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

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.

Expanding the role of Src and protein-tyrosine phosphatases balance in modulating osteoblast metabolism: lessons from mice

The widespread nature of protein phosphorylation/dephosphorylation underscores its key role in cell signaling metabolism, growth and differentiation. Tyrosine phosphorylation of cytoplasmic proteins is a critical event in the regulation of intracellular signaling pathways activated by external stimuli. An adequate balance in protein phosphorylation is a major factor in the regulation of osteoclast and osteoblast activities involved in bone metabolism. However, although phosphorylation is widely recognized as an important regulatory pathway in skeletal development and maintenance, the mechanisms involved are not fully understood. Among the putative protein-tyrosine kinases (ptk) and protein-tyrosine phosphatases (ptp) involved in this phenomenon there is increasing evidence that Src and low molecular weight-ptps play a central role in a range of osteoblast activities, from adhesion to differentiation. A role for Src in bone metabolism was first demonstrated in Src-deficient mice and has since been confirmed using low molecular weight Src inhibitors in animal models of osteoporosis. Several studies have shown that Src is important for cellular proliferation, adhesion and motility. In contrast, few studies have assessed the importance of the ptk/ptp balance in driving osteoblast metabolism. In this review, we summarize our current knowledge of the functional importance of the ptk/ptp balance in osteoblast metabolism, and highlight directions for future research that should improve our understanding of these critical signaling molecules.

The low-molecular-weight phosphotyrosine phosphatase is a negative regulator of FcγRIIA-mediated cell activation

Activation of human platelets by cross-linking of the low-affinity receptor for immunoglobulin G (FcγRIIA) is initiated by Src kinase–mediated phosphorylation of the immunoreceptor tyrosine–based activation motif (ITAM) within the receptor, but the identity of the enzyme responsible for its dephosphorylation and inactivation is unknown. Here we report that the 18-kDa low-molecular-weight phosphotyrosine phosphatase (LMW-PTP) is expressed in human platelets and undergoes subcellular redistribution upon FcγRIIA cross-linking. In vitro, LMW-PTP was found to efficiently dephosphorylate activated FcγRIIA and LAT, but not Syk or phospholipase Cγ2. In the megakaryocytic cell line DAMI, antibody-induced phosphorylation of FcγRIIA was rapid and transient. The late dephosphorylation of FcγRIIA was dramatically delayed upon reduction of LMW-PTP expression by siRNA. Strikingly, overexpression of LMW-PTP resulted in the inhibition of antibody-induced phosphorylation of FcγRIIA, and caused a more rapid dephosphorylation. In addition, overexpression of LMW-PTP inhibited activation of Syk downstream of FcγRIIA and reduced intracellular Ca2+ mobilization. These results demonstrate that LMW-PTP is responsible for FcγRIIA dephosphorylation, and is implicated in the down-regulation of cell activation mediated by this ITAM-bearing immunoreceptor.

Analyzing protein tyrosine phosphatases by phosphotyrosine analog integration

Reversible protein phosphorylation plays a central role in cellular signal transduction and is a focus of biomedical studies. However, it is a challenging task to study the effects of protein phosphorylation in the presence of protein phosphatase activities, especially for protein tyrosine phosphatases SHP1, SHP2 and LMW-PTP, which are themselves regulated by protein tyrosine phosphorylation. Expressed protein ligation, by combining chemical peptide synthesis with recombinant protein expression, allows for site-specific unnatural modifications of semisynthetic proteins. In this review, we describe how semisynthetic proteins were prepared to incorporate nonhydrolyzable phosphotyrosine analogs, and utilized in combination with site-directed mutagenesis and other means to elucidate regulatory mechanisms of protein tyrosine phosphatases.

Essential role of protein kinase C zeta in transducing a motility signal induced by superoxide and a chemotactic peptide, fMLP

Under various pathological conditions, including infection, malignancy, and autoimmune diseases, tissues are incessantly exposed to reactive oxygen species produced by infiltrating inflammatory cells. We show augmentation of motility associated with morphological changes of human squamous carcinoma SASH1 cells, human peripheral monocytes (hPMs), and murine macrophage-like cell line J774.1 by superoxide stimulation. We also disclose that motility of hPMs and J774.1 induced by a chemotactic peptide (N-formyl-methionyl-leucyl-phenylalanine [fMLP]) was inhibited by superoxide dismutase or N-acetylcystein, indicating stimulation of motility by superoxide generated by fMLP stimulation. In these cells, protein kinase C (PKC) ζ was activated to phosphorylate RhoGDI-1, which liberated RhoGTPases, leading to their activation. These events were inhibited by dominant-negative PKCζ in SASH1 cells, myristoylated PKCζ peptides in hPMs and J774.1, or a specific inhibitor of RhoGTPase in SASH1, hPMs, and J774.1. These results suggest a new approach for manipulation of inflammation as well as tumor cell invasion by targeting this novel signaling pathway.

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.

The solution structure of Escherichia coli Wzb reveals a novel substrate recognition mechanism of prokaryotic low molecular weight protein-tyrosine phosphatases

Low molecular weight protein-tyrosine phosphatases (LMW-PTPs) are small enzymes that ubiquitously exist in various organisms and play important roles in many biological processes. In Escherichia coli, the LMW-PTP Wzb dephosphorylates the autokinase Wzc, and the Wzc/Wzb pair regulates colanic acid production. However, the substrate recognition mechanism of Wzb is still poorly understood thus far. To elucidate the molecular basis of the catalytic mechanism, we have determined the solution structure of Wzb at high resolution by NMR spectroscopy. The Wzb structure highly resembles that of the typical LMW-PTP fold, suggesting that Wzb may adopt a similar catalytic mechanism with other LMW-PTPs. Nevertheless, in comparison with eukaryotic LMW-PTPs, the absence of an aromatic amino acid at the bottom of the active site significantly alters the molecular surface and implicates Wzb may adopt a novel substrate recognition mechanism. Furthermore, a structure-based multiple sequence alignment suggests that a class of the prokaryotic LMW-PTPs may share a similar substrate recognition mechanism with Wzb. The current studies provide the structural basis for rational drug design against the pathogenic bacteria.

Solution structure of a low-molecular-weight protein tyrosine phosphatase from Bacillus subtilis

The low-molecular-weight (LMW) protein tyrosine phosphatases (PTPs) exist ubiquitously in prokaryotes and eukaryotes and play important roles in cellular processes. We report here the solution structure of YwlE, an LMW PTP identified from the gram-positive bacteria Bacillus subtilis. YwlE consists of a twisted central four-stranded parallel beta-sheet with seven alpha-helices packing on both sides. Similar to LMW PTPs from other organisms, the conformation of the YwlE active site is favorable for phosphotyrosine binding, indicating that it may share a common catalytic mechanism in the hydrolysis of phosphate on tyrosine residue in proteins. Though the overall structure resembles that of the eukaryotic LMW PTPs, significant differences were observed around the active site. Residue Asp115 is likely interacting with residue Arg13 through electrostatic interaction or hydrogen bond interaction to stabilize the conformation of the active cavity, which may be a unique character of bacterial LMW PTPs. Residues in the loop region from Phe40 to Thr48 forming a wall of the active cavity are more flexible than those in other regions. Ala41 and Gly45 are located near the active cavity and form a noncharged surface around it. These unique properties demonstrate that this loop may be involved in interaction with specific substrates. In addition, the results from spin relaxation experiments elucidate further insights into the mobility of the active site. The solution structure in combination with the backbone dynamics provides insights into the mechanism of substrate specificity of bacterial LMW PTPs.

Roles of microtubules, cell polarity and adhesion in electric-field-mediated motility of 3T3 fibroblasts

Direct-current electric fields mediate motility (galvanotaxis) of many cell types. In 3T3 fibroblasts, electric fields increased the proportion, speed and cathodal directionality of motile cells. Analogous to fibroblasts' spontaneous migration, we initially hypothesized that reorientation of microtubule components modulates galvanotaxis. However, cells with intact microtubules did not reorient them in the field and cells without microtubules still migrated, albeit slowly, thus disproving the hypothesis. We next proposed that, in monolayers wounded and placed in an electric field, reorientation of microtubule organizing centers and stable, detyrosinated microtubules towards the wound edge is necessary and/or sufficient for migration. This hypothesis was negated because field exposure mediated migration of unoriented, cathode-facing cells and curtailed migration of oriented, anode-facing cells. This led us to propose that ablating microtubule detyrosination would not affect galvanotaxis. Surprisingly, preventing microtubule detyrosination increased motility speed, suggesting that detyrosination inhibits galvanotaxis. Microtubules might enhance adhesion/de-adhesion remodeling during galvanotaxis; thus, electric fields might more effectively mediate motility of cells poorly or dynamically attached to substrata. Consistent with this hypothesis, incompletely spread cells migrated more rapidly than fully spread cells. Also, overexpression of PAK4, a Cdc42-activated kinase that decreases adhesion, enhanced galvanotaxis speed, whereas its lack decreased speed. Thus, electric fields mediate fibroblast migration via participation of microtubules and adhesive components, but their participation differs from that during spontaneous motility.

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.

Low Mr phosphotyrosine protein phosphatase associates and dephosphorylates p125 focal adhesion kinase, interfering with cell motility and spreading

Low M(r) phosphotyrosine protein phosphatase interferes in vivo with the activation of several growth factor receptors and is transiently redistributed, following cell stimulation with platelet-derived growth factor, from the cytosol to the cytoskeleton. We demonstrate here that this phosphatase also participates in the regulation of cell spreading and migration, pointing to its involvement in cytoskeleton organization. Low M(r) phosphotyrosine protein phosphatase-overexpressing fibroblasts are, indeed, less spread than controls and display a significantly decreased number of focal adhesions and increased cell motility. Furthermore, p125 focal adhesion kinase is associated to, and dephosphorylated by, low M(r) phosphotyrosine protein phosphatase both in vitro and in vivo. This event is consistent with an altered association of pp60(src) with focal adhesion kinase. The activation of extracellular signal-regulated kinase, another well known event downstream of the focal adhesion kinase, is also affected. On the other hand, cells overexpressing the dominant-negative form of low M(r) phosphotyrosine protein phosphatase exhibit hyperphosphorylated focal adhesion kinase, reduced motility, and an increased number of focal adhesions, which are distributed all over the ventral cell surface. Taken together, the results reported here are in keeping with low M(r) phosphotyrosine protein phosphatase participation in FAK-mediated focal adhesion remodeling.