Targeted overexpression of luteinizing hormone causes ovary-dependent functional adenomas restricted to cells of the Pit-1 lineage

Two Hormones for One Receptor: Evolution, Biochemistry, Actions, and Pathophysiology of LH and hCG

LH and chorionic gonadotropin (CG) are glycoproteins fundamental to sexual development and reproduction. Because they act on the same receptor (LHCGR), the general consensus has been that LH and human CG (hCG) are equivalent. However, separate evolution of LHβ and hCGβ subunits occurred in primates, resulting in two molecules sharing ~85% identity and regulating different physiological events. Pituitary, pulsatile LH production results in an ~90-minute half-life molecule targeting the gonads to regulate gametogenesis and androgen synthesis. Trophoblast hCG, the "pregnancy hormone," exists in several isoforms and glycosylation variants with long half-lives (hours) and angiogenic potential and acts on luteinized ovarian cells as progestational. The different molecular features of LH and hCG lead to hormone-specific LHCGR binding and intracellular signaling cascades. In ovarian cells, LH action is preferentially exerted through kinases, phosphorylated extracellular-regulated kinase 1/2 (pERK1/2) and phosphorylated AKT (also known as protein kinase B), resulting in irreplaceable proliferative/antiapoptotic signals and partial agonism on progesterone production in vitro. In contrast, hCG displays notable cAMP/protein kinase A (PKA)-mediated steroidogenic and proapoptotic potential, which is masked by estrogen action in vivo. In vitro data have been confirmed by a large data set from assisted reproduction, because the steroidogenic potential of hCG positively affects the number of retrieved oocytes, and LH affects the pregnancy rate (per oocyte number). Leydig cell in vitro exposure to hCG results in qualitatively similar cAMP/PKA and pERK1/2 activation compared with LH and testosterone. The supposed equivalence of LH and hCG has been disproved by such data, highlighting their sex-specific functions and thus deeming it an oversight caused by incomplete understanding of clinical data.

Mouse models for the analysis of gonadotropin secretion and action

Gonadotropins are pituitary gonadotrope-derived glycoprotein hormones. They act by binding to G-protein coupled receptors on gonads. Gonadotropins play critical roles in reproduction by regulating both gametogenesis and steroidogenesis. Although biochemical and physiological studies provided a wealth of knowledge, gene manipulation techniques using novel mouse models gave new insights into gonadotropin synthesis, secretion and action. Both gain of function and loss of function mouse models for understanding gonadotropin action in a whole animal context have already been generated. Moreover, recent studies on gonadotropin actions in non-gonadal tissues challenged the central dogma of classical gonadotropin actions in gonads and revealed new signaling pathways in these non-gonadal tissues. In this Chapter, we have discussed our current understanding of gonadotropin synthesis, secretion and action using a variety of genetically engineered mouse models.

Mouse Models for the Study of Synthesis, Secretion, and Action of Pituitary Gonadotropins

Gonadotropins play fundamental roles in reproduction. More than 30years ago, Cga transgenic mice were generated, and more than 20years ago, the phenotypes of Cga null mice were reported. Since then, numerous mouse strains have been generated and characterized to address several questions in reproductive biology involving gonadotropin synthesis, secretion, and action. More recently, extragonadal expression, and in some cases, functions of gonadotropins in nongonadal tissues have been identified. Several genomic and proteomic approaches including novel mouse genome editing tools are available now. It is anticipated that these and other emerging technologies will be useful to build an integrated network of gonadotropin signaling pathways in various tissues. Undoubtedly, research on gonadotropins will continue to provide new knowledge and allow us transcend from benchside to the bedside.

Pituitary hyperplasia: case series and literature review of an under-recognised and heterogeneous condition

Pituitary hyperplasia (PH) occurs in heterogeneous settings and remains under-recognised. Increased awareness of this condition and its natural history should circumvent unnecessary trans-sphenoidal surgery. We performed an observational case series of patients referred to a single endocrinologist over a 3-year period. Four young women were identified with PH manifesting as diffuse, symmetrical pituitary enlargement near or touching the optic apparatus on MRI. The first woman presented with primary hypothyroidism and likely had thyrotroph hyperplasia given prompt resolution with thyroxine. The second and third women were diagnosed with pathological gonadotroph hyperplasia due to primary gonadal insufficiency, with histopathological confirmation including gonadal-deficiency cells in the third case where surgery could have been avoided. The fourth woman likely had idiopathic PH, though she had concomitant polycystic ovary syndrome which is a debated cause of PH. Patients suspected of PH should undergo comprehensive hormonal, radiological and sometimes ophthalmological evaluation. This is best conducted by a specialised multidisciplinary team with preference for treatment of underlying conditions and close monitoring over surgical intervention.

Animal models for aberrations of gonadotropin action

During the last two decades a large number of genetically modified mouse lines with altered gonadotropin action have been generated. These mouse lines fall into three categories: the lack-of-function mice, gain-of-function mice, and the mice generated by breeding the abovementioned lines with other disease model lines. The mouse strains lacking gonadotropin action have elucidated the necessity of the pituitary hormones in pubertal development and function of gonads, and revealed the processes from the original genetic defect to the pathological phenotype such as hypo- or hypergonadotropic hypogonadism. Conversely, the strains of the second group depict consequences of chronic gonadotropin action. The lines vary from those expressing constitutively active receptors and those secreting follicle-stimulating hormone (FSH) with slowly increasing amounts to those producing human choriogonadotropin (hCG), amount of which corresponds to 2000-fold luteinizing hormone (LH)/hCG biological activity. Accordingly, the phenotypes diverge from mild anomalies and enhanced fertility to disrupted gametogenesis, but eventually chronic, enhanced and non-pulsatile action of both FSH and LH leads to female and male infertility and/or hyper- and neoplasias in most of the gonadotropin gain-of-function mice. Elevated gonadotropin levels also alter the function of several extra-gonadal tissues either directly or indirectly via increased sex steroid production. These effects include promotion of tumorigenesis in tissues such as the pituitary, mammary and adrenal glands. Finally, the crossbreedings of the current mouse strains with other disease models are likely to uncover the contribution of gonadotropins in novel biological systems, as exemplified by the recent crossbreed of LHCG receptor deficient mice with Alzheimer disease mice.

Ovarian hyperstimulation syndrome caused by an FSH-secreting pituitary adenoma

BACKGROUND

A 40-year-old woman presented with galactorrhea and oligomenorrhea. She had a history of multiple ovarian cysts and pelvic pain.

INVESTIGATIONS

Laboratory evaluation included measurements of the levels of estradiol, follicle-stimulating hormone, luteinizing hormone, prolactin, thyroid-stimulating hormone, free endogenous T4, the glycoprotein hormone alpha subunit, cortisol, adrenocorticotropic hormone, and insulin-like growth factor I. Radiological studies included MRI of the pituitary.

DIAGNOSIS

Ovarian hyperstimulation syndrome caused by a pituitary adenoma, secreting follicle-stimulating hormone.

MANAGEMENT

The patient underwent trans-sphenoidal resection of the adenoma, with subsequent normalization of hormonal values and symptoms.

Ovarian hyperstimulation induces centrosome amplification and aneuploid mammary tumors independently of alterations in p53 in a transgenic mouse model of breast cancer

Aneuploidy and genomic instability are common features of human cancers, including breast cancer; however, mechanisms by which such abnormalities develop are not understood. The exquisite dependence of the mammary gland on hormones for growth and development as well as hormonal contributions to breast cancer risk and progression suggest that tumorigenic mechanisms in the breast should be considered in the context of hormonal stimulation. We used transgenic mice that overexpress luteinizing hormone with subsequent ovarian hyperstimulation as a model to identify mechanisms involved in hormone-induced mammary cancer. Tumor pathology in these mice is highly variable, suggesting individual tumors undergo distinct initiating or promoting events. Supporting this notion, hormone-induced tumors display considerable chromosomal instability and aneuploidy, despite the presence of functional p53. The presence of extensive centrosome amplification in tumors and hyperplastic glands prior to tumor formation suggests that alterations in the ovarian hormonal milieu dysregulate the centrosome cycle in mammary epithelial cells, leading to aneuploidy and cancer.

Expression of the high mobility group A family member p8 is essential to maintaining tumorigenic potential by promoting cell cycle dysregulation in LbetaT2 cells

The mechanism by which the HMGA protein p8 facilitates tumorigenesis may be cell cycle dysregulation. Control- (C) LbetaT2 cells, which express p8, form tumors at a rate five-times faster than p8-knockdown (p8-KD)-LbetaT2 cells. In association with this heightened tumorigenic potential, p8-expressing C-LbetaT2 cells avoid G(0)/G(1) arrest and become genetically unstable while p8-KD-LbetaT2 cells arrest in G(0)/G(1), become senescent upon overgrowth, and maintain a diploid population. These phenotypic changes correspond to altered cell cycle regulation at the G(1)-to-S transition that may be due to p8-mediated changes in expression of the Cip/Kip family members of cell cycle inhibitors, p21, p27, and p57.

Extragonadal LH/hCG action--not yet time to rewrite textbooks

Gonadotropins are indispensable in both sexes in the regulation of gonadal sex steroid production and gametogenesis. In addition to their well-established classical actions, numerous recent publications have indicated the presence and function of luteinizing hormone/chorionic gonadotropin receptors (LH/hCG-R) in a variety of extragonadal tissues. However, the physiological significance of such effects has remained unclear. We have generated two genetically modified mouse models, one with excessive production of hCG and the other with targeted disruption of LH/hCG-R gene, and used them to address the functions of LH and hCG. Numerous gonadal and extragonadal phenotypes were found in the models with the two extremes of LH/hCG action. However, when the extragonadal effects were scrutinized in greater detail, they all appeared to arise through modification of gonadal function, either through enhanced or inhibited response to LH/hCG stimulation. Hence, further evidence is needed before the extragonadal LH/hCG-R expression can be considered functionally significant.

Recent progress in studies of pituitary tumor pathogenesis

The mechanisms of tumorigenesis of the human pituitary have been elucidated to a limited extent. Classically, pituitary tumor formation was shown to be induced by thyroidectomy and estrogen administration. Molecular biological and immunohistochemical studies have revealed several aspects of pituitary tumorigenesis. Translineage cell differentiation has been shown to be induced by the aberrant expression of transcription factors and co-factors, such as Pit-1, Prop-1, and estrogen receptor. Defects or overexpression of cell cycle regulators, such as CDK inhibitors, PTTG, and GADD45gamma, result in the abnormal proliferation of pituitary cells. Recently, epigenetic regulation has been suggested to be related to pituitary tumor formation. This article presents a review and update of recent progress in studies of the development and differentiation of pituitary tumors.

Multiple sites of tumorigenesis in transgenic mice overproducing hCG

We have produced transgenic (TG) mice expressing under the ubiquitin C promoter either the glycoprotein hormone common alpha-subunit (C(alpha)) or human chorionic gonadotropin (hCG) beta-subunit. C(alpha) overexpression alone had no phenotypic effect, but the hCG(beta) expressing females, presenting with moderately elevated levels of bioactive LH/hCG, due to dimerization of the TG hCG(beta) with endogenous C(alpha), developed multiple gonadal and extragonadal neoplasias. Crosses of the C(alpha) and hCG(beta) mice (hCG(alpha)beta) had >1000-fold elevated hCG levels, due to ubiquitous transgene expression, and presented with more aggressive tumour formation. The ovaries displayed initially strong luteinisation of all somatic cell types, leading to formation of luteomas, and subsequently to germ cell tumours (teratomas). The pituitary glands of TG females were massively enlarged, up to >100 mg, developing macroprolactinomas with very high prolactin (PRL) production. This endocrine response probably induced breast cancers in the mice. In contrast to the females, similar high levels of hCG in male mice had only marginal effects in adulthood, with slight Leydig cell hyperplasia and atrophy in the seminiferous epithelium. However, clear Leydig cell adenomas were observed in postnatal mice, apparently originating from fetal Leydig cells. In conclusion, these studies demonstrate marked tumorigenic effects of supraphysiological hCG levels in female mice, but clear resistance to similar changes in males. The extragonadal tumours were induced by hCG stimulated aberrant ovarian endocrine function, rather than by direct hCG action, because gonadectomy prevented all extragonadal phenotypes despite persistent hCG elevation. The phenotypes of the TG mice apparently represent exaggerated responses to hCG/LH and/or gonadal steroids. It remains to be explored to what extent they simulate respective responses in humans to pathophysiological elevation of the same hormones.

Multiple and overlapping combinatorial codes orchestrate hormonal responsiveness and dictate cell-specific expression of the genes encoding luteinizing hormone

Normal reproductive function in mammals requires precise control of LH synthesis and secretion by gonadotropes of the anterior pituitary. Synthesis of LH requires expression of two genes [alpha-glycoprotein subunit (alphaGSU) and LHbeta] located on different chromosomes. Hormones from the hypothalamus and gonads modulate transcription of both genes as well as secretion of the biologically active LH heterodimer. In males and females, the transcriptional tone of the genes encoding alphaGSU and LHbeta reflects dynamic integration of a positive signal provided by GnRH from hypothalamic neurons and negative signals emanating from gonadal steroids. Although alphaGSU and LHbeta genes respond transcriptionally in the same manner to changes in hormonal input, different combinations of regulatory elements orchestrate their response. These hormone-responsive regulatory elements are also integral members of much larger combinatorial codes responsible for targeting expression of alphaGSU and LHbeta genes to gonadotropes. In this review, we will profile the genomic landscape of the promoter-regulatory region of both genes, depicting elements and factors that contribute to gonadotrope-specific expression and hormonal regulation. Within this context, we will highlight the different combinatorial codes that control transcriptional responses, particularly those that mediate the opposing effects of GnRH and one of the sex steroids, androgens. We will use this framework to suggest that GnRH and androgens attain the same transcriptional endpoint through combinatorial codes unique to alphaGSU and LHbeta. This parallelism permits the dynamic and coordinate regulation of two genes that encode a single hormone.

Reexpression of p8 contributes to tumorigenic properties of pituitary cells and appears in a subset of prolactinomas in transgenic mice that hypersecrete luteinizing hormone

Targeted overexpression of LH in transgenic mice causes hyperproliferation of Pit-1-positive pituitary cells and development of functional adenomas. To characterize gene expression changes associated with pituitary tumorigenesis, we performed microarray studies using Affymetrix GeneChips comparing expression profiles from pituitary tumors in LH-overexpressing mice to wild-type control pituitaries. We identified a number of candidate genes with altered expression in pituitary tumors. One of these, p8 (candidate of metastasis-1), encodes a native high-mobility group-like transcription factor previously shown to be necessary for ras-mediated transformation of mouse embryonic fibroblasts and also implicated in breast cancer progression. Herein, we show that expression of p8, normally quiescent in adult pituitary, localizes to tumor foci containing lactotropes, suggesting a linkage with their transformation. To further establish the functional significance of p8 in pituitary tumorigenesis, we constructed several clonal cell lines with reduced expression of p8 from a parent GH3 somatolactotrope cell line. These clonal derivates, along with the parent cell line, were tested for tumorigenicity by injection into athymic mice. When compared with wild-type GH3 with higher levels of p8, GH3 cells with reduced expression of p8 displayed attenuated tumor development or failed to develop tumors at all. Similar results were obtained with gonadotrope-derived cell lines that displayed reduced expression of p8. Together, these data suggest that maintenance of the transformed phenotype of pituitary GH3 cells requires expression of p8 and that it may play a similar role when reexpressed in a subset of lactotropes that form prolactinomas in vivo.