Yingfeng Deng

Adipokines as novel biomarkers and regulators of the metabolic syndrome

Over the past two decades our view of adipose tissue has undergone a dramatic change from an inert energy storage tissue to an active endocrine organ. Adipose tissue communicates with other central and peripheral organs by synthesis and secretion of a host of molecules that we generally refer to as adipokines. The levels of some adipokines correlate with specific metabolic states and have the potential to impact directly upon the metabolic homeostasis of the system. A dysregulation of adipokines has been implicated in obesity, type 2 diabetes, hypertension, cardiovascular disease, and an ever‐growing larger list of pathological changes in a number of organs. Here, we review the recent progress regarding the synthesis, secretion, and physiological function of adipokines with perspectives on future directions and potential therapeutic goals.

Spliced X-box binding protein 1 couples the unfolded protein response to hexosamine biosynthetic pathway

The hexosamine biosynthetic pathway (HBP) generates uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) for glycan synthesis and O-linked GlcNAc (O-GlcNAc) protein modifications. Despite the established role of the HBP in metabolism and multiple diseases, regulation of the HBP remains largely undefined. Here, we show that spliced X-box binding protein 1 (Xbp1s), the most conserved signal transducer of the unfolded protein response (UPR), is a direct transcriptional activator of the HBP. We demonstrate that the UPR triggers HBP activation via Xbp1s-dependent transcription of genes coding for key, rate-limiting enzymes. We further establish that this previously unrecognized UPR-HBP axis is triggered in a variety of stress conditions. Finally, we demonstrate a physiologic role for the UPR-HBP axis by showing that acute stimulation of Xbp1s in heart by ischemia/reperfusion confers robust cardioprotection in part through induction of the HBP. Collectively, these studies reveal that Xbp1s couples the UPR to the HBP to protect cells under stress.

Proangiogenic Contribution of Adiponectin toward Mammary Tumor Growth In vivo

Purpose: Adipocytes represent one of the most abundant constituents of the mammary gland. They are essential for mammary tumor growth and survival. Metabolically, one of the more important fat-derived factors (“adipokines”) is adiponectin (APN). Serum concentrations of APN negatively correlate with body mass index and insulin resistance. To explore the association of APN with breast cancer and tumor angiogenesis, we took an in vivo approach aiming to study its role in the mouse mammary tumor virus (MMTV)-polyoma middle T antigen (PyMT) mammary tumor model. Experimental Design: We compared the rates of tumor growth in MMTV-PyMT mice in wild-type and APN-null backgrounds. Results: Histology and micro-positron emission tomography imaging show that the rate of tumor growth is significantly reduced in the absence of APN at early stages. PyMT/APN knockout mice exhibit a reduction in their angiogenic profile resulting in nutrient deprivation of the tumors and tumor-associated cell death. Surprisingly, in more advanced malignant stages of the disease, tumor growth develops more aggressively in mice lacking APN, giving rise to a larger tumor burden, an increase in the mobilization of circulating endothelial progenitor cells, and a gene expression fingerprint indicative of more aggressive tumor cells. Conclusions: These observations highlight a novel important contribution of APN in mammary tumor development and angiogenesis, indicating that APN has potent angio-mimetic properties in tumor vascularization. However, in tumors deprived of APN, this antiangiogenic stress results in an adaptive response that fuels tumor growth through mobilization of circulating endothelial progenitor cells and the development of mechanisms enabling massive cell proliferation despite a chronically hypoxic microenvironment.

Identification and Characterization of a Promoter Cassette Conferring Adipocyte-Specific Gene Expression

The adipocyte-specific secretory molecule adiponectin has found widespread acceptance as a systemic marker that effectively integrates a number of signals associated with metabolic dysfunction at the level of adipose tissue. The widely used aP2 promoter cassette, which is frequently chosen to achieve adipocyte-specific expression of transgenes, conveys transcription in cell types other than adipocytes, such as macrophages and cardiomyocytes. To improve our ability to drive transgene expression in a more adipocyte-specific way, we aimed to define the minimal promoter segment from the adiponectin genomic locus. We generated a series of transgenic animals in which the expression of reporter genes and Cre recombinase was driven by 2, 4.9, and 5.4 kb of adiponectin promoter sequences. We found that the 5.4-kb adiponectin promoter fragment is the most effective cassette conveying adipocyte-specific expression of target genes. We therefore define a novel promoter cassette that ensures adipocyte-specific expression of passenger genes and may be used in the generation of transgenic mouse models to study gene function in vivo.

Xbp1s in Pomc neurons connects ER stress with energy balance and glucose homeostasis

The molecular mechanisms underlying neuronal leptin and insulin resistance in obesity and diabetes remain unclear. Here we show that induction of the unfolded protein response transcription factor spliced X-box binding protein 1 (Xbp1s) in pro-opiomelanocortin (Pomc) neurons alone is sufficient to protect against diet-induced obesity as well as improve leptin and insulin sensitivity, even in the presence of strong activators of ER stress. We also demonstrate that constitutive expression of Xbp1s in Pomc neurons contributes to improved hepatic insulin sensitivity and suppression of endogenous glucose production. Notably, elevated Xbp1s levels in Pomc neurons also resulted in activation of the Xbp1s axis in the liver via a cell-nonautonomous mechanism. Together our results identify critical molecular mechanisms linking ER stress in arcuate Pomc neurons to acute leptin and insulin resistance as well as liver metabolism in diet-induced obesity and diabetes.

The Xbp1s/GalE axis links ER stress to postprandial hepatic metabolism

Postprandially, the liver experiences an extensive metabolic reprogramming that is required for the switch from glucose production to glucose assimilation. Upon refeeding, the unfolded protein response (UPR) is rapidly, though only transiently, activated. Activation of the UPR results in a cessation of protein translation, increased chaperone expression, and increased ER-mediated protein degradation, but it is not clear how the UPR is involved in the postprandial switch to alternate fuel sources. Activation of the inositol-requiring enzyme 1 (IRE1) branch of the UPR signaling pathway triggers expression of the transcription factor Xbp1s. Using a mouse model with liver-specific inducible Xbp1s expression, we demonstrate that Xbp1s is sufficient to provoke a metabolic switch characteristic of the postprandial state, even in the absence of caloric influx. Mechanistically, we identified UDP-galactose-4-epimerase (GalE) as a direct transcriptional target of Xbp1s and as the key mediator of this effect. Our results provide evidence that the Xbp1s/GalE pathway functions as a novel regulatory nexus connecting the UPR to the characteristic postprandial metabolic changes in hepatocytes.

Inflammation and ER Stress Regulate Branched-Chain Amino Acid Uptake and Metabolism in Adipocytes

Inflammation plays a critical role in the pathology of obesity-linked insulin resistance and is mechanistically linked to the effects of macrophage-derived cytokines on adipocyte energy metabolism, particularly that of the mitochondrial branched-chain amino acid (BCAA) and tricarboxylic acid (TCA) pathways. To address the role of inflammation on energy metabolism in adipocytes, we used high fat-fed C57BL/6J mice and lean controls and measured the down-regulation of genes linked to BCAA and TCA cycle metabolism selectively in visceral but not in subcutaneous adipose tissue, brown fat, liver, or muscle. Using 3T3-L1 cells, TNFα, and other proinflammatory cytokine treatments reduced the expression of the genes linked to BCAA transport and oxidation. Consistent with this, [(14)C]-leucine uptake and conversion to triglycerides was markedly attenuated in TNFα-treated adipocytes, whereas the conversion to protein was relatively unaffected. Because inflammatory cytokines lead to the induction of endoplasmic reticulum stress, we evaluated the effects of tunicamycin or thapsigargin treatment of 3T3-L1 cells and measured a similar down-regulation in the BCAA/TCA cycle pathway. Moreover, transgenic mice overexpressing X-box binding protein 1 in adipocytes similarly down-regulated genes of BCAA and TCA metabolism in vivo. These results indicate that inflammation and endoplasmic reticulum stress attenuate lipogenesis in visceral adipose depots by down-regulating the BCAA/TCA metabolism pathway and are consistent with a model whereby the accumulation of serum BCAA in the obese insulin-resistant state is linked to adipose inflammation.

An adipo-biliary-uridine axis that regulates energy homeostasis

Uridines rise and fall: Food for thought The nucleoside uridine is well known for its role in critical cellular functions such as nucleic acid synthesis. Its role in whole-animal physiology has received comparatively little attention. In mammals, plasma uridine levels are tightly regulated, but the underlying mechanisms are unclear. Studying mouse models, Deng et al. show that plasma uridine levels are controlled by feeding behavior (see the Perspective by Jastroch and Tschöp). Fasting causes an adipocyte-mediated rise in plasma uridine, which triggers a lowering of body temperature. Feeding causes a bile-mediated drop in plasma uridine, which enhances insulin sensitivity in a leptin-dependent manner. Thus, uridine is part of a complex regulatory loop that affects energy balance and potentially contributes to metabolic disease. Science, this issue p. aaf5375; see also p. 1124 Plasma uridine levels are controlled by feeding behavior, a discovery with possible implications for metabolic disease. INTRODUCTION Uridine is a pyrimidine nucleoside that is critical for cellular function and survival. In addition to its role in RNA and DNA biosynthesis, uridine is required for glycogen deposition, protein and lipid glycosylation, extracellular matrix biosynthesis, and detoxification of xenobiotics. Plasma uridine levels are maintained within a narrow range, and most cells depend on a readily available pool of uridine in plasma to maintain basic cellular functions. Enhanced understanding of the physiological mechanisms controlling biosynthesis and clearance of this metabolite has the potential to shed light on several disease states, including diabetes, cancer, and neurological disorders. RATIONALE Despite its pivotal physiological role, uridine has received limited attention in comparison to other nucleosides such as adenosine. Studying rodent models, we set out to define the mechanisms regulating plasma uridine levels and to dissect the molecular circuitry whereby uridine governs energy homeostasis in normal and obese conditions. RESULTS One of our key findings is that plasma uridine levels are subject to tight regulation during feeding and fasting in both rodents and humans. Plasma uridine levels are elevated during fasting and drop rapidly in the postprandial state. We demonstrate that liver is the predominant biosynthetic organ and contributor to plasma uridine in the fed state, whereas the adipocyte dominates uridine biosynthetic activity in the fasted state. Both glucose and uridine levels must be maintained in the fasted state, not only as basic building blocks for macromolecule biosynthesis, but also as fuels for metabolically active cell types such as neurons. We find that the fasting-induced rise in uridine is tightly linked to a drop in core body temperature driven by a reduction in metabolic rate. The fasting-induced drop in body temperature, although small, is highly reproducible and seen in both rodents and humans. Plasma uridine homeostasis thus links thermoregulation to the fasting/refeeding cycle. Leptin signaling governs uridine-dependent thermoregulation such that leptin deficiency amplifies fasting-induced declines in core temperature. Conversely, prolonged exposure to a high-fat diet blunts the fasting-induced body temperature drop. We clarify the mechanism underlying the rapid reduction of plasma uridine upon refeeding, which involves both reduction of uridine synthesis in adipocytes and enhancement of its clearance through the bile. Uridine from the digestive tract has a different fate than uridine derived biosynthetically from the adipocyte in the fasted state. Adipose tissue–derived uridine increases plasma uridine concentrations, which in turn elicit a hypothalamic response culminating in body temperature lowering. In contrast, gut-derived uridine is never fully released into the circulation, but rather is rapidly resorbed into bile again and effectively reused as part of an enterohepatic recycling process. This minimizes the effects of postprandial uridine absorption, obviating an impact on temperature control in the fed state. CONCLUSION Our results show that plasma uridine concentrations in mammals are regulated by fasting/refeeding. Adipocytes are key contributors to uridine supply during fasting, whereas biliary excretion is the primary mechanism for uridine clearance following food intake. Bile-mediated uridine release promotes body temperature declines during fasting and enhances insulin sensitivity in a leptin-dependent manner. Because nutrient intake triggers bile release, our work identifies a metabolic regulatory model in which feeding behavior directly regulates plasma uridine homeostasis, which then alters energy balance through thermoregulation. A regulatory model of energy homeostasis during fasting/refeeding. The liver is the predominant biosynthetic organ and contributor to plasma uridine in the fed state, whereas the adipocyte dominates uridine biosynthetic activity in the fasted state. Biliary excretion is the primary mechanism for plasma uridine clearance. Because nutrient intake triggers bile release, plasma uridine levels are elevated during fasting and drop rapidly in the postprandial state. The fasting-associated increase of plasma uridine elicits a hypothalamic response culminating in body temperature lowering, whereas bile-mediated uridine release promotes a decline of plasma uridine and enhances insulin sensitivity. Uridine, a pyrimidine nucleoside present at high levels in the plasma of rodents and humans, is critical for RNA synthesis, glycogen deposition, and many other essential cellular processes. It also contributes to systemic metabolism, but the underlying mechanisms remain unclear. We found that plasma uridine levels are regulated by fasting and refeeding in mice, rats, and humans. Fasting increases plasma uridine levels, and this increase relies largely on adipocytes. In contrast, refeeding reduces plasma uridine levels through biliary clearance. Elevation of plasma uridine is required for the drop in body temperature that occurs during fasting. Further, feeding-induced clearance of plasma uridine improves glucose metabolism. We also present findings that implicate leptin signaling in uridine homeostasis and consequent metabolic control and thermoregulation. Our results indicate that plasma uridine governs energy homeostasis and thermoregulation in a mechanism involving adipocyte-dependent uridine biosynthesis and leptin signaling.

Angiopoietin-2 in white adipose tissue improves metabolic homeostasis through enhanced angiogenesis

Despite many angiogenic factors playing crucial roles in metabolic homeostasis, effects of angiopoietin-2 (ANG-2) in adipose tissue (AT) remain unclear. Utilizing a doxycycline-inducible AT-specific ANG-2 overexpression mouse model, we assessed the effects of ANG-2 in AT expansion upon a high-fat diet (HFD) challenge. ANG-2 is significantly induced, with subcutaneous white AT (sWAT) displaying the highest ANG-2 expression. ANG-2 overexpressing mice show increased sWAT vascularization and are resistant to HFD-induced obesity. In addition, improved glucose and lipid metabolism are observed. Mechanistically, the sWAT displays a healthier expansion pattern with increased anti-inflammatory macrophage infiltration. Conversely, ANG-2 neutralization in HFD-challenged wild-type mice shows reduced vascularization in sWAT, associated with impaired glucose tolerance and lipid clearance. Blocking ANG-2 causes significant pro-inflammatory and pro-fibrotic changes, hallmarks of an unhealthy AT expansion. In contrast to other pro-angiogenic factors, such as vascular endothelial growth factor-A (VEGF-A), this is achieved without any enhanced beiging of white AT. DOI: http://dx.doi.org/10.7554/eLife.24071.001

Hepatic GALE Regulates Whole Body Glucose Homeostasis by Modulating Tff3 Expression

Transcripts of key enzymes in the Leloir pathway of galactose metabolism in mouse livers are significantly increased after chronic high-fat/high-sucrose feeding. UDP-galactose-4-epimerase (GALE) is the last enzyme in this pathway that converts UDP-galactose to UDP-glucose and was previously identified as a downstream target of the endoplasmic reticulum (ER) stress effector spliced X-box binding protein 1, suggesting an interesting cross talk between galactose and glucose metabolism in the context of hepatic ER stress and whole-body metabolic fitness. However, its specific role in glucose metabolism is not established. Using an inducible and tissue-specific mouse model, we report that hepatic overexpression of Gale increases gluconeogenesis from pyruvate and impairs glucose tolerance. Conversely, genetic reduction of Gale in liver improves glucose tolerance. Transcriptional profiling identifies trefoil factor 3 (Tff3) as one of the downstream targets of GALE. Restoration of Tff3 expression corrects glucose intolerance in Gale-overexpressing mice. These studies reveal a new link between hepatic GALE activity and whole-body glucose homeostasis via regulation of hepatic Tff3 expression.