Caspase-mediated cleavage of β-catenin precedes drug-induced apoptosis in resistant cancer cells

Abstract

A delicate balance between cell death and survival pathways maintains normal physiology, which is altered in many cancers, shifting the balance toward increased survival. Several studies have established a close connection between the Wnt/β-catenin pathway and tumorigenesis, aberrant activation of which might contribute toward increased cancer cell growth and survival. Extensive research is underway to identify therapeutic agents that can induce apoptosis specifically in cancer cells with minimal collateral damage to normal cells. Although tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) can induce apoptosis specifically in tumor cells, many cancer cells develop resistance, which can be overcome by combinatorial treatment with other agents: for example, peroxisome proliferator-activated receptorγ (PPARγ) ligands. To identify the molecular target mediating combinatorial drug-induced apoptosis, we focused on β-catenin, a protein implicated in oncogenesis. Our results show that co-treatment of TRAIL-resistant cancer cells with TRAIL and the PPARγ ligand troglitazone leads to a reduction of β-catenin expression, coinciding with maximal apoptosis. Modulation of β-catenin levels via ectopic overexpression or small interference RNA-mediated gene silencing modulates drug-induced apoptosis, indicating involvement of β-catenin in regulating this pathway. More in-depth studies indicated a post-translational mechanism, independent of glycogen synthase kinase-3β activity regulating β-catenin expression following combinatorial drug treatment. Furthermore, TRAIL- and troglitazone-induced apoptosis was preceded by a cleavage of β-catenin, which was complete in a fully apoptotic population, and was mediated by caspases-3 and -8. These results demonstrate β-catenin as a promising new target of drug-induced apoptosis, which can be targeted to sensitize apoptosis-resistant cancer cells.