The acetyltransferase 60 kDa trans-acting regulatory protein of HIV type 1-interacting protein (Tip60) interacts with the translocation E26 transforming-specific leukaemia gene (TEL) and functions as a transcriptional co-repressor

(GGAA)3-Based TF-PROTACs Enable Targeted Degradation of ETV6 to Inhibit Ewing Sarcoma Growth

Ewing sarcoma is a rare pediatric cancer primarily driven by the EWS::FLI1 oncofusion transcription factor. Despite being an ideal drug target, EWS::FLI1 has proven challenging to inhibit with conventional approaches. Recent studies identified ETV6 as a vulnerability in Ewing sarcoma, where it competes with EWS::FLI1 at short GGAA repeats to restrain EWS::FLI1 function. However, no therapies targeting ETV6 have been developed. In this study, we report the discovery of a unique (GGAA)3 DNA oligonucleotide that specifically binds to ETV6 but not EWS::FLI1. We developed (GGAA)3-based TF-PROTACs, termed d(GGAA)3s, by coupling (GGAA)3 with VHL ligands. d(GGAA)3s effectively degraded endogenous ETV6 but not EWS::FLI1 proteins in Ewing sarcoma cells, thus suppressing Ewing sarcoma growth. Mechanistically, d(GGAA)3s enhanced oncogenic EWS::FLI1 transcriptional activity, inducing cellular stress and cell death. Additionally, d(GGAA)3s sensitized Ewing sarcoma cells to standard chemotherapy, suggesting their potential use in combination therapies. Beyond Ewing sarcoma, d(GGAA)3s also targeted ETV6-fusion proteins found in breast cancer, broadening their potential clinical applications. In summary, d(GGAA)3s represent a nucleotide-based approach for degrading ETV6, inhibiting Ewing sarcoma growth and targeting ETV6-dependent cancers. This strategy offers a promising therapeutic avenue for Ewing sarcoma and other malignancies involving ETV6 fusions.

Structural analysis and molecular dynamics simulation studies of HIV-1 antisense protein predict its potential role in HIV replication and pathogenesis

The functional significance of the HIV-1 Antisense Protein (ASP) has been a paradox since its discovery. The expression of this protein in HIV-1-infected cells and its involvement in autophagy, transcriptional regulation, and viral latency have sporadically been reported in various studies. Yet, the definite role of this protein in HIV-1 infection remains unclear. Deciphering the 3D structure of HIV-1 ASP would throw light on its potential role in HIV lifecycle and host-virus interaction. Hence, using extensive molecular modeling and dynamics simulation for 200 ns, we predicted the plausible 3D-structures of ASP from two reference strains of HIV-1 namely, Indie-C1 (subtype-C) and NL4-3 (subtype-B) so as to derive its functional implication through structural domain analysis. In spite of sequence and structural differences in subtype B and C ASP, both structures appear to share common domains like the Von Willebrand Factor Domain-A (VWFA), Integrin subunit alpha-X (ITGSX), and ETV6-Transcriptional repressor, thereby reiterating the potential role of HIV-1 ASP in transcriptional repression and autophagy, as reported in earlier studies. Gromos-based cluster analysis of the centroid structures also reassured the accuracy of the prediction. This is the first study to elucidate a highly plausible structure for HIV-1 ASP which could serve as a feeder for further experimental validation studies.

Tip60-mediated H2A.Z acetylation promotes neuronal fate specification and bivalent gene activation

Cell lineage specification is accomplished by a concerted action of chromatin remodeling and tissue-specific transcription factors. However, the mechanisms that induce and maintain appropriate lineage-specific gene expression remain elusive. Here, we used an unbiased proteomics approach to characterize chromatin regulators that mediate the induction of neuronal cell fate. We found that Tip60 acetyltransferase is essential to establish neuronal cell identity partly via acetylation of the histone variant H2A.Z. Despite its tight correlation with gene expression and active chromatin, loss of H2A.Z acetylation had little effect on chromatin accessibility or transcription. Instead, loss of Tip60 and acetyl-H2A.Z interfered with H3K4me3 deposition and activation of a unique subset of silent, lineage-restricted genes characterized by a bivalent chromatin configuration at their promoters. Altogether, our results illuminate the mechanisms underlying bivalent chromatin activation and reveal that H2A.Z acetylation regulates neuronal fate specification by establishing epigenetic competence for bivalent gene activation and cell lineage transition.

TIP60 in aging and neurodegeneration

Epigenetic modification of chromatin, including histone methylation and acetylation, plays critical roles in eukaryotic cells and has a significant impact on chromatin structure/accessibility, gene regulation and, susceptibility to aging, neurodegenerative disease, cancer, and other age-related diseases. This article reviews the current advances on TIP60/KAT5, a major histone acetyltransferase with diverse functions in eukaryotes, with emphasis on its regulation of autophagy, proteasome-dependent protein turnover, RNA transcription, DNA repair, circadian rhythms, learning and memory, and other neurological functions implicated in aging and neurodegeneration. Moreover, the promising therapeutic potential of TIP60 is discussed to target Alzheimer's disease and other neurological diseases.

Tip60 and p300 function antagonistically in the epigenetic regulation of HPV18 E6/E7 genes in cervical cancer HeLa cells

BACKGROUND

High-risk HPV is a causative factor of cervical cancer. HPV DNA fragments integrate into host genome resulting in the constitutive expression of HPV genes E6 and E7 under the regulation of transcription factors, such as p300 and Tip60. Interestingly, Tip60, a factor with HAT (histone acetyl transferase) activity, represses HPV18 E6/E7 genes while another HAT p300 activates the transcription of HPV18 E6/E7.

OBJECTIVE

To explore the mechanism for the opposite roles of Tip60 and p300 in the virus gene regulation, and the influence of Tip60 and p300 in histone modifications in the regulatory sequence of HPV18 genes.

METHODS

Tip60 or p300 was either knocked down or overexpressed in HeLa cells. The effects on HPV E6E7 expression were determined with RT-qPCR. The association of RNA polymerase II and the enrichment of acetylated or methylated histones in HPV promoter region were measured by ChIP assays with specific antibodies.

RESULTS

ChIP results showed that Tip60 and p300 differently affected the modifications of histone H3K9 and the deposition of nucleosomes in HPV18 long control region (LCR). HPV18 LCR in HeLa cells is bivalent chromatin carrying both the active histone H3K9 acetylation mark and the repressive histone H3K9 trimethylation mark, the balance is maintained by Tip60 and p300.

CONCLUSION(S)

Based on the roles of Tip60 and p300 in HPV gene regulation, chemical compounds targeting Tip60 or p300 are potential anti-cervical cancer drugs.

Epigenetic factor siRNA screen during primary KSHV infection identifies novel host restriction factors for the lytic cycle of KSHV

Establishment of viral latency is not only essential for lifelong Kaposi’s sarcoma-associated herpesvirus (KSHV) infection, but it is also a prerequisite of viral tumorigenesis. The latent viral DNA has a complex chromatin structure, which is established in a stepwise manner regulated by host epigenetic factors during de novo infection. However, despite the importance of viral latency in KSHV pathogenesis, we still have limited information about the repertoire of epigenetic factors that are critical for the establishment and maintenance of KSHV latency. Therefore, the goal of this study was to identify host epigenetic factors that suppress lytic KSHV genes during primary viral infection, which would indicate their role in latency establishment. We performed an siRNA screen targeting 392 host epigenetic factors during primary infection and analyzed which ones affect the expression of the viral replication and transcription activator (RTA) and/or the latency-associated nuclear antigen (LANA), which are viral genes essential for lytic replication and latency, respectively. As a result, we identified the Nucleosome Remodeling and Deacetylase (NuRD) complex, Tip60 and Tip60-associated co-repressors, and the histone demethylase KDM2B as repressors of KSHV lytic genes during both de novo infection and the maintenance of viral latency. Furthermore, we showed that KDM2B rapidly binds to the incoming viral DNA as early as 8 hpi, and can limit the enrichment of activating histone marks on the RTA promoter favoring the downregulation of RTA expression even prior to the polycomb proteins-regulated heterochromatin establishment on the viral genome. Strikingly, KDM2B can also suppress viral gene expression and replication during lytic infection of primary gingival epithelial cells, revealing that KDM2B can act as a host restriction factor of the lytic cycle of KSHV during both latent and lytic infections in multiple different cell types.

Latent viral infection of cancer cells in KSHV-associated tumors is critical for the growth and survival of the cancer. Thus, revealing how lytic viral genes get suppressed through epigenetic regulation following de novo KSHV infection, resulting in the establishment of latency, is central to understanding the pathogenesis of KSHV infection. Importantly, the epigenetic factors that we identified as suppressors of KSHV lytic genes are not only crucial for the establishment and maintenance of KSHV latency in different cell types, but also several of them can block lytic KSHV infection in oral epithelial cells. Since herpesviruses often rely on similar sets of host epigenetic factors, the characterization of these newly identified epigenetic factors in KSHV infection may help to better understand fundamental epigenetic mechanisms that may also be utilized by other herpesviruses to establish latency following primary infection.

Epigenetics and Epi-miRNAs: Potential markers/therapeutics in leukemia

Epigenetic variations can play remarkable roles in different normal and abnormal situations. Such variations have been shown to have a direct role in the pathogenesis of various diseases either through inhibition of tumor suppressor genes or increasing the expression of oncogenes. Enzymes involving in epigenetic machinery are the main actors in tuning the epigenetic-based controls on gene expressions. Aberrant expression of these enzymes can trigger a big chaos in the cellular gene expression networks and finally lead to cancer progression. This situation has been shown in different types of leukemia, where high or low levels of an epigenetic enzyme are partly or highly responsible for involvement or progression of a disease. DNA hypermethylation, different histone modifications, and aberrant miRNA expressions are three main epigenetic variations, which have been shown to play a role in leukemia progression. Epigenetic based treatments now are considered as novel and effective therapies in order to decrease the abnormal epigenetic modifications in patient cells. Different epigenetic-based approaches have been developed and tested to inhibit or reverse the unusual expression of epigenetic agents in leukemia. The reciprocal behavior of miRNAs in the regulation of epigenetic modifiers, while being regulated by them, unlocks a new opportunity in order to design some epigenetic-based miRNAs able to silence or sensitize these effectors in leukemia.

ETV6 and ETV7: Siblings in hematopoiesis and its disruption in disease

ETV6 (TEL1) and ETV7 (TEL2) are closely-related members of the ETS family of transcriptional regulators. Both ETV6 and ETV7 have been demonstrated to play key roles in hematopoiesis, particularly with regard to maintenance of hematopoietic stem cells and control of lineage-specific differentiation, with evidence of functional interactions between both proteins. ETV6 has been strongly implicated in the molecular etiology of a number of hematopoietic diseases, including as a tumor suppressor, an oncogenic fusion partner, and an important regulator of thrombopoiesis, but recent evidence has also identified ETV7 as a potential oncogene in certain malignancies. This review provides an overview of ETV6 and ETV7 and their contribution to both normal and disrupted hematopoiesis. It also highlights the key clinical implications of the growing knowledge base regarding ETV6 abnormalities with respect to prognosis and treatment.

ETV6 in hematopoiesis and leukemia predisposition

The ETV6 (also known as TEL) gene encodes a transcriptional repressor that plays a critical role in hematopoiesis and in embryonic development. While somatic ETV6 translocations and missense mutations are frequently observed in human cancers, the role of ETV6 in malignant transformation was unclear. Recently, autosomal dominant germline ETV6 mutations were discovered in families with inherited thrombocytopenia and a propensity to develop hematological malignancy, unequivocally demonstrating a role for ETV6 in leukemogenesis. Studies of germline ETV6 mutations also uncovered an important function of ETV6 in megakaryocyte development. Here we discuss our current understanding of the role of ETV6 in malignancy and in hematopoiesis.

Chromatin modifiers and the promise of epigenetic therapy in acute leukemia

Hematopoiesis is a tightly regulated process involving the control of gene expression that directs the transition from hematopoietic stem and progenitor cells to terminally differentiated blood cells. In leukemia, the processes directing self-renewal, differentiation and progenitor cell expansion are disrupted, leading to the accumulation of immature, non-functioning malignant cells. Insights into these processes have come in stages, based on technological advances in genetic analyses, bioinformatics and biological sciences. The first cytogenetic studies of leukemic cells identified chromosomal translocations that generate oncogenic fusion proteins and most commonly affect regulators of transcription. This was followed by the discovery of recurrent somatic mutations in genes encoding regulators of the signal transduction pathways that control cell proliferation and survival. Recently, studies of global changes in methylation and gene expression have led to the understanding that the output of transcriptional regulators and the proliferative signaling pathways are ultimately influenced by chromatin structure. Candidate gene, whole-genome and whole-exome sequencing studies have identified recurrent somatic mutations in genes encoding epigenetic modifiers in both acute myeloid leukemia (AML) and acute lymphoid leukemia (ALL). In contrast to the two-hit model of leukemogenesis, emerging evidence suggests that these epigenetic modifiers represent a class of mutations that are critical to the development of leukemia and affect the regulation of various other oncogenic pathways. In this review, we discuss the range of recurrent, somatic mutations in epigenetic modifiers found in leukemia and how these modifiers relate to the classical leukemogenic pathways that lead to impaired cell differentiation and aberrant self-renewal and proliferation.

Histone-modifying enzymes: their role in the pathogenesis of acute leukemia and their therapeutic potential

Histone-modifying enzymes have recently been shown to play a central role in the regulation of both normal and malignant hematopoiesis. Post-translational modifications of histones and non-histone proteins underlies a regulatory complexity affecting numerous processes including transcriptional regulation, RNA processing and DNA damage response. Insights into the functions of these enzymes as well as their role in the epigenetic alterations found in leukemia will guide the development of novel therapeutic approaches. This review discusses examples of the proteins that have been implicated in the pathogenesis of leukemia, that may serve as potential therapeutic targets.

Autoinhibition of ETV6 (TEL) DNA binding: appended helices sterically block the ETS domain

ETV6 (or TEL), a transcriptional repressor belonging to the ETS family, is frequently involved in chromosomal translocations linked with human cancers. It displays a DNA-binding mode distinct from other ETS proteins due to the presence of a self-associating PNT domain. In this study, we used NMR spectroscopy to dissect the structural and dynamic bases for the autoinhibition of ETV6 DNA binding by sequences C-terminal to its ETS domain. The C-terminal inhibitory domain (CID) contains two helices, H4 and H5, which sterically block the DNA-binding interface of the ETS domain. Importantly, these appended helices are only marginally stable as revealed by amide hydrogen exchange and (15)N relaxation measurements. The CID is thus poised to undergo a facile conformational change as required for DNA binding. The CID also dampens millisecond timescale motions of the ETS domain hypothesized to be critical for the recognition of specific ETS target sequences. This work illustrates the use of appended sequences on conserved structural domains to generate biological diversity and complements previous studies of the allosteric mechanism of ETS1 autoinhibition to reveal both common and divergent features underlying the regulation of DNA binding by ETS transcription factors.

Tip60 HAT activity mediates APP induced lethality and apoptotic cell death in the CNS of a Drosophila Alzheimer's disease model

Histone acetylation of chromatin promotes dynamic transcriptional responses in neurons that influence neuroplasticity critical for cognitive ability. It has been demonstrated that Tip60 histone acetyltransferase (HAT) activity is involved in the transcriptional regulation of genes enriched for neuronal function as well as the control of synaptic plasticity. Accordingly, Tip60 has been implicated in the neurodegenerative disorder Alzheimer's disease (AD) via transcriptional regulatory complex formation with the AD linked amyloid precursor protein (APP) intracellular domain (AICD). As such, inappropriate complex formation may contribute to AD-linked neurodegeneration by misregulation of target genes involved in neurogenesis; however, a direct and causative epigenetic based role for Tip60 HAT activity in this process during neuronal development in vivo remains unclear. Here, we demonstrate that nervous system specific loss of Tip60 HAT activity enhances APP mediated lethality and neuronal apoptotic cell death in the central nervous system (CNS) of a transgenic AD fly model while remarkably, overexpression of Tip60 diminishes these defects. Notably, all of these effects are dependent upon the C-terminus of APP that is required for transcriptional regulatory complex formation with Tip60. Importantly, we show that the expression of certain AD linked Tip60 gene targets critical for regulating apoptotic pathways are modified in the presence of APP. Our results are the first to demonstrate a functional interaction between Tip60 and APP in mediating nervous system development and apoptotic neuronal cell death in the CNS of an AD fly model in vivo, and support a novel neuroprotective role for Tip60 HAT activity in AD neurodegenerative pathology.

Function of the active site lysine autoacetylation in Tip60 catalysis

The 60-kDa HIV-Tat interactive protein (Tip60) is a key member of the MYST family of histone acetyltransferases (HATs) that plays critical roles in multiple cellular processes. We report here that Tip60 undergoes autoacetylation at several lysine residues, including a key lysine residue (i.e. Lys-327) in the active site of the MYST domain. The mutation of K327 to arginine led to loss of both the autoacetylation activity and the cognate HAT activity. Interestingly, deacetylated Tip60 still kept a substantial degree of HAT activity. We also investigated the effect of cysteine 369 and glutamate 403 in Tip60 autoacetylation in order to understand the molecular pathway of the autoacetylation at K327. Together, we conclude that the acetylation of K327 which is located in the active site of Tip60 regulates but is not obligatory for the catalytic activity of Tip60. Since acetylation at this key residue appears to be evolutionarily conserved amongst all MYST proteins, our findings provide an interesting insight into the regulatory mechanism of MYST activities.

HIV-1 Tat impairs cell cycle control by targeting the Tip60, Plk1 and cyclin B1 ternary complex

HIV-1 Tat triggers intrinsic and extrinsic apoptosis pathways in both infected and uninfected cells and plays an important role in the pathogenesis of AIDS. Knocking down Tip60, an interactive protein of Tat, leads to the impairment of cell cycle progression, indicating a key role of Tip60 in cell cycle control. We found that Tip60 interacts with Plk1 through its ZnFMYST domain, and that this interaction is enhanced in the G 2/M phase. In addition, cyclin B1 was confirmed to interact with the ZnF domain of Tip60. Immunofluorescence imaging showed that Tip60 co-localizes with both Plk1 and cyclin B1 at the centrosome during the mitotic phase and to the mid-body during cytokinesis. Further experiments revealed that Tip60 forms a ternary complex with Plk1 and cyclin B1 and acetylates Plk1 but not cyclin B1. HIV-1 Tat likely forms a quaternary complex with Tip60, cyclin B1 and Plk1. Fluorescent microscopy showed that Tat causes an unscheduled nuclear translocation of both cyclin B1 and Plk1, causing their co-localization with Tip60 in the nucleus. Tat, Tip60, cyclin B1 and Plk1 interactions provide new a mechanistic explanation for Tat-mediated cell cycle dysregulation and apoptosis.

GPR50 interacts with TIP60 to modulate glucocorticoid receptor signalling

GPR50 is an orphan G-protein coupled receptor most closely related to the melatonin receptors. The physiological function of GPR50 remains unclear, although our previous studies implicate the receptor in energy homeostasis. Here, we reveal a role for GPR50 as a signalling partner and modulator of the transcriptional co-activator TIP60. This interaction was identified in a yeast-two-hybrid screen, and confirmed by co-immunoprecipitation and co-localisation of TIP60 and GPR50 in HEK293 cells. Co-expression with TIP60 increased perinuclear localisation of full length GPR50, and resulted in nuclear translocation of the cytoplasmic tail of the receptor, suggesting a functional interaction of the two proteins. We further demonstrate that GPR50 can enhance TIP60-coactiavtion of glucocorticoid receptor (GR) signalling. In line with in vitro results, repression of pituitary Pomc expression, and induction of gluconeogenic genes in liver in response to the GR agonist, dexamethasone was attenuated in Gpr50^−/− mice. These results identify a novel role for GPR50 in glucocorticoid receptor signalling through interaction with TIP60.

Microarray analysis uncovers a role for Tip60 in nervous system function and general metabolism

BACKGROUND

Tip60 is a key histone acetyltransferase (HAT) enzyme that plays a central role in diverse biological processes critical for general cell function; however, the chromatin-mediated cell-type specific developmental pathways that are dependent exclusively upon the HAT activity of Tip60 remain to be explored.

METHODS AND FINDINGS

Here, we investigate the role of Tip60 HAT activity in transcriptional control during multicellular development in vivo by examining genome-wide changes in gene expression in a Drosophila model system specifically depleted for endogenous dTip60 HAT function.

CONCLUSIONS

We show that amino acid residue E431 in the catalytic HAT domain of dTip60 is critical for the acetylation of endogenous histone H4 in our fly model in vivo, and demonstrate that dTip60 HAT activity is essential for multicellular development. Moreover, our results uncover a novel role for Tip60 HAT activity in controlling neuronal specific gene expression profiles essential for nervous system function as well as a central regulatory role for Tip60 HAT function in general metabolism.

Characterization of a newly identified ETV6-NTRK3 fusion transcript in acute myeloid leukemia

Background

Characterization of novel fusion genes in acute leukemia is important for gaining information about leukemia genesis. We describe the characterization of a new ETV6 fusion gene in acute myeloid leukemia (AML) FAB M0 as a result of an uncommon translocation involving chromosomes 12 and 15.

Methods

The ETV6 locus at 12p13 was shown to be translocated and to constitute the 5' end of the fusion product by ETV6 break apart fluorescence in situ hybridisation (FISH). To identify a fusion partner 3' rapid amplification of cDNA-ends with polymerase chain reaction (RACE PCR) was performed followed by cloning and sequencing.

Results

The NTRK3 gene on chromosome 15 was found to constitute the 3' end of the fusion gene and the underlying ETV6-NTRK3 rearrangement was verified by reverse transcriptase PCR. No RNA of the reciprocal NTRK3-ETV6 fusion gene could be detected.

Conclusion

We have characterized a novel ETV6-NTRK3 fusion transcript which has not been previously described in AML FAB M0 by FISH and RACE PCR. ETV6-NTRK3 rearrangements have been described in secretory breast carcinoma and congenital fibrosarcoma.

Histone acetyltransferase mediated regulation of FOXP3 acetylation and Treg function

Regulatory T cells (Tregs) are required for the maintenance of immune homeostasis as first clearly described by Herman Waldmann's laboratory. Dysfunction of Treg cells also leads to fatal autoimmunity in humans and mice. Conversely, the activation of different classes of Tregs operative systemically and within the cancer microenvironment can suppress host anti-tumor immune responses and promote tumor progression. Therefore, the development of new therapeutic approaches to regulate the activity of Treg cells may have considerable clinical potential. FOXP3 is the key transcriptional regulator of Treg development and function. The activity of FOXP3 is regulated by acetylation, a process catalyzed by distinct types of histone/protein acetyltransferases (HATs) that regulate the functions of many transcription factors, independently of FOXP3, as well as non-histone proteins, in addition to their effects on chromatin accessibility. Interactions between FOXP3 and these enzymes determine the suppressive function of FOXP3. Clearly, small molecules targeting these enzymes are candidates for the regulation of Treg function in vaccines and tumor therapies.

Screening and identification of differentially expressed genes from chickens infected with Newcastle disease virus by suppression subtractive hybridization

Newcastle disease is an important viral infectious disease caused by Newcastle disease virus (NDV), which leads to severe economic losses in the poultry industry worldwide. The molecular mechanisms involved in the pathogenesis of NDV and the host-directed antiviral responses remain poorly understood. In this study, we screened and identified the differentially expressed transcripts from chicken spleen 36 h post NDV infection using suppression subtractive hybridization (SSH). From the SSH library, we obtained 140 significant differentially expressed sequence tags (ESTs), which could be divided into three categories: high homology genes (58), high homology ESTs (62) and novel ESTs (20). The 58 high homology genes could be grouped into nine clusters based on the best known function of their protein products, which involved signalling transduction (HSPC166, PDE7B, GRIA4, GARNL1), transcriptional regulation (ANP32A, LOC423724, SATB1, QKI, ETV6), cellular molecular dynamics (MYLK, MYO7A, DCTN6), cytoskeleton (LAMA4, LAMC1, COL4A1), stress response (DNAJC15, CIRBP), immune response (TIA1, TOX, CMIP), metabolism (RPS15A, RPL32, GLUT8, CYPR21, DPYD, LOC417295), oxidation-reduction (TXN, MSRB3, GCLC), and others. In addition, we found that the 20 novel ESTs provide a clue for the discovery of some new genes associated with infection. In summary, our findings demonstrate previously unrecognized changes in gene transcription that are associated with NDV infection in vivo, and many differentially expressed genes identified in the study clearly merit further investigation. Our data provide new insights into better understanding the molecular mechanism of host-NDV interaction.

Regulation of sodium-calcium exchanger activity by creatine kinase under energy-compromised conditions

Na(+)/Ca(2+) exchanger (NCX) is one of the major mechanisms for removing Ca(2+) from the cytosol especially in cardiac myocytes and neurons, where their physiological activities are triggered by an influx of Ca(2+). NCX contains a large intracellular loop (NCXIL) that is responsible for regulating NCX activity. Recent evidence has shown that proteins, including kinases and phosphatases, associate with NCX1IL to form a NCX1 macromolecular complex. To search for the molecules that interact with NCX1IL and regulate NCX1 activity, we used the yeast two-hybrid method to screen a human heart cDNA library and found that the C-terminal region of sarcomeric mitochondrial creatine kinase (sMiCK) interacted with NCX1IL. Moreover, both sMiCK and the muscle-type creatine kinase (CKM) coimmunoprecipitated with NCX1 using lysates of cardiacmyocytes and HEK293T cells that transiently expressed NCX1 and various creatine kinases. Both sMiCK and CKM were able to produce a recovery in the decreased NCX1 activity that was lost under energy-compromised conditions. This regulation is mediated through a putative PKC phosphorylation site of sMiCK and CKM. The autophosphorylation and the catalytic activity of sMiCK and CKM are not required for their regulation of NCX1 activity. Our results suggest a novel mechanism for the regulation of NCX1 activity.

The adipogenic acetyltransferase Tip60 targets activation function 1 of peroxisome proliferator-activated receptor gamma

The transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma) plays a key role in the regulation of lipid and glucose metabolism in adipocytes, by regulating their differentiation, maintenance, and function. The transcriptional activity of PPARgamma is dictated by the set of proteins with which this nuclear receptor interacts under specific conditions. Here we identify the HIV-1 Tat-interacting protein 60 (Tip60) as a novel positive regulator of PPARgamma transcriptional activity. Using tandem mass spectrometry, we found that PPARgamma and the acetyltransferase Tip60 interact in cells, and through use of chimeric proteins, we established that coactivation by Tip60 critically depends on the N-terminal activation function 1 of PPARgamma, a domain involved in isotype-specific gene expression and adipogenesis. Chromatin immunoprecipitation experiments showed that the endogenous Tip60 protein is recruited to PPARgamma target genes in mature 3T3-L1 adipocytes but not in preadipocytes, indicating that Tip60 requires PPARgamma for its recruitment to PPARgamma target genes. Importantly, we show that in common with disruption of PPARgamma function, small interfering RNA-mediated reduction of Tip60 protein impairs differentiation of 3T3-L1 preadipocytes. Taken together, these findings qualify the acetyltransferase Tip60 as a novel adipogenic factor.

Proteomic identification of the MYST domain histone acetyltransferase TIP60 (HTATIP) as a co-activator of the myeloid transcription factor C/EBPalpha

The transcription factor C/EBPalpha (CEBPA) is a key player in granulopoiesis and leukemogenesis. We have previously reported the interaction of C/EBPalpha with other proteins (utilizing mass spectrometry) in transcriptional regulation. In the present study, we characterized the association of the MYST domain histone acetyltransferase Tat-interactive protein (TIP) 60 (HTATIP) with C/EBPalpha. We show in pull-down and co-precipitation experiments that C/EBPalpha and HTATIP interact. A chromatin immunoprecipitation (ChIP) and a confirmatory Re-ChIP assay revealed in vivo occupancy of the C/EBPalpha and GCSF-R promoter by HTATIP. Reporter gene assays showed that HTATIP is a co-activator of C/EBPalpha. The co-activator function of HTATIP is dependent on its intact histone acetyltransferase (HAT) domain and on the C/EBPalpha DNA-binding domain. The resulting balance between histone acetylation and deacetylation at the C/EBPalpha promoter might represent an important mechanism of C/EBPalpha action. We observed a lower expression of HTATIP mRNA in undifferentiated U937 cells compared to retinoic acid-induced differentiated U937 cells, and correlated expression of CEBPA and HTATIP mRNA levels were observed in leukemia samples. These findings point to a functional synergism between C/EBPalpha and HTATIP in myeloid differentiation and suggest that HTATIP might be an important player in leukemogenesis.

Tip60 modulates PLAGL2-mediated transactivation by acetylation

Pleiomorphic adenoma gene (PLAG) family proteins are oncogenes involved in various malignancies including lipoblastomas, hepatoblastomas, and acute myeloid leukemia. Overexpression of PLAGL2 induces cell transformation and proliferation, but little is known about how its activities are regulated. We previously showed that transcriptional activity of PLAGL2 is negatively regulated by sumoylation. Here we report that Tip60 modulates PLAGL2 functions through acetylation. Tip60 associates with PLAGL2 through its zinc finger domain and acetylates PLAGL2. Wild-type but not the histone acetyltransferase (HAT)-minus mutant form of Tip60 enhances PLAGL2-mediated transactivation. In addition, coexpression of Tip60 and PLAGL2 completely abolishes the sumoylation of PLAGL2. Both Tip60 and DN-Ubc9 increase transactivation activity of wild-type but not the sumoylation deficient form of PLAGL2 (K250, 269, 356R), indicating that Tip60 acetylates PLAGL2 and abolishes the sumoylation of PLAGL2 possibly through modification of the same lysine residues (K250, 269, 356) within PLAGL2. Tip60 effects vary between different PLAGL2 target gene promoters, suggesting that Tip60 is a novel promoter-specific coactivator of PLAGL2. This is the first demonstration that Tip60 can function as a sumoylation inhibitor in part through its intrinsic acetyltransferase activity to regulate specific gene expression.

Expression Analysis of cPLA2 Alpha Interacting TIP60 in Diabetic KKAy and Non-Diabetic C57BL Wild-Type Mice: No Impact of Transient and Stable TIP60 Overexpression on Glucose-Stimulated Insulin Secretion in Pancreatic Beta-Cells

In the present study we investigate the expression levels of cytosolic phospholipase A2 alpha (cPLA2alpha) interacting histone acetyl transferase proteins TIP60alpha and TIP60beta in non-diabetic C57BL wild-type mice and obese type 2 diabetic KKAy model mice. The aim was to test our hypothesis that TIP60 plays a regulatory role in glucose-stimulated insulin secretion from pancreatic beta-cells.

MATERIAL AND METHODS

Ten obese diabetic KKAy mice and ten non-diabetic C57BL mice were fed a standard chow diet. After nine weeks, islet RNA was purified and used to measure TIP60 expression. We investigated the effect of TIP60alpha and TIP60beta on glucose-stimulated insulin secretion by transient and stable overexpression in the pancreatic mouse beta-cell line MIN6 and the rat beta-cell line INS-1E.

RESULTS

We found that non-diabetic C57BL mice and diabetic KKAy mice have the same level of both the alpha and beta splice forms of TIP60. Furthermore, we demonstrated that transient and stable expression of TIP60 in INS-1E cells affects neither glucose-stimulated insulin secretion, insulin output nor cell insulin content. Also susceptibility to developing gluco-toxicity was unaffected.

CONCLUSION

TIP60 over-expression does not affect glucose stimulated insulin secretion, insulin content or abnormal beta-cell function during glucotoxicity.

The interaction of ETV6 (TEL) and TIP60 requires a functional histone acetyltransferase domain in TIP60

The ets-family transcription factor ETV6 (TEL) has been shown to be the target of a large number of balanced chromosomal translocations in various hematological malignancies and in some soft tissue tumors. Furthermore, ETV6 is essential for hematopoietic stem cell function. We identified ETV6 interacting proteins using the yeast two hybrid system. One of these proteins is the HIV Tat interacting protein (TIP60), a histone acetyltransferase (HAT) containing the highly conserved MYST domain. TIP60 functions as a corepressor of ETV6 in reporter gene assays. Fluorescently tagged ETV6 and TIP60 colocalize in the nucleus and an increase in nuclear localization of ETV6 was seen when TIP60 was cotransfected. ETV6 interacts with TIP60 through a 63 amino acids region located in the central domain of ETV6 between the pointed and the ets domain. The ETV6 interacting region of TIP60 mapped to the C2HC zinc finger of the TIP60 MYST domain. The interaction of TIP60 with full length ETV6 required an intact acetyltransferase domain of TIP60. Interestingly, the MYST domains of MOZ and MORF were also able to interact with portions of ETV6. These observations suggest that MYST domain HATs regulate ETV6 transcriptional activity and may therefore play critical roles in leukemogenesis and possibly in normal hematopoietic development.

Tip60 protein isoforms and altered function in skin and tumors that overexpress ornithine decarboxylase

Elevated expression of ornithine decarboxylase (ODC) and increased synthesis of polyamines are hallmarks of epithelial tumorigenesis. The skin and tumors of K6/ODC and ODC/Ras transgenic mice, in which overexpression of ODC has been targeted to hair follicles, were found to exhibit intrinsically high histone acetyltransferase (HAT) activity. We identified Tip60 as a candidate enzyme for contributing significantly to this abnormally high HAT activity. Compared with normal littermate controls, the levels of Tip60 protein and an alternative splice variant Tip53 were found to be greater in K6/ODC mouse skin. Furthermore, skin tumors that spontaneously develop in ODC/Ras bigenic mice typically have substantially more Tip60 protein than adjacent non-tumor-bearing skin and exhibit a unique pattern of Tip60 size variants and chemically modified protein isoforms. Steady-state Tip60 and Tip53 mRNA levels were not affected in ODC-overexpressing skin and tumors, implying novel posttranscriptional regulation by polyamines. Given the diverse roles of Tip60, the overabundance of Tip60 protein is predicted to have biological consequences. Compared with normal littermate skin, we detected altered association of Tip60 with E2F1 and a subset of newly identified Tip60-interacting transcription factors in ODC transgenic mouse skin and tumors. E2F1 was shown to be bound in greater amounts to up-regulated target genes in ODC-overexpressing skin. Thus, up-regulation of Tip60 protein, influencing the expression of Tip60-regulated genes, could play a contributing role in polyamine-mediated tumor promotion. (

Phosphorylation of Tat-interactive protein 60 kDa by protein kinase C epsilon is important for its subcellular localisation

Tat-interactive protein 60 kDa is a nuclear acetyltransferase that both coactivates and corepresses transcription factors and has a definitive function in the DNA damage response. Here, we provide evidence that Tat-interactive protein 60 kDa is phosphorylated by protein kinase C epsilon. In vitro, protein kinase C epsilon phosphorylates Tat-interactive protein 60 kDa on at least two sites within the acetyltransferase domain. In whole cells, activation of protein kinase C increases the levels of phosphorylated Tat-interactive protein 60 kDa and the interaction of Tat-interactive protein 60 kDa with protein kinase C epsilon. A phosphomimetic mutant Tat-interactive protein 60 kDa has distinct subcellular localisation compared to the wild-type protein in whole cells. Taken together, these findings suggest that the protein kinase C epsilon phosphorylation sites on Tat-interactive protein 60 kDa are important for its subcellular localisation. Regulation of the subcellular localisation of Tat-interactive protein 60 kDa via phosphorylation provides a novel means of controlling Tat-interactive protein 60 kDa function.

Tip60 histone acetyltransferase acts as a negative regulator of Notch1 signaling by means of acetylation

The Notch signaling pathway appears to perform an important function in a wide variety of organisms and cell types. In our present study, we provide evidence that UV irradiation-induced Tip60 proteins reduced Notch1 activity to a marked degree. Accumulated UV irradiation-induced Tip60 suppresses Notch1 transcriptional activity via the dissociation of the Notch1-IC-CSL complex. The binding between endogenous Tip60 and Notch1-IC in UV radiation-exposed cells was verified in this study by coimmunoprecipitation. Interestingly, the physical interaction of Tip60 with Notch1-IC occurs to a more profound degree in the presence of CSL but does not exist in a trimeric complex. Using Notch1-IC and Tip60 deletion mutants, we also determined that the N terminus, which harbors the RAM domain and seven ankyrin repeats of Notch1-IC, interacts with the zinc finger and acetyl coenzyme A domains of Tip60. Furthermore, here we report that Notch1-IC is a direct target of the acetyltransferase activity of Tip60. Collectively, our data suggest that Tip60 is an inhibitor of the Notch1 signaling pathway and that Tip60-dependent acetylation of Notch1-IC may be relevant to the mechanism by which Tip60 suppresses Notch1 signaling.

Stability of Nmi protein is controlled by its association with Tip60

Tip60 exerts diverse biological functions through mechanisms that are either dependent or independent on its intrinsic histone acetyltransferase activity. In the present study, we identified Nmi (N-Myc and STATs Interactor) as a novel binding partner for Tip60 by a yeast two-hybrid screen. The association of Tip60 with Nmi was further confirmed by coimmunoprecipitation in mammalian cells. The zinc finger domain of Tip60 interacts with the NID repeats of Nmi, a region essential for the cytoplamic localization and homo- and heterodimerization of Nmi. We further showed that Nmi is an unstable protein and is targeted for proteasome-mediated degradation. The stability of Nmi can be enhanced by its association with Tip60, a process that is dependent on histone acetyltransferase activity of Tip60. The stabilization of Nmi by Tip60 is in part mediated by the translocation of Tip60 into cytoplasm to form distinct large cytoplasmic speckles. Our finding that Tip60 stabilizes Nmi through the formation of distinct cytoplasmic speckles provides a new mechanism to modulate Nmi-mediated functions.

The cloning and characterization of the histone acetyltransferase human homolog Dmel\TIP60 in Drosophila melanogaster: Dmel\TIP60 is essential for multicellular development

Chromatin packaging directly influences gene programming as it permits only certain portions of the genome to be activated in any given developmental stage, cell, and tissue type. Histone acetyltransferases (HATs) are a key class of chromatin regulatory proteins that mediate such developmental chromatin control; however, their specific roles during multicellular development remain unclear. Here, we report the first isolation and developmental characterization of a Drosophila HAT gene (Dmel\TIP60) that is the homolog of the human HAT gene TIP60. We show that Dmel\TIP60 is differentially expressed during Drosophila development, with transcript levels significantly peaking during embryogenesis. We further demonstrate that reducing endogenous Dmel\TIP60 expression in Drosophila embryonic cells by RNAi results in cellular defects and lethality. Finally, using a GAL4-targeted RNAi system in Drosophila, we show that ubiquitous or mesoderm/muscle-specific reduction of Dmel\TIP60 expression results in lethality during fly development. Our results suggest a mechanism for HAT regulation involving developmental control of HAT expression profiles and show that Dmel\TIP60 is essential for multicellular development. Significantly, our inducible and targeted HAT knockdown system in Drosophila now provides a powerful tool for effectively studying the roles of TIP60 in specific tissues and cell types during development.

Identification of transcripts modulated by ETV6 expression

Deletions at chromosome 12p12-13 are observed in 26-47% of childhood pre-B acute lymphoblastic leukaemia (ALL) cases, suggesting the presence of a tumour suppressor gene (TSG). Accumulating genetic and functional evidence points to ETV6 as being the most probable TSG targeted by the deletions. ETV6 is a ubiquitously expressed transcription factor of the ETS family with very few known targets. To understand its function and to elucidate the impact of its absence in leukaemia, we conducted a study to identify targeted genes. Following the induction of ETV6 expression, global expression was evaluated at different time points. We identified 87 modulated genes, of which 10 (AKR1C1, AKR1C3, IL18, LUM, PHLDA1, PTGER4, PTGS2, SPHK1, TP53 and VEGF) were validated by real-time quantitative reverse transcription-polymerase chain reaction. To assess the significance of the validated candidate genes in leukaemia, their expression patterns were determined, as well as that of ETV6, in pre-B ALL patients. The expression of IL18, LUM, PTGER4, SPHK1 and TP53 was significantly correlated with that of ETV6, further suggesting that ETV6 could regulate the expression of these genes in leukaemia. This work constitutes another step towards the understanding of the functions of ETV6 and the impact of its inactivation in childhood leukaemia.

Analysis of transcriptional modulation of the presenilin 1 gene promoter by ZNF237, a candidate binding partner of the Ets transcription factor ERM

DNA sequences required for the expression of the human presenilin 1 (PS1) gene have been identified between -118 and +178 flanking the major initiation site (+1) mapped in SK-N-SH cells. Several Ets sites are located both upstream as well as downstream from the +1 site, including an Ets motif present at -10 that controls 90% of transcription in SK-N-SH cells. However, in SH-SY5Y cells, transcription initiates further downstream and requires an alternative set of promoter elements including a +90 Ets motif. Ets2, ER81, ERM and Elk1 were identified by yeast one-hybrid selection in a human brain cDNA library using the -10 Ets motif as a bait. We have shown that ERM recognizes specifically Ets motifs on the PS1 promoter located at -10 as well as downstream at +90, +129 and +165 and activates PS1 transcription with promoter fragments whether or not they contain the -10 Ets site. We have now searched for ERM interacting proteins by yeast two-hybrid selection in a human brain cDNA library using the C-terminal 415 amino acid of ERM as a bait. One of the interacting proteins was ZNF237, a member of the MYM gene family. It is widely expressed in different tissues in eukaryotes under several forms derived by alternative splicing, including a large 382 amino acid form containing a single MYM domain, and 2 shorter forms of 208 and 213 amino acids respectively that do not. We show that both the 382 as well as the 208 amino acid forms are expressed in SK-N-SH cells but not in SH-SY5Y cells. Both forms interact with ERM and repress the transcription of PS1 in SH-SY5Y cells. The effect of both C-terminal and N-terminal deletions indicates that the N-terminal 120 amino acid region is required for interaction with ERM in yeast, and furthermore single amino acid mutations show that residues 112 and 114 play an important role. The repression of transcription in SH-SY5Y cells also appears to require the N-terminal potion of ZNF237 and was affected by mutation of the amino acid 112. Data from electrophoretic mobility shift assays indicate that ERM and possibly ZNF237 interact with a fragment of the PS1 promoter.

Tip60 in DNA damage response and growth control: many tricks in one HAT

The Tip60 histone acetyltransferase is part of an evolutionarily conserved multisubunit complex, NuA4, which is recruited by many transcription factors to their target promoters, where it is thought to participate in histone acetylation and transcriptional activation. These transcription factors include tumor promoters and also tumor suppressors, such as p53, which links Tip60 to DNA damage responses. Tip60 also has transcription-independent roles in DNA damage responses. First, independently from NuA4, Tip60 binds the kinases ataxia-telangiectasia mutated (ATM) and DNA-dependent protein kinase catalytic subunit (DNA-PKcs) and participates in their activation by DNA double-strand breaks. Second, NuA4 is recruited to the chromatin surrounding the breaks and, through a series of chromatin modifications, contributes to the dynamics of DNA repair. These molecular activities might endow Tip60 with multiple and potentially antagonistic biological functions.

Cellular functions of TIP60

TIP60 was originally identified as a cellular acetyltransferase protein that interacts with HIV-1 Tat. As a consequence, the role of TIP60 in transcriptional regulation has been investigated intensively. Recent data suggest that TIP60 has more divergent functions than originally thought and roles for TIP60 in many processes, such as cellular signalling, DNA damage repair, cell cycle and checkpoint control and apoptosis are emerging. TIP60 is a tightly regulated transcriptional coregulator, acting in a large multiprotein complex for a range of transcription factors including androgen receptor, Myc, STAT3, NF-kappaB, E2F1 and p53. This usually involves recruitment of TIP60 acetyltransferase activities to chromatin. Additionally, in response to DNA double strand breaks, TIP60 is recruited to DNA lesions where it participates both in the initial as well as the final stages of repair. Here, we describe how TIP60 is a multifunctional enzyme involved in multiple nuclear transactions.

ETV6: a versatile player in leukemogenesis

Alterations of the ets family transcription factor ETV6 (TEL) and the RUNT domain transcription factor RUNX1 (AML1) play pivotal roles in the leukemogenesis of various types of leukemia. While only three fusion partners of RUNX1 namely ETO, ETV6 and MTG16 have been described so far, there is a plethora of ETV6 fusion partners with about 20 partners described so far. Apart from forming fusion genes there are other genetic alterations of ETV6 including deletions, point mutations and possible alterations at the promoter level that might contribute to the malignant phenotype. This review will focus on ETV6 and on the different mechanisms that are used by this gene to cause leukemia.

Histone acetyltransferases and deacetylase in Entamoeba histolytica

In our efforts to understand how transcription may be regulated in Entamoeba histolytica, we have examined if this parasite has conserved enzymatic mechanisms for targeted acetylation and deacetylation of histones. Western blotting indicated that basic nuclear proteins in the size range of 16-23 kDa were acetylated in amebic trophozoites, suggesting histone acetylation. Single representatives of the GNAT and MYST family of histone acetyltransferases (HATs) were identified in the E. histolytica genome and their expression in amebic trophozoites was detected by reverse transcription of RNA followed by the polymerase chain reaction (RT-PCR). Full-length recombinant EhMYST protein demonstrated HAT activity with calf thymus histones and showed a preference for histone H4, similar to the yeast MYST protein, Esa1. However, ehMYST did not complement a yeast esa1 mutation. Histone deacetylase (HDAC) activity was detected in nuclear extracts from E. histolytica, and characteristically, was inhibited by trichostatin A (TSA). Consistent with the observation of HDAC activity, RT-PCR analysis demonstrated that an amebic hdac1 homolog (ehHDAC) is expressed and appropriately spliced in E. histolytica trophozoites. Our results suggest that mechanisms for histone acetylation and deacetylation are operational in E. histolytica.

Directing oncogenic fusion genes into stem cells via an SCL enhancer

TEL-TRKC is a fusion gene generated by chromosomal translocation and encodes an activated tyrosine kinase. Uniquely, it is found in both solid tumors and leukemia. However, a single exon difference (in TEL) in TEL-TRKC fusions is associated with the two sets of cancer phenotypes. We expressed the two TEL-TRKC variants in vivo by using the 3' regulatory element of SCL that is selectively active in a subset of mesodermal cell lineages, including endothelial and hematopoietic stem cells and progenitors. The leukemia form of TEL-TRKC (-exon 5 of TEL) enhanced hematopoietic stem cell renewal and initiated leukemia. In contrast, the TEL-TRKC solid tumor variant (+ TEL exon 5) elicited an embryonic lethal phenotype with impairment of both angiogenesis and hematopoiesis indicative of an effect at the level of the hemangioblasts. The ability of TEL-TRKC to repress expression of Flk1, a critical regulator of early endothelial and hematopoietic cells, depended on TEL exon 5. These data indicate that related oncogenic fusion proteins similarly expressed in a hierarchy of early stem cells can have selective, cell type-specific developmental impacts.

Nizp1, a novel multitype zinc finger protein that interacts with the NSD1 histone lysine methyltransferase through a unique C2HR motif

Haploinsufficiency of the NSD1 gene is a hallmark of Sotos syndrome, and rearrangements of this gene by translocation can cause acute myeloid leukemia. The NSD1 gene product is a SET-domain histone lysine methyltransferase that has previously been shown to interact with nuclear receptors. We describe here a novel NSD1-interacting protein, Nizp1, that contains a SCAN box, a KRAB-A domain, and four consensus C2H2-type zinc fingers preceded by a unique finger derivative, referred to herein as the C2HR motif. The C2HR motif functions to mediate protein-protein interaction with the cysteine-rich (C5HCH) domain of NSD1 in a Zn(II)-dependent fashion, and when tethered to RNA polymerase II promoters, represses transcription in an NSD1-dependent manner. Mutations of the cysteine or histidine residues in the C2HR motif abolish the interaction of Nizp1 with NSD1 and compromise the ability of Nizp1 to repress transcription. Interestingly, converting the C2HR motif into a canonical C2H2 zinc finger has a similar effect. Thus, Nizp1 contains a novel type of zinc finger motif that functions as a docking site for NSD1 and is more than just a degenerate evolutionary remnant of a C2H2 motif.

The diverse superfamily of lysine acetyltransferases and their roles in leukemia and other diseases

Acetylation of the epsilon-amino group of lysine residues, or N(epsilon)-lysine acetylation, is an important post-translational modification known to occur in histones, transcription factors and other proteins. Since 1995, dozens of proteins have been discovered to possess intrinsic lysine acetyltransferase activity. Although most of these enzymes were first identified as histone acetyltransferases and then tested for activities towards other proteins, acetyltransferases only modifying non-histone proteins have also been identified. Lysine acetyltransferases form different groups, three of which are Gcn5/PCAF, p300/CBP and MYST proteins. While members of the former two groups mainly function as transcriptional co-activators, emerging evidence suggests that MYST proteins, such as Esa1, Sas2, MOF, TIP60, MOZ and MORF, have diverse roles in various nuclear processes. Aberrant lysine acetylation has been implicated in oncogenesis. The genes for p300, CBP, MOZ and MORF are rearranged in recurrent leukemia-associated chromosomal abnormalities. Consistent with their roles in leukemogenesis, these acetyltransferases interact with Runx1 (or AML1), one of the most frequent targets of chromosomal translocations in leukemia. Therefore, the diverse superfamily of lysine acetyltransferases executes an acetylation program that is important for different cellular processes and perturbation of such a program may cause the development of cancer and other diseases.

Control of human PIRH2 protein stability: involvement of TIP60 and the proteosome

Murine PIRH2 (mPIRH2) was recently identified as a RING finger-containing ubiquitin-protein isopeptide ligase that interacts with both p53 and the human androgen receptor. mpirh2 is a p53-responsive gene that is up-regulated by UV, and mPIRH2 protein has the capacity to polyubiquitylate p53, perhaps leading to p53 destruction. mpirh2 therefore has properties similar to those of the oncogene mdm2. Here, we have identified human PIRH2 (hPIRH2) as a TIP60-interacting protein. To investigate its regulation, we characterized hPIRH2 in parallel with hPIRH2 variants possessing mutations of conserved RING finger residues. We observed that wild-type hPIRH2 is an unstable protein with a short half-life and is a target for RING domain-dependent proteasomal degradation. Accordingly, we found that hPIRH2 was ubiquitylated in cells. The TIP60-hPIRH2 association appeared to regulate hPIRH2 stability; coexpression of TIP60 enhanced hPIRH2 protein stability and altered hPIRH2 subcellular localization. These results suggest that hPIRH2 activities can be controlled, at the post-translational level, in multiple ways.

v-SRC specifically regulates the nucleo-cytoplasmic delocalization of the major isoform of TEL (ETV6)

TEL is a frequent target of chromosomal translocations in human cancer and an alleged tumor suppressor gene. TEL encodes two isoforms: a major TEL-M1 isoform as well as TEL-M43, which lacks the first 42 amino acid residues of TEL-M1. Both isoforms are potent transcriptional repressors that can inhibit RAS-induced transformation. Here we show that the v-SRC protein-tyrosine kinase relieves the repressive activity of TEL-M1, an activity that is associated with the v-SRC-induced delocalization of TEL-M1 from the nucleus to the cytoplasm. TEL-M1 delocalization requires the kinase activity of v-SRC and is not induced by oncogenic RAS or AKT. Cytoplasmic delocalization of TEL-M1 in response to v-SRC critically depends upon its unique amino-terminal domain (SRCD domain) because (i). v-SRC did not inhibit the repressive properties of TEL-M43, nor affected TEL-M43 nuclear localization; (ii). fusion of the first 52 amino acid residues of TEL-M1 to FLI-1, an ETS protein insensitive to v-SRC-induced delocalization, is sufficient to confer v-SRC-induced delocalization to this TEL/FLI-1 chimeric protein. The v-SRC-induced nucleo-cytoplasmic delocalization of TEL-M1 does not involve phosphorylation of the SRCD and does not require TEL self-association and repressive domains. Finally, enforced expression of the v-SRC-insensitive TEL-M43, but not of TEL-M1, inhibits v-SRC-induced transformation of NIH3T3 fibroblasts. These results identify a regulatory domain in TEL that specifically impinges on the subcellular localization of its major TEL-M1 isoform. They, furthermore, indicate that inhibition of TEL-M1 nuclear function is required for v-SRC to induce cellular transformation.