expression being completely blocked by aspirin/NSAIDs.
These findings further support the notion that aspirin and
NSAIDs play a protective role in the pathogenesis of esopha-
geal cancer including ESCC, and suggest that regulation of
MIF expression by aspirin and the COX-2 inhibitor, contribute
to this protective role. A few studies have looked at the asso-
ciation between MIF and COX-2 (41,42). One such study
reported that recombinant human MIF up-regulated COX-2
mRNA, and anti-MIF monoclonal antibody significantly
reduced COX activity and mRNA expression (42). It has
been shown that MIF inhibits p53 activity, and importantly,
that this regulation coincides with the induction of arachidonic
acid metabolism and COX-2 expression (43,44). The latter
supported our initial hypothesis that inhibition of MIF-induced
COX-2 activity by aspirin/NSAIDs might contribute to the
protective role of these drugs in the development of ESCC.
However, our further experiment indicates that aspirin/
NSAIDs modulate MIF expression by a COX-independent
mechanism, because addition of prostaglandin E2 in the
culture medium did not reverse the inhibitory effect of
aspirin/NSAIDs.
Further studies are required to determine the exact mechan-
isms by which aspirin/NSAIDs inhibit bile acid-induced MIF
expression. It has been demonstrated that bile acids activate
protein kinase K and the transcription factors AP-1 and NF-
kB, induce IL-8 expression and up-regulate some oncogenes
such as c-myc in esophageal adenocarcinoma cells (45-- 48).
Therefore, it would be helpful to determine whether bile acids
exert similar effects in esophageal squamous carcinoma cells
and whether aspirin/NSAIDs inhibit bile acid-induced MIF
expression by antagonizing one or more of these pathways.
In addition, it is generally accepted that cigarette smoking and
alcohol intake are closely associated with the development of
ESCC, an association that may account for 90% of ESCC cases
in the developed world (49). There is evidence that one of the
molecular targets for cigarette smoking is the p53 gene, and
damage to this gene is known to depend on the number of
cigarettes smoked per day (50). It is possible that cigarette
smoking exerts its effect on p53 through an MIF-mediated
pathway, but this hypothesis needs to be tested further.
In conclusion, MIF expression is increased in ESCC.
Whereas bile acids induce MIF expression in esophageal squa-
mous carcinoma cells, aspirin and NS398, a COX-2 inhibitor,
significantly inhibit MIF expression even in the presence of
bile acids, in a COX-independent mechanism.
Acknowledgements
This project was supported by a competitive earmarked research grant from the
Research Grants Council of Hong Kong Special Administrative Region, China
(HKU7318/01 M to H.H.-X.X.). S.T.Z. was a visiting professor sponsored by
the Croucher Foundation (Chinese Visitorship Scheme), The University of
Hong Kong. The authors thank Dr Hua He and Dr Yun Wei Sun, Department
of Medicine, The University of Hong Kong, Hong Kong, and Dr Xing Xiang
He, Department of Medicine, The Second Affiliated Hospital of Guangzhou
Medical College, Guangzhou, China, for technical assistance.
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