Integration of estrogen and Wnt signaling circuits by the polycomb group protein EZH2 in breast cancer cells.
Journal: 2007/August - Molecular and Cellular Biology
ISSN: 0270-7306
Abstract:
Essential for embryonic development, the polycomb group protein enhancer of zeste homolog 2 (EZH2) is overexpressed in breast and prostate cancers and is implicated in the growth and aggression of the tumors. The tumorigenic mechanism underlying EZH2 overexpression is largely unknown. It is believed that EZH2 exerts its biological activity as a transcription repressor. However, we report here that EZH2 functions in gene transcriptional activation in breast cancer cells. We show that EZH2 transactivates genes that are commonly targeted by estrogen and Wnt signaling pathways. We demonstrated that EZH2 physically interacts directly with estrogen receptor alpha and beta-catenin, thus connecting the estrogen and Wnt signaling circuitries, functionally enhances gene transactivation by estrogen and Wnt pathways, and phenotypically promotes cell cycle progression. In addition, we identified the transactivation activity of EZH2 in its two N-terminal domains and demonstrated that these structures serve as platforms to connect transcription factors and the Mediator complex. Our experiments indicated that EZH2 is a dual function transcription regulator with a dynamic activity, and we provide a mechanism for EZH2 in tumorigenesis.
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Mol Cell Biol 27(14): 5105-5119

Integration of Estrogen and Wnt Signaling Circuits by the Polycomb Group Protein EZH2 in Breast Cancer Cells<sup><a href="#fn2" rid="fn2" class=" fn">▿</a></sup>

+4 authors
Department of Biochemistry and Molecular Biology, Peking University Health Science Center, Beijing 100083, China
Corresponding author. Mailing address: Department of Biochemistry and Molecular Biology, Peking University Health Science Center, 38 Xue Yuan Road, Beijing 100083, China. Phone: 86-10-82805118. Fax: 86-10-82801355. E-mail: nc.ude.ukp.csh@gnahsy
B.S. and J.L. contributed equally to this work.
Received 2007 Jan 27; Revised 2007 Mar 8; Accepted 2007 Apr 24.

Abstract

Essential for embryonic development, the polycomb group protein enhancer of zeste homolog 2 (EZH2) is overexpressed in breast and prostate cancers and is implicated in the growth and aggression of the tumors. The tumorigenic mechanism underlying EZH2 overexpression is largely unknown. It is believed that EZH2 exerts its biological activity as a transcription repressor. However, we report here that EZH2 functions in gene transcriptional activation in breast cancer cells. We show that EZH2 transactivates genes that are commonly targeted by estrogen and Wnt signaling pathways. We demonstrated that EZH2 physically interacts directly with estrogen receptor α and β-catenin, thus connecting the estrogen and Wnt signaling circuitries, functionally enhances gene transactivation by estrogen and Wnt pathways, and phenotypically promotes cell cycle progression. In addition, we identified the transactivation activity of EZH2 in its two N-terminal domains and demonstrated that these structures serve as platforms to connect transcription factors and the Mediator complex. Our experiments indicated that EZH2 is a dual function transcription regulator with a dynamic activity, and we provide a mechanism for EZH2 in tumorigenesis.

Abstract

Initially discovered as epigenetic silencers during embryogenesis, polycomb group (PcG) proteins have been implicated in development and differentiation (34). The biological activities of PcG proteins are expanding and now include the regulation of various adult processes, such as lymphopoiesis, X-inactivation, and cell proliferation, and several PcG genes have been implicated in tumorigenesis (3, 5).

Initial suggestions that EZH2 is involved in cell proliferation came from the observation that EZH2 is preferentially expressed in proliferating, but not resting, mantle cell lymphoma cells (53). Subsequently, EZH2 was found to be overexpressed in metastatic prostate cancer, and knockdown of EZH2 expression inhibited cell proliferation (52). It was also observed that the EZH2 level directly correlates with the aggressiveness of breast cancer, and forced EZH2 expression in immortalized human mammary epithelial cell lines promotes anchorage-independent growth and cell invasion (3, 20).

How EZH2 promotes cell proliferation and tumor progression is still largely unknown. It is believed that EZH2 functions by forming polycomb repressive complex (PRC) with other PcG proteins (29, 35). These protein complexes are characterized by an intrinsic histone lysine methyltransferase (HMTase) activity that is mediated by the SET domain of EZH2 (25) and that targets different lysine residues on histones H1 or H3 in vitro (5, 23). Core histone methylation facilitates the establishment of a stable, repressive chromatin structure to prevent transcription initiation by prebound factors (7). In addition, several PcG proteins interact or colocalize with various non-PcG proteins, including the transcription modulators CtBP (41), E2F6 (2, 51), RYBP (12), AF9 (13), SSX (49), and the mitogen-activated protein/kinase-activated protein kinase 3pK (54). All of these proteins may contribute to the activity of PcG complexes and their ability to bind chromatin and, thus, may be involved in oncogenesis.

Despite the characterization of EZH2 as a component of transcriptional repressors, there has been evidence implying EZH2 activities that are not compatible with the transcription repression model. It is believed that PcG proteins act in conjunction with Trithorax group (TrxG) proteins to maintain repressed or active transcription states of developmentally regulated genes. However, gene ablation of E(z) in Drosophila melonagaster produced phenotypes similar to those of trithorax loss-of-function mutants (46), and E(z) loss-of-function mutations led to a complete loss of the accumulation of the homeotic selector gene products SCR, ANTP, and UBX and the segmentation gene product EN (27). More recently, it has been shown in mammalian cells that knockdown EZH2 expression by RNA interference (RNAi) led to a significant decrease, rather than increase, of G1/S-expressed cyclins (3), suggesting that EZH2 may play an activation role in the expression of these genes. Here we show that EZH2 activates rather than represses the transcription of c-Myc and cyclin D1 in breast cancer cells, and this activation activity is independent of the SET domain that is believed to mediate its transcriptional repression. We show that EZH2 interacts with components of the estrogen and Wnt pathways, integrates estrogen and Wnt signaling, and promotes cell proliferation in breast cancer cells.

Acknowledgments

We thank Joanne Balmer Green (Penn State University) for editorial assistance.

This work was supported by grants (30621002, 30393113, and 30470912 to Y.S.) from the National Natural Science Foundation of China and grants (863 Program 2006AA02Z466 and 973 Program 2005CB522404 to Y.S.) from the Ministry of Science and Technology of China.

Acknowledgments

Footnotes

Published ahead of print on 14 May 2007.

Footnotes

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