Identification of an alternatively spliced form of the Tat interactive protein (Tip60), Tip60(beta)

Post-translational modifications of epigenetic modifier TIP60: their role in cellular functions and cancer

TIP60 is a crucial lysine acetyltransferase protein that catalyzes the acetylation of histone and non-histone proteins. This enzyme plays a crucial role in maintaining genomic integrity, by participating in DNA damage repair, ensuring accurate chromosomal segregation, and regulating a myriad of cellular processes such as apoptosis, autophagy, and wound-induced cell migration. One of the primary mechanisms through which TIP60 executes these diverse cellular functions is via post-translational modifications (PTMs). Over the years, extensive studies have demonstrated the importance of PTMs in controlling protein functions. This review aims to summarize the findings on PTMs occurring on the TIP60 protein and their functional implications. We also discuss previously uncharacterized PTM sites identified on TIP60 and examine their relationship with cancer-associated mutations, with a particular focus on residues potentially modified by various PTMs, to understand the cause of deregulation of TIP60 in various cancers.

Evaluating the Cellular Roles of the Lysine Acetyltransferase Tip60 in Cancer: A Multi-Action Molecular Target for Precision Oncology

Precision (individualized) medicine relies on the molecular profiling of tumors' dysregulated characteristics (genomic, epigenetic, transcriptomic) to identify the reliance on key pathways (including genome stability and epigenetic gene regulation) for viability or growth, and then utilises targeted therapeutics to disrupt these survival-dependent pathways. Non-mutational epigenetic changes alter cells' transcriptional profile and are a key feature found in many tumors. In contrast to genetic mutations, epigenetic changes are reversable, and restoring a normal epigenetic profile can inhibit tumor growth and progression. Lysine acetyltransferases (KATs or HATs) protect genome stability and integrity, and Tip60 is an essential acetyltransferase due to its roles as an epigenetic and transcriptional regulator, and as master regulator of the DNA double-strand break response. Tip60 is commonly downregulated and mislocalized in many cancers, and the roles that mislocalized Tip60 plays in cancer are not well understood. Here we categorize and discuss Tip60-regulated genes, evaluate Tip60-interacting proteins based on cellular localization, and explore the therapeutic potential of Tip60-targeting compounds as epigenetic inhibitors. Understanding the multiple roles Tip60 plays in tumorigenesis will improve our understanding of tumor progression and will inform therapeutic options, including informing potential combinatorial regimes with current chemotherapeutics, leading to improvements in patient outcomes.

Acetylation of the NS3 helicase by KAT5γ is essential for flavivirus replication

Direct targeting of essential viral enzymes such as proteases, polymerases, and helicases has long been the major focus of antiviral drug design. Although successful for some viral enzymes, targeting viral helicases is notoriously difficult to achieve, demanding alternative strategies. Here, we show that the NS3 helicase of Zika virus (ZIKV) undergoes acetylation in its RNA-binding tunnel. Regulation of the acetylated state of K389 in ZIKV NS3 modulates RNA binding and unwinding and is required for efficient viral replication. NS3 acetylation is mediated by a specific isoform of the host acetyltransferase KAT5 (KAT5γ), which translocates from the nucleus to viral replication complexes upon infection. NS3 acetylation by KAT5γ and its proviral role are also conserved in West Nile virus (WNV), dengue virus (DENV), and yellow fever virus (YFV). Our study provides molecular insight into how a cellular acetyltransferase regulates viral helicase functions, unveiling a previously unknown target for antiviral drug development.

PRMT1 and PRMT5: on the road of homologous recombination and non-homologous end joining

DNA double-strand breaks (DSBs) are widely accepted to be the most deleterious form of DNA lesions that pose a severe threat to genome integrity. Two predominant pathways are responsible for repair of DSBs, homologous recombination (HR) and non-homologous end-joining (NHEJ). HR relies on a template to faithfully repair breaks, while NHEJ is a template-independent and error-prone repair mechanism. Multiple layers of regulation have been documented to dictate the balance between HR and NHEJ, such as cell cycle and post-translational modifications (PTMs). Arginine methylation is one of the most common PTMs, which is catalyzed by protein arginine methyltransferases (PRMTs). PRMT1 and PRMT5 are the predominate PRMTs that promote asymmetric dimethylarginine and symmetric dimethylarginine, respectively. They have emerged to be crucial regulators of DNA damage repair. In this review, we summarize current understanding and unaddressed questions of PRMT1 and PRMT5 in regulation of HR and NHEJ, providing insights into their roles in DSB repair pathway choice and the potential of targeting them for cancer therapy.

KAT5 histone acetyltransferase mutations in cancer cells

Cancer cells are characterized by accumulation of mutations due to improperly repaired DNA damage. The DNA double strand break is one of the most severe form of damage and several redundant mechanisms have evolved to facilitate accurate repair. During DNA replication and in mitosis, breaks are primarily repaired by homologous recombination which is facilitated by several genes. Key to this process is the breast cancer susceptibility genes BRCA1 and BRCA2 as well as the accessory RAD52 gene. Proper chromatin remodeling is also essential for repair and the KAT5 histone acetyltransferase facilitates histone removal at the break. Here we undertook a pan cancer analysis to investigate mutations within the KAT5 gene in cancer cells. We employed two standard artificial algorithms to classify mutations as either driver (CHASMPlus algorithm) or pathogenic (VEST4 algorithm). We find that most predicted driver and disease-causing mutations occur in the catalytic site or within key regulatory domains. In silico analysis of protein structure using AlphaFold shows that these mutations are likely to destabilize the function of KAT5 or interactions with DNA or its other partners. The data presented here, although preliminary, could be used to inform clinical strategies.

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.

PRMT5 Regulates DNA Repair by Controlling the Alternative Splicing of Histone-Modifying Enzymes

Protein arginine methyltransferase 5 (PRMT5) is overexpressed in many cancer types and is a promising therapeutic target for several of them, including leukemia and lymphoma. However, we and others have reported that PRMT5 is essential for normal physiology. This dependence may become dose limiting in a therapeutic setting, warranting the search for combinatorial approaches. Here, we report that PRMT5 depletion or inhibition impairs homologous recombination (HR) DNA repair, leading to DNA-damage accumulation, p53 activation, cell-cycle arrest, and cell death. PRMT5 symmetrically dimethylates histone and non-histone substrates, including several components of the RNA splicing machinery. We find that PRMT5 depletion or inhibition induces aberrant splicing of the multifunctional histone-modifying and DNA-repair factor TIP60/KAT5, which selectively affects its lysine acetyltransferase activity and leads to impaired HR. As HR deficiency sensitizes cells to PARP inhibitors, we demonstrate here that PRMT5 and PARP inhibitors have synergistic effects on acute myeloid leukemia cells.

TIP55, a splice isoform of the KAT5 acetyltransferase, is essential for developmental gene regulation and organogenesis

Regulation of chromatin structure is critical for cell type-specific gene expression. Many chromatin regulatory complexes exist in several different forms, due to alternative splicing and differential incorporation of accessory subunits. However, in vivo studies often utilize mutations that eliminate multiple forms of complexes, preventing assessment of the specific roles of each. Here we examined the developmental roles of the TIP55 isoform of the KAT5 histone acetyltransferase. In contrast to the pre-implantation lethal phenotype of mice lacking all four Kat5 transcripts, mice specifically deficient for Tip55 die around embryonic day 11.5 (E11.5). Prior to developmental arrest, defects in heart and neural tube were evident in Tip55 mutant embryos. Specification of cardiac and neural cell fates appeared normal in Tip55 mutants. However, cell division and survival were impaired in heart and neural tube, respectively, revealing a role for TIP55 in cellular proliferation. Consistent with these findings, transcriptome profiling revealed perturbations in genes that function in multiple cell types and developmental pathways. These findings show that Tip55 is dispensable for the pre- and early post-implantation roles of Kat5, but is essential during organogenesis. Our results raise the possibility that isoform-specific functions of other chromatin regulatory proteins may play important roles in development.

Modulation of Nuclear Receptor Function by Chromatin Modifying Factor TIP60

Nuclear receptors (NRs) are transcription factors that bind to specific DNA sequences known as hormone response elements located upstream of their target genes. Transcriptional activity of NRs can be modulated by binding of the compatible ligand and transient interaction with cellular coregulators, functioning either as coactivators or as corepressors. Many coactivator proteins possess intrinsic histone acetyltransferase (HAT) activity that catalyzes the acetylation of specific lysine residues in histone tails and loosens the histone-DNA interaction, thereby facilitating access of transcriptional factors to the regulatory sequences of the DNA. Tat interactive protein 60 (TIP60), a member of the Mof-Ybf2-Sas2-TIP60 family of HAT protein, is a multifunctional coregulator that controls a number of physiological processes including apoptosis, DNA damage repair, and transcriptional regulation. Over the last two decades or so, TIP60 has been extensively studied for its role as NR coregulator, controlling various aspect of steroid receptor functions. The aim of this review is to summarize the findings on the role of TIP60 as a coregulator for different classes of NRs and its overall functional implications. We also discuss the latest studies linking TIP60 to NR-associated metabolic disorders and cancers for its potential use as a therapeutic drug target in future.

MOZ and MORF acetyltransferases: Molecular interaction, animal development and human disease

Lysine residues are subject to many forms of covalent modification and one such modification is acetylation of the ε-amino group. Initially identified on histone proteins in the 1960s, lysine acetylation is now considered as an important form of post-translational modification that rivals phosphorylation. However, only about a dozen of human lysine acetyltransferases have been identified. Among them are MOZ (monocytic leukemia zinc finger protein; a.k.a. MYST3 and KAT6A) and its paralog MORF (a.k.a. MYST4 and KAT6B). Although there is a distantly related protein in Drosophila and sea urchin, these two enzymes are vertebrate-specific. They form tetrameric complexes with BRPF1 (bromodomain- and PHD finger-containing protein 1) and two small non-catalytic subunits. These two acetyltransferases and BRPF1 play key roles in various developmental processes; for example, they are important for development of hematopoietic and neural stem cells. The human KAT6A and KAT6B genes are recurrently mutated in leukemia, non-hematologic malignancies, and multiple developmental disorders displaying intellectual disability and various other abnormalities. In addition, the BRPF1 gene is mutated in childhood leukemia and adult medulloblastoma. Therefore, these two acetyltransferases and their partner BRPF1 are important in animal development and human disease.

A novel snRNA-like transcript affects amyloidogenesis and cell cycle progression through perturbation of Fe65L1 (APBB2) alternative splicing

FE65 proteins constitute a family of adaptors which modulates the processing of amyloid precursor protein and the consequent amyloid β production. Thus, they have been involved in the complex and partially unknown cascade of reactions at the base of Alzheimer's disease etiology. However, FE65 and FE65-like proteins may be linked to neurodegeneration through the regulation of cell cycle in post-mitotic neurons. In this work we disclose novel molecular mechanisms by which APBB2 can modulate APP processing. We show that APBB2 mRNA splicing, driven by the over-expression of a novel non-coding RNA named 45A, allow the generation of alternative protein forms endowed with differential effects on Aβ production, cell cycle control, and DNA damage response. 45A overexpression also favors cell transformation and tumorigenesis leading to a marked increase of malignancy of neuroblastoma cells. Therefore, our results highlight a novel regulatory pathway of considerable interest linking APP processing with cell cycle regulation and DNA-surveillance systems, that may represent a molecular mechanism to induce neurodegeneration in post-mitotic neurons.

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.

Phosphorylation of Tip60 by GSK-3 determines the induction of PUMA and apoptosis by p53

Activation of p53 by DNA damage results in either cell-cycle arrest, allowing DNA repair and cell survival, or induction of apoptosis. As these opposite outcomes are both mediated by p53 stabilization, additional mechanisms to determine this decision must exist. Here, we show that glycogen synthase kinase-3 (GSK-3) is required for the p53-mediated induction of the proapoptotic BH3 only-protein PUMA, an essential mediator of p53-induced apoptosis. Inhibition of GSK-3 protected from cell death induced by DNA damage and promoted increased long-term cell survival. We demonstrate that GSK-3 phosphorylates serine 86 of the p53-acetyltransferase Tip60. A Tip60(S86A) mutant was less active to induce p53 K120 acetylation, histone 4 acetylation, and expression of PUMA. Our data suggest that GSK-3 mediated Tip60S86 phosphorylation provides a link between PI3K signaling and the choice for or against apoptosis induction by p53.

The tale of protein lysine acetylation in the cytoplasm

Reversible posttranslational modification of internal lysines in many cellular or viral proteins is now emerging as part of critical signalling processes controlling a variety of cellular functions beyond chromatin and transcription. This paper aims at demonstrating the role of lysine acetylation in the cytoplasm driving and coordinating key events such as cytoskeleton dynamics, intracellular trafficking, vesicle fusion, metabolism, and stress response.

Nuclear receptor coactivators: structural and functional biochemistry

Transcription of eukaryotic cell is a multistep process tightly controlled by concerted action of macromolecules. Nuclear receptors are ligand-activated sequence-specific transcription factors that bind DNA and activate (or repress) transcription of specific sets of nuclear target genes. Successful activation of transcription by nuclear receptors and most other transcription factors requires "coregulators" of transcription. Coregulators make up a diverse family of proteins that physically interact with and modulate the activity of transcription factors and other components of the gene expression machinery via multiple biochemical mechanisms. The coregulators include coactivators that accomplish reactions required for activation of transcription and corepressors that suppress transcription. This review summarizes our current knowledge of nuclear receptor coactivators with an emphasis on their biochemical mechanisms of action and means of regulation.

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.

Involvement of TIP60 acetyltransferase in intracellular Salmonella replication

BACKGROUND

Salmonella enterica is a facultative intracellular pathogen that replicates within a membrane-bound compartment termed Salmonella containing vacuole (SCV). The biogenesis of SCV requires Salmonella type III protein secretion/translocation system and their effector proteins which are translocated into host cells to exploit the vesicle trafficking pathways. SseF is one of these effectors required for SCV formation and Intracellular Salmonella replication through unknown mechanisms.

RESULTS

In an attempt to identify host proteins that interact with SseF, we conduct a yeast two-hybrid screening of human cell cDNA library using SseF as the bait. We identified that TIP60, an acetyltransferase, interacts with SseF. We showed that the TIP60 acetylation activity was increased in the presence of SseF, and TIP60 was upregulated upon Salmonella infection. In addition, TIP60 is required for efficient intracellular Salmonella replication in macrophages.

CONCLUSION

Taken together, our data suggest that Salmonella may use SseF to exploit the host TIP60 acetyltransferase activity to promote efficient Salmonella replication inside host cells.

MYST family lysine acetyltransferase facilitates ataxia telangiectasia mutated (ATM) kinase-mediated DNA damage response in Toxoplasma gondii

The MYST family of lysine acetyltransferases (KATs) function in a wide variety of cellular operations, including gene regulation and the DNA damage response. Here we report the characterization of the second MYST family KAT in the protozoan parasite Toxoplasma gondii (TgMYST-B). Toxoplasma causes birth defects and is an opportunistic pathogen in the immunocompromised, the latter due to its ability to convert into a latent cyst (bradyzoite). We demonstrate that TgMYST-B can gain access to the parasite nucleus and acetylate histones. Overexpression of recombinant, tagged TgMYST-B reduces growth rate in vitro and confers protection from a DNA-alkylating agent. Expression of mutant TgMYST-B produced no growth defect and failed to protect against DNA damage. We demonstrate that cells overexpressing TgMYST-B have increased levels of ataxia telangiectasia mutated (ATM) kinase and phosphorylated H2AX and that TgMYST-B localizes to the ATM kinase gene. Pharmacological inhibitors of ATM kinase or KATs reverse the slow growth phenotype seen in parasites overexpressing TgMYST-B. These studies are the first to show that a MYST KAT contributes to ATM kinase gene expression, further illuminating the mechanism of how ATM kinase is up-regulated to respond to DNA damage.

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. (

The orphan nuclear receptor Rev-erbbeta recruits Tip60 and HDAC1 to regulate apolipoprotein CIII promoter

Nuclear hormone receptors function as ligand activated transcription factors. Ligand binding and modification such as acetylation have been reported to regulate nuclear hormone receptors. The orphan receptors, Rev-erbalpha and Rev-erbbeta, are members of the nuclear receptor superfamily and act as transcriptional repressors. In this study, the role of recruitment of co-factors by Rev-erbbeta and acetylation of Rev-erbbeta in modulating apolipoprotein CIII (apoCIII) transcription were investigated. Rev-erbbeta was found to transcriptionally repress apoCIII after binding to the apoCIII promoter. Tip60, a histone acetyl-transferase (HAT), was a novel binding partner for Rev-erbbeta and recruited to the apoCIII promoter by Rev-erbbeta. Tip60 was able to acetylate Rev-erbbeta and relieve the apoCIII repression mediated by Rev-erbbeta. This de-repression effect depended on acetylation of Rev-erbbeta at its RXKK motif by Tip60. In addition, histone deacetylase 1 (HDAC1) interacted with Rev-erbbeta and was recruited to the apoCIII promoter by Rev-erbbeta to antagonize Tip60's activity. Taken together, we have provided evidence that Rev-erbbeta modulates the apoCIII gene expression by recruiting different transcription co-activator or co-repressor.

Co-activation of Atrial Natriuretic Factor Promoter by Tip60 and Serum Response Factor

Tat-interactive protein 60 (Tip60) is a member of the MYST family of histone acetyltransferases (HATs). In addition to its HAT domain, Tip contains a heterochromatin-associated protein 1-like chromodomain and a zinc finger-like domain. Several alternative splice variants of Tip60 have been characterized, including full-length Tip60alpha, Tip60beta (which lacks exon V encoded by the Tip60 gene), and Tip55 (which encodes a novel 103-amino-acid C terminus). We report here that isoproteins recognized by a pan-Tip60 antibody are strongly and transiently expressed between embryonic days 8 and 11 in the embryonic mouse myocardium. A functional role for Tip60 isoproteins in cardiac myocyte differentiation is suggested by immunoprecipitation experiments showing that Tip60alpha, Tip60beta, and Tip55 can bind serum response factor (SRF) and by transient transfection assessments showing that Tip60 and SRF cooperatively activate the atrial natriuretic factor promoter. Although this combinatorial activity is inhibited by histone deacetylase 7, it was unexpectedly enhanced by point mutation of the HAT domain. Ablation of the chromodomain from Tip60beta caused derepression. These findings suggest that Tip60 modulates expression of SRF-dependent cardiac genes.

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.

Dual regulation of c-Myc by p300 via acetylation-dependent control of Myc protein turnover and coactivation of Myc-induced transcription

The c-Myc oncoprotein (Myc) controls cell fate by regulating gene transcription in association with a DNA-binding partner, Max. While Max lacks a transcription regulatory domain, the N terminus of Myc contains a transcription activation domain (TAD) that recruits cofactor complexes containing the histone acetyltransferases (HATs) GCN5 and Tip60. Here, we report a novel functional interaction between Myc TAD and the p300 coactivator-acetyltransferase. We show that p300 associates with Myc in mammalian cells and in vitro through direct interactions with Myc TAD residues 1 to 110 and acetylates Myc in a TAD-dependent manner in vivo at several lysine residues located between the TAD and DNA-binding domain. Moreover, the Myc:Max complex is differentially acetylated by p300 and GCN5 and is not acetylated by Tip60 in vitro, suggesting distinct functions for these acetyltransferases. Whereas p300 and CBP can stabilize Myc independently of acetylation, p300-mediated acetylation results in increased Myc turnover. In addition, p300 functions as a coactivator that is recruited by Myc to the promoter of the human telomerase reverse transcriptase gene, and p300/CBP stimulates Myc TAD-dependent transcription in a HAT domain-dependent manner. Our results suggest dual roles for p300/CBP in Myc regulation: as a Myc coactivator that stabilizes Myc and as an inducer of Myc instability via direct Myc acetylation.

HIV-1 Tat targets Tip60 to impair the apoptotic cell response to genotoxic stresses

HIV-1 transactivator Tat uses cellular acetylation signalling by targeting several cellular histone acetyltransferases (HAT) to optimize its various functions. Although Tip60 was the first HAT identified to interact with Tat, the biological significance of this interaction has remained obscure. We had previously shown that Tat represses Tip60 HAT activity. Here, a new mechanism of Tip60 neutralization by Tat is described, where Tip60 is identified as a substrate for the newly reported p300/CBP-associated E4-type ubiquitin-ligase activity, and Tat uses this mechanism to induce the polyubiquitination and degradation of Tip60. Tip60 targeting by Tat results in a dramatic impairment of the Tip60-dependent apoptotic cell response to DNA damage. These data reveal yet unknown strategies developed by HIV-1 to increase cell resistance to genotoxic stresses and show a role of Tat as a modulator of cellular protein ubiquitination.

cPLA2-interacting protein, PLIP, causes apoptosis and decreases G1 phase in mesangial cells

The balance between proliferation and apoptosis of mesangial cells is a critical component of proliferative glomerulonephritis. The regulation of cell proliferation and apoptosis is linked at the level of the cell cycle (Shankland SJ. Kidney Int 52: 294-308, 199). cPLA2-interacting protein (PLIP), the Tip60 splice variant, interacts with cPLA2 and enhances the susceptibility of renal mesangial cells to serum deprivation-induced apoptosis (Sheridan AM, Force T, Yoon HJ, O'Leary E, Choukroun G, Taheri MR, and Bonventre JV. Mol Cell Biol 21: 4470-4481, 2001). We report that adenoviral-driven PLIP expression results in enhanced apoptosis of non-serum-deprived mesangial cells associated with a marked decrease in G0/G1 phase cells. The effect of PLIP on the cell cycle may be independent of its interaction with cPLA2 because a mutation of PLIP that does not interact with cPLA2 also causes a decrease in G0/G1 cells. Endogenous PLIP and Tip60 protein levels are increased in cells exposed to injurious stimuli including X-irradiation and H2O2, but the intracellular localization of the splice variants may differ. Whereas PLIP localizes in the nucleus of all mesangial cells, Tip60 localizes in the cytosol of untreated mesangial cells and of cells exposed to low concentrations (50-200 microM) of H2O2. Tip60 is targeted to the nucleus of cells exposed to high concentrations (1-2 mM) of H2O2. We conclude that PLIP may cause cells to exit from the cell cycle after the S phase and may function as part of a G2/M checkpoint mechanism. Tip60 splice variants may function in both cytosolic and nuclear signaling pathways in mesangial cells.

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.

Identification of a larger form of the histone acetyl transferase Tip60

The histone acetyl transferase Tat interactive protein 60kD (Tip60) plays major roles in the cellular response to extra- or intra-cellular signalling. Tip60 activity appears to be tightly regulated in cells through post-translational modifications or subcellular localisation of the protein. In addition, several alternatively spliced forms of Tip60 have been described. We found here that in a significant proportion of cytoplasmic poly adenylated Tip60 mRNAs, the intron 1 was not excised. Translation of this mRNA would predictably lead to the production of a longer Tip60 protein, containing a 33 amino acids insertion four amino acids after the ATG. By transient transfection experiments, we could demonstrate that this protein was significantly produced. Thus, taken together, our results indicate the existence of a longer Tip60 protein. Whether this protein could be differentially regulated and could play different roles than the classical Tip60 protein is an intriguing possibility.

Expression of Tip60, an androgen receptor coactivator, and its role in prostate cancer development

Prostate cancer (CaP) is initially androgen sensitive and responsive to hormone ablation therapy. However, cancer growth recurs despite androgen deprivation in the majority of cases of advanced disease. The molecular basis of this progression still remains unknown. The significance of androgen receptor (AR) coactivator proteins in this androgen-dependent malignancy is only beginning to emerge. In the present study, we examined the role of Tat interactive protein, 60 kDa (Tip60), an AR coactivator, in CaP progression. In hormone refractory CaP biopsies, we observed a nuclear accumulation of Tip60 expression in contrast to a more diffuse distribution pattern observed in benign prostate hyperplasia and primary CaP. Furthermore, in both the prostate xenograft model CWR22 and the LNCaP CaP cell line, we observed that androgen withdrawal promoted upregulation of Tip60 as well as nuclear accumulation. In contrast, androgen exposure resulted in decreased Tip60 expression that was more closely linked to a cytoplasmic presence. Chromatin immunoprecipitation analysis revealed Tip60's recruitment to the PSA gene promoter in both androgen-dependent and -independent cell lines. Thus, in vitro and in vivo data support a possible role for Tip60 in the molecular pathway leading to the development of androgen-independent CaP following long-term androgen deprivation therapy.

The MYST family of histone acetyltransferases

Multiple chromatin modifying proteins and multisubunit complexes have been characterized in recent years. Histone acetyltransferase (HAT) activities have been the most thoroughly studied, both biochemically and functionally. This review sums up the current knowledge on a specific group of proteins that is extremely well conserved throughout evolution, the MYST family of histone acetyltransferases. These proteins play critical roles in various nuclear functions and the control of cell proliferation.

Tip60 is a co-repressor for STAT3

Tip60 (Tat-interactive protein, 60 kDa), a cellular protein with intrinsic histone acetyltransferase activity, is involved in DNA damage repair and apoptosis. Recent studies have suggested that Tip60 acts either as a co-activator or a co-repressor to modulate transcription. In this study, we demonstrate that Tip60 represses reporter gene expression when it is fused to the Gal4 DNA binding domain. We also show that Tip60 associates with histone deacetylase 7 (HDAC7) through its N-terminal zinc finger-containing region and that HDAC7 activity is required for the repressive effect of Tip60. Because endogenous Tip60 interacts with STAT3, we hypothesized that Tip60 might complex with STAT3 and HDAC7 and modulate STAT3-mediated trans-activation. Consistent with this hypothesis, the overexpression of Tip60 represses STAT3-driven reporter gene expression, which can be further potentiated by the co-transfection of HDAC7. Furthermore, interleukin-9-induced c-myc expression, which depends on STAT3 activity, is abrogated by exogenous expression of Tip60. This is the first demonstration of which Tip60 represses STAT3 activity in part through the recruitment of HDAC7.

Characterization and expression of the mouse tat interactive protein 60 kD (TIP60) gene

Tat interactive protein-60 (TIP60) is a novel histone acetyltransferase-containing protein that has been implicated in the regulation of transcription, DNA repair and apoptosis. In this report we describe the structure and expression of the mouse TIP60 gene, as well the localization of TIP60 protein at the cellular level. The gene contains 14 exons within a DNA sequence interval of 6611 bp. The assembled exons comprise a 1,539 bp DNA complementary to RNA (cDNA) having 91.7 and 78.7% homology with respective human and chick TIP60 cDNAs. Translation predicts a approximately 59 kD protein having 99.6 and 91.6% sequence homology with respective human and chick proteins. Alignment with mouse expressed sequence tag database entries indicates, similar to human and chick TIP60, the existence of an alternative splice created by removal of exon 5 that results in a 1383 bp cDNA with a predicted translation product of approximately 53 kD. Northern hybridization analysis reveals a peak of TIP60 expression during mouse embryogenesis at E11; in adult tissues TIP60 is expressed in the following order of intensity: testis>heart>brain>kidney>liver>lung, with little to no expression in spleen and skeletal muscle. Cellular localization using green fluorescent protein-TIP fusion constructs and immunohistochemistry reveal that TIP53 and TIP60 are nuclear proteins.

MOZ and MORF histone acetyltransferases interact with the Runt-domain transcription factor Runx2

The monocytic leukemia zinc finger protein MOZ and its homologue MORF have been implicated in leukemogenesis. Both MOZ and MORF are histone acetyltransferases with weak transcriptional repression domains and strong transcriptional activation domains, suggesting that they may function as transcriptional coregulators. Here we describe that MOZ and MORF both interact with Runx2 (or Cbfa1), a Runt-domain transcription factor that is known to play important roles in T cell lymphomagenesis and bone development. Through its C-terminal SM (serine- and methionine-rich) domain, MORF binds to Runx2 in vitro and in vivo. Consistent with this, the SM domain of MORF also binds to Runx1 (or AML1), a Runx2 homologue that is frequently altered by leukemia-associated chromosomal translocations. While MORF does not acetylate Runx2, its SM domain potentiates Runx2-dependent transcriptional activation. Moreover, endogenous MORF is required for transcriptional activation by Runx2. Intriguingly, Runx2 negatively regulates the transcriptional activation potential of the SM domain. Like that of MORF, the SM domain of MOZ physically and functionally interacts with Runx2. These results thus identify Runx2 as an interaction partner of MOZ and MORF and suggest that both acetyltransferases are involved in regulating transcriptional activation mediated by Runx2 and its homologues.

Tip60 is targeted to proteasome-mediated degradation by Mdm2 and accumulates after UV irradiation

Acetylation is a prominent post-translational modification of nucleosomal histone N-terminal tails, which regulates chromatin accessibility. Accordingly, histone acetyltransferases (HATs) play major roles in processes such as transcription. Here, we show that the HAT Tip60, which is involved in DNA repair and apoptosis following gamma irradiation, is subjected to proteasome-dependent proteolysis. Furthermore, we provide evidence that Mdm2, the ubiquitin ligase of the p53 tumour suppressor, interacts physically with Tip60 and induces its ubiquitylation and proteasome-dependent degradation. Moreover, a ubiquitin ligase-defective mutant of Mdm2 had no effect on Tip60 stability. Our results indicate that Mdm2 targets both p53 and Tip60, suggesting that these two proteins could be co-regulated with respect to protein stability. Consistent with this hypothesis, Tip60 levels increased significantly upon UV irradiation of Jurkat cells. Collectively, our results suggest that degradation of Tip60 could be part of the mechanism leading to cell transformation by Mdm2.

A transcriptionally [correction of transcriptively] active complex of APP with Fe65 and histone acetyltransferase Tip60

Amyloid-beta precursor protein (APP), a widely expressed cell-surface protein, is cleaved in the transmembrane region by gamma-secretase. gamma-Cleavage of APP produces the extracellular amyloid beta-peptide of Alzheimer's disease and releases an intracellular tail fragment of unknown physiological function. We now demonstrate that the cytoplasmic tail of APP forms a multimeric complex with the nuclear adaptor protein Fe65 and the histone acetyltransferase Tip60. This complex potently stimulates transcription via heterologous Gal4- or LexA-DNA binding domains, suggesting that release of the cytoplasmic tail of APP by gamma-cleavage may function in gene expression.

PLIP, a novel splice variant of Tip60, interacts with group IV cytosolic phospholipase A(2), induces apoptosis, and potentiates prostaglandin production

The group IV cytosolic phospholipase A(2) (cPLA(2)) has been localized to the nucleus (M. R. Sierra-Honigmann, J. R. Bradley, and J. S. Pober, Lab. Investig. 74:684-695, 1996) and is known to translocate from the cytosolic compartment to the nuclear membrane (S. Glover, M. S. de Carvalho, T. Bayburt, M. Jonas, E. Chi, C. C. Leslie, and M. H. Gelb, J. Biol. Chem. 270:15359-15367, 1995; A. R. Schievella, M. K. Regier, W. L. Smith, and L. L. Lin, J. Biol. Chem. 270:30749-30754, 1995). We hypothesized that nuclear proteins interact with cPLA(2) and participate in the functional effects of this translocation. We have identified a nuclear protein, cPLA(2)-interacting protein (PLIP), a splice variant of human Tip60, which interacts with the amino terminal region of cPLA(2). Like Tip60, PLIP cDNA includes the MYST domain containing a C2HC zinc finger and well-conserved similarities to acetyltransferases. Both PLIP and Tip60 coimmunoprecipitate and colocalize with cPLA(2) within the nuclei of transfected COS cells. A polyclonal antibody raised to PLIP recognizes both PLIP and Tip60. Endogenous Tip60 and/or PLIP in rat mesangial cells is localized to the nucleus in response to serum deprivation. Nuclear localization coincides temporally with apoptosis. PLIP expression, mediated by adenoviral gene transfer, potentiates serum deprivation-induced prostaglandin E(2) (PGE(2)) production and apoptosis in mouse mesangial cells from cPLA(2)(+/+) mice but not in mesangial cells derived from cPLA(2)(-/-) mice. Thus PLIP, a splice variant of Tip60, interacts with cPLA(2) and potentiates cPLA(2)-mediated PGE(2) production and apoptosis.