The two closely related RabGAPs TBC1D1 and TBC1D4 are key signaling factors of skeletal muscle substrate utilization. In mice, deficiency in both RabGAPs leads to reduced skeletal muscle glucose transport in response to insulin and lower GLUT4 abundance. Conversely, Tbc1d1 and Tbc1d4 deficiency results in enhanced lipid use as fuel in skeletal muscle, through yet unknown mechanisms. In humans, variants in TBC1D1 and TBC1D4 are linked to obesity, insulin resistance and type 2 diabetes. While the specific function in metabolism of each of the two RabGAPs remains to be determined, TBC1D1 emerges to be controlling exercise endurance and physical capacity, whereas TBC1D4 may rather be responsible for maintaining muscle insulin sensitivity, muscle contraction, and exercise. There is growing evidence that TBC1D1 also plays an important role in skeletal muscle development, since it has been found to be associated to meat production traits in several livestock species. In addition, TBC1D1 protein abundance in skeletal muscle is regulated by both, insulin receptor and insulin-like growth factor-1 (IGF-1) receptor signaling. This review focuses on the specific roles of the two key signaling factors TBC1D1 and TBC1D4 in skeletal muscle metabolism, development and exercise physiology.
Lena Espelage, Hadi Al-Hasani and Alexandra Chadt
M J Moreno-Aliaga, M M Swarbrick, S Lorente-Cebrián, K L Stanhope, P J Havel and J A Martínez
We have previously demonstrated that insulin-stimulated glucose metabolism, and not insulin per se, mediates the effects of insulin to increase the transcriptional activity of the leptin promoter in adipocytes. Here, we sought to identify the specific cis-acting DNA elements required for the upregulation of leptin gene transcription in response to insulin-mediated glucose metabolism. To accomplish this, 3T3-L1 cells and primary rat adipocytes were transfected with a series of luciferase reporter genes containing portions of the mouse leptin promoter. Using this method, we identified an element between −135 and −95 bp (relative to the transcriptional start site) that mediated transcription in response to insulin-stimulated glucose metabolism in adipocytes. This effect was abolished by incubation with 2-deoxy-d-glucose, a competitive inhibitor of glucose metabolism. Gel shift electrophoretic mobility shift assays confirmed that the stimulatory effect of insulin-mediated glucose metabolism on leptin transcription was mediated by a previously identified Sp1 site. Consistent with these findings, incubation of primary rat adipocytes with WP631, a specific inhibitor of specificity protein (Sp)1-dependent transcription, inhibited glucose- and insulin-stimulated, but not basal, leptin secretion. Together, these findings support a key role for Sp1 in the transcriptional activation of the leptin gene promoter by insulin-mediated glucose metabolism.
Purificación Gómez, Francisco Pérez-Jiménez, Carmen Marín, Juan Antonio Moreno, María José Gómez, Cecilia Bellido, Pablo Pérez-Martínez, Francisco Fuentes, Juan Antonio Paniagua and José López-Miranda
Impaired insulin action has been associated with diabetes, dyslipidemia and atherosclerotic vascular disease. The expression of insulin resistance results from the interaction of environmental and genetic factors. Human hepatic lipase (HL) is a lipolytic enzyme that plays a role in the metabolism of several lipoproteins, while insulin up-regulates the activity of HL via insulin-responsive elements in the HL promoter. We have examined the influence of −514 C/T polymorphism in the hepatic lipase gene promoter on insulin sensitivity in 59 healthy young subjects (30 males and 29 females). The volunteers were subjected to three dietary periods, each lasting four weeks. During the first period all subjects consumed a saturated fat (SFA)-enriched diet with 38% as fat (20% SFA, 12% monounsaturated fatty acids (MUFA) and 6% polyunsaturated fatty acids (PUFA)). In the second and third dietary periods, a randomized crossover design was used, consisting of a low fat, high carbohydrate diet (CHO diet) (< 10% SFA, 12% MUFA and 6% PUFA) and a high-MUFA, or Mediterranean diet, with < 10% SFA, 22% MUFA and 6% PUFA. We determined the in vivo insulin resistance using the insulin suppression test with somatostatin. Steady-state plasma glucose (SSPG) concentrations (a measure of insulin sensitivity) were significantly higher in men carriers of the −514T allele after the consumption of the SFA diet than after the CHO diet and the Mediterranean diet. This effect was not observed in women. Moreover, there were no significant differences in insulin sensitivity after the three diets in men and women with the CC genotype. In summary, our results show an improvement in insulin sensitivity in men with the −514T allele of the HL promoter polymorphism, when MUFA and carbohydrates are consumed instead of SFA fat.
NT Lam, AT Cheung, MJ Riedel, PE Light, CI Cheeseman and TJ Kieffer
Leptin suppresses insulin secretion by opening ATP-sensitive K(+) (K(ATP)) channels and hyperpolarizing beta-cells. We measured the intracellular concentration of ATP ([ATP](i)) in tumor-derived beta-cells, INS-1, and found that leptin reduced [ATP](i) by approximately 30%, suggesting that the opening of K(ATP) channels by leptin is mediated by decreased [ATP](i). A reduction in glucose availability for metabolism may explain the decreased [ATP](i), since leptin (30 min) reduced glucose transport into INS-1 cells by approximately 35%, compared to vehicle-treated cells. The twofold induction of GLUT2 phosphorylation by GLP-1, an insulin secretagogue, was abolished by leptin. Therefore, the acute effect of leptin could involve covalent modification of GLUT2. These findings suggest that leptin may inhibit insulin secretion by reducing [ATP](i) as a result of reduced glucose availability for the metabolic pathway. In addition, leptin reduced glucose transport by 35% in isolated rat hepatocytes that also express GLUT2, suggesting that glucose transport may also be altered by leptin in other glucose-responsive tissues such as the liver.
Laura Marroquí, Alejandro Gonzalez, Patricia Ñeco, Ernesto Caballero-Garrido, Elaine Vieira, Cristina Ripoll, Angel Nadal and Ivan Quesada
Leptin plays an important role in the control of food intake, energy expenditure, metabolism, and body weight. This hormone also has a key function in the regulation of glucose homeostasis. Although leptin acts through central and peripheral mechanisms to modulate glucose metabolism, the pancreatic β-cell of the endocrine pancreas is a critical target of leptin actions. Leptin receptors are present in the β-cell, and their activation directly inhibits insulin secretion from these endocrine cells. The effects of leptin on insulin occur also in the long term, since this hormone inhibits insulin gene expression as well. Additionally, β-cell mass can be affected by leptin through changes in proliferation, apoptosis, or cell size. All these different functions in the β-cell are triggered by leptin as a result of the large diversity of signaling pathways that this hormone is able to activate in the endocrine pancreas. Therefore, leptin can participate in glucose homeostasis owing to different levels of modulation of the pancreatic β-cell population. Furthermore, it has been proposed that alterations in this level of regulation could contribute to the impairment of β-cell function in obesity states. In the present review, we will discuss all these issues with special emphasis on the effects and pathways of leptin signaling in the pancreatic β-cell.
Qi Cheng, Violeta D Beltran, Stanley M H Chan, Jeremy R Brown, Alan Bevington and Terence P Herbert
The branched-chain amino acids (BCAA) leucine, isoleucine and valine, are essential amino acids that play a critical role in cellular signalling and metabolism. They acutely stimulate insulin secretion and activate the regulatory serine/threonine kinase mammalian target of rapamycin complex 1 (mTORC1), a kinase that promotes increased β-cell mass and function. The effects of BCAA on cellular function are dependent on their active transport into the mammalian cells via amino acid transporters and thus the expression and activity of these transporters likely influence β-cell signalling and function. In this report, we show that the System-L transporters are required for BCAA uptake into clonal β-cell lines and pancreatic islets, and that these are essential for signalling to mTORC1. Further investigation revealed that the System-L amino acid transporter 1 (LAT1) is abundantly expressed in the islets, and that knockdown of LAT1 using siRNA inhibits mTORC1 signalling, leucine-stimulated insulin secretion and islet cell proliferation. In summary, we show that the LAT1 is required for regulating β-cell signalling and function in islets and thus may be a novel pharmacological/nutritional target for the treatment and prevention of type 2 diabetes.
Hisashi Masuyama and Yuji Hiramatsu
Normal pregnancy is characterized by insulin resistance, which contributes to the development of gestational diabetes mellitus and preeclampsia by incompletely understood mechanisms. The constitutive androstane receptor (CAR) may participate in insulin resistance in pregnancy, and sex steroids, estradiol (E2) and progesterone, may also be involved. We applied glucose and insulin tolerance tests and measured the expression of gluconeogenic and lipogenic genes in the livers of oophorectomized mice treated with E2 and progesterone with or without CAR ligands. We also investigated how E2 and progesterone affected CAR-mediated signaling and the activity of transcription factors in gluconeogenesis in vitro. Mice with the concentrations of E2 and progesterone within normal physiological range during pregnancy exhibited increased insulin resistance along with increased expression of gluconeogenic and lipogenic genes, and CAR activation rescued the abnormal glucose metabolism. In HepG2 cells, CAR ligands suppressed the gluconeogenic and lipogenic gene expression in the presence of E2 and/or progesterone. DNA affinity immunoblotting and chromatin immunoprecipitation assay revealed that CAR ligand enhanced the recruitment of the gluconeogenic transcription factors, forkhead box O1 (FOXO1) and hepatocyte nuclear factor 4α (HNF4α), but sex steroids suppressed these recruitments on the CAR responsive element. Moreover, CAR ligand suppressed the recruitment of FOXO1 and HNF4α on their responsive element in gluconeogenic gene promoters and E2 and progesterone augmented these recruitments on their responsive element. Taken together, these findings suggest that the activation of CAR-mediated signaling may ameliorate insulin resistance under relatively high concentrations of E2 and progesterone, which were compatible with pregnancy via decreased activities of transcription factors in gluconeogenesis in combination with CAR.
Isadora C Furigo, Pryscila D S Teixeira, Paula G F Quaresma, Naira S Mansano, Renata Frazão and Jose Donato Jr
AgRP neurons are important players in the control of energy homeostasis and are responsive to several hormones. In addition, STAT5 signalling in the brain, which is activated by metabolic hormones and growth factors, modulates food intake, body fat and glucose homeostasis. Given that, and the absence of studies that describe STAT5 function in AgRP cells, the present study investigated the metabolic effects of Stat5a/b gene ablation in these neurons. We observed that STAT5 signalling in AgRP neurons regulates body fat in female mice. However, male and female STAT5-knockout mice did not exhibit altered food intake, energy expenditure or glucose homeostasis compared to control mice. The counter-regulatory response or glucoprivic hyperphagia induced by 2-deoxy-d-glucose treatment were also not affected by AgRP-specific STAT5 ablation. However, under 60% food restriction, AgRP STAT5-knockout mice had a blunted upregulation of hypothalamic Agrp mRNA expression and corticosterone serum levels compared to control mice, suggesting a possible role for STAT5 in AgRP neurons for neuroendocrine adaptations to food restriction. Interestingly, ad libitum fed knockout male mice had reduced Pomc and Ucp-1 mRNA expression compared to control group. Taken together, these results suggest that STAT5 signalling in AgRP neurons regulates body adiposity in female mice, as well as some neuroendocrine adaptations to food restriction.
He Jiang, Xiao-Ping Ye, Zhong-Yin Yang, Ming Zhan, Hai-Ning Wang, Huang-Min Cao, Hui-Jun Xie, Chun-Ming Pan, Huai-Dong Song and Shuang-Xia Zhao
There is a high incidence of metabolic syndrome among patients with primary aldosteronism (PA), which has recently been associated with an unfavorable cardiometabolic profile. However, the underlying mechanisms have not been clarified in detail. Characterizing aldosterone (Ald) target genes in adipocytes will help us to elucidate the deleterious effects associated with excess Ald. Apelin, a novel adipokine, exerts beneficial effects on obesity-associated disorders and cardiovascular homeostasis. The objective of this study was to investigate the effects of high Ald levels on apelin expression and secretion and the underlying mechanisms involved in adipocytes. In vivo, a single-dose Ald injection acutely decreased apelin serum levels and adipose tissue apelin production, which demonstrates a clear inverse relationship between the levels of plasma Ald and plasma apelin. Experiments using 3T3-L1 adipocytes showed that Ald decreased apelin expression and secretion in a time- and dose-dependent manner. This effect was reversed by glucocorticoid receptor (GR) antagonists or GR (NR3C1) knockdown; furthermore, putative HREs were identified in the apelin promoter. Subsequently, we verified that both glucocorticoids and mineralocorticoids regulated apelin expression through GR activation, although no synergistic effect was observed. Additionally, detailed potential mechanisms involved a p38 MAPK signaling pathway. In conclusion, our findings strengthen the fact that there is a direct interaction between Ald and apelin in adipocytes, which has important implications for hyperaldosteronism or PA-associated cardiometabolic syndrome and hoists apelin on the list of potent therapeutic targets for PA.
Knut R Steffensen, Soek Ying Neo, Thomas M Stulnig, Vinsensius B Vega, Safia S Rahman, Gertrud U Schuster, Jan-Åke Gustafsson and Edison T Liu
The liver X receptors α and β (LXRα and LXRβ ) are members of the nuclear receptor superfamily of proteins which are highly expressed in metabolically active tissues. They regulate gene expression of critical genes involved in cholesterol catabolism and transport, lipid and triglyceride biosynthesis and carbohydrate metabolism in response to distinct oxysterols and intermediates in the cholesterol metabolic pathway. The biological roles of the LXRs in tissues other than liver, intestine and adipose tissue are poorly elucidated. In this study we used global gene-expression profiling analysis to detect differences in expression patterns in several tissues from mice fed an LXR agonist or vehicle. Our results show that LXR plays an important role in the kidney, lung, adrenals, brain, testis and heart where several putative LXR target genes were found. The effects of the LXRs were further analysed in adrenals where treatment with an LXR agonist induced expression of adrenocorticotrophic hormone receptor, suppressed expression of uncoupling protein (UCP)-1 and UCP-3 as well as several glycolytic enzymes and led to increased serum corticosterone levels. These results indicate novel biological roles of the LXR including regulation of energy metabolism, glycolysis and steroidogenesis in the adrenals via alteration of expression profiles of putative target genes.