Identification of a hormonally regulated luteinizing hormone/human chorionic gonadotropin receptor mRNA binding protein. Increased mrna binding during receptor down-regulation

Expression and function of the luteinizing hormone choriogonadotropin receptor in human endometrial stromal cells

The human luteinising hormone choriogonadotropin receptor (LHCGR) is a G-protein coupled receptor activated by both human chorionic gonadotropin (hCG) and luteinizing hormone (LH), two structurally related gonadotropins with essential roles in ovulation and maintenance of the corpus luteum. LHCGR expression predominates in ovarian tissues where it elicits functional responses through cyclic adenosine mononucleotide (cAMP), Ca²⁺ and extracellular signal-regulated kinase (ERK) signalling. LHCGR expression has also been localized to the human endometrium, with purported roles in decidualization and implantation. However, these observations are contentious. In this investigation, transcripts encoding LHCGR were undetectable in bulk RNA sequencing datasets from whole cycling endometrial tissue and cultured human endometrial stromal cells (EnSC). However, analysis of single-cell RNA sequencing data revealed cell-to-cell transcriptional heterogeneity, and we identified a small subpopulation of stromal cells with detectable LHCGR transcripts. In HEK-293 cells expressing recombinant LHCGR, both hCG and LH elicited robust cAMP, Ca²⁺ and ERK signals that were absent in wild-type HEK-293 cells. However, none of these responses were recapitulated in primary EnSC cultures. In addition, proliferation, viability and decidual transformation of EnSC were refractory to both hCG and LH, irrespective of treatment to induce differentiation. Although we challenge the assertion that LHCGR is expressed at a functionally active level in the human endometrium, the discovery of a discrete subpopulation of EnSC that express LHCGR transcripts may plausibly account for the conflicting evidence in the literature.

Mechanism underlying the stimulation by IGF-1 of LHCGR expression in porcine granulosa cells

IGF-1 plays important roles in mammalian fertility by promoting cell growth and increasing steroid hormone secretion. Although IGF-1 significantly upregulated luteinizing hormone/choriogonadotropin receptor (LHCGR) gene expression in granulosa cells in a previous study, the mechanism was unclear. The present experiment was designed to primarily explore the regulation of LHCGR expression by IGF-1. First, based on a porcine LHCGR double-luciferase reporter experiment, c-Fos significantly inhibited the activity of the LHCGR promoter. Second, porcine granulosa cells were cultured in vitro with IGF-1, and we observed that the expression of LHCGR was significantly increased and the expression of c-Fos mRNA significantly reduced. After c-Fos overexpression in granulosa cells, IGF-1 attenuated the inhibitory effect of c-Fos on LHCGR. Furthermore, the level of LHCGR mRNA stimulated by IGF-1 in the presence of SB203580 was markedly lower than that of IGF-1 alone action. In conclusion, IGF-1 enhanced the expression of LHCGR by regulating c-Fos in granulosa cells, which may be mediated by the p38MAPK-signaling pathway.

SREBP Plays a Regulatory Role in LH/hCG Receptor mRNA Expression in Human Granulosa-Lutein Cells

CONTEXT

LH receptor (LHR) expression has been shown to be regulated posttranscriptionally by LHR mRNA binding protein (LRBP) in rodent and human ovaries. LRBP was characterized as mevalonate kinase. The gene that encodes mevalonate kinase is a member of a family of genes that encode enzymes involved in lipid synthesis and are regulated by the transcription factor sterol regulatory element binding proteins (SREBPs).

OBJECTIVE

The current study examined the regulation of LHR mRNA expression in human granulosa-lutein cells in response to alterations in cholesterol metabolism.

DESIGN

Using atorvastatin, an inhibitor of 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase to inhibit cholesterol biosynthesis, we examined its effect on LHR mRNA expression. The effect of atorvastatin on SREBP and mRNA expression as well as LHR mRNA binding protein expression was examined. Finally, the effect of atorvastatin on human chorionic gonadotropin (hCG)-stimulated progesterone production and the expression of key steroidogenic enzymes was also examined.

RESULTS

Statin treatment reduced LHR mRNA expression by increasing the levels of SREBP1a and SREBP2, leading to an increase in LRBP. RNA gel shift assay showed that increased binding of LHR mRNA to LRBP occurred in response to atorvastatin, leading to LHR mRNA degradation. The granulosa-lutein cells pretreated with atorvastatin also showed decreased responsiveness to hCG by decreasing the mRNA and protein expression of steroidogenic enzymes. Atorvastatin also attenuated LH/hCG-induced progesterone production.

CONCLUSION

These results imply that LHR mRNA expression by the human granulosa-lutein cells is regulated by cholesterol, through a mechanism involving SREBP and SREBP cleavage activating protein serving as the cholesterol sensor.

miR-122 Regulates LHR Expression in Rat Granulosa Cells by Targeting Insig1 mRNA

Luteinizing hormone/chorionic gonadotropin receptor (LHR) expression in the ovary is regulated by a messenger RNA (mRNA) binding protein, which specifically binds to the coding region of LHR mRNA. We have shown that miR-122, a short noncoding RNA, mediates LHR mRNA levels by modulating the expression of LHR mRNA-binding protein (LRBP) through the regulation of sterol regulatory element binding protein (SREBP) activation. The present results show that miR-122 regulates LRBP levels by increasing the processing of SREBP through the degradation of Insig1, the anchoring protein of SREBP. We present evidence showing that mRNA and protein levels of Insig1 undergo a time-dependent increase following the treatment of rat granulosa cells with follicle-stimulating hormone (FSH), which leads to a decrease in LRBP levels. Furthermore, overexpression of miR-122 using an adenoviral vector (AdmiR-122) abolished FSH-induced increases in Insig1 mRNA and protein. We further confirmed the role of Insig1 by showing that inhibition of Insig1 using a specific small interfering RNA prior to FSH treatment resulted in the abrogation of LHR upregulation. Silencing of Insig1 also reversed FSH-mediated decreases in SREBP and LRBP activation. These results show that decreased levels of miR-122 increase Insig1 and suppress SREBP processing in response to FSH stimulation of rat granulosa cells.

Regulation of Luteinizing Hormone Receptor mRNA Expression in the Ovary: The Role of miR-122

The expression of luteinizing hormone receptor (LHR) in the mammalian ovary is regulated in response to changes in the secretion of follicle-stimulating hormone and luteinizing hormone by the anterior pituitary, at least in part, through posttranscriptional mechanisms. The steady-state levels of LHR mRNA are maintained by controlling its rate of degradation by an RNA-binding protein designated as LHR mRNA-binding protein (LRBP). LRBP forms a complex with LHR mRNA and targets it for degradation in the p bodies. miR-122, an 18 nucleotide noncoding RNA, regulates the expression of LRBP. Thus, the levels of miR-122 determine the cellular levels of LHR mRNA expression. This phenomenon has been examined during the induction of LHR mRNA expression that occurs during follicle maturation in response to rising levels of FSH. In this situation, miR-122 and LRBP levels decrease as LHR mRNA expression undergoes downregulation in response to preovulatory LH surge. miR-122 expression as well as LRBP levels show robust increases. The mechanism of induction of LRBP by miR-122 has also been discussed.

Impact of RNA structure on ZFP36L2 interaction with luteinizing hormone receptor mRNA

ZFP36L2 (L2) destabilizes AU-rich element (ARE)-containing transcripts and has been implicated in female fertility. We have shown that only one of three putative AREs within the 3' UTR of murine luteinizing hormone receptor mRNA, ARE2197 (UAUUUAU), is capable of interacting with L2. To assess whether structural elements of ARE2197 could explain this unique binding ability, we performed whole-transcript SHAPE-MaP (selective 2' hydroxyl acylation by primer extension-mutational profiling) of the full-length mLHR mRNA. The data revealed that the functional ARE2197 is located in a hairpin loop structure and most nucleotides are highly reactive. In contrast, each of the nonbinding AREs, 2301 and 2444, contains only a pentamer AUUUA; and in ARE2301 much of the ARE sequence is poorly accessible. Because the functional mARE was also found to be conserved in humans at the sequence level (ARE 2223), we decided to investigate whether binding and structure are also preserved. Similar to mouse, only one ARE in hLHR mRNA is capable of binding to L2; and it is also located in a hairpin structure, based on our SHAPE-MaP data. To investigate the role of secondary structure in the binding, we mutated specific nucleotides in both functional AREs. Mutations in the flexible stem region proximal to the loop that enforce strong base-pairing, drastically reduced L2 binding affinity; this confirms that the structural context is critical for L2 recognition of hARE2223. Collectively, our results suggest that a combination of minimal ARE sequence, placement of the ARE in a hairpin loop, and stem flexibility mediate high-affinity L2 binding to hLHR mRNA.

LHCGR Expression During Follicle Stimulating Hormone-Induced Follicle Growth Is Negatively Regulated by Eukaryotic Initiation Factor 5A

We have shown that the transient changes in the expression of luteinizing hormone/choriogonadotropin receptor (LHCGR) messenger RNA (mRNA) during the ovarian cycle occurs, at least in part, through a posttranscriptional mechanism involving an LHCGR mRNA-binding protein (LRBP). Eukaryotic initiation factor 5A (eIF5A), an LRBP-interacting protein, participates in this process. eIF5A undergoes hypusination, a unique posttranslational modification that is necessary for its functions. This study examined the role of eIF5A in follicle-stimulating hormone (FSH)-induced LHCGR expression during follicular growth. Treatment of primary cultures of rat granulosa cells with FSH and 17β-estradiol (E2) showed a time-dependent increase in LHCGR mRNA expression. Conversely, inhibition of endogenous hypusination of eIF5A using N1-guanyl-1,7-diaminoheptane (GC7), a hypusination inhibitor, showed a greater increase in LHCGR mRNA expression over that produced by FSH and E2 alone. Further studies were carried out to determine the mechanism by which inhibition of hypusination of eIF5A causes an increase in LHCGR mRNA expression. Because LHCGR expression is negatively regulated by LRBP, the effect of inhibiting hypusination of eIF5A on LRBP expression was examined. The results showed a decrease in the expression of LRBP mRNA and protein when hypusination of eIF5A was inhibited by GC7. Because LRBP promotes LHCGR mRNA degradation, the results of this study support the notion that by inhibiting eIF5A hypusination, FSH reduces the expression of LRBP. This increases LHCGR mRNA expression by abrogating the inhibitory action of LRBP.

Infertility and ovarian follicle reserve depletion are associated with dysregulation of the FSH and LH receptor density in human antral follicles

The low take-home baby rate in older women in Australia (5.8%) undergoing IVF (5.8%) is linked to the depletion of the ovarian reserve of primordial follicles. Oocyte depletion causes an irreversible change to ovarian function. We found that the young patient FSH receptor and LH receptor expression profile on the granulosa cells collected from different size follicles were similar to the expression profile reported in natural cycles in women and sheep. This was reversed in the older patients with poor ovarian reserve. The strong correlation of BMPR1B and FSH receptor density in the young was not present in the older women; whereas, the LH receptor and BMPR1B correlation was weak in the young but was strongly correlated in the older women. The reduced fertilisation and pregnancy rate was associated with a lower LH receptor density and a lack of essential down-regulation of the FSH and LH receptor. The mechanism regulating FSH and LH receptor expression appears to function independently, in vivo, from the dose of FSH gonadotrophin, rather than in response to it. Restoring an optimum receptor density may improve oocyte quality and the pregnancy rate in older women.

Molecular regulation of LHCGR expression by miR-122 during follicle growth in the rat ovary

We have previously reported that LHCGR expression in the ovary is regulated through a post-transcriptional mechanism involving an mRNA binding protein designated as LRBP, which is regulated, at least in part, by a non-coding RNA, miR-122. Our present study examined the regulatory role of miR-122 in FSH-induced LHCGR expression during follicle development. Treatment of rat granulosa cells concurrently with FSH and 17β estradiol showed, as expected, a time-dependent increase in LHCGR mRNA levels as well as hCG-induced progesterone production. However, miR-122 expression was decreased during the early time periods, which preceded the increased expression of LHCGR mRNA. The role of miR-122 in FSH-induced LHCGR mRNA expression was then examined by overexpressing miR-122 prior to FSH stimulation by infecting granulosa cells with an adenoviral vector containing a miR-122 insert (AdmiR-122). Pretreatment with AdmiR-122 resulted in complete abrogation of FSH- mediated upregulation of LHCGR. AdmiR-122 also blocked FSH-induced decrease in LRBP expression and increased the binding of LHCGR mRNA to LRBP. Based on these results, we conclude that miR-122 plays a regulatory role in LHCGR expression by modulating LRBP levels during FSH-induced follicle growth.

Hypusination of eukaryotic initiation factor 5A via cAMP-PKA-ERK1/2 pathway is required for ligand-induced downregulation of LH receptor mRNA expression in the ovary

Luteinizing hormone receptor (LHR) mRNA expression in the ovary is regulated post-transcriptionally by an LH receptor mRNA binding protein (LRBP). Eukaryotic initiation factor 5A (EIF5A), identified as an LRBP-interacting protein plays a crucial role in LHR mRNA expression. In this study, we have demonstrated that during hCG-induced LHR downregulation, a significant upregulation of eIF5A mRNA expression and hypusination of eIF5A protein occurs in a time dependent manner. Pretreatment with H89, a specific inhibitor of PKA, and U0126, a specific inhibitor of ERK1/2 significantly inhibited both hCG-induced eIF5A mRNA expression and hypusination of eIF5A protein. Pretreatment with GC7, a specific inhibitor of eIF5A hypusination significantly abolished hCG-induced LRBP mRNA and protein expression. Furthermore, GC7 pretreatment significantly inhibited hCG-induced interaction of LRBP with LHR mRNA as assessed by RNA electrophoretic mobility gel shift assay (REMSA). GC7 treatment also reversed LHR mRNA downregulation. Taken together, these results suggest that hCG-induced LHR mRNA downregulation is mediated by cAMP-PKA-ERK1/2 signaling leading to activation of eIF5A hypusination.

miR-122 Regulates LH Receptor Expression by Activating Sterol Response Element Binding Protein in Rat Ovaries

LH/human chorionic gonadotropin receptor (LHR) undergoes down-regulation during preovulatory LH surge or in response to exposure to a supraphysiological concentration of its ligands through a posttranscriptional mechanism involving an RNA binding protein designated as LHR mRNA binding protein (LRBP). miR-122, a short noncoding RNA, has been shown to mediate the up-regulation of LRBP. In the present study, we show that inhibition of miR-122 using a locked nucleic acid (LNA)-conjugated antagomir suppressed human chorionic gonadotropin (hCG)-induced up-regulation of LRBP as well as its association with LHR mRNA, as analyzed by RNA EMSA. Most importantly, inhibition of miR-122 resulted in the abolishment of hCG-mediated LHR mRNA down-regulation. We also show that the transcription factor, sterol regulatory element binding protein (SREBP) (SREBP-1a and SREBP-2 isoforms), is an intermediate in miR-122-mediated LHR mRNA regulation. HCG-stimulated increase in the activation of both SREBP-1a and SREBP-2 was inhibited by pretreatment with the miR-122 antagomir. The inhibition of cAMP/protein kinase A (PKA) and ERK pathways, upstream activators of miR-122, abolished SREBP activation after hCG treatment. SREBP-mediated regulation of LRBP expression is mediated by recruitment of LRBP promoter element to SREBP-1a, because chromatin immunoprecipitation assay revealed that association of LRBP promoter to SREBP was increased by hCG treatment. Pretreatment with miR-122 antagomir suppressed this response. Inhibition of SREBP activation by pretreating the rats with a chemical compound, fatostatin abrogated hCG-induced up-regulation of LRBP mRNA and protein. Fatostatin also inhibited LHR-LRBP mRNA-protein complex formation and LHR down-regulation. These results conclusively show that miR-122 plays a regulatory role in LH/hCG-induced LHR mRNA down-regulation by increasing LRBP expression through the activation of SREBP pathway.

Eukaryotic initiation factor 5A plays an essential role in luteinizing hormone receptor regulation

Down-regulation of LH receptor (LHR) in the ovary by its ligand is mediated by a specific RNA-binding protein, designated LH receptor mRNA-binding protein (LRBP), through translational suppression and mRNA degradation. Using yeast 2-hybrid screens, we previously identified eukaryotic initiation factor 5A (eIF5A) as one of the proteins that interacts with LRBP during LHR mRNA down-regulation. The present study examined the role of eIF5A and its hypusination in the context of LHR mRNA down-regulation. The association of eIF5A with LRBP or LHR mRNA was determined using immunoprecipitation and RNA immunoprecipitation assays. The results showed that the association of eIF5A with the LHR mRNA-LRBP complex increased significantly during down-regulation. Furthermore, gel fractionation and the hypusination activity assay both showed increased hypusination of eIF5A during LHR mRNA down-regulation. Abolishment of hypusination by pretreatment with the chemical inhibitor GC7 prevented the association of eIF5A with LHR mRNA and LRBP. Inhibition of hypusination also reduced the extent of ligand-induced down-regulation of LHR mRNA as well as the expression of functional LHRs assessed by real-time PCR and (125)I-human chorionic gonadotropin (hCG) binding assays, respectively. The loss of human chorionic gonadotropin-mediated downstream signaling during LHR down-regulation was also restored by inhibition of hypusination of eIF5A. Thus, the present study, for the first time, reveals the crucial role of eIF5A and its hypusination in the regulation of LHR expression in the ovary.

MicroRNA miR-513a-3p acts as a co-regulator of luteinizing hormone/chorionic gonadotropin receptor gene expression in human granulosa cells

The luteinizing hormone/chorionic gonadotropin receptor (LHCGR) is essential for normal male and female reproductive processes. The spatial and temporal LHCGR gene expression is controlled by a complex system of regulatory mechanisms which are crucial for normal physiological function, especially during the female cycle. In this study, we aimed to elucidate whether microRNAs are involved in this network and play a role in regulating LHCGR expression. Computational analysis predicted that miR-513a-3p interacts with the LHCGR mRNA via three binding sites located in the 3'UTR region, enabling a synergistic action. Moreover, using a luciferase-based reporter assay we found that miR-513a-3p targets the LHCGR, resulting in a significant down-regulation of its expression. In human primary granulosa cell cultures we detected a dynamic, inversely associated expression pattern of miR-513a-3p and the LHCGR. In addition, transfection with miR-513a-3p or its specific inhibitor led to a down- or up-regulation at the LHCGR mRNA level, respectively. An increased amount of miR-513a-3p resulted in the down-regulation of the LHCGR mRNA, reflected by the attenuation of cAMP synthesis after hormonal stimulation. In conclusion, these data demonstrate that miR-513a-3p is involved in the control of the LHCGR expression by an inversely regulated mechanism at the post-transcriptional level and show for the first time that this kind of post-transcriptional process contributes to the multifaceted system of the human LHCGR regulation.

Regulation of LH receptor mRNA binding protein by miR-122 in rat ovaries

LH receptor (LHR) expression in the ovary is regulated by the RNA binding protein, (LHR mRNA binding protein [LRBP]), which has been identified as being mevalonate kinase. This study examined the role of microRNA miR-122 in LRBP-mediated LHR mRNA expression. Real-time PCR analysis of ovaries from pregnant mare serum gonadotropin/human chorionic gonadotropin (hCG)-primed female rats treated with hCG to down-regulate LHR expression showed that an increase in miR-122 expression preceded LHR mRNA down-regulation. The expression of miR-122 and its regulation was confirmed using fluorescent in situ hybridization of the frozen ovary sections using 5'-fluorescein isothiocyanate-labeled miR-122 locked nucleic acid probe. The increased expression of miR-122 preceded increased expression of LRBP mRNA and protein, and these increases were followed by LHR mRNA down-regulation. Inhibition of protein kinase A (PKA) and ERK1/2 signaling pathways by H89 and UO126, respectively, attenuated the hCG-mediated up-regulation of miR-122 levels. This was also confirmed in vitro using human granulosa cells. These results suggest the possibility that hCG-mediated miR-122 expression is mediated by the activation of cAMP/PKA/ERK signaling pathways. Inhibition of miR-122 by injection of the locked nucleic acid-conjugated antagomir of miR-122 abrogated the hCG-mediated increases in LRBP protein expression. Because it has been previously shown that miR-122 regulates sterol regulatory element-binding proteins (SREBPs) and SREBPs, in turn, regulate LRBP expression, the role of SREBPs in miR-122-mediated increase in LRBP expression was then examined. The levels of active forms of both SREBP-1a and SREBP-2 were increased in response to hCG treatment, and the stimulatory effect was sustained up to 4 hours. Taken together, our results suggest that hCG-induced down-regulation of LHR mRNA expression is mediated by activation of cAMP/PKA/ERK pathways to increase miR-122 expression, which then increases LRBP expression through the activation of SREBPs.

The Interplay between Circadian System, Cholesterol Synthesis, and Steroidogenesis Affects Various Aspects of Female Reproduction

Circadian aspect of reproduction has gained much attention in recent years. In mammals, it is very important that the timing of greatest sexual motivation is in line with the highest fertility. Peripheral clocks have been found to reside also in reproductive organs, such as the uterus and ovary. The timing signal from the suprachiasmatic nucleus is suggested to be transmitted via hormonal and neural mechanisms, and could thus mediate circadian expression of target genes in these organs. In turn, estrogens from the ovary have been found to signal back to the hypothalamus, completing the feedback loop. In this review we will focus on the interplay between clock and estrogens. Estradiol has been directly linked with expression of Per1 and Per2 in the uterus. CLOCK, on the other hand, has been shown to alter estradiol signaling. We also present the idea that cholesterol could play a vital role in the regulation of reproduction. Cholesterol synthesis itself is circadially regulated and has been found to interfere with steroidogenesis in the ovary on the molecular level. This review presents a systems view on how the interplay between circadian clock, steroidogenesis, and cholesterol synthesis affect various aspects of mammalian reproduction.

Association of luteinizing hormone receptor (LHR) mRNA with its binding protein leads to decapping and degradation of the mRNA in the p bodies

Luteinizing hormone receptor undergoes downregulation during preovulatory Luteinizing hormone surge through a post-transcriptional mechanism involving an RNA binding protein designated as LRBP. The present study examined the mechanism by which LRBP induces the degradation of Luteinizing hormone receptor mRNA, specifically the role of decapping of Luteinizing hormone receptor mRNA and the translocation of LRBP-bound Luteinizing hormone receptor mRNA to degradative machinery. Immunoprecipitation of the complex with the 5'cap structure antibody followed by real time PCR analysis showed progressive loss of capped Luteinizing hormone receptor mRNA during downregulation suggesting that Luteinizing hormone receptor mRNA undergoes decapping prior to degradation. RNA immunoprecipitation analysis confirmed dissociation of eukaryotic initiation factor 4E from the cap structure, a step required for decapping. Furthermore, RNA immunoprecipitation analysis using antibody against the p body marker protein, DCP1A showed that Luteinizing hormone receptor mRNA was associated with the p bodies, the cytoplasmic foci that contain RNA degradative enzymes and decapping complex. Immunohistochemical studies using antibodies against LRBP and DCP1A followed by confocal analysis showed colocalization of LRBP with DCP1A during downregulation. This was further confirmed by co-immunoprecipitation of LRBP with DCP1A. The association of LRBP and Luteinizing hormone receptor mRNA in the p bodies during downregulation was further confirmed by examining the association of a second p body component, rck/p54, using immunoprecipitation and RNA immunoprecipitation respectively. These data suggest that the association of LRBP with Luteinizing hormone receptor mRNA results in the translocation of the messenger ribonucleoprotein complex to the p bodies leading to decapping and degradation.

Effect of superstimulatory treatments on the expression of genes related to ovulatory capacity, oocyte competence and embryo development in cattle

Multiple ovulation (superovulation) and embryo transfer has been used extensively in cattle. In the past decade, superstimulatory treatment protocols that synchronise follicle growth and ovulation, allowing for improved donor management and fixed-time AI (FTAI), have been developed for zebu (Bos indicus) and European (Bos taurus) breeds of cattle. There is evidence that additional stimulus with LH (through the administration of exogenous LH or equine chorionic gonadotrophin (eCG)) on the last day of the superstimulatory treatment protocol, called the ‘P-36 protocol’ for FTAI, can increase embryo yield compared with conventional protocols that are based on the detection of oestrus. However, inconsistent results with the use of hormones that stimulate LH receptors (LHR) have prompted further studies on the roles of LH and its receptors in ovulatory capacity (acquisition of LHR in granulosa cells), oocyte competence and embryo quality in superstimulated cattle. Recent experiments have shown that superstimulation with FSH increases mRNA expression of LHR and angiotensin AT2 receptors in granulosa cells of follicles >8 mm in diameter. In addition, FSH decreases mRNA expression of growth differentiation factor 9 (GDF9) and bone morphogenetic protein 15 (BMP15) in oocytes, but increases the expression of both in cumulus cells, without diminishing the capacity of cumulus–oocyte complexes to generate blastocysts. Although these results indicate that superstimulation with FSH is not detrimental to oocyte competence, supplementary studies are warranted to investigate the effects of superstimulation on embryo quality and viability. In addition, experiments comparing the cellular and/or molecular effects of adding eCG to the P-36 treatment protocol are being conducted to elucidate the effects of superstimulatory protocols on the yield of viable embryos.

Structure, function and regulation of gonadotropin receptors - a perspective

Luteinizing hormone receptor and follicle stimulating hormone receptor play a crucial role in female and male reproduction. Significant new information has emerged about the structure, mechanism of activation, and regulation of expression of these receptors. Here we provide an overview of the current information on those aspects with an in-depth discussion of the recent developments in the post-transcriptional mechanism of LH receptor expression mediated by a specific LH receptor mRNA binding protein, designated as LRBP. LRBP was identified by electrophoretic gel mobility shift assay using cytosolic fractions from ovaries in the down regulated state. LRBP was purified, its binding site on LH receptor mRNA was identified and characterized. During ligand-induced down regulation, LRBP expression is increased through the cAMP/PKA and ERK signaling pathway, is translocated to translating ribosomes, binds LH receptor mRNA and forms an untranslatable ribonucleoprotein complex. This complex is then routed to the mRNA degradation machinery resulting in diminished levels of both LHR mRNA and cell surface expression of LH receptor. The studies leading to these conclusions are presented.

Luteinizing hormone receptor mRNA down-regulation is mediated through ERK-dependent induction of RNA binding protein

The ligand-induced down-regulation of LH receptor (LHR) expression in the ovaries, at least in part, is regulated by a posttranscriptional process mediated by a specific LH receptor mRNA binding protein (LRBP). The LH-mediated signaling pathways involved in this process were examined in primary cultures of human granulosa cells. Treatment with 10 IU human chorionic gonadotropin (hCG) for 12 h resulted in the down-regulation of LHR mRNA expression while producing an increase in LHR mRNA binding to LRBP as well as a 2-fold increase in LRBP levels. The activation of ERK1/2 pathway in LH-mediated LHR mRNA down-regulation was also established by demonstrating the translocation of ERK1/2 from the cytosol to the nucleus using confocal microcopy. Inhibition of protein kinase A using H-89 or ERK1/2 by U0126 abolished the LH-induced LHR mRNA down-regulation. These treatments also abrogated both the increases in LRBP levels as well as the LHR mRNA binding activity. The abolishment of the hCG-induced increase in LRBP levels and LHR mRNA binding activity was further confirmed by transfecting granulosa cells with ERK1/2 specific small interfering RNA. This treatment also reversed the hCG-induced down-regulation of LHR mRNA. These data show that LH-regulated ERK1/2 signaling is required for the LRBP-mediated down-regulation of LHR mRNA.

Molecular regulation of gonadotropin receptor expression: relationship to sterol metabolism

We have identified a specific LHR mRNA binding protein that selectively binds to the polypyrimidine-rich bipartite sequence in the coding region of the LHR mRNA and accelerates its degradation. This process has been shown to be one of the mechanisms that is responsible for the loss of the steady-state levels of LHR mRNA following the preovulatory LH surge or the down regulation of the receptor in response to the administration of a pharmacological dose of LH or hCG. The trans factor, designated as the LHR mRNA binding protein (LRBP), was purified and its identity was established as being mevalonate kinase, an enzyme involved in cholesterol biosynthesis. When mevalonate kinase expression was abolished by treating cultured luteal cells with 25-hydroxycholesterol, the ability to undergo LH-induced down regulation of LHR mRNA was completely abrogated. Examination of the crystal structure of mevalonate kinase coupled with mutagenesis of the critical residues in the catalytic site revealed that the catalytic site is in close proximity to the LHR mRNA binding site. Further studies revealed that mevalonate kinase causes LHR mRNA degradation by acting as a translational suppressor by forming an untranslatable ribonucleoprotein (RNP) complex which is then targeted for degradation. These studies show that LHR expression in the ovary is regulated by a post-transcriptional mechanism mediated by mevalonate kinase thereby linking LHR expression with cholesterol metabolism.

Expression of vascular endothelial growth factor A during ligand-induced down-regulation of luteinizing hormone receptor in the ovary

Vascular endothelial growth factor A (VEGF-A) is one of the most important regulators of ovarian angiogenesis. In this study, we examined the temporal relationship between VEGF-A and luteinizing hormone receptor (LHR) mRNA expression during ligand-induced down-regulation of LHR. Immature female rats were treated with pregnant mare's serum gonadotropin followed by 25 IU hCG 56 h later (day 0). On day 5, treatment with hCG (50 IU) to down-regulate LHR showed a temporal decrease in VEGF-A mRNA and protein levels in parallel with decreasing LHR mRNA. This effect was specific since the expression of CYP11A1 mRNA showed no decline. Examination of VEGF-A mRNA expression, using in situ hybridization histochemistry with (35)S-labeled antisense VEGF-A mRNA probe, showed intense signal in the corpora lutea on day 5. Treatment with 50 IU hCG to down-regulate LHR mRNA showed a decline in the intensity of VEGF-A mRNA in the corpora lutea. VEGF-A mRNA expression returned to control level 53 h later when the expression of LHR mRNA also recovered. These results show that the transient down-regulation of VEGF-A mRNA and protein closely parallels the ligand-induced down-regulation of LHR mRNA. The present study establishes a close association between VEGF-A and LHR mRNA expression, suggesting the possibility that VEGF-A-induced vascularization of the ovary is dictated by the expression of LHR and this might play a regulatory role in ovarian physiology.

Identification and characterization of proteins that selectively interact with the LHR mRNA binding protein (LRBP) in rat ovaries

Luteinizing hormone receptor (LHR) mRNA binding protein (LRBP), identified as mevalonate kinase, has been shown to be a trans factor mediating the post-transcriptional regulation of LHR mRNA expression in ovaries. LRBP binds to the coding region of LHR mRNA and accelerates its degradation. Our previous studies in an in vitro system showed that LRBP represses the translation of LHR mRNA by forming an untranslatable ribonucleoprotein (mRNP) complex, further suggesting that the untranslatable mRNP complex is directed to the mRNA repression/decay machinery for subsequent mRNA turnover. In the present studies, we used yeast two-hybrid system to screen a cDNA library which was constructed from LHR down-regulated ovaries. Two proteins were identified interacting with LRBP: ribosomal protein S20 (RP S20) and ubiquitin conjugating enzyme 2i (UBCE2i). Their interactions with LRBP were confirmed by the mating assay, co-immunoprecipitation analyses and in vitro sumoylation assays. Furthermore, we show that LRBP is a target for modification by SUMO2/3 but not by SUMO1, at K256 and/or K345. Mutation of both lysine residues is sufficient to abrogate the sumoylation of LRBP. These findings suggest that the direct interaction of LRBP with the translation machinery, through RP S20, may be responsible for the transition of LHR mRNA to an untranslatable complex, and that sumoylation of LRBP may play a role in targeting the untranslatable mRNP complex to the mRNA decay machinery in specific cytoplasmic foci.

Evidence for the association of luteinizing hormone receptor mRNA-binding protein with the translating ribosomes during receptor downregulation

Luteinizing hormone receptor (LHR) mRNA is post-transcriptionally regulated during ligand-induced downregulation. This process involves interaction of LHR mRNA with a specific mRNA-binding protein (LRBP), identified as mevalonate kinase (MVK), resulting in inhibition of translation followed by targeting the ribonucleoprotein complex to accelerated degradation. The present study investigated the endogenous association of LRBP with the translational machinery and its interaction with LHR mRNA during LH/hCG-induced downregulation. Ovaries were collected from rats that were injected with the ligand, hCG, to induce downregulation of LHR mRNA expression. Western blot analysis showed significantly higher levels of LRBP in polysomes from downregulated ovaries compared to controls. Western blot analysis of ribosome-rich fractions from FPLC-assisted gel filtration of post-mitochondrial supernatants confirmed the presence of LRBP in translating ribosomes isolated from the downregulated state but not from controls. The association of LRBP with LHR mRNA in the downregulated polysomes was demonstrated by immunoprecipitation with LRBP antibody followed by qPCR analysis of the associated RNA. Increased association of LHR mRNA with LRBP during downregulation was also demonstrated by subjecting the polysome-associated RNAs to oligo(dT) cellulose chromatography followed by immunoprecipitation and qPCR analysis. Additionally, analysis of in vitro translation of LHR mRNA showed increased inhibition of translation by polysomes from downregulated ovaries compared to control. This study provides strong in vivo and in vitro evidence to show that during ligand-induced downregulation, LRBP translocates to ribosomes and associates with LHR mRNA to form an untranslatable ribonucleoprotein complex and inhibits LHR mRNA translation, paving the way to its degradation.

LH/hCGR gene expression in human cumulus cells is linked to the expression of the extracellular matrix modifying gene TNFAIP6 and to serum estradiol levels on day of hCG administration

BACKGROUND

Recent studies suggest a role for luteinizing hormone and human chorionic gonadotrophin receptor (LH/hCGR) signalling in the regulation of the oocyte-cumulus oophorus cell interplay. The present study aimed at assessing the LH/hCGR gene expression in cumulus cells (CCs) surrounding oocytes in patients undergoing controlled ovarian hyperstimulation (COS) before ICSI and to relate the LH/hCGR expression to other COS quality parameters.

METHODS

CCs from single oocytes of normal responder patients were analysed by DNA microarrays. Concomitantly, estradiol levels on the day of hCG administration, CC morphology, total collected oocyte and metaphase II oocyte number were assessed in relation to LH/hCGR gene expression in CC.

RESULTS

The transcriptome analysis of CC indicated a variable expression of LH/hCGR among the patients and intra-patients. LH/hCGR mRNA expression was negatively correlated with serum estradiol level on the day of hCG administration. Eighty-five genes were significantly modulated between CCs from patients with a high and a low LH/hCGR expression. These genes are involved principally in steroid metabolism and in the ovulation process and include TNFAIP6, a gene expressed during CC-oocyte complex (COC) expansion. There were no significant differences in LH/hCGR gene expression profile between COS protocols.

CONCLUSIONS

LH/hCGR is expressed in CC under COS conditions. LH/hCGR expression level is associated with TNFAIP6 gene expression and negatively correlated with serum estradiol level on the day of hCG administration.

Anti-Mullerian hormone, its receptor, FSH receptor, and androgen receptor genes are overexpressed by granulosa cells from stimulated follicles in women with polycystic ovary syndrome

CONTEXT

In the polycystic ovary syndrome (PCOS), in addition to intrinsic thecal dysregulation leading to hyperandrogenism, a granulosa cell (GC) dysregulation may occur. Expression of anti-Müllerian hormone (AMH), FSH receptor (FSHR) and androgen receptor (AR) are suspected to be altered in PCOS GCs.

DESIGN

The aim of this prospective study was to analyze the expression of these genes at the last stages of follicular maturation in GCs from 17 patients with PCOS and 15 controls undergoing controlled ovarian hyperstimulation during a cycle with in vitro fertilization.

MATERIALS AND METHODS

On the day of oocyte retrieval, follicular fluids were collected from small follicles (SF; 8-13 mm) and large follicles (17-22 mm) in separate tubes. Total RNAs and proteins were extracted from GCs. Reverse transcription was performed and quantification of gene expression levels was achieved by real-time quantitative PCR.

RESULTS

AMH and FSHR mRNA levels were significantly higher in PCOS than in controls in GCs from both SF and large follicles. Likewise, AR and AMH receptor II mRNA levels in GCs from SF were significantly higher in PCOS compared with controls. In both PCOS patients and controls, AMH and AR mRNA levels correlated strongly, positively, and independently to FSHR mRNA levels.

CONCLUSION

Using quantitative RT-PCR, AMH, AMH receptor II, FSHR, and AR genes were shown to be overexpressed by GCs from stimulated follicles of women with PCOS undergoing controlled ovarian hyperstimulation. This could be the sign of a maturation defect or may reflect hyperandrogenism.

Regulation of luteinizing hormone receptor mRNA expression by mevalonate kinase--role of the catalytic center in mRNA recognition

We have shown that hormone-induced downregulation of luteinizing hormone receptor (LHR) in the ovary is post-transcriptionally regulated by an mRNA binding protein. This protein, later identified as mevalonate kinase (MVK), binds to the coding region of LHR mRNA, suppresses its translation, and the resulting ribonucleoprotein complex is targeted for degradation. Mutagenesis and crystallographic studies of rat MVK have established Ser146, Glu193, Asp204 and Lys13 as being crucial for its catalytic function. The present study examined the structural aspects of MVK required for LHR mRNA recognition and translational suppression. Single MVK mutants (S146A, E193Q, D204N and K13A) were overexpressed in 293T cells. Cytosolic fractions were examined for LHR mRNA binding activities by RNA electrophoretic mobility shift analysis. All the single MVK mutants showed decreased LHR mRNA binding activity compared with the wild-type MVK. Double mutants (S146A & E193Q, E193Q & D204N and E193Q & K13A) of MVK also showed a significant decrease in binding to LHR mRNA, suggesting that the residues required for catalytic function are also involved in LHR mRNA recognition. Mutation of the residues outside the catalytic site (D316A and S314A) did not cause any change in LHR mRNA binding activity of MVK when compared with wild-type MVK. To examine the biological effects of these mutants on LHR mRNA expression, a full-length capped rat LHR mRNA was synthesized and translated using a rabbit reticulocyte lysate system in the presence or absence of the MVK mutant proteins. The results showed that mutations of the active site residues of MVK abrogated the inhibitory effect on LHR mRNA translation. Therefore, these data indicate that an intact active site of MVK is required for its binding to rat LHR mRNA and for its translational suppressor function.

Regulation of luteinizing hormone receptor mRNA expression by a specific RNA binding protein in the ovary

The expression of LH receptor mRNA shows significant changes during different physiological states of the ovary. Previous studies from our laboratory have identified a post-transcriptional mechanism by which LH receptor mRNA is regulated following preovulatory LH surge or in response to hCG administration. A specific binding protein, identified as mevalonate kinase, binds to the open reading frame of LH receptor mRNA. The protein binding site is localized to nucleotides 203-220 of the LH receptor mRNA and exhibits a high degree of specificity. The expression levels of the protein show an inverse relationship to the LH receptor mRNA levels. The hCG-induced down-regulation of LH receptor mRNA can be mimicked by increasing the intracellular levels of cyclic AMP by a phosphodiesterase inhibitor. An in vitro mRNA decay assay showed that addition of the binding protein to the decay system caused accelerated LH receptor mRNA decay. Our results therefore show that LH receptor mRNA expression in the ovary is regulated post-transcriptionally by altering the rate of mRNA degradation by a specific mRNA binding protein.

A novel post-transcriptional mechanism of regulation of luteinizing hormone receptor expression by an RNA binding protein from the ovary

Luteinizing hormone/human chorionic gonadotropin (LH/hCG) receptor, a member of the rhodopsin/beta(2) adrenergic receptor subfamily of G-protein coupled receptors, is expressed primarily in the gonads and essential for the regulation of reproductive function. In the ovary, the expression of the receptor is post-transcriptionally regulated under physiological conditions. Studies from our laboratory showed that the ligand-induced down-regulation of the receptor occurs by accelerated degradation of the mRNA rather than by decreased transcription. We have identified a cytoplasmic LHR mRNA binding protein (LRBP) as a trans-acting factor in regulating LHR mRNA levels. LRBP binds to the coding region of LHR mRNA and causes accelerated degradation of mRNA. The RNA binding activity of LRBP was found to be inversely correlated to LH/hCG receptor mRNA levels. LRBP was purified to homogeneity and its identity was established as mevalonate kinase by N-terminal microsequencing and MALDI analysis. Mevalonate kinase, an enzyme involved in de novo synthesis of cholesterol, belongs to the GHMP family of kinases having a potential RNA binding fold. The expression of MVK mRNA and MVK protein levels were induced in response to hCG treatment prior to the down-regulation of LH/hCG receptor mRNA expression. A model for the post-transcriptional regulation of LH/hCG receptor in the ovary by mevalonate kinase is proposed.

Regulation of Luteinizing Hormone Receptor Expression

Our previous studies have identified a luteinizing hormone receptor (LHR) mRNA-binding protein (LRBP) that binds to the coding region (LBS) of rat LHR mRNA. The identity of LRBP was later established as mevalonate kinase (MVK). The present study examined if LRBP binding to LHR mRNA impairs translation. A full-length FLAG-tagged rat LHR mRNA was synthesized and translated in vitro. The translation product was immunoprecipitated and analyzed on SDS-PAGE. The addition of LRBP inhibited LHR mRNA translation. This inhibitory effect was reversed by an excess of wild type (wt) LBS. To determine whether this reversal of the inhibitory effect of LRBP was indeed due to the sequestration of LRBP by the wtLBS, a translation reaction was performed in the presence of mutated LBS in which all the cytidine in the wtLBS was mutated to uridine. This mutation of LBS has been shown to render it incapable of interacting with LRBP. Unlike wtLBS, the mutated LBS was unable to reverse the inhibitory effect of LRBP on LHR mRNA translation. The addition of mevalonate, which has been shown to compete for LHR mRNA binding to LRBP, also reduced the extent of translation inhibition by LRBP. Endogenous association of LHR mRNA with MVK was assessed by immunoprecipitation of the ribonucleoprotein complex with MVK antibody followed by reverse transcription-PCR of the RNA associated with the immune complex. Amplification of LHR mRNA, if any, associated with the immunoprecipitate obtained from ovarian ribonucleoprotein complex with gene-specific primers confirmed the association of LHR mRNA with MVK. Collectively, the present data support the novel function of LRBP as a translational inhibitor of LHR mRNA in the ovary.

Effect of follicle-stimulating hormone and estrogen on the expression of betaglycan messenger ribonucleic acid levels in cultured rat granulosa cells

Betaglycan (TGFbeta type III receptor) was recently identified as a coreceptor to enhance the binding of inhibin A to activin type II receptor. This inhibin/betaglycan/activin type II receptor complex prevents activins from binding to their own receptors. The present study was undertaken to identify the expression and the regulation of the betaglycan gene in cultured rat granulosa cells. Northern blot analysis indicated betaglycan mRNA transcript of approximately 6.4 kbp. The treatment of the cells with FSH increased the betaglycan mRNA level, and a concurrent treatment with estradiol brought a significant increase in betaglycan mRNA. The protein kinase A activator, 8-bromoadenosine-cAMP, also increased the expression of its mRNA. Furthermore, betaglycan mRNA was induced additively by estradiol, which was blocked by estrogen receptor antagonists [ICI 182780, (R, R)-cis-diethyltetrahydro-2,8-chrysenediol]. In the luciferase assay, FSH altered the promoter activity of betaglycan. Moreover, when FSH plus estradiol was added to the granulosa cells, a significant increase in the half-life of betaglycan mRNA transcript was seen. In summary, FSH and estradiol increased betaglycan mRNA expression, most possibly through the protein kinase A pathway and the estrogen receptor-beta. The increase of betaglycan mRNA was due to an increase in transcription and altered mRNA stability. In ovarian regulatory function, the expression of betaglycan may involve the functional antagonism of inhibin A in activin signal transduction.

Gonadotropin receptors: role of post-translational modifications and post-transcriptional regulation

This review focuses on the post-translational modifications of LH and FSH receptors and recent studies on the regulation of LH receptor expression mediated by an RNA binding protein. Both LH and FSH receptors undergo extensive post-translational modifications. N-linked glycosylation occurs co-translationally and plays a role in the maturation and processing of the receptor, while palmitoylation is involved in receptor endocytosis and post-endocytic trafficking. A third type of post-translational modification is phosphorylation and its function has been reviewed. Finally, the regulation of LH receptor at the mRNA level by an RNA binding protein is discussed in the context of ovarian function.

Regulation of luteinizing hormone/chorionic gonadotropin receptor messenger ribonucleic acid expression in the rat ovary: relationship to cholesterol metabolism

Down-regulation of LH/human chorionic gonadotropin (hCG) receptor (LHR) mRNA in the ovary after the preovulatory LH surge or the administration of a pharmacological dose of LH/hCG occurs through a posttranscriptional mechanism. A LHR mRNA-binding protein was identified as the LHR mRNA destabilizing factor, and its identity was established as mevalonate kinase (Mvk). In the present study, we determined that, in the pseudopregnant rat ovary, LHR mRNA levels began to fall 4 h after hCG injection, at which time Mvk protein levels were elevated, and this elevation was preceded by an increase in Mvk mRNA levels. When the cytosolic fractions of hCG-treated ovaries were subjected to RNA EMSA, an increase in LHR mRNA-LHR mRNA-binding protein complex formation was observed, in parallel with the increase of Mvk expression. We also found that hCG coordinately up-regulated the expression of Mvk and other sterol-responsive elements containing cholesterol biosynthesis enzymes, such as 3-hydroxy-3-methylglutaryl-coenzyme A synthase, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, and farnesyl pyrophosphate synthase. This up-regulation was transient, but the hCG-induced ovarian cholesterol depletion lasted for more than 24 h. Taken together, our results suggest that, in the ovary, LH/hCG up-regulates the expression of cholesterol biosynthesis enzymes and lipoprotein receptors to replenish cellular cholesterol, and the up-regulation of Mvk leads to a down-regulation of LHR and suppresses the LH/hCG signal cascade transiently. Thus Mvk, an enzyme involved in cholesterol biosynthesis, serves as a link between LHR mRNA expression and cellular cholesterol metabolism.

Isolation and Characterization of a Novel trans-Factor for Luteinizing Hormone Receptor mRNA from Ovary

Post-transcriptional mechanisms play a major role in regulating luteinizing hormone (LH) receptor mRNA expression in the ovary. An ovarian cytosolic protein that we have identified in rats and humans, which binds to a polypyrimidine-rich bipartitate sequence in the coding region of LHR mRNA, acts as a trans-acting factor in this process. In the present study, we isolated and characterized this LH receptor mRNA-binding protein (LRBP) from rat ovary. LRBP was purified to homogeneity by cation exchange chromatography followed by Northwestern analysis and subsequent elution of the single protein band from SDS-polyacrylamide gel. Purified LRBP was subjected to N-terminal microsequencing followed by homology search, which revealed its identity as mevalonate kinase. Purified rat mevalonate kinase antibody recognized the gel-purified LRBP on Western blots performed with one- and two-dimensional SDS-polyacrylamide gels. When recombinant mevalonate kinase produced in human embryonic kidney cells (293 cells) was tested, it showed all of the characteristics of LRBP with respect to specificity of LHR mRNA binding sequence, as examined by gel mobility shift analysis. Inhibition of LHR mRNA binding activity of mevalonate kinase in the presence of ATP and mevalonate indicates that the RNA recognition site of mevalonate kinase might involve the ATP/mevalonate binding region of the protein. Treatment of 293 cells with mevastatin to deplete cellular mevalonate resulted in an increase in LHR mRNA binding activity of mevalonate kinase. Collectively, the data support the novel function of rat mevalonate kinase as a LHR mRNA-binding protein in the post-transcriptional regulation of LH receptor expression in the ovary.

Regulation of luteinizing hormone/human chorionic gonadotropin receptor expression: a perspective

The LH/hCG receptor, a member of the G protein coupled receptor family mediates the cellular actions of LH in the ovary. A considerable amount of information regarding its structure, mechanism of activation, and regulation of expression has emerged in recent years. Here we provide a brief overview of the current information on the structural organization of the receptor and the mechanism of receptor mediated signaling as well as an in-depth discussion on recent developments pertaining to the regulation of receptor expression. Specifically, we describe studies from our laboratory showing that the posttranscriptional regulation of the receptor involves an LH/hCG receptor mRNA-binding protein. We also propose a model to explain the loss of steady-state LH/hCG receptor mRNA levels during receptor down-regulation.

Mechanisms of action of transforming growth factor beta on the expression of follicle-stimulating hormone receptor messenger ribonucleic acid levels in rat granulosa cells

The present study was undertaken to identify the mechanisms underlying the effect of transforming growth factor (TGF) beta on FSH receptor (FSH-R) in rat granulosa cells. Compared to the control, the treatment of granulosa cells with TGFbeta (10 ng/ml) increased FSH-R mRNA transcripts (5.5 and 2.4 kilobases) in a time-dependent manner, with a maximum increase of approximately 2-fold at 48 h. We then investigated whether the effect of TGFbeta on FSH-R mRNA levels was the result of increased transcription and/or altered mRNA stability. To determine whether the FSH-R 5'-flanking region plays a role in directing FSH-R mRNA expression, the proximal area of the FSH-R 5'-flanking regions were inserted into an expression vector, pGL-Basic, which contains luciferase as the receptor gene, and the resulting plasmids were transiently transfected into rat granulosa cells. The FSH (30 ng/ml) significantly enhanced the activity of 1862 base pairs of the FSH-R 5'-flanking region, but treatment with TGFbeta did not significantly influence the activity induced by FSH. On the other hand, the decay curves for FSH-R mRNA transcript in primary granulosa cells showed a significant increase in half-life after the addition of TGFbeta. Transforming growth factor beta stimulates the expression of follistatin mRNA accumulation in a dose- and time-dependent manner. Treatment with activin produced a substantial increase in FSH-R mRNA level. Concurrent treatment with follistatin neutralized this activin effect on FSH-R mRNA, as reported, although concurrent treatment with follistatin did not affect TGFbeta-induced FSH-R mRNA. Therefore, the profile of the TGFbeta effect on FSH-R mRNA granulosa cells may be caused by the increased stability of FSH-R mRNA and insensitivity to the follistatin.

Effect of IGF-1 and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the expression of LH receptors during cell differentiation in cultured granulosa cells

Ovarian granulosa cells undergo a complex differentiation process during the growth and maturation of ovarian follicle. This process includes the acquisition of cell surface LH receptor, which mediates the granulosa cell's ability to respond to circulating LH. The results of the actions of LH on the mature granulosa cell include steroidogenesis, luteinization, and ovulation. As such, induction of the LH receptor in granulosa cells is a critical step in reproductive physiology. In the present study, we attempted to assess the effects of IGF-1 and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on FSH-induced LH receptor expression in rat granulosa cells to understand the actions of these factors on normal reproductive function. Treatment with FSH, as expected, produced a substantial increase in LH receptor mRNA level, and concurrent treatment with increasing concentrations of IGF-1 brought about dose-dependent increases in FSH-induced LH receptor mRNA. On the other hand, the concurrent treatment of TCDD (10 pM) resulted in a significant decrease in LH receptor after 24 h. The decay curves for LH receptor mRNA transcript showed a significant increase in the half-life after the addition of IGF-1 and a significant decrease after addition of TCDD. These data suggests a possible role for changes in LH receptor mRNA stability in the IGF-1 and TCDD induced regulation of LH receptor in rat granulosa cells. The rates of LH receptor mRNA gene transcription, assessed by nuclear run-on transcription assay, were not increased by the addition of IGF-1, but decreased by the addition of TCDD. The data of IGF-1 present that the interface between circulating hormones and paracrine/autocrine systems could provide an important mechanism to amplify the effects of gonadotropin hormones at the local level. In addition, the endocrine-disrupting effects of TCDD are, at least in part, caused by direct action on the expression of LH receptor expression in granulosa cells.

Effect of transforming growth factor beta on the expression of luteinizing hormone receptor in cultured rat granulosa cells

The present study was undertaken in order to identify the mechanism underlying the effect of transforming growth factor-beta (TGFbeta) on LH receptor (LH-R) expression in rat granulosa cells. Treatment with FSH produced a substantial increase in LH-R mRNA level, and concurrent treatment with increasing concentrations of TGFbeta brought about dose-dependent increases in FSH-induced LH receptor mRNA. TGFbeta, either alone or in combination with FSH, did not affect intracellular cAMP levels. We then investigated whether the effect of TGFbeta and FSH on LH-R mRNA levels results in increased transcription and/or altered mRNA stability. To determine whether the LH receptor 5'-flanking region plays a role in directing LH receptor mRNA expression, the proximal area of the LH receptor 5'-flanking regions were inserted into an expression vector, pGL-Basic, which contains luciferase as the receptor gene, and the resulting plasmids were transiently transfected into rat granulosa cells. FSH (30 ng/ml) significantly enhanced the activity of 1389 base pairs of the LH receptor 5'-flanking region, but treatment with TGFbeta did not significantly influence the activity induced by FSH. On the other hand, the decay curves for LH-R mRNA transcript in primary granulosa cells showed a significant increase in half-life after the addition of TGFbeta.

Post-transcriptional regulation of luteinizing hormone receptor mRNA in the ovary by a novel mRNA-binding protein

Luteinizing hormone (LH) receptor mRNA is post-transcriptionally regulated. An ovarian cytosolic LH receptor mRNA-binding protein (LRBP) identified in our laboratory binds to a polypyrimidine-rich bipartite sequence in the coding region of LH receptor mRNA. The present studies show a role for LRBP in the regulation of LH receptor mRNA. We demonstrated that increased LH receptor mRNA degradation occurs during hormone-induced LH receptor down-regulation. Furthermore, increased degradation of LH receptor mRNA was seen when partially purified LRBP was included in an in vitro mRNA decay reaction. The LH receptor mRNA binding activity of LRBP measured by RNA electrophoretic mobility shift analysis showed an inverse relationship to LH receptor mRNA levels during different physiological states. These results suggest that LRBP is a physiological regulator of LHR mRNA expression in the ovary and provides a novel mechanism for the regulation of LH receptor expression in the ovary.

The lutropin/choriogonadotropin receptor, a 2002 perspective

Reproduction cannot take place without the proper functioning of the lutropin/choriogonadotropin receptor (LHR). When the LHR does not work properly, ovulation does not occur in females and Leydig cells do not develop normally in the male. Also, because the LHR is essential for sustaining the elevated levels of progesterone needed to maintain pregnancy during the first trimester, disruptions in the functions of the LHR during pregnancy have catastrophic consequences. As such, a full understanding of the biology of the LHR is essential to the survival of our species. In this review we summarize our current knowledge of the structure, functions, and regulation of this important receptor.

Gonadotropins decrease estrogen receptor-beta messenger ribonucleic acid stability in rat granulosa cells

We have previously shown that the preovulatory LH surge down-regulates estrogen receptor-beta (ERbeta) messenger RNA (mRNA) levels selectively in the granulosa cells of preovulatory follicles. To gain insight into the underlying mechanisms, we examined whether the LH-induced loss of ERbeta mRNA expression in rat granulosa cells is attributable to the hormone-induced changes at the level of transcription and/or mRNA degradation. When the rate of ERbeta gene transcription was assessed in cultured granulosa cells, by nuclear run-off assays, we observed only a marginal effect of hCG on ERbeta gene transcription. In contrast, when ERbeta mRNA levels were estimated in granulosa cells that were cultured in the presence of 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), an RNA synthesis inhibitor, we observed a significant inhibitory effect of human CG (hCG) on ERbeta mRNA expression at a magnitude similar to that observed in the absence of DRB. Forskolin (FSK) and 2-O-tetradecanol-phorbol-13-acetate (TPA), pharmacological agents that mimic LH actions in granulosa cells, also showed similar effects. Thus, these results suggest that LH decreases ERbeta mRNA expression in the granulosa cells of preovulatory follicles, primarily by destabilizing the preexisting ERbeta mRNA. We next determined the decay rate of the ERbeta mRNA in granulosa cells that were cultured in the presence of DRB and additional hCG, FSK, or TPA for various time periods, by estimating ERbeta mRNA levels, using semiquantitative RT-PCR assays and subsequent linear regression analyses. The half-life of the ERbeta mRNA in the presence of vehicle was 17.87 +/- 1.2 h (n = 4). hCG dramatically decreased the half-life of the ERbeta mRNA (4.85 +/- 0.49 h, n = 4). Similarly, both FSK and TPA decreased the half-life of the ERbeta mRNA to 3.57 +/- 0.31 h and 4.02 +/- 0.13 h, respectively. We extended these findings by examining whether the LH-induced down-regulation of the ERbeta mRNA is cycloheximide-sensitive. When granulosa cells were cultured in the presence of cycloheximide, a protein synthesis inhibitor, the inhibitory effects of hCG, FSK, and TPA on ERbeta mRNA levels were abolished. Similar results were obtained in the presence or absence of DRB, indicating that the hormone-induced destabilization of the ERbeta mRNA is coupled with translation processes. Taken together, our results demonstrate that LH decreases ERbeta mRNA expression, predominantly at the posttranscriptional level, in a cycloheximide-sensitive manner.

Cyclic nucleotide regulation of Na+/glucose cotransporter (SGLT1) mRNA stability. Interaction of a nucleocytoplasmic protein with a regulatory domain in the 3'-untranslated region critical for stabilization

Expression of the Na(+)-coupled glucose cotransporter SGLT1 is regulated post-transcriptionally at the level of mRNA stability. We have previously demonstrated that cAMP-dependent stabilization of the SGLT1 message was correlated with the protein phosphorylation-dependent binding of cytoplasmic proteins to a uridine-rich sequence (URE) in the 3'-untranslated region (UTR). In the present study, the regulatory role of the URE was demonstrated by inserting it into the 3'-UTR of a beta-globin reporter minigene under the control of the tetracycline-regulated promoter. The resultant chimeric globin/SGLT1 mRNA expressed after transfection into LLC-PK1 cells exhibited a decreased half-life compared with the beta-globin control, indicating that the URE serves a destabilizing function. Activation of protein kinase A stabilized the chimeric message but not the beta-globin control, indicating the presence of a regulatory stabilizing sequence within the URE. A 38-kDa nucleocytoplasmic protein was identified that recognized a 12-nucleotide binding site within the URE. A mutation in this binding site that prevented protein binding assayed in vitro by UV cross-linking also prevented protein kinase A-dependent stabilization of the chimeric message assayed in vivo. These findings identify the interaction between a 38-kDa nucleocytoplasmic protein and a regulatory uridine-rich sequence in the 3'-UTR as critical for cAMP-mediated SGLT1 message stabilization.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin on the expression of luteinizing hormone receptors during cell differentiation in cultured granulosa cells

Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD) is a common environmental pollutant causing public concern. By use of a cell culture system derived from rat granulosa cells that provides unique advantages for studying the molecular mechanisms underlying the action of TCDD, the influence of TCDD on luteinizing hormone receptor (LHR) induction was examined. Treatment with follicle-stimulating hormone (FSH) produced, as expected, a substantial increase in specific LHR expression; concurrent treatment with TCDD (10 pM) resulted in a significant decrease in LHR after 24 h. Cotreatment with 30 ng/ml FSH and increasing doses of TCDD inhibited the levels of FSH-induced LHR mRNA in a dose-dependent manner, and 1 pM TCDD inhibited FSH-induced LHR significantly after 48 h. The rate of LHR mRNA gene transcription, assessed by nuclear run-on transcription assay, was found to decrease after addition of TCDD. The decay curves for the 5.4-kb LHR mRNA transcript showed a significant decrease after addition of TCDD.

A role of insulin-like growth factor I for follicle-stimulating hormone receptor expression in rat granulosa cells

The present study was undertaken to identify the mechanisms underlying the effect of insulin-like growth factor I (IGF-I) on FSH receptor (FSHR) in rat granulosa cells. Treatment with FSH produced a substantial increase in FSHR mRNA level, as was expected, while concurrent treatment with increasing concentrations of IGF-I brought about dose-dependent increases in FSH-induced FSHR mRNA, with a maximal response 2.8-fold greater than that induced by FSH alone. IGF-I, either alone or in combination with FSH, did not affect intracellular cAMP levels, whereas it enhanced the effect of 8-bromo (Br)-cAMP on FSHR mRNA production. Taken together, these findings suggest that the ability of IGF-I to enhance FSH action concerning the induction of FSHR is exerted at sites distal to cAMP generation. We then investigated whether the effect of IGF-I and FSH on FSHR mRNA levels was the result of increased transcription and/or altered mRNA stability. The rates of FSHR mRNA gene transcription, assessed by nuclear run-on transcription assay, were not increased by the addition of IGF-I. On the other hand, the decay curves for the 2. 4-kilobase (kb) FSHR mRNA transcript in primary granulosa cells significantly altered the slope of the FSHR mRNA decay curve in the presence of IGF-I and increased the half-life of the FSHR mRNA transcript. These data suggest a possible role for changes in FSHR mRNA stability in the IGF-I-induced regulation of FSHR in rat granulosa cells. Treatment with activin produced a substantial increase in FSHR mRNA level, as was expected, and concurrent treatment with IGF-I did not affect activin-induced FSHR mRNA. Our data suggest that the IGF-I effect on FSHR expression is related to cAMP production induced by FSH and may maintain FSHR mRNA level because of prolonged FSHR mRNA stability.

Retinoic acid (RA) represses follicle stimulating hormone (FSH)-induced luteinizing hormone (LH) receptor in rat granulosa cells

The present study was undertaken to identify the mechanisms underlying the effect of retinoic acid (RA) on the luteinizing hormone receptor (LH-R) in rat granulosa cells. Treatment with FSH produced a substantial increase in LH-R mRNA level, as was expected, while concurrent treatment with increasing concentrations of RA brought about dose-dependent decreases in FSH-induced LH-R mRNA. RA, either alone or in combination with FSH, did not affect intracellular cAMP levels, while it inhibited the effect of 8-Br-cAMP on LH-R mRNA production. Whether the effect of RA and FSH on LH-R mRNA levels was the result of decreased transcription and/or altered mRNA stability was also investigated. The rate of LH receptor mRNA gene transcription, assessed by nuclear run-on transcription assay, was inhibited by the addition of RA. The effect of RA on LH-R mRNA stability was determined by measuring the decay of LH receptor mRNA under conditions known to inhibit transcription. The decay curves for the 5.4-kb LH-R mRNA transcript showed a significant decrease after the addition of RA. It may be possible that RA not only inhibits FSH-induced transcription but also stimulates the production of destabilizing factors for the LH-R mRNA. These findings assist in understanding the molecular mechanism underlying the effect of RA on reproductive function in rat granulosa cells.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Cyclic nucleotide regulation of PAI-1 mRNA stability. Identification of cytosolic proteins that interact with an a-rich sequence

Incubation of HTC rat hepatoma cells with the cyclic nucleotide analogue 8-bromo-cAMP results in a 3-fold increase in the rate of degradation of type-1 plasminogen activator-inhibitor (PAI-1) mRNA. Previous studies utilizing HTC cells stably transfected with beta-globin:PAI-1 chimeric constructs demonstrated that at least two regions within the PAI-1 3'-untranslated region mediate the cyclic nucleotide-induced destabilization of PAI-1 mRNA; one of these regions is the 3'-most 134 nucleotides (nt) of the PAI-1 mRNA (Heaton, J. H., Tillmann-Bogush, M., Leff, N. S., and Gelehrter, T. D. (1998) J. Biol. Chem. 273, 14261-14268). In the present study, ultraviolet cross-linking analyses of this region demonstrate HTC cell cytosolic mRNA-binding proteins ranging from 38 to 76 kDa, with a major complex migrating at approximately 50 kDa. RNA electrophoretic mobility shift analyses demonstrate high molecular weight multiprotein complexes that specifically interact with the 134-nt cyclic nucleotide-responsive sequence. The 50, 61, and 76 kDa and multiprotein complexes form with an A-rich sequence at the 3' end of the cyclic nucleotide-responsive region; a 38-kDa complex forms with a U-rich region at the 5' end of the 134 nt sequence. Mutation of the A-rich region prevents both the binding of the 50-, 61-, and 76-kDa proteins and formation of the multiprotein complexes, as well as cyclic nucleotide-regulated degradation of chimeric globin:PAI-1 transcripts in HTC cells. These data suggest that the proteins identified in this report play an important role in the cyclic nucleotide regulation of PAI-1 mRNA stability.