Growth hormone-releasing hormone and pituitary somatotrope proliferation

Growth hormone-releasing hormone (GHRH) is a hypothalamic hormone that is essential for normal expansion of the somatotrope lineage during pituitary development. Decreased GHRH secretion and/or action leads to impairment of this process and somatotrope hypoplasia in both humans and experimental animals. Excessive GHRH secretion and/or action result in dysregulated somatotrope proliferation, leading to hyperplasia and neoplastic transformation. Our understanding of the molecular and morphologic 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.

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.

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.

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

Elevation of circulating GH acts to feed back at the level of the hypothalamus to decrease GH-releasing hormone (GHRH) and increase somatostatin (SRIF) production. In the rat, GH-induced changes in GHRH and SRIF expression are associated with changes in pituitary GHRH receptor (GHRH-R), GH secretagogue receptor (GHS-R), and SRIF receptor subtype messenger RNA (mRNA) levels. These observations suggest that GH regulates its own synthesis and release not only by altering expression of key hypothalamic neuropeptides but also by modulating the sensitivity of the pituitary to hypothalamic input, by regulating pituitary receptor synthesis. To further explore this possibility, we examined the relationship between the expression of hypothalamic neuropeptides [GHRH, SRIF, and neuropeptide Y (NPY)] and pituitary receptors [GHRH-R, GHS-R, and SRIF receptor subtypes (sst2 and sst5)] in two mouse strains with alterations in the GH-axis; the GH receptor/binding protein gene-disrupted mouse (GHR/BP-/-) and the metallothionein promoter driven human GHRH (MT-hGHRH) transgenic mouse. In GHR/BP-/- mice, serum insulin-like growth factor I levels are low, and circulating GH is elevated because of the lack of GH negative feedback. Hypothalamic GHRH mRNA levels in GHR/BP-/- mice were 232 +/- 20% of GHR/BP+/+ littermates (P < 0.01), whereas SRIF and NPY mRNA levels were reduced to 86 +/- 2% and 52 +/- 3% of controls, respectively (P < 0.05; ribonuclease protection assay). Pituitary GHRH-R and GHS-R mRNA levels of GHR/BP-/- mice were elevated to 275 +/- 55% and 319 +/- 68% of GHR/BP+/+ values (P < 0.05, respectively), whereas the sst2 and sst5 mRNA levels did not differ from GHR/BP intact controls as determined by multiplex RT-PCR. Therefore, in the absence of GH negative feedback, both hypothalamic and pituitary expression is altered to favor stimulation of GH synthesis and release. In MT-hGHRH mice, ectopic hGHRH transgene expression elevates circulating GH and insulin-like growth factor I. In this model of GH excess, endogenous (mouse) hypothalamic GHRH mRNA levels were reduced to 69 +/- 6% of nontransgenic controls, whereas SRIF mRNA levels were increased to 128 +/- 6% (P < 0.01). NPY mRNA levels were not significantly affected by hGHRH transgene expression. Also, MT-hGHRH pituitary GHRH-R and GHS-R mRNA levels did not differ from controls. However, sst2 and sst5 mRNA levels in MT-hGHRH mice were increased to 147 +/- 18% and 143 +/- 16% of normal values, respectively (P < 0.05). Therefore, in the presence of GH negative feedback, both hypothalamic and pituitary expression is altered to favor suppression of GH synthesis and release.

Secretagogues and the somatotrope: signaling and proliferation

Somatotrope function requires consideration of both growth hormone (GH) secretion and cellular proliferation. The regulation of these processes is, to a large extent, controlled by three hypothalamic hormones: GH-releasing hormone (GHRH), somatostatin (SRIF), and an as-yet-unidentified GH secretagogue (GHS). Each binds to G protein-linked membrane receptors through which signaling occurs. Our laboratory has used a series of genetic and transgenic models with perturbations of individual components of the GH regulatory system to study both somatotrope signaling and proliferation. Impaired GHRH signaling is present in the lit mouse, which has a GHRH receptor (R) mutation, and the dw rat, which has a post-receptor signaling defect. Both models also have impaired responses to a GHS, implying an interaction between the two signaling systems. The spontaneous dwarf rat (SDR), in which a mutation of the GH gene results in total absence of the hormone, shows characteristic changes in the hypothalamic regulatory hormones due to an absence of GH feedback and alterations in the expression of each of their pituitary receptors. Treatment of SDRs with GHRH and a GHS has allowed demonstration of a stimulatory effect of GHRH on GHRH-R, GHS-R, and SRIF type 2 receptor (SSTR-2) expression and an inhibitory effect on SSTR-5 expression. GH also modifies the expression of these receptors, though its effects are seen at later time periods and appear to be indirect. Overall, the results indicate a complex regulation of GH secretion in which somatotrope receptor, as well as ligand expression, exerts an important physiological role. Both the SDR and the GH-R knockout (ko) mouse have small pituitaries and decreased somatotropes, despite elevated GHRH secretion and intact GHRH-R signaling. Introduction of the hGHRH transgene into GH-R ko mice confirmed that the proliferative effects of GHRH require GH/insulin-like growth factor-I (IGF-I) action. The results offer new insights into factors participating in somatotrope proliferation.

Modulation of pituitary somatostatin receptor subtype (sst1-5) messenger ribonucleic acid levels by changes in the growth hormone axis

The role of individual components of the hypothalamic-pituitary-GH axis in the modulation of pituitary somatostatin (SRIF) receptor subtype (sst1-5) synthesis was assessed using multiplex RT-PCR to measure receptor messenger RNA (mRNA) levels in normal rats and spontaneous dwarf rats (SDRs). In SDRs, a strain with no immunodetectable GH, pituitary sst1 and sst2 mRNA levels were elevated, sst5 mRNA levels were reduced, and sst3 and sst4 mRNA levels did not significantly differ from those in normal controls. Treatment of SDRs with GH (72 h), but not insulin-like growth factor I, significantly decreased sst2 mRNA levels and increased sst4 and sst5 mRNA levels above vehicle-treated control levels. To test whether more rapid changes in circulating GH levels could alter SRIF receptor subtype expression, normal rats were infused (iv) with GH-releasing hormone (GHRH) for 4 h in the presence or absence of SRIF antiserum. GHRH infusion increased pituitary sst1 and sst2 and decreased sst5, but had no effect on sst3 and sst4 mRNA levels. Immunoneutralization of SRIF, which produced a rise in circulating GH levels, did not alter basal or GHRH-mediated SRIF receptor subtype expression. These observations indicate that acute suppression of SRIF tone does not regulate pituitary SRIF receptor subtype mRNA levels in vivo. The possibility that elevated circulating GH concentrations induced by GHRH infusion were responsible for the observed changes in SRIF receptor subtype mRNA levels was examined by infusing SDRs with GHRH for 4 h. GHRH did not increase sst1 mRNA levels in SDRs above their already elevated value. However, GHRH infusion produced an increase in sst2 and a decrease in sst5 mRNA levels similar to those observed in normal rats, indicating that the acute effects of GHRH on SRIF receptor subtype expression are independent of circulating GH levels. Primary rat pituitary cell cultures were incubated with GHRH (10 nM) or forskolin (10 microM) for 4 h to determine whether GHRH could directly mediate SRIF receptor subtype mRNA. GHRH treatment increased sst1 and sst2 mRNA levels and decreased sst5 mRNA levels, but had no effect on sst3 and sst4, similar to the results in vivo. The effect of forskolin mimicked that of GHRH on sst1, sst2, and sst5 mRNA, suggesting that GHRH acts through cAMP to directly mediate gene transcription or mRNA stability of these SRIF receptor subtypes. In addition, forskolin reduced sst3 and sst4 expression. These results strongly suggest that rat pituitary sst1, sst2, and sst5 mRNA levels are regulated both in vivo and in vitro by GHRH. The stimulatory action of GHRH on sst1 and sst2 and the inhibitory action on sst5 indicate that these receptor subtypes have independent and unique roles in the modulation of pituitary GH release.

Glucocorticoids regulate pituitary growth hormone secretagogue receptor gene expression

Glucocorticoids regulate growth hormone (GH) secretion by modulating both hypothalamic and pituitary function. At the level of the pituitary, glucocorticoids increase GH and GH-releasing hormone receptor (GHRH-R) gene expression. To test if glucocorticoids might also regulate the pituitary expression of the recently identified GH secretagogue (GHS) receptor, GHS-R; adult male rats were adrenalectomized or sham operated, and treated with the synthetic glucocorticoid (dexamethasone, 200 microg/day) or vehicle for 8 days. Pituitary GHS-R mRNA levels were assessed by reverse transcriptase polymerase chain reaction (RT-PCR). Adrenalectomy decreased pituitary GHS-R mRNA to 45% of vehicle-treated, sham-operated rats (P < 0.05). Administration of dexamethasone increased GHS-R mRNA levels in sham-operated as well as in adrenalectomized rats (199 +/- 24% (P < 0.05) and 369 +/- 48% (P < 0.01) of vehicle-treated controls). Addition of dexamethasone to primary rat pituitary cell cultures increased GHS-R mRNA levels in a dose- and time-dependent manner while the transcriptional inhibitor, actinomycin D, completely blocked the stimulatory action of dexamethasone. Taken together, these results suggest glucocorticoids directly increase pituitary GHS-R mRNA levels by stimulating GHS-R gene transcription.

p27Kip1-deficient mice exhibit accelerated growth hormone-releasing hormone (GHRH)-induced somatotrope proliferation and adenoma formation

p27Kip1 (p27) controls cell cycle progression by binding to and inhibiting the activity of cyclin dependent kinases. Disruption of the p27 gene in mice (p27-/-) results in increased body growth with a disproportionate enlargement of the spleen, thymus, testis, ovary and pituitary. The increase in pituitary size is due to selective hyperplasia of the intermediate lobe (IL) while the anterior lobe (AL) is not overtly affected. p27 heterozygous mice (p27+/-), as well as p27-/- mice, are hypersensitive to radiation- and chemical-induced tumors compared to wildtype (p27+/+) littermates. Therefore, unlike classical tumor suppressors, only a reduction in p27 levels is necessary to predispose tissues to secondary tumor promoters. Consistent with these studies is the fact that the p27 gene sequence and mRNA levels appear normal in human pituitary adenomas while p27 protein levels are decreased. Therefore, a reduction in p27 levels could be sufficient to sensitize pituitary cells to tumorigenic factors. To test this hypothesis, metallothionein promoter-driven, human growth hormone-releasing hormone (MT-hGHRH) transgenic mice, that exhibit somatotrope hyperplasia before 9 months of age and subsequent adenoma formation with 30 - 40% penetrance, were crossbred with p27+/- mice for two successive generations to produce p27+/+, p27+/- and p27-/- mice that expressed the hGHRH transgene. At 10 - 12 weeks of age, p27-/- and p27+/+, hGHRH mice were larger than their p27+/+ littermates and displayed characteristic hyperplasia of the IL and AL, respectively. Expression of the hGHRH transgene in both p27+/- and p27-/- mice selectively expanded the population of somatotropes within the AL, where pituitaries of p27+/-, hGHRH and p27-/-, hGHRH mice were two- and fivefold larger than p27+/+, hGHRH pituitaries, respectively. There was also a synergistic effect of hGHRH transgene expression and p27-deficiency on liver, spleen and ovarian growth. At 6 - 8 months of age, 83% of p27+/-, hGHRH mice displayed macroscopic AL adenomas (>100 mg), while all pituitaries from p27+/+, hGHRH mice remained hyperplastic (<20 mg). In contrast to the dramatic effects of p27-deficiency on hGHRH-induced organ growth, elimination of p53, by crossbreeding MT-hGHRH mice to p53-deficient mice, did not augment the hyperplastic/tumorigenic effects of hGHRH transgene expression. Taken together these results demonstrate that a reduction in p27 expression is sufficient to sensitize somatotropes to the proliferative actions of excess GHRH, resulting in the earlier appearance and increased penetrance of hGHRH-induced pituitary tumors.

Increase in mRNA concentrations of pituitary receptors for growth hormone-releasing hormone and growth hormone secretagogues after neonatal monosodium glutamate treatment

Previous studies have demonstrated that neonatal monosodium glutamate (MSG) treatment destroys growth hormone releasing-hormone (GHRH) neurones within the hypothalamic arcuate nucleus, decreases serum GH and insulin-like growth factor (IGF-I) concentrations, and retards linear growth. In the present study we investigated whether expression of pituitary GH, GHRH receptors (GHRH-R), growth hormone secretagogue receptors (GHS-R) and liver IGF-I is altered in this model of GHRH deficiency. In addition, we investigated if treatment of MSG-lesioned rats with the GHRH agonist, JI-38, would 'normalise' the GH-axis. Serum GH and IGF-I concentrations were determined by RIA, GH mRNA levels were evaluated by Northern blotting, and GHRH-R, GHS-R and IGF-I mRNA levels were measured by semiquantitative RT-PCR. In accord with previous reports, neonatal MSG treatment caused 50% and 76% decreases in serum GH and IGF-I concentrations, respectively, at 8 weeks of age. The decline in circulating GH was accompanied by a 56% reduction in total pituitary GH content, which was a reflection of the decrease in total pituitary protein. However, GH concentration (per mg protein) was unaltered. Despite the maintenance of a normal GH concentration, GH mRNA concentration (per microg total RNA) was suppressed by 42%, compared to saline-treated controls (P<0.05). These data indicate that a post-transcriptional mechanism, such as a reduction in the GH secretory rate, acts to maintain intracellular GH concentrations. The fall in circulating concentrations of GH leads to a 42% decrease in liver IGF-IB mRNA levels, while liver IGF-IA transcripts showed only a 27% suppression. In contrast, pituitary GHRH-R and GHS-R mRNA levels (per microg total RNA) were increased in MSG-lesioned rats by 96% and 180% of normal values (P<0.01), respectively. Twice daily treatment of MSG-lesioned rats (for 2 weeks) with the GHRH agonist, JI-38, increased serum GH and IGF-I levels, as measured 20 h after the last agonist injection. However, GH, IGF-I, GHRH-R and GHS-R mRNA levels were not altered at this time. These results demonstrate that intermittent GHRH agonist treatment stimulates pituitary GH secretion and GH in turn stimulates hepatic IGF-I but that effects on gene expression are not sustained. Collectively, our observations demonstrate a complex interplay between transcriptional, translational and post-translational mechanisms within each level of the GH-axis following destruction of GHRH neurones by neonatal MSG treatment.

Isolated familial somatotropinomas: establishment of linkage to chromosome 11q13.1-11q13.3 and evidence for a potential second locus at chromosome 2p16-12

The majority of somatotropinomas are sporadic, although a small number occur with a familial aggregation, either as a component of an endocrine neoplasia complex that includes multiple endocrine neoplasia type 1 (MEN-1) and Carney complex (CNC) or as isolated familial somatotropinomas (IFS). IFS is defined as the occurrence of at least two cases of acromegaly or gigantism in a family that does not exhibit MEN-1 or CNC. This rare disease is associated with loss of heterozygosity (LOH) on chromosome 11q13, the locus of the MEN-1 gene, although the MEN-1 sequence and expression appear normal. These data suggest the presence of another tumor suppressor gene located at 11q13 that is important in the control of somatotrope proliferation. To establish linkage of IFS to 11q13 and to define the candidate interval of the IFS gene, we performed haplotype and allelotype analyses on two families with IFS. Collectively, allelic retention in one tumor and a recombinant haplotype in an affected individual mapped the tumor suppressor gene involved in the pathogenesis of IFS to a region of 8.6 cM between polymorphic microsatellite markers D11S1335 and INT-2 located at chromosome 11q13.1-13.3. Maximum two-point LOD scores for five markers within this region were 3.0 or more at theta = 0.0. As somatotropinomas are the predominant pituitary tumor subtype associated with CNC and arise before 30 yr of age, which is strikingly similar to the age at diagnosis for IFS, we explored the possibility that the putative CNC genes might also contribute to the pathogenesis of IFS. Although the genetic defect responsible for the complex is unknown, CNC has been mapped by linkage analysis to chromosomes 2p15-16 and 17q23-24 in different kindreds. Two-point LOD scores less than -2.0 were obtained using marker D17S949 from chromosome 17q23-24, excluding linkage. However, LOD scores of 2.5 were obtained for markers within 2p16-12; therefore, linkage of IFS to chromosome 2p cannot be excluded. This report establishes linkage of the tumor suppressor gene involved in the pathogenesis of IFS to chromosome 11q13.1-13.3 and identifies a potential second locus at chromosome 2p16-12.

Growth hormone (GH)-releasing hormone (GHRH) and the GH secretagogue (GHS), L692,585, differentially modulate rat pituitary GHS receptor and GHRH receptor messenger ribonucleic acid levels

The ability of synthetic GH secretagogues (GHSs) to elicit a maximal release of GH in vivo is dependent on an intact GH-releasing hormone (GHRH) signaling system. The role of GHRH in GHS-induced GH release has been attributed primarily to the ability of GHS to release GHRH from hypothalamic neurons. However, GHS also releases GH directly at the pituitary level. Several lines of evidence suggest that GHRH is necessary to maintain pituitary responsiveness to GHS by stimulating GHS receptor (GHS-R) synthesis. To test this hypothesis, male rats (250-290 g) were anesthetized with ketamine/xylazine (which does not alter pulsatile GH secretion) and infused i.v. with a GHRH analog ([des-NH2Tyr1,D-Ala15]hGRF-(1-29)-NH2; 10 microg/h) or saline for 4 h. Serum was analyzed for GH, pituitaries were collected, and GHS-R and GHRH receptor (GHRH-R) messenger RNA (mRNA) levels were determined by RT-PCR. GHRH infusion resulted in a 10-fold increase in circulating GH concentrations that were accompanied by an increase in GHS-R mRNA levels to 200% of those in saline-treated controls (P < 0.01). In contrast, GHRH reduced GHRH-R mRNA levels slightly, but not significantly (P < 0.07). The stimulatory effect of GHRH on GHS-R mRNA levels was independent of somatostatin tone, as pretreatment with somatostatin antiserum did not alter the effectiveness of GHRH infusion. In contrast, blockade of somatostatin actions up-regulated GHRH-R mRNA levels under basal conditions and unmasked the inhibitory effects GHRH on its own receptor mRNA. These observations suggest GHRH-R mRNA is tonically suppressed by somatostatin. The stimulatory effect of GHRH on GHS-R mRNA levels was independent of circulating GH, as GHRH infusion in spontaneous dwarf rats, which do not have immunodetectable GH, increased GHS-R mRNA levels to 150% of those in saline-treated controls (P < 0.05). To determine whether this effect occurred by a direct action on the pituitary, primary cell cultures from normal rat pituitaries were incubated with GHRH (0.01-10 nM) or forskolin (10 microM) for 4 h. These GH secretagogues did not alter GHS-R mRNA levels in vitro. However, GHRH and forskolin reduced GHRH-R mRNA levels by 40% (P < 0.05). To determine whether the synthesis of the GHS-R, like that of the GHRH-R, is negatively mediated by its own ligand, anesthetized rats were infused with the nonpeptidyl secretagogue, L-692,585 (100 microg/h) for 4 h. Neither circulating GH (at 4 h) nor GHRH-R mRNA levels were significantly altered by L-692,585, whereas GHS-R mRNA levels were reduced by 50% (P < 0.05). Taken together, these results indicate that GHRH-induced up-regulation of pituitary GHS-R synthesis in vivo is indirect and independent of both somatostatin and GH. They also demonstrate that GHS-R synthesis, like that of GHRH-R, can be rapidly down-regulated by its own ligand.

Growth hormone-releasing hormone receptor (GHRH-R) and growth hormone secretagogue receptor (GHS-R) mRNA levels during postnatal development in male and female rats

Experimental evidence suggests that differential pituitary sensitivity to hypothalamic signals exerts a role in mediating both age and sex dependent patterns of growth hormone (GH) release and synthesis. One mechanism by which pituitary sensitivity to hypothalamic GH regulators could be modified is by the differential synthesis of their pituitary receptors. In the present report we therefore studied the age and sex dependency of the expression of receptors for two known stimulators of GH release, growth hormone-releasing hormone (GHRH) and the synthetic peptidyl and non-peptidyl GH secretagogues (GHSs). Pituitary GHRH receptor (GHRH-R) and GHS receptor (GHS-R) mRNA levels were measured by reverse transcriptase-polymerase chain reaction (RT-PCR) in male and female rats at postnatal day 1, 10, 30 and 75. We also examined the age- and sex-dependent expression of the GHS-R in whole hypothalamic extracts, since the GHS-R is also expressed in a variety of nuclei within the hypothalamus and has been linked to central regulation of the GH-axis. Pituitary GHRH-R mRNA concentrations were age-dependent; the highest levels were observed in d1 pituitaries and then declined with age, reaching a nadir by d30. These results are in concordance with the age-related decline in pituitary GHRH sensitivity. In contrast, the ontogenic pattern of GHS-R expression was bimodal; GHS-R mRNA concentrations in dl and d30 pituitaries were approximately twice those at d10 and d75. These results mirror the transient increase in GHS sensitivity observed around the onset of puberty, suggesting that gonadal steroids mediate GHS-R expression. GHRH-R mRNA levels were comparable in males and females within each age while GHS-R mRNA levels were gender dependent. At d30, male GHS-R mRNA levels were 30% greater than in their female counterparts. This was reversed at d75, when females had 89% more GHS-R mRNA per pituitary and 65% more per somatotrope than did age-matched males. These sexual differences further support a role for gonadal steroids in the modulation of pituitary GHS-R synthesis. The ontogenic and gender-specific pattern of hypothalamic GHS-R expression differed from that observed for the pituitary. Hypothalamic GHS-R mRNA levels increased with age but exhibited no significant sex difference at each age tested. Taken together, these data demonstrate that changes in the levels of pituitary GHS-R mRNA, but not GHRH-R mRNA, are associated with changes in the gonadal steroid environment, thereby implicating the GHS/GHS-R signalling system as a control point in the establishment and maintenance of sexually dimorphic patterns of GH secretion.

Loss of heterozygosity on chromosome 11q13 in two families with acromegaly/gigantism is independent of mutations of the multiple endocrine neoplasia type I gene

Familial acromegaly/gigantism occurring in the absence of multiple endocrine neoplasia type I (MEN-1) or the Carney complex has been reported in 18 families since the biochemical diagnosis of GH excess became available, and the genetic defect is unknown. In the present study we examined 2 unrelated families with isolated acromegaly/gigantism. In family A, 3 of 4 siblings were affected, with ages at diagnosis of 19, 21, and 23 yr. In family B, 5 of 13 siblings exhibited the phenotype and were diagnosed at 13, 15, 17, 17, and 24 yr of age. All 8 affected patients had elevated basal GH levels associated with high insulin-like growth factor I levels and/or nonsuppressible serum GH levels during an oral glucose tolerance test. GHRH levels were normal in affected members of family A. An invasive macroadenoma was found in 6 subjects, and a microadenoma was found in 1 subject from family B. The sequence of the GHRH receptor complementary DNA in 1 tumor from family A was normal. There was no history of consanguinity in either family, and the past medical history and laboratory results excluded MEN-1 and the Carney complex in all affected and unaffected screened subjects. Five of 8 subjects have undergone pituitary surgery to date, and paraffin-embedded pituitary blocks were available for analysis. Loss of heterozygosity on chromosome 11q13 was studied by comparing microsatellite polymorphisms of leukocyte and tumor DNA using PYGM (centromeric) and D11S527 (telomeric), markers closely linked to the MEN-1 tumor suppressor gene. All tumors exhibited a loss of heterozygosity at both markers. Sequencing of the MEN-1 gene revealed no germline mutations in either family, nor was a somatic mutation found in tumor DNA from one subject in family A. The integrity of the MEN-1 gene in this subject was further supported by demonstration of the presence of MEN-1 messenger ribonucleic acid, as assessed by RT-PCR. These data indicate that loss of heterozygosity in these affected family members appears independent of MEN-1 gene changes and suggest that a novel (tissue-specific?) tumor suppressor gene(s) linked to the PYGM marker and expressed in the pituitary is essential for regulation of somatotrope proliferation.

Role of growth hormone (GH)-releasing hormone and somatostatin on leptin-induced GH secretion

Leptin is a hormone secreted by the adipocytes that regulates food intake and energy expenditure. It is known that growth hormone (GH) secretion is markedly influenced by body weight, being suppressed in obesity and cachexia, and recent data have demonstrated that GH release is regulated by leptin levels. Although one of the sites of action of leptin is likely to be the hypothalamus, since leptin receptor mRNA is particularly abundant in several hypothalamic nuclei, the mechanisms by which leptin regulates GH secretion are not yet known. The aim of the present study was to investigate whether leptin could act at the hypothalamic level modulating somatostatin and GH-releasing hormone (GHRH) expression. The administration of anti-GHRH serum (500 microl, i.v.) completely blocked leptin-induced GH release in fasting rats. In contrast, the treatment with anti-somatostatin serum (500 microl, i.v.) significantly increased GH release in this condition. Furthermore, leptin administration (10 microg, i.c.v.) to intact fasting animals reversed the inhibitory effect produced by fasting on GHRH mRNA levels in the arcuate nucleus of the hypothalamus, and increased somatostatin mRNA content in the periventricular nucleus. Finally, leptin administration (10 microgram, i.c.v.) to hypophysectomized fasting rats increased GHRH mRNA levels, and decreased somatostatin mRNA content, indicating an effect of leptin on hypothalamic GHRH- and somatostatin-producing neurons. These findings suggest a role for GHRH and somatostatin as mediators of leptin-induced GH secretion.

Growth hormone-dependent regulation of pituitary GH secretagogue receptor (GHS-R) mRNA levels in the spontaneous dwarf Rat

Growth hormone secretagogues (GHSs) are synthetic peptidyl and nonpeptidyl compounds that are believed to stimulate the release of GH by a direct effect on the pituitary somatotrope and by stimulation of growth hormone-releasing hormone (GHRH) release and the suppression of somatostatin (SRIH) tone. Recently, the receptor for these pharmacologic agents was cloned and its expression localized to the pituitary and hypothalamus. The elucidation of an unique GHS receptor (GHS-R) suggests there is a yet to be identified endogenous ligand which could exert an important role in regulation of GH secretion. It is clearly established that GH acts to regulate its own production by feeding back at the level of the hypothalamus to downregulate GHRH and upregulate SRIH synthesis and by induction of IGF-I, which acts at the pituitary to block somatotrope responsiveness to GHRH. If the endogenous GHS/GHS-R signaling system is important in regulating GH release, it might be reasoned that changes in circulating GH concentrations would also directly or indirectly (via generation of IGF-I) modify GHS-R production. To test this hypothesis we used RT-PCR to examined pituitary and hypothalamic GHS-R mRNA levels in the spontaneous dwarf rat (SDR), an animal model characterized by the absence of GH due to a point mutation in the GH gene. In the absence of GH feedback regulation, SDR pituitary GHS-R mRNA levels were 385 +/- 61% greater (p < 0.01) than those observed in normal controls while SDR hypothalamic GHS-R mRNA levels were not significantly different from those in normal rats. Three-day subcutaneous infusion of rat GH by osmotic pump reduced SDR pituitary GHS-R mRNA levels to 55 +/- 9% of vehicle-treated controls (p < 0.05) but did not significantly alter hypothalamic GHS-R mRNA levels. To test if the changes in GHS-R mRNA levels observed following GH treatment were due to elevation of circulating IGF-I concentrations, SDRs were infused with recombinant human IGF-I. Replacement of IGF-I did not significantly alter either pituitary or hypothalamic GHS-R mRNA levels, indicating that GH acts independent of circulating IGF-I to regulate pituitary GHS-R expression in the SDR model.

Hypothalamic/pituitary-axis of the spontaneous dwarf rat: autofeedback regulation of growth hormone (GH) includes suppression of GH releasing-hormone receptor messenger ribonucleic acid

In this study, the spontaneous dwarf rat (SDR) has been used to examine GHRH production and action in the selective absence of endogenous GH. This dwarf model is unique in that GH is not produced because of a point mutation in the GH gene. However, other pituitary hormones are not obviously compromised. Examination of the hypothalamic pituitary-axis of SDRs revealed that GHRH messenger RNA (mRNA) levels were increased, whereas somatostatin (SS) and neuropeptide Y (NPY) mRNA levels were decreased, compared with age- and sex-matched normal controls, as determined by Northern blot analysis (n = 5 animals/group; P < 0.05). The elevated levels of GHRH mRNA in the SDR hypothalamus were accompanied by a 56% increase in pituitary GHRH receptor (GHRH-R) mRNA, as determined by RT-PCR (P < 0.05). To investigate whether the up-regulation of GHRH-R mRNA resulted in an increase in GHRH-R function, SDR and control pituitary cell cultures were challenged with GHRH (0.001-10 nM; 15 min), and intracellular cAMP concentrations were measured by RIA. Interestingly, SDR pituitary cells were hyperresponsive to 1 and 10 nM GHRH, which induced a rise in intracellular cAMP concentrations 50% greater than that observed in control cultures (n = 3 separate experiments; P < 0.05 and P < 0.01, respectively). Replacement of GH, by osmotic minipump (10 microg/h for 72 h), resulted in the suppression of GHRH mRNA levels (P < 0.01), whereas SS and NPY mRNA levels were increased (P < 0.05), compared with vehicle-treated controls (n = 5 animals/treatment group). Consonant with the fall in hypothalamic GHRH mRNA was a decrease in pituitary GHRH-R mRNA levels. Although replacement of insulin-like growth factor-I (IGF-I), by osmotic pump (5 microg/h for 72 h), resulted in a rise in circulating IGF-I concentrations comparable with that observed after GH replacement, IGF-I treatment was ineffective in modulating GHRH, SS, or NPY mRNA levels. However, IGF-I treatment did reduce pituitary GHRH-R mRNA levels, compared with vehicle-treated controls (P < 0.05). These results further validate the role of GH as a negative regulator of hypothalamic GHRH expression, and they suggest that SS and NPY act as intermediaries in GH-induced suppression of hypothalamic GHRH synthesis. These data also demonstrate that increases in circulating IGF-I are not responsible for changes in hypothalamic function observed after GH treatment. Finally, this report establishes modulation of GHRH-R synthesis as a component of GH autofeedback regulation.

Acromegaly and Cushing's syndrome due to ectopic production of GHRH and ACTH by a thymic carcinoid tumour: in vitro responses to GHRH and GHRP-6

A 50-year-old male presented with diabetes mellitus and Cushing's syndrome associated with a large mediastinal mass. The levels of serum cortisol were high (1500-1800 nmol/l) without diurnal variation. Plasma ACTH levels (200-250 ng/l) and urinary excretion of cortisol were also increased. The levels of these hormones did not change in response to stimulation with corticotrophin releasing hormone (CRH) or suppression with high doses of dexamethasone. The patient had an elevated baseline GH level (7.3 mU/l), and the levels of immunoreactive GH-releasing hormone (GHRH) in eight plasma samples were markedly increased (600-1500 ng/l). Circulating levels of IGF-1, chromogranin A and neuropeptide Y (NPY) were also increased. Computer-assisted tomography and octreotide scintigraphy revealed a large mediastinal tumour and metastases in the left supraclavicular fossa. During treatment with octreotide, the baseline GH level was decreased (to 4.4 mU/l), while the GH pulse height was unchanged. Surgical removal of most of the tumour tissue resulted in a further decrease in the baseline serum GH level to a value (1.6 mU/l) about 20% of that before treatment, while the pulse height and mean GH were affected to a lesser extent. Postoperatively, circulating levels of cortisol and IGF-1 decreased, and the patient exhibited clinical improvement. Histological examination showed a neuroendocrine tumour with characteristics consistent with a foregut carcinoid of thymic origin. Immunoreactive GHRH, ACTH and NPY, but not immunoreactive GH, were detected in 80-90% of the tumour cells and the three peptides appeared to be co-localized. In primary culture, cells from this tumour displayed calcium influx in response to GHRH or GH releasing peptide-6 (GHRP-6), while there were not such responses by cells from another carcinoid not producing GHRH, ACTH or NPY. These results demonstrate a rare case of ectopic production of GHRH, ACTH and NPY, and indicate that the tumour cells were responsive to GHRH and GHRP-6 as well as octreotide.

Expression of a fusion gene consisting of the mouse growth hormone-releasing hormone gene promoter linked to the SV40 T-antigen gene in transgenic mice

Limited information is available concerning the regulation of growth hormone-releasing hormone (GHRH) gene expression in the hypothalamus, largely because of the lack of a suitable cellular model. In an attempt to immortalize hypothalamic GHRH-producing neurons, we have generated a transgenic mouse model which expresses the simian virus 40 (SV40) T-antigen gene (Tag) under the control of the GHRH gene promoter. The transgene contains approximately 5 kb of mouse GHRH gene sequences, including 3.5 kb of the 5'-flanking region, the entire hypothalamic exon 1 and 1.5 kb of intron 1, fused to the SV40 Tag gene. This construct was microinjected into fertilized oocytes. Fourteen of 96 mice born had integrated the transgene. These mice were fertile and showed no signs of central or peripheral tumors. The pattern of expression of the SV40 Tag gene was analyzed in four different transgenic lines by RT-PCR. The tissues tested include: hypothalamus, pituitary, cortex, cerebellum, spinal cord, adrenal, testis, spleen and lung. Transgene expression was consistently detected in the hypothalamus of all lines. In addition, SV40 Tag expression was also detected in the hypothalamus by Northern blot analysis in two of the transgenic lines. SV40 Tag expression was also detected in the testis of all transgenic lines by RT-PCR. This result was not expected since the GHRH gene sequences present in the transgene do not include the testis-specific transcription initiation site previously described. This suggests that GHRH gene expression in the mouse testis can be directed by regulatory sequences located downstream of the testis specific transcription start site. We conclude that the promoter region of the GHRH gene included in this construct contains the regulatory elements necessary to drive hypothalamic and testis expression in vivo. In addition, all mice from one of the transgenic lines developed cataracts in both eyes. SV40 Tag expression was detected not only in eyes with cataracts, but also, to a lesser extent, in eyes from other transgenic lines. Furthermore, the endogenous GHRH gene was found to be expressed in the eyes of normal mice.

Effects of antagonists of growth hormone-releasing hormone (GHRH) on GH and insulin-like growth factor I levels in transgenic mice overexpressing the human GHRH gene, an animal model of acromegaly

Transgenic mice overexpressing the human GH-releasing hormone (hGHRH) gene, an animal model of acromegaly, were used to investigate the effects of potent GHRH antagonists MZ-4-71 and MZ-5-156 on the excessive GH and insulin-like growth factor I (IGF-I) secretion caused by overproduction of hGHRH. Because metallothionein (MT)-GHRH mice express the hGHRH transgene in various tissues, including the pituitary and hypothalamus, initial experiments focused on the effectiveness of the GHRH antagonists in blocking basal and stimulated GH secretion from pituitary cells in vitro. Both MZ-4-71 and MZ-5-156 suppressed basal release of GH from superfused MT-GHRH pituitary cells, apparently by blocking the action of endogenously produced hGHRH. In addition, these antagonists effectively eliminated the response to stimulatory action of exogenous hGHRH(1-29)NH2 (30 and 100 nM). To ascertain whether MZ-4-71 and MZ-5-156 could antagonize the effect of hGHRH hyperstimulation in vivo, each antagonist was administered to MT-GHRH transgenic mice in a single iv dose of 10-200 microg. Both compounds decreased serum GH levels in transgenic mice by 39-72% at 1 h after injection. The inhibitory effect of 50 microg MZ-5-156 was maintained for 5 h. Twice daily ip administration of 100 microg MZ-5-156 for 3 days suppressed the highly elevated serum GH and IGF-I concentrations in transgenic mice by 56.8% and 39.0%, respectively. This treatment also reduced IGF-I messenger RNA levels in the liver by 21.8% but did not affect the level of GH messenger RNA in the pituitary. Our results demonstrate that GHRH antagonists MZ-4-71 and MZ-5-156 can inhibit elevated GH levels caused by overproduction of hGHRH. The suppression of circulating GH concentrations induced by the antagonists seems to be physiologically relevant, because both IGF-I secretion and synthesis also were reduced. Our findings, showing the suppression of GH and IGF-I secretion with GHRH antagonists, suggest that this class of analogs could be used for the diagnosis and therapy of disorders characterized by excessive GHRH secretion.

Homologous down-regulation of growth hormone-releasing hormone receptor messenger ribonucleic acid levels

Repeated stimulation of pituitary cell cultures with GH-releasing hormone (GHRH) results in diminished responsiveness, a phenomenon referred to as homologous desensitization. One component of GHRH-induced desensitization is a reduction in GHRH-binding sites, which is reflected by the decreased ability of GHRH to stimulate a rise in intracellular cAMP. In the present study, we sought to determine if homologous down-regulation of GHRH receptor number is due to a decrease in GHRH receptor synthesis. To this end, we developed and validated a quantitative RT-PCR assay system that was capable of assessing differences in GHRH-R messenger RNA (mRNA) levels in total RNA samples obtained from rat pituitary cell cultures. Treatment of pituitary cells with GHRH, for as little as 4 h, resulted in a dose-dependent decrease in GHRH-R mRNA levels. The maximum effect was observed with 0.1 and 1 nM GHRH, which reduced GHRH-R mRNA levels to 49 +/- 4% (mean +/- SEM) and 54 +/- 11% of control values, respectively (n = three separate experiments; P < 0.05). Accompanying the decline in GHRH-R mRNA levels was a rise in GH release; reaching 320 +/- 31% of control values (P < 0.01). Because of the possibility that the rise in medium GH level is the primary regulator of GHRH-R mRNA, we pretreated pituitary cultures for 4 h with GH to achieve a concentration comparable with that induced by a maximal stimulation with GHRH (8 micrograms GH/ml medium). Following pretreatment, cultures were stimulated for 15 min with GHRH and intracellular cAMP accumulation was measured by RIA. GH pretreatment did not impair the ability of GHRH to induce a rise in cAMP concentrations. However, as anticipated, GHRH pretreatment (10 nM) significantly reduced subsequent GHRH-stimulated cAMP to 46% of untreated controls. These data suggest that GHRH, but not GH, directly reduces GHRH-R mRNA levels. To determine whether this effect was mediated through cAMP, cultures were treated with forskolin, a direct stimulator of adenylate cyclase. Forskolin (10 microM) significantly reduced GHRH-R mRNA concentrations (37 +/- 6% of control values) indicating that GHRH acts through the cAMP-second messenger system cascade to regulate GHRH-R mRNA. The somatostatin analogue, octreotide (10 nM), which has been previously reported to decrease adenylate cyclase activity, did not affect GHRH-R mRNA levels. Taken together, these results indicate that GHRH inhibits the production of its own receptor by a receptor-mediated, cAMP-dependent reduction of GHRH-R mRNA accumulation.

Suppression of growth hormone (GH) hypersecretion due to ectopic GH-releasing hormone (GHRH) by a selective GHRH antagonist

We have recently demonstrated that a competitive antagonist of GHRH, (N-Ac-Tyr1,D-Arg2)GHRH-(1-29)NH2 (GHRH-Ant), eliminates nearly all nocturnal GH pulsatility in normal subjects, supporting the hypothesis that GH pulsatility is driven by GHRH. In this study, we compared the effects of every 12 h i.v. boluses of either GHRH-Ant or saline on 24-h GH profiles in a patient with acromegaly due to a metastatic GHRH-secreting carcinoid tumor. Bolus doses of GHRH-Ant (400 micrograms/kg, i.v.) acutely suppressed GH concentration to 30-40% of the pretreatment baseline, and this effect lasted 3-4 h. Administration of GHRH (0.33 microgram/kg, i.v.) bolus resulted in a small rise in GH, and this effect was blocked by GHRH-Ant (400 micrograms/kg). During saline treatment, the secretory patterns of both GH and ectopic GHRH were pulsatile; however, there was no correlation between changes in plasma GHRH and GH concentrations. This lack of correlation was probably due to the majority of circulating GHRH immunoreactivity consisting of nonbiologically active GHRH fragments. These data support the hypothesis that GH hypersecretion in the ectopic GHRH syndrome requires GHRH receptor occupancy and validates the use of GHRH-Ant to probe the potential involvement of endogenous GHRH in patients with acromegaly due to pituitary somatotropinoma.

The tyrosine hydroxylase-human growth hormone (GH) transgenic mouse as a model of hypothalamic GH deficiency: growth retardation is the result of a selective reduction in somatotrope numbers despite normal somatotrope function

Dwarf tyrosine hydroxylase-human GH (TH-hGH) transgenic mice carrying the hGH reporter gene targeted by the TH promoter express hGH in those regions of the hypothalamus responsible for regulation of pituitary GH secretion. Central expression of the hGH gene decreases GH-releasing hormone (GHRH) and increases somatostatin, which ultimately impacts on pituitary function by reducing the overall amount of GH produced. In the present study, we sought to determine if the reduction of pituitary GH in TH-hGH mice could be attributed to a decrease in somatotrope cell numbers and/or an impairment of somatotrope function. Pituitaries from TH-hGH or wild-type (WT) male and female mice were enzymatically dispersed, counted, and immunostained for GH, PRL, TSH, and ACTH. The total number of pituitary cells recovered from TH-hGH pituitaries was approximately one-half of that from WT controls. However, the proportion of cells that stained for GH and PRL were virtually identical (males, GH-TH-hGH, 58.1 +/- 1.0% [mean +/- SEM] vs. WT, 60.7 +/- 1.0%; PRL-TH-hGH, 43.4 +/- 2.2% vs. WT, 43.1 +/- 0.7%; females, GH-TH-hGH, 47.9 +/- 2.3% vs. WT, 41.5 +/- 3.5%; PRL-TH-hGH, 43.3 +/- 3.2% vs. WT, 47.1 +/- 3.3%). In contrast, percentages of both TSH- and ACTH-containing cells were increased in TH-hGH pituitaries relative to controls (males, TSH-TH-hGH, 15.1 +/- 2.3% vs. WT, 9.6 +/- 1.5%; ACTH-TH-hGH, 24.5 +/- 2.5% vs. WT, 10.9 +/- 0.9%; females: TSH-TH-hGH, 11.3 +/- 0.7% vs. WT, 7.5 +/- 0.6%; ACTH-TH-hGH, 19.8 +/- 1.6% vs. WT, 9.3 +/- 0.8%; P < 0.05). Calculation of the absolute number of each cell type per pituitary demonstrated TH-hGH mice to have about one-half the number of GH and PRL cells, whereas TSH and ACTH cell populations were comparable with that of their WT counterparts. Immunocytochemical localization of GH cells within pituitary sections from TH-hGH mice revealed that somatotropes were confined primarily to the lateral wings of the adenohypophysis, in contrast to the heterogeneous distribution of GH-immunostained cells in WT pituitaries. To assess the functional capacity of the somatotrope populations, pituitary cells from TH-hGH and WT mice were challenged with mouse GHRH (0.01-10 nM). The quantity of GH released (as assessed by both RIA and reverse hemolytic plaque assay) under basal and stimulated conditions did not differ among TH-hGH and WT pituitary cell cultures. Similarly, GHRH induced intracellular cAMP levels were comparable. These results indicate that proliferation of pituitary somatotropes and lactotropes is much more sensitive to changes in GHRH input than is the capability of developing regulated GH secretory function.