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

Arabidopsis ABCG14 forms a homodimeric transporter for multiple cytokinins and mediates long-distance transport of isopentenyladenine-type cytokinins

Cytokinins (CKs), primarily trans-zeatin (tZ) and isopentenyladenine (iP) types, play critical roles in plant growth, development, and various stress responses. Long-distance transport of tZ-type CKs meidated by Arabidopsis ATP-binding cassette transporter subfamily G14 (AtABCG14) has been well studied; however, less is known about the biochemical properties of AtABCG14 and its transporter activity toward iP-type CKs. Here we reveal the biochemical properties of AtABCG14 and provide evidence that it is also required for long-distance transport of iP-type CKs. AtABCG14 formed homodimers in human (Homo sapiens) HEK293T, tobacco (Nicotiana tabacum), and Arabidopsis cells. Transporter activity assays of AtABCG14 in Arabidopsis, tobacco, and yeast (Saccharomyces cerevisiae) showed that AtABCG14 may directly transport multiple CKs, including iP- and tZ-type species. AtABCG14 expression was induced by iP in a tZ-type CK-deficient double mutant (cypDM) of CYP735A1 and CYP735A2. The atabcg14 cypDM triple mutant exhibited stronger CK-deficiency phenotypes than cypDM. Hormone profiling, reciprocal grafting, and ²H₆-iP isotope tracer experiments showed that root-to-shoot and shoot-to-root long-distance transport of iP-type CKs were suppressed in atabcg14 cypDM and atabcg14. These results suggest that AtABCG14 participates in three steps of the circular long-distance transport of iP-type CKs: xylem loading in the root for shootward transport, phloem unloading in the shoot for shoot distribution, and phloem unloading in the root for root distribution. We found that AtABCG14 displays transporter activity toward multiple CK species and revealed its versatile roles in circular long-distance transport of iP-type CKs. These findings provide new insights into the transport mechanisms of CKs and other plant hormones.

Proteomics analysis reveals suppression of IL-17 signaling pathways contributed to the therapeutic effects of Jia-Wei Bu-Shen-Yi-Qi formula in a murine asthma model

BACKGROUND

Jia-Wei Bu-Shen-Yi-Qi formula (JWBSYQF), a Chinese herbal formula, is a commonly used prescription for treating asthma patients. However, the targeted proteins associated with JWBSYQF treatment remain unknown.

PURPOSE

Present study aims to evaluate the therapeutic efficacy of JWBSYQF and identify the targeted proteins in addition to functional pathways.

STUDY DESIGN

The ovalbumin (OVA)-induced murine asthma model was established to explore the therapeutic effect of JWBSYQF treatment. Proteomic profiling and quantifications were performed using data-independent acquisition (DIA) methods. Differentially expressed proteins (DEPs) were validated via western blot (WB) and immunohistochemistry (IHC).

METHODS

A murine asthma model was made by OVA sensitization and challenge, and JWBSYQF (2.25, 4.50, 9,00 g/kg body weight) or dexamethasone (1 mg/ kg body weight) were administered orally. Airway hyperresponsiveness (AHR) to methacholine (Mch), inflammatory cell counts and classification in bronchoalveolar lavage fluid (BALF), lung histopathology, and cytokine levels were measured. Furthermore, DIA proteomic analyses were performed to explore the DEPs targeted by JWBSYQF and were further validated by WB and IHC.

RESULTS

Our results exhibited that JWBSYQF attenuated AHR which was mirrored by decreased airway resistance and increased lung compliance. In addition, JWBSYQF-treated mice showed reduced inflammatory score, mucus hypersecretion, as well as reduced the number of BALF leukocytes along with decreased content of BALF Th2 inflammatory cytokines (IL-4, IL-5, IL-13) and serum IgE. Proteomics analysis identified 704 DEPs between the asthmatic mice and control group (MOD vs CON), and 120 DEPs between the JWBSYQF-treatment and the asthmatic mice (JWB-M vs MOD). A total of 33 overlapped DEPs were identified among the three groups. Pathway enrichment analysis showed that DEPs were significantly enriched in IL-17 signaling pathway, in which DEPs, Lcn2, TGF-β1, Gngt2, and Ppp2r5e were common DEPs between three experimental groups. WB and IHC results further validated expressional levels and tendency of these proteins. Our results also showed that JWBSYQF affects mitogen activated protein kinase (MAPK) and nuclear factor-κB (NF-κB) signaling pathways, that are activated by IL-17 signaling.

CONCLUSION

The present study suggested that JWBSYQF could attenuate AHR and airway inflammation in OVA-induced asthmatic mice. In addition, proteomics analysis revealed that suppression of IL-17 signaling pathways contributes to the therapeutic effects of JWBSYQF.

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.

The Interaction Mechanism of Intrinsically Disordered PP2A Inhibitor Proteins ARPP-16 and ARPP-19 With PP2A

Protein phosphatase 2A (PP2A) activity is critical for maintaining normal physiological cellular functions. PP2A is inhibited by endogenous inhibitor proteins in several pathological conditions including cancer. A PP2A inhibitor protein, ARPP-19, has recently been connected to several human cancer types. Accordingly, the knowledge about ARPP-19—PP2A inhibition mechanism is crucial for the understanding the disease development and the therapeutic targeting of ARPP-19—PP2A. Here, we show the first structural characterization of ARPP-19, and its splice variant ARPP-16 using NMR spectroscopy, and SAXS. The results reveal that both ARPP proteins are intrinsically disordered but contain transient secondary structure elements. The interaction mechanism of ARPP-16/19 with PP2A was investigated using microscale thermophoresis and NMR spectroscopy. Our results suggest that ARPP—PP2A A-subunit interaction is mediated by linear motif and has modest affinity whereas, the interaction of ARPPs with B56-subunit is weak and transient. Like many IDPs, ARPPs are promiscuous binders that transiently interact with PP2A A- and B56 subunits using multiple interaction motifs. In summary, our results provide a good starting point for future studies and development of therapeutics that block ARPP-PP2A interactions.

Protective effects of acarbose against insulitis in multiple low-dose streptozotocin-induced diabetic mice

AIMS

The development of type 1 diabetes is associated with inflammatory lesion of the pancreatic islets, known as insulitis. In this study, we focused on the protective effects of acarbose against insulitis in streptozotocin (STZ)-induced diabetic mice and the underlying mechanisms.

MAIN METHODS

The mouse models were established via intraperitoneal injection of multiple low-dose STZ. Blood glucose level and body weight were measured. The severity of insulitis and inflammatory parameters in pancreatic tissues were evaluated. Insulin levels in pancreas and serum were also assessed. In vitro, MIN6 β cells were exposed to pro-inflammatory cytokines to assess the protective effects of acarbose. Cell function and apoptosis were evaluated.

KEY FINDINGS

We found that acarbose administration by gavage reduced the severity of insulitis and improved insulin levels in the experimental diabetic mice. ELISA revealed decreased levels of the inflammatory response markers IL-1β and TNF-α in mouse pancreatic tissues following acarbose treatment. In vitro, acarbose increased cell viability, decreased cell apoptosis, and improved GSIS in MIN6 β cells exposed to pro-inflammatory cytokines. In addition, caspase-3 level and p-p53/p53 ratio in β cells were reduced by acarbose treatment.

SIGNIFICANCE

Taken together, these results revealed a novel function of acarbose in attenuating insulitis. The protective effects of acarbose elicited in vitro and in vivo were shown to be mediated, at least in part, through its anti-inflammatory action.

PP2A and tumor radiotherapy

Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase that serves as a key regulator of cellular physiology in the context of apoptosis, mitosis, and DNA damage responses. Canonically, PP2A functions as a tumor suppressor gene. However, recent evidence suggests that inhibiting PP2A activity in tumor cells may represent a viable approach to enhancing tumor sensitivity to chemoradiotherapy as such inhibition can cause cells to enter a disordered mitotic state that renders them more susceptible to cell death. Indeed, there is evidence that inhibiting PP2A can slow tumor growth following radiotherapy in a range of cancer types including ovarian cancer, liver cancer, malignant glioma, pancreatic cancer, and nasopharyngeal carcinoma. In the present review, we discuss current understanding of the role of PP2A in tumor radiotherapy and the potential mechanisms whereby it may influence this process.

Yeast Lacking the PP2A Phosphatase Regulatory Subunit Rts1 Sensitizes rad51 Mutants to Specific DNA Damaging Agents

Rts1 is a regulatory subunit of the trimeric protein phosphatase 2A phosphatase and it participates in many biological processes by modulating the phosphorylation status of proteins. Consistent with its role, mutants lacking Rts1 display multiple phenotypes. We have previously performed a high throughput screen to search for yeast haploid mutants with altered sensitivity to the anticancer drug bleomycin, which acts by damaging the DNA to produce single and double strand breaks. RTS1 was among the genes that when singly deleted cause sensitivity to bleomycin. We investigate whether Rts1 plays a role in the repair of bleomycin-induced DNA lesions. We show that deletion of the RTS1 gene in the rad51 null background, lacking Rad51 known to be involved in the repair of bleomycin-induced DNA lesions, resulted in double mutants that were sensitized to bleomycin and not to other DNA damaging agents that creates DNA adducts. We further show that Rts1 has the ability to bind to DNA and in its absence cells displayed an increase in the frequency of both spontaneous and bleomycin-induced mutations compared to the parent. This is the first report implicating Rts1 with a role in DNA damage and repair, perhaps regulating the phosphorylation status of one or more proteins involved in the repair of DNA strand breaks.

To be or not to be: PP2A as a dual player in CNS functions, its role in neurodegeneration, and its interaction with brain insulin signaling

Accumulating evidence has reached the consensus that the balance of phosphorylation state of signaling molecules is a pivotal point in the regulation of cell signaling. Therefore, characterizing elements (kinases-phosphatases) in the phosphorylation balance are at great importance. However, the role of phosphatase enzymes is less investigated than kinase enzymes. PP2A is a member of serine/threonine protein phosphatase that its imbalance has been reported in neurodegenerative diseases. Therefore, we reviewed the superfamily of phosphatases and more specifically PP2A, its regulation, and physiological functions participate in CNS. Thereafter, we discussed the latest findings about PP2A dysregulation in Alzheimer and Parkinson diseases and possible interplay between this phosphatase and insulin signaling pathways. Finally, activating/inhibitory modulators for PP2A activity as well as experimental methods for PP2A study have been reviewed.

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.

Proteomic Analysis of Hydroxysafflor Yellow A Against Cerebral Ischemia/Reperfusion Injury in Rats

Hydroxysafflor yellow A (HSYA), an active component from Chinese medicinal herb, has been applied to the prevention and treatment of cerebral ischemia/reperfusion injury (CIRI). To clarify the comprehensive mechanisms HSYA for stroke, we used label-free quantitative proteomic analysis to investigate the modulated proteins of rats subjected to CIRI and their alteration by HSYA. Neurological examination, infarct assessment, and biochemical assay were performed to validate the effects of HSYA, and the results indicated that HSYA played a significant role in brain protection. A total of 13 proteins were identified as overlapped proteins by label-free quantitative proteomic analysis. Gene Ontology and pathway analysis showed that these differentially expressed proteins were mainly enriched in the hypoxia-inducible factor 1 (HIF-1) signaling pathway. Furthermore, networks were constructed with respect to protein function interactions. The results suggested that seven proteins were identified as hub proteins between model and sham groups, while 25 proteins were identified as hub proteins between HSYA and model groups. In addition, the expressions of three overlapping proteins were validated by Western blot, and their levels were consistent with the results of label-free analysis. In conclusion, Eftud2, mTOR, Rab11, Ppp2r5e, and HIF-1 signaling pathways have been detected as key hub proteins and pathways in HSYA against CIRI through proteomic analysis. Our research has provided convincing explanations for the mechanism of HSYA against CIRI and the identified key proteins and pathways might provide novel therapeutics for CIRI.

LINC00857 expression predicts and mediates the response to platinum-based chemotherapy in muscle-invasive bladder cancer

Approximately 20% of patients with bladder cancer are diagnosed with muscle‐invasive disease (MIBC). The treatment involves radical cystectomy, but almost 50% of patients with MIBC eventually relapse and develop metastasis. The use of platinum‐based chemotherapy in the neoadjuvant setting or for metastatic patients has been shown to improve the overall survival in a subset of patients. Unfortunately, no biomarkers are available to select patients with MIBC who will benefit from chemotherapy or to monitor the efficacy of the treatment. Recently, long noncoding RNAs (lncRNAs) were shown to regulate a variety of processes involved in the development and progression of cancer, including bladder cancer. Moreover, several lncRNAs have been shown to play a role in chemotherapy resistance. Here, we analyzed lncRNA expression associated with response to platinum‐based chemotherapy in metastatic MIBC using data from the MiTranscriptome lncRNA expression database. Expression of the lncRNA,LINC00857, was found to be upregulated in tumors from patients that did not respond to platinum‐based chemotherapy. Moreover, high expression of LINC00857 is correlated with shorter recurrence‐free and overall survival of patients with MIBC. Knockdown of LINC00857 significantly decreased cell viability of bladder cancer cell lines through the induction of apoptosis. Furthermore, LINC00857 knockdown sensitized UM‐UC‐3 and T24 bladder cancer cells to cisplatin, via the negative regulation of the LMAN1 gene. Our data indicate that LINC00857 plays an important role in the regulation of response to platinum‐based chemotherapy. LINC00857 potentially could serve as a novel prognostic and predictive biomarker and might be a therapeutic target to overcome cisplatin resistance in patients with MIBC.

Transient PP2A inhibition alleviates normal tissue stem cell susceptibility to cell death during radiotherapy

Unintended outcomes of cancer therapy include ionizing radiation (IR)-induced stem cell depletion, diminished regenerative capacity, and accelerated aging. Stem cells exhibit attenuated DNA damage response (DDR) and are hypersensitive to IR, as compared to differentiated non-stem cells. We performed genomic discovery research to compare stem cells to differentiated cells, which revealed Phosphoprotein phosphatase 2A (PP2A) as a potential contributor to susceptibility in stem cells. PP2A dephosphorylates pATM, γH2AX, pAkt etc. and is believed to play dual role in regulating DDR and apoptosis. Although studied widely in cancer cells, the role of PP2A in normal stem cell radiosensitivity is unknown. Here we demonstrate that constitutively high expression and radiation induction of PP2A in stem cells plays a role in promoting susceptibility to irradiation. Transient inhibition of PP2A markedly restores DNA repair, inhibits apoptosis, and enhances survival of stem cells, without affecting differentiated non-stem and cancer cells. PP2Ai-mediated stem cell radioprotection was demonstrated in murine embryonic, adult neural, intestinal, and hematopoietic stem cells.

RNA helicase Mov10 is essential for gastrulation and central nervous system development

BACKGROUND

Mov10 is an RNA helicase that modulates access of Argonaute 2 to microRNA recognition elements in mRNAs. We examined the role of Mov10 in Xenopus laevis development and show a critical role for Mov10 in gastrulation and in the development of the central nervous system (CNS).

RESULTS

Knockdown of maternal Mov10 in Xenopus embryos using a translation blocking morpholino led to defects in gastrulation and the development of notochord and paraxial mesoderm, and a failure to neurulate. RNA sequencing of the Mov10 knockdown embryos showed significant upregulation of many mRNAs when compared with controls at stage 10.5 (including those related to the cytoskeleton, adhesion, and extracellular matrix, which are involved in those morphogenetic processes). Additionally, the degradation of the miR-427 target mRNA, cyclin A1, was delayed in the Mov10 knockdowns. These defects suggest that Mov10's role in miRNA-mediated regulation of the maternal to zygotic transition could lead to pleiotropic effects that cause the gastrulation defects. Additionally, the knockdown of zygotic Mov10 showed that it was necessary for normal head, eye, and brain development in Xenopus consistent with a recent study in the mouse.

CONCLUSIONS

Mov10 is essential for gastrulation and normal CNS development. Developmental Dynamics 247:660-671, 2018. © 2017 Wiley Periodicals, Inc.

PP2A as the Main Node of Therapeutic Strategies and Resistance Reversal in Triple-Negative Breast Cancer

Triple negative breast cancer (TNBC), is defined as a type of tumor lacking the expression of estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2). The ER, PR and HER2 are usually the molecular therapeutic targets for breast cancers, but they are ineffective for TNBC because of their negative expressions, so chemotherapy is currently the main treatment strategy in TNBC. However, drug resistance remains a major impediment to TNBC chemotherapeutic treatment. Recently, the protein phosphatase 2A (PP2A) has been found to regulate the phosphorylation of some substrates involved in the relevant target of TNBC, such as cell cycle control, DNA damage responses, epidermal growth factor receptor, immune modulation and cell death resistance, which may be the effective therapeutic strategies or influence drug sensitivity to TNBCs. Furthermore, PP2A has also been found that could induce ER re-expression in ER-negative breast cancer cells, and which suggests PP2A could promote the sensitivity of tamoxifen to TNBCs as a resistance reversal agent. In this review, we will summarize the potential therapeutic value of PP2A as the main node in developing targeting agents, disrupting resistance or restoring drug sensitivity in TNBC.

Tumor suppressive protein phosphatases in human cancer: Emerging targets for therapeutic intervention and tumor stratification

Aberrant protein phosphorylation is one of the hallmarks of cancer cells, and in many cases a prerequisite to sustain tumor development and progression. Like protein kinases, protein phosphatases are key regulators of cell signaling. However, their contribution to aberrant signaling in cancer cells is overall less well appreciated, and therefore, their clinical potential remains largely unexploited. In this review, we provide an overview of tumor suppressive protein phosphatases in human cancer. Along their mechanisms of inactivation in defined cancer contexts, we give an overview of their functional roles in diverse signaling pathways that contribute to their tumor suppressive abilities. Finally, we discuss their emerging roles as predictive or prognostic markers, their potential as synthetic lethality targets, and the current feasibility of their reactivation with pharmacologic compounds as promising new cancer therapies. We conclude that their inclusion in clinical practice has obvious potential to significantly improve therapeutic outcome in various ways, and should now definitely be pushed forward.

A regulatory subunit of protein phosphatase 2A, PPP2R5E, regulates the abundance of microtubule crosslinking factor 1

Dynamic changes in microtubule organization are regulated by numerous microtubule-associating proteins and their post-translational modification. Microtubule crosslinking factor 1 (MTCL1) is a recently identified protein that regulates microtubule organization. To obtain further insight into its functions, we searched for proteins that associate with it using mass spectrometry analysis. We found that PPP2R5E, a regulatory subunit of protein phosphatase 2A, interacted with MTCL1. Depletion of PPP2R5E reduced the abundance of MTCL1 abundance, whereas exogenous expression of PPP2R5E increased endogenous MTCL1. Furthermore, inhibition of phosphatase activity by okadaic acid reduced MTCL1, which was restored by the addition of the protease inhibitor MG132. Finally, we show that cells depleted of PPP2R5E and MTCL1 exhibited defects in microtubule organization. Our results suggest that the PPP2R5E phosphatase may contribute to microtubule organization by stabilizing MTCL1.

The effect of adenosine A1 receptor agonist and antagonist on p53 and caspase 3, 8, and 9 expression and apoptosis rate in MCF-7 breast cancer cell line

Adenosine receptor family especially A1 type is expressed in breast cancer cells in which P53 and caspase genes are wild-type. The aim of this study was to investigate the correlation between A1 receptor and either cell apoptosis or proliferation and also to recognize the relationship between this receptor and P53 and the expression of caspases 3, 8 and 9 in MCF-7 cell line. MCF-7 cells were treated intermittently with A1 receptor agonist N6-Cyclopentyladenosine (CPA) and A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) in different times to measure the expression of p53, caspase 3, 8 and 9 besides apoptosis and survival rate. Our findings indicated that DPCPX significantly induced apoptosis in MCF-7 cells while the cell viability was reduced specially 72 h after the treatment and the expression of p53 gene and caspase expressions was dramatically up-regulated. On the other hand, CPA increased the cell viability and reduced apoptosis in MCF-7 cells. Our results indicated a significant down-regulation in the MCF-7 mRNA expression of p53 and caspases 3, 8 and 9. Furthermore, DPCPX induced p53 and caspase 3, 8 and 9 expressions that consequently promotes the cell apoptosis in MCF-7 cells. Therefore, DPCPX can be considered as an anti-cancer drug.

Members of the Rusc protein family interact with Sufu and inhibit vertebrate Hedgehog signaling

Hedgehog (Hh) signaling is fundamentally important for development and adult tissue homeostasis. It is well established that in vertebrates Sufu directly binds and inhibits Gli proteins, the downstream mediators of Hh signaling. However, it is unclear how the inhibitory function of Sufu towards Gli is regulated. Here we report that the Rusc family of proteins, the biological functions of which are poorly understood, form a heterotrimeric complex with Sufu and Gli. Upon Hh signaling, Rusc is displaced from this complex, followed by dissociation of Gli from Sufu. In mammalian fibroblast cells, knockdown of Rusc2 potentiates Hh signaling by accelerating signaling-induced dissociation of the Sufu-Gli protein complexes. In Xenopus embryos, knockdown of Rusc1 or overexpression of a dominant-negative Rusc enhances Hh signaling during eye development, leading to severe eye defects. Our study thus uncovers a novel regulatory mechanism controlling the response of cells to Hh signaling in vertebrates.

The broken "Off" switch in cancer signaling: PP2A as a regulator of tumorigenesis, drug resistance, and immune surveillance

Aberrant activation of signal transduction pathways can transform a normal cell to a malignant one and can impart survival properties that render cancer cells resistant to therapy. A diverse set of cascades have been implicated in various cancers including those mediated by serine/threonine kinases such RAS, PI3K/AKT, and PKC. Signal transduction is a dynamic process involving both "On" and "Off" switches. Activating mutations of RAS or PI3K can be viewed as the switch being stuck in the "On" position resulting in continued signaling by a survival and/or proliferation pathway. On the other hand, inactivation of protein phosphatases such as the PP2A family can be seen as the defective "Off" switch that similarly can activate these pathways. A problem for therapeutic targeting of PP2A is that the enzyme is a hetero-trimer and thus drug targeting involves complex structures. More importantly, since PP2A isoforms generally act as tumor suppressors one would want to activate these enzymes rather than suppress them. The elucidation of the role of cellular inhibitors like SET and CIP2A in cancer suggests that targeting these proteins can have therapeutic efficacy by mechanisms involving PP2A activation. Furthermore, drugs such as FTY-720 can activate PP2A isoforms directly. This review will cover the current state of knowledge of PP2A role as a tumor suppressor in cancer cells and as a mediator of processes that can impact drug resistance and immune surveillance.

Deregulation of the protein phosphatase 2A, PP2A in cancer: complexity and therapeutic options

The complexity of the phosphatase, PP2A, is being unravelled and current research is increasingly providing information on the association of deregulated PP2A function with cancer initiation and progression. It has been reported that decreased activity of PP2A is a recurrent observation in many types of cancer, including colorectal and breast cancer (Baldacchino et al. EPMA J. 5:3, 2014; Cristobal et al. Mol Cancer Ther. 13:938-947, 2014). Since deregulation of PP2A and its regulatory subunits is a common event in cancer, PP2A is a potential target for therapy (Baldacchino et al. EPMA J. 5:3, 2014). In this review, the structural components of the PP2A complex are described, giving an in depth overview of the diversity of regulatory subunits. Regulation of the active PP2A trimeric complex, through phosphorylation and methylation, can be targeted using known compounds, to reactivate the complex. The endogenous inhibitors of the PP2A complex are highly deregulated in cancer, representing cases that are eligible to PP2A-activating drugs. Pharmacological opportunities to target low PP2A activity are available and preclinical data support the efficacy of these drugs, but clinical trials are lacking. We highlight the importance of PP2A deregulation in cancer and the current trends in targeting the phosphatase.

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.

Protein Phosphatase 2A Mediates Oxidative Stress Induced Apoptosis in Osteoblasts

Osteoporosis is one of the most common bone diseases, which is characterized by a systemic impairment of bone mass and fragility fractures. Age-related oxidative stress is highly associated with impaired osteoblastic dysfunctions and subsequent osteoporosis. In osteoblasts (bone formation cells), reactive oxygen species (ROS) are continuously generated and further cause lipid peroxidation, protein damage, and DNA lesions, leading to osteoblastic dysfunctions, dysdifferentiations, and apoptosis. Although much progress has been made, the mechanism responsible for oxidative stress induced cellular alternations and osteoblastic toxicity is still not fully elucidated. Here, we demonstrate that protein phosphatase 2A (PP2A), a major protein phosphatase in mammalian cells, mediates oxidative stress induced apoptosis in osteoblasts. Our results showed that lipid peroxidation products (4-HNE) may induce dramatic oxidative stress, inflammatory reactions, and apoptosis in osteoblasts. These oxidative stress responses may ectopically activate PP2A phosphatase activity, which may be mediated by inactivation of AKT/mTOR pathway. Moreover, inhibition of PP2A activity by okadaic acid might partly prevent osteoblastic apoptosis under oxidative conditions. These findings may reveal a novel mechanism to clarify the role of oxidative stress for osteoblastic apoptosis and provide new possibilities for the treatment of related bone diseases, such as osteoporosis.

LB100, a small molecule inhibitor of PP2A with potent chemo- and radio-sensitizing potential

Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase that plays a significant role in mitotic progression and cellular responses to DNA damage. While traditionally viewed as a tumor suppressor, inhibition of PP2A has recently come to attention as a novel therapeutic means of driving senescent cancer cells into mitosis and promoting cell death via mitotic catastrophe. These findings have been corroborated in numerous studies utilizing naturally produced compounds that selectively inhibit PP2A. To overcome the known human toxicities associated with these compounds, a water-soluble small molecule inhibitor, LB100, was recently developed to competitively inhibit the PP2A protein. This review summarizes the pre-clinical studies to date that have demonstrated the anti-cancer activity of LB100 via its chemo- and radio-sensitizing properties. These studies demonstrate the tremendous therapeutic potential of LB100 in a variety of cancer types. The results of an ongoing phase 1 trial are eagerly anticipated.

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.

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.

Protein phosphatase 2A is regulated by protein kinase Cα (PKCα)-dependent phosphorylation of its targeting subunit B56α at Ser41

Protein phosphatase 2A (PP2A) is a family of multifunctional serine/threonine phosphatases consisting of a catalytic C, a structural A, and a regulatory B subunit. The substrate and therefore the functional specificity of PP2A are determined by the assembly of the enzyme complex with the appropriate regulatory B subunit families, namely B55, B56, PR72, or PR93/PR110. It has been suggested that additional levels of regulating PP2A function may result from the phosphorylation of B56 isoforms. In this study, we identified a novel phosphorylation site at Ser(41) of B56α. This phosphoamino acid residue was efficiently phosphorylated in vitro by PKCα. We detected a 7-fold higher phosphorylation of B56α in failing human hearts compared with nonfailing hearts. Purified PP2A dimeric holoenzyme (subunits C and A) was able to dephosphorylate PKCα-phosphorylated B56α. The potency of B56α for PP2A inhibition was markedly increased by PKCα phosphorylation. PP2A activity was also reduced in HEK293 cells transfected with a B56α mutant, where serine 41 was replaced by aspartic acid, which mimics phosphorylation. More evidence for a functional role of PKCα-dependent phosphorylation of B56α was derived from Fluo-4 fluorescence measurements in phenylephrine-stimulated Flp293 cells. The endoplasmic reticulum Ca(2+) release was increased by 23% by expression of the pseudophosphorylated form compared with wild-type B56α. Taken together, our results suggest that PKCα can modify PP2A activity by phosphorylation of B56α at Ser(41). This interplay between PKCα and PP2A represents a new mechanism to regulate important cellular functions like cellular Ca(2+) homeostasis.

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.

Novel Ser/Thr protein phosphatases in cell death regulation

Cell death is regulated by a myriad of intracellular molecular pathways, with many involving protein phosphorylation and dephosphorylation. In this review, we will focus on Ser/Thr phosphatases-mediated regulation in cell apoptosis as well as on their potential roles in cell necrosis. The emerging functional importance of Ser/Thr protein phosphatases in cell death regulation adds new dimension to the signaling mechanisms of cellular function, physiology, and diseases.

Human papillomavirus 18 E6 inhibits phosphorylation of p53 expressed in HeLa cells

Background

In HPV infected cells p53 function is abrogated by E6 and even ectopically expressed p53 is unable to perform tumor suppressor functions. In addition to facilitating its degradation, E6 may also inhibit p53 transactivity, though the mechanisms are still poorly understood. It has been reported that inhibition of p300, an acetyltransferase responsible for p53 acetylation is inactivated by E6. Activation of overexpressed p53 to cause cell growth inhibition is facilitated by its phosphorylation. Previously, we reported that non-genotoxically overexpressed p53 in HeLa cells needs to be phosphorylated to perform its cell growth inhibitory functions. Since over expressed p53 by itself was not activated, we hypothesized an inhibitory role for E6.

Results

Majority of reports proposes E6 mediated degradation of p53 as a possible reason for its inactivation. However, results presented here for the first time demonstrate that overexpressed p53 is not directly associated with E6 and therefore free, yet it is not functionally active in HPV positive cells. Also, the stability of overexpressed p53 does not seem to be an issue because inhibition of proteasomal degradation did not increase the half-life of overexpressed p53, which is more than endogenous p53. However, inhibition of proteasomal degradation prevents the degradation of endogenous p53. These findings suggest that overexpressed p53 and endogenous p53 are differentially subjected to proteasomal degradation and the reasons for this discrepancy remain unclear. Our studies demonstrate that p53 over expression has no effect on anchorage independent cell-growth and E6 nullifies its cell growth inhibitory effect. E6 overexpression abrogates OA induced p53 occupancy on the p21 promoter and cell death as well. E6 did not decrease p53 protein but phospho-p53 level was significantly reduced.

Conclusion

We report for the first time that E6 de-activates p53 by inhibiting its phosphorylation. This prevents p53 binding to p21 promoter and thereby restraining its cell-growth inhibitory functions. Our study provides new evidence indicating that viral protein E6 inhibits p53 transactivity by mechanism independent of degradation pathway.

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.

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.