1. Ovarian cancer cell lines treated with carboplatin displayed a senescence-like phenotype
Carboplatin is a first-line chemotherapy drug for patients with ovarian cancer[23]. Many studies have proven that carboplatin could significantly inhibit the proliferation and survival of cells in early -stage ovarian cancer. However, chemoresistance to carboplatin develops quickly in patients with ovarian cancer, and is one of the most lethal risk factors due to the accelerated recurrence and shortened overall survival of patients with ovarian cancer. Carboplatin displays strong cytotoxicity and DNA toxicity, leading to inhibition of proliferation and DNA damage, two hallmarks of cellular senescence[24]. Despite reports of chemotherapy-induced senescence in cancer cells, little is known about how ovarian cancer cells respond to carboplatin-induced senescence. To determine the effect of carboplatin on the proliferation of ovarian cancer cells, we exposed OVCAR3 and A2780 cells to 5 μM and 50 μM carboplatin. According to the count results at cell passaging relative to day 1, we found that these concentrations of carboplatin significantly inhibited proliferation without inducing cell death of OVCAR3 and A2780 cells at no more than 2 d (Fig 1A). Clonogenic assays confirmed the carboplatin-induced inhibition of cell proliferation (Fig 1C). Then, 3D spheroid models were used to mimic the response of ovarian cancer cells to carboplatin in vivo. The carboplatin responses were measured by the size of the spheroids of ovarian cancer cells. As shown in Fig 1B, our results demonstrated that a significant decrease in spheroid size was detected in the 50 µM carboplatin -treated OVCAR3 and A2780 cancer spheroids (Fig 1B). Cellular senescence, including chemotherapy-induced senescence, is characterized by positive staining for SA-β-galactosidase. As expected, we observed a concentration-dependent increase in the SA-β-gal -positive OVCAR3 and A2780 cells at day 2 after carboplatin treatment, with higher levels of senescence detected in the OVCAR3 and A2780 cells treated with 50 μM carboplatin (Fig 1D), consistent with their lower levels of cell proliferation. Thus, carboplatin induced senescence in ovarian cancer cells, as shown by reduced cell proliferation, reduced clonogenic ability, reduced sphere formation and increased SA-β-galactosidase activity.
2. Carboplatin-induced senescence in OVCAR3 and A2780 cells was reversible
Cellular senescence is well-recognized as a critical antitumor process due to the inhibition of proliferation and metastasis of cancer cells. Although senescence is defined as irreversible cell cycle arrest, senescent cancer cells induced by therapy are suggested as the main drivers of cancer relapse. Therefore, to explore whether ovarian cancer cells could recover from chemotherapy-induced senescence, we changed the culture medium to carboplatin-free medium at day 3, and the cells were cultured and passaged for another 5 d (Fig 2A). First, we compared the proliferative ability among the carboplatin-treated cells, carboplatin-withdrawn cells, and untreated cells. A total of 1X106 cells per dish for each group were seeded and passaged until they reached confluence. Cell was counted at every passages. Our data indicated that ovarian cancer cells could recover, at least partially, from carboplatin-induced inhibition of proliferation (Fig 2B). Then, 2000 cells per well were seeded in 6-well plates to detect the clonogenic formation abilities. Interestingly, withdrawal of carboplatin almost doubled the clonogenic number of OVCAR3 and A2780 cells (Fig 2C). To further verify whether recovery occurred after carboplatin treatments, we evaluated ovarian cancer cell spheroids by spheroid formation assays that were performed during and after withdrawal of carboplatin treatment. The ovarian cancer cells treated with carboplatin partially recovered the spheroid- forming capacity after 2 d of treatment, and significantly more spheroids were formed, although this number was much smaller than that of the untreated ovarian cancer cells (Fig 2D). To directly confirm ovarian cancer cell recovery from chemotherapy-induced senescence, we performed SA-β-galactosidase staining. Consistently, withdrawal of carboplatin from ovarian cancer cells resulted in a dramatic decrease in positive staining of SA-β-galactosidase compared to that of the cells continuously treated with carboplatin (Fig 2E). Therefore, the senescence-like phenotype induced by carboplatin was reversible. Our data also further suggested that ovarian cancer cells that recovered from carboplatin-induced senescence displayed more tolerance to carboplatin. As shown by trypan blue staining assays, ovarian cancer cells that recovered from carboplatin treatments displayed much less cumulative cell death than normal ovarian cancer cells (Fig 2F). All these results indicate that recovered ovarian cancer cells contribute to chemoresistance and recurrence.
3. EGFR and NF-κB1 were upregulated in recovered ovarian cancer cells
Despite the large amount of knowledge already accumulated, EGFR still attracts extensive research attention. Previous studies have revealed that different deleterious stresses applied to cancer cells could stimulate EGFR and trigger activation of cascade pathways, and enhanced levels of EGFR correlated with cancer cell survival and apoptotic resistance. Therefore, we evaluated the EGFR levels in ovarian cancer cells treated with carboplatin and recovered from carboplatin treatment through immunofluorescence staining and western blotting. By using immunofluorescence staining, we observed a significantly upregulated signal in the recovered ovarian cancer cells compared to the carboplatin-treated cells (Fig 3A). Interestingly, we also found that NF-κB1, a cascade pathway activated by EGFR, was upregulated in the recovered ovarian cancer cells (Fig 3B). Then, EGFR and NF-κB1 expression was verified by western blotting, and significant upregulation was observed in the recovered ovarian cancer cells (Fig 3C, 3D). Together, all these results suggested that the potential role of the enhanced EGFR was to promote carboplatin-treated ovarian cancer cell survival and reverse the senescent state.
4. Amplified EGFR accelerated recovery from carboplatin-induced senescence
To confirm that carboplatin-induced senescence of ovarian cancer cells was attenuated by enhanced expression of EGFR, we performed EGFR depletion and overexpression in ovarian cancer cells using Lipofectamine 3000-delivered shRNA and the pLVx-EGFR-EGFP plasmid (Fig 4A). Overexpression of EGFR significantly increased the number of ovarian cancer cells, while depletion of EGFR decreased the cell number, providing strong evidence that EGFR maintained the growth of ovarian cancer cells (Fig 4B). Next, we assessed the influence of overamplified EGFR on recovery from carboplatin-induced senescence. The ovarian cancer cells transfected with the pLVx-GFP-EGFR, pLVx-shRNA-EGFR and vector plasmids were treated with 50 µM carboplatin for 2 d, changed to carboplatin-free medium and cultured for another 4 d. According to the results of SA-β-galactosidase staining, overexpression of EGFR led to a significant increase in the ovarian cancer cells that recovered from the carboplatin-induced senescence-like phenotype, while knockdown of EGFR inhibited the ability of the ovarian cancer cells to recover (Fig 4C), suggesting that EGFR might attenuate the cytotoxic effect of carboplatin on ovarian cancer cells and promote ovarian cancer cells escape from carboplatin-induced senescence, and tolerance to carboplatin (Fig 4D).
5. The involvement of NF-κB1 in EGFR promoted the recovery of carboplatin-induced senescence
We observed that both EGFR and NF-κB1 were upregulated in the recovered ovarian cancer cells. Therefore, we studied the relationship between EGFR and NF-κB1. As determined by western blotting, we found that overexpression of EGFR induced upregulation of NF-κB1, and knockdown of EGFR also decreased the expression of NF-κB1 (Fig 5A, 5B). Because NF-κB1 might be the downstream signaling pathway of EGFR, we detected whether recovery of the EGFR-overexpressing ovarian cancer cells was impeded by an NF-κB1 inhibitor. We therefore used 4 ng/mL bortezomib, an NF-κB1 inhibitor, for subsequent experiments. As expected, bortezomib significantly increased the ratio of positive SA-β-galactosidase staining in the EGFR-overexpressing ovarian cancer cells, which indicated that blockade of NF-κB1 neutralized the ability of EGFR to promote recovery from carboplatin-induced senescence. Together, these data indicated that NF-κB1 was strongly associated with EGFR during the recovery of ovarian cancer cells, and was even regulated by EGFR.