From a patient's perspective, studying primary tumors is like looking for your lost wallet under a street lamp: searching in the easiest places often is futile. With cancer, what kills most patients is not their primary tumor - in fact, autopsies have revealed that a sizable fraction of people with no clinical signs of cancer whatsoever harbor primary tumors.
Cancer death, though, comes from metastases.
Researchers are obviously well aware of that, and despite the fact that metastasis is a much more complex process than primary tumor growth, translational researchers are increasingly willing to tackle its complexities. And for whatever reasons, their work has been paying off recently. In the past week alone, four studies have come out that report new insights into the basic mechanisms of metastasis in breast cancer, and new potential targets along with them.
Two of the papers, published in the April 12, 2007, Nature and the online April 9, 2007, early edition of the Proceedings of the National Academy of Sciences, come from the same group and focus on the role of four proteins: epiregulin, COX2 and matrix metalloproteinases 1 and 2. The authors summarize in their paper that the four genes "collectively facilitate the assembly of new tumor blood vessels, the release of tumor cells into the circulation and the breaching of lung capillaries by circulating tumor cells to seed pulmonary metastasis."
Using RNA interference, the researchers silenced various combinations of the four genes in human breast cancer cells that had metastasized to the lung, and then tested the cells in mice. They found that silencing the genes made the tumors less aggressive, in terms of both growth and the ability to send cells into the circulation on their metastatic journey. Silencing all four genes also reduced angiogenesis.
Finally, when they injected cells deficient in the four genes directly into the circulatory system of mice, "they just got stuck" in the lung capillaries, said senior author Joan Massagué, chairman of the department of cancer biology and genetics at Memorial Sloan-Kettering Cancer Center in New York. "We concluded that metastatic cells use these same genes to loosen up cells in capillaries, so that the cells can penetrate the lung tissue to grow there."
Massagué and his team also tested a combination of cetuximab (Erbitux, ImClone Systems Inc.,) which targets epiregulin, and celecoxib (Celebrex, Pfizer Inc.,) which affects Cox-2, to test whether the drugs could prevent metastasis.
"We found that the combination of these two inhibitory drugs was effective, even though the drugs individually were not very effective," said Massagué. She added that the combination effect "really nailed the case that if we can inactivate these genes in concert, it will affect metastasis."
A third paper, also published in the online April 9 PNAS, focuses on the protein Akt1, which is overexpressed in many cancers and interferes with apoptosis. Researchers from Jefferson Medical College of Thomas Jefferson University in Philadelphia genetically engineered Akt1 knockout mice and crossed them with mice overexpressing the HER2-neu oncogene.
They found that Akt1 appeared to be necessary for Her2-neu to do its dirty work. Mice lacking both copies of the Akt1 gene rarely had any tumors. Moreover, the tumors that did develop did not spread. Deleting Akt1 completely blocked breast cancer metastasis to the lungs, while mice that expressed Akt1 died from lung metastasis. On a molecular level, Akt1 appears to promote metastasis by causing cancer cells to secrete the factor CXCL16, which promotes breast cancer cell migration.
Finally, in a study now available online in advance of publication in the May print issue of the Journal of Clinical Investigation, scientists from Vanderbilt University report on the use of TGF-beta inhibitors to prevent breast cancer metastases.
TGF-beta is a cytokine that also is overexpressed in many cancers (and according to their website, the "main interest" of the Massagué lab). It also is a clinical target: Genzyme Inc., for example, is currently in Phase I trials with a monoclonal antibody to block all forms of the molecule for treating renal cell carcinoma or malignant melanoma.
In their JCI paper, the Vanderbilt researchers show TGF-beta from an ironic source also promotes petastasis: from anticancer therapy. Both radiation and chemotherapy treatment increased the level of TGF-beta in the blood of mice with mammary tumors, the number of cancer cells in their blood and the development of lung metastases.
Treatment with a TGF-beta inhibitor blocked the lung metastases, and mice whose tumors did not express the receptor for TGF-beta did not develop increased incidence of lung metastases after treatment with radiation. The researchers concluded in their paper that "these data implicate TGF-beta induced by anticancer therapy as a prometastatic signal in tumor cells and provide a rationale for the simultaneous use of these therapies in combination with TGF-beta inhibitors."