Estrogen receptor positive (ER+) breast cancer exhibits relatively favourable five-year survival rates for patients receiving estrogen receptor inhibitors (ERi), aromatase inhibitors, or cell cycle inhibitors (CDK4/6i), yet therapy resistance remains a significant cause of breast cancer-related death. Studies in numerous cancers demonstrate that prolonged cytostatic therapy can select for tumour cell sub-populations with intrinsic or acquired resistance mechanisms, which consequently drive tumour recurrence. Activation of Nuclear factor-kappa B (NF-kB) has been shown to promote treatment resistance to ERi by promoting many pathways underpinning hallmarks of cancer, including cell death resistance and dysregulation of cellular metabolism. Tristetraprolin (TTP) negatively regulates NF-kB and associated NF-kB target genes, and low TTP expression in ER+ breast cancer was associated with a two-to-threefold increase in risk of recurrence, implicating TTP expression in treatment resistance.
We demonstrate that ER+ MCF-7 breast cancer cells exhibit low TTP expression concomitant with an increase in NF-kB activity compared to normal mammary epithelial cells, consistent with findings in TCGA patient data. Interrogation of TCGA and METABRIC ER+ breast cancer datasets reveal worse survival outcomes in patients with low TTP and RUNX1 expression, another negative regulator of NF-kB. In vitro time-course experiments in MCF-7 cells show the ERi, tamoxifen, induces hypo-proliferation through modulation of the cell cycle and anti-apoptotic proteins as early as ten days after exposure. Further, tamoxifen increases production of the inflammatory cytokines, IL-6 and CXCL8, important NF-kB target genes known to augment treatment resistance through enhancing cell survival and chemotherapy resistance. To combat this, we have developed a new formulation of an NF-kB inhibitor (DMAPT-NF), capable of suppressing tamoxifen-induced IL-6 and CXCL8 expression. Importantly, treatment with low dose tamoxifen together with DMAPT-NF effectively suppresses MCF-7 proliferation and is associated with increased apoptosis compared with tamoxifen alone. Notable changes in the lipid profiles of MCF-7 cells exposed to DMAPT-NF are observed, suggesting disruption of cell proliferation may involve lipid metabolism. Our findings reveal the importance of the TTP-NF-kB axis in ERi treatment resistance and have identified DMAPT-NF as a potent inhibitor of ER+ breast cancer cells, potentiating its clinical translation.