ABSTRACT: Background In broilers, heat stress can result in reduced feed consumption, digestive inefficiency, impaired metabolism, and even death. The broiler sector of the U.S. poultry industry incurs approximately $52 million in heat stress-related losses annually. The objective of this study is to characterize the effects of chronic, cyclic heat stress on the transcriptome of a metabolically active organ, the liver. Characterizing the liver transcriptome of heat-stressed broilers will help clarify the effects of heat stress on metabolism. This information will provide a platform for future investigations that further elucidate physiologic responses to heat stress and seek methods to ameliorate the negative impacts of heat. Results Transcriptome sequencing of the livers of 8 broiler males using Illumina HiSeq 2000 technology, resulted in a total of 138 million, 100 base pair single end reads, yielding 13.8 gigabases of sequence. Forty genes were differentially expressed at a significance level of P-value < 0.05 and a fold change ≥ 2 in response to chronic, cyclic heat stress (mid-point of the last day of a 7-day cyclic heat stress of 7 hours per day), with 27 down-regulated and 13 up-regulated. Two gene networks were created from the function-based Ingenuity Pathway Analysis (IPA) of the differentially expressed genes; “Cell Signaling, Molecular Transport, Small Molecule Biochemistry” and “Endocrine System Development and Function, Small Molecule Biochemistry Cell Signaling”. Members of the MAPK signaling pathway and differentially expressed genes that are associated with MAPK-related functions were prominent in the networks. Cellular proliferation and differentiation, inflammationand stress-related signaling, and apoptosis-associated genes were down-regulated in response to heat stress. Genes responsible for inhibiting feed intake and sphingolipidrelated signaling were up-regulated. Genes involved with the regulation of inflammation, stress, thyroid hormone level, and body temperature were both up- and down-regulated. Conclusions Chronic, cyclic heat stress of broilers results in metabolic changes that can be characterized through RNA-seq analysis of the liver transcriptome. The primary affected pathways included cell signaling, molecular transport, endocrine system development and signaling, and small molecule biochemistry.
