University of Athens, Faculty of Medicine, Department of Histology, Embryology, Molecular Carcinogenesis Group, Athens, Greece
Pokemon transcripts can be alternatively spliced, resulting in four mRNAs with different 5-untranslated regions. The four alternative first exons are located at -11596, -10224, -9109 and -17 bp upstream of the translation start site of Pokemon, implying that Pokemon can putatively be regulated by four alternative promoters.
- Transcription regulator
1. ZBTB7A/FBI-1 + IST (HIV)
FBI-1 binds to IST, the HIV-1 inducer of short transcripts, and mediates its function. The first and second zinc fingers are required for efficient binding. FBI-1, via the zinc finger domain, interacts as well with Tat (transactivator of transcription), a viral transactivator protein, responsible for the synthesis of HIV-1 full-length transcripts. FBI-1 partially colocalizes with Tat and enhances its activity in a dose dependent manner, therefore modulating Tat-activated HIV-1 transcription.
2. FBI-1 + adipogenesis
FBI-1 acts as a dual regulator in adipogenesis. It promotes preadipocyte differentiation process and facilitates adipogenesis. On the other hand, it is implicated in termination of the mitotic clonal expansion, in which growth-arrested preadipocytes re-enter the cell cycle and complete two rounds of cell division, followed by terminal differentiation phase.
3. FBI-1 + FASN (transcriptional activator)
Increased FBI-1 expression enhances FASN (fatty-acid synthase gene) expression, in stable cancer cells or immortalised stable cells. FASN expression appears to be low in almost all nonmalignant adult tissues, but is highly expressed in many carcinomas, where it is responsible for fatty acid synthesis. Furthermore, knockdown of FBI-1 expression leads to decreased FASN expression.
FBI-1 enhances FASN transcriptional activation through SREBP-1a (sterol-responsive element (SRE)-binding protein 1). FBI-1 and SREBP-1a form a complex and co-localize mainly in the nucleus. Specifically, FBI-1 through its ZFDBD-C-term domain (zinc finger DNA binding domain) interacts directly with the bHLH domain of SREBP-1. Sp1 binds to the proximal GC-box of the FASN promoter and acts as a major activator. It has been found that the regulatory elements GC-box and SRE/E-box of FASN promoter can be bound by all three, FBI-1, SREBP-1a and Sp1, transcription factors. This can lead to a binding competition between them.
4. FBI-1 + NF-kappaB (transcriptional activator)
FBI-1 POZ domain interacts directly with the RHD domain (Rel homology domain) of NF-kappaB p65 subunit.
FBI-1 enhances the transcription of NF-kappaB-responsive genes, like E-selectin, by increasing either the localization or stability of NF-kappaB in the nucleus. FBI-1 facilitates nuclear import and prolonged nuclear retention of the p65 subunit of NF-kappaB, either by inhibiting its export to the cytoplasm, or by increasing its stability. In addition, FBI-1 inhibits IkappaB (inhibitors of NF-kappaB) interaction with NF-kappaB, eventually enhancing NF-kappaB transcription in the nucleus.
5. FBI-1 + ADH5/FDH (transcriptional repressor)
FBI-1 POZ domain interacts with the C2H2-type zinc-finger domain of Sp1 (Specificity protein 1), thereby repressing the ADH5/FDH promoter activity.
6. ZBTB7A + ARF (transcriptional repressor)
ZBTB7A was characterized as a potential proto-oncogene that specifically represses transcription of p14ARF, a major tumour suppressor gene, (leading indirectly to p53 inactivation). ZBTB7A binds directly to p19ARF promoter (mouse homologue). ARF promoter contains several putative ZBTB7A-binding sites, rich in GC. However the one located 50 base pairs upstream from the transcription start site is indispensable for p19ARF repression.
7. FBI-1 + p21WAF1/CIP1 (transcriptional repressor)
FBI-1 represses transcriptional activation of P21CIP1, by the following ways.
Firstly, by inhibiting ARF and Hdm2 expression, and therefore p53, it indirectly affects p21WAF1/CIP1 expression. Secondly, as a GC-box-binding transcription factor, it competes with Sp1 (transcriptional activator) to bind to the GC-rich FRE/GC-box 3 element at the proximal promoter, critical for p21CIP1 transcription.
Furthermore FBI-1 competes with p53 for binding to the distal p53-binding elements of p21WAF1/CIP1 and represses its transcription.
Additionally, it interacts with corepressors such as NCoR (Nuclear receptor co-repressor), SMRT (silencing mediator for retinoid and thyroid receptors), and mSin3A and recruits HDAC complexes, leading to histone deacetylation at the proximal promoter, responsible for p21WAF1/CIP1 transcriptional repression.
8. FBI-1 + CCS-3 + p21WAF1/CIP1
FBI-1 suppresses p21WAF1/CIP1 promoter activation by more than 70% by acting on both the proximal Sp1 binding GC-box 3 element and the distal p53 responsive regulatory elements. CCS-3, (cervical cancer suppressor -3), represents an isoform of eEF1A (eukaryotic translation elongation factor 1A), involved in various processes like embryogenesis, oncogenic transformation, senescence, cell proliferation, and cytoskeletal organization. CCS-3 enhances the FBI-1 mediated p21WAF1/CIP1-transcriptional repression, by another 30-50%. The above is accomplished because CCS-3 can interact with both co-repressors, (like SMRT and NCoR independently of FBI-1) and FBI-1 POZ domain. CCS-3 is able to recruit additional transcriptional co-repressor-HDAC complexes when bound to the FBI-1 POZ-domain, therefore enhancing FBI-1 transcriptional repression potential.
Furthermore, both FBI-1 and CCS-3 independently promote cell proliferation. Co-transfection of CCS-3 and FBI-1 results in a significant increase in cell population.
9. LRF - COMP (transcriptional repressor)
LRF (mouse homolog of FBI-1) binds directly to the NRE element and specifically to a nine nucleotide (GAGGGTCCC) sequence - LRF binding element (LBE) in COMP (Cartilage oligomeric matrix protein) gene promoter. NRE represents a negative regulatory element, both necessary and sufficient to repress promoter activity. Therefore, it regulates COMP gene expression implicated in chondrogenic differentiation. Human COMP gene promoter also contains a typical consensus site (GGGGGCCTC, -1294 to -1302) for binding LRF/FBI-1, implying that FBI-1 may suppress COMP gene expression in human as LRF does in the mouse.
10. FBI1 - AR (transcriptional repressor)
The androgen receptor (AR) is important for the proliferation and functioning of prostate cells.
FBI-1 physically interacts with AR and when overexpressed, it represses AR transcriptional activity in a ligand-dependent manner. More precisely, POZ domain of FBI1 is responsible for the interaction with the LBD domain (C-terminal ligand-binding domain) of AR. Furthermore, full-length FBI-1 is indispensable for the repression of AR transcriptional activity. On the contrary, FBI-1 suppression by siRNA enhanced AR transcriptional activity.
It is known that FBI-1 can interact with NCoR and SMRT, which are co-repressors of AR. (FBI-1 exhibited a much stronger interaction with SMTR compared to that with NCoR). It was shown that FBI-1 enhances the recruitment of NCoR and SMRT to endogenous ARE elements and through this mechanism it suppresses AR transcriptional activity. This notion is further supported by the fact that FBI-1 repressive effect on AR transcriptional activity, is inhibited upon endogenous NCoR and SMRT knockdown. Furthermore it was suggested that FBI-1 could recruit a co-repressor complex harbouring HDACs, resulting in repression of AR functions in the presence of R1881 (synthetic agonist of the androgen receptor). Last, it was shown that FBI-1 can play different roles in regulating androgen-dependent (LNCaP) and -independent (PC-3) prostate cancer cells. It inhibits LNCaP cell proliferation in the presence of R1881, whereas no difference in proliferation was observed in the absence of R1881. In contrast, knockdown of FBI-1 by siRNA led to a significant increase in LNCaP cell growth. Interestingly, either in the presence or absence of R1881, PC-3 cell proliferation was dramatically enhanced upon FBI-1 overexpression.
11. Pokemon - survivin (transcriptional activator)
Tissue microarray technology has shown Pokemon overexpression in 86,8% of breast cancer tissue, but not in normal breast tissue. Furthermore Pokemon expression correlated positively with tumor size, lymph node metastasis and poor patient survival.
Survivin, a member of the inhibitor of apoptosis proteins (IAP), is implicated in cell carcinogenesis and tumor progression.
Survivin and Pokemon expression levels were found to be highly correlated in human breast cancer cells. Pokemon induces survivin expression by direct binding to the GC boxes in its promoter.
- Relation with ASF/SF2 through miRNAs
LRF is regulated by microRNA (miRNA) families.
In MEFs (Mouse Embryo Fibroblasts) it was shown that miR-28 and miR-505, which are negatively controlled by LRF, can affect ASF/SF2 (Alternative Splicing Factor/Splicing Factor 2) expression and therefore can induce apoptosis or cellular transformation. Deregulation of the LRF/miR-28/miR-505/ASF/SF2 axis affects the MEF senescence/apoptosis pathway.
- Relation with anticancer agents
Curcumin is a natural compound known for its antioxidant, anti-inflammatory and anticarcinogenic properties, since it can inhibit the proliferation and induce the apoptosis of cancer cells, such as gastric, colon, breast. Curcumin inhibits ZBTB7A transcriptional activation, by suppressing the Sp1 stimulatory effect on Pokemon promoter, as it inhibits Sp1 recruitment to the Pokemon promoter.
- Global mapping of ZBTB7A downstream target genes in HepG2 cells
ChIP-on-chip analysis performed in the human hepato-carcinoma cancer cell line HepG2, revealed 556 genes as potential direct targets of ZBTB7A. These genes are mainly divided in three categories: genes regulating metabolism, transcriptional regulators and cell signal transduction genes. Among the most ZBTB7A-targeted metabolic pathways are the aminosugar, arachidonic acid, tryptophan, pyrimidine and purine metabolic pathways. Moreover sucrose, vitamin B6, valine, leucine and isoleucine, folate, fatty acid and glycerolipid metabolism, are characterized as ZBTB7A regulatory targets. As far as signal transduction is concerned, ZBTB7A might play a regulatory role on the MAPK signalling pathway. Furthermore, four genes involved in axon guidance, a key stage in the formation of the neuronal network, are directly regulated by ZBTB7A expression in HepG2 cells. This pinpoints to a potential regulatory role for ZBTB7A in neural development.
- ZBTB7A and SUMOylation
ZBTB7A can be sumoylated as it contains an amino-acid sequence highly similar to the SUMO-1 conjugate consensus motif, ψKXE (ψ: large hydrophobic residue, K: lysine to which SUMO-1 is conjugated, X: any amino acid, and E: glutamic acid). Sumoylation of several lysine residues of ZBTB7A is important in order to achieve full repressive activity.
RANK and its ligand, RANKL (a member of the tumor necrosis factor - TNF superfamily), are critical molecules in osteoclastogenesis. RANKL also plays an important role in inflammatory bone loss. FBI-1/OCZF/LRF is highly expressed in osteoclasts and is an important regulator of RANKL-induced osteoclastogenesis. LRF siRNA inhibits osteoclast differentiation. Osteoclast formation is a crucial step in bone destruction in chronic arthritis. FBI-1/OCZF is highly expressed in osteoclasts in inflammatory arthritis, implying a role in inflammatory bone erosion. The above suggest that by enhancing osteoclastogenesis and survival of osteoclasts, FBI-1/OCZF/LRF might play important roles in bone destruction in vivo, including the bone loss and remodelling that occurs in chronic inflammatory arthropathies, such as rheumatoid arthritis.
Kalliopi Apostolopoulou ; Ioannis S Pateras ; Athanassios Kotsinas ; Vassilis G Gorgoulis
ZBTB7A (zinc finger and BTB domain containing 7A)
Atlas Genet Cytogenet Oncol Haematol. 2011-07-01
Online version: http://atlasgeneticsoncology.org/gene/42863/zbtb7a-(zinc-finger-and-btb-domain-containing-7a)