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The Journal of Clinical Endocrinology & Metabolism Vol. 84, No. 9 3241-3247
Copyright © 1999 by The Endocrine Society

Original Studies

Pituitary Somatotroph Adenoma Producing Growth Hormone (GH)-Releasing Hormone (GHRH) with an Elevated Plasma GHRH Concentration: A Model Case for Autocrine and Paracrine Regulation of GH Secretion by GHRH1

Akira Matsuno, Hideki Katakami, Naoko Sanno, Yoshio Ogino, R. Yoshiyuki Osamura, Shigeru Matsukura, Naokata Shimizu and Tadashi Nagashima

Department of Neurosurgery, Teikyo University Ichihara Hospital (A.M., T.N.), 3426–3 Anegasaki, Ichihara City, Chiba 299-0111; the Third Department of Internal Medicine, Miyazaki Medical College (H.K., S.M.), 5200 Kihara, Kiyotake Town, Miyazaki-gun, Miyazaki 889-1601; the Department of Pathology, Tokai University School of Medicine (N.Sa., R.Y.O.), Boseidai, Isehara City, Kanagawa 259-1100; and the Third Department of Internal Medicine, Teikyo University Ichihara Hospital (Y.O., N.Sh.), 3426–3 Anegasaki, Ichihara City, Chiba 299-0111, Japan

Address all correspondence and requests for reprints to: Akira Matsuno, M.D., D.M.Sc., Department of Neurosurgery, Teikyo University Ichihara Hospital, 3426–3 Anegasaki, Ichihara City, Chiba 299-0111, Japan.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Discussion
 References
 
An acromegalic patient with a pituitary somatotroph adenoma associated with an extremely elevated plasma GHRH concentration is presented. The preoperatively high concentration of plasma GHRH returned to the normal level after successful removal of the adenoma. GHRH production and GHRH gene expression were confirmed in the adenoma by studies including immunohistochemistry and in situ hybridization. Expression of GHRH receptor messenger ribonucleic acid was verified by in situ hybridization. Immunohistochemical double staining for GH and GHRH revealed their colocalization in single adenoma cells. These findings confirmed the autocrine or paracrine regulation of GH production by endogenous GHRH from the adenoma cells. GHRH synthesis in the pituitary gland has recently been demonstrated, however, there have been no previous reports of a GHRH-producing pituitary somatotroph adenoma associated with an elevated plasma GHRH concentration. The existence of this GHRH-producing adenoma suggests a possible role of locally generated GHRH in the progression of somatotroph adenomas, i.e. the monoclonally established somatotroph adenomas develop further under the influence of locally produced GHRH. The demonstration of GHRH production by this somatotroph adenoma is of importance in clarifying the autocrine or paracrine regulation of GH production and the progression of human somatotroph adenomas.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Discussion
 References
 
ACROMEGALY is usually caused by a pituitary somatotroph adenoma. The other main diseases causing acromegaly are ectopic GH-producing pancreatic tumors and tumors producing GHRH, a hypothalamic hormone that stimulates GH secretion from the anterior pituitary gland. Among them are pancreatic tumors, pulmonary tumors, and hypothalamic gangliocytomas, and the endocrine mechanism stimulating GH secretion has well been documented. GHRH production or gene expression in pituitary somatotroph adenomas has been demonstrated by several investigators (1, 2, 3, 4). Nevertheless, a GHRH-producing pituitary somatotroph adenoma associated with an elevated plasma GHRH concentration has not been reported previously, and the autocrine or paracrine regulation of GH secretion by endogenous pituitary GHRH has not well been demonstrated. Penny et al. reported plasma GHRH concentrations in 80 acromegalic patients measured by RIA and found in 3 acromegalic patients elevated GHRH levels, ranging from 92–1111 pg/mL (5). However, in their reports, the source of elevated plasma GHRH was not identified histopathologically. Thorner and his co-workers reported plasma GHRH levels in 177 acromegalic patients measured by RIA and found the highest level of 82 pg/mL, although the source of GHRH production was identified in none of the patients with elevated plasma GHRH concentrations (6). These findings may be suggestive of GHRH production by a pituitary somatotroph adenoma. We have routinely measured plasma GHRH concentrations in acromegalic patients to investigate the mutual relationship between GH and GHRH. Among them, we found an extremely rare GHRH-producing pituitary somatotroph adenoma in a 26-yr-old acromegalic man with a markedly elevated plasma GHRH concentration. In this study, this extremely rare GHRH-producing pituitary somatotroph adenoma associated with a high plasma GHRH concentration in an acromegaly patient was investigated using immunohistochemistry, in situ hybridization (ISH), and electron microscopy, with special emphasis on the endocrinological and histopathological aspects.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Discussion
 References
 
Case material

A 26-yr-old man was admitted to Teikyo University Ichihara Hospital on July 10, 1996, with a 2-yr history of changes in the features and enlarged feet. Physical examination revealed a typical acromegalic face and enlarged hands and feet.

Endocrinological findings. Basal levels of serum GH and somatomedin-C (SMC) on admission were elevated, ranging from 31.7–42.2 (normal range, <0.42) and 1400 (range, 100–315) ng/mL, respectively. Serum PRL, ACTH, cortisol, TSH, FSH, and LH were within normal limits. An oral glucose tolerance test showed no suppression of GH, with levels of 42.5, 46.8, 43.4, 36.3, 33.2, and 34.6 ng/mL at 0, 30, 60, 90, 120, and 180 min, respectively (Fig. 1Go). After TRH stimulation, serum GH showed no paradoxical rise, being 35.2, 42.1, 35.7, 33.5, 30.8, and 30.5 ng/mL at 0, 15, 30, 60, 90, and 120 min, respectively (Fig. 1Go). The GnRH provocation test also showed no paradoxical rise of serum GH, with values of 31.7, 28.7, 29.0, 30.4, 31.6, and 32.0 ng/mL at 0, 15, 30, 60, 90, and 120 min, respectively (Fig. 1Go). The GHRH provocation test showed slightly reactive secretion of GH, with values of 35.3, 45.1, 48.7, 45.9, 43.0, and 44.2 ng/mL at 0, 15, 30, 60, 90, and 120 min, respectively (Fig. 1Go). The serum GH level was suppressed by 2.5 mg of bromocriptine; it was 42.2, 29.6, 14.8, 10.8, 9.8, 18.1, and 43.7 ng/mL at 0, 1, 2, 4, 6, 12, and 24 h, respectively.



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  		Figure 1. An oral glucose tolerance test (OGTT) showed no suppression of GH. After TRH or GnRH stimulation, serum GH showed no paradoxical rise. The GHRH provocation test showed a slightly reactive secretion of GH.

 
Radiological findings. A plain craniogram revealed an enlarged sella turcica, with erosive changes in the sella floor and dorsum sellae. The heel pad was thickened to 25 mm, and cauliflower-like changes were noticeable in the fingers. T1-weighted magnetic resonance imaging (MRI) revealed an isointense mass, which also showed isointensity on T2-weighted images and homogeneous weak enhancement with gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA). This mass was invading the right cavernous sinus laterally (Fig. 2aGo). The normal pituitary gland was obviously enhanced by Gd-DTPA. No abnormal findings were observed on the chest x-ray film and abdominal computed axial tomography scan.



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  		Figure 2. a, T1-weighted MRI before surgery revealed an isointense mass, which also showed isointensity on T2-weighted images and homogeneous weak enhancement with Gd-DTPA. This mass was invading the right cavernous sinus laterally. The normal pituitary gland was obviously enhanced by Gd-DTPA. Basal levels of serum GH and SMC were elevated, ranging from 31.7–42.2 and 1400 ng/mL, respectively. The plasma GHRH concentration was shown by enzyme immunoassay to be surprisingly elevated to 1,240 pg/mL. b, Postoperative MRI showed only a small residual tumor invading the right cavernous sinus at the time when serum GH and SMC levels were 4.9 and 220 ng/mL and the plasma GHRH concentration was 5.8 pg/mL, respectively.

 
The patient was diagnosed as having acromegaly caused by a pituitary somatotroph adenoma, and transsphenoidal surgery was carried out on July 18, 1996. The tumor was removed extensively, and the normal pituitary gland was confirmed intraoperatively. Postoperative endocrine studies revealed reduced serum GH and SMC levels of 4.9 and 220 ng/mL, respectively. Postoperative MRI showed only a small residual tumor invading the right cavernous sinus (Fig. 2bGo).

Measurement of plasma GHRH concentration

The plasma GHRH concentrations before and after surgery were measured by sensitive sandwich enzyme immunoassay. The details of the procedure of the measurement have been reported previously by us (7, 8, 9, 10). The plasma GHRH concentration before surgery was shown by enzyme immunoassay to be surprisingly elevated to 1240 (normal range, 4–12) pg/mL. The GHRH concentration was remarkably reduced and normalized postoperatively to 5.8 pg/mL, at the time when serum GH and SMC levels were 4.9 and 220 ng/mL, respectively. The half-life of plasma GHRH has been calculated to be less than 10 min (11). Therefore, the fact that the plasma GHRH level was normalized after successful removal of the tumor strongly suggests GHRH production by our patient’s pituitary tumor.

Bioactivity assay of plasma GHRH

GHRH bioactivity in a preoperative plasma sample was confirmed by the dispersed primary culture system of rat pituitary cells, according to the previously reported method (12). Briefly, the patient’s plasma was purified and concentrated by C18 Sep-Pak cartridges (Millipore Corp., Milford, MA) to eliminate ethylenediamine tetraacetate and aprotinin in plasma. This plasma was reconstituted with bicarbonate-buffered {alpha}MEM (AMEM; Life Technologies, Inc., Grand Island, NY). Pituitary cells prepared from female Sprague Dawley rats were dispersed in 24-well culture plates, and after a 1-h cell attachment period, the cells were flooded with 1.0 mL culture medium. After 3 days of culture, the medium was removed and replaced with 1.0 mL AMEM containing 0.2 or 0.05 mL of the patient’s plasma. The cells were incubated for 3 h at 37 C, and rat GH concentrations were measured. The control experiments included incubation of the cells with 0.1% BSA-containing AMEM and AMEM with 10-9 mol/L GHRH.

A preliminary experiment showed that the rat GH release induced by the addition of 0.2 or 0.05 mL-Eq plasma extract from a normal subject to AMEM was not statistically different from that with 0.1% BSA-containing AMEM. Two independent culture experiments were conducted due to the limited volume of the plasma sample of the patient. The Kruskal-Wallis test was used to assess between-group differences. P < 0.05 was considered significant. A representative culture study is shown in Fig. 3Go. AMEM containing plasma extract from the patient (0.2 mL-Eq) as well as 10-9 mol/L GHRH significantly stimulated rat GH release in culture. In contrast, AMEM with plasma extract from the patient (0.05 mL-Eq) did not stimulate rat GH release. The rat GH-releasing action of plasma sample from the patient was confirmed in the separate culture experiments. GH concentrations in the medium were increased in accordance with the volume of the patient’s plasma, and therefore, the bioactivity of GHRH from the patients’ plasma has been confirmed.



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  		Figure 3. GH concentrations in the medium of primary culture of rat pituitary cells were increased in accordance with the volume of the patient’s plasma extract. AMEM, AMEM with 0.1% BSA. GHRH, AMEM with 10-9 mol/L human GHRH-(1–44)NH2. The mean ± SE of each group are shown. The number of wells in each group is shown in parentheses. a, P < 0.01 vs. AMEM or AMEM with the patient’s plasma extract (0.05 mL equivalent). b, P < 0.05 vs. AMEM or AMEM with the patient’s plasma extract (0.05 mL equivalent).

 
Histopathological examination

Immunohistochemical studies. Routinely processed paraffin sections were used for hematoxylin-eosin staining and immunohistochemical staining studies. Hematoxylin-eosin staining confirmed that the tumor was a mixed acidophilic-chromophobe pituitary adenoma (Fig. 4aGo). Paraffin sections were deparaffinized, and the endogenous peroxidase was blocked with 0.3% H2O2 in methanol. They were immunostained by the indirect peroxidase method with antibodies against human GH [rabbit, polyclonal; 1:400 diluted in BSA-0.01 mol/L phosphate-buffered saline, pH 7.4 (PBS); from DAKO Corp., Carpinteria, CA], PRL (rabbit, polyclonal; 1:600 diluted in BSA-PBS; from DAKO Corp.), TSH ß-subunit (rabbit, polyclonal; 1:3200 diluted in BSA-PBS; supplied by the NIDDK, Bethesda, MD), and glycoprotein hormone {alpha}-subunit (rabbit, polyclonal; 1:2000 diluted in BSA-PBS; from UCB-Bioproducts, Brussels, Belgium). GH-immunopositive cells and PRL-immunopositive cells were common (Fig. 4Go, b and c), whereas TSH ß-subunit and glycoprotein hormone {alpha}-subunit were negative. Paraffin sections were also immunostained with an antibody against synthetic human Pit-1 protein (13), which is a transcriptional factor for GH, PRL, and TSH (14), and positive immunoreaction for Pit-1 protein was also observed (Fig. 4dGo).



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  		Figure 4. Hematoxylin-eosin staining confirmed that the tumor was a mixed acidophilic-chromophobe pituitary adenoma (a; original magnification, x600). Immunohistochemical examination of the tumor confirmed that GH-immunopositive cells and PRL-immunopositive cells were common (b and c; original magnification, x600). A positive immunoreaction for Pit-1 protein was also observed with an antibody against synthetic human Pit-1 protein (d; original magnification, x600). Immunohistochemical examination using an anti-GHRH antibody revealed the diffuse or dot-like GHRH positivity in the adenoma cells (e; original magnification, x600). Light microscopic immunohistochemical double staining for GH and GHRH revealed their colocalization in single adenoma cells (f; GH, blue, arrows; GHRH, brown, arrowheads; original magnification, x900). GHRH mRNA was detected by ISH using a biotinylated antisense oligonucleotide probe (g; original magnification, x600). Control ISH study using a sense oligonucleotide probe yielded no hybridization signals for GHRH mRNA (g; lower left corner; original magnification, x600). The positive signals for GHRH mRNA have also been confirmed using the novel amplified ISH system (g; lower right corner; original magnification, x600). GHRH receptor mRNA was also detected by ISH using a biotinylated antisense oligonucleotide probe (h; original magnification, x600). Control ISH study using a sense oligonucleotide probe yielded no hybridization signals for GHRH receptor mRNA (h; lower left corner; original magnification, x600). Electron microscopy revealed small secretory granules of 150 nm in diameter, corresponding to those containing GHRH in adenoma cells containing the fibrous bodies, characteristic of sparsely granulated somatotroph adenomas (i; bar, 500 nm).

 
Immunohistochemical studies on GHRH. Immunohistochemical examination using an anti-GHRH antibody corresponding to residues 1–44 amide (rabbit, polyclonal; 1:1000 diluted in BSA-PBS; from UCB Bioproducts) revealed diffuse or dot-like GHRH positivity in the adenoma cells (Fig. 4eGo). After immunohistochemical staining for GHRH, the slides were washed in 0.1 mol/L glycine-HCl (pH 2.2), and anti-GH antibody was applied on the same section. Alkaline phosphatase-linked antirabbit IgG (goat; 1:50 diluted in PBS; from DAKO Corp.) was applied as the secondary antibody, and positive reactions for anti-GH antibody were developed with 5-bromo-4-chloro-3-indolyl phosphate and nitro blue tetrazolium. This light microscopic immunohistochemical double staining for GH and GHRH revealed their colocalization in single adenoma cells (Fig. 4fGo). Liver and normal pituitary gland tissues were used for the negative and positive control studies, respectively. Negative control studies also included substituting normal rabbit serum for the pituitary antibodies.

ISH studies on the expression of GHRH and GHRH receptor messenger ribonucleic acid (mRNA). The sequence of the antisense oligonucleotide probe for GHRH mRNA was 5'-AAG AAC ACC CAG AGT GGC ATC CTT CAC (3), and that of the antisense oligonucleotide probe for GHRH receptor mRNA was 5'-AAA AAG CCC AAA GTT CAC CCC GAC CGA GAG G (15). The labeling of the probes and the details of the ISH procedure have been described previously (16, 17, 18, 19). Control ISH study included studies using sense or scramble oligonucleotide probe. GHRH mRNA was detected by ISH using a biotinylated antisense oligonucleotide probe (Fig. 4gGo). Control ISH study using a sense oligonucleotide probe yielded no hybridization signals (Fig. 4gGo). The positive signals for GHRH mRNA have also been confirmed using the novel amplified ISH system (GenPoint System, DAKO Corp.; Fig. 4gGo). GHRH receptor mRNA was also detected by ISH using a biotinylated antisense oligonucleotide probe (Fig. 4hGo). Control ISH study using a sense oligonucleotide probe yielded no hybridization signals (Fig. 4hGo).

MIB-1 immunohistochemical staining index, cytokeratin immunostaining, and electron microscopic findings. To assess the proliferative activity of the tumor, the MIB-1 immunohistochemical staining index was determined to be 7%, according to the previously reported method using a commercially available anti-Ki-67 antibody, MIB-1 (mouse, monoclonal; 1:100 diluted in BSA-PBS; Immunotech S.A., Marseille, France) (20). Immunohistochemistry using an anticytokeratin antibody (CAM 5.2, mouse, monoclonal; from Becton Dickinson and Co. Immunocytometry Systems, San Jose, CA) revealed dot-like positivity for cytokeratin in most adenoma cells. Electron microscopy revealed small secretory granules of 150 nm in diameter, corresponding to those containing GHRH (21) in adenoma cells containing fibrous bodies, characteristic of sparsely granulated somatotroph adenomas (Fig. 4iGo).

Based on these cytological and histopathological findings, we concluded that the pituitary somatotroph adenoma produced and secreted GHRH.


   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Discussion
References
 
An extremely rare GHRH-producing pituitary somatotroph adenoma is reported with histopathological findings. Penny et al. reported plasma GHRH concentrations in 80 acromegalic patients measured by RIA, and found in 3 acromegalic patients elevated GHRH levels, ranging from 92–1111 pg/mL (5). However, in their reports, the source of elevated plasma GHRH was not identified histopathologically. Thorner and his co-workers reported plasma GHRH levels in 177 acromegalic patients measured by RIA and found the highest level of 82 pg/mL, although the source of GHRH production was identified in none of the patients with elevated plasma GHRH concentrations (6). These findings suggest GHRH production by somatotroph pituitary adenomas, but there have been no reports on a GHRH-producing pituitary somatotroph adenoma associated with an elevated plasma GHRH concentration. In our somatotroph adenoma, GHRH and GHRH gene expression were confirmed by histopathological studies, including immunohistochemistry, ISH, and electron microscopy, together with measurement of the plasma GHRH concentration. Electron microscopy revealed small secretory granules of 150 nm in diameter that corresponded to those containing GHRH in a sparsely granulated somatotroph adenoma (21). Immunohistochemical double staining for GH and GHRH demonstrated their colocalization in single adenoma cells. Expression of GHRH receptor mRNA in the adenoma was also confirmed by ISH. The fact that the preoperatively high concentration of plasma GHRH returned to the normal level postoperatively indicated the pituitary origin of the circulating GHRH. These findings indicated the autocrine or paracrine regulation of GH production through GHRH receptor by GHRH, which the adenoma cells produced (Fig. 5Go). This is the first clinical report of a GHRH-producing somatotroph adenoma with an elevated plasma GHRH concentration.



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  		Figure 5. Autocrine or paracrine regulation of GH production by GHRH from the somatotroph adenoma.

 
GHRH is a well known hypothalamic hormone that stimulates the release and synthesis of GH as well as the proliferation of GH-producing cells in the anterior pituitary gland (22, 23). The primary structure of GHRH was originally identified from a pancreatic carcinoid that caused acromegaly. Thereafter, several types of tumors producing GHRH were reported, including pancreatic tumors, pulmonary tumors, and hypothalamic gangliocytomas. Although recent reports have shown that several neuropeptides, including vasoactive intestinal polypeptide, neuropeptide Y, substance P, galanin, interleukin-6 and interleukin-1ß, are synthesized by pituitary cells (24, 25, 26, 27, 28, 29), GHRH synthesis in the pituitary gland is still controversial. Some investigators have reported its exclusive localization in the hypothalamus and pituitary stalk within the human central nervous system, with no positivity in the pituitary gland (30), whereas Joubert et al. suggested that GHRH synthesis occurred in the normal pituitary and in GH-producing pituitary adenomas in humans (2). They measured GH, GHRH, and somatostatin concentrations in the perifusion medium from six somatotroph adenomas and three human pituitaries under basal conditions and in the presence of 10-6 mol/L TRH or somatostatin, and their results suggested the release of hypothalamic hormones from pituitary cells. Wakabayashi et al. found GHRH gene expression in six pituitary somatotroph adenomas using RT-PCR (1). Levy et al. examined 51 human pituitary adenomas for GHRH transcripts using ISH histochemistry and identified GHRH transcripts in 13 of 17 somatotroph adenomas (3). Thapar et al. demonstrated accumulation of GHRH mRNA in 91 of a consecutive series of 100 somatotroph adenomas by ISH (4). Of 20 somatotroph adenomas studied by immunohistochemistry in their series, all of which expressed high levels of GHRH message, a definite immunoreaction for GHRH was only observed in 2 cases despite application of antigen retrieval. They suggested that technical limitations rather than failure of protein translation probably accounted for the GHRH immunonegativity of tumors expressing high levels of GHRH mRNA by ISH. In contrast with their results, it is noteworthy that GHRH protein was positively immunostained and that its message was also detected by ISH in the present adenoma. Thapar et al. also stated that GHRH receptor mRNA was detected in 10 of 10 somatotroph adenomas and that its level of expression was fairly similar in all tumors. In particular, there was no evidence of GHRH receptor down-regulation even when the level of GHRH message was high. In our adenoma, expression of GHRH receptor mRNA as well as that of GHRH mRNA were revealed by ISH. This suggested the absence of receptor desensitization in GHRH-producing somatotroph adenomas. Spada and Lania stated that patients who have somatotroph adenomas continue to show a GH response after repeated GHRH injections, whereas normal subjects do not (31). They also demonstrated the lack of receptor desensitization in neoplastic somatotrophs in vitro (32).

Another important implication of this GHRH-producing somatotroph adenoma is the possible role of locally generated GHRH in the progression of somatotroph adenomas. Pituitary somatotroph adenomas have recently been considered to be provoked by subcellular abnormalities, such as mutations of coding regions of Gs{alpha} protein (33) and those of menin gene (34). GHRH production in the adenoma cells, as shown in our studies, is thought to have significant roles in the progression of established somatotroph adenoma. As Thapar et al. stated, historically there have been two opposing hypotheses concerning the development and progression of pituitary adenomas: the hypothalamic hypothesis and the pituitary hypothesis. The hypothalamic hypothesis proposes that pituitary adenomas arise as the downstream consequence of a stimulatory imbalance between hypothalamic hormones emanating from a dysregulated hypothalamus (35, 36, 37, 38). With the recent demonstration of the monoclonality of most human pituitary adenomas, the pituitary hypothesis, which suggests that pituitary adenomas result from the monoclonal expansion of a single adenohypophyseal cell with subcellular abnormalities, has become the more widely accepted concept. The present adenoma is considered to link these two opposing theories; an established somatotroph adenoma may be developed further by locally produced GHRH through GHRH receptor. Experiments in transgenic mice expressing a human GHRH gene, which suggested existence of the hyperplasia-adenoma sequence, may also support the linkage of the two theories (39, 40).

Thapar et al. stated that GHRH transcripts preferentially accumulated in clinically aggressive tumors and concluded that overexpression of the GHRH gene is associated with the neoplastic progression and clinical aggressiveness of somatotroph adenomas (4). The present GHRH-producing tumor was a macroadenoma invading the cavernous sinus and had a MIB-1 staining index of 7%, which was a relatively high index compared with indexes in their report. These results may suggest the clinical aggressiveness of the present GHRH-producing adenoma.

The endocrinological diagnosis of GHRH-producing somatotroph adenoma may be difficult. The first step toward diagnosis is to measure the plasma GHRH concentration. As stated for extrapituitary GHRH-producing tumors, measurement of the plasma GHRH concentration is the most reliable method for differentiating between GHRH-induced acromegaly and classical acromegaly (5, 6, 21). Thorner et al. reported that the plasma GHRH concentrations of three patients with extrapituitary acromegaly ranged from 2,000–24,400 pg/mL, whereas those of normal subjects were either undetectable or ranged up to 62.5 pg/mL (6). Scheithauer et al. stated that GHRH levels of 300 pg/mL or more in acromegaly suggest the presence of an ectopic GHRH-producing tumor (41). Therefore, a high plasma GHRH concentration is suggestive of a GHRH-producing tumor, and we recommend that the plasma GHRH concentration should be measured routinely in each acromegaly patient. The half-life of plasma GHRH has been calculated to be less than 10 min (11). Therefore, the fact that the plasma GHRH level was normalized after successful removal of the tumor strongly suggests GHRH production by our patient’s pituitary tumor.

In the present patient, only a slightly reactive elevation of GH was observed in the GHRH provocation test. Human GH tumor cells respond to GHRH in a similar way as normal pituitary cells (42). Regarding extrapituitary GHRH-producing tumors, some investigators have stated that no GH response was observed after GHRH loading, and this was thought to be a hallmark for differentiating GHRH-induced acromegaly from classical acromegaly (43, 44, 45). Other recent studies have demonstrated marked GH elevation in response to GHRH injection in patients with GHRH-producing tumors (46). The magnitude of GH responses to exogenous GHRH in acromegalic patients was reported to be heterogeneous (47). Therefore, the diagnostic meaning of the lack in significant elevation of GH in the GHRH provocation test may be unclear for a GHRH-producing somatotroph adenoma.

In conclusion, an extremely rare GHRH-producing pituitary somatotroph adenoma is reported with histopathological findings. GHRH production in a somatotroph adenoma has important implication for the autocrine or paracrine regulation of GH production and the progression of human somatotroph adenomas through GHRH receptor.


   Acknowledgments
 
The authors thank Drs. Shigeyuki Tahara, Yoshiko Itoh, and Johbu Itoh for their technical and photographic assistance.


   Footnotes
 
1 This work was supported in part by Grants-in-Aid for Scientific Research 09671450 (to A.M.) and 07671146 (to H.K.) from the Ministry of Education, Science, and Culture of Japan. Back

Received December 28, 1998.

Revised June 3, 1999.

Accepted June 8, 1999.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Discussion
 References
 

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