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Original Studies |
Division of Endocrinology, Diabetes, and Metabolism, State University of New York, Buffalo, New York 14209; and Kaleida Health, Buffalo, New York 14209
Address all correspondence and requests for reprints to: Paresh Dandona, M.D., Ph.D., Diabetes-Endocrinology Center of Western New York, State University of New York, 3 Gates Circle, Buffalo, New York 14209. E-mail: pdandona{at}mfhs.edu
| Abstract |
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-nitro-L-arginine, a NOS
inhibitor.
N
-Nitro-L-arginine
inhibited the insulin-induced decrease in ICAM-1 expression in HAEC at
the messenger ribonucleic acid and protein levels. Thus, the inhibitory
effect of insulin on ICAM-1 expression is mediated by NO. We conclude
that insulin reduces the expression of the proinflammatory adhesion
molecule ICAM-1 through an increase in the expression of NOS and NO
generation and that insulin may have a potential antiinflammatory and
antiatherosclerotic effect rather than a proatherosclerotic effect. | Introduction |
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It has recently been shown that insulin is an arterial and venous vasodilator (4, 5, 6, 7). It stimulates the release of nitric oxide (NO) from human umbilical endothelial cells (8), and we have also demonstrated that insulin increases the expression of endothelial nitric oxide synthase (e-NOS) by human aortic endothelial cells (HAEC) (9), the enzyme that synthesizes NO (10). The acute release of NO induced by insulin and the increase in e-NOS expression over a longer period are consistent with the vasodilatory effect of insulin. As NO is believed to be antiatherogenic (11, 12, 13), and its inhibitor, NG-monomethyl-L-arginine (L-NMMA), may be proatherogenic in experimental animals (14), it is likely that insulin may also have an antiatherogenic role. If so, it may reduce the expression of intercellular adhesion molecule-1 (ICAM-1), the adhesion molecule whose plasma concentrations have been shown to be related to future coronary heart disease events (15, 16, 17). ICAM-1 is constitutively expressed by endothelial cells, and its expression increases during inflammation and after endotoxin challenge (18, 19, 20).
In view of the above, we have investigated this concept further to examine the possibility that insulin may inhibit the expression of ICAM-1 in endothelial cells. Leukocyte function-associated antigen-1, the ligand of ICAM-1, is found in circulating monocytes. The binding of this ligand to ICAM-1 on endothelial cells allows monocytes to adhere to these cells, leading to the initiation of the atherogenic process on the endothelial surface (21, 22, 23). Thus, the expression of ICAM-1 on the endothelial surface may be an important determinant of potential atherogenicity in a patient. This report describes the inhibitory effect of insulin on ICAM-1 expression in HAEC through a NO pathway-mediated mechanism.
| Materials and Methods |
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Human endothelial cells were harvested from human aortas and
arterial vessels by the method described by Gospodarowicz et
al. (24). The vessels were washed several times with
phosphate-buffered saline (PBS) and incubated with 0.1%
collagenase/dispase solution (Roche Molecular Biochemicals, Indianapolis, IN) in medium 199 (Life Technologies, Inc., Grand Island, NY) for 20 min. The solution
was spun, and the cells were collected and harvested in flasks coated
with FBS. The identity of the cells was confirmed by
immunohistochemical staining, which was positive for factor VIII,
Ulex europaeous, EN4, CD31, and negative for
-actin. All
experiments were performed using cultures at a passage of 56. HAEC
were allowed to reach 90% confluence and then cells were washed with
PBS and taken to endothelial cell growth medium (Clonetics,
Walkersville, MD) containing 2% charcoal/dextran-stripped FBS
(HyClone Laboratories, Inc., Logan, UT) for 24 h. On
the second day, the cells were induced with insulin (0, 100, and 1000
µU/mL) and incubated for 2 days with or without
N
-nitro-L-arginine
(L-NNA; Sigma-Aldrich Corp., St. Louis, MO).
Western blotting
Total cell lysates were prepared by washing the adherent cells with PBS followed by 1 mL boiling lysis buffer (1% SDS, 1 mmol/L sodium orthovanadate, and 10 mmol/L Tris, pH 7.4). Cells were scraped and transferred to a microcentrifuge tube, boiled for an additional 5 min, and centrifuged at 14,000 x g for 5 min. Total protein concentrations were determined using bicinchoninic acid protein assay (Pierce Chemical Co., Rockford, IL). Twenty micrograms of total cell lysate were electrophoresed on 6% SDS-polyacrylamide gels. Proteins were transferred to polyvinylidene difluoride membrane. The membrane was blocked for 1 h in 5% nonfat dry milk in 0.02% Tween/Tris-buffered saline buffer and then incubated overnight with a monoclonal antibody against e-NOS (Transduction Laboratories, Inc., Lexington, KY). The membrane was washed four times for 15 min each time with 0.02% Tween/Tris-buffered saline buffer and then incubated with peroxidase-conjugated goat antimouse Ig for 1 h. Finally, the membrane was washed and developed using supersignal chemiluminescence reagent (Pierce Chemical Co.).
ICAM-1 RT-PCR
Total ribonucleic acid (RNA) was extracted from HAEC by a single step guanidium thiocyanate-phenol-chloroform extraction method. Total RNA was treated with deoxyribonuclease I (50 U). Complementary DNA was synthesized by RT using a RETROscript first strand synthesis kit for RT-PCR (Ambion, Inc., Austin, TX). Takara human ß-actin competitive PCR (Takara, Inc., Japan) was used to correct the RNA amount for PCR among the samples. Quantum RNA 18S internal standard (Ambion, Inc., Austin, TX) or 40S ribosomal RNA S9 was used in the relative PCR as housekeeping genes. PCR primers for ICAM-1 were purchased from CLONTECH Laboratories, Inc. (Palo Alto, CA). PCR was performed in a final volume of 25 µL by adding 17.65 µL deionized H2O, 2.5 µL 10 x PCR buffer, 1.25 µL deoxy-NTP (2.5 mmol/L), 1 µL ICAM-1 primers, 2.5 µL complementary DNA, and 0.1 µL SuperTaq polymerase (Ambion, Inc.). Two-stage thermal cycling was performed (94 C for 30 s, 68 C for 2 min). The optimal number of cycles for ICAM-1 was determined, and PCR reactions were then run under denaturing conditions on 5% polyacrylamide/8 mol/L urea ready-made gels (Bio-Rad Laboratories, Inc., Hercules, CA). Gels were then fixed and stained with silver staining using Bio-Rad Laboratories, Inc., Silver Staining Plus kit.
ICAM-1 protein levels
ICAM-1 protein levels were measured by an enzyme-linked immunosorbent assay kit obtained from R&D Systems, Inc. (Minneapolis, MN). Samples were homogenized in 0.5 mL homogenization buffer [10 mmol/L Tris-HCl (pH 7.4), 1 mmol/L ethylenediamine tetraacetate, 0.05% sodium azide, 1% Tween-80, 150 mmol/L NaCl, 2 mmol/L phenylmethylsulfonylfluoride, 1 µg/mL leupeptin, 1 µg/mL pepstatin A, and 1 µg/mL aprotinin] and analyzed according to kit procedure.
Statistical analysis
Statistical analysis was carried out using SigmaStat software (Jandel Scientific, San Rafael, CA). Paired t test was used to compare the levels of e-NOS and ICAM-1 in HAEC control cells (0 µU/mL insulin) vs. those in insulin-induced HAEC (100 and 1000 µU/mL insulin with or without L-NNA).
| Results |
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| Discussion |
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ICAM-1 is an adhesion molecule of the Ig family and is expressed on the
endothelial cell surface. Its ligand, leukocyte function-associated
antigen-1, which is expressed on the monocyte membrane, allows the
monocyte to adhere to the endothelial cell surface to initiate the
inflammatory and atherogenic processes. Increased expression of ICAM-1
is induced by endotoxin in vitro and in vivo (25, 26); an increase in ICAM-1 on the endothelial cell surface is also
induced by tumor necrosis factor-
(TNF
) (27). ICAM-1 is a marker
for inflammation and atherosclerosis; its increased levels have been
shown to predict coronary events (15, 16, 17). Thus, ICAM-1 is both a
mediator of inflammation and atherosclerosis as well as a marker for
atherosclerosis-related clinical events.
The inhibitory effect of insulin on ICAM-1 expression allied with its
potent vasodilatory effect (4, 5, 6, 7) and antiplatelet effect (28) are
potentially antiatherogenic. Furthermore, some of these effects appear
to be mediated through NO release. NO, too, is considered
antiatherogenic (12, 13, 14), and its inhibition by L-NNA leads to a
proinflammatory and proatherogenic state in the vessel wall (14). These
observations support the concept that insulin is probably an unlikely
mediator of atherosclerosis in insulin-resistant states. We have to
look for another mediator of atherosclerosis in insulin resistance
states; TNF
may be one such putative molecule, as it is increased in
insulin-resistant states (29, 30) and may actually antagonize insulin
action at the adipocyte (31) and endothelial cell (32) levels. Indeed,
TNF
has been shown to increase ICAM-1 expression and to decrease NOS
expression in HAEC (27, 33). TNF
is also an established mediator of
the acute inflammation that follows endotoxin injection (34).
In conclusion, insulin inhibits the expression of ICAM-1 markedly in HAEC. This inhibition is probably mediated by increased NO release and NOS expression, as it is inhibited by a NOS inhibitor. This effect of insulin is suggestive of an antiinflammatory action of this hormone. This effect of insulin along with its vasodilatory and antiplatelet effects militate against a proatherogenic role for insulin.
| Footnotes |
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Received February 9, 2000.
Revised March 30, 2000.
Accepted April 1, 2000.
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inhibits insulin induced nitric oxide synthase and
insulin receptor autophosphorylation in human aortic endothelial cells
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. Arterioscler Thromb Vasc Biol. 15:5257.This article has been cited by other articles:
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