IL-33 facilitates endocrine resistance of breast cancer by inducing cancer stem cell properties.

Breast cancers with estrogen receptor (ER) expressions account for the majority of all clinical cases. Due to hormone therapy with tamoxifen, prognoses of patients with ER-positive breast cancer are significantly improved. However, endocrine resistance to tamoxifen is common and inevitable, leading to compromised efficacy of hormone therapy. Herein, we identify a crucial role of IL-33 in inducing endocrine resistance of breast cancer. IL-33 overexpression in breast cancer cells results in resistance to tamoxifen-induced tumor growth inhibition, while IL-33 knockdown corrects this problem. Mechanistically, IL-33 induces breast cancer stem cell properties evidenced by mammosphere formation and xenograft tumorigenesis, as well as expression of cancer stem cell genes including ALDH1A3, OCT4, NANOG and SOX2. In breast cancer patients, higher serum IL-33 levels portend advanced clinical stages, poorly differentiated cancer cells and tumor recurrence. IL-33 expression levels in patients' freshly isolated breast cancer cells predicts tamoxifen resistance and cancer stem cell features in individual patient. Collectively, IL-33 induces endocrine resistance of breast cancer by promoting cancer stem cell properties. These findings provide novel mechanisms connecting IL-33 with cancer pathogenesis and pinpoint IL-33 as a promising target for optimizing hormone therapy in clinical practice.