A Sphingosine Kinase Form 2 Knockout Sensitizes Mouse Myocardium to Ischemia/Reoxygenation Injury and Diminishes Responsiveness to Ischemic Preconditioning
Sphingosine kinase (SphK) exhibits two isoforms, SphK1 and SphK2. Both forms catalyze the synthesis of sphingosine 1-phosphate (S1P), a sphingolipid involved in ischemic preconditioning (IPC). Since the ratio of SphK1？:？SphK2 changes dramatically with aging, it is important to assess the role of SphK2 in IR injury and IPC. Langendorff mouse hearts were subjected to IR (30？min equilibration, 50？min global ischemia, and 40？min reperfusion). IPC consisted of 2？min of ischemia and 2？min of reperfusion for two cycles. At baseline, there were no differences in left ventricular developed pressure (LVDP), ±？dP/dtmax, and heart rate between SphK2 null (KO) and wild-type (WT) hearts. In KO hearts, SphK2 activity was undetectable, and SphK1 activity was unchanged compared to WT. Total SphK activity was reduced by 53%. SphK2 KO hearts subjected to IR exhibited significantly more cardiac damage ( % infarct size) compared with WT ( % infarct size); postischemic recovery of LVDP was lower in KO hearts. IPC exerted cardioprotection in WT hearts. The protective effect of IPC against IR was diminished in KO hearts which had much higher infarction sizes ( %) compared to the IPC/IR group in control hearts ( %). Western analysis revealed that KO hearts had substantial levels of phosphorylated p38 which could predispose the heart to IR injury. Thus, deletion of the SphK2 gene sensitizes the myocardium to IR injury and diminishes the protective effect of IPC. 1. Introduction Exposure of the myocardium to extended periods of ischemia results in cell injury. Much of this damage occurs upon reperfusion, and thus it is referred to as ischemia/reoxygenation (I/R) injury. Reactive oxygen species are formed upon reperfusion, and they have been implicated as contributing factors to this injury [1]. I/R injury is characterized by poor recovery of hemodynamic function upon reperfusion and the development of extensive areas of infarction [2]. Protection against I/R injury can be provided by ischemic preconditioning (IPC). IPC consists of short periods of I/R that precede long term (index) ischemia and reperfusion. The triggers for IPC are cellular agonists that are released from myocytes via pannexin-1/P2X7 channels [3] in response to brief ischemia. These agonists bind to G-protein coupled receptors initiating a protective response [4–6]. These include adenosine, bradykinin, and opioids [4, 5] and sphingosine-1-phosphate (S1P) [6]. S1P is an important intracellular signaling molecule that regulates diverse cellular events and has both cell growth and prosurvival effects [7–9]. These
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