Extended role of p21 during the exit from p21-induced G0/G1 arrest

Stress- or developmentally-induced signals can trigger G0/G1 cell cycle arrest through the upregulation of p21, a key inhibitor that halts proliferation. While p21 is widely recognized as essential for establishing this arrest, its degradation is generally assumed to be a prerequisite for the exit from arrest and cell cycle re-entry. Using a rapid p21 degradation system that allows for both endogenous p21 tracking and controlled depletion, we uncovered an extended role for p21 during the exit from G0/G1 arrest. Our results showed that removal of the arrest inducing signals did not lead to an immediate decline of p21 during the exit process; instead, p21 levels continued to maintain for a period before its gradual down-regulation. Importantly, premature depletion of p21 during this process weakened the capacity of cell cycle re-entry, particularly in cells with high levels of pre-existent p21. We found that during the release from arrest, cells with high levels of pre-existent p21 but a premature p21 depletion exhibited reduced capacity to restore KRAS/ERK activity, and supplementing KRAS/ERK activity rescued the failure of cell cycle re-entry in these cells. These findings reveal a previously unappreciated function for p21 during the exit from G0/G1 arrest, aside and distinct from its role in initiating the arrest. This new paradigm extends across multiple cell lines and stress-induced arrest contexts, offering a new framework for understanding p21 function in response to anticancer therapies and in development.