Polygalacin D suppresses esophageal squamous cell carcinoma growth and metastasis through regulating miR-142-5p/Nrf2 axis

Esophageal squamous cell carcinoma (ESCC) is a common malignancy worldwide with poor survival. High expression of nuclear factor erythroid 2-related factor 2 (Nrf2) is an antioxidant transcript factor that protects malignant cells from death. Polygalacin D (PGD), a bioactive compound isolated from Platycodongrandiflorum (Jacq.), has recently been reported to be an anti-tumor agent. This study aimed to investigate the anti-cancer effects of PGD and its underlying molecular mechanisms in human ESCC. Here, we confirmed that Nrf2 was over-expressed in clinical ESCC tissues and cell lines. PGD treatments markedly reduced Nrf2 expression in a dose- and time-dependent manner in ESCC cell lines. Importantly, we found that PGD significantly reduced proliferation, and induced G2/M cell cycle arrest and apoptosis in ESCC cells. Also, PGD dramatically triggered autophagy in ESCC cells, and autophagy inhibitor bafilomycinA1 (BafA1) greatly abrogated the inhibitory role of PGD in cell viability and apoptosis. In addition, PGD evidently provoked reactive oxygen species (ROS) accumulation in ESCC cells, and pre-treatment of ROS scavenger N-acetyl-l-cysteine (NAC) markedly abolished PGD-triggered cell death. PGD also dramatically repressed migration and invasion in ESCC cells. Mechanistic investigation revealed that Nrf2 gene was directly targeted by miR-142-5p. MiR-142-5p negatively regulated Nrf2 expression in ESCC cells. We notably found that PGD-inhibited proliferation, migration and invasion in ESCC were considerably rescued by miR-142-5p knockdown; however, ROS production, apoptosis and autophagy induced by PGD were almost eliminated when miR-142-5p was silenced. On the contrast, over-expressing miR-142-5p could remarkably promote the anti-ESCC effects of PGD. Experiments in vivo by the tumor xenograft model confirmed that miR-142-5p effectively improved the activity of PGD to repress tumor growth and lung metastasis. Both in vitro and in vivo studies showed that PGD had few side effects on normal cells and major organs. Collectively, our findings provided the first evidence that PGD could be an effective therapeutic strategy for ESCC treatment by regulating miR-142-5p/Nrf2 axis with few adverse effects.