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

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.

The role of cyclic AMP response element binding protein in transactivation of scavenger receptor class B type I promoter in transfected cells and in primary cultures of rat theca-interstitial cells

In the ovary, lutropin (LH) stimulates the selective uptake and transport of cholesterol for steroid biosynthesis from HDL particles via the scavenger receptor class B type I (SR-BI). Furthermore the expression of SR-BI mRNA in the ovary is stimulated by LH and cyclic AMP (cAMP). Since the promoter of the rat SR-BI gene is devoid of consensus cyclic AMP response element (CRE) sequences, this study examined if cAMP response element binding protein (CREB) plays a role in the transactivation of SR-BI promoter (SR-BIpr). The transactivation of SR-BIpr was examined in transfected 293T cells and human granulosa SVOG-4o cells, and in primary cultures of rat theca-interstitial cells infected with adenoviral constructs containing the SR-BIpr and a luciferase reporter gene. Dose-related increases in SR-BRpr activity ranging from 2- to 4-fold was induces by 293T cells co-transfected with the catalytic subunit of protein kinase A (cPKA). Co-transfections with CREB and cPKA produced a concentration-dependent increase ranging from 6- to 32-fold. The cAMP-mediated transactivation was significantly attenuated by co-transfection with CREB M1, a non-phosphorylatable, dominant-negative form of CREB. An increase in transactivation of SR-BIpr activity was also seen in SVOG-4o cells co-transfected with CREB. In primary cultures of rat theca-interstitial (T-I) cells infected with an adenoviral construct of SR-BIpr, forskolin produced a marked increase in promoter activity. These data indicate that stimulation of the cAMP-PKA-CREB pathway enhances rat SR-BIpr activity and substantiate the role of CREB as an intermediary in this process. The absence of canonical CRE sequences in the rat SR-BIpr suggests that the activation of SR-BI by CREB may occur either through non-canonical CRE sequences or through additional transcription factors that cooperate with CREB in the activation of SR-BI promoter activity.

Non-lactational traits of importance in dairy cows and applications for emerging biotechnologies

Dairy cattle have traditionally been selected for their ability to produce milk and milk components. The traditional single-minded approach to selection of dairy cattle has now changed and secondary traits are being included in selection indices by decreasing the emphasis on production. Greater emphasis on non-production traits reflects the industry's desire for functional dairy cattle. Six broad categories of non-lactational traits are discussed in this review. They are: type; growth, body size and composition; efficiency of feed utilisation; disease resistance, e.g. udder health as measured by somatic cell score; reproduction; and management. Most of these traits can be found within selection indices worldwide, although relative emphasis varies. The non-lactational traits mentioned above are quantitative, meaning that the phenotype in the whole animal represents the sum of lesser traits that cannot be easily measured. The physiological mechanisms that underlie quantitative traits are extremely complex. Genetic selection can be applied to quantitative traits but it is difficult to link successful genetic selection with the underlying physiological mechanisms. The importance that the bovine genome sequence will play in the future of the genetics of dairy cattle cannot be understated. Completing the bovine genome sequence is the first step towards modernising our approach to the genetics of dairy cattle. Finding genes in the genome is difficult and scanning billions of base pairs of DNA is an imperfect task. The function of most genes is either unknown or incompletely understood. Combining all of the information into a useable format is known as bioinformatics. At the present time, our capacity to generate information is great but our capacity to understand the information is small. The important information resides within subtle changes in gene expression and within the cumulative effect that these have. Traditional methods of genetic selection in dairy cattle will be used for the foreseeable future. Most non-lactational traits are heritable and will be included in selection indices if the traits have value. The long-term prognosis for genome science is good but advances will take time. Genetic selection in the genome era will be different because DNA sequence analysis may replace traditional methods of genetic selection.

Chimeric GnRH-LH receptors and LH receptors lacking C-terminus palmitoylation sites do not localize to plasma membrane rafts

Luteinizing hormone and gonadotropin releasing hormone receptors (LHR and GnRHR, respectively) are G protein-coupled receptors with important functions in reproduction. We have developed chimeric GnRHR-LHR that contain the full GnRHR coupled to various forms of the LH receptor C-terminus to explore the role of the LH receptor C-terminus in raft localization of the receptor and signaling. Addition of the full-length LHR C-terminus to GnRHR resulted in localization of the resting chimeric receptor in the bulk membrane rather than plasma membrane rafts as has been reported for the wild-type GnRHR [A. Navratil, S. Bliss, K. Berghorn, J. Haughian, T. Farmerie, J. Graham, C. Clay, M. Roberson, Constitutive localization of the gonadotropin-releasing hormone (GnRH) receptor to low density membrane microdomains is necessary for GnRH signaling to ERK, J. Biol. Chem. 278 (2003) 31593-31602]. With truncation of the LHR C-terminus, approximately 3% of chimeric receptors appeared in low density membrane fractions. Palmitoylation of sites on the LHR C-terminus appears important for raft localization. Mutations to C-terminus palmitoylation sites eliminated translocation of LH receptors from the bulk membrane to rafts upon binding of hCG although these mutant receptors retained the ability to signal via cAMP.

N-linked glycosylation facilitates processing and cell surface expression of rat luteinizing hormone receptor

The extracellular domain of the luteinizing hormone (LH) receptor has six potential N-linked glycosylation sites. Although previous studies have shown that mutation of the first three sites results in decreased ligand binding at the cell surface, the role of glycosylation in LH receptor processing is not understood. In the present study, we examined whether mutation of the first three sites has any affect on receptor synthesis, processing, and degradation of the mutant receptors. The data show that mutation of N77, N152, or N173 did not affect receptor synthesis, but did significantly reduce processing of the receptor precursor to the mature, cell surface form. Furthermore, defective processing was due to increased degradation of the precursor rather than increased turnover of cell surface receptors. Thus, lack of glycosylation decreases LH receptor processing and targets the receptors for degradation thereby leading to decreased cell surface expression. These results show that glycosylation of the LH receptor plays an important role in receptor processing and cell surface expression.

Evidence that palmitoylation of carboxyl terminus cysteine residues of the human luteinizing hormone receptor regulates postendocytic processing

Palmitoylation is a well-conserved posttranslational modification among members of the G protein-coupled receptor superfamily. The present study examined the role of palmitoylation in endocytosis and postendocytic trafficking of the human LH receptor (LHR). Palmitoylation of the LHR was determined by incorporation of [3H]palmitic acid into wild-type (WT) or mutant receptor in which the potential palmitoylation sites, C643 and C644, were mutated to glycine residues. The WT receptor showed incorporation of [3H]palmitic acid into the mature 90-kDa form of the receptor whereas mutation of the two Cys residues abrogated this incorporation, indicating that Cys 643 and C644 are the sites of palmitoylation. The role of palmitoylation on endocytosis and postendocytic processing was examined by testing the ability of the WT and mutant receptor to undergo internalization, recycling, and lysosomal degradation. Compared with the WT receptor, the mutant receptor showed increased internalization and decreased recycling, suggesting that retention of palmitic acid residues at Cys 643 and 644 promotes LHR recycling. The role of palmitoylation on receptor recycling was substantiated by demonstrating that a different mutant, D578H LHR, which is deficient in palmitoylation, also recycled less efficiently. Furthermore, the data show that palmitoylation, not the rate of internalization, determines the efficiency of recycling. The present study shows that palmitoylation of cysteine residues 643 and 644 of the human LHR is a determinant of recycling.

Steroid hormones regulate gene expression posttranscriptionally by altering the stabilities of messenger RNAs

Hormones exert powerful effects on reproductive physiology by regulating gene expression. Recent discoveries in hormone action emphasize that regulation of gene expression is not restricted to their alterations of the rate of gene transcription. On the contrary, hormonal effects on the stability of a specific mRNA can profoundly alter its steady-state concentration. The mRNAs encoding hormone receptors are commonly regulated by their own hormones to create autoregulatory feedback loops. Negative and positive autoregulatory feedback loops serve to limit or augment hormonal responses, respectively. After introducing the topics of mRNA degradation and regulated stability, this review focuses on steroid hormone effects on mRNA stabilities. Autoregulation of the mRNAs encoding estrogen, progesterone, androgen, and glucocorticoid receptors by the steroid hormones in reproductive tissues is discussed. In addition, steroid hormone effects on the stabilities of many other mRNAs that are important to reproductive biology are reviewed. These include mRNAs that encode gonadotropin hormones, integrins, growth factors, and inflammatory response proteins. Through these posttranscriptional effects, steroid hormones impact the expression of a large population of genes. Studies of the molecular mechanisms of hormonally regulated mRNA stabilities continue to identify critical mRNA sequence elements and their interactions with proteins. Increased understanding of how hormones affect mRNA stability may yield novel approaches to the therapeutic control of hormone effects, including those essential to reproductive physiology in animals.

Zebrafish gonadotropins and their receptors: I. Cloning and characterization of zebrafish follicle-stimulating hormone and luteinizing hormone receptors--evidence for their distinct functions in follicle development

In the present study, we cloned and characterized zebrafish FSH receptor (Fshr) and LH receptor (Lhr). Both fshr and lhr were abundantly expressed in the zebrafish gonads; however, they could also be detected in the kidney and liver, respectively. When overexpressed in mammalian cell lines together with a cAMP-responsive reporter gene, zebrafish Fshr responded to goldfish pituitary extract but not hCG, whereas Lhr could be activated by both. It was further demonstrated that Fshr was specific to bFSH, while Lhr could be stimulated by both bovine FSH and LH. Low level of fshr expression could be detected in the immature ovary, but the level steadily increased during vitellogenesis of the first cohort of developing follicles. In contrast, the expression of lhr could barely be detected in the immature ovary, but it became detectable at the beginning of vitellogenesis and steadily increased afterward with the peak level reached at the full-grown stage. At the follicle level, the expression of fshr was very weak in the follicles of primary growth stage but significantly increased with the follicles entering vitellogenesis. However, after reaching the maximal level in the midvitellogenic follicles, the level of fshr expression dropped slightly but significantly at the full-grown stage. In comparison, the expression of lhr obviously lagged behind that of fshr. Its expression became detectable only when the follicles started to accumulate yolk granules, but the level rose steadily afterward and reached the peak at the full-grown stage before oocyte maturation. These results suggest differential roles for Fshr and Lhr in zebrafish ovarian follicle development.

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.

Review on testicular development, structure, function, and regulation in common marmoset

BACKGROUND

The common marmoset (Callithrix jacchus) is a New World primate that has been used increasingly in toxicological evaluations including testing for testicular toxicity of pharmaceutical and environmental chemicals. Information on structural and functional characteristics of the testis in common marmosets ("marmoset" in this review) is critical for designing experiments, interpreting data collected, and determining relevance to humans in risk assessment.

METHODS

This study provides a comprehensive review on testicular development, structure, function, and regulation in common marmosets.

RESULTS

There is little information regarding testicular formation and development during gestation. Based on the overall pattern of embryonic development in marmosets, it is postulated that gonadal formation and testicular differentiation most likely takes place during gestational Week 6-12. After birth, the neonatal period of the first 2-3 weeks and the pubertal period from Months 6-12 are critical for establishment of spermatogenesis in the adult. In the adult, a nine-stage model has been used to describe the organization of seminiferous epithelium and multiple stages per tubular cross-section have been observed. Seminiferous epithelium is organized in a wave or partial-wave manner. There are on average two stages per cross-section of seminiferous tubules in adult marmoset testis. Sertoli cells in the marmoset have a uniform morphology. Marmoset spermatogenesis has a high efficiency. The prime determinant of germ cell production is proliferation and survival of spermatogonia. Sertoli cell proliferation during the neonatal period is regulated by follicle-stimulating hormone (FSH), but chorionic gonadotropin (CG), instead of luteinizing hormone (LH), is the only gonadotropin with luteinizing function in marmoset. The receptor gene for CG in marmoset is unique in that it does not have exon 10. Marmosets have a "generalized steroid hormone resistance," i.e., relatively high levels of steroid hormones in circulation and relatively low response to exogenous steroids. Blockage of FSH, CG, and testosterone production during the first 3 months after birth does not cause permanent damage to the male reproductive system. Initiation of spermatogenesis in the marmoset requires unique factors that are probably not present in other mammals. Normal male marmosets respond to estradiol injection positively (increased LH or CG levels), a pattern seen in normal females or castrated males, but not usually in normal males of other mammalian species.

CONCLUSIONS

It seems that the endocrine system including the testis in marmosets has some unique features that have not been observed in rodents, Old World primates, and humans, but detailed comparison in these features among these species will be presented in another review. Based on the data available, marmoset seems to be an interesting model for comparative studies. However, interpretation of experimental findings on the testicular effects in marmosets should be made with serious caution. Depending on potential mode of testicular actions of the chemical under investigation, marmoset may have very limited value in predicting potential testicular or steroid hormone-related endocrine effects of test chemicals in humans.