The build-up of cholesterol in the walls of arteries is a hallmark of atherosclerosis. Work with transgenic mice has revealed a specific interaction through which cholesterol deposition is initiated.
Atherosclerosis is a disease of the arterial wall that is characterized by chol-esterol accumulation and culminates in potentially life-threatening conditions such as heart attack, stroke and angina. How the process starts is poorly understood. In their paper on page 750 of this issue, however, Skålén and colleagues1 show that at least part of the explanation lies in a highly specific interaction between proteoglycans, which are components of the artery wall, and a constituent of lipoproteins called apoB.
The atherogenic process starts at an early age with the deposition in blood-vessel walls of lipids such as cholesterol, derived from lipoproteins circulating in the bloodstream, which leads to the formation of the characteristic 'fatty streaks'. Inflammatory white blood cells congregate at these damaged areas through their interaction with adhesion molecules expressed by cells in the endothelial layer, which lines the inside of blood vessels. This can then set off a cascade of inflammatory processes and further lipid deposition, leading eventually to full-blown atherosclerosis with plaque formation in the artery wall. Atherosclerosis is thus viewed as a chronic inflammatory disease of the blood-vessel wall2,3. An early event — the development of arterial damage and fatty deposits — is the capture, by cells called macrophages, of lipoproteins retained in the arterial wall, resulting in the formation of lipid-laden 'foam cells'. Although it has not been shown experimentally, the 'response-to-retention' hypothesis proposes that the retention and modification of these lipoproteins in the arterial lining is one of the initiating events that triggers the inflammatory response4.
Among the many risk factors for cardiovascular disease are raised concentrations of blood cholesterol, especially cholesterol transported by low-density lipoproteins (LDLs) containing the protein apoB100 (an apolipoprotein). Results from clinical trials in which concentrations of cholesterol in the blood stream were pharmacologically lowered established LDL cholesterol unequivocally as a culprit in atherosclerosis. But what triggers the accumulation of cholesterol-containing lipoproteins in the sub-endothelial space of the arterial wall? As long ago as 1949, Mogen Faber5 suggested that proteoglycans are involved — these are components of the extracellular matrix that are produced, for instance, by smooth-muscle cells that have migrated into the plaque. But because of a lack of suitable animal models, the effects of an interaction between proteoglycans and lipoproteins on atherogenesis have not been amenable to experimental testing in vivo — until now.
Skålén et al.1 have analysed the susceptibility to atherosclerosis of transgenic mice carrying human apoB100 genes with a range of mutations that yield proteins with reduced affinity for proteoglycans6. They show that, when fed a diet that promotes atherogenesis, these mice develop plaques more slowly than control mice expressing the normal human apoB100 protein. The LDL particles from transgenic mice carrying normal and mutated human apoB100 were transported similarly across the endothelial lining of the wall and showed the same susceptibility to oxidation, inflammatory properties and capture by macrophages. So Skålén et al. conclude that the retention of apoB100 lipoproteins by proteoglycans immediately beneath the endothelial layer is a central event in early atherogenesis.
The strengths, as well as the limitations, of this study lie in the fact that Skålén and colleagues tackled only one particular facet of atherosclerosis. In reality, many factors — both genetic and environmental — are likely to contribute to the disease. Not least, it can occur in the absence of raised levels of LDL cholesterol. A crucial question, then, is whether the interaction between apoB100 and proteoglycan is a general initiating mechanism for atherosclerosis, and it would be instructive to test its contribution to atherosclerosis induced by alterations in immune and inflammatory status. One approach would be to compare the susceptibility to atherosclerosis of normal and mutant human apoB100 transgenic mice cross-bred with mouse models of atherosclerosis caused by inflammation7.
Skålén et al.1 also show that other apolipoproteins, such as apoE, which is found in triglyceride-rich and remnant lipoproteins, can substitute for apoB100 in mediating the retention of lipoproteins in the arterial wall. Increased levels of triglyceride-rich remnant lipoproteins are often found in the serum of patients suffering from insulin resistance and type II diabetes, as well as in people carrying particular variants of apoE. In such individuals, it could be that the interaction between apoE and proteoglycans is the greater contributor to the initiation of atherosclerosis.
The findings of Skålén et al. illuminate one molecular basis for lipoprotein retention in the arterial wall. But other mechanisms of atherogenesis — which might or might not depend on lipoprotein retention — could also be operating at the same time. Even given similar levels of LDL cholesterol, not every person who develops atherosclerosis does so to the same extent. Moreover, in a given individual, atherosclerotic plaques do not develop at an equal rate and are not evenly distributed throughout the vasculature. Contributing factors might include regional disturbances of blood flow or alterations in proteoglycan structure. The raised LDL concentrations due to overexpression of apoB100 in Skålén and colleagues' transgenic mice might induce an inflammatory response, resulting in increased proteoglycan expression and congregation of white blood cells at susceptible locations. Local inflammation at such sites would intensify the modification of LDL by activated cells of the arterial wall.
Whether lipoprotein retention and modification are involved in the ensuing course of atherosclerosis also remains an open question. Atherosclerotic plaques did form in mice with proteoglycan-binding-deficient human apoB100, albeit later than in mice expressing the normal form, which would seem to show that factors other than direct LDL retention by proteoglycans dominate after atherogenesis has been initiated. A major contributory factor may be lipoprotein lipase, an enzyme that is secreted by macrophages in the artery wall and provides a high-affinity bridge between lipoproteins and proteoglycans8. It will be interesting to compare plaque development in normal and mutant apoB100 transgenic mice over a longer period. And are human subjects with mutations in the proteoglycan-interacting domain of apoB protected from atherosclerosis? It could be productive to search for these mutations in healthy octogenarians.
Even though it is clear that decreasing the concentration of cholesterol reduces the incidence and degree of atherosclerosis, most patients suffering from the disease still die from cardiovascular complications. Therapies that act directly on the arterial wall are needed, and Skålén and colleagues' results point to potential targets. One might consider, for example, using proteoglycan mimics that interfere with apoB100 retention in the wall. But great care and precision would be needed. Proteoglycans have other physiological functions that might be compromised by such an approach, and different proteoglycans might have different effects on lipoprotein retention and thus in atherogenesis9. A complementary strategy would involve modulating the ability of LDLs to interact with proteoglycans by altering their composition and particle size. Given these new findings, measurements of the ability of LDL to bind to proteoglycans might produce a helpful diagnostic test, and decreasing the strength of the interaction would be a reasonable therapeutic approach.