Katie Chan

Drug-induced mitochondrial toxicity.

Mitochondria play a critical role in generating most of the cell’s energy as ATP. They are also involved in other metabolic processes such as urea generation, haem synthesis and fatty acid β-oxidation. Disruption of mitochondrial function by drugs can result in cell death by necrosis or can signal cell death by apoptosis (e.g., following cytochrome c release). Drugs that injure mitochondria usually do so by inhibiting respiratory complexes of the electron chain; inhibiting or uncoupling oxidative phosphorylation; inducing mitochondrial oxidative stress; or inhibiting DNA replication, transcription or translation. It is important to test for mitochondrial toxicity early in drug development as impairment of mitochondrial function can induce various pathological conditions that are life threatening or can increase the progression of existing mitochondrial diseases.

Structure-activity relationships for hepatocyte toxicity and electrophilic reactivity of α,β-unsaturated esters, acrylates and methacrylates

Covalent binding of reactive electrophiles to cellular targets is a molecular interaction that has the potential to initiate severe adverse biological effects. Therefore, electrophile reactivity towards biological nucleophiles could serve as an important correlate for toxic effects such as hepatocyte death. To determine if reactivity correlates with rat hepatotoxicity, α,β‐unsaturated esters, consisting of acrylates and methacrylates, that are inherently electrophilic and exhibit widely varying degrees of reactivity were investigated. Reactivity was measured using simple assays with glutathione and butylamine as surrogates for soft thiol and hard amino biological nucleophile targets. A linear relationship was observed between hepatotoxicity and thiol reactivity only, while no amine reactivity was observed. Structure–activity relationships were also investigated, with results showing toxicity was well modeled by electronic parameters ELUMO and partial charge of the carbon atoms in the reactive center. No relationship was observed between toxicity and logP. These results suggest that differences in hepatocyte toxicity of acrylates and methacrylates can be related to their electrophilic reactivity which corresponds to their ability to deplete GSH and protein thiols. Copyright

Bioactivation of fluorotelomer alcohols in isolated rat hepatocytes

Fluorotelomer alcohols (FTOHs; C(x)F(2x+1)C(2)H(4)OH) are intermediates in the production of specialty surfactants and stain-repellent polymers. The magnitude and pathways of human exposure to FTOHs are not understood, but FTOHs are present in ambient air and house dust, and FTOH-derivatives are used in food-contact applications. Previously, electrophilic FTOH biotransformation products were detected in rat hepatocytes, and liver lesions were found in FTOH exposed rodents. To begin elucidating the mechanism(s) of action, freshly isolated rat hepatocytes were incubated with FTOHs, or FTOH biotransformation products, and toxicity was followed in the presence or absence of carbonyl scavengers and metabolic enzyme modulators. The LC(50) depended on perfluorinated chain length, with the shortest (4:2 FTOH; x=4) and longest (8:2 FTOH; x=8) FTOHs tested being more toxic than the medium chain length FTOH (6:2 FTOH; x=6); a structure-toxicity relationship that is consistent with that for 2-alkenals. For hepatocytes treated with 8:2 FTOH, cytotoxicity corresponded to depletion of glutathione (GSH), increased protein carbonylation, and lipid peroxidation. Aminobenzotriazole, a P450 inhibitor, diminished cytotoxicity for all FTOHs tested, and decreased protein carbonylation and lipid peroxidation for 8:2 FTOH, indicating that a biotransformation product was responsible for FTOH cytotoxicity. Preincubation of hepatocytes with hydralazine or aminoguanidine decreased the cytotoxicity of 8:2 FTOH, suggesting that reactive aldehyde intermediates contributed to the cytotoxicity. A GSH-reactive alpha/beta-unsaturated acid metabolite was also more toxic than the corresponding FTOH, and may have contributed to the observed effects. Overall, these results suggested that FTOH toxicity was related to electrophilic aldehydes or acids through GSH depletion and protein carbonylation. Further research into the nature of protein modification is warranted for these current-use fluorochemicals.

Application of structure–activity relationships to investigate the molecular mechanisms of hepatocyte toxicity and electrophilic reactivity of α,β-unsaturated aldehydes

Covalent binding of reactive electrophiles to cellular targets is a molecular interaction that has the potential to initiate severe adverse biological effects. Therefore, a measure for electrophilic reactivity with biological nucleophiles could serve as an important correlate to toxic effects such as hepatocyte death. To determine if electrophile reactivity correlates with rat hepatocyte cytotoxicity, the inherently electrophilic α,β‐unsaturated aldehydes were chosen for investigation. Reactivity was measured with simple assays that used glutathione, a soft nucleophile, and butylamine, a harder nucleophile, as models for protein thiol and amine nucleophilic sites, respectively. Despite their higher reactivity with thiols, a linear relationship was only observed between hepatocyte cytotoxicity and amine reactivity. Structure–activity relationships were also investigated for hepatocyte toxicity, and results showed toxicity was well modelled by log P and electronic parameters ELUMO and partial charge of the carbonyl carbon (  carb′ ). Hydrophobicity and electronic descriptors were only significant in separate distinct models, suggesting that there were simultaneously occurring mechanisms that affected toxicity. Log P was linked to the ease of oxidation by a microsomal aldehyde dehydrogenase enzyme, while the electronic descriptors and amine reactivity were linked to direct alkylation. Even with the presence of electrophile characteristics, α,β‐unsaturated aldehyde hepatocyte toxicity could not be predicted exclusively by electrophilic reactivity as oxidative metabolism was also a factor for toxicity. Copyright

Cytotoxic effects of polychlorinated biphenyl hydroquinone metabolites in rat hepatocytes

Polychlorinated biphenyls (PCBs) are persistent organic pollutants that exhibit various toxic effects in animals and exposed human populations. The molecular mechanisms of PCB toxicity have been attributed to the toxicological properties of its metabolites, such as hydroquinones, formed by cytochrome‐P‐450 oxidation. The effects of PCB hydroquinone metabolites towards freshly isolated rat hepatocytes were investigated. Hydroquinones can be oxidized to semiquinones and/or quinone metabolites. These metabolites can conjugate glutathione or can oxidize glutathione as a result of redox cycling. This depletes hepatocyte glutathione, which can inhibit cellular defence mechanisms, causing cell death and an increased susceptibility to oxidative stress. However in the following, glutathione‐depleted hepatocytes became more resistant to the hydroquinone metabolites of PCBs. This suggested that their glutathione conjugates were toxic and that there was a third type of quinone toxicity mechanism which involved a hydrogen peroxide‐accelerated autoxidation of the hydroquinones to form toxic electrophilic quinone and semiquinone–glutathione conjugates. Copyright