More recently, we have directly demonstrated that hydrazine, but not the parent INH, inhibited solubilized mitochondrial

complex II isolated from Saccharomyces cerevisiae.[18] This resulted in increased superoxide GSK-3 activity formation at complex II (due to a one-electron reduction of molecular oxygen). In addition, complex II inhibition could also create a potentially dangerous situation when the functional integrity of complex I is compromised. Under normal conditions, complex I activity might easily compensate for hydrazine-mediated inhibition of complex II, still feeding electrons into the electron transport chain and reducing ubiquinone (Fig. 3). However, if complex I is inhibited chemically or by an underlying genetic change, then this would likely lead to a collapse of energy homeostasis. To test this hypothesis, we co-exposed cultured mouse hepatocytes to INH (which alone is not toxic over a wide concentration range) and the complex I inhibitors, rotenone (3 μM), or piericidin A (30 nM), both at nontoxic concentrations.[18] This led to a massive energy crisis (rapid loss of cellular ATP) and hepatocyte demise. Pretreatment with the acyl amidase inhibitor BNPP protected against the cell injury in a concentration-dependent manner,

indicating that it was hydrazine (or acetylhydrazine), rather than the parent INH, that was responsible for the toxicity (Fig. 3). In the clinical setting, certain drugs that are co-administered with INH, and that are potential inhibitors of complex I, might similarly potentiate the hepatocellular toxicity of INH via these mechanisms. For example, efavirenz

(EFV) a widely Ridaforolimus clinical trial used non-nucleoside reverse transcriptase inhibitor, is often administered together with an antitubercular therapy in patients as part of a combined antiretroviral therapy against HIV infection. EFV has been associated with liver toxicity in patients;[64] in experimental models, EFV induced endoplasmic reticulum 上海皓元 stress, mitophagy, oxidant stress, and mitochondrial dysfunction in hepatocytes.[64-69] Earlier studies had shown that EFV decreases oxygen consumption in isolated rat liver mitochondria if the mitochondria were energized with glutamate/malate, but not with succinate, suggesting that EFV selectively compromised complex I function.[65] We recently demonstrated that EFV concentration-dependently inhibited mitochondrial complex I activity in isolated mouse liver mitochondria (Lee and Boelsterli, unpublished, 2014). Importantly, exposure of cultured mouse hepatocytes to a combination of EFV and INH (both at nontoxic concentrations if used alone) caused a rapid collapse of the cellular ATP levels and greatly potentiated the cellular toxicity of INH (Lee and Boelsterli, unpublished, 2014), further highlighting the potential for underlying mitochondrial changes to precipitate INH-induced cell injury. The host (patient) greatly contributes to the risk for developing INH-associated liver injury.