Actin-modulated nuclear shape controls adaptive reprogramming in cancer-associated fibroblasts
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Cancer-associated fibroblasts (CAFs) adapt to external cues such as therapeutic agents and extracellular matrix (ECM) stiffness in the tumor microenvironment (TME) through a reprogramming process. Here, we show that BRAF inhibitors (BRAFis) drive accumulation of nuclear β-catenin in CAFs by inducing actin-mediated deformation of the nucleus. Exposure to stiff substrates can also lead CAFs to undergo cytoskeletal reorganization and exert forces on the nucleus, allowing β-catenin to enter the nucleus and reprogram its transcriptional activities. Mechanistically, BRAFi accelerates RAS-dependent RAF kinase transactivation by binding to the BRAF and CRAF kinase domains, accelerating BRAF and CRAF homo and heterodimerization and phosphorylation. Subsequently, RAF activation initiates downstream ERK signaling, which simultaneously inactivates GSK-3β and stimulates Rho kinase (ROCK) signaling. Notably, ablating RAS and RAF isoforms as well as pharmacological blockade of ROCK activity effectively suppressed BRAFi-induced nuclear deformation and β-catenin entry in CAFs, further confirming that the ROCK-cytoskeleton axis mediates BRAFi-driven RAF activation and nuclear import of β-catenin for reprogramming. Thus, ROCK-regulated actin polymerization is a master CAF response pathway that can be stimulated by external signals to reprogram the transcriptional activity of CAFs by enhancing nuclear β-catenin transport through a noncanonical mechanical mechanism.
