Degradation of p21Cip1 through Anaphase-promoting Complex/Cyclosome and Its Activator Cdc20 (APC/CCdc20) Ubiquitin Ligase Complex-mediated Ubiquitylation Is Inhibited by Cyclin-dependent Kinase 2 in Cardiomyocytes

Cyclin-dependent kinase inhibitor p21(Cip1) plays a crucial role in regulating cell cycle arrest and differentiation. It is known that p21(Cip1) increases during terminal differentiation of cardiomyocytes, but its expression control and biological roles are not fully understood. Here, we show that the p21(Cip1) protein is stabilized in cardiomyocytes after mitogenic stimulation, due to its increased CDK2 binding and inhibition of ubiquitylation. The APC/C-Cdc20 complex is shown to be an E3 ligase mediating ubiquitylation of p21(Cip1) at the N terminus. CDK2, but not CDC2, suppressed the interaction of p21(Cip1) with Cdc20, thereby leading to inhibition of anaphase-promoting complex/cyclosome and its activator Cdc20 (APC/C-Cdc20)-mediated p21(Cip1) ubiquitylation. It was further demonstrated that p21(Cip1) accumulation caused G(2) arrest of cardiomyocytes that were forced to re-enter the cell cycle. Taken together, these data show that the stability of the p21(Cip1) protein is actively regulated in terminally differentiated cardiomyocytes and plays a role in inhibiting their uncontrolled cell cycle progression. Our study provides a novel insight on the control of p21(Cip1) by ubiquitin-mediated degradation and its implication in cell cycle arrest in terminal differentiation.