MONDAY, Dec. 8 (HealthDay News) -- A protein that may contribute to the spread of breast cancer has been identified by U.S. researchers.

The protein (called Menainv) is present in invasive cells within a breast tumor. The protein may prove useful as a biomarker for metastatic breast cancer, the scientists said.

These invasive cells move into surrounding tissue and eventually to blood vessels, which provide the cancer cells with a means of moving to other areas of the body. Menainv is not present in noninvasive cancer cells that stay put in a breast tumor.

This is the first study to identify a protein that contributes to the invasive and metastatic ability of tumor cells, rather than "bystander" proteins that show up when cancer cells are invading other tissues, according to the researchers from the Albert Einstein College of Medicine and the Massachusetts Institute of Technology.

"We have micro-needles filled with growth factors and tissue that we insert into a tumor on an anesthetized mouse. If a cell is invasive, within four hours, it will crawl into the needles. We found that mouse breast tumor cells that we engineered to contain Menainv were invasive, whereas cells that did not have Menainv were not," study primary co-author Evanthia T. Roussos, an M.D.-Ph.D. student, said in an Albert Einstein news release.

The researchers also found that tumor cells with Menainv are less likely to be responsive to newer breast cancer treatments that inhibit receptors for epidermal growth factor (EGF), which has been shown to increase a breast cancer cell's invasive potential.

Drugs that inhibit EGF may not be effective against cancer cells that express Menainv, the researchers said. That's because tumor cells with Menainv are so sensitive to EGF that even a small amount of EGF signal not blocked by the drugs may be enough to stimulate EGF receptors and promote tumor cell migration and metastasis.

The study was published in the Dec. 8 issue of Developmental Cell.

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