Changes in particle size of high density lipoproteins during incubation with very low density lipoproteins, cholesteryl ester transfer protein and lipoprotein lipase

Abstract

Previous reports have produced conflicting views of the effects of lipoprotein lipase (LPL) on the particle size distribution of high density lipoproteins (HDL). In this study we have investigated the changes in particle size of HDL promoted by the interaction of LPL, the cholesteryl ester transfer protein (CETP) and very low density lipoproteins (VLDL). When the plasma fraction of d < 1.21 g/ ml (containing all lipoprotein fractions) was incubated for 24 h with bovine milk LPL alone or with CETP alone, there was relatively little change in the particle size distribution of HDL. When both LPL and CETP were added to the lipoprotein mixture, there was a substantial reduction in the particle size of HDL. This reduction in HDL particle size was found to be a direct function of the concentration of CETP. It was also influenced by the concentrations of VLDL and LPL, although in these cases the relationships were complex. When mixtures of the plasma fraction of d = 1.006−1.21 g/ ml (this fraction includes low density lipoproteins and HDL but not VLDL) were supplemented with both LPL and CETP and incubated in the presence of varying concentrations of added VLDL, there was a progressive increase in the conversion of HDL into very small HDL particles of radius 3.7 nm as the concentration of VLDL triacylglycerol increased up to about 400 nmol/nml. However, further increases in the concentration of VLDL were accompanied by a progressive reduction in the formation of small HDL particles until, at higher VLDL concentrations, the effect was all but abolished. There was a similar enhancement in the formation of small HDL when LPL was added at low but not at high concentrations. These findings are consistent with the existence of two opposing processes. On the one hand there is likely to be a synergism between CETP and the non-esterified fatty acids (NEFA) released by LPL; this will favour a reduction in HDL particle size. On the other hand, the transfer of lipolysis products from VLDL to HDL may mask any such particle size reduction. The fact that the reduction in HDL particle size promoted by LPL, CETP and VLDL was found to be all but abolished by adding fatty acid-poor albumin to the incubation mixture is consistent with the proposition that NEFA are involved in the process. It also suggests, however, that the phenomenon may have little if any physiological significance.