Jochem W. van der Veen

Evaluating the performance of integrated approaches for hazard identification of skin sensitizing chemicals.

The currently available animal-free methods for the detection of skin sensitizing potential of chemicals seem promising. However, no single method is able to comprehensively represent the complexity of the processes involved in skin sensitization. To ensure a mechanistic basis and cover the complexity, multiple methods should be integrated into a testing strategy, in accordance with the adverse outcome pathway that describes all key events in skin sensitization. Although current majority voting testing strategies have proven effective, the performance of individual methods is not taken into account. To that end, we designed a tiered strategy based on complementary characteristics of the included methods, and compared it to a majority voting approach. This tiered testing strategy was able to correctly identify all 41 chemicals tested. In terms of total number of experiments required, the tiered testing strategy requires less experiments compared to the majority voting approach. On the other hand, this tiered strategy is more complex due the number of different alternative methods required, and predicted costs are similar for both strategies. Both the tiered and majority voting strategies provide a mechanistic basis for skin sensitization testing, but the strategy most suitable for regulatory decision-making remains to be determined.

Applicability of a keratinocyte gene signature to predict skin sensitizing potential

There is a need to replace animal tests for the identification of skin sensitizers and currently many alternative assays are being developed that have very promising results. In this study a gene signature capable of very accurate identification of sensitizers was established in the HaCaT human keratinocyte cell line. This signature was evaluated in a separate study using six chemicals that are either local lymph node (LLNA) false-positive or false-negative chemicals in addition to nine sensitizers and four non-sensitizers. Similar studies do not apply these more difficult to classify chemicals, which show the true potential for human predictions of an assay. Although the gene signature has improved prediction accuracy compared to the LLNA, the misclassified compounds were comparable between the two assays. Gene profiling also showed a sensitizer specific response of the Nrf2-keap1 and Toll-like receptor signaling pathways. After exposure to non-sensitizing chemicals that induce either of the pathways the signature misclassified all Nrf2-inducers, while the Toll-like receptor ligands were correctly classified. In conclusion, we confirm that keratinocyte based prediction assays may provide essential information on the properties of compounds. Furthermore, chemical selection is critical for assessment of the performance of in vitro alternative assays.

Gene expression changes induced by skin sensitizers in the KeratinoSens™ cell line: Discriminating Nrf2-dependent and Nrf2-independent events.

The KeratinoSens™ assay is an in vitro screen for the skin sensitization potential of chemicals. It is based on a luciferase reporter gene under the control of the antioxidant response element of the aldoketoreductase gene AKR1C2. The transferability, reproducibility, and predictivity of the KeratinoSens™ assay have been investigated in detail and it is currently under assessment at the European Center for Validation of Alternatives to animal testing (ECVAM). Here we investigate the sensitizer-induced gene expression in the KeratinoSens™ cell line at the mRNA level and discriminate Nrf2-dependent and Nrf2-independent events by using siRNA to better characterize this test system at the molecular level. The results show that (i) the sensitizer-induced luciferase signal in KeratinoSens™ cells is completely dependent on Nrf2. The same holds true for the luciferase induction observed for the false positive chemical Tween80, indicating that the false positive result is not due to recruitment of an alternative transcription factor. (ii) Luciferase induction parallels the induction of endogenous Nrf2-dependent genes, indicating that the luciferase signal is representative for the sensitizer-induced Nrf2-response. (iii) The induction by sensitizers of additional genetic markers related to heat shock proteins and cellular stress could be reproduced in the KeratinoSens™ cell line and they were shown to be Nrf2-independent. These results confirm that the KeratinoSens™ cell line is a rapid and adequate screening tool to assess the sensitizer-induced Nrf2-response in keratinocytes.

Induction of skin sensitization is augmented in Nrf2-deficient mice.

Several in vitro DNA microarray studies have shown the importance of nuclear factor (erythroid-derived 2)-like 2 (Nrf2) in skin sensitization. Nevertheless, the exact in vivo role of the Nrf2-Keap1 pathway during the induction of skin sensitization remains unknown. To study the function of Nrf2, a local lymph node assay was performed in wild-type and Nrf2-deficient mice using 2,4-dinitrochlorobenzene. The Nrf2-deficient mice show a more pronounced response, indicating that Nrf2 is involved in dampening the induction of skin sensitization.

Anchoring molecular mechanisms to the adverse outcome pathway for skin sensitization: Analysis of existing data

Abstract Allergic contact dermatitis (ACD) is a hypersensitivity immune response induced by small protein-reactive chemicals. Currently, the murine local lymph node assay (LLNA) provides hazard identification and quantitative estimation of sensitizing potency. Given the complexity of ACD, a single alternative method cannot replace the LLNA, but it is necessary to combine methods through an integrated testing strategy (ITS). In the development of an ITS, information regarding mechanisms and molecular processes involved in skin sensitization is crucial. The recently published adverse outcome pathway (AOP) for skin sensitization captures mechanistic knowledge into key events that lead to ACD. To understand the molecular processes in ACD, a systematic review of murine in vivo studies was performed and an ACD molecular map was constructed. In addition, comparing the molecular map to the limited human in vivo toxicogenomic data available suggests that certain processes are similarly triggered in mice and humans, but additional human data will be needed to confirm these findings and identify differences. To gain insight in the molecular mechanisms represented by various human in vitro systems, the map was compared to in vitro toxicogenomic data. This analysis allows for comparison of emerging in vitro methods on a molecular basis, in addition to mathematical predictive value. Finally, a survey of the current in silico, in chemico, and in vitro methods was used to indicate which AOP key event is modeled by each method. By anchoring emerging classification methods to the AOP and the ACD molecular map, complementing methods can be identified, which provides a cornerstone for the development of a testing strategy that accurately reflects the key events in skin sensitization.

Comparison of the molecular topologies of stress-activated transcription factors HSF1, AP-1, NRF2, and NF-κB in their induction kinetics of HMOX1

For cells, reacting aptly to changes in their environment is of critical importance. The protein Heme oxygenase-1 (HMOX1) plays a critical role as a guard of cellular homeostasis and is considered as a reliable indicator of cellular oxidative stress. A better insight in the regulation of HMOX1 would assist in understanding the physiological role of HMOX1 as well as improving functional interpretation of the gene as a biomarker in toxicogenomics. Remarkably, as many as four transcription factors are known to regulate the HMOX1 gene: HSF1, AP-1, NRF2, and NF-κB. To investigate induction kinetics of these transcription factors, we constructed mathematical simulation models for each of them. We included the topology of the known interactions of molecules involved in the activation of the transcription factors, and the feedback loops resulting in their down-regulation. We evaluate how the molecular circuitries associated with the different transcription factors differ in their kinetics regarding HMOX1 induction, under different scenarios of acute and less acute stress. We also evaluate the combined effect of the four transcription factors on HMOX1 expression and the resulting alleviation of stress. Overall, the results support the assumption of different biological roles for the four transcription factors, with AP-1 being a fast acting general stress response protein at the expense of efficiency, and NRF2 being important for cellular homeostasis in maintaining low levels of oxidative stress.

Human relevance of an in vitro gene signature in HaCaT for skin sensitization.

The skin sensitizing potential of chemicals is mainly assessed using animal methods, such as the murine local lymph node assay. Recently, an in vitro assay based on a gene expression signature in the HaCaT keratinocyte cell line was proposed as an alternative to these animal methods. Here, the human relevance of this gene signature is assessed through exposure of freshly isolated human skin to the chemical allergens dinitrochlorobenzene (DNCB) and diphenylcyclopropenone (DCP). In human skin, the gene signature shows similar direction of regulation as was previously observed in vitro, suggesting that the molecular processes that drive expression of these genes are similar between the HaCaT cell line and freshly isolated skin, providing evidence for the human relevance of the gene signature.

Effects of pooling RNA from samples treated with different compounds for determining class specific biomarkers and processes in toxicogenomics.

Pooling of RNA samples is generally applied to obtain samples that represent the average signal of biological replicates of a single treatment. For toxicogenomics, pooling RNA of samples treated by different compounds could in the same way summarize these compounds to a single sample with average signals per class. In this study, we investigated the efficiency of such an approach to establish class specific differences in gene expression. As an example we took skin sensitizing compounds as one class and irritating compounds as another. A direct comparison was made to separately hybridized RNA samples. We observed that pooling RNA from compounds of a class substantially increased power to detect significantly regulated genes between classes because variability between pooled samples was much lower. Within pools the vast majority of genes maintained patterns of expression compared to the separately hybridized samples, especially in regulated genes. Both designs yielded appropriate biomarkers. Biomarkers selected from the pooled and separate design performed equally in classification of compounds to their class and relevant processes were found enriched in both designs. Consequently, pooling of RNA of different compound treated samples can be applied to determine class specific biomarkers and processes at much reduced cost and with limited loss of accuracy.

The involvement of the Toll-like receptor signaling and Nrf2-Keap1 pathways in the in vitro regulation of IL-8 and HMOX1 for skin sensitization

Abstract In vitro gene profiling studies have associated the molecular pathways of Nrf2-Keap1 and Toll-like receptor (TLR) signaling with skin sensitization. In this study, the role of these pathways in the regulation of protein biomarkers for skin sensitization was further elucidated using transient gene knock-down of key components of the signaling cascades in HaCaT cells after exposure to dinitrochlorobenzene (DNCB). The effect of targeting these pathways was established through evaluation of heme oxygenase1 (HMOX1) and interleukin (IL)-8 production. These experiments showed that Nrf2 is not involved in regulating HMOX1 after exposure to DNCB, but that activation of TLR signaling moderates the expression of HMOX1. The regulation of IL-8 depended on Nrf2, but also on the Toll/interleukin-1 receptor (TIR)-domain-containing adapter-inducing interferon-β (TRIF) adaptor protein in TLR signaling. This study provides new insights into the regulation of HMOX1 and IL-8, but the exact regulating mechanisms remain to be further elucidated.

Keratinocytes, Innate Immunity and Allergic Contact Dermatitis - Opportunities for the Development of In Vitro Assays to Predict the Sensitizing Potential of Chemicals

Allergic contact dermatitis (ACD) is the most prevalent form of immunotoxicity in humans characterized by clinical manifestations such as red rashes, itchy skin and blisters. The disease is caused by skin sensitizers which are allergenic low-molecular weight chemicals. ACD is an important occupational disease that gives problems at different workplaces, including hair dressers, metal workers, construction workers, and cleaners. In addition, ACD can develop in the general population as well, since several consumer products contain skin sensitizers. Important skin sensitizers are metals (nickel, chromium), fragrances, hair dye ingredients and preservatives (Kimber et al., 2002a; Vandebriel & van Loveren, 2010).