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High-grade serous carcinoma (HGSC) is the most common and lethal ovarian cancer subtype. PARP inhibitors (PARPi) have become the mainstay of HGSC-targeted therapy, given that these tumors are driven by a high degree of genomic instability (GI) and homologous recombination (HR) defects. Nonetheless, approximately 30% of patients initially respond to treatment, ultimately relapsing with resistant disease. Thus, despite recent advances in drug development and an increased understanding of genetic alterations driving HGSC progression, mortality has not declined, highlighting the need for novel therapies. Using a small-molecule activator of protein phosphatase 2A (PP2A; SMAP-061), we investigated the mechanism by which PP2A stabilization induces apoptosis in patient-derived HGSC cells and xenograft (PDX) models alone or in combination with PARPi. We uncovered that PP2A genes essential for cellular transformation (B56α, B56γ, and PR72) and basal phosphatase activity (PP2A-A and -C) are heterozygously lost in the majority of HGSC. Moreover, loss of these PP2A genes correlates with worse overall patient survival. We show that SMAP-061–induced stabilization of PP2A inhibits the HR output by targeting RAD51, leading to chronic accumulation of DNA damage and ultimately apoptosis. Furthermore, combination of SMAP-061 and PARPi leads to enhanced apoptosis in both HR-proficient and HR-deficient HGSC cells and PDX models. Our studies identify PP2A as a novel regulator of HR and indicate PP2A modulators as a therapeutic therapy for HGSC. In summary, our findings further emphasize the potential of PP2A modulators to overcome PARPi insensitivity, given that targeting RAD51 presents benefits in overcoming PARPi resistance driven by BRCA1/2 mutation reversions.

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High-grade serous carcinoma (HGSC) is the most common and lethal ovarian cancer subtype. PARP inhibitors (PARPi) have become the mainstay of HGSC-targeted therapy, given that these tumors are driven by a high degree of genomic instability (GI) and homologous recombination (HR) defects. Nonetheless, approximately 30% of patients initially respond to treatment, ultimately relapsing with resistant disease. Thus, despite recent advances in drug development and an increased understanding of genetic alterations driving HGSC progression, mortality has not declined, highlighting the need for novel therapies. Using a small-molecule activator of protein phosphatase 2A (PP2A; SMAP-061), we investigated the mechanism by which PP2A stabilization induces apoptosis in patient-derived HGSC cells and xenograft (PDX) models alone or in combination with PARPi. We uncovered that PP2A genes essential for cellular transformation (B56α, B56γ, and PR72) and basal phosphatase activity (PP2A-A and -C) are heterozygously lost in the majority of HGSC. Moreover, loss of these PP2A genes correlates with worse overall patient survival. We show that SMAP-061–induced stabilization of PP2A inhibits the HR output by targeting RAD51, leading to chronic accumulation of DNA damage and ultimately apoptosis. Furthermore, combination of SMAP-061 and PARPi leads to enhanced apoptosis in both HR-proficient and HR-deficient HGSC cells and PDX models. Our studies identify PP2A as a novel regulator of HR and indicate PP2A modulators as a therapeutic therapy for HGSC. In summary, our findings further emphasize the potential of PP2A modulators to overcome PARPi insensitivity, given that targeting RAD51 presents benefits in overcoming PARPi resistance driven by BRCA1/2 mutation reversions.

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Karyotype analysis for Spermatogonia at passage 16, before transfection with XTNs demonstrating a normal male 40, XY karyotype.

ANOVA with multiple comparisons and Tukey's post-hoc test

Supplementary Data figures and tables Fig. S1. Genomic characterization of CR cells. Fig. S2. Karyotype analysis of CR cells. Fig. S3. Drug sensitivity profiles of CR cells. Fig. S4. Population doubling times of Parental and nab-paclitaxel resistant cells. Fig. S5. Nab-paclitaxel sensitivity of CR spheroids. Fig. S6. Dependency of CR cells on KRAS. Fig. S7. CR-based PDX tumors in mice. Fig. S8. Reverse Phase Proteomic analysis of Parental and nab-paclitaxel resistant cells. Fig. S9. Modulation of c-Myc and the effects on nab-paclitaxel sensitivity. Fig. S10. SMAP2 Responses. Table S1. Patient data Table S2. Gene mutations associated with nab-paclitaxel resistance

Western Blot analysis for RAG2 protein expression in SDR rats. Rat thymus and spleen lysates were loaded onto a 7.5% SDS-PAGE gel and probed with a RAG2 polyclonal antibody at a dilution of 1:1000.

Immunophenotyping of the SDR rat. Analysis of SDR whole blood demonstrates greatly diminished T and B cells compared to the wild-type rat. A, The wild-type rat has a significant population of circulating CD4+, CD8+, and CD4/CD8 double positive T cell populations compared to the SDR rat. B, The wild-type rat has over 20% B220/IgM double positive mature B cells, compared to 3.5% in the SDR rat. C, The SDR rat has slightly less circulating NK cells, unlike what is observed in the spleen. Panels are representative of 3 wild-type and 3 SDR rats. Left panels: wild-type rat. Right panels: SDR rat.

STR DNA Profiling for Cell Line Authentication

Supplemental data for the classical and enhanced sampling molecular dynamics simulations

Supplemental Table 1: Table of all antibodies used in studies including the company, catalog numbers and dilutions used to perform western blots.

Wnt signaling is a major driver of stemness and chemoresistance in ovarian cancer, yet the genetic drivers that stimulate its expression remain largely unknown. Unlike other cancers, mutations in the Wnt pathway are not reported in high-grade serous ovarian cancer (HGSOC). Hence, a key challenge that must be addressed to develop effective targeted therapies is to identify nonmutational drivers of Wnt activation. Using an miRNA sensor-based approach, we have identified miR-181a as a novel driver of Wnt/β-catenin signaling. miR-181ahigh primary HGSOC cells exhibited increased Wnt/β-catenin signaling, which was associated with increased stem-cell frequency and platinum resistance. Consistent with these findings, inhibition of β-catenin decreased stem-like properties in miR-181ahigh cell populations and downregulated miR-181a. The Wnt inhibitor SFRP4 was identified as a novel target of miR-181a. Overall, our results demonstrate that miR-181a is a nonmutational activator of Wnt signaling that drives stemness and chemoresistance in HGSOC, suggesting that the miR–181a–SFRP4 axis can be evaluated as a novel biomarker for β-catenin–targeted therapy in this disease.Significance:These results demonstrate that miR-181a is an activator of Wnt signaling that drives stemness and chemoresistance in HGSOC and may be targeted therapeutically in recurrent disease.

Uterine serous carcinoma (USC) is a highly aggressive endometrial cancer subtype with limited therapeutic options and a lack of targeted therapies. While mutations to PPP2R1A, which encodes the predominant protein phosphatase 2A (PP2A) scaffolding protein Aα, occur in 30% to 40% of USC cases, the clinical actionability of these mutations has not been studied. Using a high-throughput screening approach, we showed that mutations in Aα results in synthetic lethality following treatment with inhibitors of ribonucleotide reductase (RNR). In vivo, multiple models of Aα mutant uterine serous tumors were sensitive to clofarabine, an RNR inhibitor (RNRi). Aα-mutant cells displayed impaired checkpoint signaling upon RNRi treatment and subsequently accumulated more DNA damage than wild-type (WT) cells. Consistently, inhibition of PP2A activity using LB-100, a catalytic inhibitor, sensitized WT USC cells to RNRi. Analysis of The Cancer Genome Atlas data indicated that inactivation of PP2A, through loss of PP2A subunit expression, was prevalent in USC, with 88% of patients with USC harboring loss of at least one PP2A gene. In contrast, loss of PP2A subunit expression was rare in uterine endometrioid carcinomas. While RNRi are not routinely used for uterine cancers, a retrospective analysis of patients treated with gemcitabine as a second- or later-line therapy revealed a trend for improved outcomes in patients with USC treated with RNRi gemcitabine compared with patients with endometrioid histology. Overall, our data provide experimental evidence to support the use of ribonucleotide reductase inhibitors for the treatment of USC.Significance:A drug repurposing screen identifies synthetic lethal interactions in PP2A-deficient uterine serous carcinoma, providing potential therapeutic avenues for treating this deadly endometrial cancer.

Supplemental data for the investigation of growth inhibition induced by SMAP treatment in EMCA

Supplemental data evaluating for disruption of PP2A holoenzyme assembly in a cell-free binding assay system

Somatic mutation of the protein phosphatase 2A (PP2A) Aα-subunit gene PPP2R1A is highly prevalent in high-grade endometrial carcinoma. The structural, molecular, and biological basis by which the most recurrent endometrial carcinoma–specific mutation site P179 facilitates features of endometrial carcinoma malignancy has yet to be fully determined. Here, we used a series of structural, biochemical, and biological approaches to investigate the impact of the P179R missense mutation on PP2A function. Enhanced sampling molecular dynamics simulations showed that arginine-to-proline substitution at the P179 residue changes the protein's stable conformation profile. A crystal structure of the tumor-derived PP2A mutant revealed marked changes in A-subunit conformation. Binding to the PP2A catalytic subunit was significantly impaired, disrupting holoenzyme formation and enzymatic activity. Cancer cells were dependent on PP2A disruption for sustained tumorigenic potential, and restoration of wild-type Aα in a patient-derived P179R-mutant cell line restored enzyme function and significantly attenuated tumorigenesis and metastasis in vivo. Furthermore, small molecule–mediated therapeutic reactivation of PP2A significantly inhibited tumorigenicity in vivo. These outcomes implicate PP2A functional inactivation as a critical component of high-grade endometrial carcinoma disease pathogenesis. Moreover, they highlight PP2A reactivation as a potential therapeutic strategy for patients who harbor P179R PPP2R1A mutations.Significance:This study characterizes a highly recurrent, disease-specific PP2A PPP2R1A mutation as a driver of endometrial carcinoma and a target for novel therapeutic development.See related commentary by Haines and Huang, p. 4009

Antibody information and primer sequences.

Supplementary Figure from Targeting Ribonucleotide Reductase Induces Synthetic Lethality in PP2A-Deficient Uterine Serous Carcinoma

Supplementary Figure Legends and Methods

PP2A Aα-, C-, and B55α-subunit protein abundance in PPP2R1A wildtype and P179-mutant patient EMCA tumor samples

Supplemental Figure 1. Comparison of the anti-proliferative activity of SMAP across a panel of cell lines. Supplemental Figure 2. The chemical structure of SMAPs utilized in the studies detailed in the manuscript. Supplemental Figure 3. The effects of SMAPs on cell proliferation and cell survival. Supplemental Figure 4. Control studies for co-immunoprecipitation experiments. Supplemental Figure 5. Activity of SMAPs in vivo with the correlation of anti-tumor activity with SMAP-2 exposure in serum of treated mice.

Supplemental data on PPP2R1A mutation in endometrial cancer (EMCA) and clinical and genomic features of P179 mutation-positive EMCA tumors, from the cBioportal cancer database

Sequencing outcomes and PPP2R1A hotspot mutation frequency for the Case Comprehensive Cancer Center Gynecologic Tumor Biobank

Supplemental Figures 1-8.

Primary prostate cancer is generally treatable by androgen deprivation therapy, however, later recurrences of castrate-resistant prostate cancer (CRPC) that are more difficult to treat nearly always occur due to aberrant reactivation of the androgen receptor (AR). In this study, we report that CRPC cells are particularly sensitive to the growth-inhibitory effects of reengineered tricyclic sulfonamides, a class of molecules that activate the protein phosphatase PP2A, which inhibits multiple oncogenic signaling pathways. Treatment of CRPC cells with small-molecule activators of PP2A (SMAP) in vitro decreased cellular viability and clonogenicity and induced apoptosis. SMAP treatment also induced an array of significant changes in the phosphoproteome, including most notably dephosphorylation of full-length and truncated isoforms of the AR and downregulation of its regulatory kinases in a dose-dependent and time-dependent manner. In murine xenograft models of human CRPC, the potent compound SMAP-2 exhibited efficacy comparable with enzalutamide in inhibiting tumor formation. Overall, our results provide a preclinical proof of concept for the efficacy of SMAP in AR degradation and CRPC treatment.Significance: A novel class of small-molecule activators of the tumor suppressor PP2A, a serine/threonine phosphatase that inhibits many oncogenic signaling pathways, is shown to deregulate the phosphoproteome and to destabilize the androgen receptor in advanced prostate cancer. Cancer Res; 78(8); 2065–80. ©2018 AACR.

Table S1. KSEA Analysis and the Mean FC Method in LNCaP treated with SMAP. Table S2. ANOVA with multiple comparisons and Dunnett''s post-hoc test for SMAP treated LNCaP colony formation assay. Table S3. ANOVA with multiple comparisons and Dunnett''s post-hoc test for SMAP treated 22Rv1 colony formation assay. Table S4. ANOVA with multiple comparisons and Dunnett''s post-hoc test for Annexin V staining of LNCap and 22Rv1 cells. Table S5. PPI of the phosphoproteomics dataset. Table S6. Kinase Substrate Database for LNCaP treated with SMAP. Table S7. ANOVA with multiple comparisons and Tukey''s post-hoc test for SMAP treatment in LNCaP cells phosphorylated to total AR ratio protein. Table S8. ANOVA for qRT-PCR RNA for AR targets in LNCaP and 22Rv1 cells in presence of SMAPs. Table S9. ANOVA with multiple comparisons and Dunnett''s post-hoc test for LNCaP and 22Rv1 cells treated with SMAP, bortezomib, and combination. Table S10. ANOVA with multiple comparisons and Tukey''s post-hoc test for relative AR protein expression in LNCaP- Small T stably expressing lines treated with SMAP. Table S11. ANOVA with multiple comparisons and Tukey''s post-hoc test for relative AR protein expression in LNCaP cells in FBS or CSS in presence or absence of R1881. Table S12. ANOVA for qRT-PCR RNA AR targets statistical analysis for LNCaP cells in FBS or CSS in presence or absence of R1881. Table S13. ANOVA with multiple comparisons and Dunnett''s post-hoc test for LnCaP/AR xenograft treatment study: vehicle control, SMAP 400mg/kg, SMAP 100mg/kg and MDV3100 100mg/kg. Table S14. ANOVA with multiple comparisons and Dunnett''s post-hoc test for castrated LNCaP/AR tumor volumes for individual treatments groups: Vehicle control, SMAP-2 100mg/kg and SMAP-2 30mg/kg. Table S15. ANOVA with multiple comparisons and Dunnett''s post-hoc test for TUNEL positive staining quantification from castrated LNCaP/AR tumors from individual treatments groups: Vehicle control, SMAP-2 30mg/kg and SMAP-2 100mg/kg. Table S16. ANOVA with multiple comparisons and Dunnett''s post-hoc test for PCNA positive staining quantification from castrated LNCaP/AR for individual treatments groups: Vehicle control, SMAP-2 100mg/kg and SMAP-2 30mg/kg.

Supplementary Figure S1. Generation of acquired cisplatin resistant models in EOC. Supplementary Figure S2. Expression of mitotic marker p-H3-Ser-10 in response to mitotic arrest and release in cisplatin sensitive and resistant cells. Supplementary Figure S3. Si-PLK1 phenotype in OV231 and OV231-CP30 Supplementary Figure S4. Generation of acquired resistance models of Volasertib in EOC. Supplementary Figure S5. Phospho-Ser-133 CyclinB1 expression in Cisplatin/Volasertib sensitive/resistant cells. Supplementary Figure S6. Effect of Volasertib on cell viability in Sh-PLK1 OV81.2- CP40 cells.

Supplementary Figures S1-2,4, describe cell viability curves of tested cell lines. Figure S3 and S5 show bioinformatic data for resistance upregulated gene sets (S3) and cisplatin-taxane comparison analysis(S5). Figure S6 shows ECAR and OCR experiments. Figure S7 is a heatmap of combination indices of cisplatin with JQ1. Figure S8 shows apoptosis rates of cell lines. S9-12 describe cell viability dynamics of SOX9 KOs.

Supplementary Data from Targeting Ribonucleotide Reductase Induces Synthetic Lethality in PP2A-Deficient Uterine Serous Carcinoma

Supplemental data for the investigations of tumorigenic phenotypes in the UT89 and UT42 isogenic cell line model systems

Chemoresistance is driven by unique regulatory networks in the genome that are distinct from those necessary for cancer development. Here, we investigate the contribution of enhancer elements to cisplatin resistance in ovarian cancers. Epigenome profiling of multiple cellular models of chemoresistance identified unique sets of distal enhancers, super-enhancers (SE), and their gene targets that coordinate and maintain the transcriptional program of the platinum-resistant state in ovarian cancer. Pharmacologic inhibition of distal enhancers through small-molecule epigenetic inhibitors suppressed the expression of their target genes and restored cisplatin sensitivity in vitro and in vivo. In addition to known drivers of chemoresistance, our findings identified SOX9 as a critical SE-regulated transcription factor that plays a critical role in acquiring and maintaining the chemoresistant state in ovarian cancer. The approach and findings presented here suggest that integrative analysis of epigenome and transcriptional programs could identify targetable key drivers of chemoresistance in cancers.Significance:Integrative genome-wide epigenomic and transcriptomic analyses of platinum-sensitive and -resistant ovarian lines identify key distal regulatory regions and associated master regulator transcription factors that can be targeted by small-molecule epigenetic inhibitors.

Supplementary Figure 7 from KLF6-SV1 Is a Novel Antiapoptotic Protein That Targets the BH3-Only Protein NOXA for Degradation and Whose Inhibition Extends Survival in an Ovarian Cancer Model

Supplementary Figure 1 from KLF6-SV1 Is a Novel Antiapoptotic Protein That Targets the BH3-Only Protein NOXA for Degradation and Whose Inhibition Extends Survival in an Ovarian Cancer Model

Supplementary Figure 2 from KLF6-SV1 Is a Novel Antiapoptotic Protein That Targets the BH3-Only Protein NOXA for Degradation and Whose Inhibition Extends Survival in an Ovarian Cancer Model

Supplementary Figure 6 from KLF6-SV1 Is a Novel Antiapoptotic Protein That Targets the BH3-Only Protein NOXA for Degradation and Whose Inhibition Extends Survival in an Ovarian Cancer Model

Supplementary Figure 4 from KLF6-SV1 Is a Novel Antiapoptotic Protein That Targets the BH3-Only Protein NOXA for Degradation and Whose Inhibition Extends Survival in an Ovarian Cancer Model

Supplementary Figure Legends 1-7 from KLF6-SV1 Is a Novel Antiapoptotic Protein That Targets the BH3-Only Protein NOXA for Degradation and Whose Inhibition Extends Survival in an Ovarian Cancer Model

High-Grade Serous Carcinoma (HGSC) is the most common and lethal ovarian cancer subtype. PARP-inhibitors (PARPi) have become the mainstay of HGSC targeted therapy, given that these tumors are driven by a high degree of genomic instability and Homologous Recombination (HR) defects. Nonetheless, only ∼30% of patients initially respond to treatment, ultimately relapsing with resistant disease. Thus, despite recent advances in drug development and increased understanding of genetic alterations driving HGSC progression, mortality has not declined, highlighting the need for novel therapies. Using a Small Molecule Activator of Protein Phosphatase 2A (PP2A) (SMAP-061), we investigated the mechanism by which PP2A stabilization induces apoptosis in Patient-Derived HGSC cells and Xenograft (PDX) models alone or in combination with PARPi. We uncovered that PP2A genes essential for transformation (B56α,B56γ and PR72) and basal phosphatase activity (PP2A-A and -C) are heterozygously lost in the majority of HGSC. Moreover, loss of these PP2A genes correlates with worse overall patient survival. We show that SMAP-061 stabilization of PP2A inhibits the HR output by targeting RAD51, leading to chronic accumulation of DNA damage and ultimately apoptosis. Furthermore, combination of SMAP-061 and PARPi leads to enhanced apoptosis in both HR-proficient and -deficient cells and in patient-derived xenograft models. Our studies identify PP2A as novel regulator of HR and introduces PP2A activators as a potential treatment for HGSC tumors. Our studies further emphasize the potential of PP2A modulators to overcome PARPi insensitivity, given that targeting RAD51 has presented benefits in overcoming PARPi-resistance driven by BRCA1/2 mutation reversions.

The large majority of high grade serous ovarian cancer (HGSOC) patients will eventually become resistant to platinum-based chemotherapy, highlighting the urgent need to develop novel therapeutics targeting recurrent, platinum-resistant disease. To study this, our lab generated two patient derived platinum-resistant tumor spheres from platinum-sensitive primary patient cells to better understand the biology behind recurrent disease and identify novel therapeutic targets. Utilizing an artificial intelligence (AI) powered model, we screened a library that consisted of compounds predicted to target known or postulated resistance mechanisms and involved a novel therapeutic target. From this screen, we identified a TNIK kinase inhibitor as the only compound to effectively kill HGSOC spheres while sparing non-malignant fallopian tube cells. TNIK plays a key role in Wnt pathway activation, which our lab has previously shown to be critical in driving cancer stemness and epithelial ovarian cancer platinum resistance. Additionally, patient data shows that TNIK is amplified in ovarian cancer more than any other cancer type and is highly activated in ovarian cancer patients. We found that this compound was able to decrease overall Wnt activity in our platinum-resistant cells and could resensitize our platinum-resistant cells to cisplatin. This work demonstrates the abilities of using AI to expedite the drug discovery process as well as uncover novel therapeutic targets which may lead to future therapeutic strategies for HGSOC patients who no longer respond to standard of care platinum chemotherapy. Citation Format: Noah Puleo, Michele Cusato, Dylan Carvette, Chantelle Husdon, Analisa DiFeo. Unraveling the function of TRAF2 and NCK interacting kinase (TNIK) in high-grade serous ovarian 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 1679.

While the rates of new cases and deaths for high grade serous ovarian cancer (HGSC) have declined steadily in the past decade, HGSC remains the fifth leading cause of cancer death in women. As many patients succumb to chemoresistance, there is a need to uncover novel targeted drugs. One way to do so is by drug repurposing. A computational screen, DrugPredict, led to the discovery that amiodarone, an antiarrhythmic, is a potent apoptosis-inducer and Myc-degrader in numerous patient-derived ovarian cancer cell lines. In search of a more selective, but less toxic compound, structure-activity relationship was applied to identify DL78. Due to its structural differences, DL78 is considered to be independent of amiodarone. DL78 lacks hERG activity, retains Myc regulation, and gains selective anticancer properties. This compound produces significant cell death selectively in patient-derived HGSC cells, including isogenic cisplatin-resistant lines, but does not harm non-transformed fallopian tube cells. Cancer cells treated with DL78 arrest prior to anaphase and showcase microtubule abnormalities and upregulation of mitotic regulators. The prolonged arrest leads to loss of Myc, mitotic catastrophe, and apoptosis selectively in the HGSC cells, while fallopian tube cells can cycle normally. DL78’s activity is confirmed to be dependent on Myc expression because targeted inhibition of Myc decreases its potency and prevents the DL78-induced cell cycle arrest. Finally, pharmacokinetic analysis shows that DL78 saturates the tumor following intraperitoneal injection, yet drug concentrations in the blood and liver present at normal levels. Overall, these phenotypes do not show with amiodarone treatment, suggesting that structurally different DL78 performs through a different mechanism. Further classification of this tumor-selective compound may reveal targeted ovarian cancer vulnerabilities. Citation Format: Jessica Teitel, Michele Cusato, Margaret Farah, Agharnan Gandhi, Pil Lee, Andrew White, John Takyi-Williams, Bo Wen, Andre Monterio Da Rocha, Analisa DiFeo. Novel Myc-dependent compound DL78 selectively induces mitotic catastrophe and cell death in high grade serous ovarian 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 1570.

Dysregulation of cap-dependent translation is a hallmark of cancer, with key roles in supporting the transformed phenotype. The eIF4E binding proteins (4E-BP1, 2, 3) are major negative regulators of cap-dependent translation that are inactivated in tumors through inhibitory phosphorylation by oncogenic kinases (e.g., mTOR) or by downregulation. Previous studies from our group and others have linked tumor suppressive PP2A family serine/threonine phosphatases to activation of 4E-BP1. Here, we leveraged novel small molecule activators of PP2A (SMAPs) (e.g., DT-061, DT-1154) that are being developed as antitumor agents to (a) explore the role of a subset of B56-PP2As in regulation of 4E-BP activity, and (b) to evaluate the potential of B56-PP2A reactivation for restoring translation control in tumor cells. We show that SMAPs promote PP2A-dependent hypophosphorylation of 4E-BP1/4EBP2 in the presence of active upstream inhibitory kinases (mTOR, ERK, AKT), supporting a role for B56-PP2As as 4E-BP phosphatases. Unexpectedly, DT-061 also led to robust PP2A-dependent upregulation of 4E-BP1, but not 4E-BP2 or 4E-BP3. Cap-binding assays and eIF4E immunoprecipitation showed that SMAP/B56-PP2A blocks the formation of the eIF4F translation initiation complex. Bicistronic reporter assays that directly measure cap-dependent translation activity confirmed the translational consequences of these effects. siRNA knockdown pointed to B56α-PP2A as a mediator of SMAP effects on 4E-BPs, although B56β- and/or B56ε-PP2A may also play a role. 4E-BP1 upregulation involved ATF4-dependent transcription of the 4E-BP1 gene (EIF4EBP1) and the effect was partially dependent on TFE3/TFEB transcription factors. Thus, B56-PP2A orchestrates a translation repressive program involving transcriptional induction and hypophosphorylation of 4E-BP1, highlighting the potential of PP2A-based therapeutic strategies for restoration of translation control in cancer cells.

Uterine serous carcinoma (USC) is a highly aggressive endometrial cancer subtype with limited therapeutic options and a lack of targeted therapies. While mutations to PPP2R1A, which encodes the predominant protein phosphatase 2A (PP2A) scaffolding protein Aα, occur in 30% to 40% of USC cases, the clinical actionability of these mutations has not been studied. Using a high-throughput screening approach, we showed that mutations in Aα results in synthetic lethality following treatment with inhibitors of ribonucleotide reductase (RNR). In vivo, multiple models of Aα mutant uterine serous tumors were sensitive to clofarabine, an RNR inhibitor (RNRi). Aα-mutant cells displayed impaired checkpoint signaling upon RNRi treatment and subsequently accumulated more DNA damage than wild-type (WT) cells. Consistently, inhibition of PP2A activity using LB-100, a catalytic inhibitor, sensitized WT USC cells to RNRi. Analysis of The Cancer Genome Atlas data indicated that inactivation of PP2A, through loss of PP2A subunit expression, was prevalent in USC, with 88% of patients with USC harboring loss of at least one PP2A gene. In contrast, loss of PP2A subunit expression was rare in uterine endometrioid carcinomas. While RNRi are not routinely used for uterine cancers, a retrospective analysis of patients treated with gemcitabine as a second- or later-line therapy revealed a trend for improved outcomes in patients with USC treated with RNRi gemcitabine compared with patients with endometrioid histology. Overall, our data provide experimental evidence to support the use of ribonucleotide reductase inhibitors for the treatment of USC. Significance: A drug repurposing screen identifies synthetic lethal interactions in PP2A-deficient uterine serous carcinoma, providing potential therapeutic avenues for treating this deadly endometrial cancer.