By David N. Leff
Atherosclerosis, the trigger of heart disease, is a process that gradually narrows coronary arteries until growing atherosclerotic plaque ruptures, which immediately dams up arterial blood flow, and heart attack ensues.
Last month, atherosclerosis researcher MacRae Linton, at Vanderbilt University in Nashville, Tenn., noted that the NIH-sponsored National Cholesterol Education Program and Adult Treatment Panel highlighted this spectrum of risk factors, described as ¿the metabolic syndrome.¿ It offered guidelines enabling physicians to better manage hypercholesterolemia and hyperlipidemia ¿ prime perpetrators of atherosclerosis and coronary disease.
¿There¿s been a big interest concerning this metabolic syndrome,¿ Linton observed, ¿but it¿s still poorly defined.¿ As a salient example of its scope, he singled out the less-than-obvious link between diabetes and atherosclerosis:
¿Among the most important features of that syndrome,¿ Linton pointed out, ¿was that it elevated diabetes to the level of being a coronary heart disease equivalent. So diabetic patients are to be treated as if they have as much risk as individuals who are already known to have coronary disease. Their risk factors need to be managed more aggressively. That¿s because a person with diabetes, without known coronary disease, has the same risk of a heart attack as somebody else who¿s already had evidence of a coronary or any other type of atherosclerosis.
¿The interest in insulin,¿ he continued, ¿goes back to observations that individuals with adult-onset diabetes, Type II ¿ an insulin-resistant state ¿ have a dramatic increase in their risk of coronary disease. In juvenile diabetes, Type I, there¿s a complete absence of insulin production; patients have to take insulin to survive. In the adult-onset disease, Type II, there are initially normal levels of insulin, but they don¿t effectively lower blood sugar.¿
Mice Model Atherosclerosis, Plaque And All
Linton is co-senior author of a paper in today¿s just-released June 2001 issue of Nature Medicine. It¿s titled ¿Lack of macrophage fatty-acid-binding protein aP2 protects mice deficient in apolipoprotein E against atherosclerosis.¿ The article¿s counterpart senior author is molecular geneticist Gvkhan Hotamisligil, at Harvard School of Public Health in Boston.
¿The overall finding we report,¿ Linton told BioWorld Today, ¿is that the fatty-acid-binding protein aP2 plays an important role in atherosclerosis. More specifically,¿ he went on, ¿that macrophage expression of aP2 promotes atherosclerotic disease. And mice with their aP2 gene knocked out were protected from developing atherosclerosis. These KO animals were more insulin sensitive, which implicated the aP2 gene in insulin resistance. And because of insulin resistance in the metabolic syndrome, it suggested a potential role for aP2 in atherosclerosis. Then we found that aP2 is also expressed by macrophages, which are key cells in the initiation and progression of atherosclerosis.
¿What this paper shows for the first time,¿ Linton observed, ¿is that macrophage expression of aP2 promotes foam cell formation ¿ independent of effects on insulin resistance, cholesterol levels or glucose metabolism.
¿Foam cells are lipid or fat, cholesterol-loaded macrophages,¿ he explained. ¿One of the earliest events in atherosclerosis is the recruitment of macrophages to the artery wall, where they gobble up modified lipoproteins. When they do that they become engorged with cholesterol esters ¿ cholesterol molecules with fatty acids stuck to them. They look foamy with their lipid droplets, and are the first histologically recognizable stage of atherosclerotic disease. AP2 suggests a potential link between the artery wall and some of the other features of that metabolic syndrome risk-factor cluster.¿
Harvard¿s Hotamisligil picked up on this connection: ¿We¿re interested in how this cluster of diseases is occurring simultaneously,¿ he told BioWorld Today. ¿The bigger question is what is the link between these disorders? How does the metabolic syndrome contribute to heart disease? Is it metabolic problems plus inflammatory problems, occurring simultaneously?
¿These are the questions,¿ he went on, ¿that led us to develop this mouse model of atherosclerosis. We came at it initially from the diabetes dyslipidemia, insulin-resistance angle. To generate these KO mice,¿ Hotamisligil recounted, ¿we removed the entire aP2 gene from early embryos. Then we intercrossed these mice ¿ the ones missing aP2 ¿ with strains that are missing apolipoprotein E, which has a key role in cholesterol transport. It¿s actually a model of atherosclerosis in mice. They develop spontaneous hypercholesterolemia and atherosclerotic plaque, even on a normal fat-content diet of mouse chow.
¿The same aP2 gene is present in humans, as in mice,¿ Hotamisligil pointed out, ¿and expressed in a very similar way ¿ in fat tissue and in macrophage cells. So there¿s a reasonable extrapolation from mice to people in experimentation with possible clinical therapy. Short of the functional data, all of the other aspects of the regulation of the gene are very similar between humans and mice.¿
Patent-Pending Rodents Spark Drug Design
He compared his novel mice with two standard mouse models ¿ the ob/ob obese mice and the db/db diabetic models. ¿Both are mutations that result in obesity,¿ he said. ¿They are mutations that prevent complications of obesity. They are predisposing mutations, whereas ours are protective mutations, with very high utility in terms of drug development purposes.¿ Harvard has applied to patent the co-authors¿ atherosclerosis mouse models.
¿A very straightforward drug,¿ Hotamisligil suggested, ¿would be something like a small molecule, because aP2 is a small protein. Its structure is very well characterized. It has a small binding pocket that binds to fatty acids. So if one were to design synthetic molecules that bind to this pocket, and prevent other normal things from binding ¿ just like an enzyme ¿ it could constitute a very powerful inhibitor of the gene, and therefore act like the equivalent of the genetic mutation in the mice.¿ He concluded: ¿We¿re working on that now.¿
Linton surmises that a putative therapeutic against atherosclerosis might ¿possibly¿ treat diabetes as well. ¿That¿s what¿s appealing about this work,¿ he said. ¿It has the potential to affect both insulin resistance and atherosclerotic disease through one target ¿ if there were ways to reduce expression of aP2, or block its actions.¿