Phosphorylation of the inhibitory guanine-nucleotide-binding protein as a possible mechanism of inhibition by protein kinase C of agonist-induced Ca2+ mobilization in human platelet

Increases in the intracellular Ca2+ concentration of human platelets caused by receptor agonists, such as thrombin, 9,11-epithio-11,12-methanothromboxane A2 (STA2), platelet-activating factor (PAF) and arginine-vasopressin, were inhibited by prior addition of 12-O-tetradecanoylphorbol 13-acetate (TPA) in time-dependent and concentration-dependent manners. The inhibitions were mostly reversed by staurosporine, and inhibitor of protein kinase C, added 1 min before TPA. Prior treatment of platelets with thrombin or STA2, the efficacious Ca2+ mobilizer, suppressed the increase in the intracellular Ca2+ concentration of the cells to other agonists, but treatment with less efficacious PAF or vasopressin did not. The heterologous receptor desensitizations were also reversed by staurosporine. The antibody, directed against the carboxy-terminal region of the alpha subunits 1 and 2 of the inhibitory guanine-nucleotide-binding proteins (Gi1 alpha and Gi2 alpha), was raised in rabbit and was used to immunoprecipitate Gi alpha in 32P-labeled platelets. The radioactivity was detected in Gi alpha after incubation of 32P-labeled platelets with TPA, thrombin or STA2, but not in the cells incubated with PAF or vasopressin. The time-dependency or concentration-dependency of TPA-induced phosphorylation of Gi alpha was similar to the dependency of its inhibitory action on agonist-induced Ca2+ mobilization. Thus, strong activation of Ca2+/phospholipid-dependent protein kinase C by phorbol ester or agonists of certain Ca(2+)-mobilizing receptors leads to phosphorylation of the alpha subunit of guanine-nucleotide-binding protein, thereby impairing the coupling of the G protein to receptors as a feedback regulatory component of the receptor-triggered intracellular Ca(2+)-mobilizing system.

Thrombin-induced calcium oscillation in human platelets and MEG-01, a megakaryoblastic leukemia cell line

Digital imaging microscopy revealed that human platelets show periodic intracellular Ca++ elevation in response to 0.01 U/ml thrombin. MEG-01, a megakaryoblastic leukemia cell line, also responded with oscillatory intracellular Ca++ elevation (0.7-1 times/min) to thrombin (0.001-0.003U/ml). Ca++ transients appears to be fused with higher thrombin doses. With extracellular Ca++ concentrations of 0.1 mM or less, Ca++ oscillation could not be elicited, or even when present, it disappeared after a few spikings of [Ca++]i. Extracellular Ca++ concentrations of 0.3 mM or more were required to facilitate ongoing Ca++ oscillation, suggesting an important role of Ca++ influx for Ca++ oscillation.

[Evaluation of platelet intracellular calcium ion concentrations with flow cytometry]

Platelet intracellular Ca++ ([Ca++] i) was measured by flow cytometry, using a new Ca(++)-sensitive fluorescent dye, fluo 3. The acetoxymethyl derivative of fluo 3, fluo 3 AM, was incorporated into platelets most efficiently in the presence of 1.5 micrograms/ml pluronic F-127, a surfactant often used to facilitate intracellular incorporation of lipophilic agents. [Ca++] i measurement with flow cytometry proved to be more sensitive than that with ordinary fluorescence spectrophotometers, detecting cells with elevated [Ca++] i at much lower agonist concentrations. Two-dimensional analysis using forward scatter intensities and [Ca++] i more clearly defined a subset of platelets responsive to low concentrations of agonists. In normal subjects [Ca++] i hardly changed in response to low-dose thrombin (0.002 U/ml), while it invariably showed a marked increase in response to high-dose thrombin (0.02 U/ml). Thus, these dose of thrombin were used to evaluate clinically whether platelets was hyper-reactive or hypo-reactive. Of 44 patients with diabetes mellitus, 6 patients had hypo-responsive platelets, while platelets were sensitive to low concentrations of thrombin in 3 patients. Since such differences cannot be detected by conventional spectrophotometric methods, [Ca++] i measurement by flow cytometry may prove useful tool for clinical evaluation of platelet function.

Evaluation of platelet calcium ion mobilization by the use of various divalent ions

Divalent ion mobilization in human platelets was evaluated with Fura-2 fluorescence changes induced by Ca2+, Sr2+, Ba2+ and Mn2+. Extracellular Ca2+, Sr2+ and Ba2+ all entered thrombin-stimulated platelets. These divalent ions were also able to refill the intracellular Ca2+ storage sites which had been depleted of Ca2+ by ionomycin treatment, and were released from the storage sites upon thrombin stimulation. However, only the refilling of the storage sites with Ca2+ and Sr2+, but not with Ba2+, were capable of suppressing the opening state of Ca2+ channels assessed with Mn2+ influx. Efflux of intracellularly accumulated divalent ions was observed with Ca2+ and Sr2+ but not with Ba2+. These findings indicate that there are subtle differences in the Ca(2+)-binding domains of the various systems involved in Ca2+ mobilization in platelets, some of which discriminate Ba2+ while accepting Sr2+.

Characterization of prostaglandin and thromboxane receptors expressed on a megakaryoblastic leukemia cell line, MEG-01s

MEG-01s, an established human megakaryoblastic leukemia cell line, exhibited specific high-affinity binding sites for [3H]iloprost, a stable prostaglandin (PG) I2 analogue, for [3H]SQ-29548, a stable thromboxane (TX) A2 antagonist and, for [3H]PGE2/PGE1, but not for [3H]PGD2. In the MEG-01s cells, iloprost/PGI2, or PGE1 stimulated cAMP production with ED50 values practically identical to the IC50 values for the [3H] iloprost binding. STA2 and U46619, TXA2/PGH2 agonists, PGE2/PGE1, iloprost/PGI2, and thrombin elevated the intracellular concentrations of Ca2+ ([Ca2+]i), as determined by Fura-2 fluorescence signals. Elevation of [Ca2+]i by PGE2/PGE1 and iloprost, but not that by TX-agonists or thrombin, was totally dependent on the presence of extracellular Ca2+. This effect by PGE2/PGE1 was partially inhibited by prior treatment of the cells with islet-activating protein (IAP), while that by TX-agonists or by PGI2/iloprost was not affected. We tentatively conclude from these results that: (1) MEG-01s cells express (a) PGI2/PGE1 receptor(s) coupled to adenylate cyclase and Ca2+ influx, a TXA2/PGH2 receptor coupled to the phosphatidylinositol-turnover-Ca2+ system, and the PGE2/PGE1 receptor coupled to Ca2+ influx; (2) the receptors for TXA2/PGH2 and iloprost and those for PGE2/PGE1 and thrombin are coupled to IAP-insensitive and IAP-sensitive GTP-binding proteins, respectively, and function in a different manner to elevate [Ca2+]i. Thus, the MEG-01s cell line is a pertinent model for studying eicosanoid receptor-mediated signal transduction in platelet/megakaryocyte systems.

Role of sodium ion gradient in facilitating extracellular calcium influx

The effect of extracellular Na+ on Ca++ influx in activated human platelets was evaluated with two methods. One method employed computer-aided subtraction of the time course of intracellular Ca++ changes in the absence of extracellular Ca++ from that observed in the presence of extracellular Ca++. The other utilized intracellular Ca++ elevation following the addition of Ca++ to a platelet suspension already activated by thrombin in the absence of extracellular Ca++. Both methods demonstrated that depletion of extracellular Na+ significantly attenuated Ca++ influx in activated platelets. That extracellular Na+ modifies the opening state of Ca++ channels was further substantiated with an experiment with Sr++ instead of Ca++. A flow cytometric study confirmed the validity of these experiments. Neither Na+/H+ exchange nor changes in the membrane potential appeared to be related to the effect of extracellular Na+ on Ca++ influx, and the mechanism by which extracellular Na+ permits Ca++ mobilization remains to be elucidated.

Two-step mobilization of arachidonic acid in platelet activation induced by low concentrations of TP 82, a monoclonal antibody against CD9 antigen

Arachidonic acid mobilization in platelets activated by low concentrations (less than or equal to 1.6 micrograms/ml) of TP 82, a monoclonal antibody against CD9, appears to consist of two distinct phases. In the first phase, limited arachidonic acid release occurs concomitantly with a shape change induced by TP 82. This appears to be dependent upon phospholipase A2 activation, since it is well preserved in the presence of aspirin, which completely blocked both intracellular Ca2+ elevation and phosphatidic acid formation which would indicate phospholipase C activation. The Na+ Exchange was also found to participate in the first phase of arachidonic acid mobilization, since extracellular Na+ depletion and ethylisopropylamiloride, a specific inhibitor of the Na+/H+ exchanger, effectively blocked this limited mobilization of arachidonic acid. The second, much larger, phase of arachidonic acid mobilization occurs with the beginning of platelet aggregation. A limited amount of thromboxane A2 formed during the first phase of arachidonic acid release plays an important role in induction of the massive arachidonic mobilization in the second phase. Factors, as yet unidentified, also appear to work synergistically with thromboxane A2 to induce the full picture of arachidonic acid mobilization.

[Acute leukemia successfully treated with natural interferon-alpha]

Natural interferon-alpha (IFN-alpha) alone was administered to a 42-year-old man with acute leukemia, whose bone marrow revealed hypocellularity (NCC = 6 X 10(4)/microliters) and a 50% increase in blasts. Initial chemotherapy regimens, including BHAC-DMP or low dose Ara-C were ineffective. One month after starting nIFN-alpha therapy, the blasts in his bone marrow decreased below 3% and peripheral blood cell counts became normal. He has been in remission for at least 7 months.

Effects of various inhibitors on platelet activation induced by TP 82, a CD 9 monoclonal antibody

TP 82, a monoclonal antibody against CD 9 antigen, induced human platelet activation at concentrations higher than 0.4 microgram/mL in terms of aggregation, release of intracellular granule contents, production of arachidonic acid metabolites, and elevation of the intracellular Ca2+ concentration. The effects of a competitive inhibitor of ADP, acetylsalicylic acid, EGTA, and GRGDSP which blocks fibrinogen binding to IIb/IIIa complex suggested that each of released ADP, thromboxane A2, extracellular Ca2+, and close cell contact acts together to potentiate platelet activation induced by TP 82. While each of these inhibitors severely suppressed platelet activation induced by lower concentrations of the antibody (less than or equal to 0.8 microgram/mL), that induced by higher concentrations (greater than or equal to 3.2 micrograms/mL) was only partially blocked. Intracellular Ca2+ elevation was totally dependent upon the production of thromboxane A2, regardless of the antibody concentrations.

Intracellular ionized calcium mobilization of CD 9 monoclonal antibody-activated human platelets

Cytoplasmic Ca2+ mobilization in human platelets triggered by a CD 9 monoclonal antibody, TP82, was monitored by the Ca2(+)-sensitive photoprotein, aequorin and Ca2(+)-sensitive fluorophores, fura 2 and quin 2. Aequorin-indicated Ca2+ values were proportional to the concentration of TP82, which was in inverse proportion to the lag time before the onset of platelet aggregation and serotonin release. When fura 2 was used as a Ca2+ indicator, above a threshold concentration, the TP82-induced intracellular Ca2+ value remained unchanged even with increasing concentration. The findings obtained with quin 2 were compatible with the fact that the TP82-induced intracellular Ca2+ increase was largely dependent on the secondary effect of thromboxane A2. These findings may be clues to explain the marked difference in the Ca2(+)-response characteristics between the fluorescent indicators and aequorin as well as the properties of TP82-induced platelet activation.

Modification of Na+/H+ exchanger in human platelets by 1,2-dioctanoylglycerol, a protein kinase C activator

Protein kinase C activation in human platelets has a modulatory role in maintaining intracellular pH (pHi), by adjusting pHi at a particular value (7.22). Changes in pHi induced by protein kinase C appeared to be dependent upon the difference between H+ efflux catalyzed by the Na+/H+ exchanger and H+ production. The pHi recovery after acid loading was significantly facilitated by protein kinase C activation. Analysis of the rate constant for pHi recovery suggested that the turnover rate or the apparent affinity of the Na+/H+ exchanger for H+ was increased. Protein kinase C also decreased the Km value of the Na+/H+ exchanger for extracellular Na+. Thus, it is suggested that the role of protein kinase C in platelet pHi regulation is dual, adjusting the pHi value at a certain setpoint on the one hand, and increasing the rate constant of the Na+/H+ exchanger on the other.

Mastoparan, a wasp venom, activates platelets via pertussis toxin-sensitive GTP-binding proteins

Mastoparan induced limited release of serotonin from intact human platelets, while neither intracellular calcium ion elevation nor arachidonic acid mobilization was observed. Cytolysis induced by mastoparan was negligible in the concentration range that induced serotonin release. In digitonin-permeabilized cells, mastoparan induced Ca(++)-independent release of serotonin and Ca(++)-dependent arachidonic acid release. Both serotonin release and arachidonic acid release were reduced by pertussis toxin, suggesting that platelet activation induced by mastoparan is mediated by GTP-binding proteins.

Purification and partial characterization of CD9 antigen of human platelets

CD9 antigen (p24) was purified from human platelets and partially characterized. The yield was 75 micrograms from 10 units of platelet concentrates. p24 (38,000 copies/platelet) has intramolecular disulfide bond(s) and, in SDS-PAGE, consists of major 24-kDa molecule and minor 26- to 31-kDa molecules. The N-terminal sequence of p24, PVKGGTKXIKYLLFGFNFIF, indicates that the protein has not previously been characterized and amino terminus (position 12-20) is hydrophobic.

Ionomycin, a Ca++ ionophore, increases platelet volume independently of the Na+/H+ exchanger

A Ca++-ionophore, ionomycin, increased the volume of human platelets suspended in a Ca++-containing buffer. This change in cell volume was dependent upon ionomycin and extracellular Ca++ concentrations, suggesting that the volume change occurs when the intracellular Ca++ reaches a certain level (greater than uM as determined by aequorin method). The ionomycin-induced volume increase was suppressed by replacement of extracellular Na+ with membrane-impermeable N-methyl-D-glucamine or Cs+, but not with Li+, K+, or Rb+. Ethylisopropylamiloride, a potent inhibitor of the Na+/H+ exchanger, had only weak inhibitory effect, and the apparent Km for Na+ was approximately 350 mM, which is much larger than that of the Na+/H+ exchanger. It is suggested that certain mechanisms other than the Na+/H+ exchanger are responsible for ionomycin-induced volume increase.

The role of intracellular pH and Ca++ on arachidonic acid metabolism in human platelets

High concentrations of ionomycin induced arachidonic acid metabolism, while its magnitude appeared to be poorly correlated with the increase in intracellular Ca++ induced by ionomycin. Intracellular pH elevation had no direct effect on arachidonic acid release. On the other hand, the elevation in intracellular pH potentiated arachidonic acid release induced by low concentrations of ionomycin, and appeared to increase the sensitivity to Ca++ of the arachidonic acid releasing mechanism. pHi fixed at low values suppressed the formation of the product of the cyclooxygenase pathway more severely than that of 12-lipoxygenase in platelet activation induced by receptor-mediated agonists. Inhibition of the Na+/H+ exchanger, which participates in intracellular pH elevation, reduced the production of arachidonic acid metabolites, with a more profound effect on the product of the cyclooxygenase pathway. The findings suggest that intracellular Ca++ and pH act synergistically to activate the arachidonic acid releasing mechanism in platelets, and that pH also regulates the activity of the cyclooxygenase pathway.

Acetyltransferase activity and production of platelet-activating factor by human neutrophils activated with various stimuli

The acetyltransferase activity and biosynthesis of platelet-activating factor (PAF) were assessed in human neutrophils activated by 4 microM A23187, 20 ng/ml phorbol myristate acetate (PMA), and 10(-6) M n-formyl-methionyl-leucyl-phenylalanine (fMLP). All three agents elevated the acetyltransferase activity dose-dependently. There were no significant differences in the Km values for acetyl CoA between non-stimulated and stimulated cells. All three stimuli gave a similar Vmax value of acetyltransferase determined 10 min after stimulation, being more than twice as high as the control value. By contrast, the amount of PAF produced by the neutrophils differed with the stimuli, A23187 being by far the most potent. The time course of PAF synthesis, particularly when activated by PMA, did not parallel that of acetyltransferase activity; PMA-induced PAF production was negligible for the first 20 min and gradually increased to reach its plateau 60 min after stimulation, while enzyme activity was at its highest level 5 min after stimulation. These results suggest that the different stimuli activated the same acetyltransferase, and that there was an increase in the number of the enzyme molecules in the activated state. It is unlikely that a change in the biological activity of a preexisting enzyme without a change in the number of active enzyme molecules could be the cause. Certain factors other than acetyltransferase may also regulate the PAF biosynthesis induced by these stimuli.

Effects of five anion channel blockers on thrombin- and ionomycin-activated platelet functions

The inhibitory effects of anion channel blockers were evaluated on aggregation, intracellular Ca2+ rises, and the production of arachidonic acid metabolites in human platelets. Inhibitors included five anion channel blockers: phloretin, probenecid, pyridoxal phosphate, 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene (DIDS) and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS). The degree of inhibition by each of these agents was dose-dependent on thrombin-activated platelet function. These agents generally had no significant inhibitory effects on ionomycin-activated platelet functions. It is suggested that anion mobilization plays a major role in the receptor-mediated activation of platelet functions, but only a minor role in Ca2+ ionophore-induced platelet activation. It is also suggested that several agents may have properties unrelated to anion channel blockers. Phloretin may be a selective cyclooxygenase inhibitor, and probenecid may inhibit phospholipase A2. DIDS and SITS may interfere with certain aggregation-inducing mechanisms.

Correlation of intracellular and extracellular calcium ion concentrations with synergy between 1,2-dioctanoyl-sn-glycerol and ionomycin in platelet arachidonic acid mobilization

The potentiation by 1,2-dioctanoyl-sn-glycerol (DiC8) of ionomycin-induced platelet production of 12-hydroxy-5,8,10-heptadecatrienoic acid (HHT) and 12-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE) was investigated in correlation with extracellular Ca2+ concentrations and increases in [Ca2+]i, as detected with aequorin and fura-2. Extracellular Ca2+ concentrations greatly influenced the production of arachidonic acid metabolites induced by DiC8 and ionomycin, while that induced by ionomycin alone was minimally affected by variation of the extracellular Ca2+ concentration. In the synergy between ionomycin and 20 microM DiC8, the optimal concentrations of ionomycin shifted from high to low with increasing concentrations of extracellular Ca2+, suggesting that there might be a range of optimal [Ca2+]i for the production of the arachidonic acid metabolites. This hypothesis was confirmed by simultaneous measurements of [Ca2+]i increases, and the production of the arachidonic acid metabolites. With the aequorin method, the optimal concentrations of [Ca2+]i fell to between 10 microM and 20 microM, and with the fura-2 method, it fell to between 800 nM and 1800 nM. Direct measurements of [14C]arachidonic acid release suggested that the DiC8-potentiated production of arachidonic acid metabolites induced by ionomycin was attributable to increased arachidonic acid release. Since ionomycin and DiC8 induced relatively low levels of phosphatidic acid production, an indicator of phospholipase C activation, it was suggested that the increased arachidonic acid release was largely dependent upon phospholipase A2. Synergy between DiC8 and ionomycin was also observed with aggregation and serotonin release. Aggregation was induced by lower concentrations of ionomycin, and appeared to be more dependent upon extracellular Ca2+, while serotonin release required higher concentrations of ionomycin, and variations in extracellular Ca2+ affected the response minimally. These findings suggest that the mechanisms underlying the synergy between protein kinase C activation and Ca2+ mobilization differ among the three functions evaluated in this study.

Anion channels contribute to the regulation of intracellular pH in human platelets

The presence of extracellular bicarbonate potentiated platelet intracellular pH rises induced by thrombin. The effect was most remarkable in sodium-depleted buffers. This effect of bicarbonate was dose-dependent and was inhibited by anion channel blockers. Bicarbonate also potentiated the pHi recovery after acid loading, but had no effect on alkali loading. It was suggested that anion exchange, most probably that of bicarbonate/chloride, contributes to the regulation of intracellular pH in human platelets.

Establishment of a human megakaryoblastic cell line (T-33) from chronic myelogenous leukemia in megakaryoblastic crisis

A megakaryoblastic cell line, termed T-33, was established from the peripheral blood of a patient with Philadelphia chromosome-positive chronic myelogenous leukemia in megakaryoblastic crisis. T-33 cells have been maintained in RPMI 1640 medium containing 10% fetal calf serum in a single cell suspension with a doubling time of 24-36 h for over 2 years. Giemsa-banded karyotypes were female hyperdiploid with a modal chromosomal number of 51, all cells including Philadelphia chromosome. The cells showed strong positivity for periodic acid-Schiff and alpha-naphthyl acetate esterase, and weak for alpha-naphthyl butyrate esterase, but were negative for myeloperoxidase. Flow cytometric analysis of cell surface markers showed the existence of HLA-DR, MY-7, MY-9, and a platelet antigen (Yukb), and no markers for T- or B-lymphocytes. Most of the cells fixed with acetone were positive for Factor VIII, platelet glycoprotein IIb-IIIa, IIIa (Yukb), and Ib, but negative for glycophorin A and hemoglobin. Ultrastructural platelet peroxidase was demonstrated in 2-3% of cells and the percentage of positive cells increased up to 20% after the treatment with 12-O-tetradecanoylphorbol-13-acetate. The cells contained small dense granules negative for platelet peroxidase, their number increasing threefold after 12-O-tetradecanoylphorbol-13-acetate treatment. Such treated cells frequently showed a complex of the demarcation membrane in the cytoplasm. T-33 responded thrombin to exhibit calcium influx. This cell line may be useful for the study of the early stage of megakaryocytic differentiation in human megakaryopoiesis.

Synergism between subthreshold concentrations of thrombin and 12-O-tetradecanoylphorbol 13-acetate in platelet activation

A comparison was made between the time courses and interdependence of platelet aggregation, serotonin release, and cytosolic free Ca2+ concentration in the same sample of platelets loaded with [14C]-serotonin and Ca2+-sensitive photoprotein aequorin. In 100 micrograms/ml aspirin-treated platelets, neither 0.01 U/ml thrombin nor 50nM TPA, an active phorbol ester, induced significant aggregation, serotonin release, or a rise in the intracellular calcium concentration. However, when these two agents were added together, marked aggregation and release were observed without a change in the cytosolic free Ca2+ concentration. No correlation was observed between the extent of the synergistic effects and time of preincubation with TPA. Potentiatory effects of protein kinase C on receptor-mediated agonists need to be considered in platelet activation.

A first case of complete remission of beta-interferon sensitive adult T-cell leukemia

A case of complete remission of adult T-cell leukemia (ATL) induced by beta-interferon is reported. A 46-year-old male was diagnosed as ATL because of the increased number of ATL cells with deeply indented and lobulated nuclei in the peripheral blood, accompanied by elevated values of the lactic dehydrogenase, the alkaline phosphatase, and the calcium in the serum. The result of the cell surface marker analysis of peripheral blood lymphocytes was compatible with ATL and anti-ATL associated antibody (ATLA) was positive. The integration of proviral deoxyribonucleic acid (DNA) of human T-cell leukemia virus type I(HTLV-I) was proved in the peripheral blood lymphocytes using Southern blot hybridization. Since an ordinal chemotherapy was not so effective for this patient, he was treated with 1.8 X 10(7) units of recombinant beta-interferon (beta-IFN) per day for 7 days as one course. After 5 courses of treatment, a markedly favorable response was recognized, and he achieved complete remission. A lower dose of beta-IFN (9 X 10(6) units per day for 3 days as one course, one or two courses per month) has been continued and he has still been in a complete remission state for 10 months. It is concluded that beta-IFN should be used to treat ATL.

Separable function of platelet release reaction and clot retraction

Amiloride, a known Na+/H+ exchange inhibitor, inhibited platelet serotonin release in a dose-dependent manner (100 microM for 50% inhibition, and 1mM for the nearly complete inhibition), although amiloride (1mM) accelerated clot retraction when it was measured at decreased platelet concentration. On the contrary, cytochalasin B (10 micrograms/ml) accelerated platelet serotonin release, but it inhibited clot retraction. These results demonstrate that release reaction and clot retraction, both of which are important processes involved in platelet activation, can be functionally separated.