Truncated Hormone Inhibits Breast Tumor Blood Vessel Formation, Not Tumor Growth

More blood vessels, indicated by green staining, formed in tumors from wild type breast cancer cells (left) in the mammary glands of mice than from breast cancer cells expressing 16kDa-PRL, a truncated form of the hormone prolactin. Blood vessel formation is essential for supplying nutrients to a tumor.

More blood vessels, indicated by green staining, formed in tumors from wild type breast cancer cells (left) in the mammary glands of mice than from breast cancer cells expressing 16kDa-PRL, a truncated form of the hormone prolactin. Blood vessel formation is essential for supplying nutrients to a tumor.

The hormone prolactin (PRL) plays a critical role in normal breast development by stimulating the proliferation of mammary cells, the production of milk proteins, and the formation of new mammary blood vessels. Unfortunately, the same cell and vessel growth pathways controlled by PRL in normal cells also operate in breast cancer cells, and elevated plasma PRL is a risk factor for breast cancer, especially in post-menopausal women.

Most of the previous research on PRL action has focused on the full-length protein, but several cleaved products also exist. Expression of one, known as 16kDa-PRL, reduced blood vessel formation and tumor initiation of colon, prostate, and melanoma cell lines. Barbara Vonderhaar, Ph.D., scientist emeritus from the Mammary Biology and Tumorigenesis Laboratory, and her colleagues set out to investigate whether 16kDa-PRL could have a similar effect on breast cancer cell biology and, thus, potentially be a treatment for patients with breast cancer. Their results were recently published in the journal Hormones and Cancer.

The researchers started by engineering one colon and two breast cancer cell lines to continuously express 16kDa-PRL. The colon cancer line served as a control for 16kDa-PRL expression. If the system worked properly, the scientists should see the same results as those published previously. The two breast cancer lines were chosen for their differences in expression of the estrogen receptor (positive vs. negative), allowing the scientists to evaluate whether estrogen, another abundant breast hormone that can promote cancer progression, played a role in 16kDa-PRL function. Culturing the three cell lines revealed that cells producing 16kDa-PRL grew more slowly than cells that did not express truncated PRL. This suggested that the system was working properly and that 16kDa-PRL may have the same inhibitory effects on breast cancer cells as on cells from other sites, such as the colon.

The researchers next wanted to determine the level of blood vessel formation that occurred in cooperation with the various cell lines under study, since cancerous growths recruit vessels to supply needed nutrients. They mixed cells from each line with a unique 3D matrix system and then implanted the solidified matrices in mice to mimic the normal biological environment. After nine days, the matrices containing 16kDa-PRL-expressing cells, both breast and colon, exhibited significantly less blood vessel growth than wild type cells. This further confirmed the previous colon cancer cell studies and supported the notion that 16kDa-PRL might also regulate breast cancer cell behavior.

To clearly determine whether breast cancer cells responded to 16kDa-PRL in vivo, Vonderhaar and colleagues investigated the ability of truncated PRL to block tumor growth in mice. As expected, the 16kDa-PRL-producing colon cancer cells failed to generate tumors. The breast cancer cells, however, formed tumors equally well with or without 16kDa-PRL, in the presence or absence of the estrogen receptor, and regardless of whether the cells were placed under the skin or directly into the mammary gland. Interestingly, even though no difference was seen in the size of the tumors that developed, those formed by cells expressing 16kDa-PRL in the mammary gland had reduced blood vessel development compared to those from the 16kDa-PRL negative cells.

These results suggest that endothelial cells, which make up blood vessels, are universally inhibited by 16kDa-PRL, independent of the type of tissue in which they are growing. In contrast, breast cancer cells grow more slowly in the presence of 16kDa-PRL in an artificial environment but fail to be inhibited by the truncated protein when grown in the mouse. It is possible that in the hormonally rich environment of the mouse, the 16kDa-PRL growth inhibitory signals are overwhelmed by an excess of pro-growth signaling from these other hormones. Further studies into the signaling pathways controlled by 16kDa-PRL will be necessary to understand if these pathways can be exploited to treat or prevent breast tumors.

Summary Posted: 12/2010

Reference

Hormones and Cancer 2010 1(2):71-9