U. CHICAGO/YALE (US)—Social isolation and related stress could contribute to human breast cancer susceptibility. The finding is part of an ongoing effort to identify environmental contributions to cancer risk.
“There is a growing interest in relationships between the environment, emotion and disease. This study offers insight into how the social world gets under the skin,” says Gretchen Hermes, first author of the paper and a resident in the Neurosciences Research Training Program at Yale University.
The researchers at the University of Chicago and Yale found that isolation and stress result in a 3.3-fold increase in the risk of developing cancer among rats with naturally occurring mammary tumors.
The research establishes, for the first time, that isolation and stress could be a factor in human breast cancer risk, says Martha McClintock, a psychologist at Chicago and an author of a paper in the current issue of the Proceedings of the National Academy of Sciences.
Researchers at Chicago have been studying social isolation in the context of breast cancer development after identifying that many women living in high-crime neighborhoods must deal with a variety of stressors, including social isolation. In particular, African American women have been noted to have an earlier onset of breast cancer, although total incidence is similar to women from other ancestries.
“We need to use these findings to identify potential targets for intervention to reduce cancer and its psychological and social risk factors,” says McClintock, the David Lee Shillinglaw Distinguished Service Professor in Psychology and Comparative Human Development at Chicago. “In order to do that, we need to look at the problem from a variety of perspectives, including examining the sources of stress in neighborhoods as well as the biological aspects of cancer development.”
The study shows that isolation led to a higher production of a stress hormone, corticosterone, among rats that were kept alone and subjected to the disturbances of colony life as well as stressful situations, such as the smell of a predator or being briefly constrained. Additionally, the isolated rats took longer to recover from a stressful situation than rats that lived together in small groups.
The study also suggests a causal relationship between social interaction and disease by showing that living alone first causes rats to have higher stress hormones, beginning in young adulthood, become fearful, anxious and vigilant and then prone to malignancy in late-middle age. The study further showed that the stress hormone receptor entered the nucleus of mammary tumor cells in isolated rats, where gene regulation occurs, something that happened less often in the cells of the non-isolated rats.
The researchers note that rats living in isolation experienced a 135 percent increase in the number of tumors and a more than 8,000 percent increase in the tumors’ size. The impact of isolation was much larger than the impact another environmental source of tumor formation―the unlimited availability of high-energy food.
In natural situations, estrogen and progesterone produced from ovaries play a role in the majority of naturally occurring mammary and breast cancers tumors. In the rat study, tumors naturally developed in late middle age, while ovaries were no longer fully functioning, further suggesting the role of isolation and stress hormones in cancer development.
The National Institute of Environmental Health Sciences/National Cancer Institute, the U.S. Department of Defense, and the State of Connecticut Department of Mental Health and Addictive Services supported the research.