HMGA2 Maintains Oncogenic RAS-Induced Epithelial-Mesenchymal Transition in Human Pancreatic Cancer Cells

Abstract

Pancreatic cancer is a highly aggressive malignancy due to elevated mitotic activities and epithelial-mesenchymal transition (EMT). Oncogenic RAS and transforming growth factor-␤ signaling are implicated in these malignant features. The mechanisms that underlie EMT need to be addressed since it promotes tissue invasion and metastasis. The high-mobility group A protein 2 (HMGA2) is a non-histone chromatin factor that is primarily expressed in undifferentiated tissues and tumors of mesenchymal origin. However, its role in EMT in pancreatic cancer is largely unknown. Here we report that HMGA2 is involved in EMT maintenance in human pancreatic cancer cells. Specific knockdown of HMGA2 inhibited cell proliferation, leading to an epithelial-state transition that restores cell-cell contact due to E-cadherin up-regulation. Consistently, an inverse correlation between HMGA2-positive cells and E-cadherin-positive cells was found in cancer tissues. Inhibition of the RAS/ MEK pathway also induced an epithelial transition, together with HMGA2 down-regulation. Transcriptional repressors of the E-cadherin gene, such as SNAIL, decreased after HMGA2 knockdown since HMGA2 directly activated the SNAlL gene promoter. The decrease of SNAIL after RAS/MEK inhibition was suppressed by HMGA2 overexpression. Further, let-7 microRNA-mediated HMGA2 down-regulation had no effect on the prevention of the transformed phenotype in these cells. These data shed light on the importance of HMGA2 in reversibly maintaining EMT, suggesting that HMGA2 is a potential therapeutic target for the treatment of pancreatic cancer. Pancreatic cancer has the poorest prognosis among human neoplasms due to its highly aggressive and metastasizing features.1-3 Current diagnostic tools have difficulty in detecting the early stage of this disease, and therapeutic applications often remain ineffective at advanced stages. Thus, the mechanisms leading to progression of this cancer especially need to be understood at molecular levels. The epithelial-mesenchymal transition (EMT) is a physiological process, originally found in embryonic development, in which the cells lose epithelial characteristics and gain mesenchymal properties. 4 This process is accompanied by loss of cell-to-cell contact and subsequent increased cell movement. Recent reports have implicated EMT in a malignant conversion of transformed cells, which represents invasive or metastasizing properties in a variety of cancers. 5,6 In pancreatic cancer cells, EMT is also reported to be a crucial step for tumor cell migration and invasion.7 Previous studies suggest that aberrations in pathways emerging from oncogenic RAS and transforming growth factor-␤ (TGF-␤) stimulate the malignant features, including EMT.6,8 Several transcriptional factors have been identified that control EMT by repressing E-cadherin and other epithelial genes in embryonic morphogenesis and cancer metastasis.6,9 These include the Snail-related zinc-finger factors (SNAIL and SLUG), the other zinc-finger factors (SIP1/ZEB2 and ZEB1/␦EF-1), ...

Role of Ribosomal Protein RPS2 in Controlling let-7a Expression in Human Prostate Cancer

“…In this regard, ras mutations seem to be low in American males, whereas 25% of Japanese patients have the mutated ras gene and show a correlation with tumor stage and grade, suggesting ethnic differences exist in frequency of ras mutation in prostate cancer (42,43). However, in contrast to these findings, immunohistochemical studies revealed a strong epithelial staining for ras in the vast majority of prostate cancers.…”

“…Interestingly, Watanabe et al (45) found that oncogenic ras signaling can induce HMGA2 expression (i.e., highmobility group protein A2), a gene induced during epithelial mesenchymal transitions (46), and promote malignancy of pancreatic cancer cells. In this connection, 3 groups (47)(48)(49) showed that HMGA2 translation is blocked by let-7a in cancer cells.…”

HMGA2: A biomarker significantly overexpressed in high-grade ovarian serous carcinoma

“…In the study of 60 cancer cell lines, Peter's research group found that HMGA2 is one of a few gene markers that can distinguish most type I (express mesenchymal gene signature) from 0 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C12 C13 C15 C16 C17 C19 C20 C21 C23 C24 C25 C26 C27 C28 C29 C30 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C15 C13 C14 C16 C17 C18 C19 C20 0V-PSC FT EMC type II (express epithelial gene signature) cancer cell lines. 9,16 HMGA2 participates in epithelialmesenchymal transition, 25,26 a key step in tumor plasticity by converting adherent epithelial cells to motile mesenchymal cells 27 through the functional loss of E-cadherin, which is required to maintain epithelial cell-cell adhesion.…”

“…25,30 The function of HMGA2 in behaviors of aggressive tumor growth in ovarian high-grade papillary serous carcinomas deserves further investigation.…”

The microRNA network and tumor metastasis

“…Several important oncogenes have been identified as the targets of let-7, including RAS, MYC and HMGA2 (Johnson et al, 2005;Mayr et al, 2007;Chang et al, 2008). These target genes, making use of the RAS-MEK pathway contribute to EMT, though let-7 miRNA-mediated HMGA2 downregulation had no effect on the prevention of the transformed phenotype in pancreatic cancer cells (Watanabe et al, 2009). Another let-7 miRNA-mediated tumor metastasis-regulating pathway was addressed recently: Raf kinase inhibitory protein, which inhibits mitogenactivated protein kinase signaling cascades, can decrease transcription of LIN28 by Myc.…”