GLUT4, GLUT1, and GLUT8 are the dominant GLUT transcripts expressed in the murine left ventricle

Abstract

Background: The heart derives energy from a wide variety of substrates including fatty acids, carbohydrates, ketones, and amino acids. The healthy heart generates up to 30% of its ATP from glucose. Under conditions of cardiac injury or stress, the heart relies even more heavily on glucose as a source of fuel. Glucose is transported into the heart by members of the family of facilitative glucose transporters (GLUTs). While research examining the transport of glucose into the heart has primarily focused on the roles of the classical glucose transporters GLUT1 and GLUT4, little is known about the functions of more newly identified GLUT isoforms in the myocardium.

Methods: In this study the presence and relative RNA message abundance of each of the known GLUT isoforms was determined in left ventricular tissue from two commonly used inbred laboratory mouse strains (C57BL/6J and FVB/NJ) by quantitative real time PCR. Relative message abundance was also determined in GLUT4 null mice and in murine models of dilated and hypertrophic cardiomyopathy.

Results: GLUT4, GLUT1, and GLUT8 were found to be the most abundant GLUT transcripts in the normal heart, while GLUT3, GLUT10, and GLUT12 are present at relatively lower levels. Assessment of relative GLUT expression in left ventricular myocardium from mice with dilated cardiomyopathy revealed increased expression of GLUT1 with reduced levels of GLUT4, GLUT8, and GLUT12. Compensatory increase in the expression of GLUT12 was observed in genetically altered mice lacking GLUT4.

Conclusions: Glucose transporter expression varies significantly among murine models of cardiac dysfunction and involves several of the class III GLUT isoforms. Understanding how these more newly identified GLUT isoforms contribute to regulating myocardial glucose transport will enhance our comprehension of the normal physiology and pathophysiology of the heart.

Figure 1

Detection of GLUT mRNAs in the left ventricle of C57BL/6J mice. A) PCR was employed to identify each known murine member of the facilitative glucose transporter family in the left ventricle of 10-week-old male C57BL/6J mice. GLUTs-1, -3, -4, -8, -10, and −12 were amplified using cDNA derived from the left ventricular myocardium. The identity of each GLUT amplicon was confirmed by sequencing. GLUTs-2, -5, -6, and −9 were not present in the left ventricle of male C57BL/6J mice. Representative images are shown (n = 3). B) The specificity of the primers used to amplify GLUTs-2, -5, -6, and −9 were confirmed using cDNA derived from the positive control tissues: liver, intestine, and brain. The specificity of the primers for each GLUT amplicon was confirmed by sequencing.

Figure 2

Relative expression of GLUT transcripts in the left ventricle of the murine GLUT4 null model of cardiac hypertrophy. Comparative real time PCR was performed to examine the relative mRNA expression of GLUT transcripts in left ventricles derived from six-month-old GLUT4 null and C57BL/6J mice. Real time PCR demonstrates an increase in GLUT12 mRNA expression in the left ventricles of GLUT4 null mice (n = 6 mice/group). *, P < 0.05.

Figure 3

Relative expression of GLUT protein in the left ventricle of GLUT4 null mice. Western blotting was performed to examine the relative expression of GLUT1 (A), GLUT4 (B), GLUT8 (C), and GLUT12 (D) in the LVs of C57BL/6J and GLUT4 null mice. GLUT1, GLUT8, and GLUT12 protein expression is significantly elevated in the LVs of GLUT4 null mice compared to wild type controls (n = 3-4 mice/group). *, P < 0.05.

Figure 4

Relative expression of GLUT transcripts in the left ventricle of the TG9 model of dilated cardiomyopathy. Comparative real time PCR was performed to examine the relative mRNA expression of GLUT transcripts in left ventricles derived from A. 70-day-old and B. 85-day-old TG9 mice (TG9) and FVB/NJ controls (WT). Real time PCR shows that the expression levels of GLUT1 mRNA increase significantly in TG9 mice compared to age matched controls during heart failure progression. Conversely, the transcript levels for the insulin responsive glucose transporters GLUT4, GLUT8, and GLUT12 are significantly lower in heart from TG9 mice compared to FVB/NJ controls (n = 6–11 mice/group). *, P < 0.05.

Figure 5

Relative expression of GLUT transcripts in the left ventricle of a murine model of cardiac pressure overload. Comparative real time PCR was performed to analyze the relative mRNA expression of GLUT transcripts in the left ventricles derived from C57BL/6J mice in a pressure-overload induced cardiac hypertrophy setting. A) An increased left ventricle to body weight ratio shows that mice receiving TAC were experiencing compensatory cardiac hypertrophy. B) Real time PCR shows that the expression levels of GLUT transcripts were lower in mice receiving aortic constriction compared to sham operated animals (n = 6-7 mice/group). *, P < 0.05.