Search

Prolonged retention of LDL in focal, atherosclerosis-prone areas of arteries is a primary event in atherogenesis. To determine whether unrecognized LDL-binding proteins participate in this process, we generated a cDNA expression library from deendothelialized rabbit aorta, a model for early atherosclerosis that shows striking focal LDL retention in healing lesions. Library screening identified a previously unknown, highly conserved, 56kDa LDL-binding protein that we call atherin. Confocal microscopy of human arteries shows that atherin is present only in atherosclerotic lesions, not in normal intima. Within lesions, atherin is found both in the extracellular compartment and within foam cells. Essentially all extracellular atherin, as well as atherin within foam cells, co-localizes with LDL across the entire spectrum of human disease, from early lesions to advanced plaques. Our results suggest that focal arterial LDL accumulation may be initiated and maintained by binding between LDL and atherin, and that atherin may play a central role in atherogenesis by immobilizing LDL in the arterial wall.

In previous in vivo animal studies, we showed that low density lipoprotein (LDL) accumulated irreversibly at the edges of healing arterial lesions rather than being internalized and degraded. To see if similar LDL accumulation occurs in vitro, fibroblasts from normal and homozygous familial hypercholesterolemic (FH) subjects were incubated at 37 degrees C with 125I-LDL and 125I-methyl LDL; the latter is not recognized by any known LDL receptor. Normal fibroblast accumulation of LDL and methyl LDL (5 microg/ml) plateaued within 1 h at 200 and 100 ng/mg, respectively. With FH cells, both LDL and methyl LDL accumulation plateaued at 100 ng/mg. Lipoprotein accumulation by both cell types rose steeply at concentrations up to 15-25 microg/ml, and less so at higher concentrations. Except for degradation of LDL by normal cells, degradation was minimal, which indicated that much of the lipoprotein accumulation was unaccompanied by internalization. The accumulation of both lipoproteins by both cell types was greater at 37 degrees C than at 4 degrees C, and was inhibited between 43 and 75% by homologous unlabeled lipoprotein. To see if any accumulation was irreversible, cells were incubated with radiolabeled lipoproteins for 3 h (pulse), then with homologous unlabeled lipoproteins for up to 20 h (chase). About 50% of intact radiolabeled lipoprotein rapidly dissociated from cells into the medium in the first 4 h of the chase period. In contrast, between 4 and 20 h, most of the remaining intact LDL and methyl LDL appeared to be irreversibly bound, because it was released at a rate of only 0-1%/h. Thus, we conclude that, under the conditions studied, both reversible and irreversible non-internalized LDL binding play a major role in LDL accumulation by cultured cells.

99MTechnetium-labeled low density lipoprotein (99MTc-labeled LDL) was developed to detect atherosclerosis by external imaging with the gamma scintillation camera (Lees, et al. J. Nucl. Med. 1985. 26: 1056-1062; Lees, et al. Arteriosclerosis. 1988. 8: 461-470). The present study examined high affinity LDL receptor recognition and intracellular sequestration of 99MTc-labeled LDL by fibroblasts. There were no significant differences between 99MTc-labeled LDL and 125I-labeled LDL in binding parameters or percent inhibition of accumulation, which indicated that 99MTc labeling did not alter receptor recognition of LDL. At 4 degrees C the Kd (+SE) for 99MTc-labeled LDL and 125I-labeled LDL, respectively, was 1.52 +/- 0.24 and 1.45 +/- 0.14 micrograms/ml; Bmax (+/- SE) was 5.45 +/- 0.48 and 4.89 +/- 0.25 ng/well, respectively. Binding was saturated at about 2 micrograms/ml. The complete linearity of 99MTc-labeled LDL accumulation from 0-6 h and the positive slope from 6-24 h indicated that radiolabel that entered cells as 99MTc-labeled LDL was sequestered; pulse-chase experiments, which measured residual cell-associated radioactivity out to 24 h, also showed that radiolabel was trapped. Because radiolabel sequestration was essentially complete, and because 99MTc-labeled LDL was recognized by the LDL receptor equally as well as 125I-labeled LDL, it should be useful not only for imaging atherosclerosis, but also for quantitatively determining sites of utilization and degradation of LDL.

We have evaluated the efficacy of plant sterol preparations from two different sources and in two different physical forms in lowering the plasma cholesterol of a total of 46 patients with type II hyperlipoproteinemia when given in addition to appropriate diet therapy. In addition, the mechanisms of the hypocholesterolemic effect were investigated in 7 patients by a sterol balance technique. The maximal mean cholesterol lowering in response to any preparation was 12 percent, although it was much greater in some individual patients. Sterol balance data showed that plant sterols inhibit cholesterol absorption with maximal negative cholesterol balance in adults at a dose of 3 g/day of a tall oil sterol suspension. Interestingly, maximal plasma cholesterol reduction in the adult outpatients on this preparation was seen at the same dose level. Since the tall oil sterol suspension is relatively palatable and is poorly absorbed, it has potential value as an adjunct to dietary therapy in patients with mild hypercholesterolemia for whom long-term drug therapy is deemed advisable.

To determine whether damaged arterial wall selectively accumulates lipoproteins, normocholesterolemic rabbits were injected with human radiolabeled low density lipoproteins, high density lipoproteins, and/or albumin 24 hr to 12 weeks after balloon-catheter de-endothelialization of the abdominal aorta. When 125I-labeled low density lipoproteins and 99mTc-labeled albumin were injected simultaneously, the amount of 125I-low density lipoprotein present 24 hr later in abdominal aortas increased steadily, for several weeks, above the amount present at 24 hr in control animals. The increase correlated closely with the degree of re-endothelialization and correlated closely with the degree of re-endothelialization and reached an average maximum for the whole abdominal aorta of three times control when re-endothelialization was between 75 and 85% complete. By contrast, the amounts of 99mTc-albumin or 125I-labeled high density lipoprotein in balloon-damaged abdominal aortas, and the amounts of 125I-low density lipoprotein, 125I-high density lipoprotein, or 99mTc-albumin in undamaged thoracic aortas of injured animals showed no such increase. As early as 2 weeks after de-endothelialization, en face radioautographs made following injection of 125I-labeled low density lipoproteins revealed localized areas of greatest radioactivity around the leading edges of regenerating endothelial islands, broad areas of intermediate radioactivity corresponding to the de-endothelialized areas, and very like radioactivity in the re-endothelialized areas. This pattern occurred rarely with 125I-labeled high density lipoproteins and not at all with 125I-labeled albumin. The results suggest that low density lipoproteins are selectively accumulated by the healing rabbit aorta and that the accumulation is greatest in regions where the endothelium is actively regenerating.

Twenty-five patinets with well defined Type ii hyperlipoproteinemia were treated with a divided 15 g daily dose of colestipol, a bile acid sequestrant, for periods of up to 20 months. The patients were divided into 3 groups: Those with no obvious sequelae, those with arcus corneae, xanthomas, and/or xanthelasmas only, and those with atherosclerotic complications. Colestipol lowered plasma cholesterol in all 3 groups, but reduced it to normal or near-normal levels in only 9 of the 25 patients (36%). The response of plasma triglycerides was highly varible; the mean for each group was elevated by the drug. Colestipol was well-tolerated and its effect did not diminish with time. It is a useful drug in the treatment of hypercholesterolemia.