An investigation of noradrenaline uptake and release by the CATH.a cell line

Downregulated Kv4.3 expression in the RVLM as a potential mechanism for sympathoexcitation in rats with chronic heart failure

Elevated central angiotensin II (ANG II) plays a critical role in the sympathoexcitation of chronic heart failure (CHF) by stimulating upregulated ANG II type 1 receptors (AT(1)R) in the rostral ventrolateral medulla (RVLM). However, the link between enhanced ANG II signaling and alterations in the electrophysiological characteristics of neurons in the RVLM remains unclear. In the present experiments, we screened for potentially altered genes in the medulla of rats with CHF that are directly related to neuronal membrane conductance using the Rat Genome 230 2.0 Array GeneChip. We found that CHF rats exhibited a 2.1-fold reduction in Kv4.3 gene expression, one of the main voltage-gated K(+) channels, in the medulla. Real-time RT-PCR and Western blot analysis confirmed the downregulation of Kv4.3 in the RVLM of CHF rats. In intact animals, we found that microinjection of the voltage-gated potassium channel blocker, 4-aminopyridine, into the RVLM evoked a sympathoexcitation and hypertension in both normal and CHF rats. CHF rats exhibited smaller responses to 4-aminopyridine than did normal rats. Finally, we used a neuronal cell line (CATH.a neurons) to explore the effect of ANG II on Kv4.3 expression and function. We found that ANG II treatment significantly downregulated mRNA and protein expression of Kv4.3 and decreased the A-type K(+) current. Employing this cell line, we also found that the ANG II-induced inhibition of Kv4.3 mRNA expression was attenuated by the superoxide scavenger Tempol and the p38 MAPK inhibitor SB-203580. The effects of ANG II were abolished by the AT(1)R antagonist losartan. We conclude that the sympathoexcitation observed in the CHF state may be due, in part, to an ANG II-induced downregulation of Kv4.3 expression and subsequent decrease in K(+) current, thereby increasing the excitability of neurons in the RVLM. The ANG II-induced inhibition of Kv4.3 mRNA expression was mediated by ANG II-AT(1)R-ROS-p38 MAPK signaling.

Expression of metallothionein-III and cell death in differentiated catecholaminergic neuronal cells

Metallothionein (MT)-III, an isomer of metallothionein, is also known to be a growth inhibitory factor. MT-III has been reported to decrease the number of surviving neuronal cells in culture medium containing brain extract. Using differentiated catecholaminergic neuronal CATH.a cells treated with dibutyryl cyclic AMP, we examined MT-III expression and the effect of mouse forebrain extract on cell viability. Increase in MT-III expression was revealed in the differentiated cells. Moreover, treatment with mouse forebrain extract induced apoptotic cell death in differentiated CATH.a cells, accompanied by decreases in both MT-III and a neuronal differentiation marker, growth-associated protein-43, expression in surviving cells. These results imply that MT-III expression during the developmental period may be associated with the regulation of normal neural differentiation.

Differentiation-induced alterations in cyclic AMP signaling in the Cath.a differentiated (CAD) neuronal cell line

Regulation of intracellular cyclic AMP is critical to the modulation of many cellular activities, including cellular differentiation. Moreover, morphological differentiation has been linked to subsequent alterations in the cAMP signaling pathway in various cellular models. The current study was designed to explore the mechanism for the previously reported enhancement of adenylate cyclase activity in Cath.a differentiated cells following differentiation. Differentiation of Cath.a differentiated cells stably expressing the D2L dopamine receptor markedly potentiated both forskolin- and A2-adenosine receptor-stimulated cAMP accumulation. This enhancement was accompanied by a twofold increase in adenylate cyclase 6 (AC6) expression and a dramatic loss in the expression of AC9. The ability of Ca2+ to inhibit drug-stimulated cAMP accumulation was enhanced following differentiation, as was D2L dopamine receptor-mediated inhibition of Galphas-stimulated cAMP accumulation. Differentiation altered basal and drug-stimulated phosphorylation of the cAMP-response element-binding protein, which was independent of changes in protein kinase A expression. The current data suggest that differentiation of the neuronal cell model, Cath.a differentiated cells induces significant alterations in the expression and function of both the proximal and distal portions of the cAMP signaling pathway and may impact cellular operations dependent upon this pathway.

Comparison of the effect of cyclic AMP on the content and release of dopamine and 1-methyl-4-phenylpyridinium (Mpp+) in PC12 cells

1 The aim of this work was to investigate the effect of acute and chronic exposure of rat pheochromocytoma (PC12) cells to elevated cAMP, using forskolin, dibutyryl-cAMP (db-cAMP) or isobutylmethylxanthine (IBMX), on endogenous dopamine content and release and on [3H]-1-methyl-4-phenylpyridinium ([3H]-MPP+) uptake and release, under basal conditions and under KCl-stimulation. 2 Cultured PC12 cells synthetized and accumulated large amounts of dopamine, but not noradrenaline or adrenaline. The release of dopamine by the cells was markedly increased in response to 50 mM KCl. 3 Acute and chronic treatment of the cells with forskolin (30 microM), but not IBMX (100 microM), slightly increased the spontaneous release of dopamine and significantly decreased the release induced by 50 mM KCl. 4 Chronic treatment of the cells with forskolin (30 microM), but not IBMX (100 microM), markedly decreased the cellular content of dopamine. 5 Cultured PC12 cells removed and accumulated [3H]-MPP+, which, similarly to dopamine, was released by KCl. 6 Acute treatment of the cells with forskolin (30 microM) or db-cAMP (2.5 mM), but not IBMX (100 microM), slightly increased the spontaneous release, but did not affect KCl-induced release of [3H]-MPP+. On the other hand, chronic treatment of the cells with forskolin produced, on [3H]-MPP+, similar effects to those obtained for dopamine. 7 Acute and chronic treatment of the cells with reserpine (50 nM) produced similar results to those obtained with forskolin on either dopamine or [3H]-MPP+ handling. 8 In conclusion, cAMP, similarly to reserpine, increases the spontaneous release and decreases the KCl-induced release of [3H]-MPP+ and dopamine. This suggests that cAMP impairs the vesicular monoamine transporter.

Effects of short- and long-term exposure to c-AMP and c-GMP on the noradrenaline transporter

The effects of short- and long-term exposure of cells to elevated cyclic adenosine monophosphate (c-AMP), using dibutyryl-c-AMP, 8-bromo-c-AMP, cholera toxin or forskolin, or cyclic guanosine monophosphate (c-GMP), using dibutyryl-c-GMP or 8-bromo-c-GMP, on the activity and expression of the noradrenaline transporter (NAT) were examined. Short- or long-term c-GMP elevation had no effects on (3)H-noradrenaline uptake by rat PC12 phaeochromocytoma cells or human SK-N-SH-SY5Y neuroblastoma cells. Short-term c-AMP elevation (for 17 min experiment duration) caused a decrease in (3)H-noradrenaline uptake by PC12 cells, but had no effects on SK-N-SH-SY5Y cells or COS-7 cells transfected with human or rat NAT cDNA. c-AMP did not affect (3)H-nisoxetine binding to PC12 cells. Long-term (24 h) exposure to elevated c-AMP levels caused a decrease in (3)H-noradrenaline uptake and NAT mRNA in PC12 cells, but had no effects on SK-N-SH-SY5Y cells and caused a small increase in (3)H-noradrenaline uptake in COS-7 cells heterologously expressing rat or human NAT. Hence, c-AMP, but not c-GMP, causes a cell type-dependent reduction in NAT activity after short-term exposure and a reduction in NAT expression after long-term exposure.

Inhibition of tyrosinase reduces cell viability in catecholaminergic neuronal cells

The biosynthesis of dopamine (DA) in catecholaminergic neurons is regulated by tyrosine hydroxylase, which converts tyrosine into 3, 4-dihydroxyphenylalanine (L-DOPA). In melanocytes, tyrosinase catalyzes both the hydroxylation of tyrosine and the consequent oxidation of L-DOPA to form melanin. Although it has been demonstrated that tyrosinase is also expressed in the brain, the physiological role of tyrosinase in the brain is still obscure. In this study, to investigate the role of tyrosinase in catecholaminergic neuronal cells, we examined the effects of tyrosinase inhibition on the viability of CATH.a and SH-SY5Y cells using tyrosinase inhibitors-specifically, phenylthiourea (PTU) and 5-hydroxyindole (5-HI)-and the transfection of antisense tyrosinase cDNA. Both inhibitors significantly reduced the cell viability of CATH.a cells in a dose-dependent manner. PTU also specifically enhanced DA-induced cell death, but 5-HI did not. This discrepancy in cell death is probably due to the inhibitors' different mechanism of action: 5-HI inhibits the hydroxylation of tyrosine as a competitor for the substrate to induce cell death that may be due to depletion of DA, whereas PTU mainly inhibits the enzymatic oxidation of L-DOPA and DA rather than tyrosine hydroxylation to increase consequently autooxidation of DA. Indeed, the intracellular DA content in CATH.a cells was enhanced by PTU exposure. In contrast, PTU showed no enhancing effects on DA-induced cell death of SH-SY5Y cells, which express little tyrosinase. Furthermore, transfection with antisense tyrosinase cDNA into CATH.a cells dramatically reduced cell viability and significantly enhanced DA-induced cell death. These results suggest that tyrosinase controls the intracellular DA content by biosynthesis or enzymatic oxidation of DA, and the dysfunction of this activity induces cell death by elevation of intracellular DA level and consequent gradual autooxidation of DA to generate reactive oxygen species.