*2.1. Cancer*

Cancer, a fatal disease involves the malignant growth of tumors because of chromosomal alteration and lead to unregulated growth of the cells [21]. This deadly disease has a complex relationship with ROS and is involved at three different levels of cancer development, i.e., initiation, progression and promotion [22]. During the initiation stage, the ROS causes a mutation in DNA, which keeps on accumulating when the affected tissue does not get repaired [23]. The overproduction of ROS triggers the mutation in an oncogene, which potentially contributes to the initiation of cancer [24].

The cancer cells favor the excessive production of ROS in comparison to healthy cells, because of the alteration in the metabolic processes [25]. ROS-induced oxidative stresses in tumor triggers the cell signaling pathways and build resistance in tumor cells and elevate the supply of blood to tumor cells and promote their metastasis [26]. The elevated level of ROS plays a significant role in expanding tumor cells by altering the genes associated with apoptosis, cell proliferation and transcription factors [27]. Furthermore, ROS also downregulate the pro-apoptotic proteins by interfering with the Akt/PI3K and ERK cell signaling pathway, and upregulate the anti-apoptotic genes [28,29]. During the cancer progression stage, ROS interferes with cellular processes and upregulates the production of metalloproteinases by obstructing the angiogenesis process and, by anti-proteases, results in the metastasis of cancer cells [23,25,30].
