Milk accumulation decreases expression of genes involved in cell–extracellular matrix communication and is associated with induction of apoptosis in the bovine mammary gland

Forty-eight primiparous, non-pregnant, Friesian cows in mid-lactation, were used to investigate the cell–extracellular matrix (ECM) communication in mammary epithelial cell (MEC) survival during induced mammary involution and to examine cell survival and apoptotic signalling events. Cows were sacrificed and alveolar mammary tissue was obtained at 0, 6, 12, 18, 24, 36, 72 and 192 h (n = 6 per group) after termination of milking. Tissue mRNA concentrations, measured by quantitative real-time RT–PCR, of different integrins (β1, α6 and α5), down-stream signal transduction factors FAK and 14-3-3, and cell survival members BAG-1 and Bcl-x_long of the Bcl-2 family were decreased by 24 h compared to 6 h, with no further decrease to 8 days (192 h). The pro-apoptotic member of the Bcl-2 family, αBax mRNA level was increased by 8 days. Apoptosis within the epithelial cell layer surrounding the alveolar lumen, measured by in situ end-labelling (ISEL), was evident. There were a low number of ISEL nuclei, in lactating mammary tissue (6 h), which was increased by 72 h of involution. By 8 days of induced involution, there was a dramatic increase in apoptotic products within the lumen, accounting for the majority of total apoptotic cells, which was accompanied by neutrophil infiltration. In conclusion, cell–ECM communication becomes compromised in the bovine mammary gland 18–24 h after termination of milking, as indicated by a decrease in the expression of integrins and cell survival factors. The different down-regulated integrins implicate crosstalk between integrins and growth factor receptors may occur during involution of the bovine mammary gland. Pro-apoptotic factors are up-regulated by 8 days of involution.