During muscle ageing the activation of the mitogenic signalling is not sufficient to guarantee cellular duplication

Satellite cells are quiescent cells that can be induced to proliferate by a variety of stimuli such as injury and exercise, providing in this way a source of new myoblasts that repopulate the damaged muscle. It is well known that, as senescence progresses, the muscle regenerative potential progressively diminishes, but the molecular mechanisms underlying this process are not yet completely defined. Many growth factors, including Platelet Derived Growth Factor (PDGF-BB)*, have been associated to satellite cells activation, acting as potent mitogenic agents for these cells. The aim of this study is to explore if the diminished response of senescent myoblasts to growth stimuli could be due to the inability to receive and transduce hormonal signals. Herein, we demonstrate that that although PDGF-r expression is down-regulated during senescence, the receptor is fully able to be phosphorylated and to transmit the signal. Although senescent myoblasts display increased level of phosphotyrosine phosphatases (PTPs), neither the PDGF receptor (PDGF-r) phosphorylation level nor the citosolic signal transduction machinery is affected. Indeed, we demonstrated that senescent human myoblasts are able to initiate a proper mitogenic signalling cascade, since the activation of mitogen-activated protein kinases (MAPK) and phosphatydil inositole 3 kinase (PI-3K) pathways is similar in young and senescent cells. Our data underline that, despite a conserved capability to activate PDGF-r after agonist stimulation and a functional signal transduction machinery, the mitogenic signal initiated by growth factors in senescent cells does not lead to cell division, being unable to overcome the cell cycle block, likely caused by the accumulation of the inhibitor p21WAF1.

Up-regulated expression of low molecular weight protein tyrosine phosphatases in different human cancers

Protein tyrosine phosphorylation, mediated by the balanced action of tyrosine kinases and phosphatases, contributes to the regulation of the growth, migration, and invasion of normal and malignant cells. Among tyrosine phosphatases, low molecular weight protein tyrosine phosphatases (LMW-PTP) have been recognized as a possible "positive factor" in tumour onset and progression. The aim of this work was to assess whether LMW-PTP are differentially expressed in normal and malignant tissues. Using real-time PCR analysis we evaluated the expression levels of total LMW-PTP mRNA in surgical samples of breast, colon and lung cancers (63, 60, and 58, respectively), and in their paired adjacent not affected tissues. Moreover, the same analysis was carried out on a group of neuroblastomas (25 cases). Significant correlations between LMW-PTP overexpression and the most common clinical-pathological features of cancers exist. In colon cancer and neuroblastoma increased total LMW-PTP mRNA expression correlates with unfavourable outcome. While LMW-PTP mRNA expression increases in tumour samples, the relative contribution of the different isoforms does not change. Our findings indicate that LMW-PTP can be considered an oncogene as it is overexpressed in different tumour types and suggests that LMW-PTP enhanced expression is generally prognostic for a more aggressive cancer.

Insulin inhibits platelet-derived growth factor-induced cell proliferation

Cellular behavior can be considered to be the result of a very complex spatial and temporal integration of intracellular and extracellular signals. These signals arise from serum-soluble factors as well as from cell-substrate or cell-cell interactions. The current approach in mitogenesis studies is generally to analyze the effect of a single growth factor on serum-starved cells. In this context, a metabolic hormone such as insulin is found to be a mitogenic agent in many cellular types. In the present study, we have considered the effect of insulin stimulation in platelet-derived growth factor (PDGF)-activated NIH-3T3 and C2C12 cells. Our results show that insulin is able to inhibit strongly both NIH-3T3 and C2C12 cell growth induced by PDGF, one of the most powerful mitotic agents for these cell types. This inhibitory effect of insulin is due primarily to a premature down-regulation of the PDGF receptor. Thus, when NIH-3T3 or C2C12 cells are stimulated with both PDGF and insulin, we observe a decrease in PDGF receptor phosphorylation with respect to cells treated with PDGF alone. In particular, we find that costimulation with insulin leads to a reduced production of H2O2 with respect to cell stimulation with PDGF alone. The relative low concentration of H2O2 in PDGF/insulin-costimulated cell leads to a limited down-regulation of protein tyrosine phosphatases, and, consequently, to a reduced PDGF receptor phosphorylation efficiency. The latter is very likely to be responsible for the insulin-dependent inhibition of PDGF-receptor mitogenic signaling.

The in vivo tyrosine phosphorylation level of yeast immunophilin Fpr3 is influenced by the LMW-PTP Ltp1

Tyr-phosphorylation in Saccharomyces cerevisiae is essential in controlling the activity of MAP kinase regulating mating, pseudohyphal growth, and cell wall biosynthesis. Yeast serves as a model system for studying the biological function of many protein kinases and PTPs. Two LMW-PTP from yeast have been cloned, namely, Ltp1 from S. cerevisiae and Stp1 from Schizosaccharomyces pombe. The sequences of both enzymes are relatively similar to those of the mammalian LMW-PTP. Recently we showed that the yeast immunophilin Fpr3 interacts with Stp1 and its dephosphorylated state induces a growth defective phenotype. Here we show the phosphatase activity of Ltp1 on Fpr3 and we demonstrated that Tyr 184 is the residue phosphorylated on in vivo Fpr3. We also described the marked activation of Ltp1 by adenine in S. cerevisiae proteome and determined in vivo the influence of tyrosine phosphorylation on Fpr3 localization.

Expression of the Stp1 LMW-PTP and inhibition of protein CK2 display a cooperative effect on immunophilin Fpr3 tyrosine phosphorylation and Saccharomyces cerevisiae growth

Although the yeast genome does not encode bona fide protein tyrosine kinases, tyrosine-phosphorylated proteins are numerous, suggesting that besides dual-specificity kinases, some Ser/Thr kinases are also committed to tyrosine phosphorylation in Saccharomyces cerevisiae. Here we show that blockage of the highly pleiotropic Ser/Thr kinase CK2 with a specific inhibitor synergizes with the overexpression of Stp1 low-molecular-weight protein tyrosine phosphatase (PTP) in inducing a severe growth-defective phenotype, consistent with a prominent role for CK2 in tyrosine phosphorylation in yeast. We also present in vivo evidence that immunophilin Fpr3, the only tyrosine-phosphorylated CK2 substrate recognized so far, interacts with and is dephosphorylated by Spt1. These data disclose a functional correlation between CK2 and LMW-PTPs, and suggest that reversible phosphorylation of Fpr3 plays a role in the regulation of growth rate and budding in S. cerevisiae.

LMW-PTP is a positive regulator of tumor onset and growth

Low molecular weight protein tyrosine phosphatases (LMW-PTPs) are an enzyme family that plays a key role in cell proliferation control by dephosphorylating/inactivating both tyrosine kinase receptors (such as PDGF, insulin, and ephrin receptors) and docking proteins (such, as beta-catenin) endowed with both adhesion and transcriptional activity. Besides being a frequent event in human tumors, overexpression of LMW-PTP has been recently demonstrated to be sufficient to induce neoplastic transformation. We recently demonstrated that overexpression of LMW-PTP strongly potentiates the stability of cell-cell contacts at the adherens junction level, which powerfully suggests that LMW-PTP may also contribute to cancer invasivity. Focusing on mechanisms by which LMW-PTP is involved in cancer onset and progression, the emerging picture is that LMW-PTP strongly increases fibronectin-mediated cell adhesion and mobility but, paradoxically, decreases cell proliferation. Nevertheless, LMW-PTP-transfected NIH3T3 fibroblasts engrafted in nude mice induce the onset of larger fibrosarcomas, which are endowed with higher proliferation activity as compared to mock-transfected controls. Quite opposite effects have been obtained with engrafted fibroblasts transfected with a dominant-negative form of LMW-PTP. Notably, in sarcoma extracts, LMW-PTP overexpression greatly influences the ephrin A2 (EphA2) but not PDGF receptor or beta-catenin tyrosine phosphorylation. The high association of dephosphorylated EphA2 overexpression with most human cancers and our observation that cell growth stimulation by LMW-PTP overexpression is restricted to the in vivo model, strongly suggest that LMW-PTP oncogenic potential is mediated by its EphA2 tyrosine dephosphorylating activity.

ACYP1 gene possesses two alternative splicing forms that induce apoptosis

In human cell lines two products of the ACYP1 gene were detected by RT-PCR. In addition to the expected amplicon corresponding to the CT form of acylphosphatase (320 bp) a second unexpected one (400 bp) was characterized as the result of an alternative splicing in which an extra 79 bp long exon is inserted between the two known exons. This new product, indicated as CTsv, was cloned and expressed. We performed the ectopic expression of the two alternative splicing forms. Both CT and CTsv products were able to induce a proapoptotic effect when expressed in HeLa cell line, despite the fact that the CTsv protein did not show any acylphosphatase activity.

Down-regulation of platelet-derived growth factor receptor signaling during myogenesis

Cell differentiation is often associated with a block in the cell cycle. Growth factor signaling has been reported to be impaired in differentiated cells, due to the withdrawal of growth factors or to transcriptional down-regulation of their receptors. Our proposal is that the down regulation of growth factor signaling may be achieved through an alternative pathway: the decrease of growth factor receptor activation and the ensuing inhibition of intracellular pathways leading the cell to division. Here we report that platelet-derived growth factor receptor (PDGFr) signaling is down-regulated during muscle differentiation, although its expression level remains unchanged. PDGFr signaling inhibition is achieved through a decrease in the receptor tyrosine phosphorylation level, in particular of Tyr716, Tyr751, Tyr857 and Tyr1021, leading to down-regulation of intracellular signaling pathways. Furthermore, during myogenesis, the expression level of several phosphotyrosine phosphatases (PTPs) increases and most of them shift toward the reduced/activated state. We propose a causal link between the down-regulation of PDGFr tyrosine phosphorylation and the increases in PTP specific activity during myogenesis.

Lymphocyte function-associated antigen-1-mediated T cell adhesion is impaired by low molecular weight phosphotyrosine phosphatase-dependent inhibition of FAK activity

Protein tyrosine phosphorylation is one of the earliest signaling events detected in response to lymphocyte function-associated antigen-1 (LFA-1) engagement during lymphocyte adhesion. In particular, the focal adhesion kinase p125FAK, involved in the modulation and rearrangement of the actin cytoskeleton, seems to be a crucial mediator of LFA-1 signaling. Herein, we investigate the role of a FAK tyrosine phosphatase, namely low molecular weight phosphotyrosine phosphatase (LMW-PTP), in the modulation of LFA-1-mediated T cell adhesion. Overexpression of LMW-PTP in Jurkat cells revealed an impairment of LFA-1-dependent cell-cell adhesion upon T cell receptor (TCR) stimulation. Moreover, in these conditions LMW-PTP causes FAK dephosphorylation, thus preventing the activation of FAK downstream pathways. Our results also demonstrated that, upon antigen stimulation, LMW-PTP-dependent FAK inhibition is associated to a strong reduction of LFA-1 and TCR co-clustering toward a single region of T cell surface, thus causing an impairment of receptor activity by preventing changes in their avidity state. Because co-localization of both LFA-1 and TCR is an essential event during encounters of T cells with antigen-presenting cells and immunological synapse (IS) formation, we suggest an intriguing role of LMW-PTP in IS establishment and stabilization through the negative control of FAK activity and, in turn, of cell surface receptor redistribution.

Reactive oxygen species as essential mediators of cell adhesion: the oxidative inhibition of a FAK tyrosine phosphatase is required for cell adhesion

Signal transduction by reactive oxygen species (ROS; "redox signaling") has recently come into focus in cellular biology studies. The signaling properties of ROS are largely due to the reversible oxidation of redox-sensitive target proteins, and especially of protein tyrosine phosphatases, whose activity is dependent on the redox state of a low pKa active site cysteine. A variety of mitogenic signals, including those released by receptor tyrosine kinase (RTKs) ligands and oncogenic H-Ras, involve as a critical downstream event the intracellular generation of ROS. Signaling by integrins is also essential for the growth of most cell types and is constantly integrated with growth factor signaling. We provide here evidence that intracellular ROS are generated after integrin engagement and that these oxidant intermediates are necessary for integrin signaling during fibroblast adhesion and spreading. Moreover, we propose a synergistic action of integrins and RTKs for redox signaling. Integrin-induced ROS are required to oxidize/inhibit the low molecular weight phosphotyrosine phosphatase, thereby preventing the enzyme from dephosphorylating and inactivating FAK. Accordingly, FAK phosphorylation and other downstream events, including MAPK phosphorylation, Src phosphorylation, focal adhesion formation, and cell spreading, are all significantly attenuated by inhibition of redox signaling. Hence, we have outlined a redox circuitry whereby, upon cell adhesion, oxidative inhibition of a protein tyrosine phosphatase promotes the phosphorylation/activation and the downstream signaling of FAK and, as a final event, cell adhesion and spreading onto fibronectin.

Characterization of a novel Drosophila melanogaster acylphosphatase

Analysis of the Drosophila melanogaster EST database led to the characterization of a novel acylphosphatase (AcPDro2). This is coded by the CG18505 (Acyp2) gene and is clearly distinct from a previously described AcPDro coded by the CG16870 (Acyp) gene from D. melanogaster. The two proteins show a 60% homology with both vertebrate isoenzymes. All the residues involved in the catalytic mechanism are conserved. AcPDro2 is a stable enzyme with a correct globular folded structure. Its activity on benzoylphosphate shows higher K(cat) but lower K(m) with respect to AcPDro. It is possible that AcPDro and AcPDro2 genes are not the direct ancestor of MT and CT vertebrate isoenzymes.

Beta-catenin interacts with low-molecular-weight protein tyrosine phosphatase leading to cadherin-mediated cell-cell adhesion increase

Beta-catenin plays a dual role as a major constituent of cadherin-based adherens junctions and also as a transcriptional coactivator. In normal ephitelial cells, at adherens junction level, beta-catenin links cadherins to the actin cytoskeleton. The structure of adherens junctions is dynamically regulated by tyrosine phosphorylation. In particular, cell-cell adhesion can be negatively regulated through the tyrosine phosphorylation of beta-catenin. Furthermore, the loss of beta-catenin-cadherin association has been correlated with the transition from a benign tumor to an invasive, metastatic cancer. Low-molecular-weight protein tyrosine phosphatase (LMW-PTP) is a ubiquitous PTP implicated in the regulation of mitosis and cytoskeleton rearrangement. Here we demonstrate that the amount of free cytoplasmic beta-catenin is decreased in NIH3T3, which overexpresses active LMW-PTP, and this results in a stronger association between cadherin complexes and the actin-based cytoskeleton with respect to control cells. Confocal microscopy analysis shows that beta-catenin colocalizes with LMW-PTP at the plasma membrane. Furthermore, we provide evidence that beta-catenin is able to associate with LMW-PTP both in vitro and in vivo. Moreover, overexpression of active LMW-PTP strongly potentiates cadherin-mediated cell-cell adhesion, whereas a dominant-negative form of LMW-PTP induces the opposite phenotype, both in NIH3T3 and in MCF-7 carcinoma cells. On the basis of these results, we propose that the stability of cell-cell contacts at the adherens junction level is positively influenced by LMW-PTP expression, mainly because of the beta-catenin and LMW-PTP interaction at the plasma membrane level with consequent dephosphorylation.

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

Low molecular weight phosphotyrosine phosphatase (LMW-PTP) is an enzyme involved in platelet-derived growth factor-induced mitogenesis and cytoskeleton rearrangement. Our previous results demonstrated that LMW-PTP is able to bind and dephosphorylate activated platelet-derived growth factor receptor (PDGF-r), thus inhibiting cell proliferation. Here we revisit the role of LMW-PTP on activated PDGF-r dephosphorylation. We demonstrate that LMW-PTP preferentially acts on cell surface PDGF-r, excluding the internalized activated receptor pool. Many phosphotyrosine phosphatases act by site-selective dephosphorylation on several sites of PDGF-r, but until now, there has been no evidence of a direct involvement of a specific phosphotyrosine phosphatase in the dephosphorylation of the 857 kinase domain activation tyrosine. Here we report that LMW-PTP affects the kinase activity of the receptor through the binding and dephosphorylation of Tyr-857 and influences many of the signal outputs from the receptor. In particular, we demonstrate a down-regulation of phosphatidylinositol 3-kinase, Src homology phosphatase-2, and phospholipase C-gamma1 binding but not of MAPK activation. In addition, we report a slight action of LMW-PTP on Tyr-716, which directs MAPK activation through Grb2 binding. On the basis of these results, we propose a key role for LMW-PTP in PDGF-r down-regulation through the dephosphorylation of the activation loop Tyr-857, thus determining a general negative regulation of all downstream signals, with the exception of those elicited by internalized receptors.

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.

New perspectives in PDGF receptor downregulation: the main role of phosphotyrosine phosphatases

Uncontrolled activation of receptor tyrosine kinases (RTKs) is implicated in the proliferation of cancerous cells, and deficiencies in RTKs results in pathological conditions such as developmental abnormalities and immunodeficiencies. Tight regulation of RTK cascades is therefore critical for eliciting an appropriate type and level of response to external stimuli. The aim of this work is to compare different RTK downregulation mechanisms, such as ligandinduced internalisation, ubiquitin-mediated proteolysis and dephosphorylation by protein phosphotyrosine phosphatase (PTPs). We choose platelet-derived growth factor receptor (PDGF-r) in NIH3T3 cells as a model of RTK. Our data suggest that PDGF-r internalisation could be mainly considered as a positive signaling system, as it is involved in MAPK activation rather than a downregulation of the mitotic signal. Inhibition of receptor ubiquitination does not result in regulation of PDGF-r tyrosine phosphorylation and does not lead to variation of intracellular signalling pathways. The overall PDGF-r protein degradation upon PDGF stimulation does not exceed 30-40% of the total receptor; thus the receptor remains functionally active for further stimulation. On the contrary, PTP-dependent dephosphorylation of the activated receptors appears to play a crucial role. In fact, inhibition of PTP upon PDGF stimulation results in upregulation of receptor phosphorylation level, of PI3K recruitment and activation and of cell cycle rate. On the contrary, PTP-dependent dephosphorylation does not affect the endosomic pool of activated receptor. Furthermore, we demonstrate that PDGF-r downregulation by means of PTP dephosphorylation is important for both short term (2 hours) and long-lasting (up to 8 hours) PDGF-r activation. Herein we propose a revisited model of PDGF-r downregulation in which PTPs dephosphorylation retains a major role, conferring on receptor internalisation a signal transduction function.

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.

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.

Expression of the small tyrosine phosphatase (Stp1) in Saccharomyces cerevisiae: a study on protein tyrosine phosphorylation

Small tyrosine phoshatase 1 (Stp1) is a Schizosaccharomyces pombe low-molecular-mass phosphotyrosine-phosphatase 50% identical to Saccharomyces cerevisiae Ltp1. In order to investigate the role of Stp1 in yeast, a mutant was generated having the characteristic of a dominant negative molecule. Changes in protein tyrosine phosphorylation in S. cerevisiae proteome in response to Stp1 or its dominant negative mutant expression were analyzed by high-resolution two-dimensional (2-D) electrophoresis. The most remarkable result is the modification by phosphorylation on tyrosine of several proteins involved in carbohydrate metabolism. Twelve proteins were identified on the basis of their positions in the anti-phosphotyrosine immunoblot of the 2-D electrophoresis. Ten of these present tyrosyl residues that are within the consensus sequence for protein kinase CK2 (casein kinase-2). These data open the possibility for the identification of Stp1 substrates in yeast and provide hints about the nature of tyrosine phosphorylating agents in yeast and in other organisms where bona fide tyrosine kinases are lacking.

Low molecular weight protein-tyrosine phosphatase controls the rate and the strength of NIH-3T3 cells adhesion through its phosphorylation on tyrosine 131 or 132

The low molecular weight protein-tyrosine phosphatase (LMW-PTP) is an enzyme involved in platelet-derived growth factor (PDGF)-induced mitogenesis and cytoskeleton rearrangement. Our previous results demonstrated that LMW-PTP is able to bind and dephosphorylate activated PDGF receptor, thus inhibiting cell proliferation. Recently we have shown that LMW-PTP is specifically phosphorylated by c-Src in a cytoskeleton-associated fraction in response to PDGF, and this phosphorylation increases LMW-PTP activity about 20-fold. LMW-PTP strongly influences cell adhesion, spreading, and chemotaxis induced by PDGF stimulation, by regulating the phosphorylation level of p190Rho-GAP, a protein that is able to regulate Rho activity and hence cytoskeleton rearrangement. In the present study we investigate the physiological role of the two LMW-PTP tyrosine phosphorylation sites, using LMW-PTP mutants on tyrosine 131 or 132. We demonstrate that each tyrosine residue is involved in specific LMW-PTP functions. Both of them are phosphorylated during PDGF signaling. Phosphorylation on tyrosine 131 influences mitogenesis, dephosphorylating activated PDGF-R and cytoskeleton rearrangement, acting on p190RhoGAP. Phosphorylation on tyrosine 132 leads to an increase in the strength of cell substrate adhesion, down-regulating matrix metalloproteases expression, through the inhibition of Grb2/MAPK pathway. In conclusion, LMW-PTP tyrosine phosphorylation on both Tyr(131) or Tyr(132) cooperate to determine a faster and stronger adhesion to extracellular matrix, although these two events may diverge in timing and relative amount.

The inhibitory effect of the 5' untranslated region of muscle acylphosphatase mRNA on protein expression is relieved during cell differentiation

Previous experiments suggested that the upstream AUG triplet present in the 5' untranslated region (UTR) of muscle acylphosphatase mRNA is involved in the regulation of protein expression. In this paper, we study the involvement of the 5'UTR secondary structure and upstream peptide on mRNA stability and protein translation. Our data, obtained using deletion and frame-shift mutants, demonstrate that the 5'UTR controls protein expression regulating translation together with mRNA stability. Furthermore, we demonstrate that the inhibitory effect of the 5'UTR of muscle acylphosphatase is relieved during the differentiation process in agreement with previous data reporting an increase of acylphosphatase content during cell differentiation. Finally, UV cross-linking experiments show that specific mRNA-binding proteins are associated with the 5'UTR of the muscle acylphosphatase mRNA.

Acylphosphatase is a strong apoptosis inducer in HeLa cell line

Acylphosphatase (AcP) is a low-molecular-weight protein widely distributed in many vertebrate tissues with a yet unknown physiologic function. To study the in vivo behavior of AcP, HeLa cells were transiently transfected with a vector expressing the AcP/EGFP fusion protein. Analysis of the transfected cells showed a high level of cellular death in cells expressing the AcP/EGFP fusion protein with respect to control cells expressing EGFP alone. Flow cytometry and time lapse analysis of AcP/EGFP transfected cells evidenced a typical pattern of apoptosis. Surprisingly, cells transfected with a mutated form of AcP, with negligible in vitro acylphosphatase activity, undergo apoptosis as well as cells transfected with wild-type protein, suggesting that the physiologic role of AcP could be not related to this catalytic activity.

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 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.

Acylphosphatase expression during macrophage differentiation and activation of U-937 cell line

The two acylphosphatase isoenzymes (muscle type and common type) are differently involved in cell differentiation processes. In this paper we investigate the expression of the two isoenzymes during macrophage differentiation and activation. The U-937 human promonocytic cell line is a model for cell differentiation induced by the tumor promoter phorbol myristic acetate (PMA). Here we show that only the expression of the muscle type acylphosphatase increases during U-937 differentiation and macrophage activation, confirming that the two isoenzymes are differently regulated. Moreover, we determined, in the same conditions, the level of specific mRNA. Results show that after an initial two-fold decrease during PMA stimulation, the muscle type acylphosphatase mRNA levels remain constant also after the treatment with lipopolysaccharide and gamma-interferon, treatments that lead to macrophage activation. It is possible that post-transcription regulation is responsible for the regulation of muscle type acylphosphatase in the cell during differentiation and macrophage activation.

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.