Exercise-induced expression of myocardial heat shock protein 72: The impact of temperature and mechanical load

Myocardial heat shock protein 72 (HSP72) expression elicited by exercise is mediated by the activation of heat shock transcription factor (HSF1). However, the stimuli governing HSF1 activation and subsequent HSP72 gene expression and protein synthesis in the exercised heart remain unclear. We hypothesized that protein damage, caused by thermal and oxidative stress, stimulates HSF1 activation during exercise. To elucidate the independent roles of increased temperature (37°C vs. 40°C) and cardiac workload (preload 110 cm, afterload 23 cm, HL; preload 60 cm, afterload 15 cm, LL) in the upregulation of left-ventricular HSP72 associated with exercise, hearts from male Sprague-Dawley rats were isolated and perfused using an in vitro working heart preparation. Parallel experiments investigated the impact of in vivo core temperature elevation on hearts from treadmill exercised rats (60 min/3 days) in ambient temperature-manipulated (4°C and 22°C) conditions.; As an index of protein oxidation, cardiac protein carbonyls were assessed by ELISA and revealed a significant increase in warm-trained animals (p < 0.05), but no significant differences among any other experimental groups existed (p > 0.05). Nonetheless, high temperature elicited nuclear translocation and phosphorylation of HSF1 compared to low temperature groups in vitro (p < 0.05), but cold-trained animals exhibited similar levels of total and phosphorylated nuclear HSF1 as warm-trained animals (p > 0.05). Strikingly, real time RT-PCR analysis revealed that compared to normothermic exercised hearts and sedentary animals, elevated temperature elicited marked up-regulation in left-ventricular HSP72 mRNA expression (40°C HL 98-fold; 40°C LL 86-fold; WT 18-fold; p < 0.05). Consistent with these findings, high temperature groups induced significant increases in HSP72 protein synthesis, independent of cardiac workload in simulated exercise (40°C HL 3-fold; 40°C LL 2.5-fold) and warm-trained rats (4-fold; p < 0.05). These data indicate that hyperphosphorylation of HSF1 is not exclusively a result of increased temperature and cardiac workload indicating that other factors occurring with exercise contribute to the achievement of transcriptional competence.