The most challenging task during the development of successful anti-cancer drugs lies in maintaining selective toxicity towards cancer cells with minimum collateral damage to the host tissue and thus selective killing of cancer cells is a goal that is far from being realized [1]. A promising approach would be to use agents that induce selective cytotoxicity in cancer cells while offering a degree of protection to the normal cells [1]. In so doing, a clear understanding of the biological differences between normal and malignant cells is essential and scientific literature suggest that a lucrative option would be to bank upon biochemical and/or metabolic differences between the two cell types [2].

One primary aspect that differentiates a malignant cell from a normal one is its persistent pro-oxidative state. It has been well-documented that cancer cells have increased levels of intracellular reactive oxygen species (ROS) owing to increased metabolic rate, changes in oxygen metabolism from oxidative phosphorylation (OXPHOS) to glycolysis and activity of NADPH oxidases (NOX) [2]. ROS is highly oncogenic as it plays major roles in various carcinogenic processes, such as, induction of genetic alterations, cellular proliferation and resistance to apoptosis, metastasis and angiogenesis [3]. To counteract the deleterious effects of ROS and to maintain a dynamic redox balance, cancer cells have seemingly evolved a sophisticated adaptation system that essentially involves high dependency on antioxidant functions and upregulation of pro-survival molecules. This situation renders cancer cells vulnerable to further oxidative challenge by exogenous ROS-inducing agents. This cytotoxic potential of ROS could be exploited to increase its intracellular concentration to a critical threshold to trigger cancer cell death. On the other hand, normal cells tolerate oxidative challenges better owing to their lower basal ROS output and normal metabolic regulation [2,4]. These differences in redox state between normal and cancer cells may be taken advantage of to develop therapeutic targets for selective killing of malignant cells [4].

In recent years, naturally occurring polyphenols have shown promise as potential anti-cancer therapeutics alone or as an adjunctive therapy in combination with classical chemotherapy. The major advantage of using polyphenols is that they can induce selective toxicity in cancer cells, while ameliorating disease as well as drug-induced toxicities [5], [6], [7], [8], [9]. This exceptional property of the polyphenols is most likely due to their unique ability to differentially modulate cellular redox status depending on the concentration and the cellular environment [10]. A strong potential candidate for treatment of cancer is pomegranate extract. Pomegranate has made its way into Phase II clinical trials of prostate cancer [11]. Several studies have shown that pomegranate and its constituents are capable of modulating inflammation, cellular transformation and cellular senescence programs to control cell proliferation, epithelial to mesenchymal transformation and angiogenesis in different types of cancer [12]. Moreover, several studies have elucidated the therapeutic properties of pomegranate extract in treating a wide range of diseases like colitis, pancreatitis, hepatic damage, diabetic nephropathy, arthritis, etc. [13], [14], [15]. Recently we have reported that pomegranate extract protects tumor-bearing host from tumor-induced hepatotoxicities by virtue of its antioxidant properties [5]. In the present study, we posit that pomegranate polyphenols act as selective inducers of tumor apoptosis. In our study, for the first time, we present solid evidence to show that pomegranate polyphenols induce tumor cell death through ROS-mediated mechanisms. This function is totally in contradiction to the antioxidant properties of pomegranate and the pathways by which it protects normal cells upon toxic insults.

By analyzing a set of cellular parameters, including basal GSH and ROS levels, total antioxidant capacities and p65/nrf2 deficient status, we demonstrate a strong correlation between cellular redox status and cell responsiveness to a commercially available pomegranate fruit extract PFE. Thus, using PFE, as a tool, we reinstate the differential cytotoxicity of polyphenols with special reference to the “ROS threshold theory”.

Here we have also established the efficacy of pomegranate extract in retarding proliferation and migration of triple negative breast cancer (TNBC) cells. These cells contain mutations in the estrogen receptors (ER), progesterone receptors (PR) and HER2 and are the most challenging form of cancer to treat; and yet no specific molecular targets have been identified to treat the breast cancer patients belonging to this subgroup [16]. Furthermore, we evaluated the role of PFE in regressing tumor in mouse allograft models. An assessment of the systemic changes in response to tumor development and pomegranate treatment showed that while hindering tumor growth, pomegranate rendered a protective effect by restoring the overall physiological homeostasis of the host. In both in vivo and in vitro models, we acknowledge the pro-oxidant properties pomegranate polyphenols in exhibiting anti-proliferative, anti-metastatic as well as anti-migratory potential.