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therapies. The relationship between acquisition of drug resistance and invasive potential is poorly understood. Currently, invasive behaviour is thought to be driven mainly by epithelial to mesenchymal transition. Material and methods MCF7 cell line and derived resistant clones were used for this study. MCF7 Tamoxifen Resistant (MCF7TR) and LTED (Long Term Oestrogen Deprivation) were derived from MCF7 upon one-year Tamoxifen or oestrogen deprivation, respectively. LTED combination treatments were also used (LTEDT and LTEDF). Additionally, we used T47D and T47D-LTED. Stable cell lines were generated for both KRT80 over-expression and knockdown. 3D organoids invasion assay, immunofluorescence, confocal microscopy, RNA-seq, ChIP-seq, RT-qPCR and Western blot were performed. Seventy-five human breast specimens and ten metastatic lymph nodes were selected with the approval of Imperial College Healthcare NHS Trust Tissue Bank. Twenty women with suspected breast cancer were prospectively recruited and radiological exam using shear wave ultrasound was used to determine tissue stiffness in the normal and peritumoral stroma, and suspected lesion. Results and discussions In this study, we show that cells that acquire resistance to aromatase inhibitors (AI) undergo active cytoskeleton re-organisation via Keratin 80 (KRT80) and FActin remodelling. These features directly drive the invasive phenotype. Mechanistically, we show that this process is driven by epigenetic reprogramming at the type II keratin locus (chromosome 12) leading to Keratin 80 (KRT80) up-regulation. Reprogramming is dependent on de novo SREBP1 binding to a single enhancer that is activated upon chronic AI treatment. AI-treated patients show KRT80 cytoskeletal reorganisation and an increased number of KRT80 positive cells at relapse. We find that KRT80 activation and redeployment leads to increased F-actin deposition and focal adhesion. Additionally, we show that KRT80 manipulation directly contributes to changes in cellular stiffness and invasive potential. In agreement, shear-wave elasticity imaging of prospective patients show that KRT80 levels correlate with stiffer tumours in vivo. Conclusion Collectively, our data uncover an unexpected and potentially targetable link between epigenetic reprogramming and cytoskeletal changes promoting cell invasion.