Disruption of cell junctions induces apoptosis and reduces synthetic activity in lactating goat mammary gland

Although it is known that disruption of the cell junctions in the mammary gland induces a decrease in milk yield, the cellular mechanisms involved in milk secretion reduction during mammary cell junction disruption are not well understood. The aim of this study was to investigate the cellular regulations taking place after cell junction disruption in the mammary gland of goats. We performed intramammary infusions of Ca chelators to induce cell junction disruption. In a first group of 5 goats, intramammary infusions of ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) in the right gland halves and saline as a control in the left gland halves were performed after 4 consecutive milkings. A second group of 4 goats received 4 intramammary infusions of citrate solution in the right gland halves and lactose solution as a control in the left halves. Intramammary infusion of EGTA and lactose induced a disruption of cell junctions, whereas citrate infusions failed to modify mammary epithelium integrity. The effect of the infused solutions was also tested in vitro via the measurement of transepithelial resistance, confirming mammary epithelium disruption by the EGTA, lactose, and citrate solutions at high concentrations. The disruption of mammary epithelium integrity by EGTA induced a decrease in the expression of the cell junction protein E-cadherin. Both the EGTA and lactose infusions induced a decrease in milk secretion that was accompanied by cellular modifications. We observed a decrease in milk casein, which was associated with a decrease in the mRNA level of kappa-casein in the lactose-infused glands, and a decrease in milk lactose, which was associated with a downregulation of alpha-lactalbumin transcripts in both the EGTA- and lactose-treated glands. Both the EGTA and lactose infusions increased terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'-triphosphate nick-end labeling (TUNEL) in the mammary tissue, indicating an induction of apoptosis. Lactose infusion increased the mRNA level of Bax, suggesting that apoptosis was regulated at the transcriptional level. The results obtained in these experiments suggest that disruption of mammary epithelium integrity was associated with both reduced synthetic activity and apoptosis induction in the mammary gland.