COMMENTARY Lipoxin A4 is an allosteric endocannabinoid that strengthens anandamide-induced CB1 receptor activation Roger G. Pertwee1 School of Medical Sciences, Institute of Medical Sciences, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD, Scotland major advance in the field of effective by themselves produced catalepsy cannabinoid research was the in mice when they were coadministered, ii discovery of the endocannabi- ( ) LXA4 produced a marked leftward A – noid system, which is currently shift in the log concentration response thought to consist of two G protein- curve of AEA for its inhibition of for- skolin-induced stimulation of cAMP pro- coupled receptors (cannabinoid CB1 and duction by mouse CB1-HEK cells, and (iii) CB2 receptors) and endogenous com- LXA4 also augmented AEA-induced in- pounds such as arachidonoylethanolamide + (i.e., anandamide; AEA; Fig. 1) and creases in inward K currents in CB1 re- ceptor-containing Xenopus laevis oocytes. 2-arachidonoyl glycerol (2-AG) that can Pamplona et al. (9) also find that, at activate these receptors and are known a concentration at which LXA4 potenti- as endocannabinoids (1). This system of ated AEA in vitro (100 nM), it also slows receptors and endogenous agonists, which the dissociation of [3H]CP55940 from is also made up of enzymes that catalyze specific binding sites in mouse brain endocannabinoid biosynthesis or metabolic membranes, which is widely accepted to be degradation, and of processes responsible a strong indication of ligand-induced al- for the cellular uptake of endocannabi- losteric modulation (13). Other experi- noids, is thought to have numerous roles ments that Pamplona et al. (9) perform in both health and disease (2, 3). Some of show that LXA4 is present in significant these are “autoprotective” in nature and amounts in mouse hippocampus, cortex, hence beneficial, with examples including and cerebellum. Consequently, they pos- the amelioration of inflammatory pain, tulate that AEA is potentiated by LXA4 multiple sclerosis, and Parkinson disease; not only when this lipoxin is administered exogenously but also when it has been whereas a few of its other roles, for ex- Fig. 1. Upper: Structures of LXA4 and AEA. Lower: ample, in obesity, are “autoimpairing,” produced endogenously. This hypothesis Comparison of how an FAAH inhibitor (FAAH-I) fi fi and therefore unwanted. AEA, 2-AG, and a CB1 receptor allosteric enhancer could affect is supported by their ndings, rst, that and other “direct” cannabinoid receptor the potency, efficacy, and selectivity with which intracerebroventricularly injected AEA agonists are thought to trigger G protein- endogenously released AEA targets cannabinoid produces much less catalepsy in mice from CB1 receptors in the brain. Further details are pro- which the LXA4-synthesizing enzyme mediated signaling of CB1 and CB2 re- ceptors by targeting orthosteric sites on videdinthetext. 5-lipoxygenase has been genetically de- these receptors (1). There is evidence, leted than in WT mice, and, second, that this effect of AEA can also be attenuated however, that the CB1 receptor also con- at doses of 0.1 and 1 pmol, LXA4 can act “ ” in an FPR2/ALX receptor-independent by the 5-lipoxygenase inhibitor MK-886. tains one or more allosteric sites that Because, when administered alone, LXA manner to produce a set of four effects: 4 can be targeted by allosteric modulators produces behavioral effects in mice that hypolocomotion, catalepsy, hypothermia, in a manner that can enhance or reduce appear to be mediated by CB receptors, fi and antinociception in a hotplate test. 1 the ef cacy with which direct agonists it is also likely that it can increase the – These effects of LXA all appeared to be activate this receptor orthosterically (4 7). 4 ability of endogenously released AEA to CB receptor-mediated because they (i) Just as the discovery of the CB1 receptor 1 activate these receptors. could be prevented by the CB -selective prompted a search for endogenous ligands 1 It has long been known that AEA- antagonist/inverse agonist SR141716A, for this receptor (8), so too the discovery induced activation of CB receptors can (ii) were not detectable in mice from 1 that CB receptors contain allosteric sites also be enhanced by drugs that inhibit its 1 which the cannabinoid CB receptor had has prompted a need to look for an en- 1 metabolism by fatty acid amide hydrolase been genetically deleted, and (iii) are dogenous CB allosteric modulator. This (FAAH) (3). However, there are three 1 known to be induced by established CB need has now been met by Pamplona et al. 1 important differences between the ways in receptor agonists (1). Importantly, the re- which an allosteric enhancer and an (9), who, in PNAS, present evidence that sults obtained in this investigation (9) also the endogenous anti-inflammatory ligand FAAH inhibitor increase the activation of suggest that LXA4 did not induce this the CB receptor by endogenously re- lipoxin A4 (LXA4; Fig. 1) can allosterically “tetrad” of effects through direct activa- 1 enhance AEA-induced activation of leased AEA (Fig. 1). First, an FAAH in- tion of the CB1 receptor, as it did not hibitor will produce such an increase by CB1 receptors within the brain when it is share the well known ability of established administered exogenously and when it is elevating the concentration of AEA at the CB1 receptor agonists to produce a com- CB receptor, whereas an allosteric en- produced endogenously. This is a ligand plete displacement of [3H]SR141716A 1 that is already known to target the fi hancer will produce it by increasing the from speci c binding sites in mouse brain potency and/or efficacy with which AEA FPR2/ALX receptor as an agonist, mainly membranes, or to inhibit forskolin-in- outside the brain, and, like AEA and duced stimulation of cAMP production by 2-AG, to be an eicosanoid that is formed mouse CB1-transfected HEK cells. In- from arachidonic acid (10–12). stead, it most likely acted by potentiating Author contributions: R.G.P. wrote the paper. The author declares no conflict of interest. In their PNAS paper, Pamplona et al. the activation of CB1 receptors by AEA, as (9) present data showing that, when ad- (i) intracerebroventricular injections of See companion article 10.1073/pnas.1202906109. 1 ministered to mice intracerebroventricularly doses of AEA and LXA4 that were sub- E-mail: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1218529110 PNAS Early Edition | 1of2 Downloaded by guest on September 30, 2021 activates this receptor. Second, by raising investigate this apparent discrepancy, and thought to be induced or exacerbated by the levels of endogenously released AEA, indeed to explore more fully the effect of the endogenous release of AEA (2, 3), an FAAH inhibitor is expected to increase LXA4 on agonist-induced CB1 receptor could be ameliorated by administering AEA-induced activation not only of CB1 signaling. a selective inhibitor of LXA4 biosynthesis receptors but also of other receptors and The discovery that the ability of AEA to oppose any ongoing endogenous ion channels that this endocannabinoid to activate CB1 receptors appears to be potentiation of AEA by LXA4 at CB1 targets (14), whereas a highly specificCB1 allosterically enhanced by endogenously receptors that mediate one or more of allosteric enhancer would be expected produced LXA4, in at least some brain these unwanted effects. In contrast to only to increase AEA-induced activation a selective CB1 receptor antagonist, such of CB1 receptors. Third, FAAH catalyzes an inhibitor would, according to the find- the metabolism not only of AEA but Pamplona et al. present ings of Pamplona et al. (9), be expected to also of 2-AG, and of a number of non- lessen CB1-mediated effects of AEA but endocannabinoid endogenous fatty acid convincing evidence not of 2-AG, and to produce a reduction ethanolamides, the levels and lifespans of of this kind only at CB1 receptors that are which can therefore also be increased by that LXA4 is an being simultaneously exposed to AEA and inhibitors of this enzyme (2, 3). It could LXA4. In view of these possibilities, it is well be, therefore, that LXA4 is more endogenous CB1 receptor noteworthy that Pamplona et al. (9) have selective than an FAAH inhibitor as a already obtained evidence that, when it potentiator of AEA, a possibility that allosteric enhancer. is administered exogenously, LXA4 can merits further investigation. In the mean- enhance the ability of endogenously re- time, what has already been found by leased AEA to oppose β-amyloid–induced areas, has therapeutic implications that Pamplona et al. (9) is that LXA is not memory impairments in mice, raising 4 merit further investigation. More specifi- an inhibitor of FAAH or the 2-AG– the possibility that an inhibitor of LXA4 metabolizing enzyme monoacylglycerol cally, it will be important to establish metabolism might ameliorate at least lipase, and also that, when administered whether endogenous LXA4 enhances any some unwanted symptoms of Alzheimer’s of the proposed autoprotective effects of exogenously, LXA4 does not alter mouse disease. It will also be important, of brain levels of AEA. They also find that, endogenously released AEA (2, 3), and, course, to seek out any toxicological although LXA enhances the ability of if it does, whether it would be therapeuti- consequences of suppressing or augment- 4 cally beneficial to boost this effect with AEA to induce catalepsy, there is no such ing any of the actions of LXA4 through detectable potentiation of 2-AG (9). This an inhibitor of LXA4 metabolism. Impor- inhibition of its biosynthesis or metabolic tantly, such enhancement might well be may be because 2-AG activates the CB1 degradation, to obtain some indication receptor with much higher efficacy than very selective, as it is likely to be restricted of the benefit-to-risk ratios of these to effects of AEA that (i) are CB re- AEA in the absence of LXA4, and so is 1 potential therapeutic strategies.