Autophagy is an intracellular recycling and degradation process for regulating tumor progression, survival and drug resistance. Nickel compounds have been identified as human carcinogens. However, the role of nickel-induced autophagy in lung carcinogenesis has not yet been fully elucidated. In this study, we determined that hexokinase 2 (HK2), which phosphorylates glucose and regulates autophagy, is the key mediator in nickel-induced autophagy in lung bronchial epithelial cells. We attempted to investigate the effects of the antidiabetic drug metformin on HK2 expression and lung cancer chemoprevention. Our results showed that metformin decreases nickel-induced autophagy and activation of apoptosis through inhibition of HK2 gene, protein and activity. Furthermore, we demonstrated that lipocalin 2 (LCN2), which is released by neutrophils at sites of infection and inflammation is involved in HK2-driven autophagy pathway. Knockdown of endogenous HK2 and LCN2 by shRNA reduced nickel-elicited autophagy and apoptosis, illustrating that metabolic alteration and inflammatory action are important in nickel-elicited carcinogenesis. We also determined the association between nickel-induced autophagy and apoptosis. Inhibition of nickel-induced autophagy abolished apoptotic cell death in chloroquine-treated, shLC3 Beas-2B cells and Atg5−/− MFFs. From TGCA database and immunohistochemistry analysis, HK2 and LCN2 expression increased in lung squamous cell carcinoma and their related adjacent normal tissues. Taken together, our results demonstrated that metformin alleviates NiCl2-induced autophagy and apoptosis via HK2-driven LCN2 activation in human bronchial epithelial cells. This novel mechanism provides a strategy for targeting nickel-elicited lung cancer progression, as well as for preventing HK2 cumulative damage triggered by environmental carcinogens.

Nickel (Ni), which is a carcinogenic workplace hazard, increases the risk of lung cancer. Angiopoietin-like protein 4 (ANGPTL4) is a multifunctional cytokine that is involved in both angiogenesis and metastasis, but its role in lung cancer is still not clear. In this study, we assessed the role of ANGPTL4 in lung carcinogenesis under nickel exposure and investigated the effects of the antidiabetic drug metformin on ANGPTL4 expression and lung cancer chemoprevention. Our results showed that ANGPTL4 is increased in NiCl2-treated lung cells in a dose- and time-course manner. The expression of ANGPTL4 and HIF-1α induced by NiCl2 were significantly repressed after metformin treatment. The downregulation of HIF-1α expression by ROS savenger and HIF-1α inhibitor or knockdown by lentiviral shRNA infection diminished NiCl2-activated ANGPTL4 expression. Chromatin immunoprecipitation and the luciferase assay revealed that NiCl2-induced HIF-1α hypoxia response element interactions activate ANGPTL4 expression, which is then inhibited by metformin. In conclusion, the increased presence of ANGPTL4 due to HIF-1α accumulation that is caused by nickel in lung cells may be one mechanism by which nickel exposure contributes to lung cancer progression. Additionally, metformin has the ability to prevent NiCl2-induced ANGPTL4 through inhibiting HIF-1α expression and its binding activity. These results provide evidence that metformin in oncology therapeutics could be a beneficial chemopreventive agent.