therapeutic potential of melatonin ligands
TRANSCRIPT
THERAPEUTIC POTENTIAL OF MELATONIN LIGANDS
Philippe Delagrange1 and Jean A. Boutin2
1Departement des Sciences Experimentales, Institut de Recherches Servier,Suresnes, France2Pharmacologie Moleculaire et Cellulaire, Institut de Recherches Servier,Croissy sur Seine, France
Melatonin is a neurohormone that is believed to be involved in a wide range of phys-iological functions. In humans, appropriate clinical trials confirm the efficacy of mela-tonin or melatoninergic agonists for the MT1 and MT2 receptor subtypes in circadianrhythm sleep disorders only. Nevertheless, preclinical animal model studies relevant tohuman pathologies involving validated reference compounds lead to other therapeuticpossibilities. Among these is a recently developed treatment concept for depression,which has been validated by the clinical efficacy of agomelatine, an agent havingboth MT1 and MT2 agonist and 5-HT2C antagonist activity. A third melatoninbinding site has been purified and characterized as the enzyme quinone reductase 2(QR2). The physiological role of this enzyme is not yet known. Recent results obtainedby different groups suggest: (1) that inhibition of QR2 may lead to “protective” effectsand (2) that over-expression of this enzyme may have deleterious effects. The inhibi-tory effect of melatonin on QR2 observed in vitro may explain the protective effectsreported for melatonin in different animal models, such as cardiac or renal ische-mia—effects that have been attributed to the controversial antioxidant properties ofthe hormone. The development of specific ligands for each of these melatoninbinding sites is necessary to link physiological and/or therapeutic effects.
Keywords Melatonin Receptors, Melatonin Ligands, MT3, Quinone Reductase 2
INTRODUCTION
Melatonin is a neurohormone synthesized and secreted primarily bythe pineal gland in a circadian manner, with peak levels in all speciesoccurring during the period of darkness. The synthesis of melatoninwithin the pineal gland is mainly regulated by the daily and seasonalchanges in the environmental light/dark (LD) cycle. Melatonin, which isreleased into the blood circulation, is thought to transduce photoperiodic
Address correspondence to Dr. Philippe Delagrange, Departement des Sciences Experimentales,Institut de Recherches Servier, 11 rue des Moulineaux 92150, Suresnes, France. E-mail: [email protected]
Chronobiology International, 23(1&2): 413–418, (2006)Copyright # 2006 Taylor & Francis Group, LLCISSN 0742-0528 print/1525-6073 onlineDOI: 10.1080/07420520500464387
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information to all the tissues expressing melatonin receptors or otherbinding sites (Reiter, 1991).
MT1 AND MT2 MELATONIN RECEPTORS SUBTYPES
Two melatonin receptors, MT1 and MT2, have been cloned from thetissue of humans and other vertebrate species. The peculiarity of thesereceptors, compared to other G-protein–coupled receptors, is that theyare expressed in very low density, except in the pars tuberalis. Forexample, the level of expression of the MT2 receptor subtype in the supra-chiasmatic nucleus is so low that 2-[125I]-melatonin binding cannot bedetected in MT1 knockout mice, even though melatonin is able toinduce a phase-shift of the circadian neuronal activity (Liu et al., 1997).This weak expression, along with the paucity of pharmacological toolsspecific for each receptor subtype (MT1 or MT2 agonists, MT1 antagonists,and MT1 or MT2 antibodies for immunohistochemistry), does not enablethe attribution of specific roles for each subtype even after the targeted dis-ruption of each subtype in mice (Jin et al., 2003).
Melatonin is thought to exert a wide range of physiological effects andto be involved in certain pathologies (Boutin et al., 2005). In most cases,however animal models are not relevant to human pathologies and/orvalidated by reference compounds. In order to demonstrate a true thera-peutic effect, the efficacy of a substance must be proven in appropriate andcontrolled clinical trials. In the case of melatonin or selective melatoninligands, clinical trials have only confirmed their chronobiotic activity(Herxheimer and Petrie, 2002; Zammit et al., 2005) or efficacy in the treat-ment of insomnia (Zemlan et al., 2005). Thus, RozeremTM (Ramelteon,Takeda Pharmaceuticals-North America, Lincolnshire, IL, USA), a selec-tive MT1/MT2 agonist, was recently approved by the U.S. Food DrugAdministration for the treatment of sleep-onset insomnia (http://www.fda.gov).
Recently, a novel concept in the treatment of depression involving theaction of agomelatine—anMT1 andMT2 agonist and 5-HT2C antagonist—was developed and clinically validated. The effect of agomelatine wasstudied in rats subjected to chronic mild stress, a well validated model ofdepression, and compared to the effect of melatonin (Papp et al., 2003).When administered in the evening, both substances showed antidepress-ant-like activity, but when administered in the morning, only agomelatineexhibited antidepressant-like activity. The effect of the evening adminis-tration of both agomelatine and melatonin was completely inhibited by aMT1/MT2 antagonist, whereas the morning effect of agomelatine wasnot inhibited. These findings suggest that a chronobiotic effect is certainlyrequired, but, alone, it is not sufficient to achieve antidepressant activity.The antidepressant effect of agomelatine appears to involve a combined
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action, both as an agonist at the level of themelatoninMT1 andMT2 recep-tors and as an antagonist at the level of the 5-HT2C receptor (Audinot et al.,2003; Millan et al., 2003). Clinical trials confirm the efficacy of agomelatinein the treatment of major depression (Loo et al., 2002). Moreover, a ran-domized, doubled-blind, placebo-controlled discontinuation study hasshown that agomelatine is not associated with discontinuation symptomsas is the case for most antidepressants (Montgomery et al., 2004).
MT3 MELATONIN BINDING SITES
Other melatonin binding sites have been identified in the brain andperipheral tissues of different species. Among these is the melatoninbinding site named MT3 (previously ML2), first described in the Syrianhamster brain by Dubocovich (1988). In contrast to the MT1/MT2 recep-tor subtypes, MT3 has a specific pharmacological profile with affinity forprazosin and N-acetylserotonin. A specific radioligand, 2-[125I]iodo-5-methoxy-carbonylamino-N-acetyltryptamine (2-[125I]-MCA-NAT), hasbeen characterized by Molinari and colleagues (1996). This receptor dis-plays rapid association and dissociation kinetics; thus, early studies wereperformed at a non-physiological temperature (48C). The unique require-ments for the study of this site initially slowed its research. A series ofbinding experiments using methods to counteract the fast kinetic par-ameters showed melatonin binding to the MT3 site is still detectable at378C and that MT3 is present in various peripheral organs, such askidney, heart, small intestine, and liver of different species (Paul et al.,1999). Interestingly, in all these tissues, melatonin is reported to exertprotective effects against a wide variety of experimental lesions. Themechanism of action is not receptor-mediated, but generally ascribed tothe controversial antioxidant capacity of melatonin (Reiter et al., 1995;Osseni et al., 2000).
The MT3 melatonin binding site was recently purified from hamsterkidney and characterized as the human homolog of quinone reductase 2(QR2) (Nosjean et al., 2000). The main characteristics of QR2 are summar-ized in Figure 1. More recently, the generation of QR22/ 2 mice in whichall tissues are depleted of 2-[125I]-MCA-NAT binding confirmed the mol-ecular identity between QR2 andMT3 (Mailliet et al., 2004). By analogy toits homolog quinone reductase 1, QR2 was suggested to be a detoxificationenzyme; however, according to recent reports, this role is not so clear. Forexample, mice lacking the QR2 gene are less sensitive to menadione tox-icity than are wild-type mice (Long et al., 2002). In humans, Parkinson’sdisease and schizophrenia are reported to be associated with polymorph-isms of QR2 (Harada et al., 2001, 2003). This polymorphism consists ofa deletion/insertion of 29 base-pair nucleotides in the promoter regionof the QR2 gene. Human cells that express a promoter containing the
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insertion polymorphism show significantly lower QR2 gene expression(Wang and Jaiswal, 2004). It is speculated that elevated QR2 activitymight make individuals more susceptible to certain neurologic disorders,including schizophrenia and Parkinson’s disease.
Melatonin inhibits QR2 activity at concentrations ranging from 600nMto 300mM, depending on the co-substrate and substrate used in protocols(Ferry, Delagrange, and Boutin, unpublished data). These high concen-trations are similar to those reported for the antioxidant and/or protectiveeffect of melatonin. The hypothesis that the antioxidant effect of melatoninmight involve QR2 is reinforced by a recent publication on the interactionbetween resveratrol and QR2. Resveratrol is a phyto-polyphenol isolatedfrom grapes and is present in significant amounts in wine. Resveratrol isclaimed to have neuroprotective, cardioprotective, and anti-aging proper-ties like melatonin. Buryanosvkyy and colleagues (2004) reported thatresveratrol is a potent QR2 inhibitor (Kd ¼ 34nM). Resveratrol decreasesthe cellular toxicity of menadione and a similar effect is observed whenQR2 expression is suppressed with a RNAi approach. The QR2 knock-down cells exhibit increased antioxidant and detoxification enzymeexpression.
The relation between the antioxidant effects of these moleculesand QR2 needs to be tested with melatonin or MT3 ligands. MCA-NAT
FIGURE 1 Summary of QR2 main characteristics.
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has already been reported to possess protective effects against ischemia-reperfusion injury in the isolated rat heart (Lagneux et al., 2000).
CONCLUSION
According to studies involving animal models relevant to human path-ologies and appropriate clinical trials, circadian sleep disorders and insom-nia are probably the sole therapeutic indications today for selectivemelatonin MT1/MT2 ligands. Nevertheless, clinical trials confirm that sub-stances such as agomelatine, which possesses combined actions as a mela-tonin receptor agonist and as a 5-HT2C receptor antagonist, represent atotally novel therapy for depression. The characterization of a melatoninbinding site on QR2 provides new prospects and will require the develop-ment of selective, long-acting inhibitors to validate the protective effects ofMT3 ligands in humans. Moreover, the complexity of the pharmacology ofmelatonin and its modulatory activity suggest the existence of other mela-tonin binding site(s) or receptors for this neurohormone.
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