It is well known that many natural compounds have strong anti-tumor effects, such as vincristine, irinotecan, etoposide and paclitaxel, etc., and they often have the characteristics of low toxicity, high efficiency, cost-effectiveness and wide sources[21–23]. Narciclasine is abundant present in fresh bulbs of natural plants such as Lycoris radiata, tiger ear, water ghost coke, snowflake genus, summer snowflake and yellow Lycoris radiata[14]. Studies have shown that narciclasine can act highly selectively on glioblastoma multiforme and inhibit the growth of Hela tumor cells[24–25]. The results of the present study found that narciclasine at a low concentration could significantly inhibit the proliferation in GC-823, MGC-803, MKN28 and SGC-7901 cells and inhibit colony formation of the gastric cancer BGC-823 and MGC-803 cells. According to research, narciclasine is highly selective for tumor cells, and its toxic effect on normal fibroblasts is 250 times lower than that of tumor cells; therefore it is unlikely to cause apoptosis of normal fibroblasts at therapeutic doses[26]. It is noteworthy that we found narciclasine had a weak toxic effect on gastric mucosal cells GES-1, but highly selective toxic effect on gastric cancer cells, indicating that narciclasine has great potential to be developed as an anti-gastric cancer drug.
Research has shown in human breast cancer MCF-7 cells and PC-3 prostate cells that narciclasine can activate Caspase-8 and Caspase-10 receptor pathways and induce apoptosis of tumor cells[27]. In the present study, we also found that narciclasine could promote the apoptosis of gastric cancer cells, inhibit the expression of Bcl-2 protein and increase the expression of Bax and cleaved-PARP protein, suggesting that narciclasine may activate the apoptosis pathway in gastric cancer cells. In addition, we confirmed that narciclasine can enhance the autophagy of gastric cancer cells, for example by the use of autophagy inhibitors.
Several relationships co-exist between autophagy and apoptosis: autophagy and apoptosis work together to promote cell death, with autophagy and apoptosis antagonizing each other; autophagy provides cells with the energy necessary for survival and inhibits their apoptosis; and autophagy promotes the transformation of cells to apoptosis[28–29]. Cao et al[30]found that narciclasine could inhibit breast cancer cell proliferation by mediating autophagy-dependent apoptosis via AMPK-ULK1 signaling. In our study, we used 3-MA to inhibit autophagy and found that narciclasine induced a decrease in apoptosis of gastric cancer cells. Therefore, it is speculated that narciclasine promotes apoptosis of gastric cancer cells by activating autophagy. The interaction between Beclin1 and Bcl-2 is one of the common regulatory modes of autophagy and apoptosis[31]. Because there is a BH3 domain on Beclin1, Bcl-2 and Bcl-xl, other apoptosis factors can affect autophagy and apoptosis activity by binding to BH3[32]. When the expression level of Beclin1 increases, it can release the apoptosis promoting protein bak/Bax from its attachment to Bcl-2, thus promoting apoptosis. On the other hand, decreased Bcl-2 expression can also lead to more Beclin1-dependent autophagy[33–34]. Although this study confirmed that narciclasine can inhibit the expression of Bcl-2 and promote the expression of Beclin1, whether or not narciclasine mediated autophagic apoptosis of gastric cancer cells is related to the Beclin1-Bcl-2 regulatory mode needs further verification in future studies.
As early as 1975, Carrasco et al. found that narciclasine could effectively inhibit the biosynthesis of eukaryotic ribosome proteins and play an anti-tumor role in experiments on rabbit reticular cells and non-cell lines: the mechanism of action was to bind to the 60S ribosome group to inhibit the formation of chemical bonds[35]. In addition, narciclasine can also regulate the Rho / Rho kinase / LIM kinase confilin signaling pathway, strengthen the activity of GTPase RhoA and can induce the formation of actin stress fibers[24]. Akt is a serine/threonine protein kinase, also known as protein kinase B. When Akt is activated, it will regulate a large number of transcription factors and activate multiple substrates, including the target proteins of rapamycin (mTOR), thereby stimulating cell autophagy, proliferation and inhibition of apoptosis. When exploring the mechanism of narciclasine against gastric cancer, we found that narciclasine could inhibit the expression of p-Akt and p-mTOR. When we activated Akt with insulin, it could save the phosphorylation level of p-Akt and p-mTOR, and reduce the degree of autophagy and apoptosis of gastric cancer cells, suggesting that narciclasine-induced inhibition of the phosphorylation level of Akt/mTOR is one of its important mechanisms of action against gastric cancer. But the anti-tumor effect of drugs often involves multiple signaling pathways. In the future, we will clarify the anti-tumor effects of narciclasine acting through multiple pathways and multiple levels. In addition, the anti-tumor effect and mechanisms of action of narciclasine in vivo also need to be verified.