Characterization of PHA and anti-T3 induced transduction mechanisms in a human T-cell leukaemia line

The injury-induced myokine insulin-like 6 is protective in experimental autoimmune myositis

Background

The idiopathic inflammatory myopathies represent a group of autoimmune diseases that are characterized by lymphocyte infiltration of muscle and muscle weakness. Insulin-like 6 (Insl6) is a poorly characterized member of the insulin-like/relaxin family of secreted proteins, whose expression is upregulated upon acute muscle injury.

Methods

In this study, we employed Insl6 gain or loss of function mice to investigate the role of Insl6 in a T cell-mediated model of experimental autoimmune myositis (EAM). EAM models in rodents have involved immunization with human myosin-binding protein C with complete Freund’s adjuvant (CFA) emulsions and pertussis toxin.

Results

Insl6-deficiency in mice led to a worsened myositis phenotype including increased infiltration of CD4 and CD8 T cells and the elevated expression of inflammatory cytokines. Insl6-deficient mice show significant motor function impairment when tested with treadmill or Rotarod devices. Conversely, muscle-specific overexpression of Insl6 protected against the development of myositis as indicated by reduced lymphocyte infiltration in muscle, diminished inflammatory cytokine expression and improved motor function. The improvement in myositis by Insl6 could also be demonstrated by acute hydrodynamic delivery of a plasmid encoding murine Insl6. In cultured cells, Insl6 inhibits Jurkat cell proliferation and activation in response to phytohemagglutinin/phorbol 12-myristate 13-acetate stimulation. Insl6 transcript expression in muscle was reduced in a cohort of dermatomyositis and polymyositis patients.

Conclusions

These data suggest that Insl6 may have utility for the treatment of myositis, a condition for which few treatment options exist.

Activation of protein kinase C and elevation of cAMP interact synergistically to raise c-Fos and AP-1 activity in Jurkat cells

We have earlier found that in Jurkat cells activation of protein kinase C (PKC) enhances the cyclic adenosine monophosphate (cAMP) accumulation induced by adenosine receptor stimulation or activation of Gs. Here we have therefore examined the effect of the phorbol ester PMA (phorbol 12-myristate 13-acetate) which stimulates PKC and a combination of the adenosine receptor agonist NECA (5'-(N-ethyl)-carboxamido adenosine) and forskolin to raise cAMP, on the levels of c-Fos and Jun and on the binding and transcriptional activity of the transcription factor, activator protein-1 (AP-1). PMA treatment caused a concentration- and time-dependent increase in both c-Fos and Jun immunoreactivity in contrast to cAMP elevation that had only a slight effect. Both PMA and the combination of NECA and forskolin acted together either to increase (c-Fos) or decrease (Jun) protein levels as well as increasing AP-1 binding, as judged by gel-shift assay, and AP-1 transcriptional activity. Furthermore there was a clear-cut synergy between the PKC stimulator and the cAMP elevating agents. The results demonstrate that the simultaneous activation of PKC and elevation of cAMP leads to an enhanced AP-1 transcriptional activity in a T-leukemia cell line, suggesting that the previously observed interaction between the parallel signal transduction pathways may have functional consequences at the level of gene transcription.

G proteins in T cell signal transduction

This review summarises recent data on G protein implication in receptor signalling in T cells. The data show that PPI-specific PLC in T cell membranes is under G protein control. Some evidence indicates that a G protein couples PLC to TCR. Differences are revealed between the effects induced by direct G protein activators, such as GTP gamma S or AlF4-, and TCR ligands, which imply that TCR ligands may trigger some G-protein-independent signals. An analysis of the conflicting results on the action of PTX and CTX, one of the main tools in studying G proteins, has shown that the toxins produce both G protein-dependent and independent effects. The G protein which couples PLC to TCR appears insensitive to both PTX and CTX. Some findings suggest G protein involvement in signalling induced by interleukins; however, in this case the effector molecules implicate often remain unknown. Scarce data on G protein involvement in signalling from differentiation antigens, on direct G protein regulation of ion channels, and on identification of G proteins in T cells, are also discussed.

Cholera toxin-mediated inhibition of signalling in Jurkat cells is followed by, but not due to a loss of T cell receptor complex

Cholera toxin treatment of the human T cell lymphoma Jurkat resulted in inhibition of signalling via the T cell antigen receptor complex (TcR/CD3-complex). Cholera toxin specifically ADP-ribosylated the alpha-subunit of the stimulatory G-protein of the adenylate cyclase (Gs alpha), no other proteins were modified in the intact cells. ADP-ribosylation of Gs alpha and its subsequent activation led to an increase of the cyclic AMP level and in addition, to a drastic reduction of the cell-surface density of the TcR/CD3-complex. Recently, we demonstrated that the effect of cholera toxin at the receptor level is not due to an increased cAMP level (4). As inhibition of signalling is also not cAMP-mediated (8), we examined whether the modulation of the TcR/CD3-complex could be the reason for the interruption of the signalling cascade. Analyzing the time courses of the multiple cholera toxin effects in Jurkat cells at 37 degrees C, the following sequence was found: ADP-ribosylation of Gs alpha--increase of cyclic AMP level--inhibition of signalling via the TcR/CD3-complex--decrease of cell-surface density of the TcR/CD3-complex. Treatment of Jurkat cells at 20 degrees C with cholera toxin resulted in an increase of cyclic AMP and inhibition of signal transduction, while no decrease of TcR/CD3-complex density could be observed. These data imply that receptor loss from the cell-surface is not causative for the inhibition of signalling. More likely, activation of Gs uncouples signal transduction in Jurkat cells via the TcR, which by a so far unknown mechanism is followed by a loss of the receptor from the cell surface.

Stimulation of the T-cell receptors CD3 and CD2 with OKT3 and OKT11 antibodies activates a common pertussis toxin-insensitive G-protein

The association of G-proteins with the T-cell-specific receptor structures CD3 and CD2 was investigated. High-affinity GTPase activity in membrane preparations of the human leukemic T-cell line Jurkat could be induced by the monoclonal antibodies OKT3 (anti-CD3) and OKT11 (anti-CD2). When combining maximally active concentrations of OKT3 and OKT11, no additive effect was seen on GTPase activity. In mutant Jurkat cells lacking the CD3 complex but with an intact CD2 receptor, neither OKT3 nor OKT11 could stimulate GTPase activity. Activation of CD3 and CD2 by monoclonal antibodies also stimulated phospholipase C activity as measured by breakdown of membrane phosphoinositides in wild-type but not in mutant Jurkat cells. Neither GTPase nor phospholipase C activation was sensitive to pretreatment with doses of pertussis toxin (PTX) that caused ADP ribosylation of a sensitive G-protein. Our data show that the CD3 complex and the CD2 receptor may activate a common PTX-insensitive G-protein. The CD2 receptor appears to stimulate the G-protein by interacting with the CD3 complex. The data are compatible with, but do not prove, that this G-protein is involved in the activation of phospholipase C by the two receptors.

The G protein coupling T cell antigen receptor/CD3-complex and phospholipase C in the human T cell lymphoma Jurkat is not a target for cholera toxin

Intact Jurkat cells could be stimulated by monoclonal antibodies against the Tcell antigen receptor complex (OKT3 directed against the CD3 complex, BMA031 directed against constant framework epitopes in the alpha/beta heterodimer). The accumulation of inositol phosphates was inhibited by prior incubation of the cells with cholera holotoxin. The inhibitory effect of cholera toxin (CT) was not cAMP mediated because forskolin (a direct activator of adenylate cyclase) did not mimic the inhibitory effect. When measuring phospholipase C (PLC) in a cell-free assay system by using [3H]inositol-labeled membranes, the enzyme could be stimulated by the poorly hydrolyzable GTP analogue guanosine 5'-O-(thiotriphosphate (GTP gamma S). Both anti-receptor antibodies augmented the GTP gamma S stimulatory effect, while the antibodies alone had no stimulatory capacity. In membranes from CT-pretreated cells, whereas the antibodies lost their stimulatory effect on PLC as in untreated cells, whereas the antibodies lost their stimulatory capacity in the presence of GTP gamma S. These data imply that CT exerts its inhibitory effect on signaling by acting at the receptor level while the PLC regulating G protein is not a target for CT-mediated alterations. This assumption is supported by the finding that in intact Jurkat cells CT, which ADP ribosylated only the alpha-subunit of the stimulatory G protein of the adenylate cyclase, led to a loss of the T cell antigen receptor complex from the cell surface as demonstrated by a decrease of receptor density using flow cytometry analysis. Receptor loss could not be achieved by forskolin treatment or incubation of the cells with the binding subunit of the toxin alone.

Phytohemagglutinin rapidly lyses S49 T-lymphoma cells and the cytotoxicity is not mediated by generation of cAMP or increase in cytosolic calcium

Thymic-like lymphomas are very sensitive to killing by phytohemagglutinin. To investigate the mechanism of cytotoxicity, we studied the effect of PHA on cytosolic calcium [( Ca2+]i) and cAMP in the S49 mouse lymphoma cell line. PHA produced a slow continuing rise in [Ca2+]i. Estimation of cell number by Coulter counting showed that PHA induced rapid lysis of S49 cells in a dose-dependent manner. Nicardipine (10(-5) M) did not prevent PHA induced cell lysis or [Ca2+]i increase. Also ionomycin (10(-7) M) did not induce cell lysis. The data suggest that PHA induced increase in [Ca2+]i is the result rather than the cause of cell lysis. Elevated intracellular cAMP has an antiproliferative effect on S49 cells. PHA had no effect on cAMP levels in S49 cells. Also S49 cyc- clone which is deficient in Gs was susceptible to killing by PHA. These results suggest that the cytotoxic effect of PHA on S49 cells is rapid, but is not mediated by cAMP generation or an increase in [Ca2+]i, and other mechanisms should be investigated.

Pertussis toxin B-subunit-induced Ca2(+)-fluxes in Jurkat human lymphoma cells: the action of long-term pre-treatment with cholera and pertussis holotoxins

The exotoxins of Bordetella pertussis and Vibrio cholera have been used to investigate signal transduction in the human T-cell lymphoma Jurkat. Stimulation of the cells, leading to an increase in cytoplasmic free calcium, could be achieved by the anti-T-cell receptor complex antibody OKT3 and by pertussis holotoxin (PTHT), or its B-subunit (PTB), but not by cholera holotoxin (CTHT) or its B-subunit (CTB). Both holotoxins ADP-ribosylated specifically G-proteins in the plasma membrane of intact cells, while their B-subunits had no ADP-ribosyltransferase activity. Incubation of the cells with CTHT led to a state of unresponsiveness to all stimulants. CTB was without any effect, indicating that the ADP-ribosyltransferase activity of cholera toxin (located in the A-subunit of the holotoxin) was necessary for the inhibition of cellular signalling. The inhibitory effect of cholera toxin on the pertussis toxin action was not due to a blockade of pertussis toxin interaction with the cell surface, because pertussis toxin was still able to ADP-ribosylate membrane proteins in cholera toxin treated intact cells. In addition, the cholera toxin mediated inhibition was not due to elevated levels of cyclic-AMP, as forskolin (a direct activator of the adenylate cyclase) and no inhibitory effect. The stimulating effect of PTHT was independent of its ADP-ribosyltransferase activity, because it could also be obtained by the B-subunit alone. In addition, the increase of cytoplasmic free calcium after stimulation by PTHT clearly preceded the ADP-ribosylation. Pre-treatment with PTHT, PTB or OKT3, led to a long lasting increase in the level of intracellular Ca2+ in Jurkat cells, which could not, therefore, be stimulated further. Inhibition by cholera holotoxin of the stimulation by OKT3 and pertussis toxin (PTHT and PTB) imply that the mitogenic effect of pertussis toxin is perhaps mediated via the T-cell antigen receptor signalling cascade. The presented data do not support the idea that a pertussis toxin-sensitive G-protein is involved in coupling the T-cell antigen receptor to the phospholipase C.

In vivo pertussis toxin treatment attenuates some, but not all, adenosine A1 effects in slices of the rat hippocampus

In order to examine the involvement of G-proteins in mediating the different effects of adenosine A1-receptor stimulation in rat hippocampus we injected pertussis toxin (PTX) intraventricularly close to the hippocampus and examined its effect in slices 48-60 h later. The in vivo PTX treatment caused a partial (50 +/- 5%) inhibition of the [32P]ADP ribosylation produced by PTX added together with [32P]NAD in vitro. Such PTX treatment eliminated the electrophysiologically determined gamma-amino-n-butyric acid (GABA)B receptor response in the hippocampal CA1 region, but GABAA effects were unaffected. The adenosine (50 microM)-mediated hyperpolarization and decrease in input resistance as well as the adenosine-mediated inhibition of low calcium-induced bursting in pyramidal CA1 neurons were virtually abolished. The same was true for the decrease in [3H]cyclic AMP accumulation that is produced by the adenosine analogue R-N6-phenylisopropyl adenosine (R-PIA) in forskolin-treated hippocampal slices. As far as modulation of transmitter release was concerned, the R-PIA (1 microM)-induced inhibition of release of both [3H]noradrenaline (NA) and [3H]acetylcholine (ACh) evoked by field stimulation in hippocampal slices was affected hardly or not at all by pertussis toxin treatment. The inhibitory effect of adenosine on field excitatory postsynaptic potential (EPSP)s evoked in the CA1 region was unaltered by PTX pretreatment. The present results show that in vivo pertussis toxin treatment can inhibit some but not all A1-adenosine-receptor effects. This strongly suggests that closely similar A1 receptors might be coupled to G-proteins that differ in their sensitivity to PTX treatment.

CD3/T-cell receptor coupling to a pertussis and cholera toxin-insensitive G-protein

We have analyzed the effect of CD3/T-cell receptor stimulation on GTP hydrolysis and GTP binding. We show that stimulation of Jurkat, T-cell, membranes with OKT3 results in a 50% increase in GTP hydrolysis which is specifically inhibited by GDP. Pretreatment of the membranes with neither pertussis toxin nor cholera toxin inhibited the GTP hydrolysis. We also show that stimulation with OKT3 increases the binding of GTP gamma S to Jurkat membranes. These data strongly implicate the involvement of a G-protein in CD3/T-cell receptor signalling.

Regulation of receptor-mediated calcium influx across the plasma membrane in a human leukemic T-cell line: evidence of its dependence on an initial calcium mobilization from intracellular stores

It has been repeatedly shown that stimulation of a human leukemic T-cell line, JURKAT, by lectins such as phytohaemagglutinin and anti-T3 antibody (OKT3) leads to an elevation in the concentration of cytosolic free Ca2. This Ca2+ transient results from both an intracellular mobilization and an influx of Ca2+ through specific membrane channels. The objective of this study was to investigate the mechanism by which receptor-mediated influx of Ca2+ is regulated in JURKAT cells, which demonstrably lack 'voltage-dependent calcium channels'. It was found that upon increased loading with quin2 or 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate (BAPTA) there was a pronounced decline of both phytohaemagglutinin-stimulated and OKT3-stimulated influx of 45Ca2+. Using 15 microM quin2/AM or 30 microM BAPTA/AM, agonist-stimulated 45Ca2+ influx was almost totally abolished. At these concentrations of both quin2/AM or BAPTA/AM, phytohaemagglutinin and OKT3 could still induce a rise of cytosolic free Ca2+ above 200 nM. In the presence of La3+ (200 microM), which completely inhibited the agonist-induced 45Ca2+ influx, both phytohaemagglutinin and OKT3 were able to raise the concentrations of cytosolic free Ca2+ to well above 200 nM by merely mobilizing Ca2+ from intracellular stores alone. The data suggest that an agonist-induced increase in the concentration of cytosolic free Ca2+, due to mobilization from intracellular stores, could either directly or indirectly, initiate receptor-mediated Ca2+ influx across the plasma membrane in JURKAT cells.