By
From the * Institute of General Pathology, Center of Biotechnology, § Department of Biochemistry and
Molecular Biology, University of Ferrara, I-44100 Ferrara Italy
Microglial cells express a peculiar plasma membrane receptor for extracellular ATP, named
P2Z/P2X7 purinergic receptor, that triggers massive transmembrane ion fluxes and a reversible
permeabilization of the plasma membrane to hydrophylic molecules of up to 900 dalton molecule
weight and eventual cell death (Di Virgilio, F. 1995. Immunol. Today. 16:524-528). The physiological role of this newly cloned (Surprenant, A., F. Rassendren, E. Kawashima, R.A. North and G. Buell. 1996. Science (Wash. DC). 272:735-737) cytolytic receptor is unknown. In vitro
and in vivo activation of the macrophage and microglial cell P2Z/P2X7 receptor by exogenous
ATP causes a large and rapid release of mature IL-1. In the present report we investigated the
role of microglial P2Z/P2X7 receptor in IL-1
release triggered by LPS. Our data suggest that
LPS-dependent IL-1
release involves activation of this purinergic receptor as it is inhibited by
the selective P2Z/P2X7 blocker oxidized ATP and modulated by ATP-hydrolyzing enzymes
such as apyrase or hexokinase. Furthermore, microglial cells release ATP when stimulated with
LPS. LPS-dependent release of ATP is also observed in monocyte-derived human macrophages. It is suggested that bacterial endotoxin activates an autocrine/paracrine loop that drives
ATP-dependent IL-1
secretion.
With the recent cloning of the macrophage P2Z/
P2X7 receptor, purinergic receptors in immune cells
are attracting increasing interest (1, 2). Microglial cells are a
good model to investigate physiological functions of purinergic receptors in the immune system because neurons are
among the few cell types that have been unequivocally
shown to release ATP (3). A recent study from our laboratory has shown that exogenous ATP causes a large release
of IL-1 Cell Culture and Cytokine Measurement.
N9 and N13 microglial cell lines were a kind gift of Dr. Paola Ricciardi-Castagnoli
(University of Milano, Italy) and were grown in RPMI 1640 medium (PAA, Linz, Austria) supplemented with 2 mM glutamine
and 10% (heat-inactivated) FCS (Life Technologies Ltd., Paisley,
Scotland), 100 U/ml penicillin, and 100 µg/ml streptomycin as
described previously (2). Human monocytes were isolated from
buffy coats by one-step gradient (Percoll; Pharmacia Biotech SpA,
Cologno Monzese, Italy) or by adherence on plastic Petri dishes.
After isolation, cells were kept in culture for 5 d in RPMI medium containing 2 mM glutamine, 5% human serum, 100 U/ml penicillin, and 100 µg/ml streptomycin. IL-1 Measurement of Extracellular ATP.
Microglial cells (25 × 103/
well) were plated in microtiter plastic wells in culture medium
and incubated in a CO2 incubator at 37°C in the absence or presence of LPS for 24 h. At the end of this incubation, the monolayers were thoroughly rinsed with saline solution and supplemented
with 100 µl of a special diluent buffer (FireZyme Ltd., San Diego, CA) to stabilize extracellular ATP and directly placed in the
test chamber of a luminometer (FireZyme). Then, 100 µl of luciferin-luciferase solution (FireZyme) was added, and light emission was recorded. As a control, total cellular ATP content was
also routinely monitored. Under resting conditions, extracellular
ATP amounted to ~10-15% of total.
Fig. 1 shows that a 24-h incubation in the presence of 10 µg/ml LPS triggers release of IL-1
Autocrine/paracrine stimulation of purinergic receptors
can also in principle be prevented by exogenously added
ATP-consuming enzymes such as apyrase or hexokinase.
Fig. 2 A shows that apyrase completely inhibits LPS-dependent IL-1
An obvious sine qua non of this hypothesis is that microglial cells must release ATP in response to LPS. Fig. 3 A
shows that microglial cells chronically stimulated with LPS
release ATP. Because the incubation medium is changed right
before ATP determination, extracellular ATP measured in
this experiment is very likely not accumulated in the bulk
phase but continuously generated by the microglial cells. In
support of this interpretation, we consistently found very little extracellular ATP in the cell-free supernatant (not shown).
This observation is consistent with that previously reported
by Filippini et al. (14) in T lymphocytes. The LPS dose-
response curve for ATP release closely matched that for IL-1
The mechanism of IL-1 It might seem paradoxical that ADP is a better IL-1 Involvement of the P2Z/P2X7 purinergic receptor in
LPS-dependent IL-1 from N9 and N13 microglial cell lines and freshly isolated microglial cells by activating the P2Z/P2X7 receptor (2). IL-1
has a pivotal role in several chronic inflammatory diseases and in the pathogenesis of septic shock (4).
It is synthesized by activated macrophages and microglial
cells as an inactive 33-kD precursor (proIL-1
) that is
cleaved into the active 17-kD form by a cysteine protease
named IL-1
-converting enzyme (ICE) (7, 8). Mature IL-1
is released through an as yet unknown pathway. In macrophages and microglial cells, bacterial endotoxin (LPS),
the best characterized stimulant of IL-1
release, causes a
rapid and large in vitro accumulation of proIL-1
, followed by slow release of the mature form (9). Release of
processed IL-1
can be greatly accelerated by depletion of
endogenous K+ (4, 10). This very recent observation has
led to the hypothesis that in vivo an additional stimulus that
causes cytoplasmic K+ depletion must act in synergy with
LPS to induce fast IL-1
release (11). Extracellular ATP
(ATPe) is an interesting candidate to serve this role because
it causes massive K+ depletion via activation of the macrophage P2Z/P2X7 receptor (12). The present report suggests that the P2Z/P2X7 receptor is also involved in IL-1
release stimulated by LPS, presumably through an autocrine loop based on LPS-triggered ATP secretion.
and IL-6 in the supernatant of LPS (Sigma Chemical Co., St. Louis, MO) treated cells were measured with the Intertext-1
X mouse IL-1
ELISA
kit and Intertext-6X mouse IL-6 ELISA kit, respectively (Genzyme
srl, Cinisello Balsamo, Italy). All reagents used were dissolved in endotoxin-free water (Sigma) and checked for endotoxin contamination. Periodate-oxidised ATP (oATP) was synthesized by
Dr. S. Hanau as described in reference 13.
and that this is blocked
by pretreatment with the selective P2Z/P2X7 inhibitor
(13) oATP. To show that the effect of oATP is not due to
a nonspecific inhibition of cell responses, we have also
monitored IL-6 release, which is much less affected. As further proof that oATP does not have nonspecific effects, we
show that IL-1
release is restored in LPS-treated, oATPinhibited cells by the K+ ionophore nigericin, an agent
known to cause IL-1
release through a receptor-independent pathway (4, 10).
Fig. 1.
Oxidized ATP inhibits LPS-dependent release of IL-1.
N13 microglial cells were incubated in 24-well plates in RPMI medium supplemented with 10% FCS at a concentration of 2 × 106 and incubated
24 h in the presence or absence (controls) of 10 µg/ml LPS. In the experiments with oATP, cells were treated with this inhibitor (300 µM) for 2 h
and then rinsed before addition of LPS. Stimulation with nigericin (20 µM)
was performed for 30 min after removal of oATP. Closed bars, IL-1
; open
bars, IL-6. Data are averages of duplicate determinations from a single experiment repeated on three separate occasions. Similar results were obtained with the N9 cell line.
[View Larger Version of this Image (15K GIF file)]
release (the inactivated enzyme has no such effect). Surprisingly, hexokinase does not inhibit but rather
stimulates IL-1
release. The main difference between apyrase and hexokinase is that the first hydrolyzes ATP and
ADP, thus generating AMP, whereas hexokinase uses ATP
as phosphorus donor to phosphorylate glucose, thus generating glucose 6 phosphate and ADP. It is known that ADP
is an agonist at P2Z/P2X7 receptor, though less potent than
ATP (12). Thus we checked whether the potentiating effect of hexokinase is mediated by stimulation of the P2Z/
P2X7 receptor by accumulated ADP. This seems to be the case because pretreatment with oATP blocks IL-1
secretion due to the combined addition of LPS and hexokinase
(Fig. 2 A), and more importantly, exogenous ADP (ADPe)
is a much more potent stimulus than ATP (Fig. 2 B). These
experiments suggest that IL-1
release could be modulated
by ATPe and ADPe, probably released by the inflammatory
cells themselves under LPS stimulation.
Fig. 2.
Modulation of IL-1
release by apyrase and hexokinase
in N13 microglial cells. (A) Where
indicated, cells were incubated in the presence of apyrase (apy, 0.4 U/ml) or hexokinase (hex, 100 µg/ml) throughout LPS treatment
(10 µg for 24 h). As a control, the
enzymes were boiled for 30 min (b
apy and b hex) before being added
to the cell monolayers. Pretreatment with oATP (300 µM) was
peformed for 2 h; then the monolayers were rinsed and challenged with the different stimuli. (B) Cells
were first stimulated for 2 h with
LPS (10 µg/ml), and then stimulated with either 1 mM ATP or
ADP for 30 min. Other experimental conditions were as shown in Fig. 1.
[View Larger Versions of these Images (16 + 10K GIF file)]
release, as shown in Fig. 3 B. It has been shown previously that ATP is a powerful stimulus for IL-1
secretion from
macrophages (10, 11), thus suggesting that this nucleotide
might also have a role in autocrine/paracrine stimulation of
these cells. In support of this hypothesis, Fig. 4 shows that
ATP is released by human macrophages isolated from three
different subjects after stimulation with LPS.
Fig. 3.
LPS dose response for IL-1 and ATP release. Microglial
cells were plated in 24-well plates as described in Fig. 1 for IL-1
secretion or microtiter plastic wells as described in Materials and Methods for
ATP release and stimulated with LPS for 24 h in a CO2 incubator at
37°C. For measurement of ATP, release samples were processed as follows: monolayers were rinsed and 100 µl of diluent buffer (Firezyme)
were added (see Materials and Methods). Accumulation of extracellular
ATP was measured by the luciferin/luciferase assay. Data for IL-1
release are duplicates from a single experiment repeated with similar results
with three different batches of microglial cells. Data for ATP release are
means of quadruplicate determinations ± SD from a single experiment
repeated in three different occasions.
[View Larger Versions of these Images (24 + 22K GIF file)]
Fig. 4.
Stimulation with LPS triggers ATP release from human macrophages. Macrophages were isolated from three different donors (A-C ) as described in Materials and Methods and plated in microtiter plastic
wells at a concentration of 50 × 103/well. After plating, cells were stimulated for 24 h with LPS and ATP release measured as detailed in Fig. 3.
Data are mean ± SD from quadruplicate determinations.
[View Larger Version of this Image (25K GIF file)]
processing and release is a key
issue in immunology (5, 15). Rather surprisingly, recent
evidence points to a decrease in cytoplasmic K+ as a pivotal
stimulus for ICE activation and IL-1
maturation (4, 10).
However, LPS itself does not directly activate plasma membrane K+ channels, and mouse microglial cells express inwardly but not outwardly rectifying K+ channels (16), thus
raising the issue of the mechanism responsible for lowering
the cytoplasmic K+ concentration. It has been suggested
that this might be achieved by a LPS-dependent increase in
the number of voltage-dependent K+ channels in the macrophage plasma membrane (4), but typical K+ channel inhibitors blocked IL-1
release only at concentrations far
above those necessary to inhibit these channels (4). The P2Z/P2X7 receptor is a good candidate to mediate cytoplasmic K+ depletion. This receptor is typically expressed
in macrophages and macrophage-like cells (2, 17, 18), and
it is modulated by inflammatory cytokines (17, 19). A brief
stimulation with ATPe triggers massive K+ efflux (12) and
release of processed IL-1
(2, 10, 11), whereas a sustained
activation causes cell death (1, 17, 20). Our data suggest
that IL-1
release from microglial cells requires a double
stimulation: first, LPS-dependent transcription of the IL-1
gene and cytoplasmic accumulation of proIL-1
; second, paracrine/autocrine activation of the P2Z/P2X7 receptor
that causes release of the mature cytokine. Adenine nucleotides can originate from many different sources: (a) the microglial cells themselves can release ATPe, either spontaneously or under LPS stimulation; (b) injured or damaged
cells certainly release significant amounts of this nucleotide,
a process likely to occur in vivo at sites of inflammation; (c) in
the central nervous system, ATPe can be released by neurons that establish close contact with the microglial cells.
-
releasing agent than ATP, although notoriously it is a less
potent stimulus at the P2Z/P2X7 receptor. However, this
is not unexpected because ADP, in contrast with ATP, is
devoid of cytotoxic activity, and data from our laboratory
show that release of IL-1
is optimal in response to a submaximal, noncytotoxic stimulation of the P2Z/P2X7 receptor, such as that due to ADPe (D. Ferrari and F. Di Virgilio, manuscript in preparation).
release may allow the development
of new pharmacological antagonists (i.e., oATP and derivatives) to modulate the in vivo production of this cytokine
in pathological conditions such as septic shock or chronic
inflammatory diseases.
Address correspondence to Francesco Di Virgilio, Institute of General Pathology, University of Ferrara, Via Borsari, 46, I-44100 Ferrara, Italy.
Received for publication 10 October 1996
This work was supported by grants from the National Research Council of Italy (target project Clinical Application of Cancer Research, ACRO), the Ministry of Scientific Research (MURST), the Italian Association for Cancer Research (AIRC), the VIII AIDS Project, the I Tuberculosis Project, and Telethon of Italy.We are grateful to Dr. Paola Ricciardi-Castagnoli (National Research Council and University of Milano) for the gift of N9 and N13 mouse microglial cell lines.
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