Natural β-carboline alkaloids regulate the PI3K/Akt/mTOR pathway and induce autophagy in insect Sf9 cells

The β-carboline alkaloids are a large group of naturally occurring and synthetic indole alkaloids with remarkable pharmacological properties. Furthermore, these alkaloids have also been reported to be effective agents for controlling many pests and plant pathogenic nematodes. However, studies on these potential insecticidal components are scarce. The previous finding that these bioactive compounds can induce programmed cell death in cancer cell lines provided a new insight for exploration of their toxicological mechanisms on insects. In the present study, the cytotoxicity of five natural harmala alkaloids was measured, and the autophagy-inducing effect was confirmed in the Spodoptera frugiperda Sf9 cultured cell line. The results demonstrated that these alkaloids inhibited the proliferation of Sf9 cells in a dose- and time-dependent manner, and the unsaturated β-carboline alkaloids, harmine and harmol, exhibited stronger autophagy induction activity based on monodansylcadaverineand LysoTracker Red staining. Many autophagy-related genes were increased after β-carbolines treatment at the RNA level, and the protein expression of Sf-Atg8 was also confirmed to increase after treatment. In addition, the primary autophagic signaling pathway, the PI3K/Akt/mTOR pathway, was altered during the procedure. Furthermore, experiments with special inhibitors and activators were performed to confirm the effect of β-carbolines on this pathway. The results suggested that the PI3K/Akt/mTOR pathway primarily regulated harmine-induced autophagy in insect cells, and this finding may potentially benefit the application of these promising bioactivity components.