The Skp2 promoter integrates signaling through the NF-kappaB, p53, and Akt/GSK3beta pathways to regulate autophagy and apoptosis. Mol Cell. 2010;38:524-538. [PMID: 20513428 DOI: 10.1016/j.molcel.2010.03.018]

p53-dependent regulation of mitochondrial energy production by the RelA subunit of NF-κB

Aberrant activity of the nuclear factor kappaB (NF-κB) transcription factor family, which regulates cellular responses to stress and infection, is associated with many human cancers. In this study, we define a function of NF-κB in regulation of cellular respiration that is dependent upon the tumor suppressor p53. Translocation of the NF-κB family member RelA to mitochondria was inhibited by p53 by blocking an essential interaction with the HSP Mortalin. However, in the absence of p53, RelA was transported into the mitochondria and recruited to the mitochondrial genome where it repressed mitochondrial gene expression, oxygen consumption, and cellular ATP levels. We found mitochondrial RelA function to be dependent on its conserved C-terminal transactivation domain and independent of its sequence-specific DNA-binding ability, suggesting that its function in this setting was mediated by direct interaction with mitochondrial transcription factors. Taken together, our findings uncover a new mechanism through which RelA can regulate mitochondrial function, with important implications for how NF-κB activity and loss of p53 can contribute to changes in tumor cell metabolism and energy production.

The role of RelA (p65) threonine 505 phosphorylation in the regulation of cell growth, survival, and migration

RelA (p65) phosphorylation at threonine 505 acts as a negative regulator of NF-κB function. In addition to its role in regulation of cell death, a role is demonstrated for T505 phosphorylation in regulating autophagy, proliferation, and migration. NOXA is also identified as a downstream, T505-dependent effector of RelA in cell death.

The NF-κB family of transcription factors is a well-established regulator of the immune and inflammatory responses and also plays a key role in other cellular processes, including cell death, proliferation, and migration. Conserved residues in the trans-activation domain of RelA, which can be posttranslationally modified, regulate divergent NF-κB functions in response to different cellular stimuli. Using rela^−/− mouse embryonic fibroblasts reconstituted with RelA, we find that mutation of the threonine 505 (T505) phospho site to alanine has wide-ranging effects on NF-κB function. These include previously described effects on chemotherapeutic drug-induced apoptosis, as well as new roles for this modification in autophagy, cell proliferation, and migration. This last effect was associated with alterations in the actin cytoskeleton and expression of cellular migration–associated genes such as WAVE3 and α-actinin 4. We also define a new component of cisplatin-induced, RelA T505–dependent apoptosis, involving induction of NOXA gene expression, an effect explained at least in part through induction of the p53 homologue, p73. Therefore, in contrast to other RelA phosphorylation events, which positively regulate NF-κB function, we identified RelA T505 phosphorylation as a negative regulator of its ability to induce diverse cellular processes such as apoptosis, autophagy, proliferation, and migration.

Zac1 is a histone acetylation-regulated NF-κB suppressor that mediates histone deacetylase inhibitor-induced apoptosis

Histone deacetylase (HDAC) inhibitors are a class of promising anticancer reagents. They are able to induce apoptosis in embryonic carcinoma (EC) cells. However, the underlying mechanism remains poorly understood. Here we show that increased expression of zinc-finger protein regulator of apoptosis and cell-cycle arrest (Zac1) is implicated in HDAC inhibitor-induced apoptosis in F9 and P19 EC cells. By chromatin immunoprecipitation analysis we identified that increased Zac1 expression is mediated by histone acetylation of the Zac1 promoter region. Knockdown of Zac1 inhibited HDAC inhibitor-induced cell apoptosis. Moreover, HDAC inhibitors repressed nuclear factor-κB (NF-κB) activity, and this effect is abrogated by Zac1 knockdown. Consistently, Zac1 overexpression suppressed cellular NF-κB activity. Further investigation showed that Zac1 inhibits NF-κB activity by interacting with the C-terminus of the p65 subunit, which suppresses the phosphorylation of p65 at Ser468 and Ser536 residues. These results indicate that Zac1 is a histone acetylation-regulated suppressor of NF-κB, which is induced and implicated in HDAC inhibitor-mediated EC cell apoptosis.

Tissue-specific apoptotic effects of the p53 codon 72 polymorphism in a mouse model

Currently there are several dozen human polymorphisms that have been loosely associated with cancer risk. Correlating such variants with cancer risk has been challenging, primarily due to factors such as genetic heterogeneity, contributions of diet and environmental factors, and the difficulty in obtaining large sample sizes for analysis. Such difficulties can be circumvented with the establishment of mouse models for human variants. Recently, several groups have modeled human cancer susceptibility polymorphisms in the mouse. Remarkably, in each case these mouse models have accurately reflected human phenotypes, and clarified the contribution of these variants to cancer risk. We recently reported on a mouse model for the codon 72 polymorphism in p53, and found that this polymorphism regulates the ability to cooperate with NF-kB and induce apoptosis. Here-in we present evidence that this polymorphism impacts the apoptotic function of p53 in a tissue-specific manner; such tissue-specific effects of polymorphic variants represent an added challenge to human cancer risk association studies. The data presented here support the premise that modeling human polymorphisms in the mouse represents a powerful tool to assess the impact of these variants on cancer risk, progression and therapy.

p63 maintains keratinocyte proliferative capacity through regulation of Skp2-p130 levels

p63 is a master regulator of proliferation and differentiation in stratifying epithelia, and its expression is frequently altered in carcinogenesis. However, its role in maintaining proliferative capacity remains unclear. Here, we demonstrate that hypoproliferation and loss of differentiation in organotypic raft cultures of primary neonatal human foreskin keratinocytes (HFKs) depleted of the α and β isoforms of p63 result from p53-p21-mediated accumulation of retinoblastoma (Rb) family member p130. Hypoproliferation in p63-depleted HFKs can be rescued by depletion of p53, p21(CIP1) or p130. Furthermore, we identified the gene encoding S-phase kinase-associated protein 2 (Skp2), the recognition component of the SCF(Skp2) E3 ubiquitin ligase, as a novel target of p63, potentially influencing p130 levels. Expression of Skp2 is maintained by p63 binding to a site in intron 2 and mRNA levels are downregulated in p63-depleted cells. Hypoproliferation in p63-depleted cells can be restored by re-expression of Skp2. Taken together, these results indicate that p63 plays a multifaceted role in maintaining proliferation in the mature regenerating epidermis, in addition to being required for differentiation.

The codon 72 polymorphism of p53 regulates interaction with NF-{kappa}B and transactivation of genes involved in immunity and inflammation

A common polymorphism at codon 72 in the p53 tumor suppressor gene encodes either proline (P72) or arginine (R72). Several groups have reported that in cultured cells, this polymorphism influences p53's transcriptional, senescence, and apoptotic functions. However, the impact of this polymorphism within the context of a living organism is poorly understood. We generated knock-in mice with the P72 and R72 variants and analyzed the tissues of these mice for apoptosis and transcription. In the thymus, we find that the P72 variant induces increased apoptosis following ionizing radiation, along with increased transactivation of a subset of p53 target genes, which includes murine Caspase 4 (also called Caspase 11), which we show is a direct p53 target gene. Interestingly, the majority of genes in this subset have roles in inflammation, and their promoters contain NF-κB binding sites. We show that caspase 4/11 requires both p53 and NF-κB for full induction after DNA damage and that the P72 variant shows increased interaction with p65 RelA, a subunit of NF-κB. Consistent with this, we show that P72 mice have a markedly enhanced response to inflammatory challenge compared to that of R72 mice. Our data indicate that the codon 72 polymorphism impacts p53's role in inflammation.

Nuclear initiated NF-κB signaling: NEMO and ATM take center stage

A large body of literature describes elaborate NF-κB signaling networks induced by inflammatory and immune signals. Decades of research has revealed that transcriptionally functional NF-κB dimers are activated by two major pathways, canonical and non-canonical. Both pathways involve the release of NF-κB dimers from inactive cytoplasmic complexes to cause their nuclear translocation to modulate gene expression programs and biological responses. NF-κB is also responsive to genotoxic agents; however, signal communication networks that are initiated in the nucleus following DNA damage induction are less defined. Evidence in the literature supports the presence of such signaling pathways induced by multiple distinct genotoxic agents, resulting in the activation of cytoplasmic IKK complex. An example is a pathway that involves the DNA damage-responsive kinase ataxia telangiectasia mutated (ATM) and a series of post-translational modifications of NF-κB essential modulator (NEMO) in the nucleus of a genotoxin-exposed cell. Recent evidence also suggests that this nuclear-initiated NF-κB signaling pathway plays significant physiological and pathological roles, particularly in lymphocyte development and human cancer progression. This review will summarize these new developments, while identifying significant unanswered questions and providing new hypotheses that may be addressed in future studies.

IKK-dependent, NF-κB-independent control of autophagic gene expression

The induction of mammalian autophagy, a cellular catabolic bulk-degradation process conserved from humans to yeast, was recently shown to require IκB kinase (IKK), the upstream regulator of the nuclear factor (NF)-κB pathway. Interestingly, it was shown that this response did not involve NF-κB. Thus, the mechanism by which IKK promotes stimulus-induced autophagy is largely unknown. Here, we investigate the role of IKK/NF-κB in response to nutrient deprivation, the well-understood autophagy-inducing stimulus. IKK and both the classic and non-canonical pathways of NF-κB are robustly induced in response to cellular starvation. Notably, cells lacking either catalytic subunit of IKK (IKK-α or IKK-β) fail to induce autophagy in response to cellular starvation. Importantly, we show that IKK activity but not NF-κB controls basal expression of the proautophagic gene LC3. We further demonstrate that starvation induces the expression of LC3 and two other essential autophagic genes ATG5 and Beclin-1 in an IKK-dependent manner. These results indicate that the IKK complex is a central mediator of starvation-induced autophagy in mammalian cells, and suggest that this requirement occurs at least in part through the regulation of autophagic gene expression. Interestingly, NF-κB subunits are dispensable for both basal and starvation-induced expression of proautophagic genes. However, starvation-induced activation of NF-κB is not inconsequential, as increases in expression of antiapoptotic NF-κB target genes such as Birc3 are observed in response to cellular starvation. Thus, IKK likely has multiple roles in response to starvation by regulating NF-κB-dependent antiapoptotic gene expression as well as controlling expression of autophagic genes through a yet undetermined mechanism.

Regulation of the Aurora-A gene following topoisomerase I inhibition: implication of the Myc transcription factor

During the G2 phase of the cell cycle, the Aurora-A kinase plays an important role in centrosome maturation and progression to mitosis. In this study, we show in colorectal cell lines that Aurora-A expression is downregulated in response to topoisomerase I inhibition. Using chromatin immunoprecipitation assays, we have observed that the Myc transcription factor and its Max binding partner are associated with the Aurora-A promoter during the G2 phase of the cell cycle. RNA interference experiments indicated that Myc is involved in the regulation of the Aurora-A gene. Following topoisomerase I inhibition, the expression of Myc decreased whereas Mad was upregulated, and the association of Myc and Max with the promoter of the kinase was inhibited. In parallel, an increased association of Mad and Miz-1 was detected on DNA, associated with an inhibition of the recruitment of transcriptional coactivators. Interestingly, a gain of H3K9 trimethylation and HP1gamma recruitment was observed on the Aurora-A promoter following sn38 treatment, suggesting that this promoter is located within SAHF foci following genotoxic treatment. Since Aurora-A is involved in centrosome maturation, we observed as expected that topoisomerase I inhibition prevented centrosome separation but did not affect their duplication. As a consequence, this led to G2 arrest and senescence induction.These results suggest a model by which the Aurora-A gene is inactivated by the G2 checkpoint following topoisomerase I inhibition. We therefore propose the hypothesis that the coordinated overexpression of Myc and Aurora-A, together with a downregulation of Mad and Miz-1 should be tested as a prognosis signature of poor responses to topoisomerase I inhibitors.