Risk Factors in Colon Carcinogenesis:

Recent Advances in Preclinical Modeling of the Disease
The stepwise development of colorectal cancer results in part from the accumulation of specific mutations in cells of the colonic crypts. Perhaps the most important of these mutations are those resulting in loss-of-function of the tumor suppressor gene APC. These mutations are observed in as many as 85% of colorectal cancers, and appear to be responsible for initiating events in development of premalignant colorectal polyps. The frequency by which these and other mutations occur in the colon will be affected by environmental influences, particularly diet. In fact, there is abundant epidemiological evidence that dietary factors may be partly responsible for the differences in colon cancer rates between different populations. Although some of the evidence is controversial, it has been suggested that an increased risk of colorectal cancer may be related to diets that are high in saturated fat, low in fiber, or deficient in calcium, vitamin D, or folic acid.

Evaluating Diet in Animal Models
One way to investigate the role of particular dietary factors in colorectal cancer is to make use of animal models of the disease and test directly the effect of specific diets on tumor incidence. A research group led by Dr. Martin Lipkin at the Strang Cancer Prevention Center has been a leader in this line of investigation for many years. Although standard rodent models such as laboratory mice have been valuable for revealing the harmful effects of potent carcinogens, it has been much more difficult to demonstrate that dietary factors have a significant impact on cancer in these animals, possibly because the increased risk from diet may be relatively small. And while it is a significant factor over the life span of a human, it is rarely manifest during the 2 to 3 year life of a mouse.

In his investigations, Dr. Lipkin has taken two approaches to overcome this limitation. First, his research group has assayed the occurrence of colonic crypt dysplasias and focal hyperplasias as endpoints, rather than carcinomas. They found that these lesions, which are thought to represent the earliest stages of the progression from normal tissue to cancer, are induced in mice fed a “Western-style” diet that is high in fat and low in calcium and vitamin D.1 Even after 2 years, however, no adenomas or adenocarcinomas developed. More recently, Dr. Lipkin and colleagues have modified the “Western-style” mouse diet to mimic the low folic acid content of diets consumed by certain human populations at increased risk for cancer. After 18 months on this diet, there was both adenoma and carcinoma development in the colon, and 42% of the mice developed tumors.2 This result is an important proof of principle, representing the first report of the dietary induction of colon cancer in normal rodents without the presence of a chemical carcinogen or targeted gene mutation that accelerates carcinogenesis. This and similar preclinical model systems can now be used to evaluate the contributions of specific dietary components, and also the potentially beneficial effects of chemopreventive agents in conjunction with a high-risk diet.

Other Research Methods
Another means of examining the effects of chemical carcinogens, dietary components, and chemopreventive agents has been through use of cell or tissue culture model systems, in which the consequences for cell growth rate, transformation parameters, and programmed cell death can be evaluated directly. Dr. Nitin Telang at the Strang Cancer Prevention Center has previously developed several such systems based on primary cultures of either breast or colon epithelium. These have been commonly derived from wild-type tissues and represent good models of normal epithelial cells in vivo. Since cancer progression depends on a stepwise accumulation of mutations, it is equally important to determine the effect of risk factors and preventive agents on cells already bearing such mutations. Such cells may respond to various agents in a different manner relative to wild-type cells.
To address this issue, Dr. Telang and his colleague Dr. Meena Katdare, in collaboration with Dr. Levy Kopelovich at the National Cancer Institute, have developed colon epithelial cell cultures from mice bearing mutations in the APC tumor suppressor gene, typically the first gene to undergo mutation in early premalignant polyps. These cells display altered growth properties in culture, validating the idea that they represent a qualitatively distinct target population.3 Telang and Katdare and have also found that chemopreventive agents such as sulindac and 9-cis-retinoic acid inhibited some of the abnormal growth properties of the mutant cells.4 Importantly, culture systems of this sort may provide a powerful means of screening for compounds that are inhibitory to colonic cells carrying specific mutations, but may have little effect on normal epithelium.

Editor’s note:
Meena Katdare, PhD, is Senior Research Associate at The Murray Rayburn Biochemical Endocrinology Research Laboratory, Strang Cancer Prevention Center.

Martin Lipkin, MD, is Professor of Medicine at Weill Medical College of Cornell University and Director of Clinical Research, Head, Clinical Chemoprevention Laboratory, Strang Cancer Prevention Center.

Nitin Telang, PhD, is Associate Research Professor, Department of Surgery at the Weill Medical College of Cornell University and Head, Carcinogenesis and Prevention Laboratory, Strang Cancer Prevention Center.

1. Risio M, Lipkin M, Newmark H, et al. Apoptosis, cell replication, and Western-style diet-induced tumorigenesis in mouse colon. Cancer Res. 1996;56:4910-4916.

2. Newmark HL, Yang K, Lipkin M, Kopelovich L, et al. Western-style diet induces benign and malignant neoplasms in the colon of normal C57Bl/6 mice. Carcinogenesis. 2001;22:1871-1875.

3. Katdare M, Kopelovich L, Telang N. Chemopreventive agents inhibit aberrant proliferation of the aneuploid phenotype in a colon epithelial cell line established from an Apc1638[+/-] mouse. Ann NY Acad Sci. 2001;952:169-174.

4. Katdare M, Kopelovich L, Telang N. Preventive efficacy of 9-cis-retinoic acid on mutant colon epithelial cell lines established from Apc+/- 1638N and Mlh+/- /1638N+/- gene knockout mice. Proc Amer Assoc Cancer Res. 2002;43:124.