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Dysregulation of hepatic very low-density lipoprotein (VLDL) secretion contributes to the pathogenesis of metabolic diseases, such as Non-alcoholic fatty liver disease (NAFLD) and hyperlipidemia. Accumulating evidence have suggested that long non-coding RNAs (lncRNAs) had malfunctioning roles in the pathogenesis of NAFLD. However, the function of lncRNAs in controlling hepatic VLDL secretion remains largely unillustrated. Here, we identified a novel long non-coding RNA, lncRHPL, which was liver-enriched, downregulated upon high fat diet feeding, and inhibited by oleic acid treatment in primary hepatocytes. With genetic manipulation in mice and primary hepatocytes, depletion of lncRHPL induces hepatic VLDL secretion accompanied with decreased hepatic lipid contents. Conversely, lncRHPL restoration reduces VLDL secretion with an increased lipid deposition in hepatocytes. Mechanistic analyses indicate that lncRHPL binds directly to heterogeneous nuclear ribonuclear protein U (hnRNPU), thereby enhances its stability, and that hnRNPU can transcriptional activate Bmal1, leading to inhibition of VLDL secretion in hepatocytes. lncRHPL deficiency accelerates the protein degradation of hnRNPU and suppresses the transcription of Bmal1, which in turn activates VLDL secretion in hepatocytes. Taken together, lncRHPL is a novel suppressor of hepatic VLDL secretion. Activating the lncRHPL/hnRNPU/Bmal1/MTTP axis represent a potential strategy for the maintenance of intrahepatic and plasma lipid homeostasis.

Background and aims The regulation of hepatic very-low-density lipoprotein (VLDL) secretion is vital for lipid metabolism whose pathogenetic status is involved in fatty liver disease and dyslipidemia seen in hepatic steatosis. Accumulated evidence suggest that apolipoprotein E (ApoE) is closely related to hepatic VLDL secretion. Here, we report that the expression of patatin-like phospholipase domain containing protein 7 (PNPLA7) is strongly induced by hepatic steatosis and positively correlates with plasma triacylglycerol (TAG) levels in the human subjects, whereas the role of PNPLA7 in hepatic VLDL secretion is unknown. Approach and results Herein, with genetic manipulation in the mice, the deficiency of hepatic PNPLA7 expression resulted in reduced VLDL secretion accompanied by enhanced hepatic lipid accumulation and decreased hepatic ApoE expression. Furthermore, knockdown of PNPLA7 in the livers of the db/db mice also resulted in significant reduction in plasma TAG level but aggravated hepatic steatosis. Importantly, we observed that PNPLA7 interacted with ApoE and presumably at the site of endoplasmic reticulum. Mechanistically, we have shown that PNPLA7 could modulate polyubiquitination and proteasomal-mediated degradation of ApoE. Overexpressed ApoE restored the impaired VLDL-TAG metabolism in PNPLA7-knockdown primary hepatocytes. Conclusion PNPLA7 plays a critical role in regulating hepatic VLDL secretion by modulating ApoE stability through its interaction with ApoE.

Chronic hepatitis C virus (HCV) infection has a close association with type 2 diabetes mellitus. Although the mechanisms of insulin resistance in chronic hepatitis C (CHC) patients have been extensively studied, little attention has been given to the role of β-cell function in HCV-associated diabetes. Here, we analysed β-cell function in CHC patients and HCV-infected mouse model and found in addition to insulin resistance, impaired pancreatic β-cell function occurred in CHC patients and HCV-infected C/OTg mice, not only in diabetic individuals but also in individuals with impaired fasting glucose levels. Both first-phase and second-phase insulin secretion were impaired, at least partially due to the reduction of exocytosis of secretory insulin-containing granules following HCV infection. Up-regulated p38δ in HCV-infected β-cells resulted in inactivation of protein kinase D (PKD), which was responsible for impaired insulin secretory capacity of β-cells. Thus, impaired insulin secretion due to HCV infection in β-cells contributes to HCV-associated type 2 diabetes. These findings provided a new inspiration for the important prognostic and therapeutic implications in the management of CHC patients with impaired fasting glucose.

Hepatic gluconeogenesis plays a crucial role in maintaining blood glucose homeostasis in mammals. Globe knockout of suppressor of cytokine signalling-2 (SOCS2), a feedback inhibitor of cytokine signalling, has been shown resistant to high-fat-diet (HFD)-induced hepatic steatosis with impaired glucose tolerance in mice. However, the underlying mechanism of SOCS2 regulates hepatic glucose homeostasis still undefined. In the present study, we demonstrated that the hepatic SOCS2 expression is markedly reduced in fasted C57BL/6 J mice or db/db mice. Moreover, hepatic SOCS2 expression levels are induced by metformin treatment. Ablation of SOCS2 attenuates suppressing effects of metformin on gluconeogenesis in hepatocytes. Gain- and loss-of-function studies indicated that SOCS2 regulates hepatic gluconeogenic genes expression and glucose output by mediating JAK2/STAT5 signalling pathway in db/db mice. Mechanistically, we observed that SOCS2 inactivates STAT5 by attenuating the interaction between JAK2 and STAT5, which in turn reduces hepatic gluconeogenesis. The present study reveals a critical role of SOCS2 in regulating hepatic gluconeogenesis. The inhibitory effect of metformin on gluconeogenesis is mediated, at least in part, by upregulating SOCS2 and therefore reducing hepatic gluconeogenic genes expression. SOCS2 may represent a new therapeutic target for the treatment of diabetes.

BackgroundThis study aimed to systematically investigate the associations between serum sphingolipids and insulin sensitivity as well as insulin secretion. This study also aimed to reveal potential predictors for insulin sensitivity or give perceptive insight into disease processes.MethodsWe conducted a lipidomics evaluation of molecularly distinct SPs in the serum of 86 consecutive Chinese adults with or without obesity and diabetes using electrospray ionization mass spectrometry coupled with liquid chromatography. The GIR30 was measured under steady conditions to assess insulin sensitivity by the gold standard hyperinsulinemic-euglycemic clamp. We created the ROC curves to detect the serum SMs diagnostic value and establish the diagnosis of insulin sensitivity.ResultsDifferential correlation network analysis illustrated correlations amongst lipids, insulin sensitivity, insulin secretion and other clinical indexes. Total and subspecies of serum SMs and globotriaosylceramides (Gb3s) were positively related to GIR30, free FAs (FFA 16:1, FFA20:4), some long chain GM3 and complex ceramide GluCers showed strong negative correlations with GIR30. Notably, ROC curves showed that SM/Cer and SM d18:0/26:0 may be good serum lipid predictors of diagnostic indicators of insulin sensitivity close to conventional clinical indexes such as 1/HOMA-IR (all areas under the curve >0.80) based on GIR30 as standard diagnostic criteria.ConclusionsThese results provide novel associations between serum sphingolipid between insulin sensitivity measured by the hyperinsulinemic-euglycemic clamp. We further identify two specific SPs that may represent prognostic biomarkers for insulin sensitivity.

Little is known about how glycogen metabolism is manipulated during exercise. In article number 2001502, John Zhong Li, Tiemin Liu, Xingxing Kong, and coworkers identify that interferon regulatory factor 4 (IRF4) in skeletal muscle regulates glycogen metabolism and subsequent exercise capacity. This work reveals a regulatory pathway including IRF4/protein targeting to glycogen (PTG)/glycogen synthesis on controlling exercise capacity. [Image: see text]

Exercise‐induced fatigue and exhaustion are interesting areas for many researchers. Muscle glycogen is critical for physical performance. However, how glycogen metabolism is manipulated during exercise is not very clear. The aim here is to assess the impact of interferon regulatory factor 4 (IRF4) on skeletal muscle glycogen and subsequent regulation of exercise capacity. Skeletal muscle‐specific IRF4 knockout mice show normal body weight and insulin sensitivity, but better exercise capacity and increased glycogen content with unaltered triglyceride levels compared to control mice on chow diet. In contrast, mice overexpression of IRF4 displays decreased exercise capacity and lower glycogen content. Mechanistically, IRF4 regulates glycogen‐associated regulatory subunit protein targeting to glycogen (PTG) to manipulate glucose metabolism in skeletal muscle. Knockdown of PTG can reverse the effects imposed by the absence of IRF4 in vivo. These studies reveal a regulatory pathway including IRF4/PTG/glycogen synthesis on controlling exercise capacity., Muscle glycogen is critical for exercise‐induced fatigue. However, how glycogen is manipulated during exercise is not well‐known. Interferon regulatory factor 4 (IRF4) was induced by exercise. Mice without IRF4 specific in muscle show increased glycogen content pre‐exercise and subsequently affects exercise capacity. These studies reveal a feedback signaling pathway including IRF4/protein targeting to glycogen (PTG)/glycogen synthesis that controlling exercise capacity.

Non-alcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver dysfunction and a significant global health problem with substantial rise in prevalence over the last decades. It is becoming increasingly clear that NALFD is not only predominantly a hepatic manifestation of metabolic syndrome, but also involves extra-hepatic organs and regulatory pathways. Therapeutic options are limited for the treatment of NAFLD. Accordingly, a better understanding of the pathogenesis of NAFLD is critical for gaining new insight into the regulatory network of NAFLD and for identifying new targets for the prevention and treatment of NAFLD. In this review, we emphasize on the current understanding of the inter-organ crosstalk between the liver and peripheral organs that contributing to the pathogenesis of NAFLD.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Promoting development and function of brown and beige fat may represent an attractive treatment of obesity. In the current study, we show that fat Klf9 expression is markedly induced by cold exposure and a β-adrenergic agonist. Moreover, Klf9 expression levels in human white adipose tissue (WAT) are inversely correlated with adiposity, and Klf9 overexpression in primary fat cells stimulates cellular thermogenesis, which is Ucp1 dependent. Fat-specific Klf9 transgenic mice gain less weight and have smaller fat pads due to increased thermogenesis of brown and beige fat. Moreover, Klf9 transgenic mice displayed lower fasting blood glucose levels and improved glucose tolerance and insulin sensitivity under the high-fat diet condition. Conversely, Klf9 mutation in brown adipocytes reduces the expression of thermogenic genes, causing a reduction in cellular respiration. Klf9-mutant mice exhibited obesity and cold sensitivity due to impairments in the thermogenic function of fat. Finally, fat Klf9 deletion inhibits the β3 agonist–mediated induction of WAT browning and brown adipose tissue thermogenesis. Mechanistically, cold-inducible Klf9 stimulates expression of Pgc1α, a master regulator of fat thermogenesis, by a direct binding to its gene promoter region, subsequently promoting energy expenditure. The current study reveals a critical role for KLF9 in mediating thermogenesis of brown and beige fat.

BackgroundSPs are a group of ubiquitously produced lipids that are structurally involved in cell membranes and include SMs, ceramides, GM3s, etc. Sphingolipids and their substrates and constituents, FAs, are implicated in the pathogenesis of various metabolic diseases associated with obesity, diabetes, and atherosclerosis. Decreased insulin sensitivity or insulin secretion is a multifactorial condition related to obesity and diabetes. Therefore, it is likely that perturbations in serum SPs are associated with diseases. Identifying these associations may reveal useful predictors or give perceptive insight into disease processes. This study aimed to systematically investigate the associations between serum sphingolipids and insulin sensitivity as well as insulin secretion. This study also aimed to reveal potential predictors for insulin sensitivity or give perceptive insight into disease processes.MethodsWe conducted a lipidomics evaluation of molecularly distinct SPs in the serum of 86 consecutive Chinese adults with or without obesity and diabetes using electrospray ionization mass spectrometry coupled with liquid chromatography. The GIR30 was measured under steady conditions to assess insulin sensitivity by the gold standard hyperinsulinemic-euglycemic clamp, and FPIR was measured to evaluate insulin secretion by the IVGTT. We created the ROC curves to detect the serum SMs diagnostic value and establish the diagnosis of insulin sensitivity based on GIR30 derived from the glucose clamp, which is the standard method, and the cutoff point of GIR30 was set as 5.1 mg/(kg *min).ResultsDifferential correlation network analysis illustrated correlations amongst lipids, insulin sensitivity, insulin secretion and other clinical indexes. Total and subspecies of serum SMs and globotriaosylceramides (Gb3s) were positively related to GIR30, free FAs (FFA 16:1, FFA20:4), some long chain GM3 and complex ceramide GluCers showed strong negative correlations with GIR30. We also found that higher concentrations of serum free FAs (FFA 22:6, FFA 22:5, FFA 18:2, FFA 18:1) were associated with a higher risk of decreased insulin secretion. Notably, ROC curves showed that SM/Cer and SM d18:0/26:0 may be good serum lipid predictors of diagnostic indicators of insulin sensitivity close to conventional clinical indexes such as 1/HOMA-IR (all areas under the curve >0.80) based on GIR30 as standard diagnostic criteria. ConclusionsThese results provide novel associations between serum sphingolipid between insulin sensitivity measured by the hyperinsulinemic-euglycemic clamp. This suggests that the balance of SMs metabolism, rather than ceramide is correlated with the pathology of insulin resistance, obesity and T2DM. We further identify two specific SPs that may represent prognostic biomarkers for insulin sensitivity.

Chronic hepatitis C virus (HCV) infection has a close association with type 2 diabetes mellitus (T2DM). Although the mechanisms of insulin resistant in hepatitis C (CHC) patients have been extensively studied, little attention has been given to the role of beta cell function in HCV-associated diabetes. Here, we found HCV could infected the beta cell directly and impair pancreatic beta cell function in CHC and mouse model. Both first-phase and second-phase insulin secretion was impaired, at least partially due to the reduction of exocytosis of secretory insulin-containing granules following HCV infection. Upregulated p38δ in HCV-infected beta cells resulted in inactivation of protein kinase D (PKD), which was responsible for impaired insulin secretory capacity of beta cells. These findings provided a new perspective into mechanism of HCV induced type 2 diabetes and a new inspiration for the important prognostic and therapeutic implications in the management of CHC patients with glucose intolerance. Funding Statement: This work was supported by grants from the National Natural Sciences Foundation of China [grant number 81770859, 81271828, 81471079, 31271268], the National Basic Research Program of China [grant number 2015CB554304], the Youth Innovation Promotion Association, Open Research Fund Program of the State Key Laboratory of Virology of China (grant number 2018IOV003) Declaration of Interests: The authors have declared that no conflict of interest exists. Ethics Approval Statement: Human studies were approved by the Ethics Committee of the First Hospital of Jilin University and the First Affiliated Hospital of Xinjiang Medical University. The written informed consents were received from participants prior to inclusion in the study. All animal experiments were approved by the Institutional Animal Ethical Committee of Wuhan Institute of Virology, Chinese Academy of Sciences (WIV, CAS).

Histone deacetylase 9 (HDAC9) regulates hepatic gluconeogenesis by deacetylating Forkhead box O 1 (FoxO1). HDAC9 upregulation is involved in hepatitis C virus (HCV)-associated exaggerated gluconeogenesis. Herein, we found in addition to FoxO1, HDAC9 also regulates other gluconeogenic transcription factors, including peroxisomeproliferator-activated receptor-γ coactivator-1α (PGC-1α), cyclic AMP-responsive element-binding protein (CREB), and glucocorticoid receptor (GR). Unlike FoxO1, which is regulated by post-translational modification responses to HDAC9, HDAC9 regulates PGC-1α, CREB and GR by altering gene expression. Similar to PGC-1α, CREB and GR were found to be novel regulatory targets of FoxO1 by examination of the FoxO1 binding site in their promoter. PGC-1α, CREB and GR were upregulated in response to HDAC9 via FoxO1 deacetylation. These findings indicate that HDAC9-FoxO1 signalling contributes to gluconeogenesis by modulating the expression of gluconeogenic transcription factors. In particular, metabolic profiling demonstrated a clear shift towards gluconeogenesis metabolism, and HDAC9-FoxO1 signalling can be strongly induced to upregulate gluconeogenic transcription factors following HCV infection. The positive correlation between HDAC9 and gluconeogenic transcription factor expression levels in the livers of both HCV-infected patients and normal individuals further emphasizes the clinical relevance of these results. Thus, HDAC9-FoxO1 signalling axis is involved in regulating gluconeogenic transcription factors, gluconeogenesis, and HCV-induced type 2 diabetes.

A genetic variant in PNPLA3 (PNPLA3(I148M)), a triacylglycerol (TAG) hydrolase, is a major risk factor for nonalcoholic fatty liver disease (NAFLD); however, the mechanism underlying this association is not known. To develop an animal model of PNPLA3-induced fatty liver disease, we generated transgenic mice that overexpress similar amounts of wild-type PNPLA3 (PNPLA3(WT)) or mutant PNPLA3 (PNPLA3(I148M)) either in liver or adipose tissue. Overexpression of the transgenes in adipose tissue did not affect liver fat content. Expression of PNPLA3(I148M), but not PNPLA3(WT), in liver recapitulated the fatty liver phenotype as well as other metabolic features associated with this allele in humans. Metabolic studies provided evidence for 3 distinct alterations in hepatic TAG metabolism in PNPLA3(I148M) transgenic mice: increased formation of fatty acids and TAG, impaired hydrolysis of TAG, and relative depletion of TAG long-chain polyunsaturated fatty acids. These findings suggest that PNPLA3 plays a role in remodeling TAG in lipid droplets, as they accumulate in response to food intake, and that the increase in hepatic TAG levels associated with the I148M substitution results from multiple changes in hepatic TAG metabolism. The development of an animal model that recapitulates the metabolic phenotype of the allele in humans provides a new platform in which to elucidate the role of PNLPA3(I148M) in NAFLD.

OBJECTIVE—Our previous study suggests that Cidea, a member of Cide family proteins that share sequence homology with the DNA fragmentation factor and are expressed at high levels in brown adipose tissue, plays an important role in the development of obesity. Cideb, another member of Cide family protein, is highly expressed in the liver. We would like to understand the physiological role of Cideb in the regulation of energy expenditure and lipid metabolism. RESEARCH DESIGN AND METHODS—We generated Cideb-null mice by homolog recombination and then fed both wild-type and Cideb-null mice with high-fat diet (58% fat). We then characterized the animals’ adiposity index, food intake, whole-body metabolic rate, liver morphology, rate of fatty acid synthesis and oxidation, insulin sensitivity, and gene expression profile. RESULTS— Cideb-null mice had lower levels of plasma triglycerides and free fatty acids and were resistant to high-fat diet–induced obesity and live steatosis. In addition, Cideb mutant mice displayed significantly increased insulin sensitivity and enhanced rate of whole-body metabolism and hepatic fatty acid oxidation. More importantly, Cideb-null mice showed decreased lipogenesis and reduced expression levels of acetyl-CoA carboxylase, fatty acid synthase, and stearol-CoA desaturase. We further demonstrated that expression levels of sterol response element binding protein 1c was significantly decreased in Cideb-deficient mice. CONCLUSIONS—Our data demonstrate that Cideb is a novel important regulator in lipid metabolism in the liver. Cideb may represent a new therapeutic target for the treatment of obesity, diabetes, and liver steatosis.

A sequence polymorphism (rs738409, I148M) in patatin-like phospholipid domain containing protein 3 (PNPLA3) is strongly associated with nonalcoholic fatty liver disease (NAFLD), but the mechanistic basis for this association remains enigmatic. Neither ablation nor overexpression of wild-type PNPLA3 affects liver fat content in mice, whereas hepatic overexpression of the human 148M transgene causes steatosis. To determine whether the 148M allele causes fat accumulation in the liver when expressed at physiological levels, we introduced a methionine codon at position 148 of the mouse Pnpla3 gene. Knockin mice had normal levels of hepatic fat on a chow diet, but when challenged with a high-sucrose diet their liver fat levels increased 2 to 3-fold compared to wild-type littermates without any associated changes in glucose homeostasis. The increased liver fat in the knockin mice was accompanied by a 40-fold increase in PNPLA3 on hepatic lipid droplets, with no increase in hepatic PNPLA3 messenger RNA (mRNA). Similar results were obtained when the catalytic dyad of PNPLA3 was inactivated by substituting the catalytic serine with alanine (S47A). Conclusion: These data provide the first direct evidence that physiological expression of PNPLA3 148M variant causes NAFLD, and that the accumulation of catalytically inactive PNPLA3 on the surfaces of lipid droplets is associated with the accumulation of TG in the liver. (Hepatology 2015;61:108–118)

PNPLA3 (adiponutrin, calcium-independent phospholipase A(2) epsilon [iPLA(2)ε]) is an adipose-enriched, nutritionally regulated protein that belongs to the patatin-like phospholipase domain containing (PNPLA) family of lipid metabolizing proteins. Genetic variations in the human PNPLA3 gene (i.e., the rs738409 I148M allele) has been strongly and repeatedly associated with fatty liver disease. Although human PNPLA3 has triacylglycerol (TAG) hydrolase and transacylase activities in vitro, its in vivo function and physiological relevance remain controversial. The objective of this study was to determine the metabolic consequences of global targeted deletion of the Pnpla3 gene in mice. We found that Pnpla3 mRNA expression is altered in adipose tissue and liver in response to acute and chronic nutritional challenges. However, global targeted deletion of the Pnpla3 gene in mice did not affect TAG hydrolysis, nor did it influence energy/glucose/lipid homoeostasis or hepatic steatosis/injury. Experimental interventions designed to increase Pnpla3 expression (refeeding, high-sucrose diet, diet-induced obesity, and liver X receptor agonism) likewise failed to reveal differences in the above-mentioned metabolic phenotypes. Expression of the Pnpla3 paralog, Pnpla5, was increased in adipose tissue but not in liver of Pnpla3-deficient mice, but compensatory regulation of genes involved in TAG metabolism was not identified. Together these data argue against a role for Pnpla3 loss-of-function in fatty liver disease or metabolic syndrome in mice.

The upsurge in prevalence of obesity has spawned an epidemic of nonalcoholic fatty liver disease (NAFLD). Previously, we identified a sequence variant (I148M) in patatin-like phospholipase domain-containing protein 3 (PNPLA3) that confers susceptibility to both hepatic triglyceride (TG) deposition and liver injury. To glean insights into the biological role of PNPLA3, we examined the molecular mechanisms by which nutrient status controls hepatic expression of PNPLA3. PNPLA3 mRNA levels, which were low in fasting animals, increased ∼90-fold with carbohydrate feeding. The increase was mimicked by treatment with a liver X receptor (LXR) agonist and required the transcription factor SREBP-1c. The site of SREBP-1c binding was mapped to intron 1 of Pnpla3 using chromatin immunoprecipitation and electrophoretic mobility shift assays. SREBP-1c also promotes fatty acid synthesis by activating several genes encoding enzymes in the biosynthetic pathway. Addition of fatty acids (C16:0, C18:1, and C18:2) to the medium of cultured hepatocytes (HuH-7) increased PNPLA3 protein mass without altering mRNA levels. The posttranslational increase in PNPLA3 levels persisted after blocking TG synthesis with triascin C. Oleate (400 μM) treatment prolonged the half-life of PNPLA3 from 2.4 to 6.7 h. These findings are consistent with nutritional control of PNPLA3 being effected by a feed-forward loop; SREBP-1c promotes accumulation of PNPLA3 directly by activating Pnpla3 transcription and indirectly by inhibiting PNPLA3 degradation through the stimulation of fatty acid synthesis.

Cideb, a member of CIDE family proteins, has emerged as an important regulator in the development of obesity and diabetes by controlling fatty acid synthesis and VLDL secretion in hepatocytes. Here, we investigated the role of Cideb in cholesterol biosynthesis, uptake and storage in the liver by using Cideb-null mice as a model system. Cideb-null mice and wild-type mice were treated with normal diet (ND) or high cholesterol diet (HCD) for one month. The metabolic parameters of cholesterol metabolism and expression profiles of genes in cholesterol biosynthesis and storage were measured. Cideb-null mice had lower levels of plasma cholesterol and LDL when fed with both ND and HCD and increased rate of cholesterol absorption. Furthermore, the liver of Cideb-null mice has lower rates of cholesterol biosynthesis and reduced expression levels of sterol response element-binding protein (SREBP) cleavage-activation protein (SCAP), and lower levels of nuclear form of SREBP2 and its downstream target genes in cholesterol biosynthesis pathway under a normal diet treatment. On the contrary, hepatic cholesterol biosynthesis rate between wild-type and Cideb-null mice was similar after high cholesterol diet treatment. Interestingly, hepatic cholesterol storage in the liver of Cideb-null mice was significantly increased due to its increased LDL receptor (LDLR) and acyl-CoA cholesterol acyltransferase (ACAT) expression. Finally, we observed drastically reduced cholesterol levels in the heart of Cideb-null mice fed with a high cholesterol diet. Overall, our data suggest that Cideb is a novel regulator in controlling cholesterol homeostasis in the liver. Therefore, Cideb could serve as an important therapeutical target for the treatment of atherosclerosis and cardiovascular diseases.

Fsp27, a member of the Cide family proteins, was shown to localize to lipid droplet and promote lipid storage in adipocytes. We aimed to understand the biological role of Fsp27 in regulating adipose tissue differentiation, insulin sensitivity and energy balance. Fsp27(-/-) mice and Fsp27/lep double deficient mice were generated and we examined the adiposity, whole body metabolism, BAT and WAT morphology, insulin sensitivity, mitochondrial activity, and gene expression changes in these mouse strains. Furthermore, we isolated mouse embryonic fibroblasts (MEFs) from wildtype and Fsp27(-/-) mice, followed by their differentiation into adipocytes in vitro. We found that Fsp27 is expressed in both brown adipose tissue (BAT) and white adipose tissue (WAT) and its levels were significantly elevated in the WAT and liver of leptin-deficient ob/ob mice. Fsp27(-/-) mice had increased energy expenditure, lower levels of plasma triglycerides and free fatty acids. Furthermore, Fsp27(-/-)and Fsp27/lep double-deficient mice are resistant to diet-induced obesity and display increased insulin sensitivity. Moreover, white adipocytes in Fsp27(-/-) mice have reduced triglycerides accumulation and smaller lipid droplets, while levels of mitochondrial proteins, mitochondrial size and activity are dramatically increased. We further demonstrated that BAT-specific genes and key metabolic controlling factors such as FoxC2, PPAR and PGC1alpha were all markedly upregulated. In contrast, factors inhibiting BAT differentiation such as Rb, p107 and RIP140 were down-regulated in the WAT of Fsp27(-/-) mice. Remarkably, Fsp27(-/-) MEFs differentiated in vitro show many brown adipocyte characteristics in the presence of the thyroid hormone triiodothyronine (T3). Our data thus suggest that Fsp27 acts as a novel regulator in vivo to control WAT identity, mitochondrial activity and insulin sensitivity.

Obesity has become the most prevalent chronic disorder that affects large populations throughout the world. Obesity is due largely to the imbalance between energy intake and expenditure. Energy balance is regulated by interactions between various tissues such as adipose tissues, liver and skeletal muscle. Adipose tissues, which include brown adipose tissue (BAT) and white adipose tissue (WAT), play crucial roles in maintaining energy homeostasis. While WAT stores energy in the form of triglycerides; BAT increases energy expenditure through thermogenesis. Cide proteins including Cidea, Cideb and Fsp27, are expressed at high levels in BAT, liver and WAT, respectively. Cidea(-/-) mice exhibit increased lipolysis in BAT and are resistant to high fat diet-induced obesity and diabetes. Our recent data suggest that Cideb also plays an important role in the development of obesity by regulating various metabolic pathways in liver. The molecular mechanism of Cide protein regulation of metabolic networks and obesity is discussed.