Objective-The aim of this study was to investigate the molecular mechanism for changes in proteoglycan binding and LDL receptor affinity on two compositional changes in LDL that have been associated with atherosclerosis: cholesterol enrichment of the core and modification by secretory group IIA phospholipase A2 (sPLA 2 ) of the surface. Methods and Results-Transgenic mice expressing recombinant apolipoprotein (apo) B and sPLA 2 were generated.Recombinant LDL were isolated and tested for their proteoglycan and LDL receptor-binding activity. The results show site A (residues 3148-3158) in apoB100 becomes functional in sPLA 2 -modified LDL and that site A acts cooperatively with site B (residues 3359-3369), the primary proteoglycan-binding site in native LDL, in the binding of sPLA 2 -modified LDL to proteoglycans. Our results also show that cholesterol enrichment of LDL is associated with increased affinity for proteoglycans and for the LDL receptor. This mechanism is likely mediated by a conformational change of site B and is independent of site A in apoB100. Conclusion-Site

An initial event in atherosclerosis is the retention of lipoproteins within the intima of the vessel wall. Previously we identified Site B (residues 3359 -3369) in apolipoprotein (apo) B100 as the proteoglycan binding sequence in low density lipoproteins (LDLs) and showed that the atherogenicity of apoB-containing lipoproteins is linked to their affinity for artery wall proteoglycans. However, both apoB100-and apoB48-containing lipoproteins are equally atherogenic even though Site B lies in the carboxyl-terminal half of apoB100 and is absent in apoB48. If binding to proteoglycans is a key step in atherogenesis, apoB48-containing lipoproteins must bind to proteoglycans via other proteoglycan binding sites in the amino-terminal 48% of apoB. In vitro studies have identified five clusters of basic amino acids in delipidated apoB48 that bind negatively charged glycosaminoglycans. To determine which of these sites is functional on LDL particles, we analyzed the proteoglycan binding activity of recombinant human LDLs from transgenic mice or rat hepatoma cells. Substitution of neutral amino acids for the basic amino acids in Site B-Ib (residues 84 -94) abolished the proteoglycan binding activity of recombinant apoB53. Carboxyl-truncated apoB80 bound biglycan with higher affinity than apoB100 and apoB48. ApoB80 in which Site B was mutated had the same affinity for proteoglycans as apoB48. These data support the hypothesis that the carboxyl terminus of apoB100 "masks" Site B-Ib, the aminoterminal proteoglycan binding site, and that this site is exposed in carboxyl-truncated forms of apoB. The presence of a proteoglycan binding site in the aminoterminal region of apoB may explain why apoB48-and apoB100-containing lipoproteins are equally atherogenic.

The pathogenesis for atherosclerosis is still unclear, and several hypotheses have been articulated to explain the initiating events in atherogenesis. Although these hypotheses are by no means mutually exclusive, there is a growing body of recent evidence that has led to the concept that subendothelial retention of apolipoprotein B100-containing lipoproteins is the initiating event in atherogenesis. Subsequently, a series of biological responses to this retained material leads to specific molecular and cellular processes that promote lesion formation. The present review assesses some of the studies that support this concept.

Atherosclerosis is a multifactorial disease whose pathogenesis is still unclear. Mounting evidence, however, supports the concept that subendothelial retention of apoB100-containing lipoproteins is the initiating event in atherogenesis. Subsequently, a series of biological responses to this retained material leads to specific molecular and cellular processes that promote lesion formation.