Effect of nerve growth factor and fibroblast growth factor on PC12 cells: inhibition by orthovanadate

Quantitative assessment of neurite outgrowth in human embryonic stem cell-derived hN2 cells using automated high-content image analysis

Throughout development neurons undergo a number of morphological changes including neurite outgrowth from the cell body. Exposure to neurotoxic chemicals that interfere with this process may result in permanent deficits in nervous system function. Traditionally, rodent primary neural cultures and immortalized human and non-human clonal cell lines have been used to investigate the molecular mechanisms controlling neurite outgrowth and examine chemical effects on this process. The present study characterizes the molecular phenotype of hN2 human embryonic stem cell (hESC)-derived neural cells and uses automated high-content image analysis to measure neurite outgrowth in vitro. At 24h post-plating hN2 cells express a number of protein markers indicative of a neuronal phenotype, including: nestin, beta(III)-tubulin, microtubule-associated protein 2 (MAP2) and phosphorylated neurofilaments. Neurite outgrowth in hN2 cells proceeded rapidly, with a majority of cells extending one to three neurites by 48h in culture. In addition, concentration-dependent decreases in neurite outgrowth and ATP-content were observed following treatment of hN2 cells with either bisindolylmaleimide I, U0126, lithium chloride, sodium orthovanadate and brefeldin A, all of which have previously been shown to inhibit neurite outgrowth in primary rodent neural cultures. Overall, the molecular phenotype, rate of neurite outgrowth and sensitivity of hN2 cells to neurite outgrowth inhibitors were comparable to other in vitro models previously characterized in the literature. hN2 cells provide a model in which to investigate chemical effects on neurite outgrowth in a non-transformed human-derived cells and provide an alternative to the use of primary rodent neural cultures or immortalized clonal cell lines.

Altered levels of PP2A regulatory B/PR55 isoforms indicate role in neuronal differentiation

The ubiquitously expressed serine/threonine-specific protein phosphatase 2A (PP2A) is prominent in brain where it serves a wide range of functions under both physiological and pathological conditions. PP2A holoenzymes are composed of a catalytic subunit and a tightly complexed scaffolding subunit. This core enzyme associates with regulatory subunits of the B/PR55, B'/PR56/PR61, B''/PR72 and B'''/PR93/PR110 families. We previously determined distribution and expression levels of the four members of the B/PR55 family in brain, as dysregulation of this subunit family has been specifically implicated in neurodegenerative disorders including Alzheimer's disease. In the present study, we used cell lines widely used in neuroscience research to determine levels of the four PR55 isoforms by qRT-PCR under different experimental conditions. We show that PR55alpha mRNA levels are highest in both HEK293 cells and SH-SY5Y neuroblastoma cells whereas PR55beta levels are lowest. Stepwise neuronal differentiation of SH-SY5Y cells causes the selective upregulation of PR55beta, and to some extent PR55gamma and PR55delta, but not PR55alpha mRNAs. In agreement with the qRT-PCR analysis, neuronal differentiation does not alter PR55alpha protein levels, whereas interestingly, PR55beta and PR55gamma protein levels are reduced when compared to undifferentiated cells. Our data point at specific roles for distinct regulatory B/PR55 subunits of PP2A in neuron-like cells with PR55alpha being the major isoform.

The tyrosine phosphatase inhibitor orthovanadate mimics NGF-induced neuroprotective signaling in rat hippocampal neurons

Activation of the high affinity neurotrophin receptor tropomyosin-related kinase A (TrkA) by nerve growth factor (NGF) leads to phosphorylation of intracellular tyrosine residues of the receptor with subsequent activation of signaling pathways involved in neuronal survival such as the phosphoinositide-3-kinase (PI3-K)/protein kinase B (PKB/Akt) pathway and the mitogen-activated protein kinase (MAPK) cascade. In the present study, we tested whether inhibition of protein-tyrosine phosphatases (PTP) by orthovanadate could enhance tyrosine phosphorylation of TrkA thereby stimulating NGF-like survival signaling in embryonic hippocampal neurons. We found that the PTP inhibitor orthovanadate (1 microM) enhanced TrkA phosphorylation and protected neurons against staurosporine (STS)-induced apoptosis in a time-and concentration-dependent manner. Inhibition of PTP enhanced TrkA phosphorylation also in the presence of NGF antibodies indicating that NGF binding to TrkA was not required for the effects of orthovanadate. Moreover, orthovanadate enhanced phosphorylation of Akt and the MAPK Erk1/2 suggesting that the signaling pathways involved in the protective effect were similar to those activated by NGF. Accordingly, inhibition of PI3-K by wortmannin and MAPK-kinase (MEK) inhibition by UO126 abolished the neuroprotective effects. In conclusion, the results indicate that orthovanadate mimics the effect of NGF on survival signaling pathways in hippocampal neurons. Thus, PTP inhibition appears to be an appropriate strategy to trigger neuroprotective signaling pathways downstream of neurotrophin receptors.

Glucocorticoid-induced osteoporosis in the rat is prevented by the tyrosine phosphatase inhibitor, sodium orthovanadate

Glucocorticoid-induced osteoporosis is characterized by decreased osteoblast numbers and a marked impairment of new bone formation. We found that, in vitro, dexamethasone inhibits both preosteoblast proliferation and mitogenic kinase activity in response to mitogens, and that inhibition of protein tyrosine phosphatases (PTPs) using sodium orthovanadate prevents this. Therefore, dexamethasone may act by either upregulating antiproliferative PTPs or downregulating promitogenic tyrosine-phosphorylated substrates. In this study, osteoporosis was induced in 3.5-month-old rats by subcutaneous injection with methylprednisolone 3.5 mg/kg per day for 9 weeks. Rats were treated with steroid alone or in combination with 0.5 mg/mL sodium orthovanadate, administered continuously in drinking water. Steroid-treated bones were significantly (p < 0.005) osteopenic (according to dual-energy X-ray absorptiometry) and physically weaker (p < 0.05) than controls. Quantitative bone histology confirmed a significant decrease in osteoid surfaces (p < 0.001), osteoblast numbers (p < 0.05), and rate of bone formation (p < 0.001). Concomitant treatment with vanadate largely prevented the densitometric, histologic, and physical abnormalities induced by prednisolone. This study supports our finding that PTPs are central to the negative regulation of osteoblast proliferation by glucocorticoids and, furthermore, suggests that PTP inhibitors such as sodium orthovanadate should be considered as novel anabolic agents for the treatment of steroid-induced osteoporosis.

Neuroendocrine differentiation of the LNCaP prostate cancer cell line maintains the expression and function of VIP and PACAP receptors

The molecular mechanisms involved in differentiation of prostate cancer cells to a neuroendocrine (NE) cell phenotype are not well understood. Here we used the androgen-dependent human prostate cancer cell line LNCaP to perform a systematic and broad analysis of the expression, pharmacology, and functionality of vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase-activating peptide (PACAP) receptors. Reverse transcription polymerase chain reaction experiments, together with pharmacological approaches with a set of specific agonists and antagonists, demonstrated the presence of the three VIP/PACAP receptor subtypes (PAC1, VPAC1, and VPAC2 with a major role for VPAC1, acting through adenylate cyclase (AC) stimulation. An essentially similar pattern was observed by NE differentiated cells (4 days after serum deprivation) in spite of the important morphological changes observed. However, the expression of the prostate-specific antigen (PSA) decreased in NE cells (and increased again by dihydrotestosterone, DHT, treatment). The present demonstration of the induction of NE transdifferentiation in LNCaP cells by increasing concentrations of VIP adds value to previous observations on the role of cAMP in this process, an interesting topic in the comprehension of the molecular changes that are involved in the progression of prostate cancer to androgen independence.

Enhancement of arachidonic acid release and prostaglandin F(2alpha) formation by Na3VO4 in PC12 cells and GH3 cells

Both activation of phospholipase A2 causing arachidonic acid release and tyrosine phosphorylation have been proposed to be involved in neuronal functions. Previously, we reported that orthovanadate (Na3VO4), an inhibitor of tyrosine phosphatases, stimulated tyrosine phosphorylation in proteins and enhanced Ca2+-induced noradrenaline release in rat pheochromocytoma PC12 cells. However, the role of tyrosine phosphorylation on phospholipase A2 activity and/or arachidonic acid release in neuronal cells has not been well established. The effects of Na3VO4 on arachidonic acid release and prostaglandin F(2alpha) formation were investigated in two types of neuronal cell lines. In PC12 cells, addition of Na3VO4 stimulated [3H]arachidonic acid release and prostaglandin F(2alpha) formation in a concentration-dependent manner. Co-addition of 5 mM Na3VO4 enhanced ionomycin-stimulated [3H]arachidonic acid release. Na3VO4 also enhanced ionomycin-stimulated [3H]arachidonic acid release from GH3 cells, a clonal strain from rat anterior pituitary. These findings suggest that the tyrosine phosphorylation pathway regulates arachidonic acid release by phospholipase A2 and prostaglandin F(2alpha) formation in neuronal cells.

Enhancement of Ca2+-induced noradrenaline release by vanadate in PC12 cells: possible involvement of tyrosine phosphorylation

Tyrosine phosphorylation has been shown to participate in the signal cascade after receptor stimulation with neurotransmitters and neurotrophins. However, the role of tyrosine phosphorylation in the process(es) of neurotransmitter release has not been well established. The effects of orthovanadate (Na3VO4), an inhibitor of protein-tyrosine phosphatases, on cytosolic free Ca2+ concentrations ([Ca2+]i), phosphotyrosine accumulation and noradrenaline (NA) release in neurosecretory PC12 cells were investigated. Addition of Na3VO4 enhanced ionomycin-stimulated [3H]NA release in a concentration-dependent manner, although Na3VO4 alone had no effect. Na3VO4 also enhanced [3H]NA release induced by P2 receptor stimulation with adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS) or by depolarization with 50 mM KCl, which stimulated a [Ca2+]i increase. A cell permeable inhibitor of protein-tyrosine phosphatases, L-p-bromotetramisole oxalate, at 0.3 mM enhanced ionomycin-stimulated [3H]NA release, although pervanadate had no effect. Addition of 5 mM Na3VO4 stimulated phosphotyrosine accumulation in several protein bands such as p130cas, but did not increase [Ca2+]i in PC12 cells. These findings suggest that the tyrosine phosphorylation pathway regulates Ca2+-stimulated NA release without changes of [Ca2+]i in PC12 cells.

Regulation of signaling by protein-tyrosine phosphatases: potential roles in the nervous system

During neuronal development, cells respond to a variety of environmental cues through cell surface receptors that are coupled to a signaling transduction machinery based on protein tyrosine phosphorylation and dephosphorylation. Receptor and non-receptor tyrosine kinases have received a great deal of attention; however, in the last few years, receptor (plasma membrane associated) and non-receptor protein-tyrosine phosphatases (PTPs) have also been shown to play important roles in development of the nervous system. In many cases PTPs have provocative distribution patterns or have been shown to be associated with specific cell adhesion and growth factor receptors. Additionally, altering PTP expression levels or activity impairs neuronal behavior. In this review we outline what is currently known about the role of PTPs in development, differentiation and neuronal physiology.

Mediating phosphorylation events in the vanadium-induced respiratory burst of alveolar macrophages

Occupational exposure by inhalation to vanadium-containing particles such as residual oil fly ash results in respiratory tract inflammation. This inflammation, characterized by abundant neutrophilia, appears to be initiated by alveolar macrophages (AMs) encountering particles and the subsequent release of proinflammatory cytokines. Intracellular signaling events in these cells in response to particles or their components are largely unknown. We investigated two immediate responses of AMs to vanadium exposure in vitro, the production of reactive oxygen intermediates (ROI) or respiratory burst (RB), and the tyrosine phosphorylation of cellular proteins. Macrophages exposed in vitro to 100 microM vanadyl chloride/1 microCi 48V incorporated 8.3% of the metal after 30 min. Exposure of AMs to increasing concentrations of sodium metavanadate resulted in a dose-dependent increase in production of ROI as measured by dichlorofluorescin oxidation. The lowest dose yielding a significant response was 50 microM, whereas 1000 microM increased RB activity by 173%. NADPH oxidase inhibitors deoxy-D-glucose (100 mM) and diphenylene iodonium (25 microM) reduced the metavanadate-induced RB by 62 and 71%, respectively, implicating NADPH oxidase as the primary cellular source of ROI. Enhanced cerium chloride oxidation in response to metavanadate localized to the plasma membrane consistent with increased NADPH oxidase activity. Pretreatment of AMs with the epidermal growth factor receptor inhibitor, tryphostin B50 (10 microM), reduced the metavanadate-induced RB, but did not influence overall tyrosine phosphorylation. Metavanadate and H2O2 exposure greatly increased overall tyrosine phosphorylation, yielding a similar but distinguishable pattern of phosphorylation in these cells. These observations demonstrate that in vitro metavanadate exposure regulates two distinct, yet related intracellular signaling pathways important in initiating inflammatory responses in these cells: (1) activation of the NADPH oxidase complex with subsequent increased ROI synthesis, and (2) enhanced tyrosine phosphorylation of cellular proteins.

Protein tyrosine phosphatases and neural development

During neural development, cells interact dynamically with each other and with the extracellular matrix, using cell signaling to control differentiation, axonogenesis, and survival. Enzymes that regulate protein tyrosine phosphorylation often lie at the core of such cell signaling. Protein tyrosine phosphatases (PTPases) are recognized as being of central importance here, and a growing family of PTPases are now known to be expressed in embryonic neurons and glia. Both receptor-like and cytoplasmic enzymes have been identified. The receptor family includes immunoglobulin superfamily members that influence cell-cell adhesion, proteoglycans that control neurite growth, and enzymes in Drosophila that regulate axon guidance and target cell recognition. Cytoplasmic PTPases are implicated in nerve cell commitment and potentially in the regulation of cell survival. This review outlines what we currently know about PTPases in the nervous system and presents concepts concerning their possible modes of action.

Inhibition of mitogen-activated protein kinase activity and proliferation of an early osteoblast cell line (MBA 15.4) by dexamethasone: role of protein phosphatases

Chronic glucocorticoid therapy causes rapid bone loss and clinical osteoporosis. We found that although the glucocorticoid, dexamethasone, stimulated osteoblast maturation, it also inhibited proliferation of a preosteoblastic cell line, MBA-15.4. The dexamethasone-induced decline in preosteoblast proliferation correlated with a 30-40% reduction in protein kinase C/TPA-stimulated mitogen-activated protein kinase (MAPK) activity. These steroid effects only became evident after 6-24 h treatment, implying that dexamethasone acts on de novo synthesis of proteins. Because MAPK is inactivated by dephosphorylation of tyrosine and threonine residues, cells were treated concomitantly for 24 h with dexamethasone and inhibitors of tyrosine (sodium orthovanadate) and/or serine/threonine phosphatases (sodium fluoride). MAPK activity and cell proliferation were restored when MBA-15.4 cells were treated with vanadate, suggesting that dexamethasone up-regulates tyrosine phosphatase activity. Inactivation of serine/threonine phosphatases with sodium fluoride had no effect. Inhibition of the PKA pathway (which is growth inhibitory in mature osteoblasts) with H-89 did not reverse the effects of dexamethasone. Pretreatment with dexamethasone inhibited both peak- and extended activation plateau-phases of MAPK activity. Both phases were fully restored by pretreatment with vanadate, implicating more than one tyrosine phosphatase. Cycloheximide, alone or in combination with dexamethasone, prevented drop-off from plateau to basal levels, suggesting that an inducible dual-specificity phosphatase regulates the plateau-phase. We conclude that dexamethasone may inhibit preosteoblast growth via a novel tyrosine phosphatase pathway.

Selective blockade of axonogenesis in cultured hippocampal neurons by the tyrosine phosphatase inhibitor orthovanadate

Protein tyrosine phosphorylation has been implicated in several aspects of neurite outgrowth regulation. To address specific roles in early neuronal morphogenesis, hippocampal neurons in culture were treated with the tyrosine phosphatase inhibitor orthovanadate. This treatment completely suppressed axon formation, yet enhanced formation of minor neurites. The inhibition of axonogenesis was dose dependent and occurred in parallel with a marked increase in cellular phosphotyrosine immunoreactivity, which was especially concentrated within neuritic growth cones and showed partial colocalization with f-actin. Both the blockade of axonogenesis and the elevation of phosphotyrosine were completely reversible. An additional and unexpected effect of orthovanadate was the appearance of many binucleate neurons. Immunoblotting experiments using a phosphotyrosine-specific antibody revealed an orthovanadate-induced reversible hyperphosphorylation of several protein bands, especially of two at 115 and 125 kD. These data suggest a potentially important role for tyrosine phosphatases and their phosphoprotein substrates in axonogenesis.

Enhancement or induction of neurite formation by a protein tyrosine phosphatase inhibitor, 3,4-dephostatin, in growth factor-treated PC12h cells

We studied the effect of the 3,4-dihydroxy analogue of dephostatin (3,4-dephostatin), an inhibitor of protein-tyrosine phosphatase (PTPase), on the differentiation of rat pheochromocytoma PC12 cells. 3,4-Dephostatin accelerated NGF-induced neurite formation in PC12h cells, a subline of PC12 cells, whereas the inhibitor alone did not induce neurite formation. It sustained the NGF-induced tyrosine phosphorylation of several proteins, most prominently that of mitogen-activated protein (MAP) kinase. EGF alone did not induce differentiation in PC12h cells, but it induced neurite formation in the presence of 3,4-dephostatin. The inhibitor also prolonged EGF-induced tyrosine phosphorylation and activation of MAP kinase. An inactive analogue of dephostatin, 2'-O-methyl-dephostatin showed no effect on either neurite formation or MAP kinase tyrosine phosphorylation in NGF or EGF-treated PC12h cells. Thus, we demonstrated that the PTPase inhibitor could enhance growth factor-induced differentiation in PC12 cells possibly by sustaining the MAP kinase activity.

Orthovanadate induces cell death in rat dentate gyrus primary culture

The aim of this study was to define the effects of a potent inhibitor of tyrosine phosphatases, sodium orthovanadate (0.1-100 microM for up to 48 h), on dentate gyrus cells (DGC) in culture. Treatment with 100 microM orthovanadate evoked a delayed form of cell death. To examine the possible involvement of apoptosis in orthovanadate-induced cell death, biochemical and morphological alterations were compared with those of necrotic death induced by sodium azide. Phase-contrast microscopy and nuclear condensation analysis showed that orthovanadate and azide each evoked cell death by distinct pathways. TUNEL assay was positive in both cases. Application of a protein synthesis inhibitor, cycloheximide, did not prevent cytotoxicity caused by either orthovanadate or azide and potentiated the effects of vanadate. We conclude that orthovanadate-induced death of DGC bears features of apoptosis.

Chimeras of the native form or achondroplasia mutant (G375C) of human fibroblast growth factor receptor 3 induce ligand-dependent differentiation of PC12 cells

Mutations in the gene for human fibroblast growth factor receptor 3 (hFGFR3) cause a variety of skeletal dysplasias, including the most common genetic form of dwarfism, achondroplasia (ACH). Evidence indicates that these phenotypes are not due to simple haploinsufficiency of FGFR3 but are more likely related to a role in negatively regulating skeletal growth. The effects of one of these mutations on FGFR3 signaling were examined by constructing chimeric receptors composed of the extracellular domain of human platelet-derived growth factor receptor beta (hPDGFR beta) and the transmembrane and intracellular domains of hFGFR3 or of an ACH (G375C) mutant. Following stable transfection in PC12 cells, which lack platelet-derived growth factor (PDGF) receptors, all clonal cell lines, with either type of chimera, showed strong neurite outgrowth in the presence of PDGF but not in its absence. Antiphosphotyrosine immunoblots showed ligand-dependent autophosphorylation, and both receptor types stimulated strong phosphorylation of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase, an event associated with the differentiative response of these cells. In addition, ligand-dependent phosphorylation of phospholipase Cgamma and Shc was also observed. All of these responses were comparable to those observed from ligand activation, such as by nerve growth factor, of the native PC12 cells used to prepare the stable transfectants. The cells with the chimera bearing the ACH mutation were more rapidly responsive to ligand with less sustained MAPK activation, indicative of a preactivated or primed condition and consistent with the view that these mutations weaken ligand control of FGFR3 function. However, the full effect of the mutation likely depends in part on structural features of the extracellular domain. Although FGFR3 has been suggested to act as a negative regulator of long-bone growth in chrondrocytes, it produces differentiative signals similar to those of FGFR1, to which only positive effects have been ascribed, in PC12 cells. Therefore, its regulatory effects on bone growth likely result from cellular contexts and not the induction of a unique FGFR3 signaling pathway.

On the role of the low-affinity neurotrophin receptor p75LNTR in nerve growth factor induction of differentiation and AP 1 binding activity in PC12 cells

Three clones of PC12 cells that differ with respect to their nerve growth factor (NGF) receptors were examined: wild-type PC12 cells that have both trkA and p75LNTR receptors; the MR-1 clone that possesses a normal trkA receptor and a truncated form of p75LNTR without the extracellular NGF-binding part; and a new PC12 variant, called v-clone, that is partly characterized here. The v-clone had no demonstrable binding to trkA, but displayed binding to p75LNTR as assessed by chemical crosslinking. NGF did not induce any change in the tyrosine phosphorylation of phosphatidy-3'-kinase in the v-clone. NGF induced neurite extension in wild-type cells, induced it more rapidly in mR-1, but not at all in v-clone cells. The v-clone lacked the b-form of protein kinase C, but transfection with this enzyme did not restore responsiveness to NGF. Neurite extension in response to staurosporine and basic fibroblast growth factor was equal in wild-type and v-clone cells. All three clones responded to forskolin, with the mR-1 clone the most responsive. NGF stimulated AP 1 binding activity in all clones. The response was transient in the MR-1 clone but prolonged in the wild-type and v-clone cells. In the wild-type and MR-1 clone cells, AP 1 binding activity was reduced by a tyrphostin analog, whereas in the v-clone cells it was inhibited by staurosporine. NGF increased inositol (1,4,5)-trisphosphate (InsP3) formation in all clones. In the wild-type and v-clone cells the InsP3 responses were followed by [Ca2+]i increases. It is concluded that although trkA is required for differentiation in response to NGF in PC12 cells, the concomitant stimulation, by NGF, of p75LNTR may affect phospholipase C and AP 1. This may be important for the reported ability of p75LNTR to modify the phenotypic changes induced in PC12 cells by NGF.

Postnatal regulation of fibroblast growth factor ligand and receptor gene expression in rat thoracic aorta

Fibroblast growth factor (FGF)-1 and FGF-2 are potent angiogenic factors and vascular smooth muscle cell (SMC) mitogens in vivo. They function via binding to a family of structurally related cell surface receptors that possess intrinsic tyrosine kinase activity. Several studies have indicated that increased FGF and/or FGF receptor (FGFR) expression may correlate with adult SMC proliferation in vivo. In this study, we used Northern blot hybridization and reverse transcription-polymerase chain reaction assays to compare the FGF and FGFR mRNA levels in newborn rat aorta, where SMCs have a high replication index, to those in adult rat aorta, where SMCs are relatively quiescent. We found that FGF-2 and FGFR-2 mRNA expression was elevated 8.2- and 5.6-fold, respectively, in adult aorta. Increased FGF-2 protein expression in the adult aorta was confirmed by Western blot analysis. We also examined FGF and FGFR mRNA expression levels in SMC cultures derived from newborn or adult rat aorta. FGF-1 transcripts were more abundant in newborn SMCs whereas FGF-2 and FGFR-1 mRNA expression was higher in adult SMCs. Furthermore, FGF-1 and FGF-2 mRNA expression levels were altered by cell culture density and by serum treatment. We conclude that elevated FGF ligand and receptor expression does not always correlate with a high SMC proliferative index, that FGF-1 or FGF-2 may not be the primary mitogens responsible for newborn SMC growth in vivo, and that FGF-1 and FGF-2 may serve nonmitogenic functions within the mature, adult vessel wall.

The novel protein-tyrosine phosphatase PTP20 is a positive regulator of PC12 cell neuronal differentiation

A novel cytoplasmic protein-tyrosine phosphatase (PTPase) designated PTP20 was isolated from a PC12 cDNA library and shown to positively regulate the differentiation process in PC12 cells. The PTP20 open reading frame of 453 amino acids contains a single tyrosine phosphatase catalytic domain and displays closest homology to members of the PTP-PEST protein-tyrosine phosphatase family. Transient expression of PTP20 in Rat-1 cells resulted in the expression of a 50-kDa protein which exhibited PTPase activity in vitro. Expression of the 2.3-kilobase PTP20 mRNA increased during differentiation of nerve growth factor (NGF)-stimulated PC12 cells. Consistent with this observation, stable overexpression of PTP20 in PC12 cells resulted in accelerated neurite formation following NGF treatment. These findings suggest a positive regulatory role of PTP20 in NGF-dependent neuronal differentiation of PC12 cells.

Angiotensin II induction of neurite outgrowth by AT2 receptors in NG108-15 cells. Effect counteracted by the AT1 receptors

In the present study, 3-day treatment of nondifferentiated NG108-15 cells with 100 nM angiotensin II (Ang II) induces morphological differentiation of neuronal cells characterized by the outgrowth of neurites. These morphological changes are correlated with an increase in the level of polymerized tubulin and in the level of the microtubule-associated protein, MAP2c. Mediation by the AT2 receptor may be inferred since: (a) these cells contain only AT2 receptors; (b) the effects are mimicked by CGP 42112 (an AT2 receptor agonist); (c) they are not suppressed by the addition of DUP 753 (an AT1 receptor antagonist); and (d) are abolished by co-incubation with PD 123319 (an AT2 receptor antagonist). Application of Ang II in dibutyryl cAMP-differentiated cells (which contain both types of receptors) induces neurite retraction, an effect mediated by the AT1 receptor. These results indicate that the AT2 receptor of Ang II induces neuronal differentiation, which is initiated through an increase in the levels of MAP2c associated with tubulin. Moreover, our results demonstrate that the AT1 receptor inhibit the process of differentiation induced by dibutyryl cAMP, whereas the AT2 receptors potentiate this effect, illustrating negative cross-talk interaction between the two types of Ang II receptors.

Induction of neurite outgrowth by interleukin-6 is accompanied by activation of Stat3 signaling pathway in a variant PC12 cell (E2) line

PC12-E2 cells, a stable variant subcloned from native cell populations, produce neurites in a rapid, transcription-independent manner upon exposure to nerve growth factor (NGF) or basic fibroblast growth factor (bFGF). They also give a similar morphological response to interleukin-6 (IL-6), which is, however, transcription-dependent and with a slower onset, a phenomenon basically not observed in native PC12 cells. The response profile of PC12-E2 cells to NGF and bFGF is similar to that observed for native PC12 cells pre-exposed (primed) to NGF, and such cells also respond to IL-6 in a fashion indistinguishable from PC12-E2 cells. Mechanistically, NGF and bFGF induce a sustained phosphorylation and activation of ERK1 and ERK2 in both cells, while IL-6 produces only a transient and weak tyrosine phosphorylation. However, it does stimulate a prolonged and biphasic tyrosine phosphorylation and nuclear translocation of Stat3 (signal transducers and activators of transcription 3; at least 24 h) and, to a lesser extent, Stat1. Gel shift and supershift analyses confirm that IL-6 predominantly activates Stat3 (and some Stat1) and stimulates sis-inducible element binding activity. Other members of the same cytokine subfamily, including ciliary neurotrophic factor and leukemia inhibitory factor, also cause a transient initial phase of tyrosine phosphorylation and activation of Stat1 and Stat3 (up to 1 h) but fail to stimulate a second phase of response and do not produce significant neurites. These results suggest that sustained signaling of either STAT or ERK pathways in PC12-E2 cells leads to induction of neuronal differentiation. However, only the latter is effective in native PC12 cells as the activation of Stat3 and Stat1 in native PC12 cells by IL-6 fails to induce neuronal differentiation. Thus, the response of PC12-E2 cells to IL-6 suggests the constitutive expression of a required factor(s) for differentiation, that is induced in native PC12 cells by NGF or bFGF (possibly by ERK activation), but not by IL-6 via Janus kinase/STAT activation. This factor(s), which has a sufficient half-life to allow primed cells to remain responsive to IL-6 for several days, is necessary but not sufficient for differentiation (as measured by neurite proliferation) to occur.

Synergistic induction of neurite outgrowth by nerve growth factor or epidermal growth factor and interleukin-6 in PC12 cells

Native PC12 cells respond differentially to nerve growth factor (NGF) but not interleukin-6 (IL-6); PC12-E2 cells, a stable variant, respond to both stimuli (and more rapidly to NGF). Neither responds to epidermal growth factor (EGF). NGF primarily induces the RAS/extracellular signal-regulated kinase (ERK) pathway and IL-6 activates a JAK (Janus tyrosine kinase)/STAT (signal transducers and activators of transcription) response. EGF also stimulates RAS/ERK but in a transient manner. When either cell type is treated with combinations of NGF, EGF, and IL-6, at concentrations that produce modest or no response, a substantial augmentation of neurite outgrowth is observed. With PC12-E2 cells, a subthreshold concentration of IL-6 increases NGF response by approximately 2-3-fold after 1-2 days; the increase with EGF is more pronounced. Native PC12 cells show even greater synergistic effects with NGF and IL-6. The most dramatic effect was observed with low levels of EGF, where IL-6 increased the percentage of responsive cells from zero to approximately 60% after 3 days. In addition, two neural-specific transcripts, GAP-43 and SCG-10, are synergistically increased by the combinations of growth factors. Importantly, IL-6 does not enhance ERK phosphorylation in the presence of either NGF or EGF. In contrast, NGF and EGF, in the presence or absence of IL-6, cause mobility shifts of Stat3 that are consistent with serine phosphorylations. Although these modifications do not lead to activation and translocation by themselves, in the presence of the tyrosine phosphorylation induced by IL-6, they may play a role in the synergistic responses. These observations suggest a differentially regulated two-stage mechanism for the differentiative response of PC12 cells to NGF.

PC12-E2 cells: a stable variant with altered responses to growth factor stimulation

A variant cell line, designated E2, characterized by more rapid responses to nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) and markedly more robust responses to interleukin-6 and 8-Br-cAMP, has been subcloned from the rat PC12 cell line. The enhanced responsiveness to NGF in E2 cells is not due to receptor overexpression as judged by TrkA protein levels and tyrosine kinase activity, but may be associated with the increased and prolonged tyrosine phosphorylation of ERK1 (extracellular signal regulated kinase 1) and ERK2. The rapid morphological differentiation induced by different growth factors in E2 cells is mediated in a transcription-independent manner suggesting that E2 cells may constitutively express some differentiation-associated molecules that allow direct entry into the neuronal program.

Specificity of nerve growth factor signaling: differential patterns of early tyrosine phosphorylation events induced by NGF, EGF, and bFGF

The specificity of nerve growth factor (NGF) action was examined by comparing early tyrosine phosphorylation events induced by NGF, epidermal growth factor (EGF), and basic fibroblast growth factor (bFGF). In PC12 cells, administration of either the differentiation factor NGF or the mitogenic factor EGF led to tyrosine phosphorylation of multiple polypeptides in the 100-110 kDa size range associated with PI-3 kinase. However, NGF induced a more prolonged phosphorylation, relative to a transient EGF effect. In contrast, the differentiation factor bFGF failed to induce measurable tyrosine phosphorylation of PI-3 kinase-associated proteins. Similarly, NGF but not bFGF induced marked tyrosine phosphorylation of PLC gamma, another early signaling molecule, suggesting that multiple pathways exist for promoting differentiation, and/or that these signaling molecules are not essential for differentiation. TrkA signaling was also compared between PC12 cells and NIH-3T3 cells heterologously expressing trkA, where receptor activation promotes mitogenesis. In this comparison, significant differences were observed in the tyrosine phosphorylation pattern of PI-3 kinase-associated polypeptides, suggesting the existence of cell type-specific molecular interactions influencing trkA signaling. Mechanistically, NGF stimulation of PC12 cells resulted in a weak or possibly indirect association between trkA and PI-3 kinase. Furthermore, NGF did not appear to activate or substantially alter the overall level of PI-3 kinase activity, raising the possibility that ligand-induced phosphorylation may serve instead to relocalize constitutively active PI-3 kinase molecules within the cell. Taken together, data presented suggest that the temporal pattern of induced phosphorylation, the nature of induced associations with other phosphoproteins, and cell type-specific components may all contribute to the generation of NGF signaling specificity.

Identification of two alternatively spliced transcripts of STEP: a subfamily of brain-enriched protein tyrosine phosphatases

A brain-enriched protein tyrosine phosphatase termed STEP46 (striatal enriched phosphatase) was previously isolated and characterized. Immunological studies with a STEP monoclonal antibody recognized several STEP-immunoreactive proteins, and suggested that additional STEP-related polypeptides existed. This study reports the isolation of two alternatively spliced transcripts of the STEP gene. One of these, STEP20 (with a predicted molecular mass of 20 kDa) was further characterized and found to lack the conserved tyrosine phosphatase domain. Northern analysis detected a 2.8 kb STEP20 message in mouse brain. The second alternatively spliced transcript, STEP61, has a 5'-extended open reading frame that encodes a protein with a predicted molecular mass of 61 kDa and contains a single tyrosine phosphatase domain. The exon-intron organization responsible for the novel STEP20 and STEP61 sequences was determined in the mouse STEP genomic DNA. We propose that the original STEP46, along with STEP20 and STEP61, are members of a brain-enriched subfamily of protein tyrosine phosphatases, and that STEP isoforms may have distinct functions within the central nervous system.

Single step purification of biologically active recombinant rat basic fibroblast growth factor by immobilized metal affinity chromatography

The construction and use of a plasmid which allows the expression and single step purification of recombinant rat basic fibroblast growth factor (bFGF) is described. A cDNA encoding rat bFGF was subcloned into the expression plasmid pQE-9 (Qiagen) in such a way that the bFGF which is produced from the resulting construct contains 6 histidine residues near the amino terminus. The resulting plasmid, pQE-9-bFGF, was expressed in the E. coli strain M15[pREP4] and the 6 x His-bFGF was purified to homogeneity from the soluble fraction of the bacterial cell lysate in a single step by affinity chromatography on a nickel chelate resin. About 5 mg of 6 x His-bFGF was obtained from the soluble fraction from one liter of bacterial cell culture. Testing of the 6 x His-bFGF in a PC12 cell differentiation assay showed that its activity was comparable to the activities for native bFGF and recombinant bFGF purified by multistep methods.

Physiological levels of beta-amyloid increase tyrosine phosphorylation and cytosolic calcium

The a beta peptide is a neurotoxic peptide that accumulates in the brains of Alzheimer patients, but is also present in body fluids at subnanomolar levels. The potential effects of these low levels of a beta are unclear. We now show that one such action is to increase tyrosine phosphorylation in PC12 cells and olfactory neuroblasts. Application of a beta 25-35 or a beta 1-40 induces a dose-dependent increase in the tyrosine phosphorylation in both whole cells and in vitro. The increase in tyrosine phosphorylation is both rapid and sensitive, being stimulated by picomolar doses of a beta and occurring within 1 min of application. Calcium imaging experiments provide further support for the role of tyrosine phosphorylation in the action of a beta. While a beta does not alter calcium metabolism under basal conditions, the addition of a beta induces a rapid increase in cytoplasmic calcium in olfactory neuroblasts that have been treated with the tyrosine phosphatase inhibitor, sodium orthovanadate or in PC12 cells treated with nerve growth factor. These responses could be blocked by the tyrosine kinase inhibitor, herbimycin. These calcium responses displayed an obligate requirement for the presence of matrix proteins. The identification of a rapid, sensitive assay for the action of a beta may facilitate investigations of its mechanism of action.

Selective translocation of protein kinase C-delta in PC12 cells during nerve growth factor-induced neuritogenesis

The specific intracellular signals initiated by nerve growth factor (NGF) that lead to neurite formation in PC12 rat pheochromocytoma cells are as of yet unclear. Protein kinase C-delta (PKC delta) is translocated from the soluble to the particulate subcellular fraction during NGF-induced-neuritogenesis; however, this does not occur after treatment with the epidermal growth factor, which is mitogenic but does not induce neurite formation. PC12 cells also contain both Ca(2+)-sensitive and Ca(2+)-independent PKC enzymatic activities, and express mRNA and immunoreactive proteins corresponding to the PKC isoforms alpha, beta, delta, epsilon, and zeta. There are transient decreases in the levels of immunoreactive PKCs alpha, beta, and epsilon after 1-3 days of NGF treatment, and after 7 days there is a 2.5-fold increase in the level of PKC alpha, and a 1.8-fold increase in total cellular PKC activity. NGF-induced PC12 cell neuritogenesis is enhanced by 12-O-tetradecanoyl phorbol-13-acetate (TPA) in a TPA dose- and time-dependent manner, and this differentiation coincides with abrogation of the down-regulation of PKC delta and other PKC isoforms, when the cells are treated with TPA. Thus a selective activation of PKC delta may play a role in neuritogenic signals in PC12 cells.

Activation of mesangial cells by the phosphatase inhibitor vanadate. Potential implications for diabetic nephropathy

The metalion vanadate has insulin-like effects and has been advocated for use in humans as a therapeutic modality for diabetes mellitus. However, since vanadate is a tyrosine phosphatase inhibitor, it may result in undesirable activation of target cells. We studied the effect of vanadate on human mesangial cells, an important target in diabetic nephropathy. Vanadate stimulated DNA synthesis and PDGF B chain gene expression. Vanadate also inhibited total tyrosine phosphatase activity and stimulated tyrosine phosphorylation of a set of cellular proteins. Two chemically and mechanistically dissimilar tyrosine kinase inhibitors, genistein and herbimycin A, blocked DNA synthesis induced by vanadate. Vanadate also stimulated phospholipase C and protein kinase C. Downregulation of protein kinase C abolished vanadate-induced DNA synthesis. Thus, vanadate-induced mitogenesis is dependent on tyrosine kinases and protein kinase C activation. The most likely mechanism for the effect of vanadate on these diverse processes involves the inhibition of cellular phosphotyrosine phosphatases. These studies demonstrating that vanadate activates mesangial cells may have major implications for the therapeutic potential of vanadate administration in diabetes. Although vanadate exerts beneficial insulin-like effects and potentiates the effect of insulin in sensitive tissue, it may result in undesirable activation of other target cells, such as mesangial cells.

A G protein is involved in the angiotensin AT2 receptor inhibition of the T-type calcium current in non-differentiated NG108-15 cells

In non-differentiated NG108-15 cells, both angiotensin II (Ang II) (100 nM) and CGP 42112 (100 nM) decreased the T-type calcium current amplitude by 24 +/- 2% and 21 +/- 3%, respectively. cGMP is not a mediator of the Ang II effect, since loading of cells with 50 microM cGMP did not prevent the inhibitory effects of Ang II. The effects of Ang II involves a non-identified GTPase activity since incubation with GDP beta S (3 mM) completely reversed the inhibitory effect of Ang II while GTP gamma S mimicked its effect. However, Ang II binding was not affected by GTP gamma S, and the effect of Ang II was not modified in pertussis toxin-treated cells. The inhibitory effect of Ang II on the T-type Ca2+ current involves a phosphotyrosine phosphatase activity since sodium orthovanadate prevented the effects of Ang II, although microcystin-LR, a selective Ser/Thr phosphatase 1 and 2A inhibitor, did not modify the effect of Ang II. These results provide the first evidence of a modulation of membrane conductance by Ang II through the AT2 receptor and demonstrate the involvement of a phosphotyrosine phosphatase and a G protein in the AT2 transduction mechanism in NG108-15 cells. Moreover, our data suggest that phosphotyrosine phosphatase activation is proximal to receptor occupation, since sodium orthovanadate inhibits both GTPase activity and T-type current blockage induced by Ang II or CGP 42112, while GTP gamma S inhibition of the T-type calcium current is not impaired.

A neuronal protein tyrosine phosphatase induced by nerve growth factor

A new protein tyrosine phosphatase (PC12-PTP1) was identified in nerve growth factor (NGF)-treated PC12 cells. The mRNA level of PC12-PTP1 is increased 9-fold over the initial 8 h of NGF treatment and then decreases dramatically after 24 h of treatment. In rat brain, three transcripts corresponding to 1.5, 2.6, and 3.0 kilobases (kb) in size are detected by Northern blot analysis. Although the 1.5- and 2.6-kb transcripts are present in brain and other tissues, the 3-kb transcript is exclusively expressed in brain and the expression of this transcript alone increases following NGF treatment. PC12-PTP1 is a non-receptor protein tyrosine phosphatase (PTP) with a 50% sequence homology in the phosphatase domain with several other non-receptor PTPs. PC12-PTP1 fusion protein exhibits tyrosine phosphatase activity, and in vitro translation of the PC12-PTP1 transcript produces a major protein of 39 kDa. The data presented suggest that NGF regulates the expression of PC12-PTP1 during periods of neuronal growth and differentiation.

From receptor internalization to nuclear translocation. New targets for long-term pharmacology

Receptors involved in intercellular communication at the cell surface share the capacity to desensitize through molecular and cellular mechanisms. Cellular desensitization is a rapid and dynamic process whereby membrane receptors internalize in response to an excess of agonists. The internalized receptors may recycle rapidly or undergo down-regulation when following a degradative pathway. However, receptor internalization does not necessarily mean degradation; it also represents the initial step of a retrograde signalling system whereby an "interiorized" message, the ligand-receptor complex, can be transported in contrast to second messengers, along axons or in the cytoplasm leading to long-term effects in the nucleus. Such "third messengers" have to undergo nuclear translocation to serve as transcriptional regulators in the control of gene expression. The "third messengers" are thus cytoplasmic proteins, including the receptor itself, which may be associated with internalized vesicles and released by mechanisms which have not yet been elucidated. They represent already good targets for the development of new drugs, and multi-targeting and synergistic approaches are likely to increase their usefulness.