Model-based identification of TNFα-induced IKKß-mediated and IκBα-mediated regulation of NFκB signal transduction as a tool to quantify the impact of drug-induced liver injury compounds.

Drug-induced liver injury (DILI) has become a major problem for patients and for clinicians, academics and the pharmaceutical industry. To date, existing hepatotoxicity test systems are only poorly predictive and the underlying mechanisms are still unclear. One of the factors known to amplify hepatotoxicity is the tumor necrosis factor alpha (TNFα), especially due to its synergy with commonly used drugs such as diclofenac. However, the exact mechanism of how diclofenac in combination with TNFα induces liver injury remains elusive. Here, we combined time-resolved immunoblotting and live-cell imaging data of HepG2 cells and primary human hepatocytes (PHH) with dynamic pathway modeling using ordinary differential equations (ODEs) to describe the complex structure of TNFα-induced NFκB signal transduction and integrated the perturbations of the pathway caused by diclofenac. The resulting mathematical model was used to systematically identify parameters affected by diclofenac. These analyses showed that more than one regulatory module of TNFα-induced NFκB signal transduction is affected by diclofenac, suggesting that hepatotoxicity is the integrated consequence of multiple changes in hepatocytes and that multiple factors define toxicity thresholds. Applying our mathematical modeling approach to other DILI-causing compounds representing different putative DILI mechanism classes enabled us to quantify their impact on pathway activation, highlighting the potential of the dynamic pathway model as a quantitative tool for the analysis of DILI compounds.

Quantitative single-molecule imaging of TLR4 reveals ligand-specific receptor dimerization.

In humans, invading pathogens are recognized by Toll-like receptors (TLRs). Upon recognition of lipopolysaccharide (LPS) derived from the cell wall of Gram-negative bacteria, TLR4 dimerizes and can stimulate two different signaling pathways, the proinflammatory, MyD88-dependent pathway and the antiviral, MyD88-independent pathway. The balance between these two pathways is ligand-dependent, and ligand composition determines whether the invading pathogen activates or evades the host immune response. We investigated the dimerization behavior of TLR4 in intact cells in response to different LPS chemotypes through quantitative single-molecule localization microscopy. Quantitative superresolved data showed that TLR4 was monomeric in the absence of its co-receptors MD2 and CD14 in transfected HEK 293 cells. When TLR4 was present together with MD2 and CD14 but in the absence of LPS, 52% of the receptors were monomeric and 48% were dimeric. LPS from Escherichia coli or Salmonella minnesota caused the formation of dimeric TLR4 complexes, whereas the antagonistic LPS chemotype from Rhodobacter sphaeroides maintained TLR4 in monomeric form at the cell surface. Furthermore, we showed that LPS-dependent dimerization was required for the activation of NF-κB signaling. Together, these data demonstrate ligand-dependent dimerization of TLR4 in the cellular environment, which could pave the way for a molecular understanding of biased signaling downstream of the receptor.

Biased signalling is an essential feature of TLR4 in glioma cells.

A distinct feature of the Toll-like receptor 4 (TLR4) is its ability to trigger both MyD88-dependent and MyD88-independent signalling, culminating in activation of pro-inflammatory NF-κB and/or the antiviral IRF3. Although TLR4 agonists (lipopolysaccharides; LPSs) derived from different bacterial species have different endotoxic activity, the impact of LPS chemotype on the downstream signalling is not fully understood. Notably, different TLR4 agonists exhibit anti-tumoural activity in animal models of glioma, but the underlying molecular mechanisms are largely unknown. Thus, we investigated the impact of LPS chemotype on the signalling events in the human glioma cell line U251. We found that LPS of Escherichia coli origin (LPSEC) leads to NF-κB-biased downstream signalling compared to Salmonella minnesota-derived LPS (LPSSM). Exposure of U251 cells to LPSEC resulted in faster nuclear translocation of the NF-κB subunit p65, higher NF-κB-activity and expression of its targets genes, and higher amount of secreted IL-6 compared to LPSSM. Using super-resolution microscopy we showed that the biased agonism of TLR4 in glioma cells is neither a result of differential regulation of receptor density nor of formation of higher order oligomers. Consistent with previous reports, LPSEC-mediated NF-κB activation led to significantly increased U251 proliferation, whereas LPSSM-induced IRF3 activity negatively influenced their invasiveness. Finally, treatment with methyl-ß-cyclodextrin (MCD) selectively increased LPSSM-induced nuclear translocation of p65 and NF-κB activity without affecting IRF3. Our data may explain how TLR4 agonists differently affect glioma cell proliferation and migration.

Receptor-ligand interactions: binding affinities studied by single-molecule and super-resolution microscopy on intact cells.

Protein­ligand interactions play an important role in many biological processes. Notably, membrane receptors are the starting point for a huge variety of cellular signal transduction pathways. Quantifying the binding affinity of a ligand for its transmembrane receptor is of great importance as it provides information on the potency of the ligand. We developed a new experimental procedure to determine binding affinities of ligands for their membrane receptors directly on intact single cells using super-resolution imaging. Dissociation constants were determined by titrating fluorophore-labelled ligand against cells expressing the target protein and applying single-molecule imaging.