CBX8 exhibits oncogenic properties and serves as a prognostic factor in hepatocellular carcinoma

Tang et al. Cell Death and Disease (2019)10:52 https://doi.org/10.1038/s41419-018-1288-0 Cell Death & Disease ARTICLE Open Access CBX8 exhibits oncogenic properties and serves as a prognostic factor in hepatocellular carcinoma 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; Bo Tang1, Yu Tian 2,3, Yong Liao1, Zeming Li1, Shuiping Yu1, Huizhao Su1, Fudi Zhong1, Guandou Yuan1, Yan Wang1, Hongping Yu1, Stephen Tomlinson4, Xiaoqiang Qiu5 and Songqing He1 Abstract Polycomb group family is a class of proteins that have important roles in both physiological and pathological processes, and its family member Chromobox homolog 8 (CBX8) regulates cell differentiation, aging, and cell cycle progression in numerous carcinomas; however, the effects and underlying mechanisms of CBX8 in hepatocellular carcinoma (HCC) are rarely reported. We found that CBX8 expression in clinical HCC specimens correlates inversely with patient survival. In HCC cells, we found that enforced overexpression of CBX8 induces epithelial–mesenchymal transition, invasive migration, and stem cell-like traits, which are associated with increased tumor growth and metastasis in mice. Conversely, CBX8 silencing inhibits the aggressive phenotype of HCC cells that have high CBX8 expression. Mechanistically, CBX8 modulates H3K27me3 in the gene promoter of bone morphogenetic protein 4 (BMP4), which is associated with active BMP4 transcription and, consequently, the activation of Smads and mitogenactivated protein kinases. BMP4 expression reverses the effects of CBX8 silencing in inhibiting epithelial–mesenchymal transition, stemness, and metastasis. Our results establish CBX8 as a critical driver of HCC stem cell-like and metastatic behaviors and characterize its role in modulating BMP4 expression. These findings have implications for the targeting of CBX8 as an approach to HCC prognosis and treatment. Highlights 1. CBX8 expression in clinical HCC specimens correlates inversely with patient survival. 2. CBX8 promotes HCC proliferation, mobility and invasion (epithelial–mesenchymal transition, EMT), and stemness. 3. CBX8 modulates H3K27me3 in the gene promoter of BMP4, which is associated with active BMP4 transcription, and consequently activated Smads and MAP kinases. Introduction Correspondence: Xiaoqiang Qiu () or Songqing He () 1 Department of Hepatobiliary Surgery, The First Affiliated Hospital of Guangxi Medical University, Nanning, Guangxi, China 2 CAS Key Laboratory of Separation Sciences for Analytical Chemistry, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China Full list of author information is available at the end of the article. These authors contributed equally: Bo Tang, Yu Tian Edited by A. Peschiaroli Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related death and the fifth most common cancer in the world. About 500,000–1,000,000 new cases occur each year, more than 50% of which occur in China1. Although surgery, transcatheter arterial chemical embolism, radiofrequency ablation, and transplantation have been widely applied in clinical treatment, patients with HCC still have poor prognosis because of the insidious © The Author(s) 2019 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Official journal of the Cell Death Differentiation Association Tang et al. Cell Death and Disease (2019)10:52 onset, high malignancy, high invasiveness, rapid progression, and high recurrence rate of HCC2,3. Moreover, markers used for HCC prognosis prediction after resection are not satisfactory due to their poor accuracy and reproducibility. Therefore, it is important to explore novel markers to improve HCC diagnosis and treatment. The polycomb group proteins, first discovered in Drosophila, are essential regulators of cell proliferation and differentiation, which are often deregulated in human cancers and contribute to the development of cancer4,5. Polycomb proteins are mainly comprised of two complexes, polycomb repressive complex 1 and 2 (PRC1 and PRC2), whose functions are to maintain transcriptional repression. Chromobox homolog 8 (CBX8), a homolog of the Drosophila polycomb protein, is a component of PRC1, which has been shown to have a critical role in the pathogenesis of cancer. As a transcriptional repressor, CBX8 regulates numerous target genes that are important for cell growth and survival, including the tumor suppressor gene INK4a/ARF locus6, which is involved in cellfate decisions, and AF9, which is implicated in the development of acute leukemia7. Recent studies have revealed that DNA damage induces CBX8 upregulation, and CBX8 knockdown results in more severe DNA damage, indicating that CBX8 is a key regulator of DNA repair. CBX8 is upregulated in human esophageal carcinoma and participates in DNA repair to promote esophageal carcinogenesis8. CBX8 is also upregulated in colorectal cancer, and CBX8 overexpression indicates poor prognosis9. Although evidence suggests that CBX8 expression is correlated with the tumor generation and development, few studies have focused on the function and mechanism of CBX8 in HCC. Migration and invasion are important malignant biological behaviors of HCC. Increasing evidence indicates that epithelial–mesenchymal transition (EMT) is one of the key initiation steps in metastasis. EMT is characterized by increased epithelial-like molecules, decreased mesenchymal-like markers, and loss of cellular polarity and junctions10. The progression of EMT stimulates cancer cell motility, migration, and invasion properties and has been regarded as an early indicator of metastasis11. Therefore, clarifying the mechanism of EMT will help us to understand how HCC metastasizes. In this study, we determined that CBX8 expression in HCC tissues is inversely correlated with patient survival. The overexpression of CBX8 in HCC cells induces EMT, migration, invasion, and stem cell-like traits in vitro and enhances the cancer stem cell-like and metastatic capacity in vivo. Conversely, silencing of CBX8 in HCC cells inhibits these processes. These functional effects of CBX8 are exerted by its ability to control bone morphogenetic protein 4 ( (...truncated)