Protein tyrosine phosphatases as drug targets: PTP1B and beyond

Protein tyrosine phosphatase 1B (PTP1B) function, structure, and inhibition strategies to develop antidiabetic drugs

Tyrosine protein phosphatase non-receptor type 1 (PTP1B; also known as protein tyrosine phosphatase 1B) is a member of the protein tyrosine phosphatase (PTP) family and is a soluble enzyme that plays an essential role in different physiological processes, including the regulation of metabolism, specifically in insulin and leptin sensitivity. PTP1B is crucial in the pathogenesis of type 2 diabetes mellitus and obesity. These biological functions have made PTP1B validated as an antidiabetic and anti-obesity, and potentially anticancer, molecular target. Four main approaches aim to inhibit PTP1B: orthosteric, allosteric, bidentate inhibition, and PTPN1 gene silencing. Developing a potent and selective PTP1B inhibitor is still challenging due to the enzyme's ubiquitous expression, subcellular location, and structural properties. This article reviews the main advances in the study of PTP1B since it was first isolated in 1988, as well as recent contextual information related to the PTP family to which this protein belongs. Furthermore, we offer an overview of the role of PTP1B in diabetes and obesity, and the challenges to developing selective, effective, potent, bioavailable, and cell-permeable compounds that can inhibit the enzyme.

A Microfluidics-Based Mobility Shift Assay to Discover New Tyrosine Phosphatase Inhibitors

Protein tyrosine phosphatases (PTPs) play key roles in regulating tyrosine phosphorylation levels in cells. Since the discovery of PTP1B as a major drug target for diabetes and obesity, PTPs have emerged as a new and promising class of signaling targets for drug development in a variety of therapeutic areas. The routine use of generic substrate 6,8-difluoro-4-methylumbelliferyl phosphate (DiFMUP) in our hands led to the discovery of very similar and often not very selective molecules. Therefore, to increase the chances to discover novel chemical scaffolds, a side-by-side comparison between the DiFMUP assay and a chip-based mobility shift assay with a specific phosphopeptide was performed, on 1 PTP, using a focused set of compounds. Assay robustness and sensitivity were comparable for both the DiFMUP and mobility shift assays. The off-chip mobility shift assay required a longer development time because of identification, synthesis, and characterization of a specific peptide, and its cost per point was higher. However, although most potent scaffolds found with the DiFMUP assay were confirmed in the mobility shift format, the off-chip mobility shift assay led to the identification of previously unidentified chemical scaffolds with improved druglike properties.

Advances in understanding the interrelations between leptin resistance and obesity

Obesity, which has developed into a global epidemic, is a risk factor in most chronic diseases and some forms of malignancy. The discovery of leptin in 1994 has opened a new field in obesity research. Currently, we know that leptin is the primary signal from energy stores and exerts negative feedback effects on energy intake. However, most individuals with diet-induced obesity (DIO) develop leptin resistance, which is characterized by elevated circulating leptin levels and decreased leptin sensitivity. To date, though various mechanisms have been proposed to explain leptin resistance, the exact mechanisms of leptin resistance in obesity are poorly understood. Consequently, it's an important issue worth discussing regarding what the exact interrelations between leptin resistance and obesity are. Here, we review the latest advancements in the molecular mechanisms of leptin resistance and the exact interrelations between leptin resistance, obesity, and obesity-related diseases, in order to supply new ideas for the study of obesity.

Why nature really chose phosphate

Phosphoryl transfer plays key roles in signaling, energy transduction, protein synthesis, and maintaining the integrity of the genetic material. On the surface, it would appear to be a simple nucleophile displacement reaction. However, this simplicity is deceptive, as, even in aqueous solution, the low-lying d-orbitals on the phosphorus atom allow for eight distinct mechanistic possibilities, before even introducing the complexities of the enzyme catalyzed reactions. To further complicate matters, while powerful, traditional experimental techniques such as the use of linear free-energy relationships (LFER) or measuring isotope effects cannot make unique distinctions between different potential mechanisms. A quarter of a century has passed since Westheimer wrote his seminal review, 'Why Nature Chose Phosphate' (Science 235 (1987), 1173), and a lot has changed in the field since then. The present review revisits this biologically crucial issue, exploring both relevant enzymatic systems as well as the corresponding chemistry in aqueous solution, and demonstrating that the only way key questions in this field are likely to be resolved is through careful theoretical studies (which of course should be able to reproduce all relevant experimental data). Finally, we demonstrate that the reason that nature really chose phosphate is due to interplay between two counteracting effects: on the one hand, phosphates are negatively charged and the resulting charge-charge repulsion with the attacking nucleophile contributes to the very high barrier for hydrolysis, making phosphate esters among the most inert compounds known. However, biology is not only about reducing the barrier to unfavorable chemical reactions. That is, the same charge-charge repulsion that makes phosphate ester hydrolysis so unfavorable also makes it possible to regulate, by exploiting the electrostatics. This means that phosphate ester hydrolysis can not only be turned on, but also be turned off, by fine tuning the electrostatic environment and the present review demonstrates numerous examples where this is the case. Without this capacity for regulation, it would be impossible to have for instance a signaling or metabolic cascade, where the action of each participant is determined by the fine-tuned activity of the previous piece in the production line. This makes phosphate esters the ideal compounds to facilitate life as we know it.

Structure of human dual-specificity phosphatase 27 at 2.38 Å resolution

There are over 100 genes in the human genome that encode protein tyrosine phosphatases (PTPs) and approximately 60 of these are classified as dual-specificity phosphatases (DUSPs). Although many dual-specificity phosphatases are still not well characterized, novel functions have been discovered for some of them that have led to new insights into a variety of biological processes and the molecular basis for certain diseases. Indeed, as the functions of DUSPs continue to be elucidated, a growing number of them are emerging as potential therapeutic targets for diseases such as cancer, diabetes and inflammatory disorders. Here, the overexpression, purification and structure determination of DUSP27 at 2.38 Å resolution are presented.

Neuroprotection by radical avoidance: search for suitable agents

Neurodegeneration is frequently associated with damage by free radicals. However, increases in reactive oxygen and nitrogen species, which may ultimately lead to neuronal cell death, do not necessarily reflect its primary cause, but can be a consequence of otherwise induced cellular dysfunction. Detrimental processes which promote free radical formation are initiated, e.g., by disturbances in calcium homeostasis, mitochondrial malfunction, and an age-related decline in the circadian oscillator system. Free radicals generated at high rates under pathophysiological conditions are insufficiently detoxified by scavengers. Interventions at the primary causes of dysfunction, which avoid secondary rises in radical formation, may be more efficient. The aim of such approaches should be to prevent calcium overload, to reduce mitochondrial electron dissipation, to support electron transport capacity, and to avoid circadian perturbations. L-theanine and several amphiphilic nitrones are capable of counteracting excitotoxicity and/or mitochondrial radical formation. Resveratrol seems to promote mitochondrial biogenesis. Mitochondrial effects of leptin include attenuation of electron leakage. Melatonin combines all the requirements mentioned, additionally regulates anti- and pro-oxidant enzymes and is, with few exceptions, very well tolerated. In this review, the perspectives, problems and limits of drugs are compared which may be suitable for reducing the formation of free radicals.

Discovery of new potent human protein tyrosine phosphatase inhibitors via pharmacophore and QSAR analysis followed by in silico screening

A pharmacophoric model was developed for human protein tyrosine phosphatase 1B (h-PTP 1B) inhibitors utilizing the HipHop-REFINE module of CATALYST software. Subsequently, genetic algorithm and multiple linear regression analysis were employed to select an optimal combination of physicochemical descriptors and pharmacophore hypothesis that yield consistent QSAR equation of good predictive potential (r = 0.87,F-statistic = 69.13,r(BS)2 = 0.76,r(LOO)2 = 0.68). The validity of the QSAR equation and the associated pharmacophoric hypothesis was experimentally established by the identification of five new h-PTP 1B inhibitors retrieved from the National Cancer Institute (NCI) database.

Regulation of cytosolic PlA2 activity by PP1/PP2A serine/threonine phosphatases in human platelets

Platelet thromboxane A2 (TXA2) synthesis is an important pathway of platelet reactivity. We report that in thrombin-stimulated platelets, PP1/PP2A serine/threonine phosphatases regulate phospholipase A2 (cPLA2) activity, which is required for TXA2 synthesis. Two mechanisms are involved: (a) constitutively active PP1/PP2A regulate cPLA2 phosphorylation, and (b) PP1/PP2A activity mediates agonist-induced increase in cytosolic Ca2+ ([Ca2+]i). Inhibition of PP1/PP2A with okadaic acid (OA) induces cPLA2 phosphorylation but reduces Ca2+ responses: release from intracellular stores and influx through the plasma membrane, particularly that mediated by store-mediated Ca2+ entry (SMCE). A significant correlation (r = 0.64) exists between OA-regulated [Ca2+]i and TXA2 synthesis. Okadaic acid-induced decrease in SMCE and the associated TXA2 synthesis are mediated by a reduction in protein-tyrosine phosphorylation. This reduction is not due to inhibition of tyrosine kinases but rather to an OA-mediated increase in tyrosine phosphatases. This is the first study to report that PP1/PP2A phosphatases are involved in the regulation of the two key elements in eicosanoid synthesis, [Ca2+]i and cPLA2 phosphorylation. Moreover, PP1/PP2A regulation of [Ca2+]i and tyrosine phosphorylation may be important for other calcium-dependent processes and/or signal transduction mechanisms in platelets.

Genetic ablation of protein tyrosine phosphatase 1B accelerates lymphomagenesis of p53-null mice through the regulation of B-cell development

Protein tyrosine phosphatase 1B (PTP1B) is involved in multiple signaling pathways by down-regulating several tyrosine kinases. For example, gene-targeting studies in mice have established PTP1B as a critical physiologic regulator of metabolism by attenuating insulin signaling. PTP1B is an important target for the treatment of diabetes, because the PTP1B null mice are resistant to diet-induced diabetes and obesity. On the other hand, despite the potential for enhanced oncogenic signaling in the absence of PTP1B, PTP1B null mice do not develop spontaneous tumors. Because the majority of human cancers harbor mutations in p53, we generated p53/PTP1B double null mice to elucidate the role of PTP1B in tumorigenesis. We show that genetic ablation of PTP1B in p53 null mice decreases survival rate and increases susceptibility towards the development of B lymphomas. This suggested a role for PTP1B in lymphopoiesis, and we report that PTP1B null mice have an accumulation of B cells in bone marrow and lymph nodes, which contributed to the increased incidence of B lymphomas. The mean time of tumor development and tumor spectrum are unchanged in p53-/-PTP1B+/- mice. We conclude that PTP1B is an important determinant of the latency and type of tumors in a p53-deficient background through its role in the regulation of B-cell development.

The oxidative mechanism of action of ortho-quinone inhibitors of protein-tyrosine phosphatase α is mediated by hydrogen peroxide

Here, we report the identification and characterization of five ortho-quinone inhibitors of PTPalpha. We observed that the potency of these compounds in biochemical assays was markedly enhanced by the presence of DTT. A kinetic analysis suggested that they were functioning as irreversible inhibitors and that the inhibition was targeted to the catalytic site of PTPalpha. The inhibition observed by these compounds was sensitive to superoxide dismutase and catalase, suggesting that reactive oxygen species may be mediators of their inhibition. We observed that in the presence of DTT, these compounds would produce up to 2.5mM hydrogen peroxide (H(2)O(2)). The levels of H(2)O(2) produced were sufficient to completely inactivate PTPalpha. In contrast, without a reducing agent the compounds did not generate H(2)O(2) and showed little activity towards PTPalpha. In addition, these compounds inhibited PTPalpha-dependent cell spreading in NIH 3T3 cells at concentrations that were similar to their activity in biochemical assays. The biological implications of these results are discussed as they support growing evidence that H(2)O(2) is a key regulator of PTPs.

Probing protein-tyrosine phosphatase substrate specificity using a phosphotyrosine-containing phage library

Protein tyrosine phosphatases (PTPs) play important, highly dynamic roles in signaling. Currently about 90 different PTP genes have been described. The enzymes are highly regulated at all levels of expression, and it is becoming increasingly clear that substrate specificity of the PTP catalytic domains proper contributes considerably to PTP functionality. To investigate PTP substrate selectivity, we have set up a procedure to generate phage libraries that presents randomized, phosphotyrosine-containing peptides. Phages that expressed suitable substrates were selected by immobilized, substrate-trapping GST-PTP fusion proteins. After multiple rounds of selection, positive clones were confirmed by SPOT analysis, dephosphorylation by wild-type enzyme, and Km determinations. We have identified distinct consensus substrate motifs for PTP1B, Sap-1, PTP-beta, SHP1, and SHP2. Our results confirm substrate specificity for individual PTPs at the peptide level. Such consensus sequences may be useful both for identifying potential PTP substrates and for the development of peptidomimetic inhibitors.