p300/CBP-dependent and -independent transcriptional interference between NF-kappaB RelA and p53

The clinical utilization of SNIP1 and its pathophysiological mechanisms in disease

Smad intranuclear binding protein 1 (SNIP1), a highly conserved nuclear protein, functions as a transcriptional regulator and exerts a significant influence on disease progression. In addition, the N-terminal domain of SNIP1 facilitates its interaction with Smad4, a signaling protein associated with the TGF-β family, and RelA/p65, a transcription factor connected to NF-κB. This interaction further enhances the transcriptional activation of c-Myc-dependent genes. Presently, the primary emphasis in research is directed towards targeting the catalytic domain of SNIP1, as it holds promise as a potential therapeutic target for various diseases. While the significance of SNIP1 in pathological mechanisms remains uncertain, this review aims to comprehensively examine the existing literature on the association between SNIP1 and proteins implicated in the regulation of diverse clinical conditions, including cancer, inflammation, and related diseases.

Emerging role and therapeutic implications of p53 in intervertebral disc degeneration

Lower back pain (LBP) is a common degenerative musculoskeletal disease that imposes a huge economic burden on both individuals and society. With the aggravation of social aging, the incidence of LBP has increased globally. Intervertebral disc degeneration (IDD) is the primary cause of LBP. Currently, IDD treatment strategies include physiotherapy, medication, and surgery; however, none can address the root cause by ending the degeneration of intervertebral discs (IVDs). However, in recent years, targeted therapy based on specific molecules has brought hope for treating IDD. The tumor suppressor gene p53 produces a transcription factor that regulates cell metabolism and survival. Recently, p53 was shown to play an important role in maintaining IVD microenvironment homeostasis by regulating IVD cell senescence, apoptosis, and metabolism by activating downstream target genes. This study reviews research progress regarding the potential role of p53 in IDD and discusses the challenges of targeting p53 in the treatment of IDD. This review will help to elucidate the pathogenesis of IDD and provide insights for the future development of precision treatments.

Encoding and Decoding of p53 Dynamics in Cellular Response to Stresses

In the cellular response to stresses, the tumor suppressor p53 is activated to maintain genomic integrity and fidelity. As a transcription factor, p53 exhibits rich dynamics to allow for discrimination of the type and intensity of stresses and to direct the selective activation of target genes involved in different processes including cell cycle arrest and apoptosis. In this review, we focused on how stresses are encoded into p53 dynamics and how the dynamics are decoded into cellular outcomes. Theoretical modeling may provide a global view of signaling in the p53 network by coupling the encoding and decoding processes. We discussed the significance of modeling in revealing the mechanisms of the transition between p53 dynamic modes. Moreover, we shed light on the crosstalk between the p53 network and other signaling networks. This review may advance the understanding of operating principles of the p53 signaling network comprehensively and provide insights into p53 dynamics-based cancer therapy.

Histone Acetylation and Modifiers in Renal Fibrosis

Histones are the most abundant proteins bound to DNA in eukaryotic cells and frequently subjected to post-modifications such as acetylation, methylation, phosphorylation and ubiquitination. Many studies have shown that histone modifications, especially histone acetylation, play an important role in the development and progression of renal fibrosis. Histone acetylation is regulated by three families of proteins, including histone acetyltransferases (HATs), histone deacetylases (HDACs) and bromodomain and extraterminal (BET) proteins. These acetylation modifiers are involved in a variety of pathophysiological processes leading to the development of renal fibrosis, including partial epithelial-mesenchymal transition, renal fibroblast activation, inflammatory response, and the expression of pro-fibrosis factors. In this review, we summarize the role and regulatory mechanisms of HATs, HDACs and BET proteins in renal fibrosis and provide evidence for targeting these modifiers to treat various chronic fibrotic kidney diseases in animal models.

p53's Extended Reach: The Mutant p53 Secretome

p53 suppresses tumorigenesis by activating a plethora of effector pathways. While most of these operate primarily inside of cells to limit proliferation and survival of incipient cancer cells, many extend to the extracellular space. In particular, p53 controls expression and secretion of numerous extracellular factors that are either soluble or contained within extracellular vesicles such as exosomes. As part of the cellular secretome, they execute key roles in cell-cell communication and extracellular matrix remodeling. Mutations in the p53-encoding TP53 gene are the most frequent genetic alterations in cancer cells, and therefore, have profound impact on the composition of the tumor cell secretome. In this review, we discuss how the loss or dominant-negative inhibition of wild-type p53 in concert with a gain of neomorphic properties observed for many mutant p53 proteins, shapes a tumor cell secretome that creates a supportive microenvironment at the primary tumor site and primes niches in distant organs for future metastatic colonization.

TAK1 signaling regulates p53 through a mechanism involving ribosomal stress

Triple-negative breast cancer (TNBC) is among the most aggressive forms of breast cancer with limited therapeutic options. TAK1 is implicated in aggressive behavior of TNBC, while means are not fully understood. Here, we report that pharmacological blockade of TAK1 signaling hampered ribosome biogenesis (RBG) by reducing expression of RBG regulators such as RRS1, while not changing expression of ribosomal core proteins. Notably, TAK1 blockade upregulated expression of p53 target genes in cell lines carrying wild type (wt) TP53 but not in p53-mutant cells, suggesting involvement of ribosomal stress in the response. Accordingly, p53 activation by blockade of TAK1 was prevented by depletion of ribosomal protein RPL11. Further, siRNA-mediated depletion of TAK1 or RELA resulted in RPL11-dependent activation of p53 signaling. Knockdown of RRS1 was sufficient to disrupt nucleolar structures and resulted in activation of p53. TCGA data showed that TNBCs express high levels of RBG regulators, and elevated RRS1 levels correlate with unfavorable prognosis. Cytotoxicity data showed that TNBC cell lines are more sensitive to TAK1 inhibitor compared to luminal and HER2+ cell lines. These results show that TAK1 regulates p53 activation by controlling RBG factors, and the TAK1-ribosome axis is a potential therapeutic target in TNBC.

Mechanical regulation of bone homeostasis through p130Cas-mediated alleviation of NF-κB activity

Mechanical loading plays an important role in bone homeostasis. However, molecular mechanisms behind the mechanical regulation of bone homeostasis are poorly understood. We previously reported p130Cas (Cas) as a key molecule in cellular mechanosensing at focal adhesions. Here, we demonstrate that Cas is distributed in the nucleus and supports mechanical loading-mediated bone homeostasis by alleviating NF-κB activity, which would otherwise prompt inflammatory processes. Mechanical unloading modulates Cas distribution and NF-κB activity in osteocytes, the mechanosensory cells in bones. Cas deficiency in osteocytes increases osteoclastic bone resorption associated with NF-κB-mediated RANKL expression, leading to osteopenia. Upon shear stress application on cultured osteocytes, Cas translocates into the nucleus and down-regulates NF-κB activity. Collectively, fluid shear stress-dependent Cas-mediated alleviation of NF-κB activity supports bone homeostasis. Given the ubiquitous expression of Cas and NF-κB together with systemic distribution of interstitial fluid, the Cas-NF-κB interplay may also underpin regulatory mechanisms in other tissues and organs.

Role of p53 in the Regulation of the Inflammatory Tumor Microenvironment and Tumor Suppression

p53 has functional roles in tumor suppression as a guardian of the genome, surveillant of oncogenic cell transformation, and as recently demonstrated, a regulator of intracellular metabolism. Accumulating evidence has shown that the tumor microenvironment, accompanied by inflammation and tissue remodeling, is important for cancer proliferation, metastasis, and maintenance of cancer stem cells (CSCs) that self-renew and generate the diverse cells comprising the tumor. Furthermore, p53 has been demonstrated to inhibit inflammatory responses, and functional loss of p53 causes excessive inflammatory reactions. Moreover, the generation and maintenance of CSCs are supported by the inflammatory tumor microenvironment. Considering that the functions of p53 inhibit reprogramming of somatic cells to stem cells, p53 may have a major role in the inflammatory microenvironment as a tumor suppressor. Here, we review our current understanding of the mechanisms underlying the roles of p53 in regulation of the inflammatory microenvironment, tumor microenvironment, and tumor suppression.

Protection of plasma transfusion against lipopolysaccharide/D-galactosamine-induced fulminant hepatic failure through inhibiting apoptosis of hepatic cells in mice*

Fulminant hepatic failure is a severe clinical condition associated with extremely poor outcomes and high mortality. A number of studies have demonstrated the ability of plasma transfusion to successfully treat fulminant hepatic failure, but the underlying mechanisms are not well understood. The aim of the present study is to define the mechanisms of plasma transfusion treatment in lipopolysaccharide/D -galactosamine (LPS/D -GalN)-induced mice. LPS/D -GalN treatment in mice causes significant hepatic failure, including increasing serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels, histopathological changes in centrilobular necrosis and inflammatory cells, and the up-regulation of inflammation (tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6)). When LPS/D -GaIN-induced mice were treated with plasma, these changes were halted. Results showed that plasma transfusion significantly reduced mortality, and decreased the levels of AST, ALT, and inflammation factors such as TNF-α and IL-6. The expression levels of cleaved Caspase-3, BAX, and p53 were down-regulated and Bcl-2 was up-regulated, suggesting that plasma can reduce LPS/D -GalN-induced apoptosis. The protective mechanism of plasma against LPS/D -GalN-induced fulminant hepatic failure is related to the inhibition of the inflammatory response and the reduction in apoptosis through the down-regulation of the p53-induced apoptotic pathway.

Marsdeniae tenacissima extract-induced growth inhibition and apoptosis in hepatoma carcinoma cells is mediated through the p53/nuclear factor-κB signaling pathway

An extract from a traditional Chinese herb, Marsdeniae tenacissima (trade name, Xiao-Ai-Ping) has been approved for use on the Chinese market as a cancer chemotherapeutic agent for decades. Previous studies have demonstrated the cytostatic and pro-apoptotic effects of M. tenacissima extract (MTE) in multiple cancer cells. However, the contributions of MTE to the proliferation and apoptosis of hepatoma carcinoma cells and the underlying mechanisms remain unclear. In the present study, Bel-7402 cells were incubated with increasing concentrations of MTE ranging from 0–320 µl/ml to explore the effects and potential mechanisms of MTE on the proliferation and apoptosis of Bel-7402 cells. 3-(4,5-dimethylthiazol-2-yl)-5(3-carboxymethoxyphenyl)-2-(4-sulfopheny)-2H-tetrazolium, inner salt and propidium iodide (PI)-stained flow cytometry assays demonstrated that MTE significantly suppressed the proliferation of Bel-7402 cells in a dose-dependent manner by arresting the cell cycle at S phase (P<0.05). Annexin V-fluorescein isothiocyanate PI-stained flow cytometry confirmed the significantly pro-apoptotic effect of MTE at both 160 and 240 µl/ml (P<0.001). Reverse transcription-quantitative polymerase chain reaction and western blot analysis demonstrated that MTE (both 160 and 240 µl/ml) induced a significant downregulation of B-cell lymphoma (Bcl)-2 (P<0.01), upregulation of Bcl-2-associated X protein (P<0.01) and activation of caspase-3 (P<0.05). Furthermore, a significant downregulation of murine double minute-2 (MDM2) (P<0.001) and activation of p53 (P<0.001) in Bel-7402 cells following treatment with 160 or 240 µl/ml MTE was observed, accompanied by the inhibition of the nuclear factor (NF)-κB pathway (P<0.001). These results suggested that MTE inhibited growth and exhibited pro-apoptotic effects in Bel-7402 cells, which was mediated by downregulation of the MDM2-induced p53-dependent mitochondrial apoptosis pathway and blocking the NF-κB pathway. Overall, these data serve as preliminary identification of the significant roles of MTE in hepatic carcinoma cells, and suggest that MTE may be a promising candidate for hepatocellular carcinoma therapy.

Identification of two p53 isoforms from Litopenaeus vannamei and their interaction with NF-κB to induce distinct immune response

p53 is a transcription factor with capability of regulating diverse NF-κB dependent biological progresses such as inflammation and host defense, but the actual mechanism remains unrevealed. Herein, we firstly identified two novel alternatively spliced isoforms of p53 from Litopenaeus vannamei (LvΔNp53 and the full-length of p53, LvFLp53). We then established that the two p53 isoforms exerted opposite effects on regulating NF-κB induced antimicrobial peptides (AMPs) and white spot syndrome virus (WSSV) immediate-early (IE) genes expression, suggesting there could be a crosstalk between p53 and NF-κB pathways. Of note, both of the two p53 isoforms could interact directly with LvDorsal, a shrimp homolog of NF-κB. In addition, the activation of NF-κB mediated by LvDorsal was provoked by LvΔNp53 but suppressed by LvFLp53, and the increased NF-κB activity conferred by LvΔNp53 can be attenuated by LvFLp53. Furthermore, silencing of LvFLp53 in shrimp caused higher mortalities and virus loads under WSSV infection, whereas LvΔNp53-knockdown shrimps exhibited an opposed RNAi phenotype. Taken together, these findings present here provided some novel insight into different roles of shrimp p53 isoforms in immune response, and some information for us to understand the regulatory crosstalk between p53 pathway and NF-κB pathway in invertebrates.

The Double Role of p53 in Cancer and Autoimmunity and Its Potential as Therapeutic Target

p53 is a sequence-specific short-lived transcription factor expressed at low concentrations in various tissues while it is upregulated in damaged, tumoral or inflamed tissue. In normally proliferating cells, p53 protein levels and function are tightly controlled by main regulators, i.e., MDM2 (mouse double minute 2) and MDM4 proteins. p53 plays an important role due to its ability to mediate tumor suppression. In addition to its importance as a tumor suppressor, p53 coordinates diverse cellular responses to stress and damage and plays an emerging role in various physiological processes, including fertility, cell metabolism, mitochondrial respiration, autophagy, cell adhesion, stem cell maintenance and development. Interestingly, it has been recently implicated in the suppression of autoimmune and inflammatory diseases in both mice and humans. In this review based on current knowledge on the functional properties of p53 and its regulatory pathways, we discuss the potential utility of p53 reactivation from a therapeutic perspective in oncology and chronic inflammatory disorders leading to autoimmunity.

p53 and the Carcinogenicity of Chronic Inflammation

Chronic inflammation is a major cancer predisposition factor. Constitutive activation of the inflammation-driving NF-κB pathway commonly observed in cancer or developed in normal tissues because of persistent infections or endogenous tissue irritating factors, including products of secretion by senescent cells accumulating with age, markedly represses p53 functions. In its turn, p53 acts as a suppressor of inflammation helping to keep it within safe limits. The antagonistic relationship between p53 and NF-κB is controlled by multiple mechanisms and reflects cardinal differences in organismal responses to intrinsic and extrinsic cell stresses driven by these two transcription factors, respectively. This provides an opportunity for developing drugs to treat diseases associated with inappropriate activity of either p53 or NF-κB through targeting the opposing pathway. Several drug candidates of this kind are currently in clinical testing. These include anticancer small molecules capable of simultaneous suppression of p53 and activation of NF-κB and NF-κB-activating biologics that counteract p53-mediated pathologies associated with systemic genotoxic stresses such as acute radiation syndrome and side effects of cancer treatment.

Transcriptional regulation of microRNA-100, -146a, and -150 genes by p53 and NFκB p65/RelA in mouse striatal STHdh(Q7)/ Hdh(Q7) cells and human cervical carcinoma HeLa cells

MicroRNA (miRNA) genes generally share many features common to those of protein coding genes. Various transcription factors (TFs) and co-regulators are also known to regulate miRNA genes. Here we identify novel p53 and NFκB p65/RelA responsive miRNAs and demonstrate that these 2 TFs bind to the regulatory sequences of miR-100, -146a and -150 in both mouse striatal and human cervical carcinoma cells and regulate their expression. p53 represses the miRNAs while NFκB p65/RelA induces them. Further, we provide evidence that exogenous p53 inhibits NFκB p65/RelA activity by reducing its nuclear content and competing with it for CBP binding. This suggests for the existence of a functional cross-talk between the 2 TFs in regulating miRNA expression. Moreover, promoter occupancy assay reveals that exogenous p53 excludes NFκB p65/RelA from its binding site in the upstream sequence of miR-100 gene thereby causing its repression. Thus, our work identifies novel p53 and NFκB p65/RelA responsive miRNAs in human and mouse and uncovers possible mechanisms of co-regulation of miR-100. It is to be mentioned here that cross-talks between p53 and NFκB p65/RelA have been observed to define the outcome of several biological processes and that the pro-apoptotic effect of p53 and the pro-survival functions of NFκB can be largely mediated via the biological roles of the miRNAs these TFs regulate. Our observation with cell lines thus provides an important platform upon which further work is to be done to establish the biological significance of such co-regulation of miRNAs by p53 and NFκB p65/RelA.

p53 modulates NF-κB mediated epithelial-to-mesenchymal transition in head and neck squamous cell carcinoma

OBJECTIVES

To investigate the role of p53 in NF-κB mediated epithelial-to-mesenchymal (EMT) in head and neck squamous cell carcinoma (HNSCC).

MATERIALS AND METHODS

We utilized HNSCC and normal oral epithelial cell lines as our model system. We used a lentiviral shRNA system to silence the expression of p65 and p53 in these cell lines. Mutant and wild-type (WT) p53 background genotypes were analyzed. The expression of epithelial and mesenchymal markers was determined using western blotting and quantitative PCR assays. Cell morphology, growth, and invasion were determined using a 3-dimensional spheroid culture and anchorage independent growth (AIG) assays.

RESULTS

In HNSCC cells with mutant p53 we found that silencing p65 expression promoted EMT. In contrast, in the context of WT p53, ectopic p65 over-expression promoted EMT. Ablation of WT p53 in normal oral epithelial cells blocked EMT induced by p65 over-expression. We demonstrate that AIG and apoptosis induced by NF-κB activation is regulated by p53.

CONCLUSION

Our data demonstrates that p53 mutational status is critical in determining the outcome of NF-κB activation in HNSCC. In the presence of WT p53, excess p65 signal can promote EMT. Conversely, ablation of p65 in the context of mutant p53 drives EMT. These results demonstrate that p53 mutational status alters the outcome of NF-κB signaling. These results, though preliminary, demonstrate the critical role of p53 mutational status in determining the outcome of NF-κB signaling and suggest that monitoring p53 status may inform the utility of NF-κB inhibitor treatment in HNSCC.

Anti-apoptotic NF-κB and "gain of function" mutp53 in concert act pro-apoptotic in response to UVB+IL-1 via enhanced TNF production

In response to genotoxic stress, including UVB radiation, transcription factors NF-κB and p53 inevitably influence the cellular fate. Loss of p53 function has been attributed to malignant transformation and interferes with therapeutic interventions, whereas “gain of function” mutants even enhance tumor promotion. Constitutive NF-κB activation is linked to tumor maintenance and resistance against chemotherapy. The cross talk between p53 and NF-κB, however, is still under debate. Using the non-transformed keratinocyte cell line HaCaT, we shed light on the interplay between p53 and NF-κB by providing clear evidence that chronically activated NF-κB together with designated “gain of function” mutp53 promotes apoptosis via cooperative tumor necrosis factor (TNF) production in response to UVB+IL-1. Performing chromatin immunoprecipitation analysis we demonstrate that both transcription factors bind to the TNF promoter, whereas UVB-induced inhibition of Ser-Thr-phosphatase protein phosphatase 2A facilitates prolonged phosphorylation of NF-κB and the transcriptional cofactor cAMP response element–binding protein, both being required for extended TNF transcription. Thus, two major anti-apoptotic factors, NF-κB and mutp53, in concert may generate pro-apoptotic responses. As human skin is constantly exposed to UVB, causing IL-1 production as well, we hypothesize that the remarkable amount of hotspot p53 mutations within the epidermis (4%) may serve a protective function to eliminate precancerous cells at an early stage.

Chronic inflammation and cancer: potential chemoprevention through nuclear factor kappa B and p53 mutual antagonism

Activation of nuclear factor-kappa B (NF- κB) as a mechanism of host defense against infection and stress is the central mediator of inflammatory responses. A normal (acute) inflammatory response is activated on urgent basis and is auto-regulated. Chronic inflammation that results due to failure in the regulatory mechanism, however, is largely considered as a critical determinant in the initiation and progression of various forms of cancer. Mechanistically, NF- κB favors this process by inducing various genes responsible for cell survival, proliferation, migration, invasion while at the same time antagonizing growth regulators including tumor suppressor p53. It has been shown by various independent investigations that a down regulation of NF- κB activity directly, or indirectly through the activation of the p53 pathway reduces tumor growth substantially. Therefore, there is a huge effort driven by many laboratories to understand the NF- κB signaling pathways to intervene the function of this crucial player in inflammation and tumorigenesis in order to find an effective inhibitor directly, or through the p53 tumor suppressor. We discuss here on the role of NF- κB in chronic inflammation and cancer, highlighting mutual antagonism between NF- κB and p53 pathways in the process. We also discuss prospective pharmacological modulators of these two pathways, including those that were already tested to affect this mutual antagonism.

Caught in the cross fire: p53 in inflammation

The p53 transcription factor is a major tumor suppressor, whose diverse activities serve to ensure genome stability and inhibit neoplastic processes. In recent years, it is becoming increasingly clear that p53 also plays a broader role in maintaining cellular homeostasis, as well as contributing to tissue homeostasis in a non-cell-autonomous fashion. Chronic inflammation is a potential cancer-promoting condition, and as such is also within the radar of p53, which mounts a multifaceted attempt to prevent the escalation of chronic tissue imbalance into neoplasia. Recent understanding of the p53 pathway and other family members reveals a broad interaction with inflammatory elements such as reactive oxygen and nitrogen species, cytokines, infectious agents and major immune-regulatory pathways like nuclear factor-kappaB. This complex cross talk is highly dependent on p53 status, as different p53 isoforms and p53 mutants can mediate different responses and even promote chronic inflammation and associated cancer, acting in the tumor cells as well as in the stromal and immune compartments.

Unraveling regulatory programs for NF-kappaB, p53 and microRNAs in head and neck squamous cell carcinoma

In head and neck squamous cell carcinoma (HNSCC), mutations of p53 usually coexist with aberrant activation of NF-kappaB (NF-κB), other transcription factors and microRNAs, which promote tumor pathogenesis. However, how these factors and microRNAs interact to globally modulate gene expression and mediate oncogenesis is not fully understood. We devised a novel bioinformatics method to uncover interactive relationships between transcription factors or microRNAs and genes. This approach is based on matrix decomposition modeling under the joint constraints of sparseness and regulator-target connectivity, and able to integrate gene expression profiling and binding data of regulators. We employed this method to infer the gene regulatory networks in HNSCC. We found that the majority of the predicted p53 targets overlapped with those for NF-κB, suggesting that the two transcription factors exert a concerted modulation on regulatory programs in tumor cells. We further investigated the interrelationships of p53 and NF-κB with five additional transcription factors, AP1, CEBPB, EGR1, SP1 and STAT3, and microRNAs mir21 and mir34ac. The resulting gene networks indicate that interactions among NF-κB, p53, and the two miRNAs likely regulate progression of HNSCC. We experimentally validated our findings by determining expression of the predicted NF-κB and p53 target genes by siRNA knock down, and by examining p53 binding activity on promoters of predicted target genes in the tumor cell lines. Our results elucidating the cross-regulations among NF-κB, p53, and microRNAs provide insights into the complex regulatory mechanisms underlying HNSCC, and shows an efficient approach to inferring gene regulatory programs in biological complex systems.

p53-independent roles of MDM2 in NF-κB signaling: implications for cancer therapy, wound healing, and autoimmune diseases

Murine double minute-2 (MDM2) is an intracellular molecule with multiple biologic functions. It serves as a negative regulator of p53 and thereby limits cell cycle arrest and apoptosis. Because MDM2 blockade suppresses tumor cell growth in vitro and in vivo, respective MDM2 inhibition is currently evaluated as anti-cancer therapy in clinical trials. However, the anti-proliferative effects of MDM2 inhibition also impair regenerative cell growth upon tissue injury. This was so far documented for tubular repair upon postischemic acute kidney injury and might apply to wound healing responses in general. Furthermore, MDM2 has numerous p53-independent effects. As a new entry, MDM2 was identified to act as a co-transcription factor for nuclear factor-kappa-light-enhancer of activated B cells (NF-κB) at cytokine promoters. This explains the potent anti-inflammatory effects of MDM2 inhibitors in vitro and in vivo. For example, the NF-κB-antagonistic and p53-agonistic activities of MDM2 inhibitors elicit potent therapeutic effects on experimental lymphoproliferative autoimmune disorders such as systemic lupus erythematosus. In this review, we discuss the classic p53-dependent, the recently discovered p53-independent, and the NF-κB-agonistic biologic functions of MDM2. We describe its complex regulatory role on p53 and NF-κB signaling and name areas of research that may help to foresee previously unexpected effects or potential alternative indications of therapeutic MDM2 blockade.

A novel mechanism of crosstalk between the p53 and NFκB pathways: MDM2 binds and inhibits p65RelA

The inflammation regulating transcription factor NFκB and the tumor-suppressing transcription factor p53 can act as functional antagonists. Chronic inflammation (NFκB activity) may contribute to the development of cancer through the inhibition of p53 function, while, conversely, p53 activity may dampen inflammation. Here we report that the E3 ubiquitin ligase MDM2, whose gene is transcriptionally activated by both NFκB and p53, can bind and inhibit the p65RelA subunit of NFκB. The interaction is mediated through the N-terminal and the acidic/zinc finger domains of MDM2 on the one hand and through the N-terminal Rel homology domain of p65RelA on the other hand. Co-expression of MDM2 and p65RelA caused ubiquitination of the latter in the nucleus, and this modification was dependent of a functional MDM2 RING domain. Conversely, inhibition of endogenous MDM2 by small-molecule inhibitors or siRNA significantly reduced the ubiquitination of ectopic and endogenous p65RelA. MDM2 was able to equip p65RelA with mutated ubiquitin moieties capable of multiple monoubiquitination but incapable of polyubiquitination; moreover, MDM2 failed to destabilize p65RelA detectably, suggesting that the ubiquitin modification of p65RelA by MDM2 was mostly regulatory rather than stability-determining. MDM2 inhibited the NFκB-mediated transactivation of a reporter gene and the binding of NFκB to its DNA binding motif in vitro. Finally, knockdown of endogenous MDM2 increased the activity of endogenous NFκB as a transactivator. Thus, MDM2 can act as a direct negative regulator of NFκB by binding and inhibiting p65RelA.

Transcriptional cooperation between p53 and NF-κB p65 regulates microRNA-224 transcription in mouse ovarian granulosa cells

MicroRNAs (miRNAs) have been indicated to play key roles in ovarian follicular development. However, little is known about how the miRNA gene expression itself is regulated in the mammalian ovary. We previously reported that miR-224 is involved in TGF-β1-mediated follicular granulosa cell (GC) growth and estradiol (E2) production by targeting Smad4. Here, the transcriptional regulation of miR-224 expression in GCs was further investigated. Our results showed that both the tumor suppressor gene p53 and NF-κB p65 subunit suppressed the TGF-β1-induced increase in pri-miR-224 expression in GCs. ChIP assays demonstrated that TGF-β1 enhanced the binding of p53 and p65 to the proximal promoter region of GABAA receptor ε subunit (miR-224 host gene). p53 and p65 transcriptionally cooperated to inactivate the GABAA receptor ε subunit promoter. In addition, p53/p65 could up-regulate Smad4 expression by inhibiting its target miR-224 in GCs which contributed, at least partially, to the effects of miR-224 and Smad4 on GC proliferation and E2 release. Our results provide new data about the interplay between transcription factors involved in GC proliferation and function by cooperatively regulating miRNA expression.

Inhibitory effect of tumor suppressor p53 on proinflammatory chemokine expression in ovarian cancer cells by reducing proteasomal degradation of IκB

Ovarian cancer, one of inflammation-associated cancers, is the fifth leading cause of cancer deaths among women. Inflammation in the tumor microenvironment is associated with peritoneal tumor dissemination and massive ascites, which contribute to high mortality in ovarian cancer. Tumor suppressor p53 is frequently deleted or mutated in aggressive and high-grade ovarian cancer, probably aggravating cancer progression and increasing mortality. We therefore investigated the influence of p53 on proinflammatory chemokines in ovarian cancer cells. A PCR array of the chemokine network revealed that ovarian cancer cells with low or mutated p53 expression expressed high levels of proinflammatory chemokines such as CXCL1, 2, 3 and 8. Transient transfection of p53 into p53-null ovarian cancer cells downregulated proinflammatory chemokines induced by tumor necrosis factor-α (TNF), a proinflammatory cytokine abundantly expressed in ovarian cancer. Furthermore, p53 restoration or stabilization blocked TNF-induced NF-κB promoter activity and reduced TNF-activated IκB. Restoration of p53 increased ubiquitination of IκB, resulting from concurrently reduced proteasome activity followed by stability of IκB. A ubiquitination PCR array on restoration of p53 did not reveal any significant change in expression except for Mdm2, indicating that the balance between p53 and Mdm2 is more important in regulating NF-κB signaling rather than the direct effect of p53 on ubiquitin-related genes or IκB kinases. In addition, nutlin-3, a specific inducer of p53 stabilization, inhibited proinflammatory chemokines by reducing TNF-activated IκB through p53 stabilization. Taken together, these results suggest that p53 inhibits proinflammatory chemokines in ovarian cancer cells by reducing proteasomal degradation of IκB. Thus, frequent loss or mutation of p53 may promote tumor progression by enhancing inflammation in the tumor microenvironment.

Charting the NF-κB pathway interactome map

Inflammation is part of a complex physiological response to harmful stimuli and pathogenic stress. The five components of the Nuclear Factor κB (NF-κB) family are prominent mediators of inflammation, acting as key transcriptional regulators of hundreds of genes. Several signaling pathways activated by diverse stimuli converge on NF-κB activation, resulting in a regulatory system characterized by high complexity. It is increasingly recognized that the number of components that impinges upon phenotypic outcomes of signal transduction pathways may be higher than those taken into consideration from canonical pathway representations. Scope of the present analysis is to provide a wider, systemic picture of the NF-κB signaling system. Data from different sources such as literature, functional enrichment web resources, protein-protein interaction and pathway databases have been gathered, curated, integrated and analyzed in order to reconstruct a single, comprehensive picture of the proteins that interact with, and participate to the NF-κB activation system. Such a reconstruction shows that the NF-κB interactome is substantially different in quantity and quality of components with respect to canonical representations. The analysis highlights that several neglected but topologically central proteins may play a role in the activation of NF-κB mediated responses. Moreover the interactome structure fits with the characteristics of a bow tie architecture. This interactome is intended as an open network resource available for further development, refinement and analysis.

Inflammation and p53: A Tale of Two Stresses

Numerous observations indicate a strong link between chronic inflammation and cancer. This link is supported by substantial experimental evidence indicating mutual negative regulation of NF-κB, the major regulator of inflammation, and p53, the major tumor suppressor. This antagonistic relationship reflects the opposite principles of the physiological responses driven by these transcription factors, which act as sensors and mediators of intrinsic and extrinsic cell stresses, respectively. Constitutive activation of NF-κB, the underlying cause of chronic inflammation, is a common acquired characteristic of tumors. A variety of experimental methods have been used to demonstrate that constitutive activation of NF-κB reduces the tumor suppressor activity of p53, thereby creating permissive conditions for dominant oncogene-mediated transformation. Loss of p53 activity is also a characteristic of the majority of tumors and results in unleashed inflammatory responses due to loss of p53-mediated NF-κB suppression. On the other hand, in natural or pharmacological situations of enforced p53 activation, NF-κB activity, inflammation, and immune responses are reduced, resulting in different pathologies. It is likely that the chronic inflammation that is commonly acquired in various tissues of older mammals leads to general suppression of p53 function, which would explain the increased risk of cancer observed in aging animals and humans. Although the molecular mechanisms underlying reciprocal negative regulation of p53 and NF-κB remain to be deciphered, this phenomenon has important implications for pharmacological prevention of cancer and aging and for new approaches to control inflammation.

Nuclear factor-κB: a key regulator in health and disease of lungs

Rel/NF-κB transcription factors play a key role in modulating the response of immunoregulatory genes including cytokines and chemokines, cell adhesion molecules, acute phase proteins, and anti-microbial peptides. Furthermore, an array of genes important for angiogenesis, tumor invasion and metastasis is also regulated by nuclear factor-κB (NF-κB). Close association of NF-κB with inflammation and tumorigenesis makes it an attractive target for basic research as well as for pharmaceutical industries. Studies involving various animal and cellular models have revealed the importance of NF-κB in pathobiology of lung diseases. This review (a) describes structures, activities, and regulation of NF-κB family members; (b) provides information which implicates NF-κB in pathogenesis of pulmonary inflammation and cancer; and (c) discusses information about available synthetic and natural compounds which target NF-κB or specific components of NF-κB signal transduction pathway and which may provide the foundation for development of effective therapy for lung inflammation and bronchogenic carcinomas.

Interactions among oscillatory pathways in NF-kappa B signaling

BACKGROUND

Sustained stimulation with tumour necrosis factor alpha (TNF-alpha) induces substantial oscillations--observed at both the single cell and population levels--in the nuclear factor kappa B (NF-kappa B) system. Although the mechanism has not yet been elucidated fully, a core system has been identified consisting of a negative feedback loop involving NF-kappa B (RelA:p50 hetero-dimer) and its inhibitor I-kappa B-alpha. Many authors have suggested that this core oscillator should couple to other oscillatory pathways.

RESULTS

First we analyse single-cell data from experiments in which the NF-kappa B system is forced by short trains of strong pulses of TNF-alpha. Power spectra of the ratio of nuclear-to-cytoplasmic concentration of NF-kappa B suggest that the cells' responses are entrained by the pulsing frequency. Using a recent model of the NF-kappa B system due to Caroline Horton, we carried out extensive numerical simulations to analyze the response frequencies induced by trains of pulses of TNF-alpha stimulation having a wide range of frequencies and amplitudes. These studies suggest that for sufficiently weak stimulation, various nonlinear resonances should be observable. To explore further the possibility of probing alternative feedback mechanisms, we also coupled the model to sinusoidal signals with a wide range of strengths and frequencies. Our results show that, at least in simulation, frequencies other than those of the forcing and the main NF-kappa B oscillator can be excited via sub- and superharmonic resonance, producing quasiperiodic and even chaotic dynamics.

CONCLUSIONS

Our numerical results suggest that the entrainment phenomena observed in pulse-stimulated experiments is a consequence of the high intensity of the stimulation. Computational studies based on current models suggest that resonant interactions between periodic pulsatile forcing and the system's natural frequencies may become evident for sufficiently weak stimulation. Further simulations suggest that the nonlinearities of the NF-kappa B feedback oscillator mean that even sinusoidally modulated forcing can induce a rich variety of nonlinear interactions.

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.

Low-dose γ-radiation-induced oxidative stress response in mouse brain and gut: regulation by NFκB-MnSOD cross-signaling

Radiation-induced amplification of reactive oxygen species (ROS) may be a sensing mechanism for activation of signaling cascades that influence cell fate. However, the regulated intrinsic mechanisms and targets of low-dose ionizing radiation (LDIR) are still unclear. Accordingly, we investigated the effects of LDIR on NFκB signal transduction and manganese superoxide dismutase (SOD2) activity in mice brain and gut. LDIR resulted in both dose-dependent and persistent NFκB activation in gut and brain. QPCR displayed a dose- and tissue-dependent differential modulation of 88 signaling molecules. With stringent criteria, a total of 15 (2cGy), 43 (10cGy) and 19 (50cGy) genes were found to be commonly upregulated between brain and gut. SOD2 immunostaining showed a LDIR-dose dependent increase. Consistent with the NFκB results, we observed a persistent increase in SOD2 activity after LDIR. Moreover, muting of LDIR-induced NFκB attenuated SOD2 transactivation and cellular localization. These results imply that exposure of healthy tissues to LDIR results in induced NFκB and SOD2 activity and transcriptional activation of NFκB-signal transduction/target molecules. More importantly, the results suggest that NFκB initiates a feedback response through transcriptional activation of SOD2 that may play a key role in the LDIR-induced oxidative stress response and may control the switch that directs cell fate.

p53-mediated neuronal cell death in ischemic brain injury

p53 is a key modulator of cellular stress responses. It is activated in the ischemic areas of brain, and contributes to neuronal apoptosis. In various stroke models, p53 deficiency or applications of p53 inhibitors can significantly attenuate brain damage. p53-mediated neuronal apoptosis occurs through various molecular mechanisms. The transcriptional pathway is an important mechanism through which p53 induces neuronal apoptosis by up-regulating the expression of its target gene p21(WAF), Peg3/Pw1 or p53-up-regulated modulator of apoptosis (PUMA). In addition, p53 disrupts NF-kappaB binding to p300 and blocks NF-kappaB-mediated survival signaling. On the other hand, the transcription-independent pathway mechanism is also of great importance. In this pathway, p53 is translocated to mitochondrial and mediates the release of cytochrome c. In both pathways, p53 seems to play a key role in post-ischemic brain damage and has become a therapeutic target against stroke pathology.

Elevation of cyclic AMP causes an imbalance between NF-kappaB and p53 in NALM-6 cells treated by doxorubicin

We previously showed that cAMP can inhibit DNA damage-induced wild type p53 accumulation in human pre-B NALM-6 cells, leading to a profound reduction of their apoptotic response. Here, we provide evidence for the potentiation of DNA damage-induced NF-kappaB activation by cAMP. We found that inhibition of NF-kappaB activation prevents the inhibitory effect of cAMP on doxorubicin-induced apoptosis. Moreover, cAMP exerts its inhibitory effect on doxorubicin-induced apoptosis in a PKA-independent manner. The present study also shows that elevation of cAMP prolongs the phosphorylation of IkappaB and subsequent activation of NF-kappaB in doxorubicin treated NALM-6 cells in a proteasome-dependent manner. Taken together, our results demonstrate that cAMP abrogates the balance between apoptotic and antiapoptotic transcription factors that are hallmarks of DNA damage signaling.

p53 and NF-κB: different strategies for responding to stress lead to a functional antagonism

The p53 transcription factor responds to a variety of intrinsic stresses, such as DNA damage, hypoxia, and even oncogene activation. NF-κB responds to a large number of extrinsic stresses such as cytokine activation and infectious diseases. The p53 tumor suppressor limits the consequences of stress by initiating cell death, senescence, or cell cycle arrest and promotes metabolic patterns in the cell to favor oxidative phosphorylation. NF-κB, the oncogene, promotes cell division, which initiates the innate and adaptive immune responses utilizing large amounts of glucose in aerobic glycolysis, resulting in the synthesis of substrates for cell division. Thus these two transcription factors, both of which have evolved to respond to different types of stress, have adopted opposite strategies and cannot function in the same cell at the same time. On activation of one of these transcription factors, the other is inactivated. This is achieved at several places in the p53 and NF-κB pathways where regulatory proteins act on both p53 and NF-κB with opposite functional consequences. These internodal sites create core regulatory circuits essential for integrating two central pathways in cells.

Cellular stress response pathway system as a sentinel ensemble in toxicological screening

High costs, long test times, and societal concerns related to animal use have required the development of in vitro assays for the rapid and cost-effective toxicological evaluation and characterization of compounds in both the pharmaceutical and environmental arenas. Although the pharmaceutical industry has developed very effective, high-throughput in vitro assays for determining the therapeutic potential of compounds, the application of this approach to toxicological screening has been limited. A primary reason for this is that while drug candidate screens are directed to a specific target/mechanism, xenobiotics can cause toxicity through any of a myriad of undefined interactions with cellular components and processes. Given that it is not practical to design assays that can interrogate each potential toxicological target, an integrative approach is required if there is to be a rapid and low-cost toxicological evaluation of chemicals. Cellular stress response pathways offer a viable solution to the creation of a set of integrative assays as there is a limited and hence manageable set (a small ensemble of 10 or less) of major cellular stress response pathways through which cells mount a homoeostatic response to toxicants and which also participate in cell fate/death decisions. Further, over the past decades, these pathways have been well characterized at a molecular level thereby enabling the development of high-throughput cell-based assays using the components of the pathways. Utilization of the set of cellular stress response pathway-based assays as indicators of toxic interactions of chemicals with basic cellular machinery will potentially permit the clustering of chemicals based on biological response profiles of common mode of action (MOA) and also the inference of the specific MOA of a toxicant. This article reviews the biochemical characteristics of the stress response pathways, their common architecture that enables rapid activation during stress, their participation in cell fate decisions, the essential nature of these pathways to the organism, and the biochemical basis of their cross-talk that permits an assay ensemble screening approach. Subsequent sections describe how the stress pathway ensemble assay approach could be applied to screening potentially toxic compounds and discuss how this approach may be used to derive toxicant MOA from the biological activity profiles that the ensemble strategy provides. The article concludes with a review of the application of the stress assay concept to noninvasive in vivo assessments of chemical toxicants.

Inactivation of the CYLD deubiquitinase by HPV E6 mediates hypoxia-induced NF-kappaB activation

The biochemical mechanisms that underlie hypoxia-induced NF-kappaB activity have remained largely undefined. Here, we find that prolonged hypoxia-induced NF-kappaB activation is restricted to cancer cell lines infected with high-risk human papillomavirus (HPV) serotypes. The HPV-encoded E6 protein is necessary and sufficient for prolonged hypoxia-induced NF-kappaB activation in these systems. The molecular target of E6 in the NF-kappaB pathway is the CYLD lysine 63 (K63) deubiquitinase, a negative regulator of the NF-kappaB pathway. Specifically, hypoxia stimulates E6-mediated ubiquitination and proteasomal degradation of CYLD. Given the established role of NF-kappaB in human carcinogenesis, these findings provide a potential molecular/viral link between hypoxia and the adverse clinical outcomes observed in HPV-associated malignancies.

Dysregulation of apoptotic signaling in cancer: molecular mechanisms and therapeutic opportunities

Apoptosis is a tightly regulated cell suicide program that plays an essential role in the maintenance of tissue homeostasis by eliminating unnecessary or harmful cells. Defects in this native defense mechanism promote malignant transformation and frequently confer chemoresistance to transformed cells. Indeed, the evasion of apoptosis has been recognized as a hallmark of cancer. Given that multiple mechanisms function at many levels to orchestrate the regulation of apoptosis, a multitude of opportunities for apoptotic dysregulation are present within the intricate signaling network of cell. Several of the molecular mechanisms by which cancer cells are protected from apoptosis have been elucidated. These advances have facilitated the development of novel apoptosis-inducing agents that have demonstrated single-agent activity against various types of cancers cells and/or sensitized resistant cancer cells to conventional cytotoxic therapies. Herein, we will highlight several of the central modes of apoptotic dysregulation found in cancer. We will also discuss several therapeutic strategies that aim to reestablish the apoptotic response, and thereby eradicate cancer cells, including those that demonstrate resistance to traditional therapies.

Activated p53 induces NF-kappaB DNA binding but suppresses its transcriptional activation

NF-kappaB plays an important role in oncogenesis. Recently, we have demonstrated that loss of p53 function enhances DNA binding and transcriptional activities of NF-kappaB via IKKalpha and IKKbeta, and that glycolysis, activated by NF-kappaB, has an integral role in oncogene-induced cell transformation. Here, we show that ectopically expressed p53 induces acetylation and phosphorylation at Ser 536 of p65, an NF-kappaB component, and enhances DNA-binding activity of NF-kappaB. However, activated p53 suppresses transcriptional activity of NF-kappaB. Under non-stimulating conditions, p65 formed a complex with IKKalpha and IKKbeta. Activated p53 bound to p65 on DNA and disrupted binding of p65 to IKKbeta. Moreover, histone H3 kinase activity, which requires transcriptional activation of NF-kappaB, was diminished by p53. Thus, activated p53 may suppress transcriptional activity of NF-kappaB through inhibition of IKK and histone H3 kinase on DNA, suggesting a novel p53-mediated suppression system for tumorigenesis.

Enhancement of chemosensitivity toward peplomycin by calpastatin-stabilized NF-kappaB p65 in esophageal carcinoma cells: possible involvement of Fas/Fas-L synergism

Chemosensitivity to anticancer drugs was compared between two human esophageal carcinoma cell lines, T.Tn and YES-6 cells. T.Tn cells were more resistant than YES-6 cells to peplomycin (PEP) but not to the other anticancer drugs such as camptothecin, mitomycin C and cytosine arabinoside. Western blot analysis showed higher expression levels of m-calpain and activated mu-calpain in T.Tn cells than in YES-6 cells. On the other hand, YES-6 cells showed a high expression level of calpastatin, which is a calpain-specific endogenous inhibitor. To investigate whether calpain activity was involved in the chemosensitivity, T.Tn cells were transfected with calpastatin cDNA in an inducible expression vector. The induction of calpastatin was accompanied by increased chemosensitivity to PEP. The increases in calpastatin levels were followed by serial increases in the expression levels of NF-kappaB p65 and Fas. Since purified m- or mu-calpain degraded NF-kappaB p65 in vitro, it is possible that calpastatin suppressed calpain-mediated degradation of NF-kappaB p65. Fas ligand (Fas-L) protein levels increased after treatment of the parental T.Tn and calpastatin-transfected cells with PEP, suggesting the synergism between calpastatin-induced Fas and PEP-induced Fas-L. These results suggest that calpain/calpastatin expression levels are effective markers for predicting the sensitivity of human esophageal carcinoma cells to PEP.

Phosphorylation of CBP by IKKalpha promotes cell growth by switching the binding preference of CBP from p53 to NF-kappaB

CBP plays a central role in coordinating and integrating multiple signaling pathways. Competition between NF-kappaB and p53 for CBP is a crucial determinant of whether a cell proliferates or undergoes apoptosis. However, how the CBP-dependent crosstalk between these two transcription factors is regulated remains unclear. Here, we show that IKKalpha phosphorylates CBP at serine 1382 and serine 1386 and consequently increases CBP's HAT and transcriptional activities. Importantly, such phosphorylation enhances NF-kappaB-mediated gene expression and suppresses p53-mediated gene expression by switching the binding preference of CBP from p53 to NF-kappaB, thus promoting cell growth. The CBP phosphorylation also correlates with constitutive IKKalpha activation in human lung tumor tissue compared with matched nontumor lung tissue. Our results suggest that phosphorylation of CBP by IKKalpha regulates the CBP-mediated crosstalk between NF-kappaB and p53 and thus may be a critical factor in the promotion of cell proliferation and tumor growth.

A critical role for p53 in the control of NF-kappaB-dependent gene expression in TLR4-stimulated dendritic cells exposed to Genistein

Considerable research has focused on the anti-inflammatory and antiproliferative activities exhibited by the soy isoflavone genistein. We previously demonstrated that genistein suppresses TNF-alpha-induced NF-kappaB-dependent IL-6 gene expression in cancer cells by interfering with the mitogen- and stress-activated protein kinase 1 activation pathway. However, effects of isoflavones on immune cells, such as dendritic cells, remain largely unknown. Here we show that genistein markedly reduces IL-6 cytokine production and transcription in LPS-stimulated human monocyte-derived dendritic cells. More particularly, we observe that genistein inhibits IL-6 gene expression by modulating the transcription factor NF-kappaB. Examination of NF-kappaB-related events downstream of TLR4 demonstrates that genistein affects NF-kappaB subcellular localization and DNA binding, although we observe only a minor inhibitory impact of genistein on the classical LPS-induced signaling steps. Interestingly, we find that genistein significantly increases p53 protein levels. We also show that overexpression of p53 in TLR4/MD2 HEK293T cells blocks LPS-induced NF-kappaB-dependent gene transcription, indicating the occurrence of functional cross-talk between p53 and NF-kappaB. Moreover, analysis of IL-6 mRNA levels in bone marrow-derived p53 null vs wild-type dendritic cells confirms a role for p53 in the reduction of NF-kappaB-dependent gene expression, mediated by genistein.

Resistance to the skeletal muscle injury expressed by repeated treatment with compound A that has HMG-CoA reductase inhibitory activity

It has been noted that chemical-induced initial insult is sometimes no longer detected in examinations after additional consecutive treatments, suggesting that the target organs acquire resistance to the chemical toxicity. In this study, whether acquired resistance to the skeletal muscle toxicity is observed during repeated treatment of a toxic dose of Compound A that has a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitory activity was examined. F344 male rats (7-weeks old) were given a mixed diet with 0.12% Compound A (corresponding to approximately 100 mg/kg/day) for up to 56 days. Blood samples were obtained from the tail vein periodically during the dosing period, and utilized for the measurement of creatine kinase (CK) as a marker of skeletal muscle injury. In the necropsies on Days 4, 8, 11, 28, 42 and 56, the skeletal muscles from the rectus femoris were removed for histopathology or gene expression analysis. A satellite group was provided to measure the plasma concentrations of Compound A and M1, the active metabolite of Compound A. CK levels increased from Day 9 and reached approximately 30 times those of the controls on Day 12. Histopathology of the skeletal muscle on Day 11 revealed severe necrosis of the muscle fibers. However, in spite of continuous treatments to the damaged rats, the CK levels decreased after that and returned to normal levels on Day 18. No skeletal muscle injury was observed on Days 42 and 56. There were no marked differences in the exposure levels of Compound A and M1 between Days 8 (prior to CK elevation) and 28 (post CK elevation). As for the most significant changes in the gene expression analysis for the skeletal muscle on Days 42 and 56, the probe for IkappaBa, which is known as an inhibitor for nuclear factor-kappaB (NF-kappaB), increased 2-fold compared to the control. Furthermore, an increased probe for CCAAT/enhancer-binding protein (C/EBP) delta, a transcriptional factor, and a decreased probe for cAMP-response element-binding protein (CBP)/p300, a transcriptional coactivator, were also noted significantly on Day 56. These changes in the gene expression analysis suggested suppressed NF-kappaB-mediated transactivation, which was responsible for the protective effects on the muscle injury. Based on the present findings, the resistance to skeletal muscle injury observed in this study may be attributable to the suppressed NF-kappaB-mediated transactivation, but not to the decreased exposure to toxicants.

Kaposi's sarcoma-associated herpesvirus induces sustained NF-kappaB activation during de novo infection of primary human dermal microvascular endothelial cells that is essential for viral gene expression

In vitro Kaposi's sarcoma-associated herpesvirus (KSHV) infection of primary human dermal microvascular endothelial (HMVEC-d) cells and human foreskin fibroblast (HFF) cells is characterized by the induction of preexisting host signal cascades, sustained expression of latency-associated genes, transient expression of a limited number of lytic genes, and induction of several cytokines, growth factors, and angiogenic factors. Since NF-kappaB is a key molecule involved in the regulation of several of these factors, here, we examined NF-kappaB induction during de novo infection of HMVEC-d and HFF cells. Activation of NF-kappaB was observed as early as 5 to 15 min postinfection by KSHV, and translocation of p65-NF-kappaB into nuclei was detected by immunofluorescence assay, electrophoretic mobility shift assay, and p65 enzyme-linked immunosorbent assay. IkappaB phosphorylation inhibitor (Bay11-7082) reduced this activation significantly. A sustained moderate level of NF-kappaB induction was seen during the observed 72 h of in vitro KSHV latency. In contrast, high levels of ERK1/2 activation at earlier time points and a moderate level of activation at later times were observed. p38 mitogen-activated protein kinase was activated only at later time points, and AKT was activated in a cyclic manner. Studies with UV-inactivated KSHV suggested a role for virus entry stages in NF-kappaB induction and a requirement for KSHV viral gene expression in sustained induction. Inhibition of NF-kappaB did not affect target cell entry by KSHV but significantly reduced the expression of viral latent open reading frame 73 and lytic genes. KSHV infection induced the activation of several host transcription factors, including AP-1 family members, as well as several cytokines, growth factors, and angiogenic factors, which were significantly affected by NF-kappaB inhibition. These results suggest that during de novo infection, KSHV induces sustained levels of NF-kappaB to regulate viral and host cell genes and thus possibly regulates the establishment of latent infection.

Functional cross-talk between p73β and NF-κB mediated by p300

p73beta is associated with induction of apoptosis or cellular growth arrest, while NF-kappaB is closely related with promotion of resistance to programmed cell death. These biologically opposing activities between p73beta and NF-kappaB propose a regulatory mechanism of critical turning on/off in cellular apoptotic or survival responses. In this study, we demonstrate that NF-kappaB-mediated transactivation is specifically downregulated by p73beta; conversely, p73beta-transactivation is negatively regulated by functional expression of p65, NF-kappaB RelA subunit. The p73beta transactivation domain (TA) and p65 NH2-terminus are crucial for their negative regulation of p65- and p73beta-mediated transactivation, respectively. Furthermore, p65- or p73beta-interaction with p300 is reciprocally inhibited by their competitive binding to p300 in a restrict amount-dependent manner. Likewise, both p73beta-activated apoptosis and p65-dependent increase of cell viability are reciprocally repressed by p65 and p73beta, respectively. These results have important implications for p300-mediated regulatory mechanism between p73beta- and p65-transactivation, by which both p73beta and NF-kappaB could mutually affect on their biological activities. Therefore, we propose that p300 is a transactivational regulator of competitively balanced cross-talk between p73beta and p65.

Up-regulation of NFκB-responsive gene expression by ΔNp73α in p53 null cells

Transactivation domain (TAD)-truncated p73, DeltaNp73, associates with p53, resulting in suppression of p53's functions. Using p53 null cell lines, we examined whether or not DeltaNp73 can regulate gene expression in a p53-independent manner. When DeltaNp73alpha was co-transfected with a luciferase reporter plasmid with various enhancer elements, NFkappaB-responsive luciferase gene expression was selectively up-regulated by DeltaNp73alpha, but not by other p73-isoforms with TAD and DeltaNp73beta. Deletion of the TAD endowed p73alpha with the ability to enhance the responsive gene's expression, but deletion of the N-terminal proline-rich domain (PRD) rendered the TAD-deleted p73alpha inactive. Considering the inability of DeltaNp73beta, which is the C-terminus-truncated form of DeltaNp73alpha, to function, these results indicate that both the PRD and C-terminus are necessary for DeltaNp73alpha to can activate NFkappaB-responsive luciferase expression. Over-expression of p53 suppressed the TAD-truncated p73alpha-mediated luciferase expression, suggesting that p53 interferes with the TAD-truncated p73alpha-mediated activation of NFkappaB. Inhibitors for NFkappaB activation reduced the TAD-truncated p73alpha-dependent NFkappaB-responsive gene expression, indicating that TAD-truncated p73alpha activates NFkappaB as does TNFalpha. In addition to the results obtained in the reporter gene assay, TAD-truncated p73alpha stimulated the translocation of NFkappaB to the nucleus and the expression of an endogenous NFkappaB-responsive gene, Bcl-XL. Taken together, these results demonstrate that TAD-truncated p73alpha can activate NFkappaB.

Platelet-activating factor-induced NF-κB activation enhances VEGF expression through a decrease in p53 activity

We investigated the role of p53 in nuclear factor (NF)-kappaB dependent, platelet-activating factor (PAF)-induced vascular endothelial growth factor (VEGF) expression. Transfected NF-kappaB subunits in ECV304 cells increased the tumor necrosis factor-alpha promoter activity, which was completely inhibited by p53. Transfected p53 increased p53RE promoter activity, which was completely inhibited by NF-kappaB subunits, indicating that cross-regulation occurs between NF-kappaB and p53. PAF-induced increase in VEGF expression was correlated with decreased p53 activity. These data suggest that NF-kappaB-dependency of the PAF-induced increase in VEGF expression is due to decreased p53 activity, which is reciprocally regulated by increased NF-kappaB activity.

Functional inactivation of p53 by human T-cell leukemia virus type 1 Tax protein: mechanisms and clinical implications

Human T-cell leukemia virus type 1 (HTLV-I) has been implicated with the etiology of adult T-cell leukemia (ATL) and certain other clinical disorders. Although the leukemogenic mechanism of HTLV-1 is not fully understood yet, the viral Tax protein is widely regarded as a key factor in this mechanism. Tax can modulate the synthesis or function of many regulatory factors which control a wide range of normal and oncogenic cellular processes and therefore, it acts as a potent oncoprotein. In the last few years, special attention has been attracted to Tax interference with the transactivation function of p53, a tumor-suppressor protein that is involved in regulation of the cell-cycle and apoptosis and in maintaining the cellular genome integrity. p53 is mutated in approximately 60% of all human tumors. In contrast, mutant p53 is found in only small percentage of ATL patients. Nevertheless, p53 is inactive in the leukemic cells of most ATL patients and in most HTLV-1 transformed cells. By inactivating p53, Tax can immortalize the HTLV-1-infected cells and destabilize their genome. Consequently, such cells can progress toward the ultimate leukemic state by a stepwise accumulation of oncogenic mutations and other types of chromosomal aberrations. Furthermore, since p53 exists in most ATL patients in its wild-type form, its reactivation by therapeutic drugs might be an effective approach for ATL therapy. Several mechanisms have been proposed so far for Tax-induced p53 inactivation. Understanding the exact mechanism of this Tax effect is essential for designing effective means for this therapeutic approach. In this review article, we discuss the various mechanisms proposed for Tax interference with p53 functions and their clinical and therapeutic implications.

Physical interaction and mutual transrepression between CCAAT/enhancer-binding protein beta and the p53 tumor suppressor

The tumor suppressor protein p53 is not only involved in defending cells against genotoxic insults but is also implicated in differentiation processes, a function that it shares with the CCAAT/enhancer-binding protein beta (C/EBPbeta). We previously reported an up-regulation of both factors in the cycle-dependent differentiation process of human endometrial stromal cells, termed decidualization. C/EBPbeta-mediated activation of a decidualization marker, the decidual prolactin promoter, was antagonized by p53. Here we report that C/EBPbeta in turn represses the transcriptional activity of p53. Competition for limiting amounts of coactivator CREB-binding protein/p300 was ruled out as the underlying mechanism of transrepression. Physical interaction between p53 and C/EBPbeta was demonstrated in vitro and in vivo and shown to depend on the C-terminal domains of both proteins. In gel shift experiments, C/EBPbeta reduced complex formation between p53 and its response element. Conversely, p53 strongly inhibited binding of endogenous C/EBPbeta from endometrial stromal cells to the C/EBP-responsive region in the decidual prolactin promoter. The observed negative cross-talk between p53 and C/EBPbeta is likely to impact expression of their respective target genes.

RelA repression of RelB activity induces selective gene activation downstream of TNF receptors

TNF-alpha is a potent proinflammatory cytokine that regulates immune and inflammatory responses and programmed cell death. TNF-alpha stimulation causes nuclear translocation of several NF-kappaB dimers, including RelA/p50 and RelB/p50. However, contrary to RelA, RelB entering the nucleus in response to TNF-alpha cannot bind to DNA in mouse embryonic fibroblasts, strongly suggesting that RelB DNA-binding activity is modulated by additional nuclear mechanisms. Here, we demonstrate that TNF-alpha promotes the association of RelA with RelB in the nucleus and that TNF-alpha-induced RelA/RelB heterodimers do not bind to kappaB sites. Remarkably, we show that RelA serine-276, the phosphorylation of which is induced by TNF receptor ligation, is crucial for RelA/RelB complex formation and subsequent inhibition of RelB DNA binding. In the absence of RelA phosphorylation on serine-276, TNF-alpha stimulation leads to a strong increase in the expression of endogenous NF-kappaB-responsive genes, such as Bcl-xL, whose transcriptional up-regulation is mainly controlled by RelB. Our findings demonstrate that RelA has a major regulatory role serving to dampen RelB activity in response to TNF-alpha and define a previously unrecognized mechanism that represents an essential step leading to selective NF-kappaB target gene expression.

Epstein-Barr virus latent membrane protein 1 represses p53-mediated DNA repair and transcriptional activity

The latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV), a viral oncogene, is essential for transformation of resting B cells by the virus. We previously demonstrated that LMP1 could repress DNA repair in p53-wild-type and p53-deficient human epithelial cells. In this study, using a host cell reactivation (HCR) assay, we demonstrated that p53-enhanced DNA repair was repressed by LMP1 in p53-deficient cells. Moreover, we found that LMP1 was able to repress p53-dependent transcriptional activity. Regarding the mechanisms of p53 repression by LMP1, we found that LMP1 did not inhibit p53 function through direct interaction, by promoting protein degradation or reducing its DNA-binding ability. Using chimeric proteins in the reporter assay, we demonstrated that LMP1 inhibited p53 transactivation by influencing the N-terminal transactivation domain of p53. Subsequent experiments using various LMP1 deletion mutants indicated that a C-terminus-activating region of LMP1, CTAR1 or CTAR2, is responsible for the repression of p53-mediated DNA repair and p53-dependent transcription, which is correlated with the region responsible for NF-kappaB activation. Furthermore, blockage of NF-kappaB signalling by IkappaB-DeltaN was shown to abolish the repression of p53 by LMP1, suggesting that LMP1 likely repressed p53 function through the NF-kappaB pathway. Based on these results, we propose that inhibition of p53-dependent transcriptional activity and DNA repair by LMP1 results in the loss of p53 activity for maintaining genomic stability, which may contribute to the oncogenesis of LMP1 in human epithelial cells.