Pituitary somatostatin receptor signaling

Medical treatment of functional pituitary adenomas, trials and tribulations

CONTEXT

Functioning pituitary adenomas (FPAs) include most frequently prolactinomas, somatotroph or corticotroph adenomas, while thyrotroph and gonadotroph adenomas are very rare. Despite their benign histological nature (aggressive tumors are rare and malignant ones exceptional), FPAs could cause significant morbidity and increased mortality due to complications associated with hormonal excess syndromes and/or mass effect leading to compression of adjacent structures. This mini review will focus on the increasing role of medical therapy in the multimodal treatment, which also includes transsphenoidal surgery (TSS) and radiotherapy.

EVIDENCE SYNTHESIS

Most patients with prolactinomas are treated only with medications, but surgery could be considered for some patients in a specialized pituitary center, if higher chances of cure. Dopamine agonists, especially cabergoline, are efficient in reducing tumor size and normalizing prolactin. TSS is the first-line treatment for all other FPAs, but most patients require complex adjuvant treatment, including a combination of therapeutic approaches. Medical therapy is the cornerstone of treatment in all patients after unsuccessful surgery or when surgery cannot be offered and includes somatostatin receptor ligands and dopamine agonists (almost all FPAs), growth hormone receptor antagonists (acromegaly), adrenal steroidogenesis inhibitors and glucocorticoid receptor blockers (Cushing's disease). Novel medical treatments, especially for acromegaly and Cushing's disease are under research.

CONCLUSIONS

An enlarged panel of effective drugs available with increased knowledge of predictive factors for response and/or adverse effects will enhance the possibility to offer a more individualized treatment. This would not only improve disease control and prognosis, but also quality of life.

The disrupting effect of chlormequat chloride on growth hormone is associated with pregnancy

Since chlormequat chloride is widely applied as a plant growth regulator in agriculture and horticulture, its exposure through food consumption is common. We demonstrated previously that chlormequat chloride exposure during pregnancy led to embryos with bigger sizes associated with higher levels of growth hormone (GH) on gestation day 11 (GD11). However, the dose-effect relationship of chlormequat chloride at a lower dose range was not established, and the underlying mechanisms of its promoting effects on embryonic growth and development were not fully elucidated. To address these, pregnant rats were orally exposed to chlormequat chloride at 0, 0.05, 0.5 and 5 mg/kg.bw from GD0 to 11 and the embryonic growth and growth related hormones were evaluated on GD11. We found that the growth and development of the embryos was significantly promoted in a dose dependent manner by chlormequat chloride. Chlormequat chloride also increased embryonic GH, GH releasing hormone (GHRH), and somatostatin (SRIF), and inhibited the embryonic cAMP dependent protein kinase A (PKA) signaling pathway. Chlormequat chloride increased GH synthesis modulated by GHRH/SRIF-PKA-Pituitary specific transcription factor 1 (Pit-1) in the maternal rats. Intriguingly, chlormequat chloride did not show any effects on GH and PKA signaling pathways in the non-pregnant female rats. These findings together suggest that the disrupting effect of chlormequat chloride on GH is associated with pregnancy.

Investigation of imaging the somatostatin receptor by opening the blood-brain barrier with melittin - A feasibility study using positron emission tomography and [64Cu]Cu-DOTATATE

DOTATATE is a somatostatin peptide analog used in the clinic to detect somatostatin receptors which are highly expressed on neuroendocrine tumors. Somatostatin receptors are found naturally in the intestines, pancreas, lungs, and brain (mainly cortex). In vivo measurement of the somatostatin receptors in the cortex has been challenging because available tracers cannot cross the blood-brain barrier (BBB) due to their intrinsic polarity. A peptide called melittin, a main component of honeybee venom, has been shown to disrupt plasma membranes and increase the permeability of biological membranes. In this study, we assessed the feasibility of using melittin to facilitate the passage of [64Cu]Cu-DOTATATE through the BBB and its binding to somatostatin receptors in the cortex. Evaluation included in vitro autoradiography on Long Evans rat brains to estimate the binding affinity of [64Cu]Cu-DOTATATE to the somatostatin receptors in the cortex and an in vivo evaluation of [64Cu]Cu-DOTATATE binding in NMRI mice after injection of melittin. This study found an in vitro Bmax = 89 ± 4 nM and KD = 4.5 ± 0.6 nM in the cortex, resulting in a theoretical binding potential (BP) calculated as Bmax/KD ≈ 20, which is believed suitable for in vivo brain PET imaging. However, the in vivo results showed no significant difference between the control and melittin injected mice, indicating that the honeybee venom failed to open the BBB. Additional experiments, potentially involving faster injection rates are required to verify that melittin can increase brain uptake of non-BBB permeable PET tracers. Furthermore, an evaluation of whether a venom with a narrow therapeutic range can be used for clinical purposes needs to be considered.

Somatostatin Receptor Gene Functions in Growth Regulation in Bivalve Scallop and Clam

Bivalves hold an important role in marine aquaculture and the identification of growth-related genes in bivalves could contribute to a better understanding of the mechanism governing their growth, which may benefit high-yielding bivalve breeding. Somatostatin receptor (SSTR) is a conserved negative regulator of growth in vertebrates. Although SSTR genes have been identified in invertebrates, their involvement in growth regulation remains unclear. Here, we identified seven SSTRs (PySSTRs) in the Yesso scallop, Patinopecten yessoensis, which is an economically important bivalve cultured in East Asia. Among the three PySSTRs (PySSTR-1, -2, and -3) expressed in adult tissues, PySSTR-1 showed significantly lower expression in fast-growing scallops than in slow-growing scallops. Then, the function of this gene in growth regulation was evaluated in dwarf surf clams (Mulinia lateralis), a potential model bivalve cultured in the lab, via RNA interference (RNAi) through feeding the clams Escherichia coli containing plasmids expressing double-stranded RNAs (dsRNAs) targeting MlSSTR-1. Suppressing the expression of MlSSTR-1, the homolog of PySSTR-1 in M. lateralis, resulted in a significant increase in shell length, shell width, shell height, soft tissue weight, and muscle weight by 20%, 22%, 20%, 79%, and 92%, respectively. A transcriptome analysis indicated that the up-regulated genes after MlSSTR-1 expression inhibition were significantly enriched in the fat digestion and absorption pathway and the insulin pathway. In summary, we systemically identified the SSTR genes in P. yessoensis and revealed the growth-inhibitory role of SSTR-1 in bivalves. This study indicates the conserved function of somatostatin signaling in growth regulation, and ingesting dsRNA-expressing bacteria is a useful way to verify gene function in bivalves. SSTR-1 is a candidate target for gene editing in bivalves to promote growth and could be used in the breeding of fast-growing bivalves.

SSTR2 Mediates the Inhibitory Effect of SST/CST on Lipolysis in Chicken Adipose Tissue

Somatostatin shows an anti-lipolytic effect in both chickens and ducks. However, its molecular mediator remains to be identified. Here, we report that somatostatin type 2 receptor (SSTR2) is expressed at a high level in chicken adipose tissue. In cultured chicken adipose tissue, the inhibition of glucagon-stimulated lipolysis by somatostatin was blocked by an SSTR2 antagonist (CYN-154086), supporting an SSTR2-mediated anti-lipolytic effect. Furthermore, a significant pro-proliferative effect was detected in SST28-treated immortalized chicken preadipocytes (ICP-1), and this cell proliferative effect may be mediated through the MAPK/ERK signaling pathway activated by SSTR2. In summary, our results demonstrate that SSTR2 may regulate adipose tissue development by affecting the number and volume of adipocytes in chickens.

Treatment of acromegaly with oral octreotide

Acromegaly is a rare disease caused by a growth hormone excess, usually due to a secreting pituitary adenoma. Somatostatin receptor ligands (SRL) are the mainstay of medical therapy for patients with acromegaly who fail to achieve biochemical control post-operatively or are not eligible for surgical treatment. SRLs are typically administered as monthly injections and have shown to be effective in maintaining biochemical and radiological control of acromegaly. However, these injections may cause local adverse events and are associated with increased psychological burden in some patients. Oral octreotide provides a new alternative for patients responding to injectable SRLs. This new formulation has shown to have similar safety and efficacy profiles compared to injectable SRLs and may be a preferable option for some patients with acromegaly. The aim of this review is to provide an overview of the role of oral octreotide in the management of acromegaly.

Connexin 36 Mediated Intercellular Endoplasmic Reticulum Stress Transmission Induces SSTA Resistance in Growth Hormone Pituitary Adenoma

Somatostatin analogues (SSTA) are first-line pharmacological treatment choice for acromegaly, which received satisfying tumor shrinkage and normalization of growth hormone. However, there are still patients unresponsive to SSTA, and the underline mechanism remains unknown. Besides, there is no evidence regarding the role of endoplasmic reticulum stress (ERS) and its transmission in SSTA resistance, which also require investigation. Primary growth hormone adenoma cells and cell lines were treated with SSTA; autophagy double-labeled LC3 (mRFP-GFP) adenovirus transfection, flow cytometry sorting, western blotting, calcium imaging as well as immunofluorescence staining were used to determine ERS and autophagy signal transmission; xenograft and syngeneic tumor in vivo model were exploited to confirm the ERS signal transmission mediated effect. Our results revealed that SSTA induces ERS in pituitary growth hormone (GH) adenoma cells. The ERS signals can be intercellularly transmitted, leading to less responsible to SSTA treatment. Moreover, SSTA stimulates inositol triphosphate (IP3) elevation, mediating ERS intercellular transfer. In addition, connexin 36 tunnels ERS transmission, and its blocker, Quinine, exhibits a synergistic effect with SSTA treating GH adenoma. Our study provided insight into ERS intercellular transmission mediated SSTA resistance, which could be translated into clinical usage to improve SSTA efficiency in GH adenoma treatment.

The Role of Receptor-Ligand Interaction in Somatostatin Signaling Pathways: Implications for Neuroendocrine Tumors

Neuroendocrine tumors (NETs) arise from neuroendocrine cells and manifest in diverse organs. Key players in their regulation are somatostatin and its receptors (SSTR1-SSTR5). Understanding receptor-ligand interactions and signaling pathways is vital for elucidating their role in tumor development and therapeutic potential. This review highlights SSTR characteristics, localization, and expression in tissues, impacting physiological functions. Mechanisms of somatostatin and synthetic analogue binding to SSTRs, their selectivity, and their affinity were analyzed. Upon activation, somatostatin initiates intricate intracellular signaling, involving cAMP, PLC, and MAP kinases and influencing growth, differentiation, survival, and hormone secretion in NETs. This review explores SSTR expression in different tumor types, examining receptor activation effects on cancer cells. SSTRs' significance as therapeutic targets is discussed. Additionally, somatostatin and analogues' role in hormone secretion regulation, tumor growth, and survival is emphasized, presenting relevant therapeutic examples. In conclusion, this review advances the knowledge of receptor-ligand interactions and signaling pathways in somatostatin receptors, with potential for improved neuroendocrine tumor treatments.

Growth Axis Somatostatin, Growth Hormone Receptor, and Insulin-like Growth Factor-1 Genes Express and Are Affected by the Injection of Exogenous Growth Hormone in Chinemys reevesii

In this study, to explore the effect of growth hormone changes on the related genes and regulatory roles of the turtle, PCR amplification, real-time fluorescence quantitative analysis, and enzyme cutting technology were used to clone and sequence the somatostatin (SS) gene, growth hormone receptor (GHR), and insulin-like growth factor-1 (IGF-I) sequence of Chinemys reevesii. The effects of human growth hormone on the mRNA expression of growth-axis-related genes SS, GHR, and IGF-1 in different sexes were observed. The study of the SS gene in turtles using real-time fluorescence quantitative PCR showed that the SS gene was mainly expressed in the nervous system and the digestive system, with the highest expression found in the brain, while the GHR gene and the IGF-I gene were expressed in all tissues of Chinemys reevesii. The SS gene was expressed in the brain, pituitary, liver, stomach, and intestine, with the highest expression in the brain and the lowest expression in the liver. Within 4 weeks of the injection of exogenous growth hormone, the expression level of the SS gene in the brain of both sexes first increased and then decreased, showing a parabolic trend, and the expression level of the experimental group was lower than that of the control group. After the injection of growth hormone (GH), the expression of the GHR gene in the liver of both sexes showed a significant increase in the first week, decreasing to the control group level in the second week, and then gradually increasing. Finally, a significant level of difference in the expression of the GHR gene was reached at 3 and 4 weeks. In terms of the IGF-I gene, the changing trend of the expression level in the liver was the same as that of the GHR gene. After the injection of exogenous growth hormone, although the expression of the SS gene increased the inhibition of the secretion of the GHR gene by the Reeves' turtle, exogenous growth hormone could replace the synthesis of GH and GHR, accelerating the growth of the turtle. The experiments showed that the injection of recombinant human growth hormone affects the expression of SS, GHR, and IGF-1 genes, and promotes the growth of the Reeves' turtle.

Next-generation PET/CT imaging in meningioma-first clinical experiences using the novel SSTR-targeting peptide [18F]SiTATE

BACKGROUND

Somatostatin-receptor (SSTR)-targeted PET/CT provides important clinical information in addition to standard imaging in meningioma patients. [18F]SiTATE is a novel, 18F-labeled SSTR-targeting peptide with superior imaging properties according to preliminary data. We provide the first [18F]SiTATE PET/CT data of a large cohort of meningioma patients.

METHODS

Patients with known or suspected meningioma undergoing [18F]SiTATE PET/CT were included. Uptake intensity (SUV) of meningiomas, non-meningioma lesions, and healthy organs were assessed using a 50% isocontour volume of interest (VOI) or a spherical VOI, respectively. Also, trans-osseous extension on PET/CT was assessed.

RESULTS

A total of 107 patients with 117 [18F]SiTATE PET/CT scans were included. Overall, 231 meningioma lesions and 61 non-meningioma lesions (e.g., post-therapeutic changes) were analyzed. Physiological uptake was lowest in healthy brain tissue, followed by bone marrow, parotid, and pituitary (SUVmean 0.06 ± 0.04 vs. 1.4 ± 0.9 vs. 1.6 ± 1.0 vs. 9.8 ± 4.6; p < 0.001). Meningiomas showed significantly higher uptake than non-meningioma lesions (SUVmax 11.6 ± 10.6 vs. 4.0 ± 3.3, p < 0.001). Meningiomas showed significantly higher uptake than non-meningioma lesions (SUVmax 11.6±10.6 vs. 4.0±3.3, p<0.001). 93/231 (40.3%) meningiomas showed partial trans-osseous extension and 34/231 (14.7%) predominant intra-osseous extension. 59/231 (25.6%) meningioma lesions found on PET/CT had not been reported on previous standard imaging.

CONCLUSION

This is the first PET/CT study using an 18F-labeled SSTR-ligand in meningioma patients: [18F]SiTATE provides extraordinary contrast in meningioma compared to healthy tissue and non-meningioma lesions, which leads to a high detection rate of so far unknown meningioma sites and osseous involvement. Having in mind the advantageous logistic features of 18F-labeled compared to 68Ga-labeled compounds (e.g., longer half-life and large-badge production), [18F]SiTATE has the potential to foster a widespread use of SSTR-targeted imaging in neuro-oncology.

Regulation of FGF2-induced proliferation by inhibitory GPCR in normal pituitary cells

Introduction

Intracellular communication is essential for the maintenance of the anterior pituitary gland plasticity. The aim of this study was to evaluate whether GPCR-Gαi modulates basic fibroblast growth factor (FGF2)-induced proliferative activity in normal pituitary cell populations.

Methods

Anterior pituitary primary cell cultures from Wistar female rats were treated with FGF2 (10ng/mL) or somatostatin analog (SSTa, 100nM) alone or co-incubated with or without the inhibitors of GPCR-Gαi, pertussis toxin (PTX, 500nM), MEK inhibitor (U0126, 100µM) or PI3K inhibitor (LY 294002, 10 μM).

Results

FGF2 increased and SSTa decreased the lactotroph and somatotroph BrdU uptak2e (p<0.05) whereas the FGF2-induced S-phase entry was prevented by SSTa co-incubation in both cell types, with these effects being reverted by PTX, U0126 or LY294002 pre-incubation. The inhibition of lactotroph and somatotroph mitosis was associated with a downregulation of c-Jun expression, a decrease of phosphorylated (p) ERK and pAKT. Furthermore, SSTa was observed to inhibit the S-phase entry induced by FGF2, resulting in a further increase in the number of cells in the G1 phase and a concomitant reduction in the number of cells in the S phases (p< 0.05), effects related to a decrease of cyclin D1 expression and an increase in the expression of the cell cycle inhibitors p27 and p21.

Discussion

In summary, the GPCR-Gαi activated by SSTa blocked the pro-proliferative effect of FGF2 in normal pituitary cells via a MEK-dependent mechanism, which acts as a mediator of both anti and pro-mitogenic signals, that may regulate the principal effectors of the G1 to S-phase transition.

Effects of Various LED Light Spectra on Growth, Gonadal Development, and Growth-/Reproduction-Related Hormones in the Juvenile Red Spotted Grouper, Epinephelus akaara

The light spectrum is a key environmental cue involved in growth and reproduction in teleosts. This study investigated the effects of exposure on juvenile red spotted grouper exposed to white (control), red (590 nm), blue (480 nm), and green (520 nm) light-emitting diodes (LEDs) (12 h light:12 h dark) for two months. The body weight (BW), total length (TL), condition factor (CF), weight gain rate (WGR), gonadosomatic index (GSI), and hepatosomatic index (HSI) were assessed. Gonadal development was observed. The gene expression of growth-related hormones, such as growth hormone (GH), pre-pro-somatostatin-I (PSS-I), neuropeptide Y (NPY), and CCK, and of reproduction-related hormones, such as Kiss1, Kiss2, GPR54, sbGnRH, FSHβ, and LHβ, was analyzed. The results showed that the fish in the white LED group exhibited the best BW, TL, CF, WGR, and HSI after one or two months. The fish exposed to white LEDs showed the best growth after two months, but no significant differences in GH levels were detected. Contrarily, the expression levels of the PSS-I significantly increased (p < 0.05) in fish from the white group, suggesting the complex regulation of GH production and the limited effects of PSS-I on the inhibition of GH synthesis and somatic growth. The significantly increased NPY levels in the four LED groups (p < 0.05) indicated that these four LED spectra were effective in stimulating food intake and energy homeostasis. After two months, the gonads developed from chromatin nucleolar-stage oocytes to perinucleolar-stage oocytes in the four LED groups. The gene expression of Kiss2 and GPR54 in the four LED groups and of sbGnRH in the white and blue LED groups significantly increased when compared to that in the initial group (p < 0.05), while there were no significant differences in FSHβ and LHβ expression in the four LED groups. These results suggest that FSH and LH may not play important roles in gonadal development in juvenile red spotted grouper that are exposed to these four LED spectra.

The Wnt pathway protein Dvl1 targets somatostatin receptor 2 for lysosome-dependent degradation

The Somatostatin receptor 2 (Sstr2) is a heterotrimeric G protein-coupled receptor that is highly expressed in neuroendocrine tumors and is a common pharmacological target for intervention. Unfortunately, not all neuroendocrine tumors express Sstr2, and Sstr2 expression can be downregulated with prolonged agonist use. Sstr2 is rapidly internalized following agonist stimulation and, in the short term, is quantitatively recycled back to the plasma membrane. However, mechanisms controlling steady state expression of Sstr2 in the absence of agonist are less well described. Here, we show that Sstr2 interacts with the Wnt pathway protein Dvl1 in a ligand-independent manner to target Sstr2 for lysosomal degradation. Interaction of Sstr2 with Dvl1 does not affect receptor internalization, recycling, or signaling to adenylyl cyclase but does suppress agonist-stimulated ERK1/2 activation. Importantly, Dvl1-dependent degradation of Sstr2 can be stimulated by overexpression of Wnts and treatment of cells with Wnt pathway inhibitors can boost Sstr2 expression in neuroendocrine tumor cells. Taken together, this study identifies for the first time a mechanism that targets Sstr2 for lysosomal degradation that is independent of Sstr2 agonist and can be potentiated by Wnt ligand. Intervention in this signaling mechanism has the potential to elevate Sstr2 expression in neuroendocrine tumors and enhance Sstr2-directed therapies.

A Preliminary Study on the Interplay between the Serum Levels of Neurotransmitters and Thyroid Hormones for the Evaluation of the Behavioral Phenotype of Dogs

A total of 112 dogs (49 males and 63 females) belonging to different breeds (i.e., Boxer, Cirneco dell'Etna, Fonni's Dog, Labrador, Crossbreed, German Shepherd, Pit Bull, Shar-Pei, Yorkshire) were analyzed to compare the serum concentration of serotonin, dopamine, norepinephrine, prolactin, beta-endorphins, thyroxine (T4), triiodothyronine (T3), thyroid-stimulating hormone (TSH), and assess whether these parameters can be correlated with the behavioral phenotype of the investigated breeds. T4 was above or below the threshold in 61% and 14% of dogs, respectively; T3, in contrast, 41% of dogs showed values below the limit, while 26% above it. TSH was within the reference range in 58% of dogs; 94% of the dogs had prolactin in the reference range and only five animals showed values above the limit. For beta-endorphins, 49% of dogs had values above the limit, while 46% had values within the reference range. Serotonin and dopamine values below physiological limits were found in 62% and 70% of dogs, respectively. Finally, 61% of the dogs showed norepinephrine values within the reference range. The study confirmed that the assessment of the serum values of hormones and neurotransmitters in dogs could be useful to better understand the behavioral phenotype of the animal and could be useful for breeders and trainers for the selection of the most suitable subjects for specific tasks.

Whole-genome resequencing reveals recent signatures of selection in five populations of largemouth bass ( Micropterus salmoides)

Largemouth bass ( Micropterus salmoides) is an economically important fish species in North America, Europe, and China. Various genetic improvement programs and domestication processes have modified its genome sequence through selective pressure, leaving nucleotide signals that can be detected at the genomic level. In this study, we sequenced 149 largemouth bass fish, including protospecies (imported from the US) and improved breeds (four domestic breeding populations from China). We detected genomic regions harboring certain genes associated with improved traits, which may be useful molecular markers for practical domestication, breeding, and selection. Subsequent analyses of genetic diversity and population structure revealed that the improved breeds have undergone more rigorous genetic changes. Through selective signal analysis, we identified hundreds of putative selective sweep regions in each largemouth bass line. Interestingly, we predicted 103 putative candidate genes potentially subjected to selection, including several associated with growth (p sst1 and grb10), early development ( klf9, sp4, and sp8), and immune traits ( pkn2, sept2, bcl6, and ripk2). These candidate genes represent potential genomic landmarks that could be used to improve important traits of biological and commercial interest. In summary, this study provides a genome-wide map of genetic variations and selection footprints in largemouth bass, which may benefit genetic studies and accelerate genetic improvement of this economically important fish.

Second line treatment of acromegaly: Pasireotide or Pegvisomant?

Acromegaly is a chronic disease with an increased mortality in case of persistently active disease. The treatment of acromegaly is mainly based on the surgical resection of the GH secreting pituitary tumor and, in cases with persistent disease, on the medical therapy with first generation somatostatin analogues (first gen-SSAs). Data from national registries, meta-analysis and epidemiology studies showed that 24%-65% of acromegaly patients treated with first gen-SSA did not reach the control of disease, requiring second line therapies, as the second gen-SSAs and the GH receptor antagonist. According to the high efficacy of these treatments and their molecular mechanisms of action, the choice of second line therapies should be personalized. In this review, we summarize the evidence on clinical, molecular and morphological aspects that may predict the response to second line therapies, in order to integrate and translate in the clinical practice for a patient-tailored therapeutic approach.

Somatostatin, Cortistatin and Their Receptors Exert Antitumor Actions in Androgen-Independent Prostate Cancer Cells: Critical Role of Endogenous Cortistatin

Somatostatin (SST), cortistatin (CORT), and their receptors (SSTR1-5/sst5TMD4-TMD5) comprise a multifactorial hormonal system involved in the regulation of numerous pathophysiological processes. Certain components of this system are dysregulated and play critical roles in the development/progression of different endocrine-related cancers. However, the presence and therapeutic role of this regulatory system in prostate cancer (PCa) remain poorly explored. Accordingly, we performed functional (proliferation/migration/colonies-formation) and mechanistic (Western-blot/qPCR/microfluidic-based qPCR-array) assays in response to SST and CORT treatments and CORT-silencing (using specific siRNA) in different PCa cell models [androgen-dependent (AD): LNCaP; androgen-independent (AI)/castration-resistant PCa (CRPC): 22Rv1 and PC-3], and/or in the normal-like prostate cell-line RWPE-1. Moreover, the expression of SST/CORT system components was analyzed in PCa samples from two different patient cohorts [internal (n = 69); external (Grasso, n = 88)]. SST and CORT treatment inhibited key functional/aggressiveness parameters only in AI-PCa cells. Mechanistically, antitumor capacity of SST/CORT was associated with the modulation of oncogenic signaling pathways (AKT/JNK), and with the significant down-regulation of critical genes involved in proliferation/migration and PCa-aggressiveness (e.g., MKI67/MMP9/EGF). Interestingly, CORT was highly expressed, while SST was not detected, in all prostate cell-lines analyzed. Consistently, endogenous CORT was overexpressed in PCa samples (compared with benign-prostatic-hyperplasia) and correlated with key clinical (i.e., metastasis) and molecular (i.e., SSTR2/SSTR5 expression) parameters. Remarkably, CORT-silencing drastically enhanced proliferation rate and blunted the antitumor activity of SST-analogues (octreotide/pasireotide) in AI-PCa cells. Altogether, we provide evidence that SST/CORT system and SST-analogues could represent a potential therapeutic option for PCa, especially for CRPC, and that endogenous CORT could act as an autocrine/paracrine regulator of PCa progression.

Pasireotide-a novel somatostatin receptor ligand after 20 years of use

Pasireotide, a novel multireceptor-targeted somatostatin receptor ligand (SRL) is characterized by a higher affinity to somatostatin receptor type 5 than type 2, unlike first-generation SRLs. Because of the broader binding profile, pasireotide has been suggested to have a greater clinical efficacy in acromegaly than first-generation SRLs and to be efficacious in Cushing's disease. The consequence of this binding profile is the increased blood glucose level in some patients. This results from the inhibition of both insulin secretion and the incretin effect and only a modest suppression of glucagon. A monthly intramuscular formulation of long-acting release pasireotide has been approved for both acromegaly and Cushing's disease treatment. This review presents data on the efficacy and safety of pasireotide treatment mostly in patients with acromegaly and Cushing's disease. Moreover, other possible therapeutic applications of pasireotide are mentioned.

Dimerization of GPCRs: Novel insight into the role of FLNA and SSAs regulating SST2 and SST5 homo- and hetero-dimer formation

The process of GPCR dimerization can have profound effects on GPCR activation, signaling, and intracellular trafficking. Somatostatin receptors (SSTs) are class A GPCRs abundantly expressed in pituitary tumors where they represent the main pharmacological targets of somatostatin analogs (SSAs), thanks to their antisecretory and antiproliferative actions. The cytoskeletal protein filamin A (FLNA) directly interacts with both somatostatin receptor type 2 (SST₂) and 5 (SST₅) and regulates their expression and signaling in pituitary tumoral cells. So far, the existence and physiological relevance of SSTs homo- and hetero-dimerization in the pituitary have not been explored. Moreover, whether octreotide or pasireotide may play modulatory effects and whether FLNA may participate to this level of receptor organization have remained elusive. Here, we used a proximity ligation assay (PLA)-based approach for the in situ visualization and quantification of SST₂/SST₅ dimerization in rat GH3 as well as in human melanoma cells either expressing (A7) or lacking (M2) FLNA. First, we observed the formation of endogenous SST₅ homo-dimers in GH3, A7, and M2 cells. Using the PLA approach combined with epitope tagging, we detected homo-dimers of human SST₂ in GH3, A7, and M2 cells transiently co-expressing HA- and SNAP-tagged SST₂. SST₂ and SST₅ can also form endogenous hetero-dimers in these cells. Interestingly, FLNA absence reduced the basal number of hetero-dimers (-36.8 ± 6.3% reduction of PLA events in M2, P < 0.05 vs. A7), and octreotide but not pasireotide promoted hetero-dimerization in both A7 and M2 (+20.0 ± 11.8% and +44.1 ± 16.3% increase of PLA events in A7 and M2, respectively, P < 0.05 vs. basal). Finally, immunofluorescence data showed that SST₂ and SST₅ recruitment at the plasma membrane and internalization are similarly induced by octreotide and pasireotide in GH3 and A7 cells. On the contrary, in M2 cells, octreotide failed to internalize both receptors whereas pasireotide promoted robust receptor internalization at shorter times than in A7 cells. In conclusion, we demonstrated that in GH3 cells SST₂ and SST₅ can form both homo- and hetero-dimers and that FLNA plays a role in the formation of SST₂/SST₅ hetero-dimers. Moreover, we showed that FLNA regulates SST₂ and SST₅ intracellular trafficking induced by octreotide and pasireotide.

Recent advances in neuropeptide-related omics and gene editing: Spotlight on NPY and somatostatin and their roles in growth and food intake of fish

Feeding and growth are two closely related and important physiological processes in living organisms. Studies in mammals have provided us with a series of characterizations of neuropeptides and their receptors as well as their roles in appetite control and growth. The central nervous system, especially the hypothalamus, plays an important role in the regulation of appetite. Based on their role in the regulation of feeding, neuropeptides can be classified as orexigenic peptide and anorexigenic peptide. To date, the regulation mechanism of neuropeptide on feeding and growth has been explored mainly from mammalian models, however, as a lower and diverse vertebrate, little is known in fish regarding the knowledge of regulatory roles of neuropeptides and their receptors. In recent years, the development of omics and gene editing technology has accelerated the speed and depth of research on neuropeptides and their receptors. These powerful techniques and tools allow a more precise and comprehensive perspective to explore the functional mechanisms of neuropeptides. This paper reviews the recent advance of omics and gene editing technologies in neuropeptides and receptors and their progresses in the regulation of feeding and growth of fish. The purpose of this review is to contribute to a comparative understanding of the functional mechanisms of neuropeptides in non-mammalians, especially fish.

Clinical and prognostic implications of an immune-related risk model based on TP53 status in lung adenocarcinoma

TP53 mutation is the most widespread mutation in lung adenocarcinoma (LUAD). Meanwhile, p53 (encoded by TP53) has recently been implicated in immune responses. However, it is still unknown whether TP53 mutation remodels the tumour microenvironment to influence tumour progression and prognosis in LUAD. In this study, we developed a 6‐gene immune‐related risk model (IRM) to predict the survival of patients with LUAD in The Cancer Genome Atlas (TCGA) cohort based on TP53 status, and the predictive ability was confirmed in 2 independent cohorts. TP53 mutation led to a decreased immune response in LUAD. Further analysis revealed that patients in the high‐index group had observably lower relative infiltration of memory B cells and regulatory T cells and significantly higher relative infiltration of neutrophils and resting memory CD4⁺ T cells. Additionally, the IRM index positively correlated with the expression of critical immune checkpoint genes, including PDCD1 (encoding PD‐1) and CD274 (encoding PD‐L1), which was validated in the Nanjing cohort. Furthermore, as an independent prognostic factor, the IRM index was used to establish a nomogram for clinical application. In conclusion, this IRM may serve as a powerful prognostic tool to further optimize LUAD immunotherapy.

Epigenetic and post-transcriptional regulation of somatostatin receptor subtype 5 (SST5 ) in pituitary and pancreatic neuroendocrine tumors

Somatostatin receptor subtype 5 (SST₅) is an emerging biomarker and actionable target in pituitary (PitNETs) and pancreatic (PanNETs) neuroendocrine tumors. Transcriptional and epigenetic regulation of SSTR5 gene expression and mRNA biogenesis is poorly understood. Recently, an overlapping natural antisense transcript, SSTR5‐AS1, potentially regulating SSTR5 expression, was identified. We aimed to elucidate whether epigenetic processes contribute to the regulation of SSTR5 expression in PitNETs (somatotropinomas) and PanNETs. We analyzed the SSTR5/SSTR5‐AS1 human locus in silico to identify CpG islands. SSTR5 and SSTR5‐AS1 expression was assessed by quantitative real‐time PCR (qPCR) in 27 somatotropinomas, 11 normal pituitaries (NPs), and 15 PanNETs/paired adjacent (control) samples. We evaluated methylation grade in four CpG islands in the SSTR5/SSTR5‐AS1 genes. Results revealed that SSTR5 and SSTR5‐AS1 were directly correlated in NP, somatotropinoma, and PanNET samples. Interestingly, selected CpG islands were differentially methylated in somatotropinomas compared with NPs. In PanNETs cell lines, SSTR5‐AS1 silencing downregulated SSTR5 expression, altered aggressiveness features, and influenced pasireotide response. These results provide evidence that SSTR5 expression in PitNETs and PanNETs can be epigenetically regulated by the SSTR5‐AS1 antisense transcript and, indirectly, by DNA methylation, which may thereby impact tumor behavior and treatment response.

TGF-β1/Smad2/3 signaling pathway modulates octreotide antisecretory and antiproliferative effects in pituitary somatotroph tumor cells

Octreotide (OCT) is used to inhibit hormone secretion and growth in somatotroph tumors, although a significant percentage of patients are resistant. It has also been tested in nonfunctioning (NF) tumors but with poor results, with these outcomes having been associated with SSTR2 levels and impaired signaling. We investigated whether OCT inhibitory effects can be improved by TGF-β1 in functioning and nonfunctioning somatotroph tumor cells. OCT effects on hormone secretion and proliferation were analyzed in the presence of TGF-β1 in WT and SSTR2-overexpressing secreting GH3 and silent somatotroph tumor cells. The mechanism underlying these effects was assessed by studying SSTR and TGFβR signaling pathways mediators. In addition, we analyzed the effects of OCT/TGF-β1 treatment on tumor growth and cell proliferation in vivo. The inhibitory effects of OCT on GH- and PRL-secretion and proliferation were improved in the presence of TGF-β1, as well as by SSTR2 overexpression. The OCT/TGF-β1 treatment induced downregulation of pERK1/2 and pAkt, upregulation of pSmad3, and inhibition of cyclin D1. In vivo experiments showed that OCT in the presence of TGF-β1 blocked tumor volume growth, decreased cell proliferation, and increased tumor necrosis. These results indicate that SSTR2 levels and the stimulation of TGF-β1/TGFβR/Smad2/3 pathway are important for strengthening the antiproliferative and antisecretory effects of OCT.

The value of [68Ga]Ga-DOTA-TATE PET/CT in diagnosis and management of suspected pituitary tumors

Background

Gallium 68-tetraazacyclododecane-tetraacetic acid-octreotate ([68Ga]Ga-DOTA-TATE) is a selective somatostatin analogue ligand, which shows increased affinity for somatostatin receptor subtype (SSTR) 2 and has been used routinely for imaging neuroendocrine tumors with PET/CT. We investigated the utility of [68Ga]Ga-DOTA-TATE positron emission tomography/computed tomography (PET/CT) in patients with suspected pituitary pathology. We reviewed imaging for twenty consecutive patients (8 men, 12 women, mean age of 48.2, range 14–78) with suspected pituitary pathology who were referred for [68Ga]Ga-DOTA-TATE PET/CT.

Results

Nine patients presented with recurrent Cushing’s syndrome following surgical resection of pituitary adenomas due to recurrent Cushing’s disease (seven patients) and ectopic ACTH secreting tumor (2 patients). All seven patients with recurrent Cushing’s disease showed positive pituitary [68Ga]Ga-DOTA-TATE uptake while both cases of ectopic hormonal secretion had absented pituitary uptake. In 1 of these 2 patients, [68Ga]Ga-DOTA-TATE was able to localize the source of ectopic ACTH tumor.

Six patients presented de novo with Cushing’s due to ectopic ACTH secretion; [68Ga]Ga-DOTA-TATE PET/CT was able to localize ectopic tumors in six of eight patients (3 lungs, 2 pancreases, 1 mid-gut)

There was high uptake [68Ga]Ga-DOTA-TATE in 3 cases of recurrent central hyperthyroidism (SUVmax 6.6–14.3) and 2 cases of prolactinoma (SUVmax 5.5 and 11.3).

Conclusion

Absent [68Ga]Ga-DOTA-TATE activity in the pituitary fossa is useful in excluding pituitary disease in recurrent Cushing’s. Recurrent pituitary thyrotropinomas and prolactinomas showed moderate to high pituitary activity. In addition, in Cushing’s syndrome, [68Ga]Ga-DOTA-TATE is useful for detection of ectopic sources of ACTH production, especially where anatomic imaging is negative.

Cytokeratin 8/18-negative somatotroph pituitary neuroendocrine tumours (PitNETs, adenomas) show variable morphological features and do not represent a clinicopathologically distinct entity

AIMS

In somatotroph pituitary neuroendocrine tumours (adenomas), a pattern of cytokeratin (CK) 18 expression is used for tumour subclassification, with possible clinical implications. Rare somatotroph tumours do not express CK 18. We aimed to characterise this subset clinically and histologically.

METHODS AND RESULTS

Clinical and pathological data for the study were derived from a previously published data set of a cohort of 110 patients with acromegaly. Data included serum levels of insulin-like growth factor 1 (IGF1), growth hormone (GH), prolactin and thyroid-stimulating hormone (TSH), tumour diameter, tumour invasion defined by Knosp grade and immunohistochemical data concerning the expression of Ki67, p53, E-cadherin, somatostatin receptor (SSTR)1, SSTR2A, SSTR3, SSTR5 and D2 dopamine receptor. Additional immunohistochemical analysis (AE1/3, CK 8/18, vimentin, neurofilament light chain, internexin-α) was performed. CK 18 was negative in 10 of 110 (9.1%) tumours. One of these tumours was immunoreactive with CK 8/18 antibody, while the remainder expressed only internexin-α intermediate filament in patterns similar to CK 18 (perinuclear fibrous bodies). CK-negative tumours showed no significant differences with respect to biochemical, radiological or pathological features. They showed significantly higher expression of SSTR2A compared to the sparsely granulated subtype and significantly lower expression of E-cadherin compared to the non-sparsely granulated subtypes of tumours. The tumours showed divergent morphology and hormonal expression: two corresponded to densely granulated tumours and three showed co-expression of prolactin and morphology of either mammosomatotroph or somatotroph-lactotroph tumours. Four tumours showed morphology and immunoprofile compatible with plurihormonal Pit1-positive tumours.

CONCLUSIONS

CK-negative somatotroph tumours do not represent a distinct subtype of somatotroph tumours, and can be further subdivided according to their morphology and immunoprofile.

Octreotide and Pasireotide Combination Treatment in Somatotroph Tumor Cells: Predominant Role of SST2 in Mediating Ligand Effects

Simple Summary

First-generation somatostatin receptor ligands, such as octreotide, are the first-line medical therapy in acromegaly. Octreotide shows preferential binding for somatostatin receptor subtype 2 (SST₂), while the second-generation ligand, pasireotide, has high affinity for multiple SSTs. We aimed to elucidate whether pasireotide acts via other receptors than SST₂ in somatotroph tumors, and to investigate the potential role of the combination therapy octreotide plus pasireotide. We found that octreotide and pasireotide are superimposable in reducing GH secretion in cultured somatotroph tumor cells, as well as in inhibiting cell proliferation and intracellular pathway activity in rat GH4C1 cells (a model of somatotroph tumors). We did not find any additive/synergistic effect for the combination treatment. Furthermore, we observed that co-incubation with a SST₂-selective antagonist reversed the inhibitory effect of both compounds. Therefore, the two drugs act mainly via SST₂ in somatotroph tumor cells, and their combination is not superior to single agent treatment.

Abstract

First-generation somatostatin receptor ligands (fg-SRLs), such as octreotide (OCT), represent the first-line medical therapy in acromegaly. Fg-SRLs show a preferential binding affinity for somatostatin receptor subtype-2 (SST₂), while the second-generation ligand, pasireotide (PAS), has high affinity for multiple SSTs (SST₅ > SST₂ > SST₃ > SST₁). Whether PAS acts via SST₂ in somatotroph tumors, or through other SSTs (e.g., SST₅), is a matter of debate. In this light, the combined treatment OCT+PAS could result in additive/synergistic effects. We evaluated the efficacy of OCT and PAS (alone and in combination) on growth hormone (GH) secretion in primary cultures from human somatotroph tumors, as well as on cell proliferation, intracellular signaling and receptor trafficking in the rat GH4C1 cell line. The results confirmed the superimposable efficacy of OCT and PAS in reducing GH secretion (primary cultures), cell proliferation, cAMP accumulation and intracellular [Ca²⁺] increase (GH4C1 cells), without any additive effect observed for OCT+PAS. In GH4C1 cells, co-incubation with a SST₂-selective antagonist reversed the inhibitory effect of OCT and PAS on cell proliferation and cAMP accumulation, while both compounds resulted in a robust internalization of SST₂ (but not SST₅). In conclusion, OCT and PAS seem to act mainly through SST₂ in somatotroph tumor cells in vitro, without inducing any additive/synergistic effect when tested in combination.

Intrathecal administration of neuronostatin induces an antinociceptive effect in a mouse visceral pain model

Neuronostatin (NST) is a peptide encoded by the somatostatin gene that serves important physiological functions in diverse tissues. Previous studies have shown that intracerebroventricular administration of NST induces antinociceptive effects and hyperalgesic effects as determined by the tail immersion assay and formalin test, respectively. In the present study, we aimed to evaluate the effects of intrathecal (i.t.) injection of NST on nociception in a model of visceral pain, and determine possible mechanisms of action in mice. NST (1, 3, 6, or 12 nmol) was administered to mice, leading to a dose‐dependent antinociceptive effect as determined by the acetic acid‐induced writhing test in mice. NST (1 nmol) also enhanced the antinociceptive effect of morphine (2.5 and 5 μg/kg) in the spine. Naloxone and β‐funaltrexamine hydrochloride significantly antagonized the antinociceptive effect of NST. The expression of G‐protein‐coupled receptor 107 (GPR107) protein and the phosphorylation of PKA at Thr197 were increased after i.t. administration of NST, suggesting that the μ‐opioid receptor and GPR107/PKA signaling pathway are involved in the analgesic response. In conclusion, i.t. injection of NST may potentially be used as a new approach in the mediation of visceral pain.

Somatostatin Receptor as a Molecular Imaging Target in Human and Canine Cushing Disease

OBJECTIVES

Fluorescence-guided surgery may improve completeness of resection in transsphenoidal surgery for Cushing disease (CD) by enabling visualization of residual tumor tissue at the margins. In this review we discuss somatostatin receptors (SSTRs) as targets for fluorescence-guided surgery and overview existing SSTR-specific imaging agents. We also compare SSTR expression in normal pituitary and corticotrophinoma tissues from human and canine CD patients to assess canines as a translational model for CD.

METHODS

A PubMed literature search was conducted for publications containing the terms canine, somatostatin receptor, Cushing's disease, and corticotroph adenoma. SSTR expression data from each study was documented as the presence or absence of expression or, when possible, the number of tumors expressing a given SSTR subtype within a group of tumors being studied. Studies that used reverse transcription polymerase chain reaction to quantify SSTR expression were selected for additional comparative analysis.

RESULTS

SSTR5 is strongly expressed in human corticotroph adenomas and weakly expressed in surrounding pituitary parenchyma, a pattern not conclusively observed in canine patients. SSTR2 mRNA expression is similar in human normal pituitary and corticotrophinoma cells but may be significantly higher in canine normal pituitary tissue than in corticotroph tumoral tissue. Limited data were available on SSTR subtypes 1, 3, and 4.

CONCLUSIONS

Further studies must fill the knowledge gaps related to species-specific SSTR expression, so using canine CD as a translational model may be premature. We do conclude that the expression profile of SSTR5 (i.e., high local expression in pituitary adenomas relative to normal surrounding tissues) makes SSTR5 a promising molecular target for FGS.

DNA damage and growth hormone hypersecretion in pituitary somatotroph adenomas

Drivers of sporadic benign pituitary adenoma growth are largely unknown. Whole-exome sequencing of 159 prospectively resected pituitary adenomas showed that somatic copy number alteration (SCNA) rather than mutation is a hallmark of hormone-secreting adenomas and that SCNAs correlate with adenoma phenotype. Using single-gene SCNA pathway analysis, we observed that both cAMP and Fanconi anemia DNA damage repair pathways were affected by SCNAs in growth hormone-secreting (GH-secreting) somatotroph adenomas. As somatotroph differentiation and GH secretion are dependent on cAMP activation and we previously showed DNA damage, aneuploidy, and senescence in somatotroph adenomas, we studied links between cAMP signaling and DNA damage. Stimulation of cAMP in C57BL/6 mouse primary pituitary cultures using forskolin or a long-acting GH-releasing hormone (GHRH) analog increased GH production and DNA damage measured by H2AX phosphorylation and a comet assay. Octreotide, a somatostatin receptor ligand that targets somatotroph adenoma GH secretion in patients with acromegaly, inhibited cAMP and GH and reversed DNA damage induction. In vivo long-acting GHRH treatment also induced pituitary DNA damage in mice. We conclude that cAMP, which induces somatotroph proliferation and GH secretion, may concomitantly induce DNA damage, potentially linking hormone hypersecretion to SCNA and genome instability. These results elucidating somatotroph adenoma pathophysiology identify pathways for targeted treatment.

Medication for Acromegaly Reduces Expression of MUC16, MACC1 and GRHL2 in Pituitary Neuroendocrine Tumour Tissue

Acromegaly is a disease mainly caused by pituitary neuroendocrine tumor (PitNET) overproducing growth hormone. First-line medication for this condition is the use of somatostatin analogs (SSAs), that decrease tumor mass and induce antiproliferative effects on PitNET cells. Dopamine agonists (DAs) can also be used if SSA treatment is not effective. This study aimed to determine differences in transcriptome signatures induced by SSA/DA therapy in PitNET tissue. We selected tumor tissue from twelve patients with somatotropinomas, with half of the patients receiving SSA/DA treatment before surgery and the other half treatment naive. Transcriptome sequencing was then carried out to identify differentially expressed genes (DEGs) and their protein–protein interactions, using pathway analyses. We found 34 upregulated and six downregulated DEGs in patients with SSA/DA treatment. Three tumor development promoting factors MUC16, MACC1, and GRHL2, were significantly downregulated in therapy administered PitNET tissue; this finding was supported by functional studies in GH3 cells. Protein–protein interactions and pathway analyses revealed extracellular matrix involvement in the antiproliferative effects of this type of the drug treatment, with pronounced alterations in collagen regulation. Here, we have demonstrated that somatotropinomas can be distinguished based on their transcriptional profiles following SSA/DA therapy, and SSA/DA treatment does indeed cause changes in gene expression. Treatment with SSA/DA significantly downregulated several factors involved in tumorigenesis, including MUC16, MACC1, and GRHL2. Genes that were upregulated, however, did not have a direct influence on antiproliferative function in the PitNET cells. These findings suggested that SSA/DA treatment acted in a tumor suppressive manner and furthermore, collagen related interactions and pathways were enriched, implicating extracellular matrix involvement in this anti-tumor effect of drug treatment.

Mice with an RGS-insensitive Gαi2 protein show growth hormone axis dysfunction

Mice carrying an RGS-insensitive Gαi2 mutation display growth retardation early after birth. Although the growth hormone (GH)-axis is a key endocrine modulator of postnatal growth, its functional state in these mice has not been characterized. The present study was undertaken to address this issue. Results revealed that pituitary mRNA levels for GH, prolactin (PRL), somatostatin (SST), GH-releasing-hormone receptor (GHRH-R) and GH secretagogue receptor (GHS-R) were decreased in mutants compared to controls. These changes were reflected by a significant decrease in plasma levels of GH, IGF-1 and IGF-binding protein-3 (IGFBP-3). Mutants were also less responsive to GHRH and ghrelin (GhL) on GH stimulation of release from pituitary primary cell cultures. In contrast, they were more sensitive to the inhibitory effect of SST. These data provide the first evidence for an alteration of the functional state of the GH-axis in Gαi2G184S mice that likely contributes to their growth retardation.

Predictive and prognostic significance of tumour subtype, SSTR1-5 and e-cadherin expression in a well-defined cohort of patients with acromegaly

In somatotroph pituitary tumours, somatostatin analogue (SSA) therapy outcomes vary throughout the studies. We performed an analysis of cohort of patients with acromegaly from the Czech registry to identify new prognostic and predictive factors. Clinical data of patients were collected, and complex immunohistochemical assessment of tumour samples was performed (SSTR1‐5, dopamine D2 receptor, E‐cadherin, AIP). The study included 110 patients. In 31, SSA treatment outcome was evaluated. Sparsely granulated tumours (SGST) differed from the other subtypes in expression of SSTR2A, SSTR3, SSTR5 and E‐cadherin and occurred more often in young. No other clinical differences were observed. Trouillas grading system showed association with age, tumour size and SSTR2A expression. Factors significantly associated with SSA treatment outcome included age, IGF1 levels, tumour size and expression of E‐cadherin and SSTR2A. In the group of SGST, poor SSA response was observed in younger patients with larger tumours, lower levels of SSTR2A and higher Ki67. We observed no relationship with expression of other proteins including AIP. No predictive value of E‐cadherin was observed when tumour subtype was considered. Multiple additional factors apart from SSTR2A expression can predict treatment outcome in patients with acromegaly.

Epithelial-Mesenchymal Transition in the Resistance to Somatostatin Receptor Ligands in Acromegaly

Epithelial-mesenchymal transition (EMT) is a dynamic process by which epithelial cells loss their phenotype and acquire mesenchymal traits, including increased migratory and invasive capacities. EMT is involved in physiological processes, such as embryogenesis and wound healing, and in pathological processes such as cancer, playing a pivotal role in tumor progression and metastasis. Pituitary tumors, although typically benign, can be locally invasive. Different studies have shown the association of EMT with increased tumor size and invasion in pituitary tumors, and in particular with a poor response to Somatostatin Receptor Ligands (SRLs) treatment in GH-producing pituitary tumors, the main cause of acromegaly. This review will summarize the current knowledge regarding EMT and SRLs resistance in acromegaly and, based on this relation, will suggest new biomarkers and possible therapies to SRLs resistant tumors.

68 Ga DOTANOC PET/CT Scan in Primary Juvenile Nasopharyngeal Angiofibroma - A Pilot Study

OBJECTIVES/HYPOTHESIS

Somatostatin receptors (SSTRs) are highly expressed in neuroendocrine tumors and is exploited for its imaging and treatment. SSTRs expression is also demonstrated in diverse benign and malignant tumor cell types and proliferating peri-tumoral vessels. Similarly, Juvenile Nasopharyngeal Angiofibroma (JNA) expresses different SSTRs and may be utilized for its imaging and treatment using DOTA, 1-Nal3-octreotide (DOTANOC)-PET/CT scan.

STUDY DESIGN

Prospective cohort.

METHODS

Nineteen clinico-radiologically diagnosed primary JNA patients underwent a ⁶⁸ Ga-DOTANOC PET-CT scan. Using a dedicated PET/CT scanner, a low-dose head and neck spot CT scan was performed after 45 to 60 minutes of intravenous injection of 2 to 3 mCi(74-111 MBq) of DOTANOC. The primary objective was to assess the intensity and pattern of DOTANOC uptake in these patients.

RESULTS

DOTANOC expression was noted in all cases (n = 19) of primary JNA (100%). The mean (SD) DOTANOC SUVmax ratio of tumor and background was 6.9+/-1.4(range, 3.8-9.5). Intra-cranial extension in all 13/19 patients was prominently visualized due to the absence of DOTANOC uptake in the brain. Compared to the background all stages of JNA showed significant DOTANOC uptake (P < .0001). No difference in uptake between advanced-stage tumors and early tumors was noted (P = .47). A statistically non-significant negative trend was noted for decreasing uptake with increasing age (Spearman correlation coefficient, r = -0.19).

CONCLUSIONS

This first study of ⁶⁸ Ga-DOTANOC-PET/CT scan in JNA demonstrates consistent and reliable uptake activity in all patients irrespective of age and stage. This opens up possibilities to physiological diagnostic imaging with a promise of greater specificity and sensitivity and may have applications in ambivalent diagnostic situations such as the detection of recurrence.

LEVEL OF EVIDENCE

3 Laryngoscope, 131:1509-1515, 2021.

Clinical Significance of Somatostatin Receptor (SSTR) 2 in Meningioma

Somatostatin receptor (SSTR) 2, widely expressed in meningioma, is a G-protein-coupled receptor and can be activated by somatostatin or its synthetic analogs. SSTR2 is therefore extensively studied as a marker and target for the diagnosis and treatment of meningioma. Accumulating studies have revealed the crucial clinical significance of SSTR2 in meningioma. Summarizing the progress of these studies is urgently needed as it may not only provide novel and better management for patients with meningioma but also indicate the direction of future research. Pertinent literature is reviewed to summarize the recent collective knowledge and understanding of SSTR2’s clinical significance in meningioma in this review. SSTR2 offers novel ideas and approaches in the diagnosis, treatment, and prognostic prediction for meningioma, but more and further studies are required.

Diagnosing Thyrotropin-Secreting Pituitary Adenomas by Short-Term Somatostatin Analogue Test

Background: Diagnosis of thyrotropin (TSH)-secreting pituitary adenomas (TSHoma) before surgery remains a challenge, especially for microadenomas. We aimed to establish a short-term somatostatin analogue (SSA) test to differentiate TSHomas from other causes of syndromes of inappropriate secretion of TSH (IST), mainly resistance to thyroid hormone β (RTHβ). Materials and Methods: We first evaluated the sensitivity and specificity of SSA test in a training cohort (TSHoma, n = 32; RTHβ, n = 20). The test was then validated in an independent cohort (TSHoma, n = 9; RTHβ, n = 2). We finally applied the SSA test in 12 perceptively enrolled IST cases with negative imaging findings and absent thyroid hormone receptor beta (THRB) mutations or mixed hormone imbalances. Results: Both TSHoma and RTHβ patients showed a decrease of TSH at the start of the SSA test, but the velocity of the TSH suppression slowly decreased in RTHβ patients after 2 hours. The suppression ratio of TSH at 24 hours versus 2 and 0 hours was significantly greater in TSHoma patients compared with RTHβ patients (70.58% ± 18.6% vs. 6.01% ± 25.41%, p < 0.0001, 79.83% ± 12.79% vs. 51.16% ± 13.62%, p < 0.0001, respectively). The 24- versus 2-hour suppression ratio showed the best diagnostic accuracy at a cut point of 44.46% in the training cohort, with a sensitivity of 95.00%, a specificity of 93.75%, and a positive predictive value (PPV) of 88.89%. The accuracy was confirmed in the validation cohort. Three out of 12 patients in the prospective cohort showed a TSH suppression ratio greater than 44.46% and all developed microadenomas during follow-up. Conclusions: A short-term SSA test provides an alternative diagnostic approach for TSHomas. A positive SSA test result is suggestive for a TSHoma even before positive findings become apparent on pituitary imaging. However, studies including larger number of patients, especially those with RTHβ, are needed to confirm our findings.

Discovery of substituted 3H-pyrido[2,3-d]pyrimidin-4-ones as potent, biased, and orally bioavailable sst2 agonist

The discovery of a novel 3H-pyrido[2,3-d]pyrimidin-4-one series as potent and biased sst2 agonists is described. This class of molecules exhibits excellent sst2 potency and selectivity against sst1, sst3, and sst5 receptors, and they are significantly more potent at inhibiting cAMP production than inducing internalization. The orally bioavailable 6-(3-chloro-5-methylphenyl)-3-(3-fluoro-5-hydroxyphenyl)-5-({methyl[(2S)-pyrrolidin-2-ylmethyl]amino}methyl)-3H,4H-pyrido[2,3-d]pyrimidin-4-one (36) also suppresses GH secretion in GHRH-challenged rats in a dose-dependent manner.

Management of persistent acromegaly following primary therapy: The current landscape in the UK

Acromegaly is the clinical consequence of chronic exposure of the tissues to excess GH and IGF-I. It is almost exclusively the result of a GH-secreting pituitary adenoma. In addition to the somatic features, uncontrolled acromegaly is associated with a number of complications and excess mortality. Management is aimed at control of the tumour; normalization of GH and IGF-I secretion and relief of symptoms. Initial management of GH-secreting pituitary adenoma is widely accepted as endonasal trans-sphenoidal surgery, with second-line therapy where disease is uncontrolled in most cases being somatostatin analogue therapy. With the combination of surgery and somatostatin analogue therapy, control is achieved in around 75% of patients; however, this leaves a significant proportion of patients requiring multimodality therapy to achieve remission. Within the UK, the health system has finite resources, and decisions for management require consideration of efficacy and cost-effectiveness. To add to the complexity, subtle differences exist in availability of high-cost medications used in the treatment of patients with acromegaly across the devolved nations of the UK. In this article, we discuss options for the management of persistent acromegaly following initial surgery and somatostatin analogue therapy, and explore earlier use of dopaminergics and conservative management.

Abundant expression of the membrane-anchored protease-regulator RECK in the anterior pituitary gland and its implication in the growth hormone/insulin-like growth factor 1 axis in mice

The tumor suppressor gene Reversion-inducing cysteine-rich protein with Kazal motifs (Reck) encodes a membrane-anchored protease regulator expressed in multiple tissues in mouse embryos and is essential for embryonic development. In postnatal mice, however, physiological roles for the RECK protein remain unclear. We found in this study that Reck is abundantly expressed in growth hormone (GH)-producing cells (somatotrophs) in the anterior pituitary gland (AP). We also found that two types of viable Reck mutant mice, one with reduced RECK expression (Hypo mice) and the other with induced Reck deficiency from 10 days after birth (iKO mice treated with tamoxifen), exhibit common phenotypes including decreases in body size and plasma levels of insulin-like growth factor-1 (IGF1). To gain insights into the function of RECK in the AP, we characterized several somatotroph-associated molecules in the AP of these mice. Immunoreactivity of GH was greatly reduced in tamoxifen-treated iKO mice; in these mice, two membrane receptors involved in the stimulation of GH secretion [growth hormone secretagogue receptor (GHSR) and growth hormone releasing hormone receptor (GHRHR)] were decreased, however, their mRNAs were increased. Decrease in GHSR immunoreactivity and concomitant increase in its mRNA were also found in the other mutant line, Hypo. Furthermore, reduced immunoreactivity of growth hormone receptor (GHR) and concomitant increase in its mRNA was also found in the liver of Hypo mice. These results raise the possibility that RECK supports proper functioning of the GH/IGF1 axis in mice, thereby affecting their growth and metabolism.

Thyrotropin-Releasing Hormone (TRH) and Somatostatin (SST), but not Growth Hormone-Releasing Hormone (GHRH) nor Ghrelin (GHRL), Regulate Expression and Release of Immune Growth Hormone (GH) from Chicken Bursal B-Lymphocyte Cultures

It is known that growth hormone (GH) is expressed in immune cells, where it exerts immunomodulatory effects. However, the mechanisms of expression and release of GH in the immune system remain unclear. We analyzed the effect of growth hormone-releasing hormone (GHRH), thyrotropin-releasing hormone (TRH), ghrelin (GHRL), and somatostatin (SST) upon GH mRNA expression, intracellular and released GH, Ser133-phosphorylation of CREB (pCREB^S133), intracellular Ca²⁺ levels, as well as B-cell activating factor (BAFF) mRNA expression in bursal B-lymphocytes (BBLs) cell cultures since several GH secretagogues, as well as their corresponding receptors (-R), are expressed in B-lymphocytes of several species. The expression of TRH/TRH-R, ghrelin/GHS-R1a, and SST/SST-Rs (Subtypes 1 to 5) was observed in BBLs by RT-PCR and immunocytochemistry (ICC), whereas GHRH/GHRH-R were absent in these cells. We found that TRH treatment significantly increased local GH mRNA expression and CREB phosphorylation. Conversely, SST decreased GH mRNA expression. Additionally, when added together, SST prevented TRH-induced GH mRNA expression, but no changes were observed in pCREB^S133 levels. Furthermore, TRH stimulated GH release to the culture media, while SST increased the intracellular content of this hormone. Interestingly, SST inhibited TRH-induced GH release in a dose-dependent manner. The coaddition of TRH and SST decreased the intracellular content of GH. After 10 min. of incubation with either TRH or SST, the intracellular calcium levels significantly decreased, but they were increased at 60 min. However, the combined treatment with both peptides maintained the Ca²⁺ levels reduced up to 60-min. of incubation. On the other hand, BAFF cytokine mRNA expression was significantly increased by TRH administration. Altogether, our results suggest that TRH and SST are implicated in the regulation of GH expression and release in BBL cultures, which also involve changes in pCREB^S133 and intracellular Ca²⁺ concentration. It is likely that TRH, SST, and GH exert autocrine/paracrine immunomodulatory actions and participate in the maturation of chicken BBLs.

A Somatostatin Receptor Subtype-3 (SST3) Peptide Agonist Shows Antitumor Effects in Experimental Models of Nonfunctioning Pituitary Tumors

PURPOSE

Somatostatin analogues (SSA) are efficacious and safe treatments for a variety of neuroendocrine tumors, especially pituitary neuroendocrine tumors (PitNET). Their therapeutic effects are mainly mediated by somatostatin receptors SST₂ and SST₅. Most SSAs, such as octreotide/lanreotide/pasireotide, are either nonselective or activate mainly SST₂. However, nonfunctioning pituitary tumors (NFPTs), the most common PitNET type, mainly express SST₃ and finding peptides that activate this particular somatostatin receptor has been very challenging. Therefore, the main objective of this study was to identify SST₃-agonists and characterize their effects on experimental NFPT models.

EXPERIMENTAL DESIGN

Binding to SSTs and cAMP level determinations were used to screen a peptide library and identify SST₃-agonists. Key functional parameters (cell viability/caspase activity/chromogranin-A secretion/mRNA expression/intracellular signaling pathways) were assessed on NFPT primary cell cultures in response to SST₃-agonists. Tumor growth was assessed in a preclinical PitNET mouse model treated with a SST₃-agonist.

RESULTS

We successfully identified the first SST₃-agonist peptides. SST₃-agonists lowered cell viability and chromogranin-A secretion, increased apoptosis in vitro, and reduced tumor growth in a preclinical PitNET model. As expected, inhibition of cell viability in response to SST₃-agonists defined two NFPT populations: responsive and unresponsive, wherein responsive NFPTs expressed more SST₃ than unresponsive NFPTs and exhibited a profound reduction of MAPK, PI3K-AKT/mTOR, and JAK/STAT signaling pathways upon SST₃-agonist treatments. Concurrently, SSTR3 silencing increased cell viability in a subset of NFPTs.

CONCLUSIONS

This study demonstrates that SST₃-agonists activate signaling mechanisms that reduce NFPT cell viability and inhibit pituitary tumor growth in experimental models that expresses SST₃, suggesting that targeting this receptor could be an efficacious treatment for NFPTs.

A Comparative Update on the Neuroendocrine Regulation of Growth Hormone in Vertebrates

Growth hormone (GH), mainly produced from the pituitary somatotrophs is a key endocrine regulator of somatic growth. GH, a pleiotropic hormone, is also involved in regulating vital processes, including nutrition, reproduction, physical activity, neuroprotection, immunity, and osmotic pressure in vertebrates. The dysregulation of the pituitary GH and hepatic insulin-like growth factors (IGFs) affects many cellular processes associated with growth promotion, including protein synthesis, cell proliferation and metabolism, leading to growth disorders. The metabolic and growth effects of GH have interesting applications in different fields, including the livestock industry and aquaculture. The latest discoveries on new regulators of pituitary GH synthesis and secretion deserve our attention. These novel regulators include the stimulators adropin, klotho, and the fibroblast growth factors, as well as the inhibitors, nucleobindin-encoded peptides (nesfatin-1 and nesfatin-1-like peptide) and irisin. This review aims for a comparative analysis of our current understanding of the endocrine regulation of GH from the pituitary of vertebrates. In addition, we will consider useful pharmacological molecules (i.e. stimulators and inhibitors of the GH signaling pathways) that are important in studying GH and somatotroph biology. The main goal of this review is to provide an overview and update on GH regulators in 2020. While an extensive review of each of the GH regulators and an in-depth analysis of specifics are beyond its scope, we have compiled information on the main endogenous and pharmacological regulators to facilitate an easy access. Overall, this review aims to serve as a resource on GH endocrinology for a beginner to intermediate level knowledge seeker on this topic.

The human brain somatostatin interactome: SST binds selectively to P-type family ATPases

Somatostatin (SST) is a cyclic peptide that is understood to inhibit the release of hormones and neurotransmitters from a variety of cells by binding to one of five canonical G protein-coupled SST receptors (SSTR1 to SSTR5). Recently, SST was also observed to interact with the amyloid beta (Aβ) peptide and affect its aggregation kinetics, raising the possibility that it may bind other brain proteins. Here we report on an SST interactome analysis that made use of human brain extracts as biological source material and incorporated advanced mass spectrometry workflows for the relative quantitation of SST binding proteins. The analysis revealed SST to predominantly bind several members of the P-type family of ATPases. Subsequent validation experiments confirmed an interaction between SST and the sodium-potassium pump (Na+/K+-ATPase) and identified a tryptophan residue within SST as critical for binding. Functional analyses in three different cell lines indicated that SST might negatively modulate the K+ uptake rate of the Na+/K+-ATPase.

Cell specific interaction of pasireotide: review of preclinical studies in somatotroph and corticotroph pituitary cells

BACKGROUND

Pasireotide is a second-generation somatostatin (SRIF) receptor ligand (SRL), approved for medical treatment of acromegaly and Cushing's disease (CD). The molecule is a stable cyclohexapeptide synthetized based on SRIF structure. Differently from first-generation SRLs (e.g. octreotide), preferentially binding somatostatin receptor (SST) subtype 2 (SST₂), pasireotide has high affinity for multiple SSTs (SST₅ > SST₂ > SST₃ > SST₁). Interestingly, early preclinical studies demonstrated that pasireotide shows distinct functional properties compared to SRIF and first-generation SRLs when binding SSTs.

METHODS

We aimed to highlight the differential receptor-targeted action of pasireotide in the treatment of somatotroph and corticotroph adenomas, throughout the critical revision of preclinical studies carried out on acromegaly and CD models.

RESULTS

Different authors demonstrated that the antisecretory effect of pasireotide in somatotroph adenoma cell cultures is comparable to that of the SST₂-preferential agonist octreotide. Some reports even show a direct correlation between SST₂ mRNA expression and GH reduction after pasireotide treatment, thus laying for a predominant role of SST₂ in driving pasireotide efficacy in somatotropinomas in vitro. On the other hand, the inhibitory effect of pasireotide on ACTH secretion in corticotropinoma cells seems to be mainly mediated by SST₅. Indeed, most reports show a higher potency and efficacy of pasireotide compared to SST₂ preferential agonists, while functional studies confirm the pivotal role of SST₅ targeting in corticotroph cells.

CONCLUSIONS

The analysis of preclinical studies carried out in somatotroph and corticoph adenomas points out that pasireotide shows a cell-specific activity, exerting its biological effects via different SSTs in the different adenoma histotypes.

International Union of Basic and Clinical Pharmacology. CV. Somatostatin Receptors: Structure, Function, Ligands, and New Nomenclature

Somatostatin, also known as somatotropin-release inhibitory factor, is a cyclopeptide that exerts potent inhibitory actions on hormone secretion and neuronal excitability. Its physiologic functions are mediated by five G protein-coupled receptors (GPCRs) called somatostatin receptor (SST)1-5. These five receptors share common structural features and signaling mechanisms but differ in their cellular and subcellular localization and mode of regulation. SST₂ and SST₅ receptors have evolved as primary targets for pharmacological treatment of pituitary adenomas and neuroendocrine tumors. In addition, SST₂ is a prototypical GPCR for the development of peptide-based radiopharmaceuticals for diagnostic and therapeutic interventions. This review article summarizes findings published in the last 25 years on the physiology, pharmacology, and clinical applications related to SSTs. We also discuss potential future developments and propose a new nomenclature.

Toward fully exploiting the therapeutic potential of marketed pharmaceuticals: the use of octreotide and chloroquine in oncology

Pleiotropy in biological systems and their targeting allows many pharmaceuticals to be used for multiple therapeutic purposes. Fully exploiting the therapeutic properties of drugs that are already marketed would be highly advantageous. This is especially the case in the field of oncology, where the ineffectiveness of typical anticancer agents is a common issue, while the development of novel anticancer agents is a costly and particularly time-consuming process. Octreotide and chloroquine are two pharmaceuticals that exhibit profound antitumorigenic activities. However, the current therapeutic use of octreotide is restricted primarily to the management of acromegaly and neuroendocrine tumors, both of which are rare medical conditions. Similarly, chloroquine is used mainly for the treatment of malaria, which is designated as a rare disease in Western countries. This limited exploitation contradicts the experimental findings of numerous studies outlining the possible expansion of the use of octreotide to include the treatment of common human malignancies and the repositioning of chloroquine in oncology. Herein, we review the current knowledge on the antitumor function of these two agents stemming from preclinical or clinical experimentation. In addition, we present in silico evidence on octreotide potentially binding to multiple Wnt-pathway components. This will hopefully aid in the design of new efficacious anticancer therapeutic regimens with minimal toxicity, which represents an enormous unmet demand in oncology.

Clinical usefulness of 99mTc-HYNIC-TOC, 99mTc(V)-DMSA, and 99mTc-MIBI SPECT in the evaluation of pituitary adenomas

Background

The aim of this study was to evaluate the behavioral uptake and ability to diagnose pituitary adenoma (PA) using tumor-seeking radiopharmaceuticals, and to provide a semiquantitative analysis of tracer uptake in the pituitary region.

Patients and methods

The study included 33 (13 hormonally active and 20 nonfunctioning) patients with PA and 45 control participants without pituitary involvement. All patients (n=78) underwent single photon emission computed tomography (SPECT) imaging with technetium-99m-labeled hydrazinonicotinyl-tyr³-octreotide (^99mTc-HYNIC-TOC), dimercaptosuccinic acid (^99mTc(V)-DMSA) and hexakis-2-methoxyisobutylisonitrile (^99mTc-MIBI). A semiquantitative analysis of abnormal uptake was carried out by drawing identical regions of interest over the pituitary area and the normal brain on one transverse section that shows the lesion most clearly. The pituitary uptake to normal brain uptake (P/B) ratio was calculated in all cases.

Results

The result of this study confirms that the SPECT semiquantitative method, with all three tracers, showed statistically significant differences between the PA group and the controls. However, ^99mTc-HYNIC-TOC scintigraphy could have the highest diagnostic yield because of the smallest overlap between the P/B ratios between adenoma versus nonadenoma participants (the receiver operating characteristic curve P/B ratio cut-off value was 13.08). In addition, only ^99mTc-MIBI SPECT have the diagnostic potential to detect secreting PAs, with statistically significant differences between groups (P<0.001), with an receiver operating characteristic curve P/B ratio cut-off value of 16.72.

Conclusion

A semiquantitative analysis of increased focal tracer uptake in the sellar area showed that ^99mTc-HYNIC-TOC is a highly sensitive and reliable tumor-seeking agent for detecting PA, whereas ^99mTc-MIBI SPECT is a highly sensitive and specific method in differentiating hormone-secreting pituitary tumor.

Is receptor profiling useful for predicting pituitary therapy?

Medical treatment of pituitary tumours may present important challenges in the presence of resistance to first line therapy. In this setting, the availability of specific markers of responsiveness/resistance could be helpful to provide tailored patients' treatment. Pituitary receptor profiling has emerged as a potentially useful tool for predicting the response to specific pituitary-directed medical therapy, mainly somatostatin analogues and dopamine agonists. However, its utility is not always straightforward. In fact, agonist-receptor coupling to the consequent biological response is complex and sometimes jeopardizes the understanding of the molecular basis of pharmacological resistance. Defective expression of pituitary receptors, genetic alterations, truncated variants, impaired signal transduction or involvement of other proteins, such as cytoskeleton proteins or the Aryl hydrocarbon receptor interacting protein amongst others, have been linked to differential tumour phenotype or treatment responsiveness with conflicting results, keeping the debate on the utility of pituitary receptor profiling open. Why does this occur? How can we overcome the difficulties? Is there a true role for pituitary receptor profiling in the near future? All authors of this debate article agree on the need of prospective studies using standardized methods in order to assess the efficacy of receptor profiling as a reliable clinical predictive factor.

Somatostatin receptor subtype 5 modifies hypothalamic-pituitary-adrenal axis stress function

Pituitary corticotroph somatostatin receptor subtype 5 (SSTR5) signals to inhibit adrenocorticotrophin (ACTH) secretion. As ACTH deficiency results in attenuated adrenal cortisol production and an impaired stress response, we sought to clarify the role of SSTR5 in modifying the hypothalamic/pituitary/adrenal (HPA) axis. We generated Tg HP5 mice overexpressing SSTR5 in pituitary corticotrophs that produce the ACTH precursor proopiomelanocortin (POMC). Basal ACTH and corticosterone were similar in HP5 and WT mice, while HP5 mice showed attenuated ACTH and corticosterone responses to corticotrophin releasing hormone (CRH). HP5 mice exhibited attenuated corticosterone responses upon a restraint stress test and inflammatory stress following LPS injection, as well as increased anxiety-like and depressive-like behavior on open field and forced swim tests. Pituitary corticotroph CRH receptor subtype 1 (CRHR1) mRNA expression and ACTH responses to CRH were also attenuated in HP5 mice. In AtT20 cells stably overexpressing SSTR5, CRHR1 expression and cAMP response to CRH were reduced, whereas both were increased after SSTR5 KO. In elucidating mechanisms for these observations, we show that SSTR5-induced miR-449c suppresses both CRHR1 expression and function. We conclude that corticotroph SSTR5 attenuates HPA axis responses via CRHR1 downregulation, suggesting a role for SSTR5 in the pathogenesis of secondary adrenal insufficiency.

Single-Molecule Microscopy Reveals Dynamic FLNA Interactions Governing SSTR2 Clustering and Internalization

The cytoskeletal protein filamin A (FLNA) has been suggested to play an important role in the responsiveness of GH-secreting pituitary tumors to somatostatin receptor subtype 2 (SSTR2) agonists by regulating SSTR2 expression and signaling. However, the underlying mechanisms are unknown. In this study, we use fast multicolor single-molecule microscopy to image individual SSTR2 and FLNA molecules at the surface of living cells with unprecedented spatiotemporal resolution. We find that SSTR2 and FLNA undergo transient interactions, which occur preferentially along actin fibers and contribute to restraining SSTR2 diffusion. Agonist stimulation increases the localization of SSTR2 along actin fibers and, subsequently, SSTR2 clustering and recruitment to clathrin-coated pits (CCPs). Interfering with FLNA-SSTR2 binding with a dominant-negative FLNA fragment increases SSTR2 mobility, hampers the formation and alignment of SSTR2 clusters along actin fibers, and impairs both SSTR2 recruitment to CCPs and SSTR2 internalization. These findings indicate that dynamic SSTR2-FLNA interactions critically control the nanoscale localization of SSTR2 at the plasma membrane and are required for coupling SSTR2 clustering to internalization. These mechanisms explain the critical role of FLNA in the control of SSTR2 expression and signaling and suggest the possibility of targeting SSTR2-FLNA interactions for the therapy of pharmacologically resistant GH-secreting pituitary tumors.