The Effect of Increasing Levels of Hyperthermia on Autophagy in Peripheral Blood Mononuclear Cells from Young Adults

Exposure to an elevated state of hyperthermia is associated with heat‐induced cytotoxicity, thereby resulting in impaired cellular function. Cellular dysfunction during heat stress leads to the initiation of systemic inflammation and apoptotic cell death, which if left unabated, can cause tissue damage and death. To counter heat‐induced cellular injury, cells employ the vital mechanism of autophagy which acts to prevent the accumulation of toxic protein aggregates and dysfunctional organelles to promote cellular survival. In our recent ex vivo model, we demonstrated that the process of autophagy is activated during exposure to increased temperatures (37‐41°C) in peripheral blood mononuclear cells (PBMCs) from young adults which acts to suppress heat‐induced apoptotic cell death. Despite this, the dose‐dependent autophagic response to heat stress has yet to be evaluated in an in vivo human model and thus the relationship between elevations in core body temperature (T core ) and autophagy have yet to be established in humans. Therefore, we sought to extend upon our previous work by assessing if in vivo rises in T core are associated with greater autophagic activation in healthy young adults. We evaluated the hypothesis that in parallel to elevations in T core (indexed by esophageal temperature), greater autophagic initiation would occur in a dose‐dependent manner in young adults during immersion in mild to very warm water. To assess this hypothesis, on separate days 6 young adults (mean [SD] 22 [3] years; 3 women) were immersed (up to the clavicle) in water set at a temperature to clamp T core at either thermoneutral (TN, 37°C), 38°C, or 39°C for 60‐min. Autophagy was characterized in PBMCs prior to and immediately post‐water immersion, as well as following 3‐h of seated recovery. Western blot was used to assess autophagy proteins microtubule associated protein 1 light chain 3 beta (LC3)‐II and sequesterome‐1/p62 (p62). All proteins were normalized to β‐actin and reported as fold change relative to baseline. Data were compared via a two‐way repeated measures ANOVA with Fisher’s LSD (α=0.05). No changes in either LC3‐II or p62 were observed during TN, indicating no time‐dependent changes in autophagy (all p≥0.28). Immediately following the 38°C clamp, LC3‐II increased (1.56 [0.32]; p=0.02) and p62 decreased (0.52 [0.30]; p=0.049) from baseline, with both proteins returning to basal levels within 3‐h (both, p≥0.22). Similarly, LC3‐II increased (1.90 [0.70]; p=0.04) and p62 decreased (0.58 [0.21]; p=0.03) immediately after clamping T core at 39°C, however LC3‐II remained elevated above baseline at 3‐h (2.54 [1.08]; p=0.03) while p62 did not differ from pre‐immersion levels at the end of recovery (p=0.57). Given LC3‐II accumulation and p62 degradation indicate elevated autophagy, our preliminary findings indicate greater autophagy during increasing levels of hyperthermia in healthy young adults. Further, we showed increased reliance on autophagy to restore cellular homeostasis after exposure to an elevated state of hyperthermia of 39°C, although the mechanism of activation is unclear.

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