ABSTRACT
Indolealkylamines(IAAs) are the main hydrophilic substances in toad skin, mainly including free N-methyl-5-hydroxytryptamine, bufotenine, bufotenidine, dehydrobufotenine, and binding bufothionine. In this study, the LPS-activated neutrophils were used to investigate the structure-activity relationship and anti-inflammatory mechanism of the above-mentioned five monomers from the toad skin in vitro. The neutrophils were divided into the control group, model group(1 μg·mL~(-1) LPS), positive drug group(100 μg·mL~(-1) indometacin), as well as the low-(50 μg·mL~(-1)), medium-(100 μg·mL~(-1)) and high-dose(200 μg·mL~(-1)) free N-methyl-5-hydroxytryptamine, bufotenine, bufotenidine, dehydrobufotenine, and binding bufothionine groups. The levels of IL-6, TNF-α and IL-1β in the neutrophil supernatant of each group was measured by enzyme-linked immunosorbent assay(ELISA) after LPS stimulation, followed by the detection of apoptosis in each group after Annexin V/PI staining. The protein expression levels of caspase-3, Bax, Bcl-2, beclin1, LC3-I, and LC3-Ⅱ were assayed by Western blot. The results showed that IAAs reduced the excessive secretion of inflammatory cytokines caused by LPS compared with the model group. Besides, the activity of each free IAAs(N-methyl-5-hydroxytryptamine, bufotenine, bufotenidine and dehydrobufotenine), especially bufotenine, was stronger than that of the binding bufothionine. As revealed by Annexin V/PI staining, LPS delayed the early apoptosis of neutrophils compared with the control group, while bufotenine promoted the apoptosis of neutrophils in a dose-dependent manner, which might be related to the elevated expression of apoptosis-related protein Bax/Bcl-2. In addition, LPS activated the autophagy pathways in neutrophils. This study confirmed the efficacy of IAAs in reducing the secretion of inflammatory cytokines in neutrophils induced by LPS for the first time. For instance, bufotenine exerts the anti-inflammatory effect possibly by inducing the apoptosis of neutrophils.