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Laboratory of Pathology, Aichi Cancer Center Research Institute, Aichi 464-8681 [N. S., Y. I., K-I. I., H. N., T. T., K. N., M. T.]; Department of Gastrointestinal Surgery, Postgraduate School of Medicine, University of Tokyo, Tokyo 112-8688 [N. S., K. N., M. K., S. K., K-I. I.]; and First Department of Surgery [A. S.], and Department of Laboratory Medicine [T. K.], Shinshu University, Matsumoto 390-8621, Japan
| ABSTRACT |
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| Introduction |
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| Materials and Methods |
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A total of 158 specific pathogen-free male, 7-week-old MGs
(Meriones unguiculatu; MGS/Sea, Seac Yoshitomi, Ltd.,
Fukuoka, Japan), were housed in steel cages on hardwood chip bedding in
an air-conditioned biohazard room (for infection) with a 12-h
light/12-h dark cycle. They were given food (Oriental MF; Oriental
Yeast Co., Tokyo, Japan) irradiated with 30 kGy gamma rays and
autoclaved distilled water ad libitum. The experimental
design described below was approved by the Animal Care Committee of the
Aichi Cancer Center Research Institute. The animals were treated as
follows. In experiment I, 79 gerbils were divided into three groups.
They were given MNU in their drinking water at the concentration of 30
ppm for alternate weeks for a total of 5 weeks exposure. On completion
of this regimen, they were given autoclaved distilled water; 1 week
thereafter, Hp were inoculated (groups 1 and 2, n = 32 and 27, respectively). Group 3 (n = 20) received Brucella broth without Hp. At week 30, five
animals in group 1 were killed to check for bacterial infection, and at
week 21, all animals in group 2 underwent treatment for eradication. In
experiment II, 79 gerbils were divided into three groups. One week
after inoculation of Hp (groups 4 and 5, n = 32 and 27, respectively) or the Brucella broth vehicle alone
(group 6; n = 20), they were given MNU in
their drinking water at the concentration of 10 ppm for 20 weeks
continuously. At week 30, five animals in group 4 were killed to check
for bacterial infection, and at week 21, all animals in group 5
underwent Hp eradication treatment (Fig. 1)
. At the 50th experimental week, after 24 h fasting, all animals
were subjected to deep ether anesthesia, laparotomized, and
exsanguinated from the inferior vena cava, with excision of their
stomachs. Tissues were processed for microbiological and
histopathological examinations.
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30-mm2 samples
of stomach mucosa from the greater curvature, containing both fundic
and pyloric glands, were homogenized with 1 ml of Brucella
broth and used for culture of Hp. Aliquots of 100 µl were then
inoculated on segregating agar plates for Hp (Eiken Chemical Co.,
Tokyo, Japan) and incubated at 37°C under microaerobic conditions for
6 days. Anti-Hp antibodies were measured as described earlier
(5)
. Briefly, blood samples containing a small amount of
EDTA were centrifuged at 8000 rpm for 5 min to isolate sera, which were
then stored at -80°C. Using the MG sera thus obtained, anti-Hp IgG
antibody (GAP-IgG; Biomerica, Newport Beach, CA) was measured by ELISA.
The antibody titers were expressed using an A.I. The excised stomachs
were fixed in 4% paraformaldehyde in PBS (pH 7.2) or 95% ethanol
containing 1% acetic acid, cut into
16 strips, processed by
standard methods, and embedded in paraffin. Tissues were sectioned at 5
µm for staining with H&E, alcian blue-periodic acid-Schiff stain and
by immunohistochemistry for Hp (anti-Hp serum; DAKO, Copenhagen,
Denmark). Adenocarcinomas of the glandular stomach were classified into
well-differentiated lesions, characterized by tubular structures with
cellular atypia, poorly differentiated tumors, characterized by little
tendency to form glandular structures with severe cellular atypia, and
signet ring cell carcinomas, characterized by isolated tumor cells
containing abundant amounts of mucin (7)
. The two-tailed t test or the Mann-Whitney U test was applied to establish the significance of differences in body weight distributions and titers of anti-Hp antibodies. Survival curves were calculated by the Kaplan-Meier method, and the differences were evaluated using the log-rank test. The adenocarcinoma incidences were assessed by the Fishers exact probability method. Ps <0.05 were considered to be statistically significant.
| Results |
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Histologically, in all MGs in groups 1 and 4, the glandular stomach
epithelium showed hyperplastic changes with variable degrees of
multifocal cystic dilation and erosion. There was marked infiltration,
predominantly of lymphocytes and some macrophages as well as
neutrophils in the lamina propria and submucosa, with frequent
formation of lymphoid follicles. Intestinal metaplasia was also noted
in 23 among 24 gerbils in group 1 and in 25 among 26 animals in group
4. Intestinal metaplasia was not observed in any but the 2 animals that
failed to be eradicated for Hp in group 2 and was lacking in groups 3,
5, and 6. The incidences of adenocarcinomas of the glandular stomach
are summarized in Table 1
. Tumors were mostly found in the antral mucosa adjacent to the oxyntic
region and comprised well-differentiated (Fig. 2)
and poorly differentiated adenocarcinomas (Fig. 3a)
, as well as signet ring cell carcinomas (Fig. 3c)
. The incidences of adenocarcinomas in group 1 was
significantly higher than in group 2 (P < 0.01) and group 3 (P < 0.001). The incidence
in group 4 was also significantly higher than those in groups 5 and 6
(P < 0.05).
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| Discussion |
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The most important factors that may influence stomach carcinogenesis are strain diversities of Hp and host reactions. Concerning Hp strains, investigators have studied Cag A and Vac A intensively. Tomb et al. (13) elucidated the complete Hp genome sequence in 1997, and a full understanding of the pathogenicity of Hp itself should be generated in the near future.
Concerning host factors, we have emphasized the stomach mucosal condition and serological response of the host (6) . One interesting point in the MG model is the persistent Hp infection in the stomach, not found in mice and rats. A decrease in the rate of Hp infection with aging and with stomach mucosal damage in MGs and increase in Hp infection related to stomach mucosal damage in mice (14) imply important roles for mucosal conditions of the stomach with regard to Hp infection. T helper 1 cellular immune responses also contribute to Helicobacter-associated gastritis in mice and humans (15) , and DElios et al. (16) showed that Hp-specific T helper 1 effectors may play a role in generation of peptic ulcers in humans. In our previous study, the titers of anti-Hp antibodies in tumor-bearing animals were found to be higher than in those which were tumor free treated in the same manner (6) . This implies that the T helper 2 humoral response may play an important role in stomach carcinogenesis.
For application of Hp eradication to human, timing will presumably be of essential significance. In our present study, we eradicated Hp in the period of active chronic gastritis but before intestinal metaplasia or atrophic gastritis had appeared. The early phase might be expected to be most amenable to intervention. To explore the effectiveness of Hp eradication in later phases, we are now modifying our protocol to clarify what lesions occurring in the Hp-infected stomach are irreversible, true neoplastic changes.
MGs resemble humans in their susceptibility and response to Hp infection (17) . It has already been reported that similar pathological changes occur in the glandular stomachs of humans and MGs in response to Hp. In this study, we observed signet ring cell carcinomas as well as poorly differentiated adenocarcinomas and well-differentiated adenocarcinomas, whereas only well-differentiated adenocarcinomas were observed in the stomachs of MGs infected with Hp without any chemical carcinogen exposure (18) . This variety of induced stomach cancers as well as the similarity in response to Hp infection suggest advantages of this model for research on stomach carcinogenesis in humans. A recent meta-analysis indicated that Hp infection is equally associated with intestinal and diffuse types of gastric cancer (19) , in agreement with our findings.
Using the present carcinogenesis model and adding eradication at various periods, sequential histopathological and molecular biological examinations should allow detailed assessment of the risk of Hp infection in terms of gastric neoplasia and elucidation of the underlying mechanisms.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 This work was supported in part by a
Grant-in-Aid for the Second-term Comprehensive 10-year Strategy for
Cancer Control from the Ministry of Health and Welfare, Japan, and by a
Grant-in-Aid from the Ministry of Education, Science, Sports and
Culture of Japan. During the performance of this work, N. S. was an
awardee of a Research Resident Fellowship from the Foundation for the
Promotion of Cancer Research. ![]()
2 To whom requests for reprints should be
addressed, at Laboratory of Pathology, Aichi Cancer Center, Research
Institute, 1-1 Kanokoden, Chikusa-ku, Nagoya 464-8681, Japan. Phone and
Fax: 81-52-76-2971; E-mail: mtatemat{at}aichigw.aichi-cc.pref.aichi.jp ![]()
3 The abbreviations used are: Hp,
Helicobacter pylori; MG, Mongolian gerbil; MNU,
N-methyl-N-nitrosourea; A.I., arbitrary
index; i.g., intragastric. ![]()
Received 11/18/99. Accepted 1/31/00.
| REFERENCES |
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