Visualization of intermediate and transition-state structures in protein-tyrosine phosphatase catalysis

Structure-based prediction of free energy changes of binding of PTP1B inhibitors

The goals were (1) to understand the driving forces in the binding of small molecule inhibitors to the active site of PTP1B and (2) to develop a molecular mechanics-based empirical free energy function for compound potency prediction. A set of compounds with known activities was docked onto the active site. The related energy components and molecular surface areas were calculated. The bridging water molecules were identified and their contributions were considered. Linear relationships were explored between the above terms and the binding free energies of compounds derived based on experimental inhibition constants. We found that minimally three terms are required to give rise to a good correlation (0.86) with predictive power in five-group cross-validation test (q2 = 0.70). The dominant terms are the electrostatic energy and non-electrostatic energy stemming from the intra- and intermolecular interactions of solutes and from those of bridging water molecules in complexes.

Allosteric inhibition of protein tyrosine phosphatase 1B

Obesity and type II diabetes are closely linked metabolic syndromes that afflict >100 million people worldwide. Although protein tyrosine phosphatase 1B (PTP1B) has emerged as a promising target for the treatment of both syndromes, the discovery of pharmaceutically acceptable inhibitors that bind at the active site remains a substantial challenge. Here we describe the discovery of an allosteric site in PTP1B. Crystal structures of PTP1B in complex with allosteric inhibitors reveal a novel site located approximately 20 A from the catalytic site. We show that allosteric inhibitors prevent formation of the active form of the enzyme by blocking mobility of the catalytic loop, thereby exploiting a general mechanism used by tyrosine phosphatases. Notably, these inhibitors exhibit selectivity for PTP1B and enhance insulin signaling in cells. Allosteric inhibition is a promising strategy for targeting PTP1B and constitutes a mechanism that may be applicable to other tyrosine phosphatases.

The tyrosine phosphatase inhibitor orthovanadate mimics NGF-induced neuroprotective signaling in rat hippocampal neurons

Activation of the high affinity neurotrophin receptor tropomyosin-related kinase A (TrkA) by nerve growth factor (NGF) leads to phosphorylation of intracellular tyrosine residues of the receptor with subsequent activation of signaling pathways involved in neuronal survival such as the phosphoinositide-3-kinase (PI3-K)/protein kinase B (PKB/Akt) pathway and the mitogen-activated protein kinase (MAPK) cascade. In the present study, we tested whether inhibition of protein-tyrosine phosphatases (PTP) by orthovanadate could enhance tyrosine phosphorylation of TrkA thereby stimulating NGF-like survival signaling in embryonic hippocampal neurons. We found that the PTP inhibitor orthovanadate (1 microM) enhanced TrkA phosphorylation and protected neurons against staurosporine (STS)-induced apoptosis in a time-and concentration-dependent manner. Inhibition of PTP enhanced TrkA phosphorylation also in the presence of NGF antibodies indicating that NGF binding to TrkA was not required for the effects of orthovanadate. Moreover, orthovanadate enhanced phosphorylation of Akt and the MAPK Erk1/2 suggesting that the signaling pathways involved in the protective effect were similar to those activated by NGF. Accordingly, inhibition of PI3-K by wortmannin and MAPK-kinase (MEK) inhibition by UO126 abolished the neuroprotective effects. In conclusion, the results indicate that orthovanadate mimics the effect of NGF on survival signaling pathways in hippocampal neurons. Thus, PTP inhibition appears to be an appropriate strategy to trigger neuroprotective signaling pathways downstream of neurotrophin receptors.

Residue 182 influences the second step of protein-tyrosine phosphatase-mediated catalysis

Previous enzyme kinetic and structural studies have revealed a critical role for Asp181 (PTP1B numbering) in PTP (protein-tyrosine phosphatase)-mediated catalysis. In the E-P (phosphoenzyme) formation step, Asp181 functions as a general acid, while in the E-P hydrolysis step it acts as a general base. Most of our understanding of the role of Asp181 is derived from studies with the Yersinia PTP and the mammalian PTP1B, and to some extent also TC (T-cell)-PTP and the related PTPa and PTPe. The neighbouring residue 182 is a phenylalanine in these four mammalian enzymes and a glutamine in Yersinia PTP. Surprisingly, little attention has been paid to the fact that this residue is a histidine in most other mammalian PTPs. Using a reciprocal single-point mutational approach with introduction of His182 in PTP1B and Phe182 in PTPH1, we demonstrate here that His182-PTPs, in comparison with Phe182-PTPs, have significantly decreased kcat values, and to a lesser degree, decreased kcat/Km values. Combined enzyme kinetic, X-ray crystallographic and molecular dynamics studies indicate that the effect of His182 is due to interactions with Asp181 and with Gln262. We conclude that residue 182 can modulate the functionality of both Asp181 and Gln262 and therefore affect the E-P hydrolysis step of PTP-mediated catalysis.

Mechanistic studies on protein tyrosine phosphatases

The human genome encodes approximately 100 phosphatases that belong to the protein tyrosine phosphatase (PTP) superfamily. 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. Based on structure and substrate specificity, the PTP superfamily 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 beta-sheet with flanking a-helices containing a beta-loop-alpha-loop that encompasses the PTP signature motif. Site-directed mutagenesis of conserved amino acids in the Yersinia PTP and several other phosphatases in the PTP superfamily combined with detailed kinetic and mechanistic analyses have revealed a common chemical mechanism for phosphate hydrolysis despite the differences in substrate specificity. This article reviews our current knowledge of the common features important for PTP catalysis, the nature of the enzymatic transition state, and the roles of essential residues in transition stabilization. Future mechanistic studies of PTPs will focus on the use of physiological substrates to determine the molecular basis of substrate recognition and regulation, which is essential for understanding the specific functional role of PTPs in cellular signaling.

Vanadate-based transition-state analog inhibitors of Cre-LoxP recombination

Cre recombinase exchanges DNA strands at the LoxP recognition site via transphosphorylation reactions that involve pentacoordinate transition states. We demonstrate that meta-vanadate ion (VO(3)(-)) and appropriate DNA substrates assemble a transition-state analog-like complex in the Cre active site. Meta-vanadate inhibits recombination of LoxP-derived oligonucleotide substrates that contain a gap at either or both scissile phosphates, but does not inhibit reactions with intact LoxP. The 3(')-hydroxyl group of the gapped substrate is required for inhibition, suggesting that vanadate is ligated by three oxo ligands. Assembly of the inhibited complex is slow (t(1/2)=19min at 4mM NaVO(3)) and requires Cre, substrates, and meta-vanadate. Holliday junction intermediates accumulated at lower meta-vanadate concentrations, suggesting that the second strand exchange is inhibited more readily than the first. The apparent K(D) for meta-vanadate is 1.5-2mM and binding shows positive cooperativity. This methodology may have general application for mechanistic studies of recombinase/topoisomerase-mediated strand exchange reactions.

The noncatalytic domains of Lck regulate its dephosphorylation by CD45

The Src-family tyrosine kinase, Lck, contains two key regulatory phosphotyrosine residues, tyrosine 394 (Tyr-394) and tyrosine 505 (Tyr-505), both of which can be dephosphorylated by CD45. Here, the interaction of CD45 with its substrate, Lck, was determined to be complex, involving multiple interactions with both the catalytic and noncatalytic regions of Lck. CD45 preferentially dephosphorylated Tyr-394 over Tyr-505 in Lck. This was not due to sequence specificity surrounding the phosphotyrosine, but was due to the noncatalytic domains of Lck. The interactions with the noncatalytic domains of Lck and CD45 enhanced the dephosphorylation of Tyr-394 whereas intramolecular interactions within Lck reduced, but did not abolish, the dephosphorylation of Tyr-505. This demonstrates that the noncatalytic domains of Lck regulate the dephosphorylation of both Tyr-394 and Tyr-505 by CD45.

Mechanism of insulin sensitization by BMOV (bis maltolato oxo vanadium); unliganded vanadium (VO4) as the active component

Organovanadium compounds have been shown to be insulin sensitizers in vitro and in vivo. One potential biochemical mechanism for insulin sensitization by these compounds is that they inhibit protein tyrosine phosphatases (PTPs) that negatively regulate insulin receptor activation and signaling. In this study, bismaltolato oxovanadium (BMOV), a potent insulin sensitizer, was shown to be a reversible, competitive phosphatase inhibitor that inhibited phosphatase activity in cultured cells and enhanced insulin receptor activation in vivo. NMR and X-ray crystallographic studies of the interaction of BMOV with two different phosphatases, HCPTPA (human low molecular weight cytoplasmic protein tyrosine phosphatase) and PTP1B (protein tyrosine phosphatase 1B), demonstrated uncomplexed vanadium (VO(4)) in the active site. Taken together, these findings support phosphatase inhibition as a mechanism for insulin sensitization by BMOV and other organovanadium compounds and strongly suggest that uncomplexed vanadium is the active component of these compounds.

Functional characterization and crystal structure of the C215D mutant of protein-tyrosine phosphatase-1B

We have characterized the C215D active-site mutant of protein-tyrosine phosphatase-1B (PTP-1B) and solved the crystal structure of the catalytic domain of the apoenzyme to a resolution of 1.6 A. The mutant enzyme displayed maximal catalytic activity at pH approximately 4.5, which is significantly lower than the pH optimum of 6 for wild-type PTP-1B. Although both forms of the enzyme exhibited identical Km values for hydrolysis of p-nitrophenyl phosphate at pH 4.5 and 6, the kcat values of C215D were approximately 70- and approximately 7000-fold lower than those of wild-type PTP-1B, respectively. Arrhenius plots revealed that the mutant and wild-type enzymes displayed activation energies of 61 +/- 1 and 18 +/- 2 kJ/mol, respectively, at their pH optima. Unlike wild-type PTP-1B, C215D-mediated p-nitrophenyl phosphate hydrolysis was inactivated by 1,2-epoxy-3-(p-nitrophenoxy)propane, suggesting a direct involvement of Asp215 in catalysis. Increasing solvent microviscosity with sucrose (up to 40% (w/v)) caused a significant decrease in kcat/Km of the wild-type enzyme, but did not alter the catalytic efficiency of the mutant protein. Structurally, the apoenzyme was identical to wild-type PTP-1B, aside from the flexible WPD loop region, which was in both "open" and "closed" conformations. At physiological pH, the C215D mutant of PTP-1B should be an effective substrate-trapping mutant that can be used to identify cellular substrates of PTP-1B. In addition, because of its insensitivity to oxidation, this mutant may be used for screening fermentation broth and other natural products to identify inhibitors of PTP-1B.

An ecto-protein tyrosine phosphatase of Entamoeba histolytica induces cellular detachment by disruption of actin filaments in HeLa cells

Actin cytoskeleton disruption in host cells has been demonstrated for PTPases from pathogenic microorganisms. In this work, we analysed whether the secreted acid phosphatase from Entamoeba histolytica has phosphotyrosine phosphatase activity and the possibility that this activity may participate in damaging host cells. The secreted acid phosphatase of E. histolytica, which catalyses p-nitrophenyl phosphate hydrolysis at acid pH values, was found to have phosphotyrosine phosphatase activity. The enzymatic properties of phosphotyrosine phosphatase and acid phosphatase were virtually identical and included: Km values of 10 x 10(-4) M, no requirement for divalent cations, and sensitivity to molybdate, vanadate, and tungstate. The phosphotyrosyl phosphatase activity caused significant levels of cell rounding and detachment correlating with disruption of the actin stress fibres in HeLa cells. Thus, our data suggest that secreted phosphotyrosine phosphatase could play a cytotoxic role during amoebic infection.

mRNA capping enzyme requirement for Caenorhabditis elegans viability

Capping of the initiated 5' ends of RNA polymerase II products is evolutionarily and functionally conserved from yeasts to humans. The m(7)GpppN cap promotes RNA stability, processing, transport, and translation. Deletion of capping enzymes in yeasts was shown to be lethal due to rapid exonucleolytic degradation of uncapped transcripts or failure of capped but unmethylated RNA to initiate protein synthesis. Using RNA interference and Caenorhabditis elegans we have found that RNA capping is also essential for metazoan viability. C. elegans bifunctional capping enzyme was cloned, and capping activity by the expressed protein as well as growth complementation of yeast deletion strains missing either RNA triphosphatase or guanylyltransferase required terminal sequences not present in the previously isolated cel-1 clone. By RNA interference analysis we show that cel-1 is required for embryogenesis. cel-1(RNAi) embryos formed cytoplasmic granules characteristic of a phenocluster of RNA processing genes and died early in development.

The Caenorhabditis elegans mRNA 5'-capping enzyme. In vitro and in vivo characterization

Eukaryotic mRNA capping enzymes are bifunctional, carrying both RNA triphosphatase (RTPase) and guanylyltransferase (GTase) activities. The Caenorhabditis elegans CEL-1 capping enzyme consists of an N-terminal region with RTPase activity and a C-terminal region that resembles known GTases, However, CEL-1 has not previously been shown to have GTase activity. Cloning of the cel-1 cDNA shows that the full-length protein has 623 amino acids, including an additional 38 residues at the C termini and 12 residues at the N termini not originally predicted from the genomic sequence. Full-length CEL-1 has RTPase and GTase activities, and the cDNA can functionally replace the capping enzyme genes in Saccharomyces cerevisiae. The CEL-1 RTPase domain is related by sequence to protein-tyrosine phosphatases; therefore, mutagenesis of residues predicted to be important for RTPase activity was carried out. CEL-1 uses a mechanism similar to protein-tyrosine phosphatases, except that there was not an absolute requirement for a conserved acidic residue that acts as a proton donor. CEL-1 shows a strong preference for RNA substrates of at least three nucleotides in length. RNA-mediated interference in C. elegans embryos shows that lack of CEL-1 causes development to arrest with a phenotype similar to that seen when RNA polymerase II elongation activity is disrupted. Therefore, capping is essential for gene expression in metazoans.

Receptor protein tyrosine phosphatases in nervous system development

Receptor protein tyrosine phosphatases (RPTPs) are key regulators of neuronal morphogenesis in a variety of different vertebrate and invertebrate systems, yet the mechanisms by which these proteins regulate central nervous system development are poorly understood. In the past few years, studies have begun to outline possible models for RPTP function by demonstrating in vivo roles for RPTPs in axon outgrowth, guidance, and synaptogenesis. In addition, the crystal structures of several RPTPs have been solved, numerous downstream effectors of RPTP signaling have been identified, and a small number of RPTP ligands have been described. In this review, we focus on how RPTPs transduce signals from the extracellular environment to the cytoplasm, using a detailed comparative analysis of the different RPTP subfamilies. Focusing on the roles RPTPs play in the development of the central nervous system, we discuss how the elucidation of RPTP crystal structures, the biochemical analysis of phosphatase enzyme catalysis, and the characterization of complex signal transduction cascades downstream of RPTPs have generated testable models of RPTP structure and function.

K vitamins, PTP antagonism, and cell growth arrest

The main function of K vitamins is to act as co-factors for gamma-glutamyl carboxylase. However, they have also recently been shown to inhibit cell growth. We have chemically synthesized a series of K vitamin analogs with various side chains at the 2 or 3 position of the core naphthoquinone structure. The analogs with short thio-ethanol side chains are found to be more potent growth inhibitors in vitro of various tumor cell lines. Cpd 5 or [2-(2-mercaptoethanol)-3-methyl-1,4-naphthoquinone] is one of the most potent. The anti-proliferation activity of these compounds is antagonized by exogenous thiols but not by non-thiol antioxidants. This suggests that the growth inhibition is mediated by sulfhydryl arylation of cellular glutathione and cysteine-containing proteins and not by oxidative stress. The protein tyrosine phosphatases (PTP) are an important group of proteins that contain cysteine at their catalytic site. PTPs regulate mitogenic signal transduction and cell cycle progression. PTP inhibition by Cpd 5 results in prolonged tyrosine phosphorylation and activation of several kinases and transcription factors including EGFR, ERK1/2, and Elk1. Cpd 5 could activate ERK1/2 either by signaling from an activated EGFR, which is upstream in the signaling cascade, or by direct inhibition of ERK1/2 phosphatase(s). Prolonged ERK1/2 phosphorylation strongly correlates with Cpd 5-mediated growth inhibition. Cpd 5 can also bind to and inhibit the Cdc25 family of dual specific phosphatases. As a result, several Cdc25 substrates (Cdk1, Cdk2, Cdk4) involved in cell cycle progression are tyrosine phosphorylated and thereby inhibited by its action. Cpd 5 could also inhibit both normal liver regeneration and hepatoma growth in vivo. DNA synthesis during rat liver regeneration following partial hepatectomy, transplantable rat hepatoma cell growth, and glutathione-S-transferase-pi expressing hepatocytes after administration of the chemical carcinogen diethylnitrosamine, are all inhibited by Cpd 5 administration. The growth inhibitory effect during liver regeneration and transplantable tumor growth is also correlated with ERK1/2 phosphorylation induced by Cpd 5. Thus, Cpd 5-mediated inhibition of PTPs, such as Cdc25 leads to cell growth arrest due to altered activity of key cellular kinases involved in signal transduction and cell cycle progression. This prototype K vitamin analog represents a novel class of growth inhibitor based upon its action as a selective PTP antagonist. It is clearly associated with prolonged ERK1/2 phosphorylation, which is in contrast with the transient ERK1/2 phosphorylation induced by growth stimulatory mitogens.

All intermediates of the arsenate reductase mechanism, including an intramolecular dynamic disulfide cascade

The mechanism of pI258 arsenate reductase (ArsC) catalyzed arsenate reduction, involving its P-loop structural motif and three redox active cysteines, has been unraveled. All essential intermediates are visualized with x-ray crystallography, and NMR is used to map dynamic regions in a key disulfide intermediate. Steady-state kinetics of ArsC mutants gives a view of the crucial residues for catalysis. ArsC combines a phosphatase-like nucleophilic displacement reaction with a unique intramolecular disulfide bond cascade. Within this cascade, the formation of a disulfide bond triggers a reversible "conformational switch" that transfers the oxidative equivalents to the surface of the protein, while releasing the reduced substrate.

Protein tyrosine phosphatases: structure and function, substrate specificity, and inhibitor development

Protein tyrosine phosphatases (PTPs) are signaling enzymes that control a diverse array of cellular processes. Malfunction of PTP activity is associated with a number of human disorders. Recent genetic and biochemical studies indicate that PTPs represent a novel platform for drug discovery. Detailed knowledge of PTP substrate specificity and the wealth of structural data on PTPs provide a solid foundation for rational PTP inhibitor design. This review summarizes a correlation of PTP structure and function from mutagenesis experiments. The molecular basis for PTP1B and MKP3 substrate recognition is discussed. A powerful strategy is presented for creating specific and high-affinity bidentate PTP inhibitors that simultaneously bind both the active site and a unique adjacent site. Finally, recent advances in the development of potent and selective inhibitors for PTP1B and Cdc25 are described.

The catalytic mechanism of Cdc25A phosphatase

Cdc25 phosphatases are dual specificity phosphatases that dephosphorylate and activate cyclin-dependent kinases (CDKs), thereby effecting the progression from one phase of the cell cycle to the next. Despite its central role in the cell cycle, relatively little is known about the catalytic mechanism of Cdc25. In order to provide insights into the catalytic mechanism of Cdc25, we have performed a detailed mechanistic analysis of the catalytic domain of human Cdc25A. Our kinetic isotope effect results, Bronsted analysis, and pH dependence studies employing a range of aryl phosphates clearly indicate a dissociative transition state for the Cdc25A reaction that does not involve a general acid for the hydrolysis of substrates with low leaving group pK(a) values (5.45-8.05). Interestingly, our Bronsted analysis and pH dependence studies reveal that Cdc25A employs a different mechanism for the hydrolysis of substrates with high leaving group pK(a) values (8.68-9.99) that appears to require the protonation of glutamic acid 431. Mutation of glutamic acid 431 into glutamine leads to a dramatic drop in the hydrolysis rate for the high leaving group pK(a) substrates and the disappearance of the basic limb of the pH rate profile for the substrate with a leaving group pK(a) of 8.05, indicating that glutamic acid 431 is essential for the efficient hydrolysis of substrates with high leaving group pK(a). We suggest that hydrolysis of the high leaving group pK(a) substrates proceeds through an unfavored but more catalytically active form of Cdc25A, and we propose several models illustrating this. Since the activity of Cdc25A toward small molecule substrates is several orders of magnitude lower than toward the physiological substrate, cyclin-CDK, we suggest that the cyclin-CDK is able to preferentially induce this more catalytically active form of Cdc25A for efficient phosphothreonine and phosphotyrosine dephosphorylation.

Activation of p42 mitogen-activated protein kinase (MAPK), but not c-Jun NH(2)-terminal kinase, induces phosphorylation and stabilization of MAPK phosphatase XCL100 in Xenopus oocytes

Dual-specificity protein phosphatases are implicated in the direct down-regulation of mitogen-activated protein kinase (MAPK) activity in vivo. Accumulating evidence suggests that these phosphatases are components of negative feedback loops that restore MAPK activity to low levels after diverse physiological responses. Limited information exists, however, regarding their posttranscriptional regulation. We cloned two Xenopus homologs of the mammalian dual-specificity MAPK phosphatases MKP-1/CL100 and found that overexpression of XCL100 in G2-arrested oocytes delayed or prevented progesterone-induced meiotic maturation. Epitope-tagged XCL100 was phosphorylated on serine during G2 phase, and on serine and threonine in a p42 MAPK-dependent manner during M phase. Threonine phosphorylation mapped to a single residue, threonine 168. Phosphorylation of XCL100 had no measurable effect on its ability to dephosphorylate p42 MAPK. Similarly, mutation of threonine 168 to either valine or glutamate did not significantly alter the binding affinity of a catalytically inactive XCL100 protein for active p42 MAPK in vivo. XCL100 was a labile protein in G2-arrested and progesterone-stimulated oocytes; surprisingly, its degradation rate was increased more than twofold after exposure to hyperosmolar sorbitol. In sorbitol-treated oocytes expressing a conditionally active DeltaRaf-DD:ER chimera, activation of the p42 MAPK cascade led to phosphorylation of XCL100 and a pronounced decrease in the rate of its degradation. Our results provide mechanistic insight into the regulation of a dual-specificity MAPK phosphatase during meiotic maturation and the adaptation to cellular stress.

New chemically reactive dsDNAs containing single internucleotide monophosphoryldithio links: reactivity of 5'-mercapto-oligodeoxyribonucleotides

Novel modified DNA duplexes with single bridging 5'-SS-monophosphoryldithio links [-OP(=O)-O(-)-SS-CH(2)-] were synthesized by autoligation of an oligonucleotide 3'-phosphorothioate and a 5'-mercapto-oligonucleotide previously converted to a 2-pyridyldisulfide adduct. Monophosphoryldisulfide link formation is not a stringent template-dependent process under the conditions used and does not require strong binding of the reactive oligomers to the complementary strand. The modified internucleotide linkage, resembling the natural phosphodiester bond in size and charge density, is stable in water, easily undergoes thiol-disulfide exchange and can be specifically cleaved by the action of reducing reagents. DNA molecules containing an internal -OP(=O)-O(-)-SS-CH(2)- bridge are stable to spontaneous exchange of disulfide-linked fragments (recombination) even in the single-stranded state and are promising reagents for autocrosslinking with cysteine-containing proteins. The chemical and supramolecular properties of oligonucleotides with 5'-sulfhydryl groups were further characterized. We have shown that under the conditions of chemical ligation the 5'-SH group of the oligonucleotide has a higher reactivity towards N-hydroxybenzotriazole-activated phosphate in an adjacent oligonucleotide than does the OH group. This autoligation, unlike disulfide bond formation, proceeds only in the presence of template oligonucleotide, necessary to provide the activated phosphate in close proximity to the SH-, OH- or phosphate function.

Enhanced sensitivity of insulin-resistant adipocytes to vanadate is associated with oxidative stress and decreased reduction of vanadate (+5) to vanadyl (+4)

Vanadate (sodium orthovanadate), an inhibitor of phosphotyrosine phosphatases (PTPs), mimics many of the metabolic actions of insulin in vitro and in vivo. The potential of vanadate to stimulate glucose transport independent of the early steps in insulin signaling prompted us to test its effectiveness in an in vitro model of insulin resistance. In primary rat adipocytes cultured for 18 h in the presence of high glucose (15 mm) and insulin (10(-7) m), sensitivity to insulin-stimulated glucose transport was decreased. In contrast, there was a paradoxical enhanced sensitivity to vanadate of the insulin-resistant cells (EC(50) for control, 325 +/- 7.5 microm; EC(50) for insulin-resistant, 171 +/- 32 microm; p < 0.002). Enhanced sensitivity was also present for vanadate stimulation of insulin receptor kinase activity and autophosphorylation and Akt/protein kinase B Ser-473 phosphorylation consistent with more effective PTP inhibition in the resistant cells. Investigation of this phenomenon revealed that 1) depletion of GSH with buthionine sulfoximine reproduced the enhanced sensitivity to vanadate while preincubation of resistant cells with N-acetylcysteine (NAC) prevented it, 2) intracellular GSH was decreased in resistant cells and normalized by NAC, 3) exposure to high glucose and insulin induced an increase in reactive oxygen species, which was prevented by NAC, 4) EPR (electron paramagnetic resonance) spectroscopy showed a decreased amount of vanadyl (+4) in resistant and buthionine sulfoximine-treated cells, which correlated with decreased GSH and increased vanadate sensitivity, while total vanadium uptake was not altered, and 5) inhibition of recombinant PTP1B in vitro was more sensitive to vanadate (+5) than vanadyl (+4). In conclusion, the paradoxical increased sensitivity to vanadate in hyperglycemia-induced insulin resistant adipocytes is due to oxidative stress and decreased reduction of vanadate (+5) to vanadyl (+4). Thus, sensitivity of PTP inhibition and glucose transport to vanadate is regulated by cellular redox state.

Orally active insulin mimics: where do we stand now?

The war against diabetes through the development of new drugs is an ongoing continuous process to counter the alarming global increase in the prevalence of diabetes and its complications, particularly in developing countries like India. Unfortunately, the speed with which our knowledge of diabetes and its effects is expanding is not matched by the availability of new drugs. Following the identification of the insulin receptor (IR), its intrinsic kinase activity and molecular cloning, many studies have looked at IR as an ideal drug target. This review summarizes in brief the latest advancements in this field with particular reference to the current situation in respect of the development of orally active insulin mimetics in the treatment of type 2 diabetes.

The mechanism of dephosphorylation of extracellular signal-regulated kinase 2 by mitogen-activated protein kinase phosphatase 3

The mitogen-activated protein (MAP) kinase phosphatase-3 (MKP3) is a dual specificity phosphatase that specifically inactivates one subfamily of MAP kinases, the extracellular signal-regulated kinases (ERKs). Inactivation of MAP kinases occurs by dephosphorylation of Thr(P) and Tyr(P) in the TXY kinase activation motif. To gain insight into the mechanism of ERK2 inactivation by MKP3, we have carried out an analysis of the MKP3-catalyzed dephosphorylation of the phosphorylated ERK2. We find that ERK2/pTpY dephosphorylation by MKP3 involves an ordered, distributive mechanism in which MKP3 binds the bisphosphorylated ERK2/pTpY, dephosphorylates Tyr(P) first, dissociates and releases the monophosphorylated ERK2/pT, which is then subjected to dephosphorylation by a second MKP3, yielding the fully dephosphorylated ERK2. The bisphosphorylated ERK2 is a highly specific substrate for MKP3 with a k(cat)/K(m) of 3.8 x 10(6) m(-1) s(-1), which is more than 6 orders of magnitude higher than that for small molecule aryl phosphates and an ERK2-derived phosphopeptide encompassing the pTEpY motif. This strikingly high substrate specificity displayed by MKP3 may result from a combination of high affinity binding interactions between the N-terminal domain of MKP3 and ERK2 and specific ERK2-induced allosteric activation of the MKP3 C-terminal phosphatase domain.

Crystal structure of PTP-SL/PTPBR7 catalytic domain: implications for MAP kinase regulation

Protein tyrosine phosphatases PTP-SL and PTPBR7 are isoforms belonging to cytosolic membrane-associated and to receptor-like PTPs (RPTPs), respectively. They represent a new family of PTPs with a major role in activation and translocation of MAP kinases. Specifically, the complex formation between PTP-SL and ERK2 involves an unusual interaction leading to the phosphorylation of PTP-SL by ERK2 at Thr253 and the inactivating dephosphorylation of ERK2 by PTP-SL. This interaction is strictly dependent upon a kinase interaction motif (KIM) (residues 224-239) situated at the N terminus of the PTP-SL catalytic domain. We report the first crystal structure of the catalytic domain for a member of this family (PTP-SL, residues 254-549, identical with residues 361-656 of PTPBR7), providing an example of an RPTP with single cytoplasmic domain, which is monomeric, having an unhindered catalytic site. In addition to the characteristic PTP-core structure, PTP-SL has an N-terminal helix, possibly orienting the KIM motif upon interaction with the target ERK2. An unusual residue in the catalytically important WPD loop promotes formation of a hydrophobically and electrostatically stabilised clamp. This could induce increased rigidity to the WPD loop and therefore reduced catalytic activity, in agreement with our kinetic measurements. A docking model based on the PTP-SL structure suggests that, in the complex with ERK2, the phosphorylation of PTP-SL should be accomplished first. The subsequent dephosphorylation of ERK2 seems to be possible only if a conformational rearrangement of the two interacting partners takes place.

Structure and mechanism of the RNA triphosphatase component of mammalian mRNA capping enzyme

The 5' capping of mammalian pre-mRNAs is initiated by RNA triphosphatase, a member of the cysteine phosphatase superfamily. Here we report the 1.65 A crystal structure of mouse RNA triphosphatase, which reveals a deep, positively charged active site pocket that can fit a 5' triphosphate end. Structural, biochemical and mutational results show that despite sharing an HCxxxxxR(S/T) motif, a phosphoenzyme intermediate and a core alpha/beta-fold with other cysteine phosphatases, the mechanism of phosphoanhydride cleavage by mammalian capping enzyme differs from that used by protein phosphatases to hydrolyze phosphomonoesters. The most significant difference is the absence of a carboxylate general acid catalyst in RNA triphosphatase. Residues conserved uniquely among the RNA phosphatase subfamily are important for function in cap formation and are likely to play a role in substrate recognition.

Mutational and kinetic evaluation of conserved His-123 in dual specificity protein-tyrosine phosphatase vaccinia H1-related phosphatase: participation of Tyr-78 and Thr-73 residues in tuning the orientation of His-123

Active-site cysteine strategically positioned in the P-loop of protein-tyrosine phosphatases has been suggested to be further stabilized by hydrogen bonding arrays radiating out from the P-loop to neighboring residues. In this work, we investigated the structural role of histidine array in HC(X)(5)RS motif of the vaccinia H1-related protein phosphatase (VHR), using site-directed mutagenesis in conjunction with an extensive kinetic analysis. Conserved His-123 was mutated along with neighboring residues Tyr-78 and Thr-73. The increased pK(a) values of active-site Cys-124 found in Y78F and T73A mutants (6.51 and 6.75, respectively) were comparable to those of H123A and H123F mutants. Kinetic evaluation of Y78F and T73A mutants further implicates that the mutations perturb the relative position of Cys-124 within the P-loop. These results imply that Tyr-78 and Thr-73 make up an essential part of the His-123 array and structurally tune the Cys-124 position. Tyr-78 of VHR turns out to be the invariant Tyr reported in several protein-tyrosine phosphatases by a structure-based sequence alignment. Therefore, orientation of the imidazole ring of His-123 by the invariant Tyr-78 is crucial for maintaining the proper position of Cys-124 in the P-loop.

Inhibition of cell growth and spreading by stomach cancer-associated protein-tyrosine phosphatase-1 (SAP-1) through dephosphorylation of p130cas

SAP-1 (stomach cancer-associated protein-tyrosine phosphatase-1) is a transmembrane-type protein-tyrosine phosphatase that is abundant in the brain and certain cancer cell lines. With the use of a "substrate-trapping" approach, p130(cas), a major focal adhesion-associated phosphotyrosyl protein, has now been identified as a likely physiological substrate of SAP-1. Expression of recombinant SAP-1 induced the dephosphorylation of p130(cas) as well as that of two other components of the integrin-signaling pathway (focal adhesion kinase and p62(dok)) in intact cells. In contrast, expression of a substrate-trapping mutant of SAP-1 induced the hyperphosphorylation of these proteins, indicating a dominant negative effect of this mutant. Overexpression of SAP-1 induced disruption of the actin-based cytoskeleton as well as inhibited various cellular responses promoted by integrin-mediated cell adhesion, including cell spreading on fibronectin, growth factor-induced activation of extracellular signal-regulated kinase 2, and colony formation. Finally, the enzymatic activity of SAP-1, measured with an immunocomplex phosphatase assay, was substantially increased by cell-cell adhesion. These results suggest that SAP-1, by mediating the dephosphorylation of focal adhesion-associated substrates, negatively regulates integrin-promoted signaling processes and, thus, may contribute to contact inhibition of cell growth and motility.

Insulin-like growth factor-1 (IGF-1): a neuroprotective trophic factor acting via the Akt kinase pathway

Insulin-like growth factor-I (IGF-I) is a pleiotropic polypeptide with a wide range of actions in both central and peripheral nervous sytems. Over the past few years, we studied the trophic as well as neuromodulatory roles of IGF-I in the brain. Accumulated evidence indicates that IGF-I, apart from regulating growth and development, protects neurons against cell death induced by amyloidogenic derivatives, glucose or serum deprivation via the activation of intracellular pathways implicating phosphatidylinositide 3/Akt kinase, winged-helix family of transcription factor FKHRL1 phosphorylation or production of free radicals. The effects of IGF-I on neuroprotection, glucose metabolism and activity-dependent plasticity suggest the potential usefulness of this growth factor or related mimetics in the treatment of Alzheimer's disease and other neurodegenerative disorders.

Development of a robust scintillation proximity assay for protein tyrosine phosphatase 1B using the catalytically inactive (C215S) mutant

Protein tyrosine phosphatases are a class of enzymes that function to modulate tyrosine phosphorylation of cellular proteins and play an essential role in regulating cell function. PTP1B has been implicated in the negative regulation of the insulin signaling pathway by dephosphorylating the activated insulin receptor. Inhibiting this phosphatase and preventing the insulin-receptor downregulation has been suggested as a target for the treatment of Type II diabetes. A high-throughput screen for inhibitors of PTP1B was developed using a scintillation proximity assay (SPA) with GST-- or FLAG--PTP1B((1-320)) and a potent [(3)H]-tripeptide inhibitor. The problem of interference from extraneous oxidizing and alkylating agents which react with the cysteine active-site nucleophile was overcome by the use of the catalytically inactive C215S form of the native enzyme (GST--PTP1B(C215S)). The GST--PTP1B was linked to the protein A scintillation bead via GST antibody. The radiolabeled inhibitor when bound to the enzyme gave a radioactive signal that was competed away by the unknown competitive compounds. Further utility of PTP1B(C215S) was demonstrated by mixing in the same well both the catalytically inactive GST--PTP1B(C215S) and the catalytically active FLAG--CD45 with an inhibitor. Both a binding and kinetic assay was then performed in the same 96-well plate with the inhibition results determined for the PTP1B(C215S) (binding assay) and CD45 (activity assay). In this way inhibitors could be differentiated between the two phosphatases under identical assay conditions in one 96-well assay plate. The use of a mutant to reduce interference in a binding assay and compare with activity assays is also amenable for most cysteine active-site proteases.

Intramolecular interactions in protein tyrosine phosphatase RPTPmu: kinetic evidence

The receptor-like protein tyrosine phosphatase RPTPmu contains three intracellular domains: the juxtamembrane (JM) and two phosphatase domains (D1 and D2). D1 is catalytically active in vitro. The functional roles of JM and D2 are still unclear. To find out whether and how they modulate the phosphatase activity of D1, we compared the enzymatic characteristics of two constructs, containing a truncated JM and either D1 or both phosphatase domains. p-Nitrophenyl phosphate and two peptide substrates were efficiently dephosphorylated by both constructs. The specificity constant of D1 alone was up to 50% higher. D2 induces (a) decreased K(m) values for peptide substrates, (b) decreased catalytic efficiency for these substrates, (c) shifting of the optimal pH to slightly lower values, and (d) looser binding of competitive inhibitors. These data suggest that the phosphatase activity of D1 is negatively modulated and its ligand binding capacity is sensibly modified by domain D2, having possible functional significance.

Rat testicular myotubularin, a protein tyrosine phosphatase expressed by Sertoli and germ cells, is a potential marker for studying cell-cell interactions in the rat testis

The full-length cDNA encoding the entire open reading frame (ORF) of rat myotubularin (rMTM) was isolated from a rat testis expression library by PCR. Among the three approximately 2.9-kb cDNAs that were sequenced, one clone was different from the other two clones. It contained seven extra amino acids of FVVLNLQ; this short stretch of extra sequence was found between Gln(421) and Phe(422) within the SET (Suvar3-9, Enhancer-of-zeste, Trithorax) interacting domain (SID) of rMTM. The rMTM ORF had 1,713 bp encoding for a 571 amino acid polypeptide and a calculated molecular weight of 65.8 kDa. A comparison between its deduced amino acid sequence and the GenBank database using BLAST revealed a 53.1% identity with human myotubularin protein (hMTM1), which is a member of the protein tyrosine phosphatase (PTP) family associated with X-linked myotubular myopathy. A 22 amino acid peptide NH(2)-TKVNERYELCDTYPALLAVPAN was synthesized based on the deduced amino acid sequence of rMTM and used for antibody production. By using immunoblot analysis, a 66-kDa protein was indeed detected in both Sertoli and germ-cell cytosols. rMTM mRNA was found in various tissues but was predominantly expressed in the testis, ovary, and skeletal muscle. Sertoli cell rMTM expression was stimulated by germ cells and enhanced when inter-Sertoli junctions were being assembled in vitro. A drastic reduction in testicular rMTM steady-state mRNA level correlated with the depletion of germ cells from the testis in vivo following either glycerol or lonidamine treatment. These results indicate that rMTM is a rat homologue of hMTM1 that may be a useful marker in monitoring the events of cell-cell interactions in the testis.

Mechanism of phosphoanhydride cleavage by baculovirus phosphatase

Baculovirus phosphatase (BVP) is a member of the metazoan RNA triphosphatase enzyme family that includes the RNA triphosphatase component of the mRNA capping apparatus. BVP and other metazoan RNA triphosphatases belong to a superfamily of phosphatases that act via the formation and hydrolysis of a covalent cysteinyl-phosphate intermediate. Here we demonstrate the formation of a BVP phosphoenzyme upon reaction with [gamma-(32)P]ATP and identify the linkage as a thiophosphate based on its chemical lability. We surmise that the phosphate is linked to Cys(119) of BVP because replacement of Cys(119) by alanine or serine abrogates phosphoenzyme formation and phosphohydrolase activity. The catalytic cysteine is situated within a conserved phosphate-binding loop ((118)HCTHGINRTGY(128)). We show that all of the non-aliphatic side chains of the phosphate-binding loop are functionally important, insofar as mutants H118A, H121A, N124A, R125A, T126A, and Y128A were inactive in gamma phosphate hydrolysis and the T120A mutant was 7% as active as wild-type BVP. Structure-activity relationships at the essential positions of the phosphate-binding loop were elucidated by conservative substitutions. A conserved aspartic acid (Asp(60)) invoked as a candidate general acid catalyst was dispensable for phosphohydrolase activity and phosphoenzyme formation by BVP. We propose that the low pK(a) of the bridging oxygen of the beta phosphate leaving group circumvents a requirement for expulsion by a proton donor during attack by cysteine on the gamma phosphorus. In contrast, a conserved aspartic acid is essential for the phosphomonoesterase reactions catalyzed by protein phosphatases, where the serine or tyrosine leaving groups have a much higher pK(a) than does ADP.

The transport/phosphorylation of N,N'-diacetylchitobiose in Escherichia coli. Characterization of phospho-IIB(Chb) and of a potential transition state analogue in the phosphotransfer reaction between the proteins IIA(Chb) AND IIB(Chb)

Enzyme II permeases of the phosphoenolpyruvate:glycose phosphotransferase system comprise one to five separately encoded polypeptides, but most contain similar domains (IIA, IIB, and IIC). The phosphoryl group is transferred from one domain to another, with histidine as the phosphoryl acceptor in IIA and cysteine as the acceptor in certain IIB domains. IIB(Chb) is a phosphocarrier in the uptake/phosphorylation of the chitin disaccharide, (GlcNAc)(2) by Escherichia coli and is unusual because it is separately encoded and soluble. Both the crystal and solution structures of a IIB(Chb) mutant (C10S) have been reported. In the present studies, homogeneous phospho-IIB(Chb) was isolated, and the phosphoryl-Cys linkage was established by (31)P NMR spectroscopy. Rate constants for the hydrolysis of phospho-IIB(Chb) plotted versus pH gave the same shape peak reported for the model compound, butyl thiophosphate, but was shifted about 4 pH units. Evidence is presented for a stable complex between homogeneous Cys10SerIIB(Chb) (which cannot be phosphorylated) and phospho-IIA(Chb), but not with IIA(Chb). The complex (a tetramer (3)) contains equimolar quantities of the two proteins and has been chemically cross-linked. It appears to be an analogue of the transition state complex in the reaction: phospho-IIA(Chb) + IIB(Chb) <--> IIA(Chb) + phospho-IIB(Chb). This is apparently the first report of the isolation of a transition state analogue in a protein-protein phosphotransfer reaction.

Protein tyrosine phosphatase-1B in diabetes

A role for protein tyrosine phosphatases in the negative regulation of insulin signaling and a putative involvement in the insulin resistance associated with type 2 diabetes have been postulated since their discovery. The recent demonstration that mice lacking the protein tyrosine phosphatase-1B (PTP-1B) have enhanced insulin sensitivity validates this. Furthermore, when fed a high fat diet, these mice maintained insulin sensitivity and were resistant to obesity, suggesting that inhibition of PTP-1B activity could be a novel way of treating type 2 diabetes and obesity. This commentary reviews our current knowledge of PTP-1B in insulin signaling and its role in diabetes and discusses the development of potent and selective PTP-1B inhibitors.

Effects of metal ions on the activity of protein tyrosine phosphatase VHR: highly potent and reversible oxidative inactivation by Cu2+ ion

The posttranslational regulation of protein tyrosine phosphatases (PTPs) has been suggested to have a crucial role in maintaining the phosphotyrosine level in cells. Here we examined the regulatory effects of metal ions on human dual-specificity vaccinia H1-related protein tyrosine phosphatase (VHR) in vitro. Among various metal ions examined, Fe3+, Cu2+, Zn2+, and Cd2+ exerted their inactivational effects on VHR, and Cu2+ is the most potent inactivator. The VHR activity inactivated by the metal ions except Cu2+ was significantly restored by EDTA. The efficacy of Cu2+ for the VHR inactivation was about 200-fold more potent than that of H2O2. Cu2+ also inactivated other PTPs including PTP1B and SHP-1. The Cu2+-mediated inactivation at the submicromolar range was eradicated by dithiothreitol treatment. The loss of VHR activity correlated with the decreased [14C]iodoacetate labeling of active-site cysteine, suggesting that Cu2+ brought about the oxidation of the active-site cysteine. On the contrary, Zn2+ that exerted an inactivational effect at millimolar concentrations appeared not directly linked to the active-site cysteine, as indicated by the fact that [14C]iodoacetate labeling was unaffected and that the effect of Zn2+ on the Y78F mutant was increased. The reduction potential of VHR was estimated to be -331 mV by utilizing the reversibility of the redox state of VHR. Thus, we conclude that the highly potent Cu2+ inactivation of VHR is a consequence of the oxidation of the active-site cysteine and the mode of Zn2+ inactivation is distinct from that of Cu2+.

Inhibition of mitogen-activated protein kinase by a Drosophila dual-specific phosphatase

The Drosophila extracellular signal-regulated kinase (DERK) mitogen-activated protein kinase (MAPK) is involved in the regulation of multiple differentiation and developmental processes. Tight control of MAPK activity is critical for normal cell behaviour. We identified a novel Drosophila MAPK phosphatase (DMKP) cDNA from the expressed-sequence-tag database and characterized it. Analysis of the nucleotide sequence revealed an open reading frame encoding the 203-amino acid protein, with a calculated molecular mass of 23 kDa, which has a high amino acid sequence similarity with 'VH1-like' dual-specific phosphatases at the broad region near the catalytic sites. The expression of DMKP mRNA occurs from the late larval stages to adulthood in Drosophila development. The recombinant DMKP protein produced in yeast retained its phosphatase activity. When expressed in Schneider cells, DMKP dose-dependently inhibited DERK and Drosophila c-Jun N-terminal kinase activities with high selectivity towards DERK. However, DMKP did not have any affect on Drosophila p38 activity. When DMKP was expressed in yeast, it down-regulated the fus1-lacZ trans-reporter gene of the pheromone MAPK pathway without any significant effect on the high-osmolarity-glycerol-response pathway.

Protein-tyrosine phosphatases: structure, mechanism, and inhibitor discovery

Protein-tyrosine kinases (PTKs) and their associated signaling pathways are crucial for the regulation of numerous cell functions including growth, mitogenesis, motility, cell-cell interactions, metabolism, gene transcription, and the immune response. Since tyrosine phosphorylation is reversible and dynamic in vivo, the phosphorylation states of proteins are governed by the opposing actions of PTKs and protein-tyrosine phosphatases (PTPs). In this light, both PTKs and PTPs play equally important roles in signal transduction in eukaryotic cells, and comprehension of mechanisms behind the reversible pTyr-dependent modulation of protein function and cell physiology must necessarily encompass the characterization of PTPs as well as PTKs. In spite of the large number of PTPs identified to date and the emerging role played by PTPs in disease, a detailed understanding of the role played by PTPs in signaling pathways has been hampered by the absence of PTP-specific agents. Such PTP-specific inhibitors could potentially serve as useful tools in determining the physiological significance of protein tyrosine phosphorylation in complex cellular signal transduction pathways and may constitute valuable therapeutics in the treatment of several human diseases. The goal of this review is therefore to summarize current understandings of PTP structure and mechanism of catalysis and the relationship of these to PTP inhibitor development. The review is organized such that enzyme structure is covered first, followed by mechanisms of catalysis then PTP inhibitor development. In discussing PTP inhibitor development, nonspecific inhibitors and those obtained by screening methods are initially presented with the focus then shifting to inhibitors that utilize a more structure-based rationale.

3,6-Fluorescein Diphosphate: A Sensitive Fluorogenic and Chromogenic Substrate for Protein Tyrosine Phosphatases*

A highly sensitive and continuous protein tyrosine phosphatase (PTPase) assay using 3,6-fluorescein diphosphate (FDP) is described. Leukocyte phosphatase CD45 (leukocyte common antigen), protein tyrosine phosphatase-1B, and leukocyte common antigen-related protein LAR preferentially hydrolyze FDP to fluorescein monophosphate (FMP) with V(max) and K(m) values comparable with those of phosphotyrosine peptide substrates. Further hydrolysis of FMP to fluorescein was less efficient because of increased K(m) values compared with those of FDP. FMP absorbs strongly at 445 nm and fluoresces intensely near 515 nm, both of which are insensitive to pH perturbations above pH 6. Its high catalytic efficiency, coupled with the highly sensitive dual detection in the visible wavelength region and wider pH operating range, make FDP the substrate of choice for PTPase inhibitor screening in HTS format and assay miniaturization.

Specific inhibition of FGF-induced MAPK activation by the receptor-like protein tyrosine phosphatase LAR

LAR is a widely expressed receptor-like protein tyrosine phosphatase that is implicated in regulation of intracellular signaling triggered by both cell adhesion and peptide growth factors. Genetic studies revealed that LAR regulates neuron axon path finding in Drosophila and mammary gland epithelial cell differentiation in mice. The molecular mechanism underlying the tissue specific function of LAR has not been clearly understood. We investigated the role and mechanism of LAR in peptide growth factors EGF and FGF signaling in human tissue culture cells in which the expression of LAR is under the control of an inducible promoter. We found that although both EGF and FGF induce activation of mitogen-activated protein kinase (MAPK), LAR only inhibits FGF-induced MAPK activation. LAR does not interact directly with the peptide growth factor receptors, since the ligand-induced autophosphorylation of growth factor receptors was not affected by induction of LAR. The specific effect of LAR on FGF-induced MAPK activation appeared to be mediated by specific inhibition of the phosphorylation of two signal transducers that act downstream of the FGF receptor, FRS2 and a 180 kDa protein, and by prevention of their interaction with the adaptor protein GRB2. In contrast, LAR selectively inhibited the epidermal growth factor (EGF)-induced phosphorylation of p130CAS and the formation of the complex between p130CAS and GRB2 but this effect did not influence the activation of MAPK by EGF. These data suggest that LAR and similar receptor-like protein tyrosine phosphatases may contribute to the regulation of transmembrane signaling by selectively inhibiting the tyrosine phosphorylation of specific signal transducers that act downstream of the plasma membrane-associated tyrosine kinases. The consequent inhibition of the formation of signaling complexes by these proteins may contribute to the specificity of the signals generated by specific peptide growth factors as well as extracellular matrix proteins.

Structure-based design of a low molecular weight, nonphosphorus, nonpeptide, and highly selective inhibitor of protein-tyrosine phosphatase 1B

Several protein-tyrosine phosphatases (PTPs) have been proposed to act as negative regulators of insulin signaling. Recent studies have shown increased insulin sensitivity and resistance to obesity in PTP1B knockout mice, thus pointing to this enzyme as a potential drug target in diabetes. Structure-based design, guided by PTP mutants and x-ray protein crystallography, was used to optimize a relatively weak, nonphosphorus, nonpeptide general PTP inhibitor (2-(oxalyl-amino)-benzoic acid) into a highly selective PTP1B inhibitor. This was achieved by addressing residue 48 as a selectivity determining residue. By introducing a basic nitrogen in the core structure of the inhibitor, a salt bridge was formed to Asp-48 in PTP1B. In contrast, the basic nitrogen causes repulsion in other PTPs containing an asparagine in the equivalent position resulting in a remarkable selectivity for PTP1B. Importantly, this was accomplished while retaining the molecular weight of the inhibitor below 300 g/mol.

Protein phosphatases and the regulation of mitogen-activated protein kinase signalling

The magnitude and duration of signalling through mitogen- and stress-activated kinases are critical determinants of biological effect. This reflects a balance between the activities of upstream activators and a complex regulatory network of protein phosphatases. These mitogen-activated protein kinase phosphatases include both dual-specificity (threonine/tyrosine) and tyrosine-specific enzymes, and recent evidence suggests that a single mitogen-activated protein kinase isoform may be acted upon by both classes of protein phosphatase. In both cases, substrate selectivity is determined by specific protein-protein interactions mediated through noncatalytic amino-terminal mitogen-activated protein kinase binding domains. Future challenges include the determination of exactly how this network of protein phosphatases interacts selectively with mitogen-activated protein kinase signalling complexes to achieve precise regulation of these key pathways in mammalian cells.

5-Hydroxytryptamine1A receptor/Gibetagamma stimulates mitogen-activated protein kinase via NAD(P)H oxidase and reactive oxygen species upstream of src in chinese hamster ovary fibroblasts

The hypothesis of this work is that the 'serotonin' or 5-hydroxytryptamine (5-HT)(1A) receptor, which activates the extracellular signal-regulated kinase (ERK) through a G(i)betagamma-mediated pathway, does so through the intermediate actions of reactive oxygen species (ROS). Five criteria were shown to support a key role for ROS in the activation of ERK by the 5-HT(1A) receptor. (1) Antioxidants inhibit activation of ERK by 5-HT. (2) Application of cysteine-reactive oxidant molecules activates ERK. (3) The 5-HT(1A) receptor alters cellular redox properties, and generates both superoxide and hydrogen peroxide. (4) A specific ROS-producing enzyme [NAD(P)H oxidase] is involved in the activation of ERK. (5) There is specificity both in the effects of various chemical oxidizers, and in the putative location of the ROS in the ERK activation pathway. We propose that NAD(P)H oxidase is located in the ERK activation pathway stimulated by the transfected 5-HT(1A) receptor in Chinese hamster ovary (CHO) cells downstream of G(i)betagamma subunits and upstream of or at the level of the non-receptor tyrosine kinase, Src. Moreover, these experiments provide confirmation that the transfected human 5-HT(1A) receptor induces the production of ROS (superoxide and hydrogen peroxide) in CHO cells, and support the possibility that an NAD(P)H oxidase-like enzyme might be involved in the 5-HT-mediated generation of both superoxide and hydrogen peroxide.

Mechanistic basis for catalytic activation of mitogen-activated protein kinase phosphatase 3 by extracellular signal-regulated kinase

The dual specificity mitogen-activated protein kinase phosphatase MKP3 has been shown to down-regulate mitogenic signaling through dephosphorylation of extracellular signal-regulated kinase (ERK). Camps et al. (Camps, M., Nichols, A., Gillieron, C., Antonsson, B., Muda, M., Chabert, C., Boschert, U., and Arkinstall, S. (1998) Science 280, 1262-1265) had demonstrated that ERK binding to the noncatalytic amino-terminal domain of MKP3 can dramatically activate the phosphatase catalytic domain. The physical basis for this activation has not been established. Here, we provide detailed biochemical evidence that ERK activates MKP3 through the stabilization of the active phosphatase conformation, inducing closure of the catalytic "general acid" loop. In the closed conformation, this loop structure can participate efficiently in general acid/base catalysis, substrate binding, and transition-state stabilization. The pH activity profiles of ERK-activated MKP3 clearly indicated the involvement of general acid catalysis, a hallmark of protein-tyrosine phosphatase catalysis. In contrast, unactivated MKP3 did not display this enzymatic group as critical for the low activity form of the enzyme. Using a combination of Brönsted analyses, pre-steady-state and steady-state kinetics, we have isolated all catalytic steps in the reaction and have quantified the specific rate enhancement. Through protonation of the leaving group and transition-state stabilization, activated MKP3 catalyzes formation of the phosphoenzyme intermediate approximately 100-fold faster than unactivated enzyme. In addition, ERK-activated MKP3 catalyzes intermediate hydrolysis 5-6-fold more efficiently and binds ligands up to 19-fold more tightly. Consistent with ERK stabilizing the active conformation of MKP3, the chemical chaperone dimethyl sulfoxide was able to mimic this activation. A general protein-tyrosine phosphatase regulatory mechanism involving the flexible general acid loop is discussed.

Rediscovering good old friend IGF-I in the new millenium: possible usefulness in Alzheimer's disease and stroke

Much research has been done over the past two decades on the role of insulin-like growth factors I and II (IGF) in the maintenance of normal body homeostasis, especially in regard to various endocrine functions, growth and aging. For example, IGF-I is a well established promoter of tissue growth and has been used in the clinics for the treatment of growth related disorders, even being abused by athletes to enhance performance in competitions. In contrast, comparatively limited attention has been given to the potential significance of the IGFs in the central nervous system. Over the past few years, we have studied the trophic as well as neuromodulatory roles of the IGFs in the brain. IGF-I and IGF-II are potent modulators of acetylcholine release, IGF-I inhibiting release while IGF-II is a potent stimulant. Moreover, only the internalization of the IGF-I receptor complex was blocked by an inhibitor of phosphotyrosylation. This is in accordance with the differential nature of the IGF-I and IGF-II receptors, the former being a tyrosine kinase receptor while the later is a single transmembrane domain protein bearing binding sites for 6-mannose phosphate containing residues. The activation of IGF-I receptors protected neurons against cell death induced by amyloidogenic derivatives likely by an intracellular mechanism distinct from those involved in the regulation of acetylcholine release and neuronal growth. The stimulation of IGF-I receptors can activate intracellular pathways implicating a PI3/Akt kinase and CREB phosphorylation or modulate the production of free radicals. The effects, particularly those of IGF-I on key markers of the Alzheimer's (AD) brains namely cholinergic dysfunction, neuronal amyloid toxicity, tau phosphorylation and glucose metabolism suggest the potential usefulness of this growth factor in the treatment of neurodegenerative diseases. However, the poor bioavailability, enzymatic stability and brain penetration of IGF-I hamper progress in this regard. The recent development of a small, non-peptidyl mimetic of insulin able to directly activate the insulin receptor [Zhang, B., Salituro, G., Szalkowski, D., Li, Z., Zhang, Y., Royo, I., Vilella, D., Diez, M.T., Pelaez, F., Ruby, C., Kendall, R.L., Mao, X., Griffin, P., Calaycay, J., Zierath, J.R., Heck, J. V., Smith, R.G., Moller, D.E., 1999. Science, 284, 974-977] suggests that a similar strategy could be used for IGF-I and the IGF-I receptor leading to the characterization of IGF-I mimics of potential clinical usefulness.

Oxidative stress and gene regulation

Reactive oxygen species are produced by all aerobic cells and are widely believed to play a pivotal role in aging as well as a number of degenerative diseases. The consequences of the generation of oxidants in cells does not appear to be limited to promotion of deleterious effects. Alterations in oxidative metabolism have long been known to occur during differentiation and development. Experimental perturbations in cellular redox state have been shown to exert a strong impact on these processes. The discovery of specific genes and pathways affected by oxidants led to the hypothesis that reactive oxygen species serve as subcellular messengers in gene regulatory and signal transduction pathways. Additionally, antioxidants can activate numerous genes and pathways. The burgeoning growth in the number of pathways shown to be dependent on oxidation or antioxidation has accelerated during the last decade. In the discussion presented here, we provide a tabular summary of many of the redox effects on gene expression and signaling pathways that are currently known to exist.

Form, function, and regulation of protein tyrosine phosphatases and their involvement in human diseases

Protein tyrosine phosphatases (PTPs) are a family of enzymes that modulate the cellular level of tyrosine phosphorylation. Based on cellular location, they are classified as receptor like or intracellular PTPs. Structure and function studies have led to the understanding of the enzymatic mechanism of this class of enzymes. Proper targeting of PTPs is essential for many cellular signalling events including antigen induced proliferative responses of B and T cells. The physiological significance of PTPs is further unveiled through mice gene knockout studies and human genome sequencing and mapping projects. Several PTPs are shown to be critical in the pathogenesis of human diseases.