Luteinizing Hormone and Follicle-Stimulating Hormone Exhibit Different Secretion Patterns from Cultured Madin-Darby Canine Kidney Cells

Rerouting of follicle-stimulating hormone secretion and gonadal function

Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are synthesized in the same pituitary cell, i.e., gonadotrope. They both consist of a common α-subunit that is noncovalently assembled with a hormone-specific β-subunit in gonadotropes. The heterodimers exit gonadotropes through distinct modes of trafficking and secretion. The FSH is constitutively secreted, whereas LH is secreted in pulses through the regulated pathway that involves dense core granules. Based on several in vitro mutagenesis studies, the carboxy terminus heptapeptide of human LH-β subunit is identified as a gonadotrope sorting determinant. When heptapeptide is genetically fused to human FSH-β subunit and the mutant transgene expressed on a Fshb null genetic background, the rerouted FSH mutant dimer enters the LH secretory pathway, stored in dense core granules, coreleased with LH on gonadotropin releasing hormone stimulation and rescues Fshb null mice as efficiently as the constitutively secreted wild-type FSH. The rerouted FSH markedly suppresses follicle atresia and significantly enhances ovulations per cycle and prolongs the female reproductive life span. Gonadotropin rerouting is emerging as a novel paradigm to treat ovarian dysfunction in women, and may explain the origins of ovarian cyclicity as well as provide clues to understand gene and protein networks that maintain optimal ovarian function throughout the female reproductive life span.

The recombinant equine LHβ subunit combines divergent intracellular traits of human LHβ and CGβ subunits

The pituitary LHβ and placental CGβ subunits are products of different genes in primates. The major structural difference between the two subunits is in the carboxy-terminal region, where the short carboxyl sequence of hLHβ is replaced by a longer O-glycosylated carboxy-terminal peptide in hCGβ. In association with this structural deviation, there are marked differences in the secretion kinetics and polarized routing of the two subunits. In equids, however, the CGβ and LHβ subunits are products of the same gene expressed in the placenta and pituitary (LHβ), and both contain a carboxy-terminal peptide. This unusual expression pattern intrigued us and led to our study of eLHβ subunit secretion by transfected Chinese hamster ovary and Madin-Darby canine kidney cells. In continuous labeling and pulse-chase experiments, the secretion of the eLHβ subunit from the transfected Chinese hamster ovary cells was inefficient (medium recovery of 16%-25%) and slow (t1/2 > 6.5 hours). This indicated that, the secretion of the eLHβ subunit resembles that of hLHβ rather than hCGβ. In Madin-Darby canine kidney cells grown on Transwell filters, the eLHβ subunit was preferentially secreted from the apical side, similar to the hCGβ subunit secretory route (∼65% of the total protein secreted). Taken together, these data suggested that secretion of the eLHβ subunit integrates features of both hLHβ and hCGβ subunits. We propose that the evolution of this intracellular behavior may fulfill the physiological demands for biosynthesis of the LH and CG β-subunits in the pituitary and placenta, respectively.

The role of the 3' region of mammalian gonadotropin β subunit gene in the luteinizing hormone to chorionic gonadotropin evolution

CGβ subunits comprise a unique carboxyl-terminal peptide (CTP) that has multiple O-linked glycans and extends serum half-life of the protein. It has evolved by incorporating a previously untranslated region of the LHβ gene into the reading frame. Although CTP-like sequences are encrypted in the LHβ genes of several mammals, the CGβ subunit developed only in primates and equids. To study this restriction in evolution, we examined whether the cryptic CTP decoded from the bovine LHβ gene (boCTP) possesses key characteristics of the human (h) CGβ-CTP. The boCTP does not impede several crucial aspects of hormone biosynthesis, but compared to the hCGβ-CTP, the stretch lacks O-glycans and determinants for circulatory survival. O-glycan deficiency and the associated incapacity to extend serum half-life is a major drawback of the boCTP. This may explain why LH did not evolve into CG in ruminants and consequently alternative mechanisms evolved to delay luteolysis early in gestation.

Role of cortactin in dynamic actin remodeling events in gonadotrope cells

GnRH induces marked activation of the actin cytoskeleton in gonadotropes; however, the physiological consequences and cellular mechanisms responsible have yet to be fully elucidated. The current studies focus on the actin scaffolding protein cortactin. Using the gonadotrope-derived αT3-1 cell line, we found that cortactin is phosphorylated at Y(421), S(405), and S(418) in a time-dependent manner in response to the GnRH agonist buserelin (GnRHa). GnRHa induced translocation of cortactin to the leading edge of the plasma membrane where it colocalizes with actin and actin-related protein 3 (Arp3). Incubation of αT3-1 cells with the c-src inhibitor phosphoprotein phosphatase 1, blocked tyrosine phosphorylation of cortactin, reduced cortactin association with Arp3, and blunted actin reorganization in response to GnRHa. Additionally, we used RNA silencing strategies to knock down cortactin in αT3-1 cells. Knockdown of cortactin blocked the ability of αT3-1 cells to generate filopodia, lamellipodia, and membrane ruffles in response to GnRHa. We show that lamellipodia and filopodia are capable of LHβ mobilization in primary pituitary culture after GnRHa treatment, and disruption of these structures using jasplakinolide reduces LH secretion. Collectively, our findings suggest that after GnRHa activation, src activity leads to tyrosine phosphorylation of cortactin, which facilitates its association with Arp3 to engage the actin cytoskeleton. The reorganization of actin by cortactin potentially underlies GnRHa-induced secretory events within αT3-1 cells.

Synthesis and secretion of gonadotropins including structure-function correlates

The synthesis and secretion of the gonadotropic hormones involves coordination of signal transduction, gene expression, protein translation, post-translational folding and modification and finally secretion. The production of biologically active gonadotropin thus requires appropriately folded and glycosylated subunits that assemble to form the heterodimeric hormone. Here we overview recent literature on regulation of gonadotropin subunit gene expression and current understanding of the assembly and secretion of biologically active gonadotropic hormones. Finally, we discuss the therapeutic potential of understanding glycosylation function towards designing new forms of gonadotropins based on observations of physiologically relevant parameters such as age related glycosylation changes.

LIM homeodomain transcription factor Isl-1 enhances follicle stimulating hormone-beta and luteinizing hormone-beta gene expression and mediates the activation of leptin on gonadotropin synthesis

The Lin-11, Isl-1, and Mec-3 (LIM) homeodomain transcription factor Isl-1 has been reported to be involved in pituitary development in the early stages of mouse embryogenesis. Our recent studies have shown that Isl-1 is mainly located in the pituitary gonadotropes throughout pituitary development and persists to adulthood. We still do not know the physiological functions of Isl-1 expression and its related mechanisms in the pituitary gland. The aim of the present study was to examine the hypothesis that Isl-1 is involved in regulating pituitary gonadotropin hormone (FSH/LH) production by activating FSHβ and LHβ gene expressions. We have shown that Isl-1 activates FSHβ and LHβ subunit promoters and endogenous gene transcription in LβT2 cells. In addition, Isl-1 overexpression significantly increased FSH synthesis and secretion but not LH. The actions of Isl-1 were not observed when the homeodomain or LIM1 domains are mutated. This demonstrates that Isl-1 induction of FSHβ and LHβ is by both direct and indirect binding of Isl-1 to DNA sequences. Furthermore, Isl-1 expressional level was up-regulated in LβT2 cells after exposure to GnRH, activin, and leptin. However, RNA interference-induced knockdown of Isl-1 significantly reduced the effect of leptin but did not obviously influence the stimulating effects of GnRH and activin on LH and FSH production. In conclusion, the results demonstrate that the LIM-homeodomain transcription factor Isl-1 functions to increase FSHβ/LHβ gene transcription, and mediates the effects of leptin on gonadotropin synthesis.

Secretory trafficking signal encoded in the carboxyl-terminal region of the CGbeta-subunit

Although the LHbeta- and chorionic gonadotropin-beta- (CGbeta) subunits share a high degree of sequence identity (>85%) in the first 114 amino acids, there is considerable sequence divergence at their carboxy ends. The CGbeta-subunit terminates with a unique carboxyl-terminal extension (115-145; carboxyl-terminal peptide), which contains four O-linked oligosaccharides, whereas the LHbeta-subunit bears a hydrophobic heptapeptide (115-121) at its carboxy terminus. LH is released through the regulated pathway in the pituitary, whereas CG is secreted constitutively from the placenta. We previously demonstrated in rat somatotroph-derived GH(3) cells that the LH is associated primarily with a regulated routing, and although the majority of CG was released constitutively from the cells, there was a fraction that was segregated through the regulated pathway. Moreover, we showed that the LHbeta heptapeptide is a determinant for the regulated secretion of LH. Given that the primary evolutionary change between LHbeta and CGbeta occurred at the carboxy terminus, these data suggested that the presence of the CGbeta carboxyl-terminal peptide region is responsible for the constitutive secretion of CG. A CG114 mutant (CGDeltaT) was constructed and expressed in GH(3) cells. Steady-state labeling and pulse-chase experiments demonstrated that the CGDeltaT entered the regulated pathway resulting in over 4-fold increase in the intracellular pool. The secretagogue, forskolin, stimulated CGDeltaT release over 3-fold, which was accompanied by a parallel intracellular decrease, and only marginal stimulation of CG was seen. Immunofluorescence demonstrated a unique membrane pattern of staining for CGDeltaT compared with dispersed cytoplasmic puncta for CG. Stimulation with forskolin caused a significant reduction in the relative fluorescence of CGDeltaT cells compared with a minor reduction for CG. These data show that the CGDeltaT analog resembles LH in its intracellular trafficking, further supporting the hypothesis that determinants at the carboxyl-terminal end of the CGbeta-subunit evolved from the LHbeta-subunit primarily to overcome the slow release and intracellular storage of LH resulting in rapid secretion of CG from the placenta.

Gonadotrophin-releasing hormone (GnRH) release in marmosets II: pulsatile release of GnRH and pituitary gonadotrophin in adult females

Unlike other mammals, including rodents, Old World primates and humans, common marmosets and probably all other New World primates synthesise and release chorionic gonadotrophin (CG), and not luteinising hormone (LH) from pituitary gonadotrophs. However, little is known about the physiological dynamics of gonadotrophin-releasing hormone (GnRH)-regulated CG release from gonadotrophs and whether such CG release has pulsatile release characteristics similar to those of LH in other mammalian species. Consequently, we performed a series of in vivo and in vitro studies in ovariectomised laboratory rats and female marmosets to compare GnRH-induced pituitary LH and CG release characteristics, respectively. Exogenous GnRH stimulated a slower onset of release of marmoset pituitary CG, both in vivo and in vitro, and induced an approximately 400% greater increase in the duration of marmoset pituitary CG release compared to that for rat LH. Not surprisingly, hypothalamic pulsatile release of GnRH in vivo was not obviously concordant with endogenous episodic changes in circulating levels of CG in marmosets, in contrast to the clear concordance observed between in vivo GnRH and LH release previously demonstrated in rats and other mammals. Pituitary CG release in marmosets thus demonstrates considerable divergence from the timely hypothalamic GnRH-regulated LH release in other female mammals, implying potentially different physiological dynamics in gonadotrophin regulation of marmoset ovarian function.

Strategies for construction of luteinizing hormone beta subunit analogs with carboxyl terminal extensions in non-primate, non-equid mammalian species

Chorionic gonadotropins (CG) are unique because they have a carboxyl terminal peptide (CTP) extension on their beta subunits that prolongs circulatory survival. CGbeta genes from the human being and horse have evolved from ancestral luteinizing hormone (LH) beta genes by different pathways that involve deletions that change the reading frames and yield a CTP. Here we further review our previous analysis, aimed at determining whether LHbeta genes in non-primate, non-equid species inherently possess DNA sequences that encode CTP-like domains. In multiple mammalian species, simple frame-shift mutations using either the human or equine CGbeta gene as a model can be used to construct LHbeta analogs with putative CTP domains. Furthermore, DNA sequences from mammalian LHbeta genes can be aligned to maximize similarity with CGbeta genes in order to devise more refined strategies for construction of CTP-bearing LHbeta analogs as exemplified in the bovine case. Thus, mammalian LHbeta genes have DNA sequences that can be potentially expressed in order to construct CTP-bearing glycoprotein hormone analogs.

The LHbeta gene of several mammals embeds a carboxyl-terminal peptide-like sequence revealing a critical role for mucin oligosaccharides in the evolution of lutropin to chorionic gonadotropin in the animal phyla

The expression of a previously untranslated carboxylterminal sequence is associated with the ancestral lutropin (LH) beta to the beta-subunit gene evolution of choriogonadotropins (CG). The peptide extension (denoted as CTP) is rich in mucin-type O-glycans and confers new hormonal properties on CG relative to the LH. Although the LHbeta gene is conserved among mammals and only a few frameshift mutations account for the extension, it is merely seen in primates and equids. Bioinformatics identified a CTP-like sequence that is encrypted in the LHbeta gene of several mammalian species but not in birds, amphibians, or fish. We then examined whether or not decoding of the cryptic CTP in the bovine LHbeta gene (boCTP) would be sufficient to generate the LHbeta species of a ruminant with properties typical to the CGbeta subunit. The mutated bovine LHbeta-boCTP subunit was expressed and N-glycosylated in transfected Chinese hamster ovary cells. However, unlike human (h) CGbeta CTP, the cryptic boCTP was devoid of mucin O-glycans. This deficiency was further confirmed when the boCTP domain was substituted for the natural CTP in the human CGbeta subunit. Moreover, when expressed in polarized Madin-Darby canine kidney cells, this hCGbeta-boCTP chimera was secreted basolaterally rather than from the apical compartment, which is the route of the wild type hCGbeta subunit, a sorting function attributed to the O-glycans attached to the CTP. This result shows that the cryptic peptide does not orientate CG to the apical face of the placenta, to the maternal circulation as seen in primates. The absence of this function, which distinguishes CG from LH, provides an explanation as to why the LHbeta to CGbeta evolution did not occur in ruminants. We propose that in primates and equids, further natural mutations in the progenitor LHbeta gene resulted in the efficient O-glycosylation of the CTP, thus favoring the retention of an elongated reading frame.