Inhibition Of PP2A Research Articles

Progressive Insight into Tumor Promotion with Proinflammatory Cytokines

IL-1β on mouse skin [2,3]; The study of tumor promotion by both okadaic acid and TPA revealed that TNF-α is an essential tumor promoter in TNF-α-deficient mouse skin initiated with DMBA [3]; Inhibitors of PP1 and PP2A induced tumor promotion on mouse skin, in rat glandular stomach and rat liver initiated with carcinogens [4]; TNF-α, IL-1α and IL-1β induced clonal growth of v-Ha-ras-transfected BALB/3T3 cells (Bhas 42) [3]; Treatment with tumor promoters commonly released TNF-α from target organs [5]. In light of our evidence showing that TNF-α, IL-1α and IL-1β act as endogenous tumor promoters and/or cancer mediators [6,7], we specifically found a TNF-α–inducing protein (Tipα) gene in H. pylori genome [8,9]. Tipα dimer bind to nucleolin on cell surface of human gastric cancer cells as a receptor [10], and are secreted in large quantities from H. pylori of gastric cancer patients [11]. The Epithelial-Mesenchymal Transition (EMT) was induced in human gastric cancer cells by Tipα [12]. The Tipα and nucleolin complex shows a strong link between tumor promotion and clinical cancer development. Since TNF-α and IL-1 are tumor-promoting cytokines, we discussed the role of inflammasome signaling for tumor promotion using apoptosisassociated speck-like protein containing a caspase recruitment domain (ASC)-KO and caspase (CASP)-1-KO mice reported by Campanelli’s group [13].

SNP rs34172460 (S258A)Â in Gene CIP2A Correlates with Bortezomib Resistance in Multiple Myeloma Patients

Introduction: The proteasome inhibitor (PI) bortezomib is a novel anticancer drug that shows activity especially for treating patients with multiple myeloma (MM). However, its clinical efficacy has been hampered by the emergence of drug-resistance phenomena, the genetic and molecular basis of which remains elusive. We propose that the resistance to bortezomib treatment may be related to a polymorphism in a gene known to be a target for bortezomib's activity which has been identified as CIP2A which is a tumor suppressor PP2A inhibitor. We have used a high resolution melting curve (HRM) approach to screen several CIP2A SNPs containing significant genetic changes that may be different between bortezomib sensitive and resistant MM patients. In this study, we evaluated SNPs in this gene in healthy subjects and MM patients who were sensitive or resistant to bortezomib-based therapy.Methods: A panel of DNA samples was purified from MM patients (N=73) and healthy subjects (N=95). The samples were divided into three groups: bortezomib sensitive MM patients (N=61), bortezomib resistant MM patients (N=12), and healthy subjects. A single nucleotide polymorphism (SNP) of the bortezomib target genes CIP2A was selected. The PCR prime pairs for each SNP marker were designed with the software "Primer Expression 3" (Thermo Fisher Scientific). The genotype of each DNA sample was determined with the high resolution melt curve (HRM) technology on a real-time PCR instrument, StepOne Plus (Thermo Fisher Scientific).Results: The frequency of heterozygosity and homozygosity of CIP2A, SNP rs34172460(S258A) was 0.049 (3 out of 61 cases) in the bortezomib sensitive group, while it was 0.25 (3 out of 12 cases) in the bortezomib resistant group, a 5-fold increase in the frequency of this SNP. The frequency of heterozygosity and homozygosity of this same SNP in the healthy subjects was only 0.03 (3 out of 95 samples).The minor allele frequency (MAF) in the normal population is 0.046, according to the SNP data base, compatible with what we observed in both our healthy subjects and the bortezomib sensitive MM patients.Conclusion: This preliminary data suggests that heterozygotic and homozygotic mutants of rs34172460(S258A) may have an impact on resistance to bortezomib among patients with multiple myeloma. DisclosuresNo relevant conflicts of interest to declare.

The High Expression of CFTR in Ph+ Acute Leukemia Regulates the BCR-ABL and Wnt/ -Catenin Signaling Pathway Via Interaction with PP2A Protein

Purpose The cystic fibrosis transembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transporter superfamily and mediates the transportion of Cl- and HCO3-. Recent studies have found that CFTR is not only an anion channel protein, but also is used as a regulator to regulate the function of other proteins through interactions of its PDZ domain with other proteins. Here, the expression of CFTR in Ph+ acute leukemia, and the role of CFTR in the regulation of BCR-ABL and its mediated classical Wnt/β-catenin signaling were investigated.Methods The expression of CFTR protein in the normal human airway epithelial cell line HBE, Ph+ chronic granulocytic leukemia (CML) cell line K562 and Ph+ acute lymphoblastic leukemia (ALL) cell line SUP-B15, and Ph+ primary leukemic cells were examined by Western-blot assay; the effect of CFTRinh-172, a CFTR inhibitor, on BCR-ABL and Wnt/β-catenin signaling was investigated by Western-blot assay in Ph+ acute leukemia cell lines; the interaction between the CFTR, BCR-ABL and PP2A proteins in K562 cell line was analyzed by co-immunoprecipitation (co-IP)/Western-blot assay.Results 1.Compared with the normal human airway epithelial cell line HBE (a positive control, expression value of CFTR =1), CFTR protein expression by Western-blot assay in K562 (value=2.921) and SUP-B15 (value=2.042) were higher; the CFTR expression in normal mononuclear cells(MNCs) showed very low level (mean value =0.068 ± 0.005); however, in 40 cases of B-ALL primary leukemia cells, the expression of CFTR ( values = 1.613) in 20 cases of Ph+ B-ALL was higher than that in 20 cases of Ph- B-ALL (CFTR values = 0.432); similarly, in 12 cases of CML primary leukemic cells, the expression of CFTR ( values = 1.360) in 6 cases of CML-blast crisis was higher than that in 6 cases of CML-chronic phase (CFTR values = 0.060). These results demonstrated that CFTR were over-expression in Ph+ acute leukemia. 2.CFTRinh-172 down-regulated the expression of CFTR, p-BCR-ABL and Wnt/β-catenin signaling (Dvl-2 and β-catenin were significantly reduced and p-GSK3β was significantly upregulated) in K562 and SUP-B15 cell lines. Imatinib also down-regulated expression of p-BCR-ABL and Wnt/β-catenin signaling, however, had no effect on CFTR expression. 3. After CFTR was down-regulated by CFTRinh-172 and CFTR shRNA, the expression of PP2AA subunit increased and p-PP2AC(Y307)/PP2AC rate (the phosphorylation on Y307 site was responsible for PP2A inactivation) significantly reduced, meaning that the down-regulation of CFTR increased PP2A function. 4.The results of Co-IP confirmed that CFTR could combine with PP2AA subunit (rather than PP2AC subunit), and PP2AA subunit binding with CFTR were reduced after the treatment of CFTRinh-172. 5.The combination of CFTRinh-172 and PP2A inhibitor okadaic acid not only inhibited CFTRinh-172-mediated up-regulation of PP2AA subunit and significantly up-regulated p-PP2AC/PP2AC rate, and more importantly, restored CFTRinh-172-mediated down-regulation of p-BCR-ABL and β-catenin partially. The results demonstrated that CFTR directly regulate the expression and activity of BCR-ABL, and further regulate the classical Wnt/β-catenin signaling via the interaction with PP2A.Conclusion The high expression of CFTR in Ph+ acute leukemia regulates the BCR-ABL and Wnt/-catenin signaling pathway via interaction with PP2A protein. DisclosuresNo relevant conflicts of interest to declare.

Abstract A175: A phase 1 study of a novel inhibitor of protein phosphatase 2A alone or in combination with docetaxel

Abstract Background Protein phosphatase (PP2A) is a multifunctional protein involved in regulation of cell cycle, DNA-damage response, and apoptosis. LB-100, a novel small molecule inhibitor of PP2A, inhibits the growth of a broad spectrum of leukemic and solid tumor cell lines. In addition, LB100 potentiates the effectiveness of cytotoxic drugs (cisplatin, docetaxel, doxorubicin, temozolomide) and radiation without significant increases in toxicity. The predominant mechanisms responsible for potentiation are inhibition of mitotic exit and homologous recombination repair. Methods This is an open label, first-time-in-human, multicenter, phase 1 study of LB-100 in patients with advanced cancer refractory to standard therapies. The first part of the study determines the maximum tolerated dose (MTD) of LB-100 as a single agent when given intravenously days 1-3 every 21 days. Utilizing a standard 3+3 design, patients (pts) are evaluated for dose limiting toxicities (DLT) through 2 cycles. Once the single agent MTD is determined, the dose will be reduced by 2 dose levels (DL) and combined with docetaxel given on day 2. Escalation will continue until the MTD of the combination is determined. Plasma sampling for pharmacokinetics of LB-100 will be collected on days 1 and 3 of cycle 1 in the MTD confirmation cohort. Results The starting dose of LB-100 was 0.25 mg/m2 and 21 pts have enrolled in part 1 of the study through six dose levels. At DL6, pts received 2.33 mg/m2 and no DLTs have been observed. One pt with metastatic colon cancer at DL6 had a grade 2 creatinine after 2 doses that resolved with hydration. This was related to LB-100 and the study was amended to increase the volume and infusion time. One pt on DL3 with stage 4 pancreas cancer had stable disease through 15 cycles of treatment and another pt on DL5 with metastatic thymoma remains on treatment through 8 cycles. Stable disease for 4-6 cycles was also observed in breast, ovarian, carcinoid and testicular cancer patients. Conclusions Rb and p53 mutations are common in malignancies leading to chromosomal instability and overexpression of the mitotic checkpoint gene Mad2. PP2A inhibition results in synthetic lethality of cancer cells overexpressing Mad2 which may be a biomarker for LB100 responsiveness. Through 6 DLs, LB-100 has been well tolerated without any DLTs and early activity has been observed with stabilization of disease in a wide variety of cancers. Correlative studies for biomarkers of response are planned. Clinical trial identifier: NCTO1837667 Citation Format: Vincent Chung, Donald Richards, Fadi Braiteh, John S. Kovach, Aaron Scott Mansfield. A phase 1 study of a novel inhibitor of protein phosphatase 2A alone or in combination with docetaxel. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr A175.

Changes in Carboxy Methylation and Tyrosine Phosphorylation of Protein Phosphatase PP2A Are Associated with Epididymal Sperm Maturation and Motility.

Mammalian sperm contain the serine/threonine phosphatases PP1γ2 and PP2A. The role of sperm PP1γ2 is relatively well studied. Here we confirm the presence of PP2A in sperm and show that it undergoes marked changes in methylation (leucine 309), tyrosine phosphorylation (tyrosine 307) and catalytic activity during epididymal sperm maturation. Spermatozoa isolated from proximal caput, distal caput and caudal regions of the epididymis contain equal immuno-reactive amounts of PP2A. Using demethyl sensitive antibodies we show that PP2A is methylated at its carboxy terminus in sperm from the distal caput and caudal regions but not in sperm from the proximal caput region of the epididymis. The methylation status of PP2A was confirmed by isolation of PP2A with microcystin agarose followed by alkali treatment, which causes hydrolysis of protein carboxy methyl esters. Tyrosine phosphorylation of sperm PP2A varied inversely with methylation. That is, PP2A was tyrosine phosphorylated when it was demethylated but not when methylated. PP2A demethylation and its reciprocal tyrosine phosphorylation were also affected by treatment of sperm with L-homocysteine and adenosine, which are known to elevate intracellular S-adenosylhomocysteine, a feedback inhibitor of methyltransferases. Catalytic activity of PP2A declined during epididymal sperm maturation. Inhibition of PP2A by okadaic acid or by incubation of caudal epididymal spermatozoa with L-homocysteine and adenosine resulted in increase of sperm motility parameters including percent motility, velocity, and lateral head amplitude. Demethylation or pharmacological inhibition of PP2A also leads to an increase in phosphorylation of glycogen synthase kinase-3 (GSK3). Our results show for the first time that changes in PP2A activity due to methylation and tyrosine phosphorylation occur in sperm and that these changes may play an important role in the regulation of sperm function.

CIP2A overexpression induces autoimmune response and enhances JNK signaling pathway in human lung cancer.

BackgroundCancerous inhibitor of PP2A (CIP2A) is a recently characterized oncoprotein, which promotes cancer cell proliferation. But the role of CIP2A in lung cancer progression is still not well understood.MethodsThe expression level of CIP2A in lung cancer tissues was examined by immunohistochemistry. CIP2A-associated cell proliferation was performed by knock down or overexpression of CIP2A in lung cancer cells. Phospho-array was used to screen kinase candidates related to expression change of CIP2A. Western-blot and luciferase reporter assay were used to validate phospho-array results.ResultsOverexpression of CIP2A in lung cancer not only triggers immune response in lung cancer patients but also promotes lung cancer cell proliferation. By phospho-array, several kinase candidates were identified, one of which is c-Jun activated kinases (JNK). The knock down of CIP2A decreased JNK phosphorylation, and the phosphorylation of downstream transcriptional factors, ATF2 and c-Jun, whose transcriptional activity were decreased as well. Furthermore, the expression level of CIP2A also affected the phosphorylation of the upstream kinase of JNK, MKK4/MKK7. At last, treatment with JNK inhibitor partially abolished CIP2A-induced cell proliferation.ConclusionCIP2A is a tumor-associated autoantigen in lung cancer, which promote lung cancer proliferation partially through MKK4/7-JNK signaling pathway.

Microcystin-LR promotes proliferation by activating Akt/S6K1 pathway and disordering apoptosis and cell cycle associated proteins phosphorylation in HL7702 cells.

Our previous studies had shown that MC-LR inhibited PP2A activity and hyperphosphorylated PP2A substrates at 24 h exposure in HL7702 cells. Although the cytoskeleton was rearranged, the cellular effects were not observed. The purpose of the present study with HL7702 cell exposed to MC-LR for 1-72 h was to further uncover the adverse effects of MC-LR comprehensively. The results showed that there were no obvious difference in apoptosis rate and cell-cycle distribution but the cell proliferation was changed since 36 h exposure while the uptake of MC-LR and its binding to PP2A/C kept unchanged since 1h exposure. PP2A activity had not manifested continued decline compare to 24h exposure and PP2A regulator α4 was found to release its associated PP2A/C since 1h exposure. The increasing of p-Akt-T308, p-Akt-S473, p-S6K1, p-S6, and p-4E-BP1 since 1h MC-LR exposure indicated that Akt/S6K1 cascade had been activated as early as 1h MC-LR treatment. And, PI3K/Akt inhibitor (LY294002) blocked MC-LR-induced Akt/S6K1 activation and proliferation. Besides, MC-LR also led to hyperphosphorylation of c-Myc, c-Jun, Bcl-2 and Bad and activation of Cdk1. Our study indicated that MC-LR exposure promoted HL7702 cell proliferation and the main mechanism was the activation of Akt/S6K1 cascade. Meanwhile, hyperphosphorylation of Bcl-2, Bad, c-Myc and c-Jun might also be involved. And, the inhibition of PP2A was the major reason for these molecular changes.

Potential role for inhibition of protein phosphatase 2A tumor suppressor in salivary gland malignancies.

The aetiology and pathogenesis of salivary gland malignancies remain unknown. To reveal novel molecular factors behind the development of salivary gland cancer, we performed gene expression analyses from Smgb-Tag mouse salivary gland samples. The overall purpose was to apply these results for clinical use to find new approaches for both possible therapeutic targets and more accurate diagnostic tools. Smgb-Tag mouse strain, in which salivary neoplasms arise through a dysplastic phase in submandibular glands, was investigated using genome-wide microarray expression analysis, ingenuity pathway analysis, RT-PCR, and immunohistochemistry. Thirty-eight human salivary gland adenoid cystic carcinoma samples were investigated using immunohistochemistry for validation purposes. Our genome-wide study showed that Ppp2r1b, a PP2A subunit encoding tumor suppressor gene, is underexpressed in submandibular gland tumors of Smgb-Tag mice. mTOR signaling pathway was significantly enriched and mTOR linked PP2A subunit gene B55 gamma was significantly underexpressed in the analyses. Furthermore, parallel immunohistochemical analysis of three PP2A inhibitors demonstrated that two PP2A inhibitors, CIP2A and SET, are highly expressed in both dysplastic and adenocarcinomatous tumors of the Smgb-Tag mice. In addition, all 38 investigated human salivary adenoid cystic carcinoma samples stained positively for CIP2A and most for SET. Finally, p-S6 staining showed activation of mTOR pathway in human adenoid cystic carcinoma samples. Our results suggest that PP2A inhibition either via PP2A subunit underexpression or PP2A inhibitor overexpression play an important role in the formation of salivary gland malignancy, potentially due to mTOR signaling activation.

Ceramide-Initiated Protein Phosphatase 2A Activation Contributes to Arterial Dysfunction In Vivo.

Prior studies have implicated accumulation of ceramide in blood vessels as a basis for vascular dysfunction in diet-induced obesity via a mechanism involving type 2 protein phosphatase (PP2A) dephosphorylation of endothelial nitric oxide synthase (eNOS). The current study sought to elucidate the mechanisms linking ceramide accumulation with PP2A activation and determine whether pharmacological inhibition of PP2A in vivo normalizes obesity-associated vascular dysfunction and limits the severity of hypertension. We show in endothelial cells that ceramide associates with the inhibitor 2 of PP2A (I2PP2A) in the cytosol, which disrupts the association of I2PP2A with PP2A leading to its translocation to the plasma membrane. The increased association between PP2A and eNOS at the plasma membrane promotes dissociation of an Akt-Hsp90-eNOS complex that is required for eNOS phosphorylation and activation. A novel small-molecule inhibitor of PP2A attenuated PP2A activation, prevented disruption of the Akt-Hsp90-eNOS complex in the vasculature, preserved arterial function, and maintained normal blood pressure in obese mice. These findings reveal a novel mechanism whereby ceramide initiates PP2A colocalization with eNOS and demonstrate that PP2A activation precipitates vascular dysfunction in diet-induced obesity. Therapeutic strategies targeted to reducing PP2A activation might be beneficial in attenuating vascular complications that exist in the context of type 2 diabetes, obesity, and conditions associated with insulin resistance.

Overexpression of CIP2A promotes bladder cancer progression by regulating EMT.

Bladder cancer is the second most common urological malignancy worldwide. CIP2A is a newly identified inhibitor of PP2A. Recent studies have highlighted a potential role for CIP2A in promoting the proliferation of several cancer cells. However, the role of CIP2A in bladder cancer still remains unclear. The expression of CIP2A was detected by quantitative real-time polymerase chain reaction and IHC in bladder cancer tissues and bladder cancer cell lines. In addition, silencing of CIP2A with siRNA was performed in T24 cells, and the impact on proliferation, and apoptosis of T24 cells was analyzed. Our results found that CIP2A expression levels were higher in bladder cancer tissues and cell lines. Furthermore, CIP2A siRNA significantly reduced the proliferation rate of T24 cells, induced a significant population of early and late apoptosis, and could reverse EMT in T24 cells, indicates that CIP2A expression is increased in bladder cancer and implies a role of the protein in bladder cancer progression. These results suggest that CIP2A is involved in tumor progression, and thus CIP2A could represent selective targets for the targeted treatments of bladder cancer.

Abstract 273: Phosphoinositide 3-Kinase γ Regulates Cardio-protective ERK by Kinase Independent Mechanism

Phosphoinositide 3-kinase (PI3K) enzymes are critical in many cellular processes including survival. PI3Kγ, a member of the PI3K family activated by G-protein coupled receptor (GPCR), is known to be a critical player in activation of extracellular regulated kinase (ERK) signal transduction cascade, a cell survival pathway. However, the exact mechanism by which PI3Kγ plays a role in ERK activation is not clearly understood. Our studies show that PI3Kγ plays a crucial role in enhancing the tone of ERK activation as use of PI3K inhibitors reduced GPCR stimulated ERK phosphorylation in HEK293 cells. siRNA knockdown of PI3Kγ resulted in loss of ERK phosphorylation through GPCRs (β-adrenergic) as well as receptor tyrosine kinases. The role of PI3Kγ in ERK activation was further corroborated by loss of insulin stimulated ERK phosphorylation in PI3Kγ-knockout (KO) mouse embryonic fibroblasts (MEFs). Surprisingly, ERK activation in KO MEFs post-insulin stimulation was completely rescued by expression of kinase-dead PI3Kγ mutant in KO MEFs suggesting a kinase-independent role of PI3Kγ in regulating ERK function. Indepth mechanistic studies showed that PI3Kγ mediated activation of ERK by inhibiting ERK dephosphorylation following stimulation, thus stabilizing the ERK phosphorylation. PI3Kγ physically disrupts the interaction between ERK and ERK dephosphorylating phosphatase PP2A as evidenced by increase in phosphatase association with ERK in KO MEFs. Consistent with this observation, ERK activation was completely abolished in KO MEFs following carvedilol suggesting an essential role for PI3Kγ in cardio-protective ERK activation pathway. In this context, it is known that transverse aortic constriction (TAC) in mice leads to increase in ERK activation in the hearts and is also associated with concurrent up-regulation of PI3Kγ suggesting a key role for kinase-independent function of PI3Kγ in activating and maintaining the ERK signaling cascade. These indepth cellular studies and observation from our TAC studies led us to believe that kinase-dependent function of PI3Kγ may contribute to pathology while kinase-independent function may be cardio-protective through inhibition of PP2A by PI3Kγ. This novel signaling mechanism by PI3Kγ will be presented.

Abstract A89: Targeting the MYC activation and degradation pathway for the treatment of pancreatic cancer

Abstract Pancreatic cancer is a disease with a high rate of mortality, as it is generally diagnosed in the advanced stages when few effective treatments are available. Given the clinical aggressivity of this disease and lack of good treatment options, the development of new therapeutic agents is of the upmost importance. The c-MYC (MYC) oncoprotein is a critical transcription factor that regulates many diverse cellular functions by either activating or suppressing downstream target genes. High levels of MYC expression occur in a wide variety of human tumors, and animal models demonstrate MYC-induced tumorigenesis in many tissues. It has been shown that the MYC gene is frequently amplified in human pancreatic cancer and that MYC mRNA and/or protein are often overexpressed. However, what pathways contribute to this overexpression or whether MYC can be therapeutically targeted in pancreatic cancer is yet not known. Work in our laboratory has elucidated a complex signaling pathway that regulates MYC protein stability and transcriptional activity. RAS activation in early G1 leads to ERK-dependent phosphorylation of MYC at Serine62 (pS62-MYC), which stabilizes MYC. pS62-MYC is then recognized by the phosphorylation-directed Pin1 prolyl isomerase, which isomerizes Proline63 (P63) to cis, enhancing MYC’s recruitment to, and transactivation of, pro-growth target genes. Later in G1, when RAS activation decreases, GSK3β phosphorylates MYC at T58 (pT58-MYC). Following a second isomerization by Pin1, the trans-specific phosphatase PP2A-B56α removes the stabilizing S62 phosphate. pT58-MYC is then recognized by the E3 ubiquitin ligase SCFFbw7 and degraded by the proteasome. We have found that this pathway is frequently deregulated in human cancer leading to elevated pS62, decreased pT58, and increased c-Myc stability and activity. Here, we show that MYC is overexpressed and stabilized in pancreatic cancer cells. Additionally, we find aberrant MYC phosphorylation in primary human pancreatic cancer tissue, suggesting that the pathway that controls MYC phosphorylation is deregulated in human pancreatic cancer. Therefore, we hypothesized that the pathways regulating MYC phosphorylation and activation might represent new therapeutic targets. We are pursuing two different approaches to target MYC in pancreatic cancer: 1) inhibiting Pin1, the prolyl isomerase that co-activates pS62-MYC, and 2) activation of PP2A, the phosphatase that destabilizes MYC protein. We have observed decreased PP2A activity in pancreatic cancer cells, and have found that two endogenous inhibitors of PP2A, SET and CIP2A, are frequently overexpressed in pancreatic cancer cells and primary patient samples. This overexpression correlates with aberrant phosphorylation and stabilization of MYC. Furthermore, knockdown of SET or CIP2A, or inhibition of SET using a peptide mimetic (OP449), increases PP2A activity and reduces the tumorigenic potential of pancreatic cancer cells both in vitro and in vivo. In addition to increased SET and CIP2A expression in pancreatic cancer, we also observe frequent overexpression of Pin1 in human pancreatic cancer cell lines and primary patient tumor tissue and find that Pin1 knockdown or inhibition reduces the oncogenic potential of pancreatic cancer cells both in vitro and in vivo. Together, these findings suggest that MYC may play an important role in pancreatic cancer, and that targeting the pathway that regulates MYC activity and protein stability may be a viable therapeutic approach for human pancreatic cancer. Citation Format: Amy S. Farrell, Brittany Allen-Petersen, Colin J. Daniel, Xiaoyan Wang, Zhiping Wang, Dale J. Christensen, Brett Sheppard, Rosalie C. Sears. Targeting the MYC activation and degradation pathway for the treatment of pancreatic cancer. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Innovations in Research and Treatment; May 18-21, 2014; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2015;75(13 Suppl):Abstract nr A89.

Rapamycin inhibits BAFF-stimulated cell proliferation and survival by suppressing mTOR-mediated PP2A-Erk1/2 signaling pathway in normal and neoplastic B-lymphoid cells.

B-cell activating factor (BAFF) is involved in not only physiology of normal B cells, but also pathophysiology of aggressive B cells related to malignant and autoimmune diseases. Rapamycin, a lipophilic macrolide antibiotic, has recently shown to be effective in the treatment of human lupus erythematosus. However, how rapamycin inhibits BAFF-stimulated B-cell proliferation and survival has not been fully elucidated. Here, we show that rapamycin inhibited human soluble BAFF (hsBAFF)-induced cell proliferation and survival in normal and B-lymphoid (Raji and Daudi) cells by activation of PP2A and inactivation of Erk1/2. Pretreatment with PD98059, down-regulation of Erk1/2, expression of dominant negative MKK1, or overexpression of wild-type PP2A potentiated rapamycin's suppression of hsBAFF-activated Erk1/2 and B-cell proliferation/viability, whereas expression of constitutively active MKK1, inhibition of PP2A by okadaic acid, or expression of dominant negative PP2A attenuated the inhibitory effects of rapamycin. Furthermore, expression of a rapamycin-resistant and kinase-active mTOR (mTOR-T), but not a rapamycin-resistant and kinase-dead mTOR-T (mTOR-TE), conferred resistance to rapamycin's effects on PP2A, Erk1/2 and B-cell proliferation/viability, implying mTOR-dependent mechanism involved. The findings indicate that rapamycin inhibits BAFF-stimulated cell proliferation/survival by targeting mTOR-mediated PP2A-Erk1/2 signaling pathway in normal and neoplastic B-lymphoid cells. Our data highlight that rapamycin may be exploited for preventing excessive BAFF-induced aggressive B-cell malignancies and autoimmune diseases.

Roles of Serine/Threonine Phosphatases in Low-Dose Endothelial Monocyte-Activating Polypeptide-II-Induced Opening of Blood–Tumor Barrier

Previous studies have demonstrated that low-dose endothelial monocyte-activating polypeptide-II (EMAP-II) induces blood-tumor barrier (BTB) opening via RhoA/Rho kinase/PKC-α/β signaling pathway. In a recent study, we revealed that low-dose EMAP-II induced significant increases in expression levels of serine/threonine (Ser/Thr) phosphatase (PP)1 and 2A in rat brain microvascular endothelial cells (RBMECs) of BTB model. In addition, PKC-ζ/PP2A signaling pathway is involved in EMAP-II-induced BTB hyperpermeability. The present study further investigated the exact roles of PPs in this process. In an in vitro BTB model, low-dose EMAP-II (0.05nM) induced a significant increase in PP1 activity in RBMECs. There was an interaction between PKC-α/β and PP1 in RBMECs. Inhibition of PKC-α/β activity with GÖ6976 completely blocked EMAP-II-induced activation of PP1. Conversely, inhibition of PP1 activity with tautomycin had no effect on EMAP-II-induced PKC-α/β activation. Like GÖ6976, tautomycin significantly prevented EMAP-II-induced BTB hyperpermeability and MLC phosphorylation in RBMECs. Also, in this study, EMAP-II induced a marked redistribution of occludin and a significant dephosphorylation of occludin on Ser/Thr residues in RBMECs. Similar with GÖ6976 pretreatment, tautomycin pretreatment dramatically diminished EMAP-II-induced redistribution of occludin. Furthermore, pretreatment with tautomycin significantly inhibited EMAP-II-induced dephosphorylation of occludin on Ser residues. However, pretreatment with okadaic acid (an inhibitor of PP2A) significantly prevented changes in Ser-phosphorylated occludin induced by EMAP-II treatment. Collectively, this study demonstrates that low-dose EMAP-II increases BTB permeability via a RhoA/Rho kinase/PKC-α/β/PP1 signaling pathway and that PP1/PP2A-mediated Ser/Thr dephosphorylation of occludin plays an important role in EMAP-II-induced BTB hyperpermeability.

Fast Decay of CaMKII FRET Sensor Signal in Spines after LTP Induction Is Not Due to Its Dephosphorylation.

Because CaMKII is the critical Ca2+ sensor that triggers long-term potentiation (LTP), understanding its activation and deactivation is important. A major advance has been the development of a FRET indicator of the conformational state of CaMKII called Camui. Experiments using Camui have demonstrated that the open (active) conformation increases during LTP induction and then decays in tens of seconds, with the major fast component decaying with a time-constant of ~ 6 sec (tau1). Because this decay is faster if autophosphorylation of T286 is prevented (the autophosphorylation prolongs activity by making the enzyme active even after Ca2+ falls), it seemed likely that the fast decay is due to the T286 dephosphorylation. To test this interpretation, we studied the effect of phosphatase inhibitors on the single-spine Camui signal evoked by two-photon glutamate uncaging. We applied inhibitors of PP1 and PP2A, two phosphatases that are present at synapses and that have been shown to dephosphorylate CaMKII in vitro. The inhibitors increased the basal Camui activation state, indicating their effectiveness in cells. However, in no case did we find that tau1 was prolonged, contrary to what would be expected if the decay was phosphatase-dependent. This could either mean that decay was due to some unknown phosphatase or that the decay was not due to dephosphorylation. To distinguish between these possibilities, we expressed pseudo-phosphorylated Camui (T286D) (plus additional mutations [T/A] that prevented inhibitory 305/306 phosphorylation). This form had an elevated basal activation state, but was further activated during glutamate uncaging; importantly the activation state decayed with tau1 nearly the same as that of WT Camui. Therefore, the data strongly indicate that tau1 is not due to T286 dephosphorylation. We conclude that, although Camui is an excellent tool for observing CaMKII signaling, further experimentation is needed to determine how CaMKII is turned off by its dephosphorylation.

Stimulation of the cardiac myocyte Na+-K+ pump due to reversal of its constitutive oxidative inhibition.

Protein kinase C can activate NADPH oxidase and induce glutathionylation of the β1-Na(+)-K(+) pump subunit, inhibiting activity of the catalytic α-subunit. To examine if signaling of nitric oxide-induced soluble guanylyl cyclase (sGC)/cGMP/protein kinase G can cause Na(+)-K(+) pump stimulation by counteracting PKC/NADPH oxidase-dependent inhibition, cardiac myocytes were exposed to ANG II to activate NADPH oxidase and inhibit Na(+)-K(+) pump current (Ip). Coexposure to 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1) to stimulate sGC prevented the decrease of Ip. Prevention of the decrease was abolished by inhibition of protein phosphatases (PP) 2A but not by inhibition of PP1, and it was reproduced by an activator of PP2A. Consistent with a reciprocal relationship between β1-Na(+)-K(+) pump subunit glutathionylation and pump activity, YC-1 decreased ANG II-induced β1-subunit glutathionylation. The decrease induced by YC-1 was abolished by a PP2A inhibitor. YC-1 decreased phosphorylation of the cytosolic p47(phox) NADPH oxidase subunit and its coimmunoprecipitation with the membranous p22(phox) subunit, and it decreased O2 (·-)-sensitive dihydroethidium fluorescence of myocytes. Addition of recombinant PP2A to myocyte lysate decreased phosphorylation of p47(phox) indicating the subunit could be a substrate for PP2A. The effects of YC-1 to decrease coimmunoprecipitation of p22(phox) and p47(phox) NADPH oxidase subunits and decrease β1-Na(+)-K(+) pump subunit glutathionylation were reproduced by activation of nitric oxide-dependent receptor signaling. We conclude that sGC activation in cardiac myocytes causes a PP2A-dependent decrease in NADPH oxidase activity and a decrease in β1 pump subunit glutathionylation. This could account for pump stimulation with neurohormonal oxidative stress expected in vivo.

Propofol ameliorates endothelial inflammation induced by hypoxia/reoxygenation in human umbilical vein endothelial cells: Role of phosphatase A2

Hypoxia/reoxygenation (H/R) induces endothelial inflammation with augmentation of endothelial adhesion molecules over-expression. Propofol was reported to attenuate endothelial adhesion molecule expression in some situations. Here, we examined the molecular mechanism for how propofol restored H/R-mediated up-regulation of endothelial adhesion molecules in human umbilical vein endothelial cells (HUVECs). Compared with the control group, H/R up-regulated expression of Pin-1 and PP2A, increased p66Shc–Ser36 phosphorylation, induced p66Shc mitochondrial translocation, O2− accumulation and NF-κB activation, and decreased eNOS–Ser1177 phosphorylation and nitric oxide (NO) production, thus up-regulating expression of endothelial adhesion molecules and increasing mononuclear-endothelial interaction. More importantly, except that propofol had no effect on H/R-induced p66Shc–Ser36 phosphorylation, most of H/R-mediated changes were alleviated by propofol, resulting in the reduction of endothelial adhesion molecules expression and mononuclear-endothelial adhesion. Moreover, we demonstrated the protective effect of propofol on H/R-induced endothelial inflammation was similar to that of calyculin A, an inhibitor of PP2A. In contrast, FTY720, an activator of PP2A, antagonized the effect of propofol. Our data indicated that propofol down-regulated PP2A expression, leading to reduced dephosphorylation of p66Shc–Ser36 and eNOS–Ser1177, which is associated with ROS accumulation and NO reduction, resulting in inhibition of endothelial adhesion molecule expression and mononuclear-endothelial interaction.

Rapamycin inhibits Erk1/2-mediated neuronal apoptosis caused by cadmium.

Cadmium (Cd), an environmental contaminant, causes neurodegenerative disorders. Recently we have shown that rapamycin prevents Cd-induced neuronal cell death by inhibiting mTOR signaling pathway. Here we found that rapamycin exerted its prevention against Cd-induced neuronal cell death also partially via blocking Erk1/2 pathway. Inhibiting Erk1/2 with PD98059 or silencing Erk1/2 potentiated rapamycin's inhibition of Cd-induced phosphorylation of Erk1/2 and apoptosis in neuronal cells. Both PP2A and PTEN/Akt were involved in the regulation of Erk1/2 activation and cell death triggered by Cd. Inhibition of PP2A with okadaic acid or ectopic expression of dominant negative PP2A attenuated rapamycin's inhibition of Cd-induced phospho-Erk1/2 and apoptosis, whereas over-expression of wild-type PP2A enhanced rapamycin's effects; Over-expression of wild-type PTEN or dominant negative Akt, or inhibition of Akt with Akt inhibitor X strengthened rapamycin's inhibition of Cd-induced phospho-Erk1/2 and cell death. Furthermore, expression of a rapamycin-resistant and kinase-active mTOR (mTOR-T) blocked rapamycin's inhibitory effects on Cd-induced inhibition of PP2A, down-regulation of PTEN, and activation of Akt, leading to Erk1/2 activation and cell death, whereas silencing mTOR mimicked rapamycin's effects. The results uncover that rapamycin inhibits Cd activation of Erk1/2-mediated neuronal apoptosis through intervening mTOR-PP2A/PTEN signaling network.

Basal protein phosphatase 2A activity restrains cytokine expression: role for MAPKs and tristetraprolin

PP2A is a master controller of multiple inflammatory signaling pathways. It is a target in asthma; however the molecular mechanisms by which PP2A controls inflammation warrant further investigation. In A549 lung epithelial cells in vitro we show that inhibition of basal PP2A activity by okadaic acid (OA) releases restraint on MAPKs and thereby increases MAPK-mediated pro-asthmatic cytokines, including IL-6 and IL-8. Notably, PP2A inhibition also impacts on the anti-inflammatory protein – tristetraprolin (TTP), a destabilizing RNA binding protein regulated at multiple levels by p38 MAPK. Although PP2A inhibition increases TTP mRNA expression, resultant TTP protein builds up in the hyperphosphorylated inactive form. Thus, when PP2A activity is repressed, pro-inflammatory cytokines increase and anti-inflammatory proteins are rendered inactive. Importantly, these effects can be reversed by the PP2A activators FTY720 and AAL(s), or more specifically by overexpression of the PP2A catalytic subunit (PP2A-C). Moreover, PP2A plays an important role in cytokine expression in cells stimulated with TNFα; as inhibition of PP2A with OA or PP2A-C siRNA results in significant increases in cytokine production. Collectively, these data reveal the molecular mechanisms of PP2A regulation and highlight the potential of boosting the power of endogenous phosphatases as novel anti-inflammatory strategies to combat asthmatic inflammation.

Highlights from Recent Cancer Literature

Highlights from Recent Cancer Literature