EZH2-Mediated Activation of Serine Biosynthetic Pathway Is Critical for Resistance of Clonal Cells to Stress-Related Apoptosis in Low-Grade Myelodysplastic Syndrome

Background

Increased cell apoptosis mediated by P53 or activated autoimmunity was frequent in low-grade MDS especially in 5q-syndrome and those with trisomy 8. However, MDS clonal cells can gradually acquired apoptosis resistance under different stressed conditions. The mechanism of apoptosis resistance is still not fully understood. The polycomb group protein EZH2 as an anti-apoptosis protein drives tumorigenesis. The association of EZH2 with apoptosis resistance of MDS clonal cells is worth further investigating. 

Objective

The objective of this study is to investigate the role of EZH2 in the mechanism of apoptosis resistance of MDS.

Methods

The expression of EZH2 protein was analyzed by flow cytometry in 71 MDS patients. EZH2-targeted genes were identified through an integrated analysis of gene expression microarray analysis in MDS patients and cell line. EZH2-targeted genes were confirmed through chromatin immunoprecipitation and luciferase reporter gene assay. Cell functional and metabolic experiments were also performed. 

Results

We observed upregulation of the EZH2 expression in the patients with low-grade MDS especially in the individuals with trisomy 8 although downregulation in the patients with high-grade MDS. Based on the distinct EZH2 expression pattern, we identified an EZH2-targeted gene PSAT1, a serine biosynthetic critical enzyme through an integrated analysis of gene expression microarray analysis in MDS patients and cell line. Functionally, EZH2 can directly activate PSAT1 transcription by binding to its promoter and this effect is independent of its H3K27me3 enzymatic activity in vitro. Expanded sample analysis revealed the overexpression of PSAT1 in MDS patients especially in low-grade MDS. High expression of PSAT1 was associated with severe cytopenias, iron overload and activation of T lymphocytes-mediated autoimmunity. In vitro, overexpression of PSAT1 led to a significant proliferation advantage in the MDS and leukemia cell lines. Overexpression of PSAT1 could also downregulate apoptosis sensitivity to inflammatory cytokines and cytotoxic agents in vitro. However, knockdown of PSAT1 resulted in reduced cell growth capacity and increased the apoptosis sensitivity to cytotoxic agents. Further cancer metabolic experiments showed that overexpression of PSAT1 inhibited the ROS generation and induced an increase in glucose uptake, lactate and ATP production, which may make MDS clonal cells adaptive in the stress-related apoptosis environment.

Conclusion

EZH2/PSAT1 axis-mediated serine biosynthetic pathway is activated in low-grade MDS, which may confer MDS clonal cells growth advantages and resistance to stress-related apoptosis.