suggesting activation of tyrosine kinase(s). In addition, we found that P induces a perturbation of sperm membrane phospholipid metabolism resulting in an increase of synthesis of platelet-activating factor and liberation of arachidonic acid. Results of these biochemical studies indicate that P is able to stimulate several signal transduction pathways in human sperm. We have also investigated responsiveness to P in sperm of oligozoospermic subjects as well as of men undergoing an in vitro fertilization (IVF) program. Our results show that the percentage increases of intracellular calcium and acrosome reaction in response to P is significantly reduced in oligozoospermic men as well as in subjects with reduced fertilization rate. Moreover, in the latter subjects response to P is highly significant correlated to fertilization rate of oocytes. These studies indicate that a biochemical alteration of sperm in their capacity to respond to P might be responsible for reduced fertilizing ability.