Epigenetic regulation of histone lysine demethylase 4B on cancer therapy in tamoxifen-sensitive and -resistant breast cancer cells

Background: Histone lysine demethylase 4B (KDM4B), a key histone lysine demethylase, is aberrantly overexpressed in breast cancer and functions as an essential epigenetic co-activator of ERalpha signaling. Although its oncogenic role in breast tumorigenesis has been established, the functional distinctions and underlying molecular mechanisms of KDM4B in tamoxifen-sensitive versus tamoxifen-resistant states remain poorly defined. Elucidating KDM4B's contribution to the evolution of endocrine resistance will be critical for identifying novel therapeutic targets and advancing precision strategies to overcome resistance to endocrine therapy. Methods: KDM4B expression across human cancers and normal tissues was evaluated using TCGA and GTEx datasets. To investigate its functional role, we compared the effects of KDM4B modulation in ERalpha-positive MCF-7 cells and their tamoxifen-resistant derivative, MCF-7 TamR. KDM4B was suppressed either by siRNA-mediated knockdown or by pharmacological inhibition using the selective KDM4B inhibitor B3. Cell proliferation was measured through MTT and colony formation assays. Protein expression levels of ERalpha, Bcl-2, H3K9me3, Cyclin D1, c-Myc, and related signaling molecules were examined via Western blotting. Cell-cycle progression and apoptosis were analyzed using flow cytometry. Metabolic reprogramming was evaluated using the Seahorse glycolytic stress test. Results: TCGA and GTEx analyses revealed that KDM4B is significantly overexpressed in breast cancer compared with normal breast tissue. Consistently, KDM4B levels were markedly elevated in tamoxifen-resistant MCF-7 (TamR) cells relative to parental MCF-7 cells. Suppression of KDM4B through siRNA-mediated knockdown or pharmacological inhibition substantially reduced the proliferative capacity and colony-forming ability of both MCF-7 and TamR cells. KDM4B depletion restored H3K9me3 enrichment, decreased ERalpha expression, and induced G1/S cell-cycle arrest. In addition, metabolic profiling demonstrated that KDM4B inhibition attenuated glycolytic activity, indicating a role in metabolic reprogramming. Conclusion: KDM4B serves as a critical epigenetic regulator in both tamoxifen-sensitive and tamoxifen-resistant ERalpha-positive breast cancer. Inhibition of KDM4B restores repressive histone marks, suppresses ERalpha-dependent transcription, and attenuates cellular proliferation and metabolic activity. The heightened dependence of tamoxifen-resistant cells on KDM4B further underscores its potential as a promising therapeutic target for overcoming endocrine therapy resistance.