Hormonal Carcinogenesis: Estrogens and Reactive Oxygen Species

Reactive oxygen species (ROS) are known critical contributors to the process of carcinogenesis from initiation through promotion and, possibly, also dissemination by metastases. Environmental chemicals and dietary constituents have been generally considered to be the major sources of potentially carcinogenic ROS and the causes of cellular oxidative stress. However, there is growing body of evidence that hormonal estrogens may also become a source of ROS, and that the mechanism involved are analogous to P450-mediated metabolic activation responsible for the generation of ROS from xenobiotics. The potentially carcinogenic products of estrogens identified are the quinone derivatives of catechol-estrogens, that are the products aromatic hydroxylation of estrone and estradiol, and ROS produced in the course of redox cycling between quinone- and catecholestrogens. Interest in this pathway of metabolic activation of estrogens has been heightened by evidence that estrogens' role in carcinogenesis suggested by epidemiological studies can not be fully accounted for by their estrogen receptor mediated actions as mitogens.

Our previous studies focused on characterizing the pathways of catecholestrogen formation, in particular, in the hamster kidney model of estrogen induced renal cancer. We are now concentrating on establishing the relevance of the findings obtained in these and other basic biochemical and molecular biological studies to the human, in particular, the genesis of breast cancer. We are using a multidisciplinary approach encompassing molecular biology, biochemistry and cytochemistry. Thus far we have demonstrated normal human mammary ductal epithelial cells (that are the cells of origin of most breast cancer), express virtually all of the genes encoding the known enzymes required for the metabolic activation and redox cycling of estrogens. We are in the process of characterizing the regulatory untranslated portion of the transcript for most recently identified member of the P450 superfamily that can catalyze aromatic hydroxylation of estrogens, and then plan to identify and clone the gene for what promises to be a novel form of P450 that can catalyze catecholestrogen formation via a peroxidatic mechanism. This mechanism, first identified in our previous biochemical studies, has characteristics that could make it especially relevant to carcinogenesis since it utilizes as its co-factor organic hydroperoxides known to be generated under conditions of oxidative stress.

We have recently turned our attention to developing a histoculture system appropriate for examining the role of estrogen metabolism in immortalization and transformation of human mammary epithelial cells. We have decided to use histoculture, rather than cell lines for these studies, because in histoculture the three dimensional structure and cellular heterogeneity of the mammary gland is preserved. Hence, it allows for cell-cell interactions, including those between stromal and epithelial cells that are known to play a critical role both in normal hormone action and in carcinogenesis. Once characterized, this system will enable us not only to assess the effect of estrogens and of estrogen metabolism on breast tissue but also the interactions between estrogens and environmental chemicals.