Abstract
Brief periods of ischemia do not damage the heart and can actually protect against reperfusion injury caused by extended ischemia. It is not known what causes the transition from protection to irreversible damage as ischemia progresses. c-Jun N-terminal kinase-1 (JNK-1) is a stress-regulated kinase that is activated by reactive oxygen and thought to promote injury during severe acute myocardial infarction. Because some reports suggest that JNK-1 can also be protective, we hypothesized that the function of JNK-1 depends on the metabolic state of the heart at the time of reperfusion, a condition that changes progressively with duration of ischemia. Mice treated with JNK-1 inhibitors or transgenic mice wherein the JNK-1 gene was ablated were subjected to 5 or 20 min of ischemia followed by reperfusion. When JNK-1 was inactive, ischemia of only 5 min duration caused massive apoptosis, infarction, and negative remodeling that was equivalent to or greater than extended ischemia. Conversely, when ischemia was extended JNK-1 inactivation was protective. Mechanisms of the JNK-1 switch in function were investigated in vivo and in cultured cardiac myocytes. In vitro there was a comparable switch in the function of JNK-1 from protective when ATP levels were maintained during hypoxia to injurious when reoxygenation followed glucose and ATP depletion. Both apoptotic and necrotic death pathways were affected and responded reciprocally to JNK-1 inhibitors. JNK-1 differentially regulated Akt phosphorylation of the regulatory sites Ser-473 and Thr-450 and the catalytic Thr-308 site in vivo. The studies define a novel role for JNK-1 as a conditional survival kinase that protects the heart against brief but not protracted ischemia.
Highlights
When the heart is reperfused after a short period of ischemia, irreversible injury is avoided, and the consequences of ischemia reperfusion are limited to reversible defects of contractility known as myocardial stunning
Jun N-terminal kinase-1 (JNK-1) Protects against Acute Injury during Brief Ischemia-reperfusion—Multiple studies have shown that pharmacological inhibition of JNK-1 reduces myocardial infarction and apoptosis in animal models of acute myocardial infarction (AMI) [13, 21, 22, 24]
These results suggest that JNK-1 protects against brief but not extended ischemia
Summary
Acute Myocardial Infarction—Male C57B6 and JNK-1 (Ϫ/Ϫ) mice were from The Jackson Laboratory and were used at age 10 –12 weeks. MRI acquisitions employed an electrocardiogram-triggered gradient echo flow and compensated cine sequence to obtain orthogonal long axis images. Six to eight 1-mm-thick short axis slices were used to cover the entire heart, resulting in a total scan time of 30 – 45 min for each mouse at each imaging session. Total Akt kinase was assayed in lysates of isolated cardiac myocytes or left ventricle tissue using a PKB kinase assay kit (ImmuneChem Pharmaceuticals Inc., Burnaby, British Columbia, Canada). Cultured myocytes or snap-frozen left ventricles were solubilized in lysis buffer as described previously (59 – 61). Samples were subjected to Western blot analysis and probed with specific antibodies against the phospho-substrates as indicated. Antibodies were from Cell Signaling Technologies, Santa Cruz Biotechnology, or R&D Systems, Inc. Glucose and ATP were measured using kits from Sigma and PerkinElmer Life Sciences, respectively. The brightness function of Photoshop was used to improve visualization without changing the relative image density or resolution
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