Aspirin intake reduces the risk of colorectal cancer (CRC),
but the molecular mechanisms remain unknown. Based on data from various articles,
researches and literature, Cyclooxygenase-2 (COX-2) has long been suspected to
be the primary target for aspirin and NSAID-mediated CRC chemoprevention. There
are many studies that either support or deny COX-2 as the primary target, but
it is well established that COX-2 activation alone is insufficient to cause
tumorigenesis, evidenced by the fact that COX-2 transgenic mice fail to develop
tumors spontaneously. Collectively, COX-2 might function as a tumor promoter
rather than as an initiator, but the mechanism of action by which COX-2 drives
tumorigenesis remains imperfectly understood.
The epidermal growth factor receptor (EGFR), a transmembrane
receptor tyrosine kinase of the ErbB family, was elevated in up to 80% of CRC
cases and is associated with clinical outcomes.
Researchers demonstrated that COX-2 might drive intestinal
tumorigenesis by up-regulating EGFR expression in familial adenomatous polyposis
(FAP) patients, an effect that could be attenuated by regular aspirin use. This
is the first instance where a kinase activity is being linked with aspirin, and
a possible molecular mechanism for CRC inhibition is postulated using both a
kinase as well as COX-2 enzyme.
Study and Data
Familial adenomatous polyposis (FAP) patients were recruited
by the Gastroenterology and Hepatology group at Mayo Clinic, Rochester,
Minnesota. Through a protocol approved by the Mayo Clinic Rochester MN IRB, all
subjects with FAP seen at Mayo Clinic Rochester MN between January 1990 through
May and from whom polyp tissue was available were identified through a search
of clinical diagnoses and pathology reports in the electronic medical record
(EMR). The EMR of the identified FAP cases with available tissue (n = 178) was
searched to identify those with a history of NSAID use at the time that the
available polyp tissue was collected. Cigarette smoking and inflammatory bowel
diseases are independent risk factors for CRC, while FAP patients suffering
hypertension or cardiovascular diseases might take additional anticoagulant
drugs other than aspirin. Thus, exclusion criteria also included cigarette
smoking, inflammatory bowel diseases, hypertension, a history of cardiovascular
disease, and pregnancy and subjects were not on any other pharmacological
treatments. FAP patients who reported taking two or more standard (325 mg)
aspirin tablets per week within the previous 12 months were classified as
regular aspirin users (n = 25) and those reporting consumption of less aspirin
were classified as aspirin nonusers (n = 25). Individuals in the healthy control group (n =
25) were normal subjects who underwent colonoscopy screening. The gender ratio
in each group was approximately 1:1.
The experimental assays included determination of
prostaglandin production, electrophoretic mobility-shift assay, rt-pcr
analysis, reporter assays, western blot analysis abd fianlly statistical
analysis.
Results
The following results are reported:
1. COX-2 and EGFR are Positively Correlated in FAP Patients: Data strongly suggested that EGFR and COX-2
were co-localized in the polyp tissues in FAP patients. Relationship between
COX-2 and EGFR expression in colonic adenomatous polyps from FAP patients indicated that in 95% of the FAP cases (19/20)
exhibiting COX-2 overexpression, EGFR was also highly expressed. Further
quantification analysis indicated that the observed expression levels of COX-2
and EGFR were not independent of each other but were highly and positively
correlated (p = 0.0015; R = 0.7585) in adenomatous polyps from FAP patients.
2. Functional Relevance of COX-2 in the Regulation of EGFR:
Data from immunohistochemistry analysis reveal that COX-2 and EGFR were highly
expressed in intestinal adenoma epithelial cells, but not in normal intestinal
epithelial cells. Furthermore, treatment with either aspirin or celecoxib, a
well-known COX-2 inhibitor, lowered EGFR expression in intestinal adenoma
epithelial cells. Murine Embryonic
Fibroblasts (MEFs) with COX-2 gene deficiency (COX-2-/-) have a much lower EGFR
level. Furthermore, typical COX-2 activators, including inflammatory cytokine
interleukin-1 beta (IL-1b) and
bacterial lipopolysaccharide (LPS), dramatically boosted both COX-2 and EGFR
protein expression in COX-2 wildtype cells compared to knockout cells.
3. COX-2 Modulates EGFR Transcription Through Activator
Protein-1 (AP-1): Researchers hypothesized that COX-2 might modulate EGFR at
the transcriptional level by activating the Egfr promoter. Aspirin was previously reported as an
inhibitor of the activity of activator protein-1 (AP-1), one of the potential
transcription factors for Egfr. Results from the study indicated that AP-1
activity as well as AP-1 DNA binding activity was markedly attenuated in the
absence of cox-2 gene expression.
Issues Still Not
Solved
Although aspirin exposure decreases the elevated EGFR levels
in FAP patients, in the majority of individuals who were aspirin users or
non-users, the levels of EGFR were still overlapping. This is consistent with
the outcome of CRC chemoprevention in which aspirin benefits most but not all
of the overall population. Although COX-2 and EGFR were co-localized in colon
adenocarcinomas from FAP patients, a functional association between them was
not observed.
The authors conclude as “In summary, this study revealed a
previously unknown functional association between COX-2 and EGFR during
colorectal carcinogenesis, and provided an explanation as to how aspirin intake
can lower the risk of CRC in FAP patients.”
Article citation:
Li, H.; et. al. Aspirin Prevents
Colorectal Cancer by Normalizing EGFR Expression, EBioMedicine 2015.
http://dx.doi.org/ 10.1016/j.ebiom.2015.03.019
