The 5'-flanking region of the rat luteinizing hormone/chorionic gonadotropin receptor gene confers Leydig cell expression and negative regulation of gene transcription by 3',5'-cyclic adenosine monophosphate

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.

G protein-coupled receptors as targets for prolactin actions

Prolactin (PRL) is known to be involved in a wide range of biological functions including osmoregulation, lactation, reproduction, and immunomodulation. The first step in PRL action involves its interaction with a specific membrane receptor that belongs to the cytokine receptor superfamily. In spite of the lack of a kinase domain, receptors of the cytokine superfamily induce tyrosine phosphorylation of cellular substrates including the receptors. The role of PRL in female reproductive functions is well known and a direct effect on ovarian and testicular steroidogenesis has been established. In the ovary, PRL binds to a specific membrane receptor and exerts an inhibitory effect on follicular steroidogenesis. This effect is the result of an impairment involving FSH stimulation of G protein-coupled receptors (GPCR) and cyclic AMP-mediated activation of aromatase cytochrome P450 gene expression. This observation may indicate a direct connection between tyrosine phosphorylation and follicle-stimulating hormone (FSH) receptor (FSHR) transduction pathways, as is the case for growth factor receptors with intrinsic tyrosine kinase activity, which share several downstream signaling elements with GPCRs. Some studies leading to our understanding of these pathways are reviewed.

Analysis of the promoter of the luteinizing hormone/human chorionic gonadotropin receptor gene in neuroendocrine cells

We investigated the molecular basis of luteinizing hormone (LH)/human chorionic gonadotropin (hCG) receptor gene transcription in immortalized alphaT3 gonadotropes, hypothalamic GT1-7 and hippocampal HN33p neurons. Nuclear run-on transcription, as well as transfection assays with fusion constructs of luciferase and the 5'-flanking region of LH/hCG receptor gene, revealed that GT1-7 neurons transcribe more than the alphaT3 or HN33p cells. Transient transfection of truncated reporter gene constructs and gel mobility shift assays revealed that while all neuroendocrine cells use the same promoter, they contain different levels of promoter binding proteins. Higher levels of these proteins may explain increased transcription of the LH/hCG receptor gene in GT1-7 neurons compared with alphaT3 and HN33p cells.

The luteinizing hormone receptor

The luteinizing hormone receptor (LHR) is a member of the subfamily of glycoprotein hormone receptors within the superfamily of G protein-coupled receptor (GPCR)/seven-transmembrane domain receptors. Over the past eight years, major advances have been made in determining the structure and function of the LHR and its gene. The hormone-binding domain has been localized to exons 1-7 in the extracellular (EC) domain/region of the receptor, which contains several leucine-rich repeats. High-affinity binding of LH and human chorionic gonadotrophin (hCG) causes secondary hormone or receptor contacts to be established with regions of the EC loop/transmembrane module that initiate signal transduction. Models of hormone-receptor interaction have been derived from the crystal structures of hCG and of the ribonuclease inhibitor, which also contains leucine-rich repeats. Such models provide a framework for the interpretation of mutational studies and for further experiments. The extracellular domain of the receptor has been overexpressed in vitro, which will facilitate crystallographic resolution of the structure of the receptor-binding site. The transmembrane domain/loop/cytoplasmic module transduces the signal for coupling to G proteins. Several constitutive, activating mutations that cause human disease have been found in helix VI and adjacent structures. These mutations have provided valuable information about mechanisms of signal transfer and G protein coupling. The structure of the LHR gene has been elucidated, and the regulation of its transcription is beginning to be understood. Valuable insights into receptor evolution have been derived from analysis of sequence homologies, the gene structure of glycoprotein hormone receptors and other members of the GPCR family, and the glycoprotein hormone receptor-like precursors identified in several invertebrate species.

Effect of phorbol ester on the regulation of LH/hCG receptors

Granulosa cells have been used to study the regulation of LH/hCG receptor protein and mRNA expression. Phorbol 12-myristate 13-acetate (PMA) dose-dependently attenuates the increases in LH/hCG receptor mRNA and protein induced by FSH and forskolin (FSK). The presence of PMA caused a decrease in cAMP production stimulated by FSH and FSK. These results suggest that PMA-mediated decreases in cAMP are a major factor in PMA-mediated decreases in LH/hCG receptor mRNA. On the other hand, in the presence of 8-Br-cAMP, PMA significantly increased LH/hCG receptor mRNA and protein, with maximal stimulation between PMA concentrations of 3 to 30 nM (1.5 fold) with 8-Br-cAMP. These findings suggest that activation of protein kinase C by PMA attenuates the increase in cAMP accumulation induced by FSH but enhances the effect of cAMP on LH/hCG receptor expression, and that the inhibitory and stimulatory effects of PMA on LH/hCG receptor content are correlated with regulation of LH/hCG receptor mRNA levels. Since the half-life study revealed no change in the stability of the LH/hCG receptor mRNA following PMA treatment, a change in the rate of LH/hCG receptor gene transcription must be responsible for the change in the LH/hCG receptor mRNA levels.

Structural organization and characterization of the promoter region of the rat gonadotropin-releasing hormone receptor gene

The gene encoding the rat gonadotropin-releasing hormone (GnRH) receptor was isolated, and its structural organization and promoter region were characterized. The gene was found to consist of three exons that encode the receptor protein, and spanned about 20 kb. Of two genomic clones analyzed, one contained the 5'-untranslated region and the first exon, and the other contained the second and third exons. The sizes of the first, second, and third exons are 625, 217, and 1476 nt, respectively. The first intron is at least 12 kb in length and is located between nucleotides 522 and 523 of the cDNA reading frame, in the middle of the fourth transmembrane domain. The second intron is about 2.5 kb and is also located in the reading frame between nucleotides 739 and 740, separating the fifth and sixth transmembrane domains. Genomic blots in combination with cloning and sequencing suggested that a single GnRH receptor gene is present in the rat genome. Primer extension indicated that the transcription start site is located 103 nt upstream of the translational start codon. A putative TATA box is positioned 23 nt in front of the transcription initiation site. The 1.8 kb 5' flanking sequence contains an SF-1 site, an AP-1 site, CCAAT sequences, a Pit-1 binding site, and a potential CRE-like sequence. To evaluate promoter activity, the 1.8 kb and two 5' deleted fragments of 1.2 and 0.6 kb were fused to the luciferase reporter gene and transiently expressed in immortalized pituitary gonadotrophs (alphaT3-1 cells) and hypothalamic neurons (GT1-7 cells), and in nonpituitary (COS-7) cells. Luciferase gene expression was significantly increased by all three fragments in pituitary and hypothalamic cells, but not in COS-7 cells. The promoter activity of the 1.2 kb fragment was higher than that of the other fragments. Forskolin and cAMP analogs increased luciferase gene expression in both alphaT3-1 and GT1-7 cells, but activation of protein kinase C by phorbol myristate acetate had no effect. These studies indicate that positive and negative regulatory elements are present within the 1.8 kb 5' flanking sequence of the GnRH receptor. Knowledge of the genomic organization and analysis of the promoter region of the rat GnRH receptor gene will facilitate the elucidation of its transcriptional control in pituitary gonadotrophs and hypothalamic neurons.

Regulation of function of the murine luteinizing hormone receptor promoter by cis- and trans-acting elements in mouse Leydig tumor cells

The transcriptional activity of various lengths of the 5'-untranslated region (UTR) of the murine LH receptor (R) gene were studied using the luciferase reporter gene in transiently transfected mouse Leydig tumor cells (mLTC-1). Chinese hamster ovarian (CHO) and HeLa cells were used as controls. The basal transcriptional promoter activity in mLTC-1 cells resided within the first 173 base pairs (bp) of the 5'-UTR. Placing an LHR promoter fragment (bases -715/ -56) in front of the Herpes simplex virus thymidine kinase (TK) minimal promoter resulted in a 7-fold increase in luciferase activity. Deletion of bases -56 to -173 of the above construct totally abolished the increased luciferase activity, brought about by the LHR promoter sequences. Basically similar results on LHR promoter function were observed using CHO cells. In contrast, no LHR promoter activity was detected in HeLa cells, indicating a cell specific nature of its function. The first 173 bp promoter domain is GC-rich, with several SP-1 binding domains, and it bound specifically nuclear proteins isolated from mLTC-1 and HeLa cells. RNAse protection assays reconfirmed the presence of several transcription initiation sites within the first 310 bp of the 5'-UTR, also in the absence of the cognate LHR coding sequences. The most distal site at bp -310 did not function in the absence of the first 173 bp of the 5'-UTR. Other transcription initiation sites were identified closer to the translation initiation site. hCG (50 micrograms/l), 8-bromo (Br)-cAMP (100 mumol/l) and cholera toxin (100 microgram/l) displayed qualitatively similar negative effects on the LHR promoter activity in the transfected mLTC-1 cells when the constructs containing at least the first 565 bp of the LHR 5'-UTR were used, but the inhibitory effects were greatly decreased in constructs containing < or = 304 bp of the promoter region. Hence, the hCG/cAMP associated inhibitory effects interact with region(s) located mainly between bp -565 and -305 of the LHR promoter. The inhibitory role of cAMP on LHR gene expression was also confirmed at the level of hCO-binding and hCG stimulated cAMP production of mLTC-1 cells. In conclusion, the current results elucidate the cis- and trans-acting elements in the regulation of expression of the murine LHR gene in cultured mouse Leydig cells. The minimal basal promoter activity is within the first 173 nucleotides of the 5'-UTR and the structural elements of the negative LHR regulation by the cognate hormone and elevated cAMP levels are mainly located within nucleotides -305 to -565 of the 5'-UTR. The function of the murine LHR promoter is similar to, though not identical with that of the rat, but at variance with that of the human LHR gene.

Structure of the human luteinizing hormone-choriogonadotropin receptor gene: unusual promoter and 5' non-coding regions

The complete organization of the human luteinizing hormone-choriogonadotropin (LH/CG) receptor (LH/CGR) gene and the structure of 1591 bp of its 5' flanking region have been determined. This gene spans over 70 kbp and contains 11 exons. The first ten exons and part of the last exon encode the extracellular domain of the receptor while the transmembrane and intracellular domains are encoded by the remaining part of the last exon. The gene encodes a 701 amino acids long preprotein, contrary to a previous report of 699 amino acids. Primer extension experiments and polymerase chain reaction (PCR) mapping allowed definition of the transcription initiation site, which is located 1085 bp upstream from the initiation codon. The 5' non-coding region is thus unusually long. The promoter region which is different from the murine LH/CG receptor promoter, contains two putative TATA boxes at positions -34 and -47 and a CAAT box consensus sequence at position -89. A consensus sequence corresponding to a cAMP responsive element is found at position -697. Seven API consensus sequences are also found in the 5' flanking region of the gene. Southern blot experiments demonstrated an informative biallelic polymorphism within the human LH/CG receptor gene locus using BglII endonuclease. The cloning of the human LH/CGR gene and the determination of the organization and structure of its 5' flanking region allow the study of its hormonal, developmental and tissue-specific regulation. Primers and PCR conditions are described for the direct genomic sequencing of all the exons of the gene. This information should facilitate the study of pathological mutations of the receptor.

Endocrine, paracrine and autocrine regulation of testicular steroidogenesis

Testicular steroidogenesis takes place almost exclusively in Leydig cells. Some metabolism of the androgens produced by Leydig cells takes place in seminiferous tubules, especially in the immature animal (e.g. aromatization and 5 alpha- reduction). Luteinizing hormone (LH) is the main tropic regulator of Leydig cell function, without which quantitatively important androgen production is not possible. LH acts through a receptor that belongs to the seven times cell membrane spanning, G protein associated, receptor family, and cyclic AMP is the main second messenger of its signal transduction. Information about the involvement of other signal transduction systems in LH action has also emerged recently. The action of LH is under manyfold modulation by other hormones (e.g. prolactin, growth hormone and insulin), growth factors and bioactive peptides. In this modulation, various paracrine and autocrine mechanisms play an important role. Seminiferous tubules influence the development and function of adjacent Leydig cells through several growth factors. When germ cells are damaged, Leydig cells in the vicinity proliferate faster. Leydig cell morphology also depends on the germ cell composition in the neighbouring seminiferous tubules, and certain stages of the seminiferous epithelial cycle increase the Leydig cell capacity to produce testosterone. Also negative modulation of Leydig cells by Sertoli/germinal cell derived factors has been demonstrated. However, the physiological importance of the paracrine and modulatory influences of the different hormones and growth factors still remains obscure since almost all information has so far been obtained from in vitro studies. In the study of testicular steroidogenesis, the main switch of the function, LH action, is well known whereas the role of the "in house" circuits of paracrine and autocrine regulation remain to be elucidated.

The thyrotropin receptor

This chapter has outlined the complex process required for thyroid growth and function. Both events are regulated by TSHR via a multiplicity of signals, with the aid of and requirement for a multiplicity of hormones that regulate the TSHR via receptor cross-talk: insulin, IGF-I, adrenergic receptors, and purinergic receptors. Cross-talk appears to regulate G-protein interactions or activities induced by TSH as well as TSHR gene expression. The TSHR structure and its mechanism of signal transduction is being rapidly unraveled in several laboratories, since the recent cloning of the receptor. In addition, the epitopes for autoantibodies against the receptor that can subvert the normal regulated synthesis and secretion of thyroid hormones, causing hyper- or hypofunction, have been defined. Studies of regulation of the TSHR minimal promotor have uncovered a better understanding of the mechanisms by which TSH regulates both growth and function of the thyroid cell. A key novel component of this phenomenon involves TSH AMP positive and negative regulation of the TSHR. Negative transcriptional regulation is a common feature of MHC class I genes in the thyroid. Subversion of negative regulation or too little negative regulation is suggested to result in autoimmune disease. Methimazole and iodide at autoregulatory levels may be important in reversing this process and returning thyroid function to normal. Their action appears to involve factors that react with the IREs on both the TSHR and the TG promoter. Too much negative regulation, as in the case of ras transformation, results in abnormal growth without function. TTF-1 is implicated as a critical autoregulatory component in both positive and negative regulation of the TSHR and appears to be the link between TSH, the TSHR, TSHR-mediated signals, TG and TPO biosynthesis, and thyroid hormone formation. Differentially regulated expression of the TSHR and TG by cAMP and insulin depend on differences in the specificity of the TTF-1 site, that is, the lack of Pax-8 interactions with the TSHR, and the IRE sites. Single-strand binding proteins will become important in determining how TSHR transcription is controlled mechanistically.

Cloning and sequence analysis of the human beta 1-adrenergic receptor 5'-flanking promoter region

We present 3.1 kb of the nucleotide sequence from the 5'-flanking region of the human beta 1-adrenergic receptor gene. The first 1.0 kb upstream from the translational start site is composed of 72% G + C residues. The sequence was analyzed for the presence of transcriptional regulatory elements and contains putative thyroid hormone, glucocorticoid hormone and cAMP response elements. These putative hormone response elements support physiological evidence that thyroid and glucocorticoid hormones regulate beta 1AR function by affecting receptor expression in tissues such as heart and adipose, where beta 1-adrenergic receptors are important regulators of heart rate and lipolysis, respectively.

Recent perspectives on the molecular structure and regulation of the beta 2-adrenoceptor

The recent cloning of the beta 2-AR and other members of the adrenergic receptor family and related molecules has generated a flood of new information about the structure of this ubiquitous class of receptor molecules, and the ways in which their unique structures determine their function. Agonists interact with the receptor-binding pocket and somehow trigger conformational changes in the cytoplasmic portions of the receptor. These intracellular changes are then transmitted to the G-protein alpha-subunit. Phosphorylation of the receptor by several protein kinases and subsequent binding of additional cytosolic factors appear to be a major step in the control of receptor function. Changes in receptor gene expression which regulate receptor number and responsiveness provide another means to regulate transmembrance signalling. The realization that G-protein-coupled receptors are transcriptionally regulated by their own second messengers indicates that the parallels among the various members of this receptor family extend beyond their structural homologies, of 7 transmembrane segments and G-protein coupling, to include the genetic aspects of their regulation.