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1.  Female Nur77-Deficient Mice Show Increased Susceptibility to Diet-Induced Obesity 
PLoS ONE  2013;8(1):e53836.
Adipose tissue is essential in the regulation of body weight. The key process in fat catabolism and the provision of energy substrate during times of nutrient deprivation or enhanced energy demand is the hydrolysis of triglycerides and the release of fatty acids and glycerol. Nur77 is a member of the NR4A subfamily of nuclear receptors that plays an important metabolic role, modulating hepatic glucose metabolism and lipolysis in muscle. However, its endogenous role on white adipose tissue, as well as the gender dependency of these mechanisms, remains largely unknown. Male and female wild type and Nur77 deficient mice were fed with a high fat diet (45% calories from fat) for 4 months. Mice were analyzed in vivo with the indirect calorimetry system, and tissues were analyzed by real-time PCR and Western blot analysis. Female, but not male Nur77 deficient mice, gained more weight and fat mass when compared to wild type mice fed with high fat diet, which can be explained by decreased energy expenditure. The lack of Nur77 also led to a decreased pHSL/HSL ratio in white adipose tissue and increased expression of CIDEA in brown adipose tissue of female Nur77 deficient mice. Overall, these findings suggest that Nur77 is an important physiological modulator of lipid metabolism in adipose tissue and that there are gender differences in the sensitivity to deletion of the Nur77 signaling. The decreased energy expenditure and the actions of Nur77 on liver, muscle, brown and white adipose tissue contribute to the increased susceptibility to diet-induced obesity in females lacking Nur77.
PMCID: PMC3544711  PMID: 23342015
2.  Nur77 coordinately regulates expression of genes linked to glucose metabolism in skeletal muscle 
Innervation is important for normal metabolism in skeletal muscle, including insulin-sensitive glucose uptake. However, the transcription factors that transduce signals from the neuromuscular junction to the nucleus and affect changes in metabolic gene expression are not well defined. We demonstrate here that the orphan nuclear receptor Nur77 is a regulator of gene expression linked to glucose utilization in muscle. In vivo, Nur77 is preferentially expressed in glycolytic compared to oxidative muscle and is responsive to β-adrenergic stimulation. Denervation of rat muscle compromises expression of Nur77 in parallel with that of numerous genes linked to glucose metabolism, including GLUT4 and genes involved in glycolysis, glycogenolysis, and the glycerophosphate shuttle. Ectopic expression of Nur77, either in rat muscle or in C2C12 muscle cells, induces expression of a highly overlapping set of genes, including GLUT4, muscle phosphofructokinase, and glycogen phosphorylase. Furthermore, selective knockdown of Nur77 in rat muscle by shRNA or genetic deletion of Nur77 in mice reduces the expression of a battery of genes involved in skeletal muscle glucose utilization in vivo. Finally, we show that Nur77 binds the promoter regions of multiple innervation-dependent genes in muscle. These results identify Nur77 as a potential mediator of neuromuscular signaling in the control of metabolic gene expression.
PMCID: PMC2602962  PMID: 17550977
Nur77; glucose metabolism; skeletal muscle
3.  Inhibition of adipocyte differentiation by Nur77, Nurr1 and Nor1 
Members of the NR4A subgroup of nuclear receptors have been implicated in the regulation of glucose and lipid metabolism in insulin-sensitive tissues such as liver and skeletal muscle. However, their function in adipocytes is not well defined. Previous studies have reported that these receptors are rapidly upregulated following treatment of 3T3-L1 preadipocytes with an adipogenic cocktail. We show here that although Nur77 expression is acutely induced by cAMP agonists in 3T3-L1 cells, it is not induced by other adipogenic stimuli, such as PPARγ ligands, nor is it induced during the differentiation of 3T3-F442A preadipocytes, suggesting that Nur77 induction is not an obligatory feature of preadipocyte differentiation. We further demonstrate that inflammatory signals that antagonize differentiation, such as TNFα and lipopolysaccharide, acutely induce Nur77 expression both in vitro and in vivo. We also show that NR4A expression in adipose tissue is responsive to fasting/refeeding. Retroviral transduction of each of the NR4A receptors (Nur77, Nurr1 and NOR1) into either 3T3-L1 or 3T3-F442A preadipocytes potently inhibits adipogenesis. Interestingly, NR4A-mediated inhibition of adipogenesis cannot not be rescued by PPARγ overexpression or activation. Transcriptional profiling of Nur77-expressing preadipocytes led to the identification of gap-junction protein alpha 1 (Gja1) and tolloid-like 1 (Tll1) as Nur77-responsive genes. Remarkably, retroviral expression of either Gja1 or Tll1 in 3T3-L1 preadipocytes also inhibited adipocyte differentiation, implicating these genes as potential mediators of Nur77’s effects on adipogenesis. Finally, we show that Nur77 expression inhibits mitotic clonal expansion of preadipocytes, providing an additional mechanism by which Nur77 may inhibit adipogenesis.
PMCID: PMC2610364  PMID: 18945812
Nuclear Receptor; Nur77; adipogenesis; differentiation
4.  Nur77 Decreases Atherosclerosis Progression in apoE−/− Mice Fed a High-Fat/High-Cholesterol Diet 
PLoS ONE  2014;9(1):e87313.
It is clear that lipid disorder and inflammation are associated with cardiovascular diseases and underlying atherosclerosis. Nur77 has been shown to be involved in inflammatory response and lipid metabolism.
Here, we explored the role of Nur77 in atherosclerotic plaque progression in apoE−/− mice fed a high-fat/high cholesterol diet.
Methods and Results
The Nur77 gene, a nuclear hormone receptor, was highly induced by treatment with Cytosporone B (Csn-B, specific Nur77 agonist), recombinant plasmid over-expressing Nur77 (pcDNA-Nur77), while inhibited by treatment with siRNAs against Nur77 (si-Nur77) in THP-1 macrophage-derived foam cells, HepG2 cells and Caco-2 cells, respectively. In addition, the expression of Nur77 was highly induced by Nur77 agonist Csn-B, lentivirus encoding Nur77 (LV-Nur77), while silenced by lentivirus encoding siRNA against Nur77 (si-Nur77) in apoE−/− mice fed a high-fat/high cholesterol diet, respectively. We found that increased expression of Nur77 reduced macrophage-derived foam cells formation and hepatic lipid deposition, downregulated gene levels of inflammatory molecules, adhesion molecules and intestinal lipid absorption, and decreases atherosclerotic plaque formation.
These observations provide direct evidence that Nur77 is an important nuclear hormone receptor in regulation of atherosclerotic plaque formation and thus represents a promising target for the treatment of atherosclerosis.
PMCID: PMC3909091  PMID: 24498071
5.  Skeletal muscle Nur77 expression enhances oxidative metabolism and substrate utilization[S] 
Journal of Lipid Research  2012;53(12):2610-2619.
Mitochondrial dysfunction has been implicated in the pathogenesis of type 2 diabetes. Identifying novel regulators of mitochondrial bioenergetics will broaden our understanding of regulatory checkpoints that coordinate complex metabolic pathways. We previously showed that Nur77, an orphan nuclear receptor of the NR4A family, regulates the expression of genes linked to glucose utilization. Here we demonstrate that expression of Nur77 in skeletal muscle also enhances mitochondrial function. We generated MCK-Nur77 transgenic mice that express wild-type Nur77 specifically in skeletal muscle. Nur77-overexpressing muscle had increased abundance of oxidative muscle fibers and mitochondrial DNA content. Transgenic muscle also exhibited enhanced oxidative metabolism, suggestive of increased mitochondrial activity. Metabolomic analysis confirmed that Nur77 transgenic muscle favored fatty acid oxidation over glucose oxidation, mimicking the metabolic profile of fasting. Nur77 expression also improved the intrinsic respiratory capacity of isolated mitochondria, likely due to the increased abundance of complex I of the electron transport chain. These changes in mitochondrial metabolism translated to improved muscle contractile function ex vivo and improved cold tolerance in vivo. Our studies outline a novel role for Nur77 in the regulation of oxidative metabolism and mitochondrial activity in skeletal muscle.
PMCID: PMC3494265  PMID: 23028113
Nr4a; nuclear receptor; mitochondria
6.  Inhibition of Nur77/Nurr1 leads to inefficient clonal deletion of self- reactive T cells 
The Journal of Experimental Medicine  1996;183(4):1879-1892.
The Nur77/Nurr1 family of DNA binding proteins has been reported to be required for the signal transduction of CD3/T cell receptor (TCR)- mediated apoptosis in T cell hybridomas. To determine the role of this family of DNA-binding proteins in thymic clonal deletion, transgenic (Tg) mice bearing a dominant negative mutation were produced. The transgene consisted of a truncated Nur77 (deltaNur77) gene encoding the DNA-binding domain of Nur77 ligated to a TCR-beta enhancer resulting in early expression in thymocytes. Apoptosis of CD4+CD8+ thymocytes mediated by CD3/TCR signaling was greatly inhibited in the deltaNur77 Tg mice, compared with non-Tg littermates, after treatment with anti- CD3 or anti-TCR antibody in vivo and in vitro. Clonal deletion of self- reactive T cells was investigated in deltaNur77-Db/HY TCR-alpha/beta double Tg mice. There was a five-fold increase in the total number of thymocytes expressing self-reactive Db/HY TCR-alpha/beta in the deltaNur77-TCR-alpha/beta double Tg male mice. Deficient clonal deletion of self-reactive thymocytes was demonstrated by a 10-fold increase in the CD4+CD8+ thymocytes that expressed Tg TCR-alpha/beta. There was an eightfold increase in the CD8+, Db/HY TCR-alpha/beta T cells in the lymph nodes (LN) of delta Nur77-Db/HY TCR-alpha/beta double Tg compared with Db/HY TCR-alpha/beta Tg male mice. In spite of defective clonal deletion, the T cells expressing the Tg TCR were functionally anergic. In vivo analysis revealed increased activation and apoptosis of T cells associated with increased expression of Fas and Fas ligand in LN of deltaNur77-Db/HY TCR-alpha/beta double male mice. These results indicate that inhibition of Nur77/Nurr1 DNA binding in T cells leads to inefficient thymic clonal deletion, but T cell tolerance is maintained by Fas-dependent clonal deletion in LN and spleen.
PMCID: PMC2192482  PMID: 8666944
7.  Bone marrow NR4A expression is not a dominant factor in the development of atherosclerosis or macrophage polarization in mice[S] 
Journal of Lipid Research  2013;54(3):806-815.
The formation of the atherosclerotic lesion is a complex process influenced by an array of inflammatory and lipid metabolism pathways. We previously demonstrated that NR4A nuclear receptors are highly induced in macrophages in response to inflammatory stimuli and modulate the expression of genes linked to inflammation in vitro. Here we used mouse genetic models to assess the impact of NR4A expression on atherosclerosis development and macrophage polarization. Transplantation of wild-type, Nur77−/−, or Nor1−/− null hematopoetic precursors into LDL receptor (LDLR)−/− recipient mice led to comparable development of atherosclerotic lesions after high-cholesterol diet. We also observed comparable induction of genes linked to M1 and M2 responses in wild-type and Nur77-null macrophages in response to lipopolysaccharides and interleukin (IL)-4, respectively. In contrast, activation of the nuclear receptor liver X receptor (LXR) strongly suppressed M1 responses, and ablation of signal transductor and activator of transcription 6 (STAT6) strongly suppressed M2 responses. Recent studies have suggested that alterations in levels of Ly6Clo monocytes may be a contributor to inflammation and atherosclerosis. In our study, loss of Nur77, but not Nor1, was associated with decreased abundance of Ly6Clo monocytes, but this change was not correlated with atherosclerotic lesion development. Collectively, our results suggest that alterations in the Ly6Clo monocyte population and bone marrow NR4A expression do not play dominant roles in macrophage polarization or the development of atherosclerosis in mice.
PMCID: PMC3617954  PMID: 23288947
Nur77; Ly6C; nuclear receptor
8.  Orphan Nuclear Receptor Nur77 Regulates Androgen Receptor Gene Expression in Mouse Ovary 
PLoS ONE  2012;7(6):e39950.
The androgen receptor (AR) is a nuclear receptor that is expressed in growing follicles and involved in folliculogenesis and follicle growth. The orphan nuclear receptor, Nur77, also has an important role in steroid signaling and follicle maturation. We hypothesized that AR levels and androgen signaling through AR are regulated by Nur77 in the ovary. In the ovaries of Nur77 knockout mice (n = 5), real-time PCR results showed that the mRNA levels of AR and an androgen signaling target gene, Kitl, were decreased by 35% and 24%, respectively, relative to wild-type mice (n = 5), which suggested transcriptional regulation of AR by Nur77 in vivo. In cultured mouse granulosa cells and a steroidogenic human ovarian granulosa-like tumor cell line, KGN, mRNA and protein expression levels of AR were increased by overexpressing Nur77 but decreased by knocking down endogenous Nur77. Consistent with increased AR expression, chromatin immunoprecipitation showed that Nur77 bound to the NGFI-B response element (NBRE) in the AR promoter sequence. AR promoter activity was stimulated by Nur77 in HEK293T cells and attenuated in Nur77 knockout mouse granulosa cells (luciferase assay). Overexpression of Nur77 enhanced the androgenic induction of Kitl (200 nM; 48h), while knockout of Nur77 attenuated this induction. These results demonstrate that AR is regulated by Nur77 in the ovaries, and they suggest that the participation of Nur77 in androgen signaling may be essential for normal follicular development.
PMCID: PMC3386274  PMID: 22761936
9.  Limited Role of Nuclear Receptor Nur77 in Escherichia coli-Induced Peritonitis 
Infection and Immunity  2014;82(1):253-264.
Nuclear receptor Nur77 (NR4A1, TR3, or NGFI-B) has been shown to play an anti-inflammatory role in macrophages, which have a crucial function in defense against peritonitis. The function of Nur77 in Escherichia coli-induced peritoneal sepsis has not yet been investigated. Wild-type and Nur77-knockout mice were inoculated with E. coli, and bacterial outgrowth, cell recruitment, cytokine profiles, and tissue damage were investigated. We found only a minor transient decrease in bacterial loads in lung and liver of Nur77-knockout compared to wild-type mice at 14 h postinfection, yet no changes were found in the peritoneal lavage fluid or blood. No differences in inflammatory cytokine levels or neutrophil/macrophage numbers were observed, and bacterial loads were equal in wild-type and Nur77-knockout mice at 20 h postinfection in all body compartments tested. Also, isolated peritoneal macrophages did not show any differences in cytokine expression patterns in response to E. coli. In endothelial cells, Nur77 strongly downregulated both protein and mRNA expression of claudin-5, VE-cadherin, occludin, ZO-1, and β-catenin, and accordingly, these genes were upregulated in lungs of Nur77-deficient mice. Functional permeability tests pointed toward a strong role for Nur77 in endothelial barrier function. Indeed, tissue damage in E. coli-induced peritonitis was notably modulated by Nur77; liver necrosis and plasma aspartate aminotransferase (ASAT)/alanine aminotransferase (ALAT) levels were lower in Nur77-knockout mice. These data suggest that Nur77 does not play a role in the host response to E. coli in the peritoneal and blood compartments. However, Nur77 does modulate bacterial influx into the organs via increased vascular permeability, thereby aggravating distant organ damage.
PMCID: PMC3911847  PMID: 24166953
10.  Resistance of Foxp3+ Regulatory T Cells to Nur77-Induced Apoptosis Promotes Allograft Survival 
PLoS ONE  2008;3(5):e2321.
The NR4A nuclear receptor family member Nur77 (NR4A1) promotes thymocyte apoptosis during negative selection of autoreactive thymocytes, but may also function in mature extrathymic T cells. We studied the effects of over-expression of Nur77 on the apoptosis of murine peripheral T cells, including thymic-derived Foxp3+ regulatory (Treg) cells. Overexpression of Nur77 in the T cell lineage decreased numbers of peripheral CD4 and CD8 T cells by ∼80% compared to wild-type (WT) mice. However, the proportions of Treg cells were markedly increased in the thymus (61% of CD4+Foxp3+ singly positive thymocytes vs. 8% in WT) and secondary lymphoid organs (40–50% of CD4+Foxp3+ T cells vs. 7–8% in WT) of Nur77 transgenic (Nur77Tg) mice, and immunoprecipitation studies showed Nur77 was associated with a recently identified HDAC7/Foxp3 transcriptional complex. Upon activation through the T cell receptor in vitro or in vivo, Nur77Tg T cells showed only marginally decreased proliferation but significantly increased apoptosis. Fully allogeneic cardiac grafts transplanted to Nur77Tg mice survived long-term with well-preserved structure, and recipient splenocytes showed markedly enhanced apoptosis and greatly reduced anti-donor recall responses. Allografts in Nur77Tg recipients had significantly increased expression of multiple Treg-associated genes, including Foxp3, Foxp1, Tip60 and HDAC9. Allograft rejection was restored by CD25 monoclonal antibody therapy, indicating that allograft acceptance was dependent upon Treg function in Nur77Tg recipients. These data show that compared to conventional CD4 and CD8 T cells, Foxp3+ Tregs are relatively resistant to Nur77-mediated apoptosis, and that tipping the balance between the numbers of Tregs and responder T cells in the early period post-transplantation can determine the fate of the allograft. Hence, induced expression of Nur77 might be a novel means to achieve long-term allograft survival.
PMCID: PMC2386419  PMID: 18509529
11.  Muscle Mitochondrial ATP Synthesis and Glucose Transport/Phosphorylation in Type 2 Diabetes 
PLoS Medicine  2007;4(5):e154.
Muscular insulin resistance is frequently characterized by blunted increases in glucose-6-phosphate (G-6-P) reflecting impaired glucose transport/phosphorylation. These abnormalities likely relate to excessive intramyocellular lipids and mitochondrial dysfunction. We hypothesized that alterations in insulin action and mitochondrial function should be present even in nonobese patients with well-controlled type 2 diabetes mellitus (T2DM).
Methods and Findings
We measured G-6-P, ATP synthetic flux (i.e., synthesis) and lipid contents of skeletal muscle with 31P/1H magnetic resonance spectroscopy in ten patients with T2DM and in two control groups: ten sex-, age-, and body mass-matched elderly people; and 11 younger healthy individuals. Although insulin sensitivity was lower in patients with T2DM, muscle lipid contents were comparable and hyperinsulinemia increased G-6-P by 50% (95% confidence interval [CI] 39%–99%) in all groups. Patients with diabetes had 27% lower fasting ATP synthetic flux compared to younger controls (p = 0.031). Insulin stimulation increased ATP synthetic flux only in controls (younger: 26%, 95% CI 13%–42%; older: 11%, 95% CI 2%–25%), but failed to increase even during hyperglycemic hyperinsulinemia in patients with T2DM. Fasting free fatty acids and waist-to-hip ratios explained 44% of basal ATP synthetic flux. Insulin sensitivity explained 30% of insulin-stimulated ATP synthetic flux.
Patients with well-controlled T2DM feature slightly lower flux through muscle ATP synthesis, which occurs independently of glucose transport /phosphorylation and lipid deposition but is determined by lipid availability and insulin sensitivity. Furthermore, the reduction in insulin-stimulated glucose disposal despite normal glucose transport/phosphorylation suggests further abnormalities mainly in glycogen synthesis in these patients.
Michael Roden and colleagues report that even patients with well-controlled insulin-resistant type 2 diabetes have altered mitochondrial function.
Editors' Summary
Diabetes mellitus is an increasingly common chronic disease characterized by high blood sugar (glucose) levels. In normal individuals, blood sugar levels are maintained by the hormone insulin. Insulin is released by the pancreas when blood glucose levels rise after eating (glucose is produced by the digestion of food) and “instructs” insulin-responsive muscle and fat cells to take up glucose from the bloodstream. The cells then use glucose as a fuel or convert it into glycogen, a storage form of glucose. In type 2 diabetes, the commonest type of diabetes, the muscle and fat cells become nonresponsive to insulin (a condition called insulin resistance) and consequently blood glucose levels rise. Over time, this hyperglycemia increases the risk of heart attacks, kidney failure, and other life-threatening complications.
Why Was This Study Done?
Insulin resistance is often an early sign of type 2 diabetes, sometimes predating its development by many years, so understanding its causes might provide clues about how to stop the global diabetes epidemic. One theory is that mitochondria—cellular structures that produce the energy (in the form of a molecule called ATP) needed to keep cells functioning—do not work properly in people with insulin resistance. Mitochondria change (metabolize) fatty acids into energy, and recent studies have revealed that fat accumulation caused by poorly regulated fatty acid metabolism blocks insulin signaling, thus causing insulin resistance. Other studies using magnetic resonance spectroscopy (MRS) to study mitochondrial function noninvasively in human muscle indicate that mitochondria are dysfunctional in people with insulin resistance by showing that ATP synthesis is impaired in such individuals. In this study, the researchers have examined both baseline and insulin-stimulated mitochondrial function in nonobese patients with well-controlled type 2 diabetes and in normal controls to discover more about the relationship between mitochondrial dysfunction and insulin resistance.
What Did the Researchers Do and Find?
The researchers determined the insulin sensitivity of people with type 2 diabetes and two sets of people (the “controls”) who did not have diabetes: one in which the volunteers were age-matched to the people with diabetes, and the other containing younger individuals (insulin resistance increases with age). To study insulin sensitivity in all three groups, the researchers used a “hyperinsulinemic–euglycemic clamp.” For this, after an overnight fast, the participants' insulin levels were kept high with a continuous insulin infusion while blood glucose levels were kept normal using a variable glucose infusion. In this situation, the glucose infusion rate equals glucose uptake by the body and therefore measures tissue sensitivity to insulin. Before and during the clamp, the researchers used MRS to measure glucose-6-phosphate (an indicator of how effectively glucose is taken into cells and phosphorylated), ATP synthesis, and the fat content of the participants' muscle cells. Insulin sensitivity was lower in the patients with diabetes than in the controls, but muscle lipid content was comparable and hyperinsulinemia increased glucose-6-phosphate levels similarly in all the groups. Patients with diabetes and the older controls had lower fasting ATP synthesis rates than the young controls and, whereas insulin stimulation increased ATP synthesis in all the controls, it had no effect in the patients with diabetes. In addition, fasting blood fatty acid levels were inversely related to basal ATP synthesis, whereas insulin sensitivity was directly related to insulin-stimulated ATP synthesis.
What Do These Findings Mean?
These findings indicate that the impairment of muscle mitochondrial ATP synthesis in fasting conditions and after insulin stimulation in people with diabetes is not due to impaired glucose transport/phosphorylation or fat deposition in the muscles. Instead, it seems to be determined by lipid availability and insulin sensitivity. These results add to the evidence suggesting that mitochondrial function is disrupted in type 2 diabetes and in insulin resistance, but also suggest that there may be abnormalities in glycogen synthesis. More work is needed to determine the exact nature of these abnormalities and to discover whether they can be modulated to prevent the development of insulin resistance and type 2 diabetes. For now, though, these findings re-emphasize the need for people with type 2 diabetes or insulin resistance to reduce their food intake to compensate for the reduced energy needs of their muscles and to exercise to increase the ATP-generating capacity of their muscles. Both lifestyle changes could improve their overall health and life expectancy.
Additional Information.
Please access these Web sites via the online version of this summary at
The MedlinePlus encyclopedia has pages on diabetes
The US National Institute of Diabetes and Digestive and Kidney Diseases provides information for patients on diabetes and insulin resistance
The US Centers for Disease Control and Prevention has information on diabetes for patients and professionals
American Diabetes Association provides information for patients on diabetes and insulin resistance
Diabetes UK has information for patients and professionals on diabetes
PMCID: PMC1858707  PMID: 17472434
12.  Skeletal Muscle–Specific Deletion of Lipoprotein Lipase Enhances Insulin Signaling in Skeletal Muscle but Causes Insulin Resistance in Liver and Other Tissues 
Diabetes  2009;58(1):116-124.
OBJECTIVE—Skeletal muscle–specific LPL knockout mouse (SMLPL−/−) were created to study the systemic impact of reduced lipoprotein lipid delivery in skeletal muscle on insulin sensitivity, body weight, and composition.
RESEARCH DESIGN AND METHODS—Tissue-specific insulin sensitivity was assessed using a hyperinsulinemic-euglycemic clamp and 2-deoxyglucose uptake. Gene expression and insulin-signaling molecules were compared in skeletal muscle and liver of SMLPL−/− and control mice.
RESULTS—Nine-week-old SMLPL−/− mice showed no differences in body weight, fat mass, or whole-body insulin sensitivity, but older SMLPL−/− mice had greater weight gain and whole-body insulin resistance. High-fat diet feeding accelerated the development of obesity. In young SMLPL−/− mice, insulin-stimulated glucose uptake was increased 58% in the skeletal muscle, but was reduced in white adipose tissue (WAT) and heart. Insulin action was also diminished in liver: 40% suppression of hepatic glucose production in SMLPL−/− vs. 90% in control mice. Skeletal muscle triglyceride was 38% lower, and insulin-stimulated phosphorylated Akt (Ser473) was twofold greater in SMLPL−/− mice without changes in IRS-1 tyrosine phosphorylation and phosphatidylinositol 3-kinase activity. Hepatic triglyceride and liver X receptor, carbohydrate response element–binding protein, and PEPCK mRNAs were unaffected in SMLPL−/− mice, but peroxisome proliferator–activated receptor (PPAR)-γ coactivator-1α and interleukin-1β mRNAs were higher, and stearoyl–coenzyme A desaturase-1 and PPARγ mRNAs were reduced.
CONCLUSIONS—LPL deletion in skeletal muscle reduces lipid storage and increases insulin signaling in skeletal muscle without changes in body composition. Moreover, lack of LPL in skeletal muscle results in insulin resistance in other key metabolic tissues and ultimately leads to obesity and systemic insulin resistance.
PMCID: PMC2606858  PMID: 18952837
13.  Common polymorphisms within the NR4A3 locus, encoding the orphan nuclear receptor Nor-1, are associated with enhanced β-cell function in non-diabetic subjects 
BMC Medical Genetics  2009;10:77.
Neuron-derived orphan receptor (Nor) 1, nuclear receptor (Nur) 77, and nuclear receptor-related protein (Nurr) 1 constitute the NR4A family of orphan nuclear receptors which were recently found to modulate hepatic glucose production, insulin signalling in adipocytes, and oxidative metabolism in skeletal muscle. In this study, we assessed whether common genetic variation within the NR4A3 locus, encoding Nor-1, contributes to the development of prediabetic phenotypes, such as glucose intolerance, insulin resistance, or β-cell dysfunction.
We genotyped 1495 non-diabetic subjects from Southern Germany for the five tagging single nucleotide polymorphisms (SNPs) rs7047636, rs1526267, rs2416879, rs12686676, and rs10819699 (minor allele frequencies ≥ 0.05) covering 100% of genetic variation within the NR4A3 locus (with D' = 1.0, r2 ≥ 0.9) and assessed their association with metabolic data derived from the fasting state, an oral glucose tolerance test (OGTT), and a hyperinsulinemic-euglycemic clamp (subgroup, N = 506). SNPs that revealed consistent associations with prediabetic phenotypes were subsequently genotyped in a second cohort (METSIM Study; Finland; N = 5265) for replication.
All five SNPs were in Hardy-Weinberg equilibrium (p ≥ 0.7, all). The minor alleles of three SNPs, i.e., rs1526267, rs12686676, and rs10819699, consistently tended to associate with higher insulin release as derived from plasma insulin at 30 min(OGTT), AUCC-peptide-to-AUCGluc ratio and the AUCIns30-to-AUCGluc30 ratio with rs12686676 reaching the level of significance (p ≤ 0.03, all; additive model). The association of the SNP rs12686676 with insulin secretion was replicated in the METSIM cohort (p ≤ 0.03, additive model). There was no consistent association with glucose tolerance or insulin resistance in both study cohorts.
We conclude that common genetic variation within the NR4A3 locus determines insulin secretion. Thus, NR4A3 represents a novel candidate gene for β-cell function which was not covered by the SNP arrays of recent genome-wide association studies for type 2 diabetes mellitus.
PMCID: PMC2741445  PMID: 19682370
14.  NR4A1 (Nur77) Deletion Polarizes Macrophages Towards an Inflammatory Phenotype and Increases Atherosclerosis 
Circulation Research  2011;110(3):416-427.
NR4A1 (Nur77) is a nuclear receptor that is expressed in macrophages and within atherosclerotic lesions, yet its function in atherosclerosis is unknown.
Nur77 regulates the development of monocytes, particularly patrolling Ly6C− monocytes that may be involved in resolution of inflammation. We sought to determine how absence of NR4A1 in hematopoietic cells impacted atherosclerosis development.
Methods and Results
Nur77−/− chimeric mice on a Ldlr−/− background showed a 3-fold increase in atherosclerosis development when fed a Western diet for 20 weeks, despite having a drastic reduction in Ly6C− patrolling monocytes. In a second model, mice deficient in both Nur77 and ApoE (ApoE−/−Nur77−/−) also showed increased atherosclerosis after 11 weeks of Western diet. Atherosclerosis was associated with a significant change in macrophage polarization towards a pro-inflammatory phenotype, with high expression of TNFα and nitric oxide, and low expression of Arginase-I. Moreover, we found increased expression of TLR4 mRNA and protein in Nur77−/− macrophages as well as increased phosphorylation of the p65 subunit of NFκB. Inhibition of NFκB activity blocked excess activation of Nur77−/− macrophages.
We conclude that the absence of Nur77 in monocytes and macrophages results in enhanced TLR signaling and polarization of macrophages towards a pro-inflammatory M1 phenotype. Despite having fewer monocytes, Nur77−/− mice developed significant atherosclerosis when fed a Western diet. These studies indicate that Nur77 is a novel target for modulating the inflammatory phenotype of monocytes and macrophages and may be important for regulation of atherogenesis.
PMCID: PMC3309661  PMID: 22194622
monocyte; atherosclerosis; nuclear receptors; macrophage; toll-like receptors
15.  Endocrine and neurogenic regulation of the orphan nuclear receptors Nur77 and Nurr-1 in the adrenal glands. 
Molecular and Cellular Biology  1994;14(5):3469-3483.
nurr77 and nurr-1 are growth factor-inducible members of the steroid/thyroid hormone receptor gene superfamily. In order to gain insight into the potential roles of nur77 in the living organism, we used pharmacologic treatments to examine the expression of nur77 in the mouse adrenal gland. We found that nur77 and nurr-1 are induced in the adrenal gland upon treatment with pentylene tetrazole (Ptz; Metrazole). This induction is separable into distinct endocrine and neurogenic mechanisms. In situ hybridization analysis demonstrates that nur77 expression upon Ptz treatment in the adrenal cortex is localized primarily to the inner cortical region, the zona fasciculata-reticularis, with minimal induction in the zona glomerulosa. This induction is inhibitable by pretreatment with dexamethasone, indicating involvement of the hypothalamic-pituitary-adrenal axis in the activation of adrenal cortical expression. When mice were injected with adrenocorticotrophic hormone (ACTH), nur77 expression in the adrenal gland spanned all cortical layers including the zona glomerulosa, but medullary expression was not induced. Ptz also induces expression of both nur77 and nurr-1 in the adrenal medulla. Medullary induction is likely to have a neurogenic origin, as nur77 expression was not inhibitable by dexamethasone pretreatment and induction was seen after treatment with the cholinergic neurotransmitter nicotine. nur77 is also inducible by ACTH, forskolin, and the second messenger analog dibutyryl cyclic AMP in the ACTH-responsive adrenal cortical cell line Y-1. Significantly, Nur77 isolated from ACTH-stimulated Y-1 cells bound to its response element whereas Nur77 present in unstimulated cells did not. Moreover, Nur77 in ACTH-treated Y-1 cells was hypophosphorylated at serine 354 compared with that in untreated cells. These results, taken together with the previous observation that dephosphorylation of serine 354 affects DNA binding affinity in vitro, show for the first time that phosphorylation of Nur77 at serine 354 is under hormonal regulation, modulating its DNA binding affinity. Thus, ACTH regulates Nur77 in two ways: activation of its gene and posttranslational modification. A promoter analysis of nur77 induction in Y-1 cells indicates that the regulatory elements mediating ACTH induction differ from those required for induction in the adrenal medullary tumor cell line PC12 and in 3T3 fibroblasts.
PMCID: PMC358711  PMID: 8164692
16.  Myostatin Inhibition in Muscle, but Not Adipose Tissue, Decreases Fat Mass and Improves Insulin Sensitivity 
PLoS ONE  2009;4(3):e4937.
Myostatin (Mstn) is a secreted growth factor expressed in skeletal muscle and adipose tissue that negatively regulates skeletal muscle mass. Mstn−/− mice have a dramatic increase in muscle mass, reduction in fat mass, and resistance to diet-induced and genetic obesity. To determine how Mstn deletion causes reduced adiposity and resistance to obesity, we analyzed substrate utilization and insulin sensitivity in Mstn−/− mice fed a standard chow. Despite reduced lipid oxidation in skeletal muscle, Mstn−/− mice had no change in the rate of whole body lipid oxidation. In contrast, Mstn−/− mice had increased glucose utilization and insulin sensitivity as measured by indirect calorimetry, glucose and insulin tolerance tests, and hyperinsulinemic-euglycemic clamp. To determine whether these metabolic effects were due primarily to the loss of myostatin signaling in muscle or adipose tissue, we compared two transgenic mouse lines carrying a dominant negative activin IIB receptor expressed specifically in adipocytes or skeletal muscle. We found that inhibition of myostatin signaling in adipose tissue had no effect on body composition, weight gain, or glucose and insulin tolerance in mice fed a standard diet or a high-fat diet. In contrast, inhibition of myostatin signaling in skeletal muscle, like Mstn deletion, resulted in increased lean mass, decreased fat mass, improved glucose metabolism on standard and high-fat diets, and resistance to diet-induced obesity. Our results demonstrate that Mstn−/− mice have an increase in insulin sensitivity and glucose uptake, and that the reduction in adipose tissue mass in Mstn−/− mice is an indirect result of metabolic changes in skeletal muscle. These data suggest that increasing muscle mass by administration of myostatin antagonists may be a promising therapeutic target for treating patients with obesity or diabetes.
PMCID: PMC2654157  PMID: 19295913
17.  Dopamine D2 Antagonist-Induced Striatal Nur77 Expression Requires Activation of mGlu5 Receptors by Cortical Afferents 
Dopamine D2 receptor antagonists modulate gene transcription in the striatum. However, the molecular mechanism underlying this effect remains elusive. Here we used the expression of Nur77, a transcription factor of the orphan nuclear receptor family, as readout to explore the role of dopamine, glutamate, and adenosine receptors in the effect of a dopamine D2 antagonist in the striatum. First, we investigated D2 antagonist-induced Nur77 mRNA in D2L receptor knockout mice. Surprisingly, deletion of the D2L receptor isoform did not reduce eticlopride-induced upregulation of Nur77 mRNA levels in the striatum. Next, we tested if an ibotenic acid-induced cortical lesion could block the effect of eticlopride on Nur77 expression. Cortical lesions strongly reduced eticlopride-induced striatal upregulation of Nur77 mRNA. Then, we investigated if glutamatergic neurotransmission could modulate eticlopride-induced Nur77 expression. A combination of a metabotropic glutamate type 5 (mGlu5) and adenosine A2A receptor antagonists abolished eticlopride-induced upregulation of Nur77 mRNA levels in the striatum. Direct modulation of Nur77 expression by striatal glutamate and adenosine receptors was confirmed using corticostriatal organotypic cultures. Taken together, these results indicate that blockade of postsynaptic D2 receptors is not sufficient to trigger striatal transcriptional activity and that interaction with corticostriatal presynaptic D2 receptors and subsequent activation of postsynaptic glutamate and adenosine receptors in the striatum is required. Thus, these results uncover an unappreciated role of presynaptic D2 heteroreceptors and support a prominent role of glutamate in the effect of D2 antagonists.
PMCID: PMC3418524  PMID: 22912617
antipsychotic drugs; neuroleptics; Nr4a1; transcription factor; organotypic culture; glutamate receptors; adenosine receptors; striatum
18.  Fat Cell–Specific Ablation of Rictor in Mice Impairs Insulin-Regulated Fat Cell and Whole-Body Glucose and Lipid Metabolism 
Diabetes  2010;59(6):1397-1406.
Rictor is an essential component of mammalian target of rapamycin (mTOR) complex (mTORC) 2, a kinase that phosphorylates and activates Akt, an insulin signaling intermediary that regulates glucose and lipid metabolism in adipose tissue, skeletal muscle, and liver. To determine the physiological role of rictor/mTORC2 in insulin signaling and action in fat cells, we developed fat cell–specific rictor knockout (FRic−/−) mice.
Insulin signaling and glucose and lipid metabolism were studied in FRic−/− fat cells. In vivo glucose metabolism was evaluated by hyperinsulinemic-euglycemic clamp.
Loss of rictor in fat cells prevents insulin-stimulated phosphorylation of Akt at S473, which, in turn, impairs the phosphorylation of downstream targets such as FoxO3a at T32 and AS160 at T642. However, glycogen synthase kinase-3β phosphorylation at S9 is not affected. The signaling defects in FRic−/− fat cells lead to impaired insulin-stimulated GLUT4 translocation to the plasma membrane and decreased glucose transport. Furthermore, rictor-null fat cells are unable to suppress lipolysis in response to insulin, leading to elevated circulating free fatty acids and glycerol. These metabolic perturbations are likely to account for defects observed at the whole-body level of FRic−/− mice, including glucose intolerance, marked hyperinsulinemia, insulin resistance in skeletal muscle and liver, and hepatic steatosis.
Rictor/mTORC2 in fat cells plays an important role in whole-body energy homeostasis by mediating signaling necessary for the regulation of glucose and lipid metabolism in fat cells.
PMCID: PMC2874700  PMID: 20332342
19.  Regulation of the Nur77 orphan steroid receptor in activation-induced apoptosis. 
Molecular and Cellular Biology  1995;15(11):6364-6376.
T-cell receptor (TCR)-mediated apoptosis in immature thymocytes and T-cell hybridomas is calcium dependent and can be inhibited by cyclosporin A (CsA). Induction of the orphan steroid receptor Nur77 (NGFI-B) is required for activation-induced apoptosis. Here, we examined the regulation of Nur77 expression, in response to apoptotic TCR signals, which consists of kinase C and calcium pathways. We show that the major control of Nur77 induction is mediated by the calcium signaling pathway. In contrast, protein kinase C signals induce only a low level of Nur77 activity. Nur77 promoter activity parallels its protein levels. CsA decreases both Nur77 protein levels and promoter activity, and the kinetics of CsA inhibition of apoptosis correlates with a decrease in Nur77 protein levels. TCR signals and kinase C signals result in a similar level of Nur77 protein phosphorylation but mediate differential transactivation activity of Nur77. In addition, Nur77 promoter deletion analysis revealed two RSRF (related to serum-responsive factor) binding sites, which can confer calcium and CsA sensitivity on a heterologous promoter. Taken together, our data suggest that the levels of transcriptional induction of Nur77 play an important role during activation-induced apoptosis and that calcium signals regulate a novel CsA-sensitive nuclear factor required for Nur77 transcription in T cells.
PMCID: PMC230888  PMID: 7565789
20.  Regulation of NR4A by nutritional status, gender, postnatal development and hormonal deficiency 
Scientific Reports  2014;4:4264.
The NR4A is a subfamily of the orphan nuclear receptors (NR) superfamily constituted by three well characterized members: Nur77 (NR4A1), Nurr1 (NR4A2) and Nor 1 (NR4A3). They are implicated in numerous biological processes as DNA repair, arteriosclerosis, cell apoptosis, carcinogenesis and metabolism. Several studies have demonstrated the role of this subfamily on glucose metabolism, insulin sensitivity and energy balance. These studies have focused mainly in liver and skeletal muscle. However, its potential role in white adipose tissue (WAT), one of the most important tissues involved in the regulation of energy homeostasis, is not well-studied. The aim of this work was to elucidate the regulation of NR4A in WAT under different physiological and pathophysiological settings involved in energy balance such as fasting, postnatal development, gender, hormonal deficiency and pregnancy. We compared NR4A mRNA expression of Nur77, Nurr1 and Nor 1 and found a clear regulation by nutritional status, since the expression of the 3 isoforms is increased after fasting in a leptin-independent manner and sex steroid hormones also modulate NR4A expression in males and females. Our findings indicate that NR4A are regulated by different physiological and pathophysiological settings known to be associated with marked alterations in glucose metabolism and energy status.
PMCID: PMC3939456  PMID: 24584059
21.  Transcriptomic and network component analysis of glycerol kinase in skeletal muscle using a mouse model of glycerol kinase deficiency 
Molecular genetics and metabolism  2009;96(3):106-112.
Glycerol kinase (GK) is at the interface of fat and carbohydrate metabolism and has been linked to obesity and type 2 diabetes mellitus (T2DM). The purpose of this study was to investigate the role of GK in fat metabolism and insulin signaling in skeletal muscle (an important end organ tissue in T2DM). Microarray analysis determined that there were 525 genes that were differentially expressed (1.2 fold, p-value <0.05) between knockout (KO) and wild-type (WT) mice. Quantitative PCR (qPCR) confirmed the differential expression of genes including glycerol kinase (Gyk), phosphatidylinositol 3-kinase regulatory subunit, polypeptide 1 (p85 alpha) (Pik3r1), insulin-like growth factor 1 (Igf1), and growth factor receptor bound protein 2-associated protein 1 (Gab1). Network component analysis demonstrated that transcription factor activities of myogenic differentiation 1 (MYOD), myogenic regulatory factor 5 (MYF5), myogenin (MYOG), nuclear receptor subfamily 4, group A, member 1 (NUR77) are decreased in the Gyk KO whereas the activity of paired box 3 (PAX3) is increased. The activity of MYOD was confirmed using a DNA binding assay. In addition, myoblasts from Gyk KO had less ability to differentiate into myotubes compared to WT myoblasts. These findings support our previous studies in brown adipose tissue and demonstrate that the role of Gyk in muscle is due in part to its non-metabolic (moonlighting) activities.
PMCID: PMC2702540  PMID: 19121967
glycerol kinase; gene expression; skeletal muscle; network component analysis; moonlighting function
22.  Time-Resolved and Tissue-Specific Systems Analysis of the Pathogenesis of Insulin Resistance 
PLoS ONE  2010;5(1):e8817.
The sequence of events leading to the development of insulin resistance (IR) as well as the underlying pathophysiological mechanisms are incompletely understood. As reductionist approaches have been largely unsuccessful in providing an understanding of the pathogenesis of IR, there is a need for an integrative, time-resolved approach to elucidate the development of the disease.
Methodology/Principal Findings
Male ApoE3Leiden transgenic mice exhibiting a humanized lipid metabolism were fed a high-fat diet (HFD) for 0, 1, 6, 9, or 12 weeks. Development of IR was monitored in individual mice over time by performing glucose tolerance tests and measuring specific biomarkers in plasma, and hyperinsulinemic-euglycemic clamp analysis to assess IR in a tissue-specific manner. To elucidate the dynamics and tissue-specificity of metabolic and inflammatory processes key to IR development, a time-resolved systems analysis of gene expression and metabolite levels in liver, white adipose tissue (WAT), and muscle was performed. During HFD feeding, the mice became increasingly obese and showed a gradual increase in glucose intolerance. IR became first manifest in liver (week 6) and then in WAT (week 12), while skeletal muscle remained insulin-sensitive. Microarray analysis showed rapid upregulation of carbohydrate (only liver) and lipid metabolism genes (liver, WAT). Metabolomics revealed significant changes in the ratio of saturated to polyunsaturated fatty acids (liver, WAT, plasma) and in the concentrations of glucose, gluconeogenesis and Krebs cycle metabolites, and branched amino acids (liver). HFD evoked an early hepatic inflammatory response which then gradually declined to near baseline. By contrast, inflammation in WAT increased over time, reaching highest values in week 12. In skeletal muscle, carbohydrate metabolism, lipid metabolism, and inflammation was gradually suppressed with HFD.
HFD-induced IR is a time- and tissue-dependent process that starts in liver and proceeds in WAT. IR development is paralleled by tissue-specific gene expression changes, metabolic adjustments, changes in lipid composition, and inflammatory responses in liver and WAT involving p65-NFkB and SOCS3. The alterations in skeletal muscle are largely opposite to those in liver and WAT.
PMCID: PMC2809107  PMID: 20098690
23.  Partial Inhibition of Adipose Tissue Lipolysis Improves Glucose Metabolism and Insulin Sensitivity Without Alteration of Fat Mass 
PLoS Biology  2013;11(2):e1001485.
Partial inhibition of adipose tissue lipolysis does not increase fat mass but improves glucose metabolism and insulin sensitivity through modulation of fatty acid turnover and induction of fat cell de novo lipogenesis.
When energy is needed, white adipose tissue (WAT) provides fatty acids (FAs) for use in peripheral tissues via stimulation of fat cell lipolysis. FAs have been postulated to play a critical role in the development of obesity-induced insulin resistance, a major risk factor for diabetes and cardiovascular disease. However, whether and how chronic inhibition of fat mobilization from WAT modulates insulin sensitivity remains elusive. Hormone-sensitive lipase (HSL) participates in the breakdown of WAT triacylglycerol into FAs. HSL haploinsufficiency and treatment with a HSL inhibitor resulted in improvement of insulin tolerance without impact on body weight, fat mass, and WAT inflammation in high-fat-diet–fed mice. In vivo palmitate turnover analysis revealed that blunted lipolytic capacity is associated with diminution in FA uptake and storage in peripheral tissues of obese HSL haploinsufficient mice. The reduction in FA turnover was accompanied by an improvement of glucose metabolism with a shift in respiratory quotient, increase of glucose uptake in WAT and skeletal muscle, and enhancement of de novo lipogenesis and insulin signalling in liver. In human adipocytes, HSL gene silencing led to improved insulin-stimulated glucose uptake, resulting in increased de novo lipogenesis and activation of cognate gene expression. In clinical studies, WAT lipolytic rate was positively and negatively correlated with indexes of insulin resistance and WAT de novo lipogenesis gene expression, respectively. In obese individuals, chronic inhibition of lipolysis resulted in induction of WAT de novo lipogenesis gene expression. Thus, reduction in WAT lipolysis reshapes FA fluxes without increase of fat mass and improves glucose metabolism through cell-autonomous induction of fat cell de novo lipogenesis, which contributes to improved insulin sensitivity.
Author Summary
In periods of energy demand, mobilization of fat stores in mammals (i.e., adipose tissue lipolysis) is essential to provide energy in the form of fatty acids. In excess, however, fatty acids induce resistance to the action of insulin, which serves to regulate glucose metabolism in skeletal muscle and liver. Insulin resistance (or low insulin sensitivity) is believed to be a cornerstone of the complications of obesity such as type 2 diabetes and cardiovascular diseases. In this study, our clinical observation of natural variation in fat cell lipolysis in individuals reveals that a high lipolytic rate is associated with low insulin sensitivity. Furthermore, partial genetic and pharmacologic inhibition of hormone-sensitive lipase, one of the enzymes involved in the breakdown of white adipose tissue lipids, results in improvement of insulin sensitivity in mice without gain in body weight and fat mass. We undertake a series of mechanistic studies in mice and in human fat cells to show that blunted lipolytic capacity increases the synthesis of new fatty acids from glucose in fat cells, a pathway that has recently been shown by others to be a major determinant of whole body insulin sensitivity. In conclusion, partial inhibition of adipose tissue lipolysis is a plausible strategy in the treatment of obesity-related insulin resistance.
PMCID: PMC3576369  PMID: 23431266
24.  Ketogenic Essential Amino Acids Modulate Lipid Synthetic Pathways and Prevent Hepatic Steatosis in Mice 
PLoS ONE  2010;5(8):e12057.
Although dietary ketogenic essential amino acid (KAA) content modifies accumulation of hepatic lipids, the molecular interactions between KAAs and lipid metabolism are yet to be fully elucidated.
Methodology/Principal Findings
We designed a diet with a high ratio (E/N) of essential amino acids (EAAs) to non-EAAs by partially replacing dietary protein with 5 major free KAAs (Leu, Ile, Val, Lys and Thr) without altering carbohydrate and fat content. This high-KAA diet was assessed for its preventive effects on diet-induced hepatic steatosis and whole-animal insulin resistance. C57B6 mice were fed with a high-fat diet, and hyperinsulinemic ob/ob mice were fed with a high-fat or high-sucrose diet. The high-KAA diet improved hepatic steatosis with decreased de novo lipogensis (DNL) fluxes as well as reduced expressions of lipogenic genes. In C57B6 mice, the high-KAA diet lowered postprandial insulin secretion and improved glucose tolerance, in association with restored expression of muscle insulin signaling proteins repressed by the high-fat diet. Lipotoxic metabolites and their synthetic fluxes were also evaluated with reference to insulin resistance. The high-KAA diet lowered muscle and liver ceramides, both by reducing dietary lipid incorporation into muscular ceramides and preventing incorporation of DNL-derived fatty acids into hepatic ceramides.
Our results indicate that dietary KAA intake improves hepatic steatosis and insulin resistance by modulating lipid synthetic pathways.
PMCID: PMC2919399  PMID: 20706589
25.  Myeloid Cell-Restricted Insulin Receptor Deficiency Protects Against Obesity-Induced Inflammation and Systemic Insulin Resistance 
PLoS Genetics  2010;6(5):e1000938.
A major component of obesity-related insulin resistance is the establishment of a chronic inflammatory state with invasion of white adipose tissue by mononuclear cells. This results in the release of pro-inflammatory cytokines, which in turn leads to insulin resistance in target tissues such as skeletal muscle and liver. To determine the role of insulin action in macrophages and monocytes in obesity-associated insulin resistance, we conditionally inactivated the insulin receptor (IR) gene in myeloid lineage cells in mice (IRΔmyel-mice). While these animals exhibit unaltered glucose metabolism on a normal diet, they are protected from the development of obesity-associated insulin resistance upon high fat feeding. Euglycemic, hyperinsulinemic clamp studies demonstrate that this results from decreased basal hepatic glucose production and from increased insulin-stimulated glucose disposal in skeletal muscle. Furthermore, IRΔmyel-mice exhibit decreased concentrations of circulating tumor necrosis factor (TNF) α and thus reduced c-Jun N-terminal kinase (JNK) activity in skeletal muscle upon high fat feeding, reflecting a dramatic reduction of the chronic and systemic low-grade inflammatory state associated with obesity. This is paralleled by a reduced accumulation of macrophages in white adipose tissue due to a pronounced impairment of matrix metalloproteinase (MMP) 9 expression and activity in these cells. These data indicate that insulin action in myeloid cells plays an unexpected, critical role in the regulation of macrophage invasion into white adipose tissue and in the development of obesity-associated insulin resistance.
Author Summary
Obesity represents a major health burden with steadily increasing incidence. While it is associated with numerous co-morbidities, type 2 diabetes mellitus represents one of the major life-threatening, obesity-related conditions. Over the last years, it has become clear that during the course of obesity development not only does fat mass increase, but also fat composition changes qualitatively, leading to an influx of inflammatory cells, such as macrophages, into adipose tissue. Macrophages in turn secrete inflammatory mediators, which inhibit insulin action in skeletal muscle, liver, and even the central nervous system to ultimately cause insulin-resistant diabetes mellitus. However, the effect of insulin action and resistance in these inflammatory cell types themselves has not been addressed. To this end, we have generated and analyzed mice with inactivation of the insulin receptor specifically in myeloid cell-derived, inflammatory cells. Surprisingly, these animals are protected from the development of obesity-associated deterioration of glucose metabolism, thereby defining insulin action in inflammatory cells as a novel and promising target for therapeutic intervention against obesity-associated diabetes mellitus.
PMCID: PMC2865520  PMID: 20463885

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