Enhanced binding of phospholipase-A2-modified low density lipoprotein by human adipocytes

Abstract

Recognition of low density lipoprotein (LDL) by human adipocytes is not dependent on the classical LDL (apoprotein B–E) receptor. To assess whether LDL phospholipids have a role in adipocyte-LDL interactions, binding studies were carried out with human LDL modified with cobra venom phospholipase A₂ (PLA₂) and freshly isolated adipocytes and purified adipocyte plasma membranes prepared from surgical biopsies. LDL incubated with PLA₂ showed increased monoacylphospholipid content, decreased diacylphospholipid content, and increased anodic migration on agarose gel electrophoresis. LDL cholesterol, triglyceride, and protein content remained unchanged. Typically, modification of 16 and 47% of LDL phospholipids enhanced specific binding of ¹²⁵I-labelled LDL to plasma membranes progressively from 3.1 μg LDL bound/mg membrane protein (control) to 5.8 and 28.2 μg LDL bound/mg membrane protein, respectively. Nonspecific binding was not altered significantly. Excess unlabelled native LDL and high density lipoprotein (HDL₃) effectively inhibited binding of PLA₂-modified LDL. Freshly isolated adipocytes also showed enhanced binding and uptake of PLA₂-modified LDL (0.1 vs. 0.9 μg LDL/(10⁶ cells∙2 h), control vs. modified). The results demonstrate that alterations of LDL phospholipids significantly enhance LDL binding and suggest a regulatory role for phospholipids in lipoprotein–cell interaction. Furthermore, the results support the view that human adipose tissue may be involved in the metabolism of modified lipoproteins, in vivo.Key words: low density lipoprotein, adipocyte, phospholipase, lipoprotein receptors.