Neighborhood matters: Response to EZH2 methyltransferase inhibitors

Enhancer of zeste homolog 2 (EZH2) has been found to be overexpressed in ovarian and other cancers and plays critical roles in cell proliferation, angiogenesis, and inhibition of apoptosis.^1,2 Among its many functions, it tri-methylates lysine residue at the 27 position of histone H3 associated with the target gene.³ EZH2 is widely considered an important target for cancer therapy. Several approaches to block EZH2, including inhibitors (e.g., GSK126, GSK343, EI1) and siRNA incorporated into nanoparticles, are being examined.^2,4-6

It is becoming increasingly clear that 2D in vitro assays may not fully simulate the complex tumor microenvironment, likely due to differences in gene expression, lack of histological differentiation and cell–cell or cell–ECM (extracellular matrix) interactions in 2D assays. On the other hand, tumor cells grown in three-dimensional settings are morphologically and histologically closer to those growing in vivo.^7,8 Some studies have shown that cells in 3D conditions are more resistant to chemotherapy and require larger doses of drug as compared with 2D culture.⁸ Therefore, many investigators have started to use 3D cultures for in vitro drug testing. In the recent issue of Cell Cycle, Zhang and colleagues observed that GSK343, a methyltransferase inhibitor, had minimal to no effects on ovarian cancer cells under 2D culture setting. Interestingly, there was a significant decrease in cell proliferation in 3D cultures with GSK343 treatment. Treatment also changed the morphology of cells to normal epithelial-like phenotype and suppressed invasion in 3D cultures. To investigate downstream signaling, the authors checked the expression of a pro-apoptotic H3K27Me3 target gene, HRK (Harakiri), and found that it is significantly upregulated in 3D, but not in 2D settings. These observations suggest that the tumor microenvironment plays a very important role in regulating the efficacy of methyltransferase inhibitors.⁶

Although the authors have convincingly shown differences in drug efficacy in vitro; in vivo studies are essential to further develop these drugs. EZH2 has been shown to play diverse roles in cancer biology including angiogenesis and maintenance and expansion of cancer stem cell populations.^1,2 Thus EZH2 is an attractive target for cancer therapy and the current study provides an important step forward related to crosstalk between EZH2 and ECM. Differential expression of certain genes in 3D conditions might be playing an important role in sensitizing the cells to GSK343. Additional knowledge about the differences in signaling molecules will not only help to understand the exact mechanism by which 3D setting alters the response of cancer cells to the inhibitors, but may also suggest some potential therapeutic targets. Additional experiments with other cell types in the tumor microenvironment, such as fibroblasts, endothelial cells and macrophages, in a hypoxic environment to study their effects on sensitization of tumor cells could further inform mechanisms by which these inhibitors function.

Overall, the study by Zhang and colleagues provides new and important knowledge regarding the therapeutic potential of EZH2 inhibitors. Such studies coupled with additional in vivo work are essential for providing a path toward clinical development of such inhibitors. The expanding body of preclinical data regarding EZH2 strongly justifies further development of this important target.

Amatangelo MD, Garipov A, Li H, Conejo-Garcia JR, Speicher DW, Zhang R. Three-dimensional culture sensitizes epithelial ovarian cancer cells to EZH2 methyltransferase inhibition. Cell Cycle. 2013;12:2113–9. doi: 10.4161/cc.25163.