Dopamine D1 receptor-mediated toxicity in human SK-N-MC neuroblastoma cells

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.

Renal dopamine receptors, oxidative stress, and hypertension

Dopamine, which is synthesized in the kidney, independent of renal nerves, plays an important role in the regulation of fluid and electrolyte balance and systemic blood pressure. Lack of any of the five dopamine receptor subtypes (D1R, D2R, D3R, D4R, and D5R) results in hypertension. D1R, D2R, and D5R have been reported to be important in the maintenance of a normal redox balance. In the kidney, the antioxidant effects of these receptors are caused by direct and indirect inhibition of pro-oxidant enzymes, specifically, nicotinamide adenine dinucleotide phosphate, reduced form (NADPH) oxidase, and stimulation of anti-oxidant enzymes, which can also indirectly inhibit NADPH oxidase activity. Thus, stimulation of the D2R increases the expression of endogenous anti-oxidants, such as Parkinson protein 7 (PARK7 or DJ-1), paraoxonase 2 (PON2), and heme oxygenase 2 (HO-2), all of which can inhibit NADPH oxidase activity. The D5R decreases NADPH oxidase activity, via the inhibition of phospholipase D2, and increases the expression of HO-1, another antioxidant. D1R inhibits NADPH oxidase activity via protein kinase A and protein kinase C cross-talk. In this review, we provide an overview of the protective roles of a specific dopamine receptor subtype on renal oxidative stress, the different mechanisms involved in this effect, and the role of oxidative stress and impairment of dopamine receptor function in the hypertension that arises from the genetic ablation of a specific dopamine receptor gene in mice.

Methamphetamine induces dopamine D1 receptor-dependent endoplasmic reticulum stress-related molecular events in the rat striatum

Methamphetamine (METH) is an illicit toxic psychostimulant which is widely abused. Its toxic effects depend on the release of excessive levels of dopamine (DA) that activates striatal DA receptors. Inhibition of DA-mediated neurotransmission by the DA D1 receptor antagonist, SCH23390, protects against METH-induced neuronal apoptosis. The initial purpose of the present study was to investigate, using microarray analyses, the influence of SCH23390 on transcriptional responses in the rat striatum caused by a single METH injection at 2 and 4 hours after drug administration. We identified 545 out of a total of 22,227 genes as METH-responsive. These include genes which are involved in apoptotic pathways, endoplasmic reticulum (ER) stress, and in transcription regulation, among others. Of these, a total of 172 genes showed SCH23390-induced inhibition of METH-mediated changes. Among these SCH23390-responsive genes were several genes that are regulated during ER stress, namely ATF3, HSP27, Hmox1, HSP40, and CHOP/Gadd153. The secondary goal of the study was to investigate the role of DA D1 receptor stimulation on the expression of genes that participate in ER stress-mediated molecular events. We thus used quantitative PCR to confirm changes in the METH-responsive ER genes identified by the microarray analyses. We also measured the expression of these genes and of ATF4, ATF6, BiP/GRP78, and of GADD34 over a more extended time course. SCH23390 attenuated or blocked METH-induced increases in the expression of the majority of these genes. Western blot analysis revealed METH-induced increases in the expression of the antioxidant protein, Hmox1, which lasted for about 24 hours after the METH injection. Additionally, METH caused DA D1 receptor-dependent transit of the Hmox1 regulator protein, Nrf2, from cytosolic into nuclear fractions where the protein exerts its regulatory functions. When taken together, these findings indicate that SCH23390 can provide protection against neuronal apoptosis by inhibiting METH-mediated DA D1 receptor-mediated ER stress in the rat striatum. Our data also suggest that METH-induced toxicity might be a useful model to dissect molecular mechanisms involved in ER stress-dependent events in the rodent brain.

Marked changes in signal transduction upon heteromerization of dopamine D1 and histamine H3 receptors

BACKGROUND AND PURPOSE

Functional interactions between the G protein-coupled dopamine D1 and histamine H3 receptors have been described in the brain. In the present study we investigated the existence of D1-H3 receptor heteromers and their biochemical characteristics.

EXPERIMENTAL APPROACH

D1-H3 receptor heteromerization was studied in mammalian transfected cells with Bioluminescence Resonance Energy Transfer and binding assays. Furthermore, signalling through mitogen-activated protein kinase (MAPK) and adenylyl cyclase pathways was studied in co-transfected cells and compared with cells transfected with either D1 or H3 receptors.

KEY RESULTS

Bioluminescence Resonance Energy Transfer and binding assays confirmed that D1 and H3 receptors can heteromerize. Activation of histamine H3 receptors did not lead to signalling towards the MAPK pathway unless dopamine D1 receptors were co-expressed. Also, dopamine D1 receptors, usually coupled to G(s) proteins and leading to increases in cAMP, did not couple to G(s) but to G(i) in co-transfected cells. Furthermore, signalling via each receptor was blocked not only by a selective antagonist but also by an antagonist of the partner receptor.

CONCLUSIONS AND IMPLICATIONS

D1-H3 receptor heteromers constitute unique devices that can direct dopaminergic and histaminergic signalling towards the MAPK pathway in a G(s)-independent and G(i)-dependent manner. An antagonist of one of the receptor units in the D1-H3 receptor heteromer can induce conformational changes in the other receptor unit and block specific signals originating in the heteromer. This gives rise to unsuspected therapeutic potentials for G protein-coupled receptor antagonists.

Dopamine promotes striatal neuronal apoptotic death via ERK signaling cascades

Although the mechanisms underlying striatal neurodegeneration are poorly understood, we have shown that striatal pathogenesis may be initiated by high synaptic levels of extracellular dopamine (DA). Here we investigated in rat striatal primary neurons the mobilization of the mitogen-activated protein kinase (MAPK) signaling pathways after treatment with DA. Instead of observing an elevation of the archetypical pro-cytotoxic MAPKs, p-JNK and p-p38 MAPK, we found that DA, acting through D1 DA receptors, induced a sustained stimulation of the phosphorylated form of extracellular signal-regulated kinase (p-ERK) via a cAMP/protein kinase A (PKA)/Rap1/B-Raf / MAPK/ERK kinase (MEK) pathway. Blockade of D2 DA receptors, beta-adrenergic receptors or N-methyl-D-aspartate receptors with receptor-specific antagonists had no significant effect on this process. Activation of D1 DA receptors and PKA by DA caused phosphorylation and inactivation of the striatal-enriched tyrosine phosphatase, an important phosphatase for the dephosphorylation and subsequent inactivation of p-ERK in the striatum. Interestingly, p-ERK was primarily retained in the cytoplasm, with only low amounts translocated to the nucleus. The scaffold protein beta-arrestin2 interacted with both p-ERK and D1 DA receptor, triggering the cytosolic retention of p-ERK and inducing striatal neuronal apoptotic death. These data provide unique insight into a novel role of p-ERK in striatal neurodegeneration.

Catecholamines-crafty weapons in the inflammatory arsenal of immune/inflammatory cells or opening pandora's box?

It is well established that catecholamines (CAs), which regulate immune and inflammatory responses, derive from the adrenal medulla and from presynaptic neurons. Recent studies reveal that T cells also can synthesize and release catecholamines which then can regulate T cell function. We have shown recently that macrophages and neutrophils, when stimulated, can generate and release catecholamines de novo which, then, in an autocrine/paracrine manner, regulate mediator release from these phagocytes via engagement of adrenergic receptors. Moreover, regulation of catecholamine-generating enzymes as well as degrading enzymes clearly alter the inflammatory response of phagocytes, such as the release of proinflammatory mediators. Accordingly, it appears that phagocytic cells and lymphocytes may represent a major, newly recognized source of catecholamines that regulate inflammatory responses.

Overexpression of NQO1 protects human SK-N-MC neuroblastoma cells against dopamine-induced cell death

NAD(P)H quinone oxidoreductase 1 (NQO1) can metabolize dopamine-derived quinones (DAQ) and absence of NQO1 due to the NQO1*2 polymorphism has been suggested to be a risk factor for Parkinson's disease. In order to define whether NQO1 plays a protective role in dopamine toxicity, we have examined the potential role of NQO1 in the SK-N-MC human neuroblastoma cell line. SK-N-MC cells were stably transfected with NQO1 to generate stable clones with NQO1 enzymatic activity of 245 nmol/mgmin while vector control and parental cells had NQO1 activities of less than 12 nmol/mgmin. Incubation of dopamine for 24 h in both parental and vector control SK-N-MC cells resulted in 85% and 72% cell death as assessed by annexin-V/propidium iodide analysis. In agreement, 88% and 84% of parental and vector control cells, respectively underwent loss of mitochondrial membrane potential (MMP) assessed by tetramethylrhodamine ethyl ester. In contrast, NQO1-transfected cells were resistant to dopamine toxicity and both cell death and loss of MMP were markedly abrogated in NQO1-transfected SK-N-MC cells. When dopamine was added to medium, oxygen uptake could be detected indicating autoxidation with concomitant formation of oxygen radicals and quinones. However, dopamine-induced cell death was not affected by the inclusion of either superoxide dismutase or catalase suggesting that superoxide and hydrogen peroxide were not involved in toxicity. Quinones formed in medium may exert toxicity extracellularly or intracellularly but the protective role of NQO1 argues for an intracellular mechanism. In summary, transfection of SK-N-MC cells with NQO1 protects against dopamine-induced toxicity.