Yuchang Fu

Adiponectin and the metabolic syndrome : mechanisms mediating risk for metabolic and cardiovascular disease

Purpose of review Adiponectin is secreted exclusively by adipocytes, aggregates in multimeric forms, and circulates at high concentrations in blood. This review summarizes recent studies highlighting cellular effects of adiponectin and its role in human lipid metabolism and atherosclerosis. Recent findings Adiponectin is an important autocrine/paracrine factor in adipose tissue that modulates differentiation of preadipocytes and favors formation of mature adipocytes. It also functions as an endocrine factor, influencing whole-body metabolism via effects on target organs. Adiponectin multimers exert differential biologic effects, with the high-molecular-weight multimer associated with favorable metabolic effects (i.e. greater insulin sensitivity, reduced visceral adipose mass, reduced plasma triglycerides, and increased HDL-cholesterol). Adiponectin influences plasma lipoprotein levels by altering the levels and activity of key enzymes (lipoprotein lipase and hepatic lipase) responsible for the catabolism of triglyceride-rich lipoproteins and HDL. It thus influences atherosclerosis by affecting the balance of atherogenic and antiatherogenic lipoproteins in plasma, and by modulating cellular processes involved in foam cell formation. Summary Recent studies emphasize the role played by adiponectin in the homeostasis of adipose tissue and in the pathogenesis of the metabolic syndrome, type 2 diabetes, and atherosclerosis. These pleiotropic effects make it an attractive therapeutic target for obesity-related conditions.

Foam cells in atherosclerosis.

Atherosclerosis is a chronic disease characterized by the deposition of excessive cholesterol in the arterial intima. Macrophage foam cells play a critical role in the occurrence and development of atherosclerosis. The generation of these cells is associated with imbalance of cholesterol influx, esterification and efflux. CD36 and scavenger receptor class A (SR-A) are mainly responsible for uptake of lipoprotein-derived cholesterol by macrophages. Acyl coenzyme A:cholesterol acyltransferase-1 (ACAT1) and neutral cholesteryl ester hydrolase (nCEH) regulate cholesterol esterification. ATP-binding cassette transporters A1(ABCA1), ABCG1 and scavenger receptor BI (SR-BI) play crucial roles in macrophage cholesterol export. When inflow and esterification of cholesterol increase and/or its outflow decrease, the macrophages are ultimately transformed into lipid-laden foam cells, the prototypical cells in the atherosclerotic plaque. The aim of this review is to describe what is known about the mechanisms of cholesterol uptake, esterification and release in macrophages. An increased understanding of the process of macrophage foam cell formation will help to develop novel therapeutic interventions for atherosclerosis.

Adiponectin reduces lipid accumulation in macrophage foam cells

Adiponectin is one of several, important metabolically active cytokines secreted from adipocytes. Low circulating levels of this adipokine have been associated epidemiologically with obesity, insulin resistance, type II diabetes, and cardiovascular disease. To determine if adiponectin can modulate lipid metabolism in macrophages, we expressed the adiponectin gene in human THP-1 macrophage foam cells using a lentiviral vector expression system and demonstrated that macrophages transduced with the adiponectin gene had decreased lipid accumulation compared with control macrophages transduced with the LacZ gene. Macrophages transduced with the adiponectin gene also exhibited decreased oxidized low-density lipoprotein (oxLDL) uptake and increased HDL-mediated cholesterol efflux. Additional studies suggest two potential mechanisms for the reduced lipid accumulation in these adiponectin-transduced macrophage foam cells. The first mechanism involves the PPARgamma and LXR signaling pathways which up-regulate the expression of ABCA1 and promote lipid efflux from these cells. The second mechanism involves decreased lipid uptake and increased lipid hydrolysis which may result from decreased SR-AI and increased SR-BI and HSL gene activities in the transformed macrophage foam cells. We also demonstrated that the expression of two proatherogenic cytokines, MCP-1 and TNFalpha, were decreased in the adiponectin-transduced macrophage foam cells. These results suggest that adiponectin may modulate multiple pathways of lipid metabolism in macrophages. Our studies provide new insights into potential mechanisms of adiponectin-mediated alterations in lipid metabolism and macrophage foam cell formation which may impact the development of atherosclerosis.

Macrophage Adiponectin Expression Improves Insulin Sensitivity and Protects Against Inflammation and Atherosclerosis

OBJECTIVE Adiponectin is one of several important metabolically active cytokines secreted from adipose tissue. Epidemiologic studies have associated low-circulating levels of this adipokine with multiple metabolic disorders including obesity, insulin resistance, type 2 diabetes, and cardiovascular disease. To investigate adiponectin-mediated changes in metabolism in vivo, we generated transgenic mice that specifically express the gene coding for human adiponectin in mouse macrophages using the human scavenger receptor A-I gene enhancer/promoter. METHODS AND RESULTS Using this transgenic mouse model, we found that adiponectin expression was associated with reduced whole-animal body and fat-pad weight and an improved lipid accumulation in macrophages when these transgenic mice were fed with a high-fat diet. Moreover, these macrophage Ad-TG mice exhibit enhanced whole-body glucose tolerance and insulin sensitivity with reduced proinflammatory cytokines, MCP-1 and TNF-a (both in the serum and in the metabolic active macrophage), adipose tissue, and skeletal muscle under the high-fat diet condition. Additional studies demonstrated that these macrophage adiponectin transgenic animals exhibit reduced macrophage foam cell formation in the arterial wall when these transgenic mice were crossed with an LDL receptor–deficient mouse model and were fed a high-fat diet. CONCLUSIONS These results suggest that adiponectin expressed in macrophages can physiologically modulate metabolic activities in vivo by improving metabolism in distal tissues. The use of macrophages as carriers for adiponectin, a molecule with antidiabetes, anti-inflammatory, and antiatherogenic properties, provides a novel and unique strategy for studying the mechanisms of adiponectin-mediated alterations in body metabolism in vivo.

NR4A Orphan Nuclear Receptors Modulate Insulin Action and the Glucose Transport System POTENTIAL ROLE IN INSULIN RESISTANCE

After observing that expression of two NR4A orphan nuclear receptors, NR4A3 and NR4A1, was altered by insulin in cDNA microarray analyses of human skeletal muscle, we studied whether these receptors could modulate insulin sensitivity. We found that both NR4A3 and NR4A1 were induced by insulin and by thiazolidinedione drugs (pioglitazone and troglitazone) in 3T3-L1 adipocytes. Furthermore, gene expression of NR4A3 and NR4A1 was reduced in skeletal muscles and adipose tissues from multiple rodent models of insulin resistance. To determine whether NR4A3 could modulate insulin sensitivity, 3T3-L1 adipocytes were stably transduced with NR4A3 or LacZ (control) lentiviral vectors. Compared with LacZ expressing cells, hyperexpression of NR4A3 increased the ability of insulin to augment glucose transport activity, and the mechanism involved increased recruitment of GLUT4 glucose transporters to the plasma membrane. NR4A3 hyperexpression also led to an increase in insulin-mediated tyrosine phosphorylation of insulin receptor substrate-1 as well as Akt phosphorylation. Suppression of NR4A3 using lentiviral short hairpin RNA constructs reduced the ability of insulin to stimulate glucose transport and phosphorylate Insulin receptor substrate-1 and Akt. Thus, NR4A3 and NR4A1 are attractive novel therapeutic targets for potential amelioration of insulin resistance, and treatment and prevention of type 2 diabetes and the metabolic syndrome.

The magic and mystery of MicroRNA-27 in atherosclerosis

Atherosclerosis (As) is now widely appreciated to represent a chronic inflammatory reaction of the vascular wall in response to dyslipidemia and endothelial distress involving the inflammatory recruitment of leukocytes and the activation of resident vascular cells. MicroRNAs (miRNAs) are a group of endogenous, small (~22 nucleotides in length) non-coding RNA molecules, which function specifically by base pairing with mRNA of genes, thereby induce translation repressions of the genes within metazoan cells. Recently, the function of miR-27, one of the miRNAs, in the initiation and progression of atherosclerosis has been identified. In vivo and in vitro studies suggest that miR-27 may serve as a diagnostic and prognostic marker for atherosclerosis. More recently, studies have identified important roles for miR-27 in angiogenesis, adipogenesis, inflammation, lipid metabolism, oxidative stress, insulin resistance and type 2 diabetes, etc. In this review, we focus on the role of miR-27 in the development of vulnerable atherosclerotic plaques, potential as a disease biomarker and novel therapeutic target in atherosclerosis.

Adiponectin-AdipoR1/2-APPL1 signaling axis suppresses human foam cell formation: differential ability of AdipoR1 and AdipoR2 to regulate inflammatory cytokine responses.

OBJECTIVE Adiponectin is an adipokine that exerts anti-inflammatory and anti-atherogenic effects during macrophage transformation into foam cells. To further understand the signaling pathways of adiponectin involved in macrophage foam cell transformation, we investigated the roles of two adiponectin receptors (AdipoR1 and AdipoR2) and their downstream adaptor protein, phosphotyrosine interaction, PH domain and leucine zipper containing 1 (APPL1) in mediating adiponectin action on foam cell transformation. METHODS AND RESULTS Transfections were performed to overexpress or knockdown AdipoR1 or AdipoR2 genes in human THP-1 monocytes. Lentiviral-shRNAs were also used to knockdown APPL1 gene in these cells. Foam cell transformation was induced via exposure to oxidized low-density lipoprotein (oxLDL). Our results showed that both AdipoR1 and AdipoR2 were critical for transducing the adiponectin signal that suppresses lipid accumulation and inhibits transformation from macrophage to foam cell. However, AdipoR1 and AdipoR2 were found to have differential effects in diminishing proinflammatory responses. While AdipoR1 was required by adiponectin to suppress tumor necrosis factor alpha (TNFα) and monocyte chemotactic protein 1 (MCP-1) gene expression, AdipoR2 served as the dominant receptor for adiponectin suppression of scavenger receptor A type 1 (SR-AI) and upregulation of interleukin-1 receptor antagonist (IL-1Ra). Knockdown of APPL1 significantly abrogated the ability of adiponectin to inhibit lipid accumulation, SR-AI and nuclear factor-κB (NF-κB) gene expression, and Akt phosphorylation in macrophage foam cells. CONCLUSIONS In current studies, we have demonstrated that adiponectins abilty to suppress macrophage lipid accumulation and foam cell formation is mediated through AdipoR1 and AdipoR2 and the APPL1 docking protein. However, AdipoR1 and AdipoR2 exhibited a differential ability to regulate inflammatory cytokines and SR-A1. These novel data support the idea that the adiponectin-AdipoR1/2-APPL1 axis may serve as a potential therapeutic target for preventing macrophage foam cell formation and atherosclerosis.

Proinflammatory cytokine production and insulin sensitivity regulated by overexpression of resistin in 3T3-L1 adipocytes

Resistin is secreted from adipocytes, and high circulating levels have been associated with obesity and insulin resistance. To investigate whether resistin could exert autocrine effects in adipocytes, we expressed resistin gene in 3T3-L1 fibroblasts using a lentiviral vector, and selected several stably-transduced cell lines under blasticidin selection.We observed that 3T3-L1 adipocytes expressing resistin have a decreased gene expression for related transcriptional factors (CCAAT/enhancer binding protein α(C/EBPα) , peroxisome proliferator-activated receptor gamma (PPARγ), and adipocyte lipid binding protein (ALBP/aP2) which is one of target genes for the PPARγ during adipocyte differentiation,. Overexpression of resistin increased the levels of three proinflammatory cytokines, tumor necrosis factor alpha (TNFα), interleukin 6 (IL-6) and monocyte chemoattractant protein-1 (MCP-1), which play important roles for insulin resistance, glucose and lipid metabolisms during adipogenesis. Furthermore, overexpressing resistin in adipocytes inhibits glucose transport 4 (GLUT4) activity and its gene expression, reducing insulins ability for glucose uptake by 30 %.In conclusion, resistin overexpression in stably transduced 3T3-L1 cells resulted in: 1) Attenuation of programmed gene expression responsible for adipogenesis; 2) Increase in expression of proinflammatory cytokines; 3) Decrease in insulin responsiveness of the glucose transport system. These data suggest a new role for resistin as an autocrine/paracrine factor affecting inflammation and insulin sensitivity in adipose tissue.

Tristetraprolin-dependent post-transcriptional regulation of inflammatory cytokine mRNA expression by apolipoprotein A-I: role of ATP-binding membrane cassette transporter A1 and signal transducer and activator of transcription 3.

Atherosclerosis is an inflammatory disease characterized by the accumulation of macrophages in the arterial intima. The activated macrophages secreted more pro-inflammatory cytokines, such as tumor necrosis factor (TNF)-α, which promote the development of the disease. Apolipoprotein A-I (apoA-I), the major component of high density lipoprotein, is involved in reverse cholesterol transport of lipid metabolism. Recently, it has been found that apoA-I suppresses inflammation via repression of inflammatory cytokine expression; the mechanisms of the apoA-I-suppressive action, however, are not yet well characterized. In this study, we have for the first time found that apoA-I suppresses the expression of some inflammatory cytokines induced by lipopolysaccharide via a specific post-transcriptional regulation process, namely mRNA destabilization, in macrophages. Our further studies have also shown that AU-rich elements in the 3′-untranslated region of TNF-α mRNA are responsive to the apoA-I-mediated mRNA destabilization. The apoA-I-induced inflammatory cytokine mRNA destabilization was associated with increased expression of mRNA-destabilizing protein tristetraprolin through a JAK2/STAT3 signaling pathway-dependent manner. When blocking interaction of apoA-I with ATP-binding membrane cassette transporter A1 (ABCA1), a major receptor for apoA-I in macrophages, it would almost totally abolish the effect of apoA-I on tristetraprolin expression. These results present not only a novel mechanism for the apoA-I-mediated inflammation suppression in macrophages but also provide new insights for developing strategies for modulating vascular inflammation and atherosclerosis.

Antagonism of betulinic acid on LPS-mediated inhibition of ABCA1 and cholesterol efflux through inhibiting nuclear factor-kappaB signaling pathway and miR-33 expression.

ATP-binding cassette transporter A1 (ABCA1) is critical in exporting cholesterol from macrophages and plays a protective role in the development of atherosclerosis. The purpose of this study was to investigate the effects of betulinic acid (BA), a pentacyclic triterpenoid, on ABCA1 expression and cholesterol efflux, and to further determine the underlying mechanism. BA promoted ABCA1 expression and cholesterol efflux, decreased cellular cholesterol and cholesterol ester content in LPS-treated macrophages. Furthermore, we found that BA promoted ABCA1 expression via down-regulation of miR-33s. The inhibition of LPS-induced NF-κB activation further decreased miR-33s expression and enhanced ABCA1 expression and cholesterol efflux when compared with BA only treatment. In addition, BA suppressed IκB phosphorylation, p65 phosphorylation and nuclear translocation, and the transcription of NF-κB-dependent related gene. Moreover, BA reduced atherosclerotic lesion size, miR-33s levels and NF-κB activation, and promoted ABCA1 expression in apoE−/− mice. Taken together, these results reveal a novel mechanism for the BA-mediated ABCA1 expression, which may provide new insights for developing strategies for modulating vascular inflammation and atherosclerosis.