Lipolytic remnants of human VLDL produced in vitro: effect of HDL levels in the lipolysis mixtures on the apoCs to apoE ratio and metabolic properties of VLDL core remnants

To determine the role of high-density lipoprotein (HDL) as an acceptor of l ipolytic surface remnants of very low density lipoprotein (VLDL) in the met abolism of VLDL core remnants, we examined the effect of HDL levels in the VLDL Lipolysis mixture on 1) the morphology and the apoCs to E ratio in VLD L core remnants and 2) the metabolic properties of VLDL core remnants in hu man hepatoma cell line HepG2 and human hepatocytes in the primary culture. Normolipidemic VLDL was lipolyzed in vitro by purified bovine milk lipoprot ein lipase (LpL) in a lipolysis mixture containing a physiologic level of V LDL and albumin (30 mg VLDL-cholesterol (CH)/dl and 6% albumin) in the abse nce and presence of either a low HDL level (VLDL-CH:HDL CH = 3:1) or a high HDL level (VLDL-CH:HDL-CH = 1:4). Lipolysis of VLDL in either the absence or presence of HDL resulted in the hydrolysis of >85% of VLDL-triglycerides (TG) and the conversion of VLDL into smaller and denser particles. In the absence of HDL, heterogeneous spherical particles with numerous surface ves icular materials were produced. In the presence of low or high HDL, spheric al particles containing some or no detectable vesicular surface components were produced. The apoCs to apoE ratios, as determined by densitometric sca nning of the SDS polyacrylamide gradient gel, were 2.89 in control VLDL and 2.27, 0.91, and 0.22 in VLDL core remnants produced in the absence and in the presence of low and high HDL levels, respectively, In vitro lipolysis o f VLDL markedly increased binding to HepG2 cells at 4 degrees C and interna lization and degradation by human hepatocytes in primary culture at 37 degr ees C. However, the HDL-mediated decrease in the apoCs to apoE ratio had a minimal effect on binding, internalization, and degradation of VLDL core re mnants by HepG2 cells and human hepatocytes in primary culture. In order to determine whether HepG2 bound VLDL and VLDL core remnants are deficient in apoCs, I-125-labeled VLDL and VLDL core remnants were added to HepG2 cultu re medium at 4 degrees C. The bound particles were released by heparin, and the levels of I-125-labeled apoCs and apoE, relative to apoB, in the relea sed particles were examined. When compared with those initially added to cu lture medium, the VLDL and VLDL core remnants released from HepG2 cells had a markedly increased (113%) level of apoE and a reduced (30-39%), but not absent, level of apoCs. jlr We conclude that apoCs, as a minimum structural and/or functional component of VLDL and VLDL core remnants, may not have a n inhibitory effect on the binding of VLDL or VLDL core remnants to hepatic apoE receptors.