Carboxylesterase Notum Is a Druggable Target to Modulate Wnt Signaling

Abstract

Regulation of the Wnt signaling pathway is critically important for a number of cellular processes in both development and adult mammalian biology. This Perspective will provide a summary of current and emerging therapeutic opportunities in modulating Wnt signaling, especially through inhibition of Notum carboxylesterase activity. Notum was recently shown to act as a negative regulator of Wnt signaling through the removal of an essential palmitoleate group. Inhibition of Notum activity may represent a new approach to treat disease where aberrant Notum activity has been identified as the underlying cause. Reliable screening technologies are available to identify inhibitors of Notum, and structural studies are accelerating the discovery of new inhibitors. A selection of these hits have been optimized to give fit-for-purpose small molecule inhibitors of Notum. Three noteworthy examples are LP-922056 (26), ABC99 (27), and ARUK3001185 (28), which are complementary chemical tools for exploring the role of Notum in Wnt signaling.

Structural Insights into Notum Covalent Inhibition

“…Notum is a druggable target for Wnt signaling modulation. 23 We have previously determined a number of structures of Notum inhibitor complexes. 21 , 22 , 24 , 25 , 27 , 28 All of these inhibitors bind noncovalently at the Notum enzyme catalytic pocket, where the natural substrate PAM binds.…”

Small-Molecule Inhibitors of Carboxylesterase Notum

“…The most recent hit-finding strategy to discover new small-molecule inhibitors of Notum has been the application of virtual screening. There is now an abundance of structural information available for Notum [36] 16.6%, as 158 compounds showed >50% inhibition at 10 μM. These results demonstrate the value of virtual screening with well-trained docking models based on high-resolution structures.…”

“…The shorter ghrelin octanoyl (C8) lipid adopts a linear conformation in the center of the pocket (PDB: 6ZYF) [18]. There are a number of x-ray structures with small-molecule inhibitors bound to Notum, with most bound in this pocket (e.g., 15 [PDB: 6ZUV]) [35], but with significant variation in position and orientation of the ligand [36].…”

“…The determination that Notum is a negative regulator of Wnt signaling with a druggable pocket in 2015 has prompted significant efforts to identify small-molecule inhibitors of Notum carboxylesterase activity [36]. There are already a number of inhibitors of Notum (1-24) providing a few fit-for-purpose chemical tools (Table 1).…”

“…[Steadman For a discussion of small-molecule inhibitors of Notum, see [36]. ‡ There are several screening assay formats that have been used to identify and characterize inhibitors of Notum.…”