Methionine Controls ARID1A Stability and mTOR Transcription via the AKT–USP43 Axis to Enhance Milk Synthesis in Bovine Mammary Epithelial Cells

Amino acids regulate milk synthesis in bovine mammary epithelial cells through mechanistic target of rapamycin (mTOR) signaling, yet the underlying epigenetic mechanisms remain incompletely understood. This study identifies ubiquitin-specific protease 43 (USP43) as a negative regulator of milk synthesis. USP43 directly interacted with the chromatin remodeling factor AT-rich interaction domain 1A (ARID1A) and removed its K48-linked polyubiquitin chains, thereby stabilizing ARID1A protein and inhibiting mTOR transcription. Methionine stimulation induced AKT-dependent translocation of USP43 from the nucleus to the cytoplasm, which was associated with AKT-mediated USP43 phosphorylation. This process diminished the interaction between USP43 and ARID1A within the nucleus, leading to ARID1A protein degradation, mTOR transcriptional activation, and enhanced synthesis of milk proteins and lipids. Collectively, our findings delineate a USP43–ARID1A–mTOR epigenetic regulatory axis that modulates amino acid-induced milk synthesis. These findings provide a novel epigenetic perspective on how extracellular nutrients regulate milk synthesis.