Δ⁹-THC is a major psychoactive constituent of Cannabis sativa (1). It was shown that Δ⁹-THC acts as a partial agonist of both CB₁ and CB₂ receptors (19). Throughout such activation Δ⁹-THC is able to trigger many different physiological processes, i.e., regulation of gastrointestinal, liver and cardiovascular functions, pain perception along with modulation of neurotransmitters release in the nervous system (20, 21). Now, it is clearly established that, Δ⁹-THC exerts a well-known psychoactive effect, which is mediated by the activation of CB₁ receptor in the central nervous system (CNS) (22). Additionally, Δ⁹-THC by activating CB₁ receptors, located in the limbic (enhancing motivational properties of food) and hypothalamic (increasing appetite) structures, causes the orexigenic effects (Figure 1) (23). Apart from the above implications, Δ⁹-THC has also the ability to bind to eCBome receptors including GPR55 (G protein-coupled receptor 55), 5- HT_3A (serotonin receptor subunit), TRPV2, 3, 4 receptors (transient receptor potential channels of vanilloid type 2, 3, 4), which produce some of its pharmacological effects (22). Lauckner et al. revealed that Δ⁹-THC activates the GPR55 receptor in HEK293 and CHO cells, which resulted in increased intracellular calcium level (24). Another study conducted on HEK293 cells showed that Δ⁹-THC acts as a 5- HT_3A receptor antagonist, whereas several reports demonstrated that Δ⁹-THC has agonistic effects on the TRPV2, 3, 4 channels (25–28). Furthermore, Δ⁹-THC was reported to exhibit a beneficial impact on the regulation of insulin sensitivity in the insulin resistant adipocytes. The authors have demonstrated that natural extract containing Δ⁹-THC decreased the TAGs content and improved the glucose uptake in the insulin resistant 3T3-L1 cells in a concentration-dependent manner (29). In the same study it was shown that the level of tumor necrosis factor alpha (TNF-α) in the 3T3-L1 cells was substantially decreased in the presence of Δ⁹-THC, which also improved sensitivity of the cells to insulin (29). The above mentioned studies also revealed that in a differentiated 3T3-L1 cells Δ⁹-THC treatment enhanced glucose transporter type 4 (GLUT4) and insulin receptor substrate 1 and 2 (IRS-2) gene expressions, which play key roles in the insulin signaling pathway (29). However, we have to keep in mind that in the above experimental models natural extract were applied and no information upon their purity was provided, therefore, it can be considered as the limitation of such studies in the face of stating pharmacological properties of examined substances.

On the other hand, in comparison to the well-documented orexigenic effect of Δ⁹-THC, many studies have also shown a potential anti-obesity effect of this phytocannabinoid derivative. Cluny et al. (30) using the in vivo model have demonstrated that chronic administration of Δ⁹-THC to diet-induced obesity (DIO) mice for 3 and 4 weeks treatment (intraperitoneal injections of Δ⁹-THC in a dose of 2 or 4 mg/kg and vehicle) prevented weight gain due to reduced energy intake (30). This effect might have been associated with the fact that Δ⁹-THC is a partial agonist of CB₁ and CB₂ receptors. Since, Δ⁹-THC does not produce maximal stimulation of the above receptors, the background of such changes could result from blocking endogenous full agonist, i.e., anandamide from binding to CB₁ receptor, especially, when the endocannabinoid tone is high (e.g., obesity) (30, 31). However, potential positive influence of Δ⁹-THC in the prevention and treatment of obesity requires further investigation.

Δ⁹-THCV is a propyl Δ⁹-THC analog, which binds to the cannabinoid CB₁ and CB₂ receptors (32). So far, data indicate that Δ⁹-THCV is a neutral antagonist at low doses and agonist at high doses of CB₁ and CB₂ receptors (2, 32, 33). Little evidence exists upon the activation of various eCBome receptors by Δ⁹-THCV. Nevertheless, it has been established that Δ⁹-THCV acts as an agonist of human TRPV1, rat TRP channels of ankyrin type-1 (TRPA1) and TRPV2, 3, 4 channels as well as an antagonist of rat's TRP channel of melastatin type-8 (TRPM8) (26). In one study it was demonstrated that Δ⁹-THCV activated 5-HT_1A receptor in the rat brain and human 5-HT_1A receptor-transfected CHO (Chinese hamster ovary) cells, and thereby exerted antipsychotic effects (34). Interestingly, recent research has shown that this non-psychotropic phytocannabinoid has a potential therapeutic effect in the treatment of diabetes associated with obesity. Wargent et al. (35) revealed that Δ⁹-THCV increased insulin sensitivity and improved glucose tolerance in DIO mice as well as genetically obese ob/ob mice (Figure 1). However, it did not substantially affect food intake and body weight gain in the above models, whereas body fat content was decreased (35). In the same experiment, the authors investigated the impact of Δ⁹-THCV on the insulin resistant C₂C₁₂ myotubes and HL-5 hepatocytes showing that Δ⁹-THCV restored intracellular insulin signaling pathway (35). Other studies have also displayed that Δ⁹-THCV (1–10 μM) decreased lipid accumulation in the in vitro model of hepatosteatosis in HHL-5 (Human Hepatocyte Line 5) cells and adipocytes (3T3-L1 cells) (36). According to the above results, a pilot study in patients with type 2 diabetes showed that Δ⁹-THCV reduced fasting plasma glucose with parallel improvement in β-cell function as well as increased Apo A (apolipoprotein A) and adiponectin concentrations compared with placebo, which was well-tolerated in patients (37). Nonetheless, further research is necessary to confirm the potential therapeutic properties of Δ⁹-THCV in the treatment of obesity, metabolic syndrome and type 2 diabetes.

An oxidized metabolite of Δ⁹-THC, cannabinol, was isolated in 1896 by Wood and colleagues (38). CBN is a non-enzymatic oxidative breakdown product of Δ⁹-THC due to aging or light exposure (32). It was shown that CBN binds to cannabinoid receptors CB₁ and CB₂ in the peripheral organs and central nervous system (CNS), and therefore, it exerts a weak psychoactive activity (39). CBN also displays the ability to bind to expanded ECS receptors including agonistic and antagonistic effects on TRPA1 and TRPM8 channels, respectively (26). So far, studies focusing on the impact of CBN on obesity and feeding behavior have been limited. Only in one of the recent studies it has been demonstrated that CBN increased food intake in rats in a dose-dependent manner (Figure 1) (40). Probably, the hyperphagic CBN's effect was induced by its interaction with CB₁ receptor in CNS (40) but it should be confirmed by additional experiments including for example gene silencing or knockout animals. Nevertheless, further experiments are required to completely characterize the role of CBN in food consumption and body weight control.

Among phytocannabinoids we can find a propyl analog of CBD, cannabidivarin, which exerts non-psychotropic properties (32). CBDV has a weak affinity for cannabinoid receptors CB₁ and CB₂ with simultaneous stronger activation of other molecular targets related with the ECS (41). Predominantly, anticonvulsant and other therapeutic CBDV's properties are associated with activation of TRP channels (TRPV1, 2 and TRPA1) and TRPM8 inhibition (26, 42). Another pharmacological target for CBDV is GPR55 receptor, where observed effect is inhibitory (43). However, currently there is insufficient research on the ground of CBDV's effects on obesity, insulin resistance and other metabolic disturbances (Figure 1).

Another phytocannabinoid, which lacks psychotropic properties and occurs only in trace amounts in Cannabis, is cannabigerol (41). Cannabis plants produce cannabigerolic acid (CBGA), which is a direct precursor of cannabinoids: Δ⁹-THC, CBC and CBD (44, 45). Preliminary studies, using plasma membranes isolated from the CHO and HEK-293 cells expressing human CB₁ and CB₂ receptors, have shown that CBG acts as a partial agonist of both cannabinoid receptors (46). Similarly to other phytocannabinoids CBG has an ability to interact with endocannabinoidome receptors, such as TRPV1, 2, TRPA1, and TRPM8 channels (26). Moreover, one study has provided for the first time evidence upon activation of α₂-adrenoreceptors and 5-HT_1A receptor blockage by CBG in the vas deferens and brain, respectively (47). Furthermore, recent investigation has revealed that CBG serves as a novel appetite stimulant in rats and importantly, no side effects were observed during its administration (Figure 1) (48). However, extensive research is necessary to determine a more detailed mechanisms of CBG action with respect to metabolic diseases.

The discovery of cannabichromene was reported by Izzo et al. (32). It is a minor component of Cannabis plant, which presents low affinity for CB₁ and CB₂ receptors (49, 50). Nonetheless, researchers discovered another molecular target of CBC, i.e., TRP channels. De Petrocellis et al. revealed that CBC is the most potent agonist of the TRPA1 channels (26). However, it has also the ability to activate in a lower potency, TRPV3 and TRPV4 channels, and additionally inhibits TRPM8 receptors, but to a much lesser extent (26). Although CBC exhibits different medical properties, its influence on metabolism and obesity has not been found, yet (Figure 1).

Cannabidiol (CBD) is one of the most common non-psychotropic constituent of Cannabis plant (this depends on the cannabis strain). It was revealed that CBD induces a wide-range of pharmacological effects through different mechanisms. CBD functions as a negative allosteric modulator of CB₁ receptor (51). Therefore, it has therapeutic potential in the treatment of the central nervous system diseases (neurodegenerative diseases, epilepsy, anxiety, and depression) without concurrent psychotic side effects (2, 52, 53). Furthermore, it was indicated that CBD is able to block CB₁ receptor, thereby producing anti-obesity effects. Otherwise, CBD unexpectedly exhibited a high affinity for CB₂ receptor as an agonist or inverse agonist depending on the research model (in vitro or in vivo) (54, 55). An interesting result of the latest studies was the fact that CBD has greater affinity for various eCBome receptors, including GPR55, α1-adrenoreceptors, 5-HT_1A, TRPV channels and PPARγ (Figure 2) (19, 26, 42, 56, 57). Hegde et al. as well as Esposito et al. reported that CBD significantly induced the transcriptional activity of PPARγ (58, 59). It is known that PPARγ is an interesting therapeutic target due to its crucial role in regulating glucose homeostasis, lipoprotein metabolism, and inflammation (60, 61).

Moreover, recent research has focused on the therapeutic properties of CBD, including anti-inflammatory, anti-oxidant, anti-tumor, anti-convulsant and neuroprotective effects (62, 63). In the light of the above mentioned properties, CBD emerges as a potential therapeutic agent, which can be used in the treatment of diabetes and its complications, obesity, ischemia, neurodegenerative diseases as well as pain relieving and depression. Although CBD's anti-inflammatory and neuroprotective properties are well-confirmed, there are few studies that have investigated anti-obesity effects of this compound. Wierucka-Rybak et al. have examined the effect of CBD on food intake, food preferences and weight gain in rats (64). The authors administered CBD (3 mg/kg) for 3 days to rats maintained on a standard diet (SD), high fat diet (HFD), or free choice diet (FC; high sucrose). The authors revealed that CBD injections in the case of rats being on the HFD resulted in an increase in body weight despite significantly reduced food intake. On the other hand, in rats fed FC diet CBD did not cause significant change in food consumption and body weight (64). Previous studies investigating the impact of CBD on food intake showed contradictory results. One study demonstrated CBD-induced (2.5 and 5 mg/kg) decrease in body weight gain in rats (54), while other studies have shown no significant impact on food intake and body weight in mice and rats (65, 66). Importantly, pilot studies were conducted to investigate the effect of CBD on glycemic and lipid parameters in patients with type 2 diabetes (37). The findings from these studies demonstrated that CBD did not produce any improvement in glycemic and lipid control despite producing eligible changes in gut hormones (GIP—glucose dependent insulinotropic peptide) and adipokines (resistin) concentrations (37). Probably, the reason for the lack of therapeutic effects seen during CBD administration could have been too low dosage used in the study. The anti-epileptic properties of CBD were investigated in many preclinical studies and in randomized trials (67–71). Data obtained from these studies confirmed CBD's anti-seizure properties along with a good tolerance profile. Currently, the first cannabis based medicine containing CBD/Δ⁹-THC combination for the multiple sclerosis related spasticity treatment (Sativex/Nabiximols; GW Pharmaceuticals) has been approved in numerous countries, including Australia, Canada, New Zealand and in most European Union countries (72). Furthermore, a drug composed solely of CBD (Epidiolex; GW Pharmaceuticals) has been approved in June 2018 in the United States for the treatment of Dravet and Lennox-Gastaut syndromes in children (52, 73, 74).

In view of the increasing prevalence of obesity, type 2 diabetes and metabolic syndrome, the development of liver diseases occurs more often. The above mentioned diseases are associated with excessive fat accumulation resulting from upregulated FA influx and de novo lipid synthesis in different cells, e.g., hepatocytes (105). Alterations in the hepatic fatty acid oxidation are highly related with the aforementioned metabolic disturbances, which were shown in many experimental models together with discrepancies in this field. Studies conducted on NASH and liver steatosis patients showed an increase (106, 107), decrease (108), or no change (109) in the fatty acid oxidation status. On the other hand, the expected consequence of lipid accumulation in the liver would be simultaneous enhancement in hepatic oxidation of fatty acids, thus, it seems that certain factors can also influence this process as well.

In previous studies it was shown that, hepatocytes produce endocannabinoids, AEA and 2-AG, and possess CB₁ and CB₂ receptors (80, 91). A study carried out by Liu et al. revealed that liver specific CB₁ receptor knockout mice were protected from the development of both insulin resistance and hepatosteatosis but not obesity (55). Silvestri et al. revealed positive influence of CBD on the liver, namely the reduction of intracellular lipid content in the in vitro hepatosteatosis model, possibly by enhancing lipolysis and mitochondrial activity through increased fatty acids oxidation (36). The authors have shown that CBD increases the expression of selected proteins involved in upregulating lipid metabolism, i.e., catalytic subunit of 5′AMP-activated protein kinase (AMPKa2), extracellular signal-regulated kinase (ERK1/2) along with signal transducers and transcription activators (STATs) in hepatocytes (Figure 2, Table 1) (36). Concomitantly, the authors observed that CBD enhanced the level of glutathione (GSH), adenosine triphosphate (ATP), and nicotinamide adenine dinucleotide (NAD), which supported the assumption that CBD increases intracellular lipolysis and mitochondrial activity (36). Additionally, the study carried out by Wang's research group (119) indicated, in a murine model, an impact of CBD on alcohol-induced liver injury. The obtained results displayed that CBD attenuates liver steatosis (decreased triacylglyceride and fat droplet accumulation), inflammatory response [reduced the alcohol-feeding induced mRNA expressions of interleukin 1beta (IL1β), TNF-α and monocyte chemoattractant protein 1 (MCP1)], oxidative/nitrative stress (reduced lipid peroxidation, expression of reactive oxygen species generating enzyme—NADPH oxidase 2 (NOX2) and 3-nitrotyrosine production) and neutrophil infiltration in the liver (Table 1). This also confirmed the well-known, whole body anti-inflammatory and antioxidant properties of CBD (119).

Obesity is associated with chronic low-grade inflammatory state and excessive fat accumulation (120). The expression of CB₁ and CB₂ receptors, other eCBome molecular targets (i.e., TRPV1,GPR55) and ECs enzymes level in the visceral and subcutaneous adipose tissue has been found (121, 122). These additional receptors are the most promising target for major non-psychogenic constituent of Cannabis plant, which has well-established anti-inflammatory effects and potential anti-obesity properties (123). Silvestri et al. (36) pointed out that CBD, depending on the time and dose, reduced triacylglycerols accumulation in 3T3-L1 adipocytes treated with oleic acid (OA). These results emphasize a potential role of CBD on lipolysis induction (Table 1). However, more research is necessary to investigate the exact mechanism of this phenomenon (36). Similarly, Ramlugon et al. (124) revealed that CBD treatment, in a time-dependent manner, induced the mitochondrial activation and increased oxygen consumption, which may be an explanation for the reduced fat accumulation in adipocytes despite of increased glucose uptake (Figure 2) (124). Additionally, recent research has shown that CBD inhibited weight gain in rats subjected to high fat diet (HFD) for 14 days and this effect was probably mediated by CB₂ receptor (54). The authors confirmed the above effect by using a selective CB₂ antagonist-AM630, which prevented the reduction in weight gain due to CBD treatment (54). However, additional studies are required to uncover the mechanisms by which CBD induces the above metabolic changes in adipocytes.

The pancreas plays a pivotal role in the blood glucose homeostasis and whole body metabolism through insulin and glucagon secretion (125). In obesity, secretion of the above hormones is dysregulated. To date, the expression of cannabinoid receptors within the endocrine pancreas has been shown (114, 121). However, inconsistent data have been obtained regarding the expression of CB₁ and CB₂ receptors in the pancreas islet cells. Starowicz et al. (121) demonstrated the expression of CB₁ receptor in glucagon-secreting α-cells of mice, while CB₂ receptor was found inside mouse α-cells and insulin-secreting β-cells. Similarly, in the study conducted on human samples it was determined that CB₁ receptor was present in glucagon-secreting α-cells and in a lesser extent in insulin-secreting β-cells, whereas CB₂ was only located in somatostatin-secreting delta cells (114). On the other hand, other studies indicated that β-cells exhibited the expression of both cannabinoid receptors or only CB₂ receptor (84, 115). Interestingly, Levendal et al. (126) investigated the effect of CBD on β-cell function in obese rats. The authors found that in a rat model of diet-induced obesity (DIO), cannabis extract treatment reduced the weight gain (mainly fat depots) and increased energy expenditure through upregulation of protein kinase B (PKB), mitochondrial uncoupling protein 2 (UCP2), and glucose transporter 2 (GLUT2) expression in pancreatic β-cells (rats were injected subcutaneously every second day for 28 days; the first five treatments containing an equivalence of 5 mg THC/kg body weight and the remaining treatments an equivalence of 2,5 mg THC/kg body weight) (126). Moreover, studies conducted by Weiss et al. have shown that CBD treatment (mice were administered 10–20 intraperitoneal injections at a dose of 5 mg/kg CBD) decreased the frequency of type 1 diabetes incidence and development in a model of non-obese diabetic (NOD) mice (116). Furthermore, the authors reported that CBD reduced plasma level of the pro-inflammatory cytokines, i.e., TNF-α and interferon gamma (IFN-γ) together with inflammation mediators, such as nitric oxide (NO), cyclooxygenase (COX), and prostaglandin E2 (PGE2). Whereas, anti-inflammatory cytokines—interleukins IL-4, IL-10 levels were increased (Figure 2, Table 1). Hence, reduced pancreas islets inflammation was observed in the histological examination as well as lower β-cell destruction (116). Concluding, it is likely that CBD has the ability to prevent and reduce the pancreatic damage associated with obesity and insulin resistance.

Chronic inflammatory state associated with obesity reduces the ability of the adipose tissue to store incoming plasma-borne fatty acids and leads to their accumulation in other organs, including cardiac muscle (127). Then, the local inflammatory state, oxidative stress and hyperinsulinemia can develop and progress induction of insulin resistance in the heart muscle (128). Currently, many studies are focused on the effect of CBD on the lipid and glucose metabolism in the heart, which is dysregulated during obesity and subsequently can lead to the cardiomyopathy and cardiac contractile dysfunction development (117). The impact of CBD on myocardial dysfunction, oxidative/nitrative stress, inflammation, and cell death has been recently investigated by Rajesh and coworkers (117). They used a primary human cardiomyocytes subjected to a high glucose concentration and a mouse model of type 1 diabetic cardiomyopathy. It was demonstrated that CBD reduced cardiac fibrosis, myocardial oxidative/ nitrative stress, inflammation and cell death in diabetic hearts (in the first set of experiments, 1 week diabetic mice were treated with CBD 1, 10 or 20 mg/kg intraperitoneal injections for 11 weeks; in the second set of experiments 8 weeks diabetic mice were treated with CBD for 4 weeks) (Figrue 2, Table 1) (117). These effects were mediated by decreased intracellular adhesion molecule 1 (ICAM-1), vascular cell adhesion molecule 1 (VCAM-1), and TNF-α expression as well as reduced mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NFκB) activation. Moreover, CBD attenuated the high glucose-induced enhanced ROS (reactive oxygen species) generation and cell death in human cardiomyocytes (117). These findings highlight the role of CBD in the prevention or treatment of diabetic complications.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.