Oxyesterol-binding protein family (OSBP/ORP) ligand binding and natural product drug development

Abstract

A structurally-diverse class of potent anti-cancer natural product compounds – cephalostatin 1, OSW-1, ritterazine B and schweinfurthin A -exert their anti-proliferative activity through binding to members of the oxysterol-binding protein (OSBP) and OSBP-related protein (ORP) family. Based on their shared OSBP/ORP cellular targets, these natural products are referred to as the ORPphilins. The ORPphilin compounds have been shown to bind with high affinity (Ki values = 20 – 70 nM) to two members of the OSBP/ORP family; namely OSBP and ORP4. This shared tight binding is remarkable due to the structurally-diversity of the ORPphilin natural product class of molecules, and the fact these compounds were isolated from very different biological sources. The cellular function(s) of the OSBP/ORPs are yet to be fully determined, but some of the family members apparently serve as sterol sensors and/or sterol transporting proteins. There are twelve different OSBP/ORPs in humans that show very different tissue distribution patterns. The expression levels and function of many different OSBP/ORPs have been implicated in in many disease-states, including different cancers, diabetes and cardiovascular disease. Despite their connections to disease, the OSBP/ORPs remain understudied with respect to their ligand binding and resulting biological function. The OSBP/ORP have been shown to experimentally bind oxysterols and phospholipids, but the physiological ligands for the OSBP/ORP members have not been identified. We have launched a systematic study of class-wide OSBP/ORP ligand binding, which includes understanding the binding interactions of the OSBP/ORPs with the ORPphilin natural products. Our 96-well OSBP/ORP ligand binding assay allows screening of many ligands against the complete panel of OSBP/ORP proteins, and the screen will incorporate a diverse set of oxysterols, phospholipids and ORPphilin-derived compounds. Through these comprehensive binding assays and subsequent biochemical studies, we will identify putative physiological ligands for individual OSBP/ORP family members. We will also begin to develop natural product-derived anti-cancer compounds, based on the ORPphilin compound OSW-1, that selectively target the anti-cancer target protein ORP4 over the other members of the OSBP/ORPs family.