Aspirin's preventive effects on colon cancer may result from its influence on the development of blood vessels needed for tumor growth, a Vanderbilt Cancer Center researcher reports in the Friday, May 29, issue of the journal Cell.
The work was done jointly by Dr. Raymond N. DuBois, Mina Cobb Wallace Professor of Gastroenterology and Cancer Prevention and director of the division of Gastroenterology, and colleagues at the Osaka University School of Medicine in Japan. The researchers demonstrated two ways in which aspirin can inhibit angiogenesis, an important component in the development and spread of cancer.
"What we found is that NSAIDs can work in two ways -- by inhibiting the enzyme cyclooxygenase-2 (COX-2) in the tumor cells, which then prevents production of factors that prompt angiogenesis, and by inhibiting cyclooxygenase-1 (COX-1) in endothelial cells, which form the blood vessels that grow into the tumor," DuBois said.
The research builds on earlier work by DuBois and his colleagues that suggested aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) help prevent colon cancer by inhibiting COX-2, which they found to be elevated in colon cancer cells.
However, this new research suggests that COX-1 -- and the ability of aspirin and NSAIDs to target it as well -- may also be involved. These studies were carried out in an in vitro model of angiogenesis involving a chamber with colon cancer cells on the bottom and endothelial cells on the top, separated by a porous collagen membrane.
In a series of experiments, the researchers first compared colon cancer cells that overexpress COX-2 to control cells. They found that the COX-2-positive colon carcinoma cells produced angiogenic factors that, in turn, stimulated migration of the endothelial cells across the collagen gel. These COX-2-positive colon cancer cells also prompted the endothelial cells to begin to form tube-like structures, the first steps of blood vessel development. Control cells had little effect.
The researchers then studied whether these effects would be inhibited by antibodies to specific angiogenic factors, a selective COX-2 inhibitor called NS-396 or aspirin. They found antibodies which neutralized some of the angiogenic factors inhibited angiogenesis in vitro.
Then, to examine the specificity of the effects of the COX-2 inhibitor, the researchers evaluated colon cancer cells that express neither COX-1 nor -2. These cells also produced angiogenic factors that prompted endothelial cell migration and tube formation.
Again, only combination of at least two antibodies to specific angiogenic factors inhibited tube formation, while antibodies to individual factors had no effect.
Treatment with aspirin and the COX-2 inhibitor did not affect the levels of these angiogenic factors, and the COX-2 inhibitor had no inhibiting effect on tube formation. Suprisingly, however, aspirin did inhibit the angiogenic effects.
"It was really an unexpected finding," DuBois said.
"There's long been a question about how aspirin reduces the risk for developing colon cancer, and in this model system, we found two ways in which that might be happening. If we can better understand the mechanism involved, we may be able to find other ways to effectively block angiogenesis. However, we need to keep in mind that these results will have to be validated in an in vivo tumor model to fully understand the significance of these findings."
Chris Williams, a student in DuBois' lab who is pursuing his M.D./Ph.D., is following up the research with studies in animal tumor models.
Journal
Cell