Multifaceted role of EZH2 in breast and prostate tumorigenesis: epigenetics and beyond

Abstract

Overexpression of EZH2 and other PRC2 subunits, such as SUZ12, is associated with tumor progression and poor prognosis in several human malignancies. Nevertheless, the underlying mechanisms driving aberrant EZH2 expression are poorly understood. This review provides molecular insights into the essential role of EZH2 in breast and prostate tumorigenesis. We addressed the current understanding on the oncogenic role of EZH2, with an emphasis on: (1) the less known PRC2-independent role of EZH2 in gene activation, in addition to its canonical role in transcriptional silencing as a histone methyltransferase catalyzing the trimethylation of histone H3 at lysine 27; (2) causes and consequences of its deregulation in tumor cells and; (3) collaboration of EZH2 with other epigenetic and hormone receptor-mediated oncogenic signaling pathways. We also summarize how EZH2 has emerged as a promising therapeutic target in hormone-refractory cancers and the prospects for integrating EZH2 blockade with available pharmacological inhibitors.

Keywords: EZH2; H3K27 trimethylation; PRC2 complex; androgen receptor; breast cancer; estrogen receptor; prostate cancer.

Figure 1. Molecular mechanism of action of EZH2. (A) Polycomb dependent mechanism—Role in transcriptional repression. EZH2 functions as a part of mammalian PRC2 core complex consisting of EZH1/2, SUZ12, EED and RbAp46/48 (also called RBBP7/4. When recruited to the target gene promoter it catalyses the di/tri- methylation of Histone 3 at lysine 27 (H3K27me3), resulting in chromatin compaction and inaccessibility of promoter region to RNA Pol II and other proteins of the transcription machinery, which ultimately repress transcription. H3K27me3 mark also serves as a docking site for binding of PRC1 complex containing Bmi1, Ring1a, Ring1b, HPH1, HPH2, NSPC1, MEL18 and CBX proteins (-2, 4, 6, 7, 8). PRC1 catalysed mono-ubiquitination of Histone 2A at lysine 119 further contributes to target gene silencing. (B) Polycomb independent mechanism—Role in transcriptional activation. (1) In prostate cancer cells, Akt-1 mediated phosphorylation of EZH2 at Serine-21 decreases the H3K27me3 activity of EZH2. But phosphorylated EZH2 can function independent of other PRC2 proteins and may methylate androgen receptor at lysine 630 and 632, which can enhance its transcriptional activity. This is a potential mechanism for EZH2 mediated transcriptional activation via methylation of androgen receptor or other androgen receptor associated proteins (X or Y). (2) When over-expressed in ER-positive, luminal like MCF-7 breast cancer cells, EZH2 functions as a transcriptional activator by acting as a bridge to physically link ERα and Wnt signaling components β-catenin and TCF, on the Cyclin B1 and c-Myc promoters. EZH2 also associates with Mediator complex through its domain II independent of the SET domain involved in HMTase activity and enhance transcription by its interaction with RNA polymerase II. In ER-negative, basal like MDA-MB-231 cells, EZH2 forms a ternary complex with NF-kB components RelA and RelB and activates transcription of NF- kB target genes such as TNF, IL6.

Figure 2. Molecular insights into EZH2 driven breast and prostate tumorigenesis. (A) In breast cancer cells EZH2 expression is regulated by several factors such as hypoxia induced HIFα, pRB-E2F and MEK-ERK-Elk1 pathways. Genomic loss of miR101 and miR214 also up-regulates EZH2 expression. Elevated EZH2 levels leads to the transcriptional repression of several tumor suppressor genes such as FOXC1, RAD51, RKIP, CDKIC, RUNX3, CIITA etc, by PRC2 mediated H3K27 trimethylation. High EZH2 protein levels are associated with increased expression of phospho-Akt1 (Ser473) and decreased nuclear localization of phospho-BRCA1 (Ser1423). EZH2 mediated nuclear shuttling of BRCA-1 protein in ER negative basal like breast cancer cells is one of its PRC2 independent functions (others described in Fig. 1B). Nuclear retention of BRCA-1 protein leads to aneuploidy, aberrant mitosis and genomic instability, which ultimately promotes tumorigenesis. (B) In prostate cancer cells, four molecular mechanisms are reported to be responsible for EZH2-amplification or overexpression of the EZH2 gene including deletion of its negative regulator miR-101, transcriptional regulation by MYC and ETS gene family members. MYC binds upstream of EZH2 promoter and induces EZH2 expression. Interestingly MYC represses the transcription of CTDSPL, CTDSP2 and CTDSP1 which harbor miR-26a and miR-26b. Repression of miR-26a and miR-26b contribute additionally to EZH2 overexpression as miR-26a and miR-26b would be unavailable to destabilize EZH2 mRNA by binding specifically to the EZH2 3'-UTR in RISC complex. Furthermore, ETS transcriptional network also regulates the expression of EZH2. Epithelial-specific ETS factor ESE3 represses EZH2 expression whereas ERG, binds to the promoter of EZH2 and competes with ESE3 for promoter occupancy opposing its effects. EZH2 overexpression leads to H3K27 methylation associated silencing of critical tumor suppressor genes such as DAB2IP, MSMB, SLIT, TIMP-2, and TIMP-3, which contribute to increased growth, proliferation and invasive phenotype of prostate cancer cells.