The coadministration of trehalose dibehenate and monosodium urate crystals promotes an antitumor phenotype in human-derived myeloid cells.

Trehalose dibehenate (TDB), a ligand for the macrophage-inducible C-type lectin, has shown promise as an adjuvant for preventative vaccines and also as an anticancer agent in murine assays. The potential for TDB to affect the antitumor immune response of human myeloid cells, however, has not been studied. We investigated the effect of the adjuvants TDB and monosodium urate (MSU) crystals on the protumor or antitumor immune phenotype of human monocytes, macrophages and monocyte-derived dendritic cells (Mo-DCs). TDB treatment alone led to an inflammatory response in all three cell types, which was most pronounced when using human monocytes, with MSU augmenting this response. TDB also decreased cell surface markers associated with a protumorigenic phenotype, with MSU showing some ability to augment this response. Notably, a significant reduction in CD115 was observed for all antigen-presenting cells upon TDB or MSU + TDB treatment. The potential to increase the antigen-presenting capabilities of the myeloid cells was also observed upon treatment with TDB and MSU + TDB, as indicated by the upregulation of cell surface markers such as CD86 for all three cell types and a favorable ratio of interleukin (IL)-12p40 to IL-10 for monocytes stimulated with MSU + TDB. There was no significant production of IL-12p40 by Mo-DC; however, in a mixed lymphocyte assay, MSU + TDB costimulation of Mo-DC led to a significant increase in CD4+ T-cell numbers and in the IL-12p40-to-IL-10 ratio. Taken together, these findings show for the first time the potential of MSU + TDB costimulation to favor a tumor-suppressive phenotype in human-derived myeloid cells.

Occupation and motor neuron disease: a New Zealand case-control study.

OBJECTIVES: To assess associations between occupation and motor neuron disease (MND). METHODS: We conducted a population-based case-control study with cases (n=321) recruited through the New Zealand Motor Neurone Disease Association and hospital discharge data. Controls (n=605) were recruited from the Electoral Roll. Information on personal and demographic details, lifestyle factors and a full occupational history was collected using questionnaires and interviews. Associations with ever/never employed and employment duration were estimated using logistic regression stratified by sex and adjusted for age, ethnicity, socioeconomic deprivation, education and smoking. RESULTS: Elevated risks were observed for field crop and vegetable growers (OR 2.93, 95% CI 1.10 to 7.77); fruit growers (OR 2.03, 95% CI 1.09 to 3.78); gardeners and nursery growers (OR 1.96, 95% CI 1.01 to 3.82); crop and livestock producers (OR 3.61, 95% CI 1.44 to 9.02); fishery workers, hunters and trappers (OR 5.62, 95% CI 1.27 to 24.97); builders (OR 2.90, 95% CI 1.41 to 5.96); electricians (OR 3.61, 95% CI 1.34 to 9.74); caregivers (OR 2.65, 95% CI 1.04 to 6.79); forecourt attendants (OR 8.31, 95% CI 1.79 to 38.54); plant and machine operators and assemblers (OR 1.42, 95% CI 1.01 to 2.01); telecommunications technicians (OR 4.2, 95% CI 1.20 to 14.64); and draughting technicians (OR 3.02, 95% CI 1.07 to 8.53). Industries with increased risks were agriculture (particularly horticulture and fruit growing), construction, non-residential care services, motor vehicle retailing, and sport and recreation. Positive associations between employment duration and MND were shown for the occupations fruit growers, gardeners and nursery growers, and crop and livestock producers, and for the horticulture and fruit growing industry. CONCLUSIONS: This study suggests associations between MND and occupations in agriculture and several other occupations.

Promyelocytic leukemia zinc finger protein regulates interferon-mediated innate immunity.

Interferons (IFNs) direct innate and acquired immune responses and, accordingly, are used therapeutically to treat a number of diseases, yet the diverse effects they elicit are not fully understood. Here, we identified the promyelocytic leukemia zinc finger (PLZF) protein as a previously unrecognized component of the IFN response. IFN stimulated an association of PLZF with promyelocytic leukemia protein (PML) and histone deacetylase 1 (HDAC1) to induce a decisive subset of IFN-stimulated genes (ISGs). Consequently, PLZF-deficient mice had a specific ISG expression defect and as a result were more susceptible to viral infection. This susceptibility correlated with a marked decrease in the expression of the key antiviral mediators and an impaired IFN-mediated induction of natural killer cell function. These results provide new insights into the regulatory mechanisms of IFN signaling and the induction of innate antiviral immunity.

Iterative sorting reveals CD133+ and CD133- melanoma cells as phenotypically distinct populations.

BACKGROUND: The heterogeneity and tumourigenicity of metastatic melanoma is attributed to a cancer stem cell model, with CD133 considered to be a cancer stem cell marker in melanoma as well as other tumours, but its role has remained controversial. METHODS: We iteratively sorted CD133+ and CD133- cells from 3 metastatic melanoma cell lines, and observed tumourigenicity and phenotypic characteristics over 7 generations of serial xeno-transplantation in NOD/SCID mice. RESULTS: We demonstrate that iterative sorting is required to make highly pure populations of CD133+ and CD133- cells from metastatic melanoma, and that these two populations have distinct characteristics not related to the cancer stem cell phenotype. In vitro, gene set enrichment analysis indicated CD133+ cells were related to a proliferative phenotype, whereas CD133- cells were of an invasive phenotype. However, in vivo, serial transplantation of CD133+ and CD133- tumours over 7 generations showed that both populations were equally able to initiate and propagate tumours. Despite this, both populations remained phenotypically distinct, with CD133- cells only able to express CD133 in vivo and not in vitro. Loss of CD133 from the surface of a CD133+ cell was observed in vitro and in vivo, however CD133- cells derived from CD133+ retained the CD133+ phenotype, even in the presence of signals from the tumour microenvironment. CONCLUSION: We show for the first time the necessity of iterative sorting to isolate pure marker-positive and marker-negative populations for comparative studies, and present evidence that despite CD133+ and CD133- cells being equally tumourigenic, they display distinct phenotypic differences, suggesting CD133 may define a distinct lineage in melanoma.

N,N-Bis(glycityl)amines as anti-cancer drugs.

A series of N,N-bis(glycityl)amines with promising anti-cancer activity were prepared via the reductive amination of pentoses and hexoses, and subsequently screened for their ability to selectively inhibit the growth of cancerous versus non-cancerous cells. For the first time, we show that this class of compounds possesses anti-proliferative activity, and, while the selective killing of brain cancer (LN18) cells versus matched (SVG-P12) cells was modest, several of the amines, including d-arabinitylamine 1a and d-fucitylamine 1g, exhibited low micromolar IC50 values for HL60 cells. Moreover, these two amines showed good selectivity towards HL60 cells when compared to non-cancerous HEK-293 cells. The compounds also showed low micromolar inhibition of the leukaemic cell line, THP-1. The modes of action of amines 1a and 1g were then determined using yeast chemical genetics, whereby it was established that both compounds affect similar but distinct sets of biochemical pathways. Notably purine nucleoside monophosphate biosynthesis was identified as an enriched mechanism. The rapid synthesis of the amines and their unique mode of action thus make them attractive targets for further development as anti-cancer drugs.

Enhanced immunosuppression by therapy-exposed glioblastoma multiforme tumor cells.

Glioblastoma multiforme (GBM) is a highly malignant brain tumor with an extremely short time to relapse following standard treatment. Since recurrent GBM is often resistant to subsequent radiotherapy and chemotherapy, immunotherapy has been proposed as an alternative treatment option. Although it is well established that GBM induces immune suppression, it is currently unclear what impact prior conventional therapy has on the ability of GBM cells to modulate the immune environment. In this study, we investigated the interaction between immune cells and glioma cells that had been exposed to chemotherapy or irradiation in vitro. We demonstrate that treated glioma cells are more immunosuppressive than untreated cells and form tumors at a faster rate in vivo in an animal model. Cultured supernatant from in vitro-treated primary human GBM cells were also shown to increase suppression, which was independent of accessory suppressor cells or T regulatory cell generation, and could act directly on CD4(+) and CD8(+) T cell proliferation. While a number of key immunosuppressive cytokines were overexpressed in the treated cells, including IL-10, IL-6 and GM-CSF, suppression could be alleviated in a number of treated GBM lines by inhibition of prostaglandin E2. These results reveal for the first time that conventional therapies can alter immunosuppressive pathways in GBM tumor cells, a finding with important implications for the combination of immunotherapy with standard treatment.

BCL6 is a context-dependent mediator of the glioblastoma response to irradiation therapy.

Glioblastoma is a rapidly fatal brain cancer that does not respond to therapy. Previous research showed that the transcriptional repressor protein BCL6 is upregulated by chemo and radiotherapy in glioblastoma, and inhibition of BCL6 enhances the effectiveness of these therapies. Therefore, BCL6 is a promising target to improve the efficacy of current glioblastoma treatment. BCL6 acts as a transcriptional repressor in germinal centre B cells and as an oncogene in lymphoma and other cancers. However, in glioblastoma, BCL6 induced by therapy may not be able to repress transcription. Using a BCL6 inhibitor, the whole proteome response to irradiation was compared with and without BCL6 activity. Acute high dose irradiation caused BCL6 to switch from repressing the DNA damage response to promoting stress response signalling. Rapid immunoprecipitation mass spectrometry of endogenous proteins (RIME) enabled comparison of BCL6 partner proteins between untreated and irradiated glioblastoma cells. BCL6 was associated with transcriptional coregulators in untreated glioblastoma including the known partner NCOR2. However, this association was lost in response to acute irradiation, where BCL6 unexpectedly associated with synaptic and plasma membrane proteins. These results reveal the activity of BCL6 under therapy-induced stress is context-dependent, and potentially altered by the intensity of that stress.

Residual OXPHOS is required to drive primary and metastatic lung tumours in an orthotopic breast cancer model.

Background: Fast adaptation of glycolytic and mitochondrial energy pathways to changes in the tumour microenvironment is a hallmark of cancer. Purely glycolytic ρ0 tumour cells do not form primary tumours unless they acquire healthy mitochondria from their micro-environment. Here we explored the effects of severely compromised respiration on the metastatic capability of 4T1 mouse breast cancer cells. Methods: 4T1 cell lines with different levels of respiratory capacity were generated; the Seahorse extracellular flux analyser was used to evaluate oxygen consumption rates, fluorescent confocal microscopy to assess the number of SYBR gold-stained mitochondrial DNA nucleoids, and the presence of the ATP5B protein in the cytoplasm and fluorescent in situ nuclear hybridization was used to establish ploidy. MinION nanopore RNA sequence analysis was used to compare mitochondrial DNA transcription between cell lines. Orthotopic injection was used to determine the ability of cells to metastasize to the lungs of female Balb/c mice. Results: OXPHOS-deficient ATP5B-KO3.1 cells did not generate primary tumours. Severely OXPHOS compromised ρ0D5 cells generated both primary tumours and lung metastases. Cells generated from lung metastasis of both OXPHOS-competent and OXPHOS-compromised cells formed primary tumours but no metastases when re-injected into mice. OXPHOS-compromised cells significantly increased their mtDNA content, but this did not result in increased OXPHOS capacity, which was not due to decreased mtDNA transcription. Gene set enrichment analysis suggests that certain cells derived from lung metastases downregulate their epithelial-to-mesenchymal related pathways. Conclusion: In summary, OXPHOS is required for tumorigenesis in this orthotopic mouse breast cancer model but even very low levels of OXPHOS are sufficient to generate both primary tumours and lung metastases.

Aryl-functionalised α,α'-Trehalose 6,6'-Glycolipid Induces Mincle-independent Pyroptotic Cell Death.

α,α'-Trehalose 6,6'-glycolipids have long been known for their immunostimulatory properties. The adjuvanticity of α,α'-trehalose 6,6'-glycolipids is mediated by signalling through the macrophage inducible C-type lectin (Mincle) and the induction of an inflammatory response. Herein, we present an aryl-functionalised trehalose glycolipid, AF-2, that leads to the release of cytokines and chemokines, including IL-6, MIP-2 and TNF-α, in a Mincle-dependent manner. Furthermore, plate-coated AF-2 also leads to the Mincle-independent production of IL-1ß, which is unprecedented for this class of glycolipid. Upon investigation into the mode of action of plate-coated AF-2, it was observed that the treatment of WT and Mincle-/- bone marrow derived macrophages (BMDM), murine RAW264.7 cells, and human monocytes with AF-2 led to lytic cell death, as evidenced using Sytox Green and lactate dehydrogenase assays, and confocal and scanning electron microscopy. The requirement for functional Gasdermin D and Caspase-1 for IL-1ß production and cell death by AF-2 confirmed pyroptosis as the mode of action of AF-2. The inhibition of NLRP3 and K+ efflux reduced AF-2 mediated IL-1ß production and cell death, and allowed us to conclude that AF-2 leads to Capase-1 dependent NLRP3 inflammasome-mediated cell death. The unique mode of action of plate-coated AF-2 was surprising and highlights how the physical presentation of Mincle ligands can lead to dramatically different immunological outcomes.

Side population is not necessary or sufficient for a cancer stem cell phenotype in glioblastoma multiforme.

There is strong evidence for the existence of cancer stem cells (CSCs) in the aggressive brain tumor glioblastoma multiforme (GBM). These cells have stem-like self-renewal activity and increased tumor initiation capacity and are believed to be responsible for recurrence due to their resistance to therapy. Several techniques have been used to enrich for CSC, including growth in serum-free defined media to induce sphere formation, and isolation of a stem-like cell using exclusion of the fluorescent dye Hoechst 33342, the side population (SP). We show that sphere formation in GBM cell lines and primary GBM cells enriches for a CSC-like phenotype of increased self-renewal gene expression in vitro and increased tumor initiation in vivo. However, the SP was absent from all sphere cultures. Direct isolation of the SP from the GBM lines did not enrich for stem-like activity in vitro, and tumor-initiating activity was lower in sorted SP compared with non-SP and parental cells. Transient exposure to doxorubicin enhanced both CSC and SP frequency. However, doxorubicin treatment altered the cytometric profile and obscured the SP demonstrating the difficulty of identifying SP in cells under stress. Doxorubicin-exposed cells showed a transient increase in SP, but the doxorubicin-SP cells were still not enriched for a stem-like self-renewal phenotype. These data demonstrate that the GBM SP does not necessarily contribute to self-renewal or tumor initiation, key properties of a CSC, and we advise against using SP to enumerate or isolate CSC.

The novel phloroglucinol PMT7 kills glycolytic cancer cells by blocking autophagy and sensitizing to nutrient stress.

The switch from oxidative phosphorylation to glycolytic metabolism results in cells that generate fewer reactive oxygen species (ROS) and are resistant to the intrinsic induction of apoptosis. As a consequence, glycolytic cancer cells are resistant to radiation and chemotherapeutic agents that rely on production of ROS or intrinsic apoptosis. Further, the level of glycolysis correlates with tumor invasion, making glycolytic cancer cells an important target for new therapy development. We have synthesized a novel redox-active quinone phloroglucinol derivative, PMT7. Toxicity of PMT7 was in part due to loss of mitochondrial membrane potential in treated cells with subsequent loss of mitochondrial metabolic activity. Mitochondrial gene knockout ρ0 cells, a model of highly glycolytic cancers, were only half as sensitive as the corresponding wild-type cells and metabolic pathways downstream of MET were unaffected in ρ0 cells. However, PMT7 toxicity was also due to a block in autophagy. Both wild-type and ρ0 cells were susceptible to autophagy blockade, and the resistance of ρ0 cells to PMT7 could be overcome by serum deprivation, a situation where autophagy becomes necessary for survival. The stress response class III deacetylase SIRT1 was not significantly involved in PMT7 toxicity, suggesting that unlike other chemotherapeutic drugs, SIRT1-mediated stress and survival responses were not induced by PMT7. The dependence on autophagy or other scavenging pathways makes glycolytic cancer cells vulnerable. This can be exploited by induction of energetic stress to specifically sensitize glycolytic cells to other stresses such as nutrient deprivation or potentially chemotherapy.

Comprehensive genomic screens identify a role for PLZF-RARalpha as a positive regulator of cell proliferation via direct regulation of c-MYC.

The t(11;17)(q23;q21) translocation is associated with a retinoic acid (RA)-insensitive form of acute promyelocytic leukemia (APL), involving the production of reciprocal fusion proteins, promyelocytic leukemia zinc finger-retinoic acid receptor alpha (PLZF-RARalpha) and RARalpha-PLZF. Using a combination of chromatin immunoprecipitation promotor arrays (ChIP-chip) and gene expression profiling, we identify novel, direct target genes of PLZF-RARalpha that tend to be repressed in APL compared with other myeloid leukemias, supporting the role of PLZF-RARalpha as an aberrant repressor in APL. In primary murine hematopoietic progenitors, PLZF-RARalpha promotes cell growth, and represses Dusp6 and Cdkn2d, while inducing c-Myc expression, consistent with its role in leukemogenesis. PLZF-RARalpha binds to a region of the c-MYC promoter overlapping a functional PLZF site and antagonizes PLZF-mediated repression, suggesting that PLZF-RARalpha may act as a dominant-negative version of PLZF by affecting the regulation of shared targets. RA induced the differentiation of PLZF-RARalpha-transformed murine hematopoietic cells and reduced the frequency of clonogenic progenitors, concomitant with c-Myc down-regulation. Surviving RA-treated cells retained the ability to be replated and this was associated with sustained c-Myc expression and repression of Dusp6, suggesting a role for these genes in maintaining a self-renewal pathway triggered by PLZF-RARalpha.

Early epigenetic changes and DNA damage do not predict clinical response in an overlapping schedule of 5-azacytidine and entinostat in patients with myeloid malignancies.

Sequential administration of DNA methyltransferase (DNMT) inhibitors and histone deacetylase (HDAC) inhibitors has demonstrated clinical efficacy in patients with hematologic malignancies. However, the mechanism behind their clinical efficacy remains controversial. In this study, the methylation dynamics of 4 TSGs (p15(INK4B), CDH-1, DAPK-1, and SOCS-1) were studied in sequential bone marrow samples from 30 patients with myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML) who completed a minimum of 4 cycles of therapy with 5-azacytidine and entinostat. Reversal of promoter methylation after therapy was observed in both clinical responders and nonresponders across all genes. There was no association between clinical response and either baseline methylation or methylation reversal in the bone marrow or purified CD34(+) population, nor was there an association with change in gene expression. Transient global hypomethylation was observed in samples after treatment but was not associated with clinical response. Induction of histone H3/H4 acetylation and the DNA damage-associated variant histone gamma-H2AX was observed in peripheral blood samples across all dose cohorts. In conclusion, methylation reversal of candidate TSGs during cycle 1 of therapy was not predictive of clinical response to combination "epigenetic" therapy. This trial is registered with http://www.clinicaltrials.gov under NCT00101179.

The Big Picture of Glioblastoma Malignancy: A Meta-Analysis of Glioblastoma Proteomics to Identify Altered Biological Pathways.

Glioblastoma is a highly malignant cancer with no effective treatment. It is vital to elucidate the mechanisms which drive glioblastoma in order to identify therapeutic targets. The differences in protein expression between glioblastoma, grade I-III glioma, and normal brain tissue reflect the functional alterations driving malignancy. However, proteomic analysis of glioblastoma has been hampered by the heterogeneity of glioblastoma and the variety of methodology used in its study. To reduce these inconsistencies, we performed a meta-analysis of the literature published since 2015, including 14 datasets from eight papers comparing the whole proteome of glioblastoma to normal brain or grade I-III glioma. We found that 154 proteins were commonly upregulated and 116 proteins were commonly downregulated in glioblastoma compared to normal brain. Meanwhile, 240 proteins were commonly upregulated and 125 proteins were commonly downregulated in glioblastoma compared to grade I-III glioma. Functional enrichment analysis revealed upregulation of proteins involved in mRNA splicing and the immune system and downregulation of proteins involved in synaptic signaling and glucose and glutamine metabolism. The identification of these altered biological pathways provides a basis for deeper investigation in the pursuit of an effective treatment for glioblastoma.

Distinguishing and Managing Acute-Onset Complex Tic-like Behaviors in Adolescence.

Consistent with international reports,1 this group of Tourette syndrome (TS) experts has noticed a recent increase in adolescents presenting with tic-like symptoms that show a markedly atypical onset and course. These sudden-onset motor movements and vocalizations are often associated with significant impairment and disability, resulting in emergency department visits and hospitalizations for some affected youths.

The oncogene BCL6 is up-regulated in glioblastoma in response to DNA damage, and drives survival after therapy.

The prognosis for people with the high-grade brain tumor glioblastoma is very poor, due largely to low cell death in response to genotoxic therapy. The transcription factor BCL6, a protein that normally suppresses the DNA damage response during immune cell maturation, and a known driver of B-cell lymphoma, was shown to mediate the survival of glioblastoma cells. Expression was observed in glioblastoma tumor specimens and cell lines. When BCL6 expression or activity was reduced in these lines, increased apoptosis and a profound loss of proliferation was observed, consistent with gene expression signatures suggestive of anti-apoptotic and pro-survival signaling role for BCL6 in glioblastoma. Further, treatment with the standard therapies for glioblastoma-ionizing radiation and temozolomide-both induced BCL6 expression in vitro, and an in vivo orthotopic animal model of glioblastoma. Importantly, inhibition of BCL6 in combination with genotoxic therapies enhanced the therapeutic effect. Together these data demonstrate that BCL6 is an active transcription factor in glioblastoma, that it drives survival of cells, and that it increased with DNA damage, which increased the survival rate of therapy-treated cells. This makes BCL6 an excellent therapeutic target in glioblastoma-by increasing sensitivity to standard DNA damaging therapy, BCL6 inhibitors have real potential to improve the outcome for people with this disease.

Kanuka honey versus aciclovir for the topical treatment of herpes simplex labialis: a randomised controlled trial.

OBJECTIVE: To compare New Zealand medical grade kanuka honey with topical aciclovir for the treatment of herpes simplex labialis. DESIGN: Prospective parallel randomised controlled open-label superiority trial. SETTING: 76 community pharmacies across New Zealand between 10 September 2015 and 13 December 2017. PARTICIPANTS: 952 adults randomised within the first 72 hours of a herpes simplex labialis episode. INTERVENTIONS: Random assignment 1:1 to either 5% aciclovir cream or medical grade kanuka honey (90%)/glycerine (10%) cream, both applied five times daily. OUTCOME MEASURES: The primary outcome was time from randomisation to return to normal skin (stage 7). Secondary outcomes included time from randomisation to stage 4 (open wound), time from stage 4 to 7, maximal pain, time to pain resolution and treatment acceptability. RESULTS: Primary outcome variable: Kaplan-Meier-based estimates (95% CI) for the median time in days for return to normal skin were 8 (8 to 9) days for aciclovir and 9 (8 to 9) for honey; HR (95% CI) 1.06 (0.92 to 1.22), p=0.56. There were no statistically significant differences between treatments for all secondary outcome variables. No related serious adverse events were reported. CONCLUSION: There was no evidence of a difference in efficacy between topical medical grade kanuka honey and 5% aciclovir in the pharmacy-based treatment of herpes simplex labialis. TRIAL REGISTRATION NUMBER: ACTRN12615000648527;Post-results.

MSU Crystals Enhance TDB-Mediated Inflammatory Macrophage IL-1ß Secretion.

The tumour microenvironment predominantly consists of macrophages with phenotypes ranging from pro-inflammatory (M1-like) to anti-inflammatory (M2-like). Trehalose-6,6'-dibehenate (TDB) displays moderate anti-tumour activity and stimulates M1-like macrophages via the macrophage inducible C-type lectin (Mincle) resulting in IL-1ß production. In this study, we examined if monosodium urate (MSU), a known vaccine adjuvant, can boost IL-1ß production by TDB-stimulated macrophages. We investigated the effect of MSU/TDB co-treatment on IL-1ß production by GM-CSF (M1-like) and M-CSF/IL-4 (M2-like) differentiated mouse bone marrow macrophages (BMMs) and found that MSU/TDB co-treatment of GM-CSF BMMs significantly enhanced IL-1ß production in a Mincle-dependent manner. Western blot analysis showed that increased IL-1ß production by GM-CSF BMMs was associated with the induction of pro-IL-1ß expression by TDB rather than MSU. Flow cytometry analysis showed that MSU/TDB co-stimulation of GM-CSF BMMs led to greater expansion of CD86high/MHC IIhigh and CD86low/MHC IIlow subpopulations; however, only the latter showed increased production of IL-1ß. Together, these findings provide evidence of the potential to use MSU/TDB co-treatment to boost IL-1ß-mediated anti-tumour activity in M1-like tumour-associated macrophages.

Histone acetyltransferase activity of p300 is required for transcriptional repression by the promyelocytic leukemia zinc finger protein.

Histone acetyltransferase (HAT) activities of proteins such as p300, CBP, and P/CAF play important roles in activation of gene expression. We now show that the HAT activity of p300 can also be required for down-regulation of transcription by a DNA binding repressor protein. Promyelocytic leukemia zinc finger (PLZF), originally identified as a fusion with retinoic acid receptor alpha in rare cases of all-trans-retinoic acid-resistant acute promyelocytic leukemia, is a transcriptional repressor that recruits histone deacetylase-containing corepressor complexes to specific DNA binding sites. PLZF associates with p300 in vivo, and its ability to repress transcription is specifically dependent on HAT activity of p300 and acetylation of lysines in its C-terminal C2-H2 zinc finger motif. An acetylation site mutant of PLZF does not repress transcription and is functionally deficient in a colony suppression assay despite retaining its abilities to interact with corepressor/histone deacetylase complexes. This is due to the fact that acetylation of PLZF activates its ability to bind specific DNA sequences both in vitro and in vivo. Taken together, our results indicate that a histone deacetylase-dependent transcriptional repressor can be positively regulated through acetylation and point to an unexpected role of a coactivator protein in transcriptional repression.