Sung Sik Choe

Overexpression of Glucose-6-Phosphate Dehydrogenase Is Associated with Lipid Dysregulation and Insulin Resistance in Obesity†

ABSTRACT Glucose-6-phosphate dehydrogenase (G6PD) produces cellular NADPH, which is required for the biosynthesis of fatty acids and cholesterol. Although G6PD is required for lipogenesis, it is poorly understood whether G6PD in adipocytes is involved in energy homeostasis, such as lipid and glucose metabolism. We report here that G6PD plays a role in adipogenesis and that its increase is tightly associated with the dysregulation of lipid metabolism and insulin resistance in obesity. We observed that the enzymatic activity and expression levels of G6PD were significantly elevated in white adipose tissues of obese models, including db/db, ob/ob, and diet-induced obesity mice. In 3T3-L1 cells, G6PD overexpression stimulated the expression of most adipocyte marker genes and elevated the levels of cellular free fatty acids, triglyceride, and FFA release. Consistently, G6PD knockdown via small interfering RNA attenuated adipocyte differentiation with less lipid droplet accumulation. Surprisingly, the expression of certain adipocytokines such as tumor necrosis factor alpha and resistin was increased, whereas that of adiponectin was decreased in G6PD overexpressed adipocytes. In accordance with these results, overexpression of G6PD impaired insulin signaling and suppressed insulin-dependent glucose uptake in adipocytes. Taken together, these data strongly suggest that aberrant increase of G6PD in obese and/or diabetic subjects would alter lipid metabolism and adipocytokine expression, thereby resulting in failure of lipid homeostasis and insulin resistance in adipocytes.

Berberine improves lipid dysregulation in obesity by controlling central and peripheral AMPK activity

AMP-activated protein kinase (AMPK) plays an important role in regulating whole body energy homeostasis. Recently, it has been demonstrated that berberine (BBR) exerts antiobesity and antidiabetic effects in obese and diabetic rodent models through the activation of AMPK in peripheral tissues. Here we show that BBR improves lipid dysregulation and fatty liver in obese mice through central and peripheral actions. In obese db/db and ob/ob mice, BBR treatment reduced liver weight, hepatic and plasma triglyceride, and cholesterol contents. In the liver and muscle of db/db mice, BBR promoted AMPK activity and fatty acid oxidation and changed expression of genes involved in lipid metabolism. Additionally, intracerebroventricular administration of BBR decreased the level of malonyl-CoA and stimulated the expression of fatty acid oxidation genes in skeletal muscle. Together, these data suggest that BBR would improve fatty liver in obese subjects, which is probably mediated not only by peripheral AMPK activation but also by neural signaling from the central nervous system.

Down-regulation of Histone Deacetylases Stimulates Adipocyte Differentiation

Specific cell type differentiation is driven by programmed regulation of gene expression, which is the result of coordinated modulation of the transcription machinery and chromatin-remodeling factors. We present evidence here that the down-regulation of histone deacetylases is an important process during adipocyte differentiation. In 3T3-L1 cells, histone hyperacetylation was selectively induced at the promoter regions of adipogenic genes during adipocyte differentiation. Interestingly, this was accompanied by a dramatic decrease in the expression level of several histone deacetylases including HDAC1, -2, and -5 and a reduction in overall histone deacetylase enzyme activity. Inhibition of histone deacetylase activity using sodium butyrate resulted in stimulation of adipogenic gene expression and adipocyte differentiation. Consistently, HDAC1 knock-down promoted adipogenesis whereas HDAC1 overexpression attenuated adipocyte differentiation in 3T3-L1 cells. Together, these results suggest that the regulation of not only adipogenic transcription factors, but also chromatin-modifying enzymes is crucial for the execution of bona fide adipogenesis.

Increase in Glucose-6-Phosphate Dehydrogenase in Adipocytes Stimulates Oxidative Stress and Inflammatory Signals

In adipocytes, oxidative stress and chronic inflammation are closely associated with metabolic disorders, including insulin resistance, obesity, cardiovascular disease, and type 2 diabetes. However, the molecular mechanisms underlying these metabolic disorders have not been thoroughly elucidated. In this report, we demonstrate that overexpression of glucose-6-phosphate dehydrogenase (G6PD) in adipocytes stimulates oxidative stress and inflammatory responses, thus affecting the neighboring macrophages. Adipogenic G6PD overexpression promotes the expression of pro-oxidative enzymes, including inducible nitric oxide synthase and NADPH oxidase, and the activation of nuclear factor-κB (NF-κB) signaling, which eventually leads to the dysregulation of adipocytokines and inflammatory signals. Furthermore, secretory factors from G6PD-overexpressing adipocytes stimulate macrophages to express more proinflammatory cytokines and to be recruited to the adipocytes; this would cause chronic inflammatory conditions in the adipose tissue of obesity. These effects of G6PD overexpression in adipocytes were abolished by pretreatment with NF-κB inhibitors or antioxidant drugs. Thus, we propose that a high level of G6PD in adipocytes may mediate the onset of metabolic disorders in obesity by increasing the oxidative stress and inflammatory signals.

Adipose Tissue Remodeling: its Role in energy Metabolism and Metabolic Disorders

The adipose tissue is a central metabolic organ in the regulation of whole-body energy homeostasis. The white adipose tissue functions as a key energy reservoir for other organs, whereas the brown adipose tissue accumulates lipids for cold-induced adaptive thermogenesis. Adipose tissues secrete various hormones, cytokines, and metabolites (termed as adipokines) that control systemic energy balance by regulating appetitive signals from the central nerve system as well as metabolic activity in peripheral tissues. In response to changes in the nutritional status, the adipose tissue undergoes dynamic remodeling, including quantitative and qualitative alterations in adipose tissue-resident cells. A growing body of evidence indicates that adipose tissue remodeling in obesity is closely associated with adipose tissue function. Changes in the number and size of the adipocytes affect the microenvironment of expanded fat tissues, accompanied by alterations in adipokine secretion, adipocyte death, local hypoxia, and fatty acid fluxes. Concurrently, stromal vascular cells in the adipose tissue, including immune cells, are involved in numerous adaptive processes, such as dead adipocyte clearance, adipogenesis, and angiogenesis, all of which are dysregulated in obese adipose tissue remodeling. Chronic overnutrition triggers uncontrolled inflammatory responses, leading to systemic low-grade inflammation and metabolic disorders, such as insulin resistance. This review will discuss current mechanistic understandings of adipose tissue remodeling processes in adaptive energy homeostasis and pathological remodeling of adipose tissue in connection with immune response.

Chronic Activation of Liver X Receptor Induces β-Cell Apoptosis Through Hyperactivation of Lipogenesis: Liver X Receptor–Mediated Lipotoxicity in Pancreatic β-Cells

Liver X receptor (LXR)α and LXRβ play important roles in fatty acid metabolism and cholesterol homeostasis. Although the functional roles of LXR in the liver, intestine, fat, and macrophages are well established, its role in pancreatic β-cells has not been clearly defined. In this study, we revealed that chronic activation of LXR contributes to lipotoxicity-induced β-cell dysfunction. We observed significantly elevated expression of LXR in the islets of diabetic rodent models, including fa/fa ZDF rats, OLETF rats, and db/db mice. In primary pancreatic islets and INS-1 insulinoma cells, activation of LXR with a synthetic ligand, T0901317, stimulated expression of the lipogenic genes ADD1/SREBP1c, FAS, and ACC and resulted in increased intracellular lipid accumulation. Moreover, chronic LXR activation induced apoptosis in pancreatic islets and INS-1 cells, which was synergistically promoted by high glucose conditions. Taken together, we suggest lipid accumulation caused by chronic activation of LXR in β-cells as a possible cause of β-cell lipotoxicity, a key step in the development of type 2 diabetes.

Lipid-overloaded enlarged adipocytes provoke insulin resistance independent of inflammation

ABSTRACT In obesity, adipocyte hypertrophy and proinflammatory responses are closely associated with the development of insulin resistance in adipose tissue. However, it is largely unknown whether adipocyte hypertrophy per se might be sufficient to provoke insulin resistance in obese adipose tissue. Here, we demonstrate that lipid-overloaded hypertrophic adipocytes are insulin resistant independent of adipocyte inflammation. Treatment with saturated or monounsaturated fatty acids resulted in adipocyte hypertrophy, but proinflammatory responses were observed only in adipocytes treated with saturated fatty acids. Regardless of adipocyte inflammation, hypertrophic adipocytes with large and unilocular lipid droplets exhibited impaired insulin-dependent glucose uptake, associated with defects in GLUT4 trafficking to the plasma membrane. Moreover, Toll-like receptor 4 mutant mice (C3H/HeJ) with high-fat-diet-induced obesity were not protected against insulin resistance, although they were resistant to adipose tissue inflammation. Together, our in vitro and in vivo data suggest that adipocyte hypertrophy alone may be crucial in causing insulin resistance in obesity.

AMPK activation with glabridin ameliorates adiposity and lipid dysregulation in obesity

In this study, we demonstrate that activation of AMP-activated protein kinase (AMPK) with glabridin alleviates adiposity and hyperlipidemia in obesity. In several obese rodent models, glabridin decreased body weight and adiposity with a concomitant reduction in fat cell size. Further, glabridin ameliorated fatty liver and plasma levels of triglyceride and cholesterol. In accordance with these findings, glabridin suppressed the expression of lipogenic genes such as sterol regulatory element binding transcription factor (SREBP)-1c, fatty acid synthase (FAS), acetyl-CoA carboxylase (ACC), and stearoyl-CoA desaturase (SCD)-1 in white adipose tissues and liver, whereas it elevated the expression of fatty acid oxidation genes such as carnitine palmitoyl transferase (CPT)1, acyl-CoA oxidase (ACO), and peroxisome proliferator-activated receptor (PPAR)α in muscle. Moreover, glabridin enhanced phosphorylation of AMPK in muscle and liver and promoted fatty acid oxidation by modulating mitochondrial activity. Together, these data suggest that glabridin is a novel AMPK activator that would exert therapeutic effects in obesity-related metabolic disorders.

Macrophage HIF-2α Ameliorates Adipose Tissue Inflammation and Insulin Resistance in Obesity

In obesity, adipose tissue macrophages (ATMs) play a key role in mediating proinflammatory responses in the adipose tissue, which are associated with obesity-related metabolic complications. Recently, adipose tissue hypoxia has been implicated in the regulation of ATMs in obesity. However, the role of hypoxia-inducible factor (HIF)-2α, one of the major transcription factors induced by hypoxia, has not been fully elucidated in ATMs. In this study, we demonstrate that elevation of macrophage HIF-2α would attenuate adipose tissue inflammation and improve insulin resistance in obesity. In macrophages, overexpression of HIF-2α decreased nitric oxide production and suppressed expression of proinflammatory cytokines through induction of arginase 1. HIF-2α–overexpressing macrophages alleviated proinflammatory responses and improved insulin resistance in adipocytes. In contrast, knockdown of macrophage HIF-2α augmented palmitate-induced proinflammatory gene expression in adipocytes. Furthermore, compared with wild-type mice, Hif-2α heterozygous-null mice aggravated insulin resistance and adipose tissue inflammation with more M1-like ATMs upon high-fat diet (HFD). Moreover, glucose intolerance in HFD-fed Hif-2α heterozygous-null mice was relieved by macrophage depletion with clodronate treatment, implying that increase of proinflammatory ATMs is responsible for insulin resistance by haplodeficiency of Hif-2α upon HFD. Taken together, these data suggest that macrophage HIF-2α would counteract the proinflammatory responses to relieve obesity-induced insulin resistance in adipose tissue.

Anti-obesity effects of Lysimachia foenum-graecum characterized by decreased adipogenesis and regulated lipid metabolism

Lysimachia foenum-graecum has been used as an oriental medicine with anti-inflammatory effect. The anti-obesity effect of L. foenum-graecum extract (LFE) was first discovered in our screening of natural product extract library against adipogenesis. To characterize its anti-obesity effects and to evaluate its potential as an anti-obesity drug, we performed various obesity-related experiments in vitro and in vivo. In adipogenesis assay, LFE blocked the differentiation of 3T3-L1 preadipocyte in a dose-dependent manner with an IC50 of 2.5 µg/ml. In addition, LFE suppressed the expression of lipogenic genes, while increasing the expression of lipolytic genes in vitro at 10 µg/ml and in vivo at 100 mg/kg/day. The anti-adipogenic and anti-lipogenic effect of LFE seems to be mediated by the inhibition of PPARγ and C/EBPα expression as shown in in vitro and in vivo, and the suppression of PPARγ activity in vitro. Moreover, LFE stimulated fatty acid oxidation in an AMPK-dependent manner. In high-fat diet (HFD)-induced obese mice (n = 8/group), oral administration of LFE at 30, 100, and 300 mg/kg/day decreased total body weight gain significantly in all doses tested. No difference in food intake was observed between vehicle- and LFE-treated HFD mice. The weight of white adipose tissues including abdominal subcutaneous, epididymal, and perirenal adipose tissue was reduced markedly in LFE-treated HFD mice in a dose-dependent manner. Treatment of LFE also greatly improved serum levels of obesity-related biomarkers such as glucose, triglycerides, and adipocytokines leptin, adiponectin, and resistin. All together, these results showed anti-obesity effects of LFE on adipogenesis and lipid metabolism in vitro and in vivo and raised a possibility of developing LFE as anti-obesity therapeutics.