Prostate cancer is the second leading cause of cancer death in men. Numerous epidemiological studies have suggested that phytoestrogens in soybeans can reduce the risk for acquisition of hormone-dependent cancers. Since the activation of telomerase is crucial for cells to gain immortality and survival, we examined the role of genistein in the regulation of telomerase activity in human prostate cancer cells (DU-145 and PC3). We demonstrated earlier that pharmacological doses of genistein (10-100 μM) inhibited telomerase activity in DU-145 and PC3 cells in the presence of complete growth medium (IMEM containing 10% FBS). However, we observed recently that physiologically achievable doses of genistein (500 nM - 1 μM) significantly enhanced telomerase activity (by TRAP assay) in both DU-145 and PC3 cells. Since telomerase activity is dependent on the levels of TERT expression, we examined TERT transcriptional activity by using a full-length (3.3kb) hTERT promoter-luciferase construct. Similar to the results of the TRAP assay, 1 μM of genistein increased hTERT promoter activity time-dependently (∼ 4-fold by 3 days of treatment), whereas a pharmacological dose (50 μM) of genistein decreased the hTERT promoter activity to 50% of the control level. To understand the molecular mechanism for enhanced telomerase activity by genistein, we examined the expression of various transcription factors. We observed that a low dose of genistein activated STAT3 by phosphorylating the Y705 and S727 residues. This activation of STAT3 was not transient as the activation was sustained up to 7 days. Using a STAT3 luciferase reporter assay, we confirmed that STAT3 binding activity to its DNA response element increased more than 2-fold with 500 nM and 1 μM of genistein, whereas the STAT3 reporter activity was decreased with 50 μM of genistein. In successive experiments, many STAT3 responsive proteins, such as c-Myc, cyclin D1, HSP90, and PCNA were increased significantly with 1 μM of genistein treatment. Dominant-negative (DN) STAT3 decreased the expression of all these proteins dose-dependently. To elucidate the involvement of STAT3 in increased hTERT promoter activity, DU-145 cells were transfected with DN-STAT3 and kinase dead STAT3. The inactivation of STAT3 decreased hTERT promoter activity 4- to 6-fold, respectively. Utilizing a Chip assay, the physical interaction of STAT3 with the hTERT promoter was confirmed and 1 μM of genistein increased this binding 2-fold. These results demonstrate for the first time that physiologically achievable concentrations of genistein enhance telomerase activity in human prostate cancer cells via the activation of STAT3. (Supported by NIH grant DK060875).
[Proc Amer Assoc Cancer Res, Volume 47, 2006]