Biochemical changes during growth and regression of pregnancy-dependent mammary tumors of GR/A mice.

The weights of pregnancy-dependent mammary tumors (PDMT) of GR/A mice continued to increase until parturition and decreased soon after delivery; however, mitotic indices in epithelial cells and stromal cells of PDMT reached a maximum plateau on Day 18-19 of pregnancy and decreased thereafter. Growth of PDMT in progesterone-treated mice on Day 15 of pregnancy was higher than that in 17 beta-estradiol-treated mice and no treatment controls. DNA fragmentation was observed in PDMT on Day 20 of pregnancy and just after parturition. Two-dimensional gel electrophoresis of PDMT extracts revealed that five and six protein spots appeared newly on Day 20 of pregnancy and just after parturition, respectively. N-terminal amino acid sequences of two of the protein spots were identical to that of alpha-lactalbumin. PDMT on Day 15 and 20 of pregnancy and just after parturition secreted matrilysin, one of the matrix metalloproteinases, which was identified by Western blotting. However, matrilysin was not found in hormone-independent autonomous mammary tumors of the mouse. Estrogen receptor and c-fos mRNA expression levels in PDMT were high on Day 15 of pregnancy but low on Day 20 of pregnancy and just after parturition. These findings suggest that regression of PDMT is caused by apoptosis, and new proteins expressed on Day 20 may participate in the process of regression.