Impact of incretin-related agents on endothelial cell function

Hiroshi Nomoto, Hideaki Miyoshi, ＊, Akinobu Nakamura, Tatsuya Atsumi, Naoki Manda, Yoshio Kurihara,Shin Aoki, on behalf of SAIS Study Group^

1 Division of Rheumatology, Endocrinology and Nephrology, Hokkaido University Graduate School of Medicine, Sapporo, Japan

2 Manda Memorial Hospital, Sapporo, Japan

3 Kurihara Clinic, Sapporo, Japan

4 Aoki Clinic, Sapporo, Japan

INTRODUCTION

Prevention of atherosclerotic diseases, such as cardiovascular disease (CVD) and stroke is an issue that should be resolved in patients with type 2 diabetes mellitus because they have a higher risk of these conditions.1 Recent comprehensive care of patients with risk factors dramatically decreased diabetic macro- and microvascular complications,2but the mortality rate is still higher in populations with diabetes.3Therefore, detection and management of the earlier phases of atherosclerosis is important. It has been reported that endothelial cell damage precedes progressive atherosclerosis and these dysfunctions are known to be individual risk factors for CVD in type 2 diabetes mellitus.4,5For the relationship between therapeutics and endothelial cell function, numerous studies have been performed, and statins6and angiotensin II receptor blockers7 along with some anti-diabetic agents were reported to improve endothelial dysfunction.8,9

In recent years, incretin-related drugs such as dipeptidyl peptidase-4 (DPP-4) inhibitors and glucagon-like peptide-1(GLP-1) analogues, have been available to treat patients with type 2 diabetes mellitus and their anti-atherosclerotic potentials had been expected based on many in vitro studies and animal models. However, some longitudinal studies failed to verify their superiority for the prevention of CVD.10-12Moreover, their effects on endothelial cell function have been inconsistent.13We performed two different randomized controlled trials that focused on endothelial cell function, using different incretin-related agents, the DPP-4 inhibitor sitagliptin and the GLP-1 receptor agonist liraglutide. Results of these clinical trials demonstrated that both medications did not improve endothelial cell function assessed by flow-mediated dilatation. Here, we will introduce them and discuss the effects of incretin-related therapy on the early phase of atherosclerosis.

THE EFFECT OF INCRETIN-RELATED THERAPY ON ENDOTHELIAL CELL FUNCTION

We investigated whether incretin-related drugs improve endothelial cell function using flow-mediated dilatation(FMD) of the brachial artery. FMD is known to be a less-invasive tool that reflects endothelial nitric oxide bioavailability to evaluate endothelial cell function, and a reversible marker of early phase of atherosclerosis.14Additionally, a recent meta-analysis showed that improvement of FMD was significantly related to the relative risk reduction in cardiovascular events.15Thus, treatments that have favorable effects on FMD might be ideal for prevention form cardiovascular outcomes. We conducted two open-label, multicenter, prospective randomized parallel-group comparison studies comparing sitagliptin versus glimepiride (SAIS-1)16and liraglutide versus insulin glargine (SAIS-2)17(Table 1). Briefly, patients with type 2 diabetes mellitus who did not have severe atherosclerotic diseases were randomly assigned to each treatment, and underwent treatment for 26 or 14 weeks, respectively.FMD and biochemical assessment were performed before and after the treatment periods, and all FMD measurements were conducted under controlled conditions by a well-trained technician who was blinded to the study to minimize the introduction of potential confounding variables. For FMD, neither sitagliptin nor liraglutide affected endothelial cell function in this treatment periods,and neither did glimepiride and glargine (Table 2). Some previous clinical trials suggested that GLP-1 analogues and DPP-4 inhibitors improved FMD,18-20but there is some controversy including the lack of a control group,the study duration, the subject populations, and FMD measurement accuracy. Assessment of FMD is so sensitive that the results were greatly affected by many internal and environmental factors, such as patients’ background (sex,age, obesity, heart rate and smoking),21conditions (air temperature, mental/physical stress),22-24and the medications described above. There were opposing results and opinions about the relationship between incretin agents and FMD results.25,26Our studies fully controlled these confounding factors and both treatment groups were similarly controlled for glycemic metabolism. Moreover,another assessment method for endothelial dysfunction,End-PAT, also showed the same results. A recent metaanalysis revealed that efficacy of GLP-1-based therapies on FMD was confirmed only in cross-sectional studies but not in longitudinal studies.13It was reported that native GLP-1 infusion for several hours to individuals with type 2 diabetes resulted in improvement in endothelial function.27,28Although both GLP-1 and cleaved GLP-1,such as GLP-1(9–36) and GLP-1(28-36), may cause these effects, a study comparing the effect of native GLP-1 on vascular function with or without of sitagliptin to prevent an increase of the cleaved form of GLP-1 resulted in no detectable changes in both groups.29Although the existence of the GLP-1 receptor on the vascular endothelium has been suggested, recent accumulating evidence showed that the endothelium does not possess the GLP-1 receptor,30,31but there is currently no consensus. Thus, beneficial effects of incretin-related therapy on atherosclerotic changes may be mainly explained by indirect mechanisms rather than by direct action on endothelial cells.

EFFECTS ON BETA-CELL FUNCTION AND ATHEROSCLEROSIS

Incretin-related agents exert glucose-lowering effects in a blood glucose-dependent manner,32and they also play an important role in avoiding hypoglycemia by diminishing insulin secretion and preventing glucagon secretion under normal-to-hypoglycemic conditions.33Numerous in vitro and animal model experiments verified protective and proliferative effects of incretin-related agents on pancreatic beta-cells, and our prospective studies also demonstrated that both a DPP-4 inhibitor and a GLP-1 analogue showed incremental beta-cell functions, assessed by surrogate markers of insulin secretion (Table 2). Similarly, some clinical trials using incretin agents clarified incremental beta-cell function.34,35Effects of a GLP-1 analogue on beta-cell signaling include activating adenylate cyclase/protein kinase A, phosphatidylinositol 3-kinase/Akt, and the adenosine monophosphate kinase/mTOR cascades, which results in proliferation of beta-cells in rodents. However, the precise signaling of GLP-1 analogues has not been fully determined in humans.36Additionally, pancreatic beta-cells are known to be sensitive to oxidative stresses that result from relatively low expression levels of anti-oxidant enzymes, suchas superoxide dismutase, catalase, and glutathione peroxidase.37GLP-1 exerts anti-oxidant effects via suppression of nitric acid synthase expression in pancreatic beta-cells.38DPP-4 inhibitors were reported to reduce apoptosis in pancreatic beta-cells39,40and induce anti-oxidant responses,41and such mechanisms might be beneficial for protection of beta-cells from some kinds of stresses.

Table 1: Inclusion criteria and outline of the original studies

Table 2: Summary of the original studies

In addition, long-term administration of these agents showed favorable effects on anti-oxidant, anti-atherosclerotic, and lipid metabolism in our study. One of the DPP-4 inhibitors, sitagliptin, ameliorated HDL-cholesterol via increasing adiponectin, anti-oxidant responses, and serum TNF-α levels. However, the GLP-1 analogue, liraglutide,caused a reduction of oxidative stress and improvement in cardiac load and atherosclerosis (Table 2). Incretinrelated drugs including DPP-4 inhibitors have been shown to have favorable effects on atherosclerosis via mediating several pathways, attenuating TNF-α-mediated induction of plasminogen activator inhibitor-1 expression,42preventing reactive oxygen species-induced cell senescence in endothelial cell lines,43increasing serum adiponectin levels,44suppressing macrophage-related inflammation, and inhibiting DPP-4-induced smooth muscle proliferation.45.

Despite these many favorable mechanisms, many clinical trials using DPP-4 inhibitors failed to show their superiority to placebo in CVD outcomes.10-12One of the possible reasons is that these trials mainly included patients with atherosclerotic diseases. Therefore, even such potent agents may not be enough to suppress advanced atherosclerotic diseases. Earlier intervention may be beneficial for patients with early-phase atherosclerosis.

CONCLUSIONS

Accumulating evidence suggests that incretin-based therapy has anti-atherosclerotic effects and favorable effects on pancreatic beta-cells. However, the long-term results on endothelial cell function have not yet been determined. Our clinical trials provided direct evidence that these agents did not improve endothelial cell function in patients with type 2 diabetes, although their favorable effects on beta-cells and atherosclerosis are expected and will be assessed in the future.

Conflicts of interest

Author contributions

HN and HM designed the original study. HN contributed to the writing the manuscript. HM and TA revised the manuscript. All authors contributed to enrollment of patients and approved the final manuscript.

Plagiarism check

This paper was screened twice using CrossCheck to verify originality before publication.

Peer review

This paper was double-blinded and stringently reviewed by international expert reviewers.

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