Publications by authors named "Goutham Narla"

88 Publications

Genome-Wide Association Study of Pelvic Organ Prolapse Using the Michigan Genomics Initiative.

Female Pelvic Med Reconstr Surg 2021 May 22. Epub 2021 May 22.

From the Department of Obstetrics and Gynecology, University of Pennsylvania, Philadelphia, PA Department of Biostatistics School of Public Health Division of Genetic Medicine Department of Obstetrics and Gynecology, University of Michigan, Ann Arbor, MI.

Objectives: The aim of this study was to (1) replicate previously identified genetic variants significantly associated with pelvic organ prolapse and (2) identify new genetic variants associated with pelvic organ prolapse using a genome-wide association study.

Methods: Using our institution's database linking genetic and clinical data, we identified 1,329 women of European ancestry with an International Classification of Diseases, Ninth Revision (ICD-9)/ICD-10 code for prolapse, 767 of whom also had Current Procedural Terminology (CPT)/ICD-9/ICD-10 procedure codes for prolapse surgery, and 16,383 women of European ancestry older than 40 years without a prolapse diagnosis code as controls. Patients were genotyped using the Illumina HumanCoreExome chip and imputed to the Haplotype Reference Consortium. We tested 20 million single nucleotide polymorphisms (SNPs) for association with pelvic organ prolapse adjusting for relatedness, age, chip version, and 4 principal components. We compared our results with 18 previously identified genome-wide significant SNPs from the UK Biobank, Commun Biol (2020;3:129), and Obstet Gynecol (2011;118:1345-1353).

Results: No variants achieved genome-wide significance (P = 5 × 10-8). However, we replicated 4 SNPs with biologic plausibility at nominal significance (P ≤ 0.05): rs12325192 (P = 0.002), rs9306894 (P = 0.05), rs1920568 (P = 0.034), and rs1247943 (P = 0.041), which were all intergenic and nearest the genes SALL1, GDF7, TBX5, and TBX5, respectively.

Conclusions: Our replication of 4 biologically plausible previously reported SNPs provides further evidence for a genetic contribution to prolapse, specifically that rs12325192, rs9306894, rs1920568, and rs1247943 may contribute to susceptibility for prolapse. These and previously reported associations that have not yet been replicated should be further explored in larger, more diverse cohorts, perhaps through meta-analysis.
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http://dx.doi.org/10.1097/SPV.0000000000001075DOI Listing
May 2021

Development and comparison of novel bioluminescent mouse models of pancreatic neuroendocrine neoplasm metastasis.

Sci Rep 2021 May 13;11(1):10252. Epub 2021 May 13.

Department of Neuroscience and Pharmacology, University of Iowa, 2-570 Bowen Science Building, 51 Newton Road, Iowa City, IA, 52242, USA.

Pancreatic neuroendocrine neoplasms (pNENs) are slow growing cancers of increasing incidence that lack effective treatments once they become metastatic. Unfortunately, nearly half of pNEN patients present with metastatic liver tumors at diagnosis and current therapies fail to improve overall survival. Pre-clinical models of pNEN metastasis are needed to advance our understanding of the mechanisms driving the metastatic process and for the development of novel, targeted therapeutic interventions. To model metastatic dissemination of tumor cells, human pNEN cell lines (BON1 and Qgp1) stably expressing firefly luciferase (luc) were generated and introduced into NSG immunodeficient mice by intracardiac (IC) or intravenous (IV) injection. The efficiency, kinetics and distribution of tumor growth was evaluated weekly by non-invasive bioluminescent imaging (BLI). Tumors formed in all animals in both the IC and IV models. Bioluminescent Qgp1.luc cells preferentially metastasized to the liver regardless of delivery route, mimicking the predominant site of pNEN metastasis in patients. By comparison, BON1.luc cells most commonly formed lung tumors following either IV or IC administration and colonized a wider variety of tissues than Qgp1.luc cells. These models provide a unique platform for testing candidate metastasis genes and anti-metastatic therapies for pNENs.
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http://dx.doi.org/10.1038/s41598-021-89866-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8119958PMC
May 2021

Mistletoe Extract Viscum Fraxini-2 for Treatment of Advanced Hepatocellular Carcinoma: A Case Series.

Case Rep Oncol 2021 Jan-Apr;14(1):224-231. Epub 2021 Mar 1.

Department of Gastrointestinal Medical Oncology, The University of Texas MD Anderson Cancer Center, Houston, Texas, USA.

Background: Hepatocellular carcinoma (HCC) is the fourth leading cause of death from cancer worldwide, and for advanced HCC the prognosis is poor. Preliminary studies indicate mistletoe extracts may have anticancer activity for HCC.

Methods: A prospective observational case series of advanced HCC patients that chose to take a mistletoe extract called viscum fraxini-2 (VF-2) alone for treatment. Time on treatment, imaging, and laboratory values were collected for descriptive analyses.

Results: A total of 12 patients with advanced HCC enrolled onto the protocol, and 10 patients had data available for evaluation. The majority were male (10/12) with a median age of 64 (SD 11). Most patients had received sorafenib therapy (9/12) and had varying Child-Pugh classes (A-4, B-6, C-2). Treatment with VF-2 ranged from 1 to 36 weeks with a mean of 12.3 weeks (SD 12). Six patients received 8 weeks of treatment, and 3 patients received 12 or more weeks of treatment. For patients that received at least 4 weeks of treatment, the average AFP value stabilized during the first 4 weeks of treatment. Two patients experienced an AFP decrease of >30%, approximately 37 and 40% decreases at the nadir. One patient had stable disease of 9 months. Major side effects were fever, fatigue, rash, and local injection site reaction of swelling, redness, and tenderness.

Conclusion: This case series of advanced HCC indicates that mistletoe extract VF-2 may have potential biological activity against HCC for selected patients. Research is needed to identify the active compound and predictive markers of response.
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http://dx.doi.org/10.1159/000511566DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7983630PMC
March 2021

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

Mol Cancer Ther 2021 04 10;20(4):676-690. Epub 2021 Feb 10.

The University of Maryland Marlene and Stewart Greenebaum Comprehensive Cancer Center,

-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 and MV4-11 growth suppression 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.
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http://dx.doi.org/10.1158/1535-7163.MCT-20-0663DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8027945PMC
April 2021

Loss of hepatic aldolase B activates Akt and promotes hepatocellular carcinogenesis by destabilizing the Aldob/Akt/PP2A protein complex.

PLoS Biol 2020 12 4;18(12):e3000803. Epub 2020 Dec 4.

CAS Key Laboratory of Nutrition, Metabolism and Food Safety, Shanghai Institute of Nutrition and Health, Chinese Academy of Sciences (CAS), Shanghai, China.

Loss of hepatic fructose-1, 6-bisphosphate aldolase B (Aldob) leads to a paradoxical up-regulation of glucose metabolism to favor hepatocellular carcinogenesis (HCC), but the upstream signaling events remain poorly defined. Akt is highly activated in HCC, and targeting Akt is being explored as a potential therapy for HCC. Herein, we demonstrate that Aldob suppresses Akt activity and tumor growth through a protein complex containing Aldob, Akt, and protein phosphatase 2A (PP2A), leading to inhibition of cell viability, cell cycle progression, glucose uptake, and metabolism. Interestingly, Aldob directly interacts with phosphorylated Akt (p-Akt) and promotes the recruitment of PP2A to dephosphorylate p-Akt, and this scaffolding effect of Aldob is independent of its enzymatic activity. Loss of Aldob or disruption of Aldob/Akt interaction in Aldob R304A mutant restores Akt activity and tumor-promoting effects. Consistently, Aldob and p-Akt expression are inversely correlated in human HCC tissues, and Aldob down-regulation coupled with p-Akt up-regulation predicts a poor prognosis for HCC. We have further discovered that Akt inhibition or a specific small-molecule activator of PP2A (SMAP) efficiently attenuates HCC tumorigenesis in xenograft mouse models. Our work reveals a novel nonenzymatic role of Aldob in negative regulation of Akt activation, suggesting that directly inhibiting Akt activity or through reactivating PP2A may be a potential therapeutic approach for HCC treatment.
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http://dx.doi.org/10.1371/journal.pbio.3000803DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7744066PMC
December 2020

The SRG rat, a Sprague-Dawley Rag2/Il2rg double-knockout validated for human tumor oncology studies.

PLoS One 2020 7;15(10):e0240169. Epub 2020 Oct 7.

Hera BioLabs Inc., Lexington, Kentucky, United States of America.

We have created the immunodeficient SRG rat, a Sprague-Dawley Rag2/Il2rg double knockout that lacks mature B cells, T cells, and circulating NK cells. This model has been tested and validated for use in oncology (SRG OncoRat®). The SRG rat demonstrates efficient tumor take rates and growth kinetics with different human cancer cell lines and PDXs. Although multiple immunodeficient rodent strains are available, some important human cancer cell lines exhibit poor tumor growth and high variability in those models. The VCaP prostate cancer model is one such cell line that engrafts unreliably and grows irregularly in existing models but displays over 90% engraftment rate in the SRG rat with uniform growth kinetics. Since rats can support much larger tumors than mice, the SRG rat is an attractive host for PDX establishment. Surgically resected NSCLC tissue from nine patients were implanted in SRG rats, seven of which engrafted and grew for an overall success rate of 78%. These developed into a large tumor volume, over 20,000 mm3 in the first passage, which would provide an ample source of tissue for characterization and/or subsequent passage into NSG mice for drug efficacy studies. Molecular characterization and histological analyses were performed for three PDX lines and showed high concordance between passages 1, 2 and 3 (P1, P2, P3), and the original patient sample. Our data suggest the SRG OncoRat is a valuable tool for establishing PDX banks and thus serves as an alternative to current PDX mouse models hindered by low engraftment rates, slow tumor growth kinetics, and multiple passages to develop adequate tissue banks.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0240169PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7540894PMC
December 2020

Monotherapy efficacy of blood-brain barrier permeable small molecule reactivators of protein phosphatase 2A in glioblastoma.

Brain Commun 2020 11;2(1):fcaa002. Epub 2020 Jan 11.

Turku Bioscience Centre, University of Turku and Åbo Akademi University, Turku 20520, Finland.

Glioblastoma is a fatal disease in which most targeted therapies have clinically failed. However, pharmacological reactivation of tumour suppressors has not been thoroughly studied as yet as a glioblastoma therapeutic strategy. Tumour suppressor protein phosphatase 2A is inhibited by non-genetic mechanisms in glioblastoma, and thus, it would be potentially amendable for therapeutic reactivation. Here, we demonstrate that mall olecule ctivators of rotein phosphatase 2A, NZ-8-061 and DBK-1154, effectively cross the model of blood-brain barrier, and partition to mouse brain tissue after oral dosing. , small molecule activators of protein phosphatase 2A exhibit robust cell-killing activity against five established glioblastoma cell lines, and nine patient-derived primary glioma cell lines. Collectively, these cell lines have heterogeneous genetic background, kinase inhibitor resistance profile and stemness properties; and they represent different clinical glioblastoma subtypes. Moreover, small molecule activators of protein phosphatase 2A were found to be superior to a range of kinase inhibitors in their capacity to kill patient-derived primary glioma cells. Oral dosing of either of the small molecule activators of protein phosphatase 2A significantly reduced growth of infiltrative intracranial glioblastoma tumours. DBK-1154, with both higher degree of brain/blood distribution, and more potent activity against all tested glioblastoma cell lines, also significantly increased survival of mice bearing orthotopic glioblastoma xenografts. In summary, this report presents a proof-of-principle data for blood-brain barrier-permeable tumour suppressor reactivation therapy for glioblastoma cells of heterogenous molecular background. These results also provide the first indications that protein phosphatase 2A reactivation might be able to challenge the current paradigm in glioblastoma therapies which has been strongly focused on targeting specific genetically altered cancer drivers with highly specific inhibitors. Based on demonstrated role for protein phosphatase 2A inhibition in glioblastoma cell drug resistance, small molecule activators of protein phosphatase 2A may prove to be beneficial in future glioblastoma combination therapies.
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http://dx.doi.org/10.1093/braincomms/fcaa002DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7425423PMC
January 2020

Targeting UHRF1-dependent DNA repair selectively sensitizes KRAS mutant lung cancer to chemotherapy.

Cancer Lett 2020 11 16;493:80-90. Epub 2020 Aug 16.

Department of Pharmacology, Case Comprehensive Cancer Center, Case Western Reserve University School of Medicine, Cleveland, OH, 44106, USA. Electronic address:

Kirsten rat sarcoma virus oncogene homolog (KRAS) mutant lung cancer remains a challenge to cure and chemotherapy is the current standard treatment in the clinic. Hence, understanding molecular mechanisms underlying the sensitivity of KRAS mutant lung cancer to chemotherapy could help uncover unique strategies to treat this disease. Here we report a compound library screen and identification of cardiac glycosides as agents that selectively enhance the in vitro and in vivo effects of chemotherapy on KRAS mutant lung cancer. Quantitative mass spectrometry reveals that cardiac glycosides inhibit DNA double strand break (DSB) repair through suppressing the expression of UHRF1, an important DSB repair factor. Inhibition of UHRF1 by cardiac glycosides was mediated by specific suppression of the oncogenic KRAS pathway. Overexpression of UHRF1 rescued DSB repair inhibited by cardiac glycosides and depletion of UHRF1 mitigated cardiac glycoside-enhanced chemotherapeutic drug sensitivity in KRAS mutant lung cancer cells. Our study reveals a targetable dependency on UHRF1-stimulated DSB repair in KRAS mutant lung cancer in response to chemotherapy.
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http://dx.doi.org/10.1016/j.canlet.2020.08.008DOI Listing
November 2020

Unbiased Proteomic Profiling Uncovers a Targetable GNAS/PKA/PP2A Axis in Small Cell Lung Cancer Stem Cells.

Cancer Cell 2020 07 11;38(1):129-143.e7. Epub 2020 Jun 11.

Department of Pediatrics, Stanford University, 265 Campus Drive, Stanford, CA 94305-5457, USA; Department of Genetics, Stanford University, Stanford, CA 94305, USA. Electronic address:

Using unbiased kinase profiling, we identified protein kinase A (PKA) as an active kinase in small cell lung cancer (SCLC). Inhibition of PKA activity genetically, or pharmacologically by activation of the PP2A phosphatase, suppresses SCLC expansion in culture and in vivo. Conversely, GNAS (G-protein α subunit), a PKA activator that is genetically activated in a small subset of human SCLC, promotes SCLC development. Phosphoproteomic analyses identified many PKA substrates and mechanisms of action. In particular, PKA activity is required for the propagation of SCLC stem cells in transplantation studies. Broad proteomic analysis of recalcitrant cancers has the potential to uncover targetable signaling networks, such as the GNAS/PKA/PP2A axis in SCLC.
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http://dx.doi.org/10.1016/j.ccell.2020.05.003DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7363571PMC
July 2020

A Genome-Wide Pooled shRNA Screen Identifies PPP2R2A as a Predictive Biomarker for the Response to ATR and CHK1 Inhibitors.

Cancer Res 2020 08 10;80(16):3305-3318. Epub 2020 Jun 10.

Department of Radiation Oncology, The Ohio State University James Comprehensive Cancer Center and College of Medicine, Ohio.

There is currently a lack of precise predictive biomarkers for patient selection in clinical trials of inhibitors targeting replication stress (RS) response proteins ATR and CHK1. The objective of this study was to identify novel predictive biomarkers for the response to these agents in treating non-small cell lung cancer (NSCLC). A genome-wide loss-of-function screen revealed that tumor suppressor PPP2R2A, a B regulatory subunit of protein phosphatase 2 (PP2A), determines sensitivity to CHK1 inhibition. A synthetic lethal interaction between PPP2R2A deficiency and ATR or CHK1 inhibition was observed in NSCLC and and was independent of p53 status. ATR and CHK1 inhibition resulted in significantly increased levels of RS and altered replication dynamics, particularly in PPP2R2A-deficient NSCLC cells. Mechanistically, PPP2R2A negatively regulated translation of oncogene c-Myc protein. c-Myc activity was required for PPP2R2A deficiency-induced alterations of replication initiation/RS and sensitivity to ATR/CHK1 inhibitors. We conclude that PPP2R2A deficiency elevates RS by upregulating c-Myc activity, rendering cells reliant on the ATR/CHK1 axis for survival. Our studies show a novel synthetic lethal interaction and identify PPP2R2A as a potential new predictive biomarker for patient stratification in the clinical use of ATR and CHK1 inhibitors. SIGNIFICANCE: This study reveals new approaches to specifically target PPP2R2A-deficient lung cancer cells and provides a novel biomarker that will significantly improve treatment outcome with ATR and CHK1 inhibitors.
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http://dx.doi.org/10.1158/0008-5472.CAN-20-0057DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7518641PMC
August 2020

Splice of Life for Cancer: Missplicing of PPP2R5A by Mutant SF3B1 Leads to MYC Stabilization and Tumorigenesis.

Cancer Discov 2020 06;10(6):765-767

Department of Internal Medicine: Genetic Medicine, University of Michigan, Ann Arbor, Michigan. Rogel Cancer Center, University of Michigan, Ann Arbor, Michigan.

Although mutations in are the most common RNA-splicing factor mutations in cancer, determining the downstream missplicing events that drive tumorigenesis has remained challenging. Liu and colleagues present a model by which mutant SF3B1 tumors displayed high levels of oncogenic MYC activity through the missplicing of PP2A-B56α, a key post-translational regulator of MYC stability, providing a new therapeutic target and driver of SF3B1-mediated tumorigenesis..
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http://dx.doi.org/10.1158/2159-8290.CD-20-0358DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8162994PMC
June 2020

Selective PP2A Enhancement through Biased Heterotrimer Stabilization.

Cell 2020 04 20;181(3):688-701.e16. Epub 2020 Apr 20.

Division of Genetic Medicine, Department of Internal Medicine, University of Michigan, Ann Arbor, MI 48105, USA; Rogel Cancer Center, University of Michigan, Ann Arbor, MI 48109, USA. Electronic address:

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.
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http://dx.doi.org/10.1016/j.cell.2020.03.038DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7243596PMC
April 2020

Challenges and Reinterpretation of Antibody-Based Research on Phosphorylation of Tyr on PP2Ac.

Cell Rep 2020 03;30(9):3164-3170.e3

Division of Genetic Medicine, Department of Internal Medicine, Michigan Medicine, University of Michigan, Ann Arbor, MI 48109, USA. Electronic address:

Aberrant hyperphosphorylation of the protein phosphatase 2A catalytic subunit (PP2Ac) at Tyr has been associated with aggressive disease and poor clinical outcome in multiple cancers. However, the study of reversible phosphorylation at this site has relied entirely upon the use of antibodies-most prominently, the clone E155. Here, we provide evidence that the E155 and F-8 phospho-Tyr antibodies cannot differentiate between phosphorylated and unphosphorylated forms of PP2Ac. The form of PP2Ac bound by these antibodies in H358 cells is unphosphorylated at the C-terminal tail. Furthermore, these antibodies are sensitive to additional protein modifications that occur near Tyr, including Thr phosphorylation and Leu methylation, when these post-translational modifications are present. Thus, studies that used these antibodies to report PP2Ac hyperphosphorylation require reinterpretation, as these antibodies cannot be reliably used as readouts for a single PP2Ac post-translational modification (PTM) change.
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http://dx.doi.org/10.1016/j.celrep.2020.02.012DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7156886PMC
March 2020

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

J Biol Chem 2020 01 10;295(3):757-770. Epub 2019 Dec 10.

Division of Genetic Medicine, Department of Internal Medicine, University of Michigan, Ann Arbor, Michigan 48105

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.
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http://dx.doi.org/10.1074/jbc.RA119.011443DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6970930PMC
January 2020

Deregulating MYC in a model of HER2+ breast cancer mimics human intertumoral heterogeneity.

J Clin Invest 2020 01;130(1):231-246

Department of Molecular and Medical Genetics.

The c-MYC (MYC) oncoprotein is often overexpressed in human breast cancer; however, its role in driving disease phenotypes is poorly understood. Here, we investigate the role of MYC in HER2+ disease, examining the relationship between HER2 expression and MYC phosphorylation in HER2+ patient tumors and characterizing the functional effects of deregulating MYC expression in the murine NeuNT model of amplified-HER2 breast cancer. Deregulated MYC alone was not tumorigenic, but coexpression with NeuNT resulted in increased MYC Ser62 phosphorylation and accelerated tumorigenesis. The resulting tumors were metastatic and associated with decreased survival compared with NeuNT alone. MYC;NeuNT tumors had increased intertumoral heterogeneity including a subtype of tumors not observed in NeuNT tumors, which showed distinct metaplastic histology and worse survival. The distinct subtypes of MYC;NeuNT tumors match existing subtypes of amplified-HER2, estrogen receptor-negative human tumors by molecular expression, identifying the preclinical utility of this murine model to interrogate subtype-specific differences in amplified-HER2 breast cancer. We show that these subtypes have differential sensitivity to clinical HER2/EGFR-targeted therapeutics, but small-molecule activators of PP2A, the phosphatase that regulates MYC Ser62 phosphorylation, circumvents these subtype-specific differences and ubiquitously suppresses tumor growth, demonstrating the therapeutic utility of this approach in targeting deregulated MYC breast cancers.
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http://dx.doi.org/10.1172/JCI126390DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6934197PMC
January 2020

Protein phosphatase 2A controls ongoing DNA replication by binding to and regulating cell division cycle 45 (CDC45).

J Biol Chem 2019 11 27;294(45):17043-17059. Epub 2019 Sep 27.

Department of Internal Medicine, Case Comprehensive Cancer Center, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106

Genomic replication is a highly regulated process and represents both a potential benefit and liability to rapidly dividing cells; however, the precise post-translational mechanisms regulating genomic replication are incompletely understood. Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase that regulates a diverse array of cellular processes. Here, utilizing both a gain-of-function chemical biology approach and loss-of-function genetic approaches to modulate PP2A activity, we found that PP2A regulates DNA replication. We demonstrate that increased PP2A activity can interrupt ongoing DNA replication, resulting in a prolonged S phase. The impaired replication resulted in a collapse of replication forks, inducing dsDNA breaks, homologous recombination, and a PP2A-dependent replication stress response. Additionally, we show that during replication, PP2A exists in complex with cell division cycle 45 (CDC45) and that increased PP2A activity caused dissociation of CDC45 and polymerase α from the replisome. Furthermore, we found that individuals harboring mutations in the PP2A Aα gene have a higher fraction of genomic alterations, suggesting that PP2A regulates ongoing replication as a mechanism for maintaining genomic integrity. These results reveal a new function for PP2A in regulating ongoing DNA replication and a potential role for PP2A in the intra-S-phase checkpoint.
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http://dx.doi.org/10.1074/jbc.RA119.010432DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6851307PMC
November 2019

Inactivation of PP2A by a recurrent mutation drives resistance to MEK inhibitors.

Oncogene 2020 01 20;39(3):703-717. Epub 2019 Sep 20.

Department of Internal Medicine: Genetic Medicine, University of Michigan, Ann Arbor, MI, USA.

The serine/threonine Protein Phosphatase 2A (PP2A) functions as a tumor suppressor by negatively regulating multiple oncogenic signaling pathways. The canonical PP2A holoenzyme comprises a scaffolding subunit (PP2A Aα/β), which serves as the platform for binding of both the catalytic C subunit and one regulatory B subunit. Somatic heterozygous missense mutations in PPP2R1A, the gene encoding the PP2A Aα scaffolding subunit, have been identified across multiple cancer types, but the effects of the most commonly mutated residue, Arg-183, on PP2A function have yet to be fully elucidated. In this study, we used a series of cellular and in vivo models and discovered that the most frequent Aα R183W mutation formed alternative holoenzymes by binding of different PP2A regulatory subunits compared with wild-type Aα, suggesting a rededication of PP2A functions. Unlike wild-type Aα, which suppressed tumorigenesis, the R183W mutant failed to suppress tumor growth in vivo through activation of the MAPK pathway in RAS-mutant transformed cells. Furthermore, cells expressing R183W were less sensitive to MEK inhibitors. Taken together, our results demonstrate that the R183W mutation in PP2A Aα scaffold abrogates the tumor suppressive actions of PP2A, thereby potentiating oncogenic signaling and reducing drug sensitivity of RAS-mutant cells.
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http://dx.doi.org/10.1038/s41388-019-1012-2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6980487PMC
January 2020

CDK7 Inhibition Suppresses Castration-Resistant Prostate Cancer through MED1 Inactivation.

Cancer Discov 2019 11 29;9(11):1538-1555. Epub 2019 Aug 29.

Department of Cancer Biology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania.

Metastatic castration-resistant prostate cancer (CRPC) is a fatal disease, primarily resulting from the transcriptional addiction driven by androgen receptor (AR). First-line CRPC treatments typically target AR signaling, but are rapidly bypassed, resulting in only a modest survival benefit with antiandrogens. Therapeutic approaches that more effectively block the AR-transcriptional axis are urgently needed. Here, we investigated the molecular mechanism underlying the association between the transcriptional coactivator MED1 and AR as a vulnerability in AR-driven CRPC. MED1 undergoes CDK7-dependent phosphorylation at T1457 and physically engages AR at superenhancer sites, and is essential for AR-mediated transcription. In addition, a CDK7-specific inhibitor, THZ1, blunts AR-dependent neoplastic growth by blocking AR/MED1 corecruitment genome-wide, as well as reverses the hyperphosphorylated MED1-associated enzalutamide-resistant phenotype. , THZ1 induces tumor regression of AR-amplified human CRPC in a xenograft mouse model. Together, we demonstrate that CDK7 inhibition selectively targets MED1-mediated, AR-dependent oncogenic transcriptional amplification, thus representing a potential new approach for the treatment of CRPC. SIGNIFICANCE: Potent inhibition of AR signaling is critical to treat CRPC. This study uncovers a driver role for CDK7 in regulating AR-mediated transcription through phosphorylation of MED1, thus revealing a therapeutically targetable potential vulnerability in AR-addicted CRPC...
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http://dx.doi.org/10.1158/2159-8290.CD-19-0189DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7202356PMC
November 2019

Comment on "PP2A inhibition sensitizes cancer stem cells to ABL tyrosine kinase inhibitors in BCR-ABL human leukemia".

Sci Transl Med 2019 07;11(501)

School of Biomedical Sciences and Pharmacy, University of Newcastle, Callaghan, NSW 2308, Australia.

LB100 does not sensitize CML stem cells to tyrosine kinase inhibitor-induced apoptosis.
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http://dx.doi.org/10.1126/scitranslmed.aau0416DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7402218PMC
July 2019

The Sustained Induction of c-MYC Drives Nab-Paclitaxel Resistance in Primary Pancreatic Ductal Carcinoma Cells.

Mol Cancer Res 2019 09 4;17(9):1815-1827. Epub 2019 Jun 4.

Department of Oncology, Lombardi Comprehensive Cancer Center, Georgetown University Medical Center, Washington, D.C.

Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive disease with limited and, very often, ineffective medical and surgical therapeutic options. The treatment of patients with advanced unresectable PDAC is restricted to systemic chemotherapy, a therapeutic intervention to which most eventually develop resistance. Recently, nab-paclitaxel (n-PTX) has been added to the arsenal of first-line therapies, and the combination of gemcitabine and n-PTX has modestly prolonged median overall survival. However, patients almost invariably succumb to the disease, and little is known about the mechanisms underlying n-PTX resistance. Using the conditionally reprogrammed (CR) cell approach, we established and verified continuously growing cell cultures from treatment-naïve patients with PDAC. To study the mechanisms of primary drug resistance, nab-paclitaxel-resistant (n-PTX-R) cells were generated from primary cultures and drug resistance was verified , both in zebrafish and in athymic nude mouse xenograft models. Molecular analyses identified the sustained induction of in the n-PTX-R cells. Depletion of c-MYC restored n-PTX sensitivity, as did treatment with either the MEK inhibitor, trametinib, or a small-molecule activator of protein phosphatase 2a. IMPLICATIONS: The strategies we have devised, including the patient-derived primary cells and the unique, drug-resistant isogenic cells, are rapid and easily applied and platforms to better understand the mechanisms of drug resistance and for defining effective therapeutic options on a patient by patient basis.
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http://dx.doi.org/10.1158/1541-7786.MCR-19-0191DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6726538PMC
September 2019

The Highly Recurrent PP2A Aα-Subunit Mutation P179R Alters Protein Structure and Impairs PP2A Enzyme Function to Promote Endometrial Tumorigenesis.

Cancer Res 2019 Aug 29;79(16):4242-4257. Epub 2019 May 29.

Division of Genetic Medicine, Department of Internal Medicine, University of Michigan, Ann Arbor, Michigan.

Somatic mutation of the protein phosphatase 2A (PP2A) Aα-subunit gene 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 . Furthermore, small molecule-mediated therapeutic reactivation of PP2A significantly inhibited tumorigenicity . 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 mutations. SIGNIFICANCE: This study characterizes a highly recurrent, disease-specific PP2A mutation as a driver of endometrial carcinoma and a target for novel therapeutic development..
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http://dx.doi.org/10.1158/0008-5472.CAN-19-0218DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6724736PMC
August 2019

Mistletoe extract Fraxini inhibits the proliferation of liver cancer by down-regulating c-Myc expression.

Sci Rep 2019 04 23;9(1):6428. Epub 2019 Apr 23.

Department of Medicine, Case Western Reserve University & University Hospitals, Cleveland, Ohio, USA.

Mistletoe (Viscum album) is a type of parasitic plant reported to have anticancer activity including in hepatocellular carcinoma (HCC). However, the mechanism of mistletoe's anticancer activity, and its effectiveness in treating HCC are not fully understood. We report here that mistletoe extracts, including Fraxini (grown on ash trees) and Iscador Q and M (grown on oak and maple trees), exert strong antiproliferative activity in Hep3B cells, with median inhibitory concentrations (IC) of 0.5 µg/mL, 7.49 µg/mL, and 7.51 µg/mL, respectively. Results of Reversed Phase Proteomic Array analysis (RPPA) suggests that Fraxini substantially down-regulates c-Myc expression in Hep3B cells. Fraxini-induced growth inhibition (at a concentration of 1.25 μg/ml) was less pronounced in c-Myc knockdown Hep3B cells than in control cells. Furthermore, in the Hep3B xenograft model, Fraxini-treated (8 mg/kg body weight) mice had significantly smaller tumors (34.6 ± 11.9 mm) than control mice (161.6 ± 79.4 mm, p < 0.036). Similarly, c-Myc protein expression was reduced in Fraxini treated Hep3B cell xenografts compared to that of control mice. The reduction of c-Myc protein levels in vitro Hep3B cells appears to be mediated by the ubiquitin-proteasome system. Our results suggest the importance of c-Myc in Fraxini's antiproliferative activity, which warrants further investigation.
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http://dx.doi.org/10.1038/s41598-019-41444-2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6478697PMC
April 2019

Lulling the Cancer Cell into an Eternal Sleep.

Cancer Res 2019 04;79(8):1756-1757

Division of Genetic Medicine, Department of Medicine, University of Michigan, Ann Arbor, Michigan.

A hallmark of metastasis is the ability of cancer cells to undergo the epithelial-to-mesenchymal transition to invade and disseminate to distal sites. More recently, the case has been made that the critical last step in metastasis is dependent on the ability to undergo reversion to an epithelial phenotype in a process known as the mesenchymal-to-epithelial transition (MET). It is this transition in the metastatic cascade that researchers are focusing on clinically to treat disseminated disease. Shinde and colleagues identified spleen tyrosine kinase (SYK) as a critical mediator of MET that facilitated the removal of P-bodies during autophagy. Remarkably, pharmacologic inhibition of SYK inhibited the clearance of P-bodies and autophagy in preclinical models of metastasis, arresting cancer cells in an indefinite dormant state and preventing tumor cell colonization and thus the establishment of aggressive metastatic disease..
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http://dx.doi.org/10.1158/0008-5472.CAN-19-0853DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7199440PMC
April 2019

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

JCI Insight 2019 02 21;4(4). Epub 2019 Feb 21.

Division of Genetic Medicine, Department of Medicine, University of Michigan, Ann Arbor, Michigan, USA.

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.
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http://dx.doi.org/10.1172/jci.insight.125693DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6478418PMC
February 2019

Protein phosphatase 2A Aα regulates Aβ protein expression and stability.

J Biol Chem 2019 04 22;294(15):5923-5934. Epub 2019 Feb 22.

Department of Internal Medicine, Division of Genetic Medicine, University of Michigan, Ann Arbor, Michigan 48109. Electronic address:

Protein phosphatase 2A (PP2A) represses many oncogenic signaling pathways and is an important tumor suppressor. PP2A comprises three distinct subunits and forms through a highly regulated biogenesis process, with the scaffolding A subunit existing as two highly related isoforms, Aα and Aβ. PP2A's tumor-suppressive functions have been intensely studied, and PP2A inactivation has been shown to be a prerequisite for tumor formation. Interestingly, although partial loss of the Aα isoform is growth promoting, complete Aα loss has no transformative properties. Additionally, in cancer patients, Aα is found to be inactivated in a haploinsufficient manner. Using both cellular and systems, colorectal and endometrial cancer cell lines, and biochemical and cellular assays, here we examined why the complete loss of Aα does not promote tumorigenesis. CRISPR/Cas9-mediated homozygous Aα deletion resulted in decreased colony formation and tumor growth across multiple cell lines. Protein expression analysis of PP2A family members revealed that the Aα deletion markedly up-regulates Aβ protein expression by increasing Aβ protein stability. Aβ knockdown in control and Aα knockout cell lines indicated that Aβ is necessary for cell survival in the Aα knockout cells. In the setting of Aα deficiency, co-immunoprecipitation analysis revealed increased binding of specific PP2A regulatory subunits to Aβ, and knockdown of these regulatory subunits restored colony-forming ability. Taken together, our results uncover a mechanism by which PP2A Aα regulates Aβ protein stability and activity and suggests why homozygous loss of Aα is rarely seen in cancer patients.
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http://dx.doi.org/10.1074/jbc.RA119.007593DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6463732PMC
April 2019

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

J Clin Invest 2019 03 4;129(4):1641-1653. Epub 2019 Mar 4.

Department of Pharmacology.

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.
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http://dx.doi.org/10.1172/JCI123049DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6436899PMC
March 2019

Targeting PP2A in cancer: Combination therapies.

Biochim Biophys Acta Mol Cell Res 2019 01 1;1866(1):51-63. Epub 2018 Sep 1.

Division of Genetic Medicine, Michigan Medicine, University of Michigan, Ann Arbor, MI, USA. Electronic address:

The serine/threonine phosphatase PP2A regulates a vast portion of the phosphoproteome including pathways involved in apoptosis, proliferation and DNA damage response and PP2A inactivation is a vital step in malignant transformation. Many groups have explored the therapeutic venue of combining PP2A reactivation with kinase inhibition to counteract the very changes in tumor suppressors and oncogenes that lead to cancer development. Conversely, inhibition of PP2A to complement chemotherapy and radiation-induced cancer cell death is also an area of active investigation. Here we review the studies that utilize PP2A targeted agents as combination therapy in cancer. A potential role for PP2A in tumor immunity is also highlighted.
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http://dx.doi.org/10.1016/j.bbamcr.2018.08.020DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6436821PMC
January 2019

Activation of PP2A and Inhibition of mTOR Synergistically Reduce MYC Signaling and Decrease Tumor Growth in Pancreatic Ductal Adenocarcinoma.

Cancer Res 2019 01 2;79(1):209-219. Epub 2018 Nov 2.

Department of Molecular and Medical Genetics, Oregon Health and Science University, Portland, Oregon.

In cancer, kinases are often activated and phosphatases suppressed, leading to aberrant activation of signaling pathways driving cellular proliferation, survival, and therapeutic resistance. Although pancreatic ductal adenocarcinoma (PDA) has historically been refractory to kinase inhibition, therapeutic activation of phosphatases is emerging as a promising strategy to restore balance to these hyperactive signaling cascades. In this study, we hypothesized that phosphatase activation combined with kinase inhibition could deplete oncogenic survival signals to reduce tumor growth. We screened PDA cell lines for kinase inhibitors that could synergize with activation of protein phosphatase 2A (PP2A), a tumor suppressor phosphatase, and determined that activation of PP2A and inhibition of mTOR synergistically increase apoptosis and reduce oncogenic phenotypes and . This combination treatment resulted in suppression of AKT/mTOR signaling coupled with reduced expression of c-MYC, an oncoprotein implicated in tumor progression and therapeutic resistance. Forced expression of c-MYC or loss of PP2A B56α, the specific PP2A subunit shown to negatively regulate c-MYC, increased resistance to mTOR inhibition. Conversely, decreased c-MYC expression increased the sensitivity of PDA cells to mTOR inhibition. Together, these studies demonstrate that combined targeting of PP2A and mTOR suppresses proliferative signaling and induces cell death and implicates this combination as a promising therapeutic strategy for patients with PDA. SIGNIFICANCE: These findings present a combinatorial strategy targeting serine/threonine protein phosphatase PP2A and mTOR in PDA, a cancer for which there are currently no targeted therapeutic options. http://cancerres.aacrjournals.org/content/canres/79/1/209/F1.large.jpg.
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http://dx.doi.org/10.1158/0008-5472.CAN-18-0717DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6318036PMC
January 2019

Sprague Dawley -Null Rats Created from Engineered Spermatogonial Stem Cells Are Immunodeficient and Permissive to Human Xenografts.

Mol Cancer Ther 2018 11 11;17(11):2481-2489. Epub 2018 Sep 11.

Hera BioLabs Inc., Lexington, Kentucky.

The rat is the preferred model for toxicology studies, and it offers distinctive advantages over the mouse as a preclinical research model including larger sample size collection, lower rates of drug clearance, and relative ease of surgical manipulation. An immunodeficient rat would allow for larger tumor size development, prolonged dosing and drug efficacy studies, and preliminary toxicologic testing and pharmacokinetic/pharmacodynamic studies in the same model animal. Here, we created an immunodeficient rat with a functional deletion of the Recombination Activating Gene 2 () gene, using genetically modified spermatogonial stem cells (SSC). We targeted the gene in rat SSCs with TALENs and transplanted these -deficient SSCs into sterile recipients. Offspring were genotyped, and a founder with a 27 bp deletion mutation was identified and bred to homozygosity to produce the Sprague-Dawley - (SDR) knockout rat. We demonstrated that SDR rat lacks mature B and T cells. Furthermore, the SDR rat model was permissive to growth of human glioblastoma cell line subcutaneously resulting in successful growth of tumors. In addition, a human KRAS-mutant non-small cell lung cancer cell line (H358), a patient-derived high-grade serous ovarian cancer cell line (OV81), and a patient-derived recurrent endometrial cancer cell line (OV185) were transplanted subcutaneously to test the ability of the SDR rat to accommodate human xenografts from multiple tissue types. All human cancer cell lines showed efficient tumor uptake and growth kinetics indicating that the SDR rat is a viable host for a range of xenograft studies. .
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http://dx.doi.org/10.1158/1535-7163.MCT-18-0156DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6215516PMC
November 2018

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

Sci Transl Med 2018 07;10(450)

Turku Centre for Biotechnology, University of Turku and Åbo Akademi University, 20520 Turku, Finland.

Kinase inhibitor resistance constitutes a major unresolved clinical challenge in cancer. Furthermore, the role of serine/threonine phosphatase deregulation as a potential cause for resistance to kinase inhibitors has not been thoroughly addressed. We characterize protein phosphatase 2A (PP2A) activity as a global determinant of KRAS-mutant lung cancer cell resistance across a library of >200 kinase inhibitors. The results show that PP2A activity modulation alters cancer cell sensitivities to a large number of kinase inhibitors. Specifically, PP2A inhibition ablated mitogen-activated protein kinase kinase (MEK) inhibitor response through the collateral activation of AKT/mammalian target of rapamycin (mTOR) signaling. Combination of mTOR and MEK inhibitors induced cytotoxicity in PP2A-inhibited cells, but even this drug combination could not abrogate MYC up-regulation in PP2A-inhibited cells. Treatment with an orally bioavailable small-molecule activator of PP2A DT-061, in combination with the MEK inhibitor AZD6244, resulted in suppression of both p-AKT and MYC, as well as tumor regression in two KRAS-driven lung cancer mouse models. DT-061 therapy also abrogated MYC-driven tumorigenesis. These data demonstrate that PP2A deregulation drives MEK inhibitor resistance in KRAS-mutant cells. These results emphasize the need for better understanding of phosphatases as key modulators of cancer therapy responses.
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http://dx.doi.org/10.1126/scitranslmed.aaq1093DOI Listing
July 2018