Idiosyncratic Drug-Induced Liver Injury: Is Drug-Cytokine Interaction the Linchpin?

Idiosyncratic drug-induced liver injury continues to be a human health problem in part because drugs that cause these reactions are not identified in current preclinical testing and because progress in prevention is hampered by incomplete knowledge of mechanisms that underlie these adverse responses. Several hypotheses involving adaptive immune responses, inflammatory stress, inability to adapt to stress, and multiple, concurrent factors have been proposed. Yet much remains unknown about how drugs interact with the liver to effect death of hepatocytes. Evidence supporting hypotheses implicating adaptive or innate immune responses in afflicted patients has begun to emerge and is bolstered by results obtained in experimental animal models and in vitro systems. A commonality in adaptive and innate immunity is the production of cytokines, including interferon-γ (IFNγ). IFNγ initiates cell signaling pathways that culminate in cell death or inhibition of proliferative repair. Tumor necrosis factor-α, another cytokine prominent in immune responses, can also promote cell death. Furthermore, tumor necrosis factor-α interacts with IFNγ, leading to enhanced cellular responses to each cytokine. In this short review, we propose that the interaction of drugs with these cytokines contributes to idiosyncratic drug-induced liver injury, and mechanisms by which this could occur are discussed.

Synergistic Cytotoxicity from Drugs and Cytokines In Vitro as an Approach to Classify Drugs According to Their Potential to Cause Idiosyncratic Hepatotoxicity: A Proof-of-Concept Study.

Idiosyncratic drug-induced liver injury (IDILI) typically occurs in a small fraction of patients and has resulted in removal of otherwise efficacious drugs from the market. Current preclinical testing methods are ineffective in predicting which drug candidates have IDILI liability. Recent results suggest that immune mediators such as tumor necrosis factor-α (TNF) and interferon-γ (IFN) interact with drugs that cause IDILI to kill hepatocytes. This proof-of-concept study was designed to test the hypothesis that drugs can be classified according to their ability to cause IDILI in humans using classification modeling with covariates derived from concentration-response relationships that describe cytotoxic interaction with cytokines. Human hepatoma (HepG2) cells were treated with drugs associated with IDILI or with drugs lacking IDILI liability and cotreated with TNF and/or IFN. Detailed concentration-response relationships were determined for calculation of parameters such as the maximal cytotoxic effect, slope, and EC50 for use as covariates for classification modeling using logistic regression. These parameters were incorporated into multiple classification models to identify combinations of covariates that most accurately classified the drugs according to their association with human IDILI. Of 14 drugs associated with IDILI, almost all synergized with TNF to kill HepG2 cells and were successfully classified by statistical modeling. IFN enhanced the toxicity mediated by some IDILI-associated drugs in the presence of TNF. In contrast, of 10 drugs with little or no IDILI liability, none synergized with inflammatory cytokines to kill HepG2 cells and were classified accordingly. The resulting optimal model classified the drugs with extraordinary selectivity and specificity.

Bacterial-Driven Inflammation and Mutant BRAF Expression Combine to Promote Murine Colon Tumorigenesis That Is Sensitive to Immune Checkpoint Therapy.

Colorectal cancer is multifaceted, with subtypes defined by genetic, histologic, and immunologic features that are potentially influenced by inflammation, mutagens, and/or microbiota. Colorectal cancers with activating mutations in BRAF are associated with distinct clinical characteristics, although the pathogenesis is not well understood. The Wnt-driven multiple intestinal neoplasia (MinApcΔ716/+) enterotoxigenic Bacteroides fragilis (ETBF) murine model is characterized by IL17-dependent, distal colon adenomas. Herein, we report that the addition of the BRAF V600E mutation to this model results in the emergence of a distinct locus of midcolon tumors. In ETBF-colonized BRAF V600E Lgr5 CreMin (BLM) mice, tumors have similarities to human BRAF V600E tumors, including histology, CpG island DNA hypermethylation, and immune signatures. In comparison to Min ETBF tumors, BLM ETBF tumors are infiltrated by CD8+ T cells, express IFNγ signatures, and are sensitive to anti-PD-L1 treatment. These results provide direct evidence for critical roles of host genetic and microbiota interactions in colorectal cancer pathogenesis and sensitivity to immunotherapy. SIGNIFICANCE: Colorectal cancers with BRAF mutations have distinct characteristics. We present evidence of specific colorectal cancer gene-microbial interactions in which colonization with toxigenic bacteria drives tumorigenesis in BRAF V600E Lgr5 CreMin mice, wherein tumors phenocopy aspects of human BRAF-mutated tumors and have a distinct IFNγ-dominant immune microenvironment uniquely responsive to immune checkpoint blockade.This article is highlighted in the In This Issue feature, p. 1601.

The emerging role of epigenetic modifiers in repair of DNA damage associated with chronic inflammatory diseases.

At sites of chronic inflammation epithelial cells are exposed to high levels of reactive oxygen species (ROS), which can contribute to the initiation and development of many different human cancers. Aberrant epigenetic alterations that cause transcriptional silencing of tumor suppressor genes are also implicated in many diseases associated with inflammation, including cancer. However, it is not clear how altered epigenetic gene silencing is initiated during chronic inflammation. The high level of ROS at sites of inflammation is known to induce oxidative DNA damage in surrounding epithelial cells. Furthermore, DNA damage is known to trigger several responses, including recruitment of DNA repair proteins, transcriptional repression, chromatin modifications and other cell signaling events. Recruitment of epigenetic modifiers to chromatin in response to DNA damage results in transient covalent modifications to chromatin such as histone ubiquitination, acetylation and methylation and DNA methylation. DNA damage also alters non-coding RNA expression. All of these alterations have the potential to alter gene expression at sites of damage. Typically, these modifications and gene transcription are restored back to normal once the repair of the DNA damage is completed. However, chronic inflammation may induce sustained DNA damage and DNA damage responses that result in these transient covalent chromatin modifications becoming mitotically stable epigenetic alterations. Understanding how epigenetic alterations are initiated during chronic inflammation will allow us to develop pharmaceutical strategies to prevent or treat chronic inflammation-induced cancer. This review will focus on types of DNA damage and epigenetic alterations associated with chronic inflammatory diseases, the types of DNA damage and transient covalent chromatin modifications induced by inflammation and oxidative DNA damage and how these modifications may result in epigenetic alterations.

DNA methyltransferase inhibition reduces inflammation-induced colon tumorigenesis.

Chronic inflammation is strongly associated with an increased risk of developing colorectal cancer. DNA hypermethylation of CpG islands alters the expression of genes in cancer cells and plays an important role in carcinogenesis. Chronic inflammation is also associated with DNA methylation alterations and in a mouse model of inflammation-induced colon tumorigenesis, we previously demonstrated that inflammation-induced tumours have 203 unique regions with DNA hypermethylation compared to uninflamed epithelium. To determine if altering inflammation-induced DNA hypermethylation reduces tumorigenesis, we used the same mouse model and treated mice with the DNA methyltransferase (DNMT) inhibitor decitabine (DAC) throughout the tumorigenesis time frame. DAC treatment caused a significant reduction in colon tumorigenesis. The tumours that did form after DAC treatment had reduced inflammation-specific DNA hypermethylation and alteration of expression of associated candidate genes. When compared, inflammation-induced tumours from control (PBS-treated) mice were enriched for cell proliferation associated gene expression pathways whereas inflammation-induced tumours from DAC-treated mice were enriched for interferon gene signatures. To further understand the altered tumorigenesis, we derived tumoroids from the different tumour types. Interestingly, tumoroids derived from inflammation-induced tumours from control mice maintained many of the inflammation-induced DNA hypermethylation alterations and had higher levels of DNA hypermethylation at these regions than tumoroids from DAC-treated mice. Importantly, tumoroids derived from inflammation-induced tumours from the DAC-treated mice proliferated more slowly than those derived from the inflammation-induced tumours from control mice. These studies suggest that inhibition of inflammation-induced DNA hypermethylation may be an effective strategy to reduce inflammation-induced tumorigenesis.

Inflammation-induced DNA methylation of DNA polymerase gamma alters the metabolic profile of colon tumors.

BACKGROUND: Inflammation, metabolism, and epigenetic modulation are highly interconnected processes that can be altered during tumorigenesis. However, because of the complexity of these interactions, direct cause and effect during tumorigenesis have been difficult to prove. Previously, using a murine model of inflammation-induced colon tumorigenesis, we determined that the promoter of the catalytic subunit of DNA polymerase gamma (Polg) is DNA hypermethylated and silenced in inflammation-induced tumors, but not in non-inflammation-induced (mock) tumors, suggesting that inflammation can induce silencing of Polg through promoting DNA methylation during tumorigenesis. Polg is the only mitochondrial DNA polymerase and mutations in Polg cause mitochondrial diseases in humans. Because of the role of mitochondria in metabolism, we hypothesized that silencing of Polg in inflammation-induced tumors would result in these tumors having altered metabolism in comparison to mock tumors. METHODS: Inflammation-induced and mock colon tumors and colon epithelium from a mouse model of inflammation-induced colon tumorigenesis were assayed for alterations in Polg expression, mitochondria, and metabolism. Organoids derived from these tissues were used to study the direct effect of loss of Polg on mitochondria and metabolism. RESULTS: We demonstrate that inflammation-induced tumors with reduced Polg expression have decreased mitochondrial DNA content and numbers of mitochondria compared to normal epithelium or mock tumors. Tumoroids derived from mock and inflammation-induced tumors retained key characteristics of the original tumors. Inflammation-induced tumoroids had increased glucose uptake and lactate secretion relative to mock tumoroids. shRNA-mediated knockdown of Polg in mock tumoroids reduced mtDNA content, increased glucose uptake and lactate secretion, and made the tumoroids more resistant to oxidative stress. CONCLUSIONS: These results suggest that inflammation-induced DNA methylation and silencing of Polg plays an important role in the tumorigenesis process by resulting in reduced mitochondria levels and altered metabolism. An enhanced understanding of how metabolism is altered in and drives inflammation-induced tumorigenesis will provide potential therapeutic targets.

Mismatch Repair Proteins Initiate Epigenetic Alterations during Inflammation-Driven Tumorigenesis.

Aberrant silencing of genes by DNA methylation contributes to cancer, yet how this process is initiated remains unclear. Using a murine model of inflammation-induced tumorigenesis, we tested the hypothesis that inflammation promotes recruitment of epigenetic proteins to chromatin, initiating methylation and gene silencing in tumors. Compared with normal epithelium and noninflammation-induced tumors, inflammation-induced tumors gained DNA methylation at CpG islands, some of which are associated with putative tumor suppressor genes. Hypermethylated genes exhibited enrichment of repressive chromatin marks and reduced expression prior to tumorigenesis, at a time point coinciding with peak levels of inflammation-associated DNA damage. Loss of MutS homolog 2 (MSH2), a mismatch repair (MMR) protein, abrogated early inflammation-induced epigenetic alterations and DNA hypermethylation alterations observed in inflammation-induced tumors. These results indicate that early epigenetic alterations initiated by inflammation and MMR proteins lead to gene silencing during tumorigenesis, revealing a novel mechanism of epigenetic alterations in inflammation-driven cancer. Understanding such mechanisms will inform development of pharmacotherapies to reduce carcinogenesis. Cancer Res; 77(13); 3467-78. ©2017 AACR.

Calcium Contributes to the Cytotoxic Interaction Between Diclofenac and Cytokines.

Diclofenac (DCLF) is a widely used non-steroidal anti-inflammatory drug that is associated with idiosyncratic, drug-induced liver injury (IDILI) in humans. The mechanisms of DCLF-induced liver injury are unknown; however, patients with certain inflammatory diseases have an increased risk of developing IDILI, which raises the possibility that immune mediators play a role in the pathogenesis. DCLF synergizes with the cytokines tumor necrosis factor-alpha (TNF) and interferon-gamma (IFN) to cause hepatocellular apoptosis in vitro by a mechanism that involves activation of the endoplasmic reticulum (ER) stress response pathway and of the mitogen-activated protein kinases, c-Jun N-terminal kinase (JNK), and extracellular signal-regulated kinase (ERK). DCLF also causes an increase in intracellular calcium (Ca(++)) in hepatocytes, but the role of this in the cytotoxic synergy between DCLF and cytokines is unknown. We tested the hypothesis that Ca(++) contributes to DCLF/cytokine-induced cytotoxic synergy. Treatment of HepG2 cells with DCLF led to an increase in intracellular Ca(++) at 6 and 12 h, and this response was augmented in the presence of TNF and IFN at 12 h. The intracellular Ca(++) chelator BAPTA/AM reduced cytotoxicity and caspase-3 activation caused by DCLF/cytokine cotreatment. BAPTA/AM also significantly reduced DCLF-induced activation of the ER stress sensor, protein kinase RNA-like ER kinase (PERK), as well as activation of JNK and ERK. Treatment of cells with an inositol trisphosphate receptor antagonist almost completely eliminated DCLF/cytokine-induced cytotoxicity and decreased DCLF-induced activation of PERK, JNK, and ERK. These findings indicate that Ca(++) contributes to DCLF/cytokine-induced cytotoxic synergy by promoting activation of the ER stress-response pathway and JNK and ERK.

Cytotoxic Synergy Between Cytokines and NSAIDs Associated With Idiosyncratic Hepatotoxicity Is Driven by Mitogen-Activated Protein Kinases.

Nonsteroidal anti-inflammatory drugs (NSAIDs) are among the most frequent causes of idiosyncratic, drug-induced liver injury (IDILI). Mechanisms of IDILI are unknown, but immune responses are suspected to underlie them. In animal models of IDILI, the cytokines tumor necrosis factor-alpha (TNFα) and interferon-gamma (IFNγ) are essential to the pathogenesis. Some drugs associated with IDILI interact with cytokines to kill hepatocytes in vitro, and mitogen-activated protein kinases (MAPKs) might play a role. We tested the hypothesis that caspases and MAPKs are involved in NSAID/cytokine-induced cytotoxicity. NSAIDs that are acetic acid (AA) derivatives and associated with IDILI synergized with TNFα in causing cytotoxicity in HepG2 cells, and IFNγ enhanced this interaction. NSAIDs that are propionic acid (PA) derivatives and cause IDILI that is of less clinical concern also synergized with TNFα, but IFNγ was without effect. Caspase inhibition prevented cytotoxicity from AA and PA derivative/cytokine treatment. Treatment with a representative AA or PA derivative induced activation of the MAPKs c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase (ERK), and p38. Inhibition of either JNK or ERK reduced cytotoxicity from cytokine interactions with AA derivatives. In contrast, an ERK inhibitor potentiated cytotoxicity from cytokine interactions with PA derivatives. An AA derivative but not a PA derivative enhanced IFNγ-mediated activation of STAT-1, and this enhancement was ERK-dependent. These findings raise the possibility that some IDILI reactions result from drug/cytokine synergy involving caspases and MAPKs and suggest that, even for drugs within the same pharmacologic class, synergy with cytokines occurs by different kinase signaling mechanisms.

Trovafloxacin-induced replication stress sensitizes HepG2 cells to tumor necrosis factor-alpha-induced cytotoxicity mediated by extracellular signal-regulated kinase and ataxia telangiectasia and Rad3-related.

Use of the fluoroquinolone antibiotic trovafloxacin (TVX) was restricted due to idiosyncratic, drug-induced liver injury (IDILI). Previous studies demonstrated that tumor necrosis factor-alpha (TNF) and TVX interact to cause death of hepatocytes in vitro that was associated with prolonged activation of c-Jun N-terminal kinase (JNK), activation of caspases 9 and 3, and DNA damage. The purpose of this study was to explore further the mechanism by which TVX interacts with TNF to cause cytotoxicity. Treatment with TVX caused cell cycle arrest, enhanced expression of p21 and impaired proliferation, but cell death only occurred after cotreatment with TVX and TNF. Cell death involved activation of extracellular signal-related kinase (ERK), which in turn activated caspase 3 and ataxia telangiectasia and Rad3-related (ATR), both of which contributed to cytotoxicity. Cotreatment of HepG2 cells with TVX and TNF caused double-strand breaks in DNA, and ERK contributed to this effect. Inhibition of caspase activity abolished the DNA strand breaks. The data suggest a complex interaction of TVX and TNF in which TVX causes replication stress, and the downstream effects are exacerbated by TNF, leading to hepatocellular death. These results raise the possibility that IDILI from TVX results from MAPK and ATR activation in hepatocytes initiated by interaction of cytokine signaling with drug-induced replication stress.