Differential effects of interleukin-1beta and transforming growth factor-beta1 on the expression of the inflammation-associated protein, ADAMTS-1, in human decidual stromal cells in vitro

BACKGROUND

The pro-inflammatory cytokine, interleukin-1 beta (IL-1beta) promotes the proteolytic degradation of the extracellular matrix (ECM) of maternal decidua, a critical step in pregnancy that is counterbalanced by the expression of the anti-inflammatory cytokine, transforming growth factor-beta 1 (TGF-beta1). Recently, the inflammation-associated protein, ADAMTS-1, a member of the ADAMTS (A Disintegrin And Metalloproteinase with ThromboSpondin repeats) gene family of metalloproteinases has been assigned a central role in the formation and organization of tissues. In view of these observations, we have hypothesized that ADAMTS-1 contributes to the cytokine-mediated remodelling of decidual ECM.

METHODS

The spatiotemporal expression of ADAMTS-1 in human endometrium was examined by immunohistochemistry. A quantitative-competitive (QC)-PCR strategy and western blot analysis was then employed to determine whether IL-1beta and TGF-beta1 regulate ADAMTS-1 mRNA and protein expression levels in primary cultures of stromal cells isolated from first trimester decidua.

RESULTS

ADAMTS-1 expression is associated with decidualization of the endometrial stroma in vivo. IL-1beta increased whereas TGF-beta1 decreased ADAMTS-1 mRNA and protein levels in decidual stromal cell cultures in a concentration- and time-dependent manner. These regulatory effects were attenuated by function-perturbing antibodies specific for either cytokine.

CONCLUSION

IL-1beta and TGF-beta1 differentially regulate ADAMTS-1 expression in human decidual stromal cells.

Integrin-induced Tyrosine Phosphorylation of Protein-tyrosine Phosphatase-α Is Required for Cytoskeletal Reorganization and Cell Migration

Protein-tyrosine phosphatase-alpha (PTPalpha) activates Src family kinases (SFKs) to promote the integrin-stimulated early autophosphorylation of focal adhesion kinase (FAK). We report here that integrin stimulation induces tyrosine phosphorylation of PTPalpha. PTPalpha was dephosphorylated upon fibroblast detachment from the substratum and rephosphorylated when cells were plated on the integrin ligand fibronectin. alpha PTP phosphorylation occurred at Tyr789 and required SFKs (Src or Fyn/Yes), FAK, and an intact cytoskeleton. It also required active PTPalpha or constitutively active Src. These observations indicate that PTPalpha activates SFKs and that the subsequently activated SFK.FAK tyrosine kinase complex in turn phosphorylates PTPalpha. Reintroduction of wild-type PTPalpha or unphosphorylatable PTPalpha(Y789F) (but not inactive PTPalpha) into PTPalpha-null fibroblasts restored defective integrin-induced SFK activation, FAK phosphorylation, and paxillin phosphorylation. PTPalpha(Y789F) and inactive PTPalpha could not rescue delayed actin stress fiber assembly and focal adhesion formation or defective cell migration. This study distinguishes two roles of PTPalpha in integrin signaling: an early role as an activator of SFKs and FAK with no requirement for PTPalpha phosphorylation and a later downstream role in cytoskeleton-associated events for which PTPalpha phosphorylation at Tyr789 is essential.

Protein Tyrosine Phosphatase α Regulates Fyn Activity and Cbp/PAG Phosphorylation in Thymocyte Lipid Rafts

A role for the receptor protein tyrosine phosphatase alpha (PTPalpha) in immune cell function and regulation of Src family kinases was investigated using thymocytes from PTPalpha-deficient mice. PTPalpha-null thymocytes develop normally, but unstimulated PTPalpha-/- cells exhibit increased tyrosine phosphorylation of specific proteins, increased Fyn activity, and hyperphosphorylation of Cbp/PAG that promotes its association with C-terminal Src kinase. Elevated Fyn activity in the absence of PTPalpha is due to enhanced phosphorylation of Fyn tyrosines 528 and 417. Some PTPalpha is localized in lipid rafts of thymocytes, and raft-associated Fyn is specifically activated in PTPalpha-/- cells. PTPalpha is not a Cbp/PAG phosphatase, because it is not required for Cbp/PAG dephosphorylation in unstimulated or anti-CD3-stimulated thymocytes. Together, our results indicate that PTPalpha, likely located in lipid rafts, regulates the activity of raft Fyn. In the absence of PTPalpha this population of Fyn is activated and phosphorylates Cbp/PAG to enhance association with C-terminal Src kinase. Although TCR-mediated tyrosine phosphorylation was apparently unaffected by the absence of PTPalpha, the long-term proliferative response of PTPalpha-/- thymocytes was reduced. These findings indicate that PTPalpha is a component of the complex Src family tyrosine kinase regulatory network in thymocytes and is required to suppress Fyn activity in unstimulated cells in a manner that is not compensated for by the major T cell PTP and SFK regulator, CD45.

Akt phosphorylates the Y-box binding protein 1 at Ser102 located in the cold shock domain and affects the anchorage-independent growth of breast cancer cells

Akt/PKB is a serine/threonine kinase that promotes tumor cell growth by phosphorylating transcription factors and cell cycle proteins. There is particular interest in finding tumor-specific substrates for Akt to understand how this protein functions in cancer and to provide new avenues for therapeutic targeting. Our laboratory sought to identify novel Akt substrates that are expressed in breast cancer. In this study, we determined that activated Akt is positively correlated with the protein expression of the transcription/translation factor Y-box binding protein-1 (YB-1) in primary breast cancer by screening tumor tissue microarrays. We therefore questioned whether Akt and YB-1 might be functionally linked. Herein, we illustrate that activated Akt binds to and phosphorylates the YB-1 cold shock domain at Ser102. We then addressed the functional significance of disrupting Ser102 by mutating it to Ala102. Following the stable expression of Flag:YB-1 and Flag:YB-1 (Ala102) in MCF-7 cells, we observed that disruption of the Akt phosphorylation site on YB-1 suppressed tumor cell growth in soft agar and in monolayer. This correlated with an inhibition of nuclear translocation by the YB-1(Ala102) mutant. In conclusion, YB-1 is a new Akt substrate and disruption of this specific site inhibits tumor cell growth.

Insulin signaling and glucose homeostasis in mice lacking protein tyrosine phosphatase α

Studies in cultured cells have implicated protein tyrosine phosphatase alpha (PTPalpha) as a potential regulator of insulin signaling. The physiological role of PTPalpha in insulin action was investigated using gene-targeted mice deficient in PTPalpha. PTPalpha-null animals had normal body weights and circulating levels of glucose and insulin in random fed and fasted states. In glucose and insulin tolerance tests, their efficiency of blood glucose clearance was comparable to wild-type mice. Kinetics and extents of insulin-stimulated insulin receptor and IRS-1 tyrosine phosphorylation were similar in wild-type and PTPalpha(-/-) liver, muscle, and adipose tissue. However, the association of IRS-1 and PI 3-K was altered in PTPalpha(-/-) liver, with increased insulin-independent and reduced insulin-stimulated association compared to wild-type samples. This did not affect activation of the downstream signaling effector Akt. Our data indicate that PTPalpha is not a negative regulator of insulin signaling and does not perform an essential role in mediating the physiological action of insulin.

Nogo-A at CNS paranodes is a ligand of Caspr: possible regulation of K(+) channel localization

We report Nogo-A as an oligodendroglial component congregating and interacting with the Caspr-F3 complex at paranodes. However, its receptor Nogo-66 receptor (NgR) does not segregate to specific axonal domains. CHO cells cotransfected with Caspr and F3, but not with F3 alone, bound specifically to substrates coated with Nogo-66 peptide and GST-Nogo-66. Binding persisted even after phosphatidylinositol- specific phospholipase C (PI-PLC) removal of GPI-linked F3 from the cell surface, suggesting a direct interaction between Nogo-66 and Caspr. Both Nogo-A and Caspr co-immunoprecipitated with Kv1.1 and Kv1.2, and the developmental expression pattern of both paralleled compared with Kv1.1, implicating a transient interaction between Nogo-A-Caspr and K(+) channels at early stages of myelination. In pathological models that display paranodal junctional defects (EAE rats, and Shiverer and CGT(-/-) mice), distances between the paired labeling of K(+) channels were shortened significantly and their localization shifted toward paranodes, while paranodal Nogo-A congregation was markedly reduced. Our results demonstrate that Nogo-A interacts in trans with axonal Caspr at CNS paranodes, an interaction that may have a role in modulating axon-glial junction architecture and possibly K(+)-channel localization during development.

F3/contactin acts as a functional ligand for Notch during oligodendrocyte maturation

Axon-derived molecules are temporally and spatially required as positive or negative signals to coordinate oligodendrocyte differentiation. Increasing evidence suggests that, in addition to the inhibitory Jagged1/Notch1 signaling cascade, other pathways act via Notch to mediate oligodendrocyte differentiation. The GPI-linked neural cell recognition molecule F3/contactin is clustered during development at the paranodal region, a vital site for axoglial interaction. Here, we show that F3/contactin acts as a functional ligand of Notch. This trans-extracellular interaction triggers gamma-secretase-dependent nuclear translocation of the Notch intracellular domain. F3/Notch signaling promotes oligodendrocyte precursor cell differentiation and upregulates the myelin-related protein MAG in OLN-93 cells. This can be blocked by dominant negative Notch1, Notch2, and two Deltex1 mutants lacking the RING-H2 finger motif, but not by dominant-negative RBP-J or Hes1 antisense oligonucleotides. Expression of constitutively active Notch1 or Notch2 does not upregulate MAG. Thus, F3/contactin specifically initiates a Notch/Deltex1 signaling pathway that promotes oligodendrocyte maturation and myelination.

Protein tyrosine phosphatase alpha (PTP alpha) knockout mice show deficits in Morris water maze learning, decreased locomotor activity, and decreases in anxiety

Receptor PTPalpha is a widely expressed transmembrane enzyme enriched in brain. PTPalpha knockout (PTPalpha(-/-)) mice are viable and display no gross abnormalities. Brain and embryo derived fibroblast src and fyn activity is reduced to <50% in PTPalpha(-/-) mice. These protein kinases are implicated in multiple aspects of neuronal development and function. However, the effect of the loss of function of the PTPalpha gene on behavior has yet to be investigated. PTPalpha(-/-) and WT mice were tested for anxiety, swimming ability, spatial learning, cued learning, locomotor activity, and novel object recognition (NOR). PTPalpha(-/-) mice were indistinguishable from WT in swimming ability, cued learning and novel object recognition. Knockout mice showed decreased anxiety without an increase in head dips and stretch-attend movements. During Morris water maze (MWM) learning, PTPalpha(-/-) mice had increased latencies to reach the goal compared to WT on acquisition, but no memory deficit on probe trials. On reversal learning, knockout mice showed no significant effects. PTPalpha(-/-) mice showed decreased exploratory locomotor activity, but responded normally to a challenge dose of D-methamphetamine. The data suggest that PTPalpha serves a regulatory function in learning and other forms of neuroplasticity.

Protein tyrosine phosphatase alpha (PTPalpha): a Src family kinase activator and mediator of multiple biological effects

This review discusses progress made over the past 10+ years in elucidating the properties, regulation, and function of protein tyrosine phosphatase alpha (PTPalpha). It is apparent from studies in knockout mice and diverse cell lines that the major action of PTPalpha is as a positive regulator of src and src family kinases. PTPalpha dephosphorylates and activates src. In this manner it affects transformation and tumourigenesis, inhibition of proliferation and cell cycle arrest, mitotic activation of src, integrin signaling, neuronal differentiation and outgrowth, and ion channel activity. PTPalpha may well modulate additional processes, including insulin signaling, and have other targets besides src family kinases. As an important modulator of several specific cell signaling pathways, PTPalpha has promise as a target for drug discovery. Continued research on the physiological and pathological activities of PTPalpha is necessary to define the therapeutic potential of PTPalpha-directed pharmacologicals.

PRL-3 and PRL-1 promote cell migration, invasion, and metastasis

We demonstrate here that Chinese hamster ovary cells stably expressing PRL-3, a M(r) 20000 prenylated protein tyrosine phosphatase, or its relative, PRL-1, exhibit enhanced motility and invasive activity. A catalytically inactive PRL-3 mutant has significantly reduced migration-promoting activity. We observe that PRL-3 is associated with diverse membrane structures involved in cell movement. Furthermore, we show that PRL-3- and -1-expressing cells, but not control cells, induce metastatic tumor formation in mice. Thus, our results deliver the first evidence for a causative role of PRL-3 and -1 in promoting cell motility, invasion activity, and metastasis.

PTP alpha regulates integrin-stimulated FAK autophosphorylation and cytoskeletal rearrangement in cell spreading and migration

We investigated the molecular and cellular actions of receptor protein tyrosine phosphatase (PTP) alpha in integrin signaling using immortalized fibroblasts derived from wild-type and PTP alpha-deficient mouse embryos. Defects in PTP alpha-/- migration in a wound healing assay were associated with altered cell shape and focal adhesion kinase (FAK) phosphorylation. The reduced haptotaxis to fibronectin (FN) of PTP alpha-/- cells was increased by expression of active (but not inactive) PTP alpha. Integrin-mediated formation of src-FAK and fyn-FAK complexes was reduced or abolished in PTP alpha-/- cells on FN, concomitant with markedly reduced phosphorylation of FAK at Tyr397. Reintroduction of active (but not inactive) PTP alpha restored FAK Tyr-397 phosphorylation. FN-induced cytoskeletal rearrangement was retarded in PTP alpha-/- cells, with delayed filamentous actin stress fiber assembly and focal adhesion formation. This mimicked the effects of treating wild-type fibroblasts with the src family protein tyrosine kinase (Src-PTK) inhibitor PP2. These results, together with the reduced src/fyn tyrosine kinase activity in PTP alpha-/- fibroblasts (Ponniah et al., 1999; Su et al., 1999), suggest that PTP alpha functions in integrin signaling and cell migration as an Src-PTK activator. Our paper establishes that PTP alpha is required for early integrin-proximal events, acting upstream of FAK to affect the timely and efficient phosphorylation of FAK Tyr-397.

Conserved synteny between the Fugu and human PTEN locus and the evolutionary conservation of vertebrate PTEN function

Mutations of PTEN, which encodes a protein-tyrosine and lipid phosphatase, are prevalent in a variety of human cancers. The human genome 'draft' sequence still lacks organization and much of the PTEN and adjacent loci remain undefined. The pufferfish, Fugu rubripes, by virtue of having a compact genome represents an excellent template for rapid vertebrate gene discovery. Sequencing of 56 kb from the Fugu pten (fpten) locus identified four complete genes and one partial gene homologous to human genes. Genes neighboring fpten include a PAPS synthase (fpapss2) differentially expressed between non-metastatic/metastatic human carcinoma cell lines, an inositol phosphatase (fminpp1) and an omega class glutathione-S-transferase (fgsto). We have determined the order of human BAC clones at the hPTEN locus and that the locus contains hPAPSS2 and hMINPP1 genes oriented as are their Fugu orthologs. Although the human genes span 500 kb, the Fugu genes lie within only 22 kb due to the compressed intronic and intergenic regions that typify this genome. Interestingly, and providing striking evidence of regulatory element conservation between widely divergent vertebrate species, the compact 2.1 kb fpten promoter is active in human cells. Also, like hPTEN, fpten has a growth and tumor suppressor activity in human glioblastoma cells, demonstrating conservation of protein function.

Interaction of farnesylated PRL-2, a protein-tyrosine phosphatase, with the beta-subunit of geranylgeranyltransferase II

Protein of regenerating liver (PRL)-1, -2, and -3 comprise a subgroup of closely related protein-tyrosine phosphatases featuring a C-terminal prenylation motif conforming to either the consensus sequence for farnesylation, CAAX, or geranylgeranylation, CCXX. Yeast two-hybrid screening for PRL-2-interacting proteins identified the beta-subunit of Rab geranylgeranyltransferase II (betaGGT II). The specific interaction of betaGGT II with PRL-2 but not with PRL-1 or -3 occurred in yeast and HeLa cells. Chimeric PRL-1/-2 molecules were tested for their interaction with betaGGT II, and revealed that the C-terminal region of PRL-2 is required for interaction, possibly the PRL variable region immediately preceeding the CAAX box. Additionally, PRL-2 prenylation is prequisite for betaGGT II binding. As prenylated PRL-2 is localized to the early endosome, we propose that this is where the interaction occurs. PRL-2 is not a substrate for betaGGT II, as isoprenoid analysis showed that PRL-2 was solely farnesylated in vivo. Co-expression of the alpha-subunit (alpha) of GGT II, betaGGT II, and PRL-2 resulted in alpha/betaGGT II heterodimer formation and prevented PRL-2 binding. Expression of PRL-2 alone inhibited the endogenous alpha/betaGGT II activity in HeLa cells. Together, these results indicate that the binding of alphaGGT II and PRL-2 to betaGGT II is mutually exclusive, and suggest that PRL-2 may function as a regulator of GGT II activity.

Isolation of a novel protein tyrosine phosphatase inhibitor, 2-methyl-fervenulone, and its precursors from Streptomyces

High-throughput screening identified an extract from Streptomyces sp. IM 2096 with inhibitory activity toward several protein tyrosine phosphatases (PTPs). Four 1,2,4-triazine compounds 2096A-D (1-4) were isolated from this extract and their structures elucidated by interpretation of spectroscopic data and confirmed by degradation and synthesis. The novel glycocyamidine derivatives 1 and 2 are diastereomers and may interconvert. Both are inactive in the PTP inhibition assay. Compounds 1 and 2 are unstable and partially decompose to 3 and glycocyamidine (5) at room temperature. Compound 3, known as MSD-92 or 2-methyl-fervenulone, is a broad-specificity PTP inhibitor with comparable potency to vanadate. The imidazo[4, 5-e]-1,2,4-triazine (4), inactive in the PTP-inhibition assay, may be a degradation product of 3.

Calmodulin binds to and inhibits the activity of the membrane distal catalytic domain of receptor protein-tyrosine phosphatase alpha

cDNA expression library screening revealed binding between the membrane distal catalytic domain (D2) of protein-tyrosine phosphatase alpha (PTPalpha) and calmodulin. Characterization using surface plasmon resonance showed that calmodulin bound to PTPalpha-D2 in a Ca(2+)-dependent manner but did not bind to the membrane proximal catalytic domain (D1) of PTPalpha, to the two tandem catalytic domains (D1D2) of PTPalpha, nor to the closely related D2 domain of PTPepsilon. Calmodulin bound to PTPalpha-D2 with high affinity, exhibiting a K(D) approximately 3 nm. The calmodulin-binding site was localized to amino acids 520-538 in the N-terminal region of D2. Site-directed mutagenesis showed that Lys-521 and Asn-534 were required for optimum calmodulin binding and that restoration of these amino acids to the counterpart PTPepsilon sequence could confer calmodulin binding. The overlap of the binding site with the predicted lip of the catalytic cleft of PTPalpha-D2, in conjunction with the observation that calmodulin acts as a competitive inhibitor of D2-catalyzed dephosphorylation (K(i) approximately 340 nm), suggests that binding of calmodulin physically blocks or distorts the catalytic cleft of PTPalpha-D2 to prevent interaction with substrate. When expressed in cells, full-length PTPalpha and PTPalpha lacking only D1, but not full-length PTPepsilon, bound to calmodulin beads in the presence of Ca(2+). Also, PTPalpha was found in association with calmodulin immunoprecipitated from cell lysates. Thus calmodulin does associate with PTPalpha in vivo but not with PTPalpha-D1D2 in vitro, highlighting a potential conformational difference between these forms of the tandem catalytic domains. The above findings suggest that calmodulin is a possible specific modulator of PTPalpha-D2 and, via D2, of PTPalpha.

Prenylation-dependent association of protein-tyrosine phosphatases PRL-1, -2, and -3 with the plasma membrane and the early endosome

PRL-1, -2, and -3 represent a novel class of protein-tyrosine phosphatase with a C-terminal prenylation motif. Although PRL-1 has been suggested to be associated with the nucleus, the presence of three highly homologous members and the existence of a prenylation motif call for a more detailed examination of their subcellular localization. In the present study, we first demonstrate that mouse PRL-1, -2, and -3 are indeed prenylated. Examination of N-terminal epitope-tagged PRL-1, -2, and -3 expressed in transiently transfected cells suggests that PRL-1, -2, and -3 are present on the plasma membrane and intracellular punctate structures. Stable Chinese hamster ovary cells expressing PRL-1 and -3 in an inducible manner were established. When cells were treated with brefeldin A, PRL-1 and -3 accumulated in a collapsed compact structure around the microtubule-organizing center. Furthermore, PRL-1 and -3 redistributed into swollen vacuole-like structures when cells were treated with wortmannin. These characteristics of PRL-1 and -3 are typical for endosomal proteins. Electron microscope immunogold labeling reveals that PRL-1 and -3 are indeed associated with the plasma membrane and the early endosomal compartment. Expression of PRL-3 is detected in the epithelial cells of the small intestine, where PRL-3 is present in punctate structures in the cytoplasm. When cells are treated with FTI-277, a selective farnesyltransferase inhibitor, PRL-1, -2, and -3 shifted into the nucleus. Furthermore, a mutant form of PRL-2 lacking the C-terminal prenylation signal is associated with the nucleus. These results establish that the primary association of PRL-1, -2, and -3 with the membrane of the cell surface and the early endosome is dependent on their prenylation and that nuclear localization of these proteins may be triggered by a regulatory event that inhibits their prenylation.

Protein tyrosine phosphatase alpha (PTPalpha) and contactin form a novel neuronal receptor complex linked to the intracellular tyrosine kinase fyn

Glycosyl phosphatidylinositol (GPI)-linked receptors and receptor protein tyrosine phosphatases (RPTPs), both play key roles in nervous system development, although the molecular mechanisms are largely unknown. Despite lacking a transmembrane domain, GPI receptors can recruit intracellular src family tyrosine kinases to receptor complexes. Few ligands for the extracellular regions of RPTPs are known, relegating most to the status of orphan receptors. We demonstrate that PTPalpha, an RPTP that dephosphorylates and activates src family kinases, forms a novel membrane-spanning complex with the neuronal GPI-anchored receptor contactin. PTPalpha and contactin associate in a lateral (cis) complex mediated through the extracellular region of PTPalpha. This complex is stable to isolation from brain lysates or transfected cells through immunoprecipitation and to antibody-induced coclustering of PTPalpha and contactin within cells. This is the first demonstration of a receptor PTP in a cis configuration with another cell surface receptor, suggesting an additional mode for regulation of a PTP. The transmembrane and catalytic nature of PTPalpha indicate that it likely forms the transducing element of the complex, and we postulate that the role of contactin is to assemble a phosphorylation-competent system at the cell surface, conferring a dynamic signal transduction capability to the recognition element.

Catalytic activation of the membrane distal domain of protein tyrosine phosphatase epsilon, but not CD45, by two point mutations

Most, if not all, of the catalytic activity of the tandem catalytic domain-containing receptor-like protein tyrosine phosphatases (PTPs) resides in the membrane proximal domains (D1), with little to no activity associated with the membrane distal domains (D2). Two point mutations in the D2 domain of PTPalpha, which restore invariant amino acids found in the KNRY motif and WPD loop of all active D1 domains, synergistically confer D1-equivalent kinetic properties towards the phosphotyrosine analogue pNPP, and activate PTPalpha-D2 catalysed phosphopeptide hydrolysis (Lim et al., J. Biol. Chem. 273 (1998) 28986-28993; Buist et al., Biochemistry 38 (1999) 914-922). As all D2 domains lack these two D1-invariant amino acids, we have investigated whether other D2 domains are activated by such point mutations. Mutant PTPepsilon-D2, closely related to PTPalpha-D2 and belonging to a subgroup of D2 domains with minimal and conservative substitutions of D1-invariant amino acids, exhibits synergistic activation towards pNPP but not towards a phosphopeptide substrate. CD45-D2, belonging to another subgroup of D2 domains with considerable substitutions in D1-invariant amino acids, is not activated by these mutations, even in the context of a third mutation which restores the minimal essential active site sequence C(X(5))R, indicating that additional defects are sufficient to preclude catalysis. The ability of the KNRY and WPD replacements to activate PTPepsilon-D2 and PTPalpha-D2, but not CD45-D2, in conjunction with the extent and nature of their wild-type amino acid substitutions, suggests that these D2 domains are representative of two functionally distinct groups of D2 domain.

Targeted disruption of the tyrosine phosphatase PTPalpha leads to constitutive downregulation of the kinases Src and Fyn

A role for the receptor-like protein tyrosine phosphatase alpha (PTPalpha) in regulating the kinase activity of Src family members has been proposed because ectopic expression of PTPalpha enhances the dephosphorylation and activation of Src and Fyn [1] [2] [3]. We have generated mice lacking catalytically active PTPalpha to address the question of whether PTPalpha is a physiological activator of Src and Fyn, and to investigate its other potential functions in the context of the whole animal. Mice homozygous for the targeted PTPalpha allele (PTPalpha-/-) and lacking detectable PTPalpha protein exhibited no gross phenotypic defects. The kinase activities of Src and Fyn were significantly reduced in PTPalpha-/- mouse brain and primary embryonic fibroblasts, and this correlated with enhanced phosphorylation of the carboxy-terminal regulatory Tyr527 of Src in PTPalpha-/- mice. Thus, PTPalpha is a physiological positive regulator of the tyrosine kinases Src and Fyn. Increased tyrosine phosphorylation of several unidentified proteins was also apparent in PTPalpha-/- mouse brain lysates. These may be PTPalpha substrates or downstream signaling proteins. Taken together, the results indicate that PTPalpha has a dual function as a positive and negative regulator of tyrosine phosphorylation events, increasing phosphotyrosyl proteins through activation of Src and Fyn, and directly or indirectly removing tyrosine phosphate from other unidentified proteins.

Insulin secretagogues activate the secretory granule receptor-like protein-tyrosine phosphatase IAR

To investigate the potential role of protein-tyrosine phosphatases (PTPs) in regulated secretion, cellular PTP activity was measured in pancreatic beta cell lines after exposure to insulin secretagogues. A peak of elevated PTP activity was detected in whole cell lysates after 15-20 min of treatment of the cells with high KCl, glucose, or TPA, which did not appear upon treatment with control compounds. Neither was it detected in cells that do not undergo regulated secretion. The PTP activation was transient, SDS-resistant, and localized to the cytoskeleton fraction of cells. The cytoskeletal localization of IAR, a receptor-like PTP associated with secretory granules of neuroendocrine cells, suggested the possibility that IAR is the secretagogue-activated PTP. The transient expression of human IAR in betaTC3 and HIT-T15 beta cells, followed by treatment with secretagogues or control compounds and immunoprecipitation of human IAR, showed that immunoprecipitates from the secretagogue-treated cells contained an elevated PTP activity. The secretagogue-induced activation of IAR had identical kinetics to that of the endogenous PTP. Although ectopic IAR was present in membrane and cytoskeletal fractions from the cells, only the cytoskeleton-associated IAR could be activated. Thus IAR represents the endogenous secretagogue-responsive PTP, or at least a component of it, and is one of the few receptor-like PTPs for which enzymatic activation has been demonstrated. Insulin secretion is detected prior to IAR activation, suggesting that IAR is not required for immediate secretion but likely plays a role in events downstream of insulin secretion or in another pathway related to the specialized function of secretory cells.

Interconversion of the kinetic identities of the tandem catalytic domains of receptor-like protein-tyrosine phosphatase PTPalpha by two point mutations is synergistic and substrate-dependent

The two tandem homologous catalytic domains of PTPalpha possess different kinetic properties, with the membrane proximal domain (D1) exhibiting much higher activity than the membrane distal (D2) domain. Sequence alignment of PTPalpha-D1 and -D2 with the D1 domains of other receptor-like PTPs, and modeling of the PTPalpha-D1 and -D2 structures, identified two non-conserved amino acids in PTPalpha-D2 that may account for its low activity. Mutation of each residue (Val-536 or Glu-671) to conform to its invariant counterpart in PTPalpha-D1 positively affected the catalytic efficiency of PTPalpha-D2 toward the in vitro substrates para-nitrophenylphosphate and the phosphotyrosyl-peptide RR-src. Together, they synergistically transformed PTPalpha-D2 into a phosphatase with catalytic efficiency for para-nitrophenylphosphate equal to PTPalpha-D1 but not approaching that of PTPalpha-D1 for the more complex substrate RR-src. In vivo, no gain in D2 activity toward p59(fyn) was effected by the double mutation. Alteration of the two corresponding invariant residues in PTPalpha-D1 to those in D2 conferred D2-like kinetics toward all substrates. Thus, these two amino acids are critical for interaction with phosphotyrosine but not sufficient to supply PTPalpha-D2 with a D1-like substrate specificity for elements of the phosphotyrosine microenvironment present in RR-src and p59(fyn). Whether the structural features of D2 can uniquely accommodate a specific phosphoprotein substrate or whether D2 has an alternate function in PTPalpha remains an open question.

Antibodies to the protein tyrosine phosphatases IAR and IA-2 are associated with progression to insulin-dependent diabetes (IDDM) in first-degree relatives at-risk for IDDM

Insulin-dependent diabetes mellitus (IDDM) is preceded by the presence of antibodies against islet proteins including a protein tyrosine phosphatase (PTP) designated IA-2. Recently, we cloned a novel PTP named IAR which shares 43% sequence identity with IA-2 and is recognised by antibodies from a majority of patients with IDDM. The aim of the present study was to determine whether IAR antibodies (IAR Ab) or IA-2 antibodies (IA-2 Ab) are associated with progression to IDDM in first-degree relatives "at-risk" for IDDM (operationally defined as those with islet cell antibodies [ICA] > or = 20JDFU or insulin autoantibodies [IAA] > or = 100 nU/ml), and to examine combinations of IAR Ab and IA-2 Ab in these subjects. The sensitivity and specificity of these antibodies were also examined in patients with recent-onset IDDM. Using Cox's Proportional Hazards Model, the number of siblings with IDDM was associated with progression to IDDM in "at-risk" relatives, but other covariables (age, sex, number of affected offspring or parents) were not significantly associated. Using number of affected siblings as a covariable, both IAR and IA-2 antibodies were significantly associated with progression to IDDM (p < 0.005). Combinations of both antibodies, however, did not result in a significantly stronger association with progression to IDDM. The threshold of positivity for IAR Ab (0.5 units) and IA-2 Ab (3.0 units) assays was adjusted to give the same specificity (97.9%) for each assay in 144 healthy control subjects, to allow standardised comparisons. Levels of IAR Ab and IA-2 Ab were strongly correlated in 53 recent-onset IDDM patients (r = 0.70, p < 0.0001) but 11.3% had IAR Ab in the absence of IA-2 Ab and 16.9% had IA-2 Ab in the absence of IAR Ab. The sensitivity for IDDM (defined as the proportion of IDDM patients positive) was 56.6% for IAR Ab and 62.3% for IA-2 Ab. We conclude that there is considerable overlap in IA-2 Ab and IAR Ab positivity, although either antibody can occur independently in IDDM patients. Both IAR Ab and IA-2 antibodies are associated with progression to IDDM in first-degree relatives at-risk of IDDM, but the use of IAR and IA-2 antibodies in combination are not significantly more strongly associated with progression than single antibodies. IAR Ab may play an important role in the prediction of IDDM.

Solid-phase synthesis of potential protein tyrosine phosphatase inhibitors via the Ugi four-component condensation

A library of 108 alpha,alpha-difluoromethylenephosphonic acids was prepared by Ugi four-component condensation using Rink-NH2 resin, 4-[(diethoxyphosphinyl) difluoromethyl]benzoic acid, and a set of 18 aldehydes and 6 isonitriles. Following resin cleavage, the diethylphosphonate esters were hydrolyzed with trimethylsilyl bromide to yield the free acids which were assayed for inhibition of PTP alpha, PTP beta and PTP epsilon.

Physical and functional interactions between receptor-like protein-tyrosine phosphatase alpha and p59fyn

We have examined the in vivo activity of receptor-like protein-tyrosine phosphatase alpha (PTPalpha) toward p59(fyn), a widely expressed Src family kinase. In a coexpression system, PTPalpha effected a dose-dependent tyrosine dephosphorylation and activation of p59(fyn), where maximal dephosphorylation correlated with a 5-fold increase in kinase activity. PTPalpha expression resulted in increased accessibility of the p59(fyn) SH2 domain, consistent with a PTPalpha-mediated dephosphorylation of the regulatory C-terminal tyrosine residue of p59(fyn). No p59(fyn) dephosphorylation was observed with an enzymatically inactive mutant form of PTPalpha or with another receptor-like PTP, CD45. Many enzyme-linked receptors are complexed with their substrates, and we examined whether PTPalpha and p59(fyn) underwent association. Reciprocal immunoprecipitations and assays detected p59(fyn) and an appropriate kinase activity in PTPalpha immunoprecipitates and PTPalpha and PTP activity in p59(fyn) immunoprecipitates. No association between CD45 and p59(fyn) was detected in similar experiments. The PTPalpha-mediated activation of p59(fyn) is not prerequisite for association since wild-type and inactive mutant PTPalpha bound equally well to p59(fyn). Endogenous PTPalpha and p59(fyn) were also found in association in mouse brain. Together, these results demonstrate a physical and functional interaction of PTPalpha and p59(fyn) that may be of importance in PTPalpha-initiated signaling events.