Acute Exercise Increases GDF15 and Unfolded Protein Response/Integrated Stress Response in Muscle in Type 2 diabetes

CONTEXT

Regular exercise is a key prevention strategy for obesity and type 2 diabetes (T2D). Exerkines secreted in response to exercise or recovery may contribute to improved systemic metabolism. Conversely, an impaired exerkine response to exercise and recovery may contribute to cardiometabolic diseases.

OBJECTIVE

We investigated if the exercise-induced regulation of the exerkine, growth/differentiation factor 15 (GDF15) and its putative upstream regulators of the unfolded protein response (UPR)/integrated stress response (ISR) is impaired in skeletal muscle in patients with T2D compared with weight-matched glucose-tolerant men.

METHODS

Thirteen male patients with T2D and 14 age- and weight-matched overweight/obese glucose-tolerant men exercised at 70% of VO2max for 1-h. Blood and skeletal muscle biopsies were sampled before, immediately after, and 3-h into recovery. Serum and muscle transcript levels of GDF15 and key markers of UPR/ISR were determined. Additionally, protein/phosphorylation levels of key regulators in UPR/ISR were investigated.

RESULTS

Acute exercise increased muscle gene expression and serum GDF15 levels in both groups. In recovery, muscle expression of GDF15 decreased toward baseline, whereas serum GDF15 remained elevated. In both groups, acute exercise increased the expression of UPR/ISR markers, including ATF4, CHOP, EIF2K3 (encoding PERK) and PPP1R15A (encoding GADD34), of which only CHOP remained elevated 3-h into recovery. Downstream molecules of the UPR/ISR including XBP1-U, XBP1-S, and EDEM1 were increased with exercise and 3-h into recovery in both groups. The phosphorylation levels of eIF2α-Ser51, a common marker of UPR and ISR, increased immediately after exercise in controls, but decreased 3-h into recovery in both groups.

CONCLUSION

In conclusion, exercise-induced regulation of GDF15 and key markers of UPR/ISR are not compromised in patients with type 2 diabetes compared with weight-matched controls.

Intact regulation of muscle expression and circulating levels of myokines in response to exercise in patients with type 2 diabetes

Regular exercise plays an important role in the prevention and treatment of type 2 diabetes (T2D). The synthesis and secretion of myokines in response to contraction may contribute to the beneficial metabolic effects of exercise. However, some exercise-induced responses may be attenuated in T2D. Here, we investigated whether the effect of acute exercise on selected myokines are impaired in T2D. Skeletal muscle biopsies and blood samples were obtained from 13 men with T2D and 14 weight-matched, glucose-tolerant men before, immediately after and 3-h after acute exercise (60 min cycling) to examine muscle expression and plasma/serum levels of selected myokines. One-hour of exercise increased muscle expression of IL6, FGF21, ANGPTL4, CHI3L1, CTGF and CYR61, of which FGF21, ANGPTL4 and CHI3L1 increased further 3-h into recovery, whereas expression of IL6, CYR61, and CTGF returned to baseline levels. There was no immediate effect of exercise on IL15 expression, but it decreased 3-h into recovery. Plasma IL-6 increased robustly, whereas circulating levels of FGF21, ANGPTL4, IL-15, and CHI3L1 increased only modestly in response to exercise. All returned toward baseline levels 3-h into recovery except for plasma ANGPTL4, which increased further. No significant differences in these responses to exercise were observed between the groups. Our results demonstrate that muscle expression and circulating levels of selected known and putative myokines were equally regulated by acute exercise in patients with T2D and weight-matched controls. This suggests that the potential beneficial metabolic effects of these myokines are not impaired in patients with T2D.

The Pharmacogenetics of Metformin in Women with Polycystic Ovary Syndrome: A Randomized Trial

Polycystic ovary syndrome (PCOS) is the most common endocrine disorder affecting women of reproductive age. PCOS is associated with obesity, dyslipidaemia and insulin resistance, and metformin treatment may improve such metabolic features. The effect of genetic variants in key metformin transporters, their transcriptional regulators or in metformin target genes on metformin response in women with PCOS is unclear. Associations between pharmacodynamic responses to metformin (changes in weight, lipid profile, insulin sensitivity evaluated by oral glucose tolerance testing) and polymorphisms in OCT1 (rs12208357 and rs72552763), HNF1A (rs1169288 and rs2464196), MATE1 (rs2289669 and rs2252281), MATE2-K (rs12943590) and ATM (rs11212617) were studied in 40 women with PCOS randomized to 12 months of treatment with metformin 1000 mg twice daily ± oral contraceptive pills (150 μg desogestrel + 30 μg ethinylestradiol). In the entire study population, treatment was associated with reduced weight (median weight change -2.4 kg, 25th-75th percentile -5.2 to 0.3 kg, p < 0.001) and increased triglycerides (0.2 mmol/L (0.0-0.6 mmol/L), p < 0.01) without significant changes in other lipid parameters or insulin sensitivity (insulin_AUC , glucose_AUC during OGTT). None of the evaluated polymorphisms significantly affected any treatment outcome. In conclusion, the genetic variants investigated were not crucial for the clinical response to metformin in PCOS.

Intact Regulation of the AMPK Signaling Network in Response to Exercise and Insulin in Skeletal Muscle of Male Patients With Type 2 Diabetes: Illumination of AMPK Activation in Recovery From Exercise

Current evidence on exercise-mediated AMPK regulation in skeletal muscle of patients with type 2 diabetes (T2D) is inconclusive. This may relate to inadequate segregation of trimeric complexes in the investigation of AMPK activity. We examined the regulation of AMPK and downstream targets ACC-β, TBC1D1, and TBC1D4 in muscle biopsy specimens obtained from 13 overweight/obese patients with T2D and 14 weight-matched male control subjects before, immediately after, and 3 h after exercise. Exercise increased AMPK α2β2γ3 activity and phosphorylation of ACCβ Ser(221), TBC1D1 Ser(237)/Thr(596), and TBC1D4 Ser(704) Conversely, exercise decreased AMPK α1β2γ1 activity and TBC1D4 Ser(318)/Thr(642) phosphorylation. Interestingly, compared with preexercise, 3 h into exercise recovery, AMPK α2β2γ1 and α1β2γ1 activity were increased concomitant with increased TBC1D4 Ser(318)/Ser(341)/Ser(704) phosphorylation. No differences in these responses were observed between patients with T2D and control subjects. Subjects were also studied by euglycemic-hyperinsulinemic clamps performed at rest and 3 h after exercise. We found no evidence for insulin to regulate AMPK activity. Thus, AMPK signaling is not compromised in muscle of patients with T2D during exercise and insulin stimulation. Our results reveal a hitherto unrecognized activation of specific AMPK complexes in exercise recovery. We hypothesize that the differential regulation of AMPK complexes plays an important role for muscle metabolism and adaptations to exercise.

Dysregulation of muscle glycogen synthase in recovery from exercise in type 2 diabetes

AIMS/HYPOTHESIS

Insulin and exercise stimulate skeletal muscle glycogen synthase (GS) activity by dephosphorylation and changes in kinetic properties. The aim of this study was to investigate the effects of insulin, exercise and post-exercise insulin stimulation on GS phosphorylation, activity and substrate affinity in obesity and type 2 diabetes.

METHODS

Obese men with type 2 diabetes (n = 13) and weight-matched controls (n = 14) underwent euglycaemic-hyperinsulinaemic clamps in the rested state and 3 h after 60 min of cycling (70% maximal pulmonary oxygen uptake [VO2max]). Biopsies from vastus lateralis muscle were obtained before and after clamps, and before and immediately after exercise.

RESULTS

Insulin-stimulated glucose uptake was lower in diabetic patients vs obese controls with or without prior exercise. Post exercise, glucose partitioning shifted away from oxidation and towards storage in both groups. Insulin and, more potently, exercise increased GS activity (fractional velocity [FV]) and substrate affinity in both groups. Both stimuli caused dephosphorylation of GS at sites 3a + 3b, with exercise additionally decreasing phosphorylation at sites 2 + 2a. In both groups, changes in GS activity, substrate affinity and dephosphorylation at sites 3a + 3b by exercise were sustained 3 h post exercise and further enhanced by insulin. Post exercise, reduced GS activity and substrate affinity as well as increased phosphorylation at sites 2 + 2a were found in diabetic patients vs obese controls.

CONCLUSIONS/INTERPRETATION

Exercise-induced activation of muscle GS in obesity and type 2 diabetes involves dephosphorylation of GS at sites 3a + 3b and 2 + 2a and enhanced substrate affinity, which is likely to facilitate glucose partitioning towards storage. Lower GS activity and increased phosphorylation at sites 2 + 2a in type 2 diabetes in the recovery period imply an impaired response to exercise.

Human muscle fiber type-specific insulin signaling: impact of obesity and type 2 diabetes

Skeletal muscle is a heterogeneous tissue composed of different fiber types. Studies suggest that insulin-mediated glucose metabolism is different between muscle fiber types. We hypothesized that differences are due to fiber type-specific expression/regulation of insulin signaling elements and/or metabolic enzymes. Pools of type I and II fibers were prepared from biopsies of the vastus lateralis muscles from lean, obese, and type 2 diabetic subjects before and after a hyperinsulinemic-euglycemic clamp. Type I fibers compared with type II fibers have higher protein levels of the insulin receptor, GLUT4, hexokinase II, glycogen synthase (GS), and pyruvate dehydrogenase-E1α (PDH-E1α) and a lower protein content of Akt2, TBC1 domain family member 4 (TBC1D4), and TBC1D1. In type I fibers compared with type II fibers, the phosphorylation response to insulin was similar (TBC1D4, TBC1D1, and GS) or decreased (Akt and PDH-E1α). Phosphorylation responses to insulin adjusted for protein level were not different between fiber types. Independently of fiber type, insulin signaling was similar (TBC1D1, GS, and PDH-E1α) or decreased (Akt and TBC1D4) in muscle from patients with type 2 diabetes compared with lean and obese subjects. We conclude that human type I muscle fibers compared with type II fibers have a higher glucose-handling capacity but a similar sensitivity for phosphoregulation by insulin.