Probing the molecular basis for potent and selective protein-tyrosine phosphatase 1B inhibition

Protein-tyrosine phosphatases (PTPs) are important for the control of proper cellular tyrosine phosphorylation. Despite the large number of PTPs encoded in the human genome and the emerging roles played by PTPs in human diseases, a detailed understanding of the role played by PTPs in normal physiology and in pathogenic conditions has been hampered by the absence of PTP-specific inhibitors. Such inhibitors could serve as useful tools for determining the physiological functions of PTPs and may constitute valuable therapeutics in the treatment of several human diseases. However, because of the highly conserved nature of the active site, it has been difficult to develop selective PTP inhibitors. By taking an approach to tether together two small ligands that can interact simultaneously with the active site and a unique proximal noncatalytic site, we have recently acquired Compound 2 (see Fig. 1), the most potent and selective PTP1B inhibitor identified to date, which exhibits several orders of magnitude selectivity in favor of PTP1B against a panel of PTPs. We describe an evaluation of the interaction between 2 and its analogs with PTP1B and its site-directed mutants selected based on hydrogen/deuterium exchange of PTP1B backbone amides in the presence and absence of 2. We have established the binding mode of Compound 2 and identified 12 PTP1B residues that are important for the potency and selectivity of Compound 2. Although many of the residues important for Compound 2 binding are not unique to PTP1B, the combinations of all contact residues differ between PTP isozymes, which suggest that the binding surface defined by these residues in individual PTPs determines inhibitor selectivity. Our results provide structural information toward understanding of the molecular basis for potent and selective PTP1B inhibition and further establish the feasibility of acquiring potent, yet highly selective, PTP inhibitory agents.     

5-Arylidene-2-phenylimino-4-thiazolidinones as PTP1B and LMW-PTP inhibitors

As part of a project aimed at identifying effective low molecular weight nonphosphorus monoanionic inhibitors of PTPs, we have synthesized 4-[(5-arylidene-4-oxo-2-phenyliminothiazolidin-3-yl)methyl]benzoic acids (4) and evaluated their inhibitory activity against human PTP1B and LMW-PTP enzymes. The introduction of a 2-phenylimino moiety onto the 4-thiazolidinone ring was designed to enhance the inhibitor/enzyme affinity by means of further favourable interactions with residues of the active site and the surrounding loops. Some of the compounds (4a–d, f) showed interesting inhibition levels in the low micromolar range. The 5-arylidene moiety of acids 4 proved to markedly influence the potency of these inhibitors. Molecular modeling experiments inside the binding sites of both enzymes were performed.     

2-(oxalylamino)-benzoic acid is a general, competitive inhibitor of protein-tyrosine phosphatases

Protein-tyrosine phosphatases (PTPs) are critically involved in regulation of signal transduction processes. Members of this class of enzymes are considered attractive therapeutic targets in several disease states, e.g. diabetes, cancer, and inflammation. However, most reported PTP inhibitors have been phosphorus-containing compounds, tight binding inhibitors, and/or inhibitors that covalently modify the enzymes. We therefore embarked on identifying a general, reversible, competitive PTP inhibitor that could be used as a common scaffold for lead optimization for specific PTPs. We here report the identification of 2-(oxalylamino)-benzoic acid (OBA) as a classical competitive inhibitor of several PTPs. X-ray crystallography of PTP1B complexed with OBA and related non-phosphate low molecular weight derivatives reveals that the binding mode of these molecules to a large extent mimics that of the natural substrate including hydrogen bonding to the PTP signature motif. In addition, binding of OBA to the active site of PTP1B creates a unique arrangement involving Asp(181), Lys(120), and Tyr(46). PTP inhibitors are essential tools in elucidating the biological function of specific PTPs and they may eventually be developed into selective drug candidates. The unique enzyme kinetic features and the low molecular weight of OBA makes it an ideal starting point for further optimization.     

Structure-based design of selective and potent inhibitors of protein-tyrosine phosphatase beta

Protein-tyrosine phosphatases (PTPs) are considered important therapeutic targets because of their pivotal role as regulators of signal transduction and thus their implication in several human diseases such as diabetes, cancer, and autoimmunity. In particular, PTP1B has been the focus of many academic and industrial laboratories because it was found to be an important negative regulator of insulin and leptin signaling, and hence a potential therapeutic target in diabetes and obesity. As a result, significant progress has been achieved in the design of highly selective and potent PTP1B inhibitors. In contrast, little attention has been given to other potential drug targets within the PTP family. Guided by x-ray crystallography, molecular modeling, and enzyme kinetic analyses with wild type and mutant PTPs, we describe the development of a general, low molecular weight, non-peptide, non-phosphorus PTP inhibitor into an inhibitor that displays more than 100-fold selectivity for PTPbeta over PTP1B. Of note, our structure-based design principles, which are based on extensive bioinformatics analyses of the PTP family, are general in nature. Therefore, we anticipate that this strategy, here applied to PTPbeta, in principle can be used in the design and development of selective inhibitors of many, if not most PTPs.     

Discovery of di-indolinone as a novel scaffold for protein tyrosine phosphatase 1B inhibitors

A series of di-indolinone derivatives was designed and synthesized to optimize our lead compounds basing on molecular docking study as PTP1B inhibitors. Successive enzymatic assay identified the synthetic di-indolinone as novel PTP1B inhibitors with low micromole-ranged inhibitory activity and at least several-fold selectivity over other tested homologous PTPs.     

trans-Beta-nitrostyrene derivatives as slow-binding inhibitors of protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs) catalyze the hydrolysis of phosphotyrosyl (pY) proteins to produce tyrosyl proteins and inorganic phosphate. Specific PTPs inhibitors provide useful tools for studying PTP function in signal transduction processes and potential treatment for human diseases such as diabetes, inflammation, and cancer. In this work, trans-beta-nitrostyrene (TBNS) and its derivatives are found to be slow-binding inhibitors against protein tyrosine phosphatases PTP1B, SHP-1, and Yop with moderate potencies (K(I*) = 1-10 microM). Competition experiments with a substrate (pNPP) and iodoacetate indicate that TBNS is active site-directed. The mechanism of inhibition was investigated by UV-vis absorption spectroscopy, (1)H-(13)C heteronuclear single-quantum correlation NMR spectroscopy, and site-directed mutagenesis. These studies suggested a mechanism in which TBNS acts a pY mimetic and binds to the PTP active site to form an initial noncovalent E.I complex, followed by nucleophilic attack on the TBNS nitro group by Cys-215 of PTP1B to form a reversible, covalent adduct as the tighter E.I* complex. TBNS derivatives represent a new class of neutral pY mimetic inhibitors of PTPs.     

Discovery of potent, selective and orally bioavailable triaryl-sulfonamide based PTP1B inhibitors

A novel series of pTyr mimetics containing triaryl-sulfonamide derivatives (5a-r) are reported as potent and selective PTP1B inhibitors. Some of the test compounds (5o and 5p) showed excellent selectivity towards PTP1B over various PTPs, including TCPTP (in vitro). The lead compound 5o showed potent antidiabetic activity (in vivo), along with improved pharmacokinetic profile. These preliminary results confirm discovery of highly potent and selective PTP1B inhibitors for the treatment of T2DM.     

Calixarene-based phosphinic acids as inhibitors of protein tyrosine phosphatases

In the present work, the derivatives of calix[4]arene, thiacalix[4]arene, and sulfonylcalix[4]arene bearing four methylene(phenyl)phosphinic acid groups on the upper rim of the macrocycle were synthesized and studied as inhibitors of human protein tyrosine phosphatases. The inhibitory capacities of the three compounds towards PTP1B were higher than those for protein tyrosine phosphatases TC–PTP, MEG1, MEG2, and SHP2. The most potent sulfonylcalix[4]arene phosphinic acid displayed K_i value of 32?nM. The thiacalix[4]arene phosphinic acid was found to be a low micromolar inhibitor of PTP1B with selectivity over the other PTPs. The kinetic experiments showed that the inhibitors compete with the substrate for the active site of the enzyme. Molecular docking was performed to explain possible binding modes of the calixarene-based phosphinic inhibitors of PTP1B.     

Identification of protein tyrosine phosphatases with specificity for the ligand-activated growth hormone receptor

Protein tyrosine phosphatases (PTPs) play key roles in switching off tyrosine phosphorylation cascades, such as initiated by cytokine receptors. We have used substrate-trapping mutants of a large set of PTPs to identify members of the PTP family that have substrate specificity for the phosphorylated human GH receptor (GHR) intracellular domain. Among 31 PTPs tested, T cell (TC)-PTP, PTP-beta, PTP1B, stomach cancer-associated PTP 1 (SAP-1), Pyst-2, Meg-2, and PTP-H1 showed specificity for phosphorylated GHR that had been produced by coexpression with a kinase in bacteria. We then used GH-induced, phosphorylated GH receptor, purified from overexpressing mammalian cells, in a Far Western-based approach to test whether these seven PTPs were also capable of recognizing ligand-induced, physiologically phosphorylated GHR. In this assay, only TC-PTP, PTP1B, PTP-H1, and SAP-1 interacted with the mature form of the phosphorylated GHR. In parallel, we show that these PTPs recognize very different subsets of the seven GHR tyrosines that are potentially phosphorylated. Finally, mRNA tissue distribution of these PTPs by RT-PCR analysis and coexpression of the wild-type PTPs to test their ability to dephosphorylate ligand-activated GHR suggest PTP-H1 and PTP1B as potential candidates involved in GHR signaling.     

Peptidyl aldehydes as reversible covalent inhibitors of protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs) are a large family of enzymes that catalyze the hydrolytic removal of the phosphoryl group from phosphotyrosyl (pY) proteins. PTP inhibitors provide potential treatment of human diseases/conditions such as diabetes and obesity as well as useful tools for studying the function of PTPs in signaling pathways. In this work, we have shown that certain aryl-substituted aldehydes act as reversible, slow-binding inhibitors of modest potency against PTP1B, SHP-1, and a dual-specificity phosphatase, VHR. Attachment of the tripeptide Gly-Glu-Glu to the para position of cinnamaldehyde resulted in an inhibitor (Cinn-GEE) of substantially increased potency against all three enzymes (e.g., K(I) = 5.4 microM against PTP1B). The mechanism of inhibition was investigated using Cinn-GEE specifically labeled with (13)C at the aldehyde carbon and (1)H-(13)C heteronuclear single-quantum coherence spectroscopy. While Cinn-GEE alone showed a single cross-peak at delta 9.64 ((1)H) and delta 201 ((13)C), the PTP1B/Cinn-GEE complex showed three distinct cross-peaks at delta 7.6-7.8 ((1)H) and 130-137 ((13)C). Mutation of the catalytic cysteine (Cys-215 in PTP1B) into alanine had no effect on the cross-peaks, whereas mutation of a conserved active-site arginine (Arg-221 in PTP1B) to alanine abolished all three cross-peaks. Similar experiments with Cinn-GEE that had been labeled with (13)C at the benzylic position revealed a change in the hybridization state (from sp(2) to sp(3)) for the benzylic carbon as a result of binding to PTP1B. These results rule out the possibility of a free aldehyde, aldehyde hydrate, or hemithioacetal as the enzyme-bound inhibitor form. Instead, the data are consistent with the formation of an enamine between the aldehyde group of the inhibitor and the guanidine group of Arg-221 in the PTP1B active site. These aldehydes may provide a general core structure that can be further developed into highly potent and specific PTP inhibitors.     

Protein tyrosine phosphatases: prospects for therapeutics

Protein tyrosine phosphatases (PTPs) form a large family of enzymes that serve as key regulatory components in signal transduction pathways. Recent gene knockout studies in mice identify PTP1B as a promising target for anti-diabetes/obesity drug discovery. PTPs are also implicated in a wide variety of other disorders, including cancer. Significant progress has been made in identifying small molecules that simultaneously bind both the active site and a unique adjacent site that enables specific inhibition of individual PTP isoenzymes. As a consequence, there are compelling reasons to believe that PTP inhibitors may ultimately serve as powerful therapeutic weapons in our arsenal for battling human diseases.     

Bicyclic benzofuran and indole-based salicylic acids as protein tyrosine phosphatase inhibitors

Protein tyrosine phosphatases (PTPs) constitute a large and structurally diverse family of signaling enzymes that control the cellular levels of protein tyrosine phosphorylation. Malfunction of PTP activity has significant implications in many human diseases, and the PTP protein family provides an exciting array of validated diabetes/obesity (PTP1B), oncology (SHP2), autoimmunity (Lyp), and infectious disease (mPTPB) targets. However, despite the fact that PTPs have been garnering attention as novel therapeutic targets, they remain largely an untapped resource. The main challenges facing drug developers by the PTPs are inhibitor specificity and bioavailability. Work over the last ten years has demonstrated that it is feasible to develop potent and selective inhibitors for individual members of the PTP family by tethering together small ligands that can simultaneously occupy both the active site and unique nearby peripheral binding sites. Recent results with the bicyclic salicylic acid pharmacophores indicate that the new chemistry platform may provide a potential solution to overcome the bioavailability issue that has plagued the PTP drug discovery field for many years. Structural analysis of PTP-inhibitor complexes reveals molecular determinants important for the development of more potent and selective PTP inhibitors, thus offering hope in the medicinal chemistry of a largely unexploited protein class with a wealth of attractive drug targets.     

Facile fabrication of promising protein tyrosine phosphatase (PTP) inhibitor entities based on 'clicked' serine/threonine-monosaccharide hybrids

Protein tyrosine phosphatases (PTPs) are well-validated therapeutic targets for many human major diseases. The development of their potent inhibitors has therefore become a main focus of both academia and the pharmaceutical industry. We report herein a facile strategy toward the fabrication of new and competent PTP inhibitor entities by simply 'clicking' alkynyl amino acids onto diverse azido sugar templates. Triazolyl glucosyl, galactosyl, and mannosyl serine and threonine derivatives were efficiently synthesized via click reaction, which were then identified as potent CDC25B and PTP1B inhibitors selective over a panel of homologous PTPs tested. Their inhibitory activity and selectivity were found to largely lie on the structurally and configurationally diversified monosaccharide moieties whereon serinyl and threoninyl residues were introduced. In addition, MTT assay revealed the triazole-connected sugar-amino acid hybrids may also inhibit the growth of several human cancer cell lines including A549, Hela, and especially HCT-116. On the basis of such compelling evidence, we consider that this compound series could furnish promising chemical entities serving as new CDC25B and PTP1B inhibitors with potential cellular activity. Furthermore, the 'click' strategy starting from easily accessible and biocompatible amino acids and sugar templates would allow the modular fabrication of a rich library of new PTP inhibitors efficaciously and productively.     

Modulation in the expression of SHP-1, SHP-2 and PTP1B due to the inhibition of MAPKs,cAMP and neutrophils early on in the development of cerulein-induced acute pancreatitis in rats

The protein tyrosine phosphatases (PTPs) SHP-1, SHP-2 and PTP1B are overexpressed early on during the development of cerulein -induced acute pancreatitis (AP) in rats, and their levels can be modulated by some species of mitogen-activated protein kinases (MAPKs), the intracellular levels of cAMP and by general leukocyte infiltration, the latter at least for SHP-2 and PTP1B. In this study we show that cerulein treatment activates extracellular signal-regulated kinase (ERK) and c-Jun NH₂-terminal kinase (JNK) but not p38 MAPK during the early phase of cerulein-induced AP (2 h after the first injection of cerulein). Therefore, by using the MAPK inhibitors SP600125 (a specific JNK inhibitor) and PD98059 (a specific ERK inhibitor), we have unmasked the particular MAPK that underlies the modulation of the expression levels of these PTPs. JNK would act by preventing SHP-1 protein expression from increasing beyond a certain level. ERK 1/2 was the main MAPK involved in the increase in SHP-2 protein expression due to cerulein. JNK negatively modulated the SH2-domain containing PTPs. Both MAPKs played a role in the increase in PTP1B protein expression due to cerulein. Finally, by using the white blood cell inhibitors vinblastine sulfate, gadolinium chloride and FK506 (tacrolimus), we show that the macrophage activity or T-lymphocytes does not modulate the expression of any of the PTPs, although neutrophil infiltration was found to be a regulator of SHP-2 and PTP1B protein expression due to cerulein.     

Steric hindrance as a basis for structure-based design of selective inhibitors of protein-tyrosine phosphatases.

Utilizing structure-based design, we have previously demonstrated that it is possible to obtain selective inhibitors of protein-tyrosine phosphatase 1B (PTP1B). A basic nitrogen was introduced into a general PTP inhibitor to form a salt bridge to Asp48 in PTP1B and simultaneously cause repulsion in PTPs containing an asparagine in the equivalent position [Iversen, L. F., et al. (2000) J. Biol. Chem. 275, 10300-10307]. Further, we have recently demonstrated that Gly259 in PTP1B forms the bottom of a gateway that allows easy access to the active site for a broad range of substrates, while bulky residues in the same position in other PTPs cause steric hindrance and reduced substrate recognition capacity [Peters, G. H., et al. (2000) J. Biol. Chem. 275, 18201-18209]. The current study was undertaken to investigate the feasibility of structure-based design, utilizing these differences in accessibility to the active site among various PTPs. We show that a general, low-molecular weight PTP inhibitor can be developed into a highly selective inhibitor for PTP1B and TC-PTP by introducing a substituent, which is designed to address the region around residues 258 and 259. Detailed enzyme kinetic analysis with a set of wild-type and mutant PTPs, X-ray protein crystallography, and molecular modeling studies confirmed that selectivity for PTP1B and TC-PTP was achieved due to steric hindrance imposed by bulky position 259 residues in other PTPs.     

Suramin derivatives as inhibitors and activators of protein-tyrosine phosphatases

Protein-tyrosine phosphatases (PTPs) are important signaling enzymes that have emerged within the last decade as a new class of drug targets. It has previously been shown that suramin is a potent, reversible, and competitive inhibitor of PTP1B and Yersinia PTP (YopH). We therefore screened 45 suramin analogs against a panel of seven PTPs, including PTP1B, YopH, CD45, Cdc25A, VHR, PTPalpha, and LAR, to identify compounds with improved potency and specificity. Of the 45 compounds, we found 11 to have inhibitory potency comparable or significantly improved relative to suramin. We also found suramin to be a potent inhibitor (IC(50) = 1.5 microm) of Cdc25A, a phosphatase that mediates cell cycle progression and a potential target for cancer therapy. In addition we also found three other compounds, NF201, NF336, and NF339, to be potent (IC(50) < 5 microm) and specific (at least 20-30-fold specificity with respect to the other human PTPs tested) inhibitors of Cdc25A. Significantly, we found two potent and specific inhibitors, NF250 and NF290, for YopH, the phosphatase that is an essential virulence factor for bubonic plague. Two of the compounds tested, NF504 and NF506, had significantly improved potency as PTP inhibitors for all phosphatases tested except for LAR and PTPalpha. Surprisingly, we found that a significant number of these compounds activated the receptor-like phosphatases, PTPalpha and LAR. In further characterizing this activation phenomenon, we reveal a novel role for the membrane-distal cytoplasmic PTP domain (D2) of PTPalpha: the direct intramolecular regulation of the activity of the membrane-proximal cytoplasmic PTP domain (D1). Binding of certain of these compounds to PTPalpha disrupts D1-D2 basal state contacts and allows new contacts to occur between D1 and D2, which activates D1 by as much as 12-14-fold when these contacts are optimized.     

Synthesis,activity and molecular modeling of a new series of chromones as low molecular weight protein tyrosine phosphatase inhibitors

Protein tyrosine phosphatases (PTP) are crucial elements in eukaryotic signal transduction. Several reports suggested that the LMW-PTP family has oncogenic relevance. Moreover, LMW-PTP has been recognized as a negative regulator of insulin-mediated mitotic and metabolic signaling. Thus, inhibition of the LMW-PTP can be considered an attractive approach for the design of new therapeutic agents for the treatment of type II diabetes and for new antitumoral drugs. To date very few (and weak) inhibitors of LMW-PTP have been identified. On the basis of the reported weak activity of some flavonoids on phosphatases, we discovered a lead that originated a new class of highly active LMW-PTP inhibitors; these compounds inhibit also PTP-1B and are active in cellular assays. Docking experiments and SAR highlighted the possible binding mode of these compounds to the enzyme, putting the background for the future optimization of their inhibitory activity and selectivity towards the closely related enzyme PTP-1B.     

Molecular Analysis of Aedes aegypti Classical Protein Tyrosine Phosphatases Uncovers an Ortholog of Mammalian PTP-1B Implicated in the Control of Egg Production in Mosquitoes

Background

Protein Tyrosine Phosphatases (PTPs) are enzymes that catalyze phosphotyrosine dephosphorylation and modulate cell differentiation, growth and metabolism. In mammals, PTPs play a key role in the modulation of canonical pathways involved in metabolism and immunity. PTP1B is the prototype member of classical PTPs and a major target for treating human diseases, such as cancer, obesity and diabetes. These signaling enzymes are, hence, targets of a wide array of inhibitors. Anautogenous mosquitoes rely on blood meals to lay eggs and are vectors of the most prevalent human diseases. Identifying the mosquito ortholog of PTP1B and determining its involvement in egg production is, therefore, important in the search for a novel and crucial target for vector control.

Methodology/Principal Findings

We conducted an analysis to identify the ortholog of mammalian PTP1B in the Aedes aegypti genome. We identified eight genes coding for classical PTPs. In silico structural and functional analyses of proteins coded by such genes revealed that four of these code for catalytically active enzymes. Among the four genes coding for active PTPs, AAEL001919 exhibits the greatest degree of homology with the mammalian PTP1B. Next, we evaluated the role of this enzyme in egg formation. Blood feeding largely affects AAEL001919 expression, especially in the fat body and ovaries. These tissues are critically involved in the synthesis and storage of vitellogenin, the major yolk protein. Including the classical PTP inhibitor sodium orthovanadate or the PTP substrate DiFMUP in the blood meal decreased vitellogenin synthesis and egg production. Similarly, silencing AAEL001919 using RNA interference (RNAi) assays resulted in 30% suppression of egg production.

Conclusions/Significance

The data reported herein implicate, for the first time, a gene that codes for a classical PTP in mosquito egg formation. These findings raise the possibility that this class of enzymes may be used as novel targets to block egg formation in mosquitoes.     

Identification of protein tyrosine phosphatases associating with the PDGF receptor

Protein tyrosine phosphatase (PTP) in-gel assays were used to explore association of PTPs with the platelet-derived growth factor beta-receptor (PDGFbetaR). Five PTP activity bands of approximately 120, approximately 70, approximately 60, approximately 53, and approximately 45 kDa could be detected in PDGFbetaR immunoprecipitates and were identified by immunodepletion experiments as PTP-PEST, SHP-2, an active fragment of SHP-2, PTP-1B, and T-cell PTP, respectively. The PTP pattern that was obtained was similar in PDGFbetaR immunoprecipitates from HEK 293 cells overexpressing the human PDGFbetaR and from murine fibroblasts. Association of PTP-1B with the PDGFbetaR was stabilized by pretreatment of the cells with hydrogen peroxide. The epidermal growth factor receptor (EGFR) immunoprecipitated from fibroblasts, and c-Kit isolated from CHRF myeloid cells, were associated with partially overlapping but quantitatively different patterns of PTPs. PTP-PEST was the predominant PTP in EGFR immunoprecipitates, and SHP-1 appeared in c-Kit immunoprecipitates. We propose that the differential association of PTPs with different RTKs is related to their respective contributions to regulation of RTK signaling.     

Reactive oxygen species as mediators of cell adhesion

The intracellular production of reactive oxygen species (ROS) has a fundamental importance in both cell proliferation and apoptosis induction. Moreover, many experimental and epidemiological evidence indicate that ROS contribute to the initiation and promotion of carcinogenesis, and that drugs or treatments aimed to reduce the tissue content of ROS can be chemopreventive and curative against cancer. Recently, important observations on the role of ROS as physiological regulators of intracellular signaling cascades activated by growth factors through their tyrosine-kinase receptors have shed new light on the possible mechanisms that can sustain the promoting activity of ROS. The downstream effect of ROS production is the reversible oxidation of proteins. Redox sensitive proteins include protein tyrosine phosphatases (PTPs) as the active-site cysteine is the target of specific oxidation, and this modification can be reversed by intracellular reducing agents. The reversible oxidation of PTPs family member was demonstrated firstly for PTP1B during EGF signaling and then for LMW-PTP and SHP-2 during PDGF stimulation. The inhibition exerted by ROS on tyrosine-phosphatases helps the propagation of RTK signals mediated by protein tyrosine phosphorylation, generally associated with the proliferative stimulus. Our new data are consistent with a model in which ROS take a role in integrin signaling, and in which synergistic activation of Rac-1 by growth factors and adhesion molecules translates in a critical increase of intracellular oxidants up to a threshold level where inhibition of the tyrosine phosphatase LMW-PTP takes place. In seeking for potential molecular mechanisms for oxidative signaling by integrins, we found that transient oxidation/inactivation of LMW-PTP, a known negative regulator of RTK signaling, occurred during fibroblast adhesion to matrix, with a kinetic which paralleled the generation of ROS. Moreover, overexpression of LMW-PTP in NIH-3T3 fibroblasts delayed cell attachment to the substrate. Finally, constitutively high levels of intracellular ROS, as are observed in cells expressing active Rac, would attenuate anchorage dependence for growth, by substituting for integrin signaling in non adherent cells.