Targeting PP2A for cancer therapeutic modulation

Protein phosphatases play essential roles as negative regulators of kinases and signaling cascades involved in cytoskeletal organization.Protein phosphatase 2A(PP2A)is highly conserved and is the predominant serine/threonine phosphatase in the nervous system,constituting more than 70%of all neuronal phosphatases.PP2A is involved in diverse regulatory functions,including cell cycle progression,apoptosis,and DNA repair.Although PP2A has historically been identified as a tumor suppressor,inhibition of PP2A has paradoxically demonstrated potential as a therapeutic target for various cancers.LB100,a water-soluble,small-molecule competitive inhibitor of PP2A,has shown particular promise as a chemo-and radio-sensitizing agent.Preclinical success has led to a profusion of clinical trials on LB100 adjuvant therapies,including a phase I trial in extensive-stage small-cell lung cancer,a phase I/II trial in myelodysplastic syndrome,a phase II trial in recurrent glioblastoma,and a completed phase I trial assessing the safety of LB100 and docetaxel in various relapsed solid tumors.Herein,we review the development of LB100,the role of PP2A in cancer biology,and recent advances in targeting PP2A inhibition in immunotherapy.

Protein phosphatases and chromatin modifying complexes in the inflammatory cascade in acute pancreatitis

Acute pancreatitis is an inflammation of the pancreas that may lead to systemic inflammatory response syndrome and death due to multiple organ failure. Acinar cells, together with leukocytes, trigger the inflammatory cascade in response to local damage of the pancreas. Amplification of the inflammatory cascade requires up-regulation of proinflammatory cytokines and this process is mediated not only by nuclear factor κB but also by chromatinmodifying complexes and chromatin remodeling. Among the different families of histone acetyltransferases, the p300/CBP family seems to be particularly associated with the inflammatory process. cAMP activates gene expression via the cAMP-responsive element (CRE) and the transcription factor CRE-binding protein (CREB). CREB can be phosphorylated and activated by different kinases, such as protein kinase A and MAPK, and then it recruits the histone acetyltransferase co-activator CREB-binding protein (CBP) and its homologue p300. The recruitment of CBP/p300 and changes in the level of histone acetylation are required for transcription activation. Transcriptional repression is also a dynamic and essential mechanism of down-regulation of genes for resolution of inflammation, which seems to be mediated mainly by protein phosphatases (PP1, PP2A and MKP1) and histone deacetylases(HDACs) .Class HDACs are key transcriptional regulators whose activities are controlled via phosphorylationdependent nucleo/cytoplasmic shuttling. PP2A is responsible for dephosphorylation of class HDACs, triggeringnuclear localization and repression of target genes, whereas phosphorylation triggers cytoplasmic localization leading to activation of target genes. The potential benefit from treatment with phosphodiesterase inhibitors and histone deacetylase inhibitors is discussed.

PP2A interacts with KATANIN to promote microtubule organization and conical cell morphogenesis

The organization of the microtubule cytoskeleton is critical for cell and organ morphogenesis.The evolutionarily conserved microtubule-severing enzyme KATANIN plays critical roles in microtubule organization in the plant and animal kingdoms.We previously used conical cell of Arabidopsis thaliana petals as a model system to investigate cortical microtubule organization and cell morphogenesis and determined that KATANIN promotes the formation of circumferential cortical microtubule arrays in conical cells.Here,we demonstrate that the conserved protein phosphatase PP2A interacts with and dephosphorylates KATANIN to promote the formation of circumferential cortical microtubule arrays in conical cells.KATANIN undergoes cycles of phosphorylation and dephosphorylation.Using co-immunoprecipitation coupled with mass spectrometry,we identified PP2A subunits as KATANIN-interacting proteins.Further biochemical studies showed that PP2 A interacts with and dephosphorylates KATANIN to stabilize its cellular abundance.Similar to the katanin mutant,mutants for genes encoding PP2A subunits showed disordered cortical microtubule arrays and defective conical cell shape.Taken together,these findings identify PP2A as a regulator of conical cell shape and suggest that PP2A mediates KATANIN phospho-regulation during plant cell morphogenesis.

HBV Infection Promotes the Occurrence and Development of Hepatocellular Carcinoma through Impairing the Inhibitory Effect of PPP2R5A on MAPK/AKT/WNT Signaling Pathway

Reversible phosphorylation and dephosphorylation play important roles in cell function and cell signal transduction. PPP2R5A (protein phosphatase 2 regulatory subunit B’ alpha) is responsible for specifically regulating the catalytic function, substrate specificity and intracellular localization of the tumor suppressor phosphatase PP2A (serine/threonine protein phosphatase 2A). Therefore, the abnormal expression and function of PPP2R5A may be related to canceration. The aim of this study was to reveal its role in the occurrence and development of hepatocellular carcinoma (HCC). It is hoped that the results of this study can provide guidance for the prevention and treatment of HCC. The results showed that PPP2R5A inhibited the proliferation and metastasis of HCC cells, and acted as a tumor suppressor in HCC cells, but it had no significant effect on cell cycle. Further research found that PPP2R5A exerted tumor suppressor efficacy by inhibiting the MAPK/AKT/WNT signaling pathway. Combined with analysis of clinical tissue samples and TCGA database, it was found that the expression of PPP2R5A in tumor tissues of Chinese HCC patients was down-regulated and significantly correlated with the progression-free survival (PFS) of HCC patients. On the contrary, PPP2R5A showed an up-regulation trend in HCC cases in TCGA database although its effect on PFS was the same with that in Chinese HCC patients. Hepatitis B virus (HBV) infection is the main pathogenic factor of HCC in China. It was found that HBV infection reduced the content of PPP2R5A in cells. It was concluded that HBV inhibited the initiation of the protective mechanism mediated by PPP2R5A, making the occurrence and progress of HCC more “unimpeded”. This conclusion will further reveal the role of PPP2R5A in HBV-induced and HBV-unrelated HCC, therefore, providing clues for the prevention and treatment of the two types of HCC, respectively.

DIW1 encoding a cladeⅠPP2C phosphatase negatively regulates drought tolerance by de-phosphorylating TaSnRK1.1 in wheat

Drought seriously impacts wheat production(Triticum aestivum L.),while the exploitation and utilization of genes for drought tolerance are insufficient.Leaf wilting is a direct reflection of drought tolerance in plants.Clade A PP2Cs are abscisic acid(ABA)co-receptors playing vital roles in the ABA signaling pathway,regulating drought response.However,the roles of other clade PP2Cs in drought tolerance,especially in wheat,remain largely unknown.Here,we identified a gain-of-function drought-induced wilting 1(DIW1)gene from the wheat Aikang 58 mutant library by map-based cloning,which encodes a cladeⅠprotein phosphatase 2C(TaPP2C158)with enhanced protein phosphatase activity.Phenotypic analysis of overexpression and CRISPR/Cas9 mutant lines demonstrated that DIW1/TaPP2C158 is a negative regulator responsible for drought resistance.We found that TaPP2C158 directly interacts with TaSnRK1.1 and de-phosphorylates it,thus inactivating the TaSnRK1.1–Ta AREB3 pathway.TaPP2C158 protein phosphatase activity is negatively correlated with ABA signaling.Association analysis suggested that C-terminal variation of TaPP2C158 changing protein phosphatase activity is highly correlated with the canopy temperature,and seedling survival rate under drought stress.Our data suggest that the favorable allele with lower phosphatase activity of TaPP2C158 has been positively selected in Chinese breeding history.This work benefits us in understanding the molecular mechanism of wheat drought tolerance,and provides elite genetic resources and molecular markers for improving wheat drought tolerance.

Reciprocal regulation between the negative regulator PP2CG1 phosphatase and the positive regulator OST1 kinase confers cold response in Arabidopsis

Protein phosphorylation and dephosphorylation have been reported to play important roles in plant cold responses.In addition,phospho-regulatory feedback is a conserved mechanism for biological processes and stress responses in animals and plants.However,it is less well known that a regulatory feedback loop is formed by the protein kinase and the protein phosphatase in plant responses to cold stress.Here,we report that OPEN STOMATA 1(OST1)and PROTEIN PHOSPHATASE 2C G GROUP 1(PP2CG1)reciprocally regulate the activity during the cold stress response.The interaction of PP2CG1 and OST1 is inhibited by cold stress,which results in the release of OST1 at the cytoplasm and nucleus from suppression by PP2CG1.Interestingly,cold-activated OST1 phosphorylates PP2CG1 to suppress its phosphatase activity,thereby amplifying cold signaling in plants.Mutations of PP2CG1 and its homolog PP2CG2 enhance freezing tolerance,whereas overexpression of PP2CG1 decreases freezing tolerance.Moreover,PP2CG1 negatively regulates protein levels of C-REPEAT BINDING FACTORs(CBFs)under cold stress.Our results uncover a phosphor/dephosphor-regulatory feedback loop mediated by PP2CG1 phosphatase and OST1 protein kinase in plant cold responses.