Prolonged Heat Stress Altered Autophagy Signaling in Oxidative Skeletal Muscle

Abstract

Prolonged heat stress represents a continuing threat to human health and agricultural production economics. Despite this broad, negative impact little is known about underlying pathological mechanisms, which has limited the development of etiological interventions and left clinicians and producers with only cooling and rehydration strategies. The purpose of this investigation was to determine the extent to which prolonged heat stress altered autophagic signaling in oxidative skeletal muscle. We hypothesized that heat stress would induce autophagy independent of the volitional caloric restriction that accompanies heat stress. To test this hypothesis pigs were treated as follows: thermoneutral (TN; 20°C), heat stress (HS; 35°C), or were house in thermoneutral conditions but pair‐fed to the heat stress group (PFTN) for seven days. Upon euthanasia the red portion of the semitendinosus muscle was collected for subsequent measures of relative protein abundance. We determined that markers of autophagy initiation, including phosphorylated adenosine monophosphate activated protein kinase (AMPK) and Unc51‐like kinase 1 (Ulk1), were increased approximately 3‐fold (p<0.05) in HS compared to TN and PFTN. Similarly, nucleation markers, PI3 kinase class III and Beclin‐1, were increased approximately 3‐fold (p<0.05) in HS compared to TN and PFTN. We also discovered that elongation markers autophagy related gene (Atg)12 and Atg16 were increased 2‐ and 4‐fold (p<0.05), respectively, in HS compared to TN and PFTN, while Atg5 was similar between groups. Markers of autophagosome formation including microtubule‐associated protein 1A/1B‐light chain (LC3) II and the LC3 II/I ratio were increased 5‐fold (p<05) in HS and PFTN compared to TN while LC3 I was similar between groups. Sequestosome 1 (SQSTM1/p62) is consumed during autophagosomal degradation by the lysosome and is thus an inverse correlate of autophagic degradation. We discovered that relative protein abundance of p62 was increased 5‐fold (p<0.05) in HS compared to TN and PFTN. In partial support of our hypothesis, these data indicate heat stress‐mediated induction of autophagic signaling is largely independent of reduced feed intake. Further, these data also suggest a HS‐mediated stalling of autophagic degradation despite prolonged reduced feed intake.Support or Funding InformationThis work supported by USDA grants 2014‐67015‐21627 and 2011‐67003‐30007.