1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-resistant, flat-cell PC12 variants having a partial loss of transformed phenotype

cDNA microarray analysis of changes in gene expression associated with MPP+ toxicity in SH-SY5Y cells

cDNA microarray analysis of 1-methyl-4-phenyl-pyridinium (MPP+) toxicity (1 mM, 72 h) in undifferentiated SH-SY5Y cells identified 48 genes that displayed a signal intensity greater than the mean of all differentially expressed genes and a two-fold or greater difference in normalized expression. RT-PCR analysis of a subset of genes showed that c-Myc and RNA-binding protein 3 (RMB3) expression decreased by approximately 50% after 72 h of exposure to MPP+ (1 mM) but did not change after 72 h of exposure to 6-hydroxydopamine (25 microM), rotenone (50 nM), and hydrogen peroxide (600 microM). Exposure of retinoic acid (RA)-differentiated SH-SY5Y cells to MPP+ (1 mM, 72 h) also resulted in a decrease in RMB3 expression and an increase in GADD153 expression. In contrast, c-Myc expression was slightly increased in RA-differentiated cells. Collectively, these data provide new insights into the molecular mechanisms of MPP+ toxicity and show that MPP+ can elicit distinct patterns of gene expression in undifferentiated and RA-differentiated SH-SY5Y cells.

Differential Expression of Markers and Activities in a Group of PC12 Nerve Cell Clones

Sixteen clones, recently isolated from the PC12 nerve cell line, were analysed for a variety of markers and activities. Two endoplasmic reticulum (ER) luminal markers, the chaperone protein BiP and the major Ca2+ storage protein calreticulin, as well as the 40-kD rough ER membrane marker and the plus-end-directed mirotubule motor protein, kinesin, were found to be expressed at similar levels. These results suggest that the size of the ER, the function of microtubules and the capacity of the rapidly exchanging Ca2+ store do not change substantially among the clones. Other proteins expressed at comparable levels were synapsin I and IIa, members of a nerve cell-specific protein family known to bind synaptic vesicles to the cytoskeleton. In contrast, another ER membrane protein, calnexin, and the markers of secretory organelles were found to vary markedly. One clone (clone 27) completely lacked both chromogranin B and secretogranin II, the proteins contained within dense granules, and synaptophysin, a marker of clear vesicles. Other clones expressed these markers to variable and apparently mutually unrelated levels. Marked variability was observed also in the uptake of exogenous catecholamines, in their release both at rest and after stimulation, and in nerve growth factor-induced differentiation. These results provide indirect information about the mechanisms that regulate the expression of structures and activities in PC12 cells. Of particular interest is clone 27, which appears globally incompetent for regulated secretion and might therefore be a valuable tool for the study of this activity in a nerve cell.

The fetal and neonatal brain protein neuronatin protects PC12 cells against certain types of toxic insult

The protein neuronatin is expressed in the nervous system of the fetus and neonate at a much higher level than in the adult. Its function is unknown. As a result of variable splicing, neuronatin mRNA exists in two forms, alpha and beta. Wild type PC12 cells express neuronatin-alpha. We have isolated a PC12 variant, called 1.9, that retains many of the neuron-like properties of wild type PC12 cells, but it does not express neuronatin and it exhibits markedly increased sensitivity to the toxic effects of nigericin, rotenone and valinomycin. Pretreatment of the 1.9 cells with alpha-methyltyrosine, which inhibits dopamine synthesis, had little effect on the cells' sensitivity to nigericin, rotenone or valinomycin indicating that dopamine-induced oxidative stress was not involved in the toxicity of these compounds. However, flattened cell subvariants of the 1.9 cells, which do not have any neuron-specific characteristics, did not exhibit increased sensitivity to nigericin indicating that some neuronal characteristic of the 1.9 cells contributed to the toxicity of nigericin. After the neuronatin-beta gene was transfected into and expressed in the 1.9 cells, they regained wild type PC12 levels of resistance to nigericin, rotenone and valinomycin. These studies suggest that the function of neuronatin during development could be to protect developing cells from toxic insult occurring during that period.

Neuroblastoma cells provoke Schwann cell proliferation in vitro

BACKGROUND

A subset of human neuroblastomas (NBs) has the capacity to mature completely, imitating sympathetic ganglia. Previously, we showed that the neuronal population in spontaneously maturing NBs usually has a near-triploid DNA content without 1p deletions, and we concluded that the constantly diploid Schwann cells (SCs) do not belong to the neoplastic component of these tumours. We therefore hypothesised that NB cells are able to stimulate SC proliferation, and that SCs trigger NB differentiation.

PROCEDURE

We performed in vitro experiments to test this model and to test whether SCs can also influence the growth of aggressive NBs. Human SCs were co-cultivated with NB tumours and cell lines, and were harvested after defined time intervals. Proliferative activity of the SCs and the NB cells was determined by visualisation of 5-bromo-2'-deoxyuridine (BrdU) incorporation or Ki-67 staining. Neurite outgrowth and neurofilament (NF) expression were analysed immunocytochemically and apoptotic rate was determined by a terminal deoxynucleotidyl transferase-mediated dUTP-X fluorescein nick end labelling (TUNEL) assay.

RESULTS

Human NB tumours or cell lines unequivocally increased the proliferation of SCs in vitro. In cocultivated NB cells, the proliferative activity was not altered in the first days of cocultivation, although neurite outgrowth and NF expression were enhanced. However, after 10 days, the mitotic rate of neuroblastic cells decreased and the apoptotic rate showed a marked increase.

CONCLUSIONS

The results of the cocultivation experiments provide an experimental hint that the in vivo growth of SCs in NBs is caused by the neoplastic neuroblasts, and they also indicate that cells from peripheral nerves can influence the growth of aggressive NB cells if cocultivated.

Wnt-1 inhibits nerve growth factor-induced differentiation of PC12 cells by preventing the induction of some but not all late-response genes

The vertebrate Wnt-1 proto-oncogene is expressed transiently in embryonic brain and functions in the development of the central nervous system and neural crest. The role of Wnt-1 in neural crest development appears to be to increase the number of certain progenitor cells by preventing their premature differentiation. To study the mechanism by which this transient Wnt-1 expression inhibits differentiation we have constructed PC12 pheochromocytoma cells in which Wnt-1 expression levels were controlled by use of a tetracycline-responsive transactivator. Induction of Wnt-1 expression by tetracycline withdrawal was followed by activation of the Wnt-1 signalling pathway as shown by activation of the Lef-1/Tcf transcription factor. Wnt-1 expression by these cells resulted in reversible inhibition of NGF-induced neurite outgrowth, but it did not adversely affect the maintenance of previously formed NGF-induced neurites. Wnt-1 expression also partially blocked the ability of NGF to decrease the rate of cell multiplication. Wnt-1 decreased the NGF-induced expression of the late-response gene SCG10 but not of the immediate early genes, fos, Nur77 and UPAR (urokinase-type plasminogen activator receptor) nor of the late-response genes GAP-43 and collagenase. The Wnt-1 expressing PC12 cells multiplied at a greater rate when they expressed Wnt-1 than they did in the absence of Wnt-1 expression, a result that is consistent with the proposal that Wnt-1 may also act as a mitogen.

Neuroblastoma cells can actively eliminate supernumerary MYCN gene copies by micronucleus formation--sign of tumour cell revertance?

Human neuroblastoma cell lines frequently exhibit MYCN amplification and many are characterised by the presence of morphologically distinct cell types. The neuronal cells (N-cells) and the so-called flat cells (F-cells) are thought to represent manifestations of different neural crest cell lineages and are considered to be the consequence of neuroblastoma cell pluripotency. In this study, various neuroblastoma cell lines were examined for micronuclei. In F-cells of neuroblastoma cell lines with extrachromosomally amplified MYCN, we observed the frequent occurrence of micronuclei. Using fluorescence in situ hybridisation (FISH) with a MYCN specific probe, we demonstrated that these micronuclei were packed with MYCN hybridisation signals. In addition, in a minor percentage of cells, MYCN signals occurred in clusters, adhered to the nuclear membrane and aggregated in nuclear protrusions. In F-cells, a substantial reduction or lack of amplified MYCN copies was observed. These observations let us conclude that extrachromosomally amplified genes can be actively eliminated from the nucleus resulting in a dramatic loss of amplified sequences in the F-cells. Moreover, reduction or loss of amplified sequences in F-cells was shown to be accompanied by downregulation of MYCN expression, by a decrease in proliferative activity and by upregulation of molecules of the major histocompatibility complex class I (MHC I). Interestingly, F-cells are not restricted to neuroblastoma cell cultures, but also occur in cell lines of other tissue origin. All F-cells share important biological features, interpreted as cell revertance, i.e. loss of the malignant phenotype and properties. This fact, together with the demonstration that neuroblastoma cells do not differentiate into Schwann cells in vivo [1] Ambros et al. NEJM 1996, 334, 1505-1511, do not support the hypothesis that F-cells represent Schwannian/glial differentiation in vitro. We therefore postulate that the elimination of amplified MYCN gene copies in cultivated neuroblastoma cells is in line with the phenomenon of tumour cell revertance.

Elevated expression of glutathione peroxidase in PC12 cells results in protection against methamphetamine but not MPTP toxicity

In vivo administration of either 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or methamphetamine (MA) produces damage to the dopaminergic nervous system which may be due in part to the generation of reactive oxygen species (ROS). The resistance of superoxide dismutase (SOD) over-expressing transgenic mice to the effects of both MPTP and MA suggests the involvement of superoxide in the resulting neurotoxicity of both compounds. Superoxide can be converted by SOD to hydrogen peroxide, which itself can cause cellular degeneration by reacting with free iron to produce highly reactive hydroxyl radicals resulting in damage to proteins, nucleic acids and membrane phospholipids. Hydrogen peroxide has also been reported to be produced via inhibition of NADH dehydrogenase by MPP + formed during oxidation of MPTP by MAO-B and by dopamine auto-oxidation following MA-induced dopamine release from synaptic vesicles within nerve terminals. To test whether hydrogen peroxide is an important factor in the toxicity of either of these two neurotoxins, we created clonal PC12 lines expressing elevated levels of the hydrogen peroxide-reducing enzyme glutathione peroxidase (GSHPx). Elevation of GSHPx levels in PC12 was found to diminish the rise in ROS levels and lipid peroxidation resulting from MA but not MPTP treatment. Elevated levels of GSHPx also appeared to prevent decreases in transport-mediated dopamine uptake produced via MA administration as well as to attenuate toxin-induced cell loss as measured by either MTT reduction or LDH release. Our data, therefore, suggest that hydrogen peroxide production likely contributes to MA toxicity in dopaminergic neurons.

Decreased glutathione results in calcium-mediated cell death in PC12

Neuronal damage in certain cellular populations in the brain has been linked to oxidative stress accompanied by an elevation in intracellular calcium. Many questions remain about how such oxidative stress occurs and how it affects calcium homeostasis. Glutathione (GSH) is a major regulator of cellular redox status in the brain, and lowered GSH levels have been associated with dopaminergic cell loss in Parkinson's disease (PD). We found that transfection of antisense oligomers directed against glutamylcysteine synthetase (GCS), the rate-limiting enzyme in GSH synthesis, into PC12 cells resulted in decreased GSH and increased levels of ROS. Decreased GSH levels also correlated with an increase in intracellular calcium levels. Data from this study suggest that dopaminergic neurons are very sensitive to decreases in the internal oxidant buffering capacity of the cell caused by reductions in GSH levels, and that alterations in this parameter can result in disruption of calcium homeostasis and cell death. These results may be of particular significance for therapeutic treatment of PD, as those dopaminergic neurons that are spared in this disorder appear to contain the calcium binding protein, calbindin.

Rat C6 and human astrocytic tumor cells express a neuronal type of glutamate transporter

C6 glioma cells take up aspartate and glutamate by a Na(+)-dependent transporter. Using the polymerase chain reaction and degenerate oligonucleotide primers corresponding to conserved regions of previously cloned glutamate transporters, we isolated from these cells a partial cDNA clone with a sequence of the neuronal type EAAC1 glutamate transporter. The cells express a 4.4 kb message that hybridizes to this cDNA, and they do not express either of the previously described glial type glutamate transporters, GLT-1 or GLAST. The cells were sensitive to the toxic aspartate analog alanosine, which enters the cells by a glutamate transporter. Several human brain tumors examined, including some astrocytic tumors, expressed the EAAT3 glutamate transporter, which is the human homolog of the rodent EAAC1 transporter. Some of the tumors also expressed the other types of glutamate transporter.

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.

K252a-potentiation of EGF-induced neurite outgrowth from PC12 cells is not mimicked or blocked by other protein kinase activators or inhibitors

Epidermal growth factor (EGF) has recently been shown to cause certain strains of PC12 cells to extend short neurites. This EGF-induced differentiation of PC12 was found to be potentiated by the protein kinase inhibitor, K252a, in that PC12 cells treated with both EGF and K252a extended long branched neurites similar to those induced by nerve growth factor (NGF). As reported here no other protein kinase inhibitor or activator mimicked or blocked the effect of K252a on EGF-induced PC12 differentiation. Cyclic adenosine 3',5'-monophosphate (cAMP) also potentiated EGF-induced neurite outgrowth from PC12 cells, but the mechanism of this potentiation was different from that of K252a. Cells that had been exposed to EGF and then stripped of their neurons extended neurites again when retreated with EGF in the absence of RNA synthesis or when treated with NGF in the absence of RNA synthesis. Thus EGF can prime PC12 cells for either EGF or for NGF, a finding that further suggests that EGF and NGF use similar signaling pathways to induced neuronal differentiation of PC12.

PC12 variants deficient in norepinephrine transporter mRNA have wild type activities of several other related transporters

Wild type PC12 pheochromocytoma cells express a Na(+)-dependent norepinephrine transporter that operates in the uptake of catecholamines. In addition to the previously described Na(+)-dependent system A for the uptake of alpha-amino-isobutyric acid and system Gly for glycine, we have identified two other Na(+)-dependent transporter systems for amino acid uptake in these cells: 1) system beta for beta-alanine and taurine; and 2) a system for creatine. Uptake of alpha-amino-isobutyric acid, glycine, beta-alanine, and creatine is not affected in some PC12 variants that were previously shown to be deficient in catecholamine uptake and to have decreased levels of norepinephrine transporter mRNA. We have isolated two PC12 cDNA clones that are essentially identical in sequence to recently reported cDNAs for rat brain taurine and creatine transporters, respectively, and a third cDNA that appears to code for a novel transporter. mRNAs for these three transporters are present at wild type levels in those variants that express no or little norepinephrine transporter mRNA. These results support the notion that the expression of catecholamine reuptake transporters may be particularly susceptible to down-regulation.

The Wnt-1 proto-oncogene induces changes in morphology, gene expression, and growth factor responsiveness in PC12 cells

The product of the Wnt-1 proto-oncogene is a secreted glycoprotein that is normally produced in regions of the embryonic neural tube. We show here that expression of mouse Wnt-1 cDNA in the rat PC12 pheochromocytoma cell line causes a dramatic conversion from a round to a flat cell morphology. In addition, PC12 cells expressing Wnt-1 (PC12/Wnt-1) fail to extend neurites after treatment with NGF, despite the presence and activation of high affinity NGF receptors encoded by the trk gene and the induction of early response genes. Furthermore, PC12/Wnt-1 cells fail to express several neuron- and chromaffin-specific genes, indicating that PC12/Wnt-1 cells have assumed a new phenotype. Although NGF and FGF utilize similar signal transduction pathways in PC12 cells, only FGF is capable of inducing a morphological response and synthesis of transin mRNA in PC12/Wnt-1 cells.