Animal models and cell lines of pancreatic neuroendocrine tumors

Models in Pancreatic Neuroendocrine Neoplasms: Current Perspectives and Future Directions

Pancreatic neuroendocrine neoplasms (pNENs) are a heterogeneous group of tumors derived from multiple neuroendocrine origin cell subtypes. Incidence rates for pNENs have steadily risen over the last decade, and outcomes continue to vary widely due to inability to properly screen. These tumors encompass a wide range of functional and non-functional subtypes, with their rarity and slow growth making therapeutic development difficult as most clinically used therapeutics are derived from retrospective analyses. Improved molecular understanding of these cancers has increased our knowledge of the tumor biology for pNENs. Despite these advances in our understanding of pNENs, there remains a dearth of models for further investigation. In this review, we will cover the current field of pNEN models, which include established cell lines, animal models such as mice and zebrafish, and three-dimensional (3D) cell models, and compare their uses in modeling various disease aspects. While no study model is a complete representation of pNEN biology, each has advantages which allow for new scientific understanding of these rare tumors. Future efforts and advancements in technology will continue to create new options in modeling these cancers.

Targeting of Glucose Transport and the NAD Pathway in Neuroendocrine Tumor (NET) Cells Reveals New Treatment Options

(1) Background: the potency of drugs that interfere with glucose metabolism, i.e., glucose transporters (GLUT) and nicotinamide phosphoribosyltransferase (NAMPT) was analyzed in neuroendocrine tumor (NET, BON-1, and QPG-1 cells) and small cell lung cancer (SCLC, GLC-2, and GLC-36 cells) tumor cell lines. (2) Methods: the proliferation and survival rate of tumor cells was significantly affected by the GLUT-inhibitors fasentin and WZB1127, as well as by the NAMPT inhibitors GMX1778 and STF-31. (3) Results: none of the NET cell lines that were treated with NAMPT inhibitors could be rescued with nicotinic acid (usage of the Preiss-Handler salvage pathway), although NAPRT expression could be detected in two NET cell lines. We finally analyzed the specificity of GMX1778 and STF-31 in NET cells in glucose uptake experiments. As previously shown for STF-31 in a panel NET-excluding tumor cell lines, both drugs specifically inhibited glucose uptake at higher (50 μM), but not at lower (5 μM) concentrations. (4) Conclusions: our data suggest that GLUT and especially NAMPT inhibitors are potential candidates for the treatment of NET tumors.

An update on genetically engineered mouse models of pancreatic neuroendocrine neoplasms

Pancreatic neuroendocrine neoplasms (PanNENs) are rare and clinically challenging entities. At the molecular level, PanNENs' genetic profile is well characterized, but there is limited knowledge regarding the contribution of the newly identified genes to tumor initiation and progression. Genetically engineered mouse models (GEMMs) are the most versatile tool for studying the plethora of genetic variations influencing PanNENs' etiopathogenesis and behavior over time. In this review, we present the state of the art of the most relevant PanNEN GEMMs available and correlate their findings with the human neoplasms' counterparts. We discuss the historic GEMMs as the most used and with higher translational utility models. GEMMs with Men1 and glucagon receptor gene germline alterations stand out as the most faithful models in recapitulating human disease; RIP-Tag models are unique models of early-onset, highly vascularized, invasive carcinomas. We also include a section of the most recent GEMMs that evaluate pathways related to cell cycle and apoptosis, Pi3k/Akt/mTOR, and Atrx/Daxx. For the latter, their tumorigenic effect is heterogeneous. In particular, for Atrx/Daxx, we will require more in-depth studies to evaluate their contribution; even though they are prevalent genetic events in PanNENs, they have low/inexistent tumorigenic capacity per se in GEMMs. Researchers planning to use GEMMs can find a road map of the main clinical features in this review, presented as a guide that summarizes the chief milestones achieved. We identify pitfalls to overcome, concerning the novel designs and standardization of results, so that future models can replicate human disease more closely.

The evolution of PRRT for the treatment of neuroendocrine tumors; What comes next?

Lu-177 has been developed for the treatment of patients with peptide receptor radionuclide therapy (PRRT). A second generation pure no-carrier-added Lu-177 has a high specific activity and has waste disposal advantages over the first generation carrier-added Lu-177. PRRT has recently been developed for the treatment of neuroendocrine tumors (NETs). The majority of pancreatic and gastroenteric NETs (GEP-NETs) express the somatostatin receptors (SSTRs) 2 and 5. These receptors can be specifically targeted with a somatostatin peptide analogue (DOTATOC/DOTATATE) which can be chelated to a positron emission tomography (PET) emitting radioisotope such as Ga-68 for imaging or to a β-emitting radioisotope Lu-177 for therapy. A key advantage of this approach is that the receptor expression can be demonstrated by PET imaging before the patient is treated. Clinical studies in G1 and G2 GEP-NETS have demonstrated that PRRT is extremely effective in terms of progression free survival (PFS), symptom control and quality of life, with a well-established safety profile. A beneficial effect on outcome survival awaits to be confirmed. The first commercially available product Lu-177-DOTATATE was approved following the NETTER-1 trial in G1 and G2 GE-NETS. Lu-177-DOTATATE 7,4 GBq every 8 weeks for 4 cycles, together with octreotide LAR 30 mg monthly, demonstrated a median PFS of 28,4 months compared to 8,5 months for octreotide LAR 60 mg monthly. A second pivotal study COMPETE is currently in progress, comparing no carrier-added (n.c.a.) Lu-177-DOTATOC to the m-TOR inhibitor Everolimus in both GE-NETs and PNETs. Two studies, NETTER-2 and COMPOSE are currently underway in patients with high grade G2 and G3 NETs. Novel SSTR antagonists are being developed as next generation targeting molecules for SSTR2-expressing tumors. Antagonists have a higher tumor binding to receptors than agonists, opening up the potential indications for SSTR2 targeting to tumors which have a relatively lower expression of SSTR2 compared to NET such as small cell lung cancer, hepatocellular carcinoma and breast cancer. In addition to Lu-177, radioisotopes with different radiation properties such as Tb-161 and the α-emitter Ac-225 are being developed which have the potential to improve treatment efficacy across the range of G1 to G3 NETs.

Pancreatic neuroendocrine tumors: Therapeutic challenges and research limitations

Pancreatic neuroendocrine tumors (PNETs) are known to be the second most common epithelial malignancy of the pancreas. PNETs can be listed among the slowest growing as well as the fastest growing human cancers. The prevalence of PNETs is deceptively low; however, its incidence has significantly increased over the past decades. According to the American Cancer Society's estimate, about 4032 (> 7% of all pancreatic malignancies) individuals will be diagnosed with PNETs in 2020. PNETs often cause severe morbidity due to excessive secretion of hormones (such as serotonin) and/or overall tumor mass. Patients can live for many years (except for those patients with poorly differentiated G3 neuroendocrine tumors); thus, the prevalence of the tumors that is the number of patients actually dealing with the disease at any given time is fairly high because the survival is much longer than pancreatic ductal adenocarcinoma. Due to significant heterogeneity, the management of PNETs is very complex and remains an unmet clinical challenge. In terms of research studies, modest improvements have been made over the past decades in the identification of potential oncogenic drivers in order to enhance the quality of life and increase survival for this growing population of patients. Unfortunately, the majority of systematic therapies approved for the management of advanced stage PNETs lack objective response or at most result in modest benefits in survival. In this review, we aim to discuss the broad challenges associated with the management and the study of PNETs.

Parallel Signaling through IRE1α and PERK Regulates Pancreatic Neuroendocrine Tumor Growth and Survival

Master regulators of the unfolded protein response (UPR), IRE1α and PERK, promote adaptation or apoptosis depending on the level of endoplasmic reticulum (ER) stress. Although the UPR is activated in many cancers, its effects on tumor growth remain unclear. Derived from endocrine cells, pancreatic neuroendocrine tumors (PanNET) universally hypersecrete one or more peptide hormones, likely sensitizing these cells to high ER protein-folding stress. To assess whether targeting the UPR is a viable therapeutic strategy, we analyzed human PanNET samples and found evidence of elevated ER stress and UPR activation. Genetic and pharmacologic modulation of IRE1α and PERK in cultured cells, xenograft, and spontaneous genetic (RIP-Tag2) mouse models of PanNETs revealed that UPR signaling was optimized for adaptation and that inhibiting either IRE1α or PERK led to hyperactivation and apoptotic signaling through the reciprocal arm, thereby halting tumor growth and survival. These results provide a strong rationale for therapeutically targeting the UPR in PanNETs and other cancers with elevated ER stress. SIGNIFICANCE: The UPR is upregulated in pancreatic neuroendocrine tumors and its inhibition significantly reduces tumor growth in preclinical models, providing strong rationale for targeting the UPR in these cancers.

Not only stem cells, but also mature cells, particularly neuroendocrine cells, may develop into tumours: time for a paradigm shift

Stem cells are considered the origin of neoplasms in general, and malignant tumours in particular, and the stage at which the stem cells stop their differentiation determines the degree of malignancy. However, there is increasing evidence supporting an alternative paradigm. Tumours may develop by dedifferentiation from mature cells able to proliferate. Studies of gastric carcinogenesis demonstrate that mature neuroendocrine (NE) cells upon long-term overstimulation may develop through stages of hyperplasia, dysplasia, and rather benign tumours, into highly malignant carcinomas. Dedifferentiation of cells may change the histological appearance and impede the identification of the cellular origin, as seen with gastric carcinomas, which in many cases are dedifferentiated neuroendocrine tumours. Finding the cell of origin is important to identify risk factors for cancer, prevent tumour development, and tailor treatment. In the present review, we focus not only on gastric tumours, but also evaluate the role of neuroendocrine cells in tumourigenesis in two other foregut-derived organs, the lungs and the pancreas, as well as in the midgut-derived small intestine.

Progesterone arrested cell cycle progression through progesterone receptor isoform A in pancreatic neuroendocrine neoplasm

In pancreatic neuroendocrine neoplasms (Pan-NEN) progesterone signaling has been shown to have both inhibitory and stimulatory effects on cell proliferation. The ability of progesterone to inhibit tumor proliferation is of particular interest and is suggested to be mediated through the less abundantly expressed progesterone receptor (PR) isoform A (PRA). To date the mechanistic processes underlying this inhibition of proliferation remain unclear. To examine the mechanism of PRA actions, the human Pan-NEN cell line QGP-1, that endogenously expresses PR isoform B (PRB) without PRA, was transfected with PRA. PRA transfection suppressed the majority of cell cycle related genes increased by progesterone including cyclin A2 (CCNA2), cyclin B1 (CCNB1), cyclin-dependent kinase 1 (CDK1) and cyclin-dependent kinase 2 (CDK2). Importantly, following progesterone administration cell cycle distribution was shifted to S and G2/M phases in the naïve cell line but in PRA-transfected cells, this effect was suppressed. To see if these mechanistic insights were confirmed in patient samples PRA, PRB, CCNA2, CCNB, CDK1 and CDK2 immunoreactivities were assessed in Pan-NEN cases. Higher levels of cell cycle markers were associated with higher WHO grade tumors and correlations between the markers suggested formation of cyclin/CDK activated complexes in S and G2/M phases. PRA expression was associated with inverse correlation of all cell cycle markers. Collectively, these results indicate that progesterone signals through PRA negatively regulates cell cycle progression through suppressing S and G2/M phases and downregulation of cell cycle phases specific cyclins/CDKs.

Establishment of the First Well-differentiated Human Pancreatic Neuroendocrine Tumor Model

Clinical options for systemic therapy of neuroendocrine tumors (NET) are limited. Development of new drugs requires suitable representative in vitro and in vivo model systems. So far, the unavailability of a human model with a well-differentiated phenotype and typical growth characteristics has impaired preclinical research in NET. Herein, we establish and characterize a lymph node-derived cell line (NT-3) from a male patient with well-differentiated pancreatic NET. Neuroendocrine differentiation and tumor biology was compared with existing NET cell lines BON and QGP-1. In vivo growth was assessed in a xenograft mouse model. The neuroendocrine identity of NT-3 was verified by expression of multiple NET-specific markers, which were highly expressed in NT-3 compared with BON and QGP-1. In addition, NT-3 expressed and secreted insulin. Until now, this well-differentiated phenotype is stable since 58 passages. The proliferative labeling index, measured by Ki-67, of 14.6% ± 1.0% in NT-3 is akin to the original tumor (15%-20%), and was lower than in BON (80.6% ± 3.3%) and QGP-1 (82.6% ± 1.0%). NT-3 highly expressed somatostatin receptors (SSTRs: 1, 2, 3, and 5). Upon subcutaneous transplantation of NT-3 cells, recipient mice developed tumors with an efficient tumor take rate (94%) and growth rate (139% ± 13%) by 4 weeks. Importantly, morphology and neuroendocrine marker expression of xenograft tumors resembled the original human tumor.Implications: High expression of somatostatin receptors and a well-differentiated phenotype as well as a slow growth rate qualify the new cell line as a relevant model to study neuroendocrine tumor biology and to develop new tumor treatments. Mol Cancer Res; 16(3); 496-507. ©2018 AACR.

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.

Patient-derived xenograft in zebrafish embryos: a new platform for translational research in neuroendocrine tumors

Preclinical research on neuroendocrine tumors usually involves immortalized cell lines and few animal models. In the present study we described an in vivo model based on patient-derived xenografts of neuroendocrine tumor cells in zebrafish (Danio rerio) embryos, allowing a rapid analysis of the angiogenic and invasive potential. Patient-derived neuroendocrine tumor cells were transplanted in 48 hours post-fertilization Tg(fli1a:EGFP) ^y1 zebrafish embryos that express enhanced green fluorescent protein in the entire vasculature. Neuroendocrine tumor cells, stained with CM-Dil, were injected into the subperidermal (perivitelline) space, close to the developing subintestinal venous plexus. A proper control group, represented by zebrafish injected with only D-PBS, was included in this study. Angiogenic and invasive potentials of each patient-derived xenograft were evaluated by both epifluorescence and confocal microscopes. Six out of eight neuroendocrine tumor samples were successfully transplanted in zebrafish embryos. Although the implanted tumor mass had a limited size (about 100 cells for embryos), patient-derived xenografts showed pro-angiogenic (5 cases) and invasive (6 cases) behaviors within 48 hours post injection. Patient-derived xenograft in zebrafish embryos appears to be a reliable in vivo preclinical model for neuroendocrine tumors, tumors with often limited cell availability. The rapidity of this procedure makes our model a promising platform to perform preclinical drug screening and opens a new scenario for personalized treatment in patients with neuroendocrine tumors.

The MTH1 inhibitor TH588 demonstrates anti-tumoral effects alone and in combination with everolimus, 5-FU and gamma-irradiation in neuroendocrine tumor cells

Modulation of the redox system in cancer cells has been considered a promising target for anti-cancer therapy. The novel MTH1 inhibitor TH588 proved tremendous potential in terms of cancer cell eradication, yet its specificity has been questioned by recent reports, indicating that TH588 may also induce cancer cell death by alternative mechanisms than MTH1 inhibition. Here we used a panel of heterogeneous neuroendocrine tumor cells in order to assess cellular mechanisms and molecular signaling pathways implicated in the effects of TH588 alone as well as dual-targeting approaches combining TH588 with everolimus, cytotoxic 5-fluorouracil or γ-irradiation. Our results reflect that TH588 alone efficiently decreased the survival of neuroendocrine cancer cells by PI3K-Akt-mTOR axis downregulation, increased apoptosis and oxidative stress. However, in the dual-targeting approaches cell survival was further decreased due to an even stronger downregulation of the PI3K-Akt-mTOR axis and augmentation of apoptosis but not oxidative stress. Furthermore, we could attribute TH588 chemo- and radio-sensitizing properties. Collectively our data not only provide insights into how TH588 exactly kills cancer cells but also depict novel perspectives for combinatorial treatment approaches encompassing TH588.

α-Mangostin-encapsulated PLGA nanoparticles inhibit pancreatic carcinogenesis by targeting cancer stem cells in human, and transgenic (Kras(G12D), and Kras(G12D)/tp53R270H) mice

Activation of sonic hedgehog (Shh) in cancer stem cell (CSC) has been demonstrated with aggressiveness of pancreatic cancer. In order to enhance the biological activity of α-mangostin, we formulated mangostin-encapsulated PLGA nanoparticles (Mang-NPs) and examined the molecular mechanisms by which they inhibit human and KC mice (Pdx^Cre;LSL-Kras^G12D) pancreatic CSC characteristics in vitro, and pancreatic carcinogenesis in KPC (Pdx^Cre;LSLKras^G12D;LSL-Trp53^R172H) mice. Mang-NPs inhibited human and Kras^G12D mice pancreatic CSC characteristics in vitro. Mang-NPs also inhibited EMT by up-regulating E-cadherin and inhibiting N-cadherin and transcription factors Slug, and pluripotency maintaining factors Nanog, c-Myc, and Oct4. Furthermore, Mang-NPs inhibited the components of Shh pathway and Gli targets. In vivo, Mang-NPs inhibited the progression of pancreatic intraneoplasia to pancreatic ductal adenocarcinoma and liver metastasis in KPC mice. The inhibitory effects of Mang-NPs on carcinogenesis in KPC mice were associated with downregulation of pluripotency maintaining factors (c-Myc, Nanog and Oct4), stem cell markers (CD24 and CD133), components of Shh pathway (Gli1, Gli2, Patched1/2, and Smoothened), Gli targets (Bcl-2, XIAP and Cyclin D1), and EMT markers and transcription factors (N-cadherin, Slug, Snail and Zeb1), and upregulation of E-cadherin. Overall, our data suggest that Mang-NPs can inhibit pancreatic cancer growth, development and metastasis by targeting Shh pathway.

Establishment and Characterization of a Human Neuroendocrine Tumor Xenograft

Neuroendocrine tumors (NETs) are increasing in incidence yet the cause of these tumors remains unknown. Familial associations have shed light on the genetic basis of some of these tumors, but sporadic tumors seem to have primarily epigenetic dysregulation. The rarity of cell lines and animal models has been a barrier to studies of treatment modalities. We set out to develop a xenograft model of gastrointestinal NETs. Primary human NETs were collected at the time of surgery under sterile conditions and xenografted into the flanks of immunodeficient mice. Tumor growth was measured and when tumors reached 1500 mm(3), they were excised and half was re-xenografted through multiple generations. The other half was bisected; a part was frozen and a part was fixed for morphologic and immunohistochemical characterization as well as molecular validation of fidelity of a successful xenograft. Of 106 human NETs, seven were successfully engrafted of which only one tumor was successfully propagated for eight passages. Two years later, the tumor retains its neuroendocrine features and similarity to the original primary human tumor. It has retained expression of keratin as well as chromogranin A reactivity. The establishment of a NET xenograft provides a model for further study of the biological behavior of these tumors and can be used to examine the in vivo effects of various medical and targeted radiotherapeutic agents on tumor growth.

Animal models of spontaneous pancreatic neuroendocrine tumors

Pancreatic neuroendocrine tumors (PNETs) are usually low-grade neoplasms derived from the endocrine pancreas. PNETs can be functioning and cause well-described hormonal hypersecretion syndromes or non-functioning and cause only tumor mass effect. PNETs appear to be more common recently likely due to incidental detection by imaging. Although the diagnosis and management of PNETs have been evolving rapidly, much remains to be studied in the areas of molecular pathogenesis, molecular markers of tumor behavior, early detection, and targeted drug therapy. Unique challenges facing PNETs studies are long disease course, the deep location of pancreas and difficult access to pancreatic tissue, and the variety of tumors, which make animal models valuable tools for PNETs studies. Existing animal models of PNETs have provided insights into the pathogenesis and natural history of human PNETs. Future studies on animal models of PNETs should address early tumor detection, molecular markers of tumor behavior, and novel targeted therapies.

Cabozantinib and Tivantinib, but Not INC280, Induce Antiproliferative and Antimigratory Effects in Human Neuroendocrine Tumor Cells in vitro: Evidence for 'Off-Target' Effects Not Mediated by c-Met Inhibition

BACKGROUND/AIMS

The hepatocyte growth factor/transmembrane tyrosine kinase receptor c-Met has been defined as a potential target in antitumoral treatment of various carcinomas. We aimed to investigate the direct effect of c-Met inhibition on neuroendocrine tumor cells in vitro.

METHODS

The effects of the multi-tyrosine kinase inhibitors cabozantinib and tivantinib and of the highly specific c-Met inhibitor INC280 were investigated in human pancreatic neuroendocrine BON1, bronchopulmonary NCI-H727 and midgut GOT1 cells in vitro.

RESULTS

INC280, cabozantinib and tivantinib inhibited c-Met phosphorylation, respectively. However, while equimolar concentrations (10 μM) of cabozantinib and tivantinib inhibited cell viability and cell migration, INC280 had no inhibitory effect. Knockdown experiments with c-Met siRNA also did not demonstrate effects on cell viability. Cabozantinib and tivantinib caused a G2 arrest in neuroendocrine tumor cells.

CONCLUSIONS

Our in vitro data suggest that c-Met inhibition alone is not sufficient to exert direct antitumoral or antimigratory effects in neuroendocrine tumor cells. The multi-tyrosine kinase inhibitors cabozantinib and tivantinib show promising antitumoral and antimigratory effects in neuroendocrine tumor cells, which are most probably 'off-target' effects, not mediated by c-Met.

Elusive liver factor that causes pancreatic α cell hyperplasia: A review of literature

Tumors and cancers of the gastrointestinal tract and pancreas are commonly derived from precursor lesions so that understanding the physiological, cellular, and molecular mechanisms underlying the pathogenesis of precursor lesions is critical for the prevention and treatment of those neoplasms. Pancreatic neuroendocrine tumors (PNETs) can also be derived from precursor lesions. Pancreatic α cell hyperplasia (ACH), a specific and overwhelming increase in the number of α cells, is a precursor lesion leading to PNET pathogenesis. One of the 3 subtypes of ACH, reactive ACH is caused by glucagon signaling disruption and invariably evolves into PNETs. In this article, the existing work on the mechanisms underlying reactive ACH pathogenesis is reviewed. It is clear that the liver secretes a humoral factor regulating α cell numbers but the identity of the liver factor remains elusive. Potential approaches to identify the liver factor are discussed.