Increased RUNX3 expression mediates tumor‐promoting ability of human breast cancer‐associated fibroblasts

Abstract Background Cancer‐associated fibroblasts (CAFs) are a major stromal component of human breast cancers and often promote tumor proliferation, progression and malignancy. We previously established an experimental CAF (exp‐CAF) cell line equipped with a potent tumor‐promoting ability. It was generated through prolonged incubation of immortalized human mammary fibroblasts with human breast cancer cells in a tumor xenograft mouse model. Results Herein, we found that the exp‐CAFs highly express Runt‐related transcription factor 3 (RUNX3), while counterpart fibroblasts do not. In breast cancer patients, the proportion of RUNX3‐positive stromal fibroblast‐like cells tends to be higher in cancerous regions than in non‐cancerous regions. These findings suggest an association of RUNX3 with CAF characteristics in human breast cancers. To investigate the functional role of RUNX3 in CAFs, the exp‐CAFs with or without shRNA‐directed knockdown of RUNX3 were implanted with breast cancer cells subcutaneously in immunodeficient mice. Comparison of the resulting xenograft tumors revealed that tumor growth was significantly attenuated when RUNX3 expression was suppressed in the fibroblasts. Consistently, Ki‐67 and CD31 immunohistochemical staining of the tumor sections indicated reduction of cancer cell proliferation and microvessel formation in the tumors formed with the RUNX3‐suppressed exp‐CAFs. Conclusion These results suggest that increased RUNX3 expression could contribute to the tumor‐promoting ability of CAFs through mediating cancer cell growth and neoangiogenesis in human breast tumors.