Extracellular cystatin SN and cathepsin B prevent cellular senescence by inhibiting abnormal glycogen accumulation

Abstract

Cystatin SN (CST1), a known inhibitor of cathepsin B (Cat B), has important roles in tumor development. Paradoxically, Cat B is a member of the cysteine cathepsin family that acts in cellular processes, such as tumor development and invasion. However, the relationship between CST1 and Cat B, and their roles in tumor development are poorly understood. In this study, we observed that the knockdown of CST1 induced the activity of senescence-associated β-galactosidase, a marker of cellular senescence, and expression of senescence-associated secretory phenotype genes, including interleukin-6 and chemokine (C-C motif) ligand 20, in MDA-MB-231 and SW480 cancer cells. Furthermore, CST1 knockdown decreased extracellular Cat B activity, and direct Cat B inhibition, using specific inhibitors or shCat B, induced cellular senescence. Reconstitution of CST1 restored Cat B activity and inhibited cellular senescence in CST1 knockdown cells. CST1 knockdown or Cat B inhibition increased glycogen synthase (GS) kinase 3β phosphorylation at serine 9, resulting in the activation of GS and the induction of glycogen accumulation associated with cellular senescence. Importantly, CST1 knockdown suppressed cancer cell proliferation, soft agar colony growth and tumor growth in a xenograft model. These results indicate that CST1-mediated extracellular Cat B activity enhances tumor development by preventing cellular senescence. Our findings suggest that antagonists of CST1 or inhibitors of Cat B are potential anticancer agents. ⓒ The Author(s) 2017.