Polycomb protein histone methyltransferase enhancer of Zeste homologe 2 (EZH2) is frequently overexpressed in human malignancy and is implicated in cancer cell proliferation and invasion. However, it is largely unknown whether EZH2 has a role in modulating DNA damage response. Here, we show that EZH2 is an important determinant of cell fate decision in response to genotoxic stress. EZH2 depletion results in abrogation of both cell cycle G1 and G2/M checkpoints, directing DNA damage response toward predominant apoptosis in both p53-proficient and p53-deficient cancer cells, but not in normal cells. Mechanistically, EZH2 regulates DNA damage response in p53 wild-type cells mainly through transcriptional repression of FBXO32, which binds to and directs p21 for proteasome-mediated degradation, whereas it affects p53-deficient cells through regulating Chk1 activation by a distinct mechanism. Furthermore, pharmacological depletion of EZH2 phenocopies the effects of EZH2 knockdown on cell cycle checkpoints and apoptosis. These data unravel a crucial role of EZH2 in determining the cancer cell outcome following DNA damage and suggest that therapeutic targeting oncogenic EZH2 might serve as a strategy for improving conventional chemotherapy in a given malignancy. Cell Death and Differentiation (2011) 18, 1771-1779; doi:10.1038/cdd.2011.48; published online 6 May 2011Following DNA damage, mammalian cells activate cell cycle checkpoint mechanisms to induce cell cycle arrest and protect cells from apoptosis. 1 The interconnections between the pathways regulating the cell cycle checkpoints and apoptosis dictate the cellular outcome to DNA damage, 2 but whether these pathways are differentially regulated by oncogenic lesions in tumor cells to allow cancer-specific perturbation is poorly understood. Inhibition of key cell cycle checkpoint regulators such as cyclin-dependent kinase inhibitor p21 and Chk1/2 have been shown to increase the sensitivity to DNA damage in p53-proficient or p53-deficient cancer cells, respectively. 3-14 A treatment strategy to simultaneously abrogate both G1 and G2/M checkpoint and thus sensitizing both p53 wild-type and mutant tumors has yet to be developed.Polycomb protein enhancer of Zeste homologe 2 (EZH2) is a histone methyltransferase that is frequently overexpressed in a wide variety of human malignancies, 15,16 and is implicated in cell proliferation, invasion and metastasis. [17][18][19][20][21][22][23][24][25] Mechanistically, the oncogenic function of EZH2 has been attributed to associated histone H3 with trimethylated lysine 27 (H3K27Me3), leading to transcriptional repression of tumor suppressor genes, including p16(INK4a) and p19(ARF), 26 E-cadherin, 19 adrenergic receptor-b2, 27 RUNX3, 20 p57 (also called CDKN1C), 21 Bim 28 and DAB2IP. 23,24 As such, EZH2 is emerging as a crucial regulator of cell fate by affecting multiple signaling pathways. It is not clear, however, whether EZH2 overexpression in cancer cells has a role in affecting cellular response to DNA damage. This study investi...

3-Deazaneplanocin A (DzNep) is a potential epigenetic drug for the treatment of various cancers. DzNep has been reported to deplete histone methylations, including oncogenic EZH2 complex, giving rise to epigenetic modifications that reactivate many silenced tumor suppressors in cancer cells. Despite its promise as an anticancer drug, little is known about the structure-activity relationships of DzNep in the context of epigenetic modifications and apoptosis induction. In this study, a number of analogues of DzNep were examined for DzNep-like ability to induce synergistic apoptosis in cancer cells in combination with trichostatin A, a known histone deacetylase (HDAC) inhibitor. The structure-activity relationship data thus obtained provide valuable information on the structural requirements for biological activity. The studies identified three compounds that show similar activities to DzNep. Two of these compounds show good pharmacokinetics and safety profiles. Attempts to correlate the observed synergistic apoptotic activities with measured S-adenosylhomocysteine hydrolase (SAHH) inhibitory activities suggest that the apoptotic activity of DzNep might not be directly due to its inhibition of SAHH.

Aberrant epigenetic events contribute to tumorigenesis of all human cancers. Significant efforts are underway in developing new generation of epigenetic cancer therapeutics. Although clinical trials for agents targeting DNA hypermethylation and histone deacetylation have yielded promising results, developing agents that target histone methylation remains to be in the early stage. We and others have previously reported that 3-Deazaneplanocin A (DZNep) is a histone methylation inhibitor that has a wide range of anticancer effects in various human cancers. Here, focusing on acute myeloid leukemia (AML) as a model, we reported a less toxic analog of DZNep, named D9, which is shown to be efficacious in AML cell lines and patient-derived samples in vitro, as well as AML tumorigenesis in vivo. Gene expression analysis in a panel of AML cell lines treated with D9 identified a set of genes that is associated with D9 sensitivity and implicated in multiple oncogenic signaling pathways. Moreover, we show that D9 is able to deplete the leukemia stem cells (LSC) and abolish chemotherapy-induced LSC enrichment, leading to dramatic elimination of AML cell survival. Thus, D9 appears to be a robust epigenetic compound that may constitute a potential for AML therapy.