Heat acclimation increases hypoxia-inducible factor 1α and erythropoietin receptor expression: Implication for neuroprotection after closed head injury in mice

Experimental evidence indicates that long-term exposure to moderately high ambient temperature (heat acclimation, HA) mediates cross-tolerance to various types of subsequently applied stress. The transcriptional activator hypoxia-inducible factor 1 (HIF-1) has been implicated in playing a critical role in HA. It also regulates the expression of Erythropoietin (Epo), whose neuroprotective effects have been shown in a variety of brain injuries. The aim of the present study was to examine whether HA exerts a beneficial effect on the outcome of closed head injury (CHI) in mice and to explore the possible involvement of HIF-1 and Epo in this process. Heat acclimated mice and matched normothermic controls were subjected to CHI or sham surgery. Postinjury motor and cognitive parameters of acclimated mice were compared with those of controls. Mice were killed at various time points after injury or sham surgery and brain levels of HIF-1α, the inducible subunit of HIF-1, Epo, and the specific erythropoietin receptor (EpoR) were analyzed by Western immunoblotting. Motor and cognitive functions of acclimated mice were significantly better than those of controls. Heat acclimation was found to induce a significant increase in expression of nuclear HIF-1α and EpoR. The EpoR/Epo ratio was also significantly higher in acclimated mice as compared with controls. Nuclear HIF-1α and EpoR were higher in the acclimated group at 4 h after injury as well. The improved outcome of acclimated mice taken together with the basal and postinjury upregulation of the examined proteins suggests the involvement of this pathway in HA-induced neuroprotection.