CDK4 maintains cytoskeletal homeostasis by restraining actomyosin contractility

Here, we uncover a non-canonical function of CDK4 in maintaining cytoskeletal mechanical homeostasis in triple-negative breast cancer (TNBC) cells. We show that CDK4 phosphorylates the RhoA-targeting RhoGAP Myo9b at serine 1935, enhancing its GAP activity and restraining RhoA–ROCK–pMLC-dependent actomyosin contractility.

Loss or inhibition of CDK4 alters cell morphology, disrupts actin organization, and increases contractility. Elevated contractility in CDK4-deficient cells induces collapse of the intermediate filament network, characterized by Vimentin reorganization and mitochondrial redistribution, indicating a higher-order defect in cytoskeletal architecture. Furthermore, CDK4 expression negatively correlates with pMLC levels in both mouse TNBC tumors and human breast cancer samples, supporting the physiological relevance of this pathway.