Identification of Hydrocarbon Surfactants as Predominant Synthetic Ligands for L-FABP and PPARγ in Environmental Samples  

Obesity is a metabolic disorder that contributes to heart disease, diabetes, high blood pressure, high cholesterol, liver disease, sleep apnea and even certain cancers. Chemical contaminants in the environment can interact with signaling pathways involved in lipid metabolism including liver-fatty acid binding protein (L-FABP) which is involved in transport and metabolism of fatty acids within the liver and peroxisome proliferator-activated nuclear receptor γ (PPARγ) which regulates lipid metabolism and adipocyte differentiation. Interestingly, obesity has been linked in part to environmental factors, including exposure to chemical obesogens which can promote fat accumulation and obesity through various mechanisms including the disruption of L-FABP and PPARγ signaling pathways. There have been extensive efforts to identify specific ligands for L-FABP and PPARγ from complex environmental mixtures using conventional methods such as high-throughput screening and effect-directed analysis, however, they are limited in their ability to identify all potential ligands due to coelution issues and the large chemical diversity present in environmental samples.  To address this challenge, new study published in Journal Environmental Science & Technology and conducted by Dr. Yufeng Gong, Diwen Yang, Dr. Jiabao Liu, PhD candidate Holly Barrett, Jianxian Sun, and led by Professor Hui Peng from the Department of Chemistry at the University of Toronto, developed a novel method they named Protein Affinity Purification with Nontargeted Analysis (APNA) which uses tagged proteins to directly isolate ligands from environmental samples. Their study focused on identifying ligands of L-FABP and PPARγ in indoor dust and sewage sludge and managed to uncover previously unknown ligands that could disrupt lipid metabolism. The researchers employed APNA method to identify ligands of L-FABP and PPARγ in environmental samples. They overexpressed His-tagged full-length L-FABP and His-tagged PPARγ ligand binding domain in E. coli cells and used as bait to capture potential ligands from indoor dust and sewage sludge extracts. The extracts were prepared by extracting samples with methanol and reconstituting them for use.

The authors found the APNA method to be able to isolate potential ligands from both indoor dust and sewage sludge including identifying 83 nonredundant features as putative ligands for L-FABP and six features were isolated for PPARγ. The difference in large number of features isolated for L-FABP compared to PPARγ highlights its broad ligand specificity whereas the large number of features isolated for L-FABP highlights its broad ligand specificity while PPARγ is possibly has more selective binding. The team afterward used fluorescence displacement assay to confirm the binding of these isolated ligands where they employed 1-anilinonaphthalene-8-sulfonic acid (1,8-ANS) as a fluorescent probe that can be displaced by ligands binding to the protein which can be measured as a decrease in fluorescence signal. They determined the binding affinities (Kd values) of the ligands to L-FABP and observed strong binding for both natural fatty acids and synthetic hydrocarbon surfactants. Additionally, they assessed the binding of hydrocarbon surfactants to PPARγ and found they bind to PPARγ with affinities 5–20 times weaker than those to L-FABP. To further investigate the functional implications of these bindings, the researchers conducted a luciferase reporter cell assay to evaluate the activation of PPARγ transcriptional activity and found the hydrocarbon surfactants, dodecyl benzenesulfonate and dioctyl sulfosuccinate could activate PPARγ which highlight the potential of hydrocarbon surfactants to act as environmental obesogens. The team analyzed additional indoor dust and sewage sludge samples to quantify the concentrations of hydrocarbon surfactants in environmental samples and managed to detect multiple homologues and isomers of hydrocarbon surfactants such as alkylbenzenesulfonates at high concentration in both sample types. Moreover, the authors calculated the contributions of identified hydrocarbon surfactants to the total L-FABP and PPARγ binding activities in the environmental samples and found that these surfactants could explain a substantial portion of the observed activities with fatty acids the main ligands for L-FABP, whereas alkylbenzenesulfonates predominantly contributed to PPARγ activities.

In conclusion, Professor Hui Peng and colleagues successfully identified hydrocarbon surfactants found in industrial and household products and detected at high concentrations in indoor dust and sewage sludge as predominant synthetic ligands for L-FABP and PPARγ. The authors’ finding challenges the current perception of the safety of these hydrocarbon surfactants and call for reassessment of their potential health risks. Moreover, the identification of hydrocarbon surfactants as potential obesogens requires a thorough regulatory review of existing regulations and safety assessments for these chemicals specially the long-term health impacts and their role in dysregulation of lipid metabolism and obesity. Moreover, there is a need to promote safer alternatives of these surfactant obesogens and increase the awareness about the potential health risks associated with common household and industrial products containing these surfactants. Additionally, the new study highlights the importance of monitoring environmental concentrations of hydrocarbon surfactants and other potential obesogens  in indoor dust and sewage sludge to help track these contaminants for better risk assessment and management strategies.