The liberation of endogenous AA and its autocrine action seems to be implicated during TG-induced capacitative calcium influx: TG potentiates the induction of constitutively expressed mRN
Trang 1Role of three isoforms of phospholipase A2 in capacitative calcium influx in human T-cells
Aziz Hichami1, Beenu Joshi2, Anne Marie Simonin1and Naim Akhtar Khan1
1
UPRES Lipides & Nutrition, Universite´ de Bourgogne 21000 Dijon, France;2Central Jalma Research Institute for Leprosy, Agra, UP, India
The present study was conducted on human Jurkat T-cell
lines in order to elucidate the role of phospholipase A2in
capacitative calcium entry We have employed
thapsigar-gin (TG)that induces increases in [Ca2+]i by emptying
the calcium pool of endoplasmic reticulum, followed by
capacitative calcium entry We designed a Ca2+free/Ca2+
reintroduction (CFCR)protocol for the experiments,
conducted in Ca2+-free medium By employing CFCR
protocol, we observed that addition of exogenous
arachi-donic acid (AA)stimulated TG-induced capacitative
calcium influx The liberation of endogenous AA and its
autocrine action seems to be implicated during
TG-induced capacitative calcium influx: TG potentiates the induction of constitutively expressed mRNA of four PLA2 isoforms (type 1B, IV, V, VI), the inhibitors of the three PLA2isotypes (type 1B, V, VI)inhibit TG-induced release
of [3H]AA into the extracellular medium, and finally, these PLA2 inhibitors do curtail TG-stimulated capacita-tive calcium entry in these cells These results suggest that stimulation of three isoforms of PLA2 by thapsigargin liberates free AA that, in turn, induces capacitative calcium influx in human T-cells
Keywords: Jurkat cells; thapsigargin; arachidonic acid
In T-lymphocytes, a biphasic rise in concentrations of free
calcium, [Ca2+]i, is elicited by the binding of antigen or
polyclonal mitogens to the T-cell receptor [1,2] Hence, the
rise in [Ca2+]iconstitutes an essential triggering signal for
T-cell differentiation and proliferation [3] Raising [Ca2+]i
has been found to increase the activity of a transcription
factor, the nuclear factor of activated T-cells (NF-AT),
which in turn results in the expression of lacZ gene in
transfected murine T-cells [4] Calcium oscillations with
repeated spikes for a period of 100 s are sufficient to activate
the transcriptional factors in human T-cells [5] In a study
conducted on caged IP3molecules, the trains (from 0.3 to
1.5 s)of short ultraviolet pulses, which induced calcium
oscillations, promoted the activity of NF-AT in RBL-2H3
lymphocytes [6]
According to the capacitative model of calcium entry,
first, calcium is released via receptor activation from two
intracellular stores; first, from endoplasmic reticulum (ER)
and then, from the Ca2+-induced Ca2+-release (CICR)
pool Ca2+, in turn, is extruded into the extracellular medium The cells refill their intracellular emptied pools via store-operated calcium (SOC)influx by opening calcium channels [7] In Jurkat T-cells, SOC influx is brought about
by opening of Ca2+release-activated Ca2+, CRAC [8] and
Ca2+ release-activated nonselective cation, CRANC [9], channels Human Jurkat T-cells express nearly 10 000 CRAC channels per cell [3] The refilling mechanism via CRAC and CRANC channels is regulated by the calcium influx factor (CIF), which is released into the extracellular medium during calcium release in Jurkat T-cells [9,10] It has been demonstrated that human T-cells possess func-tional ER and CICR calcium pools [1,11] The mechanisms
by which SOC influx is brought about are still not well understood For example, the nature of the signal trans-duction pathway by which store depletion is linked to the opening of plasma membrane Ca2+ channels remains unknown
During the recent past, there has been an upsurge of information on the possible implication of phospholipase
A2 and polyunsaturated fatty acids in the regulation of immune cell functions [12–14], particularly the cell signal-ling mechanisms [15–19] Several studies have reported that arachidonic acid (AA)blocks agonist-stimulated sustained rise in [Ca2+]i [18–20] On the other hand, some investigators have observed that AA both reduced and increased SOC influx, induced by store depletion in lymphocytes [21] Keeping in view this discrepancy, the present study was conducted to ascertain the role of PLA2 activation and hence released AA in the capacit-ative influx of calcium in Jurkat T-cells These cells represent a good model to study the effects of AA per se
as these lymphocytes can not metabolize this fatty acid via lipoxygenase and cyclooxygenase pathways [22,23]
Correspondence to N A Khan, UPRES Lipides & Nutrition,
Universite´ de Bourgogne, Faculte´ des Sciences de la Vie, 6,
Boulevard Gabriel, 21000 Dijon, France.
Fax: + 33 3 80 39 63 30, Tel.: + 33 3 80 39 63 12,
E-mail: Naim.Khan@u-bourgogne.fr
Abbreviations: AA, arachidonic acid (20 : 4 n-6); ARC,
arachidonate-regulated, calcium; BPB, 4-bromophenacyl, bromide; CFCR, Ca2+,
free/Ca 2+ reintroduction; CRAC, Ca 2+ , release-activated Ca 2+ ;
[Ca2+] i , free, intracellular calcium concentrations; MAF,
methyl-arachidonyl, fluorophosphonate; PLA 2 , phospholipase, A 2 ; SOC,
store-operated, calcium.
(Received 11 March 2002, revised 9 September 2002,
accepted 16 September 2002)
Trang 2M A T E R I A L S A N D M E T H O D S
Chemical products
The culture medium RPMI 1640 and L-glutamine were
purchased from Biowhitaker, Belgium The Fura-2/AM
was procured from Molecular Probes (Eugene, OR, USA)
The SuperScript II Reverse Transcriptase, Platinum Taq
DNA Polymerase, random primers, oligo(dT)18 and the
oligonucleotides used as primers in the RT-PCR analysis
were purchased from Invitrogene, Life Technologies (Cergy
Pontoise, France) Agarose was from Promega
(Char-bonnie`re, France) Phospholipases A2inhibitors
arachido-nyl trifluoromethyl ketone (AACOCF3)and bromoenol
lactone (BEL)were from Cayman Chemical (Ann Arbor,
USA) Aristolochic acid and 4-bromo phenacyl bromide
were from Sigma Chemicals (St Louis, MO, USA) Methyl
arachidonyl fluorophosphonate was obtained from
Cal-biochem (Orsay, France) [3H]Arachidonic acid (specific
activity, 217 CiÆmmol)1)was purchased from Amersham
(Orsay, France) All other chemicals including arachidonic
acid (20 : 4 n-6)were obtained from Sigma Chemicals
(St Louis, MO, USA)
Cell culture
The human (Jurkat)T-cells were kindly provided by Dr
Bent Rubin, Head, UMR-CNRS Research Unit at CHR of
Toulouse (France) The cells were cultured in RPMI-1640
medium supplemented with L-glutamine and 10% foetal
calf serum at 37C in a humidified chamber containing
95% air and 5% CO2 Cell viability was assessed by the
trypan blue exclusion test Cell numbers were determined by
haemocytometer
Measurement of Ca2+signalling
The cells (2· 106ÆmL)1)were washed with NaCl/Pi
(phos-phate buffered saline), pH 7.4 The composition of NaCl/Pi
was as follows: 3.5 mM KH2PO4; 17.02 mM Na2HPO4;
136 mMNaCl The cells were then incubated with Fura-2/
AM (1 lM)for 60 min at 37C in loading buffer which
contained the following: 110 mM NaCl; 5.4 mM KCI;
25 mM NaHCO3; 0.8 mM MgCl2; 0.4 mM KH2PO4;
20 mM Hepes-Na; 0.33 mM Na2HPO4; 1.2 mM CaCl2,
and the pH was adjusted to 7.4
After loading, the cells were washed three times
(2000 g; 10 min)and remained suspended in the identical
buffer [Ca2+]i was measured according to Grynkiewicz
et al [24] The fluorescence intensities were measured in
the ratio mode in PTI spectrofluorometer at 340 nm and
380 nm (excitation filters)and 510 nm (emission filters)
The cells were continuously stirred throughout the
experimentation The test molecules were added into the
cuvettes in small volumes with no interruptions in
recordings The intracellular concentration of free Ca2+,
[Ca2+]i, were calculated by using the following equation:
[Ca2+]i¼ Kd· (R) Rmin)/(Fmax) F) · (Sf2/Sb2) A value
of 224 nM for Kd was added into the calculations Rmax
and Rminvalues were obtained by addition of ionomycin
(5 lM)and MnCl2(2 mM), respectively All the experiments
were performed at 37C Arachidonic acid (AA)was
dissolved in ethanol (0.1%, w/v)and used immediately or
kept at)20 C in ampoules, tightly sealed under a stream
of nitrogen
We designed a Ca2+-free/Ca2+-reintroduction (CFCR) protocol for the experiments, conducted in Ca2+-free (0%
Ca2+)medium Hence, we examined the role of AA on direct calcium influx First, AA and then CaCl2was added into the cuvette
Arachidonic acid release The experiment on arachidonic acid incorporation and release was performed as described elsewhere [25] In brief, Jurkat T-cells were serum-starved for 4 h before labelling with [3H]-arachidonic acid (AA, 1.5 lCi per 3· 108cells) for 2 h in RPMI 1640 serum-free medium supplemented with 0.2% fatty acid-free BSA At the end of the incubation, cells were washed twice with RPMI 1640 serum-free medium containing 0.2% BSA The cells were resuspended in 500 lL RPMI 1640 medium supplemented with 0.5% BSA at a final concentration of 12· 106 cellsÆmL)1 and treated or not (vehicle carrier contained dimethylsulfoxide, 0.1% v/v)with 5 or 15 lM of PLA2 inhibitors or vehicle for 10 min with or without TG (5 lM) Cells were centrifuged (1250 g; 3 min)and 0.4 mL
of supernatant was added to 2 mL scintillation cocktail for counting in a liquid scintillation analyzer (Packard
1900 TR, France)
RNA isolation and semiquantitative RT-PCR analysis Total RNA from cultured Jurkat T-cells was purified using trizol reagent (Invitrogene Life Technologies, Cergy Pon-toise, France)according to the manufacturer’s instructions Oligonucleotide primer pairs, used for mRNA analysis by RT-PCR, were based on the sequences of the human genes,
as described elsewhere [26] The cDNA was either used immediately for PCR or stored at)20 C until use The conditions for the PCR amplification and the assays have been described elsewhere [26] Human b-actin mRNA primers were used as internal control to normalize the data Reaction products were electrophoresed on a 1% agarose gel impregnated with ethidium bromide The RNA pattern was visualized by UV transillumination
Statistical analysis Results are shown as mean ± SEM Statistical analysis of data was carried out usingSTATISTICA(version 4.1, Statsoft, Paris, France) The significance of the differences between mean values was determined by analysis of variance one way, followed by a least-significant-difference (LSD)test
R E S U L T S
AA facilitates capacitative Ca2+influx in TG-stimulated cells
The increases in [Ca2+]ican also be achieved by employing thapsigargin [27] According to the capacitative model of calcium homeostasis, an increase in [Ca2+]iis responsible for the extrusion of free calcium into the extracellular medium, and this phenomenon is followed by SOC influx
to refill the intracellular pool [7] The thapsigargin
Trang 3(TG)-induced Ca2+spike is followed by a lowered sustained
response that is, indeed, the phase of SOC capacitative
refilling In CFCR protocol, addition of AA and then CaCl2
to 0%Ca2+medium potentiated the thapsigargin-induced
capacitative calcium influx in a dose dependent manner in
Jurkat T-cells (Fig 1) The increases by the addition of
arachidonic acid at 1 lM, 5 lMand 10 lM were,
respect-ively, 8.01 ± 0.01, 16.0 ± 0.10 and 35.2 ± 3.15 The
inset of Fig 1 shows that AA (10 lM)in 100% Ca2+
medium induced a significant increase in [Ca2+]iin
compar-ison with that in 0% Ca2+ medium (30 ± 2.10 nM,
100% Ca2+medium vs 4.1 ± 0.56 nM, 0% Ca2+medium,
P< 0.001)
TG induces the release of AA and the expression
of mRNA of different PLA2isotypes
In [3H]AA loaded cells, TG induced the release of [3H]AA
into the extracellular medium (Fig 2) We employed
aristolochic acid and 4-bromo phenacyl bromide (BPB)
which are the inhibitors of sPLA2, i.e type IB/typ V
Arachidonyl trifluoromethyl ketone (AACOCF3)and
bromoenol lactone (BEL)are the respective inhibitors of
type IV and type VI PLA2 Methyl-arachodonyl
fluoro-phosphonate (MAF)inhibits type IV and type VI PLA2
with the same selectivity We observed that aristolochic acid,
BPB and BEL, but not AACOCF3, inhibited the release of
[3H]AA, induced by TG (Fig 2) MAF inhibited the
TG-induced release [3H]AA almost with the same order of
magnitude as BEL Figure 3 shows that Jurkat T-cells
constitutively express the mRNA of four PLA2 isotypes
(type IB, type V, type IV and type VI) Interestingly,
addition of TG stimulated the induction of the four PLA2 isotypes in Jurkat T-cells
PLA2inhibitors that inhibit AA release diminish the TG-induced capacitative calcium entry Figure 4 shows that prior addition of aristolochic acid, and BEL inhibited the sustained TG-stimulated capacitative calcium entry in these cells Interestingly, AACOCF3failed
to significantly curtail the TG-induced capacitative calcium entry in these experiments (Fig 4) The decreases of delta calcium by BEL and MAF were, respectively, 15 ± 1.02 nMand 14 ± 1.2 nMvs control 35 ± 2.10 nM BPB and aristolochic acid also diminished TG-induced capacit-ative calcium entry in human T-cells (aristolochic acid,
30 ± 1.10 nMand BPB, 29 ± 1.04 nM)
AA-induced calcium influx is contributed by opening the calcium channels
We were tempted to assess whether AA-induced calcium influx is contributed by the opening of calcium channels We employed ionomycin at 100 nM as in the continuous presence of this ionophore at this concentration, the internal calcium stores are short-circuited and recovery of an elevated calcium is almost entirely due to extracellular calcium intrusion [28]
Figure 5 shows that addition of AA, during the ionomy-cin-induced spike, evoked an additive sustained response in [Ca2+]i in Jurkat T-cells Hence, ionomycin-induced sus-tained response was 80 ± 4.2 nM whereas arachidonic acid-induced response was 165 ± 5.4 nM, if the latter agent
Fig 1 Effects of extracellular calcium on arachidonic acid (AA)-facilitated capacitative calcium (SOC) influx in Jurkat T-cells Cells (4 · l0 6 per assay)were loaded with the fluorescent probe, Fura-2/AM, as described in Materials and methods The experiments were performed in 0% Ca2+ medium The arrow heads indicate the time when the test molecules, thapsigargin, TG (1 l M ) , AA (from 0 to 10 l M )and CaCl 2 (1.5 m M )were added into the cuvette During TG-induced steady-state of capacitative calcium influx, AA and then CaCl 2 were sequentially added or not into the cuvette without interruptions in the recordings The control trace shows the recording observed in the absence of AA and CaCl 2 The figure shows the single traces of observations which were reproduced (n ¼ 10), independently The inset shows the experiment conducted in the absence of thapsigargin but in 0% and 100% Ca 2+ medium (n ¼ 11).
Trang 4was added during the spike of ionomycin In order to probe
the role of different calcium channels, implicated in
AA-induced calcium influx, we employed, tyrphostin A9 (TA9),
an inhibitor of CRAC channels, diltiazem and x-conotoxin,
the respective inhibitors of L-type and N-type calcium
channels We observed that these agents did not diminish
the AA-induced increases in [Ca2+]iin these cells (results not shown)
AA interacts extracellularly
In order to assess whether AA acts extracellularly during
Ca2+ influx, we used the fatty acid free BSA at a final concentration of 0.2% (w/v)as this concentration of BSA has been shown to compete with polyunsaturated fatty acids, bound to the plasma membrane [17] Figure 6 shows that addition of BSA during the peak response of AA
Fig 2 Effects of phospholipase A 2 inhibitors on TG-induced [3H]arachidonic acid release Serum-starved Jurkat T-cells (3 · 10 8 )were labelled for two hours with 1.5 lCi of [ 3 H]AA The cells were then treated with thapsigargin (1 l M )in the presence or absence of 5 or 15 l M of AACOCF 3 , aristolochic acid, BEL, BPB, MAF or with vehicle control (dimethylsulfoxide, 0.1% final concentration)for 10 min Cells were harvested as described in Materials and methods Results are expressed as mean ± SEM of three independent experiments Data are expressed as a percentage
of the control, which was considered 100% Data are significantly different as compared to vehicle control (\P < 0.01)and TG-stimulated cells (\\P < 0.001).
Fig 3 Effects of TG on the induction of mRNA of different
phospho-lipase A 2 isoforms in Jurkat T-cells Cells were treated for two hours
with or without TG (1 l M ) Total RNA was isolated and analyzed by
RT-PCR using specific primers for human PLA 2 -IB, -V, -IV and -VI as
described in Materials and methods b-actin mRNA was used as an
internal standard The lower panel shows the histograms of three
in-dependent experiments Data are significantly different as compared to
respective constitutively expressed mRNA levels (\P < 0.001).
Fig 4 Effect of phospholipase A 2 inhibitors on TG-induced capacitative calcium influx in Jurkat T-cells Cells (4 · l0 6 per assay)were loaded with the fluorescent probe, Fura-2/AM, as described in Materials and methods The experiments were performed in 100% Ca2+medium The arrow heads indicate the time when the test molecules, thapsi-gargin, TG (1 l M )or PLA 2 inhibitors, all at 15 l M , i.e AACOCF 3 , aristolochic acid, BEL, BPB, MAF, were added into the cuvette The control trace (none)shows the recording observed in the absence of PLA 2 inhibitors The figure shows the single traces of observations which were reproduced (n ¼ 11), independently.
Trang 5abruptly diminished the AA-induced rise in [Ca2+]i in
Jurkat T-cells (AA-induced spike response, 40 ± 4.2 nMvs
BSA-induced inhibition after the addition, 20 ± 2.1 nM)
Addition of BSA alone exerted no significant perturbation
in the Fura-2 fluorescence (results not shown)
D I S C U S S I O N
Our observations that arachidonic acid (AA)induces
calcium influx in Jurkat T-cells are in accordance with the
reports of several investigators who have shown that this
fatty acid induces calcium influx in different cell lines [17,
28–31] To shed light on whether exogenous AA evoked
capacitative calcium influx, we employed thapsigargin (TG)
and conducted experiments in 0% Ca2+ buffer In these
experiments, calcium was replaced by EGTA In 0% Ca2+
buffer, addition of AA alone did not induce any increases in [Ca2+]iin these cells In the CFCR protocol, in the presence
of thapsigargin, addition of AA and then exogenous Ca2+ exerted dose dependent effects on the increases in [Ca2+]iin Jurkat T-cells These observations suggest that AA evokes capacitative calcium influx in these cells
To elucidate whether TG liberates the endogenous AA that may account for the TG-induced capacitative calcium influx, we first loaded cells with [3H]AA and then incubated
in the presence of TG We observed that TG significantly induced the liberation of free AA into the extracellular environment Ohuchi et al [32] Have also shown that TG induces an increase in the release of [3H]AA; however, the PLA2 isoform implicated is not well known, though Tornquist et al [33] have demonstrated that cPLA2might
be responsible for the liberation of AA in FRTL-5 cells We employed the inhibitors of different isoforms of PLA2 We observed that TG seemed to act on the activation of three PLA2isotypes as aristolochic acid and BPB, the inhibitors
of type IB and type V, and BEL, an inhibitor of type VI, inhibited significantly the TG-induced [3H]AA release The type IV-PLA2does not seem to play a role in the release of
AA as its inhibitor, AACOCF3, failed to significantly diminish the TG-induced release of [3H]AA in Jurkat T-cells MAF, an inhibitor of type IV and type VI, also inhibited the release of [3H]AA but its effect does not seem additive as compared to BEL Whether TG exerts its action
at the transcriptional level, we detected the expression of mRNA, encoding for these four PLA2 isotypes We observed that, in RT-PCR, Jurkat T-cells constitutively express the mRNA of four PLA2isotypes, i.e type 1B, type
IV, type V and type VI In fact, the different phospholipases detected in our study belong to secretory (type IB and type
V, sPLA2)and cytosolic (calcium-dependent-type IV, cPLA2and calcium-independent-type VI, iPLA2)families Our results on the constitutive expression of these mRNA of different PLA2isoforms are in accordance with our recent report [26] Our results on the detection of type IV cPLA2 corroborate the findings of Boilard and Surette [34] who have recently shown that this isotype of PLA2is phospho-rylated after anti-CD3 stimulation in human T-lympho-cytes Addition of TG potentiates the induction of mRNA
of these four PLA2isotypes The mechanism of action of
TG on the induction of these enzymes is not well under-stood Several investigators have shown that the generation
of [Ca2+]ioscillations by some agonists also accompanies PLA2-mediated AA release [33,35], though PLA2 can be activated independently of increases in [Ca2+]i, probably via receptor coupling of this enzyme [36] Though TG stimu-lated the induction of expression of mRNA of four PLA2 isotypes, only three of them seem to be implicated in capacitative calcium influx as the aristolochic acid and BPB (inhibitors of sPLA2)and BEL (inhibitor of iPLA2), but not AACOCF3 (inhibitor of cPLA2), curtailed the sustained TG-induced capacitative calcium entry in these cells MAF (inhibitor of iPLA2and cPLA2)diminished the TG-induced calcium with the same order of magnitude as BEL The stimulus-induced release of AA by the action of iPLA2, though does not fit with the role of iPLA2in phospholipid remodelling, but it seems to be a specific feature of these cells
as we have reported recently that an inhibitor of this isoform significantly diminished the release of AA, induced by phorbol 12-myristate 13-acetate and ionomycin in Jurkat
Fig 6 Effects of addition of BSA on AA-evoked increases in [Ca 2+ ] i in
Jurkat T-cells Cells (4 · l0 6
per assay)were loaded with the fluores-cent probe, Fura-2/AM, as described in Materials and methods The
arrow heads indicate the time when the test molecules, fatty acid free
BSA (0.2% w/v)and AA (10 l M )were added into the cuvette without
interruptions in the recordings The figure shows the single traces of
observations that were reproduced (n ¼ 12), independently.
Fig 5 Effects of arachidonic acid (AA) on ionomycin-induced calcium
influx in Jurkat T-cells Cells (4 · l0 6 per assay)were loaded with the
fluorescent probe, Fura-2/AM, as described in Materials and methods.
The arrow heads indicate the time when the test molecules, ionomycin
(100 n M )and AA (10 l M ), were added into the cuvette without
interruptions in the recordings The figure shows the single traces
of observations which were reproduced (n ¼ 8), independently.
Trang 6T-cells [26] Similarly, Roshak et al [37] have also reported
that iPLA2 is expressed in human peripheral blood
lymphocytes and Jurkat T-cells, and it plays an important
role in T-cell proliferation as its depletion by antisense
treatment resulted in marked suppression of cell division
As the addition of AA during the ionomycin-induced
response exerted an additive prolonged effect, we can state
that AA is opening the calcium channels, probably specific
to this fatty acid The AA-stimulated calcium influx is not
mediated via classical mechanisms as TA9, an inhibitor of
CRAC channels [38], and diltiazem and x-conotoxin, the
respective inhibitors of L-type and N-type calcium channels,
failed to block AA-induced calcium influx in these cells
(results not shown) Our hypothesis on the presence of
AA-specific calcium influx is contributed by the recent
reports [39,40] that have demonstrated the
arachidonate-regulated calcium (ARC)current, specific to this fatty acid
in HEK293 cells The ARC current, evoked by AA from 8
to 10 lM in patch clamp experiments, can be blocked by
La3+at 50 lMin these cells [39] Similarly, in our study, we
observed that AA-induced capacitative calcium influx was
inhibited by the preaddition of La3+(results not shown) As
the specific inhibitors of ARC channels are not available, it
is difficult to provide a direct evidence for the implication of
these channels during capacitative calcium influx in Fura-2
loaded cells
Our results on AA-evoked capacitative calcium influx are
in contradiction with the observations of Gamberuchi et al
[19] who have reported that AA, in place of stimulating,
inhibits thapsigargin-induced capacitative calcium influx in
Jurkat T-cells The difference in the observations can be
explained by the fact that these investigators determined the
increases in [Ca2+]i at a single excitation wavelength,
340 nm In fact, this approach is not very precise as, during
the increases in [Ca2+]i, there is usually spectral
displace-ment from 340 nm to another wavelength of the excitation
spectrum when using Fura-2 [24] However, in our study,
we excited the probe, in the ratio mode, at two wavelengths,
i.e 340 nm and 380 nm, in the excitation spectrum and this
method provides accurate results by eliminating any spectral
displacement during determinations the increases in [Ca2+]i
[24]
In our study, AA seems to act extracellularly as addition
of fatty acid free BSA abruptly diminished the Ca2+rise,
evoked by the former BSA is known to possess high affinity
binding sites for free fatty acids Hence, it seems that BSA
detaches the plasma membrane bound-AA and, thereby,
contributes to the lowered response of this polyunsaturated
fatty acid How AA directly opens or modulates the calcium
channels is not known However, a direct action of
arachidonic acid on N-methyl-D-aspartate
receptor-chan-nels has been proposed as the channel protein shares an
amino acid sequence homology with fatty acid binding
proteins, FABP [41] Whether ARC channels also possess
such binding sites that will bear homology with FABP
remains to be shown Nonetheless, our study is consistent
with our recent report in which docosahexaenoic acid, a
polyunsaturated fatty acid, affects the calcium channels in
an albumin reversible manner [42]
The present study demonstrates that AA facilitates
TG-induced capacitative calcium entry The sequence of events
will be as follows: thapsigargin fi PLA2activation fi
AA release fi SOC capacitative influx Hence,
arachi-donic acid will, probably, act via opening of ARC channels Though AA does act on the capacitative calcium entry, its role in the modulation of the other T-cell signalling mechanisms cannot be ruled out as the PLA2 inhibitors almost completely inhibit the release of AA This hypothesis can be supported by our recent observations in which we have shown that free AA potentiates okadaic acid-stimu-lated activation of mitogen-activated protein kinases in Jurkat T-cells [43] Our study is certainly of physiological relevance as under some pathophysiological conditions like cardiac ischemia, the concentrations of AA are increased up
to 50 lM[44] Several studies have demonstrated that PLA2, during T-cell activation, can catalyze the liberation of free arachidonic acid [34,45], and hence, free AA can modulate the proliferation and clonal selection of T-cells during an antigenic challenge In fact, polyunsaturated fatty acids have been considered as immunomodulators and one can envisage that free AA in vivo can modulate T-cell activation
in health and disease
A C K N O W L E D G E M E N T S
Authors are thankful to the Region Bourgogne (France)for the sanction of a contingent grant.
R E F E R E N C E S
1 Donnadieu, E., Bismuth, G & Trautmann, A (1992)Calcium fluxes in T-lymphocytes J Biol Chem 267, 25864–25872.
2 Lewis, R.S & Cahalan, M.D (1989)Mitogen-induced oscillations
of cytosolic Ca 2+ and transmembrane Ca 2+ current in human leukemic T-cells Cell Regul 1, 99–112.
3 Lewis, R.S & Cahalan, M.D (1995)Potassium and calcium channels in lymphocytes Annu Rev Immunol 13, 623–653.
4 Negulescu, P.A., Shastri, N & Cahalan, M.D (1994)Intracellular calcium dependence of gene expression in single T-lymphocytes Proc Natl Acad Sci USA 91, 2873–2877.
5 Dolmetsch, R.E., Xu, K & Lewis, R.S (1998)Calcium oscilla-tions increase the efficiency and specificity of gene expression Nature 392, 933–936.
6 Li, W., Llopis, J., Whitney, M., Zlokarnik, G & Tsein, R.Y (1998)Cell-permeant caged InsP3 ester shows that Ca2+spike frequency can optimize gene expression Nature 392, 936–941.
7 Putney, J.W Jr (1997)Type 3 inositol 1,4,5-trisphosphate receptor and capacitative calcium entry Cell Calcium 21, 257–261.
8 Zweifach, A & Lewis, R.S (1993)Mitogen regulated Ca2+ cur-rent of T-lymphocytes is activated by depletion of intracellular
Ca 2+ stores Proc Natl Acad Sci USA 90, 6295–6299.
9 Su, Z., Csutora, P., Huntaon, D., Shoemaker, R.L., Marchase, R.B & Blalock, J.E (2001)A store-operated nonselective cation channel in lymphocytes is activated by Ca 2+ influx factor and diacylglycerol Am J Physiol Cell Physiol 280, C1284–C1292.
10 Ramdriamampita, C & Tsien, R.Y (1993)Emptying of intracellular Ca 2+ stores releases a novel small messenger that stimulates Ca 2+ influx Nature 364, 809–814.
11 Guse, A.H., Da Silva, C.P., Berg, I., Skapenko, A.L., Weber, K., Heyer, P., Hohenegger, M., Ashamu, G.A., Schulze-Koops, H., Potter, B.V.L & Mayer, G.W (1999)Regulation of calcium sig-nalling in T lymphocytes by the second messenger cyclic ADP-ribose Nature 398, 70–73.
12 Calder, P.C (1999)Dietary fatty acids and immune system Lipids
34, S137–S140.
13 Denys, A., Hichami, A & Khan, N.A (2001)Eicosapentaenoic acid and docosahexaenoic acid modulate MAP kinase (ERK1/ ERK2)signalling in human T-cells J Lipid Res 42, 2015–2020.
Trang 714 Khan, N.A & Hichami, A (2002)Role of n-3 polyunsaturated
fatty acids in the modulation of T-cell signalling In: Recent
Advances in Research in Lipids (ed G Pandali) Transworld
Publications, India in press
15 Mcmurray, D.N., Jolly, C.A & Chapkin, R.S (2000)Effects of
dietary n-3 fatty acids on T cell activation and T cell
receptor-mediated signaling in a murine model J Infect Dis 182 (Suppl.),
S103–S107.
16 Triboulot, C., Hichami, A., Denys, A & Khan, N.A (2001)
Dietary (n-3)polyunsaturated fatty acids exert antihypertensive
effects by modulating calcium signalling in T-cells of rats J Nutr.
131, 2364–2369.
17 Chow, S.C & Jondal, M (1990)Polyinsatured free fatty acids
stimulate an increase in cytosolic Ca 2+ by mobilizing the inositol
1,4,5-trisphosphate-sensitive Ca 2+ pool in T-cells through a
mechanism independent of phosphoinositide turnover J Biol.
Chem 265, 902–907.
18 Chow, S.C., Sisfontes, L., Jondal, M & Bjo¨rkhem, I (1991)
Modification of membrane phospholipid fatty acyl composition in
a leukemic T-cell line: effects on receptor mediated intracellular
Ca2+release Biochim Biophys Acta 1092, 358–366.
19 Gamberuchi, A., Fulceri, R & Benedetti, A (1997)Inhibition of
store-dependent capacitative Ca2+ influx by unsaturated fatty
acids Cell Calcium 21, 375–385.
20 Breittmayer, J.P., Pelassy, C., Cousin, J.L., Bernard, A & Aussel,
C (1993)The inhibition by fatty acids of receptor mediated
cal-cium movements in Jurkat T-cells is due to increased calcal-cium
extrusion J Biol Chem 268, 20812–20817.
21 Khurodova, A.B & Astashkin, E.I (1994)A dual effect of
ara-chidonic acid on Ca 2+ transport system in lymphocytes FEBS
Lett 353, 167–170.
22 Goldyne, M.E., Burrish, G.E., Paubelle, P & Borgeat, P (1984)
Arachidonic acid metabolism among human mononuclear
leu-kocytes Lipoxygenase-related pathways J Biol Chem 259,
8815–8819.
23 Kurland, J.L & Bockman, R (1978)Prostaglandin E production
by human blood monocytes and mouse peritoneal macrophages.
J Exp Med 147, 952–957.
24 Grynkiewicz, G.M., Ponie, M & Tsein, R.Y (1985)A new
gen-eration of Ca2+ indicators with greatly improved fluorescence
properties J Biol Chem 260, 3440–3450.
25 Hichami, A., Boichot, E., Germain, N., Legrand, A., Moodley, I.
& Lagante, V (1995)Involvement of cyclic AMP in the effects of
phosphodiesterase IV inhibitors on arachidonate release from
mononuclear cells Eur J Pharmacol 291, 91–97.
26 Tessier, C., Hichami, A & Khan, N.A (2002)Implication of three
isoforms of PLA 2 in human T-cell proliferation FEBS Lett 520,
111–116.
27 Thastrup, O., Cullen, P.J., Drobak, B.K., Hanley, M.R &
Dawson, A.P (1990)Thapsigargin, a tumor promoter, discharges
intracellular Ca 2+ stores by specific inhibition of the endoplasmic
reticulum Ca 2+ -ATPase Proc Natl Acad Sci USA 87, 2466–
2470.
28 Pollock, W.K., Sage, S.O & Rink, T.J (1987)Stimulation of
Ca2+efflux from fura-2 loaded platelets activated by thrombin or
phorbol myristate acetate FEBS Lett 210, 132–136.
29 Hoffmann, P., Richards, D., Hoffmann-Heinroth, I., Mathias, P.,
Wey, H & Toraason, M (1995)Arachidonic disrupts calcium
dynamics in neonatal rat cardiac myocytes Cardiovas Res 30,
889–898.
30 Roudbaraki, M., Drouhault, R., Bacquart, T & Vacher, P (1992) Arachidonic acid-induced hormone released in somatotropes: involvement of calcium Neuroendocrinology 63, 244–256.
31 Soliven, B., Takeda, M., Shandy, T & Nelson, T.J (1993) Arachidonic acid and its metabolites increase Ca2+i in cultured rat oligodendrocytes Am J Physiol Cell Physiol 264, C632–C640.
32 Ohuchi, K., Sugawara, T., Watanabe, M., Hirasawa, N., Tsurufuji, S., Fujiki, H., Sugimura, T & Christensen, S.B (1987)Stimulation of arachidonic acid metabolism in rat perito-neal macrophages by thapsigargin, a non-(12-O-tetra-decanoylphorbol-13-acetate)(TPA) -type tumor promotor.
J Cancer Res Clin Oncol 113, 319–324.
33 Tornquist, K., Ekokoski, E & Forss, L (1994)Thapsigargin-induced calcium entry in FRTL-5 cell: possible dependence on phospholipase A 2 activation J Cell Physiol 160, 40–46.
34 Boilard, E & Surette, M.E (2001)Anti-CD3 and concanavalin A-induced human T cell proliferation is associated with an increased rate of arachidonate-phospholipid remodeling Lack of involve-ment of group IV and group VI phospholipase A 2 in remodeling and increased susceptibility of proliferating T cells to CoA-independent transacyclase inhibitor-induced apoptosis J Biol Chem 276, 18321–18326.
35 Tsumoda, Y & Owayng, C (1993)Differential involvement of phospholipase A 2 /arachidonic acid and phospholipase C/phos-phoinositol pathways during cholecystokinin receptor activated
Ca2+ oscillations in pancreatic acini Biochem Biophys Res Commun 194, 1194–1202.
36 Shuttleworth, T.J (1996)Arachidonic acid activates the noncap-acitative entry of Ca 2+ during [Ca 2+ ]i oscillations J Biol Chem.
271, 21720–21725.
37 Roshak, A.K., Capper, E.A., Stevenson, C., Eichman, C & Marshall, L.A (2000)Human calcium-independent phospho-lipase A 2 mediates lymphocyte proliferation J Biol Chem 275, 35692–35698.
38 Marhaba, R., Mary, F., Pelassy, C., Stanescu, A.T., Aussel, C & Breittmayer, J.P (1996)Tyrphostin A9 inhibits calcium dependent phosphorylations and calcium entry via calcium release-activated channel in Jurkat T-cells J Immunol 157, 1468–1473.
39 Mignen, O & Suttleworth, T.J (2000)I ARC , a novel arachidonic-regulated, noncapacitative Ca2+entry channel J Biol Chem 275, 9114–9119.
40 Mignen, O., Thomson, J.L & Suttleworth, T.J (2001) Reciprocal regulation of capacitative and arachidonate regulated non-capa-citative Ca2+entry channel J Biol Chem 276, 35676–35683.
41 Petrou, S., Ordwa, R.W., Singer, J.J & Walsh, J.V Jr (1993)A putative fatty acid binding domain of the NMDA receptor Trends Biol Sci 18, 41–42.
42 Bonin, A & Khan, N.A (2000)Regulation of calcium signalling
by docosahexaenoic acid in human T cells: implication of CRAC channels J Lipid Res 41, 277–284.
43 Denys, A., Hichami, A & Khan, N.A (2002)Eicosapentaenoic acid and docosahexaenoic acid modulate MAP kinase enzyme activity in human T-cells Mol Cell Biochem 232, 143–148.
44 Nakamura, K., Ichihara, K & Abiko, Y (1989)Effect of pro-pranolol on accumulation of NEFA in the ischemic perfused rat heart Eur J Pharmacol 160, 61–69.
45 Le Gouvello, S.L., Colrad, O., Theodorou, I., Bismuth, G., Tar-antino, N & Debre, P (1990)CD2 triggering stimulates a phos-pholipase A 2 activity beside the phospholipase C pathway in human T lymphocytes J Immunol 144, 2359–2364.