Short communication: Endoplasmic reticulum stress gene network expression in bovine mammary tissue during the lactation cycle.

The endoplasmic reticulum (ER) has a crucial role in cellular metabolism. Recent studies in nonruminants discovered that components of the ER stress pathway, induced during the unfolded protein response, play critical roles in regulating lipogenesis. The bovine mammary gland faces extreme metabolic stress at the onset of lactation due primarily to the increase in flux through pathways associated with milk fat and protein synthesis. Our objective was to study, via quantitative real-time PCR, the expression of the ER stress pathway components P58IPK, PERK, XBP1, ATF4, ATF3, ATF6, CHOP, MBTPS1, GRP94, and BiP in mammary tissue (n=7 cows × 5 time points) collected at -15, 1, 15, 60, and 240 d relative to parturition. Expression of P58IPK and ATF4 increased to a peak at d 60, followed by a decrease by d 240 postpartum. Despite the decrease in expression by 240 d, P58IPK remained higher than prepartal levels (d -15). Expression patterns of ATF3 and CHOP were similar and peaked at d 15, followed by a decrease through d 240, at which point CHOP expression was still greater than prepartal levels. The sharp increase in milk production postpartum (d 15) as well as apoptosis during late lactation (240 d) may have induced a pseudo unfolded protein response state. This is supported by the similar expression patterns of P58IPK and PERK. In the context of lactation, however, transcriptional changes in the ER stress pathway at different stages of the lactation cycle are a normal aspect of the tissue's adaptation to the changing physiological state.