A growing body of literature is demonstrating that Notch signaling is a more complex process than originally thought. Contradictory findings of notch-1 acting as an oncogene or a tumor suppressor revealed that its role is very specific to the cellular context. In this review we focus on the tumor suppressor role of Notch-1 signaling in neuroendocrine tumors (NETs) such as carcinoid and medullary thyroid cancers. NETs secrete various bioactive hormones that can cause debilitating symptoms. Surgery is the only potential curative treatment for the patients with NETs. Notch-1 signaling is absent in these tumors and activation of Notch-1 significantly reduces tumor growth in vitro. Therefore, identification of compound(s) that activate the Notch-1 pathway in NETs could be a potential strategy to treat patients with NETs.

Medullary thyroid cancer accounts for 5%–10% of all thyroid cancers. The majority of medullary thyroid cancers are sporadic, but 20% of cases are a result of a germline mutation in the ret proto-oncogene. Hereditary medullary thyroid cancer can be seen as part of the multiple endocrine neoplasia syndrome type 2A or 2B or as part of familial medullary thyroid cancer. This article discusses the current methods available for the diagnosis and evaluation of a patient with suspected medullary thyroid cancer. The management of medullary thyroid cancer is predominantly surgical excision, consisting of a total thyroidectomy and lymph node dissection. The extent and timing of surgical excision are discussed. Systemic therapeutic options are limited for medullary thyroid cancer, but several therapeutic targets show promise for the development of new therapies in the future.

Carcinoid tumors are neuroendocrine malignancies that frequently metastasize and secrete hormones that cause debilitating symptoms in patients. In this study we report the effects of valproic acid (VPA), a drug long used for the treatment of epilepsy, on the growth and neuroendocrine phenotype of human carcinoid cancer cells. VPA treatment of gastrointestinal and pulmonary carcinoid cells resulted in a dose-dependent inhibition of cancer cell growth. Western blot analysis revealed degradation of cyclin D1 and an increase in cyclin-dependent kinases p21 and p27 with VPA treatment. Flow cytometry confirmed that the mechanism of VPA-induced growth inhibition is G1 phase cell cycle arrest. Furthermore, VPA suppressed expression of the neuroendocrine tumor marker chromogranin A. In addition to these effects, VPA also increased levels of full-length Notch-1 and the active Notch-1 intracellular domain. Luciferase reporter assays incorporating the centromere-binding factor 1 (CBF-1) binding site and the achaete-scute complex-like 1 (ASCL-1) promoter confirmed the functional activity of VPA-induced Notch-1. Transfection of Notch-1 small-interfering RNA into carcinoid tumor cells blocked the effects of VPA on Notch-1 activation, ASCL-1 suppression, p21 induction, and cell growth inhibition. VPA also suppressed growth of carcinoid tumors in vivo in a mouse tumor xenograft experiment. These findings confirm the important role of Notch-1 in regulating the growth and neuroendocrine phenotype of carcinoid tumor cells. On the basis of this study, a clinical trial of VPA for patients with advanced carcinoid cancer will be conducted.

A multifunctional gold (Au) nanorod (NR)-based nanocarrier capable of co-delivering small interfering RNA (siRNA) against achaete-scute complex-like 1 (ASCL1) and an anticancer drug (doxorubicin (DOX)) specifically to neuroendocrine (NE) cancer cells was developed and characterized for combined chemotherapy and siRNA-mediated gene silencing. The Au NR was conjugated with (1) DOX, an anticancer drug, via a pH-labile hydrazone linkage to enable pH-controlled drug release, (2) polyarginine, a cationic polymer for complexing siRNA, and (3) octreotide (OCT), a tumor-targeting ligand, to specifically target NE cancer cells with overexpressed somatostatin receptors. The Au NR-based nanocarriers exhibited a uniform size distribution as well as pH-sensitive drug release. The OCT-conjugated Au NR-based nanocarriers (Au–DOX–OCT, targeted) exhibited a much higher cellular uptake in a human carcinoid cell line (BON cells) than non-targeted Au NR-based nanocarriers (Au–DOX) as measured by both flow cytometry and confocal laser scanning microscopy (CLSM). Moreover, Au–DOX–OCT–ASCL1 siRNA (Au–DOX–OCT complexed with ASCL1 siRNA) resulted in significantly higher gene silencing in NE cancer cells than Au–DOX–ASCL1 siRNA (non-targeted Au–DOX complexed with ASCL1 siRNA) as measured by an immunoblot analysis. Additionally, Au–DOX–OCT–ASCL1 siRNA was the most efficient nanocarrier at altering the NE phenotype of NE cancer cells and showed the strongest anti-proliferative effect. Thus, combined chemotherapy and RNA silencing using NE tumor-targeting Au NR-based nanocarriers could potentially enhance the therapeutic outcomes in treating NE cancers.

Gastrointestinal (GI) carcinoid tumors elaborate serotonin and other vasoactive substances, causing the carcinoid syndrome. Based on developmental biology data, we hypothesized that basic helix-loop-helix transcription factors, including achaete-scute complex homolog-like 1 (Ascl1)/hASH1, and the Notch signaling pathway might regulate the neuroendocrine phenotype in GI carcinoids. The aim of this study was to evaluate expression of developmental transcription factors and Notch signaling components in GI carcinoids and model their interaction in a relevant GI carcinoid cell line. Fourteen GI carcinoid tumor specimens, five paired adjacent normal tissues, fetal tissues, and tumor cell lines were analyzed by RT-PCR and immunoblot. BON carcinoid cells were further analyzed after Notch overexpression for neuroendocrine marker expression, serotonin production, and growth. The study was conducted in an academic referral center. Deidentified archival pathology specimens were examined. Among a panel of six developmental transcription factors tested, only Ascl1 mRNA was overexpressed compared with surrounding normal tissue (seven of 10 GI carcinoid tumors and in BON cells, none of five normal tissues). Ascl1 protein was also expressed in four of four carcinoid tumors and BON cells). Notch pathway ligands, receptors, and downstream effectors were widely expressed in tumor and normal specimens. Overexpression of activated Notch1 in BON cells led to induction of the Notch effector hairy and enhancer of split 1 (Hes1), loss of Ascl1, reductions in neuron-specific enolase, synaptophysin, and chromogranin A, and most significantly, an 89% decrease in serotonin concentration and equivalent reductions in serotonin-reactive cells and repression of tryptophan hydroxylase 1 mRNA. The Notch signaling pathway is a significant regulator of neuroendocrine differentiation and serotonin production in GI carcinoid tumors.