Effects of hydrostatic compression on milk production-related signaling pathways in mouse mammary epithelial cells

This study tested whether elevated intraluminal hydrostatic compression, which rises with milk accumulation, directly regulates lactation-associated signaling in mammary epithelial cells (MECs). Using a lactation culture model on Transwell inserts (prolactin + dexamethasone to induce milk protein production and tight junction formation), MECs were exposed to hydrostatic compression at 100 kPa. Sustained compression for 8 h reduced β-casein and increased the tight-junction protein claudin-4, accompanied by reduced STAT5 activation and decreased GR, Akt, and mTOR pathway activation. Acute compression for 1 h decreased STAT5 phosphorylation and strongly activated p38 MAPK, implicating a rapid stress-response route to STAT5 inactivation. In contrast, a physiologically inspired repetitive regimen (six cycles of 1 h at 100 kPa followed by 1 h release; total 12 h) increased intracellular lactoferrin and elevated secreted αs1-casein and β-casein, while activating Akt/mTOR and also activating negative regulators (STAT3 and ERK). Overall, hydrostatic compression produced time- and pattern-dependent signaling effects, supporting a model where sustained high pressure suppresses milk-production pathways, whereas cyclic pressure fluctuations can stimulate milk protein output while concurrently engaging counter-regulatory signaling.