DNA damage caused by ultraviolet light (UV), ionizing radiation (IR), or DNA damaging agents results in the phosphorylation of CDC25A (Zhao et al. 2002, Busino et al. 2003) on multiple serine residues, which inhibits CDC25A (Zhao et al. 2002) by promoting its binding to the E3 ubiquitin ligase complex SCF-BTrCP (Busino et al. 2003) and subsequent ubiquitination and proteasome-mediated degradation (Mailand et al. 2000, Busino et al. 2003, Xiao et al. 2003) in a TP53-independent manner (Mailand et al. 2000, Xiao et al. 2003).
A recombinant human CHEK1 (Chk1) forms a complex with and phosphorylates a recombinant human CDC25A (Sanchez et al. 1997). In vitro, CHEK1 phosphorylates CDC25A on serine and threonine residues but endogenous CDC25A is only phosphorylated on serine residues (Hassepass et al. 2003). Seven evolutionarily conserved serine residues in CDC25A (S76, S124, S156, S178, S185, S293, S505) correspond to the consensus CHEK1/CHEK2 phosphorylation motif Arg-X-X-Ser (Hassepass et al. 2003). CHEK1-mediated phosphorylation of CDC25A leads to a 3-fold decrease in in vitro kinase activity of CDC25A (Hassepass et al. 2003). Destruction of CDC25A in response to UV exposure is dependent on CHEK1 activity as it can be inhibited by CHEK1 inhibitors caffeine and UCN-01 (Mailand et al. 2000). CHEK1 phosphorylated residues of CDC25A include S76 (Goloudina et al. 2003, Hassepass et al. 2003, Jin et al. 2008), S124 (Zhao et al. 2002, Sorensen et al. 2003, Goloudina et al. 2003, Hassepass et al. 2003), S178 (Sorensen et al. 2003, Hassepass et al. 2003), S279 (Sorensen et al. 2003), and S293 (Sorensen et al. 2003). CHEK1-phosphorylated sites in CDC25A partially overlap with CHEK2-phosphorylated sites (Sorensen et al. 2003, Jin et al. 2008). Phosphorylation of S124, S178, S279 and S293 promotes both basal and IR-induced proteolysis of CDC25A, with quadruple serine to alanine substitution mutants exhibiting a significantly increased stability. S124 has been reported as the only site that affects CDC25A half-life when mutated to alanine alone (Sorensen et al. 2003), but also as the site whose substitution to alanine attenuates CDC25A degradation in response to IR but not UV (Goloudina et al. 2003), as well as the site whose alanine substitution has no effect on CDC25A stability upon UV treatment (Hassepass et al. 2003). The S76A substitution was reported to impair CDC25A degradation in response to both UV- (Goloudina et al. 2003, Hassepass et al. 2003) and IR-induced DNA damage (Goloudina et al. 2003). CDC25A S178A mutation did not prevent UV-induced CDC25A degradation (Hassepass et al. 2003). However, neither CDC25A S76A nor CDC25A S124A, nor the double mutant CDC25A S76A; S124A were able to prevent G1/S arrest after IR or UV, implying additional regulatory mechanisms (Goloudina et al. 2003). Ectopic expression of CDC25A S76A was initially reported to not prevent inhibition of CDK2 activity (Goloudina et al. 2003) and subsequently to prevent inhibition of CCNE:CDK2 activity (Hassepass et al. 2003).
CHEK1-mediated phosphorylation of CDC25A is required to induce the G1/S arrest and prevent radioresistant DNA synthesis upon IR treatment in HeLa cells and normal human diploid fibroblasts (Zhao et al. 2002, Sorensen et al. 2003). CHEK1 is necessary for UV- and IR-induced degradation of CDC25A, while CHEK2 depletion does not significantly contribute to stabilization of CDC25A in CHEK1-depleted human cells (Jin et al. 2008).
A recent study shows that, even in the absence of DNA damage, low cellular proliferation seen upon CHEK1 depletion is partially improved by co-depletion of CDC25A, suggesting that CHEK1 may also control normal cell cycle progression through induction of CDC25A degradation (Goto et al. 2019).
