Estradiol inhibits growth hormone receptor gene expression in rabbit liver

Role of Estrogen and Estrogen Receptor in GH-Secreting Adenomas

Acromegaly is a rare disease with several systemic complications that may lead to increased overall morbidity and mortality. Despite several available treatments, ranging from transsphenoidal resection of GH-producing adenomas to different medical therapies, complete hormonal control is not achieved in some cases. Some decades ago, estrogens were first used to treat acromegaly, resulting in a significant decrease in IGF1 levels. However, due to the consequent side effects of the high dose utilized, this treatment was later abandoned. The evidence that estrogens are able to blunt GH activity also derives from the evidence that women with GH deficiency taking oral estro-progestins pills need higher doses of GH replacement therapy. In recent years, the role of estrogens and Selective Estrogens Receptor Modulators (SERMs) in acromegaly treatment has been re-evaluated, especially considering poor control of the disease under first- and second-line medical treatment. In this review, we analyze the state of the art concerning the impact of estrogen and SERMs on the GH/IGF1 axis, focusing on molecular pathways and the possible implications for acromegaly treatment.

An acromegaly case treated with clomiphene citrate: add-on treatment in primary medical therapy

INTRODUCTION

Acromegaly is a disease with various comorbidities and hypogonadism is a common comorbidity in patients with acromegaly. Herein, we aim to present our experience with clomiphene citrate in a patient with acromegaly accompanied by hypogonadism, who declined surgery.

CASE REPORT

A 40-year-old male patient with impaired fasting glucose, hyperlipidemia, and psychosis and who complained of increasing tongue growth, snoring, enlargement of the hands, spacing between the teeth, and loss of libido for the last 6 years was followed up. Acromegaly was diagnosed, with high levels of insulin-like growth factor-1 (IGF-1) and a pituitary neuroendocrine tumor measuring 11 mm; the patient had concomitant hypogonadism. Lanreotide was started as the initial primary medical treatment. Clomiphene citrate was added to the patient's treatment. The patient, whose IGF-1 level was high during follow-up, did not want to use the intramuscular testosterone esters for hypogonadism. In the third month of clomiphene citrate treatment, IGF-1 normalization was achieved and the patient's total testosterone level increased.

DISCUSSION

Biochemical control is not always achieved with somatostatin receptor ligands and dopamine agonists in the treatment of acromegaly. Therefore, we support the use of clomiphene citrate (CC) as a cost-effective oral add-on treatment option in selected hypogonadal acromegaly cases.

Functioning Pituitary Adenomas - Current Treatment Options and Emerging Medical Therapies

Pituitary adenomas are benign tumours comprising approximately 16% of all primary cranial neoplasms. Functioning pituitary adenomas (prolactinomas, somatotroph, corticotroph, thyrotroph and rarely gonadotroph adenomas) cause complex clinical syndromes and require prompt treatment to reduce associated morbidity and mortality. Treatment approaches include transsphenoidal surgery, medical therapy and radiation. Medical therapy is the primary therapy for prolactinomas, and surgery by a skilled neurosurgeon is the first-line approach for other functioning pituitary adenomas. A multimodal treatment is frequently necessary to achieve biochemical and clinical control, especially, when surgery is not curative or when medical therapy fails. Several emerging, novel, medical treatments for acromegaly, Cushing's disease and prolactinomas are in phase II and III clinical trials and may become effective additions to the current drug armamentarium. The availability of various management options will allow an individualised treatment approach based on the unique tumour type, clinical situation and patient preference.

A multivariable prediction model for pegvisomant dosing: monotherapy and in combination with long-acting somatostatin analogues

BACKGROUND

Effective treatment of acromegaly with pegvisomant (PEGV), a growth hormone receptor antagonist, requires an appropriate dose titration. PEGV doses vary widely among individual patients, and various covariates may affect its dosing and pharmacokinetics.

OBJECTIVE

To identify predictors of the PEGV dose required to normalize insulin-like growth factor I (IGF-I) levels during PEGV monotherapy and in combination with long-acting somatostatin analogues (LA-SSAs).

DESIGN

Two retrospective cohorts (Rotterdam + Liège Acromegaly Survey (LAS), total n = 188) were meta-analyzed as a form of external replication to study the predictors of PEGV dosing in addition to LA-SSA, the LAS (n = 83) was used to study the predictors of PEGV monotherapy dosing. Multivariable regression models were used to identify predictors of the PEGV dose required to normalize IGF-I levels.

RESULTS

For PEGV dosing in combination with LA-SSA, IGF-I levels, weight, height and age, were associated with the PEGV normalization dosage (P ≤ 0.001, P ≤ 0.001, P = 0.028 and P = 0.047 respectively). Taken together, these characteristics predicted the PEGV normalization dose correctly in 63.3% of all patients within a range of ±60 mg/week (21.3% within a range of ±20 mg/week). For monotherapy, only weight was associated with the PEGV normalization dose (P ≤ 0.001) and predicted this dosage correctly in 77.1% of all patients within a range of ±60 mg/week (31.3% within a range of ±20 mg/week).

CONCLUSION

In this study, we show that IGF-I levels, weight, height and age can contribute to define the optimal PEGV dose to normalize IGF-I levels in addition to LA-SSA. For PEGV monotherapy, only the patient's weight was associated with the IGF-I normalization PEGV dosage.

Estrogens and selective estrogen receptor modulators in acromegaly

Despite recent advances in acromegaly treatment by surgery, drugs, and radiotherapy, hormonal control is still not achieved by some patients. The impairment of IGF-1 generation by estrogens in growth hormone deficient patients is well known. Patients on oral estrogens need higher growth hormone doses in order to achieve normal IGF-1 values. In the past, estrogens were one of the first drugs used to treat acromegaly. Nevertheless, due to the high doses used and the obvious side effects in male patients, this strategy was sidelined with the development of more specific drugs, as somatostatin receptor ligands and dopamine agonists. In the last 15 years, the antagonist of growth hormone receptor became available, making possible IGF-1 control of the majority of patients on this particular drug. However, due to its high cost, pegvisomant is still not available in many centers around the world. In this setting, the effect of estrogens and also of selective estrogen receptor modulators on IGF-1 control was reviewed, and proved to be an ancillary tool in the management of acromegaly. This review describes data concerning their efficacy and place in the treatment algorithm of acromegaly.

Estrogenic compounds decrease growth hormone receptor abundance and alter osmoregulation in Atlantic salmon

Exposure of Atlantic salmon smolts to estrogenic compounds is shown to compromise several aspects of smolt development. We sought to determine the underlying endocrine mechanisms of estrogen impacts on the growth hormone (GH)/insulin-like growth factor I (IGF-I) axis. Smolts in freshwater (FW) were either injected 3 times over 10 days with 2 μgg(-1) 17β-estradiol (E2) or 150μgg(-1) 4-nonylphenol (NP). Seawater (SW)-acclimated fish received intraperitoneal implants of 30 μgg(-1) E2 over two weeks. Treatment with these estrogenic compounds increased hepatosomatic index and total plasma calcium. E2 and NP reduced maximum growth hormone binding by 30-60% in hepatic and branchial membranes in FW and SW, but did not alter the dissociation constant. E2 and NP treatment decreased plasma levels of IGF-I levels in both FW and SW. In FW E2 and NP decreased plasma GH whereas in SW plasma GH increased after E2 treatment. Compared to controls, plasma chloride concentrations of E2-treated fish were decreased 5.5mM in FW and increased 10.5mM in SW. There was no effect of NP or E2 on gill sodium-potassium adenosine triphosphatase (Na(+)/K(+)-ATPase) activity in FW smolts, whereas E2 treatment in SW reduced gill Na(+)/K(+)-ATPase activity and altered the number and size of ionocytes. Our data indicate that E2 downregulates the GH/IGF-I-axis and SW tolerance which may be part of its normal function for reproduction and movement into FW. We conclude that the mechanism of endocrine disruption of smolt development by NP is in part through alteration of the GH/IGF-I axis via reduced GH receptor abundance.

Growth hormone receptor modulators

Growth hormone (GH) regulates somatic growth, substrate metabolism and body composition. Its actions are elaborated through the GH receptor (GHR). GHR signalling involves the role of at least three major pathways, STATs, MAPK, and PI3-kinase/Akt. GH receptor function can be modulated by changes to the ligand, to the receptor or by factors regulating signal transduction. Insights on the physico-chemical basis of the binding of GH to its receptor and the stoichiometry required for activation of the GH receptor-dimer has led to the development of novel GH agonists and antagonists. Owing to the fact that GH has short half-life, several approaches have been taken to create long-acting GHR agonists. This includes the pegylation, sustained release formulations, and ligand-receptor fusion proteins. Pegylation of a GH analogue (pegvisomant) which binds but not activate signal transduction forms the basis of a new successful approach to the treatment of acromegaly. GH receptors can be regulated at a number of levels, by modifying receptor expression, surface availability and signalling. Insulin, thyroid hormones and sex hormones are among hormones that modulate GHR through some of these mechanisms. Estrogens inhibit GH signalling by stimulating the expression of SOCS proteins which are negative regulators of cytokine receptor signalling. This review of GHR modulators will cover the effects of ligand modification, and of factors regulating receptor expression and signalling.

Larval exposure to 4-nonylphenol and 17beta-estradiol affects physiological and behavioral development of seawater adaptation in Atlantic salmon smolts

Population declines of anadromous salmonids are attributed to anthropogenic disturbances including dams, commercial and recreational fisheries, and pollutants, such as estrogenic compounds. Nonylphenol (NP), a xenoestrogen, is widespread in the aquatic environment due to its use in agricultural, industrial, and household products. We exposed Atlantic salmon yolk-sac larvae to waterborne 10 or 100 microg L(-1) NP (NP-L or NP-H, respectively), 2 microg L(-1) 17beta-estradiol (E2), or vehicle, for 21 days to investigate their effects on smolt physiology and behavior 1 year later. NP-H caused approximately 50% mortality during exposure, 30 days after exposure, and 60 days after exposure. Mortality rates of NP-L and E2 fish were not affected until 60 days after treatment, when they were 4-fold greater than those of controls. Treatment with NP-L or E2 as yolk-sac larvae decreased gill sodium-potassium-activated adenosine triphosphatase (Na+,K(+)-ATPase) activity and seawater (SW) tolerance during smolt development, 1 year after exposure. Exposure to NP-L and E2 resulted in a latency to enter SW and reduced preference for SW approximately 2- and 5-fold, respectively. NP-L-exposed fish had 20% lower plasma insulin-like growth factor I (IGF-I) levels and 35% lower plasma triiodothyronine (T3). Plasma growth hormone and thyroxine (T4) were unaffected. Exposure to E2 did not affect plasma levels of IGF-I, GH, T3, or T4. Both treatment groups exhibited increased plasma cortisol and decreased osmoregulatory capacity in response to a handling stressor. These results suggest that early exposure to environmentally relevant concentrations of NP, and other estrogenic compounds, can cause direct and delayed mortalities and that this exposure can have long-term, "organizational" effects on life-history events in salmonids.

Endocrine disruption of parr-smolt transformation and seawater tolerance of Atlantic salmon by 4-nonylphenol and 17beta-estradiol

Sex steroids are known to interfere with the parr-smolt transformation of anadromous salmonids, and environmental estrogens such as nonylphenol have recently been implicated in reduced returns of Atlantic salmon in the wild. To determine the endocrine pathways by which estrogenic compounds affect smolt development and seawater tolerance, groups of juvenile Atlantic salmon were injected with one of five doses (0.5, 2, 10, 40 or 150 microg g(-1)) of branched 4-nonylphenol (NP), 2 microg g(-1) of 17beta-estradiol (E(2)), or vehicle, during the parr-smolt transformation in April, and the treatment was repeated 4, 8, and 11 days after the first injection. Plasma was obtained for biochemical analysis 7 and 14 days after initiation of treatment. After 14 days of treatment, additional fish from each treatment group were exposed to seawater for 24h to assess salinity tolerance. The E(2) treatment and the highest NP dose resulted in lower salinity tolerance and decreased plasma insulin-like growth factor I (IGF-I) levels, along with elevated levels of plasma vitellogenin and total calcium. Plasma growth hormone levels were elevated at intermediate NP doses only, and not affected by E(2). After 7 days, plasma thyroxine (T(4)) levels decreased in a strong, dose-dependent manner in response to nonylphenol, but after 14 days, this suppressive effect of T(4) occurred at the highest NP dose only. Similarly, E(2) decreased plasma T(4) levels at 7, but not 14 days. Plasma 3,3',5-triodo-l-thyronine was reduced by E(2) and the highest NP dose after 7 and 14 days of treatment. Plasma cortisol levels were not affected by any of the treatments. The results indicate that the parr-smolt transformation and salinity tolerance can be compromised by exposure to estrogenic compounds. Suppression of plasma IGF-I levels is a likely endocrine pathway for the effects of estrogenic compounds on hypo-osmoregulatory capacity, and the detrimental effects of E(2) and NP on thyroid hormone levels are also likely to compromise the normal parr-smolt transformation of Atlantic salmon.

Neurophysiological regulation and target-tissue impact of the pulsatile mode of growth hormone secretion in the human

Neuroendocrine axes function as an ensemble of regulatory loci which communicate and maintain homeostasis via time-delayed blood-borne signals. The growth hormone (GH)-insulin-like growth factor I (IGF-I) feedback axis sustains a vividly pulsatile mode of interglandular signalling. Pulsatility is driven jointly by hypothalamic GH-releasing hormone (GHRH) and GH-releasing peptide (GHRP), and modulated by somatostatinergic restraint. Paradoxically, intermittent somatostatin inputs also facilitate somatotrope-cell responses to recurrent secretagogue stimuli, thereby amplifying pulsatile GH secretion. A concurrent low basal (8-12% of normal total) rate of GH release is controlled positively by GHRH and GHRP and negatively by somatostatin. Sex-steroid hormones (such as oestradiol and aromatizable androgen) and normal female and male puberty augment GH secretory-burst mass 1.8- to 3.5-fold, whereas ageing, relative obesity, physical inactivity, hypogonadism, and hypopituitarism mute the amplitude/mass of pulsatile GH output. An abrupt rise in circulating GH concentration stimulates rapid internalization of the GH receptor in peripheral target tissues, and evokes second-messenger nuclear signalling via the STAT 5b pathway. Discrete GH peaks stimulate linear (skeletal) growth and drive muscle IGF-I gene expression more effectually than basal (time-invariant) GH exposure. A brief pulse of GH can saturate the plasma GH-binding protein system and achieve prolonged plasma GH concentrations by convolution with peripheral distribution and clearance mechanisms. A single burst of GH secretion also feeds back after a short latency on central nervous system (CNS) regulatory centres via specific brain GH receptors to activate somatostatinergic and reciprocally subdue GHRH outflow. This autoregulatory loop probably contributes to the time-dependent physiologically pulsatile dynamics of the GH axis. More slowly varying systemic IGF-I concentrations may also damp GH secretory pulse amplitude by delayed negative-feedback actions. According to this simplified construct, GH pulsatility emerges due to time-ordered multivalent interfaces among GHRH/GHRP feedforward and somatostatin, GH and IGF-I feedback signals. Resultant GH pulses trigger tissue-specific gene expression, thereby promoting skeletal and muscular growth, metabolic and body compositional adaptations, and CNS reactions that jointly maintain health and homeostasis.

Mechanisms of reduced body growth in the pubertal feminized male rat: unbalanced estrogen and androgen action on the somatotropic axis

It is well known that the sex difference in body growth at puberty is modulated by a complex interplay between sex steroids and somatotropic axis; however, the exact role played by sex steroids remains a matter of controversy. The aim of this study was to assess the mechanisms by which sex steroids regulate body growth during pubertal development. Flutamide, a non-steroid-blocking androgen receptor, was subcutaneously administered to 30-d-old male Wistar rats for 4 wk. The blockade of the androgen receptor led to a marked elevation in serum testosterone and an increment in serum estradiol. Flutamide administration decreased body weight gain, serum IGF-I levels, hepatic IGF-I mRNA, and GH receptor mRNA content. There were no significant changes in serum GH concentration, pituitary GH reserve, and pituitary GH mRNA content. Flutamide lowered hypothalamic somatostatin mRNA content and augmented hypothalamic immunoreactive somatostatin stores, but did not alter hypothalamic immunoreactive GH-releasing factor stores. Our findings indicate that during pubertal development of the male rat, the imbalance between androgen and estrogen actions determines an abnormal somatic growth, which is at least partly exerted through the peripheral or hepatic modification of the somatotropic axis that occurs under the high or exclusive action of estrogens. Potential implication of coincident sex-specific regulated mode of pulsatile GH secretion cannot be excluded from this random serum GH sample study.

Actions of estrogen on pulsatile, nyctohemeral, and entropic modes of growth hormone secretion

The neuroendocrine mechanisms by which estradiol drives growth hormone (GH) secretion in the human are poorly defined. Here we investigate estrogen's specific regulation of the 24-h pulsatile, nyctohemeral, and entropic modes of GH secretion in healthy postmenopausal women. Volunteers (n = 9) received randomly ordered placebo versus estradiol-17beta (1 mg micronized steroid twice daily orally) treatment for 7-10 days and underwent blood sampling at 10-min intervals for 24 h to capture GH release profiles quantitated in a high-sensitivity chemiluminescence assay. Pulsatile GH secretion was appraised via deconvolution analysis, nyctohemeral GH rhythms by cosinor analysis, and the orderliness of GH release patterns via the approximate entropy statistic. Mean (+/-SE) 24-h serum GH concentrations approximately doubled on estrogen treatment (viz., from 0.31 +/- 0.03 to 0.51 +/- 0.07 microgram/l; P = 0.033). Concomitantly, serum insulin-like growth factor-I (IGF-I), luteinizing hormone, and follicle-stimulating hormone concentrations fell, whereas thyroid-stimulating hormone and prolactin levels rose (P < 0.01). The specific neuroendocrine action of estradiol included 1) a twofold amplified mass of GH secreted per burst, with no significant changes in basal GH release, half-life, pulse frequency, or duration; 2) an augmented amplitude and mesor of the 24-h rhythm in GH release, with no alteration in acrophase; and 3) greater disorderliness of GH release (higher approximate entropy). These distinctive and dynamic reactions to estrogen are consistent with partial withdrawal of IGF-I's negative feedback and/or accentuated central drive to GH secretion.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.

Rat growth hormone receptor/growth hormone-binding protein mRNAs with divergent 5'-untranslated regions are expressed in a tissue-specific manner

In the rat, the growth hormone receptor (GH-R) gene generates two transcripts, one encoding the transmembrane GH-R, and a shorter one encoding the GH-binding protein (GH-BP). These transcripts exhibit a high degree of heterogeneity in their 5'-untranslated regions (5'-UTRs). Some of the exons encoding these 5'-UTR variants may be flanked by distinct promoter regions whose activity would result in the tissue-specific expression of the GH-R gene. To assess this possibility, we used single-sided polymerase chain reaction (PCR) amplification to characterize 5'-UTR variants in rat GH-R cDNAs, and by using 5'-UTR-specific probes, we determined their pattern of expression in several tissues. Besides two previously described variants (V1 and V2), three new 5'-UTR variants were identified, extending 56 nucleotides (V3), 135 nucleotides (V4), and 209 nucleotides (V5) upstream of the ATG translation initiation codon. The expression of GH-R and GH-BP transcripts was clearly tissue specific. In the liver, GH-BP mRNA was the predominant transcript, whereas in other tissues, there was equivalent expression of both transcripts or predominant expression of GH-R mRNA. With respect to the tissue distribution of the 5'-UTR variants in particular, variants V1 and V5 exhibited a pattern of expression closely resembling that seen with an exon 2 probe, with the overall expression of variant V1 being much higher than that of variant V5. The V2 species was exclusively expressed in liver. Variant V3 was expressed at low levels in liver, muscle, heart, and kidney; in muscle and heart, it was preferentially associated with GH-BP transcripts. Variant V4, although present in liver, was more abundant in extrahepatic tissues and predominantly found in GH-R mRNA transcripts. Southern blot analyses were consistent with exon 2 and the exons encoding the V1 and V2 sequences being in proximity, with the other 5'-UTR sequences being encoded by exons located further upstream of exon 2. These findings support the concept that different 5'-UTR variants are the result of the different promoters acting in a tissue-specific manner. The association of specific 5'-UTR variants with either GH-R or GH-BP transcripts raises the possibility that the alternative splicing process that generates GH-BP mRNA in the rat might be controlled by the 5'-flanking region regulating the expression of specific leader exons.