Quantification of apo[a] and apoB in human atherosclerotic lesions

Lipoprotein[a] or Lp[a] is a cholesterol-rich plasma lipoprotein that is associated with increased risk for cardiovascular disease. To better understand this association we determined the amount of apo[a] and apoB as possible estimates for Lp[a] and low density lipoprotein (LDL) accumulation in atherosclerotic lesions and in plasma, from patients undergoing vascular surgery, using specific radioimmunoassays for apolipoprotein[a] and apolipoprotein B. Apo[a] and apoB were operationally divided into a loosely bound fraction obtained by extracting minced samples of plaque with phosphate-buffered saline (PBS), and a tightly bound fraction obtained by extracting the residual tissue with 6 M guanidine-HCl (GuHCl). We found that 83% of all apo[a] but only 32% of all apoB in lesions was in the tightly bound fraction. When normalized for corresponding plasma levels, apo[a] accumulation in plaques was more than twice that of apoB. All fractions of tissue apo[a], loosely bound, tightly bound, and total, correlated significantly with plasma apo[a]. However, no significant correlations were found between any of the tissue fractions and plasma apoB. If all apo[a] and apoB had been associated with intact Lp[a] or LDL particles, the calculated mass of tightly bound Lp[a] would actually have exceeded that of tightly bound LDL in five cases with plasma Lp[a] levels above 5 mg apo[a] protein/dl. When PBS and GuHCl extracts of lesions were subjected to one-dimensional electrophoresis, the major band stained for lipid and immunoblotted positively for apo[a] and apoB, suggesting the presence of some intact Lp[a] in these extracts. These results suggest that Lp[a] accumulates preferentially to LDL in plaques, and that plaque apo[a] is directly associated with plasma apo[a] levels and is in a form that is less easily removable than most of the apoB. This preferential accumulation of apo[a] as a tightly bound fraction in lesions, could be responsible for the independent association of Lp[a] with cardiovascular disease in humans.