Tribbles-1 regulates hepatic lipogenesis through posttranscriptional regulation of C/EBPα

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Abstract

Variants near the gene TRIB1 are significantly associated with several plasma lipid traits, circulating liver enzymes, and the development of coronary artery disease in humans; however, it is not clear how its protein product tribbles-1 regulates lipid metabolism. Here, we evaluated mice harboring a liver-specific deletion of Trib1 (Trib1_LSKO) to elucidate the role of tribbles-1 in mammalian hepatic lipid metabolism. These mice exhibited increased hepatic triglyceride (TG) content, lipogenic gene transcription, and de novo lipogenesis. Microarray analysis revealed altered transcription of genes that are downstream of the transcription factor C/EBPα, and Trib1_LSKO mice had increased hepatic C/EBPα protein. Hepatic overexpression of C/EBPα in WT mice phenocopied Trib1_LSKO livers, and hepatic knockout of Cebpa in Trib1_LSKO mice revealed that C/EBPα is required for the increased lipogenesis. Using ChIP-Seq, we found that Trib1_LSKO mice had increased DNA-bound C/EBPα near lipogenic genes and the Trib1 gene, which itself was transcriptionally upregulated by C/EBPα overexpression. Together, our results reveal that tribbles-1 regulates hepatic lipogenesis through posttranscriptional regulation of C/EBPα, which in turn transcriptionally upregulates Trib1. These data suggest an important role for C/EBPα in mediating the lipogenic effects of hepatic Trib1 deletion and provide insight into the association between TRIB1 and plasma lipids, and liver traits in humans.

Authors

Robert C. Bauer, Makoto Sasaki, Daniel M. Cohen, Jian Cui, Mikhaila A. Smith, Batuhan O. Yenilmez, David J. Steger, Daniel J. Rader

Muscle-specific 4E-BP1 signaling activation improves metabolic parameters during aging and obesity

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Abstract

Eukaryotic translation initiation factor 4E–binding protein 1 (4E-BP1) is a key downstream effector of mTOR complex 1 (mTORC1) that represses cap-dependent mRNA translation initiation by sequestering the translation initiation factor eIF4E. Reduced mTORC1 signaling is associated with life span extension and improved metabolic homeostasis, yet the downstream targets that mediate these benefits are unclear. Here, we demonstrated that enhanced 4E-BP1 activity in mouse skeletal muscle protects against age- and diet-induced insulin resistance and metabolic rate decline. Transgenic animals displayed increased energy expenditure; altered adipose tissue distribution, including reduced white adipose accumulation and preserved brown adipose mass; and were protected from hepatic steatosis. Skeletal muscle–specific 4E-BP1 mediated metabolic protection directly through increased translation of peroxisome proliferator–activated receptor γ coactivator-1α (PGC-1α) and enhanced respiratory function. Non–cell autonomous protection was through preservation of brown adipose tissue metabolism, which was increased in 4E-BP1 transgenic animals during normal aging and in a response to diet-induced type 2 diabetes. Adipose phenotypes may derive from enhanced skeletal muscle expression and secretion of the known myokine FGF21. Unlike skeletal muscle, enhanced adipose-specific 4E-BP1 activity was not protective but instead was deleterious in response to the same challenges. These findings indicate that regulation of 4E-BP1 in skeletal muscle may serve as an important conduit through which mTORC1 controls metabolism.

Authors

Shihyin Tsai, Joanna M. Sitzmann, Somasish G. Dastidar, Ariana A. Rodriguez, Stephanie L. Vu, Circe E. McDonald, Emmeline C. Academia, Monique N. O’Leary, Travis D. Ashe, Albert R. La Spada, Brian K. Kennedy

Estrogen receptor–α in medial amygdala neurons regulates body weight

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Abstract

Estrogen receptor–α (ERα) activity in the brain prevents obesity in both males and females. However, the ERα-expressing neural populations that regulate body weight remain to be fully elucidated. Here we showed that single-minded–1 (SIM1) neurons in the medial amygdala (MeA) express abundant levels of ERα. Specific deletion of the gene encoding ERα (Esr1) from SIM1 neurons, which are mostly within the MeA, caused hypoactivity and obesity in both male and female mice fed with regular chow, increased susceptibility to diet-induced obesity (DIO) in males but not in females, and blunted the body weight–lowering effects of a glucagon-like peptide-1–estrogen (GLP-1–estrogen) conjugate. Furthermore, selective adeno-associated virus-mediated deletion of Esr1 in the MeA of adult male mice produced a rapid body weight gain that was associated with remarkable reductions in physical activity but did not alter food intake. Conversely, overexpression of ERα in the MeA markedly reduced the severity of DIO in male mice. Finally, an ERα agonist depolarized MeA SIM1 neurons and increased their firing rate, and designer receptors exclusively activated by designer drug–mediated (DREADD-mediated) activation of these neurons increased physical activity in mice. Collectively, our results support a model where ERα signals activate MeA neurons to stimulate physical activity, which in turn prevents body weight gain.

Authors

Pingwen Xu, Xuehong Cao, Yanlin He, Liangru Zhu, Yongjie Yang, Kenji Saito, Chunmei Wang, Xiaofeng Yan, Antentor Othrell Hinton Jr., Fang Zou, Hongfang Ding, Yan Xia, Chunling Yan, Gang Shu, San-Pin Wu, Bin Yang, Yuxin Feng, Deborah J. Clegg, Richard DeMarchi, Sohaib A. Khan, Sophia Y. Tsai, Francesco J. DeMayo, Qi Wu, Qingchun Tong, Yong Xu

DNA methylation directs functional maturation of pancreatic β cells

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Abstract

Pancreatic β cells secrete insulin in response to postprandial increases in glucose levels to prevent hyperglycemia and inhibit insulin secretion under fasting conditions to protect against hypoglycemia. β cells lack this functional capability at birth and acquire glucose-stimulated insulin secretion (GSIS) during neonatal life. Here, we have shown that during postnatal life, the de novo DNA methyltransferase DNMT3A initiates a metabolic program by repressing key genes, thereby enabling the coupling of insulin secretion to glucose levels. In a murine model, β cell–specific deletion of Dnmt3a prevented the metabolic switch, resulting in loss of GSIS. DNMT3A bound to the promoters of the genes encoding hexokinase 1 (HK1) and lactate dehydrogenase A (LDHA) — both of which regulate the metabolic switch — and knockdown of these two key DNMT3A targets restored the GSIS response in islets from animals with β cell–specific Dnmt3a deletion. Furthermore, DNA methylation–mediated repression of glucose-secretion decoupling genes to modulate GSIS was conserved in human β cells. Together, our results reveal a role for DNA methylation to direct the acquisition of pancreatic β cell function.

Authors

Sangeeta Dhawan, Shuen-ing Tschen, Chun Zeng, Tingxia Guo, Matthias Hebrok, Aleksey Matveyenko, Anil Bhushan

MicroRNA-26a regulates insulin sensitivity and metabolism of glucose and lipids

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Abstract

Type 2 diabetes (T2D) is characterized by insulin resistance and increased hepatic glucose production, yet the molecular mechanisms underlying these abnormalities are poorly understood. MicroRNAs (miRs) are a class of small, noncoding RNAs that have been implicated in the regulation of human diseases, including T2D. miR-26a is known to play a critical role in tumorigenesis; however, its function in cellular metabolism remains unknown. Here, we determined that miR-26a regulates insulin signaling and metabolism of glucose and lipids. Compared with lean individuals, overweight humans had decreased expression of miR-26a in the liver. Moreover, miR-26 was downregulated in 2 obese mouse models compared with control animals. Global or liver-specific overexpression of miR-26a in mice fed a high-fat diet improved insulin sensitivity, decreased hepatic glucose production, and decreased fatty acid synthesis, thereby preventing obesity-induced metabolic complications. Conversely, silencing of endogenous miR-26a in conventional diet–fed mice impaired insulin sensitivity, enhanced glucose production, and increased fatty acid synthesis. miR-26a targeted several key regulators of hepatic metabolism and insulin signaling. These findings reveal miR-26a as a regulator of liver metabolism and suggest miR-26a should be further explored as a potential target for the treatment of T2D.

Authors

Xianghui Fu, Bingning Dong, Yan Tian, Philippe Lefebvre, Zhipeng Meng, Xichun Wang, François Pattou, Weidong Han, Xiaoqiong Wang, Fang Lou, Richard Jove, Bart Staels, David D. Moore, Wendong Huang

Calcium release channel RyR2 regulates insulin release and glucose homeostasis

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Abstract

The type 2 ryanodine receptor (RyR2) is a Ca²⁺ release channel on the endoplasmic reticulum (ER) of several types of cells, including cardiomyocytes and pancreatic β cells. In cardiomyocytes, RyR2-dependent Ca²⁺ release is critical for excitation-contraction coupling; however, a functional role for RyR2 in β cell insulin secretion and diabetes mellitus remains controversial. Here, we took advantage of rare RyR2 mutations that were identified in patients with a genetic form of exercise-induced sudden death (catecholaminergic polymorphic ventricular tachycardia [CPVT]). As these mutations result in a “leaky” RyR2 channel, we exploited them to assess RyR2 channel function in β cell dynamics. We discovered that CPVT patients with mutant leaky RyR2 present with glucose intolerance, which was heretofore unappreciated. In mice, transgenic expression of CPVT-associated RyR2 resulted in impaired glucose homeostasis, and an in-depth evaluation of pancreatic islets and β cells from these animals revealed intracellular Ca²⁺ leak via oxidized and nitrosylated RyR2 channels, activated ER stress response, mitochondrial dysfunction, and decreased fuel-stimulated insulin release. Additionally, we verified the effects of the pharmacological inhibition of intracellular Ca²⁺ leak in CPVT-associated RyR2-expressing mice, in human islets from diabetic patients, and in an established murine model of type 2 diabetes mellitus. Taken together, our data indicate that RyR2 channels play a crucial role in the regulation of insulin secretion and glucose homeostasis.

Authors

Gaetano Santulli, Gennaro Pagano, Celestino Sardu, Wenjun Xie, Steven Reiken, Salvatore Luca D’Ascia, Michele Cannone, Nicola Marziliano, Bruno Trimarco, Theresa A. Guise, Alain Lacampagne, Andrew R. Marks

Identification and validation of N-acetyltransferase 2 as an insulin sensitivity gene

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Abstract

Decreased insulin sensitivity, also referred to as insulin resistance (IR), is a fundamental abnormality in patients with type 2 diabetes and a risk factor for cardiovascular disease. While IR predisposition is heritable, the genetic basis remains largely unknown. The GENEticS of Insulin Sensitivity consortium conducted a genome-wide association study (GWAS) for direct measures of insulin sensitivity, such as euglycemic clamp or insulin suppression test, in 2,764 European individuals, with replication in an additional 2,860 individuals. The presence of a nonsynonymous variant of N-acetyltransferase 2 (NAT2) [rs1208 (803A>G, K268R)] was strongly associated with decreased insulin sensitivity that was independent of BMI. The rs1208 “A” allele was nominally associated with IR-related traits, including increased fasting glucose, hemoglobin A1C, total and LDL cholesterol, triglycerides, and coronary artery disease. NAT2 acetylates arylamine and hydrazine drugs and carcinogens, but predicted acetylator NAT2 phenotypes were not associated with insulin sensitivity. In a murine adipocyte cell line, silencing of NAT2 ortholog Nat1 decreased insulin-mediated glucose uptake, increased basal and isoproterenol-stimulated lipolysis, and decreased adipocyte differentiation, while Nat1 overexpression produced opposite effects. Nat1-deficient mice had elevations in fasting blood glucose, insulin, and triglycerides and decreased insulin sensitivity, as measured by glucose and insulin tolerance tests, with intermediate effects in Nat1 heterozygote mice. Our results support a role for NAT2 in insulin sensitivity.

Authors

Joshua W. Knowles, Weijia Xie, Zhongyang Zhang, Indumathi Chennemsetty, Themistocles L. Assimes, Jussi Paananen, Ola Hansson, James Pankow, Mark O. Goodarzi, Ivan Carcamo-Orive, Andrew P. Morris, Yii-Der I. Chen, Ville-Petteri Mäkinen, Andrea Ganna, Anubha Mahajan, Xiuqing Guo, Fahim Abbasi, Danielle M. Greenawalt, Pek Lum, Cliona Molony, Lars Lind, Cecilia Lindgren, Leslie J. Raffel, Philip S. Tsao, Eric E. Schadt, Jerome I. Rotter, Alan Sinaiko, Gerald Reaven, Xia Yang, Chao A. Hsiung, Leif Groop, Heather J. Cordell, Markku Laakso, Ke Hao, Erik Ingelsson, Timothy M. Frayling, Michael N. Weedon, Mark Walker, Thomas Quertermous

Long–echo time MR spectroscopy for skeletal muscle acetylcarnitine detection

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Abstract

Animal models suggest that acetylcarnitine production is essential for maintaining metabolic flexibility and insulin sensitivity. Because current methods to detect acetylcarnitine involve biopsy of the tissue of interest, noninvasive alternatives to measure acetylcarnitine concentrations could facilitate our understanding of its physiological relevance in humans. Here, we investigated the use of long–echo time (TE) proton magnetic resonance spectroscopy (¹H-MRS) to measure skeletal muscle acetylcarnitine concentrations on a clinical 3T scanner. We applied long-TE ¹H-MRS to measure acetylcarnitine in endurance-trained athletes, lean and obese sedentary subjects, and type 2 diabetes mellitus (T2DM) patients to cover a wide spectrum in insulin sensitivity. A long-TE ¹H-MRS protocol was implemented for successful detection of skeletal muscle acetylcarnitine in these individuals. There were pronounced differences in insulin sensitivity, as measured by hyperinsulinemic-euglycemic clamp, and skeletal muscle mitochondrial function, as measured by phosphorus-MRS (³¹P-MRS), across groups. Insulin sensitivity and mitochondrial function were highest in trained athletes and lowest in T2DM patients. Skeletal muscle acetylcarnitine concentration showed a reciprocal distribution, with mean acetylcarnitine concentration correlating with mean insulin sensitivity in each group. These results demonstrate that measuring acetylcarnitine concentrations with ¹H-MRS is feasible on clinical MR scanners and support the hypothesis that T2DM patients are characterized by a decreased formation of acetylcarnitine, possibly underlying decreased insulin sensitivity.

Authors

Lucas Lindeboom, Christine I. Nabuurs, Joris Hoeks, Bram Brouwers, Esther Phielix, M. Eline Kooi, Matthijs K.C. Hesselink, Joachim E. Wildberger, Robert D. Stevens, Timothy Koves, Deborah M. Muoio, Patrick Schrauwen, Vera B. Schrauwen-Hinderling

PPARγ ablation sensitizes proopiomelanocortin neurons to leptin during high-fat feeding

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Abstract

Activation of central PPARγ promotes food intake and body weight gain; however, the identity of the neurons that express PPARγ and mediate the effect of this nuclear receptor on energy homeostasis is unknown. Here, we determined that selective ablation of PPARγ in murine proopiomelanocortin (POMC) neurons decreases peroxisome density, elevates reactive oxygen species, and induces leptin sensitivity in these neurons. Furthermore, ablation of PPARγ in POMC neurons preserved the interaction between mitochondria and the endoplasmic reticulum, which is dysregulated by HFD. Compared with control animals, mice lacking PPARγ in POMC neurons had increased energy expenditure and locomotor activity; reduced body weight, fat mass, and food intake; and improved glucose metabolism when exposed to high-fat diet (HFD). Finally, peripheral administration of either a PPARγ activator or inhibitor failed to affect food intake of mice with POMC-specific PPARγ ablation. Taken together, our data indicate that PPARγ mediates cellular, biological, and functional adaptations of POMC neurons to HFD, thereby regulating whole-body energy balance.

Authors

Lihong Long, Chitoku Toda, Jing Kwon Jeong, Tamas L. Horvath, Sabrina Diano

Ventromedial hypothalamus–specific Ptpn1 deletion exacerbates diet-induced obesity in female mice

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Abstract

Protein-tyrosine phosphatase 1B (PTP1B) regulates food intake (FI) and energy expenditure (EE) by inhibiting leptin signaling in the hypothalamus. In peripheral tissues, PTP1B regulates insulin signaling, but its effects on CNS insulin action are largely unknown. Mice harboring a whole-brain deletion of the gene encoding PTP1B (Ptpn1) are lean, leptin-hypersensitive, and resistant to high fat diet–induced (HFD-induced) obesity. Arcuate proopiomelanocortin (POMC) neuron–specific deletion of Ptpn1 causes a similar, but much milder, phenotype, suggesting that PTP1B also acts in other neurons to regulate metabolism. Steroidogenic factor-1–expressing (SF-1–expressing) neurons in the ventromedial hypothalamus (VMH) play an important role in regulating body weight, FI, and EE. Surprisingly, Ptpn1 deletion in SF-1 neurons caused an age-dependent increase in adiposity in HFD-fed female mice. Although leptin sensitivity was increased and FI was reduced in these mice, they had impaired sympathetic output and decreased EE. Immunohistochemical analysis showed enhanced leptin and insulin signaling in VMH neurons from mice lacking PTP1B in SF-1 neurons. Thus, in the VMH, leptin negatively regulates FI, promoting weight loss, whereas insulin suppresses EE, leading to weight gain. Our results establish a novel role for PTP1B in regulating insulin action in the VMH and suggest that increased insulin responsiveness in SF-1 neurons can overcome leptin hypersensitivity and enhance adiposity.

Authors

Franck Chiappini, Karyn J. Catalano, Jennifer Lee, Odile D. Peroni, Jacqueline Lynch, Abha S. Dhaneshwar, Kerry Wellenstein, Alexandra Sontheimer, Benjamin G. Neel, Barbara B. Kahn