DNA damage caused by ionizing radiation (IR) leads to phosphorylation of CDC25A by CHEK2 (Chk2) in a TP53-independent manner (Falck et al. 2001) on serine residues S124 (Falck et al. 2001, Sorensen et al. 2003), S178 (Sorensen et al. 2003), and S293 (Sorensen et al. 2003), promoting CDC25A proteolysis (Falck et al. 2001, Sorensen et al. 2003). In vitro, CHEK2 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). Serine to alanine substitution at S124 significantly increases the half-life of CDC25A (Sorensen et al. 2003). CHEK2-phosphorylated sites in CDC25A partially overlap with CHEK1-phosphorylated sites (Sorensen et al. 2003, Jin et al. 2008).
CDC25A is almost completely downregulated 30 minutes after IR and recovers to levels seen in non-irradiated cells 4-8 h later (Falck et al. 2001). Maximum IR-induced downregulation of CDC25A (1-3 h after IR) correlates with an increased inhibitory phosphorylation of CDK2 at Y15, a reduction of the S phase-promoting CCNE:CDK2 kinase activity, and ~50% inhibition of DNA synthesis (Falck et al. 2001). Ectopic overexpression of CDC25A results in radioresistant DNA synthesis and can be mimicked by ectopic expression of CDK2 mutant with T14A and Y15F substitutions (Falck et al. 2001).
The sudy by Falck et al. 2001 reported that the increased phoshorylation of CDC25A in response to IR correlates with activating phosphorylation of CHEK2 but not CHEK1 (Chk1), but a study by Jin et al. 2008 showed that CHEK1 is also involved in CDC25A destabilization upon IR treatment (see below). Some of the cancer-associated CHEK2 variants are defective in their ability to bind and phosphorylate CDC25A (Falck et al. 2001). Irradiated ATM-defective cells isolated from ataxia-telangiectasia patients are unable to activate CHEK2 and downregulate CDC25A protein level and activity (Falck et al. 2001). In the study by Sorensen et al. 2003, however, it was shown that CHEK2 on its own was unable to induce the G1/S arrest in IR-treated HeLa cells and that CHEK1 activity was needed (Sorensen et al. 2003), which was reproduced in normal human diploid fibroblasts (Zhao et al. 2002). These findings are additionally corroborated by a study showing that CHEK2 does not phosphorylate S76 residue of CDC25A and that CHEK2 depletion does not lead to additional stabilization of CDC25A in CHEK1-depleted cells upon IR or UV treatment, while CHEK1 depletion leads to significant additional stabilization of CDC25A in CHEK2-depleted cells (Jin et al. 2008).
