Progesterone regulation of hTERT transcription in human endometrial cancer cells.

Abstract

5352

Objectives: In the normal endometrium, progesterone antagonizes the actions of estrogen and inhibits estrogen-induced cellular proliferation. Consequently, progestins have had some therapeutic benefit in the treatment of endometrial cancer. Telomerase activity is dynamic throughout the menstrual cycle and is highest during the proliferative phase under the influence of estrogen and then falls during the secretory phase under the influence of progesterone. Despite this observation, little is known regarding the role of progesterone in the control of telomerase expression. Thus, our goal was to determine progesterone’s role in regulation of telomerase activity in progesterone receptor (PR)-positive endometrial cancer cell lines. Mehods: In our lab, Ishikawa and ECC-1 cells have been previously shown to be estrogen receptor (ER) and PR-positive, and HEC-1B and RL 95-2 cells to be ER and PR-negative. Telomerase activity was assayed using a PCR-based telomeric repeat amplification protocol (TRAP) after cells were exposed to estradiol (E2), medroxyprogesterone acetate (MPA) or both. hTERT mRNA expression was assessed by real-time RT-PCR. To determine whether progesterone regulates transcripton of the hTERT gene, transient expression assays using luciferase reporter plasmids containing varying lengths of the 5’ promoter region of the hTERT gene were performed. Results: Among the PR-positive endometrial cancer cell lines, E2 (10⁻⁵ - 10⁻⁷ M) increased telomerase activity and hTERT mRNA expression in a dose-dependent manner. In contrast, MPA (10⁻⁶ M) inhibited E2 induced telomerase activity in the PR-positive endometrial cancer cell lines by 24 hours. This effect was not seen in the PR-negative endometrial cancer cell lines. hTERT mRNA levels were decreased parallel to suppression of telomerase activity. Luciferase assays demonstrated that exposure to MPA inhibited E2-induced activation of the hTERT promoter which contains two putative estrogen response elements (EREs) but no progesterone response elements (PREs). Conclusions: We provide evidence that progesterone suppresses estrogen-induced hTERT mRNA expression in PR-positive endometrial cancer cell lines. Downregulation of the PR and reactivation of telomerase activity are thought to occur in advanced stages of endometrial cancer; and thus, may be related. Further work focused on the interaction between progesterone, estrogen and telomerase may provide insight into endometrial carcinogenesis as well as the underlying molecular mechanisms of progestins as a targeted therapy for this estrogen-dependent tumor.