GLP-1R Agonists and Endothelial Dysfunction: More Than Just Glucose Lowering?

The treatment of type 2 diabetes (T2D) focuses on glycemic control to reduce microvascular and macrovascular complications. Unfortunately, while pivotal studies using conventional therapies have demonstrated that intensive glycemic control positively impacts microvascular complications, effects on cardiovascular benefit are less robust (1). There is therefore intense interest in newer antidiabetes agents, such as glucagon-like peptide 1 receptor (GLP-1R) agonists, which lower blood glucose and modify cardiovascular risk factors (lipids, adiposity, blood pressure) without increasing hypoglycemic risk (2). Whether GLP-1R agonists improve endothelial dysfunction is less clear, but this potential effect is important as endothelial dysfunction increases the risk for cardiovascular events in T2D (3).

While acute GLP-1(7-36) infusion consistently improves forearm vasodilation (a measurement of endothelial function) (4–7), effects with GLP-1R agonists are inconsistent. In a 16-week study of 20 patients with T2D, exenatide improved brachial artery flow-mediated dilation compared with glimepiride (8). In contrast, exenatide therapy for 3 months did not increase vasoreactivity by digital plethysmography compared with metformin in 50 obese, glucose-intolerant patients (9). Similarly, in a separate study involving 49 T2D participants, liraglutide did not improve forearm blood flow measured by venous occlusion plethysmography in response to graded infusions of acetylcholine or sodium nitroprusside relative to placebo or glimepiride (10). The effect of dipeptidyl peptidase-4 (DPP-4) inhibition on endothelial function is also inconsistent, with some indicating beneficial (11,12), neutral (13,14), or even detrimental (15) effects. Possible explanations for differing results between GLP-1(7-36), GLP-1R agonists, and DPP-4 inhibitors include 1) differences in study design (T2D vs. patients without diabetes, measurement during fasting vs. feeding, lack of active control for glycemic and metabolic changes) or 2) the vascular bed studied (large “macrocirculation” conduit arteries vs. small “microcirculation” vessels).

Therefore, Koska et al. (16) examined the postprandial effect of GLP-1R agonists and demonstrated that a single preprandial dose of exenatide improved endothelial function following a high-fat meal or oral glucose tolerance test in patients with impaired glucose tolerance or new-onset T2D. Whether these effects persist over time in patients with a longer duration of T2D was unclear, however. In this issue of Diabetes, Koska et al. (17) report the results of mechanistic studies examining the effect of short-term exenatide on postprandial endothelial function in patients with long-standing T2D. They also characterize the molecular mechanisms responsible for vasodilatory effects of exenatide in vitro in human aortic endothelial cells and ex vivo in human subcutaneous adipose tissue arterioles.

In this crossover study involving 36 participants with T2D, exenatide treatment for 11 days lowered fasting blood glucose, body weight, blood pressure, and total cholesterol compared with placebo. The cumulative 8-h reactive hyperemia index (RHI)—a measure of endothelial function—increased with exenatide following two sequential meals, independent of changes in HbA_1c, body weight, glucose, triglyceride, or insulin. In a second mechanistic study, acute coinfusion of the GLP-1R antagonist exenatide-9 abolished exenatide-induced increases in RHI. In the in vitro experiments, exenatide increased AMPKα phosphorylation and endothelial nitric oxide (NO) synthase (eNOS) phosphorylation and increased eNOS activity and NO production. Finally, in the ex vivo vasoreactivity studies, exendin-4 and GLP-1 increased vasodilatation in isolated subcutaneous arterioles. As hyperglycemia and products of lipolysis promote endothelial dysfunction, the authors measured the effects of high glucose and lipolysis on arteriolar vasodilatation. Exendin-4 dilated arterioles in a dose- and NO-dependent manner and also improved endothelial function during both hyperglycemia and lipolysis. These effects were reproduced with AMPKα agonism, which activates eNOS, and were inhibited with AMPKα blockade. The authors therefore conclude that exenatide stimulates AMPK-related vasodilatory pathways and NO bioactivity via direct GLP-1R–mediated mechanisms.

The study by Koska et al. (17) demonstrates that exenatide exerts a protective endothelial function effect in the postprandial period following short-term therapy in patients with T2D (diabetes duration >5 years). A potential limitation of this study was the lack of an active comparator to control for changes in glycemia, lipids, insulin, and adiposity, which are factors known to influence endothelial function. In addition, as the presence of GLP-1R expression in endothelial cells has not yet been elucidated, it would have been informative to identify a functional GLP-1R in the current experimental model. Pyke et al. (18) recently reported GLP-1R immunopositivity in vascular smooth muscle cells but not in endothelial cells of the primate kidney or heart. Recognizing that accurate localization of the GLP-1R is methodologically challenging and prone to pitfalls (19,20), extensive clarification of the presence or absence of GLP-1R expression in human endothelial cells is still required. While exenatide-9 did block the vasodilatory effects of exenatide, suggesting a direct GLP-1R–mediated mechanism, exenatide-9 is a nonselective antagonist of the GLP-1R, with weak partial agonist properties (21). As a result, the direct effects of GLP-1R agonism on endothelial function remain incompletely understood.

Nevertheless, these carefully performed studies by Koska and colleagues (16,17) support a direct role for exenatide on the endothelial function in humans and are consistent with prior studies for GLP-1(7-36) but are in contrast to many studies using GLP-1R agonists and DPP-4 inhibitors. How, then, do we interpret these differences, and how can we reconcile whether or not GLP-1R agonists directly modify endothelial function? Additional studies using active comparators to correct for changes in hormonal and metabolic factors would complement and strengthen the present findings and help to better define endothelial versus nonendothelial effects of GLP-1R agonists on vascular function (Fig. 1). Also confirmation of a functional canonical GLP-1R in endothelial cells is needed to determine whether effects of exenatide on endothelial function are GLP-1R mediated. In the absence of a canonical GLP-1R in endothelial cells, the existence of a second, as-yet unidentified, noncanonical GLP-1R, sensitive to both exenatide and exenatide-9, could also explain these observations and should be explored. Finally, future studies are needed to better define the role of GLP-1R signaling in vascular smooth muscle cells which may impact endothelial function indirectly.

In conclusion, endothelial dysfunction is a unifying pathobiological process that links diabetic macrovascular and microvascular complications. The studies by Koska and colleagues (16,17) are important as they have better defined how GLP-1R agonists impact endothelial function in humans. Moreover, novel physiological insights derived from these elegant experiments supporting the presence of direct vascular effects with GLP-1R agonists may ultimately help to better interpret the pending results of large cardiovascular and microvascular outcome studies using these agents.