Interaction between High Density and Low Density Lipoproteins during Uptake and Degradation by Cultured Human Fibroblasts

Abstract

High density lipoprotein (HDL) inhibited the binding (trypsin-releasable radioactivity), internalization (cell-associated radioactivity after trypsinization), and degradation (TCA-soluble non-iodide radioactivity) of ¹²⁵I-low density lipoprotein (¹²⁵I-LDL) by cultured normal human fibroblasts. At HDL:LDL molar ratios of 25:1 (protein ratios about 5:1), these parameters were reduced by about 25%. Unlabeled LDL was about 25 times more effective in reducing ¹²⁵I-LDL binding, implying that if HDL and LDL bind at common sites the affinity of HDL for these sites is very low or that the interaction is on some other basis. The fractional reduction in ¹²⁵I-LDL binding at a given HDL: ¹²⁵I-LDL ratio was independent of ¹²⁵I-LDL concentration and occurred equally with fibroblasts from a subject with homozygous familial hypercholesterolemia. Reciprocally, the binding, internalization, and degradation of ¹²⁵I-HDL were reduced by LDL. Preincubation of fibroblasts with HDL (or LDL) reduced the subsequent binding of ¹²⁵I-LDL (or ¹²⁵I-HDL) during a second incubation. In other studies HDL reduced the net increase in cell cholesterol content induced by incubation with LDL. HDL alone had no net effect on cell cholesterol content. These findings suggest that HDL reduces both the high affinity and the low affinity binding of LDL to human fibroblasts and that this in turn reduces the internalization and degradation of LDL. The effect of HDL on the LDL-induced changes in cell cholesterol content could be in part on this basis and in part on the basis of an HDL-stimulated release of cholesterol from the cells. These effects of HDL in vitro may be relevant to the negative correlations reported from in vivo studies between plasma HDL concentration and both body cholesterol pool size and the prevalence of clinically manifest atherosclerosis but further studies will be needed to establish this.