Biallelic inactivation of MEN1 encoding menin in pancreatic neuroendocrine tumors (PNETs) associated with the multiple endocrine neoplasia type 1 (MEN1) syndrome is well established, but how menin loss/inactivation initiates tumorigenesis is not well understood. We show that menin activates the long noncoding RNA maternally expressed gene 3 (Meg3) by histone-H3 lysine-4 trimethylation and CpG hypomethylation at the Meg3 promoter CRE site, to allow binding of the transcription factor cAMP response element-binding protein. We found that Meg3 has tumor-suppressor activity in PNET cells because the overexpression of Meg3 in MIN6 cells (insulin-secreting mouse PNET cell line) blocked cell proliferation and delayed cell cycle progression. Gene expression microarray analysis showed that Meg3 overexpression in MIN6 mouse insulinoma cells down-regulated the expression of the protooncogene c-Met (hepatocyte growth factor receptor), and these cells showed significantly reduced cell migration/invasion. Compared with normal islets, mouse or human MEN1-associated PNETs expressed less MEG3 and more c-MET. Therefore, a tumor-suppressor long noncoding RNA (MEG3) and suppressed protooncogene (c-MET) combination could elicit menin's tumor-suppressor activity. Interestingly, MEG3 and c-MET expression was also altered in human sporadic insulinomas (insulin secreting PNETs) with hypermethylation at the MEG3 promoter CRE-site coinciding with reduced MEG3 expression. These data provide insights into the β-cell proliferation mechanisms that could retain their functional status. Furthermore, in MIN6 mouse insulinoma cells, DNA-demethylating drugs blocked cell proliferation and activated Meg3 expression. Our data suggest that the epigenetic activation of lncRNA MEG3 and/or inactivation of c-MET could be therapeutic for treating PNETs and insulinomas.

The molecular pathogenesis of sporadic parathyroid adenomas is incompletely understood. The possible role of cyclin-dependent kinase inhibitor (CDKI) genes was raised by recognition of cyclin D1 as a parathyroid oncogene, identification of rare germline mutations in CDKI genes in patients with multiple endocrine neoplasia type 1; that in rodents, mutation in Cdknlb caused parathyroid tumors; and subsequently through identification of rare predisposing germline sequence variants and somatic mutation of CDKN1B, encoding p27kip1, in sporadic human parathyroid adenoma. We therefore sought to determine whether mutations/variants in the other six CDKI genes CDKN1A, CDKN1C, CDKN2A, CDKN2B, CDKN2C, and CDKN2D, encoding p21, p57, p14ARF/p16, p15, p18, and p19, respectively, contribute to the development of typical parathyroid adenomas. In a series of 85 sporadic parathyroid adenomas, direct DNA sequencing identified alterations in five adenomas (6 %): Two contained distinct heterozygous changes in CDKN1A, one germline and one of undetermined germline status; one had a CDKN2B germline alteration, accompanied by loss of the normal allele in the tumor (LOH); two had variants of CDKN2C, one somatic and one germline with LOH. Abnormalities of three of the mutant proteins were readily demonstrable in vitro. Thus, germline mutations/rare variants in CDKN1A, CDKN2B, and CDKN2C likely contribute to the development of a significant subgroup of common sporadic parathyroid adenomas, and somatic mutation in CDKN2C further suggests a direct role for CDKI alteration in conferring a selective growth advantage to parathyroid cells, providing novel support for the concept that multiple CDKIs can play primary roles in human neoplasia.

5-Androstenediol (5-AED) stimulates hematopoiesis and enhances survival in animals exposed to ionizing radiation (IR), suggesting that this steroid may act on hematopoietic progenitor cells. We used ␥-irradiated primary human CD34 ϩ hematopoietic progenitor cells to show that 5-AED protects hematopoietic cells from IR damage, as shown by enhanced cell survival, clonogenicity, proliferation, and differentiation. Unlike in tumor cells, IR did not induce nuclear factor-B (NFB) activation in primary progenitors. However, IR stimulated IB ␤ release from NFB/IB complexes and caused NFB1 (p50) degradation. 5-AED stabilized NFB1 in irradiated cells and induced NFB gene expression and NFB activation (DNA binding). 5-AED stimulated interleukin-6 and granulocyte colony-stimulating factor (G-CSF) secretion. The survival-enhancing effects of 5-AED on clonogenic cells were abrogated by small interfering RNA inhibition of NFB gene expression and by neutralization of G-CSF with antibody.

The multiple endocrine neoplasia type 1 (MEN1) syndrome is caused by germline mutations in the MEN1 gene encoding menin, with tissue-specific tumors of the parathyroids, anterior pituitary, and enteropancreatic endocrine tissues. Also, 30–40% of sporadic pancreatic endocrine tumors show somatic MEN1 gene inactivation. Although menin is expressed in all cell types of the pancreas, mouse models with loss of menin in either pancreatic α-cells, or β-cells, or total pancreas develop β-cell-specific endocrine tumors (insulinomas). Loss of widely expressed tumor suppressor genes may produce tissue-specific tumors by reactivating one or more embryonic-specific differentiation factors. Therefore, we determined the effect of menin overexpression or knockdown on the expression of β-cell differentiation factors in a mouse β-cell line (MIN6). We show that the β-cell differentiation factor Hlxb9 is posttranscriptionally upregulated upon menin knockdown, and it interacts with menin. Hlxb9 reduces cell proliferation and causes apoptosis in the presence of menin, and it regulates genes that modulate insulin level. Thus, upon menin loss or from other causes, dysregulation of Hlxb9 predicts a possible combined mechanism for β-cell proliferation and insulin production in insulinomas. These observations help to understand how a ubiquitously expressed protein such as menin might control tissue-specific tumorigenesis. Also, our findings identify Hlxb9 as an important factor for β-cell proliferation andinsulin regulation.

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Background: Germ line heterozygous loss of the MEN1 tumor suppressor gene causes tissue-specific tumors such as insulinomas. Results: GSK-3␤ and GSK-3␤-mediated phosphorylation of HLXB9, a ␤-cell differentiation factor, is elevated in insulinomas, and GSK-3␤ inhibition blocks insulinoma cell proliferation. Conclusion: GSK-3␤ and phospho-HLXB9 form a targetable mechanism of insulinoma pathogenesis. Significance: Reactivation of a tissue-specific differentiation factor accounts for tumor tissue specificity.