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

Abstract 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. Statement of translational relevance Here, we identified a subgroup of patients with triple-negative breast cancer (TNBC) with worse prognosis and eligible for therapies that target extracellular matrix proteins in the tumor stroma. Specifically, we found that expression of the matricellular protein SPARC in cancer-associated fibroblasts (CAFs) is an independent prognostic marker of poor outcome in TNBC. Furthermore, we showed that in TNBC, SPARC is expressed by different CAF subpopulations, including myofibroblasts and inflammatory fibroblasts that are involved/associated with many tumor-related processes. We then found that SPARC secreted by fibroblasts has a pro-tumor-promoting role by inhibiting TNBC cell adhesion and stimulating their motility and invasiveness. Overall, our results support the need to consider SPARC expressed/secreted by CAFs as a novel therapeutic target in TNBC in the context of treatments to modulate the tumor stroma.