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Briefings in Functional Genomics and Proteomics Advance Access originally published online on March 24, 2007
Briefings in Functional Genomics and Proteomics 2007 6(1):8-18; doi:10.1093/bfgp/elm002
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Functional analysis of the role of POK transcriptional repressors

Jose A. Costoya

Jose A. Costoya, Molecular Oncology Lab, Departamento de Fisioloxia, Facultade de Medicina, Universidade de Santiago de Compostela, Rua San Francisco s/n, 15782 Santiago de Compostela, Spain. Tel: +34 981 582658/ext.12290; Fax: +34 981 574145, E-mail: jcostoya{at}usc.es; http://www.usc.es/gom


   ABSTRACT
TOP
 ABSTRACT
PLZF: A POK PROTEIN...
 ZBTB7 A CRUCIAL REGULATOR...
 PLZP: A PLZF HOMOLOGUE...
 PROTO-ONCOGENE BCL6 IN ITS...
 CONCLUSIONS
 References
 
Transcription factors (TF) play a key role in certain mechanisms by which specific genes are expressed in a temporal and tissue-specific manner. Understanding those mechanisms is still a challenging question in biology. Their modular organization allows the possibility of classifying them based on the structure of the domains that bind DNA or interact with other proteins. Those domains not only define the different TF families but also provide insights into the biological functions played by them. Among these, the POK (Poxviruses and Zinc-finger (POZ) and Krüppel) family of transcription repressors is characterized by the presence in their structures of an amino-terminal POZ/Broad Complex, Tramtrack, and Bric à brac (BTB) domain and several Krüppel-type zinc fingers at the carboxy-terminal moiety. The POZ/BTB domain mediates homo- and heterodimerization as well as protein–protein interactions, allowing the recruitment of corepressor complexes. On the other hand, the specific zinc fingers mediate specific DNA sequences recognition and binding. In the last few years, several reports have highlighted the importance that this family of transcriptional repressors plays in different processes such as cancer, development and stem cell biology.

Keywords: POZ and Krüppel-type zinc finger domains, transcriptional repression, development, stem cell, cancer

Knowledge of the different players that intervene and regulate the mechanisms by which specific genes are expressed in a temporal or tissue-specific manner is still a thrilling issue in biology. Understanding the mechanisms the cell uses for controlling critical regulatory pathways at the central control, the nucleus, is somehow still an open field. Transcription factors (TF) play a key role in this process, and as modular proteins they can be classified mainly based on the structure of their DNA binding domain. One of these domains, designated as a zinc finger motif on the base of its requirement for zinc, is the large C2H2 Krüppel-type zinc finger group, ‘surnamed’ due to the fact that it resembles the Drosophila segmentation protein Krüppel. It represents one of the most common types of DNA binding domains. Two conserved cysteine and histidine residue pairs coordinate a single zinc atom, crucial components of the zinc finger conformation characteristic of this motif. This domain is ~25–30 amino acids long [1]. There are approximately more than 600 genes in the human genome encoding C2H2 motifs [2], suggesting that this class of TF represents a substantial portion of the genes in the human genome.

The Broad complex, Tramtrack, and Bric à brac (BTB), also known as Poxviruses and Zinc-finger (POZ) domain is an evolutionary conserved protein–protein interaction domain. In most cases this domain is associated with C2H2 zinc finger motifs in TF involved in transcriptional regulation through chromatin re-modelling [3]. Although the number of proteins harbouring this motif is lower than the ones including C2H2 zinc fingers, more than 100 proteins has been identified so far. In fact, it is estimated that 5–10% of the zinc finger proteins in humans also share this domain [2, 4]. The proteins showing this combination of motifs are defined as members of the so called POK (POZ and Krüppel) family of transcriptional repressors. Interestingly, both in human and mouse genomes, more than 40 genes are associated with those domains (Tables 1 and 2) [4].


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  		Table 1: POK members encoded in the human genomea

 

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  		Table 2: POK members encoded in the mouse genomea

 
The biological functions of POK proteins are defined on the base of the protein–protein interaction properties conferred by the BTB/POZ domain, while the Krüppel-like C2H2 zinc fingers mediate the specific binding to DNA sequences located within gene-regulatory regions (Figure 1). Thus, the BTB/POZ domain promotes homo- and hetero-dimerization and exerts their transcriptional role through their interaction with transcriptional co-factors. These co-factors include SIN3A (SIN3 homolog A, transcription regulator – yeast), SMRT (Silencing Mediator for Retinoid and Thyroid hormone receptor) and NCOR1 (Nuclear Receptor CoRepressor 1) corepressors, which in turn recruit HDACs (Histone DeACetylases). Transcription is highly dependent of how DNA is packaged. DNA can be tightly compacted and therefore prevent accessibility of TF or can be available to TF via modification of the nucleosome, fundamental subunit of chromatin composed of an octamer of four core histones surrounded by 146 bp of DNA. This architecture of chromatin is strongly influenced by post-translational modifications of these histones. Among these modifications, the acetylation of core histones is the best-characterized type of modification. Usually, increased levels of histone acetylation are associated with increased transcriptional activity (open chromatin), while decreased levels of acetylation are associated with repression of gene expression (closed chromatin). HDACs cause chromatin deacetylation and therefore contribute to transcriptional repression [5].


Figure 1
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  		Figure 1: Structure of selected POK transcription factors. These proteins characteristically have an amino-terminal POZ/BTB domain and several carboxy-terminal Krüppel-type zinc fingers. These repressors have been shown to play important roles in a number of biological processes.

 
In summary, POK proteins are able to act as a molecular ‘switch’ opening or closing the chromatin through the deacetylation of the histones, and therefore regulating the transcription of their target genes (Figure 2). As we have previously mentioned, there are many proteins associated with these domains, some of them involved in cancer, development and stem cell biology. We have selected some of them to illustrate some of the interesting features of this protein family.


Figure 2
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  		Figure 2: HDACs and corepressors are recruited by sequence-specific POK transcriptional repressors and promote a ‘closed’ chromatin conformation that prevents transcription activation.

 

   PLZF: A POK PROTEIN IMPLICATED IN DEVELOPMENT, CANCER AND STEM CELL BIOLOGY
TOP
 ABSTRACT
 PLZF: A POK PROTEIN...
ZBTB7 A CRUCIAL REGULATOR...
 PLZP: A PLZF HOMOLOGUE...
 PROTO-ONCOGENE BCL6 IN ITS...
 CONCLUSIONS
 References
 
Promyelocytic leukaemia zinc finger (PLZF), also known as zinc finger and BTB domain containing 16 (ZBTB16) or ZNF145, was initially identified by virtue of its fusion with the gene RAR{alpha} as a result of a variant t(11;17) chromosomal translocation that occurs in a small subset of acute promyelocytic leukaemia (APL) patients [6, 7]. In this genetic aberration, PLZF is fused to retinoic acid receptor {alpha} (RAR{alpha}) producing PLZF-RAR{alpha} and RAR{alpha}-PLZF fusion products.

The transcriptional repressor PLZF is paradigmatic of this family of proteins: one BTB/POZ domain at the NH2-terminal moiety and nine C2H2 Krüppel-type zinc fingers at the carboxy-terminal of the protein. There is also a central region located between both domains, a potentially regulatory region carrying PEST sequences. These regions rich in proline (P), glutamic acid (E), serine (S), and threonine (T) are implicated in the rapid intracellular degradation, usually through the ubiquitin-proteasome pathway, of proteins containing these consensus sequences [8–10]. The BTB/POZ domain allows PLZF to self-associate and also form heteromeric complexes with other proteins while the zinc fingers confer the specificity of the transcriptional repressor activity on particular promoters. The ability of the BTB/POZ domain to mediate protein–protein interaction depends on its charged pocket [11].

Many PLZF interacting proteins have been identified, among them PLZF is able to recruit transcriptional corepressors (N-CoR and SMTR/mSIN3A) and HDACs to target regulatory elements [12, 13]. In a more specific manner, ETO (Eight Twenty One) protein has also been shown to associate with PLZF in vivo and in vitro. ETO was initially described as a partner of AML-1 in the acute myelogenous leukaemia (AML)-associated translocation t(8;21) [14]. Its interaction with PLZF potentiates its transcriptional repressive activity. The amino-terminal portion of ETO bind to PLZF, while the carboxy-terminal moiety is the one able to bind N-CoR and SMRT corepressor, a crucial interaction for PLZF transcriptional repression activity mediated by recruitment of HDACs.

Initially, the study of the altered function of PLZF associated to APL, due to an aberrant and dominant negative activity of the fusion proteins (PLZF-RAR{alpha} and RAR{alpha}-PLZF) on PLZF, was an important clue for understanding its putative physiological role. However, only the generation of a Plzf mutant mice, in which the gene has been inactivated by targeted disruption, has revealed some of PLZF biological activities, such as regulation of apoptosis and cellular proliferation. A precise control of proliferation during development is crucial for the normal outgrowth and remodelling of the limb. In this respect, interdigital programmed cell death is a necessary prerequisite for the separation of the digits. Plzf –/– mice display a profound defect in this process in the developing hindlimb. Apoptosis of interdigital regions, where PLZF is normally highly expressed, is markedly reduced in Plzf –/– mutants resulting in interdigital webbing. Additionally, Plzf –/– mice show an increase in the proliferation rate in these interdigital regions. Thus, PLZF inhibits proliferation and functions as a pro-apoptotic factor in the developing limb [15].

PLZF in development: How a POK protein regulates the Hox genes
The Hox family regulates critical pathways in cell fate decision, patterning and embryogenesis. Thus, a detailed analysis and knowledge of the function of the TF implicated in their regulation is essential for understanding how these genes exert their control on the developmental processes. PLZF is one of this TF due its decisive role in Hox gene regulation. It is a key factor for patterning in the limb and axial skeletal structures. Plzf –/– mutant mice display homeotic transformations of anterior skeletal elements into posterior structures and this phenotype is accompanied with an alteration in the pattern of expression of both Hox gene complex and genes encoding Bone Morphogenetic Proteins (BMPs). Therefore, PLZF operates as a global regulator controlling the spatial activation of the Abdb HoxD (Abdb: Abdominal B) gene complex through binding to cis elements within Hox genes and recruitment of HDACs as well as Polycomb proteins on DNA. As a consequence of all this, a transition from euchromatic to a heterochromatic chromatin state occurs. This mechanism exerts by this POK member appears to take place downstream or independent of Shh (Sonic hedgehog homolog), an important mediator in pattern formation along the anterior-to-posterior (AP) limb axis, and consequently independently or downstream of a polarizing signal [16].

Lately, a new piece of evidence on PLZF role on development was unveiled. The genetic interaction between Gli3 (GLI-Krüppel family member 3) and Plzf, through the characterization of the double mutant mice Gli3–/–; Plzf –/–, shed new light on the mechanisms that lead to establish and pattern the skeletal elements along the proximal to distal axis. Gli3 and Plzf orchestrate the correct temporal and spatial distribution of chondrocyte progenitors in the proximal limb-bud independently of known proximal to distal patterning markers and overall limb-bud size. Those findings demonstrate that the formation of proximal and distal skeletal patterning is differentially regulated very early in limb development, reinforcing the Early Specification Model which proposes that distinct proximal to distal progenitor populations are specified early in an independent, non-progressive manner [17].

All these findings have clear implications not only on development but also for the pathogenesis of human APL, in which aberrant PLZF function might lend a selective growth and survival advantage to the leukaemic cells through deregulated expression of PLZF target genes. In fact, Hox genes have been implicated in the control of proliferation and differentiation of primitive haemopoietic cells [18]. The consequence of an aberrant regulation in Hox gene expression throughout myeloid haemopoietic differentiation may be further exacerbated by the fact that the fusion protein PLZF-RAR{alpha} interferes at the same time with the transcriptional role of RAR and Retinoid X Receptor (RXR), nuclear receptors which are also implicated in Hox gene regulation [19].

PLZF role in leukaemogenesis and tumourigenesis
As we previously mentioned, PLZF is the only POK member involved in one of the APL-specific reciprocal and balanced translocation involving the RAR{alpha} locus. Unlike other APL, leukaemia associated with t(11;17) translocation shows a distinctly worse prognosis, with little or no response to RA treatment [20]. The aberrant protein PLZF-RAR{alpha} is able to heterodimerize with PLZF, as it happens with other fusion proteins where RAR{alpha} fuses to other protein. Similarly the RAR{alpha} portion retained in X-RAR{alpha} is able to mediate heterodimerization with RXR, as well as binds both DNA and its specific ligand through the RAR{alpha} moiety. Thus, PLZF-RAR{alpha} is able to potentially interfere with both PLZF and RAR/RXR pathways. Although very little is known on the biochemical and biological role of the various RAR{alpha}-X fusion proteins, a discrete biochemical activity has been attributed to RAR{alpha}-PLZF. This molecule can in fact bind DNA through seven out of the nine zinc fingers which constitute the PLZF DNA binding domain, but lacks the POZ domain which is replaced by one of the RAR{alpha} transactivating domains. RAR{alpha}-PLZF can thus act as a dominant negative PLZF mutant. PLZF-RAR{alpha} expression under the human cathepsin G (hCG) promoter in cancer mouse models does not induce an APL-like disorder, but a myeloproliferative-like disorder leukaemia, which lacks the promyelocytic block and is instead reminiscent of human chronic myeloid leukaemia. On the other hand, expression under the control of the same promoter of the reciprocal translocation RAR{alpha}-PLZF induces a myeloproliferative disorder characterized by an accumulation of myeloid cells in the absence of a differentiation block, but do not progress to overt leukaemia. When the mutants harboured both fusion proteins, through the cross of PLZF-RAR{alpha} and RAR{alpha}-PLZF transgenic mouse models, in which the dual complexity of t(11;17) APL was recreated, those mice developed leukaemia with faithful APL features such as the classic block in promyelocytic differentiation [21]. Therefore, these two genetics events are concurrently required to induce an APL. The authors concluded that RAR{alpha}-PLZF is not oncogenic per se, but it is required to determine the phenotypic and clinical characteristics of the disease acting as a tumour metamorphoser. In complete agreement with the notion that RAR{alpha}-PLZF is dominant on PLZF function, inactivation of PLZF obtained by interbreeding Plzf –/– mice with PLZF-RAR{alpha} transgenic mice phenocopied the presence of RAR{alpha}-PLZF [21]. This piece of evidence emphasizes once again the crucial importance of the blockade of PLZF function in APL pathogenesis. Thus, in t(11;17) APL, the dominant negative function of the RAR{alpha}-PLZF on PLZF target genes may be critical in completing the activity of PLZF-RAR{alpha}, data also confirm with a knock in mouse model of PLZF-RAR{alpha} (Costoya JA et al. unpublished observation).

PLZF has also been implicated in the pathogenesis of a different leukaemia, myeloid leukaemia associated with AML-1/ETO fusion protein. It has been shown in fact that AML-1/ETO fusion protein blocks in vitro PLZF transcriptional repression activity through their physically interaction. This effect by AML-1/ETO is dependent on the presence of the ETO zinc finger moiety. The possible mechanism of action may be that AML-1/ETO excludes PLZF from the nuclear matrix and therefore impairs its ability to bind to its cognate DNA-binding site [14].

As we previously discussed, it has been proposed that POK proteins may also interact among them through their POZ domain. One of these interactions is the one demonstrated in vitro between PLZF and BCL6. This is a well-known proto-oncogenic POK protein implicated in the pathogenesis of a large subset of non-Hodgking lymphoma (NHLs) where it is overexpressed/misexpressed as a consequence of chromosomal translocation which juxtaposes its coding to heterologous promoters (see subsequent text). PLZF/BCL6 interaction is associated with recruitment onto multiprotein nuclear complexes, presumably involved in transcriptional silencing, and whose integrity and/or function may be altered in APL and/or NHLs pathogenesis. Nevertheless, it remains to be seen whether the PLZF/BCL6 association can be confirmed in vivo between endogenous proteins in relevant cell types and, if so, whether this association is of any functional relevance [22].

Although PLZF was classically considered as a protein mainly involved in non-solid tumours since its discovery, few references start to come out in the literature about its putative role on solid tumours. PLZF seems to be involved in melanoma tumourigenesis, there is a link between their expression levels and a better prognosis [23, 24]. The mechanism through PLZF exerts its function in this cellular compartment is still controversial. It has been postulated a possible HOXB7 repression by PLZF but both proteins are functionally independent [23], however a recent publication claims that the repression of Pre-B-cell leukaemia transcription factor 1 (Pbx1) by PLZF reduces the expression of HOXB7 target genes, including some melanoma-associated factors [24]. Similar mechanism was described for prostate cancer although in this case through the interaction with Pbx1-HOXC8. It has been proposed that this interaction with the heterocomplex may lead to androgen-independent growth in prostate cancer [25]. Other tumour type where PLZF is also involved is cervical cancer, a novel cervical cancer suppressor 3 (CCS-3), protein down-regulated in human cervical cell lines as well as in cervical tumours when compared to those from normal tissues, was identified as a PLZF interacting partner. The interaction suggests that the potential tumour suppressor activity of CCS-3 may be mediated by its interaction with PLZF [26].

PLZF as a spermatogonia-specific POK required for self-renewal and maintenance of the stem cell pool
PLZF is dynamically expressed during embryogenesis, showing a high degree of evolutionary conservation in its patterns of expression [27], has thus been essential in patterning limb development and axial structures [15–17]. Moreover, PLZF reduces cell proliferation by induction of G0/G1 cell cycle arrest. Although little is known about the molecular mechanisms behind spermatogonia self-renewing control, a couple of studies recently showed that PLZF has an essential role in spermatogonia maintenance. Lack of this gene, both demonstrated by spontaneous mutation [28] and by homologous recombination [29], results in age-dependent germ cell loss leading to testicular degeneration due the progressive reduction of self-renewal capability of the stem cell compartment. The mechanism through PLZF seems to control this process is regulating the exit from quiescence. Its inactivation derives in an inappropriate activation of meiotic checkpoints and increased apoptosis [29]. PLZF is also involved in control of the balance between quiescent and cycling pools of haemopoietic stem cell (HSC) (Costoya JA et al. unpublished observations). Therefore PLZF is required to regulate self-renewal and maintenance of the stem cell pool, acting on the chromatin remodelling and transcriptional regulation mechanisms taking place during the control of those processes.


   ZBTB7 A CRUCIAL REGULATOR IN CANCER PATHOGENESIS
TOP
 ABSTRACT
 PLZF: A POK PROTEIN...
 ZBTB7 A CRUCIAL REGULATOR...
PLZP: A PLZF HOMOLOGUE...
 PROTO-ONCOGENE BCL6 IN ITS...
 CONCLUSIONS
 References
 
The zinc finger and BTB domain containing 7 (ZBTB7), also known as Lrf, FBI-1 or Pokemon is a member of the POK family initially identified as a protein that binds specifically to HIV type I promoter element [30]. ZBTB7 POZ domains mediate its homodimerization and heterodimerization with other proteins such as BCL-6, in all these interactions where ZBTB7 full-length is required for this interaction [31]. The gene was cloned as a PLZF homologue able to physically interact with BCL-6 and enhance its oncogenic properties. In fact, coexpression of ZBTB7 and BCL-6 is predictor of clinical outcome in lymphoma [32,33]. Zbtb7 inactivation results in embryonic lethality due to severe anaemia and profoundly impaired cellular differentiation in multiples tissues (Merghoub T et al., unpublished observation). These data suggested that ZBTB7 play an important role on transcriptional control on cellular differentiation, because alteration of its function results in a clear defect at this level, common feature observed in human cancers. Furthermore, mouse embryonic fibroblast lacking Zbtb7 are completely refractory to oncogene-mediated cellular transformation. In the other hand, overexpression of ZBTB7 acts as a genuine proto-oncogene both in vitro and in vivo. In fact, the generation of a transgenic mouse model in which the gene was overexpressed in immature T and B lymphoid lineage cells developed aggressive tumours characterized as mouse precursor T-cell lymphoblastic lymphoma/leukaemia (pre-T LBL). Moreover, Zbtb7 was identified as the first ARF-specific transcriptional repressor and p19Arf inactivation is able to rescue the refractoriness to oncogenic transformation of Zbtb7 –/– cells. This repressive activity on ARF provides the mechanism through ZBTB7 plays its main oncogenic acivity. Indeed, ZBTB7 is expressed at very high levels in a subset of several human cancer types, and in diffuse large B-cell lymphoma (DLBCL) its expression level predicts clinical outcome [32].


   PLZP: A PLZF HOMOLOGUE THAT CONTROLS CELL CYCLE AND CYTOKINE PRODUCTION IN T-CELL COMPARTMENT AS WELL AS HSC HOMEOSTASIS
TOP
 ABSTRACT
 PLZF: A POK PROTEIN...
 ZBTB7 A CRUCIAL REGULATOR...
 PLZP: A PLZF HOMOLOGUE...
PROTO-ONCOGENE BCL6 IN ITS...
 CONCLUSIONS
 References
 
Several groups looking for a homologue of PLZF found a new protein called PLZP (PLZF-like zinc finger protein), also known as FAZF (Fanconi anaemia zinc finger), TZFP (testis zinc finger protein) and RoG (repressor of GATA) [34–36]. Initially, it was identified as a novel BTB/POZ protein that has three C2H2 Krüppel-type zinc finger domains; those domains display a high homology with the three most carboxy-terminal zinc finger domains of PLZF. This PLZF homolog is able to heterodimerize with FANC-C (Fanconi anaemia, complementation group C) and PLZF, both proteins implicated in human diseases. Although this member of the POK family, up to now was not directly related to human disease, it has been proposed to play a role in the pathogenesis of the autosomal-recessive bone marrow failure and/or cancer-predisposing syndrome Fanconi's anaemia as well as in APL [37]. PLZP binds similar DNA responsive elements as PLZF, probably due the high homology between their Zn finger domains. Other TF implicated in hemopoiesis have been demonstrated to also interact with PLZP; among them it is interesting to mention GATA-3. PLZP displaces GATA-3 from its specific consensus sequences, mainly located on specific cytokine promoters, producing as a result of that a repression of GATA-3-dependent transactivation of these target genes [35]. Additionally, this transcriptional repressor regulates, in this case in a direct manner, interleukin-4 (IL-4) expression in CD8 positive T-cell subpopulation through recruitment of HDAC proteins to the cytokine promoter [38]. Disruption of PLZP in mice through in-frame insertion of a lacZ reporter, without perturbing the expression of the neighbouring MLL2 gene, allowed to discover defects in cell cycle control and cytokine production in the T-cell compartment. At the same time, this inactivation of the gene altered the homeostasis of the HSCs and/or progenitor cells. PLZP regulates the balance between quiescent and cycling pools of HSCs, a function that seems to be shared with PLZF (Costoya JA et al., unpublished observations). Although the human and mouse PLZP is highly expressed in the testis, hence it was originally cloned from this tissue and named as testis zinc-finger protein (TZFP) suggesting a significant effect in spermatogenesis, inactivation of the gene showed no gross defect, Plzp mutant mice are fertile and a more detailed analysis of the testis of these animals showed unaffected proliferation and apoptosis rates, as well as the composition of the different cellular populations seemed normal within the seminiferous tubules. Interestingly, although the kinase Aurora C (Aurora kinase C; serine/threonine kinase 13) is known to be expressed in meiotically active spermatocytes and it has been reported to be transcriptionally repressed by PLZP [39], levels of expression of this kinase were no significantly up-regulated after Plzp inactivation [36].


   PROTO-ONCOGENE BCL6 IN ITS TWO ROLES: NORMAL LYMPHOID DEVELOPMENT AND LYMPHOMAGENESIS
TOP
 ABSTRACT
 PLZF: A POK PROTEIN...
 ZBTB7 A CRUCIAL REGULATOR...
 PLZP: A PLZF HOMOLOGUE...
 PROTO-ONCOGENE BCL6 IN ITS...
CONCLUSIONS
 References
 
The BCL6 gene encodes another transcriptional repressor, as we saw along this review, a common feature of almost all POK protein family explained by their peculiar molecular structure. Consequently, its POZ motif allows BCL6 to interact with N-CoR and SMTR/mSIN3A/HDAC corepressor complexes to mediate its potent transrepression activity [40]. The six Krüppel-type zinc finger domains of BCL6 recognize a consensus DNA sequence highly resembling STAT (signal transducers and activators of transcription) binding site. In fact, BCL6 binds STAT6 consensus DNA binding site and inhibits specific STAT6-dependent gene transcription [41, 42]. Moreover, Bcl6 –/– mice display an increased ability to class switch to IgE in response to IL-4 in vitro, resulting in multiorgan inflammatory disease in those animals characterized by the presence of a large number of IgE + B cells. The mechanism behind this phenotype is the role of BCL6 in modulating transcription from the germ line epsilon promoter, through its binding to target sequences for the IL-4-induced STAT6. BCL-6 regulation of Ig class switching upon STAT6 signalling was also confirmed by the rescue of the phenotype observed in the double mutants Bcl6 –/– Stat6 –/– mice [43].

Between the POZ and Krüppel-type zinc finger domains, middle portion of the protein, several MAPK phosphorylation sites were identified, all of them within PEST motifs. Thus, those PEST motifs of BCL6 play a crucial role in MAPK-induced degradation. This signalling cascade activated by antigen receptor may be essential for the control of B-cell differentiation and antibody response [44]. This POK member is regulated not only by phosphorylation, it has been reported that BCL6 can be acetylated by p300/CBP, and in this case this post-translational modification regulates also negatively BCL6 activity [45].

Disruption of Bcl6 in mice revealed that this protein is an important regulator of the immune system, the different mutant mice generated by different groups failed to develop germinal centres (GC), place where the B cells undergo their clonal expansion after challenging those mice through their immunization with a T-cell-dependent antigen [46–48]. Although the mechanism behind this phenotype is usually related to a defect in lymphoid organs and specifically B-cell compartment development, Bcl6 mutant mice show normal development of lymphoid organs and B cells. However, some experiments recently performed suggest that BCL6 may function as a negative regulator in B cell apoptosis and differentiation during GC formation [49]. In summary, the physiological role of BCL6 is as a repressor of several genes implicated in crucial pathways such as apoptosis, differentiation and Ig switching in the immune system [50]. Some of these mechanisms are clearly implicated in lymphomagenesis and BCL6 was initially identified by cloning the chromosome breakpoint involved in chromosomal translocations associated with diffuse large cell lymphoma (DLCL), the most common form of non-Hodgkin's lymphoma (NHL) [51]. Interestingly, DLCL is characterized by a high degree of heterogeneity, both in its pathological features and clinical manifestations, for this reason BCL6 has become an important marker of transit through the GC development stage as well as a prognostic factor [52]. BCL6 rearrangement had a favourable outcome in terms of overall survival and survival without disease progression and therefore it may serve as a prognostic marker in patients with this form of malignant lymphoma [53].


   CONCLUSIONS
TOP
 ABSTRACT
 PLZF: A POK PROTEIN...
 ZBTB7 A CRUCIAL REGULATOR...
 PLZP: A PLZF HOMOLOGUE...
 PROTO-ONCOGENE BCL6 IN ITS...
 CONCLUSIONS
References
 
Recent advances in understanding the molecular pathways and the biological function controlled by the different proteins belonging to the large POK family have allowed shedding new light on the genetic and biochemical events behind several complex process. Furthermore, the analysis of the genetics of the different mouse models generated for better understanding the physiological and pathological roles played by these genes is and will be facilitating the knowledge of the different links established among development, cancer and stem cell biology. Those have become more important in our days due their important role on the molecular aspects of pathogenesis of a large number of human diseases (Figure 3).


Figure 3
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  		Figure 3: POK transcriptional repressors play an essential role on crucial physiological and pathological processes such as development, stem cell biology and cancer.

 

Key Points

  • POK proteins are defined by the presence in their structure of a POZ/BTB domain associated to several Krüppel-type zinc fingers.
  • These transcriptional repressors act as a molecular ‘switch’ remodelling the chromatin.
  • PLZF is a paradigmatic member of the POK family of proteins.
  • Zbtb7 inactivation results in embryonic lethality and when it is overexpressed acts as a genuine proto-oncogene both in vitro and in vivo.
  • The analysis of the different mouse models generated allows to understand and characterize the biological functions played by the POK members.

 


   Acknowledgments
 
I thank the members of my laboratory for fruitful discussions on the topics covered in this article. I also want to thank Pier Paolo Pandolfi, Taha Merghoub, Francesco Piazza and other past and present members of the laboratory of Molecular and Developmental Biology (MADB) at the Memorial Sloan-Kettering Cancer Center, working on POK related subjects. Work in the laboratory of J.A.C. is supported by Xunta de Galicia (PGIDIT05PXIB20801PR) and Spanish Ministry of Education and Science (SAF2005-00306). J.A.C. is an Investigator of Ramón y Cajal Programme (Spanish Ministry of Education and Science).


   FOOTNOTES
 
Jose A. Costoya is group leader of the Molecular Oncology Lab at Universidade de Santiago de Compostela, Departamento de Fisioloxia in Santiago de Compostela, Spain.


   References
TOP
 ABSTRACT
 PLZF: A POK PROTEIN...
 ZBTB7 A CRUCIAL REGULATOR...
 PLZP: A PLZF HOMOLOGUE...
 PROTO-ONCOGENE BCL6 IN ITS...
 CONCLUSIONS
 References
 

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A. P. R. Sutherland, H. Zhang, Y. Zhang, M. Michaud, Z. Xie, M.-E. Patti, M. J. Grusby, and W. J. Zhang
Zinc Finger Protein Zbtb20 Is Essential for Postnatal Survival and Glucose Homeostasis
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