SPARC in cancer-associated fibroblasts is an independent poor prognostic factor in non-metastatic triple-negative breast cancer and exhibits pro-tumor activity

Purpose: Triple-negative breast cancer (TNBC) is the most aggressive breast cancer subtype and lacks specific targeted therapeutics. The current mechanistic evidence from cell-based studies suggests that the matricellular protein SPARC has a tumor-promoting role in TNBC; however, data on the clinical relevance of SPARC expression/secretion by tumor and stromal cells in TNBC are limited. Experimental Design: This study analyzed the prognostic value of tumor and stromal cell SPARC expression in a large series of 148 patients with non-metastatic TNBC and long follow-up (median: 5.4 years). Fibrosis, tumor-associated macrophage (TAM) infiltration, tumor-infiltrating lymphocyte (TIL) density, PD-L1 and PD-1 expression were assessed. Tumor and stromal cell SPARC expression was studied by immunofluorescence, western blotting, and meta-analysis of published single-cell mRNA sequencing data. The biological role of fibroblast-secreted SPARC was analyzed using cell adhesion, wound healing, Transwell-based motility and invasion, and tumor spheroid assays. Results: SPARC expression was detected in cancer cells (42.4%), cancer-associated fibroblasts (CAFs) (88.1%), TAMs (77.1%), endothelial cells (75.2%), and TILs (9.8%). Recurrence-free survival was significantly lower in patients with SPARC-expressing CAFs. SPARC expression in CAFs was an independent prognostic factor in multivariate analysis. Tumor and stromal cell SPARC expression was observed in TNBC cytosols, patient-derived xenografts, and cell lines. SPARC was expressed by different CAF subsets, including myofibroblasts and inflammatory CAFs. Fibroblast-secreted SPARC inhibited TNBC cell adhesion and stimulated their migration and invasion. Conclusions: SPARC expression in CAFs is an independent predictor of recurrence-free survival in TNBC. Patients with SPARC-expressing CAFs could be eligible for anti-SPARC-targeted therapy.