PHILADELPHIA — Overexpression or hyperactivation of ErbB cell-surface receptors drives the growth of many breast cancers. Drugs, like Herceptin, that block the receptors’ signals halt tumor progression in some patients. However, not all patients’ tumors respond, with some becoming resistant over time. Different drugs that interfere with other steps in the signaling pathway may improve the response of patients, yet little is known about these molecules.
Now, Marcelo G. Kazanietz, PhD, professor of Pharmacology at the University of Pennsylvania School of Medicine and colleagues, report that a protein called P-Rex1 is crucial for signal transmission from ErbB receptors. What’s more, they found that P-Rex1 is overexpressed in nearly 60 percent of breast cancer samples tested and patients whose tumors express P-Rex1 were more likely to develop metastasis, compared with those whose tumors did not express P-Rex1.
“We identified a downstream target of the ErbB receptors which seems to be crucial for cancer cell proliferation, migration, and metastasis,” Kazanietz says. “Understanding how this pathway works should allow us to find new drugs or therapeutic approaches in the future.”
The team’s research is featured on the cover of the December 22 issue of Molecular Cell.
A Well-Known Family
The ErbB family of receptors is well known in the cancer world. The family includes the epidermal growth factor receptor (EGFR, also known as ErbB1); ErbB2 (also known as HER2/neu), which is the target of Herceptin; as well as ErbB and ErbB4.
Previous work from Kazanietz and others suggested the receptors might rely on small proteins in the Rac pathway to help transmit their signal. To find out if that was the case, Kazanietz and colleagues examined human breast cancer cell lines and found that one Rac pathway protein, P-Rex1, is overexpressed in numerous cell lines compared with normal mammary cells. The team also found that P-Rex1 is present in some breast tumors, particularly those that express the Her2/neu receptor or estrogen receptor and belong to the luminal subtype.
“We found that about two-thirds of the patient samples had very high levels of P-Rex1 expression in tumor cells in their lymph nodes,” Kazanietz says. “There seems to be a correlation between P-Rex1 expression in the tumor cells and the capacity of these cells to metastasize. And since P-Rex1 is likely to be essential for cell migration and migration is essential for metastasis, we believe blocking this pathway could reduce the risk of metastasis.”
P-Rex1 may be important for several other cancer-promoting pathways. For example, estrogen-receptor signaling also appears to rely on P-Rex1, which means that targeted inhibitors of P-Rex1 might improve responses to anti-estrogen therapies such as tamoxifen.
The authors also found that P-Rex1 is also used by another receptor, CXCR4, which has recently shown up in many cancer studies. Despite being used in numerous pathways during cancerous growth, P-Rex1 is not expressed in many normal tissues. “That gives us a very good target. It is really cancer specific,” Kazanietz says. “What’s more, as P-Rex1 is expressed in some subtypes of breast tumors, it may be an excellent prototype for future personalized medicine.”
Preclinical data from other groups supports the importance of the Rac pathway in cancer, according to Kazanietz. And small molecule inhibitors that block proteins in the Rac pathway appear to have strong anti-cancer effects in model systems, though their use in patients still needs to be tested.
This work has been supported by grants from the National Cancer Institute and the Susan Komen Foundation for the Cure.
Maria Soledad Sosa and Cynthia Lopez-Haber from the Penn Department of Pharmacology were co-first authors on the study. Additional co-authors include Hongbin Wang and Mark A. Lemmon from Penn; Chengfeng Yang from Michigan State University, East Lansing; John M. Busillo, Jiansong Luo, and Jeffrey L. Benovic from Thomas Jefferson University, Philadelphia; Andres Klein-Szanto, from Fox-Chase Cancer Center, Philadelphia; Hiroshi Yagi and J. Silvio Gutkind from the National Institute of Dental and Craniofacial Research, Bethesda, MD; and Ramon E. Parsons from Columbia University Medical Center, New York.
Penn Medicine is one of the world’s leading academic medical centers, dedicated to the related missions of medical education, biomedical research, and excellence in patient care. Penn Medicine consists of the University of Pennsylvania School of Medicine (founded in 1765 as the nation’s first medical school) and the University of Pennsylvania Health System, which together form a $3.6 billion enterprise.
Penn’s School of Medicine is currently ranked #2 in U.S. News & World Report’s survey of research-oriented medical schools, and is consistently among the nation’s top recipients of funding from the National Institutes of Health, with $367.2 million awarded in the 2008 fiscal year.
Penn Medicine’s patient care facilities include:
- The Hospital of the University of Pennsylvania — the nation’s first teaching hospital, recognized as one of the nation’s top 10 hospitals by U.S. News & World Report.
- Penn Presbyterian Medical Center — named one of the top 100 hospitals for cardiovascular care by Thomson Reuters for six years.
- Pennsylvania Hospital — the nation’s first hospital, founded in 1751, nationally recognized for excellence in orthopaedics, obstetrics & gynecology, and psychiatry & behavioral health.
Additional patient care facilities and services include Penn Medicine at Rittenhouse, a Philadelphia campus offering inpatient rehabilitation and outpatient care in many specialties; as well as a primary care provider network; a faculty practice plan; home care and hospice services; and several multispecialty outpatient facilities across the Philadelphia region.
Penn Medicine is committed to improving lives and health through a variety of community-based programs and activities. In fiscal year 2009, Penn Medicine provided $733.5 million to benefit our community.