The mechanism for the assembly of HDL with cellular lipid by ABCA1 and helical apolipoprotein was investigated in hepatocytes. Both HepG2 cells and mouse primary culture hepatocytes produced HDL with apolipoprotein A-I (apoA-I) whether endogenously synthesized or exogenously provided. Probucol, an ABCA1 inactivator, inhibited these reactions, as well as the reversible binding of apoA-I to HepG2. Primary cultured hepatocytes of ABCA1-deficient mice also lacked HDL production regardless of the presence of exogenous apoA-I. HepG2 cells secreted apoA-I into the medium even when ABCA1 was inactivated by probucol, but it was all in a free form as HDL production was inhibited. When a lipid-free apoA-I-specific monoclonal antibody, 725-1E2, was present in the culture medium, production of HDL was suppressed, whether with endogenous or exogenously added apoA-I, and the antibody did not influence HDL already produced by HepG2 cells. We conclude that the main mechanism for HDL assembly by endogenous apoA-I in HepG2 cells is an autocrine-like reaction in which apoA-I is secreted and then interacts with cellular ABCA1 to generate HDL. -Tsujita, M., C-A. Wu, S. AbeDohmae, S. Usui, M. Okazaki, and S. Yokoyama. On the hepatic mechanism of HDL assembly by the ABCA1/apoA-I pathway. J. Lipid Res. 2005. 46: 154-162.

Objective-Regulation of plasma cholesteryl ester transfer protein (CETP) concentration was studied in lecithin-cholesterol acyltransferase (LCAT)-knockout mice. Methods and Results-LCAT-knockout mice were cross-bred with CETP transgenic mice. The offspring (nϭ63) were classified for LCAT genotype and plasma CETP levels (no CETP, low CETP, and high CETP). High density lipoprotein (HDL) decreased as LCAT decreased in each CETP-level group. In the lcat(ϩ/ϩ) and lcat(ϩ/Ϫ) mice, plasma CETP varied from 0 to 30 g/mL, whereas it was Ͻ10 g/mL in the lcat(Ϫ/Ϫ) mice. HDL cholesterol and phospholipid decreased and HDL triglyceride and apolipoprotein B increased in CETP in the lcat(ϩ/ϩ) and lcat(ϩ/Ϫ) mice, whereas there was no difference in HDL between low and high CETP. An effect of CETP on HDL was not detected in the lcat(Ϫ/Ϫ) mice because of the absence of mature HDL. Genomic DNA and mRNA of CETP were correlated and were similar in the lcat(Ϫ/Ϫ) and lcat(ϩ/ϩ) mice. Plasma CETP was correlated with its genomic DNA and mRNA, but the slope of the increase was much lower in the lcat(Ϫ/Ϫ) mice. Whereas plasma CETP mostly associates with HDL in the lcat(ϩ/ϩ) mouse, it is found free in the lcat(Ϫ/Ϫ) mouse. Key Words: cholesterol Ⅲ high density lipoprotein Ⅲ lecithin-cholesterol acyltransferase Ⅲ cholesteryl ester transfer protein Ⅲ mice H igh density lipoprotein (HDL) is given a key role in the hypothesis of a cholesterol transport pathway from the peripheral tissues to the liver. Lecithin-cholesterol acyltransferase (LCAT) plays a major role in this process by esterifying free cholesterol (FC) in circulating lipoproteins to maintain a FC gradient between the peripheral cells and the HDL particle surface and, accordingly, to promote FC efflux from the cells. 1 The importance of LCAT in HDL metabolism has been established by identification and characterization of the patients with LCAT deficiency. 2 Mutations in the human LCAT gene cause either familial LCAT deficiency or fish eye disease, which results in the decrease of plasma HDL and accumulation of cholesterol in the cell membrane in certain organs. 2 Disruption of the LCAT gene in mice also exhibits severe reduction of plasma HDL 3,4 and mimics many features of LCAT deficiency in humans. Conclusions-PlasmaThe cholesteryl ester (CE) generated by LCAT and present in the HDL core can be transported directly to the liver by selective uptake 5,6 and/or potentially as a whole particle. 7 Alternatively, HDL CE is transferred to apoB-containing lipoproteins by CE transfer protein (CETP) for liver uptake. 8 CETP is a plasma glycoprotein that mediates transfer/exchange of CE and triglycerides between HDL and apoBcontaining lipoproteins. 9,10 The heteroexchange of CE with triglycerides by CETP leads to the net CE transfer between plasma lipoproteins. 9 This reaction is also one of the key steps of cholesterol transport from the peripheral tissues to the liver. Thus, CETP is involved as much as LCAT in regulation of the plasma HDL level and remodeling of HDL particles. In fact, patients...