The putative cannabinoid receptor GPR55 affects osteoclast function in vitro and bone mass in vivo

Abstract

GPR55 is a G protein-coupled receptor recently shown to be activated by certain cannabinoids and by lysophosphatidylinositol (LPI). However, the physiological role of GPR55 remains unknown. Given the recent finding that the cannabinoid receptors CB 1 and CB2 affect bone metabolism, we examined the role of GPR55 in bone biology. GPR55 was expressed in human and mouse osteoclasts and osteoblasts; expression was higher in human osteoclasts than in macrophage progenitors. Although the GPR55 agonists O-1602 and LPI inhibited mouse osteoclast formation in vitro, these ligands stimulated mouse and human osteoclast polarization and resorption in vitro and caused activation of Rho and ERK1/2. These stimulatory effects on osteoclast function were attenuated in osteoclasts generated from GPR55 ؊/؊ macrophages and by the GPR55 antagonist cannabidiol (CBD). Furthermore, treatment of mice with this non-psychoactive constituent of cannabis significantly reduced bone resorption in vivo. Consistent with the ability of GPR55 to suppress osteoclast formation but stimulate osteoclast function, histomorphometric and microcomputed tomographic analysis of the long bones from male GPR55 ؊/؊ mice revealed increased numbers of morphologically inactive osteoclasts but a significant increase in the volume and thickness of trabecular bone and the presence of unresorbed cartilage. These data reveal a role of GPR55 in bone physiology by regulating osteoclast number and function. In addition, this study also brings to light an effect of both the endogenous ligand, LPI, on osteoclasts and of the cannabis constituent, CBD, on osteoclasts and bone turnover in vivo.A role for the endocannabinoid system (1) in the regulation of bone mass has been demonstrated recently, because mice lacking either of the cannabinoid receptors CB 1 or CB 2 have abnormal bone phenotypes. Furthermore, cannabinoid receptor agonists and inverse agonists reduce bone loss in mice following ovariectomy and have direct effects on both bone-resorbing cells (osteoclasts) and bone-forming cells (osteoblasts) in vitro (2, 3).In some systems such as the vasculature, there is considerable evidence for a role of non-CB 1 /non-CB 2 receptors in mediating some of the effects of certain cannabinoid ligands (for review, see ref. 4). Such non-CB 1 /non-CB 2 effects have been observed with a range of cannabinoid ligands including certain endocannabinoids and the phytocannabinoid-like compound O-1602; these effects are antagonized by the cannabis constituent cannabidiol (CBD) (4). Recently, the G protein-coupled receptor GPR55 has been shown to be activated by O-1602 (EC 50 ϭ 13 nM) and antagonized by CBD (IC 50 ϭ 445 nM) (5-8). In contrast, these compounds have low affinity (5-30 M) for CB 1 and CB 2 receptors (9, 10). GPR55 also is activated by the bioactive lipid, L-␣-lysophosphatidylinositol (LPI) (11,12).The physiological role(s) of GPR55 remains unknown. Given the apparent role of CB 1 and CB 2 in regulating bone mass, we examined whether GPR55 is expressed by osteoblasts and osteoc...

The orphan G protein-coupled receptor GPR55 promotes cancer cell proliferation via ERK

“…This original observation, made in cells ectopically expressing GPR55, was shortly corroborated by other reports (Lauckner et al, 2008;Henstridge et al, 2009;Kapur et al, 2009;Oka et al, 2009Oka et al, , 2010Yin et al, 2009). Moreover, LPI has been found to activate GPR55 in cells in which the receptor is endogenously expressed (large dorsal root ganglion neurons (Lauckner et al, 2008), osteoclasts (Whyte et al, 2009) and lymphoblastoid cells (Oka et al, 2010)), supporting the notion that this phospholipid may be an endogenous GPR55 ligand. Nonetheless, all the functions described so far for LPI on GPR55 come from experiments in which LPI was exogenously added to the cultured cells, and therefore evidence for the role of the naturally occurring lipid in more physiological settings is still missing.…”

“…However, the inconsistencies among the pharmacological results obtained so far (some compounds being active in some reports and inactive in others, some being agonists in some studies and antagonists in others, and so on) do not entirely clarify whether GPR55 is an actual cannabinoid receptor (Brown and Robin Hiley, 2009;Ross, 2009). GPR55 mRNA is highly expressed in the brain, the adrenal glands, parts of the gastrointestinal tract, spleen, tonsils, testes, thymus (Sawzdargo et al, 1999;Ryberg et al, 2007;Oka et al, 2010), large dorsal root ganglion neurons (Lauckner et al, 2008), osteoclasts (Whyte et al, 2009), certain microglial cells (Pietr et al, 2009), endothelial cells and mesenteric arterial smooth muscle cells (Daly et al, 2010), but very little is known about the physiological role of the receptor in these or other tissues. To date, GPR55 has been implicated in the control of pain, specifically in the mechanical hyperalgesia induced by inflammatory and neuropathic pain (Staton et al, 2008), and in the control of bone formation (Whyte et al, 2009).…”

Molecular Targets of Cannabidiol in Neurological Disorders

“…A radiolabelled synthetic analogue of Δ 9 -THC has been shown to bind GPR55 with an EC 50 of 5 nM. Further studies with CBD (0.001-1.000 μM) have shown an inhibitory effect on the agonist activity of CP 55,940 at GPR55, with an IC 50 of 445 nM [24]. Functional consequences of this inhibition have been demonstrated in rat hippocampal preparations [51].…”

“…It is clear that CBD can modulate functions involving 5-HT receptor function, although to what extent such effects are direct remains unclear, particularly as several in vivo studies did not include 5-HT receptor antagonist-only More recently, attention has turned to interactions between CBD and non-endocannabinoid G protein-coupled receptors (GPCRs) [23,24,51]. The orphan GPCR 55 (GPR55) shares structural similarities in transmembrane domains 1, 2, and 3 when compared with the cannabinoid receptors, which may indicate a binding site for cannabinoids [24,52]. A radiolabelled synthetic analogue of Δ 9 -THC has been shown to bind GPR55 with an EC 50 of 5 nM.…”

Synthesis of Novel Cannabinoid Ligands and Their Use as Anti-Glioma and Anti-Inflammatory Agents

“…11,112 The dose response curve of KM-233 in the presence of 10 μM SR141716A (Figure 2.11) shifted the curve to the right increasing the EC 50 7 fold. The shift in the dose response curve suggests that GPR55 is a target of KM-233; however, additional pharmacological probes need to be employeed such as cholera toxin (Ctx, constitutively activates Gs), inhibition of the G α12/13 -RhoA-ROCK pathway (possible GPR55 mediated pathway) with Y-27632, or decoupling G αi with Ptx and screening GPR55 selective ligands such as O-1602 or lysophosphatidylinsoitol (LPI).…”