OR34-2 SREBP1 Drives KRT80-Dependent Cytoskeletal Changes and Invasive Behaviour in Endocrine-Resistant ERα Breast Cancer

About one third of oestrogen receptor alpha breast cancer (ERα BC) patients progress to invasive metastatic disease despite targeted endocrine therapies(1). Our previous study showed how breast cancer cells developing resistance to aromatase inhibitors (AI) endogenously trigger cholesterol biosynthesis potentially through activation of sterol regulatory element binding protein 1 (SREBP1) to promote sustained oestrogen independent, ERα activation(2). Tumour cells are phenotypically characterized by an aberrant architecture and, although this feature is associated with cell migration and invasion, the mechanisms underlying the cytoskeletal reorganization are still poorly understood. In this study, we used breast cancer models to investigate the relationship between acquisition of drug resistance and invasive potential. We showed that cells acquiring resistance to AI undergo active cytoskeleton re-organisation via Keratin 80 (KRT80) and F-Actin remodelling. This process is driven by epigenetic reprogramming at the type II keratin locus leading to KRT80 upregulation. Chromatin Immunoprecipitation coupled with Next Generation Sequencing (ChIP-seq) revealed that reprogramming is dependent on de novo SREBP1 binding to a single enhancer that is activated upon chronic AI treatment.  AI-treated patients’ samples are characterized by KRT80 cytoskeletal re-organization and an increased number of KRT80 positive cells at relapse (seventy-five human breast specimens and ten metastatic lymph nodes were selected for immunostaining with the Trust Tissue Bank approval). Using confocal microscopy imaging we found that KRT80 activation and redeployment led to increased F-actin deposition and increased focal adhesion. Additionally, results from 3D organoids invasion assay showed KRT80 manipulation directly contributed to changes in cellular stiffness and invasive potential. Intriguingly, radiological exam using shear wave elasticity imaging performed on twenty prospectively recruited cancer patients confirmed that KRT80 levels correlate with stiffer tumours in vivo. Our data strongly suggested that therapy plays a direct role in shaping the biophysical properties and invasive potential of cancer cells, by inducing epigenetic rearrangements leading to KRT80 upregulation and concomitant cytoskeletal reorganization. We also described an unexpected role for intermediate filaments in promoting cancer cell invasion by showing for the first time that KRT80 promotes actin fibre formation as well as focal adhesion maturation and lamellipodia formation. The link between epigenetic and cytoskeleton reprogramming offers an intriguing axis for drug development and biomarker discovery, especially within the goal of preventing metastatic invasion in BC patients. References: (1) (EBCTCG), Lancet 386, 1341–1352 (2015). (2) Nguyen, V. et al. Nat Commun 6, 10044 (2015).