ABSTRACT: Background: The in vivo gene response associated with hyperthermia and subsequent return to homeostasis or development of heat illness is poorly understood. Early activation of gene networks in the heat stress response is likely to lead to the systemic inflammation, multi-organ functional impairment, and other pathophysiological states characteristic of heat illness. Here, we perform an unbiased global characterization of the multi-organ gene response using an in vivo model of heat stress in the conscious rat. Results: Rats were subjected to elevated temperatures until implanted thermal probes indicated a maximal core temperature of 41.8 M-BM-0C (Tc,Max). Liver, lung, kidney, and heart were harvested at Tc,Max, 24 hours, and 48 hours after heat stress in groups of experimental animals and time-matched controls kept at ambient temperature. Clinical chemistries suggested abnormal function in liver, kidney, and lung at Tc,Max, and cardiac histopathology at 48 hours supported persistent cardiac damage in 3 out of 6 animals. Microarray analysis identified 78 differentially expressed genes common to all 4 organs at Tc,Max (i.e., the consensus heat stress response). Gene set enrichment analysis of gene ontology terms identified 25 biological processes in 4 general gene ontology categories: protein folding, regulation of apoptosis, response to cytokines, and transcriptional responses. Functional analysis clustering of the 78 differentially expressed genes in the consensus heat stress response also identified functional categories of protein folding and regulation of apoptosis. Self-organizing maps identified gene-specific signatures corresponding to protein folding disorders specific to only heat-stressed rats with histopathologic evidence of cardiac injury at 48 hours. Enrichment analysis of differentially expressed proteins in heat-injured hearts at 48 hours corroborated gene enrichment analysis results. Quantitative proteomics analysis by iTRAQ demonstrated that differential protein expression was not comparable to transcript expression at Tc,Max and 24 hours. However, the profile of differentially expressed proteins closely matched the transcriptomic profile in heat-injured animals at 48 hours. Pathway analysis at both the transcript and protein levels supported catastrophic deficits in energetics and cellular metabolism, chronic proteotoxic response, and activation of the unfolded protein response. Calculation of protein super-saturation scores demonstrated an increased propensity of proteins to aggregate in the hearts of heat-injured animals at 48 hours, consistent with accumulation of misfolded proteins. Conclusions: Global transcriptomic and proteomic analysis identified networks of genes and proteins initiating an unfolded protein response, metabolic dysfunction, and mitochondrial energy crisis in animals with histopathologic evidence of persistent heat injury, providing the basis for a systems-level physiological model of heat illness and recovery. To induce the pathophysiological effects of heat stress, conscious rats were placed in an incubator at 37M-BM-:C and their core temperature was monitored. Animals were sacrificed when their core temperature reached 41.8M-BM-:C (Tc,Max), or they were returned to their normal housing environment and were allowed to recover for up to 24 or 48 hours prior to sacrifice. Time-matched controls were euthanized at times corresponding to Tc,Max, 24 hours, and 48 hours. For each condition n=6 for a total of 36 animals. Four tissues (liver, lung, kidney and heart) were analyzed from each animal for a total of 144 arrays; however a few arrays did not pass QC therefore only 140 are reported here: 34 liver, 35 lung, 36 kidney, or 35 heart.
