The promyelocytic leukemia zinc finger (PLZF) protein binds DNA in a high molecular weight complex associated with cdc2 kinase

Regulatory function of DNA methylation mediated lncRNAs in gastric cancer

As one of the most common malignancies worldwide, gastric cancer contributes to cancer death with a high mortality rate partly responsible for its out-of-control progression as well as limited diagnosis. DNA methylation, one of the epigenetic events, plays an essential role in the carcinogenesis of many cancers, including gastric cancer. Long non-coding RNAs have emerged as the significant factors in the cancer progression functioned as the oncogene genes, the suppressor genes and regulators of signaling pathways over the decade. Intriguingly, increasing reports, recently, have claimed that abnormal DNA methylation regulates the expression of lncRNAs as tumor suppressor genes in gastric cancer and lncRNAs as regulators could exert the critical influence on tumor progression through acting on DNA methylation of other cancer-related genes. In this review, we summarized the DNA methylation-associated lncRNAs in gastric cancer which play a large impact on tumor progression, such as proliferation, invasion, metastasis and so on. Furthermore, the underlying molecular mechanism and signaling pathway might be developed as key points of gastric cancer range from diagnosis to prognosis and treatment in the future.

Identification of EOMES-expressing spermatogonial stem cells and their regulation by PLZF

Long-term maintenance of spermatogenesis in mammals is supported by GDNF, an essential growth factor required for spermatogonial stem cell (SSC) self-renewal. Exploiting a transgenic GDNF overexpression model, which expands and normalizes the pool of undifferentiated spermatogonia between Plzf +/+ and Plzf lu/lu mice, we used RNAseq to identify a rare subpopulation of cells that express EOMES, a T-box transcription factor. Lineage tracing and busulfan challenge show that these are SSCs that contribute to steady state spermatogenesis as well as regeneration following chemical injury. EOMES+ SSCs have a lower proliferation index in wild-type than in Plzf lu/lu mice, suggesting that PLZF regulates their proliferative activity and that EOMES+ SSCs are lost through proliferative exhaustion in Plzf lu/lu mice. Single cell RNA sequencing of EOMES+ cells from Plzf +/+ and Plzf lu/lu mice support the conclusion that SSCs are hierarchical yet heterogeneous.

Expression of the PTEN/FOXO3a/PLZF signalling pathway in pancreatic cancer and its significance in tumourigenesis and progression

Pancreatic cancer (PC) is one of the most lethal gastrointestinal malignancies. The PTEN/AKT signalling pathway is closely related to the tumourigenesis and progression of PC. The downstream effectors, FOXO3a, PLZF and VEGF, are reported to be involved in angiogenesis, lymph node metastasis and poor survival in PC. By using tissue microarrays and immunohistochemistry, we found, that PTEN, FOXO3a and PLZF expression was significantly decreased in PC specimens compared with that in chronic pancreatitis (CP) specimens, while VEGF expression was significantly increased. Furthermore, the expression of PTEN was positively correlated with that of FOXO3a and PLZF but negatively correlated with that of VEGF. Our results suggest that the PTEN/FOXO3a/PLZF signalling pathway may negatively regulate VEGF expression in PC. Through clinical analysis of 69 PC patients, PTEN, FOXO3a and PLZF expression was found to be significantly decreased in specimens from PC patients with lymph node metastasis and poor prognosis, while VEGF expression was significantly increased. Taken together, these reaults suggest that the PTEN/FOXO3a/PLZF signalling pathway may be capable of inhibiting growth and metastasis in PC by regulating VEGF-mediated angiogenesis, which requires further in vivo and in vitro studies and can potentially be a therapeutic target for PC.

Regulation of hematopoietic development by ZBTB transcription factors

Hematopoietic development is governed by the coordinated expression of lineage- and differentiation stage-specific genes. Transcription factors play major roles in this process and their perturbation may underlie hematologic and immunologic disorders. Nearly 1900 transcription factors are encoded in the human genome: of these, 49 BTB (for broad-complex, tram-track and bric à brac)-zinc finger transcription factors referred to as ZBTB or POK proteins have been identified. ZBTB proteins, including BCL6, PLZF, ThPOK and LRF, exhibit a broad spectrum of functions in normal and malignant hematopoiesis. This review summarizes developmental and molecular functions of ZBTB proteins relevant to hematology.

Zebrafish Plzf transcription factors enhance early type I IFN response induced by two non-enveloped RNA viruses

The BTB-POZ transcription factor Promyelocytic Leukemia Zinc Finger (PLZF, or ZBTB16) has been recently identified as a major factor regulating the induction of a subset of Interferon stimulated genes in human and mouse. We show that the two co-orthologues of PLZF found in zebrafish show distinct expression patterns, especially in larvae. Although zbtb16a/plzfa and zbtb16b/plzfb are not modulated by IFN produced during viral infection, their over-expression increases the level of the early type I IFN response, at a critical phase in the race between the virus and the host response. The effect of Plzfb on IFN induction was also detectable after cell infection by different non-enveloped RNA viruses, but not after infection by the rhabdovirus SVCV. Our findings indicate that plzf implication in the regulation of type I IFN responses is conserved across vertebrates, but at multiple levels of the pathway and through different mechanisms.

The promyelocytic leukemia zinc finger protein: two decades of molecular oncology

The promyelocytic leukemia zinc finger (PLZF) protein, also known as Zbtb16 or Zfp145, was first identified in a patient with acute promyelocytic leukemia, where a reciprocal chromosomal translocation t(11;17)(q23;q21) resulted in a fusion with the RARA gene encoding retinoic acid receptor alpha. The wild-type Zbtb16 gene encodes a transcription factor that belongs to the POK (POZ and Krüppel) family of transcriptional repressors. In addition to nine Krüppel-type sequence-specific zinc fingers, which make it a member of the Krüppel-like zinc finger protein family, the PLZF protein contains an N-terminal BTB/POZ domain and RD2 domain. PLZF has been shown to be involved in major developmental and biological processes, such as spermatogenesis, hind limb formation, hematopoiesis, and immune regulation. PLZF is localized mainly in the nucleus where it exerts its transcriptional repression function, and many post-translational modifications affect this ability and also have an impact on its cytoplasmic/nuclear dissociation. PLZF achieves its transcriptional regulation by binding to many secondary molecules to form large multi-protein complexes that bind to the regulatory elements in the promoter region of the target genes. These complexes are also capable of physically interacting with its target proteins. Recently, PLZF has become implicated in carcinogenesis as a tumor suppressor gene, since it regulates the cell cycle and apoptosis in many cell types. This review will examine the major advances in our knowledge of PLZF biological activities that augment its value as a therapeutic target, particularly in cancer and immunological diseases.

Smooth muscle α-actin is a direct target of PLZF: effects on the cytoskeleton and on susceptibility to oncogenic transformation

Changes in cell morphology and rearrangements of the actin cytoskeleton are common features accompanying cell transformation induced by various oncogenes. In this study, we show that promyelocytic leukemia zinc finger protein (PLZF) binds to the promoter of smooth muscle α-actin, reducing mRNA and protein levels encoded by this gene and resulting in a reorganization of the actin cytoskeleton. In cultures of chicken embryo fibroblasts (CEF), this effect on α-actin expression is correlated with a change in cellular phenotype from spindle shaped to polygonal and flattened. This morphological change is dependent on Ras function. The polygonal, flattened CEF show a high degree of resistance to the transforming activity of several oncoproteins. Our results support the conclusion that reorganization of the actin cytoskeleton plays an important role in tumor suppression by PLZF.

PLZF/ZBTB16, a glucocorticoid response gene in acute lymphoblastic leukemia, interferes with glucocorticoid-induced apoptosis

Glucocorticoids (GCs) cause cell cycle arrest and apoptosis in lymphoid cells which is exploited to treat lymphoid malignancies. The mechanisms of these anti-leukemic GC effects are, however, poorly understood. We previously defined a list of GC-regulated genes by expression profiling in children with acute lymphoblastic leukemia (ALL) during systemic GC monotherapy and in experimental systems of GC-induced apoptosis. PLZF/ZBTB16, a transcriptional repressor, was one of the most promising candidates derived from this screen. To investigate its role in the anti-leukemic GC effects, we performed overexpression and knock-down experiments in CCRF-CEM childhood ALL cells. Transgenic PLZF/ZBTB16 alone had no detectable effect on cell proliferation or survival, but reduced sensitivity to GC-induced apoptosis but not apoptosis induced by antibodies against Fas/CD95 or 3 different chemotherapeutics. Knock-down of ZBTB16 entailed a small, but significant, increase in cell death induction by GC. Affymetrix Exon array-based whole genome expression profiling revealed that PLZF/ZBTB16 induction did not significantly alter the expression profile, however, it interfered with the regulation of numerous GC response genes, including BCL2L11/Bim, which has previously been shown to be responsible for cell death induction in CCRF-CEM cells. Thus, the protective effect of PLZF/ZBTB16 can be attributed to interference with transcriptional regulation by GC.

Inhibition of angiogenesis by the BTB domain of promyelocytic leukemia zinc finger protein

Promyelocytic leukemia zinc finger is a negative regulator of cell cycle progression. In this study, we showed that PLZF inhibits endothelial cell angiogenesis using a human umbilical vein endothelial cell system. We also focused on characterizing the specific function of the BTB domain of PLZF as a novel apoptotic and anti-angiogenic protein via deletion mapping analysis. The BTB domain directly inhibited tube formation, as well as the biological functions of angiostatic activity in vivo, and reduced the expression of p-Akt and p-eNOS, which play a significant role in angiogenesis when stimulated by VEGF. These results strongly suggest that the BTB domain could potentially modulate the apoptotic and anti-angiogenic effects of PLZF.

Posttranslational regulation of Myc by promyelocytic leukemia zinc finger protein

The promyelocytic leukemia zinc finger (PLZF) protein, a transcriptional repressor, induces cellular resistance to oncogenic transformation by diverse oncoproteins. Two point mutants of PLZF that have lost the antioncogenic activity of the wild-type protein are oncogenic in chicken embryo fibroblasts; this activity is correlated with differential effects on Myc. Wild-type PLZF represses Myc transcription without affecting total Myc protein levels and reduces the levels of phosphorylated Myc. The PLZF mutants do not alter Myc transcription or protein expression but increase the levels of phosphorylated Myc. These modifications of Myc are correlated with PLZF-induced changes in Akt and the mitogen-activated protein kinase (MAPK) pathway. Wild-type PLZF downregulates the MAPK pathway and activates Akt, resulting in reduced phosphorylation on serine 62 of Myc by Erk and on threonine 58 by glycogen synthase kinase 3beta. The mutants fail to activate Akt and only slightly downregulate phospho-Erk. We postulate that the 2 PLZF mutants are oncogenic, because they function as dominant negatives of wild-type PLZF, enhancing Myc phosphorylation and increasing Myc transcriptional and oncogenic activity. In support of this suggestion, a specific inhibitor of Myc is able to revert the transformed phenotype of PLZF mutant-expressing cells.

Nuclear LYRIC/AEG-1 interacts with PLZF and relieves PLZF-mediated repression

LYRIC/AEG-1 and its altered expression have been linked to carcinogenesis in prostate, brain and melanoma as well as promoting chemoresistance and metastasis in breast cancer. LYRIC/AEG-1 function remains unclear, although LYRIC/AEG-1 is activated by oncogenic HA-RAS, through binding of c-myc to its promoter, which in turn regulates the key components of the PI3-kinase and nuclear factor-kappaB pathways. We have identified the transcriptional repressor PLZF as an interacting protein of LYRIC/AEG through a yeast two-hybrid screen. PLZF regulates the expression of genes involved in cell growth and apoptosis including c-myc. Coexpression of LYRIC/AEG-1 with PLZF leads to a reduction in PLZF-mediated repression by reducing PLZF binding to promoters. We have confirmed that nuclear LYRIC/AEG-1 and PLZF interact in mammalian cells via the N- and C termini of LYRIC/AEG-1 and a region C terminal to the RD2 domain of PLZF. Both proteins colocalize to nuclear bodies containing histone deacetylases, which are known to promote PLZF-mediated repression. Our data suggest one mechanism for cells with altered LYRIC/AEG-1 expression to evade apoptosis and increase cell growth during tumourigenesis through the regulation of PLZF repression.

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.

PLZF-mediated control on c-kit expression in CD34(+) cells and early erythropoiesis

The promyelocytic leukemia zinc-finger protein (PLZF) is a transcription factor and c-kit is a receptor tyrosine kinase associated with human disease, particularly in hematopoietic cells. MicroRNAs (miRs) are post-transcriptional regulators of gene expression, and c-kit has been described as a target of miRs-221 and -222 in erythropoiesis. In the present study, we identified c-kit as a target of PLZF in normal and leukemic cells. Particularly, in erythropoietic (E) culture of CD34(+) progenitors, PLZF is downregulated, whereas c-kit expression at both the mRNA and protein levels inversely increases during the first days of E differentiation. In functional experiments, PLZF transfection induces c-kit downregulation, inhibits E proliferation and delays differentiation, whereas PLZF knockdown induces opposite effects, independently of miRs-221 and -222 expression. The inverse correlation between PLZF and c-kit expression was found in normal CD34(+)38(+/-) hematopoietic progenitor/stem cells and in acute myeloid leukemias of M0/M1 French-American-British subtypes, suggesting that the control of PLZF on c-kit expression may be crucial at the level of the stem cell/progenitor compartment. Altogether, our data indicate a new mechanism of regulation of c-kit expression that involves a transcriptional control by PLZF in CD34(+) cells and early erythropoiesis.

The GTPase domain of Galphao contributes to the functional interaction of Galphao with the promyelocytic leukemia zinc finger protein

Go, one of the most abundant heterotrimeric G proteins in the brain, is classified as a member of the Gi/Go family based on its homology to Gi proteins. Recently, we identified promyelocytic leukemia zinc finger protein (PLZF) as a candidate downstream effector for the alpha subunit of Go (Gαo). Activated Gαo interacts with PLZF and augments its function as a repressor of transcription and cell growth. G protein-coupled receptor-mediated Gαo activation also enhanced PLZF function. In this study, we determined that the GTPase domain of Gαo contributes to Gαo:PLZF interaction. We also showed that the Gαo GTPase domain is important in modulating the function of PLZF. This data indicates that the GTPase domain of Gαo may be necessary for the functional interaction of Gαo with PLZF.

A three-step pathway comprising PLZF/miR-146a/CXCR4 controls megakaryopoiesis

MicroRNAs (miRNAs or miRs) regulate diverse normal and abnormal cell functions. We have identified a regulatory pathway in normal megakaryopoiesis, involving the PLZF transcription factor, miR-146a and the SDF-1 receptor CXCR4. In leukaemic cell lines PLZF overexpression downmodulated miR-146a and upregulated CXCR4 protein, whereas PLZF knockdown induced the opposite effects. In vitro assays showed that PLZF interacts with and inhibits the miR-146a promoter, and that miR-146a targets CXCR4 mRNA, impeding its translation. In megakaryopoietic cultures of CD34(+) progenitors, PLZF was upregulated, whereas miR-146a expression decreased and CXCR4 protein increased. MiR-146a overexpression and PLZF or CXCR4 silencing impaired megakaryocytic (Mk) proliferation, differentiation and maturation, as well as Mk colony formation. Mir-146a knockdown induced the opposite effects. Rescue experiments indicated that the effects of PLZF and miR-146a are mediated by miR-146a and CXCR4, respectively. Our data indicate that megakaryopoiesis is controlled by a cascade pathway, in which PLZF suppresses miR-146a transcription and thereby activates CXCR4 translation.

The PLZF gene of t (11;17)-associated APL

The PLZF gene is one of five partners fused to the retinoic acid receptor alpha in acute promyelocytic leukemia. PLZF encodes a DNA-binding transcriptional repressor and the PLZF-RARalpha fusion protein like other RARalpha fusions can inhibit the genetic program mediated by the wild tpe retinoic acid receptor. However an increasing body of literature indicates an important role for the PLZF gene in growth control and development. This information suggests that loss of PLZF function might also contribute to leukemogenesis.

TIMP-1 regulates cell proliferation by interacting with the ninth zinc finger domain of PLZF

The tissue inhibitors of metalloproteinases (TIMPs) are multifunctional proteins that specifically inhibit matrix metalloproteinases (MMPs) and regulate extracellular matrix (ECM) turnover and tissue remodeling. This is directed by forming tightly bound inhibitory complexes with MMPs. Recent years have revealed important differences of various biological activities between TIMP families but molecular mechanisms are not clear. To define the molecular mechanisms of TIMP-1-dependent biological processes, we used TIMP-1 as bait in a yeast two-hybrid screen, along with a human ovary cDNA library. Further characterization revealed the ninth zinc finger domain as an interacting domain of the promyelocytic leukemia zinc finger protein (PLZF). Interaction of PLZF with TIMP-1 in mammalian cells was also confirmed by co-immunoprecipitation and with in vitro binding assays. We investigated whether TIMP-1-mediated anti-apoptotic activity could promote the growth of ovarian cancer in an experimental model system. TIMP-1 treatment was found to be more effective at increasing ovarian cancer growth when compared with PLZF in parallel experiments. Subsequently, the efficacy of a combined treatment with TIMP-1 and PLZF was investigated. In the presence of both of these proteins, TIMP-1 significantly reduced apoptosis induced by PLZF in cervical carcinoma cells. These combined results indicate that TIMP-1 functions as an anti-activator of the transcriptional repressive activity of PLZF.

A novel cervical cancer suppressor 3 (CCS-3) interacts with the BTB domain of PLZF and inhibits the cell growth by inducing apoptosis

Promyelocytic leukemia zinc finger protein (PLZF) is a sequence-specific, DNA binding, transcriptional repressor differentially expressed during embryogenesis and in adult tissues. PLZF is known to be a negative regulator of cell cycle progression. We used PLZF as bait in a yeast two-hybrid screen with a cDNA library from the human ovary tissue. A novel cervical cancer suppressor 3 (CCS-3) was identified as a PLZF interacting partner. Further characterization revealed the BTB domain as an interacting domain of PLZF. Interaction of CCS-3 with PLZF in mammalian cells was also confirmed by co-immunoprecipitation and in vitro binding assays. It was found that, although CCS-3 shares similar homology with eEF1A, the study determined CCS-3 to be an isoform. CCS-3 was observed to be downregulated in human cervical cell lines as well as in cervical tumors when compared to those from normal tissues. Overexpression of CCS-3 in human cervical cell lines inhibits cell growth by inducing apoptosis and suppressing human cyclin A2 promoter activity. These combined results suggest that the potential tumor suppressor activity of CCS-3 may be mediated by its interaction with PLZF.

PLZF-mediated control on VLA-4 expression in normal and leukemic myeloid cells

The promyelocytic leukemia zinc-finger protein (PLZF) is a transcriptional repressor. To investigate the role of PLZF in the regulation of cytoadhesion molecules involved in the mobilization of hemopoietic cells, we have analysed PLZF and very late antigen 4 (VLA-4) expression in normal and leukemic cells. In hematopoiesis, we found a negative correlation between PLZF and VLA-4 expression, except for the megakaryocytic lineage. In contrast, we observed a positive correlation between PLZF and VLA-4 expression in a panel of acute myeloid leukemia (AML) samples. In K562 cells expressing PLZF (K562-PLZF), we found that the expression of VLA-4 and c-kit was downmodulated. We have investigated the possibility for VLA-4 or the c-kit receptor to be direct target genes of PLZF in K562-PLZF cells and identified a PLZF DNA-binding site within the VLA-4 promoter. Furthermore, decrease in VLA-4 expression was associated with loss of adhesion on fibronectin-coated plates, which promotes drug-induced apoptosis of K562-PLZF cells. Our findings indicate that VLA-4 is a potential target gene of PLZF. However, in primary AMLs the control of PLZF on VLA-4 expression is lost. Altogether, we suggest that VLA-4 modulation by PLZF may represent an important step in the control of normal and leukemic cell mobilization.

Analysis of the interaction between hPFTAIRE1 and PLZF in a yeast two-hybrid system

hPFTAIRE1 is a Cdc2-related kinase family member. To search its substrates and regulatory proteins, hPFTAIRE1 was fused to LexA and used as a bait to screen a human brain LexA two-hybrid library. In this screening, seven hPFTAIRE1 interacting proteins, including promyelocytic leukemia zinc finger (PLZF), were obtained. The interaction between PLZF and hPFTAIRE1 was confirmed by beta-galactosidase assay and Leu growth activity. PLZF encodes a transcription factor belonging to the POZ/BTB domain and Krüpel zinc finger (POK) family. The highly conserved POZ/BTB domain plays a critical role in protein-protein interaction. We deleted the POZ/BTB and Krüpel zinc finger domains, respectively, and observed the interaction between hPFTAIRE1 and truncated PLZFs by liquid beta-galactosidase activity assay. A weak interaction was detected between hPFTAIRE1 and PLZF. We also observed the interaction between PLZF and another Cdc2-related kinase, PCTAIRE1. A similar result was observed. The interaction between PLZF and hPFTAIRE1 or PCTAIRE1 was confirmed by co-immunoprecipitation assay in a yeast system. PLZF is a phosphoprotein and plays multiple roles during cell growth. Our results suggest that hPFTAIRE1 and PCTAIRE1 may play important roles in the functional regulation of PLZF.

Sequence and structural analysis of BTB domain proteins

BACKGROUND

The BTB domain (also known as the POZ domain) is a versatile protein-protein interaction motif that participates in a wide range of cellular functions, including transcriptional regulation, cytoskeleton dynamics, ion channel assembly and gating, and targeting proteins for ubiquitination. Several BTB domain structures have been experimentally determined, revealing a highly conserved core structure.

RESULTS

We surveyed the protein architecture, genomic distribution and sequence conservation of BTB domain proteins in 17 fully sequenced eukaryotes. The BTB domain is typically found as a single copy in proteins that contain only one or two other types of domain, and this defines the BTB-zinc finger (BTB-ZF), BTB-BACK-kelch (BBK), voltage-gated potassium channel T1 (T1-Kv), MATH-BTB, BTB-NPH3 and BTB-BACK-PHR (BBP) families of proteins, among others. In contrast, the Skp1 and ElonginC proteins consist almost exclusively of the core BTB fold. There are numerous lineage-specific expansions of BTB proteins, as seen by the relatively large number of BTB-ZF and BBK proteins in vertebrates, MATH-BTB proteins in Caenorhabditis elegans, and BTB-NPH3 proteins in Arabidopsis thaliana. Using the structural homology between Skp1 and the PLZF BTB homodimer, we present a model of a BTB-Cul3 SCF-like E3 ubiquitin ligase complex that shows that the BTB dimer or the T1 tetramer is compatible in this complex.

CONCLUSION

Despite widely divergent sequences, the BTB fold is structurally well conserved. The fold has adapted to several different modes of self-association and interactions with non-BTB proteins.

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.

eor-1 and eor-2 are required for cell-specific apoptotic death in C. elegans

Programmed cell death occurs in every multicellular organism and in diverse cell types yet the genetic controls that define which cells will live and which will die remain poorly understood. During development of the nematode Caenorhabditis elegans, the coordinated activity of four gene products, EGL-1, CED-9, CED-4 and CED-3, results in the death of essentially all cells fated to die. To identify novel upstream components of the cell death pathway, we performed a genetic screen for mutations that abolish the death of the hermaphrodite-specific neurons (HSNs), a homologous pair of cells required for egg-laying in the hermaphrodite. We identified and cloned the genes, eor-1 and eor-2, which are required to specify the fate of cell death in male HSNs. In addition to defects in HSN death, mutation of either gene leads to defects in coordinated movement, neuronal migration, male tail development, and viability; all consistent with abnormal neuronal differentiation. eor-1 encodes a putative transcription factor related to the human oncogene PLZF. eor-2 encodes a novel but conserved protein. We propose that eor-1 and eor-2 function together throughout the nervous system to promote terminal differentiation of neurons and function specifically in male HSNs to promote apoptotic death of the HSNs.

Identification and characterization of PLZF as a prostatic androgen-responsive gene

BACKGROUND

Promyelocytic leukemia zinc finger protein (PLZF) was initially identified by virtue of its fusion with RARalpha as a result of a variant t(11;17) chromosomal translocation that occurs in a small subset of acute promyelocytic leukemia (APL) patients. PLZF has been reported to have pro-apoptotic and anti-proliferative activity both in vivo and in vitro.

METHODS

Using a modified subtractive hybridization, we identified PLZF as an androgen-responsive gene in the rat ventral prostate. Northern blot and Western blot were used to characterize the regulation of PLZF by androgens in LNCaP cells. Stable transfections of PLZF in LNCaP cells were performed to assay the effect of PLZF overexpression on LNCaP cell proliferation.

RESULTS

PLZF mRNA was transiently up-regulated by androgens in the regressed ventral prostate of castrated adult rat. PLZF was also up-regulated by androgens, at both mRNA and protein levels, in the androgen-responsive human prostate cancer cell line LNCaP. Androgen induction of PLZF mRNA was not inhibited by protein synthesis inhibitor cycloheximide but inhibited by androgen receptor antagonist bicalutamide, indicating that PLZF is a direct androgen-responsive gene. To study the functions of PLZF in androgen action, LNCaP sublines stably overexpressing PLZF were generated. PLZF overexpression inhibited LNCaP proliferation either in the presence or absence of androgen, which is consistent with the reported anti-proliferative activity of PLZF.

CONCLUSIONS

The above observations indicate that PLZF is an androgen-responsive gene with anti-proliferative activity in prostate cancer cells.

The Flt3 internal tandem duplication mutant inhibits the function of transcriptional repressors by blocking interactions with SMRT

Fms-like tyrosine kinase 3 (Flt3) is a type III receptor tyrosine kinase (RTK). Between 20% and 30% of acute myeloid leukemia (AML) patients have either an internal tandem duplication (ITD) of the juxtamembrane region or a point mutation of the Flt3 receptor leading to the constitutive activation of downstream signaling pathways and aberrant cell growth. The silencing mediator of retinoic and thyroid hormone receptors (SMRT) corepressor mediates transcriptional repression by interacting with transcription factors such as the promyelocytic leukemia zinc finger (PLZF) protein. Previous reports indicate that SMRT interaction with transcription factors can be disrupted by phosphorylation through activation of RTK pathways. We report here that the Flt3-ITD interferes with the transcriptional and biologic action of the PLZF transcriptional repressor. In the presence of Flt3-ITD, PLZF-SMRT interaction was reduced, transcriptional repression by PLZF was inhibited, and PLZF-mediated growth suppression of leukemia cells was partially blocked. Furthermore, overexpression of Flt3-ITD led to a partial relocalization of SMRT protein from the nucleus to the cytoplasm. Nuclear export was dependent on the SMRT receptor interaction domain (RID), and Flt3-ITD enhances the binding of nuclear-cytoplasm shuttling protein nuclear factor-kappaB-p65 (NFkappaB-p65) to this region. These data suggest that activating mutations of Flt3 may disrupt transcriptional repressor function resulting in aberrant gene regulation and abnormal leukemia cell growth.

Growth suppression by acute promyelocytic leukemia-associated protein PLZF is mediated by repression of c-myc expression

The transcriptional repressor PLZF was identified by its translocation with retinoic acid receptor alpha in t(11;17) acute promyelocytic leukemia (APL). Ectopic expression of PLZF leads to cell cycle arrest and growth suppression, while disruption of normal PLZF function is implicated in the development of APL. To clarify the function of PLZF in cell growth and survival, we used an inducible PLZF cell line in a microarray analysis to identify the target genes repressed by PLZF. One prominent gene identified was c-myc. The array analysis demonstrated that repression of c-myc by PLZF led to a reduction in c-myc-activated transcripts and an increase in c-myc-repressed transcripts. Regulation of c-myc by PLZF was shown to be both direct and reversible. An interaction between PLZF and the c-myc promoter could be detected both in vitro and in vivo. PLZF repressed the wild-type c-myc promoter in a reporter assay, dependent on the integrity of the binding site identified in vitro. PLZF binding in vivo was coincident with a decrease in RNA polymerase occupation of the c-myc promoter, indicating that repression occurred via a reduction in the initiation of transcription. Finally, expression of c-myc reversed the cell cycle arrest induced by PLZF. These data suggest that PLZF expression maintains a cell in a quiescent state by repressing c-myc expression and preventing cell cycle progression. Loss of this repression through the translocation that occurs in t(11;17) would have serious consequences for cell growth control.

Mechanism of SMRT Corepressor Recruitment by the BCL6 BTB Domain

BCL6 encodes a transcription factor that represses genes necessary for the terminal differentiation of lymphocytes within germinal centers, and the misregulated expression of this factor is strongly implicated in several types of B cell lymphoma. The homodimeric BTB domain of BCL6 (also known as the POZ domain) is required for the repression activity of the protein and interacts directly with the SMRT and N-CoR corepressors that are found within large multiprotein histone deacetylase-containing complexes. We have identified a 17 residue fragment from SMRT that binds to the BCL6 BTB domain, and determined the crystal structure of the complex to 2.2 A. Two SMRT fragments bind symmetrically to the BCL6 BTB homodimer and, in combination with biochemical and in vivo data, the structure provides insight into the basis of transcriptional repression by this critical B cell lymphoma protein.

PLZF is a negative regulator of retinoic acid receptor transcriptional activity

BACKGROUND: Retinoic acid receptors (RARs) are ligand-regulated transcription factors controlling cellular proliferation and differentiation. Receptor-interacting proteins such as corepressors and coactivators play a crucial role in specifying the overall transcriptional activity of the receptor in response to ligand treatment. Little is known however on how receptor activity is controlled by intermediary factors which interact with RARs in a ligand-independent manner. RESULTS: We have identified the promyelocytic leukemia zinc finger protein (PLZF), a transcriptional corepressor, to be a RAR-interacting protein using the yeast two-hybrid assay. We confirmed this interaction by GST-pull down assays and show that the PLZF N-terminal zinc finger domain is necessary and sufficient for PLZF to bind RAR. The RAR ligand binding domain displayed the highest affinity for PLZF, but corepressor and coactivator binding interfaces did not contribute to PLZF recruitment. The interaction was ligand-independent and correlated to a decreased transcriptional activity of the RXR-RAR heterodimer upon overexpression of PLZF. A similar transcriptional interference could be observed with the estrogen receptor alpha and the glucocorticoid receptor. We further show that PLZF is likely to act by preventing RXR-RAR heterodimerization, both in-vitro and in intact cells. CONCLUSION: Thus RAR and PLZF interact physically and functionally. Intriguingly, these two transcription factors play a determining role in hematopoiesis and regionalization of the hindbrain and may, upon chromosomal translocation, form fusion proteins. Our observations therefore define a novel mechanism by which RARs activity may be controlled.

Molecular pathogenesis of acute promyelocytic leukaemia and APL variants

It has been 12 years since the simultaneous discovery of the unique sensitivity of acute promyelocytic leukaemia (APL) to differentiation therapy with all-trans retinoic acid (ATRA) and the discovery that the retinoic acid receptor alpha (RARalpha) gene was rearranged in APL. Nearly 98% of cases of APL are associated with t(15;17) chromosomal translocation and fusion of the PML gene to that encoding RARalpha to yield an abnormal receptor with the capability of de-regulating gene expression in the haematopoietic cell, causing differentiation block and eventually the development of leukaemia. Since this original discovery, four other translocations were described in APL. In each of these the RARalpha gene is fused to different partner genes, all yielding aberrant nuclear receptors. These fusion proteins share in common the ability to repress rather than activate retinoic acid targets, one so strongly that the result is an ATRA-resistant form of the disease. In addition each of the partner proteins is important for normal cell growth and development. In this chapter we explore the biology of the RARalpha, the fusion proteins created in APL and the normal forms of the partner proteins. Through continued study of this disease it is hoped that novel treatments, potentially more applicable to other forms of leukaemia, may arise.

NAC1, a POZ/BTB protein present in the adult mammalian brain, triggers apoptosis after adenovirus-mediated overexpression in PC-12 cells

POZ/BTB proteins influence cellular development and in some examples act as oncoproteins. However, several POZ/BTB transcription factors have been found in terminally differentiated neurons, where their functions remain unknown. One example is NAC1, a constitutively-expressed protein that can regulate behaviors associated with cocaine use. The present study represents an initial attempt to understand the actions of NAC1 within neurons by using adenoviral-mediated gene transfer into differentiated PC-12 cells. Cell survival in PC-12 cells overexpressing NAC1 was greatly reduced compared with cells infected by a control Ad-GFP. The morphological appearance of the dying cells was consistent with programmed cell death. Fragmentation of genomic DNA occurred in PC-12 cells infected with adenoviruses encoding NAC1 but not control viruses. NAC1 over expression was followed by the down regulation of the anti-apoptotic proteins Bcl-2 and Bcl-2-xl. Concurrently, levels of the pro-apoptotic proteins Bax and p53 increased following NAC1 overexpression. These observations suggest that NAC1expression in PC-12 cells induces apoptosis by altering the expression of these upstream mediators of the execution phase of programmed cell death. These findings raise the possibility that aberrantly regulated NAC1 expression in the mammalian brain may contribute to programmed cell death.

The LIM-only protein DRAL/FHL2 interacts with and is a corepressor for the promyelocytic leukemia zinc finger protein

Members of the four-and-a-half-LIM domain (FHL) protein family, which are expressed in a tissue- and stage-specific manner, have been reported previously to function as transcriptional coactivators. One of these is the p53-inducible protein DRAL/FHL2 (where DRAL is down-regulated in rhabdomyosarcoma LIM domain protein). In this work, we identified potential binding partners for DRAL/FHL2 using an inducible yeast two-hybrid system. We present evidence of a functional interaction between the promyelocytic leukemia zinc finger protein (PLZF) and DRAL/FHL2. PLZF is a sequence-specific transcriptional repressor whose function relies on recruitment of corepressors that form part of the histone deacetylase complex involved in chromatin remodeling. DRAL/FHL2 interacts specifically with PLZF in vitro and in vivo and augments transcriptional repression mediated by PLZF. This is the first reported incidence of a bona fide FHL protein-mediated corepression and supports the notion of these proteins having a role as coregulators of tissue-specific gene expression.

Phyllopod acts as an adaptor protein to link the sina ubiquitin ligase to the substrate protein tramtrack

The RING domain protein Sina, together with Phyllopod and the F-box protein Ebi, forms a Ras-regulated E3 ubiquitin ligase complex that activates photoreceptor cell differentiation in the eye of Drosophila melanogaster. The expression of Phyllopod is induced upon Ras activation, allowing the complex to degrade the transcription repressor Tramtrack and removing its block of neuronal development in photoreceptor precursors. We show that Phyllopod functions as an adaptor in the complex, physically linking Sina with Tramtrack via separate binding domains. One 19-amino-acid domain in Phyllopod interacts with a region of Sina's SBD domain. Another domain in Phyllopod interacts with a C-terminal helix in the POZ domain of Tramtrack. This interaction is specific to the Tramtrack POZ domain and not to other POZ domain proteins present in photoreceptor precursors. Degradation of Tramtrack is dependent upon association of Sina with its cognate binding site in Phyllopod. These results illustrate how Ras signaling can modulate an E3 ligase activity not by the phosphorylation of substrate proteins but by regulating the expression of specific E3 adaptors.

PLZF induces megakaryocytic development, activates Tpo receptor expression and interacts with GATA1 protein

We investigated the expression of the PLZF gene in purified human hematopoietic progenitors induced to unilineage erythroid, granulocytic or megakaryocytic differentiation and maturation in serum-free culture. PLZF is expressed in quiescent progenitors: the expression level progressively rises through megakaryocytic development, whereas it gradually declines in erythroid and granulopoietic culture. To investigate the role of PLZF in megakaryopoiesis, we transduced the PLZF gene into the erythro-megakaryocytic TF1 cell line. PLZF overexpression upmodulates the megakaryocytic specific markers (CD42a, CD42b, CD61, PF4) and induces the thrombopoietin receptor (TpoR). The proximal promoter of the TpoR gene is activated in PLZF-expressing TF1 cells: in this promoter region, a PLZF DNA-binding site was identified by deletion constructs studies. Interestingly, PLZF and GATA1 proteins coimmunoprecipitate in PLZF-expressing TF1 cells: enforced expression of both PLZF and GATA1 in TF1 cells results in increased upregulation of megakaryocytic markers, as compared to exogenous PLZF or GATA1 alone, suggesting a functional role for the PLZF/GATA1 complex. Our data indicate that PLZF plays a significant stimulatory role in megakaryocytic development, seemingly mediated in part by induction of TpoR expression at transcriptional level. This stimulatory effect is potentiated by physical interaction of PLZF and GATA1, which are possibly assembled in a multiprotein transcriptional complex.

The effects of the Fanconi anemia zinc finger (FAZF) on cell cycle, apoptosis, and proliferation are differentiation stage-specific

FAZF, a member of the BTB/POZ family of transcriptional repressor proteins, has been shown to bind to FANCC, the protein defective in patients with the bone marrow failure syndrome Fanconi anemia complementation group C. Because bone marrow failure in Fanconi anemia has been attributed to a failure of the hematopoietic stem cell population to produce sufficient progeny, we documented the expression of FAZF in human CD34(+) hematopoietic progenitor cells. FAZF was expressed at high levels in early stages of differentiation but declined during subsequent differentiation into erythroid and myeloid lineages. Consistent with its presumed role as a transcriptional repressor, FAZF was found in the nuclear compartment, where it resides in distinct nuclear speckles at or near sites of DNA replication. Using a FAZF-inducible myeloid cell line, we found that enforced expression of FAZF was accompanied by accumulation in the G(1) phase of the cell cycle followed later by apoptosis. These results suggest an essential role for FAZF during the proliferative stages of primitive hematopoietic progenitors, possibly acting in concert with (a subset of) the Fanconi anemia proteins.

Differences in expression, actions and cocaine regulation of two isoforms for the brain transcriptional regulator NAC1

BTB/POZ proteins can influence the cell cycle and contribute to oncogenesis. Many family members are present in the mammalian CNS. Previous work demonstrated elevated NAC1 mRNA levels in the rat nucleus accumbens in response to cocaine. NAC1 acts like other BTB/POZ proteins that regulate transcription but is unusual because of the absence of identifiable DNA binding domains. cDNAs were isolated encoding two NAC1 isoforms differing by only 27 amino acids (the longer isoform contains 514 amino acids). The mRNAs for both isoforms were simultaneously expressed throughout the rat brain and peripheral tissues. Semi-quantitative reverse transcription-polymerase chain reaction analysis revealed that the mRNA of the longer isoform was more abundant than the mRNA of the shorter isoform. Western blot analysis demonstrated a similar unequal distribution between the isoforms in the CNS. The longer isoform was the more abundant of the two NAC1 proteins and the ratio between them differed throughout the rat brain. The shorter isoform was not detected in most of the examined peripheral tissues, suggesting differences from the CNS in post-transcriptional processing. Both isoforms repressed transcription in H293T cells using a Gal4-luciferase reporter system. However, the shorter isoform did not repress transcription as effectively as the longer isoform. Transfection of different ratios for both isoforms, in order to replicate the relative amounts observed throughout the CNS, supported an interaction between the isoforms. The net effect on transcriptional repression was determined by the ratio of the two NAC1 isoforms. Each isoform exhibited the subnuclear localization that is characteristic of many BTB/POZ proteins. A rapid and transient increase in the level of the shorter isoform occurred in the nucleus accumbens 2 h following a single i.p. cocaine injection. We conclude that the two isoforms of NAC1 may differentially affect neuronal functions, including the regulation of cocaine-induced locomotion.

Critical residues within the BTB domain of PLZF and Bcl-6 modulate interaction with corepressors

The PLZF (promyelocytic leukemia zinc finger) transcriptional repressor, when fused to retinoic acid receptor alpha (RARalpha), causes a refractory form of acute promyelocytic leukemia. The highly conserved N-terminal BTB (bric a brac, tramtrack, broad complex)/POZ domain of PLZF plays a critical role in this disease, since it is required for transcriptional repression by the PLZF-RARalpha fusion protein. The crystal structure of the PLZF BTB domain revealed an obligate homodimer with a highly conserved charged pocket formed by apposition of the two monomers. An extensive structure-function analysis showed that the charged pocket motif plays a major role in transcriptional repression by PLZF. We found that mutations of the BTB domain that neutralize key charged pocket residues did not disrupt dimerization, yet abrogated the ability of PLZF to repress transcription and led to the loss of interaction with N-CoR, SMRT, and histone deacetylases (HDACs). We extended these studies to the Bcl-6 protein, which is linked to the pathogenesis of non-Hodgkin's lymphomas. In this case, neutralizing the charged pocket also resulted in loss of repression and corepressor binding. Experiments with purified protein showed that corepressor-BTB interactions were direct. A comparison of the PLZF, Bcl-6, and the FAZF (Fanconi anemia zinc finger)/ROG protein shows that variations in the BTB pocket result in differential affinity for corepressors, which predicts the potency of transcriptional repression. Thus, the BTB pocket represents a molecular structure involved in recruitment of transcriptional repression complexes to target promoters.

The acute promyelocytic leukemia-associated protein, promyelocytic leukemia zinc finger, regulates 1,25-dihydroxyvitamin D(3)-induced monocytic differentiation of U937 cells through a physical interaction with vitamin D(3) receptor

Monocyte differentiation induced by 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) is interrupted during the course of acute promyelocytic leukemia (APL). One form of APL is associated with the translocation t(11;17), which joins the promyelocytic leukemia zinc finger (PLZF) and retinoic acid receptor alpha (RARalpha) genes. Because PLZF is coexpressed in the myeloid lineage with the vitamin D(3) receptor (VDR), the interplay between PLZF and VDR was examined. It was found that PLZF interacts directly with VDR. This occurred at least partly through contacts in the DNA-binding domain of VDR and the broad complex, tram-trak, bric-a-brac/pox virus zinc finger (BTB/POZ) domain of PLZF. Moreover, PLZF altered the mobility of VDR derived from nuclear extracts when bound to its cognate binding site, forming a slowly migrating DNA-protein complex. Overexpression of PLZF in a monocytic cell line abrogated 1,25(OH)(2)D(3) activation from both a minimal VDR responsive reporter and the promoter of p21(WAF1/CIP1), a target gene of VDR. Deletion of the BTB/POZ domain significantly relieved PLZF-mediated repression of 1,25(OH)(2)D(3)-dependent activation. In addition, stable, inducible expression of PLZF in U937 cells inhibited the ability of 1,25(OH)(2)D(3) to induce surface expression of the monocytic marker CD14 and morphologic changes associated with differentiation. These results suggest that PLZF may play an important role in regulating the process by which 1,25(OH)(2)D(3) induces monocytic differentiation in hematopoietic cells.

Translocations of the RARalpha gene in acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) has been recognized as a distinct clinical entity for over 40 years. Although relatively rare among hematopoietic malignancies (approximately 10% of AML cases), this disease has attracted a particularly good share of attention by becoming the first human cancer in which all-trans-retinoic acid (ATRA), a physiologically active derivative of vitamin A, was able to induce complete remission (CR). ATRA induced remission is not associated with rapid cell death, as in the case of conventional chemotherapy, but with a restoration of the 'normal' granulocytic differentiation pathway. With this remarkable medical success story APL has overnight become a paradigm for the differentiation therapy of cancer. A few years later, excitement with APL was further enhanced by the discovery that a cytogenetic marker for this disease, the t(15:17) reciprocal chromosomal translocation, involves a fusion between the retinoic acid receptor alpha (RARalpha) gene and a previously unknown locus named promyelocytic leukemia (PML). Consequence of this gene rearrangement is expression of the PML-RARalpha chimeric oncoprotein, which is responsible for the cellular transformation as well as ATRA response that is observed in APL. Since this initial discovery, a number of different translocation partner genes of RARalpha have been reported in rarer cases of APL, strongly suggesting that disruption of RARalpha underlies its pathogenesis. This article reviews various rearrangements of the RARalpha gene that have so far been described in literature, functions of the proteins encoded by the different RARalpha partner loci, and implications that these may have for the molecular pathogenesis of APL.

AML-1/ETO fusion protein is a dominant negative inhibitor of transcriptional repression by the promyelocytic leukemia zinc finger protein

The AML-1/ETO fusion protein, created by the (8;21) translocation in M2-type acute myelogenous leukemia (AML), is a dominant repressive form of AML-1. This effect is due to the ability of the ETO portion of the protein to recruit co-repressors to promoters of AML-1 target genes. The t(11;17)(q21;q23)-associated acute promyelocytic leukemia creates the promyelocytic leukemia zinc finger PLZFt/RARα fusion protein and, in a similar manner, inhibits RARα target gene expression and myeloid differentiation. PLZF is expressed in hematopoietic progenitors and functions as a growth suppressor by repressing cyclin A2 and other targets. ETO is a corepressor for PLZF and potentiates transcriptional repression by linking PLZF to a histone deacetylase-containing complex. In transiently transfected cells and in a cell line derived from a patient with t(8;21) leukemia, PLZF and AML-1/ETO formed a tight complex. In transient assays, AML-1/ETO blocked transcriptional repression by PLZF, even at substoichiometric levels relative to PLZF. This effect was dependent on the presence of the ETO zinc finger domain, which recruits corepressors, and could not be rescued by overexpression of co-repressors that normally enhance PLZF repression. AML-1/ETO also excluded PLZF from the nuclear matrix and reduced its ability to bind to its cognate DNA-binding site. Finally, ETO interacted with PLZF/RARα and enhanced its ability to repress through the RARE. These data show a link in the transcriptional pathways of M2 and M3 leukemia.

AML-1/ETO fusion protein is a dominant negative inhibitor of transcriptional repression by the promyelocytic leukemia zinc finger protein

The AML-1/ETO fusion protein, created by the (8;21) translocation in M2-type acute myelogenous leukemia (AML), is a dominant repressive form of AML-1. This effect is due to the ability of the ETO portion of the protein to recruit co-repressors to promoters of AML-1 target genes. The t(11;17)(q21;q23)-associated acute promyelocytic leukemia creates the promyelocytic leukemia zinc finger PLZFt/RARα fusion protein and, in a similar manner, inhibits RARα target gene expression and myeloid differentiation. PLZF is expressed in hematopoietic progenitors and functions as a growth suppressor by repressing cyclin A2 and other targets. ETO is a corepressor for PLZF and potentiates transcriptional repression by linking PLZF to a histone deacetylase-containing complex. In transiently transfected cells and in a cell line derived from a patient with t(8;21) leukemia, PLZF and AML-1/ETO formed a tight complex. In transient assays, AML-1/ETO blocked transcriptional repression by PLZF, even at substoichiometric levels relative to PLZF. This effect was dependent on the presence of the ETO zinc finger domain, which recruits corepressors, and could not be rescued by overexpression of co-repressors that normally enhance PLZF repression. AML-1/ETO also excluded PLZF from the nuclear matrix and reduced its ability to bind to its cognate DNA-binding site. Finally, ETO interacted with PLZF/RARα and enhanced its ability to repress through the RARE. These data show a link in the transcriptional pathways of M2 and M3 leukemia.

In-depth mutational analysis of the promyelocytic leukemia zinc finger BTB/POZ domain reveals motifs and residues required for biological and transcriptional functions

The promyelocytic leukemia zinc finger (PLZF) protein is a transcription factor disrupted in patients with t(11;17)(q23;q21)-associated acute promyelocytic leukemia. PLZF contains an N-terminal BTB/POZ domain which is required for dimerization, transcriptional repression, formation of high-molecular-weight DNA-protein complexes, nuclear sublocalization, and growth suppression. X-ray crystallographic data show that the PLZF BTB/POZ domain forms an obligate homodimer via an extensive interface. In addition, the dimer possesses several highly conserved features, including a charged pocket, a hydrophobic monomer core, an exposed hydrophobic surface on the floor of the dimer, and two negatively charged surface patches. To determine the role of these structures, mutational analysis of the BTB/POZ domain was performed. We found that point mutations in conserved residues that disrupt the dimer interface or the monomer core result in a misfolded nonfunctional protein. Mutation of key residues from the exposed hydrophobic surface suggests that these are also important for the stability of PLZF complexes. The integrity of the charged-pocket region was crucial for proper folding of the BTB/POZ domain. In addition, the pocket was critical for the ability of the BTB/POZ domain to repress transcription. Alteration of charged-pocket residue arginine 49 to a glutamine (mutant R49Q) yields a domain that can still dimerize but activates rather than represses transcription. In the context of full-length PLZF, a properly folded BTB/POZ domain was required for all PLZF functions. However, PLZF with the single pocket mutation R49Q repressed transcription, while the double mutant D35N/R49Q could not, despite its ability to dimerize. These results indicate that PLZF requires the BTB/POZ domain for dimerization and the charged pocket for transcriptional repression.

NAC-1 is a brain POZ/BTB protein that can prevent cocaine-induced sensitization in the rat

Levels of the mRNA NAC-1 are increased in the rat forebrain weeks after cocaine exposure. This long-term neuroadaptation occurs during the expression of behavioral sensitization, a model of psychostimulant-induced paranoia. NAC-1, the protein encoded by this cocaine-regulated mRNA, contains a Pox virus and zinc finger/bric-a-brac tramtrack broad complex (POZ/BTB) motif, which mediates interactions among several transcriptional regulators. The present studies demonstrate that NAC-1 acts as a transcription factor. NAC-1 was localized to the nucleus of neurons in the brain. Transfection of NAC-1 in cell culture repressed transcription of a reporter gene. NAC-1 was also able to affect the actions of other POZ/BTB proteins in mammalian two-hybrid studies; these interactions required the presence of the POZ/BTB domain. However, NAC-1 appears to be a unique POZ/BTB transcriptional regulator because it does not contain any zinc finger regions found in these other DNA-binding proteins. Adenoviral-mediated overexpression of NAC-1 protein in the rat nucleus accumbens prevented the development but not the expression of behavioral sensitization produced by repeated administration of cocaine. Thus, NAC-1 may modify the long-term behaviors of psychostimulant abuse by regulating gene transcription in the mammalian brain.

NAC-1 is a brain POZ/BTB protein that can prevent cocaine-induced sensitization in the rat

Levels of the mRNA NAC-1 are increased in the rat forebrain weeks after cocaine exposure. This long-term neuroadaptation occurs during the expression of behavioral sensitization, a model of psychostimulant-induced paranoia. NAC-1, the protein encoded by this cocaine-regulated mRNA, contains a Pox virus and zinc finger/bric-a-brac tramtrack broad complex (POZ/BTB) motif, which mediates interactions among several transcriptional regulators. The present studies demonstrate that NAC-1 acts as a transcription factor. NAC-1 was localized to the nucleus of neurons in the brain. Transfection of NAC-1 in cell culture repressed transcription of a reporter gene. NAC-1 was also able to affect the actions of other POZ/BTB proteins in mammalian two-hybrid studies; these interactions required the presence of the POZ/BTB domain. However, NAC-1 appears to be a unique POZ/BTB transcriptional regulator because it does not contain any zinc finger regions found in these other DNA-binding proteins. Adenoviral-mediated overexpression of NAC-1 protein in the rat nucleus accumbens prevented the development but not the expression of behavioral sensitization produced by repeated administration of cocaine. Thus, NAC-1 may modify the long-term behaviors of psychostimulant abuse by regulating gene transcription in the mammalian brain.

The ETO protein disrupted in t(8;21)-associated acute myeloid leukemia is a corepressor for the promyelocytic leukemia zinc finger protein

The ETO protein was originally identified by its fusion to the AML-1 transcription factor in translocation (8;21) associated with the M2 form of acute myeloid leukemia (AML). The resulting AML-1-ETO fusion is an aberrant transcriptional regulator due to the ability of ETO, which does not bind DNA itself, to recruit the transcriptional corepressors N-CoR, SMRT, and Sin3A and histone deacetylases. The promyelocytic leukemia zinc finger (PLZF) protein is a sequence-specific DNA-binding transcriptional factor fused to retinoic acid receptor alpha in acute promyelocytic leukemia associated with the (11;17)(q23;q21) translocation. PLZF also mediates transcriptional repression through the actions of corepressors and histone deacetylases. We found that ETO is one of the corepressors recruited by PLZF. The PLZF and ETO proteins associate in vivo and in vitro, and ETO can potentiate transcriptional repression by PLZF. The N-terminal portion of ETO forms complexes with PLZF, while the C-terminal region, which was shown to bind to N-CoR and SMRT, is required for the ability of ETO to augment transcriptional repression by PLZF. The second repression domain (RD2) of PLZF, not the POZ/BTB domain, is necessary to bind to ETO. Corepression by ETO was completely abrogated by histone deacetylase inhibitors. This identifies ETO as a cofactor for a sequence-specific transcription factor and indicates that, like other corepressors, it functions through the action of histone deactylase.