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653

Editorial Comment

Is NO an

Endogenous Antiatherogenic Molecule

John P. Cooke, Philip S. Tsao

E

ndothelium-derived relaxing factor is now

known to be nitric oxide (NO) or NO bound to

a carrier molecule, possibly in the form of a

nitrosothiol or nitrosoheme.

13

NO is derived from the

metabolism of L-arginine by NO synthase, which is

expressed by a number of cell types, including endothe-

lial cells, neurons, macrophages, platelets, and vascular

smooth muscle cells.

4

See pp 746, 753

NO is a potent vasodilator that activates soluble

guanylate cyclase, thereby increasing intracellular levels

of cyclic GMP. In addition, NO exerts an inhibitory

influence on cell proliferation and adhesion. The pro-

liferation of vascular smooth muscle cells or lympho-

cytes in culture is inhibited by exogenous NO donors as

well as endogenous NO.

5

-

6

Platelet adhesion and aggre-

gation as well as monocyte adhesion and chemotaxis a re

negatively regulated by this molecule.

79

These in vitro

studies indicate that NO suppresses a number of key

processes that are involved in atherogenesis. It seems

logical, therefore, that a reduction in the activity of

vascular NO would promote atherogenesis. Indeed, one

of the earliest abnormalities to occur in hypercholester-

olemic animals and hum ans is a reduction in the activity

of vascular NO, as manifested by impaired endotheli-

um-dependent vasodilation. This occurs well before any

structural changes in the vessel wall.

This impairment in endothelium-depend ent vasodila-

tion is reversible in hypercholesterolemic animal models

and humans by intravenous infusion of the NO precur-

sor L-arginine.

1013

The effect of L-arginine to normalize

endothelium-dependent vasodilation is likely due to its

metabolism to NO, since the effect is not mimicked by

D-arginine (which is not a substrate for NO syn-

thase).

10

-

12

Chronic administration of supplemental di-

etary arginine to hypercholesterolemic rabbits is also

associated with an improvement in endothelium-depen-

dent vasodilation.

14

The sustained enhancement of vas-

cular NO activity is paralleled by a striking inhibition of

intimal lesion formation in these animals.

14

Dietary

arginine also exerts an inhibitory effect on neointimal

lesion formation after balloon injury in this animal

From the Section of Vascular Medicine, Division of Cardiovas-

cular Medicine, Stanford U niversity School of M edicine, Stanford,

alif

Correspondence to John P. Cooke, MD, PhD, CVRC, Division

of Cardiovascular Medicine, Stanford University School of Medi-

cine, Stanford, CA 94305.

model.

15

Therefore, the weight of the available evidence

suggests that vascular NO exerts an inhibitory effect on

key processes in atherogenesis.

In this issue of

Arteriosclerosis and Thrombosis,

Naruse and colleagues

16

add support to this hypothesis

by demonstrating that chronic inhibition of NO synthase

accelerates atherogenesis. New Zealand White rabbits

were fed standard chow or a 2 cholesterol diet; some

animals in each group also received in their drinking

water a low (80 mg/mL) or high (160 mg/mL) dose of

nitro-L-arginine methylester

( L - N A M E ) ,

an inhibitor of

NO synthase. After 8 or 12 weeks, the animals were

killed to allow studies of vascular reactivity and histo-

morphometry. As expected, the hypercholesterolemic

animals had impaired endothelium-dependent relax-

ation. Administration of L - N A M E caused a further

impairment of endothelium-dependent relaxation in

hypercholesterolemic animals. Parallel changes were

observed in vascular structure, with the thoracic aortas

from the L-NAME-treated hypercholesterolemic ani-

mals manifesting greater lesion surface area. The data

suggest that impairment of NO synthesis promotes

a therogenes is . However , ora l adminis tra t ion of

L - N A M E augmented serum cholesterol values in the

hypercholesterolemic animals, and it is possible that this

played some role in the acceleration of lesion

development.

This confounding variable was obviated in the inves-

tigation by Cayatte and colleagues.

17

These investigators

administered a similar dose of L - N A M E by osmotic

minipump; subcutaneous delivery of

L - N A M E

was not

associated with any change in lipid profile or hemody-

namics. It is possible that the effect of the NO synthase

antagonist to alter the lipid profile is a unique effect of

oral administration and may indicate an interesting

interaction between the mesenteric NO synthase system

and lipid absorption or metabolism. Despite this and

other methodological differences between the two stud-

ies (composition of the cholesterol diet, length of treat-

ment, and type of morphometric analysis), the findings

are strikingly similar. In each case, chronic administra-

tion of NO synthase antagonists is associated with

inhibition of endothelium-dependent vasodilation and a

significant increase in the severity of intimal lesions.

The mechanisms of this intriguing effect remain un-

explained. Moreover, it is not clear whether the effect of

the NO synthase antagonist is due to inhibition of

endothelium-derived NO and/or NO elaborated from

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654 Arteriosclerosis and Thrombosis Vol 14, No 5 May 1994

other cell types (macrophages, platelets, or vascular

smooth muscle cells).

Recent studies from our laboratory support a role for

endothelium-derived NO .

18

New Zealand White rabbits

were fed normal chow or a 0.5 cholesterol diet; some

animals received supplemental dietary arginine,

whereas others received nitroarginine (the NO synthase

antagonist). After 2 weeks the animals were killed and

the thoracic aortas harvested for functional binding

studies and measurement of nitrogen oxide by chemilu-

minescence. We found that the elaboration of vascular

NO was increased in hypercholesterolemic animals that

had received supplemental arginine; vascular NO re-

lease was inhibited in animals that had received ni-

troarginine. Binding of monocytoid cells to the vessel

wall ex vivo was increased in hypercholesterolemic

animals and was increased to a greater degree in

animals that had received nitroarginine. By contrast,

there was a reduction in cells bound to the thoracic

aortas from hypercholesterolemic animals that had re-

ceived dietary arginine. To conclude, changes in vascu-

lar NO elaboration were associated with reciprocal

alterations in endothelial adhesiveness.

Is this alteration of endothelial adhesiveness due to

an inhibitory effect of N O on t he activation of adhesion

molecules and /or th e expression of chemotactic pro-

teins? Bath and colleagues

9

have found that exogenous

NO inhibits adherence of mononuclear cells to porcine

aortic endothelial cells in culture and inhibits in vitro

chemotaxis stimulated by Af-formyl-methionyl-leucyl

phenylalanine. More recently, Zeiher and colleagues

19

have reported that the NO d onor SIN-1 inhibits mRN A

expression and secretion of monocyte chemotactic pro-

tein-1 in primary cultures of human umbilical vein

endothelial cells. Thus, endogenous NO may modulate

endothelial-monocyte interactions. In addition, NO in-

terferes with the elaboration of superoxide anion and

may reduce the oxidative modification of low-density

lipoprotein by macrophages.

20

'

21

Although the available evidence suggests that vas-

cular NO inhibits de novo formation of intimal lesions,

the role of NO in the presence of preexisting lesions

remains to be determined. It is possible that in the

setting of established lesions, vascular NO could pro-

mote cell injury. Activated macrophages generate

superoxide anion and NO, which combine to form

peroxynitrite anion, a highly reactive free radical.

22

The cytotoxic effects of peroxynitrite anion are in part

mediated by membrane lipid peroxidation as well as

nitrosation of tyrosine moieties regulating enzyme

function and signal transduction.

23

Furthermore, in

large quantities, NO has cytotoxic effects mediated by

nitrosylation, mono- and poly-ADP ribosylation, and

interactions with metal-containing proteins that lead

to the inhibition of enzymes required for cellular

metabolism and DNA synthesis.

2427

However, even

these effects of NO could serve to suppress cellular

proliferation, inflammation, and lesion growth.

62 8

To

conclude, future investigations will be devised to elu-

cidate the mechanisms and conditions under which

endogenous NO inhibits atherogenesis. Knowledge of

these mechanisms may lead to new therapeutic strat-

egies for atherosclerosis and restenosis.

Acknowledgments

This work was supported in part by grants from the National

Heart, Lung, and Blood Institute (1PO1HL418638); the Uni-

versity of California Tobacco Related Disease Program

(IRT215); the Institute of Biological and Clinical Investigation

of Stanford University; and the Peninsula Community Foun-

dation. Dr Tsao is the recipient of a National Service Research

Award (1F328HL08779). Dr Cooke is the recipient of the

Vascular Academic Award from the National Heart, Lung,

and Blood Institute (1KO7HCO2660).

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Cooke

and Tsao Editorial Comment 655

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J P Cooke and P S TsaoIs NO an endogenous antiatherogenic molecule?

Print ISSN: 1079-5642. Online ISSN: 1524-4636Copyright 1994 American Heart Association, Inc. All rights reserved.

Avenue, Dallas, TX 75231is published by the American Heart Association, 7272 GreenvilleArteriosclerosis, Thrombosis, and Vascular Biology

doi: 10.1161/01.ATV.14.5.6531994;14:653-655Arterioscler Thromb Vasc Biol.

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