The I(1)-imidazoline receptor in PC12 pheochromocytoma cells reverses NGF-induced ERK activation and induces MKP-2 phosphatase

Imidazoline Receptor System: The Past, the Present, and the Future

Imidazoline receptors historically referred to a family of nonadrenergic binding sites that recognize compounds with an imidazoline moiety, although this has proven to be an oversimplification. For example, none of the proposed endogenous ligands for imidazoline receptors contain an imidazoline moiety but they are diverse in their chemical structure. Three receptor subtypes (I₁, I₂, and I₃) have been proposed and the understanding of each has seen differing progress over the decades. I₁ receptors partially mediate the central hypotensive effects of clonidine-like drugs. Moxonidine and rilmenidine have better therapeutic profiles (fewer side effects) than clonidine as antihypertensive drugs, thought to be due to their higher I₁/α ₂-adrenoceptor selectivity. Newer I₁ receptor agonists such as LNP599 [3-chloro-2-methyl-phenyl)-(4-methyl-4,5-dihydro-3H-pyrrol-2-yl)-amine hydrochloride] have little to no activity on α ₂-adrenoceptors and demonstrate promising therapeutic potential for hypertension and metabolic syndrome. I₂ receptors associate with several distinct proteins, but the identities of these proteins remain elusive. I₂ receptor agonists have demonstrated various centrally mediated effects including antinociception and neuroprotection. A new I₂ receptor agonist, CR4056 [2-phenyl-6-(1H-imidazol-1yl) quinazoline], demonstrated clear analgesic activity in a recently completed phase II clinical trial and holds great promise as a novel I₂ receptor-based first-in-class nonopioid analgesic. The understanding of I₃ receptors is relatively limited. Existing data suggest that I₃ receptors may represent a binding site at the Kir6.2-subtype ATP-sensitive potassium channels in pancreatic β-cells and may be involved in insulin secretion. Despite the elusive nature of their molecular identities, recent progress on drug discovery targeting imidazoline receptors (I₁ and I₂) demonstrates the exciting potential of these compounds to elicit neuroprotection and to treat various disorders such as hypertension, metabolic syndrome, and chronic pain.

The Use of the Selective Imidazoline I1 Receptor Agonist Carbophenyline as a Strategy for Neuropathic Pain Relief: Preclinical Evaluation in a Mouse Model of Oxaliplatin-Induced Neurotoxicity

Anti-cancer therapy based on the repeated administration of oxaliplatin is limited by the development of a disabling neuropathic syndrome with detrimental effects on the patient's quality of life. The lack of effective pharmacological approaches calls for the identification of innovative therapeutic strategies based on new targets. We focused our attention on the imidazoline I1 receptor (I1-R) and in particular on the selective I1-R agonist 2-(1-([1,1'-biphenyl]-2-yl)propan-2-yl)-4,5-dihydro-1H-imidazole) (carbophenyline). The purpose of this work was the preclinical evaluation of the efficacy of carbophenyline on oxaliplatin-induced neuropathic pain in mice. Carbophenyline, acutely per os administered (0.1-10 mg kg-1), induced a dose-dependent anti-hyperalgesic effect that was completely blocked by the pre-treatment with the I1-R antagonist 3 or the I1/α2 receptor antagonist efaroxan, confirming the I1-R-dependent mechanism. Conversely, pre-treatment with the I2-R antagonist BU224 did not block the anti-nociceptive effect evoked by carbophenyline. Repeated oral administrations of carbophenyline (1 mg kg-1) for 14 days, starting from the first day of oxaliplatin injection, counteracted the development of neuropathic pain in all behavioral tests (cold plate, Von Frey, and paw pressure tests) carried out 24 h after the last carbophenyline treatment on days 7 and 14. In the dorsal horn of the spinal cord, carbophenyline significantly decreased the oxaliplatin-induced astrocyte activation detected by immunofluorescence staining by the specific labelling with GFAP antibody. In conclusion, carbophenyline showed anti-neuropathic properties both after acute and chronic treatment with preventive effect against oxaliplatin-induced astrocyte activation in the spinal cord. Therefore, I1-R agonists emerge as a new class of candidates for the management of oxaliplatin-induced neuropathic pain.

Inhibitory effects of imidazoline receptor ligands on basal and kainic acid-induced neurotoxic signalling in mice

This in vivo study assessed the potential of the imidazoline receptor (IR) ligands moxonidine (selective I1-IR), BU224 (selective I2-IR) and LSL61122 (mixed I1/I2-IR) to dampen excitotoxic signalling induced by kainic acid (KA; 45 mg/kg) in the mouse brain (hippocampus and cerebral cortex). KA triggered a strong behavioural syndrome (seizures; maximal at 60-90 minutes) and sustained stimulation (at 72 hours with otherwise normal mouse behaviour) of pro-apoptotic c-Jun-N-terminal kinases (JNK) and calpain with increased cleavage of p35 into neurotoxic p25 (cyclin-dependent kinase 5 [Cdk5] activators) in mouse hippocampus. Pretreatment (five days) with LSL61122 (10 mg/kg), but not moxonidine (1 mg/kg) or BU224 (20 mg/kg), attenuated the KA-induced behavioural syndrome, and all three IR ligands inhibited JNK and calpain activation, as well as p35/p25 cleavage after KA in the hippocampus (effects also observed after acute IR drug treatments). Efaroxan (I1-IR, 10 mg/kg) and idazoxan (I2-IR, 10 mg/kg), postulated IR antagonists, did not antagonise the effects of moxonidine and LSL61122 on KA targets (these IR ligands showed agonistic properties inhibiting pro-apoptotic JNK). Brain subcellular preparations revealed reduced synaptosomal postsynaptic density-95 protein contents (a mediator of JNK activation) and indicated increased p35/Cdk5 complexes (with pro-survival functions) after treatment with moxonidine, BU224 and LSL61122. These results showed that I1- and I2-IR ligands (moxonidine and BU224), and especially the mixed I1/I2-IR ligand LSL61122, are partly neuroprotective against KA-induced excitotoxic signalling. These findings suggest a therapeutic potential of IR drugs in disorders associated with glutamate-mediated neurodegeneration.

Brain stem adenosine receptors modulate centrally mediated hypotensive responses in conscious rats: A review

Adenosine is implicated in the modulation of cardiovascular responses either at the peripheral or at central level in experimental animals. However, there are no dedicated reviews on the involvement of adenosine in mediating the hypotensive response of centrally administered clonidine in general and specifically in aortically barodenervated rats (ABD). The conscious ABD rat model exhibits surgically induced baroreflex dysfunction and exaggerated hypotensive response, compared with conscious sham-operated (SO) rats. The current review focuses on, the role of adenosine receptors in blood pressure (BP) regulation and their possible crosstalk with other receptors e.g. imidazoline (I₁) and alpha (α_2A) adrenergic receptor (AR). The former receptor is a molecular target for clonidine, whose hypotensive effect is enhanced approx. 3-fold in conscious ABD rats. We also discussed how the balance between the brain stem adenosine A₁ and A_2A receptors is regulated by baroreceptors and how such balance influences the centrally mediated hypotensive responses. The use of the ABD rat model yielded insight into the downstream signaling cascades following clonidine-evoked hypotension in a surgical model of baroreflex dysfunction.

Regulation of angiopoietin-1/Tie-2 receptor signaling in endothelial cells by dual-specificity phosphatases 1, 4, and 5

Background

Angiopoietin‐1 (Ang‐1) promotes survival and migration of endothelial cells, in part through the activation of mitogen‐activated protein kinase (MAPK) pathways downstream of Tie‐2 receptors. Dual‐specificity phosphatases (DUSPs) dephosphorylate phosphotyrosine and phosphoserine/phosphothreonine residues on target MAPKs. The mechanisms by which DUSPs modulate MAPK activation in Ang‐1/Tie‐2 receptor signaling are unknown in endothelial cells.

Methods and Results

Expression of various DUSPs in human umbilical vein endothelial cells exposed to Ang‐1 was measured. The functional roles of DUSPs in Ang‐1‐induced regulation of MAPK activation, endothelial cell survival, migration, differentiation, and permeability were measured using selective siRNA oligos. Ang‐1 differentially induces DUSP1, DUSP4, and DUSP5 in human umbilical vein endothelial cells through activation of the PI‐3 kinase, ERK1/2, p38, and SAPK/JNK pathways. Lack‐of‐function siRNA screening revealed that DUSP1 preferentially dephosphorylates p38 protein and is involved in Ang‐1‐induced cell migration and differentiation. DUSP4 preferentially dephosphorylates ERK1/2, p38, and SAPK/JNK proteins and, under conditions of serum deprivation, is involved in Ang‐1‐induced cell migration, several antiapoptotic effects, and differentiation. DUSP5 preferentially dephosphorylates ERK1/2 proteins and is involved in cell survival and inhibition of permeability.

Conclusions

DUSP1, DUSP4, and DUSP5 differentially modulate MAPK signaling pathways downstream of Tie‐2 receptors, thus highlighting the importance of these phosphatases to Ang‐1‐induced angiogenesis.

I1 imidazoline receptor: novel potential cytoprotective target of TVP1022, the S-enantiomer of rasagiline

TVP1022, the S-enantiomer of rasagiline (Azilect®) (N-propargyl-1R-aminoindan), exerts cyto/cardio-protective effects in a variety of experimental cardiac and neuronal models. Previous studies have demonstrated that the protective activity of TVP1022 and other propargyl derivatives involve the activation of p42/44 mitogen-activated protein kinase (MAPK) signaling pathway. In the current study, we further investigated the molecular mechanism of action and signaling pathways of TVP1022 which may account for the cyto/cardio-protective efficacy of the drug. Using specific receptor binding and enzyme assays, we demonstrated that the imidazoline 1 and 2 binding sites (I₁ & I₂) are potential targets for TVP1022 (IC₅₀ = 9.5E-08 M and IC₅₀ = 1.4E-07 M, respectively). Western blotting analysis showed that TVP1022 (1–20 µM) dose-dependently increased the immunoreactivity of phosphorylated p42 and p44 MAPK in rat pheochromocytoma PC12 cells and in neonatal rat ventricular myocytes (NRVM). This effect of TVP1022 was significantly attenuated by efaroxan, a selective I₁ imidazoline receptor antagonist. In addition, the cytoprotective effect of TVP1022 demonstrated in NRVM against serum deprivation-induced toxicity was markedly inhibited by efaroxan, thus suggesting the importance of I₁imidazoline receptor in mediating the cardioprotective activity of the drug. Our findings suggest that the I₁imidazoline receptor represents a novel site of action for the cyto/cardio-protective efficacy of TVP1022.

Novel I1-imidazoline S43126 enhance insulin action in PC12 cells

The I(1)-imidazoline receptor is a novel target for drug development for hypertension and insulin resistance, major disorders associated with type 2 diabetes. In the present study, we examined the effects of a novel imidazoline agonist S43126, on phosphorylation of protein kinase B (PKB/Akt) and extracellular signal-regulated kinase (ERK1/2) in PC12 cells. We further examined the effects of S43126 on insulin stimulated PKB and ERK phosphorylation. PC12 cells were treated with varying doses of S43126 (10(-10) to 10(-6) M) or insulin (10(-10) to 10(-6) M) or combination treatment with insulin (10(-6) M) and varying doses of S43126 (10(-6) - 10(-11) M) for 10 min. Western blot analysis of treated samples showed that S43126 increased both ERK1/2 and PKB phosphorylation by 5 fold. Combination treatment with insulin (10(-6) M) and varying doses of S43126 (10(-6) - 10(-11) M) enhanced phosphorylation of PKB and ERK1/2 above the level of insulin alone, in a dose and time dependent manner. Treatment with siRNA against Nischarin (mouse homologue of I(1)-imidazoline receptor) reduced the phosphorylation of both ERK and PKB following combination treatments. These results indicate that S43126 has the potential to augment insulin action and should be further studied as a possible candidate drug for the treatment of insulin resistance states.

Identification of IRAS/Nischarin as an I1-imidazoline receptor in PC12 rat pheochromocytoma cells

The I1-imidazoline receptor (I1R) is a proposed target for drug action relevant to blood pressure and glucose control. The imidazoline receptor antisera-selected (IRAS) gene, also known as Nischarin, has several characteristics of an I1R. To test the contribution of IRAS to I1R binding capacity and cell-signaling function, an antisense probe targeting the initiating codon of rat IRAS gene was evaluated in PC12 rat pheochromocytoma cells, a well-established model for I1R action. The density of I1R was significantly reduced by antisense compared with control transfection (Bmax = 400 +/- 16 vs. 691 +/- 29 fmol/mg protein), without significantly affecting binding affinity (Kd = 0.30 +/- 0.04 vs. 0.39 +/- 0.05 nmol/L). Thus, IRAS expression is necessary for high-affinity binding to I1R. Western blots with polyclonal anti-IRAS showed reduced IRAS expression in the major 85-kDa band relative to an actin reference, paralleling the reduction in binding site density. To determine whether reduced IRAS expression attenuated I1R cell signaling, PC12 cells transfected with antisense or sense oligo-DNA were treated with moxonidine, an I1R agonist, then cell lysates were analyzed by western blot. Dose-dependent activation of extracellular signal-regulated kinase was attenuated without affecting the potency of the agonist. In contrast, extracellular signal-regulated kinase activation by insulin was unchanged. The IRAS gene is likely to encode an I1R or a functional subunit.

Marked Insulin Resistance in Obese Spontaneously Hypertensive Rat Adipocytes Is Ameliorated by in Vivo but Not in Vitro Treatment with Moxonidine

The obese spontaneously hypertensive rat (SHROB) is a model of marked insulin resistance with normoglycemia. We sought to determine whether insulin resistance extends to adipocytes and the impact of an insulin-sensitizing imidazoline, moxonidine (4 mg/kg/days for 21 days). Gonadal adipocytes were isolated from SHROB and lean spontaneously hypertensive rat (SHR) littermates. In lean SHR adipocytes, Akt activation by 100 nM insulin peaked at 3 min at 25-fold, whereas SHROB adipocytes showed only 4-fold activation. In dose-response experiments, the maximal response (E(max)) was markedly reduced 18.8 +/- 2.3 versus 3.7 +/- 0.8. Insulin sensitivity was also attenuated, with higher concentrations required for responses (EC(50) = 3.5 +/- 0.5 versus 29 +/- 3.8 nM). Glucose uptake as determined with [(3)H]2-deoxyglucose was also less responsive to insulin in SHROB relative to lean SHR. Moxonidine had little or no effect when applied acutely in vitro, but adipocytes isolated from SHROB treated with moxonidine in vivo showed significantly improved responses to insulin, both in terms of Akt activation and facilitation of glucose uptake. Chronic but not acute moxonidine treatment partially restores insulin sensitivity in SHROB adipocytes, suggesting an indirect action of this agent.

IRAS, a candidate for I1-imidazoline receptor, mediates inhibitory effect of agmatine on cellular morphine dependence

Agmatine, an endogenous ligand for the I1-imidazoline receptor, has previously been shown to prevent morphine dependence in rats and mice. To investigate the role of imidazoline receptor antisera-selected protein (IRAS), a strong candidate for I1R, in morphine dependence, two CHO cell lines were created, in which mu opioid receptor (MOR) was stably expressed alone (CHO-mu) or MOR and IRAS were stably co-expressed (CHO-mu/IRAS). After 48 h administration of morphine (10 microM), naloxone induced a cAMP overshoot in both cell lines, suggesting cellular morphine dependence had been produced. Agmatine (0.1-2.5 microM) concentration-dependently inhibited the naloxone-precipitated cAMP overshoot when co-pretreated with morphine in CHO-mu/IRAS, but not in CHO-mu. Agmatine at 5-100 microM also inhibited the cAMP overshoot in CHO/mu and CHO-mu/IRAS. Efaroxan, an I1R-preferential antagonist, completely blocked the effect of agmatine on the cAMP overshoot at 0.1-2.5 microM in CHO-mu/IRAS, while partially reversing the effects of agmatine at 5-100 microM. L-type calcium channel blocker nifedipine entirely mimicked the effects of agmatine at high concentrations on forskolin-stimulated cAMP formation in CHO-mu and naloxone-precipitated cAMP overshoot in morphine-pretreated CHO-mu. Therefore, IRAS, in the co-transfected CHO-mu/IRAS cell line, appears necessary for low concentrations of agmatine to cause attenuation of cellular morphine dependence. An additional effect of agmatine at higher concentrations seems to relate to both transfected IRAS and some naive elements in CHO cells, and L-type voltage-gated calcium channels are not ruled out. This study suggests that IRAS mediates agmatine's high affinity effects on cellular morphine dependence and may play a role in opioid dependence.

Effect of BU98008, an imidazoline1-binding site ligand, on body temperature in mice

Previous studies using the novel imidazoline1-binding site ligand 1-(4,5-dihydro-1H-imidazol-2-yl)isoquinoline hydrochloride, BU98008, have shown it induces a hypothermic response in rodents following intraperitoneal administration. Radioligand binding data has shown that BU98008 is a highly selective imidazoline1-binding site ligand with 300 fold selectively for the imidazoline1-binding site relative to alpha2-adrenoceptors. However, alpha2-adrenoceptor agonists are known to induce hypothermia, therefore, the present study has investigated the ability of the selective alpha2-adrenoceptor antagonist, RX811059 (2-ethoxy idazoxan) and the mixed imidazoline1-binding site/alpha2-adrenoceptor antagonist, efaroxan, to attenuate the BU98008-induced hypothermia. Preliminary experiments confirmed that BU98008 induced a dose-dependent decrease in body temperature in mice at 10 and 20 mg/kg. The response was not affected by pre-treatment with RX811059 but was significantly attenuated following pre-treatment with efaroxan. These data suggest that BU98008-induced hypothermia is mediated by activation of imidazoline1-binding site. Body temperature may therefore provide a novel assay for investigating agonist and antagonist action at the imidazoline1-binding site.