National report - Voluntary exercise inhibits the formation of ultraviolet B (UVB)-induced skin cancer, according to first-ever studies conducted in mice.
The animals exposed to cancer-inducing UVB develop fewer and smaller skin tumors if they are allowed to exercise than if they are sedentary.
In one experiment, mice were exposed to UVB three times a week for 16 weeks, then stratified by body weight and randomized to cages with and without a running wheel. Mice with access to a wheel ran an average of about two miles a day, researchers estimated.
After stopping exposure to UVB, the mice without access to a wheel developed a 50 percent tumor incidence by week four, while those with access to a wheel did not reach the 50 percent tumor incidence until week seven. The median tumor-free time for the wheel group was seven weeks, double that of the group that had no access to exercise (three-and-a-half weeks). Tumor volume also was smaller in the exercise group at all times.
"Exercise inhibits tumor formation and also results in smaller tumors," Dr. Conney says. "We've seen that mice with a single exposure to UVB have enhanced apoptosis in the epidermis, and, with exercise that response is magnified. There is selectivity, since exercise did not enhance apoptosis in the non-UVB- treated mice. In a longer term study, exercising mice have more apoptosis in UVB-induced tumors than sedentary mice but there is no effect of exercise in the epidermis surrounding the tumor."
"Our studies may be the first to suggest an apoptotic mechanism for the effect of voluntary exercise in the development of skin cancer."
Dr. Conney says their ongoing work is trying to better understand the mechanism behind the enhanced apoptosis so that it can be put to better use for cancer prevention.
"In our studies, we utilized voluntary exercise; forced exercise, in some animal models, increases tumor formation, possibly because of the metabolic consequences of stress," Dr. Conney says.
The Rutgers' findings are consistent with what has been found in other animal models and with other forms of cancer. But Dr. Conney emphasizes that his study has only been conducted in mice; the field is "still on shaky ground" when it comes to translating the outcomes to humans. However, one case control study has suggested at least a weak correlation between exercise and a reduced incidence of melanoma.
Dr. Conney would like to see a proof of concept study in persons with actinic keratosis, who have a high risk for developing squamous cell carcinomas, or in individuals who have had a squamous cell carcinoma removed and are also a high-risk group. These patients could be randomized to exercise and nonexercise groups and matched as closely as possible for diet. He believes a beneficial effect of exercise might be observed over a period of three to five years.
Ellen S. Marmur M.D. calls the study "scientifically interesting and potentially very exciting. It opens the door to further lines of research."
But Dr. Marmur, chief, Division of Dermatologic and Cosmetic Surgery at Mt. Sinai Medical Center in New York City, shares the caution in extending the conclusions from mice to people.