EZH mutations or amplifications have been
EZH2 mutations or amplifications have been found in a broad spectrum of human cancers including B cell lymphoma, ovarian cancer, breast cancer, melanoma, bladder cancer, gastric cancer, and other cancers (Kim and Roberts, 2016). Given the evidence of EZH2 as a cancer driver, numerous efforts have been made that led to the development of EZH2 inhibitory compounds including EPZ-6438 (Knutson et al., 2013) and GSK126 (McCabe et al., 2012), both of which are currently used in clinical trials primarily against EZH2-mutated B cell lymphoma and advanced solid tumors (Kim and Roberts, 2016). However, mixed responses of anti-EZH2 single-agent therapies have been reported in both clinical and pre-clinical studies, particularly in the settings of solid tumors, advocating novel combination therapies for EZH2 hyperactive solid tumor patients (Kim and Roberts, 2016).
Here we found that AMPK directly phosphorylates EZH2 at Thr311 to disrupt its interaction with SUZ12 and to inhibit PRC2 enzymatic activity, which is supported by the increased expression of PRC2-repressed genes. Furthermore, the T311E-EZH2 mutant that mimics AMPK-mediated phosphorylation status suppresses tumor cell growth both in vitro and in vivo. Consistent with these results, a positive correlation between AMPK activity and pT311-EZH2 status was observed in ovarian and breast tumor samples, and we also found that higher pT311-EZH2 correlates with better survival in both ovarian and breast cancer patients.
Discussion Both anti- and pro-tumorigenic roles of AMPK have been supported by multiple lines of experimental evidence in distinct tissue/tumor settings (Zadra et al., 2015). The role of AMPK in harnessing unchecked cell growth has been underscored by the findings that AMPK suppresses protein synthesis and de novo fatty ACET receptor synthesis (Hardie et al., 2012). Given the pivotal role of PRC2 in promoting cell proliferation, our findings suggest that energy stresses like glucose deprivation and glycolysis blockade activate the AMPK kinase and subsequently relieve EZH2-mediated silencing, which promotes the expression of PRC2 target genes to inhibit cell-cycle progression, suppress cell proliferation, and promote differentiation. This is another example of AMPK as a determining pivot to connect cellular energy state and pathological development through regulation of epigenetics (Hardie et al., 2012). On the other hand, a recent study showed that AMPK catalyzes H2B phosphorylation, which contributes to AMPK-dependent transcriptional activation of a subset of genes that facilitate cell survival in response to stress (Bungard et al., 2010). In support of a pro-survival role of AMPK in conditions of metabolic stress, it has been shown that AMPK promotes autophagy by phosphorylating ULK1 (Egan et al., 2011) and accelerates fatty acid oxidation to produce ATP (Jeon et al., 2012). Taken together, AMPK governs numerous downstream signaling pathways, and depending on different cellular contexts, activation of AMPK leads to either cell proliferation suppression or cell survival under starvation. These findings once again support the complexity of AMPK’s role in dictating cellular fate under stress (Hardie, 2013, Zadra et al., 2015). Several EZH2 inhibitors that block its methyltransferase activity, such as CPI-1205, EPZ-6438, and GSK126, have been developed and are currently validated in clinical trials for non-Hodgkin’s lymphoma containing gain-of-function mutations of EZH2 or genetically defined solid tumors (Lim et al., 2015). However, acquired resistance to anti-EZH2 therapies has been reported (Kim and Roberts, 2016), suggesting effective combinational therapies are clearly warranted. Our findings that activation of AMPK suppresses PRC2/EZH2 activity suggest that agonists of AMPK might be a promising sensitizer for EZH2-targeting drugs for anti-cancer treatment. Among currently available AMPK agonists, metformin, a first-line drug for treating patients with type 2 diabetes mellitus (T2DM) (Foretz et al., 2014), has been widely used in pre-clinical studies to interrogate the effect of energy restriction on tumor cells. Intriguingly, most reports support an anti-cancer function of metformin, which is further evidenced by the observation that in the T2DM population, metformin administration is associated with decreased cancer risks (Foretz et al., 2014). It is noteworthy that metformin directly targets the mitochondrion to inhibit the complex I, resulting in reduced ATP production and subsequently activating AMPK (Pernicova and Korbonits, 2014). This indicates that metformin may exert its anti-cancer functions via other possible mechanisms. Our findings that T311-EZH2 phosphorylation by AMPK impairs PRC2 enzymatic activity (Figures S6H and S7C) advocate for a future comprehensive evaluation of response synergy between direct AMPK agonists and anti-EZH2 inhibitors for treating EZH2-overexpressing solid tumors.