cdc25 M-phase inducer

PP2A inhibition from LB100 therapy enhances daunorubicin cytotoxicity in secondary acute myeloid leukemia via miR-181b-1 upregulation

Patients with secondary acute myeloid leukemia (sAML) arising from myelodysplastic syndromes have a poor prognosis marked by an increased resistance to chemotherapy. An urgent need exists for adjuvant treatments that can enhance or replace current therapeutic options. Here we show the potential of LB100, a small-molecule protein phosphatase 2 A (PP2A) inhibitor, as a monotherapy and chemosensitizing agent for sAML using an in-vitro and in-vivo approach. We demonstrate that LB100 decreases cell viability through caspase activation and G2/M cell-cycle arrest. LB100 enhances daunorubicin (DNR) cytotoxicity resulting in decreased xenograft volumes and improved overall survival. LB100 profoundly upregulates miR-181b-1, which we show directly binds to the 3′ untranslated region of Bcl-2 mRNA leading to its translational inhibition. MiR-181b-1 ectopic overexpression further diminishes Bcl-2 expression leading to suppression of sAML cell growth, and enhancement of DNR cytotoxicity. Our research highlights the therapeutic potential of LB100, and provides new insights into the mechanism of LB100 chemosensitization.

The role of the LH subdomain in the function of the Cip/Kip cyclin-dependent kinase regulators

The Cip/Kip protein family, which includes p27, p21, and p57, modulates the activity of cyclin-dependent kinases (Cdks). A domain within these proteins, termed the kinase inhibitory domain (KID), is necessary and sufficient for Cdk inhibition. The KID consists of a cyclin-binding subdomain (termed D1) and a Cdk-binding subdomain (termed D2) joined by a 22-residue linker subdomain (termed LH). Before binding the Cdks, D1 and D2 are largely unstructured and the LH subdomain exhibits nascent helical characteristics. Curiously, although the sequence of the linker subdomain is not highly conserved within the family, its nascent helical structure is conserved. In this study, we explored the role of this structural conservation in interactions with cyclin-dependent kinase 2 (Cdk2) and cyclin A. We constructed chimeric p27-KID molecules in which the p27 LH subdomain was replaced with the corresponding segments of either p21 or p57. The chimeric molecules bind and inhibit Cdk2 in a manner similar to wild-type p27-KID. However, the extent of enthalpy/entropy compensation associated with these interactions was dramatically different, indicating different extents of LH subdomain folding upon binding. Our results indicate that the different LH subdomains, despite their sequence and thermodynamic differences, play similar roles in binding and inhibiting Cdk2/cyclin A.

Regulation of the cell cycle by protein phosphatase 2A in Saccharomyces cerevisiae

Protein phosphatase 2A (PP2A) has long been implicated in cell cycle regulation in many different organisms. In the yeast Saccharomyces cerevisiae, PP2A controls cell cycle progression mainly through modulation of cyclin-dependent kinase (CDK) at the G(2)/M transition. However, CDK does not appear to be a direct target of PP2A. PP2A affects CDK activity through its roles in checkpoint controls. Inactivation of PP2A downregulates CDK by activating the morphogenesis checkpoint and, consequently, delays mitotic entry. Defects in PP2A also compromise the spindle checkpoint and predispose the cell to an error-prone mitotic exit. In addition, PP2A is involved in controlling the G(1)/S transition and cytokinesis. These findings suggest that PP2A functions in many stages of the cell cycle and its effect on cell cycle progression is pleiotropic.

On the Slowing of S Phase in Response to DNA Damage in Fission Yeast

Eukaryotic cells slow their progression through S phase upon DNA damage. The mechanism that leads to this slowing is called the intra-S-phase checkpoint. Previous studies demonstrated that in the fission yeast Schizosaccharomyces pombe this checkpoint is mediated by a pathway that includes Rad3 (similar to human ATR and ATM) and Cds1 (similar to human Chk1 and Chk2). Here we present evidence that a major downstream target of this pathway is the cyclin-dependent kinase, Cdc2. We also present evidence suggesting that the intra-S-phase checkpoint makes a relatively minor contribution to the survival of cells with damaged DNA.

Ectopic expression of the Drosophila Cdk1 inhibitory kinases, Wee1 and Myt1, interferes with the second mitotic wave and disrupts pattern formation during eye development

Wee1 kinases catalyze inhibitory phosphorylation of the mitotic regulator Cdk1, preventing mitosis during S phase and delaying it in response to DNA damage or developmental signals during G2. Unlike yeast, metazoans have two distinct Wee1-like kinases, a nuclear protein (Wee1) and a cytoplasmic protein (Myt1). We have isolated the genes encoding Drosophila Wee1 and Myt1 and are using genetic approaches to dissect their functions during normal development. Overexpression of Dwee1 or Dmyt1 during eye development generates a rough adult eye phenotype. The phenotype can be modified by altering the gene dosage of known regulators of the G2/M transition, suggesting that we could use these transgenic strains in modifier screens to identify potential regulators of Wee1 and Myt1. To confirm this idea, we tested a collection of deletions for loci that can modify the eye overexpression phenotypes and identified several loci as dominant modifiers. Mutations affecting the Delta/Notch signaling pathway strongly enhance a GMR-Dmyt1 eye phenotype but do not affect a GMR-Dwee1 eye phenotype, suggesting that Myt1 is potentially a downstream target for Notch activity during eye development. We also observed interactions with p53, which suggest that Wee1 and Myt1 activity can block apoptosis.

The cell cycle Cdc25A tyrosine phosphatase is activated in degenerating postmitotic neurons in Alzheimer's disease

The Cdc25 phosphatases play key roles in cell-cycle progression by activating cyclin-dependent kinases. The latter are absent from neurons that are terminally differentiated in adult brain. However, accumulation of mitotic phosphoepitopes, and re-expression and activation of the M phase regulator, Cdc2/cyclin B, have been described in neurons undergoing degeneration in Alzheimer's disease (AD). To explain this atypical mitotic activation in neurons we investigated the Cdc2-activating Cdc25A phosphatase in human brain. The structural hallmarks of AD neurodegeneration, neurofibrillary tangles and neuritic plaques, were prominently immunolabeled with Cdc25A antibodies. In addition numerous neurons without visible structural alterations were also intensely stained, whereas control brain was very weakly positive. After immunoprecipitation from control and AD tissue, we found that the tyrosine dephosphorylating activity of Cdc25A against exogenous Cdc2 substrate was elevated in AD. Accordingly, Cdc25A from AD tissue displayed increased immunoreactivity with the mitotic phosphoepitope-specific antibody, MPM-2, and co-localized with MPM-2 immunoreactivity in AD neurons. These data suggest that Cdc25A participates in mitotic activation during neurodegeneration. The involvement of Cdc25A in cellular transformation, modulation of the DNA damage checkpoint, and linkage of mitogenic signaling to cell cycle machinery, also implicates one of these cell-cycle pathways in AD pathogenesis.

RNA-Mediated interference of a cdc25 homolog in Caenorhabditis elegans results in defects in the embryonic cortical membrane, meiosis, and mitosis

The CDC25 dual-specificity phosphatase family has been shown to play a key role in cell cycle regulation. The phosphatase activity of CDC25 drives the cell cycle by removing inhibitory phosphates from cyclin-dependent kinase/cyclin complexes. Although the regulation of CDC25 phosphatase activity has been elucidated both biochemically and genetically in other systems, the role of this enzyme during development is not well understood. To examine the expression pattern and function of CDC25 in Caenorhabditis elegans, we characterized a cdc25 homolog, cdc-25.1, during early embryonic development. The CDC-25.1 protein localizes to oocytes, embryonic nuclei, and embryonic cortical membranes. When the expression of CDC-25.1 was disrupted by RNA-mediated interference, the anterior cortical membrane of fertilized eggs became very fluid during meiosis and subsequent mitotic cell cycles. Mispositioning of the meiotic spindle, defects in polar body extrusion and chromosome segregation, and abnormal cleavage furrows were also observed. We conclude that CDC-25.1 is required for a very early developmental process-the proper completion of meiosis prior to embryogenesis.

The four cdc25 genes from the nematode Caenorhabditis elegans

During eukaryotic evolution, multicellular organisms have evolved multiple members of gene families that may display unique, partially overlapping, or redundant functions during development. More than 75% of the C. elegans genome has been sequenced, which represents approximately 95% of the coding sequences. This provides a unique opportunity to identify most, if not all, of the members of a given gene family. We have searched the C. elegans genome database for members of a key family of cell cycle regulators, the CDC25 phosphatases, and have identified four genes. The four C. elegans genes represent a larger family within a single organism than has been reported so far in Drosophila, mice and humans. An amino acid comparison revealed a high degree of similarity and identity within the phosphatase domain. This analysis also identified an expanded consensus sequence that can be used to discover new members of the CDC25 phosphatase family. However, the four C. elegans sequences display a few novel amino acid substitutions in the residues surrounding the invariant catalytic motif CX5R. These data demonstrate the value of genome database searching for identifying new members of known gene families, understanding genetic diversity, and for studying gene structure.

Advancement through mitosis requires rae1 gene function in fission yeast

Growth of the rae1-1 mutant of Schizosaccharomyces pombe at restrictive temperature results in accumulation of poly(A)+ RNA in the nucleus and a cell cycle arrest at the G2/M boundary. We demonstrate here that rae1 function is required for a process other than mRNA export which is essential for advancement through mitosis. Cells lacking rae1 function arrest with elevated Cdc2p kinase levels at a step before the formation of a mitotic spindle and without separation of the spindle pole bodies. Rae1p was localized to the nuclear periphery, consistent with a role in nucleocytoplasmic trafficking, which could include protein import. We propose a model where rae1 functions in cell cycle progression through trafficking of proteins required for mitosis.

S-phase and DNA-damage checkpoints: a tale of two yeasts

Many genes required for the S-phase and DNA-damage checkpoints have been identified in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. This year many checkpoint genes have been sequenced, providing new information about the mechanism of checkpoint control. Several of these genes are conserved between the two yeasts but others are species-specific.

Roles of active site residues and the NH2-terminal domain in the catalysis and substrate binding of human Cdc25

Human Cdc25 proteins are dual specific protein phosphatases that play important roles in cell cycle regulation. In this study, the catalytic mechanism and substrate binding specificity of human Cdc25A and -B proteins were investigated by site-directed and deletion mutagenesis methods. Mutations of the cysteine or the arginine residues in the active site motif abolished the Cdc25 phosphatase activity. However, the cysteine mutation in both Cdc25A and -B created enzymes that still retain the ability to bind their substrates. This allowed us to test the ability of Cdc25A and -B to bind various cyclin-Cdk complexes in vitro. While Cdc25A Cys --> Ser could interact with cyclin A-Cdk2, cyclin B-Cdc2, and cyclin E-Cdk2 strongly, Cdc25B mutant was only found to bind to cyclin A-Cdk2 at significant levels. We also identified Arg452 and Ser449 as two crucial residues that could be directly involved in the molecular interactions between Cdc25 and cyclin-Cdk proteins. Deletion mutagenesis data also indicate that the phosphatase catalytic domains of Cdc25A and -B proteins are located within their carboxyl terminus.

Cloning and characterization of a cdc25 phosphatase from mouse lymphocytes

Members of the cdc25 phosphatase family are proposed to function as important regulators of the eukaryotic cell cycle, particularly in the induction of mitotic events. A new cdc25 tyrosine phosphatase, cdc25M1, has been cloned from a mouse pre-B cell cDNA library and characterized. The cdc25M1 protein consists of 465 amino acids with a predicted relative molecular mass (M(r)) of 51,750. Over the highly conserved carboxyl terminal region, the amino acid sequence similarity to the human cdc25 C or Hs1 isoform is 89%, while the overall similarity is 67%. The phosphatase active site is located within residues 367-374. Tissue expression of the cdc25M1 was highest in mouse spleen and thymus by northern blot analysis. The cdc25M1 mRNA was detected in a number of cloned mouse lymphocyte cell lines including both CD8+ and CD4+ cells. cdc25M1 mRNA was shown to be cell cycle-regulated in T cells following interleukin-2 (IL-2)-stimulation. Accumulation of cdc25M1 mRNA occurred at 48 h after IL-2 stimulation, when lymphocytes were progressing from S phase to G2/M phase of the cell cycle. This pattern of expression is in contrast to that observed for other protein tyrosine phosphatases expressed in T lymphocytes including CD45, LRP, SHP, and PEP. The elevation in cdc25M1 mRNA level occurred concomittant to the appearance of the hyperphosphorylated form of p34cdc2 protein kinase. A purified, bacterial-expressed recombinant cdc25M1 phosphatase domain catalyzed the dephosphorylation of p-nitrophenol phosphate, as well as [32P-Tyr] and [32P-Ser/Thr]-containing substrates. Preincubation of p34cdc2 kinase with cdc25M1 activated its histone H1 kinase activity in vitro. These results suggest that cdc25M1 may be involved in regulating the proliferation of mouse T lymphocytes following cytokine stimulation, through its action on p34cdc2 kinase.

Dephosphorylation of cdc25-C by a type-2A protein phosphatase: specific regulation during the cell cycle in Xenopus egg extracts

We have examined the roles of type-1 (PP-1) and type-2A (PP-2A) protein-serine/threonine phosphatases in the mechanism of activation of p34cdc2/cyclin B protein kinase in Xenopus egg extracts. p34cdc2/cyclin B is prematurely activated in the extracts by inhibition of PP-2A by okadaic acid but not by specific inhibition of PP-1 by inhibitor-2. Activation of the kinase can be blocked by addition of the purified catalytic subunit of PP-2A at a twofold excess over the activity in the extract. The catalytic subunit of PP-1 can also block kinase activation, but very high levels of activity are required. Activation of p34cdc2/cyclin B protein kinase requires dephosphorylation of p34cdc2 on Tyr15. This reaction is catalysed by cdc25-C phosphatase that is itself activated by phosphorylation. We show that, in interphase extracts, inhibition of PP-2A by okadaic acid completely blocks cdc25-C dephosphorylation, whereas inhibition of PP-1 by specific inhibitors has no effect. This indicates that a type-2A protein phosphatase negatively regulates p34cdc2/cyclin B protein kinase activation primarily by maintaining cdc25-C phosphatase in a dephosphorylated, low activity state. In extracts containing active p34cdc2/cyclin B protein kinase, dephosphorylation of cdc25-C is inhibited, whereas the activity of PP-2A (and PP-1) towards other substrates is unaffected. We propose that this specific inhibition of cdc25-C dephosphorylation is part of a positive feedback loop that also involves direct phosphorylation and activation of cdc25-C by p34cdc2/cyclin B. Dephosphorylation of cdc25-C is also inhibited when cyclin A-dependent protein kinase is active, and this may explain the potentiation of p34cdc2/cyclin B protein kinase activation by cyclin A. In extracts supplemented with nuclei, the block on p34cdc2/cyclin B activation by unreplicated DNA is abolished when PP-2A is inhibited or when stably phosphorylated cdc25-C is added, but not when PP-1 is specifically inhibited. This suggests that unreplicated DNA inhibits p34cdc2/cyclin B activation by maintaining cdc25-C in a low activity, dephosphorylated state, probably by keeping the activity of a type-2A protein phosphatase towards cdc25-C at a high level.