Potent OGT inhibitors for the treatment of cancer and diabetic complications
First-in-class and potent OGT inhibitors
The Walker group has developed novel high throughput assays (HTS) and screened 200,000 compounds at the Institute for Chemistry and Cell Biology (ICCB) at Harvard Medical School, and identified several compound classes/cores that have the properties of OGT inhibitors. To identify other prospective leads, the scientists have evaluated 1400 commercially available compounds containing selected cores for improved OGT inhibition in vitro. So far, the Walker group has identified cell-permeable OGT inhibitors with IC50 values in 1-2 micromolar range, a 50-fold improvement over the original hits. Importantly, the Walker group possesses SAR data and several co-crystal structures of human OGT to guide the design of additional novel compounds.
O-GlcNAcation and breast cancer
Walker and collaborators have discovered a link between OGT and cancer cell growth and invasion (Caldwell et al., 2010). They demonstrated that OGT and O-GlcNAc levels are elevated in breast cancer cells. Also, they showed that OGT inhibitor (OGTi) reduces O-GlcNAcation and blocks growth and invasion of MCF-10A-ErbB2 cells in vitro.
O-GlcNAcation, PI3K/Akt signaling and insulin resistance
In a landmark study in 2008, Evans and coworkers reported a new mechanism of regulation of OGT. They discovered a unique binding site for phosphatidylinositol(3,4,5)trisphosphate (PIP3) that mapped to the C-terminus of OGT. PIP3 is important in several signaling cascades, serving as a second messenger. It was shown that upon insulin stimulation, PIP3 binds to OGT, recruiting it to the plasma membrane. OGT then decorates insulin signaling pathway proteins with sugars, impeding their activity and dampening the insulin response. One of such examples is the inhibition of insulin-stimulated phosphorylation of AktThr308. Overexpression of OGT in the liver of mice causes insulin resistance and dyslipidaemia. Abnormal O-GlcNAc modification of insulin signaling may therefore contribute to insulin resistance, obesity and type-2 diabetes.
Crystal structures of human OGT
In 2011, the Walker group reported two crystal structures of human OGT, as a binary complex with UDP (2.8Å resolution; referred to as OGT–UDP) and as a ternary complex with UDP and a well-characterized 14-residue CKII peptide substrate (1.95Å; referred to as OGT–UDP–peptide). The structures provide clues to the enzyme mechanism, show how OGT recognizes target peptide sequences, and reveal the fold of the unique domain between the two halves of the catalytic region.
Currently under study is a third crystal structure, a co-crystal of a lead OGT inhibitor and human OGT. The data show that the compound inhibits human O-GlcNAc transferase by an unprecedented mechanism.
The structure information will accelerate the rational design of biological experiments to investigate OGT’s functions. It will also help the design of inhibitors for use as cellular probes and help to assess its potential as a therapeutic target.
Intellectual Property Status: Patent(s) Pending
OGT- An essential enzyme for nutrient sensing and regulating cellular siegnaling pathways
The ability to sense and respond to nutrient levels is critical for the growth of all living systems. In eukaryotes, a major mechanism for nutrient sensing involves the essential protein glycosyltransferase OGT, which senses cellular glucose levels via UDP-GlcNAc concentrations, and responds by dynamically O-GlcNAcylating a wide range of proteins including numerous transcription factors, tumor suppressors, kinases, phosphatase and histone-modifying proteins. OGT catalyses the transfer of N-acetylglucosamine from UDP-N-acetylglucosamine (UDP-GlcNAc) to serines and threonines of protein sites. As many known O-GlcNAcylation sites are also phosphorylation sites, OGT is proposed to play a major role in modulating cellular kinase signaling cascades. OGT is also involved in widespread transcriptional regulation. Prolonged hyperglycaemia, such as occurs in diabetes, or excessive glucose uptake, such as occurs in cancer cells, results in hyper-O-GlcNAcylation of cellular proteins by OGT, and this increased O-GlcNAcylation has been linked to harmful cellular effects. Aberrant glycosylation by OGT has been linked to insulin resistance, diabetic complications, cancers and neurodegenerative diseases including Alzheimer’s. Thus, strategies to modulate OGT activity may have therapeutic value for treating diabetic complications, cancer, and other diseases.