Current Topics in Medicinal Chemistry - Volume 3, Issue 7, 2003

The human genome encodes approximately 100 phosphatases that belong to the protein tyrosine phosphatase (PTP) superfamily, whose substrates range from proteins to phosphoinositides and mRNAs. The hallmark for this superfamily is the active site sequence C(X)5R, also known as the PTP signature motif. The PTPs are key regulatory components in signal transduction pathways and the importance of PTPs in the control of cellular signaling is well established. Furthermore, there are compelling reasons to believe that PTP inhibitors may serve as novel medicinal agents for the treatment of various diseases. Based on structure and substrate specificity, the PTP super-family is divided into four distinct subfamilies: 1) pTyr specific PTPs, 2) dual specificity phosphatases, 3) Cdc25 phosphatases, and 4) LMW PTPs. The PTPs have similar core structures made of a central parallel β- sheet with flanking α-helices containing a β-loop-α loop that encompasses the PTP signature motif. Not surprisingly, they employ a common chemical mechanism for phosphate hydrolysis despite the differences in substrate specificity. Despite the conserved structural and catalytic properties, there are also sufficient differences in the active site pockets and its immediate surrounding environment among different PTPs. Further structural and mechanistic study will continue to be of considerable importance, providing a solid basis for inhibitor design.

The identification of autophosphorylation of the insulin receptor as a pivotal component in the signal transduction induced by insulin, initiated the hunt to identify the tyrosine phosphatase(s) that were responsible for regulating dephosphorylation, and thus inactivation of the receptor. Compelling evidence for the existence of an insulin receptor specific PTP has come from the remarkable phenotype of the PTP1B deficient mouse. PTP1B deficient mice display an insulin sensitive phenotype and are able to maintain glucose homeostasis with about half the level of circulating insulin. In response to insulin administration PTP1B deficient mice have a significant increase in insulin receptor phosphorylation in liver and muscle compared to wild type controls. Unexpectedly these animals were also resistant to diet induced obesity. These observations strongly support PTP1B as a negative regulator of insulin action, thereby making it an ideal therapeutic target for intervention in type 2 diabetes and obesity.

Recent studies have demonstrated that protein tyrosine phosphatase 1B (PTP1B) is involved in the down regulation of insulin signaling. Selective inhibitors of PTP1B hold much promise for the treatment of type 2 diabetes mellitus and obesity. Consequently much effort, by both industry and academia, has been devoted towards the development of PTP1B specific inhibitors. This article gives an overview of reports that have appeared in the primary scientific literature on the development of PTP1B inhibitors, starting from the days of early development up to September of 2002.

CD45 is expressed on all nucleated haematopoietic cells and was originally identified as the first and prototypic transmembrane protein tyrosine phosphatase (PTPase). CD45 has been extensively studied for over two decades as a PTPase that functions in antigen receptor signaling by dephosphorylation of Srckinases. CD45 can operate as a positive as well negative regulator of Src-family kinases. In CD45 mutant cell lines, CD45-deficient mice, and CD45-deficient human SCID patients, CD45 is required for signal transduction through antigen receptors. Our group has recently shown that CD45 can also function as a Janus kinase (JAK) tyrosine phosphatase that negatively regulates cytokine receptor signaling involved in the differentiation, proliferation, and antiviral immunity of haematopoietic cells. Moreover, a point mutation in CD45, implicated in affecting CD45 dimerization, and a genetic polymorphism that affects alternative CD45 splicing have been implicated in autoimmunity in mice and humans. CD45 is expressed in multiple isoforms and modulation of specific CD45 splice variants with antibodies can prevent transplant rejections. Moreover, loss of CD45 can affect microglia activation in a mouse model for Alzheimer's disease. Modulation of CD45 splice variants and CD45 activity might provide a unique opportunity to design drugs that turn off or turn-on antigen and cytokine receptor signaling in cancer, allergy, transplantation, or autoimmunity.

The protein-tyrosine phosphatase (PTP) CD45 serves both positive and negative signaling elements by dephosphorylating regulatory pTyr residues on Srcfamily protein-tyrosine kinases. Although its physiological participation in immune function makes it an important point of intervention for treatment of a variety of inflammatory and immune disorders, comparatively little has been reported on development of CD45 inhibitors. Frequently, when inhibitory data against CD45 is reported, the data has been generated secondarily to other target PTPs. The focus of the current review is to summarize the types of structures that have been found to inhibit CD45, even in cases the compounds themselves were designed as antagonists of other PTPs. The review's organization begins with generic broad spectrum PTP inhibitors and progresses from peptide-based inhibitors and small molecule peptide mimetics to inhibitors that have resulted from screening hits. Although potent and moderately selective CD45 inhibitors have been reported, no single dominant theme has yet emerged in the design of these CD45-directed agents.

The leukocyte common antigen-related protein, LAR, is a receptor-like protein tyrosine phosphatase (PTP) which has a wide tissue distribution. Post-translational processing cleaves the proprotein into two non-covalently associated subunits, an extracellular subunit resembling a cell adhesion molecule with three immunoglobulinlike domains and eight fibronectin III-like domains, and a phosphatase subunit containing a short extracellular domain, a transmembrane segment, and tandem cytoplasmic PTP catalytic domains. Current evidence supports a role for LAR in cadherin complexes where it associates with and dephosphorylates β-catenin, a pathway which may be critical for cadherin complex stability and cell-cell association. LAR also localizes to focal adhesions. Evidence strongly suggests that LAR is involved in axon guidance in the developing nervous system, being localized through association with α-liprins. Finally, considerable data support a role for LAR in negatively regulating the insulin receptor signaling. Now that targeting of specific PTPs for therapeutic inhibition is a reality, the clinically relevant pathways requiring LAR must be identified. Inhibition of LAR might improve insulin sensitivity in patients with insulin resistance and type 2 diabetes. Unfortunately, the LAR knockout mouse displays no improvement in insulin sensitivity but rather has defects in terminal mammary gland development and in basal forebrain cholinergic neurons. With LAR being implicated in diverse pathways, additional investigations are needed before clinical targets for therapeutic inhibition of LAR can be predicted. However, selective inhibitors of LAR would be valuable reagents to probe the function of LAR, particularly in animal studies where the most susceptible LAR-dependent pathway(s) must be determined.

This review discusses progress made over the past 10+ years in elucidating the properties, regulation, and function of protein tyrosine phosphatase alpha (PTPα). It is apparent from studies in knockout mice and diverse cell lines that the major action of PTPα is as a positive regulator of src and src family kinases. PTPα dephosphorylates and activates src. In this manner it affects transformation and tumourigenesis, inhibition of proliferation and cell cycle arrest, mitotic activation of src, integrin signaling, neuronal differentiation and outgrowth, and ion channel activity. PTPα may well modulate additional processes, including insulin signaling, and have other targets besides src family kinases. As an important modulator of several specific cell signaling pathways, PTPα has promise as a target for drug discovery. Continued research on the physiological and pathological activities of PTPα is necessary to define the therapeutic potential of PTPα-directed pharmacologicals.