Effects Of Acute Heat Stress On Hepatic Redox Status In Briolers And Its Mechanism

High ambient temperature induces the imbalance of redox status and long-term heat exposure can further lead to oxidative stress in broilers.Previous studies have shown that the decline in production performance and impairment in health of broilers caused by chronic heat stress were related to oxidative stress in the body However,there is little information about the effects of acute heat stress on the redox status,the molecular mechanism of regulating the redox status in broilers under the initial stage of heat stress is still not completely clear.In this study,A total of 144 Arbor Acres broilers were randomly divided into 4 groups with 6 replicates of 6 birds in each and subsequently distributed in different environment chambers for normal feeding.When the broilers were at 34 days old,the temperature of four environment chambers were set to 26（control）,29,32 and 35℃for 6 hours,respectively,the blood and liver samples of broilers were collected after the heat stress.This study was focus on the effects of acute heat stress on serum biochemical parameters,hepatic antioxidant capacity,Nrf2 signaling pathway,endoplasmic reticulum stress and apoptosis in broilers.The aim of this study was exploring the changes about Nrf2signaling pathway and UPR reaction which were the core of antioxidant defense system under the initial stage of heat stress,and revealing the molecular mechanism of regulating the redox status in broilers under stress conditions.The results showed that:（i）The 6 hours acute heat stress can increase the hormone secretion,up-regulate the expression of heat shock proteins,and elevate the activity of serum antioxidant enzymes in 34-day-old broilers.High temperature led to the stress condition in broilers body.With the ambient temperature increasing,the content of hormones such as CORT,T3,T4 in serum gradually increased（P<0.05）,the gene and protein expression of HSP70 in the liver of broilers was extremely significant increased（P<0.01）,and the gene and protein expression of HSP90 was significantly up-regulated（P<0.05）.Meanwhile,Acute heat stress could significantly increase the content of H₂O₂ in broilers serum（P<0.05）,the activity of antioxidant enzymes SOD and CAT also increased significantly（P<0.05）,but the content of small molecule antioxidant GSH decreased significantly（P<0.05）.（ii）The 6 hours acute heat stress can change the hepatic redox status,increase the content of reactive oxygen species,at the same time,activate antioxidant defense system elevate the antioxidant capacity in34-day-old broilers.Under the condition of acute heat stress,in broilers liver,the content of H₂O₂,and PC（which was the marker of protein oxidative damage）increased significantly（P<0.05）,and the SOD activity had a tendency to increase（P=0.07）,while the activity of CAT gradually increased with the ambient temperature increasing（P<0.05）,meanwhile,the activity of GR was also increased significantly（P<0.05）.Unlike the serum results,high temperature significantly increased the content of GSH in broilers liver.（iii）The 6 hours acute heat stress can activate the Nrf2 signal pathway,initiate the expression of a large number of downstream antioxidant genes,and play its protective role in 34-day-old broilers.By detecting the expression of target genes and proteins in the liver of broilers,it was found that under acute heat stress conditions,the mRNA expression of Nrf2 and its downstream antioxidant genes such as CAT,SOD1,SOD2,HO-1,NQO1,Prx3,GCLc,GST,etc.was significantly increased with the ambient temperature increasing（P<0.05）.Meanwhile,the WB results also showed that Nrf2 and Prx3 protein level in broilers liver was significantly up-regulated with increasing temperature（P<0.05）.（iv）The 6 hours acute heat stress can cause unfold protein response to relieve endoplasmic reticulum stress and prevent the occurrence of apoptosis in 34-day-old broilers.The endoplasmic reticulum stress markers GRP78 and GRP94 in the broilers liver were significantly upregulated in mRNA level and protein level（P<0.05）under the high temperature,and the PERK,IRE1 and XBP1,which involved in the unfolded protein response,were significantly up-regulated in mRNA level（P<0.05）.At the same time,the anti-apoptotic factor Bcl-2 mRNA expression was significantly up-regulated（P<0.05）.In addition,BAK1,BIK,Caspase-3,Caspase-9 and other apoptosis-related proteins did not change significantly in the broiler liver of each group in terms of mRNA level and protein level（P>0.05）.In summary,the results of this study indicate that under the initial stage of heat stress,The broilers can regulate the large-scale expression of various antioxidant genes and proteins with cytoprotective functions by activating the Nrf2 signaling pathway and the UPR reaction,etc.so as to increasing the activity of antioxidant enzymes and accelerating the synthesis of glutathione,enhancing the ROS scavenging ability,maintaining the appropriate redox status of broilers under acute heat stress,relieving the pressure of endoplasmic reticulum and preventing the occurrence of apoptosis.