ERα36-High Cancer-Associated Fibroblasts as an Unfavorable Factor in Triple-Negative Breast Cancer

Simple Summary The tumor microenvironment, as an important constituent of neoplastic tissue, has been a promising target for cancer therapy. Triple-negative breast cancer accounts for around 10-20% of invasive breast cancers. This study describes a new cancer-associated fibroblast subtype characterized by high ER alpha 36 levels that secretes HGF, which can impact triple-negative breast cancer. The data enlighten the importance of the stromal effect on the disease course and underlines the significance of further research on the tumor microenvironment and its role in the progression of cancer. Background: Cancer-associated fibroblasts (CAFs) are the most abundant cell type in the tumor microenvironment (TME). Estrogen receptor alpha 36 (ER alpha 36), the alternatively spliced variant of ER alpha, is described as an unfavorable factor when expressed in cancer cells. ER alpha can be expressed also in CAFs; however, the role of ER alpha 36 in CAFs is unknown. Methods: Four CAF cultures were isolated from chemotherapy-naive BC patients and characterized for ER alpha 36 expression and the NanoString gene expression panel using isolated RNA. Conditioned media from CAF cultures were used to assess the influence of CAFs on triple-negative breast cancer (TNBC) cells using a matrigel 3D culture assay. Results: We found that ER alpha 36(high) CAFs significantly induced the branching of TNBC cells in vitro (p < 0.001). They also produced a set of pro-tumorigenic cytokines compared to ER alpha 36(low) CAFs, among which hepatocyte growth factor (HGF) was the main inducer of TNBC cell invasive phenotype in vitro (p < 0.001). Tumor stroma rich in ER alpha 36(high) CAFs was correlated with high Ki67 expression (p = 0.041) and tumor-associated macrophages markers (CD68 and CD163, p = 0.041 for both). HGF was found to be an unfavorable prognostic factor in TCGA database analysis (p = 0.03 for DFS and p = 0.04 for OS). Conclusions: Breast cancer-associated fibroblasts represent distinct subtypes based on ER alpha 36 expression. We propose that ER alpha 36(high) CAFs could account for an unfavorable prognosis for TNBC patients.