Cancer is the leading cause of death among women between the ages of 30 and 54. An estimated 186,000 new cases of breast cancer will be diagnosed, and 42,000 women in the U.S. will die from breast cancer in 2001. Currently, one out of nine American women will develop breast cancer in her lifetime. This research focuses on examination of the role of steroid hormones in breast cancer etiology. An estimated 60-70% of human breast cancers are associated with sex hormone exposure. Approximately 60% of all breast cancer patients have hormone-dependent breast cancer, which contains estrogen receptors and requires estrogen for tumor growth. The chemical forms of estrogen molecules and their relative concentrations in the breast tissue can influence breast epithelial cancer cell growth. Furthermore, the ratios of various endogenous estrogen molecules can influence (either positively or negatively) other cell types in the breast tissue environment, such as stromal cells and vascular endothelial cells. Our hypothesis is that alterations in the breast cancer tissue microenvironment can influence the extent of estrogen biosynthesis and metabolism, result in altered levels of biologically active estrogens and their metabolites, and therefore influence breast tumor development and growth. Recent studies in our laboratory supported by the current grant demonstrated a relationship between CYP19 gene expression and the expression of COX genes. This significant relationship between the aromatase and cyclooxygenase enzyme systems suggests that autocrine and paracrine mechanisms may be involved in hormone-dependent breast cancer development via growth stimulation from local estrogen biosynthesis. Biochemical and molecular examination of this hypothesis in vitro will be performed in human patient breast tissue specimens and in several human breast cancer cell systems currently in use in our laboratories. The specific aims are: (1) To examine the relationship between aromatase and cyclooxygenases in human breast cell culture systems by correlating enzyme activities and by modulating cyclooxygenases using COX inhibitors or antisense approaches. (2) To investigate potential biochemical mechanisms for the interactions of these biosynthetic pathways using selective membrane EP agonists and antagonists and pharmacological agents acting through PPAR. (3) To measure cellular levels of aromatase expression and cyclooxygenases expression in human breast cancer specimens.