The axon guidance molecule semaphorin 3F is a negative regulator of tumor progression and proliferation in ileal neuroendocrine tumors.

Gastro-intestinal neuroendocrine tumors (GI-NETs) are rare neoplasms, frequently metastatic, raising difficult clinical and therapeutic challenges due to a poor knowledge of their biology. As neuroendocrine cells express both epithelial and neural cell markers, we studied the possible involvement in GI-NETs of axon guidance molecules, which have been shown to decrease tumor cell proliferation and metastatic dissemination in several tumor types. We focused on the role of Semaphorin 3F (SEMA3F) in ileal NETs, one of the most frequent subtypes of GI-NETs.SEMA3F expression was detected in normal neuroendocrine cells but was lost in most of human primary tumors and all their metastases. SEMA3F loss of expression was associated with promoter gene methylation. After increasing endogenous SEMA3F levels through stable transfection, enteroendocrine cell lines STC-1 and GluTag showed a reduced proliferation rate in vitro. In two different xenograft mouse models, SEMA3F-overexpressing cells exhibited a reduced ability to form tumors and a hampered liver dissemination potential in vivo. This resulted, at least in part, from the inhibition of mTOR and MAPK signaling pathways.This study demonstrates an anti-tumoral role of SEMA3F in ileal NETs. We thus suggest that SEMA3F and/or its cellular signaling pathway could represent a target for ileal NET therapy.

Islet Cells Serve as Cells of Origin of Pancreatic Gastrin-Positive Endocrine Tumors.

The cells of origin of pancreatic gastrinomas remain an enigma, since no gastrin-expressing cells are found in the normal adult pancreas. It was proposed that the cellular origin of pancreatic gastrinomas may come from either the pancreatic cells themselves or gastrin-expressing cells which have migrated from the duodenum. In the current study, we further characterized previously described transient pancreatic gastrin-expressing cells using cell lineage tracing in a pan-pancreatic progenitor and a pancreatic endocrine progenitor model. We provide evidence showing that pancreatic gastrin-expressing cells, found from embryonic day 12.5 until postnatal day 7, are derived from pancreatic Ptf1a(+) and neurogenin 3-expressing (Ngn3(+)) progenitors. Importantly, the majority of them coexpress glucagon, with 4% coexpressing insulin, indicating that they are a temporary subpopulation of both alpha and beta cells. Interestingly, Men1 disruption in both Ngn3 progenitors and beta and alpha cells resulted in the development of pancreatic gastrin-expressing tumors, suggesting that the latter developed from islet cells. Finally, we detected gastrin expression using three human cohorts with pancreatic endocrine tumors (pNETs) that have not been diagnosed as gastrinomas (in 9/34 pNETs from 6/14 patients with multiple endocrine neoplasia type 1, in 5/35 sporadic nonfunctioning pNETs, and in 2/20 sporadic insulinomas), consistent with observations made in mouse models. Our work provides insight into the histogenesis of pancreatic gastrin-expressing tumors.

Mechanisms of local invasion in enteroendocrine tumors: identification of novel candidate cytoskeleton-associated proteins in an experimental mouse model by a proteomic approach and validation in human tumors.

Small-intestinal neuroendocrine tumors (SI-NETs) are defined as locally invasive only after extension to the muscularis propria. To gain further insight into the molecular mechanisms, we applied a proteomic approach to an orthotopic xenograft model to identify candidate proteins evaluable in human SI-NETs. After grafting STC-1 neuroendocrine tumor cells on the caecum of nude mice, comparative proteomic studies were performed between the pre-invasive and the invasive stages, respectively 2 and 8 weeks after grafting. We identified 24 proteins displaying at least a 1.5-fold differential expression between 2 and 8 week-stages. Most were cytoskeleton-associated proteins, among which five showed decreasing expression levels (CRMP2, TCP1ε, TPM2, vimentin, desmin) and two increasing expression levels (14-3-3γ, CK8). Changes for CRMP2, TCP1ε, TPM2 and 14-3-3γ were confirmed in experimental tumors and in a series of 28 human SI-NETs. In conclusion, our results underline the relevance of proteomics to identify novel biomarkers of tissue invasion.

TRF2 inhibits a cell-extrinsic pathway through which natural killer cells eliminate cancer cells.

Dysfunctional telomeres suppress tumour progression by activating cell-intrinsic programs that lead to growth arrest. Increased levels of TRF2, a key factor in telomere protection, are observed in various human malignancies and contribute to oncogenesis. We demonstrate here that a high level of TRF2 in tumour cells decreased their ability to recruit and activate natural killer (NK) cells. Conversely, a reduced dose of TRF2 enabled tumour cells to be more easily eliminated by NK cells. Consistent with these results, a progressive upregulation of TRF2 correlated with decreased NK cell density during the early development of human colon cancer. By screening for TRF2-bound genes, we found that HS3ST4--a gene encoding for the heparan sulphate (glucosamine) 3-O-sulphotransferase 4--was regulated by TRF2 and inhibited the recruitment of NK cells in an epistatic relationship with TRF2. Overall, these results reveal a TRF2-dependent pathway that is tumour-cell extrinsic and regulates NK cell immunity.

Antitumor effect of everolimus in preclinical models of high-grade gastroenteropancreatic neuroendocrine carcinomas.

BACKGROUND/AIMS: While the range of therapeutic options for well-differentiated gastroenteropancreatic neuroendocrine tumors has recently increased with the emergence of targeted therapies, such as mTOR inhibitors, there is no recent progress in the treatment of poorly differentiated neuroendocrine carcinomas (PDNECs). Since PDNECs have been shown to strongly express mTOR pathway components, the aim of the present study was to assess the antitumor effect of the mTOR inhibitor everolimus in preclinical models of PDNECs. METHODS: The expression of mTOR pathway components and their response to everolimus were assessed in two neuroendocrine cell lines: STC-1 and GluTag. A xenograft model of intrahepatic dissemination in the nude mouse, based on the intrasplenic injection of either STC-1 and GluTag tumor cells, was used. Animals were started on everolimus treatment 3 days after injection. The effects of treatment on tumor growth, proliferative capacities, apoptosis and in situ expression of mTOR pathway components were assessed. RESULTS: The expression of mTOR pathway components was comparable in STC-1 and GluTag cells and in human PDNECs and could be inhibited in vitro by everolimus. In vivo, the tumor volume of STC-1 and GluTag xenografts was significantly reduced in treated animals (6.05 ± 1.84% as compared to 21.76 ± 3.88% in controls). Everolimus treatment also induced a significant decrease in Ki67 index and in the phosphorylation levels of the two major effectors of mTOR, p70S6K and 4E-BP1. CONCLUSION: Our experimental data suggest that mTOR inhibition could be considered a therapeutic option for high-grade gastroenteropancreatic neuroendocrine tumors.

Dependence receptor TrkC is a putative colon cancer tumor suppressor.

The TrkC neurotrophin receptor belongs to the functional dependence receptor family, members of which share the ability to induce apoptosis in the absence of their ligands. Such a trait has been hypothesized to confer tumor-suppressor activity. Indeed, cells that express these receptors are thought to be dependent on ligand availability for their survival, a mechanism that inhibits uncontrolled tumor cell proliferation and migration. TrkC is a classic tyrosine kinase receptor and therefore generally considered to be a proto-oncogene. We show here that TrkC expression is down-regulated in a large fraction of human colorectal cancers, mainly through promoter methylation. Moreover, we show that TrkC silencing by promoter methylation is a selective advantage for colorectal cell lines to limit tumor cell death. Furthermore, reestablished TrkC expression in colorectal cancer cell lines is associated with tumor cell death and inhibition of in vitro characteristics of cell transformation, as well as in vivo tumor growth. Finally, we provide evidence that a mutation of TrkC detected in a sporadic cancer is a loss-of-proapoptotic function mutation. Together, these data support the conclusion that TrkC is a colorectal cancer tumor suppressor.

Targeting the PI3K/mTOR pathway in murine endocrine cell lines: in vitro and in vivo effects on tumor cell growth.

The mammalian target of rapamycin (mTOR) inhibitors, such as rapalogues, are a promising new tool for the treatment of metastatic gastroenteropancreatic endocrine tumors. However, their mechanisms of action remain to be established. We used two murine intestinal endocrine tumoral cell lines, STC-1 and GLUTag, to evaluate the antitumor effects of rapamycin in vitro and in vivo in a preclinical model of liver endocrine metastases. In vitro, rapamycin inhibited the proliferation of cells in the basal state and after stimulation by insulin-like growth factor-1. Simultaneously, p70S6 kinase and 4EBP1 phosphorylation was inhibited. In vivo, rapamycin substantially inhibited the intrahepatic growth of STC-1 cells, irrespectively of the timing of its administration and even when the treatment was administered after cell intrahepatic engraftment. In addition, treated animals had significantly prolonged survival (mean survival time: 47.7 days in treated animals versus 31.8 days in controls) and better clinical status. Rapamycin treatment was associated with a significant decrease in mitotic index and in intratumoral vascular density within STC-1 tumors. Furthermore, the antitumoral effect obtained after treatment with a combination of rapamycin and phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 was more significant than with rapamycin alone in both cell lines. Our results suggest that the antitumor efficacy of rapamycin in neuroendocrine tumors results from a combination of antiproliferative and antiangiogenic effects. Interestingly, a more potent antitumor efficiency could be obtained by simultaneously targeting several levels of the PI3K/mTOR pathway.

Neuroendocrine tumors of Meckel's diverticulum: lessons from a single institution study of eight cases.

INTRODUCTION: Endocrine tumors of Meckel's diverticulum are rare. Their clinical and pathological characteristics are not well known, making it difficult to assess the best strategy for therapeutic management. MATERIALS AND METHODS: Eight cases of endocrine tumors of Meckel's diverticulum, submitted to surgical resection in our institution between 1977 and 2009, were studied. Clinical charts were reviewed; classification, grading, and staging were performed according to recent international recommendations. Five cases, including two associated with the carcinoid syndrome, were revealed by mesenteric mass or liver metastases; three cases were diagnosed incidentally at laparotomy or laparoscopy. RESULTS: All cases presented as typical well-differentiated midgut endocrine tumors. Five cases were associated with mesenteric lymph node metastases; three presented with liver metastases. Seven cases were classified as well-differentiated endocrine carcinomas, one as well-differentiated endocrine tumor of benign behavior. DISCUSSION: All tumors >1 cm, but one, had regional or distant disease. All patients had complete surgical resection of the primary. One patient deceased after 25 months; the others were alive after 12-101 months. CONCLUSION: In conclusion, endocrine tumors of Meckel's diverticulum are rarely symptomatic and often diagnosed at an advanced stage. All tumors measuring more than 1 cm in diameter must be resected according to oncological principles.

Netrin-1 up-regulation in inflammatory bowel diseases is required for colorectal cancer progression.

Chronic inflammation and cancer are intimately associated. This is particularly true for inflammatory bowel diseases (IBD), such as ulcerative colitis and Crohn's disease, which show a major increased risk for colorectal cancer. While the understanding of the molecular pathogenesis of IBD has recently improved, the mechanisms that link these chronic inflammatory states to colorectal cancer development are in large part unknown. One of these mechanisms is NF-kappaB pathway activation which in turn may contribute to tumor formation by providing anti-apoptotic survival signals to the epithelial cells. Based on the observation that netrin-1, the anti-apoptotic ligand for the dependence receptors DCC and UNC5H is up-regulated in colonic crypts in response to NF-kappaB, we show here that colorectal cancers from inflammatory bowel diseases patients have selected up-regulation of netrin-1. Moreover, we demonstrate that this inflammation-driven netrin-1 up-regulation is causal for colorectal cancer development as interference with netrin-1 autocrine loop in a mouse model for ulcerative colitis-associated colorectal cancer, while showing no effect on inflammation, inhibits colorectal cancer progression.

Inactivation of TIF1gamma cooperates with Kras to induce cystic tumors of the pancreas.

Inactivation of the Transforming Growth Factor Beta (TGFbeta) tumor suppressor pathway contributes to the progression of Pancreatic Ductal AdenoCarcinoma (PDAC) since it is inactivated in virtually all cases of this malignancy. Genetic lesions inactivating this pathway contribute to pancreatic tumor progression in mouse models. Transcriptional Intermediary Factor 1 gamma (TIF1gamma) has recently been proposed to be involved in TGFbeta signaling, functioning as either a positive or negative regulator of the pathway. Here, we addressed the role of TIF1gamma in pancreatic carcinogenesis. Using conditional Tif1gamma knockout mice (Tif1gamma(lox/lox)), we selectively abrogated Tif1gamma expression in the pancreas of Pdx1-Cre;Tif1gamma(lox/lox) mice. We also generated Pdx1-Cre;LSL-Kras(G12D);Tif1gamma(lox/lox) mice to address the effect of Tif1gamma loss-of-function in precancerous lesions induced by oncogenic Kras(G12D). Finally, we analyzed TIF1gamma expression in human pancreatic tumors. In our mouse model, we showed that Tif1gamma was dispensable for normal pancreatic development but cooperated with Kras activation to induce pancreatic tumors reminiscent of human Intraductal Papillary Mucinous Neoplasms (IPMNs). Interestingly, these cystic lesions resemble those observed in Pdx1-Cre;LSL-Kras(G12D);Smad4(lox/lox) mice described by others. However, distinctive characteristics, such as the systematic presence of endocrine pseudo-islets within the papillary projections, suggest that SMAD4 and TIF1gamma don't have strictly redundant functions. Finally, we report that TIF1gamma expression is markedly down-regulated in human pancreatic tumors by quantitative RT-PCR and immunohistochemistry supporting the relevance of these findings to human malignancy. This study suggests that TIF1gamma is critical for tumor suppression in the pancreas, brings new insight into the genetics of pancreatic cancer, and constitutes a promising model to decipher the respective roles of SMAD4 and TIF1gamma in the multifaceted functions of TGFbeta in carcinogenesis and development.

Angiogenesis and tumor progression in neuroendocrine digestive tumors.

BACKGROUND: Clinical observations suggest that in neuroendocrine digestive tumors a high intratumoral microvascular density is associated with good prognosis. We used an experimental orthotopic xenograft model to analyze the relations between angiogenic activity and tumor progression in this tumor subset. MATERIAL AND METHODS: We compared 2 endocrine cell lines: STC-1, a low vascular endothelial growth factor (VEGF)-producing cell line, and INS-r3, a high VEGF-producing cell line. Tumor cells were grafted in the adventitial layer of the caecal wall of nude mice, sacrificed after 8 wk. RESULTS: At 8 wk, "primary" tumors were present in all animals. STC-1 derived tumors were morphologically moderately differentiated, with high proliferative and apoptotic activities; in contrast, INS-r3 derived tumors were well differentiated, with low proliferative and apoptotic activities. VEGF was expressed in <50% grafted STC-1 cells but in >90% of grafted INS-r3 cells. Microvascular density was significantly higher in INS-r3 derived tumors than in STC-1 derived tumors. All STC-1 derived tumors (n = 8) have invaded the mucosa, in contrast to none of the INS-r3 derived tumors (n = 8); liver metastases were detected in 7/8 animals bearing STC-1 derived tumors and in 0/8 animals with INS-r3 derived tumors, despite the presence of lymph node metastases. CONCLUSIONS: Our experimental data concur with clinical findings to suggest that in well differentiated digestive neuroendocrine tumors angiogenesis is disconnected from tumor progression: the development of a highly vascular tumor microenvironment is correlated with VEGF secretion but is not associated with invasive and metastatic properties; it must therefore be regarded as an indirect marker of differentiation.