Three-fold effect of lovastatin treatment on low density lipoprotein metabolism in subjects with hyperlipidemia: increase in receptor activity, decrease in apoB production, and decrease in particle affinity for the receptor. Results from a novel triple-tracer approach.

To differentiate effects of lovastatin on low density lipoprotein (LDL) receptor activity from effects on LDL metabolic properties, LDL apolipoprotein B (apoB) turnover was studied in eight hyperlipidemic subjects during baseline and lovastatin treatment, in the latter case with LDL tracers isolated during both baseline (CLDL) and drug treatment (Rx-LDL) conditions. Lovastatin (40 mg/day) significantly lowered total plasma and LDL cholesterol levels (27% and 25%, respectively) as well as plasma triglyceride levels (30%). Using contemporaneous tracers (C-LDL before and Rx-LDL during treatment), lovastatin caused a modest increase in LDL fractional catabolic rate (FCR) (0.410+/-0.113 vs. 0.339+/-0.108 pools/day, P < 0.04 by paired t). The increase in LDL tracer FCR was higher when C-LDL tracer isolated during the untreated period was injected during lovastatin treatment (0.496+/-0.177 vs. 0.339+/-0.108 pools/day, P < 0.02). These in vivo studies in humans were confirmed by injecting LDL tracers from two patients into five guinea pigs. The C-LDL tracer was cleared consistently faster than the Rx-LDL tracer (0.082+/-0.018 vs. 0.057+/-0.015 pools/h, P< 0.001). The results demonstrate three important outcomes of lovastatin treatment in these subjects: LDL receptor activity increased by 49% (P < 0.02); LDL apoB production rate decreased by 17% (P < 0.03), and LDL particle in vivo affinity for the LDL receptor decreased by 15% (P < 0.01). The decrease in LDL particle affinity partially negated the expected effect of increased LDL receptors on LDL clearance. The present study provides an explanation for earlier observations by several investigators using contemporaneous tracers that treatment with HMG-CoA reductase inhibitors resulted in only modest increases in low density lipoprotein functional catabolic rate.

Radioiodination of low density lipoprotein initiates lipid peroxidation: protection by use of antioxidants.

It is now apparent that low density lipoprotein (LDL) is very susceptible to lipid peroxidation and that the resulting oxidized LDL has altered biological properties. Radiation, particularly of longer duration and lower intensities, initiates lipid peroxidation, yet radioiodination with 125I and 131I is a frequently used method to label LDL for biological studies. To test the possibility that this procedure alters the biological properties of LDL, native LDL was radioiodinated with 125I/131I using ICl to average specific activities of approximately 300 and approximately 100 cpm/ng protein, respectively. Lipid peroxidation was monitored by TBARS and conjugated diene formation. Biological properties were monitored by fibroblast and macrophage uptake of LDL as well as by rate of plasma clearance (FCR) in guinea pigs. 131I-labeled LDL showed enhanced indices of lipid peroxidation compared to 125I-labeled LDL and both were greater than native LDL. The FCR of 131I-labeled LDL was greater than that of 125I-labeled LDL (by 20-40%) and both increased progressively (by > 250%) when measured at 2, 6, and 13 days after iodination. The radioiodinated LDL samples were also more susceptible to pro-oxidant conditions. Thus, after exposure to Cu2+, 131I-labeled LDL showed greatly enhanced lipid peroxidation, decreased uptake by fibroblasts, increased uptake by macrophages and greatly accelerated FCR in guinea pigs. Exposure of LDL to 131I-labeled albumin produced similar changes. Protecting LDL with antioxidants such as BHT and ascorbate immediately after radioiodination generally ameliorated the adverse effects.