Interleukin-6 and pro inflammatory status in the breast tumor microenvironment

Greater than 50,000 new cases of breast cancer cases were diagnosed in Italy during 2013, with nearly 15,000 women succumbing to the disease. These epidemiological statistics highlight the overwhelming clinical dilemma of breast cancer and emphasize the need for novel therapeutic targets and prevention strategies. Countless studies in the fields of mammary gland development and breast cancer have led to an appreciation of a breast tumor microenvironment that actively contributes to the heterogeneous nature of breast cancer.

The current review will focus on the impact of IL-6 and in the breast tumor microenvironment. Excessive IL-6 has been demonstrated in primary breast tumors and breast cancer patient sera and is associated with poor clinical outcomes in breast cancer. These clinical associations are corroborated by emerging preclinical data revealing that IL-6 is a potent growth factor and promotes an epithelial-mesenchyme (EMT) phenotype in breast cancer cells to indicate that IL-6 in the breast tumor microenvironment is clinically relevant.

High serum levels of interleukin-6 correlate with poor outcome in breast cancer patients. However, few data are yet available on the relationship between IL-6 and stem/progenitor cells, which may fuel the genesis of breast cancer in vivo. Mammospheres (MS) from node invasive breast carcinoma tissues express IL-6 mRNA at higher levels than MS from matched non-neoplastic mammary glands. IL-6 mRNA is detectable only in basal-like breast carcinoma tissues; our results reveal that IL-6 triggers a Notch-3-dependent upregulation of the Notch ligand Jagged-1, whose interaction with Notch-3 promotes the growth of MS and Michigan Cancer Foundation-7 (MCF-7)-derived spheroids. IL-6 induces a Notch-3-dependent upregulation of the carbonic anhydrase IX gene and promotes a hypoxia-resistant/invasive phenotype in MCF-7 cells and MS.

In conclusion, our data support the hypothesis that IL-6 induces malignant features in Notch-3-expressing, stem/progenitor cells from human ductal breast carcinoma and normal mammary gland.