Targeting sphingosine 1-phosphate and sphingosine kinases in pancreatic cancer: mechanisms and therapeutic potential

Pancreatic cancer is known to be the most lethal cancer. Fewer new treatments are being developed for pancreatic cancer as compared to other cancers. The bioactive lipid S1P, which is mainly regulated by sphingosine kinase 1 (SK1) and sphingosine kinase 2 (SK2) enzymes, plays significant roles in pancreatic cancer initiation and exacerbation. S1P controls many signaling pathways to modulate the progression of pancreatic cancer through the G-coupled receptor S1PR1-5. Several papers reporting amelioration of pancreatic cancer via modulation of S1P levels or downstream signaling pathways have previously been published. In this paper, for the first time, we have reviewed the results of previous studies to understand how S1P and its receptors contribute to the development of pancreatic cancer, and whether S1P can be a therapeutic target. In addition, we have also reviewed papers dealing with the effects of SK1 and SK2, which are kinases that regulate the level of S1P, on the pathogenesis of pancreatic cancer. We have also listed available drugs that particularly focus on S1P, S1PRs, SK1, and SK2 for the treatment of pancreatic cancer. Through this review, we would like to suggest that the SK/S1P/S1PR signaling system can be an important target for treating pancreatic cancer, where a new treatment target is desperately warranted.

Network-based analysis predicts interacting genetic modifiers from a meta-mapping study of spike-wave discharge in mice

Absence seizures are characterized by brief lapses in awareness accompanied by a hallmark spike-and-wave discharge (SWD) electroencephalographic pattern and are common to genetic generalized epilepsies (GGEs). While numerous genes have been associated with increased risk, including some Mendelian forms with a single causal allele, most cases of GGE are idiopathic and there are many unknown genetic modifiers of GGE influencing risk and severity. In a previous meta-mapping study, crosses between transgenic C57BL/6 and C3HeB/FeJ strains, each carrying one of three SWD-causing mutations (Gabrg2tm1Spet(R43Q) , Scn8a8j or Gria4spkw1 ), demonstrated an antagonistic epistatic interaction between loci on mouse chromosomes 2 and 7 influencing SWD. These results implicate universal modifiers in the B6 background that mitigate SWD severity through a common pathway, independent of the causal mutation. In this study, we prioritized candidate modifiers in these interacting loci. Our approach integrated human genome-wide association results with gene interaction networks and mouse brain gene expression to prioritize candidate genes and pathways driving variation in SWD outcomes. We considered candidate genes that are functionally associated with human GGE risk genes and genes with evidence for coding or non-coding allele effects between the B6 and C3H backgrounds. Our analyses output a summary ranking of gene pairs, one gene from each locus, as candidates for explaining the epistatic interaction. Our top-ranking gene pairs implicate microtubule function, cytoskeletal stability and cell cycle regulation as novel hypotheses about the source of SWD variation across strain backgrounds, which could clarify underlying mechanisms driving differences in GGE severity in humans.

PP2A-based triple-strike therapy overcomes mitochondrial apoptosis resistance in brain cancer cells

Mitochondrial glycolysis and hyperactivity of the phosphatidylinositol 3-kinase-protein kinase B (AKT) pathway are hallmarks of malignant brain tumors. However, kinase inhibitors targeting AKT (AKTi) or the glycolysis master regulator pyruvate dehydrogenase kinase (PDKi) have failed to provide clinical benefits for brain tumor patients. Here, we demonstrate that heterogeneous glioblastoma (GB) and medulloblastoma (MB) cell lines display only cytostatic responses to combined AKT and PDK targeting. Biochemically, the combined AKT and PDK inhibition resulted in the shutdown of both target pathways and priming to mitochondrial apoptosis but failed to induce apoptosis. In contrast, all tested brain tumor cell models were sensitive to a triplet therapy, in which AKT and PDK inhibition was combined with the pharmacological reactivation of protein phosphatase 2A (PP2A) by NZ-8-061 (also known as DT-061), DBK-1154, and DBK-1160. We also provide proof-of-principle evidence for in vivo efficacy in the intracranial GB and MB models by the brain-penetrant triplet therapy (AKTi + PDKi + PP2A reactivator). Mechanistically, PP2A reactivation converted the cytostatic AKTi + PDKi response to cytotoxic apoptosis, through PP2A-elicited shutdown of compensatory mitochondrial oxidative phosphorylation and by increased proton leakage. These results encourage the development of triple-strike strategies targeting mitochondrial metabolism to overcome therapy tolerance in brain tumors.

PME-1 sensitizes glioblastoma cells to oxidative stress-induced cell death by attenuating PP2A-B55α-mediated inactivation of MAPKAPK2-RIPK1 signaling

Glioblastoma (GBM) is the most common primary brain tumor in adults. Current standard therapy is surgery followed by radiotherapy, with concurrent and adjuvant temozolomide chemotherapy. GBM is characterized by almost uniformly fatal outcomes, highlighting the unmet clinical need for more efficient, biomarker-guided treatments. Protein phosphatase methylesterase-1 (PME-1), a regulator of the tumor suppressive phosphatase PP2A, promotes PP2A demethylation and inactivation, and is overexpressed in 44% of GBM, associated with increased tumor grade and cellular proliferation. Here, we aimed to investigate how reactive oxygen species (ROS), a frequent by-product of radiotherapy and temozolomide chemotherapy, regulate PP2A function via its methylesterase PME-1, and how PME-1 overexpression impacts the response of GBM cells to oxidative stress. We found that in two glioblastoma cell lines, U87MG and U251MG, expression of PME-1 is positively correlated with the sensitivity of the cells to H₂O₂ or t-BHP-induced oxidative stress. Experiments using the irreversible pharmacologic PME-1 inhibitor, AMZ30, and different PME-1 mutants, revealed that the methylesterase function, the PP2A binding capacity, and the nuclear localization of PME-1 are all important for the sensitizing effect of PME-1 expression. Furthermore, we identified increased nuclear localization of the PP2A-B55α subunit, increased binding of PP2A-B55α to PME-1, and increased B55α-bound PP2A-C demethylation upon oxidative stress. Lastly, we uncovered increased stress-induced phosphorylation and activity of MAPKAPK2 and RIPK1 in PME-1 overexpressing U87MG cells, which caused the observed sensitization to t-BHP treatment. Our data reveal a novel role for PME-1 in oxidative stress-induced GBM cell death, regulating nuclear PP2A-B55α activity and MAPKAPK2-RIPK1 signaling. Patients with GBM tumors overexpressing PME-1, although having a worse prognosis due to increased cellular proliferation of the tumor, could actually be more responsive to oxidative stress-inducing therapies.

PTEN deficiency facilitates gemcitabine efficacy in cancer by modulating the phosphorylation of PP2Ac and DCK

Gemcitabine is a nucleoside analog that has been successfully used in the treatment of multiple cancers. However, intrinsic or acquired resistance reduces the chemotherapeutic potential of gemcitabine. Here, we revealed a previously unappreciated mechanism by which phosphatase and tensin homolog (PTEN), one of the most frequently mutated genes in human cancers, dominates the decision-making process that is central to the regulation of gemcitabine efficacy in cholangiocarcinoma (CCA). By investigating a gemcitabine-treated CCA cohort, we found that PTEN deficiency was correlated with the improved efficacy of gemcitabine-based chemotherapy. Using cell-based drug sensitivity assays, cell line-derived xenograft, and patient-derived xenograft models, we further confirmed that PTEN deficiency or genetic-engineering down-regulation of PTEN facilitated gemcitabine efficacy both in vitro and in vivo. Mechanistically, PTEN directly binds to and dephosphorylates the C terminus of the catalytic subunit of protein phosphatase 2A (PP2Ac) to increase its enzymatic activity, which further dephosphorylates deoxycytidine kinase (DCK) at Ser⁷⁴ to diminish gemcitabine efficacy. Therefore, PTEN deficiency and high phosphorylation of DCK predict a better response to gemcitabine-based chemotherapy in CCA. We speculate that the combination of PP2A inhibitor and gemcitabine in PTEN-positive tumors could avoid the resistance of gemcitabine, which would benefit a large population of patients with cancer receiving gemcitabine or other nucleoside analogs.

PTPA variants and impaired PP2A activity in early-onset parkinsonism with intellectual disability

The protein phosphatase 2A complex (PP2A), the major Ser/Thr phosphatase in the brain, is involved in a number of signalling pathways and functions, including the regulation of crucial proteins for neurodegeneration, such as alpha-synuclein, tau and LRRK2. Here, we report the identification of variants in the PTPA/PPP2R4 gene, encoding a major PP2A activator, in two families with early-onset parkinsonism and intellectual disability. We carried out clinical studies and genetic analyses, including genome-wide linkage analysis, whole-exome sequencing, and Sanger sequencing of candidate variants. We next performed functional studies on the disease-associated variants in cultured cells and knock-down of ptpa in Drosophila melanogaster. We first identified a homozygous PTPA variant, c.893T>G (p.Met298Arg), in patients from a South African family with early-onset parkinsonism and intellectual disability. Screening of a large series of additional families yielded a second homozygous variant, c.512C>A (p.Ala171Asp), in a Libyan family with a similar phenotype. Both variants co-segregate with disease in the respective families. The affected subjects display juvenile-onset parkinsonism and intellectual disability. The motor symptoms were responsive to treatment with levodopa and deep brain stimulation of the subthalamic nucleus. In overexpression studies, both the PTPA p.Ala171Asp and p.Met298Arg variants were associated with decreased PTPA RNA stability and decreased PTPA protein levels; the p.Ala171Asp variant additionally displayed decreased PTPA protein stability. Crucially, expression of both variants was associated with decreased PP2A complex levels and impaired PP2A phosphatase activation. PTPA orthologue knock-down in Drosophila neurons induced a significant impairment of locomotion in the climbing test. This defect was age-dependent and fully reversed by L-DOPA treatment. We conclude that bi-allelic missense PTPA variants associated with impaired activation of the PP2A phosphatase cause autosomal recessive early-onset parkinsonism with intellectual disability. Our findings might also provide new insights for understanding the role of the PP2A complex in the pathogenesis of more common forms of neurodegeneration.

Inhibiting PP2A Upregulates B7-H3 Expression and Potentially Increases the Sensitivity of Malignant Meningiomas to Immunotherapy by Proteomics

Malignant meningiomas have a high mortality rate and short survival time and currently have no effective treatment. In our study, proteomics analysis was performed to identify highly expressed proteins as therapeutic targets in malignant meningiomas. Cell Counting Kit-8 (CCK-8) assays were performed to verify the effect of LB-100 on the growth of malignant meningiomas. In addition, immunoblotting was used to verify the expression of B7-H3 and phosphorylation of STAT1 (Tyr701) in tissues and cells. Our results show that STAT1 and CD276 (B7-H3) regulated by PP2A were enriched in GO_IMMUNE_EFFECTOR_PROCESS and GO_REGULATION_OF_IMMUNE_SYSTEM_PROCESS. The immunotherapy target protein B7-H3 was confirmed to be upregulated in malignant meningiomas compared with meningothelial (p = 0.0001) and fibroblastic (p = 0.0046) meningiomas. In vitro, the PP2A inhibitor LB-100 suppressed the growth and invasion of malignant meningioma cells. Notably, the PP2A inhibitor LB-100 increased the phosphorylation of STAT1, thereby increasing the expression of the immune checkpoint protein B7-H3 in malignant meningioma cells in vitro. In conclusion, B7-H3 was found to be upregulated in malignant meningiomas. The PP2A inhibitor LB-100 increased the phosphorylation of STAT1 and B7-H3 expression, which could increase the sensitivity of malignant meningiomas to B7-H3 targeted immunotherapy.

Loss of 4E-BP converts cerebellar long-term depression to long-term potentiation

Genetic perturbances in translational regulation result in defects in cerebellar motor learning; however, little is known about the role of translational mechanisms in the regulation of cerebellar plasticity. We show that genetic removal of 4E-BP, a translational suppressor and target of mammalian target of rapamycin complex 1, results in a striking change in cerebellar synaptic plasticity. We find that cerebellar long-term depression (LTD) at parallel fiber-Purkinje cell synapses is converted to long-term potentiation in 4E-BP knockout mice. Biochemical and pharmacological experiments suggest that increased phosphatase activity largely accounts for the defects in LTD. Our results point to a model in which translational regulation through the action of 4E-BP plays a critical role in establishing the appropriate kinase/phosphatase balance required for normal synaptic plasticity in the cerebellum.

Targeting protein phosphatases for the treatment of inflammation-related diseases: From signaling to therapy

Inflammation is the common pathological basis of autoimmune diseases, metabolic diseases, malignant tumors, and other major chronic diseases. Inflammation plays an important role in tissue homeostasis. On one hand, inflammation can sense changes in the tissue environment, induce imbalance of tissue homeostasis, and cause tissue damage. On the other hand, inflammation can also initiate tissue damage repair and maintain normal tissue function by resolving injury and restoring homeostasis. These opposing functions emphasize the significance of accurate regulation of inflammatory homeostasis to ameliorate inflammation-related diseases. Potential mechanisms involve protein phosphorylation modifications by kinases and phosphatases, which have a crucial role in inflammatory homeostasis. The mechanisms by which many kinases resolve inflammation have been well reviewed, whereas a systematic summary of the functions of protein phosphatases in regulating inflammatory homeostasis is lacking. The molecular knowledge of protein phosphatases, and especially the unique biochemical traits of each family member, will be of critical importance for developing drugs that target phosphatases. Here, we provide a comprehensive summary of the structure, the "double-edged sword" function, and the extensive signaling pathways of all protein phosphatases in inflammation-related diseases, as well as their potential inhibitors or activators that can be used in therapeutic interventions in preclinical or clinical trials. We provide an integrated perspective on the current understanding of all the protein phosphatases associated with inflammation-related diseases, with the aim of facilitating the development of drugs that target protein phosphatases for the treatment of inflammation-related diseases.

Proteomics and phosphoproteomics analysis of tissues for the reoccurrence prediction of colorectal cancer

BACKGROUND

Many stage II/III colorectal cancer (CRC) patients may relapse after routine treatments. Aberrant phosphorylation can regulate pathophysiological processes of tumors, and finding characteristic protein phosphorylation is an efficient approach for the prediction of CRC relapse.

RESEARCH DESIGN AND METHODS

We compared the tissue proteome and phosphoproteome of stage II/III CRC patients between the relapsed group (n = 5) and the non-relapsed group (n = 5). Phosphopeptides were enriched with Ti⁴⁺-IMAC material. We utilized label-free quantification-based proteomics to screen differentially expressed proteins and phosphopeptides between the two groups. Gene Ontology (GO) analysis and Ingenuity Pathway Analysis (IPA) were used for bioinformatics analysis.

RESULTS

The immune response of the relapsed group (Z-score -2.229) was relatively poorer than that of the non-relapsed group (Z-score 1.982), while viability of tumor was more activated (Z-score 2.895) in the relapsed group, which might cause increased relapse risk. The phosphorylation degrees of three phosphosites (phosphosite 1362 of TP53BP1, phosphosite 809 of VCL and phosphosite 438 of STK10) might be reliable prognostic biomarkers.

CONCLUSIONS

Some promising proteins and phosphopeptides were discovered to predict the relapse risk in postoperative follow-ups.

PP2A and Its Inhibitors in Helper T-Cell Differentiation and Autoimmunity

Protein phosphatase 2A (PP2A) is a highly complex heterotrimeric Ser/Thr phosphatase that regulates many cellular processes. The role of PP2A as a tumor suppressor has been extensively studied and reviewed. However, emerging evidence suggests PP2A constrains inflammatory responses and is important in autoimmune and neuroinflammatory diseases. Here, we reviewed the existing literature on the role of PP2A in T-cell differentiation and autoimmunity. We have also discussed the modulation of PP2A activity by endogenous inhibitors and its small-molecule activators as potential therapeutic approaches against autoimmunity.

Druggable cancer phosphatases

The phosphorylation status of oncoproteins is regulated by both kinases and phosphatases. Kinase inhibitors are rarely sufficient for successful cancer treatment, and phosphatases have been considered undruggable targets for cancer drug development. However, innovative pharmacological approaches for targeting phosphatases have recently emerged. Here, we review progress in the therapeutic targeting of oncogenic Src homology region 2 domain-containing phosphatase-2 (SHP2) and tumor suppressor protein phosphatase 2A (PP2A) and select other druggable oncogenic and tumor suppressor phosphatases. We describe the modes of action for currently available small molecules that target phosphatases, their use in drug combinations, and advances in clinical development toward future cancer therapies.

Effects of carboxyl-terminal methylation on holoenzyme function of the PP2A subfamily

Phosphoprotein Phosphatases (PPPs) are enzymes highly conserved from yeast and human and catalyze the majority of the serine and threonine dephosphorylation in cells. To achieve substrate specificity and selectivity, PPPs form multimeric holoenzymes consisting of catalytic, structural/scaffolding, and regulatory subunits. For the Protein Phosphatase 2A (PP2A)-subfamily of PPPs, holoenzyme assembly is at least in part regulated by an unusual carboxyl-terminal methyl-esterification, commonly referred to as 'methylation'. Carboxyl-terminal methylation is catalyzed by Leucine carboxyl methyltransferase-1 (LCMT1) that utilizes S-adenosyl-methionine (SAM) as the methyl donor and removed by protein phosphatase methylesterase 1 (PME1). For PP2A, methylation dictates regulatory subunit selection and thereby downstream phosphorylation signaling. Intriguingly, there are four families of PP2A regulatory subunits, each exhibiting different levels of methylation sensitivity. Thus, changes in PP2A methylation stoichiometry alters the complement of PP2A holoenzymes in cells and creates distinct modes of kinase opposition. Importantly, selective inactivation of PP2A signaling through the deregulation of methylation is observed in several diseases, most prominently Alzheimer's disease (AD). In this review, we focus on how carboxyl-terminal methylation of the PP2A subfamily (PP2A, PP4, and PP6) regulates holoenzyme function and thereby phosphorylation signaling, with an emphasis on AD.

Effects of chitosan and oligochitosans on the phosphatidylinositol 3-kinase-AKT pathway in cancer therapy

Phosphatidylinositol 3-kinase (PI3K)-AKT pathway is one of the most important kinase signaling networks in the context of cancer development and treatment. Aberrant activation of AKT, the central mediator of this pathway, has been implicated in numerous malignancies including endometrial, hepatocellular, breast, colorectal, prostate, and, cervical cancer. Thus regulation and blockage of this kinase and its key target nodes is an attractive approach in cancer therapy and diverse efforts have been done to achieve this aim. Chitosan is a carbohydrate with multiple interesting applications in cancer diagnosis and treatment strategies. This bioactive polymer and its derivative oligomers commonly used in drug/DNA delivery methods due to their functional properties which improve efficiency of delivery systems. Further, these compounds exert anti-tumor roles through the stimulation of apoptosis, immune enhancing potency, anti-oxidative features and anti-angiogenic roles. Due to the importance of PI3K-AKT signaling in cancer targeting and treatment resistance, this review discusses the involvement of chitosan, oligochitosaccharides and carriers based on these chemicals in the regulation of this pathway in different tumors.

Polyomavirus Small T Antigen Induces Apoptosis in Mammalian Cells through the UNC5B Pathway in a PP2A-Dependent Manner

UNC5B is a dependence receptor that promotes survival in the presence of its ligand, netrin-1, while inducing cell death in its absence. The receptor has an important role in the development of the nervous and vascular systems. It is also involved in the normal turnover of intestinal epithelium. Netrin-1 and UNC5B are deregulated in multiple cancers, including colorectal, neuroblastoma, and breast tumors. However, the detailed mechanism of UNC5B function is not fully understood. We have utilized the murine polyomavirus small T antigen (PyST) as a tool to study UNC5B-mediated apoptosis. PyST is known to induce mitotic arrest followed by extensive cell death in mammalian cells. Our results show that the expression of PyST increases mRNA levels of UNC5B by approximately 3-fold in osteosarcoma cells (U2OS) and also stabilizes UNC5B at the posttranslational level. Furthermore, UNC5B is upregulated predominantly in those cells that undergo mitotic arrest upon PyST expression. Interestingly, although its expression was previously reported to be regulated by p53, our data show that the increase in UNC5B levels by PyST is p53 independent. The posttranslational stabilization of UNC5B by PyST is regulated by the interaction of PyST with PP2A. We also show that netrin-1 expression, which is known to inhibit UNC5B apoptotic activity, promotes survival of PyST-expressing cells. Our results thus suggest an important role of UNC5B in small-T antigen-induced mitotic catastrophe that also requires PP2A.IMPORTANCE UNC5B, PP2A, and netrin-1 are deregulated in a variety of cancers. UNC5B and PP2A are regarded as tumor suppressors, as they promote apoptosis and are deleted or mutated in many cancers. In contrast, netrin-1 promotes survival by inhibiting dependence receptors, including UNC5B, and is upregulated in many cancers. Here, we show that UNC5B-mediated apoptosis can occur independently of p53 but in a PP2A-dependent manner. A substantial percentage of cancers arise due to p53 mutations and are insensitive to chemotherapeutic treatments that activate p53. Unexpectedly, treatment of cancers having functional p53 with many conventional drugs leads to the upregulation of netrin-1 through activated p53, which is counterintuitive. Therefore, understanding the p53-independent mechanisms of the netrin-UNC5B axis, such as those involving PP2A, assumes greater clinical significance. Anticancer strategies utilizing anti-netrin-1 antibody treatment are already in clinical trials.

Retracted: Allosteric Activators of Protein Phosphatase 2A Display Broad Antitumor Activity Mediated by Dephosphorylation of MYBL2

Protein phosphatase 2A (PP2A) enzymes can suppress tumors, but they are often inactivated in human cancers overexpressing inhibitory proteins. Here, we identify a class of small-molecule iHAPs (improved heterocyclic activators of PP2A) that kill leukemia cells by allosterically assembling a specific heterotrimeric PP2A holoenzyme consisting of PPP2R1A (scaffold), PPP2R5E (B56ε, regulatory), and PPP2CA (catalytic) subunits. One compound, iHAP1, activates this complex but does not inhibit dopamine receptor D2, a mediator of neurologic toxicity induced by perphenazine and related neuroleptics. The PP2A complex activated by iHAP1 dephosphorylates the MYBL2 transcription factor on Ser241, causing irreversible arrest of leukemia and other cancer cells in prometaphase. In contrast, SMAPs, a separate class of compounds, activate PP2A holoenzymes containing a different regulatory subunit, do not dephosphorylate MYBL2, and arrest tumor cells in G1 phase. Our findings demonstrate that small molecules can serve as allosteric switches to activate distinct PP2A complexes with unique substrates.

Expression Pattern and Prognostic Utility of PME-1 in Patients with Hepatocellular Carcinoma

Purpose

Hepatocellular carcinoma (HCC) remains one of the most common malignancies. While there is lack of markers capable of predicting which patients are at risk of aggressive course of the disease. Although a few protein phosphatase methyl-esterase-1 (PME-1) tumor-promoting mechanisms have been reported, the role of PME-1 in cancer including HCC occurrence and progression remains to be elucidated. The aim of this study was to explore the expression pattern and relationship between PME-1 with the pathological parameters in patients with HCC.

Methods

PME-1 expression was assessed from HCC tissue chips via immunohistochemistry. Chi-square test was used to identify the association between PME-1 staining and clinicopathological variables of HCC patients. Kaplan–Meier analysis and Cox regression analysis were performed to draw survival curves and verify the independent prognostic factors of HCC patients, respectively.

Results

We found that PME-1 expression was significantly higher in HCC tumor tissues compared with non-tumor tissues (P < 0.001). Furthermore, high expression level of PME-1 was significantly associated with differentiation (P = 0.047), tumor number (P = 0.001), intrahepatic or extrahepatic metastasis (P = 0.018), and recurrence (P = 0.001). Kaplan–Meier analysis revealed that high expression level of PME-1 was associated with shorter survival (P < 0.001). Univariate analysis with Log-rank test revealed that PME-1 expression status was significantly correlated with overall survival (P < 0.001). Furthermore, multivariate models with Cox proportional hazards analysis showed that high expression of PME-1 was a statistically independent predictive factor of poor prognosis in HCC patients (hazard ratio, 3.429; 95% confidence interval, 1.369–8.589; P = 0.009).

Conclusion

In conclusion, these findings indicated that PME-1 expression was associated with aggressive pathological features and worse oncological outcomes, suggesting its potential therapeutic value for patients with HCC.

Reduced Expression of the PP2A Methylesterase, PME-1, or the PP2A Methyltransferase, LCMT-1, Alters Sensitivity to Beta-Amyloid-Induced Cognitive and Electrophysiological Impairments in Mice

Beta-amyloid (Aβ) is thought to play a critical role in Alzheimer's disease (AD), and application of soluble oligomeric forms of Aβ produces AD-like impairments in cognition and synaptic plasticity in experimental systems. We found previously that transgenic overexpression of the PP2A methylesterase, PME-1, or the PP2A methyltransferase, LCMT-1, altered the sensitivity of mice to Aβ-induced impairments, suggesting that PME-1 inhibition may be an effective approach for preventing or treating these impairments. To explore this possibility, we examined the behavioral and electrophysiological effects of acutely applied synthetic Aβ oligomers in male and female mice heterozygous for either a PME-1 KO or an LCMT-1 gene-trap mutation. We found that heterozygous PME-1 KO mice were resistant to Aβ-induced impairments in cognition and synaptic plasticity, whereas LCMT-1 gene-trap mice showed increased sensitivity to Aβ-induced impairments. The heterozygous PME-1 KO mice produced normal levels of endogenous Aβ and exhibited normal electrophysiological responses to picomolar concentrations of Aβ, suggesting that reduced PME-1 expression in these animals protects against Aβ-induced impairments without impacting normal physiological Aβ functions. Together, these data provide additional support for roles for PME-1 and LCMT-1 in regulating sensitivity to Aβ-induced impairments, and suggest that inhibition of PME-1 may constitute a viable therapeutic approach for selectively protecting against the pathologic actions of Aβ in AD.SIGNIFICANCE STATEMENT Elevated levels of β-amyloid (Aβ) in the brain are thought to contribute to the cognitive impairments observed in Alzheimer's disease patients. Here we show that genetically reducing endogenous levels of the PP2A methylesterase, PME-1, prevents the cognitive and electrophysiological impairments caused by acute exposure to pathologic concentrations of Aβ without impairing normal physiological Aβ function or endogenous Aβ production. Conversely, reducing endogenous levels of the PP2A methyltransferase, LCMT-1, increases sensitivity to Aβ-induced impairments. These data offer additional insights into the molecular factors that control sensitivity to Aβ-induced impairments, and suggest that inhibiting PME-1 may constitute a viable therapeutic avenue for preventing Aβ-related impairments in Alzheimer's disease.

Phosphoproteome and drug-response effects mediated by the three protein phosphatase 2A inhibitor proteins CIP2A, SET, and PME-1

Protein phosphatase 2A (PP2A) critically regulates cell signaling and is a human tumor suppressor. PP2A complexes are modulated by proteins such as cancerous inhibitor of protein phosphatase 2A (CIP2A), protein phosphatase methylesterase 1 (PME-1), and SET nuclear proto-oncogene (SET) that often are deregulated in cancers. However, how they impact cellular phosphorylation and how redundant they are in cellular regulation is poorly understood. Here, we conducted a systematic phosphoproteomics screen for phosphotargets modulated by siRNA-mediated depletion of CIP2A, PME-1, and SET (to reactivate PP2A) or the scaffolding A-subunit of PP2A (PPP2R1A) (to inhibit PP2A) in HeLa cells. We identified PP2A-modulated targets in diverse cellular pathways, including kinase signaling, cytoskeleton, RNA splicing, DNA repair, and nuclear lamina. The results indicate nonredundancy among CIP2A, PME-1, and SET in phosphotarget regulation. Notably, PP2A inhibition or reactivation affected largely distinct phosphopeptides, introducing a concept of nonoverlapping phosphatase inhibition- and activation-responsive sites (PIRS and PARS, respectively). This phenomenon is explained by the PPP2R1A inhibition impacting primarily dephosphorylated threonines, whereas PP2A reactivation results in dephosphorylation of clustered and acidophilic sites. Using comprehensive drug-sensitivity screening in PP2A-modulated cells to evaluate the functional impact of PP2A across diverse cellular pathways targeted by these drugs, we found that consistent with global phosphoproteome effects, PP2A modulations broadly affect responses to more than 200 drugs inhibiting a broad spectrum of cancer-relevant targets. These findings advance our understanding of the phosphoproteins, pharmacological responses, and cellular processes regulated by PP2A modulation and may enable the development of combination therapies.

Prolyl oligopeptidase inhibition activates autophagy via protein phosphatase 2A

Prolyl oligopeptidase (PREP) is a serine protease that has been studied particularly in the context of neurodegenerative diseases for decades but its physiological function has remained unclear. We have previously found that PREP negatively regulates beclin1-mediated macroautophagy (autophagy), and that PREP inhibition by a small-molecule inhibitor induces clearance of protein aggregates in Parkinson's disease models. Since autophagy induction has been suggested as a potential therapy for several diseases, we wanted to further characterize how PREP regulates autophagy. We measured the levels of various kinases and proteins regulating beclin1-autophagy in HEK-293 and SH-SY5Y cell cultures after PREP inhibition, PREP deletion, and PREP overexpression and restoration, and verified the results in vivo by using PREP knock-out and wild-type mouse tissue where PREP was restored or overexpressed, respectively. We found that PREP regulates autophagy by interacting with protein phosphatase 2A (PP2A) and its endogenous inhibitor, protein phosphatase methylesterase 1 (PME1), and activator (protein phosphatase 2 phosphatase activator, PTPA), thus adjusting its activity and the levels of PP2A in the intracellular pool. PREP inhibition and deletion increased PP2A activity, leading to activation of death-associated protein kinase 1 (DAPK1), beclin1 phosphorylation and induced autophagy while PREP overexpression reduced this. Lowered activity of PP2A is connected to several neurodegenerative disorders and cancers, and PP2A activators would have enormous potential as drug therapy but development of such compounds has been a challenge. The concept of PREP inhibition has been proved safe, and therefore, our study supports the further development of PREP inhibitors as PP2A activators.

PP2A inhibition is a druggable MEK inhibitor resistance mechanism in KRAS-mutant lung cancer cells

Kinase inhibitor resistance constitutes a major unresolved clinical challenge in cancer. Furthermore, the role of serine/threonine phosphatase deregulation as a potential cause for resistance to kinase inhibitors has not been thoroughly addressed. We characterize protein phosphatase 2A (PP2A) activity as a global determinant of KRAS-mutant lung cancer cell resistance across a library of >200 kinase inhibitors. The results show that PP2A activity modulation alters cancer cell sensitivities to a large number of kinase inhibitors. Specifically, PP2A inhibition ablated mitogen-activated protein kinase kinase (MEK) inhibitor response through the collateral activation of AKT/mammalian target of rapamycin (mTOR) signaling. Combination of mTOR and MEK inhibitors induced cytotoxicity in PP2A-inhibited cells, but even this drug combination could not abrogate MYC up-regulation in PP2A-inhibited cells. Treatment with an orally bioavailable small-molecule activator of PP2A DT-061, in combination with the MEK inhibitor AZD6244, resulted in suppression of both p-AKT and MYC, as well as tumor regression in two KRAS-driven lung cancer mouse models. DT-061 therapy also abrogated MYC-driven tumorigenesis. These data demonstrate that PP2A deregulation drives MEK inhibitor resistance in KRAS-mutant cells. These results emphasize the need for better understanding of phosphatases as key modulators of cancer therapy responses.

Protein phosphatase 2A as a therapeutic target in inflammation and neurodegeneration

Protein phosphatase 2A (PP2A) is a highly complex heterotrimeric enzyme that catalyzes the selective removal of phosphate groups from protein serine and threonine residues. Emerging evidence suggests that it functions as a tumor suppressor by constraining phosphorylation-dependent signalling pathways that regulate cellular transformation and metastasis. Therefore, PP2A-activating drugs (PADs) are being actively sought and investigated as potential novel anti-cancer treatments. Here we explore the concept that PP2A also constrains inflammatory responses through its inhibitory effects on various signalling pathways, suggesting that PADs may be effective in the treatment of inflammation-mediated pathologies.

Alterations of Methionine Metabolism as Potential Targets for the Prevention and Therapy of Hepatocellular Carcinoma

Several researchers have analyzed the alterations of the methionine cycle associated with liver disease to clarify the pathogenesis of human hepatocellular carcinoma (HCC) and improve the preventive and the therapeutic approaches to this tumor. Different alterations of the methionine cycle leading to a decrease of S-adenosylmethionine (SAM) occur in hepatitis, liver steatosis, liver cirrhosis, and HCC. The reproduction of these changes in MAT1A-KO mice, prone to develop hepatitis and HCC, demonstrates the pathogenetic role of MAT1A gene under-regulation associated with up-regulation of the MAT2A gene (MAT1A:MAT2A switch), encoding the SAM synthesizing enzymes, methyladenosyltransferase I/III (MATI/III) and methyladenosyltransferase II (MATII), respectively. This leads to a rise of MATII, inhibited by the reaction product, with a consequent decrease of SAM synthesis. Attempts to increase the SAM pool by injecting exogenous SAM have beneficial effects in experimental alcoholic and non-alcoholic steatohepatitis and hepatocarcinogenesis. Mechanisms involved in hepatocarcinogenesis inhibition by SAM include: (1) antioxidative effects due to inhibition of nitric oxide (NO•) production, a rise in reduced glutathione (GSH) synthesis, stabilization of the DNA repair protein Apurinic/Apyrimidinic Endonuclease 1 (APEX1); (2) inhibition of c-myc, H-ras, and K-ras expression, prevention of NF-kB activation, and induction of overexpression of the oncosuppressor PP2A gene; (3) an increase in expression of the ERK inhibitor DUSP1; (4) inhibition of PI3K/AKT expression and down-regulation of C/EBPα and UCA1 gene transcripts; (5) blocking LKB1/AMPK activation; (6) DNA and protein methylation. Different clinical trials have documented curative effects of SAM in alcoholic liver disease. Furthermore, SAM enhances the IFN-α antiviral activity and protects against hepatic ischemia-reperfusion injury during hepatectomy in HCC patients with chronic hepatitis B virus (HBV) infection. However, although SAM prevents experimental tumors, it is not curative against already established experimental and human HCCs. The recent observation that the inhibition of MAT2A and MAT2B expression by miRNAs leads to a rise of endogenous SAM and strong inhibition of cancer cell growth could open new perspectives to the treatment of HCC.

Interferon-inducible cytoplasmic lncLrrc55-AS promotes antiviral innate responses by strengthening IRF3 phosphorylation

Type I interferon (IFN-I) production is efficiently induced to ensure a potent innate immune response to viral infection. How this response can be enhanced, however, remains to be explored. Here, we identify a new cytoplasmic long non-coding RNA (lncRNA), lncLrrc55-AS, that drives a positive feedback loop to promote interferon regulatory factor 3 (IRF3) signaling and IFN-I production. We show that lncLrrc55-AS is virus-induced in multiple cell types via the IFN-JAK-STAT pathway. LncLrrc55-AS-deficient mice display a weakened antiviral immune response and are more susceptible to viral challenge. Mechanistically, lncLrrc55-AS binds phosphatase methylesterase 1 (PME-1), and promotes the interaction between PME-1 and the phosphatase PP2A, an inhibitor of IRF3 signaling. LncLrrc55-AS supports PME-1-mediated demethylation and inactivation of PP2A, thereby enhancing IRF3 phosphorylation and signaling. Loss of PME-1 phenocopies lncLrrc55-AS deficiency, leading to diminished IRF3 phosphorylation and IFN-I production. We have identified an IFN-induced lncRNA as a positive regulator of IFN-I production, adding mechanistic insight into lncRNA-mediated regulation of signaling in innate immunity and inflammation.

PP2A: A Promising Biomarker and Therapeutic Target in Endometrial Cancer

Over the last decade, the use of targeted therapies has immensely increased in the treatment of cancer. However, treatment for endometrial carcinomas (ECs) has lagged behind, although potential molecular markers have been identified. This is particularly problematic for the type II ECs, since these aggressive tumors are usually not responsive toward the current standard therapies. Therefore, type II ECs are responsible for most EC-related deaths, indicating the need for new treatment options. Interestingly, molecular analyses of type II ECs have uncovered frequent genetic alterations (up to 40%) in PPP2R1A, encoding the Aα subunit of the tumor suppressive heterotrimeric protein phosphatase type 2A (PP2A). PPP2R1A mutations were also reported in type I ECs and other common gynecologic cancers, albeit at much lower frequencies (0-7%). Nevertheless, PP2A inactivation in the latter cancer types is common via other mechanisms, in particular by increased expression of Cancerous Inhibitor of PP2A (CIP2A) and PP2A Methylesterase-1 (PME-1) proteins. In this review, we discuss the therapeutic potential of direct and indirect PP2A targeting compounds, possibly in combination with other anti-cancer drugs, in EC. Furthermore, we investigate the potential of the PP2A status as a predictive and/or prognostic marker for type I and II ECs.

The PHLPP2 phosphatase is a druggable driver of prostate cancer progression

Nowak et al. show that loss of the AKT-inactivating phosphatase PHLPP2 paradoxically blocks prostate tumor growth and metastasis. PHLPP2, they find, is critical for MYC stability, suggesting that PHLPP2 inhibitors may present a therapeutic opportunity to target MYC.

Metastatic prostate cancer commonly presents with targeted, bi-allelic mutations of the PTEN and TP53 tumor suppressor genes. In contrast, however, most candidate tumor suppressors are part of large recurrent hemizygous deletions, such as the common chromosome 16q deletion, which involves the AKT-suppressing phosphatase PHLPP2. Using RapidCaP, a genetically engineered mouse model of Pten/Trp53 mutant metastatic prostate cancer, we found that complete loss of Phlpp2 paradoxically blocks prostate tumor growth and disease progression. Surprisingly, we find that Phlpp2 is essential for supporting Myc, a key driver of lethal prostate cancer. Phlpp2 dephosphorylates threonine-58 of Myc, which renders it a limiting positive regulator of Myc stability. Furthermore, we show that small-molecule inhibitors of PHLPP2 can suppress MYC and kill PTEN mutant cells. Our findings reveal that the frequent hemizygous deletions on chromosome 16q present a druggable vulnerability for targeting MYC protein through PHLPP2 phosphatase inhibitors.

Differential Expression of Genes for Ubiquitin Ligases in Medulloblastoma Subtypes

Using publically available datasets on gene expression in medulloblastoma (MB) subtypes, we selected genes for ubiquitin ligases and identified statistically those that best predicted each of the four major MB subgroups as separate disease entities. We identify a gene coding for an ubiquitin ligase, ZNRF3, whose overexpression alone can predict the WNT subgroup for 100% in the Pfister dataset. For the SHH subgroup, we identify a gene for a regulatory subunit of the protein phosphatase 2A (PP2A), PPP2R2C, as the major predictor among the E3 ligases genes. The ubiquitin and ubiquitin-like conjugation database (UUCD) lists PPP2R2C as coding for a Cullin Ring ubiquitin ligase adaptor. For group 3 MBs, the best ubiquitin ligase predictor was PPP2R2B, a gene which codes for another regulatory subunit of the PP2A holoenzyme. For group 4, the best E3 gene predictors were MID2, ZBTB18, and PPP2R2A, which codes for a third PP2A regulatory subunit. Heatmap analysis of the E3 gene data shows that expression of ten genes for ubiquitin ligases can be used to classify MBs into the four major consensus subgroups. This was illustrated by analysis of gene expression of ubiquitin ligases of the Pfister dataset and confirmed in the dataset of Cavalli. We conclude that genes for ubiquitin ligases can be used as genetic markers for MB subtypes and that the proteins coded for by these genes should be investigated as subtype specific therapeutic targets for MB.

Protein Phosphatases-A Touchy Enemy in the Battle Against Glioblastomas: A Review

Glioblastoma (GBM) is the most common malignant tumor arising from brain parenchyma. Although many efforts have been made to develop therapies for GBM, the prognosis still remains poor, mainly because of the difficulty in total resection of the tumor mass from brain tissue and the resistance of the residual tumor against standard chemoradiotherapy. Therefore, novel adjuvant therapies are urgently needed. Recent genome-wide analyses of GBM cases have clarified molecular signaling mechanisms underlying GBM biology. However, results of clinical trials targeting phosphorylation-mediated signaling have been unsatisfactory to date. Protein phosphatases are enzymes that antagonize phosphorylation signaling by dephosphorylating phosphorylated signaling molecules. Recently, the critical roles of phosphatases in the regulation of oncogenic signaling in malignant tumor cells have been reported, and tumorigenic roles of deregulated phosphatases have been demonstrated in GBM. However, a detailed mechanism underlying phosphatase-mediated signaling transduction in the regulation of GBM has not been elucidated, and such information is necessary to apply phosphatases as a therapeutic target for GBM. This review highlights and summarizes the phosphatases that have crucial roles in the regulation of oncogenic signaling in GBM cells.

Synthesis of Novel FTY720 Analogs with Anticancer Activity through PP2A Activation

FTY720 inhibits various cancers through PP2A activation. The structure of FTY720 is also used as a basic structure for the design of sphingosine kinase (SK) inhibitors. We have synthesized derivatives using an amide chain in FTY720 with a phenyl backbone, and then compounds were screened by an MTT cell viability assay. The PP2A activity of compound 7 was examined. The phosphorylation levels of AKT and ERK, downstream targets of PP2A, in the presence of compound 7, were determined. Compound 7 may exhibit anticancer effects through PP2A activation rather than the mechanism by inhibition of SK1 in cancer cells. In the docking study of compound 7 and PP2A, the amide chain of compound 7 showed an interaction with Asn61 that was different from FTY720, which is expected to affect the activity of the compound.

Cornel Iridoid Glycoside Inhibits Tau Hyperphosphorylation via Regulating Cross-Talk Between GSK-3β and PP2A Signaling

Neurofibrillary pathology contributes to neuronal dysfunction and correlates with the clinical progression of Alzheimer's disease (AD). Tau phosphorylation is mainly regulated by a balance of glycogen synthase kinase-3β (GSK-3β) and protein phosphatase 2A (PP2A) activities. Cornel iridoid glycoside (CIG) is a main component extracted from Cornus officinalis. The purpose of this study was to investigate the effects of CIG on GSK-3β and PP2A, thus to explore the mechanisms of CIG to inhibit tau hyperphosphorylation. The rat model of tau hyperphosphorylation was established by intraventricular injection of wortmannin and GF-109203X (GFX) to activate GSK-3β. The results showed that intragastrical administration of CIG inhibited tau hyperphosphorylation in the brain of rats induced by wortmannin/GFX. The results in vivo and in vitro exhibited that CIG inhibited tau hyperphosphorylation and GSK-3β over-activation. In the mechanism of action, CIG's attenuating GSK-3β activity was found to be dependent on PI3K/AKT signaling pathway. PP2A catalytic C subunit (PP2Ac) siRNA abrogated the effect of CIG on PI3K/AKT/GSK-3β. Additionally and crucially, we also found that CIG inhibited the demethylation of PP2Ac at Leu309 in vivo and in vitro. It enhanced PP2A activity, decreased tau hyperphosphorylation, and protected cell morphology in okadaic acid (OA)-induced cell model in vitro. PP2Ac siRNA abated the inhibitory effect of CIG on tau hyperphosphorylation. Moreover, CIG inhibited protein phosphatase methylesterase-1 (PME-1) and demethylation of PP2Ac, enhanced PP2A activity, and decreased tau hyperphosphorylation in PME-1-transfectd cells. Taken together, CIG inhibited GSK-3β activity via promoting P13K/AKT and PP2A signaling pathways. In addition, CIG also elevated PP2A activity via inhibiting PME-1-induced PP2Ac demethylation to inhibit GSK-3β activity, thus regulated the cross-talk between GSK-3β and PP2A signaling and consequently inhibited tau hyperphosphorylation. These results suggest that CIG may be a promising agent for AD therapy.

ERα36 gene silencing promotes tau protein phosphorylation, inhibits cell proliferation, and induces apoptosis in human neuroblastoma SH-SY5Y cells

Neuroblastoma is the most common cancer in infants and the third most common cancer in children after leukemia and brain cancer. The purpose of our study was to investigate the effects of estrogen receptor (ER)-α36 gene silencing on tau protein phosphorylation, cell proliferation, and cell apoptosis in human neuroblastoma SH-SY5Y cells. SH-SY5Y cells were treated with estrogen or left untreated, to investigate the effects of estrogen stimulation on ERα36 and the ERK/protein B kinase (AKT) signaling pathway. ERα36 mRNA expressions were detected by quantitative RT-PCR. A phosphatase kit was used to test protein phosphatase (PP)-2A activity before and after treatment. Western blot analysis was conducted to detect protein expression of ERα36; tau protein; phosphorylated- tau (p-tau) at site Thr231 [p-tau (Thr231)]; glycogen synthase kinase (GSK)3β and its specificity sites (Tyr216 and Ser9); Cyclin Dl; proliferating cell nuclear antigen (PCNA); B-cell lymphoma (Bcl)-2; and Bcl-2-associated X protein (Bax). A cell-counting kit (CCK)-8 assay was used to determine cell viability. Cell apoptosis and rate of tumor growth and volume were determined by Annexin V-FITC/PI staining and a xenotransplanted tumor model in nude mice. Results show that without estrogen stimulation, ERα36 was inactivated. When stimulated by estrogen, expression of ERα36, PP2A, p-GSK3β (Ser9)/total protein ( t)-GSK3β, Cyclin Dl, PCNA, and Bcl-2 were up-regulated, and p-GSK3β (Tyr216)/ t-GSK3β expression was down-regulated, as was p-tau (Thr231) and Bax expression. The expression of p-ERK/ERK, p-AKT/AKT, p-methyl ethyl ketone (MEK)/MEK, and p-mammalian target of rapamycin (mTOR)/mTOR expression was up-regulated, suggesting that the ERK/AKT signaling pathway is activated. Cell proliferation was also accelerated, whereas apoptosis was inhibited with stimulation by estrogen. However, we found that the effects of silencing ERα36 on the expression of related intracellular factors had no association with estrogen. Our study demonstrates that ERα36 gene silencing can inhibit the activation of the ERK/AKT signaling pathway, increase tau protein phosphorylation, decrease cell vitality and tumorigenicity, and promote apoptosis of human neuroblastoma SH-SY5Y cells.-Wang, H.-B., Li, T., Ma, D.-Z., Zhi, H. ERα36 gene silencing promotes tau protein phosphorylation, inhibits cell proliferation, and induces apoptosis in human neuroblastoma SH-SY5Y cells.

Targeting PP2A activates AMPK signaling to inhibit colorectal cancer cells

LB-100 is a novel PP2A inhibitor. Its activity in human colorectal cancer (CRC) cells was tested. The in vitro studies demonstrated that LB-100 inhibited survival and proliferation of both established CRC cells (HCT-116 and HT-29 lines) and primary human colon cancer cells. Further, LB-100 activated apoptosis and induced G1-S cell cycle arrest in CRC cells. LB-100 inhibited PP2A activity and activated AMPK signaling in CRC cells. AMPKα1 dominant negative mutation, shRNA-mediated knockdown or complete knockout (by CRISPR/Cas9 method) largely attenuated LB-100-induced AMPK activation and HCT-116 cytotoxicity. Notably, microRNA-17-92-mediated silence of PP2A (regulatory B subunit) also activated AMPK and induced HCT-116 cell death. Such effects were again largely attenuated by AMPKα mutation, silence or complete knockout. In vivo studies showed that intraperitoneal injection of LB-100 inhibited HCT-116 xenograft growth in nude mice. Its anti-tumor activity was largely compromised against HCT-116 tumors-derived from AMPKα1-knockout cells. We conclude that targeting PP2A by LB-100 and microRNA-17-92 activates AMPK signaling to inhibit CRC cells.

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.

PP2A regulates signaling through hormonal receptors in breast cancer with important therapeutic implications

The functional inhibition of protein phosphatase 2A (PP2A) has emerged in the last years as a common alteration in breast cancer that determines poor outcome and contributes to disease progression and aggressiveness. Furthermore, expression of estrogen receptor (ER) is a high relevant molecular event with key therapeutic implications in breast cancer, and androgen receptor (AR) signaling is involved in the pathogenesis of breast cancer and represents a novel target with crescent importance in this disease. In this review, we summarize the role of the tumor suppressor PP2A in modulating ER and AR signaling in breast cancer, the molecular mechanisms involved, and its biological and therapeutic impact.