The possible involvement of protein phosphatase 1 in thrombin-induced Ca2+ influx of human platelets

Protein phosphatase 1 in tumorigenesis: is it worth a closer look?

Cancer cells take advantage of signaling cascades to meet their requirements for sustained growth and survival. Cell signaling is tightly controlled by reversible protein phosphorylation mechanisms, which require the counterbalanced action of protein kinases and protein phosphatases. Imbalances on this system are associated with cancer development and progression. Protein phosphatase 1 (PP1) is one of the most relevant protein phosphatases in eukaryotic cells. Despite the widely recognized involvement of PP1 in key biological processes, both in health and disease, its relevance in cancer has been largely neglected. Here, we provide compelling evidence that support major roles for PP1 in tumorigenesis.

The Phosphatase Inhibitor Calyculin-A Impairs Clot Retraction, Platelet Activation, and Thrombin Generation

The aim of this study was to investigate the effect of the serine/threonine protein phosphatase inhibitor, calyculin-A (CLA), on clot formation and on the procoagulant activity of human platelets. Platelet-rich plasma (PRP) samples were preincubated with buffer or CLA and subsequently platelets were activated by the protease-activated receptor 1 (PAR-1) activator, thrombin receptor activating peptide (TRAP). Clot retraction was detected by observing clot morphology up to 1 hour, phosphatidylserine- (PS-) expression was studied by flow cytometry, and thrombin generation was measured by a fluorimetric assay. For the intracellular Ca²⁺ assay, platelets were loaded with calcium-indicator dyes and the measurements were carried out using a ratiometric method with real-time confocal microscopy. CLA preincubation inhibited clot retraction, PS-expression, and thrombin formation. TRAP activation elicited Ca²⁺ response and PS-expression in a subset of platelets. The activated PRP displayed significantly faster and enhanced thrombin generation compared to nonactivated samples. CLA pretreatment abrogated PS-exposure and clot retraction also in TRAP-activated samples. As a consequence of the inhibitory effect on calcium elevation and PS-expression, CLA significantly downregulated thrombin generation in PRP. Our results show that CLA pretreatment may be a useful tool to investigate platelet activation mechanisms that contribute to clot formation and thrombin generation.

Protein kinase C inhibits the transplasma membrane influx of Ca2+ triggered by 4-aminopyridine in Jurkat T lymphocytes

4-aminopyridine (4AP) is a general blocker of voltage-dependent K+ channels. This pyridine derivative has also been shown to inhibit T cell proliferation, to modulate immune responses and to alleviate some of the symptoms associated with neurological disorders such as multiple sclerosis, myasthenia gravis and Alzheimer's disease. 4AP triggers a Ca2+ response in lymphocytes, astrocytes, neurons and muscle cells but little is known about the regulation of the 4AP response in these cells. We report that 4AP induced a non-capacitative transplasma membrane influx of Ca2+ in Jurkat T lymphocytes. The influx of Ca2+ was not affected by activation or inhibition of protein kinase A (PKA). In contrast, activation of protein kinase C (PKC) by phorbol myristyl acetate (PMA), mezerein or 1-oleoyl-2-acetyl-sn-glycerol (OAG) inhibited the influx of Ca2+ triggered by 4AP. The inhibitory effect of PKC could be prevented by prior exposure of the cells to the PKC inhibitor GF 109203X. Under these conditions, mezerein and OAG no longer inhibited the 4AP-dependent Ca2+ response. Inhibition of serine and threonine protein phosphatases PP1 and PP2A by treating the cells with calyculin A (CalA) reduced the Ca2+ response to 4AP. Okadaic acid (OA) had no effect, suggesting an involvement of PP1. A combination of CalA and OAG (or PMA) abolished the influx of Ca2+ induced by 4AP, adding further evidence to the importance of protein phosphorylation in the modulation of the 4AP response. Our data suggest that the transplasma membrane influx of Ca2+ triggered by 4AP in Jurkat T cells can be modulated by the opposite actions of PKC and protein serine and threonine phosphatase(s).

Phosphorylation sites in the integrin beta3 cytoplasmic domain in intact platelets

Protein seryl/threonyl phosphatase inhibitors such as calyculin A block inside-out and outside-in platelet signaling. Our studies demonstrate that the addition of calyculin A blocks platelet adhesion and spreading on fibrinogen, responses that depend on integrin alphaIIb beta3 signaling. We hypothesized that this reflects a change in alphaIIb beta3 structure caused by a specific state of phosphorylation. We show that addition of calyculin A leads to increased phosphorylation of the beta3 subunit, and phosphoamino acid analysis reveals that only threonine residues become phosphorylated; sequence analysis by Edman degradation established that threonine 753 became stoichiometrically phosphorylated during inhibition of platelet phosphatases by calyculin A. This region of beta3 is linked to outside-in signaling such as platelet spreading responses. The effect of calyculin A on platelet adhesion and spreading and on the phosphorylation of T-753 in beta3 is reversed by the calcium ionophore A23187, demonstrating that these effects of calyculin A are not generally toxic ones. We propose that phosphorylation of beta3 on threonine 753, a region of beta3 linked to outside-in signaling, may be a mechanism by which integrin alphaIIb beta3 function is regulated.

Protein phosphatase inhibitors exert specific and nonspecific effects on calcium influx in thapsigargin-treated human neutrophils

C2-ceramide but not inhibitors of phosphatase types 1 and 2A (okadaic acid, calyculin A, tautomycin) blocked store-regulated Ca2+ entry induced in human neutrophils by thapsigargin. This contrasts with previous results showing that both types of compounds inhibit Ca2+ influx in fmet-leu-phe-treated cells. In present studies, phosphatase inhibitors increased the rate of secondary Ca2+ influx in a temperature-dependent manner. Their mechanism of action appeared to be independent of phosphatase inhibition since the inactive congeners, norokadaone and tetraacetyl okadaic acid, also potentiated Ca2+ influx at similar concentrations. When Ca2+ stores were predischarged by thapsigargin, okadaic acid but not norokadaone acted synergistically with fMLP to inhibit subsequent Ca2+ entry. Results suggest that blockade of Ca2+ influx in neutrophils is mediated by a phosphorylation reaction that is prolonged by phosphatase inhibitors. The requisite phosphorylation occurs in fMLP-activated cells but may be absent in cells incubated with thapsigargin.

Membrane-destabilizing properties of C2-ceramide may be responsible for its ability to inhibit platelet aggregation

We have studied the effects of short-chain ceramides on platelet structure and function. N-Acetylsphingosine (C2-ceramide), a cell-permeable short-chain analogue, and N-acetyldihydrosphingosine (C2-dihydroceramide), which lacks the 4-5 double bond, have been investigated. C2-Ceramide (15 microM) inhibited ADP-induced aggregation by 50% at a platelet concentration of 1.25 x 10(8)/mL, while it took twice that concentration to inhibit aggregation by 50% when the platelet concentration was doubled. This indicates that the effect of C2-ceramide on ADP-induced platelet aggregation depends on the ratio of ceramide to total platelet lipid, with a ratio of 0.2 giving significant inhibition. C2-Ceramide at a ceramide: lipid ratio of 0.2 caused platelets to form fenestrations and pseudopodia which were longer and thinner than those caused by agonists such as ADP or thrombin. C2-Dihydroceramide had no effect on ADP-induced aggregation or platelet morphology at any ceramide:lipid ratio. Platelet lysis was induced by C2-ceramide at higher ceramide:lipid ratios (0.5), whereas C2-dihydroceramide did not induce lysis, suggesting that C2-ceramide is able to destabilize membranes. This was tested directly by assessing whether the ceramides induced leakage of 6-carboxyfluorescein from lipid vesicles. C2-Ceramide caused nearly total leakage of dye from the vesicles at a ceramide:lipid ratio of 10. The leakage caused by C2-dihydroceramide at a ceramide:lipid ratio of 10 was equal to that induced by C2-ceramide at a ratio of 0.2 (approximately 3%). The ability of the ceramides to destabilize membranes was also examined by measuring changes in fluorescence anisotropy of the fluorescent dye 1,6-diphenyl-1,3,5-hexatriene (DPH) incorporated into lipid vesicles. C2-Ceramide induced a larger decrease in anisotropy than a detergent (Triton X-100) which is known to lyse membranes. C2-Dihydroceramide did not alter membrane fluidity. The ability of C2-ceramide to cause platelet fenestrations, formation of irregular platelet pseudopodia, platelet lysis, lipid vesicle leakage, and increases in the fluidity of lipid vesicles all suggest that C2-ceramide inhibits platelet aggregation because it destabilizes the platelet membrane. C2-Dihydroceramide did not inhibit platelet aggregation and lacked the nonspecific effects on membranes that C2-ceramide possessed, suggesting that C2-dihydroceramide is not an appropriate control for the nonspecific effects of C2-ceramide.

C6-ceramide maintains elevated cytosolic calcium levels in activated platelets

The effects of cell-permeable C2 and C6-ceramides on human platelet responses were investigated. In thrombin-activated platelets, C6(5-30 microM) potentiated Ca2+ mobilization and Ca2+ influx, and decreased the rate of removal of Ca2+ from cytosol. The effect of C2 was not significant. Phorbol ester or calyculin A inhibition of thrombin-induced rises in platelet [Ca2+]i was attenuated by C6. Assays show that C6 either prolonged the generation, or retarded the metabolism of inositol trisphosphates. Previous studies indicate that protein kinase C (PKC) acts in a negative feedback manner by inhibiting phosphatidylinositol breakdown, accelerating inositol trisphosphate metabolism, and increasing Ca2+ pump activity. C6 may counter these PKC effects indirectly. The synthetic ceramides inhibited platelet aggregation weakly and had no effect on pleckstrin (p47) phosphorylation. Recently we reported that C2 but not C6 inhibits superoxide generation and store-regulated Ca2+ influx in neutrophils at similar concentrations. Cellular differences in ceramide metabolism or ceramide-sensitive enzymes and their substrates may account for the disparate results.

Aggregation-dependent signaling in human platelets is sensitive to protein serine/threonine phosphatase inhibitors

When platelets are stimulated by the addition of thrombin, a series of temporally linked signaling events are initiated. Some of the early events are needed to engage the integrin glycoprotein (GP) IIb-IIIa in a high-affinity state. This in turn leads to aggregation, which initiates a wave of events distinct from those triggered by thrombin. Platelet responses are sensitive to protein serine/threonine phosphatase inhibitors, but which events are dependent on protein phosphatase activity is not known. In the present studies, the effect of the phosphatase inhibitor calyculin A on aggregation-induced signaling was examined. The addition of 0.2 unit/mL thrombin caused aggregation-dependent redistribution of cytoskeletal proteins (actin binding protein, talin, vinculin, and alpha-actinin), glycoproteins (GPIIb-IIIa, PECAM), and signaling molecules (PI3-kinase, pp60c-src) to the cytoskeletal fraction of platelets. Addition of 1-2 microM calyculin A blocked the ability of 0.2 unit/mL thrombin to induce aggregation and the association of these molecules with the cytoskeleton. Aggregation (60-80% of control) was restored if 1 unit/mL thrombin was added, but there was no corresponding redistribution of actin binding protein, talin, vinculin, alpha-actinin, GPIIb-IIIa, PECAM, PI3-kinase, and pp60c-src to the cytoskeleton. Treatment of platelets with calyculin A resulted in an increase in the phosphorylation state of a membrane skeletal protein of 50 kDa. These data strongly suggest that platelet aggregation is dissociable from aggregation-induced signaling, which is dependent on type 1 and 2A phosphatase activities.

Characterization of a potent platelet aggregation inducer from Cerastes cerastes (Egyptian sand viper) venom

A potent, proteinaceous inducer of platelet aggregation designated as IVa, has been purified to homogeneity from Cerastes cerastes venom by molecular sieve and ion exchange chromatography. It is composed of 2 subunits with total M(r) of 62,000 as shown by native gel chromatography and chemical cross-linking with disuccinimidyl suberate. It is not clear at the present time whether both subunits are identical gene products, however, both have identical N-terminal sequences for the first 15 amino acids. The protein has a pI above 9.6. IVa (0.1 micrograms/ml) could aggregate platelets up to 80% and was inhibited by p-APMSF, leupeptin, iodoacetamide, protein kinase C inhibitor, phosphatase inhibitor, ATP and PGE1, while it was insensitive to acetylsalicylic acid, ADP scavenger system, protein kinase A inhibitor and hirudin. Protein IVa is a serine proteinase with thrombin-like activity as it hydrolysed thrombin chromogenic substrate CBS 34.47, its aggregatory activity was partially inhibited by monoclonal antibodies against GPIb and the thrombin receptor, as was the thrombin, and its ability to induce intracellular Ca2+ release was blocked by pretreating platelets with thrombin. Unlike thrombin, the IVa protein showed very weak coagulant activity as indicated by plasma recalcification time and fibrinogen clotting time although it could hydrolyse fibrinogen alpha-chains.

Cytoskeletal reorganization of human platelets induced by the protein phosphatase 1/2 A inhibitors okadaic acid and calyculin A

Okadaic acid (OA) and calyculin A (CLA), which are potent and specific inhibitors of serine/threonine protein phosphatases type 1 and 2A, have been shown to induce drastic changes in platelet morphology. The aim of this study was to analyse the molecular mechanisms of OA- or CLA-induced cytoskeletal reorganization, with a specific focus on microtubules and actin filaments. Confocal fluorescence microscopy revealed that OA or CLA altered the distribution of microtubules from marginal band arrangements to homogeneous patterns, consistent with the transmission-electron-microscopic finding that microtubules were fragmented and redistributed into pseudopod-like processes. In thrombin-activated platelets, OA or CLA induced extremely long pseudopods containing an array of microtubules and actin filaments, and a condensed mass of actin filaments in the centre of platelets. OA or CLA induced the constriction of actin filaments without an increase in filamentous (F)-actin, and also rather significantly inhibited actin polymerization in thrombin-activated platelets. Furthermore, neither OA or CLA enhanced phosphorylation of myosin light chain (MLC). By immunoprecipitation of platelet lysate with anti-alpha-tubulin antibody, a 90 kDa protein was co-precipitated with tubulin and was predominantly phosphorylated in the presence of OA. As the time-dependent phosphorylation of 90 kDa protein correlated well with the reorganization of microtubules, these data suggest that phosphorylation and dephosphorylation of this protein might play a role in the regulation of microtubule organization. These findings indicate that OA or CLA induces reorganization of microtubules and actin filaments via the phosphorylation of a microtubule-associated 90 kDa protein and an MLC-phosphorylation-independent mechanism. mechanism.

N-acetylsphingosine (C2-ceramide) inhibited neutrophil superoxide formation and calcium influx

Ceramide, a product arising from sphingomyelinase activity, has been shown to act as an intracellular second messenger in effecting growth inhibition, cellular differentiation, and apoptosis. In the present study, the relative effects of cell-permeable ceramides, N-acetylsphingosine (C2-ceramide) and N-hexanoylsphingosine (C6-ceramide), on neutrophil responses were measured. When cells were activated with fMet-Leu-Phe, C2-ceramide both potentiated (< 1 microM) and inhibited (> 1 microM) superoxide generation. C2- and C6-ceramide inhibited phorbol ester-induced superoxide release from neutrophils at IC50 values of 5 and 120 microM, respectively. C2-ceramide had no effect on semipurified protein kinase C activity. Neither ceramide affected significantly the general level of phosphorylated proteins in phorbol ester-treated cells. C2-ceramide (1-20 microM) alone did not change cytosolic free Ca2+ levels but inhibited Ca2+ and Mn2+ influx in fMet-Leu-Phe-activated neutrophils. In contrast, sphingosine enhanced Ca2+ entry; thus, ceramide conversion to sphingosine was not significant. Unlike C2-ceramide, C2-dihydroceramide failed to block superoxide generation or Ca2+ influx. Preincubation of cells with 10 nM okadaic acid reversed slightly the effects of C2-ceramide. Calyculin A, tautomycin, and much higher concentrations of okadaic acid inhibited agonist-induced Ca2+ influx. We postulate that C2-ceramide may inhibit neutrophil superoxide release by activation of type 2A protein phosphatases. Results suggest that protein phosphatase type 1 up-regulates Ca2+ entry, whereas type 2A (or a ceramide-activated subtype) forestalls Ca2+ entry by inactivating a calcium influx factor.

Platelet talin is phosphorylated by calyculin A

Calyculin A and okadaic acid, potent and cell permeable inhibitors of type 1 and type 2A protein phosphatases, inhibit platelet aggregation and secretion. However, the relationship between phosphatase inhibition and inhibition of platelet function is not well understood. We found that in unstimulated platelets, talin (P235) was phosphorylated at threonine residues by calyculin A. Furthermore, the extent of talin phosphorylation by calyculin A was closely correlated with its inhibition of thrombin-induced platelet aggregation. Since the binding of talin to platelet glycoprotein IIb/IIIa complex has been shown to be affected by its phosphorylation, these results suggest that type 1 and/or type 2A protein phosphatases may play a role in the regulation of membrane-cytoskeleton interaction through dephosphorylation of talin.

Role of type 1 and type 2A phosphatases in signal transduction of platelet-activating-factor-stimulated rabbit platelets

Calyculin A, the potent inhibitor of type 1 (PP1) and type 2A (PP2A) phosphatases, has been employed in order to investigate the role of endogenously activated PP1/PP2A in the signal-transduction pathway of platelet-activating-factor (PAF)-stimulated platelets. Calyculin A alone caused an increase in protein phosphorylation in unstimulated platelets, with the detection of a number of newly phosphorylated proteins, whereas in PAF-stimulated platelets phosphorylation of the major substrates of protein kinase C and myosin light-chain kinase were no longer transient, but phosphorylation was sustained. PP1/PP2A appear to play a role in Ca2+ homoeostasis, as inhibition of PP1/PP2A caused an inhibition of Ca2+ mobilization and Ca2+ influx through the plasma membrane in PAF-stimulated platelets. The effect of calyculin A on Ca2+ mobilization correlated with the observed inhibition of the production of the signal molecule Ins(1,4,5)P3. The release reaction (which is a Ca(2+)-dependent event) was also inhibited by calyculin A. The results are discussed in relation to the possible role of protein kinase C in mediating the events leading to the effects observed with calyculin A.

Phosphatase inhibitors suppress Ca2+ influx induced by receptor-mediated intracellular Ca2+ store depletion in human platelets

The effects of three phosphatase inhibitors including okadaic acid, calyculin A and tautomycin were evaluated on platelet Ca2+ mobilization. Calyculin A and tautomycin at appropriate concentrations appeared to have a selective inhibitory effect on thrombin-induced Ca2+ influx, but not on [Ca2+]i release from intracellular Ca2+ storage sites. In contrast, pretreatment with okadaic acid at concentrations that effectively lowered Ca2+ influx also suppressed Ca2+ release from intracellular Ca2+ stores. In a system that specifically evaluates the effects of agents on Ca2+ influx induced by the Ca(2+)-depleted state of intracellular Ca2+ storage sites, the three phosphatase inhibitors attenuated Ca2+ influx in a dose dependent manner and showed complete inhibition at appropriate concentrations. These findings suggest that protein phosphorylation/dephosphorylation plays an important role in mediating signals to open Ca2+ channels when Ca2+ depletion in intracellular Ca2+ stores is caused by thrombin. In contrast, Ca2+ influx induced by thapsigargin, a Ca(2+)-ATPase inhibitor, was only partially suppressed by pretreatment with each of the three phosphatase inhibitors. Based on these findings, we suggest that the Ca(2+)-depleted state of intracellular Ca2+ stores by thapsigargin induces the opening of Ca2+ channels via phosphatase inhibitor-insensitive pathways. All the phosphatase inhibitors, at the highest concentrations tested in the present study, only partially inhibited Mn2+ entry induced by thrombin. These findings suggest that there are at least two types of divalent ion channels on platelet plasma membranes and that one of them, that preferentially allows Mn2+ entry, is resistant to the inhibitory effects of phosphatase inhibitors.