Yong（刘勇） Liu

Variants in the Fat Mass–and Obesity-Associated ( FTO ) Gene Are Not Associated With Obesity in a Chinese Han Population

OBJECTIVE— Recently, genome-wide association studies have provided evidence that several common variants within the fat mass–and obesity-associated (FTO) gene were significantly associated with obesity in populations of European origin. However, their effects in other ethnic populations remain to be elucidated. RESEARCH DESIGN AND METHODS— In this study, we examined the association between three FTO variants (rs8050136, rs9939609, and rs9930506) and obesity and related traits in a population-based study of 3,210 unrelated Chinese Han subjects from Shanghai and Beijing. In secondary analyses, we also tested for association with type 2 diabetes and related traits. Logistics regression and generalized linear models were used to test for additive and dominant effects of the risk alleles. RESULTS— The minor allele frequencies of rs8050136, rs9939609, and rs9930506 in our population (0.12, 0.12, and 0.20, respectively) were substantially lower than those observed for populations of European descent (e.g., for CEU population of HapMap: 0.45, 0.48, and 0.45, respectively). Despite our study being sufficiently powered to detect effects similar to those previously reported, none of the FTO SNPs were found to be associated with obesity, overweight, BMI, waist circumference, or body fat percentage. In addition, none of the SNPs exhibited significant associations with fasting levels of plasma glucose, A1C, insulin, or β-cell function (estimated via homeostasis model assessment) under either an additive or a dominant model in the quantitative trait analyses. Analyses stratified by sex or geographical region did not change these observations. CONCLUSIONS— Our data do not support that the FTO common variants are major contributors of obesity or type 2 diabetes in the Chinese Han population.

Structural modulation of gut microbiota in life-long calorie-restricted mice

Calorie restriction has been regarded as the only experimental regimen that can effectively lengthen lifespan in various animal models, but the actual mechanism remains controversial. The gut microbiota has been shown to have a pivotal role in host health, and its structure is mostly shaped by diet. Here we show that life-long calorie restriction on both high-fat or low-fat diet, but not voluntary exercise, significantly changes the overall structure of the gut microbiota of C57BL/6 J mice. Calorie restriction enriches phylotypes positively correlated with lifespan, for example, the genus Lactobacillus on low-fat diet, and reduces phylotypes negatively correlated with lifespan. These calorie restriction-induced changes in the gut microbiota are concomitant with significantly reduced serum levels of lipopolysaccharide-binding protein, suggesting that animals under calorie restriction can establish a structurally balanced architecture of gut microbiota that may exert a health benefit to the host via reduction of antigen load from the gut.

Ferritin Concentrations, Metabolic Syndrome, and Type 2 Diabetes in Middle-Aged and Elderly Chinese

CONTEXT Elevated ferritin concentrations frequently cluster with well-established risk factors of diabetes including obesity, metabolic syndrome, chronic inflammation, and altered circulating adipokines. Few studies, however, have systematically evaluated the effect of these risk factors on ferritin-diabetes association, particularly in Chinese populations. OBJECTIVE We aimed to investigate, in a middle-aged and elderly Chinese population, whether elevated ferritin concentrations are associated with higher risk of metabolic syndrome and type 2 diabetes and to what extent the associations were influenced by obesity, inflammation, and adipokines. DESIGN AND METHODS We conducted a population-based, cross-sectional survey of 3,289 participants aged 50-70 yr in Beijing and Shanghai in 2005. Fasting plasma ferritin, glucose, insulin, lipid profile, glycohemoglobin, inflammatory markers, adipokines, and dietary profile were measured. RESULTS Median ferritin concentrations were 155.7 ng/ml for men and 111.9 ng/ml for women. After multiple adjustment, the odds ratios (ORs) were substantially higher for type 2 diabetes (OR 3.26, 95% confidence interval 2.36-4.51) and metabolic syndrome [OR 2.80 (95% confidence interval 2.24-3.49)] in the highest ferritin quartile compared with those in the lowest quartile. These associations remained significant after further adjustment for dietary factors, body mass index, inflammatory markers, and adipokines. CONCLUSIONS Elevated circulating ferritin concentrations were associated with higher risk of type 2 diabetes and metabolic syndrome in middle-aged and elderly Chinese independent of obesity, inflammation, adipokines, and other risk factors. Our data support the crucial role of iron overload for metabolic diseases, even in a country with relatively high prevalence of iron deficiency.

Leucine Deprivation Increases Hepatic Insulin Sensitivity via GCN2/mTOR/S6K1 and AMPK Pathways

OBJECTIVE We have previously shown that serum insulin levels decrease threefold and blood glucose levels remain normal in mice fed a leucine-deficient diet, suggesting increased insulin sensitivity. The goal of the current study is to investigate this possibility and elucidate the underlying cellular mechanisms. RESEARCH DESIGN AND METHODS Changes in metabolic parameters and expression of genes and proteins involved in regulation of insulin sensitivity were analyzed in mice, human HepG2 cells, and mouse primary hepatocytes under leucine deprivation. RESULTS We show that leucine deprivation improves hepatic insulin sensitivity by sequentially activating general control nonderepressible (GCN)2 and decreasing mammalian target of rapamycin/S6K1 signaling. In addition, we show that activation of AMP-activated protein kinase also contributes to leucine deprivation–increased hepatic insulin sensitivity. Finally, we show that leucine deprivation improves insulin sensitivity under insulin-resistant conditions. CONCLUSIONS This study describes mechanisms underlying increased hepatic insulin sensitivity under leucine deprivation. Furthermore, we demonstrate a novel function for GCN2 in the regulation of insulin sensitivity. These observations provide a rationale for short-term dietary restriction of leucine for the treatment of insulin resistance and associated metabolic diseases.

A crucial role for RACK1 in the regulation of glucose-stimulated IRE1alpha activation in pancreatic beta cells.

RACK1 dictates the response of the intracellular stress sensor IRE1α to different extracellular stimuli. Stress Control Obesity and metabolic diseases, such as diabetes, are associated with endoplasmic reticulum (ER) stress and the accumulation of unfolded proteins in the ER, which activates the unfolded protein response (UPR). One of the mediators of the UPR is inositol-requiring enzyme 1α (IRE1α), which is autophosphorylated and activated in response to ER stress. In pancreatic β cells, IRE1α promotes insulin biosynthesis in response to acute glucose stimulation but inhibits this process after chronic glucose stimulation. To determine the mechanisms that mediate these different responses of IRE1α to glucose stimulation, Qiu et al. searched for previously unidentified binding partners of IRE1α. They found that the scaffold protein RACK1 interacted with IRE1α after glucose stimulation. Protein phosphatase 2A (PP2A) remained associated with RACK1 after acute glucose stimulation but dissociated from RACK1 after chronic glucose stimulation or the induction of ER stress. The differential association of PP2A with RACK1 accounted for stimulus-specific alterations in the phosphorylation and activation state of IRE1α. Islets from db/db mice, which are obese and mildly diabetic, showed decreased RACK1 abundance, as well as increased IRE1α phosphorylation and insulin content, and overexpression of RACK1 in these islets partially reversed these increases. Thus, RACK1 differentially modulates the activation of IRE1α in response to the duration of glucose stimulation and to ER stress, and RACK1-mediated regulation of IRE1α may be altered by prolonged metabolic stress. Autophosphorylation of inositol-requiring enzyme 1α (IRE1α) is required for its activation, which elicits the cellular unfolded protein response (UPR) and is functionally connected with insulin biosynthesis in pancreatic β cells. We found that the scaffold protein receptor for activated C-kinase 1 (RACK1) interacted with IRE1α in a glucose-stimulated or endoplasmic reticulum (ER) stress–responsive manner in pancreatic β cells and primary islets. RACK1 mediated the glucose-inducible assembly of a complex containing IRE1α, RACK1, and protein phosphatase 2A (PP2A) to promote dephosphorylation of IRE1α by PP2A, thereby inhibiting glucose-stimulated IRE1α activation and attenuating IRE1α-dependent increases in insulin production. Moreover, IRE1α activation was increased and RACK1 abundance was decreased in a mouse model of diabetes. Thus, our findings demonstrate that RACK1 functions as a key component in regulating the IRE1α signaling pathway in pancreatic β cells.

Downregulation of miR-181a upregulates sirtuin-1 (SIRT1) and improves hepatic insulin sensitivity

Aims/hypothesisSirtuin-1 (SIRT1) is a potential therapeutic target to combat insulin resistance and type 2 diabetes. This study aims to identify a microRNA (miRNA) targeting SIRT1 to regulate hepatic insulin sensitivity.MethodsLuciferase assay combined with mutation and immunoblotting was used to screen and verify the bioinformatically predicted miRNAs. miRNA and mRNA levels were measured by real-time PCR. Insulin signalling was detected by immunoblotting and glycogen synthesis. Involvement of SIRT1 was studied with adenovirus, inhibitor and SIRT1-deficient hepatocytes. The role of miR-181a in vivo was explored with adenovirus and locked nucleic acid antisense oligonucleotides.ResultsmiR-181a targets the 3′ untranslated region (3′UTR) of Sirt1 mRNA through a miR-181a binding site, and downregulates SIRT1 protein abundance at the translational level. miR-181a is increased in insulin-resistant cultured hepatocytes and liver, and in the serum of diabetic patients. Overexpression of miR-181a decreases SIRT1 protein levels and activity, and causes insulin resistance in hepatic cells. Inhibition of miR-181a by antisense oligonucleotides increases SIRT1 protein levels and activity, and improves insulin sensitivity in hepatocytes. Ectopic expression of SIRT1 abrogates the effect of miR-181a on insulin sensitivity, and inhibition of SIRT1 activity or SIRT1 deficiency markedly attenuated the improvement in insulin sensitivity induced by antisense miR-181a. In addition, overexpression of miR-181a by adenovirus impairs hepatic insulin signalling, and intraperitoneal injection of locked nucleic acid antisense oligonucleotides for miR-181a improves glucose homeostasis in diet-induced obesity mice.Conclusions/interpretationmiR-181a regulates SIRT1 and improves hepatic insulin sensitivity. Inhibition of miR-181a might be a potential new strategy for treating insulin resistance and type 2 diabetes.

Abrogation of hepatic ATP-citrate lyase protects against fatty liver and ameliorates hyperglycemia in leptin receptor-deficient mice.

Hepatic steatosis is a hallmark of nonalcoholic fatty liver disease (NAFLD) and a key component of obesity‐associated metabolic dysfunctions featuring dyslipidemia, insulin resistance, and loss of glycemic control. It has yet to be completely understood how much dysregulated de novo lipogenesis contributes to the pathogenic development of hepatic steatosis and insulin resistance. ATP‐citrate lyase (ACL) is a lipogenic enzyme that catalyzes the critical reaction linking cellular glucose catabolism and lipogenesis, converting cytosolic citrate to acetyl‐coenzyme A (CoA). Acetyl‐CoA is further converted to malonyl‐CoA, the essential precursor for fatty acid biosynthesis. We investigated whether dysregulation of hepatic ACL is metabolically connected to hepatic steatosis, insulin resistance, and hyperglycemia. We found that in leptin receptor–deficient db/db mice, the expression of ACL was selectively elevated in the liver but not in the white adipose tissue. Liver‐specific ACL abrogation via adenovirus‐mediated RNA interference prominently reduced the hepatic contents of both acetyl‐CoA and malonyl‐CoA, markedly inhibited hepatic de novo lipogenesis, and protected against hepatic steatosis in db/db mice. Surprisingly, liver‐specific ACL abrogation markedly inhibited the expression of peroxisome proliferator‐activated receptor‐gamma and the entire lipogenic program in the liver. Moreover, hepatic ACL deficiency resulted in significantly down‐regulated expression of gluconeogenic genes in the liver as well as enhanced insulin sensitivity in the muscle, leading to substantially improved systemic glucose metabolism. Conclusion: These findings establish a crucial role of hepatic ACL in lipid and glucose metabolism; therefore, hepatic ACL may serve as a potential target to treat NAFLD and type 2 diabetes. (HEPATOLOGY 2009.)

Fibroblast growth factor 21 is regulated by the IRE1α-XBP1 branch of the unfolded protein response and counteracts endoplasmic reticulum stress-induced hepatic steatosis.

Background: Although both are involved in metabolic homeostasis, the interconnection between ER stress and FGF21 remains incompletely understood. Results: Directly up-regulated by the IRE1α-XBP1 pathway, FGF21 could alleviate ER stress-induced liver steatosis. Conclusion: FGF21 acts as a metabolic effector of the UPR program, exerting feedback effects upon lipid metabolism. Significance: These findings reveal a regulatory mechanism linking FGF21 actions to metabolic ER stress. Endoplasmic reticulum (ER) stress activates the adaptive unfolded protein response (UPR) and represents a critical mechanism that underlies metabolic dysfunctions. Fibroblast growth factor 21 (FGF21), a hormone that is predominantly secreted by the liver, exerts a broad range of effects upon the metabolism of carbohydrates and lipids. Although increased circulating levels of FGF21 have been documented in animal models and human subjects with obesity and nonalcoholic fatty liver disease, the functional interconnections between metabolic ER stress and FGF21 are incompletely understood. Here, we report that increased ER stress along with the simultaneous elevation of FGF21 expression were associated with the occurrence of nonalcoholic fatty liver disease both in diet-induced obese mice and human patients. Intraperitoneal administration of the ER stressor tunicamycin in mice resulted in hepatic steatosis, accompanied by activation of the three canonical UPR branches and increased the expression of FGF21. Furthermore, the IRE1α-XBP1 pathway of the UPR could directly activate the transcriptional expression of Fgf21. Administration of recombinant FGF21 in mice alleviated tunicamycin-induced liver steatosis, in parallel with reduced eIF2α-ATF4-CHOP signaling. Taken together, these results suggest that FGF21 is an integral physiological component of the cellular UPR program, which exerts beneficial feedback effects upon lipid metabolism through counteracting ER stress.

Associations of Physical Activity With Inflammatory Factors, Adipocytokines, and Metabolic Syndrome in Middle-Aged and Older Chinese People

Background— Inflammatory factors, adipocytokines, and the metabolic syndrome are important determinants of cardiometabolic disease. It remains unclear how physical activity is related to these risk factors. Our objective was to investigate single and joint associations of physical activity with inflammatory factors, adipocytokines, and the metabolic syndrome among middle-aged and older Chinese people. Methods and Results— A total of 3289 individuals (1458 men, 1831 women) 50 to 70 years of age participated in a population-based cross-sectional survey in Beijing and Shanghai, China. Levels of total physical activity were assessed with the International Physical Activity Questionnaire. High-sensitivity C-reactive protein, interleukin-6, tumor necrosis factor-&agr; receptor 2, adiponectin, and retinol-binding protein 4 were measured. The metabolic syndrome was defined using the updated National Cholesterol Education Program/Adult Treatment Panel III criteria for Asian Americans. Plasma concentrations of high-sensitivity C-reactive protein were 1.58, 1.74, and 1.27 mg/L (P=0.0138) and of adiponectin were 16.12, 16.20, and 17.21 mg/L (P=0.0078) among individuals with low, medium, and high levels of total physical activity, respectively, with adjustment for potential confounders. In the multivariable-adjusted logistic regression analyses, participants with higher levels of total physical activity had a lower risk of having the metabolic syndrome (odds ratio, 0.68; 95% confidence interval, 0.54 to 0.85; P for trend=0.001) compared with those with lower levels. Conclusions— Being physically active is associated with a better profile of inflammatory factors and adipocytokines and a reduced risk of having the metabolic syndrome among Chinese people.

Leptin contributes to the adaptive responses of mice to high-fat diet intake through suppressing the lipogenic pathway.

Background Leptin is an adipocyte-derived hormone that plays a critical role in energy homeostasis and lipid metabolism. Overnutrition-associated obesity is known to be accompanied by hyperleptinemia. However, the physiological actions of leptin in the metabolic responses to high-fat diet (HFD) intake remain to be completely elucidated. Here we characterized the metabolic features of mice fed high-fat diets and investigated the impact of leptin upon the lipogenic program which was found to be suppressed by HFD feeding through a proteomics approach. Results When maintained on two types of high-fat diets for up to 16 weeks, mice with a higher fat intake exhibited increased body fat accumulation at a greater pace, developing more severely impaired glucose tolerance. Notably, HFD feeding at 4 weeks elicited the onset of marked hyperleptinemia, prior to the occurrence of apparent insulin resistance and hyperinsulinemia. Proteomic analysis revealed dramatically decreased expression of lipogenic enzymes in the white adipose tissue (WAT) from HFD-fed mice, including ATP-citrate lyase (ACL) and fatty acid synthase (FAS). The expression of ACL and FAS in the liver was similarly suppressed in response to HFD feeding. By contrast, HFD-induced downregulation of hepatic ACL and FAS was significantly attenuated in leptin receptor-deficient db/db mice. Furthermore, in the liver and WAT of wild type animals, intraperitoneal leptin administration was able to directly suppress the expression of these two lipogenic enzymes, accompanied by reduced triglyceride levels both in the liver and serum. Conclusions These results suggest that leptin contributes to the metabolic responses in adaptation to overnutrition through suppressing the expression of lipogenic enzymes, and that the lipogenic pathway represents a key targeted peripheral component in exerting leptins liporegulatory actions.