Montserrat Rojo de la Vega

Cancer Biology Graduate Interdisciplinary Program

Arsenic-mediated autophagy blockage activates Nrf2 to promote lung cancer
Background: Chronic exposure to arsenic in drinking water is a worldwide public health problem that has been associated with an increased risk of developing cancer, particularly lung cancer. Despite many years of research, the precise carcinogenic mechanism of arsenic has not been fully elucidated. Our previous studies demonstrated that arsenic blocked autophagy (a self-degradation pathway), and resulted in prolonged activation of Nrf2, a protein that regulates the cellular antioxidant response. During autophagy, cells engulf damaged components (organelles, proteins) in an organelle called autophagosome, which then fuses with another organelle called the lysosome to form an autolysosome, where the damaged cellular components are degraded by lysosomal enzymes. When autophagy is blocked defective mitochondria accumulate, causing an excessive production of reactive oxygen species, which cause DNA mutations. Reactive oxygen species and oxidative stress are contended with by controlled Nrf2 activation, but uncontrolled or prolonged activation of Nrf2 (such as that caused by arsenic exposure) promotes carcinogenesis. We are investigating the detailed molecular mechanisms by which environmentally relevant, low doses of arsenic block autophagy to persistently activate Nrf2 and cause lung cancer.
Methods: We used a cell culture model to study how arsenic blocks autophagy to test the hypothesis that arsenic blocks autophagy by preventing the interaction of proteins involved in the fusion of the autophagosome with the lysosome. We generated fluorescently-labeled proteins to visualize with fluorescence microscopy the formation of autophagosomes, the localization of autophagosome and lysosome fusion proteins, as well as the movement and fusion of these organelles in live cells treated with arsenic. We also analyzed the interaction of these fusion proteins, as well as the protein levels of Nrf2 in arsenic-treated cell extracts.
Results: Arsenic treatment blocks autophagy by preventing the fusion of the autophagosomes and lysosomes in our cell model. Our results suggest that this blockage is due to slower movement of autophagosomes and decreased interaction of fusion proteins which results in decreased autolysosome formation. Concomitant with the disruption of autophagy, arsenic treatment increases Nrf2 protein levels. 
Conclusion: Our results show that arsenic blocked autophagy by preventing the autophagosome-lysosome fusion. These results explain how arsenic blockage causes Nrf2 activation. Further studies will test if genetic deletion of autophagosome-lysosome fusion proteins mimics the effects of arsenic treatment (mainly, decreased autolysosome formation and increased Nrf2 protein levels). Future studies will also test if both autophagy blockage and prolonged Nrf2 activation are necessary steps for arsenic-induced malignant transformation. A detailed and thorough understanding of the molecular events leading to the prolonged Nrf2 activation in arsenic-induced carcinogenesis will prove extremely valuable in the generation of preventive and therapeutic strategies, as well as in the identification of biomarkers, for the populations at risk.