Harmaline isolated from Peganum harmala suppresses growth of esophageal squamous cell carcinoma through targeting mTOR

Harmaline is a naturally occurring β‐carboline alkaloid that is isolated from Peganum harmala. It has shown efficacy in treating Parkinson's disease and has been reported to exhibit antimicrobial and anticancer properties. However, the molecular mechanism of harmaline in the context of esophageal squamous cell carcinoma (ESCC) has not been characterized. Here, we report that harmaline attenuates ESCC growth by directly targeting the mammalian target of rapamycin (mTOR). Harmaline strongly reduced cell proliferation and anchorage‐independent cell growth. Additionally, harmaline treatment induced G2/M phase cell‐cycle arrest through upregulation of p27. The results of in vitro and cell‐based assays showed that harmaline directly inhibited the activity of mTOR kinase and the phosphorylation of its downstream pathway components. Depletion of mTOR using an shRNA‐mediated strategy in ESCC cell lines indicated that reduced mTOR protein expression levels are correlated with decreased cell proliferation. Additionally, we observed that the inhibitory effect of harmaline was dependent upon mTOR expression. Notably, oral administration of harmaline suppressed ESCC patient‐derived tumor growth in vivo. Taken together, harmaline is a potential mTOR inhibitor that might be used for therapeutically treating ESCC.