Interleukin 37 reverses the metabolic cost of inflammation, increases oxidative respiration, and improves exercise tolerance

Abstract

IL-1 family member interleukin 37 (IL-37) has broad antiinflammatory properties and functions as a natural suppressor of innate inflammation. In this study, we demonstrate that treatment with recombinant human IL-37 reverses the decrease in exercise performance observed during systemic inflammation. This effect was associated with a decrease in the levels of plasma and muscle cytokines, comparable in extent to that obtained upon IL-1 receptor blockade. Exogenous administration of IL-37 to healthy mice, not subjected to an inflammatory challenge, also improved exercise performance by 82% compared with vehicle-treated mice (P = 0.01). Treatment with eight daily doses of IL-37 resulted in a further 326% increase in endurance running time compared with the performance level of mice receiving vehicle (P = 0.001). These properties required the engagement of the IL-1 decoy receptor 8 (IL-1R8) and the activation of AMP-activated protein kinase (AMPK), because both inhibition of AMPK and IL-1R8 deficiency abrogated the positive effects of IL-37 on exercise performance. Mechanistically, treatment with IL-37 induced marked metabolic changes with higher levels of muscle AMPK, greater rates of oxygen consumption, and increased oxidative phosphorylation. Metabolomic analyses of plasma and muscles of mice treated with IL-37 revealed an increase in AMP/ATP ratio, reduced levels of proinflammatory mediator succinate and oxidative stress-related metabolites, as well as changes in amino acid and purine metabolism. These effects of IL-37 to limit the metabolic costs of chronic inflammation and to foster exercise tolerance provide a rationale for therapeutic use of IL-37 in the treatment of inflammation-mediated fatigue.

Keywords: AMPK; fatigue; inflammation; interleukin 37; metabolism.

Fig. 1.

Effects of IL-37 treatment on systemic inflammation. (A) Mean ± SEM of endurance Rota-Rod run time in seconds. (B) Mean ± SEM plasma levels of IL-6. (C) Muscle content of IL-6. (D) CXCL-1. (E) IL-1Ra. (F) IL-1α. n = 5 mice per group. Statistical significance evaluated with the one-way ANOVA test. *P < 0.05; **P < 0.005; ***P < 0.001; ****P < 0.0001; ns, nonsignificant. Veh, vehicle.

Fig. 2.

Effects of IL-37 treatment on physical performance. Mice receiving daily injections of IL-37 or vehicle control were subjected to accelerating Rota-Rod training sessions on day 1 and to an exercise tolerance test on day 2 and on day 8. (A) Mean ± SEM run time in seconds in three consecutive accelerating Rota-Rod training sessions (∼1 h apart). (B) Endurance run time in seconds on day 2. n = 12 mice per group. (C) Separate experiment for endurance run time in seconds on day 2 and day 8. n = 9 mice per group. (D) Circulating levels of IL-6 evaluated 1 h after the exercise tolerance test. (E) Muscle IL-6 1 h after the exercise tolerance test. n = 5 mice per group. Data expressed as mean ± SEM. Statistical significance evaluated with the Student’s unpaired t test and with the one-way ANOVA test. *P < 0.05, **P < 0.005.

Fig. S1.

Effects of IL-37 treatment on physical performance on a treadmill. Mean ± SEM endurance run time on a treadmill in seconds. Statistical significance of differences was evaluated with the Student’s unpaired t test. **P < 0.005.

Fig. S2.

Effects of IL-37 treatment on circulating cytokines. Mean ± SEM plasma levels of IL-1β, IL-1Ra, TNFα. Statistical significance of differences between groups was evaluated with the one-way ANOVA test. *P < 0.05, ns, nonsignificant.

Fig. 3.

IL-37 activates the IL-1R8/AMPK axis. (A) In vivo levels of total AMPK, phosphorylated AMPK, and relative ratio in m. gastrocnemius (n = 8 mice per group). (B) Decline in exercise tolerance after in vivo inhibition of AMPK with a single i.p. dose of 0.2 mg/kg dorsomorphin 1 h before testing (n = 5 mice per group). (C) Western Blot and relative ratio for phospho-AMPK and total AMPK in differentiated C2C12 muscle cells stimulated with IL-37; total of four separate experiments. (D) Exercise tolerance test for WT or IL-1R8^−/− mice treated with vehicle or IL-37 (n = 5 mice per group). Data expressed as mean ± SEM. Statistical significance evaluated with the Student’s unpaired t test and with the one-way ANOVA test. *P < 0.05, ns, nonsignificant. pAMPK, phosphorylated AMPK; tAMPK, total AMPK.

Fig. 4.

IL-37 increases oxidative phosphorylation substrate flux. Oxygen consumption rate was evaluated by using isolated mitochondria from m. gastrocnemius of mice treated with IL-37 for 8 days. (A) Respiration rates under described substrate conditions. (B) Citrate synthase activity, measured as an index of mitochondrial content. Data expressed as mean ± SEM. n = 4 mice per group. Statistical significance evaluated with the Student’s unpaired t test and with the one-way ANOVA test. *P < 0.05. GM, glutamate + malate; GMD, glutamate + malate + ADP; OCR, oxygen consumption rate; Olig, oligomycin.

Fig. 5.

Metabolome changes upon treatment with IL-37. M. gastrocnemius, red blood cells, and plasma were analyzed for metabolic profiles. (A) Heatmap for metabolites in plasma. (B) Significant plasma metabolites and their respective fold change IL-37/Control. (C) Succinate levels in muscle. (D) AMP/ATP ratio in muscle. n = 5 mice per group. Statistical significance evaluated with the Student’s unpaired t test. *P < 0.05.

Fig. 6.

Changes in energy reservoirs and amino acid catabolism in muscle of IL-37–treated mice. (A) MetaboAnalyst elaboration of the metabolic pathways up-regulated in muscles of IL-37–treated mice indicated an increase in citric acid cycle, amino acid metabolism, nucleoside (purine and pyrimidine), phosphocreatine, and glutathione homeostasis. (B) Phosphocreatine synthesis, phosphocreatine/creatine ratios. (C) Purine synthesis intermediates and products in IL-37 mouse muscles in comparison with controls. (D) Amino acids can fuel energy metabolism by providing substrates for late glycolysis and citric acid cycle at different levels. Free muscle amino acid levels decreased in IL-37–treated mice, whereas pyruvate and citrate levels increased. n = 5 mice per group. Statistical significance evaluated with the Student’s unpaired t test. *P < 0.05, **P < 0.005. Creat, creatine; PCreat, phosphocreatine.