p53

Stimuli such as DNA damage can activate both the NF-κB and the p53 pathways. While p53 induces cell-cycle arrest or cell death in response to these treatments, the contribution of NF-κB to cell fate is more complex, and pathways in which it either antagonizes or cooperates with p53 have been described (Perkins, 2007). NF-κB-mediated negative regulation of p53 can contribute to tumorigenesis and has been shown to operate at a number of levels. Genetic analysis indicates that NF-κB controls the levels of the p53 E3 ubiquitin ligase Mdm2, thereby negatively regulating p53 stability ([Kashatus et al., 2006] and [Tergaonkar et al., 2002]). In addition, NF-κB-mediated upregulation of antiapoptotic gene targets can antagonize the proapoptotic functions of p53 (Perkins, 2007). There is also evidence that the p65(RelA) NF-κB subunit and p53 negatively regulate each others' activity by competing for limiting pools of the p300 and CBP coactivators, which are required for transactivation by both factors ([Ravi et al., 1998], [Webster and Perkins, 1999] and [Wadgaonkar et al., 1999]).

Of these mechanisms, coactivator competition provides a direct means through which one factor might negatively regulate the other. Originally, this was thought to occur in a concentration-dependent manner, in which the more abundant factor sequesters the limiting quantities of coactivators present in a cell in response to a particular stimulation ([Ravi et al., 1998], [Webster and Perkins, 1999] and [Wadgaonkar et al., 1999]). However, the rate-limiting step regulating these interactions was unknown. Such a regulatory step could function through covalent modification of p65, p53, or both. Indeed, phosphorylation of both proteins has been shown to affect their interaction with coactivators. In a recent issue of Molecular Cell, Huang et al. (2007) reveal a key regulatory step that not only determines the ability of p65 to interact with CBP but also controls competitive binding of p53 to this coactivator. They demonstrate that, although CBP is preferentially bound to p53 in unstimulated cells, upon TNF-α and LTβ stimulation CBP becomes phosphorylated at serines 1382 and 1386, resulting in its dissociation from p53, binding to p65, and recruitment to NF-κB-regulated promoters. This phosphorylation is therefore a toggle that regulates crosstalk between p53 and NF-κB. However, given that more than 200 cellular proteins can interact with this coactivator, it might be expected to have wider effects on CBP-dependent gene expression.

IKKα and IKKβ are two important kinases required for NF-κB activation in response to most stimuli (Ghosh and Karin, 2002). IKKβ is the kinase typically responsible for IκB phosphorylation and induction of the classical NF-κB pathway, while IKKα mediates the activation of the noncanonical pathway through phosphorylation of the p100 NF-κB subunit, which induces its processing to p52. Intriguingly, although much of the manuscript focuses on the ability of TNFα, an inflammatory cytokine and activator of the classical pathway, to induce CBP phosphorylation, it is IKKα and not IKKβ that is found to be the CBP kinase. This new function for IKKα is consistent with a growing appreciation for its ability to phosphorylate substrates unrelated to the NF-κB/IκB family, such as histone H3 and other transcription factors (Perkins, 2007). Furthermore, these results suggest that activation of IKKα must have more widespread effects on gene expression than can be attributed to the activation of the canonical and noncanonical NF-κB pathways.

How IKKα-dependent regulation of CBP recruitment to NF-κB promoters translates into target gene expression was not addressed in this article. Significantly, the CBP homolog p300 was not IKKα regulated, although competitive binding with p53 was still observed. Therefore, this pathway might be expected to only regulate NF-κB target genes uniquely dependent upon CBP activity. Furthermore, given that crosstalk between NF-κB and p53 is probably most relevant to the DNA damage response or to situations in which both NF-κB and p53 are active, it will be necessary to understand if IKKα and CBP phosphorylation has any function under these circumstances. NF-κB and p53 can also function cooperatively, either through the coregulation of specific target genes or the proapoptotic activity of NF-κB in response to certain inducers (Perkins, 2007). Moreover, IKKα has been documented to function as a positive or negative regulator of NFκB signaling in a cell type- and stimulus-specific manner. In addition, some of the cells used in this study, such as HeLa cells and immortalized MEFs, may not have truly wild-type p53 function. It will be important, therefore, that future studies address the cell type specificity of this mechanism and investigate its significance in more physiological settings.

Huang et al. (2007) also report that noncanonical pathway stimuli induce IKKα-dependent CBP phosphorylation, stimulating its association with p52 and displacement of p53. Recently, the noncanonical pathway was found to regulate p53 function through an alternative mechanism (Schumm et al., 2006). Here, it was shown that p52 could be recruited by p53 to its target promoters, where, depending upon the gene, it could either repress or stimulate p53 transcriptional activity. This suggests a coordinated network of pathways linking the noncanonical pathway with p53 function. It will be interesting to determine if downregulation of p53-dependent transcription has any role in key developmental events, such as lymphoid organogenesis, that are regulated by noncanonical NF-κB signaling. Furthermore, it can be hypothesized that aberrant activation of the noncanonical pathway in tumors could contribute to tumorigenesis through inhibition of p53 activity. Indeed, Huang et al. (2007) document constitutive activation of IKKα and phosphorylation of serine 1382/1386 of CBP in lung cancers. Significantly, a recent large-scale genomic sequencing study found multiple mutations in IKKα across many human cancers (Greenman et al., 2007). Therefore, a larger picture might be emerging in which noncanonical NF-κB pathway activity is demonstrated to be an important regulator of tumorigenesis in many types of human cancer. This study unveils an interesting and potentially significant component of this picture, providing a mechanistic basis through which noncanonical pathway signaling might play a role in oncogenesis by activating NF-κB and concomitantly suppressing the activity of a key tumor suppressor.