Background: The cJun-N-terminal-kinase (JNK) plays a central role in the cell stress response, with outcomes ranging from cell death to cell proliferation and survival, depending on the specific context. JNK is also one of the most investigated signal transducers in obesity and insulin resistance, and studies have identified new molecular mechanisms linking obesity and insulin resistance. Emerging evidence indicates that whereas JNK1 and JNK2 isoforms promote the development of obesity and insulin resistance, JNK3 activity protects from excessive adiposity. Furthermore, current evidence indicates that JNK activity within specific cell types may, in specific stages of disease progression, promote cell tolerance to the stress associated with obesity and type-2 diabetes.
Scope of review: This review provides an overview of the current literature on the role of JNK in the progression from obesity to insulin resistance, NAFLD, type-2 diabetes, and diabetes complications.
Major conclusions: Whereas current evidence indicates that JNK1/2 inhibition may improve insulin sensitivity in obesity, the role of JNK in the progression from insulin resistance to diabetes, and its complications is largely unresolved. A better understanding of the role of JNK in the stress response to obesity and type-2 diabetes, and the development of isoform-specific inhibitors with specific tissue distribution will be necessary to exploit JNK as possible drug target for the treatment of type-2 diabetes.[Hide abstract]
Background: GPRC6A, a widely expressed G-protein coupled receptor, is proposed to be a master regulator of complex endocrine networks and metabolic processes. GPRC6A is activated by multiple ligands, including osteocalcin (Ocn), testosterone (T), basic amino acids, and various cations.
Scope of review: We review the controversy surrounding GPRC6A functions. In mice, GPRC6A is proposed to integrate metabolic functions through the coordinated secretion of hormones, including insulin, GLP-1, T, and IL-6, and direct effects of this receptor to control glucose and fat metabolism in the liver, skeletal muscle, and fat. Loss-of-GPRC6A results in metabolic syndrome (MetS), and activation of GPRC6A stimulates proliferation of β-cells, increases peripheral insulin sensitivity, and protects against high fat diet (HFD) induced metabolic abnormalities in most mouse models. Bone, cardiovascular, immune, and skin functions of GPRC6A have also been identified in mice. Expression of GPRC6A is increased in prostate cancer (PCa) cells, and inhibition of GPRC6A attenuates PCa progression in mouse models. The function of GPRC6A in humans, however, is not clear. During evolution, a unique polymorphism of GPRC6A emerged mainly in humans of Asian and European decent that has been proposed to alter membrane trafficking and function. In contrast, the ancestral allele found in all other species is retained in 1%, 15%, and 40% of people of Asian, European and African descent, respectively, suggesting GPRC6A gene variants may contribute to the racial disparities in the risk of developing MetS and PCa.
Major conclusions: If the regulatory functions of GPRC6A identified in mice translate to humans, and polymorphisms in GPRC6A are found to predict racial disparities in human diseases, GPRC6A may be a new gene target to predict, prevent, and treat MetS, PCa, and other disorders impacted by GPRC6A.[Hide abstract]
Objective: Kinase Suppressor of Ras 2 (KSR2) is a molecular scaffold coordinating Raf/MEK/ERK signaling that is expressed at high levels in the brain. KSR2 disruption in humans and mice causes obesity and insulin resistance. Understanding the anatomical location and mechanism of KSR2 function should lead to a better understanding of physiological regulation over energy balance.
Methods: Mice bearing floxed alleles of KSR2 (KSR2fl/fl) were crossed with mice expressing the Cre recombinase expressed by the Nestin promoter (Nes-Cre) to produce Nes-CreKSR2fl/fl mice. Growth, body composition, food consumption, cold tolerance, insulin and free fatty acid levels, glucose, and AICAR tolerance were measured in gender and age matched KSR2-/- mice.
Results: Nes-CreKSR2fl/fl mice lack detectable levels of KSR2 in the brain. The growth and onset of obesity of Nes-CreKSR2fl/fl mice parallel those observed in KSR2-/- mice. As in KSR2-/- mice, Nes-CreKSR2fl/fl are glucose intolerant with elevated fasting and cold intolerance. Male Nes-CreKSR2fl/fl mice are hyperphagic, but female Nes-CreKSR2fl/fl mice are not. Unlike KSR2-/- mice, Nes-CreKSR2fl/fl mice respond normally to leptin and AICAR, which may explain why the degree of obesity of adult Nes-CreKSR2fl/fl mice is not as severe as that observed in KSR2-/- animals.
Conclusions: These observations suggest that, in the brain, KSR2 regulates energy balance via control of feeding behavior and adaptive thermogenesis, while a second KSR2-dependent mechanism, functioning through one or more other tissues, modulates sensitivity to leptin and activators of the energy sensor AMPK.[Hide abstract]
Objective: Recent data show that iNOS has an essential role in ER stress in obesity. However, whether iNOS is sufficient to account for obesity-induced ER stress and Unfolded Protein Response (UPR) has not yet been investigated. In the present study, we used iNOS knockout mice to investigate whether high-fat diet (HFD) can still induce residual ER stress-associated insulin resistance.
Methods: For this purpose, we used the intraperitoneal glucose tolerance test (GTT), euglycemic-hyperinsulinemic clamp, western blotting and qPCR in liver, muscle, and adipose tissue of iNOS KO and control mice on HFD.
Results: The results of the present study demonstrated that, in HFD fed mice, iNOS-induced alteration in insulin signaling is an essential mechanism of insulin resistance in muscle, suggesting that iNOS may represent an important target that could be blocked in order to improve insulin sensitivity in this tissue. However, in liver and adipose tissue, the insulin resistance induced by HFD was only partially dependent on iNOS, and, even in the presence of genetic or pharmacological blockade of iNOS, a clear ER stress associated with altered insulin signaling remained evident in these tissues. When this ER stress was blocked pharmacologically, insulin signaling was improved, and a complete recovery of glucose tolerance was achieved.
Conclusions: Taken together, these results reinforce the tissue-specific regulation of insulin signaling in obesity, with iNOS being sufficient to account for insulin resistance in muscle, but in liver and adipose tissue ER stress and insulin resistance can be induced by both iNOS-dependent and iNOS-independent mechanisms.[Hide abstract]
Objective: Fatty acid oxidation in macrophages is thought to regulate inflammatory status and insulin-sensitivity. An important unanswered question in this field is whether carnitine acetyl-transferase (CrAT) that regulates fatty acid oxidation and mitochondrial acetyl-CoA balance is required to integrate nutrient stress sensing to inflammatory response in macrophages.
Methods: Mice with myeloid lineage-specific Crat deletion were subjected to several metabolic stressors, including high-fat diet-induced obesity, fasting, and LPS-induced endotoxemia. Their metabolic homeostasis was compared to that of Crat-sufficient littermate controls. Inflammatory potential of Crat-deficient and Crat-sufficient macrophages were measured both in vitro and in vivo.
Results: Our studies revealed that ablation of CrAT in myeloid lineage cells did not impact glucose homeostasis, insulin-action, adipose tissue leukocytosis, and inflammation when animals were confronted with a variety of metabolic stressors, including high-fat diet, fasting, or LPS-induced acute endotoxemia.
Conclusions: These findings demonstrate that unlike muscle cells, substrate switch mechanisms that control macrophage energy metabolism and mitochondrial short-chain acyl-CoA pools during nutrient stress are controlled by pathways that are not solely reliant on CrAT.[Hide abstract]
Objective: Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) possess multiple bioactive isoforms that are rendered non-insulinotropic by the enzyme dipeptidyl peptidase-4 (DPP-4). Recently, some ELISA kits have been developed to specifically measure “active” GIP and GLP-1, but it is unclear if these kits can accurately quantify all bioactive forms. Therefore, it remains uncertain to what extent treatment with a DPP-4 inhibitor boosts levels of biologically active GIP and GLP-1. Thus, we evaluated our novel receptor-mediated incretin bioassays in comparison to commercially available ELISA kits using plasma samples from healthy subjects before and after DPP-4 inhibitor administration.
Methods: We utilized cell lines stably co-transfected with human GIP or GLP-1 receptors and a cAMP-inducible luciferase expression construct for the bioassays and commercially available ELISA kits. Assays were tested with synthetic GIP and GLP-1 receptor agonists and plasma samples collected from subjects during a 75 g oral glucose tolerance test (OGTT) performed before or following 3-day administration of a DPP-4 inhibitor.
Results: A GIP isoform GIP(1–30)NH2 increased luciferase activity similarly to GIP(1–42) in the GIP bioassay but was not detectable by either a total or active GIP ELISA kit. During an OGTT, total GIP levels measured by ELISA rapidly increased from 0 min to 15 min, subsequently reaching a peak of 59.2 ± 8.3 pmol/l at 120 min. In contrast, active GIP levels measured by the bioassay peaked at 15 min (43.4 ± 6.4 pmol/l) and then progressively diminished at all subsequent time points. Strikingly, at 15 min, active GIP levels as determined by the bioassay reached levels approximately 20-fold higher after the DPP-4 inhibitor treatment, while total and active GIP levels determined by ELISA were increased just 1.5 and 2.1-fold, respectively. In the absence of DPP-4 inhibition, total GLP-1 levels measured by ELISA gradually increased up to 90 min, reaching 23.5 ± 2.4 pmol/l, and active GLP-1 levels determined by the bioassay did not show any apparent peak. Following administration of a DPP-4 inhibitor there was an observable peak of active GLP-1 levels as determined by the bioassay at 15 min after oral glucose load, reaching 11.0 ± 0.62 pmol/l, 1.4-fold greater than levels obtained without DPP-4 inhibitor treatment. In contrast, total GLP-1 levels determined by ELISA were decreased after DPP-4 inhibitor treatment.
Conclusions: Our results using bioassays indicate that there is a greater increase in plasma levels of bioactive GIP than GLP-1 in subjects treated with DPP-4 inhibitors, which may be unappreciated using conventional ELISAs.[Hide abstract]