Menin promotes the Wnt signaling pathway in pancreatic endocrine cells.

Menin is a tumor suppressor protein mutated in patients with multiple endocrine neoplasia type 1. We show that menin is essential for canonical Wnt/beta-catenin signaling in cultured rodent islet tumor cells. In these cells, overexpression of menin significantly enhances TCF gene assay reporter activity in response to beta-catenin activation. Contrastingly, inhibition of menin expression with Men1 siRNA decreases TCF reporter gene activity. Likewise, multiple endocrine neoplasia type 1 disease associated missense mutations of menin abrogate the ability to increase TCF reporter gene activity. We show that menin physically interacts with proteins involved in the canonical Wnt signaling pathway, including beta-catenin, TCF3 (TCFL1), and weakly with TCF4 (TCFL2). Menin overexpression increases expression of the Wnt/beta-catenin downstream target gene Axin2, which is associated with increased H3K4 trimethylation of the Axin2 gene promoter. Moreover, inhibition of menin expression by siRNA abrogates H3K4 trimethylation and Axin2 gene expression. Based on these studies, we hypothesized that Wnt signaling could inhibit islet cell proliferation because loss of menin function is thought to increase endocrine tumor cell proliferation. TGP61 rodent islet tumor cells treated with a glycogen synthase kinase 3beta inhibitor that increases Wnt pathway signaling had decreased cell proliferation compared with vehicle-treated cells. Collectively, these data suggest that menin has an essential role in Wnt/beta-catenin signaling through a mechanism that eventually affects histone trimethylation of the downstream target gene Axin2, and activation of Wnt/beta-catenin signaling inhibits islet tumor cell proliferation.

Menin localizes to chromatin through an ATR-CHK1 mediated pathway after UV-induced DNA damage.

BACKGROUND: Menin is the tumor suppressor protein product of the gene identified in MEN1 syndrome. Evidence suggests menin binds DNA and interacts with proteins implicated in DNA damage pathways. The canonical cellular response to UV-induced DNA damage involves activation of the ataxia-telangiectasia-mutated and Rad3-related (ATR) kinase pathway. MATERIALS AND METHODS: HEK293 cells were irradiated in a UV chamber. Menin's cellular location before and after UV irradiation was investigated by extracting four separate cellular components--a soluble, two chromatin and a nuclear matrix. To block the ATR pathway, we treated with 5 microM of caffeine for 1 h before irradiation. The ATR pathway was further investigated by transiently transfecting HEK293 cells with two mammalian CHK1 expression constructs--full length CHK1 and truncated active CHK1. RESULTS: A 24-h post UV-irradiation time course was studied and demonstrated menin concentration in the chromatin peaked at 4 h. At 4 h post-irradiation, menin concentration in the chromatin increased in a dose dependent manner and demonstrated a 2.8-fold maximal increase. HEK293 cells were pretreated with caffeine, an inhibitor of the ATR. Caffeine decreased menin localization to the chromatin after UV. Constitutively active CHK1 (1-365) transfection increased chromatin-bound menin, mimicking UV irradiation. CONCLUSIONS: Menin localizes to the chromatin after UV irradiation. Caffeine blocks menin localization to the chromatin after UV-irradiation. Over expressing active CHK1 (1-365) increased chromatin-bound menin, similar to UV. The data suggest menin localization to chromatin after UV irradiation is the result of an ATR-CHK1 dependent pathway.