Direct and indirect endocrine disruption : aromatase and estrogen receptor-mediated processes in breast cancer development

Endocrine disrupting chemicals (EDCs) have been defined by the World Health Organization as: “exogenous substances or mixtures that alters function(s) of the endocrine system and causes adverse health effects in an intact organism, or its progeny, or (sub)populations”. Synthetic, as well as, naturally occurring chemicals can disrupt the endocrine system. EDCs can exert their effect through multiple mechanisms. For example, compounds can bind to a steroid hormone receptor and subsequently block or induce the response. Estrogenic effects of chemicals have been subject of extensive research during the past decades. Interactions with the estrogen receptor (ER) could possibly lead to adverse effects in the human body and therefore, in the US, companies are obliged to test their compounds for estrogenic effects before being allowed to bring the product onto the market. In Japan and Europe, legislation forcing companies to perform tests for estrogenicity is being introduced. However, recently studies on chemicals affecting estrogen biosynthesis have attracted increasing attention. By enhancing estrogen synthesis, chemicals exert an indirect estrogenic effect. Estrogens play an important role in breast cancer development. Malignant tumors of the mammary gland are the most common type of cancer in women. Approximately 60% of all breast tumors are estrogen-responsive and depend on estrogens for growth. Therefore, chemicals that show estrogenic properties might be able to interfere with breast tumor growth. Epithelial breast tumor cells express ERs on their cell membrane to which endogenous and/or exogenous estrogens can bind. Connective tissue surrounding the tumor cells are able to produce estrogens through the aromatase aromatase enzyme. Aromatase activity is in turn increased by factors that are secreted by the epithelial tumor cells. In this way, a positive feedback loop is established, resulting in rapid growth of the tumor. This thesis describes estrogenic effects of frequently used UV-filters. Our studies showed that daily exposure to UV-filters through sunscreen products could possibly lead to estrogenic effects in humans. Methyl sulfonyl PCB-metabolites inhibit aromatase activity in primary human mammary connective tissue. Furthermore, several pesticides either induce or inhibit aromatase activity in human or rat cell lines. The R2C rat Leydig cell carcinoma cell line is suitable for detection of aromatase inhibitors and the H295R human adrenocortical carcinoma cell line is suitable to test for the detection of both inducers and inhibitors of aromatase activity. Increased estrogenic effects after aromatase induction and increased aromatase gene transcription after exposure to estrogenic compounds, suggest the presence of paracrine interactions between two cell types in our co-culture of MCF-7 human malignant breast tumor cells and primary human mammary fibroblasts. Indirect estrogenic effects caused by induction of aromatase activity should be considered when testing for estrogenic effects of compounds