Monocyte/macrophage expression of ABCA1 has minimal contribution to plasma HDL levels.

Excess accumulation of cholesterol in macrophages results in foam cell production and lesion development. Recent studies have demonstrated that ATP-binding cassette protein A1 (ABCA1) is highly regulated in macrophages and mediates the efflux of cholesterol and phospholipids to apolipoproteins, a process necessary for HDL formation. The goal of this study was to determine the contribution of monocyte/macrophage ABCA1 to HDL formation in vivo. We generated mice expressing ABCA1 in macrophages and mice with selected inactivation of ABCA1 in macrophages by bone marrow transplantation in ABCA1-deficient (ABC1(-/-)) and wild-type (WT) mice. At all times, the level of HDL in ABC1(-/-) recipient mice remained low relative to WT recipient mice irrespective of the genotype of the donor macrophage ABCA1 or high-fat feeding. Expression of WT macrophage ABCA1 in ABC1(-/-) mice resulted in a small but significant increase in apoA-I levels starting 2 weeks after transplantation. No further increase in apoAI was observed up to 14 weeks after transplantation. The increase in apoAI was accompanied by a small but significant increase in HDL cholesterol 6 weeks after transplantation. The HDL formed as a consequence of the expression of WT macrophage ABCA1 migrated to the alpha position in a two-dimensional gel electrophoresis. These results demonstrate that monocyte/macrophage ABCA1 contributes to HDL formation; however, the contribution to the overall plasma HDL levels is minimal.

The full induction of human apoprotein A-I gene expression by the experimental nephrotic syndrome in transgenic mice depends on cis-acting elements in the proximal 256 base-pair promoter region and the trans-acting factor early growth response factor 1.

To identify molecular factors regulating apo A-I production in vivo, we induced in transgenic mice the experimental nephrotic syndrome, which results in elevated levels of HDL cholesterol (HDL-C), plasma apo A-I, and hepatic apo A-I mRNA. Human (h) apo A-I transgenic mice with different length 5' flanking sequences (5.5 or 0.256 kb, the core promoter for hepatic-specific basal expression) were injected with nephrotoxic (NTS) or control serum. With nephrosis, there were comparable (greater than twofold) increases in both lines of HDL-C, h-apo A-I, and hepatic h-apo A-I mRNA, suggesting that cis-acting elements regulating induced apo A-I gene expression were within its core promoter. Hepatic nuclear extracts from control and nephrotic mice footprinted the core promoter similarly, implying that the same elements regulated basal and induced expression. Hepatic mRNA levels for hepatocyte nuclear factor (HNF) 4 and early growth response factor (EGR) 1, trans-acting factors that bind to the core promoter, were measured: HNF4 mRNA was not affected, but that of EGR-1 was elevated approximately fivefold in the nephrotic group. EGR-1 knockout (EGR1-KO) mice or mice expressing EGR-1 were injected with either NTS or control serum. Levels of HDL-C, apo A-I, and hepatic apo A-I mRNA were lowest in nonnephrotic EGR1-KO mice and highest in nephrotic mice expressing EGR-1. Although in EGR1-KO mice HDL-C, apo A-I, and apo A-I mRNA levels also increased after NTS injection, they were approximately half of those in the nephrotic EGR-1-expressing mice. We conclude that in this model, basal and induced apo A-I gene expression in vivo are regulated by the trans-acting factor EGR-1 and require the same cis-acting elements in the core promoter.

Disruption of the murine lecithin:cholesterol acyltransferase gene causes impairment of adrenal lipid delivery and up-regulation of scavenger receptor class B type I.

Lecithin:cholesterol acyltransferase (LCAT) is the major determinant of the cholesteryl ester (CE) content of high density lipoprotein (HDL) in plasma. The selective uptake of HDL-CE is postulated to participate in delivery of tissue-derived cholesterol both to the liver and steroidogenic tissues. Recent studies comparing mice with similarly low levels of HDL, due to the absence of either of the two major HDL-associated apolipoproteins apoA-I and apoA-II, suggest that apoA-I is crucial in modulating this process, possibly through interaction with scavenger receptor class B type I (SR-BI). Because of the central role of LCAT in determining the size, lipid composition, and plasma concentration of HDL, we have created LCAT-deficient mice by gene targeting to examine the effect of LCAT deficiency on HDL structure and composition and adrenal cholesterol delivery. The HDL in the LCAT-deficient mice was reduced in its plasma concentration (92%) and CE content (96%). The HDL particles were heterogeneous in size and morphology and included numerous discoidal particles, mimicking those observed in LCAT-deficient humans. The adrenals of the male Lcat (-/-) mice were severely depleted of lipid stores, which was associated with a 2-fold up-regulation of the adrenal SR-BI mRNA. These studies demonstrate that LCAT deficiency, similar to apoA-I deficiency, is associated with a marked decrease in adrenal cholesterol delivery and supports the hypothesis that adrenal SR-BI expression is regulated by the adrenal cholesterol.

Expression of human lecithin:cholesterol acyltransferase in transgenic mice: effects on cholesterol efflux, esterification, and transport.

Human lecithin:cholesterol acyltransferase (LCAT) is a key enzyme in the plasma metabolism of cholesterol and is postulated to participate in a physiologic process called reverse cholesterol transport. We have used transgenic mice expressing the human LCAT transgene to study the effect of increased plasma levels of LCAT in each of the proposed steps involved in the reverse cholesterol transport pathway. High density lipoprotein (HDL) from LCAT transgenic mice was 44% more efficient than control mouse HDL in the efflux of cholesterol from human skin fibroblasts. Esterification of cell-derived cholesterol was also markedly increased in mice expressing the human LCAT transgene. The rate of plasma clearance of HDL cholesteryl ester was virtually the same in both types of animals whereas the HDL cholesteryl ester transport rate was significantly increased in mice expressing the human LCAT transgene (152.3 +/- 16.9 micrograms/h vs. 203.1 +/- 30.9 micrograms/h in control and transgenic mice, respectively). Liver cholesteryl ester uptake was significantly increased in mice expressing human LCAT (58.0 +/- 1.4 micrograms/h/g liver vs. 77.9 +/- 1.7 micrograms/h/g liver in control and transgenic mice, respectively). These studies indicate that LCAT modulates the rate by which cholesterol is effluxed from cell membranes onto HDL, esterified, and transported to the liver.

Inhibition of apolipoprotein B and triglyceride secretion in human hepatoma cells (HepG2).

Apolipoprotein B (apoB), the major protein component of triglyceride-rich lipoproteins, is assembled into a lipoprotein particle via a complex, multistep process. Recent studies indicate that triglyceride-rich lipoprotein assembly requires the activity of the heterodimeric protein, microsomal triglyceride transfer protein (MTP). We identified a novel inhibitor of apolipoprotein B secretion using the human hepatoma cell line, HepG2. CP-10447, a derivative of the hypnotic drug methaqualone (Quaalude), inhibited apoB secretion from HepG2 cells with an IC50 of approximately 5 microM. CP-10447 also inhibited apoB secretion from Caco-2 cells, a model of intestinal lipoprotein production. In experiments using [3H]glycerol as a precursor for triglyceride synthesis, CP-10447 (20 microM) inhibited radiolabeled triglyceride secretion by approximately 83% (P < 0.0001) in HepG2 cells and 76% (P < 0.05) in Caco-2 cells with no effect on radiolabel incorporation into cellular triglyceride, indicating that CP-10447 inhibited triglyceride secretion without affecting triglyceride synthesis. RNA solution hybridization assay indicated that CP-10447 did not affect apoB or apoA-I mRNA levels. Pulse-chase experiments in HepG2 cells confirmed that CP-10447 inhibited the secretion of apoB (not its synthesis) without affecting secretion of total proteins or albumin and suggested that CP-10447 stimulates the early intracellular degradation of apoB in the endoplasmic reticulum (ER). Further studies demonstrated that CP-10447 is a potent inhibitor of human liver microsomal triglyceride transfer activity (IC50 approximately 1.7 microM) in an in vitro assay containing artificial liposomes and partially purified human MTP. These data suggest that CP-10447 may inhibit apoB and triglyceride secretion by inhibiting MTP activity and stimulating the early ER degradation of apoB. CP-10447 should provide a useful tool for further study of the mechanisms of apoB secretion and triglyceride-rich lipoprotein assembly.

Increased prebeta-HDL levels, cholesterol efflux, and LCAT-mediated esterification in mice expressing the human cholesteryl ester transfer protein (CETP) and human apolipoprotein A-I (apoA-I) transgenes.

The effects of cholesteryl ester transfer protein (CETP) on the distribution of apolipoprotein (apo) A-I between high density lipoprotein (HDL) subspecies and its impact on efflux and esterification of cell-derived cholesterol was studied in transgenic mice expressing either the human apoA-I (HuAITg) or both the human apoA-I and CETP (HuAICETPTg) transgenes. The simultaneous expression of the human CETP and apoA-I transgenes induced a 2-fold increase in the proportion of human apoA-I in the prebeta-HDL fraction and 1.4- and 2.2-fold increases in the HDL3a and HDL3c fractions, respectively, at the expense of the larger HDL2b population. HuAICETPTg mouse plasma has a greater ability to cause efflux of cholesterol from 3H-labeled fibroblasts than plasma from HuAITg mice. Furthermore, the LCAT-mediated esterification of cell-derived cholesterol is increased 1.7-fold in mice expressing the human apoA-I and CETP transgenes compared to HuAITg mouse plasma. LCAT activity (measured with an exogenous substrate) was increased 1.4-fold and LCAT mRNA levels were increased 1.3-fold as a result of CETP expression. Taken together, these data indicate that in vivo, the expression of CETP resulted in an increase in the proportion of apoA-I in the prebeta-HDL fraction and a stimulation of the efflux and esterification of cell-derived cholesterol.

9-cis-retinoic acid increases apolipoprotein AI secretion and mRNA expression in HepG2 cells.

HepG2 cells were studied as a model for regulation of hepatic apolipoprotein AI (apo AI) secretion and gene expression by 9-cis-retinoic acid. HepG2 cells cultured on plastic dishes were exposed to 9-cis-retinoic acid (9-cis-RA) for 48 h with a complete media change at 24 h. Apo AI mass in cultured media was determined by ELISA, by quantitative immunoblotting and by steady-state 35S-methionine labeling. Messenger RNA levels were determined by RNase protection using probes for apo AI and the housekeeping gene, glyceraldehyde 3-phosphate dehydrogenase (G3PDH). 9-cis-RA increased secretion of apo AI by 52% at doses of 10 and 1 microM (6.3 +/- 0.6 vs. 4.2 +/- 0.3; P < 0.005; 6.1 +/- 0.3 vs. 4.0 +/- 0.7 ng of apo AI/mg cell protein, P < 0.05) and by 35% at 0.1 microM (5.5 +/- 0.6 vs. 4.1 +/- 0.4 ng apo AI/mg protein, P < 0.05, n = 4). Immunoblotting results were consistent with results from ELISA (70% increase at 10 microM 9-cis-RA, P < 0.001; 34% increase at 1 microM, P < 0.005, n = 3). Metabolically labeled apoAI in the medium was increased by 39% following steady-state labeling in the presence of 10 microM 9-cis-RA (597 +/- 7 vs. 430 +/- 13 DPM/microliters media; P < 0.001; n = 4). 9-cis-RA (10 microM) also increased HepG2 cell apo AI mRNA expression by 76% (68 700 +/- 400 vs. 38 900 +/- 2700 DPM, P < 0.01, n = 4), whereas expression of G3PDH mRNA was slightly decreased (14%, P < 0.05). Thus, 9-cis-RA stimulates apo AI expression in HepG2 cells, suggesting a role for retinoids in activating endogenous apo AI gene expression.

An alternative method for treating buccal osseous defects.

This case demonstrates the successful use of a hydroxylapatite block for maintenance of ridge contour. The procedure offers an alternative to guided tissue regeneration for treating osseous defects of the buccal cortical plate, which may occur following tooth extraction.