We are working in collaboration with the Brown/Goldstein lab to develop small molecules that aid in understanding the mechanism of cholesterol egress from lysosomes. After cholesterol is liberated from cholesteryl esters in the lysosome, this water-insoluble molecule binds to NPC-2, a soluble diffusible lysosomal protein that then transfers cholesterol to membrane-embedded NPC-1 in what is referred to as a hydrophobic hand-off.
The mechanism by which cholesterol leaves the lysosome after transfer to NPC-1 is poorly understood. U18666A is a cationic amphiphilic derivative of androstenolone, which was shown to inhibit the exit of LDL-derived cholesterol from lysosomes and thus represents a powerful tool for the study of lysosomal cholesterol transport.
Our lab has prepared over 50 derivatives of the U-compound useful for dissecting SAR and development of photoaffinity-based probes. A photoreactive derivative led to the specific cross-linking and pull-down of a few candidate proteins, one of which was characterized as NPC-1. Interestingly, the U-compound binds to the sterol-sensing domain and not the N-terminal domain that accepts the cholesterol from NPC-2 during the hydrophobic handoff.
These results were recently published in eLife. We are currently characterizing additional U-compound binding proteins and are also investigating the target of another class of very potent (low nM) small molecules that inhibit cholesterol exit from the lysosome including itraconazole and posaconazole.