Inhibitors of protein phosphatases 1 and 2A differentially prevent intrinsic and extrinsic apoptosis pathways

Identification of intracellular cavin target proteins reveals cavin-PP1alpha interactions regulate apoptosis

Caveolae are specialized domains of the plasma membrane. Formation of these invaginations is dependent on the expression of Caveolin-1 or -3 and proteins of the cavin family. In response to stress, caveolae disassemble and cavins are released from caveolae, allowing cavins to potentially interact with intracellular targets. Here, we describe the intracellular (non-plasma membrane) cavin interactome using biotin affinity proteomics and mass spectrometry. We validate 47 potential cavin-interactor proteins using a cell-free expression system and protein-protein binding assays. These data, together with pathway analyses, reveal unknown roles for cavin proteins in metabolism and stress signaling. We validated the interaction between one candidate interactor protein, protein phosphatase 1 alpha (PP1α), and Cavin-1 and -3 and show that UV treatment causes release of Cavin3 from caveolae allowing interaction with, and inhibition of, PP1α. This interaction increases H2AX phosphorylation to stimulate apoptosis, identifying a pro-apoptotic signaling pathway from surface caveolae to the nucleus.

The impact of phosphatases on proliferative and survival signaling in cancer

The dynamic and stringent coordination of kinase and phosphatase activity controls a myriad of physiologic processes. Aberrations that disrupt the balance of this interplay represent the basis of numerous diseases. For a variety of reasons, early work in this area portrayed kinases as the dominant actors in these signaling events with phosphatases playing a secondary role. In oncology, these efforts led to breakthroughs that have dramatically altered the course of certain diseases and directed vast resources toward the development of additional kinase-targeted therapies. Yet, more recent scientific efforts have demonstrated a prominent and sometimes driving role for phosphatases across numerous malignancies. This maturation of the phosphatase field has brought with it the promise of further therapeutic advances in the field of oncology. In this review, we discuss the role of phosphatases in the regulation of cellular proliferation and survival signaling using the examples of the MAPK and PI3K/AKT pathways, c-Myc and the apoptosis machinery. Emphasis is placed on instances where these signaling networks are perturbed by dysregulation of specific phosphatases to favor growth and persistence of human cancer.

Carfilzomib induces leukaemia cell apoptosis via inhibiting ELK1/KIAA1524 (Elk-1/CIP2A) and activating PP2A not related to proteasome inhibition

Enhancing the tumour suppressive activity of protein phosphatase 2A (PP2A) has been suggested to be an anti-leukaemic strategy. KIAA1524 (also termed CIP2A), an oncoprotein inhibiting PP2A, is associated with disease progression in chronic myeloid leukaemia and may be prognostic in cytogenetically normal acute myeloid leukaemia. Here we demonstrated that the selective proteasome inhibitor, carfilzomib, induced apoptosis in sensitive primary leukaemia cells and in sensitive leukaemia cell lines, associated with KIAA1524 protein downregulation, increased PP2A activity and decreased p-Akt, but not with the proteasome inhibition effect of carfilzomib. Ectopic expression of KIAA1524, or pretreatment with the PP2A inhibitor, okadaic acid, suppressed carfilzomib-induced apoptosis and KIAA1524 downregulation in sensitive cells, whereas co-treatment with the PP2A agonist, forskolin, enhanced carfilzomib-induced apoptosis in resistant cells. Mechanistically, carfilzomib affected KIAA1524 transcription through disturbing ELK1 (Elk-1) binding to the KIAA1524 promoter. Moreover, the drug sensitivity and mechanism of carfilzomib in xenograft mouse models correlated well with the effects of carfilzomib on KIAA1524 and p-Akt expression, as well as PP2A activity. Our data disclosed a novel drug mechanism of carfilzomib in leukaemia cells and suggests the potential therapeutic implication of KIAA1524 in leukaemia treatment.

Antitumor effect of forbesione isolated from Garcinia hanburyi on cholangiocarcinoma in vitro and in vivo

Cholangiocarcinoma (CCA) is a malignancy with no effective therapy and poor prognosis. Forbesione, a caged xanthone isolated from Garcinia hanburyi, has been reported to inhibit proliferation and to induce apoptosis in human CCA cell lines. The present study aimed to further explore the potential anticancer properties of forbesione by testing its effects against the hamster CCA cell line Ham-1 in vitro and in vivo. It was observed that forbesione inhibited the growth of Ham-1 cells in vitro and suppressed Ham-1 growth as allograft in hamsters by inducing cell cycle arrest at the S phase. This was mediated by decreasing the protein expression of cyclin E, cyclin A and cyclin-dependent kinase 2. In addition, increased expression of p21 and p27 was detected, which could possibly explain the reduced expression of proliferating cell nuclear antigen and of the bile duct cell marker cytokeratin 19 observed in forbesione-treated Ham-1 cells in vitro and in tumor tissues of forbesione-treated hamsters. Furthermore, forbesione induced apoptosis through multiple pathways. The death receptor pathway was activated by increased expression of Fas, Fas-associated death domain and activated caspase-3, along with decreased expression of procaspase-8 and procaspase-3. The mitochondrial pathway was driven by increased expression of B-cell lymphoma (Bcl)-2-like protein 4, activated caspase-9 and inhibitor of κB-α, along with decreased expression of Bcl-2, survivin, procaspase-9 and nuclear factor-κB/p65. The endoplasmic reticulum pathway was stimulated by increased expression of activated caspase-12 and decreased expression of procaspase-12. No side effects or toxicity were observed in forbesione-treated hamsters. Thus, forbesione is a potential drug candidate for cancer therapy that deserves further investigation.

Follicle-Stimulating Hormone (FSH)-dependent Regulation of Extracellular Regulated Kinase (ERK) Phosphorylation by the Mitogen-activated Protein (MAP) Kinase Phosphatase MKP3

Within the ovarian follicle, granulosa cells (GCs) surround and support immature oocytes. FSH promotes the differentiation and proliferation of GCs and is essential for fertility. We recently reported that ERK activation is necessary for FSH to induce key genes that define the preovulatory GC. This research focused on the phosphoregulation by FSH of ERK within GCs. FSH-stimulated ERK phosphorylation on Thr(202)/Tyr(204) was PKA-dependent, but MEK(Ser(217)/Ser(221)) phosphorylation was not regulated; rather, MEK was already active. However, treatment of GCs with the EGF receptor inhibitor AG1478, a dominant-negative RAS, an Src homology 2 domain-containing Tyr phosphatase inhibitor (NSC 87877), or the MEK inhibitor PD98059 blocked FSH-dependent ERK(Thr(202)/Tyr(204)) phosphorylation, demonstrating the requirement for upstream pathway components. We hypothesized that FSH via PKA enhances ERK phosphorylation by inhibiting the activity of a protein phosphatase that constitutively dephosphorylates ERK in the absence of FSH, allowing MEK-phosphorylated ERK to accumulate in the presence of FSH because of inactivation of the phosphatase. GCs treated with different phosphatase inhibitors permitted elimination of both Ser/Thr and Tyr phosphatases and implicated dual specificity phosphatases (DUSPs) in the dephosphorylation of ERK. Treatment with MAP kinase phosphatase (MKP3, DUSP6) inhibitors increased ERK(Thr(202)/Tyr(204)) phosphorylation in the absence of FSH to levels comparable with ERK phosphorylated in the presence of FSH. ERK co-immunoprecipitated with Myc-FLAG-tagged MKP3(DUSP6). GCs treated with MKP3(DUSP6) inhibitors blocked and PKA inhibitors enhanced dephosphorylation of recombinant ERK2-GST in an in vitro phosphatase assay. Together, these results suggest that FSH-stimulated ERK activation in GCs requires the PKA-dependent inactivation of MKP3(DUSP6).

Extracellular Signal-regulated Kinase (ERK)-dependent Phosphorylation of Y-Box-binding Protein 1 (YB-1) Enhances Gene Expression in Granulosa Cells in Response to Follicle-stimulating Hormone (FSH)

Within the ovarian follicle, immature oocytes are surrounded and supported by granulosa cells (GCs). Stimulation of GCs by FSH leads to their proliferation and differentiation, events that are necessary for fertility. FSH activates multiple signaling pathways to regulate genes necessary for follicular maturation. Herein, we investigated the role of Y-box-binding protein-1 (YB-1) within GCs. YB-1 is a nucleic acid binding protein that regulates transcription and translation. Our results show that FSH promotes an increase in the phosphorylation of YB-1 on Ser(102) within 15 min that is maintained at significantly increased levels until ∼8 h post treatment. FSH-stimulated phosphorylation of YB-1(Ser(102)) is prevented by pretreatment of GCs with the PKA-selective inhibitor PKA inhibitor (PKI), the MEK inhibitor PD98059, or the ribosomal S6 kinase-2 (RSK-2) inhibitor BI-D1870. Thus, phosphorylation of YB-1 on Ser(102) is PKA-, ERK-, and RSK-2-dependent. However, pretreatment of GCs with the protein phosphatase 1 (PP1) inhibitor tautomycin increased phosphorylation of YB-1(Ser(102)) in the absence of FSH; FSH did not further increase YB-1(Ser(102)) phosphorylation. This result suggests that the major effect of RSK-2 is to inhibit PP1 rather than to directly phosphorylate YB-1 on Ser(102) YB-1 coimmunoprecipitated with PP1β catalytic subunit and RSK-2. Transduction of GCs with the dephospho-adenoviral-YB-1(S102A) mutant prevented the induction by FSH of Egfr, Cyp19a1, Inha, Lhcgr, Cyp11a1, Hsd17b1, and Pappa mRNAs and estradiol-17β production. Collectively, our results reveal that phosphorylation of YB-1 on Ser(102) via the ERK/RSK-2 signaling pathway is necessary for FSH-mediated expression of target genes required for maturation of follicles to a preovulatory phenotype.

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.

The natural tumorcide Manumycin-A targets protein phosphatase 1α and reduces hydrogen peroxide to induce lymphoma apoptosis

Numerous compounds for treating human disease have been discovered in nature. Manumycin-A (Man-A) is a natural, well-tolerated microbial metabolite and a potent experimental tumoricide. We recently showed that Man-A stimulated reactive oxygen species (ROS) which were upstream of serine/threonine (Ser/Thr) dephosphorylation and caspase-dependent cleavage of MEK and Akt in lymphoma apoptosis. Conversely, activation-specific, Ser/Thr phosphorylation of MEK and Akt proteins was stable in Man-A-resistant tumors suggesting that stimulation of Ser/Thr PPase activity might be required for Man-A tumoricidal activity. Pre-treatment with Calyculin-A, an equipotent inhibitor of PP1 and PP2A, blocked all downstream effects of Man-A whereas, the PP2A-selective inhibitor, Okadaic acid did not, suggesting that PP1 and not PP2A played a role in Man-A action. Phosphorylation of PP1α on Thr320 inhibits its activity. Hence, we posited that if PP1α was important for Man-A action, then Man-A treatment should promote dephosphorylation of PP1α on Thr320. Indeed, T320 was only dephosphorylated in the tumors that underwent apoptosis. Lastly, stable over-expression of a constitutively active PP1α mimetic (PP1αT320A mutant), elevated basal ROS levels and enhanced Man-A-stimulated apoptosis. Taken together, we conclude that PP1α is an important proximal effector of Man-A mediated lymphoma apoptosis and that the mechanisms of Man-A action warrant further investigation.

Inhibition of protein phosphatase-1 and -2A decreases the chemosensitivity of leukemic cells to chemotherapeutic drugs

The phosphorylation of key proteins balanced by protein kinases and phosphatases are implicated in the regulation of cell cycle and apoptosis of malignant cells and influences anticancer drug actions. The efficacy of daunorubicin (DNR) in suppression of leukemic cell survival was investigated in the presence of tautomycin (TM) and calyculin A (CLA), specific membrane permeable inhibitors of protein phosphatase-1 (PP1) and -2A (PP2A), respectively. CLA (50 nM) or TM (1μM) suppressed viability of THP-1 and KG-1 myeloid leukemia cell lines to moderate extents; however, they significantly increased survival upon DNR-induced cell death. CLA increased the phosphorylation level of Erk1/2 and PKB/Akt kinases, the retinoblastoma protein (pRb), decreased caspase-3 activation by DNR and increased the phosphorylation level of the inhibitory sites (Thr696 and Thr853) in the myosin phosphatase (MP) target subunit (MYPT1) as well as in a 25kDa kinase-enhanced phosphatase inhibitor (KEPI)-like protein. TM induced enhanced phosphorylation of pRb only, suggesting that this event may be a common factor upon CLA-induced PP2A and TM-induced PP1 inhibitory influences on cell survival. Silencing PP1 by siRNA in HeLa cells, or overexpression of Flag-KEPI in MCF-7 cells coupled with inducing its phosphorylation by PMA or CLA, resulted in increased phosphorylation of pRb. Our results indicate that PP1 directly dephosphorylates pRb, while PP2A might have an indirect influence via mediating the phosphorylation level of PP1 inhibitory proteins. These data imply the importance of PP1 inhibitory proteins in controlling the phosphorylation state of key proteins and regulating drug sensitivity and apoptosis in leukemic cells.

Inhibition of protein phosphatase 2A (PP2A) prevents Mcl-1 protein dephosphorylation at the Thr-163/Ser-159 phosphodegron, dramatically reducing expression in Mcl-1-amplified lymphoma cells

Abundant, sustained expression of prosurvival Mcl-1 is an important determinant of viability and drug resistance in cancer cells. The Mcl-1 protein contains PEST sequences (enriched in proline, glutamic acid, serine, and threonine) and is normally subject to rapid turnover via multiple different pathways. One of these pathways involves a phosphodegron in the PEST region, where Thr-163 phosphorylation primes for Ser-159 phosphorylation by glycogen synthase kinase-3. Turnover via this phosphodegron-targeted pathway is reduced in Mcl-1-overexpressing BL41-3 Burkitt lymphoma and other cancer cells; turnover is further slowed in the presence of phorbol ester-induced ERK activation, resulting in Mcl-1 stabilization and an exacerbation of chemoresistance. The present studies focused on Mcl-1 dephosphorylation, which was also found to profoundly influence turnover. Exposure of BL41-3 cells to an inhibitor of protein phosphatase 2A (PP2A), okadaic acid, resulted in a rapid increase in phosphorylation at Thr-163 and Ser-159, along with a precipitous decrease in Mcl-1 expression. The decline in Mcl-1 expression preceded the appearance of cell death markers and was not slowed in the presence of phorbol ester. Upon exposure to calyculin A, which also potently inhibits PP2A, versus tautomycin, which does not, only the former increased Thr-163/Ser-159 phosphorylation and decreased Mcl-1 expression. Mcl-1 co-immunoprecipitated with PP2A upon transfection into CHO cells, and PP2A/Aα knockdown recapitulated the increase in Mcl-1 phosphorylation and decrease in expression. In sum, inhibition of PP2A prevents Mcl-1 dephosphorylation and results in rapid loss of this prosurvival protein in chemoresistant cancer cells.

A novel colon cancer gene therapy using rAAV‑mediated expression of human shRNA-FHL2

FHL2 (Four and a half LIM-only protein 2) has been identified as an oncogene in colon cancer and suppression of FHL2 induces cell differentiation and tumorigenesis in colon cancer cell lines. The aim of this study was to develop a novel and effective approach to knockdown FHL2, which can serve as a promising target of colon cancer therapy. Recombinant adeno-associated virus (rAAV) was generated bearing with FHL2-shRNA and transfected into LoVo cells. Cell cycle and growth were assessed. The interaction between FHL2 and G0/G1 cell cycle and growth was evaluated by flow cytometry, western blot analysis and WST-1 assay. We showed that suppression of FHL2 by rAAV-shRNA induced G0/G1 cell cycle arrest and inhibited cell growth. Apoptosis-related proteins and their activity was investigated at the same time. rAAV-FHL2‑shRNA activated intrinsic and extrinsic apoptotic pathways and increased cell susceptibility to apoptotic stimuli by 5-FU. Moreover, a xenograft model was established to explore rAAV-FHL2-shRNA with 5-FU mediated tumorigenesis in vivo. A strong anti-tumorigenic effect of rAAV-FHL2-shRNA was shown in nude mice and this antitumor effect was enhanced when combined with 5-FU treatment. These findings implicate FHL2 as a cell cycle and growth modulator and thus inhibit apoptosis in colon cancer cells. rAAV-shRNA-FHL2 may serve as a novel and potent therapeutic or 5-FU co-therapeutic agent for colon cancer.

Cytotoxic effects and changes in cytokine gene expression induced by microcystin-containing extract in fish immune cells--an in vitro and in vivo study

Blooms of cyanobacteria producing very toxic secondary metabolites (especially microcystins) are potent environmental stressors, hazardous not only to aquatic animals but also to public health. The purpose of this study was to investigate the effects of an extract containing microcystins on immune cells isolated from the common carp (Cyprinus carpio L.). In the present study it has been found that the extract induced apoptosis and inhibited in vitro lymphocyte proliferation. In addition, the results indicated the possible role of oxidative stress in this cytotoxicity and apoptosis. The in vivo investigations showed that the extract containing microcystins had greater suppressive effects on the essential functions of immune cells (intracellular reactive oxygen species production and lymphocyte proliferation) than the pure toxin alone. Moreover, immersion of fish in the toxic extract caused changes in the mRNA levels of various pro- and anti-inflammatory cytokines in carp leukocytes, while after exposure to the pure toxin, only IL1-β expression was markedly up-regulated. The observed modulatory effects on immune cells could have important implications for the health of planktivorous fish, which feed more frequently on toxic cyanobacteria.

Pin1-FADD interactions regulate Fas-mediated apoptosis in activated eosinophils

Abnormally long-lived eosinophils (Eos) are the major inflammatory component of allergic responses in the lungs of active asthmatics. Eos recruited to the airways after allergen exposure produce and respond to IL-5 and GM-CSF, enhancing their survival. Prosurvival signaling activates Pin1, a peptidyl-prolyl cis-trans isomerase that binds to Bax and prevents its activation. How long-lived Eos, despite the continued presence of GM-CSF or IL-5, eventually undergo apoptosis to end allergic inflammation remains unclear. In this study, we show that Pin1 location, activity, and protein interactions are jointly influenced by Fas and the prosurvival cytokine IL-5. Fas signaling strongly induced the phosphorylation of FADD at Ser(194) and Pin1 at Ser(16), as well as their nuclear accumulation. Phospho-mimic Ser(194)Glu FADD mutants accelerated Eos apoptosis compared with wild-type or Ser(194)Ala mutants. Downstream of FADD phosphorylation, caspase 8, 9, and 3 cleavage, as well as Eos apoptosis induced by Fas, were reduced by constitutively active Pin1 and enhanced by Pin1 inhibition. Pin1 was activated by IL-5, whereas simultaneous IL-5 and anti-Fas treatment modestly reduced peptidyl isomerase activity but induced Pin1 to associate with FADD after its phosphorylation at Ser(194). Mechanistically, Pin1-mediated isomerization facilitated the subsequent dephosphorylation of Ser(194) FADD and maintenance of cytoplasmic location. In vivo-activated bronchoalveolar Eos obtained after allergen challenge showed elevated survival and Pin1 activity that could be reversed by anti-Fas. Therefore, our data suggest that Pin1 is a critical link between FADD-mediated cell death and IL-5-mediated prosurvival signaling.

Protein phosphatase-dependent circadian regulation of intermediate-term associative memory

The endogenous circadian clock is a principal factor modulating memory across species. Determining the processes through which the circadian clock modulates memory formation is a key issue in understanding and identifying mechanisms to improve memory. We used the marine mollusk Aplysia californica to investigate circadian modulation of intermediate-term memory (ITM) and the mechanisms through which the circadian clock phase specifically suppresses memory using the operant learning paradigm, learning that food is inedible. We found that ITM, a temporally and mechanistically distinct form of memory, is rhythmically expressed under light-dark and constant conditions when induced by either massed or spaced training. Strong circadian regulation of ITM occurs with memory exhibited only by animals trained during the early subjective day; no apparent memory is expressed when training occurs during the late subjective day or night. Given the necessity of multiple persistent kinase cascades for ITM, we investigated whether protein phosphatase activity affected circadian modulation. Inhibition of protein phosphatases 1 and 2A blocked ITM when animals were trained during the early (subjective) day while resulting in phase-specific memory rescue when animals were trained late in the subjective day and early night. In contrast, inhibition of calcineurin did not block ITM when animals were trained during the early day and permitted ITM when animals were trained during the late subjective day, early evening, and throughout the night. These results demonstrate that levels of protein phosphatase activity are critical regulators of ITM and one mechanism through which the circadian clock regulates memory formation.

Apaf-1- and Caspase-8-independent apoptosis

Two major apoptosis pathways, the mitochondrial and death receptor pathways, are well recognized. Here we established cell lines from the fetal thymus of Apaf-1-, Caspase-9-, or Bax/Bak-deficient mice. These cell lines were resistant to apoptosis induced by DNA-damaging agents, RNA or protein synthesis inhibitors, or stress in the endoplasmic reticulum. However, they underwent efficient apoptosis when treated with kinase inhibitors such as staurosporine and H-89, indicating that these inhibitors induce a caspase-dependent apoptosis that is different from the mitochondrial pathway. CrmA, a Caspase-8 inhibitor, did not prevent staurosporine-induced apoptosis of fetal thymic cell lines, suggesting that the death receptor pathway was also not involved in this process. The staurosporine-induced cell death was inhibited by okadaic acid, a serine/threonine phosphatase inhibitor, suggesting that dephosphorylation of a proapoptotic molecule triggered the death process, or that phosphorylation of an antiapoptotic molecule could block the process. Cells of various types (fetal thymocytes, bone marrows, thymocytes, and splenocytes), but not embryonic fibroblasts, were sensitive to the noncanonical staurosporine-induced apoptosis, suggesting that the noncanonical apoptosis pathway is tissue specific.

Reactivating PP2A by FTY720 as a novel therapy for AML with C-KIT tyrosine kinase domain mutation

The tyrosine kinase domain (TKD) mutations of receptor tyrosine kinase C-KIT are associated with a poor prognosis in acute myeloid leukemia (AML). However, the underlying mechanisms are not fully understood. We found the activity of protein phosphatase 2A (PP2A), a human tumor suppressor whose dysfunction contributes to malignant cell behavior, was significantly decreased in AML subgroups harboring C-KIT/D816V and AML cell line Kasumi-1 bearing C-KIT/N822K mutation. Primary AML cells and various AML cell lines were treated with PP2A activator FTY720. FTY720 showed a toxic effect in all leukemic cells, especially for cells harboring C-KIT/TKD mutation. Furthermore, FTY720-induced toxicity in AML leukemic cells was mediated by restoration of PP2A activity, via down-regulation of PP2A inhibitor SET, dephosporylation of PP2A-C(TYR307), and up-regulation of relevant PP2A subunit A and B55α. Our research indicates that the decreased PP2A activity in AML harboring C-KIT/TKD mutation may make the restoration of PP2A activity a novel therapy for AML patients with C-KIT/TKD mutation.

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.

A protein phosphatase 1 gamma (PP1γ) of the human protozoan parasite Trichomonas vaginalis is involved in proliferation and cell attachment to the host cell

In this work, evidence for a critical role of Trichomonas vaginalis protein phosphatase 1 gamma (TvPP1γ) in proliferation and attachment of the parasite to the mammalian cell is provided. Firstly, proliferation and attachment of T. vaginalis parasites to HeLa cells was blocked by calyculin A (CA), a potent PP1 inhibitor. Secondly, it was demonstrated that the enzyme activity of native and recombinant TvPP1γ proteins was inhibited by CA. Thirdly, reverse genetic studies confirmed that antisense oligonucleotides targeted to PP1γ but not PP1α or β inhibited proliferation and attachment of trichomonads CA-treated parasites underwent cytoskeletal modifications, including a lack of axostyle typical labelling, suggesting that cytoskeletal phosphorylation could be regulated by a CA-sensitive phosphatase where the role of PP1γ could not be ruled out. Analysis of subcellular distribution of TvPP1γ by cell fractionation and electron microscopy demonstrated the association between TvPP1γ and the cytoskeleton. The expression of adhesins, AP120 and AP65, at the cell surface was also inhibited by CA. The concomitant inhibition of expression of adhesins and changes in the cytoskeleton in CA-treated parasites suggest a specific role for PP1γ -dependent dephosphorylation in the early stages of the host-parasite interaction. Molecular modelling of TvPP1γ showed the conservation of residues critical for maintaining proper folding into the gross structure common to PP1 proteins. Taken together, these results suggest that TvPP1γ could be considered a potential novel drug target for treatment of trichomoniasis.

Novel Ser/Thr protein phosphatases in cell death regulation

Cell death is regulated by a myriad of intracellular molecular pathways, with many involving protein phosphorylation and dephosphorylation. In this review, we will focus on Ser/Thr phosphatases-mediated regulation in cell apoptosis as well as on their potential roles in cell necrosis. The emerging functional importance of Ser/Thr protein phosphatases in cell death regulation adds new dimension to the signaling mechanisms of cellular function, physiology, and diseases.

Modulation of ROS/MAPK signaling pathways by okadaic acid leads to cell death via, mitochondrial mediated caspase-dependent mechanism

Okadaic acid (OA) is a specific and potent protein phosphatase inhibitor and tumor promoter. The present study establishes the role of reactive oxygen species (ROS) and mitogen activated protein kinases in cell death induced by okadaic acid. The study showed that okadaic acid is cytotoxic at 10 nM with an IC50 of 100 nM in U-937 cells. The CVDE assay and mitochondrial dehydrogenase assay showed a time dependent cytotoxicity. The phase contrast visualization of the OA treated cells showed the apoptotic morphology and was confirmed with esterase staining for plasma membrane integrity. OA activated caspases-7, 9 and 3, PARP cleavage and induced nuclear damage in a time and dose dependent manner. Compromised mitochondrial membrane potential, release of cytochrome-c and apoptosis inducing factor confirms the involvement of mitochondria. A time dependent decrease in glutathione levels and a dose dependent increase in ROS with maximum at 30 min were observed. ROS scavenger-N-acetyl cysteine, mitochondrial stabilizer-cyclosporin-A, and broad spectrum caspase inhibitor Z-VAD-FMK inhibited the OA induced caspase-3 activation, DNA damage and cell death but caspase-8 inhibitor had no effect. OA activated p38 MAPK and JNK in a time dependent manner, but not ERK½. MAP kinase inhibitors SB203580, SP600125 and PD98059 confirm the role of p38 MAPK and JNK in OA induced caspase-3 activation and cell death. Over all, our results indicate that OA induces cell death by generation of ROS, and activation of p38 MAPK and JNK, and executed through mitochondrial mediated caspase pathway.

Caveolin-1 mediates Fas-BID signaling in hyperoxia-induced apoptosis

Fas-mediated apoptosis is a crucial cellular event. Fas, the Fas-associated death domain, and caspase 8 form the death-inducing signaling complex (DISC). Activated caspase 8 mediates the extrinsic pathways and cleaves cytosolic BID. Truncated BID (tBID) translocates to the mitochondria, facilitates the release of cytochrome c, and activates the intrinsic pathways. However, the mechanism causing these DISC components to aggregate and form the complex remains unclear. We found that Cav-1 regulated Fas signaling and mediated the communication between extrinsic and intrinsic pathways. Shortly after hyperoxia (4 h), the colocalization and interaction of Cav-1 and Fas increased, followed by Fas multimer and DISC formation. Deletion of Cav-1 (Cav-1-/-) disrupted DISC formation. Further, Cav-1 interacted with BID. Mutation of Cav-1 Y14 tyrosine to phenylalanine (Y14F) disrupted the hyperoxia-induced interaction between BID and Cav-1 and subsequently yielded a decreased level of tBID and resistance to hyperoxia-induced apoptosis. The reactive oxygen species (ROS) scavenger N-acetylcysteine decreased the Cav-1-Fas interaction. Deletion of glutathione peroxidase-2 using siRNA aggravated the BID-Cav-1 interaction and tBID formation. Taken together, these results indicate that Cav-1 regulates hyperoxia/ROS-induced apoptosis through interactions with Fas and BID, probably via Fas palmitoylation and Cav-1 Y14 phosphorylation, respectively.

Okadaic acid induces morphological changes, apoptosis and cell cycle alterations in different human cell types

Okadaic acid (OA) is a marine toxin produced by dinoflagellate species which is frequently accumulated in molluscs usual in the human diet. The exact action mechanism of OA has not been described yet and the results of most reported studies are often conflicting. The aim of this work was to evaluate the OA effects on morphology, cell cycle and apoptosis induction by means of light microscopy and flow cytometry, in three different types of human cells (leukocytes, HepG2 cells and SHSY5Y cells). Cells were treated with a range of OA concentrations in the presence and absence of S9 fraction. OA induced morphological changes in all the cell types studied, and cell cycle disruption only in leukocytes and neuronal cells. SHSY5Y cells were the most sensitive to OA assault. Results obtained in the presence and absence of metabolic activation were similar, suggesting that OA acts both directly and indirectly. Furthermore, OA was found to increase the subG(1) region in the flow cytometry cell cycle analysis, suggesting induction of apoptosis. These results were confirmed by the employment of specific methodologies for studying apoptosis such as caspase 3 activation and annexin V staining. Increases in the apoptosis rate were obtained in all the cells treated in the absence of S9 fraction, accompanied by increases in caspase 3 activation, suggesting that apoptosis induced by OA is a caspase 3-dependent process. Nevertheless, in the presence of S9 fraction no apoptosis was detected, indicating a metabolic detoxifying activity, although necrosis was observed in neuroblastoma cells.

PP2A inhibitors induce apoptosis in pancreatic cancer cell line PANC-1 through persistent phosphorylation of IKKα and sustained activation of the NF-κB pathway

Serine/threonine protein phosphatase 2A (PP2A), is thought to be a cancer suppresser, as inhibition of PP2A can induce phosphorylation and activation of substrate kinases, most of which can accelerate growth. Interestingly, cantharidin potently inhibits PP2A but efficiently represses various cancer cells. In the present study, we found that PP2A inhibitors, cantharidin or Okadaic acid, inhibited cell viability and triggered apoptosis in PANC-1 pancreatic cancer cell line dependent on PP2A/IKKα/IκBα/p65 NF-κB pathway. The activation of NF-κB pathway up-regulated downstream pro-apoptotic genes, TNF-α, TRAILR1 and TRAILR2, and triggered apoptosis through the extrinsic pathway, indicating that PP2A is a potential target for pancreatic cancer treatment.

The dependence receptor UNC5H2/B triggers apoptosis via PP2A-mediated dephosphorylation of DAP kinase

The UNC5H dependence receptors promote apoptosis in the absence of their ligand, netrin-1, and this is important for neuronal and vascular development and for limitation of cancer progression. UNC5H2 (also called UNC5B) triggers cell death through the activation of the serine-threonine protein kinase DAPk. While performing a siRNA screen to identify genes implicated in UNC5H-induced apoptosis, we identified the structural subunit PR65β of the holoenzyme protein phosphatase 2A (PP2A). We show that UNC5H2/B recruits a protein complex that includes PR65β and DAPk and retains PP2A activity. PP2A activity is required for UNC5H2/B-induced apoptosis, since it activates DAPk by triggering its dephosphorylation. Moreover, netrin-1 binding to UNC5H2/B prevents this effect through interaction of the PP2A inhibitor CIP2A to UNC5H2/B. Thus we show here that, in the absence of netrin-1, recruitment of PP2A to UNC5H2/B allows the activation of DAPk via a PP2A-mediated dephosphorylation and that this mechanism is involved in angiogenesis regulation.

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.

A protein phosphatase feedback mechanism regulates the basal phosphorylation of Chk2 kinase in the absence of DNA damage

The checkpoint kinase Chk2 is an effector component of the ATM-dependent DNA damage response (DDR) pathway. The activation of Chk2 by genotoxic stress involves its phosphorylation on T68 by ATM and additional auto/transphosphorylations. Here we demonstrate that in unperturbed cells, chemical inhibition of Chk2 by VRX0466617 (VRX) enhances the phosphorylation of Chk2-T68 throughout the cell cycle phases. This event, dependent on the presence of ATM and catalytically functional Chk2, is not consequential to DNA damage, as neither gamma-H2AX nuclear foci nor increased ATM activation is detected in VRX-treated cells, suggesting the involvement of other regulatory proteins. As serine/threonine protein phosphatases (PPs) regulate the phosphorylation and deactivation of proteins of the DDR pathway, we analyzed their role in phospho-T68-Chk2 regulation. We found that intracellular inhibition of PP1 and PP2A-like activities by okadaic acid markedly raised the accumulation of Chk2-pT68 without DNA damage induction, and this phenomenon was also seen when PP1-C, PP2A-C, and Wip1/PPM1D were simultaneously knockdown by siRNA. Altogether, these data indicate a novel mechanism in undamaged cells where PPs function to maintain the balance between ATM and its direct substrate Chk2 through a regulatory circuit.

Regulation of postsynaptic gephyrin cluster size by protein phosphatase 1

The scaffolding protein gephyrin is essential for the clustering of glycine and GABA(A) receptors (GABA(A)Rs) at inhibitory synapses. Here, we provide evidence that the size of the postsynaptic gephyrin scaffold is controlled by dephosphorylation reactions. Treatment of cultured hippocampal neurons with the protein phosphatase inhibitors calyculin A and okadaic acid reduced the size of postsynaptic gephyrin clusters and increased cytoplasmic gephyrin staining. Protein phosphatase 1 (PP1) was found to colocalize with gephyrin at selected postsynaptic sites and to interact with gephyrin in transfected cells and brain extracts. Alanine or glutamate substitution of the two established serine/threonine phosphorylation sites in gephyrin failed to affect its clustering at inhibitory synapses and its ability to recruit gamma2 subunit containing GABA(A)Rs. Our data are consistent with the postsynaptic gephyrin scaffold acting as a platform for PP1, which regulates gephyrin cluster size by dephosphorylation of gephyrin- or cytoskeleton-associated proteins.

Mitochondrial mechanisms of estrogen neuroprotection

Mitochondria have become a primary focus in our search not only for the mechanism(s) of neuronal death but also for neuroprotective drugs and therapies that can delay or prevent Alzheimer's disease and other chronic neurodegenerative conditions. This is because mitochrondria play a central role in regulating viability and death of neurons, and mitochondrial dysfunction has been shown to contribute to neuronal death seen in neurodegenerative diseases. In this article, we review the evidence for the role of mitochondria in cell death and neurodegeneration and provide evidence that estrogens have multiple effects on mitochondria that enhance or preserve mitochondrial function during pathologic circumstances such as excitotoxicity, oxidative stress, and others. As such, estrogens and novel non-hormonal analogs have come to figure prominently in our efforts to protect neurons against both acute brain injury and chronic neurodegeneration.

Role of protein phosphatases and mitochondria in the neuroprotective effects of estrogens

In the present treatise, we provide evidence that the neuroprotective and mito-protective effects of estrogens are inexorably linked and involve the ability of estrogens to maintain mitochondrial function during neurotoxic stress. This is achieved by the induction of nuclear and mitochondrial gene expression, the maintenance of protein phosphatases levels in a manner that likely involves modulation of the phosphorylation state of signaling kinases and mitochondrial pro- and anti-apoptotic proteins, and the potent redox/antioxidant activity of estrogens. These estrogen actions are mediated through a combination of estrogens receptor (ER)-mediated effects on nuclear and mitochondrial transcription of protein vital to mitochondrial function, ER-mediated, non-genomic signaling and non-ER-mediated effects of estrogens on signaling and oxidative stress. Collectively, these multifaceted, coordinated action of estrogens leads to their potency in protecting neurons from a wide variety of acute insults as well as chronic neurodegenerative processes.

The phosphatase inhibitor, okadaic acid, strongly protects primary rat cortical neurons from lethal oxygen-glucose deprivation

The protein kinase-mediated actions of peptide growth factors such as IGF-1 and bFGF protect cultured neurons from being killed by the oxygen and glucose deprivations (OGD) that prevail in the 'stroked brain'. Here, we show that neuroprotection by IGF-1 is mediated by PI-3K/Akt, whereas that of bFGF is mediated by MAPK. IGF-1 and bFGF together did not further increase protection suggesting a downstream convergence of their pathways. Since protein kinases mediated the protection, a phosphatase inhibitor such as okadaic acid (OA) might be as protective as the growth factors against OGD. Here, we show that OA is actually a much more effective protector. It increased the phosphorylation of both PI-3K/Akt and MAPK, and stimulated new protein synthesis. OA also acted independently of the CREB activation and FKHRL1 and GSK-3 inactivation which have been implicated in IGF-1 actions.

Okadaic acid protects human neuroblastoma SH-SY5Y cells from 1-methyl-4-phenylpyridinium ion-induced apoptosis

1-methyl-4-phenylpyridinium ion (MPP(+)) has been shown to selectively inhibit mitochondrial function and induce a parkinsonism-like syndrome. MPP(+) stimulates the production of reactive oxygen species (ROS) and induces cell death in vitro. In this study, we investigated the protective effects of okadaic acid on MPP(+)-induced cell death in SH-SY5Y neuroblastoma cells. We found that MPP(+)-induced apoptosis and -ROS generation were blocked by okadaic acid. MPP(+)-mediated activation of AKT was also inhibited by okadaic acid. Taken together, these results demonstrate that okadaic acid protects against MPP(+)-induced apoptosis by blocking ROS stimulation and ROS-mediated signaling pathways in SH-SY5Y cells. These data indicated that okadaic acid could provide a therapeutic strategy for the treatment of neurodegenerative diseases including Parkinson's disease.

Regulation of CD95/APO-1/Fas-induced apoptosis by protein phosphatases

Triggering the CD95/APO-1/Fas receptor by CD95-L induces the assembly of the death-inducing signaling complex (DISC), which permits initiator caspases activation and progression of a signaling cascade that culminates in cellular apoptosis. Despite the CD95 receptor does not exhibit any kinase activity by itself, phosphorylation/dephosphorylation events seem important to regulate many aspects of CD95-mediated apoptosis. Here, we try to highlight particularly the importance of protein phosphatases in the modulation of the CD95 system.

Phosphorylation of murine caspase-9 by the protein kinase casein kinase 2 regulates its cleavage by caspase-8

Previous studies from our laboratory had indicated that cytochrome c-independent processing and activation of caspase-9 by caspase-8 contributed to early amplification of the caspase cascade in tumor necrosis factor (TNF)-alpha-treated murine cells. Here we show that murine caspase-9 is phosphorylated by casein kinase 2 (CK2) on a serine near the site of caspase-8 cleavage. CK2 has been shown to regulate cleavage of the pro-apoptotic Bid protein by phosphorylating serine residues near its caspase-8 cleavage site. Similarly, CK2 modification of Ser(348) on caspase-9 appears to render the protease refractory to cleavage by active caspase-8. This phosphorylation did not affect the ability of caspase-9 to autoprocess. Substitution of Ser(348) abolished phosphorylation but not cleavage, and a phospho-site mutant promoted apoptosis in TNF-alpha-treated caspase-9 knock-out mouse embryo fibroblasts. Furthermore, inhibition of CK2 activity and RNA interference-mediated knockdown of the kinase accelerated caspase-9 activation, whereas phosphatase inhibition delayed both caspase-9 activation and death in response to TNF receptor occupation. Taken together, these studies show that TNF receptor cross-linking promotes dephosphorylation of caspase-9, rendering it susceptible to processing by activated caspase-8 protein. Thus, our data suggest that modification of procaspase-9 to protect it from inappropriate cleavage and activation is yet another mechanism by which the oncogenic kinase CK2 promotes survival.

Luteinizing hormone receptor activation in ovarian granulosa cells promotes protein kinase A-dependent dephosphorylation of microtubule-associated protein 2D

The actions of LH to induce ovulation and luteinization of preovulatory follicles are mediated principally by activation of cAMP-dependent protein kinase (PKA) in granulosa cells. PKA activity is targeted to specific locations in many cells by A kinase-anchoring proteins (AKAPs). We previously showed that FSH induces expression of microtubule-associated protein (MAP) 2D, an 80-kDa AKAP, in rat granulosa cells, and that MAP2D coimmunoprecipitates with PKA-regulatory subunits in these cells. Here we report a rapid and targeted dephosphorylation of MAP2D at Thr256/Thr259 after treatment with human chorionic gonadotropin, an LH receptor agonist. This event is mimicked by treatment with forskolin or a cAMP analog and is blocked by the PKA inhibitor myristoylated-PKI, indicating a role for cAMP and PKA signaling in phosphoregulation of granulosa cell MAP2D. Furthermore, we show that Thr256/Thr259 dephosphorylation is blocked by the protein phosphatase 2A (PP2A) inhibitor, okadaic acid, and demonstrate interactions between MAP2D and PP2A by coimmunoprecipitation and microcystin-agarose pull-down. We also show that MAP2D interacts with glycogen synthase kinase (GSK) 3beta and is phosphorylated at Thr256/Thr259 by this kinase in the basal state. Increased phosphorylation of GSK3beta at Ser9 and the PP2A B56delta subunit at Ser566 is observed after treatment with human chorionic gonadotropin and appears to result in LH receptor-mediated inhibition of GSK3beta and activation of PP2A, respectively. Taken together, these results show that the phosphorylation status of the AKAP MAP2D is acutely regulated by LH receptor-mediated modulation of kinase and phosphatase activities via PKA.

Modulation of receptor phosphorylation contributes to activation of peroxisome proliferator activated receptor alpha by dehydroepiandrosterone and other peroxisome proliferators

Dehydroepiandrosterone (DHEA), a C19 human adrenal steroid, activates peroxisome proliferator-activated receptor alpha (PPARalpha) in vivo but does not ligand-activate PPARalpha in transient transfection experiments. We demonstrate that DHEA regulates PPARalpha action by altering both the levels and phosphorylation status of the receptor. Human hepatoma cells (HepG2) were transiently transfected with the expression plasmid encoding PPARalpha and a plasmid containing two copies of fatty acyl coenzyme oxidase (FACO) peroxisome-proliferator activated receptor responsive element consensus oligonucleotide in a luciferase reporter gene. Nafenopin treatment increased reporter gene activity in this system, whereas DHEA treatment did not. Okadaic acid significantly decreased nafenopin-induced reporter activity in a concentration-dependent manner. Okadaic acid treatment of primary rat hepatocytes decreased both DHEA- and nafenopin-induced FACO activity in primary rat hepatocytes. DHEA induced both PPARalpha mRNA and protein levels, as well as PP2A message in primary rat hepatocytes. Western blot analysis showed that the serines at positions 12 and 21 were rapidly dephosphorylated upon treatment with DHEA and nafenopin. Results using specific protein phosphatase inhibitors suggested that protein phosphatase 2A (PP2A) is responsible for DHEA action, and protein phosphatase 1 might be involved in nafenopin induction. Mutation of serines at position 6, 12, and 21 to an uncharged alanine residue significantly increased transcriptional activity, whereas mutation to negative charged aspartate residues (mimicking receptor phosphorylation) decreased transcriptional activity. DHEA action involves induction of PPARalpha mRNA and protein levels as well as increased PPARalpha transcriptional activity through decreasing receptor phosphorylation at serines in the AF1 region.

Activation of brain protein phosphatase-1(I) following cardiac arrest and resuscitation involving an interaction with 14-3-3 gamma

The intracellular signaling mechanisms that couple transient cerebral ischemia to cell death and neuroprotective mechanisms provide potential therapeutic targets for cardiac arrest. Protein phosphatase (PP)-1 is a major serine/threonine phosphatase that interacts with and dephosphorylates critical regulators of energy metabolism, ionic balance, and apoptosis. We report here that PP-1(I), a major regulated form of PP-1, is activated in brain by approximately twofold in vivo following cardiac arrest and resuscitation in a clinically relevant pig model of transient global cerebral ischemia and reperfusion. PP-1(I) purified to near homogeneity from either control or ischemic pig brain consisted of the PP-1 catalytic subunit, the inhibitor-2 regulatory subunit, as well as the novel constituents 14-3-3gamma, Rab GDP dissociation protein beta, PFTAIRE kinase, and C-TAK1 kinase. PP-1(I) purified from ischemic brain contained significantly less 14-3-3gamma than PP-1(I) purified from control brain, and purified 14-3-3gamma directly inhibited the catalytic subunit of PP-1 and reconstituted PP-1(I). These findings suggest that activation of brain PP-1(I) following global cerebral ischemia in vivo involves dissociation of 14-3-3gamma, a novel inhibitory modulator of PP-1(I). This identifies modulation of PP-1(I) by 14-3-3 in global cerebral ischemia as a potential signaling mechanism-based approach to neuroprotection.

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.

Differential involvement of reactive oxygen species in apoptosis caused by the inhibition of protein phosphatase 2A in Jurkat and CCRF-CEM human T-leukemia cells

A better understanding of dysregulated signaling pathways in cancer cells may suggest novel strategies to prevent tumor development and/or progression. Here we show that Jurkat and CCRF-CEM human T-leukemia cell lines were more sensitive than normal human T cells to the cytotoxic effect of inhibiting protein phosphatase 2A (PP2A). Inhibition of PP2A by okadaic acid (OA) caused T-leukemia cells to die by apoptosis, as indicated by DNA fragmentation, caspase-3 activation, loss of mitochondrial membrane potential (DeltaPsi(m)), and changes in nuclear morphology that were consistent with apoptosis. PP2A might therefore be a useful intracellular target for the treatment of T cell-derived leukemias. We also observed that reactive oxygen species (ROS) were generated in response to PP2A inhibition in T-leukemia cells. However, loss of DeltaPsi(m) that resulted from PP2A inhibition was not prevented by exogenous antioxidants (glutathione and N-acetyl-cysteine), indicating that OA-induced changes in mitochondrial membrane permeability were not a consequence of ROS production. Moreover, exogenous antioxidants protected CCRF-CEM T-leukemia cells from apoptosis caused by PP2A inhibition but failed to prevent OA-induced apoptosis in Jurkat T-leukemia cells, indicating a differential role for ROS in apoptosis caused by PP2A inhibition in two different human T-leukemia cell lines.

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.

Apoptosis of human neutrophils induced by protein phosphatase 1/2A inhibition is caspase-independent and serine protease-dependent

Protein phosphatase (PP) activity is associated with the regulation of apoptosis in neutrophils. However, the underlying regulatory mechanism(s) in apoptosis remain unclear. The type of cell death induced by okadaic acid (OA), the inhibitor of PP1 and PP2A, is characterized by apoptotic morphological changes of the cells and annexin V-positive staining without DNA fragmentation. The apoptotic effects of OA and calyculin A on neutrophils were observed at concentrations ranging from 50 to 200 nM, or 10 to 50 nM, respectively. Cyclosporine A (a PP2B specific inhibitor), however, did not exhibit any pro-apoptotic effects. OA and calyculin A, but not cyclosporine A, exhibited significant effects on protein levels and on the electrophoretic mobility of Mcl-1. zVAD-fmk, a pancaspase inhibitor, failed to inhibit the effect of OA on the caspase-3 activity, procaspase-3 processing, and the apoptotic rate of neutrophils. However, 4-(2-aminoethyl) benzenesulfonylfluoride (AEBSF), a general serine protease inhibitor, significantly abrogated the OA-induced mobility shift in procaspase-3, caspase-3 activation, and the apoptotic morphological changes in neutrophils. Moreover, OA enhanced the serine protease activity of the neutrophils. The addition of the proteinase-3 protein increased the rate of neutrophil apoptosis, which was also blocked by AEBSF but not by zVAD-fmk. These results suggest that OA induces procaspase-3 processing but that OA-induced apoptosis is caspase-independent and serine protease-dependent.

Post-translational regulation of the Drosophila circadian clock requires protein phosphatase 1 (PP1)

Phosphorylation is an important timekeeping mechanism in the circadian clock that has been closely studied at the level of the kinases involved but may also be tightly controlled by phosphatase action. Here we demonstrate a role for protein phosphatase 1 (PP1) in the regulation of the major timekeeping molecules in the Drosophila clock, TIMELESS (TIM) and PERIOD (PER). Flies with reduced PP1 activity exhibit a lengthened circadian period, reduced amplitude of behavioral rhythms, and an altered response to light that suggests a defect in the rising phase of clock protein expression. On a molecular level, PP1 directly dephosphorylates TIM and stabilizes it in both S2R(+) cells and clock neurons. However, PP1 does not act in a simple antagonistic manner to SHAGGY (SGG), the kinase that phosphorylates TIM, because the behavioral phenotypes produced by inhibiting PP1 in flies are different from those achieved by overexpressing SGG. PP1 also acts on PER, and TIM regulates the control of PER by PP1, although it does not affect PP2A action on PER. We propose a modified model for post-translational regulation of the Drosophila clock, in which PP1 is critical for the rhythmic abundance of TIM/PER while PP2A also regulates the nuclear translocation of TIM/PER.

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.