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The Role of Hormones in Breast Cancer and Metastasis

By Robin Arnette
April 2008

Distinguished Lecturer Horwitz fielded questions from the audience.
Distinguished Lecturer Horwitz fielded questions from the audience. (Photo courtesy of Steve McCaw)
Horwitz and lecture host Cidlowski.
Horwitz and lecture host Cidlowski. (Photo courtesy of Steve McCaw)

Breast cancer researcher Kathryn B. Horwitz, Ph.D., presented the most recent talk in the 2007-2008 NIEHS Distinguished Lecture Series on March 11 in Rodbell Auditorium to a capacity audience. The talk, titled "Hormonal Regulation of Breast Cancer: Stem Cells and Metastasis,". Was hosted by John Cidlowski, Ph.D., supervisory biologist in the NIEHS Laboratory of Signal Transduction.

Horwitz( Exit NIEHS Website, Distinguished Professor of Medicine & Pathology at the University of Colorado Health Science Center in Denver, Colo., was the first person to show that breast cancer cells contained progesterone receptors. She prefaced her talk by explaining why it was important to understand the roles of estrogen (ER) and progesterone receptors (PR), and hormones, in metastasis. "The problem with breast cancer is that women don't die of the primary disease of the breast; they die of their metastasis."

Metastasis is defined as the spread of a cancer from the initial site to the rest of the body. Horwitz said that the metastases that spread from the primary tumor to lymph nodes and more distant sites contain ER or PR, or both. Her lab has developed methods that help shed light on the metastatic process.

Many metastatic tumors travel to the lymph nodes, so Horwitz's research group created a model system for studying lymph node metastasis using the ER+ breast cancer cell lines T47D and MCF-7. Human breast cancer cells that were ER+/PR+ were flagged with green or red fluorescence using retroviral vectors. ER- cancer cells were used as controls. Because team members wanted to understand the effects of estrogens and progestins in their experiments, they implanted the fluorescent tumor cells into the mammary glands of immune deficient mice that had undergone an ovariectomy or ovary removal. Some of these mice were given hormone (estradiol) treatments and some were not. 

"After six weeks, you could see red or green tumor cells migrating up the lymphatic system in mice treated with estradiol, so these are clearly estrogen-dependent tumors," Horwitz explained. "In the no estradiol control mice, the tumors didn't grow and no lymph node metastases occurred. The tumor cells weren't dead, just dormant."

Horwitz's studies indicated a difference between lymph node tumors and primary tumors with regard to estrogen resistance. To study this in detail, tumors from the mammary gland and the lymph nodes of the same mouse were grown in the presence of estradiol. Some mice were kept on estradiol, while in other mice, the estradiol was withdrawn. The difference between the two would define the estrogen-regulated genes. The tumors were harvested and the DNA was extracted and placed on microarray chips.

Microarray analysis revealed that many more estrogen-regulated genes were expressed in the primary tumor than in the lymph node tumor. "Therefore, the lymph node appears to be a protected area in which the ER is working differently than it does at the primary tumor site," Horwitz said. "In other words, the microenvironment or the part of the body where the tumor resides does have an effect on estrogen regulation."

Horwitz concluded her talk with data on breast cancer stem cells, progenitor cells that undergo several rounds of asymmetric division before forming a tumor. She knew these cells expressed CD44+/CD24-/low, but her work determined that breast cancer stem cells  also contained cytokeratin 5 (CK5), were ER-/PR- even in ER+/PR+ cancers, and could differentiate into the ER+/PR+ cancer subtype. This means that if a patient has an ER+ breast cancer that contains ER- stem cells, standard therapies such as anti-estrogens or aromatase inhibitors will shrink the tumor, but they won't affect the stem cells.

"We'll have to figure out a way to kill these stem cells without affecting normal stem cells," Horwitz remarked, "but it gives us hope that we could possibly cure a disease like breast cancer by targeting the appropriate cells with the appropriate drugs."

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