Bortezomib induces in HepG2 cells IκBα degradation mediated by caspase-8

The present paper demonstrates that the proteasome inhibitor bortezomib, which behaves as an apoptotic agent in hepatoma HepG2 cells, caused in these cells a decrease in I kappa B alpha level and a consequent increase in NF-kappa B activity. The effect already appeared at 4 h of treatment and preceded the onset of apoptosis which was observed at 24 h. Our results demonstrate that bortezomib-induced I kappa B alpha degradation occurred in conjunction with the activation of caspase-8; moreover, the decrease in I kappa B alpha level was prevented in a dose-dependent manner by the addition of z-IETD, a specific inhibitor of caspase-8. Bortezomib caused the same effects in non-tumor Chang liver cells, which were not susceptible to the apoptotic effect of the drug. Our results also show that other proteases, such as caspase-3 and calpains, exerted only a limited effect on I kappa B alpha degradation. These findings suggest that caspase-8 can be involved in the control of I kappa B alpha level. In addition, the activation of caspase-8 can exert, at least in the first phase of treatment with bortezomib, a protective effect in both HepG2 and Chang liver cells, favouring the activation of the survival factor NF-kappa B