5.0 Cannabinoid activity and Gut inflammation
The GI tract is rich in components of the endocannabinoid system. In addition to the classical CB1 and CB2receptors, the gut also expresses G protein-coupled receptor 55 (GPR55), a cannabinoid-responsive non-CB1/CB2receptor, which upon activation leads to increased intracellular calcium concentration (Grill et al., 2018). This mechanism has been shown to involve Gq, G12, actin, RhoA, phospholipase C and calcium released from inositol 1,4,5-triphosphate receptor (IP3R)-gated stores (Lauckner et al., 2008). Within the GI system, CB2 is largely expressed on immune cells whereas CB1 is largely expressed on cholinergic neurons (Sibaev et al., 2009; Schmole et al., 2015). Moreover, the gut also contains enzymes of the endocannabinoid system including diacylglycerol lipase and N-acyl phosphatidylethanolamine-specific phospholipase D, FAAH and MAGL (Grill et al., 2018).
By examining the role of endocannabinoids in maintaining gut integrity, it is most apparent that these endogenous ligands have multiple regulatory roles including, maintaining the integrity of the epithelial barrier, regulating gut microbiota and keeping the immune cells tolerant to commensals (Cani et al., 2016; Karwad et al., 2017). The latter is achieved through regulation of the expansion of the regulatory T cell (Treg) subset Tr1 and the presence of CX3CR1hi, an immunosuppressive macrophage population (Acharya et al., 2017). Thus, it is unsurprising that changes in the levels of endocannabinoids and endocannabinoids-like lipids are implicated in inflammatory bowel disease (IBD) and colorectal cancer (Di Sabatino et al., 2011; Chen et al., 2015). It however remains to be determined if such variations correlate with disease progress (Grill et al., 2018).
Functional data from animal models shows alterations in the components of the endocannabinoid system during experimental intestinal inflammation (Massa et al., 2004; D’Argenio et al., 2006). It has been shown that, pharmacological activation of CB1 and CB2 attenuates experimental colitis (Kimball et al., 2006; Storr et al., 2009), while pharmacological antagonism or genetic ablation of these receptors worsens gut inflammation (Massa et al., 2004; Storr et al., 2008). This is evident from studies that demonstrated that increased levels of anandamide and 2-AG, following inhibition of FAAH or MAGL or genetic deficiency in genes that encodes these enzymes, is protective against experimental colitis such as dextran sodium sulfate (DSS)-induced colitis or trinitrobenzene sulfonic acid (TNBS)-induced colitis (Pagano et al., 2016; Shamran et al., 2017; Zhao et al., 2017).
Synthetic analogs of endocannabinoids as well as phytocannabinoids have been shown to ameliorate gut inflammation in animal models of intestinal inflammation (Grill et al., 2018). In a study by Jamontt et al. (2010), both the psychotropic Δ9-THC and the non-psychotropic cannabidiol reduced colonic injury in a rat model of TNBS-induced colitis. Cannabidiol, however, has very low affinity on CB1 and CB2, and is antagonistic on GPR55 (Ryberg et al., 2007). In addition, cannabidiol acts on PPARγ (De Filippis et al., 2010) and TRPV1 (De Petrocellis et al., 2011) expressed in the gut. It also inhibits the activity of FAAH and has been shown to ameliorate intestinal inflammation in dinitrobenzene sulfonic acid (DNBS)-induced colitis in mice (Borrelli et al., 2009). Significant reduction in intestinal inflammation by cannabidiol appears to be a combined effect with other compounds in Cannabis sativa rather than a single effect (Pagano et al., 2016). Moreover, O-1602, an analog of cannabidiol and a GPR55 agonist, has been shown to protect against intestinal damage in experimental colitis. This effect, however, was not mediated by GPR55, and by extrapolation, not by CB1 or CB2 since O-1602 lacks affinity for CB1and CB2 (Ryberg et al., 2007). Several other phytocannabinoids, endocannabinoids, endocannabinoid-like substances and synthetic endocannabinoid analogs have been demonstrated to inhibit intestinal inflammation in various models of experimental colitis. Table 1 summarizes these ligands and the mechanisms involved in the inhibition of inflammatory processes in the GIT.
By employing the use of chronic ileitis model, Leinwand et al. (2017) identified upregulation of CB2 and anandamide in actively inflamed ileum of TNFΔARE/+ mice compared with controls. In TNFΔARE/+ mice, CB2mRNA was relatively expressed 11-fold more on Tregs as compared to T effector cells, while in wild-type mice, there was a 2.4-fold increase in its expression on Tregs compared to T effector cells. The authors reported that GP-1a, a previously classified CB2receptor agonist, acts at CB2 as an inverse agonist, thus allows the receptor to resensitize. This finding is also supported by a previous study by Soethoudt et al. (2017), where GP-1a was shown to actively inhibit the reuptake of anandamide and also act as an inverse agonist at the CB2 receptors in vitro . GP-1a however did not stimulate CB1 receptors. The inverse agonism at CB2 enhances the activation of Treg-expressed CB2 receptors by endocannabinoids, with subsequent increase in Treg suppressive function and associated increase in IL-10 secretion (Leinward et al., 2017). As opposed to active inflammation, CB2R was downregulated in both chronically inflamed TNFΔARE/+ mice and in Crohn’s disease patients. Activation of CB2 receptors inhibited ileitis in mice, thus establishing the protective effect of the endocannabinoid system in intestinal inflammation (Leinward et al., 2017).
Table 1. Other cannabinoid receptor ligands in murine models of intestinal inflammation