Serine/threonine protein phosphatases

A novel transcription factor involved in plant defense endowed with protein phosphatase activity

In plants, expression of a disease-resistance character following perception of a pathogen involves massive deployment of transcription-dependent defenses. Thus, if rapid and effective defense responses have to be achieved, it is crucial that the pathogenic signal is transduced and amplified through pre-existing signaling pathways. Reversible phosphorylation of specific transcription factors, by a concerted action of protein kinases and phosphatases, may represent a mechanism for rapid and flexible regulation of selective gene expression by environmental stimuli. Here we identified a novel DNA-binding protein from tobacco plants, designated DBP1, with protein phosphatase activity, which binds in a sequence-specific manner to a cis- acting element of a defense-related gene and participates in its transcriptional regulation. This finding helps delineate a terminal event in a signaling pathway for the selective activation of early transcription-dependent defense responses in plants, and suggests that stimulus-dependent reversible phosphorylation of regulatory proteins may occur directly in a transcription protein-DNA complex.

Effect of microcystin-LR and cyanobacterial extract from Polish reservoir of drinking water on cell cycle progression, mitotic spindle, and apoptosis in CHO-K1 cells

Microcystin-LR is a cyanobacterial toxin possessing a potent tumor-promoting activity mediated through inhibition of protein phosphatases PP1 and PP2A. Because these enzymes are involved in fundamental cell processes, we decided to examine the influence of microcystin-LR on cell cycle progression, onset of anaphase, segregation of chromosomes by the mitotic spindle, and apoptosis in Chinese hamster ovary (CHO-K1) cells. Cells were incubated with 25, 50, and 100 microM of pure microcystin-LR and a cyanobacterial extract for 14, 18, and 22 h. Giemsa staining of cells treated with these toxins revealed a dose- and time-dependent increase of mitotic indices, accumulation of abnormal G(2)/M figures with hypercondensed chromosomes, abnormal anaphases with defective chromosome separation, and polyploid cells. Because spindle checkpoint is a fundamental regulatory mechanism that assures the onset of anaphase and subsequent exit from mitosis, we examined the spindle organization in microcystin-treated cells. The majority of the mitotic cells showed monopolar and multipolar mitotic spindles (multiple asters). Microtubule bundles were present in interphase cells. Our results indicate that microcystin-LR induces apoptosis and necrosis in a dose- and time-dependent manner and that the frequency of dead cells cells is positively correlated with the frequency of polyploid cells.

Protein phosphatase type 2C dephosphorylates BAD

Reversible phosphorylation modulates a cells' susceptibility to apoptosis. The phosphorylation status of BAD, a member of the Bcl-2 protein family, is an important checkpoint governing life-or-death decisions: Phosphorylation of serine residues 112, 136 and 155 on BAD prevents apoptosis. Here we report that BAD is a substrate for PP2C. Ser(155) is involved in heterodimerization with Bcl-X(L). We could demonstrate that PP1, PP2A and PP2C act on this site in vitro. However, only PP2C gives priority to P-Ser(155) compared to P-Ser(112) and P-Ser(136) on BAD. The results indicate that PP2C is an additional factor triggering the pro-apoptotic function of BAD.

Protein phosphatase 1cgamma is required in germ cells in murine testis

The protein phosphatase 1cgamma (PP1cgamma) gene is required for spermatogenesis. Males homozygous for a null mutation are sterile, and display both germ cell and Sertoli cell defects. As these two cell types are physically and functionally intimately connected in the testis, the question arises as to whether the primary site of PP1cgamma action is in Sertoli cells, germ cells, or both. We generated chimeric males by embryo aggregation to test whether wild type Sertoli cells are capable of rescuing mutant germ cells. To distinguish between the desired XY-XY chimeras and uninformative XX-XY chimeras, we designed an adaptation of the single nucleotide primer extension (SNuPE) assay. None of the XY-XY chimeras sired pups derived from mutant germ cells, indicating that the protein is required in germ cells for production of functional sperm. Analysis of a chimeric testis revealed intermediate phenotypes when compared with PP1cgamma-/- testes, suggestive of cell nonautonomous effects. We conclude that PP1cgamma is required in a cell autonomous fashion in germ cells. There may be an additional cell nonautonomous role played by this gene in testes, possibly mediated by defective signaling between germ cells and Sertoli cells.

Ectopic expression of inhibitors of protein phosphatase type 1 (PP1) can be used to analyze roles of PP1 in Drosophila development

We have identified two proteins that bind with high specificity to type 1 serine/threonine protein phosphatase (PP1) and have exploited their inhibitory properties to develop an efficient and flexible strategy for conditional inactivation of PP1 in vivo. We show that modest overexpression of Drosophila homologs of I-2 and NIPP1 (I-2Dm and NIPP1Dm) reduces the level of PP1 activity and phenotypically resembles known PP1 mutants. These phenotypes, which include lethality, abnormal mitotic figures, and defects in muscle development, are suppressed by coexpression of PP1, indicating that the effect is due specifically to loss of PP1 activity. Reactivation of I-2Dm:PP1c complexes suggests that inhibition of PP1 activity in vivo does not result in a compensating increase in synthesis of active PP1. PP1 mutants enhance the wing overgrowth phenotype caused by ectopic expression of the type II TGF beta superfamily signaling receptor Punt. Using I-2Dm, which has a less severe effect than NIPP1Dm, we show that lowering the level of PP1 activity specifically in cells overexpressing Punt is sufficient for wing overgrowth and that the interaction between PP1 and Punt requires the type I receptor Thick-veins (Tkv) but is not strongly sensitive to the level of the ligand, Decapentaplegic (Dpp), nor to that of the other type I receptors. This is consistent with a role for PP1 in antagonizing Punt by preventing phosphorylation of Tkv. These studies demonstrate that inhibitors of PP1 can be used in a tissue- and developmental-specific manner to examine the developmental roles of PP1.

Xylulose 5-phosphate mediates glucose-induced lipogenesis by xylulose 5-phosphate-activated protein phosphatase in rat liver

Carbohydrate-responsive element binding protein (ChREBP) is a transcription factor that activates lipogenic genes in liver in response to excess carbohydrate in the diet. ChREBP is regulated in a reciprocal manner by glucose and cAMP. cAMP-dependent protein kinase (protein kinase A) phosphorylates two physiologically important sites in ChREBP, Ser-196, which is located near nuclear localization signal sequence (NLS), and Thr-666, within the basic helix-loop-helix (bHLH) site, resulting in inactivation of nuclear translocation of ChREBP and of the DNA-binding activity, respectively. We demonstrate here that crude cytosolic extracts from livers of rats fed a high carbohydrate diet contained protein phosphatase (PPase) activity that dephosphorylated a peptide containing Ser-196, whereas a PPase in the nuclear extract catalyzed dephosphorylation of Ser-568 and Thr-666. All these PPases are activated specifically by xylulose 5-phosphate (Xu5P), but not by other sugar phosphates. Furthermore, addition of Xu5P elevated PPase activity to the level observed in extracts of fed liver cells. These partially purified PPases were characterized as PP2A-AB delta C by immunoblotting with specific antibodies. These results suggest that (ia) Xu5P-dependent PPase is responsible for activation of transcription of the L-type pyruvate kinase gene and lipogenic enzyme genes, and (ii) Xu5P is the glucose signaling compound. Thus, we propose that the same Xu5P-activated PPase controls both acute and long-term regulation of glucose metabolism and fat synthesis.

PNUTS, a protein phosphatase 1 (PP1) nuclear targeting subunit. Characterization of its PP1- and RNA-binding domains and regulation by phosphorylation

PNUTS, Phosphatase 1 NUclear Targeting Subunit, is a recently described protein that targets protein phosphatase 1 (PP1) to the nucleus. In the present study, we characterized the biochemical properties of PNUTS. A variety of truncation and site-directed mutants of PNUTS was prepared and expressed either as glutathione S-transferase fusion proteins in Escherichia coli or as FLAG-tagged proteins in 293T cells. A 50-amino acid domain in the center of PNUTS mediated both high affinity PP1 binding and inhibition of PP1 activity. The PP1-binding domain is related to a motif found in several other PP1-binding proteins but is distinct in that Trp replaces Phe. Mutation of the Trp residue essentially abolished the ability of PNUTS to bind to and inhibit PP1. The central PP1-binding domain of PNUTS was an effective substrate for protein kinase A in vitro, and phosphorylation substantially reduced the ability of PNUTS to bind to PP1 in vitro and following stimulation of protein kinase A in intact cells. In vitro RNA binding experiments showed that a C-terminal region including several RGG motifs and a novel repeat domain rich in His and Gly interacted with mRNA and single-stranded DNA. PNUTS exhibited selective binding for poly(A) and poly(G) compared with poly(U) or poly(C) ribonucleotide homopolymers, with specificity being mediated by distinct regions within the domain rich in His and Gly and the domain containing the RGG motifs. Finally, a PNUTS-PP1 complex was isolated from mammalian cell lysates using RNA-conjugated beads. Together, these studies support a role for PNUTS in protein kinase A-regulated targeting of PP1 to specific RNA-associated complexes in the nucleus.

Interaction between protein phosphatase 2A and members of the importin beta superfamily

While performing a yeast two-hybrid library screen to uncover novel PP2A-interacting proteins, we discovered a specific interaction between a member of the importin beta/karyopherin beta superfamily, importin 9, and the A subunit of PP2A (PR65). This interaction between importin 9 and the A subunit was confirmed by in vitro pulldown, immunoprecipitation, and microcystin-Sepharose chromatography. We also found that another family member, importin beta, interacted specifically with the A subunit of PP2A. Finally, we showed that treatment of cells with a concentration of okadaic acid known to inhibit PP2A impeded the nuclear localization of an NLS-containing protein. These results provide evidence that these importins can exist in a native complex with endogenous PP2A and that this serine/threonine phosphatase plays a role in regulating the nuclear import of NLS-containing proteins in vivo.

Caspase inhibition shifts neuroepithelioma cell response to okadaic acid from apoptosis to an apoptotic-like form of death

We have previously shown that the protein phosphatase inhibitor okadaic acid (OA) induces caspase-3 activation and apoptosis in CHP-100 human neuroepithelioma cells. Herein we provide a more general picture of the effects brought about by OA in this system, also investigating whether caspase activation is necessary for apoptosis induction. We report that incubation for 24 h with 10 nM OA induced a large fraction of the cell population to undergo premature chromosome condensation (PCC) or mitotic arrest, but not apoptosis. The former two effects were also observed after cell treatment with 20 nM OA; however, at this concentration, typical apoptotic cells were also detected, characterized by pycnotic and fragmented nuclei. Occurrence of the above-mentioned apoptotic figures turned extensive at 100 nM OA. The pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD.fmk, 100 microM) fully prevented apoptosis induced by 20 nM OA, increasing PCC incidence. Conversely, 100 nM OA induced an apoptotic-like phenotype, even in the presence of Z-VAD.fmk: in this case, however, nuclei, albeit pycnotic, displayed morphological characteristics distinct from those of typical apoptotic cells; moreover, as assessed by flow cytometry, they were largely unfragmented. The reported OA effects occurred in a setting in which neither p53 nor p21(Cip1/Waf1) was upregulated, thus ruling out a role for these proteins in apoptosis induction. On the other hand, apoptotic doses of OA induced a shift of the retinoblastoma gene product to the hypophosphorylated state and its downregulation by a caspase-dependent mechanism.

Expression of the scaffolding subunit A of protein phosphatase 2A during rat testicular development

Previously, we found that the poly(A)+ RNA of the scaffolding subunit A (alpha isoform) of protein phosphatase 2A (PP2A-Aalpha) was clearly expressed by fetal gonocytes but weakly expressed by adult single (As), paired (Apr), and aligned (Aal) A spermatogonia. The scaffolding subunit A of PP2A (PP2A-A) is the major subunit in the formation of a functional PP2A holoenzyme. In this study, we investigated the expression of PP2A-A during testicular development in more detail using in situ hybridization, immunohistochemistry, and Western blot with testes of rats of various ages from 16 days postcoitum (pc) to adulthood. The expression of PP2A-A was detected in fetal proliferative gonocytes at 16 days pc, declining thereafter during the quiescent period of the gonocytes. From the day of birth to the start of spermatogenesis (Day 4 postpartum [pp]), the number of PP2A-A-immunopositive gonocytes increased. At Day 4 pp, the first A1 spermatogonia appeared along the basement membrane; all were PP2A-A positive. In the adult, PP2A-A was upregulated during the differentiation of the As, Apr, and Aal spermatogonia to the A1 spermatogonia and expressed thereafter by all other spermatogonia. Spermatocytes from the pachytene stage onward and all spermatids in the adult testis also showed clear expression of PP2A-A. In Sertoli cells, PP2A-A was detected during their proliferative period at 19 days pc to 15 days pp. The presence of a functional enzyme was confirmed by the additional detection of the catalytic subunit C of PP2A using Western blot analyses at various ages during testicular development. This apparent pattern of expression of PP2A-A during testicular development suggests that PP2A may play an important role in the proliferation of distinct populations of testicular cells and during meiosis and sperm maturation.

Special sensitization pattern in adenosine-induced myocardial responses after thyroxine-treatment

Chronic thyroxine treatment reduces the susceptibility of atrial myocardium to adenosine. While the possible role of membrane adenosine receptors in this action is supported by several studies, the involvement of intracellular adenosine mechanisms has not been defined. The present experiments were carried out in electrically driven euthyroid and hyperthyroid guinea pig atrial myocardium. The extracellular and intracellular actions of adenosine were analyzed pharmacologically by the use of specific blockers of membrane adenosine transport and intracellular adenosine deaminase (ADA). The involvement of phosphoprotein phosphatase, phospholamban, and sarcoplasmic reticulum Ca2+ ATPase (SERCA) in the adenosine-induced responses was also studied. The major findings were as follows: i) pD(2)- and E(max)-values for adenosine-induced decrease of mechanical activity were significantly reduced after an 8-day thyroxine treatment in atrial tissues; ii) in atria of thyroxine-treated animals, membrane purine transport inhibitors (dipyridamole, NBTI) induced similar leftward shifts in concentration-response curves for adenosine in both euthyroid and hyperthyroid atrial myocardium without altering the depressed E(max) values; iii) the leftward displacement evoked by inhibitors of intracellularly located ADA (coformycin, EHNA) was more striking in hyperthyroid than euthyroid myocardia. ADA inhibitors induced a complete reversal of the maximum adenosine actions; iv) inhibition by cantharidin of phosphoprotein phosphatases (after inhibition of ADA) reduced the adenosine-induced responses. This inhibition was stronger in hyperthyroid atria; v) pharmacological elimination of sarcoplasmic reticulum Ca2+ ATPase by cyclopiazonic acid did not alter the cardiac responses to adenosine and this was independent of thyroid status. It is suggested that distinct modulation of the extra- and intracellular adenosine actions is present in eu- and hyperthyroid hearts. In the latter, a predominance of intracellular adenosine mechanisms can be proposed.

The MID1/PP2A complex: a key to the pathogenesis of Opitz BBB/G syndrome

Opitz BBB/G syndrome is a monogenic disorder that is characterized by malformations of the ventral midline. Investigations into the underlying genetic defects and the pathobiochemistry of this syndrome have already shed light on the mechanisms of both the physiological and the pathological development of the ventral midline, a complicated multistep process. Moreover, these studies have revealed the ubiquitin-dependent regulation of microtubule-associated phosphatase 2A, a central mechanism in many cellular processes. In this review, we summarize recent findings and speculate upon their implications for both medical and general research.

Increased recombination frequency showing evidence of loss of interference is associated with abnormal testicular histopathology

Nondisjunction leading to aneuploid gametes has been linked genetically to both increases and decreases in recombination frequency on the aneuploid chromosome. In the present study, we present physical evidence of increased frequency of recombination nodules as measured by Mut-S-like homologue-1 (MLH1) foci on pachytene chromosomes from sterile male mice homozygous for a mutation in the protein phosphatase 1cgamma (PP1cgamma) gene. The pattern of elevated recombination frequency in PP1cgamma mutant spermatocytes is consistent with a loss of interference. Previous studies demonstrated: (1) spermiogenesis is impaired starting at step 8 with a severe reduction in elongating and condensed spermatids; (2) spermatids and sperm exhibit elevated rates of DNA fragmentation; and (3) haploid gametes exhibit elevated levels of aneuploidy. Morphometric analysis of developing testes revealed that the first wave of meiosis proceeds at a normal rate in mutant testes, a surprising result given that the PP1 inhibitor okadaic acid has been shown to accelerate progression of spermatocytes from pachytene to the first meiotic division (MI). Evidence of abnormal testicular histopathology is apparent at 3 weeks, before the appearance of haploid gametes, eliminating the possibility that the mutant phenotype is caused by the presence of abnormal spermatids, but coincident with the appearance of the first set of mid to late pachytene spermatocytes. These observations lead us to conclude that the PP1cgamma mutation causes a complex phenotype, including subtle adverse effects on meiosis, possibly mediated by defective signaling between germ cells and Sertoli cells.

Regulation of alpha(1)-adrenoceptor-linked phosphoinositide metabolism in cultured glia: involvement of protein phosphatases and kinases

Noradrenaline-stimulated phosphoinositide breakdown in cultured glia was found to be mediated by alpha(1A)-adrenoceptors. The alpha(1A)-selective agonist A61603 was as effective as noradrenaline in eliciting 3H-inositol phosphate (IP) accumulation but was approximately 50-fold more potent. In addition, the use of selective antagonists revealed a clear rank order of potency in the ability of these drugs to reverse the effect of noradrenaline on phosphoinositide breakdown: RS17053 (alpha(1A)-selective) >>AH11110A (alpha(1B)-selective)>BMY7378 (alpha(1D)-selective). Pre-treatment of cultured glia with the protein phosphatase inhibitor okadaic acid resulted in a concentration- and time-dependent reduction in noradrenaline-evoked 3H-IP accumulation. This effect was mimicked by, but was not additive with, a phorbol ester, was reversed by protein kinase C (PKC) inhibitors and was not evident in cells which had been PKC depleted. The ability of cell extracts to dephosphorylate radiolabelled glycogen phosphorylase revealed the presence of the phosphatases PP1 and PP2A in almost equal abundance. Okadaic acid pre-treatment of intact cultures elicited a marked reduction in total phosphatase activity, particularly that mediated by PP2A. We also determined the effect of okadaic acid pre-treatment on PKC and cyclic AMP-dependent protein kinase (PKA) activities in these cells. PKC and PKA activities in cell extracts were assessed by determining the incorporation of 32P into histone and kemptide, respectively. Okadaic acid elicited increases in both Ca(2+)-dependent and Ca(2+)-independent PKC activity; in addition, increases in both initial and total PKA activities were also recorded. The effect of okadaic acid on noradrenaline-stimulated 3H-IP accumulation were not, however, mimicked by either forskolin or 8-bromo-cyclic AMP, suggesting that this event is not regulated by PKA. Our data point to roles for both PKC and PP2A in the regulation of alpha(1A)-adrenoceptor-linked phosphoinositide metabolism in cultured cortical glia.

An investigation into the effect of selenium supplementation on microcystin hepatotoxicity

Toxin-producing cyanobacteria pose a worldwide health threat to humans and animals due to their increasing presence in both drinking and recreational waters. Little work has, however, been done on a preventative therapy for anyone at risk of exposure to cyanobacterial toxins. The potential benefits of dietary supplementation of selenium, an antioxidant, to protect against the mouse liver injury induced by the toxin, microcystin-LR, has been investigated. BALB/c mice were pretreated for two weeks with sodium selenite (1.5 microg/mouse/day) before an intraperitoneal injection of microcystin-LR. Selenium-supplementation was found to provide some protection to the action of the toxin. In addition selenium pretreatment reduced the liver damage caused by lethal and sub-lethal toxin doses as reflected in liver pathology, decreased serum ALT and lipid peroxidation levels as well as prevention of glycogen loss compared to non-selenium supplemented toxin treated mice. The increased level of liver glutathione peroxidase activity following selenium-supplementation may indicate the possible route of selenium protection in the mice.

Characterization of a unique aspartate-rich protein of the SET/TAF-family in the human malaria parasite, Plasmodium falciparum, which inhibits protein phosphatase 2A

A search for physiological inhibitors of protein phosphatases led to the identification of a Plasmodium falciparum (Pf) cDNA that had the potential to code for an aspartate-rich protein and hence named ARP. The PfARP was virtually identical to its Plasmodium berghei counterpart in gene structure and protein sequence. The PfARP coding sequence contained two introns, and the predicted protein contained 269 amino acid residues. Its primary structure showed significant similarity to eukaryotic proteins of the SET and TAF-family that included two inhibitors of mammalian serine/threonine protein phosphatase 2A (PP2A), namely I1(PP2A) and I2(PP2A). Like the SET and TAF proteins, it had an extremely acidic tail. The cDNA was confirmed by recombinant expression in bacteria. Native parasitic ARP was purified and was found to be highly thermostable. PfARP specifically inhibited the parasitic PP2A at nanomolar concentrations, with no effect on PP1, PP2B, PP5, or PPJ. Expression of PfARP in HeLa cells led to elevated phosphorylation of c-Jun, and activation of transcription factors AP1 and NF-kappa B. These functional properties are also characteristic of the SET/TAF-family proteins. The ARP mRNA and protein were detectable in all the erythrocytic asexual stages of the parasite, and the protein was located mainly in the parasitic cytoplasm. Thus, PfARP is a unique cytoplasmic member of the SET/TAF-family and a candidate physiological regulator of the Plasmodium PP2A.

Usage of tautomycetin, a novel inhibitor of protein phosphatase 1 (PP1), reveals that PP1 is a positive regulator of Raf-1 in vivo

Protein phosphatase type 1 (PP1), together with protein phosphatase 2A (PP2A), is a major eukaryotic serine/threonine protein phosphatase involved in regulation of numerous cell functions. Although the roles of PP2A have been studied extensively using okadaic acid, a well known inhibitor of PP2A, biological analysis of PP1 has remained restricted because of lack of a specific inhibitor. Recently we reported that tautomycetin (TC) is a highly specific inhibitor of PP1. To elucidate the biological effects of TC, we demonstrated in preliminary experiments that treatment of COS-7 cells with 5 microm TC for 5 h inhibits endogenous PP1 by more than 90% without affecting PP2A activity. Therefore, using TC as a specific PP1 inhibitor, the biological effect of PP1 on MAPK signaling was examined. First, we found that inhibition of PP1 in COS-7 cells by TC specifically suppresses activation of ERK, among three MAPK kinases (ERK, JNK, and p38). TC-mediated inhibition of PP1 also suppressed activation of Raf-1, resulting in the inactivation of the MEK-ERK pathway. To examine the role of PP1 in regulation of Raf-1, we overexpressed the PP1 catalytic subunit (PP1C) in COS-7 cells and found that PP1C enhanced activation of Raf-1 activity, whereas phosphatase-dead PP1C blocked Raf-1 activation. Furthermore, a physical interaction between PP1C and Raf-1 was also observed. These data strongly suggest that PP1 positively regulates Raf-1 in vivo.

Protein phosphatases in plants

Phosphorylation and dephosphorylation of a protein often serve as an "on-and-off" switch in the regulation of cellular activities. Recent studies demonstrate the involvement of protein phosphorylation in almost all signaling pathways in plants. A significant portion of the sequenced Arabidopsis genome encodes protein kinases and protein phosphatases that catalyze reversible phosphorylation. For optimal regulation, kinases and phosphatases must strike a balance in any given cell. Only a very small fraction of the thousands of protein kinases and phosphatases in plants has been studied experimentally. Nevertheless, the available results have demonstrated critical functions for these enzymes in plant growth and development. While serine/threonine phosphorylation is widely accepted as a predominant modification of plant proteins, the function of tyrosine phosphorylation, desptie its overwhelming importance in animal systems, had been largely neglected until recently when tyrosine phosphatases (PTPs) were characterized from plants. This review focuses on the structure, regulation, and function of protein phosphatases in higher plants.

Expression of protein phosphatase 1 during the asexual development of Neurospora crassa

We cloned and sequenced the cDNA and the gene encoding the catalytic subunit of protein phosphatase 1 from the filamentous fungus Neurospora crassa. The gene, designated ppp-1 (phosphoprotein phosphatase 1), was mapped by restriction fragment length polymorphism to linkage group III, in the vicinity of con-7 and trp-1. The expression of the gene was monitored by reverse transcriptase and polymerase chain reactions, by Western blotting, and by protein phosphatase activity assays in synchronized cultures. Transcripts of ppp-1 were detected in the dormant conidia. The abundance of ppp-1 mRNA, Ppp-1 protein, and the activity of protein phosphatase 1 increased during germination and subsequent hyphal elongation as well as during the early stages of aerial mycelium formation.

Molecular cloning and characterization of protein phosphatase 2C of vomeronasal sensory epithelium of garter snakes

The earthworm-derived chemoattractant ES20 interacts with its G-protein-coupled receptors on the plasma membrane of vomeronasal (VN) sensory neurons of garter snakes, resulting in an increase in inositol trisphosphate [J. Biol. Chem. 269 (1994) 16867] and a rapid phosphorylation of the membrane-bound proteins, p42/44 [Biochim. Biophys. Acta 1450 (1999) 320]. The phosphorylation of p42/44 proteins are countervailingly regulated by a protein kinase and an okadaic acid-insensitive but fluoride-sensitive protein phosphatase (PPase) [J. Liu et al. (loc. cit.)]. The phosphorylation of p42/44 induced by ES20 appears to play a role in the regulation of signal transduction pathways by modulating the GTPase activity [J. Liu et al. (loc. cit.)]. A 564-bp fragment of cDNA was obtained from VN RNA of garter snakes by reverse transcription polymerase chain reaction with degenerate primers. The 564-bp fragment was amplified, cloned, and sequenced. Northern blot analysis revealed that both the VN organ (VNO) and brain contained the gene of PPase 2C. A full-length complementary 4119-bp DNA containing an open reading frame of 1146bp that encodes a protein of 382 amino acids with a molecular mass of 49,123Da was obtained from the VN cDNA library of garter snakes. The deduced amino acid sequence showed 88% amino acid identity to bovine protein phosphatase 2C alpha and 87% identity to human and rat PP2C alpha and to Mg(2+)-dependent protein phosphatase 1A of rat and rabbit. In situ hybridization revealed that the mRNA of VN protein phosphatase 2C is expressed in the vomeronasal sensory epithelium. This is the first report of the identification of a type 2C serine/threonine protein phosphatase in the VN system.

K vitamins, PTP antagonism, and cell growth arrest

The main function of K vitamins is to act as co-factors for gamma-glutamyl carboxylase. However, they have also recently been shown to inhibit cell growth. We have chemically synthesized a series of K vitamin analogs with various side chains at the 2 or 3 position of the core naphthoquinone structure. The analogs with short thio-ethanol side chains are found to be more potent growth inhibitors in vitro of various tumor cell lines. Cpd 5 or [2-(2-mercaptoethanol)-3-methyl-1,4-naphthoquinone] is one of the most potent. The anti-proliferation activity of these compounds is antagonized by exogenous thiols but not by non-thiol antioxidants. This suggests that the growth inhibition is mediated by sulfhydryl arylation of cellular glutathione and cysteine-containing proteins and not by oxidative stress. The protein tyrosine phosphatases (PTP) are an important group of proteins that contain cysteine at their catalytic site. PTPs regulate mitogenic signal transduction and cell cycle progression. PTP inhibition by Cpd 5 results in prolonged tyrosine phosphorylation and activation of several kinases and transcription factors including EGFR, ERK1/2, and Elk1. Cpd 5 could activate ERK1/2 either by signaling from an activated EGFR, which is upstream in the signaling cascade, or by direct inhibition of ERK1/2 phosphatase(s). Prolonged ERK1/2 phosphorylation strongly correlates with Cpd 5-mediated growth inhibition. Cpd 5 can also bind to and inhibit the Cdc25 family of dual specific phosphatases. As a result, several Cdc25 substrates (Cdk1, Cdk2, Cdk4) involved in cell cycle progression are tyrosine phosphorylated and thereby inhibited by its action. Cpd 5 could also inhibit both normal liver regeneration and hepatoma growth in vivo. DNA synthesis during rat liver regeneration following partial hepatectomy, transplantable rat hepatoma cell growth, and glutathione-S-transferase-pi expressing hepatocytes after administration of the chemical carcinogen diethylnitrosamine, are all inhibited by Cpd 5 administration. The growth inhibitory effect during liver regeneration and transplantable tumor growth is also correlated with ERK1/2 phosphorylation induced by Cpd 5. Thus, Cpd 5-mediated inhibition of PTPs, such as Cdc25 leads to cell growth arrest due to altered activity of key cellular kinases involved in signal transduction and cell cycle progression. This prototype K vitamin analog represents a novel class of growth inhibitor based upon its action as a selective PTP antagonist. It is clearly associated with prolonged ERK1/2 phosphorylation, which is in contrast with the transient ERK1/2 phosphorylation induced by growth stimulatory mitogens.

Sperm PP1gamma2 is regulated by a homologue of the yeast protein phosphatase binding protein sds22

Serine/threonine phosphatase PP1gamma2 is a testis-specific protein phosphatase isoform in spermatozoa. This enzyme appears to play a key role in motility initiation and stimulation. Catalytic activity of PP1gamma2 is higher in immotile compared with motile spermatozoa. Inhibition of PP1gamma2 activity causes both motility initiation and motility stimulation. Protein phosphatases, in general, are regulated by their binding proteins. The objective of this article is to understand the mechanisms by which PP1gamma2 is regulated, first by identifying its regulatory proteins. We had previously shown that a portion of bovine sperm PP1gamma2 is present in the cytosolic fraction of sperm sonicates. We purified PP1gamma2 from soluble bovine sperm extracts by immunoaffinity chromatography. Gel electrophoresis of the purified enzyme showed that it was complexed to a protein 43 M(r) x 10(-3) in size. Microsequencing revealed that this protein is a mammalian homologue of sds22, which is a yeast PP1 binding protein. Phosphatase activity measurements showed that PP1gamma2 complexed to sds22 is catalytically inactive. The complex cannot be activated by limited proteolysis. The complex is unable to bind to microcystin sepharose. This suggests that sds22 may block the microcystin binding site in PP1gamma2. A proportion of PP1gamma2 in sperm extracts, which is presumably not complexed to sds22, is catalytically active. Fluorescence immunocytochemistry was used to determine the intrasperm localization of PP1gamma2 and sds22. Both proteins are present in the tail. They are also present in distinct locations in the head. Our data suggest that PP1gamma2 binding to sds22 inhibits its catalytic activity. Mechanisms regulating sds22 binding to PP1gamma2 are likely to be important in understanding the biochemical basis underlying development and regulation of sperm function.

PNUTS (phosphatase nuclear targeting subunit) inhibits retinoblastoma-directed PP1 activity

Protein phosphatase type 1 catalytic subunit (PP1c) is a serine/threonine phosphatase involved in the dephosphorylation of many proteins in eukaryotic cells. It associates with several known targeting or regulatory subunits that directly regulate PP1c activity toward specific substrates. The recently identified Phosphatase Nuclear Targeting Subunit (PNUTS) binds to PP1c and inhibits PP1 activity toward phosphorylase a. One of the substrates of PP1c has been shown to be the cell cycle regulatory protein, Retinoblastoma (pRb). In this study, we show that PNUTS dissociates from PP1c under mildly hypoxic cell growth conditions that lead to an increase of PP1c activity toward pRb. We developed an assay that measures pRb-directed PP1c activity and show that a GST-PNUTS fusion protein inhibits phosphatase activity toward pRb when using PP1c from cell lysates, GST-PP1c, or purified PP1c. These studies suggest that PNUTS is involved in the regulation of PP1c activity toward pRb.

Heparin inhibits phosphorylation and autonomous activity of Ca(2+)/calmodulin-dependent protein kinase II in vascular smooth muscle cells

Vascular smooth muscle cell (VSMC) hyperproliferation is a characteristic feature of both atherosclerosis and restenosis seen after vascular surgery. A number of studies have shown that heparin inhibits VSMC proliferation in vivo and in culture. To test our hypothesis that heparin mediates its antiproliferative effect by altering Ca(2+) regulated pathways involved in mitogenic signaling in VSMC, we analyzed the effect of heparin on multifunctional Ca(2+)/calmodulin dependent protein kinase II (CaM kinase II) which is abundantly expressed in VSMC. Using activity assays, radioactive labeling, and immunoprecipitation it was found that heparin inhibits the overall phosphorylation of the delta-subunit of CaM kinase II which is consistent with inhibition of autophosphorylation-dependent, Ca(2+)/calmodulin-independent CaM kinase II activity. This effect was less evident in heparin-resistant cells, consistent with a role for CaM kinase II in mediating the antiproliferative effect of heparin. Finally, the effects of pharmacological inhibitors of phosphatases like okadaic acid, calyculin, and tautomycin suggest that heparin inhibits CaM kinase II phosphorylation by activating protein phosphatases 1 and 2A. These findings support the hypothesis that alterations in calcium-mediated mitogenic signaling pathways may be involved in the antiproliferative mechanism of action of heparin.

Initiation of eukaryotic DNA replication: regulation and mechanisms

The accurate and timely duplication of the genome is a major task for eukaryotic cells. This process requires the cooperation of multiple factors to ensure the stability of the genetic information of each cell. Mutations, rearrangements, or loss of chromosomes can be detrimental to a single cell as well as to the whole organism, causing failures, disease, or death. Because of the size of eukaryotic genomes, chromosomal duplication is accomplished in a multiparallel process. In human somatic cells between 10,000 and 100,000 parallel synthesis sites are present. This raises fundamental problems for eukaryotic cells to coordinate the start of DNA replication at each origin and to prevent replication of already duplicated DNA regions. Since these general phenomena were recognized in the middle of the 20th century the regulation and mechanisms of the initiation of eukaryotic DNA replication have been intensively investigated. These studies were carried out to find the essential factors involved in the process and to determine their functions during DNA replication. These studies gave rise to a model of the organization and the coordination of DNA replication within the eukaryotic cell. The elegant experiments carried out by Rao and Johnson (1970) (1), who fused cells in different phases of the cell cycle, showed that G1 cells are competent for replication of their chromosomes, but lack a specific diffusible factor required to activate their replicaton machinery and showed that G2 cells are incompetent for DNA replication. These findings suggested that eukaryotic cells exist in two states. In G1 phase, cells are competent to initiate DNA replication, which is subsequently triggered in S phase. After completion of S phase, cells in G2 are no longer able to initiate DNA replication and they require a transition through mitosis to reenable initiation of DNA replication to take place in the next S phase. The Xenopus cell-free replication system has proved a good model system in which to study DNA replication in vitro as well as the mechanism preventing rereplication within a single cell cycle (2). Studies using this system resulted in the development of a model postulating the existence of a replication licensing factor, which binds to chromatin before the G1-S transition and which is displaced during replication (2, 3). These results were supported by genetic and biochemical experiments in Saccharomyces cerevisiae (budding yeast) and Schizosaccharomyces pombe (fission yeast) (4, 5). The investigation of cell division cycle mutants and the budding yeast origin of replication resulted in the concept of a prereplicative and a postreplicative complex of initiation proteins (6-9). These three individual concepts have recently started to merge and it has become obvious that initiation in eukaryotes is generally governed by the same ubiquitous mechanisms.

Anticancer activity and protein phosphatase 1 and 2A inhibition of a new generation of cantharidin analogues

Cantharidin (Spanish Fly) is a natural toxin and an inhibitor of protein phosphatases 1 (PP1) and 2A (PP2A), which have key roles in cell cycle progression. We have synthesised two series of demethylated cantharidin analogues, one displaying an open-ring lactone configuration in solution (Novo-1 to Novo-5) similar to cantharidin, the other showing a closed-ring lactone configuration (Novo-6 to Novo-10). In the present study, these ten agents were screened for in vitro PP1 and PP2A inhibition and cellular cytotoxicity in nine cancer cell lines of haematopoietic (L1210, HL60), ovarian (A2780, ADDP), osteo (143B), and colon (HCT116, HT29, WiDr, SW480) origin and one normal colon cell line (CCD-018). The open-ring series (IC50, PPI=2.0-4.8 microM, PP2A=0.2-0.5 microM) maintained the PP2A selectivity of cantharidin (IC50, PPI=1.8 microM, PP2A=0.2 microM), although some were less potent. The closed-ring series (IC50, PPI=12.5->1000 microM, PP2A=5->1000 microM) were considerably less potent inhibitors, confirming the need of ring opening for inhibition. The cytotoxicity (IC50, 72 h, MTT assay) of cantharidin ranged from 6-15 microM, while the new analogues ranged from 14 to >1000 microM. Cytotoxicity of the agents did not consistently parallel the in vitro potency of protein phosphatase inhibition. A number of analogues showed colon cancer selectivity, particularly Novo-6, where the cytotoxicity ranged from 14-88 microM in the colon cancer cells and 275-680 microM in all other cell lines including normal colon cells. The reason for this selectivity was not apparent and may involve additional intracellular targets. Cell cycle analysis showed cantharidin to enhance cell cycle progression as evident from an increased S-phase population and enhanced DNA synthesis, culminating in G2/M arrest and apoptosis. With Novo-1 and Novo-6, the cell cycle changes paralleled the cytotoxicity responses, with the predominant effect of G2/M cell cycle arrest followed by cell death. In conclusion, we have synthesised new anticancer agents that show selective cytotoxicity in colon cancer cells while remaining inactive in normal colon cells, and which mediate their effects via the G2/M phase of the cell cycle.

Antiadrenergic effects of adenosine A(1) receptor-mediated protein phosphatase 2a activation in the heart

The ability of adenosine A(1) receptors to activate type 2a protein phosphatase (PP2a) and account for antiadrenergic effects was investigated in rat myocardial preparations. We observed that the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) significantly reduces the isoproterenol-induced increase in left ventricular developed pressure of isolated heats, and this effect is blocked by pretreatment of hearts with the PP2a inhibitor cantharidin. CPA alone or given in conjunction with isoproterenol stimulation decreases phosphorylation of phospholamban and troponin I in ventricular myocytes. These dephosphorylations are blocked by an adenosine A(1) receptor antagonist and by PP2a inhibition with okadaic acid. Adenosine A(1) receptor activation was also shown to increase carboxymethylation of the PP2a catalytic subunit (PP2a-C) and cause translocation of PP2a-C to the particulate fraction in ventricular myocytes. These results support the hypothesis that adenosine A(1) receptor activation leads to methylation of PP2a-C and subsequent translocation of the PP2a holoenzyme. Increases in localized PP2a activity lead to dephosphorylation of key cardiac proteins responsible for the positive inotropic effects of beta-adrenergic stimulation.

Relationship between protein phosphatase type-2C activity and induction of apoptosis in cultured neuronal cells

The cellular composition and concentration of fatty acids are crucial for proliferation and survival. We recently showed stimulation of protein phosphatase type-2C (PP2C) by unsaturated fatty acids. Here, we describe that treatment of cultured chick neurons with 100 microM oleic acid for 24h increased the percentage of damaged neurons to 61+/-9% compared with 25+/-4% in controls. Oleic acid-induced cell death showed features of apoptosis such as chromatin condensation, shrinkage of the nucleus, DNA fragmentation and caspase-3 activation. Extensive studies with a variety of fatty acids revealed a striking correlation between activation of PP2C and induction of apoptosis. Lipophilicity, oxidizability, and an acidic group were required for both effects. In addition, activation of PP2C and induction of apoptosis could discriminate between cis- and trans-conformation of the fatty acids. The results are in favor of PP2C playing an important, yet unidentified role in apoptosis.

Neurabins recruit protein phosphatase-1 and inhibitor-2 to the actin cytoskeleton

Inhibitor-2 (I-2) bound protein phosphatase-1 (PP1) and several PP1-binding proteins from rat brain extracts, including the actin-binding proteins, neurabin I and neurabin II. Neurabins from rat brain lysates were sedimented by I-2 and its structural homologue, I-4. The central domain of both neurabins bound PP1 and I-2, and mutation of a conserved PP1-binding motif abolished neurabin binding to both proteins. Microcystin-LR, a PP1 inhibitor, also attenuated I-2 binding to neurabins. Immunoprecipitation of neurabin I established its association with PP1 and I-2 in HEK293T cells and suggested that PP1 mediated I-2 binding to neurabins. The C terminus of I-2, although not required for PP1 binding, facilitated PP1 recruitment by neurabins, which also targeted I-2 to polymerized F-actin. Mutations that attenuated PP1 binding to I-2 and neurabin I suggested distinct and overlapping sites for these two proteins on the PP1 catalytic subunit. Immunocytochemistry in epithelial cells and cultured hippocampal neurons showed that endogenous neurabin II and I-2 colocalized at actin-rich structures, consistent with the ability of neurabins to target the PP1.I-2 complex to actin cytoskeleton and regulate cell morphology.

Interaction of protein phosphatase 1 delta with nucleolin in human osteoblastic cells

We examined the expression and cytolocalization of the protein phosphatase type 1 delta (PP1delta) isoform and nucleolin in human osteoblastic MG63 and Saos-2 cells. Cellular fractionation of MG63 cells was done and protein was prepared from each fraction. Anti-nucleolin antibody interacted with the 100- and 95-kD proteins present in the whole-cell lysate. The 100-kD protein was detected in nuclear and nucleolar fractions. The 95-kD protein was detected in cytosolic and nucleoplasmic fractions. PP1delta and nucleolin were co-localized in the nucleolus in MG63 and Saos-2 cells revealed by an immunofluorescence method. PP1delta and nucleolin were also co-immunoprecipitated with anti-nucleolin and anti-PP1delta antibodies. In the actinomycin D-treated cells, the subcellular localization of PP1delta and nucleolin was changed. Expression of PP1delta was upregulated with actinomycin D treatment. The level of 100-kD protein did not change in the actinomycin D-treated cells. However, the level of the 95-kD band increased with actinomycin D treatment. These results indicate that PP1delta was associated with nucleolin in the nucleolus of MG63 and Saos-2 cells and that nucleolin is a possible candidate substrate for PP1delta.

Regulation of focal adhesion kinase by a novel protein inhibitor FIP200

Focal adhesion kinase (FAK) is a major mediator of integrin signaling pathways. The mechanisms of regulation of FAK activity and its associated cellular functions are not very well understood. Here, we present data suggesting that a novel protein FIP200 functions as an inhibitor for FAK. We show the association of endogenous FIP200 with FAK, which is decreased upon integrin-mediated cell adhesion concomitant with FAK activation. In vitro- and in vivo-binding studies indicate that FIP200 interacts with FAK through multiple domains directly. FIP200 bound to the kinase domain of FAK inhibited its kinase activity in vitro and its autophosphorylation in vivo. Overexpression of FIP200 or its segments inhibited cell spreading, cell migration, and cell cycle progression, which correlated with their inhibition of FAK activity in vivo. The inhibition of these cellular functions by FIP200 could be rescued by coexpression of FAK. Last, we show that disruption of the functional interaction between endogenous FIP200 with FAK leads to increased FAK phosphorylation and partial restoration of cell cycle progression in cells plated on poly-L-lysine, providing further support for FIP200 as a negative regulator of FAK. Together, these results identify FIP200 as a novel protein inhibitor for FAK.

Alpha4 integrins and the immune response

The alpha4 integrins (alpha4beta1 and alpha4beta7) play multiple roles in the immune system. Alpha4 integrins impact hematopoiesis, leukocyte trafficking in immune surveillance and inflammation, and leukocyte activation and survival. To perform these functions, alpha4 integrins act as both adhesive and signaling receptors. Paxillin, a signaling adapter molecule, binds directly to the alpha4 subunit cytoplasmic domain, and its binding is regulated by serine phosphorylation of the alpha4 subunit. This regulated interaction of paxillin with the alpha4 subunit is likely to regulate the diverse functions of alpha4 integrins in the immune system. Furthermore, this protein-protein interaction may provide novel targets for the modulation of the immune response.

An attempt to promote neo-vascularization by employing a newly synthesized inhibitor of protein tyrosine phosphatase

Vascular endothelial growth factor (VEGF) and its receptors play a key role in angiogenesis. VEGF receptor-2 (VEGFR-2) has a tyrosine kinase domain, and, once activated, induces the phosphorylation of cytoplasmic signaling proteins. The phosphorylated VEGFR-2 may be a substrate for intracellular protein tyrosine phosphatases (PTPs) which prevent VEGF signaling. We synthesized a series of alpha,alpha-difluoro(phenyl)methylphosphonic acids (DFPMPAs) which inhibit the action of PTP. In this study, we test their effects on VEGF-induced angiogenesis. DFPMPA-3, the most effective inhibitor of human PTP-1B, promoted tube formation by human umbilical vein endothelial cells (HUVEC) on Matrigel more effectively than any other DFPMPAs. The inhibitor promoted the VEGF-induced proliferation and migration of HUVEC by inhibiting the dephosphorylation of VEGFR-2. Its effectiveness was proven through neo-vascularization in mice. The present findings suggest that targeting PTP to promote therapeutic neo-vascularization may be a potential strategy.

The Saccharomyces cerevisiae type 2A protein phosphatase Pph22p is biochemically different from mammalian PP2A

The Saccharomyces cerevisiae type 2A protein phosphatase (PP2A) Pph22p differs from the catalytic subunits of PP2A (PP2Ac) present in mammals, plants and Schizosaccharomyces pombe by a unique N-terminal extension of approximately 70 amino acids. We have overexpressed S. cerevisiae Pph22p and its N-terminal deletion mutant Delta N-Pph22p in the GS115 strain of Pichia pastoris and purified these enzymes to apparent homogeneity. Similar to other heterologous systems used to overexpress PP2Ac, a low yield of an active enzyme was obtained. The recombinant enzymes designed with an 8 x His-tag at their N-terminus were purified by ion-exchange chromatography on DEAE-Sephacel and affinity chromatography on Ni2+-nitrilotriacetic acid agarose. Comparison of biochemical properties of purified Pph22p and Delta N-Pph22p with purified human 8 x His PP2Ac identified similarities and differences between these two enzymes. Both enzymes displayed similar specific activities with 32P-labelled phosphorylase a as substrate. Furthermore, selected inhibitors and metal ions affected their activities to the same extend. In contrast to the mammalian catalytic subunit PP2Ac, but similar to the dimeric form of mammalian PP2A, Pph22p, but not Delta N-Pph22p, interacted strongly with protamine. Also with regard to the effects of protamine and polylysine on phosphatase activity Pph22p, but not Delta N-Pph22p, behaved similarly to the PP2Ac-PR65 dimer, indicating a regulatory role for the N-terminal extension of Pph22p. The N-terminal extension appears also responsible for interactions with phospholipids. Additionally Pph22p has different redox properties than PP2Ac; in contrast to human PP2Ac it cannot be reactivated by reducing agents. These properties make the S. cerevisiae Pph22p phosphatase a unique enzyme among all type 2A protein phosphatases studied so far.

Tight binding inhibition of protein phosphatase-1 by phosphatidic acid. Specificity of inhibition by the phospholipid

Phosphatidic acid (PA) has been identified as a bioactive lipid second messenger, yet despite extensive investigation, no cellular target has emerged as a mediator of its described biological effects. In this study, we identify the gamma isoform of the human protein phosphatase-1 catalytic subunit (PP1c gamma) as a high affinity in vitro target of PA. PA inhibited the enzyme dose-dependently with an IC(50) of 15 nm. Mechanistically, PA inhibited the enzyme noncompetitively with the kinetics of a tight binding inhibitor and a K(i) value of 0.97 +/- 0.24 nm. Together, these data describe one of the most potent in vitro effects of PA. To further elucidate the interaction between PA and PP1c gamma, structure/function analysis of the lipid was carried out using commercially available and synthetically generated analogs of PA. These studies disclosed that the lipid-protein interaction is dependent on the presence of the lipid phosphate as well as the presence of the fatty acid side chains, because lipids lacking either of these substituents resulted in complete loss of inhibition. However, the specific composition of the fatty acid side chains was not important for inhibition. Using 1-O-hexadecyl,2-oleoyl-PA, it was also shown that the carbonyl group of the sn-1 acyl linkage is not required for the lipid-protein interaction. Finally, using a lipid-protein overlay assay, it was demonstrated that PP1c gamma specifically and directly interacts with phosphatidic acid while not significantly binding other phospholipids. These results identify PA as a tight binding and specific inhibitor of PP1, and they raise the hypothesis that PP1c gamma may function as a mediator of PA action in cells. They also argue for the existence of a specific high affinity PA-binding domain on the enzyme.

Characterisation and expression of a PP1 serine/threonine protein phosphatase (PfPP1) from the malaria parasite, Plasmodium falciparum: demonstration of its essential role using RNA interference

BACKGROUND

Reversible protein phosphorylation is relatively unexplored in the intracellular protozoa of the Apicomplexa family that includes the genus Plasmodium, to which belong the causative agents of malaria. Members of the PP1 family represent the most highly conserved protein phosphatase sequences in phylogeny and play essential regulatory roles in various cellular pathways. Previous evidence suggested a PP1-like activity in Plasmodium falciparum, not yet identified at the molecular level.

RESULTS

We have identified a PP1 catalytic subunit from P. falciparum and named it PfPP1. The predicted primary structure of the 304-amino acid long protein was highly similar to PP1 sequences of other species, and showed conservation of all the signature motifs. The purified recombinant protein exhibited potent phosphatase activity in vitro. Its sensitivity to specific phosphatase inhibitors was characteristic of the PP1 class. The authenticity of the PfPP1 cDNA was further confirmed by mutational analysis of strategic amino acid residues important in catalysis. The protein was expressed in all erythrocytic stages of the parasite. Abrogation of PP1 expression by synthetic short interfering RNA (siRNA) led to inhibition of parasite DNA synthesis.

CONCLUSIONS

The high sequence similarity of PfPP1 with other PP1 members suggests conservation of function. Phenotypic gene knockdown studies using siRNA confirmed its essential role in the parasite. Detailed studies of PfPP1 and its regulation may unravel the role of reversible protein phosphorylation in the signalling pathways of the parasite, including glucose metabolism and parasitic cell division. The use of siRNA could be an important tool in the functional analysis of Apicomplexan genes.

Identification and biochemical characterisation of a protein phosphatase 5 homologue from Plasmodium falciparum

We report the identification of a new serine/threonine phosphatase from Plasmodium falciparum at the DNA and protein levels. A 1.8 kb cDNA fragment encoding the protein phosphatase was identified via PCR amplification. The sequence has a coding capacity of 594 amino acids. Immunoblot analysis of P. falciparum extracts showed that antibodies generated against the His(6)-fusion protein recognise a protein of approximately 80 kDa. The deduced amino acid sequence shares 55% identity with a mouse protein, identified as Protein Phosphatase 5 (PP5). We show that the P. falciparum PP5 homologue (PfPP5) has all structural and functional characteristics of this class of enzymes. It contains three tetratricopeptide repeats (TPR) and a nuclear targeting sequence at its N-terminus and a highly conserved C-terminal catalytic domain. Southern blot results are compatible with the existence of PfPP5 as a single copy gene. Purified recombinant protein, like the native protein enriched from P. falciparum extracts exhibited phosphatase activity that can be enhanced by both arachidonic and oleic acids, but not by myristic or stearic acid. In addition, the activity is inhibited by okadaic acid (OA) with an IC(50) of 4 nM. Immunofluorescence microscopy has localised PfPP5 preferentially to the nucleus. The function of PfPP5 is presently unclear, but like other PP5s of many eukaryotic organisms, it may have important regulatory functions in the parasite cell cycle.

Calyculin A-induced endothelial cell shape changes are independent of [Ca(2+)](i) elevation and may involve actin polymerization

Changes in endothelial cell (EC) shape result in inter-EC gap formation and subsequently regulate transendothelial passage. In this work, we investigated the effects of protein phosphorylation (induced by inhibition of protein phosphatases) on EC shape changes. Treatment of bovine pulmonary artery endothelial cells (BPAEC) with calyculin A (100 nM, an inhibitor of protein Ser/Thr phosphatases 1 and 2A) resulted in cell retraction, surface bleb formation and cell rounding. Trypan blue and electrophysiological experiments suggested that the plasma membrane of these rounded cells maintained functional integrity. Calyculin A-induced morphological changes were strongly inhibited by staurosporine, but not affected by specific inhibitors of the myosin light chain (MLC) kinase, protein kinases A, C and G, and tyrosine kinases. The calyculin A effects were not mimicked by phorbol myristate acetate, dibutyryl cAMP, 8-bromo-cGMP or ionomycin. Cytochalasin B (an inhibitor of actin polymerization) almost completely abolished such shape changes while colchicine (an inhibitor of microtubule polymerization) had no inhibitory effect at all. Ca(2+) imaging experiments showed that the morphological changes were not associated with any global or local cytosolic Ca(2+) concentration ([Ca(2+)](i)) elevation. The results suggest that calyculin A unmasked the basal activities of some protein Ser/Thr kinases other than MLC kinase and protein kinases A, C and G; these unknown kinases might cause BPAEC shape changes by a mechanism involving actin polymerization but not [Ca(2+)](i) elevation.

Identification of a potent and selective pharmacophore for Cdc25 dual specificity phosphatase inhibitors

Small molecules provide powerful tools to interrogate biological pathways but many important pathway participants remain refractory to inhibitors. For example, Cdc25 dual-specificity phosphatases regulate mammalian cell cycle progression and are implicated in oncogenesis, but potent and selective inhibitors are lacking for this enzyme class. Thus, we evaluated 10,070 compounds in a publicly available chemical repository of the National Cancer Institute for in vitro inhibitory activity against oncogenic, full-length, recombinant human Cdc25B. Twenty-one compounds had mean inhibitory concentrations of <1 microM; >75% were quinones and >40% were of the para-naphthoquinone structural type. Most notable was NSC 95397 (2,3-bis-[2-hydroxyethylsulfanyl]-[1,4]naphthoquinone), which displayed mixed inhibition kinetics with in vitro K(i) values for Cdc25A, -B, and -C of 32, 96, and 40 nM, respectively. NSC 95397 was more potent than any inhibitor of dual specificity phosphatases described previously and 125- to 180-fold more selective for Cdc25A than VH1-related dual-specificity phosphatase or protein tyrosine phosphatase 1b, respectively. Modification of the bis-thioethanol moiety markedly decreased enzyme inhibitory activity, indicating its importance for bioactivity. NSC 95397 showed significant growth inhibition against human and murine carcinoma cells and blocked G(2)/M phase transition. A potential Cdc25 site of interaction was postulated based on molecular modeling with these quinones. We propose that inhibitors based on this chemical structure could serve as useful tools to probe the biological function of Cdc25.

Induction of heme-binding protein 23/peroxiredoxin I gene expression by okadaic acid in cultured rat hepatocytes

Heme-binding protein 23 (HBP23), also termed peroxiredoxin I (Prx I), is an antioxidant protein that is induced by various oxidative stress stimuli. HBP23/Prx I has thioredoxin-dependent peroxidase activity and noncovalently binds the prooxidant heme with high affinity. To investigate the regulatory role of cellular phosphorylation and dephosphorylation events on hepatic HBP23/Prx I gene expression, primary cultures of rat hepatocytes were treated with okadaic acid (OA) which is a specific inhibitor of the serine threonine protein phosphatases 1 and 2A. In hepatocyte cultures HBP23/Prx I was highly expressed for up to 5 days and, both protein and mRNA levels of HBP23/Prx I were induced by OA. The time kinetics of OA-dependent HBP23/Prx I mRNA upregulation were coordinate to that of heme oxygenase (HO)-1, which is the inducible isoform of the rate-limiting enzyme of heme-degradation. In contrast to HO-1, however, induction of HBP23/Prx I mRNA by OA was downregulated by dibutyryl-cAMP, and was enhanced by the specific protein kinase A inhibitors KT5720 and H-89. HBP23/Prx I induction by OA occurred on the transcriptional level as determined by studies with actinomycin D and nuclear run-off assays.

Protein phosphatase 2A is the main phosphatase involved in the regulation of protein kinase B in rat adipocytes

In adipocytes, protein kinase B (PKB) has been suggested to be the enzyme that phosphorylates phosphodiesterase 3B (PDE3B), a key enzyme in insulin's antilipolytic signalling pathway. In order to screen for PKB phosphatases, adipocyte homogenates were fractionated using ion-exchange chromatography and analysed for PKB phosphatase activities. PKB phosphatase activity eluted as one main peak, which coeluted with serine/threonine phosphatases (PP)2A. In addition, adipocytes were incubated with inhibitors of PP. Incubation of adipocytes with 1 microM okadaic acid inhibited PP2A by 75% and PP1 activity by only 17%, while 1 microM tautomycin inhibited PP1 activity by 54% and PP2A by only 7%. Okadaic acid, but not tautomycin, induced the activation of both PKBalpha and PKBbeta. Finally, PP2A subunits were found in several subcellular compartments, including plasma membranes (PM) where the phosphorylation of PKB is thought to occur. In summary, our results suggest that PP2A is the principal phosphatase that dephosphorylates PKB in adipocytes.

Protein phosphatases 1 and 2A transiently associate with myosin during the peak rate of secretion from mast cells

Mast cells undergo cytoskeletal restructuring to allow secretory granules passage through the cortical actomyosin barrier to fuse with the plasma membrane and release inflammatory mediators. Protein phosphorylation is believed to regulate these rearrangements. Although some of the protein kinases implicated in this phosphorylation are known, the relevant protein phosphatases are not. At the peak rate of antigen-induced granule mediator release (2.5 min), protein phosphatases PP1 and PP2A, along with actin and myosin II, are transiently relocated to ruffles on the apical surface and a band at the peripheral edge of the cell. This leaves an area between the nucleus and the peripheral edge significantly depleted (3-5-fold) in these proteins. Phorbol 12-myristate 13-acetate (PMA) plus A23187 induces the same changes, at a time coincident with its slower rate of secretion. Coimmunoprecipitation experiments demonstrated a significantly increased association of myosin with PP1 and PP2A at the time of peak mediator release, with levels of association decreasing by 5 min. Jasplakinolide, an inhibitor of actin assembly, inhibits secretion and the cytoskeletal rearrangements. Surprisingly, jasplakinolide also affects myosin, inducing the formation of short rods throughout the cytoplasm. Inhibition of PP2A inhibited secretion, the cytoskeletal rearrangements, and led to increased phosphorylation of the myosin heavy and light chains at protein kinase C-specific sites. These findings indicate that a dynamic actomyosin cytoskeleton, partially regulated by both PP1 and PP2A, is required for mast cell secretion.

Serine (threonine) phosphatase(s) acting on cAMP-dependent phosphoproteins in mammalian mitochondria

Immunochemical and functional evidence showing the existence in the inner membrane and matrix fraction of mammalian mitochondria of serine/threonine phosphatases acting on cAMP-dependent phosphoproteins is presented. Mg(2+)-dependent Ca(2+)-inhibitable PP2C phosphatase, associated to the inner membrane, dephosphorylates the 18 kDa (NDUFS4 gene) of complex I.

Anticancer activity and protein phosphatase 1 and 2A inhibition of a new generation of cantharidin analogues

Cantharidin (Spanish Fly) is a natural toxin and an inhibitor of protein phosphatases 1 (PP1) and 2A (PP2A), which have key roles in cell cycle progression. We have synthesised two series of demethylated cantharidin analogues, one displaying an open-ring lactone configuration in solution (Novo-1 to Novo-5) similar to cantharidin, the other showing a closed-ring lactone configuration (Novo-6 to Novo-10). In the present study, these ten agents were screened for in vitro PP1 and PP2A inhibition and cellular cytotoxicity in nine cancer cell lines of haematopoietic (L1210, HL60), ovarian (A2780, ADDP), osteo (143B), and colon (HCT116, HT29, WiDr, SW480) origin and one normal colon cell line (CCD-018). The open-ring series (IC50, PPI=2.0-4.8 microM, PP2A=0.2-0.5 microM) maintained the PP2A selectivity of cantharidin (IC50, PPI=1.8 microM, PP2A=0.2 microM), although some were less potent. The closed-ring series (IC50, PPI=12.5->1000 microM, PP2A=5->1000 microM) were considerably less potent inhibitors, confirming the need of ring opening for inhibition. The cytotoxicity (IC50, 72 h, MTT assay) of cantharidin ranged from 6-15 microM, while the new analogues ranged from 14 to >1000 microM. Cytotoxicity of the agents did not consistently parallel the in vitro potency of protein phosphatase inhibition. A number of analogues showed colon cancer selectivity, particularly Novo-6, where the cytotoxicity ranged from 14-88 microM in the colon cancer cells and 275-680 microM in all other cell lines including normal colon cells. The reason for this selectivity was not apparent and may involve additional intracellular targets. Cell cycle analysis showed cantharidin to enhance cell cycle progression as evident from an increased S-phase population and enhanced DNA synthesis, culminating in G2/M arrest and apoptosis. With Novo-1 and Novo-6, the cell cycle changes paralleled the cytotoxicity responses, with the predominant effect of G2/M cell cycle arrest followed by cell death. In conclusion, we have synthesised new anticancer agents that show selective cytotoxicity in colon cancer cells while remaining inactive in normal colon cells, and which mediate their effects via the G2/M phase of the cell cycle.

Protein phosphatase 1 – targeted in many directions

Protein phosphatase 1 (PP1) is a major eukaryotic protein serine/threonine phosphatase that regulates an enormous variety of cellular functions through the interaction of its catalytic subunit (PP1c) with over fifty different established or putative regulatory subunits. Most of these target PP1c to specific subcellular locations and interact with a small hydrophobic groove on the surface of PP1c through a short conserved binding motif – the RVxF motif – which is often preceded by further basic residues. Weaker interactions may subsequently enhance binding and modulate PP1 activity/specificity in a variety of ways. Several putative targeting subunits do not possess an RVxF motif but nevertheless interact with the same region of PP1c. In addition, several ‘modulator’ proteins bind to PP1c but do not possess a domain targeting them to a specific location. Most are potent inhibitors of PP1c and possess at least two sites for interaction with PP1c, one of which is identical or similar to the RVxF motif.Regulation of PP1c in response to extracellular and intracellular signals occurs mostly through changes in the levels, conformation or phosphorylation status of targeting subunits. Understanding of the mode of action of PP1c complexes may facilitate development of drugs that target particular PP1c complexes and thereby modulate the phosphorylation state of a very limited subset of proteins.

Protein phosphatase type-1 from skeletal muscle of the freeze-tolerant wood frog

We evaluated the effects of freezing, dehydration and anoxia stresses on muscle PP-1 activity in the freeze-tolerant amphibian, Rana sylvatica. In addition, PP-1 catalytic subunit (PP-1c) was purified to homogeneity to assess the biochemical properties of the enzyme from a freeze-tolerant vertebrate. Freezing stimulated a rise in the amount of active PP-1 (70% above the control) at 20 min post-nucleation. With longer freezing (1-12 h), the amount of active enzyme returned to control levels, and the amount of total PP-1 fell, decreasing by up to 43%. This decline in total PP-1 kept the % active at a high value throughout the freeze. Anoxia exposure (12 h) reduced the active PP-1 by 60%, but had no effect on total PP-1 activity. Neither dehydration nor rehydration had any significant effect on the amounts of either total or active PP-1. PP-1 activity associated with the myofibril fraction increased, while activity associated with the glycogen pellet decreased in response to freezing and dehydration, but not anoxia. Purified frog PP-1c showed a variety of properties that are typical of the enzyme from other sources. In addition, the enzyme was strongly inhibited by AMP and weakly by ADP and ATP; the physiological relevance of inhibition by nucleotides remains to be determined. Overall, the results suggest an important role for PP-1 in signal transduction in the skeletal muscle of freeze-tolerant amphibians.