Abstract
Nickel-containing compounds are widely used in industry. Nickel is a known human carcinogen that primarily affects the lungs. Proposed mechanisms of nickel-induced carcinogenesis include disruption of cellular iron homeostasis, generation of reactive oxygen species (ROS), and induction of hypoxia signaling. However, the precise molecular mechanisms of nickel-induced malignant transformation and tumor development remain unclear. This study shows that the transcription factor Nrf2 is highly expressed in lung tumor tissue and in nickel-transformed human lung bronchial epithelial BEAS-2B cells (NiT cells). Additionally, constitutively high levels of Nrf2 play a critical role in apoptosis resistance in NiT cells. Basal ROS levels were extremely low in NiT cells and were correlated with elevated expression levels of both antioxidant enzymes (e.g. catalase and superoxide dismutases) and antiapoptotic proteins (e.g. Bcl-2 and Bcl-xL). These processes are tightly controlled by Nrf2. Autophagy inhibition, induced pharmacologically or genetically, enhanced Ni2+-induced apoptosis, indicating that the induction of autophagy is the cause of apoptosis resistance in NiT cells. Using similar approaches, we show that in NiT cells the inhibition of apoptosis decreases autophagy. We have shown that Stat3, which is up-regulated by Nrf2, controls autophagy induction in NiT cells. Colony formation and tumor growth were significantly attenuated by knockdown of Nrf2 or Bcl-2. Taken together, this study demonstrates that in NiT cells constitutively high Nrf2 expression inhibits apoptosis by up-regulating antioxidant enzymes and antiapoptotic proteins to increase autophagy via Stat3 signaling. These findings indicate that the Nrf2-mediated suppression of apoptosis and promotion of autophagy contribute to nickel-induced transformation and tumorigenesis.
Highlights
Nickel-containing compounds are widely used in industry. regulating antioxidant enzymes and antiapoptotic proteins to Nickel is a known human carcinogen that primarily affects the increase autophagy via Stat3 signaling
Note that single agent treatments with either wortmannin or 3-methyladenine did not alter cell viability or apoptosis (Fig. 3, A and B, and supplemental Fig. 1). These findings indicate that Ni2ϩ-induced autophagy in NiT cells is involved in cell survival, whereas autophagy promotes cell death in the parental BEAS-2B cells., Ni2ϩ-induced cell death was significantly enhanced in autophagy-defective beclin 1-deficient NiT cells when compared with NiT cells transfected with the control shRNA (Fig. 3C)
Our group reported that Nrf2 is highly expressed in metal-transformed cells and that this may play a critical role in metal-induced carcinogenesis [11, 12]
Summary
To generate the nickel-transformed cell line, NiT, we continuously exposed BEAS-2B cells to Ni2ϩ (50 M) for 4 months. A soft agar assay revealed that this exposure malignantly transformed the cells (Fig. 1A). An apoptosis assay showed that ϳ60% of the BEAS-2B cells were apoptotic following Ni2ϩ exposure. Ϳ40% of the NiT cells were apoptotic following the same Ni2ϩ exposure (Fig. 1, F and G). The cleavage of PARP and caspases 3 and 7, as well as the decrease in the levels of pro-caspase 8, were much more pronounced in the parental BEAS-2B cells compared with NiT cells. These results suggested that NiT cells resist cell death, including apoptosis
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
