Christian P. Fischer

Brain-derived neurotrophic factor (BDNF) and type 2 diabetes

Aims/hypothesisDecreased levels of brain-derived neurotrophic factor (BDNF) have been implicated in the pathogenesis of Alzheimer’s disease and depression. These disorders are associated with type 2 diabetes, and animal models suggest that BDNF plays a role in insulin resistance. We therefore explored whether BDNF plays a role in human glucose metabolism.Subjects and methodsWe included (Study 1) 233 humans divided into four groups depending on presence or absence of type 2 diabetes and presence or absence of obesity; and (Study 2) seven healthy volunteers who underwent both a hyperglycaemic and a hyperinsulinaemic–euglycaemic clamp.ResultsPlasma levels of BDNF in Study 1 were decreased in humans with type 2 diabetes independently of obesity. Plasma BDNF was inversely associated with fasting plasma glucose, but not with insulin. No association was found between the BDNF G196A (Val66Met) polymorphism and diabetes or obesity. In Study 2 an output of BDNF from the human brain was detected at basal conditions. This output was inhibited when blood glucose levels were elevated. In contrast, when plasma insulin was increased while maintaining normal blood glucose, the cerebral output of BDNF was not inhibited, indicating that high levels of glucose, but not insulin, inhibit the output of BDNF from the human brain.Conclusions/interpretationLow levels of BDNF accompany impaired glucose metabolism. Decreased BDNF may be a pathogenetic factor involved not only in dementia and depression, but also in type 2 diabetes, potentially explaining the clustering of these conditions in epidemiological studies.

Searching for the exercise factor: is IL-6 a candidate?

For years the search for the stimulus that initiates and maintains the change of excitability or sensibility of the regulating centers in exercise has been progressing. For lack of more precise knowledge, it has been called the ‘work stimulus’, ‘the work factor’ or ‘the exercise factor’. In other terms, one big challenge for muscle and exercise physiologists has been to determine how muscles signal to central and peripheral organs. Here we discuss the possibility that interleukin-6 (IL-6) could mediate some of the health beneficial effects of exercise. In resting muscle, the IL-6 gene is silent, but it is rapidly activated by contractions. The transcription rate is very fast and the fold changes of IL-6 mRNA is marked. IL-6 is released from working muscles into the circulation in high amounts. The IL-6 production is modulated by the glycogen content in muscles, and IL-6 thus works as an energy sensor. IL-6 exerts its effect on adipose tissue, inducing lipolysis and gene transcription in abdominal subcutaneous fat and increases whole body lipid oxidation. Furthermore, IL-6 inhibits low-grade TNF-α-production and may thereby inhibit TNF-α-induced insulin resistance and atherosclerosis development. We propose that IL-6 and other cytokines, which are produced and released by skeletal muscles, exerting their effects in other organs of the body, should be named ‘myokines’.

Supplementation with vitamins C and E inhibits the release of interleukin‐6 from contracting human skeletal muscle

Contracting human skeletal muscle is a major contributor to the exercise‐induced increase of plasma interleukin‐6 (IL‐6). Although antioxidants have been shown to attenuate the exercise‐induced increase of plasma IL‐6, it is unknown whether antioxidants inhibit transcription, translation or translocation of IL‐6 within contracting human skeletal muscle. Using a single‐blind placebo‐controlled design with randomization, young healthy men received an oral supplementation with either a combination of ascorbic acid (500 mg day−1) and RRR‐α‐tocopherol (400 i.u. day−1) (Treatment, n= 7), or placebo (Control, n= 7). After 28 days of supplementation, the subjects performed 3 h of dynamic two‐legged knee‐extensor exercise at 50% of their individual maximal power output. Muscle biopsies from vastus lateralis were obtained at rest (0 h), immediately post exercise (3 h) and after 3 h of recovery (6 h). Leg blood flow was measured using Doppler ultrasonography. Plasma IL‐6 concentration was measured in blood sampled from the femoral artery and vein. The net release of IL‐6 was calculated using Ficks principle. Plasma vitamin C and E concentrations were elevated in Treatment compared to Control. Plasma 8‐iso‐prostaglandin F2α, a marker of lipid peroxidation, increased in response to exercise in Control, but not in Treatment. In both Control and Treatment, skeletal muscle IL‐6 mRNA and protein levels increased between 0 and 3 h. In contrast, the net release of IL‐6 from the leg, which increased during exercise with a peak at 3.5 h in Control, was completely blunted during exercise in Treatment. The arterial plasma IL‐6 concentration from 3 to 4 h, when the arterial IL‐6 levels peaked in both groups, was ∼50% lower in the Treatment group compared to Control (Treatment versus Control: 7.9 pg ml−1, 95% confidence interval (CI) 6.0–10.7 pg ml−1, versus 19.7 pg ml−1, CI 13.8–29.4 pg ml−1, at 3.5 h, P < 0.05 between groups). Moreover, plasma interleukin‐1 receptor antagonist (IL‐1ra), C‐reactive protein and cortisol levels all increased after the exercise in Control, but not in Treatment. In conclusion, our results show that supplementation with vitamins C and E attenuated the systemic IL‐6 response to exercise primarily via inhibition of the IL‐6 protein release from the contracting skeletal muscle per se.

Integration of microRNA changes in vivo identifies novel molecular features of muscle insulin resistance in type 2 diabetes

BackgroundSkeletal muscle insulin resistance (IR) is considered a critical component of type II diabetes, yet to date IR has evaded characterization at the global gene expression level in humans. MicroRNAs (miRNAs) are considered fine-scale rheostats of protein-coding gene product abundance. The relative importance and mode of action of miRNAs in human complex diseases remains to be fully elucidated. We produce a global map of coding and non-coding RNAs in human muscle IR with the aim of identifying novel disease biomarkers.MethodsWe profiled >47,000 mRNA sequences and >500 human miRNAs using gene-chips and 118 subjects (n = 71 patients versus n = 47 controls). A tissue-specific gene-ranking system was developed to stratify thousands of miRNA target-genes, removing false positives, yielding a weighted inhibitor score, which integrated the net impact of both up- and down-regulated miRNAs. Both informatic and protein detection validation was used to verify the predictions of in vivo changes.ResultsThe muscle mRNA transcriptome is invariant with respect to insulin or glucose homeostasis. In contrast, a third of miRNAs detected in muscle were altered in disease (n = 62), many changing prior to the onset of clinical diabetes. The novel ranking metric identified six canonical pathways with proven links to metabolic disease while the control data demonstrated no enrichment. The Benjamini-Hochberg adjusted Gene Ontology profile of the highest ranked targets was metabolic (P < 7.4 × 10-8), post-translational modification (P < 9.7 × 10-5) and developmental (P < 1.3 × 10-6) processes. Protein profiling of six development-related genes validated the predictions. Brain-derived neurotrophic factor protein was detectable only in muscle satellite cells and was increased in diabetes patients compared with controls, consistent with the observation that global miRNA changes were opposite from those found during myogenic differentiation.ConclusionsWe provide evidence that IR in humans may be related to coordinated changes in multiple microRNAs, which act to target relevant signaling pathways. It would appear that miRNAs can produce marked changes in target protein abundance in vivo by working in a combinatorial manner. Thus, miRNA detection represents a new molecular biomarker strategy for insulin resistance, where micrograms of patient material is needed to monitor efficacy during drug or life-style interventions.

Muscle-derived interleukin-6: Lipolytic, anti-inflammatory and immune regulatory effects

Interleukin-6 (IL-6) is produced locally in working skeletal muscle and can account for the exercise-induced increase in plasma IL-6. The transcription rate for IL-6 in muscle nuclei isolated from muscle biopsies during exercise is very high and is enhanced further when muscle glycogen content is low. Furthermore, cultured human primary muscle cells can increase IL-6 mRNA when incubated with the calcium ionophore ionomycin and it is likely that myocytes produce IL-6 in response to muscle contraction. The biological roles of muscle-derived IL-6 have been investigated in studies in which human recombinant IL-6 was infused in healthy volunteers to mimic closely the IL-6 concentrations observed during prolonged exercise. Using stable isotopes, we have demonstrated that physiological concentrations of IL-6 induce lipolysis. Although we have yet to determine the precise biological action of muscle-derived IL-6, our data support the hypothesis that the role of IL-6 released from contracting muscle during exercise is to act in a hormone-like manner to mobilize extracellular substrates and/or augment substrate delivery during exercise. In addition, IL-6 inhibits low-level TNF-α production, and IL-6 produced during exercise probably inhibits TNF-α-induced insulin resistance in peripheral tissues. Hence, IL-6 produced by skeletal muscle during contraction may play an important role in the beneficial health effects of exercise

Galectin-1 interacts with the {alpha}5{beta}1 fibronectin receptor to restrict carcinoma cell growth via induction of p21 and p27.

Surface binding of galectin family members has the potential to link distinct glycan structures to growth regulation. Therefore, we addressed the antiproliferative potential of galectin-1 (Gal-1) in a panel of carcinoma cell lines. We discovered growth inhibition by Gal-1 in epithelial tumor cell lines from different origins and provide evidence that this effect requires functional interaction with the α5β1 integrin. Antiproliferative effects result from inhibition of the Ras-MEK-ERK pathway and consecutive transcriptional induction of p27. We have further identified two Sp1-binding sites in the p27 promoter as crucial for Gal-1 responsiveness. Inhibition of the Ras-MEK-ERK cascade by Gal-1 increased Sp1 transactivation and DNA binding due to reduced threonine phosphorylation of Sp1. Furthermore, Gal-1 induced p21 transcription and selectively increased p27 protein stability. Gal-1-mediated accumulation of p27 and p21 inhibited cyclin-dependent kinase 2 activity and ultimately resulted in G1 cell cycle arrest and growth inhibition. These data define a novel mechanism whereby Gal-1 regulates epithelial tumor cell homeostasis via carbohydrate-dependent interaction with the α5β1 integrin.

Plasma levels of interleukin-6 and C-reactive protein are associated with physical inactivity independent of obesity.

It is recognized that the path from physical inactivity and obesity to lifestyle‐related diseases involves low‐grade inflammation, indicated by elevated plasma levels of inflammatory markers. Interestingly, contracting skeletal muscle is a major source of circulating interleukin‐6 (IL‐6) in response to acute exercise, but with a markedly lower response in trained subjects. As C‐reactive protein (CRP) is induced by IL‐6, we hypothesized that basal levels of IL‐6 and CRP reflect the degree of regular physical activity when compared with other markers of inflammation associated with lifestyle‐related morbidity. Fasting plasma/serum levels of IL‐6, IL‐18, CRP, tumur necrosis factor‐α (TNF‐α), soluble TNF receptor II (sTNF‐RII), and adiponectin were measured in healthy non‐diabetic men and women (n=84). The amount of leisure‐time physical activity (LTPA) was assessed by interview. Obesity was associated with elevated insulin, C‐peptide, triglycerides, low‐density lipoprotein, IL‐6, CRP, and adiponectin (all P<0.05). Importantly, physical inactivity was associated with elevated C‐peptide (P=0.036), IL‐6 (P=0.014), and CRP (P=0.007) independent of obesity, age, gender, and smoking. Furthermore, the LTPA score was inversely associated with IL‐6 (P=0.017) and CRP (P=0.005), but with neither of the other markers. The results indicate that low levels of IL‐6 and CRP – not IL‐18, TNF‐α, sTNF‐RII, or adiponectin – reflect regular physical activity.

Fat-specific Protein 27 Regulates Storage of Triacylglycerol

FSP27 (fat-specific protein 27) is a member of the cell death-inducing DNA fragmentation factor-α-like effector (CIDE) family. Although Cidea and Cideb were initially characterized as activators of apoptosis, recent studies have demonstrated important metabolic roles for these proteins. In this study, we investigated the function of another member of this family, FSP27 (Cidec), in apoptosis and adipocyte metabolism. Although overexpression of FSP27 is sufficient to increase apoptosis of 293T and 3T3-L1 cells, more physiological levels of expression stimulate spontaneous lipid accumulation in several cell types without induction of adipocyte genes. Increased triacylglycerol is likely due to decreased β-oxidation of nonesterified fatty acids. Altered flux of fatty acids into triacylglycerol may be a direct effect of FSP27 function, which is localized to lipid droplets in 293T cells and 3T3-L1 adipocytes. Stable knockdown of FSP27 during adipogenesis of 3T3-L1 cells substantially decreases lipid droplet size, increases mitochondrial and lipid droplet number, and modestly increases glucose uptake and lipolysis. Expression of FSP27 in subcutaneous adipose tissue of a human diabetes cohort decreases with total fat mass but is not associated with measures of insulin resistance (e.g. homeostasis model assessment). Together, these data indicate that FSP27 binds to lipid droplets and regulates their enlargement.

Physiological roles of muscle-derived interleukin-6 in response to exercise.

Purpose of reviewTo discuss recent findings with regard to the regulation of muscle-derived interleukin-6 as well as the possible physiological and metabolic roles of interleukin-6 in response to exercise. Recent findingsContraction-induced transcription and release of interleukin-6 is primarily regulated by an altered intramuscular milieu in response to exercise. Accordingly, changes in calcium homeostasis, impaired glucose availability and increased formation of reactive oxygen species are all associated with exercise and capable of activating transcription factors known to regulate interleukin-6 synthesis. Acute interleukin-6 administration to humans increases lipolysis, fat oxidation and insulin-mediated glucose disposal. Adenosine monophosphate-activated protein kinase activation by interleukin-6 appears to play an important role in modulating some of these metabolic effects. Interleukin-6 facilitates an antiinflammatory milieu and may exert some of its biological effects via inhibition of the proinflammatory cytokine tumor necrosis factor-α. SummaryThe discovery of contracting muscle as a cytokine-producing organ opens a new paradigm: skeletal muscle is an endocrine organ that in response to contractions produces and releases ‘myokines’, which subsequently can modulate the metabolic and immunological response to exercise in several tissues. In our view, interleukin-6 may be one of several myokines.

Antioxidant Supplementation Does Not Alter Endurance Training Adaptation

BACKGROUND There is a considerable commercial market, especially within the sports community, claiming the need for antioxidant supplementation. One argument for antioxidant supplementation in sports is that physical exercise is associated with increased reactive oxygen and nitrogen species (RONS) production, which may cause cell damage. However, RONS production may also activate redox-sensitive signaling pathways and transcription factors, which subsequently, may promote training adaptation. PURPOSE Our aim was to investigate the effects of combined vitamin C and E supplementation to healthy individuals on different measures of exercise performance after endurance training. METHODS Using a double-blinded placebo-controlled design, moderately trained young men received either oral supplementation with vitamins C and E (n = 11) or placebo (n = 10) before and during 12 wk of supervised, strenuous bicycle exercise training of a frequency of 5 d x wk(-1). Muscle biopsies were obtained before and after training. RESULTS After the training period, maximal oxygen consumption, maximal power output, and workload at lactate threshold increased markedly (P < 0.01) in both groups. Also, glycogen concentration, citrate synthase, and beta-hydroxyacyl-CoA dehydrogenase activity in the muscle were significantly higher in response to training (P < 0.01) in both groups. However, there were no differences between the two groups concerning any of the physiological and metabolic variables measured. CONCLUSIONS Our results suggest that administration of vitamins C and E to individuals with no previous vitamin deficiencies has no effect on physical adaptations to strenuous endurance training.