BioWorld International Correspondent

LONDON - The mystery of exactly how the blood vessel wall interacts with fat particles circulating in the blood in order to trigger the chain of events that leads up to atherosclerosis has been solved.

A team of Swedish scientists and their American collaborator have elucidated how the molecule that coats fat particles circulating in the blood sticks to molecules called proteoglycans, which form part of the blood vessel walls.

The discovery raises the prospect of developing drugs to prevent the interaction and stop the buildup of atherosclerosis. It also will reinforce the importance of advice to follow a low-fat diet and to exercise, for those at high risk of the disease.

Jan Boren, director of the Wallenberg Laboratory for Cardiovascular Research at Goteborg University in Goteborg, Sweden, told BioWorld International: "The majority of people in the Western world die from complications of cardiovascular disease. Now, for the first time, we understand how this disease is initiated. Furthermore, we have identified a potential target for drugs that would interfere with the interaction we have discovered."

Boren and his colleagues at Goteborg University, together with their collaborator at the Gladstone Institute of Cardiovascular Disease in San Francisco, report their findings in a June 13, 2002, letter to Nature titled "Subendothelial retention of atherogenic lipoproteins in early atherosclerosis." The first author of the paper is Kristina Skalen.

In an article in the same issue of Nature, titled "A cholesterol tether," Bart Staels of the Institut Pasteur de Lille and the Universite de Lille II in France, writes: "Therapies [for cardiovascular disease] that act directly on the arterial wall are needed, and Skalen and colleagues' results point to potential targets."

The Swedish/American group set out to test the hypothesis, first proposed about 50 years ago, that in order for atherosclerosis to begin, fat particles in the blood must become retained in the blood vessel wall. That is known as the response-to-retention hypothesis.

The type of circulating fat particle that contributes most to the development of atherosclerosis is known as a low-density lipoprotein (LDL). This has a core of cholesterol, surrounded by a shell of a protein called apolipoprotein B100 (apoB100). LDL normally circulates in the bloodstream but also moves in and out of the blood vessel wall. According to the response-to-retention hypothesis, atherosclerosis begins when LDL becomes stuck to some part of the blood vessel wall and is retained there.

ApoB100 is a large protein, 4536 amino acids long. Experiments carried out by the Swedish/American team identified a sequence of 10 amino acids within apoB100 that plays a crucial role in allowing the protein to interact with proteoglycans found in the blood vessel wall. Proteoglycans are comprised of both proteins and sugars. They form the extracellular matrix of the blood vessel walls that holds the smooth muscle cells together.

The team found that, while the sugars on the proteoglycans were negatively charged, most of the 10 amino acids in the key sequence of apoB100 were positively charged, thus allowing the two molecules to bind together.

To examine the interaction further, the researchers generated a series of genetically modified mice that overexpressed human recombinant LDL that had been modified so that the sequence of 10 amino acids was no longer positively charged, but neutral. That LDL was unable to bind to proteoglycans.

The mice were fed a high-fat diet for up to 20 weeks, after which their aortas were dissected and the extent of atherosclerosis measured. The researchers looked for the presence of fatty streaks - deposits of fat that arise from macrophages that have engulfed fat particles on the endothelial wall.

"Mice expressing normal human LDL had quite a lot of these fatty streaks," Boren said. "However, mice with the same concentration of LDL particles in their bloodstream, but where the particles had been modified so that they could not interact with proteoglycans, developed significantly fewer fatty streaks. This finding allows us to conclude that the interaction between LDLs and proteoglycans is the key to the initiation of atherosclerosis."

In several other experiments, the team ruled out several alternative explanations for their finding. They showed, for example, that the modified LDL is as susceptible to oxidation as normal LDL, and that the cellular response to the modified LDL by macrophages in the blood vessel wall was identical to that made to normal LDL.

Furthermore, to double-check their result, they enriched the particles of mutant LDL with molecules of apolipoprotein E, a protein related to apoB100, which also is capable of binding to proteoglycans. Mice that had been manipulated in that way experienced a similar buildup of fatty streaks in their aortas as those with normal LDL.

Staels posed the question of whether human subjects with mutations in the proteoglycan-interacting domain of apoB are protected from atherosclerosis. He suggested that it could be useful to look for such mutations in healthy octogenarians.

Next, Boren and his group plan to study the mechanisms that are responsible for retaining LDL once an atherosclerotic lesion has been formed, and to link the retention of the LDL to the activation of macrophages in the vessel wall.