Is the GLP-1 system a viable therapeutic target for weight reduction?

Abstract

Incretin hormones are intestinally derived peptides that are known to augment glucose-stimulated insulin secretion and suppress glucagon levels. Incretin mimetics are attractive adjunctive therapy for type 2 diabetes due to its efficacy on reducing hyperglycemia with a minimal risk of hypoglycemia. In contrast to most available hypoglycemia agents that cause weight gain, incretin mimetics are associated with moderate weight loss. In this review, we focused our discussion on the actions of glucagon-like peptide 1 (GLP-1) in the brain regulation of energy expenditure and food intake. Furthermore, we reviewed the data from preclinical and clinical studies in humans and discussed the actions of GLP-1, GLP-1 analogs, dipeptidyl pepidase 4 (DPP-4) inhibitors on body weight regulation as well as mechanism by which these effects may occur. The gastrointestinal side effects common to GLP-1 based therapeutics such as nausea hamper its wide spread use. Here, we discussed theoretical possibilities for maximizing weight loss and minimizing nausea with of incretin-based therapy.

1 Introduction

Over 50 years ago, it was discovered that insulin secretion in response to a glucose load was greater with oral vs. intravenous glucose administration [1]. The authors hypothesized that this was due to hormones secreted from the gut that stimulated insulin release in response to nutrient exposure. We now know that gastric inhibitory peptide (GIP) and glucagon-like peptide-1 (GLP-1) are two such incretins. However, the predominant role of GIP seems to be related to increased adipogenesis, likely through direct actions on adipocytes [2], while GLP-1 has been found to be a very important regulator of glucose homeostasis. In fact, long acting GLP-1 derivatives and pharmaceuticals aimed at increasing circulating GLP-1 by inhibiting the cleavage enzymes (dipeptidyl pepidase 4 [DPP-4]) have been found to be effective treatments for type 2 diabetes mellitus (Table 1). While majority of the diabetes therapies cause weight gain [3, 4], GLP-1-based therapies are associated with weight loss that is actually comparable to the handful of FDA-approved drugs available to treat obesity [5]. One of the side effects of GLP-1 analogs is that they cause nausea in many patients [5]. This review will focus on the potential contribution of GLP-1 on regulation of energy homeostasis. We will discuss the CNS regional effects GLP-1 and how this could be exploited to optimize weight loss and minimize nausea.

Table 1.

Summary of major classes of GLP-1 based therapies for the treatment of type 2 diabetes

Drug	Route of administration	Dose frequency	Effect on body weight	Manufacture
GLP-1 receptor agonists
Exenatide	Subcutaneous injection	Twice daily	↓	Amylin Pharmaceuticals
Liraglutide	Subcutaneous injection	Once daily	↓	Novo Nordisk
DPP-4 inhibitors
Sitagliptin	Oral	Once daily	↔↑	Merck & Co.
Saxagliptin	Oral	Once daily	↔↑	AstraZeneca Pharmaceuticals   LP/Bristol-Myers Squibb
Vildagliptin	Oral	Once daily	↔	Novartis

2 The GLP-1 system

In response to a meal, GLP-1 is secreted into the circulation by enteroendocrine L-cells located predominantly in the mucosa of the distal intestinal tract [6, 7]. Only one GLP-1 receptor (GLP-1r), initially cloned from pancreatic islets, has been described. The receptor is also found within the CNS, heart, and lung. In general, GLP-1 is thought to be a brain-gut peptide that acts as a hormone and neurotransmitter mediating several distinct processes related to nutrient metabolism, including glucose metabolism and regulation of food intake.

3 Review of clinical data on GLP-1 and energy homeostasis

3.1 Regulation of energy homeostasis

Energy intake and energy expenditure are two essential elements for body weight regulation. Data on the effect of GLP-1 on energy expenditure in humans is limited. Higher fasting GLP-1 levels were found to be correlated with higher resting energy expenditure and fat oxidation [8]. GLP-1 infusion resulting in a four-fold increase in plasma GLP-1 and increased energy expenditure in healthy individuals [9]. However, this effect was abolished when insulin level was kept constant during a pancreatic clamp suggesting that the effect on energy expenditure was indirect. In another study, a similar dose of GLP-1 infusion lowered diet induced thermogenesis by 47% due to reduced carbohydrate oxidation but did not alter fat oxidation [10]. In contrast, treatment of a low dose (0.6 mg daily) of the long-acting GLP-1 analog liraglutide for 8 weeks had no effect on 24 h energy expenditure [11]. The combination of exenatide treatment and lifestyle modification induced greater weight loss and glucose lowering but showed similar decreases in caloric intake and increase in exercise-derived energy expenditure as compared to placebo in overweight or obese individuals with type 2 diabetes [12]. Taken together, exogenous GLP-1 or GLP-1 analogs do not seem to have a consistent or significant effect on energy expenditure in humans.

3.2 Regulation of food intake

The vast amount of literature examining GLP-1 based therapies has focused on the effect of GLP-1 on food intake. Exogenous administration of GLP-1 to humans, raising plasma levels to pharmacologic ranges, has an acute and negative effect on energy intake in humans. In a randomized, blinded, placebo-controlled crossover study, short-term intravenous (IV) infusion of GLP-1(7–36 amide) that increases plasma concentration of total GLP-1 to pharmacological levels (60–90 pmol/L) in healthy normal-weight males enhanced satiety and fullness as well as reduced spontaneous food intake by 12% as compared with placebo [13]. Similar effects of GLP-1 on hunger ratings, satiety and energy consumption were also reported in obese subjects [14] and subjects with type 2 diabetes [15]. The anorexic effect of GLP-1 was also demonstrated in healthy individuals given GLP-1 to achieve plasma concentrations closer to those seen postprandially [16], in which graded GLP-1 infusions caused a dose dependent reduction in caloric intake. In a meta-analysis on 9 published and unpublished studies that included 115 subjects, the average energy intake reduction was 12% during varying rates and durations of GLP-1 infusion [17]. In this analysis, the GLP-1 infusion rate was the only independent predictor of reduced energy intake. However, the negative effects of GLP-1 on satiety and food intake have not been universally observed [18, 19]. Proof of concept for the feasibility of using native GLP-1 for therapeutic purpose was obtained from a 6-week study of patients with type 2 diabetes [20]. GLP-1 delivered via continuous subcutaneous infusion, in addition to benefitting glucose homeostasis, significantly decreased weight by 1.9 kg. Taken together, these data suggest that pharmacological administration of GLP-1 can lead to reductions in food intake, but it remains whether the physiological changes that occur with GLP-1 during a meal in humans actually contributes to reductions in food intake. Regardless, these pharmacological effects are proving to be beneficial in patients with type 2 diabetes. Whether it is a viable option for weight loss in obese, non-diabetic patients remains to be determined.

3.3 GLP-1 based therapies and regulation of food intake

3.3.1 Long-acting GLP-1r agonists

Exenatide

Exendin-4 is a 39 amino acid peptide extracted from the venom of the Gila monster (Heloderma suspectum) with a structural homology of 53% with mammalian GLP-1 and a high affinity for GLP-1 receptors [21]. The glycine residue at position 2 of the peptide confers resistance to DPP-4 degradation. Twice daily administration of subcutaneous injection of exenatide, a synthetic exendin-4 derivative, has been shown to improve glucose control with concomitant weight loss ranging 1.5–3 kg over 30 weeks [22–24]. A 52-week open-label, uncontrolled extension of the 30-week exendin-4 treatment led to a total weight loss of 4–5 kg [25]. In another study, a 5.3 kg weight loss was achieved after 3-year treatment with 10 µg twice daily exendin-4 administration [26, 27]. Importantly, this weight loss is due to reductions in fat mass as one-year exenatide treatment led to significant reduction in body weight (6%), waist circumference, total body and trunk fat mass in addition to increased adiponectin [28]. Mild to moderate gastrointestinal (GI) side effects such as nausea, vomiting or diarrhea was reported in all trials but few subjects discontinued therapy during the trial period.

The requirement that current peptide GLP-1r agonists be given by injection has led to the development of new compounds with daily or weekly effectiveness. A long acting release formula (LAR) of exenatide and albuminexendin-4 conjugates that are partly DPP-4 resistant, have beneficial effects on glycemic control and body weight in rodents and humans, making them attractive alternatives for the treatment of type 2 diabetes [29]. In a phase II randomized control trial, exenatide LAR administered at 0.8 and 2.0 mg once weekly for 15 weeks significantly improved glucose control as compared to placebo, and at the higher dose there was comparable weight loss to previous studies using shorter acting exenatide. Mild nausea was the most frequent adverse event.

Liraglutide

Another long-acting GLP-1r agonist currently available in the clinic is liraglutide, a fatty acid acylated GLP-1 molecule that exhibits a prolonged pharmacokinetic profile after a single injection due to the non-covalent bound with albumin [30]. Liraglutide mimics all of the actions of native GLP-1 and effectively lowers blood glucose in human subjects with type 2 diabetes [31]. The circulating half-life of liraglutide is 10–14 h as compared to 60–90 min with exenatide after a single subcutaneous injection [32, 33].

The efficacy of liraglutide on glucose lowering and body weight as well as body composition changes was evaluated in the phase 3, double-blind, randomized controlled Liraglutide Effect and Action in Diabetes (LEAD) trials. After 26 or 52 weeks of 1.2–1.8 mg liraglutide once daily treatment, weight reduction of 2–2.5 kg was observed with liraglutide monotherapy (LEAD-3) [34], 0.2 kg in combination treatment with sulphonyluea (SU) (LEAD-1) [35], 1.8–2.8 kg in combination with metformin (LEAD-2) [36], 1–2 kg when added to metformin and thiazolidinediones (TZD) (LEAD-4) [37], and 1.8 kg combined with metformin and SU (LEAD-5) [38]. The higher dose of liraglutide led to larger amount of weight loss in these studies suggesting a dose dependency. Interestingly, participants who had nausea for more than 7 days also had a tendency to lose more weight compared to those who did not have nausea days in LEAD-3 [34]; this is similar to findings reported from a recent study with exenatide-LAR (Duration trial) [29]. The effectiveness of weight loss with liraglutide (1.8 mg, once daily) and exenatide (10 µg, twice daily) was similar when compared in a head-to-head trial (LEAD-6) [39]. Varying combination treatment produces similar degree of weight loss in most trials (LEAD 2–6) [34, 36–39] except for the SU combination where greater weight loss was achieved with exenatide (LEAD-1) [35]. However, this result needs to be interpreted with caution due to the lower baseline body weight in the liraglutide treated group than the exenatide treated group.

The effectiveness of liraglutide in the treatment of obesity was assessed in a randomized, double-blind, placebo-controlled 20-week trial where 564 obese individuals with fasting plasma glucose of less than 7 mmol/L and BMI ranging 30–40 kg/m² were randomized to receive one of the four doses of liraglutide (1.2–3.0 mg daily) or placebo or orlistat (120 mg daily) [40]. Treatment with liraglutide, in addition to an energy-deficit diet and exercise program, led to a dose-dependent weight loss that was significantly greater than that with placebo (all doses) and orlistat (vs. liraglutide 2.4 and 3.0 mg only). The mean weight loss with liraglutide 3.0 mg was 7.2 kg over 20 weeks. Nausea and vomiting were again more common in the liraglutide treated group but mostly transient and of mild or moderate intensity.

The reduction of weight with liraglutide was found to be primarily from reduction in fat mass rather than lean tissue mass. Body composition changes related to liraglutide therapy were evaluated by using dual-energy X-ray absorptiometry and computed tomography in LEAD-2 and LEAD-3 trials [41]. Both visceral and subcutaneous adipose tissue were significantly reduced by liraglutide treatment alone (LEAD-3) or in combination with metformin (LEAD-2). In addition, liraglutide 1.8 mg increased the liver-to-spleen attenuation ratio, possibly indicating reduced hepatic steatosis. Absolute total lean body tissue mass was also reduced in all liraglutide treatment arms in a dose-dependent manner. However, these reductions were not significantly different from the placebo control group. Together with the findings from the exenatide treatment study discussed previously [28], GLP-1 analogues induce weight loss and fat mass reduction. In addition, body weight reduction due to liraglutide treatment was also associated with a reduction in lean mass as commonly observed in subjects undergoing a weight loss intervention.

3.3.2 Increasing circulating endogenous GLP-1

The previous literature suggested that raising circulating levels of GLP-1 with exogenous administration could inhibit food intake. Another way to increase circulating GLP-1 is to inhibit the cleavage enzyme, DPP-4. In fact, DPP-4 inhibitors that raise endogenous concentrations of active GLP-1 [42] and have been shown to be effective in lowering fasting and postprandial glucose concentrations [43]. In contrast to GLP-1 based therapy, DPP-4 inhibitors have been shown to be weight neutral [5]. Thirteen randomized controlled trials evaluating the effects of DPP-4 inhibitors on HbA1c and weight loss prior to May 2007 were summarized in this meta-analysis and systematic review by Amori et al. [5]. Overall, there was a small increase in weight with DPP-4 inhibitors compared with placebo (weighted mean difference, 0.5 kg; 95% CI, 0.3– 0.7 kg). In non-inferiority trials, sitagliptin produced more weight loss than glipizide (between-treatment difference, −2.5 kg; 95% CI, −3.1, −2.0) [44] while vildagliptin had a favored weight profile compared with thiazolidinediones (TZDs) (−1.7 kg; 95% CI, −2.2 to −1.2 kg) [45, 46] but not compared with metformin [47]. The new member of the FDA approved DPP-4 inhibitors saxagliptin has shown similarly minimal effect on weight as the other members [48]. In contrast to GLP-1r agonists, DPP-4 inhibitors have not shown to have significant effect on body composition at least in rodents [49, 50]. It is still important to note that while these methods of increasing circulating GLP-1 may not cause weight loss, they do not cause the significant weight gain seen with other antidiabetic medications such as insulin and some insulin sensitizers. The respective side effects of DPP-4 inhibitors were summarized in the systematic review by Amori et al. [5].

3.4 Summary of clinical findings

While we still have much to learn regarding the physiological role of GLP-1 in body weight regulation in humans, it is clear that the long acting GLP-1r agonists are effective at lowering body weight through a reduction in food intake vs. increasing energy expenditure. The mechanism for this remains unknown. Data from the LEAD-3 trial [34] suggested greater incidence of nausea was associated with greater weight loss. This leads us to the question of whether nausea is the cause of the weight loss. We now turn to a review of basic science literature on the role of GLP-1 in body weight regulation and will explore further the potential role of GLP-1-induced nausea in the reduction of food intake.

3.5 Mechanisms for GLP-1-induced weight loss

GLP-1 is also made within a discrete population of neurons the found in the nucleus of the solitary tract (NTS) [6, 51, 52]. These GLP-1 neurons within the NTS have rich axonal innervation to the hypothalamus [52, 53]. Third ventricular (i3vt) administration of GLP-1, increases c-fos immunoreactivity, a marker for neuronal activation, in various areas of the brain known for regulating energy balance including the paraventricular (PVN) and arcuate (ARC) nucleus of the hypothalamus [54]. Further support for a role for GLP-1 in energy homeostasis is that neuronal expression of preproglucagon within the hindbrain is decreased with fasting in mice [55].

3.5.1 Mechanisms for GLP-1-induced changes in energy expenditure

If GLP-1 regulates energy expenditure, it could be through sympathetic activation. Acute GLP-1 or long-acting GLP-1 agonist administration has been shown to increase sympathetic outflow to regulate heart rate, and blood pressure [56], and lipolysis white adipose tissue [57] via central mechanisms. While this suggests that GLP-1 systemically increases sympathetic activity and thus increases in energy expenditure, the animal literature regarding the role of GLP-1 on energy expenditure conflicts. As stated above, human data shows minimal effect of GLP-1 on energy expenditure [9, 11]. In mice, the effect may depend upon diet since 1 week of ICV administration of GLP-1 prevented the decrease in energy expenditure seen with caloric restriction in lean mice [57]. Further, acute (1 week) central blockade of endogenous GLP-1r had no affect on food intake but increased body weight with a tendency for a decreased energy expenditure in C57/Bl6 mice fed a chow diet [57] while more chronic (1 month) central blockade increased energy expenditure in high fat fed C57/Bl6 mice [58]. It is unknown whether it is the timing (1 week vs. 1 month) vs. the diet (chow vs. high fat) or some other factor that leads to these discordant results. Regardless, it seems that if central GLP-1 action is beneficial for treatment of obesity, increased in energy expenditure does not make a major contribution to the negative energy balance.

3.5.2 Mechanisms for GLP-1-induced anorexia

While GLP-1 may have only minimal effects on energy expenditure, at least in obesity, it is clear that CNS GLP-1 affects the other side of the energy balance equation by decreasing food intake [59–63]. The anorectic action of GLP-1 is short-lived, reducing food intake typically only within the first few hours of the onset of the dark cycle (when rats typically eat most) [59–63]. Unlike the effects on energy expenditure, central GLP-1 administration works equally well to reduce food intake in lean vs. obese animals [60].

GLP-1 administered directly into the lateral [64], 3rd and 4th ventricles [59, 64], the paraventricular nucleus and hindbrain [65] reduces food intake. Interestingly, GLP-1 given directly into the arcuate nucleus (ARC) of the hypothalamus, a key nuclei regulating energy balance, has little effect on food intake [66].

The regional effects of GLP-1 to reduce food intake suggest that GLP-1 acts on a variety of neuronal populations to activate anorectic circuits. The GLP-1r is colocalized with POMC (anorectic neurons) but not NPY (orexigenic neurons) neurons within the ARC [66], However, the role of these cells to mediate the feeding effects of GLP-1 are not direct since administration of peptide to this brain region does not change food intake. While there are data suggesting NPY [67–69] may block GLP-1 anorectic action, neither AgRP [70], nor administration of SHU9119, a melanocortin receptor antagonist, at a dose that blocked the anorectic effects of leptin [71], blocked the anorectic effects of GLP-1. However, it remains possible that these neurons play a role in other CNS effects of GLP-1.

Recent data suggest a link between leptin and GLP-1 in reducing food intake. When sub-threshold doses of GLP-1 and leptin are given together, they potently inhibit food intake suggesting an additive effect of the two drugs [72]. Interestingly, prolonged fasting blunted the anorexic effects of both GLP-1 and exendin-4 (a long acting GLP-1 agonist), an effect that was overcome with a concomitant infusion of leptin. In addition, fasting downregulates hindbrain expression of preproglucagon and this is prevented with leptin infusion during the fast [73]. Whether these effects of leptin are direct or indirect remains unclear but do suggest that these peptides interact to regulate satiety.

3.5.3 The interplay between the peripheral and the central glp-1 systems for regulating body weight

The role for peripheral GLP-1 to regulate glucose homeostasis, and the role of the CNS GLP-1r to regulate food intake are thought to be two separate functions of the GLP-1 system. However, there is evidence for interplay of these two systems. Intravenously administered GLP-1 binds to specific areas within the CNS [74]. However, the short residence time in the circulation raises some doubt as to whether GLP-1 can directly activate CNS neurons from the blood stream or after transport into the brain. An alternative to endocrine action of GLP-1 on the CNS is that the secreted peptide engages visceral afferents which, in turn, activate the central GLP-1 system to initiate some of the physiological effects. Support for this possibility is that nodose ganglion nerves contain GLP-1r [75] and intra-portal infusion of GLP-1 increases pancreatic and vagal afferent activity [76, 77]. However, GLP-1 action in the portal vein seems to be more important for glucose homeostasis [78] rather than food intake [79]. Interestingly, recent data show that the anorectic effect of gastric distention, but not duodenal nutrient infusion, is blocked by hindbrain administration of a GLP-1r antagonist [55]. Together these data suggest a model similar to the CNS GLP-1r system, whereby activation of visceral afferent neurons by GLP-1 is heterogeneous whereby activation of the GLP-1r on the gastric vagal afferents have different physiological effects compared to activation of the GLP-1r located on hepatic visceral afferents. Explaining the physiologic basis for this heterogeneity may facilitate the identification of more specific pharmacologic targets for the GLP-1 system aimed at optimizing weight loss. It is possible to imagine that these targets may even be distinct from targets used to treat type 2 diabetes mellitus.

3.5.4 The role of nausea in GLP-1-induced weight loss

Nausea is the one of the most limiting factors for targeting GLP-1 as a therapy for obesity (and its current use as therapy for type 2 diabetes). Data from the LEAD 3 trial [34] showed an association between increased nausea and increased weight loss in type 2 diabetic patients. This would lead one to speculate that nausea may be a mechanism for GLP-1-induced weight loss.

While the effect of these agents on improving glucose homeostasis is similar between GLP-1 analogues and DPP-4 inhibitors, the GI side effects commonly reported with GLP-1 analogue use is absent with DPP-4 inhibitors [5, 48]. Exogenous GLP-1 that produces high circulating hormone levels can delay gastric emptying [80], increases gastric volume [81, 82], increase satiation and induce weight loss in patients with type 2 diabetes [20]. GLP-1r agonists decrease caloric intake and are associated with increased GI symptoms such as such as nausea and vomiting [22, 24]. DPP-4 inhibitors (e.g. vildagliptin) on the other hand did not alter satiety, gastric volume, or gastric emptying [83, 84]. The modest increase of active GLP-1 level in the postprandial period with DPP-4 inhibition has been proposed as the explanation for the lack of effect on GI motility and the desirable side effect profile associated with this class of medication. However, even when the peak active GLP-1 concentration following meal ingestion is similar to the level produced by exogenous GLP-1 that is known to cause delayed gastric emptying [84, 85], individuals treated with DPP-4 inhibitor did not have increased frequency of GI symptoms [83]. Therefore, the differential effect of GLP-1 analogues and DPP-4 inhibitors on the GI tract cannot be completely explained by the concentration of active GLP-1 levels in the circulation alone.

Another possibility is that exendin-4, a therapeutically potent GLP-1r agonist, may have distinct potency, duration of action, and altered binding to the GLP-1r compared to GLP-1 [86]. While the fact that exendin-4 may be more potent and have a longer duration of action may not be surprising, but the idea that there are differences in the ability of each agonist to bind to the GLP-1r is novel. Interestingly, it seems that this effect is specific to the CNS GLP-1r signaling as central, but not peripheral affects of exendin-4 are resistance to GLP-1r antagonism. Importantly, there may also be a greater capacity to generate visceral illness with exendin-4 vs. GLP-1.

Like the clinical data, the major adverse effect of GLP-1 administration to rodents is the induction of visceral illness. Importantly, the CNS regions responsible for these effects have been identified. Specifically, GLP-1 acts on neurons within the central nucleus of the amygdala to cause a conditioned taste aversion, a robust indicator of visceral illness [16]. Furthermore, administration of a GLP-1r antagonist, blocks visceral illness induced by lithium chloride in rats and mice [87]. It is important to note that when given directly into the PVN and 4th ventricle (which presumably hits the hindbrain neurons), GLP-1 reduces food intake without causing visceral illness. The fact that we can dissociate these effects in animal models lead to the plausibility of developing therapeutic strategies that would dissociate the anorectic effects from illness-inducing effects of GLP-1. This has considerable clinical significance since in humans treated with GLP-1r agonists for diabetes the most commonly reported side effect, and the principle factor limiting tolerance, is nausea [5].

4 Summary and conclusions

GLP-1 infusion and both short- and long-acting GLP-1r agonists in humans are associated with weight loss due to decreased food intake rather than changes in energy expenditure, an effect likely mediated by GLP-1 action in the hypothalamus and hindbrain. Conversely, DPP-4 inhibitors that moderately increase active circulating GLP-1 level are weight neutral. A major adverse effect of these GLP-1 analogue therapies is nausea, an effect that is not seen with DPP-4 inhibitors. This difference may be due to differences in receptor potency of the long acting GLP-1 analogues vs. the endogenous peptide. Regardless, GLP-1 appears to act within very specific regions of the CNS to cause visceral illness in rodents, which may be separate from regions mediating its effect on food intake. The ability to dissociate these effects leads to the possibility that therapies could be developed aimed at reducing food intake without nausea.

This unique function of GLP-1 and GLP-1 mimetics on reducing body weight make them attractive therapeutic alternatives to the limited number of weight loss medications available on the market and the weight-enhancing hypoglycemic agents for the treatment of obesity and type 2 diabetes, respectively. However, the common side effect of GLP-1 such as nausea makes it less desirable. A recent report on the use of glucagon and GLP-1 co-agonist showed potent and sustained effect on inducing satiation and lipolysis in diet induced obese mice [88]. Combination treatment with agents that have synergistic effects on food intake could enhance potency yet eliminate unwanted side effects associated with single-agent therapy. Agents that specifically target the brain regions that are responsible for the GLP-1 action on food intake but not nausea such as the paraventricular nucleus and 4th ventricle may become attractive options. Overall, a better understanding of the satiety effects of GLP-1 is needed and will have an immediate impact on clinical practice.