Impacts of Heat Stress-Induced Oxidative Stress on the Milk Protein Biosynthesis of Dairy Cows

SIMPLE SUMMARY: Heat stress (HS) of dairy cows affects milk protein synthesis partially due to a decline in appetite and dry matter intake (DMI). Several published studies indicate that HS causes oxidative stress (OS) with high levels of free radicals in tissues. Furthermore, the involvement of reactive oxygen species (ROS) in inducing apoptosis, in reducing mammary epithelial cell numbers, and in endocrine disruptions are proposed as the main mechanisms by which HS-induced OS modifies milk protein synthesis. However, challenges remain in determining the levels of apoptosis in vivo, as well as the tracking of the sources of ROS formation. Therefore, further investigations are required. ABSTRACT: Heat stress (HS) is one of the most important factors posing harm to the economic wellbeing of dairy industries, as it reduces milk yield as well as milk protein content. Recent studies suggest that HS participates in the induction of tissue oxidative stress (OS), as elevated levels of reactive oxygen species (ROS) and mitochondrial dysfunction were observed in dairy cows exposed to hot conditions. The OS induced by HS likely contributes to the reduction in milk protein content, since insulin resistance and apoptosis are promoted by OS and are negatively associated with the synthesis of milk proteins. The apoptosis in the mammary gland directly decreases the amount of mammary epithelial cells, while the insulin resistance affects the regulation of insulin on mTOR pathways. To alleviate OS damages, strategies including antioxidants supplementation have been adopted, but caution needs to be applied as an inappropriate supplement with antioxidants can be harmful. Furthermore, the complete mechanisms by which HS induces OS and OS influences milk protein synthesis are still unclear and further investigation is needed.