Anoikis: an emerging hallmark in health and diseases

Anoikis is a programmed cell death occurring upon cell detachment from the correct extracellular matrix, thus disrupting integrin ligation. It is a critical mechanism in preventing dysplastic cell growth or attachment to an inappropriate matrix. Anoikis prevents detached epithelial cells from colonizing elsewhere and is thus essential for tissue homeostasis and development. As anchorage-independent growth and epithelial-mesenchymal transition, two features associated with anoikis resistance, are crucial steps during tumour progression and metastatic spreading of cancer cells, anoikis deregulation has now evoked particular attention from the scientific community. The aim of this review is to analyse the molecular mechanisms governing both anoikis and anoikis resistance, focusing on their regulation in physiological processes, as well as in several diseases, including metastatic cancers, cardiovascular diseases and diabetes.

HNE interacts directly with JNK isoforms in human hepatic stellate cells

4-Hydroxy-2,3-nonenal (HNE) is an aldehydic end product of lipid peroxidation which has been detected in vivo in clinical and experimental conditions of chronic liver damage. HNE has been shown to stimulate procollagen type I gene expression and synthesis in human hepatic stellate cells (hHSC) which are known to play a key role in liver fibrosis. In this study we investigated the molecular mechanisms underlying HNE actions in cultured hHSC. HNE, at doses compatible with those detected in vivo, lead to an early generation of nuclear HNE-protein adducts of 46, 54, and 66 kD, respectively, as revealed by using a monoclonal antibody specific for HNE-histidine adducts. This observation is related to the lack of crucial HNE-metabolizing enzymatic activities in hHSC. Kinetics of appearance of these nuclear adducts suggested translocation of cytosolic proteins. The p46 and p54 isoforms of c-Jun amino-terminal kinase (JNKs) were identified as HNE targets and were activated by this aldehyde. A biphasic increase in AP-1 DNA binding activity, associated with increased mRNA levels of c-jun, was also observed in response to HNE. HNE did not affect the Ras/ERK pathway, c-fos expression, DNA synthesis, or NF-kappaB binding. This study identifies a novel mechanism linking oxidative stress to nuclear signaling in hHSC. This mechanism is not based on redox sensors and is stimulated by concentrations of HNE compatible with those detected in vivo, and thus may be relevant during chronic liver diseases.

Cancer-associated fibroblasts and M2-polarized macrophages synergize during prostate carcinoma progression

Inflammation is now acknowledged as an hallmark of cancer. Cancer-associated fibroblasts (CAFs) force a malignant cross talk with cancer cells, culminating in their epithelial-mesenchymal transition and achievement of stemness traits. Herein, we demonstrate that stromal tumor-associated cells cooperate to favor malignancy of prostate carcinoma (PCa). Indeed, prostate CAFs are active factors of monocyte recruitment toward tumor cells, mainly acting through stromal-derived growth factor-1 delivery and promote their trans-differentiation toward the M2 macrophage phenotype. The relationship between M2 macrophages and CAFs is reciprocal, as M2 macrophages are able to affect mesenchymal-mesenchymal transition of fibroblasts, leading to their enhanced reactivity. On the other side, PCa cells themselves participate in this cross talk through secretion of monocyte chemotactic protein-1, facilitating monocyte recruitment and again macrophage differentiation and M2 polarization. Finally, this complex interplay among cancer cells, CAFs and M2 macrophages, cooperates in increasing tumor cell motility, ultimately fostering cancer cells escaping from primary tumor and metastatic spread, as well as in activation of endothelial cells and their bone marrow-derived precursors to drive de novo angiogenesis. In keeping with our data obtained in vitro, the analysis of patients affected by prostate cancers at different clinical stages revealed a clear increase in the M2/M1 ratio in correlation with clinical values. These data, coupled with the role of CAFs in carcinoma malignancy to elicit expression of stem-like traits, should focus great interest for innovative strategies aimed at the co-targeting of inflammatory cells and fibroblasts to improve therapeutic efficacy.

Two vicinal cysteines confer a peculiar redox regulation to low molecular weight protein tyrosine phosphatase in response to platelet-derived growth factor receptor stimulation

Low molecular weight protein tyrosine phosphatase (LMW-PTP) is an enzyme involved in platelet-derived growth factor (PDGF)-induced mitogenesis and cytoskeleton rearrangement because it is able to bind and dephosphorylate the activated receptor. LMW-PTP presents two cysteines in positions 12 and 17, both belonging to the catalytic pocket; this is a unique feature of LMW-PTP among all protein tyrosine phosphatases. Our previous results demonstrated that in vitro LMW-PTP is oxidized by either H(2)O(2) or nitric oxide with the formation of a disulfide bond between Cys-12 and Cys-17. This oxidation leads to reversible enzyme inactivation because treatment with reductants permits catalytic activity rescue. In the present study we investigated the in vivo inactivation of LMW-PTP by either extracellularly or intracellularly generated H(2)O(2), evaluating its action directly on its natural substrate, PDGF receptor. LMW-PTP is oxidized and inactivated by exogenous oxidative stress and recovers its activity after oxidant removal. LMW-PTP is oxidized also during PDGF signaling, very likely upon PDGF-induced H(2)O(2) production, and recovers its activity within 40 min. Our results strongly suggest that reversibility of in vivo LMW-PTP oxidation is glutathione-dependent. In addition, we propose an intriguing and peculiar role of Cys-17 in the formation of a S-S intramolecular bond, which protects the catalytic Cys-12 from further and irreversible oxidation. On the basis of our results we propose that the presence of an additional cysteine near the catalytic cysteine could confer to LMW-PTP the ability to rapidly recover its activity and finely regulate PDGF receptor activation during both extracellularly and intracellularly generated oxidative stress.

Lactate modulates CD4+ T-cell polarization and induces an immunosuppressive environment, which sustains prostate carcinoma progression via TLR8/miR21 axis

Leukocyte infiltration plays an active role in controlling tumor development. In the early stages of carcinogenesis, T cells counteract tumor growth. However, in advanced stages, cancer cells and infiltrating stromal components interfere with the immune control and instruct immune cells to support, rather than counteract, tumor malignancy, via cell-cell contact or soluble mediators. In particular, metabolites are emerging as active players in driving immunosuppression. Here we demonstrate that in a prostate cancer model lactate released by glycolytic cancer-associated fibroblasts (CAFs) acts on CD4⁺ T cells, shaping T-cell polarization. In particular, CAFs exposure (i) reduces the percentage of the antitumoral Th1 subset, inducing a lactate-dependent, SIRT1-mediated deacetylation/degradation of T-bet transcription factor; (ii) increases T_reg cells, driving naive T cells polarization, through a lactate-based NF-kB activation and FoxP3 expression. In turn, this metabolic-based CAF-immunomodulated environment exerts a pro-invasive effect on prostate cancer cells, by activating a previously unexplored miR21/TLR8 axis that sustains cancer malignancy.

Low molecular weight protein tyrosine phosphatases: small, but smart

Low molecular weight protein tyrosine phosphatases (LMW-PTPs) are a family of 18-kDa enzymes involved in cell growth regulation. Despite very limited sequence similarity to the PTP superfamily, they display a conserved signature motif in the catalytic site. LMW-PTP associates and dephosphorylate many growth factor receptors, such as platelet-derived growth factor receptor (PDGF-r), insulin receptor and ephrin receptor, thus downregulating many of the tyrosine kinase receptor functions that lead to cell division. In particular, LMW-PTP acts on both growth-factor-induced mitosis, through dephosphorylation of activated PDGF-r, and on cytoskeleton rearrangement, through dephosphorylation of p190RhoGAP and the consequent regulation of the small GTPase Rho. LMW-PTP activity is modulated by tyrosine phosphorylation on two specific residues, each of them with specific characteristics. LMW-PTP activity on specific substrates depends also on its localization. Moreover, LMW-PTP is reversibly oxidized during growth factor signaling, leading to inhibition of its enzymatic activity. Recovery of phosphatase activity depends on the availability of reduced glutathione and involves the formation of an S-S bridge between the two catalytic site cysteines. Furthermore, studies on the redox state of LMW-PTP in contact-inhibited cells and in mature myoblasts suggest that LMW-PTP is a general and versatile modulator of growth inhibition.

The role of Cys12, Cys17 and Arg18 in the catalytic mechanism of low-M(r) cytosolic phosphotyrosine protein phosphatase

Low-M(r) phosphotyrosine protein phosphatase (PTPase), previously known as low-M(r) acid phosphatase, catalyzes the in-vitro hydrolysis of tyrosine phosphorylated proteins, low-M(r) aryl phosphates and natural and synthetic acyl phosphates. Its activity on Ser/Thr-phosphorylated proteins and on most alkyl phosphates is very poor. In this study the mechanism of benzoyl-phosphate hydrolysis was studied by means of non-mutated and mutated PTPase fusion proteins. The mechanism of benzoyl-phosphate hydrolysis catalyzed by the enzyme was compared to the known mechanism of p-nitrophenyl-phosphate hydrolysis. The results demonstrated that both hydrolytic processes proceed through common enzyme-catalyzed mechanisms. Nevertheless, the performed phosphoenzyme-trapping experiments enable us to identify Cys12 as the active-site residue that performs the nucleophilic attack at the phosphorus atom of the substrate to produce a phosphoenzyme covalent intermediate. In addition, while the role of Cys17 in the substrate binding was confirmed, its participation a second time in the step that involves the Cys12 dephosphorylation was suggested by the results of phosphoenzyme-trapping experiments. The participation of Arg18 in the substrate-binding site was demonstrated by site-directed mutagenesis that produced the conservative Lys18 and the non-conservative Met18 mutants. Both these mutants were almost inactive and not able to bind the substrate and a competitive inhibitor. Furthermore, phosphoenzyme-trapping experiments clearly excluded that Cys62 and Cys145 (that were indicated by another laboratory to be involved in the active site of the enzyme as powerful nucleophilic agents) are the residues directly involved in the formation of the phosphoenzyme covalent intermediate.

LMW-PTP is a negative regulator of insulin-mediated mitotic and metabolic signalling

To understand the physiological role of low Mr weight phosphotyrosine protein phosphatase (LMW-PTP) in insulin mediated signaling, we established clonal cell lines overexpressing the dominant negative (C12S mutant) LMW-PTP (dnLMW-PTP) from NIH3T3 murine fibroblasts expressing insulin receptor. Upon insulin stimulation we observe an association between the dnLMW-PTP and the beta-subunit of the insulin receptor. This association is dependent on the tyrosine phosphorylation of the insulin receptor since it is not observed in unstimulated cells. Furthermore, in vitro binding experiments between dnLMW-PTP and the insulin receptor reveal that the interaction is mediated by the LMW-PTP catalytic site, as indicated by competition with orthovanadate. DnLMW-PTP overexpression influences both the mitogenic and the metabolic bioeffects of insulin. In particular, in cells overexpressing dnLMW-PTP we observe an increase in the glycogenosynthesis rate and in mitosis as indicated by glucose incorporation into glycogen and thymidine incorporation into DNA, respectively. Moreover, we studied the insulin mediated signal transduction pathways starting from insulin receptor, such as the Src kinase, the p21Ras/ERK, and the PI3K routes. Our findings are consistent with a specific regulation of mitogenesis by LMW-PTP through a pathway involving c-Src kinase but independent by both PI3K and ERK. These data strongly suggest that LMW-PTP acts as a negative regulator of both mitogenetic and metabolic insulin signalling.

PDGF receptor as a specific in vivo target for low M(r) phosphotyrosine protein phosphatase

Low M(r) phosphotyrosine protein phosphatase (LMW-PTP) is a 18 kDa cytosolic enzyme widely distributed in eukaryotic cells. LMW-PTP catalyses the hydrolysis of phosphotyrosine residues and overexpression of the enzyme in normal and transformed cells inhibits cell proliferation. Site directed mutagenesis, together with crystallographic studies, have contributed to clarify the catalytic mechanism, which involves the active site signature sequence C12XXXXXR18, a main feature of all PTPase family members. In order to identify the LMW-PTP substrate/s we have expressed in NIH-3T3 cells a catalytically inert Cys12 to Ser phosphatase mutant which has preserved its capacity for substrate binding. Overexpression of the mutant phosphatase leads to enhanced cell proliferation and serum induced mitogenesis, indicating that the mutation results in the production of a dominant negative protein. Analysis of mutant LMW-PTP expressing cells has enabled us to demonstrate an association between LMW-PTP and platelet derived growth factor receptor that appears to be highly specific. Our data suggest a catalytic action of LMW-PTP on the phosphorylated platelet derived growth factor receptor.

The low M(r) protein-tyrosine phosphatase is involved in Rho-mediated cytoskeleton rearrangement after integrin and platelet-derived growth factor stimulation

The low molecular weight protein-tyrosine phosphatase (LMW-PTP) is an enzyme that is involved in the early events of platelet-derived growth factor (PDGF) receptor signal transduction. In fact, LMW-PTP is able to specifically bind and dephosphorylate activated PDGF receptor, thus modulating PDGF-induced mitogenesis. In particular, LMW-PTP is involved in pathways that regulate the transcription of the immediately early genes myc and fos in response to growth factor stimulation. Recently, we have found that LMW-PTP exists constitutively in cytosolic and cytoskeleton-associated localization and that, after PDGF stimulation, c-Src is able to bind and phosphorylate LMW-PTP only in the cytoskeleton-associated fraction. As a consequence of its phosphorylation, LMW-PTP increases its catalytic activity about 20-fold. In this study, our interest was to investigate the role of LMW-PTP phosphorylation in cellular response to PDGF stimulation. To address this issue, we have transfected in NIH-3T3 cells a mutant form of LMW-PTP in which the c-Src phosphorylation sites (Tyr(131) and Tyr(132)) were mutated to alanine. We have established that LMW-PTP phosphorylation by c-Src after PDGF treatment strongly influences both cell adhesion and migration. In addition, we have discovered a new LMW-PTP substrate localized in the cytoskeleton that becomes tyrosine-phosphorylated after PDGF treatment: p190Rho-GAP. Hence, LMW-PTP plays multiple roles in PDGF receptor-mediated mitogenesis, since it can bind and dephosphorylate PDGF receptor, and, at the same time, the cytoskeleton-associated LMW-PTP, through the regulation of the p190Rho-GAP phosphorylation state, controls the cytoskeleton rearrangement in response to PDGF stimulation.

The Src and signal transducers and activators of transcription pathways as specific targets for low molecular weight phosphotyrosine-protein phosphatase in platelet-derived growth factor signaling

The low molecular weight phosphotyrosine-protein phosphatase (LMW-PTP) is a cytosolic phosphotyrosine-protein phosphatase specifically interacting with the activated platelet-derived growth factor (PDGF) receptor through its active site. Overexpression of the LMW-PTP results in modulation of PDGF-dependent mitogenesis. In this study we investigated the effects of this tyrosine phosphatase on the signaling pathways relevant for PDGF-dependent DNA synthesis. NIH 3T3 cells were stably transfected with active or dominant negative LMW-PTP. The effects of LMW-PTP were essentially restricted to the G1 phase of the cell cycle. Upon stimulation with PDGF, cells transfected with the dominant negative LMW-PTP showed an increased activation of Src, whereas the active LMW-PTP induced a reduced activation of this proto-oncogene. We observe that c-Src binding to PDGF receptor upon stimulation is prevented by overexpression of LMW-PTP. These effects were associated with parallel changes in myc expression. Moreover, wild-type and dominant negative LMW-PTP differentially regulated STAT1 and STAT3 activation and tyrosine phosphorylation, whereas they did not modify extracellular signal-regulated kinase activity. However, these modifications were associated with changes in fos expression despite the lack of any effect on extracellular signal-regulated kinase activation. Other independent pathways involved in PDGF-induced mitogenesis, such as phosphatidylinositol 3-kinase and phospholipase C-gamma1, were not affected by LMW-PTP. These data indicate that this phosphatase selectively interferes with the Src and the STATs pathways in PDGF downstream signaling. The resulting changes in myc and fos proto-oncogene expression are likely to mediate the modifications observed in the G1 phase of the cell cycle.

Inhibitory effect of full-length human endostatin on in vitro angiogenesis

Endostatin, a C-terminal product of collagen XVIII, is a very powerful angiogenesis inhibitor. In vivo experiments in mice indicate that endostatin dramatically reduces tumor mass without causing the onset of any resistance to the treatment. Recently, a 12-aa shorter human endostatin has been purified from plasma, but is ineffective in in vitro angiogenesis assays. Here we report that the full-length human recombinant endostatin has a potent inhibitory activity in in vitro angiogenesis assays. Two powerful angiogenic factors were used to stimulate endothelial cells: FGF-2 and VEGF-165. Endostatin prevented cell growth both in the basal condition and after stimulation with FGF-2 or VEGF-165. Migration of microvascular endothelial cells toward FGF-2 or VEGF-165 was impaired, both when cells were pretreated with the inhibitor and when endostatin was added together with the growth factors. Furthermore, experiments of inhibition of proliferation performed on nonmicroendothelial cells showed that endostatin was ineffective. This study indicates that human endostatin is a potent angiogenesis inhibitor and suggests its use in human anticancer therapy.

The low Mr phosphotyrosine protein phosphatase behaves differently when phosphorylated at Tyr131 or Tyr132 by Src kinase

The low molecular weight phosphotyrosine protein phosphatase (LMW-PTP) is phosphorylated by Src and Src-related kinases both in vitro and in vivo; in Jurkat cells, and in NIH-3T3 cells, it becomes tyrosine-phosphorylated upon stimulation by PDGF. In this study we show that pp60Src phosphorylates in vitro the enzyme at two tyrosine residues, Tyr131 and Tyr132, previously indicated as the main phosphorylation sites of the enzyme, whereas phosphorylation by the PDGF-R kinase is much less effective and not specific. The effects of LMW-PTP phosphorylation at each tyrosine residue were investigated by using Tyr131 and Tyr132 mutants. We found that the phosphorylation at either residue has differing effects on the enzyme behaviour: Tyr131 phosphorylation is followed by a strong (about 25-fold) increase of the enzyme specific activity, whereas phosphorylation at Tyr132 leads to Grb2 recruitment. These differing effects are discussed on the light of the enzyme structure.

c-Src activates both STAT1 and STAT3 in PDGF-stimulated NIH3T3 cells

Treatment of cells with PDGF and EGF specifically induces STAT1 and STAT3, which became phosphorylated on tyrosine residues to form homo and heterodimers: in these configurations they translocate into the nucleus where they act as transcription activators. However little is known about the activation of STATs in growth factor receptor signal transduction. Recently it has been shown that v-Src modulates the tyrosine phosphorylation of STAT3 but not of STAT1. Here we report that the cellular Src tyrosine kinase is involved in the activation of both STAT1 and STAT3 in PDGF stimulated NIH3T3 cells. Both tyrosine phosphorylation and DNA binding activity of STAT1 and STAT3 are up-regulated in c-Src overexpressing cells, while we observe the opposite phenomenon in cells overexpressing the dominant negative Src. Furthermore, our results show that STAT1 co-immunoprecipitates with c-Src, suggesting that the activation of STATs by Src occurs via a direct interaction. Taken together, these data suggest that c-Src is involved in activation of both STAT1 and 3 in PDGF signal transduction.

Aspartic-129 is an essential residue in the catalytic mechanism of the low M(r) phosphotyrosine protein phosphatase

The crystal structure of the bovine liver low M(r) phosphotyrosine protein phosphatase suggests the involvement of aspartic acid-129 in enzyme catalysis. The Asp-129 to alanine mutant has been prepared by oligonucleotide-directed mutagenesis of a synthetic gene coding for the enzyme. The purified mutant elicited an highly reduced specific activity (about 0.04% of the activity of the wild-type) and a native-like fold, as judged by 1H NMR spectroscopy. The kinetic analysis revealed that the mutant is able to bind the substrate and a competitive inhibitor, such as inorganic phosphate. Moreover, trapping experiments demonstrated it maintains the ability to form the E-P covalent complex. The Asp-129 to alanine mutant shows extremely reduced enzyme phosphorylation (k2) and dephosphorylation (k3) kinetic constant values as compared to the wild-type enzyme. The data reported indicate that aspartic acid-129 is likely to be involved both in the first step and in the rate-limiting step of the catalytic mechanism, i.e. the nucleophilic attack of the phosphorylated intermediate.

Low molecular weight protein-tyrosine phosphatase tyrosine phosphorylation by c-Src during platelet-derived growth factor-induced mitogenesis correlates with its subcellular targeting

The low molecular weight phosphotyrosine phosphatase (LMW-PTP) is an enzyme that is involved in the early events of platelet-derived growth factor (PDGF) receptor signal transduction. Our previous results have shown that LMW-PTP is able to specifically bind and dephosphorylate activated PDGF receptor, thus modulating PDGF-induced mitogenesis. In particular LMW-PTP is involved in pathways that regulate the transcription of the immediately early genes myc and fos in response to growth factor stimulation. In this study we have established that, in nontransformed NIH3T3 cells, LMW-PTP exists constitutively in cytosolic and cytoskeleton-associated localization and that, after PDGF stimulation, c-Src is able to bind and to phosphorylate LMW-PTP only in the cytoskeleton-associated fraction. As a consequence of its tyrosine phosphorylation, LMW-PTP significantly increases its catalytic activity. After PDGF stimulation these two LMW-PTP pools act on distinct substrates, contributing in different manners to the PDGF receptor signaling. The cytoplasmic LMW-PTP fraction exerts its well known action on activated PDGF receptor. On the other hand we have now demonstrated that the cytoskeleton-associated LMW-PTP acts specifically on a few not yet identified proteins that become tyrosine-phosphorylated in response to the PDGF receptor activation. Finally, these two LMW-PTP pools markedly differ in the timing of the processes in which they are involved. The cytoplasmic LMW-PTP pool exerts its action within a few minutes from PDGF receptor activation (short term action), while tyrosine phosphorylation of cytoskeleton-associated LMW-PTP lasts for more than 40 min (long term action). In conclusion LMW-PTP is a striking example of an enzyme that exerts different functions and undergoes different regulation in consequence of its subcellular localization.

The molecular basis of the differing kinetic behavior of the two low molecular mass phosphotyrosine protein phosphatase isoforms

The low molecular mass phosphotyrosine protein phosphatase is a cytosolic enzyme of 18 kDa. Mammalian species contain a single gene that codifies for two distinct isoenzymes; they are produced through alternative splicing and thus differ only in the sequence from residue 40 to residue 73. Isoenzymes differ also in substrate specificity and in the sensitivity to activity modulators. In our study, we mutated a number of residues included in the alternative 40-73 sequence by substituting the residues present in the type 2 isoenzyme with those present in type 1 and subsequently examined the kinetic properties of the purified mutated proteins. The results enabled us to identify the molecular site that determines the kinetic characteristics of each isoform; the residue in position 50 plays the main role in the determination of substrate specificity, while the residues in both positions 49 and 50 are involved in the strong activation of the type 2 low M(r) phosphotyrosine protein phosphatase isoenzyme by purine compounds such as guanosine and cGMP. The sequence 49-50 is included in a loop whose N terminus is linked to the beta 2-strand and whose C terminus is linked to the alpha 2-helix; this loop is very near the active site pocket. Our findings suggest that this loop is involved both in the regulation of the enzyme activity and in the determination of the substrate specificity of the two low M(r) phosphotyrosine protein phosphatase isoenzymes.

In vivo inactivation of phosphotyrosine protein phosphatases by nitric oxide

The effect of NO on phosphotyrosine protein phosphatases (PTPases) has been investigated in vivo. NO production is induced in interferon-gamma and lipopolysaccharide stimulated RAW-264.7 macrophages as indicated by the increase of NO2- in the medium. Our results demonstrate an inhibition of p-nitrophenylphosphatase activity as a consequence of macrophages activation. Under the described experimental conditions, most of the hydrolysis of p-nitrophenylphosphate can be ascribed to the action of cellular PTPases. The presence of NG-mono-methyl-L-arginine, a specific inhibitor of NO synthase decreases the inactivation rate of both membrane-bound and soluble PTPases. This evidence further confirms the ability of NO to inactivate PTPases and suggests a possible role of NO in the regulation of cellular processes involving this class of phosphatases.

Low M(r) phosphotyrosine protein phosphatase interacts with the PDGF receptor directly via its catalytic site

Many proteins bind to the activated platelet derived growth factor receptor (PDGF-R) either directly or by means of adapter molecules. Up to now all these proteins were shown to transmit and amplify the signal started with PDGF-R stimulation. In a recent study our group had demonstrated that low M(r) phosphotyrosine protein phosphatase (LMW-PTP) specifically interacts with PDGF-R in NIH3T3 cells. In the present study we have attempted to clarify the modality of interaction, both in vivo and in vitro, of these two proteins, using a catalytically inactive LMW-PTP mutant. Our results indicate that LMW-PTP and PDGF-R interact directly, without the necessity of any adapter protein. This interaction leads to PDGF-R dephosphorylation and, presumably, interrupts one or more of the mitogenic pathways that originate from receptor activation.

The redox regulation of LMW-PTP during cell proliferation or growth inhibition

Low-molecular-weight protein tyrosine phosphatase (LMW-PTP) is an enzyme involved in platelet derived growth factor (PDGF)-induced mitogenesis and cytoskeleton rearrangement, because it is able to bind and dephosphorylate the activated receptor. LMW-PTP contains two cysteines located in position 12 and 17, both inside the catalytic pocket: this is a unique feature of LMW-PTP among all protein tyrosine phosphatases. LMW-PTP is oxidized and inactivated both in vitro and in vivo by exogenous oxidative stress and by endogenously generated oxidative burst produced during PDGF signaling, and recovers its activity within 40 min. The reversibility of in vivo LMW-PTP oxidation is glutathione dependent. The additional catalytic pocket cysteine, in position 17, retains an intriguing and peculiar role in the formation of a S-S intramolecular bond, which protects the catalytic Cys12 from further and irreversible oxidation. The presence of an additional cysteine near the catalytic one, confers to LMW-PTP the ability to rapidly recover its activity and finely regulate PDGF receptor activation during both extracellularly and intracellularly generated oxidative stress. In addition, redox up-regulation of LMW-PTP is involved in intracellular delivery of antiproliferative signals, as the arrest of growth induced by cell confluence and differentiation are associated with a decrease in the steady-state levels of intracellular ROS. The studies of the redox state of LMW-PTP in contact-inhibited cells and in mature myoblasts demonstrate that the protein is reduced and activated in both these conditions, thus leading to the hypothesis that LMW-PTP is a modulator of growth inhibition.

Low molecular weight protein-tyrosine phosphatase is involved in growth inhibition during cell differentiation

Low molecular weight protein-tyrosine phosphatase (LMW-PTP) is an enzyme involved in mitogenic signaling and cytoskeletal rearrangement after platelet-derived growth factor (PDGF) stimulation. Recently, we demonstrated that LMW-PTP is regulated by a redox mechanism involving the two cysteine residues of the catalytic site, which turn reversibly from reduced to oxidized state after PDGF stimulation. Since recent findings showed a decrease of intracellular reactive oxygen species in contact inhibited cells and a lower tyrosine phosphorylation level in dense cultures in comparison to sparse ones, we studied if the level of endogenous LMW-PTP is regulated by growth inhibition conditions, such as cell confluence and differentiation. Results show that both cell confluence and cell differentiation up-regulate LMW-PTP expression in C2C12 and PC12 cells. We demonstrate that during myogenesis LMW-PTP is regulated at translational level and that the protein accumulates at the plasma membrane. Furthermore, we showed that both myogenesis and cell-cell contact lead to a dramatic decrease of tyrosine phosphorylation level of PDGF receptor. In addition, we observed an increased association of the receptor with LMW-PTP during myogenesis. Herein, we demonstrate that myogenesis decreases the intracellular level of reactive oxygen species, as observed in dense cultures. As a consequence, LMW-PTP turns from oxidized to reduced form during muscle differentiation, increasing its activity in growth inhibition conditions such as differentiation. These data suggest that LMW-PTP plays a crucial role in physiological processes, which require cell growth arrest such as confluence and differentiation.

Differential role of four cysteines on the activity of a low M(r) phosphotyrosine protein phosphatase

In this paper we describe the construction of five mutants of a bovine liver low M(r) phosphotyrosine protein phosphatase (PTPase) expressed as a fusion protein with the maltose binding protein in E. coli. Almost no changes in the kinetic parameters were observed in the fusion protein with respect to the native PTPase. Using oligonucleotide-directed mutagenesis Cys-17, Cys-62 and Cys-145 were converted to Ser while Cys-12 was converted to both Ser and Ala. The kinetic properties of the mutants, using p-nitrophenyl phosphate as substrate, were compared with those of the normal protein fused with the maltose binding protein of E. coli; both of the Cys-12 mutants showed a complete loss of enzymatic activity while the specific activity of the Cys-17 mutant was greatly decreased (200-fold). The Cys-62 mutant showed a 2.5-fold decrease in specific activity, while the Cys-145 mutant remained almost unchanged. These data confirm the involvement of Cys-12 and Cys-17 in the catalytic site and suggest that Cys-62 and Cys-145 mutations may destabilise the structure of the enzyme.

LMW-PTP exerts a differential regulation on PDGF- and insulin-mediated signaling

Low-molecular-weight protein tyrosine phosphatase (LMW-PTP) is able to specifically bind and dephosphorylate activated PDGF and insulin receptors, modulating the onset of mitogenic process. LMW-PTP is present in two distinct intracellular locations. While the cytosolic LMW-PTP pool interacts directly with activated insulin or PDGF receptors, the cytoskeleton-associated LMW-PTP is tyrosine phosphorylated upon PDGF stimulation and is involved in cytoskeleton rearrangement acting on p190Rho-GAP. We investigated the differential role of LMW-PTP in PDGF- or insulin-induced mitogenesis and cytoskeleton rearrangement. Dominant negative LMW-PTP influences both PDGF- and insulin-induced mitogenesis with a different extent and it induces a decrease in cellular adhesion and chemotaxis after PDGF but not insulin treatment. PDGF but not insulin stimulation leads to tyrosine phosphorylation of LMW-PTP. We propose that the differential effect of LMW-PTP on PDGF and insulin signaling is mainly due to the fact that during insulin signaling LMW-PTP does not become phosphorylated and thus does not act on its cytoskeleton-associated substrate/s.

The role of Cys-17 in the pyridoxal 5'-phosphate inhibition of the bovine liver low M(r) phosphotyrosine protein phosphatase

Mammalian tissues contain two low M(r) phosphotyrosine protein phosphatase isoforms (type-1 and type-2) that differ in the 40-73 amino-acid sequence. Only one isoform (type-2) is strongly inhibited by pyridoxal 5'-phosphate, whereas the other is poorly inhibited by this compound. The mechanism of pyridoxal 5'-phosphate inhibition of the bovine liver enzyme (a type-2 isoform) has been studied by kinetic methods using a series of pyridoxal 5'-phosphate analogues. These studies indicate that pyridoxal 5'-phosphate interacts with the enzyme in both the phosphate and aldehyde groups. Active site-directed mutagenesis has been used to investigate the sites of pyridoxal 5'-phosphate binding. Our results indicate that Cys-17, essential for enzyme activity, interacts with the phosphate moiety of pyridoxal 5'-phosphate. On the other hand, Cys-12, which is also involved in the catalytic mechanism, does not participate in pyridoxal 5'-phosphate binding.