Protein Phosphatase 2A Regulates Apoptosis in Intestinal Epithelial Cells

GTP Signaling Links Metabolism, DNA Repair, and Responses to Genotoxic Stress

How cell metabolism regulates DNA repair is incompletely understood. Here, we define a GTP-mediated signaling cascade that links metabolism to DNA repair and has significant therapeutic implications. GTP, but not other nucleotides, regulates the activity of Rac1, a guanine nucleotide-binding protein, which promotes the dephosphorylation of serine 323 on Abl-interactor 1 (Abi-1) by protein phosphatase 5 (PP5). Dephosphorylated Abi-1, a protein previously not known to activate DNA repair, promotes nonhomologous end joining. In patients and mouse models of glioblastoma, Rac1 and dephosphorylated Abi-1 mediate DNA repair and resistance to standard-of-care genotoxic treatments. The GTP-Rac1-PP5-Abi-1 signaling axis is not limited to brain cancer, as GTP supplementation promotes DNA repair and Abi-1-S323 dephosphorylation in nonmalignant cells and protects mouse tissues from genotoxic insult. This unexpected ability of GTP to regulate DNA repair independently of deoxynucleotide pools has important implications for normal physiology and cancer treatment.

SIGNIFICANCE

A newly described GTP-dependent signaling axis is an unexpected link between nucleotide metabolism and DNA repair. Disrupting this pathway can overcome cancer resistance to genotoxic therapy while augmenting it can mitigate genotoxic injury of normal tissues. This article is featured in Selected Articles from This Issue, p. 5.

GTP signaling links metabolism, DNA repair, and responses to genotoxic stress

How cell metabolism regulates DNA repair is incompletely understood. Here, we define a GTP-mediated signaling cascade that links metabolism to DNA repair and has significant therapeutic implications. GTP, but not other nucleotides, regulates the activity of Rac1, a G protein, that promotes the dephosphorylation of serine 323 on Abl-interactor 1 (Abi-1) by protein phosphatase 5 (PP5). Dephosphorylated Abi-1, a protein previously not known to activate DNA repair, promotes non-homologous end joining. In patients and mouse models of glioblastoma, Rac1 and dephosphorylated Abi-1 mediate DNA repair and resistance to standard of care genotoxic treatments. The GTP-Rac1-PP5-Abi-1 signaling axis is not limited to brain cancer, as GTP supplementation promotes DNA repair and Abi-1-S323 dephosphorylation in non-malignant cells and protects mouse tissues from genotoxic insult. This unexpected ability of GTP to regulate DNA repair independently of deoxynucleotide pools has important implications for normal physiology and cancer treatment.

PUMA overexpression dissociates thioredoxin from ASK1 to activate the JNK/BCL-2/BCL-XL pathway augmenting apoptosis in ovarian cancer

ASK1-JNK signaling promotes mitochondrial dysfunction-mediated apoptosis, but the bridge between JNK and apoptosis is not fully understood. PUMA induces apoptosis through BAX/BAK. Our previous study suggests a therapeutic potential of PUMA for ovarian cancer. However, whether and how PUMA activates ASK1 remains unclear. Here, we found for the first time that PUMA activated ASK1 by dissociating thioredoxin (TRX) from ASK1, however, it neither interacted with ASK1 nor TRX. Furthermore, PUMA overexpression caused ROS release from mitochondrial. H₂O₂ significantly impaired the interaction of ASK1 with TRX, whereas ROS scavenger NAC effectively abrogated the H₂O₂ effect, partly rescued PUMA-interfered interaction of ASK1 with TRX, and also abolished ASK1 phosphorylation. Interestingly, PUMA could not impair the association of ASK1 with TRX-C32S or TRX-C35S, two TRX mutants which are no longer oxidized in response to ROS. We further showed that PUMA activated ASK1-JNK axis to phosphorylate BCL-2 and BCL-XL, further augmenting apoptosis of ovarian cancer cells. In vivo, PUMA adenovirus combined with paclitaxel significantly inhibited intrinsically cisplatin-resistant ovarian cancer growth, and caused phosphorylation of BCL-2 and BCL-XL. Our results from human ovarian cancer TMA chips also revealed a positive correlation between PUMA expression and the phosphorylation of BCL-2 and BCL-XL. More importantly, all patients had no distal metastasis, implying a possibly clinical significance. Collectively, our results reveal a new pro-apoptotic signal amplification mechanism for PUMA by which PUMA overexpression first induces ROS-mediated dissociation of TRX from ASK1, and then causes JNK activation-triggering BCL-2/BCL-XL phosphorylation, ultimately augmenting apoptosis in ovarian cancer.

A disorder-related variant (E420K) of a PP2A-regulatory subunit (PPP2R5D) causes constitutively active AKT-mTOR signaling and uncoordinated cell growth

Functional genomic approaches have facilitated the discovery of rare genetic disorders and improved efforts to decipher their underlying etiology. PPP2R5D-related disorder is an early childhood onset condition characterized by intellectual disability, hypotonia, autism-spectrum disorder, macrocephaly, and dysmorphic features. The disorder is caused by de novo single nucleotide changes in PPP2R5D, which generate heterozygous dominant missense variants. PPP2R5D is known to encode a B'-type (B'56δ) regulatory subunit of a PP2A-serine/threonine phosphatase. To help elucidate the molecular mechanisms altered in PPP2R5D-related disorder, we used a CRISPR-single-base editor to generate HEK-293 cells in which a single transition (c.1258G>A) was introduced into one allele, precisely recapitulating a clinically relevant E420K variant. Unbiased quantitative proteomic and phosphoproteomic analyses of endogenously expressed proteins revealed heterozygous-dominant changes in kinase/phosphatase signaling. These data combined with orthogonal validation studies revealed a previously unrecognized interaction of PPP2R5D with AKT in human cells, leading to constitutively active AKT-mTOR signaling, increased cell size, and uncoordinated cellular growth in E420K-variant cells. Rapamycin reduced cell size and dose-dependently reduced RPS6 phosphorylation in E420K-variant cells, suggesting that inhibition of mTOR1 can suppress both the observed RPS6 hyperphosphorylation and increased cell size. Together, our findings provide a deeper understanding of PPP2R5D and insight into how the E420K-variant alters signaling networks influenced by PPP2R5D. Our comprehensive approach, which combines precise genome editing, isobaric tandem mass tag labeling of peptides generated from endogenously expressed proteins, and concurrent liquid chromatography-mass spectrometry (LC-MS³), also provides a roadmap that can be used to rapidly explore the etiologies of additional genetic disorders.

Co-culture model of Caco-2/HT29-MTX cells: A promising tool for investigation of phycotoxins toxicity on the intestinal barrier

Most lipophilic phycotoxins have been involved in human intoxications but some of these toxins have never been proven to induce human gastro-intestinal symptoms, although intestinal damage in rodents has been documented. For investigating the in vitro toxicological profile of lipophilic phycotoxins on intestine, the epithelial Caco-2 cell line has been the most commonly used model. Nevertheless, considering the complexity of the intestinal epithelium, in vitro co-cultures integrating enterocyte-like and mucus-secreting cell types are expected to provide more relevant data. In this study, the toxic effects (viability, inflammation, cellular monolayer integrity, modulation of cell type proportion and production of mucus) of four lipophilic phycotoxins (PTX2, YTX, AZA1 and OA) were evaluated in Caco-2/HT29-MTX co-cultured cells. The four toxins induced a reduction of viability from 20% to 50% and affected the monolayer integrity. Our results showed that the HT29-MTX cells population were more sensitive to OA and PTX2 than Caco-2 cells. Among the four phycotoxins, OA induced inflammation (28-fold increase of IL-8 release) and also a slight increase of both mucus production (up-regulation of mucins mRNA expression) and mucus secretion (mucus area and density). For PTX2 we observed an increase of IL-8 release but weaker than OA. Intestinal cell models integrating several cell types can contribute to improve hazard characterization and to describe more accurately the modes of action of phycotoxins.

The natural anticancer agent cantharidin alters GPI-anchored protein sorting by targeting Cdc1-mediated remodeling in endoplasmic reticulum

Cantharidin (CTD) is a potent anticancer small molecule produced by several species of blister beetle. It has been a traditional medicine for the management of warts and tumors for many decades. CTD suppresses tumor growth by inducing apoptosis, cell cycle arrest, and DNA damage and inhibits protein phosphatase 2 phosphatase activator (PP2A) and protein phosphatase 1 (PP1). CTD also alters lipid homeostasis, cell wall integrity, endocytosis, adhesion, and invasion in yeast cells. In this study, we identified additional molecular targets of CTD using a Saccharomyces cerevisiae strain that expresses a cantharidin resistance gene (CRG1), encoding a SAM-dependent methyltransferase that methylates and inactivates CTD. We found that CTD specifically affects phosphatidylethanolamine (PE)-associated functions that can be rescued by supplementing the growth media with ethanolamine (ETA). CTD also perturbed endoplasmic reticulum (ER) homeostasis and cell wall integrity by altering the sorting of glycosylphosphatidylinositol (GPI)-anchored proteins. A CTD-dependent genetic interaction profile of CRG1 revealed that the activity of the lipid phosphatase cell division control protein 1 (Cdc1) in GPI-anchor remodeling is the key target of CTD, independently of PP2A and PP1 activities. Moreover, experiments with human cells further suggested that CTD functions through a conserved mechanism in higher eukaryotes. Altogether, we conclude that CTD induces cytotoxicity by targeting Cdc1 activity in GPI-anchor remodeling in the ER.

Inhibition of Rac1 ameliorates neuronal oxidative stress damage via reducing Bcl-2/Rac1 complex formation in mitochondria through PI3K/Akt/mTOR pathway

Although the neuroprotective effects of Rac1 inhibition have been reported in various cerebral ischemic models, the molecular mechanisms of action have not yet been fully elucidated. In this study, we investigated whether the inhibition of Rac1 provided neuroprotection in a diabetic rat model of focal cerebral ischemia and hyperglycemia-exposed PC-12 cells. Intracerebroventricular administration of lentivirus expressing the Rac1 small hairpin RNA (shRNA) and specific Rac1 inhibitor NSC23766 not only decreased the infarct volumes and improved neurologic deficits with a correlated significant activation of mitochondrial DNA specific proteins, such as OGG1 and POLG, but also elevated Bcl-2 S70 phosphorylation in mitochondria. Furthermore, the levels of p-PI3K, p-Akt and p-mTOR increased, while 8-OHdG, ROS production and Bcl-2/Rac1 complex formation in mitochondria reduced in both Rac1-shRNA- and NSC23766-treated rats. Moreover, to confirm our in vivo observations, inhibition of Rac1 activity by NSC23766 suppressed the interactions between Bcl-2 and Rac1 in the mitochondria of PC-12 cells cultured in high glucose conditions and protected PC-12 cells from high glucose-induced neurotoxicity. More importantly, these beneficial effects of Rac1 inhibition were abolished by PI3K inhibitor LY294002. In contrast to NSC23766 treatment, LY294002 had little effect on the decrement of p-PTEN level. Taken together, these findings revealed novel neuroprotective roles of Rac1 inhibition against cerebral ischemic reperfusion injury in vivo and high glucose-induced neurotoxicity in PC-12 cells in vitro, by reducing Bcl-2/Rac1 complex formation in mitochondria through the activation of PI3K/Akt/mTOR survival pathway.

Atractylenolide I stimulates intestinal epithelial repair through polyamine-mediated Ca2+ signaling pathway

BACKGROUND

An impairment of the integrity of the mucosal epithelial barrier can be observed in the course of various gastrointestinal diseases. The migration and proliferation of the intestinal epithelial (IEC-6) cells are essential repair modalities to the healing of mucosal ulcers and wounds. Atractylenolide I (AT-I), one of the major bioactive components in the rhizome of Atractylodes macrocephala Koidz. (AMR), possesses multiple pharmacological activities. This study was designed to investigate the therapeutic effects and the underlying molecular mechanisms of AT-I on gastrointestinal mucosal injury.

METHODS

Scratch method with a gel-loading microtip was used to detect IEC-6 cell migration. The real-time cell analyzer (RTCA) system was adopted to evaluate IEC-6 cell proliferation. Intracellular polyamines content was determined using high performance liquid chromatography (HPLC). Flow cytometry was used to measure cytosolic free Ca²⁺ concentration ([Ca²⁺]_c). mRNA and protein expression of TRPC1 and PLC-γ₁ were determined by real-time PCR and Western blotting assay respectively.

RESULTS

Treatment of IEC-6 cells with AT-I promoted cell migration and proliferation, increased polyamines content, raised cytosolic free Ca²⁺ concentration ([Ca²⁺]_c), and enhanced TRPC1 and PLC-γ₁ mRNA and protein expression. Depletion of cellular polyamines by DL-a-difluoromethylornithine (DFMO, an inhibitor of polyamine synthesis) suppressed cell migration and proliferation, decreased polyamines content, and reduced [Ca²⁺]_c, which was paralleled by a decrease in TRPC1 and PLC-γ₁ mRNA and protein expression in IEC-6 cells. AT-I reversed the effects of DFMO on polyamines content, [Ca²⁺]_c, TRPC1 and PLC-γ₁ mRNA and protein expression, and restored IEC-6 cell migration and proliferation to near normal levels.

CONCLUSION

Our data demonstrate that AT-I stimulates intestinal epithelial cell migration and proliferation via the polyamine-mediated Ca²⁺ signaling pathway. Therefore, AT-I may have the potential to be further developed as a promising therapeutic agent to treat diseases associated with gastrointestinal mucosal injury, such as inflammatory bowel disease and peptic ulcer.

Non-Hematopoietic β-Arrestin1 Confers Protection Against Experimental Colitis

β-Arrestins are multifunctional scaffolding proteins that modulate G protein-coupled receptor (GPCR)-dependent and -independent cell signaling pathways in various types of cells. We recently demonstrated that β-arrestin1 (β-arr1) deficiency strikingly attenuates dextran sodium sulfate (DSS)-induced colitis in mice. Since DSS-induced colitis is in part dependent on gut epithelial injury, we examined the role of β-arr1 in intestinal epithelial cells (IECs) using a colon epithelial cell line, SW480 cells. Surprisingly, we found that knockdown of β-arr1 in SW480 cells enhanced epithelial cell death via a caspase-3-dependent process. To understand the in vivo relevance and potential cell type-specific role of β-arr1 in colitis development, we generated bone marrow chimeras with β-arr1 deficiency in either the hematopoietic or non-hematopoietic compartment. Reconstituted chimeric mice were then subjected to DSS-induced colitis. Similar to our previous findings, β-arr1 deficiency in the hematopoietic compartment protected mice from DSS-induced colitis. However, consistent with the role of β-arr1 in epithelial apoptosis in vitro, non-hematopoietic β-arr1 deficiency led to an exacerbated colitis phenotype. To further understand signaling mechanisms, we examined the effect of β-arr1 on TNF-α-mediated NFκB and MAPK pathways. Our results demonstrate that β-arr1 has a critical role in modulating ERK, JNK and p38 MAPK pathways mediated by TNF-α in IECs. Together, our results show that β-arr1-dependent signaling in hematopoietic and non-hematopoietic cells differentially regulates colitis pathogenesis and further demonstrates that β-arr1 in epithelial cells inhibits TNF-α-induced cell death pathways.

The mechanism by which MEK/ERK regulates JNK and p38 activity in polyamine depleted IEC-6 cells during apoptosis

Polyamine-depletion inhibited apoptosis by activating ERK1/2, while, preventing JNK1/2 activation. MKP-1 knockdown by SiRNA increased ERK1/2, JNK1/2, and p38 phosphorylation and apoptosis. Therefore, we predicted that polyamines might regulate MKP1 via MEK/ERK and thereby apoptosis. We examined the role of MEK/ERK in the regulation of MKP1 and JNK, and p38 activities and apoptosis. Inhibition of MKP-1 activity with a pharmacological inhibitor, sanguinarine (SA), increased JNK1/2, p38, and ERK1/2 activities without causing apoptosis. However, pre-activation of these kinases by SA significantly increased camptothecin (CPT)-induced apoptosis suggesting different roles for MAPKs during survival and apoptosis. Inhibition of MEK1 activity prevented the expression of MKP-1 protein and augmented CPT-induced apoptosis, which correlated with increased activities of JNK1/2, caspases, and DNA fragmentation. Polyamine depleted cells had higher levels of MKP-1 protein and decreased JNK1/2 activity and apoptosis. Inhibition of MEK1 prevented MKP-1 expression and increased JNK1/2 and apoptosis. Phospho-JNK1/2, phospho-ERK2, MKP-1, and the catalytic subunit of PP2Ac formed a complex in response to TNF/CPT. Inactivation of PP2Ac had no effect on the association of MKP-1 and JNK1. However, inhibition of MKP-1 activity decreased the formation of the MKP-1, PP2Ac and JNK complex. Following inhibition by SA, MKP-1 localized in the cytoplasm, while basal and CPT-induced MKP-1 remained in the nuclear fraction. These results suggest that nuclear MKP-1 translocates to the cytoplasm, binds phosphorylated JNK and p38 resulting in dephosphorylation and decreased activity. Thus, MEK/ERK activity controls the levels of MKP-1 and, thereby, regulates JNK activity in polyamine-depleted cells.

Mitochondrial ROS and involvement of Bcl-2 as a mitochondrial ROS regulator

Mitochondria are the major intracellular source of reactive oxygen species (ROS). While excessive mitochondrial ROS (mitoROS) production induces cell injury and death, there is accumulating evidence that non-toxic low levels of mitoROS could serve as important signaling molecules. Therefore, maintenance of mitoROS at physiological levels is crucial for cell homeostasis as well as for survival and proliferation. This review describes the various mechanisms that keep mitoROS in check, with particular focus on the role of the onco-protein Bcl-2 in redox regulation. In addition to its canonical anti-apoptotic activity, Bcl-2 has been implicated in mitoROS regulation by its effect on mitochondrial complex IV activity, facilitating the mitochondrial incorporation of GSH and interaction with the small GTPase-Rac1 at the mitochondria. We also discuss some of the plausible mechanism(s) which allows Bcl-2 to sense and respond to the fluctuations in mitoROS.

Spermidine promotes adipogenesis of 3T3-L1 cells by preventing interaction of ANP32 with HuR and PP2A

We have shown previously that the polyamine spermidine is indispensable for differentiation of 3T3-L1 preadipocytes. In the present study, we examined the mechanism of spermidine function by using the polyamine biosynthesis inhibitor α-difluoromethylornithine in combination with the metabolically stable polyamine analogues γ-methylspermidine or (R,R)-α,ω-bismethylspermine. At the early phase of differentiation, spermidine-depleted 3T3-L1 cells showed decreased translation of the transcription factor C/EBPβ (CCAAT/enhancer-binding protein β), decreased PP2A (protein phosphatase 2A) activity and increased cytoplasmic localization of the RNA-binding protein HuR (human antigen R). The amount of HuR bound to C/EBPβ mRNA was reduced, whereas the amount of bound CUGBP2, an inhibitor of C/EBPβ translation, was increased. ANP32 (acidic nuclear phosphoprotein 32) proteins, which are known PP2A inhibitors and HuR ligands, bound more PP2A and HuR in spermidine-depleted than in control cells, whereas immunodepletion of ANP32 proteins from the lysate of spermidine-depleted cells restored PP2A activity. Taken together, our data shows that spermidine promotes C/EBPβ translation in differentiating 3T3-L1 cells, and that this process is controlled by the interaction of ANP32 with HuR and PP2A.

Specific targeting of caspase-9/PP2A interaction as potential new anti-cancer therapy

PURPOSE

PP2A is a serine/threonine phosphatase critical to physiological processes, including apoptosis. Cell penetrating peptides are molecules that can translocate into cells without causing membrane damage. Our goal was to develop cell-penetrating fusion peptides specifically designed to disrupt the caspase-9/PP2A interaction and evaluate their therapeutic potential in vitro and in vivo.

EXPERIMENTAL DESIGN

We generated a peptide containing a penetrating sequence associated to the interaction motif between human caspase-9 and PP2A (DPT-C9h), in order to target their association. Using tumour cell lines, primary human cells and primary human breast cancer (BC) xenografts, we investigated the capacity of DPT-C9h to provoke apoptosis in vitro and inhibition of tumour growth (TGI) in vivo. DPT-C9h was intraperitoneally administered at doses from 1 to 25 mg/kg/day for 5 weeks. Relative Tumour Volume (RTV) was calculated.

RESULTS

We demonstrated that DPT-C9h specifically target caspase-9/PP2A interaction in vitro and in vivo and induced caspase-9-dependent apoptosis in cancer cell lines. DPT-C9h also induced significant TGI in BC xenografts models. The mouse-specific peptide DPT-C9 also induced TGI in lung (K-Ras model) and breast cancer (PyMT) models. DPT-C9h has a specific effect on transformed B cells isolated from chronic lymphocytic leukemia patients without any effect on primary healthy cells. Finally, neither toxicity nor immunogenic responses were observed.

CONCLUSION

Using the cell-penetrating peptides blocking caspase-9/PP2A interactions, we have demonstrated that DPT-C9h had a strong therapeutic effect in vitro and in vivo in mouse models of tumour progression.

Copy-number variation of cancer-gene orthologs is sufficient to induce cancer-like symptoms in Saccharomyces cerevisiae

BACKGROUND

Copy-number variation (CNV), rather than complete loss of gene function, is increasingly implicated in human disease. Moreover, gene dosage is recognised as important in tumourigenesis, and there is an increasing realisation that CNVs may not be just symptomatic of the cancerous state but may, in fact, be causative. However, the identification of CNV-related phenotypes for mammalian genes is a slow process, due to the technical difficulty of constructing deletion mutants. Using the genome-wide deletion library for the model eukaryote, Saccharomyces cerevisiae, we have identified genes (termed haploproficient, HP) which, when one copy is deleted from a diploid cell, result in an increased rate of proliferation. Since haploproficiency under nutrient-sufficient conditions is a novel phenotype, we sought here to characterise a subset of the yeast haploproficient genes which seem particularly relevant to human cancers.

RESULTS

We show that, for a subset of HP genes, heterozygous deletion is sufficient to cause aberrant cell cycling and altered rates of apoptosis, phenotypes associated with cancer in mammalian cells. A majority of these yeast genes are the orthologs of mammalian cancer genes, and hence our studies suggest that CNV of these oncogenic orthologs may be sufficient to lead to tumourigenesis in human cells. Moreover, where not already implicated, this cluster of cancer-like phenotypes in this model eukaryote may be predictive of the involvement in cancer of the mammalian orthologs of these yeast HP genes. Using the yeast set as a model, we show that the response to a range of anti-cancer drugs is strongly dependent on gene dosage, such that intermediate concentrations of the drugs can actually increase a mutant's growth rate.

CONCLUSIONS

The exploitation of data on the phenotypic impact of heterozygosis in Saccharomyces cerevisiae has permitted the prediction of CNVs affecting tumourigenesis in humans. Our yeast data also suggest that the identification of CNVs in tumour cells may assist both the selection of anti-cancer drugs and the dosages at which they should be administered if they are to be a beneficial, rather than a deleterious, therapy.

Suppressions of Migration and Invasion by Cantharidin in TSGH-8301 Human Bladder Carcinoma Cells through the Inhibitions of Matrix Metalloproteinase-2/-9 Signaling

Cancer metastasis becomes an initial cause of cancer death in human population. In many cancers, it has been shown that the high levels of matrix metalloproteinase (MMP)-2 and/or MMP-9 are associated with the invasive phenotypes of cancer cells. In this study, we investigated the effects of cantharidin, a derivative of blister beetles which is one of the traditional Chinese medicines, on the adhesion, migration, and invasion of human bladder cancer TSGH-8301 cells. Cantharidin effectively suppressed TSGH-8301 cell adhesion, migration, and invasion in a concentration-dependent manner. Results from Western blotting, RT-PCR, and gelatin zymography assays indicated that cantharidin blocked the protein levels, gene expression (mRNA), and activities of MMP-2 and -9 in TSGH-8301 cells. Cantharidin also significantly suppressed the protein expressions of p-p38 and p-JNK1/2 in TSGH-8301 cells. Taken together, cantharidin was suggested to present antimetastatic potential via suppressing the levels of MMP-2 and MMP-9 expression that might be mediated by targeting the p38 and JNK1/2 MAPKs pathway in TSGH-8301 human bladder cancer cells.

Effect of okadaic acid on cultured clam heart cells: involvement of MAPkinase pathways

Okadaic acid (OA) is one of the main diarrhetic shellfish poisoning toxins and a potent inhibitor of protein phosphatases 1 and 2A. The downstream signal transduction pathways following the protein phosphatase inhibition are still unknown and the results of most of the previous studies are often conflicting. The aim of the present study was to evaluate the effects of OA on heart clam cells and to analyse its possible mechanisms of action by investigating the signal transduction pathways involved in OA cytotoxicity. We showed that OA at 1 µM after 24 h of treatment induces disorganization of the actin cytoskeleton, rounding and detachment of fibroblastic cells. Moreover, treatment of heart cells revealed a sequential activation of MAPK proteins depending on the OA concentration. We suggest that the duration of p38 and JNK activation is a critical factor in determining cell apoptosis in clam cardiomyocytes. In the opposite, ERK activation could be involved in cell survival. The cell death induced by OA is a MAPK modulated pathway, mediated by caspase 3-dependent mechanism. OA was found to induce no significant effect on spontaneous beating rate or inward L-type calcium current in clam cardiomyocytes, suggesting that PP1 was not inhibited even by the highest dose of OA.

Quantitative proteomic analysis of okadaic acid treated mouse small intestines reveals differentially expressed proteins involved in diarrhetic shellfish poisoning

Okadaic acid (OA) is a principal diarrhetic shellfish poisoning toxin produced by marine dinoflagellates. This study compared protein profiles of mice small intestines at four time points (0, 3, 6 and 24 h) after a single oral administration of 750 μg/kg OA, and identified the differentially expressed proteins using 2-D DIGE and MALDI-TOF-TOF mass spectrometry. The results showed that the toxin content of the intestines reached its peak 3h after oral administration and then decreased rapidly. OA remarkably inhibited the intestinal PP activity but it recovered to the normal levels within 6 to 24 h. Electron microscope revealed the collapse of the villous architecture and the intestinal microvilli fell off at 3 h, but were repaired within 24h. Notable damage to the intestinal ultrastructure was observed after oral administration. Comparison of the small intestine protein profiles at four time points revealed that 58 proteins were remarkably altered in abundance, and these proteins were involved in macromolecular metabolism, cytoskeleton reorganization, signal transduction, molecular chaperoning and oxidative stress, suggesting that OA toxicity in mouse intestines was complex and diverse, and that multiple proteins other than PP were involved in the diarrhetic process. Villin 1 and hnRNP F might be the key triggers inducing diarrhea in the mouse small intestines.

Microcystin congener- and concentration-dependent induction of murine neuron apoptosis and neurite degeneration

Cyanobacterial microcystins (MCs) represent a toxin group with > 100 variants, requiring active uptake into cells via organic anion-transporting polypeptides, in order to irreversibly inhibit serine/threonine-specific protein phosphatases. MCs are a human health hazard with repeated occurrences of severe poisonings. In the well-known human MC intoxication in Caruaru, Brazil (1996), patients developed signs of acute neurotoxicity, e.g., deafness, tinnitus, and intermittent blindness, as well as subsequent hepatotoxicity. The latter data, in conjunction with some animal studies, suggest that MCs are potent neurotoxins. However, there is little data to date demonstrating MC neuron-specific toxicity. MC exposure-induced cytotoxicity, caspase activity, chromatin condensation, and microtubule-associated Tau protein hyperphosphorylation (epitopes serine199/202 and serine396) were determined. Neurite degeneration was analyzed with confocal microscopy and neurite length determined using image analysis. MC-induced apoptosis was significantly increased by MC-LF and MC-LW, however, only at high concentrations (≥ 3μM), whereas significant neurite degeneration was already observed at 0.5μM MC-LF. Moreover, sustained hyperphosphorylation of Tau was observed with all MC congeners. The concentration- and congener-dependent mechanisms observed suggest that low concentrations of MC-LF and MC-LW can induce subtle neurodegenerative effects, reminiscent of Alzheimer's disease type human tauopathies, and thus should be taken more seriously with regard to potential human health effects than the apical cytotoxicity (apoptosis or necrosis) demonstrated at high MC concentrations.

PIM1-activated PRAS40 regulates radioresistance in non-small cell lung cancer cells through interplay with FOXO3a, 14-3-3 and protein phosphatases

Resistance of cancer cells to ionizing radiation plays an important role in the clinical setting of lung cancer treatment. To date, however, the exact molecular mechanism of radiosensitivity has not been well explained. In this study, we compared radioresistance in two types of non-small cell lung cancer (NSCLC) cells, NCI-H460 and A549, and investigated the signaling pathways that confer radioresistance. In radioresistant cells, exposure to radiation led to overexpression of PIM1 and reduction of protein phosphatases (PP2A and PP5), which induced translocation of PIM1 into the nucleus. Increased nuclear PIM1 phosphorylated PRAS40. Consequently, pPRAS40 made a trimeric complex with 14-3-3 and AKT-activated pFOXO3a, which then moved rapidly to the cytoplasm. Cytoplasmic retention of FOXO3a was associated with downregulation of proapoptotic genes and possibly radioresistance. On the other hand, no suppressive effect of radiation on protein phosphatases was detected and, concomitantly, protein phosphatases downregulated PIM1 in radiosensitive cells. In this setting, PIM1-activated pPRAS40, AKT-activated pFOXO3a, and their complex formation with 14-3-3 could be key regulators of the radiation-induced radioresistance in NSCLC cells.

Inhibition of Mdm2 sensitizes human retinal pigment epithelial cells to apoptosis

PURPOSE

Because recent studies indicate that blocking the interaction between p53 and Mdm2 results in the nongenotoxic activation of p53, the authors sought to investigate whether the inhibition of p53-Mdm2 binding activates p53 and sensitizes human retinal epithelial cells to apoptosis.

METHODS

Apoptosis was evaluated by the activation of caspases and DNA fragmentation assays. The Mdm2 antagonist Nutlin-3 was used to dissociate p53 from Mdm2 and, thus, to increase p53 activity. Knockdown of p53 expression was accomplished by using p53 siRNA.

RESULTS

ARPE-19 and primary RPE cells expressed high levels of the antiapoptotic proteins Bcl-2 and Bcl-xL. Exposure of these cells to camptothecin (CPT) or TNF-α/ cycloheximide (CHX) failed to induce apoptosis. In contrast, treatment with the Mdm2 antagonist Nutlin-3 in the absence of CPT or TNF-α/CHX increased apoptosis. Activation of p53 in response to Nutlin-3 also increased levels of Noxa, p53-upregulated modulator of apoptosis (PUMA), and Siva-1, decreased expression of Bcl-2 and Bcl-xL, and simultaneously increased caspases-9 and -3 activities and DNA fragmentation. Knockdown of p53 decreased the basal expression of p21Cip1 and Bcl-2, inhibited the Nutlin-3-induced upregulation of Siva-1 and PUMA expression, and consequently inhibited caspase-3 activation.

CONCLUSIONS

These results indicate that the normally available pool of intracellular p53 is predominantly engaged in the regulation of cell cycle checkpoints by p21Cip1 and does not trigger apoptosis in response to DNA-damaging agents. However, the blockage of p53 binding to Mdm2 frees a pool of p53 that is sufficient, even in the absence of DNA-damaging agents, to increase the expression of proapoptotic targets and to override the resistance of RPE cells to apoptosis.

Mitochondrial dysfunction and oxidative stress contribute to the pathogenesis of spinocerebellar ataxia type 12 (SCA12)

Spinal cerebellar ataxia type 12 (SCA12) has been attributed to the elevated expression of ppp2r2b. To better elucidate the pathomechanism of the neuronal disorder and to search for a pharmacological treatment, Drosophila models of SCA12 were generated by overexpression of a human ppp2r2b and its Drosophila homolog tws. Ectopic expression of ppp2r2b or tws caused various pathological features, including neurodegeneration, apoptosis, and shortened life span. More detailed analysis revealed that elevated ppp2r2b and tws induced fission of mitochondria accompanied by increases in cytosolic reactive oxygen species (ROS), cytochrome c, and caspase 3 activity. Transmission electron microscopy revealed that fragmented mitochondria with disrupted cristae were engulfed by autophagosomes in photoreceptor neurons of flies overexpressing tws. Additionally, transgenic flies were more susceptible to oxidative injury induced by paraquat. By contrast, ectopic Drosophila Sod2 expression and antioxidant treatment reduced ROS and caspase 3 activity and extended the life span of the SCA12 fly model. In summary, our study demonstrates that oxidative stress induced by mitochondrial dysfunction plays a causal role in SCA12, and reduction of ROS is a potential therapeutic intervention for this neuropathy.

Regulation of DNA fragmentation: the role of caspases and phosphorylation

DNA fragmentation is a hallmark of apoptosis that is induced by apoptotic stimuli in various cell types. Apoptotic signal pathways, which eventually cause DNA fragmentation, are largely mediated by the family of cysteinyl aspartate-specific protease caspases. Caspases mediate apoptotic signal transduction by cleavage of apoptosis-implicated proteins and the caspases themselves. In the process of caspase activation, reversible protein phosphorylation plays an important role. The activation of various proteins is regulated by phosphorylation and dephosphorylation, both upstream and downstream of caspase activation. Many kinases/phosphatases are involved in the control of cell survival and death, including the mitogen-activated protein kinase signal transduction pathways. Reversible protein phosphorylation is involved in the widespread regulation of cellular signal transduction and apoptotic processes. Therefore, phosphatase/kinase inhibitors are commonly used as apoptosis inducers/inhibitors. Whether protein phosphorylation induces apoptosis depends on many factors, such as the type of phosphorylated protein, the degree of activation and the influence of other proteins. Phosphorylation signaling pathways are intricately interrelated; it was previously shown that either induction or inhibition of phosphorylation causes cell death. Determination of the relationship between protein and phosphorylation helps to reveal how apoptosis is regulated. Here we discuss DNA fragmentation and protein phosphorylation, focusing on caspase and serine/threonine protein phosphatase activation.

The phosphorylation state of MRLC is polyamine dependent in intestinal epithelial cells

Cell migration is important to the integrity of the gastrointestinal tract for the normal movement of cells from crypt to villi and the healing of wounds. Polyamines are essential to cell migration, mucosal restitution, and, hence, healing. Polyamine depletion by α-difluoromethyl ornithine (DFMO) inhibited migration by decreasing lamellipodia and stress fiber formation and preventing the activation of Rho-GTPases. Polyamine depletion increased the association of the thick F-actin cortex with phosphorylated myosin regulatory light chain (pMRLC). In this study, we determined why MRLC is constitutively phosphorylated as part of the actin cortex. Inhibition of myosin light chain kinase (MLCK) decreased RhoA and Rac1 activities and significantly inhibited migration. Polyamine depletion increased phosphorylation of MRLC (Thr18/Ser19) and stabilized the actin cortex and focal adhesions. The Rho-kinase inhibitor Y27632 increased spreading and migration by decreasing the phosphorylation of MRLC, remodeling focal adhesions, and by activating Rho-GTPases. Thus phosphorylation of MRLC appears to be the rate-limiting step during the migration of IEC-6 cells. In addition, increased localization of RhoA with the actin cortex in polyamine-depleted cells appears to activate Rho-kinase. In the absence of polyamines, activated Rho-kinase phosphorylates myosin phosphatase targeting subunit 1 (MYPT1) at serine-668 leading to its inactivation and preventing the recruitment of phosphatase (protein phosphastase, PP1cδ) to the actomyosin cortex. In this condition, MRLC is constitutively phosphorylated and cycling does not occur. Thus activated myosin binds F-actin stress fibers and prevents focal adhesion turnover, Rho-GTPase activation, and the remodeling of the cytoskeleton required for migration.

Okadaic acid induces DNA fragmentation via caspase-3-dependent and caspase-3-independent pathways in Chinese hamster ovary (CHO)-K1 cells

DNA fragmentation is a hallmark of apoptosis that occurs in a variety of cell types; however, it remains unclear whether caspase-3 is required for its induction. To investigate this, we produced caspase-3 knockout Chinese hamster ovary (CHO)-K1 cells and examined the effects of gene knockout and treatment with caspase-3 inhibitors. Okadaic acid (OA) is a potent inhibitor of the serine/threonine protein phosphatases (PPs) PP1 and PP2A, which induce apoptotic cellular reactions. Treatment of caspase-3(-/-) cells with OA induced DNA fragmentation, indicating that caspase-3 is not an essential requirement. However, in the presence of benzyloxycarbonyl-Asp-Glu-Val-Asp (OMe) fluoromethylketone (z-DEVD-fmk), DNA fragmentation occurred in CHO-K1 cells but not in caspase-3(-/-) cells, suggesting that caspase-3 is involved in OA-induced DNA fragmentation that does not utilize DEVDase activity. In the absence of caspase-3, DEVDase activity may play an important role. In addition, OA-induced DNA fragmentation was reduced but not blocked in CHO-K1 cells, suggesting that caspase-3 is involved in caspase-independent OA-induced DNA fragmentation. Furthermore, OA-induced cleavage of caspase-3 and DNA fragmentation were blocked by pretreatment with the wide-ranging serine protease inhibitor 4-(2-aminoethyl)-benzenesulfonyl fluoride hydrochloride. These results suggest that serine proteases regulate DNA fragmentation upstream of caspase-3.

AMP-activated protein kinase confers protection against TNF-{alpha}-induced cardiac cell death

AIMS

Although a substantial role for 5' adenosine monophosphate-activated protein kinase (AMPK) has been established in regulating cardiac metabolism, a less studied action of AMPK is its ability to prevent cardiac cell death. Using established AMPK activators like dexamethasone (DEX) or metformin (MET), the objective of the present study was to determine whether AMPK activation prevents tumour necrosis factor-alpha (TNF-alpha) induced apoptosis in adult rat ventricular cardiomyocytes.

METHODS AND RESULTS

Cardiomyocytes were incubated with DEX, MET, or TNF-alpha for varying durations (0-12 h). TNF-alpha-induced cell damage was evaluated by measuring caspase-3 activity and Hoechst staining. Protein and gene estimation techniques were employed to determine the mechanisms mediating the effects of AMPK activators on TNF-alpha-induced cardiomyocyte apoptosis. Incubation of myocytes with TNF-alpha for 8 h has increased caspase-3 activation and apoptotic cell death, an effect that was abrogated by DEX and MET. The beneficial effect of DEX and MET was associated with stimulation of AMPK, which led to a rapid and sustained increase in Bad phosphorylation. This event reduced the interaction between Bad and Bcl-xL, limiting cytochrome c release and caspase-3 activation. Addition of Compound C to inhibit AMPK reduced Bad phosphorylation and prevented the beneficial effects of AMPK against TNF-alpha-induced cytotoxicity.

CONCLUSION

Our data demonstrate that although DEX and MET are used as anti-inflammatory agents or insulin sensitizers, respectively, their common property to phosphorylate AMPK promotes cardiomyocyte cell survival through its regulation of Bad and the mitochondrial apoptotic mechanism.

Early signalling pathways in rice roots under vanadate stress

Vanadate is beneficial to plant growth at low concentration. However, plant exposure to high concentrations of vanadate has been shown to arrest cell growth and lead to cell death. We are interested in understanding the signalling pathways of rice roots in response to vanadate stress. In this study, we demonstrated that vanadate induced rice root cell death and suppressed root growth. In addition, we found that vanadate induced ROS accumulation, increased lipid peroxidation and elicited a remarkable increase of MAPKs and CDPKs activities in rice roots. In contrast, pre-treatment of rice roots with ROS scavenger (sodium benzoate), serine/threonine protein phosphatase inhibitor (endothall), and CDPK antagonist (W7), reduced the vanadate-induced MAPKs activation. Furthermore, the expression of a MAPK gene (OsMPK3) and four tyrosine phosphatase genes (OsDSP3, OsDSP5, OsDSP6, and OsDSP10) were regulated by vanadate in rice roots. Collectively, these results strongly suggest that ROS, protein phosphatase, and CDPK may function in the vanadate-triggered MAPK signalling pathway cause cell death and retarded growth in rice roots.

Peroxynitrite-induced p38 MAPK pro-apoptotic signaling in enterocytes

Enterocyte apoptosis in necrotizing enterocolitis is partly due to the elaboration of toxic intermediates of nitric oxide (NO), such as peroxynitrite (PN). Because p38 mitogen-activated protein kinase (MAPK) and serine-threonine kinase (AKT) are well-characterized pro- and anti-apoptotic mediators, respectively, we hypothesized that PN could induce enterocyte apoptosis via activation of p38 and deactivation of AKT. To test this hypothesis, the rat intestinal cell line, IEC-6, was treated with PN. PN caused phosphorylation of p38, its upstream activator, MKK3/6, and downstream effector, transcription factor ATF-2. PN-induced apoptosis was inhibited by the p38 inhibitor, SB202190, and by p38 siRNA. PN decreased AKT phosphorylation; this effect was abrogated by pre-treatment with SB202190 or p38 siRNA. PN exposure also increased the activity of the protein phosphatase 2A (PP2A). These data demonstrate that PN-mediated apoptosis depends on the p38 pathway and that p38 mediates deactivation of AKT survival pathways possibly by the involvement of PP2A.

Epithelial cell expression of BCL-2 family proteins predicts mechanisms that regulate Helicobacter pylori-induced pathology in the mouse stomach

Corpus-predominant infection with Helicobacter pylori (HP) results in the activation of programmed cell death pathways in surface, parietal, and chief cells. At present, mechanisms that regulate these pathways to result in HP-associated pathology are not fully understood. Because it is not known which survival and death pathways are present in gastric epithelial cells, we used an antibody panel to evaluate the expression of BCL-2 family prosurvival proteins or multi-Bcl-2 homology (BH)-domains (group 1) or BH3-only (group-2) proapoptotic proteins in the stomachs of uninfected or HP-infected C57BL/6 mice. This strategy identified BCL-2, BAK, and BAD as the major prosurvival and proapoptotic proteins, in surface cells and BAD as the only BCL-2 family protein expressed in parietal cells. Chief cells express altogether different effectors, including BCL-X(L)/BCL-2, for survival but have no constitutively expressed proapoptotic proteins. In model chief cells, however, the group 1 proapoptotic protein BCL-X(S) was expressed after exposure to proinflammatory cytokines concomitant with reduced viability, demonstrating that chief cells can transcriptionally regulate the induction of proapoptotic proteins to execute apoptosis. During HP infection, no additional BCL-2 family proteins were expressed in epithelial cells, whereas those present either remained unchanged or were reduced as cell deletion occurred over time. Additional studies demonstrated that the posttranslational regulation of BAD in surface and parietal cells was negatively affected by HP infection, a result that may be directly related to an increase in apoptosis during infection. Thus, gastric epithelial cells express cell-specific prosurvival and proapoptotic pathways. From the results presented here, mechanisms that regulate HP-related changes in the survival and death profile of gastric epithelial cells can be predicted and then tested, with the ultimate goal of elucidating important therapeutic targets to inhibit the progression of HP-related pathology in the stomach.

The spinocerebellar ataxia 12 gene product and protein phosphatase 2A regulatory subunit Bbeta2 antagonizes neuronal survival by promoting mitochondrial fission

The neurodegenerative disorder spinocerebellar ataxia 12 (SCA12) is caused by CAG repeat expansion in the non-coding region of the PPP2R2B gene. PPP2R2B encodes Bbeta1 and Bbeta2, alternatively spliced and neuron-specific regulatory subunits of the protein phosphatase 2A (PP2A) holoenzyme. We show here that in PC12 cells and hippocampal neurons, cell stressors induced a rapid translocation of PP2A/Bbeta2 to mitochondria to promote apoptosis. Conversely, silencing of PP2A/Bbeta2 protected hippocampal neurons against free radical-mediated, excitotoxic, and ischemic insults. Evidence is accumulating that the mitochondrial fission/fusion equilibrium is an important determinant of cell survival. Accordingly, we found that Bbeta2 expression induces mitochondrial fragmentation, whereas Bbeta2 silencing or inhibition resulted in mitochondrial elongation. Based on epistasis experiments involving Bcl2 and core components of the mitochondrial fission machinery (Fis1 and dynamin-related protein 1), mitochondrial fragmentation occurs upstream of apoptosis and is both necessary and sufficient for hippocampal neuron death. Our data provide the first example of a proapoptotic phosphatase that predisposes to neuronal death by promoting mitochondrial division and point to a possible imbalance of the mitochondrial morphogenetic equilibrium in the pathogenesis of SCA12.

Cantharidin induces apoptosis of human multiple myeloma cells via inhibition of the JAK/STAT pathway

Multiple myeloma is an incurable B-cell malignancy requiring new therapeutic strategies in clinical settings. Interleukin (IL)-6 signaling pathways play a critical role in the pathogenesis of multiple myeloma. The traditional Chinese medicine cantharidin (CTD) has been shown to inhibit cellular proliferation and induce apoptosis of various cancer cells. The aim of this study was to investigate the possibility of CTD as a novel therapeutic agent for the patients with multiple myeloma. We investigated the in vitro effects of CTD for its antimyeloma activity, and further examined the molecular mechanisms of CTD-induced apoptosis. CTD inhibited the cellular growth of human myeloma cell lines as well as freshly isolated myeloma cells in patients. Cultivation with CTD induced apoptosis of myeloma cells in a cell-cycle-independent manner. Treatment with CTD induced caspase-3, -8, and -9 activities, and it was completely blocked by each caspase inhibitor. We further examined the effect of CTD on the IL-6 signaling pathway in myeloma cells, and found that CTD inhibited phosphorylation of STAT3 at tyrosine 705 residue as early as 1 h after treatment and down-regulated the expression of the antiapoptotic bcl-xL protein. STAT3 directly bound and activated the transcription of bcl-xL gene promoter, resulting in the induction of the expression of bcl-xL in myeloma cells. The essential role of STAT3 in CTD effects was confirmed by transfection with the constitutively active and dominant negative form of STAT3 in U266 cells. In conclusion, we have demonstrated that CTD is a promising candidate to be a new therapeutic agent in signal transduction therapy.

Raf protects against colitis by promoting mouse colon epithelial cell survival through NF-kappaB

BACKGROUND & AIMS

Raf-1 kinase is a key regulator of a number of cellular processes, which promote the maintenance of a healthy colon epithelium. This study addresses the role of Raf in epithelial cell survival in response to dextran sulfate sodium (DSS)-induced injury and inflammation.

METHODS

Inducible intestinal epithelium-specific Raf knockout mice were generated and subjected to acute colitis followed by a short recovery period. Colon sections were analyzed by in situ oligo ligation or immunostaining for Ki67, phospho-extracellular signal regulated kinase, and nuclear factor-kappaB p65. Western blot analysis and terminal deoxynucleotidyl transferase nick-end labeling assays were performed on Raf small interfering RNA-transfected young adult mouse colon cells following DSS treatment.

RESULTS

We report that Raf protects against epithelial injury and inflammation and promotes recovery from acute DSS-induced colitis by both MAPK/ERK kinase (MEK)-dependent and -independent pathways. Furthermore, we demonstrate that Raf induces novel cell survival responses through activating nuclear factor-kappaB in a MEK-independent manner.

CONCLUSIONS

These novel findings indicate a protective role for Raf in colon epithelium following ulcerative damage through inhibiting cell apoptosis and promoting proliferation with important implications for responses such as inflammation-associated carcinogenesis.

TNF-alpha/cycloheximide-induced apoptosis in intestinal epithelial cells requires Rac1-regulated reactive oxygen species

Previously we have shown that both Rac1 and c-Jun NH(2)-terminal kinase (JNK1/2) are key proapoptotic molecules in tumor necrosis factor (TNF)-alpha/cycloheximide (CHX)-induced apoptosis in intestinal epithelial cells, whereas the role of reactive oxygen species (ROS) in apoptosis is unclear. The present studies tested the hypothesis that Rac1-mediated ROS production is involved in TNF-alpha-induced apoptosis. In this study, we showed that TNF-alpha/CHX-induced ROS production and hydrogen peroxide (H(2)O(2))-induced oxidative stress increased apoptosis. Inhibition of Rac1 by a specific inhibitor NSC23766 prevented TNF-alpha-induced ROS production. The antioxidant, N-acetylcysteine (NAC), or rotenone (Rot), the mitochondrial electron transport chain inhibitor, attenuated mitochondrial ROS production and apoptosis. Rot also prevented JNK1/2 activation during apoptosis. Inhibition of Rac1 by expression of dominant negative Rac1 decreased TNF-alpha-induced mitochondrial ROS production. Moreover, TNF-alpha-induced cytosolic ROS production was inhibited by Rac1 inhibition, diphenyleneiodonium (DPI, an inhibitor of NADPH oxidase), and NAC. In addition, DPI inhibited TNF-alpha-induced apoptosis as judged by morphological changes, DNA fragmentation, and JNK1/2 activation. Mitochondrial membrane potential change is Rac1 or cytosolic ROS dependent. Lastly, all ROS inhibitors inhibited caspase-3 activity. Thus these results indicate that TNF-alpha-induced apoptosis requires Rac1-dependent ROS production in intestinal epithelial cells.

Induced TRPC1 expression increases protein phosphatase 2A sensitizing intestinal epithelial cells to apoptosis through inhibition of NF-kappaB activation

Transient receptor potential canonical-1 (TRPC1) functions as a store-operated Ca2+ channel in intestinal epithelial cells (IECs), and induced TRPC1 expression sensitizes IECs to apoptosis by inhibiting NF-kappaB activation. However, the exact mechanism by which increased TRPC1 results in NF-kappaB inactivation remains elusive. Protein phosphatase 2A (PP2A) is a widely conserved protein serine/threonine phosphatase that is implicated in the regulation of a wide array of cellular functions including apoptosis. The present study tests the hypothesis that induced TRPC1 expression inhibits NF-kappaB activation by increasing PP2A activity through Ca2+ influx in IECs. The expression of TRPC1 induced by stable transfection with the wild-type TRPC1 gene increased PP2A activity as indicated by increases in levels of PP2A proteins and their phosphatase activity. Increased levels of PP2A activity in stable TRPC1-transfected IEC-6 cells (IEC-TRPC1) were associated with decreased nuclear levels of NF-kappaB proteins and a reduction in NF-kappaB-dependent transcriptional activity, although there were no changes in total NF-kappaB protein levels. Inhibition of PP2A activity by treatment with okadaic acid or PP2A silencing with small interfering RNA not only enhanced NF-kappaB transactivation but also prevented the increased susceptibility of IEC-TRPC1 cells to apoptosis induced by treatment with tumor necrosis factor-alpha (TNF-alpha)/cycloheximide (CHX). Decreasing Ca2+ influx by exposure to the Ca2+-free medium reduced PP2A mRNA levels, destabilized PP2A proteins, and induced NF-kappaB activation, thus blocking the increased sensitivity of IEC-TRPC1 cells to TNF-alpha/CHX-induced apoptosis. These results indicate that induced TRPC1 expression increases PP2A activity through Ca2+ influx and that increased PP2A sensitizes IECs to apoptosis as a result of NF-kappaB inactivation.

Overexpression of the cis/trans isomerase PTPA triggers caspase 3-dependent apoptosis

PTPA, which possesses a peptidyl prolyl isomerase activity, was initially isolated as a protein that stimulates the weak phosphotyrosyl phosphatase activity of the Ser/Thr phosphatase PP2A. Here we show that transient overexpression of PTPA leads to cell death in a time-dependent manner in mammalian cells. PTPA-overproducing cells manifest hallmarks of apoptosis including chromatin condensation, membrane blebbing, positive staining with annexin V, dephosphorylation of Bad, and caspase-3 cleavage. Incubation of cells with the PP2A inhibitor okadaic acid does not prevent either dephosphorylation of Bad or PTPA-induced apoptosis, indicating that PTPA is unlikely to mediate its proapoptotic effect via PP2A. Moreover, we find no evidence for the involvement of either p53 or MAP kinases. Our data reveal a potential novel role for PTPA in the apoptotic process.

C-Jun N-terminal kinase regulates adenosine A1 receptor-mediated synaptic depression in the rat hippocampus

Adenosine A1 receptors are ubiquitous mediators of presynaptic inhibition of neurotransmission in the central nervous system, yet the signalling pathway linking A1 receptor activation and decreased neurotransmitter release remains poorly resolved. We tested the contribution of c-Jun N-terminal kinase (JNK) to adenosine A1 receptor-mediated depression of field excitatory postsynaptic potentials (fEPSPs) in area CA1 of the rat hippocampus. We found that inhibition of JNK with SP600125 or JNK inhibitor V, but not an inactive analogue, attenuated the depression of fEPSPs induced by adenosine, hypoxia, and the A1 receptor agonist N(6)-cyclopentyladenosine (CPA). In contrast, the JNK inhibitor SP600125 did not inhibit GABA(B)-mediated synaptic depression. In support of our electrophysiological findings, Western blot analysis showed that A1 receptor stimulation resulted in a transient increase in JNK phosphorylation in the membrane fraction of hippocampal lysates. The total amount of JNK in the membrane fraction was unchanged by CPA treatment. The increase in phosphorylated JNK induced by A1 receptor stimulation was blocked by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), indicating that A1 receptors specifically activate JNK in the hippocampus. Together with functional data indicating that JNK inhibition decreased CPA-induced paired pulse facilitation, these results suggest that JNK activation is necessary for adenosine A1 receptor-mediated synaptic depression occurring at a presynaptic locus The adenosine A1 receptor-JNK signalling pathway may represent a novel mechanism underlying inhibition of neurotransmitter release in the CNS.

Differential expression of genes associated with cell proliferation and apoptosis induced by okadaic acid during the transformation process of BALB/c 3T3 cells

Okadaic acid (OA) is a tumor promoter in two-stage carcinogenesis experiments. Nevertheless, the effects of OA on cell transformation, cell proliferation and apoptosis vary widely, and the molecular events underlying these effects of OA are not well understood. In the present study, we examined the promoting activity and the associated effects on cell growth and apoptosis mediated by OA in BALB/c 3T3 cells, and evaluated alterations of gene transcriptional expression by microarray analysis. The promoting activity of OA was estimated by a two-stage transformation assay, in which cells were treated first with a low dose of the initiator N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and then with OA for 14 days. It showed that OA, at concentrations of 7.8-31.3 ng/ml, enhanced the transformation of MNNG-treated cells. In the promotion phase, cells exposed to OA (7.8 ng/ml) grew slowly for the first 2 days and subsequently died. As determined by Hoechst 33342 fluorescent dye and Annexin-V/PI dual-colored flow cytometry, OA induced morphologically apoptotic cells and increased the percentage of early apoptotic cells. The gene expression profile induced by OA at five time points in the promotion phase was determined by use of a specific mouse toxicological microarray containing 1796 clones, and a total of 177 differentially expressed genes were identified. By gene ontology analysis, 31 of these were determined to be functionally involved with cell growth and/or maintenance. In this group, numerous genes associated with the cell proliferation and cell cycle progression were down-regulated at early and/or middle time points. Among these was a subset of genes associated with apoptosis, in which Bnip3, Cycs, Casp3 and Bag1 genes are involved in the mitochondrial pathway of apoptosis. Ier3, Mdm2 and Bnip3 genes may be p53 targets. Furthermore, real-time PCR confirmed the expression changes of five genes selected at random from the differentially expressed genes. We conclude that OA induces cell growth inhibition and apoptosis in the two-stage, MNNG-initiated transformation of BALB/c 3T3 cells. The results of gene expression profile analysis imply that multiple molecular pathways are involved in OA-induced proliferation inhibition and apoptosis. Mitochondrial and p53-associated apoptotic pathways also may contribute to OA-induced apoptosis.

Leucine carboxyl methyltransferase-1 is necessary for normal progression through mitosis in mammalian cells

Protein phosphatase 2A (PP2A) is a multifunctional phosphatase that plays important roles in many cellular processes including regulation of cell cycle and apoptosis. Because PP2A is involved in so many diverse processes, it is highly regulated by both non-covalent and covalent mechanisms that are still being defined. In this study we have investigated the importance of leucine carboxyl methyltransferase-1 (LCMT-1) for PP2A methylation and cell function. We show that reduction of LCMT-1 protein levels by small hairpin RNAs causes up to a 70% reduction in PP2A methylation in HeLa cells, indicating that LCMT-1 is the major mammalian PP2A methyltransferase. In addition, LCMT-1 knockdown reduced the formation of PP2A heterotrimers containing the Balpha regulatory subunit and, in a subset of the cells, induced apoptosis, characterized by caspase activation, nuclear condensation/fragmentation, and membrane blebbing. Knockdown of the PP2A Balpha regulatory subunit induced a similar amount of apoptosis, suggesting that LCMT-1 induces apoptosis in part by disrupting the formation of PP2A(BalphaAC) heterotrimers. Treatment with a pan-caspase inhibitor partially rescued cells from apoptosis induced by LCMT-1 or Balpha knockdown. LCMT-1 knockdown cells and Balpha knockdown cells were more sensitive to the spindle-targeting drug nocodazole, suggesting that LCMT-1 and Balpha are important for spindle checkpoint. Treatment of LCMT-1 and Balpha knockdown cells with thymidine dramatically reduced cell death, presumably by blocking progression through mitosis. Consistent with these results, homozygous gene trap knock-out of LCMT-1 in mice resulted in embryonic lethality. Collectively, our results indicate that LCMT-1 is important for normal progression through mitosis and cell survival and is essential for embryonic development in mice.

EGFR plays a pivotal role in the regulation of polyamine-dependent apoptosis in intestinal epithelial cells

Intracellular polyamine synthesis is regulated by the enzyme ornithine decarboxylase (ODC), and its inhibition by alpha-difluromethylornithine (DFMO), confers resistance to apoptosis. We have previously shown that DFMO leads to the inhibition of de novo polyamine synthesis, which in turn rapidly activates Src, STAT3 and NF-kappaB via integrin beta3 in intestinal epithelial cells. One mechanism to explain these effects involves the activation of upstream growth factor receptors, such as the epidermal growth factor receptor (EGFR). We therefore hypothesized that EGFR phosphorylation regulates the early response to polyamine depletion. DFMO increased EGFR phosphorylation on tyrosine residues 1173 (pY1173) and 845 (pY845) within 5 min. Phosphorylation declined after 10 min and was prevented by the addition of exogenous putrescine to DFMO containing medium. Phosphorylation of EGFR was concomitant with the activation of ERK1/2. Pretreatment with either DFMO or EGF for 1 h protected cells from TNF-alpha/CHX-induced apoptosis. Exogenous addition of polyamines prevented the protective effect of DFMO. In addition, inhibition of integrin beta3 activity (with RGDS), Src activity (with PP2), or EGFR kinase activity (with AG1478), increased basal apoptosis and prevented protection conferred by either DFMO or EGF. Polyamine depletion failed to protect B82L fibroblasts lacking the EGFR (PRN) and PRN cells expressing either a kinase dead EGFR (K721A) or an EGFR (Y845F) mutant lacking the Src phosphorylation site. Conversely, expression of WT-EGFR (WT) restored the protective effect of polyamine depletion. Fibronectin activated the EGFR, Src, ERKs and protected cells from apoptosis. Taken together, our data indicate an essential role of EGFR kinase activity in MEK/ERK-mediated protection, which synergizes with integrin beta3 leading to Src-mediated protective responses in polyamine depleted cells.

An endogenous serine/threonine protein phosphatase inhibitor, G-substrate, reduces vulnerability in models of Parkinson's disease

Relative neuronal vulnerability is a universal yet poorly understood feature of neurodegenerative diseases. In Parkinson's disease, dopaminergic (DA) neurons in the substantia nigra (SN) (A9) are particularly vulnerable, whereas adjacent DA neurons within the ventral tegmental area (A10) are essentially spared. Our previous laser capture microdissection and microarray study (Chung et al., 2005) demonstrated that molecular differences between these DA neurons may underlie their differential vulnerability. Here we show that G-substrate, an endogenous inhibitor of Ser/Thr protein phosphatases, exhibits higher expression in A10 compared with A9 DA neurons in both rodent and human midbrain. Overexpression of G-substrate protected dopaminergic BE(2)-M17 cells against toxins, including 6-OHDA and MG-132 (carbobenzoxy-L-leucyl- L-leucyl-L-leucinal), whereas RNA interference (RNAi)-mediated knockdown of endogenous G-substrate increased their vulnerability to these toxins. G-substrate reduced 6-OHDA-mediated protein phosphatase 2A (PP2A) activation in vitro and increased phosphorylated levels of PP2A targets including Akt, glycogen synthase kinase 3beta, and extracellular signal-regulated kinase 2 but not p38. RNAi to Akt diminished the protective effect of G-substrate against 6-OHDA. In vivo, lentiviral delivery of G-substrate to the rat SN increased baseline levels of phosphorylated Akt and protected A9 DA neurons from 6-OHDA-induced toxicity. These results suggest that inherent differences in the levels of G-substrate contribute to the differential vulnerability of DA neurons and that enhancing G-substrate levels may be a neuroprotective strategy for the vulnerable A9 (SN) DA neurons in Parkinson's disease.

Transcriptional regulation of Bim by FoxO3A mediates hepatocyte lipoapoptosis

Hepatocyte lipoapoptosis, a critical feature of nonalcoholic steatohepatitis, can be replicated in vitro by incubating hepatocytes with saturated free fatty acids (FFA). These toxic FFA induce Bim expression, which is requisite for their cytotoxicity. Because the FoxO3a transcription factor has been implicated in Bim expression, our aim was to determine if FFA induce Bim by a FoxO3a-dependent mechanism. In Huh-7 cells, the saturated FFA, palmitic and stearic acid, increased Bim mRNA 16-fold. Treatment of cells with the saturated FFA induced FoxO3a dephosphorylation (activation) and nuclear translocation and stimulated a FoxO luciferase-based reporter assay; direct binding of FoxO3a to the Bim promoter was also confirmed by a chromatin immunoprecipitation assay. A small interfering RNA-targeted knockdown of FoxO3a abrogated FFA-mediated Bim induction and apoptosis. FoxO3a was activated by a phosphatase 2A-dependent mechanism, since okadaic acid- and small interfering RNA-targeted knockdown of this phosphatase blocked FoxO3a dephosphorylation, Bim expression, and apoptosis. Consistent with these data, phosphatase 2A activity was also stimulated 3-fold by saturated FFA. Immunoprecpitation studies revealed an FFA-dependent association between FoxO3a and protein phosphatase 2A. FFA-mediated FoxO3a activation by protein phosphatase 2A was also observed in HepG2 cells and murine hepatocytes. In conclusion, saturated FFA stimulate protein phosphatase 2A activity, which activates FoxO3a, inducing expression of the intracellular death mediator Bim.

Apoptosis signal-regulating kinase 1 in amyloid beta peptide-induced cerebral endothelial cell apoptosis

A pathological hallmark of Alzheimer's disease is accumulation of amyloid-beta peptide (Abeta) in senile plaques. Abeta has also been implicated in vascular degeneration in cerebral amyloid angiopathy because of its cytotoxic effects on non-neuronal cells, including cerebral endothelial cells (CECs). We explore the role of apoptosis signal-regulating kinase 1 (ASK1) in Abeta-induced death in primary cultures of murine CECs. Abeta induced ASK1 dephosphorylation, which could be prevented by selective inhibition of protein phosphatase 2A (PP2A) but not PP2B. ASK1 dephosphorylation resulted in its dissociation from 14-3-3. ASK1, released from 14-3-3 inhibition, activated p38 mitogen-activated protein kinase (p38MAPK), leading to p53 phosphorylation. p53, a proapoptotic transcription factor, in turn transactivated the expression of Bax, a proapoptotic protein. Transfection with various dominant-negative mutants (DNs), including ASK1 DN and p38MAPK DN, suppressed Abeta-induced p38MAPK activation, p53 phosphorylation, and Bax upregulation and partially prevented CEC death. Bax knockdown using a bax small interfering RNA strategy also reduced Bax expression and subsequent CEC death. These results suggest that Abeta activates the ASK1-p38MAPK-p53-Bax cascade to cause CEC death in a PP2A-dependent manner.

Polyamine biosynthesis as a target to inhibit apoptosis of non-tumoral cells

Growing evidence suggests a role for polyamines in apoptosis, although the relationship appears to be complex. alpha-Difluoromethylornithine (DFMO), a largely used ornithine decarboxylase inhibitor, is cytostatic, hardly cytotoxic and may even increase the resistance of tumour cells to some apoptotic stimuli. This may represent a problem in cancer therapy, where the killing of tumoral cells would be a desired effect, but could be an advantage in other pathological contexts related to an excess of apoptosis, such as cardiovascular diseases, stem cell transplantation, arthritis and infections. In different cellular models, polyamine depletion following treatment with polyamine biosynthesis inhibitors appears to inhibit mitochondrial and death receptor pathways of apoptosis by affecting key proteins. These studies indicate that inhibition of polyamine biosynthesis may prevent or reduce the apoptotic response triggered by a variety of stimuli in non-tumoral cells, such as cardiac cells, stem cells, chondrocytes, macrophages and intestinal epithelial cells.

Galpha12 stimulates apoptosis in epithelial cells through JNK1-mediated Bcl-2 degradation and up-regulation of IkappaBalpha

Apoptosis is an essential mechanism for the maintenance of somatic tissues, and when dysregulated can lead to numerous pathological conditions. G proteins regulate apoptosis in addition to other cellular functions, but the roles of specific G proteins in apoptosis signaling are not well characterized. Galpha12 stimulates protein phosphatase 2A (PP2A), a serine/threonine phosphatase that modulates essential signaling pathways, including apoptosis. Herein, we examined whether Galpha12 regulates apoptosis in epithelial cells. Inducible expression of Galpha12 or constitutively active (QL)alpha12 in Madin-Darby canine kidney cells led to increased apoptosis with expression of QLalpha12, but not Galpha12. Inducing QLalpha12 led to degradation of the anti-apoptotic protein Bcl-2 (via the proteasome pathway), increased JNK activity, and up-regulated IkappaBalpha protein levels, a potent stimulator of apoptosis. Furthermore, the QLalpha12-stimulated activation of JNK was blocked by inhibiting PP2A. To characterize endogenous Galpha12 signaling pathways, non-transfected MDCK-II and HEK293 cells were stimulated with thrombin. Thrombin activated endogenous Galpha12 (confirmed by GST-tetratricopeptide repeat (TPR) pull-downs) and stimulated apoptosis in both cell types. The mechanisms of thrombin-stimulated apoptosis through endogenous Galpha12 were nearly identical to the mechanisms identified in QLalpha12-MDCK cells and included loss of Bcl-2, JNK activation, and up-regulation of IkappaBalpha. Knockdown of the PP2A catalytic subunit in HEK293 cells inhibited thrombin-stimulated apoptosis, prevented JNK activation, and blocked Bcl-2 degradation. In summary, Galpha12 has a major role in regulating epithelial cell apoptosis through PP2A and JNK activation leading to loss of Bcl-2 protein expression. Targeting these pathways in vivo may lead to new therapeutic strategies for a variety of disease processes.

Molecular basis for PP2A regulatory subunit B56alpha targeting in cardiomyocytes

Protein phosphatase 2A (PP2A) is a multifunctional protein phosphatase with critical roles in excitable cell signaling. In the heart, PP2A function is linked with modulation of beta-adrenergic signaling and has been suggested to regulate key ion channels and transporters including Na/Ca exchanger, ryanodine receptor, inositol 1,4,5-trisphosphate receptor, and Na/K ATPase. Although many of the functional roles and molecular targets for PP2A in heart are known, little is established regarding the cellular pathways that localize specific PP2A isoform activities to subcellular sites. We report that the PP2A regulatory subunit B56alpha is an in vivo binding partner for ankyrin-B, an adapter protein required for normal subcellular localization of the Na/Ca exchanger, Na/K ATPase, and inositol 1,4,5-trisphosphate receptor. Ankyrin-B and B56alpha are colocalized and coimmunoprecipitate in primary cardiomyocytes. Using multiple strategies, we identified the structural requirements on B56alpha for ankyrin-B association as a 13 residue motif in the B56alpha COOH terminus not present in other B56 family polypeptides. Finally, we report that reduced ankyrin-B expression in primary ankyrin-B(+/-) cardiomyocytes results in disorganized distribution of B56alpha that can be rescued by exogenous expression of ankyrin-B. These new data implicate ankyrin-B as a critical targeting component for PP2A in heart and identify a new class of signaling proteins targeted by ankyrin polypeptides.