Histone methyltransferases regulate the transcriptional expression of ERα and the proliferation of tamoxifen-resistant breast cancer cells

Purpose Although tamoxifen remains the frontline treatment for ER alpha-positive breast cancers, resistance to this drug limits its clinical efficacy. Most tamoxifen-resistant patients retain ER alpha expression which may support growth and progression of breast cancers. Therefore, we investigated epigenetic regulation of ER alpha that may provide a rationale for targeting ER alpha in these patients. Methods Expression levels of the mixed-lineage leukemia (MLL) family of proteins in tamoxifen-resistant breast cancer cells and publicly available breast cancer patient data sets were analyzed. Histone methylation levels in ER alpha promoter regions were assessed using chromatin immunoprecipitation. Expression levels of ER alpha and its target gene were analyzed using western blotting and real-time qPCR. Cell-cycle was analyzed by flow cytometry. Results The expression of MLL3 and SET-domain-containing 1A (SET1A) were increased in tamoxifen-resistant breast cancers. An MLL3 chromatin immunoprecipitation-sequencing data analysis and chromatin immunoprecipitation experiments for MLL3 and SET1A suggested that these proteins bound to enhancer or intron regions of the ESR1 gene and regulated histone H3K4 methylation status. Depletion of MLL3 or SET1A downregulated the expression level of ER alpha and inhibited the growth of tamoxifen-resistant breast cancer cells. Additional treatment with fulvestrant resulted in a synergistic reduction of ER alpha levels and the growth of the cells. Conclusions The enhanced expression of MLL3 and SET1A in tamoxifen-resistant breast cancer cells supported the ER alpha-dependent growth of these cells by increasing ER alpha expression. Our results suggest that targeting these histone methyltransferases might provide an attractive strategy to overcome endocrine resistance.