The growth hormone (GH)-axis of GH receptor/binding protein gene-disrupted and metallothionein-human GH-releasing hormone transgenic mice: hypothalamic neuropeptide and pituitary receptor expression in the absence and presence of GH feedback

Growth hormone receptor gene disruption

Much of our understanding of growth hormone's (GH)'s numerous activities stems from studies utilizing GH receptor (GHR) knockout mice. More recently, the role of GH action has been examined by creating mice with tissue-specific or temporal GHR disruption. To date, 37 distinct GHR knockout mouse lines have been created. Targeted tissues include fat, liver, muscle, heart, bone, brain, macrophage, intestine, hematopoietic stem cells, pancreatic β cells, and inducible multi-tissue "global" disruption at various ages. In this chapter, a summary of each mouse line is provided with background information on the generation of the mouse line as well as important physiological outcomes resulting from GHR gene disruption. Collectively, these mouse lines provide unique insights into GH action and have resulted in the development of new hypotheses about the functions ascribed to GH action in particular tissues.

Signaling mechanism of growth hormone-releasing hormone receptor

The hypothalamic peptide growth hormone-releasing hormone (GHRH) stimulates the secretion of growth hormone (GH) from the pituitary through binding and activation of the pituitary type of GHRH receptor (GHRH-R), which belongs to the family of G protein-coupled receptors with seven potential membrane-spanning domains. Splice variants of GHRH-Rs (SV) in human tumors and other extra pituitary tissues were identified and their cDNA was sequenced. Among the SVs, splice variant 1 (SV1) possesses the greatest similarity to the full-length GHRH-R and remains functional by eliciting cAMP signaling and mitogenic activity upon GHRH stimulation. A large body of work have evaluated potential clinical applications of agonists and antagonists of GHRH in diverse fields, including endocrinology, oncology, cardiology, diabetes, obesity, metabolic dysfunctions, Alzheimer's disease, ophthalmology, wound healing and other applications. In this chapter, we briefly review the expression and potential function of GHRH-Rs and their SVs in various tissues and also elucidate and summarize the activation, molecular mechanism and signalization pathways of these receptors. Therapeutic applications of GHRH analogs are also discussed.

Mice with gene alterations in the GH and IGF family

Much of our understanding of GH's action stems from animal models and the generation and characterization of genetically altered or modified mice. Manipulation of genes in the GH/IGF1 family in animals started in 1982 when the first GH transgenic mice were produced. Since then, multiple laboratories have altered mouse DNA to globally disrupt Gh, Ghr, and other genes upstream or downstream of GH or its receptor. The ability to stay current with the various genetically manipulated mouse lines within the realm of GH/IGF1 research has been daunting. As such, this review attempts to consolidate and summarize the literature related to the initial characterization of many of the known gene-manipulated mice relating to the actions of GH, PRL and IGF1. We have organized the mouse lines by modifications made to constituents of the GH/IGF1 family either upstream or downstream of GHR or to the GHR itself. Available data on the effect of altered gene expression on growth, GH/IGF1 levels, body composition, reproduction, diabetes, metabolism, cancer, and aging are summarized. For the ease of finding this information, key words are highlighted in bold throughout the main text for each mouse line and this information is summarized in Tables 1, 2, 3 and 4. Most importantly, the collective data derived from and reported for these mice have enhanced our understanding of GH action.

Growth hormone modulates hypothalamic inflammation in long-lived pituitary dwarf mice

Mice in which the genes for growth hormone (GH) or GH receptor (GHR(-/-) ) are disrupted from conception are dwarfs, possess low levels of IGF-1 and insulin, have low rates of cancer and diabetes, and are extremely long-lived. Median longevity is also increased in mice with deletion of hypothalamic GH-releasing hormone (GHRH), which leads to isolated GH deficiency. The remarkable extension of longevity in hypopituitary Ames dwarf mice can be reversed by a 6-week course of GH injections started at the age of 2 weeks. Here, we demonstrate that mutations that interfere with GH production or response, in the Snell dwarf, Ames dwarf, or GHR(-/-) mice lead to reduced formation of both orexigenic agouti-related peptide (AgRP) and anorexigenic proopiomelanocortin (POMC) projections to the main hypothalamic projection areas: the arcuate nucleus (ARH), paraventricular nucleus (PVH), and dorsomedial nucleus (DMH). These mutations also reduce hypothalamic inflammation in 18-month-old mice. GH injections, between 2 and 8 weeks of age, reversed both effects in Ames dwarf mice. Disruption of GHR specifically in liver (LiGHRKO), a mutation that reduces circulating IGF-1 but does not lead to lifespan extension, had no effect on hypothalamic projections or inflammation, suggesting an effect of GH, rather than peripheral IGF-1, on hypothalamic development. Hypothalamic leptin signaling, as monitored by induction of pStat3, is not impaired by GHR deficiency. Together, these results suggest that early-life disruption of GH signaling produces long-term hypothalamic changes that may contribute to the longevity of GH-deficient and GH-resistant mice.

Endocrine parameters and phenotypes of the growth hormone receptor gene disrupted (GHR-/-) mouse

Disruption of the GH receptor (GHR) gene eliminates GH-induced intracellular signaling and, thus, its biological actions. Therefore, the GHR gene disrupted mouse (GHR-/-) has been and is a valuable tool for helping to define various parameters of GH physiology. Since its creation in 1995, this mouse strain has been used by our laboratory and others for numerous studies ranging from growth to aging. Some of the most notable discoveries are their extreme insulin sensitivity in the presence of obesity. Also, the animals have an extended lifespan, which has generated a large number of investigations into the roles of GH and IGF-I in the aging process. This review summarizes the many results derived from the GHR-/- mice. We have attempted to present the findings in the context of current knowledge regarding GH action and, where applicable, to discuss how these mice compare to GH insensitivity syndrome in humans.

Different degrees of somatotroph ablation compromise pituitary growth hormone cell network structure and other pituitary endocrine cell types

We have generated transgenic mice with somatotroph-specific expression of a modified influenza virus ion channel, (H37A)M2, leading to ablation of GH cells with three levels of severity, dependent on transgene copy number. GH-M2(low) mice grow normally and have normal-size pituitaries but 40-50% reduction in pituitary GH content in adult animals. GH-M2(med) mice have male-specific transient growth retardation and a reduction in pituitary GH content by 75% at 42 d and 97% by 100 d. GH-M2(high) mice are severely dwarfed with undetectable pituitary GH. The GH secretory response of GH-M2(low) and GH-M2(med) mice to GH-releasing peptide-6 and GHRH was markedly attenuated. The content of other pituitary hormones was affected depending on transgene copy number: no effect in GH-M2(low) mice, prolactin and TSH reduced in GH-M2(med) mice, and all hormones reduced in GH-M2(high) mice. The effect on non-GH hormone content was associated with increased macrophage invasion of the pituitary. Somatotroph ablation affected GH cell network organization with limited disruption in GH-M2(low) mice but more severe disruption in GH-M2(med) mice. The remaining somatotrophs formed tight clusters after puberty, which contrasts with GHRH-M2 mice with a secondary reduction in somatotrophs that do not form clusters. A reduction in pituitary beta-catenin staining was correlated with GH-M2 transgene copy number, suggesting M2 expression has an effect on cell-cell communication in somatotrophs and other pituitary cell types. GH-M2 transgenic mice demonstrate that differing degrees of somatotroph ablation lead to correlated secondary effects on cell populations and cellular network organization.

Use of the metallothionein promoter-human growth hormone-releasing hormone (GHRH) mouse to identify regulatory pathways that suppress pituitary somatotrope hyperplasia and adenoma formation due to GHRH-receptor hyperactivation

Hyperactivation of the GHRH receptor or downstream signaling components is associated with hyperplasia of the pituitary somatotrope population, in which adenomas form relatively late in life, with less than 100% penetrance. Hyperplastic and adenomatous pituitaries of metallothionein promoter-human GHRH transgenic (Tg) mice (4 and > 10 months, respectively) were used to identify mechanisms that may prevent or delay adenoma formation in the presence of excess GHRH. In hyperplastic pituitaries, expression of the late G(1)/G(2) marker Ki67 increased, whereas the proportion of 5-bromo-2'-deoxyuridine-labeled cells (S phase marker) did not differ from age-matched controls. These results indicate cell cycle progression is blocked, with further evidence suggesting that enhanced p27 activity may contribute to this process. For adenomas, formation was associated with loss of p27 activity (nuclear localization and mRNA). Increased endogenous somatostatin (SST) tone may also slow the conversion from hyperplastic to adenomatous state because mRNA levels for SST receptors, sst2 and sst5, were elevated in hyperplastic pituitaries, whereas adenomas were associated with a decline in sst1 and sst5 mRNA. Also, SST-knockout Tg pituitaries were larger and adenomas formed earlier compared with those of SST-intact Tg mice. Unexpectedly, these changes were independent of changes in proliferation rate within the hyperplastic tissue, suggesting that endogenous SST controls GHRH-induced adenoma formation primarily via modulation of apoptotic and/or cellular senescence pathways, consistent with the predicted function of some of the most differentially expressed genes (Casp1, MAP2K1, TNFR2) identified by membrane arrays and confirmed by quantitative real-time RT-PCR.

Sizes of abdominal organs in adults with severe short stature due to severe, untreated, congenital GH deficiency caused by a homozygous mutation in the GHRH receptor gene

OBJECTIVE

To assess the sizes of intra-abdominal organs of adult subjects with untreated severe congenital isolated GH deficiency (IGHD) due to lack of functional GHRH receptor (GHRH-R), and to verify whether there is proportionality between size of organ and adult stature and body surface area (BSA).

SUBJECTS AND METHODS

By using ultrasound, we studied the sizes (absolute and corrected by height, weight and BSA) of the intra-abdominal organs of 18 adult subjects with IGHD (eight females, IGHD group) who have never received GH replacement therapy. They were all homozygous for the same null mutation (IVS1 + 1G --> A) in the GHRH receptor gene (GHRH-R). They were compared with normal controls from the same region.

RESULTS

After correction for BSA, subjects lacking a functional GHRH-R have normal prostate and ovaries size, small spleen and uterus, and large liver, pancreas and kidney.

CONCLUSIONS

Size of individual abdominal organs is influenced in different ways by severe and congenital lack of GH due to a GHRH-R mutation.

Model-projected mechanistic bases for sex differences in growth hormone regulation in humans

Models of physiological systems facilitate rational experimental design, inference, and prediction. A recent construct of regulated growth hormone (GH) secretion interlinks the actions of GH-releasing hormone (GHRH), somatostatin (SRIF), and GH secretagogues (GHS) with GH feedback in the rat (Farhy LS, Veldhuis JD. Am J Physiol Regul Integr Comp Physiol 288: R1649–R1663, 2005). In contrast, no comparable formalism exists to explicate GH dynamics in any other species. The present analyses explore whether a unifying model structure can represent species- and sex-defined distinctions in the human and rodent. The consensus principle that GHRH and GHS synergize in vivo but not in vitro was explicable by assuming that GHS 1) evokes GHRH release from the brain, 2) opposes inhibition by SRIF both in the hypothalamus and on the pituitary gland, and 3) stimulates pituitary GH release directly and additively with GHRH. The gender-selective principle that GH pulses are larger and more irregular in women than men was conferrable by way of 4) higher GHRH potency and 5) greater GHS efficacy. The overall construct predicts GHRH/GHS synergy in the human only in the presence of SRIF when the brain-pituitary nexus is intact, larger and more irregular GH pulses in women, and observed gender differences in feedback by GH and the single and paired actions of GHRH, GHS, and SRIF. The proposed model platform should enhance the framing and interpretation of novel clinical hypotheses and create a basis for interspecies generalization of GH-axis regulation.

Differential regulation of GHRH-receptor and GHS-receptor expression by long-term in vitro treatment of ovine pituitary cells with GHRP-2 and GHRH

GH secretion is regulated by GHRH and somatostatin via actions on their specific receptors in pituitary somatotropes. Ghrelin and synthetic analogs, GHRPs, also stimulate GH release via GHS-receptors (GHS-R). To examine the long-term effect of GHRH and/or GHRP on somatotropes, primary cultured ovine somatotropes were treated with GHRH (10(-9) and 10(-8) M) and GHRP-2 (10(-8) and 10(-7) M) for up to 2 d. After treatment, culture medium was collected for GH assay, and total RNA was extracted for RT-PCR analysis. To evaluate cell cultures used in this report, somatotrope-enriched pituitary cells were challenged by 6 h GHRH and dexamethasone (DEX) treatment. As expected, GHRH significantly decreased, whereas DEX increased, the levels of GHRHR mRNA. Combined low doses of GHRH (10(-9) M) and GHRP-2 (10(-8) M) treatment for 24 h increased accumulated GH secretion, significantly more than that induced by high doses of GHRH (10(-8) M) and GHRP-2 (10(-7) M). While levels of GHRH-R mRNA increased, GHS-R mRNA levels were decreased by low doses of GHRH and GHRP-2 for 24 h. High doses of GHRH and/or GHRP-2 for 2 d did not increase GH secretion in the second day of treatment and reduced the level of GHRH-R mRNA. High doses of GHRP-2 treatment decreased the levels of both GHRH-R and GHS-R mRNA. Low doses of GHRH and/or GHRP-2 for 2 d increased the level of GHS-R mRNA without changing GHRH-R mRNA levels. Such treatment also increased ghrelin- (10(-9) M) or ghrelin/GHRH (10(-9) M)-induced GH secretion. These results suggest that low doses of GHRP-2 and GHRH prime somatotropes for stimulation by GHRH and ghrelin.

Growth hormone receptor deficiency results in blunted ghrelin feeding response, obesity, and hypolipidemia in mice

We have previously shown that growth hormone (GH) overexpression in the brain increased food intake, accompanied with increased hypothalamic agouti-related protein (AgRP) expression. Ghrelin, which stimulates both appetite and GH secretion, was injected intracerebroventricularly to GHR-/- and littermate control (+/+) mice to determine whether ghrelin's acute effects on appetite are dependent on GHR signaling. GHR-/- mice were also analyzed with respect to serum levels of lipoproteins, apolipoprotein (apo)B, leptin, glucose, and insulin as well as body composition. Central injection of ghrelin into the third dorsal ventricle increased food consumption in +/+ mice, whereas no change was observed in GHR-/- mice. After ghrelin injection, AgRP mRNA expression in the hypothalamus was higher in +/+ littermates than in GHR-/- mice, indicating a possible importance of AgRP in the GHR-mediated effect of ghrelin. Compared with controls, GHR-/- mice had increased food intake, leptin levels, and total and intra-abdominal fat mass per body weight and deceased lean mass. Moreover, serum levels of triglycerides, LDL and HDL cholesterol, and apoB, as well as glucose and insulin levels were lower in the GHR-/- mice. In summary, ghrelin's acute central action to increase food intake requires functionally intact GHR signaling. Long-term GHR deficiency in mice is associated with high plasma leptin levels, obesity, and increased food intake but a marked decrease in all lipoprotein fractions.

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.

Insulin-like growth factor-I down-regulates ghrelin receptor (growth hormone secretagogue receptor) expression in the rat pituitary

The effects of insulin-like growth factor-I (IGF-I) on the ghrelin receptor [growth hormone secretagogue receptor (GHS-R)] gene expression and on the GH response to GHS in rat pituitary cell cultures were examined. Pituitary GHS-R mRNA levels were decreased in a dose (0.01-10 nM)- and time (4-12 h)-dependent manner by IGF-I as measured with reverse transcriptase (RT)-PCR. The basal GH secretion was not influenced by the pretreatment with IGF-I (1 nM for 8 h); however, the GH response to the receptor ligand, a synthetic GHS, KP-102 (100 nM, 15 min), was significantly reduced by pretreatment with IGF-I. Thus, the present studies indicate that IGF-I could inhibit GH secretion at least in part by regulating the expression of the GHS-R.

Expression analysis of hypothalamic and pituitary components of the growth hormone axis in fasted and streptozotocin-treated neuropeptide Y (NPY)-intact (NPY+/+) and NPY-knockout (NPY-/-) mice

In the fasted and the streptozotocin (STZ)-induced diabetic male rat, hypothalamic growth hormone (GH)-releasing hormone (GHRH) mRNA levels, and pulsatile GH release are decreased. These changes are believed to be due to a rise in hypothalamic neuropeptide Y (NPY) that inhibits GHRH expression. To directly test if NPY is required for metabolic regulation of hypothalamic neuropeptides important in GH secretion, NPY, GHRH and somatostatin (SRIH) mRNA levels were determined in fasted (48 h) and STZ-treated wild-type (NPY(+/+)) and NPY-knockout (NPY(-/-)) mice by ribonuclease protection assay. In addition, pituitary receptor mRNA levels for GHRH (GHRH-R), ghrelin (GHS-R) and SRIH (sst2) were assessed by RT-PCR. Under fed conditions the GH axis of NPY(+/+) and NPY(-/-) did not differ. In the NPY(+/+) mouse, fasting resulted in a 23% weight loss and >250% increase in NPY mRNA accompanied by a significant reduction in both GHRH and SRIH mRNA. These changes were associated with increases in pituitary expression of GHRH-R and GHS-R and a concomitant suppression of sst2. In the NPY(-/-) mouse, fasting also resulted in a 23% weight loss and comparable changes in GHRH-R and sst2, but failed to alter GHRH, SRIH and GHS-R mRNA levels. Fasting resulted in an overall increase in circulating GH, which reached significance in the fasted NPY(-/-) mouse. Induction of diabetes in NPY(+/+) mice, using a single, high-dose, STZ injection (150 mg/kg), resulted in modest weight loss (5%), and a 158% increase NPY expression which was associated with reciprocal changes in pituitary GHS-R and sst2 expression, similar to that observed in the fasted state, but no change in hypothalamic GHRH or SRIF expression was observed. Induction of diabetes in NPY(+/+) and NPY(-/-) mice, using a multiple, low-dose, STZ paradigm (5 consecutive daily injections of 40 mg/kg), did not alter body weight, hypothalamic neuropeptide expression or pituitary receptor expression, with the exception that sst2 mRNA levels were suppressed and GH levels did rise in the NPY(-/-) mouse. These observations demonstrate that NPY is not required for basal regulation of the GH axis, but is required for fasting-induced suppression of GHRH and SRIH expression, as well as fasting-induced augmentation of pituitary GHS-R mRNA. In contrast to the rat, fasting clearly did not suppress circulating GH levels in mice, but resulted in an overall rise in mean GH levels, similar to that observed in other mammalian species. The fact that many of the fasting-induced changes in the GH axis were observed in the high-dose STZ-treated mice, but were not observed in the multiple, low-dose paradigm, suggests STZ-mediated modulation of GH axis function is dependent on the severity of the catabolic state and not hyperglycemia.

A mouse with targeted ablation of the growth hormone-releasing hormone gene: a new model of isolated growth hormone deficiency

The proliferation of pituitary somatotroph cells and the synthesis and secretion of GH are under the stimulatory control of the hypothalamic peptide GHRH. GHRH is initially synthesized as pre-prohormone and then enzymatically cleaved to its mature form (44 amino acids in humans and 42 in mice). Although mutations in the GHRH receptor cause isolated GH deficiency (IGHD) both in humans and mice, mutations in the GHRH gene have never been described. To determine the consequences of generalized lack of GHRH, we have created a mouse with targeted disruption (knockout) of the GHRH gene (GHRHKO). We have substituted a portion of the gene that encodes for the initial 14 amino acids of the 1-42 GHRH with a neomycin resistance cassette. Heterozygous founder (+/-) mice were mated to obtain -/- animals. The expected Mendelian ratio was conserved (25.8% of offspring were +/+, 52.8% were +/-, and 21.4% were -/-), showing no lethality in the GHRHKO embryos. GHRHKO mice appeared normal at birth. Starting at 3 wk of age, -/- mice showed significant growth retardation. By 12 wk of age, their weight was about 60% of +/+ and +/- littermates. Growth retardation was due to IGHD, as shown by reduced pituitary GH mRNA and protein content, reduced serum IGF-I, and reduced liver IGF-I mRNA. The phenotype of the GHRHKO mice is similar to the one observed in the mouse with mutated GHRH receptor, including pituitary hypoplasia. Heterozygous mice had normal growth, although adult +/- males (but not females) had mild reduction in serum IGF-I. In conclusion, we demonstrate that ablation of the GHRH gene causes IGHD in mice. The GHRHKO mouse will be the new useful model of IGHD.

Functional modification of pituitary somatotropes in the aromatase knockout mouse and the effect of estrogen replacement

Available data on the influence of estradiol (E(2)) on GH levels remains controversial. A factor contributing to this uncertainty is a lack of knowledge of both E(2) action on somatotropes as well as the molecular mechanisms involved. In this study we investigated gene expression implicated in GH secretion in somatotropes derived from female aromatase knockout (ArKO) mice. In these mice E(2) production is blocked due to disruption of the Cyp19 gene encoding aromatase, the enzyme responsible for estrogen biosynthesis. The effect of E(2) replacement was also studied by in vivo treatment of mice with E(2) for 3 wk. It was demonstrated that somatotropes from ArKO mice had a low expression of GH, GH secretagogue receptor, GHRH receptor (GHRH-R), and pituitary-specific transcription factor (Pit-1). On the other hand, the somatotropes exhibited elevated expression of somatostatin receptors (sst1-5). Overall, these effects resulted in a reduction in GH secretion. E(2) replacement increased GHRH-R, Pit-1, and GH mRNA levels to 185%, 193%, and 157% and reduced the levels of sst1, sst2, sst4, and sst5 mRNA expression in ArKO mice, respectively. E(2) replacement did not affect the levels of pituitary estrogen (alpha and beta) and androgen receptor mRNA expression. It is concluded that the expression of important genes involved in GH synthesis in somatotropes of the female ArKO mouse are functionally down-regulated, and such a down-regulation is reversed to normal levels by E(2) replacement. The levels of GH secretagogue receptor, GHRH-R, and Pit-1 mRNA expression were also reduced, and sst1 and sst3 mRNA expression enhanced in aging ArKO and wild-type mice, resulting in a decrease in GH mRNA expression. It is suggested that aging is another important impact factor for the pituitary expression and regulation of GH mRNA in female mice.

Dwarfism, impaired skin development, skeletal muscle atrophy, delayed bone development, and impeded adipogenesis in mice lacking Akt1 and Akt2

To elucidate the functions of the serine/threonine kinase Akt/PKB in vivo, we generated mice lacking both akt1 and akt2 genes. Akt1/Akt2 double-knockout (DKO) mice exhibit severe growth deficiency and die shortly after birth. These mice display impaired skin development because of a proliferation defect, severe skeletal muscle atrophy because of a marked decrease in individual muscle cell size, and impaired bone development. These defects are strikingly similar to the phenotypes of IGF-1 receptor-deficient mice and suggest that Akt may serve as the most critical downstream effector of the IGF-1 receptor during development. In addition, Akt1/Akt2 DKO mice display impeded adipogenesis. Specifically, Akt1 and Akt2 are required for the induced expression of PPARgamma, the master regulator of adipogenesis, establishing a new essential role for Akt in adipocyte differentiation. Overall, the combined deletion of Akt1 and Akt2 establishes in vivo roles for Akt in cell proliferation, growth, and differentiation. These functions of Akt were uncovered despite the observed lower level of Akt activity mediated by Akt3 in Akt1/Akt2 DKO cells, suggesting that a critical threshold level of Akt activity is required to maintain normal cell proliferation, growth, and differentiation.

Growth hormone-induced diacylglycerol and ceramide formation via Galpha i3 and Gbeta gamma in GH4 pituitary cells. Potentiation by dopamine-D2 receptor activation

Growth hormone (GH) secretion is regulated by indirect negative feedback mechanisms. To address whether GH has direct actions on pituitary cells, lipid signaling in GH(4)ZR(7) somatomammotroph cells was examined. GH (EC(50) = 5 nm) stimulated diacylglycerol (DAG) and ceramide formation in parallel by over 10-fold within 15 min and persisting for >3 h. GH-induced DAG/ceramide formation was blocked by pertussis toxin (PTX) implicating G(i)/G(o) proteins and was potentiated 1.5-fold by activation of G(i)/G(o)-coupled dopamine-D2S receptors, which had no effect alone. Following PTX pretreatment, only PTX-resistant Galpha(i)3, not Galpha(o) or Galpha(i)2, rescued GH-induced DAG/ceramide signaling. GH-induced DAG/ceramide formation was also blocked in cells expressing Gbetagamma blocker GRK-ct. In GH(4)ZR(7) cells, GH induced phosphorylation of JAK2 and STAT5, which was blocked by PTX and mimicked by ceramide analogue C2-ceramide or sphingomyelinase treatment to increase endogenous ceramide. We conclude that in GH(4) pituitary cells, GH induces formation of DAG/ceramide via a novel Galpha(i)3/Gbetagamma-dependent pathway. This novel pathway suggests a mechanism for autocrine feedback regulation by GH of pituitary function.

A model for tissue-specific inducible insulin-like growth factor-I (IGF-I) inactivation to determine the physiological role of liver-derived IGF-I

Insulin-like growth factor-I (IGF-I) has important growthpromoting and metabolic effects and is expressed in virtually every tissue of the body. The highest expression is found in the liver, but the physiological role of liver-derived IGF-I is unknown. It has been difficult to separate the endocrine effects of liver-derived IGF-I from the autocrine/paracrine effects of locally produced IGF-I in peripheral tissues. Therefore, we have developed a mouse model with a liver-specific inducible deletion of the IGF-I gene (LI-IGF-I-/- mouse). The LI-IGF-I-/- mouse has dramatically reduced (>80%) serum IGF-I levels, demonstrating that the major part of serum IGF-I is liver-derived. Surprisingly, LI-IGFI -/- mice demonstrate a normal appendicular skeletal growth up to at least 12 mo of age despite the dramatic decrease in circulating IGF-I levels, indicating that liver-derived IGF-I is not required for appendicular skeletal growth. However, the adult axial skeletal growth is reduced in the LI-IGF-I-/- mice. Furthermore, the amount of cortical bone is reduced due to decreased radial growth of the cortical bone, while the trabecular bone mineral density is unchanged in the LI-IGFI -/- mice. The decreased levels of circulating IGF-I are associated with increased serum levels of growth hormone (GH), indicating a role for liver-derived IGFI in the negative-feedback regulation of GH secretion. Measurements of factors regulating GH secretion in the pituitary and in the hypothalamus revealed an increased expression of GH-releasing-hormone (GHRH) and GHsecretagogue (GHS) receptors in the pituitary of LI-IGFI -/- mice. This in turn results in an increased sensitivity to systemically administered GHRH and GHS, demonstrating that the regulatory action of liver-derived IGF-I on GH secretion is at the pituitary rather than at the hypothalamic level. The liver is an important metabolic organ and LI-IGF-I-/- mice are markedly hyperinsulinemic and yet normoglycemic, consistent with an adequately compensated insulin resistance. Interestingly, LI-IGF-I-/- mice display a reduced age-dependent fat mass accumulation compared with control mice. Furthermore, LI-IGF-I-/- mice have increased blood pressure attributable to increased peripheral resistance indicating a role for liver-derived IGF-I in the regulation of blood pressure. In conclusion, liver-derived IGF-I is important for carbohydrate and lipid metabolism and for the regulation of GH secretion at the pituitary level. Furthermore, it regulates adult axial skeletal growth and cortical radial growth while it is not required for appendicular skeletal growth.

Liver-derived IGF-I regulates GH secretion at the pituitary level in mice

We have reported that liver-specific deletion of IGF-I in mice (LI-IGF-I-/-) results in decreased circulating IGF-I and increased GH levels. In the present study, we determined how elimination of hepatic IGF-I modifies the hypothalamic-pituitary GH axis to enhance GH secretion. The pituitary mRNA levels of GH releasing factor (GHRF) receptor and GH secretagogue (GHS) receptor were increased in LI-IGF-I-/- mice, and in line with this, their GH response to ip injections of GHRF and GHS was increased. Expression of mRNA for pituitary somatostatin receptors, hypothalamic GHRF, somatostatin, and neuropeptide Y was not altered in LI-IGF-I-/- mice, whereas hypothalamic IGF-I expression was increased. Changes in hepatic expression of major urinary protein and the PRL receptor in male LI-IGF-I-/- mice indicated an altered GH release pattern most consistent with enhanced GH trough levels. Liver weight was enhanced in LI-IGF-I-/- mice of both genders. In conclusion, loss of liver-derived IGF-I enhances GH release by increasing expression of pituitary GHRF and GHS receptors. The enhanced GH release in turn affects several liver parameters, in line with the existence of a pituitary-liver axis.

The effect of GHRH on somatotrope hyperplasia and tumor formation in the presence and absence of GH signaling

Excessive GHRH stimulation leads to somatotrope hyperplasia and, ultimately, pituitary adenoma formation in the metallothionein promoter-driven human GHRH (hGHRH) transgenic mouse. This pituitary phenotype is similar to that observed in humans with ectopic production of GHRH. In both mice and man, GHRH hyperstimulation also results in dramatic increases in circulating GH and IGF-I. To determine whether GH/IGF-I modulates the development and growth rate of GHRH-induced pituitary tumors, pituitary growth and histology were evaluated in mice generated from cross-breeding metallothionein promoter-driven hGHRH transgenic mice with GH receptor binding protein (GHR) gene disrupted mice (GHR(-/-)). Expression of the hGHRH transgene in 2-month-old GHR intact (GHR(+)) mice resulted in the doubling of pituitary weight that was largely attributed to an increase in the number of GH-immunopositive cells. Pituitary weight of GHR(+) hGHRH mice did not significantly change between 2 and 6 months of age, whereas at 12 months, weights increased up to 100-fold those of GHR(+) pituitaries, and 70% of the glands contained grossly visible adenomas. All adenomas stained positively for GH, whereas some showed scattered PRL staining. Pituitaries of GHR(-/-) mice were half the size of those of GHR(+) mice. Although reduced in size, the histological features of GHR(-/-) mouse pituitaries were suggestive of somatotrope hyperplasia. Despite evidence of somatotrope hyperplasia, pituitaries from GHR(-/-) mice as old as 28 months of age were similar in size to those of 2-month-old mice and did not show signs of adenoma formation. Expression of the hGHRH transgene in GHR(-/-) mice did not significantly increase pituitary size between 2 and 6 months of age. However, at 12 months the majority of GHR(-/-), hGHRH pituitaries developed adenomas with mean pituitary weight and histological features similar to those of GHR(+), hGHRH mice. These observations demonstrate that intact GH signaling is not required for GHRH tumor formation. Although the majority of GHR(+), hGHRH and GHR(-/-), hGHRH pituitaries developed tumors by 12 months of age, a small subset remained morphologically indistinct from those at 2 months of age. These observations taken together with the fact that overt tumor formation is preceded by a static pituitary growth phase between 2 and 6 months, indicates that protective mechanisms are in place to maintain pituitary mass despite hGHRH hyperstimulation.