Insight into the role of low molecular weight phosphotyrosine phosphatase (LMW-PTP) on platelet-derived growth factor receptor (PDGF-r) signaling. LMW-PTP controls PDGF-r kinase activity through TYR-857 dephosphorylation

Structure-Based Design of Active-Site-Directed, Highly Potent, Selective, and Orally Bioavailable Low-Molecular-Weight Protein Tyrosine Phosphatase Inhibitors

Protein tyrosine phosphatases constitute an important class of drug targets whose potential has been limited by the paucity of drug-like small-molecule inhibitors. We recently described a class of active-site-directed, moderately selective, and potent inhibitors of the low-molecular-weight protein tyrosine phosphatase (LMW-PTP). Here, we report our extensive structure-based design and optimization effort that afforded inhibitors with vastly improved potency and specificity. The leading compound inhibits LMW-PTP potently and selectively (K_i = 1.2 nM, >8000-fold selectivity). Many compounds exhibit favorable drug-like properties, such as low molecular weight, weak cytochrome P450 inhibition, high metabolic stability, moderate to high cell permeability (Papp > 0.2 nm/s), and moderate to good oral bioavailability (% F from 23 to 50% in mice), and therefore can be used as in vivo chemical probes to further dissect the complex biological as well as pathophysiological roles of LMW-PTP and for the development of therapeutics targeting LMW-PTP.

The low molecular weight protein tyrosine phosphatase promotes adipogenesis and subcutaneous adipocyte hypertrophy

Obesity is a major contributing factor to the pathogenesis of Type 2 diabetes. Multiple human genetics studies suggest that high activity of the low molecular weight protein tyrosine phosphatase (LMPTP) promotes metabolic syndrome in obesity. We reported that LMPTP is a critical promoter of insulin resistance in obesity by regulating liver insulin receptor signaling and that inhibition of LMPTP reverses obesity-associated diabetes in mice. Since LMPTP is expressed in adipose tissue but little is known about its function, here we examined the role of LMPTP in adipocyte biology. Using conditional knockout mice, we found that selective deletion of LMPTP in adipocytes impaired obesity-induced subcutaneous adipocyte hypertrophy. We assessed the role of LMPTP in adipogenesis in vitro, and found that LMPTP deletion or knockdown substantially impaired differentiation of primary preadipocytes and 3T3-L1 cells into adipocytes, respectively. Inhibition of LMPTP in 3T3-L1 preadipocytes also reduced adipogenesis and expression of proadipogenic transcription factors peroxisome proliferator activated receptor gamma (PPARγ) and CCAAT/enhancer-binding protein alpha. Inhibition of LMPTP increased basal phosphorylation of platelet-derived growth factor receptor alpha (PDGFRα) on activation motif residue Y849 in 3T3-L1, resulting in increased activation of the mitogen-associated protein kinases p38 and c-Jun N-terminal kinase and increased PPARγ phosphorylation on inhibitory residue S82. Analysis of the metabolome of differentiating 3T3-L1 cells suggested that LMPTP inhibition decreased cell glucose utilization while enhancing mitochondrial respiration and nucleotide synthesis. In summary, we report a novel role for LMPTP as a key driver of adipocyte differentiation via control of PDGFRα signaling.

Vascular smooth muscle cell proliferation as a therapeutic target. Part 1: molecular targets and pathways

Cardiovascular diseases are a major cause of human death worldwide. Excessive proliferation of vascular smooth muscle cells contributes to the etiology of such diseases, including atherosclerosis, restenosis, and pulmonary hypertension. The control of vascular cell proliferation is complex and encompasses interactions of many regulatory molecules and signaling pathways. Herein, we recapitulated the importance of signaling cascades relevant for the regulation of vascular cell proliferation. Detailed understanding of the mechanism underlying this process is essential for the identification of new lead compounds (e.g., natural products) for vascular therapies.

Protein Tyrosine Phosphatases in Systemic Sclerosis: Potential Pathogenic Players and Therapeutic Targets

PURPOSE OF REVIEW

The pathogenesis of systemic sclerosis depends on a complex interplay between autoimmunity, vasculopathy, and fibrosis. Reversible phosphorylation on tyrosine residues, in response to growth factors and other stimuli, critically regulates each one of these three key pathogenic processes. Protein tyrosine kinases, the enzymes that catalyze addition of phosphate to tyrosine residues, are known players in systemic sclerosis, and tyrosine kinase inhibitors are undergoing clinical trials for treatment of this disease. Until recently, the role of tyrosine phosphatases-the enzymes that counteract the action of tyrosine kinases by removing phosphate from tyrosine residues-in systemic sclerosis has remained largely unknown. Here, we review the function of tyrosine phosphatases in pathways relevant to the pathogenesis of systemic sclerosis and their potential promise as therapeutic targets to halt progression of this debilitating rheumatic disease.

RECENT FINDINGS

Protein tyrosine phosphatases are emerging as important regulators of a multitude of signaling pathways and undergoing validation as molecular targets for cancer and other common diseases. Recent advances in drug discovery are paving the ways to develop new classes of tyrosine phosphatase modulators to treat human diseases. Although so far only few reports have focused on tyrosine phosphatases in systemic sclerosis, these enzymes play a role in multiple pathways relevant to disease pathogenesis. Further studies in this field are warranted to explore the potential of tyrosine phosphatases as drug targets for systemic sclerosis.

Redox regulation of the actin cytoskeleton and its role in the vascular system

The actin cytoskeleton is critical for form and function of vascular cells, serving mechanical, organizational and signaling roles. Because many cytoskeletal proteins are sensitive to reactive oxygen species, redox regulation has emerged as a pivotal modulator of the actin cytoskeleton and its associated proteins. Here, we summarize work implicating oxidants in altering actin cytoskeletal proteins and focus on how these alterations affect cell migration, proliferation and contraction of vascular cells. Finally, we discuss the role of oxidative modification of the actin cytoskeleton in vivo and highlight its importance for vascular diseases.

Inhibition of Low Molecular Weight Protein Tyrosine Phosphatase by an Induced-Fit Mechanism

The low molecular weight protein tyrosine phosphatase (LMW-PTP) is a regulator of a number of signaling pathways and has been implicated as a potential target for oncology and diabetes/obesity. There is significant therapeutic interest in developing potent and selective inhibitors to control LMW-PTP activity. We report the discovery of a novel class of LMW-PTP inhibitors derived from sulfophenyl acetic amide (SPAA), some of which exhibit greater than 50-fold preference for LMW-PTP over a large panel of PTPs. X-ray crystallography reveals that binding of SPAA-based inhibitors induces a striking conformational change in the LMW-PTP active site, leading to the formation of a previously undisclosed hydrophobic pocket to accommodate the α-phenyl ring in the ligand. This induced-fit mechanism is likely a major contributor responsible for the exquisite inhibitor selectivity.

Low molecular weight protein tyrosine phosphatase: Multifaceted functions of an evolutionarily conserved enzyme

Originally identified as a low molecular weight acid phosphatase, LMW-PTP is actually a protein tyrosine phosphatase that acts on many phosphotyrosine-containing cellular proteins that are primarily involved in signal transduction. Differences in sequence, structure, and substrate recognition as well as in subcellular localization in different organisms enable LMW-PTP to exert many different functions. In fact, during evolution, the LMW-PTP structure adapted to perform different catalytic actions depending on the organism type. In bacteria, this enzyme is involved in the biosynthesis of group 1 and 4 capsules, but it is also a virulence factor in pathogenic strains. In yeast, LMW-PTPs dephosphorylate immunophilin Fpr3, a peptidyl-prolyl-cis-trans isomerase member of the protein chaperone family. In humans, LMW-PTP is encoded by the ACP1 gene, which is composed of three different alleles, each encoding two active enzymes produced by alternative RNA splicing. In animals, LMW-PTP dephosphorylates a number of growth factor receptors and modulates their signalling processes. The involvement of LMW-PTP in cancer progression and in insulin receptor regulation as well as its actions as a virulence factor in a number of pathogenic bacterial strains may promote the search for potent, selective and bioavailable LMW-PTP inhibitors.

Regulation of Platelet Derived Growth Factor Signaling by Leukocyte Common Antigen-related (LAR) Protein Tyrosine Phosphatase: A Quantitative Phosphoproteomics Study

Intracellular signaling pathways are reliant on protein phosphorylation events that are controlled by a balance of kinase and phosphatase activity. Although kinases have been extensively studied, the role of phosphatases in controlling specific cell signaling pathways has been less so. Leukocyte common antigen-related protein (LAR) is a member of the LAR subfamily of receptor-like protein tyrosine phosphatases (RPTPs). LAR is known to regulate the activity of a number of receptor tyrosine kinases, including platelet-derived growth factor receptor (PDGFR). To gain insight into the signaling pathways regulated by LAR, including those that are PDGF-dependent, we have carried out the first systematic analysis of LAR-regulated signal transduction using SILAC-based quantitative proteomic and phosphoproteomic techniques. We haveanalyzed differential phosphorylation between wild-type mouse embryo fibroblasts (MEFs) and MEFs in which the LAR cytoplasmic phosphatase domains had been deleted (LARΔP), and found a significant change in abundance of phosphorylation on 270 phosphosites from 205 proteins because of the absence of the phosphatase domains of LAR. Further investigation of specific LAR-dependent phosphorylation sites and enriched biological processes reveal that LAR phosphatase activity impacts on a variety of cellular processes, most notably regulation of the actin cytoskeleton. Analysis of putative upstream kinases that may play an intermediary role between LAR and the identified LAR-dependent phosphorylation events has revealed a role for LAR in regulating mTOR and JNK signaling.

Protein tyrosine phosphatases as potential therapeutic targets

Protein tyrosine phosphorylation is a key regulatory process in virtually all aspects of cellular functions. Dysregulation of protein tyrosine phosphorylation is a major cause of human diseases, such as cancers, diabetes, autoimmune disorders, and neurological diseases. Indeed, protein tyrosine phosphorylation-mediated signaling events offer ample therapeutic targets, and drug discovery efforts to date have brought over two dozen kinase inhibitors to the clinic. Accordingly, protein tyrosine phosphatases (PTPs) are considered next-generation drug targets. For instance, PTP1B is a well-known targets of type 2 diabetes and obesity, and recent studies indicate that it is also a promising target for breast cancer. SHP2 is a bona-fide oncoprotein, mutations of which cause juvenile myelomonocytic leukemia, acute myeloid leukemia, and solid tumors. In addition, LYP is strongly associated with type 1 diabetes and many other autoimmune diseases. This review summarizes recent findings on several highly recognized PTP family drug targets, including PTP1B, Src homology phosphotyrosyl phosphatase 2(SHP2), lymphoid-specific tyrosine phosphatase (LYP), CD45, Fas associated phosphatase-1 (FAP-1), striatal enriched tyrosine phosphatases (STEP), mitogen-activated protein kinase/dual-specificity phosphatase 1 (MKP-1), phosphatases of regenerating liver-1 (PRL), low molecular weight PTPs (LMWPTP), and CDC25. Given that there are over 100 family members, we hope this review will serve as a road map for innovative drug discovery targeting PTPs.

SmLMWPTP, a teleost low molecular weight protein tyrosine phosphatase, inhibits the immune response of peripheral blood leukocytes in a manner that depends on the conserved P-loop

Protein tyrosine phosphatases (PTPs) are a family of enzymes that play a key role in cellular signal transduction. Low molecular weight PTPs (LMWPTPs) are a subfamily of PTPs that are characterized by the presence of a conserved phosphate-binding loop (P-loop) with the signature sequence of (V/I)CXGNXCRS. To date, very little study on teleost LMWPTPs has been documented, and, as a result, the function of LMWPTPs in fish is essentially unknown. In this study, we identified a LMWPTP from turbot (Scophthalmus maximus) and examined its biological activity and functionality. The turbot LMWPTP (SmLMWPTP) is composed of 158 residues and possesses a typical P-loop sequence in the form of (12)VCLGNICRS(20). Purified recombinant SmLMWPTP (rSmLMWPTP) exhibited apparent phosphatase activity, which was optimal at pH 5 and 50°C. The activity of SmLMWPTP was abolished when C13 and, in particular, R19 of the P-loop were mutated. SmLMWPTP expression was detected in a wide range of tissues and upregulated by bacterial and viral infection. Subcellular localization analysis showed that SmLMWPTP was secreted by peripheral blood leukocytes (PBL) into the extracellular milieu. When PBL were treated with rSmLMWPTP, the cells exhibited significant reductions in (i) proliferative and respiratory burst activity, (ii) expression levels of multiple immune relevant genes, and (iii) phagocytic activity. In contrast, the mutant SmLMWPTP bearing R19 mutation had no effect on PBL activity. Taken together, these results indicate that SmLMWPTP is a secreted PTP that exerts a negative regulatory effect on the innate immune response of PBL in a manner that depends on the structural integrity of the P-loop.

Regulation of TCR signalling by tyrosine phosphatases: from immune homeostasis to autoimmunity

More than half of the known protein tyrosine phosphatases (PTPs) in the human genome are expressed in T cells, and significant progress has been made in elucidating the biology of these enzymes in T-cell development and function. Here we provide a systematic review of the current understanding of the roles of PTPs in T-cell activation, providing insight into their mechanisms of action and regulation in T-cell receptor signalling, the phenotypes of their genetically modified mice, and their possible involvement in T-cell-mediated autoimmune disease. Our projection is that the interest in PTPs as mediators of T-cell homeostasis will continue to rise with further functional analysis of these proteins, and PTPs will be increasingly considered as targets of immunomodulatory therapies.

The role of protein tyrosine phosphatases in colorectal cancer

Colorectal cancer is one of the most common oncogenic diseases in the Western world. Several cancer associated cellular pathways have been identified, in which protein phosphorylation and dephosphorylation, especially on tyrosine residues, are one of most abundant regulatory mechanisms. The balance between these processes is under tight control by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Aberrant activity of oncogenic PTKs is present in a large portion of human cancers. Because of the counteracting role of PTPs on phosphorylation-based activation of signal pathways, it has long been thought that PTPs must act as tumor suppressors. This dogma is now being challenged, with recent evidence showing that dephosphorylation events induced by some PTPs may actually stimulate tumor formation. As such, PTPs might form a novel attractive target for anticancer therapy. In this review, we summarize the action of different PTPs, the consequences of their altered expression in colorectal cancer, and their potential as target for the treatment of this deadly disease.

PDGF: the nuts and bolts of signalling toolbox

PDGF is a growth factor and is extensively involved in multi-dimensional cellular dynamics. It switches on a plethora of molecules other than its classical pathway. It is engaged in various transitions of development; however, if the unleashed potentials lead astray, it brings forth tumourigenesis. Conventionally, it has been assumed that the components of this signalling pathway show fidelity and act with a high degree of autonomy. However, as illustrated by the PDGF signal transduction, reinterpretation of recent data suggests that machinery is often shared between multiple pathways, and other components crosstalk to each other through multiple mechanisms. It is important to note that metastatic cascade is an intricate process that we have only begun to understand in recent years. Many of the early steps of this PDGF cascade are not readily targetable in the clinic. In this review, we will unravel the paradoxes with reference to mitrons and cellular plasticity and discuss how disruption of signalling cascade triggers cellular proliferation phase transition and metastasis. We will also focus on the therapeutic interventions to counteract resultant molecular disorders.

Redox regulation of ERK1/2 activation induced by sphingosine 1-phosphate in fibroblasts: involvement of NADPH oxidase and platelet-derived growth factor receptor

BACKGROUND

Sphingosine 1-phosphate (S1P) is a sphingolipid metabolite synthesized after stimulation with growth factors or cytokines. S1P extracellular effects are mediated through specific Gi-protein coupled receptors (GPCRs). Recently, we demonstrated in NIH3T3 fibroblasts stimulated by platelet-derived growth factor (PDGF) or S1P the NADPH oxidase activation and the H(2)O(2) intracellular level increase trough the Gi protein involvement.

METHODS

NIH3T3 fibroblast cell cultures were used. Western blot and quantitative analyses by Chemidoc-Quantity-One software were performed. H(2)O(2) level was assayed by fluorescence spectrophotometric analysis, and cell proliferation by counted manually or ELISA kit.

RESULTS

This study demonstrates, in NIH 3T3 fibroblasts, a novel redox regulated mechanism of S1P-induced activation of ERK 1/2 related to NADPH oxidase activity and intracellular H(2)O(2) level increase with PDGF receptor tyrosine kinase involvement through a transactivation mechanism. This event is mediated by S1P(1) and S1P(3) receptors by Gi proteins and can contribute to S1P mitogenic signaling.

CONCLUSION

These results can be related to mechanisms of cross-talk previously identified between receptor tyrosine kinase, including PDGFreceptor, and several GPCR ligands.

GENERAL SIGNIFICANCE

The redox-sensitive ERK1/2 and PDGFr tyrosine kinase activity could be targets for therapies in diseases in which deregulation of intracellular oxidative status and the consequent alteration of S1P and/or PDGF signaling pathway are involved.

12/15-lipoxygenase-derived lipid peroxides control receptor tyrosine kinase signaling through oxidation of protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs) are regulated through reversible oxidation of the active-site cysteine. Previous studies have implied soluble reactive oxygen species (ROS), like H(2)O(2), as the mediators of PTP oxidation. The potential role(s) of peroxidized lipids in PTP oxidation have not been described. This study demonstrates that increases in cellular lipid peroxides, induced by disruption of glutathione peroxidase 4, induce cellular PTP oxidation and reduce the activity of PDGF receptor targeting PTPs. These effects were accompanied by site-selective increased PDGF beta-receptor phosphorylation, sensitive to 12/15-lipoxygenase (12/15-LOX) inhibitors, and increased PDGF-induced cytoskeletal rearrangements. Importantly, the 12/15-LOX-derived 15-OOH-eicosatetraenoic acid lipid peroxide was much more effective than H(2)O(2) in induction of in vitro PTP oxidation. Our study thus establishes that lipid peroxides are previously unrecognized inducers of oxidation of PTPs. This identifies a pathway for control of receptor tyrosine kinase signaling, which might also be involved in the etiology of diseases associated with increased lipid peroxidation.

Proteomics study of oxidative stress and Src kinase inhibition in H9C2 cardiomyocytes: a cell model of heart ischemia-reperfusion injury and treatment

Protein phosphorylation plays a crucial role in the signal transduction pathways that regulate gene expression, metabolism, cell adhesion, and cell survival in response to oxidative stress. In this study, we have used hydrogen peroxide treatment of H9C2 rat cardiomyocytes as a model of oxidative stress in heart ischemia-reperfusion injury. We show that oxidative stress induces a robust tyrosine phosphorylation of multiple proteins in this cell type. A phosphoproteomics approach using anti-phosphotyrosine affinity purification and LC-MS/MS was then used to identify the protein targets of this stress-induced phosphorylation. Twenty-three tyrosine-phosphorylated proteins were identified, with the majority known to be associated with cell-cell junctions, the actin cytoskeleton, and cell adhesion. This suggested that oxidative stress may have a profound effect on intercellular connections and the cytoskeleton to affect cell adhesion, morphology, and survival. Importantly, Src kinase was shown to be a major upstream regulator of these events. Immunofluorescence studies, fluorescence-activated cell sorting, and cell-based assays were used to demonstrate oxidative stress-induced modification of cell adhesion structures and the cytoskeleton, induced de-adhesion, and increased apoptosis, which were reversed by treatment with the Src kinase inhibitor PP1. These data demonstrate the critical role of Src kinase in oxidative stress-induced phosphorylation and cell damage in cardiomyocytes and suggest that targeting this kinase may be an effective strategy for preventing ischemia-reperfusion injury in the heart.

ACP1 genotype, glutathione reductase activity, and riboflavin uptake affect cardiovascular risk in the obese

Erythrocyte acid phosphatase (ACP locus 1), also known as low-molecular-weight protein tyrosine phosphatase, has previously been associated to glycemia, dyslipidemia, and obesity. In this study, ACP1 genotype and activity were tested in 318 women aged 19 to 83 (mean, 51.74 +/- 13.44) years. ACP1 genotype was found to directly correlate to glutathione reductase activity (P < .001) and levels of low-density lipoprotein cholesterol (P = .038). Glutathione reductase activity was in turn found to correlate to a series of cardiovascular risk factors such as systolic arterial pressure (P < .001), total cholesterol levels (P = .018), and low-density lipoprotein cholesterol levels (P = .039). A possible protective effect of ACP1 genotype AA against these cardiovascular risk factors was observed in this study. Furthermore, this work hypothesizes that nutritional riboflavin uptake becomes more crucial as body mass index increases, to counteract oxidative stress and minimize cardiovascular risk. This might be especially true in ACP1 genotypes AC, BC, and CC, which might possibly show the least endogenous protection against oxidative stress.

A PTP4A3 peptide PIMAP39 modulates TNF-alpha levels and endotoxic shock

Lipopolysaccharide (LPS) stimulation of macrophages initiates intracellular signaling pathways leading to activation of MAPK and its subsequent influence on cytokine production. We recently identified a LITAF-STAT6(B) complex regulated by p38 MAPK in response to LPS stimulation. However, the LPS-induced cascade in the p38/LITAF/TNF signaling pathway remains unclear. Here, we identified PTP4A3, a protein tyrosine phosphotase, as a novel negative regulator of LPS-induced LITAF/TNF-alpha production. PTP4A3 exerts its negative role by dephosphorylating p38 alpha MAPK in response to LPS stimulation of primary macrophages. PTP4A3 expression is upregulated in primary macrophages. Further structure-function analysis revealed that a unique short peptide (PIMAP39) derived from PTP4A3 is capable of mimicking the functionality of full-length PTP4A3 to selectively dephosphorylate p38 alpha and indirectly suppress LPS-induced LITAF-STAT6B complex when it is translocated from the cytoplasmic region to the nucleus of the cell. Treatment of mice with PIMAP39 significantly attenuates the severity of adverse host responses to LPS stimulation, and in some cases provides complete resistance to a lethal dose of LPS due to suppression of TNF-alpha production. All together, these results reveal a previously unrecognized role for the PTP4A3 pathway in response to LPS.

NADPH oxidase promotes pancreatic cancer cell survival via inhibiting JAK2 dephosphorylation by tyrosine phosphatases

BACKGROUND & AIMS

Growth factors, such as insulin-like growth factor-1 (IGF-I), protect pancreatic cancer (PaCa) cells from death. We recently showed that reactive oxygen species (ROS) produced by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase Nox4 mediate the antiapoptotic effect of growth factors. Here, we examine the mechanisms of the antiapoptotic role of NADPH oxidase. We hypothesized that ROSs produced by NADPH oxidase inhibit key protein tyrosine phosphatases (PTPs) and thus sustain the activation of kinases mediating antiapoptotic pathways in PaCa cells.

METHODS

Transfections and pharmacologic inhibition were used to assess the effects of NADPH oxidase on Janus kinase 2 (JAK2) kinase, the low molecular weight-protein tyrosine phosphatase (LMW-PTP), and apoptosis.

RESULTS

We found that 1 target of ROSs is JAK2, an important antiapoptotic kinase in PaCa cells. Both serum-induced and IGF-I biphasic JAK2 phosphorylation, with a rapid (minutes) and transient first phase, and a slow and sustained (24-72 hours) second phase. Nox4 mediated the sustained phase of JAK2 phosphorylation, which was required for the antiapoptotic effects of IGF-I and serum. Transfection experiments identified the LMW-PTP as a negative regulator of sustained JAK2 phosphorylation. Growth factors inhibited LMW-PTP through its oxidation by NADPH oxidase. LMW-PTP colocalizes with Nox4 both in PaCa cells and in human pancreatic adenocarcinoma.

CONCLUSIONS

The results suggest a novel signaling pathway, in which NADPH oxidase activation results in inhibition of PTPs, such as LMW-PTP, leading, in turn, to enhanced and sustained phosphorylation of kinases, such as JAK2, and suppression of apoptosis. This pathway mediates the prosurvival effect of ROSs and suggests new targets for pancreatic cancer treatment.

Low molecular weight protein tyrosine phosphatase (LMW-PTP) and its possible physiological functions of redox signaling in the eye lens

Low molecular weight protein tyrosine phosphatase (LMW-PTP) was cloned from human lens epithelial B3 cells (HLE B3) and the recombinant enzyme was purified to homogeneity. The pure enzyme reacted positively with anti-LMW-PTP antibody, displayed tyrosine-specific phosphatase activity and was extremely sensitive to H(2)O(2). The inactivated LMW-PTP could be regenerated by thioltransferase (TTase)/GSH system as demonstrated by both activity assay and by mass spectrometry (MS). The MS study also showed that an intramolecular disulfide bond was formed between C13 and C18 at the active site, and was reduced by the TTase/GSH system. The putative role of LMW-PTP in regulating platelet derived growth factor (PDGF)-stimulated cell signaling was demonstrated in wild type mouse lens epithelial cells (LEC) in which LMW-PTP was transiently inactivated, corroborated with the transient phosphorylation of Tyr857 at the active site of PDGF receptor and the downstream signaling components of Akt and ERK1/2. In contrast, LMW-PTP activity in PDGF-stimulated LEC from TTase(-/-) mice was progressively lost, concomitant with the high basal and sustained high phosphorylation levels at Tyr857, Akt and ERK1/2. We conclude that the reversible LMW-PTP activity regulated by ROS-mediated oxidation and TTase/GSH reduction is the likely mechanism of redox signaling in lens epithelial cells.

Reduction of low molecular weight protein-tyrosine phosphatase expression improves hyperglycemia and insulin sensitivity in obese mice

To investigate the role of low molecular weight protein-tyrosine phosphatase (LMW-PTP) in glucose metabolism and insulin action, a specific antisense oligonucleotide (ASO) was used to reduce its expression both in vitro and in vivo. Reduction of LMW-PTP expression with the ASO in cultured mouse hepatocytes and in liver and fat tissues of diet-induced obese (DIO) mice and ob/ob mice led to increased phosphorylation and activity of key insulin signaling intermediates, including insulin receptor-beta subunit, phosphatidylinositol 3-kinase, and Akt in response to insulin stimulation. The ASO-treated DIO and ob/ob animals showed improved insulin sensitivity, which was reflected by a lowering of both plasma insulin and glucose levels and improved glucose and insulin tolerance in DIO mice. The treatment did not decrease body weight or increase metabolic rate. These data demonstrate that LMW-PTP is a key negative regulator of insulin action and a potential novel target for the treatment of insulin resistance and type 2 diabetes.

A novel redox-based switch: LMW-PTP oxidation enhances Grb2 binding and leads to ERK activation

Low molecular weight-PTP has been reported as a redox-sensitive protein during both platelet-derived growth factor and integrin signalling. In response to oxidation the phosphatase undergoes a reversible inactivation, which in turn leads to the increase in tyrosine phosphorylation of its substrates and the properly executed anchorage-dependent proliferation program. Here, we report that an exogenous oxidative stress enhances LMW-PTP tyrosine phosphorylation, through oxidation/inactivation of the enzyme, thus preventing its auto-dephosphorylation activity. In particular, we observed a selective hyper-phosphorylation of Tyr132, that acts as a docking site for the adaptor protein Grb2. The redox-dependent enhancement of Grb2 recruitment to LMW-PTP ultimately leads to an improvement of ERK activation, likely triggering a prosurvival signal against the oxidant environment.

Redox regulation of platelet-derived-growth-factor-receptor: Role of NADPH-oxidase and c-Src tyrosine kinase

This study identifies some early events contributing to the redox regulation of platelet-derived growth factor receptor (PDGFr) activation and its signalling in NIH3T3 fibroblasts. We demonstrate for the first time that the redox regulation of PDGFr tyrosine autophosphorylation and its signalling are related to NADPH oxidase activity through protein kinase C (PKC) and phosphoinositide-3-kinase (PI3K) activation and H2O2 production. This event is also essential for complete PDGF-induced activation of c-Src kinase by Tyr416 phosphorylation, and the involvement of c-Src kinase on H2O2-induced PDGFr tyrosine phosphorylation is demonstrated, suggesting a role of this kinase on the redox regulation of PDGFr activation. Finally, it has been determined that not only PI3K activity, but also PKC activity, are related to NADPH oxidase activation due to PDGF stimulation in NIH3T3 cells, as it occurs in non-phagocyte cells. Therefore, we suggest a redox circuit whereby, upon PDGF stimulation, PKC, PI3K and NADPH oxidase activity contribute to complete c-Src kinase activation, thus promoting maximal phosphorylation and activation of PDGFr tyrosine phosphorylation.

Identification of MMP-15 as an anti-apoptotic factor in cancer cells

We have performed an in vitro selection for an anti-apoptotic phenotype that resembles the selection process that pre-malignant cells undergo in the initial phase of carcinogenesis in vivo. Using the cervical carcinoma cell line HeLa S3 as a model system, the selection procedure yielded cell clones that displayed increased resistance to apoptosis induced by Fas, tumor necrosis factor-related apoptosis-inducing ligand, and serum starvation. Gene expression profiling using gene family focused cDNA arrays revealed numerous genes that are differentially expressed in HeLa S3 and the resistant subclones and therefore are potentially involved in the definition of sensitivity to apoptotic stimuli. From the genes identified in this functional genomics approach we validated the anti-apoptotic activity of the membrane-anchored matrix metalloproteinase 15 (MMP-15) by means of small interfering RNA-mediated knock-down and ectopic expression in parental HeLa S3 cells and, to confirm a more general significance of our findings, in other cancer cell lines. The in vivo relevance of these findings is supported by the overexpression of MMP-15 in human lung adenocarcinoma compared with normal lung. Because MMP-15 is known to promote invasion, our results suggest that this protease connects metastasis and apoptosis resistance by an unknown regulatory mechanism. Our findings therefore strongly suggest that cancer characteristics such as metastatic potential, which are thought to evolve late in cancer progression, could be manifested early on by selection for an anti-apoptotic phenotype.

On the cross-regulation of protein tyrosine phosphatases and receptor tyrosine kinases in intracellular signaling

Intracellular signaling proteins are very often regulated by site-specific phosphorylation. For example, growth factor receptors in eukaryotic cells contain intrinsic tyrosine kinase activity and use inter- and intra-molecular interactions to recruit and orient potential protein substrates for phosphorylation. Equally important in determining the magnitude and kinetics of such a response is protein dephosphorylation, catalysed by phosphatase enzymes. A growing body of evidence indicates that certain protein tyrosine phosphatases (PTPs), like tyrosine kinases, are affected by intermolecular interactions that alter the specific activity or localization of their catalytic domains. Using a detailed kinetic modeling framework, we theoretically explore the regulation of PTPs through their association with receptor tyrosine kinases, as noted for the Src homology 2-domain-containing PTPs, SHP-1 and -2. Receptor-PTP binding, in turn, is expected to influence the phosphorylation pattern of those receptors and proteins they associate with, and we show how PTPs might serve to co- or counter-regulate parallel pathways in a signaling network.

Mechanism of platelet-derived growth factor-dependent caveolin-1 phosphorylation: relationship to sterol binding and the role of serine-80

In human vascular smooth muscle cells, inhibitors of protein kinase C activity reduced serine phosphorylation of caveolin-1 and increased sterol binding by this protein. This was measured after immunoprecipitation of caveolin-1 from cells labeled with tritiated cholesterol or the photoactivable cholesterol analogue FCBP [Fielding et al. (2002) Biochemistry 41, 4929-4937]. At the same time cellular sterol efflux was inhibited. Mutagenesis within a caveolin-1 central domain (residues 80-104) suggested a major role for serine-80 in mediating both of these effects. To perturb sterol binding, platelet-derived growth factor was added to the cells, leading to a transient loss of caveolin-1-associated sterol. Under these conditions, sterol efflux was stimulated, and caveolin-1 phosphorylation at tyrosine(14), assayed with a selective antibody, was substantially increased above baseline levels. These changes were also blocked by inhibitors of protein kinase C activity. Selective inhibitors of the platelet-derived growth factor receptor and downstream kinases were used to show that loss of sterol from caveolin-1 preceded tyrosine phosphorylation, but relipidation was dependent on phosphotyrosine hydrolysis.

Platelet derived growth factor (PDGF)-induced reactive oxygen species in the lens epithelial cells: the redox signaling

Low level of reactive oxygen species (ROS) has been shown to play an important role in host defense and mediating mitogen-stimulated cell signaling in several cell types. This study is to identify the mitogen-induced endogenous ROS generation and the range of exogenous H(2)O(2) that initiate redox signaling and cell proliferation in human lens epithelial cells (HLE B3), using platelet-derived growth factor (PDGF) as a model. To detect ROS generation, serum starved HLE cells (1.6 million) were loaded with fluorescent dye, 2',7'-dichlorofluorescin diacetate (DCFH-DA), before exposing to PDGF (1 ng ml(-1)). The fluorescence generated from the oxidant-sensitive DCFH, the intracellular product of DCFH-DA hydrolysate, was immediately measured in live cells by confocal laser light microscopy (lambda(Ex)=488 nm, lambda(Em)=522 nm, laser power=10%). PDGF-stimulated cells showed strong transient fluorescence during the 60 min while no fluorescence could be seen in the unstimulated cells. The PDGF-induced fluorescence could be suppressed with cells preloaded with N-acetyl-L-cysteine (NAC, 30 mm), catalase (1 mg ml(-1)), or D-mannitol (100mm). The ability of catalase to penetrate and function in HLE cells was confirmed by western blot, enzyme activity and immunofluorescence microscopic analyses. PDGF induced DNA synthesis within one hour as measured by (3)H-thymidine incorporation, and transiently activated the mitogen-activated protein kinases (MAPKs) of ERK1/2 and JNK. PDGF-stimulated DNA synthesis and MAPK activation were eliminated in the presence of catalase or mannitol. Low levels of H(2)O(2) (10-20 microm) mimicked PDGF in both MAPK stimulation and cell proliferation. In conclusion, the mitogenic stimulus function of PDGF in HLE cells appears to be mediated via ROS to activate MAPKs and cell proliferation, which can be mimicked by low levels of H(2)O(2). It is proposed that the physiological function of ROS, the redox signaling, is present in the HLE cells and may play an important role in the development and maintenance of the lens.

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.

Redox regulation of protein tyrosine phosphatases during receptor tyrosine kinase signal transduction

In addition to protein phosphorylation, redox-dependent post-translational modification of proteins is emerging as a key signaling system that has been conserved throughout evolution and that influences many aspects of cellular homeostasis. Both systems exemplify dynamic regulation of protein function by reversible modification, which, in turn, regulates many cellular processes such as cell proliferation, differentiation and apoptosis. In this article we focus on the interplay between phosphorylation- and redox-dependent signaling at the level of phosphotyrosine phosphatase-mediated regulation of receptor tyrosine kinases (RTKs). We propose that signal transduction by oxygen species through reversible phosphotyrosine phosphatase inhibition, represents a widespread and conserved component of the biochemical machinery that is triggered by RTKs.

Kinetic analysis of platelet-derived growth factor receptor/phosphoinositide 3-kinase/Akt signaling in fibroblasts

Isoforms of the serine-threonine kinase Akt coordinate multiple cell survival pathways in response to stimuli such as platelet-derived growth factor (PDGF). Activation of Akt is a multistep process, which relies on the production of 3'-phosphorylated phosphoinositide (PI) lipids by PI 3-kinases. To quantitatively assess the kinetics of PDGF receptor/PI 3-kinase/Akt signaling in fibroblasts, a systematic study of this pathway was performed, and a mechanistic mathematical model that describes its operation was formulated. We find that PDGF receptor phosphorylation exhibits positive cooperativity with respect to PDGF concentration, and its kinetics are quantitatively consistent with a mechanism in which receptor dimerization is initially mediated by the association of two 1:1 PDGF/PDGF receptor complexes. Receptor phosphorylation is transient at high concentrations of PDGF, consistent with the loss of activated receptors upon endocytosis. By comparison, Akt activation responds to lower PDGF concentrations and exhibits more sustained kinetics. Further analysis and modeling suggest that the pathway is saturated at the level of PI 3-kinase activation, and that the p110alpha catalytic subunit of PI 3-kinase contributes most to PDGF-stimulated 3'-PI production. Thus, at high concentrations of PDGF the kinetics of 3'-PI production are limited by the turnover rate of these lipids, while the Akt response is additionally influenced by the rate of Akt deactivation.