Exercise and Training Regulation of Autophagy Markers in Human and Rat Skeletal Muscle

Abstract

Autophagy is a key intracellular mechanism by which cells degrade old or dysfunctional proteins and organelles. In skeletal muscle, evidence suggests that exercise increases autophagosome content and autophagy flux. However, the exercise-induced response seems to differ between rodents and humans, and little is known about how different exercise prescription parameters may affect these results. The present study utilised skeletal muscle samples obtained from four different experimental studies using rats and humans. Here, we show that, following exercise, in the soleus muscle of Wistar rats, there is an increase in LC3B-I protein levels immediately after exercise (+109%), and a subsequent increase in LC3B-II protein levels 3 h into the recovery (+97%), despite no change in Map1lc3b mRNA levels. Conversely, in human skeletal muscle, there is an immediate exercise-induced decrease in LC3B-II protein levels (−24%), independent of whether exercise is performed below or above the maximal lactate steady state, which returns to baseline 3.5 h following recovery, while no change in LC3B-I protein levels or MAP1LC3B mRNA levels is observed. SQSTM1/p62 protein and mRNA levels did not change in either rats or humans following exercise. By employing an ex vivo autophagy flux assay previously used in rodents we demonstrate that the exercise-induced decrease in LC3B-II protein levels in humans does not reflect a decreased autophagy flux. Instead, effect size analyses suggest a modest-to-large increase in autophagy flux following exercise that lasts up to 24 h. Our findings suggest that exercise-induced changes in autophagosome content markers differ between rodents and humans, and that exercise-induced decreases in LC3B-II protein levels do not reflect autophagy flux level.

Autophagy and Exercise: Current Insights and Future Research Directions

“…Participants completed three work-matched experimental sessions at 51%, 59%, and 67% of peak aerobic power (Ẇ max ), representing –18%, –6% and+6% of the maximal lactate steady state (MLSS) – an important metabolic threshold delineating heavy from severe exercise intensity 59 . The findings from this study supported the intensity-independent nature of exercise-induced decreases in LC3B-II and LC3B-II/I ratio, and we observed no changes in p62 protein 47 ( Fig. 3-a ).…”

“…In contrast to the rodent data, exercise in humans decreases markers of autophagosome content, which could be due to increased autophagic flux or decreased autophagosome biogenesis. Regarding autophagy receptors, most studies have shown unchanged p62 protein levels following endurance exercise [47,52,57,58]. Schwalm et al postulated that autophagy activation was dependent on exercise intensity [53].…”

“…Some of these ex vivo protocols have been used to demonstrate exercise-induced changes in autophagy flux [46], and circadian autophagy flux changes following fasting [84], in mouse skeletal muscle. Using a similar approach, we have recently performed ex vivo autophagy flux experiments on human skeletal muscle samples [47] collected before and after exercise (▶Figs. 4-a and ▶4-b).…”

“…3-a). Protein levels of LC3B-II, and the LC3B-II/I ratio, have been shown to decrease 0 to 1 h following different types of endurance exercise (30 to 120 min at 50 to 70 % of V ̇O2peak ) and to return to baseline values after 3 to 4 h of recovery in human skeletal muscle [47,48,52,53,56]. Only one study [57] with a similar design (60 min of exercise at 60 % of V ̇O2peak ), has reported an increase in the levels of LC3A/B-II protein levels 2 h after the end of exercise, which could be related to the LC3 protein analyzed.…”

“…Schematic representation of the ex vivo autophagy flux assay previously used in human tissues[47,82,85]. Tissue samples are collected and immediately incubated with a lysosomal inhibitor (leupeptin, chloroquine, bafilomycin or control, incubated at 37 °C for 60 min).…”