Molecular Medicine |
From the Institut für Kardiovaskuläre Physiologie (A.G., R.P.B., K.N., M.A., R.B.), and Institut für Anatomie II (F.D.), Klinikum der J.W. Goethe-Universität, Frankfurt/Main, Germany.
Correspondence to Agnes Görlach, MD, Institut für Kardiovaskuläre Physiologie, Klinikum der JWG Universität, Theodor-Stern-Kai 7, 60590 Frankfurt/Main, Germany. E-mail a.goerlach{at}em.uni-frankfurt.de
| Abstract |
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Key Words: oxygen radicals endothelial function smooth muscle cells p22phox
| Introduction |
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Several reports have suggested that endothelial cells (ECs) and vascular smooth muscle cells (SMCs) express a ROS-generating, NADH/NADPH-dependent oxidase containing p22phox,5 6 7 8 9 which is a membrane-bound component of the leukocyte NADPH oxidase.10 In leukocytes, this complex enzyme is responsible for the production of superoxide anions during the respiratory burst and, in addition to p22phox, contains the membrane-bound protein gp91phox.11 On activation of the enzyme by phorbol-12-myristate-13-acetate (PMA), for example, several cytosolic proteins, including rac, p47phox, p67phox, and p40phox, translocate to the membrane and associate with the membrane-bound subunits. The assembled enzyme complex facilitates electron transfer from NADPH to molecular oxygen, leading to the generation of superoxide anions, which subsequently react to form further ROS such as hydrogen peroxide and hydroxyl radicals.12
Whereas p22phox is widely expressed in mammalian tissues,10 the other subunits, and in particular gp91phox, are thought to be specifically expressed in myeloid cells.11 The identification of p22phox in vascular cells and the observation that addition of NADPH/NADH to intact cells elicits ROS generation led to the suggestion that a NAD(P)H oxidase resembling the leukocyte enzyme is present in vascular cells.13 Because p22phox and gp91phox, which together make up cytochrome b558, are essential for NADPH oxidase activity in leukocytes, we compared the expression of these subunits in leukocytes, ECs and SMCs. Moreover, we determined the functional importance of these subunits for ROS generation in vascular cells.
| Materials and Methods |
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Immunoblotting
Western blot analysis was performed as described
previously.14 p22phox SDS-PAGE was performed using a
membrane preparation of HUVECs and SMCs. For detection of gp91phox,
purified glycosylated proteins were isolated by wheat germ agglutinin
agarose affinity purification. Antibodies were provided by Dr J.
Kreutzer (Heidelberg, Germany; p22phox) and Dr D. Roos
(Amsterdam, Netherlands; gp91phox).
Reverse TranscriptasePolymerase Chain Reaction (RT-PCR) and
Sequence Analysis
RT-PCR was performed with total RNA.16 PCR primers
were selected on the bases of the published sequences (Table
). PCR
products were gel purified and subjected to cycle sequencing
(Applied Biosystems, Perkin-Elmer).
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Screening of a HUVEC cDNA Library
A HUVEC cDNA library (gift of Dr R. Bowditch, Oklahoma City, OK)
was screened with full-length human p22phox or gp91phox cDNA probes
(provided by Dr M. Dinauer, Indianapolis, IN). Positive clones were
amplified by PCR and sequenced.
Organ Chamber Experiments
Organ chamber experiments were performed as
described14 using phenylephrine-precontracted
mouse aortic segments from
gp91phox-/- and wild-type
mice. The phenylephrine concentration was adjusted (0.03 to
0.3 µmol/L) to obtain identical preconstriction levels (
80%
of the initial KCl constriction). Relaxations to acetylcholine (ACh) in
the presence or absence of the ROS scavenger tiron (5 mmol/L) and
relaxations to sodium nitroprusside (SNP) were recorded.
Statistical Analysis
All values are mean±SEM. Statistical significance was tested
using ANOVA for repeated measures followed by Newman-Keuls test.
An expanded Materials and Methods section is available online at http://www.circresaha.org.
| Results |
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To exclude the possibility that the contamination of cell cultures with
leukocytes contributed to PMA-induced ROS generation in HUVECs,
measurements were repeated using different cell passages. Basal and
PMA-induced chemiluminescence was not affected by passage (P) number
(for basal lucigenin signal, P0, 327±129; P1, 234±94; and P2,
231±131 CPM/coverslip; for PMA-induced lucigenin signal, P0,
1082±373; P1, 1147±144; and P2, 1275±445 CPM/coverslip; n=3,
P=NS). Using the ROS-sensitive fluorophore
H2DCFDA, a 2.5-fold increase in
fluorescence in response to stimulation with PMA was observed
in all ECs studied (basal, 1.1±0.3, and PMA, 2.6±0.6 light units/min,
P<0.02, n=7 each group; Figure 2
).
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PMA-Stimulated Oxygen Radical Formation in Intact Segments Requires
the Presence of Endothelium
In intact rat aortic segments, PMA as well as NADPH increased ROS
generation (Figure 3
). Mechanical
denudation of the endothelium abolished the response to
PMA, whereas the response to NADPH remained detectable. Pretreatment
with the flavin inhibitor DPI attenuated the responses to
both PMA (Figure 3C
) and NADPH (data not shown).
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Expression of p22phox and gp91phox in Vascular Cells
p22phox was detected by RT-PCR in the monocytic cell line THP1,
which is known to express the leukocyte NADPH oxidase, and in HUVECs,
the EC line EA.Hy926, HSMCs, and the human colon carcinoma cell line
CaCo-1 (Figure 4
, top). Screening of a
HUVEC cDNA library with a human p22phox probe revealed 5 positive
clones. The HUVEC library was positive for endothelial
NO synthase (eNOS) as a marker enzyme for ECs, but negative for tumor
necrosis factor-
(TNF-
), which is expressed in leukocytes,
demonstrating that the HUVEC cDNA library was free of contaminating
leukocytes (Figure 4
, bottom). Sequence analysis of 84%
of the p22phox coding sequence revealed that p22phox in HUVECs and
HSMCs is identical to the human leukocyte form. The p22phox sequence
was also identical in rat SMCs and rat leukocytes (data not shown).
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RT-PCR performed with human leukocyte gp91phox primers led to PCR
fragments of expected sizes in THP1 cells as well as in HUVECs and
EA.Hy926 cells. No gp91phox PCR product could be obtained using
HSMC or CaCo-1 cell cDNA (Figure 4
, top). Screening of the HUVEC
cDNA library with a human gp91phox probe revealed 1 positive clone.
Sequence analysis of cloned and PCR-amplified cDNA composing
the complete gp91phox coding sequence as well as 148 bp of the 5'
flanking sequence and 208 bp of the 3' flanking region demonstrated
that gp91phox is identical in HUVECs and human leukocytes. To exclude
the possibility of gp91phox expression in HSMCs being lost during
culture, mRNA from the media of human umbilical arteries was isolated
and subjected to gp91phox RT-PCR. In accordance with the results
obtained in HSMCs, gp91phox was not detectable (data not shown).
An isoform of gp91phox termed mitogenic oxidase-1 (mox1)
was recently identified in SMCs and CaCo-1 cells.17 RT-PCR
with primers derived from the human mox1 sequence (Table
)
resulted in PCR products of the predicted size in HSMCs and CaCo-1
cells (Figure 4
, top). However, no mox1 PCR product was
obtained from HUVECs, EA.Hy926 cells, or THP1 cells.
Western blot analysis demonstrated the presence of p22phox
protein in membrane but not cytoplasmic fractions prepared from HUVECs
and HSMCs (Figure 5A
). Because gp91phox
is extensively glycosylated and the protein appears as a smear rather
than a distinct band in SDS-PAGE, it is difficult to detect small
amounts of the protein in whole-cell lysates. To circumvent this
problem, Western blotting was performed using glycoproteins
isolated from polymorphonuclear neutrophils (PMNs), HUVECs, and
HSMCs. gp91phox protein was detected in PMNs and HUVECs, but not in
HSMCs (Figure 5B
).
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gp91phox Is Involved in Vascular ROS Generation
To investigate whether gp91phox is involved in vascular ROS
formation, lucigenin-enhanced chemiluminescence was assessed in aortic
segments from
gp91phox-/-,
eNOS-/- and wild-type
mice (Figure 6
). PMA-stimulated ROS
production was detectable in aortae from wild-type and
eNOS-/- mice and was
abolished by endothelial denudation. In contrast, PMA
did not elicit ROS generation in endothelium-intact or
endothelium-denuded aortic segments from
gp91phox-/- mice.
Lucigenin chemiluminescence in unstimulated aortic preparations or
after the addition of NADPH was similar in the 3 strains, suggesting
that the PMA-induced, but not the NADPH-mediated ROS formation requires
a functional gp91phox.
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ACh-Mediated Relaxation Is Increased in gp91phox Knockout
Mice
Compared with responses obtained using aortic segments from
wild-type mice, the ACh-induced relaxation of segments from
gp91phox-/- mice was more
pronounced. The ROS scavenger tiron (5 mmol/L) enhanced the
ACh-induced relaxation of aortic segments from wild-type, but not from
gp91phox-/- mice, so that
there was no longer a difference in the ACh-induced relaxation of
segments from the 2 strains. Endothelium-independent
relaxations to SNP were similar in aortic segments from both groups
(Figure 7
).
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| Discussion |
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In contrast to the ubiquitously distributed p22phox,10 gp91phox expression has been assumed to be specific to myeloid cells.18 In the present study, we have demonstrated that gp91phox is expressed in ECs, but not in SMCs, and that the endothelial gp91phox is identical to that isolated from leukocytes.
The initial concept that gp91phox is myeloid-specific was strengthened by the demonstration of binding sites for several "myeloid-specific" transcription factors such as Pu.1 and Elf-1 in its promoter region.19 However, Elf-1 protein is constitutively expressed in ECs and has been proposed to play a role in the control of gene expression of eNOS.20 Thus, the presence of "myeloid-specific" transcription factors may account for the expression of gp91phox in ECs, but not in SMCs. The recent identification of mox1, a gp91phox homologue, in colon carcinoma cells and SMCs17 suggests that the composition of the NADPH oxidase is fundamentally different in ECs and SMCs. Indeed, although we found evidence for the presence of mox1 in HSMCs, we could not detect mox1 cDNA in the myeloid cell line THP1 or in ECs. It is therefore tempting to speculate that in SMCs mox1, instead of gp91phox, associates with p22phox to constitute an NADPH oxidase.
Although the expression of gp91phox mRNA in ECs has been reported previously,5 6 7 the expression of functionally relevant amounts of protein has been questioned.5 In addition to demonstrating the presence of gp91phox protein in ECs, our results indicate that this protein is functionally active. Indeed, gp91phox appears to be crucial for the burstlike ROS generation observed in response to PMA, as knockout of the gp91phox gene abolished ROS generation in endothelium-intact aortic segments.
The exact mechanism by which PMA activates the endothelial NADPH oxidase remains to be elucidated. However, in leukocytes, PMA directly activates protein kinase C, which leads to the phosphorylation of p47phox,21 possibly to the activation of rac,22 and to the subsequent activation of the oxidase. Although p47phox, p67phox, and rac are present in ECs (A.G. and R.P.B., unpublished observation, 2000), the role played by these proteins in the specific signaling pathways leading to the activation of the oxidase in ECs has not been established.
To measure ROS formation, we mainly used lucigenin-enhanced chemiluminescence, although the validity of this method has recently been questioned.23 We obtained similar results using compound 5, one of the imidazopyrazinone-type chemiluminescence enhancers that do not undergo redox cycling.24 Moreover, PMA-stimulated ROS formation was also observed by using DCF fluorescence and by using cells derived from different passages. Such experiments indicate that PMA-induced ROS formation in ECs was not due to an artifact associated with the use of lucigenin and did not reflect contamination of EC cultures with leukocytes.
PMA increases [Ca2+]i and activates eNOS, which is another important source of ROS under certain conditions.25 Because PMA stimulated ROS generation in wild-type and in eNOS-/- mice to a similar extent, we could exclude a substantial contribution of eNOS-derived ROS to the PMA response. The specificity of this effect was further demonstrated by the finding that the addition of NADPH increased chemiluminescence in ECs and SMCs as well as in aortic segments from gp91phox-/- mice. A number of studies have demonstrated increased ROS formation in vascular segments in response to NADPH and, to an even greater extent, in response to NADH.13 26 Our finding that NADPH, but not PMA, stimulated ROS formation in aortae from gp91phox-/- mice clearly indicates that NADPH does not activate the gp91phox-containing leukocyte/EC NADPH oxidase. Whether the NADPH-elicited chemiluminescence originates from ROS generated by other oxidases or by auto-oxidation of endogenous redox cyclers such as ubiquinones, or can be attributed to redox cycling of the chemiluminescence enhancers on the outer surface of the cell membrane, remains to be determined.
The amount of ROS generated by ECs after stimulation with PMA was significantly lower than that measured in leukocytes. This is not unexpected, because ROS production is determined by the amount of oxidase protein in the cell27 and ECs express only 1/100 of p22phox and gp91phox protein detected in leukocytes.
ECs are able to generate low levels of ROS under basal conditions. However, basal chemiluminescence was not significantly different in aortic segments from wild-type and gp91phox-/- mice, suggesting that the gp91phox-containing NADPH oxidase might not contribute significantly to basal ROS generation. However, it cannot be excluded that the chemiluminescence assay used was not sensitive enough to detect small differences in basal aortic ROS production between wild-type and gp91phox-/- mice.
Aortic segments from gp91phox-/- mice exhibited a more pronounced endothelium-dependent relaxation to ACh than that observed in aortae from wild-type mice. An antioxidant, tiron, selectively enhanced endothelium-dependent relaxation in wild-type mice, indicating that the reduced scavenging of endothelial NO by O2 in the aorta from gp91phox-/- mice could account for the enhanced relaxation. ACh is known to increase [Ca2+]i in ECs, release arachidonic acid, and facilitate the activation of protein kinase C, all mechanisms known to activate the leukocyte NADPH oxidase.11 It is, however, unclear whether ACh, phenylephrine, or both are required to stimulate gp91phox-dependent ROS generation in ECs.
In conclusion, our data demonstrate that the endothelium is a major generator of vascular ROS and that the formation of endothelial ROS can be acutely modulated by activation of protein kinase C. One prominent source of endothelial ROS is the gp91phox-containing NADPH oxidase, which is similar to that found in leukocytes but distinct from the enzyme in vascular SMCs. However, this endothelial NADPH oxidase generates ROS in a concentration sufficient to affect the bioavailability of endothelium-derived NO.
| Acknowledgments |
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| Footnotes |
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This manuscript was sent to Donald D. Heistad, Consulting Editor, for review by expert referees, editorial decision, and final disposition.
Received January 10, 2000; accepted May 15, 2000.
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