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 Previous Article | Table of Contents | Next Article 
Blood, Vol. 94 No. 5 (September 1), 1999:
pp. 1555-1560
Sickle Cell Acute Chest Syndrome: Pathogenesis and Rationale for
Treatment
By
Marie J. Stuart and
B.N. Yamaja Setty
From the Department of Pediatrics and the Cardeza Foundation for
Hematologic Research, Thomas Jefferson University, Philadelphia, PA.
 |
ABSTRACT |
Acute chest syndrome (ACS) is a leading cause of death in sickle
cell disease (SCD). Our previous work showed that hypoxia enhances the
ability of sickle erythrocytes to adhere to human microvessel
endothelium via interaction between very late activation antigen-4
(VLA4) expressed on sickle erythrocytes and the endothelial adhesion
molecule vascular cell adhesion molecule-1 (VCAM-1). Additionally,
hypoxia has been shown to decrease the production of nitric oxide (NO)
which inhibits VCAM-1 upregulation. Based on these observations, we
hypothesize that during ACS, the rapidly progressive clinical course
that can occur is caused by initial hypoxia-induced pulmonary
endothelial VCAM-1 upregulation that is not counterbalanced by
production of cytoprotective mediators, including NO, resulting in
intrapulmonary adhesion. We assessed plasma NO metabolites and soluble
VCAM-1 in 36 patients with SCD and 23 age-matched controls. Patients
with SCD were evaluated at baseline (n = 36), in vaso-occlusive
crisis (VOC; n = 12), and during ACS (n = 7). We observed marked
upregulation of VCAM-1 during ACS (1,290 ± 451 ng per mL; mean ± 1 SD) with values significantly higher than controls (P < .0001) or patients either in steady state or VOC (P < .01).
NO metabolites were concomitantly decreased during ACS (9.2 ± 1.5 nmol/mL) with values lower than controls (22.2 ± 5.5), patients
during steady state (21.4 ± 5.5), or VOC (14.2 ± 1.2) (P < .0001). Additionally, the ratio of soluble VCAM-1 to NO metabolites
during ACS (132.9 ± 46.5) was significantly higher when compared with
controls (P < .0001) or patients either in steady state or
VOC (P < .0001). Although hypoxia enhanced in vitro sickle
erythrocyte-pulmonary microvessel adhesion, NO donors inhibited this
process with concomitant inhibition of VCAM-1. We suggest that in ACS
there is pathologic over expression of endothelial VCAM-1. Our
investigations also provide a rationale for the therapeutic use in ACS
of cytoprotective modulators including NO and dexamethasone, which
potentially exert their efficacy by an inhibitory effect on VCAM-1 and
concomitant inhibition of sickle erythrocyte-endothelial adhesion.
© 1999 by The American Society of Hematology.
| |
INTRODUCTION |
SICKLE CELL DISEASE (SCD) is a disorder
whose protean manifestations are caused by the substitution of a single
base in the gene encoding the human  -globin subunit.1
Its reach is worldwide, affecting predominantly people of equatorial
African descent, although it is also found in persons of Mediterranean, Indian, and Middle Eastern lineage. Although the episodic and unpredictable vaso-occlusive crisis (VOC) or pain crisis is the hallmark of SCD, the acute chest syndrome (ACS) is the second most
common cause of hospitalization and is a leading cause of both
mortality and morbidity in this disease entity.2,3 Although clinically ACS is often a self-limited acute pulmonary illness whose
typical features are chest pain, cough, dyspnea, fever, and pulmonary
infiltrates on chest x-ray, the illness may rapidly progress to
life-threatening respiratory insufficiency and total "white out"
on chest radiographs. Multiple etiologic factors underlie this
syndrome, including infection; hypoventilation (after surgery, rib
infarcts, or narcotic administration for VOC); and pulmonary fat
embolism from infarcted bone marrow.2-7 We propose and
demonstrate preliminary observations linking these disparate etiologies
of ACS into a more unifying hypothesis.
Our previous work has shown that hypoxia markedly enhances the ability
of sickle erythrocytes to adhere to both macrovascular and
microvascular endothelium via an interaction between the integrin  41, otherwise known as the very late
activation antigen-4 (VLA-4), expressed on sickle reticulocytes and the
endothelial adhesion receptor, vascular cell adhesion molecule-1
(VCAM-1), which was observed to be upregulated by hypoxia.8
The pulmonary microcirculation is particularly vulnerable to changes in
oxygenation. We propose that the rapidly progressive pulmonary findings
in ACS are mediated in part by hypoxia and cytokine-induced pulmonary
endothelial cell VCAM-1 upregulation and that this process is not
counterbalanced by the release of cytoprotective mediators (including
nitric oxide [NO]) that normally inhibits this endothelial VCAM-1
upregulation. The net result of this imbalance is massive
intrapulmonary sickle red blood cell (RBC)-endothelial adhesion with
its attendant pulmonary findings.
| |
MATERIALS AND METHODS |
Materials.
Mouse monoclonal antibodies against VCAM-1 [CD106, (clone 1G11)], the
isotypic control antibody (clone B-Z1), and goat anti-mouse IgG
conjugated with alkaline phosphatase were obtained from Immunotec, Inc
(Westbrook, ME), Harlan Bioproducts for Science, Inc (Indianapolis, IN), and Sigma Immunochemicals (St Louis, MO), respectively.
51Cr-Sodium chromate (400 to 1,200 mCi/mg) was purchased
from New England Nuclear (Boston, MA). 3-Morpholinosydnonimine
hydrochloride (SIN-1), S-nitrosoglutathione (GSNO), oleic acid, sodium
nitrite, nitrate reductase from Aspergillus niger, NADPH,
adenosine, theophylline, indomethacin, and nordihydroguaiaretic acid
(NDGA) were purchased from Sigma Chemical Co (St Louis, MO).
2,3-Diaminonaphthalene was obtained from Aldrich Chemical Co
(Milwaukee, WI). Tissue culture supplies were purchased from Clonetics
Corp (San Diego, CA).
Patients.
The study population included 23 normal controls (age ranged from 2.75 to 30 years) and 36 patients with homozygous SS disease (age ranged
from 2 to 23 years). Thirty-six patients were evaluated in steady
state, and 12 were studied both during steady state and VOC. Seven were
studied during episodes of ACS, 5 of whom were also evaluated during
steady state. VOC was defined as an admission for pain which required
parenteral narcotics, the patient being afebrile at the time of
admission. ACS was defined as the development of a new infiltrate on
chest radiography in combination with fever, respiratory symptoms, or
chest pain, with the blood specimen being obtained before therapeutic
transfusion or exchange. No infectious etiology was ascertained as the
cause of ACS in this latter patient group.
Blood was drawn by a well-trained phlebotomist using a 2-syringe
technique. This study was reviewed and approved by the Institutional Review Committee for the protection of human subjects at St
Christopher's Hospital for Children (Philadelphia, PA). Blood was
drawn by venipuncture from patients and control donors after informed
consent. For minors, patient assent where appropriate was obtained in
addition to parental permission. For analyses of soluble VCAM-1 and NO
metabolites, blood was collected in acid-citrate-dextrose tubes
containing NDGA, indomethacin, adenosine, and theophylline added to
final concentrations of 25 µmol/L, 30 µmol/L, 100 mmol/L, and 1 mmol/L, respectively. Plasma was separated by centrifugation of
anticoagulated blood samples at 735g for 10 minutes at room
temperature (to remove the cellular elements of blood) followed by a
second spin for 10 minutes at 11,750g at 4°C. All plasma
samples were stored frozen at  80°C until assayed for both
soluble VCAM-1 and NO metabolites. To prepare RBCs for the adhesion
assay, blood was collected using sodium heparin as anticoagulant and
the assay performed within 2 to 24 hours.
Culture of endothelial cells.
Both human umbilical vein endothelial cells (HUVECs) and human lung
microvascular endothelial cells (HLMECs) were obtained from Clonetics
and cultured in Clonetics endothelial cell growth medium supplemented
with 10 ng/mL human recombinant epidermal growth factor, 1 µg/mL
hydrocortisone, 50 µg/mL gentamicin, 50 ng/mL amphotericin B, 12 µg/mL bovine brain extract, and 2% (for HUVECs) or 5% (for HLMECs)
fetal bovine serum. Cells were passaged using trypsin-EDTA as detailed
in the protocol provided by the supplier (Clonetics), and cells from
passages between 2 and 4 were used in the experiments described below.
RBC adherence assay.
Endothelial cells were plated at a density of 100,000 cells per well
into wells of 12-well plates, and grown to confluence. Confluent
endothelial cell monolayers were exposed to either room air (normoxia)
or 1% O2 (hypoxia) for 24 hours, as previously described,8 in the presence or the absence of the indicated concentrations of the NO donors, 3-morpholinosydnonimine hydrochloride (SIN-1; 10 and 100 µmol/L) or S-nitrosoglutathione (GSNO; 0.1 and 1.0 mmol/L), or 30 µmol/L oleic acid. Cell monolayers were then tested in
the static adherence assay of Hebbel et al.9 In brief,
51Cr-labeled RBCs (0.5 mL, 2% hematocrit)
were layered on washed endothelial monolayers. Incubations for 45 minutes at 37°C were conducted in the absence of plasma and the
nonadherent RBCs removed. The monolayers were washed 5 times with 0.5 mL Hanks' balanced salt solution containing 1.3 mmol/L
CaCl2, 0.5 mmol/L MgCl2, and 0.5% bovine serum
albumin buffered with 5 mmol/L
N-2-hydroxy-ethylpiperazine-N'-2-ethanesulfonic acid. Adherent
RBCs were determined by 51Cr release after cell lysis.
Oleic acid was provided from 1,000X stocks prepared in ethanol. The
control cells in the oleic acid study were treated with an equivalent
amount of vehicle. The final ethanol concentration in the assay was
constant at 0.1%. Individual data points represent the mean of
triplicate values.
Measurement of soluble VCAM-1 in plasma.
Plasma levels of soluble VCAM-1 were measured using a commercially
available ELISA Kit (R&D Systems, Minneapolis, MN) according to the
manufacturer's instructions.
Measurement of NO metabolites (nitrate and nitrite) in plasma.
Plasma samples were ultra-filtered using ultra-free MC microcentrifuge
filters (Millipore, Bedford, MA) at 9,880g for 30 minutes. Nitrate present in the ultra-filtrate was reduced using nitrate reductase and the total nitrite was then assayed fluorometrically using
2,3-diaminonaphthalene.10 Standards from 10 to 1,000 pmol were measured for each assay, with the assay being reproducible and
linear over this range.
Analysis of expression of VCAM-1 on endothelium by enzyme-linked
immunosorbent assay (ELISA).
Confluent HLMECs or HUVECs grown in wells of 12-well plate were exposed
to hypoxia in the presence or the absence of SIN-1 (10 and 100 µmol/L) or 30 µmol/L oleic acid for 24 hours and assayed for the
surface expression of VCAM-1 using a fluorescence ELISA.8
Data analyses.
Statistical evaluation was performed using the SigmaStat Statistical
Package (Jandel Scientific, San Rafael, CA). All results are presented
as the mean ± 1 SD. Because analyses of the data related to plasma
sVCAM-1, NO metabolites, and the ratios of these plasma markers showed
a nonparametric distribution, statistically significant changes in the
levels of these plasma markers and their ratio were analyzed using the
Kruskal-Wallis Test (for overall comparison). If the P value
for this overall comparison was statistically significant at P < .05, group-wise comparisons were subsequently performed using the
Mann-Whitney test. One-way analysis of variance followed by Dunnett's
test were used to compare the effects of NO donors on hypoxia-induced
sickle RBC-HLMEC adhesion and VCAM-1 expression on HLMECs. The paired
Student's t-test was used to assess the effects of oleic acid
on hypoxia-induced sickle RBC-HUVEC adhesion and VCAM-1 expression on HUVECs.
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RESULTS |
Plasma levels of soluble VCAM-1.
As depicted in Fig 1A, levels in the normal
controls and patients with SCD in steady state, VOC, and ACS were 524 ± 104 (mean ± 1 SD), 850 ± 189, 802 ± 152, and 1,290 ± 451 ng/mL, respectively. Because the overall comparison of
sVCAM-1 levels was statistically significant (P < .0001),
further group-wise analyses of the data were performed. Although
significant differences were observed between the controls and the
various patient groups (P < .0001), there were no differences
in the levels between the SCD patient in steady state and VOC. However,
during ACS, levels of VCAM-1 were significantly increased when compared
with both controls (P < .0001) or patients with SCD either in
steady state (P < .01) or during VOC (P < .01).
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| Fig 1.
Plasma levels of sVCAM-1 (A) and NO metabolites (B) from
control donors are compared with those from patients with SCD during
steady state, VOC, and ACS. Bars with open circles represent mean ± 1 SD values.
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Plasma levels of NO metabolites.
Figure 1B shows the levels of NO metabolites in control donors and
patients with SCD. Although the mean value in the control population
was 22.2 ± 5.5 (mean ± 1 SD) nmol/mL, levels in SCD patients in
steady state, VOC, and ACS were 21.4 ± 5.5, 14.2 ± 1.2, and 9.2 ± 1.5 nmol/mL, respectively. Since the overall comparison of NO
metabolite levels was statistically significant (P < .0001), further group-wise analyses of the data were performed. Although no
differences were noted between controls and SCD patients in steady
state, levels were significantly reduced during VOC (P < .0001). During ACS, an even further reduction in levels was noted when
compared with either control values (P < .0001) or those obtained during VOC (P < .0001).
Ratio of soluble VCAM-1 to NO metabolites.
Figure 2A shows the sVCAM-1 to nitrate
ratios in individual controls and in the various patient groups. While
the mean ratio in the control population was 28.3 ± 9.6 (mean ± 1 SD), levels in SCD patients in steady state, VOC, and ACS were 53.2 ± 23.9, 53.2 ± 18.1, and 132.9 ± 46.5, respectively. Because the overall comparison was statistically
significant (P < .0001), further group-wise analyses of data
were performed. Although significant differences were observed between
control and the various patient groups (P < .0001), there
were no differences between the SCD patients in steady state and VOC.
However, during ACS, the ratio was significantly elevated when compared
to both controls (P < .0001) or patients with SCD, either in
steady state (P < .0001) or VOC (P < .0001). In the
5 patients who were studied both at baseline and during ACS, paired
data (Fig 2B) showed a significant increase in the sVCAM-1 to NO ratio
during ACS (P < .03).
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| Fig 2.
Ratio of soluble VCAM-1 to NO metabolites from control
donors are compared with those from patients with SCD during steady
state, VOC, and ACS (A). Bars with open circles represent mean ± 1 SD
values. (B) The data in 5 patients evaluated both during steady state
and ACS.
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Effects of NO donors and oleic acid on hypoxia-induced sickle
erythrocyte endothelial adhesion and VCAM-1 expression.
To test the hypothesis that NO functioned as a modulator of
hypoxia-induced sickle RBC-endothelial cell adhesion, human pulmonary microvascular endothelium was exposed to hypoxia in the presence or
absence of varying concentrations of the NO donors SIN-1 and GSNO, and
cell monolayers were subsequently evaluated for adhesion. Hypoxia
enhanced sickle RBC adhesion to the microendothelium by 45% ± 31%
(mean ± 1 SD, N = 8, P < .01, Fig 3A). The NO donor SIN-1 inhibited this
hypoxia-induced adhesion in a dose-dependent manner with mean
inhibitory effects of 88% (P < .05) and 100% (P < .01) noted at SIN-1 concentrations of 10 and 100 µmol/L, respectively. GSNO (0.1 and 1 mmol/L), the other NO donor evaluated, also induced similar inhibitory effects on hypoxia-induced adhesion (data not shown). In concomitant experiments, pulmonary microvascular endothelial cells were assessed for VCAM-1 expression by ELISA. Results
presented in Fig 3B show that hypoxia upregulated VCAM-1 expression by
127% ± 15% (mean ± 1 SD). This hypoxia-induced receptor expression was inhibited by 69% and 88% (P < .05) in the
presence of 10 and 100 µmol/L SIN-1, respectively, thus paralleling
the inhibitory effects of this NO donor on functional adhesion.
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| Fig 3.
Effects of the NO donor, SIN-1, on hypoxia-induced sickle
RBC adherence (A) and VCAM-1 expression (B) on HLMECs. Endothelial cell
monolayers were subjected to hypoxia in the presence or absence of the
indicated concentrations of SIN-1 for 24 hours and then assessed for
sickle RBC adherence and VCAM-1 expression. Values presented are the
means ± 1 SD from 8 (RBC adherence) or 3 (VCAM-1 expression)
experiments. Differences observed in RBC adhesion (A) and VCAM-1
expression (B) between normoxic, hypoxic, and hypoxic plus
SIN-1-treated groups were significantly different as assessed by
one-way analysis of variance (P < .01).
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Finally, the effects of oleic acid on sickle RBC-endothelial adhesion
and VCAM-1 expression on HUVECs are depicted in
Fig 4. While hypoxia increased basal
adhesion by 42% ± 9% (mean ± 1 SD), 30 µmol/L oleic acid
further enhanced hypoxia-induced adhesion by 50% (P < .005, N = 5). Concomitant with this increase in functional adhesion,
hypoxia-induced VCAM-1 expression on HUVECs was further upregulated by
42% in the presence of oleic acid (P < .02, N = 4).
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| Fig 4.
Effects of oleic acid on hypoxia-induced sickle RBC
adherence (A) and VCAM-1 expression (B) on HUVECs. Endothelial cell
monolayers were subjected to hypoxia in the absence (control) or
presence of 30 µmol/L oleic acid (oleic acid) for 24 hours and then
assessed for sickle RBC adherence and VCAM-1 expression.
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DISCUSSION |
The rate of polymerization of sickle hemoglobin and the capillary
transit time of sickle erythrocytes play pivotal roles in the
pathophysiology of the vaso-occlusive event in SCD.11
Research into the latter process achieved prominence after the
observation of Hebbel et al,9 who showed increased
adherence of sickle erythrocytes to vascular endothelium, thus
effectively prolonging capillary transit time. The molecular
interactions that underlie this adherence process have been the subject
of intensive investigation.12 To date, the main receptors
on the microendothelium that have been shown to participate in
adherence include VCAM-1 and CD36. The major erythrocyte
counter-receptors include the integrin complex 41 and CD36, respectively, with the
presence of these receptors, for the most part, on the surface of
sickle reticulocytes rather than on the mature RBC. The adhesinogenic
potential of sickle cells is further enhanced by several plasmatic
factors such as thrombospondin, fibronectin, and von Willebrand factor,
which act as ligands facilitating the process of adhesion.
Because tissue hypoxia is an integral part of the pathophysiology of
SCD, our previous studies centered around the effects of hypoxia on
sickle RBC-endothelial adherence. We have shown that hypoxia enhances
adherence, and that the specific receptor pair involved in modulating
hypoxia-induced adherence is the interaction between the sickle
reticulocyte integrin complex 41 and
VCAM-1 present on the microendothelial cell surface.8
VCAM-1, which belongs to the Ig superfamily, contains 7 Ig-like
domains, the ligand binding site for the integrin
41 being located within the
NH2-terminal first domain.13 VCAM-1 is
expressed only at low levels on vascular endothelial cells under basal
conditions, being upregulated by cytokines such as interleukin-1 and
tumor necrosis factor-, endothelin-1, and hypoxia8,14-17
(thus markedly increasing sickle RBC-endothelial adhesion via the
previously noted 41: VCAM-1 coupling). A
soluble form of VCAM-1 has also been described.
A variety of modifying mechanisms, including the signaling molecule NO,
is called into play, in vivo, to counteract the detrimental consequences that could potentially result from unopposed endothelial VCAM-1 upregulation. Besides the well-known effects of NO, which induces smooth muscle cell relaxation and inhibition of platelet activation via stimulation of guanylyl cyclase,18 NO has
also been shown to decrease cytokine-induced endothelial activation by
repression of VCAM-1 gene transcription.19 While this
anti-VCAM-1 effect was initially postulated to enhance the
anti-atherogenic and anti-inflammatory effects of this potent signaling
molecule (by inhibition of leukocyte and monocyte adhesion to the
vessel wall), our parallel studies elucidating the crucial role of
VCAM-1 in mediating hypoxia-induced erythrocyte-endothelial adhesion makes NO a prime cytoprotective mediator in inhibition of
hypoxia-induced RBC vascular adhesion.
There are, however, unique circumstances that are called into play in
sickle cell disease, and in particular in the ACS that disrupts the
balance between this major cytoprotective mediator and the
counter-regulatory forces that cause endothelial activation and VCAM-1
upregulation (Fig 5). The pulmonary
microcirculation is particularly vulnerable to changes in oxygenation
such that infection or hypoventilation of a segment or lobe could
result in fairly rapid and extensive hypoxia-induced changes, including vasoconstriction, local cytokine and endothelin-1 release, and subsequent VCAM-1 upregulation.3,17,19 Moreover, in sickle cell disease there are a number of factors that could blunt the response of the cytoprotective molecule NO. Although hypoxia itself inhibits NO production by decreasing protein levels of constituitive NO
synthase in endothelial cells,20 sickled erythrocytes
specifically have been shown to inhibit NO by a similar
mechanism.21 Additionally, exposure of endothelial cells to
sickled RBCs results in a 4- to 8-fold increase in transcriptional
induction of the gene encoding endothelin-1 in vitro, which could lead
to further feedback vasoconstriction and VCAM-1
upregulation.22 Most recently, sickled RBCs have also been
shown to upregulate nuclear NFkB levels, leading to endothelial cell
activation (including VCAM-1 upregulation) and tissue factor
expression.23
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| Fig 5.
Factors in the ACS that contribute to intrapulmonary
erythrocyte-endothelial adhesion are depicted. Hypoxia induces cytokine
and endothelial-1 release, which singly and in combination upregulate
VCAM-1. NO acts as a cytoprotective mediator that inhibits both
endothelin-1 production and endothelial VCAM-1 expression under normal
circumstances. However, during the pathophysiology of sickle cell ACS,
sickled RBCs and hypoxia can both inhibit NO production (by decreasing
cNOS transcription), leading to unopposed VCAM-1 upregulation and
consequent RBC-endothelial adhesion.
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The findings we present in this report support our hypothesis. While
the levels of soluble VCAM-1 were elevated to a similar extent in
patients with SCD in both steady state and vaso-occlusive crisis
(signifying some expected degree of endothelial cell activation in this
disorder), VCAM-1 levels in the ACS were significantly higher than in
the other patient groups. In parallel with this finding, we demonstrate
that in patients with ACS, plasma levels of the NO metabolites were
most significantly decreased. While previous published studies on the
production of NO metabolites in SCD have provided discrepant
results,24,25 no previous evaluation of these metabolites
has been documented in ACS, except for a very recent abstract that
supports our present findings.26 Our results show an
unequivocal decrease during ACS of this crucial cytoprotective
mediator. Additionally, when the ratio of the plasma markers (VCAM-1 to
NO metabolites) was calculated, the highest ratios were observed in
patients with ACS, with significant increases over baseline in those
patients assessed both during steady state and and during ACS.
To further test our hypothesis that NO could function as a regulator of
hypoxia-induced sickle erythrocyte-endothelial adhesion to the
pulmonary microvessels, we evaluated the effects of 2 NO donors SIN-1
and GSNO on in vitro hypoxia-induced adhesion using cells from this
circulatory bed. We have shown that these NO donors (at concentrations
of NO that theoretically could be generated in vivo under conditions of
inflammation)19 reverse hypoxia-induced sickle
erythrocyte-endothelial adhesion in parallel with inhibition of
hypoxia-induced upregulation of VCAM-1. Thus, the dramatic beneficial
effects that have been previously reported27 for inhaled NO
in 2 patients with severe ACS could be due not only to a reduction in
pulmonary ventilation-perfusion mismatch and its effect on augmenting
the oxygen affinity of sickle erythrocytes in vivo,28 but
also to its specific anti-adhesiogenic potential under conditions of
hypoxia (Fig 5).
Our final studies on oleic acid were performed since previous reports
have documented pulmonary fat embolism as a cause of ACS7
and since elevated levels of secretory phospholipase A2 have been observed in this syndrome.29 These findings
suggest a potential relationship between free fatty acids and ACS; in fact, oleic acid (which has been found to be increased in
ACS)30 has been used in an animal model to simulate acute
lung injury, with improvement of gas exchange after inhalation of
NO.31,32 In keeping with our hypothesis, we show that this
fatty acid also upregulated endothelial VCAM-1 expression and enhanced
sickle RBC-endothelial adhesion. Additionally, a recent report suggests a beneficial effect of dexamethasone in modifying the severity of
ACS.33 While the salutary effect of dexamethasone in this report was attributed to its inhibition of phopholipase A2
and to its anti-inflammatory properties, this steroid also prevents the
cytokine-induced expression of VCAM-1 on endothelium.34
In summary, we have shown that during the ACS a marked upregulation of
endothelial cell VCAM-1 occurs, with a concomitant significant decrease
in NO metabolite production. These findings, together with our in vitro
demonstration that NO donors reverse hypoxia-induced sickle
RBC-endothelial adhesion by downregulation of hypoxia-induced VCAM-1,
suggests that in ACS massive intrapulmonary sickle
erythrocyte-endothelial adhesion occurs due to hypoxia- and
cytokine-induced VCAM-1 upregulation that is not counterbalanced by the
compensatory production of cytoprotective mediators including NO. The
role of the other cellular elements of blood, particularly neutrophils
(that could also adhere to the pulmonary microvasculature primed by
ACS-induced hypoxemia), needs further elucidation, as do other
mechanisms of RBC endothelial adhesion, including the recently
described property of phosphatidylserine positive erythrocytes to
adhere to vascular endothelium.35
| |
FOOTNOTES |
Submitted January 13, 1999; accepted May 10, 1999.
Supported by Grants No. HL51497 and 1P60HL62148 from the National
Heart, Lung and Blood Institute, National Institutes of Health.
The publication costs of this
article were defrayed in part by
page charge payment. This article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. section
1734 solely to indicate this fact.
Address reprint requests to Marie J. Stuart, MD, Department of
Pediatrics, College Bldg #727, Thomas Jefferson University Medical
College, 1025 Walnut St, Philadelphia, PA 19107.
| |
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ABSTRACT
INTRODUCTION