Treatment of pituitary-dependent Cushing's disease with the multireceptor ligand somatostatin analog pasireotide (SOM230): a multicenter, phase II trial

CONTEXT

There is currently no medical therapy for Cushing's disease that targets the pituitary adenoma. Availability of such a medical therapy would be a valuable therapeutic option for the management of this disorder.

OBJECTIVE

Our objective was to evaluate the short-term efficacy of the novel multireceptor ligand somatostatin analog pasireotide in patients with de novo, persistent, or recurrent Cushing's disease.

DESIGN

We conducted a phase II, proof-of-concept, open-label, single-arm, 15-d multicenter study.

PATIENTS

Thirty-nine patients with either de novo Cushing's disease who were candidates for pituitary surgery or with persistent or recurrent Cushing's disease after surgery without having received prior pituitary irradiation.

INTERVENTION

Patients self-administered sc pasireotide 600 microg twice daily for 15 d.

MAIN OUTCOME MEASURE

Normalization of urinary free cortisol (UFC) levels after 15 d treatment was the main outcome measure.

RESULTS

Of the 29 patients in the primary efficacy analysis, 22 (76%) showed a reduction in UFC levels, of whom five (17%) had normal UFC levels (responders), after 15 d of treatment with pasireotide. Serum cortisol levels and plasma ACTH levels were also reduced. Steady-state plasma concentrations of pasireotide were achieved within 5 d of treatment. Responders appeared to have higher pasireotide exposure than nonresponders.

CONCLUSIONS

Pasireotide produced a decrease in UFC levels in 76% of patients with Cushing's disease during the treatment period of 15 d, with direct effects on ACTH release. These results suggest that pasireotide holds promise as an effective medical treatment for this disorder.

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.

Consensus statement: medical management of acromegaly

In November 2003, the Pituitary Society and the European Neuroendocrine Association sponsored a consensus workshop in Seville to address challenging issues in the medical management of acromegaly. Participants comprised 70 endocrinologists and neurosurgeons with international expertise in managing patients with acromegaly. All participants participated in the workshop proceedings, and the final document written by the scientific committee reflects the consensus opinion of the interactive deliberations. The meeting was supported by an unrestricted educational grant from Ipsen. No pharmaceutical representatives participated in the program planning or in the scientific deliberations.

Diagnosis and treatment of acromegaly complications

The Pituitary Society in conjunction with the European Neuroendocrine Association held a consensus workshop to develop guidelines for diagnosis and treatment of the co-morbid complications of acromegaly. Fifty nine pituitary specialists (endocrinologists, neurosurgeons and cardiologists) assessed the current published literature on acromegaly complications in light of recent advances in maintaining tight therapeutic control of GH hypersecretion. The impact of elevated GH levels on cardiovascular disease, hypertension, diabetes, sleep apnea, colon polyps, bone disease, reproductive disorders, and neuropsychologic complications were considered. Guidelines are proposed for effective management of these complications in the context of overall acromegaly control. When appropriate, requirements for prospective evidence-based studies and surveillance database development are enunciated. Effective management of co-morbid acromegaly complications will lead to improved morbidity and mortality in acromegaly.

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.

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.

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.

Enhanced growth of mice lacking the cyclin-dependent kinase inhibitor function of p27(Kip1)

SUMMARY

Disruption of the cyclin-dependent kinase-inhibitory domain of p27 enhances growth of mice. Growth is attributed to an increase in cell number, due to increased cell proliferation, most obviously in tissues that ordinarily express p27 at the highest levels. Disruption of p27 function leads to nodular hyperplasia in the intermediate lobe of the pituitary. However, increased growth occurs without an increase in the amounts of either growth hormone or IGF-I. In addition, female mice were infertile. Luteal cell differentiation is impaired, and a disordered estrus cycle is detected. These results reflect a disturbance of the hypothalamic-pituitary-ovarian axis. The phenotypes of these mice suggest that loss of p27 causes an alteration in cell proliferation that can lead to specific endocrine dysfunction.

Autofeedback suppression of growth hormone (GH) secretion in transgenic mice expressing a human GH reporter targeted by tyrosine hydroxylase 5'-flanking sequences to the hypothalamus

Transgenic mice expressing a tyrosine hydroxylase-human (h) GH fusion gene in the hypothalamus exhibit a dwarf phenotype. The GH feedback mechanism(s) underlying the growth retardation in these animals was investigated by assessing peptide and messenger RNA (mRNA) levels of the hormones of the hypothalamic-GH-IGF-I axis. Pituitary GH content, hypothalamic GH-releasing hormone (GHRH) and somatostatin (SRIH) content, and serum IGF-I levels were measured by RIA. mRNA levels of hypothalamic GHRH and SRIH and of pituitary GH and the GHRH receptor were measured by Northern blot hybridization. Transgenic mice of both sexes and their wild-type littermates were studied at 2-4 months of age. The pituitary GH content was markedly reduced by 85% in male and by 87% in female transgenic mice compared to that in wild-type controls (P < 0.01 for both). The pituitary GH mRNA content was also decreased by 73% (P = 0.002) in transgenic male mice. Circulating IGF-I levels were significantly reduced by 66% and 68% in male and female transgenic mice, respectively (P = 0.001). The hypothalamic GHRH content was significantly reduced by 19% and 33% (P < 0.05) in male and female transgenic mice, respectively. No significant difference was detected, however, in the hypothalamic SRIH content between wild-type and transgenic mice. Hypothalamic GHRH mRNA levels were significantly decreased by 35% (P = 0.002) in transgenic male mice compared to those in wild-type littermates. In contrast, SRIH mRNA was not significantly changed. An even greater reduction (61%; P = 0.003) was observed in pituitary GHRH receptor mRNA in transgenic mice. These data indicate that the GH deficiency and dwarf phenotype of the tyrosine hydroxylase-hGH transgenic mouse can be attributed primarily to impaired hypothalamic GHRH production. The mechanism of GH feedback inhibition appears to involve direct suppression of GHRH gene expression by locally produced hGH in the hypothalamus.

Discordant effects of endogenous and exogenous somatostatin on growth hormone-releasing hormone secretion from perifused mouse hypothalami

The role of somatostatin (SRIF) on growth hormone-releasing hormone (GRH) secretion has been controversial because of discordant findings that may be model dependent. We have examined possible explanations for these findings by altering endogenous and exogenous SRIF tone in a mouse hypothalamic perifusion system. Four mediobasal hypothalamic fragments were perifused in a single chamber for 6 h. After a 2-hour equilibration period, test substances were introduced and maintained throughout the perifusion. After an additional 2 h, fragments were submaximally stimulated with 30 mM K+. Depletion of tissue SRIF by 10(-3) M cysteamine increased K(+)-stimulated GRH release 2-fold without altering basal GRH secretion. Removal of endogenous SRIF tone by anti-SRIF serum also augmented the GRH response to K+. Perifusion of SRIF at concentrations ranging from 10(-12) to 10(-8) M significantly increased the GRH response to K+ in a dose-dependent manner. A significant increase was also observed during the perifusion of 10(-9) M octreotide. Simultaneous perifusion with anti-SRIF serum and 10(-9) M octreotide (to which the antibody does not bind) resulted in a response of GRH to K+ that was similar to that observed with anti-SRIF serum alone. Combined perifusion with cysteamine and 10(-9) M SRIF also resulted in a GRH response to K+ that did not differ from the response observed during cysteamine alone. The enhancement of GRH secretion by reduction of endogenous SRIF tone or tissue content implies an inhibitory role of endogenous SRIF on GRH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone-releasing hormone immunoreactivity in mouse placenta, maternal blood, and amniotic fluid: molecular characterization and secretion from primary cell cultures in vitro

The GH-releasing hormone (GRH) gene, along with those of many other hypothalamic hormones, is abundantly expressed in mouse and rat placenta. The presence of GRH immunoreactivity (GRH-IR) is described in mouse placenta, maternal blood, and amniotic fluid, and its molecular form has been characterized using HPLC. Two different molecular forms of mouse GRH-IR (mGRH-IR) were detected in the mouse hypothalamus and one in placenta. Twenty-five percent of mGRH-IR in the hypothalamus corresponded to mGRH(1-42)OH, whereas the remainder, and all of the mGRH-IR in placenta, had a retention time consistent with the GRH precursor. High levels of mGRH-IR were detected in both maternal plasma and amniotic fluid. In addition, a mouse placental cell primary culture system was established to study the regulation of mGRH-IR release. Turnover of mGRH in placental cells was rapid, resulting in a 24-h media content of 10 times that present in cells. Both 1-oleoyl-2-acetyl-sn-glycerol and 1,2-dioctanoyl-sn-glycerol significantly stimulated the release of mGRH-IR from cultured placental cells into the incubation media but had no effect on total peptide synthesis. These results suggest that the release of mGRH-IR from placental cells is mediated, at least in part, by the activation of protein kinase C. The HPLC elution profiles of mGRH-IR released from placental cells under basal and 1-oleoyl-2-acetyl-sn-glycerol-stimulated conditions were similar to those in placental tissue. Although the biological function of mGRH-IR in placental, maternal plasma, and amniotic fluid is not yet clear, the presence of mGRH-IR in these tissues and circulating fluids suggests the possibility that mGRH-IR may exert an important role in both fetal and maternal physiology.

A neuroendocrine overview of aging

The neuroendocrine changes associated with aging are numerous, and tend to vary quantitatively, if not qualitatively between species. The extent to which neuroendocrine dysfunction contributes to the undesirable features of aging remains to be fully determined. Nevertheless, although the aging process itself may not have a neuroendocrine basis, identification and correction of the associated neuroendocrine dysfunction may be important in enhancing the quality of life during this period.

Somatotroph hyperplasia without pituitary adenoma associated with a long standing growth hormone-releasing hormone-producing bronchial carcinoid

Acromegaly is most often associated with a pituitary somatotroph adenoma. While multiple lines of evidence suggest an intrinsic somatic cell defect in adenoma formation, the role of hypothalamic hormones in pituitary tumorigenesis remains unclear. We describe the functional and morphological features of the pituitary of a patient with a long-standing ectopic GH-releasing hormone (GHRH)-producing tumor and acromegaly. This 28-yr-old woman with a documented 10-yr history of a disseminated bronchial carcinoid was evaluated for clinical features of acromegaly. Elevated serum GH (88 micrograms/L) was not suppressed after glucose ingestion and was paradoxically stimulated by TRH, but did not respond to GHRH or GnRH administration. Serum insulin-like growth factor-1 (730 micrograms/L; normal, < 333 micrograms/L), insulin-like growth factor-binding protein-3 (9.5 mg/L; normal, 2-4.2 mg/L), and GHRH (26.1 micrograms/L; normal, < 20 ng/L) were elevated. Magnetic resonance imaging revealed a diffusely enlarged pituitary gland. Octreotide treatment for 4 months resulted in suboptimal clinical and biochemical responses. Examination of the transsphenoidally resected pituitary by light microscopy revealed diffuse somatotroph hyperplasia, with intact reticulin network and preservation of the acinar architecture. Electron microscopy showed active somatotrophs interspersed with other cell types. In situ hybridization revealed very strong positivity for GH mRNA, whereas fewer cells contained GHRH and somatostatin mRNA signals. Dispersed pituitary cells secreted GH into culture medium. GH release was stimulated by GHRH and GHRH plus TRH, but not by TRH alone; GH was suppressed by octreotide in vitro. We conclude that sustained exposure to ectopic GHRH leads to somatotroph hyperplasia, but, at least in this case, was not sufficient for adenomatous transformation.

Development of neuroepithelial tumors of the adrenal medulla in transgenic mice expressing a mouse hypothalamic growth hormone-releasing hormone promoter-simian virus-40 T-antigen fusion gene

Expression of the mouse GH-releasing hormone (GRH) gene is restricted to neurons within the hypothalamus and to placenta. In an attempt to generate immortalized mouse hypothalamic neurons expressing GRH, the proximal 872-nucleotide segment of the 5'-flanking region of the hypothalamic mouse GRH gene was cloned by polymerase chain reaction and ligated to a 2.7-kilobase DNA sequence encoding the simian virus-40 (SV40) T-antigen, so that regulation of SV40 T-antigen expression was dependent on sequences within the mGRH 5'-flanking region. This region contains both TATA and CAAT boxes. The mouse GRH/SV40 T-antigen fusion gene was injected into 1-cell mouse embryos, and SV40 T-antigen incorporation in the mouse genome was found in 11 of 77 live births (3 males and 8 females). Although no evidence of hypothalamic tumors was found, all mice that expressed the transgene also developed tumors originating in the adrenal medulla. Gene copy number varied from 1-20 and was inversely proportional to survival, which ranged from 7-16 weeks. Corticosterone levels were normal. The male transgenic mice were fertile, and their progeny expressed the transgene and developed similar tumors. Microscopic examination of the tumors revealed a primitive neuroectodermal neoplasm that exhibited hematogenous and lymph node metastases and contained 100 ng norepinephrine, 2.85 ng epinephrine, and 1.1 ng dopamine/mg tumor tissue. Primary culture of dispersed tumor cells released norepinephrine into the medium (180 pg/ml.24 h). Cell lines from 2 tumors were established and exhibited characteristics similar to those of mixed neuroblastoma or primitive neuroectodermal tumors. In conclusion, the proximal 872 nucleotides of the hypothalamic mouse GRH promoter contain elements directing tissue-specific expression limited to early adrenal neuroectodermal cells. Other GRH DNA sequences appear to be required for restricted expression of mouse GRH within the hypothalamus.

Impaired growth hormone-releasing hormone signal transduction in the dwarf (dw) rat is independent of a generalized defect in the stimulatory G-protein, Gs alpha

The homozygous dwarf (dw) rat pituitary is characterized by a 95% reduction in GH content as well as a 75-80% reduction in the number of somatotrophs. The nature of the mutation responsible for this phenotype is unknown. Previous investigations from our laboratory indicate that dw somatotrophs exhibit decreased sensitivity and a reduced GH secretory response to GH-releasing hormone in vitro, accompanied by a decreased generation of cAMP. We hypothesized that dw rats have a defect in the pathway linking the GH-releasing hormone receptor to adenylyl cyclase and focused on the expression and function of the stimulatory G-protein, Gs alpha. When corrected for differences in pituitary size and cell number, GH mRNA content was reduced by 78% compared to that in controls. However, there was no difference in Gs alpha mRNA content or size in dw pituitaries. Similarly, there was no difference in the content or size of mRNA for the pituitary transcription factor pit-1 in dw pituitaries. Immunoblot analysis of pituitary membrane proteins using a Gs alpha-specific antibody revealed no differences in size, quantity, or relative distribution of Gs alpha peptides between control and dw pituitaries. In addition, cholera toxin effectively ribosylated dw Gs alpha, and there were no differences in the size, quantity, or relative distribution of ribosylated membrane proteins between dw and control pituitaries. Finally, to examine for mutations in other regions of the Gs alpha-coding sequence, we cloned the full-length Gs alpha cDNA from dw rat pituitaries by polymerase chain reaction. The sequence of this clone was identical to that of normal rat Gs alpha cDNA. These results indicate that 1) 52-kilodalton Gs alpha appears to predominate in both normal and dw pituitary; 2) the content, function, and sequence of Gs alpha in adult dw rats are normal; and 3) a generalized Gs alpha regulatory or structural mutation as the cause of the observed phenotype can be excluded. The results also demonstrate that there is no decrease in pit-1 expression in the adult dw pituitary.

Obesity-associated decrease in growth hormone-releasing hormone gene expression: a mechanism for reduced growth hormone mRNA levels in genetically obese Zucker rats

The secretion of growth hormone (GH) is impaired in the genetically obese Zucker rat where GH gene expression and plasma GH levels are depressed; however, the underlying mechanism of this abnormality remains unclear. We have evaluated the potential causative role of hypothalamic GH-releasing hormone (GHRH) and somatostatin (SRIH) gene expression in the onset of the decreased GH mRNA levels by studying both GHRH and SRIH mRNA and peptide levels in obese and lean rats at 5 weeks of age when the decrease in GH mRNA is first detected. At that age both GHRH content and GHRH mRNA were significantly reduced in obese rats as compared to lean controls; hypothalamic SRIH content was also decreased in obese rats, but SRIH mRNA levels did not differ. Since GHRH is capable of stimulating GH gene expression, the decreased GHRH mRNA level could be a critical factor in causing the attenuation in GH gene expression and consequent diminution of circulating plasma GH.

Differential effects of central and peripheral administration of growth hormone (GH) and insulin-like growth factor on hypothalamic GH-releasing hormone and somatostatin gene expression in GH-deficient dwarf rats

The roles of GH and insulin-like growth factor-I (IGF-I) in the regulation of hypothalamic GH-releasing hormone (GRH) and somatostatin (SRIH) gene expression were investigated in the GH-deficient dwarf (dw) rat, in which endogenous feedback signals are lacking. Adult male and female dw rats were treated with GH or IGF-I by systemic (sc) administration or intracerebroventricular (icv) infusion, and hypothalamic GRH and SRIH mRNA were determined by Northern blotting and densitometric analysis. Systemic sc injection of rGH (75 micrograms every 12 h for 3 days) decreased GRH mRNA levels in both sexes. However, systemic sc injection of human IGF-I (150 micrograms every 12 h for 3 days) did not affect GRH mRNA levels in either sex despite significant stimulation of body weight gain. The use of a continuous sc infusion, which normalized serum IGF-I levels, and prolongation of the treatment period to 7 days also failed to change GRH mRNA levels. SRIH mRNA was unaffected by systemic administration of either GH or IGF-I. Continuous icv infusion of GH (1 microgram/h for 7 days) decreased GRH mRNA levels in both sexes, but did not alter SRIH mRNA levels. Continuous icv infusion of IGF-I (100 ng/h for 7 days) decreased GRH mRNA in both sexes. In contrast, SRIH mRNA levels were increased in both sexes. IGF-I decreased GRH mRNA levels at icv infusion rates of 100 and 300 ng/h and stimulated SRIH mRNA levels at infusion rates of 30 and 100 ng/h. Food intake was unaffected at these infusion rates. Changes in GRH and SRIH mRNA levels in response to systemic or central GH and IGF-I administration were similar in both sexes, except that the decrease in GRH mRNA levels produced by the icv infusion of IGF-I was greater in female than in male rats. The results provide evidence for a direct inhibitory feedback effect of GH in the central nervous system on the regulation of hypothalamic GRH gene expression that is independent of peripheral IGF-I. IGF-I feedback, in contrast, appears to originate with central nervous system-derived IGF-I and exhibits a stimulatory effect on SRIH mRNA as well as an inhibitory effect on GRH mRNA.

Mouse hypothalamic growth hormone-releasing hormone and somatostatin responses to probes of signal transduction systems

Signal transduction mechanisms involved in mouse growth hormone-releasing hormone (GRH) and somatostatin (SRIH) release were investigated using an in vitro perifusion system. Hypothalamic fragments were exposed to depolarizing agents, protein kinase A and C activators, and a calcium ionophore. The depolarizing agents, KCl (60 mM) and veratridine (50 microM), induced similar patterns of GRH and SRIH release. Somatostatin release in response to both agents was twofold greater than that of GRH. Forskolin (10 microM and 100 microM), an adenylate cyclase activator, stimulated both GRH and SRIH release, though with different secretory profiles. The SRIH response was prolonged and persisted beyond removal of the drug from the system, while the GRH response was brief, ending even prior to forskolin removal. Neither GRH nor SRIH were stimulated by 1,9-dideoxy-forskolin (100 microM), a forskolin analog with cAMP-independent actions. A23187 (5 microM), a calcium ionophore, stimulated the release of SRIH to a much greater extent than that of GRH. The GRH and SRIH secretory responses to PMA (1 microM), a protein kinase C activator, were similar, though delayed. The results suggest that 1) GRH and SRIH secretion are regulated by both protein kinase A and C pathways, and 2) depolarizing agents are important for the release of both hormones.

Stimulatory effect of thyroid hormone on growth hormone gene expression in a human pituitary cell line

A cell line (GX) derived from a pituitary tumor of a boy with gigantism, which exhibits morphological characteristics of epithelial cells, has been established and maintained in culture for over 2 yr. Initially, GX cells secreted GH which was stimulated by T3. However, after 2 yr in culture, GH production in GX cells is no longer detectable, though GH messenger RNA (mRNA) synthesis has been maintained. The GX cell line is a somatomammotroph line as indicated by the presence of both GH and PRL mRNAs. GX cells were used to study the effect of T3 on GH gene expression. A nearly 3-fold increase in GH mRNA accumulation was observed in GX cells in response to the addition of T3 to the culture medium. This increase was also observed in serum-free medium, indicating that the effect of T3 does not require the presence of other hormones or serum factors. Neither PRL nor beta-actin mRNA accumulation was stimulated by T3 in GX cells. Nuclear runoff experiments demonstrated that the stimulatory effect of T3 on GH mRNA accumulation is related to transcriptional activation of the human GH gene.

Differential sensitivity of growth hormone-releasing hormone and somatostatin release from perifused mouse hypothalamic fragments in response to glucose deficiency

The effects of glucose deficiency on growth hormone (GH)-releasing hormone (GRH) and somatostatin (SRIH) release from mouse hypothalamic fragments were investigated using an in vitro perifusion system. Fragments were perifused with Krebs-Ringer bicarbonate solution (KRB) containing 5.6 mM glucose for 3 h followed by reduced glucose concentrations in KRB for the next 2 h. GRH release was simulated by 0.7-2.8 mM glucose in an inverse concentration-dependent manner. In contrast, SRIH release was not stimulated by glucose at concentrations of 2.8 and 1.4 mM; only at 0.7 mM was there a modest stimulation of SRIH release that was comparable to the effect of 2.8 mM glucose on GRH release. The maximal stimulation of GRH and SRIH release by 0.7 mM glucose was 221 and 150%, respectively, of controls. Glucose concentrations of 11.2 and 22.4 mM inhibited GRH release but did not alter SRIH release. The glucose analog 2-deoxy-D-glucose (2-DG; 5.6-39.2 mM) also stimulated GRH release in a dose-dependent manner, and SRIH release was less sensitive to 2-DG than was GRH. The maximal stimulation of GRH and SRIH release by 39.2 mM 2-DG was 190 and 147%, respectively, of controls. Increases in GRH and SRIH release stimulated by 30 mM KCl 1 h after exposure to low glucose or 2-DG were not significantly different from those after exposure to 5.6 mM glucose. However, the SRIH response to K(+)-induced depolarization was much greater than that of GRH. The glucose intermediate pyruvate (4.9 and 9.8 mM) partially inhibited both GRH and SRIH release induced by 0.7 mM glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of effect of drinking water chlorine on lipid and thyroid metabolism in healthy humans

Animal studies and a single human epidemiological study have suggested that chlorine in drinking water may raise the level of blood cholesterol. The purpose of this study was to determine whether a 4-week exposure to drinking water chlorine (1.5 L per day) at a concentration of 20 ppm (ppm = mg/L) under controlled conditions would alter circulating parameters of lipid metabolism in healthy humans. Thirty men and thirty women each completed an 8-week protocol during which diet (600 mg cholesterol per day, 40% calories as fat) and other factors known to affect lipid metabolism were controlled. For the first 4 weeks of the protocol, all subjects consumed distilled water. For the second 4 weeks, half of the subjects were assigned randomly to drink 1.5 L per day of chlorinated water (20 ppm), while the others continued drinking distilled water. Four blood samples were collected from each subject at the end of each 4-week study period. Compared to the control group, those subjects given chlorine showed no significant changes in total plasma cholesterol, triglycerides, HDL-cholesterol, LDL-cholesterol, or apolipoproteins A1, A2, or B. There was a trend toward low serum thyroxine and triiodothyronine levels in men given chlorine, though thyroid-stimulating hormone levels were unchanged. This trend, if real, was not clinically significant. Thus, short-term exposure to chlorinated drinking water at 20 ppm appears to have no significant impact on parameters of lipid or thyroid metabolism in healthy humans.

Effects of drinking water monochloramine on lipid and thyroid metabolism in healthy men

The purpose of this study was to determine whether a 4-week consumption of 1.5L per day of drinking water containing monochloramine at a concentration of 2 ppm (ppm = mg/L) or 15 ppm under controlled conditions would alter parameters of lipid or thyroid metabolism in healthy men. Forty-eight men completed an 8-week protocol during which diet (600 mg cholesterol per day, 40% calories as fat) and other factors known to affect lipid metabolism were controlled. During the first 4 weeks of the protocol, all subjects consumed distilled water. During the second 4 weeks, one-third of the subjects were assigned randomly to drink 1.5 L per day of water containing 2 ppm of monochloramine, to drink 1.5 L per day of water containing 15 ppm monochloramine, or to continue drinking distilled water. Four blood samples were collected from each subject at the end of each 4-week study period. Subjects drinking monochloramine at a concentration of 2 ppm showed no significant changes in total cholesterol, triglycerides, HDL cholesterol, LDL cholesterol, apolipoproteins A1, A2, or B when compared to the distilled water group. Parameters of thyroid function also were unchanged by exposure to monochloramine at this concentration. However, subjects drinking monochloramine at a concentration of 15 ppm experienced an increase in the level of apolipoprotein B. Other parameters of lipid and thyroid metabolism did not change. We conclude that consumption of drinking water containing 2 ppm of monochloramine does not alter parameters of lipid and thyroid metabolism in healthy men. Consumption of water containing 15 ppm monochloramine may be associated with increased levels of plasma apolipoprotein B.

Congenital gigantism due to growth hormone-releasing hormone excess and pituitary hyperplasia with adenomatous transformation

The cause of gigantism in most patients is a GH-secreting pituitary tumor. In this report, a case of congenital gigantism due to probable central hypersection of GH-releasing hormone (GHRH) is described. Normal at birth (4.4 kg; 53 cm), our 7-yr-old male patient grew progressively thereafter to attain a height of 182 cm and a weight of 99.4 kg at the time of our evaluation. The markedly increased baseline plasma levels of GH (730 micrograms/L) did not suppress during a standard 3-h oral glucose tolerance test, but did increase 54% after iv infusion of GHRH. Baseline plasma levels of insulin-like growth factor-I, PRL, and immunoreactive GHRH were also markedly increased. Computed imaging of the head showed a large, partially cystic sellar and suprasellar mass. Extensive imaging studies did not localize a potential source of GHRH. Preoperative treatment with octreotide and bromocriptine for 4 months resulted in a 25% reduction of suprasellar tissue mass. The pituitary tissue removed at transsphenoidal and transfrontal operations showed massive somatotroph, lactotroph, and mammosomatotroph hyperplasia. Areas of GH- and PRL-secreting cell adenomatous transformation were also evident. No histological or immunohistochemical evidence of a pituitary source of GHRH was found. The peripheral plasma immunoreactive GHRH concentration remained unaffected by pharmacological and surgical interventions. We suspect that a congenital hypothalamic regulatory defect may be responsible for the GHRH excess in this case.

Inhibitory effects of ethanol on the growth hormone (GH)-releasing hormone-GH-insulin-like growth factor-I axis in the rat

Ethanol administration decreases GH secretion in humans and experimental animals. The mechanism of these inhibitory effects was investigated by evaluating the spontaneous secretory pattern of GH in chronically cannulated unanesthetized rats, plasma insulin-like growth factor-I (IGF-I) concentrations, and hypothalamic GH-releasing hormone (GHRH) and somatostatin, and pituitary GH mRNA levels. Body weight gain was reduced in ethanol (5%)-liquid diet-fed rats (n = 6) for 6 days compared to that in both isocalorically pair-fed controls (n = 6) and ad libitum-fed animals (n = 6). Spontaneous GH secretion was markedly decreased (by 75-90%) in ethanol-fed rats compared to that in pair-fed and ad libitum-fed groups, while pulsatile pattern of GH release was preserved, with secretory bursts occurring every 180-220 min in all groups. Mean 6-h plasma GH levels in ethanol-, pair-, and ad libitum-fed animals were: 18.8 +/- 4.5, 113.3 +/- 14.9, and 179.6 +/- 30.1 ng/ml, respectively (P < 0.01, ethanol vs. each control). Plasma IGF-I concentrations were decreased in the ethanol-fed rats (338 +/- 16 ng/ml) compared to those in pair-fed (427 +/- 39 ng/ml; P < 0.05) and ad libitum-fed (769 +/- 25 ng/ml; P < 0.01) rats. Ethanol treatment decreased GHRH mRNA levels to 9% of those in ad libitum-fed (P < 0.01) and 20% of those in pair-fed (P < 0.05) animals, whereas it did not significantly alter somatostatin or GH mRNA levels. The results indicate that the effects of ethanol inhibit GH secretion primarily at the hypothalamic level, resulting in impaired GHRH gene expression. Since the GHRH-GH-IGF-I axis has an important role in growth regulation, the growth retardation seen in experimental models of alcohol abuse may be a consequence at least in part of the suppressive effects of ethanol on this axis.

Growth hormone-releasing factor analogs with hydrophobic residues at position 19. Effects on growth hormone releasing activity in vitro and in vivo, stability in blood plasma in vitro, and secondary structure

To test the hypothesis that replacing Ala19 in growth hormone-releasing factor (GRF) with more hydrophobic residues will increase growth hormone releasing activity, four GRF analogs were prepared and tested. The molecules were made by substituting Val, Ile, or Leu at position 19 of [Thr2,Ala15,Leu27]bGRF(1-29)NH2. The compounds were evaluated for growth hormone (GH) releasing activity in vitro (rat anterior pituitary cells) and in vivo (steers). Additionally, their half-life in vitro was determined in bovine plasma, and their secondary structure was examined by circular dichroism. In pituitary cells, peptides with substitutions at position 19 had the following potencies: Ala (native), 0.37; Val, 1.16; Ile, 0.37; Leu, 0.043. When assayed in steers as a single iv bolus, over a 2-h period, the compounds gave the following integrated GH response: Ala, 2.75; Val, 2.67; Ile, 2.57; Leu, 1.55. Only the Leu analog was statistically different from the other three (p = 0.05). In bovine plasma, the half-lives (hours) were as follows: Ala, 4.9; Val, 6.6; Ile, 12.3; Leu, 14.7. In phosphate buffer the compounds were calculated to have the following percent helical content: Ala, 26; Val, 21; Ile, 27; Leu, 32. For these analogs, helicity in aqueous buffer is inversely related to their in vitro activity. Using a linear multiple regression model, the plasma half-life of the analogs positively correlated (r2 = 0.999) with both the hydrophobicity of the residue at position 19 and the helicity of the analog. Although the Val analog had both increased inherent activity and increased plasma stability in vitro compared to the Ala analog, in this study we were unable to demonstrate an increase in activity in vivo. The in vivo GH releasing activity of the analogs was not simply related to a combination of their intrinsic GH releasing activity and their in vitro plasma half-life. This suggests that in vivo additional factors are moderating the expression of activity.

Regulation of growth hormone secretion

The secretion of growth hormone (GH) is controlled by a complex regulatory system. The primary control is exerted by two neuroendocrine hormones, GH-releasing hormone and somatostatin, though other hypothalamic neuropeptides directly and indirectly participate in this process. The regulation of each of these neurohormones is now being clarified at both physiologic and molecular levels, as are their respective roles in the generation of pulsatile GH secretion and in GH feedback regulation. Considerable information has been amassed concerning signal transduction systems mediating the effects of hypothalamic hormones on GH secretion. Although multiple second messengers have been implicated, the adenylate cyclase-cyclic AMP-protein kinase A system appears to exert a predominant role. The developmental regulation of the somatotropes and of GH gene expression is also of importance in determining the GH responses to releasing and inhibiting hormones. The availability of several rodent strains with genetic disorders of growth associated with impaired GH secretion, along with the development of transgenic models, has permitted a more detailed analysis of the role of cytokines and growth factors on both somatotrope biology and hormone secretion. Finally, knowledge gained from studies in animals is permitting a better understanding of the mechanisms involved in physiologic GH secretion and altered GH secretion associated with specific disease states in humans.

Morphologic effects of hGRH gene expression on the pituitary, liver, and pancreas of MT-hGRH transgenic mice. An in situ hybridization analysis

Morphologic changes in the pituitary, liver, and pancreas of mice with the metallothionein-human growth hormone--releasing hormone (MT-hGRH) transgene were analyzed by in situ hybridization histochemistry (ISH). There was progression from somatotroph hyperplasia to neoplasia in pituitaries of transgenic mice. Pituitary neoplasms were present between 9 to 12 months of age in some mice. Magnetic resonance imaging (MRI) readily identified enlarged pituitaries in MT-hGRH transgenic mice. Serum mouse GH and hGRH levels were marked elevated in MT-hGRH transgenic mice. In situ hybridization histochemistry showed mRNA for hGRH in liver, pituitary, pancreas, spleen, and in most other tissues examined. Combined ISH and immunohistochemistry in the pituitary gland showed that some of the GH cells also produced hGRH, and ultrastructural immunohistochemical analysis of pituitaries showed that GH and hGRH were localized in the same cell and within the same secretory granules. Liver cells of MT-hGRH transgenic mice showed evidence of hypertrophy, and the pancreatic islets were hyperplastic with significant increases in the islet cell areas. The morphologic changes in the liver were distinctive enough to separate control littermates from MT-hGRH transgenic mice in all cases. The enlarged pancreatic islets had increased numbers of insulin-producing cells. Immunoreactive hGRH and hGRH mRNA were both localized in islet cells, and an intense hybridization signal of hGRH mRNA, but only weak staining for hGRH protein, were detected in the liver of transgenic mice. These results indicate that excessive hGRH production leads to distinct morphologic changes in various organs in MT-hGRH transgenic mice and that there is temporal progression from hyperplasia to adenomatous somatotrophs in pituitaries with chronic stimulation by hGRH that involves paracrine, endocrine, and autocrine mechanisms.