Suramin Induces Phosphorylation of the High-Affinity Nerve Growth Factor Receptor in PC12 Cells and Dorsal Root Ganglion Neurons

Abstract: Suramin is a polysulfonated naphthylurea with demonstrated antineoplastic activity. Toxicity includes adrenal insufficiency and peripheral neuropathy. Although the mechanism of antitumor activity is unknown, inhibition of binding of growth factors to their receptors has been suggested. Growth factors inhibited by suramin include platelet-derived growth factor, fibroblast growth factor, transforming growth factor, epidermal growth factor, insulin-like growth factor, and nerve growth factor (NGF). In these studies, suramin was shown to be cytotoxic to PC12 cells in a dose-dependent manner. At lower doses and in surviving cells, we observed the induction of neurite outgrowth. To determine the mechanism of suramin-induced neurite outgrowth, PC12 cells were exposed to suramin and/or NGF for various time periods and treated cells were analyzed, by western blot analysis, for expression of tyrosine phosphoproteins. There was a similarity in the pattern of tyrosine-phosphorylated proteins in PC12 cells stimulated with suramin or NGF. Of particular interest was the rapid phosphorylation (by 1 min) of the high-affinity NGF (TrkA) receptor. Activation of other members of the signal-transduction cascade (Shc, p21 ^ras , Raf-1, ERK-1) revealed similar phosphorylation levels induced by suramin and NGF. Parallel studies were performed in rat dorsal root ganglion cultures; suramin potentiated neurite outgrowth and activated the NGF receptor on these cells. This finding of specific patterns of tyrosine phosphorylation of cellular proteins in response to suramin treatment demonstrated that suramin is a partial agonist for the NGF receptor in both PC12 cells and dorsal root ganglion neurons.     

Disulfide Bond Formation Between Nerve Growth Factor and the Nerve Growth Factor Receptor from Embryonic Sensory Neurons

Recent studies with sympathetic neurons using radiolabeled nerve growth factor have indicated that a high-molecular-weight covalent complex is formed. This complex is between the nerve growth factor and the high-affinity (type I) receptor and occurs through the formation of a disulfide bond. Studies presented in the present article demonstrate a similar complex is formed on chicken embryonic sensory neurons. The formation of this complex is inhibited by the addition of unlabeled nerve growth factor, metabolic energy inhibitors (dinitrophenol and NaF), and of sulfhydryl reagents. On the other hand, formation of this complex is not inhibited by temperature, or by the addition of insulin or epidermal growth factor. The receptor involved in the covalent complex formation is the high-affinity (type I) receptor. The molecular weight of this complex is approximately 232,000 daltons. Evidence indicates that this covalent complex may be required for the biological activity of the nerve growth factor.     

Relationship of Intracellular Calcium to Dependence on Nerve Growth Factor in Dorsal Root Ganglion Neurons in Cell Culture

During development, neural crest-derived sensory neurons require nerve growth factor (NGF) for survival, but lose this dependency postnatally. Similarly, dissociated embryonic sensory neurons lose their NGF dependence during the first 3 weeks in cell culture. It has been hypothesized that, in sympathetic neurons, intracellular levels of calcium are related to trophic factor dependence. In vitro during the period in which embryonic-day-15 sensory neurons become independent of NGF, intracellular calcium concentrations progressively increased in parallel to the decline in NGF dependence. This elevation of intracellular calcium was directly related to the absolute age of the neurons, not to the length of time in culture. Without NGF, immature sensory, i.e., dependent, neurons survived in the presence of high extracellular potassium, a condition that produces elevated intracellular calcium. In another paradigm, measurements of intracellular calcium were determined in NGF-dependent neurons "committed to die" after NGF withdrawal. These measurements were determined prior to the time that extensive morphological changes, consistent with cell death, were noted by phase-contrast microscopy. No elevation in intracellular calcium was found in these dying neurons, but rather, a small decrease was observed prior to the disintegration of the neurons. These findings support the hypothesis that trophic factor dependence of neurons may be inversely related to levels of intracellular calcium.     

Properties of the Sodium Extrusion Mechanism Controlled by Nerve Growth Factor in Chick Embryo Dorsal Root Ganglionic Cells

Abstract: Cell dissociates from embryonic chick dorsal root ganglia, incubated for 6 h with ²²Na⁺, accumulated four to six times more radioactivity in the absence than in the presence of Nerve Growth Factor (NGF). The accumulation of radioactivity paralleled the external Na⁺ concentration, indicating that the cells may have been reaching equilibrium with the medium. Delayed presentation of NGF to ²²Na⁺-loaded cells caused a rapid loss of radioactivity, even with extracellular ²²Na⁺ still present, demonstrating that NGF caused an overall efflux of Na⁺ rather than an accelerated equilibration. The Na⁺ exclusion from ²²Na⁺-loaded cells was dependent upon NGF concentration. Use of nutrient-rich medium, serum, and certain hormones and other proteins did not prevent the Na+ accumulation in the absence of NGF or its reversion by delayed NGF administration. Incubation of the ganglionic cells with ouabain or dinitrophenol during the ²²Na⁺ loading period (no NGF) increased the rate, but not the magnitude, of loading. The same incubation carried out in a Na+-free medium and followed by ²²Na⁺ presentation resulted in fast radioactive loading that was identical to that occurring in drug-free, NGF-deprived cells and was not prevented by presentation of NGF together with the ²²Na⁺. These data are consistent with a model in which NGF acts through a Na⁺ pump rather than by restricting Na+ influxes.     

Ganglioside GM1 Antibodies and B-Cholera Toxin Bind Specifically to Embryonic Chick Dorsal Root Ganglion Neurons but Do Not Modulate Neurite Regeneration

Polyclonal antibodies to ganglioside GM1 have been prepared and characterised by direct and competitive enzyme-linked immunoassay. An immunoglobulin fraction was prepared from a rabbit antisera showing high specificity and antibody titre for GM1 relative to the other major brain gangliosides. The anti-GM1 immunoglobulin fraction and B-cholera toxin specifically labelled neurons in primary cultures of embryonic chick dorsal root ganglia and there was a good correlation between the relative increase in binding of anti-GM1 immunoglobulin and B-cholera toxin following neuraminidase treatment of a variety of cell types. At antibody concentrations that show saturable binding to endogenous ganglioside in the neuronal membrane, the anti-GM1 immunoglobulin fraction did not interfere with the nerve growth factor (NGF)-mediated fibre outgrowth and neuronal survival as indexed by measurement of neurofilament protein levels. Similarly, at levels in excess of those shown to stimulate thymocyte proliferation, B-cholera toxin was also without effect. These data are not consistent with GM1 in the neuronal membrane functioning as a receptor molecule for NGF and/or other differentiation factors present in the tissue culture media.     

Activation of Mitogen-Activated Protein Kinase by Epidermal Growth Factor in Hippocampal Neurons and Neuronal Cell Lines

Abstract: Epidermal growth factor (EGF) functions in a bimodal capacity in the nervous system, acting as a mitogen in neuronal stem cells and a neurotrophic factor in differentiated adult neurons. Thus, it is likely that EGF signal transduction, as well as receptor expression, differs among various cell types and possibly in the same cell type at different stages of development. We used hippocampal neuronal cell lines capable of terminal differentiation to investigate changes in EGF receptor expression, DNA synthesis, and stimulation of mitogen-activated protein (MAP) kinase by EGF before and after differentiation. H19-7, the line that was most representative of hippocampal neurons, was mitogenically responsive to EGF only before differentiation and increased in EGF binding after differentiation. MAP kinase was stimulated by EGF in both undifferentiated and differentiated cells, as well as in primary hippocampal cultures treated with either EGF or glutamate. These results indicate that the activation of MAP kinase by EGF is an early signaling event in both mitotic and postmitotic neuronal cells. Furthermore, these studies demonstrate the usefulness of hippocampal cell lines as a homogeneous neuronal system for studies of EGF signaling or other receptor signaling mechanisms in the brain.     

Different Epidermal Growth Factor Receptor (EGFR) Agonists Produce Unique Signatures for the Recruitment of Downstream Signaling Proteins

The EGF receptor can bind seven different agonist ligands. Although each agonist appears to stimulate the same suite of downstream signaling proteins, different agonists are capable of inducing distinct responses in the same cell. To determine the basis for these differences, we used luciferase fragment complementation imaging to monitor the recruitment of Cbl, CrkL, Gab1, Grb2, PI3K, p52 Shc, p66 Shc, and Shp2 to the EGF receptor when stimulated by the seven EGF receptor ligands. Recruitment of all eight proteins was rapid, dose-dependent, and inhibited by erlotinib and lapatinib, although to differing extents. Comparison of the time course of recruitment of the eight proteins in response to a fixed concentration of each growth factor revealed differences among the growth factors that could contribute to their differing biological effects. Principal component analysis of the resulting data set confirmed that the recruitment of these proteins differed between agonists and also between different doses of the same agonist. Ensemble clustering of the overall response to the different growth factors suggests that these EGF receptor ligands fall into two major groups as follows: (i) EGF, amphiregulin, and EPR; and (ii) betacellulin, TGFα, and epigen. Heparin-binding EGF is distantly related to both clusters. Our data identify differences in network utilization by different EGF receptor agonists and highlight the need to characterize network interactions under conditions other than high dose EGF.     

Identification of an Activator of the Microtubule-Associated Protein 2 Kinases ERK1 and ERK2 in PC12 Cells Stimulated with Nerve Growth Factor or Bradykinin

Treatment of PC12 pheochromocytoma cells with nerve growth factor (NGF) or bradykinin leads to the activation of extracellular signal-regulated kinases ERK1 and ERK2, two isozymes of microtubule-associated protein 2 (MAP) kinase that are present in numerous cell lines and regulated by diverse extracellular signals. The activation of MAP kinase is associated with its phosphorylation on tyrosine and threonine residues, both of which are required for activity. In the present studies, we have identified a factor in extracts of PC12 cells treated with NGF or bradykinin, named MAP kinase activator, that, when reconstituted with inactive MAP kinase from untreated cells, dramatically increased MAP kinase activity. Activation of MAP kinase in vitro by this factor required MgATP and was associated with the phosphorylation of a 42- (ERK1) and 44-kDa (ERK2) polypeptide. Incorporation of 32P into ERK1 and ERK2 occurred primarily on tyrosine and threonine residues and was associated with a single tryptic peptide, which is identical to one whose phosphorylation is increased by treatment of intact PC12 cells with NGF. Thus, the MAP kinase activator identified in PC12 cells is likely to be a physiologically important intermediate in the signaling pathways activated by NGF and bradykinin. Moreover, stimulation of the activator by NGF and bradykinin suggests that tyrosine kinase receptors and guanine nucleotide-binding protein-coupled receptors are both capable of regulating these pathways.     

The Differential Role of Protein Kinase C Isozyme in the Rapid Induction of Neurofilament Phosphorylation by Nerve Growth Factor and Phorbol Esters in PC12 Cells

We examined the short-term regulation of the phosphorylation of the mid-sized neurofilament subunit (NF-M) by kinases which were activated in rat pheochromocytoma (PC12) cells by nerve growth factor (NGF) and/or 12-O-tetradecanoylphorbol 13-acetate (TPA). We found that NGF and TPA, alone or in combination, increased (a) the incorporation of [32P]Pi into NF-M and (b) the rate of conversion of NF-M from a poorly phosphorylated to a more highly phosphorylated form. This was not due to increased synthesis of NF-M, because NGF alone did not increase NF-M synthesis and TPA alone or TPA and NGF together inhibited the synthesis of NF-M. Further, an increase in calcium/phospholipid-dependent kinase (PKC) activity resulting from the treatment of PC12 cells with NGF and TPA was observed concomitant with the increased phosphorylation of NF-M. This PKC activity was determined to be derived from the PKC alpha and PKC beta isozymes. Finally, when PC12 cells were rendered PKC-deficient by treatment with 1 muM TPA for 24 h, NGF maintained the ability to induce an increase in NF-M phosphorylation, though not to the level attained in cells which were not PKC-deficient. These data suggest that NGF with or without TPA stimulates NF-M phosphorylation as a result of a complex series of events which include PKC-independent and PKC-dependent pathways.     

Phosphorylation of Glutathione S-Transferase P1 (GSTP1) by Epidermal Growth Factor Receptor (EGFR) Promotes Formation of the GSTP1-c-Jun N-terminal kinase (JNK) Complex and Suppresses JNK Downstream Signaling and Apoptosis in Brain Tumor Cells

Under normal physiologic conditions, the glutathione S-transferase P1 (GSTP1) protein exists intracellularly as a dimer in reversible equilibrium with its monomeric subunits. In the latter form, GSTP1 binds to the mitogen-activated protein kinase, JNK, and inhibits JNK downstream signaling. In tumor cells, which frequently are characterized by constitutively high GSTP1 expression, GSTP1 undergoes phosphorylation by epidermal growth factor receptor (EGFR) at tyrosine residues 3, 7, and 198. Here we report on the effect of this EGFR-dependent GSTP1 tyrosine phosphorylation on the interaction of GSTP1 with JNK, on the regulation of JNK downstream signaling by GSTP1, and on tumor cell survival. Using in vitro and in vivo growing human brain tumors, we show that tyrosine phosphorylation shifts the GSTP1 dimer-monomer equilibrium to the monomeric state and facilitates the formation of the GSTP1-JNK complex, in which JNK is functionally inhibited. Targeted mutagenesis and functional analysis demonstrated that the increased GSTP1 binding to JNK results from phosphorylation of the GSTP1 C-terminal Tyr-198 by EGFR and is associated with a >2.5-fold decrease in JNK downstream signaling and a significant suppression of both spontaneous and drug-induced apoptosis in the tumor cells. The findings define a novel mechanism of regulatory control of JNK signaling that is mediated by the EGFR/GSTP1 cross-talk and provides a survival advantage for tumors with activated EGFR and high GSTP1 expression. The results lay the foundation for a novel strategy of dual EGFR/GSTP1 for treating EGFR+ve, GSTP1 expressing GBMs.