Receptor-associated resistance to growth hormone-releasing factor in dwarf "little" mice

Systemic growth hormone deficiency causes mechanical and thermal hypersensitivity during early postnatal development

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Systemic GHD causes behavioral hypersensitivity at P7 and P14, but not P21.

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Primary afferent sensitization is observed in GHRHr KOs.

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Knockout of GHRHr changes DRG gene expression that is observed throughout development.

Injury during early postnatal life causes acute alterations in afferent function and DRG gene expression, which in addition to producing short-term sensitivity has the potential to influence nociceptive responses in adulthood. We recently discovered that growth hormone (GH) is a key regulator of afferent sensitization and pain-related behaviors during developmental inflammation of the skin. Peripheral injury caused a significant reduction in cutaneous GH levels, which corresponded with the observed hypersensitivity. However, it has yet to be determined whether GH deficiency (GHD) is sufficient to drive peripheral sensitization in uninjured animals. Here, we found that systemic GHD, induced by knockout of the GH release hormone receptor (GHRHr), was able to induce behavioral and afferent hypersensitivity to peripheral stimuli specifically during early developmental stages. GHD also produced an upregulation of many receptors and channels linked to nociceptive processing in the DRGs at these early postnatal ages (P7 and P14). Surprisingly, P21 GHRHr knockouts also displayed significant alterations in DRG gene expression even though behavioral and afferent hypersensitivity resolved. These data support previous findings that GH is a key modulator of neonatal hypersensitivity. Results may provide insight into whether GH treatment may be a therapeutic strategy for pediatric pain.

Partial correction of the dwarf phenotype by non-viral transfer of the growth hormone gene in mice: Treatment age is critical

Non-viral transfer of the growth hormone gene to different muscles of immunodeficient dwarf (lit/scid) mice is under study with the objective of improving phenotypic correction via this particular gene therapy approach. Plasmid DNA was administered into the exposed quadriceps or non-exposed tibialis cranialis muscle of lit/scid mice followed by electroporation, monitoring several growth parameters. In a 6-month bioassay, 50μg DNA were injected three times into the quadriceps muscle of 80-day old mice. A 50% weight increase, with a catch-up growth of 21%, together with a 16% increase for nose-to-tail and tail lengths (catch-up=19-21%) and a 24-28% increase for femur length (catch-up=53-60%), were obtained. mIGF1 serum levels were ~7-fold higher than the basal levels for untreated mice, but still ~2-fold lower than in non-dwarf scid mice. Since treatment age was found to be particularly important in a second bioassay utilizing 40-day old mice, these pubertal mice were compared in a third bioassay with adult (80-day old) mice, all treated twice with 50μg DNA injected into each tibialis cranialis muscle, via a less invasive approach. mIGF1 concentrations at the same level as co-aged scid mice were obtained 15days after administration in pubertal mice. Catch-up growth, based on femur length (77%), nose-to-tail (36%) and tail length (39%) increases was 40 to 95% higher than those obtained upon treating adult mice. These data pave the way for the development of more effective pre-clinical assays in pubertal dwarf mice for the treatment of GH deficiency via plasmid-DNA muscular administration.

Hormonal regulation of Cyp4a isoforms in mouse liver and kidney

Mouse Cyp4a subfamily, including Cyp4a10, Cyp4a12a, Cyp4a12b and Cyp4a14, demonstrate a gender- and strain-specific expression in liver and kidney. In C57BL/6 mouse liver and kidney, Cyp4a12a and 4a12b are male-predominant, whereas Cyp4a14 is female-predominant. Cyp4a10 is female-predominant in liver, but shows no gender difference in kidney. The present study was aimed to determine whether sex hormones and/or growth hormone (GH) secretion patterns are responsible for the gender-specific Cyp4a expression in C57BL/6 mice. Gonadectomized mice, GH-releasing hormone receptor-deficient little (lit/lit) mice and hypophysectomized mice were used with replacement of sex hormones or GH in male or female secretion patterns. Both androgens and male-pattern GH regulated the gender-divergent Cyp4a10, 4a12a and 4a12b in liver, whereas androgens played an exclusive role in regulating Cyp4a10 and 4a12a in kidney. In contrast, Cyp4a12b was increased by male-pattern GH but not androgens in kidney. The female-predominant Cyp4a14 in liver and kidney was due to a combined effect of male-pattern GH and androgens. In addition, estrogens played a minor role in regulation of Cyp4a isoforms through an indirect pathway. In conclusion, gender-divergent Cyp4a mRNA expression in liver is caused by male-pattern GH secretion pattern and androgens, whereas in kidney, Cyp4a mRNA expression is primarily regulated by androgens.

Growth hormone response to growth hormone-releasing peptide-2 in growth hormone-deficient little mice

OBJECTIVE

To investigate a possible direct, growth hormone-releasing, hormone-independent action of a growth hormone secretagogue, GHRP-2, in pituitary somatotroph cells in the presence of inactive growth hormone-releasing hormone receptors.

MATERIALS AND METHODS

The responses of serum growth hormone to acutely injected growth hormone-releasing P-2 in lit/lit mice, which represent a model of GH deficiency arising from mutated growth hormone-releasing hormone-receptors, were compared to those observed in the heterozygous (lit/+) littermates and wild-type (+/+) C57BL/6J mice.

RESULTS

After the administration of 10 mcg of growth hormone-releasing P-2 to lit/lit mice, a growth hormone release of 9.3±1.5 ng/ml was observed compared with 1.04±1.15 ng/ml in controls (p<0.001). In comparison, an intermediate growth hormone release of 34.5±9.7 ng/ml and a higher growth hormone release of 163±46 ng/ml were induced in the lit/+ mice and wild-type mice, respectively. Thus, GHRP-2 stimulated growth hormone in the lit/lit mice, and the release of growth hormone in vivo may be only partially dependent on growth hormone-releasing hormone. Additionally, the plasma leptin and ghrelin levels were evaluated in the lit/lit mice under basal and stimulated conditions.

CONCLUSIONS

Here, we have demonstrated that lit/lit mice, which harbor a germline mutation in the Growth hormone-releasing hormone gene, maintain a limited but statistically significant growth hormone elevation after exogenous stimulation with GHRP-2. The present data probably reflect a direct, growth hormone-independent effect on Growth hormone S (ghrelin) stimulation in the remaining pituitary somatotrophs of little mice that is mediated by growth hormone S-R 1a.

Abrogation of growth hormone secretion rescues fatty liver in mice with hepatocyte-specific deletion of JAK2

Non-alcoholic fatty liver disease is associated with multiple comorbid conditions, including diabetes, obesity, infection, and malnutrition. Mice with hepatocyte-specific disruption of growth hormone (GH) signaling develop fatty liver (FL), although the precise mechanism underlying this finding remains unknown. Because GH signals through JAK2, we developed mice bearing hepatocyte-specific deletion of JAK2 (referred to herein as JAK2L mice). These mice were lean, but displayed markedly elevated levels of GH, liver triglycerides (TGs), and plasma FFAs. Because GH is known to promote lipolysis, we crossed GH-deficient little mice to JAK2L mice, and this rescued the FL phenotype. Expression of the fatty acid transporter CD36 was dramatically increased in livers of JAK2L mice, as was expression of Pparg. Since GH signaling represses PPARγ expression and Cd36 is a known transcriptional target of PPARγ, we treated JAK2L mice with the PPARγ-specific antagonist GW9662. This resulted in reduced expression of liver Cd36 and decreased liver TG content. These results provide a mechanism for the FL observed in mice with liver-specific disruption in GH signaling and suggest that the development of FL depends on both GH-dependent increases in plasma FFA and increased hepatic uptake of FFA, likely mediated by increased expression of CD36.

Abrogation of growth hormone secretion rescues fatty liver in mice with hepatocyte-specific deletion of JAK2

Non-alcoholic fatty liver disease is associated with multiple comorbid conditions, including diabetes, obesity, infection, and malnutrition. Mice with hepatocyte-specific disruption of growth hormone (GH) signaling develop fatty liver (FL), although the precise mechanism underlying this finding remains unknown. Because GH signals through JAK2, we developed mice bearing hepatocyte-specific deletion of JAK2 (referred to herein as JAK2L mice). These mice were lean, but displayed markedly elevated levels of GH, liver triglycerides (TGs), and plasma FFAs. Because GH is known to promote lipolysis, we crossed GH-deficient little mice to JAK2L mice, and this rescued the FL phenotype. Expression of the fatty acid transporter CD36 was dramatically increased in livers of JAK2L mice, as was expression of Pparg. Since GH signaling represses PPARγ expression and Cd36 is a known transcriptional target of PPARγ, we treated JAK2L mice with the PPARγ-specific antagonist GW9662. This resulted in reduced expression of liver Cd36 and decreased liver TG content. These results provide a mechanism for the FL observed in mice with liver-specific disruption in GH signaling and suggest that the development of FL depends on both GH-dependent increases in plasma FFA and increased hepatic uptake of FFA, likely mediated by increased expression of CD36.

Liver-derived IGF-I contributes to GH-dependent increases in lean mass and bone mineral density in mice with comparable levels of circulating GH

The relative contributions of circulating and locally produced IGF-I in growth remain controversial. The majority of circulating IGF-I is produced by the liver, and numerous mouse models have been developed to study the endocrine actions of IGF-I. A common drawback to these models is that the elimination of circulating IGF-I disrupts a negative feedback pathway, resulting in unregulated GH secretion. We generated a mouse with near total abrogation of circulating IGF-I by disrupting the GH signaling mediator, Janus kinase (JAK)2, in hepatocytes. We then crossed these mice, termed JAK2L, to GH-deficient little mice (Lit). Compound mutant (Lit-JAK2L) and control (Lit-Con) mice were treated with equal amounts of GH such that the only difference between the two groups was hepatic GH signaling. Both groups gained weight in response to GH but there was a reduction in the final weight of GH-treated Lit-JAK2L vs. Lit-Con mice. Similarly, lean mass increased in both groups, but there was a reduction in the final lean mass of Lit-JAK2L vs. Lit-Con mice. There was an equivalent increase in skeletal length in response to GH in Lit-Con and Lit-JAK2L mice. There was an increase in bone mineral density (BMD) in both groups, but Lit-JAK2L had lower BMD than Lit-Con mice. In addition, GH-mediated increases in spleen and kidney mass were absent in Lit-JAK2L mice. Taken together, hepatic GH-dependent production of IGF-I had a significant and nonredundant role in GH-mediated acquisition of lean mass, BMD, spleen mass, and kidney mass; however, skeletal length was dependent upon or compensated for by locally produced IGF-I.

Increased thrombosis susceptibility and altered fibrin formation in STAT5-deficient mice

To explore the effect(s) of growth hormone signaling on thrombosis, we studied signal transduction and transcription factor 5 (STAT5)-deficient mice and found markedly reduced survival in an in vivo thrombosis model. These findings were not explained by a compensatory increase in growth hormone secretion. There was a modest increase in the activity of several procoagulant factors, but there was no difference in the rate or magnitude of thrombin generation in STAT5-deficient mice relative to control. However, thrombin-triggered clot times were markedly shorter, and fibrin polymerization occurred more rapidly in plasma from STAT5-deficient mice. Fibrinogen depletion and mixing studies indicated that the effect on fibrin polymerization was not due to intrinsic changes in fibrinogen, but resulted from changes in the concentration of a circulating plasma inhibitor. While thrombin-triggered clot times were significantly shorter in STAT5-deficient animals, reptilase-triggered clot times were unchanged. Accordingly, while the rate of thrombin-catalyzed release of fibrinopeptide A was similar, the release of fibrinopeptide B was accelerated in STAT5-deficient plasma versus control. Taken together, these studies demonstrated that the loss of STAT5 resulted in a decrease in the concentration of a plasma inhibitor affecting thrombin-triggered cleavage of fibrinopeptide B. This ultimately resulted in accelerated fibrin polymerization and greater thrombosis susceptibility in STAT5-deficient animals.

The role of liver-derived insulin-like growth factor-I

IGF-I is expressed in virtually every tissue of the body, but with much higher expression in the liver than in any other tissue. Studies using mice with liver-specific IGF-I knockout have demonstrated that liver-derived IGF-I, constituting a major part of circulating IGF-I, is an important endocrine factor involved in a variety of physiological and pathological processes. Detailed studies comparing the impact of liver-derived IGF-I and local bone-derived IGF-I demonstrate that both sources of IGF-I can stimulate longitudinal bone growth. We propose here that liver-derived circulating IGF-I and local bone-derived IGF-I to some extent have overlapping growth-promoting effects and might have the capacity to replace each other (= redundancy) in the maintenance of normal longitudinal bone growth. Importantly, and in contrast to the regulation of longitudinal bone growth, locally derived IGF-I cannot replace (= lack of redundancy) liver-derived IGF-I for the regulation of a large number of other parameters including GH secretion, cortical bone mass, kidney size, prostate size, peripheral vascular resistance, spatial memory, sodium retention, insulin sensitivity, liver size, sexually dimorphic liver functions, and progression of some tumors. It is clear that a major role of liver-derived IGF-I is to regulate GH secretion and that some, but not all, of the phenotypes in the liver-specific IGF-I knockout mice are indirect, mediated via the elevated GH levels. All of the described multiple endocrine effects of liver-derived IGF-I should be considered in the development of possible novel treatment strategies aimed at increasing or reducing endocrine IGF-I activity.

The role of endocrine insulin-like growth factor-I and insulin in breast cancer

Epidemiologic studies demonstrate that breast cancer is the most common type of cancer diagnosed in women and is a significant cause of morbidity and mortality. While there are many risk factors known to be associated with increased breast cancer risk, this review will focus specifically on circulating IGF-I, hyperinsulinemia, and type 2 diabetes. Their effects on promoting breast cancer development, progression, and adverse outcomes have been demonstrated in both animal and human studies, suggesting that the IGF system is a potential target for breast cancer therapy. In addition, in the clinical setting, emphasizing metabolic risk modifications to patients including weight loss, dietary changes, and diabetes control may also play an important role in breast cancer risk reduction.

Sex differences in thrombosis in mice are mediated by sex-specific growth hormone secretion patterns

Sex differences in thrombosis are well described, but their underlying mechanism(s) are not completely understood. Coagulation proteins are synthesized in the liver, and liver gene expression is sex specific and depends on sex differences in growth hormone (GH) secretion--males secrete GH in a pulsatile fashion, while females secrete GH continuously. Accordingly, we tested the hypothesis that sex-specific GH secretion patterns cause sex differences in thrombosis. Male mice were more susceptible to thrombosis than females in the thromboplastin-induced pulmonary embolism model and showed shorter clotting times ex vivo. GH-deficient little (lit) mice were protected from thrombosis, and pulsatile GH given to lit mice restored the male clotting phenotype. Moreover, pulsatile GH administration resulted in a male clotting phenotype in control female mice, while continuous GH caused a female clotting phenotype in control male mice. Expression of the coagulation inhibitors Proc, Serpinc1, Serpind1, and Serpina5 were strongly modulated by sex-specific GH patterns, and GH modulated resistance to activated protein C. These results reveal what we believe to be a novel mechanism whereby sex-specific GH patterns mediate sex differences in thrombosis through coordinated changes in the expression of coagulation inhibitor genes in the liver.

Studies on the Small Body Size Mouse Developed by Mutagen N-Ethyl-N-nitrosourea

Mutant mouse which show dwarfism has been developed by N-ethyl-N-nitrosourea (ENU) mutagenesis using BALB/c mice. The mutant mouse was inherited as autosomal recessive trait and named Small Body Size (SBS) mouse. The phenotype of SBS mouse was not apparent at birth, but it was possible to distinguish mutant phenotype from normal mice 1 week after birth. In this study, we examined body weight changes and bone mineral density (BMD), and we also carried out genetic linkage analysis to map the causative gene(s) of SBS mouse. Body weight changes were observed from birth to 14 weeks of age in both affected (n = 30) and normal mice (n = 24). BMD was examined in each five SBS and normal mice between 3 and 6 weeks of age, respectively. For the linkage analysis, we produced backcross progeny [(SBS × C57BL/6J) F₁ × SBS] N₂ mice (n = 142), and seventy-four microsatellite markers were used for primary linkage analysis. Body weight of affected mice was consistently lower than that of the normal mice, and was 43.7% less than that of normal mice at 3 weeks of age (P < 0.001). As compared with normal mice at 3 and 6 weeks of age, BMD of the SBS mice was significantly low. The results showed 15.5% and 14.1% lower in total body BMD, 15.3% and 8.7% lower in forearm BMD, and 29.7% and 20.1% lower in femur BMD, respectively. The causative gene was mapped on chromosome 10. The map order and the distance between markers were D10Mit248 - 2.1 cM - D10Mit51 - 4.2 cM - sbs - 0.7 cM - D10Mit283 - 1.4 cM - D10Mit106 - 11.2 cM - D10Mit170.

The role of hypothalamic hormones in the control of growth hormone secretion and of growth

GHRH and somatostatin have major integrative roles in the control of GH secretion. Alterations in the secretion of each hypothalamic hormone have profound effects on GH secretion. On the basis of current information, it appears that disturbances in GHRH secretion provide a most convincing argument for the pathophysiological role of this hypothalamic hormone in clinically recognized disorders of GH secretion. Thus, the potential use of GHRH and its agonists and antagonists in the treatment of patients with both deficient and excessive GH secretion is based on a solid framework of physiological and pathophysiological studies.

Role of endogenous ghrelin in growth hormone secretion, appetite regulation and metabolism

Ghrelin, a 28-amino acid hormone that is acylated post-translation, is the endogenous ligand for the growth hormone (GH) secretagogue (GHS) receptor (GHS-R). The highest concentrations of ghrelin are found in the stomach; however ghrelin peptide is also present in hypothalamic nuclei known to be important in the control of GH and feeding behavior. Exogenous ghrelin potently stimulates pituitary GH release through a mechanism that is dependent, in part, on endogenous GH-releasing hormone. Whether endogenous ghrelin plays a role in the control of GH secretion and growth is not clear and ghrelin deficient animals appear to grow normally. In contrast, experimental animal and clinical data suggest that abnormalities in GHS-R signaling could impact growth. Ghrelin or other GHS are clinically useful for GH-testing and limited data suggest that they might be useful in the treatment of some patients with GH deficiency. Substantial data have implicated ghrelin as an important regulator of feeding behavior and energy equilibrium. Ghrelin has a potent orexigenic effect in both animals and humans and this effect is mediated through hypothalamic neuropeptide Y (NPY) and Agouti-related peptide (AgRP). Appetite simulation coupled with other metabolic effects promotes weight gain during chronic treatment with ghrelin. These metabolic effects are in part mediated through an increase in respiratory quotient (VQ). Presence of ghrelin appears to be necessary for the development of obesity in some animal models. Whether abnormalities in ghrelin signaling are involved in human obesity is not yet known.

Endocrine regulation of gender-divergent mouse organic anion-transporting polypeptide (Oatp) expression

Several examples of gender-divergent pharmacokinetics exist in humans and experimental animals, and one reason for these variations may be gender differences in transporter expression. Organic anion transporting polypeptides (Oatp) are transporters involved in hepatic and renal uptake of many organic compounds. In mouse livers, Oatp1a1 is male-predominant, whereas Oatp1a4 is female-predominant. However, in kidneys, Oatp1a1 and Oatp3a1 are both female-predominant. The purpose of the present study was to determine whether sex hormones and/or growth hormone (GH) secretion patterns are responsible for the gender-specific Oatp expression in mice. Gonadectomized mice, GH-releasing hormone receptor-deficient little (lit/lit) mice, and hypophysectomized mice were used with replacement of sex hormones or GH in male or female secretion patterns. Androgens increased Oatp1a1 mRNA in liver and kidney, whereas male-pattern GH administration increased Oatp1a1 mRNA in livers but not in kidneys. Hepatic Oatp1a4 mRNA levels were decreased by both androgens and male-pattern GH administration. In kidneys, Oatp3a1 mRNA expression was only induced by androgen treatment. In conclusion, gender-divergent Oatp expression in liver is caused by male-pattern GH secretion pattern and androgens. In kidney, gender-divergent Oatp expression is exclusively caused by stimulation by androgens.

Inactivating mutations of G protein-coupled receptors and diseases: Structure-function insights and therapeutic implications

Since the discovery of the first rhodopsin mutation that causes retinitis pigmentosa in 1990, significant progresses have been made in elucidating the pathophysiology of diseases caused by inactivating mutations of G protein-coupled receptors (GPCRs). This review aims to compile the compelling evidence accumulated during the past 15 years demonstrating the etiologies of more than a dozen diseases caused by inactivating GPCR mutations. A generalized classification scheme, based on the life cycle of GPCRs, is proposed. Insights gained through detailed studies of these naturally occurring mutations into the structure-function relationship of these receptors are reviewed. Therapeutic approaches directed against the different classes of mutants are being developed. Since intracellular retention emerges as the most common defect, recent progresses aimed at correcting this defect through membrane permeable pharmacological chaperones are highlighted.

Mice severely deficient in growth hormone have normal hematopoiesis

OBJECTIVE

Many studies suggest that growth hormone (GH) is important for hematopoietic stem cell (HSC) function. The objective of this study is to determine if the genetic absence of GH reduces hematopoietic function and recovery, by testing various points in hematopoiesis, from numbers and functional abilities of primitive stem cells to the maintenance of normal numbers of differentiated cells.

MATERIALS AND METHODS

Analyses were conducted on blood and bone marrow to compare GH-deficient C57BL/6J-Ghrhr(lit) / Ghrhr(lit) (lit/lit) mice with their normal (lit/+) littermates. Flow cytometric analysis was used to measure numbers of HSC and progenitor cells based on antigenic markers. Spleen colony-forming units (CFU-S) were examined to determine function of common myeloid progenitor (CMP) cells. Competitive repopulation assays were conducted to test whether normally functional HSCs are produced and supported in the lit/lit hematopoietic environment.

RESULTS

The lit/lit mutant mice produced HSC and progenitor cells at least as well as their lit/+ control littermates. In CFU-S assays, the CMP from the lit/lit mice functioned as well as those from the lit/+ controls. Marrow cells from lit/lit mice repopulated irradiated recipients long-term better than did marrow cells from C57BL/6J(+/+) controls; thus, HSC produced in the absence of GH can replenish irradiated recipients. When lit/lit mice were used as irradiated recipients, they supported HSC function as well as lit/+ control recipients did; thus, the lit/lit hematopoietic environment can support normal hematopoiesis.

A germ line mutation that delays prostate cancer progression and prolongs survival in a murine prostate cancer model

Circulating insulin-like growth factor-I (IGF-I) levels have been shown to be related to risk of prostate cancer in epidemiologic studies. While specific genetic loci responsible for interindividual variation in circulating IGF-I levels in normal men have not been identified, candidate genes include those involved in the growth hormone (GH)-IGF-I axis such as the hypothalamic factors GH releasing hormone (GHRH) and somatostatin and their receptors. To investigate the role of the GH-IGF-I axis on in vivo prostate carcinogenesis and neoplastic progression, we generated mice genetically predisposed to prostate cancer (the TRAMP model) to be homozygous for lit, a mutation that inactivates the GHRH receptor (GHRH-R) and reduces circulating levels of GH and IGF-I. The lit mutation significantly reduced the percentage of the prostate gland showing neoplastic changes at 35 weeks of age (P=0.0005) and was also associated with improved survival (P<0.01). These data provide an example of a germ line mutation that reduces risk in an experimental prostate carcinogenesis model. The results suggest that prostate carcinogenesis and progression may be influenced by germ line variation of genes encoding signalling molecules in the GH-IGF-I axis.

Diurnal variation in growth hormone receptor messenger ribonucleic acid in liver and skeletal muscle of lit/+ and lit/lit mice

The present study investigated the diurnal variation in GH receptor (GHR) mRNA in liver and skeletal muscle of 3-month-old GH-deficient and -sufficient mice using quantitative real-time RT-PCR. lit/lit (GH deficient) or lit/+ (GH sufficient) mice were fed ad libitum and lights were on between 0600 and 2000. Tissues were collected at 0800-1000, 1200-1400 and 2000-2200. Hepatic GHR mRNA levels of lit/+ mice at 0800-1000 were significantly lower than those at 1200-1400 and 2000-2200. There was no significant variation in hepatic GHR mRNA of lit/lit mice. In skeletal muscle, GHR mRNA levels of both lit/+ and lit/lit mice at 1200-1400 were significantly higher than those at 0800-1000 and 2000-2200. There was also a diurnal change in hepatic IGF-I mRNA levels of lit/+ but not of lit/lit mice; the levels were lowest at 0800-1000 in lit/+ mice. On the other hand, there was no variation in IGF-I mRNA levels in skeletal muscle. These results suggest that 1) there is a diurnal variation in GHR expression in liver and skeletal muscle and the pattern of the variation is tissue specific; 2) GH deficiency blunted the diurnal variation in GHR mRNA in liver but not that in skeletal muscle; 3) IGF-I mRNA expression in liver is more closely related to GHR mRNA expression than that in skeletal muscle.

Regulation of mouse renal CYP2J5 expression by sex hormones

Mouse CYP2J5 is abundant in kidney and active in the metabolism of arachidonic acid to epoxyeicosatrienoic acids. Western blots of microsomes prepared from mouse kidneys demonstrate that after puberty, CYP2J5 protein is present at higher levels in male mice than in female mice. Northern analysis reveals that CYP2J5 transcripts are more abundant in adult male versus female kidneys, indicating that gender differences in renal CYP2J5 expression are regulated at a pretranslational level. Castration of male mice results in decreased renal CYP2J5 expression, and treatment of castrated male mice or female mice with 5alpha-dihydrotestosterone increases expression to levels that approximate those in intact male mice. In contrast, treatment of ovariectomized female mice or castrated male mice with 17beta-estradiol causes a further reduction in CYP2J5 expression. Growth hormone-deficient (lit/lit) mice respond similarly to castration and 5alpha-dihydrotestosterone treatment, indicating that the androgen effects are not mediated by alterations in the growth hormone secretory pattern. Mice that lack a functional androgen receptor (Tfm hemizygous) have reduced levels of renal CYP2J5 and do not respond to 5alpha-dihydrotestosterone treatment. Similarly, wild-type male mice treated with flutamide, an androgen antagonist, exhibit reduced renal CYP2J5 levels. Female estrogen receptor-alpha knockout (alphaERKO) mice, which are known to have elevated circulating testosterone levels, have significantly increased renal CYP2J5 expression compared with wild-type female mice, and these differences are abrogated by ovariectomy or treatment with flutamide. Based on these data, we conclude that the renal expression of CYP2J5 is up-regulated by androgen and down-regulated by estrogen.

Growth hormone-releasing hormone and pituitary development, hyperplasia and tumorigenesis

Growth hormone-releasing hormone (GHRH) is essential for expansion of the somatotrope lineage during pituitary development, and excessive GHRH secretion and/or action results in unregulated somatotrope proliferation and neoplastic transformation. Our understanding of the molecular and morphological bases for these effects from both animal and clinical studies has greatly increased during the past decade. However, many features of the cellular pathways remain to be defined, including the interaction of other genes in the multistep process of somatotrope tumorigenesis.

Antagonism of endogenous growth hormone-releasing hormone (GHRH) leads to reduced proliferation and apoptosis in MDA231 breast cancer cells

GHRH, in addition to stimulating the release of growth hormone (GH) from the pituitary, is a trophic factor for pituitary somatotrophs. Growth hormone-releasing hormone is also expressed in the gonads, gastrointestinal tract, pancreas, thymus, and lymphocytes, as well as in tumors of the pancreas, lung, central nervous system, and breast. Since GHRH has mitogenic effects, we examined the hypothesis that GHRH is an autocrine/paracrine growth factor in neoplastic breast tissue. The effect of disrupting endogenous GHRH on cell growth and apoptosis of MDA231 cells was examined through the use of a competitive GHRH antagonist, [N-acetyl-Tyr1, D-Arg2] fragment 1-29Amide (GHRHa). Cell proliferation was determined by direct cell counting and tritiated thymidine incorporation. Apoptosis was analyzed by examination of DNA laddering and nuclear condensation. GHRHa resulted in a dose-dependent, transient, and reversible decrease in cell number, proliferation rate, and tritiated thymidine uptake. Conversely, GHRHa led to a marked and dose-dependent increase in both DNA laddering and nuclear condensation. These results indicate that disruption of endogenous GHRH action in MDA231 cells results in both decreased cellular proliferation and increased apoptosis. Taken together, the findings suggest that endogenous GHRH acts as an autocrine/paracrine factor in the regulation of growth of at least some breast cancer cell types.

Pitx2 is required at multiple stages of pituitary organogenesis: pituitary primordium formation and cell specification

Analysis of an allelic series in mice revealed that the Pitx2 homeobox gene is required at multiple stages of pituitary development. It is necessary for initiating expansion of Rathke's pouch and maintaining expression of the fetal-specific transcription factors Hesx1 and Prop1. At later stages Pitx2 is necessary for specification and expansion of the gonadotropes and Pit1 lineage within the ventral and caudomedial anterior pituitary. Mechanistically, this is due to the dependence of several critical lineage-specific transcription factors, Pit1, Gata2, Egr1 and Sf1, on a threshold level of PITX2. The related Pitx1 gene has a role in hormone gene transcription, and it is important late in ontogeny for the final expansion of the differentiated cell types. Pitx1 and Pitx2 have overlapping functions in the expansion of Rathke's pouch, revealing the sensitivity of pituitary organogenesis to the dosage of the PITX family. The model developed for PITX gene function in pituitary development provides a better understanding of the etiology of Rieger syndrome and may extend to other PITX-sensitive developmental processes.

The little mutation suppresses DEN-induced hepatocarcinogenesis in mice and abrogates genetic and hormonal modulation of susceptibility

In mice, the sex difference in susceptibility to hepatocarcinogenesis results from the tumor promoting activity of testosterone and from the inhibition of tumor promotion by ovarian hormones. We investigated the role of growth hormone in the sex-dependent regulation of susceptibility, because sex hormones are known to regulate the temporal pattern of growth hormone secretion and subsequent sex differences in liver gene expression. We found that in both males and females, wild-type mice developed significantly more tumors than growth hormone-deficient, C57BL/6J-lit/lit (B6-lit/lit) mutant mice following perinatal treatment with the carcinogen N,N-diethylnitrosamine (DEN). B6 wild-type males developed 36-59-fold more liver tumors per animal than age matched B6-lit/lit males and wild-type females developed 11-fold more tumors than B6-lit/lit females. We bred the little mutation onto the more susceptible C57BR/cdJ (BR) and C3H/HeJ (C3H) strains to assess the effect of growth hormone deficiency on hepatocarcinogenesis on additional genetic backgrounds. Growth hormone deficiency suppressed liver tumor development to <1% in males of each strain and in BR strain females. In B6 and C3H females, growth hormone deficiency caused 2-4-fold reductions in the volume fraction of the liver occupied by preneoplastic lesions. Furthermore, in contrast to wild-type strains, neither gonadectomy nor strain background significantly affected susceptibility in lit/lit mice, as mean liver tumor multiplicities ranged from 0 to 0.24 +/- 0.44 and the volume fraction of preneoplastic lesions ranged from 0.21 +/- 0.22 to 0.61 +/- 1.9%. These results demonstrate that both strain and sex hormonal effects on susceptibility to liver carcinogenesis are dependent on wild-type levels of growth hormone.

Skeletal growth acceleration with growth hormone secretagogues in transgenic growth retarded rats: pattern-dependent effects and mechanisms of desensitization

The transgenic growth retarded (Tgr) rat is the first genetic model of growth hormone (GH) deficiency whose growth can be accelerated with exogenous GH secretagogues (GHSs). In this study, we have demonstrated that GHS-receptor (GHS-R) mRNA expression in the arcuate nucleus of Tgr rats was not significantly different to that in wild-type littermates. We have confirmed that GHS-induced elevation in body weight gain was accompanied by acceleration of skeletal growth, and that the effects of the GHS, GHRP-6, were both dose- and pattern-dependent. The growth response with continuous infusion of GHRP-6 was transient, accompanied by suppression of GH and corticosterone responses to bolus injection of GHRP-6. This desensitization occurred without downregulation of arcuate GHS-R mRNA expression, but was accompanied by elevated periventricular somatostatin mRNA expression. In contrast, pulsatile (3-hourly) infusion of GHRP-6 produced sustained growth and GH responses, which were accompanied by suppression of corticosterone responses and elevated arcuate GH-releasing factor (GRF) mRNA expression. Skeletal growth was further accelerated by coinfusion of GRF, but significant depletion of pituitary GH stores suggested that this growth rate may not be sustainable. These experiments confirm the importance of the Tgr rat for investigating the growth promoting potential of the GHSs in the context of GH-deficient dwarfism, and suggest that elevated somatostatin expression may mediate the suppression of the GRF-GH and hypothalamo-pituitary-adrenal axes following continuous GHRP-6 treatment.

Growth hormone (GH)-independent stimulation of adiposity by GH secretagogues

Growth hormone secretagogues (GHSs) stimulate growth hormone (GH) secretion, which is lipolytic. Here we compared the effects of twice daily s.c. treatment of GH and the GHS, ipamorelin, on body fat in GH-deficient (lit/lit) and in GH-intact (+/lit and +/+) mice. In +/lit and lit/lit mice ipamorelin induced a small (15%) increase in body weight by 2 weeks, that was not further augmented by 9 weeks. GH treatment markedly enhanced body weight in both groups. Ipamorelin also increased fat pad weights relative to body weight in both lit/lit and +/lit mice. Two weeks GHS treatment (ipamorelin or GHRP-6) also increased relative body fat, quantified by in vivo dual energy X-ray absorpiometry (DEXA) in GH-intact mice. GH decreased relative fat mass in lit/lit mice and had no effect in GH-intact mice. Treatment with GHS, but not GH, increased serum leptin and food intake in GH-intact mice. Thus, GHSs increase body fat by GH-independent mechanisms that may include increased feeding.

Induction of mammary gland development in estrogen receptor-alpha knockout mice

Mammary glands from the estrogen receptor-a knockout (alphaERKO) mouse do not undergo ductal morphogenesis or alveolar development. Disrupted ERalpha signaling may result in reduced estrogen-responsive gene products in the mammary gland or reduced mammotropic hormones that contribute to the alphaERKO mammary phenotype. We report that circulating PRL is reduced in the female alphaERKO mouse. Implantation of an age-matched, heterozygous ERalpha pituitary isograft under the renal capsule of 25-day-old or 12-week-old alphaERKO mice increased circulating PRL and progesterone levels, and induced mammary gland development. Grafted alphaERKO mice also possessed hypertrophied corpora lutea demonstrating that PRL is luteotropic in the alphaERKO ovary. By contrast, ovariectomy at the time of pituitary grafting prevented mammary gland development in alphaERKO mice despite elevated PRL levels. Hormone replacement using pellet implants demonstrated that pharmacological doses of estradiol induced limited mammary ductal elongation, and estradiol in combination with progesterone stimulated lobuloalveolar development. PRL alone or in combination with progesterone or estradiol did not induce alphaERKO mammary growth. Estradiol and progesterone are required for the structural development of the alphaERKO mammary gland, and PRL contributes to this development by inducing ovarian progesterone levels. Therefore, the manifestation of the alphaERKO mammary phenotype appears due to the lack of direct estrogen action at the mammary gland and an indirect contributory role of estrogen signaling at the hypothalamic/pituitary axis.

Stimulation of mitogen-activated protein kinase pathway in rat somatotrophs by growth hormone-releasing hormone

Growth hormone-releasing hormone (GHRH) is an important regulator of somatotroph development and function. However, GHRH signaling is still not completely understood. Signaling through the mitogen-activated protein kinase (MAPK) pathway has been observed in a wide variety of cell types but has not been explored as a mediator of GHRH action. In this study, we examined the phosphorylation of MAPK pathway intermediates in response to GHRH. After treatment of the GH4 rat somatotroph cell line with rGHRH (10(7) M) for 2.5 min, there was robust phosphorylation of MAPK not seen in vehicle-treated cells. Treatment of HeLa cells with GHRH resulted in no activation of MAPK, but activation was conferred by transfection with the GHRH receptor cDNA. MAPK activation by GHRH was dose dependent from 1 to 100 nM, was evident at 2.5 min, peaked at 5 min, and returned to baseline by 20 min. Pretreatment of GH4 cells with somatostatin analog BIM23014 or the MEK1 inhibitor PD98095 prevented the activation of MAPK. Finally, treatment with GHRH increased GH4 proliferation in culture, and this response was prevented by pretreatment with BIM23014 and PD98095. These results indicate that GHRH activates the MAPK pathway. Furthermore, activation of MAPK may mediate, at least in part, the effects of GHRH on somatotroph cell line proliferation. The findings support the concept that multiple pathways mediate the effects of GHRH.

Structure and function of the growth-hormone-releasing hormone receptor

Growth-hormone-releasing hormone (GHRH) stimulates growth hormone (GH) secretion and GH synthesis and is also thought to cause somatotroph proliferation. Specific high-affinity binding sites for GHRH have been demonstrated on pituitary membranes using iodinated GHRH analogs. The complementary DNA encoding for the human GHRH receptor (GHRH-R) was recently cloned. The open reading frame was shown to extend 1269 bp and thus to encode a protein of 423 amino acids with a predicted molecular weight of 47 kDa. Expression is restricted to specific tissues. Analysis of the genomic structure revealed that the human GHRH-R gene spans 15 kb and consists of 13 exons. The 5'-flanking region of the human GHRH-R gene was recently characterized. Transcriptional regulation of the GHRH-R is discussed in this review. Mechanisms of signal transduction for control of GH transcription and secretion are presented. Furthermore, the role of the GHRH-R in proliferation and differentiation of the somatotrophic pituitary cell as well as in disease is examined.

The mutant growth hormone-releasing hormone (GHRH) receptor of the little mouse does not bind GHRH

The little mouse is a dwarf strain characterized by low levels of GH, pituitary hypoplasia, and an unresponsiveness to treatment with exogenous GHRH. The defect has been mapped to a missense mutation in the GHRH receptor gene that abolishes the function of the receptor, but the mechanism of this inactivation is unknown. Receptor function might be affected at the level of protein expression, maturation and processing, localization to the cell surface, ligand binding, or signaling. In this study, Western blots, using antiserum raised against the GHRH receptor and immunoprecipitation analysis of epitope-tagged receptors, demonstrate that both wild-type and mutant receptor proteins are expressed at equivalent levels in transfected cells. Immunofluorescence analysis of intact and permeabilized cells expressing the epitope-tagged receptors suggests that wild-type and little mouse receptors are similarly localized to the cell surface. A species homologous binding assay was developed and used to show that 125I-mouse GHRH binds with high affinity to the wild-type mouse receptor but not to the little mutant receptor. Consistent with this, the mutant receptor fails to stimulate intracellular cAMP accumulation. Our results demonstrate that the little mutation does not dramatically affect the expression level, glycosylation, or cellular localization of the receptor protein but that it blocks specific GHRH binding, and therefore, signaling does not take place.

Mutations in the growth hormone releasing hormone receptor: a new form of dwarfism in humans

We describe a recently identified new form of dwarfism due to isolated growth hormone (GH) deficiency, secondary to inactivating mutations in the GH-releasing hormone receptor (GHRHR) gene. The identical nonsense mutations in the extracellular domain of the GHRHR (E72X or E50X, depending on whether the signal peptide is included in the numbering) has been independently described in three families residing on or originating from the Indian subcontinent (Pakistan, the Bombay region, and Delft near Sri Lanka). Another inactivating mutation, involving the donor splice site of intron 1, has been identified in a population in north-eastern Brazil. Genetic transmission is autosomal recessive; the gene is located on the short arm of chromosome 7. Affected subjects have severe isolated GH deficiency and postnatal growth failure, with a mean adult height of 130 cm for men and 114 cm for women (7-8 standard deviations below the norm). Dwarfism is proportional; a characteristic feature is relative microcephaly, which results in a 'miniaturized adult', eumorphic aspect. Bone age and puberty are delayed, but fertility appears normal. This new syndrome corresponds to the human homologue of the previously identified 'little mouse'.

Molecular and cell biology of the growth hormone-releasing hormone receptor

The GHRH receptor is a seven transmembrane G-protein-linked receptor found predominantly in the pituitary gland. It is essential for normal somatotroph proliferation and for the synthesis and secretion of GH. Significant amounts of GHRH receptor are also found in the hypothalamus, kidney and placenta. Transcription of the GHRH receptor gene promoter is enhanced by Pit-1 and by glucocorticoids but is inhibited by oestrogen. Recently, mutations involving severe GHRH receptor truncations have been associated with human type 1 GHD. Studies of chimeric receptors and of GHRH receptor cross-linking sites have shown that the N-terminal extracellular domain of the GHRH receptor is required for hormone binding, but that key sites for ligand specificity and signalling are associated with the transmembrane helices and intervening loops. Evidence from the ovine GHRH receptor suggests that the C-terminus has an inhibitory function and may be involved in down-regulation via internalization and phosphorylation. A better understanding of the GHRH receptor may lead to new therapies for the treatment of GH disorders.

Interactions of gender, growth hormone, and phenobarbital induction on murine Cyp2b expression

The interactions of gender, growth hormone, and phenobarbital induction on Cyp2b expression were examined in phenotypically normal (lit/+) and growth-hormone deficient "little" (lit/lit) mice. Using an immunocrossreactive monoclonal antibody designed to identify rat CYP2B1 and 2B2 proteins, we observed three hepatic Cyp2b proteins in control (lit/+) females, but only two proteins, one at trace levels, in control males. Phenobarbital administration to lit/+ mice increased the expression of the two Cyp2b isoforms in the males by 3- to 4-fold, but produced an approximately 75% reduction in the female-expressed proteins. Whereas growth hormone depletion (lit/lit) had no effect on the expression profile of Cyp2b proteins in females, it had a de-repressive effect in males, resulting in the expression of three proteins at concentrations now comparable to those observed in female liver. Generally, phenobarbital had no inductive effects in the lit/lit mice of both sexes. In all groups, transcript levels measured by a CYP2B1 probe were in agreement with the protein findings. In contrast, Cyp2b mRNA identified by an oligonucleotide probe for CYP2B2 were repressed completely by growth hormone in both sexes, and was expressed as a female-predominant transcript in the lit/lit mice. In spite of an apparent high degree of sequence homology between the rat CYP2B and murine Cyp2b gene families, the present findings highlight fundamental differences in their constitutive and gender-dependent expression, growth hormone regulation, and phenobarbital inducibility.

Endocrine effects of IGF-I on normal and transformed breast epithelial cells: potential relevance to strategies for breast cancer treatment and prevention

Insulin-like growth factors (IGFs) are mitogenic and anti-apoptotic peptides that influence the proliferative behavior of many cell types, including normal and transformed breast epithelial cells. IGF-I has properties of both a tissue growth factor and a systemic hormone: there is evidence that IGF bioactivity in tissues is influenced not only by local factors such as tissue expression of IGFs, IGF binding proteins (IGFBPs), and IGFBP proteases, but also by factors that regulate whole-body IGF physiology and circulating IGF-I levels. Experimental evidence that interventions that reduce circulating IGF-I levels reduce proliferation of breast neoplasms has raised interest in the possibility of developing novel endocrine therapies that target the growth hormone/IGF-I axis. Furthermore, influences of the growth hormone/IGF-I axis on normal breast epithelial cells may underlie recent epidemiological observations that suggest that premenopausal women with high circulating IGF-I level are at increased risk for breast cancer. These studies suggest that the growth hormone/IGF-I axis deserves investigation as a possible target for novel breast cancer prevention strategies.

Ultrasensitive in vivo bioassay detects bioactive human growth hormone in transduced primary human keratinocytes

An improved in vivo body weight gain bioassay for the potency determination of human growth hormone (hGH) has been set up in "little" mice (lit/lit), a mutant derived from the C57BL/6J strain. This improved assay now has a detection limit of the order of 0.05 micrograms/mouse/day, which corresponds to a sensitivity about 20-fold higher than that of the most sensitive in vivo assay reported up to now: the tibia test in hypophysectomized rats or mice. This sensitivity was achieved mainly by introduction of a careful pre-assay selection and of a three injections per day schedule. The utilization of these conditions in a 2x2 factorial assay design allowed the potency determination of recombinant DNA-derived hGH (rec-hGH) in bacterial extracts with acceptable accuracy and precision, together with the greatest economy of material, only 0.24 mg of unknown and standard hormone preparation being sufficient for an entire 10-animal assay. This contrasts to a minimum of 2.7 mg that are necessary for an economical assay in hypophysectomized rats. The same assay procedure was also used to demonstrate the in vivo bioactivity of hGH secreted into a culture medium from transduced human primary keratinocytes. The growth curve constructed with n = 8 little mice presented a highly significant correlation (r = 0.939, p < 0.001) and a slope = 0.016 g/mouse/day. It was thus possible to prove, for the first time, the in vivo bioactivity of rec-hGH secreted by transplantable human epidermal cells, utilized as an experimental model for somatic gene therapy.

Hypothalamic growth hormone secretagogue-receptor (GHS-R) expression is regulated by growth hormone in the rat

Synthetic GH secretagogues (GHSs) act via a receptor (GHS-R) distinct from that for GH-releasing hormone (GHRH). We have studied the hypothalamic expression and regulation of this receptor by in situ hybridization using a homologous riboprobe for rat GHS-R. GHS-R mRNA is prominently expressed in arcuate (ARC) and ventromedial nuclei (VMN) and in hippocampus, but not in the periventricular nucleus. Little or no specific hybridization could be observed in the pituitary under the conditions that gave strong signals in the hypothalamus. No sex difference in GHS-R expression was found in ARC or hippocampus, though expression in VMN was lower in males than in females. Compared with GHRH and neuropeptide Y (NPY), GHS-R was expressed in a distinct region of ventral ARC, and in regions of VMN not expressing GHRH or NPY. GHS-R expression was highly sensitive to GH, being markedly increased in GH-deficient dw/dw dwarf rats, and decreased in dw/dw rats treated with bovine GH (200 microg/day) for 6 days. Similar changes were observed in GHRH expression, whereas NPY expression was reduced in dw/dw rats and increased by bGH treatment. Continuous sc infusion of GHRP-6 in normal female rats did not alter ARC or VMN GHS-R expression. Our data implicate ARC and VMN cells as major hypothalamic targets for direct GHS action. The sensitivity of ARC GHS-R expression to modulation by GH suggests that GHS-Rs may be involved in feedback regulation of GH.

Hypothalamic targets for growth hormone secretagogues

Various novel growth hormone (GH) secretagogues have been developed. GH secretagogues release GH directly from the pituitary via a pathway distinct from that involving GH-releasing hormone (GHRH). However, they also act centrally to activate hypothalamic neurones, and require an intact GHRH system for potent in vivo activity. Both normal and transgenic growth-retarded (Tgr) rats release GH in response to GH secretagogues, and their responses are sensitive to the pattern of secretagogue administration. GH secretagogues are not completely specific for GH release, but also activate the adrenocorticotrophin-adrenal axis, implying that they have additional central actions. The recent cloning of an endogenous receptor for GH secretagogues now makes it possible to identify central targets for their action. An endogenous receptor implies the existence of an endogenous ligand, but its site of production, relationship to the xenobiotic pharmacological agents and its underlying physiological relevance remain unclear.

Construction of a 3-Mb contig and partial transcript map of the central region of mouse chromosome 11

We report the establishment of a high-resolution genetic map, a physical map, and a partial transcript map of the Ames dwarf critical region on mouse chromosome 11. A contig of 24 YACs and 13 P1 clones has been assembled and spans approximately 3 Mb from Flt4 to Tcf7. A library of approximately 1000 putative transcript clones from the region was prepared using exon amplification and pituitary cDNA selection. Ten novel transcripts were partially characterized, including a member of the olfactory receptor family, an alpha-tubulin-related sequence, and a novel member of the cdc2/CDC28-like kinase family, Clk4. The location of Prop1, the gene responsible for Ames dwarfism, has been localized within the contig. This contig spans a region of mouse chromosome 11 that exhibits linkage conservation with human chromosome 5q23-q35. The strength of the genetic map and genomic resources for this region suggest that comparative DNA sequencing of this region could reveal the genes responsible for other mouse mutants and human genetic diseases.

The growth-hormone-releasing hormone receptor: signal transduction, gene expression, and physiological function in growth regulation

We now summarize key issues that we have investigated and highlight additional areas that need to be addressed. We are interested in two basic aspects of the GHRH pathway, those occurring in the brain, involving the synthesis of GHRH, and those occurring in the pituitary, involving signaling by GHRH. We have a long-term interest in the activity and regulation of the hypothalamic neurosecretory cells that synthesize GHRH. With respect to human disease, it is interesting that, despite the primal role played by GHRH in growth-hormone secretion, no mutations in the GHRH gene have yet been identified in association with growth disorders. Focusing on the downstream signaling components of the GHRH pathway, we now know quite a lot about the structure of the GHRH receptor and about some aspects of the signal transduction pathways that mediate the actions of GHRH. With respect to human disease, we have found that in an animal model, the little mouse, a mutation of the GHRH receptor results in growth-hormone deficiency and a dwarf phenotype, and there are ongoing attempts in several laboratories to try to identify similar inactivating mutations in the GHRH receptor in patients with isolated growth-hormone deficiency. Conversely, there is also substantial interest in whether activating mutations in this receptor might be identified in patients with growth-hormone-secreting pituitary tumors. We are also interested in whether there are additional receptors that might mediate some of the extrapituitary actions of GHRH. Finally, a major direction we are taking in the laboratory at the present time is toward understanding the developmental, hormonal, and tissue-specific regulation of the GHRH receptor gene.

New insights into the regulation of somatotrope function using genetic and transgenic models

Growth hormone (GH) secretion is under the control of the hypothalamic hormones GH-releasing hormone (GHRH) and somatostatin (SRIF), and is regulated by feedback effects of GH and insulin-like growth factor (IGF-1). GHRH and SRIF act on somatotropes by binding to G-protein-coupled receptors. GHRH activates the stimulatory G protein (Gs), leading primarily to activation of adenylyl cyclase and protein kinase A. SRIF activates the inhibitory G protein (Gi). Several animal models enable the study of various disorders of GH secretion in vivo. Genetic models of impaired GH secretion include the little (lit) mouse, the dwarf (dw) rat, the fatty (fa) rat, and the high-growth (hg) mouse. Transgenic models of impaired and excessive GH secretion, respectively, include the tyrosine hydroxylase-human GH (TH-hGH) transgenic mouse and the metallothionein-human GHRH transgenic mouse. These models encompass a wide spectrum of disorders of GH secretion, involving defects of hypothalamic regulation, feedback control at the pituitary level, or the mechanism of GHRH action in the somatotrope. They may provide insights into our understanding of human GH secretory disorders.

The molecular pathology of pituitary hormone deficiency and resistance

In this chapter, we have reviewed the fast-moving area of the molecular pathology of pituitary hormone deficiencies and resistance. Examples have been described affecting all levels of pituitary function, i.e. the releasing hormone, its receptor, the pituitary hormone and its receptor, and the development of the pituitary gland. Other examples in these genes, and in those in which no mutation has yet been found, will undoubtedly be discovered in the next few years, throwing light on the structural basis of the gene product's function and allowing a greater understanding of endocrine physiology and pathophysiology. The main reason for this rapid progress in knowledge is the recent technological advances in mutation detection, which bring this activity within the grasp of the majority of reasonably equipped laboratories. Technological advancement, however is not all that it takes to carry out this work. The conditions caused by genetic damage such as we have described are rare, and there is clearly a requirement for great awareness on the part of the clinical endocrinologist. Patients in whom it is suspected that mutations such as these may occur require careful clinical and biochemical work-up. Indeed, in many instances, careful thought has to go into deciding what the phenotype of a particular mutation might be. Thus, the requirement for close collaboration between clinical and molecular endocrinologists has to be the important message for the future in this area of research.