US EPA

1391116 

Journal Article 

Phosphorylation Of Nrf2 By Casein Kinase 2 (CK2) Regulates Activation And Degradation 

Pi, J; Bai, Y; Reece, J; William, J; Liu, D; Freeman, M; Fahl, W; Shugar, D; Waalkes, M 

2005 

1 

Toxicological Sciences
ISSN: 1096-6080
EISSN: 1096-0929 

TOX/5000951 

84 

1-S 

English 

Phosphorylation of transcription factor Nrf2 is a potential mechanism of activation. In this study, we identified two phosphorylated types of Nrf2 and provide evidence that CK2-mediated Nrf2 phosphorylation plays a critical role in activation and proteasome-mediated degradation. Human Nrf2 has a predicted molecular mass (MM) of 66 kDa. However, western blot showed two bands at 96 and 116 kDa are increased in response to Nrf2 inducers, like arsenite (As), in several human cell lines. Over-expression of Nrf2 by transfection results in two similar bands. Nuclear fractions from As-exposed cells treated with lambda-phosphatase or PP1 showed the loss of the two higher MM bands and the appearance of a lower MM band, suggesting both the higher MM forms of Nrf2 are phosphorylated. In vitro kinase assay using Nrf2 immunoprecipitation products as the substrate for purified CK2 indicated Nrf2 is a CK2 substrate and CK2 mediates two steps of Nrf2 phosphorylation. Although the mechanism of CK2 activation is unknown, our results support a role in which calmodulin (CaM) binding regulates CK2 activity. CaM and Ca2+ dramatically inhibits CK2 activity in vitro, suggesting Ca2+ mobilization may affect local CK2 activity. Accordingly, we found cold shock in Ca2+-free media decrease cellular Ca2+ levels and caused Nrf2 hyper-phosphorylation, which in turn was prevented by CK2 specific inhibitors. Gel-shift assay showed the hyper-phopsphorylated type Nrf2 (HP-Nrf2) can not bind to the gamma-GCSh antioxidant response element, indicating phosphorylated-Nrf2, but not HP-Nrf2, has transcriptional activity. In contrast, HP-Nrf2 is more susceptible to proteasome-mediated degradation.