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Jonathan M Mudry, Julie Massart, Ferenc L M Szekeres and Anna Krook

TWIST proteins are important for development of embryonic skeletal muscle and play a role in the metabolism of tumor and white adipose tissue. The impact of TWIST on metabolism in skeletal muscle is incompletely studied. Our aim was to assess the impact of TWIST1 and TWIST2 overexpression on glucose and lipid metabolism. In intact mouse muscle, overexpression of Twist reduced total glycogen content without altering glucose uptake. Expression of TWIST1 or TWIST2 reduced Pdk4 mRNA, while increasing mRNA levels of Il6, Tnfα, and Il1β. Phosphorylation of AKT was increased and protein abundance of acetyl CoA carboxylase (ACC) was decreased in skeletal muscle overexpressing TWIST1 or TWIST2. Glycogen synthesis and fatty acid oxidation remained stable in C2C12 cells overexpressing TWIST1 or TWIST2. Finally, skeletal muscle mRNA levels remain unaltered in ob/ob mice, type 2 diabetic patients, or in healthy subjects before and after 3 months of exercise training. Collectively, our results indicate that TWIST1 and TWIST2 are expressed in skeletal muscle. Overexpression of these proteins impacts proteins in metabolic pathways and mRNA level of cytokines. However, skeletal muscle levels of TWIST transcripts are unaltered in metabolic diseases.

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Cathy A Guo and Shaodong Guo

The heart is an insulin-dependent and energy-consuming organ in which insulin and nutritional signaling integrates to the regulation of cardiac metabolism, growth and survival. Heart failure is highly associated with insulin resistance, and heart failure patients suffer from the cardiac energy deficiency and structural and functional dysfunction. Chronic pathological conditions, such as obesity and type 2 diabetes mellitus, involve various mechanisms in promoting heart failure by remodeling metabolic pathways, modulating cardiac energetics and impairing cardiac contractility. Recent studies demonstrated that insulin receptor substrates 1 and 2 (IRS-1,-2) are major mediators of both insulin and insulin-like growth factor-1 (IGF-1) signaling responsible for myocardial energetics, structure, function and organismal survival. Importantly, the insulin receptor substrates (IRS) play an important role in the activation of the phosphatidylinositide-3-dependent kinase (PI-3K) that controls Akt and Foxo1 signaling cascade, regulating the mitochondrial function, cardiac energy metabolism and the renin–angiotensin system. Dysregulation of this branch in signaling cascades by insulin resistance in the heart through the endocrine system promotes heart failure, providing a novel mechanism for diabetic cardiomyopathy. Therefore, targeting this branch of IRS→PI-3K→Foxo1 signaling cascade and associated pathways may provide a fundamental strategy for the therapeutic and nutritional development in control of metabolic and cardiovascular diseases. In this review, we focus on insulin signaling and resistance in the heart and the role energetics play in cardiac metabolism, structure and function.

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Almas R Juma, Pauliina E Damdimopoulou, Sylvia V H Grommen, Wim J M Van de Ven and Bert De Groef

Pleomorphic adenoma gene 1 (PLAG1) belongs to the PLAG family of zinc finger transcription factors along with PLAG-like 1 and PLAG-like 2. The PLAG1 gene is best known as an oncogene associated with certain types of cancer, most notably pleomorphic adenomas of the salivary gland. While the mechanisms of PLAG1-induced tumorigenesis are reasonably well understood, the role of PLAG1 in normal physiology is less clear. It is known that PLAG1 is involved in cell proliferation by directly regulating a wide array of target genes, including a number of growth factors such as insulin-like growth factor 2. This is likely to be a central mode of action for PLAG1 both in embryonic development and in cancer. The phenotype of Plag1 knockout mice suggests an important role for PLAG1 also in postnatal growth and reproduction, as PLAG1 deficiency causes growth retardation and reduced fertility. A role for PLAG1 in growth and reproduction is further corroborated by genome-wide association studies in humans and domestic animals in which polymorphisms in the PLAG1 genomic region are associated with body growth and reproductive traits. Here we review the current evidence for PLAG1 as a regulator of growth and fertility and discuss possible endocrine mechanisms involved.

Free access

Sharon H Chou and Christos Mantzoros

Leptin, as a key hormone in energy homeostasis, regulates neuroendocrine function, including reproduction. It has a permissive role in the initiation of puberty and maintenance of the hypothalamic–pituitary–gonadal axis. This is notable in patients with either congenital or acquired leptin deficiency from a state of chronic energy insufficiency. Hypothalamic amenorrhea is the best-studied, with clinical trials confirming a causative role of leptin in hypogonadotropic hypogonadism. Implications of leptin deficiency have also emerged in the pathophysiology of hypogonadism in type 1 diabetes. At the other end of the spectrum, hyperleptinemia may play a role in hypogonadism associated with obesity, polycystic ovarian syndrome, and type 2 diabetes. In these conditions of energy excess, mechanisms of reproductive dysfunction include central leptin resistance as well as direct effects at the gonadal level. Thus, reproductive dysfunction due to energy imbalance at both ends can be linked to leptin.

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Tamiki Hikake, Shinji Hayashi, Taisen Iguchi and Tomomi Sato

IGF1 knockout (IGF1KO) mice show a reduced number of prolactin (PRL) producing cells (PRL cells); however, the role of IGF1 in PRL cell proliferation and differentiation in immature mice is unclear. In this study, ontogenic changes in the percentages of PRL cells, GH producing cells (GH cells), and 5-bromo-2′-deoxyuridine (BrdU)-labeled cells in the anterior pituitary of male IGF1KO mice during the postnatal period were investigated. The percentage of PRL cells in IGF1KO mice was significantly lower at day 20 compared with that in wild-type (WT) mice, while GH cells in IGF1KO mice were significantly increased from day 10. From days 5 to 20, the percentage of BrdU-labeled cells in WT and IGF1KO mice was similar. PRL cells and GH cells are thought to originate from the same progenitor cells, therefore, PRL cells in IGF1KO mice are not able to differentiate because progenitor cells have already committed to be GH cells. However, IGF1, 17β-estradiol (E2), epidermal growth factor (EGF), or IGF1 plus E2 treatments increased the PRL cell number in the pituitaries in vitro of 10-day-old WT and IGF1KO mice. This fact suggests that these factors are involved in PRL cell proliferation and differentiation. In addition, the increase of PRL cells in IGF1KO mice stimulated by E2 or EGF was less than that of WT mice. Thus, IGF1 plays a crucial role in PRL cell proliferation and differentiation in mouse pituitaries by regulating the differentiation of progenitor cells and mediating the actions of E2 and EGF.

Free access

Bohan Wang, I Stuart Wood and Paul Trayhurn

The effect of hypoxia on the expression and secretion of major adipokines by human preadipocytes has been examined. Hypoxia (1% O2) led to an increase in the HIF-1α transcription factor subunit in cultured preadipocytes, as did incubation with the hypoxia mimetic CoCl2. Leptin mRNA was essentially undetectable in preadipocytes incubated under normoxia (21% O2), but exposure to 1% O2, or CoCl2, for 4 or 24 h resulted in an induction of leptin gene expression (measured by real-time PCR). Immunoreactive leptin was not detected in the medium from normoxic preadipocytes, but was present in the medium from the hypoxic cells. Hypoxia stimulated expression of the GLUT-1 facilitative glucose transporter gene and the vascular endothelial growth factor (VEGF) gene in preadipocytes, as in adipocytes. PPARγ and aP2 mRNA levels, markers of adipocyte differentiation, were reduced by hypoxia in both cell types. In marked contrast to adipocytes, interleukin-6 (IL-6), angiopoietin-like protein 4, and plasminogen activator inhibitor-1 expression by preadipocytes was not stimulated by low O2 tension. Consistent with the gene expression results, VEGF release into the medium from preadipocytes was increased by hypoxia, but there was no change in IL-6 secretion. It is concluded that hypoxia induces human preadipocytes to synthesize and secrete leptin. Preadipocytes and adipocytes differ in their responsiveness to low O2 tension, maturation of the response to hypoxia developing on differentiation.

Free access

Ruben Rodriguez, Jacqueline N Minas, Jose Pablo Vazquez-Medina, Daisuke Nakano, David G Parkes, Akira Nishiyama and Rudy M Ortiz

Obesity is associated with the inappropriate activation of the renin-angiotensin system (RAS), which increases arterial pressure, impairs insulin secretion and decreases peripheral tissue insulin sensitivity. RAS blockade reverses these detriments; however, it is not clear whether the disease state of the organism and treatment duration determine the beneficial effects of RAS inhibition on insulin secretion and insulin sensitivity. Therefore, the objective of this study was to compare the benefits of acute vs chronic angiotensin receptor type 1 (AT1) blockade started after the onset of obesity, hyperglycemia and hypertension on pancreatic function and peripheral insulin resistance. We assessed adipocyte morphology, glucose intolerance, pancreatic redox balance and insulin secretion after 2 and 11 weeks of AT1 blockade in the following groups of rats: (1) untreated Long-Evans Tokushima Otsuka (lean control; n = 10), (2) untreated Otsuka Long-Evans Tokushima Fatty (OLETF; n = 12) and (3) OLETF + ARB (ARB; 10 mg olmesartan/kg/day by oral gavage; n = 12). Regardless of treatment duration, AT1 blockade decreased systolic blood pressure and fasting plasma triglycerides, whereas chronic AT1 blockade decreased fasting plasma glucose, glucose intolerance and the relative abundance of large adipocytes by 22, 36 and 70%, respectively. AT1 blockade, however, did not improve pancreatic oxidative stress or reverse impaired insulin secretion. Collectively, these data show that AT1 blockade after the onset of obesity, hyperglycemia and hypertension improves peripheral tissue insulin sensitivity, but cannot completely reverse the metabolic derangement characterized by impaired insulin secretion once it has been compromised.

Free access

Susan Kralisch, Anke Tönjes, Kerstin Krause, Judit Richter, Ulrike Lossner, Peter Kovacs, Thomas Ebert, Matthias Blüher, Michael Stumvoll and Mathias Fasshauer

Rather than a traditional growth factor, fibroblast growth factor-21 (FGF21) is considered to be a metabolic hormone. In the current study, we investigated serum FGF21 levels in the self-contained population of Sorbs. Serum FGF21 concentrations were quantified by ELISA and correlated with IGF1 as well as metabolic, renal, hepatic, inflammatory, and cardiovascular parameters in 913 Sorbs from Germany. Moreover, human IGF1 protein secretion was investigated in FGF21-stimulated HepG2 cells. Median FGF21 serum concentrations were 2.1-fold higher in subjects with type 2 diabetes mellitus (141.8 ng/l) compared with controls (66.7 ng/l). Furthermore, nondiabetic subjects with FGF21 levels below the detection limit of the ELISA showed a more beneficial metabolic profile compared with subjects with measurable FGF21. Moreover, FGF21 was significantly lower in female compared with male subjects after adjustment for age and BMI. In multiple regression analyses, circulating FGF21 concentrations remained independently and positively associated with gender, systolic blood pressure, triglycerides, and γ glutamyl transferase whereas a negative association was observed with IGF1 in nondiabetic subjects. Notably, FGF21 significantly inhibited IGF1 secretion into HepG2 cell culture supernatants in preliminary in vitro experiments. FGF21 serum concentrations are associated with facets of the metabolic syndrome, hepatocellular function, as well as GH status.

Free access

BD Rodgers, M Bernier and MA Levine

Adipocyte beta-adrenergic sensitivity is compromised in animal models of obesity and type 2 diabetes. Although changes in the membrane concentrations of G-protein alpha subunits (Galpha) have been implicated, it remains to be determined how these changes are affected by insulin resistance in the different animal models. Because previous studies used young animals, we measured the concentrations of Galpha and Gbeta subunits in epididymal fat from aged (48 weeks old) db/db mice and from their lean littermates to more closely reproduce the model of type 2 diabetes mellitus. Levels of immunoreactive Galphas, Galphai(1/2), Galphao and Galphaq/11 were all significantly greater in adipocyte membranes from the db/db mice than in membranes from their lean non-diabetic littermate controls. Levels of Galphai(1) and Galphai(2) were also individually determined and although they appeared to be slightly higher in db/db membranes, these differences were not significant. Although the levels of both Galphas isoforms were elevated, levels of the 42 and 46 kDa proteins rose by approximately 42% and 20% respectively, indicating differential protein processing of Galphas. By contrast, levels of Galphai3 were similar in the two groups. The levels of common Gbeta and Gbeta2 were also elevated in db/db mice, whereas Gbeta1 and Gbeta4 levels were not different. To determine whether these changes were due to insulin resistance per se or to elevated glucocorticoid production, G-protein subunit levels were quantified in whole cell lysates from 3T3-L1 adipocytes that were stimulated with different concentrations of either insulin or corticosterone. Although none of the subunit levels was affected by insulin, the levels of both Galphas isoforms were increased equally by corticosterone in a concentration-dependent manner. Since glucocorticoids are known regulators of Galphas gene expression in many cell types and in adipocytes from diabetic rodents, the results presented herein appear to more accurately reflect diabetic pathophysiology than do those of previous studies which report a decrease in Galphas levels. Taken together, these results indicate that most of the selective changes in G-protein subunit production in adipocytes from this animal model of type 2 diabetes may not be due to diminished insulin sensitivity, but may be due to other endocrine or metabolic abnormalities associated with the diabetic phenotype.

Free access

Jennifer A Crookshank, Daniel Serrano, Gen-Sheng Wang, Christopher Patrick, Baylie S Morgan, Marie-France Paré and Fraser W Scott

It is unknown whether there is a gene signature in pancreas which is associated with type 1 diabetes (T1D). We performed partial pancreatectomies on 30-day preinsulitic, diabetes-prone BioBreeding (BBdp) rats to prospectively identify factors involved in early prediabetes. Microarrays of the biopsies revealed downregulation of endoplasmic reticulum (ER) stress, metabolism and apoptosis. Based on these results, additional investigations compared gene expression in control (BBc) and BBdp rats age ~8, 30 and 60 days using RT-qPCR. Neonates had increased ER stress gene expression in pancreas. This was associated with decreased insulin, cleaved caspase-3 and Ins1 whereas Gcg and Pcsk2 were increased. The increase in ER stress was not sustained at 30 days and decreased by 60 days. In parallel, the liver gene profile showed a similar signature in neonates but with an early decrease of the unfolded protein response (UPR) at 30 days. This suggested that changes in the liver precede those in the pancreas. Tnf and Il1b expression was increased in BBdp pancreas in association with increased caspase-1, cleaved caspase-3 and decreased proinsulin area. Glucagon area was increased in both 30-day and 60-day BBdp rats. Increased colocalization of BIP and proinsulin was observed at 60 days in the pancreas, suggesting insulin-related ER dysfunction. We propose that dysregulated metabolism leads to ER stress in neonatal rats long before insulitis, creating a microenvironment in both pancreas and liver that promotes autoimmunity.