Down-regulation of epidermal growth factor receptors by nerve growth factor in PC12 cells is p140(trk)-, Ras-, and Src-dependent

Nerve growth factor in metabolic complications and Alzheimer's disease: Physiology and therapeutic potential

As the population ages, obesity and metabolic complications as well as neurological disorders are becoming more prevalent, with huge economic burdens on both societies and families. New therapeutics are urgently needed. Nerve growth factor (NGF), first discovered in 1950s, is a neurotrophic factor involved in regulating cell proliferation, growth, survival, and apoptosis in both central and peripheral nervous systems. NGF and its precursor, proNGF, bind to TrkA and p75 receptors and initiate protein phosphorylation cascades, resulting in changes of cellular functions, and are associated with obesity, diabetes and its complications, and Alzheimer's disease. In this article, we summarize changes in NGF levels in metabolic and neuronal disorders, the signal transduction initiated by NGF and proNGF, the physiological and pathophysiological relevance, and therapeutic potential in treating chronic metabolic diseases and cognitive decline.

Design, synthesis and biological evaluation of 2,3-dihydro-5,6-dimethoxy-1H-inden-1-one and piperazinium salt hybrid derivatives as hAChE and hBuChE enzyme inhibitors

2,3-Dihydro-5,6-dimethoxy-2-[4-(4-alkyl-4-methylpiperazinium-1-yl)benzylidine]-1H-inden-1-one halide salt derivatives as a novel donepezil hybrid analogs with the property of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) enzyme inhibition were designed and synthesized via N-alkylation reaction of 2,3-dihydro-5,6-dimethoxy-2-[4-(4-methylpiperazin-1-yl)benzylidene]-1H-inden-1-one with some alkyl halides. Biological tests demonstrated that most of the synthesized compounds have moderate to good inhibitory activities effect on cholinesterase enzymes. Among them, 10e showed the best profile as a selected compound for inhibition of hAChE (IC50 = 0.32) and hBuChE (IC50 = 0.43 μM) enzymes. Kinetic analysis and molecular docking led to a better understanding of this compound. Kinetic studies disclosed that 10e inhibited acetylcholinesterase in mixed-type and butyrylcholinesterase in non-competitive type. The toxicity results showed that 10e is less toxic than donepezil and has better inhibitory activity against hBuChE when compared to donepezil or Galantamine. Other performed experiments revealed that 10e has an anti-β amyloid effect which is capable of reducing ROS, LDH and MDA also possing positive effect on TAC. On the other hand, it has shown a good anti-inflammation effect.

Reactive oxygen species, Ki-Ras, and mitochondrial superoxide dismutase cooperate in nerve growth factor-induced differentiation of PC12 cells

Nerve growth factor (NGF) induces terminal differentiation in PC12, a pheochromocytoma-derived cell line. NGF binds a specific receptor on the membrane and triggers the ERK1/2 cascade, which stimulates the transcription of neural genes. We report that NGF significantly affects mitochondrial metabolism by reducing mitochondrial-produced reactive oxygen species and stabilizing the electrochemical gradient. This is accomplished by stimulation of mitochondrial manganese superoxide dismutase (MnSOD) both transcriptionally and post-transcriptionally via Ki-Ras and ERK1/2. Activation of MnSOD is essential for completion of neuronal differentiation because 1) expression of MnSOD induces the transcription of a neuronal specific promoter and neurite outgrowth, 2) silencing of endogenous MnSOD by small interfering RNA significantly reduces transcription induced by NGF, and 3) a Ki-Ras mutant in the polylysine stretch at the COOH terminus, unable to stimulate MnSOD, fails to induce complete differentiation. Overexpression of MnSOD restores differentiation in cells expressing this mutant. ERK1/2 is also downstream of MnSOD, as a SOD mimetic drug stimulates ERK1/2 with the same kinetics of NGF and silencing of MnSOD reduces NGF-induced late ERK1/2. Long term activation of ERK1/2 by NGF requires SOD activation, low levels of hydrogen peroxide, and the integrity of the microtubular cytoskeleton. Confocal immunofluorescence shows that NGF stimulates the formation of a complex containing membrane-bound Ki-Ras, microtubules, and mitochondria. We propose that active NGF receptor induces association of mitochondria with plasma membrane. Local activation of ERK1/2 by Ki-Ras stimulates mitochondrial SOD, which reduces reactive oxygen species and produces H(2)O(2). Low and spatially restricted levels of H(2)O(2) induce and maintain long term ERK1/2 activity and ultimately differentiation of PC12 cells.

Integrin alpha9 beta1 is a receptor for nerve growth factor and other neurotrophins

The integrin alpha9beta1 is a multifunctional receptor that interacts with a variety of ligands including vascular cell adhesion molecule 1, tenascin C and osteopontin. We found that this integrin is a receptor for nerve growth factor (NGF) and two other neurotrophins, brain-derived neurotrophic factor and NT3, using a cell adhesion assay with the alpha9SW480 cell line. Interaction of alpha9beta1 with NGF was confirmed in an ELISA assay by direct binding to purified integrin. alpha9beta1 integrin binds to neurotrophins in a manner similar to another common neurotrophin receptor, p75(NTR) (NGFR), although alpha9beta1 activity is correlated with induction of pro-survival and pro-proliferative signaling cascades. This property of alpha9beta1 resembles the interaction of NGF with a high affinity receptor, TrkA, however, this integrin shows a low affinity for NGF. NGF induces chemotaxis of cells expressing alpha9beta1 and their proliferation. Moreover, alpha9beta1 integrin is a signaling receptor for NGF, which activates the MAPK (Erk1/2) pathway. The alpha9beta1-dependent chemotactic ability of NGF appears to result from the activation of paxillin.

You can't go home again: transcriptionally driven alteration of cell signaling by NGF

Here we review findings indicating that neurotrophins such as NGF promote changes in gene transcription that in turn influence the ways that cells subsequently respond to trophic factors. As a result, initial responses of "naïve" cells to NGF and other trophic agents differ from those of cells with prior NGF exposure. We discuss specific examples based on reports in the literature as well as on data derived from a serial analysis of gene expression (SAGE) study of NGF-promoted transcriptional changes in PC12 pheochromocytoma cells.

Loss of cell adhesion impairs the early response of TrkA to nerve growth factor (NGF)

Within the in vitro model of nerve growth factor (NGF)-dependent differentiation of PC12 cells, loss of adhesion is known to interfere with certain neurotrophic responses and not with others. The present analysis of early NGF signaling shows that the upstream activation of TrkA, as well as the recruitment of the adaptor protein Shc, are dramatically attenuated upon loss of cell adhesion, while the downstream activation of ERK1/2 is unaffected. That loss of cell adhesion interferes with the primary signaling response to NGF (i.e. autophosphorylation of TrkA) may explain why suspended PC12 cells fail to morphologically differentiate in response to NGF. The possibility that established adhesion-driven mechanisms underlie the full responsiveness of TrkA to NGF is discussed.

Establishment and characterization of pheochromocytoma tumor models expressing different levels of trkA receptors

To date experimental in vivo pheochromocytoma (PC) models have not been available. A major in vitro PC model consists of PC12 cells that respond to nerve growth factor (NGF) by differentiation, mediated by the trkA receptor. We report the establishment of PC12 tumor models expressing low and high levels of trkA receptor in CD1 nude mice. The tumors are characterized by their responsiveness to NGF, karyotype, presence of enolase, and chromaffin granules, as well as dopamine release. These novel PC models facilitate research on the role of the trkA receptor in cancer and the development of trkA-selective anti-cancer agents.

A quantitative bioassay for nerve growth factor, using PC12 clones expressing different levels of trkA receptors

Nerve growth factor (NGF) is a neurotrophin required for differentiation, development, and survival of the sympathetic nervous system, with many of its biological effects being mediated via trkA receptors. There is a need for a standard quantitative bioassay for NGF, to be used in basic research and in pharmaceutical studies. The objective of the present research was to develop a selective, quantitative, and reliable bioassay for NGF, using a morphological criterion: neurite cell outgrowth. In addition, we aimed to apply the aforementioned bioassay to measure NGF administered to mice. Pheochromocytoma PC12 cell variants including wild-type cultures, and a trkA-overexpressing stable transfectant PC12-6.24-I, PC12nnr5, and PC12EN lacking trkA receptors, were used. Dose-response curves were generated with NGF beta-subunit (2.5S) purified from mouse submaxillary glands. Our results demonstrated that the bioassay was sensitive to 0.3-20 ng/mL, and selective, as neurite outgrowth was not seen by any other growth factor other than NGF. In addition, variant clones PC12nnr5 and PC12EN, lacking trkA receptors, did not respond to NGF. The bioassay detected NGF in serum of mice injected with NGF. This novel developed bioassay can serve as a model system for various neuroscience purposes.

Differences in survival-promoting effects and intracellular signaling properties of BDNF and IGF-1 in cultured cerebral cortical neurons

Brain-derived neurotrophic factor (BDNF) and insulin-like growth factor-1 (IGF-1) act on various neurons of the CNS as neurotrophic factors promoting neuronal differentiation and survival. We examined the survival-promoting effects of BDNF and IGF-1 on serum deprivation-induced death in cultured cerebral cortical neurons, and compared the intracellular signaling pathways stimulated by BDNF and IGF-1 in the neurons. We found that the survival-promoting effect of BDNF was much weaker than that of IGF-1 in serum deprivation-induced death of cultured cortical neurons. We found no differences in the levels of phosphatidylinositol 3-kinase (PtdIns3-K) activity or Akt (also called PKB) phosphorylation induced by BDNF and IGF-1 in the cultured cortical neurons, although many reports suggest that PtdIns3-K and Akt are involved in survival promotion. In addition, phosphorylation signals of mitogen-activated protein kinase (MAPK) and cAMP responsive element-binding protein (CREB), which have also been reported to be involved in survival promotion, were stimulated by BDNF much more potently than by IGF-1. These results show that there may be, as yet unidentified, intracellular signaling pathways other than the PtdIns3-K-Akt, MAPK and CREB signaling, to regulate survival promotion. These unidentified signaling pathways may be responsible for the distinct strengths of the survival-promoting effects of BDNF and IGF-1.

The Wilms' tumor gene product WT1 mediates the down-regulation of the rat epidermal growth factor receptor by nerve growth factor in PC12 cells

Recently, we characterized the rat epidermal growth factor receptor (EGFR) promoter and demonstrated that TCC repeat sequences are required for the down-regulation of EGFR by nerve growth factor (NGF) in PC12 cells. In this study, we report that the Wilms' tumor gene product WT1, a zinc finger transcription factor, is able to enhance the activity of the rat EGFR promoter in cotransfection assays. Gel mobility shift assays demonstrate that WT1 binds to the TCC repeat sequences of the rat EGFR promoter. Overexpression of WT1 resulted in up-regulation of the expression levels of endogenous EGFR in PC12 cells. Interestingly, NGF down-regulated the expression levels of WT1 and EGFR in PC12 cells, but not in the p140(trk)-deficient variant PC12nnr5 cells or in cells expressing either dominant-negative Ras or dominant-negative Src. Most importantly, we evaluated the inhibitory effect of antisense WT1 RNA on EGFR expression, and we found that antisense WT1 RNA could substantially reduce EGFR repression in either histochemical staining study or immunoblot analysis. These results indicate that NGF-induced down-regulation of the EGFR in PC12 cells is mediated through WT1 and that WT1 may play an important role in the differentiation of nerve cells.

A differential role of extracellular signal-regulated kinase in stimulated PC12 pheochromocytoma cell movement

Rat pheochromocytoma PC12 cells have been widely used as a cell system for study of growth factor-stimulated cell functions. We report here that nerve growth factor (NGF) stimulated both chemotaxis (directional migration) and chemokinesis (random migration) of PC12 cells. Treatment with a MEK1/2-specific inhibitor (PD98059) or expression of a dominant negative variant of Ras differentially inhibited NGF-stimulated chemotaxis but not chemokinesis of PC12 cells. Priming of PC12 cells with NGF resulted in reduced extracellular signal-regulated kinase (ERK) activation and loss of chemotactic, but not chemokinetic, response. In addition, NGF stimulation of ERK is known to involve an early transient phase of activation followed by a late sustained phase of activation; in contrast, epidermal growth factor (EGF) elicits only early transient ERK activation. We observed that like NGF, EGF also stimulated both chemotaxis and chemokinesis, and treatment with PD98059 abolished the EGF-stimulated chemotaxis. Therefore, the early transient phase of ERK activation functioned in signaling chemotaxis; the late sustained phase of ERK activation did not seem to have an essential role. In addition, our results suggested that chemotactic signaling required a threshold level of ERK activation; at below threshold level of ERK activation, chemotaxis would not occur.

A role for Src kinase in spontaneous epileptiform activity in the CA3 region of the hippocampus

Members of the Src family of nonreceptor protein tyrosine kinases (PTKs) have been implicated in the regulation of cellular excitability and synaptic plasticity. We have investigated the role of these PTKs in in vitro models of epileptiform activity. Spontaneous epileptiform discharges were induced in vitro in the CA3 region of rat hippocampal slices by superfusion with the potassium channel blocker 4-aminopyridine in Mg(2+)-free medium. In hippocampal slices treated in this fashion, Src kinase activity was increased and the frequency of epileptiform discharges could be greatly reduced by inhibitor of the Src family of PTKs, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2), but not by the inactive structural analog 4-amino-7-phenylpyrazol[3,4-d]pyrimidine (PP3). 4-Amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine also reduced epileptiform activity induced by either 4-aminopyridine or Mg(2+)-free medium alone. These observations demonstrate a role for Src family PTKs in the pathophysiology of epilepsy and suggest potential therapeutic targets for antiepileptic therapy.

Chemiluminescent immunodetection protocols with 1,2-dioxetane substrates. 4

Chemiluminescent 1,2-dioxetane enzyme substrates provide a highly sensitive and versatile detection method for immunoblots and other membrane-based detections. 1,2-Dioxetane substrates, coupled with either alkaline phosphatase or beta-galactosidase enzyme labels, generate glow light emission kinetics, with a signal duration that is significantly longer than most enhanced luminol/horseradish peroxidase chemiluminescent detection systems. The long-lived, high-intensity light signal is ideal for imaging using a variety of formats, including X-ray film, photographic film, chemiluminescence phosphor imaging screens, and the rapidly expanding selection of camera imaging systems.

Adenosine A(2A) receptor mRNA regulation by nerve growth factor is TrkA-, Src-, and Ras-dependent via extracellular regulated kinase and stress-activated protein kinase/c-Jun NH(2)-terminal kinase

We have shown previously that nerve growth factor (NGF) down-regulates adenosine A(2A) receptor (A(2A)AR) mRNA in PC12 cells. To define cellular mechanisms that modulate A(2A)AR expression, A(2A)AR mRNA and protein levels were examined in three PC12 sublines: i) PC12nnr5 cells, which lack the high affinity NGF receptor TrkA, ii) srcDN2 cells, which overexpress kinase-defective Src, and iii) 17.26 cells, which overexpress a dominant-inhibitory Ras. In the absence of functional TrkA, Src, or Ras, NGF-induced down-regulation of A(2A)AR mRNA and protein was significantly impaired. However, regulation of A(2A)AR expression was reconstituted in PC12nnr5 cells stably transfected with TrkA. Whereas NGF stimulated the mitogen-activated protein kinases p38, extracellular regulated kinase 1 and 2 (ERK1/ERK2), and stress-activated protein kinase/c-Jun NH(2)-terminal kinase (SAPK/JNK) in PC12 cells, these kinases were activated only partially or not at all in srcDN2 and 17.26 cells. Inhibiting ERK1/ERK2 with PD98059 or inhibiting SAPK/JNK by transfecting cells with a dominant-negative SAPKbeta/JNK3 mutant partially blocked NGF-induced down-regulation of A(2A)AR expression in PC12 cells. In contrast, inhibiting p38 with SB203580 had no effect on the regulation of A(2A)AR mRNA and protein levels. Treating SAPKbeta/JNK3 mutant-transfected PC12 cells with PD98059 completely abolished the NGF-induced decrease in A(2A)AR mRNA and protein levels. These results reveal a role for ERK1/ERK2 and SAPK/JNK in regulating A(2A)AR expression.

Heterologous upregulation of nerve growth factor-TrkA receptors in PC12 cells by pituitary adenylate cyclase-activating polypeptide (PACAP)

The capacity for the neurotrophic factor PACAP38 to regulate expression of nerve growth factor (NGF)-trkA receptors in PC12 cells has been examined. Treatment of PC12 cells with 5 nM PACAP38 for 48 h elicited a 2.5-fold increase in 125I-NGF binding sites. FACS and Western analysis of trkA receptor protein indicate an abundance of receptors. The PACAP38-selective antagonist PACAP 6-38 blocked trkA receptor upregulation elicited by PACAP38. The expression of epidermal growth factor receptors was not affected by PACAP38 suggesting that upregulation of trkA represents a selective effect of this neurotrophic peptide. Similarly, expression of the pan-neurotrophin binding receptor p75 was not altered by PACAP38 treatment. In addition to effects on trkA observed in wild-type PC12 cells, PACAP38 stimulated an increase in the level of expressed human trkA receptors stably transfected into PC12 cells. PACAP38 provoked an increase in basal and NGF-stimulated phosphorylation of trkA. Enhanced phosphorylation of trkA was detected as early as 6 h following addition of PACAP38 and was maximal at 48 h. Increased incorporation of phosphate occurs on both serine and tyrosine residues of trkA. These results suggest that PACAP38 is able to promote upregulation of trkA receptors, an event associated with elevated serine/tyrosine phosphorylation of trkA.

Ectopic p21(WAF1) expression induces differentiation-specific cell cycle changes in PC12 cells characteristic of nerve growth factor treatment

Nerve growth factor treatment of PC12 cells results in neuronal differentiation, a process accompanied by induction of the Cdk inhibitor p21(WAF1). To determine the role of p21 in differentiation, PC12 clones containing an inducible p21 construct were utilized to induce growth arrest. Expression of p21 led to accumulation of cyclins D1 and E and to a decrease in cyclins A and B. Levels of Cdc2 and Cdk4 also decreased after p21 induction. Initially, thymidine incorporation into DNA was dramatically inhibited; however, low levels of incorporation were observed during prolonged p21 expression. Fluorescence-activated cell sorter analysis revealed that this low level of DNA synthesis resulted in the generation of polyploid cells. Results from Western blots were consistent with phosphorylation of p21 protein coincident with the resumption of DNA synthesis. Finally, treatment of p21-arrested populations with epidermal growth factor, a known PC12 mitogen, resulted in neurite extension, a key feature of neuronal differentiation. Overall, cell cycle changes following p21 overexpression in PC12 cells closely mimic distinctive events previously shown to occur during differentiation. These results suggest that the mechanism by which nerve growth factor induces the many cellular changes associated with growth arrest during differentiation is through p21(WAF1) induction.

Nerve growth factor treatment prevents the increase in superoxide produced by epidermal growth factor in PC12 cells

Stimulation of pheochromocytoma (PC12) cells with the mitogen epidermal growth factor (EGF) produced a rapid and robust accumulation of intracellular reactive oxygen species (ROS), an accumulation which, in other systems, has been shown to be essential for mitogenesis. Brief pretreatment of the cells with nerve growth factor (NGF) suppressed the EGF-mediated ROS increase. EGF failed to produce elevations in ROS in a PC12 variant stably expressing a dominant-negative p21(ras) construct (PC12-N17) or in cells pretreated with the MEK inhibitor PD098059. NGF failed to suppress the increase in ROS in the PC12 variant nnr5, which lacks p140(trk) receptors. The suppression of the increase in ROS by NGF was restored in nnr5 cells stably expressing p140(trk) (nnr5-trk), but NGF failed to prevent the increase in ROS in nnr cells expressing mutant p140(trk) receptors that lack binding sites for Shc and phospholipase Cgamma. Among several inhibitors of superoxide-generating enzymes, only the lipoxygenase inhibitor, nordihydroguaiaretic acid reduced EGF-mediated ROS accumulation. The inhibitory action of NGF on ROS production was mimicked by the nitric oxide donor, sodium nitroprusside, and was blocked by an inhibitor of nitric-oxide synthetase, L-nitroarginine methyl ester. These results suggest a novel mechanism for the rapid interruption of mitogenic signaling by the neurotrophin NGF.

Transcriptional down-regulation of epidermal growth factor receptors by nerve growth factor treatment of PC12 cells

Treatment of PC12 cells with nerve growth factor leads to a decrease in the number of epidermal growth factor receptors on the cell membrane. The mRNA for the epidermal growth factor receptor decreases in a comparable fashion. This decrease appears due to a decrease in the transcription of the epidermal growth factor receptor gene because first, there is no difference in the stability of the epidermal growth factor receptor mRNA, second, newly transcribed epidermal growth factor receptor mRNA is decreased in nerve growth factor-differentiated cells, and third, constructs containing the promoter region of the epidermal growth factor receptor gene are transcribed much less readily in nerve growth factor-differentiated cells than in untreated cells. The decreases in mRNA are not seen in the p140(trk)-deficient variant PC12nnr5 cells nor in cells containing either dominant-negative Ras or dominant-negative Src. Treatment with nerve growth factor also increases the cellular content of GCF2, a putative transcription factor inhibitory for the transcription of the epidermal growth factor receptor gene. The increase in GCF2, like the decrease in the epidermal growth factor receptor mRNA, is not seen in PC12nnr5 cells nor in cells expressing either dominant-negative Ras or dominant-negative Src. The results suggest that nerve growth factor-induced down-regulation of the epidermal growth factor receptor is under transcriptional control, is p140(trk)-, Ras-, and Src-dependent, and may involve transcriptional repression by GCF2.