Deficiency in the Heat Stress Response Could Underlie Susceptibility to Metabolic Disease

Abstract

Heat treatment (HT) effectively prevents insulin resistance and glucose intolerance in rats fed a high-fat diet (HFD). The positive metabolic actions of heat shock protein 72 (HSP72), which include increased oxidative capacity and enhanced mitochondrial function, underlie the protective effects of HT. The purpose of this study was to test the ability of HSP72 induction to mitigate the effects of consumption of a short-term 3-day HFD in rats selectively bred to be low-capacity runners (LCRs) and high-capacity runners (HCRs)-selective breeding that results in disparate differences in intrinsic aerobic capacity. HCR and LCR rats were fed a chow or HFD for 3 days and received a single in vivo HT (41°C, for 20 min) or sham treatment (ST). Blood, skeletal muscles, liver, and adipose tissues were harvested 24 h after HT/ST. HT decreased blood glucose levels, adipocyte size, and triglyceride accumulation in liver and muscle and restored insulin sensitivity in glycolytic muscles from LCR rats. As expected, HCR rats were protected from the HFD. Importantly, HSP72 induction was decreased in LCR rats after only 3 days of eating the HFD. Deficiency in the highly conserved stress response mediated by HSPs could underlie susceptibility to metabolic disease with low aerobic capacity.

Figure 1

Skeletal muscle insulin responsiveness in EDL and soleus muscles of HCR/LR rats. Insulin-stimulated glucose uptake in EDL muscle (A) and soleus muscle (E) and phosphorylation of Akt (B and F) and AS160 (C and G) in HCR/LCR rats fed either a chow diet (C) or a 3-day HFD and receiving a single in vivo ST (37°C) or HT (41°C). D and H: Representative blots of phosphorylated (p)-Akt/Akt and p-AS160/AS160 in EDL and soleus muscles. White bars represent the basal condition, and black bars represent the insulin-stimulated condition. *P < 0.05–0.001 denotes significantly different from basal condition; #P < 0.05, ##P < 0.01 denotes that HCR/LCR rats are significantly different; †P < 0.05 denotes that LCR HFD-ST rats had significantly lower insulin-stimulated glucose uptake than LCR C-ST rats and LCR HFD-HT rats determined by one-way ANOVA. Values are reported as the mean ± SE. N = 3–6 animals/group.

Figure 2

Heat shock response in skeletal muscle of HCR/LCR rats. HSP72 and HSP25 expression in the EDL (A and C) and soleus (B and D) muscles of male HCR/LCR rats after HT. Rats were fed a chow diet or underwent a 3-day HFD challenge and received either a single in vivo ST (37°C) or HT (41°C). Protein levels were normalized to α-tubulin protein levels. White bars represent chow-ST, black bars represent chow-HT, white-hatched bars represent HFD-ST, and gray-hatched bars represent HFD-HT. *P < 0.01, **P < 0.001 denotes significant main effect of treatment; †P < 0.05 denotes a significant diet × treatment interaction determined by two-way ANOVA performed in HCR/LCR rats separately; #P < 0.05 denotes significantly different between HCR/LCR rats by one-way ANOVA. Values are reported as the mean ± SE. N = 5–6 samples/group.

Figure 3

Glucose transport and HSP response in primary myotubes isolated from HCR/LCR rats with decreased induction of HSP72. Insulin-stimulated glucose uptake 24 h after a single in vitro HT (42°C, 30 min) of primary myotubes isolated from adult male HCR (A) and LCR rats (B). Primary myotubes were isolated, transfected with siCTL or siHSP72 to disrupt the induction of HSP72, received a single HT 24 h after transfection; and 24 h after HT underwent insulin-stimulated glucose uptake experiments. Western blots of HSP72 (C and D) and HSP25 (E and F) expression after ST (37°C, 30 min) or HT and transfection with siHSP72 or siCTL in primary myotubes isolated from HCR/LCR rats. Protein levels were normalized to β-actin protein levels. *P < 0.01, **P < 0.001 denotes significantly increased from basal condition or ST; †P < 0.05, ††P < 0.001 denotes significantly different from siCTL determined by one-way ANOVA. Values are reported as the mean ± SE. N = 6 samples/condition.

Figure 4

Triglyceride content of white and red gastrocnemius muscles of HCR/LCR rats. Intramuscular triglyceride content of white (A) and red (B) gastrocnemius muscles of male HCR/LCR rats after HT. Rats were fed a chow diet or underwent a 3-day HFD challenge and received either a single in vivo ST (37°C) or HT (41°C). White bars represent chow-ST, black bars represent chow-HT, white-hatched bars represent HFD-ST, and gray-hatched bars represent HFD-HT. *P < 0.01 denotes a significant main effect of HT; †P < 0.05 denotes a significant diet × treatment interaction whereby triglyceride content of HFD-ST LCR rats is significantly greater than chow-ST LCR rats and HFD-HT LCR rats determined by two-way ANOVA. There was a trend (P = 0.062) toward a main effect of diet in LCR rats. Values are reported as the mean ± SE. N = 4–6 samples/group.

Figure 5

Liver and adipose tissue response to HT in HCR/LCR rats. Triglyceride content (A) and HSP72 (B) and HSP25 expression in liver (C); representative images of cross-sections of eWAT (D); cross-sectional area of adipocytes (E); and HSP72 (F) and HSP25 (G) expression in eWAT of HCR/LCR rats. Rats were fed a chow diet or underwent a 3-day HFD challenge and received either a single in vivo ST (37°C) or HT (41°C). Protein levels were normalized to β-actin protein levels. White bars represent chow-ST, black bars represent chow-HT, white-hatched bars represent HFD-ST, and gray-hatched bars represent HFD-HT. *P < 0.05, **P < 0.01, ***P < 0.001 denote a significant main effect of treatment; δP < 0.01 denotes a significant main effect of diet; †P < 0.01 denotes a significant diet × treatment interaction determined by two-way ANOVA performed in HCR/LCR rats separately; #P < 0.05 and ##P < 0.01 denote significantly different from HCR rats of corresponding treatment group by one-way ANOVA. Values are reported as the mean ± SE. N = 4–6 animals/ group.