Autophagy and Heat Shock Responses in Human Peripheral Blood Mononuclear Cells During Physiologically Relevant Heat Stress Conditions

Abstract

When exposed to stress caused by elevated internal body temperatures, proteins become denatured or misfolded, leading to toxic protein aggregation. To protect the cell under heat stress, misfolded proteins are tagged or refolded by heat shock proteins (HSP). Under conditions of extreme heat stress, the process of autophagy has previously been shown to protect human cells, but the exact temperatures at which autophagic activation occurs is largely unknown. Further, the interplay of HSP and autophagy responses as a function of increasing levels of hyperthermia have yet to be examined. Thus, the purpose of this study was to examine threshold changes in the autophagy and heat shock response to increasing levels of in vitro heat stress characteristic to physiologically relevant temperature conditions representative of normal (37°C), moderate (39°C), and severe (41°C) heat stress conditions in humans. We hypothesized that autophagy and HSP responses in human peripheral blood mononuclear cells (PBMCs) would increase in a dose‐dependent manner to heat stress in young adults. To test our hypothesis, whole blood was collected from 20 young (23±4 years; 10 women) participants. PBMCs were isolated immediately (baseline) and after 90‐min of whole‐blood heating in 37, 39, and 41°C water baths. Using western blot and qRT‐PCR techniques, the interplay of autophagy and HSP was characterized through measurements of microtubule associated protein 1 light chain 3 beta (LC3B), p62/sequesterome‐1 (p62), and heat shock protein 70 (HSP70) proteins and their associated genes. All values are reported as fold‐change relative to baseline values after being normalized to β‐actin. Statistical analysis was completed using a one‐way repeated measures ANOVA to asses fold change in proteins and genes between each heat condition. In the simulated thermoneutral conditions of 37°C, increased autophagic activity was demonstrated only through decreased levels of p62 protein (0.73±0.40, p<0.01) but no significant changes (p>0.05) in protein synthesis and gene expression in HSP70 (1.11±0.29) or LC3B (0.87±0.34) were observed. Subsequently, responses of HSP and autophagy increased at moderate heat stress (39°C) with increased p62 (2.31±2.02, p<0.01) and LC3B gene expression (1.86±1.21, p<0.01), an increased protein response in HSP70 (1.19±0.37, p=0.03), a decreased protein response in p62 (0.66±0.29, p<0.01), and no significant protein response in LC3B (1.04±0.24, p>0.05). At the highest level of heat stress (41°C), increases in protein synthesis and gene expression were observed in HSP70 (1.38±0.49, 2.77±2.15 respectively; all p<0.01) and LC3B (1.39±0.57, 2.32±1.80 respectively; all p<0.01) concomitant with decreased p62 protein synthesis (0.73±0.28, p<0.01) indicating an increase in the HSP response and autophagic flux. Taken together, our findings demonstrate that in human PBMCs, the HSP and autophagy systems work synergistically in responding to increases in the level of heat stress occurring within the human system.Support or Funding InformationCanadian Institutes of Health Research