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Advances in Brief |
Laboratory of Molecular Medicine, Human Genome Center, Institute of Medical Science, The University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, Japan [Y-M. L., K. O., S. S., H. I., M. F., N. M., Y. N., Y. F.]; SNP Research Center, Riken (Institute of Physical and Chemical Research), 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, Japan [T. Ta., T. Ts.]; and Department of Internal Medicine, Shin Kong Wu Ho-Su Memorial Hospital, Taipei, Taiwan [Y-M. L., K-C. Y.]
| ABSTRACT |
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| Introduction |
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Various cytogenetic abnormalities play critical roles in leukemogenesis. For example, translocations involving chromosomal band 11q23, e.g., t(4;11), t(6;11), t(9;11), and t(11;19), are observed in 10% of patients with acute lymphoblastic leukemia and more than 5% of myeloid leukemias (10) . The MLL (ALL-1, HRX, TRX) gene, located at the breakpoint on band 11q23, is cleaved by these translocations, and its fusion to specific genes on partner chromosomes results in production of chimeric proteins. The AF17 gene at chromosome 17q21 is a fusion partner of a less frequent translocation of the MLL gene, t(11;17)(q23;q21). The predicted amino acid sequence of AF17 contains three zinc-finger domains at the NH2 terminus, and a leucine-zipper dimerization motif located 3' of the fusion point. Although AF17 is thought to function as a transcriptional regulator (10) , its role in leukemogenesis remains to be explained.
In this study, we report identification of AF17 as a possible downstream target of the ß-catenin-Tcf/LEF transcriptional complex. We also document its involvement in cell-cycle regulation and discuss its possible role in the mechanisms of colorectal carcinogenesis.
| Materials and Methods |
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cDNA Microarray and Selection of Target Genes.
Fabrication of the cDNA microarray slides and construction of
recombinant adenovirus have been described elsewhere (8
, 11)
. Duplicate sets of cDNA microarray slides containing 9216
cDNA spots in all were used for each analysis of expression profiles to
reduce experimental fluctuation. Briefly, SW480 cells were infected
with adenovirus expressing APC, Ad-APC, or control virus,
Ad-LacZ. Total RNAs were extracted 72 h after infection,
mRNA was purified from the samples, and T7-based RNA
amplification was carried out. Aliquots (5-µg) of amplified RNA from
SW480/Ad-APC and SW480/Ad-LacZ were labeled with Cy5-dCTP and
Cy3-dCTP, respectively (Amersham Pharmacia Biotech, Piscataway, NJ).
Hybridization, washing, and detection were carried out as described
previously (11)
. Genes were excluded from further
investigation when the intensities of both Cy3 and Cy5 were below
250,000 fluorescence units. Those with Cy3/Cy5 signal ratios greater
than 2.0 were selected for further
evaluation.
RT-PCR.
We carried out semiquantitative RT-PCR using cDNA reversely transcribed
from 0.2 µg of total RNA from each cell line. The PCR exponential
phase was determined on 2032 cycles to allow comparison among cDNAs
developed from identical reactions. GAPDH served as an
internal control. Primers for human AF17 were H-AF17F
(5'-GGAGACCTCTGAGAGCAGC-3'), H-AF17R (5'-GGAGTACTTGTCCTCCTCTG-3'); for
human GAPDH, H-GAPDH3F (5'-ACAACAGCCTCAAGATCATCAG-3'),
H-GAPDH3R (5'-GGTCCACCACTGACACGTTG-3'); and for mouse homologue of
AF17, M-Af17F (5'-GAGGTGCCCACTAGGACAG-3'), M-Af17R
(5'-GCACAATTCCAGGCTTGGAG-3'). All of the reactions were carried
out in 25-µl volumes and amplified for 3 min at 94°C for initial
denaturation, followed by 2032 cycles at 94°C for 30 s, 58°C
for 30 s, and 72°C for 1 min on a GeneAmp PCR system 9700 (PE
Applied Biosystems, Foster City, CA).
Real-time quantitative RT-PCR (TaqMan PCR; Perkin-Elmer) was carried out using 7700 Sequence Detector (Perkin-Elmer) according to the manufacturers recommendations. Quantification and equalization of the amount of cDNA was achieved by amplifying GAPDH as an internal control (TaqMan GAPDH Control Reagents). Primers and the probe for AF17 were AF17F (5'-TCGCTTGGCAACAACACAAG-3'), AF17R (5'-TGGTCTGGGCAGTGAGGACT-3'), and AF17-Probe (5'-Fam-CAGCAGTAGCAGCAGCAGGCGGA-Tamra-3').
Construction of Plasmids.
The entire coding region of AF17 was amplified by RT-PCR
with primers AF17-forward (5'-AGGAATTCCATGGGAGTATGAAGGAGATGGTAG-3') and
AF17-reverse (5'-TACTCGAGGATATAGCCTTTTTCCTGGTTGGCTG-3'). The
product was digested with EcoRI and XhoI and
cloned into the appropriate sites of pcDNA3.1(+) (Invitrogen, Carlsbad,
CA) and pFlag-CMV-5a (Sigma Chemical Co., St. Louis, MO).
Immunocytochemical Staining.
NHDF cells were transfected with pFlag-CMV-5a/AF17 using
FuGENE 6 (Boehringer Mannheim, Mannheim, Germany) according to the
manufacturers instructions and fixed with PBS containing 4%
paraformaldehyde. Fixed cells were incubated with a mouse anti-Flag
antibody (Sigma Chemical Co.) and stained by a rhodamine-conjugated
antimouse secondary antibody (ICN Biomedicals, Inc., Costa Mesa, CA).
After nuclei were stained with 4',6'-diamidine-2'-phenylindole
dihydrochloride (Boehringer Mannheim), the preparations were observed
under a Nikon Eclipse E800 fluorescence microscope.
Colony-formation Assay (Anchorage-dependent Growth Assay).
We transiently transfected either pcDNA3.1(+)/AF17, expressing AF17, or
empty vector into NIH3T3 cells and treated the cultures with G418 (0.8
mg/ml) for 2 weeks. Cells that survived were fixed with 100% methanol
and stained by Giemsas solution (Merck, Darmstadt, Germany).
Growth Analysis.
NIH3T3 and DLD1 cells stably expressing AF17 were established by
transfecting NIH3T3 and DLD1 cells with pcDNA3.1(+)/AF17 plasmid using
FuGENE 6. Control and AF17-expressing NIH3T3 and DLD1 cells were seeded
on 6-cm plates (1 x 105
cells/plate) and counted with a hemacytometer every day.
Cell-cycle Analysis.
To examine whether AF17 has a role in cell-cycle progression, SW480
cells expressing abundant amounts of AF17 were transfected with sense
(5'-ATGAAGGAGATGGTAG-3') or antisense (5'-CTACCATCTCCTTCAT-3')
S-oligonucleotide, the latter being designed to suppress
expression of AF17, by Lipofectin (Life Technologies, Inc., Rockville,
MD) according to the manufacturers recommendations and maintained for
an additional 24 h. RT-PCR and FACS were performed on a FACScan
flow cytometer using CycleTEST reagents and the manufacturers
protocol (Becton Dickinson, San Jose, CA). To examine the role of AF17
further, DLD1 cells expressing low levels of AF17 were transfected with
pcDNA3.1(+)/AF17, and we selected stable clones (DLD1-AF17-1 and
DLD-AF17-2) expressing high levels of AF17. These cells, together with
DLD1-vector (mock) cells as controls, were growth-arrested in
G1 phase by incubation with 5 µg/ml aphidicolin
(Sigma Chemical Co.) for 36 h and released from
G1 by removal of aphidicolin. FACS was performed
0, 4, 8, and 12 h later. We also treated those cells with
-irradiation (10 Gy) and analyzed the population at each phase of
the cell cycle by FACS 12 h and 24 h after irradiation.
Statistics.
Assessment of statistical differences for Figs. 3
and 4
were determined
by Fishers Protected Least Significant Difference test and
Students t test, respectively. P < 0.05 was considered statistically significant. Statistical
analyses were performed using StatView software.
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| Results |
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Localization of AF17 in Mammalian Cells.
Flag-tagged AF17 protein was detected in the nuclei of transfected
cells by immunocytochemical staining (Fig. 2)
. Similar results were obtained when we transfected the plasmid into
COS7 and LoVo cells (data not shown).
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Cell-cycle Regulation.
We performed FACS using SW480 cells transfected with an antisense
S-oligonucleotide designed to suppress expression of AF17
(Fig. 4A)
. A larger proportion of SW480 cells were arrested at the
G2-M phase after this treatment in comparison
with cells treated with control S-oligonucleotide (16.2%
versus 6.7%; P < 0.05; Fig. 4B
).
To confirm a role of AF17 in G2-M progression, we
cloned DLD1 cells that stably expressed high levels of AF17 (DLD1-AF17
cells) and synchronized them at the G1 phase by
aphidicolin treatment. Overexpression of AF17 significantly shortened
the G2-M transition time as shown in Fig. 4C
(12.9% versus 26.6%; P < 0.05). In addition, 24 h after cells were exposed to
-irradiation (10 Gy), about 20% more of the DLD1-AF17 cells
arrested at the G2-M phase had progressed to
G0/G1 than had DLD1-mock
cells (43.9% remaining in G2-M versus
64.2%; P < 0.05; Fig. 4D
). These
results indicated that AF17 accelerated cell-cycle progression by
promoting G2-M transition. All of the results
were confirmed in at least two independent experiments in triplicate
plates.
| Discussion |
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Because molecules of the Tcf/LEF family interact with the consensus sequence 5'-CTTTGWW-3', we searched for this binding motif in the 5' flanking region of AF17 and found one located about 523529 bp upstream of the transcription-initiation site. Although we carried out reporter assays using plasmids containing a DNA fragment encompassing the motif and various deletion mutants of it, we were unable to detect any significant difference in the reporter activities among them. Thus, the putative binding element did not affect the elevated transcription of AF17, a result that suggested two possibilities: either AF17 may be regulated only indirectly by the ß-catenin-Tcf complex or ß-catenin may regulate AF17 through other binding motifs coupled with a different transcriptional factor. The latter notion is consistent with a recent report (13) that the WISP1 gene, a known target gene of ß-catenin, is transactivated by cyclic AMP-responsive element binding protein through its binding site but not through putative Tcf/LEF-binding elements.
The AF17 gene was initially isolated as a fusion partner of the MLL gene in t(11;17)(q23;q21) translocations present in some acute myeloid leukemias (10) . Although chromosomal translocations are usually associated with overexpression or activation of oncogenes, Prasad et al. (10) proposed a model in which MLL rearrangements would result in loss of function of that gene because most of the partner genes encode unrelated proteins except for similarities between AF9 and ENL and between AF10 and AF17 (14 , 15) . However, reciprocal translocations can produce two chimeric proteins. Thus, in addition to inactivation of or interference with MLL, t(11;17) translocations might confer oncogenic activity through abnormal activation of AF17 and produce a malignant phenotype in leukemic cells. AF10 bears significant homology to AF17 within their respective cysteine-rich domains at the NH2 termini and leucine zipper domains toward the COOH termini, although they diverge outside those regions (15) . The first part of the cysteine-rich region in each case contains conserved zinc-finger domains known as LAP/PHD-finger. The remainder contains a cluster of 12 conserved cysteines and histidines. These cysteine-rich domains show similarity with part of BR140, a bromodomain- and PHD finger-containing protein that is homologous to the TAF250 subunit of transcription factor TFIID. Therefore, AF17 and AF10 may both function as transcriptional repressors or activators (16) .
Our experiments have supported a view that AF17, like c-myc and cyclin D1, is involved in cell-cycle progression and is regulated by ß-catenin (12 , 17) . Although the mechanisms by which AF17 regulates the cell cycle are not clear at present, accelerated progression of the G2-M boundary could result from abrogation of a checkpoint. If that is the case, cells that overexpress AF17 may accumulate genetic alterations in addition to conferring a growth advantage. Additional investigations of its functions and isolation of its target molecules will help to clarify the role of AF17 in colorectal carcinogenesis. Such studies may also provide clues for identifying predictive and prognostic markers for diagnosis and for developing more effective therapeutic strategies for specific cancers.
| FOOTNOTES |
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1 Supported in part by Research for the Future
Program Grant #96L00102 from the Japan Society for the Promotion of
Science. ![]()
2 To whom requests for reprints should addressed,
at Laboratory of Molecular Medicine, Human Genome Center, Institute of
Medical Science, The University of Tokyo, 4-6-1 Shirokanedai,
Minato-ku, Tokyo 108-8639, Japan. Phone: 81-3-5449-5373; Fax:
81-3-5449-5406; E-mail: furukawa{at}ims.u-tokyo.ac.jp ![]()
3 The abbreviations used are: APC, adenomatous
polyposis coli; MLL, myeloid/lymphoid or mixed lineage leukemia; FACS,
fluorescence-activated cell sorter analysis; Tcf, T-cell factor; LEF,
lymphoid enhancer factor; RT-PCR, reverse transcription-PCR; GAPDH,
glyceraldehydes-3-phosphate dehydrogenase. ![]()
Received 3/21/01. Accepted 7/18/01.
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