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We examined the mechanism by which estrogen regulates telomerase activity in Caov-3 human ovarian cancer cell lines, which express ER, to determine whether the regulation affects the expression and/or phosphorylation of the telomerase catalytic subunit (hTERT). 17β-Estradiol (E2) induced telomerase activity and hTERT expression. Transient expression assays using luciferase reporter plasmids containing various fragments of hTERT promoter showed that the estrogen-responsive element appeared to be partially responsible for the E2-induced activation of the hTERT promoter. Either pretreatment with a phosphatidylinositol 3-kinase (PI3K) inhibitor, LY294002, or transfection with a dominant-negative Akt attenuated the E2-induced activation of the hTERT promoter. In addition, estrogen induced the phosphorylation of IκB inhibitor protein via the Akt cascade, and cotransfection with a dominant-negative subunit of NFκB attenuated the response of the ERE-deleted hTERT promoter to E2. Moreover, E2 induced the phosphorylation of hTERT, the association of 14-3-3 protein and NFκB with hTERT, and nuclear accumulation of hTERT in an Akt-dependent manner. These results indicate that E2 induces telomerase activity not only by transcriptional regulation of hTERT via an ERE-dependent mechanism and a PI3K/Akt/NFκB cascade, but also by post-transcriptional regulation via Akt-dependent phosphorylation of hTERT. Thus, the phosphorylation of Akt is a key event in the induction of telomerase activity by E2 in human ovarian cancer cells.