Activating ligands of Uncoupling protein 1 identified by rapid membrane protein thermostability shift analysis

Abstract

Objective: Uncoupling protein 1 (UCP1) catalyses mitochondrial proton leak in brown adipose tissue to facilitate nutrient oxidation for heat production, and may combat metabolic disease if activated in humans. During the adrenergic stimulation of brown adipocytes, free fatty acids generated from lipolysis activate UCP1 via an unclear interaction. Here, we set out to characterise activator binding to purified UCP1 to clarify the activation process, discern novel activators and the potential to target UCP1.

Methods: We assessed ligand binding to purified UCP1 by protein thermostability shift analysis, which unlike many conventional approaches can inform on the binding of hydrophobic ligands to membrane proteins. A detailed activator interaction analysis and screening approach was carried out, supported by investigations of UCP1 activity in liposomes, isolated brown fat mitochondria and UCP1 expression-controlled cell lines.

Results: We reveal that fatty acids and other activators influence UCP1 through a specific destabilising interaction, behaving as transport substrates that shift the protein to a less stable conformation of a transport cycle. Through the detection of specific stability shifts in screens, we identify novel activators, including the over-the-counter drug ibuprofen, where ligand analysis indicates that UCP1 has a relatively wide structural specificity for interacting molecules. Ibuprofen successfully induced UCP1 activity in liposomes, isolated brown fat mitochondria and UCP1-expressing HEK293 cells but not in cultured brown adipocytes, suggesting drug delivery differs in each cell type.

Conclusions: These findings clarify the nature of the activator-UCP1 interaction and demonstrate that the targeting of UCP1 in cells by approved drugs is in principle achievable as a therapeutic avenue, but requires variants with more effective delivery in brown adipocytes.

Keywords: Brown adipose tissue; Differential scanning fluorimetry; Energy expenditure; Ligand binding; Mitochondrial carrier; Proton transport; Thermal stability assay.