Pessa, J. C.; Paavolainen, O.; Puustinen, M. C.; Hastbacka, H. S.; Da Silva, A. J.; Pihlstrom, S.; Gramolelli, S.; Bostrom, P.; Hartiala, P.; Peuhu, E.; Joutsen, J.; Sistonen, L.

Phenotypic plasticity is a hallmark of breast carcinogenesis that facilitates the acquisition of invasive properties via epithelial-mesenchymal transition (EMT). Transforming growth factor-beta (TGF-{beta}), a key EMT-inducing cytokine, drives pro-metastatic gene programs through downstream transcription factors. Among mammalian stress-protective transcription factors, heat shock factor 2 (HSF2) has been associated with cancer progression, but the mechanisms regulating HSF2 expression and activity are unknown. Here, we demonstrate that TGF-{beta} stimulation downregulates HSF2 to enable activation of an invasive phenotype. Remarkably, ectopic expression of HSF2 in breast cancer cells inhibited TGF-{beta}-mediated effects on gene expression and cellular properties. By using both in vitro cell models and in vivo zebrafish xenografts, we found that a temporal downregulation of HSF2 is a prerequisite for EMT activation, while ectopically sustained HSF2 promotes rapid cell proliferation and survival. Analyses of human patient tissues corroborated our results by showing that HSF2 is dynamically regulated during breast cancer progression. Specifically, HSF2 expression and nuclear co-localization with the proliferation marker Ki67 are dramatically increased already in ductal carcinoma in situ. Altogether, our findings expand the pathological landscape of HSF2, demonstrating that dynamic regulation of HSF2 is an inherent property of malignant progression and characterizes the distinct stages of breast cancer.