CAA-derived IL-6 promoted the PD-L1 expression of breast cancer via STAT3/miR-497a-5p signaling.

[BACKGROUND] Adipocytes are engaged in the development and progression of breast cancer (BC). Cancer-associated adipocytes (CAAs) are specific adipocytes located at the invasive front of BC that modulate tumor behaviors. This study aimed to investigate the effect of CAA-derived IL-6 in regulating BC progression.

[METHODS] Human BC specimens and adipocytes co-cultured with BC cells were used to explore the characteristics of CAAs. Adipocytes and 4T1 cells co-implanted in mouse model and tail vein metastasis model were used to explore the effect of CAAs on malignant progression and immunosuppressive tumor microenvironment (TME) of BC in vivo. The functional assays, flow cytometry, ELISA, miRNAs sequencing and dual-luciferase reporter assay were implemented to investigate the role of CAA-derived IL-6 in regulating programmed cell death protein ligand 1 (PD-L1) expression.

[RESULTS] CAAs were present at the invasive front of BC with a de-differentiated fibroblast phenotype. CAAs enhanced the malignant behaviors of 4T1 BC cells in vitro, and promoted 4T1 tumor growth and lung metastasis with decreased CD8T cells and upregulated Tregs. The IHC results of both human BC specimens and xenografts showed that CAAs could upregulate PD-L1 expression in BC. Besides, CAAs could secrete abundant IL-6 and thus enhanced PD-L1 expression in 4T1 cells and tumors. More importantly, CAA-derived IL-6 could activate STAT3, while STAT3 blockade in CAA-cultured 4T1 cells upregulated miRNA-497a-5p expression and downregulated PD-L1 expression. Dual-luciferase reporter assay indicated that PD-L1 was a direct target of miRNA-497a-5p.

[CONCLUSIONS] Our study demonstrated that CAAs promoted the malignant behaviors of BC and enhanced immunosuppression in TME. Specifically, CAA-derived IL-6 promoted the PD-L1 expression of BC via STAT3/miR-497a-5p signaling, thereby contributing to BC progression.