Induction of hTERT expression and phosphorylation by estrogen via Akt cascade in human ovarian cancer cell lines.

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). 17beta-Estradiol (E(2)) 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 E(2)-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 E(2)-induced activation of the hTERT promoter. In addition, estrogen induced the phosphorylation of IkappaB inhibitor protein via the Akt cascade, and cotransfection with a dominant-negative subunit of NFkappaB attenuated the response of the ERE-deleted hTERT promoter to E(2). Moreover, E(2) induced the phosphorylation of hTERT, the association of 14-3-3 protein and NFkappaB with hTERT, and nuclear accumulation of hTERT in an Akt-dependent manner. These results indicate that E(2) induces telomerase activity not only by transcriptional regulation of hTERT via an ERE-dependent mechanism and a PI3K/Akt/NFkappaB 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 E(2) in human ovarian cancer cells.