Activation of nuclear receptors (NR), for example the retinoid-X-receptors (RXR) or the liver-X-receptors (LXR), plays a crucial role as the molecular initiating event in the adverse outcome pathway for liver steatosis. The downstream biological consequences of NR interactions are still not fully understood, especially with multi-receptor-activating compounds and their mixtures. While the default assumption for mixture risk assessment is dose addition, the potential of combinations of synthetic RXR agonists to exert synergistic effects has been shown in the context of NR activation studies. The fact that RXR and LXR are heterodimerization partners raises the question whether combinations of LXR and RXR agonists may cause synergistic effects. Compounds with defined properties were chosen to examine their interactions regarding the activation of RXR and LXR, as well as the steatosis-related key events target gene activation and triglyceride accumulation, using the human HepaRG liver cell model. Synergistic effects were determined for cellular triglyceride accumulation, especially at high compound concentrations, as evaluated using five different mathematical models. Altered LXRα activation in the presence of RXR agonists was observed, and synergistic effects on LXR target genes were identified as a presumably underlying mechanism of the observed synergistic effect. These findings challenge the general validity of dose addition as the default assumption for mixture effects, and point toward the need for a mode of action-based risk assessment for chemical mixtures.
Keywords: Adverse outcome pathway; Liver steatosis; Liver-X-receptor; Mixtures; Nuclear receptor activation; Retinoid-X-receptor.