Inhibition by staurosporine of mitogen-induced calcium mobilisation in human T lymphoblasts

Quantitative and qualitative signals determine T-cell cycle entry and progression

Cell growth and proliferation as well as cell cycle arrest and apoptosis all play integral roles in the cellular immune response. The signals that lead to cytokine production by antigen- or mitogen-stimulated T cells have been studied in detail. However, it is not fully understood how these signals promote cell cycle entry and progression to DNA synthesis in T lymphocytes, especially in primary cells. We used a model distinguishing between competence and progression phases to examine quantitative and qualitative differences in signal transduction that resulted in cell cycle entry and G1 phase arrest or led to DNA synthesis in human T cells. Resting peripheral blood T cells were rendered competent by stimulation with submitogenic concentrations of phytohemagglutinin (PHA) or they were stimulated to proliferate using mitogenic concentrations of PHA. The competent state (that is, the capacity to proliferate in response to exogenous IL-2) was characterized by calcium mobilization, a protein kinase C-dependent internalization of CD3, increased mitogen-activated protein kinase (MAPK) activity, transient translocation of AP-1 transcription factors to the nucleus, expression of immediate early genes, activation of G1-phase cyclin-dependent kinases, and increased CD25 (IL-2Ralpha) expression. However, all of these events were of lesser magnitude in T cells rendered competent than in T cells stimulated to proliferate. Furthermore, the mitogenic stimulus induced a different pattern of MAPK activation and sustained translocation of AP-1 to the nucleus with concomitant IL-2 production. The data indicate that quantitative and qualitative differences in early signaling events distinguish the acquisition of the competent state or the induction of cytokine production with a commitment to T-cell proliferation.

Hypoxia inhibits macrophage migration

The chemokine monocyte chemoattractant protein (MCP)-1 plays a role in regulating the lymphocyte and macrophage infiltrate in ovarian cancer, but macrophages also accumulate in necrotic areas of the tumors where there is little MCP-1 expression (Negus, R. P. M. et al., Am. J. Pathol. 1997. 150: 1723-1734). Necrotic regions are likely to be hypoxic. In this study we show that hypoxia inhibits MCP-1-induced migration of THP-1 monocytic cells and human macrophages. In contrast, lymphocytes from peripheral blood migrate normally to an MCP-1 gradient in hypoxic conditions. The inhibition of monocyte migration by hypoxia is rapid and reversible. At the exposure times studied (30-90 min) hypoxia does not affect expression of the MCP-1 receptor CCR2B and cells exposed to hypoxia still respond to MCP-1 with an elevation of intracellular calcium. Although hypoxia is known to modulate gene expression, the inhibition of migration reported here was not due to the production of soluble factors, and mRNA expression of macrophage migration inhibitory factor was unchanged. Hypoxia-induced inhibition of chemotaxis was not limited to MCP-1. Hypoxia also inhibited the chemotactic response to macrophage inflammatory protein-1alpha, RANTES and the chemoattractant N-formyl-met-leu-phe, but hypoxic cells were still able to phagocytose opsonized red blood cells. We suggest that inhibition of migration by hypoxia is not due to gene regulation but is a reflection of metabolic changes in the cell. Transient hypoxia may regulate the distribution of macrophages in tumors and other inflammatory conditions.

Regulation of adenylyl cyclase activity in human peripheral blood mononuclear cells: effects of protein kinase inhibitors and of a calcium ionophore

In a previous paper we presented evidence for a negative regulation of adenylyl cyclase activity by tyrosine protein kinase(s) in the human leukemic T cell line Jurkat. In order to examine this point in non malignant cells, we conducted the present study in human peripheral blood mononuclear cells (PBMC). In these cells, staurosporine, a broad spectrum protein kinase inhibitor, enhanced not only the receptor-mediated, induced by prostaglandin E2 (PGE2), but also the direct (forskolin-induced) stimulation of adenylyl cyclase activity. Herbimycin A, a specific protein tyrosine kinase inhibitor, reproduced only in part the effect of staurosporine, whereas bisindolylmaleimide, the most specific protein kinase C (PKC) inhibitor known at present time, was ineffective. All these observations were made both in the absence and presence of isobutylmethylxanthine, a phosphodiesterase inhibitor, indicating that the effects of staurosporine and herbimycin A on cAMP accumulation were not due to phosphodiesterase inhibition. The calcium ionophore A 23187 also enhanced the PGE2-induced cAMP accumulation, and this effect was not additive to that of staurosporine, but additive to that of herbimycin A. These results confirm and extend those obtained in Jurkat cells. Taken together, they indicate that in human PBMC the adenylyl cyclase activity is negatively regulated by tyrosine kinase(s) and not by PKC, and positively regulated by Ca2+. They also suggest that the major enhancement by staurosporine of the PGE2-induced cAMP accumulation, although chiefly mediated by protein tyrosine kinase inhibition, also depends on another, presently undetermined, effect of the drug simulating that of Ca2+.

Effects of staurosporine on the capacitative regulation of the state of the Ca2+ reserves in activated Jurkat T lymphocytes

Staurosporine (Stp) is an inhibitor of protein kinase C (PKC) that has been used to address the role of this enzyme in a variety of cells. However, Stp can also inhibit protein tyrosine kinases (PTK). We have investigated the effects of Stp on the InsP3-(using mAb C305 directed against the beta chain of the T cell receptor (TcR/CD3 complex) and the thapsigargin (Tg)-dependent release and influx of Ca2+ in human (Jurkat) T cells. The addition of Stp (200 nM) during the sustained phase of the TcR-dependent Ca2+ response resulted in a rapid inhibition of the influx of Ca2+ that was not seen when Ca2+ mobilization was triggered by Tg (1 microM). When the cells were preincubated with Stp (200 nM), there was an inhibition of the mAb C305- but not the Tg-dependent Ca2+ response. The effect of Stp was not the result of the inhibition of PKC as shown by down-regulation of PKC and with the use of the specific PKC inhibitor bis-indolyl maleimide GF 109203X. The effect of Stp on the entry of Ca2+ in activated (mAb C305) Jurkat lymphocytes was dose-related and was not the result of a direct inhibition of plasma membrane Ca2+ channels based on an absence of effect on the Tg-dependent entry of Ca2+ and the use of Ca2+ channel blockers (econazole and Ni2+). These blockers terminated the influx of Ca2+ but the Tg-sensitive Ca2+ reserves were not refilled in marked contrast to the effect of Stp. Quantification of InsP3 revealed that the addition of Stp resulted in an approximate 40% reduction in mAb C305-activated Jurkat cells. The effects of Stp can be explained as follows. Stp decreases the mAb C305-induced production of InsP3 by inhibiting the TcR/CD3-dependent activation of PTK associated with the stimulation of phospholipase C-gamma 1. A decrease in [InsP3] without a return to baseline is sufficient to close the InsP3 Ca2+ channel, endoplasmic Ca2+ ATPases use the incoming Ca2+ to refill the Ca2+ pools and that terminates the capacitative entry of Ca2+. A simple kinetic model reproduced the experimental data.

Inhibition of CD28-mediated T cell costimulation by the phosphoinositide 3-kinase inhibitor wortmannin

T lymphocyte activation requires at least two signals, one via the antigen-specific T cell receptor and a second via the surface molecule CD28 which provides signals critical to interleukin-2 (IL-2) production and T cell proliferation. We have previously shown (Ward S. G., Westwick, J., Hall N. and Sansom D. M. Eur. J. Immunol. 1993. 23: 2572) that CD28 stimulates phosphoinositide (PI) 3-kinase activity, indicating that D-3 phosphoinositides may act as mediators of CD28-induced T cell costimulation. Here, we report that immunoprecipitation of CD28 molecules from Jurkat cells stimulated with the CD28-ligand B7, results in a ligand-dependent association of CD28 with PI 3-kinase. This association correlates with the appearance of PI 3-kinase enzymatic activity in CD28 immunoprecipitates and the formation of D-3 phosphoinositides. Consistent with the hypothesis that D-3 phosphoinositides are important mediators of CD28 signaling, treatment of T cells with the PI 3-kinase inhibitor wortmannin, inhibited both T cell proliferation and production of IL-2, but not the response of T cells to exogenous IL-2. Hence, abrogation of PI 3-kinase activity by wortmannin, appears sufficient to disrupt the costimulatory pathway utilized by CD28, indicating a central role for this enzyme in the CD28 signaling pathway.

Regulation of T-cell-receptor-stimulated bivalent-cation entry in Jurkat E6 cells: role of protein kinase C

Stimulation of Jurkat E6 cells with anti-CD3 antibody results in a characteristic rise in [Ca2+]i which is due to both the release of Ca2+ from intracellular stores and the entry of external Ca2+. Individual components of the [Ca2+]i increase were investigated by measuring intracellular Ca2+ release in the absence of external Ca2+ and determining influx of bivalent cations by following the entry of Mn2+. The increase in [Ca2+]i induced by anti-CD3 antibody in the presence or absence of extracellular Ca2+ could be inhibited by the non-selective kinase inhibitor staurosporine, which also inhibits anti-CD3-stimulated phospholipase C activity. Staurosporine also inhibits the influx of bivalent cations induced by anti-CD3 antibody, but not that induced by depletion of intracellular Ca2+ stores using thapsigargin. The effect of staurosporine was compared with that of Ro 31-8425, a potent and selective inhibitor of protein kinase C (PKC). Ro 31-8425, at concentrations up to 10 microM, has no inhibitory effect on the anti-CD3 antibody-induced [Ca2+]i increase or phospholipase C activity. These studies are consistent with the concept that augmentation of [Ca2+]i by stimulated T-cell receptors requires activation of a kinase, probably a tyrosine kinase such as p56lck, ZAP-70 or p59fyn, and is independent of PKC. Phorbol esters inhibit the anti-CD3-stimulated [Ca2+]i increase and phospholipase C activity, showing that this can be negatively regulated by PKC. A small potentiation of the anti-CD3 antibody-induced [Ca2+]i rise in the presence of extracellular Ca2+ was detected in the presence of Ro 31-8425; this suggests that T-cell-receptor ligation can also limit the increase in [Ca2+]i via PKC activation.

Effects of lymphokines and mitogens on a histamine derivative-induced intracellular calcium mobilization and inositol phosphate production

Histamine trifluoromethyl-toluidine derivative (HTMT), a novel immunosuppressive agent, stimulates H1, H2 and HTMT receptors in lymphocytes. HTMT receptors are different from the classical H1, H2 or H3 receptors. Stimulation of HTMT receptors results in increased intracellular concentrations of calcium ([Ca2+]i) and inositol phosphate (IP) in human peripheral blood lymphocytes. In the present study, we investigated the effects of lymphokines [interleukin-4 (IL-4), interleukin-2 (IL-2)] and other pharmacologic agents [lipopolysaccharide (LPS), phorbol 12-myristate 13-acetate (PMA)] on HTMT-induced Ca2+ and IP responses in non-rosetted cells. HTMT caused enhanced [Ca2+]i and IP responses when the cells were pretreated with IL-4. The effects of IL-4 were concentration dependent and became maximal after the cells were incubated with IL-4 for 48 hr. Inhibitors of protein synthesis, but not of RNA synthesis, blocked the effects of IL-4 on HTMT-induced responses. LPS was more potent than IL-4 in augmenting CA2+ mobilization induced by HTMT. However, the effects of LPS were not altered by inhibitors of either protein synthesis or RNA transcription. This indicated that LPS may act differently than IL-4 on the HTMT response. IL-2 and PMA did not affect HTMT-induced [Ca2+]i and IP responses. The effects of IL-4 and LPS were agonist specific. They did not affect the Ca2+ mobilization induced by PAF. The data indicate that the response to HTMT can be regulated by IL-4 and LPS. Although the in vivo importance of these receptors is not yet clear, the receptor is likely a contributor to immune and/or inflammatory regulation.

Integrated system for the screening of the specificity of protein kinase inhibitors

Tyrosine protein kinases (TPKs) play a major role in the transformation of cells. They are currently used as molecular targets for new generations of anticancer compounds. Numerous TPKs have been described from various tissues using either classical molecular biochemical techniques or cloning strategies. As a natural extension of these discoveries, a large number of "specific" inhibitors have been described in the literature. The major problem with these inhibitors is that there is no simple way to compare their specificity and/or selectivity from one report to another. We have set up a simple, straightforward technique to compare the inhibitory potency of 14 classical inhibitors towards six well-described and at least partially purified protein kinases. This technique is based on a new assay, easy to carry out and non-restrictive in terms of the type of protein substrate used. It permits direct comparisons between the results obtained from various sources. Data obtained showed that, when assessed in this integrated system, specificity and selectivity of many "classical" inhibitors are often weak, thus demonstrating that a universal technique such as ours is essential for the molecular screening of new protein kinase inhibitors. Compounds showing specificity for this panel of protein kinases will be more easily targeted to some defined types of oncogene and of transformed cells.

Effects of inorganic and organic mercury on intracellular calcium levels in rat T lymphocytes

The importance of cytosolic free calcium level ([Ca2+]i) in lymphocyte activation prompted us to investigate changes in [Ca2+]i in T cells caused by mercury compounds, which have been shown to have immunomodulatory and immunotoxic properties. Using fura-2 as fluorescent Ca2+ indicator, we found that both methyl-mercury (MeHg; 0.02-2 microM) and inorganic mercury (HgCl2; 0.01-1 microM) increased [Ca2+]i in lymphocytes from rat spleen in a concentration-dependent manner. The effect of MeHg was rapid and the increase of Ca2+ level was sustained in time, while HgCl2 caused a slow rise in [Ca2+]i. The effects of mercury compounds did not appear to be associated with alterations of membrane integrity, since there was no significant difference in the extent of MnCl2 quench between control and mercury-treated cells. However, HgCl2 (1 microM) and MeHg (2 microM) appeared to cause membrane damage at longer incubation times (15 min). When cells were incubated in Ca(2+)-free medium (in the presence of 1 mM EDTA) MeHg still increased [Ca2+]i, though to a lesser extent, while HgCl2 had no effect. Heparin, an inhibitor of inositol 1,4,5-trisphosphate-induced Ca2+ mobilization partially blocked this rise of [Ca2+]i, while carbonyl cyanide m-chlorophenylhydraxone (CCCP), an inhibitor of mitochondrial function, had a lesser effect. When added together, heparin and CCCP almost completely block the response to MeHg. These results suggest that MeHg and HgCl2 exert their effects of [Ca2+]i in different ways: MeHg-induced increases in [Ca2+]i are due to influx from outside the cells as well as to mobilization from intracellular stores, possibly the endoplasmic reticulum, and, to a minor extent, the mitochondria; on the other hand, HgCl2 causes only Ca2+ influx from the extracellular medium.

Multiple effects of staurosporine, a kinase inhibitor, on thymocyte functions. Comparison with the effect of tyrosine kinase inhibitors

The effects of staurosporine, a protein kinase inhibitor, on the signal transduction and proliferation of thymocytes were studied. Signal transduction in response to Concanavalin A (Con A) as well as Concanavalin A (Con A)-induced augmentation of [3H]inositol incorporation into phospholipids were inhibited by staurosporine (> or = 10(-8) M). Staurosporine inhibited thymocyte proliferation in response to Con A in the presence or absence of the phorbol ester, phorbol myristate acetate (TPA) (10 nM). This inhibition was observed regardless of whether staurosporine was added together with Con A or 3 hr later. High concentrations of staurosporine (> 10(-8) M) inhibited thymocyte proliferation induced by the calcium ionophore A23187 and the phorbol ester TPA, whereas lower concentrations of the inhibitor (< or = 10(-8) M) enhanced thymidine incorporation in response to these activators. This dual effect of staurosporine was also observed in the presence of the staurosporine-related kinase inhibitor, K252a. In contrast, the tyrosine kinase inhibitor, tyrphostin AG490, inhibited the response to A23187 and TPA at all concentrations of the inhibitor and no augmentation was seen. Interleukin 2 (IL-2)-driven mitogenesis in IL-2-dependent cells was also inhibited by staurosporine. We suggest that the inhibition of thymocyte proliferation by staurosporine results from inhibition of both protein kinase C and tyrosine kinase: the augmentation of the response to A23187 and TPA results from inhibition of protein kinase C. Inhibition of signal transduction as well as inhibition of IL-2-driven mitogenesis result from inhibition of tyrosine kinase.

The effect of endotoxemia on concanavalin A induced alterations in cytoplasmic free calcium in rat spleen cells as determined with Fluo-3

The effects of acute (3 h) and chronic (30 h) in vivo infusions of Escherichia coli endotoxin on the Ca2+ homeostasis of rat spleen cells was investigated. Conditions were established for obtaining reliable estimates of [Ca2+]i in these cells using the newly-developed Ca2+ indicator Fluo-3. The resting [Ca2+]i of splenocytes and T lymphocyte-enriched preparations were 119 +/- 35 and 102 +/- 31 nM, respectively. Treatment of the cells with concanavalin A (Con A) resulted in a rapid increase in [Ca2+]i. The magnitude of the increase was positively correlated with the concentration of Con A, whereas the time required to reach the maximum [Ca2+]i was inversely related to the amount of Con A. The peak [Ca2+]i was attained more rapidly in splenocytes (i.e. less than or equal to 30 s) than in the T cell-enriched fraction (i.e. 1.5-2.0 min). Both the resting [Ca2+]i and the Con A-induced increase in [Ca2+]i were similar to values previously reported for other lymphocyte cell types using different Ca2+ indicators, thereby supporting the values obtained with Fluo-3. Infusions of saline or endotoxin prior to the isolation of the cells did not result in significant alterations of either resting [Ca2+]i or the cells' response to Con A. Since chronic infusions of endotoxin have previously been shown to cause a reduction in blastogenic responsiveness of splenocytes to Con A, these data suggest that the endotoxin-induced lesion occurs distal to the mobilization of intracellular Ca2+.

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.