Epigenetic Regulation Of Tumor Suppressor Gene PTEN And Its Anticancer Mechanisms

Tumor suppressor gene PTEN is a major negative regulator of PI3K/Akt cellular survival pathway. Overexpression of PTEN by adenoviral transfection increases doxorubicin-induced apoptosis. While doxorubicin-induced apoptosis can be potentiated by HDAC inhibitor TSA, the mechanisms underlying this process remain unclear. The aim of this study was to investigate whether changes in PTEN expression is involved in TSA/doxorubicin-induced apoptosis. We found that doxorubicin-induced apoptosis was enhanced by TSA, while small interfering RNA (siRNA) targeting the PTEN gene inhibited TSA/doxorubicin-induced apoptosis. Also, TSA promoted Egr-1 expression, which is the main transcription factor of PTEN, and this resulted in upregulation of PTEN expression, which consequently potentiated apoptosis. Moreover, we showed that histone acetyltransferase p300 was able to synergistically activate PTEN transcription with Egr-1, implicating the role of histone acetylation in regulation of PTEN expression. Thus, TSA promoted doxorubicin-induced apoptosis through a mechanism involving the stimulation of Egr-1 expression, acetylation of core histones at the PTEN promoter, and the consequent induction of PTEN transcription. These findings provide a theoretical basis for therapeutic application of combined treatments of TSA/doxorubicin for cancer.Previous studies implicated the anticancer effect of Vitamin D; but the mechanisms underlying this action have not been fully explored. Here we showed that 1,25-dihydroxyvitamin D3 (VD3, the active form of Vitamin D) significantly promoted the apoptosis in undifferentiated gastric cancer cell line HGC-27, and this was accompanied by a concurrent increase in PTEN expression upon VD3 treatment. Contrarily, knockdown of PTEN expression by stable transfection of PTEN siRNA remarkably decreased the apoptosis ratio. We further demonstrated that VD3 induced PTEN expression through Vitamin D receptor (VDR). In addition, our evidence showed that VDR, Egr-1 and p300 induced PTEN expression in a synergistic fashion. Furthermore, we identified that HDAC inhibitors TSA/NaBu and methylation inhibitor 5-Aza played important roles in Vitamin D-induced apoptosis through PTEN upregulation. Data presented in this report are suggestive of the potential benefic of Vitamin D in gastric cancer therapies in association with the uses of TSA/NaBu and 5-Aza.All together, we have identified in this thesis that tumor suppressor gene PTEN is an effective target in gastric cancer therapy. These findings provide a therapeutic application through combined treatments of TSA/doxorubicin or VD3 for cancer.