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Short Communications |

From the Department of Surgery,*
Emory University Schoolof Medicine, Atlanta; and the School of MechanicalEngineering,
Georgia Institute of Technology,Atlanta, Georgia
| Abstract |
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The major active component of cigarette smoke is nicotine.5 Nicotine has been shown to have a variety of effects on vascular biology that may contribute to atherosclerosis. At levels similar to those in the blood plasma of habitual smokers, nicotine has been shown to induce changes in expression of various atherosclerosis-related genes in endothelial cells including endothelial nitric oxide synthase, angiotensin I-converting enzyme, tissue-type plasminogen activator, platelet-derived growth factor, and basic fibroblast growth factor.6-8 Nicotine has also been shown to cause morphological changes in endothelial cells, increased endothelial cell death, and enhanced transendothelial transport of plasma macromolecules.7,9 However, the mechanisms of increased endothelial turnover and permeability because of nicotine have not yet been completely elucidated.
The role of nicotine in tumorigenesis has not been as clearly defined as its role in vascular disease.10 It has been demonstrated that nicotine may enhance proliferation and inhibit apoptosis of certain types of human cancer cell lines.10,11 Another important mechanism by which nicotine may contribute to tumor growth is by the enhancement of angiogenesis, a process necessary for tumor growth and metastasis.12 It has been demonstrated that nicotine produces a proliferative response in endothelial cells and thus it is possible that it may also enhance angiogenesis and metastasis, although this has not directly been shown.13
One common factor that could contribute to both increased endothelial permeability and turnover related to vascular disease and increased angiogenesis and tumor growth in cancer is vascular endothelial growth factor (VEGF). It has been demonstrated that VEGF is expressed in high levels in human atherosclerotic lesions whereas it is expressed in very low levels in nondiseased arteries.14,15 Furthermore, VEGF is expressed in the majority of cancers and blocking its activity has been shown to inhibit growth of experimental tumors in vivo.16
Despite the known correlations between cigarette smoking and vascular disease and cancer, there is little research on the effects of nicotine on VEGF expression, an important factor in the progression of both diseases. This study was therefore undertaken to determine the effects of nicotine and the major product of its metabolism, cotinine, on the expression of VEGF in endothelial cells. To this end, we used an intact porcine artery ex vivo perfusion culture model. Our results demonstrated, for the first time, that nicotine and cotinine, in doses similar to those seen in the plasma of habitual smokers, significantly increase endothelial cell VEGF mRNA and protein expression.
| Materials and Methods |
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Dextran, phosphate-buffered saline solution, Tris-buffered saline solution, Tri-Reagent, ß-actin monoclonal antibodies, nicotine, cotinine, and Tween 20 were obtained from Sigma Chemical Co. (St. Louis, MO). Dulbeccos modified Eagles medium was from Life Technologies, Inc. (Grand Island, NY), antibiotic-anti-mycotic was from Mediatech Inc. (Herndon, VA), and the one-step reverse transcriptase-polymerase chain reaction (RT-PCR) kit was from Promega Corp. (Madison, WI). The protein assay kit and precast polyacrylamide gels were obtained from Bio-Rad Laboratories (Hercules, CA). Antibodies against VEGF were from Santa Cruz Biotechnology (Santa Cruz, CA), the horseradish peroxidase-conjugated anti-rabbit secondary antibodies and the Enhanced Chemiluminescence kit were from Amersham Life Sciences (Buckinghamshire, England). VEGF and ß-actin primers were synthesized by Operon Technologies Inc. (Alameda, CA).
Vascular Perfusion Culture
The vascular perfusion culture system and methods have been described in detail previously.17 Porcine common carotid arteries were harvested from 6- to 8-month-old male and female domestic pigs and mounted on an adjustable cannula. Branches were then ligated and the vessels were stretched to their physiological length. The perfusion culture systems were assembled, filled with culture medium (Dulbeccos modified Eagles medium with 5% 207-kd dextran to increase the viscosity) and placed in standard cell culture incubators. Lastly, the flow loop tubing was connected to the pump, the pressure transducers were connected to the monitors, and the flow was initiated. Vessels were cultured for 24 hours with 100 mmHg of pressure, 150 ml/minute flow, and as controls, with 10-7 mol/L nicotine, or with 10-7 mol/L cotinine.
After culturing, vessels were removed from the systems and a ring was
cut from the middle of each vessel for histological analysis. The
endothelial cells were removed from the remaining portions of each
vessel for mRNA and protein analysis as described previously by gently
scraping the luminal surface with a scalpel blade and resuspending the
cells in Tri-Reagent.6
Great care was taken to not disrupt
the internal elastic lamina during scraping to avoid smooth muscle
contamination. Purity of endothelial cells insolated in this manner was
determined to be
95 to 100% by culturing cells for 1 week after
removal from vessels and examining for smooth muscle cell and
fibroblast contamination morphologically (data not shown).
RT-PCR
Total RNA was isolated from endothelial cells from cultured
vessels using Tri-Reagent. RNA concentrations were determined by
absorption at 260 nm. A one-step RT-PCR kit was used according to the
manufacturers instructions. The same amount of total RNA (0.2 µg)
was used from each sample. The upstream VEGF primer was
5'-ATGCGGATCAAACCTCACC-3' and downstream primer was
5'-ATCTGGTTCCCGAAACGCTG-3'.18
ß-actin was used as an
internal RNA loading control for each sample with an upstream primer of
5'-CTTCCTGGGCATGGAATCCT-3' and a downstream primer of
5'-GATCTTGATCTTCATCGTGCT-3'. The thermal cycle conditions used were as
follows: 45 minutes at 48°C (reverse transcription), 2 minutes at
94°C (denaturation), followed by PCR cycling of 94°C for 30 seconds
(denaturation), 60°C for 1 minute (annealing), and 68°C for 2
minutes (extension), and lastly a final extension for 7 minutes at
68°C. Using these conditions, the linear ranges of amplification for
VEGF and ß-actin products were determined by performing RT-PCR for
both with 0, 8, 15, 20, 24, 26, 28, 30, 32, 36, and 38 cycles. The
middle of the linear range for both VEGF and ß-actin was determined
to be
24 cycles (data not shown). PCR was thus performed for 24
cycles for all experiments. Controls were performed with no RT-enzyme
for both VEGF and ß-actin to demonstrate the lack of DNA
contamination in samples. VEGF and ß-actin RT-PCR products were
combined and were resolved on a 2% agarose gel stained with ethidium
bromide. Semiquantitative analysis was performed using an
AlphaImager gel documentation system and software (Alpha Innotech
Co., San Leandro, CA). The density of each VEGF band was divided by the
density of its respective ß-actin band to account for variations in
loading.
Western Blotting
Endothelial cell protein from vessels after culturing was isolated using Tri-Reagent and resuspended in 50 µl of 10 mol/L urea. Protein concentrations were determined using a protein assay kit. The same amount of endothelial protein (3 µg) from each sample was resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis on 12% polyacrylamide gels and subsequently transferred to nitrocellulose membranes. Membranes were blocked and blotted for VEGF with a polyclonal rabbit anti-human VEGF primary antibody and detected with a horseradish peroxidase-conjugated secondary antibody. Subsequently, membranes were gently washed and then reblotted for ß-actin using a monoclonal ß-actin antibody and a horseradish peroxidase-conjugated secondary antibody. Blots were developed using enhanced chemiluminescence and analyzed with an AlphaImager gel documentation system and analysis software.
Immunohistochemistry
After culturing, a 4-mm long ring was cut from the middle of each vessel and fixed overnight in 10% neutral buffered formalin. Sections were paraffin-processed, embedded, and 5-µm thick sections were cut. Immunohistochemical staining was performed for VEGF using a monoclonal anti-VEGF antibody. Color was developed using the avidin-biotin complex immunoperoxidase procedure as described previously.19 Sections were counterstained with hematoxylin.
Statistics
Statistical differences were determined using the two-tailed Students t-test and analysis of variance, with significance considered to be P < 0.05 unless otherwise noted. Results are reported as mean ± SEM.
| Results |
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20 to 24 kd in molecular mass, corresponding to the correct
size as stated by the antibody manufacturer. The average VEGF/ß-actin
optical density ratio for endothelial cells from control vessels was
0.744 ± 0.040 whereas for nicotine-treated vessels the ratio was
1.128 ± 0.095 (n = 4) and for endothelial
cells from cotinine-treated vessels the ratio was 1.239 ± 0.16
(n = 4) (Figure 2B)
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| Discussion |
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10 times higher than nicotine
concentrations because of the long half-life of cotinine compared to
that of nicotine.22
In this study we used an ex
vivo perfusion culture system with intact porcine common carotid
arteries cultured with physiological pressure and flow levels. This
model was chosen because it is more physiological than cell culture
models because of retention of different cell types, and allows
nicotine and cotinine to be studied separately, which cannot be done
in vivo because cotinine is produced by nicotine metabolism
in the liver. Deeper studies are warranted including combining nicotine
and cotinine to investigate possible synergisms in their effects as
well as dose-response studies. Such studies are currently underway in
our laboratory. The increased endothelial VEGF expression because
of nicotine and cotinine shown here could have important implications
in both vascular disease and cancer. Previous clinical studies on the effects of smoking on VEGF expression have been inconsistent. One study showed no difference in plasma VEGF levels between smokers and non-smokers.23 However, in another study, the majority of the control patients with detectable plasma VEGF levels were smokers.24 Furthermore, it was shown in this same study that smoking acutely raises VEGF levels in 22.6% of habitual smokers. In another study, patients with lung cancer that smoked had an average of 33% more plasma VEGF compared to patients with lung cancer that did not smoke.25 Likewise, a recent study demonstrated that nicotine caused a significant increase in serum VEGF levels in a mouse model as well as an increase in tumor growth and atherosclerosis.26
Nicotine has been shown to alter the expression of other growth factors in vascular cells. It has been shown, for example, that nicotine induces basic fibroblast growth factor and platelet-derived growth factor release in endothelial cells and increases proliferation.7,8 Cotinine has also been shown to stimulate basic fibroblast growth factor production in smooth muscle cells.27 In fact, cotinine has also been shown to be a more powerful mitogen for smooth muscle cells than nicotine.28 Likewise, the results presented here demonstrated that at the same concentration, cotinine caused a larger increase in endothelial cell VEGF expression than nicotine. Taken together, these data may suggest that cotinine is even more harmful than nicotine and should be studied more widely.
The exact mechanisms by which nicotine and cotinine may up-regulate
VEGF expression in endothelial cells is not known at this time.
However, it seems likely that it may involve interactions between
nicotine and the nicotine-sensitive acetylcholine receptors (nAChR),
which have recently been shown to be present on endothelial
cells.29
Cotinine may also have interacted through nAChR
because it has been shown to bind to the
7 nAChR subtype in
Xenopus oocytes.30
Additionally, it has been
demonstrated in vitro that blocking of the nAChR with
hexamethonium in endothelial cells treated with nicotine abolished the
increase in DNA synthesis seen in cells treated with nicotine
alone.13
It was also shown in vivo that
blocking nAChR with hexamethonium in a nicotine-treated mouse hind-limb
ischemia model eliminated the increase in capillary formation seen with
nicotine treatment alone, possibly indicating a decrease in
nicotine-stimulated VEGF secretion.26
Nicotine has been
shown in cultured bovine adrenal medullary chromaffin cells to activate
the fibroblast growth factor-2 (FGF-2) gene through nAChR via tyrosine
phosphorylation of cytoplasmic and nuclear proteins including 50- to
55-kd promoter-binding factors.31
It is possible that
nicotine and cotinine stimulation of VEGF production in endothelial
cells may use a similar pathway. This is a current area of
investigation in our laboratory.
VEGF was only recently shown to be present in human atherosclerotic plaques.14 Since then, studies have shown it to be involved in the genesis of vascular lesions in research models. Recently, it was demonstrated in hypercholesterolemic mice and rabbits that low VEGF doses cause an increase in plaque macrophage levels and endothelial cell content relative to controls without VEGF treatment.32 VEGF also increases the permeability of endothelial cells and may thus contribute to the atherosclerotic process by increasing transport into the vessel wall of low-density lipoprotein, fibrin, and other atherogenic macromolecules. One mechanism by which VEGF may increase vascular permeability is by increasing endothelial turnover.15,33 VEGF may also exert its influence on vascular permeability by regulating endothelial tight junction molecules ZO-1 and occludin.34,35 Increased endothelial VEGF expression because of nicotine and cotinine from cigarette smoking may thus contribute vascular disease progression in part by increasing endothelial permeability and turnover.
In addition to its role in atherogenesis, VEGF has been shown to be important in tumor growth and metastasis. Elevated plasma VEGF levels have been shown in patients with lung cancer and prostate cancer, and VEGF is expressed in high levels in most tumors.16,25,36 VEGF is known to be a powerful stimulator of angiogenesis, a process necessary for the growth of tumors.16 Blocking of VEGF in animal models of cancer with monoclonal VEGF antibodies has been demonstrated to suppress tumor growth.37 Clinical studies have also shown a positive correlation between tumor VEGF expression and metastasis in several forms of carcinomas.38,39 It has also been demonstrated in mice that inhibitors of the VEGF-receptor tyrosine kinases significantly reduced lung metastasis in a renal cell carcinoma model.40 The increase in endothelial VEGF expression shown in the present study could therefore be an important mechanism by which nicotine and cotinine increase cancer growth and metastasis.
In conclusion, we have demonstrated that nicotine and cotinine cause a significant increase in endothelial cell VEGF expression. This was demonstrated in intact porcine common carotid arteries using a vascular perfusion culture model that retains in vivo cell types and structure and is thus more physiological than cell culture models. Increased VEGF expression because of nicotine and cotinine may have important implications in vascular disease by increasing endothelial turnover and permeability to atherogenic macromolecules such as low-density lipoprotein as well as increasing tumor growth and metastasis because of increased angiogenesis.
| Acknowledgements |
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| Footnotes |
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Supported by the National Institutes of Health/National Heart, Lung, and Blood Institute (grants HL61943-01, HL65916, and HL60135 to C. C.) and the Engineering Research Center Program of the National Science Foundation (award number EEC-9731643).
Accepted for publication October 25, 2001.
| References |
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