The expression of thyrotrophin-releasing hormone receptor 1 messenger ribonucleic acid in human pituitary adenomas

Growth impairment in children with atrophic autoimmune thyroiditis and pituitary hyperplasia

BACKGROUND

Atrophic autoimmune thyroiditis (AAT) is a rare phenotype of autoimmune thyroiditis (AT) in pediatric age. AAT occurs without thyroid enlargement leading to a delay in its diagnosis. Growth impairment is infrequent in autoimmune thyroiditis, if timely diagnosed. Prolonged severe hypothyroidism is a rare cause of pituitary hyperplasia (PH) in childhood. Loss of thyroxine negative feedback causes a TRH-dependent hyperplasia of pituitary thyrotroph cells resulting in adenohypophysis enlargement. A transdifferentiation of pituitary somatotroph cells into thyrotroph cells could explain growth failure in those patients.

METHODS

Twelve patients were retrospectively evaluated at five Italian and Polish Centres of Pediatric Endocrinology for height growth impairment. In all Centres, patients underwent routine clinical, biochemical and radiological evaluations.

RESULTS

At the time of first assessment, the 75% of patients presented height growth arrest, while the remaining ones showed growth impairment. The study of thyroid function documented a condition of hypothyroidism, due to AT, in the entire cohort, although all patients had no thyroid enlargement. Thyroid ultrasound showed frankly atrophic or normal gland without goiter. Cerebral MRI documented symmetrical enlargement of the adenohypophysis in all patients and a homogeneous enhancement of the gland after the administration of Gadolinium-DPTA. Replacement therapy with levothyroxine was started and patients underwent close follow-up every 3 months. During the 12 months of follow-up, an improvement in terms of height growth has been observed in 88% of patients who continued the follow-up. Laboratory findings showed normalization of thyroid function and the control brain MRI documented complete regression of PH to a volume within the normal range for age and sex.

CONCLUSIONS

This is the largest pediatric cohort with severe autoimmune primary hypothyroidism without goiter, but with pituitary hyperplasia in which significant growth impairment was the most evident presenting sign. AAT phenotype might be correlated with this specific clinical presentation. In youths with growth impairment, hypothyroidism should always be excluded even in the absence of clear clinical signs of dysthyroidism.

Investigation of responsiveness to thyrotropin-releasing hormone in growth hormone-producing pituitary adenomas

Objective. The aim of this study was to investigate how the paradoxical response of GH secretion to TRH changes according to tumor volumes. Methods. Patients with newly diagnosed acromegaly were classified as either TRH responders or nonresponders according to the results of a TRH stimulation test (TST), and their clinical characteristics were compared according to responsiveness to TRH and tumor volumes. Results. A total of 41 acromegalic patients who underwent the TST were included in this study. Between TRH responders and nonresponders, basal GH, IGF-I levels, peak GH levels, and tumor volume were not significantly different, but the between-group difference of GH levels remained near significant over the entire TST time. ΔGHmax-min during the TST were significantly different according to the responsiveness to TRH. Peak GH levels and ΔGHmax-min during the TST showed significantly positive correlations with tumor volume with higher levels in macroadenomas than in microadenomas. GH levels over the entire TST time also remained significantly higher in macroadenomas than in microadenomas. Conclusion. Our data demonstrated that the paradoxical response of GH secretion to TRH in GH-producing pituitary adenomas was not inversely correlated with tumor volumes.

Cortistatin-17 and somatostatin-14 display the same effects on growth hormone, prolactin, and insulin secretion in patients with acromegaly or prolactinoma

CONTEXT

Cortistatin binds all somatostatin receptor subtypes but also has particular central actions; moreover, a specific cortistatin receptor has also been discovered.

OBJECTIVE

We compared the endocrine effects of cortistatin-17 with those of somatostatin-14 in patients with acromegaly (ACRO) or prolactinoma (PRLOMA). Normal subjects (NS) were studied as control group.

DESIGN

All subjects underwent the following tests: 1) saline, 2) somatostatin-14 (2.0 microg/kg.h iv, 0-120 min) and 3) cortistatin-17 (2.0 microg/kg.h iv, 0-120 min) infusion.

RESULTS

Cortistatin-17 and somatostatin-14 inhibited GH secretion to the same extent in ACRO (P < 0.05) and NS (P < 0.01). Cortistatin-17 and somatostatin-14 inhibited PRL secretion in PRLOMA (P < 0.05), to some extent in ACRO (P value not significant), but not in NS. Insulin secretion was inhibited by both cortistatin-17 and somatostatin-14 to the same extent in all groups (P < 0.05).

CONCLUSIONS

Cortistatin-17 and somatostatin-14 display the same effects on GH, PRL, and insulin secretion in patients with ACRO or PRLOMA.

Mechanisms of TRH-induced GH release (paradoxical response) in human somatotroph adenoma cells

The mechanisms of paradoxical TRH response in human somatotroph adenoma cells were investigated using intracellular calcium measurement and static incubation assay. Intracellular calcium measurement revealed that TRH induces a biphasic response: a transient increase followed by a sustained plateau. The transient phase was due to the calcium release from IP(3)-regulated intracellular calcium store and the subsequent sustained phase was due to the calcium influx through the voltage-gated calcium channels. The signal transduction mechanism of the calcium plateau involved protein kinase C. These calcium responses, especially the second phase, was responsible for the TRH-induced GH release.

Somatotroph to thyrotroph cell transdifferentiation during experimental hypothyroidism - a light and electron-microscopy study

Somatotroph and thyrotroph pituitary cells share a common precursor cell expressing the transcription factor Pit1 in ontogeny. Cells expressing both thyrotropin (TSH) and growth-hormone (GH) are found in adult rat pituitary and in human pituitary adenomas in acromegaly, and these tumors contain both thyrotropin-releasing hormone (TRH) and the TRH receptors (TRHR). It has been shown that stimulation of TSH expression in primary hypothyroidism promotes changes suggestive of somatotroph to thyrotroph cell transdifferentiation. We tested this hypothesis and the role of TRH in experimental primary hypothyroidism in rats. Adult female Long-Evans rats, 6 months old, were administered the antithyroid drug methimazole (0.1% w/v) in the drinking water for 42 days. Animals were sacrificed by perfusion fixation under anaesthesia at weekly intervals and pituitary tissue processed in acrylic resin for immunofluorescence and immuno-electronmicroscopy for TSH, GH and TRHR. In the hypothyroid rat pituitary immunofluorescent somatotrophs were greatly reduced in number and gradually replaced by thyrotrophs during methimazole administration. Colocalization of GH and TSH in the same cell was noted. Immunoelectronmicroscopy demonstrated the development of enlarged thyrotrophs with dilated rough endoplasmic reticulum containing an electron-dense material and intracisternal granules, both of which are immunoreactive for TSH ('thyroidectomy cells'). The somatotrophs showed reduced GH immunoreactivity and also the presence of TSH-type, small-size secretory granules. This suggests that the greatly increased number of TSH-cells in methimazole-induced-hypothyroidism is due, at least partially, to the transdifferentiation of somatotroph into thyrotroph cells. TRHR immunofluorescence was expressed in many somatotrophs in normal rat pituitary and unlike immunoreactive GH, its expression was enhanced during hypothyroidism. The number of TRHR-immunoreactive cells increased in parallel with the number of TSH-immunoreactive cells. This indicates a role for TRH stimulation in the transdifferentiation process. Taken together, these data suggest that, in addition to the cell mutation mechanism involving an early totipotential progenitor cell, transdifferentiation of existing somatotroph cells also plays a part in the pathogenesis of multihormonal GH-secreting adenomas.

Preoperative identification of clearly separated double pituitary adenomas

OBJECTIVE

Double pituitary adenomas are extremely rare. They can be divided into contiguous and clearly separated types. Most contiguous tumours are surgically removed as one tumour and the co-existence of different adenoma types can be confirmed by histological methods. In contrast, detailed preoperative neuroimaging studies can suggest the co-existence of separated multiple adenomas. In patients with multiple adenomas, surgical failure may result when one adenoma is missed during surgery. Among 600 surgical cases we encountered four patients with clearly separated double pituitary adenomas; all were highly suspect on preoperative MRI studies.

PATIENTS AND RESULTS

All four patients manifested acromegalic symptoms; one patient also exhibited hyperprolactinemia and two had familial pituitary adenomas unrelated to multiple endocrine neoplasia type I (MEN-1). All underwent transsphenoidal surgery and histology confirmed the diagnosis of GH-producing plus gonadotroph adenoma in two cases and of two GH-producing adenomas each in the other two patients.

CONCLUSION

Although the pathogenesis of our double adenomas remains unknown, genetic abnormalities may be involved because two patients had familial pituitary adenomas unrelated to MEN-1. When preoperative MRI is suggestive of double adenomas, careful surgical exploration is necessary to avoid missing the other adenoma because the risk of surgical failure is high, especially in patients with functioning adenomas.

Biochemical markers of acromegaly: GH vs. IGF-I

The development of sensitive and specific growth hormone (GH) and insulin-like growth factor I (IGF-I) assays opened a new page in the diagnosis and surveillance of acromegaly. Currently, it is possible to make an accurate pre-operative diagnosis even in patients with virtually no typical clinical signs of the disease and to monitor the efficacy of therapeutic intervention with a high degree of precision. This review briefly discusses the performance parameters of GH and IGF-I as diagnostic and surveillance tools in patients with acromegaly. In brief, whereas GH-based parameters may offer the advantage of disclosing dysregulation of GH secretion, a single plasma IGF-I measurement provides the most comprehensive assessment of both the overall GH output as well as the pattern of GH presentation to the peripheral tissues. Judicious use of both biochemical markers allows accurate and early diagnosis, precise assessment of the efficacy of therapeutic interventions, permits adjustment in the dose of medication and allows for unequivocal demonstration of a final cure.

Effects of food restriction on peroxisome proliferator-activated receptor-gamma and glucocorticoid receptor signaling in adipose tissues of normal rats

In adipocytes, peroxisome proliferator-activated receptor (PPAR)-gamma activates adipocyte differentiation and glucocorticoid (GC) stimulates the expression of PPAR-gamma mRNA. The local tissue concentrations of GC, in turn, are modulated by 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1). To clarify the change of energy metabolism in condition of reduced energy intake, we investigated whether food restriction alters the adipocyte size and levels of PPAR-gamma, GC receptor (GR), and 11beta-HSD1 mRNA expression in the white adipose tissues of normal rats. Male Wistar rats weighing 340 g were housed under free feeding or 20% reduction of food intake for 2 or 14 days. We found that 2-day food restriction did not cause any change in the mean size or number of adipocytes in the omentum, while 14-day food restriction decreased the size and increased the number of adipocytes. In addition, the levels of PPAR-gamma2, GR, and 11beta-HSD1 mRNA expression in the omentum were lower in the food-restricted rats after 2 days, while they did not differ after 14 days. Also, after both 2 and 14 days, plasma concentrations of free fatty acid (FFA) were higher in the food-restricted rats than in control rats. Finally, plasma concentrations of adrenocorticotropin (ACTH) and corticosterone were the same in the both groups after 2 days, although they were higher in the food-restricted rats after 14 days. These results suggest that adipocyte differentiation in the omentum of food-restricted rats is attenuated after 2 days but recovers after 14 days, resulting in an increase in the number of small adipocytes. It is also likely that lipolysis induced during the 14-day period of food restriction decreased the size of adipocytes. Further, food restriction may affect the efficiency of local GC effects by altering GR and 11beta-HSD1 mRNA expression. Also, higher levels of plasma GC and recovery of GR and 11beta-HSD1 mRNA expression may contribute to the recovery of the levels of PPAR-gamma2 mRNA expression in the omentum and result in the recovery of adipocyte differentiation.

Gene expression study of thyrotropin releasing hormone (TRH) receptor using RT-PCR: relationship to clinical and immunohistochemical phenotypes in a series of human pituitary adenomas

Thyrotropin releasing hormone receptor 1 (TRHR1) and 2 (TRHR2) mRNAs were examined using reverse transcription polymerase chain reaction (RT-PCR). These data were then correlated with endocrinological and histological characteristics in 65 human pituitary adenomas including one non tumorous pituitary gland, to clarify the role of TRHR1 and 2 gene expression in human pituitary adenomas, especially as it relates to the paradoxical stimulatory effects of TRH found in pituitary adenomas. TRHR mRNA was not identified in four ACTH cell adenomas, whereas TRHR mRNA expression was found in 12/23 acromegaly specimens, 7/8 prolactinomas, 3/3 TSH cell adenomas, and 22/27 clinically nonfunctioning adenomas. Specimens obtained from patients expressing the TRHR gene were found to express TRHR1 and TRHR2 or TRHR1 alone, whereas no cases were identified in which TRHR2 alone was expressed. In examining the relationship between GH, PRL, TSH, or gonadotropin subunit response to TRH administration and TRHR gene expression, TRHR mRNA was found to be absent in 9 out of 10 GH cell adenomas without paradoxical GH response to TRH (non-responder), whereas TRHR genes were shown to be expressed in 10 out of 12 GH cell adenomas with paradoxical GH response to TRH (responders) (chi2 = 11.73, p = 0.0009). However, there was no significant correlation between TRHR gene expression and responsiveness to PRL or gonadotropins to TRH administration in PRL cell adenomas (chi2 = 0.16, p = 0.87) or gonadotroph cell adenomas (chi2 = 0.0006, p = 1), respectively. We concluded that the existence of the TRH receptor in adenoma cells plays an important role in the paradoxical GH response to TRH administration in GH cell adenomas. It should be noted that the PRL response to TRH in prolactinoma or an abnormal response of gonadotropin and/or its subunits to TRH in gonadotroph cell adenomas is considered to be due to a mechanism other than direct TRH action in adenoma cells. However, further studies are required to elucidate the role of TRHR2 in pituitary adenomas.

Cytochemical and molecular biological aspects of the pituitary and pituitary adenomas--cell differentiation and transcription factors

The anterior pituitary is composed of several cell types, each responsible for the production of specific hormones. Each hormone secreting cells is defined by the activation of its respective hormone genes in a temporally and spatially regulated manner. Recent development in cytochemistry and molecular biology have provided various aspects of human pituitary adenomas, i.e., functional differentiation and classification. The molecular factors that determine hormone production have now been identified as transcription factors. Many novel transcription factors that play a role in anterior pituitary development are implicated. In this review, we focus on the transcriptional factors roles on functional differentiation of the pituitary cells and adenomas and the contribution of cytochemistry and recent development in molecular biological techniques.