The Transcription Factor Pokemon: A New Key Player in Cancer Pathogenesis
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Review
The Transcription Factor Pokemon: A New Key Player
in Cancer Pathogenesis
Takahiro Maeda, Robin M. Hobbs, and Pier Paolo Pandolfi
Cancer Biology and Genetics Program, Department of Pathology, Sloan-Kettering Institute, Memorial Sloan-Kettering
Cancer Center, New York, New York
Abstract in mouse resulted in embryonic lethality due to severe anemia
and profoundly impaired cellular differentiation in multiple
Learning how critical cell regulatory pathways are controlled
tissues.1 We hypothesized a potential role for Pokemon in
may lead to new opportunities for cancer treatment. We
recently identified the transcription factor Pokemon as a oncogenesis based on its observed role in regulating differenti-
central regulator of the important tumor suppressor ARF. ation, a core facet of a transformed phenotype. To examine this
Pokemon is overexpressed in multiple human cancers and hypothesis, we tested the effect of targeted deletion of Pokemon
cells lacking Pokemon are refractory to oncogenic transfor- on oncogenic transformation of primary mouse embryo fibro-
mation. These findings suggest that Pokemon may offer an blasts (MEF). In addition, we examined the expression of
effective new target for cancer therapeutics. (Cancer Res 2005; POKEMON in a variety of human cancers (8).
Using Pokemon wild-type and null MEFs, we tested whether
65(19): 8575-8)
loss of Pokemon function would modulate cell growth or
transformation upon oncogenic stimuli. Strikingly, Pokemon null
POKEMON is a member of the POK (POZ and Krüppel) family MEFs were almost completely resistant to the effects of all the
of transcriptional repressors, which consists of an NH2-terminal oncogenic combinations tried, including adenovirus E1A +
POZ/BTB domain and COOH-terminal Krüppel-type zinc fingers RasV12, Myc+RasV12, SV40 large T antigen + RasV12 or BCL6 +
(Fig. 1). The POZ/BTB domain mediates homodimerization and RasV12. The cells failed to acquire any proliferative advantage and
heterodimerization plus recruitment of corepressor/HDAC com- were unable to be transformed (as indicated by colony formation
plexes to these proteins (1), whereas the COOH-terminal zinc in soft agar). Moreover, MEFs coinfected with Pokemon and the
fingers mediate specific DNA recognition and binding. Currently, various oncogenes displayed a marked proliferative advantage as
>40 POK proteins have been identified in the human genome well as the ability to form colonies in soft agar. We therefore
(http://btb.uhnres.utoronto.ca). Recent reports have uncovered concluded that Pokemon is critical for oncogenic transformation
essential roles for POK proteins in development (2, 3), of MEFs and is able to act as a proto-oncogene in cooperation
differentiation (4, 5), and oncogenesis (6–8). For instance, with other classic oncogenes.
promyelocytic leukemia zinc finger (PLZF)–null mice display We subsequently attempted to identify Pokemon consensus
severe defects in limb development and germ stem cell DNA binding sequences by CAST analysis. This screening
maintenance (2, 4). Th-POK (T-helper-inducing POZ/Krüppel-like selected a specific GC–rich sequence for Pokemon binding,
factor, also known as cKrox) has been recently reported as a which shows a certain similarity to the consensus sequence for
master regulator of T-cell lineage commitment (3). Concerning the transcription factor Sp1. An essentially identical DNA
roles for POK proteins in oncogenesis, B-cell lymphoma 6 (BCL6) binding site was independently characterized by Hernandez et
and PLZF, have been implicated in the pathogenesis of non- al. using the human POKEMON protein (FBI-1; ref. 12). Because
Hodgkin’s lymphoma and acute promyelocytic leukemia, respec- POK proteins are known to recruit corepressor complexes
tively (5, 6). Moreover, another POK protein family member through the POZ domain, thereby acting as transcriptional
hypermethylated in cancer-1 (HIC1) is known to be hyper- repressors, we hypothesized that Pokemon might exert its
methylated and transcriptionally silent in human cancers. HIC1 oncogenic activity through the direct repression of potent
heterozygous mice develop spontaneous malignant tumors in tumor suppressor or proapoptotic genes. We located several
multiple tissues (7). putative Pokemon binding sites in the tumor suppressor ARF
POKEMON, which stands for POK erythroid myeloid ontogenic promoter and so tested whether Pokemon would repress its
factor (also known as LRF, OCZF, or FBI-1) was originally activity. Taking advantage of ARF-luciferase reporters and
identified as a protein that binds specifically to a HIV type 1 chromatin immunoprecipitation assays, we discovered that
promoter element (9) and was subsequently cloned as a Pokemon directly binds the p19 Arf promoter in vivo and is
homologue of PLZF that can physically interact with BCL6 able to repress its activity. In the absence of Pokemon, p19 Arf
(10). The Rat homologue was also identified as a key protein expression was found markedly elevated upon both culture
involved in osteoclast differentiation (11). Pokemon inactivation shock and oncogenic transformation. Interestingly, expression of
the p16Ink4a gene, another tumor suppressor gene encoded in
the same Ink4a-Arf locus (13), was not elevated in Pokemon-
null MEFs, showing the specificity of Pokemon repressive
Requests for reprints: Pier Paolo Pandolf i, Cancer Biology and Genetics Program,
activity. Furthermore, the growth defects and refractoriness to
Department of Pathology, Sloan-Kettering Institute, Memorial Sloan-Kettering Cancer
Center, 1275 York Avenue, New York, NY 10021. Phone: 212-639-6168; Fax: 212-717-
3102; E-mail: p-pandolf i@ski.mskcc.org.
I2005 American Association for Cancer Research.
1
doi:10.1158/0008-5472.CAN-05-1055 Merghoub and Pandolf i, unpublished observations.
www.aacrjournals.org 8575 Cancer Res 2005; 65: (19). October 1, 2005
Downloaded from cancerres.aacrjournals.org on March 3, 2015. © 2005 American Association for Cancer
Research.Cancer Research
Figure 1. Structure of the Pokemon protein and its proposed role in oncogenesis. Top, Pokemon protein consists of an NH2-terminal POZ domain and
COOH-terminal Zinc fingers. Possible functions of each domain are indicated. (bottom ) overexpression (depicted in red) of Pokemon results in ARF repression,
which leads to cellular transformation. Conversely, loss (or down-regulation, depicted in blue) of Pokemon causes cellular senescence, apoptosis, and blockage
of differentiation.
oncogenic transformation in Pokemon-null MEFs were fully immature T and B lymphoid lineage cells using a lckEA
reverted by p19 Arf loss. These findings showed that Pokemon is enhancer/promoter transgenic construct. Strikingly, lckEA-Poke-
a specific repressor of p19 Arf and that loss of Pokemon causes mon mice from two independent transgenic lines developed fatal
aberrant ARF up-regulation, resulting in premature senescence thymic lymphomas accompanied by lymphadenopathy, spleno-
and unresponsiveness to oncogenic stimuli. Interestingly, our megaly, hepatomegaly, and tumor infiltration into bone marrow.
preliminary study using Pokemonflox/flox MEFs (established from Because Pokemon proved to be an essential factor for
conditional Pokemon knockout mice) showed that the ‘‘acute’’ oncogenesis both in vitro and in vivo, we went on to investigate
loss of Pokemon upon Cre expression also produces prolifer- POKEMON expression in human cancers by immunohisto-
ative defects and a premature senescence phenotype in MEFs.2 chemistry using a monoclonal antibody specific for POKEMON.
When exploring the oncogenic role of Pokemon in vivo, we A total of 130 cases of diffuse large B cell lymphomas (DLBCL)
were particularly interested in mouse lymphoma models for the and 290 cases of follicular lymphomas were examined for
following reasons. First, Pokemon-null embryos show defects in POKEMON protein expression. Significantly, POKEMON was
B-cell development. Second, Pokemon is generally expressed in expressed in 60% to 80% of DLBCL and follicular lymphoma
the germinal center of lymphoid tissues and interacts with BCL6, cases. Interestingly, POKEMON and BCL6 double-positive cases
which is also present in germinal center and misexpressed in showed a higher proliferative index (high Ki-67 positivity),
human B-cell lymphomas. Lastly, preliminary studies indicated suggesting a possible functional crosstalk between POKEMON
that Pokemon is highly expressed in a subset of human T-cell and BCL6 in lymphomagenesis. Regarding POKEMON expression
lymphomas, whereas Pokemon is expressed at a low level in in T-cell malignancies, our recent analysis of >80 cases of T-cell
CD3-positive T cells of adult thymus. We therefore generated a lymphomas revealed that POKEMON is highly expressed particu-
transgenic mouse model in which Pokemon is overexpressed in larly in anaplastic large cell lymphoma and angioimmunoblastic
lymphoma cases.2
The ARF/p53 pathway is frequently abrogated in human
cancers. However, whereas mutations in the ARF/p53 pathway
2
Maeda and Pandolf i, unpublished observations. are relatively rare in DLBCL compared with solid tumors (14),
Cancer Res 2005; 65: (19). October 1, 2005 8576 www.aacrjournals.org
Downloaded from cancerres.aacrjournals.org on March 3, 2015. © 2005 American Association for Cancer
Research.The Transcription Factor Pokemon
both Arf and p53 mutant mice commonly develop lymphomas Although we found Pokemon to be aberrantly overexpressed
(15, 16). Our real-time reverse transcription-PCR analysis of a in human cancer, little is known about the mechanism by
cohort of DLBCL cases revealed that high Pokemon gene which it becomes overexpressed in cancer. Thus far, genetic
expression generally correlated with low expression of the evidence directly linking POKEMON with human cancer is
p14 ARF gene, which underscores the potential importance of lacking (e.g., chromosomal translocations and mutations of the
ARF suppression by Pokemon in DLBCL. Given that Pokemon POKEMON locus). However, the genomic locus where the
is also overexpressed in solid tumors such as colon cancer POKEMON gene resides (chromosome 19p13.3) is a gene-rich
and bladder cancer in which the normal function of the ARF/ region that is found frequently mutated in human cancers.
p53 pathway is frequently lost, it is likely that Pokemon has Interestingly, one recent study showed that the t(14;19)
multiple additional target genes by which it can exert its (q32;p13.3) translocation is one of the more common cryptic
oncogenic activity. In this respect, it is worth noting that translocations involving the immunoglobulin heavy chain gene
Kaiso, another POK family member, specifically binds to in B-cell non-Hodgkin’s lymphoma (21). It is also possible that
methylated DNA sequences and further enhances repression POKEMON up-regulation would be due in some instances to
of target genes (17). It is tempting to speculate that Pokemon the aberrant activation of upstream regulatory pathways. In this
also exerts its oncogenic function through epigenetic control of respect, Astier et al. recently reported that POKEMON (FBI-1) is
the target gene (i.e., promoter hypermethylation). Furthermore, induced upon fibronectin-mediated h1-integrin ligation in
recent reports have uncovered a functional correlation between precursor B leukemia cells (22). The authors speculate that
POZ/BTB domain–containing proteins and Cullin-based E3 up-regulation of Pokemon facilitates cellular proliferation upon
ligases. POZ domain protein family acts as substrate-specific fibronectin binding. Further studies will be necessary to
adaptors of Cullin3-based E3 ubiquitin ligase complexes and precisely determine how POKEMON expression is regulated
plays a role in substrate targeting of cullin-based E3 ligase both in normal and cancerous cells.
complexes (18). Pokemon may therefore be able to regulate In closing, our findings suggest that POKEMON could be
targets or pathways through protein ubiquitination. Whereas an attractive therapeutic target for human cancer therapy in
>40 POK proteins have been identified in human, little is view of its essential role in oncogenic transformation.
known about the ‘‘functional crosstalk’’ among these proteins. Indeed, the possibility of a strategy to elicit functional
Given that POK proteins can form heterodimers/homodimers blockade of POKEMON has been proposed recently by A.
through the POZ/BTB domain (e.g., PLZF/BCL6 and Pokemon/ Melnick et al. (23). This group developed a blocking peptide
BCL6) and these proteins could share similar DNA consensus for the POK family protein BCL6 that specifically binds the
sequences (e.g., PLZF and FAZF; ref. 19), functional networks ‘‘lateral groove’’ of its POZ domain and abrogates corepressor
that are governed by POK family proteins are likely more interaction, resulting in the effective blockade of BCL6
dynamic and complicated than originally anticipated. Further- function both in vitro and in vivo. A similar strategy may
more, it is interesting to hypothesize possible interactions/ also be useful for the knockdown of POKEMON function in
crosstalk between POK proteins and other POZ/BTB containing cancer cells. Indeed, we are now testing whether inhibition
protein families, which contain >100 proteins in human. The of Pokemon function could lead to the eradication or
POZ/BTB domain is normally found as a single copy in prevention of malignant transformation and progression both
proteins that also contain one or two other domain types in vitro and in vivo using a conditional Pokemon mouse
such as Zinc finger (POK proteins), BACK-kelch, voltage-gated knockout model.
potassium channel T1, and MATH domains (20). The
multitude of potential interaction partners for POK proteins
would be expected to lead to a great diversity in their cellular Acknowledgments
functions beyond simple target gene repression. Received 3/29/2005; revised 7/18/2005; accepted 7/19/2005.
References and Th2- type inflammation. Nat Genet 1997;16: inducer of short transcripts. Mol Cell Biol 1997;17:
161–70. 3786–98.
1. Melnick A, Carlile G, Ahmad KF, et al. Critical residues 6. Koken MH, Reid A, Quignon F, et al. Leukemia- 10. Davies JM, Hawe N, Kabarowski J, et al. Novel BTB/
within the BTB domain of PLZF and Bcl-6 modulate associated retinoic acid receptor a fusion partners, POZ domain zinc-finger protein, LRF, is a potential
interaction with corepressors. Mol Cell Biol 2002;22: PML and PLZF, heterodimerize and colocalize to target of the LAZ-3/BCL-6 oncogene. Oncogene 1999;
1804–18. nuclear bodies. Proc Natl Acad Sci U S A 1997;94: 18:365–75.
2. Barna M, Hawe N, Niswander L, Pandolfi PP. Plzf 10255–60. 11. Kukita A, Kukita T, Ouchida M, Maeda H, Yatsuki
regulates limb and axial skeletal patterning. Nat Genet 7. Chen WY, Zeng X, Carter MG, et al. Heterozygous H, Kohashi O. Osteoclast-derived zinc finger (OCZF)
2000;25:166–72. disruption of Hic1 predisposes mice to a gender- protein with POZ domain, a possible transcriptional
3. He X, Dave VP, Zhang Y, et al. The zinc finger dependent spectrum of malignant tumors. Nat Genet repressor, is involved in osteoclastogenesis. Blood
transcription factor Th-POK regulates CD4 versus 2003;33:197–202. 1999;94:1987–97.
CD8 T-cell lineage commitment. Nature 2005;433: 8. Maeda T, Hobbs RM, Merghoub T, et al. Role 12. Pessler F, Hernandez N. Flexible DNA binding of the
826–33. of the proto-oncogene Pokemon in cellular trans- BTB/POZ-domain protein FBI-1. J Biol Chem 2003;278:
4. Costoya JA, Hobbs RM, Barna M, et al. Essential role of formation and ARF repression. Nature 2005;433: 29327–35.
Plzf in maintenance of spermatogonial stem cells. Nat 278–85. 13. Quelle DE, Zindy F, Ashmun RA, Sherr CJ. Alternative
Genet 2004;36:653–9. 9. Pessler F, Pendergrast PS, Hernandez N. Purification reading frames of the INK4a tumor suppressor gene
5. Ye BH, Cattoretti G, Shen Q, et al. The BCL-6 and characterization of FBI-1, a cellular factor that encode two unrelated proteins capable of inducing cell
proto-oncogene controls germinal-centre formation binds to the human immunodeficiency virus type 1 cycle arrest. Cell 1995;83:993–1000.
www.aacrjournals.org 8577 Cancer Res 2005; 65: (19). October 1, 2005
Downloaded from cancerres.aacrjournals.org on March 3, 2015. © 2005 American Association for Cancer
Research.Cancer Research
14. Sanchez-Beato M, Sanchez-Aguilera A, Piris MA. Cell N-CoR mediates DNA methylation-dependent repres- Anastasi J, Le Beau MM. Identification of novel cryptic
cycle deregulation in B-cell lymphomas. Blood 2003;101: sion through a methyl CpG binding protein Kaiso. Mol translocations involving IGH in B-cell non-Hodgkin’s
1220–35. Cell 2003;12:723–34. lymphomas. Cancer Res 2002;62:5523–7.
15. Donehower LA, Harvey M, Slagle BL, McArthur 18. Pintard L, Willems A, Peter M. Cullin-based ubiquitin 22. Astier AL, Xu R, Svoboda M, et al. Temporal gene
MJ, Montgomery CA, Jr., Butel JS. Bradley. Mice ligases: Cul3-BTB complexes join the family. EMBO J expression profile of human precursor B leukemia
deficient for p53 are developmentally normal but 2004;23:1681–7. cells induced by adhesion receptor: identification of
susceptible to spontaneous tumours. Nature 1992;356: 19. Hoatlin ME. Zhi Y, Ball H, et al. A novel BTB/POZ pathways regulating B-cell survival. Blood 2003;101:
215–21. transcriptional repressor protein interacts with the 1118–27.
16. Kamijo T, Bodner S, van de Kamp E, Randle DH, Fanconi anemia group C protein and PLZF. Blood 23. Polo JM, Dell’Oso T, Ranuncolo SM, et al. Specific
Sherr CJ. Tumor spectrum in ARF-deficient mice. 1999;94:3737–47. peptide interference reveals BCL6 transcriptional and
Cancer Res 1999;59:2217–22. 20. Stogios PJ, Prive GG. Genome Biol. In press. oncogenic mechanisms in B-cell lymphoma cells. Nat
17. Yoon HG, Chan DW, Reynolds AB, Qin J, Wong J. 21. Gozzetti A, Davis EM, Espinosa R III, Fernald AA, Med 2004;10:1329–35.
Cancer Res 2005; 65: (19). October 1, 2005 8578 www.aacrjournals.org
Downloaded from cancerres.aacrjournals.org on March 3, 2015. © 2005 American Association for Cancer
Research.The Transcription Factor Pokemon: A New Key Player in
Cancer Pathogenesis
Takahiro Maeda, Robin M. Hobbs and Pier Paolo Pandolfi
Cancer Res 2005;65:8575-8578.
Updated version Access the most recent version of this article at:
http://cancerres.aacrjournals.org/content/65/19/8575
Cited Articles This article cites by 22 articles, 10 of which you can access for free at:
http://cancerres.aacrjournals.org/content/65/19/8575.full.html#ref-list-1
Citing articles This article has been cited by 8 HighWire-hosted articles. Access the articles at:
http://cancerres.aacrjournals.org/content/65/19/8575.full.html#related-urls
E-mail alerts Sign up to receive free email-alerts related to this article or journal.
Reprints and To order reprints of this article or to subscribe to the journal, contact the AACR Publications
Subscriptions Department at pubs@aacr.org.
Permissions To request permission to re-use all or part of this article, contact the AACR Publications
Department at permissions@aacr.org.
Downloaded from cancerres.aacrjournals.org on March 3, 2015. © 2005 American Association for Cancer
Research.You can also read
