PP2A:B56epsilon is required for Wnt/beta-catenin signaling during embryonic development

Differential Synonymous Codon Selection in the B56 Gene Family of PP2A Regulatory Subunits

Protein phosphatase 2A (PP2A) functions as a tumor suppressor and consists of a scaffolding, catalytic, and regulatory subunit. The B56 gene family of regulatory subunits impart distinct functions onto PP2A. Codon usage bias (CUB) involves the selection of synonymous codons, which can affect gene expression by modulating processes such as transcription and translation. CUB can vary along the length of a gene, and differential use of synonymous codons can be important in the divergence of gene families. The N-termini of the gene product encoded by B56α possessed high CUB, high GC content at the third codon position (GC3), and high rare codon content. In addition, differential CUB was found in the sequence encoding two B56γ N-terminal splice forms. The sequence encoding the N-termini of B56γ/γ, relative to B56δ/γ, displayed CUB, utilized more frequent codons, and had higher GC3 content. B56α mRNA had stronger than predicted secondary structure at their 5' end, and the B56δ/γ splice variants had long regions of weaker than predicted secondary structure at their 5' end. The data suggest that B56α is expressed at relatively low levels as compared to the other B56 isoforms and that the B56δ/γ splice variant is expressed more highly than B56γ/γ.

Regulation and role of the PP2A-B56 holoenzyme family in cancer

Protein phosphatase 2A (PP2A) inactivation is common in cancer, leading to sustained activation of pro-survival and growth-promoting pathways. PP2A consists of a scaffolding A-subunit, a catalytic C-subunit, and a regulatory B-subunit. The functional complexity of PP2A holoenzymes arises mainly through the vast repertoire of regulatory B-subunits, which determine both their substrate specificity and their subcellular localization. Therefore, a major challenge for developing more effective therapeutic strategies for cancer is to identify the specific PP2A complexes to be targeted. Of note, the development of small molecules specifically directed at PP2A-B56α has opened new therapeutic avenues in both solid and hematological tumors. Here, we focus on the B56/PR61 family of PP2A regulatory subunits, which have a central role in directing PP2A tumor suppressor activity. We provide an overview of the mechanisms controlling the formation and regulation of these complexes, the pathways they control, and the mechanisms underlying their deregulation in cancer.

Subgroup-Specific Diagnostic, Prognostic, and Predictive Markers Influencing Pediatric Medulloblastoma Treatment

Medulloblastoma (MB) is the most common malignant central nervous system tumor in pediatric patients. Mainstay of therapy remains surgical resection followed by craniospinal radiation and chemotherapy, although limitations to this therapy are applied in the youngest patients. Clinically, tumors are divided into average and high-risk status on the basis of age, metastasis at diagnosis, and extent of surgical resection. However, technological advances in high-throughput screening have facilitated the analysis of large transcriptomic datasets that have been used to generate the current classification system, dividing patients into four primary subgroups, i.e., WNT (wingless), SHH (sonic hedgehog), and the non-SHH/WNT subgroups 3 and 4. Each subgroup can further be subdivided on the basis of a combination of cytogenetic and epigenetic events, some in distinct signaling pathways, that activate specific phenotypes impacting patient prognosis. Here, we delve deeper into the genetic basis for each subgroup by reviewing the extent of cytogenetic events in key genes that trigger neoplastic transformation or that exhibit oncogenic properties. Each of these discussions is further centered on how these genetic aberrations can be exploited to generate novel targeted therapeutics for each subgroup along with a discussion on challenges that are currently faced in generating said therapies. Our future hope is that through better understanding of subgroup-specific cytogenetic events, the field may improve diagnosis, prognosis, and treatment to improve overall quality of life for these patients.

Role of protein phosphatase 2A in kidney disease (Review)

Kidney disease affects millions of people worldwide and is a financial burden on the healthcare system. Protein phosphatase 2A (PP2A), which is involved in renal development and the function of ion-transport proteins, aquaporin-2 and podocytes, is likely to serve an important role in renal processes. PP2A is associated with the pathogenesis of a variety of different kidney diseases including podocyte injury, inflammation, tumors and chronic kidney disease. The current review aimed to discuss the structure and function of PP2A subunits in the context of kidney diseases. How dysregulation of PP2A in the kidneys causes podocyte death and the inactivation of PP2A in renal carcinoma tissues is discussed. Inhibition of PP2A activity prevents epithelial-mesenchymal transition and attenuates renal fibrosis, creating a favorable inflammatory microenvironment and promoting the initiation and progression of tumor pathogenesis. The current review also indicates that PP2A serves an important role in protection against renal inflammation. Understanding the detailed mechanisms of PP2A provides information that can be utilized in the design and application of novel therapeutics for the treatment and prevention of renal diseases.

Unravelling the Mechanistic Role of Quinazolinone Pharmacophore in the Inhibitory Activity of Bis-quinazolinone Derivative on Tankyrase-1 in the Treatment of Colorectal Cancer (CRC) and Non-small Cell Lung Cancer (NSCLC): A Computational Approach

In recent years, tankyrase inhibition has gained a great focus as an anti-cancer strategy due to their modulatory effect on WNT/β-catenin pathway implicated in many malignancies, including colorectal cancer (CRC) and non-small cell lung cancer (NSCLC). Based on the structural homology in the catalytic domain of PARP enzymes, bis-quinazolinone 5 (Cpd 5) was designed to be a potent selective tankyrase inhibitor. In this study, we employed molecular dynamics simulations and binding energy analysis to decipher the underlying mechanism of TNK-1 inhibition by Cpd 5 in comparison with a known selective tankyrase, IWR-1. The Cpd 5 had a relatively higher ΔG_bind than IWR-1 from the thermodynamics analysis, revealing the better inhibitory activity of Cpd 5 compared to IWR-1. High involvement of solvation energy (ΔG_sol) and the van der Waals energy (ΔE_vdW) potentiated the affinity of Cpd 5 at TNK-1 active site. Interestingly, the keto group and the N3 atom of the quinazolinone nucleus of Cpd 5, occupying the NAM subsite, was able to form H-bond with Gly1185, thereby favoring the better stability and higher inhibitory efficacy of Cpd 5 relative to IWR-1. Our analysis proved that the firm binding of Cpd 5 was achieved by the quinazolinone groups via the hydrophobic interactions with the side chains of key site residues at the two subsite regions: His1201, Phe1188, Ala1191, and Ile1192 at the AD subsite and Tyr1224, Tyr1213, and Ala1215 at the NAM subsite. Thus, Cpd 5 is dominantly bound through π-π stacked interactions and other hydrophobic interactions. We believe that findings from this study would provide an important rationale towards the structure-based design of improved selective tankyrase inhibitors in cancer therapy.

Protein phosphatase 2A - structure, function and role in neurodevelopmental disorders

Neurodevelopmental disorders (NDDs), including intellectual disability (ID), autism and schizophrenia, have high socioeconomic impact, yet poorly understood etiologies. A recent surge of large-scale genome or exome sequencing studies has identified a multitude of mostly de novo mutations in subunits of the protein phosphatase 2A (PP2A) holoenzyme that are strongly associated with NDDs. PP2A is responsible for at least 50% of total Ser/Thr dephosphorylation in most cell types and is predominantly found as trimeric holoenzymes composed of catalytic (C), scaffolding (A) and variable regulatory (B) subunits. PP2A can exist in nearly 100 different subunit combinations in mammalian cells, dictating distinct localizations, substrates and regulatory mechanisms. PP2A is well established as a regulator of cell division, growth, and differentiation, and the roles of PP2A in cancer and various neurodegenerative disorders, such as Alzheimer's disease, have been reviewed in detail. This Review summarizes and discusses recent reports on NDDs associated with mutations of PP2A subunits and PP2A-associated proteins. We also discuss the potential impact of these mutations on the structure and function of the PP2A holoenzymes and the etiology of NDDs.

Tissue-Specific Regulation of the Wnt/β-Catenin Pathway by PAGE4 Inhibition of Tankyrase

Spatiotemporal control of Wnt/β-catenin signaling is critical for organism development and homeostasis. The poly-(ADP)-ribose polymerase Tankyrase (TNKS1) promotes Wnt/β-catenin signaling through PARylation-mediated degradation of AXIN1, a component of the β-catenin destruction complex. Although Wnt/β-catenin is a niche-restricted signaling program, tissue-specific factors that regulate TNKS1 are not known. Here, we report prostate-associated gene 4 (PAGE4) as a tissue-specific TNKS1 inhibitor that robustly represses canonical Wnt/β-catenin signaling in human cells, zebrafish, and mice. Structural and biochemical studies reveal that PAGE4 acts as an optimal substrate decoy that potently hijacks substrate binding sites on TNKS1 to prevent AXIN1 PARylation and degradation. Consistently, transgenic expression of PAGE4 in mice phenocopies TNKS1 knockout. Physiologically, PAGE4 is selectively expressed in stromal prostate fibroblasts and functions to establish a proper Wnt/β-catenin signaling niche through suppression of autocrine signaling. Our findings reveal a non-canonical mechanism for TNKS1 inhibition that functions to establish tissue-specific control of the Wnt/β-catenin pathway.

Comparative phosphorylation map of Dishevelled 3 links phospho-signatures to biological outputs

Background

Dishevelled (DVL) is an essential component of the Wnt signaling cascades. Function of DVL is controlled by phosphorylation but the molecular details are missing. DVL3 contains 131 serines and threonines whose phosphorylation generates complex barcodes underlying diverse DVL3 functions. In order to dissect the role of DVL phosphorylation we analyzed the phosphorylation of human DVL3 induced by previously reported (CK1ε, NEK2, PLK1, CK2α, RIPK4, PKCδ) and newly identified (TTBK2, Aurora A) DVL kinases.

Methods

Shotgun proteomics including TiO₂ enrichment of phosphorylated peptides followed by liquid chromatography tandem mass spectrometry on immunoprecipitates from HEK293T cells was used to identify and quantify phosphorylation of DVL3 protein induced by 8 kinases. Functional characterization was performed by in-cell analysis of phospho-mimicking/non-phosphorylatable DVL3 mutants and supported by FRET assays and NMR spectroscopy.

Results

We used quantitative mass spectrometry and calculated site occupancies and quantified phosphorylation of > 80 residues. Functional validation demonstrated the importance of CK1ε-induced phosphorylation of S268 and S311 for Wnt-3a-induced β-catenin activation. S630–643 cluster phosphorylation by CK1, NEK2 or TTBK2 is essential for even subcellular distribution of DVL3 when induced by CK1 and TTBK2 but not by NEK2. Further investigation showed that NEK2 utilizes a different mechanism to promote even localization of DVL3. NEK2 triggered phosphorylation of PDZ domain at S263 and S280 prevents binding of DVL C-terminus to PDZ and promotes an open conformation of DVL3 that is more prone to even subcellular localization.

Conclusions

We identify unique phosphorylation barcodes associated with DVL function. Our data provide an example of functional synergy between phosphorylation in structured domains and unstructured IDRs that together dictate the biological outcome.

Novel functions of the ubiquitin-independent proteasome system in regulating Xenopus germline development

In most species, early germline development occurs in the absence of transcription with germline determinants subject to complex translational and post-translational regulations. Here, we report for the first time that early germline development is influenced by dynamic regulation of the proteasome system, previously thought to be ubiquitously expressed and to serve 'housekeeping' roles in controlling protein homeostasis. We show that proteasomes are present in a gradient with the highest levels in the animal hemisphere and extending into the vegetal hemisphere of Xenopus oocytes. This distribution changes dramatically during the oocyte-to-embryo transition, with proteasomes becoming enriched in and restricted to the animal hemisphere and therefore separated from vegetally localized germline determinants. We identify Dead-end1 (Dnd1), a master regulator of vertebrate germline development, as a novel substrate of the ubiquitin-independent proteasomes. In the oocyte, ubiquitin-independent proteasomal degradation acts together with translational repression to prevent premature accumulation of Dnd1 protein. In the embryo, artificially increasing ubiquitin-independent proteasomal degradation in the vegetal pole interferes with germline development. Our work thus reveals novel inhibitory functions and spatial regulation of the ubiquitin-independent proteasome during vertebrate germline development.

Benzene-induced mouse hematotoxicity is regulated by a protein phosphatase 2A complex that stimulates transcription of cytochrome P4502E1

Chronic benzene exposure is associated with hematotoxicity and the development of aplastic anemia and leukemia. However, the signaling pathways underlying benzene-induced hematotoxicity remain to be defined. Here, we investigated the role of protein phosphatase 2A (PP2A) in the regulation of benzene-induced hematotoxicity in a murine model. Male mice with a hepatocyte-specific homozygous deletion of the Ppp2r1a gene (encoding PP2A Aα subunit) (HO) and matched wildtype (WT) mice were exposed to benzene via inhalation at doses of 1, 10, and 100 ppm for 28 days. Peripheral white blood cell counts and activation of bone marrow progenitors were attenuated in the HO mice, indicating that Ppp2r1a deletion protects against benzene-induced hematotoxicity. Moreover, elevation of urinary S-phenyl mercapturic acid, a benzene metabolite, was much greater in WT mice than in HO mice. Real-time exhalation analysis revealed more exhaled benzene but fewer benzene metabolites in HO mice than in WT mice, possibly because of the down-regulation of Cyp2e1, encoding cytochrome P4502E1, in hepatocytes of the HO mice. Loss-of-function screening disclosed that PP2A complexes containing the B56α subunit participate in regulating Cyp2e1 expression. Notably, PP2A-B56α suppression in HepG2 cells resulted in persistent β-catenin phosphorylation at Ser³³-Ser³⁷-Thr⁴¹ in response to CYP2E1 agonists. In parallel, nuclear translocation of β-catenin was inhibited, concomitant with a remarkable decrease of Cyp2e1 expression. These findings support the notion that a regulatory cascade comprising PP2A-B56α, β-catenin, and Cyp2e1 is involved in benzene-induced hematotoxicity, providing critical insight into the role of PP2A in responses to the environmental chemicals.

CK1ε and p120-catenin control Ror2 function in noncanonical Wnt signaling

Canonical and noncanonical Wnt pathways share some common elements but differ in the responses they evoke. Similar to Wnt ligands acting through the canonical pathway, Wnts that activate the noncanonical signaling, such as Wnt5a, promote Disheveled (Dvl) phosphorylation and its binding to the Frizzled (Fz) Wnt receptor complex. The protein kinase CK1ε is required for Dvl/Fz association in both canonical and noncanonical signaling. Here we show that differently to its binding to canonical Wnt receptor complex, CK1ε does not require p120‐catenin for the association with the Wnt5a co‐receptor Ror2. Wnt5a promotes the formation of the Ror2–Fz complex and enables the activation of Ror2‐bound CK1ε by Fz‐associated protein phosphatase 2A. Moreover, CK1ε also regulates Ror2 protein levels; CK1ε association stabilizes Ror2, which undergoes lysosomal‐dependent degradation in the absence of this kinase. Although p120‐catenin is not required for CK1ε association with Ror2, it also participates in this signaling pathway as p120‐catenin binds and maintains Ror2 at the plasma membrane; in p120‐depleted cells, Ror2 is rapidly internalized through a clathrin‐dependent mechanism. Accordingly, downregulation of p120‐catenin or CK1ε affects late responses to Wnt5a that are also sensitive to Ror2, such as SIAH2 transcription, cell invasion, or cortical actin polarization. Our results explain how CK1ε is activated by noncanonical Wnt and identify p120‐catenin and CK1ε as two critical factors controlling Ror2 function.

hnRNP I regulates neonatal immune adaptation and prevents colitis and colorectal cancer

The intestinal epithelium plays a critical role in host-microbe homeostasis by sensing gut microbes and subsequently initiating proper immune responses. During the neonatal stage, the intestinal epithelium is under immune repression, allowing the transition for newborns from a relatively sterile intra-uterine environment to one that is rich in foreign antigens. The mechanism underlying such immune repression remains largely unclear, but involves downregulation of IRAK1 (interleukin-1 receptor-associated kinase), an essential component of toll-like receptor-mediated NF-κB signaling. We report here that heterogeneous nuclear ribonucleoprotein I (hnRNPI), an RNA binding protein, is essential for regulating neonatal immune adaptation. We generated a mouse model in which hnRNPI is ablated specifically in the intestinal epithelial cells, and characterized intestinal defects in the knockout mice. We found that loss of hnRNPI function in mouse intestinal epithelial cells results in early onset of spontaneous colitis followed by development of invasive colorectal cancer. Strikingly, the epithelium-specific hnRNPI knockout neonates contain aberrantly high IRAK1 protein levels in the colons and fail to develop immune tolerance to environmental microbes. Our results demonstrate that hnRNPI plays a critical role in establishing neonatal immune adaptation and preventing colitis and colorectal cancer.

Precisely controlled host-microbe interactions in the gastrointestinal tract are crucial for human overall health and well-being. Dysregulated host responses to gut microbiota are the major cause of autoimmune diseases, inflammatory disorders and cancers. The intestinal epithelium lines the gastrointestinal tract and plays a critical role in sensing gut microbes and subsequently developing a balance of immune tolerance and active immune responses. During the neonatal stage, the immune system in the gastrointestinal tract must be temporally suppressed to accommodate the large number of newly arrived microbes. This process is known as neonatal immune adaptation, and is critical for the establishment of proper host- microbe interactions. We studied the function of hnRNPI in the intestinal epithelium by genetically ablating it in the intestinal epithelial cells of mouse. We found that loss of hnRNPI in intestinal epithelial cells disrupts neonatal immune adaptation, resulting in spontaneous colitis and early onset of invasive colorectal cancer. We show that hnRNPI is required for the neonatal immune suppression through decreasing the protein level of IRAK1, an essential component of toll-like receptor-mediated NF-κB signaling. Our studies demonstrate a critical role of hnRNPI in establishing neonatal immune adaptation and preventing colitis and colorectal cancer.

p120-catenin in canonical Wnt signaling

Canonical Wnt signaling controls β-catenin protein stabilization, its translocation to the nucleus and the activation of β-catenin/Tcf-4-dependent transcription. In this review, we revise and discuss the recent results describing actions of p120-catenin in different phases of this pathway. More specifically, we comment its involvement in four different steps: (i) the very early activation of CK1ɛ, essential for Dvl-2 binding to the Wnt receptor complex; (ii) the internalization of GSK3 and Axin into multivesicular bodies, necessary for a complete stabilization of β-catenin; (iii) the activation of Rac1 small GTPase, required for β-catenin translocation to the nucleus; and (iv) the release of the inhibitory action caused by Kaiso transcriptional repressor. We integrate these new results with the previously known action of other elements in this pathway, giving a particular relevance to the responses of the Wnt pathway not required for β-catenin stabilization but for β-catenin transcriptional activity. Moreover, we discuss the possible future implications, suggesting that the two cellular compartments where β-catenin is localized, thus, the adherens junction complex and the Wnt signalosome, are more physically connected that previously thought.

Activation of CK1ɛ by PP2A/PR61ɛ is required for the initiation of Wnt signaling

Canonical Wnt signaling induces the stabilization of β-catenin, its translocation to the nucleus and the activation of target promoters. This pathway is initiated by the binding of Wnt ligands to the Frizzled receptor, the association of the LRP5/6 co-receptor and the formation of a complex comprising Dvl-2, Axin and protein kinases CK1α, ɛ, γ and GSK3. Among these, activation of CK1ɛ, constitutively bound to LRP5/6 through p120-catenin, is required for the association of the rest of the components. We describe here that CK1ɛ is activated by the PP2A/PR61ɛ phosphatase. Binding of Wnt ligands promotes the interaction of LRP5/6-associated CK1ɛ with Frizzled-bound PR61ɛ regulatory subunit, facilitating the access of PP2A catalytic subunit to CK1ɛ and its activation, what enables the recruitment of Dvl-2 to the receptor complex and the initiation of the Wnt pathway. Our results uncover the mechanism of activation of the canonical Wnt pathway by its ligands.

Protein Phosphatase 2A as a Therapeutic Target in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a heterogeneous malignant disorder of hematopoietic progenitor cells in which several genetic and epigenetic aberrations have been described. Despite progressive advances in our understanding of the molecular biology of this disease, the outcome for most patients is poor. It is, therefore, necessary to develop more effective treatment strategies. Genetic aberrations affecting kinases have been widely studied in AML; however, the role of phosphatases remains underexplored. Inactivation of the tumor-suppressor protein phosphatase 2A (PP2A) is frequent in AML patients, making it a promising target for therapy. There are several PP2A inactivating mechanisms reported in this disease. Deregulation or specific post-translational modifications of PP2A subunits have been identified as a cause of PP2A malfunction, which lead to deregulation of proliferation or apoptosis pathways, depending on the subunit affected. Likewise, overexpression of either SET or cancerous inhibitor of protein phosphatase 2A, endogenous inhibitors of PP2A, is a recurrent event in AML that impairs PP2A activity, contributing to leukemogenesis progression. Interestingly, the anticancer activity of several PP2A-activating drugs (PADs) depends on interaction/sequestration of SET. Preclinical studies show that pharmacological restoration of PP2A activity by PADs effectively antagonizes leukemogenesis, and that these drugs have synergistic cytotoxic effects with conventional chemotherapy and kinase inhibitors, opening new possibilities for personalized treatment in AML patients, especially in cases with SET-dependent inactivation of PP2A. Here, we review the role of PP2A as a druggable tumor suppressor in AML.

The small leucine-rich repeat secreted protein Asporin induces eyes in Xenopus embryos through the IGF signalling pathway

Small leucine-rich repeat proteoglycan (SLRP) family proteins play important roles in a number of biological events. Here, we demonstrate that the SLRP family member Asporin (ASPN) plays a crucial role in the early stages of eye development in Xenopus embryos. During embryogenesis, ASPN is broadly expressed in the neuroectoderm of the embryo. Overexpression of ASPN causes the induction of ectopic eyes. By contrast, blocking ASPN function with a morpholino oligonucleotide (ASPN-MO) inhibits eye formation, indicating that ASPN is an essential factor for eye development. Detailed molecular analyses revealed that ASPN interacts with insulin growth factor receptor (IGFR) and is essential for activating the IGF receptor-mediated intracellular signalling pathway. Moreover, ASPN perturbed the Wnt, BMP and Activin signalling pathways, suggesting that ASPN thereby creates a favourable environment in which the IGF signal can dominate. ASPN is thus a novel secreted molecule essential for eye induction through the coordination of multiple signalling pathways.

A Nonsynonymous/Synonymous Substitution Analysis of the B56 Gene Family Aids in Understanding B56 Isoform Diversity

Gene duplication leads to the formation of gene families, wherein purifying or neutral selection maintains the original gene function, while diversifying selection confers new functions onto duplicated genes. The B56 gene family is highly conserved; it is encoded by one gene in protists and fungi, and five genes in vertebrates. B56 regulates protein phosphatase 2A (PP2A), an abundant heterotrimeric serine/threonine phosphatase that functions as a tumor suppressor and consists of a scaffolding “A” and catalytic “C” subunit heterodimer bound to a regulatory “B” subunit. Individual regulatory B56 subunits confer disparate functions onto PP2A in various cell-cell signaling pathways. B56 proteins share a conserved central core domain, but have divergent N- and C-termini which play a role in isoform specificity. We carried out a nonsynonymous/synonymous substitution analysis to better understand the divergence of vertebrate B56 genes. When five B56 paralogs from ten vertebrate species were analyzed, the gene family displayed purifying selection; stronger purifying selection was revealed when individual B56 isoforms were analyzed separately. The B56 core experienced stronger purifying selection than the N- and C-termini, which correlates with the presence of several contacts between the core and the AC heterodimer. Indeed, the majority of the contact points that we analyzed between B56 and the AC heterodimer experienced strong purifying selection. B56 subfamilies showed distinct patterns of selection in their N- and C-termini. The C-terminus of the B56-1 subfamily and the N-terminus of the B56-2 subfamily exhibited strong purifying selection, suggesting that these termini carry out subfamily-specific functions, while the opposite termini exhibited diversifying selection and likely carry out isoform-specific functions. We also found reduced synonymous substitutions at the N- and C-termini when grouping B56 genes by species but not by isoform, suggesting species-specific codon bias may have a role in regulating B56 gene expression.

Evolving molecular era of childhood medulloblastoma: time to revisit therapy

Currently medulloblastoma is treated with a uniform therapeutic approach based on histopathology and clinico-radiological risk stratification, resulting in unpredictable treatment failure and relapses. Improved understanding of the biological, molecular and genetic make-up of these tumors now clearly identifies it as a compendium of four distinct subtypes (WNT, SHH, group 3 and 4). Advances in utilization of the genomic and epigenomic machinery have now delineated genetic aberrations and epigenetic perturbations in each subgroup as potential druggable targets. This has resulted in endeavors to profile targeted therapy. The challenge and future of medulloblastoma therapeutics will be to keep pace with the evolving novel biological insights and translating them into optimal targeted treatment regimens.

Evolutionary Analysis of the B56 Gene Family of PP2A Regulatory Subunits

Protein phosphatase 2A (PP2A) is an abundant serine/threonine phosphatase that functions as a tumor suppressor in numerous cell-cell signaling pathways, including Wnt, myc, and ras. The B56 subunit of PP2A regulates its activity, and is encoded by five genes in humans. B56 proteins share a central core domain, but have divergent amino- and carboxy-termini, which are thought to provide isoform specificity. We performed phylogenetic analyses to better understand the evolution of the B56 gene family. We found that B56 was present as a single gene in eukaryotes prior to the divergence of animals, fungi, protists, and plants, and that B56 gene duplication prior to the divergence of protostomes and deuterostomes led to the origin of two B56 subfamilies, B56αβε and B56γδ. Further duplications led to three B56αβε genes and two B56γδ in vertebrates. Several nonvertebrate B56 gene names are based on distinct vertebrate isoform names, and would best be renamed. B56 subfamily genes lack significant divergence within primitive chordates, but each became distinct in complex vertebrates. Two vertebrate lineages have undergone B56 gene loss, Xenopus and Aves. In Xenopus, B56δ function may be compensated for by an alternatively spliced transcript, B56δ/γ, encoding a B56δ-like amino-terminal region and a B56γ core.

PP2A-B56ϵ complex is involved in dephosphorylation of γ-H2AX in the repair process of CPT-induced DNA double-strand breaks

Phosphorylation of histone H2AX (γ-H2AX) in response to DNA double-strand breaks (DSBs) should be eliminated from the sites of DNA damage to fulfill the DNA repair and release cells from the growth arrest. Previous study showed that protein phosphatase 2A (PP2A) interact with γ-H2AX that lead to the dephosphorylation of γ-H2AX. Here, we examined the effects of suppression of PP2A regulatory subunits on dephosphorylation of γ-H2AX in human embryonic kidney epithelial cells (HEK) treated by topoisomerase I inhibitor camptothecin (CPT). We found that cells with suppression of B55α or B56ϵ were more sensitive to DNA damage agents. Suppression of B56ϵ led to persistence of γ-H2AX, resulting in prolonged DSBs repair and increased chromatin instability measured by comet assay. In addition, the deficiency of B56ϵ impaired the cell cycle regulation and the DNA repair pathway of homologous recombination (HR). Notably, we detected that PP2A B56ϵ subunit was involved directly in dephosphorylation of γ-H2AX and translocated from cytoplasm to nucleus upon the treatment of CPT. Our findings demonstrate that PP2A holoenzyme containing B56ϵ is responsible for the dephosphorylation of γ-H2AX and regulation of DNA repair of DSBs induced by CPT.

HSP105 recruits protein phosphatase 2A to dephosphorylate β-catenin

The Wnt/β-catenin pathway causes accumulation of β-catenin in the cytoplasm and its subsequent translocation into the nucleus to initiate the transcription of the target genes. Without Wnt stimulation, β-catenin forms a complex with axin (axis inhibitor), adenomatous polyposis coli (APC), casein kinase 1α (CK1α), and glycogen synthase kinase 3β (GSK3β) and undergoes phosphorylation-dependent ubiquitination. Phosphatases, such as protein phosphatase 2A (PP2A), interestingly, also are components of this degradation complex; therefore, a balance must be reached between phosphorylation and dephosphorylation. How this balance is regulated is largely unknown. Here we show that a heat shock protein, HSP105, is a previously unidentified component of the β-catenin degradation complex. HSP105 is required for Wnt signaling, since depletion of HSP105 compromises β-catenin accumulation and target gene transcription upon Wnt stimulation. Mechanistically, HSP105 depletion disrupts the integration of PP2A into the β-catenin degradation complex, favoring the hyperphosphorylation and degradation of β-catenin. HSP105 is overexpressed in many types of tumors, correlating with increased nuclear β-catenin protein levels and Wnt target gene upregulation. Furthermore, overexpression of HSP105 is a prognostic biomarker that correlates with poor overall survival in breast cancer patients as well as melanoma patients participating in the BRIM2 clinical study.

Regulation of nuclear-cytoplasmic shuttling and function of Family with sequence similarity 13, member A (Fam13a), by B56-containing PP2As and Akt

Recent genome-wide association studies reveal that the FAM13A gene is associated with human lung function and a variety of lung diseases, including chronic obstructive pulmonary disease, asthma, lung cancer, and pulmonary fibrosis. The biological functions of Fam13a, however, have not been studied. In an effort to identify novel substrates of B56-containing PP2As, we found that B56-containing PP2As and Akt act antagonistically to control reversible phosphorylation of Fam13a on Ser-322. We show that Ser-322 phosphorylation acts as a molecular switch to control the subcellular distribution of Fam13a. Fam13a shuttles between the nucleus and cytoplasm. When Ser-322 is phosphorylated by Akt, the binding between Fam13a and 14-3-3 is enhanced, leading to cytoplasmic sequestration of Fam13a. B56-containing PP2As dephosphorylate phospho-Ser-322 and promote nuclear localization of Fam13a. We generated Fam13a-knockout mice. Fam13a-mutant mice are viable and healthy, indicating that Fam13a is dispensable for embryonic development and physiological functions in adult animals. Intriguingly, Fam13a has the ability to activate the Wnt pathway. Although Wnt signaling remains largely normal in Fam13a-knockout lungs, depletion of Fam13a in human lung cancer cells causes an obvious reduction in Wnt signaling activity. Our work provides important clues to elucidating the mechanism by which Fam13a may contribute to human lung diseases.

Pediatric medulloblastoma - update on molecular classification driving targeted therapies

As advances in the molecular and genetic profiling of pediatric medulloblastoma evolve, associations with prognosis and treatment are found (prognostic and predictive biomarkers) and research is directed at molecular therapies. Medulloblastoma typically affects young patients, where the implications of any treatment on the developing brain must be carefully considered. The aim of this article is to provide a clear comprehensible update on the role molecular profiling and subgroups in pediatric medulloblastoma as it is likely to contribute significantly toward prognostication. Knowledge of this classification is of particular interest because there are new molecular therapies targeting the Shh subgroup of medulloblastomas.

Stabilization of speckle-type POZ protein (Spop) by Daz interacting protein 1 (Dzip1) is essential for Gli turnover and the proper output of Hedgehog signaling

The Hedgehog (Hh) pathway is essential for embryonic development and adult tissue homeostasis. The Gli/Cubitus interruptus (Ci) family of transcription factors acts at the downstream end of the pathway to mediate Hh signaling. Both Hh-dependent and -independent Gli regulatory mechanisms are important for the output of Hh signaling. Daz interacting protein 1 (Dzip1) has bipartite positive and negative functions in the Hh pathway. The positive Hh regulatory function appears to be attributed to a requirement for Dzip1 during ciliogenesis. The mechanism by which Dzip1 inhibits Hh signaling, however, remains largely unclear. We recently found that Dzip1 is required for Gli turnover, which may account for its inhibitory function in Hh signaling. Here, we report that Dzip1 regulates Gli/Ci turnover by preventing degradation of speckle-type POZ protein (Spop), a protein that promotes proteasome-dependent turnover of Gli proteins. We provide evidence that Dzip1 regulates the stability of Spop independent of its function in ciliogenesis. Partial knockdown of Dzip1 to levels insufficient for perturbing ciliogenesis, sensitized Xenopus embryos to Hh signaling, leading to phenotypes that resemble activation of Hh signaling. Importantly, overexpression of Spop was able to restore proper Gli protein turnover and rescue phenotypes in Dzip1-depleted embryos. Consistently, depletion of Dzip1 in Drosophila S2 cells destabilized Hh-induced BTB protein (HIB), the Drosophila homolog of Spop, and increased the level of Ci. Thus, Dzip1-dependent stabilization of Spop/HIB is evolutionarily conserved and essential for proper regulation of Gli/Ci proteins in the Hh pathway.

Maternal Dead-End1 is required for vegetal cortical microtubule assembly during Xenopus axis specification

Vertebrate axis specification is an evolutionarily conserved developmental process that relies on asymmetric activation of Wnt signaling and subsequent organizer formation on the future dorsal side of the embryo. Although roles of Wnt signaling during organizer formation have been studied extensively, it is unclear how the Wnt pathway is asymmetrically activated. In Xenopus and zebrafish, the Wnt pathway is triggered by dorsal determinants, which are translocated from the vegetal pole to the future dorsal side of the embryo shortly after fertilization. The transport of dorsal determinants requires a unique microtubule network formed in the vegetal cortex shortly after fertilization. However, molecular mechanisms governing the formation of vegetal cortical microtubule arrays are not fully understood. Here we report that Dead-End 1 (Dnd1), an RNA-binding protein required for primordial germ cell development during later stages of embryogenesis, is essential for Xenopus axis specification. We show that knockdown of maternal Dnd1 specifically interferes with the formation of vegetal cortical microtubules. This, in turn, impairs translocation of dorsal determinants, the initiation of Wnt signaling, organizer formation, and ultimately results in ventralized embryos. Furthermore, we found that Dnd1 binds to a uridine-rich sequence in the 3'-UTR of trim36, a vegetally localized maternal RNA essential for vegetal cortical microtubule assembly. Dnd1 anchors trim36 to the vegetal cortex in the egg, promoting high concentrations of Trim36 protein there. Our work thus demonstrates a novel and surprising function for Dnd1 during early development and provides an important link between Dnd1, mRNA localization, the microtubule cytoskeleton and axis specification.

Impaired expression of protein phosphatase 2A subunits enhances metastatic potential of human prostate cancer cells through activation of AKT pathway

Background:

Protein phosphatase 2A (PP2A) is a dephosphorylating enzyme, loss of which can contribute to prostate cancer (PCa) pathogenesis. The aim of this study was to analyse the transcriptional and translational expression patterns of individual subunits of the PP2A holoenzyme during PCa progression.

Methods:

Immunohistochemistry (IHC), western blot, and real-time PCR was performed on androgen-dependent (AD) and androgen-independent (AI) PCa cells, and benign and malignant prostate tissues for all the three PP2A (scaffold, regulatory, and catalytic) subunits. Mechanistic and functional studies were performed using various biochemical and cellular techniques.

Results:

Through immunohistochemical analysis we observed significantly reduced levels of PP2A-A and -B′γ subunits (P<0.001 and P=0.0002) in PCa specimens compared with benign prostate. Contemporarily, there was no significant difference in PP2A-C subunit expression between benign and malignant tissues. Similar to the expression pattern observed in tissues, the endogenous levels of PP2A-A and B′γ subunits were abrogated from the low metastatic to high metastatic and AD to AI cell line models, without any change in the catalytic subunit expression. Furthermore, using in vitro studies we demonstrated that PP2A-Aα scaffold subunit has a role in dampening AKT, β-catenin, and FAK (focal adhesion kinase) signalling.

Conclusion:

We conclude that loss of expression of scaffold and regulatory subunits of PP2A is responsible for its altered function during PCa pathogenesis.

Cortical rotation and messenger RNA localization in Xenopus axis formation

In Xenopus eggs, fertilization initiates a rotational movement of the cortex relative to the cytoplasm, resulting in the transport of critical determinants to the future dorsal side of the embryo. Cortical rotation is mediated by microtubules, resulting in activation of the Wnt/β-catenin signaling pathway and expression of organizer genes on the dorsal side of the blastula. Similar cytoplasmic localizations resulting in β-catenin activation occur in many chordate embryos, suggesting a deeply conserved mechanism for patterning early embryos. This review summarizes the experimental evidence for the molecular basis of this model, focusing on recent maternal loss-of-function studies that shed light on two main unanswered questions: (1) what regulates microtubule assembly during cortical rotation and (2) how is Wnt/β-catenin signaling activated dorsally? In addition, as these processes depend on vegetally localized molecules in the oocyte, the mechanisms of RNA localization and novel roles for localized RNAs in axis formation are discussed. The work reviewed here provides a beginning framework for understanding the coupling of asymmetry in oogenesis with the establishment of asymmetry in the embryo.

Dietary supplementation with the probiotic Lactobacillus fermentum I5007 and the antibiotic aureomycin differentially affects the small intestinal proteomes of weanling piglets

Antibiotics have long been used in animal production and medication to alleviate weaning stress. However, due to the concerns over food safety and human health, its use in animal production has been prohibited in many countries. Therefore, there is growing interest in developing alternative additives, such as a probiotic Lactobacillus. In this study, a proteomic approach coupled with biochemical analysis was applied to investigate alterations of proteomes in the small intestinal mucosa of weanling piglets after a 13-d period of feeding with supplemental L. fermentum I5007 or aureomycin (an antibiotic). We indentified 27 differentially expressed protein spots that participated in 7 key biological processes, including: 1) energy metabolism; 2) lipid metabolism; 3) protein synthesis; 4) cell structure and mobility; 5) cellular proliferation and apoptosis; 6) immune response; and 7) stress response and detoxification. Both L. fermentum I5007 and aureomycin decreased the expression of proteins related to apoptosis, stress response, and increased the expression of proteins related to detoxification in the gastrointestinal (GI) tract of weanling piglets. L. fermentum I5007 exhibited additional effects in alleviating weaning stress syndrome by enhancing the levels of proteins involved in energy metabolism, lipid metabolism, cell structure and mobility, protein synthesis, and immune response, thereby facilitating cellular proliferation and depressing apoptosis. In contrast, aureomycin reduced the levels of proteins related to energy metabolism, protein synthesis, cell structure, motility, and immunity. These novel findings have important implications for understanding the mechanisms whereby L. fermentum I5007 can improve the GI health of postweaning piglets.

Phosphatases: the new brakes for cancer development?

The phosphatidylinositol 3-kinase (PI3K) pathway plays a pivotal role in the maintenance of processes such as cell growth, proliferation, survival, and metabolism in all cells and tissues. Dysregulation of the PI3K/Akt signaling pathway occurs in patients with many cancers and other disorders. This aberrant activation of PI3K/Akt pathway is primarily caused by loss of function of all negative controllers known as inositol polyphosphate phosphatases and phosphoprotein phosphatases. Recent studies provided evidence of distinct functions of the four main phosphatases—phosphatase and tensin homologue deleted on chromosome 10 (PTEN), Src homology 2-containing inositol 5′-phosphatase (SHIP), inositol polyphosphate 4-phosphatase type II (INPP4B), and protein phosphatase 2A (PP2A)—in different tissues with respect to regulation of cancer development. We will review the structures and functions of PTEN, SHIP, INPP4B, and PP2A phosphatases in suppressing cancer progression and their deregulation in cancer and highlight recent advances in our understanding of the PI3K/Akt signaling axis.

The antagonistic action of B56-containing protein phosphatase 2As and casein kinase 2 controls the phosphorylation and Gli turnover function of Daz interacting protein 1

The Hedgehog (Hh) pathway is evolutionarily conserved and plays critical roles during embryonic development and adult tissue homeostasis. Defective Hh signaling has been linked to a wide range of birth defects and cancers. Hh family proteins regulate the expression of their downstream target genes through the control of proteolytic processing and the transcriptional activation function of Gli transcription factors. Although Hh-dependent regulation of Gli has been studied extensively, other Gli regulatory mechanisms remain relatively unappreciated. Here we report our identification of a novel signaling cascade that controls the stability of Gli proteins. This cascade consists of Daz interacting protein 1 (Dzip1), casein kinase 2 (CK2), and B56 containing protein phosphatase 2As (PP2As). We provide evidence that Dzip1 is involved in a novel Gli turnover pathway. We show that CK2 directly phosphorylates Dzip1 at four serine residues, Ser-664/665/706/714. B56-containing PP2As, through binding to a domain located between amino acid residue 474 and 550 of Dzip1, dephosphorylate Dzip1 on these CK2 sites. Our mutagenesis analysis further demonstrates that the unphosphorylatable form of Dzip1 is more potent in promoting Gli turnover. Consistently, we found that the stability of Gli proteins was decreased upon CK2 inhibition and increased by inhibition of B56-containing PP2As. Thus, reversible phosphorylation of Dzip1, which is controlled by the antagonistic action of CK2 and B56-containing PP2As, has an important impact on the stability of Gli transcription factors and Hh signaling.

Norcantharidin impairs medulloblastoma growth by inhibition of Wnt/β-catenin signaling

Medulloblastoma is one of the leading causes of morbidity and mortality in pediatric cancer. Wnt-active tumors, an independent molecular subgroup in medulloblastoma, are characterized by a distinct pattern of genomic aberrations. We assessed the anticancer activity of cantharidin and norcantharidin against medulloblastoma, as cell lines in vitro and in athymic nude mice in vivo. Cantharidin and norcantharidin treatment impaired the growth of DAOY and UW228 medulloblastoma cells and promoted the loss of β-catenin activation and the β-catenin nuclearization linked to N-cadherin impairment in vitro. Intra-peritoneal administration of norcantharidin inhibited the growth of intra-cerebellum tumors in orthotopic xenograft nude mice. Analysis of the xenograft tissues revealed enhanced neuronal differentiation and reduced β-catenin expression. Our findings suggest that norcantharidin has potential therapeutic applications in the treatment of medulloblastoma as a result of its ability to cross the blood-brain barrier and its impairment of Wnt-β-catenin signaling.

Cdc55 coordinates spindle assembly and chromosome disjunction during meiosis

During meiosis, two consecutive nuclear divisions follow a single round of deoxyribonucleic acid replication. In meiosis I, homologues are segregated, whereas in meiosis II, sister chromatids are segregated. This requires that the sequential assembly and dissolution of specialized chromosomal factors are coordinated with two rounds of spindle assembly and disassembly. How these events are coupled is unknown. In this paper, we show, in budding yeast, that the protein phosphatase 2A regulatory subunit Cdc55 couples the loss of linkages between chromosomes with nuclear division by restraining two other phosphatases, Cdc14 and PP2A(Rts1). Cdc55 maintains Cdc14 sequestration in the nucleolus during early meiosis, and this is essential for the assembly of the meiosis I spindle but not for chromosomes to separate. Cdc55 also limits the formation of PP2A holocomplexes containing the alternative regulatory subunit Rts1, which is crucial for the timely dissolution of sister chromatid cohesion. Therefore, Cdc55 orders passage through the meiotic divisions by ensuring a balance of phosphatases.

A genetic variant in a PP2A regulatory subunit encoded by the PPP2R2B gene associates with altered breast cancer risk and recurrence

A recent candidate gene association study identified a single nucleotide polymorphism (SNP) in the PPP2R2B gene (rs319217, A/G) that manifests allelic differences in the cellular responses to treatment with chemotherapeutic agents (Vazquez et al., Nat Rev Drug Discov 2008;7:979-87). This gene encodes a regulatory subunit of protein phosphatase 2A (PP2A), one of the major Ser/Thr phosphatases implicated in the negative control of cell growth and division. Given the tumor suppressor activities of PP2A, here we evaluate whether this genetic variant associates with the age of diagnosis and recurrence of breast cancer in women. To investigate the linkage disequilibrium in the vicinity of this SNP, PPP2R2B haplotypes were analyzed using HapMap data for 90 Caucasians. It is found that the A variant of rs319217 tags a haplotype that appears tobe under positive selection in the Caucasian population, implying that this SNP is functional. Subsequently, associations with cellular responses were investigated using data reported by the NCI anticancer drug screen and associations with breast cancer clinical variables were analyzed in a cohort of 819 Caucasian women. The A allele associates with a better response of tumor derived cell lines, lower risk of breast cancer recurrence, later time to recurrence, and later age of diagnosis of breast cancer in Caucasian women. Taken together these results indicate that the A variant of the rs319217 SNP is a marker of better prognosis in breast cancer.

The phosphoprotein phosphatase family of Ser/Thr phosphatases as principal targets of naturally occurring toxins

Phosphoprotein phosphatases (PPPs) constitute one of three otherwise unrelated families of enzymes that specialize in removing the phosphate group from phosphorylated serine and threonine residues. The involvement of PPP enzymes in the regulation of processes such as gene expression, DNA replication, morphogenesis, synaptic transmission, glycogen metabolism, and apoptosis has underscored their potential as targets for the treatment of a variety of conditions such as cancer, diabetes, or Alzheimer's disease. Interestingly, PPP enzymes also constitute the physiological target of multiple naturally occurring toxins, including microcystins from cyanobacteria and cantharidin from beetles. This review is devoted to the PPP family of enzymes--with a focus on the human PPPs--and the naturally occurring toxins that are known to potently impair their activity. The interaction of the toxins with the enzymes is evaluated in atomic detail to obtain insight on two complementary aspects: (1) which specific structural differences within the similarly folded catalytic core of the PPP enzymes explain their diverse sensitivities to toxin inhibition and (2) which structural features presented by the various toxins account for the differential inhibitory potency towards each PPP. These analyses take advantage of numerous site-directed mutagenesis studies, structure-activity evaluations, and recent crystallographic structures of PPPs bound to different toxins.

Functions of B56-containing PP2As in major developmental and cancer signaling pathways

Members of the B'/B56/PR61 family regulatory subunits of PP2A determine the subcellular localization, substrate specificity, and catalytic activity of PP2A in a wide range of biological processes. Here, we summarize the structure and intracellular localization of B56-containing PP2As and review functions of B56-containing PP2As in several major developmental/cancer signaling pathways.

Association of Tat with promoters of PTEN and PP2A subunits is key to transcriptional activation of apoptotic pathways in HIV-infected CD4+ T cells

Apoptosis in HIV-1-infected CD4+ primary T cells is triggered by the alteration of the PI3K and p53 pathways, which converge on the FOXO3a transcriptional activator. Tat alone can cause activation of FOXO3a and of its proapoptotic target genes. To understand how Tat affects this pathway, we carried out ChIP-Chip experiments with Tat. Tat associates with the promoters of PTEN and two PP2A subunit genes, but not with the FOXO3a promoter. PTEN and PP2A encode phosphatases, whose levels and activity are increased when Tat is expressed. They counteract phosphorylation of Akt1 and FOXO3a, and so activate transcriptional activity of FOXO3a. FOXO3a promotes increased transcription of Egr-1, which can further stimulate the transcription of PTEN, thereby reinforcing the pathway that leads to FOXO3a transcriptional activation. RNAi experiments support the role of PTEN and PP2A in the initiation of the Tat-mediated cascade, which is critical to apoptosis. The increased accumulation of PTEN and PP2A subunit mRNAs during Tat expression is more likely to be the result of increased transcription initiation and not relief of promoter-proximal pausing of RNAPII. The Tat-PTEN and -PP2A promoter interactions provide a mechanistic explanation of Tat-mediated apoptosis in CD4+ T cells.

HIV infection leads to the depletion of CD4+ T cells, the major viral cell target. The destruction of these cells can occur because of cytopathic effect or apoptosis. HIV Tat is one of the proteins that can contribute to the apoptotic process of both infected and uninfected cells, as it is released in the plasma and enter uninfected cells. Tat expression in CD4+ T-cells is linked to increased transcriptional activity of FOXO3a, a factor that targets the transcription of pro-apoptotic genes. The mechanism by which Tat leads to activation apoptotic pathways is by associating with the promoters of the phospatases PTEN and PP2A and by increasing their levels. The increased amount of these proteins leads to a decreased amount of pAKt1 and increased amount of non-phosphorylated FOXO3a, which migrates from the cytoplasm to the nucleus and increases the transcription of its proapoptotic target genes. These results, together with experiments that silence PTEN and PP2A and measure their activities, identify the association of Tat with PTEN and PP2A promoters as the initiating event of Tat-mediated apoptosis.

PP2A:B56{epsilon}, a substrate of caspase-3, regulates p53-dependent and p53-independent apoptosis during development

Protein phosphatase 2A (PP2A) is one of the most abundantly expressed serine/threonine protein phosphatases. A large body of evidence suggests that PP2A is a tumor suppressor and plays critical roles in regulating apoptosis. PP2A is a heterotrimeric protein complex. Its substrate specificity, localization, and activity are regulated by regulatory subunits of PP2A. A recent study has demonstrated that single nucleotide polymorphism in B56ε (PPP2R5E), a B56 family regulatory subunit of PP2A, is associated with human soft tissue sarcoma. This raises the possibility that B56ε is involved in tumorigenesis and plays important roles in regulating apoptosis. However, this hypothesis has not been tested experimentally. Our previous studies revealed that B56ε regulates a number of developmental signaling pathways during early embryonic patterning. Here we report novel functions of B56ε in regulating apoptosis. We provide evidence that B56ε has both anti- and pro-apoptotic functions. B56ε suppresses p53-independent apoptosis during neural development, but triggers p53-dependent apoptosis. Mechanistically, B56ε regulates the p53-dependent apoptotic pathway solely through controlling the stability of p53 protein. In addition to its function in regulating apoptosis, we show that B56ε undergoes proteolytic cleavage. The cleavage of B56ε is mediated by caspase-3 and occurs on the carboxyl side of an evolutionarily conserved N-terminal "DKXD" motif. These results demonstrate that B56ε, a substrate of caspase-3, is an essential regulator of apoptosis. So far, we have identified an alternative translation isoform and a caspase cleavage product of B56ε. The significance of post-transcriptional regulation of B56ε is discussed.

Predominant actions of cytosolic BSU1 and nuclear BIN2 regulate subcellular localization of BES1 in brassinosteroid signaling

During brassinosteroid (BR) signaling in Arabidopsis, BSU1 (bri1 SUPPRESSOR1) phosphatase and BIN2 (BRASSINOSTEROID INSENSITIVE2) kinase regulate the signal intensity by determining the phosphorylation status of the transcription factors BZR1 (BRASSINAZOLE RESISTANT1) and BES1 (bri1 EMS SUPPRESSOR1). Here, we report hat BIN2 and BSU1 are nucleocytoplasmic regulators that modulate the subcellular localization of BES1, with differential activities between the nucleus and the cytosol. In our experiments, the nuclear BIN2 induced phosphorylation and nuclear export of BES1 more efficiently than cytosolic BIN2.The cytoplasmic BSU1 mediated the dephosphorylation and nuclear translocation of BES1 more efficiently than the nuclear one. BSU1 compromised the dwarf phenotype of bri1-5, a weak allele of BRI1 (BR-INSENSITIVE 1) receptor kinase, more effectively when localized in the cytosol than in the nucleus in transgenic plants. In conclusion, the predominance of cytosolic BSU1 and nuclear BIN2 might be required for the efficient subcellular localization of BES1 in BR signaling.

Expression of the genes siamois, engrailed-2, bmp4 and myf5 during Xenopus development in presence of the marine toxins okadaic acid and palytoxin

The present investigation examines the effects of the marine toxins, okadaic acid (OA) and palytoxin (PTX), on some genes involved in the neural and muscular specification and patterning of Xenopus laevis. The RT-PCR analyses performed at different stages of embryonic and larval development (stages 11-47) demonstrated that both toxins induce an over-expression of the genes siamois and engrailed-2 and a different behaviour in bmp4 and myf5. Indeed, OA provoked a significant increase in bmp4 in the earliest stage (11) examined, a down-regulation from stages 12 to 17, and a renewed increase from the beginning of hatching onwards (stages 35-47). In contrast, myf5 was up-regulated in all stages up to 35. PTX induced an over-expression of both bmp4 and myf5 during the embryonic and early larval development stages. The results show that PTX induces an increase in expression levels in all tested genes, while the response to OA seems to be more stage-dependent, with the embryonic development stage more sensitive to the toxin than the larval stages.

PR55 alpha, a regulatory subunit of PP2A, specifically regulates PP2A-mediated beta-catenin dephosphorylation

A central question in Wnt signaling is the regulation of beta-catenin phosphorylation and degradation. Multiple kinases, including CKI alpha and GSK3, are involved in beta-catenin phosphorylation. Protein phosphatases such as PP2A and PP1 have been implicated in the regulation of beta-catenin. However, which phosphatase dephosphorylates beta-catenin in vivo and how the specificity of beta-catenin dephosphorylation is regulated are not clear. In this study, we show that PP2A regulates beta-catenin phosphorylation and degradation in vivo. We demonstrate that PP2A is required for Wnt/beta-catenin signaling in Drosophila. Moreover, we have identified PR55 alpha as the regulatory subunit of PP2A that controls beta-catenin phosphorylation and degradation. PR55 alpha, but not the catalytic subunit, PP2Ac, directly interacts with beta-catenin. RNA interference knockdown of PR55 alpha elevates beta-catenin phosphorylation and decreases Wnt signaling, whereas overexpressing PR55 alpha enhances Wnt signaling. Taken together, our results suggest that PR55 alpha specifically regulates PP2A-mediated beta-catenin dephosphorylation and plays an essential role in Wnt signaling.

hnRNP I inhibits Notch signaling and regulates intestinal epithelial homeostasis in the zebrafish

Regulated intestinal stem cell proliferation and differentiation are required for normal intestinal homeostasis and repair after injury. The Notch signaling pathway plays fundamental roles in the intestinal epithelium. Despite the fact that Notch signaling maintains intestinal stem cells in a proliferative state and promotes absorptive cell differentiation in most species, it remains largely unclear how Notch signaling itself is precisely controlled during intestinal homeostasis. We characterized the intestinal phenotypes of brom bones, a zebrafish mutant carrying a nonsense mutation in hnRNP I. We found that the brom bones mutant displays a number of intestinal defects, including compromised secretory goblet cell differentiation, hyperproliferation, and enhanced apoptosis. These phenotypes are accompanied by a markedly elevated Notch signaling activity in the intestinal epithelium. When overexpressed, hnRNP I destabilizes the Notch intracellular domain (NICD) and inhibits Notch signaling. This activity of hnRNP I is conserved from zebrafish to human. In addition, our biochemistry experiments demonstrate that the effect of hnRNP I on NICD turnover requires the C-terminal portion of the RAM domain of NICD. Our results demonstrate that hnRNP I is an evolutionarily conserved Notch inhibitor and plays an essential role in intestinal homeostasis.

The structure of the PP2A regulatory subunit B56 gamma: the remaining piece of the PP2A jigsaw puzzle

The PP2A serine/threonine phosphatase regulates a plethora of cellular processes. In the cell the predominant form of the enzyme is a heterotrimer, formed by a core dimer composed of a catalytic and a scaffolding subunit, which assemble together with one of a range of different regulatory B subunits. Here, we present the first structure of a free non-complexed B subunit, B56 gamma. Comparison with the recent structures of a heterotrimeric complex and the core dimer reveals several significant conformational changes in the interface region between the B56 gamma and the core dimer. These allow for an assembly scheme of the PP2A holoenzyme to be put forth where B56 gamma first complexes with the scaffolding subunit and subsequently binds to the catalytic subunit and this induces the formation of a binding site for the invariant C-terminus of the catalytic subunit that locks in the complex as a last step of assembly.

The 48-kDa alternative translation isoform of PP2A:B56epsilon is required for Wnt signaling during midbrain-hindbrain boundary formation

Alternative translation is an underappreciated post-transcriptional regulation mechanism. Although only a small number of genes are found to be alternatively translated, most genes undergoing alternative translation play important roles in tumorigenesis and development. Protein phosphatase 2A (PP2A) is involved in many cellular events during tumorigenesis and development. The specificity, localization, and activity of PP2A are regulated by B regulatory subunits. B56epsilon, a member of the B56 regulatory subunit family, is involved in multiple signaling pathways and regulates a number of developmental processes. Here we report that B56epsilon is alternatively translated, leading to the production of a full-length form and a shorter isoform that lacks the N-terminal 76 amino acid residues of the full-length form. Alternative translation of B56epsilon occurs through a cap-dependent mechanism. We provide evidence that the shorter isoform is required for Wnt signaling and regulates the midbrain/hindbrain boundary formation during Xenopus embryonic development. This demonstrates that the shorter isoform of B56epsilon has important biological functions. Furthermore, we show that the N-terminal sequence of B56epsilon, which is not present in the shorter isoform, contains a nuclear localization signal, whereas the C terminus of B56epsilon contains a nuclear export signal. The shorter isoform, which lacks the N-terminal nuclear localization signal, is restricted to the cytoplasm. In contrast, the full-length form can be localized to the nucleus in a cell type-specific manner. The finding that B56epsilon is alternatively translated adds a new level of regulation to PP2A holoenzymes.

The function of PP2A/B56 in non-metazoan multicellular development

DB56, the Dictyostelium B56 homolog, displayed high sequence homology to other eukaryotic B56 subunits of the PP2A and a specific association with the PP2A catalytic subunit. Cells lacking DB56, psrA(-), displayed higher PP2A phosphatase activity compared with the wild type, approximately 10 hr of delayed expression of ecmA and ecmB prestalk markers, and inefficient culmination. The prespore marker cotB declined as wild-type cells culminate, but no such decline was observed from psrA(-) cells. Interestingly, psrA(-) cells exhibited higher GSK3 kinase activity. Furthermore, the expression of the dominant negative GSK3 mutant (K84/85M) in psrA(-) cells improved both prestalk and prespore expression patterns similarly to wild-type cells. However, culmination was not restored in psrA(-) cells expressing dominant negative GSK3, which suggests that PP2A/DB56 has an extra target during terminal differentiation. This report shows that PP2A/DB56 controls not only metazoan development, but also non-metazoan cell fate decision processes.

A tumor suppressor role for PP2A-B56alpha through negative regulation of c-Myc and other key oncoproteins

Loss or inhibition of the serine/threonine protein phosphatase 2A (PP2A) has revealed a critical tumor suppressor function for PP2A. However, PP2A has also been shown to have important roles in cell cycle progression and survival. Therefore, PP2A is not a typical tumor suppressor. This is most likely due to the fact that PP2A represents a large number of different holoenzymes. Further understanding of PP2A function(s), and especially its tumor suppressor activity, will depend largely on our ability to determine specific targets for these different PP2A holoenzymes and to gain an understanding of how these targets confer tumor suppressor activity or contribute to cell cycle progression and cell survival. Recent work has identified c-Myc as a target of the PP2A holoenzyme, PP2A-B56alpha. This holoenzyme also negatively regulates beta-catenin expression and modulates the anti-apoptotic activity of Bcl2, thus characterizing PP2A-B56alpha as a tumor suppressor PP2A holoenzyme. This review will focus on the role of PP2A-B56alpha in regulating c-Myc and will place this tumor suppressor activity of PP2A within the context of its other tumor suppressor functions. Finally, the mechanism(s) through which PP2A-B56alpha tumor suppressor activity may be lost in cancer will be discussed.

Wnt signaling: the good and the bad

Since the first Wnt gene was identified in 1982, the functions and mechanisms of Wnt signaling have been extensively studied. Wnt signaling is conserved from invertebrates to vertebrates and regulates early embryonic development as well as the homeostasis of adult tissues. In addition, both embryonic stem cells and adult stem cells are regulated by Wnt signaling. Deregulation of Wnt signaling is associated with many human diseases, particularly cancers. In this review, we will discuss in detail the functions of many components involved in the Wnt signal transduction pathway. Then, we will explore what is known about the role of Wnt signaling in stem cells and cancers.

Protein phosphatase 2A regulatory subunits and cancer

The serine/threonine protein phosphatase (PP2A) is a trimeric holoenzyme that plays an integral role in the regulation of a number of major signaling pathways whose deregulation can contribute to cancer. The specificity and activity of PP2A are highly regulated through the interaction of a family of regulatory B subunits with the substrates. Accumulating evidence indicates that PP2A acts as a tumor suppressor. In this review we summarize the known effects of specific PP2A holoenzymes and their roles in cancer relevant pathways. In particular we highlight PP2A function in the regulation of MAPK and Wnt signaling.