Acute environmental hypoxia induces LC3 lipidation in a genotype‐dependent manner

Abstract

Hypoxia-induced muscle wasting is a phenomenon often described with prolonged stays at high altitude, which has been attributed to altered protein metabolism. We hypothesized that acute normobaric hypoxia would induce a negative net protein balance by repressing anabolic and activating proteolytic signaling pathways at rest and postexercise and that those changes could be partially genetically determined. Eleven monozygotic twins participated in an experimental trial in normoxia and hypoxia (10.7% O2). Muscle biopsy samples were obtained before and after a 20-min moderate cycling exercise. In hypoxia at rest, autophagic flux was increased, as indicated by an increased microtubule-associated protein 1 light chain 3 type II/I (LC3-II/I) ratio (+25%) and LC3-II expression (+60%) and decreased p62/SQSTM1 expression (-25%; P<0.05), whereas exercise reversed those changes to a level similar to that with normoxia except for p62/SQSTM1, which was further decreased (P<0.05). Hypoxia also increased Bnip3 (+34%) and MAFbx (+18%) mRNA levels as well as REDD1 expression (+439%) and AMP-activated protein kinase phosphorylation (+22%; P<0.05). Among the molecular responses to hypoxia and/or exercise, high monozygotic similarity was found for REDD1, LC3-II, and LC3-II/I (P<0.05). Our results indicate that environmental hypoxia modulates protein metabolism at rest and after moderate exercise by primarily increasing markers of protein breakdown and, more specifically, markers of the autophagy-lysosomal system, with a modest genetic contribution.

Emerging role for regulated in development and DNA damage 1 (REDD1) in the regulation of skeletal muscle metabolism

“…These data are in contrast to earlier human work in which simulated hypoxia increased REDD1 mRNA as well as phosphorylation of p70S6K1 (Thr 389 ) and Akt (Ser 473 ) compared with the time-matched normoxic condition (20). These studies highlight the lack of consistency of the effects of increased REDD1 on mTORC1 substrates within human muscle; however, the hypoxia protocols did differ, and feeding status at the time of biopsy was not clearly specified in either study (20,81). Last, in COPD patients, a decrease in the phosphorylation of Akt (Thr 308 ), p70S6K1 (Thr 389 ), and GSK3␤ (Ser 9 ) was observed.…”

“…2) (26). While evidence of this mechanism was elucidated in nonmuscle cells in culture, similar results were reported in the soleus of male rats housed at simulated altitude (i.e., hypoxic conditions) for ϳ2 wk (35 (81). Interestingly, in this latter study, the induction of REDD1 was associated with increased autophagy in the hypoxic state, consistent with the emerging role of REDD1 in the regulation of autophagy (81,104).…”

“…While evidence of this mechanism was elucidated in nonmuscle cells in culture, similar results were reported in the soleus of male rats housed at simulated altitude (i.e., hypoxic conditions) for ϳ2 wk (35 (81). Interestingly, in this latter study, the induction of REDD1 was associated with increased autophagy in the hypoxic state, consistent with the emerging role of REDD1 in the regulation of autophagy (81,104). These data are in contrast to earlier human work in which simulated hypoxia increased REDD1 mRNA as well as phosphorylation of p70S6K1 (Thr 389 ) and Akt (Ser 473 ) compared with the time-matched normoxic condition (20).…”

“…In mice, the increase in REDD1 protein appeared to coincide with repressed mTORC1 signaling, whereas in rats, mTORC1 signaling was increased in parallel with elevated REDD1 protein (51,99). In humans, a single 20-min bout of submaximal aerobic exercise on a cycle ergometer at an intensity of 1.2 W/kg (ϳ50% of the subjects' V O 2 max ) failed to change REDD1 protein in the vastus lateralis, wheereas mTORC1 signaling was blunted (81). Potential explanations for these discordant results include sex (male vs. female), species (human vs. mouse vs. rat), type of exercise (treadmill vs. cycle ergometer), the source of the REDD1 antibody (Proteintech vs. Bio-Connect), the time that measurements were made (immediately vs. hours postexercise), and a difference in exercise acclimation (2 vs. 10 days for the animal studies).…”

Heat shock response and autophagy—cooperation and control

“…Similar to resistance training, a short (20 min) sub-maximal aerobic exercise (cycle ergometer corresponding to 81% VO 2 max) exerts no significant effect on autophagy regulation in human skeletal muscle. 195 Finally, future research is needed to investigate the effect of exercise (intensity and type) on autophagy regulation in humans and to determine the role of autophagy under physical performance. A key question is to identify, besides exercise, optimal strategies including nutritional modifications that help autophagy work more effectively.…”