Jane Cheeseman

Activation of Peroxisome Proliferator–Activated Receptor (PPAR)δ Promotes Reversal of Multiple Metabolic Abnormalities, Reduces Oxidative Stress, and Increases Fatty Acid Oxidation in Moderately Obese Men

OBJECTIVE— Pharmacological use of peroxisome proliferator–activated receptor (PPAR)δ agonists and transgenic overexpression of PPARδ in mice suggest amelioration of features of the metabolic syndrome through enhanced fat oxidation in skeletal muscle. We hypothesize a similar mechanism operates in humans. RESEARCH DESIGN AND METHODS— The PPARδ agonist (10 mg o.d. GW501516), a comparator PPARα agonist (20 μg o.d. GW590735), and placebo were given in a double-blind, randomized, three-parallel group, 2-week study to six healthy moderately overweight subjects in each group. Metabolic evaluation was made before and after treatment including liver fat quantification, fasting blood samples, a 6-h meal tolerance test with stable isotope fatty acids, skeletal muscle biopsy for gene expression, and urinary isoprostanes for global oxidative stress. RESULTS— Treatment with GW501516 showed statistically significant reductions in fasting plasma triglycerides (−30%), apolipoprotein B (−26%), LDL cholesterol (−23%), and insulin (−11%), whereas HDL cholesterol was unchanged. A 20% reduction in liver fat content (P < 0.05) and 30% reduction in urinary isoprostanes (P = 0.01) were also observed. Except for a lowering of triglycerides (−30%, P < 0.05), none of these changes were observed in response to GW590735. The relative proportion of exhaled CO2 directly originating from the fat content of the meal was increased (P < 0.05) in response to GW501516, and skeletal muscle expression of carnitine palmitoyl-transferase 1b (CPT1b) was also significantly increased. CONCLUSIONS— The PPARδ agonist GW501516 reverses multiple abnormalities associated with the metabolic syndrome without increasing oxidative stress. The effect is probably caused by increased fat oxidation in skeletal muscle.

Downregulation of Adipose Tissue Fatty Acid Trafficking in Obesity A Driver for Ectopic Fat Deposition

OBJECTIVE Lipotoxicity and ectopic fat deposition reduce insulin signaling. It is not clear whether excess fat deposition in nonadipose tissue arises from excessive fatty acid delivery from adipose tissue or from impaired adipose tissue storage of ingested fat. RESEARCH DESIGN AND METHODS To investigate this we used a whole-body integrative physiological approach with multiple and simultaneous stable-isotope fatty acid tracers to assess delivery and transport of endogenous and exogenous fatty acid in adipose tissue over a diurnal cycle in lean (n = 9) and abdominally obese men (n = 10). RESULTS Abdominally obese men had substantially (2.5-fold) greater adipose tissue mass than lean control subjects, but the rates of delivery of nonesterified fatty acids (NEFA) were downregulated, resulting in normal systemic NEFA concentrations over a 24-h period. However, adipose tissue fat storage after meals was substantially depressed in the obese men. This was especially so for chylomicron-derived fatty acids, representing the direct storage pathway for dietary fat. Adipose tissue from the obese men showed a transcriptional signature consistent with this impaired fat storage function. CONCLUSIONS Enlargement of adipose tissue mass leads to an appropriate downregulation of systemic NEFA delivery with maintained plasma NEFA concentrations. However the implicit reduction in adipose tissue fatty acid uptake goes beyond this and shows a maladaptive response with a severely impaired pathway for direct dietary fat storage. This adipose tissue response to obesity may provide the pathophysiological basis for ectopic fat deposition and lipotoxicity.

Regulation of human metabolism by hypoxia- inducible factor

The hypoxia-inducible factor (HIF) family of transcription factors directs a coordinated cellular response to hypoxia that includes the transcriptional regulation of a number of metabolic enzymes. Chuvash polycythemia (CP) is an autosomal recessive human disorder in which the regulatory degradation of HIF is impaired, resulting in elevated levels of HIF at normal oxygen tensions. Apart from the polycythemia, CP patients have marked abnormalities of cardiopulmonary function. No studies of integrated metabolic function have been reported. Here we describe the response of these patients to a series of metabolic stresses: exercise of a large muscle mass on a cycle ergometer, exercise of a small muscle mass (calf muscle) which allowed noninvasive in vivo assessments of muscle metabolism using 31P magnetic resonance spectroscopy, and a standard meal tolerance test. During exercise, CP patients had early and marked phosphocreatine depletion and acidosis in skeletal muscle, greater accumulation of lactate in blood, and reduced maximum exercise capacities. Muscle biopsy specimens from CP patients showed elevated levels of transcript for pyruvate dehydrogenase kinase, phosphofructokinase, and muscle pyruvate kinase. In cell culture, a range of experimental manipulations have been used to study the effects of HIF on cellular metabolism. However, these approaches provide no potential to investigate integrated responses at the level of the whole organism. Although CP is relatively subtle disorder, our study now reveals a striking regulatory role for HIF on metabolism during exercise in humans. These findings have significant implications for the development of therapeutic approaches targeting the HIF pathway.

Fasted to Fed Trafficking of Fatty Acids in Human Adipose Tissue Reveals a Novel Regulatory Step for Enhanced Fat Storage

CONTEXT Absence or excess of adipose tissue are both associated with metabolic complications, implying the importance of well-functioning adipose tissue present in normal amounts. Adipose tissue sequesters dietary fat and thus protects other tissues from excess fat exposure, especially after meals. OBJECTIVE The objective of the study was the use of an integrative physiological technique to quantify trafficking of fatty acids (FAs) in adipose tissue over a 24 h period. METHODS Adipose tissue FA handling was studied in response to three meals in eight healthy men by the combination of arteriovenous blood sampling, tissue blood flow, and specific labeling of FA tracing of exogenous and endogenous fat by stable isotope methodology. RESULTS The efficiency of adipose tissue FA uptake increased robustly with each meal. Chylomicron-triglyceride was the dominating source of FA. Adipose tissue fractional extraction of chylomicron-triglyceride increased from 21 +/- 4 to 47 +/- 8% (P = 0.03) between the first and last meal. Although adipose tissue lipoprotein lipase action increased with time (2-fold), there was an even greater increase in FA reesterification (3-fold), which led to a reduced spillover of chylomicron-derived FA, from 77 +/- 15 to 34 +/- 7% (P = 0.04) comparing the end of the first and the third meal period. Increased uptake of very low-density lipoprotein-derived FA was observed, but spillover of very low-density lipoprotein-derived FA was seen only in the fasting state. CONCLUSION Human adipose tissue has a significant potential to up-regulate fat storage during a normal day that goes beyond increased lipoprotein lipase activation. The adaptation toward increasing fat storage may provide an explanation for the beneficial properties of normal amounts of adipose tissue.

The effects of APOE on brain activity do not simply reflect the risk of Alzheimer's disease.

Possession of the APOE-ε4 allele is the best established genetic risk factor for sporadic Alzheimers disease (AD), while the ε2 allele may confer protection against the disease. Previous functional magnetic resonance imaging (fMRI) studies have shown an effect of APOE genotype on brain function, typically by comparing only ε4 carriers with noncarriers. Here we included a wide range of genotype groups to determine how closely the effects of APOE on brain function are related to differences in relative risk for AD. We used functional magnetic resonance imaging (fMRI) to compare the pattern of activation during an episodic encoding task and during a counting Stroop task in 76 adults, aged 32 to 55, with different APOE genotypes (23 ε2/ε3, 20 ε3/ε3, 26 ε3/ε4, and 7 ε4/ε4). Strikingly, participants with an increased risk (ε4 carriers) and with a decreased risk (ε2 carriers) for AD both showed increased activation, relative to ε3 homozygotes, during both tasks. The increased activation was due to decreased deactivation or paradoxical activation of nontask-related regions of the brain, which suggests an intrinsic effect of APOE on the differentiation of functional cortical networks. These results question the often assumed link between APOE, the blood oxygenation level dependent (BOLD) response, and AD risk.

Resolving TYK2 locus genotype-to-phenotype differences in autoimmunity

Resolving TYK2 locus genotype-to-phenotype differences reveals an immune signaling optimum that may be exploited therapeutically for treating autoimmune diseases. TYK2’s balancing act Determining the biological consequences of the thousands of genetic variants that contribute to common diseases for the purpose of improving health care is challenging. Genetic variants that influence autoimmune diseases have been identified in the tyrosine kinase 2 (TYK2) gene, but conflicting evidence regarding their biological impact obscures the therapeutic potential of TYK2. By resolving this conflict, Dendrou et al. have revealed a genetic effect that drives an optimal degree of immune signaling: low enough to be protective against autoimmunity but high enough to prevent immunodeficiency. These findings indicate that TYK2 may be a potential drug target in a number of autoimmune conditions. Thousands of genetic variants have been identified, which contribute to the development of complex diseases, but determining how to elucidate their biological consequences for translation into clinical benefit is challenging. Conflicting evidence regarding the functional impact of genetic variants in the tyrosine kinase 2 (TYK2) gene, which is differentially associated with common autoimmune diseases, currently obscures the potential of TYK2 as a therapeutic target. We aimed to resolve this conflict by performing genetic meta-analysis across disorders; subsequent molecular, cellular, in vivo, and structural functional follow-up; and epidemiological studies. Our data revealed a protective homozygous effect that defined a signaling optimum between autoimmunity and immunodeficiency and identified TYK2 as a potential drug target for certain common autoimmune disorders.

Cohort Profile: The Oxford Biobank

Major progress has been made over the past decade in the understanding of the genetic background to chronic metabolic disease such as type 2 diabetes (T2D) and atherosclerotic cardiovascular disease (CVD). These disorders show a significant degree of heritability and disease pathogenesis that rely on the combination of a multitude of unfavourable genotypes on which over-nutrition, lack of physical exercise, obesity and smoking augment the phenotype. Currently, the number of common genetic variants robustly associated with CVD and T2D are increasing with the increasing size of discovery cohorts; for CVD, the number now exceeds 50 variants and for T2D and glycaemic traits, the corresponding number is about 75. Combining several genome-wide association studies (GWAS) datasets which include information on highly relevant intermediate phenotypes has potentially helped in discovery and replication of several disease loci and identification of novel pathways and pleiotropic genes. However, little is known about the functional consequences of most of the identified gene variants. The use of well-characterized bioresources, in which investigations into intermediate phenotypes can be performed, will be invaluable in order to provide mechanistic insight into these poorly characterized genes and thus promote translational research. To this end the Oxford Biobank (OBB) was set up with the primary goal of establishing a local cohort accessible for genomic translational research. The resource is built to enable studies on physiological consequences of genetic mechanisms of disease. A leading principle has been to seek informed consent from participants to be reapproached for future discrete projects. Therefore, based on the information gathered during a baseline visit, ‘recruit-by-genotype’ (RbG) and ‘recruit-by-phenotype’ (RbP) projects allow for detailed investigations of associations between genotypes and biomarkers, or monitoring of more detailed physiological processes. The OBB serves as a resource for researchers to investigate mechanisms leading to increased T2D and CVD susceptibility and to explore novel therapeutic targets in the prevention and treatment of chronic non-communicable diseases.

Context-specific regulation of monocyte surface IL7R expression and soluble receptor secretion by a common autoimmune risk allele

IL-7 is a key factor in T-cell immunity and IL7R polymorphisms are implicated in autoimmune pathogenesis. We previously reported an expression quantitative trait locus (eQTL) at rs931555, 59 to IL7R, associated with stimulated monocyte IL7R mRNA expression. Unlike in T-cells, a role for IL7R in monocyte biology is poorly described. Here we detail replication and characterization of this eQTL at protein level across cell subsets and conditions in a separate cohort. We find rs6897932, a non-synonymous IL7R polymorphism associated with susceptibility to Multiple Sclerosis, Ankylosing Spondylitis and Primary Biliary Cirrhosis, is the key determinant of monocyte IL7R surface expression and soluble IL7R (sIL7R) and functions in a context-specific manner. Monocyte surface IL7R is markedly induced by LPS and TNF stimulation under the genotypic regulation of rs6897932, whereas no effect of this allele was observed on CD4+, CD8+ or CD56+ cell surface IL7R or in unstimulated monocytes. LPS-induced monocyte release of sIL7R was strongly associated with both rs6897932 genotype and expression of the splicing factor gene DDX39A. After induction of IL7R expression, human monocytes display a robust and pleiotropic transcriptional response to exogenous IL-7. Monocytes from the synovial fluid of patients with Spondyloarthritis were similarly found to express high levels of surface IL7R. These data demonstrate that disease-associated genetic variants in the IL7R gene critically impact monocyte IL7R and sIL7R expression following innate immune stimulation, suggesting a previously unappreciated key role for monocytes in IL-7 pathway biology and IL7R-associated diseases.IL-7 is a key factor in T-cell immunity and IL7R polymorphisms are implicated in autoimmune pathogenesis. IL7R mRNA is induced in stimulated monocytes in a genetically determined manner, yet a role for IL7R in monocyte biology remains unexplored. Here we characterize genetic regulation of IL7R at the protein level across multiple cell subsets and conditions in healthy individuals. We find monocyte surface and soluble IL7R (sIL7R) protein are markedly expressed in response to lipopolysaccharide (LPS). We further demonstrate alleles of rs6897932, a non-synonymous IL7R polymorphism associated with susceptibility to Multiple Sclerosis, Ankylosing Spondylitis and Primary Biliary Cirrhosis, form the key determinant of both surface IL7R and sIL7R in the context of inflammation. No effect of this allele was observed in unstimulated monocytes or across lymphoid subsets. Production of sIL7R by monocytes greatly exceeded that of CD4+ T-cells, and was strongly associated with both rs6897932 genotype and expression of the splicing factor gene DDX39A. Stimulated monocytes were sensitive to exogenous IL-7, which elicits a defined transcriptional signature. Flow cytometry and single-cell sequencing of synovial fluid derived monocytes from patients with spondyloarthritis showed an enlarged subset of IL7R+ monocytes with a unique transcriptional profile that markedly overlaps that induced by IL-7 in-vitro and shows similarity to the previously described ‘Mono4’ subset. These data demonstrate disease-associated genetic variants at IL7R specifically impact monocyte surface IL7R and sIL7R following innate immune stimulation, suggesting a previously unappreciated key role for monocytes in IL-7 pathway biology and IL7R-associated diseases.