Direct Interaction between Nrf2 and p21^Cip1/WAF1 Upregulates the Nrf2-Mediated Antioxidant Response

In response to oxidative stress, Nrf2 and p21^Cip1/WAF1 are both upregulated to protect cells from oxidative damage. Nrf2 is constantly ubiquitinated by a Keap1 dimer that interacts with a weak-binding ²⁹DLG motif and a strong-binding ⁷⁹ETGE motif in Nrf2, resulting in degradation of Nrf2. Modification of the redox-sensitive cysteine residues on Keap1 disrupts the Keap1-²⁹DLG binding, leading to diminished Nrf2 ubiquitination and activation of the antioxidant response. However, the underlying mechanism by which p21 protects cells from oxidative damage remains unclear. Here we present molecular and genetic evidence suggesting that the antioxidant function of p21 is mediated through activation of Nrf2 by stabilizing the Nrf2 protein. The ¹⁵⁴KRR motif in p21 directly interacts with the ²⁹DLG and ⁷⁹ETGE motifs in Nrf2 and thus competes with Keap1 for Nrf2 binding, compromising ubiquitination of Nrf2. Furthermore, the physiological significance of our findings was demonstrated in vivo using p21-deficient mice.