Pertussis toxin provides evidence for two independent signalling pathways leading to the activation of the nerve growth factor gene

Interleukin 1alpha (IL-1alpha) induced activation of p38 mitogen-activated protein kinase inhibits glucocorticoid receptor function

Previous studies have demonstrated that interleukinalpha (IL-1alpha) inhibits glucocorticoid receptor (GR) nuclear translocation and dexamethasone (Dex)-induced gene transcription. Given that IL-1alpha is a potent activator of the p38 mitogen-activated protein kinase (MAPK) signal transduction pathway and p38 MAPK has been associated with reduced GR function, we examined the role of p38 MAPK in IL-1alpha-mediated inhibition of GR function in mouse fibroblast cells stably transfected with a GR-mediated reporter gene construct (LMCAT cells). Treatment of LMCAT cells with IL-1alpha (1000 U/ml) for 24 h inhibited Dex (50 nM)-induced GRE-CAT activity by approximately 35%. When cells were cotreated for 24 h with IL-1alpha plus SB-203580 (0.5-1 microM), a selective p38 inhibitor, IL-1alpha's inhibitory effect on GR function as determined by Dex-induced GRE-CAT activity was reversed. Using gel mobility shift assay, SB-203580 was also found to reverse IL-1alpha inhibition of GR-GRE binding. Further confirming the role of p38 pathways, pretreatment of LMCAT cells with antisense oligonucleotides targeted to p38 MAPK completely abrogated IL-1alpha inhibition of Dex-induced GRE-CAT activity. Taken together, these results demonstrate that activation of p38 MAPK pathways are involved in IL-1alpha-mediated inhibition of GR function. In addition, these findings extend the intracellular targets of p38 to include the GR and indicate that p38 inhibitors may have special utility in immunologic and/or neuropsychiatric disorders associated with impaired GR-mediated feedback inhibition.

The G-protein inhibitor, pertussis toxin, inhibits the secretion of brain-derived neurotrophic factor

Secretion of neurotrophins is critical for the delivery of neurotrophic support. Brain-derived neurotrophic factor is targeted to a regulated secretory pathway in neurons as well as the neurosecretory AtT-20 cells. Here, we show that pertussis toxin, which inactivates Gi and Go G proteins, inhibits up to 50% of the regulated release of brain derived neurotrophic factor by AtT-20 cells. To determine whether pertussis toxin-sensitive G proteins may regulate brain-derived neurotrophic factor release in vivo, the effect of intraocular pertussis toxin was assessed on the isthmo-optic nucleus in the developing chick visual system. The isthmo-optic nucleus projects axons from the midbrain to innervate retinal amacrine cells and depends on target-derived brain-derived neurotrophic factor between embryonic days 13 and 17 (E13-17). During this period approximately 50% of isthmo-optic neurons are eliminated by programmed cell death. Intraocular pertussis toxin administered at E13 increased cell death of isthmo-optic neurons by 42%, whereas injections at E19 had no effect. Co-injection of brain-derived neurotrophic factor with pertussis toxin rescued approximately 50% of isthmo-optic neurons from enhanced cell death, although overall retinal brain derived neurotrophic factor protein levels were unaffected by pertussis toxin. Retrograde transport of exogenous 125I-labeled brain derived neurotrophic factor from the retina to the midbrain was increased by co-administration of pertussis toxin, possibly owing to diminished competition from endogenously released brain-derived neurotrophic factors for the receptors that mediate retrograde axonal transport. These data suggest that the release of a major fraction of brain-derived neurotrophic factor in the secretory pathway in vitro and in vivo is regulated by the activity of pertussis toxin-sensitive G proteins.

Pharmacological modulation of nerve growth factor synthesis: a mechanistic comparison of vitamin D receptor and beta(2)-adrenoceptor agonists

Increasing nerve growth factor (NGF) in the PNS is a rational strategy for treating certain neurodegenerative disorders. The present studies were undertaken to compare two compounds, a vitamin D(3) analogue (CB1093) with minimal calcaemic effects, and clenbuterol, a long-acting beta(2)-adrenoceptor agonist, both of which induce NGF synthesis in vivo. Clenbuterol caused significant increases in both NGF mRNA and protein in 3T3 cells; with maxima at 10 nM and at 8-12 h exposure. Effects of clenbuterol on NGF mRNA were antagonized by propranolol. Mobility shift assays on whole cell extracts showed that clenbuterol increased AP1 binding in 3T3 cells prior to increasing NGF synthesis. Clenbuterol was without effect on NGF mRNA levels in L929 cells, whereas CB1093 caused significant increases in both NGF mRNA and protein levels in both 3T3 and L929 cells. Stimulation was almost maximal at 24 h exposure and was sustained for at least 72 h. The magnitude of the increase was much greater in L929 (700% increase) than in 3T3 cells (80%). Binding to the vitamin D nuclear receptor (VDR), which acts as a transcription factor itself, was increased as early as 30 min after exposure to of CB1093 and maintained up to 24 h. Increased VDR binding preceded increased NGF mRNA. A 150% increase in AP-1 binding was also evident. This study demonstrates that CB1093 and clenbuterol stimulate NGF levels in vitro and that AP-1 binding could be a commonality between the mechanism of NGF induction of these two compounds.

Expression of the nerve growth factor gene is controlled by the microtubule network

Colchicine, nocodazol, and vinblastine, three microtubule-disrupting drugs, were shown to increase the levels of both nerve growth factor (NGF) mRNA and cell-secreted NGF protein in L929 cells, with levels of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) or amyloid precursor protein (APP) mRNAs remaining unaffected. Northern blot analysis demonstrated that colchicine also increased NGF mRNA levels in rat primary astrocytes and mouse skin fibroblasts. The specificity of the effects observed was assessed by the fact that the microtubule-stabilizing agent Taxotere, a semisynthetic compound structurally related to taxol, suppressed the effects of colchicine, whereas lumicolchicine, a colchicine derivative that has no action on the microtubule network, had no influence on NGF expression. Likewise, the disruption of the microfilament network by cytochalasin B did not increase NGF mRNA levels in L929 cells. Furthermore, the increase in NGF gene expression observed following microtubule disruption depended on a cascade of events involving at least one protein kinase, which is not down-regulated by phorbol ester, and on a pertussis toxin sensitive step. These results support the concept that tubulin and/or the microtubule cytoskeleton play an active role in the regulation of the NGF gene.

Interactions between second messenger pathways influence NGF synthesis in mouse primary astrocytes

Primary mouse brain astrocytes were stimulated with phorbol 12-myristate 13-acetate (PMA), serum, forskolin and ionophore A23187, in order to investigate the effect of distinct signalling pathways on the expression of the nerve growth factor (NGF) gene and of proto-oncogenes encoding transcription factors of the Fos and Jun families. PMA, and to a lesser extent serum, induced a marked accumulation of NGF transcripts, in agreement with published observations [Brain Res., 570 (1992) 316-322]. The effect of A23187 was less pronounced and that of forskolin barely detectable. No relationship was observed between the expression of NGF gene and that of c-fos, fos-B, fra-1, jun-B proto-oncogenes. In contrast, changes in the levels of NGF transcripts were associated with corresponding modifications of the levels of c-jun transcripts, a fact which suggests that the c-Jun protein exerts a regulatory role on the expression of the NGF gene. In these cells, however, the regulation of NGF synthesis appears complex, since a pretreatment with forskolin or ionophore A23187 interfered with the promoting effect elicited by PMA or serum in inducing an early decline of the levels of NGF transcripts. This phenomenon was accompanied by a corresponding decrease in the amounts of cell-secreted NGF in cells treated with forskolin and PMA. A23187 had a much more striking effect on the production of mature NGF since this compound maintained the level of cell-secreted NGF to basal values, irrespective of the presence of PMA. A similar inhibitory effect was observed with thapsigargin, another compound able to increase the cytosolic concentration of calcium.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of glioma cell death by 1,25(OH)2 vitamin D3: towards an endocrine therapy of brain tumors?

The secosteroid 1,25-dihydroxyvitamin D3 (1,25 (OH)2D3) is the major biologically active metabolite of vitamin D. Antitumor activity of this hormone has been observed on several cell lines and on breast cancer in vivo. The purpose of this in vitro study was to determine the possible effect of 1,25(OH)2D3 on glioma cells. Two glioma cell lines from rat (C6) or human (GHD) origin were cultured in the presence of 1,25(OH)2D3. The sensitivity of these cells to 1,25 (OH)2D3 was assessed with a colorimetric MTT assay. A cytotoxic effect of 1,25(OH)2D3 was detected at concentrations around 10(-8) M. A lag period of 3 days was required between the onset of the treatment and the observation of the effects. However, the continuous presence of 1,25(OH)2D3 is not required since cell death occurred even when C6 cells were challenged for 24 hr with 1,25(OH)2D3 and then cultured in the absence of the hormone. In addition, 1,25(OH)2D3 regulates the expression of its own receptors in C6 glioma. These results provide to our knowledge the first evidence for a cytotoxic effect of 1,25(OH)2D3 on rat and human glioma cells and could offer both an experimental model to study a programmed cell death in a brain-derived cell line and a new strategy for the inhibition of glioma growth in vivo.

Complex interactions among second messenger pathways, steroid hormones, and protooncogenes of the Fos and Jun families converge in the regulation of the nerve growth factor gene

Regulation of the expression of the nerve growth factor (NGF) gene has been reported previously to be mediated via the protooncogene c-fos. Activation of the protein kinase C pathway and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] has also been reported to increase the pool of NGF transcripts in L929 fibroblasts. Here we show that activation of the cyclic AMP second messenger pathway antagonized the effect of phorbol 12-myristate 13-acetate (PMA) or serum on NGF synthesis, whereas it enhanced that of 1,25(OH)2D3. A positive effect was also observed when serum, PMA, and 1,25(OH)2D3 were added together, but dexamethasone reduced this enhancement. There was no close correlation between the increase in c-fos mRNA and that in NGF mRNA, suggesting that expression of the c-fos protooncogene is not necessarily followed by induction of the NGF gene. Rather, these two genes are simultaneously, and not sequentially, induced after forskolin treatment. It appears that regulation of the NGF gene depends on a repertoire of multiple regulatory AP-1 complexes arising from activation of the second messenger pathways. This suggests that NGF gene expression is under the control of a complex interplay among second messenger pathways, protooncogenes, and steroid hormones such as 1,25(OH)2D3 and glucocorticoids.