Phosphorylation-triggered CUEDC2 degradation promotes UV-induced G1 arrest through APC/CCdh1 regulation

DNA damage triggers cell cycle arrest to provide a time window for DNA repair. Failure of arrest could lead to genomic instability and tumorigenesis. DNA damage-induced G₁ arrest is generally achieved by the accumulation of Cyclin-dependent kinase inhibitor 1 (p21). However, p21 is degraded and does not play a role in UV-induced G₁ arrest. The mechanism of UV-induced G₁ arrest thus remains elusive. Here, we have identified a critical role for CUE domain-containing protein 2 (CUEDC2) in this process. CUEDC2 binds to and inhibits anaphase-promoting complex/cyclosome-Cdh1 (APC/C^Cdh1), a critical ubiquitin ligase in G₁ phase, thereby stabilizing Cyclin A and promoting G₁–S transition. In response to UV irradiation, CUEDC2 undergoes ERK1/2-dependent phosphorylation and ubiquitin-dependent degradation, leading to APC/C^Cdh1-mediated Cyclin A destruction, Cyclin-dependent kinase 2 inactivation, and G₁ arrest. A nonphosphorylatable CUEDC2 mutant is resistant to UV-induced degradation. Expression of this stable mutant effectively overrides UV-induced G₁–S block. These results establish CUEDC2 as an APC/C^Cdh1 inhibitor and indicate that regulated CUEDC2 degradation is critical for UV-induced G₁ arrest.DNA damage induced by various genotoxic stresses can jeopardize genomic integrity. UV light is the most pervasive environmental DNA-damaging agent, and accumulating evidence indicates that overexposure to UV light would increase the risk of skin cancer development. To maintain genomic stability, DNA damage response triggers cell cycle arrest, especially G₁ arrest, which allows time for DNA repair and prevents aberrant replication of damaged DNA (1). Timely down-regulation of cell cycle promoters and rapid accumulation of cell cycle inhibitors are critical for DNA damage-induced G₁ arrest. Earlier studies have indicated that the DNA damage-induced G₁ arrest is mainly achieved by protein 53 (p53) activation and the subsequent p21 accumulation. However, Cyclin-dependent kinase inhibitor 1 (p21) is degraded following UV irradiation and does not play a role in this process (2). Thus, the molecular mechanism underlying UV-induced G₁ arrest is not fully understood. Understanding the regulation of UV-induced G₁ arrest will ultimately help to develop novel strategies for skin cancer prevention and therapy.The anaphase-promoting complex or cyclosome (APC/C), a multisubunit E3 ubiquitin ligase, is an important regulator of protein degradation during the cell cycle. Activation of APC/C requires the association of either cell division cycle protein 20 (Cdc20) or Cdc20 homolog 1 (Cdh1), two related coactivators that recognize specific substrates containing the destruction box (D-box) or the lysine(K)-glutamic acid(E)-asparagine(N) (KEN) motif (3–5). Cdc20 functions in early mitosis, whereas Cdh1 has crucial functions in both late mitosis and G₁ by targeting multiple cell cycle regulators, such as Cyclin A, Cyclin B1, and S-phase kinase-associated protein 2 (Skp2), for degradation (3, 4, 6–9). The destruction of Cyclin A and Skp2 prevents Cyclin-dependent kinase 2 (CDK2) activation and premature entry into S phase. To enter S phase, APC/C^Cdh1 must be turned off to allow for the reaccumulation of Cyclin A and Skp2 (10–12). However, how APC/C^Cdh1 is switched off is not fully understood. Recent studies have indicated that APC/C^Cdh1 is activated in response to DNA damage stress including UV irradiation and is crucial for maintaining genomic integrity (13–16). The underlying mechanism for APC/C^Cdh1 activation in DNA damage response also remains largely unknown.CUE-domain-containing protein 2 (CUEDC2) plays critical roles in several important signaling pathways (17–21). Our recent work has demonstrated that CUEDC2 is phosphorylated by CDK1 and promotes spindle checkpoint inactivation through releasing APC/C^Cdc20 from checkpoint inhibition during mitosis (19). In the current study, we show that CUEDC2 exists in nonphosphorylated form in G₁ phase, and inhibits APC/C^Cdh1 activity through binding to Cdh1 in a KEN-box–dependent manner. Upon UV treatment, ERK1/2 mediates CUEDC2 phosphorylation and triggers its degradation. Destruction of CUEDC2 releases APC/C^Cdh1 activity, resulting in Cyclin A destruction, CDK2 inactivation, and G₁ arrest. A nonphosphorylatable stable CUEDC2 mutant overrides UV-induced G₁ arrest. Collectively, our results identify CUEDC2 as a regulator of APC/C^Cdh1 and implicate its regulated degradation as an important mechanism for UV-induced G₁ arrest.