Lipoprotein(a) Link between Structure Angelo

M.

Scanu,

and Pathology

MD

It is now established that high plasma levels of Iigoprotein(a) are associated with an increased risk of atherosclerotic cardiovascular disease. However, the mechanisms underlying this increased risk have not been elucidated. Lipoprotein(a) represents a class of lipoprotein particles having a cholesteryl ester-rich low-density lipoprotein (LDL)-like structure with a protein moiety represented by apolipoprotein B,m covalently linked to upolipoprotein (a), the specific marker of lipoprotein(a). Lipoprotein(a) particles with a triglyceride-rich core have also been described. Apolipoprotein(a) is a glycoprotein conmining about 30% carbohydrates by weight, with a polypeptide chain highly polymorphic in size (300-700 kDa) and structurally similar to plasminogen. There appears to be LI relation between apo&oprotein(a) size and lipoprotein(a) species. From a number of studies, it is becoming apparent that lipoprotein(a) can transverse the endothelium and accumulate in the arterial initima either extra- or intracellularly. Immunochemical evidence has ulso indicated that apolipoprotein(a) in the artery wall is colocalized with fibrin(ogen), suggesting thclt this complex&on may have an &erogenic potential by promoting the transformation of resident macrophages into foam cells. This mighr also occur by the chemical modijication of lipoprotein(a) by the action of either oxygen-free radicals, malondialdehyde, or interactions with matrix components. These findings invite the speculation that much of the apolipoprotein B detected in atherosclerotic lesions is contributed by lipoprotein(a). The role that lipoprotein(a) size and density heterogeneity and apoligoprotein(a) polymorphism might phy in the intima accumulation of apolipoprotein B is not established. Ann Epidemiol f992;2:407-412. Lipoprotein (a), plasma lipoproteins, cardiovascular thrombosis, jibrinolysis.

KEY WORDS:

risk, atherosclerosis,

INTRODUCTION Lipoprotein(a) or Lp(a), discovered more than 25 years ago by Khre Berg (l), has become the focus of attention only relatively recently following the elucidation of its structural features, which has led to the suggestion that this lipoprotein may have atherogenic and thrombogenic potentials (2-4). 1 provide an overview of the current knowledge of the structure and biology of Lp(a) and also examine the experimental evidence favoring the notion that this lipoprotein must be considered a cardiovascular pathogen.

LIPOPROTEIN(a) Lp(a) represents a class of lipoprotein particles that have in common a protein moiety consisting of apolipoprotein Bloo (apoBlo0) disulfide linked to apolipoprotein(a) or ape(a), the specific marker of Lp(a). The variable content in lipid and apoB,oo-ape(a)

From the Departments of Medicine, Biochemistry, and Molecular Biology, University of Chicago, Chicago, IL. Address reprint requests to: Angelo M. Scanu, MD, University ofchicago, 5841 S. Maryland (Box 231), Chicago, IL 60637. Actepted September 4, 1991. 0 1992 Elsevier Science Publishing Co.,

Inc.

1047-2797192/$05.00

408

SCFGW

AEP Vol. 2,No.4 july 1992:407-412

LIPOPROTEIN(a)

stoichiometry (usually 1 : 1 on a molar basis but occasionally 1 :2) accounts for the marked size and density heterogeneity of Lp(a). This heterogeneity can occur among individuals and within the same individual (for recent reviews see [2, 4, 51). The better

recognized

Lp(a)

particles

are those

having

a low-density

lipoprotein

(LDL)-

like structure and thus a cholesteryl ester-rich core. I refer to them as CE-Lp(a). additional species of Lp(a), comparatively less abundant in the plasma, is called Lp(a)

because

the apoBiae-ape(a)

tein (VLDL)-like CE-Lp(a) remains

complex

triglyceride-rich to be established.

is affiliated

with very-low-density

An TG-

lipopro-

particles (2, 5-7). Their precise correlation Due to the important density heterogeneity,

with it is

not possible to assign precise density limits to Lp(a) particles and thus establish a universal method of ultracentrifugal separation for them. Usually, the isolation of pure Lp(a)

requires

properties

APOLIPOPROTEIN(

a combination

of methods

that take advantage

of the unique

structural

of this class of lipoproteins.

a)

Ape(a)

is a glycoprotein

that contains

a high percentage

of carbohydrates

(about

30%

by weight). The polypeptide chain of ape(a) has a remarkable size heterogeneity that varies from 300 to 800 kDa due to differences in the number of kringle 4 domains (from

l-40).

The

size polymorphism

of the ape(a)

isoforms

is under

the alleles of the ape(a) gene located in the long arm of chromosome to the plasminogen gene (2, 3). Each of the kringle 4 domains glycosylated than the single kringle 4 of plasminogen, tant structural and functional significance. Compared

the control

of

6 (q26-27) next of ape(a) is more

a fact that might to plasminogen,

have imporape(a) lacks

kringles 1, 2, and 3 and has a single copy of kringle 5 (exhibiting about 90% homology to that of plasminogen) and a protease region that has the same catalytic triad as that of plasminogen.

An

important

difference

between

ape(a)

and

plasminogen

is the

absence in the former of a suitable activation site. In consequence, ape(a) is a giant zymogen incapable of inducing clot lysis (8), contrary to the active enzyme plasmin that is generated from the activation of plasminogen. Recent studies established that the ape(a) size polymorphism relates to Lp(a) d ensity in that the low-molecularweight species of ape(a) has a preferential affinity for the light Lp(a) fraction and vice versa. It follows that in every instance, only one copy of the apoBieo-ape(a) complex is affiliated

APO( a)Biee-APO(

with

each

Lp(a)

particle,

at least in the case of CE-Lp(a).

a) COMPLEX

In the circulation, ape(a) in Lp(a) is all bound to apoBioe. Free ape(a) lacks lipophilic properties (6) and, being hydrophilic, it can circulate in the blood in a lipid-free form. However, very small amounts of ape(a) are usually detected in the plasma but their artifactual derivation cannot be excluded. It is also possible that lipid-free ape(a) may be more prone to the action of proteolytic enzymes in the plasma with formation of proteolytic fragments amenable to ready removal from the circulation. In general, ape(a) is likely to exert its action when linked to apoBiee. Thus, the latter protein is likely to have an influence on ape(a) function by ensuring a “proper” structural configuration.

AEP Vol. 2, No. 4

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LIPOPROTEIN (a)

PHYSIOLOGIC CONSIDERATIONS The organ that synthesizes Lp(a) in adults is the liver. Observations in cell culture and ex viva studies suggest that apoBrae and ape(a) are synthesized separately, then linked together and assembled into a lipoprotein particle prior to secretion. The real nature of nascent Lp(a) has not been well defined, as results from cell culture studies are not amenable to direct extrapolation to the living system. The factors that regulate the rates of synthesis and secretion of Lp(a) are also not established (2-4). It has been suggested that CE-Lp(a) and TG-Lp(a) may be under the control of different regulatory events and be dependent on the flux of cholesterol and triglycerides in the hepatocyte. The factors that control the levels of Lp(a) in the plasma are also not well established. A partial influence by the ape(a) gene has been suggested; however, it is likely that a number of post-transcriptional events, including intravascular remodeling steps, might also be operative. The role of catabolism on plasma Lp(a) levels is controversial (2-4). However, based on the majority of information gathered from cell culture systems and from in viva turnover studies, the LDL receptor that is key in the uptake of apoB1001eo-containing particles is unlikely to be important in Lp(a) degradation. This appears also to be true for the scavenger receptor, which may have a role only in pathologic processes involving chemically modified Lp(a) (9). Moreover, the participation of a nonreceptor-mediated pathway in the clearance of Lp(a) from plasma has been noted in recent studies ( 10). This issue is analyzed when I discuss Lp(a) as a pathogen.

ATHEROSCLEROTIC POTENTIAL OF LIPOPROTEIN(a) Only high plasma levels of Lp(a), that is, above 25 to 30 mg/dL of total Lp(a) or 5.5 to 7.0 mg/dL of Lp(a) protein, have been implicated in the cardiovascular pathogenicity of Lp(a) (2-4). The role of “normal” levels of Lp(a) remains to be established. The current experimental evidence derived both from cell culture and from in viva turnover studies, as well as the finding that intact Lp(a) can transverse the endothelium permitting Lp(a) to accumulate in the extracellular space (11, 12), suggests that the following series of events may be responsible for pathology to occur: (1) Lp(a) leaves the bloodstream and accumulates in the intima space of the artery wall; (2) Lp(a) undergoes chemical modification by interacting with oxygen-free radicals, proteoglycans, glycosaminoglycans, or other matrix components as well as fibrin; and (3) chemically modified Lp(a) is taken up by the scavenger receptor of resident macrophages, which are then transformed into foam cells, a first step toward the atherosclerotic process. The recent observations of anti-Lp(a)-reacting material in the intima of blood vessels as well as the concentration of large amounts of fibrin-bound Lp(a) in the area of the atherosclerotic plaque (11-13) are supportive of this hypothesis. However, this might be an oversimplistic view, since Lp(a) could act as a chemical reactant at the level of the endothelial cells of the arterial wall and could set in motion humoral reactions that may contribute to the atherogenic process. Moreover, one has to be reminded that LDL particles have a well-documented atherogenic potential and that LDL and Lp(a) might act in a synergistic way.

410

AEP Vol. 2,No.4 1992:407-412

SCiXlU

July

LIPOPROTEIN(a)

THROMBOGENIC

POTENTIAL

Based

OF LIPOPROTEIN(a)

on structural

considerations,

Lp(a)

may compete

with

the functions

of plas-

minogen in the fibrinolytic system (2-4, 14, 15). Ample support for this hypothesis has come from in vitro studies showing that Lp(a) competes for the binding of plasminogen to fibrin(ogen) and fragments and intereferes with the process of activation of plasminogen to plasmin. There is also a delay of clot lysis in the presence of Lp(a). Moreover,

Lp(a)

endothelial formation

has been

shown

to compete

for the binding

of Lp(a) over plasminogen, fibrin formation thrombogenic state will ensue. This imbalance the whole plasma structural alteration thrombogenesis

because it might or a dysfunction

also deserves

GENERAL CONSIDERATIONS OF LIPOPROTEIN(a) The

ON

epidemiologic

independent evidence

of plasminogen

to the

cells, platelets, and macrophages. Taken together, this experimental inmay indicate that whenever on a tissue site there is a functional imbalance

careful

THE

evidence

only occur at focal of the endothelium.

arterial sites, attending The role of TG-Lp(a)

ATHEROTHROMBOTIC

is strongly

in the previous

a in

inquiry.

supportive

risk factor for atherosclerotic examined

would prevail over fibrinolysis and a might not be detectable on the level of

of the

cardiovascular

two sections,

ROLE

notion

disease

that

Lp(a)

(ASCVD).

it is apparent

that

is an

From the

Lp(a)

has the

necessary structural characteristics for participating in both atherogenic and thrombogenie processes whenever it is “in excess.” However, based on the views examined in the previous

two sections,

consideration

must also be given to the status of endothelial

function in terms of transport and permeability. For instance, one could envisage a scenario where in the presence of severe endothelial artery injury or dysfunction, Lp(a) may readily enter the arterial wall regardless of plasma Lp(a) levels or plasmatissue gradient when

and accumulate

the plasma

recent

studies

levels

showing

in the subendothelial

of Lp(a) that

are “normal.”

patients

normal plasma levels of Lp(a) (16). corroborated by future studies, plasma tors of existing

atherothrombotic

with

intima

In keeping

three-vessel

of the arterial with

coronary

wall even

this hypothesis disease

can

are have

If these preliminary observations were to be Lp(a) levels might not be always good indica-

disease.

It also follows

that these false-negative

cases

may represent a confounding factor in the analysis of epidemiologic results. The same considerations might apply to black subjects who, although having plasma levels of Lp(a) much higher than those in white subjects, are thought to experience a similar incidence

LIPOPROTEIN(a)

of ASCVD.

IN PREVENTIVE

CARDIOLOGY

The emerging notion that Lp( a) is a cardiovascular pathogen has stimulated questions about the need of determining plasma Lp(a) levels in the general population, considering the fact that this parameter cannot be estimated from the values of total and LDL cholesterol. Usually plasma levels of Lp(a) do not vary significantly throughout life except for the emergence of disease states, particularly those affecting the liver, kidney, and thyroid gland. Credible measurement of plasma Lp(a) can also be carried out in children. In practice, until techniques are standardized and enter the general

AEP Vol. 2, No. 4

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SCXIU LIPOPROTEIN (a)

July1992: 407-412

chemistry laboratory, it would be best to limit the measurement of plasma Lp(a) to subjects with a personal and/or family history of ASCVD or other dyslipoproteinemias. For instance, subjects with heterozygous -familial hypercholesterolemia who present with ASCVD have been shown to have elevated plasma levels of Lp(a) (17). Similarly, the association of high plasma Lp(a) levels and low plasma high-density lipoprotein concentration is often attended by premature ASCVD. On these premises, in the Lipid Clinic of the University of Chicago, we determine plasma Lp(a) levels in any normo- or hypercholesterolemic subject presenting with a personal or family history of ASCVD. In terms of Lp(a) subspecies and ape(a) isoforms, we are not yet in the position of recommending these analyses on a clinical level, since their usefulness above that of total plasma Lp(a) has not been clearly established. This, however, must remain an open question since the atherothrombogenic potentials of Lp(a) may differ among subjects with single-band and double-band ape(a) phenotypes.

THERAPEUTIC

APPROACHES

TO LIPOPROTEIN(a)

Whereas, by a judicious use of diets alone or a combination of drugs, it is now possible to readily control the majority of dyslipoproteinemias, this is not the case for Lp(a). Drugs of the statin family, gemfibrozil, probucol, and ion-exchange resins have had little effect on the levels of plasma Lp(a) (18). Nicotinic acid or niacin, in dosages up to 3 to 4 g daily, can effect a 20 to 30% reduction of plasma Lp(a) but only in subjects with high levels of Lp(a) and not in all of them (19). Considering that niacin treatment does not result in a normalization of the plasma Lp(a) levels and the potential side effects of a long-term use of this drug, caution must be used. At this time, no drugs that have been designed take into account the structure and biology of Lp(a). One may speculate that agents affecting the linkage between apoBle0 and ape(a) would be beneficial, since Lp(a) would be transformed into LDL particles amenable to management while the released lipid-free ape(a) might undergo proteolytic fragmentation by plasma and tissue enzymes. While waiting for the availability of a specific and effective pharmacology for Lp(a), it is advisable to direct the efforts at correcting all of the correctable risk factors by an appropriate balancing of diet and exercise and adding hypolipidemic agents whenever dictated by the underlying dyslipoproteinemia. If the suggested atherogenic role of oxidized Lp(a) can be proved, the use of an antioxidant drug like probucol may be indicated. The original work by the author and his colleagues mentioned in this review was supported by US Public Health Service-National 18577. Ms. Sue Hutchison provided

Heart, invaluable

Lung, and Blood Institute help in preparing

program project

grant

this manuscript.

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