Dynamic changes in neutrophil cytoskeleton during priming and subsequent surface stimulated functions

The naturally occurring inositol polyphosphate, inositol hexakisphosphate (InsP6, phytic acid) primes human neutrophils for enhanced responses to stimulatory agonists. Unlike other primers InsP6 has no effect at high doses (up to 500 microM) on the neutrophil's basal rate of superoxide production (O2-). Pretreatment of the cells for 2 min with InsP6 (100 microM) substantially enhances fMet-Leu-Phe (FMLP) (10(-7) M)-induced respiratory burst. In investigations of the dynamic aspect of neutrophil actin during priming and stimulation with FMLP the content of F-actin has been measured with and NBD (nitrobenz-2-oxa-1,3-diazole)-phallacidin assay. The induced assembly of F-actin is rapid (peaks at approximately 45 s) followed by depolymerization. In contrast, priming with InsP6, with or without FMLP stimulation, results in a sustained assembly of F-actin as visualized with a rhodamine-phalloidin staining technique. These changes in macromolecular status and distribution of F-actin during InsP6 priming occur in the absence of any other demonstrable functional responses and thus InsP6 may be a useful tool to follow early events in neutrophil functions or to monitor the presence of unprimed and preprimed subpopulations in the circulation. Since InsP6 is present in high concentrations in neutrophils (up to 100 microM), its release from damaged or effete cells may have a modulatory role on neutrophil functions.

Priming action of inositol hexakisphosphate (InsP6) on the stimulated respiratory burst in human neutrophils

After priming by a number of different host, bacterial and chemical agents, human neutrophils may be stimulated to produce a greater respiratory burst than would be elicited by the stimulus alone. Other neutrophil functions may be similarly enhanced by pre-exposure to a priming agent. We describe here a new extracellular role for inositol hexakisphosphate (InsP6) as a priming agent for a variety of human neutrophil functional responses. Preincubation of the cells with InsP6 alone (up to 250 microM) has no stimulatory effect upon the basal production of reactive oxygen intermediates but the response to a subsequent stimulus (FMLP, PMA or phagocytic particles) is substantially enhanced. Levels 100-200% higher than 'stimulus only' controls have been recorded. Peak enhancement of the FMLP-induced oxidative response occurs after 1-2 min preincubation with InsP6 and the effect is dose-dependent (maximum at approx. 100 microM InsP6). As others have shown FMLP stimulation of superoxide anion production has no external Ca2+ dependence but the presence of low levels of Ca2+ and Mg2+ (0.1 mM) during priming appears to be an essential requirement for full expression. Reports of intracellular concentrations of InsP6 in mammalian cells in the 30-100 microM range suggest that the local release of this inositol polyphosphate from damaged or effect cells could have a physiologically important modulatory role on neutrophil functions.

Enhancer, repressor, and promoter specificities combine to regulate the rat alpha-fetoprotein gene

The upstream transcription control region of the rat alpha-fetoprotein (AFP) gene was analyzed using transient expression of CAT genes in HepG2 cells which express the gene; H4C3 cells which repress the AFP gene but express the albumin gene; and four nonexpressing cell lines. Deletion analysis based on the DNA sequence resolved three upstream enhancers corresponding to the mouse AFP enhancers, but showed additional weak effects from flanking sequences. Quantitative experiments demonstrated that the three enhancers were additive when acting through a single promoter and did not confirm the presence of a distal upstream repressor. All three enhancers stimulated the AFP, albumin, or thymidine kinase (tk) promoter in HepG2, but only the tk and albumin promoters in H4C3. Deletion of a proximal repressor region near the AFP promoter allowed expression in H4C3 cells with the AFP promoter. Thus, the liver-specific developmental repressor is near the AFP promoter, and H4C3 cells provide an in vitro system for analysis of this repressor in transfection assays. The repressor region also blocked expression of the SV40 enhancer through the AFP promoter in hepatic and nonhepatic cell lines, but when this enhancer was combined with an AFP promoter from which the repressor region was deleted, the combination showed expression in all six cell lines studied. AFP expression results from a combination of enhancer, promoter, and repressor activities, and the repressor is functional with a heterologous enhancer in a variety of cells.

Electrokinetic properties and surface membrane sialic acid status of platelets in idiopathic thrombocytopenic purpura (ITP)

Continuous flow electrophoresis has been used to investigate heterogeneity in the circulating platelets from patients with idiopathic thrombocytopenia (ITP) and from age- and sex-matched control donors. The ITP patients' platelet counts ranged from 15 to 80 X 10(9)/l and they had much higher mean volumes and significantly higher (two-fold) total cell and neuraminidase-labile surface sialic acid contents expressed per unit cell than normal. After volume normalization the mean difference between the two groups was not statistically significant. The relationships of these findings to abnormal thrombopoiesis and to molecular recognition for reticuloendothelial system (RES) removal are discussed.

Basal and induced prothrombinase expression in platelets from patients with essential thrombocythaemia (ET)

Both the basal and the collagen plus thrombin-stimulated prothrombinase activities have been measured in platelets from eight patients with essential thrombocythaemia (ET). On a mean basis these activities were significantly lower than the values recorded for a group of age and sex matched control subjects. When the Ca2+ ionophore A23187 was substituted for the agonist mixture the induced expression of prothrombinase activity was essentially the same for the patient and control groups. It is suggested that the defect in prothrombinase expression seen with ET platelets may reside in either membrane signal transduction processes concerned with Ca2+ mobilization or in Ca2+ sensitive cytoskeletal control of membrane phospholipid topography.

Electrokinetic, analytical and functional heterogeneity of circulating human platelets: separation of subpopulations by continuous flow electrophoresis after taxol stabilization

A continuous flow electrophoresis procedure has been developed to study platelet subpopulation heterogeneity with separations based upon surface electrical charge differences. Taxol at low concentrations has been used to transiently stabilize the cells during the separations. At a concentration of 10(-5) M taxol has no effect upon a wide range of physical, analytical and enzymatic properties and does not compromise agonist-induced activation responses (aggregation and secretion). A typical normal platelet subpopulation profile extends over 15-20 fractions with mobilities from -0.97 to -0.78 microns per s per volt per cm. Platelet size (resistive particle counter volumes) differed significantly across the profile, the most electronegative cells being the larger, and the least electronegative the smaller platelets. Total platelet sialic acid content and surface neuraminidase-labile sialic acid correlated positively with electronegativity, but the surface -SH group status had an inverse relationship with the least electronegative smaller platelets, having twice as many surface DTNB-titratable - SH groups as the most electrophoretically mobile and larger cells. Normalisation of analytical and enzymatic data to cell volumes revealed that the smaller less electronegative platelets were substantially richer in all constituents and properties than the larger more electronegative platelets. These smaller cells showed higher activities for lysosomal enzymes, and their functions (capacity to transport 5-hydroxytryptamine and adenosine across the plasma membrane and responsiveness to thrombin expressed by synthesis of thromboxane B2 (TXB2) or release of 5HT) were greater than the larger more electronegative cells. No significant differences were observed, however, in the subpopulations by optical aggregometry using six different agonists each at three different concentrations. This free flow electrophoresis separation of platelets, which can be carried out on a preparative scale, may have some advantages over the conventional density gradient separations of subpopulations for investigating clinical states affecting thrombopoietic regulation or platelet losses from the circulation due to vessel wall disease, prosthetic implants or during extracorporeal circuitry.

High incorporation of [3H]inositol into phosphoinositides of human platelets during reversible electropermeabilisation

A new method for high incorporation of [3H]inositol into human platelets is described. The method involves incorporation of [3H]inositol during reversible electropermeabilisation by high voltage discharge, followed by resealing the cells during incubation at 37 degrees C. Between 10- and 20-fold increase of isotope uptake is achieved compared to control intact cells. Permeabilised resealed platelets maintain good responses to thrombin and collagen. Analysis of the incorporation of the label amongst the phosphoinositides shows 70% to be in PI, 20% in PIP, and 10% in PIP2. Stimulation with thrombin and analysis of the formation of IP1, IP2 and IP3 shows the labelling to occur in a hormone-sensitive pool. These studies indicate that reversible electropermeabilisation can be used to achieve good uptake of non-membrane penetrating substances such as inositol.

Reversible electropermeabilisation of human and rat blood platelets: evaluation of morphological and functional integrity 'in vitro' and 'in vivo'

A high-voltage discharge procedure has been developed for permeabilising the plasma membranes of both human and rat blood platelets. The cells can be resealed by incubation at 37 degrees C, show less than 4% loss of lactate dehydrogenase (LDH) implying minimal cell lysis and also have well maintained morphological and functional integrity. The prototype apparatus used at field strengths between 6 and 8 kV/cm produces membrane pores which allow free diffusion of low molecular weight substances such as adenine nucleotides, inositol phosphate and fluorescent dyes. Two properties, namely Ca2+-induced secretion of granule stored 5-hydroxytryptamine (5HT) and inositol 1,4,5-trisphosphate (IP3)-induced release of intracellularly sequestered 45Ca, which are both well expressed immediately after permeabilisation, are essentially abolished after resealing. The efficiency of permeabilisation and resealing can be simply monitored by shifts in 'apparent platelet volume' using a resistive particle counter (Coulter). Permeabilised platelets show a shift in modal volumes from a control range 4-7 fl to 10-15 fl. Resealing restores these modal volumes to the original control range. Encapsulation of the fluorochrome, Lucifer yellow (Mr 550), during permeabilisation revealed that after resealing greater than 85% of rat platelets, and close to 100% human platelets, contained the encapsulated dye. The initial rates and % aggregation responses of both human and rat platelets to collagen, thrombin and the thromboxane A2-mimetic U46619 remained essentially normal after permeabilisation and resealing further illustrating the maintenance of functional competence following treatment. Resealed rat platelets reinfused into the circulation after labelling with [111In]indium oxine gave survival curves similar to those of control platelets. Therefore, this reversible permeabilisation procedure may allow the use of autologous or heterologous platelets as carrier vehicles for the delivery of drugs and other agents 'in vivo'.

Action of guanosine 5'-[beta-thio]diphosphate on thrombin-induced activation and Ca2+ mobilization in saponin-permeabilized and intact human platelets

The non-hydrolysable guanine analogues guanosine 5'-[gamma-thio]triphosphate (GTP[S]) and guanosine 5'-[beta-thio]diphosphate (GDP[S]) have been used extensively (as promoters and inhibitors respectively) to probe the importance of G-protein function. We report on the use of GDP[S] in permeabilized and intact platelets. The stimulatory analogue GTP[S] (9-60 microM) induces shape change, aggregation and 5-hydroxy[14C]-tryptamine secretion when added to saponin (12-14 micrograms/ml)-permeabilized platelets, but not to intact platelets. In line with the activation responses in permeabilized cells, GTP[S] induces an increase in [32P]-phosphatidic acid, which is indicative of phospholipase C activity. GDP[S] (greater than 400 microM) totally inhibits GTP[S] (90 microM)-stimulated phospholipase C activity and functional responses in saponized platelets. GDP[S] (1 mM) was also effective at inhibiting low-dose thrombin (0.1 unit/ml)-induced aggregation and secretion responses (without affecting shape change) in permeabilized platelets with inhibition of [32P]-phosphatidic acid formation. At higher doses of thrombin (greater than 0.5 unit/ml), both functional responses and [32P]phosphatidic acid formation are restored in the presence of GDP[S]. Studies on intact cells revealed that GDP[S] was as effective at inhibiting low-dose thrombin-induced functional responses as in the permeabilized cells, but there was no inhibition of [32P]phosphatidic acid formation, indicating that the agent is nonmembrane-penetrating. This reflected the fact that GDP[S] has additional inhibitory sites on the surface of platelets. In Fura-2-loaded cells GDP[S] inhibited thrombin-induced Ca2+ mobilization, as measured by Fura-2 fluorescence, in a dose-dependent manner. In studies with and without Ca2+ present on the outside, the effect of GDP[S] was to block Ca2+ influx. These studies indicate that, although GDP[S] is a valuable tool in studying G-protein function in permeabilized cells, it also has inhibitory activities on the surface of platelets, and one of these has been identified as an effect on the Ca2+-influx channel after agonist stimulation.

Introduction of antibody (PL/IM 430) to a 100 kDa protein into permeabilised platelets inhibits intracellular sequestration of Ca2+

A monoclonal antibody (PL/IM 430), previously found to inhibit the uptake of Ca2+ into highly purified platelet intracellular membrane vesicles (Hack, N., Wilkinson, J.M. and Crawford, N. 1988, Biochem. J. 250, 355-361) has been introduced into saponin-permeabilised platelets. At a saponin concentration (20-25 micrograms/ml) commensurate with total LDH release, sequestration of Ca2+ into intracellular non-mitochondrial stores is inhibited by the antibody (approximately 50% inhibition at 20 micrograms/ml IgG). At higher saponin concentrations when intracellular binding of 125I-labelled mAb is maximum, inhibition of Ca2+ sequestration approaches 70%. The inhibition is specific, control studies with non-platelet directed mouse IgG and mAbs which immunoblot platelet antigens other than the 100 kDa protein did not affect the Ca2+ sequestration. No effect of the antibody were observed against IP3-induced release of prestored Ca2+, either in permeabilised platelets or with isolated intracellular membrane vesicles. The mAb PL/IM 430 appears to bind only to the Ca2+ translocating channel protein associated with the intracellular membrane (Ca2+ + Mg2+) ATPase and not to Ca2+ channels responsive to IP3.

Platelet surface charge heterogeneity: characterization of human platelet subpopulations separated by high voltage continuous flow electrophoresis

Washed formol-fixed normal human platelets have been separated into surface charge-dependent subpopulations using high voltage continuous flow electrophoresis. The procedure is highly reproducible and the heterogeneity profile extends over 20-25 fraction tubes on the anodal side of the entry port to the separation chamber. Fractions have been subdivided into subpopulation pools A, B and C which have mean mobilities by analytical cytopherometry extending over the range 0.81-0.91 micron/s/V/cm from the least (C) to the most (A) electronegative cells. Coulter volume differences across the profile from 5.0 to 12.8 fl correlated well with electrophoretic mobilities whereas buoyant density appeared to be an independent parameter. Analysis of surface neuraminidase-labile sialic acid of the platelets in pools A and C correlated well with differences in electrophoretic mobility, whereas a similar relationship for the alkaline phosphatase-labile phosphate moieties (also believed to be contributory to cell surface electrokinetic properties) could not be established even though in both cases the profiles of the enzyme-treated platelets showed significant shifts towards the cathode when compared with untreated cells. Titration of surface DTNB-reactive sulphydryl (-SH) groups revealed an inverse relationship between electronegativity and membrane -SH group status. This electrophoretic expression of subpopulation heterogeneity within the circulating platelet pool may have advantages in studying clinical conditions where the profiles may reflect cell surface interactions 'in vivo'.

Albumin and alpha-fetoprotein gene expression and DNA methylation in rat hepatoma cell lines

To define systems for the study of gene control and differentiation in vitro, we analyzed albumin and alpha-fetoprotein (AFP) gene expression and gene methylation in a series of rat hepatoma-derived cell lines and controls. These cell lines had several specific phenotypes: adult (high albumin and low AFP mRNA), fetal (high albumin, high AFP), embryonic (low albumin, high AFP), or undifferentiated (no albumin or AFP). The adult hepatocyte phenotype is marked by a novel 2.2-kb AFP gene transcript and high DNA methylation. In general, tumor cell lines had higher albumin and AFP gene methylation than hepatocytes in vivo. Levels of total DNA methylation did not determine the methylation patterns of specific genes, except for one cell line with hypermethylated and one with hypomethylated DNA. 5'-Hypomethylation of the AFP gene correlated with gene activity in all cases; the albumin gene showed a similar relationship, but with some exceptions. Only adult hepatocytes, not cell lines, have a unique 3'-region of AFP gene demethylation.

Characterization of a monoclonal antibody ITI-Pl 1 directed against human platelet membrane glycoprotein IIb using extracts of whole platelets and platelet surface and intracellular membranes

A monoclonal antibody (mAb) termed ITI-Pl 1 has been prepared by the hybridoma procedure. Using immuno-absorption and crossed immunoelectrophoresis of Triton X-100 extracts of untreated and EDTA-treated human platelets it was shown to be directed against the surface membrane glycoprotein IIb (GP IIb). This mAb binds to whole platelets independently of ADP-stimulation and the presence of Ca2+-ions. It saturates at around 870 ng/10(8) cells corresponding to approximately 35,800 molecules/platelet. ITI-Pl 1 did not significantly inhibit GP IIb-IIIa dependent functions such as platelet aggregation or fibrinogen binding. Immunofluorescence could be demonstrated using ITI-Pl 1 and intact normal platelets, but not with platelets from a Glanzmann's thrombasthenia patient. Crossed immuno-electrophoresis with platelet extracts from four different thrombasthenic patients gave a line precipitate in the intermediate gel with 125I-labelled ITI-Pl 1 and autoradiography indicating trace amounts of free GP IIb or the GP IIb-IIIa complex. The epitope on GP IIb detected by ITI-Pl 1 is not destroyed by neuraminidase treatment. Thus the mAb also interacts with neuraminidase-treated GP IIb-IIIa complex in highly purified platelet surface membrane fractions as well as with GP IIb-IIIa from untreated internal membranes isolated by continuous flow electrophoresis.

Changes in the distribution and organization of platelet actin induced by diamide and its functional consequences

Exposure of blood platelets to diamide (azodicarboxylic acid-bis-dimethylamide) results in oxidation of sulphydryl groups present in the cytoskeleton and other proteins. This results in dramatic changes in functional behaviour of the cells. The distribution and level of organization of the major cytoskeletal protein actin has been studied analytically by the DNase-I inhibition assay and morphologically by electron microscopy (EM) of Triton X-100 treated platelets adherent to EM grids. Exposure to diamide results in a redistribution of actin within the cell reflected in an increase in cytoskeletal F-actin and a concomitant decrease in cytosolic actin. The magnitude of these changes depends upon the concentration of diamide and the time of exposure. Diamide also alters platelet aggregatory functions in response to certain stimuli. Treatment of normal human platelets with 0.1 mM diamide proceeds via disaggregation (5 min exposure to diamide), inhibition of aggregation (30 min exposure), to finally a normalization of the aggregation response after 60-120 min incubation with diamide. In parallel with the return to full functional response the distribution of F-actin between the cytoskeleton and cytoplasmic compartments returns to the control pattern. Incubation of the platelets with 0.5 mM diamide for 60 or more minutes leads to total inhibition of the aggregatory ability. In these cells the cytoskeleton associated F-actin remains significantly elevated and the structural organization of the cytoskeleton is markedly altered. In contrast to the network of filaments subadjacent to the surface membrane seen in unstimulated platelets, the cytoskeleton now shows electron dense zones in the more central parts of the cytoplasm. This diamide-induced structural reorganization of platelet cytoskeletal elements, associated with the inhibition of functional responses, emphasizes the dynamic nature of the membrane-cytoskeletal axis and its importance in the expression of shape changes and aggregatory phenomena in response to surface stimuli.

Actin in B16 melanoma cells of differing metastatic potential. Effects of trypsin and serum

Actin is present in cells in monomeric and polymeric (filamentous) forms. Filamentous actin is distributed in Triton-soluble (cytosolic) and Triton-insoluble (cytoskeletal core) fractions. We have used the DNase 1 inhibition assay and immunofluorescence to investigate the distribution of actin in monomeric and polymeric forms in cloned B16 murine melanoma cell lines of low and high metastatic capacity. The protease trypsin caused rounding up and detachment of both cell lines within 5 min. This was associated with almost complete depolymerization of cytosolic actin filaments but the Triton-insoluble cytoskeleton was not quantitatively affected by trypsin treatment. There were quantitative differences between the clones in their response to incubation in the presence or absence of 10% serum. The highly metastatic cell line contained 35% more actin when incubated in the presence of 10% serum, almost completely distributed to the Triton-insoluble cytoskeleton, an effect not seen in the low metastatic cells.

Inositol 1,4,5-trisphosphate (IP3) induced rapid formation of thromboxane B2 in saponin-permeabilised human platelets: mechanism of IP3 action

The mechanism of IP3-induced activation of saponin-permeabilised platelets has been examined. Saponin permeabilization resulted in the leakage of low-Mr substances into and from the cells without loss of cytoplasmic proteins. Addition of IP3 rapidly induced a dose-related formation of thromboxane B2 and release into the medium, leading to the responses of shape change, aggregation and [14C]5HT release. These responses were inhibited by the thromboxane A2 receptor antagonist AH23848. The IP3-induced release of 45Ca from intracellular stores was not affected by indomethacin. Synthesis of thromboxane was inhibited if Ca2+ elevation was prevented by using Ca-EGTA buffers during permeabilization. These studies indicate that IP3-induced activation was due to Ca2+ mobilisation leading to phospholipase activation and thromboxane synthesis.

A sensitive procedure for quantifying the phagocytic competence of blood granulocytes

The phagocytic activity of blood granulocytes can be quantitatively assayed by ingestion of opsonised paraffin oil droplets containing the dye "Oil Red-O" (Stossel, T.P., Mason, R.J., Hartwig, J., and Vaughan, M. (1972) J. Clin. Invest. 51: 615-624). We have modified this assay by incorporating [3H]glycerol into the oil droplets which allows a more sensitive and reproducible measurement of the phagocytic competence of blood granulocytes even at very low cell counts. Comparative studies after one day storage of the blood at 4 degrees C is feasible since they retain 84% of the phagocytic capacity measured when isolated from fresh blood.

Effect of transformation by Rous sarcoma virus on the character and distribution of actin in Rat-1 fibroblasts: a biochemical and microscopical study

Actin has been measured in subcellular fractions from Rat-1 fibroblasts and in Rous sarcoma virus-transformed Rat-1 cells (VIT), using the DNase 1 inhibition assay. The transformed cells showed a significant shift in the actin monomer (G)in equilibrium with polymer (F) equilibrium within the cell cytosol, and a significant increase in actin in the Triton-insoluble cytoskeletal core in comparison with untransformed cells. This incorporation of actin into the cytoskeletal core fraction is associated with a change in filamentous actin assemblies from 'stress fibre' patterns to punctate filament aggregates. These differences have been correlated with changes in morphology, in actin, vinculin and alpha-actinin distribution, in adhesion plaque formation and with the production of pp60v-src-associated protein kinase activity in the transformed cells. Changes in actin distribution and its polymerization in response to src-gene expression may play an important role in the determination of the transformed cell characteristics.