AbstractBackgroundThe tumor immune microenvironment (TIME) generated by cancer-infiltrating immune cells has a crucial role in promoting or suppressing breast cancer progression. However, whether the steroid receptor coactivator-3 (SRC-3) modulates TIME to progress breast cancer is unclear. Therefore, the present study evaluates whether SRC-3 generates a tumor-promoting TIME in breast tumors using a syngeneic immune-intact mouse model of breast cancer.MethodsWe employed E0771 and 4T1 breast cancer in immune-intact syngeneic female C57BL/6 and BALB/c mice, respectively. SI-2, a specific small-molecule inhibitor of SRC-3, was administered daily (2.5 mg/kg) to E0771 and 4T1 breast tumor-bearing immune-intact mice. In addition, SRC-3 knockdown (KD)-E0771 and SRC-3 KD-4T1 cells and their parental breast cancer cells were injected into their syngeneic immune-intact female mice versus immune-deficiency mice to validate that the host immune system is required for breast tumor suppression by SRC-3 KD in immune-intact mice. Furthermore, tumor-infiltrating immune cells (such as CD4+, CD8+, CD56+, and Foxp3+ cells) in E0771 and 4T1 breast cancers treated with SI-2 and in SRC-3 KD E0771 and 4T1 breast cancers were determined by immunohistochemistry. Additionally, cytokine levels in SI-2-treated and SRC-3 KD E0771 breast tumors and their control cancers were defined with a Mouse Cytokine Array.ResultsSRC-3 inhibition by SI-2 significantly suppressed the progression of breast cancer cells (E0771 and 4T1) into breast cancers in immune-intact syngeneic female mice. SRC-3 KD-E0771 and -4T1 breast cancer cells did not produce well-developed tumors in immune-intact syngeneic female mice compared to their parental cells, but SRC-3 KD breast cancers were well developed in immune-defective host mice. SRC-3 inhibition by SI-2 and SRC-3 KD effectively increased the numbers of cytotoxic immune cells, such as CD4+ and CD8+ T cells and CD56+ NK cells, and Interferon γ (Ifng) in breast cancers compared to vehicle. However, SI-2 treatment reduced the number of tumor-infiltrating CD4+/Foxp3+ regulatory T (Treg) cells compared to vehicle treatment. In addition, SRC-3 inhibition by SI-2 and SRC-3 KD increased C-X-C motif chemokine ligand 9 (Cxcl9) expression in breast cancer to recruit C-X-C motif chemokine receptor 3 (Cxcr3)-expressing cytotoxic immune cells into breast tumors.ConclusionsSRC-3 is a critical immunomodulator in breast cancer, generating a protumor immune microenvironment. SRC-3 inhibition by SI-2 or SRC-3 KD activates the Cxcl9/Cxcr3 axis in breast tumors and enhances the antitumor immune microenvironment to suppress breast cancer progression.