Receptor-mediated gonadotropin action in ovary. Differential effects of various gangliosides and cholera enterotoxin on 125I-choriogonadotropin binding, production of adenosine 3':5'-monophosphate and steroidogenesis in rat ovarian cells

Molecular architecture and biorecognition processes of the cystine knot protein superfamily: part I. The glycoprotein hormones

In this review article, the reader is introduced to recent advances in our knowledge on a subset of the cystine knot superfamily of homo- and hetero-dimeric proteins, from the perspective of the endocrine glycoprotein hormone family of proteins: follitropin (FSH), Iutropin (LH), thyrotropin. (TSH) and chorionic gonadotropin (CG). Subsequent papers will address the structure-function behaviour of other members of this increasingly significant family of proteins, including various members of the transforming growth factor-beta (TGF-beta) family of proteins, the activins, inhibins, bone morphogenic growth factor, platelet derived growth factor-beta, nerve growth factor and more than 35 other proteins with similar topological features. In the present review article, specific emphasis has been placed on advances with the glycoprotein hormones (GPHs) that have facilitated greater insight into their physiological functions, molecular structures and most importantly the basis of the molecular recognition events that lead to the formation of hetero-dimeric structures as well as their specific and selective recognition by their corresponding receptors and antibodies. Thus, this review article focuses on the structural motifs involved in receptor recognition and the current techniques available to identify these regions, including the role of immunological methodology, peptide fragment design and synthesis and mutagenesis to delineate their structure-function relationships and molecular recognition behaviour.

Inhibition of luteal steroidogenesis by two LHRH antagonists (Nal-Glu and Nal-Arg antagonists) in the pregnant rat

We have examined the effect of two LHRH antagonists (Nal-Glu and Nal-Arg antagonists) on the basal progesterone (P4), pregnenolone (P5) and 20 alpha-dihydroprogesterone (20 alpha-DHP) production by luteal cells obtained from day 8 pregnant rats. A dose of 0.1 mmol/l of Nal-Glu or Nal-Arg attenuated basal P4 production by luteal cells after 12, 24 or 48 h of incubation. P5, a precursor for P4 synthesis was also reduced by both doses of Nal-Glu or Nal-Arg (0.1 mmol or 0.1 mumol/l) after 24 h of incubation. A period of 12 h was not sufficient to inhibit P5 production by luteal cells incubated with both doses of Nal-Glu or with the lower dose of Nal-Arg. The higher dose of Nal-Glu and Nal-Arg remained effective in attenuating P5 production by luteal cells after 48 h of incubation. We measured the production of a metabolite of P4, i.e., 20 alpha-DHP to assess whether this suppression in P4 production is due to an enhancement in the P4 metabolism by increasing the activity of 20 alpha-hydroxydehydrogenase. However, instead, we observed (i) a decrease in the production of 20 alpha-DHP by the higher dose of Nal-Glu and Nal-Arg after 12, 24 or 48 h of incubation and (ii) a decrease or no change in the production of 20 alpha-DHP by the lower dose of Nal-Glu or Nal-Arg at all time periods of incubation. Based on these observations we conclude that LHRH antagonists exert a direct effect on the corpus luteum and suppress luteal steroidogenesis. This suppression in luteal steroidogenesis could be due to a decrease in the activity of any one of these enzymes of the steroidogenic pathway, viz., cholesterol side chain cleavage (P450scc), a rate limiting enzyme involved in the synthesis of P5 from cholesterol, or 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD), which catalyzes the oxidation of P5 to P4 or due to a decrease in activity of both enzymes.

Granulosa cell luteinizing hormone receptor expression is modulated by ganglioside-specific ligands

The ganglioside GM1 (Gal beta 1-->3GalNAc beta 1-->4[NeuAc alpha 2-->3] Gal beta 1-->4Glc beta 1-->1Cer) was synthesized during granulosa cell development in vitro, and the effect of the interaction between cell-surface GM1 and its ligands on the luteinizing hormone (LH) receptor expression was investigated. GM1 synthesis, demonstrated by metabolic labeling of glycosphingolipids with [3H]galactose and binding studies using the 125I-B-subunit of cholera toxin, a specific ligand for GM1, was increased in follicle-stimulating hormone (FSH)-treated granulosa cells. When granulosa cells were cultured for 72 h in a medium containing the B-subunit of cholera toxin, FSH-induced LH-receptor contents determined by measuring the binding of 125I-deglycosylated human chorionic gonadotropin to intact cells, was augmented. The stimulatory effect of the B-subunit was dependent on the FSH concentration and culture duration. The augmentation was observed after culture for 48 h, and marked increases were evident after 72 h, which coincided with an increase of the 125I-B-subunit binding capacity. Scatchard analysis of the LH-receptor binding indicated that treatment with the B-subunit increased the number of LH-binding sites (6580 sites/cell after treatment with 20 ng/ml FSH; 11,290 sites/cell after FSH plus 100 ng/ml B-subunit), but did not alter the binding affinity. A specific antibody against GM1 mimicked the stimulatory effect of the B-subunit. The augmentation was not accompanied by granulosa cell proliferation. These findings suggest that binding of exogenous or possible endogenous ligands to cell-surface GM1 produces signals and modulates the cellular behavior during granulosa cell development.

Establishment of heterogeneity among blood vessels: hormone-influenced appearance of hepatic lipase in specific subsets of the ovarian microvasculature

We used biochemical and structural approaches to analyze the influence of gonadotropic hormones on the association of hepatic lipase with specific subsets of ovarian blood vessels. Western blotting was used to detect this enzyme in effluent collected from heparin-perfused ovaries of nonhormone-treated immature rats and those primed with pregnant mare's serum gonadotropin (PMSG) alone or in combination with human chorionic gonadotropin (hCG). The effects of these hormones on hepatic lipase distribution among ovarian blood vessels was assessed before and after hCG and/or PMSG treatment by immunofluorescence and immunogold cytochemistry. For the latter, immunoreagents and fixative were delivered directly to chilled, unfixed ovaries by in situ vascular perfusion. Data from biochemical and structural analyses indicated that hepatic lipase was absent from nonhormone-treated ovaries. As shown by Western blotting of ovarian effluent, the enzyme appeared following treatment with PMSG and PMSG-hCG; it increased in amount in a time-dependent manner, with a transient decline in the early hours after hCG injection. Enzyme levels paralleled growth and vascularization of follicles and corpora lutea; the fall tended to coincide with early events in luteal angiogenesis. Immunogold microscopy showed that hepatic lipase was abundant in thin-walled blood vessels of theca interna of follicles, corpora lutea, and interstitial cells but sparse in those of the stroma. Moreover, during neovascularization of differentiating corpora lutea, vascular sprouts arising from hepatic lipase-laden thecal vessels appeared to lose, then regain, the enzyme as development progressed. Our findings thus suggest 1) that hormones influence the establishment of endothelial cell heterogeneity within the microvasculature of a single organ and 2) that development of novel endothelial cell properties in specific subsets of blood vessels underlies compartmentalization of function within a tissue.

Enhancement of ganglioside GM3 synthesis in okadaic-acid-treated granulosa cells

Okadaic acid is a potent inhibitor of type-2A (PP2A) and type-1 (PP1) protein phosphatases and has been proved to be a valuable tool for studies on the protein phosphorylation. We have investigated the effects of okadaic acid on rat granulosa cells in order to determine whether the regulation of ganglioside synthesis involves protein phosphorylation via inhibition of PP2A and PP1. Granulosa cells expressed luteinizing hormone (LH) receptors, measured as the binding of 125I-deglycosylated human chorionic gonadotropin (hCG) to intact cells, and synthesized the gangliosides NeuAc alpha 2-->3Gal beta 1-->4Glc beta 1-->1Cer (GM3) and Gal beta 1-->3GalNAc beta 1-->4[NeuAc alpha 2-->3]Gal beta 1-->4Glc beta 1-->1Cer (GM1), demonstrated by metabolic labeling of glycosphingolipids with [3H]galactose, in response to follicle-stimulating hormone (FSH). When FSH-stimulated granulosa cells were treated with 10 nM okadaic acid for 15 h, down-regulation of LH receptors, dissociation of LH receptor-effector coupling and significant decreases of intracellular and extracellular 3',5'-cyclic adenosine monophosphate (cAMP) levels were observed. The okadaic acid-induced desensitization to gonadotropin in granulosa cells was accompanied by increased ganglioside synthesis. The amount of 3H-labeled ganglioside GM3, the major ganglioside (about 95% of the total) synthesized by mature granulosa cells, was enhanced in okadaic acid-desensitized cells (to 215% of the control value) and in those desensitized by hCG (by 354%), forskolin (by 190%) and 12-O-tetradecanoylphorbol 13-acetate (by 143%). The results of this study suggest that an increase in the phosphorylation state of cells is accompanied by enhancement of ganglioside synthesis.

Biphasic effects of exogenous ganglioside GM3 on follicle-stimulating hormone-dependent expression of luteinizing hormone receptor in cultured granulosa cells

The major ganglioside NeuAc alpha 2-->3Gal beta 1-->4Glc beta 1-->1Cer (GM3) present in cultured rat granulosa cells was examined for potential function in the expression of luteinizing hormone (LH) receptor on the cell surface in response to follicle-stimulating hormone (FSH). Synthesis of GM3 was stimulated concentration-dependently by FSH, and the stimulation was enhanced synergistically by insulin, as revealed by metabolic labeling of glycosphingolipids with [3H]galactose. When granulosa cells were cultured in the media containing GM3 (0.2-20 microM), biphasic changes in FSH-dependent expression of LH receptor were observed, as measured by the binding of 125I-deglycosylated human choriogonadotropin to the intact cells. Exogenous GM3 suppressed expression of LH receptor in the cells treated with a low dose of FSH (20 ng/ml), which was characterized by a low GM3 level, to 30% of control at 10 microM, with a half-maximal inhibitory concentration of 8 microM. In contrast, in the cells treated with a high dose of FSH (100 ng/ml) and insulin, which was characterized by a high GM3 level, expression of LH receptor was enhanced by exogenous GM3, to 148% of control at 10 microM, with a half-maximal effective concentration of 2 microM. Exogenous GM3 produced concomitant changes in the levels of extracellular cAMP. These effects of exogenous GM3 were not accompanied by changes in granulosa cell proliferation. Exogenous GM3 also modulated the LH receptor expression by the synergistic action of 12-O-tetradecanoylphorbol 13-acetate with insulin, with no significant changes in cellular DNA contents, suggesting that exogenous GM3 does not modulate directly the action of FSH at its receptor sites.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of neuraminidase and gangliosides on ovarian LH/hCG receptors during rat development

Ovaries of neonatal rats are not endowed with specific LH/hCG receptors up to 6-8 days of age. Treatment of ovarian membranes of the neonatal rat with neuraminidase results in a specific binding of radioactively labeled hCG, while an increase of hormone binding is observed after neuraminidase treatment of ovarian membranes of the 21-day-old rat. These changes in hormone receptor sites in the ovary are dependent on the neuraminidase concentration used and are due to a receptor with a dissociation constant (KD) of about 10(-9) M. The KD of the receptor in the LH/hCG sensitive ovary without neuraminidase treatment is about 10(-10) M. These results indicate the presence of two different LH/hCG receptors in the ovarian membrane. The unmasking effect of neuraminidase onto LH/hCG receptors indicate that ganglioside-like structures are responsible for the masking of receptors in the neonatal, insensitive rat ovary and also in the 21-day-old sensitive ovary. Ganglioside preparations are able to inhibit the binding, and the fractionation of ovary gangliosides results in a fraction with a rather high inhibition potency of LH/hCG binding to the receptor. It is hypothesized that the masked receptor in the sensitive period represent a store of receptors for the reconstitution of the ovarian cells with active receptors after internalization of the hormone-receptor complex. Thus the masking of the receptors in the early postnatal rat ovary could be a prerequisite for the female differentiation of hypothalamic centers. The observed neuraminidase effect in vitro could reflect a physiologic situation. Neuraminidase was found in the ovary, and during early postnatal development the neuraminidase activity pattern coincides with that of the ovarian LH/hCG receptor changes.

Receptor-mediated gonadotropin action in the ovary. Inhibitory actions of concanavalin A and wheat-germ agglutinin on gonadotropin-stimulated cyclic AMP and progesterone responses in ovarian cells

Pretreatment of ovarian cells with concanavalin A and wheat-germ agglutinin blocked the gonadotropin-induced cyclic AMP and progesterone responses and this effect was time- and concentration-dependent. Basal production of either cyclic AMP or progesterone, however, was not affected by treatment of cells with lectin. The effect of concanavalin A on gonadotropin-mediated cyclic AMP and progesterone responses was blocked by alpha-methyl D-mannoside and alpha-methyl d-glucoside. Similarly the inhibitory effect of wheat-germ agglutinin was reversed by N-acetyl-D-glucosamine. Pretreatment of ovarian cells with concanavalin A or wheat-germ agglutinin had no effect on protein synthesis in the ovary as monitored by [3H]proline incorporation studies. Concanavalin A and wheat-germ agglutinin did not affect steroid production in response to dibutyryl cyclic AMP and 8-bromo cyclic AMP, indicating that the inhibitory action of lectin was occurring at a step before cyclic AMP formation. Lectins specific for L-fucose, D-galactose and N-acetyl-D-galactosamine, gorse seed agglutinin, peanut agglutinin and Dolichos biflorus agglutinin respectively, did not interfere with gonadotropin-induced cyclic AMP and progesterone responses. The present studies suggest that gonadotropin receptors may be glycoprotein in nature or closely associated with glycoprotein structures with the carbohydrate chain containing N-acetyl-D-glucosamine, mannose and possibly N-acetylneuraminic acid.

Receptor-mediated gonadotropin action in the ovary. Action of cytoskeletal element-disrupting agents on gonadotropin-induced steroidogenesis in rat luteal cells

The role of the cellular cytoskeletal system of microtubules and microfilaments on gonadotropin-stimulated progesterone production by isolated rat luteal cells has been investigated. Exposure of luteal cells to human choriogonadotropin resulted in a stimulation of cyclic AMP (4-7-fold) and progesterone (3-4-fold) responses.l Incubation of cells with the microfilament modifier cytochalasin B inhibited the gonadotropin-induced steroidogenesis in a dose- and time-dependent manner. The effect of cytochalasin B on basal production of steroid was less pronounced. Cytochalasin B also inhibited the accumulation of progesterone in response to lutropin, cholera enterotoxin, dibutyryl cyclic AMP and 8-bromo cyclic AMP. The inhibition of steroidogenesis by cytochalasin B was not due to (a) inhibition of 125I-labelled human choriogonadotropin binding to luteal cells, (b) inhibition of gonadotropin-stimulated cyclic AMP formation or (c) a general cytotoxic effect and/or inhibition of protein biosynthesis. Cytochalasin D, like cytochalasin B, inhibited gonadotropin- and 8-bromo cyclic AMP-stimulated steroidogenesis. Although cytochalasin B also blocked the transport of 3-O-methyl-glucose into luteal cells, cytochalasin D was without such an effect. Increasing glucose concentration in the medium, or using pyruvate as an alternative energy source, failed to reverse the inhibitory effect of cytochalasin B. The anti-microtubular agent colchicine failed to modulate synthesis and release of progesterone by luteal cells in response to human choriogonadotropin. These studies suggest that the cellular microfilaments may be involved in the regulation of gonadotropin-induced steroidogenesis. In contrast, microtubules appear to be not directly involved in this process.