The effect of non-obesogenic high fat diet on cardiac Epac expression: implications for excitation-contraction coupling

Non-obesogenic high-fat diet can directly trigger cardiac changes without obesity, by altering cardiac metabolism and /or function (Littlejohns et al., 2014). These changes include an increase in cardiomyocyte percentage shortening and were associated with an increase in diastolic calcium during staircase response. Excitation-contraction coupling is regulated by sympathetic stimulation involving cAMP signalling. Recent evidence has shown that in addition to protein kinase A (PKA), cAMP/guanine nucleotide exchange factor directly activated by cAMP (Epac) is also important in mediating cAMP signalling (Okumura et al., 2014). The aim of this study was to determine whether a non-obesogenic high-fat diet alters the relative protein expression of Epac. Consequently, we measured the expression of Epac in hearts from normal and high fat diet. Male C57/BL 6 mice aged 6 weeks fed a high-fat diet (45% calories from fat and 0.15% cholesterol) for approximately 20 weeks had an increase in blood cholesterol but no evidence of cardiac hypertrophy or insulin sensitivity with little increase in body weight (Littlejohns et al., 2014). Extracted ventricular tissue from normal diet (control) and high-fat diet groups were used to detect the relative protein expression of the two Epac isoforms (Epac1 and Epac2) using western blotting. RIPA buffer was used to extract proteins and the expression was normalised to GAPDH. For western blots, monoclonal mouse antibodies for Epac1& 2 (dilution 1:1000) and rabbit GAPDH (1:5000) were purchased from Cell Signaling Technology. Data are presented as mean±SEM (n = 4 hearts/group) and were analysed using unpaired t-test. High fat diet was associated with a significant (p<0.05) increase in the relative level of Epac1 protein expression compared to normal diet (1.03 ±0.01 vs. 0.4±0.08). However, high-fat diet did not change Epac2 expression compared to normal diet (0.58 ±0.08 vs. 0.6±0.04). These data show that a high-fat diet increases the expression of Epac1 which can be involved in altering excitation-contraction coupling in cardiomyocytes.