A novel AhR ligand, 2AI, protects the retina from environmental stress

Retinal pigmented epithelium (RPE) cells are important for maintaining intercellular homeostasis in the retina. These cells form a barrier through the formation of tight junctions between neighboring pigmented epithelial cells, controlling the amount of nutrients, ions, and fluids between the neuroretina and the choroid1,2. One of the more significantly noted features of the RPE is the capacity to phagocytose and metabolize outer segments that are shed by the light-sensitive rod and cone photoreceptors3,4. Dysregulation of this function has a potential to play a role in the degeneration of the retina5. Overall, it has been ascertained that the functional disruption and atrophy of the RPE is a key factor in the progression of degenerative conditions in the retina, leading to the death of other cell types in the retina, including the rod and cone photoreceptors, resulting in significant vision loss6,7. Therefore, developing strategies to maintain the function and cellular homeostasis of the RPE is a significant point of investigation with regards to preventing retinal degeneration in humans. In this context, the Aryl hydrocarbon receptor (AhR) has been implicated to play a role in maintaining retinal homeostasis8,9. This transcription factor is a ligand-dependent Per-ARNT-Sim (PAS)/bHLH transcription factor that has been originally identified as the receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin (referred to as dioxin) and then cloned in the 1990’s10. Since then, AhR is critical for cellular responses to environmental stimuli through the induction of detoxification factors such as the cytochrome P450 enzymes11. AhR is most notably known to be activated in response to environmental stressors such as xenobiotic stimuli. It has also been shown to be active in response to endogenous metabolic products such as those found in the tryptophan oxidation pathway such as 6-formylindolo[3,2-b]carbazole (FICZ) and kynurenic acid12. It has also been shown that the knock-out of AhR in mice can lead to the degeneration of RPE cells in an age-dependent fashion8,9, suggesting a critical role for AhR in protecting RPE cells from chronic environmental stress. This indicates that molecules that activate AhR in degenerative conditions might promote retinal homeostasis. To study this we utilized in vitro assays to characterize the role of AhR signaling in RPE cell homeostasis. Canonical polyaromatic hydrocarbon ligands of AhR are not suitable drug candidates due to their numerous cytotoxic effects10. Based on previously known natural indole based ligands of AhR13, we identified a novel indole containing synthetic AhR-ligand 2,2′-aminophenyl indole (2AI) that potently induces the expression of the cytochrome P450, family 1a1, members (CYP1A1 and CYP1B1), and maintains RPE-cell viability in the presence of 4-hydroxynonenal (4HNE). Finally, we identified the omega-7 monounsaturated fatty acid commonly known as palmitoleic acid, as a downstream effector of 2AI, which we show to be protective against 4HNE treatment in human RPE cells and light-mediated toxicity in the murine retina.