GPR has been implicated in neuropathic and inflammatory pain

GPR55 has been implicated in neuropathic and inflammatory pain (Staton et al., 2008) and bone remodeling (Whyte et al., 2009). Thus delineating the pharmacology of this receptor and the discovery of selective agonists and antagonists merits further study and could lead to new therapeutics.
Acknowledgments This work was supported from NIH/NIDA — DA023204 and DA05274. The authors would like to acknowledge Dr. Patricia H. Reggio, Dow P. Hurst and Evangelia Kotsikorou from UNC-Greensboro for their input into structural features of GPR55 and its ligands. We wish to thank Jahan Marcu and Pingwei Zhao, members of the Abood laboratory, for their critical reading during the preparation of this manuscript.
Introduction The endocannabinoid (EC) system controls appetite, food intake and Pyoluteorin balance mainly through the CB1 receptor (Pagotto et al., 2006). The EC system plays also important actions on glucose homeostasis and insulin sensitivity, which are independent of body weight and food intake modulatory actions (Williams and Kirkham, 1999, Williams et al., 1998). One of the main sites of action of CB1 antagonists at peripheral level is the WAT. The interaction between leptin and cannabinoids was established more than ten years ago, when Di Marzo and colleagues discovered that endocannabinoids in the hypothalamus activate CB1 receptors to maintain food intake and are integrated in the neural circuitry by leptin (Di Marzo et al., 2001). Leptin suppresses the synthesis of the endocannabinoid anandamide (AEA) in WAT. When suppression of endocannabinoid tone is prevented by systemic CB1 receptor activation, leptin fails to exert some of its actions on lipid metabolism in WAT (Buettner et al., 2008). Additionally, CB1 receptor knockout mice have significantly less fat mass and a lower caloric intake than wild-type animals and are resistant to diet induced obesity (Cota et al., 2003). Pharmacological evidence suggests that endocannabinoids may be acting through other uncharacterized receptors. Several groups have recently suggested that GPR55, a seven-transmembrane G protein-coupled receptor identified in 1999 (Sawzdargo et al., 1999), might be a novel type of cannabinoid receptor (Johns et al., 2007, Ryberg et al., 2007, Lauckner et al., 2008), although this assumption is still under dispute. GPR55 has been shown to be involved in a large variety of biological actions such as neutrophil recruitment (Schicho et al., 2011), nociception (Schuelert and McDougall, 2011), cancer cell proliferation (Pineiro et al., 2011, Andradas et al., 2011), and bone mass regulation through the stimulation of osteoclast function, an effect that was attenuated in mice lacking GPR55 and by a GPR55 antagonist (Whyte et al., 2009). GPR55 also exerts some metabolic actions on insulin secretion (Romero-Zerbo et al., 2011) and gut movement (Lin et al., 2011). Importantly, GPR55 may be a clinically relevant target because GPR55 expression in WAT is increased in obese subjects and even more in obese patients with type 2 diabetes (Moreno-Navarrete et al., 2012). Visceral adipose tissue expression of GPR55 is correlated with weight, body mass index and percent of fat mass (Moreno-Navarrete et al., 2012). Consistently, lysophosphatidylinositol (LPI), the endogenous ligand of GPR55, shows increased circulating levels in obese patients and a positive correlation with fat percentage and body mass index in women (Moreno-Navarrete et al., 2012).
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Discussion Cannabinoids play a key role in adipocyte metabolism (Di Marzo et al., 2011, Kunos et al., 2008) and it has been demonstrated that there is a strong metabolic interaction between cannabinoids and leptin (Di Marzo et al., 2001, Buettner et al., 2008). Furthermore, GPR55 is expressed in the WAT and its expression is markedly up-regulated in the visceral adipose tissue of obese and diabetic patients (Moreno-Navarrete et al., 2012). We therefore evaluated the effect of fasting and chronic food restriction, and found a marked increase in gWAT GPR55 and CB1 mRNA expression in comparison with fed states. Our results indicated that the increased expression of both receptors after fasting was reversed after refeeding and leptin challenge and that circulating levels of LPI presented an inverse pattern to that exhibited by GPR55 receptor levels. Thus, it seems that the ligand and its receptor are inversely modulated by nutritional status. These changes on feeding status might be mediated by leptin, as it was able to reverse WAT GPR55 expression and circulating LPI levels. However it is also important to highlight that there are different fasted-associated alterations such as stress that might be also affecting GPR55 levels. The expression of both GPR55 and CB1 in gWAT during pregnancy showed a clear reduction particularly at the end of this period, when serum leptin levels are higher (Garcia et al., 2000). Since pregnancy is a hyperleptinemic state, a plausible hypothesis is that leptin might represent an important factor modulating the expression of both CB1 and GPR55 in gWAT. However, it is also clear that gestation is a leptin resistant state (Mounzih et al., 1998) and that levels of multiple hormones are severely altered. Therefore, the fact that other factors different to leptin modulate GPR55 and LPI levels cannot be discarded. The correlation in gene expression changes in the adipose tissue, and the previously reported cannabinoid-binding properties of both receptors, suggest that GPR55 and CB1 could work in a coordinated fashion to mediate endocannabinoid control of adipose tissue physiology. It is also important to highlight that WAT GPR55 gene expression was decreased in leptin deficient mice (Moreno-Navarrete et al., 2012), a result that seems controversial with the current findings. However, it is worthy to note that ob/ob mice exhibit severe endocrine alterations, including hypogonadism, GH deficiency and altered thyroid hormone levels (Sone and Osamura, 2001), which, as discussed below, differentially modulate GPR55 expression in WAT.