G1 checkpoint is compromised in mouse ESCs due to functional uncoupling of p53-p21Waf1 signaling

<div><p></p><p>Mouse embryonic stem cells (mESCs) lack of G1 checkpoint despite that irradiation (IR) activates ATM/ATR-mediated DDR signaling pathway. The IR-induced p53 localizes in the nuclei and up-regulates <i>p21/Waf1</i> transcription but that does not lead to accumulation of p21/Waf1 protein. The negative control of the p21Waf1 expression appears to occur at two levels of regulation. First, both <i>p21/Waf1</i> gene transcription and the p21/Waf1 protein content increase in mESCs treated with histone-deacetylase inhibitors, implying its epigenetic regulation. Second, proteasome inhibitors cause the p21/Waf1 accumulation, indicating that the protein is a subject of proteasome-dependent degradation in ESСs. Then, the dynamics of IR-induced p21Waf1 protein show its accumulation at long-term time points (3 and 5 days) that coincides with an increase in the proportion of G1-phase cells, down-regulation of <i>Oct4</i> and <i>Nanog</i> pluripotent gene transcription and activation of endoderm-specific genes <i>sox17</i> and <i>afp</i>. In addition, nutlin-dependent stabilization of p53 in mESC was also accompanied by the accumulation of p21/Waf1 as well as restoration of G1 checkpoint and an onset of differentiation. Thus, the lack of functional p21/Waf1 is indispensable for maintaining self-renewal and pluripotency of mESCs.</p></div