Steatohepatitis-inducing drugs trigger cytokeratin cross-links in hepatocytes. Possible contribution to Mallory-Denk body formation

Animal models of NAFLD from the pathologist's point of view

Fatty liver disease is a multifactorial world-wide health problem resulting from a complex interplay between liver, adipose tissue and intestine and initiated by alcohol abuse, overeating, various types of intoxication, adverse drug reactions and genetic or acquired metabolic defects. Depending on etiology fatty liver disease is commonly categorized as alcoholic or non-alcoholic. Both types may progress from simple steatosis to the necro-inflammatory lesion of alcoholic (ASH) and non-alcoholic steatohepatitis (NASH), respectively, and finally to cirrhosis and hepatocellular carcinoma. Animal models are helpful to clarify aspects of pathogenesis and progression. Generally, they are classified as nutritional (dietary), toxin-induced and genetic, respectively, or represent a combination of these factors. Numerous reviews are dealing with NASH animal models designed to imitate as closely as possible the metabolic situation associated with human disease. This review focuses on currently used mouse models of NASH with particular emphasis on liver morphology. Despite metabolic similarities most models (except those with chemically or genetically induced porphyria or keratin 18-deficiency) fail to develop the morphologic key features of NASH, namely hepatocyte ballooning and formation of histologically and immunohistochemically well-defined Mallory-Denk-Bodies (MDBs). Although MDBs are not universally detectable in ballooned hepatocytes in NASH their experimental reproduction and analysis may, however, significantly contribute to our understanding of important pathogenic aspects of NASH despite the obvious differences in etiology.

Customised in vitro model to detect human metabolism-dependent idiosyncratic drug-induced liver injury

Drug-induced liver injury (DILI) has a considerable impact on human health and is a major challenge in drug safety assessments. DILI is a frequent cause of liver injury and a leading reason for post-approval drug regulatory actions. Considerable variations in the expression levels of both cytochrome P450 (CYP) and conjugating enzymes have been described in humans, which could be responsible for increased susceptibility to DILI in some individuals. We herein explored the feasibility of the combined use of HepG2 cells co-transduced with multiple adenoviruses that encode drug-metabolising enzymes, and a high-content screening assay to evaluate metabolism-dependent drug toxicity and to identify metabolic phenotypes with increased susceptibility to DILI. To this end, HepG2 cells with different expression levels of specific drug-metabolism enzymes (CYP1A2, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, GSTM1 and UGT2B7) were exposed to nine drugs with reported hepatotoxicity. A panel of pre-lethal mechanistic parameters (mitochondrial superoxide production, mitochondrial membrane potential, ROS production, intracellular calcium concentration, apoptotic nuclei) was used. Significant differences were observed according to the level of expression and/or the combination of several drug-metabolism enzymes in the cells created ad hoc according to the enzymes implicated in drug toxicity. Additionally, the main mechanisms implicated in the toxicity of the compounds were also determined showing also differences between the different types of cells employed. This screening tool allowed to mimic the variability in drug metabolism in the population and showed a highly efficient system for predicting human DILI, identifying the metabolic phenotypes associated with increased DILI risk, and indicating the mechanisms implicated in their toxicity.

A myriad of pathways to NASH

Nonalcoholic steatohepatitis (NASH) is defined histopathologically by the presence of macrovesicular steatosis, cellular ballooning, and inflammation. NASH represents a complex multifactorial disease that typically occurs within the context of the metabolic syndrome. NASH lacks homogeneity, and other forms of NASH can present atypically. Less than 50% of patients with NASH respond to pharmacologic treatment, which speaks to this heterogeneity. The authors discuss drugs, disease entities, and nutritional states that can cause or exacerbate underlying NASH indirectly through worsening insulin resistance or directly by interfering with lipid metabolism, promoting oxidative injury, or activating inflammatory pathways.

Central role of mitochondria in drug-induced liver injury

A frequent mechanism for drug-induced liver injury (DILI) is the formation of reactive metabolites that trigger hepatitis through direct toxicity or immune reactions. Both events cause mitochondrial membrane disruption. Genetic or acquired factors predispose to metabolite-mediated hepatitis by increasing the formation of the reactive metabolite, decreasing its detoxification, or by the presence of critical human leukocyte antigen molecule(s). In other instances, the parent drug itself triggers mitochondrial membrane disruption or inhibits mitochondrial function through different mechanisms. Drugs can sequester coenzyme A or can inhibit mitochondrial β-oxidation enzymes, the transfer of electrons along the respiratory chain, or adenosine triphosphate (ATP) synthase. Drugs can also destroy mitochondrial DNA, inhibit its replication, decrease mitochondrial transcripts, or hamper mitochondrial protein synthesis. Quite often, a single drug has many different effects on mitochondrial function. A severe impairment of oxidative phosphorylation decreases hepatic ATP, leading to cell dysfunction or necrosis; it can also secondarily inhibit ß-oxidation, thus causing steatosis, and can also inhibit pyruvate catabolism, leading to lactic acidosis. A severe impairment of β-oxidation can cause a fatty liver; further, decreased gluconeogenesis and increased utilization of glucose to compensate for the inability to oxidize fatty acids, together with the mitochondrial toxicity of accumulated free fatty acids and lipid peroxidation products, may impair energy production, possibly leading to coma and death. Susceptibility to parent drug-mediated mitochondrial dysfunction can be increased by factors impairing the removal of the toxic parent compound or by the presence of other medical condition(s) impairing mitochondrial function. New drug molecules should be screened for possible mitochondrial effects.

Novel participation of transglutaminase-2 through c-Jun N-terminal kinase activation in sphingosylphosphorylcholine-induced keratin reorganization of PANC-1 cells

Sphingosylphosphorylcholine (SPC) is found at increased levels in the malignant ascites of tumor patients and induces perinuclear reorganization of keratin 8 (K8) filaments that contribute to the viscoelasticity of metastatic cancer cells. In this study, we investigated the role and molecular mechanisms of Tgase-2 in SPC-induced K8 phosphorylation and perinuclear reorganization in PANC-1 cells (PAN(WT)), and in PANC-1 cells that stably expressed shTgase-2 or Tgase-2 (PAN(shTg2) and PAN(Tg2)). SPC induces the expression of Tgase-2 in a time- and dose-dependent manner. Gene silencing of Tgase-2 or cystamine suppressed the SPC-induced phosphorylation and perinuclear reorganization of K8 and suppressed the SPC-induced migration of PANC-1 cells. An inhibitor of c-Jun N-terminal kinase (JNK), SP600125, suppressed the SPC-induced phosphorylation of serine 431 in K8 and keratin reorganization. Next, we examined the effect of Tgase-2 on JNK activation of serine 431 phosphorylation in K8. Tgase-2 gene silencing suppressed the expression of active form JNK (pJNK). Constitutive or tetracyclin-induced conditional expression of Tgase-2 increased the levels of pJNK. Tgase-2 was coimmunoprecipitated with K8 and JNK. In addition, K8 was coimmunoprecipitated with Tgase-2 and JNK. JNK was also coimmunoprecipitated with K8 and Tgase-2. Overall, these results suggest that Tgase-2 is involved in SPC-induced phosphorylation and perinuclear reorganization of K8 by activating JNK and forming a triple complex with K8 and JNK. Therefore, SPC-induced Tgase-2 might be a new target for modulating keratin reorganization, metastasis of cancer cells and JNK activation.

Drug-induced toxicity on mitochondria and lipid metabolism: mechanistic diversity and deleterious consequences for the liver

Numerous investigations have shown that mitochondrial dysfunction is a major mechanism of drug-induced liver injury, which involves the parent drug or a reactive metabolite generated through cytochromes P450. Depending of their nature and their severity, the mitochondrial alterations are able to induce mild to fulminant hepatic cytolysis and steatosis (lipid accumulation), which can have different clinical and pathological features. Microvesicular steatosis, a potentially severe liver lesion usually associated with liver failure and profound hypoglycemia, is due to a major inhibition of mitochondrial fatty acid oxidation (FAO). Macrovacuolar steatosis, a relatively benign liver lesion in the short term, can be induced not only by a moderate reduction of mitochondrial FAO but also by an increased hepatic de novo lipid synthesis and a decreased secretion of VLDL-associated triglycerides. Moreover, recent investigations suggest that some drugs could favor lipid deposition in the liver through primary alterations of white adipose tissue (WAT) homeostasis. If the treatment is not interrupted, steatosis can evolve toward steatohepatitis, which is characterized not only by lipid accumulation but also by necroinflammation and fibrosis. Although the mechanisms involved in this aggravation are not fully characterized, it appears that overproduction of reactive oxygen species by the damaged mitochondria could play a salient role. Numerous factors could favor drug-induced mitochondrial and metabolic toxicity, such as the structure of the parent molecule, genetic predispositions (in particular those involving mitochondrial enzymes), alcohol intoxication, hepatitis virus C infection, and obesity. In obese and diabetic patients, some drugs may induce acute liver injury more frequently while others may worsen the pre-existent steatosis (or steatohepatitis).

Strategies for the early detection of drug-induced hepatic steatosis in preclinical drug safety evaluation studies

Hepatic steatosis is characterized by the accumulation of lipid droplets in the liver. Although relatively benign, simple steatosis can eventually lead to the development of steatohepatitis, a more serious condition characterized by fibrosis, cirrhosis, and eventual liver failure if the underlying cause is not eliminated. According to the "two hit" theory of steatohepatitis, the initial hit involves fat accumulation in the liver, and a second hit leads to inflammation and subsequent tissue injury. Because some xenobiotics target liver fatty acid metabolism, especially mitochondrial β-oxidation, it is important to avoid potential drug candidates that can contribute to either the initiation of liver steatosis or progression to the more injurious steatohepatitis. The gold standard for the detection of these types of hepatic effects is histopathological examination of liver tissue. In animal studies, these examinations are slow, restricted to a single sampling time, and limited tissue sections. Recent literature suggests that rapid in vitro screening methods can be used early in the drug R&D process to identify compounds with steatotic potential. Further, progress in the identification of potential serum or plasma protein biomarkers for these liver changes may provide additional in vivo tools to the preclinical study toxicologist. This review summarizes recent developments for in vitro screening and in vivo biomarker detection for steatotic drug candidates.

Mitochondrial involvement in drug-induced liver injury

Mitochondrial dysfunction is a major mechanism of liver injury. A parent drug or its reactive metabolite can trigger outer mitochondrial membrane permeabilization or rupture due to mitochondrial permeability transition. The latter can severely deplete ATP and cause liver cell necrosis, or it can instead lead to apoptosis by releasing cytochrome c, which activates caspases in the cytosol. Necrosis and apoptosis can trigger cytolytic hepatitis resulting in lethal fulminant hepatitis in some patients. Other drugs severely inhibit mitochondrial function and trigger extensive microvesicular steatosis, hypoglycaemia, coma, and death. Milder and more prolonged forms of drug-induced mitochondrial dysfunction can also cause macrovacuolar steatosis. Although this is a benign liver lesion in the short-term, it can progress to steatohepatitis and then to cirrhosis. Patient susceptibility to drug-induced mitochondrial dysfunction and liver injury can sometimes be explained by genetic or acquired variations in drug metabolism and/or elimination that increase the concentration of the toxic species (parent drug or metabolite). Susceptibility may also be increased by the presence of another condition, which also impairs mitochondrial function, such as an inborn mitochondrial cytopathy, beta-oxidation defect, certain viral infections, pregnancy, or the obesity-associated metabolic syndrome. Liver injury due to mitochondrial dysfunction can have important consequences for pharmaceutical companies. It has led to the interruption of clinical trials, the recall of several drugs after marketing, or the introduction of severe black box warnings by drug agencies. Pharmaceutical companies should systematically investigate mitochondrial effects during lead selection or preclinical safety studies.