Abstract
BACKGROUND: This study investigated the effects of chronic heat stress on liver inflammatory injury and its potential mechanisms in broilers. Chickens were randomly assigned to the 1-week control group (Control 1), 1-week heat stress group (HS1), 2-week control group (Control 2), and a 2-week heat stress group (HS2) with 15 replicates per group. Broilers in the heat stress groups were exposed to heat stress (35 ± 2 °C) for 8 h/d for 7 or 14 consecutive days, and the rest of 26 hours/day were kept at 23 ± 2 °C like control group broilers. Growth performance and liver inflammatory injury were examined for the analysis of liver injury. RESULTS: The results showed that heat stress for 2 weeks decreased the growth performance, reduced the liver weight (P < 0.05) and liver index (P < 0.05), induced obvious bleeding and necrosis points. Liver histological changes found that the heat stress induced the liver infiltration of neutrophils and lymphocytes in broilers. Serum levels of AST and SOD were enhanced in HS1 (P < 0.01, P < 0.05) and HS2 (P < 0.01, P < 0.05) group, compared with control 1 and 2 group broilers. The MDA content in HS1 group was higher than that of in control 1 group broilers (P < 0.05). Both the gene and protein expression levels of HSP70, TLR4 and NF-κB in the liver were significantly enhanced by heat stress. Furthermore, heat stress obviously enhanced the expression of IL-6, TNF-α, NF-κB P65, IκB and their phosphorylated proteins in the livers of broilers. In addition, heat stress promoted the activation of NLRP3 with increased NLRP3, caspase-1 and IL-1β levels. CONCLUSIONS: These results suggested that heat stress can cause liver inflammation via activation of the TLR4-NF-κB and NLRP3 signaling pathways in broilers. With the extension of heat stress time, the effect of heat stress on the increase of NF-κB and NLRP3 signaling pathways tended to slow down.