Macrophage ATP-binding cassette transporter A1 (ABCA1), scavenger receptor class B type I (SR-BI), and ABCG1 have been shown to promote cholesterol efflux to extracellular acceptors in vitro and influence atherosclerosis in mice, but their roles in mediating reverse cholesterol transport (RCT) from macrophages in vivo are unknown. Using an assay of macrophage RCT in mice, we found that primary macrophages lacking ABCA1 had a significant reduction in macrophage RCT in vivo, demonstrating the importance of ABCA1 in promoting macrophage RCT, however substantial residual RCT exists in the absence of macrophage ABCA1. Using primary macrophages deficient in SR-BI expression, we found that macrophage SR-BI, which was shown to promote cholesterol efflux in vitro, does not contribute to macrophage RCT in vivo. To investigate whether macrophage ABCG1 is involved in macrophage RCT in vivo, we used ABCG1-overexpressing, -knockdown, and -knockout macrophages. We show that increased macrophage ABCG1 expression significantly promoted while knockdown or knockout of macrophage ABCG1 expression significantly reduced macrophage RCT in vivo. Finally, we show that there was a greater decrease in macrophage RCT from cells where both ABCA1 and ABCG1 expression were knocked down than from ABCG1-knockdown cells. These results demonstrate that ABCA1 and ABCG1, but not SR-BI, promote macrophage RCT in vivo and are additive in their effects.

Reverse cholesterol transport (RCT) is a term used to describe the efflux of excess cellular cholesterol from peripheral tissues and its return to the liver for excretion in the bile and ultimately the feces. It is believed to be a critical mechanism by which HDL exert a protective effect on the development of atherosclerosis. In this paradigm, cholesterol is effluxed from arterial macrophages to extracellular HDLbased acceptors through the action of transporters such as ABCA1 and ABCG1. After efflux to HDL, cholesterol may be esterified in the plasma by the enzyme lecithin:cholesterol acyltransferase and is ultimately transported from HDL to the liver, either directly via the scavenger receptor BI or after transfer to apolipoprotein B-containing lipoproteins by the cholesteryl ester transfer protein. Methods for assessing the integrated rate of macrophage RCT in animals have provided insights into the molecular regulation of the process and suggest that the dynamic rate of macrophage RCT is more strongly associated with atherosclerosis than the steady-state plasma concentration of HDL cholesterol. Promotion of macrophage RCT is a potential therapeutic approach to preventing or regressing atherosclerotic vascular disease, but robust measures of RCT in humans will be needed in order to confidently advance RCT-promoting therapies in clinical development.-

Background-Liver X receptors (LXRs) are ligand-activated transcription factors involved in the control of lipid metabolism and inflammation. Synthetic LXR agonists have been shown to inhibit the progression of atherosclerosis in mice, but the mechanism is uncertain. LXR agonism upregulates the genes encoding ATP binding cassette transporters A1 (ABCA1) and G1 (ABCG1) in macrophages, thus promoting efflux of cholesterol; it also upregulates liver and intestinal ABCG5 and ABCG8, helping to promote biliary and fecal excretion of cholesterol. Thus, LXR agonism may inhibit atherosclerosis through promotion of reverse cholesterol transport (RCT) in vivo, but this has not been proven. We previously described an in vivo method to trace the movement of cholesterol from 3 H-cholesterol-labeled J774 macrophages into plasma, into liver, and ultimately into the bile and feces as free cholesterol or bile acids. In the present study we used this approach to test the hypothesis that administration of the synthetic LXR agonist GW3965 would increase the rate of macrophage RCT in vivo. Methods and Results-Three different mouse models-wild-type C57BL/6 mice, LDLR/apobec-1 double knockout mice, and human apolipoprotein (apo)B/cholesteryl ester transfer protein (CETP) double transgenic mice-were treated with either vehicle or GW3965. Mice were injected intraperitoneally with 3 H-cholesterol-labeled and cholesterol-loaded macrophages and monitored for the appearance of 3 H-tracer in plasma, liver, and feces. Administration of GW3965 significantly increased the levels of 3 H-tracer in plasma and feces in all 3 mouse models. Conclusions-These

The enzyme acyl coenzyme A:cholesterol acyltransferase 1 (ACAT1) mediates sterol esterification, a crucial component of intracellular lipid homeostasis. Two enzymes catalyze this activity in Saccharomyces cerevisiae (yeast), and several lines of evidence suggest multigene families may also exist in mammals. Using the human ACAT1 sequence to screen data bases of expressed sequence tags, we identified two novel and distinct partial human cDNAs. Full-length cDNA clones for these ACAT related gene products (ARGP) 1 and 2 were isolated from a hepatocyte (HepG2) cDNA library. ARGP1 was expressed in numerous human adult tissues and tissue culture cell lines, whereas expression of ARGP2 was more restricted. In vitro microsomal assays in a yeast strain deleted for both esterification genes and completely deficient in sterol esterification indicated that ARGP2 esterified cholesterol while ARGP1 did not. In contrast to ACAT1 and similar to liver esterification, the activity of ARGP2 was relatively resistant to a histidine active site modifier. ARGP2 is therefore a tissue-specific sterol esterification enzyme which we thus designated ACAT2. We speculate that ARGP1 participates in the coenzyme A-dependent acylation of substrate(s) other than cholesterol. Consistent with this hypothesis, ARGP1, unlike any other member of this multigene family, possesses a predicted diacylglycerol binding motif suggesting that it may perform the last acylation in triglyceride biosynthesis.

Background Inflammation is proposed to impair reverse cholesterol transport (RCT), a major atheroprotective function of HDL. This study presents the first integrated functional evidence that inflammation retards numerous components of RCT. Methods and Results We employed sub-acute endotoxemia in the rodent macrophage-to-feces RCT model to assess the effects of inflammation on RCT in vivo, and performed proof of concept experimental endotoxemia studies in humans. Endotoxemia (3mg/kg, SQ) reduced 3H-cholesterol movement from macrophage to plasma and 3H-cholesterol associated with HDL fractions. At 48h bile and fecal counts were markedly reduced consistent with downregulation of hepatic expression of ABCG5, ABCG8 and ABCB11 biliary transporters. Low dose LPS (0.3mg/kg, SQ) also reduced bile and fecal counts, as well as expression of biliary transporters, but in the absence of effects on plasma or liver counts. In vitro, LPS impaired 3H-cholesterol efflux from human macrophages to apoA-I and serum coincident with reduced expression of the cholesterol transporter, ABCA1. During human (3ng/kg; n=20) and murine endotoxemia (3mg/kg, SQ), ex vivo macrophage cholesterol efflux to acute phase HDL was attenuated. Conclusions We provide the first in vivo evidence that inflammation impairs RCT at multiple steps in the RCT pathway, particularly cholesterol flux through liver to bile and feces. Attenuation of RCT and HDL efflux function, independent of HDL-cholesterol levels, may contribute to atherosclerosis in chronic inflammatory states including obesity, metabolic syndrome and type-2 diabetes.

Diacylglycerol esterification provides an excellent target for the pharmacological reduction of triglyceride accumulation in several human disease states. We have used Saccharomyces cerevisiae as a model system to study this critical component of triglyceride synthesis. Recent studies of an oleaginous fungus, Mortierella ramanniana, identified a new family of enzymes with in vitro acyl-CoA:diacylglycerol acyltransferase activity. We show here that DGA1, the sole member of this gene family in yeast, has a physiological role in triglyceride synthesis. Metabolic labeling of DGA1 deletion strains with triglyceride precursors detected significant reductions in triglyceride synthesis. Triglyceride synthesis was virtually abolished in four different growth conditions when DGA1 was deleted in concert with LRO1, an enzyme that esterifies diacylglycerol from a phospholipid acyl donor. The relative contributions of the two enzymes depended on growth conditions. The residual synthesis was lost when ARE2, encoding an acyl-CoA: sterol acyltransferase, was deleted. In vitro microsomal assays verified that DGA1 and ARE2 mediate acyl-CoA: diacylglycerol acyltransferase reactions. Three enzymes can thus account for diacylglycerol esterification in yeast. Yeast strains deficient in both diacylglycerol and sterol esterification showed only a slight growth defect indicating that neutral lipid synthesis is dispensable under common laboratory conditions.A common element to diabetes, atherosclerosis and obesity, is the subcellular and extracellular accumulation of neutral lipids (1-3). The synthesis of these neutral lipids (triglyceride and steryl ester) results in both storage and detoxification of the alcohol and acyl substrates to these reactions. The reactions thus represent a pivotal component of lipid homeostasis in all eukaryotes and of the pathophysiology of some of the most prevalent human disease syndromes of the western world (reviewed in Refs. 4 -6).Significant progress has been made recently toward the complete identification of the enzymes that mediate the intracellular synthesis of cholesteryl esters and triglyceride (TG). 1 In mammals, these reactions are catalyzed, in part, by the three members of the O-acyltransferase gene family (5, 7-12). The ACAT1 and ACAT2 genes encode a ubiquitous and a tissuespecific acyl-CoA:cholesterol O-acyltransferase (ACAT), respectively (9, 10, 12), while the DGAT1 gene encodes an acylcoenzyme A (CoA) diacylglycerol O-acyltransferase (DGAT (9,11,13,14)). Although induced mutant mice for DGAT1 exhibit reduced body fat and are resistant to diet-induced obesity, these animals have normal serum TG levels, indicating that DGAT1-independent mechanism(s) for TG synthesis must also exist (15,16). Such a mechanism may be mediated, at least in part, by DGAT2, a human protein recently identified based on sequence similarity to two acyl-CoA:diacylglycerol acyltransferases purified from the oleaginous fungus Mortierella ramanniana (16 -18). The two M. ramanniana enzymes are 53% identical and show no se...

Cholesterol efflux occurs by different pathways, including transport mediated by specific proteins. We determined the effect of enriching cells with free cholesterol (FC) on the release of FC to human serum. Loading Fu5AH cells with FC had no effect on fractional efflux, whereas enriching mouse peritoneal macrophages (MPMs) resulted in a doubling of fractional efflux. Efflux from cholesterolnormal MPM and Fu5AH cells to 15 human sera correlated well with HDL parameters. However, these relationships were reduced or lost with cholesterol-loaded MPMs. Using macrophages from scavenger receptor class B type I (SR-BI)-, ABCA1-, and ABCG1-knockout mice, together with inhibitors of SR-BI-and ABCA1-mediated efflux, we were able to quantitate efflux upon loading macrophages with excess cholesterol and to establish the contributions of the various efflux pathways in cholesterol-normal and -enriched cells. The removal of ABCA1 had essentially no effect on the total efflux when cell cholesterol levels were normal. However, in cholesterol-enriched cells, the removal of ABCA1 reduced efflux by 50%. Approximately 20% of the efflux stimulated by FC-loading MPM is attributable to ABCG1. The SR-BI contribution to efflux was small. Another pathway that is present in all cells is aqueous diffusion. Our studies demonstrate that this mechanism is one of the major contributors to efflux, particularly in cholesterol-normal cells.-

Triglyceride (TG), a water-insoluble energy-rich lipid, is secreted by the liver as part of very low density lipoproteins (VLDLs) to supply energy to extrahepatic tissues. Overproduction of VLDL is associated with increased risk of cardiovascular heart disease; this has renewed an interest in factors that affect hepatic TG production. The TG production rate is determined by measuring temporal increases in plasma TG under conditions in which TG hydrolysis by lipoprotein lipase (LPL) is inhibited. The nonionic detergent, Triton WR-1339 (Triton), has commonly been used to inhibit LPL for this purpose. Triton, in addition to inhibition of TG hydrolysis, has properties that have the potential to adversely influence lipoprotein metabolism. Another nonionic detergent, poloxamer 407 (P-407), also inhibits LPL. In these studies, we demonstrate that P-407 is comparable to Triton in the determination of TG production but without the unwanted side effects of Triton. Supplementary key words non-ionic detergents • hepatic lipids • lipoproteinsTriglyceride (TG) is an energy-rich compound, primarily stored in liver and adipose, and is mobilized in response to various metabolic signals. In plasma, TG, which is water insoluble, circulates as the neutral lipid core of lipoproteins, mainly chylomicrons, which carry dietary fat and are secreted by the small intestine, and very low density lipoproteins (VLDLs), which carry TG from the liver. Overproduction of VLDL has been associated with a number of disease states that result in an increased risk of cardiovascular heart disease; this has renewed an interest in factors that affect hepatic TG (lipoprotein) production (1).In the early 1950s, it was noted that intravenous injection of certain nonionic detergents resulted in milky serum that lasted up to 48 h (2). This was later shown to be due to the inhibition of TG hydrolysis by lipoprotein lipase (LPL) (3). Since then, lipolysis inhibition has been used to determine hepatic TG production rates, with Triton WR-1339 (also known as Tyloxapol) being widely used. Using this technique, the TG production rate is calculated from the increase in TG over time following detergent injection. Although this method is the basis for most studies on TG production in animals, there is considerable variation in its implementation. Variables include whether mice are fasted, fed chow or a fat-free diet, and what the plasma sampling period is (0 to 300 min) over which TG production rates are determined ( Table 1 ).In addition to inhibition of LPL, Triton has a number of other physiologic effects related to lipoprotein metabolism. Triton has been shown to cause dissociation of apolipoprotein A-I (apoA-I) and apoC-II from HDL (13). Triton is rapidly taken up by the liver, where it accumulates in the lysosomes, causes autophagic vacuole formation (14, 15), and is excreted in bile, possibly explaining a reduction in biliary phospholipid and cholesterol output (16). These hepatic and plasma effects of Triton may affect hepatic TG production, especial...