Protein tyrosine phosphatases: a diverse family of intracellular and transmembrane enzymes

Differential regulation of multiple hepatic protein tyrosine phosphatases in alloxan diabetic rats

The involvement of tyrosine phosphorylation in insulin action led us to hypothesize that increased activity of protein tyrosine phosphatases (PTPases) might contribute to insulin resistance in alloxan diabetes in the rat. Hepatic PTPase activity was measured using two artificial substrates phosphorylated on tyrosine: reduced, carboxyamidomethylated, and maleylated lysozyme (P-Tyr-RCML) and myelin basic protein (P-Tyr-MBP), as well as an autophosphorylated 48-kD insulin receptor tyrosine kinase domain (P-Tyr-IRKD). Rats that were made alloxan diabetic exhibited a significant increase in hepatic membrane (detergent-soluble) PTPase activity measured with P-Tyr-MBP, without a change in activity measured with P-Tyr-RCML or the P-Tyr-IRKD. The PTPase active with P-Tyr-MBP behaved as a high molecular weight peak during gel filtration chromatography. Characterization of this enzyme indicated it shared properties with CD45, the prototype for a class of transmembrane, receptor-like PTPases. Our results indicate that alloxan diabetes in the rat is associated with an increase in the activity of a large, membrane-associated PTPase which accounts for only a small proportion of insulin receptor tyrosine dephosphorylation. Nonetheless, increased activity of this PTPase may oppose tyrosine kinase-mediated insulin signal transmission, thus contributing to insulin resistance.

The cDNA cloning, nucleotide sequence and expression of an intracellular protein tyrosine phosphatase from mouse testis

The PTP-1 cDNA encoding an intracellular protein tyrosine phosphatase (PTPase) was isolated and sequenced from a mouse testis cDNA library. This PTP-1 cDNA was found to contain an open reading frame of 1,296 nucleotides as well as 5' (83 nucleotides) and 3' (289 nucleotides) non-coding regions. The deduced sequence of 432 amino acids of mouse PTPase-1 exhibited 93% and 83% identity to that of rat PTPase-1 and human PTPase-1B, respectively. Thus, this PTP-1 is a mouse homologue of human PTP-1B and rat PTP-1. Northern blot analysis indicated that PTP-1 mRNAs were most abundant in testis, and were detected in sizes of 4.4 Kb, 2.4 Kb and 2.2 Kb, 2.0 Kb. The PTP-1 transcripts of 4.4 Kb and 2.0 Kb, but not 2.4 Kb and 2.2 Kb, were also present in kidney, spleen, muscle, liver, heart and brain. Genomic blot analysis showed that a single copy of the PTP-1 gene is contained in the mouse genome and that introns are present in mammalian PTP-1 genes.

Overexpression of a synthetic phosphotyrosine protein phosphatase gene inhibits normal and transformed cell growth

We studied the level of the cytosolic phosphotyrosine protein phosphatase (PTPase) (originally termed low-M(r) acid phosphatase) in normal NIH/3T3 and in v-erbB-transformed fibroblasts. The level of the enzyme, assayed by ELISA, was inversely related to cell proliferation, normally growing cells had less enzyme than their contact-inhibited counterparts and v-erbB transformants had less enzyme than normal NIH/3T3. In order to overexpress the enzyme and study its effects in normal and transformed cells, we transfected a synthetic gene coding for the PTPase in control NIH/3T3 and v-erbB transformants. The overexpressed enzyme was recognized by antibodies raised against the native enzyme and, in cells overexpressing the PTPase, we observed a marked dephosphorylation of tyrosyl residues of cellular proteins. Cell proliferation, in both normal and v-erbB transformants overexpressing the PTPase, was measured. We observed that PTPase overexpression was accompanied by significantly reduced thymidine incorporation in both cell types, either serum-starved or serum-stimulated. The ability of transformed v-erbB cells to grow in soft agar was also markedly decreased by overexpression of the enzyme. Taken together, our results indicate that overexpression of PTPase might interfere with mitogenic signalling pathways in both normal and transformed cells, and propose a role for PTPase in the control of cell proliferation.

Cytokinetic failure and asynchronous nuclear division in BHK cells overexpressing a truncated protein-tyrosine-phosphatase

Previous work has shown that a T-cell protein-tyrosine-phosphatase truncated in its carboxyl-terminal domain (delta C11.PTP) has full enzymatic activity but no longer localizes in the particulate fraction of the cell. Two baby hamster kidney (BHK) cell lines overexpressing the truncated protein are markedly multinucleate, a state likely caused by a failure in cytokinesis. Nuclei within syncytial cells overexpressing delta C11.PTP display a remarkable asynchronous entry into mitosis. The effects require tyrosine phosphatase activity because expression of an inactive form of the truncated enzyme yields cells indistinguishable from the parental cell line. Redistribution of the enzyme from the particulate to the soluble fraction is apparently important to these observed effects because cells overexpressing the full-length, wild-type enzyme are morphologically similar to controls. Further, when these cells contain more than one nucleus, their syncytial nuclei undergo mitosis synchronously.

Importance of tyrosine phosphatases in the effects of cell-cell contact and microenvironments on EGF-stimulated tyrosine phosphorylation

We have compared the EGF responses of A431 cells when grown as monolayers at a variety of cell densities or as multicellular spheroids in order to investigate the effects of cell contact and 3-dimensional structure on signal transduction. Proliferation of the A431 squamous carcinoma cell line grown in our laboratory was unaffected by EGF when grown in monolayer culture. As 3-dimensional, multicellular spheroids, however, growth was stimulated by EGF. The maximum volume attainable in the presence of EGF was more than 30 times that in its absence. EGF-dependent tyrosine phosphorylation was compared under these conditions by immunohistochemistry and Western blotting. In initial experiments using published procedures, tyrosine phosphorylation was density-dependent in monolayers and undetectable in spheroids. However, the density-dependence was abolished by the addition of high concentrations of protein tyrosine phosphatase inhibitors (1 mM Zn++ and VO4(3)-). The density dependence of EGF-stimulated tyrosine phosphorylation in monolayers was, therefore, largely the result of changes in phosphatase activity rather than kinase. Using high concentrations of phosphatase inhibitors, phosphotyrosine was clearly visible by immunohistochemistry in the outermost cells of spheroids, but it was still not visible in the spheroid center. The lack of response within the spheroid was not related to the presence of EGF receptor nor diffusion of EGF. In companion experiments, we showed that staining for EGF receptor was present homogeneously throughout the spheroid and that EGF penetrated to its center under the conditions of the experiment. Thus, although an increase in tyrosine phosphatase activity was a major factor affecting tyrosine phosphorylation in the outer cells, other factors were important in the inner cells. We concluded that an increase of tyrosine phosphatase activity was the most important component of the adaptation of the EGF signal transduction system to high cell density in monolayer cultures. In spheroids, tyrosine phosphatases are also enhanced, but other factors, such as autocrine synthesis of TGF-alpha and possibly the cellular distribution of EGF receptors and cell shape, play a role.

The role of protein phosphatases in synaptic transmission, plasticity and neuronal development

In the past year significant advances have been made in our understanding of the role of protein dephosphorylation in the control of neuronal function. Molecular cloning has identified a large number of serine/threonine and tyrosine protein phosphatases in the nervous system. Many of these enzymes are selectively enriched in the nervous system, some are localized to specific neurons, and yet others are expressed only during specific periods of neuronal development. The availability of purified protein phosphatases and selective inhibitors has facilitated the analysis of these enzymes and their role in the regulation of neurotransmitter receptors and ion channels.

Epidermal growth factor receptor: elements of intracellular communication

While EGF has an important function in cell growth regulation, the molecular mechanisms by which intracellular signal connect the EGF: receptor complex on the plasma membrane with the initiation of DNA synthesis and mitogenesis is not well understood. The discovery that rasGAP, PI-3 kinase and PLC-gamma 1 are substrates for the EGF receptor tyrosine kinase has provided a beginning in understanding the biochemistry underlying growth factor receptor transduction.

Insulin receptor and epidermal growth factor receptor dephosphorylation by three major rat liver protein-tyrosine phosphatases expressed in a recombinant bacterial system

Protein-tyrosine phosphatases (PTPases) play an essential role in the regulation of signal transduction mediated by reversible protein-tyrosine phosphorylation. In order to characterize individual rat hepatic PTPases that might have specificity for autophosphorylated receptor tyrosine kinases, we isolated cDNA segments encoding three PTPases (PTPase 1B, LAR and LRP) that are expressed in insulin-sensitive liver and skeletal muscle tissue, and evaluated their catalytic activity in vitro. The intrinsic PTPase activities of the full-length PTPase 1B protein and the cytoplasmic domains of LAR and LRP were studied by expression of recombinant cDNA constructs in the inducible bacterial vector pKK233-2 using extracts of a host strain of Escherichia coli that lacks endogenous PTPase activity. Each of the cloned cDNAs dephosphorylated a cognate phosphopeptide derived from the regulatory region of the insulin receptor. Despite having only 30-39% sequence identity in their catalytic domains, LAR and PTPase 1B had similar relative activities between the peptide substrate and intact insulin receptors, and also displayed similar initial rates of simultaneous dephosphorylation of insulin and epidermal growth factor (EGF) receptors. In contrast, LRP exhibited a higher rate of dephosphorylation of both intact receptors relative to the peptide substrate, and also dephosphorylated EGF receptors more rapidly than insulin receptors. These studies indicate that three PTPases with markedly divergent structures have the catalytic potential to dephosphorylate both insulin and EGF receptors in intact cells and that redundant PTPase activity may occur in vivo. For these PTPases to have specific physiological actions in intact cells, they must be influenced by steric effects of the additional protein segments of the native transmembrane enzymes, cellular compartmentalization and/or interactions with regulatory proteins.

Characterization of hematopoietic intracellular protein tyrosine phosphatases: description of a phosphatase containing an SH2 domain and another enriched in proline-, glutamic acid-, serine-, and threonine-rich sequences

Protein tyrosine phosphatases (PTPases) are a family of enzymes important in cellular regulation. Characterization of two cDNAs encoding intracellular PTPases expressed primarily in hematopoietic tissues and cell lines has revealed proteins that are potential regulators of signal transduction. One of these, SHP (Src homology region 2 [SH2]-domain phosphatase), possesses two tandem SH2 domains at the amino terminus of the molecule. SH2 domains have previously been described in proteins implicated in signal transduction, and SHP may be one of a family of nonreceptor PTPases that can act as direct antagonists to the nonreceptor protein tyrosine kinases. The SH2 domains of SHP preferentially bind a 15,000-Mr protein expressed by LSTRA cells. LSTRA cells were shown to express SHP protein by immunoprecipitation, thus demonstrating a potential physiological interaction. The other PTPase, PEP (proline-, glutamic acid-, serine-, and threonine-rich [PEST]-domain phosphatase), is distinguished by virtue of a large carboxy-terminal domain of approximately 500 amino acids that is rich in PEST residues. PEST sequences are found in proteins that are rapidly degraded. Both proteins have been expressed by in vitro transcription and translation and in bacterial expression systems, and both have been demonstrated to have PTPase activity. These two additional members of the PTPase family accentuate the variety of PTPase structures and indicate the potential diversity of function for intracellular tyrosine phosphatases.

G Protein Activation of a Hormone-Stimulated Phosphatase in Human Tumor Cells

The growth-inhibiting peptide hormone somatostatin stimulates phosphotyrosine phosphatase activity in the human pancreatic cell line MIA PaCa-2. This hormonal activation was mediated by a pertussis toxin-sensitive guanosine 5'-triphosphate-binding protein (G protein) in the membranes of these cells. Activation of this G protein by somatostatin stimulated the dephosphorylation of exogenous epidermal growth factor receptor prepared from A-431 cells in vitro. This pathway may mediate the antineoplastic action of somatostatin in these cells and in human tumors and could represent a general mechanism of G protein coupling that is utilized by normal cells in the hormonal control of cell growth.

Differential expression of a novel murine non-receptor protein tyrosine phosphatase during differentiation of P19 embryonal carcinoma cells

Protein phosphorylation on tyrosine residues is one of the major mechanisms of cell signal transduction and is regulated by protein tyrosine kinases and protein tyrosine phosphatases. Here we report the molecular cloning of an additional member of the protein tyrosine phosphatase-family from differentiated murine P19 embryonal carcinoma cells. This non-receptor protein tyrosine phosphatase, P19-PTP, does not contain regulatory sequences, homologous to the ones found in other non-receptor PTPases. P19-PTP is differentially expressed during in vitro differentiation of P19 EC cells, in that P19-PTP mRNA could only be detected in embryoid bodies, derived from P19 cells.

Continuous spectrophotometric assay of protein tyrosine phosphatase using phosphotyrosine

A continuous activity assay for protein tyrosine phosphatases (PTPs), employing phosphotyrosine (P-Tyr) as a substrate, has been developed and applied to measure the activities of two purified enzymes, namely, the full length T-cell protein tyrosine phosphatase (TC PTP) and its truncated form (TC delta C11 PTP). The reaction was followed by changes in ultraviolet absorption and fluorescence resulting from the dephosphorylation of P-Tyr. Both enzymes obey Michaelis-Menten kinetics, with Km = 304 microM, Vmax = 62,000 units/mg for TC PTP and Km = 194 microM, Vmax = 73,000 units/mg for TC delta C11 PTP. The D- and L-forms of P-Tyr are equally effective as substrates. The optimum pH for both enzymes is 4.75. The known effectors of PTPs have the predicted effects on catalytic activity.

Characterization of hematopoietic intracellular protein tyrosine phosphatases: description of a phosphatase containing an SH2 domain and another enriched in proline-, glutamic acid-, serine-, and threonine-rich sequences

Protein tyrosine phosphatases (PTPases) are a family of enzymes important in cellular regulation. Characterization of two cDNAs encoding intracellular PTPases expressed primarily in hematopoietic tissues and cell lines has revealed proteins that are potential regulators of signal transduction. One of these, SHP (Src homology region 2 [SH2]-domain phosphatase), possesses two tandem SH2 domains at the amino terminus of the molecule. SH2 domains have previously been described in proteins implicated in signal transduction, and SHP may be one of a family of nonreceptor PTPases that can act as direct antagonists to the nonreceptor protein tyrosine kinases. The SH2 domains of SHP preferentially bind a 15,000-Mr protein expressed by LSTRA cells. LSTRA cells were shown to express SHP protein by immunoprecipitation, thus demonstrating a potential physiological interaction. The other PTPase, PEP (proline-, glutamic acid-, serine-, and threonine-rich [PEST]-domain phosphatase), is distinguished by virtue of a large carboxy-terminal domain of approximately 500 amino acids that is rich in PEST residues. PEST sequences are found in proteins that are rapidly degraded. Both proteins have been expressed by in vitro transcription and translation and in bacterial expression systems, and both have been demonstrated to have PTPase activity. These two additional members of the PTPase family accentuate the variety of PTPase structures and indicate the potential diversity of function for intracellular tyrosine phosphatases.

Cloning and expression of a cytosolic megakaryocyte protein-tyrosine-phosphatase with sequence homology to retinaldehyde-binding protein and yeast SEC14p

Protein tyrosine phosphorylation is important in the regulation of cell growth, the cell cycle, and malignant transformation. We have cloned a cDNA that encodes a cytosolic protein-tyrosine-phosphatase (PTPase), MEG2, from MEG-01 cell and human umbilical vein endothelial cell cDNA libraries. The 4-kilobase cDNA sequence of PTPase MEG2 corresponds in length to the mRNA transcript detected by Northern blotting. The predicted open reading frame encodes a 68-kDa protein composed of 593 amino acids and has no apparent signal or transmembrane sequences, suggesting that it is a cytosolic protein. The C-terminal region has a PTPase catalytic domain that has 30-40% amino acid identity to other known PTPases. The N-terminal region has 254 amino acids that are 28% identical to cellular retinaldehyde-binding protein and 24% identical to yeast SEC14p, a protein that has phosphatidylinositol transfer activity and is required for protein secretion through the Golgi complex in yeast. Recombinant PTPase MEG2 expressed in Escherichia coli possesses PTPase activity. PTPase MEG2 mRNA was detected in 12 cell lines tested, which suggests that this phosphatase is widely expressed. The structure of PTPase MEG2 implies that a tyrosine phosphatase could participate in the transfer of hydrophobic ligands or in functions of the Golgi apparatus.

Modification of cell proliferation with inhibitors

Recent developments on mechanisms that control cell multiplication, using molecular biology, are renewing interest in inhibitors and activators. A great deal of information has been gained in the past through the use of chemicals that modify passage through the cell cycle. The kinds of inhibitors, their sites of action that disrupt functions essential for proliferation, their usefulness in synchronizing cultures and, importantly, their therapeutic value, have been the subject of many investigations.

Effects of flavonoids on immune and inflammatory cell functions

No doubt can remain that the flavonoids have profound effects on the function of immune and inflammatory cells as determined by a large number and variety of in vitro and some in vivo observations. That these ubiquitous dietary chemicals may have significant in vivo effects on homeostasis within the immune system and on the behavior of secondary cell systems comprising the inflammatory response seems highly likely but more work is required to strengthen this hypothesis. Ample evidence indicates that selected flavonoids, depending on structure, can affect (usually inhibit) secretory processes, mitogenesis, and cell-cell interactions including possible effects on adhesion molecule expression and function. The possible action of flavonoids on the function of cytoskeletal elements is suggested by their effects on secretory processes. Moreover, evidence indicates that certain flavonoids may affect gene expression and the elaboration and effects of cytokines and cytokine receptors. How all of these effects are mediated is not yet clear but one important mechanism may be the capacity of flavonoids to stimulate or inhibit protein phosphorylation and thereby regulate cell function. Perhaps the counterbalancing effect of cellular protein tyrosine phosphatases will also be found to be affected by flavonoids. Some flavonoid effects can certainly be attributed to their recognized antioxidant and radical scavenging properties. A potential mechanism of action that requires scrutiny, particularly in relation to enzyme inhibition, is the redox activity of appropriately configured flavonoids. Finally, in a number of cell systems it seems that resting cells are not affected significantly by flavonoids but once a cell becomes activated by a physiological stimulus a flavonoid-sensitive substance is generated and interaction of flavonoids with that substance dramatically alters the outcome of the activation process.

A gene encoding a putative tyrosine phosphatase suppresses lethality of an N-end rule-dependent mutant

The N-end rule relates the in vivo half-life of a protein to the identity of its N-terminal residue. In the yeast Saccharomyces cerevisiae, mutational inactivation of the N-end rule pathway is neither lethal nor phenotypically conspicuous. We have used a "synthetic lethal" screen to isolate a mutant that requires the N-end rule pathway for viability. An extragenic suppressor of this mutation was cloned and found to encode a 750-residue protein with strong sequence similarities to protein phosphotyrosine phosphatases. This heat-inducible gene was named PTP2. Null ptp2 mutants grow slowly, are hypersensitive to heat, and are viable in either the presence or absence of the N-end rule pathway. We discuss possible connections between dephosphorylation of phosphotyrosine in proteins and the N-end rule pathway of protein degradation.

Protein tyrosine phosphatase containing SH2 domains: characterization, preferential expression in hematopoietic cells, and localization to human chromosome 12p12-p13

Protein tyrosine phosphorylation has been implicated in the growth and functional responses of hematopoietic cells. Recently, approaches have been developed to characterize the protein tyrosine phosphatases that may contribute to regulation of protein tyrosine phosphorylation. One novel protein tyrosine phosphatase was expressed predominantly in hematopoietic cells. Hematopoietic cell phosphatase encodes a 68-kDa protein that contains a single phosphatase conserved domain. Unlike other known protein tyrosine phosphatases, hematopoietic cell phosphatase contains two src homology 2 domains. We also cloned the human homolog, which has 95% amino acid sequence identity. Both the murine and human gene products have tyrosine-specific phosphatase activity, and both are expressed predominantly in hematopoietic cells. Importantly, the human gene maps to chromosome 12 region p12-p13. This region is associated with rearrangements in approximately 10% of cases of acute lymphocytic leukemia in children.

Covalent modification and active site-directed inactivation of a low molecular weight phosphotyrosyl protein phosphatase

Covalent modification experiments were conducted in order to identify active site residues of the 18-kDa cytoplasmic phosphotyrosyl protein phosphatases. The enzyme was inactivated by diethyl pyrocarbonate, phenylglyoxal, cyclohexanedione, iodoacetate, iodoacetamide, phenylarsine oxide, and certain epoxides in a manner consistent with the modification of active site residues. Phenylglyoxal and cyclohexanedione both bind to the active site in a rapid preequilibrium process and thus act as active site-directed inhibitors. The pH dependencies of the inactivation by iodoacetate and by iodoacetamide were examined in detail and compared with rate data for the alkylation of glutathione as a model compound. The enzyme inactivation data permitted the determination of pKa values of two reactive cysteines at or near the active site. Although phosphomycin is simply a competitive inhibitor of the enzyme, it was found that 1,2-epoxy-3-(p-nitrophenoxy)propane (EPNP) and (R)- and (S)-benzylglycidol act as irreversible covalent inactivators, consistent with the importance of a hydrophobic moiety on the substrate in controlling substrate specificity. EPNP exhibits characteristics of an active site-directed inactivator, with a preequilibrium binding constant somewhat smaller than that of phosphate ion. The pH dependencies of inactivation of EPNP and (S)-benzylglycidol are identical to that observed for iodoacetamide and similar to that for iodoacetate, suggesting that they modify similar groups. Sequencing of the tryptic digests of the EPNP-labeled enzyme indicates that Cys-62 and Cys-145 are labeled. Phenylarsine oxide acts as a very slow, tight-binding inhibitor of the enzyme. The results are interpreted in terms of an active site model that incorporates a histidine-cysteine ion pair, similar to that present in papain.

The nontransmembrane tyrosine phosphatase PTP-1B localizes to the endoplasmic reticulum via its 35 amino acid C-terminal sequence

We report the first intracellular characterization of an endogenous nontransmembrane protein tyrosine phosphatase (PTP). Using affinity-purified polyclonal antibodies, we have identified PTP-1B as a 50 kd serine phosphoprotein in immunoprecipitation and immunoblotting assays. Surprisingly, indirect immunofluorescence experiments indicate that PTP-1B is localized predominantly in the endoplasmic reticulum (ER). Subcellular fractionation is consistent with this localization and establishes that PTP-1B is tightly associated with microsomal membranes, with its phosphatase domain oriented towards the cytoplasm. The C-terminal 35 amino acids of PTP-1B are both necessary and sufficient for targeting to the ER. The finding of a tyrosine phosphatase on the ER suggests new possibilities for cellular events controlled by tyrosine phosphorylation.

Role of cAMP in mediating effects of fasting on dephosphorylation of insulin receptor

We studied the effect of fasting on phosphotyrosine phosphatase (PTPase) activities in particulate (PF) and cytosolic (CF) fractions of rat adipocytes and liver. PTPase activity was assessed using [32P]tyrosine insulin receptor (IR). In adipocytes, 48 h fasting significantly inhibited PTPase activity. Dephosphorylation of IR by PF and CF PTPases was reduced by 80 and 65%, respectively. Similar reductions of lesser magnitude were observed in fasted rat livers. The effect of fasting was completely reversed by either refeeding or by incubating "fasted" adipocytes for 2 h in tissue culture medium containing 5 mM glucose. Neither 20 mM glucose nor the presence of insulin influenced phosphatase activity. Because fasting is accompanied by elevated protein kinase C (PKC) and adenosine 3',5'-cyclic monophosphate (cAMP) levels, we examined their influence on adipocyte PTPases. Neither activation (1 microM 12-O-tetradecanoylphorbol-13-acetate) nor inhibition (20 microM sphingosine) of PKC affected PTPase activity. In contrast, cAMP (2 mM) significantly inhibited PTPase activity (80% inhibition at 2 h), and its effect was prevented by a cAMP antagonist RpcAMP. Fasting- and cAMP-induced inhibition of PTPase activity was restored by incubating PF with trypsin (4 micrograms/ml for 5 min), which separated the putative inhibitors from the phosphatases. We conclude that fasting-induced inhibition of PTPases is mediated by elevated cAMP levels, most likely by activating phosphatase inhibitors.

Protein tyrosine phosphatase containing SH2 domains: characterization, preferential expression in hematopoietic cells, and localization to human chromosome 12p12-p13

Protein tyrosine phosphorylation has been implicated in the growth and functional responses of hematopoietic cells. Recently, approaches have been developed to characterize the protein tyrosine phosphatases that may contribute to regulation of protein tyrosine phosphorylation. One novel protein tyrosine phosphatase was expressed predominantly in hematopoietic cells. Hematopoietic cell phosphatase encodes a 68-kDa protein that contains a single phosphatase conserved domain. Unlike other known protein tyrosine phosphatases, hematopoietic cell phosphatase contains two src homology 2 domains. We also cloned the human homolog, which has 95% amino acid sequence identity. Both the murine and human gene products have tyrosine-specific phosphatase activity, and both are expressed predominantly in hematopoietic cells. Importantly, the human gene maps to chromosome 12 region p12-p13. This region is associated with rearrangements in approximately 10% of cases of acute lymphocytic leukemia in children.

Tyrosine kinases and their interactions with signalling proteins

Recent progress has been made in identifying signal transduction pathways controlled by receptor protein-tyrosine kinases. The receptors for nerve growth factor and hepatocyte growth factor have been identified as the Trk and Met tyrosine kinases. The stimulation of intracellular signal transduction pathways by activated receptors appears to involve the association of SH2-containing cytoplasmic signalling proteins with autophosphorylated receptors.

The angiotensin AT2 receptor stimulates protein tyrosine phosphatase activity and mediates inhibition of particulate guanylate cyclase

The signalling mechanism and cellular targets of the AT2 receptor are still unknown. We report that angiotensin II (Ang II) inhibits basal and atrial natriuretic peptide stimulated particulate guanylate cyclase (pGC) activity through AT2 receptors in rat adrenal glomerulosa and PC12W cells. This inhibition is blocked by the phosphotyrosine phosphatase (PTPase) inhibitor orthovanadate but not by the Ser/Thr phosphatase inhibitor okadaic acid, suggesting the involvement of a PTPase in this process. Moreover, Ang II induces a rapid, transient and orthovanadate sensitive dephosphorylation of phosphotyrosine containing proteins in PC12W cells. Our findings suggest that AT2 receptors signal through stimulation of a PTPase and that this mechanism is implicated in the regulation of pGC activity. This observation is also the first example of hormonal inhibition of basal pGC activity.

Isolation of a src homology 2-containing tyrosine phosphatase

Tyrosine phosphorylation is controlled by the opposing actions of tyrosine kinases and phosphotyrosine phosphatases (PTPs). src homology 2 domains (SH2) are found in several types of signaling proteins, including some tyrosine kinases. These domains bind phosphotyrosyl proteins and thus help promote signal transduction. Using mixed oligonucleotide-directed polymerase chain reactions, two previously undescribed rat PTP cDNA fragments were generated. Through subsequent screening of rat megakaryocyte and human erythroleukemia libraries, we obtained a full-length coding sequence for one of these fragments. This cDNA, SH-PTP1, encodes a tyrosine phosphatase containing two highly conserved SH2 domains. SH-PTP1, with a 2.4-kilobase mRNA, a predicted open reading frame of 595 amino acids, and a structure suggesting a nontransmembrane protein, is expressed primarily in hematopoietic and epithelial cells. When expressed in Escherichia coli, SH-PTP1 possesses PTP activity. The structure of SH-PTP1 establishes an additional branch of the tyrosine phosphatase family and suggests mechanisms through which tyrosine phosphatases might participate in signal transduction pathways.

Flow cytometric analysis of opposite effects of a monokine on proliferation and differentiation of murine B lymphocytes

A 35,000 mw factor able to replace macrophages (FRM) in the induction of the in vitro antibody response to sheep erythrocytes has been isolated from the supernatant of murine resident peritoneal macrophage cultures. Purified FRM, when added at the outset of cultures, induced B cells to generate an antigen-specific antibody response. The signals provided by FRM in the process of B cell activation were analyzed using a polyclonal model. Cell cycle analysis by multiparameter flow cytometry after acridine orange staining showed that FRM, on its own, did not trigger the transition of B cells from the G0 to the G1 stage of the cell cycle. In addition, FRM affected neither the basal intracellular free calcium level ([Ca2+]i) nor the rapid increase in [Ca2+]i induced by crosslinking of membrane immunoglobulin (mIgM) with anti-mu antibodies. In parallel with its positive effect on B cell differentiation, FRM markedly reduced both proliferation and cell cycle progression of B cells stimulated with anti-mu plus interleukin 4 (IL-4). Indeed, the addition of FRM to such cultures led to a preferential accumulation of cells in the early G1 compartment of the cell cycle and to a decreased frequency of cells in all other phases including G1B, S and G2/M.

Cloning and expression of an inducible lymphoid-specific, protein tyrosine phosphatase (HePTPase)

Tyrosine phosphorylation and dephosphorylating events have been shown to be central to the process of growth regulation and signal transduction. We report here, the identification of a new gene with a tyrosine phosphatase domain (EC 3.1.3.48) which is expressed exclusively in thymus and spleen. A cDNA of 2760 bp encodes a 339-amino acid, intracellular, single-domain tyrosine phosphatase. When expressed as a glutathionine-S-transferase fusion protein, efficient lysis of p-nitrophenyl phosphate is noted, indicating in vitro enzymatic activity of the cloned gene product. Normal mouse lymphocytes increase mRNA expression 10-15-fold upon stimulation with phytohemagglutinin, concanavalin A, lipopolysaccharide or anti-CD3 monoclonal antibody. This new hematopoietic tyrosine phosphatase, (HePTP), may play a role in the regulation of T and B lymphocyte development and signal transduction.

Protein-tyrosine phosphatases and the regulation of insulin action

Protein-tyrosine phosphatases (PTPases) play an important role in the regulation of insulin action by dephosphorylating the active (autophosphorylated) form of the insulin receptor and attenuating its tyrosine kinase activity. PTPases can also modulate post-receptor signalling by catalyzing the dephosphorylation of cellular substrates of the insulin receptor kinase. Dramatic advances have recently been made in our understanding of PTPases as an extensive family of transmembrane and intracellular proteins that are involved in a number of pathways of cellular signal transduction. Identification of the PTPase(s) which act on various components of the insulin action cascade will not only enhance our understanding of insulin signalling but will also clarify the potential involvement of PTPases in the pathophysiology of insulin-resistant disease states. This brief review provides a summary of reversible tyrosine phosphorylation events in insulin action and available data on candidate PTPases in liver and skeletal muscle that may be involved in the regulation of insulin action.

Inhibition of protein tyrosine phosphatase activity by diamide is reversed by epidermal growth factor in fibroblasts

Diamide (azodicarboxylic acid bis(dimethylamide] inhibits protein tyrosine phosphatase activity in fibroblasts without altering protein tyrosine kinase activity associated with the epidermal growth factor receptor. The loss of protein tyrosine phosphatase activity caused by diamide is reversed by 2-mercaptoethanol or epidermal growth factor.

Elevated protein tyrosine phosphorylation in the optic tract of the chick embryo

Antibodies specific for protein phosphotyrosyl residues were used to localize sites of protein tyrosine kinase activity in the optic tract of the developing chick by immunoperoxidase staining. In the stage 34 (day 8) chick embryo, phosphotyrosine-modified proteins were abundant within outgrowing neuronal processes in the optic nerve head and nerve fiber layer of the retina, and in the developing stratum opticum at the surface of the optic tectum. These sites corresponded to regions where migrating growth cones and fasciculating bundles of some, but not all, retinal ganglion cell axons were located. Phosphotyrosine-modified proteins were also abundant in and highly restricted to the process-rich layers of the embryonic optic tectum. Phosphotyrosine immunoreactivity decreased dramatically in the corresponding regions of the optic tract of the adult chicken, indicating that protein tyrosine phosphorylation occurred principally in developing, rather than mature, neuronal processes. These findings are in accord with the idea that protein tyrosine phosphorylation may be important in cell-cell or cell-substratum interactions of ganglion cell axons.

Three receptor-linked protein-tyrosine phosphatases are selectively expressed on central nervous system axons in the Drosophila embryo

We describe the isolation of seven different protein-tyrosine phosphatase (PTPase) cDNAs from Drosophila embryos, three of which are primarily expressed in the central nervous system (CNS). The CNS-specific PTPases include the previously sequenced DLAR, as well as two novel PTPases (denoted DPTP10D and DPTP99A), which have extracellular domains consisting of multiple fibronectin type III repeats. Each of the Drosophila sequences is most closely related to a different human PTPase. The three PTPase mRNAs are expressed in different patterns of cells in the ventral nerve cord, and all three proteins are restricted to axons. DLAR and DPTP99A are apparently expressed on most or all axons, while DPTP10D is primarily localized to the anterior commissure and its junctions with the longitudinal tracts.

A gene deleted in Kallmann's syndrome shares homology with neural cell adhesion and axonal path-finding molecules

Kallmann's syndrome (clinically characterized by hypogonadotropic hypogonadism and inability to smell) is caused by a defect in the migration of olfactory neurons, and neurons producing hypothalamic gonadotropin-releasing hormone. A gene has now been isolated from the critical region on Xp22.3 to which the syndrome locus has been assigned: this gene escapes X inactivation, has a homologue on the Y chromosome, and shows an unusual pattern of conservation across species. The predicted protein has significant similarities with proteins involved in neural cell adhesion and axonal pathfinding, as well as with protein kinases and phosphatases, which suggests that this gene could have a specific role in neuronal migration.

Cyclin A, cell cycle control and oncogenesis

One of the most fundamental questions in biology is how a cell is able to regulate its division cycle. Initially it was thought that in mammalian cells control over entry into the cell cycle is exerted at a restriction point in G1; once past this point the cell would be free to undergo all the steps needed until the following division. Hence, for many years research on tumorigenesis focused on the mitogenic activation of quiescent cells by growth factors, peptide hormones and oncogene products (for reviews see [1, 2]). These studies investigated the initial steps required to induce a quiescent, nondividing cell to proliferate, and led to the identification of many growth factor receptors, of both the tyrosine kinase family and the G-protein coupled family. Receptors bearing protein tyrosine phosphatase or serine kinase catalytic domains were also identified via this route (for reviews see [3, 4, 5]). However more recent studies on the cooperation between different growth factors for mitogenesis have shown that multiple requirements exist for a cell to proceed through the entire division cycle. Indeed studies in several different organisms, pioneered by investigators working with Ascomycetes [6, 7, 8], have now clearly shown that the eukaryotic cell cycle proceeds through multiple check-points. Furthermore, it now appears that many of the regulatory elements and even pathways have been conserved throughout evolution. In this review we discuss the possible involvement of one of the transducing molecules, cyclin A, in abnormal cell proliferation.