Cell Biology:
Sphingosine 1-Phosphate (S1P)
Carrier-dependent Regulation of
Endothelial Barrier: HIGH DENSITY
LIPOPROTEIN (HDL)-S1P PROLONGS
ENDOTHELIAL BARRIER
ENHANCEMENT AS COMPARED
WITH ALBUMIN-S1P VIA EFFECTS ON
LEVELS, TRAFFICKING, AND
SIGNALING OF S1P1
J. Biol. Chem. 2012, 287:44645-44653.
doi: 10.1074/jbc.M112.423426 originally published online November 7, 2012
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This article cites 38 references, 22 of which can be accessed free at
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Brent A. Wilkerson, G. Daniel Grass, Shane
B. Wing, W. Scott Argraves and Kelley M.
Argraves
THE JOURNAL OF BIOLOGICAL CHEMISTRY VOL. 287, NO. 53, pp. 44645–44653, December 28, 2012
© 2012 by The American Society for Biochemistry and Molecular Biology, Inc. Published in the U.S.A.
Sphingosine 1-Phosphate (S1P) Carrier-dependent
Regulation of Endothelial Barrier
HIGH DENSITY LIPOPROTEIN (HDL)-S1P PROLONGS ENDOTHELIAL BARRIER
ENHANCEMENT AS COMPARED WITH ALBUMIN-S1P VIA EFFECTS ON LEVELS,
TRAFFICKING, AND SIGNALING OF S1P1 *□
S
Received for publication, September 27, 2012 Published, JBC Papers in Press, November 7, 2012, DOI 10.1074/jbc.M112.423426
Brent A. Wilkerson1, G. Daniel Grass2, Shane B. Wing, W. Scott Argraves, and Kelley M. Argraves3
From the Department of Regenerative Medicine and Cell Biology, Medical University of South Carolina,
Charleston, South Carolina 29425
Sphingosine 1-phosphate (S1P) is a blood-borne lysosphingolipid that acts to promote endothelial cell (EC) barrier function.
In plasma, S1P is associated with both high density lipoproteins
(HDL) and albumin, but it is not known whether the carriers
impart different effects on S1P signaling. Here we establish that
HDL-S1P sustains EC barrier longer than albumin-S1P. We
showed that the sustained barrier effects of HDL-S1P are
dependent on signaling by the S1P receptor, S1P1, and involve
persistent activation of Akt and endothelial NOS (eNOS), as well
as activity of the downstream NO target, soluble guanylate
cyclase (sGC). Total S1P1 protein levels were found to be higher
in response to HDL-S1P treatment as compared with albuminS1P, and this effect was not associated with increased S1P1
mRNA or dependent on de novo protein synthesis. Several
pieces of evidence indicate that long term EC barrier enhancement activity of HDL-S1P is due to specific effects on S1P1 trafficking. First, the rate of S1P1 degradation, which is proteasomemediated, was slower in HDL-S1P-treated cells as compared with
cells treated with albumin-S1P. Second, the long term barrier-promoting effects of HDL-S1P were abrogated by treatment with the
recycling blocker, monensin. Finally, cell surface levels of S1P1 and
levels of S1P1 in caveolin-enriched microdomains were higher
after treatment with HDL-S1P as compared with albumin-S1P.
Together, the findings reveal S1P carrier-specific effects on S1P1
and point to HDL as the physiological mediator of sustained S1P1PI3K-Akt-eNOS-sGC-dependent EC barrier function.
A major physiological function of the vascular endothelium
is to act as a selective barrier, regulating the exchange of liquids
and solutes and the passage of cells (1). This critical role is
evident in various pathological processes that cause increased
vascular permeability including hypertension and stroke, myocardial infarction, ischemia-reperfusion, acute lung injury, sepsis, inflammatory disorders, tumor invasion, diabetic retinopathy, and macular edema (2, 3).
Sphingosine 1-phosphate (S1P)4 is a blood-borne lysosphingolipid that acts to enhance the barrier function of the vascular
endothelium (4, 5). Considering the importance of vascular
barrier regulation in physiology and disease, defining the mechanisms by which S1P enhances integrity of the endothelial cell
barrier is an area of active research (6 –12).
In blood, S1P is associated predominantly with apolipoprotein M (apoM)-containing HDL and to a lesser extent with
albumin (13, 14). However, most studies of S1P bioactivity have
employed albumin as the S1P carrier, thus raising questions
whether the observed effects of albumin-S1P can be extended
to HDL-S1P and whether there are carrier-specific effects on
S1P signaling. Here we evaluate the influence of albumin-S1P
and HDL-S1P on endothelial cell barrier function and discover
that the two have distinct effects on the persistence of S1P-dependent barrier enhancement that relate to differential effects
on the trafficking and stabilization of the S1P receptor, S1P1,
and its signaling.
* This work was supported, in whole or in part, by National Institutes of Health
EXPERIMENTAL PROCEDURES
Culture of Human Umbilical Vein Endothelial Cells—Human umbilical vein endothelial cells (HUVECs, Cascade Bio-
Grants HL094883 and HL080404 (to K. M. A.) and HL061873 (to W. S. A.).
This article contains supplemental Table 1 and Figs. 1– 4.
1
Supported by National Institutes of Health Training Grant T32HL007260
and by a fellowship from the American Heart Association (Grant
10PRE3910006).
2
Supported by a United States Department of Defense predoctoral fellowship (Grant W81XWH-10-1-0083).
3
To whom correspondence should be addressed: Dept. of Regenerative
Medicine and Cell Biology, Medical University of South Carolina, 173 Ashley Ave., Charleston, SC 29425-2204. Tel.: 843-792-3535; Fax: 843-7920664; E-mail: argravek@musc.edu.
□
S
DECEMBER 28, 2012 • VOLUME 287 • NUMBER 53
4
The abbreviations used are: S1P, sphingosine 1-phosphate; EC, endothelial
cell; HUVEC, human umbilical vein endothelial cell; TEER, trans-endothelial
electrical resistance; ECIS, electric cell-substrate impedance sensing;
apoM, apolipoprotein M; CEM, caveolin-enriched microdomain; eNOS,
endothelial NOS; sGC, soluble guanylate cyclase; EBM, endothelial basal
medium-2; DMSO, dimethyl sulfoxide; ANOVA, analysis of variance; ODQ,
1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one; L-NAME, N-nitro-L-arginine
methyl ester.
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Background: S1P promotes endothelial barrier and is associated with HDL and albumin in blood.
Results: HDL-S1P sustained barrier longer than albumin-S1P, reduced S1P receptor (S1P1) degradation, increased S1P1 cell
surface and lipid raft levels, and persistently activated PI3K-Akt and eNOS.
Conclusion: HDL-S1P persistently activates S1P-S1P1-PI3K-Akt-eNOS signaling.
Significance: The findings highlight the basis for HDL-S1P as a mediator of sustained endothelial barrier.
HDL-S1P Prolongs Endothelial Barrier Function
44646 JOURNAL OF BIOLOGICAL CHEMISTRY
teau (⬃1400 –1600 ohms, ⬃48 h). The medium was then
replaced with serum-free EBM, resulting in a drop in impedance. When impedance reached a minimum plateau (⬃800 –
1000 ohms, 24 – 48 h), effectors (e.g. HDL-S1P) were added to
culture medium, and the TEER response was measured for up
to 20 h. The maximum volume of each effector added did not
exceed one-tenth of the 400-l volume of culture medium in
each well. In studies evaluating specific effectors, controls
included treatments with matched volumes of fatty acid-free
serum albumin, HDL storage buffer, or vehicle buffers.
ECIS impedance values were first normalized by dividing
each value by the level of impedance measured just prior to the
addition of effectors. To quantify differences in barrier activity
in response to effectors, the area under the normalized impedance traces was calculated in KaleidaGraph Version 4.0.3 (Synergy Software, Reading, PA) using the “Integrate-Area” macro.
Integrated impendence values for effectors (e.g. albumin-S1P or
HDL-S1P) were divided by integrated mean impendence values
for control agents (e.g. S1P free albumin in PBS) for the specified period of time.
Phospho-Akt, Phospho-ERK1/2, and Phospho-eNOS Detection—
Bio-Plex phospho-Akt and phospho-ERK1/2 detection was
carried out as described previously (9). To detect phosphoeNOS, cells were extracted in lysis buffer (1% Nonidet P-40,
20 mM Tris, 137 mM NaCl, and Roche Applied Science
Minitab protease inhibitor mixture) plus 100 nM okadaic
acid, and the extracts were subjected to immunoblot analysis
using antibodies to phospho-eNOS (serine 1177) and eNOS
(BD Pharmingen).
S1P1 Immunoblot Analysis—HUVECs were seeded into
6-well plates (Corning, Lowell, MA) at 1.5–3 ⫻ 105 cells/well
and grown to confluence. The medium was then replaced with
serum-free EBM. After 48 h of serum starvation, HDL or albumin containing equal molar amounts of S1P was added to culture medium (control wells received equal volumes of S1P-free
vehicle). HUVECs were lysed in 200 l of ice-cold lysis buffer.
Lysates were subjected to centrifugation at 7500 ⫻ g for 10 min
at 4 °C, and protein levels in the supernatants were measured
using the Bio-Rad DC protein assay. Aliquots were subjected to
SDS-PAGE and transferred to PVDF membranes (Santa Cruz
Biotechnology, Inc.; Santa Cruz, CA). Membranes were
blocked in TBS, pH 7.4, containing 5% milk and incubated with
rabbit anti-human S1P1 (H-60) (sc-25489; Santa Cruz Biotechnology) in TBS, 0.1% Tween 20 overnight at 4 °C. After washing,
the membranes were incubated with horseradish peroxidaseconjugated donkey anti-rabbit IgG secondary antibody (Jackson ImmunoResearch Laboratories, West Grove, PA) in TBS,
0.1% Tween 20. Detection was achieved using Amersham Biosciences ECL Plus reagents (GE Healthcare). To control for
protein loading, blots were probed using rabbit anti-human
cytochrome oxidase-IV (AB16056; Abcam, Cambridge, MA),
actin (A2668, Sigma), or GAPDH (AB37168, Abcam).
Cell Surface S1P1 Analysis—HUVECs were grown to confluence in 100-mm plates and then serum-starved 48 h. Following
the indicated treatments, HUVEC surface proteins were isolated using the Pierce cell surface protein isolation kit (Pierce).
Immunoblot analysis was performed on the cell surface fractions using antibodies to S1P1 (Santa Cruz Biotechnology) and
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logics, Inc.) were maintained in humidified 5% CO2, 95% air in
endothelial growth medium-2 (Lonza; Basel, Switzerland), containing 2% fetal bovine serum. Serum starvation and experiments measuring S1P responses were carried out in serum-free
endothelial basal medium-2 (EBM) (Lonza) with 0.5⫻ penicillin streptomycin glutamine (Invitrogen) and 0.5⫻ GlutaMAX
(Invitrogen). Unless specified otherwise, experimentation described in the present studies used confluent passage 2–5
HUVEC monolayers serum-starved 48 h.
Reagents—S1P (D-erythro-S1P) was purchased from Avanti
Polar Lipids, Inc. (Alabaster, AL). The S1P1 antagonist W146
was purchased from Avanti Polar Lipids and dissolved in acidified dimethyl sulfoxide (DMSO), 4 mg/ml fatty acid-free
bovine serum albumin (BSA). The protein synthesis inhibitor
cycloheximide was obtained from Sigma-Aldrich and dissolved
in DMSO. The recycling inhibitor monensin was purchased
from Sigma-Aldrich and dissolved in methanol. The PI3K
inhibitor LY294002 was purchased from R&D Systems, Inc.
(Minneapolis, MN) and dissolved in DMSO. The endothelial
NOS (eNOS) inhibitor L-NAME was purchased from Alexis
Biochemicals (San Diego, CA) and dissolved in serum-free
EBM. The inhibitor of soluble guanylate cyclase 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) was purchased from
Cayman Chemical (Ann Arbor, MI) and dissolved in DMSO.
Reconstitution of S1P in Albumin and Quantification of
S1P—100 M stock solutions of S1P were prepared fresh daily
by dissolving S1P into Dulbecco’s PBS (PBS) containing 4
mg/ml fatty acid-free BSA. Measurement of S1P levels in stock
albumin-S1P solutions and HDL was performed by the Lipidomics Core Facility at the Medical University of South Carolina
using LC-MS-MS methods for detection of S1P with subpicomolar sensitivity (15).
Purification of HDL—HDL (1.063–1.21 g/ml) was purified by
density gradient ultracentrifugation of EDTA-plasma (16), typically collected from 2–3 healthy, normolipidemic donors. HDL
fractions were dialyzed against a PBS, 0.03 mM EDTA solution
(HDL storage buffer). Protein levels of HDL preparations were
determined by Bio-Rad DC protein assay (Bio-Rad).
S1P Fortification of HDL—In some cases, purified HDL was
fortified with exogenous S1P to bolster S1P concentrations to
physiological levels. S1P fortification of HDL was achieved by
incubating HDL with lyophilized S1P in HDL storage buffer
followed by dialysis against HDL storage buffer. S1P and protein contents of the resulting HDL preparations were determined by LC-MS-MS and Bio-Rad DC protein assay, respectively. In all cases, findings made using fortified HDL were
similar to those obtained using native, unaltered HDL.
Trans-endothelial Electrical Resistance Measurement and
Analysis—Endothelial barrier integrity was measured in real
time as trans-endothelial electrical resistance (TEER). TEER of
confluent monolayers of HUVECs was measured using an
Applied Biophysics ECIS 1600 instrument (Applied Biophysics,
Troy, NY) as described previously (9). Cells were seeded at a
density of 1.5 ⫻ 105 cells/well onto 8W10E⫹ electric cell-substrate impedance sensing (ECIS) arrays (Applied Biophysics)
coated with human plasma fibronectin (Invitrogen) at 100
g/ml in 0.15 M NaCl, 0.01 M Tris, pH 8.0. Cells were cultured to
confluence until electrical impedance reached a maximal pla-
HDL-S1P Prolongs Endothelial Barrier Function
FIGURE 1. HDL-S1P sustains endothelial barrier activity longer than albumin-S1P. Endothelial cells were treated with HDL-S1P, albumin-S1P (each
containing 150 nM S1P), or S1P-free vehicle (no S1P). In A, each of the TEER tracings is an average from four replicate wells. Wells treated with HDL-S1P received
fatty acid-free serum albumin, wells treated with albumin-S1P received HDL storage buffer, and control wells received matched volumes of fatty acid-free
serum albumin and HDL storage buffer. ⍀ indicates ohms. B shows integrated impedance values (i.e. area under impedance traces), which are integrated
impendence values for effectors divided by integrated mean impendence values for control agents for the specified period of time. Student’s t test was used
to test for significant differences in mean integrated impedance. Asterisks indicate p ⬍ 0.01.
DECEMBER 28, 2012 • VOLUME 287 • NUMBER 53
were designed using the computer programs mFold (19) and
Primer3 (20). PCR was performed on a Bio-Rad iCycler using
primer pairs (400 nM) together with template cDNAs and iQ SYBR
Green supermix (Quanta Biosciences, Gaithersburg, MD). Cycling conditions used were 40 cycles of denaturation (95 °C for
30 s), annealing (60 °C for 30 s), and elongation (72 °C for 30 s).
Controls were performed using RNA and cDNA synthesis reaction components minus reverse transcriptase. Real-Time PCR
Miner was used for calculation of efficiency and fluorescence
threshold crossing-cycle (CT) values (21). Average CT values and
efficiencies of replicate reactions were normalized to the internal
control gene GAPDH. Relative expression differences of S1P1
(fold difference) were calculated using the Pfaffl equation (22).
Plots and Statistical Analysis—Statistical tests and plots were
done using KaleidaGraph. Error bars depict S.D. In dot plots,
bars represent means. For comparison of one- or two-treatment groups, Student’s t-tests were used with assumption of
equal variance for unpaired data unless unequal variance was
identified (i.e. F test, p ⬍ 0.05). Two-sided p values are reported
except where it is otherwise indicated. In cases where three or
more groups were compared, one-way ANOVA (␣ ⫽ 0.05) and
Tukey’s pairwise tests were used. For ANOVA, two-sided p
values are reported. y indicate p ⬍ 0.05.
RESULTS
HDL-S1P Sustains Endothelial Cell Barrier Function Longer
than Albumin-S1P—TEER analysis was used to compare the
endothelial barrier-enhancing activities of HDL and albumin,
each containing equal molar amounts of S1P (150 nM). As
shown in Fig. 1, within minutes of treatment, both albumin-S1P
and HDL-S1P elicited increases in impedance, reaching similar
maximal levels after ⬃30 min. Following this initial period,
impedance levels in albumin-S1P-treated cultures steadily
declined, reaching the level of S1P-free vehicle control at ⬃4 h
(Fig. 1), whereas the level of impedance induced by HDL-S1P
remained above the vehicle control base line for at least 20 h
(Fig. 1A) and the difference was statistically significant over that
period (p ⫽ 0.003; Fig. 1B). The inability of albumin-S1P to
promote barrier after 4 h was not due to a reduction in S1P
levels in the medium at later time points because LC-MS-MS
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rabbit anti-human von Willebrand factor (Dako, Carpentaria,
CA); the latter was used to normalize for protein loading.
Analysis of S1P1 in Membrane Fractions Prepared by Discontinuous Gradient Ultracentrifugation—HUVECs were lysed
(17), and the lysates were subjected to ultracentrifugation over
discontinuous 0 – 40% OptiPrep gradients (18). 12 density fractions were collected, and aliquots were assayed by immunoblot
using rabbit antibodies to S1P1 (Santa Cruz Biotechnology) and
caveolin-1 (BD Pharmingen).
Immunofluorescent Labeling of S1P1—HUVECs were seeded
in 4-well glass chamber slides (Nalge Nunc; Rochester, NY) and
serum-starved 48 h. Following incubation with indicated effectors, cells were fixed for 20 min in 3% paraformaldehyde,
washed in PBS, and permeabilized in PBS, 0.1% Triton-X-100,
0.01% azide for 30 min and then blocked in PBS containing 3%
BSA and 5% donkey serum. Cultures were labeled with antibody to S1P1 and Alexa Fluor 488-conjugated donkey anti-rabbit secondary (Invitrogen). Nuclei were stained with Draq5 (5
M; Axxora, San Diego, CA). Slides were analyzed using a Leica
SP-5 confocal microscope system (Leica Microsystems Inc.,
Exton, PA). Analysis of micrographs contains data representative of at least five random fields. Immunofluorescence micrographs were collected using identical microscopy settings in a
single session. Montages of primary images were adjusted
equally using “Levels” in Photoshop. Background signal in
empty, acellular space was subtracted using the black level
slider. Brightness was increased using the white level slider to provide clarity of details in the original images. The gamma slider was
not independently adjusted. “Rainbow RGB” look-up table was
applied using ImageJ to demonstrate relative quantitative differences in the distribution of S1P1 immunofluorescence.
S1P1 mRNA Quantification by RT-PCR—RNA was isolated
from HUVECs using RNA STAT-60 (Tel-Test, Inc., Friendswood, TX). Contaminating genomic DNA was digested by
treatment with TURBO DNA-free DNase (Ambion, Inc., Austin, TX). Reverse transcription of total RNA was performed
using reverse transcriptase and the SuperScript first-strand
synthesis system containing random hexamers (Invitrogen).
Oligonucleotide primers used for quantitative reverse transcriptase PCR are described in supplemental Table 1. Primers
HDL-S1P Prolongs Endothelial Barrier Function
analysis of the culture medium from endothelial cells treated
with either C17-S1P-fortified HDL or C17-S1P-fortified albumin
showed no discernable reduction in S1P over the 6-h time course,
irrespective of the carrier (supplemental Fig. 2A). Furthermore,
exposure of the cells to HDL-S1P followed by washing the cell layer
and replacement of the medium with HDL-S1P-free medium
showed that HDL-S1P must be continuously present to achieve
maximal barrier enhancement (supplemental Fig. 2B).
HDL-sustained Endothelial Cell Barrier Activity Is Dependent on S1P1 Signaling Involving Akt and eNOS Activation—Previous studies have established that HDL promotes endothelial
barrier enhancement via S1P signaling dependent on the S1P1
receptor and components of the PI3K-Akt pathway (9). To
determine whether the sustained endothelial barrier enhancement activity of HDL was mediated via the S1P-S1P1-PI3K-Akt
signaling pathway, we first assessed the effect of the S1P1 antagonist, W146, on HDL-S1P-stimulated barrier occurring ⬃4 h
following initiation of HDL-S1P treatment (i.e. the period of
time at which albumin-S1P-induced barrier effects have
waned). W146 was therefore added at 4 h following HDL-S1P
treatment. As shown in Fig. 2A, S1P1 antagonism eliminated
the sustained barrier effects of HDL-S1P.
44648 JOURNAL OF BIOLOGICAL CHEMISTRY
We also evaluated the effect of the PI3K inhibitor, LY294002,
on HDL-S1P-stimulated barrier occurring ⬃4 h following initiation of HDL-S1P treatment. PI3K inhibition completely
reduced the level of HDL-induced barrier to levels below S1Pfree vehicle treatments (Fig. 2A).
We next evaluated the level of Akt activation during the
phase of HDL-S1P-stimulated barrier enhancement. At 4.5 and
6 h after treatment, HDL-S1P elevated phospho-Akt levels over
those measured in response to either albumin-S1P or S1P free
control treatments (Fig. 2B). By contrast, HDL-S1P treatment
only elicited a short-lived effect (⬍30 min) on the activation
Erk1/2, with no activation apparent in the 0.5– 6-h window of
observation as compared with S1P-free control (Fig. 2C). These
findings emphasize that the sustained HDL-S1P barrier effects
are dependent on PI3K-Akt activation. These studies also
revealed that S1P1 inhibition reduced the levels of total Akt
(Fig. 2B), suggesting that S1P1 signaling sustains Akt levels in
addition to augmenting Akt activation.
Although HDL-S1P did not stimulate Erk1/2 activation at
6 h, S1P1 antagonist treatment augmented Erk1/2 activation at
6 h, suggesting that persistent HDL-S1P-S1P1 signaling might
be eliciting a suppressive effect on Erk1/2 activation (Fig. 2C).
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FIGURE 2. HDL-S1P-sustained endothelial barrier activity is dependent on S1P1 signaling involving Akt activation. A shows ECIS analysis of endothelial
cells treated for ⬃4 h with HDL-S1P, albumin-S1P (each containing 150 nM S1P), or S1P-free vehicle (no S1P) followed by addition (arrow) of the S1P1 antagonist
W146 (3.3 M), the PI3K inhibitor LY294002 (50 nM), or vehicle. The asterisks indicate that the level of HDL-induced endothelial cell barrier is significantly
reduced (p ⬍ 0.0001) 2 h following S1P1 antagonism as determined by one-way ANOVA and Tukey’s post hoc test (␣ ⫽ 0.05). The TEER tracings shown
represent mean normalized impedance values of 2–3 replicate wells for each treatment. These results are representative of data from at least three independent experiments. ⍀ indicates ohms. B and C, show multiplex bead array analysis showing the temporal effects of albumin-S1P and HDL-S1P (150 nM S1P)
treatments on the phosphorylation of Akt and ERK (p-AKT and p-ERK), respectively. Left panels show mean -fold changes in Akt/ERK (n ⫽ 2) at 5 min, 30 min, 3 h,
4.5 h, and 6 h relative to base-line Akt/ERK levels measured in S1P-free control treatments at each time point. Right panels show independent Akt/ERK assays
6 h after the indicated treatments each having 3– 4 replicates. Asterisks indicate significant changes in Akt/ERK (p ⬍ 0.05) as determined by one-way ANOVA and
Tukey’s pairwise tests (␣ ⫽ 0.05).
HDL-S1P Prolongs Endothelial Barrier Function
This is consistent with other findings showing that S1P-S1P1
signaling inhibits endothelial cell Erk1/2 activation through
induction of MAP kinase phosphatase-3 (23). Our studies also
revealed that S1P1 inhibition reduced the levels of total Erk1/2
and that HDL-S1P diminished this effect (Fig. 2C), indicating
that S1P1 signaling sustains total Erk1/2 levels, while inhibiting
activation at 6 h.
HDL-S1P signaling in endothelial cells also involves eNOS
activation via phosphorylation of Ser-1177 (24). Active eNOS
catalyzes production of nitric oxide (NO), a modulator of both
vascular tone and endothelial barrier (25, 26). To determine
whether eNOS activity was required for the sustained EC barrier-promoting effects of HDL-S1P, the eNOS/NO synthesis
inhibitor, L-NAME, was used. L-NAME was found to cause
dose-dependent inhibition of the sustained effects of HDL-S1P
on EC barrier (Fig. 3A).
We next measured changes in phosphorylated eNOS and
total eNOS in endothelial cells after 6 h of treatment with HDLS1P or albumin-S1P. The results show that HDL-S1P, but not
albumin-S1P, mediates prolonged Ser-1177 phosphorylation of
eNOS for at least 6 h (Fig. 3, C and D, left panel). HDL-S1Pinduced phosphorylation of eNOS at Ser-1177 was reduced by
treatment with the S1P1 antagonist, W146 (Fig. 3C). In addition
to augmenting eNOS activation, HDL-S1P treatment was also
found to increase the levels of total eNOS (Fig. 3D, right panel),
which is consistent with findings of others who showed that
HDL promotes eNOS protein stability (27).
DECEMBER 28, 2012 • VOLUME 287 • NUMBER 53
Because soluble guanylate cyclase (sGC) activation occurs in
response NO and has been implicated in promoting barrier
(28), we sought to determine whether sGC activity was required
for the sustained EC barrier-promoting effects of HDL-S1P. We
found that the sGC inhibitor, ODQ, elicited a dose-dependent
inhibition of the sustained effects of HDL-S1P on EC barrier
(Fig. 3B).
S1P1 Protein Levels Are Higher in Response to HDL-S1P as
Compared with Albumin-S1P—We were next interested to
determine whether the differential effects of HDL-S1P and
albumin-S1P on S1P1-mediated barrier enhancement were
reflective of alterations in S1P1 expression. In other studies,
albumin-S1P induces time-dependent loss of S1P1 protein (29,
30). Using S1P1 immunoblot analysis, we observed that both
albumin-S1P and HDL-S1P treatment elicited a time-dependent decrease in total S1P1 levels (Fig. 4A). However, in endothelial cells treated with HDL-S1P, there were higher levels of
S1P1 protein at 2 and 6 h after treatment as compared with cells
treated with albumin-S1P (Fig. 4A). In fact, levels of S1P1 were
⬃2-fold higher in endothelial cells treated with HDL-S1P as
compared with albumin-S1P (p ⫽ 0.0129) at 6 h after treatment
(Fig. 4B). Specificity of rabbit anti-S1P1 used for immunoblot
analysis is shown in supplemental Fig. 1.
Effects of HDL-S1P on S1P1 Levels Are Not Due to Alterations
in S1P1 Transcription or Protein Synthesis—Quantitative PCR
analysis showed that the effect of HDL-S1P treatment on S1P1
protein levels did not involve an increase in S1P1 mRNA (Fig. 5A).
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FIGURE 3. HDL-S1P-sustained endothelial barrier activity is dependent on eNOS activity. A, ECIS analysis of endothelial cells treated for 3 h with HDL-S1P,
albumin-S1P (each containing 150 nM S1P), or S1P-free vehicle followed by the addition of the eNOS inhibitor L-NAME (1–3 mM) or vehicle (arrow). Each of the
TEER tracings shown is an average of 2–3 replicates per condition and is representative of data from three independent experiments. Significant differences in
normalized impedance at 8 h after S1P stimulation were identified by one-way ANOVA and Tukey’s pairwise tests (␣ ⫽ 0.05). Asterisks indicate p ⬍ 0.038. ⍀
indicates ohms. B and C show anti-eNOS and phospho-eNOS (p-eNOS) (phospho-serine 1177 eNOS) immunoblot analysis of endothelial cell extracts isolated
6 h after the addition of the indicated effectors. Plots in B and C show -fold differences in eNOS and phospho-eNOS protein levels as determined by densitometric analysis and normalized to GAPDH levels. Significant differences in phospho-eNOS and eNOS levels were identified by one-way ANOVA and Tukey’s
pairwise tests (␣ ⫽ 0.05). alb-S1P, albumin-S1P. D shows an ECIS analysis of endothelial cells treated for 3 h with HDL-S1P (containing 150 nM S1P) followed by
the addition of the sGC inhibitor ODQ (30 –3000 nM) or vehicle (arrow). Each of the TEER tracings shown is an average of 2–3 replicates per condition and is
representative of data from two independent experiments.
HDL-S1P Prolongs Endothelial Barrier Function
FIGURE 4. HDP-S1P induces a less rapid decrease in levels of S1P1 than
albumin-S1P. A, anti-S1P1 immunoblot analysis of extracts from endothelial
cells treated for 0, 2, or 6 h with albumin-S1P (alb-S1P) or HDL-S1P (each
containing 150 nM S1P). As a loading control, filters were also probed with
-actin antiserum. B, densitometric analysis of immunoblots of extracts of
endothelial cells treated for 6 h with albumin-S1P or HDL-S1P. The plotted
values represent S1P1 protein levels in experimental replicates normalized
using cytochrome oxidase-IV (COXIV) as a reference protein. Student’s t test
(␣ ⫽ 0.05) showed that S1P1 levels were significantly higher (p ⫽ 0.0129) 6 h
following HDL-S1P treatment as compared with albumin-S1P.
The possibility that the higher levels of S1P1 protein observed
under conditions of HDL-S1P treatment were due to augmented protein synthesis were discounted by findings showing
that the level of barrier enhancement by HDL-S1P remained
44650 JOURNAL OF BIOLOGICAL CHEMISTRY
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FIGURE 5. HDL-S1P reduces the rate of S1P1 degradation as compared
with albumin-S1P. A, quantitative PCR analysis of S1P1 transcript levels in
endothelial cells were treated with HDL-S1P, albumin-S1P (each containing
150 nM S1P), or S1P-free vehicle for 6 h. B, ECIS analysis of endothelial cells
treated with cycloheximide (5 g/ml) during treatments with either albuminS1P or HDL-S1P (150 nM S1P). The lower panel shows integrated impedance
values for the indicated time intervals. These values were obtained by dividing the integrated impedance values for each treatment by the mean integrated impedance values from cells treated with no S1P and no cycloheximide. ⍀ indicates ohms. C, immunoblot analysis of endothelial cells treated
with cycloheximide (0.5 g/ml) for 1 h and then up to 6 h with albumin-S1P
(alb-S1P), HDL-S1P, or S1P-free vehicle. Diminishing levels of cytochrome oxidase-IV (COXIV) with time indicate the efficacy of cycloheximide at inhibiting
de novo synthesis. S1P1 levels at 3 and 6 h were determined by densitometric
analysis of immunoblots from four experiments as exemplified in the blot
shown in C. Plotted values are the percentage of S1P1 in HDL-S1P-treated and
albumin-S1P-treated cells relative to S1P1 levels in vehicle-treated controls
(gray). The p value was determined using a one-sided Student’s t test. D, antiS1P1 immunoblot analysis of endothelial cells treated with albumin-S1P,
HDL-S1P, or S1P-free vehicle ⫾ the proteasome inhibitor MG132 (20 M).
greater than that of albumin-S1P under conditions of cycloheximide treatment (Fig. 5B).
HDL-S1P Reduces the Rate of S1P1 Degradation as Compared
with Albumin-S1P—Hla and colleagues (30) have demonstrated that S1P induces S1P1 proteolysis via -arrestin-mediated internalization, polyubiquitylation, and subsequent proteasomal degradation. To compare and contrast the effects of
albumin-S1P versus HDL-S1P on S1P1 turnover, we first performed anti-S1P1 immunoblot analysis on extracts of endothelial cells treated for different periods of time with either HDLS1P or albumin-S1P, under conditions of protein synthesis
inhibition (Fig. 5C). Densitometric analysis of the anti-S1P1
reactive 45-kDa bands in four separate experiments was performed. Both albumin-S1P and HDL-S1P induced time-dependent decreases in S1P1 levels. The rate of S1P1 loss for each
treatment was examined from the fits of the densitometric data
to exponential functions. The results show that the reduction of
S1P1 over time in response to HDL-S1P treatment is less than
that occurring in response to albumin-S1P treatment (p ⫽
0.037 at 6 h) (Fig. 5C). The half-life of S1P1 in response to
HDL-S1P was 4.4 h as compared with 2.7 h in response to
albumin-S1P.
We next evaluated the effect of proteasome inhibition on the
loss of S1P1 in response to albumin-S1P and HDL-S1P treatments. Proteasomal inhibition was found to block all loss of
S1P1 in response to HDL-S1P and albumin-S1P (Fig. 5D).
Taken together, both HDL-S1P-induced degradation and albumin-S1P-induced S1P1 degradation are dependent on the proteasome; however, HDL-S1P induces less S1P1 degradation as
compared with albumin-S1P.
Cell Surface Levels of S1P1 Are Higher after Treatment with
HDL-S1P as Compared with Albumin-S1P—A number of studies have shown that S1P induces internalization of S1P1 (30 –
32). Thus, we were interested to know whether HDL-S1P and
albumin-S1P differentially influenced cell surface levels of
S1P1. Similar to what others have shown when using albuminS1P (32), within 15 min of treatment with HDL-S1P, there is an
apparent alteration in the subcellular distribution of the receptor (i.e. more discrete punctate staining) suggestive of internalization (Fig. 6A). In cells treated for 6 h, there was a greater
degree of cell border and diffuse cellular S1P1 immunolabeling
in HDL-S1P-treated versus albumin-S1P-treated cells (Fig. 6B)
suggestive of differential effects on the cell surface localization
of S1P1.
Using cell surface biotinylated endothelial cells, we found
that levels of cell surface S1P1 were higher in endothelial cells
after 3 h of treatment with HDL-S1P as compared with albumin-S1P (Fig. 6C). These findings, together with the evidence
that HDL-S1P reduces degradation of S1P1 as compared with
albumin-S1P, suggest that the sustained barrier effects of HDLS1P may involve recycling of S1P1 to the cell surface. Indeed,
when cells were treated with the recycling blocker, monensin,
the long term barrier-promoting effects of HDL-S1P (i.e. ⬎3.5
h) were abrogated (p ⫽ 0.01; Fig. 6, D and E). Thus, the contribution of S1P1 recycling to HDL-S1P enhanced barrier is
apparent after the albumin-S1P effect on barrier is diminished
(Fig. 1). Accordingly, it can be concluded that differences in
HDL-S1P Prolongs Endothelial Barrier Function
FIGURE 7. HDL-S1P treatment increases S1P1 levels in caveolin-1-enriched microdomains as compared with albumin-S1P. Following a 3-h
treatment with HDL-S1P or albumin-S1P (each containing 150 nM S1P),
extracts were made and subjected to density gradient ultracentrifugation.
Fractions 1–12 were collected from top (lowest density) to bottom of the
density gradient and subjected to immunoblot analysis using anti-S1P1 and
anti-caveolin-1. Data are representative of two experiments.
determine whether HDL-S1P and albumin-S1P elicit differential effects on targeting of S1P1 to CEMs. We found that following 3 h of treatment with HDL-S1P, S1P1 was present in the
CEM fractions, whereas there was little or no S1P1 detected in
these fractions in response to albumin-S1P (Fig. 7).
S1P1 recycling underlie the disparate effects of HDL-S1P versus
albumin-S1P on barrier.
HDL-S1P Induces Localization of S1P1 in Caveolin-enriched
Microdomains—Because S1P-S1P1 signaling occurs within
caveolin-enriched microdomains (CEMs) (6), we sought to
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FIGURE 6. Cell surface levels of S1P1 are higher after treatment with HDLS1P as compared with albumin-S1P. A, anti-S1P1 immunofluorescence
analysis of endothelial cells treated for 15 min with HDL-S1P, albumin-S1P
(each containing 150 nM S1P), or S1P-free vehicle. B, confocal immunofluorescence analysis of S1P1 in endothelial cells treated for 6 h with HDL-S1P, albumin-S1P (each containing 150 nM S1P), or S1P-free vehicle with a quantitative
look-up table applied to the image. Arrowheads point to S1P1-rich cell borders. C, anti-S1P1 immunoblot analysis of biotinylated cell surface fractions
from endothelial cells treated 3 h with HDL-S1P or albumin-S1P (each containing 150 nM S1P). Densitometric S1P1 values in each sample were normalized to the level of von Willebrand factor (vWF) and plotted to show fold
differences in levels of surface S1P1 in HDL-S1P-treated cells relative to levels
in albumin-S1P-treated cells. The plotted data contain values from two independent experiments. p was determined by one-sided, single group Student’s t test compared against the value of 1. D, ECIS analysis of endothelial
cells treated ⫾ HDL-S1P in the presence of the G protein-coupled receptor
recycling inhibitor monensin (285 nM) or vehicle. ⍀ indicates ohms. E, shows
integrated impedance at the 3.5– 6-h time interval after dividing mean integrated impedance values from cells treated with no S1P. The p value was
determined by Student’s t test.
DISCUSSION
Here we show that albumin-S1P and HDL-S1P have distinct
effects on the persistence of S1P-dependent endothelial cell
barrier enhancement such that HDL-S1P sustains endothelial
cell barrier longer than albumin-S1P. Our findings indicate that
HDL-S1P elicits specific effects on S1P1 trafficking that prolong S1P-S1P1 signaling involving persistent activation of Akt
and eNOS. Consistently, we observed that the duration of the
barrier promotion elicited by HDL-S1P lasted at least through
20 h after treatment (longer time points were not evaluated),
whereas the barrier-promoting effects of albumin-S1P subsided within 4 h of treatment.
Previous studies established that both HDL-S1P and albumin-S1P stimulated relatively short duration activation of
Erk1/2 and PI3K-Akt pathways as well as activation of eNOS (9,
13, 24). Here we showed that HDL-S1P but not albumin-S1P
elicited a persistent activation of PI3K-Akt and eNOS, lasting
for at least 6 h. By contrast, the duration of Erk1/2 activation
stimulated by HDL-S1P treatment lasted less than 30 min.
These findings together with evidence that inhibition of S1P1,
PI3K-Akt, eNOS, or sGC blocked the sustained barrier effects
elicited by HDL-S1P indicate that the HDL-S1P-S1P1-PI3KAkt-eNOS-sGC pathway mediates sustained endothelial barrier activity. Furthermore, HDL-S1P augmented levels of total
eNOS, whereas albumin-S1P had no effect on total eNOS levels, which is a means by which the flux through the pathway
may be amplified. Our findings also highlight a new role of S1P1
signaling that involves its ability to maintain basal Erk1/2 and
Akt levels.
Mechanistically, we also find that relative to albumin-S1P,
HDL-S1P acts to reduce the rate of S1P1 protein degradation
and promote recycling of internalized S1P1, leading to
increased levels of the receptor on the endothelial cell surface.
In addition to increasing cell surface S1P1 levels, HDL-S1P also
acts to promote retention of S1P1 within CEMs, sites of S1P1
signaling (6). Previous studies have shown that within 5 min of
S1P stimulation, S1P1 is recruited to CEMs (6). Our studies
show that CEMs contain higher S1P1 levels 3 h after treatment
with HDL-S1P as compared with treatment with albumin-S1P.
Together, our findings suggest that enhanced S1P1 recycling
underlies the sustaining effects of HDL-S1P on endothelial cell
barrier. Although S1P1 recycling is known to be initiated in
HDL-S1P Prolongs Endothelial Barrier Function
44652 JOURNAL OF BIOLOGICAL CHEMISTRY
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The critical role of S1P in supporting vascular barrier integrity in vivo under basal conditions has been illustrated in animal
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