NF-κΒ activator 1

Therapeutic potential of cannabinoid receptor 2 in the treatment of diabetes mellitus and its complications

Abstract

The biological eﬀects of endocannabinoid system are mediated by two types of receptors, cannabinoid 1 (CB1) and cannabinoid 2 receptor (CB2). They play a pivotal role in the management of pain, inﬂammation, cancer, obesity and diabetes mellitus. CB2 receptor activity downregulation is hallmark of inﬂammation and oXidative stress. Strong evidence display the relation between activation of CB2 receptors with decrease in the pro-in- ﬂammatory cytokines and pro-apoptotic factors. Numerous in vitro and in vivo studies have been validated to conﬁrm the role of CB2 receptor in the management of obesity, hyperlipidemia and diabetes mellitus by regulating glucose and lipid metabolism. Activation of CB2 receptor has led to reduction of inﬂammatory cytokines; tumor necrosis factor-alpha (TNF-α), Interleukin 6 (IL-6), Nuclear factor kappa beta (NF-κβ) and also ameli- oration of reactive oXygen species and reactive nitrogen species playing role in apoptosis. Many studies con- ﬁrmed the role of CB2 receptors in the insulin secretion via facilitating calcium entry into the pancreatic β-cells. CB2 receptors also displayed improvement in the neuronal and renal functions by decreasing the oXidative stress and downregulating inﬂammatory cascade. The present review addresses, potential role of CB2 receptor acti- vation in management of diabetes and its complications. It also includes the role of CB2 receptors as an anti- oXidant, anti-apoptotic and anti-inﬂammatory for the treatment of DM and its complications. Also, an in- formative summary of CB2 receptor agonist drugs is provided with their potential role in the reduction of glucose levels, increment in the insulin levels, decrease in the hyperglycaemic oXidative stress and inﬂammation.

1. Introduction

Diabetes Mellitus (DM), a metabolic disorder aﬀects large popula- tion and increases the socio-economic burden. Patients suﬀering from diabetes are greatly subjected to the development of diabetic compli- cations which signiﬁcantly contribute to morbidity and mortality. Emerging evidences demonstrate that the endocannabinoid system modulate cell signaling targets and regulates various functions in DM and its complications. Research in the 1990s has revealed the presence of the endocannabinoid system (ECS) in humans (Di Marzo and Fontana, 1995), which has gained more attention from the research community. ECS consists of three major constituents; endocannabinoid molecules, cannabinoid receptors, and enzymes. The endogenous ECS consist of endocannabinoids molecules such as Anandamide (AEA) and 2-arachidonoylglycerol (2-AG) (Demuth and Molleman, 2006) which are responsible for maintaining signaling with cannabinoid receptors. ECS consists of two G Protein-coupled receptor (GPCR) (Dalton et al.,(CB2). Fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) (Basavarajappa, 2007) enzymes are mainly involved in enzy- matic degradation of AEA and 2 –AG respectively.

CB2 receptors are present mainly in the periphery, primarily to immune cells also expressed in all hematopoietic cells such as lym- phocytes, natural killer cells, macrophages, and neutrophils as well as kidney and pancreas (Svíženská et al., 2008). Due to the absence of CB2 receptors in brain, they does not shows any psychotropic eﬀects (Dhopeshwarkar and Mackie, 2014). Many pre-clinical pieces of evi-
dence shows that activation of CB2 receptor regulates a number of pathophysiological processes (Aghazadeh Tabrizi et al., 2016) and in- volved in the management of many disease conditions such as pain (Shang and Tang, 2017), inﬂammation (Turcotte et al., 2016), athero- sclerosis (Carbone et al., 2014), diabetes (Basha and Sankaranarayanan, 2014), cancer (Elbaz et al., 2017) and cardiovascular diseases (Steﬀens and Pacher, 2012). Clinical study of CB2 receptor agonist also eﬀec- tively decreased neuropathic pain (Gertsch et al., 2008).

Recent evidences suggest that inﬂammation, secretion plays an important role in the pathophysiology (Domingueti et al., 2016) of DM and its complications. Activation of CB2 receptor reduces pro-inﬂammatory mediators such as tumor necrosis factor alpha (TNF-α), IL-6 (Interleukin 6), NF-κβ (Nuclear factor kappa beta) (Béla Horváth et al., 2012a), Reactive oXygen species and reactive ni-
trogen species generation and also increases insulin secretion in pan- creatic β-cells via Ca2+ signal regulation (Juan-Picó et al., 2006). Thus, the current review provides the role of CB2 signaling and the eﬀect of CB2 agonists in the management of DM and its complications; diabetic nephropathy, neuropathy and cardiomyopathy.

2.2. Role of CB2 receptor in diabetes mellitus

DM is a metabolic disorder in which increased plasma glucose level is characterized by insulin resistance or lack of insulin, and obesity is the major risk factor for the development of Type 2 Diabetes Mellitus (T2DM). Inﬂammation and oXidative stress play an important role in the pathogenesis of DM and its complications (Kaur and C, 2012). The ECS regulates food intake, glucose homeostasis, inﬂammation, oXida- tive stress and insulin secretion (Gruden et al., 2016; Nogueiras et al., 2008). The recent reports conﬁrmed that CB2 receptors are present in human and rat pancreatic β-cells. (Bermúdez-Silva et al., 2008; Schuel et al., 2009). Sankaranarayanan et al. studied the role of natural sesquiterpene, β-caryophyllene in STZ induced diabetic rats. They pro- vided treatment at dose 200 mg/kg orally for 42 days and found a signiﬁcant reduction in plasma glucose level and elevation of the insulin secretion. This anti-diabetic activity of β-caryophyllene was observed due to CB2 receptor activation which to resulted in insulin se- cretion and glucose metabolism. β-caryophyllene treatment also improved oXidative stress and circulatory proinﬂammatory cytokines type of G protein-coupled receptor (GPCR) aid in the regulation of the intracellular mechanisms by coupling with Gi/o proteins (Howlett, 2005). CB2 receptors are expressed throughout the body, eﬀecting all the systems. CB2 receptors are present peripherally, speciﬁcally in the levels (TNF-α, IL-6) in diabetic rats (Basha and Sankaranarayanan, pancreas, immune cells, spleen, thymus, bone marrow, lungs, hemato- poietic cells such as lymphocytes, natural killer cells, macrophages, and neutrophils (Herkenham et al., 1990). CB2 receptors act by inhibiting adenylyl cyclase activity to produce cyclic adenosine monophosphate (cAMP) as well as induce mitogen-activated protein kinase (MAPK) activation (Bouaboula et al., 1996).

The signaling pathway of the CB2 receptor is dependent on the lo- calization of the receptor, cells, and type of G proteins involved in the transduction. They inhibit the activity of adenylyl cyclase through their Gi/Goα subunits, which are also known to couple the MAPK pathway.

2.1. Therapeutic potential of CB2 receptors

Recent preclinical research is demonstrating the major role of CB2 receptor activation by agonists in a variety of animal models involving central and peripheral systems. CB2 agonists have been reported to display its anti-inﬂammatory, anti-diabetic, anti-cancer, im- munomodulatory, neuroprotective, anti-atherosclerotic properties and are also useful in the management of neuropathic pain and osteo- porosis. CB2 receptor-mediated anti-inﬂammatory activity is carried out by inhibition of inﬂammatory cytokines such as TNF-α, IL-6, Nf-κβ, and MatriX metallopeptidase 9 (MMP-9). Administration of β-caryophyllene for 4 weeks at 50 mg/kg i.p. dose against rotenone-induced Parkinson’s disease in rats have shown neuroprotective activity via inhibition of pro-inﬂammatory cytokines (IL-1 β, IL-6, Nf-κβ, and COX) and oXida- tive stress (Javed et al., 2016). Presence of CB2 receptors in the immune cells show immunomodulation in the inﬂammatory processes, thus playing an important role in the management of DM and rheumatoid arthritis. Numerous results of in vivo studies have revealed that, CB2 receptor agonists inhibit the oXidative stress (Bėla Horváth et al., 2012) and prevents cell death or necrosis. Therefore we can conclude that CB2 agonist can be suitable candidate for the management of DM and its complications (Javid et al., 2016).

2.3. Role of CB2 receptor in the mediating insulin secretion

Presence of CB2 receptors in human pancreas have shown en- dogenous cannabinoid signaling which play a key role in endocrine secretion. Human islets of Langerhans and rat pancreas also express CB2 receptor protein and mRNA. It is also conﬁrmed that CB2 receptor are coupled to the stimulation of insulin secretion (Li et al., 2011). Ripoll et al. suggested that cannabinoid receptors are involved in the regula-
tion of the calcium signaling and increase the insulin secretion in pancreatic β-cells. Real-time polymerase chain reaction (PCR) has conﬁrmed that CB2 exists in β-cells of the human pancreas.

Activation of CB2 receptors by an agonist facilitates the calcium

entry into β-cells further causing insulin release. This is one of the most signiﬁcant contributing factors in the development of anti-diabetic drugs. An important observation from an in vitro study by Zhang et al.
was that, Trans caryophyllene dose-dependently (0.1, 0.5, 1.0 μM) in- creases insulin secretion in MIN-6 cell line. Researchers have also de-
monstrated that inhibition of CB2 receptor by a speciﬁc RNA inter- ference decreased the insulin secretion activity of trans caryophyllene. Results of in vivo antidiabetic studies have demonstrated that, CB2 agonist signiﬁcantly increases the insulin secretion in STZ-induced Type 1 DM (T1DM) (Basha and Sankaranarayanan, 2016) and high-fat diet/ low dose STZ induced T2DM (Zhang et al., 2016). Overall in vitro and in vivo studies exhibit the strong involvement in the activation of the CB2 receptors and insulin secretion which support the proposed hypothesis. As shown in Table no 2, diﬀerent in vitro and in vivo studies sug-
gested that CB2 agonist increases insulin secretion.

2.4. Role of CB2 receptor in the regulation of diabetic nephropathy

Diabetic nephropathy develops in approXimately 40% of the pa- tients with T1DM and T2DM. It is the leading cause of chronic kidney disorder and cardiovascular mortality (Reutens and Atkins, 2011). Diabetic nephropathy is characterized by increased plasma glucose level which in turn increases glomerular permeability to proteins and excessive extracellular matriX deposition in the mesangium. Eventually, it results in glomerulosclerosis and progressive renal impairment. In- ﬂammation, glomerular hypertension, and advanced glycation end products (AGE) play an important role in the pathophysiology of dia- betic nephropathy (Barutta et al., 2011). In diabetic nephropathy, ac- tivation of pro-inﬂammatory macrophages and release of pro-inﬂammatory markers such as TNF-α, Nf-κβ, IL-6 and extracellular matriX protein (Transforming growth factor-beta (TGF-β)) (Suryavanshi and Kulkarni, 2017). Presence of full ECS in the kidney is recently gaining attention of researchers for the treatment of kidney diseases through activation CB2 receptor (Francois and Lecru, 2018). Hyperglycemia leads to an increase in oXidative stress, advanced glycation end pro- ducts (AGE) and angiotensin II formation. Also, there is an imbalance between activation and down-regulation of CB1 and CB2 receptors. Several studies have been proposed that, high glucose and high albumin levels decrease the expression of CB2 receptors in the glomerulus, proXimal tubule cells (Jenkin et al., 2013) and whole kidney (Jenkin et al., 2016). An important implication of an in vivo study conducted by Federica et al. reveals that the diabetic nephropathy was induced by STZ administration in mice. Treatment of CB2 receptor agonist (AM1241) at the dose of 3 mg/kg consequently for 14 weeks. After treatment with AM1241 showed decreased urinary albumin excretion, podocyte protein loss, and glomerular monocyte inﬁltration (Barutta et al., 2011). Thus, it can conﬁrm that the potential role of CB2 receptor activation has a protective role in diabetic nephropathy. In vivo studies have shown that hyperglycemia induces the loss of nephrin and in- creases the albuminuria due to down-regulation of CB2 receptors which may lead to renal ﬁbrosis, proteinuria and ﬁnally progression of dia- betic nephropathy (Francois and Lecru, 2018). Downregulation of CB2 receptors are also involved in the increase of pro-inﬂammatory cyto- kines and oXidative stress. This mechanism also triggers the progression of diabetic nephropathy. In another in vivo study by Zoja et al. on BTBR ob/ob mice was treated by CB2 receptor agonist (HU910) at the dose of
10 mg/kg i.p. for 21 weeks in diabetic mice has resulted in the decrease albuminuria in association with the amelioration of the defective ne- phrin expression on podocytes, limited mesangial expansion and sclerosis. The treatment also attenuated glomerular inﬂammation dis- played a renoprotective eﬀect by activation of CB2 receptor activation. Horváth and co-workers demonstrated that β-caryophyllene (1, 3, 10 mg/kg i. p. for 3 days) have shown dose-dependent protection of kidney from cisplatin-induced nephrotoXicity in rats. β-caryophyllene also activates the CB2 receptor and decreased the renal inﬂammatory response (Monocyte Chemoattractant Protein-1 (MCP-1), Macrophage Inﬂammatory Protein 2 (MIP-2), TNF-α, IL-1β, intercellular adhesion molecules- 1(ICAM-1) (Béla Horváth et al., 2012b). The results from the in vivo studies conﬁrmed that activation of CB2 receptor plays an im- portant role in the management of diabetic nephropathy.

2.5. Role of CB2 receptor in diabetic neuropathy

Diabetic neuropathy is highly complicated and prevalent type of diabetic complication. It has been reported that approXimately 20–30 million people suﬀer from diabetic neuropathy (Bayram et al., 2016). The pathological outcomes of diabetic neuropathy may cause noci- ception, decreased motility, ranging from socio-economic burden to high mortality. Molecular pathways are associated with diabetic neu- ropathy such as oXidative stress (Javed et al., 2016), inﬂammatory cytokines, hexosamine pathway, polyol pathway (Dewanjee et al., 2018). Many studies demonstrated that ECS modulates pain perception through endocannabinoid ligands such as AEA and 2-AG (Woodhams et al., 2015). CB2 receptors present peripherally are not associated with psychotropic eﬀects and these receptors are participate in the mediating analgesic eﬀects (Ibrahim et al., 2003). CB2 receptor agonist AM1241 was evaluated against STZ-induced diabetic neuropathy and it was observed that stimulation of CB2 receptors inhibit the pain transmission in STZ induced neuropathic pain and also inhibits the cyclooXygenase (COX) and nitric oXide synthases (NOSs) (Bujalska, 2008). An im- portant in vivo study carried out by Mohab et al. has shown activation of CB2 by agonist, AM1241 (100, 300, 1000 and 3000 μg/kg) dose dependently inhibits the experimental neuropathic pain without having any of CNS side eﬀects.

CB2 agonist JWH-015 and JWH-133 (0.15, 0.5, 1 and 3 mg/kg i. p.) showed signiﬁcant anti-nociceptive eﬀect by reducing mechanical al- lodynia in the db/db mice and study also suggested that, agonistic ac- tivity of CB2 receptor is suitable in the use of painful diabetic neuropathy (McDonnell et al., 2017). Mehr et al. reported that, spinally administered CB2 agonist WIN 55, 212–2 in diabetic rats showed sig- niﬁcant anti-nociceptive eﬀect and improved nerve conduction velo- city, as well as reduction in sciatic nerve necrosis (Jahanabadi et al., 2016).

2.6. Role of CB2 receptor in diabetic cardiomyopathy

Diabetic cardiomyopathy is a leading cause of disability and mor- tality among the diabetic patients and is associated with increased plasma glucose level which results in the myocardium ﬁbrosis, hyper- trophy, collagen deposition and abnormalities in the cardiac myocytes. Increased myocardial reactive oXygen species and generation of in- ﬂammatory cytokines signiﬁcantly trigger diabetic cardiomyopathy (Gilca et al., 2017). ECS is present in the heart and CB1 and CB2 re- ceptors expressed are in the myocardium (Kaschina, 2016). CB2 acti- vation leads to decreased inﬂammatory response which results in the decreased reactive oXygen species and nitrogen species. Targeting these species is shown as a potential therapeutic target for diabetic cardio- myopathy (Steﬀens and Pacher, 2012). As only few researches have been done in the exploration of CB2 in diabetic cardiomyopathy. Fur- ther, future studies need to be done in exploring potential role of CB2 receptor in the management of diabetic cardiomyopathy.

2.7. Role of CB2 receptor in diabetic retinopathy

ECS expression is found in the retina and visual brain and canna- binoid receptors; CB1 and CB2 are expressed in the human eye (Bouchard et al., 2016), but the role of CB2 receptors in diabetic re- tinopathy is still unexplored. Retinal molecular and physiological ab- normalities have been found to be associated with diabetes, in- ﬂammation and oXidative stress. Utilization of CB2 receptor as an anti- inﬂammatory and an anti-oXidant will escalate a potential approach for the treatment of diabetic retinopathy.

3. Conclusion

Numerous pre-clinical evidences have reported that ECS plays an important role in the development of DM and its complications. CB1 receptor also display their intrinsic activity in the treatment of DM and obesity but outlie due to its adverse eﬀects. Inversely, CB2 receptors are not involved in the production of any psychotropic eﬀects. Recent re- ports have also demonstrated that potential role of CB2 in the treatment of inﬂammation, oXidative stress, and DM. Accelerating the insulin secretion by CB2 receptors have proved beneﬁcial in the management of DM. Findings of the review revealed that CB2 agonists are a better treatment option for DM and its complications. We hope that this novel approach to treat DM and its complications by CB2 agonist, will further boost clinical research. It is suggested that CB2 receptor agonists will be a better therapeutic preference NF-κΒ activator 1 for the treatment of DM and complications in the near future.