Tumor-associated macrophages play a critical role in breast tumor development and progression; however, it is still unclear what effector molecular mechanisms they employ to impact tumorigenesis. Ferritin is the primary intracellular iron storage protein and is also abundant in circulation. In breast cancer patients, ferritin is detected at higher levels in both serum and tumor lysates, and its increase correlates with poor clinical outcome. In this study, we have performed a comprehensive examination of the distribution of ferritin in normal and malignant breast tissue at different stages in tumor development. We demonstrated decreased ferritin expression in cancer cells but increased infiltration of ferritin-rich CD68-positive macrophages with increased histological grade. Interestingly, ferritin stained within the stroma surrounding tumor tissue suggesting local release within the breast. In cell culture, primary macrophages, but not breast cancer cells, were capable of ferritin secretion and this secretion was further increased in response to the inflammatory cytokines TNFα and IL-1β. We next examined the possible functional significance of extracellular ferritin in a breast cancer cell culture model. Ferritin stimulated the proliferation of the human breast cancer cell lines MCF7 and T47D in an iron-independent manner suggesting a direct and novel function for this protein in tumorigenesis. To provide further support to this concept, we measured the inflammatory biomarker serum ferritin in advanced breast cancer patients receiving combinational therapeutics (n=66). Serum ferritin was elevated in approximately 50% of the patients and this elevation was predictive of progression-free survival (P=0.004, median 8.30 vs. 23.90 months) and overall survival (P<0.0001, median 12.73 vs. 69.57 months). When patients were stratified based on their levels for serum ferritin and the hepatic inflammatory biomarker C-reactive protein (CRP), elevation in serum ferritin alone was predictive of shorter survival while elevation in CRP alone was not. Moreover, serum ferritin and CRP showed independent prognostic value in a multivariate model. Overall, the independent prognostic value of serum ferritin indicates that its increase is not caused exclusively by inflammation-induced secretion by the liver and that it may have direct functional significance on breast tumors. In conclusion, the identification of tumor-associated macrophages as the major site of ferritin expression and release within breast tumors as well as the tumorigenic effects of extracellular ferritin introduces novel therapeutic targets and molecular diagnostic tools for breast cancer. The targeting of ferritin's functionality or secretion within the tumor microenvironment may have clinical efficacy regardless of tumor site or molecular subtype.

Citation Format: Ahmed A. Alkhateeb, Bing Han, Kim Leitzel, Suhail Ali, Allan Lipton, James Connor. Extracellular ferritin is a tumorigenic factor and is secreted by tumor-associated macrophages: A functional link between inflammation and tumorigenesis. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1538. doi:10.1158/1538-7445.AM2013-1538