New Functions of Long Noncoding Rnas During EMT and Tumor Progression Chunlai Li

Cancer Cancer Research Highlights Research

New Functions of Long Noncoding RNAs during EMT and Tumor Progression Chunlai Li

Epithelial-to-mesenchymal transition (EMT) is orches- dysregulated in colorectal carcinoma and correlates with trated by aberrant activation of EMT transcription factors poor survival and prognosis. They present a new functional and expression of downstream target genes resulting in mechanism illustrating how the SATB2-AS1–SATB2–Snail increased tumor initiation, metastatic potential, and thera- axis is involved in epigenetic modiﬁcation that regulates peutic resistance. In this issue of Cancer Research,Wangand colorectal carcinoma progression. colleagues show that the long noncoding RNA SATB2-AS1 is See related article by Wang et al., p. 3542

The epithelial-to-mesenchymal transition (EMT) is a dynamic plasm, nucleus, or nucleolus. The distinct cellular localization of a process, in which epithelial cells acquire mesenchymal features. It lncRNA determines its molecular function: cytoplasmic lncRNAs is characterized by loss of intercellular contacts and cellular are responsible for posttranscriptional regulation including polarity, and acquisition of motility and invasive properties (1). mRNA stability and protein modification (6); nuclear lncRNAs EMT is crucial during embryonic development, wound healing, have functions at the transcriptional level such as histone mod- tissue regeneration, and organ fibrosis. In addition to the funda- ification, alternative splicing, or direct transcriptional regula- mental role EMT plays in development, it is also involved in tion (7). Accumulating evidence demonstrate that dysregulated several pathologic processes, including tumor initiation and expression of lncRNAs could serve as biomarkers or therapeutic progression, and resistance to cancer therapies (2). targets in different oncogenic settings. Recent studies indicate that EMT is mediated by a set of EMT-activating transcription certain lncRNAs control EMT by regulating upstream signaling factors (EMT-TF), which include three protein families: Snail pathways or by directly acting on EMT-TFs (4). (Snail/SNAI1 and Slug/SNAI2), basic helix-loop-helix (TWIST1, In this issue of Cancer Research, Wang and colleagues report the TWIST2, and TCF3), and zinc-finger E-box-binding homeo- discovery of a new lncRNA that suppresses colorectal carcinoma box (ZEB1 and ZEB2). Several signaling pathways such as the progression by inhibiting EMT (8). The expression of lncRNAs in TGFb/SMAD, WNT/b-catenin, NOTCH, and receptor tyrosine colorectal carcinoma tissues with or without metastasis was kinase signaling pathways have been implicated in the activation determined to identify lncRNAs that play a role in colorectal of EMT. These pathways induce the expression of various EMT-TFs carcinoma metastasis. A long noncoding antisense transcript upon exposure to ligands in the tumor microenvironment (1). SATB2-AS1 (AK056625) was found to be significantly down- Over the past decade, noncoding RNAs [miRNAs and long regulated in colorectal carcinoma tissues with metastasis. These noncoding RNAs (lncRNA)] have emerged as novel regulators of findings were confirmed by measuring SATB2-AS1 expression in the expression and function of EMT-TF in different pathologic colorectal carcinoma tissues and a panel of colorectal carcinoma conditions (3, 4). Compared with miRNAs, our current knowl- cell lines, as well as an unpaired colorectal carcinoma population edge of lncRNA remain limited. Advances in massive sequencing from The Cancer Genome Atlas. The decreased expression of technology and mega-analyses have led to tens of thousands of SATB2-AS1 was associated with advanced stage and poor overall lncRNAs being discovered and annotated, and nearly 20,000 survival; SATB2-AS1 expression could therefore serve as an inde- lncRNAs have been identified with potential functional rele- pendent prognostic factor in patients with colorectal carcinoma. vance (5). LncRNAs are involved in various important biological Gain- and loss-of-function studies revealed SATB2-AS1 functions processes such as cellular homeostasis, stem cell differentiation, as a tumor suppressor as it inhibited cell proliferation, migration, development, and disease. LncRNAs are localized in the cyto- and invasion in vitro, and suppressed tumorigenesis and metastasis in vivo. Mechanistically, SATB2-AS1 upregulated the expression of the tumor suppressor special AT-rich sequence-binding protein Department of Experimental Therapeutics, The University of Texas MD 2 (SATB2). SATB2 expression positively correlated with SATB2- Anderson Cancer Center, Houston, Texas. AS1 expression in colorectal carcinoma. SATB2 is a nuclear matrix Corresponding Author: Chunlai Li, University of Texas MD Anderson Cancer DNA-binding protein that specifically binds to genomic nuclear Center, 1901 East Road, South Campus Research Building 4 (4SCR4.1130), matrix attachment regions (MAR) and participates in chromatin Houston, TX 77054. Phone: 713-563-5629; Fax: 713-745-1710; E-mail: [email protected] remodeling and transcriptional regulation (9). The nuclear localization of both SATB2 and SATB2-AS1 suggested that SATB2-AS1 Cancer Res 2019;79:3536–8 could be involved in the epigenetic modification of SATB2 by doi: 10.1158/0008-5472.CAN-19-1205 recruiting chromatin remodeling complexes to the SATB2 pro- 2019 American Association for Cancer Research. moter. Indeed, the authors demonstrated that SATB2-AS1

3536 Cancer Res; 79(14) July 15, 2019

recruited histone acetyltransferase p300 to the SATB2 promoter models of mammary gland morphogenesis and cutaneous and increased H3K9ac and H3K27ac levels at the SATB2 promoter wound healing demonstrate that the impaired EMT-TFs promote region, which led to transcriptional activation of SATB2. EMT when PTEN polyubiquitination is inhibited (10). These ENCODE data analysis also showed high levels of the epigenetic findings highlight the contrasting roles of lncRNAs in EMT. modification marks, H3K4me3, H3K9ac, H3K27ac, and p300, Although the authors show that the SATB2-AS1–SATB2–Snail within the SATB2 promoter region. The physical association of axis is involved in the progression of colorectal carcinoma, some SATB2-AS1 and p300 was validated using immunoprecipitation important questions arise from this study. First, additional studies and RNA pulldown, and p300 occupancy on the SATB2 promoter are needed to connect EMT-inducing signals (such as TGFb, WNT, region was demonstrated using chromatin immunoprecipitation and NOTCH signaling) to SATB2-AS1 activation and to investi- (ChIP). The authors therefore propose that SATB2-AS1 can serve gate the potential regulation of SATB2-AS1 by these signaling as a scaffold to recruit p300 to the SATB2 promoter and regulate its pathways that are active in specific oncogenic contexts. Second, a transcriptional activity. Overexpression of SATB2-AS1 or inhibi- set of miRNAs have been identified that regulate EMT-TFs during tion of histone deacetylases (HDAC) using trichostatin A development and tissue homeostasis; in addition, miRNAs can increased SATB2 expression, whereas inhibition of histone acet- target lncRNAs and modulate their expression (3, 4). Thus, the ylation using the acetyltransferase inhibitor C646 abolished this cross-talk between miRNAs and lncRNAs in EMT needs to be effect. In addition, loss-of-function and histologic analyses vali- explored further. Third, a central mechanism of action of lncRNAs dated that the activation of SATB2 by SATB2-AS1 was mediated by is mediated through interaction with proteins to regulate protein p300. Inhibition of p300 or SATB2 restored colorectal carcinoma function and interaction networks or guide the localization growth, migration, and invasion. of ribonucleoprotein complexes to specific subcellular compart- SATB2 is mainly expressed by epithelial cells of the colon and ments (6, 7). Thus, besides p300, other proteins could be inter- rectum (9), and its lower expression is positively correlated with acting with SATB2-AS1 to promote colorectal carcinoma enrichment of metastasis signatures and EMT. Wang and collea- initiation and metastasis through alternative oncogenic signaling gues found that ectopic overexpression of both SATB2-AS1 and pathways. Similarly, other EMT-TFs or drivers could be involved SATB2 led to decreased mRNA and decreased protein levels of in SATB2-AS1–dependent EMT and colorectal carcinoma progres- Snail, a central regulator of EMT. Sequence analysis revealed that sion. Answering these questions could deepen our understanding three potential SATB2-binding regions with AT-rich sequences of the complex nature and diverse factors that contribute to the resided within the Snail promoter; the specificity of these binding aggressiveness of colorectal carcinoma. sites was verified by ChIP-qPCR analysis. The recruitment of In conclusion, this study reveals a new link between lncRNAs HDACs to target gene promoters is a typical characteristic of and EMT mediated by epigenetic modulation. It also provides MAR-binding proteins such as SATB2 (9), therefore the associa- new insights into the regulatory mechanism of lncRNAs and tion between HDACs and SATB2/Snail was determined. Immu- potential targeting sites in colorectal carcinoma progression. noprecipitation and ChIP analyses verified that HDAC1 inter- Accumulating evidence indicates that conventional therapies acted with SATB2; furthermore, HDAC1 was significantly often fail to eradicate solid tumors that have active EMT, enriched on the Snail promoter at the same region as SATB2. The therefore development of RNA-targeting therapeutics may lead luciferase reporter assay also showed that depletion of SATB2 to improved responses in the clinic. Preclinical studies have significantly enhanced Snail transcriptional activity. Notably, already demonstrated the therapeutic efficacy of antisense ectopic expression of Snail partially restored the growth-inhibit- oligonucleotides that target cancer-associated lncRNAs. In addi- ing phenotypes caused by SATB2-AS1 or SATB2 introduction. tion, splice-switching oligonucleotides could be used to resect Similarly, the suppression of HDAC1 antagonized EMT inhibi- lncRNA exons that encode essential functional domains. Alter- tion induced by SATB2-AS1/SATB2 ectopic expression in colo- natively, steric-blocking oligonucleotides may prevent the asso- rectal carcinoma cells. ciation of lncRNAs with their interacting partners. Finally, a Wang and colleagues provide new evidence of a tumor sup- deeper understanding of the mechanisms regulating EMT and pressor lncRNA that suppresses colorectal carcinoma progression the emergence of new EMT regulators such as lncRNAs can by inhibiting master drivers of EMT. In contrast, a recent study promote development of new therapeutic tools and strategies reported another lncRNA glucose aroused for EMT activation (GAEA) to inhibit EMT and prevent tumor metastasis and resistance to that promotes EMT progression through enhancing the stability of therapy in human cancers. EMT-TFs (10). GAEA binds and activates the E3 ligase, MEX3C, which can catalyze K27-linked polyubiquitination of PTEN. Unexpectedly, PTENK27-polyUb switches its phosphoinositide/ Disclosure of Potential Conflicts of Interest protein tyrosine phosphatase activity to protein serine/ No potential conflicts of interest were disclosed. threonine phosphatase activity, thus dephosphorylates the phos- phoserine/threonine residues of TWIST1, SNAI1, and YAP1, Received April 15, 2019; accepted April 15, 2019; published online July 15, increasing the stabilization of these EMT-TFs. Genetic mouse 2019.

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3538 Cancer Res; 79(14) July 15, 2019 Cancer Research

Chunlai Li

Cancer Res 2019;79:3536-3538.

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