The effects of aromatase overexpression on mammary growth and gene expression in the aromatase x transforming growth factor alpha double transgenic mice

The transforming growth factor alpha (TGFα) and its receptor (EGFR) are expressed in many breast cancers. Typically, the progression of estrogen dependent primary breast cancers into a hormone-independent state, due to the loss of the estrogen receptor, is associated with increased levels of TGFα and EGFR, leading to aggressive breast carcinomas. The relationship between breast tumorigenesis and TGFα is evident in the transgenic mice overexpressing TGFα in the mammary glands. In the aromatase transgenic mice, the mammary glands exhibit preneoplastic developments but do not form frank tumors. To test the interactions between growth factor overexpression with tissue estrogen, we have crossed the aromatase transgenic mice with the TGFα transgenic mice to produce a double transgenic strain. The histological data for the mammary glands of aromatase x TGFα double transgenic mice show that these mice develop hyperplastic changes similar to the aromatase parental strain but no tumors are formed. Consistently, the expression of cyclin D1 and PCNA is diminished in the double transgenic strain as compared to the parental strains. In addition, the expression of TGFα, EGF and EGFR are also decreased in the double transgenic strain, suggesting that continuous estrogen presence in the tissue due to aromatase overexpression downregulates the expression of EGFR and its ligands.