ABSTRACT
Involution of the mammary gland following weaning is divided into two distinct phases. Initially, milk stasis results in the induction of local factors that cause apoptosis in the alveolar epithelium. Secondly after a prolonged absence of suckling, the consequent decline in circulating lactogenic hormone concentrations initiates remodeling of the mammary gland to the virgin-like state. We have shown that immediately following weaning TGFβ3 mRNA and protein is rapidly induced in the mammary epithelium and that this precedes the onset of apoptosis. Unilateral inhibition of suckling and hormonal reconstitution experiments showed that TGFβ3 induction is regulated by milk stasis and not by the circulating hormonal concentration. Directed expression of TGFβ3 in the alveolar epithelium of lactating mice using a β-lactoglobulin promoter mobilized SMAD4 translocation to the nucleus and caused apoptosis of these cells, but not tissue remodeling. Transplantation of neonatal mammary tissue derived from TGFβ3 null mutant mice into syngenic hosts resulted in a significant inhibition of cell death compared to wild-type mice upon milk stasis. These results provide direct evidence that TGFβ3 is a local mammary factor induced by milk stasis that causes apoptosis in the mammary gland epithelium during involution.
