Under normal circumstances, the hormone prolactin is responsible for stimulating breast tissue growth and differentiation at several times during a woman's life -- during puberty, pregnancy, and lactation. Unfortunately, it can also stimulate the growth of breast cancers that may arise, according to Charles V. Clevenger, MD, PhD, an assistant professor of pathology and laboratory medicine. Additionally, recent results from Clevenger's laboratory suggest that prolactin's role in the disease may be much greater than previously thought.
Scientists examining breast cancer tissues in the past estimated that between 20 and 60 percent of these cancers expressed the receptor for prolactin, meaning that they would be responsive to the stimulatory effects of the hormone. Using improved techniques, Clevenger's team has found that a much higher proportion of breast cancers -- more than 95 percent -- express the prolactin receptor. Also, prolactin is known to be secreted by the pituitary gland, and for many years it was assumed that this gland was the only source for the hormone. Clevenger's laboratory, however, has shown that breast tissue itself produces prolactin in significant quantities. "Not only do most mammary tissues make their own prolactin, but so do most breast cancers," Clevenger notes.
Coupled with the newly demonstrated prevalence of the prolactin receptor, the findings suggest that prolactin is likely serving to stimulate the growth of these cancers in most cases. While the news may not appear to be positive, it does present an opportunity for the development of important new therapies.
"If we can find a way to block prolactin from binding to its receptor, we may be able to induce regression in breast tumors," Clevenger says. "They might stop growing, and they might even die." And, unlike some other hormone-based therapies for breast cancer, such as the new drug herceptin, which is useful in only a minority of patients, treatments focusing on prolactin and its receptor would benefit the majority of women with breast cancers.