Inhibition of EZH2 via activation of SAPK/JNK and reduction of p65 and DNMT1 as a novel mechanism in inhibition of human lung cancer cells by polyphyllin I

BACKGROUND: Polyphyllin I (PPI), a bioactive phytochemical extracted from the Rhizoma of Paris polyphylla, has been reported to exhibit anti-cancer activity. However, the detailed mechanism underlying this remains to be elucidated. METHODS: Cell viability and cell cycle distribution were measured using a 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) and flow cytometry assays, respectively. The expression of enhancer of zeste homolog 2 (EZH2) mRNA was measured by quantitative real time PCR (qRT-PCR). Western blot analysis was performed to examine the phosphorylation and protein expression of stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK), p65, DNA methyltransferase 1 (DNMT1) and EZH2. Exogenous expression of p65, DNMT1, and EZH2 were carried out by transient transfection assays. Promoter activity of EZH2 gene was determined using Secrete-Pair Dual Luminescence Assay Kit. A xenografted tumor model in nude mice and bioluminescent imaging system were used to further test the effect of PPI in vivo. RESULTS: We showed that PPI significantly inhibited growth and induced cell cycle arrest of non-small cell lung cancer (NSCLC) cells in a dose-dependent manner. Mechanistically, we found that PPI increased the phosphorylation of SAPK/JNK, reduced protein expression of p65 and DNMT1. The inhibitor of SAPK/JNK (SP600125) blocked the PPI-inhibited p65 and DNMT1 protein expression. Interestingly, exogenously expressed p65 overcame PPI-inhibited protein expression of DNMT1. Moreover, PPI reduced EZH2 protein, mRNA, and promoter activity; overexpression of EZH2 resisted the PPI-inhibited cell growth, and intriguingly, negative feedback regulation of SAPK/JNK signaling. Finally, exogenous expression of DNMT1 antagonized the PPI-suppressed EZH2 protein expression. Consistent with this, PPI inhibited tumor growth, protein expression levels of p65, DNMT1 and EZH2, and increased phosphorylation of SAPK/JNK in vivo. CONCLUSION: Our results show that PPI inhibits growth of NSCLC cells through SAPK/JNK-mediated inhibition of p65 and DNMT1 protein levels, subsequently; this results in the reduction of EZH2 gene expression. The interactions among p65, DNMT1 and EZH2, and feedback regulation of SAPK/JNK by EZH2 converge on the overall responses of PPI. This study reveals a novel mechanism for regulating EZH2 gene in response to PPI and suggests a new strategy for NSCLC associated therapy.