Gastrin Induces Nuclear Export and Proteasome Degradation of Menin in Enteric Glial Cells.

BACKGROUND & AIMS: The multiple endocrine neoplasia, type 1 (MEN1) locus encodes the nuclear protein and tumor suppressor menin. MEN1 mutations frequently cause neuroendocrine tumors such as gastrinomas, characterized by their predominant duodenal location and local metastasis at time of diagnosis. Diffuse gastrin cell hyperplasia precedes the appearance of MEN1 gastrinomas, which develop within submucosal Brunner's glands. We investigated how menin regulates expression of the gastrin gene and induces generation of submucosal gastrin-expressing cell hyperplasia. METHODS: Primary enteric glial cultures were generated from the VillinCre:Men1FL/FL:Sst-/- mice or C57BL/6 mice (controls), with or without inhibition of gastric acid by omeprazole. Primary enteric glial cells from C57BL/6 mice were incubated with gastrin and separated into nuclear and cytoplasmic fractions. Cells were incubated with forskolin and H89 to activate or inhibit protein kinase A (a family of enzymes whose activity depends on cellular levels of cyclic AMP). Gastrin was measured in blood, tissue, and cell cultures using an ELISA. Immunoprecipitation with menin or ubiquitin was used to demonstrate post-translational modification of menin. Primary glial cells were incubated with leptomycin b and MG132 to block nuclear export and proteasome activity, respectively. We obtained human duodenal, lymph node, and pancreatic gastrinoma samples, collected from patients who underwent surgery from 1996 through 2007 in the United States or the United Kingdom. RESULTS: Enteric glial cells that stained positive for glial fibrillary acidic protein (GFAP+) expressed gastrin de novo through a mechanism that required PKA. Gastrin-induced nuclear export of menin via cholecystokinin B receptor (CCKBR)-mediated activation of PKA. Once exported from the nucleus, menin was ubiquitinated and degraded by the proteasome. GFAP and other markers of enteric glial cells (eg, p75 and S100B), colocalized with gastrin in human duodenal gastrinomas. CONCLUSIONS: MEN1-associated gastrinomas, which develop in the submucosa, might arise from enteric glial cells through hormone-dependent PKA signaling. This pathway disrupts nuclear menin function, leading to hypergastrinemia and associated sequelae.

Cellular proliferative fraction measured with topoisomerase IIalpha predicts malignancy in endocrine pancreatic tumors.

CONTEXT: Endocrine pancreatic tumors (EPTs) are rare lesions with varying biological behavior. Establishing malignancy is a challenge for clinicians and pathologists. OBJECTIVE: To establish the role of proliferative, apoptotic, angiogenic, and hormonal markers as predictors of malignancy in EPTs. DESIGN: Paraffin-embedded EPT samples were studied for prognostic markers. PATIENTS: Twenty-one consecutive patients with a diagnosis of EPT. MAIN OUTCOME MEASURES: The proliferative fraction (topoisomerase IIalpha), microvascular density (CD34), vascular endothelial growth factor expression, and estrogen receptor-beta (ERbeta) expression were studied by immunohistochemistry on all EPTs. Apoptosis was also assessed with terminal deoxynucleotidyl transferase nick-end labeling. RESULTS: We identified 13 benign and 8 malignant tumors. Topoisomerase IIalpha was significantly increased in malignant tumors (P =.001), while there were no differences in apoptosis, microvascular density, or vascular endothelial growth factor expression in association with malignancy. No correlation could be identified between microvascular density and vascular endothelial growth factor expression, and ERbeta was not detected. A receiver operating characteristic curve for topoisomerase IIalpha disclosed that above a labeling index of 13, the test had 88% sensitivity and 100% specificity for predicting malignancy. CONCLUSION: Cellular proliferation measured with topoisomerase IIalpha is a simple prognostic marker for malignancy in EPTs, unlike apoptosis, angiogenesis, or the presence of ERbeta, which were not associated with malignant behavior. These findings designate a defined field for future research on endocrine pancreatic carcinogenesis and a possible target for chemotherapeutic agents.

Immunocytochemical localization of prohormone convertase 1/3 and 2 in pancreatic islet cells and islet cell tumors.

Peptide hormones are synthesized as bigger prohormones, which are processed posttranslationally into smaller active hormones. Proinsulin and proglucagon are processed into insulin and glucagon by prohormone convertase (PC) 1/3 and 2. The current study was performed to test a hypothesis that there may be some difference in immunoreactive PC levels between normal islet cells and islet cell tumors, as the latter contain more prohormones than the former. All islet cell tumors, including insulinomas, gastrinomas, glucagonomas, pancreatic polypeptide-omas (PP-omas), and nonfunctioning islet cell tumors, contain fewer PCs than normal islet cells. The smaller PC levels in islet cell tumors may be responsible for the higher levels of prohormones in islet cell tumors, and the smaller levels of PCs in islet cell tumors may be another distinguishing characteristic of islet cell tumors.

In vivo demonstration of enzyme activity in endocrine pancreatic tumors: decarboxylation of carbon-11-DOPA to carbon-11-dopamine.

METHODS: We used PET to characterize the uptake and decarboxylation of 11C-L-DOPA in vivo in two patients with endocrine pancreatic tumors: one glucagonoma and one gastrinoma. RESULTS: With L-DOPA labeled with 11C in the beta position, in which the radioactive label follows the molecule through decarboxylation to dopamine, significant uptake was observed in the tumors. With L-DOPA labeled in the carboxyl group, in which the label is rapidly eliminated from the tissue as 11CO2 if decarboxylation takes place, an almost complete lack of uptake is noted. CONCLUSION: This study shows that, using selective position labeling, an in vivo action of enzymatic activity can be observed with PET and that significant decarboxylation occurs in the tested endocrine pancreatic tumors. Also, marked retention of radioactivity occurs after treatment with somatostatin analogs. It is hypothesized that this is a reflection of a reduction of exocytosis which is induced by this treatment.