Type 2 diabetes mellitus (T2DM) has been related to alterations of oxidative metabolism in insulin-responsive tissues. Overt T2DM can present with acquired or inherited reductions of mitochondrial ...oxidative phosphorylation capacity, submaximal ADP-stimulated oxidative phosphorylation and plasticity of mitochondria and/or lower mitochondrial content in skeletal muscle cells and potentially also in hepatocytes. Acquired insulin resistance is associated with reduced insulin-stimulated mitochondrial activity as the result of blunted mitochondrial plasticity. Hereditary insulin resistance is frequently associated with reduced mitochondrial activity at rest, probably due to diminished mitochondrial content. Lifestyle and pharmacological interventions can enhance the capacity for oxidative phosphorylation and mitochondrial content and improve insulin resistance in some (pre)diabetic cases. Various mitochondrial features can be abnormal but are not necessarily responsible for all forms of insulin resistance. Nevertheless, mitochondrial abnormalities might accelerate progression of insulin resistance and subsequent organ dysfunction via increased production of reactive oxygen species. This Review discusses the association between mitochondrial function and insulin sensitivity in various tissues, such as skeletal muscle, liver and heart, with a main focus on studies in humans, and addresses the effects of therapeutic strategies that affect mitochondrial function and insulin sensitivity.
Muscle insulin resistance is a key feature of obesity and type 2 diabetes and is strongly associated with increased intramyocellular lipid content and inflammation. However, the cellular and ...molecular mechanisms responsible for causing muscle insulin resistance in humans are still unclear. To address this question, we performed serial muscle biopsies in healthy, lean subjects before and during a lipid infusion to induce acute muscle insulin resistance and assessed lipid and inflammatory parameters that have been previously implicated in causing muscle insulin resistance. We found that acute induction of muscle insulin resistance was associated with a transient increase in total and cytosolic diacylglycerol (DAG) content that was temporally associated with protein kinase (PKC)θ activation, increased insulin receptor substrate (IRS)-1 serine 1101 phosphorylation, and inhibition of insulin-stimulated IRS-1 tyrosine phosphorylation and AKT2 phosphorylation. In contrast, there were no associations between insulin resistance and alterations in muscle ceramide, acylcarnitine content, or adipocytokines (interleukin-6, adiponectin, retinol-binding protein 4) or soluble intercellular adhesion molecule-1. Similar associations between muscle DAG content, PKCθ activation, and muscle insulin resistance were observed in healthy insulin-resistant obese subjects and obese type 2 diabetic subjects. Taken together, these data support a key role for DAG activation of PKCθ in the pathogenesis of lipid-induced muscle insulin resistance in obese and type 2 diabetic individuals.
Epidemiological studies have shown that increased circulating branched-chain amino acids (BCAAs) are associated with insulin resistance and type 2 diabetes (T2D). This may result from altered energy ...metabolism or dietary habits.
We hypothesized that a lower intake of BCAAs improves tissue-specific insulin sensitivity.
This randomized, placebo-controlled, double-blinded, crossover trial examined well-controlled T2D patients receiving isocaloric diets (protein: 1 g/kg body weight) for 4 wk. Protein requirements were covered by commercially available food supplemented ≤60% by an AA mixture either containing all AAs or lacking BCAAs. The dietary intervention ensured sufficient BCAA supply above the recommended minimum daily intake. The patients underwent the mixed meal tolerance test (MMT), hyperinsulinemic-euglycemic clamps (HECs), and skeletal muscle and white adipose tissue biopsies to assess insulin signaling.
After the BCAA− diet, BCAAs were reduced by 17% during fasting (P < 0.001), by 13% during HEC (P < 0.01), and by 62% during the MMT (P < 0.001). Under clamp conditions, whole-body and hepatic insulin sensitivity did not differ between diets. After the BCAA− diet, however, the oral glucose sensitivity index was 24% (P < 0.01) and circulating fibroblast-growth factor 21 was 21% higher (P < 0.05), whereas meal-derived insulin secretion was 28% lower (P < 0.05). Adipose tissue expression of the mechanistic target of rapamycin was 13% lower, whereas the mitochondrial respiratory control ratio was 1.7-fold higher (both P < 0.05). The fecal microbiome was enriched in Bacteroidetes but depleted of Firmicutes.
Short-term dietary reduction of BCAAs decreases postprandial insulin secretion and improves white adipose tissue metabolism and gut microbiome composition. Longer-term studies will be needed to evaluate the safety and metabolic efficacy in diabetes patients.
This trial was registered at clinicaltrials.gov as NCT03261362.
The rs738409(G) single nucleotide polymorphism (SNP) in the patatin-like phospholipase domain-containing 3 (
) gene associates with increased risk and progression of nonalcoholic fatty liver disease ...(NAFLD). As the recently described severe insulin-resistant diabetes (SIRD) cluster specifically relates to NAFLD, this study examined whether this SNP differently associates with hepatic lipid content (hepatocellular lipids HCL) and insulin sensitivity in recent-onset diabetes.
A total of 917 participants in the German Diabetes Study (GDS) underwent genotyping, hyperinsulinemic-euglycemic clamps with stable isotopic tracer dilution, and MRS.
The G allele associated positively with HCL (β = 0.36,
< 0.01), independent of age, sex, and BMI across the whole cohort, but not in the individual clusters. Those with SIRD exhibited lowest whole-body insulin sensitivity compared with those with severe insulin-deficient (SIDD), moderate obesity-related (MOD), moderate age-related (MARD), and severe autoimmune diabetes (SAID) clusters (all
< 0.001). Interestingly, the SIRD group presented with higher prevalence of the rs738409(G) SNP compared with other clusters and the glucose-tolerant control group (
< 0.05). HCL was higher in the SIRD group (median 13.6% 1st quartile 5.8; 3rd quartile 19.1 compared with the MOD (6.4 % 2.1; 12.4,
< 0.05), MARD (3.0% 1.0; 7.9,
< 0.001), SAID (0.4% 0.0; 1.5,
< 0.001), and glucose-tolerant (0.9% 0.4; 4.9),
< 0.001) group. Although the
polymorphism did not directly associate with whole-body insulin sensitivity in SIRD, the G-allele carriers had higher circulating free fatty acid concentrations and greater adipose tissue insulin resistance compared with noncarriers (both
< 0.001).
Members of the SIRD cluster are more frequently carriers of the rs738409(G) variant. The SNP-associated adipose tissue insulin resistance and excessive lipolysis may contribute to their NAFLD.
Aims/hypothesis
Emerging evidence suggests that in addition to hyperglycaemia, dyslipidaemia could represent a contributing pathogenetic factor to diabetic neuropathy, while obesity and insulin ...resistance play a role in the development of diabetic cardiac autonomic neuropathy (CAN) characterised by reduced heart rate variability (HRV), particularly in type 2 diabetes. We hypothesised that distinct lipid metabolites are associated with diminished HRV in recent-onset type 2 diabetes rather than type 1 diabetes.
Methods
We analysed 127 plasma lipid metabolites (11 acylcarnitines, 39 NEFA, 12 sphingomyelins (SMs), 56 phosphatidylcholines and nine lysophosphatidylcholines) using MS in participants from the German Diabetes Study baseline cohort recently diagnosed with type 1 (
n
= 100) and type 2 diabetes (
n
= 206). Four time-domain HRV indices (number of normal-to-normal (NN) intervals >50 ms divided by the number of all NN intervals pNN50; root mean square of successive differences RMSSD; SD of NN intervals SDNN; and SD of differences between adjacent NN intervals) and three frequency-domain HRV indices (very-low-frequency VLF, low-frequency LF and high-frequency HF power spectrum) were computed from NN intervals recorded during a 3 h hyperinsulinaemic–euglycaemic clamp at baseline and in subsets of participants with type 1 (
n
= 60) and type 2 diabetes (
n
= 95) after 5 years.
Results
In participants with type 2 diabetes, after Bonferroni correction and rigorous adjustment, SDNN was inversely associated with higher levels of diacyl-phosphatidylcholine (PCaa) C32:0, PCaa C34:1, acyl-alkyl-phosphatidylcholine (PCae) C36:0, SM C16:0 and SM C16:1. SD of differences between NN intervals was inversely associated with PCaa C32:0, PCaa C34:1, PCaa C34:2, PCae C36:0 and SM C16:1, and RMSSD with PCae C36:0. For VLF power, inverse associations were found with PCaa C30:0, PCaa C32:0, PCaa C32:1, PCaa C34:2 and SM C16:1, and for LF power inverse associations were found with PCaa C32:0 and SM C16:1 (
r
= −0.242 to
r
= −0.349;
p
≤ 0.0005 for all correlations). In contrast, no associations of lipid metabolites with measures of cardiac autonomic function were noted in participants recently diagnosed with type 1 diabetes. After 5 years, HRV declined due to ageing rather than diabetes, whereby prediction analyses for lipid metabolites were hampered.
Conclusions/interpretation
Higher plasma levels of specific lipid metabolites are closely linked to cardiac autonomic dysfunction in recent-onset type 2 diabetes but not type 1 diabetes, suggesting a role for perturbed lipid metabolism in the early development of CAN in type 2 diabetes.
Graphical abstract
Nonalcoholic fatty liver disease is associated with hepatic insulin resistance and may result primarily from increased hepatic de novo lipogenesis (PRIM) or secondarily from adipose tissue lipolysis ...(SEC). We studied mice with hepatocyte- or adipocyte-specific SREBP-1c overexpression as models of PRIM and SEC. PRIM mice featured increased lipogenic gene expression in the liver and adipose tissue. Their selective, liver-specific insulin resistance was associated with increased C18:1-diacylglycerol content and protein kinase Cε translocation. SEC mice had decreased lipogenesis mediated by hepatic cholesterol responsive element-binding protein and featured portal/lobular inflammation along with total, whole-body insulin resistance. Hepatic mitochondrial respiration transiently increased and declined with aging along with higher muscle reactive oxygen species production. In conclusion, hepatic insulin resistance originates from lipotoxicity but not from lower mitochondrial capacity, which can even transiently adapt to increased peripheral lipolysis. Peripheral insulin resistance is prevented during increased hepatic lipogenesis only if adipose tissue lipid storage capacity is preserved.
Advanced glycation end-products (AGEs) form at an accelerated rate in diabetes and are associated with the development of diabetic complications. So far, the effect of AGEs on proteases was mostly ...investigated with substrates modified non-specifically with hexoses amongst others in combination with proteases of broad specificity. Here, we investigated the effect of AGEs on the proinsulin processing peptidases preprotein convertase 1 (PC1) and carboxypeptidase E (CPE) via two ways: - Measuring of activity towards peptide substrates. These contained chemically defined AGEs carboxymethyllysine (CML) or methylglyoxyl-hydroimidazolone-1 (MGH1) or unmodified amino acids. - Recombinant human proinsulin was incubated with methylglyoxal for 24 h and was processed by addition of PC1 and CPE in vitro. Formation of insulin was compared to unmodified proinsulin. In parallel formation of AGE-modified proinsulin was quantified.
Artificial peptide substrates were synthesized by solid phase peptide synthesis and purified by HPLC. A protected building block was used for MGH1 incorporation while CML was introduced on the resin. Protease activity towards artificial substrates was measured by fluorescence assay or LC MS/MS. Proinsulin, insulin and AGE-modified proinsulin were quantified by LC MS.
Strong activity of PC1 and CPE was seen towards arginine containing substrates. Activity was completely absent towards modified amino acids MGH1 and CML for both, PC1 and CPE. Incubation of proinsulin with MG resulted in MGH1 modified proinsulin. At low MG (µM) concentrations this was associated with a 25 % reduction of insulin formation while high MG concentrations (95 µM) led to a 75 % reduction in insulin formation.
Here we demonstrate for the first time the effect of substrate modifications with AGEs on physiologically important proteases PC1 and CPE. The latter are not only involved in the processing of proinsulin but a range of endocrinologically and neurophysiologically important peptide hormones.
Disclosure
W. Mier: None. B. Beijer: None. J.M. Szendroedi: Consultant; Boehringer Ingelheim International GmbH. T.H. Fleming: None.
Funding
DFG (SFB 1118)
Fasting hyperglycemia in type 2 diabetes mellitus (T2DM) results from elevated endogenous glucose production (EGP), which is mostly due to augmented hepatic gluconeogenesis. Insulin-resistant humans ...exhibit impaired insulin-dependent suppression of EGP and excessive hepatic lipid storage (steatosis), which relates to abnormal supply of free fatty acids (FFA) and energy metabolism. Only two glucose-lowering drug classes, the biguanide metformin and the thiazolidendiones (TZDs), exert insulin- and glucagon-independent hepatic effects. Preclinical studies suggest that metformin inhibits mitochondrial complex I. TZDs, as peroxisome proliferator-activated receptor (PPAR) γ-agonists, predominantly reduce the flux of FFA and cytokines from adipose tissue to the liver, but could also directly inhibit mitochondrial complex I. Although both metformin and TZDs improve fasting hyperglycemia and EGP in clinical trials, only TZDs decrease steatosis and peripheral insulin resistance. More studies are required to address their effects on hepatocellular energy metabolism with a view to identifying novel targets for the treatment of T2DM.
Aims/hypothesis
The individual risk of progression of diabetic peripheral neuropathy is difficult to predict for each individual. Mutations in proteins that are responsible for the process of ...myelination are known to cause neurodegeneration and display alteration in experimental models of diabetic neuropathy. In a prospective observational human pilot study, we investigated myelin-specific circulating mRNA targets, which have been identified in vitro, for their capacity in the diagnosis and prediction of diabetic neuropathy. The most promising candidate was tested against the recently established biomarker of neural damage, neurofilament light chain protein.
Methods
Schwann cells were cultured under high-glucose conditions and mRNAs of various myelin-specific genes were screened intra- and extracellularly. Ninety-two participants with type 2 diabetes and 30 control participants were enrolled and evaluated for peripheral neuropathy using neuropathy deficit scores, neuropathy symptom scores and nerve conduction studies as well as quantitative sensory testing at baseline and after 12/24 months of a follow-up period. Magnetic resonance neurography of the sciatic nerve was performed in 37 individuals. Neurofilament light chain protein and four myelin-specific mRNA transcripts derived from in vitro screenings were measured in the serum of all participants. The results were tested for associations with specific neuropathic deficits, fractional anisotropy and the progression of neuropathic deficits at baseline and after 12 and 24 months.
Results
In neuronal Schwann cells and human nerve sections, myelin protein zero was identified as the strongest candidate for a biomarker study. Circulating mRNA of myelin protein zero was decreased significantly in participants with diabetic neuropathy (
p
< 0.001), whereas neurofilament light chain protein showed increased levels in participants with diabetic neuropathy (
p
< 0.05). Both variables were linked to altered electrophysiology, fractional anisotropy and quantitative sensory testing. In a receiver-operating characteristic curve analysis myelin protein zero improved the diagnostic performance significantly in combination with a standard model (diabetes duration, age, BMI, HbA
1c
) from an AUC of 0.681 to 0.836 for the detection of diabetic peripheral neuropathy. A follow-up study revealed that increased neurofilament light chain was associated with the development of a hyperalgesic phenotype (
p
< 0.05), whereas decreased myelin protein zero predicted hypoalgesia (
p
< 0.001) and progressive loss of nerve function 24 months in advance (HR of 6.519).
Conclusions/interpretation
This study introduces a dynamic and non-invasive assessment strategy for the underlying pathogenesis of diabetic peripheral neuropathy. The diagnosis of axonal degeneration, associated with hyperalgesia, and demyelination, linked to hypoalgesia, could benefit from the usage of neurofilament light chain protein and circulating mRNA of myelin protein zero as potential biomarkers.
Graphical abstract
Aims/hypothesis
The euglycaemic–hyperinsulinaemic clamp is the gold-standard method for measuring insulin sensitivity, but is less suitable for large clinical trials. Thus, several indices have been ...developed for evaluating insulin sensitivity from the oral glucose tolerance test (OGTT). However, most of them yield values different from those obtained by the clamp method. The aim of this study was to develop a new index to predict clamp-derived insulin sensitivity (
M
value) from the OGTT-derived oral glucose insulin sensitivity index (OGIS).
Methods
We analysed datasets of people that underwent both a clamp and an OGTT or meal test, thereby allowing calculation of both the
M
value and OGIS. The population was divided into a training and a validation cohort (
n
= 359 and
n
= 154, respectively). After a stepwise selection approach, the best model for
M
value prediction was applied to the validation cohort. This cohort was also divided into subgroups according to glucose tolerance, obesity category and age.
Results
The new index, called PREDIcted M (PREDIM), was based on OGIS, BMI, 2 h glucose during OGTT and fasting insulin. Bland–Altman analysis revealed a good relationship between the
M
value and PREDIM in the validation dataset (only 9 of 154 observations outside limits of agreement). Also, no significant differences were found between the
M
value and PREDIM (equivalence test:
p
< 0.0063). Subgroup stratification showed that measured
M
value and PREDIM have a similar ability to detect intergroup differences (
p
< 0.02, both
M
value and PREDIM).
Conclusions/interpretation
The new index PREDIM provides excellent prediction of
M
values from OGTT or meal data, thereby allowing comparison of insulin sensitivity between studies using different tests.
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