Expression of cdc25A and cdc25B phosphatase in breast carcinoma

CDC25B partners with PP2A to induce AMPK activation and tumor suppression in triple negative breast cancer

Cell division cycle 25 (CDC25) dual specificity phosphatases positively regulate the cell cycle by activating cyclin-dependent kinase/cyclin complexes. Here, we demonstrate that in addition to its role in cell cycle regulation, CDC25B functions as a regulator of protein phosphatase 2A (PP2A), a major cellular Ser/Thr phosphatase, through its direct interaction with PP2A catalytic subunit. Importantly, CDC25B alters the regulation of AMP-activated protein kinase signaling (AMPK) by PP2A, increasing AMPK activity by inhibiting PP2A to dephosphorylate AMPK. CDC25B depletion leads to metformin resistance by inhibiting metformin-induced AMPK activation. Furthermore, dual inhibition of CDC25B and PP2A further inhibits growth of 3D organoids isolated from patient derived xenograft model of breast cancer compared to CDC25B inhibition alone. Our study identifies CDC25B as a regulator of PP2A, and uncovers a mechanism of controlling the activity of a key energy metabolism marker, AMPK.

Cyclin E expression is associated with high levels of replication stress in triple-negative breast cancer

Replication stress entails the improper progression of DNA replication. In cancer cells, including breast cancer cells, an important cause of replication stress is oncogene activation. Importantly, tumors with high levels of replication stress may have different clinical behavior, and high levels of replication stress appear to be a vulnerability of cancer cells, which may be therapeutically targeted by novel molecularly targeted agents. Unfortunately, data on replication stress is largely based on experimental models. Further investigation of replication stress in clinical samples is required to optimally implement novel therapeutics. To uncover the relation between oncogene expression, replication stress, and clinical features of breast cancer subgroups, we immunohistochemically analyzed the expression of a panel of oncogenes (Cyclin E, c-Myc, and Cdc25A,) and markers of replication stress (phospho-Ser33-RPA32 and γ-H2AX) in breast tumor tissues prior to treatment (n = 384). Triple-negative breast cancers (TNBCs) exhibited the highest levels of phospho-Ser33-RPA32 (P < 0.001 for all tests) and γ-H2AX (P < 0.05 for all tests). Moreover, expression levels of Cyclin E (P < 0.001 for all tests) and c-Myc (P < 0.001 for all tests) were highest in TNBCs. Expression of Cyclin E positively correlated with phospho-RPA32 (Spearman correlation r = 0.37, P < 0.001) and γ-H2AX (Spearman correlation r = 0.63, P < 0.001). Combined, these data indicate that, among breast cancers, replication stress is predominantly observed in TNBCs, and is associated with expression levels of Cyclin E. These results indicate that Cyclin E overexpression may be used as a biomarker for patient selection in the clinical evaluation of drugs that target the DNA replication stress response.

Aberrant localization of signaling proteins in skin cancer: Implications for treatment

Aberrant subcellular localization of signaling proteins can provide cancer cells with advantages such as resistance to apoptotic cell death, increased invasiveness and more rapid proliferation. Nuclear to cytoplasmic shifts in tumor-promoting proteins can lead to worse patient outcomes, providing opportunities to target cancer-specific processes. Herein, we review the significance of dysregulated protein localization with a focus on skin cancer. Altered localization of signaling proteins controlling cell cycle progression or cell death is a common feature of cancer. In some instances, aberrant subcellular localization results in an acquired prosurvival function. Taking advantage of this knowledge reveals novel targets useful in the development of cancer therapeutics.

Characterization of immortalized human mammary epithelial cell line HMEC 2.6

Primary human mammary epithelial cells have a limited life span which makes it difficult to study them in vitro for most purposes. To overcome this problem, we have developed a cell line that was immortalized using defined genetic elements, and we have characterized this immortalized non-tumorigenic human mammary epithelial cell line to establish it as a potential model system. human mammary epithelial cells were obtained from a healthy individual undergoing reduction mammoplasty at SIU School of Medicine. The cells were transduced with CDK4R24C followed by transduction with human telomerase reverse transcriptase. Post all manipulation, the cells displayed a normal cell cycle phase distribution and were near diploid in nature, which was confirmed by flow cytometry and karyotyping. In vitro studies showed that the cells were anchorage dependent and were non-invasive in nature. The cell line expressed basal epithelial markers such as cytokeratin 7, CD10, and p63 and was negative for the expression of estrogen receptor and progesterone receptor. Upon G-band karyotyping, the cell line displayed the presence of a few cytogenic abnormalities, including trisomy 20 and trisomy 7, which are also commonly present in other immortalized mammary cell lines. Furthermore, the benign nature of these cells was confirmed by multiple in vitro and in vivo experiments. Therefore, we think that this cell line could serve as a good model to understand the molecular mechanisms involved in the development and progression of breast cancer and to also assess the effect of novel therapeutics on human mammary epithelial cells.

Phosphatases and solid tumors: focus on glioblastoma initiation, progression and recurrences

Phosphatases and cancer have been related for many years now, as these enzymes regulate key cellular functions, including cell survival, migration, differentiation and proliferation. Dysfunctions or mutations affecting these enzymes have been demonstrated to be key factors for oncogenesis. The aim of this review is to shed light on the role of four different phosphatases (PTEN, PP2A, CDC25 and DUSP1) in five different solid tumors (breast cancer, lung cancer, pancreatic cancer, prostate cancer and ovarian cancer), in order to better understand the most frequent and aggressive primary cancer of the central nervous system, glioblastoma.

The Expression and Clinical Outcome of pCHK2-Thr68 and pCDC25C-Ser216 in Breast Cancer

Checkpoint kinase 2 (CHK2) and cell division cycle 25C (CDC25C) are two proteins involved in the DNA damage response pathway, playing essential roles in maintaining genome integrity. As one of the major hallmarks of abnormal cellular division, genomic instability occurs in most cancers. In this study, we identified the functional expression of pCHK2-Thr68 and pCDC25C-Ser216 in breast cancer, as well as its association with breast cancer survival. Tissue microarray analysis using immunohistochemistry was constructed to identify the expression of pCHK2-Thr68 and pCDC25C-Ser216 in 292 female breast cancer patients. The relationship among protein expression, clinicopathological factors (e.g., human epidermal growth factor receptor 2 (HER 2), tumor size, tumor-node-metastasis (TNM) classification), and overall survival of the breast cancer tissues were analyzed using Pearson’s χ-square (χ²) test, Fisher’s exact test, multivariate logistic regression and Kaplan–Meier survival analysis. Significantly higher expressions of pCHK2-Thr68 and pCDC25C-Ser216 were observed in the nucleus of the breast cancer cells compared to the paracancerous tissue (pCHK2-Thr68, 20.38% vs. 0%; pCDC25C-Ser216, 82.26% vs. 24.24%). The expression of pCHK2-Thr68 and pCDC25C-Ser216 in breast cancer showed a positive linear correlation (p = 0.026). High expression of pCHK2-Thr68 was associated with decreased patient survival (p = 0.001), but was not an independent prognostic factor. Our results suggest that pCHK2-Thr68 and pCDC25C-Ser216 play important roles in breast cancer and may be potential treatment targets.

A Drosophila-centric view of protein tyrosine phosphatases

Most of our knowledge on protein tyrosine phosphatases (PTPs) is derived from human pathologies and mouse knockout models. These models largely correlate well with human disease phenotypes, but can be ambiguous due to compensatory mechanisms introduced by paralogous genes. Here we present the analysis of the PTP complement of the fruit fly and the complementary view that PTP studies in Drosophila will accelerate our understanding of PTPs in physiological and pathological conditions. With only 44 PTP genes, Drosophila represents a streamlined version of the human complement. Our integrated analysis places the Drosophila PTPs into evolutionary and functional contexts, thereby providing a platform for the exploitation of the fly for PTP research and the transfer of knowledge onto other model systems.

Expression of CDC25A and CDC25B phosphatase proteins in human retinoblastoma and its correlation with clinicopathological parameters

BACKGROUND

CDC25 proteins play a pivotal role in controlling cell proliferation during development and tumorigenesis. The aim of the study is to elucidate the role of CDC25A and CDC25B proteins in retinoblastoma and their association with the clinical and histopathological parameters.

METHODS

One hundred and nine prospective cases of primary enucleated retinoblastomas were included in the present study. Expression of CDC25A and CDC25B proteins was investigated by immunohistochemistry, western blotting and mRNA expression by reverse-transcriptase PCR.

RESULTS

Immunohistochemistry showed CDC25A expression in (57/109) 52.29%, whereas CDC25B expressed in (69/109) 63.30% cases. Western blotting confirmed the immunoreactivity results on representative cases. mRNA expression of CDC25A and CDC25B was found in 29/60 (48.33%) and 35/60 (58.33%) cases, respectively. Expression of CDC25A and CDC25B showed significant correlation with poor tumour differentiation and tumour invasion (p<0.05). There was a statistically significant difference in the overall survival of patients with CDC25B expression (p=0.0270).

CONCLUSIONS

Our results suggest that expression of CDC25B may be used as a potential prognostic marker in the pathogenesis of retinoblastoma. These findings demonstrate an important role of CDC25 phosphatase proteins and inhibition of these proteins may have therapeutic potential in retinoblastoma.

Crosstalk between kinases, phosphatases and miRNAs in cancer

Reversible phosphorylation of proteins, performed by kinases and phosphatases, is the major post translational protein modification in eukaryotic cells. This intracellular event represents a critical regulatory mechanism of several signaling pathways and can be related to a vast array of diseases, including cancer. Cancer research has produced increasing evidence that kinase and phosphatase activity can be compromised by mutations and also by miRNA silencing, performed by small non-coding and endogenously produced RNA molecules that lead to translational repression. miRNAs are believed to target about one-third of human mRNAs while a single miRNA may target about 200 transcripts simultaneously. Regulation of the phosphorylation balance by miRNAs has been a topic of intense research over the last years, spanning topics going as far as cancer aggressiveness and chemotherapy resistance. By addressing recent studies that have shown miRNA expression patterns as phenotypic signatures of cancers and how miRNA influence cellular processes such as apoptosis, cell cycle control, angiogenesis, inflammation and DNA repair, we discuss how kinases, phosphatases and miRNAs cooperatively act in cancer biology.

The regulatory roles of phosphatases in cancer

The relevance of potentially reversible post-translational modifications required for controlling cellular processes in cancer is one of the most thriving arenas of cellular and molecular biology. Any alteration in the balanced equilibrium between kinases and phosphatases may result in development and progression of various diseases, including different types of cancer, though phosphatases are relatively under-studied. Loss of phosphatases such as PTEN (phosphatase and tensin homologue deleted on chromosome 10), a known tumour suppressor, across tumour types lends credence to the development of phosphatidylinositol 3-kinase inhibitors alongside the use of phosphatase expression as a biomarker, though phase 3 trial data are lacking. In this review, we give an updated report on phosphatase dysregulation linked to organ-specific malignancies.

SMG-1 suppresses CDK2 and tumor growth by regulating both the p53 and Cdc25A signaling pathways

The DNA damage response is coordinated by phosphatidylinositol 3-kinase-related kinases, ATM, ATR, and DNA-PK. SMG-1 is the least studied stress-responsive member of this family. Here, we show that SMG-1 regulates the G 1/S checkpoint through both a p53-dependent, and a p53-independent pathway. We identify Cdc25A as a new SMG-1 substrate, and show that cells depleted of SMG-1 exhibit prolonged Cdc25A stability, failing to inactivate CDK2 in response to radiation. Given an increased tumor growth following depletion of SMG-1, our data demonstrate a novel role for SMG-1 in regulating Cdc25A and suppressing oncogenic CDK2 driven proliferation, confirming SMG-1 as a tumor suppressor.

MicroRNA-148a is down-regulated in human pancreatic ductal adenocarcinomas and regulates cell survival by targeting CDC25B

MicroRNAs (miRNAs: short non-coding RNAs) are emerging as a class of potential novel tumor markers, as their dysregulation is being increasingly reported in various types of cancers. In the present study, we investigated the transcription status of miRNA-148a (miR-148a) in human pancreatic ductal adenocarcinoma (PDAC) and its role in the regulation of the dual specificity protein phosphatase CDC25B. We observed that miR-148a exhibited a significant 4-fold down-regulation in PDAC as opposed to normal pancreatic ductal cells. In addition, we observed that stable lentiviral-mediated overexpression of miR-148a in the pancreatic cancer cell line IMIM-PC2, inhibited tumor cell growth and colony formation. Furthermore, CDC25B was identified as a potential target of miR-148a by in silico analysis using PicTar, Targetscan and miRanda in conjunction with gene ontology analysis. The proposed interaction between miR-148a and the 3' untranslated region (UTR) of CDC25B was verified by in-vitro luciferase assays. We demonstrate that the activity of a luciferase reporter containing the 3'UTR of CDC25B was repressed in the presence of miR-148a mimics, confirming that miR-148a targets the 3'UTR of CDC25B. Finally, CDC25B was down-regulated at the protein level in miR-148a overexpressing IMIM-PC2-cells, and in transiently transfected pancreatic cell lines (as detected by Western blot analysis), as well as in patient tumor samples (as detected by immunohistochemistry). In summary, we identified CDC25B as a novel miR-148a target which may confer a proliferative advantage in PDAC.

Differential expression of CDC25 phosphatases splice variants in human breast cancer cells

BACKGROUND

CDC25 phosphatases control cell cycle progression by activating cyclin dependent kinases. The three CDC25 isoforms encoding genes are submitted to alternative splicing events which generate at least two variants for CDC25A and five for both CDC25B and CDC25C. An over-expression of CDC25 was reported in several types of cancer, including breast cancer, and is often associated with a poor prognosis. Nevertheless, most of the previous studies did not address the expression of CDC25 splice variants. Here, we evaluated CDC25 spliced transcripts expression in anti-cancerous drug-sensitive and resistant breast cancer cell lines in order to identify potential breast cancer biomarkers.

METHODS

CDC25 splice variants mRNA levels were evaluated by semi-quantitative RT-PCR and by an original real-time RT-PCR assay.

RESULTS

CDC25 spliced transcripts are differentially expressed in the breast cancer cell lines studied. An up-regulation of CDC25A2 variant and an increase of the CDC25C5/C1 ratio are associated to the multidrug-resistance in VCREMS and DOXOR breast cancer cells, compared to their sensitive counterpart cell line MCF-7. Additionally, CDC25B2 transcript is exclusively over-expressed in VCREMS resistant cells and could therefore be involved in the development of certain type of drug resistance.

CONCLUSIONS

CDC25 splice variants could represent interesting potential breast cancer prognostic biomarkers.

Cdc25A regulates matrix metalloprotease 1 through Foxo1 and mediates metastasis of breast cancer cells

Cdc25A is a cell cycle-activating phosphatase, and its overexpression in breast cancers has been shown to correlate with poor prognosis. Most recent studies related to Cdc25A and tumor progression have focused on its role in regulating cell cycle progression. However, less is known about how Cdc25A modulates the metastasis of breast cancer cells. In this study, we revealed that Cdc25A enhances Foxo1 stability by dephosphorylating Cdk2, and Foxo1 was shown to directly regulate transcription of the metastatic factor MMP1. Further studies have shown that overexpression of Cdc25A in breast cancer cells enhances metastasis, whereas its downmodulation inhibits metastasis in mouse models, and the effects of Cdc25A on breast cancer cell metastasis are independent of cell proliferation and apoptosis. Furthermore, we have demonstrated that aberrant Cdc25A in breast cancer patient samples directly correlates with the metastatic phenotype. Further insights into this critical role of Cdc25A in the metastasis of breast cancer cells and the trial of an anti-Cdc25A strategy in mouse models may reveal its therapeutic potential in prevention and treatment of breast cancer cell dissemination.

Mice with the CHEK2*1100delC SNP are predisposed to cancer with a strong gender bias

The CHEK2 kinase (Chk2 in mouse) is a member of a DNA damage response pathway that regulates cell cycle arrest at cell cycle checkpoints and facilitates the repair of dsDNA breaks by a recombination-mediated mechanism. There are numerous variants of the CHEK2 gene, at least one of which, CHEK2*1100delC (SNP), associates with breast cancer. A mouse model in which the wild-type Chk2 has been replaced by a Chk2*1100delC allele was tested for elevated risk of spontaneous cancer and increased sensitivity to challenge by a carcinogenic compound. Mice homozygous for Chk2*1100delC produced more tumors than wild-type mice, whereas heterozygous mice were not statistically different. When fractionated by gender, however, homozygous and heterozygous mice developed spontaneous tumors more rapidly and to a far greater extent than wild-type mice, indicative of a marked gender bias in mice harboring the variant allele. Consistent with our previous data showing elevated genomic instability in mouse embryonic fibroblasts (MEFs) derived from mice homozygous for Chk2*1100delC, the level of Cdc25A was elevated in heterozygous and homozygous MEFs and tumors. When challenged with the carcinogen 7,12-dimethylbenz[a]anthracene, all mice, regardless of genotype, had a reduced lifespan. Latency for mammary tumorigenesis was reduced significantly in mice homozygous for Chk2*1100delC but unexpectedly increased for the development of lymphomas. An implication from this study is that individuals who harbor the variant CHEK2*1100delC allele not only are at an elevated risk for the development of cancer but also that this risk can be further increased as a result of environmental exposure.

TGF-beta regulates the expression of transcription factor KLF6 and its splice variants and promotes co-operative transactivation of common target genes through a Smad3-Sp1-KLF6 interaction

KLF6 (Krüppel-like factor 6) is a transcription factor and tumour suppressor with a growing range of biological activities and transcriptional targets. Among these, KLF6 suppresses growth through transactivation of TGF-beta1 (transforming growth factor-beta1). KLF6 can be alternatively spliced, generating lower-molecular-mass isoforms that antagonize the full-length WT (wild-type) protein and promote growth. A key target gene of full-length KLF6 is endoglin, which is induced in vascular injury. Endoglin, a homodimeric cell membrane glycoprotein and TGF-beta auxiliary receptor, has a pro-angiogenic role in endothelial cells and is also involved in malignant progression. The aim of the present work was to explore the effect of TGF-beta on KLF6 expression and splicing, and to define the contribution of TGF-beta on promoters regulated by co-operation between KLF6 and Sp1 (specificity protein 1). Using co-transfection, co-immunoprecipitation and fluorescence resonance energy transfer, our data demonstrate that KLF6 co-operates with Sp1 in transcriptionally regulating KLF6-responsive genes and that this co-operation is further enhanced by TGF-beta1 through at least two mechanisms. First, in specific cell types, TGF-beta1 may decrease KLF6 alternative splicing, resulting in a net increase in full-length, growth-suppressive KLF6 activity. Secondly, KLF6-Sp1 co-operation is further enhanced by the TGF-beta-Smad (similar to mothers against decapentaplegic) pathway via the likely formation of a tripartite KLF6-Sp1-Smad3 complex in which KLF6 interacts indirectly with Smad3 through Sp1, which may serve as a bridging molecule to co-ordinate this interaction. These findings unveil a finely tuned network of interactions between KLF6, Sp1 and TGF-beta to regulate target genes.

Overexpression of CDC25B and LAMC2 mRNA and protein in esophageal squamous cell carcinomas and premalignant lesions in subjects from a high-risk population in China

Molecular events associated with the initiation and progression of esophageal squamous cell carcinoma (ESCC) remain poorly understood but likely hold the key to effective early detection approaches for this almost invariably fatal cancer. CDC25B and LAMC2 are two promising early detection candidates emerging from new molecular studies of ESCC. To further elucidate the role of these two genes in esophageal carcinogenesis, we did a series of studies to (a) confirm RNA overexpression, (b) establish the prevalence of protein overexpression, (c) relate protein overexpression to survival, and (d) explore their potential as early detection biomarkers. Results of these studies indicated that CDC25B mRNA was overexpressed (>/=2-fold overexpression in tumor compared with normal) in 64% of the 73 ESCC cases evaluated, whereas LAMC2 mRNA was overexpressed in 89% of cases. CDC25B protein expression was categorized as positive in 59% (144 of 243) of ESCC cases on a tumor tissue microarray, and nonnegative LAMC2 patterns of protein expression were observed in 82% (225 of 275) of cases. Multivariate-adjusted proportional hazard regression models showed no association between CDC25B protein expression score and risk of death [hazard ratio (HR) for each unit increase in expression score, 1.00; P = 0.90]; however, several of the LAMC2 protein expression patterns strongly predicted survival. Using the cytoplasmic pattern as the reference (the pattern with the lowest mortality), cases with a diffuse pattern had a 254% increased risk of death (HR, 3.52; P = 0.007), cases with no LAMC2 expression had a 169% increased risk of death (HR, 2.69; P = 0.009), and cases with a peripheral pattern had a 130% greater risk of death (HR, 2.30; P = 0.02). CDC25B protein expression scores in subjects with esophageal biopsies diagnosed as normal (n = 35), dysplastic (n = 23), or ESCC (n = 32) increased significantly with morphologic progression. For LAMC2, all normal and dysplastic patients had a continuous pattern of protein expression, whereas all ESCCs showed alternative, noncontinuous patterns. This series of studies showed that both CDC25B and LAMC2 overexpress RNA and protein in a significant majority of ESCC cases. The strong relation of LAMC2 pattern of protein expression to survival suggests a role in prognosis, whereas the association of CDC25B with morphologic progression indicates a potential role as an early detection marker.

Proteomics-based identification of autoantibody against CDC25B as a novel serum marker in esophageal squamous cell carcinoma

This study was aimed to identify tumor proteins that elicit a humoral response in patients with esophageal squamous cell carcinoma (ESCC). Autologous sera of 15 newly diagnosed patients with ESCC and age- and gender-matched 15 healthy controls were analyzed individually for antibody-based reactivity against proteins from 15 homogenized ESCC tissue mixture resolved by two-dimensional PAGE. One protein spot, which reacted with sera from ESCC patients but not with those from controls, was identified to be CDC25B by mass spectrometry and Western blotting. High expression of CDC25B was detected in ESCC cell lines and primary tumor tissues, but not in normal esophageal tissues. In addition, CDC25B expression was significantly higher in tumor tissue of patients with sera positive CDC25B-Abs than that of patients without CDC25B-Abs. Finally, anti-CDC25B antibodies were readily detectable in sera from 45 of 124 (36.29%) patients with ESCC, 13 of 150 (8.67%) patients with other types of cancer and 0 of 102 (0%) of healthy individuals. Thus, CDC25B autoantibodies may have clinical utility in ESCC screening and diagnosis.

Variations in intracellular levels of TATA binding protein can affect specific genes by different mechanisms

We previously showed that reduced intracellular levels of the TATA binding protein (TBP), brought about by tbp heterozygosity in DT40 cells, resulted in a mitotic delay reflecting reduced expression of the mitotic regulator cdc25B but did not significantly affect overall transcription. Here we extend these findings in several ways. We first provide evidence that the decrease in cdc25B expression reflects reduced activity of the cdc25B core promoter in the heterozygous (TBP-het) cells. Strikingly, mutations in a previously described repressor element that overlaps the TATA box restored promoter activity in TBP-het cells, supporting the idea that the sensitivity of this promoter to TBP levels reflects a competition between TBP and the repressor for DNA binding. To determine whether cells might have mechanisms to compensate for fluctuations in TBP levels, we next examined expression of the two known vertebrate TBP homologues, TLP and TBP2. Significantly, mRNAs encoding both were significantly overexpressed relative to levels observed in wild-type cells. In the case of TLP, this was shown to reflect regulation of the core promoter by both TBP and TLP. Together, our results indicate that variations in TBP levels can affect the transcription of specific promoters in distinct ways, but overall transcription may be buffered by corresponding alterations in the expression of TBP homologues.

CDC25 phosphatases in cancer cells: key players? Good targets?

Cell division cycle 25 (CDC25) phosphatases regulate key transitions between cell cycle phases during normal cell division, and in the event of DNA damage they are key targets of the checkpoint machinery that ensures genetic stability. Taking only this into consideration, it is not surprising that CDC25 overexpression has been reported in a significant number of human cancers. However, in light of the significant body of evidence detailing the stringent complexity with which CDC25 activities are regulated, the significance of CDC25 overexpression in a subset of cancers and its association with poor prognosis are proving difficult to assess. We will focus on the roles of CDC25 phosphatases in both normal and abnormal cell proliferation, provide a critical assessment of the current data on CDC25 overexpression in cancer, and discuss both current and future therapeutic strategies for targeting CDC25 activity in cancer treatment.

Inhibiting transient protein-protein interactions: lessons from the Cdc25 protein tyrosine phosphatases

Transient protein-protein interactions have key regulatory functions in many of the cellular processes that are implicated in cancerous growth, particularly the cell cycle. Targeting these transient interactions as therapeutic targets for anticancer drug development seems like a good idea, but it is not a trivial task. This Review discusses the issues and difficulties that are encountered when considering these transient interactions as drug targets, using the example of the cell division cycle 25 (Cdc25) phosphatases and their cyclin-dependent kinase (CDK)-cyclin protein substrates.