Luteinizing hormone-releasing hormone enhances polyphosphoinositide breakdown in rat granulosa cells

Luteinizing hormone (LH) stimulates both intracellular calcium ion ([Ca2+]i) mobilization and transmembrane cation influx in single ovarian (granulosa) cells: recruitment as a cellular mechanism of LH-[Ca2+]i dose response

The gonadotropic hormones, LH and FSH, activate adenylyl cyclase in their respective target cells and thereby initiate many biochemical responses. In addition to stimulating cAMP production, both LH and FSH promote agonist-specific increases in the cytoplasmic concentration of free calcium ions ([Ca2+]i) in gonadal cells. Here, we have applied single cell fluorescence video microscopy with the Ca2+-sensitive dye fura-2 to investigate the mechanism(s) by which LH induces a rise in the [Ca2+]i in individual (swine) granulosa cells collected from single Graafian follicles. Stimulation with LH induced a rapid onset, biphasic, spike- and plateau-like [Ca2+]i signal in responsive granulosa cells. The cellular mechanisms mediating this biphasic LH-stimulated increase in [Ca2+]i were examined by external Ca2+ removal and via the manganese (Mn2+) quench technique, which showed that LH triggers initial intracellular Ca2+ mobilization followed by delayed transmembrane Ca2+ influx. Single cell Ca2+ assessment of the LH dose-response mechanism(s) revealed that higher concentrations of LH progressively recruit a larger number of responding individual granulosa cells. Further analyses disclosed a marked [Ca2+]i response heterogeneity among individual granulosa cells harvested from the same Graafian follicle. In addition, the percentage of cells responding to LH [but not to an alternative putative agonist of the phospholipase C (PLC) pathway, viz. endothelin-1] with a biphasic [Ca2+]i rise increased with maturational development of the follicle. Pretreatment of granulosa cells with a specific PLC inhibitor, U-73122 (but not with its inactive congener U-73343), significantly reduced the percentage of cells responding to a LH challenge from 78% to 25% (P < 0.0001) and prolonged the time required to achieve a half-maximal value of the [Ca2+]i transient, viz. from 22 +/- 1.5 sec (n = 27 cells) to 39 +/- 7.2 sec (n = 12 cells; P = 0.002). In cell population studies, LH stimulated in a concentration- and time-dependent manner the accumulation of inositol phosphate in porcine granulosa cells. In summary, the present single cell investigations in mature granulosa cells demonstrate that LH drives initial intracellular Ca2+ mobilization followed by transmembrane divalent cation influx. The PLC inhibitor U-73122 antagonizes this action of LH. By analyzing [Ca2+]i responses in individual living granulosa cells, we further show that, despite within-follicle diversity, the LH dose biphasic [Ca2+]i response arises via the recruitment of a larger number of responding gonadal cells rather than by increased [Ca2+]i signal amplitude. Finally, the percentage of individual LH (but not endothelin-1)-responding granulosa cells increases with follicular maturation. Collectively, these data highlight the potential importance of the LH-stimulatable, PLC-transduced [Ca2+]i signaling mechanism in the later stages of granulosa cell differentiation.

Parvalbumin expression during developmental differentiation of the rat ovary

Parvalbumin (PV) is a high-affinity Ca(2+)-binding protein which is expressed in a limited number of vertebrates tissues and restricted to a few distinct cell types. It has been shown by biochemical methods to be present in the adult rat ovary, but cellular localizations or developmental appearance have not been described until now. This study describes the presence of PV and its transcripts, analyzed by immunohistochemistry and in situ hybridization, respectively, during the postnatal development of the rat ovary: 13 developmental stages between day 1 and day 83 were examined. In ovaries 1-16 days old, neither PV mRNA nor PV was observed in any caaaaaaaaaaaaaay. By contrast, starting from day 18 postpartum, both PV mRNA and PV were detected in low amounts, simultaneously with the onset of differentiation of secondary intestitial gland cells in the ovarian interfollicular stroma. PV and its transcripts were primarily detected in conspicuous patches of interstitial gland tissue and in the differentiated thecal cells around the large follicles, and PV appeared to be fully expressed 33 days after birth. The parallel time courses of PV mRNA and PV accumulation during developmental differentiation, and the appearance of a steroid-producing cellular phenotype as well as the strict cellular colocalization of these two features, strongly suggest involvement of PV in the steroid metabolism of these cells, as earlier proposed for the Leydig cells of the testis. According to this hypothesis, we also show that suppression of gonadotrophic hormone production by hypophysectomy of adult rats totally suppresses PV production in parallel with the disappearance of the morphological features typical of steroid-producing cells in the remaining interstitial tissue of the ovary.

Arachidonic acid and cell signalling in the ovary and placenta

Arachidonic acid (AA) and its metabolites make up a diverse group of signalling molecules important to mediation of metabolic and endocrine function of ovarian and placental cell membranes. This paper reviews recent literature examining AA and eicosanoid involvement in the functional dynamics of follicular development, ovulation and corpus luteum function. The putative roles of AA metabolites in establishment and maintenance of pregnancy are reviewed with reference to decidualization, trophoblast invasion and implantation, maintenance of perfusion of the feto-placental unit and lipid transfer. Finally, recent evidence implicating AA metabolism in mediation of enzyme activity following hormone-receptor coupling within various cells types comprising the placental membranes is reviewed.

Calretinin in rat ovary: an in situ hybridization and immunohistochemical study

Calretinin is a cytosolic calcium-binding protein of the calmodulin superfamily, with high homology with calbindin D28k. The only cells in which calretinin has been described so far are neurons, in the central nervous system and in retina. In the present work, we describe the expression of the calretinin gene in the interstitial cells of rat ovary. Immunohistochemistry, using a calretinin-specific antibody, allowed to detect the protein from 19 days after birth. Western blot from ovary homogenates confirmed the labelling of a 29 kDa band, the size of calretinin. In situ hybridization confirmed immunochemical data; calretinin transcripts were clearly shown in the same cell population. This represents the first description of calretinin outside the nervous system. Its function in ovary remains to be determined.

Diacylglycerol rather than Ca2+ mediates GnRH inhibition of FSH induced steroidogenesis in ovarian granulosa cells

Treatment of cultured granulosa cells with PLC or GnRH stimulated the rapid generation of DAG and phosphoinositide turnover. The PKC activators PLC (3 mU/ml) and TPA (10(-7)M) or the decapeptide GnRH (10(-6)M) elicited similar inhibitory responses on FSH or cAMP stimulated granulosa cell steroidogenesis. Mobilization of intracellular Ca2+ with A23187 (10(-8)M) was followed by a slight increase in the steroidogenic activity of cultured granulosa cells, whereas elevation of extracellular K+ (50 mM) largely augmented the steroid biosynthetic activity of the granulosa cells. These results suggest that the inhibitory effect of GnRH on granulosa cell steroidogenesis is mediated by generation of DAG, rather than by increases in intracellular Ca2+ concentrations.

Intracellular calcium and the signaling mechanism of luteinizing hormone-releasing hormone in rat granulosa cells

OBJECTIVE

The purpose of these studies was to determine the source(s) of the increase in intracellular free calcium in response to luteinizing hormone-releasing hormone in ovarian granulosa cells.

STUDY DESIGN

Rat granulosa cells were cultured and loaded with fura-2-acetoxy-methyl ester, a fluorescent calcium indicator dye, and intracellular free calcium measured by microspectrofluorometry. The source of the luteinizing hormone-releasing hormone induced increase in intracellular Ca++ was investigated with various calcium channel blockers (verapamil, diltiazem, and nifedipine), high K+ buffer, and perifusion with media lacking Ca++.

RESULTS

All three voltage-sensitive calcium channel blockers (10(-5) mol/L) tested were ineffective in blocking the luteinizing hormone-releasing hormone induced intracellular Ca++ increase. Treatment with high K+ buffer also had no effect. Perifusion with media lacking calcium resulted in a gradual loss of the luteinizing hormone-releasing hormone response, an effect that was accelerated by repeated stimulation with hormone. Transient replacement of extracellular Ca++ failed to restore the response but continued perifusion with Ca(++)-replete media allowed a luteinizing hormone-releasing hormone response 10 minutes later.

CONCLUSIONS

The luteinizing hormone-releasing hormone induced intracellular Ca++ increase does not appear to result from the opening of voltage-sensitive or K(+)-dependent Ca++ channels. To the contrary, this response likely results from the release of Ca++, primarily from intracellular stores.

Intragonadal signalling mechanisms in the control of steroid hormone production

In the gonads, there are two recognized signal transduction mechanisms which operate in the processing of hormonal stimuli. The gonadotropins, follicle stimulating hormone and luteinizing hormone, act primarily through the generation of cyclic AMP. Several other hormonal regulators in the ovary and the testis, such as gonadotropin releasing hormone and prostaglandin F2 alpha stimulate inositol lipid metabolism following receptor binding. This triggers a cascading mechanism which ultimately results in the generation of increased cytosolic free calcium levels, enhanced protein kinase C activity, and liberation of arachidonic acid. There is also evidence that luteinizing hormone shares in the activation of this pathway. In this review, the significance of these signal transduction pathways is discussed in relation to the effects of various hormones on steroid biosynthesis in the gonads.

Prostaglandin F2 alpha and gonadotropin-releasing hormone increase intracellular free calcium in rat granulosa cells

Changes in cytosolic free calcium concentration ([Ca2+]i) in response to prostaglandin F2 alpha (PGF2 alpha) and gonadotropin-releasing hormone (GnRH) were measured in single rat granulosa cells, using the calcium-sensitive fluorescent dye, fura-2AM. In 90 out of 135 granulosa cells (67%), there was a 3- to 4-fold increase in resting [Ca2+]i within 30 s of administration of PGF2 alpha (10(-6) M). The resting [Ca2+]i returned to pre-stimulation levels in approximately 80 s. Granulosa cells were responsive to PGF2 alpha at concentrations ranging from 10(-7) M to 10(-4) M (n = 7). Within this range of concentrations, the magnitude of the calcium response did not differ. In another series of experiments, the majority (93%, n = 57) of the granulosa cells which responded to PGF2 alpha also responded to GnRH. Neither the magnitude of the [Ca2+]i response nor the time to response differed between PGF2 alpha and GnRH. Furthermore, simultaneous treatment of granulosa cells with both hormones did not generate a larger response than with either hormone alone. During perifusion with low calcium media, the characteristic [Ca2+]i response to PGF2 alpha decreased, and was eliminated within 10 min (n = 9). Similar observations were made in response to GnRH under these conditions. These data confirm that PGF2 alpha and GnRH stimulate a transient increase in [Ca2+]i in rat granulosa cells, the source of which may be shared intracellular stores.

Calcium-independent phospholipid/diolein-dependent phosphorylation of a soluble ovarian Mr 80,000 substrate protein: biochemical characteristics

Soluble ovarian extracts were incubated with protein kinase effectors in the presence of [gamma 32P]ATP and proteins were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Autoradiograms revealed phosphorylation of an ovarian Mr = 80,000 substrate in the presence of EGTA ([ethylenebis(oxyethylenenitrilo)]tetraacetic acid), phosphatidylserine and 1,2-diolein. In contrast to a classical response pattern to C-kinase effectors, the ovarian Mr = 80,000 phosphorylation was inhibited by 2 x 10(-7) M or greater free Ca2+. The ovarian Mr = 80,000 substrate was distinguished from the myristoylated acidic Mr = 80,000 C-kinase substrate of brain tissue on the basis of heat stability and phosphorylative response to effectors. Phosphorylation of the exogenous substrate myelin basic protein by DEAE-resolved ovarian kinase showed the variant effector dependence, maximal in the presence of EGTA, phosphatidylserine and 1,2-diolein. Finally, the effect of Ca2+ on ovarian Mr = 80,000 [32P]phosphate content could not be accounted for by post-phosphorylation activities, or by DEAE-resolvable or hydroxylapatite-resolvable inhibitory activities.

Tumor necrosis factor-alpha inhibits follicle-stimulating hormone-induced differentiation in cultured rat granulosa cells

We have investigated the effects of TNF-alpha on FSH-induced LH receptor expression, cAMP and progesterone production in cultured rat granulosa cells. TNF-alpha (0.5-100 ng/ml) inhibits the stimulating action of FSH on LH receptor formation in a dose-dependent manner with an IC50 of 1 ng/ml and an almost complete suppression of LH receptor induction for 50-100 ng/ml TNF-alpha. The inhibitory effect of TNF-alpha is not due to variations in cell number or viability but rather to a reduction of the LH receptor content per cell with no change in binding affinity (KD = 0.8 x 10(-10)M). TNF-alpha also inhibits the FSH-induced cAMP production but at a lower extent, with a maximum reduction of 60% for 100 ng/ml TNF-alpha. Moreover, TNF-alpha impairs the LH receptor formation induced by forskolin, cholera toxin or 8-Bromo-cAMP, indicating that the cytokine also acts at a step distal to FSH receptor and to cAMP formation. Finally, TNF-alpha decreases dramatically the progesterone synthesis that is stimulated by FSH, with a reduction to undetectable levels on and after 10 ng/ml TNF-alpha. These results suggest that TNF-alpha may drastically reduce the capacity of granulosa cells to differentiate upon FSH stimulation and to respond to LH during the physiological ovarian follicular maturation. Such anti-gonadotropic action of TNF-alpha on granulosa cell differentiation may be also relevant to the alteration of ovarian function during physiopathological processes like inflammatory or infection diseases.

Differential role of protein kinase C in the action of luteinizing hormone-releasing hormone on hormone production in rat ovarian cells

This study was undertaken to determine the involvement of arachidonic acid and protein kinase C in the actions of luteinizing hormone-releasing hormone on steroid and prostaglandin formation in the ovary. In primary culture of rat granulosa cells, treatment with 3 x 10(-7) mol/L melittin stimulates progesterone and prostaglandin E2 accumulation after a 5-hour culture period. Concomitant treatment of the cells with melittin and luteinizing hormone-releasing hormone or 12-0-tetradecanoylphorbol 13-acetate further enhances the stimulatory action of either luteinizing hormone-releasing hormone or 12-0-tetradecanoylphorbol 13-acetate by itself on prostaglandin E2 production. In contrast, no synergistic effects are observed on progesterone production by the same treatments. Treatment with luteinizing hormone-releasing hormone for 24 hours significantly decreases follicle-stimulating hormone-induced progesterone production by approximately 50%. Treatment of the cells with either follicle-stimulating hormone or luteinizing hormone-releasing hormone stimulates prostaglandin E2 production at least tenfold in the same cultures. When follicle-stimulating hormone and luteinizing hormone-releasing hormone are present concomitantly, they synergistically enhance prostaglandin E2 formation (p less than 0.01). Similar effects are observed with the phorbol ester, 12-0-tetradecanoylphorbol 13-acetate, which causes a dose-dependent inhibition of progesterone production by follicle-stimulating hormone whereas follicle-stimulating hormone-stimulated prostaglandin E2 formation is enhanced. Thus luteinizing hormone-releasing hormone-induced activation of protein kinase C may play multiple roles (stimulatory or inhibitory) in hormone production in the ovary.

Inhibition of follicle-stimulating hormone- and adenosine-3',5'-cyclic monophosphate-induced progesterone production by calcium and protein kinase C in the rat ovary

In this study we examined the effects of A23187 (a calcium ionophore) and 12-O-tetradecanoyl phorbol-13-acetate, a known activator of protein kinase C, on progesterone production. Granulosa cells obtained from pregnant mare serum gonadotropin-primed rats were maintained in primary culture. Treatment with follicle-stimulating hormone (0.5 microgram/ml), 8-bromo-adenosine-3',5'-cyclic monophosphate (2 mmol/L), or cholera toxin (0.1 microgram/ml) for 5 hours or 24 hours markedly stimulated progesterone production. The concomitant presence of A23187 attenuated the elevated levels of progesterone induced by follicle-stimulating hormone, 8-bromo-adenosine-3',5'-cyclic monophosphate, or cholera toxin, with or without the presence of a phosphodiesterase inhibitor, 1-methyl-3-isobutylxanthine (0.2 mmol/L). Likewise, treatment of the cells with 12-O-tetradecanoyl phorbol-13-acetate suppressed follicle-stimulating hormone-induced progesterone production, whether or not 1-methyl-3-isobutylxanthine was present in the cultures. The effect of 12-O-tetradecanoyl phorbol-13-acetate was not mimicked by phorbol-13-monoacetate or 4 alpha-phorbol-12, 13-didecanoate. These results indicate that both A23187 and 12-O-tetradecanoyl phorbol-13-acetate inhibit follicle-stimulating hormone-induced progesterone production, in part at a step or steps beyond adenosine-3',5'-cyclic monophosphate generation and degradation. They further support a role of calcium and protein kinase C in the intraovarian action of luteinizing hormone-releasing hormone.

Role of protein kinase C in luteinizing hormone-releasing hormone (LHRH)-stimulated progesterone production in rat granulosa cells

Treatment of rat granulosa cells with LHRH or the phorbol ester TPA stimulated progesterone (P) production during a 5-h incubation. The concomitant presence of a potent inhibitor of protein kinase C attenuated the LHRH effect by ca. 60% and completely blocked the stimulatory effect of TPA. Addition of TPA (10(-9), 10(-8) or 10(-7) M) or dioctanylglycerol (100 micrograms/ml) increased P production; these stimulatory effects were not potentiated by the concomitant presence of a calcium ionophore (A23187, 10(-7) or 10(-6)M). These data support the hypothesis that protein kinase C activation have a role in the steroidogenic action of LHRH in the ovary.

Luteinizing hormone-releasing hormone (LHRH)-induced arachidonic acid release in rat granulosa cells. Role of calcium and protein kinase C

In rat granulosa cells prelabeled with [3H]arachidonic acid, addition of LHRH, A23187 or 12-0-tetradecanoyl phorbol-13-acetate (TPA) enhanced the release of [3H]arachidonic acid into the culture medium. The effect of A23187 was significant as early as 5 min and the lowest effective dose was 5 X 10(-8)/M. On the other hand, TPA was effective only at dosages greater than 10(-6)M. These results suggest that the stimulatory effect of LHRH on arachidonic acid release is coupled more tightly to a Ca2+-dependent rather than a protein kinase C-mediated pathway.

Calcium-dependent actions of gonadotropin-releasing hormone agonist and luteinizing hormone upon cyclic AMP and progesterone production in rat ovarian granulosa cells

The Ca2+ dependency of the direct stimulatory effect of the gonadotropin-releasing hormone (GnRH) agonist analog [D-Ser(t-Bu)6]des-Gly10-GnRH-N-ethylamide (GnRHa) on progesterone production was investigated and compared to that of luteinizing hormone (LH) in rat granulosa cells from preovulatory follicles. Removal of extracellular Ca2+ by EGTA, or the use of the Ca2+ channel blockers verapamil and La3+, resulted in complete inhibition of GnRHa-induced progesterone production and a partial inhibition of LH-stimulated progesterone production (80, 80 and 50% inhibition respectively for EGTA, verapamil and La3+). Removal of extracellular Ca2+ increased the ED50 for LH-induced cAMP production by four-fold (from 80 to 330 ng/ml) and decreased maximal nucleotide formation by 44%. LH-induced cAMP production was also inhibited partially by verapamil (35%) at 10(-4) M drug concentration. GnRHa had no effect on cAMP production in the presence or absence of Ca2+. GnRHa and LH were found to have maximal effects on progesterone production at about 0.5 mM of Ca2+ in the incubation medium. On the other hand the stimulatory effect of dibutyryl cAMP [Bu)2cAMP) on progesterone production showed little dependency on extracellular Ca2+. The calmodulin antagonist trifluoperazine (TFP) caused concentration-dependent inhibition of the stimulatory action of GnRHa and LH on progesterone production with IC50 values of 3 and 8 microM, respectively. The stimulatory effect of (Bu)2cAMP on progesterone synthesis was attenuated by verapamil and TFP. These results indicate that the direct stimulatory effect of GnRH on ovarian progesterone production is absolutely dependent on Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Luteinizing hormone stimulates the formation of inositol trisphosphate and cyclic AMP in rat granulosa cells. Evidence for phospholipase C generated second messengers in the action of luteinizing hormone

The following studies were conducted to determine whether luteinizing hormone (LH), a hormone which increases cellular levels of cyclic AMP, also provokes increases in 'second messengers' derived from inositol lipid metabolism (i.e. inositol phosphates and diacylglycerol). Rat granulosa cells isolated from mature Graafian follicles were prelabelled for 3 h with myo-[2-3H]inositol. LH provoked rapid (5 min) and sustained (up to 60 min) increases in the levels of inositol mono-, bis, and trisphosphates (IP, IP2 and IP3, respectively). Time course studies revealed that IP3 was formed more rapidly than IP2 and IP following LH treatment. The response to LH was concentration-dependent with maximal increases at LH concentrations of 1 microgram/ml. LiCl (2-40 mM) enhanced the LH-provoked accumulation of all [3H]inositol phosphates, presumably by inhibiting the action of inositol phosphate phosphatases. The effectiveness of LH, however, was dependent on the concentration of lithium employed; maximal increases in IP were observed at 10 mM-LiCl, whereas maximal increases in IP2 and IP3 were observed at 20 mM- and 40 mM-LiCl, respectively. The stimulatory effects of LH on inositol phosphate and progesterone accumulation were also compared with changes in cyclic nucleotide levels. LH rapidly increased levels of inositol phosphates, progesterone and cyclic AMP, but transiently reduced levels of cyclic GMP. These results demonstrate that LH increases both cyclic AMP and inositol trisphosphate (and presumably diacylglycerol) in rat granulosa cells. Our findings suggest that two messenger systems exist to mediate the action of LH in granulosa cells.

Gonadotropin releasing hormone enhances polyphosphoinositide hydrolysis in rat pituitary cells

Addition of gonadotropin releasing hormone to myo-[2-3H]inositol-prelabeled rat pituitary cells in primary culture evoked a dose-dependent increase of the accumulation of [3H]inositol phosphates with a rise of inositol triphosphate within 30 sec of stimulation, followed by a rise in inositol diphosphate and inositol monophosphate. Inositol phosphate accumulation was enhanced up to 5-to-8-fold and was time-dependent between up to 15 min incubation without further increase beyond this time period. Without preincubation with LiCl2, there was no measurable increase of GnRH-induced inositol phosphate accumulation compared to controls. The presence of calcium in the incubation medium did not affect the increase of inositol phosphates. These data give evidence, that polyphosphoinositide breakdown may be an early step in the action of gonadotropin releasing hormone on gonadotropin secretion.