O-GlcNAc transferase (OGT) is a promiscuous covalent regulatory enzyme that affects diverse cellular processes by attaching O-linked N-acetylglucosamine (O-GlcNAc) residues to intracellular proteins including kinases, phosphatases, transcription factors, and histones. Though recent evidence suggests that OGT may be an anti-cancer target, developing a specific inhibitor that does not interfere with cell surface glycans is a significant challenge. In a recent HTS effort at Harvard Medical School Rodrigo F. Ortiz-Meoz, Suzanne Walker, and colleagues identified just such a molecule.
OGT acts as a nutrient sensor and recent studies have suggested that it contributes to reprogramming of cellular metabolism to enable rapid growth during tumorigenesis. Therefore development of specific inhibitors is of potential therapeutic value. Rodrigo et. al, had previously developed a glycosoltransferase assay method for OGT based on displacement of a fluorescent UDP-GlcNAc tracer from the enzyme using a fluorescence polarization readout. Based on the hits from their initial screen, they screened a focused set of quinolinone-6-sulfonamide (Q6S) compounds and produced analogs of the most potent hits. From these efforts, they produced OSMI-1, which inhibited full length human OGT enzymatic activity with an IC50 value of 2.7µM. Surprisingly, the potency of OSMI-1 inhibition was insensitive to UDP-GlcNAc, indicating that it was not competitive with the donor molecule.
OSMI-1 inhibited global O-GlcNAcylation in Chinese hamster ovary (CHO) cells within 2 hours; similar results were observed in several other mammalian cell lines. Further analysis revealed OSMI-1-dependent reduction of specific cellular markers of OGT. The investigators then analyzed the specificity of OSMI-1 using a panel of lectins to profile cell surface glycans and found that it had little effect on the lectin binding pattern on CHO cells in contrast with control inhibitors. OSMI-1 decreased cell viability by about 50%, though it was not clear that this was due to OGT inhibition.
Overall the researchers were able to describe a cell-permeable small molecules OGT inhibitor that was identified through HTS approaches. By using biased library and follow-up medicinal chemistry, they were able to increase the potency of initial screening hits by more than 100-fold and identify a compound that inhibits OGT in cells with on-target activity in cells. Optimizing and continuously using this scaffold may lead to more potent OGT inhibitors that can be implemented in animal model studies of OGT.
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