Development of an adverse outcome pathway network for nephrotoxicity

Adverse outcome pathways (AOPs) were introduced in modern toxicology to provide evidence-based representations of the events and processes involved in the progression of toxicological effects across varying levels of the biological organisation to better facilitate the safety assessment of chemicals. AOPs offer an opportunity to address knowledge gaps and help to identify novel therapeutic targets. They also aid in the selection and development of existing and new in vitro and in silico test methods for hazard identification and risk assessment of chemical compounds. However, many toxicological processes are too intricate to be captured in a single, linear AOP. As a result, AOP networks have been developed to aid in the comprehension and placement of associated events underlying the emergence of related forms of toxicity-where complex exposure scenarios and interactions may influence the ultimate adverse outcome. This study utilised established criteria to develop an AOP network that connects thirteen individual AOPs associated with nephrotoxicity (as sourced from the AOP-Wiki) to identify several key events (KEs) linked to various adverse outcomes, including kidney failure and chronic kidney disease. Analysis of the modelled AOP network and its topological features determined mitochondrial dysfunction, oxidative stress, and tubular necrosis to be the most connected and central KEs. These KEs can provide a logical foundation for guiding the selection and creation of in vitro assays and in silico tools to substitute for animal-based in vivo experiments in the prediction and assessment of chemical-induced nephrotoxicity in human health.

Proposal of an in silico profiler for categorisation of repeat dose toxicity data of hair dyes

This study outlines the analysis of 94 chemicals with repeat dose toxicity data taken from Scientific Committee on Consumer Safety opinions for commonly used hair dyes in the European Union. Structural similarity was applied to group these chemicals into categories. Subsequent mechanistic analysis suggested that toxicity to mitochondria is potentially a key driver of repeat dose toxicity for chemicals within each of the categories. The mechanistic hypothesis allowed for an in silico profiler consisting of four mechanism-based structural alerts to be proposed. These structural alerts related to a number of important chemical classes such as quinones, anthraquinones, substituted nitrobenzenes and aromatic azos. This in silico profiler is intended for grouping chemicals into mechanism-based categories within the adverse outcome pathway paradigm.

Drug-induced liver injury: mechanisms, types and biomarkers

Drug-induced liver injury is a ubiquitous issue in clinical settings and pharmaceutical industry. Hepatotoxicity elicited by drugs may be intrinsic or idiosyncratic, both which are driven by different molecular mechanisms. Recently, a unifying mechanistic model of drug-induced liver injury has been introduced. According to this model, drug-induced hepatotoxicity relies on 3 consecutive steps, namely an initial cellular insult that leads to the occurrence of mitochondrial permeability transition, which in turn ultimately burgeons into the onset of cell death. Clinically, drug-induced liver injury can be manifested in a number of acute and chronic conditions, including hepatitis, cholestasis, steatosis and fibrosis. These pathologies can be diagnosed and monitored by addressing well-established physical, clinical chemistry and histopathological biomarkers. In the last few years, several novel read-outs of drug-induced liver injury have been proposed, involving genetic, epigenetic, transcriptomic, proteomic and metabolomic parameters. These new concepts and recent developments in the field of drug-induced liver injury are revised in the current paper.

Primary hepatocyte cultures for pharmaco-toxicological studies: at the busy crossroad of various anti-dedifferentiation strategies

Continuously increasing understanding of the molecular triggers responsible for the onset of diseases, paralleled by an equally dynamic evolution of chemical synthesis and screening methods, offers an abundance of pharmacological agents with a potential to become new successful drugs. However, before patients can benefit of newly developed pharmaceuticals, stringent safety filters need to be applied to weed out unfavourable drug candidates. Cost effectiveness and the need to identify compound liabilities, without exposing humans to unnecessary risks, has stimulated the shift of the safety studies to the earliest stages of drug discovery and development. In this regard, in vivo relevant organotypic in vitro models have high potential to revolutionize the preclinical safety testing. They can enable automation of the process, to match the requirements of high-throughput screening approaches, while satisfying ethical considerations. Cultures of primary hepatocytes became already an inherent part of the preclinical pharmaco-toxicological testing battery, yet their routine use, particularly for long-term assays, is limited by the progressive deterioration of liver-specific features. The availability of suitable hepatic and other organ-specific in vitro models is, however, of paramount importance in the light of changing European legal regulations in the field of chemical compounds of different origin, which gradually restrict the use of animal studies for safety assessment, as currently witnessed in cosmetic industry. Fortunately, research groups worldwide spare no effort to establish hepatic in vitro systems. In the present review, both classical and innovative methodologies to stabilize the in vivo-like hepatocyte phenotype in culture of primary hepatocytes are presented and discussed.

Connexin 43 hemichannels contribute to the propagation of apoptotic cell death in a rat C6 glioma cell model

Gap junctions (GJs) have been demonstrated to communicate cell death signals from apoptotic to healthy cells, thereby spatially extending apoptosis. Before being incorporated into GJs, hemichannels (hemi-GJs) are normally closed but recent evidence suggests that they can be opened by various messengers and conditions, thereby forming a pore through which molecules can enter or leave the cell potentially leading to cell death. The aim of this study was to determine the contribution of GJs and hemichannels in the communication of apoptosis toward surrounding cells. We induced apoptosis in C6 glioma cells stably transfected with connexin (Cx)43, with cytochrome C (cytC) using in situ electroporation and found that healthy surrounding cells underwent apoptotic transformation. Work with various cell death markers, wild-type (WT) and Cx43-expressing cells, inhibitors of GJs and/or hemichannels, and Cx43 gene silencing showed that GJs contribute to the spread of apoptosis in a zone next to where apoptosis was triggered whereas hemichannels also promoted cell death beyond this area. Buffering cytoplasmic Ca(2+) changes inhibited the spread of apoptosis in both cases. We conclude that Cx43 hemichannels, in concert with their GJ counterparts, play a role in communicating cytC-induced apoptotic cell death messages.

Inhibition of NF-kappaB activation by the histone deacetylase inhibitor 4-Me2N-BAVAH induces an early G1 cell cycle arrest in primary hepatocytes

OBJECTIVE

Benzoylaminoalkanohydroxamic acids, including 5-(4-dimethylaminobenzoyl)aminovaleric acid hydroxamide (4-Me(2)N-BAVAH), are structural analogues of Trichostatin A, a naturally occurring histone deacetylase inhibitor (HDACi). 4-Me(2)N-BAVAH has been shown to induce histone hyperacetylation and to inhibit proliferation in Friend erythroleukaemia cells in vitro. However, the molecular mechanisms have remained unidentified.

MATERIALS AND METHODS

In this study, we evaluated the effects of 4-Me(2)N-BAVAH on proliferation in non-malignant cells, namely epidermal growth factor-stimulated primary rat hepatocytes.

RESULTS AND CONCLUSION

We have found that 4-Me(2)N-BAVAH inhibits HDAC activity at non-cytotoxic concentrations and prevents cells from responding to the mitogenic stimuli of epidermal growth factor. This results in an early G(1) cell cycle arrest that is independent of p21 activity, but instead can be attributed to inhibition of cyclin D1 transcription through a mechanism involving inhibition of nuclear factor-kappaB activation. In addition, 4-Me(2)N-BAVAH delays the onset of spontaneous apoptosis in primary rat hepatocyte cultures as evidenced by down-regulation of the pro-apoptotic proteins Bid and Bax, and inhibition of caspase-3 activation.

[The role of intercellular communication via "gap junctions" in disease]

Gap junctional intercellular communication plays an important role in the maintenance of cellular homeostasis. The flow of chemical messengers through gap junctions, gap junctional intercellular communication, is essential in processes such as electrical coupling, embryonic development and adaptive tissue response. Gap junctions are formed by connexin proteins. Mutational alterations in the connexin genes are associated with the occurrence of multiple diseases, such as peripheral neuropathy, cardiovascular disease, dermatological disease, hereditary deafness and cataract. Consequently, modulation of gap junctional intercellular communication is a potential pharmacological target. Future research, based, for example, on the recent developments in genetics, may clarify gap junction physiology. This will in turn provide promising perspectives for the development of targeted drugs.

Major phase I biotransformation pathways of Trichostatin a in rat hepatocytes and in rat and human liver microsomes

Phase I biotransformation of Trichostatin A (TSA), a histone deacetylase inhibitor with promising antifibrotic and antitumoral properties, was investigated in rat and human liver microsomes and in suspensions of rat hepatocytes. TSA (50 micro M) was readily and completely metabolized by rat hepatocytes in suspension (2 x 10(6) cells/ml), whereafter its phase I metabolites were separated by high-performance liquid chromatography and detected with simultaneous UV and electrospray ionization mass spectrometry (ESI-MS). ESI tandem mass spectrometry (ESI-MS/MS) was used to identify the metabolites. Two major phase I biotransformation pathways in rat hepatocytes were shown to be N-demethylation and reduction of the hydroxamic acid function to its corresponding amide. N-monodemethylated TSA and TSA amide were preferentially formed during the first 20 min of exposure, and N-monodemethylated TSA amide appeared as the main metabolite after a 30 min incubation period. At this time, virtually all TSA had been metabolized. Trichostatic acid, N-monodemethylated Trichostatic acid, and N-didemethylated TSA were identified as minor metabolites. Longer incubation led to the formation of N-didemethylated TSA amide as the main metabolite. Lower concentrations of TSA (5 and 25 micro M) formed relatively higher amounts of N-demethylated, nonreduced metabolites. Incubations of TSA with rat and human microsomal suspensions, however, led to an incomplete biotransformation with the formation of two major metabolites, N-mono- and N-didemethylated TSA. Traces of Trichostatic acid, TSA amide, N-mono- and N-didemethylated TSA amide were also detected. This study is the first to show that TSA undergoes intensive phase I biotransformation in rat hepatocytes. This has important consequences for its potential development as a drug, since rapid biotransformation resulting in a short exposure to the pharmacologically active parent compound, and a complex mixture of metabolites is usually not desired. Further biotransformation studies of TSA and structural analogs with antitumoral and antifibrotic properties need to be performed in cultured intact hepatocytes, in particular since one of the major phase I biotransformation pathways is catalyzed by nonmicrosomal enzymes.