Shed proteoglycans in tumor stroma.

Cancer cell behavior is not only governed by tumor cell-autonomous properties but also by the surrounding tumor stroma. Cancer-associated fibroblasts, blood vessels, immune cells and the extracellular matrix of the tumor microenvironment have a profound influence on tumor progression. Proteoglycans control various normal and pathological processes, modulating cell proliferation and motility, cell-matrix interactions, immune cell recruitment and angiogenesis. They are major mediators of cancer cell behavior though a dynamic interplay with extracellular matrix components. During cancer progression, their altered expression can promote the activation of several signaling cascades regulating crucial functional properties of cancer cells. Notably, the function of cell surface proteoglycans can be altered by ectodomain shedding, which converts membrane-bound coreceptors into soluble paracrine effector molecules. In this review, we highlight the importance of proteoglycans and their soluble counterparts in cancer progression and the consequences of their interactions with the adjacent stroma. The dynamic interplay among shed proteoglycans and proteolytic enzymes has a significant impact both on tumor cells and their surrounding stroma, with important implications for the diagnosis of this disease and for novel therapeutic approaches. Graphical Abstract Syndecan shedding. The mechanism of shedding involves the proteolytic cleavage of their ectodomain near the plasma membrane by metzincin enzymes, such as metalloproteinases. N-acetylglucosamine-alpha-L-iduronic acid/beta-D-glucuronic acid (HS) chains can be additionally cleaved by heparanase. Syndecan core protein can be further processed by intramembrane enzymatic cleavage. Syndecans are in a dynamic interplay with the extracellular matrix and several receptor-tyrosine-kinases (RTKs) and various growth factors, for which they act as co-receptors, thus mediating numerous signaling pathways.