Is an early calcium flux necessary to stimulate lymphocytes?

Enhancing Effect of Electromagnetic Exposure on Calciumionophore (A23187), but Not IL-1, Induced Txa2 Release by Human Neutrophils

The exposure of human polymorphonuclears to an extremely low frequency (3 Hz) electromagnetic field for 1 hour had an enhancing effect on thromboxane A2 release stimulated by A23187 calcium ionophore. On the contrary, IL-1 stimulation of TxA2 production was not affected by an electromagnetic field, suggesting that interleukin-1 influence on thromboxane synthesis is not due to a calcium ionophore-like action.

Role of ion channels in lymphocytes

Ion channels, and ion fluxes in general, appear to regulate a wide variety of processes important to lymphocyte function in normal and disease states. These include resting ionic homeostasis and the more complex signaling events involved in activation, proliferation, cytotoxic function, and volume regulation. The wider application of patch-clamp and microfluorimetry techniques to lymphocytes has helped to clarify some issues and raised many more. It seems likely that rapid progress will be made in our understanding of these areas through a combination of immunological, biochemical, and electrophysiological approaches.

Early increase in intracellular calcium during photodynamic permeabilization

The purpose of this study was to determine if there was an early increase in intracellular Ca++ which preceded generalized lysis of thymocytes during photodynamic permeabilization. A method was developed that facilitated the simultaneous measurement in real time of permeabilization of the thymocyte cell membrane to Ca++, Mn++, and ethidium bromide during photodynamic action. Quin-2 loaded cells were illuminated in the presence of erythrosin B and the change in the fluorescence emission of the calcium-quin-2 complex was used to determine how soon and to what extent intracellular Ca++ changed following illumination. In the presence of extracellular manganese, the same system was used to determine how soon the cells became permeable to Mn++ or quin-2. It was determined that the fluorescence emission of the ethidium bromide-DNA complex was strong enough to be measured in the presence of the calcium-quin-2 complex. This enabled the concomitant determination of the elapsed time following illumination before ethidium bromide entered the cell. It was established that increased intracellular Ca++ was an early event in the photodynamic permeabilization of thymocytes that preceded permeabilization of the cell membrane to ethidium bromide, Mn++ or quin-2, or lysis.

Differential Sensitivity of CD4+ and CD8+ T Lymphocytes to Phorbol Myristate Acetate upon Anti-CD3 Stimulation: Evidence for a Distinct Signaling Pathway

The effects of suboptimal concentrations of Phorbol Myristate Acetate (PMA) in combination with submitogenic concentrations of OKT3 antibody on CD4+ and CD8+ cell activation were investigated. Upon stimulation with OKT3 (25 pg/ml) and PMA (0.25 – 0.75 ng/ml), the majority of CD4+ cells entered the cell cycle whereas most of CD8+ cells remained in G0/G1 phase. Under the same conditions of OKT3 stimulations, both CD4+ and CD8+ cells failed to produce IL2 in the absence of PMA. In the presence of PMA (0,25 ng/ml), CD4+ produced measurable amounts of IL2 (0,5 – 2,3 U/ml) whereas IL2 production by CD8+ cells remained below the detection limit. Expression of TAC antigen (CD25) was found to parallel IL2 production and cell proliferation in both subsets whereas changes in [Ca2+] mobilization following OKT3 stimulation were similar in both subsets.

Interestingly, the elevation of intracellular cyclic adenosine 3':5' monophosphate (cAMP) was not equally distributed between CD4+ and CD8+ subpopulations. Furthermore, the kinetics of protein kinase (PKC) translocation was markedly prolonged in membranes of CD4+ cells compared with CD8+ cells suggesting a differential involvement that may operate under distinct regulatory signaling mechanisms.

Calcium uptake and efflux during the yeast to mycelium transition in Sporothrix schenckii

A study was made of calcium metabolism during germ tube formation in Sporothrix schenckii yeast cells. A net efflux of calcium was observed very early in the transformation process and remained constant thereafter. The efflux of calcium in yeast cells induced to form germ tubes was twice that observed in yeast cells not induced to form germ tubes. Two peaks of calcium uptake were observed in germ tube forming yeast cells at 30 and 300 minutes following inoculation, while non-induced yeast cells, a continuous increase in uptake was observed which ultimately reached higher values than the ones obtained in germ tube forming cells. Substances which affect calcium metabolism in other cells such as cobalt ions, ionophore A23187 and compound R24571 were observed to inhibit germ tube formation and calcium uptake. In addition, ionophore A23187 was found to increase calcium efflux to approximately twice the control values. The inhibition of germ tube formation brought about by substances which inhibit calcium uptake or increase efflux suggests that the intracellular calcium concentration in these cells must be precisely regulated for the yeast to mycelium transition to occur.

Effect of concanavalin A on serotonin transport into blood platelets: possible involvement of protein kinase C

Possible involvement of protein kinases in the serotonin (5-HT) transport system in platelets and the inhibitory effect of concanavalin A (Con A) on platelet 5-HT uptake were investigated. Staurosporine and K-252a, highly active inhibitors of protein kinases, did not inhibit 5-HT transport, but they antagonized the inhibitory effect of Con A on 5-HT uptake. KT5720, a protein kinase A inhibitor that has no effect on protein kinase C, neither affected 5-HT transport nor antagonized the inhibitory effect of Con A on 5-HT uptake. The Con A effect on 5-HT uptake was also antagonized by LaCl3, a Ca++ entry blocker. When the activity of Ca++ transport into platelets was estimated, Con A was shown to have a stimulative effect, which was antagonized by alpha-methyl-D-mannoside, a specific antagonist of Con A binding to cell membrane glycoproteins. Furthermore, Con A was shown to stimulate the protein kinase C activity of platelets, which phosphorylates a 40-kDa platelet protein; the Con A effects were antagonized by alpha-methyl-D-mannoside, staurosporine and K-252a, but not by KT5720. We suggest that the activation of protein kinase C and phosphorylation of 40-kDa protein might be involved in the inhibitory effect of Con A on platelet 5-HT transport.

In vitro adsorption losses of arachidonic acid and calcium ionophore A23187

Arachidonic acid (AA) is often utilized in in vitro studies to label cellular pools of AA or to elicit cellular responses dependent on eicosanoid production. Because of the hydrophobic nature of AA, organic diluents such as ethanol or dimethyl sulfoxide are utilized in preparing concentrated solutions. The fate of AA when added to aqueous medium is not generally considered because of the dilution of the AA, although some investigators utilize bovine serum albumin (BSA) to solubilize as well as to trap AA and its hydrophobic metabolites. These experiments demonstrate a rapid and progressive decline in AA concentration when added to aqueous media in tissue baths and in glass test tubes. The extent of the decline was greater in the tissue baths than in the test tubes. The calcium ionophore A23187, which is used to stimulate AA metabolism, is also hydrophobic, and its concentration also decreased when added to aqueous media. The decline in the concentration of both AA and A23187 was due to adsorption to the container walls. The presence of 1% BSA in the aqueous solution attenuated and even eliminated the decline in the concentration, indicating binding of the two agents to the protein. However, the presence of BSA in culture medium inhibited the A23187-induced stimulation of AA metabolites in baboon aortic smooth muscle cells. These results underscore the complexities arising from the in vitro use of hydrophobic substances in biological systems.

Complex Ca2+ flux inhibition as primary mechanism of staurosporine-induced impairment of T cell activation

The inhibitory effect of the highly effective drug staurosporine on the early activation signal Ca2+ flux was investigated via multiparameter flow cytometry in human peripheral blood T lymphocytes. Staurosporine has been reported to be a specific inhibitor of protein kinase C. However, we show that it inhibits the Ca2+ influx in anti-CD3 and phytohemagglutinin-stimulated human CD4+ and CD8+ lymphocytes at concentrations between 1.0 and 10.0 ng/ml. Staurosporine decreases the number of Ca2+-positive CD4+ and CD8+ lymphocytes as well as the Ca2+ influx per cell; the drug also delays the time of the maximum response to polyclonal stimulation. In addition, we demonstrate that staurosporine affects the primary Ca2+ response via inhibition of the release of the membrane-bound Ca2+ from the endoplasmic reticulum in CD4+ and CD8+ lymphocytes. Binding studies of the anti-CD3 antibody to T lymphocytes indicate normal binding capacities in the presence of staurosporine. With respect to the classical scheme of T cell activation via phospholipase C, our data suggest that staurosporine may inhibit T cell activation primarily by its effect on the early Ca2+ flux signal.

Treatment of intact hepatocytes with the calcium ionophore A23187 perturbs both the synthesis and the degradation of the second messenger cyclic AMP. Actions on adenylate cyclase and cyclic AMP phosphodiesterase activities

The presence of the calcium ionophore A23187 augmented glucagon's ability to elevate intracellular cyclic AMP concentrations in intact hepatocytes. However, when the cyclic AMP phosphodiesterase inhibitor 1-isobutyl-3-methylxanthine (IBMX) was added to prevent the degradation of cyclic AMP then the presence of A23187 attenuated the ability of glucagon to increase intracellular cyclic AMP concentrations. Treatment of intact hepatocytes with A23187 led to a dose-dependent persistent inhibition of the glucagon-stimulated adenylate cyclase activity expressed by a membrane fraction isolated from such ionophore-treated hepatocytes. In hepatocytes where glucagon-stimulated adenylate cyclase activity was desensitized then A23187-treatment of hepatocytes failed to exert any inhibitory action on adenylate cyclase. Treatment of isolated membranes directly with A23187 did not elicit any changes in glucagon-stimulated adenylate cyclase activity. Such actions of A23187 were blunted when Ca2+ (2.5 mM) was not added to the extracellular medium. It is suggested that treatment of hepatocytes with A23187 leads to the functional uncoupling of glucagon-stimulated adenylate cyclase activity in a manner which appears to mimic the desensitization process. A23187-treatment also exerted an overall inhibitory effect on the cyclic AMP phosphodiesterase activity displayed by intact hepatocytes. Thus treatment of hepatocytes with A23187 exerted a profound effect on cyclic AMP metabolism in these cells.

Comparison of patterns of expression of tumour necrosis factor, lymphotoxin and interleukin-6 mRNA

The expression of the mRNA encoding tumour necrosis factor, lymphotoxin and interleukin-6 by peripheral blood mononuclear cells was analysed. Unstimulated cells contained no detectable mRNA for these cytokines, however each mRNA was transiently expressed after stimulation with either the combination of phytohaemagglutinin and phorbol ester or the single stimulus of lipopolysaccharide. The dual stimulus yielded the stronger signal. The cytokine mRNA's had short half lives, but were stabilised following protein synthesis inhibition. Cyclosporin A completely blocked induction of lymphotoxin and partially inhibited induction of TNF and IL-6 mRNA. The features of regulation described in this paper suggest these genes belong within the "early" set of genes expressed following immune cell activation.

C-fos gene activation in murine thymocytes by a mechanism independent of protein kinase C or a Ca2+ signal

The accumulation of c-fos mRNA in mouse thymocytes was compared when the cells were stimulated by concanavalin A (Con A), the Ca2+ ionophore A23187 or the phorbol ester, TPA, either separately or by combinations of these mitogens. The c-fos response to mitogenic concentrations of Con A could not be attributed either to the rise in [Ca2+]i it induces or to activation of protein kinase C. Thus, although Con A causes the breakdown of phosphatidylinositol 4,5-bisphosphate in these cells, neither of the signals which can be generated by this response was responsible for the activation of the c-fos gene by Con A. This implies that some other unidentified signal generated by Con A activates the c-fos gene.

Analysis of prostaglandin E2 effect on T lymphocyte activation. Abrogation of prostaglandin E2 inhibitory effect by the tumor promotor 12.0 tetradecanoyl phorbol-13 acetate

We have investigated the inhibitory potential of prostaglandin E2 (PGE2) with respect to intracellular messengers implicated in the signaling system of T-lymphocyte activation pathway. Using the fluorescent indicator Quin 2, it is demonstrated that PGE2 inhibits the increase in cytosolic-free calcium concentration [Ca2+]i. Reconstitution of calcium mobilization in the presence of PGE2 by the calcium ionophore A23187 results in a partial restoration of both interleukin 2 (IL2) production and cell proliferation and has no effect on the inhibition of transferrin receptor expression. In contrast, the treatment of cell cultures with the tumor promotor 12.0 tetra decanoyl phorbol-13-acetate (TPA) abrogates the suppressor activity of PGE2. When T lymphocyte stimulation is provided by the combination of A23187 and TPA, the PGE2 inhibitory effect does not occur. These data also indicate that the down regulation of transferrin receptor by PGE2 is proximal to protein kinase C activation and is not associated with decreased expression of the functional IL2 receptor.

Permeabilization of thymocytes by photon activation of erythrosin

Thymocytes previously loaded with quin 2 were rapidly permeabilized by the photon activation of erythrosin and the rate of permeabilization monitored by measuring fluorimetrically the increasing saturation of quin 2 with calcium. The extent of permeabilization was assessed also by the loss of [3H]quin 2 from the thymocytes and penetration of the cells by eosin and trypan blue. Lactate dehydrogenase leakage from the permeabilized cells was markedly delayed compared to the rapid increase in permeability to calcium and quin 2. The rate of permeabilization was dependent upon the concentration of erythrosin, the duration of illumination, the presence of oxygen, and the temperature. These results are consistent with the rapid photochemical generation of highly reactive singlet oxygen which alters thymocyte membrane structure and permeability.

Effect of cytochalasins on cytosolic-free calcium concentration and phosphoinositide metabolism in leukocytes

Cytochalasins are routinely used to stimulate a variety of functions in eukaryotic cells even though their precise mode of action remains to be elucidated. In the present work we used the fluorescent Ca2+ indicator quin2 to study the effect of various cytochalasins, cytochalasins A, B, C, D, E (CA, CB, CC, CD, CE) and dihydrocytochalasin B (dhCB) on the intracellular Ca2+ concentration ([Ca2+]i) in various types of leukocytes, viz, neutrophils and lymphocytes. In human neutrophils, cytochalasins increase [Ca2+]i mainly by releasing Ca2+ from membrane-bound, intracellular stores. Thus, in order to readily appreciate the effect of cytochalasins on [Ca2+ )i, these cells must be loaded with low intracellular quin2 concentrations. On the other hand, in peripheral blood lymphocytes, splenocytes and thymocytes, the increase in [Ca2+]i is predominantly due to an increased Ca2+ influx from the extracellular medium. In addition, we found that in neutrophils these drugs prolong the increase in [Ca2+]i induced by chemotactic peptides, probably by increasing the cell permeability to Ca2+. Finally, in thymocytes, cytochalasins potentiate the production of inositol phosphates induced by the polyclonal mitogen concanavalin A (conA).

The role of calcium in stimulation of activated T lymphocytes with interleukin 2

In a study of the role of Ca++ in the stimulation of activated T lymphocytes with interleukin 2 (IL-2) it was found that IL-2-induced proliferation can occur independently of extracellular calcium. Further, there was no correlation between triggering of DNA synthesis and an increase in free cytoplasmic calcium. However, IL-2 induced an increased uptake of 45Ca++ from the extracellular medium. Since there is no increase in free cytoplasmic calcium, it must be assumed that this is caused by an increase in membrane-associated calcium. Further, the calcium channel-blocking agent, verapamil, and TMB-8, a putative inhibitor of mobilization of calcium from intracellular pools, both exerted a dose-dependent inhibition of IL-2-induced DNA synthesis in activated T lymphocytes. We conclude that calcium is not a second messenger in activated T lymphocytes stimulated by IL-2, but our results indicate that calcium may play a role at membrane level.

Mitogen dose-dependent effect of weak pulsed electromagnetic field on lymphocyte blastogenesis

The effects of pulsed extremely low frequency electromagnetic fields on human peripheral blood lymphocyte mitogenesis induced by phytohaemoagglutinin, concanavalin A or calcium ionophore A23187 were studied. The dependence of the field effect on mitogen concentrations was investigated. Field exposure produced strong inhibition of DNA synthesis when optimal doses of mitogens were used, confirming our previous findings. Opposite effects were observed at suboptimal concentration of mitogens. Experiments performed by exposing cell cultures to the field for short periods indicated that a field application of at least 6 h is needed to influence irreversibly lymphocyte blastogenesis.

Early transmembrane events in alloimmune cytotoxic T-lymphocyte activation as revealed by stopped-flow fluorometry

We have studied early transmembrane events in mouse alloimmune cytotoxic T-lymphocyte (LC7, H-2b) activation by specific target cells (mouse mastocytoma P815, H-2d) and a mitogenic lectin, Con A, by using stopped-flow fluorometry with three different fluorescent probes. After binding to target cells (P815), cytotoxic T lymphocytes (LC7) first increased their membrane fluidity and, then, calcium was released from intracellular stores. After that, there was a calcium influx from the external medium into the T lymphocytes. This calcium influx was blocked by calcium antagonists (verapamil or diltiazem). The same sequence of events was also observed in the activation of T lymphocytes (LC7) by Con A and in the response of specific target cells (P815) after cytotoxic T lymphocytes (LC7) binding. Nonspecific (syngeneic) target cells (mouse lymphoma EL-4, H-2b) did not cause any early transmembrane events in cytotoxic T lymphocytes (LC7, H-2b).

Oxygen free radicals play a role in cellular differentiation: an hypothesis

Evidence from a variety of sources supports the view that oxygen free radicals play a role in cellular differentiation. It is postulated that cellular differentiation is accompanied by changes in the redox state of cells. Differentiated cells have a relatively more prooxidizing or less reducing intracellular environment than the undifferentiated or dedifferentiated cells. Changes in the redox balance during differentiation appear to be due to an increase in the rate of O2- generation. Differentiated cells, in general, exhibit higher rates of cyanide-resistant respiration, cyanide-insensitive SOD activity, and peroxide concentration and lower levels of GSH as compared to undifferentiated cells. The effects of free radicals on cellular differentiation may be mediated by the consequent changes in ionic composition.

Calcium ionophore A23187 induces interleukin 2 reactivity in human T cells

In the present study the activation of purified human T lymphocytes by the calcium ionophore A23187 was analysed in the light of current concepts of receptor-linked inositol lipid metabolism. It was found that A23187 was only slightly mitogenic, with a narrow optimum at 400-500 nM. The proliferation could be blocked by anti-Tac ascites at 10(-3) dilution, suggesting an interleukin 2 (IL-2)-dependent pathway of activation. However, an unexpectedly large proportion of A23187-stimulated cells expressed the IL-2 receptor. Reculturing the cells with exogenous IL-2 after removal of A23187 resulted in strongly enhanced proliferation. Phorbol myristic acetate (PMA) at non-mitogenic concentrations exerted an extremely strong synergistic effect on A23187-induced cell proliferation, which was, again, mediated via an IL-2-dependent pathway. Supernatants of A23187-stimulated T cells did not contain detectable amounts of IL-2. Combination of PMA and A23187 resulted in considerable IL-2 production. It is concluded that A23187 induces the expression of IL-2 receptors without concurrent stimulation of IL-2 production, thus allowing only low levels of proliferation. Addition of exogenous IL-2 or of PMA restores the imbalance between the occurrence of IL-2 and its receptor and results in high rates of proliferation.

A combination of calcium ionophore and 12-O-tetradecanoyl-phorbol-13-acetate (TPA) stimulates the growth of purified resting B cells

In this study we investigated whether the calcium ionophores A23187 and ionomycin can act synergistically with the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) to stimulate the growth of resting B lymphocytes purified from human tonsil cells. Ionomycin, A23187, and TPA added separately to cultures at doses of 0.4-1.6 micrograms/ml, 0.2-0.8 micrograms/ml, and 0.05-0.25 ng/ml respectively, did not induce DNA synthesis in resting B lymphocytes. In contrast, calcium ionophores at concentrations of 0.4-1.6 micrograms/ml ionomycin and 0.2-0.8 micrograms/ml A23187, in the presence of 0.05-4 ng/ml TPA, induced marked DNA synthesis and B-cell proliferation, as shown by analyses of incorporation of [3H]thymidine, growth kinetics, and the percentage of cells in the S and G2 + M phases of the cell cycle. These results show that the synergistic effects of calcium ionophores and TPA can bypass the requirement for antigen and exogenous growth factors in B-cell activation. These observations are similar to those obtained from studies of T lymphocytes by other workers.

Embryonic induction and cation concentrations in amphibian embryos

Explanted ectoderm from early gastrulae of Triturus alpestris was treated with the Na-K ionophore gramicidin (10(-9) to 10(-5) M) and the Ca-ionophore A 23187 (10(-7) to 10(-5) M). The ectoderm developed almost exclusively to atypical epidermis as in the control explants. When the ectoderm was treated with ouabain (10(-4) M), intracellular Na+ increased about 4.4-fold and K+ was reduced by half. Mesenchyme cells in small number differentiated in about 40% of the ouabain-treated explants. The time course of total Na+ and K+ ion concentrations was measured over a period of 72 h in ectoderm of T. alpestris after induction with vegetalizing factor and in control explants. In the first 15 h after explantation, no significant differences between control and induced explants were found. Thereafter, the steady state concentration of K+ decreased in the induced explants, whereas the steady-state concentration of Na+ slightly increased. The membrane resting potential recorded intracellularly of ectoderm sandwiches from early gastrula stages was found to be -41.3 mV in control and -59.3 mV in induced explants. From the specific conductances and permeabilities of non-induced and induced cells it is concluded that the induction process leads to a differentiation of the cell membrane, which acquires the characteristics of ionic selectivity. Ectoderm from Ambystoma mexicanum forms neural or neuroid tissue, mesenchyme and melanophores after explantation in salt solution in up to 50% of the explants without any additions. Isolated Ambystoma ectoderm is therefore not suitable for test experiments.

Antigen-dependent increase in cytosolic free calcium in specific human T-lymphocyte clones

Calcium has been implicated as an intracellular messenger in the cellular response to various external stimuli. Exposure of lymphocytes to various mitogens and lectins results in rapid transmembrane calcium fluxes and increased cytoplasmic calcium concentrations ([Ca2+]i). It is not clear, however, whether the mechanisms by which these non-physiological stimuli activate cells are related to those involved in antigen-specific activation. We have now used antigen-specific T-cell clones to study changes in [Ca2+]i associated with specific activation and show here that these cells respond specifically in the presence of antigen and antigen-presenting cells (APC) with increased [Ca2+]i and that this increased [Ca2+]i shows the same genetic restrictions as are seen in the proliferation assay. The kinetics of the [Ca2+]i response to antigen indicate that antigen undergoes a time-dependent processing step as a prerequisite for recognition by T cells, as has been shown for T-cell proliferative responses, but that the [Ca2+]i response to processed antigen is extremely rapid. The close correlation between changes in [Ca2+]i and cell activation resulting in proliferation suggests that Ca2+ may act as an intracellular messenger in antigen-specific responses.

The influence of antibiotics on the immune system. II. Modulation of fish leukocyte responses in culture

We report the immunomodulating effect of two tetracycline analogues (oxytetracycline and doxycycline) on mitogenic and allogeneic stimulation of carp (Cyprinus carpio) leukocytes in vitro. Both drugs interfered with 3H-thymidine incorporation into the DNA of phytohaemagglutinin-stimulated pronephric leukocytes in a dose-dependent manner. Low concentrations of oxytetracycline delayed the mitogenic response, but did not reduce it. In addition, several other antibiotics were tested for their possible interference with blastogenesis.

The free cytoplasmic Ca2+ levels in duchenne muscular dystrophy lymphocytes

An increased cellular Ca2+ content has been associated with Duchenne muscular dystrophy (DMD). However, estimates of the free cytoplasmic Ca2+ concentration ([Ca2+]i) in cells of DMD patients were not available. We compared the [Ca2+]i levels of normal and DMD peripheral blood lymphocytes and Epstein-Barr virus-transformed lymphoblasts using the novel probe, quin 2, an internally trapped fluorescent indicator. The [Ca2+]i levels of normal and DMD cells were not significantly different.

Kinetics of long-term (72 hr) calcium content during mitogen activation of cultured human T lymphocytes

In vitro stimulation of human blood lymphocytes with mitogen resulted in an increased intracellular content of Ca2+ per unit cell volume. This increase in Ca2+ content of lectin-activated cells reached a maximum after 24 hr of culture and thereafter slowly declined. Brief treatment of cells at 24 hr of culture with the Ca2+ ionophore A23187 in combination with EGTA resulted in a larger release of Ca2+ from cells in mitogen-stimulated cultures than from cells in control cultures. This indicates that the Ca2+ is accumulated intracellularly but is readily exchangeable. At 24 hr of culture the increase in cellular Ca2+ correlated well with the proliferative response as measured by 3H-thymidine incorporation. Ca2+ influx at 24 and 48 hr of culture was markedly enhanced in the mitogenically stimulated cells as compared either to cells cultured for 1 and 72 hr or cells cultured without mitogen.

Mitogen-induced phosphorylation of cytosolic proteins in rabbit T- and B-lymphocytes

The addition of anti-immunoglobulin (anti-Ig) to purified rabbit B-cells or concanavalin A (Con A) to purified rabbit T-cells within minutes resulted in the phosphorylation of a number of cytosolic proteins. Two-dimensional (2-D) electrophoresis and autoradiography of 32P-labeled cell sonicates was used to identify proteins whose phosphorylation was enhanced by these mitogens. Two proteins, pp58 and pp90, were phosphorylated 1.5 min after addition of anti-Ig to B-cells. Four other proteins, pp60, pp65, pp67 and pp95, were phosphorylated at later times. Three of these proteins were also phosphorylated after addition of Con A to purified T-cells. These phosphoproteins do not correspond to any previously described cytoplasmic proteins. Although all of these phosphoproteins were present in the cytosolic fraction, pp58 may be associated with the cytoskeleton. Protein pp58 is also distinguished from the rest by its absence from 2-D gels run under non-reducing conditions. Treatment of the B-cells with F(ab')2 fragments of anti-Ig stimulated phosphorylation but Fab' fragments did not--indicating that receptor cross-linking is required to induce phosphorylation. Both pp58 and pp90 contained phosphoserine, but neither phosphothreonine nor phosphotyrosine. Quantitatively the 32P-labeling of pp58 was 2.7-fold over background at 10 min after anti-Ig addition. The identification of these phosphoproteins, which may play a role in activational cascades or in cytoskeletal rearrangements, hopefully will help to clarify the interrelationships between cyclic nucleotide dependent and independent kinases in lymphocyte activation.

A role for Ca2+ in the effect of very low frequency electromagnetic field on the blastogenesis of human lymphocytes

The DNA synthesis of lymphocytes triggered by phytohemagglutinin or phorbol-myristate-acetate is strongly reduced by the externally applied electromagnetic field (ELF). Ca2+ uptake by stimulated lymphocytes is also reduced by ELF. The effect appears to be synergistic with that of the well-known calcium blocker agent, verapamil.

Early steps of lymphocyte activation bypassed by synergy between calcium ionophores and phorbol ester

Although it has been proposed that the activation of T lymphocytes is mediated by an early rise in cytosolic calcium concentration, it has not been possible to mimic antigen- or mitogen-induced mouse lymphocyte activation by calcium ionophores that bypass receptor-mediated processes. There is now evidence from other systems that the rise in cytosolic calcium which follows receptor triggering is preceded by the breakdown of phosphatidylinositol bisphosphate into 1,2-diacylglycerol and inositol trisphosphate. The latter is known to cause release of calcium from intracellular stores. The cellular target for the former is now widely accepted to be protein kinase C. Therefore, ligand-induced cellular response follows a rise in cytosolic calcium concentration and protein kinase C activation. Here we confirm that the calcium ionophores A23187 and ionomycin do not activate mouse T lymphocytes. However, either one in combination with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA), which is structurally related to 1,2-diacylglycerol, induces in lymphoid cell populations the expression of receptors for interleukin-2 (IL-2), the secretion of IL-2 and cell proliferation as measured by 3H-thymidine uptake. The growth-promoting effect of IL-2 on an exogenous IL-2-dependent clone could not be substituted for by ionomycin either alone or with TPA. Thus, the combination of calcium ionophores and TPA bypasses the requirement for antigen- or lectin-induced signal at the onset of lymphocyte activation.

Interleukin 2 stimulates T cell proliferation using a calcium flux

A preparation enriched in rat interleukin 2 caused enhanced DNA synthesis in an interleukin 2-dependent mouse cytotoxic T cell line, in lectin transformed mouse splenocytes and in rat thymocytes. The enhanced proliferation due to interleukin 2 could be abrogated by chelating calcium from the culture medium or blocking calcium entry into the cells. Compounds which interfere with the function of calmodulin also inhibited proliferation. The addition of interleukin 2 to IL-2 dependent cells caused an increase in the intracellular concentration of calcium ions, as measured using Quin 2. The requirement for IL-2 by blasts and thymocytes could be replaced by calcium ionophore. The results implicate a calcium flux as an essential component of the action of interleukin 2 on its target cells.

Effects of Cd2+ upon Ca2+ fluxes and proliferation in concanavalin A-stimulated lymphocytes

The mitogenic response of human peripheral blood lymphocytes to the lectin concanavalin A (conA) is inhibited by micromolar concentrations of CdCl2. This inhibition is partially relieved by an increase in the external Ca2+ concentration (from 0.6 to 2.2 mM). The initial rate of conA-induced 45Ca2+ influx is unaltered by CdCl2, although the level of 45Ca2+ accumulation increases. The basal rate of 45Ca2+ entry is not measurably disturbed by CdCl2 (100 microM). The steady-state efflux of 45Ca2+ and the calmodulin-activated (Ca2+ + Mg2+)-ATPase activity of erythrocyte ghosts are inhibited by CdCl2 (10 microM). Thus, the mechanism behind the Cd2+-induced suppression of the mitogenic response to conA is not due to alteration of mitogen-stimulated Ca2+ influx. We suggest that Cd2+ competes with Ca2+ for intracellular Ca2+-binding molecules, such as calmodulin, essential for the induction of cell proliferation.

Ion channels in lymphocytes

The advent of the gigaohm-seal recording technique has enabled the study of the electrical properties of small cells, such as individual lymphocytes. Recent studies using this technique in combination with standard immunological and biochemical techniques indicate that cells of the immune system may utilize ion channels, similar in properties to those described in nerve and muscle, in the process of activation. For example, potassium channels may be required for T-lymphocyte mitogenesis and calcium channels for antibody production. This article summarizes these recent reports.

Voltage-gated potassium channels are required for human T lymphocyte activation

The calcium channel blockers, verapamil and diltiazem, inhibit phytohemagglutinin (PHA)-induced mitogenesis at concentrations that block the T lymphocyte K channel currents. K channel blockers also inhibit the allogeneic mixed lymphocyte response in a dose-dependent manner with the same potency sequence as for block of K currents. K channel blockers inhibit PHA-stimulated mitogenesis only if added during the first 20-30 h after PHA addition, but not later, indicating a requirement for functional K channels during this period. We investigated the effect of K channel blockers on various aspects of protein synthesis for two reasons: first, protein synthesis appears to be necessary for the events leading to DNA synthesis, and second, the increase in the protein synthetic rate commences during the first 24-48 h after PHA addition. PHA-induced total protein synthesis was reduced to the level in unstimulated T lymphocytes by K channel blockers in a dose-dependent manner with the same potency sequence as for the block of K currents and inhibition of [3H]thymidine incorporation. Two-dimensional gel electrophoresis demonstrated that although the synthesis of the majority of proteins was reduced by K channel blockers to the level in unstimulated T cells, some proteins continued to be synthesized at an enhanced rate compared with resting cells. Two proteins, S and T, detected by two-dimensional gel electrophoresis in unstimulated T lymphocytes, appeared to be reduced in intensity in gels of PHA-treated T lymphocytes, in contrast to the increased synthesis of the remaining proteins. 4-Aminopyridine (4-AP), at concentrations that inhibit protein synthesis, prevented the apparent PHA-induced reduction of proteins S and T. These proteins may play a role in maintaining the T lymphocyte in a resting state and may be related to the translation inhibitory factors reported to be present at a higher specific activity in quiescent T lymphocytes than in PHA-activated T cells. The expression of the IL-2 receptor (Tac) during T lymphocyte activation was not altered by K channel blockers, whereas the production of interleukin 2 (IL-2) was reduced to the level in unstimulated T lymphocytes. Exogenous IL-2 partially relieved the inhibition of mitogenesis by low, but not by high, concentrations of 4-AP. These experiments clarify the role of K channels in T lymphocyte activation and suggest that functional K channels are required either for protein synthesis or for events leading to protein synthesis.

Direct mitogenic effect of ionophore A23187 on isolated human T helper lymphocytes

The Ca2+ ionophore A23187 induces only a weak mitogenic response in cultures of unfractionated mononuclear leukocytes from human blood. When a comitogen, 12-O-tetradecanoyl-phorbol-13-acetate, which by itself is nonmitogenic, is added, a greatly increased cell proliferation is obtained. Highly purified T lymphocytes respond by proliferation to A23187 alone. Studies on functional subsets of T cells, fractionated by using the monoclonal antibodies OKT4/OKT8, revealed that A23187 is a strong mitogen for pure T helper cells (T4+). This suggests that increased cytosolic Ca2+ directly triggers a proliferative response in T helper cells with no apparent need for accessory cells.

Is cytoplasmic Ca2+ in lymphocytes elevated in cystic fibrosis?

An increased cytoplasmic Ca2+ concentration ( [Ca2+]i) has been implicated in the pathogenesis of cystic fibrosis. We compared the [Ca2+]i levels of normal and cystic fibrosis peripheral blood lymphocytes and Epstein-Barr virus-transformed lymphoblasts using quin 2, an internally trapped indicator. The [Ca2+]i levels of normal and cystic fibrosis cells were not significantly different. The ionophore-releasable intracellular Ca2+ stores were also comparable in both types of individual.

Effects of the skin mitogens tumor-promotor 12-O-tetradecanoylphorbol 13-acetate and divalent-cation-ionophore A23187 on ion fluxes and membrane potential in a murine epidermal cell line (HEL30) and in 3T3 fibroblasts

The transmembrane potential of HEL30 keratinocytes and 3T3 fibroblasts has been determined by measuring the distribution of labelled triphenylmethylphosphonium bromide. The tumor-promotor 12-O-tetradecanoylphorbol 13-acetate (1-5 microM) induces hyperpolarization in 3T3 cells but does not exert any effect on the membrane potential of keratinocytes, whereas the divalent cation ionophore A23187 (0.5 - 1 microM) hyperpolarizes keratinocytes and probably also 3T3 cells. Studies on Na+ and Rb+ fluxes, as well as with different inhibitors, indicate that the hyperpolarizing effect is the consequence of an increased Na+ influx which in turn stimulates the Na+/K+-dependent ATPase. No causal relationship seems to exist between the change of the membrane potential and arachidonic acid release (and subsequent prostaglandin synthesis) which is induced by both drugs in both cell lines. Since the induction of the arachidonic cascade (by both agents) as well as the stimulation of Na+ influx (by A23187) are found to be critically dependent on extracellular Ca2+ and are inhibited by 'Ca2+-blockers', it is concluded that both reactions are triggered by the same event (Ca2+ translocation) but proceed independently of each other. The release of arachidonic acid is already stimulated under conditions where a measurable influx of Ca2+ is not yet observed. This indicates a local mobilization of Ca2+, perhaps across the plasma membrane. It is concluded that monovalent cation fluxes and changes of the membrane potential are not critically involved in the stimulation of the arachidonic acid cascade and cellular proliferation by agents which induce epidermal hyperplasia in vivo.

The effect of ionophore A23187 on albumin internalization in cultured human umbilical vein endothelial cells

The effects of calcium and the calcium ionophore A23187 on endocytosis were studied in cultured human umbilical vein endothelial cells using iodinated human albumin to measure bulk phase endocytosis. In the absence of the ionophore, varying the levels of extracellular calcium did not affect endocytosis. In the presence of 10 microM A23187, the endocytic clearance of albumin decreased approx. 50% when exposed to physiological concentrations of extracellular calcium, but increased approx. 50% at lower calcium concentrations. Since the ionophore is known to alter cellular calcium levels, these results are compatible with a role for intracellular calcium in the modulation of endothelial cell endocytosis.

Mechanism of activation of pyruvate dehydrogenase by mitogens in pig lymphocytes

The activity of pyruvate dehydrogenase in extracts of pig mesenteric lymphocytes was measured under different preincubation conditions. The mitogens concanavalin A and ionophore A23187 both increased pyruvate dehydrogenase activity. In both cases activation required extracellular Ca2+. Digitonin-permeabilized cells required 0.5 microM free Ca2+ for half-maximal activation of pyruvate dehydrogenase. The stimulation by concanavalin A in intact cells was probably not due to changes in effectors of pyruvate dehydrogenase kinase. This evidence suggests that activation of pyruvate dehydrogenase is by Ca2+ activation of pyruvate dehydrogenase phosphatase and supports the view that the cytoplasmic free [Ca2+] rises to something less than 1 microM on stimulation with mitogens.

Large effects of preparative techniques on lymphocyte cyclic AMP content

When a cell suspension is formed by disruption of a pig lymph node into medium, large and transient increases in intracellular cyclic AMP occur. Similar effects are observed when pig lymphocytes are centrifuged and the cell pellets resuspended, or when the cells are subjected to rapid temperature changes. These observations define the conditions required to manipulate the cells while maintaining a stable cyclic AMP concentration. Under these conditions, neither concanavalin A nor ionophore A23187 at mitogenic concentrations have any early effect on cyclic AMP in pig lymphocytes, but small increases in cyclic AMP (less than 2-fold) were observed at supramitogenic concentrations of concanavalin A (50 microgram/ml) or A23187 (500nM). Mouse thymocytes show qualitatively similar but much smaller changes in cyclic AMP concentration in response to experimental manipulations, and no response to mitogenic or supramitogenic concentrations of concanavalin A below the cytotoxic value.

Calcium-dependent activation of lymphocytes by ionophore, A23187, and a phorbol ester tumor promoter

The phorbol ester tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) and the calcium ionophore, A23187, have similar effects on many different cells. For example, both show mitogenic and comitogenic activities for lymphocytes. It had been suggested that some of TPA's effects are due to its ability to act as a calcium ionophore. In order to test this idea, we compared the ability of TPA and ionophore to synergize with concanavalin A (Con A) in a two-phase system of lymphocyte mitogenesis. We found that ionophore was most comitogenic with Con A when present in the early phase of stimulation. TPA was only comitogenic when present in the late phase. Ionophore and TPA could not replace one another in the system. However, both ionophore and TPA together could replace Con A and stimulate DNA synthesis when they were presented to the cells in the sequential order of ionophore followed by TPA. Both compounds required the presence of external calcium to be effective.

Limits to the early increase in free cytoplasmic calcium concentration during the mitogenic stimulation of lymphocytes

Three aspects of the calcium hypothesis we have proposed previously [Metcalfe, Pozzan, Smith & Hesketh (1980) Biochem. Soc. Symp. 45, 1-26] for the control of mitogenic stimulation of lymphocytes are examined in studies on the mitogenic action of the Ca2+ ionophore A23187 and its effect on cap formation. (1) Pig lymphocytes that were mitogenically stimulated by continuous incubation with 3H-labelled A23187 for 48 h contained between 3 and 15 amol of ionophore per cell. Lymphocytes exposed to 3H-labelled A23187 for 2h before washing the cells and resuspending them in ionophore-free medium were only stimulated mitogenically at 48h if the residual ionophore associated with the cells after washing was in the concentration range 3-15 amol per cell. When the cells were washed repeatedly after 2h incubation with ionophore to reduce the cell-associated ionophore below the critical concentration range, no mitogenic stimulation occurred as a result of short-term exposure to any ionophore concentration. Re-addition of ionophore to within the indicated range of cell-associated concentrations restored mitogenic stimulation at 48h. We conclude that large, short-term Ca2+ fluxes into the cells induced by the ionophore cannot generate a mitogenic signal that commits the cells to enter the cell cycle. (2) Further experiments with the ionophore showed that detectable mitogenic stimulation at 48h required a minimum of 3h exposure to optimal ionophore concentrations, and that maximal stimulation required at least 20h exposure. This is consistent with the view that a prolonged increase in the free cytoplasmic calcium concentration is required to stimulate the maximum proportion of the cells into the cell cycle. (3) Mouse splenic lymphocytes treated for short periods with very high ionophore concentrations (30 microM) in the presence of various external Ca2+ concentrations showed significant inhibition of cap formation of surface immunoglobulin receptors in the range 1-10 microM-Ca2+ in normal or depolarizing medium. We conclude that mitogens at optimal concentrations for the stimulation of lymphocytes do not cause any early increase in the free cytoplasmic Ca2+ concentration above 10 microM.