Investigation on the Epigenetic Regulation of Autophagy

Autophagy (self-digestion) is a physiological process involved in the turnover of proteins or intracellular organelles. It becomes an alternative energy source, thus serving as a temporary survival mechanism during starvation. In contrast, total destruction of the cells by autophagy is served as type II programmed cell death. Autophagic cell death has been reported to be activated in cancer cells in response to various anticancer therapies. However, the mechanism responsible for the cell fate decision is still unclear. Although autophagy is commonly seen as a cytoplasmic event, recent studies have unveiled a transcriptional and epigenetic network that regulates autophagy. Enhancer of zeste homolog 2 (EZH2) is an epigenetic enzyme that mediates gene silencing through tri-methylation of histone H3 lysine 27 (H3K27-me3). Our preliminary results showed that EZH2 was downregulated during rapamycin-induced autophagy. Inhibition of EZH2 by GSK343 induced autophagy, indicating that EZH2 may represent a novel epigenetic mechanism for autophagy regulation. Therefore, Aim 1 of this project is to study the role of EZH2 in regulating autophagy. Microarray data analysis indicated that 78kDa glucose-regulated protein (GRP78) may be the gene target responsible for the regulation of autophagy by EZH2. GRP78 is a major ER chaperone that binds to unfolded protein to prevent further accumulation, thus promoting cell survival. It has been established as a novel necessary component of stress-induced autophagy. Aim 2 of this project is to study the regulation of GRP78 by EZH2 and its role in autophagy. Our preliminary data showed that inhibition of autophagy enhanced GSK343-induced cytotoxicity, suggesting that GSK343 induces cytoprotective autophagy. Because the role of autophagy in promoting cell survival and death is still controversial, elucidating the role of EZH2-regulated autophagy can get more insights into cell fate decision during autophagy. Therefore, Aim 3 of this project is to study the role of EZH2-regulated autophagy in determining cancer cell survival or death. Triple-negative breast cancer (TNBC) is a subtype of human breast cancers lacking expression of epidermal growth factor receptor 2 (HER2), estrogen receptor (ER) and progesterone receptor (PR). TNBC are more aggressive than other disease subtypes, and no molecular targeted agents are currently available for their treatment. Our preliminary results showed that EZH2 was upregulated in TNBC patients, implying that EZH2 might be an attracting molecular target for treating TNBC. In addition, the synergistic effect of GSK343 and an autophagy inhibitor 3-MA on inhibiting the cell viability of a TNBC cell line, MDA-MB-231, was observed. Therefore, Aim 4 of this project is to study the anti-TNBC effect of GSK343 in combination with the autophagy inhibitor. The overall goal of this project is to elucidate the novel epigenetic mechanism for the regulation of autophagy by EZH2 and to translate it into therapeutic strategy for TNBC.