Cytochrome P450 modulates the therapeutic actions of tamoxifen, as evidenced in novel breast cancer models

Regulation of cytochrome P450s in breast cancer and their role for tumour growth and anticancer chemotherapy are studied. Mammary cancer can develop for many reasons, one of which is the exposure to environmental carcinogens and/or steroid hormones. The cytochrome P450 enzyme family not only catalyses the metabolism of a wide range of carcinogens but is also involved in metabolism of steroids. This process alters their steroidogenic properties, a mechanism important for mammary carcinogenesis.

At the centre of this research stand cytochrome P450 1B1 (CYP1B1) and cytochrome P450 CYP1A1. Unlike many other P450s, these isoforms are expressed extrahepatically. CYP1B1 protein is found to be overexpressed in tumours compared with the corresponding healthy tissues. Special regulatory mechanisms are likely to cause this difference.

In the present study we employed TaqMan analysis, immunoblotting and reporter assays to investigate the expression patterns of CYP1B1 and CYP1A1 in a panel of breast cancer cell lines derived from different stages of mammary carcinomas. Furthermore, we investigated the expression of these P450s in cell lines derived from primary human mammary epithelial cells that have been transfected with various combinations of oncogenes and telomerase. In the transformed human mammary epithelial cells we found that the expression of CYP1B1, CYP1A1 and their inducibility by TCDD was differentially affected by the different oncogenes. Presently, we investigate the regulatory mechanisms that cause this response.

In a second investigation, we analysed the relevance of P450 expression for mammary-tumour development and tumour therapy. For this purpose we have developed MCF-7-derived cell lines in which the expression of CYP1A1 and CYP1B1 could be switched on/off by treatment with low doses of doxicycline. We demonstrated that expression of these P450s altered the effects of estrogens and antiestrogens on cell cycle and apoptotic markers. The MCF-7-derived cell lines were grown in xenografts. P450 expression was induced by doxicycline in the drinking water. We were able to demonstrate that P450 expression in our xenograft-model was tightly regulated by tetracycline. In future, animals will be treated with or without tamoxifen. Subsequently, the effects of P450 expression on tumour growth, angiogenesis and apoptosis will be measured. It is anticipated that the results of these investigations will greatly enhance our understanding about the aetiology of breast cancer and may provide strategies to improve treatment.