Targeting SET/I(2)PP2A oncoprotein functions as a multi-pathway strategy for cancer therapy

Biomarkers and Target-Specific Small-Molecule Drugs in Alzheimer's Diagnostic and Therapeutic Research: From Amyloidosis to Tauopathy

Alzheimer's disease (AD) is the most common type of human dementia and is responsible for over 60% of diagnosed dementia cases worldwide. Abnormal deposition of β-amyloid and the accumulation of neurofibrillary tangles have been recognised as the two pathological hallmarks targeted by AD diagnostic imaging as well as therapeutics. With the progression of pathological studies, the two hallmarks and their related pathways have remained the focus of researchers who seek for AD diagnostic and therapeutic strategies in the past decades. In this work, we reviewed the development of the AD biomarkers and their corresponding target-specific small molecule drugs for both diagnostic and therapeutic applications, underlining their success, failure, and future possibilities.

Merkel Cell Polyomavirus targets SET/PP2A complex to promote cellular proliferation and migration

Merkel Cell Carcinoma (MCC) is a rare neuroendocrine skin cancer. In our previous work, we decoded genes specifically deregulated by MCPyV early genes as opposed to other polyomaviruses and established functional importance of NDRG1 in inhibiting cellular proliferation and migration in MCC. In the present work, we found the SET protein, (I2PP2A, intrinsic inhibitor of PP2A) upstream of NDRG1 which was modulated by MCPyV early genes, both in hTERT-HK-MCPyV and MCPyV-positive (+) MCC cell lines. Additionally, MCC dermal tumour nodule tissues showed strong SET expression. Inhibition of the SET-PP2A interaction in hTERT-HK-MCPyV using the small molecule inhibitor, FTY720, increased NDRG1 expression and inhibited cell cycle regulators, cyclinD1 and CDK2. SET inhibition by shRNA and FTY720 also decreased cell proliferation and colony formation in MCPyV(+) MCC cells. Overall, these results pave a path for use of drugs targeting SET protein for the treatment of MCC.

Oncoprotein SET-associated transcription factor ZBTB11 triggers lung cancer metastasis

Metastasis is the major cause of lung cancer-related death, but the mechanisms governing lung tumor metastasis remain incompletely elucidated. SE translocation (SET) is overexpressed in lung tumors and correlates with unfavorable prognosis. Here we uncover SET-associated transcription factor, zinc finger and BTB domain-containing protein 11 (ZBTB11), as a prometastatic regulator in lung tumors. SET interacts and collaborates with ZBTB11 to promote lung cancer cell migration and invasion, primarily through SET-ZBTB11 complex-mediated transcriptional activation of matrix metalloproteinase-9 (MMP9). Additionally, by transcriptional repression of proline-rich Gla protein 2 (PRRG2), ZBTB11 links Yes-associated protein 1 (YAP1) activation to drive lung tumor metastasis independently of SET-ZBTB11 complex. Loss of ZBTB11 suppresses distal metastasis in a lung tumor mouse model. Overexpression of ZBTB11 is recapitulated in human metastatic lung tumors and correlates with diminished survival. Our study demonstrates ZBTB11 as a key metastatic regulator and reveals diverse mechanisms by which ZBTB11 modulates lung tumor metastasis.

Targeting phosphatases: From molecule design to clinical trials

Phosphatase is a kind of enzyme that can dephosphorylate target proteins, which can be divided into serine/threonine phosphatase and tyrosine phosphatase according to its mode of action. Current evidence showed multiple phosphatases were highly correlated with diseases including various cancers, demonstrating them as potential targets. However, currently, targeting phosphatases with small molecules faces many challenges, resulting in no drug approved. In this case, phosphatases are even regarded as "undruggable" targets for a long time. Recently, a variety of strategies have been adopted in the design of small molecule inhibitors targeting phosphatases, leading many of them to enter into the clinical trials. In this review, we classified these inhibitors into 4 types, including (1) molecular glues, (2) small molecules targeting catalytic sites, (3) allosteric inhibition, and (4) bifunctional molecules (proteolysis targeting chimeras, PROTACs). These molecules with diverse strategies prove the feasibility of phosphatases as drug targets. In addition, the combination therapy of phosphatase inhibitors with other drugs has also entered clinical trials, which suggests a broad prospect. Thus, targeting phosphatases with small molecules by different strategies is emerging as a promising way in the modulation of pathogenetic phosphorylation.

SET-PP2A complex as a new therapeutic target in KMT2A (MLL) rearranged AML

KMT2A-rearranged (KMT2A-R) is an aggressive and chemo-refractory acute leukemia which mostly affects children. Transcriptomics-based characterization and chemical interrogation identified kinases as key drivers of survival and drug resistance in KMT2A-R leukemia. In contrast, the contribution and regulation of phosphatases is unknown. In this study we uncover the essential role and underlying mechanisms of SET, the endogenous inhibitor of Ser/Thr phosphatase PP2A, in KMT2A-R-leukemia. Investigation of SET expression in acute myeloid leukemia (AML) samples demonstrated that SET is overexpressed, and elevated expression of SET is correlated with poor prognosis and with the expression of MEIS and HOXA genes in AML patients. Silencing SET specifically abolished the clonogenic ability of KMT2A-R leukemic cells and the transcription of KMT2A targets genes HOXA9 and HOXA10. Subsequent mechanistic investigations showed that SET interacts with both KMT2A wild type and fusion proteins, and it is recruited to the HOXA10 promoter. Pharmacological inhibition of SET by FTY720 disrupted SET-PP2A interaction leading to cell cycle arrest and increased sensitivity to chemotherapy in KMT2A-R-leukemic models. Phospho-proteomic analyses revealed that FTY720 reduced the activity of kinases regulated by PP2A, including ERK1, GSK3β, AURB and PLK1 and led to suppression of MYC, supporting the hypothesis of a feedback loop among PP2A, AURB, PLK1, MYC, and SET. Our findings illustrate that SET is a novel player in KMT2A-R leukemia and they provide evidence that SET antagonism could serve as a novel strategy to treat this aggressive leukemia.

Protein Phosphatase 2A as a Therapeutic Target in Pulmonary Diseases

New disease targets and medicinal chemistry approaches are urgently needed to develop novel therapeutic strategies for treating pulmonary diseases. Emerging evidence suggests that reduced activity of protein phosphatase 2A (PP2A), a complex heterotrimeric enzyme that regulates dephosphorylation of serine and threonine residues from many proteins, is observed in multiple pulmonary diseases, including lung cancer, smoke-induced chronic obstructive pulmonary disease, alpha-1 antitrypsin deficiency, asthma, and idiopathic pulmonary fibrosis. Loss of PP2A responses is linked to many mechanisms associated with disease progressions, such as senescence, proliferation, inflammation, corticosteroid resistance, enhanced protease responses, and mRNA stability. Therefore, chemical restoration of PP2A may represent a novel treatment for these diseases. This review outlines the potential impact of reduced PP2A activity in pulmonary diseases, endogenous and exogenous inhibitors of PP2A, details the possible PP2A-dependent mechanisms observed in these conditions, and outlines potential therapeutic strategies for treatment. Substantial medicinal chemistry efforts are underway to develop therapeutics targeting PP2A activity. The development of specific activators of PP2A that selectively target PP2A holoenzymes could improve our understanding of the function of PP2A in pulmonary diseases. This may lead to the development of therapeutics for restoring normal PP2A responses within the lung.

Oncoprotein SET dynamically regulates cellular stress response through nucleocytoplasmic transport in breast cancer

SETβ is the predominant isoform of oncoprotein SE translocation (SET) in various breast cancer cell lines. Interactome-transcriptome analysis has shown that SETβ is intimately associated with cellular stress response. Among various exogenous stimuli, formaldehyde (FA) causes distinct biological effects in a dose-dependent manner. In response to FA at different concentrations, SET dynamically shuttles between the nucleus and cytoplasm, performing diverse biofunctions to restore homeostasis. At a low concentration, FA acts as an epidermal growth factor (EGF) and activates the HER2 receptor and downstream signaling pathways in HER2+ breast cancer cells, resulting in enhanced cell proliferation. Nucleocytoplasmic transport of SETβ is controlled by the PI3K/PKCα/CK2α axis and depletion or blockade of the transport of SETβ suppresses EGF-induced activation of AKT and ERK. SETβ also inhibits not only stress-induced activation of p38 MAPK signaling pathway, but also assembly of stress granules by hindering formation of the G3BP1-RNA complex. Our findings suggest that SET functions as an important regulator which modulates cellular stress signaling pathways dynamically.

Circular MTHFD2L RNA-encoded CM-248aa inhibits gastric cancer progression by targeting the SET-PP2A interaction

The available targeted therapies for gastric cancer (GC) are still limited, so it is important to discover novel molecules as potential treatment options. Proteins or peptides encoded by circular RNAs (circRNAs) are increasingly reported to play essential roles in malignancies. The aim of the present study was to identify an undiscovered protein encoded by circRNA and explore its key role and molecular mechanism in GC progression. CircMTHFD2L (hsa_circ_0069982) was screened and validated as a downregulated circRNA with coding potential. The protein encoded by circMTHFD2L, named CM-248aa, was identified for the first time by immunoprecipitation and mass spectrometry. CM-248aa was significantly downregulated in GC, while its low expression was associated with advanced tumor-node-metastasis (TNM) stage and histopathological grade. Low expression of CM-248aa could be an independent risk factor for poor prognosis. Functionally, CM-248aa, instead of circMTHFD2L suppressed the proliferation and metastasis of GC in vitro and in vivo. Mechanistically, CM-248aa competitively targeted the acidic domain of SET nuclear oncogene (SET) and acted as an endogenous inhibitor of the SET-protein phosphatase 2A interaction to promote dephosphorylation of AKT, extracellular signal-regulated kinase, and P65. Our discovery revealed that CM-248aa could be a potential prognostic biomarker and endogenous therapeutic option for GC.

A direct interaction between RhoGDIα/Tau alleviates hyperphosphorylation of Tau in Alzheimer's disease and vascular dementia

RhoGDIα is an inhibitor of RhoGDP dissociation that involves in Aβ metabolism and NFTs production in Alzheimer's disease (AD) by regulating of RhoGTP enzyme activity. Our previous research revealed that RhoGDIα, as the target of Polygala saponin (Sen), might alleviate apoptosis of the nerve cells caused by hypoxia/reoxygenation (H/R). To further clarify the role of RhoGDIα in the generation of NFTs, we explored the relationship between RhoGDIα and Tau. We found out that RhoGDIα and Tau can bind with each other and interact by using coimmunoprecipitation (Co-IP) and GST pulldown methods in vitro. This RhoGDIα-Tau partnership was further verified by using immunofluorescence colocalization and fluorescence resonance energy transfer (FRET) approaches in PC12 cells. Using the RNA interference (RNAi) technique, we found that the RhoGDIα may be involved in an upstream signaling pathway for Tau. Subsequently, in Aβ25-35- and H/R-induced PC12 cells, forced expression of RhoGDIα via cDNA plasmid transfection was found to reduce the hyperphosphorylation of Tau, augment the expression of bcl-2 protein, and inhibit the expression of Bax protein (reducing the Bax/bcl-2 ratio) and the activity of caspase-3. In mouse AD and VaD models, forced expression of RhoGDIα via injection of a viral vector (pAAV-EGFP-RhoGDIα) into the lateral ventricle of the brain alleviated the pathological symptoms of AD and VaD. Finally, GST pulldown confirmed that the binding sites on RhoGDIα for Tau were located in the range of the ΔC33 fragment (aa 1-33). These results indicate that RhoGDIα is involved in the phosphorylation of Tau and apoptosis in AD and VaD. Overexpression of RhoGDIα can inhibit the generation of NFTs and delay the progress of these two types of dementia.

The Effects of Protein Phosphatase 2A Activation with Novel Tricyclic Sulfonamides on Hepatoblastoma

BACKGROUND

The tumor suppressor, protein phosphatase 2A (PP2A), is downregulated in hepatoblastoma. We aimed to examine the effects of two novel compounds of the tricyclic sulfonamide class, ATUX-3364 (3364) and ATUX-8385 (8385), designed to activate PP2A without causing immunosuppression, on human hepatoblastoma.

METHODS

An established human hepatoblastoma cell line, HuH6, and a human hepatoblastoma patient-derived xenograft, COA67, were treated with increasing doses of 3364 or 8385, and viability, proliferation, cell cycle and motility were investigated. Cancer cell stemness was evaluated by real-time PCR and tumorsphere forming ability. Effects on tumor growth were examined using a murine model.

RESULTS

Treatment with 3364 or 8385 significantly decreased viability, proliferation, cell cycle progression and motility in HuH6 and COA67 cells. Both compounds significantly decreased stemness as demonstrated by decreased abundance of OCT4, NANOG, and SOX2 mRNA. The ability of COA67 to form tumorspheres, another sign of cancer cell stemness, was significantly diminished by 3364 and 8385. Treatment with 3364 resulted in decreased tumor growth in vivo.

CONCLUSION

Novel PP2A activators, 3364 and 8385, decreased hepatoblastoma proliferation, viability, and cancer cell stemness in vitro. Animals treated with 3364 had decreased tumor growth. These data provide evidence for further investigation of PP2A activating compounds as hepatoblastoma therapeutics.

The SET oncoprotein promotes estrogen-induced transcription by facilitating establishment of active chromatin

SET is a multifunctional histone-binding oncoprotein that regulates transcription by an unclear mechanism. Here we show that SET enhances estrogen-dependent transcription. SET knockdown abrogates transcription of estrogen-responsive genes and their enhancer RNAs. In response to 17β-estradiol (E2), SET binds to the estrogen receptor α (ERα) and is recruited to ERα-bound enhancers and promoters at estrogen response elements (EREs). SET functions as a histone H2 chaperone that dynamically associates with H2A.Z via its acidic C-terminal domain and promotes H2A.Z incorporation, ERα, MLL1, and KDM3A loading and modulates histone methylation at EREs. SET depletion diminishes recruitment of condensin complexes to EREs and impairs E2-dependent enhancer-promoter looping. Thus, SET boosts E2-induced gene expression by establishing an active chromatin structure at ERα-bound enhancers and promoters, which is essential for transcriptional activation.

The repression of oncoprotein SET by the tumor suppressor p53 reveals a p53-SET-PP2A feedback loop for cancer therapy

The oncoprotein SET is frequently overexpressed in many types of tumors and contributes to malignant initiation and progression through multiple mechanisms, including the hijacking of the tumor suppressors p53 and PP2A. Targeting aberrant SET represents a promising strategy for cancer intervention. However, the mechanism by which endogenous SET is regulated in cancer cells remains largely unknown. Here, we identified the tumor suppressor p53 as a key regulator that transcriptionally repressed the expression of SET in both normal and cancer cells. In addition, p53 stimulated PP2A phosphatase activity via p53-mediated transcriptional repression of SET, whereby SET-mediated inhibition of PP2A was alleviated. Moreover, targeting the interaction between SET and PP2A catalytic subunit (PP2Ac) with FTY720 enhanced stress-induced p53 activation via PP2A-mediated dephosphorylation of p53 on threonine 55 (Thr55). Therefore, our findings uncovered a previously unknown p53-SET-PP2A regulatory feedback loop. To functionally potentiate this feedback loop, we designed a combined therapeutic strategy by simultaneously administrating a p53 activator and SET antagonist in cancer cells and observed a dramatic synergistic effect on tumor suppression. Our study reveals mechanistic insight into the regulation of the oncoprotein SET and raises a potential strategy for cancer therapy by stimulating the p53-SET-PP2A feedback loop.

Thirty years of SET/TAF1β/I2PP2A: from the identification of the biological functions to its implications in cancer and Alzheimer's disease

The gene encoding for the protein SE translocation (SET) was identified for the first time 30 years ago as part of a chromosomal translocation in a patient affected by leukemia. Since then, accumulating evidence have linked overexpression of SET, aberrant SET splicing, and cellular localization to cancer progression and development of neurodegenerative tauopathies such as Alzheimer's disease. Molecular biology tools, such as targeted genetic deletion, and pharmacological approaches based on SET antagonist peptides, have contributed to unveil the molecular functions of SET and its implications in human pathogenesis. In this review, we provide an overview of the functions of SET as inhibitor of histone and non-histone protein acetylation and as a potent endogenous inhibitor of serine-threonine phosphatase PP2A. We discuss the role of SET in multiple cellular processes, including chromatin remodelling and gene transcription, DNA repair, oxidative stress, cell cycle, apoptosis cell migration and differentiation. We review the molecular mechanisms linking SET dysregulation to tumorigenesis and discuss how SET commits neurons to progressive cell death in Alzheimer's disease, highlighting the rationale of exploiting SET as a therapeutic target for cancer and neurodegenerative tauopathies.

Vascular smooth muscle RhoA counteracts abdominal aortic aneurysm formation by modulating MAP4K4 activity

Whether a small GTPase RhoA plays a role in the pathology of abdominal aortic aneurysm (AAA) has not been determined. We show here that RhoA expression is reduced in human AAA lesions, compared with normal areas. Furthermore, incidence of AAA formation is increased in vascular smooth muscle cell (VSMC)-specific RhoA conditional knockout (cKO) mice. The contractility of the aortic rings and VSMCs from RhoA cKO mice is reduced, and expression of genes related to the VSMC contractility is attenuated by loss of RhoA. RhoA depletion activates the mitogen-activated protein (MAP) kinase signaling, including MAP4K4, in the aorta and VSMCs. Inhibition of MAP4K4 activity by DMX-5804 decreases AAA formation. Set, a binding protein to active RhoA, functions as an activator of MAP4K4 by sequestering PP2A, an inhibitor of MAP4K4, in the absence of RhoA. In conclusion, RhoA counteracts AAA formation through inhibition of MAP4K4 in cooperation with Set.

PP2A is activated by cytochrome c upon formation of a diffuse encounter complex with SET/TAF-Iβ

Intrinsic protein flexibility is of overwhelming relevance for intermolecular recognition and adaptability of highly dynamic ensemble of complexes, and the phenomenon is essential for the understanding of numerous biological processes. These conformational ensembles-encounter complexes-lack a unique organization, which prevents the determination of well-defined high resolution structures. This is the case for complexes involving the oncoprotein SET/template-activating factor-Iβ (SET/TAF-Iβ), a histone chaperone whose functions and interactions are significantly affected by its intrinsic structural plasticity. Besides its role in chromatin remodeling, SET/TAF-Iβ is an inhibitor of protein phosphatase 2A (PP2A), which is a key phosphatase counteracting transcription and signaling events controlling the activity of DNA damage response (DDR) mediators. During DDR, SET/TAF-Iβ is sequestered by cytochrome c (Cc) upon migration of the hemeprotein from mitochondria to the cell nucleus. Here, we report that the nuclear SET/TAF-Iβ:Cc polyconformational ensemble is able to activate PP2A. In particular, the N-end folded, globular region of SET/TAF-Iβ (a.k.a. SET/TAF-Iβ ΔC)-which exhibits an unexpected, intrinsically highly dynamic behavior-is sufficient to be recognized by Cc in a diffuse encounter manner. Cc-mediated blocking of PP2A inhibition is deciphered using an integrated structural and computational approach, combining small-angle X-ray scattering, electron paramagnetic resonance, nuclear magnetic resonance, calorimetry and molecular dynamics simulations.

PPP2CA Is a Novel Therapeutic Target in Neuroblastoma Cells That Can Be Activated by the SET Inhibitor OP449

Neuroblastoma (NB) is the most common extracranial solid tumor in childhood and has a poor prognosis in high-risk cases, requiring novel therapies. Pathways that depend on phospho-signaling maintain the aggressiveness of NB. Protein phosphatase 2 (PP2A) with its catalytic subunit PPP2CA is a major phosphatase in cancer cells, including NB. We show that reduction of PPP2CA by knock-down decreased growth of NB cells and that complete ablation of PPP2CA by knock-out was not tolerated. Thus, NB cells are addicted to PPP2CA, an addiction augmented by MYCN activation. SET, a crucial endogenous inhibitor of PP2A, was overexpressed in poor-prognosis NB. The SET inhibitor OP449 effectively decreased the viability of NB cells, independent of their molecular alterations and in line with a tumor suppressor function of PPP2CA. The contrasting concentration-dependent functions of PPP2CA as an essential survival gene at low expression levels and a tumor suppressor at high levels are reminiscent of other genes showing this so-called Goldilocks phenomenon. PP2A reactivated by OP449 decreased activating phosphorylation of serine/threonine residues in the AKT pathway. Conversely, induced activation of AKT led to partial rescue of OP449-mediated viability inhibition. Dasatinib, a kinase inhibitor used in relapsed/refractory NB, and OP449 synergized, decreasing activating AKT phosphorylations. In summary, concomitantly reactivating phosphatases and inhibiting kinases with a combination of OP449 and dasatinib are promising novel therapeutic approaches to NB.

Pre-Clinical Study Evaluating Novel Protein Phosphatase 2A Activators as Therapeutics for Neuroblastoma

BACKGROUND

Protein phosphatase 2A (PP2A) functions as an inhibitor of cancer cell proliferation, and its tumor suppressor function is attenuated in many cancers. Previous studies utilized FTY720, an immunomodulating compound known to activate PP2A, and demonstrated a decrease in the malignant phenotype in neuroblastoma. We wished to investigate the effects of two novel PP2A activators, ATUX-792 (792) and DBK-1154 (1154).

METHODS

Long-term passage neuroblastoma cell lines and human neuroblastoma patient-derived xenograft (PDX) cells were used. Cells were treated with 792 or 1154, and viability, proliferation, and motility were examined. The effect on tumor growth was investigated using a murine flank tumor model.

RESULTS

Treatment with 792 or 1154 resulted in PP2A activation, decreased cell survival, proliferation, and motility in neuroblastoma cells. Immunoblotting revealed a decrease in MYCN protein expression with increasing concentrations of 792 and 1154. Treatment with 792 led to tumor necrosis and decreased tumor growth in vivo.

CONCLUSIONS

PP2A activation with 792 or 1154 decreased survival, proliferation, and motility of neuroblastoma in vitro and tumor growth in vivo. Both compounds resulted in decreased expression of the oncogenic protein MYCN. These findings indicate a potential therapeutic role for these novel PP2A activators in neuroblastoma.

Second-Generation JK-206 Targets the Oncogenic Signal Mediator RHOA in Gastric Cancer

Ras homologous A (RHOA), a signal mediator and a GTPase, is known to be associated with the progression of gastric cancer (GC), which is the fourth most common cause of death in the world. Previously, we designed pharmacologically optimized inhibitors against RHOA, including JK-136 and JK-139. Based on this previous work, we performed lead optimization and designed novel RHOA inhibitors for greater anti-GC potency. Two of these compounds, JK-206 and JK-312, could successfully inhibit the viability and migration of GC cell lines. Furthermore, using transcriptomic analysis of GC cells treated with JK-206, we revealed that the inhibition of RHOA might be associated with the inhibition of the mitogenic pathway. Therefore, JK-206 treatment for RHOA inhibition may be a new therapeutic strategy for regulating GC proliferation and migration.

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.

Bi-Functional Peptides as a New Therapeutic Tool for Hepatocellular Carcinoma

Background: The interfering peptides that block protein–protein interactions have been receiving increasing attention as potential therapeutic tools. Methods: We measured the internalization and biological effect of four bi-functional tumor-penetrating and interfering peptides into primary hepatocytes isolated from three non-malignant and 11 hepatocellular carcinomas. Results: These peptides are internalized in malignant hepatocytes but not in non-malignant cells. Furthermore, the degree of peptide internalization correlated with receptor expression level and tumor aggressiveness levels. Importantly, penetration of the peptides iRGD-IP, LinTT1-IP, TT1-IP, and RPARPAR-IP induced apoptosis of the malignant hepatocytes without effect on non-malignant cells. Conclusion: Receptor expression levels correlated with the level of peptide internalization and aggressiveness of the tumor. This study highlights the potential to exploit the expression of tumor-penetrating peptide receptors as a predictive marker of liver tumor aggressiveness. These bi-functional peptides could be developed for personalized tumor treatment.

Deregulation of protein phosphatase 2A inhibitor SET is associated with malignant progression in breast cancer

To understand the mechanism underlying metastasis, identification of a mechanism-based and common biomarker for circulating tumour cells (CTCs) in heterogenous breast cancer is needed. SET, an endogenous inhibitor of protein phosphatase 2A, was overexpressed in all subtypes of invasive breast carcinoma tissues. Treatment with SET-targeted siRNAs reduced the motility of MCF-7 and MDA-MB-231 cells in transwell assay. SET knockdown reduced the number of mammospheres by 60–70% in MCF-7 and MDA-MB-231 cells, which was associated with the downregulation of OCT4 and SLUG. Hence, we analysed the presence of SET-expressing CTCs (SET-CTCs) in 24 breast cancer patients. CTCs were enriched using a size-based method and then immunocytochemically analysed using an anti-SET antibody. SET-CTCs were detected in 6/6 (100%) patients with recurrent breast cancer with a median value of 12 (12 cells/3 mL blood), and in 13/18 (72.2%) patients with stage I–III breast cancer with a median value of 2.5, while the median value of healthy controls was 0. Importantly, high numbers of SET-CTCs were correlated with lymph node metastasis in patients with stage I–III disease. Our results indicate that SET contributes to breast cancer progression and can act as a potential biomarker of CTCs for the detection of metastasis.

Targeting of SET/I2PP2A oncoprotein inhibits Gli1 transcription revealing a new modulator of Hedgehog signaling

The Hedgehog (Hh)/Gli signaling pathway controls cell proliferation and differentiation, is critical for the development of nearly every tissue and organ in vertebrates and is also involved in tumorigenesis. In this study, we characterize the oncoprotein SET/I2PP2A as a novel regulator of Hh signaling. Our previous work has shown that the zebrafish homologs of SET are expressed during early development and localized in the ciliated organs. In the present work, we show that CRISPR/Cas9-mediated knockdown of setb gene in zebrafish embryos resulted in cyclopia, a characteristic patterning defect previously reported in Hh mutants. Consistent with these findings, targeting setb gene using CRISPR/Cas9 or a setb morpholino, reduced Gli1-dependent mCherry expression in the Hedgehog reporter zebrafish line Tg(12xGliBS:mCherry-NLS). Likewise, SET loss of function by means of pharmacological inhibition and gene knockdown prevented the increase of Gli1 expression in mammalian cells in vitro. Conversely, overexpression of SET resulted in an increase of the expression of a Gli-dependent luciferase reporter, an effect likely attributable to the relief of the Sufu-mediated inhibition of Gli1. Collectively, our data support the involvement of SET in Gli1-mediated transcription and suggest the oncoprotein SET/I2PP2A as a new modulator of Hedgehog signaling.

Strategies for Targeting Serine/Threonine Protein Phosphatases with Small Molecules in Cancer

Among numerous posttranslational regulation patterns, phosphorylation is reversibly controlled by the balance of kinases and phosphatases. The major form of cellular signaling involves the reversible phosphorylation of proteins on tyrosine, serine, or threonine residues. However, altered phosphorylation levels are found in diverse diseases, including cancer, making kinases and phosphatases ideal drug targets. In contrast to the success of prosperous kinase inhibitors, design of small molecules targeting phosphatase is struggling due to past bias and difficulty. This is especially true for serine/threonine phosphatases, one of the largest phosphatase families. From this perspective, we aim to provide insights into serine/threonine phosphatases and the small molecules targeting these proteins for drug development, especially in cancer. Through highlighting the modulation strategies, we aim to provide basic principles for the design of small molecules and future perspectives for the application of drugs targeting serine/threonine phosphatases.

The interaction of SET and protein phosphatase 2A as target for cancer therapy

In cancer cells, tumor suppressor proteins loss-of-function are usually the result of genetic mutations. Protein Phosphatase 2A is a tumor suppressor that inactivates several signaling pathways through removal of phosphate residues important for other proteins stability and/or activation. Different from other tumor suppressors, PP2A is, in many cancer types, inactivated by endogenous inhibitors. In physiological conditions, these inhibitors are important to balance PP2A activity. However, in cancer cells, overexpression of these inhibitors can keep PP2A inactive, resulting in sustained activation of mitogenic signaling pathways and transcription factors, metabolic reprogramming, with the resulting cancer progression and the resistance to anti-cancer therapies. One of these endogenous inhibitors is the protein SET (SE Translocation). SET is a multifunctional protein, which high expression has been associated with several types of cancer, as well as other diseases such as Alzheimer's disease. Disruption of the interaction between SET and PP2A to rescue the activity of PP2A may represent a new therapeutic strategy and opportunity for cancer treatment. This review brings up-to-date advances on the interactions between SET and PP2A and their biological consequences. Moreover, we review reported inhibitors of SET-PP2A interaction under investigation as therapeutic opportunities for the treatment of cancers.

Proteomic Analysis on Anti-Proliferative and Apoptosis Effects of Curcumin Analog, 1,5-bis(4-Hydroxy-3-Methyoxyphenyl)-1,4-Pentadiene-3-One-Treated Human Glioblastoma and Neuroblastoma Cells

Curcumin analogs with excellent biological properties have been synthesized to address and overcome the poor pharmacokinetic profiles of curcumin. This study aims to investigate the cytotoxicity, anti-proliferative, and apoptosis-inducing ability of curcumin analog, MS13 on human glioblastoma U-87 MG, and neuroblastoma SH-SY5Y cells, and to examine the global proteome changes in these cells following treatment. Our current findings showed that MS13 induced potent cytotoxicity and anti-proliferative effects on both cells. Increased caspase-3 activity and decreased bcl-2 concentration upon treatment indicate that MS13 induces apoptosis in these cells in a dose- and time-dependent manner. The label-free shotgun proteomic analysis has defined the protein profiles in both glioblastoma and neuroblastoma cells, whereby a total of nine common DEPs, inclusive of glyceraldehyde 3-phosphate dehydrogenase (GAPDH), alpha-enolase (ENO1), heat shock protein HSP 90-alpha (HSP90AA1), Heat shock protein HSP 90-beta (HSP90AB1), Eukaryotic translation initiation factor 5A-1 (EFI5A), heterogenous nuclear ribonucleoprotein K (HNRNPK), tubulin beta chain (TUBB), histone H2AX (H2AFX), and Protein SET were identified. Pathway analysis further elucidated that MS13 may induce its anti-tumor effects in both cells via the common enriched pathways, “Glycolysis” and “Post-translational protein modification.” Conclusively, MS13 demonstrates an anti-cancer effect that may indicate its potential use in the management of brain malignancies.

SET levels contribute to cohesion fatigue

Chromosome instability (CIN) is a major hallmark of cancer cells and believed to drive tumor progression. Several cellular defects including weak centromeric cohesion are proposed to promote CIN, but the molecular mechanisms underlying these defects are poorly understood. In a screening for SET protein levels in various cancer cell lines, we found that most of the cancer cells exhibit higher SET protein levels than nontransformed cells, including RPE-1. Cancer cells with elevated SET often show weak centromeric cohesion, revealed by MG132-induced cohesion fatigue. Partial SET knockdown largely strengthens centromeric cohesion in cancer cells without increasing overall phosphatase 2A (PP2A) activity. Pharmacologically increased PP2A activity in these cancer cells barely ameliorates centromeric cohesion. These results suggest that compromised PP2A activity, a common phenomenon in cancer cells, may not be responsible for weak centromeric cohesion. Furthermore, centromeric cohesion in cancer cells can be strengthened by ectopic Sgo1 overexpression and weakened by SET WT, not by Sgo1-binding-deficient mutants. Altogether, these findings demonstrate that SET overexpression contributes to impaired centromeric cohesion in cancer cells and illustrate misregulated SET-Sgo1 pathway as an underlying mechanism.

Inhibition of the PP2A activity by the histone chaperone ANP32B is long-range allosterically regulated by respiratory cytochrome c

Repair of injured DNA relies on nucleosome dismantling by histone chaperones and de-phosphorylation events carried out by Protein Phosphatase 2A (PP2A). Typical histone chaperones are the Acidic leucine-rich Nuclear Phosphoprotein 32 family (ANP32) members, e.g. ANP32A, which is also a well-known PP2A inhibitor (a.k.a. I₁PP2A). Here we report the novel interaction between the endogenous family member B—so-called ANP32B—and endogenous cytochrome c in cells undergoing camptothecin-induced DNA damage. Soon after DNA lesions but prior to caspase cascade activation, the hemeprotein translocates to the nucleus to target the Low Complexity Acidic Region (LCAR) of ANP32B; in a similar way, our group recently reported that the hemeprotein targets the acidic domain of SET/Template Activating Factor-Iβ (SET/TAF-Iβ), which is another histone chaperone and PP2A inhibitor (a.k.a. I₂PP2A). The nucleosome assembly activity of ANP32B is indeed unaffected by cytochrome c binding. Like ANP32A, ANP32B inhibits PP2A activity and is thus herein referred to as I₃PP2A. Our data demonstrates that ANP32B-dependent inhibition of PP2A is regulated by respiratory cytochrome c, which induces long-distance allosteric changes in the structured N-terminal domain of ANP32B upon binding to the C-terminal LCAR. In agreement with the reported role of PP2A in the DNA damage response, we propose a model wherein cytochrome c is translocated from the mitochondria into the nucleus upon DNA damage to modulate PP2A activity via its interaction with ANP32B.

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Respiratory cytochrome c interacts with ANP32B under DNA damage in the nucleus.

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Cytochrome c binding to ANP32B LCAR restores ANP32B-mediated PP2A inhibition.

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Cytochrome c emerges as a DNA Damage Response regulator.

SET knockdown attenuated phenotype modulation and calcium channel associated markers of airway smooth muscle cells in asthmatic mice

Background

Dysfunctional phenotype modulation and calcium channels in airway smooth muscle cells (ASMCs) are important characteristics of airway remodeling in chronic asthma. However, the mechanisms underlying these pathological processes remain unclear. SET (I2PP2A, inhibitor-2 of protein phosphatase 2A) has many significant functions and is involved in various physiological and pathological processes. This study aimed to determine the function of SET in chronic asthma.

Methods

BALB/c mice were sensitized by ovalbumin injection and repeated inhalation of ovalbumin. The Penh value was measured using the Buxco whole body plethysmography system. A short hairpin RNA of the SET gene was designed and transfected into ASMCs derived from asthmatic mice. Flow cytometry of Annexin-V/propidium iodide staining was used for evaluating cell apoptosis. Western blot was adopted to measure the expression levels of ASMCs phenotype modulation markers and calcium channel-associated proteins.

Results

The results showed that shRNA targeting SET significantly decreased the expression of SET, and enhanced the apoptosis of ASMCs. SET knockdown promoted the expression of contractile phenotype markers such as α-SMA (alpha smooth muscle Actin), SM-MHC (smooth muscle Myosin heavy chain), and calponin, and inhibited the expression of synthetic phenotype markers including vimentin and CD44. The expression of the calcium channel-related proteins STIM1 (Stromal interaction molecule 1) and Orai1 were also inhibited after SET knockdown.

Conclusions

These data demonstrated that SET participated in the development of airway dysfunction in asthma, suggesting that the silencing of SET may be a new therapeutic target for the treatment of asthma patients.

Expression of Phosphorylated BRD4 Is Markedly Associated with the Activation Status of the PP2A Pathway and Shows a Strong Prognostic Value in Triple Negative Breast Cancer Patients

Simple Summary

The use of BRD4 inhibitors has emerged as a novel therapeutic approach in a wide variety of tumors including the triple negative breast cancer. Moreover, PP2A has been proposed as the phosphatase involved in regulating BRD4 phosphorylation and stabilization. Our aim was to evaluate for the first time the clinical impact of BRD4 phosphorylation in triple negative breast cancer patients and as well as its potential linking with the PP2A activation status in this disease. Our findings are special relevant since they suggest the prognostic value of BRD4 phosphorylation levels, and the potential clinical usefulness of PP2A inhibition markers to anticipate response to BRD4 inhibitors.

Abstract

The bromodomain-containing protein 4 (BRD4), a member of the bromodomain and extra-terminal domain (BET) protein family, has emerged in the last years as a promising molecular target in many tumors including breast cancer. The triple negative breast cancer (TNBC) represents the molecular subtype with the worst prognosis and a current therapeutic challenge, and TNBC cells have been reported to show a preferential sensitivity to BET inhibitors. Interestingly, BRD4 phosphorylation (pBRD4) was found as an alteration that confers resistance to BET inhibition and PP2A proposed as the phosphatase responsible to regulate pBRD4 levels. However, the potential clinical significance of pBRD4, as well as its potential correlation with the PP2A pathway in TNBC, remains to be investigated. Here, we evaluated the expression levels of pBRD4 in a series of 132 TNBC patients. We found high pBRD4 levels in 34.1% of cases (45/132), and this alteration was found to be associated with the development of patient recurrences (p = 0.007). Interestingly, BRD4 hyperphosphorylation predicted significantly shorter overall (p < 0.001) and event-free survival (p < 0.001). Moreover, multivariate analyses were performed to confirm its independent prognostic impact in our cohort. In conclusion, our findings show that BRD4 hyperphosphorylation is an alteration associated with PP2A inhibition that defines a subgroup of TNBC patients with unfavorable prognosis, suggesting the potential clinical and therapeutic usefulness of the PP2A/BRD4 axis as a novel molecular target to overcome resistance to treatments based on BRD4 inhibition.

Exploring and exploiting plant cyclic peptides for drug discovery and development

Ever since the discovery of insulin, natural peptides have become an important resource for therapeutic development. Decades of research has led to the discovery of a long list of peptide drugs with broad applications in clinics, from antibiotics to hypertension treatment to pain management. Many of these US FDA-approved peptide drugs are derived from microorganisms and animals. By contrast, the great potential of plant cyclic peptides as therapeutics remains largely unexplored. These macrocyclic peptides typically have rigid structures, good bioavailability and membrane permeability, making them appealing candidates for drug development and engineering. In this review, we introduce the three major classes of plant cyclic peptides and summarize their potential medical applications. We discuss how we can leverage the genome information of many different plants to quickly search for new cyclic peptides and how we can take advantage of the insights gained from their biosynthetic pathways to transform the process of production and drug development. These recent developments have provided a new angle for exploring and exploiting plant cyclic peptides, and we believe that many more peptide drugs derived from plants are about to come.

New Cytotoxic Cerebrosides from the Red Sea Cucumber Holothuria spinifera Supported by In-Silico Studies

Bioactivity-guided fractionation of a methanolic extract of the Red Sea cucumber Holothuria spinifera and LC-HRESIMS-assisted dereplication resulted in the isolation of four compounds, three new cerebrosides, spiniferosides A (1), B (2), and C (3), and cholesterol sulfate (4). The chemical structures of the isolated compounds were established on the basis of their 1D NMR and HRMS spectral data. Metabolic profiling of the H. spinifera extract indicated the presence of diverse secondary metabolites, mostly hydroxy fatty acids, diterpenes, triterpenes, and cerebrosides. The isolated compounds were tested for their in vitro cytotoxicities against the breast adenocarcinoma MCF-7 cell line. Compounds 1, 2, 3, and 4 displayed promising cytotoxic activities against MCF-7 cells, with IC50 values of 13.83, 8.13, 8.27, and 35.56 µM, respectively, compared to that of the standard drug doxorubicin (IC50 8.64 µM). Additionally, docking studies were performed for compounds 1, 2, 3, and 4 to elucidate their binding interactions with the active site of the SET protein, an inhibitor of protein phosphatase 2A (PP2A), which could explain their cytotoxic activity. This study highlights the important role of these metabolites in the defense mechanism of the sea cucumber against fouling organisms and the potential uses of these active molecules in the design of new anticancer agents.

Disclosing the Interactome of Leukemogenic NUP98-HOXA9 and SET-NUP214 Fusion Proteins Using a Proteomic Approach

The interaction of oncogenes with cellular proteins is a major determinant of cellular transformation. The NUP98-HOXA9 and SET-NUP214 chimeras result from recurrent chromosomal translocations in acute leukemia. Functionally, the two fusion proteins inhibit nuclear export and interact with epigenetic regulators. The full interactome of NUP98-HOXA9 and SET-NUP214 is currently unknown. We used proximity-dependent biotin identification (BioID) to study the landscape of the NUP98-HOXA9 and SET-NUP214 environments. Our results suggest that both fusion proteins interact with major regulators of RNA processing, with translation-associated proteins, and that both chimeras perturb the transcriptional program of the tumor suppressor p53. Other cellular processes appear to be distinctively affected by the particular fusion protein. NUP98-HOXA9 likely perturbs Wnt, MAPK, and estrogen receptor (ER) signaling pathways, as well as the cytoskeleton, the latter likely due to its interaction with the nuclear export receptor CRM1. Conversely, mitochondrial proteins and metabolic regulators are significantly overrepresented in the SET-NUP214 proximal interactome. Our study provides new clues on the mechanistic actions of nucleoporin fusion proteins and might be of particular relevance in the search for new druggable targets for the treatment of nucleoporin-related leukemia.

Potential roles of PP2A-Rac1 signaling axis in pancreatic β-cell dysfunction under metabolic stress: Progress and promise

Recent estimates by the International Diabetes Federation suggest that the incidence of diabetes soared to an all-time high of 463 million in 2019, and the federation predicts that by 2045 the number of individuals afflicted with this disease will increase to 700 million. Therefore, efforts to understand the pathophysiology of diabetes are critical for moving toward the development of novel therapeutic strategies for this disease. Several contributors (oxidative stress, endoplasmic reticulum stress and others) have been proposed for the onset of metabolic dysfunction and demise of the islet β-cell leading to the pathogenesis of diabetes. Existing experimental evidence revealed sustained activation of PP2A and Rac1 in pancreatic β-cells exposed to metabolic stress (diabetogenic) conditions. Evidence in a variety of cell types implicates modulatory roles for specific signaling proteins (α4, SET, nm23-H1, Pak1) in the functional regulation of PP2A and Rac1. In this Commentary, I overviewed potential cross-talk between PP2A and Rac1 signaling modules in the onset of metabolic dysregulation of the islet β-cell leading to impaired glucose-stimulated insulin secretion (GSIS), loss of β-cell mass and the onset of diabetes. Potential knowledge gaps and future directions in this fertile area of islet biology are also highlighted. It is hoped that this Commentary will provide a basis for future studies toward a better understanding of roles of PP2A-Rac1 signaling module in pancreatic β-cell dysfunction, and identification of therapeutic targets for the treatment of islet β-cell dysfunction in diabetes.

Selective PP2A Enhancement through Biased Heterotrimer Stabilization

Impairment of protein phosphatases, including the family of serine/threonine phosphatases designated PP2A, is essential for the pathogenesis of many diseases, including cancer. The ability of PP2A to dephosphorylate hundreds of proteins is regulated by over 40 specificity-determining regulatory "B" subunits that compete for assembly and activation of heterogeneous PP2A heterotrimers. Here, we reveal how a small molecule, DT-061, specifically stabilizes the B56α-PP2A holoenzyme in a fully assembled, active state to dephosphorylate selective substrates, such as its well-known oncogenic target, c-Myc. Our 3.6 Å structure identifies molecular interactions between DT-061 and all three PP2A subunits that prevent dissociation of the active enzyme and highlight inherent mechanisms of PP2A complex assembly. Thus, our findings provide fundamental insights into PP2A complex assembly and regulation, identify a unique interfacial stabilizing mode of action for therapeutic targeting, and aid in the development of phosphatase-based therapeutics tailored against disease specific phospho-protein targets.

CIP2A Constrains Th17 Differentiation by Modulating STAT3 Signaling

Cancerous Inhibitor of Protein Phosphatase 2A (CIP2A) is an oncogene and a potential cancer therapy target protein. Accordingly, a better understanding of the physiological function of CIP2A, especially in the context of immune cells, is a prerequisite for its exploitation in cancer therapy. Here, we report that CIP2A negatively regulates interleukin (IL)-17 production by Th17 cells in human and mouse. Interestingly, concomitant with increased IL-17 production, CIP2A-deficient Th17 cells had increased strength and duration of STAT3 phosphorylation. We analyzed the interactome of phosphorylated STAT3 in CIP2A-deficient and CIP2A-sufficient Th17 cells and indicated together with genome-wide gene expression profiling, a role of Acylglycerol Kinase (AGK) in the regulation of Th17 differentiation by CIP2A. We demonstrated that CIP2A regulates the strength of the interaction between AGK and STAT3, and thereby modulates STAT3 phosphorylation and expression of IL-17 in Th17 cells.

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.

Protocadherin 7 is overexpressed in castration resistant prostate cancer and promotes aberrant MEK and AKT signaling

BACKGROUND

Castrate resistant prostate cancer (CRPC) accounts for almost all prostate cancer (PCa) deaths. Aberrant activation of ERK/MEK and PI3K/AKT signaling pathways plays an important role in subsets of patients with CRPC. The role of protocadherin 7 (PCDH7) in modulating these signaling pathways is investigated for the first time in PCa in the present investigation.

METHODS

PCDH7 expression was analyzed in CRPC/neuroendocrine prostate cancer (NEPC) dataset. Protein expression was assessed by Western blotting and immunohistochemistry, and messenger RNA (mRNA) by quantitative real-time polymerase chain reaction. Small hairpin ribonucleic acid was used to knockdown PCDH7. Colony formation, cell migration, and invasion studies were done using standard protocols.

RESULTS

PCDH7 amplification/mRNA upregulation was observed in 41% of patients in CRPC/NEPC dataset. PCDH7 was also overexpressed in CRPC cells. Increased PCDH protein expression was observed during tumor progression in PCa tissues and in TRAMP mice. Epidermal growth factor treatment resulted in aberrant activation of ERK/AKT. Knockdown of PCDH7 decreased ERK, AKT, and RB phosphorylation and reduced colony formation, decreased cell invasion, and cell migration.

CONCLUSIONS

These data show for the first time that PCDH7 is overexpressed in a large number of patients with CRPC and suggest that PCDH7 may be an attractive target in subsets of patients with CRPC for whom there is no cure to-date.

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.

Novel secretome-to-transcriptome integrated or secreto-transcriptomic approach to reveal liquid biopsy biomarkers for predicting individualized prognosis of breast cancer patients

Background

Presently, a 50-gene expression model (PAM50) serves as a breast cancer (BC) subtype classifier that is insufficient to distinguish, within each single PAM50-classified subtype, patient subpopulations having different prognosis. There is a pressing need for inexpensive and minimally invasive biomarker tests to easily and accurately predict individuals’ clinical outcomes and response to treatments. Although quantitative proteomic approaches have been developed to identify/profile proteins secreted (secretome) from various cancer cell lines in vitro, missing are the clinicopathological relevance and the associated prognostic value of these secretomic identifications.

Methods

To discover biomarkers to predict individualized prognosis we introduce a new multi-omics (secreto-transcriptomics) method that identifies, in their oncogenically secreted states, candidate markers of BC subtypes whose genes bear patient-specific mRNA expression alterations of prognostic significance. First, we used label-free quantitative (LFQ) proteomics to identify the proteins showing BC-subtypic secretion from a series of BC cell lines representing major BC-subtypes. To determine and externally validate the prognostic value of these secreted proteins, we developed a secreto-transcriptomic approach that discovered a PAM50-subtypic Secretion-Correlated mRNA Expression Pattern (SeCEP) wherein the PAM50-subtypic secretion of select proteins statistically correlated with cis-mRNA expression of their encoding genes in patients of the corresponding PAM50-subtypes. Kaplan-Meier analysis of SeCEP genes was used to identify new liquid biopsy biomarkers for predicting individualized prognosis.

Results

The mRNA expression-to-secretion correlation (SeCEP) pinpointed multiple genes that are fully translated into the oncogenically active secretome in a PAM50-subtypic manner. Further, multiple SeCEP genes in distinct combinations or panels of multiple SeCEP genes were identified as ‘systems prognostic markers’ that showed mRNA co-overexpression patterns in the distinct subpopulations of PAM50-subtypic patients with poor prognosis or high-risk of relapse. Thus, our secreto-transcriptomic approach statistically linked BC subtypic secretome genes with patient-specific information about their mRNA expression alterations and significantly improved the sensitivity and specificity in patient stratification in the context of clinical outcomes or prognosis.

Conclusions

By combining LFQ secretome screening with proteo-transcriptomic retrospective analysis of patient data our integrated multi-omics approach bypasses costly, tedious, genome-wide fishing and predictive modeling that are commonly required to distinguish a few prognostically altered genes from thousands of other non-BC related genes in a genome.

Electronic supplementary material

The online version of this article (10.1186/s12920-019-0530-7) contains supplementary material, which is available to authorized users.

Cyclin-dependent kinase 1-mediated phosphorylation of SET at serine 7 is essential for its oncogenic activity

SE translocation (SET), an inhibitor of protein phosphatase 2A (PP2A), plays important roles in mitosis and possesses oncogenic activity in several types of cancer. However, little is known regarding its regulation. Here we reveal a novel phosphorylation site of SET isoform 1, and we have determined its biological significance in tumorigenesis. We found that the mitotic kinase cyclin-dependent kinase 1 (CDK1) phosphorylates SET isoform 1 in vitro and in vivo at serine 7 during antitubulin drug-induced mitotic arrest and normal mitosis. SET deletion resulted in massive multipolar spindles, chromosome misalignment and missegregation, and centrosome amplification during mitosis. Moreover, mitotic phosphorylation of SET isoform 1 is required for cell migration, invasion, and anchorage-independent growth in vitro and tumorigenesis in xenograft animal models. We further documented that SET phosphorylation affects Akt activity. Collectively, our findings suggest that SET isoform 1 promotes oncogenesis in a mitotic phosphorylation-dependent manner.

Deregulation of SET is Associated with Tumor Progression and Predicts Adverse Outcome in Patients with Early-Stage Colorectal Cancer

SET nuclear proto-oncogene (SET) deregulation is a novel molecular target in metastatic colorectal cancer (CRC). However, its role in CRC progression and its potential clinical impact in early-stage CRC patients remain unknown. Here, we studied the biological effects of SET on migration using wound-healing and transwell assays, and anchorage-independent cell growth using soft agar colony formation assays after ectopic SET modulation. SET was analyzed by immuno-staining in 231 early-stage CRC patients, and miR-199b expression was quantified by real-time PCR in a set of CRC patients. Interestingly, SET enhances cell migration, markedly affects the colony-forming ability, promotes epithelial to mesenchymal transition, and induces the expression of the MYC proto-oncogene (c-MYC) in CRC cells. SET overexpression was detected in 15.4% of cases and was associated with worse Eastern Cooperative Oncology Group (ECOG) status (p = 0.021) and relapse in stage-II CRC patients (p = 0.008). Moreover, SET overexpression predicted shorter overall survival (p < 0.001) and time to metastasis (p < 0.001), and its prognostic value was particularly evident in elderly patients. MiR-199b downregulation was identified as a molecular mechanism to deregulate SET in patients with localized disease. In conclusion, SET overexpression is a common alteration in early-stage CRC, playing an oncogenic role associated with progression and aggressiveness, and portends a poor outcome. Thus, SET emerges as a novel potential molecular target with clinical impact in early-stage in CRC.

Targeting SET to restore PP2A activity disrupts an oncogenic CIP2A-feedforward loop and impairs triple negative breast cancer progression

Background

Triple-negative breast cancer (TNBC) remains difficult to be targeted. SET and cancerous inhibitor of protein phosphatase 2A (CIP2A) are intrinsic protein-interacting inhibitors of protein phosphatase 2A (PP2A) and frequently overexpressed in cancers, whereas reactivating PP2A activity has been postulated as an anti-cancer strategy. Here we explored this strategy in TNBC.

Methods

Data from The Cancer Genome Atlas (TCGA) database was analyzed. TNBC cell lines were used for in vitro studies. Cell viability was examined by MTT assay. The apoptotic cells were examined by flow cytometry and Western blot. A SET-PP2A protein-protein interaction antagonist TD19 was used to disrupt signal transduction. In vivo efficacy of TD19 was tested in MDA-MB-468-xenografted animal model.

Findings

TCGA data revealed upregulation of SET and CIP2A and positive correlation of these two gene expressions in TNBC tumors. Ectopic SET or CIP2A increased cell viability, migration, and invasion of TNBC cells. Notably ERK inhibition increased PP2A activity. ERK activation is known crucial for Elk-1 activity, a transcriptional factor regulating CIP2A expression, we hypothesized an oncogenic feedforward loop consisting of pERK/pElk-1/CIP2A/PP2A. This loop was validated by knockdown of PP2A and ectopic expression of Elk-1, showing reciprocal changes in loop members. In addition, ectopic expression of SET increased pAkt, pERK, pElk-1 and CIP2A expressions, suggesting a positive linkage between SET and CIP2A signaling. Moreover, TD19 disrupted this CIP2A-feedforward loop by restoring PP2A activity, demonstrating in vitro and in vivo anti-cancer activity. Mechanistically, TD19 downregulated CIP2A mRNA via inhibiting pERK-mediated Elk-1 nuclear translocation thereby decreased Elk-1 binding to the CIP2A promoter.

Interpretation

These findings suggested that a novel oncogenic CIP2A-feedforward loop contributes to TNBC progression and targeting SET to disrupt this oncogenic CIP2A loop showed therapeutic potential in TNBC.

PP2A holoenzymes, substrate specificity driving cellular functions and deregulation in cancer

PP2A is a highly conserved eukaryotic serine/threonine protein phosphatase of the PPP family of phosphatases with fundamental cellular functions. In cells, PP2A targets specific subcellular locations and substrates by forming heterotrimeric holoenzymes, where a core dimer consisting of scaffold (A) and catalytic (C) subunits complexes with one of many B regulatory subunits. PP2A plays a key role in positively and negatively regulating a myriad of cellular processes, as it targets a very sizable fraction of the cellular substrates phosphorylated on Ser/Thr residues. This review focuses on insights made toward the understanding on how the subunit composition and structure of PP2A holoenzymes mediates substrate specificity, the role of substrate modulation in the signaling of cellular division, growth, and differentiation, and its deregulation in cancer.

The concept of the okadaic acid class of tumor promoters is revived in endogenous protein inhibitors of protein phosphatase 2A, SET and CIP2A, in human cancers

PURPOSE

The okadaic acid class of tumor promoters, which are inhibitors of protein phosphatases 1 and 2A (PP1 and PP2A), induced tumor promotion in mouse skin, rat glandular stomach, and rat liver. Endogenous protein inhibitors of PP2A, SET and CIP2A, were up-regulated in various human cancers, so it is vital to review the essential mechanisms of tumor promotion by the okadaic acid class compounds, together with cancer progression by SET and CIP2A in humans.

RESULTS AND DISCUSSION

The first part of this review introduces the okadaic acid class compounds and the mechanism of tumor promotion: (1) inhibition of PP1 and PP2A activities of the okadaic acid class compounds; (2) some topics of tumor promotion; (3) TNF-α gene expression as a central mediator in tumor promotion; (4) exposure to the okadaic acid class of tumor promoters in relation to human cancer. The second part emphasizes the overexpression of SET and CIP2A in cancer progression, and the anticancer activity of SET antagonists as follows: (5) isolation and characterization of SET; (6) isolation and characterization of CIP2A; (7) progression of leukemia with SET; (8) progression of breast cancer with SET and CIP2A; (9) progression of lung cancer with SET; (10) anti-carcinogenic effects of SET antagonists OP449 and FTY720; and also (11) TNF-α-inducing protein of Helicobacter pylori, which is a clinical example of the okadaic acid pathway.

CONCLUSIONS

The overexpression of endogenous protein inhibitors of PP2A, SET and CIP2A, is tightly linked to the progression of various human cancers, as well as Alzheimer's disease.

SET alpha and SET beta mRNA isoforms in chronic lymphocytic leukaemia

Alteration in RNA splicing is implicated in carcinogenesis and progression. Mutations in spliceosome genes and alternative splicing of other genes have been noted in chronic lymphocytic leukaemia (CLL), a common B cell malignancy with heterogeneous outcomes. We previously demonstrated that differences in the amount of SET oncoprotein (a physiological inhibitor of the serine/threonine phosphatase, PP2A) is associated with clinical aggressiveness in patients with CLL. It is unknown if alternative splicing of gene transcripts regulating kinases and phosphatases affects disease pathobiology and CLL progression. We show here for the first time that mRNA levels of the alternatively spliced SET isoforms, SETA and SETB (SETα and SETβ), significantly correlate with disease severity (overall survival and time-to-first-treatment) in CLL patients. In addition, we demonstrate that relative increase of SETA to SETB mRNA can discriminate patients with a more aggressive disease course within the otherwise favourable CLL risk classifications of IGHV mutated and favourable hierarchical fluorescence in situ hybridisation groups. We validate our finding by showing comparable relationships of SET mRNA with disease outcomes using samples from an independent CLL cohort from a separate institution. These findings indicate that alternative splicing of SET, and potentially other signalling cascade molecules, influences CLL biology and patient outcomes.

p53 Function Is Compromised by Inhibitor 2 of Phosphatase 2A in Sonic Hedgehog Medulloblastoma

Medulloblastomas, the most common malignant pediatric brain tumors, have been genetically defined into four subclasses, namely WNT-activated, Sonic Hedgehog (SHH)-activated, Group 3, and Group 4. Approximately 30% of medulloblastomas have aberrant SHH signaling and thus are referred to as SHH-activated medulloblastoma. The tumor suppressor gene TP53 has been recently recognized as a prognostic marker for patients with SHH-activated medulloblastoma; patients with mutant TP53 have a significantly worse outcome than those with wild-type TP53. It remains unknown whether p53 activity is impaired in SHH-activated, wild-type TP53 medulloblastoma, which is about 80% of the SHH-activated medulloblastomas. Utilizing the homozygous NeuroD2:SmoA1 mouse model with wild-type Trp53, which recapitulates human SHH-activated medulloblastoma, it was discovered that the endogenous Inhibitor 2 of Protein Phosphatase 2A (SET/I2PP2A) suppresses p53 function by promoting accumulation of phospho-MDM2 (S166), an active form of MDM2 that negatively regulates p53. Knockdown of I2PP2A in SmoA1 primary medulloblastoma cells reduced viability and proliferation in a p53-dependent manner, indicating the oncogenic role of I2PP2A. Importantly, this mechanism is conserved in the human medulloblastoma cell line ONS76 with wild-type TP53. Taken together, these findings indicate that p53 activity is inhibited by I2PP2A upstream of PP2A in SHH-activated and TP53-wildtype medulloblastomas. IMPLICATIONS: This study suggests that I2PP2A represents a novel therapeutic option and its targeting could improve the effectiveness of current therapeutic regimens for SHH-activated or other subclasses of medulloblastoma with wild-type TP53.