Stimulation of the NF-kappaB pathway can have proapoptotic or antiapoptotic consequences, and one mechanism that determines the outcome is the nuclear distribution of RelA. Certain stress stimuli induce nucleolar accumulation of RelA thereby mediating apoptosis, whereas others induce nucleoplasmic accumulation and inhibition of apoptosis. Here we investigated the mechanisms that regulate the nuclear distribution of RelA, specifically, the role of the ubiquitin/proteasome system. We found that stress-induced nucleolar translocation of RelA is preceded by ubiquitination of the protein. We also found that chemical proteasome inhibitors induce the ubiquitination and nucleolar translocation of RelA and that this is required for the apoptotic response to these agents. We show that the RelA nucleolar localization signal (amino acids 27-30) is a critical domain for ubiquitination of the protein but that the lysine residue within this motif is not a direct target. We show that RelA binds COMMD1, the rate-limiting component of the RelA ubiquitin ligase complex, in response to stress. Furthermore, we show that overexpression of COMMD1 promotes stress-mediated nucleolar targeting of RelA, whereas knockdown of COMMD1 blocks this effect, causing RelA to remain in the nucleoplasm. These data identify a new role for COMMD1 in regulating the nuclear/nucleolar distribution of RelA and suggest that ubiquitination acts as a signal for transport of RelA to the nucleolus. These findings have relevance to the design of chemopreventative/anticancer agents that act by targeting RelA to the nucleolar compartment.
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