Metallothionein expression and apoptosis in pregnancy-dependent and-independent mouse mammary tumors

Background. A metal binding protein, metallothionein (MT), may be involved in the regulation of carcinogenesis and apoptosis in addition to various ph ysiological processes. Materials and Methods: MT and sex hormone receptors (estrogen and progesterone receptors, ER and PR) expressions, and apoptosis were investigated immunohistochemically in pregnancy-dependent (PDMT) and -independent mammary tumors (PIMT) in GR/A mice in order to evaluate the po ssible role of MT in the genesis and regression of tumors. Results: In PDMT and PIMT, MT was localized in the nucleus and/or cytoplasm of tumor cells. In PDMT, MT expression and apoptotic figures were increased during the reg ression period after parturition. MT and ER expressions in PIMT were approx imately the same as those in the growing PDMT, while PR expression was lowe r in PIMT than in the PDMT. A significant correlation was observed between MT expression and apoptosis in PDMT, but not in PIMT. Conclusion: The prese nt study suggests that MT involvement in the PDMT regression is associated with apoptosis.