Inhibition of Cytotoxic T Lymphocyte-mediated Lysis and Cellular Proliferation by Isoquinoline Sulfonamide Protein Kinase Inhibitors

Rapid degradation of ABCA1 protein following cAMP withdrawal and treatment with PKA inhibitor suggests ABCA1 is a short-lived protein primarily regulated at the transcriptional level

Objectives

ATP-binding cassette transporter A1 (ABCA1) is a key player in the reverse cholesterol transport (RCT) and HDL biogenesis. Since RCT is compromised as a result of ABCA1 dysfunction in diabetic state, the objective of this study was to investigate the regulation of ABCA1 in a stably transfected 293 cells expressing ABCA1 under the control of cAMP response element.

Methods

To delineate transcriptional and posttranscriptional regulation of ABCA1, 293 cells were stably transfected with the full length ABCA1 cDNA under the control of CMV promoter harboring cAMP response element. cAMP-mediated regulation of ABCA1 and cholesterol efflux were studied in the presence of 8-Br-cAMP and after withdrawal of 8-Br-cAMP. The mechanism of cAMP-mediated transcriptional induction of the ABCA1 gene was studied in protein kinase A (PKA) inhibitors-treated cells.

Results

The transfected 293 cells expressed high levels of ABCA1, while non-transfected wild-type 293 cells showed very low levels of ABCA1. Treatments of transfected cells with 8-Br-cAMP increased ABCA1 protein by 10-fold and mRNA by 20-fold. Cholesterol efflux also increased in parallel. Withdrawal of 8-Br-cAMP caused time-dependent rapid diminution of ABCA1 protein and mRNA, suggesting ABCA1 regulation at the transcriptional level. Treatment with PKA inhibitors abolished the cAMP-mediated induction of the ABCA1 mRNA and protein, resulting dampening of ABCA1-dependent cholesterol efflux.

Conclusions

These results demonstrate that transfected cell line mimics cAMP response similar to normal cells with natural ABCA1 promoter and suggest that ABCA1 is a short-lived protein primarily regulated at the transcriptional level to maintain cellular cholesterol homeostasis.

Synthesis of pyrido[2,1-a]isoquinolin-4-ones and oxazino[2,3-a]isoquinolin-4-ones: new inhibitors of mitochondrial respiratory chain

Benzo[a]quinolizine is an important heterocyclic framework that can be found in numerous bioactive compounds. The general scheme for the synthesis of these compounds was based on the preparation of the appropriate dihydroisoquinolines by Bischler-Napieralski cyclization with good yields, followed by the Pemberton method to form the oxazinones or pyridones derivatives via acyl-ketene imine cyclocondensation. All the synthesized compounds were assayed in vitro for their ability to inhibit mitochondrial respiratory chain. Most of the tested compounds were able to inhibit the integrated electron transfer chain, measured as NADH oxidation, which includes complexes I, III and IV, in the low micromolar range. Oxazino[2,3-a]isoquinolin-4-ones displayed greater activity than their pyrido[2,1-a]isoquinolin-4-ones analogs. Indeed, the presence of a furan ring in C₂ position of oxazino[2,3-a]isoquinolin-4-ones provided the compound (1g) with the most potent biological activity. Therefore, these compounds and especially the oxazinone derivatives are in the tendency of the new less toxic antitumor agents that target mitochondrial electron transport chain in a middle range potency.

Role of LAT in the granule-mediated cytotoxicity of CD8 T cells

Linker for activation of T cells (LAT) is a transmembrane adaptor protein that is essential to bridge T cell receptor (TCR) engagement to downstream signaling events. The indispensable role of LAT in thymocyte development and T cell activation has been well characterized; however, the function of LAT in cytotoxic-T-lymphocyte (CTL) cytotoxicity remains unknown. We show here that LAT-deficient CTLs failed to upregulate FasL and produce gamma interferon after engagement with target cells and had impaired granule-mediated killing. We further dissected the effect of the LAT deletion on each step of granule exocytosis. LAT deficiency led to altered synapse formation, subsequently causing unstable T cell-antigen-presenting cell (APC) conjugates. Microtubule organizing center polarization and granule reorientation were also impaired by LAT deficiency, leading to reduced granule delivery. Despite these defects, granule release was still observed in LAT-deficient CTLs due to residual calcium flux and phospholipase C (PLC) activity. Our data demonstrated that LAT-mediated signaling intricately regulates CTL cytotoxicity at multiple steps.

ERK activation is only one role of PKC in TCR-independent cytotoxic T cell granule exocytosis

Cytotoxic T cells (CTLs) kill target cells by releasing lytic agents via regulated exocytosis. Three signals are known to be required for exocytosis: an increase in intracellular Ca(2+), activation of protein kinase C (PKC) and activation of extracellular signal regulated signal kinase (ERK). ERK activation required for exocytosis depends on activity of PKC. The simplest possibility is that the sole effect of PKC required for exocytosis is ERK activation. Testing this requires dissociating ERK and PKC activation. We did this using TCR-independent stimulation of TALL-104 human leukemic CTLs. When cells are stimulated with thapsigargin and PMA, agents that increase intracellular Ca(2+) and activate PKC, respectively, PKC-dependent ERK activation is required for lytic granule exocytosis. Expressing a constitutively active mutant MAP kinase kinase activates ERK independent of PKC. However, activating ERK without PKC does not support lytic granule exocytosis, indicating that there are multiple effects of PKC required for granule exocytosis.

Protein kinase C activity is required for cytotoxic T cell lytic granule exocytosis, but the theta isoform does not play a preferential role

CTLs kill virus-infected, tumor, and transplanted targets via secretion of lytic agents including perforin and granzymes. Knowledge of the signals controlling this important process remains vague. We have tested the idea that protein kinase C (PKC)theta, a member of the novel PKC (nPKC) family, which has been shown to play a preferential role in critical Th cell functions, plays a similar, preferential role in CTL lytic granule exocytosis using T acute lymphoblastic leukemia-104 (TALL-104) human leukemic CTLs as a model. We provide evidence consistent with the idea that PKC activity is important for the degranulation step of lytic granule exocytosis, as opposed to upstream events. In contrast with previous work, our results with pharmacological agents suggest that conventional PKCs (cPKCs) and nPKCs may participate. Our results suggest that stimulation with soluble agents that bypass the TCR and trigger granule exocytosis activates PKCalpha and PKCtheta, which can both accumulate at the site of contact with a target cell, although PKCtheta did so more often. Finally, using a novel assay that detects granule exocytosis specifically in transfected, viable cells, we find that overexpression of constitutively active mutants of PKCalpha or PKCtheta can synergize with increases in intracellular [Ca(2+)] to promote granule exocytosis. Taken together, our results lend support for the idea that PKCtheta does not play a preferential role in CTL granule exocytosis.

Cross-talk with Ca2+ Influx Does Not Underlie the Role of Extracellular Signal-regulated Kinases in Cytotoxic T Lymphocyte Lytic Granule Exocytosis

One important mechanism cytotoxic T lymphocytes use to kill target cells is exocytosis of lytic granules that contain cytotoxic agents such as perforin and granzyme. Ca(2+) influx and activation of protein kinase C have been known for many years to be key signals for granule exocytosis. Recent work has suggested that activation of extracellular signal-regulated kinases (ERK), members of the mitogen-activated protein kinase (MAP kinase) family, may be a third required signal. We surmised that the involvement of ERK in lytic granule exocytosis could be mediated through cross-talk with Ca(2+) influx, rather than constituting an independent signal. We tested this idea using TALL-104 human leukemic CTLs as a model system and discovered the following. 1) ERK inhibition caused a modest decrease in the amplitude of increases in intracellular Ca(2+) concentration, but this effect cannot account for the profound inhibition of granule exocytosis. 2) Ca(2+) influx can activate ERK in TALL-104 cells, but this effect does not contribute to ERK activation stimulated by solid phase anti-CD3 monoclonal antibodies. We conclude that cross-talk between ERK signaling and Ca(2+) does not mediate the role of ERK in CTL lytic granule exocytosis.

Enhancement of cytolytic activity of human peripheral blood lymphocytes by sodium periodate (IO4) possible involvement of protein kinase C

Sodium periodate (IO4) exerts a number of biological effects including the enhancement of lymphocyte activation. In this study, we investigated its effects on cytotoxicity of human peripheral blood lymphocytes (PBL) and explored the mechanism whereby it exerted these effects. In vitro treatment of human PBL with IO4 augmented their cytotoxicity against K562 myelogenous leukemia cells. IO4 oxidative treatment increased the frequency of effector-to-target cell binding. It also increased cellular ATP levels in effector cells, suggesting that the post-binding cytolytic functions of these cells were also enhanced after treatment with IO4. Moreover, IO4 treatment significantly increased the protein kinase C (PKC) activity of effector cells and induced the translocation of activity in the membrane fraction from the cytosol. H-7, a potent PKC inhibitor, significantly reduced this enhancement of membrane-associated PKC activity at 10 microM and significantly reduced the enhanced cytotoxicity of PBL at the same concentration. These results indicated that IO4 enhanced the binding capacity and post-binding cytolytic functions of PBL and that PKC activation was one mechanism to explain the IO4-induced cellular activation.

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.

T cell activation by a Trypanosoma brucei brucei-derived lymphocyte triggering factor is dependent on tyrosine protein kinases but not on protein kinase C and A

Trypanosoma brucei brucei releases a lymphocyte-triggering factor (TLTF) that activates CD8+ T cells. We here study second messenger mechanisms in this activation, i.e. the effects of protein kinase C (PKC), protein kinase A (PKA) and tyrosine kinases (TPK) inhibitors on TLTF-induced interferon-gamma (IFN-gamma) secretion and proliferation in lymphoid cell cultures. The effects were compared to those obtained by phytohemagglutinin (PHA) stimulation. Rat spleen mononuclear cells (MNC) and spleen MNC from a mutant mouse strain possessing CD8+ T cells but lacking CD4+ T cells were used as responder cells. Although both the PKC and the PKA inhibitors suppressed PHA-induced IFN-gamma secretion and proliferation of rat MNC and mouse CD8+ CD4- MNC, they had no effect on the same TLTF-induced responses. The TPK inhibitor genistein, however, strongly suppressed TLTF-induced activation of both types of responder cells to IFN-gamma secretion and the TLTF-induced proliferation of mouse CD8+ CD4- MNC. The suppressive effects of the drugs could be overcome by ionomycin and tetradecanoylphorbol acetate, which show that the effects were not due to drug nonspecific cellular toxicity of the drugs. We conclude that TLTF activates CD8+ T cells through pathways other than the PKC- or PKA-dependent signal transduction, and that TPK may be involved in the triggering.

Effects of protein kinase inhibitors on the mitogenic activity of human hepatocyte growth factor on rat hepatocytes in primary culture

To evaluate the role of protein phosphorylation reactions in signal transduction of human hepatocyte growth factor (hHGF), now known to be the same protein as the scatter factor and tumor cytotoxic factor, we examined the effects of various inhibitors of protein kinases on the mitogenic activity of hHGF on rat hepatocytes in primary culture. Genistein, a specific inhibitor of tyrosine kinase, dose-dependently inhibited the effect of hHGF in stimulating DNA synthesis of hepatocytes. By contrast, 1-(5-isoquinolinesulfonyl)-2- methylpiperazine (H7), a specific inhibitor of protein kinase C, potentiated the stimulatory effect of hHGF on DNA synthesis of hepatocytes. H7 was effective at over 2 micrograms/ml and potentiated the effect of hHGF over 2-fold at 20 micrograms/ml. On the other hand, an inhibitor of Ca++/calmodulin-dependent protein kinase inhibited both the basal and hHGF-stimulated DNA synthesis in the cells, whereas an inhibitor of cyclic nucleotide-dependent protein kinases had little effect on the action of hHGF. These results suggest that tyrosine phosphorylation is required for stimulation of hepatocyte DNA synthesis by hHGF and that the action of hHGF is negatively regulated by protein kinase C activation.

Evidence for protein kinase C--independent pathways mediating phorbol ester induced plasmacytoid differentiation of B chronic lymphocytic leukemia cells

The effects of phorbol esters on many cell types are known to be mediated through activation of the protein kinase C (PKC) signal transduction pathway. By using the specific inhibitor of this enzyme 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine dihydrochloride (H7) we have assessed the role of PKC activation in phorbol ester (phorbol 12-myristate 13-acetate, PMA)-induced plasmacytoid differentiation of B chronic lymphocytic leukemia cells (B-CLL) as a model of terminal differentiation of human B lymphocytes. H7 affected a dose-dependent inhibition of PMA-induced thymidine and uridine uptake with ID50 values of 41 microM and 32 microM, respectively. A comparable ID50 value (34 microM) was obtained for H7 inhibition of B-CLL PKC activity in a cell-free system. PMA-induced changes in cell morphology, expression of CD20, CD37 and FMC7 surface antigens together with increased secretion of immunoglobulin were variably abrogated by H7 suggesting that PKC activation is more important in B cell activation/DNA synthesis than in the differentiative response. Consistent with this, expression of a sizable proportion of PMA-inducible genes was not significantly affected by H7. These data are consistent with the existence of a PMA-activated, PKC-independent signal transduction pathway which may be important, though by itself apparently insufficient, for eliciting full terminal differentiation in B lymphocytes.

Modulation of cellular processes by H7, a non-selective inhibitor of protein kinases

H7 has been described as a potent inhibitor of protein kinase C (PKC) and has been widely used to investigate the regulatory role of this enzyme in intact cell systems. In this comparative study between H7 and the microbial alkaloid, staurosporine, we found that the former inhibited rat brain PKC and cAMP dependent protein kinase with IC50 values of 18 and 16 microM respectively whereas the latter was a much more potent inhibitor of both kinases with IC50 values of 9.5 nM and 42 nM respectively. H7, at concentrations up to 100 microM, failed to block cellular events induced by phorbol esters, agents which specifically stimulate PKC, yet was a potent inhibitor of IL-2 induced T cell proliferation with an IC50 value of 19 microM. In contrast, staurosporine was a potent inhibitor of both phorbol ester induced p47 phosphorylation in platelet (I50 value = 540 nM) and also CD3 and CD4 down-regulation in T cells (I50 values 200 nM and 50 nM respectively). Staurosporine was also a potent inhibitor of IL-2 induced T cell proliferation I50 value = 9 nM). These results provide a strong argument against the use of H7 to probe for PKC involvement in cellular processes.

The involvement of protein kinase C, calcium, and 5-lipoxygenase in the production of tumor necrosis factor by a cloned interleukin-3 dependent cell line with natural cytotoxic activity

The cloned interleukin-3 dependent cell line, M1-A5 was studied to determine whether protein kinase C, calcium mobilization, and 5-lipoxygenase activity were involved in the signal transduction pathways required for the production of TNF. TNF release was stimulated by 10 ng/ml phorbol myristate acetate (PMA), 2 microM calcium ionophore A23187, and 1 microgram/ml lipopolysaccharide (LPS) with synergism seen between PMA and A23187. All signals were blocked by phloretin and the PMA signal was blocked by H-7, both drugs acting as protein kinase C inhibitors. Desensitization of protein kinase C by PMA (1 microgram/ml for 24 h) provided evidence that both PMA- and LPS-stimulated TNF production were protein kinase C-dependent while A23187-stimulated TNF production was not. Both the calcium chelator, EGTA, and the intracellular calcium antagonist, TMB-8, inhibited TNF production stimulated by all agents, indicating that TNF stimulation by all agents was calcium dependent. Finally, the 5-lipoxygenase inhibitors, ketoconazole and L-656,224, but not the cyclo-oxygenase inhibitor ASA, inhibited TNF stimulated by all agents. These findings indicate that, although TNF production by M1-A5 cells can be stimulated either by a calcium/protein kinase C- or by a calcium-dependent signal, there is a convergence of signals at the level of 5-lipoxygenase activation.

Transmembrane signalling associated with ganglioside-induced CD4 modulation

Ganglioside (GM1) treatment of CD4+ human CEM lymphoma cells stimulated transient phosphoinositide (PI) breakdown, production of inositol phosphates (IP), protein phosphorylation and rapid decrease of CD4 surface expression. A comparison between the actions of GM1 and other agents that affect these signal transduction pathways demonstrated a distinct mechanism for GM1-induced decrease of CD4. GM1 stimulated both phospholipase C activity and protein phosphorylation but had no effect on either cellular cAMP levels or tyrosine kinase activity. Phorbol myristate acetate (PMA) stimulated protein phosphorylation and caused a significant decrease in surface display of CD4. Both of these processes were blocked by pretreating cells with the protein kinase C (PKC) inhibitor H7. These results demonstrate that GM1 stimulates PI turnover and induces a rapid decrease of CD4 surface expression by processes that do not activate adenylate cyclase or tyrosine kinase. They further demonstrate that the mechanism for GM1-induced decrease of CD4 is distinct from the CD4 internalization processes mediated by PKC activity.