Regulation Of Gonadotropin Receptors Research Articles

Differential Regulation of Gonadotropin Receptors ( fshr and lhcgr ) by Estradiol in the Zebrafish Ovary Involves Nuclear Receptors that are Likely Located on the Plasma Membrane

Differential Regulation of Gonadotropin Receptors ( fshr and lhcgr ) by Estradiol in the Zebrafish Ovary Involves Nuclear Receptors that are Likely Located on the Plasma Membrane

Differential expression and regulation of gonadotropins and their receptors in the Japanese eel, Anguilla japonica

Eel species have a striking life cycle with a blockade of puberty until the oceanic migration. We report the first molecular data on eel gonadotropin receptors. The partial sequences cloned covered two-third of the open reading frame and included most of the extracellular and transmembrane domains. Phylogenetic analysis partitioned the two eel gonadotropin receptors into the two teleost FSHR and LHR clusters, respectively. Real-time quantitative RT-PCR was used to quantify the expression of eel gonadotropins and their receptors. Similar levels of pituitary FSH-β and LH-β transcripts were found in the immature previtellogenic female eels. In contrast, ovarian FSHR mRNA level was at 100- to 185-fold higher than that of LHR. This revealed that FSHR rather LHR would mediate gonadotropin stimulation of the early stages of ovarian growth. Chronic treatment with fish pituitary homogenates, applied to induce eel sexual maturation, stimulated pituitary LH-β but suppressed FSH-β transcripts. In the ovaries, both FSHR and LHR mRNA were significantly increased in experimentally matured eels. Treatments with sexual steroids showed a stimulatory effect of estradiol-17β (E 2) on pituitary LH-β mRNA levels, while FSH-β transcripts were suppressed by E 2 or testosterone (T). In contrast, neither E 2 nor T-treatment had any significant effect on ovarian FSHR nor LHR transcripts. This suggests that steroid feedbacks may be responsible for the opposite regulation of pituitary gonadotropins in experimentally matured eels, but are not involved in the regulation of gonadotropin receptors. In conclusion, these are the first data on the sequence, expression and regulation of gonadotropin receptors in the eel. They provide new foundation for basic and applied research on eel reproduction.

Gonadotropins and ovarian gonadotropin receptors during the perimenopausal transition period

As an early sign of perimenopause, the menstrual cycles become irregular. Associated with this irregularity, serum gonadotropin levels increase. There are studies correlating these changes to ovarian follicular morphology and number, but no reports are available, to our knowledge, correlating the increases in gonadotropin levels to respective cellular receptors. In fact, the regulation of gonadotropin receptors is poorly understood in the human. The action of gonadotropins on gonadal function is mediated through specific binding of the hormones to receptors located on the surface of target cells followed by activation of intracellular second messenger systems. In the ovary, the target cells for FSH are granulose cells of ovarian follicles, while the action of LH is mediated through binding of the ligand to theca, granulosa and luteal cells. The cell-surface receptors for gonadotropins are located respectively on these cell types. The molecular nature of FSH receptors and LH receptors has been characterized in many species, and their structure is well-known both on the protein and nucleic acid level. Despite the vast amount of studies in rodents, there is limited information available on the regulation of gonadotropin receptors in the human. In a recent study, serum LH and FSH levels were correlated to respective receptor levels in ovarian tissue in order to better understand gonadotropin action and the physiology of the menopause in the human. High serum gonadotropin levels in perimenopause suggested the existence of low ovarian gonadotropin receptor levels. In most patients who are still experiencing cyclic menstrual bleeding no FSH receptors were detected possibly due to severe disturbances in follicular FSH receptor synthesis prior to ovarian follicle exhaustion.

Regulation of gonadotropin receptors, gonadotropin responsiveness, and cell multiplication by somatomedin-C and insulin in cultured pig Leydig cells.

We have investigated the effects of insulin and somatomedin-C/insulinlike growth factor I(Sm-C) in purified porcine Leydig cells in vitro on gonadotrophins (hCG) receptor number, hCG responsiveness (cAMP and testosterone production), and thymidine incorporation into DNA. Leydig cells cultured in a serum-free medium containing transferrin, vitamin E, and insulin (5 micrograms/ml) maintained fairly constant both hCG receptors and hCG responsiveness. When they were cultured for 3 days in the same medium without insulin, there was a dramatic decline (more than 80%) in both hCG receptor number and hCG responsiveness. However the cAMP but not the testosterone response to forskolin was normal. Both insulin and Sm-C at nanomolar concentrations prevent the decline of both hCG receptors and hCG-induced cAMP production. This effect of both peptides was dose dependent with an ED50 of about 1 ng/ml and 5 ng/ml for SM-C and insulin, respectively. Insulin and Sm-C had no additive effect on these parameters. At nanomolar concentrations, Sm-C and insulin enhanced hCG-induced testosterone production but the effect of Sm-C was significantly higher than that of insulin. However, the effect of insulin at higher concentrations (5 micrograms/ml) was significantly higher than that of Sm-C at 50 ng/ml. In contrast, at nanomolar concentrations only Sm-C stimulated [3H]-thymidine incorporation into DNA and cell multiplication, the stimulatory effect of insulin on these parameters, was seen only at micromolar concentrations. These results indicate that both Sm-C and insulin acting through their own receptors increase Leydig cell steroidogenic responsiveness to hCG by increasing hCG receptor number and improving some step beyond cAMP formation. In contrast, the mitogenic effects of insulin are mediated only through Sm-C receptors.

Regulation of Gonadotropin Receptors on Cultured Porcine Leydig and Sertoli Cells: Effect of Potassium Depletion*

We have examined the role of the NaK-ATPase pump activity on the ligand-induced down-regulation of gonadotropin receptors in cultured porcine Leydig and Sertoli cells. In both cells, inhibition of the NaK pump by ouabain produced a depletion of intracellular K+ levels (ID50, 10(-7) M) after a lag period of about 8 h. In the absence of ligand, the number of FSH receptors in ouabain-treated Sertoli cells was unaffected or slightly reduced, whereas a 2-fold increase in the number of human CG (hCG)/LH receptors with small changes in the binding affinity was observed in Leydig cells treated by ouabain. The effect of ouabain was dose dependent. Differences were also observed in the down-regulation process of gonadotropin receptors in ouabain-treated cells. The hCG-induced receptor loss in Leydig cells was completely reversed by ouabain whereas the drug had no effect on ligand-induced loss of FSH receptors in Sertoli cells. Similar results were observed when the cells were incubated in K+-free medium. Kinetics studies with labeled hCG have shown that ouabain treatment slows down significantly the rate of [125I]iodo-hCG internalization (t 1/2, 18 h; control cells, t 1/2, 6 h), but had no effect on the degradation of internalized hormone. The internalization of receptor-bound [125I]iodo-hCG was also reduced when Leydig cells were incubated in K+-free medium, but was restored when this medium was supplemented with rubidium. The influence of the NaK pump on the receptor regulation of a ligand common to both types of cells, such as epidermal growth factor, was studied under the same experimental conditions. Neither ouabain nor K+-free medium were able to prevent the epidermal growth factor-induced reduction of receptor levels in Leydig and Sertoli cells. Thus, it appears that modulation of ligand-induced receptor loss by depletion of cellular K+ levels is not dependent on the cell type, but on the ligand-receptor complex. The data also show a striking difference in the dynamics of gonadotropin-receptor interaction of two structurally related hormones.

Hormonal regulation of gonadotropin receptors and steroidogenesis in cultured fetal rat tests.

Gonadotropic activation of the adult rat testis in vitro and in vivo is followed by down-regulation of luteinizing hormone receptors and decreased androgen responses to subsequent hormonal stimulation. In contrast, treatment of cultured fetal testes with gonadotropins and dibutyryl adenosine 3',5'-monophosphate enhanced steroidogenic responsiveness and did not cause the luteinizing hormone-receptor loss and desensitization that is characteristic of the adult gonad. The analysis of gonadotropin receptors and action in cultured fetal testis cells facilitates developmental studies of gonadal function, and has revealed significant differences in the responses of fetal and adult Leydig cells to gonadotropic regulation.

Photoperiod and regulation of gonadotropin receptors.

Photoperiod and regulation of gonadotropin receptors.

Regulation of gonadotropin receptors and steroidogenesis in cultured porcine Leydig cells.

Cultures of porcine Leydig cells (>80% pure) were used to study the regulation of LH/hCG receptors and Leydig cell responsiveness over several days. These cells contained approximately 60,000 receptors/cell, with an apparent Kd of 1.5 × 10-9 M. Binding of hCG to receptor was time and dose dependent (saturation at 4–6 h), and maximal binding was maintained for 24 h. After removal of hCG, radioactivity was released into the medium. This spontaneous release (50% released by 8 h) was unaffected by the presence of cold hormone in the medium. About 80% of the released material was inactive in a binding assay and had the properties of monoiodotyrosine. Throughout the period of culture, hCG receptor was maintained at or above the original level. The addition of hCG during the culture caused a dose-and time-dependent down-regulation of receptor. After removal of the hormone, hCG receptors increased, reaching 50% of control within 24 h. Most of the bound radioactive hormone could be released by a brief acid treatment (pH 4 for 4 min). At 15 min of binding, 80% of the hormone could be released, with a decrease to less than 30% released by 2 h of binding. The acid-releasable [125I]iodo-hCG remained constant from 2–8 h of binding even though the total cell-associated radioactivity increased until 6–8 h. After saturation binding, the cells were allowed to release the bound radioactivity. These cells were rechallenged with radioactive hormone and were capable of rebinding hormone, although not to the level originally attained. These data syggested that in a culture system, the hCG receptors in Leydig cells reached a steady state turnover rate where internalization of gonadotropin receptors was balanced by the appearance of new receptors on the cell surface. Cycloheximide caused an increase in hCG binding, suggesting that receptor synthesis was not the limiting or controlling step in the steady state turnover at this early time period.

Regulation of gonadotropin receptors and gonadotropin responses in a clonal strain of Leydig tumor cells by epidermal growth factor.

The MA-10 line is a clonal strain of Leydig tumor cells that has receptors for human choriogonadotropin (hCG) and mouse epidermal growth factor (mEGF). These cells respond to hCG, cholera toxin, and 8-Br-adenosine 3':5'-monophosphate with increased steroid production. It is reported herein that exposure of the MA-10 cells to mEGF results in a substantial (80 to 90%) reduction in the number of hCG receptors per cell. The loss of hCG receptors is accompanied by a corresponding reduction in the ability of hCG to stimulate steroidogenesis. The steroidogenic responses to cholera toxin and 8-Br-adenosine 3':5'-monophosphate, however, are not affected. Other results presented show that mEGF is not a mitogen for these cells.

Regulation of Gonadotropin Receptors by Luteinizing Hormone in Granulosa Cells1

In association with luteinization, LH induces a decrease in the content of receptors for FSH and LH and an increase in that for PRL. To elucidate if the mechanism by which LH regulates its own receptors involved occupancy of sites and/or loss of receptors the effects of a luteinizing dose of LH were examined in the preovulatory follicles of immature hypophysectomized rats primed with estradiol and FSH. The measurable LH receptor content declined by 82% 24 h after LH administration. Serum concentration of the hormone declined by 24 h to 1.4% of the concentration measured 2 h after LH administration. Administration of iodinated LH to demonstrate occupancy of sites in vivo, resulted in a decline in the amount of hormone bound in vivo, over a period of time. This decline in occupancy paralleled the decrease in the number of available sites as measured in vitro. Furthermore, a large dose of highly purified hFSH administered in lieu of LH induced luteinization and an associated loss of gonadotropin receptors. These results indicate that luteinizing doses of LH and FSH induce a loss in gonadotropin receptors by mechanisms other than occupancy.