A 9-kDa matricellular SPARC fragment released by cathepsin D exhibits pro-tumor activity in the triple-negative breast cancer microenvironment

Extracellular matrix (ECM) remodeling by proteases results in the release of protein fragments that promote tumor progression and metastasis. The protease cathepsin D (cath-D), a marker of poor prognosis in triple-negative breast cancer (TNBC), is aberrantly secreted in the tumor microenvironment. Using degradomic analyses by TAILS, we discovered that the matricellular protein SPARC is a substrate of extracellular cath-D. In vitro, cath-D induced limited proteolysis of SPARC C-terminal extracellular Ca2+ binding domain at acidic pH, leading to the production of SPARC fragments (34-, 27-, 16-, 9-, and 6-kDa). Similarly, cath-D secreted by TNBC cells cleaved fibroblast- and cancer cell-derived SPARC at the tumor pericellular acidic pH. SPARC cleavage also occurred in TNBC tumors. Among these fragments, only the 9-kDa SPARC fragment inhibited TNBC cell adhesion and spreading on fibronectin, and stimulated their migration, endothelial transmigration, and invasion. Our study establishes a novel crosstalk between proteases and matricellular proteins in the tumor microenvironment through limited proteolysis of SPARC, revealing a novel targetable 9-kDa bioactive SPARC fragment for new TNBC treatments.