In the absence of oxidative stress, the abundance of the transcription factor Nrf2 is kept low by proteasomal degradation following Keap1-mediated ubiquitination. Reactive oxygen species modify cysteine residues in Keap1, allowing Nrf2 to accumulate and activate transcription of genes with an antioxidant response element. Lau et al. identified p62, a protein that directs ubiquitinated or aggregated proteins to the autophagosome, as a binding partner for Keap1. This interaction required Arg380, Arg415, and Arg483 in Keap1; these arginine residues are also necessary for the association of Nrf2 with Keap1, suggesting that p62 could compete with Nrf2 for binding to Keap1. Overexpression of wild-type p62, but not of a p62 mutant unable to bind to Keap1, increased the abundance of Nrf2 and its transcriptional activity. Imaging of cells transfected with fluorescently tagged p62 and Keap1 indicated that coexpression of p62 caused Keap1 to localize to cytoplasmic aggregates. Biochemical analyses revealed that coexpression of p62 increased ubiquitination of Keap1 while decreasing that of Nrf2, thereby increasing its stability in the absence of oxidative stress. Deficiencies in either Beclin or Atg5, both of which are regulatory proteins in the autophagic pathway, result in impaired autophagy and increased p62 abundance. In cells from Beclin+/– or Atg5–/– mice, endogenous Keap1 was distributed into cytoplasmic aggregates, reminiscent of the effect of overexpression of p62 in transfected cells. In a separate study, Komatsu et al. (see also Rusten and Stenmark) also found that p62 induces the aggregation of Keap1, thereby increasing the transcriptional activity of Nrf2. Thus, in addition to oxidative stress, Nrf2 can also be activated by impaired autophagy, which occurs in diseases such as cancer and neurodegenerative disorders.
A. Lau, X.-J. Wang, F. Zhao, N. F. Villeneuve, T. Wu, T. Jiang, Z. Sun, E. White, D. D. Zhang, A noncanonical mechanism of Nrf2 activation by autophagy deficiency: Direct interaction between Keap1 and p62. Mol. Cell. Biol. 30, 3275–3285 (2010). [Abstract] [Full Text]
M. Komatsu, H. Kurokawa, S. Waguri, K. Taguchi, A. Kobayashi, Y. Ichimura, Y.-S. Sou, I. Ueno, A. Sakamoto, K. I. Tong, M. Kim, Y. Nishito, S.-i. Iemura, T. Natsume, T. Ueno, E. Kominami, H. Motohashi, K. Tanaka, M. Yamamoto, The selective autophagy substrate p62 activates the stress responsive transcription factor Nrf2 through inactivation of Keap1. Nat. Cell Biol. 12, 213–223 (2010). [PubMed]
T. E. Rusten, H. Stenmark, p62, an autophagy hero or culprit? Nat. Cell Biol. 12, 207–209 (2010). [PubMed]