The Endocannabinoids Anandamide and Virodhamine Modulate the Activity of the Candidate Cannabinoid Receptor GPR55

Abstract

The role of cannabinoid receptors in inflammation has been the topic of many research endeavors. Despite this effort, to date the involvement of the endocannabinoid system (ECS) in inflammation remains obscure. The ambiguity of cannabinoid involvement may be explained by the existence of cannabinoid receptors, other than CB1 and CB2, or a consequence of interaction of endocannabinoids with other signaling systems. GPR55 has been proposed to be a cannabinoid receptor; however the interaction of the endocannabinoid system with GPR55 remains elusive. Consequently this study set about to examine the effects of the endocannabinoids, anandamide (AEA) and virodhamine, on GPR55 mediated signaling. Specifically, we assessed changes in β-arrestin2 (βarr2) distribution and GPR55 receptor internalization following activation by lysophosphatidylinositol (LPI), the synthetic cannabinoid ligand SR141716A, and new selective synthetic GPR55 agonists. Data obtained from the experiments presented herein demonstrate that AEA and virodhamine modulate agonist-mediated recruitment of βarr2. AEA and virodhamine act as partial agonists; enhancing the agonist effect at low concentrations and inhibiting it at high concentrations. Furthermore, both virodhamine and AEA significantly attenuated agonist-induced internalization of GPR55. These effects are attributed to the expression of GPR55, and not CB1 and CB2 receptors, as we have established negligible expression of CB1 and CB2 in these GPR55-transfected U2OS cells. The identification of select endocannabinoids as GPR55 modulators will aide in elucidating the function of GPR55 in the ECS.

From Phytocannabinoids to Cannabinoid Receptors and Endocannabinoids: Pleiotropic Physiological and Pathological Roles Through Complex Pharmacology

“…This indicates that this receptor possesses not only amino acid sequence but also ligand binding and signaling profiles different from those of CB1 and CB2. Indeed, there is controversy regarding the effects of endocannabinoids and congeners at GRP55, due to the different results obtained with different functional assays in various biological systems (786). It is possible that different biased signaling properties of these ligands, and tissue and celldependent coupling of GPR55 with different G proteins and effector systems, or with CB1 or CB2 in heteromers, all contribute to this heterogeneity of results (110,537,592).…”

The Endocannabinoid System and its Modulation by Phytocannabinoids

“…In particular: 1) the orphan GPCR, GPR55, has been suggested to act as target for both cannabinoids, that is, THC and CBD, which seem to act as agonist and antagonist, respectively, for this receptor [42], and N-palmitoylethanolamine, which seems to be an agonist [42]. In fact, there is controversial evidence that anandamide and 2-AG may also activate GPR55, and the recent finding of CB1-GPR55 heteromers might explain why some authors have found that the 2 endocannabinoids directly activate this orphan GPCR and most others have not [43][44][45][46]. Another orphan GPCR, GPR18, is instead activated by N-arachidonoyl-glycine and by a synthetic CBD analogue known as abnormal-cannabidiol [47,48].…”

Mitochondria: a possible nexus for the regulation of energy homeostasis by the endocannabinoid system?

“…For example, SR141716 (also known as rimonabant) has been shown to act both as a CB1R antagonist and as an inverse agonist (Table 1) (15,72). Notably, endocannabinoids can also mediate some of their biological effects through alternative molecular targets such as the orphan G proteincoupled receptor GPR55 or the transient receptor potential cation channel (TRPV1) (90,127).…”