Association between the increase of cAMP content and the trans-synaptic induction of tyrosine hydroxylase in rat adrenal medulla. Studies with dexamethasone and reserpine

Does the developmental neurotoxicity of chlorpyrifos involve glial targets? Macromolecule synthesis, adenylyl cyclase signaling, nuclear transcription factors, and formation of reactive oxygen in C6 glioma cells

The widespread use of chlorpyrifos (CPF) has raised major concerns about its potential to cause fetal or neonatal neurobehavioral damage, even at doses that do not evoke acute toxicity. CPF has been shown to inhibit replication of brain cells, to elicit alterations in neurotrophic signaling governing cell differentiation and apoptosis, and to evoke oxidative stress. However, the specific cell types targeted by CPF have not been clarified, an issue of vital importance in establishing the boundaries of the critical period in which the developing brain is vulnerable. In the current study, we evaluated the effects of CPF on C6 glioma cells, a well-established glial model. In undifferentiated C6 cells, CPF inhibited DNA synthesis in a concentration-dependent manner, with greater potency than had been seen previously with neuronal cell lines. Just as found after in vivo CPF treatment or with neuronal cell lines, the effects on cell replication were independent of cholinergic stimulation, as cholinergic antagonists did not block CPF-induced inhibition. CPF interfered with cell signaling mediated through adenylyl cyclase at the level of G-protein function; the effects again were greater in undifferentiated C6 cells but were still detectable in differentiating cells. In contrast, differentiation enhanced the ability of CPF to elicit the formation of reactive oxygen species and to evoke deficits in Sp1, a nuclear transcription factor essential for differentiation. These results indicate that glial-type cells are targeted by CPF through the same multiple mechanisms that have been demonstrated for the effects of CPF on brain development in vivo. Because glial development continues long after the conclusion of neurogenesis, and given that CPF targets events in both glial cell replication and the later stages of differentiation, the vulnerable period for developmental neurotoxicity of CPF is likely to extend well into childhood.

Inducible cAMP early repressor can modulate tyrosine hydroxylase gene expression after stimulation of cAMP synthesis

Members of the CREB/CREM/ATF family of transcription factors either enhance or repress transcription after binding to the cAMP response elements (CREs) of numerous genes. The rat gene for tyrosine hydroxylase (TH) bears a canonical CRE, at base pairs -38 through -45 from the transcription initiation site, that is essential for basal and cAMP-stimulated transcription (Kim, K.-S., Lee, M. K., Carroll, J., and Joh, T. H. (1993) J. Biol. Chem. 268, 15689-15695; Lazaroff, M., Patankar, S., Yoon, S. O., and Chikaraishi, D. M. (1995) J. Biol. Chem. 270, 21579-21589). The current study identifies CRE-binding proteins induced in pharmacological paradigms characterized by TH activation. PC12- and rat adrenal gland-derived nuclear proteins retarded a TH-CRE oligonucleotide in gel mobility shift assays with virtually identical patterns. These differed substantially from patterns exhibited by extracts from locus ceruleus or from neuroblastoma (SK-N-BE()C) and locus ceruleus-derived (CATH.a) cell lines. Forskolin stimulation of PC12 cells and reserpine treatment of rats increased, in nuclear extracts derived from cells and adrenal glands, respectively, the amount of a fast moving CRE/protein complex that was supershifted by an anti-CREM antibody. Subsequent Western, Northern, and polymerase chain reaction analyses indicated that a specific member of the CREM family, the inducible cAMP early repressor (ICER), was strongly induced in both systems. Cotransfection of PC12 cells with TH2400CAT plasmid and the expression vector pCMV-ICER-Ib demonstrated that ICER efficiently represses the transcriptional activity of the TH gene promoter. In addition, PKA-stimulated transcriptional activity of the promoter was effectively suppressed by ICER. These results suggest that ICER can modulate cAMP-stimulated transcription of the TH gene and provide a model accounting for rapid reversal of increased TH transcription following elevations in cAMP.

Regulation of guanosine triphosphate cyclohydrolase and tetrahydrobiopterin levels and the role of the cofactor in tyrosine hydroxylation in primary cultures of adrenomedullary chromaffin cells

Selective modification of the tetrahydrobiopterin levels in cultured chromaffin cells were followed by changes in the rate of tyrosine hydroxylation. Addition of sepiapterin, an intermediate on the salvage pathway for tetrahydrobiopterin synthesis, rapidly increased intracellular levels of tetrahydrobiopterin and elevated the rate of tyrosine hydroxylation in the intact cell. Tyrosine hydroxylation was also enhanced when tetrahydrobiopterin was directly added to the incubation medium of intact cells. When the cultured chromaffin cells were treated for 72 h with N-acetylserotonin, an inhibitor of sepiapterin reductase, tetrahydrobiopterin content and the rate of tyrosine hydroxylation were decreased. Addition of sepiapterin or N-acetylserotonin had no consistent effect on total extractable tyrosine hydroxylase activity or on catecholamine content in the cultured chromaffin cells. Three-day treatment of chromaffin cell cultures with compounds that increase levels of cyclic AMP (forskolin, cholera toxin, theophylline, dibutyryl- and 8-bromo cyclic AMP) increased total extractable tyrosine hydroxylase activity and GTP-cyclohydrolase, the rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin. Tetrahydrobiopterin levels and intact cell tyrosine hydroxylation were markedly increased after 8-bromo cyclic AMP. The increase in GTP-cyclohydrolase and tetrahydrobiopterin induced by 8-bromo cyclic AMP was blocked by the protein synthesis inhibitor cycloheximide. Agents that deplete cellular catecholamines (reserpine, tetrabenazine, and brocresine) increased both total tyrosine hydroxylase and GTP-cyclohydrolase activities, although treating the cultures with reserpine or tetrabenazine resulted in no change in cellular levels of cyclic AMP. Brocresine and tetrabenazine increased tetrahydrobiopterin levels, but the addition of reserpine to the cultures decreased catecholamine and tetrahydrobiopterin content and resulted in a decreased rate of intact cell tyrosine hydroxylation in spite of the increased activity of the total extractable enzyme. These data indicate that in cultured chromaffin cells GTP-cyclohydrolase activity like tyrosine hydroxylase activity is regulated by both cyclic AMP-dependent and cyclic AMP-independent mechanisms and that the intracellular level of tetrahydrobiopterin is one of the many factors that control the rate of tyrosine hydroxylation.

Alternative modes of enkephalin biosynthesis regulation by reserpine and cyclic AMP in cultured chromaffin cells

Exposure of bovine chromaffin cells in primary culture to 5 microM reserpine or 25 microM forskolin results in an increase in enkephalin peptide levels within 24-48 hr; 25 microM forskolin (or cholera toxin at 50 micrograms/ml) causes a 1.5- to 2-fold increase in enkephalin peptide levels, which is maximal after 48 hr of exposure and is totally blocked by addition of cycloheximide (0.5 microgram/ml). Reserpine (5 microM) elicits a 1.5- to 2-fold increase in enkephalin peptide levels within 24 hr, which is only partially blocked by cycloheximide. Chromatographic analysis of cellular extracts shows that forskolin increases levels of both [Met]enkephalin pentapeptide and high molecular weight enkephalin-containing peptides, while reserpine causes an increase in [Met]enkephalin pentapeptide and a concomitant decrease in high molecular weight enkephalin-containing peptides, suggesting enhanced conversion of enkephalin precursor(s) to the mature polypeptide hormone. Measurement of preproenkephalin messenger RNA (mRNAenk) by RNA blot hybridization with a cDNA probe for mRNAenk reveals that forskolin and cholera toxin cause a relatively rapid (less than 17 hr) 3- to 5-fold increase in mRNAenk, while exposure to reserpine elicits a gradual decrease in enkephalin mRNA (a 50%-80% decline) beginning within 24 hr and continuing over a 72-hr period. These results suggest that forskolin and reserpine differentially regulate enkephalin biosynthesis in cultured chromaffin cells, the former by increasing, presumably via a cAMP-dependent mechanism, cellular mRNA coding for preproenkephalin and the latter by a post-translational increase in proenkephalin processing.

Effect of carbachol and 56 mm-potassium chloride on the cyclic AMP-mediated induction of tyrosine hydroxylase in neuroblastoma cells in culture

Tyrosine hydroxylase (TH) activity is increased two- to threefold in neuroblastoma cell line NBP2 maintained in culture for 48 h in the presence of either the inhibitor of cyclic AMP-phosphodiesterase (PDE), 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (RO 20-1724), or the activator of adenylate cyclase, prostaglandin E1 (PGE1). Cyclic AMP levels are elevated 70-80% and 30-40% throughout the 48-h treatment with RO 20-1724 and PGE1, respectively. Carbachol does not affect either basal TH activity or cyclic AMP levels in the cells. However, the cholinergic agonist delays the induction of TH elicited by either RO 20-1724 or PGE1. This delay is prevented by atropine. The elevation in cyclic AMP levels elicited by either RO 20-1724 or PGE1 is blocked for 1 h or 15 min, respectively, after treatment with carbachol. Cyclic AMP levels then begin to rise until they reach those levels observed in the presence of RO 20-1724 or PGE1 alone by 12 h or 1 h of treatment, respectively. Time course studies demonstrate that this transient inhibition of the elevation of cyclic AMP is associated with a 48-h delay in the induction of TH elicited by either RO 20-1724 or PGE1. In contrast, the induction elicited by 8-bromo cyclic AMP is unaffected by carbachol. A depolarizing concentration (56 mM) of KCl produces a 24-h delay in the induction of TH elicited by RO 20-1724, without affecting the concomitant elevation of cyclic AMP produced by the PDE inhibitor. Furthermore, 56 mM-KCl inhibits the induction of TH elicited by 8-bromo cyclic AMP. It thus appears that carbachol delays the induction of TH by transiently inhibiting the elevation of cyclic AMP, whereas potassium depolarization delays the induction of TH by inhibiting a process with a site of action that is distal to the elevation of cyclic AMP.