Editorial
Lipoprotein(a) and Atherosclerosis
Shun IshibashiLphages. Together with the observation that apo(a) was deposited almost exclusively in the extracellular matrixes, theauthors speculated that apo(a) stimulates the phenotypictransformation of vascular SMCs. In this regard, impaired
activation of transforming growth factor-␤ (TGF-␤) wasproposed to explain the mitogenic effects of apo(a) on
SMCs.10 Because of its structural resemblance to plasminogen, apo(a) has inhibitory effects on the production ofplasmin by competing with plasminogen. Because latent
TGF-␤ is converted to its active form by means of plasmin,
the activation of TGF-␤ is suppressed in the presence ofincreased concentrations of apo(a). Active TGF-␤ has inhibitory effects on the proliferation and migration of SMCs.
Therefore, apo(a) may stimulate the proliferation and migration of SMCs through inhibiting TGF-␤ activation as a whole,thus contributing to the development of atherosclerosis.
Neither phenotypic transformation nor proliferation of SMCs
has, however, been directly demonstrated in the apo(a)transgenic mice.
In addition to the mitogenic effects of apo(a) on SMCs,apo(a) is believed to possess thrombogenic properties. For thereasons mentioned above, apo(a) is considered to inhibit
fibrinolysis by competing with plasminogen. Given that thispresumption is correct, mural thrombi, once generated, areexpected to be resistant to fibrinolytic degradation, resulting
in exaggerated responses of wound repair. However, compelling in vivo evidence for the thrombogenic properties ofapo(a) is lacking. It will be of great interest to test these
attractive, but still elusive, hypotheses in the apo(a) transgenic rabbits.
This animal model is also useful for investigating theproperty of association between apo(a) and rabbit apoB-100.
In humans, apo(a) is covalently associated with Cys-4326 of
apoB-100. In mice, the association of apo(a) with apoB-100
is not covalent because mouse apoB-100 lacks Cys-4326.11 Inrabbits, apo(a) is associated with rabbit apoB-100 more
tightly than with mouse apoB-100 because nondenaturing
polyacrylamide gel electrophoresis (PAGE) showed that mostof apo(a) was in Lp(a).7–9 However, the association might not
be covalent because apo(a) and apoB-100 migrated differently under nonreducing conditions of SDS-PAGE. Thisfinding is consistent with the fact that rabbit apoB-100 also
lacks Cys-4326. In apo(a) yeast artificial chromosome transgenic rabbits, as reported by Rouy et al,7 ⬇20% of plasma
apo(a) is covalently associated with rabbit apoB-100 via acysteine residue other than Cys-4326. It would also beinteresting to know which is more atherogenic, free apo(a) orLp(a). Introduction of the apo(a) transgene into human
apoB-100 transgenic mice did not significantly potentiate thedevelopment of aortic fatty lesions in the absence or presenceof the LDL receptor.5,6,12 Probably the atherogenic effects of

ipoprotein(a) [Lp(a)] is composed of 1 particle of LDL
attached to apo(a), a glycoprotein that has striking
homology to plasminogen and tremendous size heterogeneity owing to the presence of multiple repeats ofplasminogen-like kringle 4 modules (see References 1 and 2for a review). Since Lp(a) was discovered by Berg, numerous
cross-sectional and prospective studies have revealed associations between high plasma levels of Lp(a) and atherosclerotic vascular diseases, such as coronary heart disease andstroke (see References 1 and 3 for a review). Althoughseveral pathogenic roles for Lp(a) have been proposed, it has
not been completely elucidated how high Lp(a) levels areassociated with vascular diseases. Transgenic animal models
give invaluable means to provide answers to the intractablequestion of whether high Lp(a) causes atherosclerosis and notvice versa. Lawn and colleagues4 first reported the generationof apo(a) transgenic mice in 1992. Fatty streak lesion formation in the aortic sinus of these mice was substantially
increased compared with that of the nontransgenic controls
when fed an atherogenic diet. However, another laboratory
failed to reproduce the atherogenic effects of the overexpressed apo(a) with or without the overexpressed human
apoB-100, even though the same transgene was used.5,6 It
may be possible that a certain strain of mouse is resistant tothe atherogenic effects of apo(a). In this context, apo(a)transgenic rabbits have emerged as an alternative model inwhich the atherogenicity of apo(a) as well as that of Lp(a) canbe reasonably reevaluated. Recently, 2 lines of apo(a) transgenic rabbits have been reported independently.7,8 In thecurrent issue of Arteriosclerosis, Thrombosis, and VascularBiology, Fan and colleagues9 have demonstrated that transgenic expression of apo(a) in the liver aggravates atherosclerosis in rabbits fed a high-cholesterol diet.

See page 88
In contrast to mice whose atherosclerosis is normallylimited to the area of the aortic sinus, the sites of lesions in thecholesterol-fed rabbits were more widely distributed in theapo(a) transgenic line.9 Lesion formation was increased in allof the sites examined: the aorta, carotid, iliac, and coronary
arteries. This diffuse spatial distribution may be related to theincreased incidence of coronary, peripheral vascular, andcerebrovascular diseases in patients with increased Lp(a)levels. It is of particular interest that atherosclerotic lesions
were mostly composed of vimentin-positive immature ordedifferentiated smooth muscle cells (SMCs), not of macroFrom the Department of Metabolic Diseases, Faculty of Medicine,
University of Tokyo, Tokyo, Japan.
Correspondence to Shun Ishibashi, MD, PhD, Department of Metabolic Diseases, Faculty of Medicine, University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8655, Japan. E-mail ishibash-tky@umin.ac.jp(Arterioscler Thromb Vasc Biol. 2001;21:1-2.)© 2001 American Heart Association, Inc.

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apoB-100 may have overwhelmed the subtle effects of apo(a)and masked the difference.
New biological functions have been increasingly assignedto Lp(a) and/or apo(a): chemotactic activity on monocytes,
angiogenic effects because of resemblance to angiostatin,presence of platelet-activating factor acetylhydrolase activity,
enhancement of the expression of intercellular adhesion
molecule-1 on endothelial cells, and so forth. Their physiological relevance can be tested in vivo by using apo(a)transgenic rabbits.
Besides the mystery of bona fide functions of Lp(a),extreme polymorphism of apo(a) and huge variation of theplasma levels of Lp(a) (up to 1000-fold) have fascinatedmany clinical geneticists. The most polymorphic region ofapo(a) is the kringle 4 – encoding region. The first (type 1)
and last 8 (types 3 through 10) repeats of the kringle 4
tandem array are present in only 1 copy. The number oftype 2 kringle 4 sequences varies from 3 to 43 and is
responsible for the size heterogeneity.13 Ninety percent of
the variation in plasma levels is accounted for by theapo(a) gene. Forty percent to 70% of the variation isexplained by the size of the apo(a) isoform, which isinversely related to the plasma levels of Lp(a), which arelargely determined by the difference in the production ratein the liver. Moreover, there is a great difference in thedistribution of plasma levels of Lp(a) among different
ethnic groups. For example, African-Americans havehigher plasma levels of Lp(a) than do whites. The isoformconcentration relationship can be easily tested by generating transgenic animals with the use of constructs that aredesigned to express different isoforms.
More important, it remains unknown whether pharmacological intervention to lower Lp(a) levels inhibits the progression of atherosclerosis. Currently, sex steroid hormones andnicotinic acids are known to significantly decrease plasma
levels of Lp(a). Given the difficulties encountered in clinicalstudies, the apo(a) transgenic rabbit should be an excellent
animal model in which the antiatherogenic efficacy of thesecompounds can be tested preclinically.

References
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The Metabolic and Molecular Bases of Inherited Disease. New York,
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2. Hobbs H, White AL. Lipoprotein(a): intrigues and insights. Curr Opin
Lipidol. 1999;10:225–236.
3. Djurovic S, Berg K. Epidemiology of Lp(a) lipoprotein: its role inatherosclerotic/thrombotic disease. Clin Genet. 1997;52:281–292.
4. Lawn R, Wade DP, Hammer RE, Chiesa G, Verstuyft JG, Rubin EM.
Atherosclerosis in transgenic mice expressing human apolipoprotein(a).
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5. Mancini F, Newland DL, Mooser V, Murata J, Marcovina S, Young SG,Hammer RE, Sanan DA, Hobbs HH. Relative contributions of apolipoprotein(a) and apolipoprotein-B to the development of fatty lesions in
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artificial chromosome transgenic rabbits: lipoprotein(a) assembly withhuman and rabbit apolipoprotein B. J Biol Chem. 1998;273:1247–1251.
8. Fan J, Challah M, Shimoyamada H, Shiomi M, Marcovina S, Watanabe
T. Defects of the LDL receptor in WHHL transgenic rabbits lead to amarked accumulation of plasma lipoprotein(a). J Lipid Res. 2000;41:1004 –1012.
9. Fan J, Shimoyamada H, Challah M, Sun H, Marcovina S, Honda K,Watanabe T. Transgenic rabbits expressing human apolipoprotein(a)develop more extensive atherosclerotic lesions in response to a cholesterol-rich diet. Arterioscler Thromb Vasc Biol. 2000;21:88 –94.
10. Grainger D, Kemp PR, Liu AC, Lawn RM, Metcalfe JC. Activation oftransforming growth factor-␤ is inhibited in transgenic apolipoprotein(a)mice. Nature. 1994;370:460 – 462.
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RE. Reconstitution of lipoprotein(a) by infusion of human low density
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KEY WORDS: lipoprotein(a)transgenic 䡲 rabbit䡲 apolipoprotein(a) 䡲 atherosclerosis 䡲