Exercise training modifies skeletal muscle clock gene expression but not 24-hour rhythmicity in substrate metabolism of men with insulin resistance

Twenty-four hour rhythmicity in whole-body substrate metabolism, skeletal muscle clock gene expression and mitochondrial respiration is compromised upon insulin resistance. With exercise training known to ameliorate insulin resistance, our objective was to test if exercise training can reinforce diurnal variation in whole-body and skeletal muscle metabolism in men with insulin resistance. In a single-arm longitudinal design, 10 overweight and obese men with insulin resistance performed 12 weeks of high-intensity interval training recurrently in the afternoon (between 14.00 and 18.00 h) and were tested pre- and post-exercise training, while staying in a metabolic research unit for 2 days under free-living conditions with regular meals. On the second days, indirect calorimetry was performed at 08.00, 13.00, 18.00, 23.00 and 04.00 h, muscle biopsies were taken from the vastus lateralis at 08.30, 13.30 and 23.30 h, and blood was drawn at least bi-hourly over 24 h. Participants did not lose body weight over 12 weeks, but improved body composition and exercise capacity. Exercise training resulted in reduced 24-h plasma glucose levels, but did not modify free fatty acid and triacylglycerol levels. Diurnal variation of muscle clock gene expression was modified by exercise training with period genes showing an interaction (time × exercise) effect and reduced mRNA levels at 13.00 h. Exercise training increased mitochondrial respiration without inducing diurnal variation. Twenty-four-hour substrate metabolism and energy expenditure remained unchanged. Future studies should investigate alternative exercise strategies or types of interventions (e.g. diet or drugs aiming at improving insulin sensitivity) for their capacity to reinforce diurnal variation in substrate metabolism and mitochondrial respiration. KEY POINTS: Insulin resistance is associated with blunted 24-h flexibility in whole-body substrate metabolism and skeletal muscle mitochondrial respiration, and disruptions in the skeletal muscle molecular circadian clock. We hypothesized that exercise training modifies 24-h rhythmicity in whole-body substrate metabolism and diurnal variation in skeletal muscle molecular clock and mitochondrial respiration in men with insulin resistance. We found that metabolic inflexibility over 24 h persisted after exercise training, whereas mitochondrial respiration increased independent of time of day. Gene expression of Per1-3 and Rorα in skeletal muscle changed particularly close to the time of day at which exercise training was performed. These results provide the rationale to further investigate the differential metabolic impact of differently timed exercise to treat metabolic defects of insulin resistance that manifest at a particular time of day.

Twenty-four hour rhythmicity in mitochondrial network connectivity and mitochondrial respiration; a study in human skeletal muscle biopsies of young lean and older individuals with obesity

OBJECTIVE

Mitochondrial network dynamics may play role in metabolic homeostasis. Whether mitochondrial network dynamics are involved in adaptations to day-night fluctuations in energy supply and demand is unclear. Here we visualized and quantified the mitochondrial network morphology in human skeletal muscle of young healthy lean and older individuals with obesity over the course of 24 hours METHODS: Muscle biopsies taken at 5 timepoints over a 24-hour period obtained from young healthy lean and older metabolically impaired obese males were analyzed for mitochondrial network integrity with confocal laser scanning microscopy. Variation of level of fragmentation over the course of the day were aligned with variation of mitochondrial respiration over the day RESULTS: Young healthy lean individuals displayed a day-night rhythmicity in mitochondrial network morphology, which aligned with the day-night rhythmicity of mitochondrial respiratory capacity, with a more fused network coinciding with higher mitochondrial respiratory capacity. In the older individuals with obesity, the mitochondrial network was more fragmented overall compared to young healthy lean individuals and completely lacked 24 hour rhythmicity, which was also true for the mitochondrial respiratory capacity CONCLUSIONS: Our data shows a paralleled rhythmicity between mitochondrial network morphology and mitochondrial oxidative capacity, which oscillates over the course of a mimicked real-life day in human skeletal muscle of young, healthy lean individuals. In older individuals with obesity, the lack of a 24-hour rhythmicity in mitochondrial network connectivity was also aligned with a lack in respiratory capacity. This suggests that 24-hour rhythmicity in mitochondrial network connectivity is a determinant of rhythmicity in mitochondrial respiratory capacity. Thus, restoring mitochondrial network integrity may promote mitochondrial respiratory capacity and hence contribute to blunting the metabolic aberrations in individuals with a disturbed 24-hour rhythmicity in metabolism, like older individuals with obesity.

Effects of SGLT2 inhibitor dapagliflozin in patients with type 2 diabetes on skeletal muscle cellular metabolism

OBJECTIVE

SGLT2 inhibitors increase urinary glucose excretion and have beneficial effects on cardiovascular and renal outcomes; the underlying mechanism may be metabolic adaptations due to urinary glucose loss. Here, we investigated the cellular and molecular effects of 5 weeks of dapagliflozin treatment on skeletal muscle metabolism in type 2 diabetes patients.

METHODS

Twenty-six type 2 diabetes mellitus patients were randomized to a 5-week double-blind, cross-over study with 6-8-week wash-out. Skeletal muscle acetylcarnitine levels, intramyocellular lipid (IMCL) content and phosphocreatine (PCr) recovery rate were measured by magnetic resonance spectroscopy (MRS). Ex vivo mitochondrial respiration was measured in skeletal muscle fibers using high resolution respirometry. Intramyocellular lipid droplet and mitochondrial network dynamics were investigated using confocal microscopy. Skeletal muscle levels of acylcarnitines, amino acids and TCA cycle intermediates were measured. Expression of genes involved in fatty acid metabolism were investigated.

RESULTS

Mitochondrial function, mitochondrial network integrity and citrate synthase and carnitine acetyltransferase activities in skeletal muscle were unaltered after dapagliflozin treatment. Dapagliflozin treatment increased intramyocellular lipid content (0.060 (0.011, 0.110) %, p = 0.019). Myocellular lipid droplets increased in size (0.03 μm² (0.01-0.06), p < 0.05) and number (0.003 μm^-2 (-0.001-0.007), p = 0.09) upon dapagliflozin treatment. CPT1A, CPT1B and malonyl CoA-decarboxylase mRNA expression was increased by dapagliflozin. Fasting acylcarnitine species and C4-OH carnitine levels (0.4704 (0.1246, 0.8162) pmoles∗mg tissue^-1, p < 0.001) in skeletal muscle were higher after dapagliflozin treatment, while acetylcarnitine levels were lower (-40.0774 (-64.4766, -15.6782) pmoles∗mg tissue^-1, p < 0.001). Fasting levels of several amino acids, succinate, alpha-ketoglutarate and lactate in skeletal muscle were significantly lower after dapagliflozin treatment.

CONCLUSION

Dapagliflozin treatment for 5 weeks leads to adaptive changes in skeletal muscle substrate metabolism favoring metabolism of fatty acid and ketone bodies and reduced glycolytic flux. The trial is registered with ClinicalTrials.gov, number NCT03338855.

One-leg inactivity induces a reduction in mitochondrial oxidative capacity, intramyocellular lipid accumulation and reduced insulin signalling upon lipid infusion: a human study with unilateral limb suspension

AIMS/HYPOTHESIS

Physical inactivity, low mitochondrial function, increased intramyocellular lipid (IMCL) deposition and reduced insulin sensitivity are common denominators of chronic metabolic disorders, like obesity and type 2 diabetes. Yet, whether low mitochondrial function predisposes to insulin resistance in humans is still unknown.

METHODS

Here we investigated, in an intervention study, whether muscle with low mitochondrial oxidative capacity, induced by one-legged physical inactivity, would feature stronger signs of lipid-induced insulin resistance. To this end, ten male participants (age 22.4 ± 4.2 years, BMI 21.3 ± 2.0 kg/m²) underwent a 12 day unilateral lower-limb suspension with the contralateral leg serving as an active internal control.

RESULTS

In vivo, mitochondrial oxidative capacity, assessed by phosphocreatine (PCr)-recovery half-time, was lower in the inactive vs active leg. Ex vivo, palmitate oxidation to ¹⁴CO₂ was lower in the suspended leg vs the active leg; however, this did not result in significantly higher [¹⁴C]palmitate incorporation into triacylglycerol. The reduced mitochondrial function in the suspended leg was, however, paralleled by augmented IMCL content in both musculus tibialis anterior and musculus vastus lateralis, and by increased membrane bound protein kinase C (PKC) θ. Finally, upon lipid infusion, insulin signalling was lower in the suspended vs active leg.

CONCLUSIONS/INTERPRETATION

Together, these results demonstrate, in a unique human in vivo model, that a low mitochondrial oxidative capacity due to physical inactivity directly impacts IMCL accumulation and PKCθ translocation, resulting in impaired insulin signalling upon lipid infusion. This demonstrates the importance of mitochondrial oxidative capacity and muscle fat accumulation in the development of insulin resistance in humans.

TRIAL REGISTRATION

ClinicalTrial.gov NCT01576250.

FUNDING

PS was supported by a 'VICI' Research Grant for innovative research from the Netherlands Organization for Scientific Research (Grant 918.96.618).

Adipocyte differentiation-related protein and OXPAT in rat and human skeletal muscle: involvement in lipid accumulation and type 2 diabetes mellitus

SETTING

A disordered lipid metabolism is implicated in the development of skeletal muscle insulin resistance. Lipid droplet proteins of the PAT [perilipin, adipocyte differentiation-related protein (ADRP), and TIP47] family have been shown to regulate lipid accumulation and intracellular metabolism in other tissues.

OBJECTIVE

This study aimed to explore the role of the PAT proteins OXPAT and ADRP in skeletal muscle lipid metabolism and their putative role in modulating insulin sensitivity.

DESIGN

Muscle OXPAT and ADRP protein content was examined during the development of insulin resistance in Zucker diabetic fatty (ZDF) rats and in type 2 diabetes patients and BMI-matched control subjects. Furthermore, we examined the effect of 8 wk of insulin sensitizing by rosiglitazone on muscle OXPAT and ADRP content.

RESULTS

OXPAT and ADRP protein expression is muscle fiber type specific in humans and rats, with highest protein content in fibers containing most intramyocellular lipids (IMCL). Muscle OXPAT and ADRP protein content was 2- to 3-fold higher in ZDF rats during the progression of type 2 diabetes than in lean normoglycemic control rats, which was paralleled by high IMCL levels. Muscle OXPAT and ADRP content, as well as IMCL level, was not different between type 2 diabetes patients and control subjects. ADRP content was negatively associated with insulin-stimulated glucose uptake (r = -0.50; P = 0.017). Interestingly, rosiglitazone treatment decreased muscle OXPAT (-29%) and ADRP (-28%) content in diabetes patients, without affecting IMCL.

CONCLUSIONS

These results indicate involvement of OXPAT and ADRP in muscular lipid accumulation and type 2 diabetes.

Genetic analysis of heat shock proteins in maize

A genetic analysis of heat shock protein (HSP) synthesis was performed in seedling leaf tissue of two maize inbred lines, their F1 hybrid and F2 progeny. Protein synthesis following a high temperature treatment was visualized by [(35)S]-methionine in vivo labelling and two-dimensional gel electrophoresis. The parental lines' HSP synthesis patterns revealed both qualitative and quantitative polymorphisms implicative of differences in HSP structural genes and regulatory factors. The F1 hybrid HSP profile indicated that synthesis of all parental HSPs conformed to dominant inheritance patterns, including complete dominance, over-dominance and co-dominance. Alleles for six low-molecularweight HSPs in F2 progeny assorted according to typical 3∶1 Mendelian ratios for dominant gene expression. There is evidence for unlinked gene loci of four different HSP gene pairs, but data for three other HSP gene pairs were inconclusive, perhaps reflecting linkage for one pair and complex regulatory factor interactions for the other two pairs of genes. These results clearly indicate the existence of genetic variability in HSP synthesis and emphasize the potential of partitioning their roles in thermal tolerance using genetic and molecular analyses.

Genotype-specific heat shock proteins in two maize inbreds

Leaf blade tissue of maize inbred lines B73 and Mo17 was analyzed for intraspecific genetic variability in the heat shock response. The maize inbreds were characterized for acquired thermal tolerance and patterns of heat shock protein synthesis. The leakage conductivity assay of membrane stability during stress indicated that Mol7 possesses greater potential than B73 to acquire thermal tolerance. Poly(A)(+) RNA, extracted from leaf blades, was translated in vitro in the presence of (35)S-methionine and the translation products separated by twodimensional gel electrophoresis. Major genotypic differences were observed in the translation products. Mo 17 synthesized twelve unique heat shock proteins in the 15-18 kD range, but B73 synthesized only three unique heat shock proteins in the same range. DNA polymorphisms were observed between the maize lines using (32)P labeled heat shock protein gene probes.

A study of donor alloreactivity, which may predict acute graft-versus-host disease in HLA identical bone marrow transplantations for early leukaemia

Recent studies performed in Seattle, USA have suggested that pretransplant assignment of high or low donor alloreactivity may predict acute graft-versus-host disease (aGvHD) after allogeneic HLA identical marrow transplantation for acute leukaemia. The effect of such pretransplant assignment was studied in a Scandinavian population of 114 consecutive transplantations for acute and chronic leukaemias in 1st remission (n = 74) or chronic phase (n = 40) performed between 1975 and 1989. The selected cut-off value for discriminating between donors of high and low responding capacity (DRC) was based on distribution plots of results from the pretransplant mixed lymphocyte culture (MLC) and chosen as the median value (80% normalized response). Then 57 donors were assigned with high DRC and 57 donors assigned with low DRC. Kaplan-Meier estimates of the probability of patients to develop Grade II or higher aGvHD in receipt of high or low responder donor transplants were compared by univariant analysis. The patients in first remission or chronic phase transplanted with bone marrow from donors assigned as high or low responders had a 36.1% and 10.6% risk for aGvHD, respectively, a difference found to be significant by log rank test (chi-squared = 10.1, d.f. = 1, P = 0.0015). Subsequent studies of the cellular and humoral requirements for this predictive response of donor cells, by blocking with cytokine specific antibodies, addition of excess of recombinant human cytokines and scanning of lymphocyte subsets during the response, showed that the response against pool cells mostly depended upon IL-2 responding cells with the phenotype CD3+, CD4+, CD8-, CD25+, CD16-. It is concluded that prospective studies of alloreactivity as a risk factor should be performed to confirm the above findings.