A RelA(p65) Thr505 phospho-site mutation reveals an important mechanism regulating NF-κB-dependent liver regeneration and cancer

Post-translational modifications of nuclear factor (NF)-κB subunits provide a mechanism to differentially regulate their activity in response to the many stimuli that induce this pathway. However, the physiological significance of these modifications is largely unknown, and it remains unclear if these have a critical role in the normal and pathological functions of NF-κB in vivo. Among these, phosphorylation of the RelA(p65) Thr505 residue has been described as an important regulator of NF-κB activity in cell lines, but its physiological significance was not known. Therefore, to learn more about the role of this pathway in vivo, we generated a knockin mouse with a RelA T505A mutation. Unlike RelA knockout mice, the RelA T505A mice develop normally but exhibit aberrant hepatocyte proliferation following liver partial hepatectomy or damage resulting from carbon tetrachloride (CCl4) treatment. Consistent with these effects, RelA T505A mice exhibit earlier onset of cancer in the N-nitrosodiethylamine model of hepatocellular carcinoma. These data reveal a critical pathway controlling NF-κB function in the liver that acts to suppress the tumour-promoting activities of RelA.

The NF-κB subunit c-Rel regulates Bach2 tumour suppressor expression in B-cell lymphoma

The REL gene, encoding the NF-κB subunit c-Rel, is frequently amplified in B-cell lymphoma and functions as a tumour-promoting transcription factor. Here we report the surprising result that c-rel-/- mice display significantly earlier lymphomagenesis in the c-Myc driven, Eμ-Myc model of B-cell lymphoma. c-Rel loss also led to earlier onset of disease in a separate TCL1-Tg-driven lymphoma model. Tumour reimplantation experiments indicated that this is an effect intrinsic to the Eμ-Myc lymphoma cells but, counterintuitively, c-rel-/- Eμ-Myc lymphoma cells were more sensitive to apoptotic stimuli. To learn more about why loss of c-Rel led to earlier onset of disease, microarray gene expression analysis was performed on B cells from 4-week-old, wild-type and c-rel-/- Eμ-Myc mice. Extensive changes in gene expression were not seen at this age, but among those transcripts significantly downregulated by the loss of c-Rel was the B-cell tumour suppressor BTB and CNC homology 2 (Bach2). Quantitative PCR and western blot analysis confirmed loss of Bach2 in c-Rel mutant Eμ-Myc tumours at both 4 weeks and the terminal stages of disease. Moreover, Bach2 expression was also downregulated in c-rel-/- TCL1-Tg mice and RelA Thr505Ala mutant Eμ-Myc mice. Analysis of wild-type Eμ-Myc mice demonstrated that the population expressing low levels of Bach2 exhibited the earlier onset of lymphoma seen in c-rel-/- mice. Confirming the relevance of these findings to human disease, analysis of chromatin immunoprecipitation sequencing data revealed that Bach2 is a c-Rel and NF-κB target gene in transformed human B cells, whereas treatment of Burkitt's lymphoma cells with inhibitors of the NF-κB/IκB kinase pathway or deletion of c-Rel or RelA resulted in loss of Bach2 expression. These data reveal a surprising tumour suppressor role for c-Rel in lymphoma development explained by regulation of Bach2 expression, underlining the context-dependent complexity of NF-κB signalling in cancer.

Using body temperature, food and water consumption as biomarkers of disease progression in mice with Eμ-myc lymphoma

Background:

Non-invasive biomarkers of disease progression in mice with cancer are lacking making it challenging to implement appropriate humane end points. We investigated whether body temperature, food and water consumption could be used to predict tumour burden.

Methods:

Thirty-six male, wild-type C57Bl/6 mice were implanted with subcutaneous RFID temperature sensors and inoculated with Eμ-myc tumours that infiltrate lymphoid tissue.

Results:

Decrease in body temperature over the course of the study positively predicted post-mortem lymph node tumour burden (R²=0.68, F(1,22)=44.8, P<0.001). At experimental and humane end points, all mice that had a mean decrease in body temperature of 0.7 °C or greater had lymph nodes heavier than 0.5 g (100% sensitivity), whereas a mean decrease in body temperature <0.7 °C always predicted lymph nodes lighter than 0.5 g (100% specificity). The mean decrease in food consumption in each cage also predicted mean post-mortem lymph node tumour burden at 3 weeks (R²=0.89, F(1,3)=23.2, P=0.017).

Conclusion:

Temperature, food and water consumption were useful biomarkers of disease progression in mice with lymphoma and could potentially be used more widely to monitor mice with other forms of cancer.

Proenkephalin assists stress-activated apoptosis through transcriptional repression of NF-kappaB- and p53-regulated gene targets

The respective pro- and antiapoptotic functions of the transcription factors p53 and nuclear factor kappaB (NF-kappaB), and their potential impact on tumorigenesis and response to tumor therapy are well recognized. The capacity of the RelA(p65) subunit of NF-kappaB to specify a pro-apoptotic outcome in response to some stimuli is less well recognized, but needs to be understood if rational manipulation of the NF-kappaB pathway is to be deployed in cancer therapy. In this report, we provide evidence that the growth-responsive nuclear protein, proenkephalin (Penk), is required, in part, for apoptosis induction, in response to activation or overexpression of p53 and RelA(p65). We describe UV-C-inducible physical associations between endogenous Penk and p53 and RelA(p65) in mammalian cell lines. Depletion of Penk by RNA interference (RNAi) substantially preserves viable cell number following exposure to UV-C irradiation or hydrogen peroxide and confers transient protection in cells exposed to the genotoxin etoposide. In virally transformed and human tumor cell lines, overexpression of nuclear Penk with overabundant or activated p53, or RelA(p65) even in the absence of p53, enhances apoptosis to the point of synergy. We have further shown that Penk depletion by RNAi substantially derepresses transcription of a range of antiapoptotic gene targets previously implicated in repression-mediated apoptosis induction by NF-kappaB and p53. Physical association of endogenous Penk with the transcriptional co-repressor histone deacetylase suggests that it may be a component of a transcriptional repression complex that contributes to a pro-apoptotic outcome, following activation of the NF-kappaB and p53 pathways, and could therefore help to facilitate an antitumor response to a broad range of agents.

Regulation of ATR-dependent pathways by the FHA domain containing protein SNIP1

The forkhead associated (FHA) domain-containing protein Smad nuclear interacting protein 1 (SNIP1) has multiple cellular functions, including the ability to interact with DNA-binding transcription factors and transcriptional coactivators. Moreover, we have demonstrated previously that SNIP1 regulates cyclin D1 expression and promoter activity. Here, we identify a new function for SNIP1 as a regulator of ATR checkpoint kinase-dependent pathways in human U-2 OS osteosarcoma cells: SNIP1 is required for p53 induction in response to ultraviolet light treatment and selectively regulates the phosphorylation of known ATR target proteins, including p53, Chk1 and the histone variant H2AX. These activities are independent of its ability to regulate cyclin D1 expression. Significantly, SNIP1 is also required for ATR-dependent functions of the human p14(ARF) tumour suppressor, including its ability to modulate the activity of the RelA(p65) NF-kappaB subunit. This, together with its other described functions, suggests that SNIP1 could have an important role during tumorigenesis and cancer therapy.

Post-translational modifications regulating the activity and function of the nuclear factor kappa B pathway

The diverse cellular and biological functions of the nuclear factor kappa B (NF-kappaB) pathway, together with the catastrophic consequences of its aberrant regulation, demand specific and highly regulated control of its activity. As described in this review, regulation of the NF-kappaB pathway is brought about through multiple post-translational modifications that control the activity of the core components of NF-kappaB signaling: the IkappaB kinase (IKK) complex, the IkappaB proteins and the NF-kappaB subunits themselves. These regulatory modifications, which include phosphorylation, ubiquitination, acetylation, sumoylation and nitrosylation, can vary, depending on the nature of the NF-kappaB-inducing stimulus. Moreover, they frequently have distinct, sometimes antagonistic, functional consequences and the same modification can have different effects depending on the context. Given the important role of NF-kappaB in human health and disease, understanding these pathways will not only provide valuable insights into mechanism and function, but could also lead to new drug targets and the development of diagnostic and prognostic biomarkers for many pathological conditions.

Good cop, bad cop: the different faces of NF-kappaB

Complexes formed from the nuclear factor kappaB (NF-kappaB) family of transcription factors are ubiquitously expressed and are induced by a diverse array of stimuli. This results in their becoming activated in a wide variety of different settings. While the functions of NF-kappaB in many of these contexts have been the subject of intense research and are now well established, it is also clear that there is great diversity in the effects and consequences of NF-kappaB activation. NF-kappaB subunits do not necessarily regulate the same genes, in an identical manner, in all of the different circumstances in which they are induced. This review will discuss the different functions of NF-kappaB, the pathways that modulate NF-kappaB subunit activity and, in contrast to its more commonly thought of role as a promoter of cancer cell growth and survival, the ability of NF-kappaB, under some circumstances, to behave as a tumor suppressor.

Post-translational modification of RelA(p65) NF-kappaB

Stimulation with diverse agents activates the NF-kappaB (nuclear factor kappaB) transcription factor, affecting inflammatory and immune responses, proliferation, differentiation, apoptosis and tumourigenesis. Determining how NF-kappaB elicits such distinct responses is essential to understanding NF-kappaB function in diseased tissues. Recent developments illustrating that post-translational modification of NF-kappaB subunits influences their nuclear role are discussed. These observations suggest that diagnosis and new therapies based on reprogramming NF-kappaB activity could be more efficient than total NF-kappaB inhibition.

Regulation of NF-κB by atypical activators and tumour suppressors

The RelA(p65) NF-κB (nuclear factor κB) subunit is typically thought of as being antiapoptotic and tumour-promoting. However, in our laboratory, we have discovered that RelA can inhibit, rather than induce, antiapoptotic gene expression when activated by certain chemotherapeutic drugs, UV light or through the action of the ARF tumour suppressor. These observations explain why RelA can sometimes facilitate rather than inhibit apoptosis and also exhibits tumour-suppressor characteristics in vivo. A better understanding of these processes and an ability to analyse RelA function in tumours could lead to improved cancer diagnosis, choice of therapy and, ultimately, development of new drugs.

SNIP1 inhibits NF-kappa B signaling by competing for its binding to the C/H1 domain of CBP/p300 transcriptional co-activators

SNIP1 is a 396-amino acid nuclear protein shown to be an inhibitor of the TGF-beta signal transduction pathway and to be important in suppressing transcriptional activation dependent on the co-activators CBP and p300. In this report we show that SNIP1 potently inhibits the activity of NF-kappa B, which binds the C/H1 domain of CBP/p300, but does not interfere with the activity of transcription factors such as p53, which bind to other domains of p300, or factors such as VP16, which are independent of these co-activators. Inhibition of NF-kappa B activity is a function of the N-terminal domain of SNIP1 and involves competition of SNIP1 and the NF-kappa B subunit, RelA/p65, for binding to p300, similar to the mechanism of inhibition of Smad signaling by SNIP1. Immunohistochemical staining shows that expression of SNIP1 is strictly regulated in development and that it colocalizes, in certain tissues, with nuclear staining for RelA/p65 and for p300, suggesting that they may regulate NF-kappa B activity in vivo in a spatially and temporally controlled manner. These data led us to suggest that SNIP1 may be an inhibitor of multiple transcriptional pathways that require the C/H1 domain of CBP/p300.

UV stimulation induces nuclear factor kappaB (NF-kappaB) DNA-binding activity but not transcriptional activation

The cellular response to DNA-damaging agents is partly mediated by DNA-binding transcription factors such as p53 and nuclear factor kappaB (NF-kappaB). Typically NF-kappaB activation is associated with resistance to apoptosis. Following stimulation with UV light however, NF-kappaB activation has been shown to be required for programmed cell death. To study this effect further and to determine the relationship between NF-kappaB and p53 function, we have examined the effect of UV light on U2OS cells. UV stimulation resulted in the activation of NF-kappaB DNA-binding and the induction of p53. Surprisingly, and in contrast with tumour necrosis factor alpha stimulation, this UV- induced NF-kappaB was transcriptionally inert. These observations suggest a model in which the NF-kappaB switch from an anti-apoptotic to a pro-apoptotic role within the cell results from modulation of its ability to stimulate gene expression, possibly as a result of the ability of p53 to sequester transcriptional co-activator proteins such as p300/CREB (cAMP-response-element-binding protein)-binding protein.

The Rel/NF-kappa B family: friend and foe

The members of the Rel/NF-kappa B family of transcription factors form one of the first lines of defense against infectious diseases and cellular stress. These proteins initiate a highly coordinated response in multiple cell types that effectively counteracts the threat to the health of the organism. Conversely, disruption of the regulatory mechanisms that control the specificity and extent of this response, which results in aberrant activation of NF-kappa B, can be one of the primary causes of a wide range of human diseases. Thus, targeting NF-kappa B might lead to the development of new pharmaceutical reagents that could provide novel treatments for many inflammatory diseases and cancer.

A novel transcriptional repression domain mediates p21(WAF1/CIP1) induction of p300 transactivation

The transcriptional coactivators p300 and CREB binding protein (CBP) are important regulators of the cell cycle, differentiation, and tumorigenesis. Both p300 and CBP are targeted by viral oncoproteins, are mutated in certain forms of cancer, are phosphorylated in a cell cycle-dependent manner, interact with transcription factors such as p53 and E2F, and can be found complexed with cyclinE-Cdk2 in vivo. Moreover, p300-deficient cells show defects in proliferation. Here we demonstrate that transcriptional activation by both p300 and CBP is stimulated by coexpression of the cyclin-dependent kinase inhibitor p21(WAF/CIP1). Significantly this stimulation is independent of both the inherent histone acetyltransferase (HAT) activity of p300 and CBP and of the previously reported carboxyl-terminal binding site for cyclinE-Cdk2. Rather, we describe a previously uncharacterized transcriptional repression domain (CRD1) within p300. p300 transactivation is stimulated through derepression of CRD1 by p21. Significantly p21 regulation of CRD1 is dependent on the nature of the core promoter. We suggest that CRD1 provides a novel mechanism through which p300 and CBP can switch activities between the promoters of genes that stimulate growth and those that enhance cell cycle arrest.

Inhibition of the RelA(p65) NF-kappaB subunit by Egr-1

Induction of transcription from the human immunodeficiency virus 1 long terminal repeat by the RelA (p65) NF-kappaB subunit has been shown to be dependent upon an interaction with the zinc finger DNA-binding domain of Sp1. It was unknown, however, whether NF-kappaB could also interact with other zinc finger-containing transcription factors. In this study we demonstrate that the early growth response transcription factor Egr-1, whose DNA-binding domain shares a high degree of homology with that of Sp1, can also interact with RelA in vitro and regulate NF-kappaB transcriptional activity in vivo. Similar to the interaction with Sp1, the Rel homology domain of RelA interacts with the zinc finger domain of Egr-1. Surprisingly, and in contrast to Sp1, Egr-1 specifically represses RelA transcriptional activity through its zinc finger domain. Moreover, the interaction between RelA and the Egr-1 zinc fingers is mutually exclusive with DNA binding suggesting a model in which Egr-1 directly sequesters NF-kappaB from its target promoters. Because Egr-1 is induced by many of the same stimuli that activate NF-kappaB, this novel transcriptional regulatory mechanism has many implications for the involvement of both factors in cellular processes such as apoptosis and the response to stress and infection.

Transcriptional cross talk between NF-kappaB and p53

Many cellular stimuli result in the induction of both the tumor suppressor p53 and NF-kappaB. In contrast to activation of p53, which is associated with the induction of apoptosis, stimulation of NF-kappaB has been shown to promote resistance to programmed cell death. These observations suggest that a regulatory mechanism must exist to integrate these opposing outcomes and coordinate this critical cellular decision-making event. Here we show that both p53 and NF-kappaB inhibit each other's ability to stimulate gene expression and that this process is controlled by the relative levels of each transcription factor. Expression of either wild-type p53 or the RelA(p65) NF-kappaB subunit suppresses stimulation of transcription by the other factor from a reporter plasmid in vivo. Moreover, endogenous, tumor necrosis factor alpha-activated NF-kappaB will inhibit endogenous wild-type p53 transactivation. Following exposure to UV light, however, the converse is observed, with p53 downregulating NF-kappaB-mediated transcriptional activation. Both p53 and RelA(p65) interact with the transcriptional coactivator proteins p300 and CREB-binding protein (CBP), and we demonstrate that these results are consistent with competition for a limiting pool of p300/CBP complexes in vivo. These observations have many implications for regulation of the transcriptional decision-making mechanisms that govern cellular processes such as apoptosis. Furthermore, they suggest a previously unrealized mechanism through which dysregulated NF-kappaB can contribute to tumorigenesis and disease.

Requirements for enhanced transgene expression by untranslated sequences from the human cytomegalovirus immediate-early gene

BACKGROUND

The cytomegalovirus immediate early (CMV IE) promoter has been widely used for heterologous expression. Further enhancements of gene expression from this potent promoter may allow for the development of improved gene transfer strategies. We aimed to determine whether inclusion of the first exon (5' untranslated) and first intron of the CMV IE gene would increase heterologous transgene expression in primary target cells and to determine the sequences required for any observed increases.

MATERIALS AND METHODS

Comparisons of reporter gene expression were made following transient transfection of vascular smooth muscle cells (VSMCs) with plasmids containing the first exon and intron from the CMV IE gene or deletional mutations. Comparisons were also made using a heterologous promoter (RSV).

RESULTS

Gene expression from the CMV IE promoter was increased 5.7-fold in VSMC with the inclusion of the first exon and intron. Similar increases were seen with other target cells and from the heterologous RSV promoter. This increase was associated with an increase in steady-state mRNA. Deletion analyses demonstrated that the enhancement was dependent on the presence of the 5' portion of the first exon while deletion of large segments within the intron was associated with similar levels of expression compared with the parental plasmid.

CONCLUSIONS

Inclusion of the first exon and intron from the CMV IE gene increases expression from the CMV IE promoter. This enhancement is seen with the heterologous RSV promoter and is associated with an increase in steady-state mRNA. Deletion analyses suggest that this enhancement is associated with inclusion of sequences within the 5' portion of the first exon and inclusion of an intron.

Cell cycle regulation of the transcriptional coactivators p300 and CREB binding protein

To respond to changes in its environment, the cell utilizes mechanisms that integrate extracellular signals with specific changes in gene expression. To better understand these critical regulatory mechanisms, research has focused, for the most part, on the identification of sequence-specific DNA-binding proteins, such as the nuclear factor kappaB (NF-kappaB) or activator protein 1 (AP-1) families of transcription factors, that interact with the promoter and enhancer elements of genes induced or repressed during cellular activation. More recently, however, it has become apparent that non-DNA-binding transcriptional coactivators, such as p300 and CREB binding protein (CBP), previously thought to function primarily as "bridging" proteins between DNA-bound transcription factors and the basal transcription complex, play a critical regulatory role as integrators of diverse signalling pathways with the selective induction of gene expression. In this commentary, we shall discuss the implications of a particular aspect of this growing and expanding field: how cell cycle regulation of p300 and CBP impacts our understanding of cellular differentiation, the response to DNA damage, and oncogenesis.

Achieving transcriptional specificity with NF-kappa B

Nuclear Factor-Kappa B (NF-kappa B) was first identified by Sen and Baltimore (1986, Cell 46, 705-716) as a constitutively active transcription factor binding the kappa light chain immunoglobulin enhancer in B cells. Shortly afterwards, the same researchers found NF-kappa B to be present in other cell types in an inactive cytoplasmic form which upon cellular stimulation could be induced to translocate to the nucleus and bind DNA. Subsequently, it has been demonstrated that NF-kappa B performs a critical role as a regulator of the immune system, the response to stress, apoptosis, viral replication and is involved in many diseases, leading to it becoming one of the most intensively studied transcription factors of the last decade. The pivotal role played by NF-kappa B is illustrated not only by the great diversity of genes that it regulates, but also by the large variety of stimuli leading to its activation. This article will address how NF-kappa B, a ubiquitously expressed transcription factor composed of dimers formed from five subunits, differentially regulates the expression of such a diverse array of genes with different functions, in different cell types and at different times. Recent research indicates that this behavioral diversity arises from a delicately balanced network of protein: protein interactions: NF-kappa B activity is determined not only through its regulated nuclear localization but is also dependent on the cellular context in which it is found.

Distinct domains of adenovirus E1A interact with specific cellular factors to differentially modulate human immunodeficiency virus transcription

Transcription of human immunodeficiency virus (HIV) type 1 and other viruses is regulated by the transcription factor NF-kappaB, which interacts with the multifunctional cellular protein p300. p300, originally identified by its ability to bind adenovirus early region 1A (E1A), has also been shown to regulate HIV transcription through its interaction with NF-kappaB. The 13S form of E1A activates HIV gene expression, while the 12S form represses its transcription. In this report, we have investigated whether these divergent effects of E1A are dependent upon common or distinct cellular cofactors, including p300, pRb, and the TATA box-binding protein (TBP). Unlike activation in the absence of E1A, cooperative stimulation of HIV gene expression by 13S E1A and RelA was independent of the ability of E1A to bind p300 but was critically dependent on the E1A CR3 region which associates with TBP. In contrast, inhibition of basal HIV gene expression by the 12S form of E1A was dependent on p300 but not pRb or TBP. Interestingly, mutation of the CR2 region of 12S E1A responsible for pRb binding abolished the repression of HIV transcription stimulated by tumor necrosis factor alpha, suggesting that repression of cytokine-activated transcription involves cofactors different from those used in unstimulated cells. Repression and activation of HIV transcription by different forms of E1A are mediated by distinct sets of cellular cofactors. These findings suggest that E1A has evolved to interact by alternative mechanisms with a transcriptional coactivator and its associated cofactors to differentially modulate cellular and viral gene expression.

Regulation of NF-kappaB by cyclin-dependent kinases associated with the p300 coactivator

The nuclear factor kappaB (NF-kappaB) transcription factor is responsive to specific cytokines and stress and is often activated in association with cell damage and growth arrest in eukaryotes. NF-kappaB is a heterodimeric protein, typically composed of 50- and 65-kilodalton subunits of the Rel family, of which RelA(p65) stimulates transcription of diverse genes. Specific cyclin-dependent kinases (CDKs) were found to regulate transcriptional activation by NF-kappaB through interactions with the coactivator p300. The transcriptional activation domain of RelA(p65) interacted with an amino-terminal region of p300 distinct from a carboxyl-terminal region of p300 required for binding to the cyclin E-Cdk2 complex. The CDK inhibitor p21 or a dominant negative Cdk2, which inhibited p300-associated cyclin E-Cdk2 activity, stimulated kappaB-dependent gene expression, which was also enhanced by expression of p300 in the presence of p21. The interaction of NF-kappaB and CDKs through the p300 and CBP coactivators provides a mechanism for the coordination of transcriptional activation with cell cycle progression.

A cooperative interaction of human T-cell leukemia virus type 1 Tax with the p21 cyclin-dependent kinase inhibitor activates the human immunodeficiency virus type 1 enhancer

Interactions between the Tax transactivator of human T-cell leukemia virus type 1 (HTLV-1) and a cell cycle regulatory protein have been examined. We report cooperative stimulation of human immunodeficiency virus type 1 gene expression by Tax and a regulator of cell cycle progression, the p21 cyclin-dependent kinase inhibitor (CKI). This cooperativity results from the effect of p21 on transcriptional coactivation by Tax-induced NF-kappaB. This effect was abrogated by a mutation in Tax which specifically eliminated NF-kappaB induction, was inhibitable by IkappaB-alpha, and was markedly reduced in human immunodeficiency virus reporter plasmids with mutant kappaB sites. These studies demonstrate that transcriptional activation by Tax is influenced by cell cycle regulatory proteins.

Role of the p21 cyclin-dependent kinase inhibitor in limiting intimal cell proliferation in response to arterial injury

Arterial injury induces a series of proliferative, vasoactive, and inflammatory responses that lead to vascular proliferative diseases, including atherosclerosis and restenosis. Although several factors have been defined which stimulate this process in vivo, the role of specific cellular gene products in limiting this response is not well understood. The p21 cyclin-dependent kinase inhibitor affects cell cycle progression, senescence, and differentiation in transformed cells, but its expression in injured blood vessels has not been investigated. In this study, we report that p21 protein is induced in porcine arteries following balloon catheter injury and suggest that p21 is likely to play a role in limiting arterial cell proliferation in vivo. Vascular endothelial and smooth muscle cell growth was arrested through the ability of p21 to inhibit progression through the G1 phase of the cell cycle. Following injury to porcine arteries, p21 gene product was detected in the neointima and correlated inversely with the location and kinetics of intimal cell proliferation. Direct gene transfer of p21 using an adenoviral vector into balloon injured porcine arteries inhibited the development of intimal hyperplasia. Taken together, these findings suggest that p21, and possibly related cyclin-dependent kinase inhibitors, may normally regulate cellular proliferation following arterial injury, and strategies to increase its expression may prove therapeutically beneficial in vascular diseases.

The p21 cyclin-dependent kinase inhibitor suppresses tumorigenicity in vivo

The p21 gene encodes a cyclin-dependent kinase inhibitor that affects cell-cycle progression, but the potential of this gene product to serve as a tumour suppressor in vivo has not been established. In this report, we show that the growth of malignant cells in vitro and in vivo is inhibited by expression of p21. Expression of p21 resulted in an accumulation of cells in G0/G1, altered morphology, and cell differentiation, but apoptosis was not induced. Introduction of p21 with adenoviral vectors into malignant cells completely suppressed their growth in vivo and also reduced the growth of established pre-existing tumours. Gene transfer of p21 may provide a molecular genetic approach to arresting cancer cell growth by committing malignant cells irreversibly to a pathway of terminal differentiation.

Cytokine induction of nuclear factor kappa B in cycling and growth-arrested cells. Evidence for cell cycle-independent activation

Nuclear factor kappa B (NF-kappa B) is a pleiotropic transcription factor which regulates the expression of a large number of cellular and viral genes. Induction of NF-kappa B has been shown previously to occur during cell cycle transition from G0 to G1, but the relationship of cytokine induction of this transcription factor to cell cycling has not been directly addressed. Here we examine the inductions of NF-kappa B in serum-deprived and cycling cells in response to tumor necrosis factor-alpha (TNF-alpha). In 3T3 fibroblasts deprived of serum, and in the temperature-sensitive G2 phase mutant carcinoma line FT210, we find that NF-kappa B DNA binding activity is rapidly induced upon addition of TNF-alpha. In addition, NF-kappa B induction in cycling cells occurs without a significant change in cell cycle distribution. These data reveal that NF-kappa B is rapidly induced by TNF-alpha in both proliferating and arrested cells and suggest that distinct activation pathways can lead to cell cycle-dependent or -independent induction of NF-kappa B.

A proliferative p53-responsive element mediates tumor necrosis factor alpha induction of the human immunodeficiency virus type 1 long terminal repeat

Transforming mutants of the p53 tumor suppressor gene can positively regulate transcription from several promoters that do not contain known p53 binding sites. Here, we report the identification of a novel p53 binding site in the human immunodeficiency virus long terminal repeat that specifically mediates mutant p53 transactivation. This DNA element was bound by endogenous Jurkat p53 when these cells were stimulated by tumor necrosis factor. Mutation of this sequence inhibited p53 transactivation and tumor necrosis factor inducibility of the human immunodeficiency virus type 1 long terminal repeat. In addition, this DNA element was found to be sufficient to confer mutant p53 responsiveness on a heterologous minimal promoter. It has been hypothesized that transforming mutants of p53 represent a proliferative conformational stage that can be adopted by the native protein under stimulation by growth factors. The data presented suggest that proliferative and antiproliferative p53 conformations recognize different DNA binding sites in order to mediate distinct biological functions. Thus, transforming mutants of p53 that fold into the proliferative conformation would favor proliferative over antiproliferative functions.

Transcription factor AP-2 regulates human immunodeficiency virus type 1 gene expression

Human immunodeficiency virus type 1 (HIV-1) gene expression is regulated by an enhancer region composed of multiple potential cis-acting regulatory sites. Here, we describe binding sites for the transcription factor AP-2 in the HIV-1 long terminal repeat which modulate HIV enhancer function. One site is embedded within the two previously described kappa B elements, and a second site is detected further downstream. DNase I footprinting and electrophoretic mobility shift assay experiments demonstrated that AP-2 binds to the site between the kappa B elements. Interestingly, AP-2 and NF-kappa B bind to this region in a mutually exclusive manner. Mutations which disrupt this AP-2-binding site lower basal levels of transcription but do not affect NF-kappa B-mediated induction by tumor necrosis factor alpha in Jurkat T leukemia cells.

An interaction between the DNA-binding domains of RelA(p65) and Sp1 mediates human immunodeficiency virus gene activation

Induction of human immunodeficiency virus type 1 (HIV-1) gene expression in stimulated T cells has been attributed to the activation of the transcription factor NF-kappa B. The twice-repeated kappa B sites within the HIV-1 long terminal repeat are in close proximity to three binding sites for Sp1. We have previously shown that a cooperative interaction of NF-kappa B with Sp1 is required for the efficient stimulation of HIV-1 transcription. In this report, we define the domains of each protein responsible for this effect. Although the transactivation domains seemed likely to mediate this interaction, we find, surprisingly, that this interaction occurs through the putative DNA-binding domains of both proteins. Sp1 specifically interacted with the amino-terminal region of RelA(p65). Similarly, RelA bound directly to the zinc finger region of Sp1. This interaction was specific and resulted in cooperative DNA binding to the kappa B and Sp1 sites in the HIV-1 long terminal repeat. Furthermore, the amino-terminal region of RelA did not associate with several other transcription factors, including MyoD, E12, or Kox15, another zinc finger protein. These findings suggest that the juxtaposition of DNA-binding sites promotes a specific protein interaction between the DNA-binding regions of these transcription factors. This interaction is required for HIV transcriptional activation and may provide a mechanism to allow for selective activation of kappa B-regulated genes.

Inhibition of phorbol ester-induced cellular adhesion by competitive binding of NF-kappa B in vivo

Adhesive interactions between cells are essential for the organization and function of differentiated tissues and organs and are mediated by inducible cell surface glycoproteins. In normal tissues, cell adhesion molecules contribute to immune regulation, inflammation, and embryogenesis. Additionally, they play an important role in a variety of pathogenic processes. Cell adhesion molecule expression can be induced by stimuli known to activate NF-kappa B, a ubiquitous transcription factor found in a variety of cell types. To investigate the role of NF-kappa B in cell adhesion molecule expression, we treated HL-60 cells with a double-stranded oligonucleotide which specifically inhibits NF-kappa B-mediated transcription. This treatment resulted in the inhibition of phorbol 12-myristate 13-acetate (PMA)-induced cellular adhesion, morphological changes, and the expression of leukocyte integrin CD11b. In a similar fashion, expression of intercellular adhesion molecule 1 on human endothelial cells induced by PMA was specifically inhibited by the NF-kappa B antagonist. We suggest that NF-kappa B activation is a necessary event for the PMA-induced differentiation of HL-60 cells and the expression of certain activation is a necessary event for the PMA-induced differentiation of HL-60 cells and the expression of certain adhesion molecules. Furthermore, the inhibition of transcription factor functions by this generally applicable mechanism can be used to define their role in cellular differentiation and function.

A cooperative interaction between NF-kappa B and Sp1 is required for HIV-1 enhancer activation

The human immunodeficiency virus (HIV-1) long terminal repeat (LTR) contains two binding sites for NF-kappa B in close proximity to three binding sites for the constitutive transcription factor, Sp1. Previously, stimulation of the HIV enhancer in response to mitogens has been attributed to the binding of NF-kappa B to the viral enhancer. In this report, we show that the binding of NF-kappa B is not by itself sufficient to induce HIV gene expression. Instead, a protein-protein interaction must occur between NF-kappa B and Sp1 bound to an adjacent site. Cooperativity both in DNA binding and in transcriptional activation of NF-kappa B and Sp1 was confirmed by electrophoretic mobility shift gel analysis, DNase footprinting, chemical cross-linking and transfection studies in vivo. With a heterologous promoter, we find that the interaction of NF-kappa B with Sp1 is dependent on orientation and position, and is not observed with other elements, including GATA, CCAAT or octamer. An increase in the spacing between the kappa B and Sp1 elements virtually abolishes this functional interaction, which is not restored when these sites are brought back into the same helical position. Several other promoters regulated by NF-kappa B also contain kappa B in proximity to Sp1 binding sites. These findings suggest that an interaction between NF-kappa B and Sp1 is required for inducible HIV-1 gene expression and may serve as a regulatory mechanism to activate specific viral and cellular genes.

Human T cell leukemia virus type I Tax and phorbol 12-myristate 13-acetate induce expression of the A20 zinc finger protein by distinct mechanisms involving nuclear factor kappa B

A20 was originally identified as a primary tumor necrosis factor alpha (TNF)-responsive gene, which encodes a 790-amino acid zinc finger protein. A20 is expressed in a wide variety of cell lines, including fibroblasts, in which A20 expression protects cells from TNF cytotoxicity. An analysis of A20 expression in lymphocytic and monocytic cells lines revealed that A20 protein expression correlates with lymphocyte activation and monocyte differentiation. A20 expression was also induced in Jurkat T cells expressing the human T cell leukemia virus type I Tax protein. Transient transfection studies demonstrated that stimulation of A20 transcription by TNF, phorbol 12-myristate 13-acetate (PMA), and Tax was mediated by two kappa B elements within the A20 promoter. Accordingly, DNA electrophoretic mobility shift assays confirmed inducible binding of nuclear factor kappa B (NF-kappa B) to a promoter fragment containing both A20 kappa B elements. Analysis of individual A20 kappa B sites revealed that both kappa B sites were required for TNF or PMA activation of the A20 promoter; however, Tax activation required only one kappa B site. Overexpression of NF-kappa B subunits activated the wild type A20 promoter, but did not activate mutated forms containing single kappa B sites. Thus, Tax activation of A20 transcription occurs through a mechanism distinct from PMA and TNF, possibly due to differential activation of NF-kappa B complexes or transcriptional cofactors.

Dimerization of NF-KB2 with RelA(p65) regulates DNA binding, transcriptional activation, and inhibition by an I kappa B-alpha (MAD-3)

Inducible expression of human immunodeficiency virus (HIV) is regulated by a cellular transcription factor, nuclear factor kappa B (NF-kappa B). NF-kappa B is composed of distinct subunits; five independent genes, NFKB1(p105), NFKB2(p100), RelA(p65), c-rel and relB, that encode related proteins that bind to kappa B DNA elements have been isolated. We have previously found that NFKB2(p49/p52) acts in concert with RelA(p65) to stimulate the HIV enhancer in Jurkat T-leukemia cells. Here we examine the biochemical basis for the transcriptional regulation of HIV by NFKB2. Using Scatchard analysis, we have determined the dissociation constants of homodimeric p49 and heterodimeric p49/p65 for binding to the HIV kappa B site. p49 has a approximately 18-fold-lower affinity for the HIV kappa B site (KD = 69.1 pM) than does the approximately 50-kDa protein NFKB1(p50) derived from p105 (KD = 3.9 pM). In contrast, the affinity of heterodimeric NFKB2(p49)/RelA(p65) for this site is approximately 6-fold higher (KD = 11.8 pM) than that of p49 alone. Consistent with these findings, in vitro transcription was stimulated 18-fold by the addition of preformed, heterodimeric NFKB2(p49)/RelA(p65) protein. Transcriptional activation of the HIV enhancer was also subject to regulation by recently cloned I kappa B-alpha(MAD-3). Recombinant I kappa B-alpha(MAD-3) inhibited the DNA binding activity of p65, p49/p65, and p50/p65 but stimulated the binding of NFKB2(p49) or NFKB1(p50). Functional activation of an HIV reporter plasmid by p49/p65 in transiently transfected Jurkat T-leukemia cells was also inhibited by coexpression of MAD-3. These data suggest that binding of the NFKB2 subunit to the HIV enhancer is facilitated by RelA(p65) and that this NFKB2(p49)/p65 heterodimeric complex mediates transcriptional activation which is subject to regulation by MAD-3.

Specific NF-kappa B subunits act in concert with Tat to stimulate human immunodeficiency virus type 1 transcription

NF-kappa B is a protein complex which functions in concert with the tat-I gene product to stimulate human immunodeficiency virus (HIV) transcription. To determine whether specific members of the NF-kappa B family contribute to this effect, we have examined the abilities of different NF-kappa B subunits to act with Tat-I to stimulate transcription of HIV in Jurkat T-leukemia cells. We have found that the p49(100) DNA binding subunit, together with p65, can act in concert with Tat-I to stimulate the expression of HIV-CAT plasmid. Little effect was observed with 50-kDa forms of p105 NF-kappa B or rel, in combination with p65 or full-length c-rel, which do not stimulate the HIV enhancer in these cells. These findings suggest that the combination of p49(100) and p65 NF-kappa B can act in concert with the tat-I gene product to stimulate the synthesis of HIV RNA.

Related subunits of NF-kappa B map to two distinct loci associated with translocations in leukemia, NFKB1 and NFKB2

The chromosomal locations of the human genes NFKB1 and NFKB2, which encode two alternative DNA binding subunits of the NF-kappa B complex, p105 and p49/p100, respectively, have been determined. p105 was assigned to 4q21.1-q24 and p49/p100 to chromosome 10 by Southern blot analysis of panels of human/Chinese hamster cell hybrids. The locations were confirmed by fluorescence in situ hybridization and mapped with greater resolution to 4q23 and 10q24, respectively. These results demonstrate that these members of the NF-kappa B family are unlinked. Interestingly, p49/p100 as well as p105 maps to regions associated with certain types of acute lymphoblastic leukemia.

Distinct combinations of NF-kappa B subunits determine the specificity of transcriptional activation

The nuclear factor that binds to the kappa light-chain enhancer of B cells (NF-kappa B) is a transcription factor that regulates the expression of a variety of cellular and viral genes. NF-kappa B is composed of distinct subunits, and at least four independent genes (p105, p100, p65, and c-rel) have been isolated that encode related proteins that bind kappa B sites. Because it is possible that specific interactions of different subunits can allow selective gene activation, we have characterized the specificity of transcriptional activation by various combinations of these subunits. When tested alone, an approximately 49-kDa form (p49) of the p100 protein bound weakly to kappa B, but p49 associated with p65 to bind efficiently to this site. Furthermore, p49 acted in combination with either p65 or a Rel/VP16 fusion protein to activate kappa B-dependent transcription in Jurkat T leukemia cells. The p49/p65 or p49/Rel combination stimulated transcription mediated by the canonical kappa B site but did not stimulate reporter genes containing interleukin 2 receptor alpha or major histocompatibility complex kappa B elements, despite its ability to bind to these sites. Transactivation mediated by the p49/p100 and p65 NF-kappa B proteins is therefore sensitive to minor changes in the sequence of the kappa B site. Specificity determined by the association of NF-kappa B subunits provides a mechanism to selectively regulate variant kappa B sites associated with different cellular and viral genes.

Cloning of an NF-kappa B subunit which stimulates HIV transcription in synergy with p65

The transcription factor NF-kappa B is a protein complex which comprises a DNA-binding subunit and an associated transactivation protein (of relative molecular masses 50,000 (50K) and 65K, respectively). Both the 50K and 65K subunits have similarity with the rel oncogene and the Drosophila maternal effect gene dorsal. The 50K DNA-binding subunit was previously thought to be a unique protein, derived from the 105K gene product (p105). We now report the isolation of a complementary DNA that encodes an alternative DNA-binding subunit of NF-kappa B. It is more similar to p105 NF-kappa B than other family members and defines a new subset of rel-related genes. It is synthesized as approximately 100K protein (p100) that is expressed in different cell types, contains cell cycle motifs and, like p105, must be processed to generate a 50K form. A 49K product (p49) can be generated independently from an alternatively spliced transcript; it has specific kappa B DNA-binding activity and can form heterodimers with other rel proteins. In contrast to the approximately 50K protein derived from p105, p49 acts in synergy with p65 to stimulate the human immunodeficiency virus (HIV) enhancer in transiently transfected Jurkat cells. p49/p100 NF-kappa B could therefore be important in the regulation of HIV and other kappa B-containing genes.

Detection in non-erythroid cells of a factor with the binding characteristics of the erythroid cell transcription factor EF1

The erythroid transcription factor erythroid factor-1 (EF1) plays a critical role in the transcription of erythroid-specific genes. Here we report the presence of a factor with the mobility and sequence-specific DNA-binding characteristics of EF1 at low abundance in a wide variety of non-erythroid cell types. This is the first report of an EF1-like activity in non-erythroid cells and indicates that this factor may play a role in the regulation of genes expressed in such cells.

The purification of an erythroid protein which binds to enhancer and promoter elements of haemoglobin genes

An erythroid nuclear protein (EF1), originally detected as a protein binding within the nuclease hypersensitive site upstream of the chicken beta H-globin gene, has been purified. This protein of 37,000-39,000 molecular weight binds to three sites within the hypersensitive region: one between the CCAAT and TATA boxes, the second (further upstream) next to a NF1 binding site, and the third adjacent to a regulatory element found in a number of beta-globin genes. The EF1 protein also binds to an erythroid-specific promoter element of the mouse alpha-globin gene and to two sites within the chicken beta A-globin enhancer. These six EF1-binding sites are related by the consensus sequence A/TGATAA/GG/C. A minor protein of molecular weight 72,000 which co-purifies with EF1 also binds to the same sequences.