Nongenomic Activity and Subsequent c-fos Induction by Estrogen Receptor Ligands Are Not Sufficient to Promote Deoxyribonucleic Acid Synthesis in Human Endometrial Adenocarcinoma Cells

Estrogen 17β-estradiol (E2) rapidly modulates several signaling pathways related to cell growth, preservation, and differentiation. The physiological role of these nongenomic effects with regard to downstream outcomes, and the relationship with transcriptional estrogen activity are unclear. Furthermore, the ability of selective estrogen receptor modulators (SERMs) to trigger nongenomic actions is largely unknown. To determine whether estrogen receptor (ER) ligands exert nongenomic activity in endometrial adenocarcinoma cells, and whether this activity affects transcription and DNA synthesis, we challenged human Ishikawa cells with E2 or partial ER agonists 4-hydroxytamoxifen (OHT) and raloxifene (ral). Serum-starved Ishikawa cells exposed for 5 min to 0.1 nm E2 showed induced phosphorylation of MAPK (ERK1/2). Ral and 4-OHT each at 1 nm also stimulated ERK in a rapid transient manner. E2 and 4-OHT induced proto-oncogene c-fos mRNA expression in Ishikawa cells within 30 min, but ral had no effect. In contrast to nongenomic action, only E2 stimulated expression of an estrogen response element (ERE)-driven luciferase (LUC) reporter gene. To examine DNA synthesis, [3H]-thymidine incorporation was measured in serum-starved cultures exposed to E2 or partial agonists for 2 d. E2 at 1 nm stimulated thymidine uptake in an ERK-dependent manner, but 1 nm 4-OHT, 1 nm ral, and 0.1-nm concentrations of E2 had no significant effects. Taken together, these data indicate that both nongenomic and direct transcriptional ER effects are likely required to promote DNA synthesis.