Mitogen-Activated Protein Kinase Phosphatases: No Longer Undruggable?

Phosphatases and kinases maintain an equilibrium of dephosphorylated and phosphorylated proteins, respectively, that are required for critical cellular functions. Imbalance in this equilibrium or irregularity in their function causes unfavorable cellular effects that have been implicated in the development of numerous diseases. Protein tyrosine phosphatases (PTPs) catalyze the dephosphorylation of protein substrates on tyrosine residues, and their involvement in cell signaling and diseases such as cancer and inflammatory and metabolic diseases has made them attractive therapeutic targets. However, PTPs have proved challenging in therapeutics development, garnering them the unfavorable reputation of being undruggable. Nonetheless, great strides have been made toward the inhibition of PTPs over the past decade. Here, we discuss the advancement in small-molecule inhibition for the PTP subfamily known as the mitogen-activated protein kinase (MAPK) phosphatases (MKPs). We review strategies and inhibitor discovery tools that have proven successful for small-molecule inhibition of the MKPs and discuss what the future of MKP inhibition potentially might yield.

Defining the structure-activity relationship for a novel class of allosteric MKP5 inhibitors.

Mitogen-activated protein kinase (MAPK) phosphatase 5 (MKP5) is responsible for regulating the activity of the stress-responsive MAPKs and has been put forth as a potential therapeutic target for a number of diseases, including dystrophic muscle disease a fatal rare disease which has neither a treatment nor cure. In previous work, we identified Compound 1 (3,3-dimethyl-1-((9-(methylthio)-5,6-dihydrothieno[3,4-h]quinazolin-2-yl)thio)butan-2-one) as the lead compound of a novel class of MKP5 inhibitors. In this work, we explore the structure-activity relationship for inhibition of MKP5 through modifications to the scaffold and functional groups present in 1. A series of derivative compounds was designed, synthesized, and evaluated for inhibition of MKP5. In addition, the X-ray crystal structures of six enzyme-inhibitor complexes were solved, further elucidating the necessary requirements for MKP5 inhibition. We found that the parallel-displaced π-π interaction between the inhibitor three-ring core and Tyr435 is critical for modulating potency, and that modifications to the core and functionalization at the C-9 position are essential for ensuring proper positioning of the core for this interaction. These results lay the foundation from which more potent MKP5 allosteric inhibitors can be developed for potential therapeutics towards the treatment of dystrophic muscle disease.

An allosteric site on MKP5 reveals a strategy for small-molecule inhibition.

The mitogen-activated protein kinase (MAPK) phosphatases (MKPs) have been considered "undruggable," but their position as regulators of the MAPKs makes them promising therapeutic targets. MKP5 has been suggested as a potential target for the treatment of dystrophic muscle disease. Here, we identified an inhibitor of MKP5 using a p38α MAPK-derived, phosphopeptide-based small-molecule screen. We solved the structure of MKP5 in complex with this inhibitor, which revealed a previously undescribed allosteric binding pocket. Binding of the inhibitor to this pocket collapsed the MKP5 active site and was predicted to limit MAPK binding. Treatment with the inhibitor recapitulated the phenotype of MKP5 deficiency, resulting in activation of p38 MAPK and JNK. We demonstrated that MKP5 was required for TGF-ß1 signaling in muscle and that the inhibitor blocked TGF-ß1-mediated Smad2 phosphorylation. TGF-ß1 pathway antagonism has been proposed for the treatment of dystrophic muscle disease. Thus, allosteric inhibition of MKP5 represents a therapeutic strategy against dystrophic muscle disease.