Downregulation of CASTOR1 Inhibits Heat-Stress-InducedApoptosis and Promotes Casein and Lipid Synthesis in MammaryEpithelial Cells

Heat stress is one of the most important factors limiting the milk yields of dairy animals. This decline can beattributed to the heat-stress-induced apoptosis of mammary epithelial cells (MECs). The cytosolic arginine sensor for mTORC1subunit 1 (CASTOR1) is a crucial upstream regulator of the mechanistic target of rapamycin complex 1 (mTORC1) signaling,which has close connections with apoptosis. However, the specific roles of CASTOR1 in regulating the apoptosis and lactation ofMECs are still obscure. In the present study, we found that heat stress promotes apoptosis and CASTOR1's expression in HC11cells. Downregulation of CASTOR1 inhibits heat-stress-induced apoptosis through a ROS-independent pathway. In addition,silencing of CASTOR1 promotes cell proliferation, cell cycle progression, and milk component synthesis, and overexpressing ofCASTOR1 reverses these observations. Furthermore, we found that silencing of CASTOR1 contributes to the nuclear transport ofSREBP1 and promotes lipid synthesis. This study demonstrates the pivotal roles of CASTOR1 in heat-stress-induced apoptosis andmilk component synthesis in MECs.