Alpha-melanocyte-stimulating hormone through melanocortin-4 receptor inhibits nitric oxide synthase and cyclooxygenase expression in the hypothalamus of male rats

The Melanocortin System: A Promising Target for the Development of New Antidepressant Drugs

Major depression is one of the most prevalent mental disorders, causing significant human suffering and socioeconomic loss. Since conventional antidepressants are not sufficiently effective, there is an urgent need to develop new antidepressant medications. Despite marked advances in the neurobiology of depression, the etiology and pathophysiology of this disease remain poorly understood. Classical and newer hypotheses of depression suggest that an imbalance of brain monoamines, dysregulation of the hypothalamic-pituitary-adrenal axis (HPAA) and immune system, or impaired hippocampal neurogenesis and neurotrophic factors pathways are cause of depression. It is assumed that conventional antidepressants improve these closely related disturbances. The purpose of this review was to discuss the possibility of affecting these disturbances by targeting the melanocortin system, which includes adrenocorticotropic hormone-activated receptors and their peptide ligands (melanocortins). The melanocortin system is involved in the regulation of various processes in the brain and periphery. Melanocortins, including peripherally administered non-corticotropic agonists, regulate HPAA activity, exhibit anti-inflammatory effects, stimulate the levels of neurotrophic factors, and enhance hippocampal neurogenesis and neurotransmission. Therefore, endogenous melanocortins and their analogs are able to complexly affect the functioning of those body’s systems that are closely related to depression and the effects of antidepressants, thereby demonstrating a promising antidepressant potential.

Activation of Melanocortin-4 Receptor by a Synthetic Agonist Inhibits Ethanolinduced Neuroinflammation in Rats

BACKGROUND

High ethanol intake induces a neuroinflammatory response resulting in the subsequent maintenance of chronic alcohol consumption. The melanocortin system plays a pivotal role in the modulation of alcohol consumption. Interestingly, it has been shown that the activation of melanocortin-4 receptor (MC4R) in the brain decreases the neuroinflammatory response in models of brain damage other than alcohol consumption, such as LPS-induced neuroinflammation, cerebral ischemia, glutamate excitotoxicity, and spinal cord injury.

OBJECTIVES

In this work, we aimed to study whether MC4R activation by a synthetic MC4R-agonist peptide prevents ethanol-induced neuroinflammation, and if alcohol consumption produces changes in MC4R expression in the hippocampus and hypothalamus.

METHODS

Ethanol-preferring Sprague Dawley rats were selected offering access to 20% ethanol on alternate days for 4 weeks (intermittent access protocol). After this time, animals were i.p. administered an MC4R agonist peptide in the last 2 days of the protocol. Then, the expression of the proinflammatory cytokines interleukin 6 (IL-6), interleukin 1-beta (IL-1β), and tumor necrosis factor-alpha (TNF-α) were measured in the hippocampus, hypothalamus and prefrontal cortex. It was also evaluated if ethanol intake produces alterations in the expression of MC4R in the hippocampus and the hypothalamus.

RESULTS

Alcohol consumption increased the expression of MC4R in the hippocampus and the hypothalamus. The administration of the MC4R agonist reduced IL-6, IL-1β and TNF-α levels in hippocampus, hypothalamus and prefrontal cortex, to those observed in control rats that did not drink alcohol.

CONCLUSION

High ethanol consumption produces an increase in the expression of MC4R in the hippocampus and hypothalamus. The administration of a synthetic MC4R-agonist peptide prevents neuroinflammation induced by alcohol consumption in the hippocampus, hypothalamus, and prefrontal cortex. These results could explain the effect of α-MSH and other synthetic MC4R agonists in decreasing alcohol intake through the reduction of the ethanol-induced inflammatory response in the brain.

Activation of Melanocortin-4 Receptor Inhibits Both Neuroinflammation Induced by Early Exposure to Ethanol and Subsequent Voluntary Alcohol Intake in Adulthood in Animal Models: Is BDNF the Key Mediator?

The concept that neuroinflammation induced by excessive alcohol intake in adolescence triggers brain mechanisms that perpetuate consumption has strengthened in recent years. The melanocortin system, composed of the melanocortin 4 receptor (MC4R) and its ligand α-melanocyte-stimulating hormone (α-MSH), has been implicated both in modulation of alcohol consumption and in ethanol-induced neuroinflammation decrease. Chronic alcohol consumption in adolescent rats causes a decrease in an α-MSH release by the hypothalamus, while the administration of synthetic agonists of MC4R causes a decrease in neuroinflammation and a decrease in voluntary alcohol consumption. However, the mechanism that connects the activation of MC4R with the decrease of both neuroinflammation and voluntary alcohol consumption has not been elucidated. Brain-derived neurotrophic factor (BDNF) has been implicated in alcohol drinking motivation, dependence and withdrawal, and its levels are reduced in alcoholics. Deficiencies in BDNF levels increased ethanol self-administration in rats. Further, BDNF triggers important anti-inflammatory effects in the brain, and this could be one of the mechanisms by which BDNF reduces chronic alcohol intake. Interestingly, MC4R signaling induces BDNF expression through the activation of the cAMP-responsive element-binding protein (CREB). We hypothesize that ethanol exposure during adolescence decreases the expression of α-MSH and hence MC4R signaling in the hippocampus, leading to a lower BDNF activity that causes dramatic changes in the brain (e.g., neuroinflammation and decreased neurogenesis) that predispose to maintain alcohol abuse until adulthood. The activation of MC4R either by α-MSH or by synthetic agonist peptides can induce the expression of BDNF, which would trigger several processes that lead to lower alcohol consumption.

Oxytocin Neurons Enable Melanocortin Regulation of Male Sexual Function in Mice

The melanocortin pathway has been implicated in both metabolism and sexual function. When the melanocortin 4 receptor (MC4R) is knocked out globally, male mice display obesity, low sexual desire, and copulatory difficulties; however, it is unclear whether these phenotypes are interdependent. To elucidate the neuronal circuitry involved in sexual dysfunction in MC4R knockouts, we re-expressed the MC4R in these mice exclusively on Sim1 neurons (tbMC4RSim1 mice) or on a subset of Sim1 neurons, namely oxytocin neurons (tbMC4Roxt mice). The groups were matched at young ages to control for the effects of obesity. Interestingly, young MC4R null mice had no deficits in sexual motivation or erectile function. However, MC4R null mice were found to have an increased latency to reach ejaculation compared to control mice, which was restored in both tbMC4RSim1 and tbMC4Roxt mice. These results indicate that melanocortin signaling via the MC4R on oxytocin neurons is important for normal ejaculation independent of the male's metabolic health.

NDP-MSH reduces oxidative damage induced by palmitic acid in primary astrocytes

Recent findings relate obesity to inflammation in key hypothalamic areas for body weight control. Hypothalamic inflammation has also been related to oxidative stress. Palmitic acid (PA) is the most abundant free fatty acid found in food, and in vitro studies indicate that it triggers a pro-inflammatory response in the brain. Melanocortins are neuropeptides with proven anti-inflammatory and neuroprotective action mediated by melanocortin receptor 4 (MC4R), but little is known about the effect of melanocortins on oxidative stress. The aim of this study was to investigate whether melanocortins could alleviate oxidative stress induced by a high fat diet (HFD) model. We found that NDP-MSH treatment decreased PA-induced reactive oxygen species production in astrocytes, an effect blocked by the MC4R inhibitor JKC363. NDP-MSH abolished nuclear translocation of Nrf2 induced by PA and blocked the inhibitory effect of PA on superoxide dismutase (SOD) activity and glutathione levels while it also per se increased activity of SOD and γ-glutamate cysteine ligase (γ-GCL) antioxidant enzymes. However, HFD reduced hypothalamic MC4R and brain derived neurotrophic factor mRNA levels, thereby preventing the neuroprotective mechanism induced by melanocortins.

Melanocortin 4 receptor activation protects striatal neurons and glial cells from 3-nitropropionic acid toxicity

α-Melanocyte stimulating hormone (α-MSH) is a melanocortin which exerts potent anti-inflammatory and anti-apoptotic effects. Melanocortin 4 receptors (MC4R) are abundantly expressed in the brain and we previously demonstrated that [Nle(4), D-Phe(7)]melanocyte-stimulating hormone (NDP-MSH), an α-MSH analogue, increased expression of brain derived-neurotrophic factor (BDNF), and peroxisome proliferator-activated receptor-γ (PPAR-γ). We hypothesized that melanocortins could affect striatal cell survival through BDNF and PPAR-γ. First, we determined the expression of these factors in the striatum. Acute intraperitoneal administration (0.5 mg/kg) of α-MSH increased the levels of BDNF mRNA in rat striatum but not in rat cerebral cortex. Also, protein expression of PPAR-γ and MC4R was increased by acute treatment with α-MSH in striatum but not in cortex. No changes were observed by 48 h treatment. Next, we evaluated melanocortins effect on neuron and glial survival. 3-nitropropionic acid (3-NP), which is known to induce striatal degeneration, was used to induce cell death in the rat striatal cell line ST14A expressing mutant human huntingtin (Q120) or in ST14A cells expressing normal human huntingtin (Q15), in primary cultured astrocytes, and in BV2 cells. NDP-MSH protected Q15 cells, astrocytes and BV2 cells from death by 3-NP whereas it did not fully protect Q120 cells. Protection of Q15 cells and astrocytes was blocked by a MC4R specific inhibitor (JKC-363) and a PPAR-γ antagonist (GW9662). The BDNF receptor antagonist (ANA-12) abolished NDP-MSH protective effect in astrocytes but not in Q15 cells. We demonstrate for the first time that melanocortins, acting through PPAR-γ and BDNF, protect neurons and glial cells from 3-NP toxicity.

New Implications for the Melanocortin System in Alcohol Drinking Behavior in Adolescents: The Glial Dysfunction Hypothesis

Alcohol dependence causes physical, social, and moral harms and currently represents an important public health concern. According to the World Health Organization (WHO), alcoholism is the third leading cause of death worldwide, after tobacco consumption and hypertension. Recent epidemiologic studies have shown a growing trend in alcohol abuse among adolescents, characterized by the consumption of large doses of alcohol over a short time period. Since brain development is an ongoing process during adolescence, short- and long-term brain damage associated with drinking behavior could lead to serious consequences for health and wellbeing. Accumulating evidence indicates that alcohol impairs the function of different components of the melanocortin system, a major player involved in the consolidation of addictive behaviors during adolescence and adulthood. Here, we hypothesize the possible implications of melanocortins and glial cells in the onset and progression of alcohol addiction. In particular, we propose that alcohol-induced decrease in α-MSH levels may trigger a cascade of glial inflammatory pathways that culminate in altered gliotransmission in the ventral tegmental area and nucleus accumbens (NAc). The latter might potentiate dopaminergic drive in the NAc, contributing to increase the vulnerability to alcohol dependence and addiction in the adolescence and adulthood.

Exchange factors directly activated by cAMP mediate melanocortin 4 receptor-induced gene expression

G_s protein-coupled receptors regulate many vital body functions by activation of cAMP response elements (CRE) via cAMP-dependent kinase A (PKA)-mediated phosphorylation of the CRE binding protein (CREB). Melanocortin 4 receptors (MC4R) are prototypical G_s-coupled receptors that orchestrate the hypothalamic control of food-intake and metabolism. Remarkably, the significance of PKA for MC4R-induced CRE-dependent transcription in hypothalamic cells has not been rigorously interrogated yet. In two hypothalamic cell lines, we observed that blocking PKA activity had only weak or no effects on reporter gene expression. In contrast, inhibitors of exchange factors directly activated by cAMP-1/2 (EPAC-1/2) mitigated MC4R-induced CRE reporter activation and mRNA induction of the CREB-dependent genes c-fos and thyrotropin-releasing hormone. Furthermore, we provide first evidence that extracellular-regulated kinases-1/2 (ERK-1/2) activated by EPACs and not PKA are the elusive CREB kinases responsible for MC4R-induced CREB/CRE activation in hypothalamic cells. Overall, these data emphasize the pivotal role of EPACs rather than PKA in hypothalamic gene expression elicited by a prototypical G_s-coupled receptor.

[Nle4, D-Phe7]-α-MSH Inhibits Toll-Like Receptor (TLR)2- and TLR4-Induced Microglial Activation and Promotes a M2-Like Phenotype

α-melanocyte stimulating hormone (α-MSH) is an anti-inflammatory peptide, proved to be beneficial in many neuroinflammatory disorders acting through melanocortin receptor 4 (MC4R). We previously determined that rat microglial cells express MC4R and that NDP-MSH, an analog of α-MSH, induces PPAR-γ expression and IL-10 release in these cells. Given the great importance of modulation of glial activation in neuroinflammatory disorders, we tested the ability of NDP-MSH to shape microglial phenotype and to modulate Toll-like receptor (TLR)-mediated inflammatory responses. Primary rat cultured microglia were stimulated with NDP-MSH followed by the TLR2 agonist Pam₃CSK₄ or the TLR4 agonist LPS. NDP-MSH alone induced expression of the M2a/M2c marker Ag1 and reduced expression of the M2b marker Il-4rα and of the LPS receptor Tlr4. Nuclear translocation of NF-κB subunits p65 and c-Rel was induced by LPS and these effects were partially prevented by NDP-MSH. NDP-MSH reduced LPS- and Pam₃CSK₄-induced TNF-α release but did not affect TLR-induced IL-10 release. Also, NDP-MSH inhibited TLR2-induced HMGB1 translocation from nucleus to cytoplasm and TLR2-induced phagocytic activity. Our data show that NDP-MSH inhibits TLR2- and TLR4-mediated proinflammatory mechanisms and promotes microglial M2-like polarization, supporting melanocortins as useful tools for shaping microglial activation towards an alternative immunomodulatory phenotype.

Melanocortin peptides: potential targets in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease resulting in loss of self-tolerance with multiple organs, such as the kidney, skin, joints, and the central nervous system (CNS), being targeted. Numerous immunosuppressant therapies are currently being used for the treatment of SLE, but their clinical utility is somewhat variable because of the clinical heterogeneity. Melanocortins are a family of peptides derived from the common precursor protein pro-opiomelanocortin. These multifunctional peptides activate five subtypes of melanocortin receptors expressed on immune, skin, muscle, bone, and kidney cells and cells within the CNS. Melanocortin peptides have demonstrated a variety of biologic actions including immunomodulation, melanogenesis, and renoprotection. This review aims to introduce the melanocortin system and explore the mechanisms by which they may be beneficial in diseases such as SLE.

Melanocortin 4 receptor activates ERK-cFos pathway to increase brain-derived neurotrophic factor expression in rat astrocytes and hypothalamus

Melanocortins are neuropeptides with well recognized anti-inflammatory and anti-apoptotic effects in the brain. Of the five melanocortin receptors (MCR), MC4R is abundantly expressed in the brain and is the only MCR present in astrocytes. We have previously shown that MC4R activation by the α-melanocyte stimulating hormone (α-MSH) analog, NDP-MSH, increased brain-derived neurotrophic factor (BDNF) expression through the classic cAMP-Protein kinase A-cAMP responsive element binding protein pathway in rat astrocytes. Now, we examined the participation of the mitogen activated protein kinases pathway in MC4R signaling. Rat cultured astrocytes treated with NDP-MSH 1 µM for 1 h showed increased BDNF expression. Inhibition of extracellular signal-regulated kinase (ERK) and ribosomal p90 S6 kinase (RSK), an ERK substrate, but not of p38 or JNK, prevented the increase in BDNF expression induced by NDP-MSH. Activation of MC4R increased cFos expression, a target of both ERK and RSK. ERK activation by MC4R involves cAMP, phosphoinositide-3 kinase (PI3K) and the non receptor tyrosine kinase, Src. Both PI3K and Src inhibition abolished NDP-MSH-induced BDNF expression. Moreover, we found that intraperitoneal injection of α-MSH induces BDNF and MC4R expression and activates ERK and cFos in male rat hypothalamus. Our results show for the first time that MC4R-induced BDNF expression in astrocytes involves ERK-RSK-cFos pathway which is dependent on PI3K and Src, and that melanocortins induce BDNF expression and ERK-cFos activation in rat hypothalamus.

αMSH blunts endotoxin-induced MuRF1 and atrogin-1 upregulation in skeletal muscle by modulating NF-κB and Akt/FoxO1 pathway

Alpha melanocyte stimulating hormone (αMSH) has been shown to have anti-inflammatory and anticachectic actions. We hypothesized that αMSH administration could attenuate the effect of lipopolysaccharide (LPS) on the skeletal muscle through modifications in IGF-Akt-FoxO1 pathway, or/and in serum corticosterone. Adult male Wistar rats were injected with LPS and/or αMSH. αMSH administration reduced LPS-induced increase in liver TNFα and serum nitrites as well as NF-κB activation in skeletal muscle. In contrast, αMSH was not able to prevent the stimulatory effect of LPS on serum concentration of ACTH and corticosterone. LPS decreased serum levels of IGF-I and IGFBP3 and their expression in the liver (P < 0.01). However IGFBP3 expression in the gastrocnemius was increased by LPS. Treatment with αMSH prevented the effects of LPS on IGFBP3 but not on IGF-I. In the gastrocnemius αMSH blocked LPS-induced decrease in pAkt as well as the increase in pNF-κB(p65), FoxO1, atrogin-1, and MuRF1 levels. These results suggest that αMSH blunts skeletal muscle response to endotoxin by downregulating atrogenes and FoxO1 at least in part by controlling NF-κB activation and Akt signalling, but not through modifications in the secretion of corticosterone or IGF-I.

Astrocytes: new targets of melanocortin 4 receptor actions

Astrocytes exert a wide variety of functions with paramount importance in brain physiology. After injury or infection, astrocytes become reactive and they respond by producing a variety of inflammatory mediators that help maintain brain homeostasis. Loss of astrocyte functions as well as their excessive activation can contribute to disease processes; thus, it is important to modulate reactive astrocyte response. Melanocortins are peptides with well-recognized anti-inflammatory and neuroprotective activity. Although melanocortin efficacy was shown in systemic models of inflammatory disease, mechanisms involved in their effects have not yet been fully elucidated. Central anti-inflammatory effects of melanocortins and their mechanisms are even less well known, and, in particular, the effects of melanocortins in glial cells are poorly understood. Of the five known melanocortin receptors (MCRs), only subtype 4 is present in astrocytes. MC4R has been shown to mediate melanocortin effects on energy homeostasis, reproduction, inflammation, and neuroprotection and, recently, to modulate astrocyte functions. In this review, we will describe MC4R involvement in anti-inflammatory, anorexigenic, and anti-apoptotic effects of melanocortins in the brain. We will highlight MC4R action in astrocytes and discuss their possible mechanisms of action. Melanocortin effects on astrocytes provide a new means of treating inflammation, obesity, and neurodegeneration, making them attractive targets for therapeutic interventions in the CNS.

Single administration of tripeptide α-MSH(11-13) attenuates brain damage by reduced inflammation and apoptosis after experimental traumatic brain injury in mice

Following traumatic brain injury (TBI) neuroinflammatory processes promote neuronal cell loss. Alpha-melanocyte-stimulating hormone (α-MSH) is a neuropeptide with immunomodulatory properties, which may offer neuroprotection. Due to short half-life and pigmentary side-effects of α-MSH, the C-terminal tripeptide α-MSH(11-13) may be an anti-inflammatory alternative. The present study investigated the mRNA concentrations of the precursor hormone proopiomelanocortin (POMC) and of melanocortin receptors 1 and 4 (MC1R/MC4R) in naive mice and 15 min, 6, 12, 24, and 48 h after controlled cortical impact (CCI). Regulation of POMC and MC4R expression did not change after trauma, while MC1R levels increased over time with a 3-fold maximum at 12 h compared to naive brain tissue. The effect of α-MSH(11-13) on secondary lesion volume determined in cresyl violet stained sections (intraperitoneal injection 30 min after insult of 1 mg/kg α-MSH(11-13) or 0.9% NaCl) showed a considerable smaller trauma in α-MSH(11-13) injected mice. The expression of the inflammatory markers TNF-α and IL-1β as well as the total amount of Iba-1 positive cells were not reduced. However, cell branch counting of Iba-1 positive cells revealed a reduced activation of microglia. Furthermore, tripeptide injection reduced neuronal apoptosis analyzed by cleaved caspase-3 and NeuN staining. Based on the results single α-MSH(11-13) administration offers a promising neuroprotective property by modulation of inflammation and prevention of apoptosis after traumatic brain injury.

Effect of NDP-α-MSH on PPAR-γ and -β expression and anti-inflammatory cytokine release in rat astrocytes and microglia

Brain inflammation plays a central role in numerous brain pathologies. Microglia and astrocytes are the main effector cells that become activated when an inflammatory process takes place within the central nervous system. α-melanocyte-stimulating hormone (α-MSH) is a neuropeptide with proven anti-inflammatory properties. It binds with highest affinity to the melanocortin receptor 4 (MC4R), which is present in astrocytes and upon activation triggers anti-inflammatory pathways. The aim of this research was to identify anti-inflammatory mediators that may participate in the immunomodulatory effects of melanocortins in glial cells. Since peroxisome proliferator-activated receptors (PPARs) have recently been implicated in the modulation of inflammation, we investigated the effect of an α-MSH analog, [Nle(4), D-Phe(7)]-α-MSH (NDP-α-MSH), on PPAR-β and PPAR-γ gene and protein expression in rat primary astrocytes and microglia. We initially demonstrated that rat primary microglia express MC4R and showed that treatment with NDP-α-MSH increases PPAR-γ protein levels and strongly decreases PPAR-β levels in both astrocytes and microglia. We also showed that extracellular signal-regulated kinase 1/2 (ERK1/2)-mediated signaling is partially involved in these effects in a cell-specific fashion. Finally, we showed that NDP-α-MSH stimulates the release of the anti-inflammatory cytokines IL-10 and TGF-β from microglia and astrocytes, respectively. The presented data suggest a role for IL-10 and TGF-β in the protective action of melanocortins and a connection between MC4R pathway and that of the nuclear receptor PPAR-γ. This is the first report providing evidence that MC4R is expressed in rat primary microglia and that melanocortins modulate PPAR levels in glial cells. Our findings provide new insights into the mechanisms underlying the activation of glial MC4R and open perspectives for new therapeutic strategies for the treatment of inflammation-mediated brain diseases.

Melanocortins in brain inflammation: the role of melanocortin receptor subtypes

The melanocortins (MC) are released from neurons and paracrine cells in the CNS where they are involved in important physiological functions, including regulation of body temperature and immune responses. MC bind to melanocortin receptors, a class of cell surface G-protein-coupled receptors. Of the five subtypes of MC receptors that have been cloned in mammals, the MC1, MC3, MC4 and MC5 receptors are expressed in brain tissues. Expression of MC receptors in both brain cells and cells of the immune system suggests direct involvement of MC in regulation of inflammatory processes in the brain. The binding of MC to MC receptors induces activation of adenylate cyclase, increase in intracellular cAMP level and, consequently, inhibition of the nuclear transcription factor kappaB (NF-κB) signalling. Inflammatory processes contribute to development of severe CNS diseases, both in acute and chronic conditions. Thus far, the anti-inflammatory effects of MC in the CNS have been mainly studied using peptides that are relatively unselective for individual MC receptor subtypes. Consequently, these studies do not allow identification of specific MC receptor(s) involved in the regulation of inflammatory processes. However, recently synthesized ligands selective for individual MC receptors indicated that both MC4 and MC3 agonists are promising anti-inflammatory agents in treatment of brain inflammation.

Effect of periaqueductal gray melanocortin 4 receptor in pain facilitation and glial activation in rat model of chronic constriction injury

OBJECTIVES

Substantial evidence shows that spinal melanocortin 4 receptor (MC4R) may participate in regulation of central sensitization and chronic pain condition induced by peripheral nerve injury. Periaqueductal gray (PAG) is an important component of descending pain facilitatory system and takes part in spinal nociceptive information. This research will choose PAG to discuss the effect of MC4R in pain facilitation induced by chronic constriction injury (CCI) and further discuss its effect in glial activity and inflammatory factor levels in nerve injury.

METHODS

Behavior tests (von Frey test and hot-plate test), semi-quantitative reverse transcription polymerase chain reaction (RT-PCR), and immunohistochemistry were used in this research.

RESULTS

PAG injection of HS014 (a selective inhibitor of MC4R), not only significantly reduced the established mechanical allodynia and thermal hyperalgesia, but also delayed the development of pain facilitation. Semi-quantitative RT-PCR analysis revealed that MC4R and proopiomelanocortin (POMC) expression in PAG was significantly increased after CCI, but agouti-related protein (AgRP) expression decreased. Immunohistochemistry analysis showed that protein levels of astrocytic marker (GFAP), microglial marker (OX-42), tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, and IL-6 were significantly increased, but there was little change of the protein levels of IL-10 following CCI. Furthermore, blockade of MC4R decreased immunoreactivity of glia cells and protein levels of pro-inflammatory cytokines, and increased protein levels of anti-inflammatory cytokine IL-10 after CCI.

DISCUSSION

This research suggests that activation of MC4R in PAG after peripheral nerve injury participates in pain facilitation by regulating the glial activation and inflammatory cytokines secretion.

Melanocortin 4 receptor activation induces brain-derived neurotrophic factor expression in rat astrocytes through cyclic AMP-protein kinase A pathway

Melanocortin 4 receptors (MC4R) are mainly expressed in the brain. We previously showed that the anti-inflammatory action of α-melanocyte-stimulating hormone (α-MSH) in rat hypothalamus and in cultured astrocytes involved MC4R activation. However, MC4R mechanisms of action remain undetermined. Since brain-derived neurotrophic factor (BDNF) may be mediating MC4R hypothalamic anorexigenic actions, we determined melanocortin effects on BDNF expression in rat cultured astrocytes and certain mechanisms involved in MC4R signaling. α-MSH and its analogue NDP-MSH, induced production of cAMP in astrocytes. This effect was completely blocked by the MC4R antagonist, HS024. We found that NDP-MSH increased BDNF mRNA and protein levels in astrocytes. The effect of NDP-MSH on BDNF expression was abolished by the adenylate cyclase inhibitor SQ22536, and decreased by the PKA inhibitor Rp-cAMP. Since melanocortins are immunomodulators, we investigated their actions with bacterial lipopolysaccharide (LPS) and interferon-γ (IFN-γ) stimulus. Although both α-MSH and LPS+IFN-γ increased cAMP responding element binding protein (CREB) activation, LPS+IFN-γ did not modify BDNF expression. On the other hand, α-MSH did not modify basal or LPS+IFN-γ-induced nuclear factor-κB activation. Our results show for the first time that MC4R activation in astrocytes induces BDNF expression through cAMP-PKA-CREB pathway without involving NF-κB.

Reduced cAMP, Akt activation and p65-c-Rel dimerization: mechanisms involved in the protective effects of mGluR3 agonists in cultured astrocytes

In recent decades, astrocytes have emerged as key pieces in the maintenance of normal functioning of the central nervous system. Any impairment in astroglial function can ultimately lead to generalized disturbance in the brain, thus pharmacological targets associated with prevention of astrocyte death are actually promising. Subtype 3 of metabotropic glutamate receptors (mGluR3) is present in astrocytes, its activation exerting neuroprotective roles. In fact, we have previously demonstrated that mGluR3 selective agonists prevent nitric oxide (NO)-induced astrocyte death. However, mechanisms responsible for that cytoprotective property are still subject to study. Although inhibition of adenylyl cyclase by mGluR3 activation was extensively reported, the involvement of reduced cAMP levels in the effects of mGluR3 agonists and the association between cAMP decrease and the downstream pathways activated by mGluR3 remain neglected. Thus, we studied intracellular signaling mediating anti-apoptotic actions of mGluR3 in cultured rat astrocytes exposed to NO. In the present work, we showed that the cytoprotective effect of mGluR3 agonists (LY379268 and LY404039) requires both the reduction of intracellular cAMP levels and activation of Akt, as assessed by MTT and TUNEL techniques. Moreover, dibutyryl-cAMP impairs Akt phosphorylation induced by LY404039, indicating a relationship between mGluR3-reduced cAMP levels and PI3K/Akt pathway activation. We also demonstrated, by co-immunoprecipitation followed by western-blot, that the mGluR3 agonists not only induce per se survival-linked interaction between members of the NF-κB family p65 and c-Rel, but also impede reduction of levels of p65-c-Rel dimers caused by NO, suggesting a possible anti-apoptotic role for p65-c-Rel. All together, these data suggest that mGluR3 agonists may regulate cAMP/Akt/p65-c-Rel pathway, which would contribute to the protective effect of mGluR3 against NO challenge in astrocytes. Our results widen the knowledge about mechanisms of action of mGluR3, potential targets for the treatment of neurodegenerative disorders where a pathophysiological role for NO has been established.

Expression and prognostic significance of iNOS in uveal melanoma

Uveal melanoma (UM) is the most common primary intraocular tumor in adults. Disease metastasis occurs in half of the patients and is uniformly fatal despite systemic therapy. Inducible nitric oxide synthase (iNOS) is associated with disease progression in various malignancies including cutaneous melanoma. In this retrospective cohort, we examined the prognostic value of iNOS in UM by performing immunohistochemistry on paraffin-embedded sections of primary tumors (90 patients) and matched primary and metastatic hepatic tumors (19 patients) with complete histopathological and clinical data. We show that iNOS is expressed in UM (57% of the patients) and high iNOS levels significantly (p = 0.04; hazard ratio (HR) = 2.3) predict disease-specific survival (DSS) as assessed by Kaplan-Meier analysis and univariate Cox's proportional hazards regression model. Furthermore, high iNOS expression in the UM primary tissue was significantly associated with metastatic disease and vice versa. Expression of iNOS in hepatic metastases significantly (p = 0.02) predicted a shortened survival as assessed by Kaplan-Meier analysis. However, iNOS did not appear to be a significant (p = 0.16; HR = 1.9) factor in the multivariate Cox's regression analysis performed together with the clinical parameters tumor diameter, tumor cell type, and tumor location in which only tumor diameter predicted DSS. In conclusion, iNOS predicts DSS in UM and may play a role in disease progression but it is not an independent prognostic factor.

Central interleukin-10 attenuates lipopolysaccharide-induced changes in food intake, energy expenditure and hypothalamic Fos expression

Lipopolysaccharide (LPS) is often used to mimic acute infection and induces hypophagia, the selective partitioning of fat for energy, and fever. Interleukin-10 (IL-10) is an anti-inflammatory cytokine expressed in the brain which attenuates LPS-induced hypophagia; however the potential sites of interaction within the brain have not been investigated. Hypothalamic orexin (ORX) and melanin-concentrating hormone (MCH) regulate energy expenditure and food intake although the regulation of these neuropeptides through the interactions between central IL-10 and the inflammatory consequences of peripheral LPS have not been investigated. The present study in the rat investigated during the dark phase of the light-dark cycle the ability of central IL-10 (250 ng, i.c.v.) to attenuate the changes in food intake, energy substrate partitioning, and central Fos expression within the hypothalamus to peripheral LPS (100 microg/kg, i.p.); Fos expression changes specifically within ORX and MCH neurons were also investigated. Central IL-10 attenuated the peripheral LPS-induced hypophagia, reduction in motor activity, fever and reduction in respiratory exchange ratio. Central IL-10 also attenuated peripheral LPS-induced increases in Fos expression within ORX neurons and decreases in Fos expression within unidentified cells of the caudal arcuate nucleus. In contrast, both IL-10 and LPS injection independently decreased Fos expression within MCH neurons. The present study provides further insight into the interactions within the brain between the anti-inflammatory cytokine IL-10, the inflammatory consequences of LPS, and neuropeptides known to regulate energy expenditure and food intake.

Role of alpha-melanocyte stimulating hormone and melanocortin 4 receptor in brain inflammation

Inflammatory processes contribute widely to the development of neurodegenerative diseases. The expression of many inflammatory mediators was found to be increased in central nervous system (CNS) disorders suggesting that these molecules are major contributors to neuronal damage. Melanocortins are neuropeptides that have been implicated in a wide range of physiological processes. The melanocortin alpha-melanocyte stimulating hormone (alpha-MSH) has pleiotropic functions and exerts potent anti-inflammatory actions by antagonizing the effects of pro-inflammatory cytokines and by decreasing important inflammatory mediators. Five subtypes of melanocortin receptors (MC1R-MC5R) have been identified. Of these, the MC4 receptor is expressed predominantly throughout the CNS. Evidence of effectiveness of selective MC4R agonists in modulating inflammatory processes and their low toxicity suggest that these molecules may be useful in the treatment of CNS disorders with an inflammatory component. This review describes the involvement of the MC4R in central anti-inflammatory effects of melanocortins and discusses the potential value of MC4R agonists for the treatment of inflammatory-related disorders.

Role of metabotropic glutamate receptors in the control of neuroendocrine function

Glutamate exerts its effects through binding and activation of two classes of specific receptors: ionotropic (iGluRs) and metabotropic (mGluRs). Group I mGluR includes mGluR1 and mGluR5 subtypes, group II includes mGluR2 and mGluR3 subtypes and group III includes the subtypes mGluR 4, 6, 7 and 8. Glutamate and its receptors are found in all key hypothalamic areas critically involved in reproduction and neuroendocrine function. To date, considerable data support an important role for iGluRs in the control of neuroendocrine function; however, the role of mGluRs as regulators of hypothalamic-pituitary function has not been clearly elucidated. mGluRs could be exerting a fine tune on the release of hypothalamic factors that regulate hormone release such as Substance P, GABA, alpha-MSH and CRH. Group II mGluR exert a direct inhibitory effect on anterior pituitary prolactin and GH secretion. Moreover, some group II mGluR agonists, like LY 354,740 and LY 379,268, can modulate PRL secretion from the anterior pituitary through their actions as dopamine receptor agonists. Evidence suggests a role for group III mGluR subtypes in stress-related behavioral disorders. Several reports indicate that selective ligands for mGluR subtypes have potential for the treatment of a wide variety of neurological and psychiatric disorders, including depression, anxiety disorders, schizophrenia, epilepsy and Alzheimer's disease among others. Since converging lines of evidence suggest a role for mGluRs subtypes in neuroendocrine regulation of hormone secretion, mGluRs neuroendocrine actions must be taken in consideration to insure proper treatment of these diseases. Moreover, discovery of selective agonists provides an opportunity to investigate the physiological role of mGluR subtypes and to directly test the neuroendocrine actions of mGluRs. Finally, mGluRs selective agonists may have an impact in the treatment of conditions involving chronic stress, such as depression and anxiety disorders, since they regulate neuroendocrine stress circuits involving the HPA axis and stress-sensitive hormones such as oxytocin and prolactin. This review aims to provide a survey of our current understanding of the effects of mGluR activation on neuroendocrine function.

Activation of melanocortin 4 receptors reduces the inflammatory response and prevents apoptosis induced by lipopolysaccharide and interferon-gamma in astrocytes

Alpha-MSH exerts an immunomodulatory action in the brain and may play a neuroprotective role acting through melanocortin 4 receptors (MC4Rs). In the present study, we show that MC4Rs are constitutively expressed in astrocytes as determined by immunocytochemistry, RT-PCR, and Western blot analysis. alpha-MSH (5 microm) reduced the nitric oxide production and the expression of inducible nitric oxide synthase (iNOS) induced by bacterial lipopolysaccharide (LPS, 1 microg/ml) plus interferon-gamma (IFN-gamma, 50 ng/ml) in cultured astrocytes after 24 h. alpha-MSH also attenuated the stimulatory effect of LPS/IFN-gamma on prostaglandin E(2) release and cyclooxygenase-2 (COX-2) expression. Treatment with HS024, a selective MC4R antagonist, blocked the antiinflammatory effects of alpha-MSH, suggesting a MC4R-mediated mechanism in the action of this melanocortin. In astrocytes, LPS/IFN-gamma treatment reduced cell viability, increased the number of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling-positive cells and activated caspase-3. alpha-MSH prevented these apoptotic events, and this cytoprotective effect was abolished by HS024. LPS/IFN-gamma decreased Bcl-2, whereas it increased Bax protein expression in astrocytes, thus increasing the Bax/Bcl-2 ratio. Alpha-MSH produced a shift in Bax/Bcl-2 ratio toward astrocyte survival because it increased Bcl-2 expression and also prevented the effect of LPS/IFN-gamma on Bax and Bcl-2 expression. In summary, these findings suggest that alpha-MSH, through MC4R activation, attenuates LPS/IFN-gamma-induced inflammation by decreasing iNOS and COX-2 expression and prevents LPS/IFN-gamma-induced apoptosis of astrocytes by modulating the expression of proteins of the Bcl-2 family.

Role of nitric oxide in alpha-melanocyte-stimulating hormone-induced hypotension in the nucleus tractus solitarii of the spontaneously hypertensive rats

Pro-opiomelanocortin (POMC) is expressed in the nucleus tractus solitarii (NTS) of the brainstem, where nitric oxide (NO) plays an important role in cardiovascular regulation. The POMC-derived neuropeptides and their receptors are important regulators of energy homeostasis and cardiovascular functions in the central nervous system. In this study, we investigated the cardiovascular effect of alpha-melanocyte-stimulating hormone (alpha-MSH), a POMC-derived neuropeptide, and its relationship with NO pathway in the NTS of spontaneously hypertensive rats (SHR). Unilateral microinjection of alpha-MSH (0.3-300 pmol) into the NTS resulted in a dose-dependent hypotension and bradycardia in urethane-anesthetized SHR. The alpha-MSH-induced hypotension was abolished by pretreatment with the antagonist of melanocortin-3/4 receptor (MC-3/4R), Ac-Nle-c[Asp-His-D-Nal(2')-Arg-Trp-Lys]-NH2 (SHU9119). Blockade of cAMP/protein kinase A (PKA), the downstream effector of melanocortin receptors, by previous injection of N-[2-(4-bromocinnamylamino)ethyl]-5-isoquinoline (H89) also ablated the cardiovascular effect of alpha-MSH. To elucidate the role of NO pathway in alpha-MSH-evoked hypotension, pretreatment with Nomega-nitro-L-arginine methyl ester, a universal inhibitor of nitric-oxide synthase (NOS), partially reversed the depressor and bradycardic effects of alpha-MSH. Furthermore, previous application of the inducible NOS (iNOS) inhibitor, aminoguanidine, but not the neuronal NOS inhibitor, 7-nitroindazole, attenuated the cardiovascular effect of alpha-MSH. Histological analysis revealed the colocalization of MC-4R, but not MC-3R, with iNOS in the NTS of SHR. In summary, intra-NTS injection of alpha-MSH induces hypotension and bradycardia of SHR via MC-4R signaling, which activates cAMP/PKA and iNOS.

α-MSH and γ-MSH modulate early release of hypothalamic PGE2 and NO induced by IL-1β differently

Interleukin-1beta (IL-1beta) stimulates corticotropin-releasing hormone (CRH) secretion in hypothalamus, which involves the release of prostaglandins (PGE2) and nitric oxide (NO). We have demonstrated that melanocortins can inhibit the early effects of IL-1beta on the HPA axis by acting on the central nervous system (CNS). Our study investigated whether alpha-melanocyte stimulating hormone (alpha-MSH) and gamma-MSH could inhibit IL-1beta-induced PGE2 and NO release in hypothalamus in the rapid activation of the HPA axis. An i.c.v. injection of 12.5 ng/microl of IL-1beta significantly increased the release of PGE2 and NOS activity in the hypothalamus. Treatment with alpha-MSH (0.1 microg/microl) inhibited the effect of IL-1beta on PGE2 release. Also, gamma-MSH (1 microg/microl) eliminated the increase in NOS activity induced by IL-1beta. Our data indicate the modulatory role of melanocortins in the early hypothalamic response to IL-1beta, with different regulation of PGE2 and NO release.

Melanocortin-4 receptor-mediated inhibition of apoptosis in immortalized hypothalamic neurons via mitogen-activated protein kinase

The melanocortin-4 receptor (MC4R) is a seven transmembrane member of the melanocortin receptor family. The GT1-1 cell line exhibits endogenous expression of MC4R. In this study, GT1-1 cells were used to study MC4R signaling pathways and to examine the effects of melanocortin receptor agonist NDP-MSH on apoptosis. MC4R mRNA expression was demonstrated by RT-PCR. Functional melanocortin receptor expression was implied by specific binding of NDP-MSH and cAMP production. NDP-MSH-stimulated GnRH release in a dose-dependent manner. Serum deprivation-induced apoptosis in GT1-1 cells, and the NDP-MSH inhibited this effect. The melanocortin receptor antagonist SHU9119 blocked the antiapoptotic actions of NDP-MSH, and the MAP kinase inhibitor PD98059 significantly attenuated the antiapoptotic effect. NDP-MSH-stimulated ERK1/2 phosphorylation in a dose-dependent manner. ERK1/2 phosphorylation could be abolished by SHU9119. In GT1-1 cells, melanocortin receptor activation causes ERK1/2 phosphorylation. In these cells, MC4R activation is also associated with antiapoptotic effects.

The melanocortin system in leukocyte biology

The melanocortin system is composed of the melanocortin peptides, adrenocorticotropic hormone and alpha-, beta-, and gamma-melanocyte-stimulating hormone, the melanocortin receptors (MCRs), and the endogenous antagonists agouti- and agouti-related protein. Melanocortin peptides exert multiple effects upon the host, including anti-inflammatory and immunomodulatory effects. Leukocytes are a source of melanocortins and a major target for these peptides. Because of reduced translocation of the nuclear factor NF-kappaB to the nucleus, MCR activation by their ligands causes a collective reduction of the most important molecules involved in the inflammatory process. This review examines how melanocortin peptides and their receptors participate in leukocyte biology.

Alpha-melanocyte-stimulating hormone gene therapy reverses carbon tetrachloride induced liver fibrosis in mice

BACKGROUND

Hepatic fibrosis represents a process of healing and scarring in response to chronic liver injury. Effective therapies are lacking. We have previously demonstrated that alpha-melanocyte-stimulating hormone (alpha-MSH) gene therapy protects against thioacetamide-induced acute liver failure in mice. Recent reports showed that collagen metabolism is a novel target of alpha-MSH. Therefore, the aim of this study is to investigate whether alpha-MSH gene therapy possesses anti-hepatic fibrogenic effect in mice.

METHODS

Liver fibrosis was induced in mice by administering carbon tetrachloride (CCl4) continuously for 10 weeks. Alpha-MSH expression plasmid was delivered via electroporation after liver fibrosis had been established. Histopathology, reverse-transcription polymerase chain reaction (RT-PCR), immunoblotting, and gelatin zymography were used to investigate its possible mechanisms of action.

RESULTS

Alpha-MSH gene therapy reversed established liver fibrosis in CCl4-treated mice. RT-PCR revealed that alpha-MSH gene therapy attenuated the liver TGF-beta1, collagen alpha1, and cell adhesion molecule mRNA upregulation. Following gene transfer, both the activation of alpha-smooth muscle actin (alpha-SMA) and cyclooxygenase-2 (COX-2) was significantly attenuated. Further, alpha-MSH significantly increased matrix metalloproteinase (MMP) activity with tissue inhibitors of matrix metalloproteinase (TIMP) inactivation.

CONCLUSIONS

We have demonstrated that alpha-MSH gene therapy reversed established liver fibrosis in mice. It also prevented the upregulated fibrogenic and proinflammatory gene response after CCl4 administration. Its collagenolytic effect may be attributed to MMP and TIMP modulation. In summary, alpha-MSH gene therapy may be an effective therapeutic modality against liver fibrosis with potential clinical use.

The MC3 receptor binding affinity of melanocortins correlates with the nitric oxide production inhibition in mice brain inflammation model

Melanocortins possess strong anti-inflammatory effects acting in the central nervous system via inhibition of the production of nitric oxide (NO) during brain inflammation. To shed more light into the role of melanocortin (MC) receptor subtypes involved we synthesized and evaluated some novel peptides, modified in the melanocyte-stimulating hormone (MSH) core structure, natural MCs and known MC receptor selective peptides - MS05, MS06. Since the study included both selective, high affinity binders and the novel peptides, it was possible to do the correlation analysis of binding activities and the NO induction-related anti-inflammatory effect of the peptides. beta-MSH, gamma1-MSH, gamma2-MSH, alpha-MSH, MS05, Ac-MS06 and Ac-[Ser12]MS06 caused dose dependent inhibition of the lipopolysaccharide (LPS)-induced increase of NO overproduction in the mice forebrain whereas MSH core modified peptides Ac-[Asp9,Ser12]MS06, [Asp9]alpha-MSH and [Asp16]beta-MSH were devoid of this effect in doses up to 10 nmol per mouse. When the minimal effective dose required for inhibition of NO production was correlated with the in vitro binding activity to MC receptor subtypes a strong and significant correlation was found for the MC3 receptor (r = 0.90; p = 0.0008), whereas weak correlation was present for the other receptors. Our results suggest that the MC3 receptor is the major player in mediating the anti-inflammatory activity of MCs in the central nervous system.

Electroporative alpha-MSH gene transfer attenuates thioacetamide-induced murine hepatic fibrosis by MMP and TIMP modulation

Hepatic fibrosis represents a process of healing and scarring in response to chronic liver injury. alpha-Melanocyte-stimulating hormone (alpha-MSH) is a 13-amino-acid peptide with potent anti-inflammatory effects. We have previously demonstrated that alpha-MSH gene therapy protects against thioacetamide (TAA)-induced acute liver failure. Therefore, the aim of this study is to investigate whether alpha-MSH gene therapy possesses antihepatic fibrogenic effect. Liver fibrosis was induced by long-term TAA administration in mice. alpha-Melanocyte-stimulating hormone expression plasmid was delivered via electroporation after liver fibrosis was established. Our results showed that alpha-MSH gene therapy attenuated liver fibrosis in TAA-treated mice. Reverse transcription polymerase chain reaction revealed that alpha-MSH gene therapy attenuated the liver transforming growth factor-beta1, collagen alpha1 and cell adhesion molecule mRNA upregulation. Following gene transfer, the expression of alpha-smooth muscle actin and cyclooxygenase-2 were both significantly attenuated. Further, alpha-MSH significantly increased matrix metalloproteinase (MMP), while tissue inhibitors of matrix metalloproteinase (TIMPs) were inactivated. In summary, alpha-MSH gene therapy reversed established liver fibrosis in mice and prevented the upregulated fibrogenic and pro-inflammatory gene responses after TAA administration. Its collagenolytic effect might be attributed to MMP and TIMP modulation. Hence, alpha-MSH gene therapy may be an effective therapeutic modality against liver fibrosis with potential clinical use.

Gene transfer of pro-opiomelanocortin prohormone suppressed the growth and metastasis of melanoma: involvement of alpha-melanocyte-stimulating hormone-mediated inhibition of the nuclear factor kappaB/cyclooxygenase-2 pathway

Pro-opiomelanocortin (POMC) is a prohormone of various neuropeptides, including corticotropin, alpha-melanocyte-stimulating hormone (alpha-MSH), and beta-endorphin (beta-EP). POMC neuropeptides are potent inflammation inhibitors and immunosuppressants and may exert opposite influences during tumorigenesis. However, the role of POMC expression in carcinogenesis remains elusive. We evaluated the antineoplastic potential of POMC gene delivery in a syngenic B16-F10 melanoma model. Adenovirus-mediated POMC gene delivery in B16-F10 cells increased the release of POMC neuropeptides in cultured media, which differentially regulated the secretion of pro- and anti-inflammatory cytokines in lymphocytes. POMC gene transfer significantly reduced the anchorage-independent growth of melanoma cells. Moreover, pre- or post-treatment with POMC gene delivery effectively retarded the melanoma growth in mice. Intravenous injection of POMC-transduced B16-F10 cells resulted in reduced foci formation in lung by 60 to 70% of control. The reduced metastasis of POMC-transduced B16-F10 cells could be attributed to their attenuated migratory and adhesive capabilities. POMC gene delivery reduced the cyclooxygenase-2 (COX-2) expression and prostaglandin (PG) E(2) synthesis in melanoma cells and tumor tissues. In addition, application of NS-398, a selective COX-2 inhibitor, mimicked the antineoplastic functions of POMC gene transfer in melanoma. The POMC-mediated COX-2 down-regulation was correlated with its inhibition of nuclear factor kappaB (NFkappaB) activities. Exogenous supply of alpha-MSH inhibited NFkappaB activities, whereas application of the alpha-MSH antagonist growth hormone-releasing peptide-6 (GHRP-6) abolished the POMC-induced inhibition of NFkappaB activities and melanoma growth in mice. In summary, POMC gene delivery suppresses melanoma via alpha-MSH-induced inhibition of NFkappaB/COX-2 pathway, thereby constituting a novel therapy for melanoma.