Epigenetic Regulation of the lncRNA MEG3 and Its Target c-MET in Pancreatic Neuroendocrine Tumors

Abstract

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.

Pro-oncogenic Roles of HLXB9 Protein in Insulinoma Cells through Interaction with Nono Protein and Down-regulation of the c-Met Inhibitor Cblb (Casitas B-lineage Lymphoma b)

“…c-MET amplification and mutations have not been observed in the exome sequencing efforts of PNETs and insulinomas (5-7). However, increased c-MET level in human PNETs and insulinomas has been observed (16,17). We previously showed in mouse insulinoma MIN6 ␤-cells that c-Met mRNA could be down-regulated by a menin target, the long non-coding RNA Meg3, and a commonly used c-Met inhibitor (PHA-665752) suppressed the growth and migration of mouse insulinoma MIN6 ␤-cells (17).…”

“…Formalin-fixed paraffinembedded pancreas tissue sections from pancreatic islet tumor-bearing18-month-old Men1 ϩ/Ϫ mice (32), 12-monthold Men1 f/f (with germ-line homozygous floxed Men1), and 12-month-old pancreatic islet tumor-bearing RIP-Cre-Men1 f/f mice (␤-cell-specific Men1 knock-out; germ-line homozygous floxed Men1 with conditional expression of Cre-recombinase in ␤-cells from the Rat insulin promoter) (33) were stained for insulin using standard IF procedures. For IHC, formalin-fixed paraffin-embedded pancreas sections were processed as previously described (10,17) for HB9, HB9-PO4, Cblb, and c-Met staining. After IF and IHC, images were captured on the BZ9000 microscope (Keyence).…”

“…Aberrant activation of hepatocyte growth factor/MET signaling (through c-MET mutation or increased expression) has been observed in a variety of tumors (15). Although activating c-MET mutations have not been reported in PNETs, they show increased c-MET expression (16,17). c-Met knock-out mice are embryonic lethal, but conditional loss of c-Met in ␤-cells results in smaller islets and impaired insulin secretion (18 -20).…”

MEG3 long non-coding RNA prevents cell growth and metastasis of osteosarcoma

“…A number of studies have demonstrated that lncRNA MEG3, a tumor suppressor gene, was down-regulated in various cancers and was associated with poor prognosis (14)(15)(16)(17)(18). A previous study indicated that the expression of lncRN A MEG3 was clearly lower in osteosarcoma tissues compared with adjacent non-tumor tissues (19).…”

“…Zhu et al suggested that MEG3 functions as a tumor suppressor in hepatoma cells through interacting with p53 protein (18). In addition to these studies, the role of MEG3 has been investigated in gallbladder cancer, NSCLC, neuroendocrine tumor, gastric cancer, thyroid carcinoma and so on (25,(14)(15)(16)(17). However, to the best of our knowledge, the potential roles of MEG3 in OS remain largely unknown.…”