Background The Multiple Endocrine Neoplasia, type 1 (MEN1) locus encodes the nuclear tumor suppressor protein menin. MEN1 mutations frequently cause neuroendocrine tumors (NETs) such as gastrinomas, remarkable for their predominant duodenal location and local metastasis at the time of diagnosis. Diffuse gastrin cell hyperplasia precedes the appearance of MEN1 gastrinomas, which develop within submucosal Brunner’s glands. We show here that loss of menin in enteric glial cells induces gastrin expression. Aim To determine how menin regulates gastrin gene expression and induces the generation of submucosal gastrin-expressing cell hyperplasia. Methods Primary enteric glial cultures were generated from the Villin-Cre:Men1FL/FL:Sst−/− mice with or without inhibition of gastric acid using omeprazole. In addition, primary enteric glial cells from wild type mice were treated with gastrin and were separated into nuclear and cytoplasmic fractions. Forskolin and H89 treatments were used to activate or inhibit protein kinase A activity. Immunoprecipitation with menin or ubiquitin was used to demonstrate posttranslational modification of menin. Primary glial cells were treated with Leptomycin b and MG132 to block nuclear export and proteasome activity, respectively. Results Gfap+ enteric glial cells expressed gastrin de novo through a feedforward PKA-dependent mechanism. Gastrin-induced nuclear export of menin through Cckbr-mediated PKA activation. Once exported menin was ubiquitinated and degraded by the proteasome. Gfap and other enteric glial markers co-localized with gastrin in human duodenal gastrinomas. Conclusion Collectively, these results suggest that MEN1-associated gastrinomas, which develop in the submucosa might arise from enteric glial cells through hormone-dependent PKA signaling that abrogates menin function leading to hypergastrinemia and associated sequelae.

Gastric carcinoids are slow growing neuroendocrine tumors arising from enterochromaffin-like (ECL) cells in the corpus of stomach. Although most of these tumors arise in the setting of gastric atrophy and hypergastrinemia, it is not understood what genetic background predisposes development of these ECL derived tumors. Moreover, diffuse microcarcinoids in the mucosa can lead to a field effect and limit successful endoscopic removal. Objective To define the genetic background that creates a permissive environment for gastric carcinoids using transgenic mouse lines. Design: The multiple endocrine neoplasia 1 gene locus (Men1) was deleted using Cre recombinase expressed from the Villin promoter (Villin-Cre) and was placed on a somatostatin null genetic background. These transgenic mice received omeprazole-laced chow for 6 months. The direct effect of gastrin and the gastrin receptor antagonist YM022 on expression and phosphorylation of the cyclin inhibitor p27Kip1 was tested on the human AGSE and mouse STC-1 cell lines. Results The combination of conditional Men1 deletion in the absence of somatostatin lead to the development of gastric carcinoids within 2 years. Suppression of acid secretion by omeprazole accelerated the timeline of carcinoid development to 6 months in the absence of significant parietal cell atrophy. Carcinoids were associated with hypergastrinemia, and correlated with increased Cckbr expression and nuclear export of p27Kip1 both in vivo and in gastrin-treated cell lines. Loss of p27Kip1 was also observed in human gastric carcinoids arising in the setting of atrophic gastritis. Conclusion Gastric carcinoids require threshold levels of hypergastrinemia, which modulates p27Kip1 cellular location and stability.

In colorectal cancer (CRC), APC-mediated induction of unregulated cell growth involves post-translational mechanisms that prevent proteasomal degradation of proto-oncogene β-catenin (CTNNB1) and its eventual translocation to the nucleus. However, about 10 percent of colorectal tumors also exhibit increased CTNNB1 mRNA. Here we show in CRC that increased expression of ZNF148, the gene coding for transcription factor ZBP-89, correlated with reduced patient survival. Tissue arrays showed that ZBP-89 protein was overexpressed in the early stages of CRC. Conditional deletion of Zfp148 in a mouse model of Apc-mediated intestinal polyps demonstrated that ZBP-89 was required for polyp formation due to induction of Ctnnb1 gene expression. ChIP and EMSA identified a ZBP-89 binding site in the proximal promoter of CTNNB1. Recipricolly, siRNA-mediated reduction of CTNNB1 expression also decreased ZBP-89 protein. ChIP identified TCF DNA binding sites in the ZNF148 promoter through which Wnt signaling regulates ZNF148 gene expression. Suppression of either ZNF148 or CTNNB1 reduced colony formation in WNT-dependent, but not WNT-independent cell lines. Therefore, the increase in intracellular β–catenin protein initiated by APC mutations is sustained by ZBP-89-mediated feedforward induction of CTNNB1 mRNA.

Neuroendocrine tumors (NETs) were initially identified as a separate entity in the early 1900s as a unique malignancy that secretes bioactive amines. GI-NETs are the most frequent type, and represent a unique subset of NETs because at least 75% of these tumors represent gastrin stimulation of the enterochromaffin-like cell located in the body of the stomach. Purpose of Review To understand the specific role of gastrin in the generation of Gastric NETs (G-NETs). Recent Findings We review here the origin of enterochromaffin cells gut and the role of hypergastrinemia in gastric enteroendocrine tumorigenesis. We describe generation of the first genetically engineered mouse model of gastrin-driven G-NETs that mimics the human phenotype. The common mechanism observed in both the hypergastrinemic mouse model and human carcinoids is translocation of the cyclin-dependent inhibitor p27kip to the cytoplasm and its subsequent degradation by the proteasome. Summary Therapies that block degradation of p27kip, the CCKBR2 gastrin receptor or gastrin peptide are likely to facilitate treatment.

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