LOW FERROPORTIN LEVELS INCREASE BREAST CANCER RECCURENCE RISK

August 4, 2010 — An iron-regulating protein known as ferroportin might play an important role in the clinical behavior of breast cancer, according to new biological experiments and analyses of clinical datasets.

The experiments and analyses are described in a paper published August 4 online in Science Translational Medicine.

Investigators from Wake Forest University Baptist Medical Center in Winston-Salem, North Carolina, found that low levels of ferroportin are associated with the most aggressive breast cancers.

In examining previously published datasets, the study authors also found that there is a metastasis-free survival advantage in breast cancer patients whose tumors have increased ferroportin gene expression.

Ferroportin, the only known protein to eliminate iron from cells, is "strikingly decreased in breast cancer tissue," say the authors, led by senior author Frank M. Torti, MD, MPH, director of the Comprehensive Cancer Center at Wake Forest.

They also note that "many cancers exhibit increased iron requirements" — probably because of the need for iron as a cofactor to sustain cancer cell growth and proliferation.

In short, the authors suggest that not enough ferroportin, with its ability to help eliminate iron from cells, contributes to more aggressive breast cancers and worse clinical outcomes.

The potential importance of iron in the proliferation of cancer is not news, say the authors. "Agents that deplete iron are currently under investigation as anticancer therapies," they point out.

However, what is news is that ferroportin seems to be, in the words of the authors, "a critical determinant of outcome in breast cancer."

Ferroportin might also be important in other tumor types, including hepatocellular carcinoma, prostate cancer, and leukemia, report the authors.

Biological Evidence

The Wake Forest investigators conducted a series of biological tests as a starting point in their evaluation of ferroportin.

First, they explored whether or not ferroportin is present in normal human breast epithelial cells, and if its levels are altered in breast cancer.

To do so, Dr. Torti and his team compared ferroportin protein levels in mammary epithelial cells with "variable malignant potential" from a woman with no breast cancer, from a woman with metastatic disease, and from a woman with inflammatory breast cancer.

The authors found that ferroportin levels were "reduced in all aggressive breast cancer cell lines when compared to their counterparts with little or no malignant potential."

Also, the researchers found that, in terms of the genetics of these cell lines, ferroportin mRNA levels were lower in the more malignant cells than in the nonmalignant cells. In other words, a stronger genetic expression of the protein was associated with healthier breast epithelial cells.

The researchers also noted, in other experiments, that a reduction of ferroportin in breast cancer cells was associated with an increase in the "labile iron pool" or metabolically available iron (which breast cancer cells can use for growth).

To determine whether ferroportin is decreased in human breast cancer tissue (as opposed to breast cancer cells that were examined earlier), the Wake Forest researchers performed immunohistochemical analysis.

A total of 154 samples of breast tissue from breast cancer patients and 6 samples from normal breasts were stained with antiferroportin antibody.

The authors report that 70% of normal samples received the highest staining intensity score, but that only 9% of the cancer samples received this score (P = .0015, Fisher's exact test); the result was "consistent with a reduction in ferroportin protein levels in human breast cancer tissue, as well as in breast cancer cell lines" say the authors.

The authors also used data from a cohort of 251 breast cancer patients to relate molecular subtypes — such as luminal A, luminal B, and basal — and their disease outcomes with the genetic expression of ferroportin in the patients. "Ferroportin was significantly differentially expressed among the subtypes, with lowest expression levels in the poor-prognosis subtypes," report the authors.

Does Ferroportin Expression Predict Clinical Outcome?

The preliminary evidence gathered by the researchers led to a pivotal question.

They explain in their paper that "the remarkably consistent decrease in ferroportin protein levels in malignant breast tissue and the association of decreased ferroportin gene expression with molecular subtypes of breast cancer with poor prognosis led us to ask whether ferroportin levels were related to breast cancer outcome."

So they performed an analysis using 4 large breast cancer datasets in the public domain that had both patient outcomes and microarray profiles.

"In all 4 studies, low ferroportin gene expression was associated with a statistically significant and clinically substantial reduction in metastasis-free survival," they report.

The "most dramatic effect" was seen in a Norwegian study in which the 8-year disease-free survival rates were separated by more than 30% — 77% for those with high ferroportin compared with 43% for those with low ferroportin (Proc Natl Acad Sci USA.2003;100:8418-8423).

What role ferroportin might have in elucidating the prognosis or even the therapy of breast cancer remains to be seen, conclude the authors. They point out that such matters are "best addressed in appropriately designed prospective trials." The authors did not indicate that any such trials are currently underway.

The study was funded by the National Institute of Diabetes and Digestive and Kidney Diseases.

Sci Transl Med. Published online August 4, 2010.