Heat stress and LPS activate multiple signaling pathways to regulate cytokine expression in macrophages

Abstract

Animals exposed to heat stress (HS) have suppressed response to immune stimulants such as bacteria lipopolysaccharide. This has implication for health and production of livestock because an effective immune response is critical for maintenance of health and productive function, even during heat stress. Using the RAW 264.6 macrophage model, we show that HS induces differential regulation of inflammatory cytokine expression in response to LPS in macrophages cultured under normal (C, 37 oC) or HS (41.5 oC) conditions. Expression of cytokines (TNFα, IL-1β and IL-10) and enzymes (INOS and Arginase) was downregulated in HS condition, whereas IL-6 was upregulated. Chronic HS (24hr) treatment resulted in higher level of phosphorylated IKBα than in control. Treatment of cells with LPS after HS led to an accelerated degradation of IKBα. Heat stress also resulted in reduced expression of total and phosphorylated p65 NFκB protein. HS caused reduced P44/42 MAPK and JNK phosphorylation. This suggests that multiple signaling pathways that are implicated in transcriptional regulation of inflammatory cytokine levels are suppressed under HS, and this may explain the suppression of expression of most cytokines under HS. However, HS and LPS treatment individually and additively induced phosphorylation of C-fos, a member of the AP-1 transcription factor complex implicated in the regulation of IL-6 expression. Therefore, increased activation of C-fos may explain the induction of IL-6 expression under HS. These results show that complex signaling pathways regulate immune response in animals exposed to HS, and their knowledge may help guide potential mitigation strategies against HS.