Ovarian cancer remains the most common cause of gynecological-related mortalities in post-menopausal women. 75% of ovarian cancers were diagnosed at the advanced stage. Debulking surgery and platinum-based chemotherapy are the first lines of treatment provided to ovarian cancer patients [29, 30]. A combination of platinum, along with paclitaxel, olaprib, bevacizumab, and niraparib, are some of the drugs prescribed for treating ovarian cancer. In the initial stage, the patient responds to these drugs, whereas later, due to platinum resistance, 80% of the patients are prone to relapses of cancer, and their survival is questionable [31]. At present, targeted therapies are the new hope for treating ovarian cancer, but the recurrence rate of cancer is still in considerable numbers [32].
Nicotinamide adenine dinucleotide phosphate oxidase (NOX) is an enzyme with multiple subunits expressed in various tissues throughout the body. The prime function of NOX is to catalyze the generation of free radicals to facilitate adaptive immunity [33–35]. NOX were classified into two categories based on their activity: phagocyte-derived and non-phagocyte-derived NADPH oxidase. Overexpression of non-phagocytotic-derived NADPH oxidase, specifically the NOX-1 subtype, was observed in various cancer cells [36–38]. Ovarian cancer cells are also reported to overexpress NOX-1; hence, inhibiting NOX-1 may suppress cancer cell proliferation [39]. Apocynin, a NOX inhibitor, had been reported to suppress intestinal tumorigenesis in obese mice [40], prostate cancer [41, 42], breast cancer [43], and bladder cancer [44]. In this study, we investigated the anticancer effect of apocycin against ovarian cancer cells and normal epithelial cells. Apocynin treatment significantly inhibited the cell proliferation of ovarian cancer cells (A2780), whereas a only minimal percentage of cell death was observed in normal epithelial vero cells. This confirms the targeted cell toxicity of apocynin.
Oxidative stress created due to a reactive oxygen species imbalance imparts a dual response in cells. At the physiological level, ROS is the prime signaling pathway that regulates the growth, proliferation, metabolism, and apoptosis of cells [45, 46], whereas excessive generation of ROS during pathogenic conditions hinders normal cellular functions [47, 48]. Generation and inhibition of these reactive oxygen species play a vital role in treating various inflammatory diseases, including cancer [49, 50]. Induction of ROS generation in ovarian cancer is proposed to render an anticancer effect by causing membrane lipid peroxidation, altering the genetic material, and inducing apoptosis in cancer cells [51]. Our study also correlated with the previous findings that apocynin treatment significantly increased lipid peroxidation in the A2780 cells via decreasing the enzymatic antioxidant superoxide and the non-enzymatic antioxidant glutathione levels, thereby causing apoptosis.
Further, to confirm the apocynin-induced free radical generation-mediated apoptosis in A2780 cells, the apocynin-treated cells were stained with DCHF-DA stain, Rhodamine 123, and AO/EtBr stain. Our staining proves apocynin treatment significantly increased reactive oxygen species generation, thereby decreasing mitochondrial membrane potential and leading to cellular apoptosis. The mitochondrial disruption triggers the apoptotic proteins, both the pro-apoptotic proteins Bid, Bax, andcaspases, and the anti-apoptotic protein Bcl2. The anti-apoptotic protein Bcl2 prevents the mitochondrial release of apoptosis-inducing factors and also caspase activities. Hence, targeting Bcl2 may promote apoptosis in cancer cells [52]. In our study, apocynin treatment significantly decreased the levels of Bcl2 protein and also increased the levels of pro-apoptotic proteins Bax and caspases. The increased cell death observed in AO/EtBr-stained apocynin-treated A2780 cells may be due to the increase in the pro-apoptotic protein and the decrease in the anti-apoptotic protein Bcl2 levels.
Anti-cancer treatment disrupts the homeostasis of the cells by generating reactive oxygen species, thereby damaging the nuclear content [53, 54]. The oxidative stress created causes a damaged DNA response, which shifts the cells to cell cycle arrest and eventually leads to apoptosis [55]. Our flow cytometric analysis of apocynin-treated cells confirmed the cell cycle arrest in the A2780 cell. Apocynin treatment retained an increased number of A2780 cells in the G0/G1phase, and a decreased number of cells were observed in the S and G2/M phases compared to the control cells. Apart from inhibition of cell proliferation and induction of apoptosis, apocynin treatment also inhibited cell migration of A2780, as evidenced by our wound scratch assay.