Metabolism
Abstract
Nonalcoholic fatty liver disease is strongly associated with hepatic insulin resistance and type 2 diabetes mellitus, but the molecular signals linking hepatic fat accumulation to hepatic insulin resistance are unknown. Three days of high-fat feeding in rats results specifically in hepatic steatosis and hepatic insulin resistance. In this setting, PKCε, but not other isoforms of PKC, is activated. To determine whether PKCε plays a causal role in the pathogenesis of hepatic insulin resistance, we treated rats with an antisense oligonucleotide against PKCε and subjected them to 3 days of high-fat feeding. Knocking down PKCε expression protects rats from fat-induced hepatic insulin resistance and reverses fat-induced defects in hepatic insulin signaling. Furthermore, we show that PKCε associates with the insulin receptor in vivo and impairs insulin receptor kinase activity both in vivo and in vitro. These data support the hypothesis that PKCε plays a critical role in mediating fat-induced hepatic insulin resistance and represents a novel therapeutic target for type 2 diabetes.
Authors
Varman T. Samuel, Zhen-Xiang Liu, Amy Wang, Sara A. Beddow, John G. Geisler, Mario Kahn, Xian-man Zhang, Brett P. Monia, Sanjay Bhanot, Gerald I. Shulman
Abstract
SH2B1 (previously named SH2-B), a cytoplasmic adaptor protein, binds via its Src homology 2 (SH2) domain to a variety of protein tyrosine kinases, including JAK2 and the insulin receptor. SH2B1-deficient mice are obese and diabetic. Here we demonstrated that multiple isoforms of SH2B1 (α, β, γ, and/or δ) were expressed in numerous tissues, including the brain, hypothalamus, liver, muscle, adipose tissue, heart, and pancreas. Rat SH2B1β was specifically expressed in neural tissue in SH2B1-transgenic (SH2B1Tg) mice. SH2B1Tg mice were crossed with SH2B1-knockout (SH2B1KO) mice to generate SH2B1TgKO mice expressing SH2B1 only in neural tissue but not in other tissues. Systemic deletion of the SH2B1 gene resulted in metabolic disorders in SH2B1KO mice, including hyperlipidemia, leptin resistance, hyperphagia, obesity, hyperglycemia, insulin resistance, and glucose intolerance. Neuron-specific restoration of SH2B1β not only corrected the metabolic disorders in SH2B1TgKO mice, but also improved JAK2-mediated leptin signaling and leptin regulation of orexigenic neuropeptide expression in the hypothalamus. Moreover, neuron-specific overexpression of SH2B1 dose-dependently protected against high-fat diet–induced leptin resistance and obesity. These observations suggest that neuronal SH2B1 regulates energy balance, body weight, peripheral insulin sensitivity, and glucose homeostasis at least in part by enhancing hypothalamic leptin sensitivity.
Authors
Decheng Ren, Yingjiang Zhou, David Morris, Minghua Li, Zhiqin Li, Liangyou Rui
Extrapancreatic incretin receptors modulate glucose homeostasis, body weight, and energy expenditure
Abstract
The incretin hormones glucagon-like peptide–1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) control glucose homeostasis through well-defined actions on the islet β cell via stimulation of insulin secretion and preservation and expansion of β cell mass. We examined the importance of endogenous incretin receptors for control of glucose homeostasis through analysis of Glp1r–/–, Gipr–/–, and double incretin receptor knockout (DIRKO) mice fed a high-fat (HF) diet. DIRKO mice failed to upregulate levels of plasma insulin, pancreatic insulin mRNA transcripts, and insulin content following several months of HF feeding. Both single incretin receptor knockout and DIRKO mice exhibited resistance to diet-induced obesity, preservation of insulin sensitivity, and increased energy expenditure associated with increased locomotor activity. Moreover, plasma levels of plasminogen activator inhibitor–1 and resistin failed to increase significantly in DIRKO mice after HF feeding, and the GIP receptor agonist [D-Ala2]GIP, but not the GLP-1 receptor agonist exendin-4, increased the levels of plasma resistin in studies of both acute and chronic administration. These findings extend our understanding of how endogenous incretin circuits regulate glucose homeostasis independent of the β cell via control of adipokine secretion and energy expenditure.
Authors
Tanya Hansotia, Adriano Maida, Grace Flock, Yuichiro Yamada, Katsushi Tsukiyama, Yutaka Seino, Daniel. J. Drucker
Abstract
Cyclic nucleotide phosphodiesterase 3B (PDE3B) has been suggested to be critical for mediating insulin/IGF-1 inhibition of cAMP signaling in adipocytes, liver, and pancreatic β cells. In Pde3b-KO adipocytes we found decreased adipocyte size, unchanged insulin-stimulated phosphorylation of protein kinase B and activation of glucose uptake, enhanced catecholamine-stimulated lipolysis and insulin-stimulated lipogenesis, and blocked insulin inhibition of catecholamine-stimulated lipolysis. Glucose, alone or in combination with glucagon-like peptide–1, increased insulin secretion more in isolated pancreatic KO islets, although islet size and morphology and immunoreactive insulin and glucagon levels were unchanged. The β3-adrenergic agonist CL 316,243 (CL) increased lipolysis and serum insulin more in KO mice, but blood glucose reduction was less in CL-treated KO mice. Insulin resistance was observed in KO mice, with liver an important site of alterations in insulin-sensitive glucose production. In KO mice, liver triglyceride and cAMP contents were increased, and the liver content and phosphorylation states of several insulin signaling, gluconeogenic, and inflammation- and stress-related components were altered. Thus, PDE3B may be important in regulating certain cAMP signaling pathways, including lipolysis, insulin-induced antilipolysis, and cAMP-mediated insulin secretion. Altered expression and/or regulation of PDE3B may contribute to metabolic dysregulation, including systemic insulin resistance.
Authors
Young Hun Choi, Sunhee Park, Steven Hockman, Emilia Zmuda-Trzebiatowska, Fredrik Svennelid, Martin Haluzik, Oksana Gavrilova, Faiyaz Ahmad, Laurent Pepin, Maria Napolitano, Masato Taira, Frank Sundler, Lena Stenson Holst, Eva Degerman, Vincent C. Manganiello
Abstract
Obesity and weight gain are characterized by increased adipose tissue mass due to an increase in the size of individual adipocytes and the generation of new adipocytes. New adipocytes are believed to arise from resident adipose tissue preadipocytes and mesenchymal progenitor cells. However, it is possible that progenitor cells from other tissues, in particular BM, could also contribute to development of new adipocytes in adipose tissue. We tested this hypothesis by transplanting whole BM cells from GFP-expressing transgenic mice into wild-type C57BL/6 mice and subjecting them to a high-fat diet or treatment with the thiazolidinedione (TZD) rosiglitazone (ROSI) for several weeks. Histological examination of adipose tissue or FACS of adipocytes revealed the presence of GFP+ multilocular (ML) adipocytes, whose number was significantly increased by ROSI treatment or high-fat feeding. These ML adipocytes expressed adiponectin, perilipin, fatty acid–binding protein (FABP), leptin, C/EBPα, and PPARγ but not uncoupling protein–1 (UCP-1), the CD45 hematopoietic lineage marker, or the CDllb monocyte marker. They also exhibited increased mitochondrial content. Appearance of GFP+ ML adipocytes was contemporaneous with an increase in circulating levels of mesenchymal and hematopoietic progenitor cells in ROSI-treated animals. We conclude that TZDs and high-fat feeding promote the trafficking of BM-derived circulating progenitor cells to adipose tissue and their differentiation into ML adipocytes.
Authors
Joseph T. Crossno, Susan M. Majka, Todd Grazia, Ronald G. Gill, Dwight J. Klemm
Abstract
Type 1 diabetes (T1D) is characterized by immune responses against several autoantigens expressed in pancreatic β cells. T cells specific for proinsulin and islet-specific glucose-6-phosphatase catalytic subunit–related protein (IGRP) can induce T1D in NOD mice. However, whether immune responses to multiple autoantigens are caused by spreading from one to another or whether they develop independently of each other is unknown. As cytotoxic T cells specific for IGRP were not detected in transgenic NOD mice tolerant to proinsulin, we determined that immune responses against proinsulin are necessary for IGRP-specific T cells to develop. On the other hand, transgenic overexpression of IGRP resulted in loss of intra-islet IGRP-specific T cells but did not protect NOD mice from insulitis or T1D, providing direct evidence that the response against IGRP is downstream of the response to proinsulin. Our results suggest that pathogenic proinsulin-specific immunity in NOD mice subsequently spreads to other antigens such as IGRP.
Authors
Balasubramanian Krishnamurthy, Nadine L. Dudek, Mark D. McKenzie, Anthony W. Purcell, Andrew G. Brooks, Shane Gellert, Peter G. Colman, Leonard C. Harrison, Andrew M. Lew, Helen E. Thomas, Thomas W.H. Kay
Abstract
The gut hormone ghrelin targets the brain to promote food intake and adiposity. The ghrelin receptor growth hormone secretagogue 1 receptor (GHSR) is present in hypothalamic centers controlling energy metabolism as well as in the ventral tegmental area (VTA), a region important for motivational aspects of multiple behaviors, including feeding. Here we show that in mice and rats, ghrelin bound to neurons of the VTA, where it triggered increased dopamine neuronal activity, synapse formation, and dopamine turnover in the nucleus accumbens in a GHSR-dependent manner. Direct VTA administration of ghrelin also triggered feeding, while intra-VTA delivery of a selective GHSR antagonist blocked the orexigenic effect of circulating ghrelin and blunted rebound feeding following fasting. In addition, ghrelin- and GHSR-deficient mice showed attenuated feeding responses to restricted feeding schedules. Taken together, these data suggest that the mesolimbic reward circuitry is targeted by peripheral ghrelin to influence physiological mechanisms related to feeding.
Authors
Alfonso Abizaid, Zhong-Wu Liu, Zane B. Andrews, Marya Shanabrough, Erzsebet Borok, John D. Elsworth, Robert H. Roth, Mark W. Sleeman, Marina R. Picciotto, Matthias H. Tschöp, Xiao-Bing Gao, Tamas L. Horvath
Abstract
TLR4 is the receptor for LPS and plays a critical role in innate immunity. Stimulation of TLR4 activates proinflammatory pathways and induces cytokine expression in a variety of cell types. Inflammatory pathways are activated in tissues of obese animals and humans and play an important role in obesity-associated insulin resistance. Here we show that nutritional fatty acids, whose circulating levels are often increased in obesity, activate TLR4 signaling in adipocytes and macrophages and that the capacity of fatty acids to induce inflammatory signaling in adipose cells or tissue and macrophages is blunted in the absence of TLR4. Moreover, mice lacking TLR4 are substantially protected from the ability of systemic lipid infusion to (a) suppress insulin signaling in muscle and (b) reduce insulin-mediated changes in systemic glucose metabolism. Finally, female C57BL/6 mice lacking TLR4 have increased obesity but are partially protected against high fat diet–induced insulin resistance, possibly due to reduced inflammatory gene expression in liver and fat. Taken together, these data suggest that TLR4 is a molecular link among nutrition, lipids, and inflammation and that the innate immune system participates in the regulation of energy balance and insulin resistance in response to changes in the nutritional environment.
Authors
Hang Shi, Maia V. Kokoeva, Karen Inouye, Iphigenia Tzameli, Huali Yin, Jeffrey S. Flier
Abstract
Obesity is associated with chronic low-grade inflammation. Thus, at metabolically relevant sites, including adipose tissue and muscle, there is abnormal production of proinflammatory cytokines such as TNF-α. Here we demonstrate that eNOS expression was reduced, with a concomitant reduction of mitochondrial biogenesis and function, in white and brown adipose tissue and in the soleus muscle of 3 different animal models of obesity. The genetic deletion of TNF receptor 1 in obese mice restored eNOS expression and mitochondrial biogenesis in fat and muscle; this was associated with less body weight gain than in obese wild-type controls. Furthermore, TNF-α downregulated eNOS expression and mitochondrial biogenesis in cultured white and brown adipocytes and muscle satellite cells of mice. The NO donors DETA-NO and SNAP prevented the reduction of mitochondrial biogenesis observed with TNF-α. Our findings demonstrate that TNF-α impairs mitochondrial biogenesis and function in different tissues of obese rodents by downregulating eNOS expression and suggest a novel pathophysiological process that sustains obesity.
Authors
Alessandra Valerio, Annalisa Cardile, Valeria Cozzi, Renata Bracale, Laura Tedesco, Addolorata Pisconti, Letizia Palomba, Orazio Cantoni, Emilio Clementi, Salvador Moncada, Michele O. Carruba, Enzo Nisoli
Abstract
The forkhead transcription factor FoxO1 has been identified as a negative regulator of insulin/IGF-1 signaling. Its function is inhibited by phosphorylation and nuclear exclusion through a PI3K-dependent pathway. However, the structure/function relationship of FoxO1 has not been elucidated completely. In this study, we carried out mutation analysis of the FoxO1 coactivator–interacting LXXLL motif (amino acids 459–463). Expression of a 3A/LXXAA mutant, in which 3 Akt phosphorylation sites (T24, S253, and S316) and 2 leucine residues in the LXXLL motif (L462 and L463) were replaced by alanine, decreased both Igfbp-1 and G6Pase promoter activity and endogenous Igfbp-1 and G6Pase gene expression in simian virus 40–transformed (SV40-transformed) hepatocytes. Importantly, mutagenesis of the LXXLL motif eliminated FoxO1 interaction with the nicotinamide adenine dinucleotide–dependent (NAD-dependent) deacetylase sirtuin 1 (Sirt1), sustained the acetylated state of FoxO1, and made FoxO1 nicotinamide and resveratrol insensitive, supporting a role for this motif in Sirt1 binding. Furthermore, intravenous administration of adenovirus encoding 3A/LXXAA FoxO1 into Leprdb/db mice decreased fasting blood glucose levels and improved glucose tolerance and was accompanied by reduced G6Pase and Igfbp-1 gene expression and increased hepatic glycogen content. In conclusion, the LXXLL motif of FoxO1 may have an important role for its transcriptional activity and Sirt1 binding and should be a target site for regulation of gene expression of FoxO1 target genes and glucose metabolism in vivo.
Authors
Jun Nakae, Yongheng Cao, Hiroaki Daitoku, Akiyoshi Fukamizu, Wataru Ogawa, Yoshihiko Yano, Yoshitake Hayashi
Copyright © 2025 American Society for Clinical Investigation
ISSN: 0021-9738 (print), 1558-8238 (online)