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From the Departments of Clinical Cancer Prevention,*
Thoracic/Head and Neck Medical Oncology,§
and
Pathology,
University of Texas M. D.
Anderson Cancer Center, Houston, Texas; and the Center Laboratory for
Tumor Biology,
Cancer Institute and Hospital,
Chinese Academy of Medical Science, Beijing, China
| Abstract |
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| Introduction |
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Retinoids exert their biological effects by binding to specific nuclear
retinoid receptors that belong to the steroid hormone receptor
superfamily. These receptors are
ligand-activated, DNA-binding, trans-acting,
transcription-modulating proteins.7
The retinoid receptors
include retinoic acid (RA) receptors (RARs) and retinoid X receptors
(RXRs), each of which includes three subtypes (
, ß, and
).7
We recently demonstrated that growth inhibition of
esophageal cancer cell lines by RA was associated with expression and
RA-induced up-regulation of RAR-ß. Cell lines that failed to express
RAR-ß were resistant to RA, and only these cell lines were able to
form colonies in soft agar.8
These findings implied that
the in vivo status of RAR-ß expression may be important
for the use of retinoids in chemoprevention and chemotherapeutic
clinical trials of esophageal cancer. In this study, we used in
situ hybridization to detect RAR-ß expression in normal,
dysplastic, and malignant esophageal tissue specimens to determine its
involvement in esophageal tumorigenesis.
| Materials and Methods |
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|
|
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Tissue specimens taken from 16 normal mucosae, 30 moderate or severe dysplastic lesions, and 157 esophageal tumors were obtained from the Cancer Institute and Hospital of China and Guangzhou Nanfang Hospital, China, respectively. These specimens included 71 distant normal squamous mucosae and 29 distant columnar mucosae. All samples were routinely fixed in 10% buffered formalin, embedded in paraffin, and cut into 4-µm sections. One of each of these sections was stained with hematoxylin and eosin for classification.
In Situ Hybridization
Levels of RAR-
, -ß, and -
and RXR-
mRNAs were measured
by using a previously described method of nonradioactive in
situ hybridization.9,10
The quality and specificity
of the digoxigenin-labeled anti-sense and sense riboprobes were
determined using Northern blotting, and the specificity of the binding
of antisense riboprobes was verified using negative control sections.
Briefly, the tissue sections first underwent the treatment with 0.2 N
HCl and proteinase K, respectively, after deparaffinization and
rehydration. The slides were then postfixed with 4% paraformaldehyde
and acetylated in freshly prepared 0.25% acetic anhydride in a 0.1
mol/L triethanolamine buffer. The slides were then prehybridized at
42°C with a hybridization solution containing 50% deionized
formamide, 2x standard saline citrate, 2x Denhardts solution, 10%
dextran sulfate, 400 µg/ml yeast tRNA, 250 µg/ml salmon sperm DNA,
and 20 mmol/L dithiothreitol in diethylpyrocarbonate-treated water.
Next the slides were incubated in 50 µl per slide hybridization
solution containing 20 ng of a freshly denatured dig-cRNA probe at
42°C for 4 hours. After that, the slides were washed for 2 hours in
2x SSC containing 2% normal sheep serum (NSS) and 0.05% Triton X-100
and then for 20 minutes at 42°C in 0.1x SSC. For color reaction, the
slides were incubated for 30 minutes at 23°C in 0.1 mol/L maleic acid
and 0.15 mol/L NaCl (pH 7.5, buffer 1) containing 2% NSS and 0.3%
Triton X-100 and then incubated overnight at 4°C with a sheep
anti-digoxigenin antibody. After washing in buffer 1 twice, the color
was developed in a chromogen solution (45 µl of nitroblue tetrazolium
and 35 µl of an X-phosphate solution in 10 ml of buffer 2, which
consisted of 0.1 mol/L Tris, 0.1 mol/L NaCl, and 0.05 mol/L
MgCl2 (pH 9.5) for 4 hours with occasional observation for
color development. The slides were then mounted with a coverglass in
Aqua mounting medium (Fisher, Houston, TX).
Review and Scoring of Sections
The stained sections were reviewed and scored independently by two
pathologists (H.Q., X-C. X.) with an Olympus microscope. The
sections were signed as positively or negatively staining. The positive
staining means 10% or more epithelial cells stained positive.
Statistical analysis was performed using the chi-squared
(
2) test to determine the association between normal or
distant normal tissues and tumors. P values were generated
using Statistica version 3.0a for Macintosh computer (StatSoft, Tulsa,
OK).
| Results |
|---|
|
|
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, RAR-
, and RXR-
were
expressed in all 16 normal esophageal mucosae; RAR-ß was expressed in
14 of the 16 samples (Table 2
|
|
|
, RAR-
,
and RXR-
were expressed in most (96100%) of the specimens,
whereas RAR-ß was expressed in only 17 (56.7%) of 30 (Table 2
For the study of receptor expression in esophageal cancer specimens, we
first analyzed 40 cases from northern and southern China. The
preliminary results showed that RAR-
, RAR-
, and RXR-
were
expressed in almost all of these 40 specimens (data not shown). We
therefore decided to exclude RAR-
and RAR-
from subsequent study
and only investigate the expression of RAR-ß and RXR-
(as control)
in the 157 tumors. In distant normal squamous mucosae
(N = 71) and distant columnar mucosa specimens
(N = 29) that were also collected from the 157
patients with tumors, 96% of the specimens expressed RAR-ß. In the
157 tumors themselves, RAR-ß was expressed in only 84 specimens
(53.5%) (Table 2
and Figure 1
). The differences in RAR-ß expression
levels were significant by
2
test between normal and
abnormal tissues (Table 2)
.
We further analyzed RAR-ß expression in the three histological
subtypes of esophageal cancer (ie, SCC, adenocarcinoma, and
adenosquamous carcinoma). No difference in the positivity of RAR-ß
expression was detected among these subtypes of esophageal cancers.
RAR-ß expression, however, was associated with the degree of squamous
cell differentiation: RAR-ß mRNA was expressed in 62% (26/42) of
well-differentiated SCCs and in 54% (27/50) of moderately
differentiated SCCs, but in only 29% (4/14) of poorly differentiated
SCCs. The degree of squamous differentiation was defined by a
pathologists diagnosis in the individual cases and confirmed by the
authors (H.Q., X-C. X.). The differences in RAR-ß expression
were significant between well-differentiated and poorly differentiated
SCCs (P < 0.03) (Table 3
and Figure 1B
).
|
| Discussion |
|---|
|
|
|---|
Various studies have clearly demonstrated that altered expression of
retinoid receptors is associated with malignant transformation in human
cells.8-15
Altered expression of RAR-ß is a common
event in different types of tumors, including head and neck, lung, and
breast tumors,9-14
although lost expression of RAR-
or
RAR-
has also been reported.15
A recent paper of
ours8
showed that the sensitivity of esophageal cancer
cells to RA was correlated not only with the constitutive expression
but also with RA-induced up-regulation of RAR-ß. Cell lines that
failed to express RAR-ß were resistant to RA and could form colonies
in soft agar. Two recent clinical trials of 13-cis RA in
patients with advanced esophageal cancer were
unsuccessful,16,17
indicating that loss of RAR-ß
expression could contribute to resistance to treatment with
13-cis RA in this cancer. These results suggest that RA
chemotherapy of esophageal cancer may be less effective. Nevertheless,
our current data are suggestive of a potential benefit of RA in
premalignant esophageal carcinogenesis, ie, RA chemoprevention. We
showed that RAR-ß is expressed in 96% of high-risk normal tissue
specimens and 57% of dysplastic lesions. It could be inferred that
these and the in vitro data suggest that RA chemopreventive
approaches should target high-risk populations that express RAR-ß.
Retinoids modulate epithelial cell growth and differentiation and suppress carcinogenesis in vitro and in vivo.1-4 Physiologically, retinoids can prevent abnormal squamous differentiation of epithelial cells in nonkeratinizing tissues and are able to reverse squamous metaplasia that develops during vitamin A deficiency.1 In animal experiments and clinical trials, retinoids can restore premalignant and malignant lesions to the normal nonkeratinizing phenotype and responsiveness to normal growth-control mechanisms, thereby suppressing carcinogenesis and preventing squamous cell carcinoma development.1,2 These effects may be mediated by RAR-ß, because lung cancer cells expressing a transfected RAR-ß exhibited decreased tumorigenicity in nude mice18 and transgenic mice expressing antisense RAR-ß2 developed lung cancer.19
Although loss of RAR-ß expression appears to be a common event in esophageal squamous carcinomas, adenocarcinomas, and other cancers,8-15 the underlying mechanism is largely unknown.15 Loss of heterozygosity on chromosome 3p was detected in only approximately 30% of esophageal cancers,20 and there was no correlation with RAR-ß expression (our unpublished data). The RAR-ß gene promoter includes a retinoic acid response element (RARE) that can be activated by retinoids through RAR-RXR heterodimers. In a recent study, Liu et al21 found that the loss of all-trans RA-induced ßRARE transcriptional activation may be responsible for the loss of RAR-ß gene expression. Inactivation of nuclear retinoid receptor coactivators or activation of their corepressors may also account for altered expression of RAR-ß. Finally, RAR-ß expression is dependent on cellular levels of RA. Loss of RAR-ß expression in premalignant oral lesions correlated with a low cellular level of RA.22 All together this indicates that multiple mechanisms may be involved in the loss of RAR-ß gene expression, which is an early event in carcinogenesis in head and neck, lung, and esophageal tissues and may be a useful intermediate biomarker in chemoprevention studies.
| Acknowledgements |
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| Footnotes |
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Supported in part by National Cancer Institute grant CA74835 and by grants from the Physicians Referral Service and the Office of the Vice President for Cancer Prevention, University of Texas M. D. Anderson Cancer Center and from the Chinese National Scientific Research Foundation (96-906-01-02) and China NSFC (39870838).
Accepted for publication July 2, 1999.
| References |
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-2a in patients with advanced esophageal carcinoma. Cancer 1999, 85:1213-1217[Medline]
-2a in patients with advanced esophageal cancer: a phase II trial. Invest. New Drugs 1996, 14:391394
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