Targeting Tyrosine Phosphatases: Time to End the Stigma

Abstract

Protein tyrosine phosphatases (PTPs) are a family of enzymes essential for numerous cellular processes, and several PTPs have been validated as therapeutic targets for human diseases. Historically, the development of drugs targeting PTPs has been highly challenging, leading to stigmatization of these enzymes as undruggable targets. Despite these difficulties, efforts to drug PTPs have persisted, and recent years have seen an influx of new probes providing opportunities for biological examination of old and new PTP targets. Here we discuss progress towards drugging PTPs with special emphasis on the development of selective probes with biological activity. We describe the development of new small-molecule orthosteric, allosteric, and oligomerization-inhibiting PTP inhibitors and discuss new studies targeting the receptor PTP (RPTP) subfamily with biologics.

Keywords: allosteric; biologic; drug target; inhibitor; protein tyrosine phosphatase; small molecule.

Figure 1. Key Figure. Recent approaches for developing PTP-targeting drugs

Tyrosine phosphorylation occurs when PTPs hydrolytically remove phosphate (P) from Tyr amino acids (depicted as turquoise hexagon). The reaction involves transient covalent interaction with the PTP active-site nucleophile (Cys in Class I, II and III PTPs; Asp in aspartate-based PTPs; depicted in yellow) (a–c) Approaches in small-molecule PTP inhibitor development. (a) Orthosteric inhibitors bind to the enzyme active-site, and typically compete with substrate for binding. (b) Allosteric inhibitors bind outside of the enzyme active-site, inducing or stabilizing a catalytically unfavorable enzyme conformation. (c) Oligomerization inhibitors are being used to disrupt trimerization of PRL proteins. (d–e) Approaches in RPTP-targeted biologics development. (d) The RPTP CD45 has been the object of radioimmunotherapy strategies, which involve conjugation of an antibody to a radioactive agent for specific delivery of radiation to hematopoietic cells and tissues. (e) RPTPσ is being targeted with decoy biologics that mimic regions of the protein. A cell-penetrating wedge peptide mimetic targets the RPTPσ intracellular region. A decoy protein of the extracellular RPTPσ Ig1&2 domains targets RPTPσ by disrupting interactions with extracellular matrix proteoglycans (depicted as gray bar).

Figure 2. Targeting PTPs with orthosteric small-molecule inhibitors

(a) Reversible competitive inhibitors bind to enzymes with rapid association and dissociation rates at the site of substrate binding, and thus compete with substrate for binding to the enzyme. (b) Bidentate inhibitors consist of two chemical moieties, and bind to the enzyme active-site and a proximal secondary site (in some cases this can occur within the active-site). (c) Uncompetitive inhibitors bind to an enzyme after formation of an enzyme-substrate complex, preventing completion of catalysis. (d) Irreversible inhibitors modify the enzyme active-site, rendering the enzyme non-functional.

Figure 3. Targeting RPTPσ with biologics

(a) Schematic of RPTPσ protein domains. RPTPσ consists of extracellular, transmembrane, and intracellular regions. The extracellular region consists of amino-terminal immunoglobulin-like domains (Ig1–Ig3) and multiple fibronectin type III (FNIII) repeats. As described in (b–c), the Ig1&2 domains mediate RPTPσ interactions with proteoglycans in the extracellular matrix (ECM). The intracellular region consists of a juxtamembrane helix-loop-helix “wedge” motif and two PTP domains (D1 and D2). (b) Targeting RPTPσ with a cell-penetrating wedge peptide mimetic (ISP). In neuronal cells, RPTPσ Ig1&2 domains act as a receptor for chondroitin sulfate proteoglycans (CSPG), an ECM component that is abundant in scar tissue and inhibits axon regeneration. Treatment with ISP, a wedge peptide mimetic conjugated to a cell-penetrating transactivator of transcription (TAT) peptide, alleviates CSPG-mediated inhibition of axon extension into CSPG-rich scars. The mechanism of action of ISP is currently unknown, although it likely inhibits RPTPσ function. (c) Targeting RPTPσ with an Ig1&2 decoy protein. On the surface of fibroblast-like synoviocytes (FLS), RPTPσ Ig1&2 domains bind to the heparan sulfate proteoglycan syndecan-4, which inhibits RPTPσ by inducing oligomerization. Treatment with recombinant Ig1&2 protein disrupts the interaction between RPTPσ and syndecan-4, inhibiting the invasiveness and cartilage attachment of FLS through RPTPσ catalytic activity.