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Mary A. Dwyer, Ryan Michalek, Peter Saloupis, Donald McDonnell, Goldis Malek; Oxidized Lipids Activate Aryl Hydrocarbon Receptor (AhR) and Differentially Regulate Metabolic Pathways in Retinal Pigment Epithelial Cells (RPE). Invest. Ophthalmol. Vis. Sci. 2011;52(14):3328.
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© ARVO (1962-2015); The Authors (2016-present)
AhR is a ligand-dependent transcription factor activated by environmental oxidants and plays a critical role in regulating the cellular response to toxicants. Previously we demonstrated that AhR and its binding partner, AhR nuclear translocator, are expressed in human RPE cells. We found that cigarette smoke pro-oxidants are agonists for AhR increasing expression of target genes involved in toxin metabolism and decreasing mitochondrial function. Significantly, since cigarette smoking is an established risk factor for age-related macular degeneration (AMD) in which mitochondrial dysfunction may play a central role. Here we investigated the effect of oxidized lipids on AhR activity and cell energy metabolic pathways.
Post-confluent ARPE19 cells were treated with native and oxidized lipids for 24 hours. AhR activity was determined using a luciferase-based reporter assay. Expression of AhR-regulated genes in treated cells and cells transfected with siRNA for AhR was determined by real-time PCR. Effect of AhR ligands on the activity of mitochondrial β-oxidation and glycolysis pathways was determined by monitoring palmitate oxidation and the conversion of [5-3H] glucose to [3H]H2O. Effects on expression of the glucose transporter, glycolysis enzyme hexokinase and several fatty acid oxidation genes was also determined.
Native and oxidized low density lipoprotein (LDL), arachidonic acid (AA), lipid peroxidation products HNE, HHE, and to a lesser extent docosahexanoic acid and oleic acid activated AhR and up-regulated AhR target genes including CYP1A1, CYP1B1, CYP2S1 and IL8. Decreased target gene expression was seen with AhR knockdown in treated cells. LDL and AA treatment resulted in an increase in glycolysis while oxLDL and HNE resulted in an increase in lipid oxidation.
Lipids are relevant AhR ligands. Studies show that abnormal accumulation of oxidized lipids occurs in Bruch’s membrane and sub-RPE deposits and may be involved in the pathogenesis of AMD. The high metabolic activity of photoreceptor outer segments and formation of free radicals are thought to implicate a high turnover rate of oxidatively modified lipids in RPE, including lipid peroxidation production of dietary poly-unsaturated fatty acids. Therefore, lipids are also potential injury stimuli to the RPE. AhR may be the signaling pathway through which these lipid oxidants drive deregulation of cell metabolic pathways resulting in RPE injury.
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