In Vitro Assessment of Mitochondrial Toxicity to Predict Drug-Induced Liver Injury

Comparison of HepaRG and HepG2 cell lines to model mitochondrial respiratory adaptations in non‑alcoholic fatty liver disease

Although some clinical studies have reported increased mitochondrial respiration in patients with fatty liver and early non‑alcoholic steatohepatitis (NASH), there is a lack of in vitro models of non‑alcoholic fatty liver disease (NAFLD) with similar findings. Despite being the most commonly used immortalized cell line for in vitro models of NAFLD, HepG2 cells exposed to free fatty acids (FFAs) exhibit a decreased mitochondrial respiration. On the other hand, the use of HepaRG cells to study mitochondrial respiratory changes following exposure to FFAs has not yet been fully explored. Therefore, the present study aimed to assess cellular energy metabolism, particularly mitochondrial respiration, and lipotoxicity in FFA‑treated HepaRG and HepG2 cells. HepaRG and HepG2 cells were exposed to FFAs, followed by comparative analyses that examained cellular metabolism, mitochondrial respiratory enzyme activities, mitochondrial morphology, lipotoxicity, the mRNA expression of selected genes and triacylglycerol (TAG) accumulation. FFAs stimulated mitochondrial respiration and glycolysis in HepaRG cells, but not in HepG2 cells. Stimulated complex I, II‑driven respiration and β‑oxidation were linked to increased complex I and II activities in FFA‑treated HepaRG cells, but not in FFA‑treated HepG2 cells. Exposure to FFAs disrupted mitochondrial morphology in both HepaRG and HepG2 cells. Lipotoxicity was induced to a greater extent in FFA‑treated HepaRG cells than in FFA‑treated HepG2 cells. TAG accumulation was less prominent in HepaRG cells than in HepG2 cells. On the whole, the present study demonstrates that stimulated mitochondrial respiration is associated with lipotoxicity in FFA‑treated HepaRG cells, but not in FFA‑treated HepG2 cells. These findings suggest that HepaRG cells are more suitable for assessing mitochondrial respiratory adaptations in the developed in vitro model of early‑stage NASH.

In silico approaches in organ toxicity hazard assessment: current status and future needs in predicting liver toxicity

Hepatotoxicity is one of the most frequently observed adverse effects resulting from exposure to a xenobiotic. For example, in pharmaceutical research and development it is one of the major reasons for drug withdrawals, clinical failures, and discontinuation of drug candidates. The development of faster and cheaper methods to assess hepatotoxicity that are both more sustainable and more informative is critically needed. The biological mechanisms and processes underpinning hepatotoxicity are summarized and experimental approaches to support the prediction of hepatotoxicity are described, including toxicokinetic considerations. The paper describes the increasingly important role of in silico approaches and highlights challenges to the adoption of these methods including the lack of a commonly agreed upon protocol for performing such an assessment and the need for in silico solutions that take dose into consideration. A proposed framework for the integration of in silico and experimental information is provided along with a case study describing how computational methods have been used to successfully respond to a regulatory question concerning non-genotoxic impurities in chemically synthesized pharmaceuticals.

Drug-Induced Alterations of Mitochondrial DNA Homeostasis in Steatotic and Nonsteatotic HepaRG Cells

Although mitochondriotoxicity plays a major role in drug-induced hepatotoxicity, alteration of mitochondrial DNA (mtDNA) homeostasis has been described only with a few drugs. Because it requires long drug exposure, this mechanism of toxicity cannot be detected with investigations performed in isolated liver mitochondria or cultured cells exposed to drugs for several hours or a few days. Thus, a first aim of this study was to determine whether a 2-week treatment with nine hepatotoxic drugs could affect mtDNA homeostasis in HepaRG cells. Previous investigations with these drugs showed rapid toxicity on oxidative phosphorylation but did not address the possibility of delayed toxicity secondary to mtDNA homeostasis impairment. The maximal concentration used for each drug induced about 10% cytotoxicity. Two other drugs, zalcitabine and linezolid, were used as positive controls for their respective effects on mtDNA replication and translation. Another goal was to determine whether drug-induced mitochondriotoxicity could be modulated by lipid overload mimicking nonalcoholic fatty liver. Among the nine drugs, imipramine and ritonavir induced mitochondrial effects suggesting alteration of mtDNA translation. Ritonavir toxicity was stronger in nonsteatotic cells. None of the nine drugs decreased mtDNA levels. However, increased mtDNA was observed with five drugs, especially in nonsteatotic cells. The mtDNA levels could not be correlated with the expression of key factors involved in mitochondrial biogenesis, such as peroxisome proliferator-activated receptor-γ coactivator 1α (PGC1α), PGC1β, and AMP-activated protein kinase α-subunit. Hence, drug-induced impairment of mtDNA translation might not be rare, and increased mtDNA levels could be a frequent adaptive response to slight energy shortage. Nevertheless, this adaptation could be impaired by lipid overload.