Expression and activity of cell cycle regulators during proliferation and programmed cell death in the mammary gland

In the mammary gland distinct phases of proliferation, differentiation and programmed cell death of epithelial cells occur at defined stages of development. Here we show that the expression and activity of cell cycle regulators during normal and preneoplastic proliferation and programmed cell death are remarkably similar. In all cases we found elevated levels of a protein kinase A activity and of transcription factor AP-1, cFos and JunD being the major components of the AP-1 DNA binding complex. A correlation between cFos and JunD expression and chromosomal DNA fragmentation during programmed cell death was observed. Several genes associated with G1, including cyclin D1, D2 and D3 and c-fos, c-jun, junB, JunD, c-myc and p53, are induced in proliferating and in apoptotic mouse mammary tissue. Whereas the expression of these genes correlated with active proliferation of epithelial cells in terminal end buds during puberty, very little proliferation or DNA synthesis, but, instead, extensive apoptosis of epithelial cells, was observed during involution. Our results suggest that a G1-like state is associated with programmed cell death of mammary epithelial cells in vivo and that apoptosis occurs without S-phase induction.