RSK3 switches cell fate: from stress-induced senescence to malignant progression

TGFβ signaling induces several cell phenotypes including cellular senescence, a stable form of cell cycle arrest accompanied by a secretory program, and epithelial-mesenchymal transition (EMT) in normal epithelial cells. During carcinogenesis cells lose the ability to undergo senescence in response to TGFβ but they maintain an EMT, which can contribute to tumor progression and resistance. A screen with active kinases in HMECs upon TGFβ treatment identified that the serine threonine kinase RSK3, or RPS6KA2, reverted TGFβ-induced senescence. Interestingly, RSK3 expression decreased in response to TGFβ in a SMAD3-dependent manner, and its constitutive expression rescued SMAD3-induced premature senescence, indicating that decrease of RSK3 itself contributes to TGFβ-induced senescence. Mechanistically, using transcriptomic analyses and affinity purification coupled to mass spectrometry-based proteomics, we unveiled that RSK3 regulates senescence by inhibiting NF-κΒ pathway through the decrease in proteasome-mediated IκBα degradation. Strikingly, senescent TGFβ-treated HMEC display features of epithelial to mesenchymal transition (EMT) and during RSK3-induced senescence escape HMEC conserve EMT features. Importantly, RSK3 expression correlates with EMT and invasion, and anti-correlates with senescence and NF-κΒ in human claudin-low breast tumors and its expression accelerates formation of breast invasive tumors in the mouse mammary gland. We conclude that RSK3 switches cell fate from senescence to malignancy in response to TGFβ signals. hTERT-immortalized Human Mammary Epithelial Cells (HMECT) were transduced with either pWZL control or pWZL encoding RSK3 vectors and treated or not with TGFb. Three independent experiments have been performed.