The activity of circulating dipeptidyl peptidase-4 is associated with subclinical left ventricular dysfunction in patients with type 2 diabetes mellitus

Eighty-three consecutive patients of Caucasian origin, aged >30 years, affected by T2DM, and evaluated at the University of Navarra Clinic between March 2004 and August 2005, were considered for inclusion in this retrospective cross-sectional study. The presence of diabetes was re-evaluated according to the American Diabetes Association criteria [21] (glycated haemoglobin [HbA1c] ≥ 6.5% or fasting glucose ≥ 126 mg/dl or 2 h plasma glucose ≥ 200 mg/dl during an OGTT or use of hypoglycemic medication). All of them were free from clinically apparent cardiovascular disease. Coronary and valve heart disease were excluded by electrocardiographic and echocardiographic evaluation at rest, and by exercise/scintigraphy/echo-stress test.

Fifty-nine age- and sex-matched Caucasian patients coming to the University of Navarra Clinic for a routine medical work-up were included as controls. None of them presented medical history of DM or clinically apparent cardiovascular disease.

According to institutional guidelines, all participants gave written informed consent to participate. The study was carried out in accordance with the Helsinki Declaration and the Ethical Committee of the University of Navarra Clinic approved it.

Concomitant hypertension was defined as systolic blood pressure (SBP) of ≥ 140 mmHg and/or diastolic blood pressure (DBP) of ≥ 90 mmHg and/or the presence of previous chronic antihypertensive treatment. Hypercholesterolemia was diagnosed if the fasting serum total cholesterol was ≥ 200 mg/dL and hypertriglyceridemia was diagnosed if serum triglyceride levels were ≥ 150 mg/dL. Obesity was defined as body mass index (BMI) ≥ 30 kg/m². Estimated glomerular filtration rate (GFR) was determined using the abbreviated Modification of Diet in Renal Disease (MDRD) equation and the urinary albumin-to-creatine ratio was measured. Chronic kidney disease (CKD) was diagnosed if GFR < 60 ml/min/1.73 m² and/or microalbuminuria was present defined as albumin-to-creatinine ratio between 30 and 300 mg/g.

Two-dimensional echocardiographic imaging, targeted M-mode recordings and Doppler ultrasound measurements were obtained in each patient. LV end-diastolic and systolic volumes (LVEDV and LVESV, respectively), interventricular septum thickness (IVST), posterior wall thickness (PWT) and relative wall thickness (RWT) were calculated as previously reported [22]. RWT > 0.42 was considered indicative of LV concentric remodeling [23]. LVEDV and LVESV were indexed by body surface area (BSA) (LVEDVi and LVESVi, respectively). LV mass index (LVMI) was calculated by dividing LVM by BSA. The presence of LV hypertrophy (LVH) was established when LVMI was above 115 g/m² in men and above 95 g/m² in women in accordance with the American Society of Echocardiography’s Guidelines [23]. The value of LVM directly measured from echocardiograms was divided by that predicted by an equation to predict compensatory LVM as previously described [22] and LVM was expressed as a percentage of predicted, representing the excess relative to the “compensatory” value (i.e. 100% of predicted). Inappropriate LVM was defined as more than 128% of the predicted value as previously described [22]. LV growth was established if LV concentric remodeling and/or LV hypertrophy and/or inappropriate LVM were present. Two-dimensional estimation of left atrial volume (LAV) was performed at LV end-systole. LAV was indexed to body surface area (LAVi). LA enlargement was defined as LAVi ≥ 29 mL/m² [23].

The following pulsed Doppler measurements of the mitral flow were obtained: maximum early transmitral flow velocity in diastole (E), maximum late transmitral velocity flow in diastole (A), the deceleration time of the early mitral filling wave (DT), and isovolumic relaxation time (IVRT). Tissue Doppler imaging of the lateral mitral annulus was used for measuring the early (e’) and late (a’) mitral annulus velocities throughout the cardiac cycle. Echocardiography evidence of LVDD was established if E/e’ ratio >15. If E/e’ was between 8-15, values of e’ < 9 cm/s and mitral inflow E/A ratio age-corrected abnormal values [24] were considered. In that case, the presence of at least two abnormal measurements was taken into account in order to increase the likelihood of LVDD diagnosis following the European Society of Cardiology’s Guidelines [25].

LV stroke work was calculated as the product of stroke volume and end-systolic pressure as previously described [22]. As a measure of contractility, LV stroke index (LVSWi) was calculated as LV stroke work divided by LVEDV, as previously described [26]. LVEF and subendocardial fractional shortening (FS) were calculated as previously reported [22]. Midwall fractional shortening (MFS) was calculated in accordance with the American Society of Echocardiography’s Guidelines [23]. Circumferential end-systolic stress (cESS) was calculated as previously described [22], to correct MFS (cESS corrected-MFS). mESS was calculated as previously reported [22], and corrected by the LVESVi (mESS/LVESVi). Evidence of LVSD was determined if LVEF < 50%. If LVEF was between 50-55%, LVSD was considered if MFS was lower than 15% in women and 14% in men following the American Society of Echocardiography’s Guidelines [23]. Finally, evidence of LVD was considered if either LVDD and/or LVSD were present.

Venous blood samples were withdrawn from the left antecubital vein at the time of the clinical studies and stored at -40°C. Plasma DPP4a was measured in duplicate by using the DPP4-Glo™ Protease Assay (Promega). A reference standard curve was measured in each different run by using a purified DPP4 enzyme (BPS Bioscience) with known activity. DPP4a was measured in the absence or the presence of valine pyrrolidide, a specific DPP4 inhibitor, to test the specificity of the enzymatic assay. In our samples, valine pyrrolidide inhibited the assayed activity by >95%. The intra-assay and inter-assay coefficients of variation were 1.86% and 9.95%, respectively and the sensitivity of the technique has been established as the detection of the cleaving activity of 0.5 ng of recombinant DPP4.

Amino-terminal pro-brain natriuretic peptide (NT-proBNP) was measured in serum samples by ELISA (Biomedica Gruppe). The sensitivity was 5 fmol of NT-proBNP/mL. The inter-assay and intra-assay coefficients of variation were lower than 10%.

Continuous variables were reported as mean values ± one standard deviation or, if not normally distributed, as median and interquartile range, whereas categorical variables were reported as numbers and percentages. Differences between non-diabetic subjects and diabetic patients were tested by Student’s t-test for unpaired data once normality was demonstrated (Kolmogorov-Smirnov test); otherwise, a nonparametric test (Mann-Whitney U-test) was used. Differences in continuous variables between more than two groups were tested by one-way ANOVA followed by a Student-Newman-Keuls test once normality was checked (Shapiro-Wilks test); otherwise, the nonparametric Kruskal-Wallis test followed by a Mann-Whitney U test (adjusting the α-level by Bonferroni inequality) was used. Categorical variables were analysed by the χ² test or Fisher’s exact test when necessary. Multiple regression analyses were performed to assess the independent relationship between circulating DPP4a and echocardiographic parameters of LV systolic and diastolic function after adjustment for relevant covariates: age, sex, HbA1c, SBP, presence of CKD, anti-hypertensive treatment and anti-diabetic treatment. Logistic regression analysis was performed to derive odds ratio and 95% confidence intervals adjusted for covariates. Statistical significance was defined as two-sided p < 0.05. The statistical analysis was done using the SPSS software (15.0 version; SPSS Inc., Chicago, Illinois, USA).

The demographic and clinical parameters evaluated in non-diabetic subjects and in patients with T2DM are presented in Table 1. As compared with non-diabetics, T2DM patients exhibited higher body mass index (BMI), and decreased diastolic and mean blood pressure values. As expected, the percentage of HbA1c and the fasting glucose levels in blood were significantly increased in T2DM patients as compared with non-diabetic subjects. In addition, the presence of hypertension was similar in both groups although the prevalences of hypercholesterolemia and obesity were lower and higher, respectively, in patients with T2DM than in non-diabetic subjects. As expected, more patients in the diabetic group were under treatment with cardiovascular drugs (including anti-hypertensive medications) than in the non-diabetic group.

Table 2 shows the echocardiographic parameters assessed in the population according to the presence or absence of T2DM. Compared with non-diabetic subjects, T2DM patients exhibited higher prevalence of LV concentric remodeling and inappropriate LVM. The prevalence of LVH and LA enlargement was similar in the 2 groups of subjects. In addition, parameters assessing LV diastolic and systolic function were altered in T2DM patients as compared with non-diabetic subjects. Therefore, the prevalence of LVDD and LVSD was higher in patients with T2DM than in non-diabetic subjects. Finally, the prevalence of LVD (considered as the presence of LVDD and/or LVSD) was increased in T2DM patients as compared with non-diabetic subjects (44.6% vs 6.8%, p < 0.001).

Plasma DPP4a was higher in T2DM patients as compared with non-diabetic subjects (5208 ± 957 vs 5855 ± 1632 pmol/min/mL, p < 0.05). In addition, compared with non-diabetic subjects, patients with T2DM exhibited higher levels of NT-proBNP (234 ± 136 vs 348 ±180 fmol/mL, p < 0.01).

Table 3 shows the clinical features of patients with T2DM classified according to tertiles of plasma DPP4a. Age, gender, BMI, blood pressure, HbA1c, fasting glucose, comorbidities and treatment were similar among the three groups of patients.

As shown in Table 4, parameters of LV and LA morphology did not change across the three groups of T2DM patients categorized according to plasma DPP4a.

Interestingly, T2DM patients with the highest values of plasma DPP4a (third tertile) exhibited increased values of E/e’ ratio as compared with patients showing lower DPP4a (first tertile) (Figure 1A). In addition, T2DM patients in the second and third DPP4a tertiles showed lower values of E/A ratio than patients in the first DPP4a tertile (Figure 1B). Moreover, T2DM patients in the third tertile of DPP4a had lower values of LVSWi (Figure 1C), LVEF (Figure 1D) and MFS (Figure 1E) as compared with patients in the first tertile. In accordance with these differences, the prevalence of both LVDD and LVSD progressively increased across tertiles of plasma DPP4a in diabetic patients (Figure 2). Finally, the prevalence of LVD was 13%, 39% and 71% in patients from the first, second and third tertiles of plasma DPP4a, respectively (χ² = 16.2, p < 0.001).

NT-proBNP levels did not change across the three tertiles of DPP4 activity (first to third DPP4 tertiles: 338 ± 196 vs 345 ± 180 vs 372 ± 191 fmol/mL, p = 0.823).

Associations between plasma DPP4a and parameters of LV diastolic and systolic function were analyzed in T2DM patients by multivariate linear regression analysis. After adjusting for potential confounding factors (age, gender, HbA1c, SBP, presence of CKD, anti-hypertensive treatment and anti-diabetic treatment), plasma DPP4a was directly correlated with the E/e’ ratio (β = 0.307, p = 0.036). In addition, an inverse correlation was detected between plasma DPP4a and the E/A ratio independently of gender, HbA1c, SBP, presence of CKD, anti-hypertensive treatment and anti-diabetic treatment (β = -0.295, p = 0.036). However, the multiple regression analysis evaluating the association between plasma DPP4a and the E/A ratio rendered a non-significant β coefficient after adjustment for age. Furthermore, plasma DPP4a was inversely associated with LVSWi (β = -0.210, p = 0.025), LVEF (β = -0.291, p = 0.040) and MFS (β = -0.365, p = 0.008) independently of all the considered potential confounding factors (age, gender, HbA1c, SBP, presence of CKD, anti-hypertensive treatment and anti-diabetic treatment).

No associations of NT-proBNP levels with parameters assessing LV systolic and diastolic function were identified in T2DM patients.

Multiple logistic regression analysis confirmed the previous observations since an increase of 100 pmol/min/mL in plasma DPP4a was significantly associated with a higher risk of LVDD (Table 5) and of LVSD (Table 6) in T2DM patients, independently of all the above considered confounding factors. Thus, an increase of 100 pmol/min/min plasma DPP4a was independently associated with an increased frequency of LVD with an adjusted odds ratio of 1.10 (95% CI, 1.04 to 1.15, p = 0.001). By the same token, T2DM patients in the third tertile of plasma DPP4a had an adjusted odds ratio for LVD of 8.18 (95% CI, 2.19 to 30.6, p = 0.002).

Some limitations of the current study must be recognized. First, data here presented are relevant to a selected sample from a single centre. Second, the cross-sectional design of the study does not allow for causal interpretation of the relationships found. Third, unfortunately, we could not determine concentrations of active (GLP-1 [7‐36]), as samples were not collected in tubes with a DPP4 inhibitor. Therefore, future studies should establish how much of plasma DPP4a is related to GLP-1 (7-36) concentration and whether the active GLP-1 peptide may be influencing the associations found between plasma DPP4a and LVD. Fourth, whereas LVSD was assessed using conventional Doppler echocardiography, in accordance with the American Society of Echocardiography’s Guidelines [23], we are aware that more refined methods (e.g. speckle tracking) would allow for a more detailed characterization of LV contraction. Fifth, it must be recognized that inclusion of untreated diabetics and patients treated with different classes of drugs may have confounded the findings here obtained and their interpretation. However, as mentioned above, the pharmacological treatment did not seem to affect the associations of plasma DPP4a with parameters of LV function.

In summary, findings from the current study show that the activity of circulating DPP4 is associated with subclinical LVD in T2DM patients with no coronary or valve heart disease. Albeit preliminary and descriptive in nature, these results set the stage for further experimental studies aimed to test the potential involvement of an excess of DPP4 in the pathogenesis of LVD in diabetes. Furthermore, adequate prospective studies should be considered in order to explore the usefulness of plasma DPP4a as a diagnostic biomarker and a therapeutic target for HF in patients with T2DM. These aspects can be particularly relevant taking into account that, although some meta-analysis indicate that inhibition of DPP4a with gliptins may decrease the risk of HF and other adverse cardiovascular events in T2DM patients [47‐49], it has been recently reported that DPP4 inhibition with saxagliptin is associated with increased risk of hospitalization for HF in T2DM patients [50].