Different roles of ROS and Nrf2 in Cr(VI)-induced inflammatory responses in normal and Cr(VI)-transformed cells

Abstract

Hexavalent chromium (Cr(VI)) is classified as a human carcinogen. Cr(VI) has been associated with adenocarcinomas and squamous cell carcinoma of the lung. The present study shows that acute Cr(VI) treatment in human bronchial epithelial cells (BEAS-2B) increased inflammatory responses (TNF-α, COX-2, and NF-κB/p65) and expression of Nrf2. Cr(VI)-induced generation of reactive oxygen species (ROS) are responsible for increased inflammation. Despite the fact that Nrf2 is a master regulator of response to oxidative stress, silencing of Nrf2 in the acute Cr(VI) treatment had no effect on Cr(VI)-induced inflammation. In contrast, in Cr(VI)-transformed (CrT) cells, Nrf2 is constitutively activated. Knock-down of this protein resulted in decreased inflammation, while silencing of SOD2 and CAT hadno effect in the expression of these inflammatory proteins. Results obtained from the knock-down of Nrf2 in CrT cells are very different from the results obtained in the acute Cr(VI) treatment.In BEAS-2B cells, knock-down of Nrf2 had no effect inthe inflammation levels, while inCrT cells a decrease in the expression of inflammation markers was observed. These results indicate that before transformation, ROS plays a critical role while Nrf2 not in Cr(VI)-induced inflammation, whereas after transformation (CrT cells), Nrf2 is constitutively activated and this protein maintains inflammation while ROS not. Constitutively high levels of Nrf2 in CrT binds to the promoter regions of COX-2 and TNF-α, leading to increased inflammation. Collectively, our results demonstrate that before cell transformation ROS are important in Cr(VI)-induced inflammation and after transformation a constitutively high level of Nrf2 is important.

Retracted: p62 as a therapeutic target for inhibition of autophagy in prostate cancer

“…42 Our prior study using nucleotide sequence analysis showed the presence of six putative ARE sites on the TNF-α promotor. 43 ChIP assay demonstrated that Nrf2 binds to the fourth ARE region (1374-1366) on TNF-α gene promoter, leading to the TNF-α expression, while other AREs did not exhibit any significant binding. 43 Other potential pathways include MAP kinase, as TNF-α is also known to be able to activate pro-survival MAP kinase and NF-kB pathways.…”

“…43 ChIP assay demonstrated that Nrf2 binds to the fourth ARE region (1374-1366) on TNF-α gene promoter, leading to the TNF-α expression, while other AREs did not exhibit any significant binding. 43 Other potential pathways include MAP kinase, as TNF-α is also known to be able to activate pro-survival MAP kinase and NF-kB pathways. 44,45 Future studies to understand the value of approaches to directly target TNF-α alone and in combination with autophagy targeting would be important.…”

Dual Roles of Oxidative Stress in Metal Carcinogenesis

“…112 In our recent studies, we showed that in arsenic-, Cd-, Cr(VI)-, and Ni-transformed cells, Nrf2 is constitutively activated, and this activation upregulates antioxidant and antiapoptotic proteins through the binding of Nrf2 to the ARE sites of these target proteins. 14,15,113,114 The outcome is decreased levels of ROS and the development of apoptosis resistance, an environment favorable for progression of transformed cells and tumor formation. When Nrf2 becomes constitutively activated, its inducible nature is lost in metal-transformed cells.…”

“…After transformation, however, a constitutively high level of Nrf2 is critical in maintaining the inflammation, while ROS has no role. 113 In chromium-transformed cells, the constitutive activation of Nrf2 is responsible for the production of inflammatory regulatory proteins such as cyclooxygenase-2 (COX-2), tumor necrosis factor alpha (TNF-α), and nuclear factor-κB (NF-κB). 119…”

“…This process promotes cell survival, proliferation, and carcinogenesis of transformed cells. 14,15,19,113 Based on the two roles of ROS presented in our previous studies, the following approach is proposed to find a ‘double-edged sword’ to prevent metal-induced carcinogenesis. In the first stage of metal carcinogenesis or prior to cell transformation, up-regulation of Nrf2 may reduce metal-induced ROS generation and prevent metal-induced cell transformation.…”