Inactivation of CDK4/6, CDK2, and ERK in G1-phase triggers differentiation commitment.

Terminal cell differentiation, a process vital for tissue development and regeneration where progenitor cells acquire specialized functions and permanently exit the cell cycle, remains poorly understood at the molecular level. Using live-cell imaging and adipogenesis as a model, we show that the initial stage involves a variable number of cell divisions, driven by redundant CDK4/6 or CDK2 activation. Afterwards, a delayed decrease in cyclin D1 and an increase in p27 levels reduce CDK4/6 and CDK2 activity. This results in G1 lengthening and the induction of PPARG, the master regulator of adipogenesis. PPARG then induces p21, and later p18, ultimately causing irreversible inactivation of CDK4/6 and CDK2, and thus, permanent cell cycle exit. However, contrary to expectation, CDK inactivation alone is not sufficient to trigger differentiation commitment; ERK inactivation is also necessary. Our study reveals that the coordinated activation and subsequent delayed inactivation of CDK4/6, CDK2, and ERK are crucial for irreversible cell cycle exit and differentiation commitment in terminal cell differentiation.