Growth hormone releasing hormone-sensitive adenylate cyclase activity in growth hormone-producing pituitary adenoma: correlation to the response of plasma growth hormone to growth hormone releasing hormone in patients with acromegaly

The correlation between response of plasma GH to GHRH and the GHRH-induced stimulation of the intracellular adenylate cyclase (AC) activity in pituitary adenoma cell membranes in acromegalic patients was investigated. Each peak plasma GH level after iv administration of GHRH ranged from 1.1 to 13.8 times the basal level in 13 acromegalic patients. On the other hand, the maximal stimulation of intracellular AC activity (cAMP production) induced by GHRH varied from 1.4 to 6.4 times the control level in each GH-producing pituitary adenoma cell membrane. A significant positive correlation (r = 0.89, P less than 0.005) between plasma GH response to GHRH and intracellular cAMP production stimulated by GHRH was observed in nine of the acromegalic patients. In contrast, the response of plasma GH to GHRH was significantly blunted, despite a fairly large production of intracellular cAMP stimulated by GHRH, in the other four acromegalic patients. These results suggest that GHRH-induced GH release from GH-producing pituitary adenomas of patients with acromegaly may be regulated not only by GHRH receptor-adenylate cyclase system but also modified by several other factors including somatostatin and Sm-C.     

The role of the GH/IGF system in pituitary tumorigenesis.

Pituitary adenomas are mostly benign tumours that originate from differentiated anterior pituitary cells. Altered expression of growth factors or their receptors could enhance clonal expansion of pituitary adenoma cells. GHRH overstimulation or an activating point mutation in the Gs a-subunit leads to increased GH secretion and tumour formation. In contrast, IGF-I suppresses basal and GHRH-stimulated GH secretion in pituitary adenoma cells, whereas prolactin secretion is unaffected. Somatostatin analogues and pegvisomant, a novel growth hormone-receptor antagonist, results in a reduction of serum IGF-I levels and clinical improvement in patients suffering from pituitary adenoma. Thus, this review focuses on the role of the growth hormone/insulin-like growth factor system in pituitary tumorigenesis with particular focus on the genetic alterations described in pituitary adenomas up to now.     

Clinical studies with GHRH in man

GHRH was isolated from two GHRH-secreting pancreatic tumors which resulted in clinical acromegaly. Over 98% of acromegalic patients have a pituitary adenoma; however, acromegaly may occasionally result from ectopic or eutopic GHRH secretion. Administration of GHRH to normal adults stimulates growth hormone (GH) secretion; it may also stimulate GH release in some adults with GH deficiency in childhood and in a majority of GH-deficient children. Continuous infusion of GHRH to normal men stimulates GH secretion which augments naturally occurring GH pulses. GHRH is effective when administered subcutaneously and intranasally, but requires 30- and 300-fold higher doses, respectively. Intermittent subcutaneous GHRH therapy promotes acceleration of linear growth in GH-deficient children and appears promising as a treatment for these children.     

Growth hormone-releasing hormone and somatostatin in normal and tumoral human pituitaries.

In order to further understand the role of endogenous pituitary neuropeptides in pituitary hormonal content and secretion, GHRH, SRIH and GH contents were quantified in GH adenomas obtained from acromegalic patients with plasma GH levels either high (greater than 5 micrograms/l, range 11 to 550 micrograms/l, n = 11) or in the normal range (less than 5 micrograms/l, range 1 to 3.3 micrograms/l, n = 4). Values were compared to those found in normal human pituitaries. No relationship was found between GHRH content and plasma GH or between SRIH and GH content when considering together adenomas and normal pituitaries. Results showed that there is a positive relationship between GHRH and GH content: when GHRH content is high, GH content is also high (normal pituitaries and GH adenomas of acromegalic patients with high plasma GH) and when GHRH content is low, GH content is also low (GH adenomas of acromegalic patients with plasma GH in the normal range). Conversely, SRIH content is negatively related to plasma GH levels: when SRIH is present, plasma GH is in the normal range; when SRIH is undetectable, plasma GH is high.     

Abnormalities of the human growth hormone gene and protooncogenes in some human pituitary adenomas

Hypersecretion of human GH (hGH) or PRL by human pituitary adenomas is not under normal homeostatic control despite normal receptor function mediating the regulatory effects of hypothalamic peptides for these trophic hormones. This implies that the defects underlying hormonal hypersecretion may not reside at the plasma membrane of the adenoma cell; instead, dysregulation may reside at the hormone gene level. To investigate this, genomic DNA derived from a prolactinoma and a hGH-secreting adenoma were digested with the restriction endonuclease EcoRI and the methylation sensitive restriction endonuclease HpaII and hybridized with the 32P-labeled genomic hGH (2.6 kilobase) probe. Our data revealed hypomethylation of genes of the hGH family (hGH and chorionic somatomammotropin) in the absence of gross abnormalities such as gene translocation. In a similar analysis using a 32P-labeled probe consisting of the EcoRI-BamHI (500 base pair) fragment in the 5'-flanking region upstream of the first exon of the hGH gene, hypomethylation of this specific site of the hGH genes was observed. These results are consistent with the concept that hypomethylation of genes is involved in gene expression. At the same time, protooncogene abnormalities in these adenomas were investigated to delineate any genetic basis for their neoplastic growth. Genomic DNA of adenomas were subjected to Southern blotting analysis using a panel of protooncogene probes. Amplification of the v-fos gene was observed in one prolactinoma. The significance of this observation is discussed.     

Acromegaly in a patient with normal pituitary gland and somatotropic adenoma located in the sphenoid sinus

Ectopic acromegaly is a very rare clinical entity occurring in less than 1% of acromegalic patients. In most cases it is caused by GHRH or rarely GH-secreting neoplasms. Even rarer are ectopic pituitary adenomas located in the sphenoid sinus or nasopharynx that originate from pituitary remnants in the craniopharyngeal duct. This dissertation presents the difficulties in visualizing GH-secreting adenoma located in the sphenoid sinus. A 55-year-old man had somatic features of acromegaly for several years. MRI imaging revealed a slightly asymmetric pituitary gland (14 yen 4 mm) without focal lesions. Simultaneously, a spherical mass, 10 mm in diameter, corresponding with ectopic microadenoma was demonstrated on the upper wall of the sphenoid sinus. The serum GH level was 4.3 mg/l, IGF-1 = 615 mg/l, and a lack of GH suppression with oral glucose was proven. After preliminary treatment with a long-acting somatostatin analogue, transsphenoidal pituitary tumour removal was performed. Histopathological, electron microscopical and immunohistochemical analysis revealed densely granulated somatotropic pituitary adenoma: GH(+), PRL(-), ACTH(-), TSH(-), FSH(-), LH(-), MIB1 < 1%, SSTR3(+) and SSTR5(+). Post-surgical evaluation showed normal pituitary MRI scans, GH and IGF-1 levels 0.18 mug/l and 140 mg/l, respectively, as well as normal GH suppression with oral glucose. The careful analysis of possible pituitary embryonic malformations points out their significance for proper localization of extrapituitary adenomas.     

Immunohistochemical examination of pituitary adenomas. Comparison to clinical and endocrinological findings

A comparison was made with the data of 62 cases of pituitary adenoma, evaluated pre- and postoperatively, including as well the results of immunohistochemical hormone examination (also for calcitonin). Prolactin was found in 18 of the 21 adenomas carrying the preoperative diagnosis of prolactinoma, whereas cells containing other hormones (growth hormone, LH, FSH, TSH, ACTH, beta-endorphin), were only occasionally present. The growth hormone was strongly positive in the adenoma tissue in 16 of the 17 cases of acromegaly. 5 of these adenomas were accompanied by a marked hyperprolactinemia and also contained many prolactin cells. 6 of the 19 adenomas diagnosed as being 'inactive' contained hormone-positive cells, but only a very small number of cells. ACTH was found in 3 of the 4 pituitary adenomas of patients with Cushing's disease. 2 of these were also positive for beta-endorphin. The tissue of 1 gonadotrophic adenoma (with elevated FSH in serum) gave positive results with an anti-LH antiserum. Calcitonin was not found in any adenoma. The preoperative serum prolactin levels did not quantitatively correlate with the percentage of prolactin-positive cells.     

Plasma alpha-subunit levels during the treatment of pituitary adenomas with the somatostatin analog (SMS 201-995)

The effect os SMS 201-995 (Sandostatin), a long-acting somatostatin analog, on different types of pituitary adenomas including alpha-subunit elevation is illustrated in this report. Treatment induced a fall in hCG levels in a woman with a pituitary adenoma producing only alpha-subunit. In 3 acromegalic patients, there was only a partial drop in GH and alpha-hCG. The same effect was observed in a woman with menopausal FSH and LH levels. SMS reduced plasma TSH and alpha-hCG in a case of thyrotropic adenoma. Two patients exhibiting FSH- and alpha-hCG-secreting adenomas did not respond to acute administration of SMS 201-995. More patients have to be treated before a definitive statement can be made on the usefulness of somatostatin analogs in the management of different types of pituitary adenomas.     

Human growth hormone association and binding study in vitro: high performance size exclusion chromatography and radioimmunoassay-measured levels in plasma of normal and acromegalic subjects

Human growth hormone (hGH), like other protein hormones, consists of several molecular forms both in the pituitary and in plasma. In recent years carrier proteins have been detected and studied for growth hormone, using different experimental approaches. In the present study we have attempted to investigate whether hGH could be separated and identified in molecular or aggregated forms using high performance size exclusion chromatography and a small plasma sample, without particular treatment, in order to investigate specific hGH-binding proteins in normal as well as in acromegalic subjects. Results showed that it was possible to observe binding or/and the formation of a complex between free hormone and other molecules that could be specific binding proteins. Equilibrium was reached in a few minutes (7-10 min) and was reversible, as observed with labelled hormone using low and high concentrations of hGH in the incubation medium. At 37 degrees C the associated form at equilibrium was 30% of the total (measured as percent of total radioactivity with labelled hormone) in a plasma medium in which the original growth hormone was absent. Acromegalic plasma demonstrated that the percent of the associated form (namely the 80-kDa form) was less than that in normal humans. This may be due to the fact that capacity binding and competition between labelled and non-labelled growth hormone were favorable to the non-labelled form, if only for the high concentrations in this type of pathology. Therefore our results seem to be in agreement with the hypothesis that acromegalic subjects do not lack this aggregation capacity.     

Chronic orchidectomy does not influence the sensitivity of the pituitary somatotropes to varying doses of GHRH administered intravenously to the adult male rhesus monkey

In the present study, the pituitary growth hormone (GH) response to graded doses of GH-releasing hormone (GHRH) was determined in intact (n = 3) and chronically orchidectomized (n = 3) adult rhesus monkeys (Mucaca mulatta). GHRH in doses of 0, 6.25, 12.5 and 25 microg/kg BW was infused through a teflon cannula implanted in the saphenous vein. Blood samples were collected 60 min before and 90 min after the injection of the neurohormone at 15 min intervals. All bleedings were carried out under ketamine hydrochloride anesthesia. The plasma levels of GH were determined by using AutoDELFIA time-resolved flouroimmunoassay, whereas plasma levels of testosterone and estradiol were determined using specific radioimmunoassay systems. The GH responses to GHRH were not significantly different between intact and chronically orchidectomized monkeys at any of the dose levels tested (p > 0.05). The administration of GHRH resulted in a significant (p < 0.05) stimulation of GH secretion at all the doses tested and in both the groups studied. In both intact and orchidectomized animals, the greatest response was observed at 6.25 microg/kg and no further increase was noted with the higher doses of GHRH. In conclusion, the present study suggests that chronic orchidectomy does not influence the sensitivity of the pituitary somatotropes to GHRH stimulation implying that the responsiveness of the pituitary somatotropes to GHRH is independent of testicular steroid modulation.     

UPDATE ON THE CLINICOPATHOLOGY OF PITUITARY ADENOMAS

Objective: Pituitary adenomas are the third most common central nervous system tumors and arise from the anterior pituitary within the pituitary fossa.Methods: Literature review and discussion.Results: The signs and symptoms of patients with pituitary adenomas vary from ‘mass effects’ caused by a large adenoma to features secondary to excess pituitary hormones produced by the functioning pituitary adenoma. Detailed histopathologic assessment, based on novel classifications and the latest World Health Organization guidelines, helps to categorize pituitary adenomas into different subtypes and identify features that, in some cases, help to predict their behavior. Most of the pituitary tumors occur sporadically without known genetic predisposition, but in a significant minority of cases, somatic mutations can be identified in the GNAS and USP8 genes. A small proportion of the cases have germline genetic defects or embryonic mutations leading to mosaicism. Genes with germ-line mutations predisposing to pituitary adenomas include AIP, GPR101, MEN1, CDKN1B, PRKAR1A, PRKAR2A, DICER1, NF1, and SDHx, whereas more recently, CABLES1 has also been implicated.Conclusion: Understanding the pathogenesis of pituitary adenomas will allow clinicians to correlate the pathologic and genetic features with clinical data, helping decisions on the best management of these tumors.Abbreviations: ACTH = adrenocorticotropic hormone; AIP = aryl hydrocarbon receptor-interacting protein; αSU = alpha-subunit; EGFR = epithelial growth factor receptor; ER = estrogen receptor; FSH = follicle-stimulating hormone; GH = growth hormone; GHRH = growth hormone–releasing hormone; IGF-1 = insulin-like growth factor 1; LH = luteinizing hormone; MEN1 = multiple endocrine neoplasia 1; MRI = magnetic resonance imaging; NFPA = nonfunctioning pituitary adenoma; PRL = prolactin; TSH = thyroid-stimulating hormone; USP8 = ubiquitin-specific peptidase 8; WHO = World Health Organization     

Hypopituitarism following pituitary irradiation for acromegaly

Endocrine evaluation is reported in 8 acromegalic patients who received 5,500 rad to the pituitary from a linear accelerator. There was a mean decrease in hGH levels of 72%. Plasma testosterone levels were low in 1 of the 6 male patients prior to pituitary irradiation and were below normal in all male patients on the final evaluation (3.1 +/- 0.2 SD years postirradiation). Deficiency of TSH secretion developed in 2 patients following irradiation. This rather high incidence of postirradiation partial hypopituitarism was not anticipated, and is thought to be related to radiation necrosis of the normal pituitary tissue which surrounds the adenoma.     

Acromegaly with 'normal' serum growth hormone levels. Clinical features, diagnosis and results of transsphenoidal microsurgery.

Among 216 consecutive patients with growth hormone secreting pituitary adenomas who underwent primary neurosurgical treatment at the University of Erlangen-Nürnberg, 8 cases of acromegaly with 'normal' basal growth hormone levels (less than or equal to 5 ng/ml) were seen. They all had the typical clinical features of acromegaly, exhibited an abnormal growth hormone secretion following an oral glucose load, and had markedly elevated somatomedin C levels. The GRH- and TRH/GnRH-tests were not found helpful in establishing the diagnosis. Neuroradiology could demonstrate a pituitary adenoma in all of the patients. Following transsphenoidal microsurgical resection of the tumours, growth hormone secretion during oral glucose tolerance testing was normalised in 7 of the 8 patients. Immunohistology and explant culture studies documented growth hormone secreting pituitary adenomas in all cases. The authors conclude that even the finding of repetitive 'normal' (less than or equal to 5 ng/ml) serum GH levels does not exclude active acromegaly and when the clinical diagnosis of acromegaly is suspected, dynamic endocrine testing may reveal abnormal secretion patterns of GH in these cases. Transsphenoidal microsurgical resection of a pituitary adenoma offers a good chance of clinical and endocrinological remission in these cases.     

Paradoxical prolactin response to growth hormone-releasing hormone in a patient with hyperprolactinemia and empty sella.

In a 30-year-old woman with amenorrhea due to hyperprolactinemia, serum PRL increased to twice the basal amount in response to growth hormone-releasing hormone (GHRH). Roentgenological studies revealed no pituitary adenoma but empty sella. Bromocriptine therapy normalized serum PRL and made the paradoxical response to GHRH disappear. The paradoxical response did not occur in any of eight other patients with hyperprolactinemia due to prolactinoma. Although this case is rare, GHRH stimulates PRL as well as GH release remarkably in some cases with hyperprolactinemia without a GH-producing tumor.     

Growth hormone response to a growth hormone-releasing hormone stimulation test in a population-based study following cranial irradiation of childhood brain tumors

Children with brain tumors are at high risk of developing growth hormone deficiency (GHD) after cranial irradiation (CI) if the hypothalamus/pituitary (HP) axis falls within the fields of irradiation. The biological effective dose (BED) of irradiation to the HP region was determined, since BED gives a means of expressing the biological effect of various irradiation treatment schedules in a uniform way. Hypothalamic versus pituitary damage as cause of GHD was distinguished in 62 patients by comparing the growth hormone (GH) peak response to an insulin tolerance test (ITT)/arginine stimulation test and the GH response to a growth hormone-releasing hormone (GHRH) stimulation test. Peak GH response to a GHRH test was significantly higher (median 7.3 mU/l; range: 0.5--79.0 mU/l) than that of an ITT/arginine test (median 4.7 mU/l; range: 0.01--75.0 mU/l) (p = 0.017). Peak GH after a GHRH test was significantly inversely correlated to follow-up time (r(s) = -0.46, p < 0.0001) and to BED (R(s) = -0.28, p = 0.03), and both were found to be of significance in a multivariante regression analysis. We speculate that a significant number of patients developed hypothalamic radiation-induced damage to the GHRH secreting neurons, and secondary to this the pituitary gland developed decreased responsiveness to GHRH following CI in childhood.     

Growth hormone-releasing factor (GRF) stimulation test in the diagnosis of pituitary adenomas

The studies aimed at evaluation of pituitary reserve of growth hormone following stimulation with GRF have been carried out in a group of 33 patients (11 women and 22 men, of age between 25 and 62 years) with pituitary tumors. The studied material included cases with pituitary adenoma characterized by excessive secretion of growth hormone (somatotropinoma), prolactin (prolactinoma) or alpha subunits of glycoprotein hormones (alphoma), and those with hormonally inactive adenoma. The GRF stimulation tests were carried out in hospitalized patients after overnight fast between 8.00 and 10.00 a.m. Blood samples for hormonal determinations were taken before the test, and after 15, 30, 60, 90 and 120 minutes following intravenous administration of 100 micrograms of GRF 1-29. Besides growth hormone, also the blood serum concentrations of other pituitary hormones were determined in the patients studied, both in the basal state and during the dynamic tests. In patients with acromegaly the results of the determinations of growth hormone following stimulation with GRF showed considerable individual variability. In 5 cases there was an increase in blood serum growth hormone concentration. No response to GRF was noted in the remaining 8 cases. In adenoma cases of prolactinoma type, growth hormone concentration began to rise already at the 15-th minute of the test in most cases. In three cases of prolactinoma associated with acromegaly no response to GRF was observed. The cases of alphoma-type adenoma were usually characterized by the secretion of pituitary hormones other than growth hormone.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pituitary mammosomatotroph adenomas develop in old mice transgenic for growth hormone-releasing hormone

It has been shown that mice transgenic for human growth hormone-releasing hormone (GRH) develop hyperplasia of pituitary somatotrophs and mammosomatotrophs, cells capable of producing both growth hormone and prolactin, by 8 months of age. We now report for the first time that old GRH-transgenic mice, 16 to 24 months of age, develop pituitary mammosomatotroph adenomas. These findings provide conclusive evidence that protracted stimulation of secretory activity can cause proliferation, hyperplasia and adenoma of adenohypophysial cells.