Hans-Georg Joost

The extended GLUT-family of sugar/polyol transport facilitators: nomenclature, sequence characteristics, and potential function of its novel members (review).

During the last 2 years, several novel genes that encode glucose transporter-like proteins have been identified and characterized. Because of their sequence similarity with GLUT1, these genes appear to belong to the family of solute carriers 2A ( SLC2A, protein symbol GLUT). Sequence comparisons of all 13 family members allow the definition of characteristic sugar/polyol transporter signatures: (1) the presence of 12 membrane-spanning helices, (2) seven conserved glycine residues in the helices, (3) several basic and acidic residues at the intracellular surface of the proteins, (4) two conserved tryptophan residues, and (5) two conserved tyrosine residues. On the basis of sequence similarities and characteristic elements, the extended GLUTfamily can be divided intothree subfamilies, namely class I (the previously known glucose transporters GLUT1-4), class II (the previously known fructose transporter GLUT5, the GLUT7, GLUT9 and GLUT11), and class III (GLUT6, 8, 10, 12, and the myoinositol transporter HMIT1). Functional characteristics have been reported for some of the novel GLUTs. Like GLUT1-4, they exhibit a tissue/cell-specific expression (GLUT6, leukocytes, brain; GLUT8, testis, blastocysts, brain, muscle, adipocytes; GLUT9, liver, kidney; GLUT10, liver, pancreas; GLUT11, heart, skeletal muscle). GLUT6 and GLUT8 appear to be regulated by sub-cellular redistribution, because they are targeted to intracellular compartments by dileucine motifs in a dynamin dependent manner. Sugar transport has been reported for GLUT6, 8, and 11; HMIT1 has been shown to be a H + /myo-inositol co-transporter. Thus, the members of the extended GLUT family exhibit a surprisingly diverse substrate specificity, and the definition of sequence elements determining this substrate specificity will require a full functional characterization of all members.

Identification of Serum Metabolites Associated With Risk of Type 2 Diabetes Using a Targeted Metabolomic Approach

Metabolomic discovery of biomarkers of type 2 diabetes (T2D) risk may reveal etiological pathways and help to identify individuals at risk for disease. We prospectively investigated the association between serum metabolites measured by targeted metabolomics and risk of T2D in the European Prospective Investigation into Cancer and Nutrition (EPIC)-Potsdam (27,548 adults) among all incident cases of T2D (n = 800, mean follow-up 7 years) and a randomly drawn subcohort (n = 2,282). Flow injection analysis tandem mass spectrometry was used to quantify 163 metabolites, including acylcarnitines, amino acids, hexose, and phospholipids, in baseline serum samples. Serum hexose; phenylalanine; and diacyl-phosphatidylcholines C32:1, C36:1, C38:3, and C40:5 were independently associated with increased risk of T2D and serum glycine; sphingomyelin C16:1; acyl-alkyl-phosphatidylcholines C34:3, C40:6, C42:5, C44:4, and C44:5; and lysophosphatidylcholine C18:2 with decreased risk. Variance of the metabolites was largely explained by two metabolite factors with opposing risk associations (factor 1 relative risk in extreme quintiles 0.31 [95% CI 0.21–0.44], factor 2 3.82 [2.64–5.52]). The metabolites significantly improved T2D prediction compared with established risk factors. They were further linked to insulin sensitivity and secretion in the Tübingen Family study and were partly replicated in the independent KORA (Cooperative Health Research in the Region of Augsburg) cohort. The data indicate that metabolic alterations, including sugar metabolites, amino acids, and choline-containing phospholipids, are associated early on with a higher risk of T2D.

The central melanocortin system directly controls peripheral lipid metabolism

Disruptions of the melanocortin signaling system have been linked to obesity. We investigated a possible role of the central nervous melanocortin system (CNS-Mcr) in the control of adiposity through effects on nutrient partitioning and cellular lipid metabolism independent of nutrient intake. We report that pharmacological inhibition of melanocortin receptors (Mcr) in rats and genetic disruption of Mc4r in mice directly and potently promoted lipid uptake, triglyceride synthesis, and fat accumulation in white adipose tissue (WAT), while increased CNS-Mcr signaling triggered lipid mobilization. These effects were independent of food intake and preceded changes in adiposity. In addition, decreased CNS-Mcr signaling promoted increased insulin sensitivity and glucose uptake in WAT while decreasing glucose utilization in muscle and brown adipose tissue. Such CNS control of peripheral nutrient partitioning depended on sympathetic nervous system function and was enhanced by synergistic effects on liver triglyceride synthesis. Our findings offer an explanation for enhanced adiposity resulting from decreased melanocortin signaling, even in the absence of hyperphagia, and are consistent with feeding-independent changes in substrate utilization as reflected by respiratory quotient, which is increased with chronic Mcr blockade in rodents and in humans with loss-of-function mutations in MC4R. We also reveal molecular underpinnings for direct control of the CNS-Mcr over lipid metabolism. These results suggest ways to design more efficient pharmacological methods for controlling adiposity.

Novel biomarkers for pre-diabetes identified by metabolomics

Type 2 diabetes (T2D) can be prevented in pre‐diabetic individuals with impaired glucose tolerance (IGT). Here, we have used a metabolomics approach to identify candidate biomarkers of pre‐diabetes. We quantified 140 metabolites for 4297 fasting serum samples in the population‐based Cooperative Health Research in the Region of Augsburg (KORA) cohort. Our study revealed significant metabolic variation in pre‐diabetic individuals that are distinct from known diabetes risk indicators, such as glycosylated hemoglobin levels, fasting glucose and insulin. We identified three metabolites (glycine, lysophosphatidylcholine (LPC) (18:2) and acetylcarnitine) that had significantly altered levels in IGT individuals as compared to those with normal glucose tolerance, with P‐values ranging from 2.4 × 10−4 to 2.1 × 10−13. Lower levels of glycine and LPC were found to be predictors not only for IGT but also for T2D, and were independently confirmed in the European Prospective Investigation into Cancer and Nutrition (EPIC)‐Potsdam cohort. Using metabolite–protein network analysis, we identified seven T2D‐related genes that are associated with these three IGT‐specific metabolites by multiple interactions with four enzymes. The expression levels of these enzymes correlate with changes in the metabolite concentrations linked to diabetes. Our results may help developing novel strategies to prevent T2D.

GOAT links dietary lipids with the endocrine control of energy balance

Central nervous system nutrient sensing and afferent endocrine signaling have been established as parallel systems communicating metabolic status and energy availability in vertebrates. The only afferent endocrine signal known to require modification with a fatty acid side chain is the orexigenic hormone ghrelin. We find that the ghrelin O-acyl transferase (GOAT), which is essential for ghrelin acylation, is regulated by nutrient availability, depends on specific dietary lipids as acylation substrates and links ingested lipids to energy expenditure and body fat mass. These data implicate the ghrelin-GOAT system as a signaling pathway that alerts the central nervous system to the presence of dietary calories, rather than to their absence as is commonly accepted.

The glucose transporter families SGLT and GLUT: molecular basis of normal and aberrant function

Glucose enters eucaryotic cells via 2 different types of membrane associated carrier proteins, the Na+-coupled glucose transporters (SGLT) and glucose transporter facilitators (GLUT). Three members of the SGLT family function as sugar transporters (SGLT1 and SGLT2) or sensors (SGLT3). The human GLUT family consists of 14 members, of which 11 have been shown to catalyze sugar transport. The individual isotypes exhibit different substrate specificity, kinetic characteristics, and expression profiles, thereby allowing a tissue-specific adaptation of glucose uptake through regulation of their gene expression. Furthermore, some transporters (eg, GLUT4 and GLUT8) are regulated by their subcellular distribution. In addition to catalyzing glucose entry into cells, some isotypes (eg, GLUT2) seem to be involved in the mechanisms of glucosensing of pancreatic beta-cells, neuronal, or other cells, thereby playing a major role in the hormonal and neural control. Targeted disruption in mice has helped to elucidate the physiologic function of some isotypes (GLUT1, GLUT2, GLUT4). Furthermore, several congenital defects of sugar metabolism are caused by aberrant transporter genes (eg, the glucose-galactose malabsorption syndrome, SGLT1; the glucose transporter 1 deficiency syndrome; and the Fanconi-Bickel syndrome, GLUT2). In addition, a malfunction of glucose transporter expression or regulation (GLUT4) appears to contribute to the insulin resistance syndrome.

Plasma Fetuin-A Levels and the Risk of Type 2 Diabetes

OBJECTIVE—The liver-secreted protein fetuin-A induces insulin resistance in animals, and circulating fetuin-A is elevated in insulin resistance and fatty liver in humans. We investigated whether plasma fetuin-A levels predict the incidence of type 2 diabetes in a large prospective, population-based study. RESEARCH DESIGN AND METHODS—A case-cohort study within the European Prospective Investigation into Cancer and Nutrition (EPIC)-Potsdam study comprising 27,548 subjects was designed. We randomly selected a subcohort of 2,500 individuals of whom 2,164 were diabetes free at baseline and had anamnestic, anthropometrical, and metabolic data for analysis. Of the 849 incident diabetic case subjects identified in the full cohort during 7 years of follow-up, 703 remained for analyses after similar exclusions. RESULTS—Plasma fetuin-A levels were positively associated with diabetes risk after adjustment for age (relative risk [RR] for extreme quintiles 1.75 [95% CI 1.32–2.31]; RR for 10 μg/ml 1.04 [1.03–1.06]). The association remained significant after adjustment for sex, BMI, waist circumference, and lifestyle risk factors (RR for 10 μg/ml 1.03 [1.01–1.06]). Adjustment for glucose, triglycerides, HDL cholesterol, A1C, γ-glutamyltransferase, or high-sensitivity C-reactive protein or mutual adjustment for these biomarkers did not appreciably change this result (RR for 10 μg/ml full adjusted model 1.05 [1.02–1.07]). Furthermore, fetuin-A was associated with increased diabetes risk particularly in individuals with elevated plasma glucose. CONCLUSIONS—Our data suggest that fetuin-A is an independent risk factor of type 2 diabetes.

Plasma Fetuin-A Levels and the Risk of Myocardial Infarction and Ischemic Stroke

Background— Fetuin-A, a protein almost exclusively secreted by the liver, induces insulin resistance and subclinical inflammation in rodents. Circulating fetuin-A levels are elevated in humans with metabolic syndrome and insulin resistance, conditions that are associated with increased risk of cardiovascular disease. Methods and Results— We investigated the association between fetuin-A levels and the risk of future myocardial infarction (MI) and ischemic stroke (IS) in a case-cohort study based on the European Prospective Investigation into Cancer and Nutrition (EPIC)–Potsdam Study comprising 27 548 middle-aged subjects of the general population. Fetuin-A levels were measured in plasma of 227 individuals who developed MI, in 168 who developed IS, and in 2198 individuals who remained free of cardiovascular events during a mean follow-up of 8.2±2.2 years. Individuals in the highest compared with the lowest quintile of plasma fetuin-A had significantly increased risks of MI (relative risk, 3.80; 95% confidence interval, 2.37 to 6.10; P for trend <0.0001) and IS (relative risk, 3.93; 95% confidence interval, 2.17 to 7.12; P for trend <0.0001) after adjustment for sex and age. Additional adjustment for smoking status, body mass index, waist circumference, alcohol consumption, educational attainment, physical activity, hypertension, diabetes mellitus, total and high-density lipoprotein cholesterol, and C-reactive protein only moderately attenuated these risks (MI: relative risk, 3.25; 95% confidence interval, 2.01 to 5.28; P for trend <0.0001; IS: relative risk, 3.78; 95% confidence interval, 2.06 to 6.94; P for trend <0.0001). Conclusions— Our data provide evidence for a link between high plasma fetuin-A levels and an increased risk of MI and IS. Therefore, more research is warranted to determine the role of fetuin-A in the pathophysiology of cardiovascular disease.

GLUT8, a Novel Member of the Sugar Transport Facilitator Family with Glucose Transport Activity

GLUT8 is a novel glucose transporter-like protein that exhibits significant sequence similarity with the members of the sugar transport facilitator family (29.4% of amino acids identical with GLUT1). Human and mouse sequence (86.2% identical amino acids) comprise 12 putative membrane-spanning helices and several conserved motifs (sugar transporter signatures), which have previously been shown to be essential for transport activity, e.g. GRK in loop 2, PETPR in loop 6, QQLSGVN in helix 7, DRAGRR in loop 8, GWGPIPW in helix 10, and PETKG in the C-terminal tail. An expressed sequence tag (STS A005N15) corresponding with the 3′-untranslated region of GLUT8 has previously been mapped to human chromosome 9. COS-7 cells transfected with GLUT8 cDNA expressed a 42-kDa protein exhibiting specific, glucose-inhibitable cytochalasin B binding (KD = 56.6 ± 18 nm) and reconstitutable glucose transport activity (8.1 ± 1.4 nmol/(mg protein × 10 s)versus 1.1 ± 0.1 in control transfections). In human tissues, a 2.4-kilobase pair transcript was predominantly found in testis, but not in testicular carcinoma. Lower amounts of the mRNA were detected in most other tissues including skeletal muscle, heart, small intestine, and brain. GLUT8 mRNA was found in testis from adult, but not from prepubertal rats; its expression in human testis was suppressed by estrogen treatment. It is concluded that GLUT8 is a sugar transport facilitator with glucose transport activity and a hormonally regulated testicular function.

Specificity Determinants of Substrate Recognition by the Protein Kinase DYRK1A

DYRK1A is a dual-specificity protein kinase that is thought to be involved in brain development. We identified a single phosphorylated amino acid residue in the DYRK substrate histone H3 (threonine 45) by mass spectrometry, phosphoamino acid analysis, and protein sequencing. Exchange of threonine 45 for alanine abolished phosphorylation of histone H3 by DYRK1A and by the related kinases DYRK1B, DYRK2, and DYRK3 but not by CLK3. In order to define the consensus sequence for the substrate specificity of DYRK1A, a library of 300 peptides was designed in variation of the H3 phosphorylation site. Evaluation of the phosphate incorporation into these peptides identified DYRK1A as a proline-directed kinase with a phosphorylation consensus sequence (RPX(S/T)P) similar to that of ERK2 (PX(S/T)P). A peptide designed after the optimal substrate sequence (DYRKtide) was efficiently phosphorylated by DYRK1A (K m = 35 μm) but not by ERK2. Both ERK2 and DYRK1A phosphorylated myelin basic protein, whereas only ERK2, but not DYRK1A, phosphorylated the mitogen-activated protein kinase substrate ELK-1. This marked difference in substrate specificity between DYRK1A and ERK2 can be explained by the requirement for an arginine at the P −3 site of DYRK substrates and its presumed interaction with aspartate 247 conserved in all DYRKs.