Impaired postprandial skeletal muscle vascular responses to a mixed meal challenge in normoglycaemic people with a parent with type 2 diabetes

AIMS/HYPOTHESIS

Microvascular blood flow (MBF) increases in skeletal muscle postprandially to aid in glucose delivery and uptake in muscle. This vascular action is impaired in individuals who are obese or have type 2 diabetes. Whether MBF is impaired in normoglycaemic people at risk of type 2 diabetes is unknown. We aimed to determine whether apparently healthy people at risk of type 2 diabetes display impaired skeletal muscle microvascular responses to a mixed-nutrient meal.

METHODS

In this cross-sectional study, participants with no family history of type 2 diabetes (FH-) for two generations (n = 18), participants with a positive family history of type 2 diabetes (FH+; i.e. a parent with type 2 diabetes; n = 16) and those with type 2 diabetes (n = 12) underwent a mixed meal challenge (MMC). Metabolic responses (blood glucose, plasma insulin and indirect calorimetry) were measured before and during the MMC. Skeletal muscle large artery haemodynamics (2D and Doppler ultrasound, and Mobil-O-graph) and microvascular responses (contrast-enhanced ultrasound) were measured at baseline and 1 h post MMC.

RESULTS

Despite normal blood glucose concentrations, FH+ individuals displayed impaired metabolic flexibility (reduced ability to switch from fat to carbohydrate oxidation vs FH-; p < 0.05) during the MMC. The MMC increased forearm muscle microvascular blood volume in both the FH- (1.3-fold, p < 0.01) and FH+ (1.3-fold, p < 0.05) groups but not in participants with type 2 diabetes. However, the MMC increased MBF (1.9-fold, p < 0.01), brachial artery diameter (1.1-fold, p < 0.01) and brachial artery blood flow (1.7-fold, p < 0.001) and reduced vascular resistance (0.7-fold, p < 0.001) only in FH- participants, with these changes being absent in FH+ and type 2 diabetes. Participants with type 2 diabetes displayed significantly higher vascular stiffness (p < 0.001) compared with those in the FH- and FH+ groups; however, vascular stiffness did not change during the MMC in any participant group.

CONCLUSIONS/INTERPRETATION

Normoglycaemic FH+ participants display impaired postprandial skeletal muscle macro- and microvascular responses, suggesting that poor vascular responses to a meal may contribute to their increased risk of type 2 diabetes. We conclude that vascular insulin resistance may be an early precursor to type 2 diabetes in humans, which can be revealed using an MMC.

Epigenetic modifications of the Zfp/ZNF423 gene control murine adipogenic commitment and are dysregulated in human hypertrophic obesity

AIMS/HYPOTHESIS

Subcutaneous adipocyte hypertrophy is associated with insulin resistance and increased risk of type 2 diabetes, and predicts its future development independent of obesity. In humans, subcutaneous adipose tissue hypertrophy is a consequence of impaired adipocyte precursor cell recruitment into the adipogenic pathway rather than a lack of precursor cells. The zinc finger transcription factor known as zinc finger protein (ZFP) 423 has been identified as a major determinant of pre-adipocyte commitment and maintained white adipose cell function. Although its levels do not change during adipogenesis, ectopic expression of Zfp423 in non-adipogenic murine cells is sufficient to activate expression of the gene encoding peroxisome proliferator-activated receptor γ (Pparγ; also known as Pparg) and increase the adipogenic potential of these cells. We investigated whether the Zfp423 gene is under epigenetic regulation and whether this plays a role in the restricted adipogenesis associated with hypertrophic obesity.

METHODS

Murine 3T3-L1 and NIH-3T3 cells were used as fibroblasts committed and uncommitted to the adipocyte lineage, respectively. Human pre-adipocytes were isolated from the stromal vascular fraction of subcutaneous adipose tissue of 20 lean non-diabetic individuals with a wide adipose cell size range. mRNA levels were measured by quantitative real-time PCR, while methylation levels were analysed by bisulphite sequencing. Chromatin structure was analysed by micrococcal nuclease protection assay, and DNA-methyltransferases were chemically inhibited by 5-azacytidine. Adipocyte differentiation rate was evaluated by Oil Red O staining.

RESULTS

Comparison of uncommitted (NIH-3T3) and committed (3T3-L1) adipose precursor cells revealed that Zfp423 expression increased (p < 0.01) in parallel with the ability of the cells to differentiate into mature adipocytes owing to both decreased promoter DNA methylation (p < 0.001) and nucleosome occupancy (nucleosome [NUC] 1 p < 0.01; NUC2 p < 0.001) in the 3T3-L1 compared with NIH-3T3 cells. Interestingly, non-adipogenic epigenetic profiles can be reverted in NIH-3T3 cells as 5-azacytidine treatment increased Zfp423 mRNA levels (p < 0.01), reduced DNA methylation at a specific CpG site (p < 0.01), decreased nucleosome occupancy (NUC1, NUC2: p < 0.001) and induced adipocyte differentiation (p < 0.05). These epigenetic modifications can also be initiated in response to changes in the pre-adipose cell microenvironment, in which bone morphogenetic protein 4 (BMP4) plays a key role. We finally showed that, in human adipocyte precursor cells, impaired epigenetic regulation of zinc nuclear factor (ZNF)423 (the human orthologue of murine Zfp423) was associated with inappropriate subcutaneous adipose cell hypertrophy. As in NIH-3T3 cells, the normal ZNF423 epigenetic profile was rescued by 5-azacytidine exposure.

CONCLUSIONS/INTERPRETATION

Our results show that epigenetic events regulate the ability of precursor cells to commit and differentiate into mature adipocytes by modulating ZNF423, and indicate that dysregulation of these mechanisms accompanies subcutaneous adipose tissue hypertrophy in humans.

Glucose and fatty acids synergistically and reversibly promote beta cell proliferation in rats

AIMS/HYPOTHESIS

The mechanisms underlying pancreatic islet mass expansion have attracted considerable interest as potential therapeutic targets to prevent or delay the onset of type 2 diabetes. While several factors promoting beta cell proliferation have been identified, in the context of nutrient excess the roles of glucose or NEFA in relation to insulin resistance remain unclear. Here we tested the hypothesis that glucose and NEFA synergistically and reversibly promote beta cell proliferation in the context of nutrient-induced insulin resistance.

METHODS

Using 72 h infusions of glucose (GLU) or the oleate-enriched lipid emulsion ClinOleic (CLI), singly or in combination, we assessed beta cell proliferation, islet mass and insulin sensitivity in male Lewis rats. The effects of nutrients and endogenous circulating factors were examined in isolated and transplanted islets. Reversibility was studied 3 and 6 days after the end of the infusion.

RESULTS

GLU infusions modestly stimulated beta cell proliferation, CLI alone had no effect and GLU+CLI infusions markedly stimulated beta cell proliferation. Insulin sensitivity was equally decreased in GLU and GLU+CLI infusions. GLU+CLI infusions also stimulated beta cell proliferation in islets transplanted under the kidney capsule, albeit to a lesser extent compared with endogenous islets. Ex vivo, the combination of glucose and NEFA enhanced beta cell proliferation in rat and human islets independently from secreted insulin, and serum from GLU+CLI-infused rats potentiated the effect of glucose. Glucose tolerance, beta cell proliferation and islet mass were all restored to normal levels 6 days after termination of the infusion.

CONCLUSIONS/INTERPRETATION

Glucose and NEFA synergistically and reversibly promote beta cell proliferation in part via direct action on the beta cell and independently from secreted insulin.

Effects of exercise training on intrahepatic lipid content in humans

Non-alcoholic fatty liver (NAFL) is the most common liver disorder in western society. Various factors may play a role in determining hepatic fat content, such as delivery of lipids to the liver, de novo lipogenesis, hepatic lipid oxidation, secretion of intrahepatic lipids to the circulation or a combination of these. If delivery of lipids to the liver outweighs the sum of hepatic lipid oxidation and secretion, the intrahepatic lipid (IHL) content starts to increase and NAFL may develop. NAFL is closely related to obesity and insulin resistance and a fatty liver increases the vulnerability to type 2 diabetes development. Exercise training is a cornerstone in the treatment and prevention of type 2 diabetes. There is a large body of literature describing the beneficial metabolic consequences of exercise training on skeletal muscle metabolism. Recent studies have started to investigate the effects of exercise training on liver metabolism but data is still limited. Here, first, we briefly discuss the routes by which IHL content is modulated. Second, we review whether and how these contributing routes might be modulated by long-term exercise training. Third, we focus on the effects of acute exercise on IHL metabolism, since exercise also might affect hepatic metabolism in the physically active state. This will give insight into whether the effect of exercise training on IHL could be explained by the accumulated effect of acute bouts of exercise, or whether adaptations might occur only after long-term exercise training. The primary focus of this review will be on observations made in humans. Where human data is missing, data obtained from well-accepted animal models will be used.

Regulation of glucose homeostasis and insulin action by ceramide acyl-chain length: A beneficial role for very long-chain sphingolipid species

In a recent study, we showed that in response to high fat feeding C57BL/6, 129X1, DBA/2 and FVB/N mice all developed glucose intolerance, while BALB/c mice displayed minimal deterioration in glucose tolerance and insulin action. Lipidomic analysis of livers across these five strains has revealed marked strain-specific differences in ceramide (Cer) and sphingomyelin (SM) species with high-fat feeding; with increases in C16-C22 (long-chain) and reductions in C>22 (very long-chain) Cer and SM species observed in the four strains that developed HFD-induced glucose intolerance. Intriguingly, the opposite pattern was observed in sphingolipid species in BALB/c mice. These strain-specific changes in sphingolipid acylation closely correlated with ceramide synthase 2 (CerS2) protein content and activity, with reduced CerS2 levels/activity observed in glucose intolerant strains and increased content in BALB/c mice. Overexpression of CerS2 in primary mouse hepatocytes induced a specific elevation in very long-chain Cer, but despite the overall increase in ceramide abundance, there was a substantial improvement in insulin signal transduction, as well as decreased ER stress and gluconeogenic markers. Overall our findings suggest that very long-chain sphingolipid species exhibit a protective role against the development of glucose intolerance and hepatic insulin resistance.

The relationship between BMI and insulin resistance and progression from single to multiple autoantibody positivity and type 1 diabetes among TrialNet Pathway to Prevention participants

AIMS/HYPOTHESIS

The incidence of type 1 diabetes is increasing at a rate of 3-5% per year. Genetics cannot fully account for this trend, suggesting an influence of environmental factors. The accelerator hypothesis proposes an effect of metabolic factors on type 1 diabetes risk. To test this in the TrialNet Pathway to Prevention (PTP) cohort, we analysed the influence of BMI, weight status and insulin resistance on progression from single to multiple islet autoantibodies (Aab) and progression from normoglycaemia to diabetes.

METHODS

HOMA1-IR was used to estimate insulin resistance in Aab-positive PTP participants. Cox proportional hazards models were used to evaluate the effects of BMI, BMI percentile (BMI%), weight status and HOMA1-IR on the progression of autoimmunity or the development of diabetes.

RESULTS

Data from 1,310 single and 1,897 multiple Aab-positive PTP participants were included. We found no significant relationships between BMI, BMI%, weight status or HOMA1-IR and the progression from one to multiple Aabs. Similarly, among all Aab-positive participants, no significant relationships were found between BMI, weight status or HOMA1-IR and progression to diabetes. Diabetes risk was modestly increased with increasing BMI% among the entire cohort, in obese participants 13-20 years of age and with increasing HOMA1-IR in adult Aab-positive participants.

CONCLUSIONS/INTERPRETATION

Analysis of the accelerator hypothesis in the TrialNet PTP cohort does not suggest a broad influence of metabolic variables on diabetes risk. Efforts to identify other potentially modifiable environmental factors should continue.

A short leucocyte telomere length is associated with development of insulin resistance

AIMS/HYPOTHESIS

A number of studies have shown that leucocyte telomere length (LTL) is inversely associated with insulin resistance and type 2 diabetes mellitus. The aim of the present longitudinal cohort study, utilising a twin design, was to assess whether shorter LTL predicts insulin resistance or is a consequence thereof.

METHODS

Participants were recruited between 1997 and 2000 through the population-based national Danish Twin Registry to participate in the GEMINAKAR study, a longitudinal evaluation of metabolic disorders and cardiovascular risk factors. Baseline and follow-up measurements of LTL and insulin resistance over an average of 12 years were performed in a subset of the Registry consisting of 338 (184 monozygotic and 154 dizygotic) same-sex twin pairs.

RESULTS

Age at baseline examination was 37.4 ± 9.6 (mean ± SD) years. Baseline insulin resistance was not associated with age-dependent changes in LTL (attrition) over the follow-up period, whereas baseline LTL was associated with changes in insulin resistance during this period. The shorter the LTL at baseline, the more pronounced was the increase in insulin resistance over the follow-up period (p < 0.001); this effect was additive to that of BMI. The co-twin with the shorter baseline LTL displayed higher insulin resistance at follow-up than the co-twin with the longer LTL.

CONCLUSIONS/INTERPRETATION

These findings suggest that individuals with short LTL are more likely to develop insulin resistance later in life. By contrast, presence of insulin resistance does not accelerate LTL attrition.

Insulin treatment promotes tyrosine phosphorylation of PKR and inhibits polyIC induced PKR threonine phosphorylation

Tyrosine phosphorylation of insulin receptor beta (IRβ) in insulin treated HepG2 cells is inversely correlated to ser(51) phosphorylation in the alpha-subunit of eukaryotic initiation factor 2 (eIF2α) that regulates protein synthesis. Insulin stimulates interaction between IRβ and PKR, double stranded RNA-dependent protein kinase, also known as EIF2AK2, and phosphorylation of tyrosine residues in PKR, as analyzed by immunoprecipitation and pull down assays using anti-IRβ and anti-phosphotyrosine antibodies, recombinant IRβ and immunopurified PKR. Further polyIC or synthetic double stranded RNA-induced threonine phosphorylation or activation of immunopurified and cellular PKR is suppressed in the presence of insulin treated purified IRβ and cell extracts. Acute, but not chronic, insulin treatment enhances tyrosine phosphorylation of IRβ, its interaction with PKR and tyrosine phosphorylation of PKR. In contrast, lipopolysaccharide that stimulates threonine phosphorylation of PKR and eIF2α phosphorylation and AG 1024, an inhibitor of the tyrosine kinase activity of IRβ, reduces PKR association with the receptor, IRβ in HepG2 cells. These findings therefore may suggest that tyrosine phosphorylated PKR plays a role in the regulation of insulin induced protein synthesis and in maintaining insulin sensitivity, whereas, suppression of polyIC-mediated threonine phosphorylation of PKR by insulin compromises its ability to fight against virus infection in host cells.