Cloning of the mouse somatostatin receptor subtype 5 gene: promoter structure and function

Improved pasireotide response in USP8 mutant corticotroph tumours in vitro

Cushing's disease is a rare but devastating and difficult to manage condition. The somatostatin analogue pasireotide is the only pituitary-targeting pharmaceutical approved for the treatment of Cushing's disease but is accompanied by varying efficacy and potentially severe side effects. Finding means to predict which patients are more likely to benefit from this treatment may improve their management. More than half of corticotroph tumours harbour mutations in the USP8 gene, and there is evidence of higher somatostatin receptor 5 (SSTR5) expression in the USP8-mutant tumours. Pasireotide has a high affinity for SSTR5, indicating that these tumours may be more sensitive to treatment. To test this hypothesis, we examined the inhibitory action of pasireotide on adrenocorticotrophic hormone synthesis in primary cultures of human corticotroph tumour with assessed USP8 mutational status and in immortalized murine corticotroph tumour cells overexpressing human USP8 mutants frequent in Cushing's disease. Our in vitro results demonstrate that pasireotide exerts a higher antisecretory response in USP8-mutant corticotroph tumours. Overexpressing USP8 mutants in a murine corticotroph tumour cell model increased endogenous somatostatin receptor 5 (Sstr5) transcription. The murine Sstr5 promoter has two binding sites for the activating protein 1 (AP-1) and USP8 mutants possibly to mediate their action by stimulating AP-1 transcriptional activity. Our data corroborate the USP8 mutational status as a potential marker of pasireotide response and describe a potential mechanism through which USP8 mutants may regulate SSTR5 gene expression.

Identification of the receptors for somatostatin (SST) and cortistatin (CST) in chickens and investigation of the roles of cSST28, cSST14, and cCST14 in inhibiting cGHRH1-27NH2-induced growth hormone secretion in cultured chicken pituitary cells

Somatostatin receptors (SSTRs) are proposed to mediate the actions of somatostatin (SST) and its related peptide, cortistatin (CST), in vertebrates. However, the identity, functionality, and tissue expression of these receptors remain largely unknown in most non-mammalian vertebrates including birds. In this study, five SSTRs (named cSSTR1, cSSTR2, cSSTR3, cSSTR4, cSSTR5) were cloned from chicken brain by RT-PCR. Using a pGL3-CRE-luciferase reporter system, we demonstrated that activation of each cSSTR expressed in CHO cells by cSST28, cSST14 and cCST14 treatment could inhibit forskolin-induced luciferase activity of CHO cells, indicating the functional coupling of all cSSTRs to Gi protein(s). Interestingly, cSSTR1-4 expressed in CHO cells could be activated by cSST28, cSST14 and cCST14 with high potencies, suggesting that they may function as the receptors common for these peptides. In contrast, cSSTR5 could be potently activated by cSST28 only, indicating that it is a cSST28-specific receptor. Using RT-PCR, wide expression of cSSTRs was detected in chicken tissues including pituitary. In accordance with their expression in pituitary, cSST28, cSST14, and cCST14 were demonstrated to inhibit basal and novel cGHRH1-27NH2-induced GH secretion in cultured chicken pituitary cells dose-dependently (0-10nM) by Western blot analysis, suggesting the involvement of cSSTR(s) common for these peptides in mediating their inhibitory actions. Collectively, our study establishes a molecular basis to elucidate the roles of SST/CST in birds and provide insights into the roles of SST/CST in vertebrates, such as their conserved actions on pituitary.

Progesterone-inducible cytokeratin 5-positive cells in luminal breast cancer exhibit progenitor properties

Progestins play a deleterious role in the onset of breast cancer, yet their influence on existing breast cancer and tumor progression is not well understood. In luminal estrogen receptor (ER)- and progesterone receptor (PR)-positive breast cancer, progestins induce a fraction of cells to express cytokeratin 5 (CK5), a marker of basal epithelial and progenitor cells in the normal breast. CK5(+) cells lose expression of ER and PR and are relatively quiescent, increasing their resistance to endocrine and chemotherapy compared to intratumoral CK5(-)ER(+)PR(+) cells. Characterization of live CK5(+) cells has been hampered by a lack of means for their direct isolation. Here, we describe optical (GFP) and bioluminescent (luciferase) reporter models to quantitate and isolate CK5(+) cells in luminal breast cancer cell lines utilizing the human KRT5 gene promoter and a viral vector approach. Using this system, we confirmed that the induction of GFP(+)/CK5(+) cells is specific to progestins, is dependent on PR, can be blocked by antiprogestins, and does not occur with other steroid hormones. Progestin-induced, fluorescence-activated cell sorting-isolated CK5(+) cells had lower ER and PR mRNA, were slower cycling, and were relatively more invasive and sphere forming than their CK5(-) counterparts in vitro. Repeated progestin treatment and selection of GFP(+) cells enriched for a persistent population of CK5(+) cells, suggesting that this transition can be semi-permanent. These data support that in PR(+) breast cancers, progestins induce a subpopulation of CK5(+)ER(-)PR(-) cells with enhanced progenitor properties and have implications for treatment resistance and recurrence in luminal breast cancer.

Adrenaline but not noradrenaline is a determinant of exercise-induced lipid mobilization in human subcutaneous adipose tissue

The relative contribution of noradrenaline (norepinephrine) and adrenaline (epinephrine) in the control of lipid mobilization in subcutaneous adipose tissue (SCAT) during exercise was evaluated in men treated with a somatostatin analogue, octreotide. Eight lean and eight obese young men matched for age and physical fitness performed 60 min exercise bouts at 50% of their maximal oxygen consumption on two occasions: (1) during i.v. infusion of octreotide, and (2) during placebo infusion. Lipolysis and local blood flow changes in SCAT were evaluated using in situ microdialysis. Infusion of octreotide suppressed plasma insulin and growth hormone levels at rest and during exercise. It blocked the exercise-induced increase in plasma adrenaline while that of noradrenaline was unchanged. Plasma natriuretic peptides (NPs) level was higher at rest and during exercise under octreotide infusion in lean men. Under placebo, no difference was found in the exercise-induced increase in glycerol between the probe perfused with Ringer solution alone and that with phentolamine (an alpha-adrenergic receptor antagonist) in lean subjects while a greater increase in glycerol was observed in the obese subjects. Under placebo, propranolol infusion in the probe containing phentolamine reduced by about 45% exercise-induced glycerol release; this effect was fully suppressed under octreotide infusion while noradrenaline was still elevated and exercise-induced lipid mobilization maintained in both lean and obese individuals. In conclusion, blockade of beta-adrenergic receptors during exercise performed during infusion of octreotide (blocking the exercise-induced rise in adrenaline but not that of noradrenaline) does not alter the exercise-induced lipolysis. This suggests that adrenaline is the main adrenergic agent contributing to exercise-induced lipolysis in SCAT. Moreover, it is the combined action of insulin suppression and NPs release which explains the lipolytic response which remains under octreotide after full local blockade of fat cell adrenergic receptors. For the moment, it is unknown if results apply specifically to SCAT and exercise only or if conclusions could be extended to all forms of lipolysis in humans.

Pituitary tumors arising from glycoprotein hormone alpha-subunit-deficient mice contain transcription factors and receptors present in thyrotropes

Glycoprotein-hormone alpha-subunit deficient (alphaSUnull) mice are hypothyroid and hypogonadal due to the absence of functional TSH, LH and FSH, despite normal production of the corresponding beta subunits. Pituitary tumors spontaneously developing in alphaSUnull mice were propagated in hypothyroid mice. The purpose of the current studies was to compare the gene expression profile of these alphaSUnull tumors with previously characterized TtT-97 thyrotropic tumors. A group of animals bearing each tumor type was treated with thyroid hormone (T4) prior to tumor removal. Both tumor types equally expressed TSHbeta mRNA, which significantly decreased when exposed to T4, whereas alpha-subunit mRNA was absent in alphaSUnull tumors. Northern blot analysis was performed using cDNA probes for the following transcription factors: Pit1, GATA2, pLIM, Msx1, Ptx1 and Ptx2. Both tumors were found to contain identical transcripts with similar responses to T4, with the exception of Pit1. In contrast to the signal pattern seen in TtT-97, only two bands were seen in alphaSUnull tumors, which were similar in size to those in alphaTSH cells, a thyrotropic cell line that lacks TSHbeta-subunit expression and Pit1 protein. However, western blot analysis revealed a protein band in the alphaSUnull tumors consistent with Pit1, while this signal was absent in alphaTSH cells. Northern blot analysis was also performed with specific cDNA probes for the following receptors: TRbeta1, TRbeta2, TRalpha1, non-T3 binding alpha2, RXRgamma and Sst5. Similarly-sized transcripts were found in both types of tumor, although the signal for Sst5 was seen in T4-treated alphaSUnull tumors only with a more sensitive RT-PCR analysis. The overall similarity between the two tumor types renders the alphaSUnull tumor as a suitable thyrotropic tumor model.

Distinct functional properties of native somatostatin receptor subtype 5 compared with subtype 2 in the regulation of ACTH release by corticotroph tumor cells

In a series of human corticotroph adenomas, we recently found predominant mRNA expression of somatostatin (SS) receptor subtype 5 (sst5). After 72 h, the multiligand SS analog SOM230, which has a very high sst5 binding affinity, but not Octreotide (OCT), significantly inhibited basal ACTH release. To further explore the role of sst5 in the regulation of ACTH release, we conducted additional studies with mouse AtT-20 cells. SOM230 showed a 7-fold higher ligand binding affinity and a 19-fold higher potency in stimulating guanosine 5'-O-(3-thiotriphosphate) binding in AtT-20 cell membranes compared with OCT. SOM230 potently suppressed CRH-induced ACTH release, which was not affected by 48-h dexamethasone (DEX) pretreatment. However, DEX attenuated the inhibitory effects of OCT on ACTH release, whereas it increased the inhibitory potency of BIM-23268, an sst5-specific analog, on ACTH release. Quantitative PCR analysis showed that DEX lowered sst(2A+2B) mRNA expression significantly after 24 and 48 h, whereas sst5 mRNA levels were not significantly affected by DEX treatment. Moreover, Scatchard analyses showed that DEX suppressed maximum binding capacity (B(max)) by 72% when 125I-Tyr3-labeled OCT was used as radioligand, whereas B(max) declined only by 17% when AtT-20 cells were treated with [125I-Tyr11]SS-14. These data suggest that the sst5 protein, compared with sst2, is more resistant to glucocorticoids. Finally, after SS analog preincubation, compared with OCT both SOM230 and BIM-23268 showed a significantly higher inhibitory effect on CRH-induced ACTH release. In conclusion, our data support the concept that the sst5 receptor might be a target for new therapeutic agents to treat Cushing's disease.

Receptor-Mediated Tumor Targeting with Radiopeptides. Part 1. General Concepts and Methods: Applications to Somatostatin Receptor-Expressing Tumors

Radiolabeled peptides have become important tools in nuclear oncology, both as diagnostics and more recently also as therapeutics. They represent a distinct sector of the molecular targeting approach, which in many areas of therapy will implement the old "magic bullet" concept by specifically directing the therapeutic agent to the site of action. In this three-part review, we present a comprehensive overview of the literature on receptor-mediated tumor targeting with the different radiopeptides currently studied. Part I summarizes the general concepts and methods of targeting, the selection of radioisotopes, chelators, and the criteria of peptide ligand development. Then, the >400 studies on the application to somatostatin/somatostatin-release inhibiting factor receptor-mediated tumor localization and treatment will be reviewed, demonstrating that peptide radiopharmaceuticals have gained an important position in clinical medicine.

The role of somatostatin analogs in Cushing's disease

Somatostatin (SRIF) has been proposed to be of therapeutic interest in the medical treatment of Cushing's disease. While in vitro data demonstrate the presence of SRIF-receptor subtype (sst) expression in corticotroph adenomas, the current clinically available SRIF-analog Octreotide, predominantly targeting sst(2), is ineffective in lowering ACTH levels in Cushing's disease and only appears to inhibit the release of ACTH in Nelson's syndrome. In the present review, current knowledge on the physiological role of SRIF in the regulation of ACTH secretion by the anterior pituitary gland, as well as by corticotroph tumor cells is summarized. In addition, the role of glucocorticoids in regulating sst-mediated inhibition of ACTH release by corticotroph adenoma cells is discussed. Recently, it was reported that the novel multiligand SRIF-analog SOM230 might have the potential to be of therapeutic interest for Cushing's disease. On the basis of the potent suppressive effects on ACTH release by SRIF-analogs with high binding affinity to sst(5) and the observation that sst(5) expression and action is relatively resistant to glucocorticoid treatment, including the recent observation that sst(5) is the predominant sst expressed in human corticotroph adenomas, it is hypothesized that sst(5) may be a new therapeutic target for the control of ACTH and cortisol hypersecretion in patients with pituitary dependent Cushing's disease.

Somatostatin receptors

In 1972, Brazeau et al. isolated somatostatin (somatotropin release-inhibiting factor, SRIF), a cyclic polypeptide with two biologically active isoforms (SRIF-14 and SRIF-28). This event prompted the successful quest for SRIF receptors. Then, nearly a quarter of a century later, it was announced that a neuropeptide, to be named cortistatin (CST), had been cloned, bearing strong resemblance to SRIF. Evidence of special CST receptors never emerged, however. CST rather competed with both SRIF isoforms for specific receptor binding. And binding to the known subtypes with affinities in the nanomolar range, it has therefore been acknowledged to be a third endogenous ligand at SRIF receptors. This review goes through mechanisms of signal transduction, pharmacology, and anatomical distribution of SRIF receptors. Structurally, SRIF receptors belong to the superfamily of G protein-coupled (GPC) receptors, sharing the characteristic seven-transmembrane-segment (STMS) topography. Years of intensive research have resulted in cloning of five receptor subtypes (sst(1)-sst(5)), one of which is represented by two splice variants (sst(2A) and sst(2B)). The individual subtypes, functionally coupled to the effectors of signal transduction, are differentially expressed throughout the mammalian organism, with corresponding differences in physiological impact. It is evident that receptor function, from a physiological point of view, cannot simply be reduced to the accumulated operations of individual receptors. Far from being isolated functional units, receptors co-operate. The total receptor apparatus of individual cell types is composed of different-ligand receptors (e.g. SRIF and non-SRIF receptors) and co-expressed receptor subtypes (e.g. sst(2) and sst(5) receptors) in characteristic proportions. In other words, levels of individual receptor subtypes are highly cell-specific and vary with the co-expression of different-ligand receptors. However, the question is how to quantify the relative contributions of individual receptor subtypes to the integration of transduced signals, ultimately the result of collective receptor activity. The generation of knock-out (KO) mice, intended as a means to define the contributions made by individual receptor subtypes, necessarily marks but an approximation. Furthermore, we must now take into account the stunning complexity of receptor co-operation indicated by the observation of receptor homo- and heterodimerisation, let alone oligomerisation. Theoretically, this phenomenon adds a novel series of functional megareceptors/super-receptors, with varied pharmacological profiles, to the catalogue of monomeric receptor subtypes isolated and cloned in the past. SRIF analogues include both peptides and non-peptides, receptor agonists and antagonists. Relatively long half lives, as compared to those of the endogenous ligands, have been paramount from the outset. Motivated by theoretical puzzles or the shortcomings of present-day diagnostics and therapy, investigators have also aimed to produce subtype-selective analogues. Several have become available.

Suppression of insulin secretion is associated with weight loss and altered macronutrient intake and preference in a subset of obese adults

PURPOSE

Hyperinsulinemia is a common feature of many obesity syndromes. We investigated whether suppression of insulin secretion, without dietary or exercise intervention, could promote weight loss and alter food intake and preference in obese adults.

METHODS

Suppression of insulin secretion was achieved using octreotide-LAR 40 mg IM q28d for 24 weeks in 44 severely obese adults (89% female, 39% minority). Oral glucose tolerance testing was performed before and after treatment, indices of beta-cell activity (CIRgp), insulin sensitivity (CISI), and clearance (CP/I AUC) were computed, and leptin levels, 3-day food records and carbohydrate-craving measurements were obtained. DEXA evaluations were performed pre- and post-therapy in an evaluable subgroup.

RESULTS

For the entire cohort, significant insulin suppression was achieved with simultaneous improvements in insulin sensitivity, weight loss, and body mass index (BMI). Leptin, fat mass, total caloric intake, and carbohydrate craving significantly decreased. When grouped by BMI response, high responders (HR; DeltaBMI<-3 kg/m(2)) and low responders (LR; DeltaBMI between -3 and -0.5) exhibited higher suppression of CIRgp and IAUC than nonresponders (NR; DeltaBMI-0.5). CISI improved and significant declines in leptin and fat mass occurred only in HR and LR. Conversely, both leptin and fat mass increased in NR. Carbohydrate intake was markedly suppressed in HR only, while carbohydrate-craving scores decreased in HR and LR. For the entire cohort, DeltaBMI correlated with DeltaCISI, Deltafat mass, and Deltaleptin. DeltaFat mass also correlated with DeltaIAUC and DeltaCISI.

CONCLUSIONS

In a subcohort of obese adults, suppression of insulin secretion was associated with loss of body weight and fat mass and with concomitant modulation of caloric intake and macronutrient preference.

Somatostatin analogs and radiopeptides in cancer therapy

Since the discovery of somatostatin (sst) in 1973, numerous chemical and biological studies have been carried out to develop sst analogs with enhanced resistance to proteases and prolonged activity. Three highly potent sst analogs-octreotide, lanreotide, and vapreotide-are now available in the clinic, and demonstrate efficacy in the treatment of tumors of the pituitary and the gastroenteropancreatic tract. The most striking effect is the control of hormone hypersecretion associated with these tumors. Available data on growth suppression in patients indicate a limited antiproliferative action, tumor shrinkage is observed in 10-20% patients, and tumor stabilization in about half of the patients for duration of 8-16 months. Eventually, however, all patients escape from sst analog therapy with regard to both hormone hypersecretion and tumor growth, the only exception being observed in acromegalic patients who do not experience tachyphylaxis even after more than 10 years of daily octreotide injection. The mechanism underlying the escape phenomenon is not yet clarified. Regarding the molecular mechanisms involved in sst antineoplastic activity, both indirect and direct effects via specific somatostatin receptors (SSTRs) expressed in the target cells have be described. Direct action may result from blockade of mitogenic growth signal or induction of apoptosis following interaction with SSTRs. Indirect effects may be the result of reduced or inhibited secretion of growth-promoting hormones and growth factors that stimulate the growth of various types of cancer; also, inhibition of angiogenesis or influence on the immune system are important factors. Five SSTR subtypes have been identified so far, which are variably expressed in a variety of tumors such as gastroenteropancreatic (GEP) tumors, pituitary tumors, and carcinoid tumors. Although all five SSTR subtypes are linked to adenylate cyclase, they are now known to affect multiple other cellular signaling systems and hence they differentially participate in the regulation of the various cellular processes. The finding of several laboratories that SSTR-expressing tumors frequently contain two or more SSTR subtypes, and the recent discovery that SSTR subtypes might form homo/heterodimers to create a novel receptor with different functional characteristics, expand the array of selective SSTR activation pathways and subsequent intracellular signaling cascades. This may lead to improved clinical protocols that take into account possible synergistic interactions between the SSTR subtypes present on the same cancer cell. Radiolabeled sst analogs, such as [(111)In]-[diethylenetriamine pentaacetic acid (DTPA)-D-Phe(1)]-octreotide (OcreoScan), have proved to be very useful for tumor scintigraphy and internal radiotherapy of SSTR overexpressing tumors. The recent introduction of the metal chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) considerably improved the stability of the radioconjugates, making possible the incorporation of a variety of radionuclides, such as (90)Y for receptor-mediated radionuclide therapy or (68)Ga for positron emission tomography (PET). Another promising area is the development of sst conjugates incorporating cytotoxic anticancer drugs.

Identification of an upstream pituitary-active promoter of human somatostatin receptor subtype 5

Somatostatin receptor subtype 5 (sst5) has been linked to inhibition of PRL and insulin secretion. We characterized the genomic structure of the human sst5. The transcription start site was located 94 nucleotides upstream of the initiator ATG codon. Sequence analysis of 5'-inverse PCR products revealed the presence of a 6.1-kb intron in the 5'-untranslated region. RT-PCR analysis indicated tissue-specific activation of the newly identified upstream promoter in pituitary, but not in small intestine, lung, or placenta. A -1741 promoter directed significant levels of luciferase expression in GH(4) rat pituitary cells, Skut-1B endometrium cells, and JEG3 chorion carcinoma cells, which was absent in COS-7 monkey kidney cells. A minimal -101 promoter was sufficient to allow tissue-specific expression. Its activity in COS-7 cells was not enhanced by cotransfection of the pituitary-specific transcription factor Pit-1. Analysis of deletion constructs revealed a GC-rich region immediately upstream of the transcription start site, which is necessary for promoter activity. Somatostatin led to a significant inhibition, and forskolin and thyroid hormone to a significant stimulation of pituitary-specific promoter activity. Further mapping suggested a cAMP-responsive element located between -101 and the transcription start site, and thyroid hormone-responsive elements between -1741 and -1269 and between -317 and -101. These studies identified an upstream promoter of the sst5 gene with tissue-specific activity.

Mutational analysis of the mouse somatostatin receptor type 5 gene promoter

We have previously characterized the structure of the murine somatostatin receptor type 5 gene (sst5). Initial transient transfection studies in pituitary somatolactotropes (GH(3)) mapped the promoter activity of this gene to a region 290 bp upstream of the transcription start site. The current study identifies the sst5 promoter region critical for basal activity. A series of deletions was generated, and promoter activity was localized to a region between -83 and -19. Similar promoter deletion patterns were evident in five pituitary cell types. Seven 10-bp transversion mutations encompassing the region between -83 and -19 were generated, and functional activity was assessed. Promoter activity was reduced by the mutations spanning -67 to -47 compared with the wild-type construct. Another mutation between -26 and -17 resulted in promoter activity reduction in GH(3) cells, but not TtT-97 thyrotropes. Deoxyribonuclease I protection analysis of the sst5 promoter region between -208/+47 was performed using GH(3) and TtT-97 nuclear extracts. The most striking protected regions, located between -61 and -41 and -25 and -3, correlated with functionally important regions identified by transfection studies. In summary, the mouse sst5 gene promoter has been characterized, and functional activity and nuclear factor interactions were mapped to two specific promoter regions. The region between -67 and -47 appears to contain a nucleotide sequence critical for basal transcriptional regulation of the mouse sst5 gene in pituitary cells.

Use of somatostatin receptor ligands in obesity and diabetic complications

Somatostatin (SMS) is a potent inhibitory molecule. It inhibits both exocrine and endocrine secretory functions of the pancreas, suppresses growth hormone secretion and reduces the level of insulin-like growth factor-1. Long-acting somatostatin analogues were currently investigated for potential clinical benefits in two settings: (a) control of hyperinsulinaemia in obesity and (b) control of an excess of pro-angiogenic factors in diabetes-associated retinal complications. In two randomized, controlled trials the long-acting somatostatin analogue octreotide retarded progression of the microvascular complications in pre-proliferative and advanced stages of diabetic retinopathy. Inhibition of the early phase of insulin secretion by use of octreotide in patients with hypothalamic obesity resulted in weight loss and improved quality of life. Efficacy of octreotide correlated to residual beta-cell activity prior to the treatment. Obesity and diabetes mellitus are the most common chronic metabolic disorders in the world. The use of somatostatin analogues addressing the various hormonal imbalances of these disorders may provide a novel concept for their pharmacological treatment.

The effect of thyroid hormone and a long-acting somatostatin analogue on TtT-97 murine thyrotropic tumors

Thyroid hormone inhibits thyrotropin (TSH) production and thyrotrope growth. Somatostatin has been implicated as a synergistic factor in the inhibition of thyrotrope function. We have previously shown that pharmacological doses of thyroid hormone (levothyroxine [LT4]) inhibit growth of murine TtT-97 thyrotropic tumors in association with upregulation of somatostatin receptor type 5 (sst5) mRNA and somatostatin receptor binding. In the current study, we examined the effect of physiological thyroid hormone replacement alone or in combination with the long-acting somatostatin analogue, Sandostatin LAR, on thyrotropic tumor growth, thyrotropin growth factor-beta (TSH-beta), and sst5 mRNA expression, as well as somatostatin receptor binding sites. Physiological LT4 replacement therapy resulted in tumor shrinkage in association with increased sst5 mRNA levels, reduced TSH-beta mRNA levels and enhanced somatostatin receptor binding. Sandostatin LAR alone had no effect on any parameter measured. However, Sandostatin LAR combined with LT4 synergistically inhibited TSH-beta mRNA production and reduced final tumor weights to a greater degree. In this paradigm, Sandostatin LAR required a euthyroid status to alter thyrotrope parameters. These data suggest an important interaction between the somatostatinergic system and thyroid hormone in the regulation of thyrotrope cell structure and function.