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Retinol binding protein (RBP) is a member of the lipocalin family of proteins that is responsible for transporting vitamin A (retinol) in the blood to the target cells of the body. RBP is released from the liver as a complex with transthyretin (TTR) and the role of the complex is to prevent RBP being excreted by the kidney. RBP binds to a specific cell receptor (STRA6) which facilitates the uptake of retinol into the cell. Studies have suggested that elevated levels of serum RBP may be involved in preventing cellular responses to insulin which in turn leads to insulin resistance and eventually type 2 diabetes. This suggests that RBP could be a viable therapeutic target for treating type 2 diabetes. In this study a variety of structure-based ligand design (SBLD) methods, including virtual high-throughput screening (VFITS, using the docking tool eHiTS), de novo ligand design (using SPROUT) and shape comparison software (ROCS) were used along with X-ray crystallographic data, to design a variety of non-retinoid ligands targeted at RBP. Several small libraries of ligands were synthesised and the ligands were examined for their ability to bind to RBP along with their ability to disrupt the interactions of RBP with both TTR and STRA6. A number of ligands displayed a high binding affinity for RBP with ligands 13 and 19 being two of the most potent (KD = 200 and 343 nM respectively). Several ligands were found to disrupt the interaction between RBP and TTR with ligand 26 (EC50 = 891 nM) having the greatest effect. The study identified several ligands that disrupted the interaction between RBP and STRA6 with ligands 23 and 31 (97% and 100% respectively) having the greatest effect. Rationalisation of the results using the predicted binding poses of each ligand identified Tyr90 as being a key residue involved in making a hydrogen bond interaction with the ligand which resulted in the disruption of the RBP—STRA6 interaction. This study underlines the usefulness of SBLD for designing novel ligands that displayed a high affinity for RBP and as a consequence the ligands prevented RBP from binding to TTR and STRA6. |
