Abstract-The potential role of anti-inflammatory cytokines in the modulation of the atherosclerotic process remains unknown. Interleukin (IL)-10 has potent deactivating properties in macrophages and T cells and modulates many cellular processes that may interfere with the development and stability of the atherosclerotic plaque. IL-10 is expressed in human atherosclerosis and is associated with decreased signs of inflammation. In the present study, we show that IL-10 -deficient C57BL/6J mice fed an atherogenic diet and raised under specific pathogen-free conditions exhibit a significant 3-fold increase in lipid accumulation compared with wild-type mice. Interestingly, the susceptibility of IL-10 -deficient mice to atherosclerosis was exceedingly high (30-fold increase) when the mice were housed under conventional conditions. Atherosclerotic lesions of IL-10 -deficient mice showed increased T-cell infiltration, abundant interferon-␥ expression, and decreased collagen content. In vivo, transfer of murine IL-10 achieved 60% reduction in lesion size. These results underscore the critical roles of IL-10 in both atherosclerotic lesion formation and stability. Moreover, IL-10 appears to be crucial as a protective factor against the effect of environmental pathogens on atherosclerosis. The full text of this article is available at http://www.circresaha.org. (Circ Res. 1999;85:e17-e24.)

The metabolic syndrome (visceral obesity, insulin resistance, type 2 diabetes, and dyslipidemia) resembles Cushing's Syndrome, but without elevated circulating glucocorticoid levels. An emerging concept suggests that the aberrantly elevated levels of the intracellular glucocorticoid reamplifying enzyme 11␤-hydroxysteroid dehydrogenase type 1 (11␤-HSD-1) found in adipose tissue of obese humans and rodents underlies the phenotypic similarities between idiopathic and "Cushingoid" obesity.

Plasma HDL cholesterol levels are inversely related to risk for atherosclerosis. The ATP-binding cassette, subfamily A, member 1 (ABCA1) mediates the rate-controlling step in HDL particle formation, the assembly of free cholesterol and phospholipids with apoA-I. ABCA1 is expressed in many tissues; however, the physiological functions of ABCA1 in specific tissues and organs are still elusive. The liver is known to be the major source of plasma HDL, but it is likely that there are other important sites of HDL biogenesis. To assess the contribution of intestinal ABCA1 to plasma HDL levels in vivo, we generated mice that specifically lack ABCA1 in the intestine. Our results indicate that approximately 30% of the steady-state plasma HDL pool is contributed by intestinal ABCA1 in mice. In addition, our data suggest that HDL derived from intestinal ABCA1 is secreted directly into the circulation and that HDL in lymph is predominantly derived from the plasma compartment. These data establish a critical role for intestinal ABCA1 in plasma HDL biogenesis in vivo.Introduction HDL particles mediate the transport of cholesterol from peripheral tissues to the liver in a process termed reverse cholesterol transport (1, 2), which is postulated to explain, at least in part, their ability to protect against foam cell formation and atherosclerosis. Despite the widespread interest in HDL as a potential therapeutic target (3), the origins of plasma HDL are still elusive. The ATP-binding cassette, subfamily A, member 1 (ABCA1) mediates the rate-controlling step in HDL particle formation by promoting the efflux of cholesterol and phospholipids to apoA-I (4, 5). Mutations in ABCA1 cause Tangier disease (6-8), characterized by near absence of HDL cholesterol and increased risk for atherosclerosis (9-11). ABCA1 is widely expressed throughout the body (12, 13); however, the contributions of ABCA1 in specific tissues to HDL levels and reverse cholesterol transport are still being unraveled, and only recently the role of hepatic ABCA1 in homeostasis of HDL levels was elucidated.Overexpression of hepatic ABCA1 raises HDL cholesterol levels (14, 15), and liver-specific deletion of ABCA1 results in a substantial (∼80%) decrease in plasma HDL cholesterol in chow-fed mice (16). Similarly, a 50% knockdown of hepatic ABCA1 expression by adenovirus-mediated RNA interference in mice is associated with a 40% decrease in HDL cholesterol (17). These results indicate that the liver is the single most important source of plasma HDL in vivo but also suggest the existence of additional, extrahepatic sites of HDL biogenesis.The intestine, along with the liver, is an important site for the synthesis and secretion of apoA-I, the principal apoprotein of HDL,

In obese humans and rodents there is increased expression of the key glucocorticoid (GC) regenerating enzyme, 11␤-hydroxysteroid dehydrogenase type 1 (11␤-HSD1), in adipose tissue. This increased expression appears to be of pathogenic importance because transgenic mice overexpressing 11␤-HSD1 selectively in adipose tissue exhibit a full metabolic syndrome with visceral obesity, dyslipidemia, insulinresistant diabetes, and hypertension. In this model, while systemic plasma GC levels are unaltered, GC delivery to the liver via the portal vein is increased. 11␤-HSD1 is most highly expressed in liver where inhibition or deficiency of its activity improves glucose and lipid homeostasis. To determine the potential contribution of elevated intrahepatic GCs alone toward development of insulin-resistant syndromes we generated transgenic mice expressing increased 11␤-HSD1 activity selectively in the liver under transcriptional control of hepatic regulatory sequences derived from the human apoE gene (apoE-HSD1). Transgenic lines with 2-and 5-fold-elevated 11␤-HSD1 activity exhibited mild insulin resistance without altered fat depot mass. ApoE-HSD1 transgenic mice exhibited fatty liver and dyslipidemia with increased hepatic lipid synthesis͞flux associated with elevated hepatic LXR␣ and PPAR␣ mRNA levels as well as impaired hepatic lipid clearance. Further, apoE-HSD1 transgenic mice have a marked, transgene-dose-associated hypertension paralleled by incrementally increased liver angiotensinogen expression. These data suggest that elevated hepatic expression of 11␤-HSD1 may relate to the pathogenesis of specific fatty liver, insulin-resistant, and hypertensive syndromes without obesity in humans as may occur in, for example, myotonic dystrophy, and possibly, the metabolically obese, normal-weight individual.

؊/؊ mice have improved glucose tolerance. These data suggest that 11␤-HSD-1 deficiency produces an improved lipid profile, hepatic insulin sensitization, and a potentially atheroprotective phenotype.

The peroxisome proliferator-activated receptor-␣ (PPAR␣) controls gene expression in response to a diverse class of compounds collectively referred to as peroxisome proliferators. Whereas most known peroxisome proliferators are of exogenous origin and include hypolipidemic drugs and other industrial chemicals, several endogenous PPAR␣ activators have been identified such as fatty acids and steroids. The latter finding and the fact that PPAR␣ modulates target genes encoding enzymes involved in lipid metabolism suggest a role for PPAR␣ in lipid metabolism. This was investigated in the PPAR␣-deficient mouse model. Basal levels of total serum cholesterol, high density lipoprotein cholesterol, hepatic apolipoprotein A-I mRNA, and serum apolipoprotein A-I in PPAR␣-deficient mice are significantly higher compared with wild-type controls. Treatment with the fibrate Wy 14,643 decreased apoA-I serum levels and hepatic mRNA levels in wild-type mice, whereas no effect was detected in the PPAR␣-deficient mice. Administration of the fibrate Wy 14,643 to wild-type mice results in marked depression of hepatic apolipoprotein C-III mRNA and serum triglycerides compared with untreated controls. In contrast, PPAR␣-deficient mice were unaffected by Wy 14,643 treatment. These studies demonstrate that PPAR␣ modulates basal levels of serum cholesterol, in particular high density lipoprotein cholesterol, and establish that fibrate-induced modulation in hepatic apolipoprotein A-I, C-III mRNA, and serum triglycerides observed in wild-type mice is mediated by PPAR␣.

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors, which heterodimerize with the retinoid X receptor and bind to peroxisome proliferator response elements in the promoters of regulated genes. Despite the wealth of information available on the function of PPARK K and PPARQ Q, relatively little is known about the most widely expressed PPAR subtype, PPARN N. Here we show that treatment of insulin resistant db/db mice with the PPARN N agonist L-165 041, at doses that had no effect on either glucose or triglycerides, raised total plasma cholesterol concentrations. The increased cholesterol was primarily associated with high density lipoprotein (HDL) particles, as shown by fast protein liquid chromatography analysis. These data were corroborated by the chemical analysis of the lipoproteins isolated by ultracentrifugation, demonstrating that treatment with L-165 041 produced an increase in circulating HDL without major changes in very low or low density lipoproteins. White adipose tissue lipoprotein lipase activity was reduced following treatment with the PPARN N ligand, but was increased by a PPARQ Q agonist. These data suggest both that PPARN N is involved in the regulation of cholesterol metabolism in db/db mice and that PPARN N ligands could potentially have therapeutic value. z 2000 Federation of European Biochemical Societies.

A common feature of many metabolic pathways is their control by retinoid X receptor (RXR) heterodimers. Dysregulation of such metabolic pathways can lead to the development of atherosclerosis, a disease influenced by both systemic and local factors. Here we analyzed the effects of activation of RXR and some of its heterodimers in apolipoprotein E ؊͞؊ mice, a well established animal model of atherosclerosis. An RXR agonist drastically reduced the development of atherosclerosis. In addition, a ligand for the peroxisome proliferator-activated receptor (PPAR)␥ and a dual agonist of both PPAR␣ and PPAR␥ had moderate inhibitory effects. Both RXR and liver X receptor (LXR) agonists induced ATP-binding cassette protein 1 (ABC-1) expression and stimulated ABC-1-mediated cholesterol efflux from macrophages from wild-type, but not from LXR␣ and ␤ double ؊͞؊, mice. Hence, activation of ABC-1-mediated cholesterol efflux by the RXR͞LXR heterodimer might contribute to the beneficial effects of rexinoids on atherosclerosis and warrant further evaluation of RXR͞LXR agonists in prevention and treatment of atherosclerosis. R etinoid X receptors (RXRs) are ubiquitously expressed nuclear receptors that heterodimerize with a number of other receptors (e.g., thyroid hormone receptor, vitamin D receptor, retinoic acid receptors, etc.; refs. 1 and 2). The elucidation of RXRs biological activity has advanced significantly through the characterization of rexinoids, high-affinity selective synthetic ligands for RXRs (1, 2). The use of rexinoids led to the demonstration that RXRs are active and permissive signaling molecules in heterodimers with the farnesol X receptor (FXR) [or bile acid receptor (BAR)], the liver X receptors (LXRs), and the peroxisome proliferatoractivated receptors (PPARs). The simultaneous activation of several permissive heterodimers underlies in part RXRs pleiotropic functions, affecting numerous receptor signaling pathways, and ranges from the control of cell proliferation, differentiation, and apoptosis (3) to the regulation of glucose and lipid metabolism (4). This pivotal role of the various permissive RXR heterodimers is in fact an emerging theme in multiple metabolic pathways. Heterodimers between RXR and PPAR␥, PPAR␣, LXR␣, and FXR͞BAR respectively inf luence glucose, triglyceride, cholesterol, and bile acid homeostasis. Dysregulation of these homeostatic control pathways can result in common metabolic disorders such as obesity, type 2 diabetes, and hyperlipidemia, which are often complicated by the development of atherosclerosis.In view of the potential implication of RXRs in various metabolic pathways implicated in atherosclerosis, we studied whether modulating the activity of these receptors or of some of their heterodimers affects the development of atherosclerosis in apolipoprotein (apo)E-deficient mice. We demonstrate here that the administration of rexinoids significantly attenuates atherosclerosis development, and we show that this effect might be linked to the activation of reverse cholesterol transpor...