Disruption of cell adhesion and caspase-mediated proteolysis of β-and γ-catenins and APC protein in paclitaxel-induced apoptosis

Cell adhesion is important in the regulation of cell proliferation, migration, survival, and apoptosis. The major components of cell adhesion are the cadherin family of proteins, α-, β- and γ-catenins, and cytoskeletons. In addition, β-catenin, when associated with adenomatous polyposis coil (APC) protein, an oncosuppressor, is implicated in the regulation of β-catenin/APC-related signaling pathways. To examine the correlation between impairment of cell adhesion events and apoptosis, we used human non-small-cell lung cancer H460 and H520 cell lines as models to determine whether paclitaxel-induced apoptosis is associated with disruption of the components of cell adhesion and their functions. Paclitaxel treatment resulted in cells rounding up and losing contact with their neighboring cells, suggesting that the drug does indeed affect cell adhesion and related events. Western blot analysis revealed that paclitaxel caused a time- and concentration-dependent cleavage of β-catenin, γ-catenin, and APC protein, but not α-catenin or E-cadherin. These cleavages of β-catenin and γ-catenin were apoptosis-dependent, not mitosis-dependent. Paclitaxel treatment led to the proteolysis and activation of caspase-3 and -7, but not caspase-1. Furthermore, paclitaxel-induced apoptosis and cleavage of β-catenin and γ-catenin were inhibited by the pan-caspase inhibitor Z-VAD-FMK and partially inhibited by the caspase-3 inhibitor Z-DEVD-FMK but were not affected by the caspase-1 inhibitor AC-YVAD-CMK. Although the pan-caspase inhibitor blocked the cleavage of β-catenin as well as DNA fragmentation, it did not affect paclitaxel-induced M-phase arrest and only partially prevented cell-growth inhibition. Biochemical studies revealed that cleaved β-catenin was detected only in the Triton X-100 insoluble fraction, suggesting that it might localize in nuclear and/or membrane structures. Interestingly, the paclitaxel-induced β-catenin fragment lost its ability to bind to E-cadherin, α-catenin, or APC protein and to serve as a substrate for tyrosine kinase. All our data demonstrate that the caspase-mediated cleavage of β-catenin, γ-catenin, and APC protein might contribute to paclitaxel-induced apoptosis.