In vitro and in vivo studies on potentiation of curcumin-induced lysosomal-dependent apoptosis upon silencing of cathepsin C in colorectal cancer cells

Cathepsins are lysosomal acid hydrolases that make crucial contributions to tumor progression through a variety of signaling mechanisms, including autophagy, cell survival, chemotherapeutic resistance, and metastasis. Herein, we report that cathepsin C (CTSC) silencing upregulates the anticancer potential of curcumin in colorectal cancer cells (CRCs) both in vitro and in athymic mice xenografts. Curcumin treatment enhances CTSC level in CRCs; however, CTSC silencing with subsequent curcumin treatment (sequential treatment) induces ER stress and autophagic dysregulation accompanied by lysosomal permeabilization and ROS generation. This lysosomal permeabilization triggered the cytosolic CTSB mediated BID-dependent mitochondrial membrane permeabilization and thereby caspase-dependent apoptosis. This phenotype can be rescued by CTSB inhibition and NAC, which further supported the involvement of ROS and CTSB in apoptosis following sequential treatment. Indeed, the sequential CTSC silencing and curcumin treatment also significantly curtailed tumor volume as well as ameliorated cytosolic cyt c and tBID protein levels in tumor tissues compared to those in control and individual treatments of CTSC targeting and on curcumin treatment in nude mice xenografts. The results reveal that CTSC can controls the curcumin-induced cytotoxic insult through autophagy maintenance both in vitro and in athymic mice xenografts, thereby providing an insight into the role of CTSC in chemoprevention of CRCs.