Differential regulation of GDNF, neurturin, and their receptors in primary cultures of rat glial cells

Glial cell line-derived neurotrophic factor (GDNF) and neurturin (NTN) bind to GFR alpha-1 and GFR alpha-2 receptors, respectively, and their neurotrophic activity is mediated by the tyrosine kinase receptor, Ret. All these molecules were found to be expressed in primary cultures of rat glial cells, which were largely composed of astrocytes and maintained in serum-free medium. Although GDNF, NTN and Ret mRNA levels were at the limit of detection, RNase protection assays revealed relatively high amounts of GFR alpha-1 and GFR alpha transcripts. To characterize signals controlling their expression, glial cells were exposed to serum or treated with hormones acting through nuclear receptors and by activators of the cAMP or protein kinase C (PKC)-dependent pathways. Retinoic acid or 1,25-dihydroxyvitamin D3 appeared ineffective. In contrast, the 5-fold increase in GFR alpha-2 mRNA after 24 hr of treatment with 10(-10) M of tri-iodothyronine, suggests a physiological role of thyroid hormone in the regulation of this receptor in vivo. The serum induced a 7-fold increase in GFR alpha-1 mRNA levels. These changes may be mediated by the cAMP or PKC pathways because both forskolin and TPA up-regulated the GFR alpha-1 gene. Interestingly, only TPA led to a coordinated increase in the levels of GDNF, GFR alpha-1 and GFR alpha-2 mRNAs. On the other hand, NTN transcripts remained constant, irrespective of the culture conditions. Taken together, these results indicate that GDNF family ligands and their receptors are regulated in glial cells by common or independent transductional pathways, which could modulate their specific expression during brain development or in the case of trauma.

Different mechanisms mediate the rejection of porcine neurons and endothelial cells transplanted into the rat brain

In order to investigate the early cellular responses mediating xenograft rejection in the brain, porcine aortic endothelial cells (PAEC) or porcine fetal mesencephalic neurons (PNEU) were transplanted into the striatum of LEW.1A rats. PAEC were detected with a specific anti-beta1 integrin antibody, and PNEU with an anti-porcine neurofilament antibody, or an antibody recognizing the NeuN antigen. PAEC grafts were massively infiltrated within 24 h by OX42-positive cells, which may correspond to polymorphonuclear (PMN) cells or macrophages. At that moment, the graft contained numerous cells expressing the inducible isoform of NO-synthase (iNOS). Infiltration by ED1-positive macrophages was effective after three days. The beta1-integrin labeling decreased from that time-point to day 7 post-implantation, and vanished after 11 days. Although some OX8-positive cells were present around the graft as soon as 3 days after transplantation, cells expressing the T-cell receptor (TCR)-beta chain infiltrated the graft after 7 days and their number remained low. A strong, diffuse OX8-and ED1-positive immunoreactive material remained in the scar up to the third week. In striking contrast, PNEU grafts remained poorly infiltrated by OX42- or ED1-positive cells during the first two weeks. A massive infiltration by macrophages and TCRbeta-positive lymphocytes occurred after 3 weeks. Natural killer (NK) cells were more scarce. The inflammation territory enlarged, and blood vessels were overloaded with macrophages or lymphocytes. Nevertheless, the graft contained NeuN-positive nuclei and neurites harbouring the porcine neurofilament protein. Hence, rejection was not completed at this time-point. These results suggest that the rapid rejection of PAEC is mainly driven by macrophages and possibly PMN cells, unlike PNEU, whose rejection is delayed and also involves lymphocytes. Differences in immunogenicity of grafted cells and/or patterns of production of pro-inflammatory cytokines may account for these contrasted rejection kinetics.

Rat oligodendrocytes express the vitamin D(3) receptor and respond to 1,25-dihydroxyvitamin D(3)

The present study investigates the presence of vitamin D receptor (VDR) in cells of the rat oligodendrocyte (OL) lineage. VDR transcripts were detected by in situ hybridization in a fraction of rat OL in secondary cultures. The VDR protein was shown to be co-localized in cells that are also recognized by an anti-myelin basic protein (MBP) antibody. Likewise, in vivo, VDR-positive cells were found in the brain white matter, such as the internal capsule of the striatum or the corpus callosum but also in the spinal cord. At least part of these positive cells in vivo correspond to OL, since they were co-stained by an anti-carbonic anhydrase II antiserum. Northern blot analyses of the CG-4 OL cell line demonstrated that the VDR transcripts are already found in the O-2A precursors. There was a two-fold increase in the relative abundance of these transcripts in differentiated OL or in type-2 astrocytes. 1, 25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] increased the pool of transcripts encoding its own receptor, the VDR. The hormone also enhanced the abundance of the mRNA of the nerve growth factor (NGF) and of its low-affinity receptor, the p75(NTR) protein. By contrast, the hormone had no effect on the levels of MBP or proteolipid protein (PLP) mRNA. This finding suggests that unlike retinoic acid (RA) or thyroid hormone, 1,25-(OH)(2)D(3) has no regulatory action on the synthesis of myelin proteins.

Increased translation efficiency and antizyme-dependent stabilization of ornithine decarboxylase in amino acid-supplemented human colon adenocarcinoma cells, Caco-2

The mechanisms of the response of ornithine decarboxylase(ODC), the rate-limiting enzyme in polyamine biosynthesis, to amino acid supplementation were studied in the human colon adenocarcinoma cell line, Caco-2. Supplementation of serum-deprived, subconfluent Caco-2 cells with any one of a series of amino acids (10 mM) resultedin increased ODC activity, reaching a maximum of approx. 12.5-fold after approx. 4 h, over control cells either not supplemented or supplemented with iso-osmolar D-mannitol. Glycine, L-asparagine and L-serine, as well as their D-enantiomers, were the strongest effectors and acted in a concentration-dependent manner; millimolar concentrations of most of these amino acids being sufficient to significantly increase ODC activity. In contrast, supplementation with D-methionine, L-lysine, L-aspartate or L-glutamate had little or no effect on ODC activity, whereas supplemental L-methionine, L-arginine, L-ornithine or L-cysteine was inhibitory. Polyamine assays showed that the putrescine content of cells varied in accordance with the changes in ODC activity. Western-blot and Northern-blot analyses revealed specifically increased levels of ODC protein but not mRNA,respectively, in response to supplementation with an ODC-inducing amino acid. Suppression of the increase in cycloheximide-treated cellsconfirmed a requirement for protein synthesis. Pulse-labelling of cellswith [(35)S]methionine showed a 3-fold increase in thesynthesis of ODC protein after 4 h of supplementation with glycineor L-serine. Supplemental glycine also augmented, reversibly, the half-life of ODC by almost 4-fold and simultaneously decreased the activity of putrescine-induced free antizyme. These results suggest that translational, but not transcriptional, regulation of ODC takes part in ODC induction by amino acids in Caco-2 cells. However, it also appears to occur in concert with decreased enzyme in activation and/or degradation.

Intracerebral implantation of NGF-releasing biodegradable microspheres protects striatum against excitotoxic damage

Intrastriatal implantation of genetically modified cells synthesizing nerve growth factor (NGF) constitutes one way to obtain a long-term supply of this neurotrophic factor and a neuronal protection against an excitotoxic lesion. We have investigated if NGF-loaded poly(d,l-lactide-co-glycolide) microspheres could represent an alternative to cell transplantations. These microspheres can be implanted stereotaxically and locally release the protein in a controlled and sustained way. In order to test this paradigm, the NGF release kinetics were characterized in vitro using radiolabeled NGF, immunoenzymatic assay, and PC-12 cells bioassay and then in vivo after implantation in the intact rat striatum. These microspheres were thus implanted into the rat striatum 7 days prior to infusing quinolinic acid. Control animals were either not treated or implanted with blank microspheres. The extent of the lesion and the survival of ChAT-, NADPH-d-, and DARPP-32-containing neurons were analyzed. In vitro studies showed that microspheres allowed a sustained release of bioactive NGF for at least 1 month. Microspheres implanted in the intact striatum still contained NGF after 2.5 months and they were totally degraded after 3 months. After quinolinic acid infusion, the lesion size in the group treated with NGF-releasing microspheres was reduced by 40% when compared with the control groups. A marked neuronal sparing was noted, principally concerning the cholinergic interneurons, but also neuropeptide Y/somatostatin interneurons and GABAergic striatofuge neurons. These results indicate that implantation of biodegradable NGF-releasing microspheres can be used to protect neurons from a local excitotoxic lesion and that this strategy may ultimately prove to be relevant for the treatment of various neurological diseases.

1,25-dihydroxyvitamin D3 regulates the synthesis of gamma-glutamyl transpeptidase and glutathione levels in rat primary astrocytes

Astrocytes play a pivotal role in CNS detoxification pathways, where glutathione (GSH) is involved in the elimination of oxygen and nitrogen reactive species such as nitric oxide. We have previously demonstrated that the specific activity of gamma-glutamyl transpeptidase (gamma-GT), an enzyme of central significance in GSH metabolism, is regulated in vivo in astrocytes by 1,25-dihydroxyvitamin D3 (1,25-D3). The aim of the present work was to investigate, in primary cultures of newborn rat astrocytes, the effects of this hormone on gamma-GT synthesis and on GSH and nitrite levels after lipopolysaccharide (LPS) treatment. This study demonstrates that both gamma-GT gene expression and specific activity, induced by LPS, are potentiated by 1,25-D3. In contrast, 1,25-D3 does not regulate the expression of other enzymes involved in astrocyte detoxification processes, such as superoxide dismutase or GSH peroxidase. In parallel, 1,25-D3 enhanced intracellular GSH pools and significantly reduced nitrite production induced by LPS. Taken together, these results suggest that gamma-GT, GSH, and 1,25-D3 play a fundamental role in astrocyte detoxification pathways.

Antizyme-dependent and -independent mechanisms are responsible for increased spermidine transport in amino acid-restricted human cancer cells

Amino acid deprivation can inhibit tumour cell proliferation. Since polyamines are required for cell growth, we hypothesised that their regulatory pathways can respond to amino acid restriction. We report here that exposure of human colon adenocarcinoma Caco-2 cells to a medium restricted for a single amino acid, but not for D-glucose, activates spermidine transport. The increase was rapid and seemed transient with a maximum 4-6 hr after amino acid removal. Kinetics showed that the maximal velocity of transport was solely increased in L-methionine- or L-leucine-deprived cells, indicating increased number of transporters. The intracellular level of complex of ornithine decarboxylase (ODC) with antizyme, a negative regulator of polyamine transport, was decreased by 16-29% in amino acid-deprived cells. However, exposure to limited amounts of amino acid increased transport without altering the ODC-antizyme complex level. We propose that antizyme-independent mechanisms, sensitive to the amino acid concentration, also participate to the control of spermidine transport.

Delta9-tetrahydrocannabinol induces apoptosis in C6 glioma cells

delta9-Tetrahydrocannabinol (THC), the major active component of marijuana, induced apoptosis in C6.9 glioma cells, as determined by DNA fragmentation and loss of plasma membrane asymmetry. THC stimulated sphingomyelin hydrolysis in C6.9 glioma cells. THC and N-acetylsphingosine, a cell-permeable ceramide analog, induced apoptosis in several transformed neural cells but not in primary astrocytes or neurons. Although glioma C6.9 cells expressed the CBI cannabinoid receptor, neither THC-induced apoptosis nor THC-induced sphingomyelin breakdown were prevented by SR141716, a specific antagonist of that receptor. Results thus show that THC-induced apoptosis in glioma C6.9 cells may rely on a CBI receptor-independent stimulation of sphingomyelin breakdown.

1,25-Dihydroxyvitamin D3 regulates the expression of VDR and NGF gene in Schwann cells in vitro

The vitamin D receptor (VDR) is a nuclear receptor that mediates the effect of the active metabolite of vitamin D3, the 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3). To investigate the potential role of this hormone in the peripheral nervous system, we have studied the VDR expression in Schwann cells. The VDR mRNA was detected by Northern blot analysis in rat primary cultures of Schwann cells, and its levels were strongly increased in the presence of 1,25-(OH)2D3. Using the mouse Schwann cell line, MSC80, we showed that concentrations as low as 10(-10) M of hormone stimulated the expression of the VDR gene and strongly increased the amounts of activated VDR, capable of binding to the specific vitamin D responsive element (VDRE). We also found that 1,25-(OH)2D3 stimulated the expression of the nerve growth factor gene in MSC80. These data suggest a role for the hormone in the peripheral nervous system, possibly as a mediator active in trauma.

Selective sites for polyamine binding to rabbit intestinal brush-border membranes

The intestinal polyamine transporters have not yet been identified. Our aim was to characterize specific polyamine binding sites in rabbit intestinal brush-border membranes (IBBM) as a starting step for identification of polyamine transporters. This was investigated at 4 degrees and at low membrane concentration. Saturation isotherms for [3H]putrescine (PUT) binding indicated a single population of sites (puT) with a dissociation equilibrium constant Kd of 3.8 microM and a density of sites Bmax of 58 pmol/mg of protein. [3H]spermidine (SPD) binding also involved only one class of sites (spD), albeit with a lower affinity (Kd = 106 microM) and higher abundance (Bmax = 1240 pmol/mg of protein) than puT. On the contrary, [14C]spermine (SPM) bound two classes of sites (spM1 and spM2) differing in their affinity (Kd = 2.5 and 31.4 microM) and abundance (Bmax = 467 and 1617 pmol/mg of protein, respectively). Membrane association of SPM at 4 degrees was much faster than that of SPD and PUT, both of which proceeded at a similar rate. In contrast to PUT and SPD dissociation, SPM dissociation at 23 degrees did not follow a first-order reaction. Specifically bound [3H]PUT, unlike [3H]SPD and [14C]SPM, dissociated at 23 degrees independently of the addition of nonradioactive polyamine. Methylglyoxal-bis-(guanylhydrazone) was an extremely potent inhibitor of PUT binding (Ki = 3.2 +/- 1.5 nM), but as with PUT and cadaverine (CAD), it did not alter [3H]SPD and [14C]SPM binding substantially. The intestinal brush-border membrane may contain at least three sites specific for polyamine binding and exhibiting different ligand selectivity. Site puT might be associated with the transport system already described for intestinal uptake of PUT.

Vitamin D receptor stable transfection restores the susceptibility to 1,25-dihydroxyvitamin D3 cytotoxicity in a rat glioma resistant clone

Recently, 1,25-dihydroxyvitamin D3 (1,25-D3) and less hypercalcemic analogs were shown to exert a delayed cytotoxic effect on rat C6 glioma cells. 1,25-D3 induces in these cells a programmed cell death, accompanied by the induction of c-myc, p53 and gadd 45 genes. The involvement of the intracellular vitamin D receptor (VDR) remained to be determined. In this lethal process, we have investigated its role in a subclone of C6 cells, which was isolated on the basis of its resistance to 1,25-D3, and in which VDR expression was not detected either at the mRNA or protein levels. The stable transfection of a rat VDR cDNA into this clone restored its susceptibility to the cytotoxic effects of 1,25-D3. This phenomenon was accompanied by a dramatic upregulation of c-myc mRNA expression, as already described in a C6-sensitive clone. These results provide the first evidence that VDR expression, if not sufficient, is necessary to mediate 1,25-D3 cytotoxic effect in C6 glioma cells. Since VDR mRNA expression has been already reported in human brain tumors, our data imply that the identification of VDR expression could become a prerequisite in any strategy of glioma treatment with vitamin D analogs.

Low affinity NGF receptor expression in the central nervous system during experimental allergic encephalomyelitis

In the central nervous system (CNS), p75, or low-affinity nerve growth factor receptor (LNGFR), is assumed to play a critical role in mediating the effects of neurotrophins on neuronal survival. Recent studies have shown that nerve growth factor (NGF) can act also on immune cells through its binding to p75. Using immunohistochemistry, we have investigated the expression of the p75 receptor in the CNS during chronic relapsing experimental allergic encephalomyelitis (EAE) of the Lewis rat, an animal model of multiple sclerosis (MS). We report here a sequential expression of p75, first in Purkinje cells during the first attack, and secondly on both endothelial and perivascular cells in the latter stages of the disease. Moreover, starting from the second attack, p75 was also expressed on glial ensheathing cells, likely myelinating cells, located primarily in the dorsal roots. These data suggest that during EAE, LNGFR may play an important role in leukocyte-endothelial cell interactions and in the maintenance of Purkinje cells survival.

Epigenetic control of programmed cell death: inhibition by 5-azacytidine of 1,25-dihydroxyvitamin D3-induced programmed cell death in C6.9 glioma cells

In mammalian DNA cytosine methylation occurs specifically at CpG dinucleotide. Although the full array of function of DNA methylation is yet to be elucidated, it is well established that DNA methylation is an important mechanism involved in gene expression, DNA replication and cancer. Rat glioma C6.9 cells undergo programmed cell death (PCD) after treatment with 1,25-dihydroxyvitamin D3 (1,25-D3). Hence, these cells were used to study whether DNA methylation was involved in the control of PCD. We found that 1,25-D3-mediated PCD of C6.9 cells was suppressed by exposure of the cells to the DNA demethylating agents 5-azacytidine (5-AzaC) and 5-aza-2'-deoxycytidine. This effect remains detectable several cell divisions following removal of 5-AzaC and, therefore, involves DNA methylation as an epigenetic regulatory mechanism of PCD. Accordingly, internucleosomal fragmentation, a feature of apoptosis that is detected in 1,25-D3-treated cells, is no longer observable after treatment of these cells with 5-AzaC. However, 5-AzaC does not totally suppress the responsiveness of C6.9 cells to 1,25-D3 since the induction of the c-myc gene remains unaffected. These results suggest that a change in DNA methylation pattern could suppress 1,25-D3-mediated PCD through the expression of previously hypermethylated genes such as proto-oncogenes with death-repressor activity, endogenous virus sequences or even genes inducing change in the differentiated state of these cells.

Expression of inducible nitric oxide synthase during rat brain inflammation: regulation by 1,25-dihydroxyvitamin D3

This study, based on in situ hybridization and immunolabeling experiments, presents the time-course and cellular distribution of inducible NO synthase (iNOS) expression in a rat model of brain inflammation. Both intrahippocampal injection of lipopolysaccharide (LPS) or of buffer (stab lesion) induce an early, transient, and restricted expression of iNOS mRNA and immunoreactivity in the rat CNS. The topographic and phenotypic characteristics of iNOS-producing cells are distinct. After stab lesion, iNOS mRNAs, expressed at 5 h mainly in cells in the interventricular junction and in a few cells in brain parenchyma, were no more detectable from 15 h onwards, whereas the protein was faintly expressed in parenchymal cells at 15 h and 24 h. In contrast, after LPS injection, iNOS-mRNAs were detected from 5 to 24 h. iNOS-immunoreactivity was highly induced and sequentially observed first in choroid plexus and ependymal cells at 5 h, in monocytes and activated/reactive microglia at 15 h and 24 h, and finally in astrocytes at 72 h. In order to investigate potential regulatory effects of 1,25-dihydroxyvitamin D3 (1,25-D3) on iNOS expression, we have delivered this hormone with LPS or buffer into the rat hippocampus. 1,25-D3 significantly inhibits iNOS expression, at both the mRNA and immunoreactive protein levels, 15 h and 24 h after LPS injection, in the cells of the monocyte lineage. Moreover, 72 h after LPS injection, the addition of 1,25-D3 leads to a 6-fold increase in the number of macrophages around the lesion site, that correlates with a decrease in the proportion of apoptotic cells. Since 1,25-D3 can be produced by activated macrophages/microglia and since NO stimulates 1,25-D3 synthesis by macrophages, our results support the hypothesis that this hormone might be synthesized endogenously during CNS inflammatory reactions, thus explaining the transient and restricted iNOS expression observed after LPS intracerebral injection.

Differentially expressed genes in C6.9 glioma cells during vitamin D-induced cell death program

C6.9 rat glioma cells undergo a cell death program when exposed to 1, 25-dihydroxyvitamin D3 (1,25-D3). As a global analytical approach, we have investigated gene expression in C6.9 engaged in this cell death program using differential screening of a rat brain cDNA library with probes derived from control and 1,25-D3-treated cells. Using this methodology we report the isolation of 61 differentially expressed cDNAs. Forty-seven cDNAs correspond to genes already characterized in rat cells or tissues. Seven cDNAs are homologous to yeast, mouse or human genes and seven are not related to known genes. Some of the characterized genes have been reported to be differentially expressed following induction of programmed cell death. These include PMP22/gas3, MGP and beta-tubulin. For the first time, we also show a cell death program induced up-regulation of the c-myc associated primary response gene CRP, and of the proteasome RN3 subunit and TCTP/mortalin genes. Another interesting feature of this 1,25-D3 induced-cell death program is the down-regulated expression of transcripts for the microtubule motor dynein heavy chain/MAP 1C and of the calcium-binding S100beta protein. Finally 15 upregulated cDNAs encode ribosomal proteins suggesting a possible involvement of the translational apparatus in this cell program. Alternatively, these ribosomal protein genes could be up-regulated in response to altered rates of cellular metabolism, as has been demonstrated for most of the other isolated genes which encode proteins involved in metabolic pathways. Thus, this study presents to our knowledge the first characterization of genes which are differentially expressed during a cell death program induced by 1, 25-D3. Therefore, this data provides new information on the fundamental mechanisms which participate in the antineoplastic effects of 1,25-D3 and on the machinery of a cell death program in a glioma cell line.

Regulation of nerve growth factor secretion and mRNA expression by bacterial lipopolysaccharide in primary cultures of rat astrocytes

The present work was undertaken to study the effect of bacterial lipopolysaccharide (LPS), a potent activator of the host inflammatory response, on the synthesis of nerve growth factor (NGF) by newborn rat brain astrocytes. Treatment of primary rat astroglial cells cultured in chemically defined medium with LPS resulted in a dose-dependent accumulation of NGF mRNA, and an increased release of NGF protein in the cell medium. NGF mRNA levels were maximal after 24 hr of stimulation (8-fold increase), whereas extracellular NGF peaked after 72 hours of treatment (17-fold increase). This dramatic increase of extracellular NGF was abrogated if cells were treated with actinomycin D or cycloheximide, a fact which implies that the accumulation of extracellular NGF by LPS-treated cells requires DNA transcription and RNA translation. Stimulation of NGF synthesis and secretion was: (i) unaffected by treatment with the protein kinase C inhibitor bisindolylmaleimide, and (ii) prevented by forskolin and 3-isobutyl-1-methylxanthine, two agents which increase cAMP levels. Inhibition of LPS effect was also obtained with apigenin, a proposed inhibitor of the mitogen-activated protein kinase pathway. Results thus show that LPS stimulates NGF synthesis by astroglial cells through a mechanism that is independent of protein kinase C (PKC), antagonized by cAMP-elevating agents, and probably mediated by the mitogen-activated protein kinase cascade. The data raise the possibility that LPS exerts stimulatory effects on NGF synthesis that are independent of those elicited by astrocyte-derived inflammatory lymphokines such as IL-1beta, TNF alpha or TGF beta1.

1,25-Dihydroxyvitamin D3 inhibits the expression of inducible nitric oxide synthase in rat central nervous system during experimental allergic encephalomyelitis

The inducible form of nitric oxide synthase (iNOS) generates nitric oxide of which the excessive production is associated with central nervous system (CNS) inflammatory diseases. The investigation of iNOS expression during experimental allergic encephalomyelitis (EAE) of the Lewis rat demonstrated iNOS immunoreactivity and mRNA both during inflammatory bursts (days 12 and 23 post-immunization) and during the remission phase (day 18). iNOS expression was region-specific and expanded with time along a caudo-rostral axis, thus, correlating with the development of inflammatory infiltrates. Whereas cells of the monocyte/macrophage lineage continuously contributed to iNOS expression, astrocytes only expressed iNOS immunoreactivity or mRNA during the relapse (day 23). In order to investigate possible regulatory effects of 1,25-dihydroxyvitamin D3 (1,25-D3) on iNOS expression, rats were treated with the hormone after the beginning of clinical signs (days 11, 13, 19, 21 and 23 post-immunization), and areas of the CNS were examined at day 23. 1,25-D3 exerted a drastic inhibitory effect on iNOS expression, both at the protein and the mRNA levels. However, this effect was region-specific, and was most pronounced in the cerebellum and brainstem, but non-existent in cerebral cortex. iNOS down-regulation occurred in macrophages, activated microglia and astrocytes. The inhibition of iNOS expression in some CNS structures could account for the improvement of clinical signs observed in EAE-rats treated with 1,25-D3. Since 1,25-D3 can be synthesized by activated macrophages or microglia, our results support the hypothesis that this hormone might be implicated in the control of the CNS-specific immune responses. 1,25-D3 or its analogues could, thus, be of therapeutic value in the management of iNOS-associated diseases of the CNS.

Noradrenaline inhibits the programmed cell death induced by 1,25-dihydroxyvitamin D3 in glioma

The rat glioma cell line C6.9 has been recently reported to respond to 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) by the induction of a programmed cell death. Since, in vivo, glial cells are thought to be exposed to several neurotransmitters, we investigated the possibility of a neurotransmitter-mediated inhibition of this active cell death process. Noradrenaline and the beta-adrenoceptor agonist isoproterenol showed significant inhibition of the 1,25(OH)2D3-induced programmed cell death. The beta-adrenoceptor antagonist propanolol reversed this inhibition, while the alpha-adrenoceptor antagonist yohimbin was devoid of any effect. This suggests that the efficiency of antiproliferative vitamin D-related therapies could be influenced by endogenous levels of noradrenaline.

1,25-Dihydroxyvitamin D3 induces programmed cell death in a rat glioma cell line

1,25-Dihydroxyvitamin D3 (1,25(OH)2D3), a seco-steroid hormone with potential antitumoral activities, has been recently reported to exert cytotoxic effects on C6 glioma cells. However, the molecular mechanisms which trigger this cell death remain unknown. We show here that this 1,25(OH)2D3-induced cell death is dependent upon protein synthesis and is accompanied by the expression of c-myc, p53, and gadd45 genes. Two other genes, coding for interleukin-6 and vaso-endothelial growth factor, are also upregulated after addition of 1,25(OH)2D3. This programmed cell death can be suppressed when cells are treated with forskolin, a drug which increases intracellular cAMP concentration, or with genistein, an inhibitor of tyrosine protein kinases. However, in spite of the demonstration of fragmented DNA in 1,25(OH)2D3-treated cells, the C6.9 cells used in this study do not show the classical morphological features of apoptosis. These results provide the first evidence for the existence of a programmed cell death triggered by 1,25(OH)2D3 in glioma cells and may provide a basis for the development of new therapeutic strategies. In addition, these data also suggest that the treatment of C6.9 cells with 1,25(OH)2D3 may be a useful model to study the molecular mechanisms involved in the programmed cell death of a cell of glial origin.

Differential expression of M-CSF, LIF, and TNF-alpha genes in normal and malignant rat glial cells: regulation by lipopolysaccharide and vitamin D

The effect of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) on the expression of macrophage colony-stimulating factor (M-CSF), leukemia inhibitory factor (LIF), and tumor necrosis factor-alpha (TNF-alpha) genes in primary rat astrocytes and C6 glioma cells was examined. The results show that the hormone differentially regulates the cytokine mRNA in the two cell types. 1,25-(OH)2D3 augments M-CSF and LIF mRNA in C6 glioma cells, while lipopolysaccharide (LPS) has minimal effects. When LPS and 1,25-(OH)2D3 are used in combination, a strong synergistic effect upon the induction of M-CSF and LIF genes is observed. No TNF-alpha transcript has been detected in C6 glioma cells under any stimulus conditions used. In contrast, 1,25-(OH)2D3 has no pronounced effect on M-CSF, LIF, and TNF-alpha transcripts in primary astrocytes when used as a sole stimulus, while treatment with LPS strongly enhances the levels of the three cytokines. However, when 1,25-(OH)2D3 is used in combination with LPS, a partial reduction in LPS-induced levels of M-CSF and TNF-alpha mRNA is observed. The overall results indicate that genes coding for some inflammatory cytokines obey distinct regulatory mechanisms in C6 cells and in primary astrocytes. They also suggest that 1,25-(OH)2D3, by altering the response of astrocytes to an inflammatory stimulus, could participate in the regulation of the CNS immune response.

1,25-Dihydroxyvitamin D3 regulates gamma 1 transpeptidase activity in rat brain

gamma-Glutamyl transpeptidase (gamma-GT), primarily described as a kidney enzyme, is also expressed in several cell types of the central nervous system (CNS). It is involved in the glutathione cycle and in cysteine transport. Here we report that the specific activity of this enzyme is transiently increased in the rat brain, following a treatment with 1,25-dihydroxyvitamin D3 (1,25-D3), the active form of vitamin D. In vitro experiments showed that this positive regulatory effect does not affect endothelial cells of the brain microvessels, but does affect pericytes and parenchymal astrocytes. Changes in the specific activity of gamma-GT were not correlated with any important modification of brain amino acid concentrations. Since gamma-GT is though to participate in the scavenging of reactive oxygen species, these data suggest that 1,25-D3 could be an effector controlling detoxification processes in the brain.

1,25-Dihydroxyvitamin D3 regulates the expression of the low-affinity neurotrophin receptor

1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3) is known to regulate the expression of neurotrophins [45,46]. Here, we report that 1,25-(OH)2D3 does not influence the expression of truncated or full-length forms of trkB and trkC receptors mRNAs in primary cultures of astrocytes and in C6 glioma cells. In contrast, low concentrations of 1,25-(OH)2D3 increased low-affinity neurotrophin receptor (P75NTR) mRNA and protein levels in C6 glioma cells. Putative vitamin D responsive elements (VDRE) in the P75NTR promoter have been investigated by transfecting plasmids containing sequences from P75NTR promoter fused to a cat reporter gene. A region between -610 and -860 bp upstream from the translation start codon was found to respond to 1,25-(OH)2D3. Interestingly, 1,25-(OH)2D3 does not regulate P75NTR in primary cultures of astrocytes even at concentration as high as 10(-7) M. Since long-term treatment of 1,25-(OH)2D3 induces cell death in C6 glioma cells but not in primary astrocytes [41], the possible involvement of P75NTR in 1,25-(OH)2D3-induced cell death is discussed. Finally, in-vivo studies show that treatment of 15-day-old and adult rats with 1,25-(OH)2D3 leads to a decrease in the level of P75NTR mRNA in the spinal cord but does not influence its expression in dorsal root ganglion or sciatic nerve. These results suggest that 1,25-(OH)2D3 may have a role in the specific regulation of P75NTR in vivo.

1,25-Dihydroxyvitamin D3, an inducer of glial cell line-derived neurotrophic factor

Glial cell line-derived neurotrophic factor (GDNF) has significant therapeutic potentials, in particular for neurodegenerative disorders. To determine factors that would enhance GDNF expression, we analysed the effect of 1,25-(OH)2 D3 in C6 glioma cells. Treatment of C6 cells with 10(-7) M, 1,25-(OH)2 D3 for 48 h elicited an 18.5-fold increase in the level of GDNF mRNA. In addition, our results indicate that 1,25-(OH)2 D3 is effective at concentrations as low as 10(-10) M and that retinoic acid has additive effects. These data indicate that 1,25-(OH)2 D3 is a potent inducer of GDNF expression and suggest that 1,25-(OH)2 D3 may contribute to the regulation of GDNF in vivo.