Tumor Suppressor Protein Phosphatase 2A Research Articles

Abstract 4717: Small molecule modulation of PP2A selectively induces oncogenic cytotoxicity through chronic and irreversible integrated stress response

Abstract High-grade serous cancer (HGSC) is the most lethal gynecological malignancy and accounts for approximately 70% of all ovarian carcinomas. Despite recent advancements in drug discovery and a deeper understanding of the genetic mechanisms that drive HGSC oncogenesis, the mortality rate has remained stagnant over the past three decades, with a 5-year survival rate below 30%. Due to the prevalence of homologous recombination defects (HRD) in over 50% of HGSC tumors, the clinical management of patients harboring these genetic vulnerabilities has vastly relied on PARP inhibitors (PARPi). Nonetheless, about 80% of these patients ultimately relapse with refractory resistant and more aggressive disease, having no effective therapeutic alternatives currently available. Therefore, there is an urgent need for novel targeted therapies to be tailored towards the treatment of HGSC. Recently, we unveiled that 95% of HGSC tumors present with a widespread loss-of-heterozygosity in genes essential to form the tumor suppressor Protein Phosphatase 2A (PP2A) heterotrimer, second only to TP53. These deleterious alterations have been established to inhibit PP2A function, preventing it from counteracting oncogenic signatures that drive tumorigenesis. Our studies revealed that a Small Molecule Activator of PP2A (SMAP) synergizes with PARPi to induce synthetic lethality in HGSC cells while displaying favorable tolerability profiles in normal tissues. Mechanistically, SMAP activates distinct translational responses and stress signatures depending on the cellular context, driving selective oncogenic cytotoxicity through a chronic and irreversible integrated stress response (ISR), independent of PERK activation. In malignant contexts, SMAP-mediated activation of PP2A specifically drives the dephosphorylation of TFE3 to transcriptionally activate ATF4 and CHOP. Consequently, as cancer cells are incapable of activating adaptive homeostatic recovery mechanisms in response to SMAP, chronic ISR is induced, ultimately leading to cell death. Conversely, non-transformed cells leverage their intrinsic ISR plasticity to adapt, thereby restoring basal functions that allow cell survival under SMAP-mediated stress conditions. Collectively, our data highlights a new role of PP2A in HGSC and further underscore SMAP's selective cytotoxicity in cancer cells, independent of oncogenic genetic vulnerabilities or HRD profiles, by leveraging ISR plasticity and intrinsic adaptive homeostatic mechanisms. Thus, this class of PP2A modulators holds tremendous promise for broader therapeutic applications across diverse patient populations and tumor histological subtypes beyond HGSC, while mitigating standard-of-care therapy-associated toxicity. Citation Format: Rita A. Avelar, Riya Gupta, Grace Carvette, Felipe da Veiga Leprevost, Jose Colina, Jessica Teitel, Goutham Narla, Analisa DiFeo. Small molecule modulation of PP2A selectively induces oncogenic cytotoxicity through chronic and irreversible integrated stress response [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4717.

Abstract IA012: The role of the serine/threonine phosphatase PP2A in endometrial cancer development and progression: lessons learned from mutations and small molecules

Abstract Uterine cancer is the most common gynecologic malignancy in the United States with approximately 60,000 new diagnoses each year. While many cases of uterine endometrial carcinoma (EMCA) have favorable outcomes with recurrence-free long-term survival, outcomes for the high-grade, treatment-refractory histological subtypes, including uterine serous carcinoma (USC) and uterine carcinosarcoma (UCS), remain an important clinical problem. Uterine serous carcinomas and uterine carcinosarcomas tumors are the most aggressive subtypes of uterine cancer, displaying a high propensity to invade the peritoneal cavity and to metastasize to distant sites. Accordingly, 50-70% of high-grade tumors exhibit extrauterine spread at time of initial surgery and have limited treatment options. The inherent aggressiveness of these tumors suggests that they are intrinsically equipped to disseminate and propagate tumors at secondary sites. The molecular drivers of this phenotype, however, have yet to be completely characterized. A lack of established and validated molecular drivers of the high-grade subtypes, especially those that are responsible for the high rate of metastasis, has limited the use of targeted treatment strategies. However, we and other groups have shown that PPP2R1A, the scaffolding subunit of the tumor suppressor protein phosphatase 2A, is mutated in up to 40% of USC patients, and these recurrent mutations are found at both the primary and metastatic sites. These mutations are heterozygous, and two specific mutations: P179R and S256F, occur exclusively within these high-grade subtypes of uterine cancer (i.e., are not found in any other cancer type). PP2A, is a family of serine/threonine phosphatases, where the active heterotrimeric form of the enzyme is comprised of a catalytic (C) subunit, a scaffolding (A) subunit, and a substrate directing regulatory (B) subunit. In aggregate, PP2A is considered a tumor suppressor protein, but certain heterotrimers have tumor promoting roles. Here we present data related to the molecular and biological basis through which these recurrent pathogenic mutations drive endometrial cancer development and progression using disease relevant cellular and in vivo model systems. Additionally, building from previously published work from our group, we have developed a novel series of small molecule molecular glues (PMGs) that specifically correct the mutant heterotrimer driving tumor regressions in both cellular and patient derived xenograft models of high-grade endometrial cancer. Collectively, this data suggests that recurrent pathogenic mutations in the PPP2R1A gene drive endometrial cancer pathogenesis and these mutations can be therapeutically targeted using a series of pharmaceutically tractable small molecules. Citation Format: Goutham Narla. The role of the serine/threonine phosphatase PP2A in endometrial cancer development and progression: lessons learned from mutations and small molecules [abstract]. In: Proceedings of the AACR Special Conference on Endometrial Cancer: Transforming Care through Science; 2023 Nov 16-18; Boston, Massachusetts. Philadelphia (PA): AACR; Clin Cancer Res 2024;30(5_Suppl):Abstract nr IA012.

P17.12.B FEASIBILITY OF PROLYL OLIGOPEPTIDASE (PREP) AS TUMOR BIOMARKER IN GLIOBLASTOMA PATIENTS: A PILOT STUDY

Abstract BACKGROUND Prolyl oligopeptidase (PREP) is a serine protease that hydrolyzes biologically active oligopeptides on their carboxyl side. PREP has been connected with multiple physiological and pathological conditions. Moreover, increased PREP activities have been found in tumors, although its involvement in the modulation of cancer mechanism is still unclear. Thus, the identification of PREP and clarification of its clinical significance are important for improving the diagnosis and treatment of brain tumors. The purpose of this study was to analyze the expression of PREP and its diagnostic and prognostic value in patients with Glioblastoma (GBM). Materials and METHODS In this study, GBM patients and controls were enrolled and clinical data and PREP expression, from serum and biopsies, were collected. The relationships between PREP, Protein phosphatase 2A (PP2A), STAT3 and Transforming growth factor-β (TGF- β) were also analyzed from clinical data. RESULTS Our study found that PREP was significantly upregulated in the serum samples and in biopsy from GBM patients compared with controls. This upregulation was negatively correlated with survival, but significantly correlated with age and Mindbomb Homolog-1 Index (MIB1). Also, PREP expression was inversely correlated with tumor suppressor PP2A in biopsy from GBM patients, while PREP protein levels resulted to be positively correlated with STAT3 and TGF-β protein expression. CONCLUSION s: The detection of PREP in serum and biopsies have the potential to serve as diagnostic and prognostic biomarker in GBM patient. The findings may provide diagnostic and therapeutic target for GBM.

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

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

Abstract 4727: RAS and PP2A activities converge on phosphoregulation of epigenetic complexes in cancer

Abstract RAS-mediated human cell transformation requires inhibition of the tumor suppressor Protein Phosphatase 2A (PP2A). Both RAS and PP2A mediate their effects by phosphoregulation, but phosphoprotein targets in which RAS and PP2A activities converge in human cancers have not been systematically analyzed. Here, based on mass spectrometry phosphoproteome data, we discover that phosphosites co-regulated by RAS and PP2A are enriched on proteins involved in epigenetic gene regulation. As examples, RAS and PP2A co-regulate the same phosphorylation sites on HDAC1/2, KDM1A, MTA1/2, RNF168 and TP53BP1. Mechanistically, we validate co-regulation of NuRD chromatin repressor complex by RAS and PP2A. Consistent with their known synergistic effects in cancer, RAS activation and PP2A inhibition resulted in epigenetic reporter de-repression and activation of oncogenic transcription. Notably, transcriptional de-repression by PP2A inhibition was associated with increased euchromatin and decrease in global DNA methylation. In RAS-driven cancers, PP2A activity is inhibited by nuclear PP2A inhibitor proteins PME-1 and SET. We further demonstrate that these PP2A inhibitors drive oncogenic transcription, but that their transcriptional targets are highly diversified. Whereas PME-1 controls RAS and MYC-driven transcription, SET controls TP53 targets and G2/M checkpoint genes. We further provide evidence that these nuclear PP2A inhibitors significantly differ in their roles in regulating DNA methylation. Collectively the results indicate that epigenetic protein complexes involved in oncogenic gene expression constitute a significant point of convergence for RAS hyperactivity and PP2A inhibition in cancer. Further, the results provide a rich source for future understanding of phosphorylation as a previously unappreciated layer of regulation of epigenetic gene regulation in cancer, and in other RAS/PP2A-regulated cellular processes. Citation Format: Mukund Sharma, Anna Aakula, Francesco Tabaro, Henrik Honkanen, Jesse Kamila, Matthieu Schapira, Cheryl Arrowsmith, Matti Nykter, Jukka K. Westermarck. RAS and PP2A activities converge on phosphoregulation of epigenetic complexes in cancer. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4727.

RAS and PP2A activities converge on epigenetic gene regulation.

RAS-mediated human cell transformation requires inhibition of the tumor suppressor protein phosphatase 2A (PP2A). However, the phosphoprotein targets and cellular processes in which RAS and PP2A activities converge in human cancers have not been systematically analyzed. Here, we discover that phosphosites co-regulated by RAS and PP2A are enriched on proteins involved in epigenetic gene regulation. As examples, RAS and PP2A co-regulate the same phosphorylation sites on HDAC1/2, KDM1A, MTA1/2, RNF168, and TP53BP1. We validate RAS- and PP2A-elicited regulation of HDAC1/2 chromatin recruitment, of RNF168-TP53BP1 interaction, and of gene expression. Consistent with their known synergistic effects in cancer, RAS activation and PP2A inhibition resulted in epigenetic reporter derepression and activation of oncogenic transcription. Transcriptional derepression by PP2A inhibition was associated with an increase in euchromatin and a decrease in global DNA methylation. Collectively, the results indicate that epigenetic protein complexes constitute a significant point of convergence for RAS hyperactivity and PP2A inhibition in cancer. Furthermore, the work provides an important resource for future studies focusing on phosphoregulation of epigenetic gene regulation in cancer and in other RAS/PP2A-regulated cellular processes.

Abstract 3340: Small molecule mediated stabilization of PP2A modulates the homologous recombination pathway and potentiates DNA damage-induced cell death

Abstract High Grade Serous Carcinoma (HGSC) is the most lethal ovarian cancer subtype and accounts for approximately 60% of all ovarian tumors. Despite recent advances in drug development and increased understanding of genetic alterations that drive HGSC progression, mortality has not declined, highlighting the need for novel therapies. PARP inhibitors (PARPi) have become the mainstay of HGSC targeted therapy research given that these tumors are driven by a high degree of genomic instability resulting from the combination of fast DNA replication rates and numerous defects in the DNA-damage response (DDR) pathway. Nonetheless, only ~25% of these patients initially respond to treatment and a significant percentage eventually relapses with resistant disease. Here, we discovered that a Small Molecule Activator of Protein Phosphatase 2A (PP2A) (SMAP-061) induces apoptosis in both established and patient-derived HGSC cell lines as well as in genetically distinct Patient-Derived Xenograft (PDX) mouse models. Interestingly, we also uncovered that several genes that make-up the heterotrimer PP2A tumor suppressor protein are heterozygously lost in more than 95% of HGSC tumors, second only to p53. Mechanistically, we show that stabilization of PP2A protein by SMAP-061 inhibits the Homologous Recombination (HR) pathway via the direct inhibition of RAD51, ultimately leading to chronic accumulation of DNA damage and thus programmed cell death. Furthermore, we found that SMAP-061’s ability to inhibit HR potentiated the effects of PARP inhibition and resulted in synergistic cell death in both HR proficient and deficient models. These studies emphasize the potential of PP2A activators to expand the patient population that can benefit from PARPi therapies and possibly overcome PARPi resistance. In sum, our data highlights a new role of PP2A in regulating the DDR pathway in HGSC and supports the use of SMAPs in both HR proficient and deficient HGSC tumors. Citation Format: Rita A. Avelar, Amy Armstrong, Goutham Narla, Analisa DiFeo. Small molecule mediated stabilization of PP2A modulates the homologous recombination pathway and potentiates DNA damage-induced cell death [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3340.

Potential of phenothiazines to synergistically block calmodulin and reactivate PP2A in cancer cells.

Phenothiazines (PTZ) were developed as inhibitors of monoamine neurotransmitter receptors, notably dopamine receptors. Because of this activity they have been used for decades as antipsychotic drugs. In addition, they possess significant anti-cancer properties and several attempts for their repurposing were made. However, their incompletely understood polypharmacology is challenging. Here we examined the potential of the PTZ fluphenazine (Flu) and its mustard derivative (Flu-M) to synergistically act on two cancer associated targets, calmodulin (CaM) and the tumor suppressor protein phosphatase 2A (PP2A). Both proteins are known to modulate the Ras- and MAPK-pathway, cell viability and features of cancer cell stemness. Consistently, we show that the combination of a CaM inhibitor and the PP2A activator DT-061 synergistically inhibited the 3D-spheroid formation of MDA-MB-231 (K-Ras-G13D), NCI-H358 (K-Ras-G12C) and A375 (B-raf-V600E) cancer cells, and increased apoptosis in MDA-MB-231. We reasoned that these activities remain combined in PTZ, which were the starting point for PP2A activator development, while several PTZ are known CaM inhibitors. We show that both Flu and Flu-M retained CaM inhibitory activity in vitro and in cells, with a higher potency of the mustard derivative in cells. In line with the CaM dependence of Ras plasma membrane organization, the mustard derivative potently reduced the functional membrane organization of oncogenic Ras, while DT-061 had a negligible effect. Like DT-061, both PTZ potently decreased c-MYC levels, a hallmark of PP2A activation. Benchmarking against the KRAS-G12C specific inhibitor AMG-510 in MIA PaCa-2 cells revealed a higher potency of Flu-M than combinations of DT-061 and a CaM inhibitor on MAPK-output and a strong effect on cell proliferation. While our study is limited, our results suggest that improved PTZ derivatives that retain both, their CaM inhibitory and PP2A activating properties, but have lost their neurological side-effects, may be interesting to pursue further as anti-cancer agents.

PPP2R4 dysfunction promotes KRAS-mutant lung adenocarcinoma development and mediates opposite responses to MEK and mTOR inhibition.

KRAS-mutant lung adenocarcinomas represent the largest molecular subgroup of non-small cell lung cancers (NSCLC) and are notorious for their dismal survival perspectives. To gain more insights in etiology and therapeutic response, we focused on the tumor suppressor Protein Phosphatase 2A (PP2A) as a player in KRAS oncogenic signaling. We report that the PP2A activator PTPA (encoded by PPP2R4) is commonly affected in NSCLC by heterozygous loss and low-frequent loss-of-function mutation, and this is specifically associated with poorer overall survival of KRAS-mutant lung adenocarcinoma patients. Reduced or mutant PPP2R4 expression in A549cells increased anchorage-independent growth in vitro and xenograft growth in vivo, correlating with increased Ki67 and c-MYC expression. Moreover, KrasG12D-induced lung tumorigenesis was significantly accelerated in Ppp2r4 gene trapped mice as compared to Ppp2r4 wild-type. A confined kinase inhibitor screen revealed that PPP2R4-depletion induced resistance against selumetinib (MEK inhibitor), but unexpectedly sensitized cells for temsirolimus (mTOR inhibitor), in vitro and in vivo. Our findings underscore a clinically relevant role for PTPA loss-of-function in KRAS-mutant NSCLC etiology and kinase inhibitor response.

SWATH-MS proteomics of PANC-1 and MIA PaCa-2 pancreatic cancer cells allows identification of drug targets alternative to MEK and PI3K inhibition

Pancreatic cancer remains one of the most lethal diseases with dismal five-year survival rates. Although mutant KRas protein-driven activation of downstream MAPK Raf/MEK/ERK and PI3K/Akt signaling pathways represent major oncogenic alterations, signaling blockade with MEK and PI3K inhibitors has shown that intrinsic resistance may hamper the effectiveness of this targeted approach. However, there have been no mass spectrometry-based proteomic studies for in-depth comparison of protein expression differences between pancreatic cancer cells with sensitivity and resistance to MEK and PI3K kinase inhibitors. In this work, we compared PANC-1 and MIA PaCa-2 pancreatic cancer cells which are, respectively, resistant and sensitive to MEK- and PI3K-targeted therapy. We conducted a label-free data-independent acquisition mass spectrometry (SWATH-MS) study with extensive peptide fractionation to quantitate 4808 proteins and analyze differential expression of 743 proteins between resistant and sensitive cells. This allowed identification of the tumor suppressor protein phosphatase 2A (PP2A) and proteins from mitochondrial respiratory complex I implicated in oxidative phosphorylation as alternative candidate drug targets for cells resistant to MEK and PI3K inhibition. PP2A activator DT-061 decreased viability of PANC-1 cells and this was accompanied by reduced expression of c-Myc. PANC-1 cells also showed response to metformin and the novel complex I inhibitor IACS-010759. These findings provide insights into the distinct cellular proteomes and point out alternative pharmacological targets for MEK and PI3K inhibition-resistant pancreatic cancer cells.

PP2A-activating Drugs Enhance FLT3 Inhibitor Efficacy through AKT Inhibition-Dependent GSK-3β-Mediated c-Myc and Pim-1 Proteasomal Degradation.

Fms-like tyrosine-like kinase 3 internal tandem duplication (FLT3-ITD) is present in acute myeloid leukemia (AML) in 30% of patients and is associated with short disease-free survival. FLT3 inhibitor efficacy is limited and transient but may be enhanced by multitargeting of FLT3-ITD signaling pathways. FLT3-ITD drives both STAT5-dependent transcription of oncogenic Pim-1 kinase and inactivation of the tumor-suppressor protein phosphatase 2A (PP2A), and FLT3-ITD, Pim-1, and PP2A all regulate the c-Myc oncogene. We studied mechanisms of action of cotreatment of FLT3-ITD-expressing cells with FLT3 inhibitors and PP2A-activating drugs (PADs), which are in development. PADs, including FTY720 and DT-061, enhanced FLT3 inhibitor growth suppression and apoptosis induction in FLT3-ITD-expressing cell lines and primary AML cells in vitro and MV4-11 growth suppression in vivo PAD and FLT3 inhibitor cotreatment independently downregulated c-Myc and Pim-1 protein through enhanced proteasomal degradation. c-Myc and Pim-1 downregulation was preceded by AKT inactivation, did not occur in cells expressing myristoylated (constitutively active) AKT1, and could be induced by AKT inhibition. AKT inactivation resulted in activation of GSK-3β, and GSK-3β inhibition blocked downregulation of both c-Myc and Pim-1 by PAD and FLT3 inhibitor cotreatment. GSK-3β activation increased c-Myc proteasomal degradation through c-Myc phosphorylation on T58; infection with c-Myc with T58A substitution, preventing phosphorylation, blocked downregulation of c-Myc by PAD and FLT3 inhibitor cotreatment. GSK-3β also phosphorylated Pim-1L/Pim-1S on S95/S4. Thus, PADs enhance efficacy of FLT3 inhibitors in FLT3-ITD-expressing cells through a novel mechanism involving AKT inhibition-dependent GSK-3β-mediated increased c-Myc and Pim-1 proteasomal degradation.

Abstract 2981: Biased activation of PP2A through selective heterotrimer binding and stabilization

Abstract Development of molecularly targeted therapies have shifted our clinical approach to cancer allowing for an integrated treatment approach where tumor genotype and molecular profiles are paired in a clinically actionable manner with specific targeted therapies. This approach maximizes efficacy and minimizes toxicity for each patient. While the vast majority of molecularly targeted therapies have focused on the development of kinase inhibition, an equally effective yet underexplored strategy exists in the activation or regulation of phosphatases. The tumor suppressor Protein Phosphatase 2A (PP2A) negatively regulates multiple oncogenic drivers common to a broad range of human malignancies including MYC, AKT, and ERK; and has been demonstrated as a critical tumor suppressor. In cancer, PP2A is inactivated through several mechanisms including increased expression of endogenous inhibitors, post-translational modifications of the catalytic subunit, and somatic mutation in its catalytic, regulatory or scaffold subunits. The vast mechanisms employed to dysregulate and inhibit PP2A in cancer, highlights it's potential as an endogenous molecular target to combat malignant growth. Our laboratory has been involved in the development of a novel series of small molecule activators of PP2A (SMAPs). Using structural, biochemical, cellular, and in vivo model systems we reveal how the lead SMAP, DT-061, specifically stabilizes a B56 containing PP2A holoenzyme, in its fully assembled, active state, to direct enzymatic activity towards specific oncogenic targets, including its well-known substrate c-MYC. Our structure identifies a unique interfacial small molecule binding pocket that involves all three PP2A subunits to prevent dissociation of select B56 subunit containing holoenzymes. Elucidating the mechanism of SMAPs allowed us to explore appropriate clinical placement for this targeting strategy and investigate potential mechanisms of resistance. For example, in vivo xenograft studies of cancer-associated Aα mutants, most notably R183W, demonstrates that these mutations render malignant cells resistant to these SMAPs and further support the mechanism of SMAPs described. 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. Citation Format: Daniel Leonard, Wei Huang, Sudeh Izadmehr, Caitlin M. O'Connor, Danica D. Wiredja, Zhizhi Wang, Nilesh Zaware, Daniela M. Schlatzer, Stefan Schuchner, Janna Kiselar, Yinghua Chen, Nikhil Vasireddi, Abbey L. Perl, Matthew D. Glasky, Wenqing Xu, David L. Brautigan, Egon Ogris, Derek J. Taylor, Goutham Narla. Biased activation of PP2A through selective heterotrimer binding and stabilization [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2981.

The Curcumin Analogue, EF-24, Triggers p38 MAPK-Mediated Apoptotic Cell Death via Inducing PP2A-Modulated ERK Deactivation in Human Acute Myeloid Leukemia Cells.

Curcumin (CUR) has a range of therapeutic benefits against cancers, but its poor solubility and low bioavailability limit its clinical use. Demethoxycurcumin (DMC) and diphenyl difluoroketone (EF-24) are natural and synthetic curcumin analogues, respectively, with better solubilities and higher anti-carcinogenic activities in various solid tumors than CUR. However, the efficacy of these analogues against non-solid tumors, particularly in acute myeloid leukemia (AML), has not been fully investigated. Herein, we observed that both DMC and EF-24 significantly decrease the proportion of viable AML cells including HL-60, U937, and MV4-11, harboring different NRAS and Fms-like tyrosine kinase 3 (FLT3) statuses, and that EF-24 has a lower half maximal inhibitory concentration (IC50) than DMC. We found that EF-24 treatment induces several features of apoptosis, including an increase in the sub-G1 population, phosphatidylserine (PS) externalization, and significant activation of extrinsic proapoptotic signaling such as caspase-8 and -3 activation. Mechanistically, p38 mitogen-activated protein kinase (MAPK) activation is critical for EF-24-triggered apoptosis via activating protein phosphatase 2A (PP2A) to attenuate extracellular-regulated protein kinase (ERK) activities in HL-60 AML cells. In the clinic, patients with AML expressing high level of PP2A have the most favorable prognoses compared to various solid tumors. Taken together, our results indicate that EF-24 is a potential therapeutic agent for treating AML, especially for cancer types that lose the function of the PP2A tumor suppressor.

MASTL: A novel therapeutic target for Cancer Malignancy.

Targeting mitotic kinases is an emerging anticancer approach with promising preclinical outcomes. Microtubule‐associated serine/threonine kinase like (MASTL), also known as Greatwall (Gwl), is an important mitotic kinase that regulates mitotic progression of normal or transformed cells by blocking the activity of tumor suppressor protein phosphatase 2A (PP2A). MASTL upregulation has now been detected in multiple cancer types and associated with aggressive clinicopathological features. Apart, an aberrant MASTL activity has been implicated in oncogenic transformation through the development of chromosomal instability and alteration of key oncogenic signaling pathways. In this regard, recent publications have revealed potential role of MASTL in the regulation of AKT/mTOR and Wnt/β‐catenin signaling pathways, which may be independent of its regulation of PP2A‐B55 (PP2A holoenzyme containing a B55‐family regulatory subunit). Taken together, MASTL kinase has emerged as a novel target for cancer therapeutics, and hence development of small molecule inhibitors of MASTL may significantly improve the clinical outcomes of cancer patients. In this article, we review the role of MASTL in cancer progression and the current gaps in this knowledge. We also discuss potential efficacy of MASTL expression for cancer diagnosis and therapy.

Protein phosphatase 2A activation as a therapeutic strategy for managing MYC-driven cancers.

The tumor suppressor protein phosphatase 2A (PP2A) is a serine/threonine phosphatase whose activity is inhibited in most human cancers. One of the best-characterized PP2A substrates is MYC proto-oncogene basic helix-loop-helix transcription factor (MYC), whose overexpression is commonly associated with aggressive forms of this disease. PP2A directly dephosphorylates MYC, resulting in its degradation. To explore the therapeutic potential of direct PP2A activation in a diverse set of MYC-driven cancers, here we used biochemical assays, recombinant cell lines, gene expression analyses, and immunohistochemistry to evaluate a series of first-in-class small-molecule activators of PP2A (SMAPs) in Burkitt lymphoma, KRAS-driven non-small cell lung cancer, and triple-negative breast cancer. In all tested models of MYC-driven cancer, the SMAP treatment rapidly and persistently inhibited MYC expression through proteasome-mediated degradation, inhibition of MYC transcriptional activity, decreased cancer cell proliferation, and tumor growth inhibition. Importantly, we generated a series of cell lines expressing PP2A-dependent phosphodegron variants of MYC and demonstrated that the antitumorigenic activity of SMAPs depends on MYC degradation. Collectively, the findings presented here indicate a pharmacologically tractable approach to drive MYC degradation by using SMAPs for the management of a broad range of MYC-driven cancers.

PP2A Activators Enhance Efficacy of FLT3 Inhibitors in FLT3-ITD Acute Myeloid Leukemia Cells through AKT Inactivation-Dependent Pim-1 and c-Myc Proteasomal Degradation

Introduction fms-like tyrosine like kinase 3 internal tandem duplication (FLT3-ITD), present in acute myeloid leukemia (AML) cells of 30% of patients, results in constitutive and aberrant FLT3 signaling and, clinically, short disease-free survival. Efficacy of FLT3 inhibitors is limited and transient, but may be enhanced by dual targeting of FLT3-ITD signaling pathways. The tumor suppressor protein phosphatase 2A (PP2A) is inhibited in cells with FLT3-ITD. The oncogenic serine/threonine kinase Pim-1 is transcriptionally upregulated and also stabilized by PP2A inhibition in cells with FLT3-ITD. Pim-1 contributes directly to FLT3-ITD proliferative and anti-apoptotic effects, and also phosphorylates and stabilizes FLT3-ITD in a positive feedback loop. Moreover FLT3-ITD, PP2A and Pim-1 all regulate the transcription factor c-Myc. PP2A-activating drugs enhance efficacy of FLT3 inhibitors. We sought to identify mechanisms underlying the efficacy of this combination. Methods Ba/F3-ITD and MV4-11 cells, with FLT3-ITD, and blasts from patients with AML with FLT3-ITD were cultured with the FLT3 inhibitors gilteritinib (15 nM) or quizartinib (1 nM) and/or the PP2A-activating drugs FTY720 (2-4 µM) or DT-061 (10 µM), or with DMSO control. Pim-1, c-Myc, p-AKT (S473 and T308) and AKT protein expression was measured by immunoblotting, along with p-STAT5 (Y694), STAT5, p-PP2A (Y307) and PP2A expression. To study post-translational regulation, cells were cultured with cycloheximide (100 µg/mL) with and without the proteasome inhibitor MG-132 (20 µM). Ubiquitinated c-Myc was measured by co-immunoprecipitation and immunoblotting with c-Myc and ubiquitin antibodies. Ba/F3-ITD cells were stably transfected with estrogen receptor (ER)-c-Myc, kinase-dead Pim-1 or myristoylated AKT plasmids or corresponding empty vectors. Apoptosis was detected by Annexin V and propidium iodine staining, measured by flow cytometry. Cells were also cultured with the pan-Pim kinase inhibitor AZD1208 (1 µM), the Myc inhibitor 10058-F4 (100 µM) or the pan-AKT inhibitor MK-2206 (5 µM). Results Concurrent treatment of Ba/F3-ITD and MV4-11 cells and primary AML cells with FLT3-ITD with a FLT3 inhibitor (gilteritinib or quizartinib) and a PP2A-activating drug (FTY720 or DT-061) decreased growth and increased apoptosis induction, relative to treatment with single drugs. Concurrent FLT3 inhibitor and PP2A-activating drug treatment decreased expression of both Pim-1 and c-Myc protein. Concurrent treatment decreased Pim-1 half-life from 15 to 5 minutes, and c-Myc half-life from 30 to 5 minutes, while half-lives were restored by concurrent treatment with the proteasome inhibitor MG-132. Concurrent treatment was also shown to increase c-Myc ubiquitination. Effects of concurrent treatment on Pim-1 and c-Myc were independent, as transfection with kinase-dead Pim-1 or treatment with Pim inhibitor AZD1208 did not alter c-Myc downregulation, and c-Myc overexpression or treatment with Myc inhibitor 10058-F4 did not alter Pim-1 downregulation. Concurrent treatment with FLT3 inhibitor and PP2A-activating drug did not alter expression of c-Myc deubiquitinases, but rapidly decreased AKT S473 and T308 phosphorylation. FLT3 inhibitor and PP2A activator co-treatment did not induce downregulation or increased turnover of Pim-1 and c-Myc protein or apoptosis in cells with constitutive AKT activation caused by transfection of myristoylated AKT. Moreover, AKT inhibition downregulated Pim-1 and c-Myc protein expression, decreased Pim-1 and c-Myc protein half-lives from 15 to 5 minutes and 30 to 10 minutes, respectively, and induced apoptosis of cells with FLT3-ITD, replicating the effects of FLT3 inhibitor and PP2A activator co-treatment. Conclusion PP2A activators enhance the efficacy of FLT3 inhibitors in AML cells with FLT3-ITD through AKT inactivation-dependent increased Pim-1 and c-Myc proteasomal degradation, which is a novel mechanism. The data support further preclinical and clinical testing of this dual targeting approach to treatment of AML with FLT3-ITD. Disclosures Baer: Takeda: Research Funding; Incyte: Research Funding; Kite: Research Funding; Forma: Research Funding; AI Therapeutics: Research Funding; Abbvie: Research Funding; Astellas: Research Funding.

The NMR‐based characterization of the FTY720‐SET complex reveals an alternative mechanism for the attenuation of the inhibitory SET‐PP2A interaction

The su(var)3-9, enhancer of zeste, trithorax (SET)/inhibitor 2 of protein phosphatase 2A (PP2A) oncoprotein binds and inhibits PP2A, composed of various isoforms of scaffolding, regulatory, and catalytic subunits. Targeting SET with a sphingolipid analog drug fingolimod (FTY720) or ceramide leads to the reactivation of tumor suppressor PP2A. However, molecular details of the SET-FTY720 or SET-ceramide, and mechanism of FTY720-dependent PP2A activation, remain unknown. Here, we report the first in solution examination of the SET-FTY720 or SET-ceramide complexes by NMR spectroscopy. FTY720-ceramide binding resulted in chemical shifts of residues residing at the N terminus of SET, preventing its dimerization or oligomerization. This then released SET from PP2ACα, resulting in PP2A activation, while monomeric SET remained associated with the B56γ. Our data also suggest that the PP2A holoenzyme, composed of PP2A-Aβ, PP2A-B56γ, and PP2ACα subunits, is selectively activated in response to the formation of the SET-FTY720 complex in A549 cells. Various PP2A-associated downstream effector proteins in the presence or absence of FTY720 were then identified by stable isotope labeling with amino cells in cell culture, including tumor suppressor nonmuscle myosin IIA. Attenuation of FTY720-SET association by point mutations of residues that are involved in FTY720 binding or dephosphorylation of SET at Serine 171, enhanced SET oligomerization and the formation of the SET-PP2A inhibitory complex, leading to resistance to FTY720-dependent PP2A activation.-De Palma, R. M., Parnham, S. R., Li, Y., Oaks, J. J., Peterson, Y. K., Szulc, Z. M., Roth, B. M., Xing, Y., Ogretmen, B. The NMR-based characterization of the FTY720-SET complex reveals an alternative mechanism for the attenuation of the inhibitory SET-PP2A interaction.

RABL6A inhibits tumor-suppressive PP2A/AKT signaling to drive pancreatic neuroendocrine tumor growth.

Hyperactivated AKT/mTOR signaling is a hallmark of pancreatic neuroendocrine tumors (PNETs). Drugs targeting this pathway are used clinically, but tumor resistance invariably develops. A better understanding of factors regulating AKT/mTOR signaling and PNET pathogenesis is needed to improve current therapies. We discovered that RABL6A, a new oncogenic driver of PNET proliferation, is required for AKT activity. Silencing RABL6A caused PNET cell-cycle arrest that coincided with selective loss of AKT-S473 (not T308) phosphorylation and AKT/mTOR inactivation. Restoration of AKT phosphorylation rescued the G1 phase block triggered by RABL6A silencing. Mechanistically, loss of AKT-S473 phosphorylation in RABL6A-depleted cells was the result of increased protein phosphatase 2A (PP2A) activity. Inhibition of PP2A restored phosphorylation of AKT-S473 in RABL6A-depleted cells, whereas PP2A reactivation using a specific small-molecule activator of PP2A (SMAP) abolished that phosphorylation. Moreover, SMAP treatment effectively killed PNET cells in a RABL6A-dependent manner and suppressed PNET growth in vivo. The present work identifies RABL6A as a new inhibitor of the PP2A tumor suppressor and an essential activator of AKT in PNET cells. Our findings offer what we believe is a novel strategy of PP2A reactivation for treatment of PNETs as well as other human cancers driven by RABL6A overexpression and PP2A inactivation.

Direct activation of PP2A for the treatment of tyrosine kinase inhibitor-resistant lung adenocarcinoma.

Although tyrosine kinase inhibitors (TKIs) have demonstrated significant efficacy in advanced lung adenocarcinoma (LUAD) patients with pathogenic alterations in EGFR, most patients develop acquired resistance to these agents via mechanisms enabling the sustained activation of the PI3K and MAPK oncogenic pathways downstream of EGFR. The tumor suppressor protein phosphatase 2A (PP2A) acts as a negative regulator of these pathways. We hypothesize that activation of PP2A simultaneously inhibits the PI3K and MAPK pathways and represents a promising therapeutic strategy for the treatment of TKI-resistant LUAD. After establishing the efficacy of small molecule activators of PP2A (SMAPs) in a transgenic EGFRL858R model and TKI-sensitive cell lines, we evaluated their therapeutic potential in vitro and in vivo in TKI-resistant models. PP2A activation resulted in apoptosis, significant tumor growth inhibition, and downregulation of PI3K and MAPK pathways. Combination of SMAPs and TKI afatinib resulted in an enhanced effect on the downregulation of the PI3K pathway via degradation of the PP2A endogenous inhibitor CIP2A. An improved effect on tumor growth inhibition was observed in a TKI-resistant xenograft mouse model treated with a combination of both agents. These collective data support the development of PP2A activators for the treatment of TKI-resistant LUAD.

Concurrent FLT3 Inhibitor and PP2A Activating Drug Treatment Induces Synergistic Cytotoxicity in Acute Myeloid Leukemia Cells with FLT3 Internal Tandem Duplication through Proteasomal Degradation of Pim-1 and c-Myc

Introduction: In 30% of acute myeloid leukemia (AML) patients, internal tandem duplication of fms-like tyrosine kinase 3 (FLT3-ITD) causes constitutive and aberrant FLT3 signaling, and these patients have short relapse-free and overall survival. FLT3 inhibitors have limited and transient efficacy, but their efficacy may be enhanced by combination with other drugs targeting FLT3 signaling. FLT3 activation also inhibits the tumor suppressor protein phosphatase 2A (PP2A). FLT3 inhibitors and PP2A-activating drugs have been shown to induce synergistic cytotoxicity in cells with FLT3-ITD. To address mechanisms underlying this effect, we studied effects of combination therapy on the oncogenic serine/threonine kinase Pim-1 and the transcription factor c-Myc, both of which are upregulated in cells with FLT3-ITD and are also PP2A substrates.Methods: Ba/F3-ITD and MV4-11 cells and AML patient blasts with FLT3-ITD were cultured with a FLT3 inhibitor, gilteritinib (ASP2215) or quizartinib (AC220), and/or the PP2A-activating drug fingolimod (FTY720) at pharmacologically relevant concentrations, or DMSO control. Drug combination effects were measured by combination index determined by the Chou-Talalay method using CompuSyn software. Apoptosis was measured by Annexin V/propidium iodide staining detected by flow cytometry. c-Myc and GAPDH control mRNA was measured by real-time polymerase chain reaction. Pim-1 kinase, c-Myc, phospho-c-MycSer62, phospho-c-MycThr58, phospho-STAT5Tyr694, STAT5, phospho-PP2ATyr307, PP2A, phospho-BADSer112 and BAD levels were measured by immunoblotting. Cycloheximide treatment was used to assess protein stability. Protein expression and stability were measured with and without the proteasome inhibitor MG-132. Pim-1 kinase was inhibited with the pan-Pim inhibitor AZD1208. Ba/F3-ITD cells were infected with pMX-Flag-K67M kinase-dead (KD) Pim-1 and empty pMX retroviral vectors and with pBABE-ER-cMYC and with empty pBABE-ER retroviral vectors.Results: Concurrent treatment with 15 nM gilteritinib or 1 nM quizartinib and FTY720 2 µM in cell lines and 4 µM in patient samples decreased growth and increased apoptosis of cells with FLT3-ITD, relative to single drug treatments, and produced synergistic cytotoxicity. FLT3 inhibition was confirmed by decrease in phospho-STAT5 and PP2A activation by decreased phospho-PP2A. Concurrent treatment decreased expression of both Pim-1 and c-Myc protein, but not c-Myc mRNA, in Ba/F3-ITD and MV4-11 cells and AML patient blasts with FLT3-ITD, relative to single drug treatments. Additionally, selective decrease in phospho-MycSer62, a stable c-Myc phosphoprotein that is dephosphorylated by PP2A, was seen, with persistence of phospho-c-MycThr58. FLT3 inhibitor and PP2A activator combination treatment was found to decrease stability of c-Myc and Pim-1 protein, in relation to single drugs. Moreover, pretreatment with the proteasome inhibitor MG-132 abrogated downregulation of Pim-1 and c-Myc protein expression and decrease in Pim-1 and c-Myc protein stability in Ba/F3-ITD cells treated with FLT3 inhibitor and PP2A activator. Pretreatment with the pan-Pim kinase inhibitor AZD1208, with Pim-1 inhibition confirmed by decreased phospho-BADS112 had no effect on c-Myc downregulation, and c-Myc was similarly downregulated in Pim-1 kinase-dead cells as in parental and empty-vector cells, demonstrating that combination treatment effects on c-Myc are not Pim-1 kinase-dependent. Additionally, FLT3 inhibitor and PP2A-activating drug combination induced apoptosis in 30% of cells with c-Myc overexpression, compared to 60% of parental and empty vector-infected cells. Finally, c-Myc overexpression did not abrogate Pim-1 downregulation by combination treatment.Conclusions: Concurrent FLT3 inhibitor and PP2A activating drug treatment induces synergistic cytotoxicity in AML cells with FLT3 internal tandem duplication through proteasomal degradation of Pim-1 and c-Myc, and effects on Pim-1 and c-Myc are independent. The data support in vivo testing of FLT3 inhibitor and PP2A-activating drug combinations and development of a clinical trial. DisclosuresNo relevant conflicts of interest to declare.