Enhanced p62-NRF2 Feedback Loop due to Impaired Autophagic Flux Contributes to Arsenic-Induced Malignant Transformation of Human Keratinocytes

Chronic exposure to arsenic induces a variety of cancers, particularly in the skin. Autophagy is a highly conserved process which plays a dual role in tumorigenesis. In the present study, we found that chronic exposure to an environmentally relevant dose of arsenite induced malignant transformation of human keratinocytes (HaCaT) with dysregulated autophagy as indicated by an increased number of autophagosomes, activation of mTORC1 pathway, and elevated protein levels of p62 and LC3II. Meanwhile, arsenite-transformed cells showed lower intracellular levels of reactive oxygen species compared with control. Silencing p62 ameliorated elevation in mRNA levels of NRF2 downstream genes (AKR1C1 and NQO1) and malignant phenotypes (acquired invasiveness and anchor-independent growth) induced by chronic arsenite exposure. On the other hand, silencing NRF2 abrogated the increase in mRNA and protein levels of p62 and malignant phenotypes induced by arsenite. In response to acute arsenite exposure, impaired autophagic flux with an increase in p62 protein level and interrupted autophagosome-lysosome fusion was observed. The increase in p62 protein levels in response to arsenite was not completely dependent on NRF2 activation and at least partially attributed to protein degradation. Our data indicate that accumulation of p62 by impaired autophagic flux is involved in the activation of NRF2 and contributes to skin tumorigenesis due to chronic arsenite exposure.