MRTFA-SRF driven gene expression promotes cellular stiffness by controlling cytoplasmic Ca2+ and K+ levels in cancer cells
Cellular stiffness, the inverse of compliance, is a biophysical property of cells that fundamentally impacts homeostatic functions and pathological states and understanding how these properties are controlled can provide novel insights into tissue homeostasis and disease mechanisms. A prominent regulator of cellular stiffness is the filamentous actin (F-actin) cytoskeleton that forms aligned stress fibers during cellular contraction. It has been recently shown that F-actin polymerization and the subsequent cortical stiffening of cancer cells is regulated by the myocardin-related transcription factor A (MRTFA) and this results in dramatic changes in the ability of cancer cells to evade anti-tumor immune surveillance. However, the precise mechanism by which MRTFA regulated cortical stiffness remained unknown. Here, we demonstrate that cancer cell stiffening occurs as a consequence of MRTFA binding to its cofactor serum response factor (SRF). Specifically, MRTFA-SRF promotes the expression of Ca2+ and K+ handling genes and the flux of these ions are critically needed for cortical stiffening. Finally, we show the results of a series of rigorous gain-of-function and loss-of-function studies that reveal the large conductance K+ channel auxiliary subunit, KCNMB1, as an MRTFA-SRF target that regulates cell stiffness and F-actin organization. Collectively, these findings point to previously unrecognized MRTFA-SRF targets in cellular stiffness, including a novel druggable ion channel. Overall design: To investigate the role of MRTFA-SRF binding on cancer cell stiffness phenotypes, using 2 cell lines with doxycycline-inducible overexpression of wildtype MRTFA or SRF-binding deficient MRTFA (Y238A), data is from 3 separate replicate sets.