Mass Spectrometry-Based Screening Reveals Inhibitors of Cholesterol 25-Hydroxylase.

Cholesterol 25-hydroxylase (CH25H) metabolizes cholesterol to 25-hydroxycholestrol (25HC) and plays a pathological role in osteoarthritis, Alzheimer's Disease, and other diseases. As an ER-resident transmembrane diiron lipid oxidase, CH25H remains a challenging enzyme to study, and no small molecule inhibitors of CH25H are available. As a first step toward developing CH25H inhibitors, we established a mass spectrometry-based cellular assay monitoring CH25H-mediated production of 25HC. Screening of this assay across a focused library of over 100 small molecules containing either an iron-coordinating moiety or a sterane ring system revealed three potent inhibitors of cellular CH25H activity (U73343, Ciclopirox, and phenanthroline). We additionally developed a secondary assay of CH25H function monitoring a transcriptional response confirmed to result from 25HC production. Finally, U73343 but not the iron-binding hits showed strong selectivity versus related diiron lipid oxidases. Overall, our work establishes a series of cell-based assays monitoring CH25H function and nominates first-in-class cell-active inhibitors of this disease-relevant enzyme.