Intracellular Signaling Pathway of Cardiac Apoptosis in the Prediabetic Heart

Apoptosis leads cardiac dysfunction and heart failure, which are more frequently in people with diabetes than in the general population. However, in impaired glucose tolerance (IGT), which characterizes prediabetic state, apoptosis has not been evaluated in heart. Moreover, although CaMKII is related with cardiac apoptosis, the connection with IGT is unknown. Thus, the present study aimed to evaluate apoptosis in IGT heart and its putative link with CaMKII activity.IGT was induced by a fructose-rich diet (control, CD; and fructose, FRD; rats or mice). Echocardiography, biochemical studies, reactive oxygen species (ROS), Ca2+i measurements, mitochondrial swelling and mitochondria membrane potential measurements were performed.FRD rats showed decreased contractility and increased hypertrophy (echocardiography) associated with increased CaMKII activity (P-CaMKII 191.6±18.3), and ROS (185.4±28.6%) vs CD rats (100%). TUNEL positive nuclei and Bax/Bcl2 ratio was increased in FRD vs CD rats (273.6±39.7%). Mitochondria from FRD rats showed significant more swelling (ΔDO 0.34±0.05 CD vs 0.53±0.03 FRD) and enhanced membrane depolarization than CD mitochondria. Myocytes from FRD rats showed a significant increase in sarcoplasmic reticulum (SR) Ca2+ leak vs CD myocytes. FRD SR-AIP mice (which express the CaMKII inhibitory peptide [AIP] at the SR membranes) showed less TUNEL positive nuclei than their matched FRD control mice. FRD control mice co-treated with the ROS scavenger, tempol, showed less apoptosis than the one induced by fructose alone. SR Ca2+ leak was also prevented in either FRD SR-AIP mice or CD mice co-treated with tempol. Mitochondria swelling was also prevented in S2814A mice, which ryanodine receptor (RyR2) cannot be phosphorylated by CaMKII.The results would indicate that the signaling apoptotic cascade in IGT hearts involves mitochondria damage by SR Ca2+ leak produced by CaMKII-dependent phosphorylation of RyR2. CaMKII would be activated by both, Ca2+ and ROS.