Methods
Experimental animals
Male cardiomyopathic (J2N-k; n = 8) and normal (J2N-n; n = 64) hamsters were purchased from Japan SLC Inc., Japan, at the age of 20–27 weeks. J2N-k hamsters were generated from BIO 14.6 hamsters, whose cardiomyopathy are caused by δ-sarcoglycan mutation and showed exacerbating cardiac function [
13]. J2N-k has its origin in BIO 14.6, and thus both of two lineages are innately almost similar; however, CPK levels in J2N-k linage were more rapidly elevated and showed 2 months shorter lifespan as compared with the BIO14.6 linage. The average lifespan of J2N-k is about 37 weeks, and approximately 90% of the cause of death was HF. J2N-k hamsters are recognized as established model of dilated cardiomyopathy (DCM), with mutations also detected in patients with DCM [
14]. These hamsters were randomized to receive PBS (HF group) or liraglutide (purchased from Novo nordisk pharma, Denmark). Alzet osmotic pumps (2006) contained about 230 μl of PBS or liraglutide were implanted at the age of 31 weeks. Liraglutide was administered at 20 μg/kg/day (HF-L group) or 100 μg/kg/day (HF-H group) concentrations for 42 days. In the energy-complementary experiments, 10% glucose solution was administrated to hamsters instead of drinking water for 31–37 weeks along with high-dose liraglutide treatment (HF-H-G group).
Measurement of cardiac functions and tissue sampling
All hamsters (37 weeks old) were anesthetized with 0.75 mg/kg medetomidine, 4 mg/kg midazolam, and 5 mg/kg butorphanol IP. After an echocardiographic evaluation, a catheter was inserted into the left ventricle to measure blood pressure in the LV. After cardiac function measurements, blood samples were collected, HbA1c was measured using Banalyst Ace (USIO INC. Japan), and the rest were stored at − 80 °C. After weighing the hearts and lungs, samples were frozen in liquid nitrogen and stored at − 80 °C or fixed with formalin.
Western blot analysis
The heart tissue was homogenized in RIPA buffer containing NaF, trypsin inhibitor, leupeptin, glycerophosphate, and orthovanadate. Samples of heart tissue lysate were resolved on SDS-PAGE according to a standard protocol. After being transferred to the membranes, the samples were immunoblotted with primary antibodies, followed by secondary antibodies conjugated to a horseradish peroxidase. Bands were revealed using ECL select western blotting detection reagents (GE Healthcare, Buckinghamshire, UK), and band density was quantified using the Image J software. The following primary antibodies were used: CD36 (#sc-9154) and actin antibodies (#sc-1616) were obtained from Santa Cruz biotechnology (Santa Cruz, CA). GLUT-4 antibody (#2213), cleaved caspase-3 antibody (#9661), phospho-AMPKα (Thr172) antibody (#2535), and LC3A/B (1/2) antibody (#12741), were purchased from Cell Signaling (Danvers, MA).
ATP measurement
An ATP measurement kit for tissue (TA-100; Toyo B-net, Tokyo, Japan) was used to measure the ATP concentration of heart tissues according to the manufacturer’s protocol. The chemiluminescence by the luciferase reaction was used to measure ATP concentration.
FFA measurement
Serum samples were obtained without fasting. Serum FFA levels were measured using Free fatty acid fluorometric assay kit (700310, Cayman, MI, USA) according to manufacturer’s instructions.
Quantification of collagen content in the heart tissue
For conventional light microscopy, hearts were fixed in 4% neutral buffered formaldehyde solution and embedded in paraffin. Sections (8-μm thick) were stained following the Sirius red staining procedure to detect collagens. Red area and whole heart muscle sections were quantified using Image J.
Respiratory analysis
ARCO-2000 (Arco system, Japan) was used to measure oxygen consumption (VO2), carbon dioxide production (VCO2), respiratory quotient (RQ), carbohydrate (CHO) and fat (FAT) consumption. It consists of mass spectrometer for respiratory analysis and multi-process monitoring for small animals. After a few days of habituation, data were obtained every 2 min for 2 days. Data were divided into light and dark phases: light phase, 8:00 a.m. to 8:00 p.m., and dark phase, 8 p.m. to 8:00 a.m.
Statistics
Data comparison in each group was performed with one-way analysis of variance (ANOVA) or Student’s t test with step-down method. Multiple comparisons data were analyzed using the Bonferroni method after ANOVA for all pairwise comparisons. A p-value of < 0.05 was deemed statistically significant. All statistical analyses were performed using the SPSS software.
Discussion
The major findings in this study were as follows: (1) GLP-1 analog exacerbated cardiac dysfunction and (2) adequate glucose loading as energy substrate prevented HF deterioration.
The energy starvation hypothesis as a cause of HF has been drawing renewed attention recently [
15,
16]. Our study suggested that, in line with “An Engine Out of Fuel” hypothesis [
16], the cardiac muscle treated with liraglutide showed energy-starved expression, increasing the LC3 II/I ratio that resulted in autophagy occurrence, recycling system against energy depletion.
Previous studies revealed that failing hearts reduced FFA uptake and oxidation and, in contrast, elevated the glucose uptake in advanced HF [
17‐
19], which were supported in our study, as observed by the increased CHO/FAT consumption rate in dilated cardiomyopathy. Although the mechanism remains uncertain in failing hearts, metabolic preference of substrates has been generally considered to change from lipid utilization to carbohydrate utilization [
16,
20], although several researches elucidated malnutrition expressing that lower levels of low-density lipoprotein (LDL) cholesterol were correlated with worse HF prognosis. Clinical researches clearly showed that the lower the cholesterol level, the worse the prognosis in patients with HF [
21]. Although lipid-lowering therapy with HMG-CoA reductase is recommended for patients with acute myocardial infarction, poor nutrition has a worse outcome in patients with HF, and low LDL levels were associated with worse outcomes than those in high LDL levels, known as the “lipid paradox.” Impaired myocardial energy production is associated with decreased ATP and phosphocreatine concentrations. This depletion appears to result from mitochondrial oxidation compromise in both fatty acids and carbohydrates, accompanied with a compensatory increase in glucose uptake and glycolysis [
20]. However, effects and metabolism of using anti-diabetic drugs on failing heart have not been sufficiently concluded in previous studies. In our study, the GLP-1 analog was found to augment the features of energetic states in HF, resulting in the upregulation of carbohydrate consumption and reduced lipid consumption.
Although liraglutide treatment compromised failing heart, it still exerts protective effects when adequate amount of carbohydrate was supplied to the hearts. High CHO consumption and high RQ were 0.99 in the light phase and 0.98 in the dark phase in liraglutide with adequate glucose group, resulting in the highest cardiac function performance as compared with all HF groups, including the PBS group. Increased glucose/fat rate but not total calories in energy substrate saved the failing heart from starvation, prevented fibrosis, cardiac muscle thinning, and heart enlargement. In regard to protein expression, GLUT-4, the main glucose transporter in cardiac muscles, was upregulated in this study. This result was consistent with other studies demonstrating that GLUT-4 was induced by DPP4 inhibitors and liraglutide in the cardiac tissues, effectively allowing glucose into the tissues [
22,
23]. Our result is also compatible with reports from Ramírez et al. [
24] that sitagliptin increased glucose assimilation in detriment of fatty acid.
Originally, GLP-1 is secreted from the intestines after a meal ingestion and responds to the high glucose concentration in the blood and ends up letting glucose into the muscles and liver [
25]. GLP-1 analog augments metabolic reliance on glucose in cardiac muscles and other organs, increasing insulin concentration and GLUT-4 expression in tissues and insulin secretion from the pancreas [
26]. Interestingly, Hausenloy et al. [
27] reported that the cardioprotective effect of GLP-1 for myocardial infarction was glucose-dependent, and GLP-1 and sitagliptin, a DPP4 inhibitor, did not reduce the infarct size with abnormal glucose level of 5 mmol/l, but high glucose level of 11 mmol/l in ex vivo and > 7–8 mmol/l in vivo study. Our results are in consistent with their findings that the cardioprotective effect of GLP-1 may occur only when the glucose concentration level is high. Kyhl et al. also reported that liraglutide treatment lacked any favorable alterations after acute MI in non-diabetic rats [
28], this result might be caused by expected relatively low to normal level of glucose concentration.
In this study, dilated and weakened cardiac muscles exerted increased glucose dependency more than healthy hearts; moreover, glycemic reliance is much more reinforced by GLP-1 analog in a failing heart. Liraglutide-treated failing heart was considered to have glucose shortage as a fuel because of the fact that adequate glucose supply dramatically improved cardiac function and their early mortality. As HbA1c levels between high-dose liraglutide and PBS groups were not significant, no apparent hypoglycemia has occurred. However, there may have been a relative shortage in failing hearts due to the shift in energy utilization from lipids to carbohydrates. In fact, liraglutide is clinically used against obesity for non-diabetic patients [
29‐
31], and no severe hypoglycemia has been reported. Therefore, GLP-1 agonist is recognized to reduce the glucose levels only when in hyperglycemia such as immediately after the meal. When the blood glucose level is low, GLP-1 cannot stimulate insulin secretion and inhibit glucagon secretion to maintain normoglycemia [
32]. Therefore, our findings do not support the idea that apparent hypoglycemia caused HF exacerbation. Failing cardiac muscle demand more glucose; furthermore, liraglutide may boost this glucose dependency in terms of RQ upregulation and CHO consumption in the high-dose liraglutide group.
In this study, the influence of food and drink intake quantity according to liraglutide use cannot be excluded. GLP-1 reportedly inhibits gastric motility and emptying and suppresses appetite with body weight and fat tissue loss [
33]. However, energy intake was almost equal in all groups, and energy consumption was relatively higher in the liraglutide-treated group as compared with the HF group. Therefore, shortage of total energy intake was not the mechanism to explain the HF worsening in this experiment.
A concern on another important substrate for heart muscles and lipid regulation has been reported. Although CD36, a lipid transporter, was significantly upregulated in cardiac tissues treated with liraglutide that relatively reduced serum FFA levels, FAT consumption in gas analysis was reduced, suggesting that fat consumption was decreased in reality. In fact, when glucose was additionally administered, cardiac dysfunction improved. This implies that liraglutide induced GLUT-4 in cardiac muscles, and glucose was effectively and preferably used for cardiac fertilization.
Increase serum FFA is thought to correlate with HF severity caused by the hyperadrenergic state [
16]; however, the relatively low FFA level was observed in failing hearts treated with high-dose liraglutide. GLP-1 is reported to act on enterocyte, the cell responsible for lipid absorption and chylomicron assembly and secretion. Activation of GLP-1R signaling including liraglutide rapidly lowers plasma concentration of ApoB, chylomicrons, and triglycerides in vivo [
33,
34]. Therefore, low FFA levels may be caused by the inhibitive effects of liraglutide on lipid absorption and released into the blood. Hamsters treated with high-dose liraglutide might not be able to use fatty substrate due to suppressed absorption and smaller amount of fat intake in the HF-H-G group. This lipid substrate depletion in the body might be one of the reasons of increased glucose feasibility in the high liraglutide groups.
Another explanation can be made based on the Randle cycle hypothesis, known as the “glucose-fatty acid cycle” to describe the association between fuel flux and fuel selection in tissues. Originally, lipid utilization inhibits glucose metabolism in tissues and has been known to play a role in insulin resistance; therefore, glucose utilization has also been reported to inhibit lipid metabolism in muscles and vice versa, thereby increasing malonyl-CoA that signals glucose utilization and controls long-chain fatty acid entry and oxidation in the mitochondria [
35]. In this article, we focused metabolism of heart muscles, however, liraglutide has pleiotropic effects in many organs such as digestive system, skeletal muscle and brain. We have to consider those extracardiac effects, especially liver and skeletal muscle, which have strong relations regarding glucose flow in the body. Reduced glucose production in liver and increased glucose uptake in skeletal muscle may cause the limitation of glucose use in the heart.
It is reported that liraglutide increases 6–10 bpm heart rate by stimulation of the sinus node, and it could be through sympathetic nervous system in patients [
36]. In our experiments, however, we could not reproduce heart rate upregulation, it may be possibly caused by anesthesia which affects heart rate when we performed cardiac catheterization and echocardiogram to obtain data including heart rate.
We showed exacerbated fibrosis in liraglutide administration along with HF, contradicted reports also exist. Zhang et al. [
37] demonstrated that liraglutide treatment attenuated angiotensin II-induced tissue fibrosis, Zhao [
38] found liraglutide alleviates cardiac fibrosis in STZ-induced diabetic cardiomyopathy. These conflictions might arise from the differences in animal model; pressure loaded hypertensive heart disease, high glucose and high fat induced cardiomyopathy and/or HF with reduced ejection fraction. Aoyama et al. also noted that DPP4 inhibition alleviates shortage of GLP-1 induced by thoracic aortic constriction (TAC) and reported its protective effect. However, TAC model is pressure loaded model with induced hypertrophy of the heart, difference of etiology compared to present study may result in different conclusion [
39].
Certainly, diabetic patients have been known to be highly at risk of developing heart failure [
40]. Most clinical articles reported that DPP-4 inhibitors and GLP-1 analogs are neutral for adverse effects on HF; however, several remarkable studies have already been reported, and results of recent large-scale clinical trials are attracting public attention. The SAVOR-TIMI53 trial resulted in increased hospitalization for HF due to saxagliptin use (hazard ratio [HR]: 1.27, 95% confidence interval [CI] 1.07–1.51). Although the TECOS study was completely neutral (HR: 1.00, 95% CI 0.83–1.20), DPP 4 inhibitor in the EXAMINE trial tended to increase the hospitalization for HF (HR: 1.19: 95% CI 0.90–1.58). Moreover, a population-based retrospective cohort study described that sitagliptin use was not associated with increased risk of hospitalization or death, but was associated with increased risk of HF-related hospitalization in T2DM patients with pre-existing HF [
41].
GLP-1 receptor antagonists have been expected beneficial effects extend to the cardiovascular system [
42,
43]. However, in the FIGHT trial, a randomized clinical trial that investigated whether liraglutide improves the prognosis in patients with advanced HF, the primary end-point was that liraglutide could not improve the mortality [
8]. However, especially regarding death or rehospitalization for HF, GLP-1 analog tended to worsen (HR: 1.30: 95% CI 0.92–1.83), although no statistically significant difference was observed. No favorable effects were observed for advanced HF with reduced LVEF. Moreover, it might be some differences between DPP4 inhibitor and GLP-1 agonist in HF patients. In another clinical cohort study, which figured that the use of GLP-1 agonists was associated with an increased risk of HF hospitalization compared to DPP4 inhibitors [
44].
In experimental studies, saxagliptin impairs cardiac contraction by inhibiting Ca
2+/calmodulin-dependent protein kinase II-phospholamban-sarcoplasmic reticulum Ca
2+-ATPase 2a axis and Na
+–Ca
2+ exchanger function in Ca
2+ extrusion [
45]. Genetic DPP-4 depletion exhibits impaired cardiac function and accelerated cardiac fibrosis in TAC-operated high fat-fed mice [
46]. Exendin-4, a GLP-1 receptor agonist, can increase the heart rate and blood pressure [
47]. These findings may be complicit in malice.
Therefore, many studies including clinical trials are still investigating the possibility and mechanisms whether incretin-related drugs may deteriorate cardiac function; however, the truthful and precise mechanism remains to be clarified, and their potential association with serious adverse effects including HF still exists. In this study, carbohydrates and lipids were investigated as energy sources for the heart and showed that GLP-1 analog deteriorates cardiac function in failing hearts. Furthermore, the deterioration of HF has been clarified to be prevented by adequate glucose supplementation with liraglutide administration. Failing heart may shift from lipid to carbohydrate utilization, and incretin-related drugs may result in insufficient carbohydrate supply to the heart and fail to exert protective effects to the cardiac function.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.