Abstract 5460: Targeting tumors by inhibiting mitochondrial electron transport

Abstract

Recently, we demonstrated that the monoterpene, citral, is potent inhibitor of cell proliferation. Here, we are demonstrating that the α,β-unsaturated carbonyl functionality of citral is responsible for its biological activity causing substantial increase in intracellular oxygen radicals. The resulting oxidative stress activates p53. Neutralization of oxygen radicals by N-acetylcysteine attenuates the cytotoxic effect of citral and inhibits activation of p53. Inhibition of p53 by pifithrin-α inhibits citral-induced apoptosis. Oxidative stress caused by citral initiates downstream events causing apoptosis of cancer cells. With the identification of α,β-unsaturated carbonyl group as the obligatory functionality for the anti-cancer effects of citral, we surveyed other small molecule drugs that shared this chemical entity. Cinnamaldehyde, nonenal, perillaldehyde, curcumin, and plumbagin have the α,β-unsaturated carbonyl functional groups and have demonstrated anti-cancer effects. Using curcumin and plumbagin as prototypes, we were able to demonstrate similarities in the cytotoxic mechanism with citral. The oxidative stress caused by curcumin and plumbagin is the initiating step required for cancer cell apoptosis. Fluorescent Lifetime Imaging Microscopy (FLIM) showed that exposure to plumbagin results in an increase in free NADH levels and a decrease in the NADH/FAD ratio. Additionally, in silico modeling showed that plumbagin and curcumin docked to the active site of complex I and III of the mitochondrial electron transport chain. Using the Seahorse analyzer, our experiments showed that plumbagin and curcumin inhibited the oxygen consumption rate in treated cells. These results indicate that the α,β-unsaturated carbonyl containing compounds produce their cytotoxic effects by interfering with the electron transport chain. The cancer cells respond to the surge in oxygen radicals by increasing the expression of Nrf-2, the transcription factor that regulates major anti-oxidant responses. This compensatory mechanism is a form of chemoresistance that allows cancer cells to evade the cytotoxic effects of agents such as citral. We will present data from strategies to inhibit the Nrf-2-mediated compensation and thereby enhance the cytotoxic activity of citral, curcumin and plumbagin. The data obtained through these experiments is providing important information that will allow designing of novel agents that can interfere with the electron transport chain and inhibit tumor growth by inducing oxidative stress. Note: This abstract was not presented at the meeting. Citation Format: Arvinder Kapur, Amruta P. Nayak, Thomas Beres, Kavya Rathi, Mildred Felder, Amani Gillette, Spencer Ericksen, Melissa Skala, Lisa Barroilhet, Manish S. Patankar. Targeting tumors by inhibiting mitochondrial electron transport [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5460. doi:10.1158/1538-7445.AM2017-5460