Capsaicin (CAP), the pungent ingredient of chili peppers, inhibits growth of various solid cancers via TRPV1 as well as TRPV1-independent mechanisms. Recently, we showed that TRPV1 regulates intracellular calcium level and chromogranin A secretion in pancreatic neuroendocrine tumor (NET) cells. In the present study, we characterize the role of the TRPV1 agonist - CAP - in controlling proliferation and apoptosis of pancreatic BON and QGP-1 NET cells. We demonstrate that CAP reduces viability and proliferation, and stimulates apoptotic death of NET cells. CAP causes mitochondrial membrane potential loss, inhibits ATP synthesis and reduces mitochondrial Bcl-2 protein production. In addition, CAP increases cytochrome c and cleaved caspase 3 levels in cytoplasm. CAP reduces reactive oxygen species (ROS) generation. The antioxidant N-acetyl-l-cysteine (NAC) acts synergistically with CAP to reduce ROS generation, without affecting CAP-induced toxicity. TRPV1 protein reduction by 75% reduction fails to attenuate CAP-induced cytotoxicity. In summary, these results suggest that CAP induces cytotoxicity by disturbing mitochondrial potential, and inhibits ATP synthesis in NET cells. Stimulation of ROS generation by CAP appears to be a secondary effect, not related to CAP-induced cytotoxicity. These results justify further evaluation of CAP in modulating pancreatic NETs in vivo.
Keywords: 2′,7′-dichlorofluorescein diacetate; 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; 5-bromo-2′-deoxyuridine; Apoptosis; BrdU; CAP; CPZ; Capsaicin; DCFDA; MTT; N-acetyl-l-cysteine; NAC; NET; Neuroendocrine tumor; Proliferation; ROS; TRPV1; capsaicin; capsazepine; neuroendocrine tumor; reactive oxygen species; tbHP; tert-butyl Hydrogen Peroxide; transient receptor potential vanilloid channel 1.
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