Identification of Daxx interacting with p73, one of the p53 family, and its regulation of p53 activity by competitive interaction with PML

DAXX-ATRX regulation of p53 chromatin binding and DNA damage response

DAXX and ATRX are tumor suppressor proteins that form a histone H3.3 chaperone complex and are frequently mutated in cancers with the alternative lengthening of telomeres (ALT). Here, we show that DAXX and ATRX knock-out (KO) U87-T cells that have acquired ALT-like features have defects in p53 chromatin binding and DNA damage response. RNA-seq analysis revealed that p53 pathway is among the most perturbed. ChIP-seq and ATAC-seq revealed a genome-wide reduction in p53 DNA-binding and corresponding loss of chromatin accessibility at many p53 response elements across the genome. Both DAXX and ATRX null cells showed a depletion of histone H3.3 and accumulation of γH2AX at many p53 sites, including subtelomeres. These findings indicate that loss of DAXX or ATRX can compromise p53 chromatin binding and p53 DNA damage response in ALT-like cells, providing a link between histone composition, chromatin accessibility and tumor suppressor function of p53.

Reciprocal regulation of Daxx and PIK3CA promotes colorectal cancer cell growth

Upregulation of death-domain-associated protein (Daxx) is strongly associated with diverse cancer types. Among these, the clinicopathological significance and molecular mechanisms of Daxx overexpression in colorectal cancer (CRC) remain unknown. Here, we showed that Daxx expression was increased in both clinical CRC samples and CRC cell lines. Daxx knockdown significantly reduced proliferation activity in CRC cells and tumor growth in a xenograft model. Further studies revealed that Daxx expression could be attenuated by either treatment with the PIK3CA inhibitor PIK-75 or PIK3CA depletion in CRC cells. Conversely, expression of PIK3CA constitutively active mutants could increase Daxx expression. These data suggest that PIK3CA positively regulates Daxx expression. Consistently, the expression levels of PIK3CA and Daxx were positively correlated in sporadic CRC samples. Interestingly, Daxx knockdown or overexpression yielded decreased or increased levels of PIK3CA, respectively, in CRC cells. We further demonstrated that Daxx activates the promoter activity and expression of PIK3CA. Altogether, our results identify a mechanistic pathway of Daxx overexpression in CRC and suggest a reciprocal regulation between Daxx and PIK3CA for CRC cell growth.

The Human CMV IE1 Protein: An Offender of PML Nuclear Bodies

PML nuclear bodies (PML-NBs) are SUMOylation-dependent, highly complex protein assemblies that accumulate in the interchromosomal territories of the cell nucleus. Research of the last two decades revealed that many viruses have evolved effector proteins that modify PML-NBs. This correlates with antagonization of individual PML-NB components which act as host cell restriction factors. The multifunctional immediate-early protein IE1 of human cytomegalovirus directly interacts with the PML protein resulting in a disruption of the dot-like structure of PML-NBs. This review summarizes recent advances on the functional consequences of PML-NB modification by IE1. In particular, we describe that PML exerts a novel co-regulatory role during the interferon response which is abrogated by IE1. Via binding to PML, IE1 is able to compromise both intrinsic antiviral defense mechanisms and classical innate immune responses. These interactions of IE1 with innate host defenses are crucial for the onset of lytic replication and, consequently, may represent promising targets for antiviral strategies.

Identification of the C-terminal domain of Daxx acts as a potential regulator of intracellular cholesterol synthesis in HepG2 cells

Daxx is a highly conserved nuclear transcriptional factor, which has been implicated in many nuclear processes including transcription and cell cycle regulation. Our previous study demonstrated Daxx also plays a role in regulation of intracellular cholesterol content. Daxx contains several domains that are essential for interaction with a growing number of proteins. To delineate the underlying mechanism of hypocholesterolemic activity of Daxx, we constructed a set of plasmids which can be used to overexpress different fragments of Daxx and transfected to HepG2 cells. We found that the C- terminal region Daxx626-740 clearly reduced intracellular cholesterol levels and inhibited the expression of SREBPs and SCAP. In GST pull-down experiments and Double immunofluorescence assays, Daxx626-740 was demonstrated to bind directly to androgen receptor (AR). Our findings suggest that the interaction of Daxx626-740 and AR abolishes the AR-mediated activation of SCAP/SREBPs pathway, which suppresses the de novo cholesterol synthesis. Thus, C-terminal domain of Daxx acts as a potential regulator of intracellular cholesterol content in HepG2 cells.

DNA damage-induced regulatory interplay between DAXX, p53, ATM kinase and Wip1 phosphatase

Death domain-associated protein 6 (DAXX) is a histone chaperone, putative regulator of apoptosis and transcription, and candidate modulator of p53-mediated gene expression following DNA damage. DAXX becomes phosphorylated upon DNA damage, however regulation of this modification, and its relationship to p53 remain unclear. Here we show that in human cells exposed to ionizing radiation or genotoxic drugs etoposide and neocarzinostatin, DAXX became rapidly phosphorylated in an ATM kinase-dependent manner. Our deletion and site-directed mutagenesis experiments identified Serine 564 (S564) as the dominant ATM-targeted site of DAXX, and immunofluorescence experiments revealed localization of S564-phosphorylated DAXX to PML nuclear bodies. Furthermore, using a panel of human cell types, we identified the p53-regulated Wip1 protein phosphatase as a key negative regulator of DAXX phosphorylation at S564, both in vitro and in cells. Consistent with the emerging oncogenic role of Wip1, its DAXX-dephosphorylating impact was most apparent in cancer cell lines harboring gain-of-function mutant and/or overexpressed Wip1. Unexpectedly, while Wip1 depletion increased DAXX phosphorylation both before and after DNA damage and increased p53 stability and transcriptional activity, knock-down of DAXX impacted neither p53 stabilization nor p53-mediated expression of Gadd45a, Noxa, Mdm2, p21, Puma, Sesn2, Tigar or Wip1. Consistently, analyses of cells with genetic, TALEN-mediated DAXX deletion corroborated the notion that neither phosphorylated nor non-phosphorylated DAXX is required for p53-mediated gene expression upon DNA damage. Overall, we identify ATM kinase and Wip1 phosphatase as opposing regulators of DAXX-S564 phosphorylation, and propose that the role of DAXX phosphorylation and DAXX itself are independent of p53-mediated gene expression.

PML IV/ARF interaction enhances p53 SUMO-1 conjugation, activation, and senescence

Promyelocytic leukemia protein (PML) nuclear bodies (NBs) recruit multiple partners, including p53 and many of its regulators. NBs are believed to facilitate several posttranslational modifications and are key regulators of senescence. PML, the organizer of NBs, is expressed as a number of splice variants that all efficiently recruit p53 partners. However, overexpression of only one of them, PML IV, triggers p53-driven senescence. Here, we show that PML IV specifically binds ARF, a key p53 regulator. Similar to ARF, PML IV enhances global SUMO-1 conjugation, particularly that of p53, resulting in p53 stabilization and activation. ARF interacts with and stabilizes the NB-associated UBC9 SUMO-conjugating enzyme, possibly explaining PML IV-enhanced SUMOylation. These results unexpectedly link two key tumor suppressors, highlighting their convergence for global control of SUMO conjugation, p53 activation, and senescence induction.

How the TP53 family proteins TP63 and TP73 contribute to tumorigenesis: regulators and effectors

In mammals, the p53 family comprises two additional members, p63 and p73 (hereafter referred to as TP53, TP63, and TP73, respectively). The usage of two alternative promoters produces protein variants either with (transactivating [TA] isoforms) or without (ΔN isoforms) the N-terminal transactivation domain (TAD). In general, the TA proteins exert TP53-like tumor-suppressive activities through their ability to activate a common set of target genes. The ΔN proteins can act as dominant-negative inhibitors of the transcriptionally active family members. Additionally, they possess intrinsic-specific biological activities due to the presence of alternative TADs, and as a result of engaging a different set of regulators. This review summarizes the current understanding of upstream regulators and downstream effectors of the TP53 family proteins, with particular emphasis on those that are relevant for their role in tumorigenesis. Furthermore, we highlight the existence of networks and cross-talks among the TP53 family members, their modulators, as well as the transcriptional targets.

Dynamic regulation of steroid hormone receptor transcriptional activity by reversible SUMOylation

Transcription complexes containing steroid hormone receptors (SRs) have been well characterized at selected canonical target genes. More recently, the advent of whole genome technologies has allowed for complete SR transcriptome analyses in diverse cell types and in response to a variety of cellular stimuli. These types of studies have revealed little overlap between the tissue or cell type-specific transcriptomes of a given SR, suggesting that all SRs are highly context-dependent transcription factors. However, the mechanisms controlling SR promoter selectivity have not been fully elucidated. Many factors may influence SR promoter selectivity, including chromatin structure, cofactor availability, and posttranslational modifications to SRs and/or their numerous coregulators; this review focuses on the impact that covalent attachment of small ubiquitin-like modifier (SUMO) moieties to SRs (i.e., SUMOylation) have on the transcriptional regulation of SR target genes.

A novel dual signaling axis for NSP 5a3a induced apoptosis in head and neck carcinoma

NSP 5a3a is a novel structural protein found to be over-expressed in certain cancer cell lines in-vitro such as Hela, Saos-2, and MCF-7 while barely detectable levels in normal body tissues except for Testis. This particular isoform has been known to interact with cyto- nuclear proteins B23, known to be involved in multi-faceted cellular processes such as cell division, apoptosis, ribosome biogenesis, and rRNA processing, as well as with hnRNP-L, known to be involved with RNA metabolism and rRNA processing. A previous preliminary investigation of NSP 5a3a as a potential target in Head and Neck Carcinoma revealed a novel p73 dependent mechanism through which NSP 5a3a induced apoptosis in Head and Neck cell lines when over-expressed in-vitro. Our present investigation further elucidated a novel dual axis signaling point by which NSP 5a3a induces apoptosis in Head and Neck cell line HN30 through p73-DAXX and TRAF2-TRADD. Interestingly, this novel mechanism appears independent of canonical caspases involved in the intrinsic mitochondrial pathway as well as those in the death receptor pathway thru TRAF2 and TRADD.

NSP 5a3a: a potential novel cancer target in head and neck carcinoma

NSP 5a3a along with three other distinct though similar splice variants were initially identified corresponding to locus HCMOGT-1 on chromosome 17p11.2 [1]. Secondary structure analysis of the novel structural protein (NSP) isoforms revealed similarity to Spectrin like proteins containing coiled coil domains [1]. The NSP 5a3a isoform had been found to be highly expressed in-vitro in particular cancer cell lines while very low to un-detectable levels in normal body tissues [1]. Subsequent investigation of this isoform revealed its novel interaction with B23 [2], a multifunctional nucleolar protein involved in ribosome biogenesis, rRNA transcription, mitosis, cell growth control, and apoptosis [3]. Subsequent investigation, elucidated NSP 5a3a's potential involvement in cellular processes such as ribosome biogenesis and rRNA processing by validating NSP 5a3a's novel interaction with B23 and ribonuclear protein hnRNP-L possibly implicating NSP 5a3a's involvement in cellular activities such as RNA metabolism and processing [4]. In this preliminary investigation, we wanted to observe the effect that over-expressing NSP 5a3a may have on cell cycle and its potential application in cancer treatment in aggressive cancers such as head and neck carcinomas. Over-expressed NSP 5a3a in HN30 cells induced a significant degree of apoptosis, an average of a 10.85 fold increase compared to controls 3 days post-transfection. This effect was more significant then the apoptosis observed between Fadu cells over-expressing NSP 5a3a and its controls. Though, the apoptosis induced in the WI38 control cell line showed an average of a 13.2 fold increase between treated and controls comparable to the HN30 cell line 3 days post-transfection. Molecular analysis indentified a novel p73 dependent mechanism independent of p53 and caspase 3 activity through which NSP 5a3a is inducing apoptosis. We propose NSP 5a3a as a potential therapeutic target for site directed cancer treatment in perhaps certain head and neck carcinomas by induction of apoptosis.

Daxx-beta and Daxx-gamma, two novel splice variants of the transcriptional co-repressor Daxx

Daxx is involved in transcriptional control and apoptosis. It comprises several domains, including a regulatory C terminus that is responsible for the interaction with numerous proteins such as p53, promyelocytic leukemia protein (PML), and Hsp27. Here, we describe the identification and characterization of two novel variants of Daxx termed Daxx-β and Daxx-γ, which are generated by alternative splicing. Alternative splicing results in a truncated regulatory C terminus in both proteins. As a consequence, Daxx-β and Daxx-γ show a markedly decreased affinity to PML, which in turn is associated with a different subnuclear localization of these proteins compared with Daxx. Although Daxx is localized mainly in PML-oncogenic domains (PODs) Daxx-β and Daxx-γ display a distinct distribution pattern. Furthermore, Daxx-β and Daxx-γ show a decreased affinity to p53 also due to the truncated C terminus. We provide evidence that the p53 recruitment into PODs is Daxx isoform-dependent. The decreased affinity of Daxx-β/-γ to p53 and PML results in a diffuse localization of p53 throughout the nucleus. In contrast to Daxx, Daxx-β and Daxx-γ are unable to repress p53-mediated transcription. Therefore, alternative splicing of Daxx might indicate an additional level in the cellular apoptosis network.

NSP 5a3a: a potential novel cancer target in head and neck carcinoma

NSP 5a3a along with three other distinct though similar splice variants were initially identified corresponding to locus HCMOGT-1 on chromosome 17p11.2 [1]. Secondary structure analysis of the novel structural protein (NSP) isoforms revealed similarity to Spectrin like proteins containing coiled coil domains [1]. The NSP 5a3a isoform had been found to be highly expressed in-vitro in particular cancer cell lines while very low to un-detectable levels in normal body tissues [1]. Subsequent investigation of this isoform revealed its novel interaction with B23 [2], a multifunctional nucleolar protein involved in ribosome biogenesis, rRNA transcription, mitosis, cell growth control, and apoptosis [3]. Subsequent investigation, elucidated NSP 5a3a's potential involvement in cellular processes such as ribosome biogenesis and rRNA processing by validating NSP 5a3a's novel interaction with B23 and ribonuclear protein hnRNP-L possibly implicating NSP 5a3a's involvement in cellular activities such as RNA metabolism and processing [4]. In this preliminary investigation, we wanted to observe the effect that over-expressing NSP 5a3a may have on cell cycle and its potential application in cancer treatment in aggressive cancers such as head and neck carcinomas. Over-expressed NSP 5a3a in HN30 cells induced a significant degree of apoptosis, an average of a 10.85 fold increase compared to controls 3 days post-transfection. This effect was more significant then the apoptosis observed between Fadu cells over-expressing NSP 5a3a and its controls. Though, the apoptosis induced in the WI38 control cell line showed an average of a 13.2 fold increase between treated and controls comparable to the HN30 cell line 3 days post-transfection. Molecular analysis indentified a novel p73 dependent mechanism independent of p53 and caspase 3 activity through which NSP 5a3a is inducing apoptosis. We propose NSP 5a3a as a potential therapeutic target for site directed cancer treatment in perhaps certain head and neck carcinomas by induction of apoptosis.

A novel cytoplasmic adaptor for retinoic acid receptor (RAR) and thyroid receptor functions as a Derepressor of RAR in the absence of retinoic acid

In most mammalian cells, the retinoic acid receptor (RAR) is nuclear rather than cytoplasmic, regardless of its cognate ligand, retinoic acid (RA). In testis Sertoli cells, however, RAR is retained in the cytoplasm and moves to the nucleus only when RA is supplied. This led us to identify a protein that regulates the translocation of RAR. From yeast two-hybrid screening, we identified a novel RAR-interacting protein called CART1 (cytoplasmic adaptor for RAR and TR). Systematic interaction assays using deletion mutants showed that the C-terminal CoRNR box of CART1 was responsible for the interaction with the NCoR binding region of RAR and TR. Such interaction was impaired in the presence of ligand RA, as further determined by GST pulldown assays in vitro and immunoprecipitation assays in vivo. Fluorescence microscopy showed that unliganded RAR was captured by CART1 in the cytoplasm, whereas liganded RAR was liberated and moved to the nucleus. Overexpression of CART1 blocked the transcriptional repressing activity of unliganded apoRAR, mediated by corepressor NCoR in the nucleus. CART1 siRNA treatment in a mouse Sertoli cell line, TM4, allowed RAR to move to the nucleus and blocked the derepressing function of CART1, suggesting that CART1 might be a cytoplasmic, testis-specific derepressor of RAR.

p53-family proteins and their regulators: hubs and spokes in tumor suppression

The tumor suppressor p53 is a central hub in a molecular network controlling cell proliferation and death in response to potentially oncogenic conditions, and a wide array of covalent modifications and protein interactions modulate the nuclear and cytoplasmic activities of p53. The p53 relatives, p73 and p63, are entangled in the same regulatory network, being subject at least in part to the same modifications and interactions that convey signals on p53, and actively contributing to the resulting cellular output. The emerging picture is that of an interconnected pathway, in which all p53-family proteins are involved in the response to oncogenic stress and physiological inputs. Therefore, common and specific interactors of p53-family proteins can have a wide effect on function and dysfunction of this pathway. Many years of research have uncovered an impressive number of p53-interacting proteins, but much less is known about protein interactions of p63 and p73. Yet, many interactors may be shared by multiple p53-family proteins, with similar or different effects. In this study we review shared interactors of p53-family proteins with the aim to encourage research into this field; this knowledge promises to unveil regulatory elements that could be targeted by a new generation of molecules, and allow more efficient use of currently available drugs for cancer treatment.

Interplay between Herpesvirus Infection and Host Defense by PML Nuclear Bodies

In recent studies we and others have identified the cellular proteins PML, hDaxx, and Sp100, which form a subnuclear structure known as nuclear domain 10 (ND10) or PML nuclear bodies (PML-NBs), as host restriction factors that counteract herpesviral infections by inhibiting viral replication at different stages. The antiviral function of ND10, however, is antagonized by viral regulatory proteins (e.g., ICP0 of herpes simplex virus; IE1 of human cytomegalovirus) which induce either a modification or disruption of ND10. This review will summarize the current knowledge on how viral replication is inhibited by ND10 proteins. Furthermore, herpesviral strategies to defeat this host defense mechanism are discussed.

Nuclear localization of dengue virus capsid protein is required for DAXX interaction and apoptosis

Dengue virus capsid protein (DENVC) localizes to both the cytoplasm and nucleus of dengue virus-infected cells. DENV C contains three nuclear localization signals (NLS), (6)KKAR(9), (73)KKSK(76), and the bipartite signal (85)RKeigrmlnilnRRRR(100). Stable HepG2 cells constitutively expressing DENV C, DENV C (Delta 85-100) and DENV C (Delta 73-100) were constructed to clarify whether nuclear translocation of DENV C affected apoptosis in liver cell line. While the wild-type DENV C could translocate into the nuclei of HepG2 cells, the mutant DENV Cs were restricted to the cytoplasm. The loss of nuclear localization of both mutant DENV Cs resulted in the disruption of their interactions with the apoptotic protein Daxx. Interestingly, upon treatment with anti-Fas antibody, the HepG2 cells expressing the wild-type DENV C showed significantly more apoptosis compared with the HepG2 cells expressing either mutant DENV C. To identify the amino acids required for DAXX interaction and apoptosis, substitution mutations either (K73A/K74A) or (R85A/K86A) were introduced into the C-terminal region of DENV C, and tested whether these mutations affected its interaction with Daxx and apoptosis. The results demonstrate that (73)KK and (85)RK of DENV C are important for its nuclear localization, interaction with DAXX and induction of apoptosis. This work is the first to demonstrate that nuclear localization of DENV C is required for DAXX interaction and apoptosis.

Daxx inhibits muscle differentiation by repressing E2A-mediated transcription

The basic helix-loop-helix (HLH) E2A transcription factors bind to DNA as homodimers or as heterodimers formed with other basic HLH factors, activate gene expression, and promote differentiation of muscle, lymphoid, neuronal, and other cell types. These E2A functions can be inhibited by the Id proteins, HLH factors that sequester E2A in non-DNA binding dimers. Here we describe the direct interaction of E2A with Daxx, a broadly expressed non-HLH protein previously associated with apoptosis and transcriptional repression. Daxx inhibits E2A function, but not via an Id-like mechanism; rather, it recruits histone deacetylase activity to E2A-dependent promoters. Increased Daxx expression during muscle differentiation inhibits E2A-dependent expression of key myogenic genes and reduces myotube formation, while decreased Daxx expression promotes myotube formation. These results identify a new mechanism for limiting E2A activity and establish a link between Daxx-mediated gene regulation and control of cellular differentiation.

Daxx is a transcriptional repressor of CCAAT/enhancer-binding protein beta

CCAAT/enhancer-binding Protein beta (C/EBPbeta) is a member of the bZIP transcription factor family that is expressed in various tissues, including cells of the hematopoietic system. C/EBPbeta is involved in tissue-specific gene expression and thereby takes part in fundamental cellular processes such as proliferation and differentiation. Here, we show that the activity of C/EBPbeta is negatively regulated by the transcriptional co-repressor Daxx. C/EBPbeta was found to directly interact with Daxx after overexpression as well as on the endogenous level. Glutathione S-transferase pulldown assays showed that Daxx binds via amino acids 190-400 to the C-terminal part of C/EBPbeta. Co-expression of C/EBPbeta changed the sub-nuclear Daxx distribution pattern from predominantly POD-localized to nucleoplasmic. Daxx suppressed basal and p300-enhanced transcriptional activity of C/EBPbeta. Furthermore, Daxx decreased the C/EBPbeta-dependent phosphorylation of p300, which in turn was associated with a diminished level of p300-mediated C/EBPbeta acetylation. Co-expression of promyelocytic leukemia protein abrogated the repressive effect of Daxx on C/EBPbeta as well as the direct interaction of Daxx and C/EBPbeta, presumably by re-recruiting Daxx to PML-oncogenic domains. In acute promyelocytic leukemia (APL) cells, C/EBPbeta activity is known to be required for all-trans-retinoic acid-induced cell differentiation and disease remission. We show that all-trans-retinoic acid as well as arsenic trioxide treatment leads to a reduced C/EBPbeta fraction associated with Daxx suggesting a relief of Daxx-dependent C/EBPbeta repression as an important molecular event leading to APL cell differentiation. Overall, our data identify Daxx as a new negative regulator of C/EBPbeta and provide first clues for a link between abrogation of Daxx-C/EBPbeta complex formation and APL remission.

Daxx inhibits stress-induced apoptosis in cardiac myocytes

The role of the death-associated protein Daxx in modulation of apoptosis induced in cardiac myocytes by oxidative stress was studied. Exposure of cultured cardiomyocyte-like cells to oxidative stress or simulated hypoxia increased the level of accumulated ROS and apoptosis. Under conditions of sub-apoptotic stimulation of cardiac myocytes, there was no increase in the level of the Daxx protein, but it translocated from the nucleus to the cytoplasm. Daxx overexpression protected the cells from apoptosis, while they were sensitised to cell death following its down-regulation by siRNA. Moreover, lowering the level of the Daxx protein sensitised cardiac myocytes to spontaneous apoptosis, suggesting that the protein may also have a pro-survival role under physiological conditions. Finally, it was shown that DJ-1, a protein suggested previously to sequester Daxx in the nucleus under conditions of oxidative stress (thereby preventing its cytosolic translocation), was localised solely in the cytoplasm of cardiac myocytes. This indicates that the protein does not modulate the apoptosis regulatory activity of Daxx in cardiac myocytes by its nuclear sequestration. Taken together, Daxx plays a protective role in cultured cardiomyocyte-like cells, at least under the conditions used.

New insights into the role of the subnuclear structure ND10 for viral infection

Nuclear domains 10 (ND10), alternatively termed PML nuclear bodies (PML-NBs) or PML oncogenic domains (PODs), have been discovered approximately 15 years ago as a nuclear substructure that is targeted by a variety of viruses belonging to different viral families. This review will summarize the most important structural and functional characteristics of ND10 and its major protein constituents followed by a discussion of the current view regarding the role of this subnuclear structure for various DNA and RNA viruses with an emphasis on herpesviruses. It is concluded that accumulating evidence argues for an involvement of ND10 in host antiviral defenses either via mediating an intrinsic immune response against specific viruses or via acting as a component of the cellular interferon pathway.

Cataloging and organizing p73 interactions in cell cycle arrest and apoptosis

We have compiled the p73-mediated cell cycle arrest and apoptosis pathways. p73 is a member of the p53 family, consisting of p53, p63 and p73. p73 exists in several isoforms, presenting different domain structures. p73 functions not only as a tumor suppressor in apoptosis but also as differentiator in embryo development. p53 mutations are responsible for half of the human cancers; p73 can partially substitute mutant p53 as tumor suppressor. The pathways we assembled create a p73-centered network consisting of 53 proteins and 176 interactions. We clustered our network into five functional categories: Upregulation, Activation, Suppression, Transcriptional Activity and Degradation. Our literature searches led to discovering proteins (c-Jun and pRb) with apparent opposing functional effects; these indicate either currently missing proteins and interactions or experimental misidentification or functional annotation. For convenience, here we present the p73 network using the molecular interaction map (MIM) notation. The p73 MIM is unique amongst MIMs, since it further implements detailed domain features. We highlight shared pathways between p53 and p73. We expect that the compiled and organized network would be useful to p53 family-based studies.

Daxx represses RelB target promoters via DNA methyltransferase recruitment and DNA hypermethylation

The apoptosis-modulating protein Daxx functions as a transcriptional repressor that binds to and suppresses the activity of nuclear factor-kappaB member RelB, among other transcription factors. The mechanism by which Daxx represses RelB target genes remains elusive. In this report, we demonstrate that Daxx controls epigenetic silencing of RelB target genes by DNA methylation. Daxx potently represses the RelB target genes dapk1, dapk3, c-flip, and birc3 (ciap2) at both the mRNA and protein levels. Recruitment of Daxx to target gene promoters, and its ability to repress them, is RelB-dependent, as shown by experiments using relB(-/-) cells. Importantly, methylation of target promoters is decreased in daxx(-/-) cells compared with daxx(+/+) cells, and stable transfection of daxx(-/-) cells with Daxx restores DNA methylation. Furthermore, Daxx recruits DNA methyl transferase 1 (Dnmt1) to target promoters, resulting in synergistic repression. The observation that Daxx functions to target DNA methyltransferases onto RelB target sites in the genome provides a rare example of a gene-specific mechanism for epigenetic silencing. Given the documented role of several of the RelB-regulated genes in diseases, particularly cancer, the findings have implications for developing therapeutic strategies based on epigenetic-modifying drugs.

Daxx mediates SUMO-dependent transcriptional control and subnuclear compartmentalization

SUMO (small ubiquitin-related modifier) modification is emerging as an important post-translational control in transcription. In general, SUMO modification is associated with transcriptional repression. Although many SUMO-modified transcription factors and co-activators have been identified, little is known about the mechanism underlying SUMOylation-elicited transcriptional repression. Here, we summarize that SUMO modification of transcription factors such as androgen receptor, glucocorticoid receptor, Smad4 and CBP [CREB (cAMP-response-element-binding protein)-binding protein] co-activator results in the recruitment of a transcriptional co-repressor Daxx, thereby causing transcriptional repression. Such a SUMO-dependent recruitment of Daxx is mediated by the interaction between the SUMO moiety of SUMOylated factors and Daxx SUMO-interacting motif. Interestingly, the transrepression effect of Daxx on these SUMOylated transcription factors can be relieved by SUMOylated PML (promyelocytic leukaemia) via altering Daxx partition from the targeted gene promoter to PML nuclear bodies. Because Daxx SUMO-interacting motif is a common binding site for SUMOylated factors, a model of competition for Daxx recruitment between SUMOylated PML and SUMOylated transcription factors was proposed. Together, our findings strongly suggest that Daxx functions as a SUMO reader in the SUMO-dependent regulation of transcription and subnuclear compartmentalization.

Effect of Daxx on cholesterol accumulation in hepatic cells

AIM: To study the effect of Daxx on cholesterol accumulation in hepatic cells.

METHODS: Sprague Dawley (SD) rats were fed a normal or high fat diet for 6 wk, and serum lipids and Daxx expression of hepatic tissues were measured by immunoblot assays. HepG₂ cells were transfected with the pEGFP-C1/Daxx or pEGFP-C1 plasmid. Cells stably transfected with Daxx were identified by RT-PCR analysis. Total cholesterol levels were determined by high performance liquid chromatography. Activated-SREBP and caveolin-1 were assayed by western blotting.

RESULTS: Hepatic Daxx protein was higher in normal rats than in high fat diet-fed rats. Noticeable negative correlations were seen between Daxx and LDL-C (γ = -7.56, P = 0.018), and between Daxx and TC (γ = -9.07, P = 0.01), respectively. The total cholesterol of HepG₂/GFP-Daxx cells was lower than that of control cells or HepG₂/GFP cells (9.28 ± 0.19 vs 14.36 ± 4.45 or 13.94 ± 2.62, both P < 0.05). Furthermore, in HepG₂/GFP cells, the expression of activated SREBP was lower than that of control cells, whereas caveolin-1 expression was higher.

CONCLUSION: Overexpression of Daxx in HepG₂ cells decreased intracellular cholesterol accumulation, which might be associated with inhibition of SREBP activity and an increase in caveolin-1 expression.

Daxx contains two nuclear localization signals and interacts with importin α3

Daxx plays a major role in several important signaling pathways including transcription and cell death. It has been postulated that Daxx regulates both events from the nucleus; however, the mechanism by which Daxx is localized in the nucleus remains obscure. Here we show that nuclear localization of Daxx is controlled by two independent signals and importin 3. Domain analysis reveals that Daxx contains two separate nuclear localizing domains. Site-directed mutagenesis reveals that the basic aa sequence RLKRK at residues 227-231 (NLS1) is responsible for nuclear localization of N-terminal domain, while aa sequence KKSRKEKK at residues 630-637 (NLS2) is responsible for nuclear localization of the C-terminal domain. Mutations of a NLS consensus sequence RKKRR at residues 391-395 and several other basic aa clusters have no effect on Daxx nuclear localization. In full-length Daxx, NLS1 contributes partially to nuclear localization, while NLS2 plays a major role. Markedly, it is essential to disrupt both NLS1 and NLS2 in order to completely block nuclear localization of the full-length protein and to prevent its association with PML nuclear bodies. Furthermore, Daxx interacts selectively with importin alpha3 through its NLS1 and NLS2 sequences. Conversely, importin alpha3 utilizes two NLS-binding sites for Daxx interaction, suggesting that the importin/mediates nuclear import of Daxx. Finally, we show that nuclear localization of Daxx is essential for its transcriptional effects on GR and p53. Together, these data unveil a molecular mechanism that controls nuclear localization of Daxx and support a nuclear role of Daxx in transcriptional regulation.

Physical interactions and functional coupling between Daxx and sodium hydrogen exchanger 1 in ischemic cell death

Daxx, a death domain-associated protein, is implicated in ischemic cell death. To clarify the mechanism of cell death mediated by Daxx, a yeast two-hybrid assay was performed. Sodium hydrogen exchanger isoform 1 (NHE1) was identified as a Daxx-interacting protein. During ischemic stress, Daxx translocates from the nucleus to the cytoplasm, where it colocalizes with NHE1. Daxx binds to the ezrin/radixin/moesin-interacting domain of NHE1, in competition with ezrin. Consistent with this finding, transfection of the constitutively cytoplasmic mutant, Daxx(W621A), inhibited ezrin-mediated Akt-1 activation. Moreover, transfection of Daxx(W621A), but not the Daxx(S667A) mutant that is confined to the nucleus, accelerated pH(i) recovery from an acid load, indicating that the cytoplasmic protein activates NHE1. Based on the results, we propose that ischemic insult triggers the nucleocytoplasmic translocation of Daxx, following which cytoplasmic Daxx stimulates the NHE1 transporter activity and suppresses activation of the NHE1-ezrin-Akt-1 pathway. Our data support a novel molecular function of Daxx as an upstream regulator of NHE1 in ischemic cell death.

Regulation of c-met expression by transcription repressor Daxx

The protooncogene c-met encodes the tyrosine kinase receptor for the hepatocyte growth factor/scatter factor (HGF/SF). While overexpression of c-met is documented in many types of tumors, the mechanism of c-met regulation remains elusive. Here, we demonstrate Daxx as a repressor of c-met transcription. The expression of c-met is elevated in Daxx knockout mouse cells and is reversed by Daxx reconstitution. C-met promoter analysis of Daxx-/- cells reveled changes in chromatin acetylation, but not in DNA methylation. Daxx binds to the mouse c-met promoter and Daxx-binding region is sufficient for transcription repression, while HDAC2 is associated with c-met promoter mostly in Daxx+/+ cells, pointing to Daxx-dependent HDAC2 recruitment as a potential mechanism of c-met repression. HGF-induced cell mobility and invasion confirmed augmented activity of c-Met/HGF pathway in Daxx-/- cells. Finally, inverse correlation between Daxx and c-Met in cancer cell lines and in metastatic breast cancer specimens suggests potential function of Daxx as a c-met repressor during cancer progression.

Daxx-like protein of Drosophila interacts with Dmp53 and affects longevity and Ark mRNA level

Daxx-like protein (DLP), the Drosophila homolog of Daxx, binds Drosophila melanogaster p53 (Dmp53) through its C-terminal region. We generated DLP mutants and found that although DLP expression is developmentally regulated, it is not essential for the execution of the developmental program. The effects DLP mutations show in the loss of heterozygosity assay and on phenotypes resulting from Dmp53 overexpression indicate a genetic interaction between DLP and Dmp53. In contrast to Dmp53 mutants, however, loss of DLP does not result in radiosensitivity indicating that it does not play an essential role in the activation of Dmp53-dependent response after ionizing radiation, and DLP is also not required for the irradiation-induced activation of reaper. In contrast, DLP is involved in the transcriptional regulation of Ark, because Ark mRNA level is decreased in DLP mutants and increased upon ectopic overexpression of DLP. Interestingly, DLP mutants have reduced longevity and reduced female fertility. Altogether, our data suggest complex functions for DLP, which include an anti-apoptotic effect exerted through repression of some Dmp53 functions, and activation of some proapoptotic genes.

Inhibition of NF-kappaB acetylation and its transcriptional activity by Daxx

We propose a biochemical mechanism by which Daxx modulates NF-kappaB transcriptional activity. Both chromatin immunoprecipitation (ChIP) assay and electrophoretic mobility shift assay (EMSA) have confirmed Daxx-mediated repression of transcriptional competence of NF-kappaB in HeLa cells. Overexpression of Daxx repressed the expression of NF-kappaB-regulated genes such as I kappa B alpha and IL8. Co-immunoprecipitation assay revealed the existence of intermolecular association between endogenous Daxx and p65 subunit of NF-kappaB stimulated by TNFalpha. Here, we suggest that Daxx-mediated repression of NF-kappaB transactivation correlates with the inhibition of p65 acetylation by Daxx. Based on the finding that the Daxx binding N-terminal side of p65 includes the major sites of acetylation mediated by p300/CBP, we further propose that the physical interaction between Daxx and p65 provides a functional framework for the inhibition of p65 acetylation by p300/CBP and subsequent repression of NF-kappaB transcriptional activity.

Adenovirus type 5 early region 1B 156R protein promotes cell transformation independently of repression of p53-stimulated transcription

Early region 1B (E1B) of adenovirus type 5 (Ad5) encodes at least five different polypeptides generated by alternative splicing of a common mRNA precursor. Two of these gene products, E1B-19K and E1B-55K, are individually capable of cooperating with the Ad5 E1A proteins to completely transform rodent cells in culture. Substantial evidence suggests that these two E1B proteins contribute to cell transformation by antagonizing growth arrest and apoptosis. Here, we performed genetic and biochemical analyses to assess the attributes of the remaining E1B proteins (E1B-156R, E1B-93R, and E1B-84R). Our results show that E1B-156R, which comprises the 79 amino-terminal and 77 carboxy-terminal amino acids of E1B-55K, also enhances focal transformation of primary rat cells in cooperation with E1A. Since E1B-156R seemed unable to relocalize p53 and inhibit its transactivating function, it must be assumed that it contributes to transformation independently of repression of p53-stimulated transcription. Furthermore, we discovered that E1B-156R contains a functional transcriptional repression domain and binds Ad5 E4orf6 and the cellular apoptosis regulator Daxx. While the ability to bind E4orf6 could indicate further biological functions of E1B-156R in viral infection, the interaction with Daxx might also be linked to its transforming potential. Taken together, these analyses introduce E1B-156R as a novel transformation-promoting E1B protein that acts without repressing p53 transactivation. Moreover, identification of the interaction partners E4orf6 and Daxx provides a first glance of E1B-156R's potential functions.

Ets-1-dependent expression of vascular endothelial growth factor receptors is activated by latency-associated nuclear antigen of Kaposi's sarcoma-associated herpesvirus through interaction with Daxx

Vascular endothelial growth factor (VEGF) and its receptors are highly expressed in Kaposi's sarcoma (KS) lesion and play a key role in angiogenesis. Latency-associated nuclear antigen (LANA) of Kaposi's sarcoma-associated herpesvirus (KSHV/HHV8) has multiple functions related to viral latency and KSHV-induced oncogenesis. In this report, we have identified Daxx as a LANA-binding protein by co-immunoprecipitation analysis of HeLa cells stably expressing LANA. LANA associated with Daxx in a PEL cell line infected with KSHV. LANA and Daxx also bound in vitro, suggesting direct interaction. From the results of binding assays, a region containing the Glu/Asp-rich domain within LANA, and a central region including the second paired amphipathic helix within Daxx contributed to the interaction. To address the physiological significance of this interaction, we focused on a Daxx-mediated VEGF receptor gene regulation. We found that Daxx repressed Ets-1-dependent Flt-1/VEGF receptor-1 gene expression, and that LANA inhibited the repression by Daxx in a reporter assay. Analyses of flow cytometry and real-time PCR revealed that expression of VEGF receptor-1 and -2 in LANA-expressing human umbilical vein endothelial cells (HUVECs) significantly increased. Co-immunoprecipitation and immunoblotting experiments suggested that LANA-bound Daxx to inhibit the interaction between Daxx and Ets-1. Chromatin immunoprecipitation assays showed that Daxx associated with VEGF receptor-1 promoter in HUVECs, and that LANA expression reduced this association. These results suggested that LANA contributes to a high expression of VEGF receptors in KS lesion by interfering with the interaction between Daxx and Ets-1.

Critical role for Daxx in regulating Mdm2

The tumour suppressor p53 induces apoptosis or cell-cycle arrest in response to genotoxic and other stresses. In unstressed cells, the anti-proliferative effects of p53 are restrained by mouse double minute 2 (Mdm2), a ubiquitin ligase (E3) that promotes p53 ubiquitination and degradation. Mdm2 also mediates its own degradation through auto-ubiquitination. It is unclear how the cis- and trans-E3 activities of Mdm2, which have opposing effects on cell fate, are differentially regulated. Here, we show that death domain-associated protein (Daxx) is required for Mdm2 stability. Downregulation of Daxx decreases Mdm2 levels, whereas overexpression of Daxx strongly stabilizes Mdm2. Daxx simultaneously binds to Mdm2 and the deubiquitinase Hausp, and it mediates the stabilizing effect of Hausp on Mdm2. In addition, Daxx enhances the intrinsic E3 activity of Mdm2 towards p53. On DNA damage, Daxx dissociates from Mdm2, which correlates with Mdm2 self-degradation. These findings reveal that Daxx modulates the function of Mdm2 at multiple levels and suggest that the disruption of the Mdm2-Daxx interaction may be important for p53 activation in response to DNA damage.

Physical and functional interactions between Daxx and STAT3

Signal transducer and activator of transcription 3 (STAT3) play key roles in the intracellular signaling pathways of the interleukin (IL)-6 family of cytokines, which exhibit a diverse set of cellular responses, including cell proliferation and differentiation. Dysregulated IL-6/STAT3 signaling is involved in the pathogenesis of several diseases, for example autoimmune diseases and tumors. Type I interferon (IFN) induces the expression of proapoptotic genes and has been used in the clinical treatment of several tumors. In the present study, we found that type I IFN suppressed IL-6/STAT3-mediated transcription and gene expression. Furthermore, a type I IFN-induced protein, Daxx, also suppressed STAT3-mediated transcriptional activation, while overexpression of Daxx inhibited IL-6/STAT3-mediated gene expression. Importantly, small-interfering RNA-mediated reduction of Daxx expression enhanced IL-6/leukemia inhibitory factor (LIF)-induced STAT3-dependent transcription. Co-immunoprecipitation studies revealed a physical interaction between Daxx and STAT3 in transiently transfected 293T cells. We further found that Daxx and STAT3 were co-localized in the nucleus. These results indicate that Daxx may serve as a transcriptional regulator of type I IFN-mediated suppression of the IL-6/STAT3 signaling pathway.

Regulation of the p53-MDM2 pathway by 14-3-3 σ and other proteins

The 14-3-3 sigma (sigma) protein, a unique member of 14-3-3 family, is a negative regulator of the cell cycle and is induced by p53 to initiate cell cycle checkpoint control after DNA damage. Among the 14-3-3 family members, 14-3-3 sigma is uniquely induced by p53 and has a positive feedback effect on p53 activity in response to DNA damage. Although 14-3-3 sigma is linked to p53-regulated cell cycle checkpoint control, the detailed mechanisms of cell cycle regulation by 14-3-3 sigma remain unclear. Decreased expression of 14-3-3 sigma was reported in several types of carcinomas, suggesting that the negative regulatory role of 14-3-3 sigma in the cell cycle is compromised during tumorigenesis. Given the fact that p53's tumor suppressive function is lost in almost half of all human cancers and that 14-3-3 sigma's activity is linked to the p53 network, a perspective regarding the p53/14-3-3 sigma relationship is needed for cancer research. Here we discuss the mechanisms by which 14-3-3 sigma-stabilizes p53 with the hope that these insights may be applied to develop targeted therapeutic strategies for cancer treatment.

Physiological and functional interactions between Tcf4 and Daxx in colon cancer cells

Daxx, a human cell death-associated protein, was isolated as a Tcf4-interacting protein, using a yeast two-hybrid screen. Co-immunoprecipitation in HEK-293T cells and yeast two-hybrid screen in Y190 cells were performed to identify the interaction between Tcf4 with Daxx and to map the binding regions of Tcf4. In the nucleus, Daxx reduced DNA binding activity of Tcf4 and repressed Tcf4 transcriptional activity. Overexpression of Daxx altered the expression of genes downstream of Tcf4, including cyclin D1 and Hath-1, and induced G1 phase arrest in colon cancer cells. A reduction in Daxx protein expression was also observed in colon adenocarcinoma tissue when compared with normal colon tissue. This evidence suggests a possible physiological function of Daxx, via interaction with Tcf4, to regulate proliferation and differentiation of colon cells.

Evidence for the interaction of the regulatory protein Ki-1/57 with p53 and its interacting proteins

Ki-1/57 is a cytoplasmic and nuclear phospho-protein of 57 kDa and interacts with the adaptor protein RACK1, the transcription factor MEF2C, and the chromatin remodeling factor CHD3, suggesting that it might be involved in the regulation of transcription. Here, we describe yeast two-hybrid studies that identified a total of 11 proteins interacting with Ki-1/57, all of which interact or are functionally associated with p53 or other members of the p53 family of proteins. We further found that Ki-1/57 is able to interact with p53 itself in the yeast two-hybrid system when the interaction was tested directly. This interaction could be confirmed by pull down assays with purified proteins in vitro and by reciprocal co-immunoprecipitation assays from the human Hodgkin analogous lymphoma cell line L540. Furthermore, we found that the phosphorylation of p53 by PKC abolishes its interaction with Ki-1/57 in vitro.

Daxx: death or survival protein?

The death domain-associated protein (Daxx) was originally cloned as a CD95 (FAS)-interacting protein and modulator of FAS-induced cell death. Daxx accumulates in both the nucleus and the cytoplasm; in the nucleus, Daxx is found associated with the promyelocytic leukaemia (PML) nuclear body and with alpha-thalassemia/mental retardation syndrome protein (ATRX)-positive heterochromatic regions. In the cytoplasm, Daxx has been reported to interact with various proteins involved in cell death regulation. Despite a significant number of studies attempting to determine Daxx function in apoptotic and non-apoptotic cell death, its precise role in this process is only partially understood. Here, we critically review the current understanding of Daxx function and shed new light on this interesting field.

SUMO modification negatively modulates the transcriptional activity of CREB-binding protein via the recruitment of Daxx

Small ubiquitin-like modifier (SUMO) modification is emerging as an important control in transcription regulation. Here, we show that CREB-binding protein (CBP), a versatile transcriptional coactivator for numerous transcription factors in response to diverse signaling events, can be modified by SUMO-1 at lysine residues 999, 1034, and 1057 both in vitro and in vivo. Mutation of the SUMO acceptor lysine residues either individually or in combination enhanced CBP transcriptional activity, and expression of a SUMO protease SENP2 potentiated the transcriptional activity of CBP wild-type but not its sumoylation mutant, indicating that SUMO modification negatively regulates CBP transcriptional activity. Furthermore, we demonstrated an interaction of SUMO-1-modified CBP with the transcriptional corepressor Daxx and an essential role of Daxx in mediating SUMO-dependent transcriptional regulation of CBP through histone deacetylase 2 recruitment. Together, our findings indicate that SUMO modification and subsequent recruitment of Daxx represent a previously undescribed mechanism in modulating CBP transcriptional potential.

Daxx is required for stress-induced cell death and JNK activation

Daxx has been implicated in the modulation of apoptosis in response to various stimuli. In the nucleus, Daxx interacts and colocalizes with the promyelocytic leukemia protein (PML) into the PML-nuclear body. Moreover, overexpressed Daxx positively modulates FAS-ligand and TGFbeta-induced apoptosis. However, recent reports indicate that Daxx can also act as an antiapoptotic factor. As most studies on the role of Daxx in cell death have been conducted using tumour cell lines, we analysed the function of Daxx in physiological settings. We found that Daxx is induced upon exposure to ultraviolet (UV) irradiation and hydrogen peroxide treatment. We employed RNA interference to downregulate Daxx in primary fibroblasts. Remarkably, Daxx-depleted cells are resistant to cell death induced by both UV irradiation and oxidative stress. Furthermore, the downregulation of Daxx results in impaired MKK/c-Jun-N-terminal kinase (JNK) activation. This is the first evidence that Daxx promotes cell death and JNK activation in physiological conditions.

p73-induced apoptosis: A question of compartments and cooperation

The transcriptionally active forms of p73 are capable of inducing apoptosis, and the isoforms termed TAp73 are important players when E2F and its oncogenic activators induce programmed cell death. However, the conditions under that TAp73 can kill a cell remain to be clarified. Recently, it has been found that p73 proteins are not merely floating in the nucleoplasm but rather can associate with specific compartments in the cell. Examples of intranuclear compartments associated with p73 proteins include the PML oncogenic domains and the nuclear matrix. In addition, p73 is found in the cytoplasm. It remains to be seen whether p73 might also associate with mitochondria, in analogy with p53. The relocalization of p73 is expected to be mediated by specific binding partners, mostly other proteins. Here, we discuss the possibility that the compartmentalization of p73, and the cooperation with the corresponding binding partners, might decide about its apoptosis-inducing activity.

Daxx overexpression in T-lymphoblastic Jurkat cells enhances caspase-dependent death receptor- and drug-induced apoptosis in distinct ways

The role of Daxx, in particular, its ability to promote or hinder apoptosis, still remains controversial. In order to elucidate the functional relevance of Daxx in apoptosis signaling of malignant lymphocytes, Jurkat T-cells were stably transfected with a Daxx-expressing vector or with the respective Daxx-negative control vector. We thus demonstrate that ectopic expression of Daxx substantially increases the rate of apoptosis upon incubation with death receptor agonists such as Fas and TRAIL as well as upon incubation with the cytotoxic drug doxorubicin (DOX). Analysis of the molecular changes induced in the extrinsic and intrinsic apoptosis pathways reveals that augmentation of apoptosis by Daxx overexpression is conveyed by distinctly different mechanisms. Although enforced apoptosis caused by ectopic Daxx expression is caspase-dependent in both cases, major differences between Fas/TRAIL-induced apoptosis and doxorubicin-induced apoptosis are observed in expression patterns of X-linked inhibitor of apoptosis (XIAP), p53, Bid, ZIP kinase, and prostate apoptosis response gene 4 (Par-4). Moreover, we could show that addition of a CD95 blocking antibody to the clones treated with doxorubicin was able to increase apoptosis as compared to doxorubicin treatment alone and was accompanied by an enhancement of the mitochondrial branch of apoptosis. In conclusion, we here outline the major molecular mechanisms underlying the apoptosis-promoting effect of Daxx in neoplastic lymphocytes and demonstrate fundamental molecular differences elicited by the overexpression of Daxx in the extrinsic and intrinsic signaling pathways.

A sumoylation site in PML/RARA is essential for leukemic transformation

Pathogenesis of acute promyelocytic leukemia (APL) has been proposed to involve transcriptional repression through enhanced corepressors binding onto RARA moieties of PML/RARA homodimers. Unexpectedly, we show that the K160 sumoylation site in the PML moiety of PML/RARA is required for efficient immortalization/differentiation arrest ex vivo, implying that RARA homodimerization is insufficient to fully immortalize primary hematopoietic progenitor cells. Similarly, PML/RARAK160R transgenic mice develop myeloproliferative syndromes, but never APL. The Daxx repressor no longer binds PML/RARAK160R, but fusion of these two proteins restores the differentiation block ex vivo. Thus, transcriptional repression dependent on a specific sumoylation site in PML is critical for the APL phenotype, while forced RARA dimerization could control expansion of the myeloid compartment.

Daxx mediates the small ubiquitin-like modifier-dependent transcriptional repression of Smad4

Daxx has been shown to function as an apoptosis regulator and transcriptional repressor via its interaction with various cytoplasmic and nuclear proteins. Here, we showed that Daxx interacts with Smad4 and represses its transcriptional activity via the C-terminal domain of Daxx. In vitro and in vivo interaction studies indicated that the binding of Smad4 to Daxx depends on Smad4 sumoylation. Substitution of Smad4 SUMO conjugation residue lysine 159, but not 113, to arginine not only disrupted Smad4-Daxx interaction but also relieved Daxx-elicited repression of Smad4 transcriptional activity. Furthermore, chromatin immunoprecipitation analyses revealed the recruitment of Daxx to an endogenous, Smad4-targeted promoter in a Lys(159) sumoylation-dependent manner. Finally, down-regulation of Daxx expression by RNA interference enhanced transforming growth factor beta-induced transcription of reporter and endogenous genes through a Smad4-dependent, but not K159R-Smad4-dependent, manner. Together, these results indicate that Daxx suppresses Smad4-mediated transcriptional activity by direct interaction with the sumoylated Smad4 and identify a novel role of Daxx in regulating transforming growth factor beta signaling.

Negative modulation of androgen receptor transcriptional activity by Daxx

The transcriptional activity of the androgen receptor (AR) modulated by positive or negative regulators plays a critical role in controlling the growth and survival of prostate cancer cells. Although numerous positive regulators have been identified, negative regulators of AR are less well understood. We report here that Daxx functions as a negative AR coregulator through direct protein-protein interactions. Overexpression of Daxx suppressed AR-mediated promoter activity in COS-1 and LNCaP cells and AR-mediated prostate-specific antigen expression in LNCaP cells. Conversely, downregulation of endogenous Daxx expression by RNA interference enhances androgen-induced prostate-specific antigen expression in LNCaP cells. In vitro and in vivo interaction studies revealed that Daxx binds to both the amino-terminal and the DNA-binding domain of the AR. Daxx proteins interfere with the AR DNA-binding activity both in vitro and in vivo. Moreover, sumoylation of AR at its amino-terminal domain is involved in Daxx interaction and trans-repression. Together, these findings not only provide a novel role of Daxx in controlling AR transactivation activity but also uncover the mechanism underlying sumoylation-dependent transcriptional repression of the AR.

Negative regulation of p53 functions by Daxx and the involvement of MDM2

In normal cells p53 activity is tightly controlled and MDM2 is a known negative regulator. Here we show that via its acidic domain, Daxx binds to the COOH-terminal domain of p53, whose positive charges are critical for this interaction, as Lys to Arg mutations preserved, but Lys to Ala or Ser to Glu mutations abolished Daxx-p53 interaction. These results thus implicate acetylation and phosphorylation of p53 in regulating its binding to Daxx. Interestingly, whereas Daxx did not bind to p53 in cells as assessed by immunoprecipitation, MDM2 expression restored p53-Daxx interaction, and this correlated with deacetylation of p53. In p53/MDM2-null mouse embryonic fibroblasts (DKO MEF), Daxx repressed p53 target promoters whose p53-binding elements were required for the repression. Coexpression of Daxx and MDM2 led to further repression. p53 expression in DKO MEF induced apoptosis and Daxx expression relieved this effect. Similarly, in HCT116 cells, Daxx conferred striking resistance to 5-fluorouracil-induced apoptosis. As p53 is required for 5-fluorouracil-induced cell death, our data show that Daxx can suppress cell death induced by p53 overexpression and p53-dependent stress response. Collectively, our data reveal Daxx as a novel negative regulator of p53. Importantly, posttranslational modifications of p53 inhibit Daxx-p53 interaction, thereby relieving negative regulation of p53 by Daxx.

The transcriptional repressor hDaxx potentiates p53-dependent apoptosis

p53 and its homologues p73 and p63 are transcription factors that play an essential role in modulating cell cycle arrest and cell death in response to several environmental stresses. The type and intensity of these responses, which can be different depending on the inducing stimulus and on the overall cellular context, are believed to rely on the activation of defined subsets of target genes. The proper activation of p53 family members requires the coordinated action of post-translational modifications and interaction with several cofactors. In this study, we demonstrate that the multifunctional protein hDaxx interacts with p53 and its homologues, both in vitro and in vivo, and modulates their transcriptional activity. Moreover, we show that hDaxx, which has been implicated in several apoptotic pathways, increases the sensitivity to DNA damage-induced cell death and that this effect requires the presence of p53. Although hDaxx represses p53-dependent transcription of the p21 gene, it does not affect the activation of proapoptotic genes, and therefore acts by influencing the balance between cell cycle arrest and proapoptotic p53 targets. Our results therefore underline the central role of hDaxx in modulating the apoptotic threshold upon several stimuli and identify it as a possible integrating factor that coordinates the response of p53 family members.