Ciliary neuropeptidergic signaling dynamically regulates excitatory synapses in postnatal neocortical pyramidal neurons

Primary cilia are compartmentalized sensory organelles present on the majority of neurons in the mammalian brain throughout adulthood. Recent evidence suggests that cilia regulate multiple aspects of neuronal development, including the maintenance of neuronal connectivity. However, whether ciliary signals can dynamically modulate postnatal circuit excitability is unknown. Here we show that acute cell-autonomous knockdown of ciliary signaling rapidly strengthens glutamatergic inputs onto cultured rat neocortical pyramidal neurons and increases spontaneous firing. This increased excitability occurs without changes to passive neuronal properties or intrinsic excitability. Further, the neuropeptide receptor somatostatin receptor 3 (SSTR3) is localized nearly exclusively to excitatory neuron cilia both in vivo and in culture, and pharmacological manipulation of SSTR3 signaling bidirectionally modulates excitatory synaptic inputs onto these neurons. Our results indicate that ciliary neuropeptidergic signaling dynamically modulates excitatory synapses and suggest that defects in this regulation may underlie a subset of behavioral and cognitive disorders associated with ciliopathies.

Neuroprotective effect of the somatostatin receptor 5 agonist L-817,818 on retinal ganglion cells in experimental glaucoma

Somatostatin plays important roles in modulating neuronal functions by activating the five specific G-protein coupled receptors (sst1-sst5). Previous studies have demonstrated that sst5 were expressed in retinal ganglion cells (RGCs) and sst5 agonist attenuated the α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid-induced retinal neurotoxicity. In this study, we investigated effects and underlying mechanisms of the sst5 agonist L-817,818 on RGC injury induced by elevated intraocular pressure (COH) in experimental glaucoma. Our results showed that intraperitoneal administration of L-817,818 significantly reduced RGC loss and decreased the number of terminal deoxynucleotidyl transferase mediated dUTP nick-end labeling (TUNEL)-positive RGCs in COH retinas, suggesting that L-817,818 may attenuate RGC apoptosis. Consistently, in COH retinas with L-817,818 administration, both the down-regulated mRNA and protein levels of anti-apoptotic Bcl-2 and the up-regulated mRNA and protein levels of pro-apoptotic Bax were partially reversed. L-817,818 administration downregulated the expression of apoptosis-related proteins caspase-9 and caspase-3 in COH retinas. In addition, L-817,818 administration reduced the concentrations of reactive oxygen species/reactive nitrogen species and malondialdehyde, and ameliorated the functions of mitochondrial respiratory chain complex (MRCC). Our results imply that administration of the sst5 agonist L-817,818 reduces RGC loss in COH rats through decreasing RGC apoptosis, which is mediated by regulating Bcl-2/Bax balance, reducing oxidative stress and rescuing activities of MRCC. Activation of sst5 may provide neuroprotective roles for RGCs in glaucoma.

Clinical and Radiological Predictors of Biochemical Response to First-Line Treatment With Somatostatin Receptor Ligands in Acromegaly: A Real-Life Perspective

BACKGROUND

First-generation somatostatin receptor ligands (fg-SRLs) represent the first-line medical treatment for acromegaly, recommended in patients with persistent disease after neurosurgery, or when surgical approach is not feasible. Despite the lack of strong recommendations from guidelines and consensus statements, data from national Registries report an increasing use of medical therapy as first-line treatment in acromegaly.

OBJECTIVE

We retrospectively evaluated the potential role of a large number of clinical and radiological parameters in predicting the biochemical response to 6-month treatment with fg-SRLs, in a cohort of naïve acromegaly patients referred to a single tertiary center for pituitary diseases.

METHODS

Univariable and multivariable logistic regression and linear regression analyses were performed. Biochemical response was defined based on IGF-1 levels, represented as both categorical (tight control, control, >50% reduction) and continuous (linear % reduction) variables.

RESULTS

Fifty-one patients (33 females, median age 57 years) were included in the study. At univariable logistic regression analysis, we found that younger age (≤ 40 years; OR 0.04, p=0.045) and higher BMI (OR 0.866, p=0.034) were associated with a lower chance of achieving >50% IGF-1 reduction. On the contrary, higher IGF-1 xULN values at diagnosis (OR 2.304, p=0.007) and a T2-hypointense tumor (OR 18, p=0.017) were associated with a significantly higher likelihood of achieving >50% IGF-1 reduction after SRL therapy. Of note, dichotomized age, IGF1 xULN at diagnosis, and T2-hypointense signal of the tumor were retained as significant predictors by our multivariable logistic regression model. Furthermore, investigating the presence of predictors to the linear % IGF-1 reduction, we found a negative association with younger age (≤ 40 years; β -0.533, p<0.0001), while a positive association was observed with both IGF-1 xULN levels at diagnosis (β 0.330, p=0.018) and the presence of a T2-hypointense pituitary tumor (β 0.466, p=0.019). All these variables were still significant predictors at multivariable analysis.

CONCLUSIONS

Dichotomized age, IGF-1 levels at diagnosis, and tumor T2-weighted signal are reliable predictors of both >50% IGF-1 reduction and linear % IGF-1 reduction after 6 month fg-SRL treatment in naïve acromegaly patients. These parameters should be considered in the light of an individualized treatment for acromegaly patients.

Somatostatin receptor expression and patients' response to targeted medical treatment in pituitary tumors: evidences and controversies

BACKGROUND

Somatostatin receptors (SSTs) are widely co-expressed in pituitary tumors. SST₂ and SST₅ are the most represented SST subtypes. First-generation somatostatin receptor ligands (SRLs) mainly target SST₂, while pasireotide, a multi-receptor ligand, shows high binding affinity for both SST₅ and SST₂. Therefore, SRLs are routinely used as medical treatment for GH-, TSH-, and ACTH-secreting pituitary tumors.

METHODS

Critical revision of literature data correlating SST expression with patients' response to SRLs.

RESULTS

SST₂ expression in somatroph tumors directly correlates with GH and IGF-1 decrease after first-generation SRL treatment. SST₂ immunohistochemistry represents a valuable tool to predict biochemical response to first-generation SRLs in acromegalic patients. Pasireotide seems to exert its biological effects via SST₂ in unselected patients. However, in those subjects resistant to first-generation SRLs, harbouring tumors with negligible SST₂ expression, pasireotide can act throughout SST₅. More than somatotroph tumors, TSH-omas represent the paradigm of tumors showing a satisfactory response to SRLs. This is probably due to the high SST₂ expression observed in nearly 100% of cases, as well as to the balanced amount of SST₅. In corticotroph tumors, pasireotide mainly act via SST₅, although there is a need for translational studies correlating its efficacy with SST expression in this peculiar tumor histotype.

CONCLUSIONS

The assumption "more target receptor, more drug efficacy" is not straightforward for SRLs. The complex pathophysiology of SSTs, and the technical challenges faced to translate research findings into clinical practice, still need our full commitment to make receptor evaluation a worthwhile procedure for individualizing treatment decisions.

Normal Insulin-like Growth Factor 1 During Somatostatin Receptor Ligand Treatment Predicts Surgical Cure in Acromegaly

CONTEXT

Treatment with somatostatin receptor ligands (SRLs) is often given before pituitary surgery to patients with acromegaly.

OBJECTIVE

To study whether the response to treatment with SRLs is predictive of surgical outcome.

DESIGN

Retrospective, observational study.

SETTING

Tertiary care center.

PATIENTS

We investigated 272 patients with acromegaly who had been treated with an SRL between 1990 and 2018.

INTERVENTION

All patients underwent pituitary surgery performed by a skilled neurosurgeon.

MAIN OUTCOME MEASURE

Outcome of pituitary surgery in patients who had normalization of insulin-like growth factor 1 (IGF-1) levels during SRL therapy in comparison with patients who did not normalize IGF-1 levels.

RESULTS

Normalization of IGF-1 levels during SRL treatment occurred in 62 patients (22.8%) and was similar for the 3 different types of SRL (P = .88). Surgical remission occurred in 59.6% of the patients. Patients who normalized IGF-1 levels during SRL treatment had a higher probability of surgical cure than patients without IGF-1 normalization (83.9% vs 52.4%, respectively; P < .001). Multivariate analysis confirmed that lack of cavernous sinus invasion, small maximum tumor diameter, and IGF-1 normalization during SRL therapy were the only factors independently associated with a favorable surgical outcome.

CONCLUSIONS

Our study demonstrates that the normalization of IGF-1 levels during treatment with SRLs is an independent predictive factor of a favorable surgical outcome. The underlying mechanisms remain unclear, but an optimal response to medical therapy may be a characteristic of less aggressive tumors that are more likely to be entirely removed at surgery.

Cost-Effectiveness and Efficacy of a Novel Combination Regimen in Acromegaly: A Prospective, Randomized Trial

CONTEXT

Combination therapy with somatostatin receptor ligand (SRL) plus pegvisomant for patients with acromegaly is recommended after a maximizing dose on monotherapy. Lower-dose combination regimens are not well studied.

OBJECTIVE

To compare cost-effectiveness and efficacy of 3 lower-dose combination regimens in controlled and uncontrolled acromegaly.

DESIGN AND SETTING

Prospective, randomized, open-label, parallel arm study at a tertiary referral pituitary center.

PATIENTS

Adults with acromegaly regardless of response to prior SRL and biochemical control status at baseline, stratified by an SRL dose required for insulin-like growth factor (IGF)-I normalization during any 3-month period within 12 months preceding enrollment.

INTERVENTION

Combination therapy for 24 to 32 weeks on arm A, high-dose SRL (lanreotide 120 mg/octreotide long-acting release [LAR] 30 mg) plus weekly pegvisomant (40-160 mg/week); arm B, low-dose SRL (lanreotide 60 mg/octreotide LAR 10 mg) plus weekly pegvisomant; or arm C, low-dose SRL plus daily pegvisomant (15-60 mg/day).

MAIN OUTCOME MEASURE

Monthly treatment cost in each arm in participants completing ≥ 24 weeks of therapy.

RESULTS

Sixty patients were enrolled and 52 were evaluable. Fifty of 52 (96%) demonstrated IGF-I control regardless of prior SRL responsiveness (arm A, 14/15 [93.3%]; arm B, 22/23 [95.7%]; arm C, 14/14 [100%]). Arm B was least costly (mean, $9837 ± 1375 per month), arm C was most expensive (mean, $22543 ± 11158 per month), and arm A had an intermediate cost (mean, $14261 ± 1645 per month). Approximately 30% of patients required pegvisomant dose uptitration. Rates of adverse events were all < 10%.

CONCLUSIONS

Low-dose SRL plus weekly pegvisomant represents a novel dosing option for achieving cost-effective, optimal biochemical control in patients with uncontrolled acromegaly requiring combination therapy.

Multivariable Prediction Model for Biochemical Response to First-Generation Somatostatin Receptor Ligands in Acromegaly

CONTEXT

First-generation somatostatin receptor ligands (fg-SRLs) represent the mainstay of medical therapy for acromegaly, but they provide biochemical control of disease in only a subset of patients. Various pretreatment biomarkers might affect biochemical response to fg-SRLs.

OBJECTIVE

To identify clinical predictors of the biochemical response to fg-SRLs monotherapy defined as biochemical response (insulin-like growth factor (IGF)-1 ≤ 1.3 × ULN (upper limit of normal)), partial response (>20% relative IGF-1 reduction without normalization), and nonresponse (≤20% relative IGF-1 reduction), and IGF-1 reduction.

DESIGN

Retrospective multicenter study.

SETTING

Eight participating European centers.

METHODS

We performed a meta-analysis of participant data from 2 cohorts (Rotterdam and Liège acromegaly survey, 622 out of 3520 patients). Multivariable regression models were used to identify predictors of biochemical response to fg-SRL monotherapy.

RESULTS

Lower IGF-1 concentration at baseline (odds ratio (OR) = 0.82, 95% confidence interval (CI) 0.72-0.95 IGF-1 ULN, P = .0073) and lower bodyweight (OR = 0.99, 95% CI 0.98-0.99 kg, P = .038) were associated with biochemical response. Higher IGF-1 concentration at baseline (OR = 1.40, (1.19-1.65) IGF-1 ULN, P ≤ .0001), the presence of type 2 diabetes (oral medication OR = 2.48, (1.43-4.29), P = .0013; insulin therapy OR = 2.65, (1.02-6.70), P = .045), and higher bodyweight (OR = 1.02, (1.01-1.04) kg, P = .0023) were associated with achieving partial response. Younger patients at diagnosis are more likely to achieve nonresponse (OR = 0.96, (0.94-0.99) year, P = .0070). Baseline IGF-1 and growth hormone concentration at diagnosis were associated with absolute IGF-1 reduction (β = 0.90, standard error (SE) = 0.02, P ≤ .0001 and β  = 0.002, SE = 0.001, P = .014, respectively).

CONCLUSION

Baseline IGF-1 concentration was the best predictor of biochemical response to fg-SRL, followed by bodyweight, while younger patients were more likely to achieve nonresponse.

Patient reported outcome data from acromegaly patients treated with injectable somatostatin receptor ligands (SRLs) in routine clinical practice

BACKGROUND

Acromegaly patients managed on Somatostatin receptor ligands (SRLs), the most common first-line pharmacotherapy for acromegaly, may still experience acromegaly symptoms such as headache, sweating, fatigue, soft tissue swelling, and joint pain, even those with normal IGF-1. Additionally, treatment with SRLs may cause injection site reactions and other side effects such as gastro-intestinal (GI) symptoms. This study utilized patient-reported outcome measures to examine the burden associated with acromegaly and its treatment for patients receiving a stable dose of long-acting SRLs in routine clinical practice.

METHODS

US acromegaly patients on a stable dose of SRL seen by their treating healthcare provider in the past 12 months completed a one-time online survey including the Acro-TSQ, an acromegaly-specific tool for assessing symptom burden and treatment satisfaction and convenience.

RESULTS

One hundred five patients were enrolled (mean age 49.9 years, 79.1% female). Patients experienced numerous symptoms, including > 80% who experienced joint pain, acro-fog, swelling of soft tissue, and fatigue/weakness. Many symptoms occurred constantly, while some occurred at the end of the injection cycle, even among those with IGF-1 < = 1.0 ULN. Injection site reactions were common. Patients were moderately satisfied with their current treatment; symptoms and side effects often affected daily activities. On average, patients reported > 3 acromegaly provider visits/year.

CONCLUSIONS

Despite receiving a stable dose of SRL and regular visits with an acromegaly healthcare provider, US acromegaly patients in routine clinical practice, and even the subgroup with normal IGF-1, report significant burden of disease and treatment.

Observed discordance between outcomes reported by acromegaly patients and their treating endocrinology medical provider

BACKGROUND

Acromegaly patients, even those with IGF-1 values within the normal range receiving somatostatin receptor ligands (SRLs), often suffer from significant symptoms. It is not known to what extent patients' medical providers are aware of the frequency and severity of acromegaly symptoms or level of treatment satisfaction with SRLs. This study sought to examine the concordance between outcomes reported by acromegaly patients treated with long-acting SRLs and those perceived by their medical provider.

METHODS

US acromegaly patients on a stable dose of SRL and seen by their medical provider in the past year completed an online survey which included the Acro-TSQ. Their medical providers were interviewed about the perception of their patient's symptoms, level of control, and general health, and completed relevant portions of the Acro-TSQ. Concordance between patient and medical provider reported data was examined.

RESULTS

Medical providers reported that their patients experienced acromegaly symptoms on a regular basis, however, there was poor agreement between patients and medical providers on the frequency, severity, and pattern of symptoms, as well as on the severity of injection site reactions and multiple domains of the Acro-TSQ, with patients generally reporting symptoms and injection site reactions more often and with higher severity than medical providers.

CONCLUSIONS

Medical providers were aware that their patients who were receiving a stable dose of SRL regularly experienced acromegaly symptoms. Addressing discordance in patient- and medical provider-reported frequency and severity of acromegaly symptoms and injection site reactions by facilitating better communication may improve care of acromegaly patients.

Effects of Pegvisomant and Pasireotide LAR on Vertebral Fractures in Acromegaly Resistant to First-generation SRLs

PURPOSE

Osteopathy is an emerging complication of acromegaly. In somatostatin receptor ligands (SRL)-resistant patients, pegvisomant (PegV) and pasireotide LAR (Pasi) are used for acromegaly treatment, but their effect on skeletal health is still not defined.

METHODS

In a longitudinal retrospective international study, we evaluated incidence of radiological vertebral fractures (VFs) in 55 patients with acromegaly resistant to first-generation SRL.

RESULTS

At study entry, prevalent VFs occurred in 23 patients (41.8%). Biochemical acromegaly control was reached in 66.7% of patients on PegV and in 66.7% of patients on Pasi. During the follow-up, incident VFs (iVFs) were detected in 16 patients (29.1%). Occurrence of iVFs was associated with prevalent VFs (P = .002), persistence of active acromegaly (P = .01) and higher value of insulin-like growth factor 1 (IGF-1) during follow-up (P = .03). Among patients with active disease at last visit, iVFs occurred less frequently in patients on treatment with Pasi (25%) compared to PegV (77.8% P = .04), independently of the IGF-1 values (P = .90). In patients who reached biochemical control, 22.7% on PegV and 12.5% on Pasi had iVFs (P = .40). Among both treatment groups, the presence of pre-existent VFs was the main determinant for iVFs.

CONCLUSION

Our data show for the first time that patients with biochemically active disease treated with Pasi had lower risk of iVFs versus those treated with PegV. It also confirms that the presence of pre-existent VFs was the main determinant for iVFs. Additional studies on larger populations and with longer follow-up are needed to confirm our data and disclose the mechanisms underlying our findings.

Updates in the Medical Treatment of Pituitary Adenomas

Pituitary adenomas represent approximately 15% of brain tumors; incidence is significantly on the increase due to widespread use of magnetic resonance imaging. Surgery remains the first-line treatment for most tumors overall. The role of dopaminergic agonists (DAs) and somatostatin receptor ligands (SRLs) in the treatment of pituitary adenomas is quite well established for prolactinomas and growth hormone (GH) excess. However, over the last decade new multi-receptor binding SRLs are increasingly used for treatment of acromegaly and Cushing's disease. SRLs/DA chimeric compounds seem to have enhanced potency and efficacy when compared to that of individual SRLs or DA receptor agonists according to preclinical data. However, following negative results, more research is needed to determine if this interesting mechanism will translate into positive clinical effects for acromegaly patients. Furthermore, new agents that block adrenal steroidogenesis have been developed in phase III clinical trials for Cushing's disease and several new compounds working at the pituitary level and/or blocking the glucocorticoid receptor are also in development. Combination therapy of drugs with similar or different mechanisms (possibly synergistic) are also on the increase. A growing awareness regarding all mechanisms involved in both control of pituitary secretion and cellular proliferation might allow for sole medical treatment of pituitary adenomas, especially macroadenomas, rather than surgery and/or radiation therapy, in the future. Moreover, the underlying decision on how to treat patients with pituitary adenomas should be individualized on a case-by-case basis with not only a goal of tumor shrinkage and biochemical control, but also of improving patients' quality of life.

Novel Drug-Like Somatostatin Receptor 4 Agonists are Potential Analgesics for Neuropathic Pain

Somatostatin released from the capsaicin-sensitive sensory nerves mediates analgesic and anti-inflammatory effects via the somatostatin sst4 receptor without endocrine actions. Therefore, sst4 is considered to be a novel target for drug development in pain including chronic neuropathy, which is an emerging unmet medical need. Here, we examined the in silico binding, the sst4-linked G-protein activation on stable receptor expressing cells (1 nM to 10 μM), and the effects of our novel pyrrolo-pyrimidine molecules in mouse inflammatory and neuropathic pain models. All four of the tested compounds (C1-C4) bind to the same binding site of the sst4 receptor with similar interaction energy to high-affinity reference sst4 agonists, and they all induce G-protein activation. C1 is the more efficacious (γ-GTP-binding: 218.2% ± 36.5%) and most potent (EC50: 37 nM) ligand. In vivo testing of the actions of orally administered C1 and C2 (500 µg/kg) showed that only C1 decreased the resiniferatoxin-induced acute neurogenic inflammatory thermal allodynia and mechanical hyperalgesia significantly. Meanwhile, both of them remarkably reduced partial sciatic nerve ligation-induced chronic neuropathic mechanical hyperalgesia after a single oral administration of the 500 µg/kg dose. These orally active novel sst4 agonists exert potent anti-hyperalgesic effect in a chronic neuropathy model, and therefore, they can open promising drug developmental perspectives.

Structural and functional analysis of two novel somatostatin receptors identified from topmouth culter (Erythroculter ilishaeformis)

In the present study, we cloned and characterized two somatostatin (SS) receptors (SSTRs) from topmouth culter (Erythroculter ilishaeformis) designated as EISSTR6 and EISSTR7. Analysis of EISSTR6 and EISSTR7 signature motifs, 3D structures, and homology with the known members of the SSTR family indicated that the novel receptors had high similarity to the SSTRs of other vertebrates. EISSTR6 and EISSTR7 mRNA expression was detected in 17 topmouth culter tissues, and the highest level was observed in the pituitary. Luciferase reporter assay revealed that SS14 significantly inhibited forskolin-stimulated pCRE-luc promoter activity in HEK293 cells transiently expressing EISSTR6 and EISSTR7, indicating that the receptors can be activated by SS14. We also identified phosphorylation sites important for the functional activity of EISSTR6 and EISSTR7 by mutating Ser^{23, 43, 107, 196, 311} and Ser^{7, 29, 61, 222, 225} residues, respectively, to Ala, which significantly reduced the inhibitory effects of SS14 on the CRE promoter mediated by EISSTR6 and EISSTR7. Furthermore, treatment of juvenile topmouth culters with microcystin-LR or 17β-estradiol significantly affected EISSTR6 and EISSTR7 transcription in the brain, liver and spleen, suggesting that these receptors may be involved in the pathogenic mechanisms induced by endocrine disruptors. Our findings should contribute to the understanding of the structure-function relationship and evolution of the SSTR family.

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.

Treatment escape reduces the effectiveness of cabergoline during long-term treatment of acromegaly in monotherapy or in association with first-generation somatostatin receptor ligands

BACKGROUND

Few studies evaluated the use of cabergoline (CAB) for acromegaly treatment in monotherapy or in combination with first-generation somatostatin receptor ligands (SRLs).

AIM

To evaluate the efficacy, predictors of response and safety of CAB treatment in acromegaly both in monotherapy and in combination with SRLs.

METHODS

We retrospectively collected demographic, biochemical, tumour and treatment data. Short-term disease control was defined as random GH level < 1.0 μg/L and normal age-matched IGF-I level after 3-6 months of treatment with the higher dose used. Long-term disease control was defined as maintenance of normal GH and IGF-I levels at the last visit (at least 9 months of treatment).

RESULTS

Eighty-two patients were studied. The median total time of treatment in monotherapy or in combination with SRLs was 14 months (3-124) and 34 months (3-88), respectively. Short-term disease control was observed in 6 (21%) patients in the monotherapy group and in 20 (32%) in the combination group. Treatment escape was observed in 1 patient after 16 months of CAB monotherapy and in 6 (30%) patients with combination therapy (after a median of 38 months), resulting in long-term disease control of 18% and 23%, respectively. Hyperprolactinemia was a predictor of response to monotherapy and pretreatment GH level to combination treatment.

CONCLUSION

We presented the results of the largest single-centre study with CAB in monotherapy and in combination with SRL. The efficacy of CAB in acromegaly seems to be lower than that of other drugs, and treatment escape may occur after a long-term follow-up.

Cellular effects of AP102, a somatostatin analog with balanced affinities for the hSSTR2 and hSSTR5 receptors

BACKGROUND

Somatostatin analogs (SSAs) are first-line medical therapy for the treatment of acromegaly and neuroendocrine tumors that express somatostatin receptors (SSTR). Somatostatin suppresses secretion of a large number of hormones through the stimulation of the five SSTR. However, unbalanced inhibition of secretion as observed with the highly potent SSAs pasireotide causes hyperglycaemia mainly by inhibiting insulin secretion. In contrast, AP102 a new SSAs has neutral effect on blood glucose while suppressing GH secretion. Our objective was to establish the cellular effects of AP102 on SSTR2 and SSTR5 that may explain the differences observed between AP102 and other SSAs.

METHODS

We compared the binding and agonist activity of AP102 with somatostatin-14, octreotide and pasireotide in HEK293 cells transfected with human SSTR2 and SSTR5 receptors. SSAs signal transduction effects (cAMP concentrations) were measured in forskolin-treated cells in the presence of SSAs. Proliferation and apoptotic effects were determined and binding assays were performed using ¹²⁵I- somatostatin-14.

RESULTS

AP102 has comparable affinity and agonist effect to octreotide at SSTR2 (IC50's of 112 pM and 244 pM, respectively; EC50's of 230 pM and 210 pM, respectively) in contrast to pasireotide that exhibits a 12-27 fold higher IC50 (3110 pM) and about 5-fold higher EC50 (1097 pM). At SSTR5, AP102 has much higher affinity and stimulating effect than octreotide (IC50's of 773 pM and 16,737 pM, respectively; EC50's of 8526 pM and 26,800 pM), and an intermediate affinity and agonist effect between octreotide and pasireotide. AP102, octreotide and pasireotide have variable anti-proliferative effects on HEK cells transfected with SSTR2 and SSTR5.

CONCLUSION

AP102 is a new SSA that better reduces signaling at SSTR2 than SSTR5 and prevents cell proliferation at both receptors. The euglycaemic effect of AP102 observed in preclinical studies may be related to this intermediate agonistic potency between pasireotide and octreotide at SSTR2 and SSTR5.

Does combination of "cold" and "hot" somatostatin analogs prolong survival of patients with neuroendocrine neoplasms?

A number of detected neuroendocrine neoplasms (NENs) has been on the increase due to our awareness of the NENs risk and the development of different imaging techniques. Therapy of NENs involves surgery, chemotherapy, "cold" somatostatin analogs (SSA), peptide receptor radionuclide therapy (PRRT) and kinase inhibitors in pancreatic NENs. The aim of this study is to assess the efficacy of SSA in combination with "hot" somatostatin analogs, and the survival rate of our patients with advanced NENs. Seventy nine patients with metastatic NEN and positive somatostatin receptor scintigraphy (SRS) were enrolled in the study. Every patient was supposed to receive a dose of 7.4 GBq/m² PRRT in 4-5 cycles every 4-9 weeks. Response to the therapy was assessed according to Response Evaluation Criteria In Solid Tumors (RECIST). SSA were administered one month after the last cycle of PRRT and have been continued during the whole follow up period. Median observation time was 33 months (IQR 13.6-55.6), median time to progression was 28 months (IQR 12.1-39.2) and median time to event was 28 months (IQR 12.1-39.2). Overall survival for this group of patients was 60 months. PFS was 39 months and EFS was equal to 33 months. In our group of patients not many serious adverse events were observed. PRRT using radiolabelled somatostatin analogs followed by therapy with "cold" somatostatin analogs is a promising treatment option for patients with metastatic or inoperable somatostatin receptor-positive NENs with the possibility of survival prolongation.

T2-weighted MRI signal intensity as a predictor of hormonal and tumoral responses to somatostatin receptor ligands in acromegaly: a perspective

T2-weighted MRI signal intensity of GH-secreting pituitary adenomas is gaining recognition as a marker of disease characteristics and may be a predictor of response to treatment of acromegaly. Adenomas that are T2-hypointense are more common, are smaller and are less likely to invade the cavernous sinus compared to the T2-iso and hyperintense tumors. T2-hypointense tumors are also accompanied by higher IGF1 values at baseline. When presurgical somatostatin receptor ligand (SRL) therapy is administered, T2-hypointense adenomas have better hormonal responses and have greater tumor shrinkage. Adjuvant SRL therapy of patients with T2-hypointense tumors that are uncured by surgery is also associated with a better hormonal response. We review the studies that have dealt with the T2-weighted signal intensity of GH-secreting pituitary tumors and elaborate on the details and nuances of this promising avenue of research.

Somatostatin receptor ligands in acromegaly: clinical response and factors predicting resistance

INTRODUCTION

Somatostatin (SST) receptor ligands (SRL), in particular those of first generation (Octreotide and Lanreotide), are widely used in medical treatment of acromegaly, but they assure biochemical control of disease (and the possibility of an improvement of clinical symptoms and tumor shrinkage), only in a subset of patients.

DISCUSSION

The mechanisms underlying the so called "SRL resistance" are various and involve in particular SST receptor expression and molecular pathways of signal transduction. Different predictors of SRL response have been reported, including clinical and biochemical features (gender, age, growth hormone and insulin-like growth factor-I levels at diagnosis), and tumor characteristic (both at preoperative magnetic resonance imaging study and histopathology) as well as expression of SST receptors. In some cases, only a "partial resistance" to SST can be detected, probably due to the presence of other impaired molecular mechanisms involved in signal transduction, which compromise specific pathways and not others. This may explain some cases of dissociated response between biochemical control and tumor shrinkage.

Cabergoline in acromegaly

Acromegaly, a rare disease due to growth hormone (GH) hypersecretion by a pituitary adenoma, is associated with severe comorbidity and premature death if not adequately treated. The usual first-line treatment is surgery. Various drugs, including somatostatin receptor ligands, dopamine agonists and GH receptor antagonists, are now available for use if surgery fails to suppress GH/IGF-I hypersecretion. Cabergoline, now the preferred dopamine agonist for treating hyperprolactinemia, is also used off-label for treating acromegaly. Cabergoline monotherapy is reported to normalize IGF-I levels in more than one-third of patients with acromegaly. When a somatostatin receptor ligand proves ineffective, cabergoline add-on therapy normalizes the IGF-I level in 40-50% of patients. Finally, when combined with the GH receptor antagonist pegvisomant in patients with mild uncontrolled disease, cabergoline helps to achieve normal IGF-I levels while avoiding the need for high-dose pegvisomant. Cabergoline is also inexpensive and well tolerated; in particular, it does not appear to promote heart valve disease.

New medical therapies on the horizon: oral octreotide

Somatostatin analog treatment is first line medical treatment in patients with acromegaly. This drug is currently mainly administered by monthly depot preparations of octreotide and lanreotide. With the innovative transient permeability enhancer, a technology enabling the absorption of drug molecules via transient opening of the tight junctions of the gut epithelium, it is possible to achieve therapeutic octreotide levels after oral ingestion. The present review summarized the preclinical work and the recently reported phase I and III study on oral octreotide capsules in patients with acromegaly. Maintenance of control in 155 participating patients was achieved in 65% at the end of core period. Once controlled on oral octreotide, the response was maintained to the end of the extension phase in 85%. Side effects were comparable to currently available preparations. There was a profound suppression of growth hormone levels, and significant symptom reduction. Currently available parental somatostatin analogs are generally well tolerated and are able to achieve longstanding biochemical control in patients with somatostatin sensitive tumors. Potential advantages of an oral alternative is the lack injection-related side effects, but there will be an ongoing need for a very strict compliance with the 2 daily dose regimen and fasting around drug administrations. A second phase III study is currently being conducted. The potential place in the treatment of acromegaly is discussed.

The role of combination medical therapy in the treatment of acromegaly

PURPOSE

Uncontrolled acromegaly results in approximately 2-fold excess mortality. Pituitary surgery is first-line therapy, and medical treatment is indicated for persistent disease. While cabergoline and pegvisomant are used in select patients, somatostatin receptor ligands (SRLs) remain the cornerstone of medical treatment. Management of patients poorly responsive to SRLs is therefore, challenging. The purpose of this review is to highlight the options for combination medical therapy in the treatment of acromegaly, with an emphasis on efficacy and safety.

METHODS

All original articles/abstracts detailing combination medical therapy in acromegaly were identified from a PubMed search.

RESULTS

Studies reviewed included retrospective and open-label prospective studies. While the combination of SRL and cabergoline was generally well tolerated, a lower baseline insulin-like growth factor-1 (IGF-1) level was the best predictor of efficacy; this combination may be most effective in patients with mildly elevated IGF-1. SRL-pegvisomant combination normalized IGF-1 in the majority of patients; continued efficacy despite individual drug dosing reduction was also reported. The risk of significant liver enzyme elevation was, however, higher than that reported with SRL monotherapy; close monitoring is recommended. Data on pegvisomant-cabergoline combination is limited, but this may be an option in the setting of SRL intolerance. Reports on temozolomide used in combination with other medical therapies in patients with aggressive GH-secreting tumors are also summarized.

CONCLUSION

While more prospective, randomized controlled trials on long-term efficacy and safety are needed, combination medical therapy remains a treatment strategy that should be considered for acromegaly patients poorly responsive to SRLs.

Somatostatin receptor ligands in the treatment of acromegaly

First-generation somatostatin receptors ligands (SRL) are the mainstay in the medical treatment of acromegaly, however the percentage of patients controlled with these drugs significantly varies in the different studies. Many factors are involved in the resistance to SRL. In this review, we update the physiology of somatostatin and its receptors (sst), the use of SRL in the treatment of acromegaly and the factors involved in the response to these drugs. The SRL act through interaction with the sst, which up to now have been characterized as five subtypes. The first-generation SRL, octreotide and lanreotide, are considered sst2 specific and have biochemical response rates varying from 20 to 70%. Tumor volume reduction can be found in 36-75% of patients. Several factors may determine the response to these drugs, such as sst, AIP, E-cadherin, ZAC1, filamin A and β-arrestin expression in the somatotropinomas. In patients resistant to first-generation SRL, alternative medical treatment options include: SRL high dose regimens, SRL in combination with cabergoline or pegvisomant, or the use of pasireotide. Pasireotide is a next-generation SRL with a broader pattern of interaction with sst. In the light of the recent increase of treatment options in acromegaly and the deeper knowledge of the determinants of response to the current first-line therapy, a shift from a trial-and-error treatment to a personalized one could be possible.

Antisecretory Effects of Chimeric Somatostatin/Dopamine Receptor Ligands on Gastroenteropancreatic Neuroendocrine Tumors

OBJECTIVES

The recent finding that gastroenteropancreatic neuroendocrine tumors expressed the dopaminergic D2 receptor in addition to somatostatin (sst) receptors suggested that multiple targeting approaches might decrease hormone hypersecretion more effectively than sst agonists alone.

METHODS

To test this hypothesis, (i) we measured the expression of sst receptor type 2 (sst2 receptor) and D2 receptor in 11 gastroenteropancreatic neuroendocrine tumors and (ii) we compared the ability of lanreotide, cabergoline, their combination, and sst/D2 chimeric ligands to decrease chromogranin A (CgA), gastrin, or serotonin release in primary cultures derived from these tumors.

RESULTS

Moderate to high positivity was observed for sst2 receptor and D2 receptor, the latter being more expressed in pancreatic tumors. Lanreotide decreased CgA secretion in all cultures, but only 3 tumors responded to cabergoline. No additivity was observed in lanreotide. BIM 23A781 decreased CgA release to the same extent as lanreotide, whereas the other chimeric ligands were less efficient. However, BIM 23A781 was 50 times less potent than lanreotide. Similar patterns were found for gastrin or serotonin.

CONCLUSION

No improvement was brought by the sst/D2 combination or chimeric ligands. Factors that underlie these tissue-specific differences remain to be elucidated.

Octreotide and pasireotide (dis)similarly inhibit pituitary tumor cells in vitro

Somatostatin analogs (SSA) are the mainstay of pharmacological treatment for pituitary adenomas. However, some patients escape from therapy with octreotide, a somatostatin receptor 2 (sst2)-preferring SSA, and pasireotide, a novel multi-sst-preferring SSA, may help to overcome this problem. It has been proposed that correspondence between sst1-sst5 expression pattern and SSA-binding profile could predict patient's response. To explore the cellular/molecular features associated with octreotide/pasireotide response, we performed a parallel comparison of their in vitro effects, evaluating sst1-sst5 expression, intracellular Ca²⁺ signaling ([Ca²⁺]_i), hormone secretion and cell viability, in a series of 85 pituitary samples. Somatotropinomas expressed sst5>sst2, yet octreotide reduced [Ca²⁺]_i more efficiently than pasireotide, while both SSA similarly decreased growth hormone release/expression and viability. Corticotropinomas predominantly expressed sst5, but displayed limited response to pasireotide, while octreotide reduced functional endpoints. Non-functioning adenomas preferentially expressed sst3 but, surprisingly, both SSA increased cell viability. Prolactinomas mainly expressed sst1 but were virtually unresponsive to SSA. Finally, both SSA decreased [Ca²⁺]_i in normal pituitaries. In conclusion, both SSA act in vitro on pituitary adenomas exerting both similar and distinct effects; however, no evident correspondence was found with the sst1-sst5 profile. Thus, it seems plausible that additional factors, besides the simple abundance of a given sst, critically influence the SSA response.

Somatostatin Agonist Pasireotide Promotes a Physiological State Resembling Short-Day Acclimation in the Photoperiodic Male Siberian Hamster (Phodopus sungorus)

The timing of growth in seasonal mammals is inextricably linked to food availability. This is exemplified in the Siberian hamster (Phodopus sungorus), which uses the annual cycle of photoperiod to optimally programme energy expenditure in anticipation of seasonal fluctuations in food resources. During the autumn, energy expenditure is progressively minimised by physiological adaptations, including a 30% reduction in body mass, comprising a reduction in both fat and lean tissues. However, the mechanistic basis of this adaptation is still unexplained. We hypothesised that growth hormone (GH) was a likely candidate to underpin these reversible changes in body mass. Administration of pasireotide, a long-acting somatostatin receptor agonist developed for the treatment of acromegaly, to male hamsters under a long-day (LD) photoperiod produced a body weight loss. This comprised a reduction in lean and fat mass, including kidneys, testes and brown adipose tissue, typically found in short-day (SD) housed hamsters. Furthermore, when administered to hamsters switched from SD to LD, pasireotide retarded the body weight increase compared to vehicle-treated hamsters. Pasireotide did not alter photoperiod-mediated changes in hypothalamic energy balance gene expression but altered the expression of Srif mRNA expression in the periventricular nucleus and Ghrh mRNA expression in the arcuate nucleus consistent with a reduction in GH feedback and concurrent with reduced serum insulin-like growth factor-1. Conversely, GH treatment of SD hamsters increased body mass, which included increased mass of liver and kidneys. Together, these data indicate a role for the GH axis in the determination of seasonal body mass of the Siberian hamster.

Pasireotide in Acromegaly: An Overview of Current Mechanistic and Clinical Data

BACKGROUND

Acromegaly is an insidious neuroendocrine disorder caused by hypersecretion of growth hormone (GH) by a somatotroph adenoma. Somatostatin receptor ligands (SRLs) are recommended as first-line medical therapy in patients for whom surgery has failed or is contraindicated. There are 5 known somatostatin receptor subtypes (SSTRs), 2 of which, i.e. SSTR2 and SSTR5, are expressed by a majority of somatotroph adenomas. The currently available SRLs, i.e. octreotide and lanreotide, primarily bind to SSTR2. Pasireotide (SOM230) is a new multireceptor-targeted SRL which has a broader binding profile and an increased affinity for SSTR1, 2, 3, and 5.

METHODS

PubMed searches were performed to identify all of the available published English language data on pasireotide with regard to the mechanism of action, in vitro effects, and clinical data.

RESULTS

Preclinical studies have demonstrated that pasireotide has a broader range of functional activity than octreotide. Recently, the efficacy of pasireotide in attenuating GH and insulin-like growth factor 1 (IGF-1) levels in patients with acromegaly has been evaluated in phase III clinical trials. Pasireotide demonstrated superiority over octreotide in achieving biochemical control (i.e. GH ≤2.5 µg/l and age- and sex-matched IGF-1 normalization) in patients with acromegaly, as well as significant efficacy in treating patients who were previously inadequately controlled on the maximum allowed doses of octreotide and lanreotide. Pasireotide-induced hyperglycemia was the most concerning adverse event but was reversible upon discontinuation of pasireotide.

CONCLUSION

The clinical data support pasireotide as a promising new therapy for the treatment of acromegaly, and the long-acting formulation was recently approved in the US and Europe for the treatment of acromegaly.

Cell-based assays and animal models for GPCR drug screening

The family of G protein-coupled receptors (GPCRs) remains a central focus of basic pharmacology and drug discovery efforts. Convenient methods to assess the efficacy of potentially therapeutic reagents for GPCRs are strongly required for high-throughput screening (HTS) assay. We recently developed a rapid, sensitive, and quantitative method for detecting potential chemicals that act on GPCRs using split luciferase complementation. In principle, this is based on the detection of interactions of GPCR with β-arrestin, which translocates to the activated GPCRs. This method can facilitate the construction of HTS systems in a multi-well plate format. Particularly, the method is compatible with single-cell imaging and animal models and even deeper tissues such as organs, because of its high sensitivity, suggesting that promising candidates from HTS assay can be moved easily to the next phase for additional analysis. This system can contribute to the effective evaluation of potentially therapeutic reagents and expedite the development of new drugs for GPCRs.

Selective agonists of somatostatin receptor subtype 1 or 2 injected peripherally induce antihyperalgesic effect in two models of visceral hypersensitivity in mice

Somatostatin interacts with five G-protein-coupled receptor (sst1-5). Octreotide, a stable sst2≫3≥5 agonist, exerts a visceral anti-hyperalgesic effect in experimental and clinical studies. Little is known on the receptor subtypes involved. We investigated the influence of the stable sst1-5 agonist, ODT8-SST and selective receptor subtype peptide agonists (3 or 10μg/mouse) injected intraperitoneally (ip) on visceral hypersensitivity in mice induced by repeated noxious colorectal distensions (four sets of three CRD, each at 55mmHg) or corticotropin-releasing factor receptor 1 agonist, cortagine given between two sets of graded CRD (15, 30, 45, and 60mmHg, three times each pressure). The mean visceromotor response (VMR) was assessed using a non-invasive manometry method and values were expressed as percentage of the VMR to the 1st set of CRD baseline or to the 60mmHg CRD, respectively. ODT8-SST (10μg) and the sst2 agonist, S-346-011 (3 and 10μg) prevented mechanically induced visceral hypersensitivity in the three sets of CRD, the sst1 agonist (10μg) blocked only the 2nd set and showed a trend at 3μg while the sst4 agonist had no effect. The selective sst2 antagonist, S-406-028 blocked the sst2 agonist but not the sst1 agonist effect. The sst1 agonist (3 and 10μg) prevented cortagine-induced hypersensitivity to CRD at each pressure while the sst2 agonist at 10μg reduced it. These data indicate that in addition to sst2, the sst1 agonist may provide a novel promising target to alleviate visceral hypersensitivity induced by mechanoreceptor sensitization and more prominently, stress-related visceral nociceptive sensitization.

The somatostatin receptor 4 agonist J-2156 reduces mechanosensitivity of peripheral nerve afferents and spinal neurons in an inflammatory pain model

Somatostatin (SST) is a peptide hormone that regulates the endocrine system and affects neurotransmission via interaction with G protein-coupled SST receptors and inhibition of the release of different hormones. The aim of this study was to investigate whether the analgesic properties of the selective SSTR4 agonist J-2156 are mediated via peripheral and/or spinal receptors. Effect on mechanical hyperalgesia in the Complete Freund׳s Adjuvant (CFA) model was measured after intraperitoneal application of J-2156. Electrophysiological neuronal recordings were conducted 24 h after injection of CFA or vehicle into the paw of Wistar rats. Mechanosensitivity of peripheral afferents of the saphenous nerve as well as of spinal wide dynamic range (WDR) and nociceptive-specific (NS) neurons were measured after systemic or spinal application of J-2156. In CFA animals J-2156 dose dependently reduced hyperalgesia in behavioral studies. The minimal effective dose was 0.1 mg/kg. Mechanosensitivity of peripheral afferents and spinal neurons was significantly reduced by J-2156. NS neurons were dose dependently inhibited by J-2156 while in WDR neurons only the highest concentration of 100 µM had an effect. In sham controls, J-2156 had no effect on neuronal activity. We demonstrated that J-2156 dose-dependently reduces peripheral and spinal neuronal excitability in the CFA rat model without affecting physiological pain transmission. Given the high concentration of the compound required to inhibit spinal neurons, it is unlikely that the behavioral effect seen in CFA model is mediated centrally. Overall these data demonstrated that the analgesic effect of J-2156 is mediated mainly via peripheral SST4 receptors.

Treatment of Cushing's disease: a mechanistic update

Cushing's disease (CD) is characterized by an ACTH-producing anterior corticotrope pituitary adenoma. If hypothalamus-pituitary-adrenal (HPA) axis physiology is disrupted, ACTH secretion increases, which in turn stimulates adrenocortical steroidogenesis and cortisol production. Medical treatment plays an important role for patients with persistent disease after surgery, for those in whom surgery is not feasible, or while awaiting effects of radiation. Multiple drugs, with different mechanisms of action and variable efficacy and tolerability for controlling the deleterious effects of chronic glucocorticoid excess, are available. The molecular basis and clinical data for centrally acting drugs, adrenal steroidogenesis inhibitors, and glucocorticoid receptor antagonists are reviewed, as are potential novel molecules and future possible targets for CD treatment. Although progress has been made in the understanding of specific corticotrope adenoma receptor physiology and recent clinical studies have detected improved effects with a combined medical therapy approach, there is a clear need for a more efficacious and better-tolerated medical therapy for patients with CD. A better understanding of the molecular mechanisms in CD and of HPA axis physiology should advance the development of new drugs in the future.

Comparison of somatostatin receptor agonist and antagonist for peptide receptor radionuclide therapy: a pilot study

Preclinical and clinical studies have indicated that somatostatin receptor (sst)-expressing tumors demonstrate higher uptake of radiolabeled sst antagonists than of sst agonists. In 4 consecutive patients with advanced neuroendocrine tumors, we evaluated whether treatment with (177)Lu-labeled sst antagonists is feasible.

METHODS

After injection of approximately 1 GBq of (177)Lu-DOTA-[Cpa-c(DCys-Aph(Hor)-DAph(Cbm)-Lys-Thr-Cys)-DTyr-NH2] ((177)Lu-DOTA-JR11) and (177)Lu-DOTATATE, 3-dimensional voxel dosimetry analysis based on SPECT/CT was performed. A higher tumor-to-organ dose ratio for (177)Lu-DOTA-JR11 than for (177)Lu-DOTATATE was the prerequisite for treatment with (177)Lu-DOTA-JR11.

RESULTS

Reversible minor adverse effects of (177)Lu-DOTA-JR11 were observed. (177)Lu-DOTA-JR11 showed a 1.7-10.6 times higher tumor dose than (177)Lu-DOTATATE. At the same time, the tumor-to-kidney and tumor-to-bone marrow dose ratio was 1.1-7.2 times higher. All 4 patients were treated with (177)Lu-DOTA-JR11, resulting in partial remission in 2 patients, stable disease in 1 patient, and mixed response in the other patient.

CONCLUSION

Treatment of neuroendocrine tumors with radiolabeled sst antagonists is clinically feasible and may have a significant impact on peptide receptor radionuclide therapy.

Filamin A (FLNA) plays an essential role in somatostatin receptor 2 (SST2) signaling and stabilization after agonist stimulation in human and rat somatotroph tumor cells

Somatostatin receptor type 2 (SST2) is the main pharmacological target of medical therapy for GH-secreting pituitary tumors, but molecular mechanisms regulating its expression and signaling are largely unknown. The aim of this study was to investigate the role of cytoskeleton protein filamin A (FLNA) in SST2 expression and signaling in somatotroph tumor cells. We found a highly variable expression of FLNA in human GH-secreting tumors, without a correlation with SST2 levels. FLNA silencing in human tumoral cells did not affect SST2 expression and localization but abolished the SST2-induced reduction of cyclin D1 (-37% ± 15% in control cells, P < .05 vs basal) and caspase-3/7 activation (+63% ± 31% in control cells, P < .05 vs basal). Overexpression of a FLNA dominant-negative mutant that specifically prevents SST2-FLNA binding reduced SST2 expression after prolonged agonist exposure (-55% ± 5%, P < .01 vs untreated cells) in GH3 cells. Moreover, SST2-induced apoptotic effect (77% ± 54% increase of caspase activity, P < .05 vs basal) and SST2-mediated ERK1/2 inhibition (48% ± 17% reduction of ERK1/2 phosphorylation, P < .01 vs basal) were abrogated in cells overexpressing another FLNA mutant that prevents FLNA interaction with partner proteins but not with SST2, suggesting a scaffold function of FLNA in somatotrophs. In conclusion, these data demonstrate that FLNA is involved in SST2 stabilization and signaling in tumoral somatotrophs, playing both a structural and functional role.

Orexin-1 receptor mediates the increased food and water intake induced by intracerebroventricular injection of the stable somatostatin pan-agonist, ODT8-SST in rats

Intracerebroventricular (icv) injection of the stable somatostatin pan-agonist, ODT8-SST induces a somatostatin 2 receptor (sst2) mediated robust feeding response that involves neuropeptide Y and opioid systems in rats. We investigated whether the orexigenic system driven by orexin also plays a role. Food and water intake after icv injection was measured concomitantly in non-fasted and non-water deprived rats during the light phase. In vehicle treated rats (100% DMSO, icv), ODT8-SST (1μg/rat, icv) significantly increased the 2-h food and water intake compared to icv vehicle plus saline (5.1±1.0g vs. 1.2±0.4g and 11.3±1.9mL vs. 2.5±1.2mL, respectively). The orexin-1 receptor antagonist, SB-334867 (16μg/rat, icv) completely inhibited the 2-h food and water intake induced by icv ODT8-SST. In contrast, the icv pretreatment with the selective somatostatin sst2 antagonist, S-406-028, established to block the orexigenic effect of icv ODT8-SST, did not modify the increased food and water intake induced by icv orexin-A (10.7μg/rat). These data indicate that orexin-1 receptor signaling system is part of the brain neurocircuitry contributing to the orexigenic and dipsogenic responses induced by icv ODT8-SST and that orexin-A stimulates food intake independently from brain sst2 activation.

Fine-tuning somatostatin receptor signalling by agonist-selective phosphorylation and dephosphorylation: IUPHAR Review 5

The biological actions of somatostatin are mediated by a family of five GPCRs, named sst1 to sst5 . Somatostatin receptors exhibit equally high-binding affinities to their natural ligand somatostatin-14 and largely overlapping distributions. The overexpression of somatostatin receptors in human tumours is the molecular basis for diagnostic and therapeutic application of the stable somatostatin analogues octreotide, lanreotide and pasireotide. The efficiency of somatostatin receptor signalling is tightly regulated and ultimately limited by the coordinated phosphorylation and dephosphorylation of intracellular carboxyl-terminal serine and threonine residues. Here, we review and discuss recent progress in the generation and application of phosphosite-specific antibodies for human sst2 and sst5 receptors. These phosphosite-specific antibodies are unique tools to monitor the spatial and temporal dynamics of receptors phosphorylation and dephosphorylation. Using a combined approach of phosphosite-specific antibodies and siRNA knock-down screening, relevant kinases and phosphatases were identified. Emerging evidence suggests distinct mechanisms of agonist-selective fine-tuning for individual somatostatin receptors. The recently uncovered differences in phosphorylation and dephosphorylation of these receptors may hence be of physiological significance in mediating responses to acute, persistent or repeated stimuli in a variety of target tissues.

Growth hormone values after an oral glucose load do not add clinically useful information in patients with acromegaly on long-term somatostatin receptor ligand treatment

The optimal method of assessing GH status in acromegalic patients receiving medical therapy with somatostatin analogs (SSA) has been matter of debate. The aim of the study has been to investigate whether OGTT may add information in patients with discordant random GH (GHr) and IGF values. Moreover, we evaluated the association of GH nadir with the prevalence of co-morbidities observed in acromegalic patients on SSA therapy. We evaluated 130 patients with proven diagnosis of acromegaly on SSA. The patients were subdivided in three groups: patients with controlled disease (both safe random GH and normal IGF-I, group A, 20.0 %), patients with uncontrolled disease (both high random GH and IGF-I, group B, 34.6 %), and patients with discordant random GH and IGF-I values (group C, 35.4 %). A high concordance rate for GH nadir with random GH and IGF-I was observed in group B, while a significant reduced concordance rate has been observed in group A (100 % sensitivity, 64.5 % specificity). By contrast, in group C, we observed concordant results between GH nadir and IGF-I only in 14/59 patients. In group A, the prevalence of diabetes was lower than in group B or C. Safe random GH was the only single criteria associated with a lower prevalence of diabetes. Discrepant IGF-I and either GH nadir or random GH values are frequently observed in acromegalic patients treated with SSA. Concordant IGF-I and random GH may influence the prevalence of metabolic complications. GH nadir measurement may help to interpret discrepancies between random GH and IGF-I data only in few cases.

Preoperative normalization of cortisol levels in Cushing's disease after medical treatment: consequences for somatostatin and dopamine receptor subtype expression and in vitro response to somatostatin analogs and dopamine agonists

CONTEXT

Corticotroph pituitary adenomas often highly express the dopamine 2 receptor (D₂R) and somatostatin receptor subtype 5 (sst₅). The sst₂ expression is relatively low, likely resulting from downregulating effects of high cortisol levels. This may explain why the sst₂-preferring somatostatin analog octreotide, compared with the multi-receptor-targeting somatostatin analog pasireotide, is generally ineffective in Cushing's disease.

OBJECTIVE

Our objective was to compare sst and D₂R expression levels between adenomas from patients with elevated and normalized preoperative urinary free cortisol excretion.

PATIENTS AND DESIGN

Corticotroph adenoma tissue was examined from patients from group 1 (n = 22; elevated preoperative urinary free cortisol) and group 2 (n = 11; mean duration of preoperative normocortisolism 10 weeks). Somatotroph adenoma tissue from 10 acromegalic patients was examined to compare receptor expression profiles.

MAIN OUTCOME MEASURES

We evaluated receptor mRNA and protein expression levels and effects of octreotide, pasireotide, and cabergoline on ACTH secretion by cultured human corticotroph adenoma cells.

RESULTS

The sst₂ mRNA expression in group 2 was 10-fold higher than in group 1 (P < .01), even comparable to that in somatotroph adenomas. There were no statistically significant differences in sst₅ and D₂R mRNA expression or in sst₂, sst₅, and D₂R protein expression between both groups of corticotroph adenomas. In responders, octreotide (n = 2 out of 4; -30.5% ± 10.4%) was less potent than pasireotide (n = 5 out of 6; -47.0% ± 4.2%) and cabergoline (n = 3 out of 4; -41.9% ± 3.1%) with respect to inhibition of ACTH secretion by adenomas from group 2.

CONCLUSIONS

After achieving normocortisolism induced by medical therapy, cortisol-mediated sst₂ downregulation on corticotroph adenomas appears to be a reversible process at the mRNA but not at the protein level. Octreotide remains less potent than pasireotide and cabergoline with respect to in vitro inhibition of ACTH secretion. Whether sustained normocortisolism induced by medical therapy induces re-expression of functional sst₂ protein in corticotroph adenomas and whether this increases the ACTH-lowering potency of octreotide remains to be established.

Growth hormone granulation pattern and somatostatin receptor subtype 2A correlate with postoperative somatostatin receptor ligand response in acromegaly: a large single center experience

Acromegaly is associated with serious morbidity and mortality, if not well controlled. Approved somatostatin receptor ligands (SRLs) are a mainstay of medical therapy and exhibit preferential affinity for somatostatin receptor (SSTR) subtype 2. Our objective was to assess whether characteristic features of individual growth hormone (GH)-secreting adenomas at diagnosis, correlated with SRL sensitivity, using defined tumor markers. A retrospective review of 86 consecutive acromegaly surgeries (70 patients) performed between January 2006 and December 2011 was undertaken. Patients with any preoperative medical treatment were excluded. Response to SRL therapy was defined as normalization of insulin-like growth factor 1 (IGF1) and random GH < 1.0 ng/dl. Immunohistochemical staining pattern: sparsely granulated, densely granulated, mixed growth hormone-prolactin (GH/PRL) and SSRT2 positivity (+) were correlated with clinicopathologic features, adenoma recurrence, and SRL treatment response. Two-tailed t test, univariate ANOVA, Kruskal-Wallis and bivariate correlation were performed using PAWS 18. The cohort eligible for analysis comprised 59 patients (41 female and 18 male). Based on pre-surgery adenoma imaging dimensions, 81.3% (48) were macroadenomas and average maximum tumor diameter was 18.1 ± 9.9 mm. Patients on SRLs were followed for 13.4 ± 15.8 (mean ± SD) months. Sparsely granulated adenomas were significantly larger at diagnosis, exhibited lower SSTR2 positivity and had a lower rate of biochemical normalization to SRLs. Densely granulated adenomas were highly responsive to SRLs. Overall, patients with SSTR2A+ adenomas responded more favorably to SRL treatment than those with SSTR2A- adenomas. Eighty-one percent of patients with SSTR2A+ adenomas were biochemically controlled (both GH and IGF1) on SRL treatment, e.g. a much higher normalization rate than that reported in the unselected acromegaly population (20-30%). Detailed knowledge of adenoma GH granularity and the immunohistochemical SSTR2A+ status is a predictor of SRL response. These immunoreactive markers should be assessed routinely on surgical specimens to assess subsequent SRL responsiveness and potential need for adjunctive therapy after surgery.

Molecular and functional properties of densely and sparsely granulated GH-producing pituitary adenomas

OBJECTIVE

GH-producing pituitary adenomas display two distinct morphological patterns of cytoplasmic GH-containing secretory granules, namely the densely and sparsely granulated somatotroph adenoma subtype. It is unknown whether these morphological variants reflect distinct pathophysiological entities at the molecular level.

METHODS

In 28 GH-producing adenoma tissues from a consecutive set of patients undergoing pituitary surgery for acromegaly, we studied the GH granulation pattern, the expression of somatostatin receptor subtypes (SSTR) as well as the calcium, cAMP and ZAC1 pathways in primary adenoma cell cultures.

RESULTS

The expression of GSP oncogene was similar between densely and sparsely granulated somatotroph adenoma cells. There were no differences in the calcium, cAMP and ZAC1 pathways as well as in their regulation by SSTR agonists. SSTR2 was exclusively expressed in densely but not in sparsely granulated tumours (membrane expression 86 vs 0%; cytoplasmic expression 67 vs 0%). By contrast, expression of SSTR5 was only found in sparsely but not in densely granulated somatotroph adenomas (membrane expression 29 vs 0%; cytoplasmic expression 57 vs 0%).

CONCLUSIONS

Our results indicate that different granulation patterns in GH-producing adenomas do not reflect differences in pathways and factors pivotal for somatotroph differentiation and function. In vitro, the vast majority of both densely and sparsely granulated tumour cells were responsive to SSTR activation at the molecular level. Sparsely granulated adenomas lacking SSTR2, but expressing SSTR5, might be responsive to novel SSTR agonists with increased affinity to SSTR5.

Octreotide stimulates somatostatin receptor-induced apoptosis of SW480 colon cancer cells by activation of glycogen synthase kinase-3β, A Wnt/β-catenin pathway modulator

BACKGROUND/AIMS

Peptide hormone somatostatin and its receptors (SSTRs) have a wide range of physiological functions and play a role in the treatment of numerous human diseases, including colorectal cancer. Octreotide, a somatostatin-analog peptide, inhibits growth of colonic cancer SW480 cells through Wnt/β-catenin pathway modulation. However, the specific octreotide-stimulating SSTR subtypes and the signal-transduction mechanism responsible for the negative regulation of Wnt/β-catenin pathway by octreotide have not been fully elucidated.

METHODOLOGY

Octreotide-induced apoptosis in SW480 colon cancer cells mediated by SSTR2,SSTR5-dependent regulation of the Wnt/β-catenin pathway components GSK-3β and β-catenin was investigated. Cell apoptosis of SW480 cells was measured by apoptosis-DNA ladder assay. SSTR1, SSTR2, SSTR3, SSTR4, and SSTR5 mRNA expression levels were confirmed by RT-PCR; β-catenin, TCF-4, cyclin D1, c-Myc, and GSK-3β protein levels were examined by Western blot. The distribution of β-catenin in the cell was analyzed with immunocytochemistry.

RESULTS

Octreotide treatment increased SSTR2,SSTR5-induced apoptosis of SW480 colon cancer cells, promoted the plasma membrane accumulation of β-catenin, inactivated T-cell factor-dependent transcription, and downregulated Wnt target genes cyclin D1 and c-Myc. Further, octreotide treatment mediated the activation of GSK-3.

CONCLUSIONS

These preliminary findings showed the negative regulation of the Wnt/β-catenin pathway by peptide hormone G protein-coupled receptors SSTRs.

The novel somatostatin receptor 2/dopamine type 2 receptor chimeric compound BIM-23A758 decreases the viability of human GOT1 midgut carcinoid cells

The majority of neuroendocrine tumors (NETs) of the gastroenteropancreatic system coexpress somatostatin receptors (SSTRs) and dopamine type 2 receptors (D2R), thus providing a rationale for the use of novel SSTR2/D2R chimeric compounds in NET disease. Here we investigate the antitumor potential of the SSTR2/D2R chimeric compounds BIM-23A760 and BIM-23A758 in comparison to the selective SSTR2 agonist BIM-23023 and the selective D2R agonist BIM-53097 on human NET cell lines of heterogeneous origin. While having only minor effects on human pancreatic and bronchus carcinoid cells (BON1 and NCI-H727), BIM-23A758 induced significant antitumor effects in human midgut carcinoid cells (GOT1). These effects involved apoptosis induction as well as inhibition of mitogen-activated protein kinase and Akt signaling. Consistent with their antitumor response to BIM-23A758, GOT1 cells showed relatively high expression levels of SSTR2 and D2R mRNA. In particular, GOT1 cells highly express the short transcript variant of D2R. In contrast to BIM-23A758, the SSTR2/D2R chimeric compound BIM-23A760 as well as the individual SSTR2 and D2R agonistic compounds BIM-23023 and BIM-53097 induced no or only minor antitumor responses in the examined NET cell lines. Taken together, our findings suggest that the novel SSTR2/D2R chimeric compound BIM-23A758 might be a promising substance for the treatment of NETs highly expressing SSTR2 and D2R. In particular, a sufficient expression of the short transcript variant of DR2 might play a pivotal role for effective treatment.

SSTR2 is the functionally dominant somatostatin receptor in human pancreatic β- and α-cells

Somatostatin-14 (SST) inhibits insulin and glucagon secretion by activating G protein-coupled somatostatin receptors (SSTRs), of which five isoforms exist (SSTR1-5). In mice, the effects on pancreatic β-cells are mediated by SSTR5, whereas α-cells express SSTR2. In both cell types, SSTR activation results in membrane hyperpolarization and suppression of exocytosis. Here, we examined the mechanisms by which SST inhibits secretion from human β- and α-cells and the SSTR isoforms mediating these effects. Quantitative PCR revealed high expression of SSTR2, with lower levels of SSTR1, SSTR3, and SSTR5, in human islets. Immunohistochemistry showed expression of SSTR2 in both β- and α-cells. SST application hyperpolarized human β-cells and inhibited action potential firing. The membrane hyperpolarization was unaffected by tolbutamide but antagonized by tertiapin-Q, a blocker of G protein-gated inwardly rectifying K⁺ channels (GIRK). The effect of SST was mimicked by an SSTR2-selective agonist, whereas a SSTR5 agonist was marginally effective. SST strongly (>70%) reduced depolarization-evoked exocytosis in both β- and α-cells. A slightly weaker inhibition was observed in both cell types after SSTR2 activation. SSTR3- and SSTR1-selective agonists moderately reduced the exocytotic responses in β- and α-cells, respectively, whereas SSTR4- and SSTR5-specific agonists were ineffective. SST also reduced voltage-gated P/Q-type Ca²⁺ currents in β-cells, but normalization of Ca²⁺ influx to control levels by prolonged depolarizations only partially restored exocytosis. We conclude that SST inhibits secretion from both human β- and α-cells by activating GIRK and suppressing electrical activity, reducing P/Q-type Ca²⁺ currents, and directly inhibiting exocytosis. These effects are mediated predominantly by SSTR2 in both cell types.

Somatostatin analogs and chimeric somatostatin-dopamine molecules differentially regulate human growth hormone and prolactin gene expression and secretion in vitro

We tested effects of selective somatostatin receptor 2 (SST2) agonist BIM-23120, SST5 agonist BIM-23206 and chimeric somatostatin-dopamine molecules (SRIF/DA) BIM-23A760 and BIM-23A761 on GH and PRL secretion and gene expression in human GH/PRL-secreting pituitary tumors in vitro. In "responders" group BIM-23120 suppressed GH levels by 26±4%, BIM-23206 by 31±5%, BIM-23A760 by 23±4%, BIM-23A761 by 39±8% and D(2)-dopamine agonist BIM-53097 by 31±5%. Using real-time PCR we demonstrated that GH inhibition was not accompanied by decreased GH mRNA levels. PRL secretion was inhibited by BIM-23A760 (29±5%), BIM-23A761 (34±4%), BIM-23206 (26±4%) and BIM-53097 (36±2%). SRIF/DA and BIM-53097 also suppressed PRL mRNA levels. Concluding, SST2 and SST5 agonists and SRIF/DA inhibit GH secretion, but do not suppress GH gene transcription. SRIF/DA and BIM-53097 inhibit both PRL secretion and PRL gene expression. SST5 agonist inhibits PRL secretion, but does not suppress PRL gene expression. D(2) affinity is crucial in SRIF/DA action on PRL gene expression.

A transplantable phosphorylation probe for direct assessment of G protein-coupled receptor activation

The newly developed multireceptor somatostatin analogs pasireotide (SOM230), octreotide and somatoprim (DG3173) have primarily been characterized according to their binding profiles. However, their ability to activate individual somatostatin receptor subtypes (sst) has not been directly assessed so far. Here, we transplanted the carboxyl-terminal phosphorylation motif of the sst(2) receptor to other somatostatin receptors and assessed receptor activation using a set of three phosphosite-specific antibodies. Our comparative analysis revealed unexpected efficacy profiles for pasireotide, octreotide and somatoprim. Pasireotide was able to activate sst(3) and sst(5) receptors but was only a partial agonist at the sst(2) receptor. Octreotide exhibited potent agonistic properties at the sst(2) receptor but produced very little sst(5) receptor activation. Like octreotide, somatoprim was a full agonist at the sst(2) receptor. Unlike octreotide, somatoprim was also a potent agonist at the sst(5) receptor. Together, we propose the application of a phosphorylation probe for direct assessment of G protein-coupled receptor activation and demonstrate its utility in the pharmacological characterization of novel somatostatin analogs.

Somatostatin modulates generation of inspiratory rhythms and determines asphyxia survival

Breathing and the activity of its generator (the pre-Bötzinger complex; pre-BötC) are highly regulated functions. Among neuromodulators of breathing, somatostatin (SST) is unique: it is synthesized by a subset of glutamatergic pre-BötC neurons, but acts as an inhibitory neuromodulator. Moreover, SST regulates breathing both in normoxic and in hypoxic conditions. Although it has been implicated in the neuromodulation of breathing, neither the locus of SST modulation, nor the receptor subtypes involved have been identified. In this study, we aimed to fill in these blanks by characterizing the SST-induced regulation of inspiratory rhythm generation in vitro and in vivo. We found that both endogenous and exogenous SST depress all preBötC-generated rhythms. While SST abolishes sighs, it also decreases the frequency and increases the regularity of eupnea and gasping. Pharmacological experiments showed that SST modulates inspiratory rhythm generation by activating SST receptor type-2, whose mRNA is abundantly expressed in the pre-Bötzinger complex. In vivo, blockade of SST receptor type-2 reduces gasping amplitude and consequently, it precludes auto-resuscitation after asphyxia. Based on our findings, we suggest that SST functions as an inhibitory neuromodulator released by excitatory respiratory neurons when they become overactivated in order to stabilize breathing rhythmicity in normoxic and hypoxic conditions.

DG3173 (somatoprim), a unique somatostatin receptor subtypes 2-, 4- and 5-selective analogue, effectively reduces GH secretion in human GH-secreting pituitary adenomas even in Octreotide non-responsive tumours

OBJECTIVE

Somatostatin analogues (SSA) reduce autonomous GH secretion by activating somatostatin receptors (sst) 2 and 5 in 50-60% of acromegalic patients. However, by inhibiting insulin secretion these SSA reduce glucose tolerance. DG3173 is a novel SSA with additional binding to sst4 and low insulin-suppressing activity. We investigated the effect of DG3173, including its relation to specific tumour characteristics, on GH secretion in human somatotroph adenoma cell cultures (hSA) in comparison with Octreotide.

METHODS

Twenty-seven hSA were characterised immunohistochemically for their hormone- and sst-expression, granularity and pre-surgical therapy with SSA. GH was determined in supernatants of hSA treated with DG3173 or Octreotide in time- (n=6) and dose-response (n=21) experiments. A positive response was defined as GH suppression to below 80% of baseline.

RESULTS

In the dose-response experiments DG3173 suppressed GH secretion in more adenomas than Octreotide (10/21 vs 5/21), including 38% (6/16) of Octreotide non-responders. In responders the extent of GH suppression and IC(50) were comparable for both SSA. The response-rate of both SSA was higher in monohormonal vs bihormonal adenomas, yet GH declined similarly in both groups. Neither pre-surgical SSA (n=6) nor tumour morphology was related to the GH response. However, semi-quantitative analysis indicated a small but significant negative correlation between the GH response to Octreotide and the immunoreactivity scores of sst2 expression.

CONCLUSIONS

DG3173 equalled Octreotide in suppressing GH secretion in hSA. Since DG3173 suppressed GH in some Octreotide-non-responsive adenomas, its clinical effectiveness will be worth testing. Moreover, its reduced insulin-suppressive potency would make it a valuable alternative to Octreotide.

Effects of somatostatin analogs on glucose homeostasis in rats

Pasireotide (SOM230) is a multireceptor-targeted somatostatin analog with high binding affinity for sstr(1,2,3) and sstr(5). The effects of pasireotide and octreotide on blood glucose, insulin, and glucagon levels in rats were evaluated alone and in combination. Single-dose s.c. pasireotide acutely elevated plasma glucose, whereas single-dose s.c. octreotide had no or a small hypoglycemic effect. Glucose elevation with s.c. pasireotide was transient with tachyphylaxis after repeated or continuous administration. Pasireotide and octreotide caused similar inhibitory effects on insulin secretion, whereas pasireotide had a weaker inhibitory effect on glucagon secretion than octreotide. Continuous infusion of pasireotide or injection of pasireotide long-acting release (LAR) resulted in only small and transient elevations of plasma glucose. Based on these results, and differences in the sstr binding affinity of pasireotide vs octreotide, it was hypothesized that the sstr(5) vs sstr(2) receptor activation ratio is the main driver of hyperglycemia after pasireotide. The results also suggest that stronger activation of sstr(2) may counteract the hyperglycemic effect. Indeed, co-administration of octreotide, which has a high affinity for sstr(2), with a hyperglycemic dose of pasireotide did not cause significant changes in plasma glucose levels. In conclusion, although pasireotide and octreotide inhibited insulin to a similar degree, only pasireotide administration was associated with hyperglycemia. The strong glucagon inhibitory effect exhibited by octreotide but not pasireotide may explain this observation. The lack of hyperglycemia during co-administration of pasireotide and octreotide may be explained by the greater activation of sstr(2) compared with pasireotide alone, causing the insulin-glucagon balance to shift within the normoglycemic range. Extrapolation of these data to humans must account for species differences in islet cell sstr expression.

Homologous and heterologous in vitro regulation of pituitary receptors for somatostatin, growth hormone (GH)-releasing hormone, and ghrelin in a nonhuman primate (Papio anubis)

Secretion of GH by pituitary somatotrophs is primarily stimulated by GHRH and ghrelin and inhibited by somatostatin through the activation of specific receptors [GHRH receptor (GHRH-R), GH secretagogue receptor (GHS-R) and somatostatin receptors (sst1-5), respectively]. However, we have shown that somatostatin, at low doses, can also stimulate GH release, directly and specifically, in primary pituitary cultures from a nonhuman primate (baboons, Papio anubis) and pigs. To determine whether somatostatin, GHRH, and ghrelin can also regulate the expression of their receptors in primates, pituitary cultures from baboons were treated for 4 h with GHRH or ghrelin (10(-8) m) or with high (10(-7) m) and low (10(-15) m) doses of somatostatin, and GH release and expression levels of all receptors were measured. GHRH/ghrelin decreased the expression of their respective receptors (GHRH-R and GHS-R). Both peptides increased sst1, only GHRH decreased sst5 expression, whereas sst2 expression remained unchanged. The effects of GHRH/ghrelin were completely mimicked by forskolin (adenylate cyclase activator) and phorbol 12-myristate 13-acetate (protein kinase C activator), respectively, indicating the regulation of receptor subtype levels by GHRH and ghrelin involved distinct signaling pathways. In contrast, high-dose somatostatin did not alter GH release but increased sst1, sst2, and sst5 expression, whereas GHRH-R and GHS-R expression were unaffected. Interestingly, low-dose somatostatin increased GH release and sst1 mRNA but decreased sst5 and GHRH-R expression, similar to that observed for GHRH. Altogether, our data show for the first time in a primate model that the primary regulators of somatotroph function (GHRH/ghrelin/somatostatin) exert both homologous and heterologous regulation of receptor synthesis which is dose and subtype dependent and involves distinct signaling pathways.

Pharmacological characterization of a recombinant, fluorescent somatostatin receptor agonist

Somatostatin (SST) is a peptide neurotransmitter/hormone found in several mammalian tissue types. Apart from its natural importance, labeled SST/analogues are utilized in clinical applications such as targeting/diagnosis of neuroendocrine tumors. We report on the development and characterization of a novel, recombinant, fluorescent somatostatin analogue that has potential to elucidate somatostatin-activated cell signaling. SST was genetically fused with a monomeric-red fluorescent protein (mRFP) as the fluorescent label. The attachment of SST to mRFP had no detectable effect on its fluorescent properties. This analogue's potency to activate the endogenous and transfected somatostatin receptors was characterized using assays of membrane potential and Ca(2+) mobilization and immunocytochemistry. SST-mRFP was found to be an effective somatostatin receptor agonist, able to trigger the membrane hyperpolarization, mobilization of the intracellular Ca(2+) and receptor-ligand internalization in cells expressing somatostatin receptors. This complex represents a novel optical reporter due to its red emission spectral band suitable for in vivo imaging and tracking of the somatostatin receptor signaling pathways, affording higher resolution and sensitivity than those of the state-of-the-art radiolabeling bioassays.

Suppression of ANP secretion by somatostatin through somatostatin receptor type 2

Somatostatin is a cyclic-14 amino acid peptide which mainly distributed in digestive system and brain. Somatostatin receptor (SSTR) is a G-protein coupled receptor and all five SSTR subtypes are expressed in cardiomyocytes. The aim of this study was to investigate the effect of somatostatin on atrial natriuretic peptide (ANP) secretion and its signaling pathway. Somatostatin (0.01 and 0.1nM) decreased ANP secretion in isolated beating rat atrium in a dose-dependent manner. But atrial contractility and translocation of extracellular fluid were not changed. Somatostatin-induced decrease in ANP secretion was significantly attenuated by the pretreatment with CYN 154806 (SSTR type 2 antagonist; 0.1μM), but not by BIM 23056 (SSTR type 5 antagonist; 0.1μM) and urantide (urotensin II receptor antagonist; 0.1μM). When pretreated with an agonist for SSTR type 2 (Seglitide, 0.1nM) and SSTR type 5 (L 817818, 0.1nM), only Seglitide reduced ANP secretion similar to that of somatostatin. The suppressive effect of somatostatin on ANP secretion was attenuated by the pretreatment with an inhibitor for adenylyl cyclase (MDL-12330A, 5μM) or protein kinase A (KT 5720, 0.1μM). In diabetic rat atria, the suppressive effect of somatostatin on ANP secretion and concentration was attenuated. Real time-PCR and western blot shows the decreased level of SSTR type 2 mRNA and protein in diabetic rat atria. These data suggest that somatostatin decreased ANP secretion through SSTR type 2 and an attenuation of suppressive effect of somatostatin on ANP secretion in diabetic rat atria is due to a down-regulation of SSTR type 2.