Triptolide Promotes Senescence of Prostate Cancer Cells Through Histone Methylation and Heterochromatin Formation

Background: Triptolide is a medicinal herb-derived diterpene triepoxide with potent anti-tumor activity, mainly ,correlated with its ability to inhibit and inactivate subunits of RNA polymerase II, thereby suppressing global gene transcription. Epigenetic imbalance including histone methylation are well known to play important roles in prostate cancer (PCa) onset and progression. The goal of this study was to investigate whether triptolide might exert anti-PCa influence by reshaping the histone methylation landscape. Methods: Triptolide-treated PCa cell lines were analyzed by RT-qPCR and western blotting for expression of histone demethylases and associated markers. Detection of senescence was achieved using senescence associated ß-galactosidase staining and analyses of apoptosis and cell cycle were performed by flow cytometry. Senescence­associated heterochromatin foci were detected by immunofluorescence while chromatin immunoprecipitation associated with qPCR (CHIP-qPCR) was applied to assess accumulation of histone markers on promoters of target genes. Cell viability was determined using the CCK-8 assay. Results: We found triptolide to enhance H3K27me3 levels by down-regulating JMJD3 and UTX and also H3K9me3 through up-regulation of SUV39H1. Furthermore, it up-regulated expression of HP1α. Thereby, heterochromatin formation and deposition on promoters of E2F1-target genes was promoted, correlating with suppression of gene transcription, decreased cell viability and induction of a senescence-like phenotype in PCa cells. Conclusions: Our results indicate that triptolide exerts anti-tumor effects including PCa cell senescence at least partially through increasing the levels of repressive histone H3 methylation and formation of a repressive chromatin state in PCa cells. Further studies of its potential as an epigenetic anti-PCa drug appear warranted.