Mechanisms of Pathogenesis in Drug Hepatotoxicity Putting the Stress on Mitochondria

Chelatable iron mobilization and ROS formation. In panel A, oxidative metabolism causes formation of O₂^•− (i.e., superoxide anion) and H₂O₂. Superoxide dismutase (SOD) converts some O₂^•− to H₂O₂. In the presence of chelatable iron, O₂ reduces Fe³⁺ to Fe²⁺, and Fe²⁺ reacts with H₂O₂ to form hydroxyl radical (OH^•). Ferrireductases may also reduce Fe³⁺ to Fe²⁺. Hydroxyl radicals react with lipids to form alkyl radicals (L·) to initiate an oxygen-dependent chain reaction generating lipid peroxides (LOOH) and peroxyl radicals (LOO·). Iron also catalyzes a chain reaction generating alkoxyl radicals (LO·) and more LOO·. Nitric oxide synthase (NOS) catalyzes formation of NO^•, which reacts with O₂^•− to form ONOO⁻. ONOO⁻ decomposes to nitrogen dioxide (NO₂·) and OH ^•. In panel B, the cytosol of mouse hepatocytes was loaded with calcein, and the cells were incubated in culture medium containing calce-in free acid with no further addition (Control), bafilomycin (Baf), bafilomycin plus desferal (Baf+Dsf) or bafilomycin plus starch-desferal (sBaf+Dsf). Note decreased cytosolic calcein fluorescence after bafilomycin, which was blocked by desferal and starch-desferal (see text for details). In panel C, mitochondria and lysosomes of rat hepatocytes were loaded with calcein, and the cells were exposed to bafilomycin (Baf) or bafilomycin in the presence of Ru360 (Baf+Ru), an inhibitor of the mitochondrial calcium uniporter. Note quenching of mitochondrial calcein fluorescence that was suppressed by Ru360. Arrows identify representative lysosomes whose fluorescence increased rather than decreased after bafilomycin (see text for details). In panel D, ferrireductase reduces Fe³⁺ to Fe²⁺ in the lysosomal/endosomal compartment. Bafilomycin, as well as ATP deficiency, inhibits the vacuolar proton-pumping ATPase to cause luminal alkalinization and Fe²⁺ release by apparent Fe²⁺-H⁺ exchange. Fe²⁺ released to the cytosol is taken up by mitochondria via the calcium uniporter as a first hit promoting injury. Mitochondrial oxidative stress by tBH ^- provides a second hit of O₂^•− and H₂O₂ production, and OH^• is formed by the Fenton reaction. OH^• leads to onset of the CsA-sensitive MPT that causes mitochondrial swelling and uncoupling of oxidative phosphorylation (see text for details).