Free fatty acids induce mammary epithelial cell death in ketotic dairy cows via endoplasmic reticulum stress

Ketosis is one of the most common metabolic disorders in dairy cows during the peripartum period. The major economic losses caused by ketosis are mainly attributable to reduced milk yield. However, the mechanisms by which metabolic stress leads to mammary gland dysfunction have remained incompletely understood.

A hallmark of ketosis is the state of negative energy balance induced by metabolic stress, which is characterized by high and sustained free fatty acid (FFA) concentrations in the circulation. While the hepatic response to elevated FFA is well documented, the direct impact on the mammary gland—the organ responsible for milk production—has been less clear.

A team of researchers from China Agricultural University, Heilongjiang Bayi Agricultural University, and collaborating institutions has uncovered a key mechanism: high concentrations of FFA induce apoptosis of bovine mammary epithelial cells via endoplasmic reticulum (ER) stress signaling. Their findings are published in the Journal of Integrative Agriculture.

"Excessive apoptosis leads to a decline in the number of functional cells in the mammary gland of dairy cows," explains corresponding author Chuang Xu, a professor at China Agricultural University. "Our previous studies revealed an activation of apoptosis in mammary epithelial cells and a subsequent decrease in milk yield in ketotic dairy cows. Here, we investigated whether the ER stress pathway plays a role in this process."

The team first compared mammary gland tissues from healthy cows and cows with clinical ketosis. The protein abundances of key ER stress markers, including GRP78, ATF6, the ratios of p-IRE1/IRE1 and p-PERK/PERK, and CHOP were significantly greater in the mammary glands of dairy cows in the clinical ketosis group.

To establish causality, the researchers treated bovine mammary epithelial cells (MAC-T cells) with increasing concentrations of FFA. "We found that treatment with FFA led to increased activation of the ER stress pathway, as evidenced by elevated levels of key ER stress markers, and a corresponding increase in cell apoptosis," shares Xu.

"We then used both an activator and an inhibitor of ER stress to test whether this pathway is essential for FFA-induced apoptosis," says co-corresponding author Xudong Sun, a professor at Heilongjiang Bayi Agricultural University. "Pretreatment with tunicamycin (Tun), an ER stress activator, aggravated ER stress and apoptosis in MAC-T cells induced by FFA. In contrast, pretreatment with Tauroursodeoxycholate (TUDCA), an ER stress inhibitor, attenuated ER stress induced by FFA and also attenuated the apoptosis in MAC-T cells."

Notably, inhibition of ER stress reduced apoptosis without affecting other key cellular functions, demonstrating that ER stress is a critical mediator in FFA-induced apoptosis.

"Our data confirmed that ER stress is a key mediator linking elevated FFA to mammary cell death in dairy cows with ketosis," adds Xu. "Timely resolution of ER stress may help counteract the negative effects of ketosis on the mammary gland."

The study opens the door to potential new therapeutic strategies. ER stress inhibitors such as TUDCA may offer a promising therapeutic approach for cows undergoing metabolic disorders that elevate circulating FFA such as ketosis.

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Contact the author: Xudong Sun, E-mail: sunxudong0323@163.com; Chuang Xu, E-mail: xuchuang@cau.edu.cn; xuchuang7175@163.com

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