Thiazolidinediones Downregulate Wnt/β-Catenin Signaling Via Multiple Mechanisms in Breast Cancer Cells

Background Thiazolidinediones (TZDs) have been demonstrated to possess antitumor effects in breast cancer cells, although the mechanisms are not well established. We sought to better define TZDs' antitumor effects and molecular mechanisms to permit rational utilization of these agents. Materials and methods We studied the effects of TZDs on DNA synthesis (BrdU enzyme-linked immunosorbent assay), gene expression (microarray, quantitative reverse transcription-polymerase chain reaction, and immunoblot), serine phosphorylation and localization of β-catenin (nuclear/cytoplasmic fractionation and immunoblot), transactivation activity of β-catenin/T cell factor 4 (TCF4) (luciferase assay with wild-type and mutant TCF4 responsive element), and β-catenin/TCF4 complex (immunoprecipitation) in human breast cancer cells MDA-MB-231 and T47D. Results Troglitazone (TG) down-regulated DNA synthesis in MDA-MB-231 and T47D and decreased mRNA expression of the Wnt co-receptors frizzled-1 and low-density lipoprotein-related protein 6 (LRP6) as well as protein level of LRP6. TG also targets downstream Wnt signaling molecules in T47D cells. TG down-regulates p-β-catenin (S33/S37/T41) and promotes translocation of β-catenin into the nucleus. However, TG inhibits β-catenin-mediated transactivation by down-regulating TCF4 protein levels, thereby inhibiting β-catenin/TCF4 complex formation. Finally, we found that inhibition of Akt augments TG-mediated down-regulation of DNA synthesis and this result was accompanied with the decreased protein levels of Wnt signaling molecules: LRP6, β-catenin, and TCF4. Conclusion These results suggest that the Wnt/β-catenin signaling pathway, which plays an important role in breast cancer pathogenesis, may be a target for TZD treatment. Moreover, a combination of TZDs and a specific Akt inhibitor may serve as a new approach to target Wnt/β-catenin directly and via PI3K/Akt action on glycogen synthase-3β.