br The actions of GLP on the vasculature

The actions of GLP-1 (9–36) on the vasculature/endothelium The previous sections have ascribed the vascular and/or endothelial actions of GLP-1/GLP-1R agonists to potential direct actions through a canonical GLP-1R, but as mentioned previously, whether VSMCs and ECs express a full-length GLP-1R remains equivocal. On the contrary, accumulating evidence demonstrates that the DPP-4-induced GLP-1 cleavage product, GLP-1(9–36), may contain its own independent biological activity that influences vascular/endothelial function [56]. For example, a 48 h continuous infusion of GLP-1(9–36) (1.5 pmol/kg/min) in dogs with right ventricular pacing-induced dilated cardiomyopathy reduced left ventricular end diastolic pressure, and increased left ventricular contractility, coronary flow, and myocardial glucose uptake during a hyperinsulinemic-euglycemic clamp [43]. Such actions match observations in response to a 48 h continuous infusion of native GLP-1 [42,43], and suggest that these actions of native GLP-1 may potentially be due to GLP-1(9–36) modifying vascular/endothelial function. In addition, treatment with both native GLP-1 and GLP-1(9–36) induced vasodilation in mesenteric Kinase Inhibitor Library australia from C57BL/6J male mice in a concentration dependent manner, whereas exendin-4 treatment failed to induce vasodilation [7]. Of interest, GLP-1(9–36)-induced vasodilation still occurred in mesenteric arteries isolated from Glp1r deficient mice, and this vasodilation could be prevented via pretreatment with the DPP-4 inhibitor sitagliptin. Further evidence for direct endothelial actions of GLP-1 (9–36) involve improvements in human aortic EC (HAEC) viability during treatment with 0.3 nM GLP-1(9–36), in response to both simulated hypoxia (48 h)/reoxygenation (7 h) injury and hydrogen peroxide treatment (700 μM) [6]. In contrast, treatment with the DPP-4 resistant exendin-4 did not improve HAEC viability following hydrogen peroxide treatment. Likewise, transient hyperglycemia (25 mM for 6 h)-induced ROS in HAECs was completely abrogated via concurrent treatment with 100 pM GLP-1(9–36) [21]. Not all studies support endothelial/vascular actions of GLP-1/GLP-1(9–36) that positively influence blood flow, as subcutaneous injection in the lower abdomen with native GLP-1 or GLP-1(9–36) produced no effect on the resistive index using Doppler ultrasound imaging of the superior mesenteric artery in 8 volunteers lying in the supine position [9]. Whether this is due to species-specific differences, or whether GLP-1R expression and/or signal transduction pathways elicited by GLP-1/GLP-1(9–36) differ in coronary versus intestinal vascular endothelium remains unknown and an important question for future investigation. Furthermore, the complexity of GLP-1(9–36) action is exemplified by studies demonstrating that a secondary cleavage product derived from GLP-1(9–36), GLP-1(28–36), localizes to mitochondria and may also produce its own independent biological actions [53]. Importantly, these potential biological actions for GLP-1(9–36) and GLP-1(28–36) are more pronounced in stressed metabolic states such as insulin resistance and/or obesity [53,54]. Hence, further research is required to ascertain the true physiological relevance of both GLP-1(9–36) and GLP-1(28–36) in the human endocrine and cardiovascular system, or whether such actions are merely pharmacological phenomena seen at high concentrations of these peptides often considered to be biologically inert. In addition, further research is necessary to identify the potential receptors/signaling pathways transduced via these peptides, and whether preventing GLP-1(28–36) formation from GLP-1(9–36) can attenuate the biological actions of GLP-1(9–36).
GLP-1R agonists and cardiovascular outcomes Although preclinical and clinical studies demonstrate salutary actions for GLP-1R agonists against hypertension/atherosclerosis (described above), as well as MI/heart failure (reviewed extensively in [1,17,51,56,57]), it was not until 2015 that we got a clearer picture on the impact of chronic use of these drugs on cardiovascular outcomes in T2D subjects. The initial findings were modest, as the ELIXA (Evaluation of Lixisenatide in Acute Coronary Syndrome) cardiovascular outcomes trial demonstrated that once-daily lixisenatide treatment (maximum dose of 20 μg) had neutral actions on major adverse cardiovascular event (MACE) rates in 6068 patients with T2D that had a previous MI or were hospitalized for unstable angina within the previous 180-days prior to randomization. Conversely, the findings from both the LEADER and SUSTAIN 6 cardiovascular outcomes trial produced exciting evidence that GLP-1R agonists may reduce cardiovascular risk in T2D subjects [36,37]. The LEADER trial involved 9340 patients (∼80% had established cardiovascular disease) with a median follow-up of 3.8 years. The reported findings demonstrated that liraglutide treatment (median daily dose of 1.78 mg SQ) reduced MACE rates in T2D patients with high cardiovascular risk, with 219 patients dying from cardiovascular causes in the liraglutide arm versus 278 in the placebo arm (4.7% versus 6.0% death rate from cardiovascular causes) [37]. Liraglutide treatment was also associated with a trend (P =  0.14) to reduced hospitalization rates for heart failure. Likewise, results from the SUSTAIN 6 trial demonstrated in 3297 patients with T2D that once-weekly treatment with semaglutide (0.5 or 1.0 mg) for 2 years reduced MACE rates for a primary outcome composite of cardiovascular death, nonfatal MI, or nonfatal stroke, with 108 (semaglutide) versus 146 (placebo) patients experiencing an event [36]. An initial glance of lixisenatide’s neutral effect on cardiovascular outcomes suggests that a drug-class effect may not be present for GLP-1R agonists in subjects with T2D. However, both liraglutide and semaglutide have prolonged durations of action versus lixisenatide [38,41], which may lead to more pronounced GLP-1R activation and overall cardioprotection. Moreover, the T2D subjects in the ELIXA trial were at higher risk of experiencing a subsequent cardiovascular event, due to having a serious cardiovascular event within the previous 180 days prior to starting lixisenatide or placebo treatment. When combined with other differences in trial-specific design, it is currently difficult to make generalized assertions regarding the GLP-1R agonist drug class and cardioprotection in T2D subjects. Though future clinical studies, ideally carrying out direct head-to-head comparisons within the same trial, will help advance our knowledge in this area further. It should also be noted that these cardiovascular outcomes trials are not designed to assess improvement or worsening of heart failure status in patients with T2D, as they do not assess the proper endpoints (e.g. 6-minute treadmill test, maximum myocardial ventilation oxygen consumption, LV ejection fraction, etc.). In 300 patients with advanced heart failure, liraglutide treatment resulted in numerically more deaths and HF rehospitalization events (though not significant) in the FIGHT (A Randomized Trial of Liraglutide for High-Risk Heart Failure Patients with Reduced Ejection Fraction) trial [35]. Thus, further study in larger T2D patient cohorts comorbid for heart failure, or having heart failure without T2D, are required before the appropriate conclusions can be made regarding the role of GLP-1R agonists in this setting.