Redox cycling of endogenous copper by thymoquinone leads to ROS-mediated DNA breakage and consequent cell death: putative anticancer mechanism of antioxidants

A Ahmad,S Azim,S M Hadi,H Y Khan,F H Sarkar,M F Ullah,H Zubair,A Sohail

doi:10.1038/cddis.2013.172

Abstract

Plant-derived dietary antioxidants have attracted considerable interest in recent past for their chemopreventive and cancer therapeutic abilities in animal models. Thymoquinone (TQ) is the major bioactive constituent of volatile oil of Nigella sativa and has been shown to exert various pharmacological properties, such as anti-inflammatory, cardiovascular, analgesic, anti-neoplastic, anticancer and chemopreventive. Although several mechanisms have been suggested for the chemopreventive and anticancer activity of TQ, a clear mechanism of action of TQ has not been elucidated. TQ is a known antioxidant at lower concentrations and most of the studies elucidating the mechanism have centered on the antioxidant property. However, recent publications have shown that TQ may act as a prooxidant at higher concentrations. It is well known that plant-derived antioxidants can switch to prooxidants even at low concentrations in the presence of transition metal ions such as copper. It is well established that tissue, cellular and serum copper levels are considerably elevated in various malignancies. Copper is an important metal ion present in the chromatin and is closely associated with DNA bases, particularly guanine. Using human peripheral lymphocytes and comet assay, we first show that TQ is able to cause oxidative cellular DNA breakage. Such a DNA breakage can be inhibited by copper-chelating agents, neocuproine and bathocuproine, and scavengers of reactive oxygen species. Further, it is seen that TQ targets cellular copper in prostate cancer cell lines leading to a prooxidant cell death. We believe that such a prooxidant cytotoxic mechanism better explains the anticancer activity of plant-derived antioxidants.

Highlights

TQ belongs to a family of quinones that can undergo enzymatic or non-enzymatic redox cycling with their corresponding semiquinone radicals to generate superoxide anion radicals.[6]
We have shown that various classes of plant polyphenols are able to cause oxidative breakage of cellular DNA either alone or in the presence of transition metal ions such as copper.[14,15,16,17,18]
As a further proof of concept, we show that TQ inhibits proliferation and induces apoptosis in prostate cancer cell lines by reactive oxygen species through the redox cycling of endogenous copper

Summary

Introduction

TQ belongs to a family of quinones that can undergo enzymatic or non-enzymatic redox cycling with their corresponding semiquinone radicals to generate superoxide anion radicals.[6]. The antioxidant/prooxidant ability of TQ depends on the milieu where it is present.[12] Compared with normal cells, preneoplastic cells and neoplastic cells have been shown to contain elevated levels of copper[13] and may be more sensitive to electron transfer with antioxidants to generate reactive oxygen species (ROS). DNA damage induced by antioxidants in the presence of redox-active metal Cu(II) may be an important pathway through which preneoplatic cells and neoplastic cells can be killed while normal cells survived.[14]. We have shown that various classes of plant polyphenols are able to cause oxidative breakage of cellular DNA either alone or in the presence of transition metal ions such as copper.[14,15,16,17,18] We have hypothesized that the cytotoxic action

Methods

Results

Conclusion