Doxorubicin promotes transcriptional upregulation of Cdc25B in cancer cells by releasing Sp1 from the promoter.

Cdc25B phosphatases have a key role in G2/M cell-cycle progression by activating the CDK1-cyclinB1 complexes and functioning as important targets of checkpoints. Overexpression of Cdc25B results in a bypass of the G2/M checkpoint and illegitimate entry into mitosis. It can also cause replicative stress, which leads to genomic instability. Thus, fine-tuning of the Cdc25B expression level is critical for correct cell-cycle arrest in response to DNA damage. In response to genotoxic stress, Cdc25B is mainly regulated by post-transcriptional mechanisms affecting either Cdc25B protein stability or translation. Here, we show that upon DNA damage Cdc25B can be regulated at the transcriptional level. Although ionizing radiation downregulates Cdc25B in a p53-dependent pathway, doxorubicin transcriptionally upregulates Cdc25B in p53-proficient cancer cells. We show that in the presence of wild-type p53, doxorubicin activates the Cdc25B promoter by preventing the binding of Sp1 and increasing the binding of NF-Y on the Cdc25B promoter, thus preventing p53 from downregulating this promoter. Our results highlight the mechanistically distinct regulation of the three Cdc25 phosphatases by checkpoint signalling following doxorubicin treatment.

Cdc25B is negatively regulated by p53 through Sp1 and NF-Y transcription factors.

Cdc25B phosphatases function as key players in G2/M cell cycle progression by activating the CDK1-cyclinB1 complexes. They also have an essential role in recovery from the G2/M checkpoint activated in response to DNA damage. Overexpression of Cdc25B results in bypass of the G2/M checkpoint and illegitimate entry into mitosis, and also causes replicative stress, leading to genomic instability. Thus, fine-tuning of Cdc25B expression level is critical for correct cell cycle progression and G2 checkpoint recovery. However, the transcriptional regulation of Cdc25B remains largely unknown. Earlier studies have shown that the tumor suppressor p53 overexpression transcriptionally represses Cdc25B; however, the molecular mechanism of this repression has not yet been elucidated, although it was suggested to occur through the induction of p21. Here we show that Cdc25B is downregulated by the basal level of p53 in multiple cell types. This downregulation also occurs in p21-/- cell lines, indicating that p21 is not required for p53-mediated regulation of Cdc25B. Deletion and mutation analyses of the Cdc25B promoter revealed that downregulation by p53 is dependent on the presence of functional Sp1/Sp3 and NF-Y binding sites. Furthermore, chromatin immunoprecipitation analyses show that p53 binds to the Cdc25B promoter and mediates transcriptional attenuation through the Sp1 and NF-Y transcription factors. Our results suggest that the inability to downregulate Cdc25B after loss of p53 might contribute to tumorigenesis.

A caspase-dependent cleavage of CDC25A generates an active fragment activating cyclin-dependent kinase 2 during apoptosis.

The cellular level of the CDC25A phosphatase is tightly regulated during both the normal and genotoxic-perturbed cell cycle. Here, we describe a caspase-dependent cleavage of this protein at residue D223 in non-genotoxic apoptotic conditions. This specific proteolysis generates a catalytically active C-terminal fragment that localizes to the nuclear compartment. Accumulation of this active CDC25A fragment leads to reduced inhibitory phosphorylation of the CDC25A substrate cyclin-dependent kinase 2 (CDK2) on Tyr15. Moreover, CDK2 was found stably associated with this fragment, as well as with an ectopically expressed CDC25A224-525 truncation mutant that mimicks the cleavage product. Ectopic expression of this mutant induced CDK2 Tyr15 dephosphorylation, whereas its catalytically inactive version did not. Finally, this 224-525 mutant initiated apoptosis when transfected into HeLa cells, whereas its catalytic inactive form did not. Altogether, this study demonstrates for the first time that caspase-dependent cleavage of CDC25A is a central step linking CDK2 activation with non-genotoxic apoptotic induction.

Nuclear localization of Cdc25 is regulated by DNA damage and a 14-3-3 protein.

DNA damage activates a cell-cycle checkpoint that prevents mitosis while DNA repair is under way. The protein Chk1 enforces this checkpoint by phosphorylating the mitotic inducer Cdc25. Phosphorylation of Cdc25 by Chk1 creates a binding site in Cdc25 for 14-3-3 proteins, but it is not known how 14-3-3 proteins regulate Cdc25. Rad24 is a 14-3-3 protein that is important in the DNA-damage checkpoint in fission yeast. Here we show that Rad24 controls the intracellular distribution of Cdc25. Elimination of Rad24 causes nuclear accumulation of Cdc25. Activation of the DNA-damage checkpoint causes the net nuclear export of Cdc25 by a process that requires Chk1, Rad24 and nuclear-export machinery. Mutation of a putative nuclear-export signal in Rad24 impairs the nuclear exclusion of Rad24, the damage-induced nuclear export of Cdc25 and the damage checkpoint. Thus, Rad24 appears to function as an attachable nuclear-export signal that enhances the nuclear export of Cdc25 in response to DNA damage.

Replication checkpoint enforced by kinases Cds1 and Chk1.

Cdc2, the kinase that induces mitosis, is regulated by checkpoints that couple mitosis to the completion of DNA replication and repair. The repair checkpoint kinase Chk1 regulates Cdc25, a phosphatase that activates Cdc2. Effectors of the replication checkpoint evoked by hydroxyurea (HU) are unknown. Treatment of fission yeast with HU stimulated the kinase Cds1, which appears to phosphorylate the kinase Wee1, an inhibitor of Cdc2. The protein kinase Cds1 was also required for a large HU-induced increase in the amount of Mik1, a second inhibitor of Cdc2. HU-induced arrest of cell division was abolished in cds1 chk1 cells. Thus, Cds1 and Chk1 appear to jointly enforce the replication checkpoint.

Negative regulation of Cdc18 DNA replication protein by Cdc2.

Fission yeast Cdc18, a homologue of Cdc6 in budding yeast and metazoans, is periodically expressed during the S phase and required for activation of replication origins. Cdc18 overexpression induces DNA rereplication without mitosis, as does elimination of Cdc2-Cdc13 kinase during G2 phase. These findings suggest that illegitimate activation of origins may be prevented through inhibition of Cdc18 by Cdc2. Consistent with this hypothesis, we report that Cdc18 interacts with Cdc2 in association with Cdc13 and Cig2 B-type cyclins in vivo. Cdc18 is phosphorylated by the associated Cdc2 in vitro. Mutation of a single phosphorylation site, T104A, activates Cdc18 in the rereplication assay. The cdc18-K9 mutation is suppressed by a cig2 mutation, providing genetic evidence that Cdc2-Cig2 kinase inhibits Cdc18. Moreover, constitutive expression of Cig2 prevents rereplication in cells lacking Cdc13. These findings identify Cdc18 as a key target of Cdc2-Cdc13 and Cdc2-Cig2 kinases in the mechanism that limits chromosomal DNA replication to once per cell cycle.

Cig2, a B-type cyclin, promotes the onset of S in Schizosaccharomyces pombe.

Cdc2, a catalytic subunit of cyclin-dependent kinases, is required for both the G1-to-S and G2-to-M transitions in the fission yeast Schizosaccharomyces pombe. Cdc13, a B-type cyclin, is required for the M-phase induction function of Cd2. Two additional B-type cyclins, Cig1 and Cig2, have been identified in S. pombe, but none of the B-type cyclins are individually required for the onset of S. We report that Cdc13 is important for DNA replication in a strain lacking Cig2. Unlike deltacdc13 cells, double-mutant deltacdc13 deltacig2 cells are defective in undergoing multiple rounds of DNA replication. The conclusion that Cig2 promotes S is further supported by the finding that Cig2 protein and Cig2-associated kinase activity appear soon after the completion of M and peak during S, as well as the observation that S is delayed in deltacig2 cells as they recover from a G1 arrest induced by nitrogen starvation. These studies indicate that Cig2 is the primary S-phase-promoting cyclin in S. pombe but that Cdc13 can effectively substitute for Cig2 in deltacig2 cells. These observations also suggest that the gradual increase in the activity of Cdc2-Cdc13 kinase can be sufficient for the correct temporal ordering of S and M phases in deltacig2 cells.

Purification and characterization of the low molecular weight protein tyrosine phosphatase, Stp1, from the fission yeast Schizosaccharomyces pombe.

Genetic screening in fission yeast has identified a gene named stp1+ that rescues cdc25-22 [Mondesert et al. (1994) J. Biol. Chem. 269, 27996-27999]. This gene encodes a 17.4 kDa protein that is 42% identical to members of the low molecular weight protein tyrosine phosphatases (low M(r)PTPases) previously known to exist only in mammalian species. A simple and efficient purification procedure was developed to obtain the homogeneous recombinant yeast low M(r)PTPase, Stp1, in large quantities suitable for kinetic and structural studies. Authentic Stp1 was produced as judged by amino terminal protein sequencing and electrospray ionization mass spectrometry analyses. Stp1 was shown to possess intrinsic phosphatase activity toward both aryl phosphates (such as phosphotyrosine) and alkyl phosphates (such as phosphoserine). Stp1 also dephosphorylated phosphotyrosyl peptide/protein substrates. The yeast enzyme was 6-fold slower than the mammalian enzymes, which made it amenable to pre-steady-state stopped-flow spectroscopic kinetic analysis at 30 degrees C and pH 6.0. Burst kinetics was observed with Stp1 using p-nitrophenyl phosphate as a substrate, suggesting that the rate-limiting step corresponds to the decomposition of the phosphoenzyme intermediate. Interestingly, the bovine heart low M(r)PTPase was capable of removing phosphate groups from both phosphotyrosyl and phosphoseryl/threonyl protein substrates with comparable efficiencies. The low M(r)PTPases, like the Cdc25 family of phosphatases, may represent a new group of dual specificity phosphatases which may be involved in cell cycle control.

Low molecular weight protein-tyrosine phosphatases are highly conserved between fission yeast and man.

Cdc25 protein phosphatase dephosphorylates tyrosine 15 of Cdc2, thereby activating Cdc2/cyclin B kinase, which then brings about mitosis. A fission yeast (Schizosaccharomyces pombe) cDNA expression library was screened for clones that rescue cdc25-22. In addition to the cdc25+ and pyp3+ protein-tyrosine phosphatase genes, a third gene was discovered. This gene, named stp1+ (small tyrosine phosphatase), encodes a approximately 17.5-kDa protein that is approximately 42% identical to members of an unusual class of small (approximately 18 kDa) cytosolic phosphatases previously known to exist only in mammalian species. The biological functions of these proteins are unknown, but they have vigorous protein-tyrosine phosphatase activity in vitro and have a sequence motif, Cys-X5-Arg, that is present at the active sites of all known types of protein-tyrosine phosphatases. Sequence homology between S. pombe Stp1 and its mammalian homologs is particularly high in the active site region of the proteins. Rescue of cdc25-22 by overproduction of Stp1 protein is probably due to an ability of Stp1 to dephosphorylate tyrosine 15 of Cdc2. Disruption of stp1+ causes no obvious phenotype. The fact that Stp1 homologs are highly conserved between yeast and man suggests that they have important functions.