N-Hydroxyphthalimide exhibits antitumor activity by suppressing mTOR signaling pathway in BT-20 and LoVo cells

Background: N-Hydroxyphthalimide (NHPI), an important chemical raw material, was found to have potent and selective anti-proliferative effect on human breast carcinoma BT-20 cells, human colon adenocarcinoma LoVo and HT-29 cells during our screening for anticancer compounds. The purpose of this study is to assess the antitumor efficacy of NHPI in vitro and in vivo and to explore the underlying antitumor mechanism. Methods: Cell cytotoxicity of NHPI was evaluated using MTS assay and cell morphological analysis. After NHPI treatment, cell cycle, apoptosis and mitochondrial membrane potential were analyzed using flow cytometer. The subcellular localization of eukaryotic initiation factor 4E (eIF4E) was analyzed by immunofluorescence assay. The antitumor efficacy of NHPI in vivo was tested in BT-20 xenografts. The underlying antitumor mechanisms of NHPI in vitro and in vivo were investigated with western blot analysis in NHPI-treated cancer cells and tumor tissues. Statistical significance was determined using Student's t-test. Results: In vitro, NHPI selectively inhibited the proliferation and induced G2/M phase arrest in BT-20 and LoVo cells, which was attributed to the inhibition of cyclin B1 and cdc2 expressions. Furthermore, NHPI induced apoptosis via mitochondrial pathway. Of note, NHPI effectively inhibited mammalian target of rapamycin (mTOR) complex 1 (mTORC1) and mTOR complex 2 (mTORC2) signaling, and overcame the feedback activation of Akt and extracellular signal-regulated kinase (ERK) caused by mTORC1 inhibition in BT-20 and LoVo cells. In vivo, NHPI inhibited tumor growth and suppressed mTORC1 and mTORC2 signaling in BT-20 xenografts with no obvious toxicity. Conclusions: We found for the first time that NHPI displayed antitumor activity which is associated with the inhibition of mTOR signaling pathway. Our findings suggest that NHPI may be developed as a promising candidate for cancer therapeutics by targeting mTOR signaling pathway and as such warrants further exploration.