Many pesticides, synthetic chemicals used in production agriculture to control insects (insecticides), fungi (fungicides) and weeds (herbicides), have been shown to have estrogenic activity in mammalian systems, including neoplasias of the breast. Epidemiological evidence from studies of rural communities have suggested that direct human expc, sure to these agents may account, at least in part, for the increased incidences for breast, endometrial and ovarian cancers in these sub-populations. With the pervasive use of these chemicals in production agriculture, exposure is not isolated to rural communities but may also manifest in urban societies via run-off in drinking water or chemical residues, remaining on vegetables and fruits. While pesticide-induced carcinogenesis has been suggested based on both in vivo and in vitro evidence, a consensus on whether exposure to relatively weak estrogenic pesticides is in fact physiologically relevant or even ultimately harmful has yet to be reached. Moreover, the dual actions of some endocrine-active pesticides as potential agonists and antagonists have resulted in a general confusion in the literature regarding the role that these compounds may play in cancer progression. While evidence mounts on both sides of the debate concerning the relevance of endocrine-active pesticides on cancer progression, it is becoming increasingly clear that endogenous cellular mechanisms exist which may augment the actions of pesticides, in addition to the fact that pesticides are more often found in mixtures not singularly, suggesting that low potency of one chemical by itself may be a poor measure of what can occur in vivo following pesticide exposure. An alternate explanation for the reported varied effects of endocrine-active pesticides on the mechanisms regulating cancer progression, particularly those influencing estrogen-sensitive pathways, is the fact that most cancer cell populations are heterogeneous and not homogeneous. Even in presumably homogeneous cancer cell lines, differential responses among individual cells have been observed and can mirror the heterogeneous nature of primary tumors. These differentially responsive subpopulations within a population of cancer ceils have been implicated as being responsible for the development of tumoral chemotherapeutic "resistance," and the subsequent recurrence of primary tumors post-treatment. However, the role of endocrine-active pesticides as potential effectors and/or differential regulators of individual cancer cells within a population have yet to be examined. The case for endocrine-active pesticides as variable modulators of cancer development may be greatly impacted by findings that might demonstrate differential responsiveness among individual cells within a population exposed to such agents alone or in relevant mixtures. The goal of this study is to examine how estrogenic and non-estrogenic endocrine-active pesticides alone or in combination (i.e., mixtures) may augment or inhibit endocrine responses in breast cancer cells, and whether these interactions might result in a differential selection (directly or indirectly) for an invasive phenotype. The rationale for this is that by understanding how pesticide mixtures may alter the endocrine behavior of cancerous cells, we may better understand the implications of pesticide exposure in relation to cancer risk. Moreover, by clarifying the responses of cells to pesticides within heterogeneous cancer cell societies, we may better appreciate how/why tumors become resistant or highly sensitive to chemotherapeutic or other targeted endocrine agents. The specific hypothesis to be tested is that pesticides alone and in combination can re-model breast cancer cell populations. To pursue the research objectives of this application, the following four Specific Aims have been formulated: 1. To examine the estrogen receptor- and non-estrogen receptor-mediated effects of pesticides (alone and in combination) on estrogen receptor expression and estrogen-sensitive gene transcription in breast cancer cells. 2. To elucidate whether endogenous cellular pathways may influence the actions of pesticides (alone or in mixtures) on estrogen-regulated gene transcription in breast cancer cells. 3. To determine whether endocrine-active pesticides (alone or in mixtures) may differentially regulate the responsiveness of subpopulations of cells within cancer cell societies. 4. To evaluate whether selected subpopulations of cancer cells (e.g., tamoxifen resistant phenotypes) are more or less responsive to endocrine-active pesticides (alone or in mixtures).