Liming Pei

Synthetic LXR ligand inhibits the development of atherosclerosis in mice

The nuclear receptors LXRα and LXRβ have been implicated in the control of cholesterol and fatty acid metabolism in multiple cell types. Activation of these receptors stimulates cholesterol efflux in macrophages, promotes bile acid synthesis in liver, and inhibits intestinal cholesterol absorption, actions that would collectively be expected to reduce atherosclerotic risk. However, synthetic LXR ligands have also been shown to induce lipogenesis and hypertriglyceridemia in mice, raising questions as to the net effects of these compounds on the development of cardiovascular disease. We demonstrate here that the nonsteroidal LXR agonist GW3965 has potent antiatherogenic activity in two different murine models. In LDLR−/− mice, GW3965 reduced lesion area by 53% in males and 34% in females. A similar reduction of 47% was observed in male apoE−/− mice. Long-term (12-week) treatment with LXR agonist had differential effects on plasma lipid profiles in LDLR−/− and apoE−/− mice. GW3965 induced expression of ATP-binding cassettes A1 and G1 in modified low-density lipoprotein-loaded macrophages in vitro as well as in the aortas of hyperlipidemic mice, suggesting that direct actions of LXR ligands on vascular gene expression are likely to contribute to their antiatherogenic effects. These observations provide direct evidence for an atheroprotective effect of LXR agonists and support their further evaluation as potential modulators of human cardiovascular disease.

Hyperlipidemic effects of dietary saturated fats mediated through PGC-1β coactivation of SREBP

The PGC-1 family of coactivators stimulates the activity of certain transcription factors and nuclear receptors. Transcription factors in the sterol responsive element binding protein (SREBP) family are key regulators of the lipogenic genes in the liver. We show here that high-fat feeding, which induces hyperlipidemia and atherogenesis, stimulates the expression of both PGC-1beta and SREBP1c and 1a in liver. PGC-1beta coactivates the SREBP transcription factor family and stimulates lipogenic gene expression. Further, PGC-1beta is required for SREBP-mediated lipogenic gene expression. However, unlike SREBP itself, PGC-1beta reduces fat accumulation in the liver while greatly increasing circulating triglycerides and cholesterol in VLDL particles. The stimulation of lipoprotein transport upon PGC-1beta expression is likely due to the simultaneous coactivation of the liver X receptor, LXRalpha, a nuclear hormone receptor with known roles in hepatic lipid transport. These data suggest a mechanism through which dietary saturated fats can stimulate hyperlipidemia and atherogenesis.

LXR-Dependent Gene Expression Is Important for Macrophage Survival and the Innate Immune Response

The liver X receptors (LXRs) are nuclear receptors with established roles in the regulation of lipid metabolism. We now show that LXR signaling not only regulates macrophage cholesterol metabolism but also impacts antimicrobial responses. Mice lacking LXRs are highly susceptible to infection with the intracellular bacteria Listeria monocytogenes (LM). Bone marrow transplant studies point to altered macrophage function as the major determinant of susceptibility. LXR-null macrophages undergo accelerated apoptosis when challenged with LM and exhibit defective bacterial clearance in vivo. These defects result, at least in part, from loss of regulation of the antiapoptotic factor SPalpha, a direct target for regulation by LXRalpha. Expression of LXRalpha or SPalpha in macrophages inhibits apoptosis in the setting of LM infection. Our results demonstrate that LXR-dependent gene expression plays an unexpected role in innate immunity and suggest that common nuclear receptor pathways mediate macrophage responses to modified lipoproteins and intracellular pathogens.

Autoregulation of the human liver X receptor alpha promoter.

ABSTRACT Previous work has implicated the nuclear receptors liver X receptor α (LXRα) and LXRβ in the regulation of macrophage gene expression in response to oxidized lipids. Macrophage lipid loading leads to ligand activation of LXRs and to induction of a pathway for cholesterol efflux involving the LXR target genes ABCA1 andapoE. We demonstrate here that autoregulation of the LXRα gene is an important component of this lipid-inducible efflux pathway in human macrophages. Oxidized low-density lipoprotein, oxysterols, and synthetic LXR ligands induce expression of LXRα mRNA in human monocyte-derived macrophages and human macrophage cell lines but not in murine peritoneal macrophages or cell lines. This is in contrast to peroxisome proliferator-activated receptor γ (PPARγ)-specific ligands, which stimulate LXRα expression in both human and murine macrophages. We further demonstrate that LXR and PPARγ ligands cooperate to induce LXRα expression in human but not murine macrophages. Analysis of the human LXRα promoter led to the identification of multiple LXR response elements. Interestingly, the previously identified PPAR response element (PPRE) in the murine LXRα gene is not conserved in humans; however, a different PPRE is present in the human LXR 5′-flanking region. These results have implications for cholesterol metabolism in human macrophages and its potential to be regulated by synthetic LXR and/or PPARγ ligands. The ability of LXRα to regulate its own promoter is likely to be an integral part of the macrophage physiologic response to lipid loading.

NR4A orphan nuclear receptors are transcriptional regulators of hepatic glucose metabolism

Hepatic glucose production is crucial for glucose homeostasis, and its dysregulation contributes to the pathogenesis of diabetes. Here, we show that members of the NR4A family of ligand-independent orphan nuclear receptors are downstream mediators of cAMP action in the hormonal control of gluconeogenesis. Hepatic expression of Nur77, Nurr1 and NOR1 is induced by the cAMP axis in response to glucagon and fasting in vivo and is increased in diabetic mice that exhibit elevated gluconeogenesis. Adenoviral expression of Nur77 induces genes involved in gluconeogenesis, stimulates glucose production both in vitro and in vivo, and raises blood glucose levels. Conversely, expression of an inhibitory mutant Nur77 receptor antagonizes gluconeogenic gene expression and lowers blood glucose levels in db/db mice. These results outline a previously unrecognized role for orphan nuclear receptors in the transcriptional control of glucose homeostasis.

Nuclear Receptors: Decoding Metabolic Disease

Nuclear receptors (NR) are a superfamily of ligand‐activated transcription factors that regulate development, reproduction, and metabolism of lipids, drugs and energy. The importance of this family of proteins in metabolic disease is exemplified by NR ligands used in the clinic or under exploratory development for the treatment of diabetes mellitus, dyslipidemia, hypercholesterolemia, or other metabolic abnormalities. Genetic studies in humans and rodents support the notion that NRs control a wide variety of metabolic processes by regulating the expression of genes encoding key enzymes, transporters and other proteins involved in metabolic homeostasis. Current knowledge of complex NR metabolic networks is summarized here.

Induction of NR4A Orphan Nuclear Receptor Expression in Macrophages in Response to Inflammatory Stimuli

Oxidized lipids and inflammatory cytokines are believed to play a causal role in atherosclerosis through the regulation of gene expression in macrophages and other cells. Previous work has implicated the nuclear receptors peroxisome proliferator-activated receptor and liver X receptor in the control of lipid-dependent gene expression and inflammation. Here we demonstrate that expression of a third group of nuclear receptors, the NR4A ligand-independent orphan receptors, is highly inducible in macrophages by diverse inflammatory stimuli. Treatment of macrophages with lipopolysaccharide (LPS), cytokines, or oxidized lipids triggers the transcriptional induction of Nur77 (NR4A1), Nurr1 (NR4A2), and NOR1 (NR4A3) expression. Several lines of evidence point to the NF-κB signaling pathway as a principal mediator of inducible NR4A expression in macrophages. Analysis of the murine and human Nur77 promoters revealed two highly conserved NF-κB response elements. Mutation of these elements inhibited LPS-dependent expression of the Nur77 promoter in transient transfection assays. Furthermore, induction of Nur77 expression by LPS was severely compromised in fibroblasts lacking the three NF-κB subunits, Nfkb1, c-Rel, and RelA. Consistent with its ability to be induced by oxidized lipids, Nur77 was expressed in macrophages within human atherosclerotic lesions. These results identified NR4A nuclear receptors as potential transcriptional mediators of inflammatory signals in activated macrophages.

Fat’s loss is bone’s gain

Osteoporosis, characterized by low bone mass and structural deterioration of bone tissue with an increased susceptibility to fractures, is a major public health threat to the elderly. Bone mass homeostasis in adults is maintained locally by the balance between osteoblastic bone formation and osteoclastic bone resorption. Haploinsufficiency of PPARgamma, a key transcription factor implicated previously in adipogenesis, lipid metabolism, and glucose homeostasis, has now been shown to promote osteogenesis through enhanced osteoblast formation. These findings support a reciprocal relationship between the development of bone and fat, and may prompt further exploration of the PPAR pathway as a potential target for intervention in osteoporosis.

Impaired Development of Atherosclerosis in Hyperlipidemic Ldlr−/− and ApoE−/− Mice Transplanted With Abcg1−/− Bone Marrow

Objective—The lungs of Abcg1−/− mice accumulate macrophage foam cells that contain high levels of unesterified and esterified cholesterol, consistent with a role for ABCG1 in facilitating the efflux of cholesterol from macrophages to high-density lipoprotein (HDL) and other exogenous sterol acceptors. Based on these observations, we investigated whether loss of ABCG1 affects foam cell deposition in the artery wall and the development of atherosclerosis. Methods and Results—Bone marrow from wild-type or Abcg1−/− mice was transplanted into Ldlr−/− or ApoE−/− mice. After administration of a high-fat/high-cholesterol diet, plasma and tissue lipid levels and atherosclerotic lesion size were quantified and compared. Surprisingly, transplantation of Abcg1−/− bone marrow cells resulted in a significant reduction in lesion size in both mouse models, despite the fact that lipid levels increased in the lung, spleen, and kidney. Lesions of Ldlr−/− mice transplanted with Abcg1−/− cells contained increased numbers of apoptotic cells. Consistent with this observation, in vitro studies demonstrated that Abcg1−/− macrophages were more susceptible to oxidized low-density lipoprotein (ox-LDL)-dependent apoptosis than Abcg1+/+ cells. Conclusions—Diet-induced atherosclerosis is impaired when atherosclerotic-susceptible mice are transplanted with Abcg1−/− bone marrow. The demonstration that Abcg1−/− macrophages undergo accelerated apoptosis provides a mechanism to explain the decrease in the atherosclerotic lesions.

Insulin Resistance and Altered Systemic Glucose Metabolism in Mice Lacking Nur77

OBJECTIVE Nur77 is an orphan nuclear receptor with pleotropic functions. Previous studies have identified Nur77 as a transcriptional regulator of glucose utilization genes in skeletal muscle and gluconeogenesis in liver. However, the net functional impact of these pathways is unknown. To examine the consequence of Nur77 signaling for glucose metabolism in vivo, we challenged Nur77 null mice with high-fat feeding. RESEARCH DESIGN AND METHODS Wild-type and Nur77 null mice were fed a high-fat diet (60% calories from fat) for 3 months. We determined glucose tolerance, tissue-specific insulin sensitivity, oxygen consumption, muscle and liver lipid content, muscle insulin signaling, and expression of glucose and lipid metabolism genes. RESULTS Mice with genetic deletion of Nur77 exhibited increased susceptibility to diet-induced obesity and insulin resistance. Hyperinsulinemic-euglycemic clamp studies revealed greater high-fat diet–induced insulin resistance in both skeletal muscle and liver of Nur77 null mice compared with controls. Loss of Nur77 expression in skeletal muscle impaired insulin signaling and markedly reduced GLUT4 protein expression. Muscles lacking Nur77 also exhibited increased triglyceride content and accumulation of multiple even-chained acylcarnitine species. In the liver, Nur77 deletion led to hepatic steatosis and enhanced expression of lipogenic genes, likely reflecting the lipogenic effect of hyperinsulinemia. CONCLUSIONS Collectively, these data demonstrate that loss of Nur77 influences systemic glucose metabolism and highlight the physiological contribution of muscle Nur77 to this regulatory pathway.