Activation of Nrf2 by microcystin-LR provides advantages for liver cancer cell growth.

Microcystin-LR (MC-LR) is a potent heptapeptide hepatotoxin at high doses, but its underlying mechanism of promoting liver cell proliferation at low doses is unclear. The transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) is key in mediating the protective antioxidant response against various environmental toxicants, but emerging data suggest that constitutive activation of Nrf2 contributes to a malignant phenotype. The purpose of this study was to characterize the interactions and effects of Nrf2 activation on cell proliferation induced by MC-LR treatment. Treatment of HepG2 and Hep3B cells with MC-LR resulted in significant increases in Nrf2-ARE binding activities in the nuclear fractions and upregulation of its downstream genes HO-1 and NQO1. A possible mechanism may be that MC-LR binds to the cytosolic regulator protein Keap1 to liberate Nrf2. Nrf2 knockdown inhibited MC-LR-induced cell proliferation and cell cycle progression. Together, these results indicate that MC-LR-induced upregulation of Nrf2 in cancer cells promotes liver cancer cell growth and suggest a positive role of Nrf2 in tumorigenesis.