Their study, published in PNAS, found that isolation led to 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 work also indicates 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 further found that rats living in isolation experienced a 135 percent increase in the number of tumors and a more than 8,000 percent increase in their 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 team has been studying social isolation in the context of breast cancer development after having found that 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 other and its psychological and social risk factors," said Martha McClintock, a professor of psychology and comparative human development at the university. "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."