Myosin VI orchestrates estrogen-driven gene expression in breast cancer cells.

Approximately 75% of breast cancer cases are estrogen receptor (ER)-positive, with endocrine therapies forming the foundation of treatment. However, therapeutic resistance remains a major clinical challenge, necessitating the identification of new molecular targets. Myosin VI (MVI), a motor protein increasingly linked to cancer, directly interacts with the ER and plays a key role in the spatial regulation of RNA Polymerase II. Notably, expression levels of MVI and ER are positively correlated across breast cancer tissues. Here, we present a multidisciplinary investigation combining advanced imaging, genomic profiling, and phenotypic characterisation to elucidate the interplay between MVI and the ER. We demonstrate that MVI nuclear localisation is dynamically regulated by estrogen signalling and ER activity. Conversely, ER nuclear localisation requires active MVI, suggesting a reciprocal regulatory mechanism. Furthermore, MVI influences subnuclear architecture, modulating ER transcriptional activity and downstream gene expression programmes that govern cell proliferation and migration. Importantly, pharmacological inhibition of MVI enhances the effect of hormone therapy, resulting in greater disruption of ER function than monotherapy alone. Moreover, MVI inhibition also suppresses the activity of hormone-resistant ER mutants, highlighting its potential to overcome therapy resistance. Our findings establish MVI as a critical regulator of ER nuclear dynamics and gene expression, supporting its candidacy as a novel therapeutic target in ER-positive breast cancer.