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Estradiol (E2), epidermal growth factor (EGF) and acetaminophen (APAP) each exert mitogenic activity in estrogen receptor-positive (ER+) MCF-7 human breast cancer cells. Antiestrogens inhibit the mitogenic activity of each of these compounds, suggesting that the ER plays a role. EGF is an established activator of the Erk/MAPK signal transduction pathway, and E2 reportedly activates this pathway as well. In addition, both E2 and EGF have been reported to induce ER phosphorylation, a modification of the receptor that is important in ER signaling. Therefore, the first goal of this research project was to determine the extent to which all three mitogens affect Erk/MAPK activation. In agreement with other reports, EGF induced robust Erk/MAPK activation; however, E2 did not stimulate Erk/MAPK. APAP had no effect on Erk1/2 activation. The next goal was to determine the extent to which phosphorylation of endogenous ERα is altered by these three mitogens in MCF-7 cells. In accordance with established findings, E2 induced significant ERα phosphorylation, as assessed by an upshift of the total ERα population and detection of phosphorylation of ERα at serine 118 in Western blotting. However, neither EGF nor APAP evoked phosphorylation of endogenous ERα. Although E2 did not induce Erk1/2 activation, the Erk/MAPK pathway has been implicated in E2-induced ERα phosphorylation. The third objective of this project was to identify signaling pathways that play a role in E2-induced ERα phosphorylation through the use of a variety of kinase inhibitors as well as RNA-mediated interference (RNAi). The results of these studies suggest that the basal activity of Erk1/2 plays a role is E2-mediated ERα phosphorylation. Finally, in an attempt to rectify conflicting reports regarding the effect of E2 on Erk/MAPK activation, more in-depth studies were undertaken to assess various factors that may impact experimental results. Although some experimental variables that may impact Erk/MAPK activation were identified, these studies could not completely resolve why some reports describe E2-induced Erk1/2 activation and others do not.