The protein phosphatase 2A regulatory subunit B55α is a modulator of signaling and microRNA expression in acute myeloid leukemia cells

Protein phosphatase 2A modulation and connection with miRNAs and natural products

Protein phosphatase 2A (PP2A), a heterotrimeric holoenzyme (scaffolding, catalytic, and regulatory subunits), regulates dephosphorylation for more than half of serine/threonine phosphosites and exhibits diverse cellular functions. Although several studies on natural products and miRNAs have emphasized their impacts on PP2A regulation, their connections lack systemic organization. Moreover, only part of the PP2A family has been investigated. This review focuses on the PP2A-modulating effects of natural products and miRNAs' interactions with potential PP2A targets in cancer and non-cancer cells. PP2A-modulating natural products and miRNAs were retrieved through a literature search. Utilizing the miRDB database, potential PP2A targets of these PP2A-modulating miRNAs for the whole set (17 members) of the PP2A family were retrieved. Finally, PP2A-modulating natural products and miRNAs were linked via a literature search. This review provides systemic directions for assessing natural products and miRNAs relating to the PP2A-modulating functions in cancer and disease treatments.

The role of miR-222-2p in exosomes secreted by hexavalent chromium-induced premature senescent hepatocytes as a SASP component

With the development of world industrialization, the environmental pollution of hexavalent chromium [Cr(VI)] is becoming an increasingly serious problem. In particular, the mechanisms by which long-term and low-dose exposure to Cr(VI) leading the development of related cancers are not well understood. As senescent cells gradually lose their ability to proliferate and divide, they will not be malignantly transformed. However, Senescence-associated secretory phenotype (SASP) released by senescent cells into the cellular microenvironment can act on neighboring cells. Since SASP has a bidirectional regulatory role in the malignant transformation of cells. Hence, It is very necessary to identified the composition and function of SASP which secreted by Cr(VI) induced senescent L02 hepatocytes (S-L02). Exosomes, a vesicle-like substances released extracellularly after the fusion of intracellular multivesicular bodies with cell membrane, are important components of SASP and contain a large number of microRNAs (miRNAs). By establishing Cr(VI)-induced S-L02 model, we collected the exosomes from the supernatants of S-L02 and L02 culture medium respectively, and screened out the highly expressed miRNAs in the exosomes of S-L02, namely the new SASP components. Among them, the increase of miR-222-5p was the most significant. It was validated that as SASP, miR-222-5p can inhibit the proliferation of L02 and S-L02 hepatocytes and at the same time accelerate the proliferation and migration ability of HCC cells. Further mechanistic studies revealed that miR-222-5p attenuated the regulatory effect of protein phosphatase 2A subunit B isoform R2-α (PPP2R2A) on Akt via repressing its target gene PPP2R2A, causing reduced expressions of forkhead box O3 (FOXO3a), p27 and p21, and finally increasing the proliferation of HCC cells after diminishing the negative regulation of on cell cycle. This study certainly provides valuable laboratory evidence as well as potential therapeutic targets for the prevention and further personalized treatment of Cr(VI)-associated cancers.

miR-142: A Master Regulator in Hematological Malignancies and Therapeutic Opportunities

MicroRNAs (miRNAs) are a type of non-coding RNA whose dysregulation is frequently associated with the onset and progression of human cancers. miR-142, an ultra-conserved miRNA with both active -3p and -5p mature strands and wide-ranging physiological targets, has been the subject of countless studies over the years. Due to its preferential expression in hematopoietic cells, miR-142 has been found to be associated with numerous types of lymphomas and leukemias. This review elucidates the multifaceted role of miR-142 in human physiology, its influence on hematopoiesis and hematopoietic cells, and its intriguing involvement in exosome-mediated miR-142 transport. Moreover, we offer a comprehensive exploration of the genetic and molecular landscape of the miR-142 genomic locus, highlighting its mutations and dysregulation within hematological malignancies. Finally, we discuss potential avenues for harnessing the therapeutic potential of miR-142 in the context of hematological malignancies.

Transcriptomics analyses reveal the effects of Pentagamaboronon-0-ol on PI3K/Akt and cell cycle of HER2+ breast cancer cells

Introduction

Monoclonal antibodies and targeted therapies against HER2+ breast cancer has improved overall and disease-free survival in patients; however, encountering drug resistance causes recurrence, necessitating the development of newer HER2-targeted medications. A curcumin analog PGB-0-ol showed most cytotoxicity against HCC1954 HER2+ breast cancer cells than against other subtypes of breast cancer cells.

Objective

Here, we employed next-generation sequencing technology to elucidate the molecular mechanism underlying the effect of PGB-0-ol on HCC1954 HER2+ breast cancer cells.

Methods

The molecular mechanism underlying the action of PGB-0-ol on HCC1954 HER2+ breast cancer cells was determined using next-generation sequencing technologies. Additional bioinformatics studies were performed, including gene ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway, disease-gene, and drug-gene associations, network topology analysis (NTA), and gene set enrichment analysis (GSEA).

Results

We detected 2,263 differentially expressed genes (DEGs) (1,459 upregulated and 804 downregulated) in the PGB-0-ol- and DMSO-treated HCC1954 cells. KEGG enrichment data revealed the control of phosphatidylinositol signaling system, and ErbB signaling following PGB-0-ol treatment. Gene ontology (GO) enrichment analysis demonstrated that these DEGs governed cell cycle, participated in the mitotic spindle and nuclear membrane, and controlled kinase activity at the molecular level. According to the NTA data for GO enrichment, GSEA data for KEGG, drug-gene and disease-gene, PGB-0-ol regulated PI3K/Akt signaling and cell cycle in breast cancer. Overall, our investigation revealed the transcriptomic profile of PGB-0-ol-treated HCC1954 breast cancer cells following PGB-0-ol therapy. Bioinformatics analyses showed that PI3K/Akt signaling and cell cycle was modulated. However, further studies are required to validate the findings of this study.

PP2A is a therapeutically targetable driver of cell fate decisions via a c-Myc/p21 axis in human and murine acute myeloid leukemia

Dysregulated cellular differentiation is a hallmark of acute leukemogenesis. Phosphatases are widely suppressed in cancers but have not been traditionally associated with differentiation. In this study, we found that the silencing of protein phosphatase 2A (PP2A) directly blocks differentiation in acute myeloid leukemia (AML). Gene expression and mass cytometric profiling revealed that PP2A activation modulates cell cycle and transcriptional regulators that program terminal myeloid differentiation. Using a novel pharmacological agent, OSU-2S, in parallel with genetic approaches, we discovered that PP2A enforced c-Myc and p21 dependent terminal differentiation, proliferation arrest, and apoptosis in AML. Finally, we demonstrated that PP2A activation decreased leukemia-initiating stem cells, increased leukemic blast maturation, and improved overall survival in murine Tet2-/-Flt3ITD/WT and human cell-line derived xenograft AML models in vivo. Our findings identify the PP2A/c-Myc/p21 axis as a critical regulator of the differentiation/proliferation switch in AML that can be therapeutically targeted in malignancies with dysregulated maturation fate.

PP2A-B55: substrates and regulators in the control of cellular functions

PP2A is a major serine/threonine phosphatase class involved in the regulation of cell signaling through the removal of protein phosphorylation. This class of phosphatases is comprised of different heterotrimeric complexes displaying distinct substrate specificities. The present review will focus on one specific heterocomplex, the phosphatase PP2A-B55. Herein, we will report the direct substrates of this phosphatase identified to date, and its impact on different cell signaling cascades. We will additionally describe its negative regulation by its inhibitors Arpp19 and ENSA and their upstream kinase Greatwall. Finally, we will describe the essential molecular features defining PP2A-B55 substrate specificity that confer the correct temporal pattern of substrate dephosphorylation. The main objective of this review is to provide the reader with a unique source compiling all the knowledge of this particular holoenzyme that has evolved as a key enzyme for cell homeostasis and cancer development.

Hsa-MiR-590-3p Promotes the Malignancy Progression of Pancreatic Ductal Carcinoma by Inhibiting the Expression of p27 and PPP2R2A via G1/S Cell Cycle Pathway

Objective

To investigate the effect of miR-590-3p on the malignant biological behavior of pancreatic cancer, and to explore the target genes and pathways directly affected by miR-590-3p, to provide new therapeutic ideas and targets for the study of the diagnosis and treatment of pancreatic cancer.

Methods

We used qRT-PCR to measure miR-590-3p expression quantities. We used cell cycle, CCK-8, clonal formation to verify the change of proliferation capacity of PC cells. We used transwell assay to detect the migration and invasion of PC cells. We used the bioinformatics tool TargetScan (http://www.targetscan.org) to identify the possible target genes of miR-590-3p. Immunohistochemistry revealed the clinicopathological significance of PPP2R2A, p27 and miR-590-3p in the expression of pancreatic cancer. Western blot was used to detect the expression changes of PPP2R2A, p27 and G1/S cell cycle pathway-related proteins CDK2, cyclinE2 and p21 after transfection of mimics and inhibitors of miR-590-3p.

Results

According to our study, hsa-miR-590-3p expression was significantly higher in PC tissues than that in paired normal pancreas, which was associated with PC tumor size (P=0.042) and preoperative CA19-9 level (P=0.046) of PC patients. Its overexpression promoted PC cell proliferation, invasion and migration following with the p27 and PPP2R2A protein downregulation in Capan-2, PANC-1 and BxPC-3 cells, and vice versa. Bioinformatics analysis and dual-luciferase reporter assay further confirmed that p27 and PPP2R2A were direct target genes of miR-590-3p. The negative relationship of miR-590-3p with p27 and PPP2R2A was also observed in PC tissues.

Conclusion

MiR-590-3p promotes the proliferation, migration and invasion of pancreatic cancer cells. MiR-590-3p directly downregulated p27 and PPP2R2A and via the G1/S cell cycle pathway to promote the development of pancreatic cancer.

Reduced methylation of PP2Ac promotes ethanol-induced lipid accumulation through FOXO1 phosphorylation in vitro and in vivo

Although disturbance of the methionine cycle and sequent decrease in hepatic methylation capacity are known to be important factors in the development of alcoholic liver injury, the underlying mechanisms are not fully understood. Here, we investigated the importance of the methylation of protein phosphatase 2A (PP2A) in alcoholic liver disease (ALD). We found that the severity of ethanol-induced liver injury and the extent of demethylation of PP2A catalytic C subunit (PP2Ac) were reduced after treatment with betaine, a methyl donor involved in the methionine-homocysteine cycle. These results suggest that PP2Ac methylation is decreased due to a broad decrease in hepatic methylation capacity after exposure to ethanol. Moreover, we found that the reduction in PP2Ac methylation led to increased degradation of the regulatory Bα subunit, thus promoting the phosphorylation and nuclear exclusion of Forkhead box O1 (FOXO1) and reducing FOXO1 transcriptional activity. Ultimately, the reduced activity of FOXO1 led to increased expression of TXNIP, which caused hepatic lipid accumulation. Our findings suggest that the reduction of PP2A methylation, a result of decrease hepatic methylation capacity, played an important role in ethanol-induced lipid accumulation via down-regulation of PP2A/Bα and FOXO1 phosphorylation.

Repression of sphingosine kinase (SK)-interacting protein (SKIP) in acute myeloid leukemia diminishes SK activity and its re-expression restores SK function

Previous studies have shown that sphingosine kinase interacting protein (SKIP) inhibits sphingosine kinase (SK) function in fibroblasts. SK phosphorylates sphingosine producing the potent signaling molecule sphingosine-1-phosphate (S1P). SKIP gene (SPHKAP) expression is silenced by hypermethylation of its promoter in acute myeloid leukemia (AML). However, why SKIP activity is silenced in primary AML cells is unclear. Here, we investigated the consequences of SKIP down-regulation in AML primary cells and the effects of SKIP re-expression in leukemic cell lines. Using targeted ultra-HPLC-tandem MS (UPLC-MS/MS), we measured sphingolipids (including S1P and ceramides) in AML and control cells. Primary AML cells had significantly lower SK activity and intracellular S1P concentrations than control cells, and SKIP-transfected leukemia cell lines exhibited increased SK activity. These findings show that SKIP re-expression enhances SK activity in leukemia cells. Furthermore, other bioactive sphingolipids such as ceramide were also down-regulated in primary AML cells. Of note, SKIP re-expression in leukemia cells increased ceramide levels 2-fold, inactivated the key signaling protein extracellular signal-regulated kinase, and increased apoptosis following serum deprivation or chemotherapy. These results indicate that SKIP down-regulation in AML reduces SK activity and ceramide levels, an effect that ultimately inhibits apoptosis in leukemia cells. The findings of our study contrast with previous results indicating that SKIP inhibits SK function in fibroblasts and therefore challenge the notion that SKIP always inhibits SK activity.

The Role of MYC and PP2A in the Initiation and Progression of Myeloid Leukemias

The MYC transcription factor is one of the best characterized PP2A substrates. Deregulation of the MYC oncogene, along with inactivation of PP2A, are two frequent events in cancer. Both proteins are essential regulators of cell proliferation, apoptosis, and differentiation, and they, directly and indirectly, regulate each other’s activity. Studies in cancer suggest that targeting the MYC/PP2A network is an achievable strategy for the clinic. Here, we focus on and discuss the role of MYC and PP2A in myeloid leukemias.

The non-coding RNome after splenectomy

Splenectomy is a common surgical procedure performed in millions of people worldwide. Epidemiologic data show that splenectomy is followed by infectious (sepsis) and non-infectious complications, with unknown mechanisms. In order to explore the role of the non-coding transcripts involved in these complications, we analysed a panel of circulating microRNAs (miRNAs), which were previously reported to be deregulated in sepsis, in the plasma of splenectomized patients. MiR-223 was overexpressed immediately and late after splenectomy, while miR-146a was overexpressed immediately after splenectomy, returning latter to basal levels; and miR-16, miR-93, miR-26a and miR-26b were overexpressed only late after splenectomy, suggesting similarities with sepsis. We also explored the non-coding (nc)RNome of circulating peripheral blood leucocytes by performing a ncRNA full genome profiling. We observed a reorganization of the ncRNoma after splenectomy, characterized by up-regulation of miRNAs and down-regulation of transcribed pyknons (T-PYKs). Pathway analysis revealed that deregulated miRNAs control pathways involved in immunity, cancer and endothelial growth. We checked the expression of the ncRNAs in 15 immune cell types from healthy donors and observed that plasma miRNAs, cellular miRNAs and T-PYKs have a cell-specific expression pattern and are abundant in different types of immune cells. These findings suggest that the ncRNAs potentially regulate the immune changes observed after splenectomy.

Regulation of the phosphoprotein phosphatase 2A system and its modulation during oxidative stress: A potential therapeutic target?

Phosphoprotein phosphatases are of growing interest in the pathophysiology of many diseases and are often the neglected partner of protein kinases. One family member, PP2A, accounts for dephosphorylation of ~55-70% of all serine/threonine phosphosites. Interestingly, dysregulation of kinase signalling is a hallmark of many diseases in which an increase in oxidative stress is also noted. With this in mind, we assess the evidence to support oxidative stress-mediated regulation of the PP2A system In this article, we first present an overview of the PP2A system before providing an analysis of the regulation of PP2A by endogenous inhibitors, post translational modification, and miRNA. Next, a detailed critique of data implicating reactive oxygen species, ischaemia, ischaemia-reperfusion, and hypoxia in regulating the PP2A holoenzyme and associated regulators is presented. Finally, the pharmacological targeting of PP2A, its endogenous inhibitors, and enzymes responsible for its post-translational modification are covered. There is extensive evidence that oxidative stress modulates multiple components of the PP2A system, however, most of the data pertains to the catalytic subunit of PP2A. Irrespective of the underlying aetiology, free radical-mediated attenuation of PP2A activity is an emerging theme. However, in many instances, a dichotomy exists, which requires clarification and mechanistic insight. Nevertheless, this raises the possibility that pharmacological activation of PP2A, either through small molecule activators of PP2A or CIP2A/SET antagonists may be beneficial in modulating the cellular response to oxidative stress. A better understanding of which, will have wide ranging implications for cancer, heart disease and inflammatory conditions.

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.

The impact of phosphatases on proliferative and survival signaling in cancer

The dynamic and stringent coordination of kinase and phosphatase activity controls a myriad of physiologic processes. Aberrations that disrupt the balance of this interplay represent the basis of numerous diseases. For a variety of reasons, early work in this area portrayed kinases as the dominant actors in these signaling events with phosphatases playing a secondary role. In oncology, these efforts led to breakthroughs that have dramatically altered the course of certain diseases and directed vast resources toward the development of additional kinase-targeted therapies. Yet, more recent scientific efforts have demonstrated a prominent and sometimes driving role for phosphatases across numerous malignancies. This maturation of the phosphatase field has brought with it the promise of further therapeutic advances in the field of oncology. In this review, we discuss the role of phosphatases in the regulation of cellular proliferation and survival signaling using the examples of the MAPK and PI3K/AKT pathways, c-Myc and the apoptosis machinery. Emphasis is placed on instances where these signaling networks are perturbed by dysregulation of specific phosphatases to favor growth and persistence of human cancer.

Novel B55α-PP2A mutations in AML promote AKT T308 phosphorylation and sensitivity to AKT inhibitor-induced growth arrest

Activation of the Protein Kinase B (PKB), or AKT pathway has been shown to correlate with acute myeloid leukemia (AML) prognosis. B55α-Protein Phosphatase 2A (PP2A) has been shown to dephosphorylate AKT at Thr-308 rendering it inactive. In fact, low expression of the PP2A regulatory subunit B55α was associated with activated phospho-AKT and correlated with inferior outcomes in AML. Despite this fact, no studies have specifically demonstrated a mechanism whereby B55α expression is regulated in AML. In this study, we demonstrate novel loss of function mutations in the PPP2R2A gene identified in leukemic blasts from three AML patients. These mutations eliminate B55α protein expression thereby allowing constitutive AKT activation. In addition, leukemic blasts with PPP2R2A gene mutation were more sensitive to treatment with the AKT inhibitor MK2206, but less responsive to the PP2A activator FTY720. Using leukemia cell lines, we further demonstrate that B55α expression correlates with AKT Thr-308 phosphorylation and predicts responsiveness to AKT inhibition and PP2A activation. Together our data illustrate the importance of the B55α-PP2A-AKT pathway in leukemogenesis. Screening for disruptions in this pathway at initial AML diagnosis may predict response to targeted therapies against AKT and PP2A.

miRNAs in acute myeloid leukemia

MicroRNAs (miRNAs) are small, non-coding RNAs found throughout the eukaryotes that control the expression of a number of genes involved in commitment and differentiation of hematopoietic stem cells and tumorigenesis. Widespread dysregulation of miRNAs have been found in hematological malignancies, including human acute myeloid leukemia (AML). A comprehensive understanding of the role of miRNAs within the complex regulatory networks that are disrupted in malignant AML cells is a prerequisite for the development of therapeutic strategies employing miRNA modulators. Herein, we review the roles of emerging miRNAs and the miRNAs regulatory networks in AML pathogenesis, prognosis, and miRNA-directed therapies.

Activation of protein phosphatase 2A in FLT3+ acute myeloid leukemia cells enhances the cytotoxicity of FLT3 tyrosine kinase inhibitors

Constitutive activation of the receptor tyrosine kinase Fms-like tyrosine kinase 3 (FLT3), via co-expression of its ligand or by genetic mutation, is common in acute myeloid leukemia (AML). In this study we show that FLT3 activation inhibits the activity of the tumor suppressor, protein phosphatase 2A (PP2A). Using BaF3 cells transduced with wildtype or mutant FLT3, we show that FLT3-induced PP2A inhibition sensitizes cells to the pharmacological PP2A activators, FTY720 and AAL(S). FTY720 and AAL(S) induced cell death and inhibited colony formation of FLT3 activated cells. Furthermore, PP2A activators reduced the phosphorylation of ERK and AKT, downstream targets shared by both FLT3 and PP2A, in FLT3/ITD⁺ BaF3 and MV4-11 cell lines. PP2A activity was lower in primary human bone marrow derived AML blasts compared to normal bone marrow, with blasts from FLT3-ITD patients displaying lower PP2A activity than WT-FLT3 blasts. Reduced PP2A activity was associated with hyperphosphorylation of the PP2A catalytic subunit, and reduced expression of PP2A structural and regulatory subunits. AML patient blasts were also sensitive to cell death induced by FTY720 and AAL(S), but these compounds had minimal effect on normal CD34+ bone marrow derived monocytes. Finally, PP2A activating compounds displayed synergistic effects when used in combination with tyrosine kinase inhibitors in FLT3-ITD⁺ cells. A combination of Sorafenib and FTY720 was also synergistic in the presence of a protective stromal microenvironment. Thus combining a PP2A activating compound and a FLT3 inhibitor may be a novel therapeutic approach for treating AML.

MicroRNA-191-5p exerts a tumor suppressive role in renal cell carcinoma

Renal cell carcinoma (RCC) is a common tumor of the urinary system. Previously, miR-191-5p has been reported to be associated with various types of cancer; however, its specific functions in RCC have not been investigated to date. In the present study, the expression of miR-191-5p in the 786-O and ACHN cell lines was detected in vitro by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The results of RT-qPCR revealed that miR-191-5p was significantly downregulated in the two cell lines compared with the 293T cell line. miR-191-5p was also significantly downregulated in RCC tissue compared with paired normal tissue. In addition, the effects of miR-191-5p on cell proliferation, migration, invasion and apoptosis were examined by CCK-8, MTT, wound scratch, Transwell and flow cytometry assays. Downregulation of miR-191-5p was observed to promote cell proliferation, migration and invasion, as well as to repress the cell apoptosis of 786-O and ACHN cells. Therefore, the current study suggests that miR-191-5p functions as a tumor suppressor in RCC. Further studies are required to uncover the underlying signaling pathway of miR-191-5p and its potential role as a biomarker for early detection and prognosis prediction, and as a therapeutic target of RCC.

The emerging role of FTY720 (Fingolimod) in cancer treatment

FTY720 (Fingolimod) is a clinically approved immunomodulating therapy for multiple sclerosis that sequesters T-cells to lymph nodes through functional antagonism of sphingosine-1-phosphate 1 receptor. FTY720 also demonstrates a proven efficacy in multiple in vitro and in vivo cancer models, suggesting a potential therapeutic role in cancer patients. A potential anticancer mechanism of FTY720 is through the inhibition of sphingosine kinase 1, a proto-oncogene with in vitro and clinical cancer association. In addition, FTY720's anticancer properties may be attributable to actions on several other molecular targets. This study focuses on reviewing the emerging evidence regarding the anticancer properties and molecular targets of FTY720. While the clinical transition of FTY720 is currently limited by its immune suppression effects, studies aiming at FTY720 delivery and release together with identifying its key synergetic combinations and relevant patient subsets may lead to its rapid introduction into the clinic.

Therapeutic targeting of PP2A

Protein phosphatase 2A (PP2A) is a major serine/threonine phosphatase that regulates many cellular processes. Given the central role of PP2A in regulating diverse biological functions and its dysregulation in many diseases, including cancer, PP2A directed therapeutics have become of great interest. The main approaches leveraged thus far can be categorized as follows: 1) inhibiting endogenous inhibitors of PP2A, 2) targeted disruption of post translational modifications on PP2A subunits, or 3) direct targeting of PP2A. Additional insight into the structural, molecular, and biological framework driving the efficacy of these therapeutic strategies will provide a foundation for the refinement and development of novel and clinically tractable PP2A targeted therapies.

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.

The serine/threonine protein phosphatase 2A controls autoimmunity

Protein phosphatase 2A (PP2A) is the first serine/threonine phosphatase recognized to contribute to human and murine lupus immunopathology. PP2A expression in SLE is controlled both epigenetically and genetically, and it is increased in patients with SLE, which contributes to decreased IL-2 production, decreased CD3ζ and increased FcRγ expression on the surface of T cells, increased CREMα expression, hypomethylation of genes associated with SLE pathogenesis, and increased IL-17 production. β regulatory subunit of PP2A regulates IL-2 deprivation-induced T cell death and is decreased in SLE patients. A mouse overexpressing PP2A_c in T cells displays peripheral granulocytosis, elevated IL-17 production, and develops glomerulonephritis when challenged. A mouse which lacks PP2A_c only in regulatory T cells develops severe autoimmunity and multiorgan inflammation because of loss of restraint on mTORC1 and inability of Foxp3+ cells to regulate conventional T cells. Targeting PP2A in T cell subsets may be therapeutic for SLE and other autoimmune diseases.

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.

Haploinsufficiency of the Insulin Receptor in the Presence of a Splice-Site Mutation in Ppp2r2a Results in a Novel Digenic Mouse Model of Type 2 Diabetes

Insulin resistance in mice typically does not manifest as diabetes due to multiple compensatory mechanisms. Here, we present a novel digenic model of type 2 diabetes in mice heterozygous for a null allele of the insulin receptor and an N-ethyl-N-nitrosourea-induced alternative splice mutation in the regulatory protein phosphatase 2A (PP2A) subunit PPP2R2A. Inheritance of either allele independently results in insulin resistance but not overt diabetes. Doubly heterozygous mice exhibit progressive hyperglycemia, hyperinsulinemia, and impaired glucose tolerance from 12 weeks of age without significant increase in body weight. Alternative splicing of Ppp2r2a decreased PPP2R2A protein levels. This reduction in PPP2R2A containing PP2A phosphatase holoenzyme was associated with decreased serine/threonine protein kinase AKT protein levels. Ultimately, reduced insulin-stimulated phosphorylated AKT levels were observed, a result that was confirmed in Hepa1-6, C2C12, and differentiated 3T3-L1 cells knocked down using Ppp2r2a small interfering RNAs. Altered AKT signaling and expression of gluconeogenic genes in the fed state contributed to an insulin resistance and hyperglycemia phenotype. This model demonstrates how genetic changes with individually small phenotypic effects interact to cause diabetes and how differences in expression of hypomorphic alleles of PPP2R2A and potentially other regulatory proteins have deleterious effects and may therefore be relevant in determining diabetes risk.

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.

Upregulation of miR-556-5p promoted prostate cancer cell proliferation by suppressing PPP2R2A expression

The prognosis and survival rate of prostate cancer are very poor. Previous studies have shown that miR-556-5p have emerged as important regulators in cancer cell biological processes. The role of miR-556-5p in prostate cancer remains unclear. In this study, expression of miR-556-5p in prostate cancer cell lines and tissues was upregulated. Result of MTT assays, colony formation and anchorage-independent growth assays demonstrated that overexpression of miR-556-5p promoted prostate cancer cell growth. Additionally, PPP2R2A was identified as a direct target of miR-556-5p. Ectopic expression of miR-556-5p led to downregulation of PPP2R2A protein, which resulted in the downregulation of p27, upregulation of cyclin D1. Taken together, our data provide compelling evidence that miR-556-5p functions as an onco-miRNA and participates in prostate cancer carcinogenesis by suppressing PPP2R2A expression.

The Interplay between PP2A and microRNAs in Leukemia

Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase family whose members have been implicated in tumor suppression in many cancer models. In many cancers, loss of PP2A activity has been associated with tumorigenesis and drug resistance. Loss of PP2A results in failure to turn off survival signaling cascades that drive drug resistance such as those regulated by protein kinase B. PP2A is responsible for modulating function and controlling expression of tumor suppressors such as p53 and oncogenes such as BCL2 and MYC. Thus, PP2A has diverse functions regulating cell survival. The importance of microRNAs (miRs) is emerging in cancer biology. A role for miR regulation of PP2A is not well understood; however, recent studies suggest a number of clinically significant miRs such as miR-155 and miR-19 may include PP2A targets. We have recently found that a PP2A B subunit (B55α) can regulate a number of miRs in acute myeloid leukemia cells. The identification of a miR/PP2A axis represents a novel regulatory pathway in cellular homeostasis. The ability of miRs to suppress specific PP2A targets and for PP2A to control such miRs can add an extra level of control in signaling that could be used as a rheostat for many signaling cascades that maintain cellular homeostasis. As such, loss of PP2A or expression of miRs relevant for PP2A function could promote tumorigenesis or at least result in drug resistance. In this review, we will cover the current state of miR regulation of PP2A with a focus on leukemia. We will also briefly discuss what is known of PP2A regulation of miR expression.

The Basic Biology of PP2A in Hematologic Cells and Malignancies

Reversible protein phosphorylation plays a crucial role in regulating cell signaling. In normal cells, phosphoregulation is tightly controlled by a network of protein kinases counterbalanced by several protein phosphatases. Deregulation of this delicate balance is widely recognized as a central mechanism by which cells escape external and internal self-limiting signals, eventually resulting in malignant transformation. A large fraction of hematologic malignancies is characterized by constitutive or unrestrained activation of oncogenic kinases. This is in part achieved by activating mutations, chromosomal rearrangements, or constitutive activation of upstream kinase regulators, in part by inactivation of their anti-oncogenic phosphatase counterparts. Protein phosphatase 2A (PP2A) represents a large family of cellular serine/threonine phosphatases with suspected tumor suppressive functions. In this review, we highlight our current knowledge about the complex structure and biology of these phosphatases in hematologic cells, thereby providing the rationale behind their diverse signaling functions. Eventually, this basic knowledge is a key to truly understand the tumor suppressive role of PP2A in leukemogenesis and to allow further rational development of therapeutic strategies targeting PP2A.