Abstract
BackgroundCardiac Ca2+/calmodulin-dependent protein kinase II (CaMKII) activation plays a critical role in cardiomyocyte (CM) apoptosis and arrhythmia. Functional ATP-sensitive potassium (KATP) channels are essential for cardiac protection during ischemia. In cultured CMs, L5 low-density lipoprotein (LDL) induces apoptosis and QTc prolongation. L5 is a highly electronegative and atherogenic aberrant form of LDL, and its levels are significantly higher in patients with cardiovascular-related diseases. Here, the role of L5 in cardiac injury was studied by evaluating the effects of L5 on CaMKII activity and KATP channel physiology in CMs.MethodsCultured neonatal rat CMs (NRCMs) were treated with a moderate concentration (ie, 7.5 μg/mL) of L5 or L1 (the least electronegative LDL subfraction). NRCMs were examined for apoptosis and viability, CaMKII activity, and the expression of phosphorylated CaMKIIδ and NOX2/gp91phox. The function of KATP and action potentials (APs) was analyzed by using the patch-clamp technique.ResultsIn NRCMs, L5 but not L1 significantly induced cell apoptosis and reduced cell viability. Furthermore, L5 decreased Kir6.2 expression by more than 50%. Patch-clamp analysis showed that L5 reduced the KATP current (IKATP) density induced by pinacidil, a KATP opener. The partial recovery of the inward potassium current during pinacidil washout was susceptible to subsequent inhibition by the IKATP blocker glibenclamide. Suppression of IKATP by L5 significantly prolonged the AP duration. L5 also significantly increased the activity of CaMKII, the phosphorylation of CaMKIIδ, and the expression of NOX2/gp91phox. L5-induced apoptosis was prevented by the addition of the CaMKII inhibitor KN93 and the reactive oxygen species scavenger Mn (III)TBAP.ConclusionsL5 but not L1 induces CM damage through the activation of the CaMKII pathway and increases arrhythmogenicity in CMs by modulating the AP duration. These results help to explain the harmful effects of L5 in cardiovascular-related disease.
Highlights
Cardiac Ca2+/calmodulin-dependent protein kinase II (CaMKII) activation plays a critical role in cardiomyocyte (CM) apoptosis and arrhythmia
L5 induces cell injury in neonatal rat CMs (NRCMs) Cell apoptosis was examined in NRCMs treated with L5 or L1 by using Hoechst staining, Annexin V-Alexa 488, and propidium iodide (PI) staining
In L5-treated cells, only a few were stained by PI, which indicated the early apoptosis of NRCMs (Fig. 1a)
Summary
Cardiac Ca2+/calmodulin-dependent protein kinase II (CaMKII) activation plays a critical role in cardiomyocyte (CM) apoptosis and arrhythmia. Functional ATP-sensitive potassium (KATP) channels are essential for cardiac protection during ischemia. In cultured CMs, L5 low-density lipoprotein (LDL) induces apoptosis and QTc prolongation. The role of L5 in cardiac injury was studied by evaluating the effects of L5 on CaMKII activity and KATP channel physiology in CMs. Low-density lipoprotein (LDL) can be resolved into five charge-defined subfractions, L1 to L5. L5 is the most negatively charged subfraction of circulating LDL and has been shown to induce cell apoptosis [3, 4]. L5 is nearly undetectable in healthy individuals, its concentrations are significantly higher in patients with cardiovascularrelated diseases. Functional ATP-sensitive potassium channels (KATP) are essential for ischemic preconditioning, post-ischemia cardiac protection, and regulation of atrial and ventricular rhythm during cardiac ischemia [12, 13]. The effect of L5 on KATP current (IKATP) has not been studied
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