Louise Fritsche

How Insulin Receptor Substrate Proteins Regulate the Metabolic Capacity of the Liver - Implications for Health and Disease

The liver plays a key role in glucose homeostasis, lipid and energy metabolism. Its function is primarily controlled by the anabolic hormone insulin and its counterparts glucagon, catecholamines and glucocorticoids. Dysregulation of this homeostatic system is a major cause for development of the metabolic syndrome and type 2 diabetes mellitus. The features of the underlying dynamic molecular network that coordinates systemic nutrient homeostasis are less clear. But recently, considerable progress has been made in elucidating molecular pathways and potential factors involved in the regulation of energy and lipid metabolism and affected in diabetic states. In this review we will focus on important stations in the complex network of molecules that control the balance between glucose production, glucose utilization and regulation of lipid metabolism. Special attention will be paid to the insulin receptor substrate (IRS) proteins with the two major isoforms IRS-1 and IRS-2 as a critical node in hepatic insulin signalling. IRS proteins act as docking molecules to connect tyrosine kinase receptor activation to essential downstream kinase cascades, including activation of the PI-3 kinase or MAPK cascade. IRS-1 and IRS-2 are complementary key players in the regulation of hepatic insulin signalling and expression of genes involved in gluconeogenesis, glycogen synthesis and lipid metabolism. The function of IRS proteins is regulated by their expression levels and posttranslational modifications. This regulation within the dynamic molecular network that coordinates systemic nutrient homeostasis will be outlined in detail under the following conditions: after feeding, during fasting and during exercise. Dysfunction of IRS proteins initially leads to post-prandial hyperglycemia, increased hepatic glucose production, and dysregulated lipid synthesis and is discussed as major pathophysiological mechanism for the development of insulin resistance and type 2 diabetes mellitus. Understanding the molecular regulation and the pathophysiological modifications of IRS proteins is crucial in order to identify new sites for potential intervention to treat or prevent hepatic insulin resistance and type 2 diabetes mellitus.

Insulin-induced serine phosphorylation of IRS-2 via ERK1/2 and mTOR: studies on the function of Ser675 and Ser907

The identity of specific serine phosphorylation residues of insulin receptor substrate (IRS)-2 and their impact on insulin signal transduction are largely unknown. Ser(675) and Ser(907) of mouse IRS-2 are adjacent to PI 3-kinase or Grb2 binding domains, respectively. Using monoclonal phosphosite-specific antibodies, we demonstrated the phosphorylation of both serines after stimulation of Fao hepatoma cells with insulin, anisomycin, or phorbol esters. Phosphorylation of both sites was a late and prolonged event during insulin treatment and was also detected in liver tissue of insulin-treated as well as refed mice. Inhibition and siRNA-mediated knockdown of ERK1/2 indicated that the insulin-induced phosphorylation of Ser(907) was ERK dependent. Phosphorylation of Ser(907) did not prevent the insulin-induced association of IRS-2 with Grb2, but phosphorylation of the adjacent Tyr(911) was proved to be crucial in HEK 293 cells expressing IRS-2 Ala mutants. The insulin-induced phosphorylation of Ser(675) was prevented by inhibition and siRNA-mediated knockdown of mTOR but not of p70(S6K1). Mutation of Ser(675) to Ala did not affect downstream insulin signaling but increased the half-life of the protein, suggesting an involvement of phospho-Ser(675) in an accelerated degradation of IRS-2. Moreover, the insulin-induced degradation of IRS-2 was blocked by inhibition of mTOR. We conclude that the two novel insulin-dependent serine phosphorylation sites of IRS-2 were not involved in the regulation of the adjacent PI 3-kinase and Grb2 binding domains but might be implicated in the ERK- and mTOR-mediated negative feedback control.

Gestational Diabetes Impairs Human Fetal Postprandial Brain Activity

CONTEXT Gestational diabetes (GDM) influences the fetal phenotype. OBJECTIVE In the present study, our aim was to determine the effect of GDM specifically on fetal brain activity. DESIGN Pregnant participants underwent an oral glucose tolerance test (OGTT, 75 g). At 0, 60, and 120 minutes, maternal metabolism was determined, and fetal auditory evoked fields were recorded with a fetal magnetoencephalographic device. SETTING All measurements were performed at the fMEG Center in Tübingen. PARTICIPANTS Twelve women with GDM and 28 normal glucose-tolerant (NGT) pregnant women participated on a voluntary basis. INTERVENTIONS OGTT (75 g, 120 minutes) was used in this study. MAIN OUTCOMES AND MEASURES Fetal auditory evoked response latencies were determined for this study. RESULTS In the fetuses of NGT women, latencies decreased between 0 and 60 minutes from 260 ± 90 to 206 ± 74 ms (P = .008) and remained stable until 120 minutes (206 ± 74 vs 230 ± 79, P =.129). In fetuses of women with GDM, there was no change in response latencies during OGTT (P = .11). Sixty minutes after glucose ingestion, fetal latencies in the GDM group were longer than in the NGT group (296 ± 82 vs 206 ± 74 ms, P = .001). Linear regression revealed a significant effect of maternal glucose, insulin levels, and insulin sensitivity on response latencies after 60 minutes. CONCLUSIONS Fetal postprandial brain responses were slower in the offspring of women with GDM. This might indicate that gestational diabetes directly affects fetal brain development and may lead to central nervous insulin resistance in the fetus.

Olive oil aroma extract modulates cerebral blood flow in gustatory brain areas in humans

BACKGROUND Low- and high-fat meals affect homeostatic and gustatory brain areas differentially. In a previous study, we showed that a high-fat meal decreased cerebral blood flow (CBF) in homeostatic brain areas (hypothalamus), whereas a low-fat meal increased CBF in gustatory regions (anterior insula). OBJECTIVE The aim of this study was to investigate the long-lasting effect of fat-free flavor-active compounds of olive oil on the brain and whether those aroma components can trigger fat-associated brain responses in homeostatic and gustatory regions. DESIGN Eleven healthy male subjects participated in a functional magnetic resonance imaging study. On 2 measurement days, subjects consumed single-blinded a plain low-fat yogurt or low-fat yogurt mixed with a fat-free aroma extract of olive oil. Resting CBF was measured before and 30 and 120 min after yogurt intake. Hunger was rated before each measurement. Blood samples were collected at 6 time points. RESULTS The extract-containing yogurt elicited higher CBF in the frontal operculum 30 and 120 min after a meal. Furthermore, the activity change in the anterior insula after 30 min correlated positively with the glucose change in the extract condition only. No effects were observed in the hypothalamus. CONCLUSIONS The anterior insula and the frontal operculum are regarded as the primary taste cortex. Modulation of the frontal operculum by the yogurt containing the olive oil extract suggests that it might be possible to simulate fat-triggered sensations in the brain on the gustatory level, possibly by ingredients the body implicitly associates with fat. This trial was registered at clinicaltrials.gov as NCT01716286.

Gestational diabetes alters the fetal heart rate variability during an oral glucose tolerance test: a fetal magnetocardiography study

Gestational diabetes mellitus (GDM) potentially harms the child before birth. We previously found GDM to be associated with developmental changes in the central nervous system. We now hypothesise that GDM may also impact on the fetal autonomic nervous system under metabolic stress like an oral glucose tolerance test (OGTT).

Effects of resveratrol supplementation on liver fat content in overweight and insulin-resistant subjects: A randomized, double-blind, placebo-controlled clinical trial

We performed the largest randomized, placebo‐controlled clinical trial to date (N = 112, 12‐week intervention) to investigate the effects and safety of resveratrol supplementation on liver fat content and cardiometabolic risk parameters in overweight and obese and insulin‐resistant subjects. At baseline the variability in liver fat content was very large, ranging from 0.09% to 37.55% (median, 7.12%; interquartile range, 3.85%‐12.94%). Mean (SD) liver fat content was 9.22 (6.85) % in the placebo group and 9.91 (7.76) % in the resveratrol group. During the study liver fat content decreased in the placebo group (−0.7%) but not in the resveratrol group (−0.03%) (differences between groups: P = .018 for the intention‐to‐treat [ITT] population; N = 54, resveratrol, N = 54, placebo and P = .0077 for the per protocol [PP] population). No effects of resveratrol supplementation on cardiometabolic risk parameters were observed. Resveratrol supplementation was well tolerated and safe.

Genetic determination of body fat distribution and the attributive influence on metabolism: Genetics of Fat Distribution Impact on Metabolism

Genome‐wide association studies (GWAS) have identified single‐nucleotide polymorphisms (SNPs) associated with estimates of body fat distribution. Using predefined risk allele scores, the correlation of these scores with precisely quantified body fat distribution assessed by magnetic resonance (MR) imaging techniques and with metabolic traits was investigated.

FTO Genotype Interacts with Improvement in Aerobic Fitness on Body Weight Loss During Lifestyle Intervention.

Objective: Not every participant responds with a comparable body weight loss to lifestyle intervention, despite the same compliance. Genetic factors may explain parts of this difference. Variation in fat mass and obesity-associated gene (FTO) is the strongest common genetic determinant of body weight. The aim of the present study was to evaluate the impact of FTO genotype differences in the link between improvement of fitness and reduction of body weight during a lifestyle intervention. Methods: We genotyped 292 healthy subjects for FTO rs8050136. Participants underwent a 9-month lifestyle intervention. Before and after intervention, aerobic fitness was tested by bicycle (VO2max) and treadmill spiroergometry (individual anaerobic threshold (IAT), subgroup of N = 192). Results: Participants lost body weight (p < 0.0001) independent of FTO genotype (p = 0.5). There was a significant correlation between improvement in VO2max and decrease in body weight (p < 0.0001). FTO genotype interacted with this relationship (p = 0.0042 for VO2max, p = 0.0049 for IAT). When stratifying the cohort according to their improvement in VO2max, FTO obesity-risk A-allele carriers in the higher quartiles of improvement in fitness lost significantly less body weight. Conclusions: Our data reveal that genetic variation in FTO impacts on body weight reduction during lifestyle intervention only in subjects with marked improvement in aerobic fitness.

BMI-independent effects of gestational diabetes on human placenta.

Purpose Recently, alterations in maternal lipid metabolism were associated with gestational diabetes mellitus (GDM). However, detailed plasma lipid profiles and their relevance for placental and fetal metabolism are currently not understood. Methods Maternal and placental lipid profiles were characterized in women with GDM and women with normal glucose tolerance (NGT). Inflammatory gene expression was compared in placentas and primary term trophoblasts between the groups. In addition, trophoblasts were stimulated with nonesterified fatty acids (NEFAs), and effects on gene expression were quantified. Finally, placental macrophage content and cord blood concentrations of inflammatory parameters and NEFAs were compared between women with GDM and women with NGT with similar body mass index (BMI). Results Palmitate and stearate levels were elevated in both maternal plasma and placental tissue of women with GDM. Placental GDM-associated elevations of IL6, IL8, and TLR2 expression were reflected in trophoblasts derived from women with GDM. Stimulation of primary trophoblasts with palmitate led to increased mRNA expression and protein release of the cytokine IL6 and the chemokine IL8. In line with this, elevated amounts of CD68-positive cells were quantified in the placental tissue of women with GDM. No GDM-associated elevations in a range of inflammatory parameters and NEFAs in cord blood of NGT vs GDM neonates was found. Conclusions GDM, independently of BMI, altered maternal plasma NEFAs and the placental lipid profile. GDM was associated with trophoblast and whole-placenta lipoinflammation; however, this was not accompanied by elevated concentrations of inflammatory cytokines or NEFAs in neonatal cord blood.

Maternal whole blood cell miRNA-340 is elevated in gestational diabetes and inversely regulated by glucose and insulin

The number of pregnancies complicated by gestational diabetes (GDM) is increasing worldwide. To identify novel characteristics of GDM, we studied miRNA profiles of maternal and fetal whole blood cells (WBCs) from GDM and normal glucose tolerant (NGT) pregnant women matched for body mass index and maternal age. After adjustment for maternal weight gain and pregnancy week, we identified 29 mature micro-RNAs (miRNAs) up-regulated in GDM, one of which, i.e., miRNA-340, was validated by qPCR. mRNA and protein expression of PAIP1, a miRNA-340 target gene, was found down-regulated in GDM women, accordingly. In lymphocytes derived from the mothers’ blood and treated in vitro, insulin increased and glucose reduced miRNA-340 expression. In fetal cord blood samples, no associations of miRNA-340 with maternal GDM were observed. Our results provide evidence for insulin-induced epigenetic, i.e., miRNA-dependent, programming of maternal WBCs in GDM.