Lipid peroxidation-independent mechanisms of vitamin e-mediated protection against cyclosporine a-induced hepatocellular toxicity and apoptosis.

Cyclosporine A(CsA) produces oxidative stress and apoptosis in rat hepatocytes, but it is not known whether membrane lipid peroxidation plays a role in CsA toxicity. The objective of the present study was to determine whether iron-catalyzed hydroxyl or membrane alkoxyl radical formation is causally involved in the prooxidative cell injury and apoptosis. As previously reported, cultured rat hepatocytes exposed to CsA exhibited concentration-dependent signs of apoptotic cell injury, including chromatin condensation and fragmentation, increased caspase-3 activity, and release of cytosolic lactate dehydrogenase. Addition of the ferric iron chelator desferrioxamine or the novel potent lipophilic iron chelator CGP72670 did not protect against CsA-induced cell death, indicating that iron-catalyzed lipid peroxidation is unlikely to be involved in CsA-mediated apoptosis. In contrast, the classical chain-breaking lipid peroxidation inhibitor alpha-tocopherol was able to rescue cells from CsA-induced cytotoxicity and apoptosis. alpha-Tocopherol not only effectively inhibited the production of thiobarbituric acid-reactive substances and the formation of reactive oxygen species, but it also prevented proteins from being oxidized and forming mixed disulfides. Furthermore, alpha-tocopherol inhibited the cellular uptake of extracellular calcium (Ca-45(2+)) into cells, similar to the reducing agent dithiothreitol. By decreasing the extracellular calcium concentrations or by adding calcium channel blockers ( diltiazem, nifedipine) or a cell-permeable calcium chelator Bis-(o-aminophenoxy)ethane- N, N, N', N'-tertraacetic acid (BAPTA), both CsA-induced caspase-3 activation and apoptosis were inhibited, indicating a pivotal role of Ca2+ in mediating CsA-induced cell injury. These results suggest that alpha-tocopherol protects from CsA-mediated apoptosis and cytotoxictiy by preventing the oxidation of redox-sensitive Ca2+-ATPase. Thus, it is the attenuation of increased calcium influx and, hence, the inhibition of caspase-3 activation, rather than the downstream inhibition of lipid peroxidation, that is a key mechanism of the protection provided by alpha-tocopherol against CsA-induced cell injury.