Developmental regulation of Bcl-2 family protein expression in the involuting mammary gland

Epithelial cells within the mammary gland undergo developmental programmes of proliferation and apoptosis during the pregnancy cycle. After weaning, s ecretory epithelial cells are removed by apoptosis. To determine whether me mbers of the Bcl-2 gene family could be involved in regulating this process , we have examined whether changes in their expression occur during this de velopmental apoptotic program in vivo. Bax and Bcl-x were evenly expressed throughout development. However, expression of Bak and Bad was increased du ring late pregnancy and lactation, and the proteins were present during the time of maximal apoptotic involution, Thereafter, their levels declined. I n contrast, Bcl-w was expressed in pregnancy and lactation but was downregu lated at the onset of apoptosis, Bcl-2 was not detected in lactating or ear ly involuting mammary gland. Thus, the pro-apoptotic proteins Bax, Bak and Bad, as well as the death-suppressors Bcl-x, Bcl-2 and Bcl-w, are synthesis ed in mouse mammary gland, and dynamic changes in the expression profiles o f these proteins occurs during development. To determine if changes in Bak and Bcl-w expression could regulate mammary apoptosis, their effect on cultured mouse mammary epithelial cells was exam ined in transient transfection assays. Enforced expression of Bah induced r apid mammary apoptosis, which could be suppressed by coexpression of Bcl-w. In extracts of mammary tissue in vivo, Bak heterodimerized with Bcl-x wher eas Bar associated with Bcl-w, but Bak/Bcl-w heterodimers were not detected . Thus, Bak and Bcl-w may regulate cell death through independent pathways. These results support a model in which mammary epithelial cells are primed for apoptosis during the transition from pregnancy to lactation by de novo expression of the death effecters Bah and Bad. It is suggested that these p roteins are prevented from triggering apoptosis by anti-apoptotic Bcl-2 fam ily proteins until involution, when the levels of Bcl-w decline. Our study provides evidence that regulated changes in the expression of cel l death genes may contribute to the developmental control of mammary apopto sis.