DPP4 Activity, Hyperinsulinemia, and Atherosclerosis

Abstract

Context

Obesity and type 2 diabetes are associated with chronic hyperinsulinemia, elevated plasma levels of dipeptidyl peptidase-4 (DPP4), and a pro-atherosclerotic milieu.

Evidence Acquisition

PubMed search of the term “insulin and atherosclerosis,” “hyperinsulinemia,” “atherosclerosis,” or “cardiovascular outcomes” cross-referenced with “DPP4.” Relevant research and review articles were reviewed.

Evidence Synthesis

Hyperinsulinemia in the setting of insulin resistance promotes vascular inflammation, vascular smooth muscle cell growth, pathological cholesterol profile, hypertension, and recruitment of immune cells to the endothelium, all contributing to atherosclerosis. DPP4 has pleiotropic functions and its activity is elevated in obese humans. DPP4 mirrors hyperinsulinemia’s atherogenic actions in the insulin resistant state, and genetic deletion of DPP4 protects rodents from developing insulin resistance and improves cardiovascular outcomes. DPP4 inhibition in pro-atherosclerotic preclinical models results in reduced inflammation and oxidative stress, improved endothelial function, and decreased atherosclerosis. Increased incretin levels may have contributed to but do not completely account for these benefits. Small clinical studies with DPP4 inhibitors demonstrate reduced carotid intimal thickening, improved endothelial function, and reduced arterial stiffness. To date, this has not been translated to cardiovascular risk reduction for individuals with type 2 diabetes with prior or exaggerated risk of cardiovascular disease.

Conclusion

DPP4 may represent a key link between central obesity, insulin resistance, and atherosclerosis. The gaps in knowledge in DPP4 function and discrepancy in cardiovascular outcomes observed in preclinical and large-scale randomized controlled studies with DPP4 inhibitors warrant additional research.

Despite incredible pharmacologic and technologic advancements in diabetes management over recent decades, atherosclerotic cardiovascular disease (CVD) remains the leading cause of death in people living with type 2 diabetes (T2DM). Diabetes promotes atherosclerosis by multiple mechanisms, and insulin’s pathogenic role in atherosclerosis has long been recognized, shifting the paradigm of diabetes treatment toward a regimen targeting insulin resistance and vascular health. Insulin in healthy states dilates blood vessels and maintains vascular health and function. Conversely, in insulin-resistant states, insulin’s vasodilatory effects are diminished or lost, but its effects on cell proliferation and growth remain. Importantly, this endothelial dysfunction predates and predicts later coronary artery disease (1).

Dipeptidyl peptidase-4 (DPP4) is a serine protease that cleaves peptides with specific N-terminal properties (2). DPP4 is widely expressed in many cell types including endothelial cells, fibroblasts, and lymphocytes and resides in the cell membrane predominantly in dimer form. The catalytically active DPP4 is liberated from the plasma membrane producing a soluble circulating form that lacks the intracellular and transmembrane regions but accounts for a substantial proportion of DPP4 activity in human plasma (3,4). DPP4 has many substrates leading to its pleiotropic functions. The DPP4 inhibitor drug class has gained widespread recognition for its role in glucose metabolism by preventing the degradation of glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP) (5). Apart from its role in glucose regulation, DPP4 has wide-ranging functions relevant to atherosclerosis including immune activation, signaling, and lipid storage (6,7).

Despite strong preclinical evidence implicating both hyperinsulinemia and DPP4 activity in the pathogenesis of atherosclerosis, multiple large-scale clinical trials involving DPP4 inhibitors have failed to decrease major adverse cardiovascular outcomes in people with T2DM. In this review, we discuss the mechanisms of insulin-induced atherosclerosis, examine the relationship between DPP4 and insulin resistance and the contribution of DPP4 to atherogenesis, and review preclinical and clinical data on DPP4 inhibitors’ impact on atherosclerosis and CVD prevention. We conducted a PubMed search of the term “insulin and atherosclerosis,” “hyperinsulinemia,” “atherosclerosis,” or “cardiovascular outcomes” cross-referenced with “DPP4.” The authors reviewed and adjudicated research articles and included relevant primary research and review articles.

Insulin Resistance, Hyperinsulinemia, and Mitochondrial Oxidative Stress

Insulin is an anabolic hormone. In the presence of a positive energy balance, insulin contributes to nutrient-induced insulin resistance (8,9). Insulin facilitates fat accumulation in adipose tissue and ectopic depots by inhibiting lipolysis and fat oxidation while stimulating lipogenesis. Excess adiposity causes insulin resistance and leads to chronic hyperinsulinemia. Chronic hyperinsulinemia further promotes ectopic fat accumulation, which aggravates oxidative stress and insulin resistance, forming a vicious cycle potentially leading to the development of T2DM and its associated cardiovascular complications. Compellingly, in the absence of diabetes, large-scale studies or meta-analyses show an association between estimates of insulin resistance and atherosclerosis (10,11) even after accounting for confounding variables like glucose (12,13). In mice fed a chow diet, chronic exposure to excess insulin (glargine 50 units/kg daily × 8 weeks) induced T2DM, fat accumulation and oxidative stress in the liver and increased cholesterol content in mitochondria (14).

Among many factors contributing to the development of insulin resistance, mitochondria-derived oxidative stress plays a critical role. Inasmuch as basal reactive oxygen species (ROS) are needed for the full activation of insulin signaling (15), oxidative stress is also necessary for insulin resistance development, which fails to occur without it even in the presence of ectopic fat accumulation (16,17). When the production of mitochondrial ROS is blocked, induction of insulin resistance in mice by obesity or high-fat diet (HFD) can be prevented (18-20).

While mitochondrial ROS promote insulin resistance, hyperinsulinemia in turn reduces mitochondria numbers and function leading to a deleterious cycle of increased ROS and insulin resistance. In humans and mice with insulin resistance or human cells exposed to chronic hyperinsulinemia, levels of mitochondrial DNA, mitochondrial number/mass, and expression of mitochondrion-related genes are all reduced (8,21,22). Even in mice with type 1 diabetes, prolonged exposure to insulin decreases mitochondrial production and induces insulin resistance (23). Further supporting insulin’s role in mitochondrial biogenesis suppression, deletion of adipose tissue insulin receptor increased adipose tissue mitochondrial biogenesis (24). Similarly, inhibition of insulin signaling with a phosphoinositide-3-kinase inhibitor during the nonfeeding phase reverses HFD-induced suppression of mitochondrial biogenesis and increases overall insulin sensitivity (8). Chronic exposure to insulin also suppresses the autophagy-dependent removal of aged/dysfunctional mitochondria, which limits mitochondrial ROS production. Autophagy plays a pivotal role in the development of insulin resistance (25) and is decreased in the presence of insulin resistance/hyperinsulinemia (26) and inhibited by nutrients and insulin (27,28).

Insulin Resistance, Hyperinsulinemia, and Atherosclerosis

Endothelium expresses abundant insulin receptors as well as the insulin-like growth factor I (IGF-1) receptors and the hybrid insulin/IGF-1 receptors (29-31). At physiological concentrations, insulin binds and activates the insulin receptors exclusively, but at supra-physiological or pharmacological concentrations insulin also stimulates the IGF-1 and the hybrid insulin/IGF-1 receptors (29). Under normal physiology insulin’s vascular actions help maintain vascular health and blood vessel tone and ensure adequate tissue perfusion. Chronic hyperinsulinemia, along with the associated insulin resistance, may result in a multitude of untoward health outcomes including atherosclerotic diseases, hypertension, and tissue ischemia (32).

As represented in Figure 1, chronic hyperinsulinemia, coupled with insulin resistance, contributes to atherosclerosis through multiple mechanisms including (i) increased de novo lipogenesis, fat accumulation, and ectopic fat deposition (33-35), leading to atherogenic lipid profile, systemic and vascular inflammation, mitochondrial dysfunction, and increased oxidative stress (36,37); (ii) enhanced very low density lipoprotein production and low density lipoprotein (LDL) cholesterol transport into vascular smooth muscle cells (VSMCs) (38); fat deposition in the arterial wall causes inflammation and directly promotes atherogenesis; and angiopoietin-2 is an endothelium-derived glycoprotein (39,40) that likely mediates insulin-associated endothelial inflammation (41); (iii) stimulation of endothelial production of adhesion molecules and monocyte adherence (30); (iv) hypertension due to increased secretion of endothelin-1 (42), increased renal Na⁺ reabsorption (43,44), and upregulation of the renin-angiotensin system (45,46); the latter also contributes to inflammation and oxidative stress in addition to increasing vascular tone (47,48); and (v) VSMC growth and proliferation (49-51).

Figure 1.

Chronic hyperinsulinemia contributes to atherosclerosis development via multiple mechanisms.Abbreviations: LDL, low density lipoprotein; RAS, renin angiotensin system; ROS, reactive oxygen species; VLDL, very low density lipoprotein; VSMC, vascular smooth muscle cells.

The DPP4 Link Between Insulin Resistance and Atherosclerosis: Preclinical Evidence

Since its discovery in 1966 (52), DPP4 has been implicated in metabolic disarrays and atherosclerosis development. Mice lacking DPP4 have improved glucose tolerance (53) and are protected against obesity and insulin resistance (7). In healthy humans, increased plasma DPP4 activity independently predicts the onset of metabolic syndrome (54), prediabetes, and T2DM (55) as well as increased carotid-intimal thickness (56). In the following discussion, we provide evidence linking DPP4 to insulin resistance/hyperinsulinemia and atherosclerosis. Detailed molecular mechanisms are reviewed elsewhere (57).

DPP4, obesity, and T2DM

DPP4 has been regarded as an adipokine and likely contributes to insulin resistance and diabetes pathogenesis. In humans with a wide range of insulin sensitivities, DPP4 expression and release are higher in visceral adipose tissue (VAT) than subcutaneous adipose tissue, obese subjects release more DPP4 from adipose tissue than lean controls, and plasma DPP4 levels positively correlate with the amount of VAT, adipocyte size, and adipose tissue macrophage content (58). Interestingly, insulin-sensitive individuals with obesity have significantly lower circulating DPP4 than do weight-matched insulin-resistant patients (58), suggesting a link between DPP4 and insulin resistance. In obese humans, serum DPP4 concentrations are elevated and correlate significantly with adipocyte size and insulin resistance markers, and fully differentiated adipocytes exhibit a substantially higher release of DPP4 compared with preadipocytes or macrophages (59). DPP4 protein expression in VAT is 5-fold higher than subcutaneous adipose tissue in obese subjects, and addition of soluble DPP4 to fat, skeletal muscle, or VSMCs impairs insulin signaling in vitro (59). Patients with T2DM tend to have elevated fasting plasma DPP4 activity, which correlates with hemoglobin A1c and fasting glucose levels (60,61). While it was also reported that plasma DPP4 activity is reduced in middle-aged and elderly obese patients with T2DM (62), the use of metformin and/or thiazolidinedione might have confounded the observation as both are effective in reducing plasma DPP4 activity (63-66).

DPP4 and atherosclerosis

Genetic deletion of DPP4 improves cardiovascular outcomes after transverse aortic constriction, doxorubicin administration, or myocardial infarction in mice (67,68). Strong evidence has confirmed that systemic inhibition of DPP4 attenuates atherosclerosis in atherosclerosis-prone rodent models. Treatment of HFD-fed male LDLr^−/− mice with alogliptin for 12 weeks decreased aortic plaque with a marked reduction in plaque macrophages and inflammation as well as VAT macrophage content (69). These changes were accompanied by improved markers of insulin resistance and reduced blood pressure. In the LDLr^−/− femoral artery injury mouse model, alogliptin treatment for 14 days significantly reduced intimal hyperplasia and the intima/media ratio, along with reduced cell proliferation and inflammation (70). Well-validated data from pro-atherosclerotic apolipoprotein E (apoE) knock-out (apoE^−/−) mice similarly compellingly shows that DPP4 inhibitors markedly prevent atherosclerosis development, possibly via AMPK- and mitogen-activated protein kinase-dependent mechanisms (71). Treatment of diabetic apoE-deficient mice with alogliptin for 24 weeks reduced atherosclerotic lesions, attenuated diabetes-augmented interleukin (IL)-6 and IL-1β expression in the atherosclerotic plaques and inhibited toll-like receptor 4–mediated upregulation of IL-6, IL-1β, and other proinflammatory cytokines by mononuclear cells (72). Administration of des-fluoro-sitagliptin to HFD-fed apoE-deficient mice for 16 weeks significantly improved endothelial dysfunction, increased endothelial nitric oxide synthase (NOS) phosphorylation, and reduced atherosclerotic lesion area (73).

Inhibition of DPP4 increases endogenous GLP-1 and GIP levels approximately 2-fold, so it is likely that DPP4 inhibitors indirectly mediate anti-atherosclerosis effects via prolonged elevation of plasma GLP-1 and GIP levels (74). Ample evidence has confirmed that incretin hormones have salutary anti-atherosclerosis effects in apoE^−/− mice (75). Infusion of GLP-1 receptor agonist liraglutide via osmotic pump to apoE^−/− mice for 4 weeks significantly retarded atherosclerotic lesions in the aortic wall and suppressed foam cell formation (76). While vildagliptin markedly suppressed atherosclerosis and foam cell formation, GLP-1 and GIP blockade completely reversed this effect in nondiabetic and substantially attenuated the response in diabetic apoE-null mice (77). Still, DPP4 inhibition confers additional benefit outside of increasing GLP-1 and GIP. In cultured human monocytic cells and coronary artery endothelial cells DPP4 inhibition with des-fluoro-sitagliptin clearly augmented GLP-1 activity in macrophages and endothelial cells (73).

After adjusting for other cardiovascular risk factors, individuals undergoing coronary artery bypass grafting with high combined plasma DPP4 activity and hyperinsulinemia had higher cardiac mortality compared to those with low/intermediate DPP4 and insulin levels (78). Ex vivo experiments with human internal mammary arteries and saphenous veins obtained from humans with coronary artery disease (with and without diabetes) revealed that treatment with an DPP4 inhibitor orally or administered to vessels ex vivo restored physiological insulin signaling, reversed vascular insulin responses, reduced vascular oxidative stress, and improved endothelial function (78). The detrimental effects of insulin on vascular redox state and endothelial function as well as the insulin-sensitizing effect of DPP4 inhibition were also validated in HFD-fed apoE^−/− mice (78).

Somewhat paradoxically in light of its abundant atherosclerotic properties, DPP4 also possesses antithrombotic properties via cleaving N-terminal Gly-Pro from the fibrin α-chain, thus inhibiting fibrin polymerization and clot formation (79). Loss of DPP4 activity is related to a prothrombogenic status of endothelial cells (80). Acute myocardial infarction significantly reduces DPP4 expression and activity within the infarcted area and in the coronary microvasculature (80). While the suppression of DPP4 activity in the setting of ischemia likely reduces the antithrombogenic effects, the widespread use of DPP4 inhibitors has not been associated with increases in atherosclerotic events in humans.

DPP4 and endothelial (dys)function

DPP4 is widely expressed in endothelial cells (81-83), including those in plaque neovessels (81), and endothelial cell derived DPP4 contributes to at least 25% of plasma DPP4 activity (84). Interestingly, high glucose concentrations increase endothelial cell-derived DPP4 activity and production but only in certain vascular beds. For example, exposure of human glomerular and dermal microvascular but not aortic endothelial cells to high glucose (22 mM) significantly increased DPP4 activity (85,86). This effect is mediated through undefined cellular mechanisms as incubation of purified DPP4 with high glucose does not affect the enzyme activity (85).

DPP4 inhibition attenuates endothelial cell dysfunction and atherogenesis in T2DM. Treatment of patients with T2DM for 4 weeks with vildagliptin improved endothelium-dependent vasodilatation (87) and in Goto-Kakizaki rats linagliptin administration for 4 weeks improved cerebrovascular function and remodeling and restored insulin-mediated cerebral perfusion independent of glycemic control or insulin levels (88), likely due to endothelin-1 system attenuation (89). Evidence suggests that both GLP-1-dependent and -independent pathways contribute to these salutary actions. Two weeks of sitagliptin treatment improved endothelium-dependent relaxation in renal arteries, restored renal blood flow, and reduced systolic blood pressure in spontaneously hypertensive rats and these actions were reversed by inhibition of GLP-1 receptor, adenylyl cyclase, protein kinase A, AMPK-α, or NOS (90). On the contrary, treatment of human endothelial cells with sitagliptin inhibited tumor necrosis factor α induction of plasminogen activator inhibitor-1, intercellular adhesion molecule-1, and vascular cell adhesion molecule-1 messenger RNA and protein expression and nuclear factor kappa B messenger RNA expression independent of GLP-1 (91). Alogliptin dose-dependently relaxed phenylephrine-preconstricted mouse aortic rings ex vivo through a Src-Akt-endothelial NOS pathway, and this was not affected by GLP-1 receptor antagonism (92). It is unclear whether it is the DPP4 anchored in the endothelial cell membrane, circulating soluble DPP4, or both that explains these benefits. Additionally, DPP4 inhibition increases circulating endothelial progenitor cells in patients with T2DM (93), likely via stromal cell-derived factor 1α (93), which is a chemokine that stimulates bone marrow mobilization of endothelial progenitor cells (94) and a physiological substrate of DPP4 (95).

DPP4, immune modulation, and inflammation

DPP4 is involved in both innate and adaptive immunity and is widely expressed in many types of immune cells including CD4(+) and CD8(+) T cells, B cells, natural killer cells, dendritic cells, and macrophages and modulates immunity and inflammation (96). DPP4 is highly expressed in bone marrow-derived CD11b(+) cells and alogliptin treatment effectively downregulates proinflammatory genes in these cells (69). DPP4 expression on CD4(+) and CD8(+) T cells, and its serum levels and enzymatic activities are higher in patients with T2DM compared with nondiabetic controls (66). Both obese humans and rodents demonstrate increased DPP4 expression in VAT macrophage populations, and DPP4 expression is increased during macrophage differentiation and activation (97). In humans with aortic atherosclerosis, the frequency of high-expression DPP4 monocytes in the peripheral blood and DPP4 expression level on circulating monocytes were increased, even after adjusting for obesity, and the atherosclerotic burden positively correlated with monocyte DPP4 expression (98).

Advanced glycation end-products (AGEs) are important inducers of oxidative stress and inflammation, and the crosstalk between the AGEs and receptor for AGEs (RAGE) axis and the DPP4-incretin system contributes to the pathogenesis of diabetic vascular complications (99). Serum levels of AGEs independently correlate with circulating levels of DPP4 in humans and AGEs enhance DPP4 expression in and release from proximal tubular epithelial cells (100). DPP4 dose-dependently increases ROS generation and RAGE gene expression (101) while DPP4 inhibition with linagliptin prevents AGE-induced ROS generation, RAGE, intercellular adhesion molecule-1, and plasminogen activator inhibitor-1 gene expression in endothelial cells (101).

DPP4 inhibition prevents foam cell formation and pro-inflammatory cytokine release, prominent mediators of atherosclerosis. In rabbits fed a high cholesterol diet, anagliptin suppressed the atherosclerotic lesions by >50% and reduced coronary artery macrophage positive areas by 75%, along with a 66% reduction of vascular DPP4 activity and 90% decrease in gene expression of proinflammatory cytokines in the carotid arteries (102). In macrophages isolated from humans and mice with type 1 diabetes, teneligliptin blocked oxidized LDL uptake and foam cell formation (103). Teneligliptin attenuated the harmful effects of AGEs and diminishing expression of the enzyme acyl-coA:cholesterol acyltransferase 1, which catalyzes cholesterol ester accumulation, and CD36, the scavenger receptor responsible for oxidized LDL uptake (103). Additional study also suggests the involvement of PKC inhibition in DPP4-mediated attenuation of foam cell formation (104). Furthermore, treatment of human macrophages with des-fluoro-sitagliptin suppressed proinflammatory cytokines (73).

DPP4 and mitochondria function

DPP4 inhibition is associated with improved mitochondria function and biogenesis in multiple tissues in insulin resistant rodents. Both vildagliptin and sitagliptin decreased plasma insulin, total cholesterol, and oxidative stress levels; attenuated cardiac dysfunction; and prevented cardiac mitochondrial dysfunction in rats receiving HFD (105). DPP4 inhibition also improves brain oxidative stress and mitochondrial function in HFD-fed rats (106,107). In an ischemic heart failure mouse model, DPP4 inhibition improved exercise capacity, mitochondrial biogenesis and mitochondrial oxidative phosphorylation capacity in skeletal muscle (108). It is likely that these actions are secondary to both direct inhibition of DPP4 as well as increased GLP-1 receptor signaling (108). Both GLP-1 receptor agonists (109,110) and DPP4 inhibitors (111) are known to stimulate β-cell autophagy and protect β-cells from glucolipotoxicity-induced cell death.

DPP4 inhibitor clinical data—a conundrum

T2DM is a chronic metabolic disease that results in multisystem, including vascular, complications, and the majority of patients eventually succumb to the atherosclerotic complications of the disease (112). Consistent with the preclinical evidence of DPP4 involvement in atherosclerosis pathogenesis, many clinical studies have shown that DPP4 inhibition might potentially confer anti-atherogenic actions in patients with T2DM. DPP4 inhibition improves both fasting and postprandial lipemia (113). In patients with T2DM, DPP4 inhibitor treatment for 4 to 16 weeks effectively lowered plasma levels of oxidative stress, inflammatory cytokines, and adhesion molecules (114,115); improved postprandial plasma triglyceride and apolipoprotein B-48–containing triglyceride-rich lipoprotein particle metabolism (116-118); and increased LDL size (119). Even in healthy humans administration of alogliptin for 7 days significantly suppressed the postprandial elevation in serum triglycerides, apolipoprotein B-48, and remnant lipoprotein cholesterol, along with improved brachial artery flow-mediated dilation (120).

Clinical trial data are summarized in Table 1. Intima-media thickness (IMT) and pulse wave velocity (PWV) are both indicators of arterial health and independently predict cardiovascular outcomes. In patients with T2DM free of apparent CVD both the SPIKE trial with insulin-treated participants (121) and the SPEAD-A trial with insulin-naïve participants (122), DPP4 inhibition attenuated the progression of carotid artery IMT. In the PROLOGUE study, 24 months of sitagliptin conferred no benefits on the progression of carotid IMT in people with T2DM not receiving insulin treatment beyond that achieved with conventional treatment (123). In a subgroup analysis of the PROLOGUE study, significant inhibitory effects of sitagliptin on mean and maximum internal carotid artery IMT were found in the primary but not the secondary prevention group, suggesting there is a favorable effect of DPP-4 inhibitor on carotid atherosclerosis in people with T2DM who have not experienced previous cardiovascular events (124). The RELEASE study examined the effect of 26 weeks of linagliptin on aortic PWV in people with early stage T2DM who were treatment naïve and found that linagliptin significantly decreased aortic PWV, indicating improved arterial stiffness (125).

Table 1.

Atherosclerosis and cardiovascular outcomes studies with DPP4 inhibitors

Abbreviation	Title	Drug (daily dose) vs comparator	Duration	Outcomes	Results
SPIKE (121)	Sitagliptin Preventive Study of Intima-Media Thickness Evaluation	Sitagliptin (25-100 mg) vs conventional therapy	104 weeks	Carotid IMT	Reduction
SPEAD-A (122)	Study of Preventive Effects of Alogliptin on Diabetic Atherosclerosis	Alogliptin (25 mg) vs conventional therapy	24 months	Carotid IMT	Reduction
PROLOGUE (123)	Program of Vascular Evaluation under Glucose Control by DPP-4 Inhibitor	Sitagliptin (25-100 mg) vs conventional therapy	24 months	Carotid IMT	No reduction
RELEASE (125)	Off Target Effects of Linagliptin Monotherapy on Arterial Stiffness in Early Diabetes	Linagliptin (5 mg) vs placebo	26 weeks	Aortic PWV	Reduction
SAVOR-TIMI 53 (126)	Saxagliptin Assessment of Vascular Outcomes Recorded in Patients with Diabetes Mellitus—Thrombolysis in Myocardial Infarction 53	Saxagliptin (5 mg) vs placebo	Median 2.1 years	MACE	Noninferior; increased heart failure hospitalizations
EXAMINE (127)	Examination of Cardiovascular Outcomes with Alogliptin versus Standard of Care	Alogliptin (25 mg) vs placebo	Median 18 months	MACE	Noninferior
TECOS (128)	Trial Evaluating Cardiovascular Outcomes with Sitagliptin	Sitagliptin (50-100 mg) vs placebo	Median 3 years	MACE, or hospitalization for unstable angina	Noninferior
CARMELINA (129)	Cardiovascular and Renal Microvascular Outcomes Study with Linagliptin in Patients with Type 2 Diabetes Mellitus	Linagliptin (5 mg) vs placebo	Median 2.2 years	MACE	Noninferior
CAROLINA (130)	Cardiovascular Outcome Study of Linagliptin Versus Glimepiride in Type 2 Diabetes	Linagliptin (5 mg) vs glimepiride (1-4 mg)	Median 6.3 years	MACE	Noninferior

Abbreviations: IMT, intima-media thickness; MACE, major adverse cardiovascular events, including cardiovascular death, nonfatal myocardial infarction and, nonfatal stroke; PWV, pulse wave velocity.

Surprising in the context of abundant pre-clinical data showing DPP4 inhibitors’ vasoprotective effects and clinical data demonstrating the ability of DPP4 inhibitors to attenuate carotid artery IMT progression and improve arterial stiffness in people with T2DM, large clinical studies examining the safety of DPP4 inhibitors in participants with T2DM with pre-existing or high risk of CVD have failed to demonstrate a cardiovascular risk reduction. SAVOR-TIMI 53 examined the effects of saxagliptin vs placebo in 16 492 participants with pre-existing or increased risk of CVD and a 10.3 year median T2DM duration. It demonstrated noninferiority for composite cardiovascular death, myocardial infarction, or ischemic stroke (126). Unexpectedly, more patients in the saxagliptin group required hospitalization for heart failure [hazard ratio 1.27 (95% confidence interval, 1.07-1.51); P = 0.007], which was not observed in other studies within the class. Evaluating the secondary prevention effect, the EXAMINE (alogliptin) investigators randomized 5390 participants with median duration of diabetes of 7 years and recent acute coronary syndrome and again demonstrated noninferiority compared to placebo for cardiovascular death, nonfatal myocardial infarction, or nonfatal stroke over a median follow-up of 18 months (127). TECOS (sitagliptin) evaluated secondary prevention in individuals with pre-existing CVD and a mean diabetes duration of 11.6 years and also showed noninferiority (over median follow-up of 3.0 years) for combined cardiovascular death, nonfatal myocardial infarction, nonfatal stroke, or hospitalization for unstable angina (128). The CARMELINA study (linagliptin) found similar noninferior cardiovascular composite results in 6980 individuals at high risk of CVD and high incidence of renal disease with a ~15-year mean diabetes duration over a median trial duration of 2.2 years (129). Finally, CAROLINA (linagliptin) uniquely involved head-to-head comparison of a DPP4 inhibitor with the sulfonylurea glimepiride. In participants with only a 6.3-year mean duration of diabetes and 43% with prior macrovascular disease, linagliptin demonstrated noninferiority compared to glimepiride in regards to composite cardiovascular outcomes, despite a clear reduction in hypoglycemia incidence, with no significant difference in hemoglobin A1c over a median trial length of 6.3 years (130). As these studies involved participants with long diabetes durations and either prior CVD or high risk for CVD, whether there is a specific window in which DPP4 inhibitors confer cardioprotective effects (ie, only in pre-diabetes/early diabetes and before cardiovascular complications) remains to be clearly defined.

Conclusions

Since the regulatory approval of first DPP4 inhibitor in 2006, millions of people have been treated with DPP4 inhibitors. While DPP4 has pleiotropic actions due to its large number of substrates, this class of drugs has been well tolerated. Although many preclinical studies have examined DPP4 action in experimental models of insulin resistance and diabetes, our fundamental understanding of key mechanisms transducing important actions of DPP4 inhibitors remains limited.

T2DM accelerates atherosclerosis. Humans with insulin resistance develop chronic hyperinsulinemia, which facilitates the development of ectopic fat deposition, dyslipidemia, oxidative distress, inflammation, mitochondria dysfunction, endothelial dysfunction, and hypertension, all contributing to the atherosclerotic process. DPP4 activities and levels are increased in humans with insulin resistance and T2DM. Substantial preclinical and clinical evidence has demonstrated a critical role of this enzyme in the regulation of lipid metabolism, oxidative stress, inflammation, endothelial function, mitochondrial function, and insulin action, providing a new link between hyperinsulinemia, insulin resistance, and atherosclerosis in obesity and T2DM (Fig. 2). Many studies, both in laboratory animals and humans, show that DPP4 inhibition attenuates endothelial dysfunction, inflammation, and the atherosclerotic process, likely via both DPP4 inhibition and increased circulating incretin concentrations; however, available phase IV clinical studies have not associated the use of DPP4 inhibitors with reduced cardiovascular events in T2DM. Limitations of the studies include differences in pathophysiology between animals and humans, study duration variation, heterogeneity in study populations, timing variation of the use of DPP4 inhibitors in the disease process, and, most important, lack of unified and clear evidence that the degrees of DPP4 enzyme activities were suppressed similarly during the trials. In addition, there is a lack of direct (particularly clinical) evidence that DPP4 is directly involved in insulin resistance-associated atherosclerosis. Further preclinical and clinical studies with DPP4 inhibitors are warranted to reconcile these discrepancies.

Figure 2.

Possible mechanisms underlying DPP4-mediated atherosclerosis in the state of insulin resistance. Abbreviations: ROS, reactive oxygen species; VSMC, vascular smooth muscle cells.

Additional Information

Disclosures: The authors have no conflict of interest.

Data Availability

Data sharing is not applicable to this article as no data sets were generated or analyzed during the current study.