Altered genetic response to beta-adrenergic receptor activation in late passage C6 glioma cells

Cyclic AMP-dependent down regulation of ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) in rat C6 glioma

In this communication, we demonstrate that an increase in intracellular cAMP by 1) addition of dibutyrylic cAMP (dbcAMP), a membrane-permeable cAMP-analogue, or 2) activation of the β-adrenoceptor with (-)-isoproterenol, down regulates the levels of ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) mRNA, NPP1 protein and ecto-NPPase activity in rat C6 glioma cells. DbcAMP and (-)-isoproterenol inhibit NPP1 expression in a time and dose-dependent manner. After 48h of stimulation, 1mM dbcAMP or 5μM (-)-isoproterenol decreases the amount of NPP1 protein by 75±3% and 81±1% respectively. Contrary to down regulation of NPP1, we observe an up regulation of glial fibrillary acidic protein (GFAP), a differentiation marker for astrocytic cells. Using specific inhibitors and activators, we have shown that Ca(2+), PKA, PI 3-K/PKB/GSK-3, Epac/Rap1/PP2A and MAP kinase modules are not involved in the inhibition of NPP1 gene expression. The transcription factor c-jun is significantly reduced while c-fos becomes up regulated after cAMP elevation. However an electrophoretic mobility shift assay with the activator protein-1 motif present in the promoter of the rat NPP1 gene indicates that this motif is not involved in the cAMP-dependent inhibition of NPP1 expression. In conclusion, these results indicate that intracellular cAMP levels regulate the expression of NPP1 in rat C6 glioma cells by a signalling pathway that is different from the GFAP signal transduction pathway.

Upregulation of the glutamine transporter through transactivation mediated by cAMP/protein kinase A signals toward exacerbation of vulnerability to oxidative stress in rat neocortical astrocytes

In the present study, we have evaluated the possible functionality in astrocytes of the glutamine (Gln) transporter (GlnT) known to predominate in neurons for the neurotransmitter pool of glutamate. Sustained exposure to the adenylyl cyclase activator forskolin for 24 h led to a significant increase in mRNA expression of GlnT among different membrane transporters capable of transporting Gln, with an increase in [(3)H]Gln accumulation sensitive to a system A transporter inhibitor, in cultured rat neocortical astrocytes, but not neurons. Forskolin drastically stimulated GlnT promoter activity in a manner sensitive to a protein kinase A (PKA) inhibitor in rat astrocytic C6 glioma cells, while deletion mutation analysis revealed that the stimulation was mediated by a cAMP responsive element (CRE)/activator protein-1 (AP-1) like site located on GlnT gene promoter. Forskolin drastically stimulated the promoter activity in a fashion sensitive to a PKA inhibitor in C6 glioma cells transfected with a CRE or AP-1 reporter plasmid, in association with the phosphorylation of CRE binding protein on serine133. Transient overexpression of GlnT significantly exacerbated the cytotoxicity of hydrogen peroxide in cultured astrocytes. These results suggest that GlnT expression is upregulated by cAMP/PKA signals for subsequent exacerbation of the vulnerability to oxidative stress in astrocytes.

Fluctuation of the SP/non-SP phenotype in the C6 glioma cell line

Using the C6 glioma cell as a paradigm, we found that (i) the clonogenicity of C6 cells is several orders of magnitude higher than the percentage of SP cells; (ii) non-SP cells are able to generate SP cells, and conversely SP cells generate non-SP cells; (iii) non-SP sorted cells behave as tumorigenic cells. Hence, in C6 cells cultured in serum-containing medium, SP cells can be generated from non-SP cells. This dynamic equilibrium explains in C6 cells the maintenance of the SP phenotype with cell passaging and demonstrates the existence of tumorigenic non-SP cells.

Tumor necrosis factor-α mediates the proliferation of rat C6 glioma cells via β-adrenergic receptors

In the present study, we observed that isoproterenol, a beta-adrenergic receptor (beta-AR) agonist, stimulated rat C6 glioma cell proliferation, while propranolol, a beta-AR blocker, greatly reduced the proliferative effect of TNF-alpha on C6 cells. The gene and protein expressions of both beta1- and beta2-ARs were enhanced in C6 cells after TNF-alpha treatment, and the increase in beta-AR was due to an increased number of binding sites and not due to increase in receptor affinity. We further showed that protein kinase C (PKC) was involved in the TNF-alpha-induced beta-AR expression. Collectively, our results indicate that TNF-alpha-induced proliferation in C6 glioma cells might be via the induction and activation of beta-ARs.

Selective inhibition of anandamide cellular uptake versus enzymatic hydrolysis—a difficult issue to handle

There is considerable debate at present as to whether the uptake of anandamide (AEA) into cells is by a facilitated transport process or by passive diffusion driven by fatty acid amide hydrolase (FAAH). The possibility that both processes occur, but to different extents depending upon the cell type used, has been difficult to investigate pharmacologically since available compounds show little selectivity between inhibition of AEA uptake and inhibition of FAAH. Recently, three compounds, UCM707 [N-(Fur-3-ylmethyl)arachidonamide], OMDM-1 and OMDM-2 [the 1'-(S)- and 1'-(R)-enantiomers of the 1'-4-hydroxybenzoyl analogue of oleoylethanolamide], selective for the uptake process, have been described and we have used these compounds, together with AM404 [(N-(4-hydroxyphenyl) arachidonoyl amide)] and VDM11 [(5Z,8Z,11Z,14Z)-N-(4-Hydroxy-2-methylphenyl)-5,8,11,14-eicosatetraenamide]), with the initial aim of determining which mechanism of uptake predominates in C6 glioma and RBL-2H3 cells. AM404 and VDM11 were both found to decrease the uptake of 2 microM AEA into cells (IC50 values 6-11 microM), but they also inhibited rat brain FAAH (IC50 values 1-6 microM). However, when using a different FAAH assay protocol, VDM11 was a much less potent FAAH inhibitor (IC50>50 microM) regardless of the cell type and animal species used. In contrast, we confirmed that UCM707, OMDM-1 and OMDM-2 were weak inhibitors of FAAH (IC50 values >50 microM) under all conditions used. However, their potency as inhibitors of AEA cellular accumulation appears to be largely dependent on the cell type and assay conditions used. In particular, the potency of UCM707 (IC50 value > or =25 microM) was considerably lower than the submicromolar potency previously reported for U937 cells. It is concluded that the cause/effect relationship between AEA uptake and hydrolysis cannot be investigated uniquely by using supposedly selective inhibitors of each process.

Inhibition of C6 glioma cell proliferation by anandamide, 1-arachidonoylglycerol, and by a water soluble phosphate ester of anandamide: variability in response and involvement of arachidonic acid

It has previously been shown that the endocannabinoids anandamide and 2-arachidonoylglycerol (2-AG) inhibit the proliferation of C6 glioma cells in a manner that can be prevented by a combination of capsazepine (Caps) and cannabinoid (CB) receptor antagonists. It is not clear whether the effect of 2-AG is due to the compound itself, due to the rearrangement to form 1-arachidonoylglycerol (1-AG) or due to a metabolite. Here, it was found that the effects of 2-AG can be mimicked with 1-AG, both in terms of its potency and sensitivity to antagonism by Caps and CB receptor antagonists. In order to determine whether the effect of Caps could be ascribed to actions upon vanilloid receptors, the effect of a more selective vanilloid receptor antagonist, SB366791 was investigated. This compound inhibited capsaicin-induced Ca(2+) influx into rVR1-HEK293 cells with a pK(B) value of 6.8+/-0.3. The combination of SB366791 and CB receptor antagonists reduced the antiproliferative effect of 1-AG, confirming a vanilloid receptor component in its action. 1-AG, however, showed no direct effect on Ca(2+) influx into rVR1-HEK293 cells indicative of an indirect effect upon vanilloid receptors. Identification of the mechanism involved was hampered by a large inter-experimental variation in the sensitivity of the cells to the antiproliferative effects of 1-AG. A variation was also seen with anandamide, which was not a solubility issue, since its water soluble phosphate ester showed the same variability. In contrast, the sensitivity to methanandamide, which was not sensitive to antagonism by the combination of Caps and CB receptor antagonists, but has similar physicochemical properties to anandamide, did not vary between experiments. This variation greatly reduces the utility of these cells as a model system for the study of the antiproliferative effects of anandamide. Nevertheless, it was possible to conclude that the antiproliferative effects of anandamide were not solely mediated by either its hydrolysis to produce arachidonic acid or its CB receptor-mediated activation of phospholipase A(2) since palmitoyltrifluoromethyl ketone did not prevent the response to anandamide. The same result was seen with the fatty acid amide hydrolase inhibitor palmitoylethylamide. Increasing intracellular arachidonic acid by administration of arachidonic acid methyl ester did not affect cell proliferation, and the modest antiproliferative effect of umbelliferyl arachidonate was not prevented by a combination of Caps and CB receptor antagonists.

Differential expression of small heat shock proteins in reactive astrocytes after focal ischemia: possible role of beta-adrenergic receptor

Small heat shock proteins (sHSPs), a family of HSPs, are known to accumulate in the CNS, mainly in astrocytes, in several pathological conditions such as Alexander's disease, Alzheimer's disease, and Creutzfeldt-Jakob disease. sHSPs may act not only as molecular chaperones, protecting against various stress stimuli, but may also play a physiological role in regulating cell differentiation and proliferation. In the present study, we have demonstrated that transient focal ischemia in rats dramatically induced HSP27 but not alpha B-crystallin (alphaBC), both of which are members of sHSPs, in reactive astrocytes. In contrast, in vitro chemical ischemic stress induced both HSP27 and alphaBC in cultured glial cells to the same extent. Dibutyryl cAMP (dBcAMP) and isoproterenol, a beta-adrenergic receptor (betaAR) agonist, enhanced HSP27 expression but suppressed alphaBC, and changed the shape of the cells to a stellate form. dBcAMP and isoproterenol inhibited cell proliferation under normal conditions. An increase in betaAR-like immunoreactivity was also observed in reactive astrocytes in vivo. These results, together with recent findings that betaAR plays an important role in glial scar formation in vivo, raise the possibility that betaAR activation modulates sHSP expression after focal ischemia and is involved in the transformation of astrocytes to their reactive form.

Activation of adenylate cyclase results in down-regulation of c-jun mRNA expression in rat C6 glioma cells

To investigate the possible mechanisms involved in forskolin-induced c-jun mRNA decrease in rat C6 glioma cells, we examined effects of a PKA inhibitor (H-89), a L-type Ca2+ channel blocker (nimodipine), a calmodulin activation inhibitor (calmidazolium chloride) and a Ca2+/calmodulin-dependent protein kinase II inhibitor (KN-62) on forskolin-induced c-jun mRNA down-regulation. H-89 caused a reversal of forskolin-induced c-jun mRNA decrease. Furthermore, nimodipine, KN-62 and calmidazolium chloride partially blocked forskolin-induced c-jun mRNA down-regulation. Our results suggest that activation of adenylate cyclase appears to be involved in a down-regulation of c-jun mRNA expression through a PKA pathway. In addition, L-type calcium channels, calmodulin and Ca2+/calmodulin-dependent protein kinase II may be partially involved in c-jun mRNA down-regulation induced by forskolin.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Selective induction of tumor necrosis factor receptor type II gene expression by tumor necrosis factor-alpha in C6 glioma cells

Using reverse transcription-polymerase chain reaction (RT-PCR) technique, the levels of tumor necrosis factor receptors gene expression in C6 glioma cells upon induction with tumor necrosis factor-alpha (TNF-alpha) were analysed. In control cells, the level of mRNA for tumor necrosis factor receptor type II (TNF-R2; 75/80 kDa) was much lower than that of tumor necrosis factor receptor type I (TNF-R1; 55/60 kDa). Upon exposure to TNF-alpha, the TNF-R2 mRNA level was greatly increased, while the TNF-R1 mRNA level remained unchanged even after 48 h. The induction of TNF-R2 gene expression by TNF-alpha was dose-dependent and seemed to be unique to TNF-alpha, as IL-6 had no effect. Since TNF-R2 was reported to mediate mitogenic effect in many other cell types, it is likely that the reported proliferative effect of TNF-alpha on astrocytes and C6 glioma cells was mediated by this TNF receptor subtype.

Cytotoxic effects of 1 alpha,25-dihydroxyvitamin D3 and synthetic vitamin D3 analogues on a glioma cell line

1 alpha,25-Dihydroxyvitamin D3 (1 alpha,25(OH)2D3) has recently been reported to exert a toxic effect on both rat and human glioma cell lines. However the potential clinical use of 1 alpha,25(OH)2D3 in the treatment of glioma is impaired by its potent hypercalcemic effects. We have therefore investigated the effects on glioma cell growth of several vitamin D3 analogues which have previously been shown to be less calcemic in vivo than 1 alpha,25(OH)2D3. The present study shows that several analogues are able to induce, in vitro, the death of rat glioma cells (C6.9). The compound KH 1060 appears to be the most effective in the induction of cell death, while MC 1288 and CB 1093 are as potent as 1 alpha,25(OH)2D3. EB 1089 was somewhat less effective than 1 alpha,25(OH)2D3 and MC 903, which is currently used in the treatment of psoriasis, has only a weak activity on C6.9 cells. The effective doses used are around 10(-9) M for 1 alpha,25(OH)2D3 and 10(-10) M for KH 1060. Interestingly, the toxic effect exerted by 1 alpha,25(OH)2D3 and its analogues is accompanied by several of the biochemical features of apoptosis, such as DNA fragmentation and induction of the c-myc protooncogene. These findings, together with the fact that the therapies currently available for glioma are only palliative, suggest that 1 alpha,25(OH)2D3 analogues such as KH 1060, EB 1089 or CB 1093, alone or in combination with other therapeutic approaches, could be of potential interest in the treatment of brain glial tumors.

Expression of immediate early genes after treatment of human astrocytoma cells with radiation and taxol

PURPOSE

The promising chemotherapeutic agent, taxol, has been shown to sensitize the G18 line of human astrocytoma cells to ionizing radiation. The present studies were performed to identify specific changes in gene expression associated with this altered sensitivity.

METHODS AND MATERIALS

The radioresistant, grade 3 human astrocytoma cell line, G18, was exposed for varying periods of time to treatment with taxol, tetradecanoyl phorbol acetate (TPA), serum, isoproterenol, dibutyryl cyclic adenosine monophosphate, or ionizing radiation alone or in combination with taxol pretreatment. Ribonucleic acid samples from the cells were monitored for the expression of a group of immediate early genes (IEGs), including c-fos, c-jun, TIS1, TIS7, TIS8, TIS11 and TIS21, by northern blot hybridization analysis.

RESULTS

Transient immediate early gene induction was observed after treatment of G18 cells with tetradecanoyl phorbol acetate, serum, isoproterenol, or ionizing radiation, but not after treatment with taxol. Of the seven immediate early genes analyzed, all but TIS7 were found to be inducible by one or more of the treatments. Only TIS8 (also known as egr-1 or zif268) was significantly inducible by radiation, and this transient induction was decreased by at least four-fold by pretreatment for 24 hr with a dose of taxol that was previously shown to block 96.5% of the cells in G2/M and enhance radiosensitivity.

CONCLUSION

The products of immediate early genes, which are induced transiently in cells in response to a variety of treatments, including growth factors, neurotransmitters, and irradiation with UV light or X rays, are thought to initiate a cascade of genetic responses to alterations in cellular environment. The present results demonstrate a dramatic attenuation in one immediate early gene response in association with a treatment that enhances radiosensitivity in a refractory human brain tumor line.