EZH2-targeted PROTACs target the EZH2/FOXM1 axis and reduce breast cancer cell growth

Breast cancer (BCa) remains the second leading cause of cancer-related mortalities in women, and acquired resistance to hormone therapies, such as tamoxifen, an estrogen receptor inhibitor, is a major hurdle in the treatment of luminal BCa. Another subtype, triple negative BCa (TNBC), is associated with aggressive disease and poor prognosis. The enhancer of zeste homolog 2 (EZH2), the methyltransferase component of the polycomb repressive complex 2 (PRC2), is overexpressed in BCa and has been implicated in tamoxifen resistance. In addition to its PRC2-dependent canonical transcription repressive role through catalyzing histone 3 lysine 27 trimethylation (H3K27me3), evidence suggests that EZH2 can function noncanonically, in a methyltransferase-independent manner, as a transcription activator through interacting with hormone receptors and oncogenic transcription factors. Unlike methyltransferase inhibitors, proteolysis targeting chimeras (PROTAC), which target EZH2 and interacting proteins for degradation, can suppress both activating and repressive functions of EZH2. Previous studies have suggested that PROTACs can be leveraged to inhibit TNBC cell growth. In this study, we expand our scope to test whether EZH2 targeted PROTACs can effectively inhibit luminal BCa cell growth. We find that EZH2-targeted PROTACs, MS177 and MS8815, effectively inhibited the growth luminal BCa cells, including those with acquired tamoxifen resistance, to a much greater degree when compared to methyltransferase inhibitors. Similarly, PROTACs uniquely reduced the expression of genes involved in cell cycle progression, including forkhead box M1 (FOXM1) target genes, in BCa cell lines. Likewise, promoter regions with EZH2 binding in the absence of H3K27me3 were enriched with FOXM1 target genes in both luminal BCa and TNBC cell lines, suggesting a regulatory mechanism independent of hormone receptor status. EZH2 PROTAC treatment reduced FOXM1 protein expression and increased its degradation. In clinical samples, EZH2 mRNA expression tightly correlated with FOXM1 and FOXM1 target genes. Together, this study suggests that EZH2 targeted PROTACs represent a promising avenue of research for the future treatment of BCa, including in the setting of tamoxifen resistance.