Cellular antioxidant enzyme activity and biomarkers for oxidative stress are affected by heat stress.

Heat stress (HS) causes oxidative stress and cellular changes in an attempt to detoxify the harmful effects of reactive oxygen species (ROS). However, how ROS affect different organs in chickens under acute and chronic HS is relatively unknown. We investigated the cellular enzyme activity and biomarker changes in the liver and Pectoralis (P) major muscle in broiler chickens subjected to both acute and chronic HS. Forty-eight broiler chickens at 14 days old were randomly assigned to either 25 °C (control) or 35 °C (heat-stressed) for 12 days. Five birds per treatment at 1 and 12 days post-HS were euthanized, and the liver and P. major muscle were sampled. Superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione (GSH), glutathione reductase (GR), glutathione S-transferase (GST) activity as well as 8-hydroxy-2'-deoxyguanosine (8-OHdG), advanced glycation end product (AGE), malondialdehyde (MDA), and protein carbonyl (PCO) were analyzed as biomarkers for DNA, carbohydrate, lipid, and protein oxidation, respectively. The SOD, CAT, and GSH-GPx activity levels in the liver and the P. major muscle changed under HS; however, some of the changes were tissue-specific or dependent on the duration of the HS. There were increased liver 8-OHdG during chronic HS and also increased liver AGE levels during both acute and chronic HS indicating significant carbohydrate and DNA oxidations. In the P. major muscle, we observed significant increases in lipid peroxidation and protein oxidation which may reflect that this tissue is less resilient to oxidative damage under heat stress. We show that heat stress caused tissue-specific changes to levels of oxidation biomarkers in chicken.

Expression of genes that encode cellular oxidant/antioxidant systems are affected by heat stress.

Heat stress causes critical molecular dysfunction that affects productivity in chickens. Thus, the purpose of this study was to evaluate the effect of heat stress (HS) on the expression of select genes in the oxidation/antioxidation machinery in the liver of chickens. Chickens at 14 days of age were randomly assigned to two treatment groups and kept under either a constant normal temperature (25 °C) or high temperature (35 °C) in individual cages for 12 days. mRNA expression of Nrf2, oxidants NADPH(NOX): [NOX1, NOX2, NOX3, NOX4, NOX5 and DUOX2], and antioxidants [SOD1, CAT, GR, GPx1, NQO1] in the liver were analyzed at 1 and 12 days post-HS. We show that, HS changes the mRNA expression of oxidants thereby increasing cellular reactive oxygen species (ROS). Additionally, persistent HS up-regulates SOD which converts superoxides to hydrogen peroxide. We further demonstrated the dynamic relationship between catalase, GSH peroxidase (GPx) and NADPH under both acute and chronic heat stress. The pentose phosphate pathway could be important under HS since it generates NADPH which serves as a cofactor for GPx. Also, methionine, a precursor of cysteine has been shown to have reducing properties and thereby makes for an alternative fuel for redox processes. Genes in the ROS and antioxidant generation pathways may provide insight into nutritional intervention strategies, especially the use of methionine and/or cysteine when birds are suffering from heat stress.

Effect of heat stress on protein utilization and nutrient transporters in meat-type chickens.

The aim of this study was to investigate the effect of heat stress (HS) on digestibility of protein and fat and the expression of nutrient transporters in broilers. Forty-eight male Cobb500 chicks were used in this study. At day 14, birds were randomly divided into two groups and kept under either constant normal temperature (25 °C) or high temperature (35 °C) in individual cages. Five birds per treatment at 1 and 12 days post-treatment were euthanized, and Pectoralis major (P. major) and ileum were sampled for gene expression analysis. At day 33, ileal contents were collected and used for digestibility analysis. The total consumption and retention of protein and fat were significantly lower in the HS group compared to the control group. Meanwhile, the retention of crude protein per BWG was significantly higher in the HS group compared to the control group. In P. major and ileum tissues at day 1, transporters FATP1 and SGLT1 were down-regulated in the HS group. Meanwhile, FABP1 and PepT1 were down-regulated only in the ileum of the HS group. The converse was shown in P. major. The nutrient transporter FABP1 at day 12 post-HS was down-regulated in the P. major and ileum, but GLUT1 and PepT2 were down-regulated only in the ileum, and PepT1 was down-regulated only in the P. major compared with the control group. These changes in nutrient transporters suggest that high ambient temperature might change the ileum and P. major lipids, glucose, and oligopeptide transporters.

Effects of oregano and/or rosemary extracts on growth performance, digestive enzyme activities, cecal bacteria, tight junction proteins, and antioxidants-related genes in heat-stressed broiler chickens.

The study examined the impact of adding oregano extract and/or rosemary to broiler diets to counteract the growth inhibition caused by heat stress (HS). It also investigated the effects on the activity of digestive enzymes, microbiological composition, and the expression of antioxidant and tight junction-related proteins. Three hundred- and fifty-day-old male broilers, were randomly assigned to 7 treatment groups, with each group comprising 5 replicates, and each replicate containing 10 chicks in a cage. The diets were: 1) a basal diet, 2) a diet supplemented with 50 mg/kg of rosemary, 3) a diet supplemented with 100 mg/kg of rosemary, 4) a diet supplemented with 50 mg/kg of oregano, 5) a diet supplemented with 100 mg/kg of oregano, 6) a combination diet containing 50 mg/kg each of rosemary and oregano, and 7) a combination diet containing 100 mg/kg each of rosemary and oregano. Dietary oregano extract enhanced the growth and feed utilization of heat-stressed birds, especially at a concentration of 50 mg/kg. Moreover, oregano extract improved jejunal protease and amylase activities. The extracts of rosemary and oregano significantly reduced IgG and IgM levels. Dietary 50 mg oregano extract significantly upregulated intestinal integrity-related genes including jejunal CLDNI, ZO-1, ZO-2, and MUC2. Dietary 50 mg oregano extract significantly downregulated hepatic NADPH oxidase 4 (NOX4) and nitric oxide synthase 2 (NOS2) expressions. Our results suggest that incorporating oregano leaf extract into the diet at a concentration of 50 mg/kg improves the growth performance of broilers exposed to heat stress. This improvement could be attributed to enhanced gut health and the modulation of genes associated with oxidative stress and tight junction proteins.

Molecular and Cellular Responses of DNA Methylation and Thioredoxin System to Heat Stress in Meat-Type Chickens.

Heat stress (HS) causes molecular dysfunction that adversely affects chicken performance and increases mortality. The responses of chickens to HS are extremely complex. Thus, the aim of this study was to evaluate the influence of acute and chronic exposure to HS on the expression of thioredoxin-peroxiredoxin system genes and DNA methylation in chickens. Chickens at 14 d of age were divided into two groups and reared under either constant normal temperature (25 °C) or high temperature (35 °C) in individual cages for 12 days. Five birds per group at one and 12 days post-HS were euthanized and livers were sampled for gene expression. The liver and Pectoralis major muscle were sampled for cellular analysis. mRNA expression of thioredoxin and peroxiredoxins (Prdx) 1, 3, and 4 in the liver were down-regulated at 12 days post-HS compared to controls. The liver activity of thioredoxin reductase (TXNRD) and levels of peroxiredoxin1 (Prdx1) at 12 days post-HS were significantly decreased. The results reveal that there was a significant decrease in DNA methylation at 12 days post HS in liver tissues. In conclusion, pathway of thioredoxin system under HS may provide clues to nutritional strategies to mitigate the effect of HS in meat-type chicken.

Modulation of Heat-Shock Proteins Mediates Chicken Cell Survival against Thermal Stress.

Heat stress is one of the most challenging environmental stresses affecting domestic animal production, particularly commercial poultry, subsequently causing severe yearly economic losses. Heat stress, a major source of oxidative stress, stimulates mitochondrial oxidative stress and cell dysfunction, leading to cell damage and apoptosis. Cell survival under stress conditions needs urgent response mechanisms and the consequent effective reinitiation of cell functions following stress mitigation. Exposure of cells to heat-stress conditions induces molecules that are ready for mediating cell death and survival signals, and for supporting the cell's tolerance and/or recovery from damage. Heat-shock proteins (HSPs) confer cell protection against heat stress via different mechanisms, including developing thermotolerance, modulating apoptotic and antiapoptotic signaling pathways, and regulating cellular redox conditions. These functions mainly depend on the capacity of HSPs to work as molecular chaperones and to inhibit the aggregation of non-native and misfolded proteins. This review sheds light on the key factors in heat-shock responses for protection against cell damage induced by heat stress in chicken.