Heat Stress Induces Downregulation of Hippocampal Superoxide Dismutase-1; a Possible Mechanism for Heat-Related Neuronal Cell Death

Abstract

Exertional heat injury represents a major risk for people working or exercising in hot environments. Currently, no pharmacological therapies are available and little is known about the molecular response to heat stress in the brain. In the first part of this study, we examined gene expression changes associated with hyperthermia in the hippocampus of pigs. Twelve pigs were kept at either 70ºF (Control) or 90 ºF (heat-stressed) for 4, 8, 12 and 24 h. The pigs were monitored hourly for heart rate, respiratory rate and body temperature. At 4, 8, 12 and 24 h, the hippocampus was excised from 3 pigs in each group and total RNA was obtained for use in differential display PCR. A total of 31 differentially expressed cDNAs were isolated. Eight genes were chosen for further confirmation by real-time PCR analysis. Only slight increases in the expression of three different HSP genes were reported after heat stress at all sample times. DNA polymerase ? p12 gene expression was stimulated to 2.55 fold after 12 h while it was downregulated after both 4 and 8 h. Two genes encoding 26S proteasome subunits, PSMD10 and PSMB9 were also upregulated after heat stress at all sample times. Superoxide dismutase-1 gene was significantly inhibited after both 8 and 12 hours of heat stress to 0.39 and 0.45 fold respectively. Sodium channel voltage-gated B1 subunit gene was inhibited after heat stress. This inhibition was only significant after 24 h. In conclusion, most of the hippocampal genes that have altered expression by heat stress are playing a role in cellular protection against hyperthermic insult and some altered genes may play a role in heat-related brain pathogenesis. More understanding about the heat-induced molecular changes in the brain could help in the development of effective therapeutic strategies for treatment of heat-related illness. In the second part of this study, we utilized immortalized hippocampal neuronal cells (HT-22) to investigate the effect of elevated heat (43°C/30 min) on bothSOD-1 gene expression and translation as well as enzyme activity. The effect of heat stress on accumulation of ROS, cell number and induction of apoptosis were measured. Also, the effectiveness of pretreatment with the SOD mimetic EUK-134 to protect against heat-induced cell death was evaluated. We found that in HT-22 cells, heat stress decreased the SOD-1 mRNA, protein, and activity as well as cell viability. Apoptotic cell death is involved in heat-induced neuronal cell death with overproduction of ROS. All aspects of heat-induced cell death and apoptosis were abolished by pretreatment with EUK-134. Thus heat-induced downregulation of SOD-1 in HT-22 cells disturbed the cellular antioxidant defense mechanism leading to accumulation of ROS and cell death.