GLUT1 deficiency in cardiomyocytes does not accelerate the transition from compensated hypertrophy to heart failure

Abstract

The aim of this study was to determine whether endogenous GLUT1 induction and the increased glucose utilization that accompanies pressure overload hypertrophy (POH) are required to maintain cardiac function during hemodynamic stress, and to test the hypothesis that lack of GLUT1 will accelerate the transition to heart failure. To determine the contribution of endogenous GLUT1 to the cardiac adaptation to POH, male mice with cardiomyocyte-restricted deletion of the GLUT1 gene (G1KO) and their littermate controls (Cont) were subjected to transverse aortic constriction (TAC). GLUT1 deficiency reduced glycolysis and glucose oxidation by 50%, which was associated with a reciprocal increase in fatty acid oxidation (FAO) relative to controls. Four weeks after TAC, glycolysis increased and FAO decreased by 50% in controls, but were unchanged in G1KO hearts relative to shams. G1KO and controls exhibited equivalent degrees of cardiac hypertrophy, fibrosis, and capillary density loss after TAC. Following TAC, in vivo left ventricular developed pressure was decreased in G1KO hearts relative to controls, but+dP/dt was equivalently reduced in Cont and G1KO mice. Mitochondrial function was equivalently impaired following TAC in both Cont and G1KO hearts. GLUT1 deficiency in cardiomyocytes alters myocardial substrate utilization, but does not substantially exacerbate pressure-overload induced contractile dysfunction or accelerate the progression to heart failure.

Keywords: Cardiac hypertrophy; Cardiac metabolism; Glucose transport.

Figure 1. GLUT1 deletion in cardiomyocytes

A- GLUT1 protein levels in muscle, liver and hearts of control and G1KO mice. B- Cytoplasmic and sarcolemmal levels of GLUT1 and GLUT4 proteins in cardiomyocytes isolated from control and G1KO mice. C- Glucose uptake in isolated cardiomyocytes from 10-week old control and G1KO hearts. D- Western Blot for GLUT1 and GLUT4 in control and G1KO mice 4 weeks after TAC. E- Densitometric analysis of GLUT1 normalized by Coomassie Blue. F- Densitometric analysis of GLUT4 normalized by Coomassie Blue. Data are expressed as means±SEM. Significant differences were determined by Student's t-test or by ANOVA followed by Tukey multiple comparison test, using a significance level of p<0.05, n=5 mice/group. (*) significantly different vs. control sham, (#) Significantly different vs. control TAC.

Figure 2. Lack of GLUT1 does not exaggerate pathological remodeling after TAC (5 hearts per group)

A- Masson's Trichrome staining of heart sections and quantification of percentage of fibrotic area. B- Endothelin 1 staining and quantification of vascularization. Arrows are highlighting the blood vessels. C- Heart weight to tibia length ratios. D- Lung weight to tibia length ratios. Data are expressed as means±SEM. E- mRNA expression of hypertrophy markers 4 weeks after TAC. Control (Cont) sham is represented as a dashed line. Data is represented as fold change vs. Cont Sham. Significant differences were determined by ANOVA followed by Tukey multiple comparison test, using a significance level of p<0.05. (*) significantly different vs. sham, (#) Significantly different vs. control TAC.

Figure 3. Left Ventricle Catheterization (6 hearts per group)

A- +dP/dt (rate of rise of left ventricular pressure). B- -dP/dt (rate of fall of left ventricular pressure). C- TAU (Isovolumic Relaxation Constant). D- LVEDP (Left Ventricular End Diastolic Pressure). E- LVDevP (Left Ventricular Developed Pressure). F- Heart rate. Data are expressed as means±SEM. Significant differences were determined by ANOVA followed by Tukey multiple comparison test, using a significance level of p<0.05, n ≥6. (*) significantly different vs. sham, (#) significantly different vs. control TAC.

Figure 4. Echocardiography (6 hearts per group)

A- Fractional shortening. B- Ejection fraction. C- LVIDd (Left Ventricular Internal Diameter in Diastole). D- LVIDs (Left Ventricular Internal Diameter in Systole). E- LVPWd (Left Ventricular Posterior Wall Thickness in Diastole). F- LVPWs (Left Ventricular Posterior Wall Thickness in Systole). G- IVSd (Interventricular Septum Thickness in Diastole). H- IVSs (Interventricular Septum Thickness in Systole). Data are expressed as means±SEM. Significant differences were determined by ANOVA followed by Tukey multiple comparison test, using a significance level of p<0.05, n =6. (*) significantly different vs. sham, (#) significantly different vs. control TAC.

Figure 5. Cardiac substrate metabolism and function in isolated working hearts (6 hearts per group)

A- glycolysis, B- glucose oxidation (GLOX) C- palmitate oxidation (POX), D- cardiac power, E- oxygen consumption (MVO₂), in isolated working hearts from control and G1KO mice 4 weeks after TAC or sham surgery. Hearts were perfused with 5 mmol/L glucose and 0.4 mmol/L palmitate; n=6 hearts per group for metabolism, MVO₂ and efficiency and n=12 per group for cardiac function (pooled data from glucose and FA perfusions). Data are expressed as means±SEM. Significant differences were determined by ANOVA followed by Tukey multiple comparison test, using a significance level of p<0.05. (*) significantly different vs. sham, (#) significantly different vs. control TAC.

Figure 6. Mitochondrial function is equivalently reduced in control and G1KO mice after TAC (5 to 7 hearts per group)

Mitochondrial respiration, ATP synthesis rates, and ATP/O ratios were measured in saponinpermeabilized cardiac fibers. A- Palmitoyl–carnitine or B- Succinate (in the presence of Rotenone) were used as substrates. Significant differences were determined by ANOVA followed by Tukey multiple comparison test, using a significance level of p<0.05. Data are expressed as means±SEM. (*) significantly different vs. sham.