Signaling pathways as specific pharmacologic targets for neuroendocrine tumor therapy: RET, PI3K, MEK, growth factors, and Notch

Pancreatic Neuroendocrine Neoplasms: Updates on Genomic Changes in Inherited Tumour Syndromes and Sporadic Tumours Based on WHO Classification

Pancreatic neuroendocrine neoplasms (PanNENs) are the neuroendocrine neoplasms with greatest rate of increase in incidence. Approximately 10% of PanNENs arise as inherited tumour syndromes which include multiple endocrine neoplasia type 1, multiple endocrine neoplasia type 4, von Hippel-Lindau syndrome, neurofibromatosis type1, tuberous sclerosis complex 1/2, Cowden syndrome, and Glucagon cell hyperplasia and neoplasia as well as familial insulinomatosis. In sporadic PanNENs, driver mutations in MEN1, DAXX/ATRX and mTOR pathway genes are associated with development and progression in pancreatic neuroendocrine tumours. The other changes are in VEGF pathway, Notch pathway, germline mutations in MUTYH, CHEK2, BRCA2, PHLDA3 as well as other genetic alterations. On the other hand, pancreatic neuroendocrine carcinomas share similar genetic alterations with ductal adenocarcinomas, e.g., TP53, RB1 or KRAS. In addition, microRNA and changes in immune microenvironment were noted in PanNENs. Updates on these genetic knowledges contribute to the development of management strategies for patients with PanNENs.

Medullary thyroid cancer: molecular factors, management and treatment

Medullary thyroid cancer (MTC) is an infrequent neuroendocrine tumor, which amounts to 3–5% of all thyroid malignancies. Approximately 75–80% of MTCs are sporadic neoplasms. The rest of 20–25% are familial cases that belong to multiple endocrine neoplasia (MEN) syndromes, specifically MEN2 and MEN3. These cases of familial MTC are attributed to an activating germline mutation of a tyrosine kinase receptor gene, the rearranged during transfection (RET) proto-oncogene, located on chromosome 10q11.21. These mutations are also found in some cases of sporadic MTC. This review sets forth in summary the accepted guidelines and approaches regarding diagnosis, management, and treatment of MTC. Surgical resection is the standard care, and an early, prophylactic intervention is performed in genetic cases. Further investigation and understanding of the molecular pathways involved in the growth and advancement of MTC is required in order to provide efficient therapy in cases of progressive disease.

Updates on the Role of Molecular Alterations and NOTCH Signalling in the Development of Neuroendocrine Neoplasms

Neuroendocrine neoplasms (NENs) comprise a heterogeneous group of rare malignancies, mainly originating from hormone-secreting cells, which are widespread in human tissues. The identification of mutations in ATRX/DAXX genes in sporadic NENs, as well as the high burden of mutations scattered throughout the multiple endocrine neoplasia type 1 (MEN-1) gene in both sporadic and inherited syndromes, provided new insights into the molecular biology of tumour development. Other molecular mechanisms, such as the NOTCH signalling pathway, have shown to play an important role in the pathogenesis of NENs. NOTCH receptors are expressed on neuroendocrine cells and generally act as tumour suppressor proteins, but in some contexts can function as oncogenes. The biological heterogeneity of NENs suggests that to fully understand the role and the potential therapeutic implications of gene mutations and NOTCH signalling in NENs, a comprehensive analysis of genetic alterations, NOTCH expression patterns and their potential role across all NEN subtypes is required.

RET Kinase-Regulated MicroRNA-153-3p Improves Therapeutic Efficacy in Medullary Thyroid Carcinoma

Background: Medullary thyroid carcinoma (MTC) presents a disproportionate number of thyroid cancer deaths due to limited treatment options beyond surgery. Gain-of-function mutations of the human REarranged during Transfection (RET) proto-oncogene have been well-established as the key driver of MTC tumorigenesis. RET has been targeted by tyrosine kinase inhibitors (TKIs), such as cabozantinib and vandetanib. However, clinical results have been disappointing, with regular dose reductions and inevitable progression. This study aimed to identify RET-regulated microRNAs (miRNAs) and explore their potential as novel therapeutic targets. Methods: Small RNA sequencing was performed in MTC TT cells before and after RET inhibition to identify RET-regulated miRNAs of significance. In vitro gain-of-function studies were performed to investigate cellular and molecular effects of potential miRNAs on cell phenotypes. Systemic delivery of miRNA in MTC xenografts using EDV™ nanocells, targeted to epidermal growth factor receptor on tumor cells, was employed to assess the therapeutic potential and possible modulation of TKI responses. Results: The study demonstrates the tumor suppressive role of a specific RET-regulated miRNA, microRNA-153-3p (miR-153-3p), in MTC. Targeted intravenous delivery of miR-153-3p impeded the tumor growth in MTC xenografts. Furthermore, combined treatment with miR-153-3p plus cabozantinib caused greater growth inhibition and appeared to reverse cabozantinib resistance. Mechanistically, miR-153-3p targets ribosomal protein S6 kinase B1 (RPS6KB1) of mTOR signaling and reduced downstream phosphorylation of Bcl-2 associated death promoter. Conclusion: This study provides evidence to establish systemic miRNA replacement plus TKIs as a novel therapeutic for patients with metastatic, progressive MTC.

Epigenetic regulation of RET receptor tyrosine kinase and non-coding RNAs in MTC

Medullary thyroid carcinoma (MTC) is an aggressive and rare cancer with limited treatment options for metastatic disease. Due to this, there is a need for a better understanding of MTC biology in the hope of improved treatments. One area of improved understanding of cancer biology is epigenetics. Epigenetics is defined as cellular processes which alter gene expression independent of changes in the primary DNA sequence. These processes include modifications such as DNA methylation, microRNA deregulation and post-translational histone modifications, all of which have been implicated in tumorigenesis of MTC. Transcription of the main driver of MTC - the REarranged during Transfection (RET) proto-oncogene can also be modulated by epigenetic alterations. This review will present a review of MTC and its epigenetic links with a particular focus on targeting epigenetic mechanisms as novel therapeutic strategies.

Pasireotide in the treatment of neuroendocrine tumors: a review of the literature

Somatostatin analogs have an important role in the medical therapy of neuroendocrine tumors (NETs). Octreotide and lanreotide, both somatostatin analogs binding with high affinity for the somatostatin receptor (SSTR)2, can control symptoms in functional NETs. In addition, these compounds, because of their antiproliferative effects, can stabilize growth of well-differentiated NETs. Pasireotide is a novel multireceptor-targeted somatostatin analog with high affinity for SSTR1, 2, 3, and 5. This review provides an overview of the state of the art of pasireotide in the treatment of NETs, with the aim of addressing clinical relevance and future perspectives for this molecule in the management of NETs.

Notch pathway inhibition targets chemoresistant insulinoma cancer stem cells

Insulinomas (INS) are the most common neuroendocrine pancreatic tumours in humans and dogs. The long-term prognosis for malignant INS is still poor due to a low success rate of the current treatment modalities, particularly chemotherapy. A better understanding of the molecular processes underlying the development and progression of INS is required to develop novel targeted therapies. Cancer stem cells (CSCs) are thought to be critical for the engraftment and chemoresistance of many tumours, including INS. This study was aimed to characterise and target INS CSCs in order to develop novel targeted therapies. Highly invasive and tumourigenic human and canine INS CSC-like cells were successfully isolated. These cells expressed stem cell markers (OCT4, SOX9, SOX2, CD133 and CD34), exhibited greater resistance to 5-fluorouracil (5-FU) and demonstrated a more invasive and tumourigenic phenotype in vivo compared to bulk INS cells. Here, we demonstrated that Notch-signalling-related genes (NOTCH2 and HES1) were overexpressed in INS CSC-like cells. Protein analysis showed an active NOTCH2-HES1 signalling in INS cell lines, especially in cells resistant to 5-FU. Inhibition of the Notch pathway, using a gamma secretase inhibitor (GSI), enhanced the sensitivity of INS CSC-like cells to 5-FU. When used in combination GSI and 5-FU, the clonogenicity in vitro and the tumourigenicity in vivo of INS CSC-like cells were significantly reduced. These findings suggested that the combined strategy of Notch signalling inhibition and 5-FU synergistically attenuated enriched INS CSC populations, providing a rationale for future therapeutic exploitation.

Targeted therapy of gastroenteropancreatic neuroendocrine tumours: preclinical strategies and future targets

Molecular targeted therapy of advanced neuroendocrine tumours (NETs) of the gastroenteropancreatic (GEP) system currently encompasses approved therapy with the mammalian target of rapamycin (mTOR) inhibitor everolimus and the multi-tyrosinkinase inhibitor sunitinib. However, clinical efficacy of these treatment strategies is limited by low objective response rates and limited progression-free survival due to tumour resistance. Further novel strategies for molecular targeted therapy of NETs of the GEP system are needed. This paper reviews preclinical research models and signalling pathways in NETs of the GEP system. Preclinical and early clinical data on putative novel targets for molecular targeted therapy of NETs of the GEP system are discussed, including PI3K, Akt, mTORC1/mTORC2, GSK3, c-Met, Ras-Raf-MEK-ERK, embryogenic pathways (Hedgehog, Notch, Wnt/beta-catenin, TGF-beta signalling and SMAD proteins), tumour suppressors and cell cycle regulators (p53, cyclin-dependent kinases (CDKs) CDK4/6, CDK inhibitor p27, retinoblastoma protein (Rb)), heat shock protein HSP90, Aurora kinase, Src kinase family, focal adhesion kinase and epigenetic modulation by histone deacetylase inhibitors.

CXCR4/CXCL12/CXCR7 axis is functional in neuroendocrine tumors and signals on mTOR

Objective

To evaluate the possible crosstalk between C-X-C chemokine receptor 4 (CXCR4)/C-X-C motif chemokine 12 (CXCL12)/C-X-C chemokine receptor 7 (CXCR7) axis with the mammalian target of rapamycin (mTOR) pathway in neuroendocrine tumors (NETs).

Methods

Sixty-one human NETs were included into the study. CXCR4/CXCL12/CXCR7 axis and mTOR pathway were assessed by qRT-PCR and immunohistochemistry (IHC). The effect of mTOR inhibitor, RAD001, was evaluated on CXCR4 pathway through proliferation and p-Erk and p-AKT induction. Results: CXCR4/CXCL12/CXCR7 axis and p-mTOR were found to be active and correlated with grading, Ki67 index and tumor stage. mTOR pathway activation significantly correlated with poor prognosis. In human NET cells, CXCL12 induced mTOR signalling while AMD3100 (CXCR4-antagonist) impaired it. The mTOR-antagonist, RAD001, impaired the CXCL12-dependent induction of CXCR4 downstream effectors. Combination of AMD3100 and RAD001 potentiate cell growth inhibition.

Conclusions

CXCR4/CXCL12/CXCR7 axis is active in NETs and signals on mTOR. CXCR4 might be considered a prognostic factor in NETs. Combined treatment with AMD3100 and RAD001 may provide clinical benefits in NET patients with drug-resistant.

Inhibition of Peripheral Synthesis of Serotonin as a New Target in Neuroendocrine Tumors

: The carcinoid syndrome represents a set of signs and symptoms associated with neuroendocrine tumors (NETs) that occur primarily when metastases are developed in the liver, resulting in the worsening of quality of life. Serotonin plays a central role in the physiology of carcinoid syndrome by promoting intestinal motility. Somatostatin analogs (SSAs) have widely demonstrated their efficacy as symptomatic relievers of carcinoid syndrome, but this control is ephemeral, being reduced by approximately 50% within the first year. The exact mechanisms of resistance to SSAs are not fully understood, but it is believed that serotonin might be involved. Patients with carcinoid syndrome present with a significant increase in serotonin plasma levels and, consequently, in the soluble urinary metabolite 5-hydroxyindole acetic acid. Telotristat etiprate is a potent inhibitor of tryptophan hydroxylase, a rate-limiting enzyme in the synthesis of serotonin, that has demonstrated in the phase III TELESTAR clinical trial a significant improvement in the control of bowel movements in patients with NETs who have carcinoid syndrome and who have progressed to an SSA. Based on these results, telotristat etiprate has emerged as a potential new option in the treatment algorithm of symptomatic control of functioning NETs. However, some issues need to be clarified, such as the safety profile of the drug outside clinical trials, the benefit in quality of life, and the possible impact on tumor growth, as well as its role within sequencing or combination treatment strategies with pre-existing drugs effective in NET treatment.

IMPLICATIONS FOR PRACTICE

This article reviews the literature about carcinoid syndrome, which affects patients diagnosed with neuroendocrine tumors. Few articles have been published about this syndrome and its pathophysiology. Somatostatin analogs provide symptomatic relief; however, patients may become refractory to this strategy, usually within the first year of treatment. In this context, as an agent with an innovative mechanism of action, telotristat etiprate has demonstrated activity in a phase III trial, and findings may offer a path to an improve quality of life and prolonged survival for certain patients.

Notch Signaling in Neuroendocrine Tumors

Carcinoids and neuroendocrine tumors (NETs) are a heterogeneous group of tumors that arise from the neuroendocrine cells of the GI tract, endocrine pancreas, and the respiratory system. NETs remain significantly understudied with respect to molecular mechanisms of pathogenesis, particularly the role of cell fate signaling systems such as Notch. The abundance of literature on the Notch pathway is a testament to its complexity in different cellular environments. Notch receptors can function as oncogenes in some contexts and tumor suppressors in others. The genetic heterogeneity of NETs suggests that to fully understand the roles and the potential therapeutic implications of Notch signaling in NETs, a comprehensive analysis of Notch expression patterns and potential roles across all NET subtypes is required.

Quantitative analysis of receptor tyrosine kinase-effector coupling at functionally relevant stimulus levels

A major goal of current signaling research is to develop a quantitative understanding of how receptor activation is coupled to downstream signaling events and to functional cellular responses. Here, we measure how activation of the RET receptor tyrosine kinase on mouse neuroblastoma cells by the neurotrophin artemin (ART) is quantitatively coupled to key downstream effectors. We show that the efficiency of RET coupling to ERK and Akt depends strongly on ART concentration, and it is highest at the low (∼100 pM) ART levels required for neurite outgrowth. Quantitative discrimination between ERK and Akt pathway signaling similarly is highest at this low ART concentration. Stimulation of the cells with 100 pM ART activated RET at the rate of ∼10 molecules/cell/min, leading at 5-10 min to a transient peak of ∼150 phospho-ERK (pERK) molecules and ∼50 pAkt molecules per pRET, after which time the levels of these two signaling effectors fell by 25-50% while the pRET levels continued to slowly rise. Kinetic experiments showed that signaling effectors in different pathways respond to RET activation with different lag times, such that the balance of signal flux among the different pathways evolves over time. Our results illustrate that measurements using high, super-physiological growth factor levels can be misleading about quantitative features of receptor signaling. We propose a quantitative model describing how receptor-effector coupling efficiency links signal amplification to signal sensitization between receptor and effector, thereby providing insight into design principles underlying how receptors and their associated signaling machinery decode an extracellular signal to trigger a functional cellular outcome.

Article Commentary: Medullary Thyroid Cancer: An Update for Surgeons

Medullary thyroid carcinoma is uncommon but accounts for approximately 4 to 5 per cent of all thyroid cancers. Although most cases are sporadic, close to one-fourth of cases result from germline mutations in the RET proto-oncogene. These mutations are clinically important because they predict the earliest age of medullary thyroid cancer diagnosis and clinical aggressiveness, which guide individualized management. This review covers the presentation, diagnosis, workup, current management, and future directions of the management of medullary thyroid carcinoma. Today's chance for cure depends on early and appropriate surgical resection. Further investigation of the cellular signaling pathways shown to be essential for the growth and spread of medullary thyroid carcinoma remains an active field with hope for providing targeted systemic therapy for patients with progressive disease.

Aspirin inhibits cell viability and mTOR downstream signaling in gastroenteropancreatic and bronchopulmonary neuroendocrine tumor cells

AIM: To investigate the effect of aspirin on neuroendocrine tumor (NET) cell growth and signaling in vitro.

METHODS: Human pancreatic BON1, bronchopulmonary NCI-H727 and midgut GOT1 neuroendocrine tumor cells were treated with different concentrations of aspirin (from 0.001 to 5 mmol/L), and the resulting effects on metabolic activity/cell proliferation were measured using cell proliferation assays and SYBR-DNA-labeling after 72, 144 and 216 h of incubation. The effects of aspirin on the expression and phosphorylation of several critical proteins that are involved in the most common intracellular growth factor signaling pathways (especially Akt protein kinase B) and mammalian target of rapamycin (mTOR) were determined by Western blot analyses. Propidium iodide staining and flow cytometry were used to evaluate changes in cell cycle distribution and apoptosis. Statistical analysis was performed using a 2-tailed Student’s t-test to evaluate the proliferation assays and cell cycle analyses. The results are expressed as the mean ± SD of 3 or 4 independently performed experiments. Statistical significance was set at P < 0.05.

RESULTS: Treatment with aspirin suppressed the viability/proliferation of BON1, NCI-H727 and GOT1 cells in a time- and dose-dependent manner. Significant effects were observed at starting doses of 0.5-1 mmol/L and peaked at 5 mmol/L. For instance, after treatment with 1 mmol/L aspirin for 144 h, the viability of pancreatic BON1 cells decreased to 66% ± 13% (P < 0.05), the viability of bronchopulmonary NCI-H727 cells decreased to 53% ± 8% (P < 0.01) and the viability of midgut GOT1 cells decreased to 89% ± 6% (P < 0.01). These effects were associated with a decreased entry into the S phase, the induction of the cyclin-dependent kinase inhibitor p21 and reduced expression of cyclin-dependent kinase 4 and cyclin D3. Aspirin suppressed mTOR downstream signaling, evidenced by the reduced phosphorylation of the mTOR substrates 4E binding protein 1, serine/threonine kinase P70S6K and S6 ribosomal protein and inhibited glycogen synthase kinase 3 activity. We observed the (compensatory) activation of tuberous sclerosis 2, the serine/threonine specific protein kinase AKT and extracellular signal-regulated kinases.

CONCLUSION: Aspirin demonstrates promising anticancer properties for NETs in vitro. Further preclinical and clinical studies are needed.

Specific glycogen synthase kinase-3 inhibition reduces neuroendocrine markers and suppresses neuroblastoma cell growth

OBJECTIVE

Neuroblastoma is a common neuroendocrine (NE) tumor that presents in early childhood, with a high incidence of malignancy and recurrence. The glycogen synthase kinase-3 (GSK-3) pathway is a potential therapeutic target, as this pathway has been shown to be crucial in the management of other NE tumors. However, it is not known which isoform is necessary for growth inhibition. In this study, we investigated the effect of the GSK-3 inhibitor AR-A014418 on the different GSK-3 isoforms in neuroblastoma.

METHODS

NGP and SH-5Y-SY cells were treated with 0-20 μM of AR-A014418 and cell viability was measured by MTT assay. Expression levels of NE markers CgA and ASCL1, GSK-3 isoforms, and apoptotic markers were analyzed by western blot.

RESULTS

Neuroblastoma cells treated with AR-A014418 had a significant reduction in growth at all doses and time points (P<0.001). A reduction in growth was noted in cell lines on day 6, with 10 μM (NGP-53% vs. 0% and SH-5Y-SY-38% vs. 0%, P<0.001) treatment compared to control, corresponding with a noticeable reduction in tumor marker ASCL1 and CgA expression.

CONCLUSION

Treatment of neuroblastoma cell lines with AR-A014418 reduced the level of GSK-3α phosphorylation at Tyr279 compared to GSK-3β phosphorylation at Tyr216, and attenuated growth via the maintenance of apoptosis. This study supports further investigation to elucidate the mechanism(s) by which GSK-3α inhibition downregulates the expression of NE tumor markers and growth of neuroblastoma.

Neuroendocrine phenotype alteration and growth suppression through apoptosis by MK-2206, an allosteric inhibitor of AKT, in carcinoid cell lines in vitro

Carcinoids are neuroendocrine malignancies characterized by their overproduction of various bioactive hormones that lead to the carcinoid syndrome. We have shown previously that AKT serves as a key regulator of growth and phenotypic expression of tumor markers in carcinoids by the genetic depletion of AKT expression. However, no small-molecule inhibitor of AKT kinase activity has been developed until recently. MK-2206, a novel allosteric inhibitor of AKT, is currently undergoing clinical trials for the treatment of solid tumors. In this study, we explored the effect of MK-2206 on carcinoid cell proliferation and bioactive hormone production in vitro in two carcinoid cell lines - pancreatic carcinoid BON and bronchopulmonary H727. Treatment with MK-2206 effectively suppressed AKT phosphorylation at serine 473 and significantly reduced cell proliferation in a dose-dependent manner. Most importantly, MK-2206 treatment resulted in a significant reduction in ASCL1, CgA, and NSE expression, collectively recognized as markers of neuroendocrine tumor malignancy. Furthermore, MK-2206-treated cells showed an increase in levels of cleaved PARP and cleaved caspase-3, with a concomitant reduction in levels of Mcl-1 and XIAP, indicating that the antiproliferative effect of MK-2206 occurs through the induction of apoptosis. In conclusion, MK-2206 suppresses carcinoid tumor growth, and alters its neuroendocrine phenotype, indicating that this drug may be beneficial for patients with carcinoid syndrome. These studies merit further clinical investigation.