Estrogen biosynthesis in adipose tissue increases with age and obesity, and has been implicated in the development of endometrial cancer and breast cancer. In normal human adipose tissue, expression of the CYP19 gene which encodes aromatase P450, the enzyme responsible for estrogen biosynthesis, is regulated by a distal promoter, namely promoter I.4. Stimulation of expression in adipose stromal cells by members of the type 1 cytokine family, i.e. interleukin (IL)-6, IL-11, leukemia inhibitory factor (LIF) and oncostatin M (OSM), is mediated via a Jak-STAT3 signaling pathway and a GAS element upstream of promoter I.4. In contrast, aromatase expression in breast adipose tissue proximal to tumor is increased three- to four-fold to the utilization of another promoter, namely promoter II, proximal to the translation initiation site. In the present report, we show that prostaglandin (PG) E2 is the most potent factor which stimulates aromatase expression via cyclic AMP and promoter II. PGE2 acts via EP1 and EP2 receptor subtypes to stimulate both the PKC and PKA pathways. The combined stimulation of both of these pathways results in the maximal expression of promoter II-specific CYP19 transcripts. Because PGE2 is a major secretory product both of breast tumor epithelial cells and fibroblasts, as well as of macrophages infiltrating the tumor site, then this could be the mechanism whereby estrogen biosynthesis is stimulated in breast sites adjacent to a tumor, leading in turn to increased growth and development of the tumor itself.