Tight Junction Protein Abundance and Apoptosis During Involution of Rat Mammary Glands

To examine tight junction protein abundance and apoptosis of epithelial cells at the onset of involution in rodent mammary glands, milk accumulation and mammary engorgement were induced by teat-sealing with an adhesive for 0, 6, 12, 18, 24, and 36h (n=6 per group) at peak lactation. In non-sealed control glands, histological analysis confirmed a lactating phenotype, indicating suckling by pups throughout the experiment. In contrast, alveoli of teat-sealed glands were distended within 6h, with maximal luminal size observed by 12h of non-suckling. By 18h following teat-sealing, an involuting phenotype was observed, indicated by alveolar lumina engorged with milk vesicles and increased leukocytes. Relative to non-sealed glands, mammary apoptosis was increased in engorged glands 18h following teat-sealing. The abundance of ZO-1 and occludin proteins was decreased in engorged glands by 12 and 18h, respectively, following teat-sealing. In contrast, the claudin-1 22kDa band was increased by 6h and peaked at 12-18h, whereas the 28kDa band declined by 36h, relative to controls. There were no temporal changes in ZO-1, occludin, and claudin-1 22kDa proteins within control glands, although there were minor differences in claudin-1 28kDa. These data indicate that intramammary milk accumulation due to cessation of milk removal is associated with mammary apoptosis. The apoptotic event is preceded by a rapid loss of abundance of ZO-1, occludin and an initial increase in claudin-1. The loss of cell-cell communication may initiate involution and apoptosis of mammary epithelial cells and is a localized intramammary event, occurring only in non-suckled glands. J. Cell. Physiol. 232: 2075-2082, 2017. (c) 2016 Wiley Periodicals, Inc.