CDK4 Restricts Triple-Negative Breast Cancer Cell Migration via Phosphorylation-Driven Activation of Myo9b RhoGAP Function

Cyclin-Dependent Kinase 4 (CDK4) is a key regulator of cell cycle progression, driving the G0/G1-to-S phase transition through phosphorylation of Retinoblastoma 1 (RB1). Clinically, CDK4/6 inhibitors are under investigation in Triple Negative Breast Cancer (TNBC), a subtype characterized by invasiveness, aggressiveness and poor prognosis. While CDK4 is primarily targeted for its role in proliferation, emerging evidence suggests it may also regulate other cellular processes. In particular, the mechanisms by which CDK4 could influence cancer cell migration, remain largely unexplored, particularly in highly heterogenous cell line like MDA-MB-231. This study investigates whether CDK4 contributes to the regulation of TNBC cells migration and identifies the pathways involved in MDA-MB-231 cells, independently of its role in proliferation. We demonstrate that loss or inhibition of CDK4, using respectively CRISPR/Cas9 mediated CDK4 knockout and pharmacological CDK4/6 inhibitor, leads to enhanced migration capacities and reorganization of actin subcellular networks. Mechanistically, the absence of CDK4 results in decreased phosphorylation of Myo9b at serine 1935 (S1935), which enhances RhoA signaling, a key driver of cytoskeletal dynamics, leading to polarity defects and increased cell migration. These findings reveal a non-canonical function of CDK4 in limiting TNBC cell migration through the CDK4/CyclinD-Myo9b-RhoA signaling axis. This work highlights the broader cellular roles of CDK4 beyond its established function in proliferation and suggest that inhibition of Myo9b-RhoA pathway could reduce metastatic behaviour in TNBC treated with CDK4/6i, thereby informing future co-therapeutic strategies against aggressive cancer subtypes.