The dephosphorylation characteristics of the receptors for epidermal growth factor and platelet-derived growth factor in Swiss 3T3 cell membranes suggest differential regulation of receptor signalling by endogenous protein-tyrosine phosphatases

alpha-Hemolysin-induced dephosphorylation of EGF receptor of A431 cells is carried out by rPTPsigma

Earlier we have shown that the epidermal growth factor receptor was unable to retain its phospho Tyr signal after the assembly of staphylococcal alpha-hemolysin (alpha-HL). However, the nature of the protein tyrosine phosphatase (PTPase) or its identity is not known. In this report, we demonstrate that the alpha-HL elevates the activity of receptor like protein tyrosine phosphatase sigma (rPTPsigma). The alpha-HL induced dephosphorylation is prominent only in intact A431 cells. The PTPase activity is not inhibited if the alpha-HL treatment precedes PTPase inhibitor treatments. The anti-EGFr immunoprecipitates have exhibited higher PTPase activity after alpha-HL treatment of A431 cells. Interestingly, PTPase activity of anti-EGFr immunoprecipitates from the A431 cells expressing the antisense message of rPTPsigma has not increased despite alpha-HL treatment, confirming the role of rPTPsigma in the dephosphorylation of EGFr. The studies presented here will be useful in understanding the process of signal modulation by the assembly of alpha-HL.

Identification of epidermal growth factor receptor as a target of Cdc25A protein phosphatase

Cdc25A, a dual-specificity protein phosphatase, plays a critical role in cell cycle progression. Although cyclin-dependent kinases are established substrates, Cdc25A may also affect other proteins. We have shown here that Cdc25A interacts with epidermal growth factor receptor (EGFR) both physically and functionally in Hep3B human hepatoma cells. Cdc25A inhibitor Cpd 5, a vitamin K analog, inhibited Cdc25A activity in the Cdc25A-EGFR immunocomplex and consequently caused prolonged EGFR tyrosine phosphorylation. Both purified GST-Cdc25A protein and endogenous Hep3B cellular Cdc25A dephosphorylated tyrosine-phosphorylated EGFR, and Cpd 5 antagonized the phosphatase activity of Cdc25A. A functional Cdc25A-EGFR interaction was seen in NR-6 fibroblasts expressing ectopic EGFR but not with a receptor lacking the C terminus or a mutated kinase domain. These data link the cell cycle control Cdc25A phosphatase to an EGFR-linked mitogenic signaling pathway specifically involving EGFR dephosphorylation.

Perinuclear localization of the protein-tyrosine phosphatase SHP-1 and inhibition of epidermal growth factor-stimulated STAT1/3 activation in A431 cells

The SH2 domain protein-tyrosine phosphatase SHP-1 has been shown earlier to bind to the epidermal growth factor receptor and to have the capacity for receptor dephosphorylation. New bi- and tricistronic expression vectors (pNRTIS-21 and pNRTIS-33, respectively) based on the tetracycline system were constructed and employed to generate stable cell lines with inducible expression of SHP-1. Inducible overexpression of SHP-1 in A431 cells led to attenuation of epidermal growth factor (EGF) receptor autophosphorylation and of EGF-induced DNA binding of 'signal transducers and activators of transcription' (STAT) 1 and 3. SHP-1 was localized in the cytoplasm with an enrichment in the perinuclear compartment. Association of SHP-1 with perinuclear structures may form the basis for a partial cofractionation with nuclei observed in different types of transfected cells and also with endogenous SHP-1 in U-937 cells. Treatment of SHP-1-overexpressing A431 cells or of HaCaT human keratinocytes expressing SHP-1 endogenously with the Ca2+-ionophore A23187 resulted in partial nuclear accumulation of SHP-1. Thus, SHP-1 may interact with substrates or regulatory proteins in perinuclear or nuclear structures.

Redox regulation of signal transduction in mammalian cells

This mini-review addresses the mechanism of ultraviolet-light-induced activation of receptor tyrosine kinases. The experimental approach into this mechanism revealed the existence of redox regulation of signal transduction in mammalian cells. It is postulated that, in addition to responsiveness to oxidative attacks from outside, redox regulation of specific redox-sensitive proteins likely represents an important physiological mechanism.

The transmembrane protein tyrosine phosphatase RPTPsigma modulates signaling of the epidermal growth factor receptor in A431 cells

Attenuation of epidermal growth factor receptor signaling by the ganglioside G(M3) has previously been found to involve activation of an unknown protein-tyrosine phosphatase (PTP). In transient expression experiments we tested different PTPs for activation towards EGF receptor by G(M3). The transmembrane PTP RPTPsigma but not RPTPalpha or the SH2-domain PTP SHP-1 exhibited elevated activity towards EGF receptor in G(M3)-treated cells. The possible relevance of RPTPsigma for regulation of EGF receptor signaling activity was further explored in stable A431 cells lines inducibly expressing RPTPsigma or RPTPsigma antisense RNA. RPTPsigma expression clearly reduced EGF receptor phosphorylation. Also, soft agar colony formation of respective cell lines was reduced upon RPTPsigma expression whereas RPTPsigma antisense RNA expression augmented both, EGF receptor phosphorylation and soft agar colony formation. In addition, RPTPsigma antisense RNA expression rendered A431 cells resistant to inhibition of EGF receptor phosphorylation by G(M3). We propose that RPTPsigma participates in EGF receptor dephosphorylation in A431 cells, becomes activated by G(M3) via an unknown mechanism and is thereby capable to mediate attenuation of EGF receptor phosphorylation by G(M3).

Phosphotyrosine 1173 mediates binding of the protein-tyrosine phosphatase SHP-1 to the epidermal growth factor receptor and attenuation of receptor signaling

The protein-tyrosine phosphatase SHP-1 binds to and dephosphorylates the epidermal growth factor receptor (EGFR), and both SH2 domains of SHP-1 are important for this interaction (Tenev, T., Keilhack, H., Tomic, S., Stoyanov, B., Stein-Gerlach, M., Lammers, R., Krivtsov, A. V., Ullrich, A., and Böhmer, F. D. (1997) J. Biol. Chem. 272, 5966-5973). We mapped the EGFR phosphotyrosine 1173 as the major binding site for SHP-1 by a combination of phosphopeptide activation, phosphopeptide competition, and receptor YF mutant analysis. Mutational conversion of the EGFR sequence 1171-1176 AEYLRV into the high affinity SHP-1 binding sequence LEYLYL of the erythropoietin receptor (EpoR) led to a highly elevated SHP-1 binding to the mutant EGFR (EGFR1171-1176EpoR) and in turn to an enhanced dephosphorylation of the receptor. SHP-1 expression interfered with EGF-dependent mitogen-activated protein kinase stimulation, and this effect was more pronounced in case of EGFR1171-1176EpoR. Reduced SHP-1 binding to the EGFR Y1173F mutant resulted in a reduced receptor dephosphorylation by coexpressed SHP-1 and less interference with EGF-dependent mitogen-activated protein kinase stimulation. The effects of receptor mutations on SHP-1 binding were, however, stronger than those on receptor dephosphorylation by SHP-1. Therefore, receptor dephosphorylation may be the result of the combined activity of receptor-bound SHP-1 and SHP-1 bound to an auxiliary docking protein.

Decreased PDGF receptor kinase activity in fibroblasts contracting stressed collagen matrices

Fibroblasts cultured in mechanically stressed collagen matrices proliferate, whereas cells in floating collagen matrices become quiescent. Previous research indicated that one factor contributing to cell quiescence in floating matrices was reduced receptor autophosphorylation in response to PDGF stimulation (i.e., PDGF receptor desensitization). To learn more about the mechanism of PDGF receptor desensitization, we analyzed changes in PDGF receptor autophosphorylation and receptor kinase activity after stressed collagen matrices were switched to floating conditions, which results in rapid cell contraction and dissipation of mechanical stress. PDGF receptor desensitization occurred during contraction stimulated by serum but not in the absence of serum, and desensitization was prevented by inhibitors of contraction but not by inhibitors of the contraction-activated cyclic AMP signaling pathway. Receptor desensitization resulted from decreased receptor kinase activity rather than from elevated protein tyrosine phosphatase activity, and only receptors unoccupied at the time of contraction were affected. After contraction, radiolabeled PDGF binding to the cells was decreased, which suggested that receptor desensitization resulted from a contraction-dependent change in receptor availability or affinity.

Protein phosphorylation in response to PDGF stimulation in cultured neurons and astrocytes

Platelet-derived growth factor (PDGF) is an important growth factor for a variety of cells, including neurons and glial cells. PDGF signal transduction pathways have been studied primarily in mesenchyme-derived cells (such as fibroblasts and smooth muscle cells). However, little is known about these pathways in the central nervous system (CNS). It is believed that phosphorylation is a critical aspect of several steps in the signal transduction pathway. In this study, neurons and type 1 astrocytes in vitro were radiolabeled with 32P-orthophosphate (32P-Pi). The cells were lysed, and labeled proteins were separated by two-dimensional gel electrophoresis. Autoradiograms of PDGF-stimulated and control samples were compared. We found that in neurons and type 1 astrocytes in vitro, PDGF-BB greatly enhances protein phosphorylation while PDGF-AA has less of an effect on protein phosphorylation. Furthermore, because PDGF signal transduction pathways are likely to affect the cytoskeleton, we studied changes in actin-binding proteins induced by PDGF-BB. We found that PDGF-BB alters the expression, migration pattern and/or avidity of some actin-binding proteins in neurons. In conclusion, protein phosphorylation is up-regulated by PDGF in mouse cortical neurons and type 1 astrocytes in vitro. PDGF's effects on phosphorylation of cytoskeletal proteins might be a important mechanism by which PDGF affects the development and normal functions of central nervous system cells.

Growth inhibition of human lung adenocarcinoma cells by antibodies against epidermal growth factor receptor and by ganglioside GM3: involvement of receptor-directed protein tyrosine phosphatase(s)

Growth of the EGF receptor-expressing non-small-cell lung carcinoma cell line H125 seems to be at least partially driven by autocrine activation of the resident EGF receptors. Thus, the possibility of an EGF receptor-directed antiproliferative treatment was investigated in vitro using a monoclonal antibody (alpha EGFR ior egf/r3) against the human EGF receptor and gangliosides which are known to possess antiproliferative and anti-tyrosine kinase activity. The moderate growth-inhibitory effect of alpha EGFR ior egf/r3 was strongly potentiated by the addition of monosialoganglioside GM3. Likewise, the combination of alpha EGFR ior egf/r3 and GM3 inhibited EGF receptor autophosphorylation activity in H125 cells more strongly than either agent alone. A synergistic inhibition of EGF receptor autophosphorylation by alpha EGFR ior egf/r3 and GM3 was also observed in the human epidermoid carcinoma cell line A431. In both cell lines, the inhibition of EGF receptor autophosphorylation by GM3 was prevented by pretreatment of the cells with pervanadate, a potent inhibitor of protein tyrosine phosphatases (PTPases). Also, GM3 accelerated EGF receptor dephosphorylation in isolated A431 cell membranes. These findings indicate that GM3 has the capacity to activate EGF receptor-directed PTPase activity and suggest a novel possible mechanism for the regulation of cellular PTPases.

Activation of platelet-derived growth factor (PDGF) receptor dephosphorylation in intact Swiss 3T3 cells by elevators of intracellular Ca2+ and cAMP

We investigated the modulation of platelet-derived growth factor (PDGF) receptor dephosphorylation in Swiss 3T3 cells using a novel assay permitting monitoring of receptor dephosphorylation in intact cells. PDGF treatment of the cells reduced the receptor dephosphorylation rate to 41%, the elevators of intracellular Ca2+, A23187 and thapsigargin increasing it to 227 and 138%, respectively. The cAMP elevators forskolin and isobutylmethylxanthine also accelerated PDGF receptor dephosphorylation. The involvement of Ca(2+)- and cAMP-dependent protein kinases in the regulation of PDGF receptor dephosphorylation is suggested.

Association of SH2 domain protein tyrosine phosphatases with the epidermal growth factor receptor in human tumor cells. Phosphatidic acid activates receptor dephosphorylation by PTP1C

The SH2 domain protein tyrosine phosphatases (PTPases) PTP1C and PTP1D were found associated with epidermal growth factor (EGF) receptor which was purified from A431 cell membranes by several steps of chromatography. Both PTPases also associated with the EGF receptor upon exposure of immunoprecipitated receptor to lysates of MCF7 mammary carcinoma cells. The associated PTPases had little activity toward the bound receptor when it was autophosphorylated in vitro. Receptor dephosphorylation could, however, be initiated by treatment of the receptor-PTPase complex with phosphatidic acid (PA). When autophosphorylated EGF receptor was exposed to lysates of PTP1C or PTP1D overexpressing 293 cells, the association of PTP1C but not of PTP1D was enhanced in the presence of PA. In intact A431 cells, an association of PTP1C and PTP1D with the EGF receptor was detectable by coimmunoprecipitation experiments. PA treatment reduced the phosphorylation state of ligand activated EGF receptors in A431 cells and in 293 cells overexpressing EGF receptors together with PTP1C but not in 293 cells overexpressing EGF receptors alone or together with PTP1D. We conclude that PTP1C but not PTP1D participates in dephosphorylation of activated EGF receptors. A possible role of PA for physiological modulation of EGF receptor signaling is discussed.

Physicochemical characterization of the cytoplasmic domain of the epidermal growth factor receptor and evidence for conformational changes associated with its activation by ammonium sulphate

The physiochemical properties of the purified cytoplasmic domain of the epidermal growth factor (EGF) receptor, its self-phosphorylation and peptide phosphorylation activities, and its activation by ammonium sulphate have been studied. Highly efficient purification procedures for the isolation of the recombinant cytoplasmic domain (Met644-Ala1186) of the EGF receptor, expressed in the baculovirus/insect cell system, are described. Physicochemical characterization of the protein included investigation of its isoelectric and hydrodynamic properties, stability, oligomeric status, and secondary structure using far-u.v. circular dichroism. The recombinant protein was not recognized by anti-phosphotyrosine antibodies, unless first self-phosphorylated in vitro. Tryptic phosphopeptide maps of self-phosphorylated recombinant cytoplasmic domain and the EGF-stimulated A431-membrane receptor were very similar, suggesting that the recombinant had similar self-phosphorylation capacity and specificity. The preparations were characterized by high specific activity towards peptide tyrosine phosphorylation. Although the cytoplasmic domain was isolated as a homogeneously monomeric protein, storage at 4 degrees C led to slow, spontaneous aggregation with reduction in specific activity. Both high activity and monomeric state were maintained by storage below 0 degree C. The dependence of the initial rate of self-phosphorylation on protein concentration was consistent with cross-phosphorylation but not with the known oligomerization-induced activation of holoreceptor. The peptide phosphorylation activity was stimulated by Mn2+, Mg2+ and (NH4)2SO4 at high concentrations. The substrate specificity of (NH4)2SO4 activation was studied using synthetic peptides. Self-phosphorylation was inhibited by (NH4)2SO4 in the range 0-0.25 M but activated at 1.0-1.5 M, possibly as a result of ionic and hydrophobic protein interactions respectively. Phosphopeptide maps of cytoplasmic domain phosphorylated in the presence of high (NH4)2SO4 showed that the protein was more extensively phosphorylated than in the absence of salt, or than the native receptor. Far-u.v. circular-dichroism spectra of the cytoplasmic domain changed dramatically at 1 M (NH4)2SO4, raising the possibility that (NH4)2SO4 activates the kinase catalytic domain by inducing conformational changes.