Featured ArticlesVolume 4 | No. 1 | January 2015
|Combined Gs and Gq signaling of GPR40 is associated with robust incretin secretagogue actionHauge and colleagues compare a series of GPR40 agonists for their signal transduction and binding properties. They find that GPR40, which generally is considered to be a Gq coupled receptor, can signal efficiently through Gs in response to some synthetic agonists. These Gq + Gs agonists stimulate incretin
secretion robustly both ex vivo and in vivo.|
Abstract | PDF
Objectives: GPR40 (FFAR1), a clinically proven anti-diabetes target, is a Gq-coupled receptor for long chain fatty acids (LCFA) stimulating insulin secretion directly and mediating a major part of the dietary triglyceride-induced secretion of the incretins GLP-1 and GIP. In phase-II studies the GPR40 agonist TAK-875 decreased blood glucose but surprisingly without stimulating incretins.
Methods and results: Here we find that GPR40 can signal through not only Gq and IP3 but also Gs and cAMP when stimulated with certain agonists such as AM-1638 and AM-5262 in contrast to the endogenous LCFA ligands and agonists such as TAK-875 and AM-837, which only signal through Gq. In competition binding against [3H]AM-1638 and [3H]L358 the Gq + Gs and the Gq-only agonists either competed for or showed positive cooperativity by increasing the binding of the two different radio-ligands, in opposite ways. Nevertheless, both the Gq-only and the Gq + Gs agonists all docked surprisingly well into the binding site for TAK-875 in the X-ray structure of GPR40. In murine intestinal primary cell-cultures the endogenous LCFAs and the Gq-only agonists stimulated GLP-1 secretion with rather poor efficacy as compared with the high efficacy Gq + Gs GPR40 agonists and a prototype GPR119 agonist. Similarly, in fasting both male and female mice the Gq + Gs agonists showed significantly higher efficacy than the Gq-only agonists in respect of increasing plasma GLP-1 and plasma GIP in a GPR40-dependent manner.
Conclusions: It is concluded that stimulation of GPR40 by endogenous LCFAs or by Gq-only synthetic agonists result in a rather limited incretin response, whereas Gq + Gs GPR40 agonists stimulate incretin secretion robustly.[Hide abstract]
|Neonatal overnutrition causes early alterations in the central response to peripheral ghrelinMice raised in small litters become overweight during lactation and remain overweight as adults. Collden and colleagues show that neonatal overnutrition alters the ability of peripheral ghrelin to act on the hypothalamus during an important period of growth and development. The findings support the concept that neuroendocrine defects during critical periods of development can lead to lifelong metabolic perturbations.|
Abstract | PDF
Objective: Excess nutrient supply and rapid weight gain during early life are risk factors for the development of obesity during adulthood. This metabolic malprogramming may be mediated by endocrine disturbances during critical periods of development. Ghrelin is a metabolic hormone secreted from the stomach that acts centrally to promote feeding behavior by binding to growth hormone secretagogue receptors in the arcuate nucleus of the hypothalamus. Here, we examined whether neonatal overnutrition causes changes in the ghrelin system.
Methods: We used a well-described mouse model of divergent litter sizes to study the effects of postnatal overfeeding on the central and peripheral ghrelin systems during postnatal development.
Results: Mice raised in small litters became overweight during lactation and remained overweight with increased adiposity as adults. Neonatally overnourished mice showed attenuated levels of total and acyl ghrelin in serum and decreased levels of Ghrelin mRNA expression in the stomach during the third week of postnatal life. Normalization of hypoghrelinemia in overnourished pups was relatively ineffective at ameliorating metabolic outcomes, suggesting that small litter pups may present ghrelin resistance. Consistent with this idea, neonatally overnourished pups displayed an impaired central response to peripheral ghrelin. The mechanisms underlying this ghrelin resistance appear to include diminished ghrelin transport into the hypothalamus.
Conclusions: Early postnatal overnutrition results in central resistance to peripheral ghrelin during important periods of hypothalamic development. Because ghrelin signaling has recently been implicated in the neonatal programming of metabolism, these alterations in the ghrelin system may contribute to the metabolic defects observed in postnatally overnourished mice.[Hide abstract]
|Maternal high-fat diet and obesity compromise fetal hematopoiesisMultiple lines of evidence present prenatal development as a period of global susceptibility for diet-induced metabolic injury, fetal programming and
postnatal organ dysfunction. Kamimae-Lanning and colleagues now identify the hematopoietic compartment as a previously unrecognized target for nutritional
developmental programming. Metabolic cues impact stem cell development, lineage specification, and early formation and reconstitution of the blood ystem.|
Abstract | PDF
Objective: Recent evidence indicates that the adult hematopoietic system is susceptible to diet-induced lineage skewing. It is not known whether the developing hematopoietic system is subject to metabolic programming via in utero high-fat diet (HFD) exposure, an established mechanism of adult disease in several organ systems. We previously reported substantial losses in offspring liver size with prenatal HFD. As the liver is the main hematopoietic organ in the fetus, we asked whether the developmental expansion of the hematopoietic stem and progenitor cell (HSPC) pool is compromised by prenatal HFD and/or maternal obesity.
Methods: We used quantitative assays, progenitor colony formation, flow cytometry, transplantation, and gene expression assays with a series of dietary manipulations to test the effects of gestational high-fat diet and maternal obesity on the day 14.5 fetal liver hematopoietic system.
Results: Maternal obesity, particularly when paired with gestational HFD, restricts physiological expansion of fetal HSPCs while promoting the opposing cell fate of differentiation. Importantly, these effects are only partially ameliorated by gestational dietary adjustments for obese dams. Competitive transplantation reveals compromised repopulation and myeloid-biased differentiation of HFD-programmed HSPCs to be a niche-dependent defect, apparent in HFD-conditioned male recipients. Fetal HSPC deficiencies coincide with perturbations in genes regulating metabolism, immune and inflammatory processes, and stress response, along with downregulation of genes critical for hematopoietic stem cell self-renewal and activation of pathways regulating cell migration.
Conclusions: Our data reveal a previously unrecognized susceptibility to nutritional and metabolic developmental programming in the fetal HSPC compartment, which is a partially reversible and microenvironment-dependent defect perturbing stem and progenitor cell expansion and hematopoietic lineage commitment.[Hide abstract]
|High fat diet-induced modifications in membrane lipid and protein signaturesKahle and colleagues describe comprehensive changes in membrane lipid and protein signatures prior to and throughout the development of high fat diet-mediated hepatic insulin resistance. These changes likely modulate the properties of membranes, in particular those of mitochondria, which are highly abundant in hepatocytes.|
Abstract | PDF
Objective: Excess lipid intake has been implicated in the pathophysiology of hepatosteatosis and hepatic insulin resistance. Lipids constitute approximately 50% of the cell membrane mass, define membrane properties, and create microenvironments for membrane-proteins. In this study we aimed to resolve temporal alterations in membrane metabolite and protein signatures during high-fat diet (HF)-mediated development of hepatic insulin resistance.
Methods: We induced hepatosteatosis by feeding C3HeB/FeJ male mice an HF enriched with long-chain polyunsaturated C18:2n6 fatty acids for 7, 14, or 21 days. Longitudinal changes in hepatic insulin sensitivity were assessed via the euglycemic-hyperinsulinemic clamp, in membrane lipids via t-metabolomics- and membrane proteins via quantitative proteomics-analyses, and in hepatocyte morphology via electron microscopy. Data were compared to those of age- and litter-matched controls maintained on a low-fat diet.
Results: Excess long-chain polyunsaturated C18:2n6 intake for 7 days did not compromise hepatic insulin sensitivity, however, induced hepatosteatosis and modified major membrane lipid constituent signatures in liver, e.g. increased total unsaturated, long-chain fatty acid-containing acyl-carnitine or membrane-associated diacylglycerol moieties and decreased total short-chain acyl-carnitines, glycerophosphocholines, lysophosphatidylcholines, or sphingolipids. Hepatic insulin sensitivity tended to decrease within 14 days HF-exposure. Overt hepatic insulin resistance developed until day 21 of HF-intervention and was accompanied by morphological mitochondrial abnormalities and indications for oxidative stress in liver. HF-feeding progressively decreased the abundance of protein-components of all mitochondrial respiratory chain complexes, inner and outer mitochondrial membrane substrate transporters independent from the hepatocellular mitochondrial volume in liver.
Conclusions: We assume HF-induced modifications in membrane lipid- and protein-signatures prior to and during changes in hepatic insulin action in liver alter membrane properties - in particular those of mitochondria which are highly abundant in hepatocytes. In turn, a progressive decrease in the abundance of mitochondrial membrane proteins throughout HF-exposure likely impacts on mitochondrial energy metabolism, substrate exchange across mitochondrial membranes, contributes to oxidative stress, mitochondrial damage, and the development of insulin resistance in liver.[Hide abstract]
|Fructose ingestion acutely stimulates circulating
FGF21 levels in humansThe findings of Dushay and colleagues indicate an unanticipated role of human FGF21 in fructose metabolism. Following fructose ingestion, FGF21 serum levels rise rapidly.|
Abstract | PDF
Objective: Fibroblast growth factor 21 (FGF21) is a hormone with pleiotropic metabolic activities which, in rodents, is robustly regulated by fasting and ketogenic diets. In contrast, similar dietary interventions have either no or minimal effects on circulating FGF21 in humans. Moreover, no intervention or dietary challenge has been shown to acutely stimulate circulating FGF21 in either humans or animals. Recent animal data suggest that the transcription factor Carbohydrate Responsive-Element Binding Protein (ChREBP) stimulates hepatic FGF21 expression and that fructose may activate hepatic ChREBP more robustly than glucose. Here, we examined whether fructose ingestion can acutely stimulate FGF21 in humans.
Methods: We measured serum FGF21, glucose, insulin, and triglyceride levels in ten lean, healthy adults and eleven adults with the metabolic syndrome following oral ingestion of 75 g of glucose, fructose, or a combination of the two sugars.
Results: FGF21 levels rose rapidly following fructose ingestion, achieved a mean 3.4-fold increase at two hours (P < 0.01), and returned to baseline levels within five hours. In contrast, FGF21 did not increase in the first two hours following ingestion of a glucose load, although more modest increases were observed after three to four hours. Both baseline and fructose-stimulated FGF21 levels were 2-3 fold elevated in subjects with metabolic syndrome.
Conclusions: Fructose ingestion acutely and robustly increases serum FGF21 levels in humans in a pattern consistent with a hormonal response. While FGF21 appears to be critical for the adaptive response to fasting or starvation in rodents, these findings suggest that in humans, FGF21 may play an important role in fructose metabolism.[Hide abstract]
|Novel functional role of astrocytesIntroduction of a high-fat diet to mice results in a period of voracious feeding before mechanisms prevail to restore energy intake to an isocaloric level. Buckman and colleagues show that NFκB-mediated activation of astrocytes contributes to this homeostatic response.|
Abstract | PDF
Objective: Introduction of a high-fat diet to mice results in a period of voracious feeding, known as hyperphagia, before homeostatic mechanisms prevail to restore energy intake to an isocaloric level. Acute high-fat diet hyperphagia induces astrocyte activation in the rodent hypothalamus, suggesting a potential role of these cells in the homeostatic response to the diet. The objective of this study was to determine physiologic role of astrocytes in the acute homeostatic response to high-fat feeding.
Methods: We bred a transgenic mouse model with doxycycline-inducible inhibition of NFkappaB (NFκB) signaling in astrocytes to determine the effect of loss of NFκB-mediated astrocyte activation on acute high-fat hyperphagia. ELISA was used to measure the levels of markers of astrocyte activation, glial-fibrillary acidic protein (GFAP) and S100B, in the medial basal hypothalamus.
Results: Inhibition of NFκB signaling in astrocytes prevented acute high-fat diet-induced astrocyte activation and resulted in a 15% increase in caloric intake (P < 0.01) in the first 24 h after introduction of the diet.
Conclusions: These data reveal a novel homeostatic role for astrocytes in the acute physiologic regulation of food intake in response to high-fat feeding.[Hide abstract]
|HDAC4 regulates cognitive, metabolic and endocrine functions through its expression in osteoblastsOsteocalcin, a hormone secreted by osteoblasts, influences a wide range of physiological functions such as spatial learning, memory, male fertility and insulin secretion. Makinistoglu and Karsenty demonstrate that HDAC4 regulates Osteocalcin expression and many of the hormone's endocrine functions. Through its expression in osteoblasts, HDAC4 also regulates appetite that is not affected by osteocalcin.|
Abstract | PDF
Objective: The recently described endocrine functions of osteoblasts raise questions about their transcriptional regulation. Thus far, this aspect of osteoblast biology has been addressed only by examining the role of transcription factors binding to specific cis-acting elements in the promoter of the Osteocalcin gene.
Methods: In contrast, the role of chromatin remodeling enzymes, such as histone deacetylases (HDACs), in this process has not as yet been thoroughly understood.
Results: Here we show that through its expression in osteoblasts, one class II HDAC molecule, HDAC4, favors Osteocalcin expression, and as a result, the physiological functions regulated by osteocalcin such as spatial learning, memory, male fertility and insulin secretion. Molecular and genetic evidence indicates that through its expression in osteoblasts HDAC4 fulfills these long-range functions in part by stabilizing the transcription factor ATF4. Remarkably, through its expression in osteoblasts, HDAC4 also enhances appetite, a physiological function that is not regulated by osteocalcin.
Conclusions: These results provide a more in depth molecular understanding of the regulation of the endocrine functions of the osteoblast, and suggest the existence of additional hormones synthesized by osteoblasts that also regulate appetite.[Hide abstract]
|Glucose-regulated insulin production in the liver improves glycemic control in type 1 diabetic miceType 1 diabetes results from insulin deficiency secondary to autoimmune destruction of β-cells. In an innovative approach Zhang and Dong chose the liver as an insulin-producing surrogate. The authors show that hepatic insulin production from an enhanced glucose-responsive system is capable of correcting insulin deficiency and reversing hyperglycemia in type 1 diabetic mice.|
Abstract | PDF
Objective: Type 1 diabetes results from autoimmune destruction of beta-cells in the pancreas. Our objective is to reconstitute a glucose-responsive system in the liver to regulate hepatic insulin production for improving glycemic control in type 1 diabetes.
Methods: We have cloned the glucose-responsive element (GRE) from the promoter of acetyl-CoA carboxylase (ACC), an enzyme that catalyzes the rate-limiting step in fatty acid synthesis in the liver in response to glucose. To increase the amplitude of glucose induction, we quadruplicated the GRE DNA by gene duplication. The resulting GRE multimer (4×GRE) was tested for its ability to drive rat proinsulin cDNA expression in hepatocytes and insulin-deficient diabetic mice.
Results: We showed that this GRE multimer-directed glucose-responsive system produced insulin in hepatocytes in a glucose-dependent manner. When delivered into the liver by adenovirus-mediated gene transfer, this glucose-responsive insulin production system was able to reverse hyperglycemia to a normal range without causing hypoglycemia after glucose challenge or overnight fasting. Insulin vector-treated diabetic mice exhibited significantly improved blood glucose profiles in response to glucose tolerance, correlating with insulin production in the liver. We recapitulated these findings in streptozotocin-induced diabetic CD1 mice and autoimmune non-obese diabetic mice.
Conclusion: Our data characterized the GRE motif from the ACC promoter as a potent glucose-responsive element, and provided proof-of-concept that the 4×GRE-mediated hepatic insulin production is capable of correcting insulin deficiency and improving glycemic control in type 1 diabetes.[Hide abstract]