
[Cancer Research 60, 262-265, January 15, 2000]
© 2000 American Association for Cancer Research
Mechanism for Inactivation of the KIP Family Cyclin-dependent Kinase Inhibitor Genes in Gastric Cancer Cells1
Jong-Yeon Shin,
Hyun-Seok Kim,
Jinseu Park,
Jae-Bong Park and
Jae-Yong Lee2
Department of Biochemistry, College of Medicine [J-Y. S., H-S. K., J-B. P., J-Y. L.], Department of Genetic Engineering [J. P.], and Institute of Environment & Life Science [J. P., J-B. P., J-Y. L.], Hallym University, Chunchon, Kangwon-do 200-702, South Korea
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ABSTRACT
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The mechanism for inactivation of the KIP family cyclin-dependent
kinase inhibitor (CDKI) genes, the p21,
p27, and p57 genes, in gastric cancer
cells was tested by treating the cells with either the DNA
demethylation agent, 5-aza-2'-deoxycytidine or the histone deacetylase
inhibitor, n-butyric acid or trichostatin A. RNA
expression of the gene was determined by reverse transcription PCR. The
p21 gene was activated only by histone deacetylase
inhibitor. The p57 gene was activated by histone
deacetylase inhibitors in all of the gastric cancer cell lines and by
5-aza-2'-deoxycytidine in five of eight gastric cell lines. However,
the p27 gene was not inactivated in gastric cancer cell
lines. The methylation status of the promoter of the p21
and p57 genes was also tested by digestion with the
methylation-sensitive restriction enzymes and a subsequent PCR. The
promoter of the p21 gene has no methylation. The
promoter of the p57 gene is, however, methylated in five
of eight gastric cancer cell lines as expected from the result of the
treatment with 5-aza-2'-deoxycytidine. Formation of the inactive
chromatin through histone deacetylation seems to be the general
mechanism for inactivation of both the p21 and the
p57 genes in gastric cancer cells. Hypermethylation of
promoter region seems to be an alternative pathway for inactivation of
the p57 gene.
 |
Introduction
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Cell cycle progression is regulated by interactions between
cyclins and
CDKs3
(1
, 2)
. Especially, the transition of
G1 to S phase is known to be regulated by a
family of negative cell cycle regulators, CDKIs. CDKIs are classified
in two families, the CIP/KIP family and the INK4 family, based on
primary sequence comparisons (3, 4, 5)
. The CIP/KIP family
CDKIs, p21CIP1/WAF1,
p27KIP1, and p57KIP2 share
common sequence motifs that mediate interaction between CDKI and
cyclin-CDK complexes (6, 7, 8)
.
p21CIP1/WAF1 was the first CDKI to be identified,
and it is known to be induced by p53, transforming growth factor ß,
differentiation, and cellular senescence (9)
.
p27KIP1 is known to be involved in
G1 arrest induction by cell-to-cell contact,
cyclin AMP-inducing agents, and rapamycin (8)
. In
contrast to ubiquitous expression of
p21CIP1/WAF1 and
p27KIP1,
p57KIP2 is expressed at high levels in
specific embryonic and adult tissue (7)
. Three genes,
p21, p27, and p57, have been
investigated in different kinds of human tumors and, unlike the INK4
family, only a few genetic alterations have been found. This suggests
that the mutational inactivation of these CDKIs is
infrequent (10)
, but gene inactivation by alternative
mechanisms seems to be the general pathway.
Two known mechanisms, gene inactivation by methylation in promoter
region and changes to an inactive chromatin by histone deacetylation,
seem to be the best candidate mechanisms for inactivation of CIP/KIP
family CDKI genes because these two mechanisms are recently
and frequently reported as the mechanism for the inactivation of
specific genes. Histone acetylation results in the separation of DNA
from histones, allowing nucleosomal DNA to become more accessible to
transcription factors. The resulting histone hyperacetylation is
correlated to the transcriptionally active state. Histone deacetylation
allows the formation of normal nucleosome structure. This is
referred to as transcriptionally inactive. The level of histone
acetylation depends on the activity of histone acetyl
transferases and HDACs. A number of genes and proteins have been
identified for having activity of histone acetyl transferase or of
HDAC. Naturally occurring compounds, such as n-butyric acid
and TSA, are reported to inhibit the HDAC. They seem to induce general
histone acetylation through a noncompetitive and nonspecific inhibition
of the HDAC (11
, 12)
. These compounds are known to induce
the growth arrest and the differentiation in a variety of cancer cell
types by activating the transcription of the p21 gene
through Sp1 binding sites in its promoter (11)
.
Transcriptional repression by DNA methylation of promoter and 5'
regulatory sequences is expected to be the other pathway to inactivate
the CIP/KIP CDKI genes. Changes in DNA methylation patterns
are known to occur during tumorigenesis (13)
. CpG islands
near promoters and 5' regulatory region are usually unmethylated in
normal somatic cells. In contrast, widespread methylation of CpG
islands occurs on autosomal genes and leads to the silencing of the
genes during oncogenic transformation (14)
. Promoters
silenced by methylation can be reactivated by treatment with
5-aza-2'-deoxycytidine, which is a well-established inhibitor of DNA
methylation (15)
. We hypothesized that abnormal DNA
methylation and histone deacetylation might be the mechanism of
inactivation of the CIP/KIP genes in gastric cancer cells
lines. In this study, we report that histone deacetylation is a general
mechanism for inactivation of the
p21CIP1/WAF1 and
p57KIP2 genes and methylation in promoter
region is an alternative mechanism for inactivation of the
p57KIP2 genes in gastric cancer cell lines.
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Materials and Methods
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Cell Lines and Cell Culture.
Eight gastric cancer cell lines (SNU-1, SNU-5, SNU-16, SNU-620,
SNU-638, SNU-668, SNU-719, and KATO-III) that were established from
gastric carcinomas (16, 17, 18)
were obtained from
Korea Cell Lines Bank. Gastric cancer cells were cultured in RPMI 1640
(Life Technologies, Inc.) supplemented with 10% (vol/vol) fetal bovine
serum (Life Technologies, Inc.) and maintained at 37°C and 5%
CO2.
Treatment of the Cells with HDAC Inhibitors or Demethylation
Agents.
To test the inactivation of the CIP/KIP family CDKI genes,
gastric cancer cells (2 x 106
cells) in 100-mm-diameter dishes were treated with either the HDAC
inhibitor, n-butyric acid (5 mM) or
TSA (0.3 µM), or the demethylation agent,
5-aza-2'-deoxycytidine (1 µM), at 24 h
after seeding. The medium containing drugs was removed 24 h later,
and cells were washed twice with PBS. The cells treated with
n-butyric acid or TSA were then harvested and used for RNA
preparation or stored at -80°C. The cells treated with
5-aza-2'-deoxycytidine were allowed to grow in RPMI 1640 with fetal
bovine serum for 48 h and then were harvested for RNA preparation.
RT-PCR Analysis.
Total RNA was isolated from 2 x 106 cells using RNAzol B (Cinna Scientific). cDNA
was synthesized by incubation at 52°C for 20 min in a 20-µl
reaction containing RNA (100 ng), antisense primers (see the primers
listed below), 10 mM deoxynucleotide triphosphates
(Boehringer Mannheim), 1 x reaction buffer (provided by
Takara Shuzo Co.) and Avian Myeloblastosis Virus reverse transcriptase
(Takara Shuzo Co.). cDNA was amplified using primer sets specific for
the p21 gene (sense: 5'-CAGGGGACAGCAGAGGAAGA-3'; antisense:
5'-GGGCGGCCAGGGTATGTAC-3'); the p27 gene (sense:
5'-ATGTCAAACGTGCGAGTGTC-3'; antisense: 5'-TCTGTAGTAGAACTCGGGCAA-3');
and the p57 gene (sense: 5'-TCGCTGCCCGCGTTTGCGCA-3';
antisense: 5'-CCGAGTCGG TGTCCACTTCGG-3'). The PCR reaction was
performed as follows: 95°C for 8 min, 40 cycles of 95°C for 1 min,
at different annealing temperatures for 1 min and 72°C for 1 min, and
followed by incubation at 72°C for 10 min. Annealing temperatures for
p21, p27, and p57 were 58, 62, and
62°C, respectively. PCR products were resolved on 2% agarose gels.
PCR-based Methylation Analysis.
A PCR analysis relying on the inability of HpaII to cut
methylated sequence was used to analyze the p21 gene. The
sites examined were two HpaII sites in the promoter of
p21CIP1/WAF1. DNA (1 µg) was digested for
5 h, with 10 units of enzyme per µg of DNA. The digested
DNA (100 ng) was amplified with primers flanking the restriction sites
under the following conditions: at 98°C for 5 min, 35 cycles of
98°C for 1 min, at 70°C for 1 min, and at 74°C for 1 min,
followed by incubation at 74°C for 10 min. The primer set used for
methylation analysis of p21 promoter was
5'-GCCTGCTGGAACTCGGCCAGGCTCAGCTGC-3' (sense) and 5'-GAGGCGACCCGCGCTC
GGCCCAGCGCGCCG-3' (antisense). PCR products were resolved on 2%
agarose gels. A PCR assay relying on the partial inability of
SacII to cut methylated sequence was used to analyze
p57KIP2 genes. Two SacII sites
in the promoter p57KIP2 were
examined. DNA (1 µg) was digested for 5 h, with 10 units of
enzyme per µg of DNA. The digested DNA (100 ng) was amplified with
pfu DNA polymerase (Stratagene) and primers flanking the
restriction sites (sense: 5'-CCCGAGCTGGCAGCGGCGGGTCCAAGCCTC-3';
antisense: 5'-TGCTGGCTAGCTCG-CTCGCT-CAGGCCTGGC-3') under the
following conditions: at 98°C for 5 min, 35 cycles of 98°C for 1
min, at 71°C for 30 s, and at 74°C for 1 min, followed by
incubation at 74°C for 10 min. PCR products were resolved on 1%
agarose gels.
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Results and Discussion
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To elucidate the mechanism of inactivation of p21, a
KIP family CDKI gene, in gastric cancer cells, eight gastric
cancer cell lines were treated with either the HDAC inhibitor,
n-butyric acid, or the demethylation agent,
5-aza-2'-deoxycytidine. RNA was isolated from cells and RNA expression
was analyzed by RT-PCR. RNA expression of the p21 gene was
activated only by the HDAC inhibitor, n-butyric acid, but
not by 5-aza-2'-deoxycytidine in eight gastric cancer cell lines (Fig. 1A
). The result suggests that the p21 gene is
inactivated by formation of an inactive chromatin complex through
histone deacetylation in gastric cancer cells. Methylation in the
promoter of the p21 gene does not seem to be involved in
inactivation of p21 in gastric cancer cells. The presence of
methylation in the promoter of the p21 gene was further
tested by digestion with a methylation-sensitive restriction enzyme and
a subsequent PCR reaction. The GC-rich region in the six
consecutive Sp1 binding sites of the p21 promoter was
digested either with methylation-sensitive HpaII or with
methylation-insensitive MspI. Resulting DNA was subjected
for a PCR reaction (Fig. 1B
). Only undigested DNA will
provide 110 bp of the PCR amplification product across the Sp1 binding
sites. Digestion with either MspI or HpaII got
rid of the PCR products (Fig. 1C
). The result showed that
the promoter of the p21 gene is not methylated in gastric
cancer cells. This confirms that methylation is not the mechanism for
inactivation of p21 in gastric cancer cells.

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Fig. 1. RNA expression of the p21 gene in gastric
cancer cells after treatment with the demethylation agent and the HDAC
inhibitor and a PCR-based methylation analysis for the
p21 promoter in gastric cancer cells. A,
analysis of the p21 RNA by RT-PCR. Eight gastric cancer
cell lines were treated with 1 µM of
5-aza-2'-deoxycytidine (A) or and 5 mM of
n-butyric acid (B). RNA was isolated from
the treated cells and the untreated cells (C), amplified
by RT-PCR, and separated in 2% agarose gel as described in
"Materials and Methods." Arrow on the left, the size
of the RT-PCR product, 335 bp. B, a schematic map of the
p21 promoter for restriction sites and the position of a
primer set. Arrow, the location of the transcription
initiation site of the p21 gene. Six Sp1 binding sites
() are located in a GC-rich region within 180 bp upstream of the
transcription initiation site of the p21 gene.
-124 and -111, the recognition sites of
the restriction enzyme HpaII or MspI.
Two arrows indicate a primer set for PCR and the size of
PCR product, 110 bp. C, a PCR-based methylation analysis
for the p21 promoter in gastric cancer cells. The
methylation-sensitive restriction enzyme HpaII
(H) and the methylation-insensitive restriction enzymes
MspI (M) that have the same
recognition sequence were used to test the methylation status of the
promoter of the p21 gene. DNA from eight gastric cancer
cell lines was digested with either HpaII
(H) or MspI (M), and the
resulting DNA was subjected for PCR. One hundred ng of the digested or
the undigested (U) DNA was amplified by PCR. M
(to the left of the lane labels), a DNA molecular weight
marker.
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To test the mechanism for inactivation of the p57 gene in
gastric cancer cells, eight gastric cancer cell lines were treated with
either the HDAC inhibitors, n-butyric acid or TSA, or the
demethylation agent, 5-aza-2'-deoxycytidine. RNA was isolated from the
cells and RNA expression was analyzed by RT-PCR. RNA expression of the
p57 gene was activated by HDAC inhibitors, either
n-butyric acid or TSA in all of the eight gastric cancer
cells. SNU1 was an exception in that TSA activated the p57
gene, but n-butyric acid did not. An appropriate explanation
for this is not currently available except that it may be a clonal
variation. Treatment with 5-aza-2'-deoxycytidine also activated the
p57 gene in five of eight gastric cancer cell lines (Fig. 2
). The result suggests that the p57 gene seems to be
inactivated by the formation of an inactive chromatin complex
containing HDAC. Methylation of the promoter of the p57 gene
is also involved in inactivation of the p57 gene. The
presence of methylation in the promoter of the p57 gene was
further tested by digestion with a methylation-sensitive restriction
enzyme and a subsequent PCR reaction. Because the promoter of
p57 is highly GC-rich and has many MspI sites, a
rare cutting and partially methylation-sensitive restriction enzyme,
SacII was selected. Two SacII sites are located
at -240 bp and -45 bp from the transcription initiation site. A
primer set amplifies DNA across two SacII sites of the
p57 promoter (Fig. 3A
). Only undigested DNA will give a PCR product of 655 bp.
DNA was isolated from untreated and 5-aza-2'-deoxycytidine-treated
gastric cancer cells and digested with SacII. The resulting
DNA was subjected to PCR (Fig. 3A
). DNA from five untreated
gastric cancer cell lines provided the PCR amplification products, but
DNA from three untreated gastric cancer cell lines did not (Fig. 3B
). DNA from all of the 5-aza-2'-deoxycytidine-treated
gastric cancer cells did not make the PCR product. This indicates that
five gastric cancer cell lines have the methylation in the promoter of
the p57 gene. Treatment with 5-aza-2'-deoxycytidine removed
the methylation from the promoter so that SacII cleaved the
promoter, and PCR could not make the amplification product. The five
gastric cell lines that yielded the PCR product were
SNU-5, SNU-16, SNU-668, SNU-719, and KATO-III. These cell lines were
exactly the same cell lines whose silent p57 gene was
activated by treating with 5-aza-2'-deoxycytidine as shown in Fig. 2
.
All of the eight gastric cancer cell lines are established from gastric
carcinomas. Six of them are established from metastatic ascites, and
two of them are from primary carcinoma. Four of these five gastric
tumor cell lines that are found to be methylated in the p57
promoter were established originally from metastatic tumors. Two of
three tumors that are found to have no methylation in the
p57 promoter were from primary tumors (16, 17, 18)
.
This suggests that alteration in DNA methylation is acquired before
tumors become metastatic. DNA methylation is an earlier event than
metastasis in tumorigenesis. This explanation is in accord with the
hypothetical model on molecular alterations in tumorigenesis of
colorectal tumor proposed by Vogelstein. The extent of
differentiation of original tumors and the extent of attachment in the
growth of tumor cell lines were not correlated with the methylation
status of the p57 gene in gastric tumor cell lines. The
result suggests that formation of an inactive chromatin with histone
and HDAC is a general mechanism for the inactivation of the
p57 gene in gastric cancer cell lines because all of the
gastric cell lines were activated by treating with a HDAC inhibitor.
Methylation of the p57 gene seems to be an alternative way
to form an inactive chromatin in gastric cancer cells.

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Fig. 2. RNA expression of the p57 gene in gastric
cancer cells treated with the demethylation agent and the HDAC
inhibitor. Eight gastric cancer cell lines were treated with
5-aza-2'-deoxycytidine (1 µM; A), or
n-butyric acid (5 mM; B), or
TSAT. RNA was isolated from the untreated
(C) and the drug-treated cells, was amplified by RT-PCR,
and separated in 2% agarose gel as described in "Materials and
Methods." The size of the RT-PCR product was 287 bp
(arrow). M, a DNA molecular weight
marker.
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Fig. 3. A PCR-based methylation analysis for the
p57 promoter in gastric cancer cell. A, a
schematic map of the p57 promoter for restriction sites
and the position of primer set. Arrow, the location of
the transcription initiation site of the p57 gene.
-240 and -45, the recognition sites of
SacII. The size of PCR product was 655 bp.
B, a PCR-based methylation analysis of the
p57 promoter in gastric cancer cells treated with or
without 5-aza-2'-deoxycytidine. The methylation-sensitive restriction
enzyme, SacII, was used to test the methylation status
of the promoter of the p57 gene. Gastric cancer cell
lines were incubated for 72 h in the presence (+) or the absence
(-) of 1 µM 5-aza-2'-deoxycytidine
(5-aza-dc). DNA was extracted and digested with
SacII. One hundred ng of the digested DNA was amplified
by PCR with the primer sets. M, a DNA molecular weight
marker.
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To test whether the p27 gene, another KIP family
CDKI gene, can also be activated, gastric cancer cells were
treated with the same reagent, either the HDAC inhibitors,
n-butyric acid and TSA, or a demethylation agent,
5-aza-2'-deoxycytidine. RNA was isolated and analyzed by RT-PCR.
Interestingly, considerable amount of RNA was expressed from the
p27 gene even in untreated gastric cancer cells. RT-PCR
provided almost the same amount of amplification product for the
untreated cells, the HDAC inhibitor-treated cells and the
5-aza-2'-deoxycytidine-treated cells (Fig. 4
). The results suggest that p27 is not inactivated in gastric
cancer cells. There should be a specific reason for this. Although the
exact reason for this is currently unavailable, the following
explanation is possible: (a) p27 may play a pivotal role for
cell survival and growth of gastric cancer cells; therefore, its
expression is necessary although it acts against the growth of gastric
cancer cells; (b) the p27 gene may be mutated in
these gastric cancer cells; therefore, the expression of the
p27 RNA will produce the nonfunctional p27 protein. However,
the latter explanation seems to be unlikely because our recent result
showed no mutation in gastric cancer cell lines, and recent reports for
p27 mutation analysis do not support this (19)
.

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Fig. 4. RNA expression of p27 in eight gastric
cancer cells treated with the demethylation agent or the HDAC
inhibitor. Gastric cancer cells were treated with 1 µM
5-aza-2'-deoxycytidine (A) or 5 mM
n-butyric acid (B). RNA samples from
untreated (C) and drug-treated cells were isolated and
analyzed by RT-PCR. M, a DNA molecular weight marker.
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Inactivation of the p21 gene and the p57 gene
seem to be necessary to a cancer cell inasmuch as all of the gastric
cancer cells show very low or no expression of RNA from either
gene. Because expression of either the p21 gene or
the p57 gene can cause the growth arrest, cancer cells seem
to have the alteration in both genes to have advantage in their
growth. Mutational alteration is rare in both genes. therefore,
another possible mechanism is nonmutational inactivation because many
developmentally regulated genes are inactivated without having mutation
in adult tissues. Two known mechanisms are methylation of the gene and
histone deacetylation. Although recent reports suggest that these
mechanisms are overlapped (in that the binding of
MeCP2, a methyl cytosine-binding protein, to
methyl cytosine recruits the HDAC to inactivate the genes), they
are not the same. Inactivation by histone deacetylation seems to be
broader than the methylation-mediated inactivation. The histone
deacetylation-mediated inactivation can include the
methylation-mediated inactivation. However, the histone
deacetylation-mediated inactivation does not necessarily involve the
methylation of DNA. Our result supports this. The p57 gene
is activated in all of the eight gastric cancer cell lines by the HDAC
inhibitor, but it is activated in five of eight cell lines by
5-aza-2'-deoxycytidine. In the inactivation of p21, the
methylation of DNA was not even involved. Methylation of the promoter
is probably an alternative system to recruit the histone deacetylation.
MeCP2, a methyl cytosine-specific binding
protein, first binds the methyl cytosine and recruits the proteins
including the HDAC to make an inactive chromatin complex as recent
reports have described (20, 21, 22)
. In unmethylated DNA,
recruitment of the HDAC by some transcription repressors seems to be
involved. Transcription repressors like Mad, RB, YY1, and nuclear
receptors are reported to recruit the HDAC (23, 24, 25)
. What
is the corresponding factor in the p21 gene and the
p57 gene? Sp1 binding sites are the common elements that
exist in the promoters of both genes. Unfortunately, no such
information is available for Sp1 or Sp1-binding proteins. Further
characterization of the promoter and of the promoter-binding proteins
in these genes will provide better understanding of the details of
molecular level mechanisms of gene inactivation.
 |
FOOTNOTES
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1 Supported by The Hallym Academy of Science,
Hallym University; a grant of the 19971998 Korean National Cancer
Control Program, Ministry of Health and Welfare; and a grant of
Molecular Medicine Research Group Program from the Ministry of Science
and Technology, Republic of Korea. 
2 To whom requests for reprints should be
addressed, at Department of Biochemistry, Hallym University College of
Medicine, 1 Okchon-dong, Chunchon, Kangwon-do 200-702, South Korea.
Phone: 82361-240-1625; Fax: 82361-244-8425; E-mail: jyolee{at}sun.hallym.ac.kr 
3 The abbreviations used are: CDK,
cyclin-dependent kinase; CDKI, CDK inhibitor; RT-PCR, reverse
transcription PCR; TSA, trichostatin A; 5-aza-dC,
5-aza-2'-deoxycytidine; HDAC, histone deacetylase. 
Received 9/16/99.
Accepted 11/30/99.
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[Abstract]
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