insulin resistance is a classic risk factor for the development of type 2 diabetes mellitus ( t2 dm ) . a significant predisposition to insulin resistance obesity and its associated comorbidities are reaching never - seen - before levels worldwide . with sufficient increases in basal and nutrient - stimulated insulin levels , systemic insulin levels can be increased by one of two general mechanisms : increased insulin secretion from individual beta cells ( that is , beta cell function ) and increased total numbers of beta cells ( that is , beta cell mass ) . reduced beta cell function in t2 dm is clearly an early and important aspect underlying the pathophysiology of t2 dm ; hence , addressing this dysfunction is a valid therapeutic strategy for t2 dm . ( little is known of the effects of the therapeutics discussed in this review on alpha cell function . as dysregulated glucagon secretion has also been noted as a key issue in t2 dm pathophysiology , these are clearly important questions to address , but are outside of the scope of this review . ) the first t2 dm therapeutics to target the beta cells were the sulfonylurea compounds , which act directly on the atp - sensitive potassium channels of the beta cell to enhance membrane depolarization , stimulating exocytosis of insulin granules . while still used clinically and excellent drugs in some circumstances , sulfonylureas act in a glucose - independent manner , increasing the potential risk of hypoglycemia . furthermore , sulfonylureas and other long - established therapeutics , including the insulin - sensitizer metformin , do not prevent the continuing loss of beta cell function observed in t2 dm . however , weight loss from metformin may indirectly benefit beta cell function and limit beta cell loss in t2 dm . a different aspect of beta cell dysfunction is being addressed by recent t2 dm therapeutics : the lack of potentiation of insulin secretion in response to an oral glucose / nutrient challenge . this potentiation occurs because of peptide hormones termed incretins that are secreted from the digestive tract and act on the pancreas and other peripheral tissues to elicit biological effects , including augmenting nutrient - stimulated insulin secretion . the incretin receptors belong to a superfamily of transmembrane proteins termed as g protein - coupled receptors ( gpcrs ) that promote intracellular signal transduction pathways mediated by guanine nucleotide - binding proteins ( g proteins ) . there are four major classes of g proteins : gs , gi , gq , and g12 . these proteins are classified based on the sequence identity of their alpha subunit , which confers similar downstream signaling mechanisms among subfamily members . gs is the sole member of its subfamily not restricted to olfactory / taste sensory cells , while similar members of the gi subfamily are gi1 , gi2 , gi3 , go1 , go2 and gz . the gs and gi subfamilies also include specialized members : golf is a gs subfamily member restricted to olfactory cells , while gt and ggust are gi subfamily members restricted to the retina and taste cells , respectively . while taste and olfactory cell signaling is an interesting focus of current obesity and diabetes research ( for example , see work from dotson et al . ) , only the more ubiquitously expressed g proteins will be discussed in this review . the gs and gi subfamily members all signal by modulating the levels of the intracellular second - messenger cyclic amp ( camp ) , either stimulating ( gs ) or inhibiting ( gi ) the production of camp by adenylate cyclase . camp has also been shown to regulate beta cell proliferation and survival , potentially linking these receptors to regulation of beta cell mass . the gq class of g proteins is also stimulatory toward insulin secretion , but acts through separate second - messenger pathways : inositol trisphosphate and diacylglycerol , which are formed when gq - activated phospholipase c- cleaves phosphatidylinositol bisphosphate not as much is known regarding the role of g12 on beta cell function , but its actions through the small g protein , rho , appear to be important for promoting the exocytosis process itself . drugs targeting the glucagon - like peptide 1 ( glp-1 ) receptor , a gs - coupled incretin receptor , are some of the newer t2 dm therapeutics , having been used clinically for nearly a decade . the stable glp-1 analogs , such as exenatide ( also known as byetta , amylin pharmaceuticals , san diego , ca , usa ) , are resistant to inactivating cleavage by dipeptidyl peptidase 4 , which rapidly degrades endogenous glp-1 . in addition , endogenous glp-1 can be stabilized by treatment with dipeptidyl peptidase inhibitors , such as sitagliptin ( also known as januvia , merck , whitehouse station , nj , usa ) . several other gq- and gs - coupled incretin receptors are being explored as t2 dm therapeutic targets and have been reviewed elsewhere . even with these successes , incretin - based therapeutics do not work in all individuals and safety concerns , including pancreatitis and gastrointestinal distress , in addition to possible side effects such as weight changes lead to non - compliance . these concerns raise the possibility of targeting potential dysfunction in other components of beta cell signaling pathways . endogenous inhibitors of beta cell function have long been known , even before the identity of their receptors and associated g proteins were established . for example , alpha adrenergic receptor ( -ar ) agonists such as epinephrine and norepinephrine are released from the sympathetic nervous system as part of the fight - or - flight response , one aspect of which is to maintain higher glucose levels in the bloodstream . -ars are expressed in beta cells , and agonists of the 2-ar subfamily , which are gi - coupled receptors , rapidly reduce glucose - stimulated insulin secretion . another long - established inhibitor of beta cell function , prostaglandin e2 ( pge2 ) , is produced by cyclooxygenase ( cox)-catalyzed metabolism of arachidonic acid and acts in an autocrine and/or paracrine fashion to modulate beta cell growth and insulin secretion . interestingly , receptors for adrenergic agonists and pge2 , as well as their associated g proteins , have all been linked with diabetic beta cell pathophysiology . insulin - responsive tissues besides the pancreas also play critical roles in the pathophysiology of t2 dm . the pancreatic circulation empties directly into the portal vein , meaning that the liver is the first stop for hormones secreted from the pancreas , including insulin and glucagon . insulin acts on the liver to signal the fed state of the body , promoting glucose storage into glycogen and blocking gluconeogenesis and lipolysis , blocking hepatic glucose and free fatty acid production . it is well accepted that hepatic fuel production is dysfunctional in t2 dm , as insulin - resistant alpha cells secrete glucagon in an unregulated manner , further exacerbating the phenotype of liver insulin resistance . in addition , muscle and adipose tissue are stimulated by insulin to increase glucose uptake for use as fuel and storage as glycogen and triglycerides , respectively . inefficient muscle and adipose glucose uptake is also observed in t2 dm and contributes to the pathology of hyperglycemia . there is much debate surrounding the impact of enhanced beta cell function and mass to the development of insulin resistance in t2 dm . this discussion is beyond the scope of this review , but has been well described elsewhere . specific gpcrs , including melatonin receptor isoform 2 ( mt2 ) , have been shown to have important roles in regulating insulin sensitivity . metabolic syndrome is often linked to obesity and visceral fat accumulation known to perturb blood glucose regulation and increase insulin resistance ; however , obesity by itself is not a definitive predictor of metabolic syndrome as some obese individuals do not present with typical etiologies . a diagnosis of metabolic syndrome is a significant risk factor , though , for the development of t2 dm , cardiovascular disease , neurodegeneration , polycystic ovarian syndrome , cancer and non - alcoholic fatty liver disease , among others . therefore , developing anti - obesity therapeutics as a preventative for these significant , chronic diseases is a major focus of current research . several effective medical treatments for obesity have been removed from the market or are strictly regulated due to unacceptable side effects . fenfluramine / phentermine ( also known as phen - fen ) was pulled from the us market in 1997 . sibutramine ( also known as reductil , meridia and sibutrex , abbott labs , abbott park , il , usa ) is a schedule iv - controlled substance and was pulled from the us market in 2010 . dextromethamphetamine ( also known as desoxyn , abbot labs ) is a methamphetamine derivative and a schedule ii - controlled substance . furthermore , the few widely available medical treatments such as orlistat ( also known as xenical , roche , basel , switzerland ; and alli , glaxosmithkline , research triangle park , nc , usa ) have only modest effects at best . surgical techniques such as gastric bypass and sleeve gastrectomy are often quite effective , but are also major procedures that have continuously decreasing , yet still not insignificant , peri- and post - operative morbidity and mortality risks . a safe , effective medical treatment to augment healthy lifestyle changes in the efforts to combat obesity and metabolic syndrome is sorely needed . one inhibitory gpcr in particular , cannabinoid receptor type 1 ( cb1 ) has been demonstrated as a potentially effective anti - obesity pharmacological target . in this review , we will discuss gi subfamily members that have been linked to beta cell dysfunction , insulin resistance or both . next , we will discuss a select number of inhibitory gpcrs that have been shown to have significant pre - clinical or clinical evidence of efficacy as anti - t2 dm and/or anti - obesity targets . these include gpcrs whose major effect is in the beta cell , as potential targets to maintain beta cell function and/or mass ; and gpcrs with primary effects in other insulin - responsive tissues , as targets to improve insulin resistance and/or obesity . it is our hope that this review will stimulate further research into these relatively under - studied proteins as potentially useful therapeutic targets for the obesity and t2 dm that is becoming a worldwide public health problem . beta cell dysfunction and insulin resistance are early and necessary events in the development of t2 dm , and gi signaling has been shown to be an important regulator of both these events . the gi family inhibitor , pertussis toxin ( ptx ) , was originally named islet - activating protein due to its ability to promote glucose- and hormone - stimulated insulin secretion long before its mechanism of action was known . even so , no insulin secretion phenotypes had been reported in mouse lines deficient in individual gi subfamily members , until those lacking only the ptx - insensitive gi subfamily alpha subunit , gz , were studied ( since then , other gi subfamily members that are sensitive to ptx have been linked with regulating beta cell function in vivo , explaining the effect of ptx on insulin secretion : these will be discussed in more detail below ) . by sequence and functional similarity , gz is clearly a member of the gi subfamily , but it is unique in that its alpha subunit ( gz ) lacks the c - terminal cysteine residue that is the substrate for ptx - mediated adp ribosylation and inactivation . gz - null mice exhibit improved insulin secretion response and glucose clearance after glucose challenge , correlating with constitutively increased camp levels and improved glucose - stimulated insulin secretion from their isolated islets . these effects the slow gz gtp hydrolysis and deactivation rate were posited as a potential explanation for constitutive effects , although endogenous beta cell gpcrs have been linked specifically with gz . agonists activating the prostaglandin e receptor 3 ( ep3 ) isoform of the pge2 receptor block glucose - stimulated insulin secretion from insulinoma cells and isolated mouse islets in a ptx - insensitive manner , suggesting coupling of the ep3 receptor specifically to gz and not other gi proteins . in addition , the modulation of endocytosis following exocytosis ( a required event in the maintenance of insulin secretion ) by norepinephrine is also ptx - insensitive . a synthetic peptide mimicking the c - terminus of the gz blocked this norepinephrine effect . this is not unexpected , as gz has little - to - no expression in the liver , adipose tissue or skeletal muscle , and although expressed in the brain , does not seem to regulate appetite or energy metabolism . even so , gz - null mice are completely protected from the development of glucose intolerance upon high - fat diet feeding , primarily through a significant augmentation of beta cell mass and maintenance of beta cell function . these results suggest gz as an emerging target to improve both defects of the diabetic islet : reduced beta cell mass and decreased beta cell function . other gi subfamily members , including both gi and go isoforms , have been implicated in directly inhibiting exocytosis from beta cells , which , together with their subcellular localization , indicates a role for gi proteins in the regulation of a distal step in the stimulated secretion pathway . in studies in an insulinoma cell line , norepinephrine inhibited the refilling of the readily releasable pool of secretory granules , an effect that was abolished by a dual gi1/2-blocking peptide . perplexingly , mice deficient in gi2 expression did not have noticeably improved insulin secretion , but instead had reduced insulin sensitivity . furthermore , expression of constitutively active gi2 in mouse liver and adipose tissue enhanced glucose tolerance . among other effects , a direct effect of gi2 signaling on glut4 membrane translocation through a phosphoinositide 3-kinase - dependent mechanism may be responsible for these phenotypes . early studies with go - null mice indicated changes in the nervous system structure and aberrant heart calcium channel signaling , but no effect on blood glucose levels . in more recent work , one of the two splice variants of go , go2 , mice lacking go2 , but not those lacking go1 or any gi isoforms have augmented glucose - stimulated insulin secretion and improved glucose tolerance , just as observed with gz - null mice . moreover , these results help support earlier findings that targeting ptx - sensitive proteins , such as go2 , can potentiate insulin secretion but in a different manner than gz . these results are important , as they suggest that the gi subfamily members in the beta cell are not redundant , as has been suggested in other systems and that specific isoforms and/or their associated receptors might be useful t2 dm therapeutic targets . of note , no small - molecule pharmaceutical agents in use clinically target a gpcr / g protein pathway directly at the g protein level ( although some have been proposed hypothetically and/or tested in vitro ) . in addition , biased gpcr ligands can preferentially effect one versus another g protein - dependent or -independent downstream signaling pathway , but these are still receptor - based drugs and not g protein - based . thus , understanding more about the endogenous gpcrs that these inhibitory g proteins couple to will likely be very important in pharmaceutically targeting their activation . many metabolites of the -6 polyunsaturated fatty acid ( pufa ) arachidonic acid have been linked directly with inflammation . e - series prostaglandins , comprising a major family of arachidonate metabolites , were posited as playing a role in t2d over 30 years ago . pge2 in particular has long been known as a physiological inhibitor of insulin secretion , acting via autocrine or paracrine mechanisms . this means that islet cells themselves , including beta cells , express the enzymes required for synthesis of pge2 , including phospholipase a2 , which cleaves arachidonate from plasma membrane phospholipids ; cox , which convert the fatty acid into the intermediate pgh2 ; and pge synthases ( ptges ) , which convert pgh2 to pge2 . many groups ( including the american diabetes association ) have recommended a high -3 pufa diet as a means to prevent disease . yet , there still exists some controversy as to the benefits of -3 pufas on chronic disease as well as the impact of high -3 pufas in the context of a diet that remains high in -6 pufas . notable exceptions include two groups that generated transgenic mice expressing a caenorhabditis elegans desaturase that converts -6 pufas to the -3 form . these mice robustly increased -3 pufa concentrations ( and reduced -6 pufas to near zero ) in all tissues , with improved insulin secretion from isolated islets . inhibition of cox ( which , in beta cells , is primarily cox-2 ) can completely block pge2 production along with that of numerous other metabolites downstream of pgh2 and the other cox intermediates . the upregulation of cox-2 expression and/or activity has been clearly linked with the diabetic state . the potential utility of cox-2 inhibitors in t2 dm is overshadowed by concerns of significant cardiovascular risks , especially in a population at greater intrinsic risk of cardiovascular disease . interestingly , after nearly a half century , there has been a recent resurgence in studies on the utility of salicylate , which acts directly at the site of inflammation to reduce cox-1 and cox-2 expression , as a t2d therapeutic . recently , a specific pge2 receptor isoform , ep3 , has been suggested as an emerging target for t2 dm therapeutics . ep3 receptor expression and activity was significantly upregulated in diabetic mouse islets , as was the production of pge2 . targeting the ep3 receptor with a specific antagonist , l798,106 , interestingly , ep3 signaling was found to oppose the action of the glp-1 receptor , discussed above as a significant potentiator of glucose - stimulated insulin secretion . specifically , the maximal effect of glp-1 on insulin secretion was blunted by ep3 activation , and a combination of an ep3 antagonist and glp-1 agonist additively promoted insulin secretion from diabetic mouse islets . the mechanisms mediating dysfunctional ep3 signaling in diabetic beta cells and potential ways to target ep3 in t2 dm therapy are shown on the left half of figures 1 . although the ep3 receptor plays critical roles in islet biology , extrapancreatic function may be equally important in t2 dm pathophysiology ; in particular , in mediating t2 dm complications . the migratory response of vascular smooth muscle cells is required for the dysfunctional vascular remodeling observed with cardiovascular disease . genetic deletion or pharmaceutical inhibition of ep3 results altered polarity and directional migration of vascular smooth muscle cells , suggesting that blockade of ep3 might protect from dysfunctional vascular remodeling . in addition , mice lacking ep3 have reduced baseline and stimulated mean arterial pressures , and pharmacological inhibition of ep3 blocks angiotensin 2-mediated vasoconstriction . thus , it appears that pge2 signaling through ep3 may play a significant role in the progression of t2 dm and its associated comorbidities . mice lacking ep3 develop obesity , insulin resistance , glucose intolerance and eat considerably more than wild - type controls . part of this phenotype can be explained by increased light cycle eating , most likely due to unstable sleep patterns , as pge2 ( acting through hypothalamic ep3 ) may act as a somnogen . furthermore , high levels of pge2 and increased signaling through ep3 both augment nitric oxide synthase expression , an enzyme critical in the brain development of newborns . this signaling cascade is suggested to play a significant role in connecting brain circulation and synaptic activity in perinatal development . if ep3 antagonists are pursued as therapeutics for t2 dm and/or metabolic disease , a peripherally restricted ep3 antagonist might be necessary . interestingly , an ep3 antagonist , dg-041 , completed a european phase ii clinical trial by decode genetics testing its effect in treating peripheral vascular disease , which is typically caused and/or complicated by hypertension . the results of this trial were never published , so questions remain as to whether this drug was effective as an anti - hypertensive agent . recently , dg-041 treatment in mice was found to limit atherothrombosis without altering hemostasis , suggesting that dg-041 can decrease clot formation without increasing bleeding risk , critical in the prevention of cardiovascular disease . dysfunctional -ar signaling was noted in t2 dm at least 40 years ago . in an early clinical study of six diabetic individuals , -ar blockade with -ar antagonists were never much pursued as anti - diabetic agents , though , perhaps due to the systemic effects of -ar signaling , including smooth muscle contraction , inhibition of neurotransmitter release , transient hypertension followed by sustained hypotension , and constriction of certain veins and arteries , including those leading to and from the heart , among others . this possible thinking that -ar signaling was too pleiotropic to be targeted therapeutically in t2 dm changed in 2010 , when a single nucleotide polymorphism in the 2a - ar gene , adra2a , was discovered as a susceptibility factor for t2d : rs553668 . in two different studies , including over 5000 individuals , rosengren et al . showed that individuals with a specific rs553668 genotype trended toward elevated fasting plasma glucose levels and reduced serum insulin levels . furthermore , these individuals were significantly more prone to a hypoinsulinemic response during glucose tolerance testing , indicating a defect in the insulin secretion process itself . when human pancreatic islet preparations were genotyped , those with the specific rs553668 variant had reduced insulin granule docking and secreted less insulin in response to glucose due to increased expression of 2a - ar at the membrane . treatment of isolated islets with an 2a - ar antagonist , yohimbine , corrected the insulin secretory dysfunction in these islets . a clinical trial initiated in the us is currently enrolling patients with this specific genotypic risk variant in a randomized , controlled trial of yohimbine as a t2d therapeutic . a more recent meta - analysis of four prospective studies confirmed the linkage of rs553668 with fasting glucose and t2 dm risk , and found another variant of adra2a , rs17186196 , associated with fasting glucose levels , fasting insulin levels and whole - body insulin sensitivity . furthermore , rs491589 was associated with systolic and diastolic blood pressure , while rs36022820 correlated with body mass index . additional adra2a variants have been linked with hypertension , a significant risk factor for cardiovascular disease , a common complication of t2 dm . in a study of 70 hypertensive patients , 35 with and 35 without t2 dm , individuals with a genotype encoding for a small deletion in adra2a were significantly more likely to have hypertension , regardless of t2 dm status . a potential physiological mechanism for this association was the significantly lower high - density lipoprotein and higher low - density lipoprotein in individuals homozygous for the deletion phenotype . overall , these results confirm the utility of targeting the 2a - ar to improve beta cell function in t2 dm in a specific population , and also indicate its possible utility as a target for insulin resistance , obesity and/or t2 dm complications . the mechanisms mediating dysfunctional 2a - ar activity in a specific subset of diabetic beta cells and potential ways to target 2a - ar in t2 dm therapy are shown on the right half of figure 1 . it has been demonstrated in humans that disruption of the circadian rhythm promotes obesity and diabetes , and leads to perturbations in plasma glucose and insulin concentrations . for instance , melatonin ( n - acetyl-5-methoxytryptamine ) is secreted from the pineal gland and has a major role in the regulation of circadian rhythms . melatonin acts through two receptors , mt1 ( gene name : mtnr1a ) and mt2 ( gene name : mtnr1b ) . activation of mt1 or mt2 receptors in the beta cell inhibit insulin secretion . however , it is not melatonin antagonists that have been proposed as obesity / t2 dm therapeutics , but rather melatonin agonists . this is because mt1/mt2 receptor signaling also reduces dyslipidemia , reduces hepatic glucose output , decreases adiposity and improves metabolic function . thus , stimulating melatonin signaling has been proposed as a treatment for metabolic syndrome . altered melatonin receptor expression or loss - of - function mutations and altered melatonin secretion itself melatonin was shown to function by reducing hepatic gluconeogenesis and phosphorylation and activation of akt in the hypothalamus . most of the recent research into the links between the melatonin receptors and diabetes is focused on mt2 . independent genome - wide association studies in european populations identified single nucleotide polymorphisms near mtnr1b that were associated with fasting plasma glucose , reduced beta cell function , and significantly increased t2 dm risk : rs1387153 and rs10830963 . expression of these two single nucleotide polymorphisms is also significantly associated with the development of gestational diabetes , as well as elevated fasting glucose and reduced insulin secretion after parturition . a diagnosis of gestational diabetes is one of the highest predictors for the later development of t2 dm . a clinical trial was registered in late 2012 to test the effects of 3 months of melatonin treatment on glucose tolerance in individuals with different mtnr1b variants . cb1 is a gi - coupled receptor located in the central and peripheral nervous system as well as other insulin - responsive tissues . endogenous cb1 ligands include anandamide ( which is responsible for pleasurable effects , such as that from eating chocolate ) and 2-arachidonoyl glyceride . natural and synthetic cannabinoids such as tetrahydrocannabinol , the main active compound of the psychoactive drug cannabis , and its synthetic analog , dronabinol , are potent cb1 agonists . in the brain , cb1 is highly expressed in the hypothalamus , which regulates appetite , and the medulla , which contains the chemoreceptor trigger zone , regulating nausea and vomiting , and where it is involved in the regulation of normal brain aging . this explains why tetrahydrocannabinol is such an effective drug at combatting the nausea and loss of appetite associated with chemotherapy . despite potential side effects such as nausea , the appetite - suppressive effects of central cb1 stimulated research into antagonists of the receptor as potential anti - obesity therapeutics . several trials of the systemic cb1 antagonist , rimonabant , as a weight - loss agent were initiated in both europe and north america . overall , rimonabant was superior to both sibutramine and orlistat , with a nearly 5-kg average weight loss over a 1-year period ( vs 4.5  kg for sibutramine and 2.5  kg for orlistat ) . also , t2 dm patients treated with rimonabant in addition to metformin or sulfonylurea had significantly lower hba1c levels than those treated with metformin or sulfonylurea alone . not surprisingly , nausea was a common adverse effect in the rimonabant group ( 13% , as compared with 4% with placebo ) , as well as were a number of severe psychological alterations . specifically , 15% percent of rimonabant - treated patients withdrew from the trials , and nearly half of these dropouts were attributed to mood alterations such as depression and increased suicide ideation . these outcomes halted any ongoing rimonabant trials , and the cb1 antagonist field made little progress over the next few years . perhaps due to the importance of cb1 in mediating appetite in the central nervous system , these include the neutral cb1 antagonists , which have a weaker affinity for receptor , but also peripherally restricted cb1 antagonists , which can not cross the blood - brain barrier and thus have no direct central nervous system effects . insulin - responsive tissues such as the liver , pancreas , adipose and muscle also express cb1 , and antagonism of these receptors promotes insulin signaling , blocks lipogenesis , promotes fatty acid oxidation and stimulates energy - requiring processes such as mitochondrial biogenesis and cell growth and differentiation . cb1 receptors are also located on afferent and efferent nerve fibers ; thus , the insulin - responsive state of the body is transmitted to the brain , where it can send signals to decrease food intake . overall , a requirement for direct central nervous system activation of cb1 receptors does not seem to exist in order to elicit positive metabolic changes , and perhaps , weight loss . several recent publications show strong pre - clinical efficacy of these peripherally restricted cb1 antagonists . in recent years , we have learned much about the role of inhibitory g proteins and gpcrs in mediating dysfunctional signaling pathways in metabolic diseases such as t2 dm and confirmed their efficacy in pre - clinical models and select human studies . a number of other inhibitory gpcrs not discussed above may also have therapeutic potential ( for example , serotonin 2a , ghrelin , npy and gpr41 , among others ; reviewed by ahren ) . yet , we still have a long way to go before demonstrating that any of these proteins are suitable targets for t2 dm and obesity in the general population .