Induction of dopamine biosynthesis by l-DOPA in PC12 cells: implications of L-DOPA influx and cyclic AMP

Determination of intracellular dopamine by liquid chromatography-fluorescence detection with post-column derivatization using the König reaction

Dopamine is an important neurotransmitter, and the disruption of dopaminergic homeostasis causes various neurological diseases such as Parkinson's disease. Analysis of intracellular dopamine levels is important to understand the pathology of neurological diseases. We have developed a new method for the fluorometric detection of dopamine by adopting the König reaction, which is commonly used for the detection of cyanide, thiocyanate, and selenocyanate, and demonstrated that it can be applied to the determination of intracellular dopamine levels. The present method only requires a conventional LC system with isocratic elution and post-column derivatization and is simple to perform. The LOD, LOQ, and linearity range were 10.8 nM, 32.8 nM, and 0.05-10 μM, respectively, with accuracies of 101.8-106.3 % and precisions within 5 %, which are sufficient for the quantification of intracellular dopamine. We also determined dopamine levels in PC12 cells and found that the levels increased and decreased when the cells were exposed to L-dopa and cyanide, respectively, possibly because of the conversion of L-dopa into dopamine and the depletion of intracellular dopamine by exposing cells to cyanide, respectively. These results suggest the applicability of the present method, and that this new use of the König reaction offers a reliable and useful means of quantifying intracellular dopamine.

Development of Stilbenoid and Chalconoid Analogues as Potent Tyrosinase Modulators and Antioxidant Agents

A number of stilbenoid and chalconoid derivatives were prepared by straightforward methods, and their ability to modulate tyrosinase activity and to scavenge free radicals were evaluated in vitro. The cell-free in vitro evaluation revealed two diarylpropanes, 24 and 25, as potent tyrosinase inhibitors, whereas diarylpropenoic acids seemed to enhance the enzymatic activity. An in silico evaluation of the binding affinity of the selected compounds with the crystal structure of tyrosinase was also conducted in order to obtain better insight into the mechanism. Representative synthetic compounds with inhibitory and activating properties were further evaluated in melanoma cell lines B16F1 and B16F10 for their ability to moderate tyrosinase activity and affect melanin production. Dihydrostilbene analogues I and II, exhibited a stronger anti-melanogenic effect than kojic acid through the inhibition of cellular tyrosinase activity and melanin formation, while diarylpropanoic acid 44 proved to be a potent melanogenic factor, inducing cellular tyrosinase activity and melanin formation. Moreover, the antioxidant evaluation disclosed two analogues (29 and 11) with significant free-radical-scavenging activity (12.4 and 20.3 μM), which were 10- and 6-fold more potent than ascorbic acid (122.1 μΜ), respectively.

Localization and Absolute Quantification of Dopamine in Discrete Intravesicular Compartments Using NanoSIMS Imaging

The absolute concentration and the compartmentalization of analytes in cells and organelles are crucial parameters in the development of drugs and drug delivery systems, as well as in the fundamental understanding of many cellular processes. Nanoscale secondary ion mass spectrometry (NanoSIMS) imaging is a powerful technique which allows subcellular localization of chemical species with high spatial and mass resolution, and high sensitivity. In this study, we combined NanoSIMS imaging with spatial oversampling with transmission electron microscopy (TEM) imaging to discern the compartments (dense core and halo) of large dense core vesicles in a model cell line used to study exocytosis, and to localize ¹³C dopamine enrichment following 4–6 h of 150 μM ¹³C L-3,4-dihydroxyphenylalanine (L-DOPA) incubation. In addition, the absolute concentrations of ¹³C dopamine in distinct vesicle domains as well as in entire single vesicles were quantified and validated by comparison to electrochemical data. We found concentrations of 87.5 mM, 16.0 mM and 39.5 mM for the dense core, halo and the whole vesicle, respectively. This approach adds to the potential of using combined TEM and NanoSIMS imaging to perform absolute quantification and directly measure the individual contents of nanometer-scale organelles.

Repeated treatments with the D1 dopamine receptor agonist SKF-38393 modulate cell viability via sustained ERK-Bad-Bax activation in dopaminergic neuronal cells

The D₁ dopamine receptor agonist, SKF-38393, induces cytotoxicity in striatal dopaminergic neurons via an extracellular signal-regulated kinase (ERK) signaling cascade. However, the underlying mechanism remains unclear. We hypothesized that repeated activation of dopaminergic receptors by agonists could lead to neuronal cell death. This study investigated the effects of SKF-38393 on dopaminergic neuronal cell death in a 6-hydroxydopamine-lesioned rat model of Parkinson's disease (PD) and PC12 cells. In the PD model, SKF-38393 administration (3 and 10 mg/kg per day, s.c.) for 8 weeks significantly increased the number of tyrosine hydroxylase-immunopositive neuronal cells in nigrostriatal regions. SKF-38393 administration for 8 weeks induced phosphorylation of sustained ERK1/2 and Bad (Bcl-2-associated death promoter) at Ser155 (BadSer155), and augmented Bax (Bcl-2-associated X protein) expression. However, SKF-38393 only increased Bad phosphorylation at Ser112 (BadSer112) when administered for 4 weeks. In PC12 cells, toxic levels of SKF-38393 (20 and 50 μM) rapidly induced formation of neurite-like processes, but not in the presence of an adenylyl cyclase inhibitor (MDL-12330 A). SKF-38393 (20 and 50 μM) induced sustained ERK1/2 and BadSer155 phosphorylation as well as caspase-3 activation. At a non-toxic level (5 μM), SKF-38393 produced only transient ERK1/2 and BadSer112 phosphorylation. Repeated treatments with SKF-38393 (5 μM) for 1-3 days activated BadSer112. Repeated treatments for 4-7 days induced sustained ERK1/2 and BadSer155 phosphorylation as well as Bax and caspase-3 activation. These results suggest that SKF-38393 induces neurotoxicity by activation of the sustained ERK-Bad-Bax system. These findings contribute to an understanding of the adverse effects of D₁ dopamine receptor agonists in patients with PD.

Effects of asarinin on dopamine biosynthesis and 6-hydroxydopamine-induced cytotoxicity in PC12 cells

This study investigated the effects of asarinin on dopamine biosynthesis and 6-hydroxydopamine (6-OHDA)-induced cytotoxicity in rat adrenal pheochromocytoma (PC12) cells. Treatment with asarinin (25-50 μM) increased intracellular dopamine levels and enhanced L-DOPA-induced increases in dopamine levels. Asarinin (25 μM) induced cyclic AMP-dependent protein kinase A (PKA) signaling, leading to increased cyclic AMP-response element binding protein (CREB) and tyrosine hydroxylase (TH) phosphorylation, which in turn stimulated dopamine production. Asarinin (25 μM) also activated transient phosphorylation of extracellular signal-regulated kinase (ERK1/2) and Bad phosphorylation at Ser 112, both of which have been shown to promote cell survival. In contrast, asarinin (25 μM) inhibited sustained ERK1/2, Bax, c-Jun N-terminal kinase (JNK1/2) and p38 mitogen-activated protein kinase (p38MAPK) phosphorylation and caspase-3 activity, which were induced by 6-OHDA (100 μM). These results suggest that asarinin induces dopamine biosynthesis via activation of the PKA-CREB-TH system and protects against 6-OHDA-induced cytotoxicity by inhibiting the sustained activation of the ERK-p38MAPK-JNK1/2-caspase-3 system in PC12 cells.

Anti-Melanogenic Properties of Greek Plants. A Novel Depigmenting Agent from Morus alba Wood

In therapeutic interventions associated with melanin hyperpigmentation, tyrosinase is regarded as a target enzyme as it catalyzes the rate-limiting steps in mammalian melanogenesis. Since many known agents have been proven to be toxic, there has been increasing impetus to identify alternative tyrosinase inhibitors, especially from natural sources. In this study, we investigated 900 extracts from Greek plants for potential tyrosinase inhibitive properties. Among the five most potent extracts, the methanol extract of Morus alba wood (MAM) demonstrated a significant reduction in intracellular tyrosinase and melanin content in B16F10 melanoma cells. Bioassay-guided isolation led to the acquisition of twelve compounds: oxyresveratrol (1), kuwanon C (2), mulberroside A (3), resorcinol (4), dihydrooxyresveratol (5), trans-dihydromorin (6), 2,4,3′-trihydroxydihydrostilbene (7), kuwanon H (8), 2,4-dihydroxybenzaldehyde (9), morusin (10), moracin M (11) and kuwanon G (12). Among these, 2,4,3′-trihydroxydihydrostilbene (7) is isolated for the first time from Morus alba and constitutes a novel potent tyrosinase inhibitor (IC₅₀ 0.8 ± 0.15). We report here for the first time dihydrooxyresveratrol (5) as a potent natural tyrosinase inhibitor (IC₅₀ 0.3 ± 0.05). Computational docking analysis indicated the binding modes of six tyrosinase inhibitors with the aminoacids of the active centre of tyrosinase. Finally, we found both MAM extract and compounds 1, 6 and 7 to significantly suppress in vivo melanogenesis during zebrafish embryogenesis.

Multiple treatments with L-3,4-dihydroxyphenylalanine modulate dopamine biosynthesis and neurotoxicity through the protein kinase A-transient extracellular signal-regulated kinase and exchange protein activation by cyclic AMP-sustained extracellular signal-regulated kinase signaling pathways

Multiple treatments with L-3,4-dihydroxyphenylalanine (L-DOPA; 20 µM) induce neurite-like outgrowth and reduce dopamine biosynthesis in rat adrenal pheochromocytoma (PC) 12 cells. We therefore investigated the effects of multiple treatments with L-DOPA (MT-LD) on cell survival and death over a duration of 6 days by using PC12 cells and embryonic rat midbrain primary cell cultures. MT-LD (10 and 20 µM) decreased cell viability, and both types of cells advanced to the differentiation process at 4-6 days. MT-LD induced cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) phosphorylation and exchange protein activation by cAMP (Epac) expression at 1-3 days, which led to transient extracellular signal-regulated kinase (ERK1/2) phosphorylation in both cells. In these states, MT-LD activated cAMP-response element binding protein (CREB; Ser133) and tyrosine hydroxylase (Ser40) phosphorylation in PC12 cells, which led to an increase in intracellular dopamine levels. In contrast, MT-LD induced prolonged Epac expression at 4-5 days in both cells, which led to sustained ERK1/2 phosphorylation. In these states, the dopamine levels were decreased in PC12 cells. In addition, MT-LD induced c-Jun N-terminal kinase1/2 phosphorylation and cleaved caspase-3 expression at 4-6 days in both cells. These results suggest that MT-LD maintains cell survival via PKA-transient ERK1/2 activation, which stimulates dopamine biosynthesis. In contrast, at the later time period, MT-LD induces differentiation via both prolonged Epac and sustained ERK1/2 activation, which subsequently leads to the cell death process. Our data demonstrate that L-DOPA can cause neurotoxicity by modulating the Epac-ERK pathways in neuronal and PC12 cells.

The roles of cyclic AMP-ERK-Bad signaling pathways on 6-hydroxydopamine-induced cell survival and death in PC12 cells

The roles of cyclic AMP (cAMP)-ERK1/2-Bad signaling pathways in 6-hydroxydopamine (6-OHDA)-induced cell survival and death were investigated. In PC12 cells, 6-OHDA (10-100μM) concentration-dependently increased the intracellular levels of cAMP mediated by the Ca(2+)-CaMKII-adenylyl cyclase system. 6-OHDA at the non-toxic level (10μM) induced transient ERK1/2 phosphorylation and BadSer112 phosphorylation, which maintained cell survival. In contrast, the high levels of cAMP induced by toxic levels (50 and 100μM) of 6-OHDA induced sustained ERK1/2 phosphorylaton and BadSer155 phosphorylation. The cells then moved to cell death process through Bcl2 phosphorylation and caspase-3 activation. BadSer155 phosphorylation by 6-OHDA was inhibited by PKA (H89) and MEK (U0126) inhibitors, indicating that it was mediated via the cAMP-PKA-sustained ERK1/2 system. In SK-N-BE(2)C cells, the non-toxic level of 6-OHDA also showed transient ERK1/2 phosphorylation and BadSer112 phosphorylation, and toxic levels of 6-OHDA exhibited sustained ERK1/2 phosphorylation and BadSer155 phosphorylation. These results suggest that ERK1/2 phosphorylation by 6-OHDA shows biphasic functions on cell survival and death in PC12 cells. It is, therefore, proposed that the cAMP-ERK1/2-Bad signaling pathways incurred by toxic levels of 6-OHDA play a role in dopamine neuron death of animal models of Parkinson's disease.

Modulation of PC12 cell viability by forskolin-induced cyclic AMP levels through ERK and JNK pathways: an implication for L-DOPA-induced cytotoxicity in nigrostriatal dopamine neurons

The intracellular levels of cyclic AMP (cAMP) increase in response to cytotoxic concentrations of L-DOPA in PC12 cells, and forskolin that induces intracellular cAMP levels either protects PC12 cells from L-DOPA-induced cytotoxicity or enhances cytotoxicity in a concentration-dependent manner. This study investigated the effects of cAMP induced by forskolin on cell viability of PC12 cells, relevant to L-DOPA-induced cytotoxicity in Parkinson's disease therapy. The low levels of forskolin (0.01 and 0.1 μM)-induced cAMP increased dopamine biosynthesis and tyrosine hydroxylase (TH) phosphorylation, and induced transient phosphorylation of ERK1/2 within 1 h. However, at the high levels of forskolin (1.0 and 10 μM)-induced cAMP, dopamine biosynthesis and TH phosphorylation did not increase, but rapid differentiation in neurite-like formation was observed with a steady state. The high levels of forskolin-induced cAMP also induced sustained increase in ERK1/2 phosphorylation within 0.25-6 h and then led to apoptosis, which was apparently mediated by JNK1/2 and caspase-3 activation. Multiple treatment of PC12 cells with nontoxic L-DOPA (20 μM) for 4-6 days induced neurite-like formation and decreased intracellular dopamine levels by reducing TH phosphorylation. These results suggest that the low levels of forskolin-induced cAMP increased dopamine biosynthesis in cell survival via transient ERK1/2 phosphorylation. In contrast, the high levels of forskolin-induced cAMP induced differentiation via sustained ERK1/2 phosphorylation and then led to apoptosis. Taken together, the intracellular levels of cAMP play a dual role in cell survival and death through the ERK1/2 and JNK1/2 pathways in PC12 cells.

Post-translational modification of glutamic acid decarboxylase 67 by intermittent hypoxia: evidence for the involvement of dopamine D1 receptor signaling

Intermittent hypoxia (IH) associated with sleep apnea leads to cardio-respiratory morbidities. Previous studies have shown that IH alters the synthesis of neurotransmitters including catecholamines and neuropeptides in brainstem regions associated with regulation of cardio-respiratory functions. GABA, a major inhibitory neurotransmitter in the CNS, has been implicated in cardio-respiratory control. GABA synthesis is primarily catalyzed by glutamic acid decarboxylase (GAD). In this study, we tested the hypothesis that IH like its effect on other transmitters also alters GABA synthesis. The impact of IH on GABA synthesis was investigated in pheochromocytoma 12 cells, a neuronal cell line which is known to express active form of GAD67 in the cytosolic fraction and also assessed the underlying mechanisms contributing to IH-evoked response. Exposure of cell cultures to IH decreased GAD67 activity and GABA level. IH-evoked decrease in GAD67 activity was caused by increased cAMP - protein kinase A (PKA) - dependent phosphorylation of GAD67, but not as a result of changes in either GAD67 mRNA or protein expression. PKA inhibitor restored GAD67 activity and GABA levels in IH treated cells. Pheochromocytoma 12 cells express dopamine 1 receptor (D1R), a G-protein coupled receptor whose activation increased adenylyl cyclase activity. Treatment with either D1R antagonist or adenylyl cyclase inhibitor reversed IH-evoked GAD67 inhibition. Silencing D1R expression with siRNA reversed cAMP elevation and GAD67 inhibition by IH. These results provide evidence for the role of D1R-cAMP-PKA signaling in IH-mediated inhibition of GAD67 via protein phosphorylation resulting in down-regulation of GABA synthesis.

Mechanisms of L-DOPA-induced cytotoxicity in rat adrenal pheochromocytoma cells: implication of oxidative stress-related kinases and cyclic AMP

L-DOPA therapy for Parkinson's disease has a double-edge effect on nigrostriatal dopaminergic neurons: L-DOPA increases the intracellular level of dopamine, but it induces neuron cytotoxicity in a concentration-dependent manner. To investigate the molecular signaling mechanisms that underlie the concentration-dependent effects of L-DOPA on cell viability, the activities of mitogen-activated protein kinases (MAPKs) and apoptotic enzymes were measured in rat adrenal pheochromocytoma (PC12) cells in the presence of a low concentration (20 muM) and high concentrations (100-200 muM) of L-DOPA. At the low concentration, L-DOPA was not cytotoxic and its presence increased the activities of extracellular signal-regulated kinase (ERK)1/2, p38 MAPK, BadSer112, Bcl-2, and caspase-12. At the high concentrations, L-DOPA was cytotoxic and stimulated the activities of ERK1/2, p38 MAPK, c-Jun N-terminal kinase (JNK)1/2, BadSer155, caspase-12 and caspase-3. The increased levels of ERK1/2 and BadSer155 in the presence of high concentrations of L-DOPA did not protect against L-DOPA-mediated cytotoxicity. In addition, the levels of L-type Ca(2+) channel-sensitive intracellular cyclic AMP (cAMP) and Ca(2+) were elevated in the presence of L-DOPA, and the increase in the levels of intracellular cAMP may also play a role in cellular viability, since cAMP levels and cytotoxicity increased in parallel with L-DOPA concentrations and the addition of forskolin in the medium increased cytotoxicity in a concentration-dependent manner. These results suggest that, at a low and non-toxic concentration, L-DOPA may promote cell survival by increasing the activities of ERK1/2, BadSer112 and Bcl-2, while, at high concentrations, L-DOPA activates the caspase-3 cell death enzyme through the JNK1/2 and p38 MAPK signaling pathways as well as endoplasmic reticulum stress that activates caspase-12. Intracellular cAMP levels may also play a role here. The results may lead to an effective therapy for Parkinson's disease.

Catalponol enhances dopamine biosynthesis and protects against L-DOPA-induced cytotoxicity in PC12 cells

The effects of catalponol (1) on dopamine biosynthesis and L-DOPA-induced cytotoxicity in PC12 cells were investigated. Catalponol at concentration ranges of 1-5 microM increased the intracellular levels of dopamine at 12-48 h. Catalponol at concentrations of up to 10 microM did not alter cell viability. Tyrosine hydroxylase (TH) activity was enhanced by 1 at 3 microM in a time-dependent manner, but aromatic L-amino acid decarboxylase activity was not. Catalponol also increased the intracellular levels of cyclic AMP and TH phosphorylation. In addition, catalponol at 3 microM associated with L-DOPA (20-50 microM) further enhanced the increases in dopamine levels induced by L-DOPA (50-100 microM) at 24 h. Catalponol at 2-5 microM inhibited L-DOPA (100-200 microM)-induced cytotoxicity at 48 h. These results suggest that 1 enhanced dopamine biosynthesis by inducing TH activity and protected against L-DOPA-induced cytotoxicity in PC12 cells, which was mediated by the increased levels of cyclic AMP.

Effects of scoparone on dopamine biosynthesis and L-DOPA-induced cytotoxicity in PC12 cells

The effects of scoparone on dopamine biosynthesis and L-DOPA-induced cytotoxicity in PC12 cells were investigated. PC12 cells treated with scoparone at concentrations of 100-200 microM showed a 128-136% increase in dopamine levels over the course of 24 hr. Scoparone significantly increased the secretion of dopamine into the culture medium. Under the same conditions, the activities of tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC) were enhanced by treatment with 200 microM scoparone for 6-48 hr, but the activity of TH was regulated for a longer period than that of AADC. The intracellular levels of cyclic AMP and Ca(2+) were increased by treatment with 200 microM scoparone. The levels of TH mRNA and the phosphorylation of cyclic AMP-response element-binding protein (CREB) were also significantly increased by treatment with 200 microM scoparone. In addition, scoparone at a concentration of 200 microM stimulated the activities of cyclic AMP-dependent protein kinase (PKA), protein kinase C (PKC), and Ca(2+)/calmodulin kinase II (CaMK II). Finally, pretreatment with 200 microM scoparone reduced the cytotoxicity induced by L-DOPA (20-100 microM) at 24 hr. These results suggest that scoparone enhances dopamine biosynthesis by regulating TH activity and TH gene expression, which is mediated by the PKA, CREB, PKC, and CaMK II pathways, and protects cells from L-DOPA-induced cytotoxicity by inducing cyclic AMP-PKA systems in PC12 cells.