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(Received for publication, August 25, 1995) From the
To define the molecular mechanisms of cross-regulation among
chemoattractant receptors, we stably co-expressed, in a rat basophilic
leukemia (RBL-2H3) cell line, epitope-tagged receptors for the
chemoattractants formylmethionylleucylphenylalanine (fMLP), a peptide
of the fifth component of the complement system (C5a), and
interleukin-8 (IL-8). All the expressed receptors underwent homologous
phosphorylation and desensitization upon agonist stimulation. When
co-expressed, epitope-tagged C5a receptor (ET-C5aR) and epitope-tagged
IL-8 receptor (ET-IL-8RA) were cross-phosphorylated by activation of
the other. Activation of epitope-tagged fMLP receptor (ET-FR) also
cross-phosphorylated ET-C5aR and ET-IL-8RA, but ET-FR was totally
resistant to cross-phosphorylation. Similarly, C5a and IL-8 stimulation
of [
Leukocytes migrate to sites of inflammation where they
participate in host-defensive and/or tissue-destructive activities via
activation of chemoattractant receptors. Upon stimulation by
proinflammatory agents such as a peptide component of the fifth
complement system (C5a), ( Cross-desensitization studies of
chemoattractant receptors using Ca
Figure 1:
Immunoprecipitation of epitope-tagged
chemoattractant receptors expressed in RBL-2H3 cells. A,
We also determined whether ligand cross-reactivity
could result in an apparent receptor cross-phosphorylation. Single
transfected RBL-2H3 cells expressing either ET-C5aR or ET-IL-8RA were
Figure 2:
Homologous and cross-desensitization of
peptide chemoattractant receptors stimulated
[
Figure 3:
Cross-desensitization of chemoattractant
receptors mediated Ca
It was
determined whether a depletion of the intracellular calcium pool caused
by the first ligand could account for the attenuation of Ca
Figure 4:
Measurement of IP
Despite a large body of evidence indicating that
chemoattractant-mediated inflammatory responses are regulated by
desensitization, little is known about the molecular events governing
this process. Wilde et al.(10) reported that
C5a-stimulated GTPase activity was desensitized in membranes from
neutrophils pretreated with fMLP. These results were confirmed by our
previous work, which further indicated that exposure of neutrophils to
fMLP cross-desensitized C5a, IL-8, platelet-activating factor, and
leukotriene B mediated-GTP The extent of fMLP-mediated phosphorylation of
ET-C5aR and ET-IL-8RA mirrored the ones previously obtained upon
exposure of these receptors to the protein kinase C activator, phorbol
12-myristate 13-acetate(6, 7) . Interestingly,
receptor cross-phosphorylation cannot explain the cross-desensitization
of formylpeptide receptor-mediated Ca In
summary, we have developed a system to stably co-express two G
protein-coupled receptors and study their cross-regulation. The results
presented herein indicate that chemoattractant receptor-mediated
inflammatory response are regulated at multiple sites. One is at the
level of receptor phosphorylation affecting receptor/G protein
coupling. The second is at a site distal to that, presumably involving
the activity of phospholipase C. Cross-desensitization at different
levels of the signaling cascades may be used by the receptors to
control each other's activity at sites of inflammation where
multiple chemoattractants are present. It will be important to
determine if receptor cross-desensitization at the level of PLC occurs
more generally than the subgroup of chemoattractant receptors studied
here.
Volume 270,
Number 46,
Issue of November 17, 1995 pp. 27829-27833
©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
EVIDENCE FOR A ROLE FOR INHIBITION OF PHOSPHOLIPASE C ACTIVITY (*)
S]guanosine
5`-3-O-(thio)triphosphate (GTP
S) binding and
Ca
mobilization were cross-desensitized by each other
and by fMLP. Stimulation of [
S]GTP
S binding
by fMLP was also not cross-desensitized by C5a or IL-8, however,
Ca
mobilization was, suggesting a site of inhibition
distal to G protein activation. Consistent with this desensitization of
Ca
mobilization, inositol 1,4,5-trisphosphate release
in RBL-2H3 cells expressing both ET-C5aR and ET-FR revealed that fMLP
and C5a cross-desensitized each other's ability to stimulate
phosphoinositide hydrolysis. Taken together, these results indicate
that receptor cross-phosphorylation correlates directly with
desensitization at the level of G protein activation. The ET-FR was
resistant to this process. Of note, cross-desensitization of ET-FR at
the level of phosphoinositide hydrolysis and Ca
mobilization was demonstrated in the absence of receptor
phosphorylation. This suggests a new form of chemoattractant
cross-regulation at a site distal to receptor/G protein coupling,
involving the activity of phospholipase C.
)formylpeptides (fMLP),
interleukin-8 (IL-8), platelet-activating factor, or leukotriene
B
chemoattractant receptors couple to guanine nucleotide
binding regulatory proteins (G proteins) to induce cellular
responses(1) . Prolonged stimulation of these receptors results
in desensitization. Originally, two types of desensitization were
described: homologous and heterologous(2) . Homologous
desensitization is specific for a receptor and its agonist.
Heterologous desensitization refers to a process whereby activation of
one type of receptor results in the desensitization of different
receptors. Homologous desensitization occurs as a result of
phosphorylation of the active form of a receptor by a receptor kinase,
whereas heterologous desensitization affects active and inactive
receptor forms by kinases activated by second
messengers(2, 3) . mobilization as a
measurement of receptor activation have led us to the description of a
novel type of desensitization whose specificity falls between
heterologous and homologous desensitization(4, 21) .
This type of desensitization was defined as cross-inhibition of
Ca
mobilization among a particular class of
chemoattractant receptors, i.e. those for peptide but not for
lipid chemotactic factors(4, 21) . Other studies have
shown that phosphorylation of the cytoplasmic domains of G
protein-coupled receptors, followed by their uncoupling from G
proteins, can be responsible for
desensitization(2, 5) . However, experiments in human
neutrophils indicated that component(s) distal from receptor/G-protein
may also be involved in chemoattractant receptor
cross-desensitization(4) . To better define the multiple types
of receptor desensitization, we developed a model system, a rat
basophilic leukemia cell line (RBL-2H3), in which chemoattractant
receptors can be expressed and induced to elicit cellular responses
similar to those in neutrophils. Using this model, we recently showed
that agonist-stimulation of the chemoattractant receptors for fMLP,
C5a, IL-8, and platelet-activating factor expressed in RBL-2H3 cells
resulted in phosphorylation and desensitization of these
receptors(6, 7) . (
)In the present work, we
sought to better define the mechanism(s) of cross-desensitization of
chemoattractant receptors. For that purpose, chemoattractant receptors
were co-expressed in RBL-2H3 cells and studied for their ability to
undergo and/or mediate cross-phosphorylation and correlate this with
consequent GTPS binding, generation of inositol trisphosphates,
and mobilization of intracellular calcium. The results presented here
demonstrate that receptor phosphorylation and modification of a
downstream component(s) of the chemoattractants signaling cascade
participate in different forms of chemoattractant receptor
cross-regulation.
Materials
[P]Orthophosphate
(8,500-9,120 Ci/mmol), myo-[2-
H]inositol
(24.4 Ci/mmol), and [S]GTP
S (1300 Ci/mmol)
were purchased from DuPont NEN.
I-IL-8 and
[
H]Inositol 1,4,5-trisphosphate (IP)
assay kits were from Amersham. IL-8 (monocyte derived) was purchased
from Genzyme. Monoclonal 12CA5 antibody was from BabCo. PSV2neo was
from American Type Culture Collection. Geneticin (G418) and all tissue
culture reagents were purchased from Life Technologies, Inc. Protein
G-agarose and protease inhibitors were from Boehringer Mannheim. fMLP,
indo-1 acetoxymethyl ester, and pluronic acid were from Molecular
Probes. C5a, GDP, GTP, GTPS, and ATP were purchased from Sigma.
Thapsigargin and ionomycin were from Calbiochem. All other reagents
were from commercial sources.
Construction of Epitope-tagged
Receptors
Nucleotides encoding a nine-amino acid epitope
sequence (YPYDVPDYA) was inserted between the N-terminal initiator
methionine and the second amino acid of each cDNA by polymerase chain
reaction as described previously(6, 7) .Cell Culture and Transfection
RBL-2H3
cells were maintained as monolayer cultures in Earle's modified
Eagle's medium supplemented with 15% fetal bovine serum, 2
mM glutamine, penicillin (100 units/ml), and streptomycin (100
µg/ml) (6) . These cells (1 
10
cells)
were transfected by electroporation with either pRK5 or pCDNA3
containing the receptor cDNAs (20 µg). Geneticin-resistant cells
were selected by subculturing the transfected cells in growth medium
supplemented with geneticin (1 mg/ml), and cell surface expression of
the receptors was monitored by fluorescence-activated cell sorter
analysis as described previously(6, 7) . For double transfectants, RBL-2H3 cells expressing one receptor
were electroporated in the presence of pRK5 vectors (20 µg)
containing the second receptor cDNA. Double transfectants were isolated
by fluorescence-activated cell sorter using fluorescein isothiocyanate
ligand for the second receptor. The receptors co-expressed in RBL-2H3
cells displayed pharmacological and functional properties similar to
those obtained with single expression of the receptors (K
values were 80 nM,
8
pM,
2 nM for ET-FR, ET-C5aR, and ET-IL-8RA,
respectively)(6, 7) .
Phosphorylation of the Epitope-tagged
Receptors
Phosphorylation of ET receptors was performed as
described previously(6, 7) . Briefly,
RBL-2H3 cells (2.5 10
) expressing each combination
of receptors were subcultured overnight in 60-mm tissue culture dishes.
The following day, the cells were rinsed twice with 5 ml of
phosphate-free Dulbecco's modified Eagle's medium and
incubated in the same medium supplemented with
[P]orthophosphate (150 µCi/dish) for 90 min
to metabolically label the intracellular ATP pool. Then, labeled cells
were stimulated with or without agonists (IL-8, 100 nM; C5a,
100 nM; or fMLP, 1 µM) for 5-7 min at 37
°C. The phosphorylated receptors were immunoprecipitated with the
12CA5 antibody, analyzed by SDS-electrophoresis, and visualized by
autoradiography(6, 7) .
GTP
Cells were treated with
appropriate concentrations of stimulants, and membranes were prepared
as already described(7) .S Binding
[S]GTP
S binding, using 10-20 µg
of membrane preparations, was carried out as described
previously.
Calcium Measurement
Cells (3
10) were removed, washed with HEPES-buffered Hank's
balanced salt solution, and loaded with 1 µM indo
I-acetoxymethyl ester in the presence of 1 µM pluronic
acid for 30 min at room temperature. Then, the cells were washed and
resuspended in 1.5 ml of buffer. Intracellular calcium increase in the
presence of different ligands at the indicated doses (fMLP, 100
nM; C5a, 10 nM; IL-8, 10 nM) was measured as
described(6) .Inositol Phosphate Extraction and
Measurement
RBL-2H3 cells expressing both ET-FR and ET-C5aR
were plated in 60-mm dishes (2.5 106 cells/dish) and
equilibrated in serum-free medium for 1 h. Cells were then treated with
or without agonists in the same medium at 37 °C. The reactions were
terminated by addition of an equal volume of ice-cold 15% (w/v)
trichloroacetic acid, and the samples were kept on ice for 30 min.
Inositol phosphates were extracted three times with 10 volumes of
water-saturated diethyl ether and neutralized to pH 7.5 with 1 M NaHCO(9) . 50 µl of each extract was
used to determine the IP mass using the Amersham
radioreceptor binding assay kit.
Phosphorylation and Immunoprecipitation of ET
Receptors in RBL-2H3 Cells
RBL-2H3 cells expressing
epitope-tagged receptors for FR and C5aR, IL-8RA and C5aR, or FR and
IL-8RA were P labeled and treated with different ligands
(1 µM fMLP, 0.1 µM C5a, or 0.1 µM IL-8) and immunoprecipitated with 12CA5 antibody. As shown in Fig. 1, ET-C5aR (
45 kDa) was phosphorylated by C5a (lanes 3 and 6) and cross-phosphorylated by fMLP (lane 2) and IL-8 (lane 5). ET-IL-8RA (
70 kDa)
was phosphorylated by IL-8 (lanes 5 and 8) and
cross-phosphorylated by C5a (lane 6) and fMLP (lane
9). In contrast, ET-FR (
65 kDa) was phosphorylated by fMLP (lanes 2 and 9) but resistant to
cross-phosphorylation by either C5a (lane 3) or IL-8 (lane
8) stimulation. The identity of these phosphorylated bands as the
respective receptors has been previously demonstrated by
immunoprecipitation of iodinated and phosphorylated receptors in the
presence and absence of the epitope tag peptide(6) .
Since ET-IL-8RA and ET-FR migrate as broad overlapping bands in
these SDS gels, double-transfected RBL-2H3 cell lines with one wild
type and the other epitope-tagged receptors were prepared to more
clearly resolve cross-phosphorylation of these two receptors. As shown
in Fig. 1B, RBL-2H3 cells expressing ET-IL-8RA and wild
type FR and vice versa showed similar results to ET-IFR cells
when stimulated with either IL-8 (0.1 µM) (Fig. 1B, lanes 11 and 14) or fMLP (1
µM) (Fig. 1B, lanes 12 and 15).
P-labeled double-transfected RBL-2H3 cells (2.5
10/60-mm plate) expressing epitope-tagged receptors for FR
and C5aR (ET-FCR), IL-8RA and C5aR (ET-ICR), or IL-8RA and FR (ET-IFR)
were incubated for 5 min with (lanes 2, 3, 5, 6, 8, and 9) or without (lanes 1, 4, and 7) stimulants. Cells were
lysed, immunoprecipitated with 12CA5, and analyzed by
SDS-polyacrylamide gel electrophoresis and autoradiography. This
experiment was repeated five times with similar results. B,
RBL-2H3 cells expressing either ET-FR and IL-8RA (lanes
10-12) or ET-IL-8RA and FR (lanes 13-15) were
stimulated in the presence or absence of either fMLP or IL-8, and
receptor phosphorylation was assessed as described
above.
P labeled, treated with C5a (0.1 µM), IL-8
(0.1 µM), or fMLP (1 µM), and
immunoprecipitated. Only homologous phosphorylation was observed for
each receptor (i.e. ET-C5aR only by C5a and ET-IL-8RA only by
IL-8) (data not shown), indicating that ligand cross-reactivity does
not occur in these receptors.
GTP
To determine the effect of cross-phosphorylation
in receptor cross-desensitization, agonist-stimulated
[S Binding in Cross-desensitized
Membranes
S]GTP
S binding was measured in membranes
prepared from double transfectant RBL-2H3 cells pretreated with fMLP (1
µM), C5a (100 nM), or IL-8 (100 nM). As
shown in Fig. 2, pretreatment of cells with fMLP (A and C), C5a (A and B), or IL-8 (B and C) resulted in homologous desensitization (50-70%) of
agonist-induced [
S]GTP
S binding to
membranes. Membranes from cells pretreated with fMLP (ET-FCR (Fig. 2A) and ET-IFR (Fig. 2C)) showed
a
40 and
22% decrease in C5a and IL-8-stimulated
[
S]GTP
S binding, respectively, whereas no
decrease was observed for fMLP stimulation in cells pretreated with
either C5a or IL-8 (Fig. 2, A and C). Both,
ET-C5aR and ET-IL-8RA cross-desensitized (
28 and
25%,
respectively) each other's ability to stimulate
[
S]GTP
S binding in ET-ICR cells pretreated
with either C5a or IL-8 (Fig. 2B).
S]GTP
S binding. Double-transfected RBL-2H3
cells were treated with fMLP (1 µM), C5a (100
nM), or IL-8 (100 nM) for 5 min. Membranes were
prepared and assayed for agonist-stimulated
[
S]GTP
S binding. The data shown are the
means of three different experiments performed in triplicate. The
values are represented as percentage of maximum stimulation, which is
defined as the maximal increase above basal of
[
S]GTP
S bound to control membranes
(untreated cells) after 10 min of reaction. Basal activities were
0.2-0.3 pmol of [
S]GTP
S bound/mg
of protein). Maximum stimulation was 0.23 ± 0.008 (fMLP) and
0.19 ± 0.010 (C5a) pmol of
S-GTP
S bound/mg of
protein for untreated ET-FCR cells (panel A), 0.2 ±
0.011 (C5a) and 0.21 ± 0.02 (IL-8) pmol of
[
S]GTP
S bound/mg of protein for untreated
ET-ICR cells (panel B), and 0.19 ± 0.008 (fMLP) and
0.22 ± 0.0132 (IL-8) pmol of [
S]GTP
S
bound/mg of protein for untreated ET-IFR cells (panel C).
Specific activity was
380-450 cpm/fmol of
GTP
S.
Cross-desensitization of Receptor-induced
Ca
Ca Mobilization
mobilization was also measured to determine the relationship
between cross-phosphorylation and cross-desensitization of
receptor-mediated cellular responses. As shown in Fig. 3, all
three receptors induced Ca
mobilization in response
to agonist stimulation. Ca
mobilization in response
to an EC
dose of fMLP (100 nM), C5a (10
nM), or IL-8 (10 nM) was homologously desensitized by
a first dose of the same ligand (data not shown). C5a-induced
Ca
mobilization was cross-desensitized by
pretreatment of the cells with a first dose of either fMLP (Fig. 3A) or IL-8 (Fig. 3B).
IL-8-induced Ca
mobilization was similarly
cross-desensitized by pretreatment of the cells with a first dose of
either C5a (Fig. 3B) or fMLP (Fig. 3C).
However, in contrast to fMLP-stimulated
[
S]GTP
S binding, fMLP-induced
Ca
mobilization was also cross-desensitized by
pretreatment of the cells with a first dose of either C5a (Fig. 3A) or IL-8 (Fig. 3C).
mobilization.
Double-transfected RBL-2H3 cells (3
10 cells/assay)
were loaded with indo-1 and stimulated with fMLP (100 nM), C5a
(10 nM), or IL-8 (10 nM). Cells were rechallenged 3
min later with the same concentration of ligand. Traces are
representative of three experiments.
mobilization in response to a second stimuli. Treatment of ET-FCR
cells with 2 µM thapsigargin before stimulation (301
± 21 nM), 3 min after the first ligand (362 ± 13
nM) and after the second ligand (388 ± 17 nM),
followed by 10 µM ionomycin (621 ± 76 nM)
showed no significant change in the intracellular Ca
pool. These results indicate that the cross-desensitization of
receptor-mediated Ca
mobilization was not due to an
impairment of the intracellullar Ca
storage.
Cross-desensitization of Receptor-mediated IP
The ability of fMLP and C5a to stimulate
PIP Generation
hydrolysis in control versus desensitized
cells was determined by measuring the intracellular concentration of
IP. As shown in Fig. 4, pretreatment of RBL-2H3
cells expressing both fMLPR and C5aR with an EC dose of
either fMLP (1 µM) or C5a (100 nM) decreased by
85-95% the ability of the receptors to mediate intracellular
increase of IP
levels.
concentration in control and desensitized RBL-2H3 cells. RBL-2H3 cells
(2.5 10 cells) expressing epitope-tagged FR and
C5aR were treated with 1 µM fMLP (fMLP-treated), 100
nM C5a (C5a-treated), or in the absence of stimulants
(untreated cells) for 10 min at 37 °C in serum-free medium. Cells
were then rechallenged for 10 s with fMLP, C5a, or buffer, and IP was extracted as described under ``Experimental
Procedures.'' The IP concentration in each extract was
determined using the IPH assay system from
Amersham. Data are means ± S.E. of four separate determinations
performed in duplicate.
S binding in membranes. In contrast,
receptors for formylpeptide were resistant to this type of
cross-desensitization due presumably to the absence of the necessary
phosphorylation site (see below)(4) . Heterologous
phosphorylation of chemoattractant receptors by second
messenger-activated kinases (such as PKC) followed by their uncoupling
from G protein has been thought to be the molecular mechanism
responsible for chemoattractant
cross-desensitization(4, 6) . However, Tardif et
al.(11) reported that fMLP stimulation of HL-60 cells did
not induce C5aR phosphorylation. The data presented in the work
reported here clearly indicate that fMLP stimulation resulted in
phosphorylation of both C5aR and IL-8RA in double-transfected RBL-2H3
cells (Fig. 1), and both C5a and IL-8-mediated GTP
S binding
were desensitized under such conditions. The failure of Tardif et
al.(11) to find such cross-phosphorylation in HL-60 cells
is not understood.
These
results suggest that fMLP cross-desensitization of ET-C5aR and
ET-IL8RA, as well as C5a of ET-IL8RA and IL-8 of ET-C5aR, may be
mediated by receptor phosphorylation by PKC. Indeed, the PKC inhibitor
staurosporine inhibited fMLP-mediated phosphorylation of ET-C5aR in the
ET-FCR cell line (data not shown). fMLP has been shown to increase PKC
activity in neutrophils and several other cell
lines(12, 13, 14) . Molecular cloning has
revealed that the receptor for fMLP lacks sequence motif for PKC
phosphorylation (RXXSXRX). This likely
explains its resistance to PKC-mediated phosphorylation(15) .
Neither C5a nor IL-8 pretreatment resulted in cross-desensitization of
fMLP-mediated GTPS binding, which correlated with its resistance
to cross-phosphorylation. Taken together, these results are in
agreement with the current concept that receptor phosphorylation leads
to desensitization and indicate that PKC-mediated phosphorylation
results in one form of chemoattractant receptor cross-desensitization
at the level of receptor/G protein activation.
mobilization by
other chemoattractants since the formylpeptide receptors are totally
resistant to the heterologous phosphorylation. Thus, the formylpeptide
receptors provide an important tool to determine the downstream site(s)
for chemoattractant receptor cross-desensitization. The chemoattractant
receptors studied here are coupled to phospholipase C and mediate
intracellular signals via stimulation of phosphatidylinositol
hydrolysis and production of IP
and
diacylglycerol(1) . It has been shown that IP plays
a pivotal role in stimulating intracellular Ca mobilization(16) . Thus, cross-desensitization of
fMLP-mediated Ca
mobilization could be at the level
of PIP
hydrolysis or IP activity. Indeed,
cAMP-mediated phosphorylation of the receptor for intracellular
generated IP markedly decreases its ability to stimulate
Ca release(17) . Therefore, the possibility
existed that cross-desensitization of fMLP-induced intracellular
Ca
mobilization reflected either a decrease in the
level of intracellular IP
produced or desensitization of
the receptor for IP. As shown in fig. 4, fMLP stimulated
IP production was decreased by 90% in cells pretreated
with C5a. These results indicate that the cross-desensitization of
Ca
mobilization in response to fMLP by the other
chemoattractants is likely due to a decrease in the level of
IP
production. There are several possible explanations for
diminished IP production. A depletion of the pool of
PIP prior hydrolysis or stimulation of
phosphatidylinositol-3 kinase activity is one explanation. Against this
hypothesis is that pretreatment of neutrophils with C5a decreased
fMLP-induced IP production with no significant change in
the level of PIP(18) . Moreover, in neutrophils
that have been cross-desensitized by fMLP, purinergic receptor ability
to stimulate PLC and Ca release is normal, indicating
adequate IP
receptor and PIP level(4) .
A second explanation for diminished IP production is a
decrease in the catalytic activity of the phospholipase C either by
modification of the enzyme or its activating components. Both 
and
subunits of G protein have been shown to activate PLC in
reconstituted systems(19) . Chemoattractant receptors couple to
G
and mediate PLC activation via
G subunits(1, 19) .
subunits are known
to be isoprenylated and methylated(20) . It has recently been
shown that demethylation of the
subunit, which does not
affect receptor-mediated GTP
S binding to G protein, caused a
10-fold decrease in
-mediated activation of PLC and, thus,
production of IP
(8) . Therefore, it is possible
that C5a and IL-8-mediated cross-desensitization of FR-induced
Ca mobilization may be due to either a demethylation
or other modification of
subunits, rendering them less
effective in activating PLC. Modification of PLC itself could also
result in its diminished activity. In any case, the
cross-desensitization of formylpeptide receptor as presented is likely
due to a modification in its ability to activate PLC. C5a- and
IL-8-induced Ca
mobilization and IP
production are also inhibited in cells pretreated with fMLP.
Since all three chemoattractant receptors studied here apparently
utilize the same signal transduction pathways, the downstream effect
observed with the fMLP receptor likely plays a role in the attenuation
of C5a- and IL-8-induced responses in addition to the impairment of
receptor/G protein coupling due to receptor phosphorylation.
), inositol
1,4,5-trisphosphate; GTPS, guanosine
5`-3`-O-(thio)triphosphate; G protein, GTP-regulatory protein;
PKC, protein kinase C; PLC, phospholipase C.
)
©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
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M. Lenoir, B. Djerdjouri, and A. Perianin Stroma Cell-Derived Factor 1{alpha} Mediates Desensitization of Human Neutrophil Respiratory Burst in Synovial Fluid from Rheumatoid Arthritic Patients J. Immunol., June 1, 2004; 172(11): 7136 - 7143. [Abstract] [Full Text] [PDF]
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 B. Heit, S. Tavener, E. Raharjo, and P. Kubes An intracellular signaling hierarchy determines direction of migration in opposing chemotactic gradients J. Cell Biol., October 14, 2002; 159(1): 91 - 102. [Abstract] [Full Text] [PDF]
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M. Honczarenko, Y. Le, A. M. Glodek, M. Majka, J. J. Campbell, M. Z. Ratajczak, and L. E. Silberstein CCR5-binding chemokines modulate CXCL12 (SDF-1)-induced responses of progenitor B cells in human bone marrow through heterologous desensitization of the CXCR4 chemokine receptor Blood, September 18, 2002; 100(7): 2321 - 2329. [Abstract] [Full Text] [PDF]
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G. G. Vaday, S. Franitza, H. Schor, I. Hecht, A. Brill, L. Cahalon, R. Hershkoviz, and O. Lider Combinatorial signals by inflammatory cytokines and chemokines mediate leukocyte interactions with extracellular matrix J. Leukoc. Biol., June 1, 2001; 69(6): 885 - 892. [Abstract] [Full Text] [PDF]
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H. Sano, D. K. Hsu, L. Yu, J. R. Apgar, I. Kuwabara, T. Yamanaka, M. Hirashima, and F.-T. Liu Human Galectin-3 Is a Novel Chemoattractant for Monocytes and Macrophages J. Immunol., August 15, 2000; 165(4): 2156 - 2164. [Abstract] [Full Text] [PDF]
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R. M. Richardson, B. C. Pridgen, B. Haribabu, and R. Snyderman Regulation of the Human Chemokine Receptor CCR1. CROSS-REGULATION BY CXCR1 AND CXCR2 J. Biol. Chem., March 24, 2000; 275(13): 9201 - 9208. [Abstract] [Full Text] [PDF]
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C. Dahlgren, T. Christophe, F. Boulay, P. N. Madianos, M. J. Rabiet, and A. Karlsson The synthetic chemoattractant Trp-Lys-Tyr-Met-Val-DMet activates neutrophils preferentially through the lipoxin A4 receptor Blood, March 1, 2000; 95(5): 1810 - 1818. [Abstract] [Full Text] [PDF]
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E. F. Foxman, E. J. Kunkel, and E. C. Butcher Integrating Conflicting Chemotactic Signals: The Role of Memory in Leukocyte Navigation J. Cell Biol., November 1, 1999; 147(3): 577 - 588. [Abstract] [Full Text] [PDF]
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H. Ali, R. M. Richardson, B. Haribabu, and R. Snyderman Chemoattractant Receptor Cross-desensitization J. Biol. Chem., March 5, 1999; 274(10): 6027 - 6030. [Full Text] [PDF]
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I. Sabroe, A. Hartnell, L. A. Jopling, S. Bel, P. D. Ponath, J. E. Pease, P. D. Collins, and T. J. Williams Differential Regulation of Eosinophil Chemokine Signaling Via CCR3 and Non-CCR3 Pathways J. Immunol., March 1, 1999; 162(5): 2946 - 2955. [Abstract] [Full Text] [PDF]
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J. W. Peacock and F. R. Jirik TCR Activation Inhibits Chemotaxis Toward Stromal Cell-Derived Factor-1: Evidence for Reciprocal Regulation Between CXCR4 and the TCR J. Immunol., January 1, 1999; 162(1): 215 - 223. [Abstract] [Full Text] [PDF]
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R. M. Richardson, B. C. Pridgen, B. Haribabu, H. Ali, and R. Snyderman Differential Cross-regulation of the Human Chemokine Receptors CXCR1 and CXCR2. EVIDENCE FOR TIME-DEPENDENT SIGNAL GENERATION J. Biol. Chem., September 11, 1998; 273(37): 23830 - 23836. [Abstract] [Full Text] [PDF]
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 M.C. Grimm, A. Ben-Baruch, D.D. Taub, O.M.Z. Howard, J.H. Resau, J.M. Wang, H. Ali, R. Richardson, R. Snyderman, and J.J. Oppenheim Opiates Transdeactivate Chemokine Receptors: delta and µ Opiate Receptor-mediated Heterologous Desensitization J. Exp. Med., July 20, 1998; 188(2): 317 - 325. [Abstract] [Full Text] [PDF]
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 R. Yu and P. M. Hinkle Signal Transduction, Desensitization, and Recovery of Responses to Thyrotropin-Releasing Hormone after Inhibition of Receptor Internalization Mol. Endocrinol., May 1, 1998; 12(5): 737 - 749. [Abstract] [Full Text]
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H. Ali, S. Sozzani, I. Fisher, A. J. Barr, R. M. Richardson, B. Haribabu, and R. Snyderman Differential Regulation of Formyl Peptide and Platelet-activating Factor Receptors. ROLE OF PHOSPHOLIPASE Cbeta 3 PHOSPHORYLATION BY PROTEIN KINASE A J. Biol. Chem., May 1, 1998; 273(18): 11012 - 11016. [Abstract] [Full Text] [PDF]
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R. M. Richardson, H. Ali, B. C. Pridgen, B. Haribabu, and R. Snyderman Multiple Signaling Pathways of Human Interleukin-8 Receptor A. INDEPENDENT REGULATION BY PHOSPHORYLATION J. Biol. Chem., April 24, 1998; 273(17): 10690 - 10695. [Abstract] [Full Text] [PDF]
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E. F. Foxman, J. J. Campbell, and E. C. Butcher Multistep Navigation and the Combinatorial Control of Leukocyte Chemotaxis J. Cell Biol., December 1, 1997; 139(5): 1349 - 1360. [Abstract] [Full Text] [PDF]
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ABSTRACT
