Upregulation of NOX 2 and Nrf 2 Promotes 5 Fluorouracil Resistance of Human Colon Carcinoma (HCT 116) Cells

Abstract

Altered expression of cellular redox genes and proteins contributes to invasion, metastasis, and drug resistance in cancer. NADPH oxidase (NOX) isoforms are the pro_oxidant enzymes that generate ROS as a primary product. Dysregulation of NOX activity and expression alters ROS generation, which either directly or indirectly modulates cell death and survival signaling during the progression of cancer. Nuclear factor erythroid 2_related factor 2 (Nrf_2) is an inducible transcription factor, which transcribes an array of antioxidant genes and protects cancer cells from the oxidative stress. Both NOXs and Nrf_2 participate in the regulation of cellular redox homeostasis; but their dysregulation promotes oxidative stress, which contributes to the progression of different types of cancer. Indeed, the role of NOX isoforms and Nrf_ 2 in developing the drug resistance in cancer is largely unknown. In the present study, we have explored the association of NOX isoforms and Nrf_2 signaling with the MDR1 gene expression in colon carcinoma cells (HCT_116/R). The MDR1 gene was overexpressed to develop resistant HCT_116/R cells and the NOX activation and ROS generation were monitored. We also assessed the role of NOX isoforms and Nrf_2 in the 5_fluorouracil (5_FU) mediated apoptotic cell death of HCT_116/R cells. The HCT_116/R cells demonstrated higher expression of HIF_1α, Nrf_2, and HO_1 and were highly resistant to 5_ FU; they also displayed upregulated expression and activity of NOX_2, as well as elevated ROS levels. Interestingly, the treatment with HDC, a specific NOX_2 inhibitor, reduced the ROS levels in HCT_116/R cells. The treatment with HDC and ML_385 (specific inhibitor of Nrf_2) augmented the 5_FU_mediated apoptotic cell death of HCT_116/R cells, which suggests that NOX_2 and Nrf_2 are involved in the development of the chemoresistant phenotype of these cells. Taken together, NOX_2 and Nrf_2 are associated with developing drug resistance of colorectal cancer cells and might be potential targets to overcome drug resistance during cancer therapy