Elevation of anticancer drug-induced cytotoxicity by velpatasvir through downregulation of claudin-14 expression in human colorectal cancer cells.

Claudin-14 (CLDN14) is a tight junction protein that contributes to the development of chemoresistant phenotypes in human colorectal cancer (CRC) cells. CLDN14 may represent a potential therapeutic target for CRC. In this study, we identified velpatasvir (VEL), a clinically approved hepatitis C virus nonstructural protein 5 A (NS5A) inhibitor, as a potent suppressor of CLDN14 protein expression using in silico screening and Western blot analysis. CLDN14 expression was not decreased by other NS5A inhibitors, including ledipasvir, pibrentasvir, and daclatasvir. Quartz crystal microbalance analysis revealed that VEL directly binds to recombinant CLDN14 protein with a dissociation constant of 1.8 ± 0.2 μM. VEL treatment did not affect CLDN14 mRNA levels, suggesting that CLDN14 expression is regulated at a post-transcriptional level. Pharmacological inhibition of clathrin-mediated endocytosis and lysosomal degradation significantly reversed the VEL-induced reduction in CLDN14 protein levels, suggesting the involvement of endocytosis-lysosomal degradation pathway. Functionally, VEL increased paracellular permeability to mineral ions and enhanced the transepithelial flux of aqueous fluorescent tracers. In DLD-1 spheroids, VEL attenuated intracellular oxidative stress and reduced the expression of nuclear factor erythroid 2-related factor 2 (Nrf2), a central regulator of redox homeostasis. Consequently, VEL promoted the intracellular accumulation of doxorubicin and significantly potentiated its cytotoxic effects. Moreover, VEL enhanced the antitumor efficacy of other chemotherapeutic agents, including oxaliplatin and SN-38, the active metabolite of irinotecan. These findings suggest that VEL enhances anticancer drug sensitivity in CRC cells through lysosome-dependent downregulation of CLDN14 protein and suppression of Nrf2-dependent oxidative stress responses.